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Variation in Surface Ionization Potentials of Pristine and Hydrated BiVO4
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† School of Biological and Chemical Sciences, Queen Mary University, London E1 4NS, United Kingdom
‡ Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
*E-mail: [email protected]
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The Journal of Physical Chemistry Letters

Cite this: J. Phys. Chem. Lett. 2015, 6, 12, 2379–2383
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https://doi.org/10.1021/acs.jpclett.5b00966
Published June 7, 2015

Copyright © 2015 American Chemical Society. This publication is licensed under CC-BY.

Abstract

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Bismuth vanadate (BiVO4) is a promising material for photoelectrochemical water splitting and photocatalytic degradation of organic moieties. We evaluate the ionization potentials of the (010) surface termination of BiVO4 using first-principles simulations. The electron removal energy of the pristine termination (7.2 eV) validates recent experimental reports. The effect of water absorption on the ionization potentials is considered using static models as well as structures obtained from molecular dynamics simulations. Owing to the large molecular dipole of H2O, adsorption stabilizes the valence band edge (downward band bending), thereby increasing the ionization potentials. These results provide new understanding to the role of polar layers on complex oxide semiconductors, with importance for the design of efficient photoelectrodes for water splitting.

Copyright © 2015 American Chemical Society

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Enormous efforts and resources have been put into the study of photoelectrochemical water splitting on semiconductor surfaces since the seminal work on TiO2 electrodes in the 1970s. (1-3) No single material has been found to achieve water oxidation and reduction under visible light at a rate that is commercially viable. A common architecture for the water splitting process, called Z-scheme or photosynthetic cell, (4, 5) is a tandem system composed by an n-type photoanode and a p-type photocathode, allowing a better efficiency for the whole process. Oxidation of water requires the participation of four electrons and is consequently limited by the kinetics of the carriers, which can be sensitive to the surface structure and surface potential of the photoanode.

Bismuth vanadate (BiVO4) is one of the most promising metal oxides to be used as a photoanode in the Z-scheme and as a photocatalyst for degradation of organic compounds. (6-8) BiVO4 exists in three polymorphs: orthorhombic pucherite, monoclinic clinobisvanite and tetragonal dreyerite. While the optical band gap of the orthorhombic phase is larger (around 2.9 eV), the tetragonal and monoclinic phases have similar band gaps (between 2.3 and 2.5 eV). The monoclinic phase is the thermodynamically most stable and exhibits the best photocatalytic properties, as well as a higher hole mobility in comparison to the tetragonal phase. (9)

Scheme 1 shows the alignment of energy levels for the monoclinic BiVO4 phase based on recent data from photoemission spectroscopy. (10) The valence band maximum (VBM) is in a favorable position for water oxidation. A small overpotential (electrochemical bias) is required to reduce water since the conduction band minimum (CBM) is below the H+/H2 potential. A recent study showed that quantum-sized BiVO4 can decompose water in H2O and O2 without the use of a cocatalyst, which can be understood considering the destabilization of the CBM energy (decrease in electron affinity) level due to quantum confinement effects. (11)

Scheme 1

Scheme 1. Alignment of Energy Levels of Monoclinic BiVO4 with Respect to the Vacuum Levela
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Scheme aThe values were taken from ref 10 based on photoemission experiments.

In spite of its attractive features, in the absence of an extra catalyst, crystalline BiVO4 shows a modest photoelectrochemical performance, with small current densities and low overall conversion efficiencies. (7, 12) Poor transport properties, significant electron–hole recombination and slow O2 evolution are the main limiting factors associated with its poor catalytic behavior. Weak hole localization (large hole-polaron) and a small electron polaron have been suggested, which could explain the slow electron mobility and the significant electron–hole recombination. (9, 13, 14) Different strategies like facet engineering; morphology control, doping, and the use cocatalyst have been explored to obtain better efficiencies. (15-17)

The combination of experiment and density functional theory (DFT) calculations have shown that the (010) surface of BiVO4 is the most stable with a significant area exposed to the solvent. (18-20) Water absorption on semiconductor surfaces affects the electronic structure and consequently the photocatalytic and electrocatalytic properties. In this Letter, we quantify the effects of the water at the contact with the (010) surface of BiVO4 on the ionization potentials using electronic structure and molecular dynamic simulations. Our results provide an understanding of the role of solvent on the electronic structure of semiconductor surfaces, which have implications for the design of optimal photoanodes. At the same time, absorbed water layers can serve as models for polar layers deposited on the oxide surfaces.

The quantitative prediction of absolute electronic energy levels is challenging for solid-state electronic structure modeling. (21) The most common approach consists of the alignment of the electronic bands with respect to the vacuum level using slab (pseudo-2D periodic) models. (21) To analyze the alignment of the energy levels and compare with the most recent experimental data, we consider the (010) surface using slab models that contain stoichiometric (BiVO4)4n units with n = 1,2,3,4. We focused on the most stable termination, consisting of charge-neutral quadrupolar layers, which expose the oxygen atoms. The surface energies are 0.30 and 0.20 J/m2 values with PBEsol and PBE exchange-correlation functionals, respectively (Supporting Information Table S1). The small surface energies confirm the high stability of the (010) surface in agreement with previous studies. (20)

Recent photoemission experiments placed the upper valence band 7.27 eV below the vacuum and the lower conduction band at 4.79 eV (Scheme 1). (10) The quasi-particle band gap obtained from X-ray emission (XES) and absorption (XAS) is 2.48 eV; (10) previous X-ray experiments reported a value of 2.38 eV. (22) It is well-known that gradient-corrected DFT functionals underestimate the bands gaps of insulators. The converged gap for the surface model is 2.04 eV (2.02 eV) with PBE (PBEsol), which is similar to the values obtained for the bulk material (Table 1). In contrast, for the same geometry, the band gaps obtained with the hybrid functionals HSE06 and PBE0 (25% exact exchange) are 3.15 and 3.10 eV, respectively. Kweon et al. found only 5% of exact exchange is required to obtain a band gap of 2.5 eV for BiVO4. (14) This unusual feature may be related to the chemical makeup of the band edge states: a smaller derivative discontinuity is found for metal oxides formed of post-transition metals with valence ns2 orbitals (here Bi 6s2).

Table 1. Calculated Energy Gaps, Ionization Potentials (IP), and Electron Affinities (EA) in eV for the (110) Surface of BiVO4 Formed of n Bilayersa
slab modelsEgapIPBiVO4EABiVO4
nPBEPBEsolPBEPBEsolPBEPBEsol
12.08 (3.48)b2.067.18 (8.10)b7.235.18 (4.62)b5.10
22.042.027.197.245.225.15
32.042.027.187.245.225.15
42.042.027.197.245.225.15
a

Values are compared for two exchange-correlation treatments (PBE and PBEsol) within density functional theory.

b

Values obtained with the HSE06 hybrid functional (fully optimized structure).

The comparison between the experimental data and the calculated band edge positions (Table 1) indicates that the underestimation of the electronic band gap is related to the under-stabilization of the CBM. The good agreement of the ionization potentials with these functionals shows that the VBM orbitals are well described with these GGA functionals. By alignment of the core levels with the periodic solid, the predicted bulk ionization potentials are 7.24 and 7.30 eV with PBE and PBEsol functionals, respectively (Table S2). The corresponding electron affinities are 5.21 and 5.27 eV, only slightly shifted from the values obtained for the surface models (5.10 and 5.18 eV) as a consequence of the small band bending energies.

The evaluation of the energy level alignments is more involved for interfaces and electrode models. (23) Considering the good agreement between the ionization potentials computed with the PBE functional and the experimental values, we used this functional to evaluate the effect of the water at the contact with the (010) surface on the ionization potentials. We considered four models. I – bismuth: one water molecule interacting with the Bi surface atoms. II – oxygen: one water molecule interacting with the O surface atoms at typical hydrogen bond distances. III – monolayer: molecular dynamics simulations of BiVO4 in contact with liquid water, where two molecules are stabilized by a hydrogen bond resembling the dimer of water (cell and supercell models were considered, with similar geometries and energetics, Figure 1). IV – frozen liquid: the semiconductor surface in contact with 14 molecules of water (to approximate the density of liquid water for a slab model an interlayer spacing of 15 Å), and a vacuum of 15 Å (see Figure 1).

In order to analyze the structure of water interacting with the semiconductor surface, molecular dynamics of model III were performed at 298 K. Ten snapshots from the dynamics of the monolayer model were considered in order to evaluate the effect of water mobility and surface relaxation on the ionization potentials (more details can be found in the Supporting Information). More comprehensive molecular dynamics simulations and dynamic local structure analysis will be performed in future work.

Figure 1

Figure 1. Models for the interaction between a layer of water and the BiVO4 (010) surface: (a) the bulk crystal structure; (b) the (010) surface terminated with vacuum; (c) the (010) surface in contact with water. The Bi–O polyhedra are shaded pink, with the V–O polyhedral shaded blue.

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The absorption of one water molecule on the Bi and V sites stabilizes the system by −0.48 and −0.22 eV, respectively (models I and II). The absorption energy of the dimer (model III) is −0.98 eV; the process is thermodynamically favorable. For the monolayer model, a distorted hexagonal structure of hydrogen bonds on the surface is found, where water absorbs on the Bi sites. The second water molecule is located at hydrogen bond distances of 1.74 and 1.85 Å from the O–V (Figure 1). The frozen-liquid model shows a similar pattern for the water absorbed on the surface, but with Bi···OH2 and VO···H–OH distances larger about 0.1 Å. On the other hand, the water–water distance is shorter because of the effect of the surrounding water molecules.

The first step of the water oxidation process is the H dissociation. Consequently, the presence of a second molecule interacting with the adsorbed water could be relevant for the mechanism of water oxidation on BiVO4 surfaces. Our molecular dynamics simulations for the monolayer and liquid water did not show any dissociative event. Earlier DFT calculations also reported the nondissociative nature of the water absorption on pristine BiVO4 surfaces. (19, 24, 25)

Table 2. Calculated Properties for the BiVO4 Surfaces: Monolayer and Frozen-Liquid Modelsa
 EgapIPBiVO4ΔIPBiVO4IPBiVO4/H2OWeBiVO4/H2O – WeBiVO4e0UVBM
monolayer2.187.350.176.530.812.90
frozen-liquid2.167.320.146.221.112.88
a

All quantities are in eV.

IPBiVO4 is the ionization potential associated with the surface in contact with vacuum, and the IPBiVO4/H2O corresponds to the ionization potential of the surface in contact with water. ΔIPBiVO4 is the difference between the IPBiVO4 of the slab model and the ionization potential of the bare surface (Table 1). WeBiVO4/H2O – WeBiVO4 is the work to transport an electron from the semiconductor to the solution. e0UVBM is the electrochemical potential with reference to the hydrogen electrode (Supporting Information).

Table 2 shows the effect of water on the ionization potentials for models III and IV. The interaction between water and the semiconductor slightly increases the IPBiVO4 ionization potential with respect to the bare surface (Table 1) from 7.18 eV to 7.35 and 7.32 eV for the monolayer and frozen-liquid models, respectively. This is a consequence of the stabilization of the upper valence band because of the interaction with the solvent. At the same time, the band gap increases by about 0.1 eV for both models. While the ionization potentials of the surface in contact with vacuum are similar, the ionization potential associated with the surface hydrated surface (IPBiVO4/H2O) changes from 6.53 to 6.22 eV from the monolayer to the frozen-liquid. Consequently, it is more difficult to bring an electron to the hydrated surface when the concentration of molecules of water is increased.

Understanding the effect of water on the ionization potentials is useful to analyze the electronic density of states and the chemical nature of the band edge orbitals (Figure 2). As for the bulk material and the pristine (010) surface, (26) the valence band is dominated by the 2p oxygen orbitals, and the conduction band has a significant contribution from the V 3d orbitals. In the case of hydrated surface models, a group of additional states corresponding to the combination of O 2s of water appears around −7 eV. This band is displaced about 2 eV from the band composed by the O 2s of the semiconductor due to the difference in chemical environment. The O 2s band is broader for the frozen-liquid because of the distribution of local H2O environments and some contributions from the H 1s orbitals from the hydrogen bond network. The orbitals from absorbed water molecules are more stable than the others coming from the nonabsorbed water molecules. Another interesting feature of the frozen-liquid is the broader conduction band; while the orbitals close to the lower conduction band are basically V 3d, the higher energy orbitals are a combination of Bi 6p and O 2p.

Upon water absorption, a fraction of the highest-occupied electron density is transferred from the surface to the subsurface semiconductor layers (see Figure 2). This process stabilizes the upper valence band and explains why the ionization potentials for the monolayer and frozen-liquid models increase with respect to the bare surface model. There is a small influence of water on the lower conduction band because these orbitals have a small contribution from the surface. The addition of a polar layer on the surface of BiVO4 could potentially shift the energy levels without an important contribution of the orbitals of the layer to the band edge orbitals. The use of polar layers like FeOOH, NiO and NiOOH, which accelerate the O2 release kinetics, could behave similarly. (7, 27)

Figure 2

Figure 2. Electronic density of states scaled with respect to the semicore Bi 5d band (not shown) for (a) a pristine slab model, (b) monolayer (model III), and (c) frozen-liquid (model IV). The highest occupied state is set to −IPBiVO4 (eV). The electron density from the highest occupied (HOCO) and lowest unoccupied (LUCO) crystal orbitals are represented in the right panel.

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The dynamics of the water at the contact with the surface includes the desorption–absorption processes. The mobility of water molecules could displace the states from the absorbed water to the edge of the band and could also help also to stabilize the trap states. Recent experiments show the role of the hole and electron trap states in the photophysics of BiVO4. (28) To provide insight into the effect of the dynamics of water into the ionization potentials, 10 snapshots from the molecular dynamics were analyzed. During the dynamics, all atoms were allowed to relax, and then the effect of the surface relaxation was taken into account as well as the dynamics of the water molecules. The energy levels were aligned with respect to the calculated vacuum level in each case (Figure 3).

Figure 3

Figure 3. Ionization potentials (IP) with respect to the vacuum level calculated for 10 snapshots (ordered by time) obtained from the 298 K dynamics of a water monolayer on the surface of BiVO4 (PBE functional). All values are in eV. The vertical lines represent the work to transport an electron from the bare material to the hydrated surface (WeBiVO4/H2O – WeBiVO4).

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The band gaps of the considered snapshots are in general smaller than those obtained from the static models, which can be associated with the destabilization of upper valence orbitals (decrease in ionization potential) due to the deviation from the equilibrium geometry (Table 2). The calculated ionization potentials are still larger than those obtained for the bare surfaces (Table 1). IPBiVO4/H2O depends strongly on the structure of water; consequently, their values show larger oscillations than the obtained for other properties. The effect of orientational disorder on the evaluation of ionization potentials is discussed in refs 29 and 30. In all cases, the IPBiVO4/H2O values are smaller than the obtained for the static monolayer model and closer to the values obtained for the frozen-liquid. Some orientational disorder of the water dipoles is required to reproduce the instantaneous changes in the electrostatic potential during the liquid dynamics. The work to bring an electron from the bare to the hydrated surfaces (WeBiVO4/H2O – WeBiVO4) also oscillates significantly from 0.5 to 1.2 eV, which is also correlated to the variations of the water structure during the dynamics.

The IPBiVO4 values are in good agreement with the data reported by Kim and Choi (7) for unmodified BiVO4 (7.2–7.5 eV at pH = 7). They observed an increase of the flat band potential using layers of FeOOH and NiOOH. Our calculations suggest that the deposition of polar layers on BiVO4 has an impact on the electronic structure of the semiconductor. The increasing of the ionization potential can be associated with a more efficient electron–hole separation, which is one of the effects of polar oxide layers. (7, 27, 31) Consequently, more efficient BiVO4-based materials could be designed tuning the surface ionization potentials using polar layers. Given the layered structure of this material, a polar substitution (e.g., F incorporation) in a subsurface layer could be used to provide a chemically robust modification, which we aim to explore in future studies.

In conclusion, first-principles electronic structure calculations validate the measured ionization potential of bismuth vanadate, and provide new insights into the role of water on the surface electronic structure. The main effect of the interaction between water and the BiVO4 surface is the stabilization of the upper valence band. As a consequence, the ionization potentials increase with respect to the bare surfaces. This effect was found for all considered models, as well as when molecular dynamics allowed the motion of surface atoms and water molecules. These results can contribute to a better understanding of the behavior of photoanodes that are mostly in contact with water and the effect of polar layers deposited on semiconductor surfaces.

Supporting Information

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Additional material is provided in the Supporting Information, including computational details and description of models. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b00966.

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Author Information

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  • Corresponding Author
    • Rachel Crespo-Otero - School of Biological and Chemical Sciences, Queen Mary University, London E1 4NS, United Kingdom;  Email: [email protected]
  • Author
    • Aron Walsh - Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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We acknowledge useful discussions with K. T. Butler regarding surface potentials, and valuable suggestions from Jonathan M. Skelton and E. Lora da Silva. The work was funded by the EPSRC (Grant No. EP/K004956/1) and the ERC (Grant No. 277757). The calculations used the ARCHER supercomputer through membership of the UK’s HPC Materials Chemistry Consortium (EPSRC Grant No. EP/L000202).

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    A comprehensive approach to understanding the electronic structure of monoclinic scheelite bismuth vanadate (ms-BiVO4), including both valence band (VB) and conduction band (CB) orbital character, is presented. D. functional theory (DFT) calcns. are directly compared to exptl. data obtained via X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, resonant inelastic X-ray spectroscopy (RIXS), and XPS to provide a complete portrait of the total and partial d. of states (DOS) near the bandgap. DFT calcns. are presented to confirm the VB max. and CB min. are comprised primarily of O 2p and V 3d orbitals, resp. Predicted triplet d-manifold splitting of V 3d CB states, arising from lone pair-induced lattice distortions, is quantified by V L- and O K-edge XAS. Furthermore, the partial contributions to the total DOS within both the CB and VB, detd. by RIXS, are found to be in excellent agreement with DFT calcns. Energy levels are placed relative to the vacuum level by photoemission spectroscopy, which provides a measure of the work function and electron affinity of the investigated BiVO4 thin film. The implications of the fundamental electronic structure of ms-BiVO4 on its photocatalytic behavior, as well as considerations for improvements by substitutional incorporation of addnl. elements, are discussed.
  11. 11
    Sun, S.; Wang, W.; Li, D.; Zhang, L.; Jiang, D. Solar Light Driven Pure Water Splitting on Quantum Sized BiVO4 without Any Cocatalyst ACS Catal. 2014, 4, 3498– 3503
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    11
    Solar light driven pure water splitting on quantum sized BiVO4 without any cocatalyst
    Sun, Songmei; Wang, Wenzhong; Li, Dezhi; Zhang, Ling; Jiang, Dong
    ACS Catalysis (2014), 4 (10), 3498-3503CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Photocatalytic water splitting is the most promising process to convert solar energy into high purity chem. fuel (hydrogen), which has received significant attention in recent years. Only several photocatalysts have been reported in the literature for pure water splitting under visible light. Herein we report for the first time quantum sized BiVO4 can decomp. pure water into H2 and O2 simultaneously under simulated solar light irradn. without any cocatalysts or sacrificial reagents. By electrochem. measurement, we demonstrate that the significantly different photocatalytic activity of the quantum sized BiVO4 arises from the neg. shift of conduction band edge by a quantum confinement effect and a decreased overpotential for water redn. Although the generated H2 and O2 are nonstoichiometric in the present study, these findings establish the great potential of using quantum sized BiVO4 photocatalyst and solar energy for overall water splitting.
  12. 12
    Ma, Y.; Pendlebury, S. R.; Reynal, A.; Le Formal, F.; Durrant, J. R. Dynamics of Photogenerated Holes in Undoped BiVO4 Photoanodes for Solar Water Oxidation Chem. Sci. 2014, 5, 2964– 2973
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    Dynamics of photogenerated holes in undoped BiVO4 photoanodes for solar water oxidation
    Ma, Yimeng; Pendlebury, Stephanie R.; Reynal, Anna; Le Formal, Florian; Durrant, James R.
    Chemical Science (2014), 5 (8), 2964-2973CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The dynamics of photogenerated holes in undoped BiVO4 photoanodes for water splitting were studied using transient absorption spectroscopy, correlated with photoelectrochem. and transient photocurrent data. Transient absorption signals of photogenerated holes were identified using electron/hole scavengers and applied elec. bias in a complete photoelectrochem. cell. The yield of long-lived (0.1-1 s) photogenerated holes is obsd. to correlate as a function of applied elec. bias with the width of the space charge layer, as detd. by electrochem. impedance spectroscopy. The transient absorption decay time const. of these long-lived holes is also obsd. to be dependent upon the applied bias, assigned to kinetic competition between water oxidn. and recombination of these surface accumulated holes with bulk electrons across the space charge layer. The time const. for this slow recombination measured with transient absorption spectroscopy is shown to match the time const. of back electron transfer from the external circuit detd. from chopped light transient photocurrent measurements, thus providing strong evidence for these assignments. The yield of water oxidn. detd. from these measurements, including consideration of both the yield of long-lived holes, and the fraction of these holes which are lost due to back electron/hole recombination, is obsd. to be in good agreement with the photocurrent d. measured for BiVO4 photoanodes as a function of bias under continuous irradn. Overall these results indicate two distinct recombination processes which limit photocurrent generation in BiVO4 photoanodes: firstly rapid (≤microseconds) electron/hole recombination, and secondly recombination of surface-accumulated holes with bulk BiVO4 electrons. This second 'back electron transfer' recombination occurs on the milliseconds-seconds timescale, and is only avoided at strong anodic biases where the potential drop across the space charge layer provides a sufficiently large energetic barrier to prevent this recombination process.
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    Kweon, K. E.; Hwang, G. S. Surface Structure and Hole Localization in Bismuth Vanadate: A First Principles Study Appl. Phys. Lett. 2013, 103, 131603
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    13
    Surface structure and hole localization in bismuth vanadate. A first principles study
    Kweon, Kyoung E.; Hwang, Gyeong S.
    Applied Physics Letters (2013), 103 (13), 131603/1-131603/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The monoclinic and tetragonal phases of BiVO4 were found to exhibit significantly different photocatalytic activities for water splitting. To assess a possible surface effect on the phase-dependent behavior, we calc. and compare the geometries and electronic structures of the monoclinic and tetragonal BiVO4 (001) surfaces using hybrid d. functional theory. The relaxed at. configurations of these 2 surfaces are found to be nearly identical, while an excess hole shows a relatively stronger tendency to localize at the surface than the bulk in both phases. Possible factors for the phase-dependent photocatalytic activity of BiVO4 are discussed. (c) 2013 American Institute of Physics.
  14. 14
    Kweon, K. E.; Hwang, G. S.; Kim, J.; Kim, S.; Kim, S. Electron Small Polarons and Their Transport in Bismuth Vanadate: A First Principles Study Phys. Chem. Chem. Phys. 2014, 17, 256– 260
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    Park, Y.; McDonald, K. J.; Choi, K.-S. Progress in Bismuth Vanadate Photoanodes for Use in Solar Water Oxidation Chem. Soc. Rev. 2013, 42, 2321– 2337
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    Progress in bismuth vanadate photoanodes for use in solar water oxidation
    Park, Yiseul; McDonald, Kenneth J.; Choi, Kyoung-Shin
    Chemical Society Reviews (2013), 42 (6), 2321-2337CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Harvesting energy directly from sunlight as nature accomplishes through photosynthesis is a very attractive and desirable way to solve the energy challenge. Many efforts have been made to find appropriate materials and systems that can utilize solar energy to produce chem. fuels. One of the most viable options is the construction of a photoelectrochem. cell that can reduce water to H2 or CO2 to carbon-based mols. Bismuth vanadate (BiVO4) has recently emerged as a promising material for use as a photoanode that oxidizes water to O2 in these cells. Significant advancement in the understanding and construction of efficient BiVO4-based photoanode systems has been made within a short period of time owing to various newly developed ideas and approaches. In this review, the crystal and electronic structures that are closely related to the photoelectrochem. properties of BiVO4 are described first, and the photoelectrochem. properties and limitations of BiVO4 are examd. Subsequently, the latest efforts toward addressing these limitations in order to improve the performances of BiVO4-based photoanodes are discussed. These efforts include morphol. control, formation of composite structures, compn. tuning, and coupling oxygen evolution catalysts. The discussions and insights provided in this review reflect the most recent approaches and directions for general photoelectrode developments and they will be directly applicable for the understanding and improvement of other photoelectrode systems.
  16. 16
    Huang, Z.-F.; Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Nanostructured Bismuth Vanadate-Based Materials for Solar-Energy-Driven Water Oxidation: A Review on Recent Progress Nanoscale 2014, 6, 14044– 14063
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    16
    Nanostructured bismuth vanadate-based materials for solar-energy-driven water oxidation: a review on recent progress
    Huang, Zhen-Feng; Pan, Lun; Zou, Ji-Jun; Zhang, Xiangwen; Wang, Li
    Nanoscale (2014), 6 (23), 14044-14063CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    A review. Water oxidn. is the key step for both photocatalytic water splitting and CO2 redn., but its efficiency is very low compared with the photocatalytic redn. of water. Bismuth vanadate (BiVO4) is the most promising photocatalyst for water oxidn. and has become a hot topic for current research. However, the efficiency achieved with this material to date is far away from the theor. solar-to-hydrogen conversion efficiency, mainly due to the poor photo-induced electron transportation and the slow kinetics of oxygen evolution. Fortunately, great breakthroughs have been made in the past five years in both improving the efficiency and understanding the related mechanism. This review is aimed at summarizing the recent exptl. and computational breakthroughs in single crystals modified by element doping, facet engineering, and morphol. control, as well as macro/mesoporous structure construction, and composites fabricated by homo/hetero-junction construction and co-catalyst loading. We aim to provide guidelines for the rational design and fabrication of highly efficient BiVO4-based materials for water oxidn.
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    Sun, S.; Wang, W. Advanced Chemical Compositions and Nanoarchitectures of Bismuth Based Complex Oxides for Solar Photocatalytic Application RSC Adv. 2014, 4, 47136– 47152
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    Advanced chemical compositions and nanoarchitectures of bismuth based complex oxides for solar photocatalytic application
    Sun, Songmei; Wang, Wenzhong
    RSC Advances (2014), 4 (88), 47136-47152CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    A Review. Bismuth based complex oxides have attracted considerable interest due to their great potential to harvest solar light to solve the current environmental and energy crisis. Bismuth based complex oxides have excellent photo-oxidn. ability for org. contaminant degrdn. and water oxidn. via a photocatalytic process. Many efforts have been made to improve their photocatalytic performance, esp. on the BiVO4, Bi2WO6 and Bi2MoO6 materials, which have been mostly studied in the past few years. Significant progress in understanding the fundamentals and improving the photocatalytic performance has been made due to the various new developed concepts and approaches in recent years. In this review, we present a comprehensive overview on the fundamentals and recent advances of BiVO4, Bi2WO6 and Bi2MoO6 photocatalysts. After the anal. of the structure-property relationships, the strategies that have been employed to enhance their photocatalytic performance are discussed in detail, including morphol. control, surface modification, doping and construction of composite material. Furthermore, remarks on the challenges and perspectives of research directions are proposed for further development of the highly efficient bismuth based complex oxide photocatalysts.
  18. 18
    Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li, C. Spatial Separation of Photogenerated Electrons and Holes among {010} and {110} Crystal Facets of BiVO4 Nat. Commun. 2013, 4, 1432
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    18
    Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4
    Li Rengui; Zhang Fuxiang; Wang Donge; Yang Jingxiu; Li Mingrun; Zhu Jian; Zhou Xin; Han Hongxian; Li Can
    Nature communications (2013), 4 (), 1432 ISSN:.
    Charge separation is crucial for increasing the activity of semiconductor-based photocatalysts, especially in water splitting reactions. Here we show, using monoclinic bismuth vanadate crystal as a model photocatalyst, that efficient charge separation can be achieved on different crystal facets, as evidenced by the reduction reaction with photogenerated electrons and oxidation reaction with photogenerated holes, which take place separately on the {010} and {110} facets under photo-irradiation. Based on this finding, the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} facets respectively, resulting in much higher activity in both photocatalytic and photoelectrocatalytic water oxidation reactions, compared with the photocatalyst with randomly distributed cocatalysts. These results show that the photogenrated electrons and holes can be separated between the different facets of semiconductor crystals. This finding may be useful in semiconductor physics and chemistry to construct highly efficient solar energy conversion systems.
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    Yang, J.; Wang, D.; Zhou, X.; Li, C. A Theoretical Study on the Mechanism of Photocatalytic Oxygen Evolution on BiVO4 in Aqueous Solution Chemistry 2013, 19, 1320– 1326
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    A Theoretical Study on the Mechanism of Photocatalytic Oxygen Evolution on BiVO4 in Aqueous Solution
    Yang, Jingxiu; Wang, Donge; Zhou, Xin; Li, Can
    Chemistry - A European Journal (2013), 19 (4), 1320-1326CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The oxygen evolution reaction (OER) is regarded as one of the key issues in achieving efficient photocatalytic water splitting. Monoclinic scheelite BiVO4 is a visible-light-responsive semiconductor which has proved to be effective for oxygen evolution. Recently, the synthesis of a series of monoclinic BiVO4 single crystals was reported, and it was found that the (010), (110), and (011) facets are highly exposed and that the photocatalytic O2 evolution activity depends on the degree of exposure of the (010) facets. To explore the properties of and photocatalytic water oxidn. reaction on different facets, DFT calcns. were performed to investigate the geometric structure, optical properties, electronic structure, water adsorption, and the whole OER free-energy profiles on BiVO4 (010) and (011) facets. The calcd. results suggest both favorable and unfavorable factors for OER on the (010) and the (011) facets. Due to the combined effects of the above-mentioned factors, different facets exhibit quite different photocatalytic activities.
  20. 20
    Zhao, Z.; Li, Z.; Zou, Z. Structure and Energetics of Low-Index Stoichiometric Monoclinic Clinobisvanite BiVO4 Surfaces RSC Adv. 2011, 1, 874– 883
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    20
    Structure and energetics of low-index stoichiometric monoclinic clinobisvanite BiVO4 surfaces
    Zhao, Zongyan; Li, Zhaosheng; Zou, Zhigang
    RSC Advances (2011), 1 (5), 874-883CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    In the present work, d. functional theory calcns. were employed to study the surface properties of several low-index stoichiometric monoclinic clinobisvanite BiVO4 surfaces. Their surface properties were systemically calcd. and described in details, and the similarities and differences between these surfaces were compared and analyzed. Finally, on the basis of calcd. surface energies, the equil. crystal shape of monoclinic clinobisvanite BiVO4 was detd., and its av. surface energy was estd. The calcd. results indicated that the dangling bond d. of the bismuth atom dets. not only the surface energy, but also the surface relaxation.
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    Walsh, A.; Butler, K. T. Prediction of Electron Energies in Metal Oxides Acc. Chem. Res. 2014, 47, 364– 372
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    Payne, D. J.; Robinson, M. D. M.; Egdell, R. G.; Walsh, A.; McNulty, J.; Smith, K. E.; Piper, L. F. J. The Nature of Electron Lone Pairs in BiVO4 Appl. Phys. Lett. 2011, 98, 212110
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    The nature of electron lone pairs in BiVO4
    Payne, D. J.; Robinson, M. D. M.; Egdell, R. G.; Walsh, A.; McNulty, J.; Smith, K. E.; Piper, L. F. J.
    Applied Physics Letters (2011), 98 (21), 212110/1-212110/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The electronic structure of BiVO4 was studied by x-ray photoelectron, x-ray absorption, and x-ray emission spectroscopies, in comparison with d. functional theory calcns. Results confirm both the direct band gap of 2.48 eV and that the Bi 6s electrons hybridize with O 2p to form antibonding lone pair states at the top of the valence band. The results highlight the suitability of combining s2 and d0 cations to produce photoactive ternary oxides. (c) 2011 American Institute of Physics.
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    Cheng, J.; Sprik, M. Alignment of Electronic Energy Levels at Electrochemical Interfaces Phys. Chem. Chem. Phys. 2012, 14, 11245– 11267
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    Alignment of electronic energy levels at electrochemical interfaces
    Cheng, Jun; Sprik, Michiel
    Physical Chemistry Chemical Physics (2012), 14 (32), 11245-11267CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The position of electronic energy levels in a phase depends on the surface potentials at its boundaries. Bringing two phases in contact at an interface will alter the surface potentials shifting the energy levels relative to each other. Calcg. such shifts for electrochem. interfaces requires a combination of methods from computational surface science and phys. chem. The problem is closely related to the computation of potentials of electrochem. inactive electrodes. These so-called ideally polarizable interfaces are impossible to cross for electrons. In this perspective the authors review two d. functional theory based methods that were developed for this purpose, the work function method and the H insertion method. The key expressions of the two methods are derived from the formal theory of abs. electrode potentials. As an illustration of the work function method the authors review the computation of the potential of zero charge of the Pt(111)-H2O interface as recently published by a no. of groups. The example of the H insertion method is from the authors' own work on the rutile TiO2(110)-H2O interface at the point of zero proton charge. The calcns. are summarized in level diagrams aligning the electronic energy levels of the solid electrode (Fermi level of the metal, valence band max. and conduction band min. of the semiconductor) to the band edges of liq. H2O and the std. potential for the redn. of the hydroxyl radical. All potentials are calcd. at the same level of d. functional theory using the std. H electrode as common energy ref. Comparison to expt. identifies the treatment of the valence band of H2O as a potentially dangerous source of error for application to electrocatalysis and photocatalysis.
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    Oshikiri, M.; Boero, M. Water Molecule Adsorption Properties on the BiVO4 (100) Surface 2006, 4, 9188– 9194
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    Oshikiri, M.; Boero, M.; Matsushita, A.; Ye, J. Water Molecule Adsorption Properties on Surfaces of MVO4 (M = In, Y, Bi) Photo-Catalysts J. Electroceramics 2007, 22, 114– 119
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    Walsh, A.; Yan, Y.; Huda, M. N.; Al-Jassim, M. M.; Wei, S.-H. Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals Chem. Mater. 2009, 21, 547– 551
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    26
    Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals
    Walsh, Aron; Yan, Yanfa; Huda, Muhammad N.; Al-Jassim, Mowafak M.; Wei, Su-Huai
    Chemistry of Materials (2009), 21 (3), 547-551CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The authors report the 1st-principles electronic structure of BiVO4, a promising photocatalyst for H generation. BiVO4 is a direct band gap semiconductor, despite having band extrema away from the Brillouin zone center. Coupling between Bi 6s and O 2p forces an upward dispersion of the valence band at the zone boundary; however, a direct gap is maintained via coupling between V 3d, O 2p, and Bi 6p, which lowers the conduction band min. These interactions result in sym. hole and electron masses. Implications for the design of ambipolar metal oxides are discussed.
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    Zhong, M.; Hisatomi, T.; Kuang, Y.; Zhao, J.; Liu, M.; Iwase, A.; Jia, Q.; Nishiyama, H.; Minegishi, T.; Nakabayashi, M. Surface Modification of the CoOx Loaded BiVO4 Photoanodes with Ultrathin p-Type NiO Layers for the Improved Solar Water Oxidation J. Am. Chem. Soc. 2015, 137, 5053– 5060
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    Ravensbergen, J.; Abdi, F. F.; van Santen, J. H.; Frese, R. N.; Dam, B.; van de Krol, R.; Kennis, J. T. M. Unraveling the Carrier Dynamics of BiVO4: A Femtosecond to Microsecond Transient Absorption Study J. Phys. Chem. C 2014, 118, 27793– 27800
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    28
    Unraveling the Carrier Dynamics of BiVO4: A Femtosecond to Microsecond Transient Absorption Study
    Ravensbergen, Janneke; Abdi, Fatwa F.; van Santen, Judith H.; Frese, Raoul N.; Dam, Bernard; van de Krol, Roel; Kennis, John T. M.
    Journal of Physical Chemistry C (2014), 118 (48), 27793-27800CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Bi vanadate (BiVO4) is a promising semiconductor material for photoelectrochem. H2O splitting showing good visible light absorption and a high photochem. stability. To improve the performance of BiVO4, it is of key importance to understand its photophysics upon light absorption. Here the authors study the carrier dynamics of BiVO4 prepd. by the spray pyrolysis method using broadband transient absorption spectroscopy (TAS), in thin films as well as in a photoelectrochem. (PEC) cell under H2O-splitting conditions. The use of a dual-laser setup consisting of electronically synchronized Ti:sapphire amplifiers enable one to measure the femtosecond to microsecond time scales in a single expt. The authors propose a model of carrier dynamics that includes relaxation and trapping rates for electrons and holes. Hole trapping occurs in multiple phases, with the majority of the photogenerated holes being trapped with a time const. of 5 ps and a small fraction of this hole trapping taking place within the instrument response of 120 fs. The induced absorption band that represents the trapped holes is modulated by an oscillation of 63 cm-1, which is assigned to the coupling of holes to a phonon mode. Electrons to undergo a relaxation with a time const. of 40 ps, followed by deeper trapping on the 2.5 ns time scale were found. On time scales longer than 10 ns, trap-limited recombination that follows a power law is found, spanning time scales up to microseconds. Finally, the authors observe no spectral or kinetic differences by applying a bias voltage to the PEC cell, indicating that the effect of a voltage and the charge transfer processes between BiVO4 and the electrolyte occurs on longer time scales. Results therefore provide new insights into the carrier dynamics of BiVO4 and further expand the application window of TAS as an anal. tool for photoanode materials.
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    Otani, M.; Hamada, I.; Sugino, O.; Morikawa, Y.; Okamoto, Y.; Ikeshoji, T. Electrode Dynamics from First Principles J. Phys. Soc. Jpn. 2008, 77, 024802
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    Electrode dynamics from first principles
    Otani, Minoru; Hamada, Ikutaro; Sugino, Osamu; Morikawa, Yoshitada; Okamoto, Yasuharu; Ikeshoji, Tamio
    Journal of the Physical Society of Japan (2008), 77 (2), 024802/1-024802/6CODEN: JUPSAU; ISSN:0031-9015. (Physical Society of Japan)
    The study of electrode dynamics was a major topic in the field of electrochem. for a century. Electrode dynamics consist of electron transfer reactions that give rise to, or are caused by, a bias voltage, and are influenced by surface catalysis, electrolyte soln., transport of electrons and ions. The 1st-principles mol. dynamics simulation of the electrochem. system was hampered by the difficulty to describe the bias voltage and the complex soln.-electrode interface structure. Here the authors use a new algorithm called the effective screening medium to characterize the biased interface between Pt and liq. H2O, revealing the microscopic details of the 1st, Volmer, step of the Pt-catalyzed hydrogen evolution reaction. By clarifying the important roles played by both the H2O and the bias, the authors show why this reaction occurs so efficiently at the interface. The simulations make a significant step towards a deeper understanding of electrochem. reactions.
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    Schnur, S.; Groß, A. Challenges in the First-Principles Description of Reactions in Electrocatalysis Catal. Today 2011, 165, 129– 137
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    Challenges in the first-principles description of reactions in electrocatalysis
    Schnur, Sebastian; Gross, Axel
    Catalysis Today (2011), 165 (1), 129-137CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    In spite of the strong relevance of reactions in electrocatalysis, in particular for the electrochem. energy conversion and storage, the no. of theor. studies addressing electrocatalytic reactions from 1st principles is still limited. This is due to the fact that there are two factors adding considerable complexity to the theor. treatment: the presence of the electrolyte at the electrode surface and varying electrode potentials. Still, there are promising approaches to cope with these problems allowing a realistic theor. description of reactions in electrocatalysis. It will be demonstrated that ab initio mol. dynamics simulations based on periodic d. functional theory calcns. can contribute to an understanding of the structures and reactions at H2O/metal interfaces. To model varying electrode potentials, an explicit counter electrode was implemented in a periodic d. functional theory code, and 1st preliminary results using this implementation will be presented.
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    Eisenberg, D.; Ahn, H. S.; Bard, A. J. Enhanced Photoelectrochemical Water Oxidation on Bismuth Vanadate by Electrodeposition of Amorphous Titanium Dioxide J. Am. Chem. Soc. 2014, 136, 14011– 14014
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    31
    Enhanced photoelectrochemical water oxidation on bismuth vanadate by electrodeposition of amorphous titanium dioxide
    Eisenberg, David; Ahn, Hyun S.; Bard, Allen J.
    Journal of the American Chemical Society (2014), 136 (40), 14011-14014CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    N-BiVO4 is a promising semiconductor material for photoelectrochem. water oxidn. Although most thin-film syntheses yield discontinuous BiVO4 layers, back redn. of photo-oxidized products on the conductive substrate has never been considered as a possible energy loss mechanism in the material. We report that a 15 s electrodeposition of amorphous TiO2 (a-TiO2) on W:BiVO4/F:SnO2 blocks this undesired back redn. and dramatically improves the photoelectrochem. performance of the electrode. Water oxidn. photocurrent increases by up to 5.5 times, and its onset potential shifts neg. by ∼500 mV. In addn. to blocking soln.-mediated recombination at the substrate, the a-TiO2 film-which is found to lack any photocatalytic activity in itself-is hypothesized to react with surface defects and deactivate them toward surface recombination. The proposed treatment is simple and effective, and it may easily be extended to a wide variety of thin-film photoelectrodes.

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Cite this: J. Phys. Chem. Lett. 2015, 6, 12, 2379–2383
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https://doi.org/10.1021/acs.jpclett.5b00966
Published June 7, 2015

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  • Abstract

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    Scheme 1

    Scheme 1. Alignment of Energy Levels of Monoclinic BiVO4 with Respect to the Vacuum Levela
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    Scheme aThe values were taken from ref 10 based on photoemission experiments.

    Figure 1

    Figure 1. Models for the interaction between a layer of water and the BiVO4 (010) surface: (a) the bulk crystal structure; (b) the (010) surface terminated with vacuum; (c) the (010) surface in contact with water. The Bi–O polyhedra are shaded pink, with the V–O polyhedral shaded blue.

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    Figure 2

    Figure 2. Electronic density of states scaled with respect to the semicore Bi 5d band (not shown) for (a) a pristine slab model, (b) monolayer (model III), and (c) frozen-liquid (model IV). The highest occupied state is set to −IPBiVO4 (eV). The electron density from the highest occupied (HOCO) and lowest unoccupied (LUCO) crystal orbitals are represented in the right panel.

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    Figure 3

    Figure 3. Ionization potentials (IP) with respect to the vacuum level calculated for 10 snapshots (ordered by time) obtained from the 298 K dynamics of a water monolayer on the surface of BiVO4 (PBE functional). All values are in eV. The vertical lines represent the work to transport an electron from the bare material to the hydrated surface (WeBiVO4/H2O – WeBiVO4).

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  • References

    This article references 31 other publications.

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      For electrochem. decompn. of H2O, a p.d. of >1.23 V is necessary between the anode and the cathode. This p.d. is equal to the energy of radiation with wavelength of ∼1000 nm. This energy, in the form of visible light, can be used effectively in an electrochem. system to decomp. H2O. A novel type of electrochem. cell was developed, in which a TiO2 electrode was connected with a Pt-black electrode through an external load. The direction of current revealed that oxidn. occurred at the TiO2 electrode and redn. at the Pt-black electrode. To increase the efficiency of the process, more reducible species, such as dissolved O2 or Fe3+, must be added in the Pt electrode compartment. The use of a p-type semiconductor electrode instead of Pt leads to more effective electrochem. photolysis of H2O.
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      A review. The application of photocatalysis in environment remediation as well as in the generation of useful fuels from the redn. of water (hydrogen) and of carbon dioxide (methanol, carbon monoxide and/or methane) was investigated largely in the last four decades. A significant part (12-13%) of the literature on the generation of such fuels is found in patents. Accordingly, the present article presents a selection of the patent literature on the theme. Photocatalysts, whether pure or doped, solid solns. or composites, reported in patents are reviewed along with the corresponding preparative methods and the photocatalytic performance. The absorption of light by such materials was extended toward the red side of the spectrum, so that a better use of solar irradn. was obtained, but the expected improvement of the catalytic effect has not always been achieved. The causes of these results and the way for improving the performance in the various steps of the process (e.g. avoiding charge recombination or catalyst corrosion) were documented. The correct use of the term water splitting and the fundamentals of photochem. hydrogen evolution in the presence of a sacrificial electron donor (e.g., alcs.) are discussed. Quant. data about the amt. of hydrogen evolved or carbon-based fuels produced are indicated whenever available.
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      ACS Catalysis (2013), 3 (7), 1486-1503CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A review. Water splitting on illuminated semiconductors has long been studied as a potential means of converting solar energy into chem. energy as H2, a clean and renewable energy carrier. Photocatalytic H2O splitting through 2-step photoexcitation using 2 different semiconductor powders and a reversible donor/acceptor pair (so-called shuttle redox mediator) is one of the possible forms of artificial photosynthesis. This system was inspired by natural photosynthesis in green plants and is called the Z-scheme. The development of Z-scheme H2O splitting systems has relied on both finding a new semiconductor photocatalyst that efficiently works in the presence of a shuttle redox mediator and creating active sites to promote surface chem. reactions while suppressing backward reactions involving redox mediators. This review article describes the historical development of photocatalytic H2O splitting systems driven by the Z-scheme principle.
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      Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting Chem. Soc. Rev. 2014, 43, 7520– 7535
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      Chemical Society Reviews (2014), 43 (22), 7520-7535CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Photocatalytic and photoelectrochem. water splitting under irradn. by sunlight has received much attention for prodn. of renewable hydrogen from water on a large scale. Many challenges still remain in improving energy conversion efficiency, such as utilizing longer-wavelength photons for hydrogen prodn., enhancing the reaction efficiency at any given wavelength, and increasing the lifetime of the semiconductor materials. This introductory review covers the fundamental aspects of photocatalytic and photoelectrochem. water splitting. Controlling the semiconducting properties of photocatalysts and photoelectrode materials is the primary concern in developing materials for solar water splitting, because they det. how much photoexcitation occurs in a semiconductor under solar illumination and how many photoexcited carriers reach the surface where water splitting takes place. Given a specific semiconductor material, surface modifications are important not only to activate the semiconductor for water splitting but also to facilitate charge sepn. and to upgrade the stability of the material under photoexcitation. In addn., reducing resistance loss and forming p-n junction have a significant impact on the efficiency of photoelectrochem. water splitting. Correct evaluation of the photocatalytic and photoelectrochem. activity for water splitting is becoming more important in enabling an accurate comparison of a no. of studies based on different systems. In the latter part, recent advances in the water splitting reaction under visible light will be presented with a focus on non-oxide semiconductor materials to give an overview of the various problems and solns.
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      Kim, T. W.; Choi, K.-S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting Science 2014, 343, 990– 994
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      Kim, Tae Woo; Choi, Kyoung-Shin
      Science (Washington, DC, United States) (2014), 343 (6174), 990-994CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Bismuth vanadate (BiVO4) has a band structure that is well-suited for potential use as a photoanode in solar water splitting, but it suffers from poor electron-hole sepn. Here, we demonstrate that a nanoporous morphol. (sp. surface area of 31.8 square meters per g) effectively suppresses bulk carrier recombination without addnl. doping, manifesting an electron-hole sepn. yield of 0.90 at 1.23 V (V) vs. the reversible hydrogen electrode (RHE). We enhanced the propensity for surface-reaching holes to instigate water-splitting chem. by serially applying two different oxygen evolution catalyst (OEC) layers, FeOOH and NiOOH, which reduces interface recombination at the BiVO4/OEC junction while creating a more favorable Helmholtz layer potential drop at the OEC/electrolyte junction. The resulting BiVO4/FeOOH/NiOOH photoanode achieves a photocurrent d. of 2.73 milliamps per square centimeter at a potential as low as 0.6 V vs. RHE.
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      ACS Catalysis (2014), 4 (9), 3013-3019CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Bismuth-based photocatalysts, Bi2WO6, BiVO4, and coupled Bi2WO6/TiO2-P25, have been synthesized by a facile hydrothermal method, characterized, and evaluated for the first time for the selective photooxidn. of methane to methanol. Several conditions were used in order to better comprehend the reaction mechanism. The obtained BiVO4 is, among the others, the most promising photocatalyst for this reaction, displaying higher CH3OH selectivity and being more stable than the others. When Bi2WO6 was coupled with TiO2, the methane conversion increased; however, overoxidn. of CH4 to CO2 predominates. A similar effect is obsd. when electron scavengers such as O2 or Fe3+ were introduced in the photoreactor as a result of the formation of highly oxidant radicals.
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      Cooper, J. K.; Gul, S.; Toma, F. M.; Chen, L.; Glans, P.-A.; Guo, J.; Ager, J. W.; Yano, J.; Sharp, I. D. Electronic Structure of Monoclinic BiVO4 Chem. Mater. 2014, 26, 5365– 5373
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      Electronic Structure of Monoclinic BiVO4
      Cooper, Jason K.; Gul, Sheraz; Toma, Francesca M.; Chen, Le; Glans, Per-Anders; Guo, Jinghua; Ager, Joel W.; Yano, Junko; Sharp, Ian D.
      Chemistry of Materials (2014), 26 (18), 5365-5373CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A comprehensive approach to understanding the electronic structure of monoclinic scheelite bismuth vanadate (ms-BiVO4), including both valence band (VB) and conduction band (CB) orbital character, is presented. D. functional theory (DFT) calcns. are directly compared to exptl. data obtained via X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, resonant inelastic X-ray spectroscopy (RIXS), and XPS to provide a complete portrait of the total and partial d. of states (DOS) near the bandgap. DFT calcns. are presented to confirm the VB max. and CB min. are comprised primarily of O 2p and V 3d orbitals, resp. Predicted triplet d-manifold splitting of V 3d CB states, arising from lone pair-induced lattice distortions, is quantified by V L- and O K-edge XAS. Furthermore, the partial contributions to the total DOS within both the CB and VB, detd. by RIXS, are found to be in excellent agreement with DFT calcns. Energy levels are placed relative to the vacuum level by photoemission spectroscopy, which provides a measure of the work function and electron affinity of the investigated BiVO4 thin film. The implications of the fundamental electronic structure of ms-BiVO4 on its photocatalytic behavior, as well as considerations for improvements by substitutional incorporation of addnl. elements, are discussed.
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      Sun, S.; Wang, W.; Li, D.; Zhang, L.; Jiang, D. Solar Light Driven Pure Water Splitting on Quantum Sized BiVO4 without Any Cocatalyst ACS Catal. 2014, 4, 3498– 3503
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      Solar light driven pure water splitting on quantum sized BiVO4 without any cocatalyst
      Sun, Songmei; Wang, Wenzhong; Li, Dezhi; Zhang, Ling; Jiang, Dong
      ACS Catalysis (2014), 4 (10), 3498-3503CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Photocatalytic water splitting is the most promising process to convert solar energy into high purity chem. fuel (hydrogen), which has received significant attention in recent years. Only several photocatalysts have been reported in the literature for pure water splitting under visible light. Herein we report for the first time quantum sized BiVO4 can decomp. pure water into H2 and O2 simultaneously under simulated solar light irradn. without any cocatalysts or sacrificial reagents. By electrochem. measurement, we demonstrate that the significantly different photocatalytic activity of the quantum sized BiVO4 arises from the neg. shift of conduction band edge by a quantum confinement effect and a decreased overpotential for water redn. Although the generated H2 and O2 are nonstoichiometric in the present study, these findings establish the great potential of using quantum sized BiVO4 photocatalyst and solar energy for overall water splitting.
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      Ma, Y.; Pendlebury, S. R.; Reynal, A.; Le Formal, F.; Durrant, J. R. Dynamics of Photogenerated Holes in Undoped BiVO4 Photoanodes for Solar Water Oxidation Chem. Sci. 2014, 5, 2964– 2973
      12
      Dynamics of photogenerated holes in undoped BiVO4 photoanodes for solar water oxidation
      Ma, Yimeng; Pendlebury, Stephanie R.; Reynal, Anna; Le Formal, Florian; Durrant, James R.
      Chemical Science (2014), 5 (8), 2964-2973CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The dynamics of photogenerated holes in undoped BiVO4 photoanodes for water splitting were studied using transient absorption spectroscopy, correlated with photoelectrochem. and transient photocurrent data. Transient absorption signals of photogenerated holes were identified using electron/hole scavengers and applied elec. bias in a complete photoelectrochem. cell. The yield of long-lived (0.1-1 s) photogenerated holes is obsd. to correlate as a function of applied elec. bias with the width of the space charge layer, as detd. by electrochem. impedance spectroscopy. The transient absorption decay time const. of these long-lived holes is also obsd. to be dependent upon the applied bias, assigned to kinetic competition between water oxidn. and recombination of these surface accumulated holes with bulk electrons across the space charge layer. The time const. for this slow recombination measured with transient absorption spectroscopy is shown to match the time const. of back electron transfer from the external circuit detd. from chopped light transient photocurrent measurements, thus providing strong evidence for these assignments. The yield of water oxidn. detd. from these measurements, including consideration of both the yield of long-lived holes, and the fraction of these holes which are lost due to back electron/hole recombination, is obsd. to be in good agreement with the photocurrent d. measured for BiVO4 photoanodes as a function of bias under continuous irradn. Overall these results indicate two distinct recombination processes which limit photocurrent generation in BiVO4 photoanodes: firstly rapid (≤microseconds) electron/hole recombination, and secondly recombination of surface-accumulated holes with bulk BiVO4 electrons. This second 'back electron transfer' recombination occurs on the milliseconds-seconds timescale, and is only avoided at strong anodic biases where the potential drop across the space charge layer provides a sufficiently large energetic barrier to prevent this recombination process.
    13. 13
      Kweon, K. E.; Hwang, G. S. Surface Structure and Hole Localization in Bismuth Vanadate: A First Principles Study Appl. Phys. Lett. 2013, 103, 131603
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      Surface structure and hole localization in bismuth vanadate. A first principles study
      Kweon, Kyoung E.; Hwang, Gyeong S.
      Applied Physics Letters (2013), 103 (13), 131603/1-131603/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The monoclinic and tetragonal phases of BiVO4 were found to exhibit significantly different photocatalytic activities for water splitting. To assess a possible surface effect on the phase-dependent behavior, we calc. and compare the geometries and electronic structures of the monoclinic and tetragonal BiVO4 (001) surfaces using hybrid d. functional theory. The relaxed at. configurations of these 2 surfaces are found to be nearly identical, while an excess hole shows a relatively stronger tendency to localize at the surface than the bulk in both phases. Possible factors for the phase-dependent photocatalytic activity of BiVO4 are discussed. (c) 2013 American Institute of Physics.
    14. 14
      Kweon, K. E.; Hwang, G. S.; Kim, J.; Kim, S.; Kim, S. Electron Small Polarons and Their Transport in Bismuth Vanadate: A First Principles Study Phys. Chem. Chem. Phys. 2014, 17, 256– 260
      There is no corresponding record for this reference.
    15. 15
      Park, Y.; McDonald, K. J.; Choi, K.-S. Progress in Bismuth Vanadate Photoanodes for Use in Solar Water Oxidation Chem. Soc. Rev. 2013, 42, 2321– 2337
      15
      Progress in bismuth vanadate photoanodes for use in solar water oxidation
      Park, Yiseul; McDonald, Kenneth J.; Choi, Kyoung-Shin
      Chemical Society Reviews (2013), 42 (6), 2321-2337CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Harvesting energy directly from sunlight as nature accomplishes through photosynthesis is a very attractive and desirable way to solve the energy challenge. Many efforts have been made to find appropriate materials and systems that can utilize solar energy to produce chem. fuels. One of the most viable options is the construction of a photoelectrochem. cell that can reduce water to H2 or CO2 to carbon-based mols. Bismuth vanadate (BiVO4) has recently emerged as a promising material for use as a photoanode that oxidizes water to O2 in these cells. Significant advancement in the understanding and construction of efficient BiVO4-based photoanode systems has been made within a short period of time owing to various newly developed ideas and approaches. In this review, the crystal and electronic structures that are closely related to the photoelectrochem. properties of BiVO4 are described first, and the photoelectrochem. properties and limitations of BiVO4 are examd. Subsequently, the latest efforts toward addressing these limitations in order to improve the performances of BiVO4-based photoanodes are discussed. These efforts include morphol. control, formation of composite structures, compn. tuning, and coupling oxygen evolution catalysts. The discussions and insights provided in this review reflect the most recent approaches and directions for general photoelectrode developments and they will be directly applicable for the understanding and improvement of other photoelectrode systems.
    16. 16
      Huang, Z.-F.; Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Nanostructured Bismuth Vanadate-Based Materials for Solar-Energy-Driven Water Oxidation: A Review on Recent Progress Nanoscale 2014, 6, 14044– 14063
      16
      Nanostructured bismuth vanadate-based materials for solar-energy-driven water oxidation: a review on recent progress
      Huang, Zhen-Feng; Pan, Lun; Zou, Ji-Jun; Zhang, Xiangwen; Wang, Li
      Nanoscale (2014), 6 (23), 14044-14063CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      A review. Water oxidn. is the key step for both photocatalytic water splitting and CO2 redn., but its efficiency is very low compared with the photocatalytic redn. of water. Bismuth vanadate (BiVO4) is the most promising photocatalyst for water oxidn. and has become a hot topic for current research. However, the efficiency achieved with this material to date is far away from the theor. solar-to-hydrogen conversion efficiency, mainly due to the poor photo-induced electron transportation and the slow kinetics of oxygen evolution. Fortunately, great breakthroughs have been made in the past five years in both improving the efficiency and understanding the related mechanism. This review is aimed at summarizing the recent exptl. and computational breakthroughs in single crystals modified by element doping, facet engineering, and morphol. control, as well as macro/mesoporous structure construction, and composites fabricated by homo/hetero-junction construction and co-catalyst loading. We aim to provide guidelines for the rational design and fabrication of highly efficient BiVO4-based materials for water oxidn.
    17. 17
      Sun, S.; Wang, W. Advanced Chemical Compositions and Nanoarchitectures of Bismuth Based Complex Oxides for Solar Photocatalytic Application RSC Adv. 2014, 4, 47136– 47152
      17
      Advanced chemical compositions and nanoarchitectures of bismuth based complex oxides for solar photocatalytic application
      Sun, Songmei; Wang, Wenzhong
      RSC Advances (2014), 4 (88), 47136-47152CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      A Review. Bismuth based complex oxides have attracted considerable interest due to their great potential to harvest solar light to solve the current environmental and energy crisis. Bismuth based complex oxides have excellent photo-oxidn. ability for org. contaminant degrdn. and water oxidn. via a photocatalytic process. Many efforts have been made to improve their photocatalytic performance, esp. on the BiVO4, Bi2WO6 and Bi2MoO6 materials, which have been mostly studied in the past few years. Significant progress in understanding the fundamentals and improving the photocatalytic performance has been made due to the various new developed concepts and approaches in recent years. In this review, we present a comprehensive overview on the fundamentals and recent advances of BiVO4, Bi2WO6 and Bi2MoO6 photocatalysts. After the anal. of the structure-property relationships, the strategies that have been employed to enhance their photocatalytic performance are discussed in detail, including morphol. control, surface modification, doping and construction of composite material. Furthermore, remarks on the challenges and perspectives of research directions are proposed for further development of the highly efficient bismuth based complex oxide photocatalysts.
    18. 18
      Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li, C. Spatial Separation of Photogenerated Electrons and Holes among {010} and {110} Crystal Facets of BiVO4 Nat. Commun. 2013, 4, 1432
      18
      Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4
      Li Rengui; Zhang Fuxiang; Wang Donge; Yang Jingxiu; Li Mingrun; Zhu Jian; Zhou Xin; Han Hongxian; Li Can
      Nature communications (2013), 4 (), 1432 ISSN:.
      Charge separation is crucial for increasing the activity of semiconductor-based photocatalysts, especially in water splitting reactions. Here we show, using monoclinic bismuth vanadate crystal as a model photocatalyst, that efficient charge separation can be achieved on different crystal facets, as evidenced by the reduction reaction with photogenerated electrons and oxidation reaction with photogenerated holes, which take place separately on the {010} and {110} facets under photo-irradiation. Based on this finding, the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} facets respectively, resulting in much higher activity in both photocatalytic and photoelectrocatalytic water oxidation reactions, compared with the photocatalyst with randomly distributed cocatalysts. These results show that the photogenrated electrons and holes can be separated between the different facets of semiconductor crystals. This finding may be useful in semiconductor physics and chemistry to construct highly efficient solar energy conversion systems.
    19. 19
      Yang, J.; Wang, D.; Zhou, X.; Li, C. A Theoretical Study on the Mechanism of Photocatalytic Oxygen Evolution on BiVO4 in Aqueous Solution Chemistry 2013, 19, 1320– 1326
      19
      A Theoretical Study on the Mechanism of Photocatalytic Oxygen Evolution on BiVO4 in Aqueous Solution
      Yang, Jingxiu; Wang, Donge; Zhou, Xin; Li, Can
      Chemistry - A European Journal (2013), 19 (4), 1320-1326CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The oxygen evolution reaction (OER) is regarded as one of the key issues in achieving efficient photocatalytic water splitting. Monoclinic scheelite BiVO4 is a visible-light-responsive semiconductor which has proved to be effective for oxygen evolution. Recently, the synthesis of a series of monoclinic BiVO4 single crystals was reported, and it was found that the (010), (110), and (011) facets are highly exposed and that the photocatalytic O2 evolution activity depends on the degree of exposure of the (010) facets. To explore the properties of and photocatalytic water oxidn. reaction on different facets, DFT calcns. were performed to investigate the geometric structure, optical properties, electronic structure, water adsorption, and the whole OER free-energy profiles on BiVO4 (010) and (011) facets. The calcd. results suggest both favorable and unfavorable factors for OER on the (010) and the (011) facets. Due to the combined effects of the above-mentioned factors, different facets exhibit quite different photocatalytic activities.
    20. 20
      Zhao, Z.; Li, Z.; Zou, Z. Structure and Energetics of Low-Index Stoichiometric Monoclinic Clinobisvanite BiVO4 Surfaces RSC Adv. 2011, 1, 874– 883
      20
      Structure and energetics of low-index stoichiometric monoclinic clinobisvanite BiVO4 surfaces
      Zhao, Zongyan; Li, Zhaosheng; Zou, Zhigang
      RSC Advances (2011), 1 (5), 874-883CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      In the present work, d. functional theory calcns. were employed to study the surface properties of several low-index stoichiometric monoclinic clinobisvanite BiVO4 surfaces. Their surface properties were systemically calcd. and described in details, and the similarities and differences between these surfaces were compared and analyzed. Finally, on the basis of calcd. surface energies, the equil. crystal shape of monoclinic clinobisvanite BiVO4 was detd., and its av. surface energy was estd. The calcd. results indicated that the dangling bond d. of the bismuth atom dets. not only the surface energy, but also the surface relaxation.
    21. 21
      Walsh, A.; Butler, K. T. Prediction of Electron Energies in Metal Oxides Acc. Chem. Res. 2014, 47, 364– 372
      There is no corresponding record for this reference.
    22. 22
      Payne, D. J.; Robinson, M. D. M.; Egdell, R. G.; Walsh, A.; McNulty, J.; Smith, K. E.; Piper, L. F. J. The Nature of Electron Lone Pairs in BiVO4 Appl. Phys. Lett. 2011, 98, 212110
      22
      The nature of electron lone pairs in BiVO4
      Payne, D. J.; Robinson, M. D. M.; Egdell, R. G.; Walsh, A.; McNulty, J.; Smith, K. E.; Piper, L. F. J.
      Applied Physics Letters (2011), 98 (21), 212110/1-212110/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The electronic structure of BiVO4 was studied by x-ray photoelectron, x-ray absorption, and x-ray emission spectroscopies, in comparison with d. functional theory calcns. Results confirm both the direct band gap of 2.48 eV and that the Bi 6s electrons hybridize with O 2p to form antibonding lone pair states at the top of the valence band. The results highlight the suitability of combining s2 and d0 cations to produce photoactive ternary oxides. (c) 2011 American Institute of Physics.
    23. 23
      Cheng, J.; Sprik, M. Alignment of Electronic Energy Levels at Electrochemical Interfaces Phys. Chem. Chem. Phys. 2012, 14, 11245– 11267
      23
      Alignment of electronic energy levels at electrochemical interfaces
      Cheng, Jun; Sprik, Michiel
      Physical Chemistry Chemical Physics (2012), 14 (32), 11245-11267CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The position of electronic energy levels in a phase depends on the surface potentials at its boundaries. Bringing two phases in contact at an interface will alter the surface potentials shifting the energy levels relative to each other. Calcg. such shifts for electrochem. interfaces requires a combination of methods from computational surface science and phys. chem. The problem is closely related to the computation of potentials of electrochem. inactive electrodes. These so-called ideally polarizable interfaces are impossible to cross for electrons. In this perspective the authors review two d. functional theory based methods that were developed for this purpose, the work function method and the H insertion method. The key expressions of the two methods are derived from the formal theory of abs. electrode potentials. As an illustration of the work function method the authors review the computation of the potential of zero charge of the Pt(111)-H2O interface as recently published by a no. of groups. The example of the H insertion method is from the authors' own work on the rutile TiO2(110)-H2O interface at the point of zero proton charge. The calcns. are summarized in level diagrams aligning the electronic energy levels of the solid electrode (Fermi level of the metal, valence band max. and conduction band min. of the semiconductor) to the band edges of liq. H2O and the std. potential for the redn. of the hydroxyl radical. All potentials are calcd. at the same level of d. functional theory using the std. H electrode as common energy ref. Comparison to expt. identifies the treatment of the valence band of H2O as a potentially dangerous source of error for application to electrocatalysis and photocatalysis.
    24. 24
      Oshikiri, M.; Boero, M. Water Molecule Adsorption Properties on the BiVO4 (100) Surface 2006, 4, 9188– 9194
      There is no corresponding record for this reference.
    25. 25
      Oshikiri, M.; Boero, M.; Matsushita, A.; Ye, J. Water Molecule Adsorption Properties on Surfaces of MVO4 (M = In, Y, Bi) Photo-Catalysts J. Electroceramics 2007, 22, 114– 119
      There is no corresponding record for this reference.
    26. 26
      Walsh, A.; Yan, Y.; Huda, M. N.; Al-Jassim, M. M.; Wei, S.-H. Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals Chem. Mater. 2009, 21, 547– 551
      26
      Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals
      Walsh, Aron; Yan, Yanfa; Huda, Muhammad N.; Al-Jassim, Mowafak M.; Wei, Su-Huai
      Chemistry of Materials (2009), 21 (3), 547-551CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The authors report the 1st-principles electronic structure of BiVO4, a promising photocatalyst for H generation. BiVO4 is a direct band gap semiconductor, despite having band extrema away from the Brillouin zone center. Coupling between Bi 6s and O 2p forces an upward dispersion of the valence band at the zone boundary; however, a direct gap is maintained via coupling between V 3d, O 2p, and Bi 6p, which lowers the conduction band min. These interactions result in sym. hole and electron masses. Implications for the design of ambipolar metal oxides are discussed.
    27. 27
      Zhong, M.; Hisatomi, T.; Kuang, Y.; Zhao, J.; Liu, M.; Iwase, A.; Jia, Q.; Nishiyama, H.; Minegishi, T.; Nakabayashi, M. Surface Modification of the CoOx Loaded BiVO4 Photoanodes with Ultrathin p-Type NiO Layers for the Improved Solar Water Oxidation J. Am. Chem. Soc. 2015, 137, 5053– 5060
      There is no corresponding record for this reference.
    28. 28
      Ravensbergen, J.; Abdi, F. F.; van Santen, J. H.; Frese, R. N.; Dam, B.; van de Krol, R.; Kennis, J. T. M. Unraveling the Carrier Dynamics of BiVO4: A Femtosecond to Microsecond Transient Absorption Study J. Phys. Chem. C 2014, 118, 27793– 27800
      28
      Unraveling the Carrier Dynamics of BiVO4: A Femtosecond to Microsecond Transient Absorption Study
      Ravensbergen, Janneke; Abdi, Fatwa F.; van Santen, Judith H.; Frese, Raoul N.; Dam, Bernard; van de Krol, Roel; Kennis, John T. M.
      Journal of Physical Chemistry C (2014), 118 (48), 27793-27800CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Bi vanadate (BiVO4) is a promising semiconductor material for photoelectrochem. H2O splitting showing good visible light absorption and a high photochem. stability. To improve the performance of BiVO4, it is of key importance to understand its photophysics upon light absorption. Here the authors study the carrier dynamics of BiVO4 prepd. by the spray pyrolysis method using broadband transient absorption spectroscopy (TAS), in thin films as well as in a photoelectrochem. (PEC) cell under H2O-splitting conditions. The use of a dual-laser setup consisting of electronically synchronized Ti:sapphire amplifiers enable one to measure the femtosecond to microsecond time scales in a single expt. The authors propose a model of carrier dynamics that includes relaxation and trapping rates for electrons and holes. Hole trapping occurs in multiple phases, with the majority of the photogenerated holes being trapped with a time const. of 5 ps and a small fraction of this hole trapping taking place within the instrument response of 120 fs. The induced absorption band that represents the trapped holes is modulated by an oscillation of 63 cm-1, which is assigned to the coupling of holes to a phonon mode. Electrons to undergo a relaxation with a time const. of 40 ps, followed by deeper trapping on the 2.5 ns time scale were found. On time scales longer than 10 ns, trap-limited recombination that follows a power law is found, spanning time scales up to microseconds. Finally, the authors observe no spectral or kinetic differences by applying a bias voltage to the PEC cell, indicating that the effect of a voltage and the charge transfer processes between BiVO4 and the electrolyte occurs on longer time scales. Results therefore provide new insights into the carrier dynamics of BiVO4 and further expand the application window of TAS as an anal. tool for photoanode materials.
    29. 29
      Otani, M.; Hamada, I.; Sugino, O.; Morikawa, Y.; Okamoto, Y.; Ikeshoji, T. Electrode Dynamics from First Principles J. Phys. Soc. Jpn. 2008, 77, 024802
      29
      Electrode dynamics from first principles
      Otani, Minoru; Hamada, Ikutaro; Sugino, Osamu; Morikawa, Yoshitada; Okamoto, Yasuharu; Ikeshoji, Tamio
      Journal of the Physical Society of Japan (2008), 77 (2), 024802/1-024802/6CODEN: JUPSAU; ISSN:0031-9015. (Physical Society of Japan)
      The study of electrode dynamics was a major topic in the field of electrochem. for a century. Electrode dynamics consist of electron transfer reactions that give rise to, or are caused by, a bias voltage, and are influenced by surface catalysis, electrolyte soln., transport of electrons and ions. The 1st-principles mol. dynamics simulation of the electrochem. system was hampered by the difficulty to describe the bias voltage and the complex soln.-electrode interface structure. Here the authors use a new algorithm called the effective screening medium to characterize the biased interface between Pt and liq. H2O, revealing the microscopic details of the 1st, Volmer, step of the Pt-catalyzed hydrogen evolution reaction. By clarifying the important roles played by both the H2O and the bias, the authors show why this reaction occurs so efficiently at the interface. The simulations make a significant step towards a deeper understanding of electrochem. reactions.
    30. 30
      Schnur, S.; Groß, A. Challenges in the First-Principles Description of Reactions in Electrocatalysis Catal. Today 2011, 165, 129– 137
      30
      Challenges in the first-principles description of reactions in electrocatalysis
      Schnur, Sebastian; Gross, Axel
      Catalysis Today (2011), 165 (1), 129-137CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      In spite of the strong relevance of reactions in electrocatalysis, in particular for the electrochem. energy conversion and storage, the no. of theor. studies addressing electrocatalytic reactions from 1st principles is still limited. This is due to the fact that there are two factors adding considerable complexity to the theor. treatment: the presence of the electrolyte at the electrode surface and varying electrode potentials. Still, there are promising approaches to cope with these problems allowing a realistic theor. description of reactions in electrocatalysis. It will be demonstrated that ab initio mol. dynamics simulations based on periodic d. functional theory calcns. can contribute to an understanding of the structures and reactions at H2O/metal interfaces. To model varying electrode potentials, an explicit counter electrode was implemented in a periodic d. functional theory code, and 1st preliminary results using this implementation will be presented.
    31. 31
      Eisenberg, D.; Ahn, H. S.; Bard, A. J. Enhanced Photoelectrochemical Water Oxidation on Bismuth Vanadate by Electrodeposition of Amorphous Titanium Dioxide J. Am. Chem. Soc. 2014, 136, 14011– 14014
      31
      Enhanced photoelectrochemical water oxidation on bismuth vanadate by electrodeposition of amorphous titanium dioxide
      Eisenberg, David; Ahn, Hyun S.; Bard, Allen J.
      Journal of the American Chemical Society (2014), 136 (40), 14011-14014CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      N-BiVO4 is a promising semiconductor material for photoelectrochem. water oxidn. Although most thin-film syntheses yield discontinuous BiVO4 layers, back redn. of photo-oxidized products on the conductive substrate has never been considered as a possible energy loss mechanism in the material. We report that a 15 s electrodeposition of amorphous TiO2 (a-TiO2) on W:BiVO4/F:SnO2 blocks this undesired back redn. and dramatically improves the photoelectrochem. performance of the electrode. Water oxidn. photocurrent increases by up to 5.5 times, and its onset potential shifts neg. by ∼500 mV. In addn. to blocking soln.-mediated recombination at the substrate, the a-TiO2 film-which is found to lack any photocatalytic activity in itself-is hypothesized to react with surface defects and deactivate them toward surface recombination. The proposed treatment is simple and effective, and it may easily be extended to a wide variety of thin-film photoelectrodes.

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