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Nanoporous Titanium Oxynitride Nanotube Metamaterials with Deep Subwavelength Heat Dissipation for Perfect Solar Absorption

  • Morteza Afshar
    Morteza Afshar
    Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic
    Department of Physical Chemistry, Faculty of Science, Palacký University, 17. listopadu 1192/12, 779 00 Olomouc, Czech Republic
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  • , 
  • Andrea Schirato
    Andrea Schirato
    Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano, Italy
    Istituto Italiano di Tecnologia, via Morego 30, 16163, Genoa, Italy
    Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States
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  • , 
  • Luca Mascaretti
    Luca Mascaretti
    Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic
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    Orcidhttps://orcid.org/0000-0001-8997-7018
  • , 
  • S. M. Hossein Hejazi
    S. M. Hossein Hejazi
    Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic
    CEET, Nanotechnology Centre, VŠB-Technical University of Ostrava, 17 Listopadu 2172/15, Ostrava-Poruba 708 00, Czech Republic
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  • , 
  • Mahdi Shahrezaei
    Mahdi Shahrezaei
    Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic
    Department of Physical Chemistry, Faculty of Science, Palacký University, 17. listopadu 1192/12, 779 00 Olomouc, Czech Republic
    More by Mahdi Shahrezaei
  • , 
  • Giuseppe Della Valle
    Giuseppe Della Valle
    Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano, Italy
    Istituto di Fotonica e Nanotecnologie - Consiglio Nazionale delle Ricerche, Piazza Leonardo da Vinci, 32, I-20133 Milano, Italy
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    Orcidhttps://orcid.org/0000-0003-0117-2683
  • , 
  • Paolo Fornasiero
    Paolo Fornasiero
    Department of Chemical and Pharmaceutical Sciences, INSTM and ICCOM-CNR, University of Trieste, via L. Giorgieri 1, Trieste 34127, Italy
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    Orcidhttps://orcid.org/0000-0003-1082-9157
  • , 
  • Štěpán Kment
    Štěpán Kment
    Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic
    CEET, Nanotechnology Centre, VŠB-Technical University of Ostrava, 17 Listopadu 2172/15, Ostrava-Poruba 708 00, Czech Republic
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    Orcidhttps://orcid.org/0000-0002-6381-5093
  • , 
  • Alessandro Alabastri*
    Alessandro Alabastri
    Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States
    *E-mail: [email protected]
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    Orcidhttps://orcid.org/0000-0001-6180-8052
  • , and 
  • Alberto Naldoni*
    Alberto Naldoni
    Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic
    Department of Chemistry and NIS Centre, University of Turin, Turin 10125, Italy
    *E-mail: [email protected]
    More by Alberto Naldoni
    Orcidhttps://orcid.org/0000-0001-5932-2125
Cite this: ACS Photonics 2023, 10, 9, 3291–3301
Publication Date (Web):September 8, 2023
https://doi.org/10.1021/acsphotonics.3c00731

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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We report a quasi-unitary broadband absorption over the ultraviolet–visible–near-infrared range in spaced high aspect ratio, nanoporous titanium oxynitride nanotubes, an ideal platform for several photothermal applications. We explain such an efficient light–heat conversion in terms of localized field distribution and heat dissipation within the nanopores, whose sparsity can be controlled during fabrication. The extremely large heat dissipation could not be explained in terms of effective medium theories, which are typically used to describe small geometrical features associated with relatively large optical structures. A fabrication-process-inspired numerical model was developed to describe a realistic space-dependent electric permittivity distribution within the nanotubes. The resulting abrupt optical discontinuities favor electromagnetic dissipation in the deep sub-wavelength domains generated and can explain the large broadband absorption measured in samples with different porosities. The potential application of porous titanium oxynitride nanotubes as solar absorbers was explored by photothermal experiments under moderately concentrated white light (1–12 Suns). These findings suggest potential interest in realizing solar-thermal devices based on such simple and scalable metamaterials.

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 Special Issue

Published as part of the ACS Photonics virtual special issue “Photonics for Energy”.

Introduction

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Simple and cost-effective broadband absorbers are highly desirable for developing large-scale solar-thermal technologies. Perfect light absorbers based on metamaterials have emerged as a promising solution to achieve satisfactory energy harvesting by efficiently absorbing the energy of incoming electromagnetic waves. (1,2) Metamaterials are a class of engineered artificial materials typically consisting of arrays of subwavelength structures arranged in a periodic or nonperiodic manner. (3) The interaction of the electromagnetic waves with the subwavelength structures of metamaterials, also called nanoresonators, determines their behavior and enables nanoscale light energy localization. By properly designing morphological parameters, the material, and the periodicity of the nanoresonators, it has been possible to reach desired permittivity (ε) and therefore to produce broadband absorbers. (4−6) However, the main challenge in manufacturing metamaterials resides in the high cost, time-consuming, and intricate nature of the fabrication process. (7−9) Hence, broadband absorbers must be developed based on Earth-abundant materials and industrially scalable preparation methods.
Broadband absorbers can be achieved effectively using plasmonic metamaterials because they exhibit excellent light confinement capabilities, originating from the plasmonic resonance they support. (6,10−13) However, nanostructures made by the most conventional plasmonic materials (such as gold and silver) present limitations, such as low melting temperature, low chemical stability (in the case of silver), and high cost, which hinder their practical applications in plasmonic devices. (4,14,15) To overcome these drawbacks, refractory plasmonic materials based on transition metal nitrides, particularly titanium nitride (TiN), have recently emerged as potential candidates for plasmonic applications. (14) TiN has attracted significant attention due to its similar optical properties to gold, high thermal durability, chemical stability, mechanical hardness, adjustable permittivity, and good impedance matching to air within the visible range. (4,16−18) Notably, TiN or titanium oxynitride (TiOxNy, x + y ≤ 1) in numerous morphologies, such as TiN nanotube (NT) arrays, cylindrical nanocavities, and hollow nanospheres, can be readily synthesized through cost-effective thermal nitridation process of titanium dioxide (TiO2) in the presence of ammonia (NH3). (19−24) Moreover, Ti-based materials are abundantly available in the Earth’s crust, making them economical and sustainable candidates for large-scale fabrication.
Recent reports have confirmed that the 3D structure and morphology of TiO2 NT arrays with a high surface area can be effectively manipulated through the use of diverse electrolytes or anodization parameters, including voltage, time, and temperature, (25−28) allowing a fine control on the optical properties of NTs. (29,30) The distance between TiO2 NTs is a crucial factor in determining their photoelectrochemical performance, as spaced NTs featuring a higher air volume within the arrays exhibited greater light absorption compared to close-packed ones. (27,28,31) However, these works focused on optimizing the optical response of spaced (27,28) or close-packed (30) TiO2 NTs for water-splitting applications. A detailed optical analysis of plasmonic TiOxNy as a solar absorber material for photothermal applications is still missing.
In this work, titanium oxynitride nanotubes were experimentally and numerically studied as scalable broadband solar absorbers. The dimensional parameters and nanosized porosity were tuned by the synthesis process of electrochemical anodization and thermal nitridation in an ammonia atmosphere. The so-obtained nanotubes revealed exceptional broadband absorption capability, and numerical simulations showed that such nearly perfect absorption arise from the plasmonic character of the array and, crucially, from the presence of nanovoids in the nanotube walls introduced by the nitridation step. The fragmented electric permittivity associated with the nanotubes’ porosity induces intense hot spots and extremely localized electromagnetic dissipation within the porous regions, enabling large and broadband absorption. Such a heat dissipation mechanism cannot be captured through conventional effective medium theories, typically employed to reproduce large-scale optical effects associated with relatively small geometrical features.
To study such phenomena, we investigated a series of spaced TiOxNy NTs with different lengths (0.6–3.2 μm), density, and degrees of nitridation as broadband solar absorbers. TiO2 NTs were prepared by electrochemical anodization and subsequently converted to TiOxNy by thermal treatment under an NH3 atmosphere. Nanoporous TiOxNy NT arrays demonstrated nearly unitary broadband absorption, i.e., >99%, in the 300–1100 nm range. Numerical simulations revealed that such an effect was related to light-trapping phenomena in the nanovoids in the tube walls. Fine control of the anodization parameters and nitridation temperature directly affected the NTs geometry (such as length and spacing) and oxide/nitride fraction, thus leading to optimal light absorption. As a result, the photothermal properties of the so-obtained TiOxNy NTs were investigated by temperature measurements and solar steam generation experiments. In particular, under moderately concentrated light, the optimized nanotubes generated 235 °C and a solar-steam evaporation rate of 18 kg h–1 m–2. This work introduces the possibility of realizing cost-effective and scalable metamaterial absorbers based on titanium oxynitride that could be employed in large-scale or decentralized solar-thermal devices.

Results and Discussion

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The realization of NT-based broadband solar absorbers was first addressed by optimizing the tube lengths (1–6.8 μm) and diameter (85–245 nm) through anodization parameters. To this purpose, the effect of the applied voltage in the range of 20–60 V for 6 h was thoroughly studied, which led to highly spaced and aligned TiO2 NTs with a fairly regular arrangement (Figure S1). Such anodization parameters were chosen to ensure mechanical stability and a high morphological quality of the tube structure (Figure S2). The so-obtained TiO2 NTs exhibited a strong light absorption only for light energies >3.2 eV, i.e., higher than the electronic bandgap of TiO2 (Figure S3). In order to achieve perfect absorption, the NTs were thermally treated in air at 450 °C and then in NH3 at two different temperatures, i.e., 700 and 900 °C, which led to plasmonic TiOxNy NT arrays. Hereinafter, NTs anodized at 20, 25, and 30 V and nitridated at 700 °C will be labeled as samples #1, #2, and #3, while those anodized at 60 V and nitridated at 700 and 900 °C will be referred to as samples #4 and #5, respectively.
Figure 1a shows a schematic overview of the preparation of TiOxNy NTs, while Methods and Supporting Information provide a detailed description of the fabrication process and properties of as-anodized and nitridated NTs. As a result of the nitridation process, the NTs morphology revealed a high degree of porosity characterized by nanovoids distributed throughout the tube walls (Figure 1b). The formation of porosity in the nitridation process is initiated by the partial decomposition of NH3 into N2 and H2 gases at an elevated temperature. Subsequently, oxygen atoms in TiO2 are extracted by hydrogen, resulting in oxygen vacancies that can be replaced by nitrogen. In addition, the imbalance of diffusion rates of ions, i.e., the Kirkendall effect, leads to the formation of NTs with a high degree of porosity. (32,33) Since the size of these voids (between ∼3 and 40 nm) is significantly smaller than the wavelength of the incident light, they can play a crucial role in enhancing NTs absorption via plasmonic effects. (34) On the other hand, the unique properties of nanoporous materials enable them to reduce the permittivity of a mixture compared to its original material and can further minimize the permittivity difference at the air-absorber interface. As a result, an antireflection effect can be obtained, making nanoporous materials close to a blackbody absorber. (34−37)

Figure 1

Figure 1. Nanoporous TiOxNy NT arrays for broadband perfect absorption. (a) Schematic summary of morphological changes of NTs during conversion of TiO2 to TiOxNy. (b) SEM (left) and TEM (middle and right) images of NT arrays anodized at 60 V for 6 h and nitridated at 900 °C. (c) Sketch of the numerical model mimicking the NTs morphology, where the tube porosity is explicitly accounted for by creating a random space-dependent 3D map of permittivity, shown here, mixing voids and metal. The degree of porosity (i.e., metal content against voids) is controlled numerically by a threshold parameter (ths). (d) Simulated absorption spectra for an exemplary NT array on a Ti substrate by varying the threshold parameter ths, i.e., for increasing metal-to-void ratio. Numerical calculations considered the following geometrical parameters: outer diameter, 236 nm; wall thickness, 22.5 nm; length, 3.18 μm; array periodicity, 543 nm.

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A numerical model was developed to better understand the role of NTs’ porosity on their light absorption. For an accurate description of the light-matter interactions within the system, electromagnetic simulations employed an original approach to account for the peculiar nanometric features across individual NTs. Specifically, when considering the materials’ optical properties, a wavelength- and space-dependent map of permittivity εNT(λ,r) was associated with the NT numerical domain. Such a 3D map, defined as a combination of the permittivities of air (εAir = 1) and of the plasmonic material (TiN, taken from ref. (24)), was built numerically to exhibit spatial features over tens of nanometers, i.e., comparable to the typical size of the pores in the experimental structures. This strategy enabled de facto mimicking of the NT nanoporosity within the electromagnetic problem, by mixing voids and metal on a deep sub-wavelength scale across the tube wall. Moreover, the degree of porosity related to the metal-to-void volume ratio could be adjusted via a threshold parameter, ths. In formulas, by starting from a 3D normalized random pattern c(r) defined in each point r of the NT domain, the porous permittivity of the tube was assigned based on the value of ths as follows:
εNT(λ,r)={εTiN(λ)εairif⁡c(r)≥thsif⁡c(r)<ths
(1)
with εair (εTiN) the air (TiN) permittivity, so that setting ths = 0 provides a NT composed of air only, while ths = 1 corresponds to the case of a fully compact metallic structure. For intermediate values of ths, the corresponding metal-to-void volume ratio can be numerically computed based on the specific random function c(r) considered in the simulation and the NT total volume. Further details and technical aspects of implementing our approach are provided in Methods.
An exemplary set of porous permittivity maps εNT, featuring different degrees of porosity as a function of the threshold value, is displayed in Figure 1c. Despite being associated with the same NT numerical domain, such distributions describe substantially different light–matter interaction scenarios. The extreme case of ths = 0.1 provides a NT mostly made of air with nanometric fragments of metal, whereas εNT with ths = 0.9 is essentially equal to εTiN everywhere in the NT (the metal-to-void volume fraction for the considered tube is computed to be larger than 99%). Intermediate values of ths are associated with permittivity spatial distributions mixing the metal content and air, with the former increasing compared with the latter for increasing thresholds. Importantly, the numerical results of our model indicate that varying εNT, associated with different degrees of nanostructure porosity, dramatically impacts the optical response of the corresponding NT array. In particular, Figure 1d reports the absorption spectra computed over a broad spectral range (300–1500 nm) for a NT array with fixed geometrical parameters, where the porous permittivity of the individual tubes changes with ths according to the schematics depicted in Figure 1c. In these conditions, when the threshold is lower than 0.2, i.e., the NT is mainly made of air, a relatively flat absorption is obtained over the entire spectrum, ranging from 80% to values as low as 50%. On the other hand, when ths exceeds 0.7, and the metal content is much larger than the voids fraction, absorption is almost unitary at short wavelengths but abruptly drops by ∼20% close to 700 nm. At longer wavelengths, the numerical model predicts oscillations in absorption, to be possibly ascribed to a combination of longitudinal modes supported by the single NTs, featuring a pronounced aspect ratio (an outer diameter of 236 nm and a length of 3.18 μm are set in the simulations) and coupling modes between neighbors in the array. The most intriguing results, however, are obtained for thresholds close to 0.5 (corresponding to metal-to-void volume ratios approximately equal to 50%, depending on the specific simulated system). For those values, the computed absorption approaches unity over the entire spectral range under consideration. This observation indicates a non-negligible mixing of metal and air at the nanoscale as the key element to promote an almost unitary absorption system over a broad bandwidth. Additionally, the existence of an optimum for ths suggests that tailoring the degree of porosity across the NTs is pivotal in engineering similar perfect solar absorbers.
Apart from nanosized porosity resulting from the nitridation process and the significant impact of porosity on the light absorption behavior, various morphological and chemical characteristics were modified through the nitridation. The crystal structure of the as-prepared, annealed, and nitridated nanotubes was investigated by X-ray diffraction (XRD, Figure 2a). The XRD pattern of the as-prepared TiO2 nanotubes exhibited only the characteristic peak of metallic Ti originating from the substrate, thus confirming the amorphous nature of the as-anodized samples. Upon annealing at 450 °C, the nanotubes crystallized to the anatase phase with a limited rutile fraction (evidenced by a small peak at 2θ ≈ 32°). Subsequent nitridation at 700 °C promoted a partial phase transformation to TiOxNy, as suggested by the nearly fading peaks of TiO2 and Ti2O3 together with the onset of the pattern of the cubic Ti(O,N) structure (space group Fm3̅m). By increasing the nitridation temperature to 900 °C, the peaks associated with the oxide phases (TiO2 and Ti2O3) disappeared. Meanwhile, the peaks associated with the Ti(O,N) structure increased in intensity and shifted to lower diffraction angles and additional reflections appeared, which could be associated with the tetragonal Ti2N phase. This is further illustrated in Figure 2b, which clearly shows the shift for the (111) and (200) peaks with the nitridation temperature. Such an observation was further confirmed by retrieving the lattice parameter of the nitridated samples using Bragg’s law for the cubic crystal systems (Table S1). In particular, the average lattice parameter increased from ∼4.18 to ∼4.23 Å for the samples nitridated at 700 and 900 °C, respectively (see the Supporting Information for further details). These results, therefore, suggest the increase of nitrogen content with the annealing temperature. For the sake of simplicity and because a residual degree of oxidation could not be ruled out, the nitridated nanotubes are termed TiOxNy, regardless of the nitridation temperature.

Figure 2

Figure 2. Impact of thermal treatment on chemical composition of NTs. (a) X-ray diffraction patterns of (blue) as-prepared and (gray) air-annealed TiO2 nanotube arrays and nitridated samples at different temperatures of (red) 700 and (green) 900 °C. (b) A magnified section of the patterns that emphasizes the shift of the (111) and (200) crystallographic orientations by increasing the nitridation temperature.

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The combination of chemical and morphological alterations can significantly impact the optical properties of NT arrays (Figure 3). In particular, upon nitridation at 700 °C, the overall morphology of NTs was preserved (Figures 3a–d and S4a–c), while a shrinkage in diameter and length was observed because of a sintering effect (Figure 3f) in contrast to that of their as-anodized counterparts (Figure S1j). For instance, the average length and diameter of the as-anodized NTs at 60 V decreased from 6.8 μm and 245 nm to 3.3 μm and 234 nm, respectively, after nitridation at this temperature. Moreover, TEM images revealed a roughening effect at the NT wall with numerous nanopores at the upper part of the NTs wall, which can be attributed to diffusion kinetics of the O and N atoms in the nitridation treatment, while the same did not occur at the lower part of the NT wall (Figure S4c).

Figure 3

Figure 3. Morphology and absorption spectra of NT arrays. (a–e) Cross-sectional SEM images and sketches of the geometries (the geometrical parameters considered in the simulations are set based on SEM analysis of the samples) simulated in the numerical model mimicking the experimental TiOxNy NT arrays (threshold value set to 0.5 in calculations) anodized for 6 h at (a) 20, (b) 25, (c) 30, and (d, e) 60 V and nitridated at (a–d) 700 and (e) 900 °C. (f) Effects of the applied potential and nitridation temperature on the average diameter (right-hand side axis) and length (left-hand side axis) of NT arrays. (g, h) Experimental and calculated optical absorption of NTs, respectively.

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On the other hand, nitridation at 900 °C did not affect the overall NTs morphology, but produced a slight bend of the tubes toward each other (Figures 1b and 3e). In addition, the degree of reduction in the average diameter of NTs was higher compared to those nitridated at 700 °C, and it shrank from 245 nm in as-anodized NTs to 207 nm in the sample nitridated at 900 °C (Figure 3f). The average length of NTs experienced a slight reduction in comparison with samples nitridated at 700 °C. Also, the NTs’ surface became entirely porous wherein the upper region demonstrated a greater level of porosity in comparison to the lower region (Figure 1b) and with bigger pores than nitridation at 700 °C (Figure S4c).
The experimental optical characterization (Figure 3g) of samples #5, #4, and #3 demonstrates broadband and almost unitary absorption in the wavelength region 300–1100 nm. Such a broad absorption bandwidth can be attributed to various factors, including NTs porosity, length and diameter (density) of NTs, and the length of the light pathway inside the NTs. The NT walls exhibit a gradual decrease in porosity (Figure S4c) and a concurrent increase in thickness (e.g., sample #4, with a thickness range of 30 to 110 nm) from top to bottom. This morphological variation results in a slight increase in the permittivity of NT arrays from the upper to lower portions and a further antireflection effect. (38,39) In addition, after light passes through the air-absorber interface without reflection, nanovoids in the NT arrays allow for significant plasmonic field confinement in deep subwavelength volumes, promoting efficient electromagnetic dissipation.
Absorption is also noticeably influenced by the length and diameter (i.e., density) of the NTs. Generally, in addition to porosity, the fact that (i) NTs exhibit different lengths and especially diameters (hence, changing the center-to-center distances) in each specific sample (e.g., Figure 1b); and (ii) periodicity is broken on a large scale, i.e., NTs are randomly arranged within the array, leads to the scrambling of resonances and consequently the occurrence of broadband absorption spectra. (40−42) Specifically, shorter NTs (e.g., samples #1 and #2) displayed high absorption properties at shorter wavelengths, whereas longer NTs extended absorption effectively at longer wavelengths. Longer NTs exhibited significantly lower reflectance and a flatter spectral profile over the entire spectral range than their shorter counterparts, making them a promising candidate for achieving near-unity absorption across the whole solar emission range of wavelengths. This is because the domain extension among the NTs is an additional factor to enhance absorption, which increases with increasing length of NTs. In other words, the multiple scattering of light between and inside of NTs and across the nanosized porous NT network, which significantly prolongs the optical path and increases the possibility of light absorption, leads to ultrahigh absorption. (10,12,38) While in shorter NTs, light energy can be reflected from the interface between the NTs and substrate because of a lack of sufficient light pathway and inadequate light dissipation.
Additionally, it was observed that the absorption of a sample decreased as the diameter of NTs reduced, and the sample with the shortest diameter or highest density of NTs, #1, demonstrated the lowest absorption. This effect can be attributed to the lower air density through the regions between NTs and the higher permittivity difference at the interface of NT arrays and air. The broadband absorption resulting from the combination of these factors gives an even higher absorption compared to previous reports on TiN nanoarrays. (4,43,44) For example, a 94% absorption was attained in ref (45) by coating TiN on Si nanopillars and optimizing the effect of geometrical parameters of the nanostructure arrays.
To support the experimental observations, the model previously introduced was employed to compute the optical response of the different samples, and the average geometrical parameters estimated from the SEM analysis were first used to build the numerical NT domain for each of them.
The approach outlined before to create porous optical properties across the NT was then applied to model the electromagnetic interaction with the nanostructures sketched in Figure 3a–e (right panels), which show the considered single porous NT and the corresponding array for the five samples. The computed absorption spectra for all of the samples, reported in Figure 3h, reasonably agree with the experimental findings (Figure 3g). Almost perfectly flat absorption is obtained over the whole spectrum for the five nanostructure arrays. The trend with varying NT dimensions is also qualitatively retrieved by the numerical results, capturing the decrease in absorption at longer wavelengths for sample #1. The observed mismatch with experiments (in particular, the oscillations across the simulated spectra, absent in the measured ones) is expected to be due to the fact that models consider an infinite, perfectly periodic array of precisely identical NTs, thus neglecting the size dispersion and random spatial arrangement of the nanostructures within the experimental arrays.
Furthermore, our model, termed nanoporous medium theory (NPMT), ascertained the origin of such a high broadband absorption, rationalized in terms of the nanometric discontinuities introduced by porosity in the tube permittivity. To show that, the results of our simulations were compared with the outcome of similar calculations using effective medium theory (EMT) to account for nanoporosity across the tube. Specifically, the electromagnetic problem was solved by pursuing either our NPMT modeling approach or an EMT-based one (implementing Bruggeman formulas, (46) see Methods) for the geometrical parameters of sample #4 and values of the threshold equal to 0.3, 0.5, and 0.7 as illustrative cases. Figure 4 summarizes the main results of such a comparison for an exemplary wavelength of 532 nm. Specifically, for each value of ths, Figure 4a,b displays, from left to right, (i) the permittivity (imaginary part) ε′′, (ii) the electric field enhancement |E|/E0, and (iii) the dissipated power density Qdiss across an individual NT. While NPMT (Figure 4a) explicitly accounts for the NT porosity through the nanometric features of εNT, the EMT-based description (Figure 4b) assigns to the NT an effective permittivity, neglecting any nanoscale discontinuity. This results in a homogeneous permittivity (see the left insets for each threshold in Figure 4b), weighted over the relative content of the plasmonic material. Contrary to EMT, the description proposed in Figure 4a introduces pronounced and highly localized field enhancements over spatial scales much shorter than the geometrical size of the NT (compare the middle insets for each threshold in Figure 4a,b). Such localized features induce a scrambling effect of the tube resonant mode patterns and increase the electromagnetic dissipation in deeper regions along the height of the nanostructure itself (compare the right insets for each threshold in Figure 4a,b).

Figure 4

Figure 4. Modeling porosity across NTs: failure of the effective medium theory. (a) From left to right, a 3D map of a single NT permittivity (imaginary part, ε′′), the spatial distribution of electric field enhancement factor (|E|/E0), and dissipation power density (Qdiss) calculated at a representative wavelength of 532 nm by varying the threshold parameter from 0.3 (left), 0.5 (middle), to 0.7 (right). Numerical results refer to sample #4 (outer diameter 293 nm, wall thickness 30.5 nm, length 3.3 μm, array periodicity 586 nm). (b) Same as (a) when effective medium theory (EMT) is employed in the simulations to define an effective uniform permittivity across the individual NT, mixing air and TiN permittivities. EMT-based calculations considered the metal-to-void ratio set by the threshold value used in the corresponding simulations in (a). (c) Numerically computed absorption spectra for the exemplary NT array considered (geometrical parameters for sample #4) for varying values of the threshold ths, by applying the original modeling approach here proposed (NPMT, solid curves) and EMT (Bruggeman formalism, dotted curves), respectively.

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Besides discrepancies in the NT local optical properties, the two modeling approaches also predict substantially different global nanostructure responses. In particular, Figure 4c reports the simulated absorption spectra for the three threshold values of porosity considered (ths = 0.3, 0.5, and 0.7, corresponding to metal-to-void volume ratios of ∼4.7%, 51%, and 95%) when either NPMT (solid lines) or EMT (dotted) is employed. The most pronounced differences between the two models’ results are observed for the lowest degree of porosity (ths = 0.3, light blue curves). Our approach predicts a relatively flat (besides a slight dip around 450 nm) spectrum and values for absorption larger than 80% over the entire bandwidth. Conversely, EMT provides a spectrum featuring a peak at ∼500 nm, abruptly decaying around ∼600 nm and reaching values as low as 50% at longer wavelengths. According to the effective description of Bruggeman formulas, such an optical response is that of an array of NTs with homogeneous metal-like permittivity, the plasmonic character of which is rather poor (ε′ is small), given the large fraction of air present. On the contrary, when the porous system is predominantly made of metal (i.e., for ths = 0.7 when the air content in the porous NT is <5%, purple lines), the two modeling approaches lead to almost identical results. Given the small number of voids in the permittivity map, our porous NT is essentially equivalent to a compact tube with optical properties close to those of pristine TiN (compare ε′′ in the left insets for ths = 0.7 in Figure 4a,b). As such, the obtained spectra exhibit identical features: an almost unitary absorption until ∼600 nm and resonant modes providing oscillations excited for longer wavelengths (Figure 1d). Finally, for the intermediate value of ths = 0.5 (yellow curves), although the two simulations provide a similar high absorption until ∼1000 nm, the EMT-based spectrum drops at longer wavelengths, decreases to 80% and oscillates. The same does not occur when NPMT is used, leading to a quasi-unitary absorption up to 1500 nm. Based on this comparison, performed for the optimal threshold value used in Figure 3, we conclude that a porous εNT with nanoscale discontinuities is required to reproduce the broadband quasi-perfect absorption observed experimentally.
The exceptional absorption properties of TiOxNy NTs make them ideal for photothermal applications. In this context, the photothermal behavior of NT arrays was investigated (Figure 5) by employing an infrared (IR) sensor owing to its noncontact nature and high accuracy. (47) For this purpose, the heat generation of each sample was determined by placing the IR sensor on the backside of the sample while it was being illuminated with white light from an LED source (emission spectrum reported in Figure S5a) at different intensities, i.e., 1–12 Suns, where 1 Sun = 100 mW cm–2 (Figure 5a). By measuring the temperature of the samples over a period of time (2 min) by turning on and off the LED lamp, heating and cooling curves were generated. An example of heating and cooling curves is shown for sample #5 in Figure 5b. The sample temperature rose within ∼15 s, reached a steady temperature after ∼60 s, and quickly decreased when the light was switched off. As expected, the temperature measured under irradiation increased with an increase in the incident light intensity. The maximum temperature (Tmax) recorded for each sample was extracted from the corresponding heating–cooling curve at different irradiation intensities after 60 s of irradiation (Figure S5b). Figure 5c displays the results of this analysis under illumination at 12 Suns in comparison to the spectrally averaged solar absorptance (α). The α is a fundamental parameter in determining the fraction of incident solar radiation that is absorbed by NT arrays and can be calculated by using the following equation:
α−=∫300nm1100nmA(λ)Ss(λ)dλIsolar
(2)
in which A(λ) is the absorptance, and the spectral solar irradiance (AM 1.5G) is represented by Ss(λ), the total irradiance by Isolar, and the integration is performed over the experimentally measured wavelength range, i.e., 300–1100 nm. The results of this calculation (Figure 5c) indicate that all five samples demonstrated significantly high levels of solar absorption with values of α as high as 0.96. The highest value of α, greater than 0.99, was achieved in the case of NTs anodized at 30 and 60 V (samples #3, #4, and #5), and the value of this parameter gradually reduced as the anodization voltage decreased, reaching a minimum of 0.96 for sample #1, featuring NTs with the shortest length. On the other hand, these results also demonstrate that sample #5 generated the highest temperature compared to the other samples. This is likely due to its chemical composition and high absorption. Moreover, longer TiOxNy NTs showed a higher temperature increase than their shorter counterparts, which can be ascribed to their higher surface area and higher light absorption.

Figure 5

Figure 5. Photothermal applications of NT arrays. (a) The schematic setup for temperature measurement via an IR sensor. (b) Heating–cooling cycles of #5 NT arrays, anodized at 60 V for 6 h and nitridated at 900 °C, under different irradiation intensities of an LED lamp. (c) (gray, left-hand side axis) Maximum temperature (Tmax) extracted from the heating and cooling curves of different samples under 12 Suns irradiation of an LED lamp in comparison with the averaged solar absorptance of NTs (blue, right-hand side axis). (d) The schematic diagram of the steam generation setup. (e) Comparison of solar steam generation performance among sample #5, Ti plate, and water.

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One of the most promising applications of photothermal materials is the solar steam generation, which enhances the evaporation of water that occurs naturally under solar light by employing a photothermal material. (48,49) The so-obtained water vapor can be collected into clean water, thus ensuring a simple approach to water desalination, purification, or distillation, even in remote locations. The solar steam generation performance of the samples was investigated by employing a custom setup (Figure 5d). (17) Briefly, in each experiment, the sample was enclosed within a polytetrafluoroethylene (PTFE) cell together with 800 μL of water and a thermocouple, and the PTFE cell was placed on a high-precision balance and illuminated under solar-simulated light at different intensities (1.4–14 Suns) for 25 min to monitor the mass change of water (see Methods for additional details). In the absence of illumination, no measurable water evaporation occurred, while at high irradiation conditions, a substantial steam flow was continuously observed at the water surface (see Figure S6), except for water-only conditions. The presence of TiOxNy NTs at the bottom of the water volume accelerated the kinetics of its evaporation, as also occurred by increasing the light intensity for a specific experimental condition, as expected (Figure S7). Such an observation clearly confirmed the photothermal effect of TiOxNy NTs.
The performance in solar steam generation experiments can be evaluated from weight change data by computing the evaporation rate ṁ (kg m–2 h–1) based on this equation:
m˙=ΔmΔt
(3)
where Δm refers to the mass change of water during a specific time interval Δt. Figure 5e presents an overview of the recorded evaporation rates for sample #5 and Ti foil, as well as for pure water. Sample #5 was chosen as a representative case that exhibited the highest temperature among all investigated nanotubes (Figure 5c). The water evaporation rate for all of the other samples was, indeed, slightly lower than that of sample #5 (Figure S8). The results (Figure 5e) indicate a negligible enhancement of the water evaporation rate for all samples at lower irradiation intensities, while at higher intensities the evaporation rate increased with a nonlinear trend. Such a nonlinear increase may be rationalized in terms of the exponential growth of light penetration through the bulk water on top of the NTs, and consequently higher conversion of light to heat and to the exponential increment of water vapor saturation pressure with temperature. (50) Therefore, a synergy of these effects could increase the kinetics of water evaporation. In particular, the lowest evaporation rate was found for the water-only conditions (4.97 kg m–2 h–1 at 14 Suns), while the maximum value (18.20 kg m–2 h–1 at 14 Suns) was observed for sample #5, i.e., 3.66 and 2.13 times higher than that of pure water and Ti foil, respectively. In addition, it is also higher than the evaporation rate obtained in our previous study on TiN nanocavity arrays (∼15 kg m–2 h–1). (17) Overall, the results obtained from different solar-thermal experiments, presented in Figure 5, suggest the possibility of employing TiOxNy NTs as scalable photothermal materials in various technologies.

Conclusions

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In summary, TiOxNy NT arrays were experimentally and numerically investigated as promising broadband solar absorbers. The dimensional parameters of the NTs were explored by tuning the conditions of electrochemical anodization and post-synthesis thermal treatments in NH3 atmosphere. Nearly unitary absorption over a wide wavelength range (300–1100 nm) was achieved by a combination of morphological features of the tubes, i.e., optimized length/diameter and nanosized porosity, and the plasmonic nature of TiOxNy. An original modeling approach was developed to rationalize such a high and broadband absorption, enabling us to accurately mimic the porous features of NTs at the nanometric scale. By numerically creating a map of permittivity mixing voids and metal across the individual NTs of the array, high and localized field enhancements were predicted on spatial scales much shorter than the geometrical size of the tubes, scrambling resonances, and promoting broadband electromagnetic absorption.
Contrary to the conventional description of porous media based on effective medium theories, our simulations reproduced the experimental observations, suggesting new insights into the light–matter interactions in porous nanostructures. Moreover, solar-thermal experiments demonstrated that longer TiOxNy NTs generated higher temperatures and more steam under different irradiation intensities, ascertaining their potential as efficient photothermal materials. These findings propose the development of a simple and cost-effective metamaterial with near-unity absorption and demonstrate the significance of TiOxNy NTs as promising candidates for various technological applications.

Methods

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Sample Preparation

Spaced TiO2 NTs were prepared by electrochemical anodization in a two-electrode electrochemical cell, where a Pt sheet served as the counter electrode and a Ti foil (0.25 mm thick) was used as the working electrode. First, a commercial Ti foil was ultrasonically cleaned and degreased in acetone, ethanol, and distilled water for about 15 min successively and then dried under a nitrogen flow. Second, the foil was anodized in diethylene glycol (DEG) electrolyte consisting of 0.5 wt % NH4HF2 and 3.6 wt % H2O at different direct current (DC) voltages of 20, 25, and 30 V for 6 h at 50 °C as well as 60 V for 6 h at 40 °C. After anodization, the sample was completely washed with ethanol and water to remove the residual electrolyte and then dried in the nitrogen stream. Later on, in order to prepare crystalline TiO2 NTs, the NTs were annealed in air at 450 °C for 2 h by heating and cooling ramp of 2 °C/min. Subsequently, the NTs were also annealed under NH3 flow (10 mL/min) for 1 h at 700 and 900 °C (heating and cooling ramp of 5 °C/min) to obtain TiOxNy NTs, and the obtained samples are denoted as #1, #2, and #3 to refer to samples that underwent anodization at 20, 25, and 30 V and nitridation at 700 °C, while labels #4 and #5 were used to refer samples anodized at 60 V and nitridated at 700 and 900 °C, respectively.

Characterization

The morphology of the samples was characterized by scanning electron microscopy (SEM, Hitachi FE-SEM 4800) and transmission electron microscopy (TEM JEOL 2010). The crystalline structure was investigated by an X-ray diffractometer (XRD, PANalytical, Almelo, The Netherlands) with Co–Kα radiation (λ = 0.179 nm) operated in a Bragg–Brentano geometry. The optical properties were investigated by reflectance measurements with a Specord250 Plus spectrometer equipped with an integrating sphere (Analytik Jena GmbH, Germany) in the 300–1100 nm range and with a vacuum Fourier-transform infrared (FTIR) Vertex 80v spectrometer in the 1330–25000 nm range. The absorptance (A) was retrieved by reflectance (R) spectra as A = 1 – R by neglecting the transmittance (T) due to the opaque nature of Ti substrates in all the investigated spectral ranges.

Temperature Measurement with IR Sensor

In order to accurately measure the temperature of the samples under irradiation, Fourier-transform infrared (FTIR) spectroscopy (Vertex 80v spectrometer, Bruker) was used to measure the samples back-surface reflectance at room temperature, which was then converted to emissivity by Kirchhoff law and averaged over the wavelength range corresponding to the IR sensor sensitivity range (8–14 μm; see Table S2). The samples were illuminated from the front side with a LED lamp (see the emission spectrum in Figure S5a), while an IR sensor (Omega OS-MINI802-D-C4) was placed at a 10 cm distance from the backside of the samples, thus allowing a noncontact measurement of the temperature. The samples were irradiated under different intensities (1–12 Suns, 1 Sun = 100 mW cm–2) by focusing the LED light with two lenses (i.e., a fused silica plano-convex lens, Thorlabs LA4984, and a Fresnel lens, Thorlabs FRP251). The light intensity was measured by a thermopile detector (Standa 11UP19K-30 H-H5) before each experiment.

Water Evaporation Experiments

The solar-to-heat performance was evaluated by water evaporation experiments under solar-simulated light. The samples were placed within a thermally insulating polytetrafluoroethylene (PTFE) cell made of two parts to firmly hold the Ti substrate on which the NTs were grown. The top part of the cell had a cylindrical through-hole with a 1 cm diameter acting as a water reservoir and allowing illumination of the sample surface. A type K thermocouple (RS PRO, 0.075 mm diameter) was positioned on the top surface of the sample and the cell was placed on a high-precision electronic scale (Kern and Sohn GmbH, 0.1 mg accuracy). Afterward, 800 μL (800 mg) of deionized water was poured into the reservoir, and the cell was illuminated with a 1000 W solar simulator (Sciencetech A4 Lightline C250) equipped with an AM 1.5G filter and a Fresnel lens to focus the light to a circular spot, which matched the hole diameter of the top part of the PTFE cell (1 cm). Prior to experiments, the light power was measured with a thermopile detector (Standa 11UP19K-30 H-H5). Each water evaporation test for NT samples and pure water (without NT sample and as a comparison) was carried out with three replications by monitoring weight change during 25 min under different light illumination intensities from 1.4 to 14 Suns (1 Sun = 100 mW cm–2).

Numerical Modeling

The numerical results presented in the main text have been obtained by employing a Finite Element Method (FEM)-based commercial software (COMSOL Multiphysics 6.0) to develop a three-dimensional (3D) model of the porous NTs under investigation. Full-vectorial electromagnetic simulations have been performed to determine the optical response of each system, treated as an infinite, perfectly periodic array of NTs lying on a Ti substrate and embedded in air. Maxwell’s equations are solved across the array unit cell, the geometrical parameters of which (that is, the NT length, external radius, and wall thickness along with the center-to-center distance between neighbors, assuming a squared lattice) were estimated from SEM analysis of the five experimental samples. A monochromatic linearly polarized plane wave at normal incidence has been considered, and ports formalism in the frequency domain has been employed to compute the electromagnetic behavior of the NT array in the spectral range between 300 and 1100 nm. Floquet periodic boundary conditions (BCs) were set at the lateral sides of the unit cell to simulate an infinite array, and perfectly matched layers (PMLs, with scattering BCs beyond such domains) were defined at the top and bottom of the numerical geometry to avoid spurious reflection effects and ensure the vanishing of the electric field. An air refractive index has been considered as unitary, while Ti permittivity was taken from ref (51). In modeling the porous titanium oxynitride NTs, the space-dependent 3D permittivity εNT introduced in the main text (see eq 1) was built, with TiN permittivity from ref (24) associated with the metallic phase, and air properties associated with voids. The content of O, alongside the small presence of TiO2 and Ti2O3 in the NT material, was neglected in defining the metal content permittivity upon the assumption that TiN gives the prominent contribution to the plasmonic character of the tubes. To numerically create the nanoporosity of the permittivity map, εNT, a fictive stationary diffusion problem was set across the NT numerical domain, built as a regular tube-shaped geometry. This was done by means of the Transport of Diluted Species module embedded in COMSOL, to be solved upstream, before the electromagnetic study. The solver computes a concentration field c fulfilling the equation ∇c = 0, where a spatial gradient is created by imposing fixed concentrations at the outer and inner surfaces of the tube. These concentrations are defined, respectively, as a spatial random function, R(x,y,z), varying between 0 and 1 with uniform probability, and its complementary, 1 – R(x,y,z). These BCs enforce diffusion to occur also in the radial direction across the structure. A no-flux BC is imposed at the top and bottom surfaces of the NT. Solving for such a diffusion problem provides with a 3D random pattern ranging between 0 and 1 encoded in the concentration c, to be used to create the permittivity map for the electromagnetic study featuring deep subwavelength porosity. To tailor the degree of porosity given a threshold parameter value ths, the corresponding permittivity across the porous NT is defined according to eq 1 from the main text. Importantly, since such porosity of the permittivity is built from the numerical solution of the transport problem defined for variable c, the degree of porosity (i.e., the pore size) is directly controlled by the mesh of the model. A fine mesh was thus built across the nanoporous NT, with 40 edge elements along the perimeter of the tube transverse sections and a 20 nm maximum element size (comparable to the experimental pore size) along the longitudinal tube dimension.
Besides such an original approach, as mentioned in comments on Figure 4 from the main text, Effective Medium Theory (EMT) was employed to analyze the role of nanoporosity. The Bruggeman formalism was implemented, (46) providing εNT as a homogeneous effective permittivity, written as a combination of the optical properties of the plasmonic phase (TiN) and those of air, weighted over the metal-to-void volume ratio f (corresponding therefore to a volume fraction of air of 1 – f). In formulas, εNT solves the following equation: (46)
0=fεTiN−εNTεTiN+2εNT+(1−f)εair−εNTεair+2εNT

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphotonics.3c00731.

  • Detailed synthesis procedure, SEM and TEM images, dimensional parameters, and optical properties of TiO2 NTs; Additional details of nitridation procedure and lattice parameters of nitridated NTs; SEM and TEM images of NTs nitridated at 700 °C; Spectrum of the white LED; Additional details of photothermal experiments; Emissivity of nitridated NTs (PDF)

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Author Information

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  • Corresponding Authors
    • Alessandro Alabastri - Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States;  Orcidhttps://orcid.org/0000-0001-6180-8052;  Email: [email protected]
    • Alberto Naldoni - Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic;  Department of Chemistry and NIS Centre, University of Turin, Turin 10125, Italy;  Orcidhttps://orcid.org/0000-0001-5932-2125;  Email: [email protected]
  • Authors
    • Morteza Afshar - Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic;  Department of Physical Chemistry, Faculty of Science, Palacký University, 17. listopadu 1192/12, 779 00 Olomouc, Czech Republic
    • Andrea Schirato - Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano, Italy;  Istituto Italiano di Tecnologia, via Morego 30, 16163, Genoa, Italy;  Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States
    • Luca Mascaretti - Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic;  Orcidhttps://orcid.org/0000-0001-8997-7018
    • S. M. Hossein Hejazi - Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic;  CEET, Nanotechnology Centre, VŠB-Technical University of Ostrava, 17 Listopadu 2172/15, Ostrava-Poruba 708 00, Czech Republic
    • Mahdi Shahrezaei - Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic;  Department of Physical Chemistry, Faculty of Science, Palacký University, 17. listopadu 1192/12, 779 00 Olomouc, Czech Republic
    • Giuseppe Della Valle - Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano, Italy;  Istituto di Fotonica e Nanotecnologie - Consiglio Nazionale delle Ricerche, Piazza Leonardo da Vinci, 32, I-20133 Milano, Italy;  Orcidhttps://orcid.org/0000-0003-0117-2683
    • Paolo Fornasiero - Department of Chemical and Pharmaceutical Sciences, INSTM and ICCOM-CNR, University of Trieste, via L. Giorgieri 1, Trieste 34127, Italy;  Orcidhttps://orcid.org/0000-0003-1082-9157
    • Štěpán Kment - Czech Advanced Technology and Research Institute, Regional Centre of Advanced Technologies and Materials Department, Palacký University Olomouc, Šlechtitelů 27, Olomouc 78371, Czech Republic;  CEET, Nanotechnology Centre, VŠB-Technical University of Ostrava, 17 Listopadu 2172/15, Ostrava-Poruba 708 00, Czech Republic;  Orcidhttps://orcid.org/0000-0002-6381-5093
  • Author Contributions

    M.A. and A.S. contributed equally to this paper.

  • Funding

    A.N. acknowledges support from the Project CH4.0 under the MIUR program “Dipartimenti di Eccellenza 2023–2027” (CUP: D13C2200352001). M.A., L.M., A.N., and S.K. gratefully acknowledge the support from the Operational Programme Research, Development and Education - European Regional Development Fund, Project No. CZ.02.1.01/0.0/0.0/15_003/0000416 and the funding from Czech Science Foundation, Project GACR – EXPRO, 19-27454X. FTIR experiments were performed at the CzechNanoLab Research Infrastructure supported by the Ministry of Education, Youth and Sport of the Czech Republic (MEYS CR, Grant LM2023051). A.S. and G.D.V. acknowledge support from the METAFAST project that received funding from the European Union Horizon 2020 Research and Innovation Program under Grant Agreement No. 899673. This work reflects only the author’s view, and the European Commission is not responsible for any use that may be made of the information it contains.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors express their gratitude to E. Ioannou and J. Hošek for conducting SEM analyses, J. Stráská for performing TEM measurements, and I. Medřík for carrying out the nitridation process.

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    Plasmonic and metamaterial structures as electromagnetic absorbers
    Cui, Yanxia; He, Yingran; Jin, Yi; Ding, Fei; Yang, Liu; Ye, Yuqian; Zhong, Shoumin; Lin, Yinyue; He, Sailing
    Laser & Photonics Reviews (2014), 8 (4), 495-520CODEN: LPRAB8; ISSN:1863-8880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Electromagnetic absorbers have drawn increasing attention in many areas. A series of plasmonic and metamaterial structures can work as efficient narrowband absorbers due to the excitation of plasmonic or photonic resonances, providing a great potential for applications in designing selective thermal emitters, biosensing, etc. In other applications such as solar-energy harvesting and photonic detection, the bandwidth of light absorbers is required to be quite broad. Under such a background, a variety of mechanisms of broadband/multiband absorption have been proposed, such as mixing multiple resonances together, exciting phase resonances, slowing down light by anisotropic metamaterials, employing high loss materials and so on.
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  7. 7
    Bilal, R. M. H.; Saeed, M. A.; Choudhury, P. K.; Baqir, M. A.; Kamal, W.; Ali, M. M.; Rahim, A. A. Elliptical Metallic Rings-Shaped Fractal Metamaterial Absorber in the Visible Regime. Sci. Rep. 2020, 10 (1), 14035,  DOI: 10.1038/s41598-020-71032-8
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    7
    Elliptical metallic rings-shaped fractal metamaterial absorber in the visible regime
    Bilal, R. M. H.; Saeed, M. A.; Choudhury, P. K.; Baqir, M. A.; Kamal, W.; Ali, M. M.; Rahim, A. A.
    Scientific Reports (2020), 10 (1), 14035CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
    Achieving the broadband response of metamaterial absorbers has been quite challenging due to the inherent bandwidth limitations. Herein, the investigation was made of a unique kind of visible light metamaterial absorber comprising elliptical rings-shaped fractal metasurface using tungsten metal. It was found that the proposed absorber exhibits av. absorption of over 90% in the visible wavelength span of 400-750 nm. The features of perfect absorption could be obsd. because of the localized surface plasmon resonance that causes impedance matching. Moreover, in the context of optoelectronic applications, the absorber yields absorbance up to ∼ 70% even with the incidence obliquity in the range of 0°-60° for transverse elec. polarization. The theory of multiple reflections was employed to further verify the performance of the absorber. The obtained theor. results were found to be in close agreement with the simulation results. In order to optimize the results, the performance was analyzed in terms of the figure of merit and operating bandwidth. Significant amt. of absorption in the entire visible span, wide-angle stability, and utilization of low-cost metal make the proposed absorber suitable in varieties of photonics applications, in particular photovoltaics, thermal emitters and sensors.
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  8. 8
    Kenney, M.; Grant, J.; Shah, Y. D.; Escorcia-Carranza, I.; Humphreys, M.; Cumming, D. R. S. Octave-Spanning Broadband Absorption of Terahertz Light Using Metasurface Fractal-Cross Absorbers. ACS Photonics 2017, 4 (10), 2604– 2612,  DOI: 10.1021/acsphotonics.7b00906
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    8
    Octave-Spanning Broadband Absorption of Terahertz Light Using Metasurface Fractal-Cross Absorbers
    Kenney, Mitchell; Grant, James; Shah, Yash D.; Escorcia-Carranza, Ivonne; Humphreys, Mark; Cumming, David R. S.
    ACS Photonics (2017), 4 (10), 2604-2612CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Synthetic fractals inherently carry spatially encoded frequency information that renders them as an ideal candidate for broadband optical structures. Nowhere is this more true than in the terahertz (THz) band where there is a lack of naturally occurring materials with valuable optical properties. One example are perfect absorbers that are a direct step towards the development of highly sought after detectors and sensing devices. Metasurface absorbers that can be used to substitute for natural materials suffer from poor broadband performance, while those with high absorption and broadband capability typically involve complex fabrication and design and are multilayered. Here, we demonstrate a polarization-insensitive ultrathin (∼λ/6) planar metasurface THz absorber composed of supercells of fractal crosses capable of spanning one optical octave in bandwidth, while still being highly-efficient. A sufficiently thick polyimide interlayer produces a unique absorption mechanism based on Salisbury screen and anti-reflection responses, which lends to the broadband operation. Exptl. peak absorption exceeds 93%, while the av. absorption is 83% from 2.82 THz to 5.15 THz. This new ultrathin device architecture, achieving an absorption-bandwidth of one optical octave, demonstrates a major advance towards a synthetic metasurface blackbody absorber in the THz band.
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  9. 9
    Nguyen, T. Q. M.; Nguyen, T. K. T.; Le, D. T.; Truong, C. L.; Vu, D. L.; Nguyen, T. Q. H. Numerical Study of an Ultra-Broadband and Wide-Angle Insensitive Perfect Metamaterial Absorber in the UV-NIR Region. Plasmonics 2021, 16 (5), 1583– 1592,  DOI: 10.1007/s11468-021-01424-7
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    9
    Numerical Study of an Ultra-Broadband and Wide-Angle Insensitive Perfect Metamaterial Absorber in the UV-NIR Region
    Nguyen, Thi Quynh Mai; Nguyen, Thi Kim Thu; Le, Dac Tuyen; Truong, Chi Lam; Vu, Dinh Lam; Nguyen, Thi Quynh Hoa
    Plasmonics (2021), 16 (5), 1583-1592CODEN: PLASCS; ISSN:1557-1955. (Springer)
    Developing a simple structure using low-cost material that enables both large-scale fabrication and broadband absorption response is highly desirable but very challenging for achieving high-performance metamaterial absorber. Herein, we propose and numerically investigate an ultra-broadband and wide-angle insensitive perfect metamaterial absorber in the UV to near-IR (UV-NIR) region based on a simple metal-dielec.-metal structure. The proposed absorber structure consists of a periodic array of a tungsten hexagonal prism and a tungsten ground plane sepd. by a silicon dioxide dielec. substrate. The proposed absorber achieves an ultra-broadband absorption response in the range of 275-1000 nm with an absorptivity above 90% and a relative bandwidth of 106.8% at normal incidence, which covers from the UV to NIR region. The absorption efficiency is maintained with the figure of merit ηOBW higher than 90% for a wide incident angle up to 40o for transverse elec. (TE) polarization and 65o for transverse magnetic (TM) polarization. The effects of structural parameters and different metallic materials on the absorption performance are presented. In addn., the phys. mechanism is analyzed using the surface d. and distributions of elec. and magnetic fields that are attributed to both localized surface plasmon (LSP) and propagating surface plasmon (PSP) resonances. Owing to outstanding merits of simple structure, low cost, and high absorption performance, the designed absorber can be suitable for many applications in the UV-NIR spectrum such as thermal emitters and solar cells.
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  10. 10
    Zhou, L.; Tan, Y.; Ji, D.; Zhu, B.; Zhang, P.; Xu, J.; Gan, Q.; Yu, Z.; Zhu, J. Self-Assembly of Highly Efficient, Broadband Plasmonic Absorbers for Solar Steam Generation. Sci. Adv. 2016, 2 (4), e1501227  DOI: 10.1126/sciadv.1501227
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    10
    Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation
    Zhou, Lin; Tan, Yingling; Ji, Dengxin; Zhu, Bin; Zhang, Pei; Xu, Jun; Gan, Qiaoqiang; Yu, Zongfu; Zhu, Jia
    Science Advances (2016), 2 (4), e1501227/1-e1501227/8CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
    The study of ideal absorbers, which can efficiently absorb light over a broad range of wavelengths, is of fundamental importance, as well as crit. for many applications from solar steam generation and thermophotovoltaics to light/thermal detectors. As a result of recent advances in plasmonics, plasmonic absorbers have attracted a lot of attention. However, the performance and scalability of these absorbers, predominantly fabricated by the top-down approach, need to be further improved to enable widespread applications. We report a plasmonic absorber which can enable an av. measured absorbance of ∼99% across the wavelengths from 400 nm to 10 μm, the most efficient and broadband plasmonic absorber reported to date. The absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by a one-step deposition process. Because of its efficient light absorption, strong field enhancement, and porous structures, which together enable not only efficient solar absorption but also significant local heating and continuous stream flow, plasmonic absorber-based solar steam generation has over 90% efficiency under solar irradn. of only 4-sun intensity (4 kW m-2). The pronounced light absorption effect coupled with the high-throughput self-assembly process could lead toward large-scale manufg. of other nanophotonic structures and devices.
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  11. 11
    Richardson, H. H.; Carlson, M. T.; Tandler, P. J.; Hernandez, P.; Govorov, A. O. Experimental and Theoretical Studies of Light-to-Heat Conversion and Collective Heating Effects in Metal Nanoparticle Solutions. Nano Lett. 2009, 9 (3), 1139– 1146,  DOI: 10.1021/nl8036905
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    11
    Experimental and Theoretical Studies of Light-to-Heat Conversion and Collective Heating Effects in Metal Nanoparticle Solutions
    Richardson, Hugh H.; Carlson, Michael T.; Tandler, Peter J.; Hernandez, Pedro; Govorov, Alexander O.
    Nano Letters (2009), 9 (3), 1139-1146CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors perform a set of expts. on photoheating in a H2O droplet contg. Au nanoparticles (NPs). Using photocalorimetric methods, the authors det. efficiency of light-to-heat conversion (η) which turns out to be remarkably close to 1, (0.97 < η < 1.03). Detailed studies reveal a complex character of heat transfer in an optically stimulated droplet. The main mechanism of equilibration is due to convectional flow. Theor. modeling is performed to describe thermal effects at both nano- and millimeter scales. Theory shows that the collective photoheating is the main mechanism. For a large concn. of NPs and small laser intensity, an averaged temp. increase (at the millimeter scale) is significant (∼7°), whereas on the nanometer scale the temp. increase at the surface of a single NP is small (∼0.02°). In the opposite regime, i.e., a small NP concn. and intense laser irradn., an opposite picture: a temp. increase at the millimeter scale is small (∼0.1°) but a local, nanoscale temp. has strong local spikes at the surfaces of NPs (∼3°) were found. These studies are crucial for the understanding of photothermal effects in NPs and for their potential and current applications in nano- and biotechnologies.
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  12. 12
    Zhu, M.; Li, Y.; Chen, F.; Zhu, X.; Dai, J.; Li, Y.; Yang, Z.; Yan, X.; Song, J.; Wang, Y.; Hitz, E.; Luo, W.; Lu, M.; Yang, B.; Hu, L. Plasmonic Wood for High-Efficiency Solar Steam Generation. Adv. Energy Mater. 2018, 8 (4), 1701028,  DOI: 10.1002/aenm.201701028
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  13. 13
    Tian, L.; Xin, Q.; Zhao, C.; Xie, G.; Akram, M. Z.; Wang, W.; Ma, R.; Jia, X.; Guo, B.; Gong, J. R. Nanoarray Structures for Artificial Photosynthesis. Small 2021, 17 (38), 2006530,  DOI: 10.1002/smll.202006530
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    13
    Nanoarray Structures for Artificial Photosynthesis
    Tian, Liangqiu; Xin, Qi; Zhao, Chang; Xie, Guancai; Akram, Muhammad Zain; Wang, Wenrong; Ma, Renping; Jia, Xinrui; Guo, Beidou; Gong, Jian Ru
    Small (2021), 17 (38), 2006530CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Conversion and storage of solar energy into fuels and chems. by artificial photosynthesis has been considered as one of the promising methods to address the global energy crisis. However, it is still far from the practical applications on a large scale. Nanoarray structures that combine the advantages of nanosize and array alignment have demonstrated great potential to improve solar energy conversion efficiency, stability, and selectivity. This article provides a comprehensive review on the utilization of nanoarray structures in artificial photosynthesis of renewable fuels and high value-added chems. First, basic principles of solar energy conversion and superiorities of using nanoarray structures in this field are described. Recent research progress on nanoarray structures in both abiotic and abiotic-biotic hybrid systems is then outlined, highlighting contributions to light absorption, charge transport and transfer, and catalytic reactions (including kinetics and selectivity). Finally, conclusions and outlooks on future research directions of nanoarray structures for artificial photosynthesis are presented.
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  14. 14
    Patsalas, P.; Kalfagiannis, N.; Kassavetis, S.; Abadias, G.; Bellas, D. V.; Lekka, Ch.; Lidorikis, E. Conductive Nitrides: Growth Principles, Optical and Electronic Properties, and Their Perspectives in Photonics and Plasmonics. Mater. Sci. Eng. R Rep. 2018, 123, 1– 55,  DOI: 10.1016/j.mser.2017.11.001
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  15. 15
    Yalavarthi, R.; Henrotte, O.; Kment, Š.; Naldoni, A. Determining the Role of Pd Catalyst Morphology and Deposition Criteria over Large Area Plasmonic Metasurfaces during Light-Enhanced Electrochemical Oxidation of Formic Acid. J. Chem. Phys. 2022, 157 (11), 114706,  DOI: 10.1063/5.0102012
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    15
    Determining the role of Pd catalyst morphology and deposition criteria over large area plasmonic metasurfaces during light-enhanced electrochemical oxidation of formic acid
    Yalavarthi, Rambabu; Henrotte, Olivier; Kment, Stepan; Naldoni, Alberto
    Journal of Chemical Physics (2022), 157 (11), 114706CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The use of metal composites based on plasmonic nanostructures partnered with catalytic counterparts has recently emerged as a promising approach in the field of plasmon-enhanced electrocatalysis. Here, we report on the role of the surface morphol., size, and anchored site of Pd catalysts coupled to plasmonic metasurfaces formed by periodic arrays of multimetallic Ni/Au nanopillars for formic acid electro-oxidn. reaction (FAOR). We compare the activity of two kinds of metasurfaces differing in the positioning of the catalytic Pd nanoparticles. In the first case, the Pd nanoparticles have a polyhedron crystal morphol. with exposed (200) facets and were deposited over the Ni/Au metasurfaces in a site-selective fashion by limiting their growth at the electromagnetic hot spots (Ni/Au-Pd@W). In contrast, the second case consists of spherical Pd nanoparticles grown in soln., which are homogeneously deposited onto the Ni/Au metasurface (Ni/Au-Pd@M). Ni/Au-Pd@W catalytic metasurfaces demonstrated higher light-enhanced FAOR activity (61%) in comparison to the Ni/Au-Pd@M sample (42%) for the direct dehydrogenation pathway. Moreover, the site-selective Pd deposition promotes the growth of nanoparticles favoring a more selective catalytic behavior and a lower degree of CO poisoning on Pd surface. The use of cyclic voltammetry, energy-resolved incident photon to current conversion efficiency, open circuit potential, and electrochem. impedance spectroscopy highlights the role of plasmonic near fields and hot holes in driving the catalytic enhancement under light conditions. (c) 2022 American Institute of Physics.
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  16. 16
    Guler, U.; Ndukaife, J. C.; Naik, G. V.; Nnanna, A. G. A.; Kildishev, A. V.; Shalaev, V. M.; Boltasseva, A. Local Heating with Lithographically Fabricated Plasmonic Titanium Nitride Nanoparticles. Nano Lett. 2013, 13 (12), 6078– 6083,  DOI: 10.1021/nl4033457
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    16
    Local Heating with Lithographically Fabricated Plasmonic Titanium Nitride Nanoparticles
    Guler, Urcan; Ndukaife, Justus C.; Naik, Gururaj V.; Nnanna, A. G. Agwu; Kildishev, Alexander V.; Shalaev, Vladimir M.; Boltasseva, Alexandra
    Nano Letters (2013), 13 (12), 6078-6083CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Titanium nitride is considered a promising alternative plasmonic material and is known to exhibit localized surface plasmon resonances within the near-IR biol. transparency window. Here, local heating efficiencies of disk-shaped nanoparticles made of titanium nitride and gold are compared in the visible and near-IR regions numerically and exptl. with samples fabricated using e-beam lithog. Results show that plasmonic titanium nitride nanodisks are efficient local heat sources and outperform gold nanodisks in the biol. transparency window, dispensing the need for complex particle geometries.
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  17. 17
    Mascaretti, L.; Schirato, A.; Zbořil, R.; Kment, Š.; Schmuki, P.; Alabastri, A.; Naldoni, A. Solar Steam Generation on Scalable Ultrathin Thermoplasmonic TiN Nanocavity Arrays. Nano Energy 2021, 83, 105828,  DOI: 10.1016/j.nanoen.2021.105828
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    17
    Solar steam generation on scalable ultrathin thermoplasmonic TiN nanocavity arrays
    Mascaretti, Luca; Schirato, Andrea; Zboril, Radek; Kment, Stepan; Schmuki, Patrik; Alabastri, Alessandro; Naldoni, Alberto
    Nano Energy (2021), 83 (), 105828CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    Plasmonic-based solar absorbers exhibit complete light absorption in a sub-Μm thickness, representing an alternative to mm-thick carbon-based materials most typically employed for solar-driven steam generation. In this work, we present the scalable fabrication of ultrathin plasmonic titanium nitride (TiN) nanocavity arrays that exhibit 90% broadband solar light absorption within ∼ 250 nm from the illuminated surface and show a fast non-linear increase of performance with light intensity. At 14 Suns TiN nanocavities reach ∼ 15 kg h-1 m-2 evapn. rate and ∼ 76% thermal efficiency, a steep increase from ∼ 0.4 kg h-1 m-2 and ∼ 20% under 1.4 Suns. Electromagnetic, thermal and diffusion modeling of our system reveals the contribution of each material and reactor component to heat dissipation and shows that a quasi-two-dimensional heat dissipation regime significantly accelerates water evapn. Our approach to ultrathin plasmonic absorbers can boost the performance of devices for evapn./desalination and holds promise for a broader range of phase sepn. processes.
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  18. 18
    Li, Y.; Lin, C.; Wu, Z.; Chen, Z.; Chi, C.; Cao, F.; Mei, D.; Yan, H.; Tso, C. Y.; Chao, C. Y. H.; Huang, B. Solution-Processed All-Ceramic Plasmonic Metamaterials for Efficient Solar-Thermal Conversion over 100–727 C. Adv. Mater. 2021, 33 (1), 2005074,  DOI: 10.1002/adma.202005074
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    18
    Solution-Processed All-Ceramic Plasmonic Metamaterials for Efficient Solar-Thermal Conversion over 100-727°C
    Li, Yang; Lin, Chongjia; Wu, Zuoxu; Chen, Zhongying; Chi, Cheng; Cao, Feng; Mei, Deqing; Yan, He; Tso, Chi Yan; Chao, Christopher Y. H.; Huang, Baoling
    Advanced Materials (Weinheim, Germany) (2021), 33 (1), 2005074CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Low-cost and large-area solar-thermal absorbers with superior spectral selectivity and excellent thermal stability are vital for efficient and large-scale solar-thermal conversion applications, such as space heating, desalination, ice mitigation, photothermal catalysis, and concg. solar power. Few state-of-the-art selective absorbers are qualified for both low- (<200 C) and high-temp. (>600 C) applications due to insufficient spectral selectivity or thermal stability over a wide temp. range. Here, a high-performance plasmonic metamaterial selective absorber is developed by facile soln.-based processes via assembling an ultrathin (≈120 nm) titanium nitride (TiN) nanoparticle film on a TiN mirror. Enabled by the synergetic in-plane plasmon and out-of-plane Fabry-Perot resonances, the all-ceramic plasmonic metamaterial simultaneously achieves high, full-spectrum solar absorption (95%), low mid-IR emission (3% at 100 C), and excellent stability over a temp. range of 100-727 C, even outperforming most vacuum-deposited absorbers at their specific operating temps. The competitive performance of the soln.-processed absorber is accompanied by a significant cost redn. compared with vacuum-deposited absorbers. All these merits render it a cost-effective, universal soln. to offering high efficiency (89-93%) for both low- and high-temp. solar-thermal applications.
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  19. 19
    Moon, G. D.; Joo, J. B.; Dahl, M.; Jung, H.; Yin, Y. Nitridation and Layered Assembly of Hollow TiO 2 Shells for Electrochemical Energy Storage. Adv. Funct. Mater. 2014, 24 (6), 848– 856,  DOI: 10.1002/adfm.201301718
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    19
    Nitridation and layered assembly of hollow TiO2 shells for electrochemical energy storage
    Moon, Geon Dae; Joo, Ji Bong; Dahl, Michael; Jung, Heejung; Yin, Yadong
    Advanced Functional Materials (2014), 24 (6), 848-856CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The nitridation of hollow TiO2 nanoshells and their layered assembly into electrodes for electrochem. energy storage are reported. The nitridated hollow shells are prepd. by annealing TiO2 shells, produced initially using a sol-gel process, under an NH3 environment at different temps. ranging from 700 to 900 !!!C, then assembled to form a robust monolayer film on a water surface through a quick and simple assembly process without any surface modification to the samples. This approach facilitates supercapacitor cell design by simplifying the electrochem. electrode structure by removing the need to use any org. binder or carbon-based conducting materials. The areal capacitance of the as-prepd. electrode is obsd. to be ≈180 times greater than that of a bare TiO2 electrode, mainly due to the enhanced elec. cond. of the TiN phase produced through the nitridation process. Furthermore, the electrochem. capacitance can be enhanced linearly by constructing an electrode with multilayered shell films through a repeated transfer process (0.8 to 7.1 mF cm-2, from one monolayer to 9 layers). Addnl., the high elec. cond. of the shell film makes it an excellent scaffold for supporting other psuedocapacitive materials (e.g., MnO2), producing composite electrodes with a specific capacitance of 743.9 F g-1 at a scan rate of 10 mV s-1 (based on the mass of MnO2) and a good cyclic stability up to 1000 cycles.
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  20. 20
    Wei, Q.; Kuhn, D. L.; Zander, Z.; DeLacy, B. G.; Dai, H.-L.; Sun, Y. Silica-Coating-Assisted Nitridation of TiO2 Nanoparticles and Their Photothermal Property. Nano Res. 2021, 14 (9), 3228– 3233,  DOI: 10.1007/s12274-021-3427-7
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    20
    Silica-coating-assisted nitridation of TiO2 nanoparticles and their photothermal property
    Wei, Qilin; Kuhn, Danielle L.; Zander, Zachary; DeLacy, Brendan G.; Dai, Hai-Lung; Sun, Yugang
    Nano Research (2021), 14 (9), 3228-3233CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
    Nanoparticles of refractory compds. represent a class of stable materials showing a great promise to support localized surface plasmon resonances (LSPRs) in both visible and near IR (NIR) spectral regions. It is still challenging to rationally tune the LSPR band because of the difficulty to control the d. of charge carriers in individual refractory nanoparticles and maintain the dispersity of nanoparticles in the processes of synthesis and applications. In this work, controlled chem. transformation of titanium dioxide (TiO2) nanoparticles encapsulated with mesoporous silica (SiO2) shells to titanium nitride (TiN) via nitridation reaction at elevated temps. is developed to tune the d. of free electrons in the resulting titanium-oxide-nitride (TiOxNy) nanoparticles. Such tunability enables a flexibility to support LSPR-based optical absorption in the synthesized TiOxNy@SiO2 core-shell nanoparticles across both the visible and NIR regions. The silica shells play a crucial role in preventing the sintering of TiOxNy nanoparticles in the nitridation reaction and maintaining the stability of TiOxNy nanoparticles in applications. The LSPR-based broadband absorption of light in the TiOxNy@SiO2 nanoparticles exhibits strong photothermal effect with photo-to-thermal conversion efficiency as high as ∼ 76%. [graphic not available: see fulltext].
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  21. 21
    Li, C.; Shi, J.; Zhu, L.; Zhao, Y.; Lu, J.; Xu, L. Titanium Nitride Hollow Nanospheres with Strong Lithium Polysulfide Chemisorption as Sulfur Hosts for Advanced Lithium-Sulfur Batteries. Nano Res. 2018, 11 (8), 4302– 4312,  DOI: 10.1007/s12274-018-2017-9
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    Titanium nitride hollow nanospheres with strong lithiumpolysulfide chemisorption as sulfur hosts for advanced lithium-sulfur batteries
    Li, Chuanchuan; Shi, Jingjing; Zhu, Lin; Zhao, Yingyue; Lu, Jun; Xu, Liqiang
    Nano Research (2018), 11 (8), 4302-4312CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
    Lithium-sulfur batteries are promising electrochem. energy storage devicesbecause of their high theor. specific capacity and energy d. An ideal sulfur host should possess high cond. and embrace the phys. confinement or strong chemisorption to dramatically suppress the polysulfide dissoln. Herein, uniform TiN hollow nanospheres with an av. diam. of ∼160 nm have been reported as highly efficient lithium polysulfide reservoirs for high-performance lithium-sulfur batteries. Combining the high cond. and chem. trappingof lithium polysulfides, the obtained S/TiN cathode of 70 wt.% sulfur content in the composite delivered an excellent long-life cycling performance at 0.5C and 1.0C over 300 cycles. More importantly, a stable capacity of 710.4 mAh·g-1 could bemaintained even after 100 cycles at 0.2C with a high sulfur loading of 3.6 mg·cm-1. The nature of the interactions between TiN and lithium polysulfide species wasinvestigated by XPS studies. Theor. calcns. were also carried out and the results revealed a strong binding between TiN and the lithium polysulfide species. It is expected that this class of conductive and polar materials would pave a new way for the high-energy lithium-sulfur batteries in the future.[Figure not available: see fulltext.].
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  22. 22
    Zukalova, M.; Prochazka, J.; Bastl, Z.; Duchoslav, J.; Rubacek, L.; Havlicek, D.; Kavan, L. Facile Conversion of Electrospun TiO 2 into Titanium Nitride/Oxynitride Fibers. Chem. Mater. 2010, 22 (13), 4045– 4055,  DOI: 10.1021/cm100877h
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    22
    Facile Conversion of Electrospun TiO2 into Titanium Nitride/Oxynitride Fibers
    Zukalova, Marketa; Prochazka, Jan; Bastl, Zdenek; Duchoslav, Jiri; Rubacek, Lukas; Havlicek, David; Kavan, Ladislav
    Chemistry of Materials (2010), 22 (13), 4045-4055CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Nanocryst. fibrous TiO2 (anatase) was prepd. by electrostatic spinning from ethanolic soln. of Ti(IV) butoxide, acetylacetone, and poly(vinylpyrrolidone) employing the Nanospider industrial process. These titania fibers were smoothly converted into cubic titanium oxynitride, TiOxNy fibers (a = 4.1930 Å) during 4 h at 600° in ammonia atm. The obtained material is convertible back into TiO2 fibers by heat treatment in air at 500°. The TiO2 fibers, which were reformed in this way, contain anatase as the main phase. Their follow-up reaction with NH3 at 600° 2 h leads to a less cryst. oxynitride material with a ≈ 4.173 Å, which is close to that of cubic TiO. Three subsequent cycles of this transformation were demonstrated. The described conversions are specific for electrospun anatase fibers only. At the same exptl. conditions, other forms of nanocryst. anatase do not react with ammonia yielding cubic phases. An almost perfectly stoichiometric titanium nitride, TiN (a = 4.2290 Å) contg. only 0.2 wt.% O, was prepd. from TiOxNy fibers in NH3 at temps. up to 1000°. This TiN material maintains the morphol. of fibers and is composed of nanocrystals of a similar size as those of the precursor.
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  23. 23
    Qin, P.; Huang, C.; Gao, B.; Pi, C.; Fu, J.; Zhang, X.; Huo, K.; Chu, P. K. Ultrathin Carbon Layer-Encapsulated TiN Nanotubes Array with Enhanced Capacitance and Electrochemical Stability for Supercapacitors. Appl. Surf. Sci. 2020, 503, 144293,  DOI: 10.1016/j.apsusc.2019.144293
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    23
    Ultrathin carbon layer-encapsulated TiN nanotubes array with enhanced capacitance and electrochemical stability for supercapacitors
    Qin, Ping; Huang, Chao; Gao, Biao; Pi, Chaoran; Fu, Jijiang; Zhang, Xuming; Huo, Kaifu; Chu, Paul K.
    Applied Surface Science (2020), 503 (), 144293CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Although Ti nitride (TiN) is promising as electrode material in supercapacitors due to the high cond., there are drawbacks such as the brittleness, low capacitance, and chem. stability. Herein, 3-dimensional (3D) C-encapsulated mesoporous Ti nitride nanotubes (TiN/C NTs) arrays with C doping are prepd. by 1-step nitridation of anodic TiO2 NTs with org. electrolyte as the C source. In TiN/C NTs, 3-dimensional C matrix not only serves as a protective layer and mech. support to mitigate electrochem. oxidn. and structural collapse, but also provides a conductive network to facilitate electron transfer. The capacitance retention of the TiN NTs electrodes increases from 72.5 to 92.2% for 4000 cycles after C encapsulating. Also, the C doping increases the active charge storage sites of the TiN NTs. The TiN/C NTs electrode exhibits a large volumetric capacitance of 121 F cm-3 (0.83 A cm-3), which is one time larger than that of the pure TiN NTs (69 F cm-3). The sym. all-solid-state device assembled with 2 TiN/C NTs electrodes and polyvinyl alc. electrolyte shows a large volumetric capacitance of 8.3 F cm-3. This finding provides a good potential application in flexible supercapacitor.
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  24. 24
    Naldoni, A.; Kudyshev, Z. A.; Mascaretti, L.; Sarmah, S. P.; Rej, S.; Froning, J. P.; Tomanec, O.; Yoo, J. E.; Wang, D.; Kment, Š.; Montini, T.; Fornasiero, P.; Shalaev, V. M.; Schmuki, P.; Boltasseva, A.; Zbořil, R. Solar Thermoplasmonic Nanofurnace for High-Temperature Heterogeneous Catalysis. Nano Lett. 2020, 20 (5), 3663– 3672,  DOI: 10.1021/acs.nanolett.0c00594
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    24
    Solar Thermoplasmonic Nanofurnace for High-Temperature Heterogeneous Catalysis
    Naldoni, Alberto; Kudyshev, Zhaxylyk A.; Mascaretti, Luca; Sarmah, Smritakshi P.; Rej, Sourav; Froning, Jens P.; Tomanec, Ondrej; Yoo, Jeong Eun; Wang, Di; Kment, Stepan; Montini, Tiziano; Fornasiero, Paolo; Shalaev, Vladimir M.; Schmuki, Patrik; Boltasseva, Alexandra; Zboril, Radek
    Nano Letters (2020), 20 (5), 3663-3672CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Most of existing solar thermal technologies require highly concd. solar power to operate at 300-600°. Here, thin films of refractory plasmonic TiN cylindrical nanocavities manufd. via flexible and scalable process are presented. The fabricated TiN films show polarization-insensitive 95% broadband absorption in the visible and near-IR spectral ranges and act as plasmonic "nanofurnaces" capable of reaching temps. above 600° under moderately concd. solar irradn. (~ 20 Suns). The demonstrated structures can be used to control nanometer-scale chem. with zeptoliter (10-21 L) volumetric precision, catalyzing C-C bond formation and melting inorg. deposits. Also shown is the possibility to perform solar thermal CO oxidn. at rates of 16 mol h-1 m-2 and with a solar-to-heat thermoplasmonic efficiency of 63%. Access to scalable, cost-effective refractory plasmonic nanofurnaces opens the way to the development of modular solar thermal devices for sustainable catalytic processes.
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  25. 25
    Riboni, F.; Nguyen, N. T.; So, S.; Schmuki, P. Aligned Metal Oxide Nanotube Arrays: Key-Aspects of Anodic TiO 2 Nanotube Formation and Properties. Nanoscale Horiz 2016, 1 (6), 445– 466,  DOI: 10.1039/C6NH00054A
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    25
    Aligned metal oxide nanotube arrays: key-aspects of anodic TiO2 nanotube formation and properties
    Riboni, Francesca; Nguyen, Nhat Truong; So, Seulgi; Schmuki, Patrik
    Nanoscale Horizons (2016), 1 (6), 445-466CODEN: NHAOAW; ISSN:2055-6764. (Royal Society of Chemistry)
    Over the past ten years, self-aligned TiO2 nanotubes have attracted tremendous scientific and technol. interest due to their anticipated impact on energy conversion, environment remediation and biocompatibility. In the present manuscript, we review fundamental principles that govern the self-organized initiation of anodic TiO2 nanotubes. We start with the fundamental question: why is self-organization taking place. We illustrate the inherent key mechanistic aspects that lead to tube growth in various different morphologies, such as ripple-walled tubes, smooth tubes, stacks and bamboo-type tubes, and importantly the formation of double-walled TiO2 nanotubes vs. single-walled tubes, and the drastic difference in their phys. and chem. properties. We show how both double- and single-walled tube layers can be detached from the metallic substrate and exploited for the prepn. of robust self-standing membranes. Finally, we show how by selecting specific growth approaches to TiO2 nanotubes desired functional features can be significantly improved, e.g., enhanced electron mobility, intrinsic doping, or crystn. into pure anatase at high temps. can be achieved. Finally, we briefly outline the impact of property, modifications and morphol. on functional uses of self-organized nanotubes for most important applications.
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  26. 26
    Lee, K.; Mazare, A.; Schmuki, P. One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes. Chem. Rev. 2014, 114 (19), 9385– 9454,  DOI: 10.1021/cr500061m
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    26
    One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes
    Lee, Kiyoung; Mazare, Anca; Schmuki, Patrik
    Chemical Reviews (Washington, DC, United States) (2014), 114 (19), 9385-9454CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review of different synthesis approaches to produce TiO2 nanotubes and TiO2 nanotube arrays, phys. and chem. properties of TiO2 nanotubes and techniques to modify them. The most explored and prospective applications of nanotubular TiO2 are discussed.
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    Ozkan, S.; Mazare, A.; Schmuki, P. Critical Parameters and Factors in the Formation of Spaced TiO2 Nanotubes by Self-Organizing Anodization. Electrochim. Acta 2018, 268, 435– 447,  DOI: 10.1016/j.electacta.2018.02.120
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    27
    Critical parameters and factors in the formation of spaced TiO2 nanotubes by self-organizing anodization
    Ozkan, Selda; Mazare, Anca; Schmuki, Patrik
    Electrochimica Acta (2018), 268 (), 435-447CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    Self-organized TiO2 nanotube arrays can be grown under a wide range of electrochem. conditions. In the present work, we evaluate the occurrence of spacing between tubes and the connection of this effect to organization of tubes on two-size scales. The results show that tube-spacing is initiated in the very early stages of anodization between individual pore morphologies. Furthermore, the spacing, as well as the organization on two-size scales can be controlled by changing the anodization conditions, e.g., electrolyte compn., applied voltage and temp. Namely, adjustment of H2O content, electrode temp. and voltage can lead to spaced nanotubes, and allow to control spacing. Finally, we draw conclusions on the possible mechanism relevant to the growth of spaced tubes.
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    Wawrzyniak, J.; Grochowska, K.; Karczewski, J.; Kupracz, P.; Ryl, J.; Dołȩga, A.; Siuzdak, K. The Geometry of Free-Standing Titania Nanotubes as a Critical Factor Controlling Their Optical and Photoelectrochemical Performance. Surf. Coat. Technol. 2020, 389, 125628,  DOI: 10.1016/j.surfcoat.2020.125628
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    28
    The geometry of free-standing titania nanotubes as a critical factor controlling their optical and photoelectrochemical performance
    Wawrzyniak, Jakub; Grochowska, Katarzyna; Karczewski, Jakub; Kupracz, Piotr; Ryl, Jacek; Dolega, Anna; Siuzdak, Katarzyna
    Surface and Coatings Technology (2020), 389 (), 125628CODEN: SCTEEJ; ISSN:0257-8972. (Elsevier B.V.)
    Titanium dioxide nanotubes are regarded as one of the most important functional materials and due to their unique electronic properties, chem. stability and photocorrosion resistance, they find applications in, for example, highly efficient photocatalysis or perovskite solar cells. Nevertheless, modification of TiO2 nanotubes is required to overcome their main drawback, i.e. large energy bandgap (>3.2 eV) limiting their ability to capture solar light. In this work, we report the changes in optical and photoelectrochem. properties of well-sepd. TiO2 nanotubes that are tuned by varying the geometry of the material. The ordered tubular titania is formed via anodization in the presence of fluoride ions in diethylene glycol at elevated temp. Length, inner diam., wall thickness, and sepn. distance are described in function of synthesis parameters such as applied voltage and duration. The morphol. and optical properties are characterized by means of SEM and UV-Vis spectroscopy techniques, resp., while cyclic voltammetry, linear voltammetry and chronoamperometry are used to det. electrochem./photoelectrochem. activity in different light conditions. The obtained results suggest a link between sp. surface area, the width of the band-gap, and photoactivity, each of which could be individually optimized via anodization conditions. Moreover, the behavior of the Mott-Schottky plot before and after 3 min of irradn. is studied indicating the pos. shift of the flat band position and an increase in donor d. values for all the obtained materials. The Mott-Schottky anal. was correlated with the linear voltammetry scans suggesting the important role of surface trapped holes. Presented in here results significantly supplement the current state-of-art regarding sepd. TiO2 nanotubes that are considered as not fully investigated and unappreciated class of titania materials which due to the exposure of inner and outer wall can be used for further modifications.
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    Varghese, O. K.; Paulose, M.; Grimes, C. A. Long Vertically Aligned Titania Nanotubes on Transparent Conducting Oxide for Highly Efficient Solar Cells. Nat. Nanotechnol. 2009, 4 (9), 592– 597,  DOI: 10.1038/nnano.2009.226
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    Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells
    Varghese, Oomman K.; Paulose, Maggie; Grimes, Craig A.
    Nature Nanotechnology (2009), 4 (9), 592-597CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Dye-sensitized solar cells consist of a random network of titania nanoparticles that serve both as a high-surface-area support for dye mols. and as an electron-transporting medium. Despite achieving high power conversion efficiencies, their performance is limited by electron trapping in the nanoparticle film. Electron diffusion lengths can be increased by transporting charge through highly ordered nanostructures such as titania nanotube arrays. Although titania nanotube array films have been shown to enhance the efficiencies of both charge collection and light harvesting, it has not been possible to grow them on transparent conducting oxide glass with the lengths needed for high-efficiency device applications (tens of micrometers). Here, we report the fabrication of transparent titania nanotube array films on transparent conducting oxide glass with lengths between 0.3 and 33.0 μm using a novel electrochem. approach. Dye-sensitized solar cells contg. these arrays yielded a power conversion efficiency of 6.9%. The incident photon-to-current conversion efficiency ranged from 70 to 80% for wavelengths between 450 and 650 nm.
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  30. 30
    Tesler, A. B.; Altomare, M.; Schmuki, P. Morphology and Optical Properties of Highly Ordered TiO 2 Nanotubes Grown in NH 4 F/ o -H 3 PO 4 Electrolytes in View of Light-Harvesting and Catalytic Applications. ACS Appl. Nano Mater. 2020, 3 (11), 10646– 10658,  DOI: 10.1021/acsanm.0c01859
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    30
    Morphology and Optical Properties of Highly Ordered TiO2 Nanotubes Grown in NH4F/o-H3PO4 Electrolytes in View of Light-Harvesting and Catalytic Applications
    Tesler, Alexander B.; Altomare, Marco; Schmuki, Patrik
    ACS Applied Nano Materials (2020), 3 (11), 10646-10658CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    Highly ordered titanium dioxide nanotube (TiO2 NT) arrays were grown by electrochem. anodization of titanium in a molten NH4F/o-H3PO4 electrolyte. NTs with various diams., lengths, wall thicknesses, and intertube distances could be obtained by tuning key anodization parameters such as the applied potential, anodization time, electrolyte temp., concn. of NH4F, and H2O content. The morphol. and optical properties were characterized by SEM and UV-vis spectroscopy techniques. We show that all aforementioned parameters have a strong influence on the nanostructured morphol. and optical characteristics (reflectivity) of the formed nanotubular layers. Their optical features were simulated numerically to support the exptl. measurements. We show that the optical features of anodic TiO2 nanotube layers result from the overlay of the individual optical properties of various "structural elements", e.g., the NT barrier layer, top opening morphol., intertube spacing, and thermally formed oxide sublayer. Our results provide tools for "a priori" design with nanoscale precision of TiO2 structures with advanced optical features for light-harvesting and catalytic applications, e.g., in sensing, photocatalytic self-cleaning processes, solar hydrogen generation, or photovoltaics.
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    Ozkan, S.; Nguyen, N. T.; Mazare, A.; Schmuki, P. Optimized Spacing between TiO 2 Nanotubes for Enhanced Light Harvesting and Charge Transfer. ChemElectroChem. 2018, 5 (21), 3183– 3190,  DOI: 10.1002/celc.201801136
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    Optimized Spacing between TiO2 Nanotubes for Enhanced Light Harvesting and Charge Transfer
    Ozkan, Selda; Nguyen, Nhat Truong; Mazare, Anca; Schmuki, Patrik
    ChemElectroChem (2018), 5 (21), 3183-3190CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We investigate two distinctly different anodic TiO2 nanotubular morphologies, spaced and close-packed arrays. While the close-packed tubular arrays are formed in ethylene glycol, spaced nanotubes (NTs), which have a regular gap between individual NTs, grow in a diethylene glycol or DMSO-based electrolyte. Depending on the electrolyte used for anodization, the morphol., crystal structure, and chem. compn. of the resulting nanotubular layer vary from one another. This influences the electrochem. and photoelectrochem. activity of the NTs. Overall, we find that spaced NTs can not only provide enhanced charge-transfer characteristics but can also show beneficial light absorption characteristics when used as a photo- or light-harvesting electrode.
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    Liu, Y.; Wang, Y.; Zhang, Y.; You, Z.; Lv, X. Mechanism on Reduction and Nitridation of Micrometer-sized Titania with Ammonia Gas. J. Am. Ceram. Soc. 2020, 103 (6), 3905– 3916,  DOI: 10.1111/jace.17067
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    Mechanism on reduction and nitridation of micrometer-sized titania with ammonia gas
    Liu, Yongjie; Wang, Yue; Zhang, Yu; You, Zhixiong; Lv, Xuewei
    Journal of the American Ceramic Society (2020), 103 (6), 3905-3916CODEN: JACTAW; ISSN:0002-7820. (Wiley-Blackwell)
    Ammonia gas can be simultaneously used as a reductant and nitrogen source to prep. TiN from titania. In this work, the mechanisms on redn. and nitridation of micrometer-sized anatase with ammonia gas have been investigated, using both thermodn. and exptl. studies. The thermodn. anal. indicated that redn. and nitridation of TiO2 by NH3 was feasible. Anatase will undergo different paths to form TiN, depending on the reaction temp. Upon heating, NH3 was seen to partially decomp. into N2 and H2, although the actual NH3 decompn. ratio was less than the theor. value. The exptl. results indicated that the obtained titanium nitride was non-stoichiometric (TiNxO1-x, x ≤ 1), as it contained a certain amt. of oxygen. Based on the phase transformation and XPS anal., the redn. and nitridation routes were deduced: TiO2 reacted with NH3 to form TiNxO1-x directly, at lower temps., and followed the path TiO2 → TinO2n-1 → TiNxO1-x, at higher temps. TinO2n-1 was detd. to be Ti4O7 and Ti3O5 at 1100°C and 1200°C, resp. Reaction temp. and time significantly affected the oxygen and nitrogen contents in TiNxO1-x, with the lattice parameter of roasted products gradually increasing-approaching those of pure TiN-with an increase in reaction temp. and holding time. At the same time, the content of oxygen in TiNxO1-x decreased, and its nitrogen content correspondingly increased.
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    Anderson, B. D.; Tracy, J. B. Nanoparticle Conversion Chemistry: Kirkendall Effect, Galvanic Exchange, and Anion Exchange. Nanoscale 2014, 6 (21), 12195– 12216,  DOI: 10.1039/C4NR02025A
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    Nanoparticle conversion chemistry: Kirkendall effect, galvanic exchange, and anion exchange
    Anderson, Bryan D.; Tracy, Joseph B.
    Nanoscale (2014), 6 (21), 12195-12216CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    A review. Conversion chem. is a rapidly maturing field, where chem. conversion of template nanoparticles (NPs) into new compns. is often accompanied by morphol. changes, such as void formation. The principles and examples of three major classes of conversion chem. reactions are reviewed: the Kirkendall effect for metal NPs, galvanic exchange, and anion exchange, each of which can result in void formation in NPs. These reactions can be used to obtain complex structures that may not be attainable by other methods. During each kind of conversion chem. reaction, NPs undergo distinct chem. and morphol. changes, and insights into the mechanisms of these reactions will allow for improved fine control and prediction of the structures of intermediates and products. Conversion of metal NPs into oxides, phosphides, sulfides, and selenides often occurs through the Kirkendall effect, where outward diffusion of metal atoms from the core is faster than inward diffusion of reactive species, resulting in void formation. In galvanic exchange reactions, metal NPs react with noble metal salts, where a redox reaction favors redn. and deposition of the noble metal (alloying) and oxidn. and dissoln. of the template metal (dealloying). In anion exchange reactions, addn. of certain kinds of anions to solns. contg. metal compd. NPs drives anion exchange, which often results in significant morphol. changes due to the large size of anions compared to cations. Conversion chem. thus allows for the formation of NPs with complex compns. and structures, for which numerous applications are anticipated arising from their novel catalytic, electronic, optical, magnetic, and electrochem. properties.
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    Koya, A. N.; Zhu, X.; Ohannesian, N.; Yanik, A. A.; Alabastri, A.; Proietti Zaccaria, R.; Krahne, R.; Shih, W.-C.; Garoli, D. Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis. ACS Nano 2021, 15 (4), 6038– 6060,  DOI: 10.1021/acsnano.0c10945
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    Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis
    Koya, Alemayehu Nana; Zhu, Xiangchao; Ohannesian, Nareg; Yanik, A. Ali; Alabastri, Alessandro; Proietti Zaccaria, Remo; Krahne, Roman; Shih, Wei-Chuan; Garoli, Denis
    ACS Nano (2021), 15 (4), 6038-6060CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A review. The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the UV up to the IR. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large sp. surface area, distinctive optical properties, high elec. cond., and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compds., and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.
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    Plawsky, J. L.; Kim, J. K.; Schubert, E. F. Engineered Nanoporous and Nanostructured Films. Mater. Today 2009, 12 (6), 36– 45,  DOI: 10.1016/S1369-7021(09)70179-8
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    Engineered nanoporous and nanostructured films
    Plawsky, Joel L.; Kim, Jong Kyu; Schubert, E. Fred
    Materials Today (Oxford, United Kingdom) (2009), 12 (6), 36-45CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
    A review. Nanoporous and nanostructured films have become increasingly important to the microelectronics and photonics industries. They provide a route to low dielec. const. materials that will enable future generations of powerful microprocessors. They are the only route to achieving materials with refractive indexes less than 1.2, a key feature for the future development of photonic crystal devices, enhanced omnidirectional reflectors, enhanced anti-reflection coatings and black-body absorbers. In addn., these films exhibit tremendous potential for sepns., catalytic, biomedical and heat transfer applications. This article will review two primary techniques for manufg. these films, evapn. induced self-assembly and oblique or glancing angle deposition, and will discuss some of the film properties crit. to their use in the microelectronics and photonics industries.
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  36. 36
    Lu, J. Y.; Nam, S. H.; Wilke, K.; Raza, A.; Lee, Y. E.; AlGhaferi, A.; Fang, N. X.; Zhang, T. Localized Surface Plasmon-Enhanced Ultrathin Film Broadband Nanoporous Absorbers. Adv. Opt. Mater. 2016, 4 (8), 1255– 1264,  DOI: 10.1002/adom.201600078
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    Localized Surface Plasmon-Enhanced Ultrathin Film Broadband Nanoporous Absorbers
    Lu, Jin You; Nam, Sang Hoon; Wilke, Kyle; Raza, Aikifa; Lee, Yoonkyung E.; Al Ghaferi, Amal; Fang, Nicholas X.; Zhang, TieJun
    Advanced Optical Materials (2016), 4 (8), 1255-1264CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Ultrathin lossy films have attracted much attention due to their strong interference persisting inside the lossy dielec. film on a reflective substrate. Here, a plasmon-enhanced ultrathin film broadband absorber is proposed by combining the ultrathin film absorber with localized surface plasmon resonances. This concept can be realized by patterning nanoholes on an absorber comprised of an absorptive ultrathin Ge film and a reflective Au layer, where the localized surface plasmon mode is activated by metallic pore-shaped holes. The plasmonic enhancement is resulting from the pore-shape localized resonance mode, which increases the optical path length through scattering and concs. the incident light field near the interface of Ge/Au. The exptl. characterization results of a nanoporous ultrathin film absorber, which is fabricated with a scalable laser interference lithog. approach, demonstrate its superior light absorption performance. Several materials, such as Ag, Al, and Cu, are proposed as an alternative to Au, and they can also provide plasmonic enhancement to ultrathin films. Furthermore, through an efficient way to optimize the structural dimensions of the nanoporous ultrathin film absorber, a trilayer system of TiO2/Ge/Au achieves the total solar absorptance over 89.3% with a wavelength range of 400-1100 nm.
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    Mirror-backed Dark Alumina: A Nearly Perfect Absorber for Thermoelectronics and Thermophotovotaics
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    Scientific Reports (2016), 6 (), 19984CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    We present here a broadband, wide-angle, and polarization-independent nearly perfect absorber consisting of mirror-backed nanoporous alumina. By electrochem. anodizing the disordered multicomponent aluminum and properly tailoring the thickness and air-filling fraction of nanoporous alumina, according to the Maxwell-Garnet mixt. theory, a large-area dark alumina can be made with excellent photothermal properties and absorption larger than 93% over a wide wavelength range spanning from near-IR to UV light, i.e. 250 nm-2500 nm. The measured absorption is orders of magnitude greater than other reported anodized porous alumina, typically semi-transparent at similar wavelengths. This simple yet effective approach, however, does not require any lithog., nano-mixt. deposition, pre- and post-treatment. Here, we also envisage and theor. investigate the practical use of proposed absorbers and/or photothermal converters in integrated thermoelectronic and/or thermophotovoltaic energy conversion devices, which make efficient use of the entire spectrum of ambient visible to near-IR radiation.
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    Raut, H. K.; Ganesh, V. A.; Nair, A. S.; Ramakrishna, S. Anti-Reflective Coatings: A Critical, in-Depth Review. Energy Environ. Sci. 2011, 4 (10), 3779,  DOI: 10.1039/c1ee01297e
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    Anti-reflective coatings: a critical, in-depth review
    Raut, Hemant Kumar; Ganesh, V. Anand; Nair, A. Sreekumaran; Ramakrishna, Seeram
    Energy & Environmental Science (2011), 4 (10), 3779-3804CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A review. Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-elec. equipments. Extensive research in optical and biol. reflectance minimization as well as the emergence of nanotechnol. over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature ("biomimetics") has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and assocd. challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.
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    Wang, W.; Qi, L. Light Management with Patterned Micro- and Nanostructure Arrays for Photocatalysis, Photovoltaics, and Optoelectronic and Optical Devices. Adv. Funct. Mater. 2019, 29 (25), 1807275,  DOI: 10.1002/adfm.201807275
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    Ulusoy Ghobadi, T. G.; Ghobadi, A.; Odabasi, O.; Karadas, F.; Ozbay, E. Subwavelength Densely Packed Disordered Semiconductor Metasurface Units for Photoelectrochemical Hydrogen Generation. ACS Appl. Energy Mater. 2022, 5 (3), 2826– 2837,  DOI: 10.1021/acsaem.1c03363
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    40
    Subwavelength Densely Packed Disordered Semiconductor Metasurface Units for Photoelectrochemical Hydrogen Generation
    Ulusoy Ghobadi, T. Gamze; Ghobadi, Amir; Odabasi, Oguz; Karadas, Ferdi; Ozbay, Ekmel
    ACS Applied Energy Materials (2022), 5 (3), 2826-2837CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    For most semiconductors, esp. the visible-light-absorbing ones, the carrier diffusion length is significantly shorter than the light penetration depth, limiting their photoactivities. This limitation could be mitigated through the use of subwavelength semiconductor-based metasurfaces and metamaterials. In this paper, a large-scale compatible metasurface photocathode, made of densely packed disordered p-type chromium oxide (CrOX), is developed to be utilized in photoelectrochem. (PEC) hydrogen generation. For this purpose, first, tightly packed random Cr nanorods are fabricated using an oblique angle deposition technique. Afterward, an annealing step is applied to the sample to transform these metallic units into a semiconducting p-type CrOX-based metasurface. Based on the exptl. characterization results and numerical simulations, the proposed design can provide strong light-matter interactions in an ultra-broadband-wavelength range, mainly due to its multidimensional random geometry and ultrasmall gap sizes. Finally, to substantiate the activity of the CrOX nanorods, a core-crown geometry is developed where the NiOX capping layer catalyzes the hydrogen evolution reaction (HER). The proposed heterostructure metasurface absorber can impose photocurrent values as large as 50μA cm-2 with a photocurrent spectral response extended up to 500 nm. Moreover, the electrode shows outstanding operation under light irradn. for 9 h. This work demonstrates a simple, scalable design strategy to fabricate low-cost and stable photocathodes for PEC hydrogen evolution.
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    Lithography-Free Random Bismuth Nanostructures for Full Solar Spectrum Harvesting and Mid-Infrared Sensing
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    Advanced Optical Materials (2020), 8 (4), 1901203CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A lithog.-free, double-functional single bismuth (Bi) metal nanostructure is designed, fabricated, and characterized for ultrabroadband absorption in the visible (vis) and near-IR (NIR) ranges, and for a narrowband response with ultrahigh refractive index sensitivity in the mid-IR (MIR) range. To achieve a large-scale fabrication of the design in a lithog.-free route, the oblique-angle deposition approach is used to obtain densely packed and randomly spaced/oriented Bi nanostructures. It is shown that this fabrication technique can provide a bottom-up approach to controlling the length and spacing of the design. The characterization findings reveal a broadband absorbance above 0.8 in vis and NIR, and a narrowband absorbance centered around 6.54μm. Dense architecture and extraordinary permittivity of Bi provide strong field confinement in ultrasmall gaps between nanostructures, and this can be utilized for a sensing application. An ultrahigh sensitivity of 2151 nm refractive-index unit (RIU-1) is acquired, which is, as far as it is known, the exptl. highest sensitivity attained so far. The simple and large-scale compatible fabrication route of the design together with the extraordinary optical response of Bi coating makes this design promising for many optoelectronic and sensing applications.
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    Influence of order-to-disorder transitions on the optical properties of the aluminum plasmonic metasurface
    Zhang, Feifei; Tang, Feng; Xu, Xiaolun; Adam, Pierre-Michel; Martin, Jerome; Plain, Jerome
    Nanoscale (2020), 12 (45), 23173-23182CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    To mimic the optical influence of disorder in condensed matter, the effect of uniform disorder on plasmonic resonances were investigated numerically and exptl. on aluminum (Al) nanoparticle arrays. Resorting to the analog of a plasmonic periodic array to a crystal on the sharp optical spectrum and its anisotropy, the disorder in the transition from crystal to glass (with broadened spectrum and isotropy) is imitated by three kinds of Al plasmonic metasurfaces: varying the displacement, size and rotation of each Al nanoparticle in the periodic array. The random variation on the location or size of each Al nanodisk in the plasmonic crystal induces broadening and redn. of their plasmonic resonances without significantly shifting its wavelength. Moreover, by rotating each Al nanorod in the plasmonic crystal by a random angle, the polarization dependence of plasmonic resonances is progressively decreased by increasing the rotation disorder. Thanks to these three kinds of Al metasurfaces, an enlightened understanding of the random physics in the solid state and the influence of manufg. deviation in nanophotonics is supported.
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    Huo, D.; Zhang, J.; Wang, H.; Ren, X.; Wang, C.; Su, H.; Zhao, H. Broadband Perfect Absorber with Monolayer MoS2 and Hexagonal Titanium Nitride Nano-Disk Array. Nanoscale Res. Lett. 2017, 12 (1), 465,  DOI: 10.1186/s11671-017-2232-4
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    Broadband Perfect Absorber with Monolayer MoS2 and Hexagonal Titanium Nitride Nano-disk Array
    Huo Dewang; Zhang Jingwen; Wang Hao; Ren Xiaoxuan; Wang Chao; Su Hang; Zhao Hua; Zhang Jingwen; Zhao Hua
    Nanoscale research letters (2017), 12 (1), 465 ISSN:1931-7573.
    A broadband metamaterial absorber (MA) composed of hexagonal-arranged single-sized titanium nitride (TiN) nano-disk array and monolayer molybdenum disulfide (MoS2) is studied using finite-difference time-domain (FDTD) simulations. The structure of TiN nano-disk array/dielectric silica (SiO2)/aluminum (Al) is adopted in our design. By optimizing the dimension parameters of the structure, an average absorption of 96.1% is achieved from 400 to 850 nm. In addition, by inserting a monolayer MoS2 which has high absorption at the short wavelength side underneath the TiN nano-disk array, an average absorption of 98.1% over the entire visible regime from 400 to 850 nm was achieved, with a peak absorption near 100% and absorption over 99% from 475 to 772 nm. Moreover, the absorber presented in this paper is polarization insensitive. This compact and unique design with TiN nano-disk/monolayer MoS2/ SiO2/Al structure may have great potential for applications in photovoltaics and light trapping.
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    Wang, Jianguo; Zhang, Weili; Zhu, Meiping; Yi, Kui; Shao, Jianda
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    A broadband metamaterial absorber (MA) based on the titanium nitride (TiN) nano-disk array is studied using finite difference time domain simulations. The semiconducting indium tin oxide (ITO) thin film is introduced as the space layer in this sandwiched structure. Utilizing the sym. geometry of the MA structure, polarization insensitivity of the broadband absorption was gained. The absorber with TiN nano-disk array shows a peak absorbance of 99 % and larger than 98 % from 560 to 675 nm by numerical simulation. This compact design may have potential applications in the plasmonic sensing and photovoltaic devices.
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    Dongare, P. D.; Alabastri, A.; Neumann, O.; Nordlander, P.; Halas, N. J. Solar Thermal Desalination as a Nonlinear Optical Process. Proc. Natl. Acad. Sci. U. S. A. 2019, 116 (27), 13182– 13187,  DOI: 10.1073/pnas.1905311116
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    Solar thermal desalination as a nonlinear optical process
    Dongare, Pratiksha D.; Alabastri, Alessandro; Neumann, Oara; Nordlander, Peter; Halas, Naomi J.
    Proceedings of the National Academy of Sciences of the United States of America (2019), 116 (27), 13182-13187CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The ever-increasing global need for potable water requires practical, sustainable approaches for purifying abundant alternative sources such as seawater, high-salinity processed water, or underground reservoirs. Evapn.-based solns. are of particular interest for treating high salinity water, since conventional methods such as reverse osmosis have increasing energy requirements for higher concns. of dissolved minerals. Demonstration of efficient water evapn. with heat localization in nanoparticle solns. under solar illumination has led to the recent rapid development of sustainable, solar-driven distn. methods. Given the amt. of solar energy available per square meter at the Earth's surface, however, it is important to utilize these incident photons as efficiently as possible to maximize clean water output. Here we show that merely focusing incident sunlight into small "hot spots" on a photothermally active desalination membrane dramatically increases- by more than 50%- the flux of distd. water. This large boost in efficiency results from the nearly exponential dependence of water vapor satn. pressure on temp., and therefore on incident light intensity. Exploiting this inherent but previously unrecognized optical nonlinearity should enable the design of substantially higher-throughput solar thermal desalination methods. This property provides a mechanism capable of enhancing a far wider range of photothermally driven processes with supralinear intensity dependence, such as light-driven chem. reactions and sepn. methods.
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    Palm, K. J.; Murray, J. B.; Narayan, T. C.; Munday, J. N. Dynamic Optical Properties of Metal Hydrides. ACS Photonics 2018, 5 (11), 4677– 4686,  DOI: 10.1021/acsphotonics.8b01243
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    Dynamic Optical Properties of Metal Hydrides
    Palm, Kevin J.; Murray, Joseph B.; Narayan, Tarun C.; Munday, Jeremy N.
    ACS Photonics (2018), 5 (11), 4677-4686CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Metal hydrides often display dramatic changes in optical properties upon hydrogenation. These shifts make them prime candidates for many tunable optical devices, such as optical hydrogen sensors and switchable mirrors. While some of these metals, such as palladium, have been well studied, many other promising materials have only been characterized over a limited optical range and lack direct in situ measurements of hydrogen loading, limiting their potential applications. Further, there have been no systematic studies that allow for a clear comparison between these metals. In this work, we present such a systematic study of the dynamically tunable optical properties of Pd, Mg, Zr, Ti, and V throughout hydrogenation with a wavelength range of 250-1690 nm. These measurements were performed in an environmental chamber, which combines mass measurements via a quartz crystal microbalance with ellipsometric measurements in up to 7 bar of hydrogen gas, allowing us to det. the optical properties during hydrogen loading. In addn., we demonstrate a further tunability of the optical properties of titanium and its hydride by altering annealing conditions, and we investigate the optical and gravimetric hysteresis that occurs during hydrogenation cycling of palladium. Finally, we demonstrate several nanoscale optical and plasmonic structures based on these dynamic properties. We show structures that, upon hydrogenation, demonstrate 5 orders of magnitude change in reflectivity, resonance shifts of >200 nm, and relative transmission switching of >3000%, suggesting a wide range of applications.
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  • Abstract

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    Figure 1

    Figure 1. Nanoporous TiOxNy NT arrays for broadband perfect absorption. (a) Schematic summary of morphological changes of NTs during conversion of TiO2 to TiOxNy. (b) SEM (left) and TEM (middle and right) images of NT arrays anodized at 60 V for 6 h and nitridated at 900 °C. (c) Sketch of the numerical model mimicking the NTs morphology, where the tube porosity is explicitly accounted for by creating a random space-dependent 3D map of permittivity, shown here, mixing voids and metal. The degree of porosity (i.e., metal content against voids) is controlled numerically by a threshold parameter (ths). (d) Simulated absorption spectra for an exemplary NT array on a Ti substrate by varying the threshold parameter ths, i.e., for increasing metal-to-void ratio. Numerical calculations considered the following geometrical parameters: outer diameter, 236 nm; wall thickness, 22.5 nm; length, 3.18 μm; array periodicity, 543 nm.

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    Figure 2

    Figure 2. Impact of thermal treatment on chemical composition of NTs. (a) X-ray diffraction patterns of (blue) as-prepared and (gray) air-annealed TiO2 nanotube arrays and nitridated samples at different temperatures of (red) 700 and (green) 900 °C. (b) A magnified section of the patterns that emphasizes the shift of the (111) and (200) crystallographic orientations by increasing the nitridation temperature.

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    Figure 3

    Figure 3. Morphology and absorption spectra of NT arrays. (a–e) Cross-sectional SEM images and sketches of the geometries (the geometrical parameters considered in the simulations are set based on SEM analysis of the samples) simulated in the numerical model mimicking the experimental TiOxNy NT arrays (threshold value set to 0.5 in calculations) anodized for 6 h at (a) 20, (b) 25, (c) 30, and (d, e) 60 V and nitridated at (a–d) 700 and (e) 900 °C. (f) Effects of the applied potential and nitridation temperature on the average diameter (right-hand side axis) and length (left-hand side axis) of NT arrays. (g, h) Experimental and calculated optical absorption of NTs, respectively.

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    Figure 4

    Figure 4. Modeling porosity across NTs: failure of the effective medium theory. (a) From left to right, a 3D map of a single NT permittivity (imaginary part, ε′′), the spatial distribution of electric field enhancement factor (|E|/E0), and dissipation power density (Qdiss) calculated at a representative wavelength of 532 nm by varying the threshold parameter from 0.3 (left), 0.5 (middle), to 0.7 (right). Numerical results refer to sample #4 (outer diameter 293 nm, wall thickness 30.5 nm, length 3.3 μm, array periodicity 586 nm). (b) Same as (a) when effective medium theory (EMT) is employed in the simulations to define an effective uniform permittivity across the individual NT, mixing air and TiN permittivities. EMT-based calculations considered the metal-to-void ratio set by the threshold value used in the corresponding simulations in (a). (c) Numerically computed absorption spectra for the exemplary NT array considered (geometrical parameters for sample #4) for varying values of the threshold ths, by applying the original modeling approach here proposed (NPMT, solid curves) and EMT (Bruggeman formalism, dotted curves), respectively.

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    Figure 5

    Figure 5. Photothermal applications of NT arrays. (a) The schematic setup for temperature measurement via an IR sensor. (b) Heating–cooling cycles of #5 NT arrays, anodized at 60 V for 6 h and nitridated at 900 °C, under different irradiation intensities of an LED lamp. (c) (gray, left-hand side axis) Maximum temperature (Tmax) extracted from the heating and cooling curves of different samples under 12 Suns irradiation of an LED lamp in comparison with the averaged solar absorptance of NTs (blue, right-hand side axis). (d) The schematic diagram of the steam generation setup. (e) Comparison of solar steam generation performance among sample #5, Ti plate, and water.

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    1. 1
      Chang, C.-C.; Kort-Kamp, W. J. M.; Nogan, J.; Luk, T. S.; Azad, A. K.; Taylor, A. J.; Dalvit, D. A. R.; Sykora, M.; Chen, H.-T. High-Temperature Refractory Metasurfaces for Solar Thermophotovoltaic Energy Harvesting. Nano Lett. 2018, 18 (12), 7665– 7673,  DOI: 10.1021/acs.nanolett.8b03322
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      High-Temperature Refractory Metasurfaces for Solar Thermophotovoltaic Energy Harvesting
      Chang, Chun-Chieh; Kort-Kamp, Wilton J. M.; Nogan, John; Luk, Ting S.; Azad, Abul K.; Taylor, Antoinette J.; Dalvit, Diego A. R.; Sykora, Milan; Chen, Hou-Tong
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      Solar energy promises a viable soln. to meet the ever-increasing power demand by providing a clean, renewable energy alternative to fossil fuels. For solar thermophotovoltaics (STPV), high-temp. absorbers and emitters with strong spectral selectivity are imperative to efficiently couple solar radiation into photovoltaic cells. Here, we demonstrate refractory metasurfaces for STPV with tailored absorptance and emittance characterized by in situ high-temp. measurements, featuring thermal stability up to at least 1200 °C. Our tungsten-based metasurface absorbers have close-to-unity absorption from visible to near-IR and strongly suppressed emission at longer wavelengths, while our metasurface emitters provide wavelength-selective emission spectrally matched to the band-edge of InGaAsSb photovoltaic cells. The projected overall STPV efficiency is as high as 18% when a fully integrated absorber/emitter metasurface structure is employed, which is comparable to the efficiencies of the best currently available com. single-junction PV cells and can be further improved to potentially exceed those in mainstream photovoltaic technologies. Our work opens a path forward for high-performance STPV systems based on refractory metasurface structures.
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    2. 2
      Zhou, J.; Liu, Z.; Liu, G.; Pan, P.; Liu, X.; Tang, C.; Liu, Z.; Wang, J. Ultra-Broadband Solar Absorbers for High-Efficiency Thermophotovoltaics. Opt. Express 2020, 28 (24), 36476,  DOI: 10.1364/OE.411918
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      2
      Ultra-broadband solar absorbers for high-efficiency thermophotovoltaics
      Zhou, Jin; Liu, Zhengqi; Liu, Guiqiang; Pan, Pingping; Liu, Xiaoshan; Tang, Chaojun; Liu, Zhongmin; Wang, Junqiao
      Optics Express (2020), 28 (24), 36476-36486CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
      Metamaterial absorbers have attracted great attention over the past few years and exhibited a promising prospect in solar energy harvesting and solar thermophotovoltaics (STPVs). In this work, we introduce a solar absorber scheme, which enables efficient solar irradiance harvesting, superb thermal robustness and high solar thermal energy conversion for STPV systems. The optimum structure demonstrates an av. absorbance of 97.85% at the spectral region from 200 nm to 2980 nm, indicating the near-unity absorption in the main energy range of the solar radiance. The solar-thermal conversion efficiencies surpassing 90% are achieved over an ultra-wide temp. range (100-800°C). Meanwhile, the anal. indicates that this metamaterial has strong tolerance for fabrication errors. By utilizing the simple two-dimensional (2D) titanium (Ti) gratings, this design is able to get beyond the limit of costly and sophisticated nanomanufg. techniques. These impressive features can hold the system with wide applications in metamaterial and other optoelectronic devices.
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    3. 3
      Cai, W.; Shalaev, V. Optical Properties of Metal-Dielectric Composites. In Optical Metamaterials; Springer New York: New York, NY, 2010; pp 11– 37.  DOI: 10.1007/978-1-4419-1151-3_2 .
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      There is no corresponding record for this reference.
    4. 4
      Li, W.; Guler, U.; Kinsey, N.; Naik, G. V.; Boltasseva, A.; Guan, J.; Shalaev, V. M.; Kildishev, A. V. Refractory Plasmonics with Titanium Nitride: Broadband Metamaterial Absorber. Adv. Mater. 2014, 26 (47), 7959– 7965,  DOI: 10.1002/adma.201401874
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      4
      Refractory Plasmonics with Titanium Nitride: Broadband Metamaterial Absorber
      Li, Wei; Guler, Urcan; Kinsey, Nathaniel; Naik, Gururaj V.; Boltasseva, Alexandra; Guan, Jianguo; Shalaev, Vladimir M.; Kildishev, Alexander V.
      Advanced Materials (Weinheim, Germany) (2014), 26 (47), 7959-7965CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A refractory TiN metamaterial absorber with efficient, broadband light absorption is designed and fabricated. The resulting 240-nm-thick metamaterial absorber exhibits polarization independent broad absorption over the whole visible range of 400-800 nm at a large incident angle (up to 70°). Most importantly, the fabricated TiN metamaterial absorber shows vastly improved thermal stability, illustrating the promise for the high temp. applications.
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    5. 5
      Chaudhuri, K.; Alhabeb, M.; Wang, Z.; Shalaev, V. M.; Gogotsi, Y.; Boltasseva, A. Highly Broadband Absorber Using Plasmonic Titanium Carbide (MXene). ACS Photonics 2018, 5 (3), 1115– 1122,  DOI: 10.1021/acsphotonics.7b01439
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      5
      Highly Broadband Absorber Using Plasmonic Titanium Carbide (MXene)
      Chaudhuri, Krishnakali; Alhabeb, Mohamed; Wang, Zhuoxian; Shalaev, Vladimir M.; Gogotsi, Yury; Boltasseva, Alexandra
      ACS Photonics (2018), 5 (3), 1115-1122CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Control of light transmission and reflection through nanostructured materials led to demonstration of metamaterial absorbers that have augmented the performance of energy harvesting applications of several optoelectronic and nanophotonic systems. A broadband plasmonic metamaterial absorber is fabricated using 2-dimensional Ti carbide (Ti3C2Tx) MXene. Arrays of nanodisks made of Ti3C2Tx exhibit strong localized surface plasmon resonances at near-IR frequencies. By exploiting the scattering enhancement at the resonances and the optical losses inherent to Ti3C2Tx MXene, high-efficiency absorption (∼90%) for a wide wavelength window of incident illumination (∼1.55μm) was achieved.
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    6. 6
      Cui, Y.; He, Y.; Jin, Y.; Ding, F.; Yang, L.; Ye, Y.; Zhong, S.; Lin, Y.; He, S. Plasmonic and Metamaterial Structures as Electromagnetic Absorbers: Plasmonic and Metamaterial Absorbers. Laser Photonics Rev. 2014, 8 (4), 495– 520,  DOI: 10.1002/lpor.201400026
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      6
      Plasmonic and metamaterial structures as electromagnetic absorbers
      Cui, Yanxia; He, Yingran; Jin, Yi; Ding, Fei; Yang, Liu; Ye, Yuqian; Zhong, Shoumin; Lin, Yinyue; He, Sailing
      Laser & Photonics Reviews (2014), 8 (4), 495-520CODEN: LPRAB8; ISSN:1863-8880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Electromagnetic absorbers have drawn increasing attention in many areas. A series of plasmonic and metamaterial structures can work as efficient narrowband absorbers due to the excitation of plasmonic or photonic resonances, providing a great potential for applications in designing selective thermal emitters, biosensing, etc. In other applications such as solar-energy harvesting and photonic detection, the bandwidth of light absorbers is required to be quite broad. Under such a background, a variety of mechanisms of broadband/multiband absorption have been proposed, such as mixing multiple resonances together, exciting phase resonances, slowing down light by anisotropic metamaterials, employing high loss materials and so on.
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    7. 7
      Bilal, R. M. H.; Saeed, M. A.; Choudhury, P. K.; Baqir, M. A.; Kamal, W.; Ali, M. M.; Rahim, A. A. Elliptical Metallic Rings-Shaped Fractal Metamaterial Absorber in the Visible Regime. Sci. Rep. 2020, 10 (1), 14035,  DOI: 10.1038/s41598-020-71032-8
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      7
      Elliptical metallic rings-shaped fractal metamaterial absorber in the visible regime
      Bilal, R. M. H.; Saeed, M. A.; Choudhury, P. K.; Baqir, M. A.; Kamal, W.; Ali, M. M.; Rahim, A. A.
      Scientific Reports (2020), 10 (1), 14035CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
      Achieving the broadband response of metamaterial absorbers has been quite challenging due to the inherent bandwidth limitations. Herein, the investigation was made of a unique kind of visible light metamaterial absorber comprising elliptical rings-shaped fractal metasurface using tungsten metal. It was found that the proposed absorber exhibits av. absorption of over 90% in the visible wavelength span of 400-750 nm. The features of perfect absorption could be obsd. because of the localized surface plasmon resonance that causes impedance matching. Moreover, in the context of optoelectronic applications, the absorber yields absorbance up to ∼ 70% even with the incidence obliquity in the range of 0°-60° for transverse elec. polarization. The theory of multiple reflections was employed to further verify the performance of the absorber. The obtained theor. results were found to be in close agreement with the simulation results. In order to optimize the results, the performance was analyzed in terms of the figure of merit and operating bandwidth. Significant amt. of absorption in the entire visible span, wide-angle stability, and utilization of low-cost metal make the proposed absorber suitable in varieties of photonics applications, in particular photovoltaics, thermal emitters and sensors.
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    8. 8
      Kenney, M.; Grant, J.; Shah, Y. D.; Escorcia-Carranza, I.; Humphreys, M.; Cumming, D. R. S. Octave-Spanning Broadband Absorption of Terahertz Light Using Metasurface Fractal-Cross Absorbers. ACS Photonics 2017, 4 (10), 2604– 2612,  DOI: 10.1021/acsphotonics.7b00906
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      8
      Octave-Spanning Broadband Absorption of Terahertz Light Using Metasurface Fractal-Cross Absorbers
      Kenney, Mitchell; Grant, James; Shah, Yash D.; Escorcia-Carranza, Ivonne; Humphreys, Mark; Cumming, David R. S.
      ACS Photonics (2017), 4 (10), 2604-2612CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Synthetic fractals inherently carry spatially encoded frequency information that renders them as an ideal candidate for broadband optical structures. Nowhere is this more true than in the terahertz (THz) band where there is a lack of naturally occurring materials with valuable optical properties. One example are perfect absorbers that are a direct step towards the development of highly sought after detectors and sensing devices. Metasurface absorbers that can be used to substitute for natural materials suffer from poor broadband performance, while those with high absorption and broadband capability typically involve complex fabrication and design and are multilayered. Here, we demonstrate a polarization-insensitive ultrathin (∼λ/6) planar metasurface THz absorber composed of supercells of fractal crosses capable of spanning one optical octave in bandwidth, while still being highly-efficient. A sufficiently thick polyimide interlayer produces a unique absorption mechanism based on Salisbury screen and anti-reflection responses, which lends to the broadband operation. Exptl. peak absorption exceeds 93%, while the av. absorption is 83% from 2.82 THz to 5.15 THz. This new ultrathin device architecture, achieving an absorption-bandwidth of one optical octave, demonstrates a major advance towards a synthetic metasurface blackbody absorber in the THz band.
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    9. 9
      Nguyen, T. Q. M.; Nguyen, T. K. T.; Le, D. T.; Truong, C. L.; Vu, D. L.; Nguyen, T. Q. H. Numerical Study of an Ultra-Broadband and Wide-Angle Insensitive Perfect Metamaterial Absorber in the UV-NIR Region. Plasmonics 2021, 16 (5), 1583– 1592,  DOI: 10.1007/s11468-021-01424-7
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      9
      Numerical Study of an Ultra-Broadband and Wide-Angle Insensitive Perfect Metamaterial Absorber in the UV-NIR Region
      Nguyen, Thi Quynh Mai; Nguyen, Thi Kim Thu; Le, Dac Tuyen; Truong, Chi Lam; Vu, Dinh Lam; Nguyen, Thi Quynh Hoa
      Plasmonics (2021), 16 (5), 1583-1592CODEN: PLASCS; ISSN:1557-1955. (Springer)
      Developing a simple structure using low-cost material that enables both large-scale fabrication and broadband absorption response is highly desirable but very challenging for achieving high-performance metamaterial absorber. Herein, we propose and numerically investigate an ultra-broadband and wide-angle insensitive perfect metamaterial absorber in the UV to near-IR (UV-NIR) region based on a simple metal-dielec.-metal structure. The proposed absorber structure consists of a periodic array of a tungsten hexagonal prism and a tungsten ground plane sepd. by a silicon dioxide dielec. substrate. The proposed absorber achieves an ultra-broadband absorption response in the range of 275-1000 nm with an absorptivity above 90% and a relative bandwidth of 106.8% at normal incidence, which covers from the UV to NIR region. The absorption efficiency is maintained with the figure of merit ηOBW higher than 90% for a wide incident angle up to 40o for transverse elec. (TE) polarization and 65o for transverse magnetic (TM) polarization. The effects of structural parameters and different metallic materials on the absorption performance are presented. In addn., the phys. mechanism is analyzed using the surface d. and distributions of elec. and magnetic fields that are attributed to both localized surface plasmon (LSP) and propagating surface plasmon (PSP) resonances. Owing to outstanding merits of simple structure, low cost, and high absorption performance, the designed absorber can be suitable for many applications in the UV-NIR spectrum such as thermal emitters and solar cells.
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    10. 10
      Zhou, L.; Tan, Y.; Ji, D.; Zhu, B.; Zhang, P.; Xu, J.; Gan, Q.; Yu, Z.; Zhu, J. Self-Assembly of Highly Efficient, Broadband Plasmonic Absorbers for Solar Steam Generation. Sci. Adv. 2016, 2 (4), e1501227  DOI: 10.1126/sciadv.1501227
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      10
      Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation
      Zhou, Lin; Tan, Yingling; Ji, Dengxin; Zhu, Bin; Zhang, Pei; Xu, Jun; Gan, Qiaoqiang; Yu, Zongfu; Zhu, Jia
      Science Advances (2016), 2 (4), e1501227/1-e1501227/8CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
      The study of ideal absorbers, which can efficiently absorb light over a broad range of wavelengths, is of fundamental importance, as well as crit. for many applications from solar steam generation and thermophotovoltaics to light/thermal detectors. As a result of recent advances in plasmonics, plasmonic absorbers have attracted a lot of attention. However, the performance and scalability of these absorbers, predominantly fabricated by the top-down approach, need to be further improved to enable widespread applications. We report a plasmonic absorber which can enable an av. measured absorbance of ∼99% across the wavelengths from 400 nm to 10 μm, the most efficient and broadband plasmonic absorber reported to date. The absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by a one-step deposition process. Because of its efficient light absorption, strong field enhancement, and porous structures, which together enable not only efficient solar absorption but also significant local heating and continuous stream flow, plasmonic absorber-based solar steam generation has over 90% efficiency under solar irradn. of only 4-sun intensity (4 kW m-2). The pronounced light absorption effect coupled with the high-throughput self-assembly process could lead toward large-scale manufg. of other nanophotonic structures and devices.
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    11. 11
      Richardson, H. H.; Carlson, M. T.; Tandler, P. J.; Hernandez, P.; Govorov, A. O. Experimental and Theoretical Studies of Light-to-Heat Conversion and Collective Heating Effects in Metal Nanoparticle Solutions. Nano Lett. 2009, 9 (3), 1139– 1146,  DOI: 10.1021/nl8036905
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      11
      Experimental and Theoretical Studies of Light-to-Heat Conversion and Collective Heating Effects in Metal Nanoparticle Solutions
      Richardson, Hugh H.; Carlson, Michael T.; Tandler, Peter J.; Hernandez, Pedro; Govorov, Alexander O.
      Nano Letters (2009), 9 (3), 1139-1146CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors perform a set of expts. on photoheating in a H2O droplet contg. Au nanoparticles (NPs). Using photocalorimetric methods, the authors det. efficiency of light-to-heat conversion (η) which turns out to be remarkably close to 1, (0.97 < η < 1.03). Detailed studies reveal a complex character of heat transfer in an optically stimulated droplet. The main mechanism of equilibration is due to convectional flow. Theor. modeling is performed to describe thermal effects at both nano- and millimeter scales. Theory shows that the collective photoheating is the main mechanism. For a large concn. of NPs and small laser intensity, an averaged temp. increase (at the millimeter scale) is significant (∼7°), whereas on the nanometer scale the temp. increase at the surface of a single NP is small (∼0.02°). In the opposite regime, i.e., a small NP concn. and intense laser irradn., an opposite picture: a temp. increase at the millimeter scale is small (∼0.1°) but a local, nanoscale temp. has strong local spikes at the surfaces of NPs (∼3°) were found. These studies are crucial for the understanding of photothermal effects in NPs and for their potential and current applications in nano- and biotechnologies.
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    12. 12
      Zhu, M.; Li, Y.; Chen, F.; Zhu, X.; Dai, J.; Li, Y.; Yang, Z.; Yan, X.; Song, J.; Wang, Y.; Hitz, E.; Luo, W.; Lu, M.; Yang, B.; Hu, L. Plasmonic Wood for High-Efficiency Solar Steam Generation. Adv. Energy Mater. 2018, 8 (4), 1701028,  DOI: 10.1002/aenm.201701028
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    13. 13
      Tian, L.; Xin, Q.; Zhao, C.; Xie, G.; Akram, M. Z.; Wang, W.; Ma, R.; Jia, X.; Guo, B.; Gong, J. R. Nanoarray Structures for Artificial Photosynthesis. Small 2021, 17 (38), 2006530,  DOI: 10.1002/smll.202006530
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      13
      Nanoarray Structures for Artificial Photosynthesis
      Tian, Liangqiu; Xin, Qi; Zhao, Chang; Xie, Guancai; Akram, Muhammad Zain; Wang, Wenrong; Ma, Renping; Jia, Xinrui; Guo, Beidou; Gong, Jian Ru
      Small (2021), 17 (38), 2006530CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Conversion and storage of solar energy into fuels and chems. by artificial photosynthesis has been considered as one of the promising methods to address the global energy crisis. However, it is still far from the practical applications on a large scale. Nanoarray structures that combine the advantages of nanosize and array alignment have demonstrated great potential to improve solar energy conversion efficiency, stability, and selectivity. This article provides a comprehensive review on the utilization of nanoarray structures in artificial photosynthesis of renewable fuels and high value-added chems. First, basic principles of solar energy conversion and superiorities of using nanoarray structures in this field are described. Recent research progress on nanoarray structures in both abiotic and abiotic-biotic hybrid systems is then outlined, highlighting contributions to light absorption, charge transport and transfer, and catalytic reactions (including kinetics and selectivity). Finally, conclusions and outlooks on future research directions of nanoarray structures for artificial photosynthesis are presented.
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    14. 14
      Patsalas, P.; Kalfagiannis, N.; Kassavetis, S.; Abadias, G.; Bellas, D. V.; Lekka, Ch.; Lidorikis, E. Conductive Nitrides: Growth Principles, Optical and Electronic Properties, and Their Perspectives in Photonics and Plasmonics. Mater. Sci. Eng. R Rep. 2018, 123, 1– 55,  DOI: 10.1016/j.mser.2017.11.001
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      There is no corresponding record for this reference.
    15. 15
      Yalavarthi, R.; Henrotte, O.; Kment, Š.; Naldoni, A. Determining the Role of Pd Catalyst Morphology and Deposition Criteria over Large Area Plasmonic Metasurfaces during Light-Enhanced Electrochemical Oxidation of Formic Acid. J. Chem. Phys. 2022, 157 (11), 114706,  DOI: 10.1063/5.0102012
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      15
      Determining the role of Pd catalyst morphology and deposition criteria over large area plasmonic metasurfaces during light-enhanced electrochemical oxidation of formic acid
      Yalavarthi, Rambabu; Henrotte, Olivier; Kment, Stepan; Naldoni, Alberto
      Journal of Chemical Physics (2022), 157 (11), 114706CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The use of metal composites based on plasmonic nanostructures partnered with catalytic counterparts has recently emerged as a promising approach in the field of plasmon-enhanced electrocatalysis. Here, we report on the role of the surface morphol., size, and anchored site of Pd catalysts coupled to plasmonic metasurfaces formed by periodic arrays of multimetallic Ni/Au nanopillars for formic acid electro-oxidn. reaction (FAOR). We compare the activity of two kinds of metasurfaces differing in the positioning of the catalytic Pd nanoparticles. In the first case, the Pd nanoparticles have a polyhedron crystal morphol. with exposed (200) facets and were deposited over the Ni/Au metasurfaces in a site-selective fashion by limiting their growth at the electromagnetic hot spots (Ni/Au-Pd@W). In contrast, the second case consists of spherical Pd nanoparticles grown in soln., which are homogeneously deposited onto the Ni/Au metasurface (Ni/Au-Pd@M). Ni/Au-Pd@W catalytic metasurfaces demonstrated higher light-enhanced FAOR activity (61%) in comparison to the Ni/Au-Pd@M sample (42%) for the direct dehydrogenation pathway. Moreover, the site-selective Pd deposition promotes the growth of nanoparticles favoring a more selective catalytic behavior and a lower degree of CO poisoning on Pd surface. The use of cyclic voltammetry, energy-resolved incident photon to current conversion efficiency, open circuit potential, and electrochem. impedance spectroscopy highlights the role of plasmonic near fields and hot holes in driving the catalytic enhancement under light conditions. (c) 2022 American Institute of Physics.
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    16. 16
      Guler, U.; Ndukaife, J. C.; Naik, G. V.; Nnanna, A. G. A.; Kildishev, A. V.; Shalaev, V. M.; Boltasseva, A. Local Heating with Lithographically Fabricated Plasmonic Titanium Nitride Nanoparticles. Nano Lett. 2013, 13 (12), 6078– 6083,  DOI: 10.1021/nl4033457
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      16
      Local Heating with Lithographically Fabricated Plasmonic Titanium Nitride Nanoparticles
      Guler, Urcan; Ndukaife, Justus C.; Naik, Gururaj V.; Nnanna, A. G. Agwu; Kildishev, Alexander V.; Shalaev, Vladimir M.; Boltasseva, Alexandra
      Nano Letters (2013), 13 (12), 6078-6083CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Titanium nitride is considered a promising alternative plasmonic material and is known to exhibit localized surface plasmon resonances within the near-IR biol. transparency window. Here, local heating efficiencies of disk-shaped nanoparticles made of titanium nitride and gold are compared in the visible and near-IR regions numerically and exptl. with samples fabricated using e-beam lithog. Results show that plasmonic titanium nitride nanodisks are efficient local heat sources and outperform gold nanodisks in the biol. transparency window, dispensing the need for complex particle geometries.
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    17. 17
      Mascaretti, L.; Schirato, A.; Zbořil, R.; Kment, Š.; Schmuki, P.; Alabastri, A.; Naldoni, A. Solar Steam Generation on Scalable Ultrathin Thermoplasmonic TiN Nanocavity Arrays. Nano Energy 2021, 83, 105828,  DOI: 10.1016/j.nanoen.2021.105828
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      17
      Solar steam generation on scalable ultrathin thermoplasmonic TiN nanocavity arrays
      Mascaretti, Luca; Schirato, Andrea; Zboril, Radek; Kment, Stepan; Schmuki, Patrik; Alabastri, Alessandro; Naldoni, Alberto
      Nano Energy (2021), 83 (), 105828CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      Plasmonic-based solar absorbers exhibit complete light absorption in a sub-Μm thickness, representing an alternative to mm-thick carbon-based materials most typically employed for solar-driven steam generation. In this work, we present the scalable fabrication of ultrathin plasmonic titanium nitride (TiN) nanocavity arrays that exhibit 90% broadband solar light absorption within ∼ 250 nm from the illuminated surface and show a fast non-linear increase of performance with light intensity. At 14 Suns TiN nanocavities reach ∼ 15 kg h-1 m-2 evapn. rate and ∼ 76% thermal efficiency, a steep increase from ∼ 0.4 kg h-1 m-2 and ∼ 20% under 1.4 Suns. Electromagnetic, thermal and diffusion modeling of our system reveals the contribution of each material and reactor component to heat dissipation and shows that a quasi-two-dimensional heat dissipation regime significantly accelerates water evapn. Our approach to ultrathin plasmonic absorbers can boost the performance of devices for evapn./desalination and holds promise for a broader range of phase sepn. processes.
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    18. 18
      Li, Y.; Lin, C.; Wu, Z.; Chen, Z.; Chi, C.; Cao, F.; Mei, D.; Yan, H.; Tso, C. Y.; Chao, C. Y. H.; Huang, B. Solution-Processed All-Ceramic Plasmonic Metamaterials for Efficient Solar-Thermal Conversion over 100–727 C. Adv. Mater. 2021, 33 (1), 2005074,  DOI: 10.1002/adma.202005074
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      18
      Solution-Processed All-Ceramic Plasmonic Metamaterials for Efficient Solar-Thermal Conversion over 100-727°C
      Li, Yang; Lin, Chongjia; Wu, Zuoxu; Chen, Zhongying; Chi, Cheng; Cao, Feng; Mei, Deqing; Yan, He; Tso, Chi Yan; Chao, Christopher Y. H.; Huang, Baoling
      Advanced Materials (Weinheim, Germany) (2021), 33 (1), 2005074CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Low-cost and large-area solar-thermal absorbers with superior spectral selectivity and excellent thermal stability are vital for efficient and large-scale solar-thermal conversion applications, such as space heating, desalination, ice mitigation, photothermal catalysis, and concg. solar power. Few state-of-the-art selective absorbers are qualified for both low- (<200 C) and high-temp. (>600 C) applications due to insufficient spectral selectivity or thermal stability over a wide temp. range. Here, a high-performance plasmonic metamaterial selective absorber is developed by facile soln.-based processes via assembling an ultrathin (≈120 nm) titanium nitride (TiN) nanoparticle film on a TiN mirror. Enabled by the synergetic in-plane plasmon and out-of-plane Fabry-Perot resonances, the all-ceramic plasmonic metamaterial simultaneously achieves high, full-spectrum solar absorption (95%), low mid-IR emission (3% at 100 C), and excellent stability over a temp. range of 100-727 C, even outperforming most vacuum-deposited absorbers at their specific operating temps. The competitive performance of the soln.-processed absorber is accompanied by a significant cost redn. compared with vacuum-deposited absorbers. All these merits render it a cost-effective, universal soln. to offering high efficiency (89-93%) for both low- and high-temp. solar-thermal applications.
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    19. 19
      Moon, G. D.; Joo, J. B.; Dahl, M.; Jung, H.; Yin, Y. Nitridation and Layered Assembly of Hollow TiO 2 Shells for Electrochemical Energy Storage. Adv. Funct. Mater. 2014, 24 (6), 848– 856,  DOI: 10.1002/adfm.201301718
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      Nitridation and layered assembly of hollow TiO2 shells for electrochemical energy storage
      Moon, Geon Dae; Joo, Ji Bong; Dahl, Michael; Jung, Heejung; Yin, Yadong
      Advanced Functional Materials (2014), 24 (6), 848-856CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The nitridation of hollow TiO2 nanoshells and their layered assembly into electrodes for electrochem. energy storage are reported. The nitridated hollow shells are prepd. by annealing TiO2 shells, produced initially using a sol-gel process, under an NH3 environment at different temps. ranging from 700 to 900 !!!C, then assembled to form a robust monolayer film on a water surface through a quick and simple assembly process without any surface modification to the samples. This approach facilitates supercapacitor cell design by simplifying the electrochem. electrode structure by removing the need to use any org. binder or carbon-based conducting materials. The areal capacitance of the as-prepd. electrode is obsd. to be ≈180 times greater than that of a bare TiO2 electrode, mainly due to the enhanced elec. cond. of the TiN phase produced through the nitridation process. Furthermore, the electrochem. capacitance can be enhanced linearly by constructing an electrode with multilayered shell films through a repeated transfer process (0.8 to 7.1 mF cm-2, from one monolayer to 9 layers). Addnl., the high elec. cond. of the shell film makes it an excellent scaffold for supporting other psuedocapacitive materials (e.g., MnO2), producing composite electrodes with a specific capacitance of 743.9 F g-1 at a scan rate of 10 mV s-1 (based on the mass of MnO2) and a good cyclic stability up to 1000 cycles.
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    20. 20
      Wei, Q.; Kuhn, D. L.; Zander, Z.; DeLacy, B. G.; Dai, H.-L.; Sun, Y. Silica-Coating-Assisted Nitridation of TiO2 Nanoparticles and Their Photothermal Property. Nano Res. 2021, 14 (9), 3228– 3233,  DOI: 10.1007/s12274-021-3427-7
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      Silica-coating-assisted nitridation of TiO2 nanoparticles and their photothermal property
      Wei, Qilin; Kuhn, Danielle L.; Zander, Zachary; DeLacy, Brendan G.; Dai, Hai-Lung; Sun, Yugang
      Nano Research (2021), 14 (9), 3228-3233CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
      Nanoparticles of refractory compds. represent a class of stable materials showing a great promise to support localized surface plasmon resonances (LSPRs) in both visible and near IR (NIR) spectral regions. It is still challenging to rationally tune the LSPR band because of the difficulty to control the d. of charge carriers in individual refractory nanoparticles and maintain the dispersity of nanoparticles in the processes of synthesis and applications. In this work, controlled chem. transformation of titanium dioxide (TiO2) nanoparticles encapsulated with mesoporous silica (SiO2) shells to titanium nitride (TiN) via nitridation reaction at elevated temps. is developed to tune the d. of free electrons in the resulting titanium-oxide-nitride (TiOxNy) nanoparticles. Such tunability enables a flexibility to support LSPR-based optical absorption in the synthesized TiOxNy@SiO2 core-shell nanoparticles across both the visible and NIR regions. The silica shells play a crucial role in preventing the sintering of TiOxNy nanoparticles in the nitridation reaction and maintaining the stability of TiOxNy nanoparticles in applications. The LSPR-based broadband absorption of light in the TiOxNy@SiO2 nanoparticles exhibits strong photothermal effect with photo-to-thermal conversion efficiency as high as ∼ 76%. [graphic not available: see fulltext].
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    21. 21
      Li, C.; Shi, J.; Zhu, L.; Zhao, Y.; Lu, J.; Xu, L. Titanium Nitride Hollow Nanospheres with Strong Lithium Polysulfide Chemisorption as Sulfur Hosts for Advanced Lithium-Sulfur Batteries. Nano Res. 2018, 11 (8), 4302– 4312,  DOI: 10.1007/s12274-018-2017-9
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      Titanium nitride hollow nanospheres with strong lithiumpolysulfide chemisorption as sulfur hosts for advanced lithium-sulfur batteries
      Li, Chuanchuan; Shi, Jingjing; Zhu, Lin; Zhao, Yingyue; Lu, Jun; Xu, Liqiang
      Nano Research (2018), 11 (8), 4302-4312CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
      Lithium-sulfur batteries are promising electrochem. energy storage devicesbecause of their high theor. specific capacity and energy d. An ideal sulfur host should possess high cond. and embrace the phys. confinement or strong chemisorption to dramatically suppress the polysulfide dissoln. Herein, uniform TiN hollow nanospheres with an av. diam. of ∼160 nm have been reported as highly efficient lithium polysulfide reservoirs for high-performance lithium-sulfur batteries. Combining the high cond. and chem. trappingof lithium polysulfides, the obtained S/TiN cathode of 70 wt.% sulfur content in the composite delivered an excellent long-life cycling performance at 0.5C and 1.0C over 300 cycles. More importantly, a stable capacity of 710.4 mAh·g-1 could bemaintained even after 100 cycles at 0.2C with a high sulfur loading of 3.6 mg·cm-1. The nature of the interactions between TiN and lithium polysulfide species wasinvestigated by XPS studies. Theor. calcns. were also carried out and the results revealed a strong binding between TiN and the lithium polysulfide species. It is expected that this class of conductive and polar materials would pave a new way for the high-energy lithium-sulfur batteries in the future.[Figure not available: see fulltext.].
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    22. 22
      Zukalova, M.; Prochazka, J.; Bastl, Z.; Duchoslav, J.; Rubacek, L.; Havlicek, D.; Kavan, L. Facile Conversion of Electrospun TiO 2 into Titanium Nitride/Oxynitride Fibers. Chem. Mater. 2010, 22 (13), 4045– 4055,  DOI: 10.1021/cm100877h
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      22
      Facile Conversion of Electrospun TiO2 into Titanium Nitride/Oxynitride Fibers
      Zukalova, Marketa; Prochazka, Jan; Bastl, Zdenek; Duchoslav, Jiri; Rubacek, Lukas; Havlicek, David; Kavan, Ladislav
      Chemistry of Materials (2010), 22 (13), 4045-4055CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Nanocryst. fibrous TiO2 (anatase) was prepd. by electrostatic spinning from ethanolic soln. of Ti(IV) butoxide, acetylacetone, and poly(vinylpyrrolidone) employing the Nanospider industrial process. These titania fibers were smoothly converted into cubic titanium oxynitride, TiOxNy fibers (a = 4.1930 Å) during 4 h at 600° in ammonia atm. The obtained material is convertible back into TiO2 fibers by heat treatment in air at 500°. The TiO2 fibers, which were reformed in this way, contain anatase as the main phase. Their follow-up reaction with NH3 at 600° 2 h leads to a less cryst. oxynitride material with a ≈ 4.173 Å, which is close to that of cubic TiO. Three subsequent cycles of this transformation were demonstrated. The described conversions are specific for electrospun anatase fibers only. At the same exptl. conditions, other forms of nanocryst. anatase do not react with ammonia yielding cubic phases. An almost perfectly stoichiometric titanium nitride, TiN (a = 4.2290 Å) contg. only 0.2 wt.% O, was prepd. from TiOxNy fibers in NH3 at temps. up to 1000°. This TiN material maintains the morphol. of fibers and is composed of nanocrystals of a similar size as those of the precursor.
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    23. 23
      Qin, P.; Huang, C.; Gao, B.; Pi, C.; Fu, J.; Zhang, X.; Huo, K.; Chu, P. K. Ultrathin Carbon Layer-Encapsulated TiN Nanotubes Array with Enhanced Capacitance and Electrochemical Stability for Supercapacitors. Appl. Surf. Sci. 2020, 503, 144293,  DOI: 10.1016/j.apsusc.2019.144293
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      23
      Ultrathin carbon layer-encapsulated TiN nanotubes array with enhanced capacitance and electrochemical stability for supercapacitors
      Qin, Ping; Huang, Chao; Gao, Biao; Pi, Chaoran; Fu, Jijiang; Zhang, Xuming; Huo, Kaifu; Chu, Paul K.
      Applied Surface Science (2020), 503 (), 144293CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Although Ti nitride (TiN) is promising as electrode material in supercapacitors due to the high cond., there are drawbacks such as the brittleness, low capacitance, and chem. stability. Herein, 3-dimensional (3D) C-encapsulated mesoporous Ti nitride nanotubes (TiN/C NTs) arrays with C doping are prepd. by 1-step nitridation of anodic TiO2 NTs with org. electrolyte as the C source. In TiN/C NTs, 3-dimensional C matrix not only serves as a protective layer and mech. support to mitigate electrochem. oxidn. and structural collapse, but also provides a conductive network to facilitate electron transfer. The capacitance retention of the TiN NTs electrodes increases from 72.5 to 92.2% for 4000 cycles after C encapsulating. Also, the C doping increases the active charge storage sites of the TiN NTs. The TiN/C NTs electrode exhibits a large volumetric capacitance of 121 F cm-3 (0.83 A cm-3), which is one time larger than that of the pure TiN NTs (69 F cm-3). The sym. all-solid-state device assembled with 2 TiN/C NTs electrodes and polyvinyl alc. electrolyte shows a large volumetric capacitance of 8.3 F cm-3. This finding provides a good potential application in flexible supercapacitor.
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    24. 24
      Naldoni, A.; Kudyshev, Z. A.; Mascaretti, L.; Sarmah, S. P.; Rej, S.; Froning, J. P.; Tomanec, O.; Yoo, J. E.; Wang, D.; Kment, Š.; Montini, T.; Fornasiero, P.; Shalaev, V. M.; Schmuki, P.; Boltasseva, A.; Zbořil, R. Solar Thermoplasmonic Nanofurnace for High-Temperature Heterogeneous Catalysis. Nano Lett. 2020, 20 (5), 3663– 3672,  DOI: 10.1021/acs.nanolett.0c00594
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      24
      Solar Thermoplasmonic Nanofurnace for High-Temperature Heterogeneous Catalysis
      Naldoni, Alberto; Kudyshev, Zhaxylyk A.; Mascaretti, Luca; Sarmah, Smritakshi P.; Rej, Sourav; Froning, Jens P.; Tomanec, Ondrej; Yoo, Jeong Eun; Wang, Di; Kment, Stepan; Montini, Tiziano; Fornasiero, Paolo; Shalaev, Vladimir M.; Schmuki, Patrik; Boltasseva, Alexandra; Zboril, Radek
      Nano Letters (2020), 20 (5), 3663-3672CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Most of existing solar thermal technologies require highly concd. solar power to operate at 300-600°. Here, thin films of refractory plasmonic TiN cylindrical nanocavities manufd. via flexible and scalable process are presented. The fabricated TiN films show polarization-insensitive 95% broadband absorption in the visible and near-IR spectral ranges and act as plasmonic "nanofurnaces" capable of reaching temps. above 600° under moderately concd. solar irradn. (~ 20 Suns). The demonstrated structures can be used to control nanometer-scale chem. with zeptoliter (10-21 L) volumetric precision, catalyzing C-C bond formation and melting inorg. deposits. Also shown is the possibility to perform solar thermal CO oxidn. at rates of 16 mol h-1 m-2 and with a solar-to-heat thermoplasmonic efficiency of 63%. Access to scalable, cost-effective refractory plasmonic nanofurnaces opens the way to the development of modular solar thermal devices for sustainable catalytic processes.
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    25. 25
      Riboni, F.; Nguyen, N. T.; So, S.; Schmuki, P. Aligned Metal Oxide Nanotube Arrays: Key-Aspects of Anodic TiO 2 Nanotube Formation and Properties. Nanoscale Horiz 2016, 1 (6), 445– 466,  DOI: 10.1039/C6NH00054A
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      25
      Aligned metal oxide nanotube arrays: key-aspects of anodic TiO2 nanotube formation and properties
      Riboni, Francesca; Nguyen, Nhat Truong; So, Seulgi; Schmuki, Patrik
      Nanoscale Horizons (2016), 1 (6), 445-466CODEN: NHAOAW; ISSN:2055-6764. (Royal Society of Chemistry)
      Over the past ten years, self-aligned TiO2 nanotubes have attracted tremendous scientific and technol. interest due to their anticipated impact on energy conversion, environment remediation and biocompatibility. In the present manuscript, we review fundamental principles that govern the self-organized initiation of anodic TiO2 nanotubes. We start with the fundamental question: why is self-organization taking place. We illustrate the inherent key mechanistic aspects that lead to tube growth in various different morphologies, such as ripple-walled tubes, smooth tubes, stacks and bamboo-type tubes, and importantly the formation of double-walled TiO2 nanotubes vs. single-walled tubes, and the drastic difference in their phys. and chem. properties. We show how both double- and single-walled tube layers can be detached from the metallic substrate and exploited for the prepn. of robust self-standing membranes. Finally, we show how by selecting specific growth approaches to TiO2 nanotubes desired functional features can be significantly improved, e.g., enhanced electron mobility, intrinsic doping, or crystn. into pure anatase at high temps. can be achieved. Finally, we briefly outline the impact of property, modifications and morphol. on functional uses of self-organized nanotubes for most important applications.
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    26. 26
      Lee, K.; Mazare, A.; Schmuki, P. One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes. Chem. Rev. 2014, 114 (19), 9385– 9454,  DOI: 10.1021/cr500061m
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      One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes
      Lee, Kiyoung; Mazare, Anca; Schmuki, Patrik
      Chemical Reviews (Washington, DC, United States) (2014), 114 (19), 9385-9454CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review of different synthesis approaches to produce TiO2 nanotubes and TiO2 nanotube arrays, phys. and chem. properties of TiO2 nanotubes and techniques to modify them. The most explored and prospective applications of nanotubular TiO2 are discussed.
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    27. 27
      Ozkan, S.; Mazare, A.; Schmuki, P. Critical Parameters and Factors in the Formation of Spaced TiO2 Nanotubes by Self-Organizing Anodization. Electrochim. Acta 2018, 268, 435– 447,  DOI: 10.1016/j.electacta.2018.02.120
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      Critical parameters and factors in the formation of spaced TiO2 nanotubes by self-organizing anodization
      Ozkan, Selda; Mazare, Anca; Schmuki, Patrik
      Electrochimica Acta (2018), 268 (), 435-447CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Self-organized TiO2 nanotube arrays can be grown under a wide range of electrochem. conditions. In the present work, we evaluate the occurrence of spacing between tubes and the connection of this effect to organization of tubes on two-size scales. The results show that tube-spacing is initiated in the very early stages of anodization between individual pore morphologies. Furthermore, the spacing, as well as the organization on two-size scales can be controlled by changing the anodization conditions, e.g., electrolyte compn., applied voltage and temp. Namely, adjustment of H2O content, electrode temp. and voltage can lead to spaced nanotubes, and allow to control spacing. Finally, we draw conclusions on the possible mechanism relevant to the growth of spaced tubes.
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    28. 28
      Wawrzyniak, J.; Grochowska, K.; Karczewski, J.; Kupracz, P.; Ryl, J.; Dołȩga, A.; Siuzdak, K. The Geometry of Free-Standing Titania Nanotubes as a Critical Factor Controlling Their Optical and Photoelectrochemical Performance. Surf. Coat. Technol. 2020, 389, 125628,  DOI: 10.1016/j.surfcoat.2020.125628
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      28
      The geometry of free-standing titania nanotubes as a critical factor controlling their optical and photoelectrochemical performance
      Wawrzyniak, Jakub; Grochowska, Katarzyna; Karczewski, Jakub; Kupracz, Piotr; Ryl, Jacek; Dolega, Anna; Siuzdak, Katarzyna
      Surface and Coatings Technology (2020), 389 (), 125628CODEN: SCTEEJ; ISSN:0257-8972. (Elsevier B.V.)
      Titanium dioxide nanotubes are regarded as one of the most important functional materials and due to their unique electronic properties, chem. stability and photocorrosion resistance, they find applications in, for example, highly efficient photocatalysis or perovskite solar cells. Nevertheless, modification of TiO2 nanotubes is required to overcome their main drawback, i.e. large energy bandgap (>3.2 eV) limiting their ability to capture solar light. In this work, we report the changes in optical and photoelectrochem. properties of well-sepd. TiO2 nanotubes that are tuned by varying the geometry of the material. The ordered tubular titania is formed via anodization in the presence of fluoride ions in diethylene glycol at elevated temp. Length, inner diam., wall thickness, and sepn. distance are described in function of synthesis parameters such as applied voltage and duration. The morphol. and optical properties are characterized by means of SEM and UV-Vis spectroscopy techniques, resp., while cyclic voltammetry, linear voltammetry and chronoamperometry are used to det. electrochem./photoelectrochem. activity in different light conditions. The obtained results suggest a link between sp. surface area, the width of the band-gap, and photoactivity, each of which could be individually optimized via anodization conditions. Moreover, the behavior of the Mott-Schottky plot before and after 3 min of irradn. is studied indicating the pos. shift of the flat band position and an increase in donor d. values for all the obtained materials. The Mott-Schottky anal. was correlated with the linear voltammetry scans suggesting the important role of surface trapped holes. Presented in here results significantly supplement the current state-of-art regarding sepd. TiO2 nanotubes that are considered as not fully investigated and unappreciated class of titania materials which due to the exposure of inner and outer wall can be used for further modifications.
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    29. 29
      Varghese, O. K.; Paulose, M.; Grimes, C. A. Long Vertically Aligned Titania Nanotubes on Transparent Conducting Oxide for Highly Efficient Solar Cells. Nat. Nanotechnol. 2009, 4 (9), 592– 597,  DOI: 10.1038/nnano.2009.226
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      Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells
      Varghese, Oomman K.; Paulose, Maggie; Grimes, Craig A.
      Nature Nanotechnology (2009), 4 (9), 592-597CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Dye-sensitized solar cells consist of a random network of titania nanoparticles that serve both as a high-surface-area support for dye mols. and as an electron-transporting medium. Despite achieving high power conversion efficiencies, their performance is limited by electron trapping in the nanoparticle film. Electron diffusion lengths can be increased by transporting charge through highly ordered nanostructures such as titania nanotube arrays. Although titania nanotube array films have been shown to enhance the efficiencies of both charge collection and light harvesting, it has not been possible to grow them on transparent conducting oxide glass with the lengths needed for high-efficiency device applications (tens of micrometers). Here, we report the fabrication of transparent titania nanotube array films on transparent conducting oxide glass with lengths between 0.3 and 33.0 μm using a novel electrochem. approach. Dye-sensitized solar cells contg. these arrays yielded a power conversion efficiency of 6.9%. The incident photon-to-current conversion efficiency ranged from 70 to 80% for wavelengths between 450 and 650 nm.
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    30. 30
      Tesler, A. B.; Altomare, M.; Schmuki, P. Morphology and Optical Properties of Highly Ordered TiO 2 Nanotubes Grown in NH 4 F/ o -H 3 PO 4 Electrolytes in View of Light-Harvesting and Catalytic Applications. ACS Appl. Nano Mater. 2020, 3 (11), 10646– 10658,  DOI: 10.1021/acsanm.0c01859
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      Morphology and Optical Properties of Highly Ordered TiO2 Nanotubes Grown in NH4F/o-H3PO4 Electrolytes in View of Light-Harvesting and Catalytic Applications
      Tesler, Alexander B.; Altomare, Marco; Schmuki, Patrik
      ACS Applied Nano Materials (2020), 3 (11), 10646-10658CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      Highly ordered titanium dioxide nanotube (TiO2 NT) arrays were grown by electrochem. anodization of titanium in a molten NH4F/o-H3PO4 electrolyte. NTs with various diams., lengths, wall thicknesses, and intertube distances could be obtained by tuning key anodization parameters such as the applied potential, anodization time, electrolyte temp., concn. of NH4F, and H2O content. The morphol. and optical properties were characterized by SEM and UV-vis spectroscopy techniques. We show that all aforementioned parameters have a strong influence on the nanostructured morphol. and optical characteristics (reflectivity) of the formed nanotubular layers. Their optical features were simulated numerically to support the exptl. measurements. We show that the optical features of anodic TiO2 nanotube layers result from the overlay of the individual optical properties of various "structural elements", e.g., the NT barrier layer, top opening morphol., intertube spacing, and thermally formed oxide sublayer. Our results provide tools for "a priori" design with nanoscale precision of TiO2 structures with advanced optical features for light-harvesting and catalytic applications, e.g., in sensing, photocatalytic self-cleaning processes, solar hydrogen generation, or photovoltaics.
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      Ozkan, S.; Nguyen, N. T.; Mazare, A.; Schmuki, P. Optimized Spacing between TiO 2 Nanotubes for Enhanced Light Harvesting and Charge Transfer. ChemElectroChem. 2018, 5 (21), 3183– 3190,  DOI: 10.1002/celc.201801136
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      Optimized Spacing between TiO2 Nanotubes for Enhanced Light Harvesting and Charge Transfer
      Ozkan, Selda; Nguyen, Nhat Truong; Mazare, Anca; Schmuki, Patrik
      ChemElectroChem (2018), 5 (21), 3183-3190CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We investigate two distinctly different anodic TiO2 nanotubular morphologies, spaced and close-packed arrays. While the close-packed tubular arrays are formed in ethylene glycol, spaced nanotubes (NTs), which have a regular gap between individual NTs, grow in a diethylene glycol or DMSO-based electrolyte. Depending on the electrolyte used for anodization, the morphol., crystal structure, and chem. compn. of the resulting nanotubular layer vary from one another. This influences the electrochem. and photoelectrochem. activity of the NTs. Overall, we find that spaced NTs can not only provide enhanced charge-transfer characteristics but can also show beneficial light absorption characteristics when used as a photo- or light-harvesting electrode.
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      Liu, Y.; Wang, Y.; Zhang, Y.; You, Z.; Lv, X. Mechanism on Reduction and Nitridation of Micrometer-sized Titania with Ammonia Gas. J. Am. Ceram. Soc. 2020, 103 (6), 3905– 3916,  DOI: 10.1111/jace.17067
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      Mechanism on reduction and nitridation of micrometer-sized titania with ammonia gas
      Liu, Yongjie; Wang, Yue; Zhang, Yu; You, Zhixiong; Lv, Xuewei
      Journal of the American Ceramic Society (2020), 103 (6), 3905-3916CODEN: JACTAW; ISSN:0002-7820. (Wiley-Blackwell)
      Ammonia gas can be simultaneously used as a reductant and nitrogen source to prep. TiN from titania. In this work, the mechanisms on redn. and nitridation of micrometer-sized anatase with ammonia gas have been investigated, using both thermodn. and exptl. studies. The thermodn. anal. indicated that redn. and nitridation of TiO2 by NH3 was feasible. Anatase will undergo different paths to form TiN, depending on the reaction temp. Upon heating, NH3 was seen to partially decomp. into N2 and H2, although the actual NH3 decompn. ratio was less than the theor. value. The exptl. results indicated that the obtained titanium nitride was non-stoichiometric (TiNxO1-x, x ≤ 1), as it contained a certain amt. of oxygen. Based on the phase transformation and XPS anal., the redn. and nitridation routes were deduced: TiO2 reacted with NH3 to form TiNxO1-x directly, at lower temps., and followed the path TiO2 → TinO2n-1 → TiNxO1-x, at higher temps. TinO2n-1 was detd. to be Ti4O7 and Ti3O5 at 1100°C and 1200°C, resp. Reaction temp. and time significantly affected the oxygen and nitrogen contents in TiNxO1-x, with the lattice parameter of roasted products gradually increasing-approaching those of pure TiN-with an increase in reaction temp. and holding time. At the same time, the content of oxygen in TiNxO1-x decreased, and its nitrogen content correspondingly increased.
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    33. 33
      Anderson, B. D.; Tracy, J. B. Nanoparticle Conversion Chemistry: Kirkendall Effect, Galvanic Exchange, and Anion Exchange. Nanoscale 2014, 6 (21), 12195– 12216,  DOI: 10.1039/C4NR02025A
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      Nanoparticle conversion chemistry: Kirkendall effect, galvanic exchange, and anion exchange
      Anderson, Bryan D.; Tracy, Joseph B.
      Nanoscale (2014), 6 (21), 12195-12216CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      A review. Conversion chem. is a rapidly maturing field, where chem. conversion of template nanoparticles (NPs) into new compns. is often accompanied by morphol. changes, such as void formation. The principles and examples of three major classes of conversion chem. reactions are reviewed: the Kirkendall effect for metal NPs, galvanic exchange, and anion exchange, each of which can result in void formation in NPs. These reactions can be used to obtain complex structures that may not be attainable by other methods. During each kind of conversion chem. reaction, NPs undergo distinct chem. and morphol. changes, and insights into the mechanisms of these reactions will allow for improved fine control and prediction of the structures of intermediates and products. Conversion of metal NPs into oxides, phosphides, sulfides, and selenides often occurs through the Kirkendall effect, where outward diffusion of metal atoms from the core is faster than inward diffusion of reactive species, resulting in void formation. In galvanic exchange reactions, metal NPs react with noble metal salts, where a redox reaction favors redn. and deposition of the noble metal (alloying) and oxidn. and dissoln. of the template metal (dealloying). In anion exchange reactions, addn. of certain kinds of anions to solns. contg. metal compd. NPs drives anion exchange, which often results in significant morphol. changes due to the large size of anions compared to cations. Conversion chem. thus allows for the formation of NPs with complex compns. and structures, for which numerous applications are anticipated arising from their novel catalytic, electronic, optical, magnetic, and electrochem. properties.
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    34. 34
      Koya, A. N.; Zhu, X.; Ohannesian, N.; Yanik, A. A.; Alabastri, A.; Proietti Zaccaria, R.; Krahne, R.; Shih, W.-C.; Garoli, D. Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis. ACS Nano 2021, 15 (4), 6038– 6060,  DOI: 10.1021/acsnano.0c10945
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      34
      Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis
      Koya, Alemayehu Nana; Zhu, Xiangchao; Ohannesian, Nareg; Yanik, A. Ali; Alabastri, Alessandro; Proietti Zaccaria, Remo; Krahne, Roman; Shih, Wei-Chuan; Garoli, Denis
      ACS Nano (2021), 15 (4), 6038-6060CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the UV up to the IR. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large sp. surface area, distinctive optical properties, high elec. cond., and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compds., and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.
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    35. 35
      Plawsky, J. L.; Kim, J. K.; Schubert, E. F. Engineered Nanoporous and Nanostructured Films. Mater. Today 2009, 12 (6), 36– 45,  DOI: 10.1016/S1369-7021(09)70179-8
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      Engineered nanoporous and nanostructured films
      Plawsky, Joel L.; Kim, Jong Kyu; Schubert, E. Fred
      Materials Today (Oxford, United Kingdom) (2009), 12 (6), 36-45CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
      A review. Nanoporous and nanostructured films have become increasingly important to the microelectronics and photonics industries. They provide a route to low dielec. const. materials that will enable future generations of powerful microprocessors. They are the only route to achieving materials with refractive indexes less than 1.2, a key feature for the future development of photonic crystal devices, enhanced omnidirectional reflectors, enhanced anti-reflection coatings and black-body absorbers. In addn., these films exhibit tremendous potential for sepns., catalytic, biomedical and heat transfer applications. This article will review two primary techniques for manufg. these films, evapn. induced self-assembly and oblique or glancing angle deposition, and will discuss some of the film properties crit. to their use in the microelectronics and photonics industries.
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    36. 36
      Lu, J. Y.; Nam, S. H.; Wilke, K.; Raza, A.; Lee, Y. E.; AlGhaferi, A.; Fang, N. X.; Zhang, T. Localized Surface Plasmon-Enhanced Ultrathin Film Broadband Nanoporous Absorbers. Adv. Opt. Mater. 2016, 4 (8), 1255– 1264,  DOI: 10.1002/adom.201600078
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      Localized Surface Plasmon-Enhanced Ultrathin Film Broadband Nanoporous Absorbers
      Lu, Jin You; Nam, Sang Hoon; Wilke, Kyle; Raza, Aikifa; Lee, Yoonkyung E.; Al Ghaferi, Amal; Fang, Nicholas X.; Zhang, TieJun
      Advanced Optical Materials (2016), 4 (8), 1255-1264CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Ultrathin lossy films have attracted much attention due to their strong interference persisting inside the lossy dielec. film on a reflective substrate. Here, a plasmon-enhanced ultrathin film broadband absorber is proposed by combining the ultrathin film absorber with localized surface plasmon resonances. This concept can be realized by patterning nanoholes on an absorber comprised of an absorptive ultrathin Ge film and a reflective Au layer, where the localized surface plasmon mode is activated by metallic pore-shaped holes. The plasmonic enhancement is resulting from the pore-shape localized resonance mode, which increases the optical path length through scattering and concs. the incident light field near the interface of Ge/Au. The exptl. characterization results of a nanoporous ultrathin film absorber, which is fabricated with a scalable laser interference lithog. approach, demonstrate its superior light absorption performance. Several materials, such as Ag, Al, and Cu, are proposed as an alternative to Au, and they can also provide plasmonic enhancement to ultrathin films. Furthermore, through an efficient way to optimize the structural dimensions of the nanoporous ultrathin film absorber, a trilayer system of TiO2/Ge/Au achieves the total solar absorptance over 89.3% with a wavelength range of 400-1100 nm.
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    37. 37
      Farhat, M.; Cheng, T.-C.; Le, K. Q.; Cheng, M. M.-C.; Bağcı, H.; Chen, P.-Y. Mirror-Backed Dark Alumina: A Nearly Perfect Absorber for Thermoelectronics and Thermophotovotaics. Sci. Rep. 2016, 6 (1), 19984,  DOI: 10.1038/srep19984
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      Mirror-backed Dark Alumina: A Nearly Perfect Absorber for Thermoelectronics and Thermophotovotaics
      Farhat, Mohamed; Cheng, Tsung-Chieh; Le, Khai. Q.; Cheng, Mark Ming-Cheng; Bagci, Hakan; Chen, Pai-Yen
      Scientific Reports (2016), 6 (), 19984CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      We present here a broadband, wide-angle, and polarization-independent nearly perfect absorber consisting of mirror-backed nanoporous alumina. By electrochem. anodizing the disordered multicomponent aluminum and properly tailoring the thickness and air-filling fraction of nanoporous alumina, according to the Maxwell-Garnet mixt. theory, a large-area dark alumina can be made with excellent photothermal properties and absorption larger than 93% over a wide wavelength range spanning from near-IR to UV light, i.e. 250 nm-2500 nm. The measured absorption is orders of magnitude greater than other reported anodized porous alumina, typically semi-transparent at similar wavelengths. This simple yet effective approach, however, does not require any lithog., nano-mixt. deposition, pre- and post-treatment. Here, we also envisage and theor. investigate the practical use of proposed absorbers and/or photothermal converters in integrated thermoelectronic and/or thermophotovoltaic energy conversion devices, which make efficient use of the entire spectrum of ambient visible to near-IR radiation.
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    38. 38
      Raut, H. K.; Ganesh, V. A.; Nair, A. S.; Ramakrishna, S. Anti-Reflective Coatings: A Critical, in-Depth Review. Energy Environ. Sci. 2011, 4 (10), 3779,  DOI: 10.1039/c1ee01297e
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      Anti-reflective coatings: a critical, in-depth review
      Raut, Hemant Kumar; Ganesh, V. Anand; Nair, A. Sreekumaran; Ramakrishna, Seeram
      Energy & Environmental Science (2011), 4 (10), 3779-3804CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      A review. Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-elec. equipments. Extensive research in optical and biol. reflectance minimization as well as the emergence of nanotechnol. over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature ("biomimetics") has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and assocd. challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.
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    39. 39
      Wang, W.; Qi, L. Light Management with Patterned Micro- and Nanostructure Arrays for Photocatalysis, Photovoltaics, and Optoelectronic and Optical Devices. Adv. Funct. Mater. 2019, 29 (25), 1807275,  DOI: 10.1002/adfm.201807275
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      Ulusoy Ghobadi, T. G.; Ghobadi, A.; Odabasi, O.; Karadas, F.; Ozbay, E. Subwavelength Densely Packed Disordered Semiconductor Metasurface Units for Photoelectrochemical Hydrogen Generation. ACS Appl. Energy Mater. 2022, 5 (3), 2826– 2837,  DOI: 10.1021/acsaem.1c03363
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      40
      Subwavelength Densely Packed Disordered Semiconductor Metasurface Units for Photoelectrochemical Hydrogen Generation
      Ulusoy Ghobadi, T. Gamze; Ghobadi, Amir; Odabasi, Oguz; Karadas, Ferdi; Ozbay, Ekmel
      ACS Applied Energy Materials (2022), 5 (3), 2826-2837CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      For most semiconductors, esp. the visible-light-absorbing ones, the carrier diffusion length is significantly shorter than the light penetration depth, limiting their photoactivities. This limitation could be mitigated through the use of subwavelength semiconductor-based metasurfaces and metamaterials. In this paper, a large-scale compatible metasurface photocathode, made of densely packed disordered p-type chromium oxide (CrOX), is developed to be utilized in photoelectrochem. (PEC) hydrogen generation. For this purpose, first, tightly packed random Cr nanorods are fabricated using an oblique angle deposition technique. Afterward, an annealing step is applied to the sample to transform these metallic units into a semiconducting p-type CrOX-based metasurface. Based on the exptl. characterization results and numerical simulations, the proposed design can provide strong light-matter interactions in an ultra-broadband-wavelength range, mainly due to its multidimensional random geometry and ultrasmall gap sizes. Finally, to substantiate the activity of the CrOX nanorods, a core-crown geometry is developed where the NiOX capping layer catalyzes the hydrogen evolution reaction (HER). The proposed heterostructure metasurface absorber can impose photocurrent values as large as 50μA cm-2 with a photocurrent spectral response extended up to 500 nm. Moreover, the electrode shows outstanding operation under light irradn. for 9 h. This work demonstrates a simple, scalable design strategy to fabricate low-cost and stable photocathodes for PEC hydrogen evolution.
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    41. 41
      Soydan, M. C.; Ghobadi, A.; Yildirim, D. U.; Duman, E.; Bek, A.; Erturk, V. B.; Ozbay, E. Lithography-Free Random Bismuth Nanostructures for Full Solar Spectrum Harvesting and Mid-Infrared Sensing. Adv. Opt. Mater. 2020, 8 (4), 1901203,  DOI: 10.1002/adom.201901203
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      Lithography-Free Random Bismuth Nanostructures for Full Solar Spectrum Harvesting and Mid-Infrared Sensing
      Soydan, Mahmut Can; Ghobadi, Amir; Yildirim, Deniz Umut; Duman, ElifSariguel; Bek, Alpan; Erturk, Vakur Behcet; Ozbay, Ekmel
      Advanced Optical Materials (2020), 8 (4), 1901203CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A lithog.-free, double-functional single bismuth (Bi) metal nanostructure is designed, fabricated, and characterized for ultrabroadband absorption in the visible (vis) and near-IR (NIR) ranges, and for a narrowband response with ultrahigh refractive index sensitivity in the mid-IR (MIR) range. To achieve a large-scale fabrication of the design in a lithog.-free route, the oblique-angle deposition approach is used to obtain densely packed and randomly spaced/oriented Bi nanostructures. It is shown that this fabrication technique can provide a bottom-up approach to controlling the length and spacing of the design. The characterization findings reveal a broadband absorbance above 0.8 in vis and NIR, and a narrowband absorbance centered around 6.54μm. Dense architecture and extraordinary permittivity of Bi provide strong field confinement in ultrasmall gaps between nanostructures, and this can be utilized for a sensing application. An ultrahigh sensitivity of 2151 nm refractive-index unit (RIU-1) is acquired, which is, as far as it is known, the exptl. highest sensitivity attained so far. The simple and large-scale compatible fabrication route of the design together with the extraordinary optical response of Bi coating makes this design promising for many optoelectronic and sensing applications.
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      Zhang, F.; Tang, F.; Xu, X.; Adam, P.-M.; Martin, J.; Plain, J. Influence of Order-to-Disorder Transitions on the Optical Properties of the Aluminum Plasmonic Metasurface. Nanoscale 2020, 12 (45), 23173– 23182,  DOI: 10.1039/D0NR06334G
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      Influence of order-to-disorder transitions on the optical properties of the aluminum plasmonic metasurface
      Zhang, Feifei; Tang, Feng; Xu, Xiaolun; Adam, Pierre-Michel; Martin, Jerome; Plain, Jerome
      Nanoscale (2020), 12 (45), 23173-23182CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      To mimic the optical influence of disorder in condensed matter, the effect of uniform disorder on plasmonic resonances were investigated numerically and exptl. on aluminum (Al) nanoparticle arrays. Resorting to the analog of a plasmonic periodic array to a crystal on the sharp optical spectrum and its anisotropy, the disorder in the transition from crystal to glass (with broadened spectrum and isotropy) is imitated by three kinds of Al plasmonic metasurfaces: varying the displacement, size and rotation of each Al nanoparticle in the periodic array. The random variation on the location or size of each Al nanodisk in the plasmonic crystal induces broadening and redn. of their plasmonic resonances without significantly shifting its wavelength. Moreover, by rotating each Al nanorod in the plasmonic crystal by a random angle, the polarization dependence of plasmonic resonances is progressively decreased by increasing the rotation disorder. Thanks to these three kinds of Al metasurfaces, an enlightened understanding of the random physics in the solid state and the influence of manufg. deviation in nanophotonics is supported.
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      Huo, D.; Zhang, J.; Wang, H.; Ren, X.; Wang, C.; Su, H.; Zhao, H. Broadband Perfect Absorber with Monolayer MoS2 and Hexagonal Titanium Nitride Nano-Disk Array. Nanoscale Res. Lett. 2017, 12 (1), 465,  DOI: 10.1186/s11671-017-2232-4
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      Broadband Perfect Absorber with Monolayer MoS2 and Hexagonal Titanium Nitride Nano-disk Array
      Huo Dewang; Zhang Jingwen; Wang Hao; Ren Xiaoxuan; Wang Chao; Su Hang; Zhao Hua; Zhang Jingwen; Zhao Hua
      Nanoscale research letters (2017), 12 (1), 465 ISSN:1931-7573.
      A broadband metamaterial absorber (MA) composed of hexagonal-arranged single-sized titanium nitride (TiN) nano-disk array and monolayer molybdenum disulfide (MoS2) is studied using finite-difference time-domain (FDTD) simulations. The structure of TiN nano-disk array/dielectric silica (SiO2)/aluminum (Al) is adopted in our design. By optimizing the dimension parameters of the structure, an average absorption of 96.1% is achieved from 400 to 850 nm. In addition, by inserting a monolayer MoS2 which has high absorption at the short wavelength side underneath the TiN nano-disk array, an average absorption of 98.1% over the entire visible regime from 400 to 850 nm was achieved, with a peak absorption near 100% and absorption over 99% from 475 to 772 nm. Moreover, the absorber presented in this paper is polarization insensitive. This compact and unique design with TiN nano-disk/monolayer MoS2/ SiO2/Al structure may have great potential for applications in photovoltaics and light trapping.
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      Wang, J.; Zhang, W.; Zhu, M.; Yi, K.; Shao, J. Broadband Perfect Absorber with Titanium Nitride Nano-Disk Array. Plasmonics 2015, 10 (6), 1473– 1478,  DOI: 10.1007/s11468-015-9962-x
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      Broadband Perfect Absorber with Titanium Nitride Nano-disk Array
      Wang, Jianguo; Zhang, Weili; Zhu, Meiping; Yi, Kui; Shao, Jianda
      Plasmonics (2015), 10 (6), 1473-1478CODEN: PLASCS; ISSN:1557-1955. (Springer)
      A broadband metamaterial absorber (MA) based on the titanium nitride (TiN) nano-disk array is studied using finite difference time domain simulations. The semiconducting indium tin oxide (ITO) thin film is introduced as the space layer in this sandwiched structure. Utilizing the sym. geometry of the MA structure, polarization insensitivity of the broadband absorption was gained. The absorber with TiN nano-disk array shows a peak absorbance of 99 % and larger than 98 % from 560 to 675 nm by numerical simulation. This compact design may have potential applications in the plasmonic sensing and photovoltaic devices.
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      Chirumamilla, M.; Chirumamilla, A.; Yang, Y.; Roberts, A. S.; Kristensen, P. K.; Chaudhuri, K.; Boltasseva, A.; Sutherland, D. S.; Bozhevolnyi, S. I.; Pedersen, K. Large-Area Ultrabroadband Absorber for Solar Thermophotovoltaics Based on 3D Titanium Nitride Nanopillars. Adv. Opt. Mater. 2017, 5 (22), 1700552,  DOI: 10.1002/adom.201700552
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      Boyd, R. W.; Gehr, R. J.; Fischer, G. L.; Sipe, J. E. Nonlinear Optical Properties of Nanocomposite Materials. Pure Appl. Opt. J. Eur. Opt. Soc. Part A 1996, 5 (5), 505– 512,  DOI: 10.1088/0963-9659/5/5/005
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      Mascaretti, L.; Schirato, A.; Montini, T.; Alabastri, A.; Naldoni, A.; Fornasiero, P. Challenges in Temperature Measurements in Gas-Phase Photothermal Catalysis. Joule 2022, 6 (8), 1727– 1732,  DOI: 10.1016/j.joule.2022.06.019
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      Tao, P.; Ni, G.; Song, C.; Shang, W.; Wu, J.; Zhu, J.; Chen, G.; Deng, T. Solar-Driven Interfacial Evaporation. Nat. Energy 2018, 3 (12), 1031– 1041,  DOI: 10.1038/s41560-018-0260-7
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      Zhang, P.; Liao, Q.; Yao, H.; Huang, Y.; Cheng, H.; Qu, L. Direct Solar Steam Generation System for Clean Water Production. Energy Storage Mater. 2019, 18, 429– 446,  DOI: 10.1016/j.ensm.2018.10.006
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      Dongare, P. D.; Alabastri, A.; Neumann, O.; Nordlander, P.; Halas, N. J. Solar Thermal Desalination as a Nonlinear Optical Process. Proc. Natl. Acad. Sci. U. S. A. 2019, 116 (27), 13182– 13187,  DOI: 10.1073/pnas.1905311116
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      Solar thermal desalination as a nonlinear optical process
      Dongare, Pratiksha D.; Alabastri, Alessandro; Neumann, Oara; Nordlander, Peter; Halas, Naomi J.
      Proceedings of the National Academy of Sciences of the United States of America (2019), 116 (27), 13182-13187CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The ever-increasing global need for potable water requires practical, sustainable approaches for purifying abundant alternative sources such as seawater, high-salinity processed water, or underground reservoirs. Evapn.-based solns. are of particular interest for treating high salinity water, since conventional methods such as reverse osmosis have increasing energy requirements for higher concns. of dissolved minerals. Demonstration of efficient water evapn. with heat localization in nanoparticle solns. under solar illumination has led to the recent rapid development of sustainable, solar-driven distn. methods. Given the amt. of solar energy available per square meter at the Earth's surface, however, it is important to utilize these incident photons as efficiently as possible to maximize clean water output. Here we show that merely focusing incident sunlight into small "hot spots" on a photothermally active desalination membrane dramatically increases- by more than 50%- the flux of distd. water. This large boost in efficiency results from the nearly exponential dependence of water vapor satn. pressure on temp., and therefore on incident light intensity. Exploiting this inherent but previously unrecognized optical nonlinearity should enable the design of substantially higher-throughput solar thermal desalination methods. This property provides a mechanism capable of enhancing a far wider range of photothermally driven processes with supralinear intensity dependence, such as light-driven chem. reactions and sepn. methods.
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      Palm, K. J.; Murray, J. B.; Narayan, T. C.; Munday, J. N. Dynamic Optical Properties of Metal Hydrides. ACS Photonics 2018, 5 (11), 4677– 4686,  DOI: 10.1021/acsphotonics.8b01243
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      Dynamic Optical Properties of Metal Hydrides
      Palm, Kevin J.; Murray, Joseph B.; Narayan, Tarun C.; Munday, Jeremy N.
      ACS Photonics (2018), 5 (11), 4677-4686CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Metal hydrides often display dramatic changes in optical properties upon hydrogenation. These shifts make them prime candidates for many tunable optical devices, such as optical hydrogen sensors and switchable mirrors. While some of these metals, such as palladium, have been well studied, many other promising materials have only been characterized over a limited optical range and lack direct in situ measurements of hydrogen loading, limiting their potential applications. Further, there have been no systematic studies that allow for a clear comparison between these metals. In this work, we present such a systematic study of the dynamically tunable optical properties of Pd, Mg, Zr, Ti, and V throughout hydrogenation with a wavelength range of 250-1690 nm. These measurements were performed in an environmental chamber, which combines mass measurements via a quartz crystal microbalance with ellipsometric measurements in up to 7 bar of hydrogen gas, allowing us to det. the optical properties during hydrogen loading. In addn., we demonstrate a further tunability of the optical properties of titanium and its hydride by altering annealing conditions, and we investigate the optical and gravimetric hysteresis that occurs during hydrogenation cycling of palladium. Finally, we demonstrate several nanoscale optical and plasmonic structures based on these dynamic properties. We show structures that, upon hydrogenation, demonstrate 5 orders of magnitude change in reflectivity, resonance shifts of >200 nm, and relative transmission switching of >3000%, suggesting a wide range of applications.
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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphotonics.3c00731.

    • Detailed synthesis procedure, SEM and TEM images, dimensional parameters, and optical properties of TiO2 NTs; Additional details of nitridation procedure and lattice parameters of nitridated NTs; SEM and TEM images of NTs nitridated at 700 °C; Spectrum of the white LED; Additional details of photothermal experiments; Emissivity of nitridated NTs (PDF)


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