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Site-Selective Deposition of a Cobalt Cocatalyst onto a Plasmonic Au/TiO2 Photoanode for Improved Water Oxidation

  • Megumi Okazaki
    Megumi Okazaki
    Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
    Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan
    More by Megumi Okazaki
    Orcidhttp://orcid.org/0000-0003-1167-9453
  • , 
  • Yoshiki Suganami
    Yoshiki Suganami
    Research Institute for Electronic Science, Hokkaido University, North-21 West-10, Kita-ku, Sapporo 001-0021, Japan
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  • , 
  • Naoki Hirayama
    Naoki Hirayama
    Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
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  • , 
  • Hiroko Nakata
    Hiroko Nakata
    Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
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  • , 
  • Tomoya Oshikiri
    Tomoya Oshikiri
    Research Institute for Electronic Science, Hokkaido University, North-21 West-10, Kita-ku, Sapporo 001-0021, Japan
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    Orcidhttp://orcid.org/0000-0002-1268-0256
  • , 
  • Toshiyuki Yokoi
    Toshiyuki Yokoi
    Nanospace Catalysis Unit, Institute of Innovative Research, Tokyo Institute of Technology, 4259-S2-5, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
    More by Toshiyuki Yokoi
    Orcidhttp://orcid.org/0000-0002-3315-3172
  • , 
  • Hiroaki Misawa*
    Hiroaki Misawa
    Research Institute for Electronic Science, Hokkaido University, North-21 West-10, Kita-ku, Sapporo 001-0021, Japan
    Center for Emergent Functional Matter Science, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan
    *(H.M.) Email: [email protected]
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    Orcidhttp://orcid.org/0000-0003-1070-387X
  • , and 
  • Kazuhiko Maeda*
    Kazuhiko Maeda
    Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
    *(K.M.) Email: [email protected]
    More by Kazuhiko Maeda
    Orcidhttp://orcid.org/0000-0001-7245-8318
Cite this: ACS Appl. Energy Mater. 2020, 3, 6, 5142–5146
Publication Date (Web):May 27, 2020
https://doi.org/10.1021/acsaem.0c00857

Copyright © 2020 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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Abstract

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Plasmonic Au/TiO2 thin film works as a stable water oxidation photoanode. Here we show that site-selective deposition of nanosized CoOx as a water oxidation cocatalyst at the edge of Au nanoparticles on the TiO2 film improves the photoelectrochemical water oxidation activity. The nanosized CoOx was deposited by a photoassisted electrochemical method onto the Au/TiO2 thin film. The CoOx loading amount was controllable by changing the amount of electric charge that flowed during the deposition, which influenced the photoelectrochemical performance. Under visible light, the optimized CoOx/Au/TiO2 thin film generated stable photoanodic current, which was ∼3 times higher than that obtained using Au/TiO2.

KEYWORDS:

Photoanode materials that are active for water oxidation under visible light play a vital role in constructing artificial photosynthetic schemes such as overall water splitting, (1,2) and CO2/N2 fixation systems. (3−5) Sensitization of a wide-gap metal oxide with a “catalytic photosensitizer” that simultaneously functions as a light-absorbing center and a water oxidation catalyst is an effective approach that enables one to obtain a visible-light-absorbing photoanode material for water oxidation. (1,5,6)

Among them, nanosized plasmonic metal/semiconductor hybrid systems are of particular interest, as they can utilize a wide range of visible light and near-infrared light owing to their light-harvesting effect induced by localized surface plasmon resonance (LSPR) of metal nanoparticles. (7,8) Of course, plasmonic metal nanoparticles not only serve as light-absorbing units but also promote plasmon-induced charge separation at the metal/semiconductor interface, followed by reduction/oxidation reactions. (9,10) For example, Au-loaded metal oxide thin films (e.g., TiO2 and SrTiO3) have been shown to function as stable photoanodes for water oxidation under irradiation of light with wavelength longer than 550 nm in the presence of an (electro)chemical bias without noticeable degradation of activity. (11,12) The use of a TiO2 photonic crystal or nanotunnels as scaffolds of Au nanoparticles has been shown to improve the efficiency. (13−15)

However, one of the biggest issues in the LSPR-based photosystem is the very short lifetime of plasmon-induced hot carriers. (16−18) Also, hot electrons injected into the conduction band of an adjacent semiconductor should move to an external circuit without recombination with hot holes existing in the plasmonic metal and/or surface states of TiO2. Therefore, prompt spatial separation of the hot holes is a straightforward way to improve the plasmon-induced charge separation efficiency, which will eventually contribute to more efficient water oxidation. Several attempts have been made to improve the consumption of hot holes by introducing a water oxidation promoter such as nanoparticulate cobalt or iridium species. (19−21)

Recently, Murakoshi et al. have visualized the active sites for plasmon-induced electron transfer reaction on a Au/TiO2 electrode by means of photoelectrochemical polymerization of pyrrole. (22) According to that report, the oxidative polymerization occurred at the edge of Au nanoparticles on the Au/TiO2 electrode. A subsequent report by Li et al. has also demonstrated that the active site for water oxidation over plasmonic Au/TiO2 was the interface between Au and TiO2 by means of surface photovoltage imaging with Kelvin probe force microscopy and theoretical calculations. (23) Although the localized hot holes at the Au/TiO2 interface have a strong ability to oxidize water into O2, the intrinsic catalytic property of the interfacial site for water oxidation may not be optimal. As mentioned above, introducing an additional water oxidation cocatalyst into the Au/TiO2 system has been made. (19−21) However, the distribution of the introduced water oxidation cocatalysts was uncontrolled. Moskovits et al. have developed a solar water-splitting device based on a gold nanorod array in which nanosized Pt/TiO2 and CoOx were deposited separately. (24) According to that study, CoOx on the Au nanorod was claimed to work as a water oxidation site. Nevertheless, the impact of “site-selective” deposition of a water oxidation cocatalyst onto the Au/TiO2 interface has not been investigated.

In this work, we demonstrate that site-selective deposition of a CoOx cocatalyst at the edge of Au nanoparticles on the TiO2 film is actually possible by a photoassisted electrodeposition technique. The resulting CoOx/Au/TiO2 thin film showed enhanced photoelectrochemical water oxidation performance by a factor of ∼3, as compared to Au/TiO2.

The preparation procedure of the CoOx/Au/TiO2 thin film is illustrated in Scheme 1. First, a 30 nm thick TiO2 layer was formed by an atomic layer deposition (ALD) technique on a FTO substrate. We chose an ALD method for the preparation of a TiO2 support layer, because the planar morphology of the ALD-derived oxide should be convenient for analyzing the morphological characteristics of Au and CoOx that are located on TiO2. Then, a 12 nm thick Au layer was deposited on the TiO2 film by vapor deposition, followed by annealing at 573 K in air to generate Au nanoparticle islands. Finally, CoOx was loaded on the Au/TiO2 thin film by a photoelectrochemical deposition method. The quantity of cobalt deposited was controlled with respect to the photocurrent that flowed during the photodeposition. Unless otherwise stated, the amount of electron that flowed during the preparation was ∼1.0 × 10–4 C. The detail can be found in the Supporting Information.

Scheme 1

Scheme 1. Preparation of CoOx-Deposited Au/TiO2 Photoanode
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The deposition of TiO2 on FTO and Au on TiO2/FTO was confirmed by X-ray photoelectron spectroscopy (XPS), as shown in Supporting Information Figure S1. The crystal phase of the deposited TiO2 was mainly rutile, as indicated by Raman spectroscopy (Figure S2). However, XPS measurement did not detect any clear signal of cobalt species despite 200 times scans with several electrode samples, suggesting that the concentration of deposited cobalt on Au/TiO2/FTO was very low. Inductively coupled plasma–mass spectrometry (ICP-MS) analysis indicated that the prepared film contained approximately 0.1 nmol of cobalt. Considering the photodeposition condition, cobalt species appear to be in the form of oxide and/or oxyhydroxide. (25) Therefore, we represent the deposited cobalt species as CoOx for simplicity.

Figure 1 shows extinction spectra of Au/TiO2 and CoOx/Au/TiO2. The plasmon resonance band of Au/TiO2 was confirmed in the visible-light region, which exhibited a maximum at approximately 670 nm. The maximum wavelengths were shifted to longer wavelengths (at around 700 nm) upon CoOx deposition. This red shift can be explained by a contact of CoOx with a higher refractive index than air at the edge of Au particles on TiO2. (26)

Figure 1

Figure 1. Extinction spectra of Au/TiO2 and CoOx/Au/TiO2 thin films.

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The location of the CoOx introduced was investigated by high-resolution scanning electron microscopy (SEM). As shown in Figure 2, CoOx was mainly deposited at the edge of Au nanoparticles. The thickness of the CoOx deposited was increased with an increase in electric charge flow and were estimated to be approximately 6 and 10 nm, respectively, under 1.0 × 10–4 and 2.0 × 10–4 C charge flow conditions. It shows that the amount of CoOx deposited was increased with an increase in electric charge flow during the photoassisted deposition. It has been reported by previous works that hot holes should be accumulated at the interface of Au nanoparticles and TiO2 support under visible-light irradiation. (23) Therefore, it is reasonable to consider that CoOx could be successfully loaded at the edge of Au nanoparticles on the TiO2 film under plasmonic excitation with 580 nm wavelength. We also attempted energy dispersive X-ray spectroscopy measurement to visualize the existence of cobalt species in the CoOx/Au/TiO2 thin film, but the results were unsuccessful due to charge-up effect and damage of the film sample in the measurement.

Figure 2

Figure 2. SEM images of (A) Au/TiO2 and CoOx/Au/TiO2 thin films prepared with electric charge of (B) 1.0 × 10–4 and (C) 2.0 × 10–4 C, respectively. The inset images indicate their enlarged views.

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The as-prepared CoOx/Au/TiO2 thin film generated clear photoresponse under visible light (λ > 450 nm) upon anodic polarization. As shown in Figure 3A, the anodic photocurrent generated from the CoOx/Au/TiO2 thin film was ∼3 times higher than that of Au/TiO2, with good stability determined by current–time curves. Unmodified TiO2 thin film had almost no ability to generate photocurrent because it could not effectively absorb visible light. Photoanode performance of the CoOx/Au/TiO2 thin film was also compared with those of Au/TiO2 and TiO2 by taking current–voltage curves. As shown in Figure S3, the CoOx/Au/TiO2 thin film again showed enhanced photoresponse, with a photocurrent onset potential of approximately −0.7 V vs Ag/AgCl, which was almost the same as that of Au/TiO2.

Figure 3

Figure 3. Photoelectrochemical performance. (A) Current–time curves for modified TiO2 thin films recorded in controlled-potential photoelectrolysis at +0.10 V vs Ag/AgCl (i.e., +1.07 V vs RHE) under visible light (λ > 450 nm) with the light intensity of 180 mW. The experiment was conducted in 0.2 M K3PO4 solution (pH 13) using the optimized CoOx/Au/TiO2 thin film anode, in which CoOx deposition was done under 1.0 × 10–4 C condition. Irradiation area: 3.0 cm–2. (B) IPCEs of the CoOx/Au/TiO2 thin film electrode recorded in aqueous 0.1 M KOH solution (pH 13) at 0 V vs Ag/AgCl as a function of the wavelength of the incident light. (C) IPCEs of the CoOx/Au/TiO2 electrodes at 580 nm as a function of the amount of electric charge flowed during the photoassited CoOx deposition. The error bars were estimated at two points using the optimal CoOx/Au/TiO2 and Au/TiO2 (CoOx-free specimen) thin films prepared at different batches.

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Figure 3B shows IPCEs of the CoOx/Au/TiO2 thin film at different wavelengths. The change in IPCE recorded at different wavelengths was similar to that of the extinction spectrum (Figure 1). This means that photocurrent generation was derived from plasmonic photoexcitation. The IPCE of the CoOx/Au/TiO2 thin film was found to be sensitive to the electric charge that flowed during the photodeposition of CoOx, as shown in Figure 3C. The data point at zero electric charge corresponds to Au/TiO2. The IPCE increased with increasing electric charge flow, to reach a maximum at around 1.0 × 10–4 C, beyond which it decreased.

Thus, it was shown that the plasmonic water photooxidation was enhanced by selectively depositing a suitable concentration of CoOx cocatalyst at the edge of Au nanoparticles on the TiO2 film. However, an excess loading of CoOx, as demonstrated by SEM (Figure 2), led to lower performance, most likely due to a decrease in active surface area for water oxidation, which has been usually seen in heterogeneous (photo)catalysis. (27) The larger CoOx deposits also buried the interface between Au and TiO2, thereby hindering the migration of holes from the active Au/TiO2 interface to the external surface of CoOx through the inside. This eventually leads to the lower water oxidation performance. It is also noted that loading CoOx by an impregnation method was not as effective as the photoassisted deposition (Figure S4), most likely because the impregnation method did not provide site-selective deposition of CoOx (Figure S5). Nevertheless, the performance of the impregnated specimen was a little higher than the unmodified Au/TiO2. Therefore, the randomly deposited CoOx had the functionality as a cocatalyst to some extent, and a “catalytic sensitizer” effect of CoOx for water oxidation with a TiO2 support (6) might be also at play.

The optimized CoOx/Au/TiO2 thin film photoanode worked stably under simulated sunlight, as shown in Figure 4. This result shows that the deposited CoOx cocatalyst was stable during the photoelectrolysis of water. During the 3 h of photoelectrolysis, product analysis by gas chromatography indicated that O2 was evolved with almost no sign of deactivation. The amount of O2 evolved was similar to one-fourth of the amount of electrons passing through the external circuit. There was an induction period at the initial stage of the reaction (∼1 h) due to a time lag of gas diffusion from the solution to the gas phase, as reported in some previous works. (28,29) After irradiation was stopped, O2 evolution continued for another ∼2 h, which is further confirmation of the effect of diffusion on the gas detection. A maximum solar-to-hydrogen energy conversion efficiency was 0.052%.

Figure 4

Figure 4. Time courses of (A) O2 evolution and (B) current generation in controlled-potential photoelectrolysis at −0.31 V vs Ag/AgCl (i.e., +0.66 V vs RHE) under simulated sunlight (100 mW cm–2). The experiment was conducted in 0.2 M K3PO4 solution (pH 13) using the optimized CoOx/Au/TiO2 thin film anode. Irradiation area: 3.0 cm2. Error range of O2 detection was approximately 10%.

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In conclusion, we succeeded in modifying a plasmonic Au/TiO2 film with a CoOx cocatalyst site-selectively by a photoassisted electrochemical method. The amount of the CoOx cocatalyst deposited at the interface between Au and TiO2 was controllable by changing the amount of electric charge that flowed during the photodeposition. Upon the site-selective deposition of nanosized CoOx with ∼6 nm thickness, an IPCE for water oxidation was improved by a factor of ∼3. However, deposition of >10 nm thick CoOx reduced the performance.

We have previously reported the Au film–TiO2–Au nanoparticle (ATA) water-splitting system, (30) which gives higher IPCEs (>1%) under visible light. The present method, which enabled for site-selective CoOx deposition onto the water oxidation center, could be a candidate for the improvement of IPCEs for water-splitting reaction for the ATA system as well as for other Au/semiconductor-based plasmonic artificial photosynthetic nanodevices, because the catalytic reaction of water oxidation ends the oxidation half-cycle in any of these devices. The use of TiO2 nanostructures having high surface area (13) as scaffolds of Au nanoparticles and additional CoOx may be another approach to improving efficiency. A standalone, bias-free photoelectrochemical water-splitting system can in principle be constructed using other metal oxide semiconductor supports that have more negative flat-band potential. These possibilities are currently under investigation in our groups.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.0c00857.

  • Experimental section; additional characterization and photoelectrochemical data (Figures S1–S6) (PDF)

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  • Corresponding Authors
  • Authors
    • Megumi Okazaki - Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan;  Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan;  Orcidhttp://orcid.org/0000-0003-1167-9453
    • Yoshiki Suganami - Research Institute for Electronic Science, Hokkaido University, North-21 West-10, Kita-ku, Sapporo 001-0021, Japan
    • Naoki Hirayama - Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
    • Hiroko Nakata - Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
    • Tomoya Oshikiri - Research Institute for Electronic Science, Hokkaido University, North-21 West-10, Kita-ku, Sapporo 001-0021, Japan;  Orcidhttp://orcid.org/0000-0002-1268-0256
    • Toshiyuki Yokoi - Nanospace Catalysis Unit, Institute of Innovative Research, Tokyo Institute of Technology, 4259-S2-5, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan;  Orcidhttp://orcid.org/0000-0002-3315-3172
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We acknowledge financial support from JSPS KAKENHI (Grant Nos. JP16H06441, JP19H02511, JP18H05205, JP17H01041, JP17H05245, JP17H05459, JP16H06506, JP15K04589, and JP18K05053), the Nanotechnology Platform (Hokkaido University), and the Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials (Five-Star Alliance) of MEXT (Grant Nos. 20181032 and 20191034). M.O. acknowledges support in the form of a JSPS Fellowship for Young Scientists (Grant No. JP19J21858). We thank Profs. Keigo Kamata and Michikazu Hara (Tokyo Tech) for assistance in Raman spectroscopy measurements.

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    A review. Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chem. energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. A review of recent progress in using plasmonic metallic nanostructures in the field of photocatalysis is presented. The focus is on plasmon-enhanced water splitting on composite photocatalysts contg. semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. The areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward are discussed.
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  8. 8
    Ueno, K.; Misawa, H. Surface Plasmon-Enhanced Photochemical Reactions. J. Photochem. Photobiol., C 2013, 15, 31– 52,  DOI: 10.1016/j.jphotochemrev.2013.04.001
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    Surface plasmon-enhanced photochemical reactions
    Ueno, Kosei; Misawa, Hiroaki
    Journal of Photochemistry and Photobiology, C: Photochemistry Reviews (2013), 15 (), 31-52CODEN: JPPCAF; ISSN:1389-5567. (Elsevier B.V.)
    A review. The electromagnetic field enhancement effect based on the excitation of localized surface plasmon resonance was developed for various photochem. reaction systems, such as nano-lithog., photovoltaic cells, photocatalysis, and water splitting systems. As with most points characteristic of these surface plasmon-enhanced photochem. reactions, spatially selective photochem. reactions can be induced and photons can be efficiently utilized, a concept that could contribute to the development of green nanotechnol. Electromagnetic field enhancement effects based on plasmon excitation have contributed not only to phys. processes, such as excitation efficiency, but also to chem. processes, such as photo-induced electron transfer reactions. This review article describes advanced studies on a wide variety of surface plasmon-enhanced photochem. reactions.
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  9. 9
    Zhao, G.; Kozuka, H.; Yoko, T. Sol—Gel Preparation and Photoelectrochemical Properties of TiO2 Films Containing Au and Ag Metal Particles. Thin Solid Films 1996, 277, 147– 154,  DOI: 10.1016/0040-6090(95)08006-6
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    9
    Sol-gel preparation and photoelectrochemical properties of TiO2 films containing Au and Ag metal particles
    Zhao, Gaoling; Kozuka, Hiromitsu; Yoko, Toshinobu
    Thin Solid Films (1996), 277 (1-2), 147-154CODEN: THSFAP; ISSN:0040-6090. (Elsevier)
    TiO2 film electrodes with a TiO2 overlayer contg. dispersed Au or Ag metal particles were prepd. by the sol-gel method, and the effect of the metal particles on the photoelectrochem. properties of the TiO2 electrodes were investigated. An increase in the anodic photocurrent in the visible region was obsd. for both the Au and Ag particle dispersed electrodes, which was thought to result from the surface plasma resonance of the metal particles. The introduction of Au metal particles, however, reduced the anodic photocurrent in the UV region, resulting in a decrease of the anodic photocurrent under the illumination of xenon lamp light. On the other hand, for the Ag particle dispersed electrodes, the anodic photocurrent in the UV region increased and then decreased with an increasing amt. of Ag particles. These effects of the dispersed metal particles on the photoelectrochem. properties of the TiO2 electrodes were explained on the basis of the band models.
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  10. 10
    Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. Plasmon-Assisted Photocurrent Generation from Visible to near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode. J. Phys. Chem. Lett. 2010, 1, 2031– 2036,  DOI: 10.1021/jz1006675
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    10
    Plasmon-Assisted Photocurrent Generation from Visible to Near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode
    Nishijima, Yoshiaki; Ueno, Kosei; Yokota, Yukie; Murakoshi, Kei; Misawa, Hiroaki
    Journal of Physical Chemistry Letters (2010), 1 (13), 2031-2036CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Nanoparticles of noble metals exhibit localized surface plasmons (LSPs) assocd. with the enhancement of an electromagnetic field due to its localization in nanometric domains at the surface of nanoparticles. The authors demonstrate the plasmonic photoelec. conversion from visible to near-IR wavelength without deteriorating photoelec. conversion by using electrodes in which Au nanorods are elaborately arrayed on the surface of a TiO2 single crystal.
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  11. 11
    Nishijima, Y.; Ueno, K.; Kotake, Y.; Murakoshi, K.; Inoue, H.; Misawa, H. Near-Infrared Plasmon-Assisted Water Oxidation. J. Phys. Chem. Lett. 2012, 3, 1248– 1252,  DOI: 10.1021/jz3003316
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    11
    Near-Infrared Plasmon-Assisted Water Oxidation
    Nishijima, Yoshiaki; Ueno, Kosei; Kotake, Yuki; Murakoshi, Kei; Inoue, Haruo; Misawa, Hiroaki
    Journal of Physical Chemistry Letters (2012), 3 (10), 1248-1252CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    We report the stoichiometric evolution of oxygen via water oxidn. by irradiating a plasmon-enhanced photocurrent generation system with near-IR light (λ: 1000 nm), in which gold nanostructures were arrayed on the surface of TiO2 electrode. It is considered that multiple electron holes generated by plasmon-induced charge excitation led to the effective recovery of water oxidn. after the electron transfer from gold to TiO2. The proposed system contg. a gold nanostructured TiO2 electrode may be a promising artificial photosynthetic system using near-IR light.
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  12. 12
    Zhong, Y.; Ueno, K.; Mori, Y.; Shi, X.; Oshikiri, T.; Murakoshi, K.; Inoue, H.; Misawa, H. Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy. Angew. Chem., Int. Ed. 2014, 53, 10350– 10354,  DOI: 10.1002/anie.201404926
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    12
    Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy
    Zhong, Yuqing; Ueno, Kosei; Mori, Yuko; Shi, Xu; Oshikiri, Tomoya; Murakoshi, Kei; Inoue, Haruo; Misawa, Hiroaki
    Angewandte Chemie, International Edition (2014), 53 (39), 10350-10354CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A plasmon-induced water splitting system that operates under irradn. by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate (SrTiO3) single-crystal substrate. The water splitting system contains two soln. chambers to sep. hydrogen (H2) and oxygen (O2). To promote water splitting, a chem. bias was applied by regulating the pH values of the chambers. The quantity of H2 evolved from the surface of platinum, which was used as a redn. co-catalyst, was twice the quantity of O2 evolved from an Au-nanostructured surface. Thus, the stoichiometric evolution of H2 and O2 was clearly demonstrated. The hydrogen-evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon-induced charge sepn. at the Au/SrTiO3 interface promotes water oxidn. and the subsequent redn. of a proton on the backside of the SrTiO3 substrate. The chem. bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.
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  13. 13
    Zhang, Z.; Zhang, L.; Hedhili, M. N.; Zhang, H.; Wang, P. Plasmonic Gold Nanocrystals Coupled with Photonic Crystal Seamlessly on TiO2 Nanotube Photoelectrodes for Efficient Visible Light Photoelectrochemical Water Splitting. Nano Lett. 2013, 13, 14– 20,  DOI: 10.1021/nl3029202
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    13
    Plasmonic Gold Nanocrystals Coupled with Photonic Crystal Seamlessly on TiO2 Nanotube Photoelectrodes for Efficient Visible Light Photoelectrochemical Water Splitting
    Zhang, Zhonghai; Zhang, Lianbin; Hedhili, Mohamed Nejib; Zhang, Hongnan; Wang, Peng
    Nano Letters (2013), 13 (1), 14-20CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    A visible light responsive plasmonic photocatalytic composite material is designed by rationally selecting Au nanocrystals and assembling them with the TiO2-based photonic crystal substrate. The selection of the Au nanocrystals is so that their surface plasmonic resonance (SPR) wavelength matches the photonic band gap of the photonic crystal and thus that the SPR of the Au receives remarkable assistance from the photonic crystal substrate. The design of the composite material is expected to significantly increase the Au SPR intensity and consequently boost the hot electron injection from the Au nanocrystals into the conduction band of TiO2, leading to a considerably enhanced water splitting performance of the material under visible light. A proof-of-concept example is provided by assembling 20 nm Au nanocrystals, with a SPR peak at 556 nm, onto the photonic crystal which is seamlessly connected on TiO2 nanotube array. Under visible light illumination (>420 nm), the designed material produced a photocurrent d. of ∼150 μA cm-2, which is the highest value ever reported in any plasmonic Au/TiO2 system under visible light irradn. due to the photonic crystal-assisted SPR. This work contributes to the rational design of the visible light responsive plasmonic photocatalytic composite material based on wide band gap metal oxides for photoelectrochem. applications.
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  14. 14
    Takakura, R.; Oshikiri, T.; Ueno, K.; Shi, X.; Kondo, T.; Masuda, H.; Misawa, H. Water Splitting Using a Three-Dimensional Plasmonic Photoanode with Titanium Dioxide Nano-Tunnels. Green Chem. 2017, 19, 2398– 2405,  DOI: 10.1039/C6GC03217F
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    14
    Water splitting using a three-dimensional plasmonic photoanode with titanium dioxide nano-tunnels
    Takakura, Ryohei; Oshikiri, Tomoya; Ueno, Kosei; Shi, Xu; Kondo, Toshiaki; Masuda, Hideki; Misawa, Hiroaki
    Green Chemistry (2017), 19 (10), 2398-2405CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    In this study, we developed a three-dimensional plasmonic photoanode using titanium dioxide nano-tunnels (TNTs) loaded with gold nanoparticles (Au-NPs) for water splitting, to enhance the reaction efficiency. We also optimized the procedure of loading Au-NPs on complex three-dimensional structures. We discuss the correlation between the plasmon-induced charge sepn. obtained from photoelectrochem. measurement and the morphol. of Au-NPs obsd. by transmission electron microscopy. We have successfully deposited well-dispersed Au-NPs on the walls of TNTs using HAu(OH)4 as a precursor. The amt. of Au-NPs on the TNTs was estd. to be approx. 15-fold larger than that on the thin film titanium dioxide substrate, and the particle size remained small. Photoelectrochem. water splitting was achieved by using a two-electrode system rather than a three-electrode system. Furthermore, stoichiometric water splitting was confirmed by estg. the amts. of the evolved H2 and O2 gases under visible light irradn.
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  15. 15
    Mi, Y.; Wen, L.; Xu, R.; Wang, Z.; Cao, D.; Fang, Y.; Lei, Y. Constructing a AZO/TiO2 Core/Shell Nanocone Array with Uniformly Dispersed Au NPs for Enhancing Photoelectrochemical Water Splitting. Adv. Energy Mater. 2016, 6, 1501496,  DOI: 10.1002/aenm.201501496
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  16. 16
    Harutyunyan, H.; Martinson, A. B.; Rosenmann, D.; Khorashad, L. K.; Besteiro, L. V.; Govorov, A. O.; Wiederrecht, G. P. Anomalous Ultrafast Dynamics of Hot Plasmonic Electrons in Nanostructures with Hot Spots. Nat. Nanotechnol. 2015, 10, 770– 774,  DOI: 10.1038/nnano.2015.165
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    16
    Anomalous ultrafast dynamics of hot plasmonic electrons in nanostructures with hot spots
    Harutyunyan, Hayk; Martinson, Alex B. F.; Rosenmann, Daniel; Khorashad, Larousse Khosravi; Besteiro, Lucas V.; Govorov, Alexander O.; Wiederrecht, Gary P.
    Nature Nanotechnology (2015), 10 (9), 770-774CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    The interaction of light and matter in metallic nanosystems is mediated by the collective oscillation of surface electrons, called plasmons. After excitation, plasmons are absorbed by the metal electrons through inter- and intraband transitions, creating a highly non-thermal distribution of electrons. The electron population then decays through electron-electron interactions, creating a hot electron distribution within a few hundred femtoseconds, followed by a further relaxation via electron-phonon scattering on the timescale of a few picoseconds. In the spectral domain, hot plasmonic electrons induce changes to the plasmonic resonance of the nanostructure by modifying the dielec. const. of the metal. Here, we report on the observation of anomalously strong changes to the ultrafast temporal and spectral responses of these excited hot plasmonic electrons in hybrid metal/oxide nanostructures as a result of varying the geometry and compn. of the nanostructure and the excitation wavelength. In particular, we show a large ultrafast, pulsewidth-limited contribution to the excited electron decay signal in hybrid nanostructures contg. hot spots. The intensity of this contribution correlates with the efficiency of the generation of highly excited surface electrons. Using theor. models, we attribute this effect to the generation of hot plasmonic electrons from hot spots. We then develop general principles to enhance the generation of energetic electrons through specifically designed plasmonic nanostructures that could be used in applications where hot electron generation is beneficial, such as in solar photocatalysis, photodetectors and nonlinear devices.
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  17. 17
    Boerigter, C.; Aslam, U.; Linic, S. Mechanism of Charge Transfer from Plasmonic Nanostructures to Chemically Attached Materials. ACS Nano 2016, 10, 6108– 6115,  DOI: 10.1021/acsnano.6b01846
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    17
    Mechanism of Charge Transfer from Plasmonic Nanostructures to Chemically Attached Materials
    Boerigter, Calvin; Aslam, Umar; Linic, Suljo
    ACS Nano (2016), 10 (6), 6108-6115CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Plasmonic metal nanoparticles can efficiently convert the energy of visible photons into the energy of hot charge carriers within the nanoparticles. These energetic charge carriers can transfer to mols. or semiconductors, chem. attached to the nanoparticles, where they can induce photochem. transformations. Classical models of photoinduced charge excitation and transfer in metals suggest that the majority of the energetic charge carriers rapidly decay within the metal nanostructure before they are transferred into the neighboring mol. or semiconductor, and therefore, the efficiency of charge transfer is low. Herein, we present exptl. evidence that calls into question this conventional picture. We demonstrate a system where the presence of a mol., adsorbed on the surface of a plasmonic nanoparticle, significantly changes the flow of charge within the excited plasmonic system. The nanoparticle-adsorbate system experiences high rates of direct, resonant flow of charge from the nanoparticle to the mol., bypassing the conventional charge excitation and thermalization process taking place in the nanoparticle. This picture of charge transfer suggests that the yield of extd. hot electrons (or holes) from plasmonic nanoparticles can be significantly higher than the yields expected based on conventional models. We discuss a conceptual phys. framework that allows us to explain our exptl. observations. This anal. points us in a direction toward mol. control of the charge transfer process using interface and local field engineering strategies.
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  18. 18
    Liu, J. G.; Zhang, H.; Link, S.; Nordlander, P. Relaxation of Plasmon-Induced Hot Carriers. ACS Photonics 2018, 5, 2584– 2595,  DOI: 10.1021/acsphotonics.7b00881
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    18
    Relaxation of Plasmon-Induced Hot Carriers
    Liu, Jun G.; Zhang, Hui; Link, Stephan; Nordlander, Peter
    ACS Photonics (2018), 5 (7), 2584-2595CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Plasmon-induced hot carrier generation has attracted much recent attention due to its promising potential in photocatalysis and other light harvesting applications. A theor. model for hot carrier relaxation in metallic nanoparticles was developed using a fully quantum mech. jellium model. Following pulsed illumination, nonradiative plasmon decay results in a highly nonthermal distribution of hot electrons and holes. Using coupled master equations, the time-dependent evolution of this carrier distribution in the presence of electron-electron, electron-photon, and electron-phonon scattering was calcd. Electron-electron relaxation is the dominant scattering mechanism and results in efficient carrier multiplication where the energy of the initial hot electron-hole pair is transferred to other multiple electron-hole pair excitations of lower energies. During this relaxation, a small but finite fraction of electrons scatter into luminescent states where they can recombine radiatively with holes by emission of photons. The energy of the emitted photons is found to follow the energies of the electrons and thus red shifts monotonically during the relaxation process. When the energies of the electrons approach the Fermi level, electron-phonon interaction becomes dominant and results in heating of the nanoparticle. The model was generalized to continuous-wave excitation, and nonlinear effects become important when the illumination intensity increases. When the temporal spacing between incident photons is shorter than the relaxation time of the hot carriers, the authors predict that the luminescence will blueshift with increasing illumination power. The effect of the photonic d. of states (Purcell factor) on the luminescence spectra are discussed.
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  19. 19
    Lee, J.; Mubeen, S.; Ji, X.; Stucky, G. D.; Moskovits, M. Plasmonic Photoanodes for Solar Water Splitting with Visible Light. Nano Lett. 2012, 12, 5014– 5019,  DOI: 10.1021/nl302796f
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    19
    Plasmonic Photoanodes for Solar Water Splitting with Visible Light
    Lee, Joun; Mubeen, Syed; Ji, Xiulei; Stucky, Galen D.; Moskovits, Martin
    Nano Letters (2012), 12 (9), 5014-5019CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We report a plasmonic water splitting cell in which 95% of the effective charge carriers derive from surface plasmon decay to hot electrons, as evidenced by fuel prodn. efficiencies up to 20-fold higher at visible, as compared to UV, wavelengths. The cell functions by illuminating a dense array of aligned gold nanorods capped with TiO2, forming a Schottky metal/semiconductor interface which collects and conducts the hot electrons to an non-illuminated platinum counter electrode where hydrogen gas evolves. The resultant pos. charges in the Au nanorods function as holes and are extd. by an oxidn. catalyst which electrocatalytically oxidizes water to oxygen gas.
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  20. 20
    Kim, J.; Son, H. Y.; Nam, Y. S. Multilayered Plasmonic Heterostructure of Gold and Titania Nanoparticles for Solar Fuel Production. Sci. Rep. 2018, 8, 10464,  DOI: 10.1038/s41598-018-28789-w
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    20
    Multilayered Plasmonic Heterostructure of Gold and Titania Nanoparticles for Solar Fuel Production
    Kim Jeonga; Son Ho Yeon; Nam Yoon Sung; Nam Yoon Sung
    Scientific reports (2018), 8 (1), 10464 ISSN:.
    Solar fuel production via photoelectrochemical (PEC) water splitting has attracted great attention as an approach to storing solar energy. However, a wide range of light-harvesting materials is unstable when exposed to light and oxidative conditions. Here we report a robust, multilayered plasmonic heterostructure for water oxidation using gold nanoparticles (AuNPs) as light-harvesting materials via localized surface plasmon resonance (LSPR). The multilayered heterostructure is fabricated using layer-by-layer self-assembly of AuNPs and TiO2 nanoparticles (TNPs). Plasmon-induced hot electrons are transferred from AuNPs to TNPs over the Au/TiO2 Schottky barrier, resulting in charge separation of hot carriers. Plasmonic photoanodes for water oxidation are completed by incorporating a Co-based oxygen-evolving catalyst on the multilayered heterostructure to scavenge hot holes. Light absorption capability and PEC properties of the photoanodes are investigated as a function of the number of AuNP/TNP bilayers. The PEC properties exhibits dependence on the number of the bilayers, which is affected by charge transport within the multilayered heterostructures. Photocurrent density and decrease in impedance by irradiation indicates significant photoactivity by LSPR excitation.
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  21. 21
    Hung, S.-F.; Xiao, F.-X.; Hsu, Y.-Y.; Suen, N.-T.; Yang, H.-B.; Chen, H. M.; Liu, B. Iridium Oxide-Assisted Plasmon-Induced Hot Carriers: Improvement on Kinetics and Thermodynamics of Hot Carriers. Adv. Energy Mater. 2016, 6, 1501339,  DOI: 10.1002/aenm.201501339
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  22. 22
    Minamimoto, H.; Toda, T.; Futashima, R.; Li, X.; Suzuki, K.; Yasuda, S.; Murakoshi, K. Visualization of Active Sites for Plasmon-Induced Electron Transfer Reactions Using Photoelectrochemical Polymerization of Pyrrole. J. Phys. Chem. C 2016, 120, 16051– 16058,  DOI: 10.1021/acs.jpcc.5b12727
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    22
    Visualization of Active Sites for Plasmon-Induced Electron Transfer Reactions Using Photoelectrochemical Polymerization of Pyrrole
    Minamimoto, Hiro; Toda, Takahiro; Futashima, Ryo; Li, Xiaowei; Suzuki, Kentaro; Yasuda, Satoshi; Murakoshi, Kei
    Journal of Physical Chemistry C (2016), 120 (29), 16051-16058CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Spatially selective deposition of conductive polymer was obsd. at the Au nanostructures supported on TiO2 electrodes via plasmon-induced photopolymn. of pyrrole monomers. The reactions were triggered by the excitation of localized surface plasmon resonance under near-IR light illumination to the plasmon-active Au nanostructures. The morphol. characteristics of the deposited polypyrrole prove the localization of the reaction-active sites in the plasmon-induced oxidn.-reaction system. In addn., the estn. of reaction characteristics provides information on the spatial distribution and the electrochem. potential of the holes to contribute to the reaction. The unique polymer-growing process obsd. in the present system provides information on the mechanism of plasmon-induced oxidn. reaction occurring at the active sites.
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  23. 23
    Wang, S.; Gao, Y.; Miao, S.; Liu, T.; Mu, L.; Li, R.; Fan, F.; Li, C. Positioning the Water Oxidation Reaction Sites in Plasmonic Photocatalysts. J. Am. Chem. Soc. 2017, 139, 11771– 11778,  DOI: 10.1021/jacs.7b04470
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    23
    Positioning the Water Oxidation Reaction Sites in Plasmonic Photocatalysts
    Wang, Shengyang; Gao, Yuying; Miao, Shu; Liu, Taifeng; Mu, Linchao; Li, Rengui; Fan, Fengtao; Li, Can
    Journal of the American Chemical Society (2017), 139 (34), 11771-11778CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Plasmonic photocatalysis, stemming from the effective light absorbance and confinement of surface plasmons, provides a pathway to enhance solar energy conversion. Although the plasmonic hot electrons in water redn. have been extensively studied, exactly how the plasmonic hot holes participate in the water splitting reaction has not yet been well understood. In particular, where the plasmonic hot holes participate in water oxidn. is still illusive. Herein, taking Au/TiO2 as a plasmonic photocatalyst prototype, we investigated the plasmonic hot holes involved in water oxidn. The reaction sites are positioned by photodeposition together with element mapping by electron microscopy, while the distribution of holes is probed by surface photovoltage imaging with Kelvin probe force microscopy. We demonstrated that the plasmonic holes are mainly concd. near the gold-semiconductor interface, which is further identified as the reaction site for plasmonic water oxidn. D. functional theory also corroborates these findings by revealing the promotion role of interfacial structure (Ti-O-Au) for oxygen evolution. Furthermore, the interfacial effect on plasmonic water oxidn. is validated by other Au-semiconductor photocatalytic systems (Au/SrTiO3, Au/BaTiO3, etc.).
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  24. 24
    Mubeen, S.; Lee, J.; Singh, N.; Kramer, S.; Stucky, G. D.; Moskovits, M. An Autonomous Photosynthetic Device in Which All Charge Carriers Derive from Surface Plasmons. Nat. Nanotechnol. 2013, 8, 247– 251,  DOI: 10.1038/nnano.2013.18
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    24
    An autonomous photosynthetic device in which all charge carriers derive from surface plasmons
    Mubeen, Syed; Lee, Joun; Singh, Nirala; Kraemer, Stephan; Stucky, Galen D.; Moskovits, Martin
    Nature Nanotechnology (2013), 8 (4), 247-251CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Solar conversion to electricity or to fuels based on electron-hole pair prodn. in semiconductors is a highly evolved scientific and com. enterprise. Recently, it was posited that charge carriers either directly transferred from the plasmonic structure to a neighboring semiconductor (such as TiO2) or to a photocatalyst, or induced by energy transfer in a neighboring medium, could augment photoconversion processes, potentially leading to an entire new paradigm in harvesting photons for practical use. The strong dependence of the wavelength at which the local surface plasmon can be excited on the nanostructure makes it possible, in principle, to design plasmonic devices that can harvest photons over the entire solar spectrum and beyond. So far, however, most such systems show rather small photocatalytic activity in the visible as compared with the UV. Here, the authors report an efficient, autonomous solar H2O-splitting device based on a Au nanorod array in which essentially all charge carriers involved in the oxidn. and redn. steps arise from the hot electrons resulting from the excitation of surface plasmons in the nanostructured Au. Each nanorod functions without external wiring, producing 5 × 1013 H2 mols. per cm2 per s under 1 sun illumination (AM 1.5 and 100 mW cm-2), with unprecedented long-term operational stability.
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    Bledowski, M.; Wang, L.; Ramakrishnan, A.; Betard, A.; Khavryuchenko, O. V.; Beranek, R. Visible-Light Photooxidation of Water to Oxygen at Hybrid TiO2-Polyheptazine Photoanodes with Photodeposited Co-Pi (CoOx) Cocatalyst. ChemPhysChem 2012, 13, 3018– 3024,  DOI: 10.1002/cphc.201200071
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    Visible-Light Photooxidation of Water to Oxygen at Hybrid TiO2-Polyheptazine Photoanodes with Photodeposited Co-Pi (CoOx) Cocatalyst
    Bledowski, Michal; Wang, Lidong; Ramakrishnan, Ayyappan; Betard, Angelique; Khavryuchenko, Oleksiy V.; Beranek, Radim
    ChemPhysChem (2012), 13 (12), 3018-3024, S3018/1-S3018/5CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A cobalt oxide-based oxygen-evolving cocatalyst (Co-Pi) is photodeposited by visible-light irradn. onto nanocryst. TiO2-polyheptazine (TiO2-PH) hybrid photoelectrodes in a phosphate buffer. The Co-Pi cocatalyst couples effectively to photoholes generated in the surface polyheptazine layer of the TiO2-PH photoanode, as evidenced by complete photooxidn. of water to oxygen under visible-light (λ>420 nm) irradn. at moderate bias potentials. In addn., the presence of the cocatalyst also reduces significantly the recombination of photogenerated charges, particularly at low bias potentials, which is ascribed to better photooxidn. kinetics resulting in lower accumulation of holes. This suggests that further improvements of photoconversion efficiency can be achieved if more effective catalytic sites for water oxidn. are introduced to the surface structure of the hybrid photoanodes.
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    Mulvaney, P.; Pérez-Juste, J.; Giersig, M.; Liz-Marzán, L. M.; Pecharromán, C. Drastic Surface Plasmon Mode Shifts in Gold Nanorods Due to Electron Charging. Plasmonics 2006, 1, 61– 66,  DOI: 10.1007/s11468-005-9005-0
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    The color of small gold rods changes dramatically when electrons are injected by chem. reductants. The longitudinal and transverse plasmon modes are both found to blue-shift, and the shift is larger for rods with larger aspect ratios. The color changes are visible to the eye for rods with aspect ratios around 2-3. It is found that the surface plasmon band is damped when charging becomes high. The effects are in qual. agreement with a model in which the gold plasma frequency increases due to an increase in electron d.
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    Maeda, K. Photocatalytic Water Splitting Using Semiconductor Particles: History and Recent Developments. J. Photochem. Photobiol., C 2011, 12, 237– 268,  DOI: 10.1016/j.jphotochemrev.2011.07.001
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    Photocatalytic water splitting using semiconductor particles: History and recent developments
    Maeda, Kazuhiko
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    A review. Overall water splitting to produce H2 and O2 over a semiconductor photocatalyst using solar energy is a promising process for the large-scale prodn. of clean, recyclable H2. Numerous attempts have been made to develop photocatalysts that function under visible-light irradn. to efficiently utilize solar energy. In general, overall water splitting over a photocatalyst particle can be achieved by modifying the photocatalyst with a suitable cocatalyst to provide an active redox site. Therefore, the development of active photocatalytic materials has relied on both photocatalysts and cocatalysts. This review article describes the historical development of water-splitting photocatalysts.
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    Efficient solar hydrogen production from neutral electrolytes using surface-modified Cu(In,Ga)Se2 photocathodes
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    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (16), 8300-8307CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The effects of a phosphate buffer electrolyte and surface modification with thin conductor layers on the photoelectrochem. properties of CdS and Pt-modified polycryst. Cu(In,Ga)Se2 (CIGS) photocathodes were investigated. The photocurrent obtained from Pt/CdS/CIGS electrodes, in which the CIGS layer was fabricated by co-evapn. using a three stage method, clearly increased in a phosphate buffer electrolyte soln. as a result of promotion of the hydrogen evolution reaction. The half-cell solar-to-hydrogen efficiency (HC-STH) of this device reached a max. of 5.4% at 0.30 VRHE even under neutral conditions. Furthermore, significant enhancement of the hydrogen evolution reaction on a CIGS photocathode by surface modification with thin conductor layers was obsd. The enhancement was due to the promoted charge transfer between the underlying photocathode and water through the Pt catalyst. The HC-STH of a CIGS photocathode modified with a conductive Mo/Ti layer (Pt/Mo/Ti/CdS/CIGS) was as high as 8.5% at 0.38 VRHE, a value that exceeds those previously reported for photocathodes based on polycryst. thin films.
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    Hirayama, N.; Nakata, H.; Wakayama, H.; Nishioka, S.; Kanazawa, T.; Kamata, R.; Ebato, Y.; Kato, K.; Kumagai, H.; Yamakata, A.; Oka, K.; Maeda, K. Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode. J. Am. Chem. Soc. 2019, 141, 17158– 17165,  DOI: 10.1021/jacs.9b06570
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    Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode
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    Journal of the American Chemical Society (2019), 141 (43), 17158-17165CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Mixed-anion compds. (e.g., oxynitrides and oxysulfides) are potential candidates as photoanodes for visible-light water oxidn., but most of them suffer from oxidative degrdn. by photogenerated holes, leading to low stability. Here we show an exceptional example of a stable, mixed-anion water-oxidn. photoanode that consists of an oxyfluoride, Pb2Ti2O5.4F1.2, having a band gap of ca. 2.4 eV. Pb2Ti2O5.4F1.2 particles, which were coated on a transparent conductive glass (FTO) support and were subject to postdeposition of a TiO2 overlayer, generated an anodic photocurrent upon band gap photoexcitation of Pb2Ti2O5.4F1.2 (λ <520 nm) with a rather neg. photocurrent onset potential of ca. -0.6 V vs NHE, which was independent of the pH of the electrolyte soln. Stable photoanodic current was obsd. even without loading a water oxidn. promoter such as CoOx. Nevertheless, loading CoOx onto the TiO2/Pb2Ti2O5.4F1.2/FTO electrode further improved the anodic photoresponse by a factor of 2-3. Under AM1.5G simulated sunlight (100 mW cm-2), stable water oxidn. to form O2 was achieved using the optimized Pb2Ti2O5.4F1.2 photoanode in the presence of an applied potential smaller than 1.23 V, giving a Faradaic efficiency of 93% and almost no sign of deactivation during 4 h of operation. This study presents the first example of photoelectrochem. water splitting driven by visible-light excitation of an oxyfluoride that stably works, even without a water oxidn. promoter, which is distinct from ordinary mixed-anion photoanodes that usually require a water oxidn. promoter.
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    Enhanced water splitting under modal strong coupling conditions
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    Strong coupling between plasmons and optical modes, such as waveguide or resonator modes, gives rise to a splitting in the plasmon absorption band. As a result, two new hybrid modes are formed that exhibit near-field enhancement effects. These hybrid modes have been exploited to improve light absorption in a no. of systems. Here we show that this modal strong coupling between a Fabry-Pe´rot nanocavity mode and a localized surface plasmon resonance (LSPR) facilitates water splitting reactions. We use a gold nanoparticle (Au-NP)/TiO2/Au-film structure as a photoanode. This structure exhibits modal strong coupling between the Fabry-Pe´rot nanocavity modes of the TiO2 thin film/Au film and LSPR of the Au NPs. Electronic excitation of the Au NPs is promoted by the optical hybrid modes across a broad range of wavelengths, followed by a hot electron transfer to TiO2. A key feature of our structure is that the Au NPs are partially inlaid in the TiO2 layer, which results in an enhancement of the coupling strength and water-oxidn. efficiency. We observe an 11-fold increase in the incident photon-to-current conversion efficiency with respect to a photoanode structure with no Au film. Also, the internal quantum efficiency is enhanced 1.5 times under a strong coupling over that under uncoupled conditions.
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  • Abstract

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    Scheme 1

    Scheme 1. Preparation of CoOx-Deposited Au/TiO2 Photoanode
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    Figure 1

    Figure 1. Extinction spectra of Au/TiO2 and CoOx/Au/TiO2 thin films.

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    Figure 2

    Figure 2. SEM images of (A) Au/TiO2 and CoOx/Au/TiO2 thin films prepared with electric charge of (B) 1.0 × 10–4 and (C) 2.0 × 10–4 C, respectively. The inset images indicate their enlarged views.

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    Figure 3

    Figure 3. Photoelectrochemical performance. (A) Current–time curves for modified TiO2 thin films recorded in controlled-potential photoelectrolysis at +0.10 V vs Ag/AgCl (i.e., +1.07 V vs RHE) under visible light (λ > 450 nm) with the light intensity of 180 mW. The experiment was conducted in 0.2 M K3PO4 solution (pH 13) using the optimized CoOx/Au/TiO2 thin film anode, in which CoOx deposition was done under 1.0 × 10–4 C condition. Irradiation area: 3.0 cm–2. (B) IPCEs of the CoOx/Au/TiO2 thin film electrode recorded in aqueous 0.1 M KOH solution (pH 13) at 0 V vs Ag/AgCl as a function of the wavelength of the incident light. (C) IPCEs of the CoOx/Au/TiO2 electrodes at 580 nm as a function of the amount of electric charge flowed during the photoassited CoOx deposition. The error bars were estimated at two points using the optimal CoOx/Au/TiO2 and Au/TiO2 (CoOx-free specimen) thin films prepared at different batches.

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    Figure 4

    Figure 4. Time courses of (A) O2 evolution and (B) current generation in controlled-potential photoelectrolysis at −0.31 V vs Ag/AgCl (i.e., +0.66 V vs RHE) under simulated sunlight (100 mW cm–2). The experiment was conducted in 0.2 M K3PO4 solution (pH 13) using the optimized CoOx/Au/TiO2 thin film anode. Irradiation area: 3.0 cm2. Error range of O2 detection was approximately 10%.

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    This article references 30 other publications.

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      Oshikiri, T.; Ueno, K.; Misawa, H. Plasmon-Induced Ammonia Synthesis through Nitrogen Photofixation with Visible Light Irradiation. Angew. Chem., Int. Ed. 2014, 53, 9802– 9805,  DOI: 10.1002/anie.201404748
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      5
      Plasmon-Induced Ammonia Synthesis through Nitrogen Photofixation with Visible Light Irradiation
      Oshikiri, Tomoya; Ueno, Kosei; Misawa, Hiroaki
      Angewandte Chemie, International Edition (2014), 53 (37), 9802-9805CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We have successfully developed a plasmon-induced technique for ammonia synthesis that responds to visible light through a strontium titanate (SrTiO3) photoelectrode loaded with gold (Au) nanoparticles. The photoelectrochem. reaction cell was divided into two chambers to sep. the oxidized (anodic side) and reduced (cathodic side) products. To promote NH3 formation, a chem. bias was applied by regulating the pH value of these compartments, and ethanol was added to the anodic chamber as a sacrificial donor. The quantity of NH3 formed at the ruthenium surface, which was used as a co-catalyst for SrTiO3, increases linearly as a function of time under irradn. with visible light at wavelengths longer than 550 nm. The NH3 formation action spectrum approx. corresponds to the plasmon resonance spectrum. We deduced that plasmon-induced charge sepn. at the Au/SrTiO3 interface promotes oxidn. at the anodic chamber and subsequent nitrogen redn. on the cathodic side.
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    6. 6
      Tanaka, H.; Uchiyama, T.; Kawakami, N.; Okazaki, M.; Uchimoto, Y.; Maeda, K. Water Oxidation through Interfacial Electron Transfer by Visible Light Using Cobalt-Modified Rutile Titania Thin-Film Photoanode. ACS Appl. Mater. Interfaces 2020, 12, 9219– 9225,  DOI: 10.1021/acsami.9b20793
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      6
      Water oxidation through interfacial electron transfer by visible light using cobalt-modified rutile titania thin-film photoanode
      Tanaka, Hideyuki; Uchiyama, Tomoki; Kawakami, Nozomi; Okazaki, Megumi; Uchimoto, Yoshiharu; Maeda, Kazuhiko
      ACS Applied Materials & Interfaces (2020), 12 (8), 9219-9225CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      TiO2 is a good photoanode material for water oxidn. to form O2; however, UV light (λ < 400 nm) is necessary for this system to operate. In this work, cobalt species were introduced onto a rutile TiO2 thin film grown on a fluorine-doped tin oxide (FTO) substrate for visible-light activation of TiO2 and to construct water oxidn. sites. TiO2 thin films were prepd. on the FTO surface by the thermohydrolysis of TiCl4, followed by annealing at 723 K in air; the loading of the cobalt species was achieved simply by immersing TiO2/FTO into an aq. Co(NO3)2 soln. at room temp., followed by heating at 423 K in air. Physicochem. analyses revealed that the cobalt species deposited on the TiO2 film was α-Co3(OH)4(NO3)2 and that the cobalt-modified TiO2 thin-film electrode had a visible-light absorption band that extended to 700 nm due to interfacial electron transitions from the cobalt species to the conduction band of TiO2. Upon anodic polarization in the presence of visible light, the cobalt-modified TiO2 thin-film electrode generated an anodic photocurrent with an onset potential of +0.1 V vs RHE, which was consistent with that of pristine rutile TiO2. Product anal. during the controlled potential photoelectrolysis in the presence of an applied bias smaller than 1.23 V under visible light showed that water oxidn. to O2 occurred on the cobalt-modified TiO2/FTO. This study demonstrates that a visible-light-driven photoelectrochem. cell for water oxidn. can be constructed through the use of earth-abundant metals without the need for a complicated prepn. procedure.
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    7. 7
      Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911– 921,  DOI: 10.1038/nmat3151
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      7
      Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy
      Linic, Suljo; Christopher, Phillip; Ingram, David B.
      Nature Materials (2011), 10 (12), 911-921CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      A review. Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chem. energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. A review of recent progress in using plasmonic metallic nanostructures in the field of photocatalysis is presented. The focus is on plasmon-enhanced water splitting on composite photocatalysts contg. semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. The areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward are discussed.
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    8. 8
      Ueno, K.; Misawa, H. Surface Plasmon-Enhanced Photochemical Reactions. J. Photochem. Photobiol., C 2013, 15, 31– 52,  DOI: 10.1016/j.jphotochemrev.2013.04.001
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      8
      Surface plasmon-enhanced photochemical reactions
      Ueno, Kosei; Misawa, Hiroaki
      Journal of Photochemistry and Photobiology, C: Photochemistry Reviews (2013), 15 (), 31-52CODEN: JPPCAF; ISSN:1389-5567. (Elsevier B.V.)
      A review. The electromagnetic field enhancement effect based on the excitation of localized surface plasmon resonance was developed for various photochem. reaction systems, such as nano-lithog., photovoltaic cells, photocatalysis, and water splitting systems. As with most points characteristic of these surface plasmon-enhanced photochem. reactions, spatially selective photochem. reactions can be induced and photons can be efficiently utilized, a concept that could contribute to the development of green nanotechnol. Electromagnetic field enhancement effects based on plasmon excitation have contributed not only to phys. processes, such as excitation efficiency, but also to chem. processes, such as photo-induced electron transfer reactions. This review article describes advanced studies on a wide variety of surface plasmon-enhanced photochem. reactions.
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    9. 9
      Zhao, G.; Kozuka, H.; Yoko, T. Sol—Gel Preparation and Photoelectrochemical Properties of TiO2 Films Containing Au and Ag Metal Particles. Thin Solid Films 1996, 277, 147– 154,  DOI: 10.1016/0040-6090(95)08006-6
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      9
      Sol-gel preparation and photoelectrochemical properties of TiO2 films containing Au and Ag metal particles
      Zhao, Gaoling; Kozuka, Hiromitsu; Yoko, Toshinobu
      Thin Solid Films (1996), 277 (1-2), 147-154CODEN: THSFAP; ISSN:0040-6090. (Elsevier)
      TiO2 film electrodes with a TiO2 overlayer contg. dispersed Au or Ag metal particles were prepd. by the sol-gel method, and the effect of the metal particles on the photoelectrochem. properties of the TiO2 electrodes were investigated. An increase in the anodic photocurrent in the visible region was obsd. for both the Au and Ag particle dispersed electrodes, which was thought to result from the surface plasma resonance of the metal particles. The introduction of Au metal particles, however, reduced the anodic photocurrent in the UV region, resulting in a decrease of the anodic photocurrent under the illumination of xenon lamp light. On the other hand, for the Ag particle dispersed electrodes, the anodic photocurrent in the UV region increased and then decreased with an increasing amt. of Ag particles. These effects of the dispersed metal particles on the photoelectrochem. properties of the TiO2 electrodes were explained on the basis of the band models.
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    10. 10
      Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. Plasmon-Assisted Photocurrent Generation from Visible to near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode. J. Phys. Chem. Lett. 2010, 1, 2031– 2036,  DOI: 10.1021/jz1006675
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      10
      Plasmon-Assisted Photocurrent Generation from Visible to Near-Infrared Wavelength Using a Au-Nanorods/TiO2 Electrode
      Nishijima, Yoshiaki; Ueno, Kosei; Yokota, Yukie; Murakoshi, Kei; Misawa, Hiroaki
      Journal of Physical Chemistry Letters (2010), 1 (13), 2031-2036CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Nanoparticles of noble metals exhibit localized surface plasmons (LSPs) assocd. with the enhancement of an electromagnetic field due to its localization in nanometric domains at the surface of nanoparticles. The authors demonstrate the plasmonic photoelec. conversion from visible to near-IR wavelength without deteriorating photoelec. conversion by using electrodes in which Au nanorods are elaborately arrayed on the surface of a TiO2 single crystal.
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    11. 11
      Nishijima, Y.; Ueno, K.; Kotake, Y.; Murakoshi, K.; Inoue, H.; Misawa, H. Near-Infrared Plasmon-Assisted Water Oxidation. J. Phys. Chem. Lett. 2012, 3, 1248– 1252,  DOI: 10.1021/jz3003316
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      11
      Near-Infrared Plasmon-Assisted Water Oxidation
      Nishijima, Yoshiaki; Ueno, Kosei; Kotake, Yuki; Murakoshi, Kei; Inoue, Haruo; Misawa, Hiroaki
      Journal of Physical Chemistry Letters (2012), 3 (10), 1248-1252CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      We report the stoichiometric evolution of oxygen via water oxidn. by irradiating a plasmon-enhanced photocurrent generation system with near-IR light (λ: 1000 nm), in which gold nanostructures were arrayed on the surface of TiO2 electrode. It is considered that multiple electron holes generated by plasmon-induced charge excitation led to the effective recovery of water oxidn. after the electron transfer from gold to TiO2. The proposed system contg. a gold nanostructured TiO2 electrode may be a promising artificial photosynthetic system using near-IR light.
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    12. 12
      Zhong, Y.; Ueno, K.; Mori, Y.; Shi, X.; Oshikiri, T.; Murakoshi, K.; Inoue, H.; Misawa, H. Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy. Angew. Chem., Int. Ed. 2014, 53, 10350– 10354,  DOI: 10.1002/anie.201404926
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      12
      Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy
      Zhong, Yuqing; Ueno, Kosei; Mori, Yuko; Shi, Xu; Oshikiri, Tomoya; Murakoshi, Kei; Inoue, Haruo; Misawa, Hiroaki
      Angewandte Chemie, International Edition (2014), 53 (39), 10350-10354CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A plasmon-induced water splitting system that operates under irradn. by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate (SrTiO3) single-crystal substrate. The water splitting system contains two soln. chambers to sep. hydrogen (H2) and oxygen (O2). To promote water splitting, a chem. bias was applied by regulating the pH values of the chambers. The quantity of H2 evolved from the surface of platinum, which was used as a redn. co-catalyst, was twice the quantity of O2 evolved from an Au-nanostructured surface. Thus, the stoichiometric evolution of H2 and O2 was clearly demonstrated. The hydrogen-evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon-induced charge sepn. at the Au/SrTiO3 interface promotes water oxidn. and the subsequent redn. of a proton on the backside of the SrTiO3 substrate. The chem. bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.
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    13. 13
      Zhang, Z.; Zhang, L.; Hedhili, M. N.; Zhang, H.; Wang, P. Plasmonic Gold Nanocrystals Coupled with Photonic Crystal Seamlessly on TiO2 Nanotube Photoelectrodes for Efficient Visible Light Photoelectrochemical Water Splitting. Nano Lett. 2013, 13, 14– 20,  DOI: 10.1021/nl3029202
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      13
      Plasmonic Gold Nanocrystals Coupled with Photonic Crystal Seamlessly on TiO2 Nanotube Photoelectrodes for Efficient Visible Light Photoelectrochemical Water Splitting
      Zhang, Zhonghai; Zhang, Lianbin; Hedhili, Mohamed Nejib; Zhang, Hongnan; Wang, Peng
      Nano Letters (2013), 13 (1), 14-20CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      A visible light responsive plasmonic photocatalytic composite material is designed by rationally selecting Au nanocrystals and assembling them with the TiO2-based photonic crystal substrate. The selection of the Au nanocrystals is so that their surface plasmonic resonance (SPR) wavelength matches the photonic band gap of the photonic crystal and thus that the SPR of the Au receives remarkable assistance from the photonic crystal substrate. The design of the composite material is expected to significantly increase the Au SPR intensity and consequently boost the hot electron injection from the Au nanocrystals into the conduction band of TiO2, leading to a considerably enhanced water splitting performance of the material under visible light. A proof-of-concept example is provided by assembling 20 nm Au nanocrystals, with a SPR peak at 556 nm, onto the photonic crystal which is seamlessly connected on TiO2 nanotube array. Under visible light illumination (>420 nm), the designed material produced a photocurrent d. of ∼150 μA cm-2, which is the highest value ever reported in any plasmonic Au/TiO2 system under visible light irradn. due to the photonic crystal-assisted SPR. This work contributes to the rational design of the visible light responsive plasmonic photocatalytic composite material based on wide band gap metal oxides for photoelectrochem. applications.
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    14. 14
      Takakura, R.; Oshikiri, T.; Ueno, K.; Shi, X.; Kondo, T.; Masuda, H.; Misawa, H. Water Splitting Using a Three-Dimensional Plasmonic Photoanode with Titanium Dioxide Nano-Tunnels. Green Chem. 2017, 19, 2398– 2405,  DOI: 10.1039/C6GC03217F
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      14
      Water splitting using a three-dimensional plasmonic photoanode with titanium dioxide nano-tunnels
      Takakura, Ryohei; Oshikiri, Tomoya; Ueno, Kosei; Shi, Xu; Kondo, Toshiaki; Masuda, Hideki; Misawa, Hiroaki
      Green Chemistry (2017), 19 (10), 2398-2405CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      In this study, we developed a three-dimensional plasmonic photoanode using titanium dioxide nano-tunnels (TNTs) loaded with gold nanoparticles (Au-NPs) for water splitting, to enhance the reaction efficiency. We also optimized the procedure of loading Au-NPs on complex three-dimensional structures. We discuss the correlation between the plasmon-induced charge sepn. obtained from photoelectrochem. measurement and the morphol. of Au-NPs obsd. by transmission electron microscopy. We have successfully deposited well-dispersed Au-NPs on the walls of TNTs using HAu(OH)4 as a precursor. The amt. of Au-NPs on the TNTs was estd. to be approx. 15-fold larger than that on the thin film titanium dioxide substrate, and the particle size remained small. Photoelectrochem. water splitting was achieved by using a two-electrode system rather than a three-electrode system. Furthermore, stoichiometric water splitting was confirmed by estg. the amts. of the evolved H2 and O2 gases under visible light irradn.
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    15. 15
      Mi, Y.; Wen, L.; Xu, R.; Wang, Z.; Cao, D.; Fang, Y.; Lei, Y. Constructing a AZO/TiO2 Core/Shell Nanocone Array with Uniformly Dispersed Au NPs for Enhancing Photoelectrochemical Water Splitting. Adv. Energy Mater. 2016, 6, 1501496,  DOI: 10.1002/aenm.201501496
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      There is no corresponding record for this reference.
    16. 16
      Harutyunyan, H.; Martinson, A. B.; Rosenmann, D.; Khorashad, L. K.; Besteiro, L. V.; Govorov, A. O.; Wiederrecht, G. P. Anomalous Ultrafast Dynamics of Hot Plasmonic Electrons in Nanostructures with Hot Spots. Nat. Nanotechnol. 2015, 10, 770– 774,  DOI: 10.1038/nnano.2015.165
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      16
      Anomalous ultrafast dynamics of hot plasmonic electrons in nanostructures with hot spots
      Harutyunyan, Hayk; Martinson, Alex B. F.; Rosenmann, Daniel; Khorashad, Larousse Khosravi; Besteiro, Lucas V.; Govorov, Alexander O.; Wiederrecht, Gary P.
      Nature Nanotechnology (2015), 10 (9), 770-774CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      The interaction of light and matter in metallic nanosystems is mediated by the collective oscillation of surface electrons, called plasmons. After excitation, plasmons are absorbed by the metal electrons through inter- and intraband transitions, creating a highly non-thermal distribution of electrons. The electron population then decays through electron-electron interactions, creating a hot electron distribution within a few hundred femtoseconds, followed by a further relaxation via electron-phonon scattering on the timescale of a few picoseconds. In the spectral domain, hot plasmonic electrons induce changes to the plasmonic resonance of the nanostructure by modifying the dielec. const. of the metal. Here, we report on the observation of anomalously strong changes to the ultrafast temporal and spectral responses of these excited hot plasmonic electrons in hybrid metal/oxide nanostructures as a result of varying the geometry and compn. of the nanostructure and the excitation wavelength. In particular, we show a large ultrafast, pulsewidth-limited contribution to the excited electron decay signal in hybrid nanostructures contg. hot spots. The intensity of this contribution correlates with the efficiency of the generation of highly excited surface electrons. Using theor. models, we attribute this effect to the generation of hot plasmonic electrons from hot spots. We then develop general principles to enhance the generation of energetic electrons through specifically designed plasmonic nanostructures that could be used in applications where hot electron generation is beneficial, such as in solar photocatalysis, photodetectors and nonlinear devices.
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    17. 17
      Boerigter, C.; Aslam, U.; Linic, S. Mechanism of Charge Transfer from Plasmonic Nanostructures to Chemically Attached Materials. ACS Nano 2016, 10, 6108– 6115,  DOI: 10.1021/acsnano.6b01846
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      17
      Mechanism of Charge Transfer from Plasmonic Nanostructures to Chemically Attached Materials
      Boerigter, Calvin; Aslam, Umar; Linic, Suljo
      ACS Nano (2016), 10 (6), 6108-6115CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Plasmonic metal nanoparticles can efficiently convert the energy of visible photons into the energy of hot charge carriers within the nanoparticles. These energetic charge carriers can transfer to mols. or semiconductors, chem. attached to the nanoparticles, where they can induce photochem. transformations. Classical models of photoinduced charge excitation and transfer in metals suggest that the majority of the energetic charge carriers rapidly decay within the metal nanostructure before they are transferred into the neighboring mol. or semiconductor, and therefore, the efficiency of charge transfer is low. Herein, we present exptl. evidence that calls into question this conventional picture. We demonstrate a system where the presence of a mol., adsorbed on the surface of a plasmonic nanoparticle, significantly changes the flow of charge within the excited plasmonic system. The nanoparticle-adsorbate system experiences high rates of direct, resonant flow of charge from the nanoparticle to the mol., bypassing the conventional charge excitation and thermalization process taking place in the nanoparticle. This picture of charge transfer suggests that the yield of extd. hot electrons (or holes) from plasmonic nanoparticles can be significantly higher than the yields expected based on conventional models. We discuss a conceptual phys. framework that allows us to explain our exptl. observations. This anal. points us in a direction toward mol. control of the charge transfer process using interface and local field engineering strategies.
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    18. 18
      Liu, J. G.; Zhang, H.; Link, S.; Nordlander, P. Relaxation of Plasmon-Induced Hot Carriers. ACS Photonics 2018, 5, 2584– 2595,  DOI: 10.1021/acsphotonics.7b00881
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      18
      Relaxation of Plasmon-Induced Hot Carriers
      Liu, Jun G.; Zhang, Hui; Link, Stephan; Nordlander, Peter
      ACS Photonics (2018), 5 (7), 2584-2595CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Plasmon-induced hot carrier generation has attracted much recent attention due to its promising potential in photocatalysis and other light harvesting applications. A theor. model for hot carrier relaxation in metallic nanoparticles was developed using a fully quantum mech. jellium model. Following pulsed illumination, nonradiative plasmon decay results in a highly nonthermal distribution of hot electrons and holes. Using coupled master equations, the time-dependent evolution of this carrier distribution in the presence of electron-electron, electron-photon, and electron-phonon scattering was calcd. Electron-electron relaxation is the dominant scattering mechanism and results in efficient carrier multiplication where the energy of the initial hot electron-hole pair is transferred to other multiple electron-hole pair excitations of lower energies. During this relaxation, a small but finite fraction of electrons scatter into luminescent states where they can recombine radiatively with holes by emission of photons. The energy of the emitted photons is found to follow the energies of the electrons and thus red shifts monotonically during the relaxation process. When the energies of the electrons approach the Fermi level, electron-phonon interaction becomes dominant and results in heating of the nanoparticle. The model was generalized to continuous-wave excitation, and nonlinear effects become important when the illumination intensity increases. When the temporal spacing between incident photons is shorter than the relaxation time of the hot carriers, the authors predict that the luminescence will blueshift with increasing illumination power. The effect of the photonic d. of states (Purcell factor) on the luminescence spectra are discussed.
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    19. 19
      Lee, J.; Mubeen, S.; Ji, X.; Stucky, G. D.; Moskovits, M. Plasmonic Photoanodes for Solar Water Splitting with Visible Light. Nano Lett. 2012, 12, 5014– 5019,  DOI: 10.1021/nl302796f
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      19
      Plasmonic Photoanodes for Solar Water Splitting with Visible Light
      Lee, Joun; Mubeen, Syed; Ji, Xiulei; Stucky, Galen D.; Moskovits, Martin
      Nano Letters (2012), 12 (9), 5014-5019CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We report a plasmonic water splitting cell in which 95% of the effective charge carriers derive from surface plasmon decay to hot electrons, as evidenced by fuel prodn. efficiencies up to 20-fold higher at visible, as compared to UV, wavelengths. The cell functions by illuminating a dense array of aligned gold nanorods capped with TiO2, forming a Schottky metal/semiconductor interface which collects and conducts the hot electrons to an non-illuminated platinum counter electrode where hydrogen gas evolves. The resultant pos. charges in the Au nanorods function as holes and are extd. by an oxidn. catalyst which electrocatalytically oxidizes water to oxygen gas.
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    20. 20
      Kim, J.; Son, H. Y.; Nam, Y. S. Multilayered Plasmonic Heterostructure of Gold and Titania Nanoparticles for Solar Fuel Production. Sci. Rep. 2018, 8, 10464,  DOI: 10.1038/s41598-018-28789-w
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      20
      Multilayered Plasmonic Heterostructure of Gold and Titania Nanoparticles for Solar Fuel Production
      Kim Jeonga; Son Ho Yeon; Nam Yoon Sung; Nam Yoon Sung
      Scientific reports (2018), 8 (1), 10464 ISSN:.
      Solar fuel production via photoelectrochemical (PEC) water splitting has attracted great attention as an approach to storing solar energy. However, a wide range of light-harvesting materials is unstable when exposed to light and oxidative conditions. Here we report a robust, multilayered plasmonic heterostructure for water oxidation using gold nanoparticles (AuNPs) as light-harvesting materials via localized surface plasmon resonance (LSPR). The multilayered heterostructure is fabricated using layer-by-layer self-assembly of AuNPs and TiO2 nanoparticles (TNPs). Plasmon-induced hot electrons are transferred from AuNPs to TNPs over the Au/TiO2 Schottky barrier, resulting in charge separation of hot carriers. Plasmonic photoanodes for water oxidation are completed by incorporating a Co-based oxygen-evolving catalyst on the multilayered heterostructure to scavenge hot holes. Light absorption capability and PEC properties of the photoanodes are investigated as a function of the number of AuNP/TNP bilayers. The PEC properties exhibits dependence on the number of the bilayers, which is affected by charge transport within the multilayered heterostructures. Photocurrent density and decrease in impedance by irradiation indicates significant photoactivity by LSPR excitation.
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    21. 21
      Hung, S.-F.; Xiao, F.-X.; Hsu, Y.-Y.; Suen, N.-T.; Yang, H.-B.; Chen, H. M.; Liu, B. Iridium Oxide-Assisted Plasmon-Induced Hot Carriers: Improvement on Kinetics and Thermodynamics of Hot Carriers. Adv. Energy Mater. 2016, 6, 1501339,  DOI: 10.1002/aenm.201501339
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      There is no corresponding record for this reference.
    22. 22
      Minamimoto, H.; Toda, T.; Futashima, R.; Li, X.; Suzuki, K.; Yasuda, S.; Murakoshi, K. Visualization of Active Sites for Plasmon-Induced Electron Transfer Reactions Using Photoelectrochemical Polymerization of Pyrrole. J. Phys. Chem. C 2016, 120, 16051– 16058,  DOI: 10.1021/acs.jpcc.5b12727
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      22
      Visualization of Active Sites for Plasmon-Induced Electron Transfer Reactions Using Photoelectrochemical Polymerization of Pyrrole
      Minamimoto, Hiro; Toda, Takahiro; Futashima, Ryo; Li, Xiaowei; Suzuki, Kentaro; Yasuda, Satoshi; Murakoshi, Kei
      Journal of Physical Chemistry C (2016), 120 (29), 16051-16058CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Spatially selective deposition of conductive polymer was obsd. at the Au nanostructures supported on TiO2 electrodes via plasmon-induced photopolymn. of pyrrole monomers. The reactions were triggered by the excitation of localized surface plasmon resonance under near-IR light illumination to the plasmon-active Au nanostructures. The morphol. characteristics of the deposited polypyrrole prove the localization of the reaction-active sites in the plasmon-induced oxidn.-reaction system. In addn., the estn. of reaction characteristics provides information on the spatial distribution and the electrochem. potential of the holes to contribute to the reaction. The unique polymer-growing process obsd. in the present system provides information on the mechanism of plasmon-induced oxidn. reaction occurring at the active sites.
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    23. 23
      Wang, S.; Gao, Y.; Miao, S.; Liu, T.; Mu, L.; Li, R.; Fan, F.; Li, C. Positioning the Water Oxidation Reaction Sites in Plasmonic Photocatalysts. J. Am. Chem. Soc. 2017, 139, 11771– 11778,  DOI: 10.1021/jacs.7b04470
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      23
      Positioning the Water Oxidation Reaction Sites in Plasmonic Photocatalysts
      Wang, Shengyang; Gao, Yuying; Miao, Shu; Liu, Taifeng; Mu, Linchao; Li, Rengui; Fan, Fengtao; Li, Can
      Journal of the American Chemical Society (2017), 139 (34), 11771-11778CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Plasmonic photocatalysis, stemming from the effective light absorbance and confinement of surface plasmons, provides a pathway to enhance solar energy conversion. Although the plasmonic hot electrons in water redn. have been extensively studied, exactly how the plasmonic hot holes participate in the water splitting reaction has not yet been well understood. In particular, where the plasmonic hot holes participate in water oxidn. is still illusive. Herein, taking Au/TiO2 as a plasmonic photocatalyst prototype, we investigated the plasmonic hot holes involved in water oxidn. The reaction sites are positioned by photodeposition together with element mapping by electron microscopy, while the distribution of holes is probed by surface photovoltage imaging with Kelvin probe force microscopy. We demonstrated that the plasmonic holes are mainly concd. near the gold-semiconductor interface, which is further identified as the reaction site for plasmonic water oxidn. D. functional theory also corroborates these findings by revealing the promotion role of interfacial structure (Ti-O-Au) for oxygen evolution. Furthermore, the interfacial effect on plasmonic water oxidn. is validated by other Au-semiconductor photocatalytic systems (Au/SrTiO3, Au/BaTiO3, etc.).
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    24. 24
      Mubeen, S.; Lee, J.; Singh, N.; Kramer, S.; Stucky, G. D.; Moskovits, M. An Autonomous Photosynthetic Device in Which All Charge Carriers Derive from Surface Plasmons. Nat. Nanotechnol. 2013, 8, 247– 251,  DOI: 10.1038/nnano.2013.18
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      24
      An autonomous photosynthetic device in which all charge carriers derive from surface plasmons
      Mubeen, Syed; Lee, Joun; Singh, Nirala; Kraemer, Stephan; Stucky, Galen D.; Moskovits, Martin
      Nature Nanotechnology (2013), 8 (4), 247-251CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Solar conversion to electricity or to fuels based on electron-hole pair prodn. in semiconductors is a highly evolved scientific and com. enterprise. Recently, it was posited that charge carriers either directly transferred from the plasmonic structure to a neighboring semiconductor (such as TiO2) or to a photocatalyst, or induced by energy transfer in a neighboring medium, could augment photoconversion processes, potentially leading to an entire new paradigm in harvesting photons for practical use. The strong dependence of the wavelength at which the local surface plasmon can be excited on the nanostructure makes it possible, in principle, to design plasmonic devices that can harvest photons over the entire solar spectrum and beyond. So far, however, most such systems show rather small photocatalytic activity in the visible as compared with the UV. Here, the authors report an efficient, autonomous solar H2O-splitting device based on a Au nanorod array in which essentially all charge carriers involved in the oxidn. and redn. steps arise from the hot electrons resulting from the excitation of surface plasmons in the nanostructured Au. Each nanorod functions without external wiring, producing 5 × 1013 H2 mols. per cm2 per s under 1 sun illumination (AM 1.5 and 100 mW cm-2), with unprecedented long-term operational stability.
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    25. 25
      Bledowski, M.; Wang, L.; Ramakrishnan, A.; Betard, A.; Khavryuchenko, O. V.; Beranek, R. Visible-Light Photooxidation of Water to Oxygen at Hybrid TiO2-Polyheptazine Photoanodes with Photodeposited Co-Pi (CoOx) Cocatalyst. ChemPhysChem 2012, 13, 3018– 3024,  DOI: 10.1002/cphc.201200071
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      25
      Visible-Light Photooxidation of Water to Oxygen at Hybrid TiO2-Polyheptazine Photoanodes with Photodeposited Co-Pi (CoOx) Cocatalyst
      Bledowski, Michal; Wang, Lidong; Ramakrishnan, Ayyappan; Betard, Angelique; Khavryuchenko, Oleksiy V.; Beranek, Radim
      ChemPhysChem (2012), 13 (12), 3018-3024, S3018/1-S3018/5CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A cobalt oxide-based oxygen-evolving cocatalyst (Co-Pi) is photodeposited by visible-light irradn. onto nanocryst. TiO2-polyheptazine (TiO2-PH) hybrid photoelectrodes in a phosphate buffer. The Co-Pi cocatalyst couples effectively to photoholes generated in the surface polyheptazine layer of the TiO2-PH photoanode, as evidenced by complete photooxidn. of water to oxygen under visible-light (λ>420 nm) irradn. at moderate bias potentials. In addn., the presence of the cocatalyst also reduces significantly the recombination of photogenerated charges, particularly at low bias potentials, which is ascribed to better photooxidn. kinetics resulting in lower accumulation of holes. This suggests that further improvements of photoconversion efficiency can be achieved if more effective catalytic sites for water oxidn. are introduced to the surface structure of the hybrid photoanodes.
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    26. 26
      Mulvaney, P.; Pérez-Juste, J.; Giersig, M.; Liz-Marzán, L. M.; Pecharromán, C. Drastic Surface Plasmon Mode Shifts in Gold Nanorods Due to Electron Charging. Plasmonics 2006, 1, 61– 66,  DOI: 10.1007/s11468-005-9005-0
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      26
      Drastic surface plasmon mode shifts in gold nanorods due to electron charging
      Mulvaney, Paul; Perez-Juste, Jorge; Giersig, Michael; Liz-Marzan, Luis M.; Pecharroman, Carlos
      Plasmonics (2006), 1 (1), 61-66CODEN: PLASCS; ISSN:1557-1955. (Springer)
      The color of small gold rods changes dramatically when electrons are injected by chem. reductants. The longitudinal and transverse plasmon modes are both found to blue-shift, and the shift is larger for rods with larger aspect ratios. The color changes are visible to the eye for rods with aspect ratios around 2-3. It is found that the surface plasmon band is damped when charging becomes high. The effects are in qual. agreement with a model in which the gold plasma frequency increases due to an increase in electron d.
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    27. 27
      Maeda, K. Photocatalytic Water Splitting Using Semiconductor Particles: History and Recent Developments. J. Photochem. Photobiol., C 2011, 12, 237– 268,  DOI: 10.1016/j.jphotochemrev.2011.07.001
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      27
      Photocatalytic water splitting using semiconductor particles: History and recent developments
      Maeda, Kazuhiko
      Journal of Photochemistry and Photobiology, C: Photochemistry Reviews (2011), 12 (4), 237-268CODEN: JPPCAF; ISSN:1389-5567. (Elsevier B.V.)
      A review. Overall water splitting to produce H2 and O2 over a semiconductor photocatalyst using solar energy is a promising process for the large-scale prodn. of clean, recyclable H2. Numerous attempts have been made to develop photocatalysts that function under visible-light irradn. to efficiently utilize solar energy. In general, overall water splitting over a photocatalyst particle can be achieved by modifying the photocatalyst with a suitable cocatalyst to provide an active redox site. Therefore, the development of active photocatalytic materials has relied on both photocatalysts and cocatalysts. This review article describes the historical development of water-splitting photocatalysts.
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    28. 28
      Kumagai, H.; Minegishi, T.; Sato, N.; Yamada, T.; Kubota, J.; Domen, K. Efficient Solar Hydrogen Production from Neutral Electrolytes Using Surface-Modified Cu(In, Ga)Se2 Photocathodes. J. Mater. Chem. A 2015, 3, 8300– 8307,  DOI: 10.1039/C5TA01058F
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      28
      Efficient solar hydrogen production from neutral electrolytes using surface-modified Cu(In,Ga)Se2 photocathodes
      Kumagai, Hiromu; Minegishi, Tsutomu; Sato, Naotoshi; Yamada, Taro; Kubota, Jun; Domen, Kazunari
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (16), 8300-8307CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The effects of a phosphate buffer electrolyte and surface modification with thin conductor layers on the photoelectrochem. properties of CdS and Pt-modified polycryst. Cu(In,Ga)Se2 (CIGS) photocathodes were investigated. The photocurrent obtained from Pt/CdS/CIGS electrodes, in which the CIGS layer was fabricated by co-evapn. using a three stage method, clearly increased in a phosphate buffer electrolyte soln. as a result of promotion of the hydrogen evolution reaction. The half-cell solar-to-hydrogen efficiency (HC-STH) of this device reached a max. of 5.4% at 0.30 VRHE even under neutral conditions. Furthermore, significant enhancement of the hydrogen evolution reaction on a CIGS photocathode by surface modification with thin conductor layers was obsd. The enhancement was due to the promoted charge transfer between the underlying photocathode and water through the Pt catalyst. The HC-STH of a CIGS photocathode modified with a conductive Mo/Ti layer (Pt/Mo/Ti/CdS/CIGS) was as high as 8.5% at 0.38 VRHE, a value that exceeds those previously reported for photocathodes based on polycryst. thin films.
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    29. 29
      Hirayama, N.; Nakata, H.; Wakayama, H.; Nishioka, S.; Kanazawa, T.; Kamata, R.; Ebato, Y.; Kato, K.; Kumagai, H.; Yamakata, A.; Oka, K.; Maeda, K. Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode. J. Am. Chem. Soc. 2019, 141, 17158– 17165,  DOI: 10.1021/jacs.9b06570
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      29
      Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode
      Hirayama, Naoki; Nakata, Hiroko; Wakayama, Haruki; Nishioka, Shunta; Kanazawa, Tomoki; Kamata, Ryutaro; Ebato, Yosuke; Kato, Kosaku; Kumagai, Hiromu; Yamakata, Akira; Oka, Kengo; Maeda, Kazuhiko
      Journal of the American Chemical Society (2019), 141 (43), 17158-17165CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Mixed-anion compds. (e.g., oxynitrides and oxysulfides) are potential candidates as photoanodes for visible-light water oxidn., but most of them suffer from oxidative degrdn. by photogenerated holes, leading to low stability. Here we show an exceptional example of a stable, mixed-anion water-oxidn. photoanode that consists of an oxyfluoride, Pb2Ti2O5.4F1.2, having a band gap of ca. 2.4 eV. Pb2Ti2O5.4F1.2 particles, which were coated on a transparent conductive glass (FTO) support and were subject to postdeposition of a TiO2 overlayer, generated an anodic photocurrent upon band gap photoexcitation of Pb2Ti2O5.4F1.2 (λ <520 nm) with a rather neg. photocurrent onset potential of ca. -0.6 V vs NHE, which was independent of the pH of the electrolyte soln. Stable photoanodic current was obsd. even without loading a water oxidn. promoter such as CoOx. Nevertheless, loading CoOx onto the TiO2/Pb2Ti2O5.4F1.2/FTO electrode further improved the anodic photoresponse by a factor of 2-3. Under AM1.5G simulated sunlight (100 mW cm-2), stable water oxidn. to form O2 was achieved using the optimized Pb2Ti2O5.4F1.2 photoanode in the presence of an applied potential smaller than 1.23 V, giving a Faradaic efficiency of 93% and almost no sign of deactivation during 4 h of operation. This study presents the first example of photoelectrochem. water splitting driven by visible-light excitation of an oxyfluoride that stably works, even without a water oxidn. promoter, which is distinct from ordinary mixed-anion photoanodes that usually require a water oxidn. promoter.
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    30. 30
      Shi, X.; Ueno, K.; Oshikiri, T.; Sun, Q.; Sasaki, K.; Misawa, H. Enhanced Water Splitting under Modal Strong Coupling Conditions. Nat. Nanotechnol. 2018, 13, 953– 958,  DOI: 10.1038/s41565-018-0208-x
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      30
      Enhanced water splitting under modal strong coupling conditions
      Shi, Xu; Ueno, Kosei; Oshikiri, Tomoya; Sun, Quan; Sasaki, Keiji; Misawa, Hiroaki
      Nature Nanotechnology (2018), 13 (10), 953-958CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
      Strong coupling between plasmons and optical modes, such as waveguide or resonator modes, gives rise to a splitting in the plasmon absorption band. As a result, two new hybrid modes are formed that exhibit near-field enhancement effects. These hybrid modes have been exploited to improve light absorption in a no. of systems. Here we show that this modal strong coupling between a Fabry-Pe´rot nanocavity mode and a localized surface plasmon resonance (LSPR) facilitates water splitting reactions. We use a gold nanoparticle (Au-NP)/TiO2/Au-film structure as a photoanode. This structure exhibits modal strong coupling between the Fabry-Pe´rot nanocavity modes of the TiO2 thin film/Au film and LSPR of the Au NPs. Electronic excitation of the Au NPs is promoted by the optical hybrid modes across a broad range of wavelengths, followed by a hot electron transfer to TiO2. A key feature of our structure is that the Au NPs are partially inlaid in the TiO2 layer, which results in an enhancement of the coupling strength and water-oxidn. efficiency. We observe an 11-fold increase in the incident photon-to-current conversion efficiency with respect to a photoanode structure with no Au film. Also, the internal quantum efficiency is enhanced 1.5 times under a strong coupling over that under uncoupled conditions.
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