Facet-Engineered BiVO4 Photocatalysts for Water Oxidation: Lifetime Gain Versus Energetic Loss

A limiting factor to the efficiency of water Oxygen Evolution Reaction (OER) in metal oxide nanoparticle photocatalysts is the rapid recombination of holes and electrons. Facet-engineering can effectively improve charge separation and, consequently, OER efficiency. However, the kinetics behind this improvement remain poorly understood. This study utilizes photoinduced absorption spectroscopy to investigate the charge yield and kinetics in facet-engineered BiVO4 (F-BiVO4) compared to a non-faceted sample (NF-BiVO4) under operando conditions. A significant influence of preillumination on hole accumulation is observed, linked to the saturation and, thus, passivation of deep and inactive hole traps on the BiVO4 surface. In DI-water, F-BiVO4 shows a 10-fold increase in charge accumulation (∼5 mΔOD) compared to NF-BiVO4 (∼0.5 mΔOD), indicating improved charge separation and stabilization. With the addition of Fe(NO3)3, an efficient electron acceptor, F-BiVO4 demonstrates a 30-fold increase in the accumulation of long-lived holes (∼45 mΔOD), compared to NF-BiVO4 (∼1.5 mΔOD) and an increased half-time, from 2 to 10 s. Based on a simple kinetic model, this increase in hole accumulation suggests that facet-engineering causes at least a 50-100 meV increase in band bending in BiVO4 particles, thereby stabilizing surface holes. This energetic stabilization/loss results in a retardation of OER relative to NF-BiVO4. This slower catalysis is, however, offset by the observed increase in density and lifetime of photoaccumulated holes. Overall, this work quantifies how surface faceting can impact the kinetics of long-lived charge accumulation on metal oxide photocatalysts, highlighting the trade-off between lifetime gain and energetic loss critical to optimizing photocatalytic efficiency.

Prediction of Feasibility of Polaronic OER on (110) Surface of Rutile TiO2

The polaronic effects at the atomic level hold paramount significance for advancing the efficacy of transition metal oxides in applications pertinent to renewable energy. The lattice-distortion mediated localization of photoexcited carriers in the form of polarons plays a pivotal role in the photocatalysis. This investigation focuses on rutile TiO2, an important material extensively explored for solar energy conversion in artificial photosynthesis, specifically targeting the generation of green H2 through photoelectrochemical (PEC) H2O splitting. By employing Hubbard-U corrected and hybrid density functional theory (DFT) methods, we systematically probe the polaronic effects in the catalysis of oxygen evolution reaction (OER) on the (110) surface of rutile TiO2. Theoretical understanding of polarons within the surface, coupled with simulations of OER at distinct titanium (Ti) and oxygen (O) active sites, reveals diverse polaron formation energies within the lattice sites with strong preference for bulk and surface bridge (Ob) oxygen sites. Moreover, we provide the evidence for the facilitative role of polarons in OER. We find that hole polarons situated at the equatorial oxygen sites near the Ti-active site, along with bridge site hole polarons distal from the Ob active site yield a small reduction in OER overpotential by ~0.06 eV and ~0.12 eV, respectively. However, subsurface, equatorial, and bridge site hole polarons significantly reduce the Ti-active site OER overpotential by ~0.4 eV through the peroxo-type oxygen pathway. We also observe that the presence of hole polarons stabilizes the *OH, *O, and *OOH intermediate species compared to the scenario without hole polarons. Overall, this study provides a detailed mechanistic insight into polaron-mediated OER, offering a promising avenue for improving the catalytic activity of transition metal oxide-based photocatalysts catering to renewable energy requisites.

Insight into the Crystal Facet Effect of {101} and {100} Facets of CeVO4 in the Photochemical Property and Photocatalysis

To investigate the photochemical property of specific crystal facets, two well-defined CeVO₄ dodecahedrons with exposed {101} and {100} facets are prepared, which have distinguishing appearances and unequal {101}/{100} area ratios (A_{101}/A_{100}), i.e., compressed dodecahedra (CeVO₄ CD, A_{101}/A_{100} ≈ 1) and elongated dodecahedra (CeVO₄ ED, A_{101}/A_{100} ≈ 0.3). During the visible-light-irradiated process, the {101} and {100} facets are certified to selectively deposit photogenerated holes (h⁺) and electrons (e^-), thus exhibiting the photooxidability and photoreducibility, respectively. Meanwhile, a surface heterojunction could form at the adjacent facet interface and facilitate the spatial separation of carriers. Benefiting from the large exposure extent of the {101} facet and the rational A_{101}/A_{100} (∼1), the CeVO₄ CD shows a superior photocatalytic performance for the degradation of tetracycline to the CeVO₄ ED. Finally, simulation calculations reveal that the energy deviations of the valence band (VB) and conduction band (CB) between CeVO₄{101} and CeVO₄{100} impel the photogenerated h⁺ and e^- to transfer in opposite directions, resulting in the facet-dependent photoactivity of the CeVO₄ dodecahedron.

The role of crystal facets and disorder on photo-electrosynthesis

Photoelectrochemistry has the potential to play a crucial role in the storage of solar energy and the realisation of a circular economy. From a chemical viewpoint, achieving high conversion efficiencies requires subtle control of the catalyst surface and its interaction with the electrolyte. Traditionally, such control has been hard to achieve in the complex multinary oxides used in PEC devices and consequently the mechanisms by which surface exposed facets influence light-driven catalysts are poorly understood. Yet, this understanding is critical to further improve conversion yields and fine-tune reaction selectivities. Here, we review the impact that crystal facets and disorder have on photoelectrochemical reactivity. In particular, we discuss how the crystal orientation influences the energetics of the surface, the existence of defects and the transport of reactive charges, ultimately dictating the PEC activity. Moreover, we evaluate how facet stability dictates the tendency of the solid to undergo reconstructions during catalytic processes and highlight the experimental and computational challenges that must be overcome to characterise the role of the exposed facets and disorder in catalytic performance.

Bimolecular Self-Trapped Exciton Formation in Bismuth Vanadate

Bismuth vanadate (BiVO₄) is a promising photoanode material for solar-driven water splitting, and knowledge of the photocarrier dynamics in BiVO₄ could offer guidance to propel the development of the photoanode performance. Herein, we uncovered the nature of various photogenerated transient species in BiVO₄ and extracted their respective dynamics. We found spectral and dynamic evidence that the electrons in the conduction band collapsed into severely localized small electron polarons on a subpicosecond time scale, while the holes in the valence band remained delocalized and accounted for the photoconductivity. In the following tens to hundreds of picoseconds, the electron polaron captured the hole to form a self-trapped exciton via a bimolecular reaction mechanism, and in consequence, the hole was immobilized. Our finding suggests that exciton dissociation strategies should be taken into account in the design of the BiVO₄-based water-splitting applications in order to enhance charge transport and suppress charge recombination.

Influence of Excess Charge on Water Adsorption on the BiVO4(010) Surface

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO₄(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO₄(010) surface stabilizes the formation of a small electron polaron on the VO₄ tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V⁴⁺ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure.

Effect of water-oil ratio on the photocatalytic performance of visible light-active BiVO4 nanoparticles prepared by inverse microemulsion-calcination method

Nowadays, nano-photocatalysts (NPs) have become the research focus in the field of photocatalysis due to their excellent photocatalytic activity, and microemulsion is an effective method to prepare high-efficiency nano-photocatalysts. Here, BiVO₄ NPs with high efficiency under visible light were prepared by a combination of reverse microemulsion method and calcination method. XRD, SEM, TEM, XPS, DRS, PL, BET and other characterization tests were used to comprehensively explore the influence of water-oil ratio on the physicochemical properties of the catalysts. The results show that BiVO₄ NPs of monoclinic scheelite with high crystallization degree can be obtained by this method. The microscopic morphology, specific surface area and total pore volume of BiVO₄ NPs are significantly affected by the water-oil ratio. It is difficult to obtain BiVO₄ NPs with small particle size and uniform dispersion under the condition of too low or too high water-oil ratio. Meanwhile, the photogenerated carrier recombination efficiency of the catalyst is significantly improved, thus reducing the photocatalytic activity of the catalyst. Strikingly, the BiVO₄ NPs obtained under the condition of water-oil ratio is 20 exhibited well-dispersed nanospheres with diameters ranging from 80 to 100 nm. It has the highest photocatalytic activity due to its high crystallinity, large specific surface area and total pore volume and relatively low photogenerated carrier recombination efficiency. Under visible light irradiation, the degradation efficiency of RhB can reach 97.69% in 100 min, and the rate constant is 0.03253 min^-1.

Solution-based synthesis of wafer-scale epitaxial BiVO4 thin films exhibiting high structural and optoelectronic quality

We demonstrate a facile approach to solution-based synthesis of wafer-scale epitaxial bismuth vanadate (BiVO₄) thin films by spin-coating on yttria-stabilized zirconia. Epitaxial growth proceeds via solid-state transformation of initially formed polycrystalline films, driven by interface energy minimization. The (010)-oriented BiVO₄ films are smooth and compact, possessing remarkably high structural quality across complete 2'' wafers. Optical absorption is characterized by a sharp onset with a low sub-band gap response, confirming that the structural order of the films results in correspondingly high optoelectronic quality. This combination of structural and optoelectronic quality enables measurements that reveal a strong optical anisotropy of BiVO₄, which leads to significantly increased in-plane optical constants near the fundamental band edge that are of particular importance for maximizing light harvesting in semiconductor photoanodes. Temperature-dependent transport measurements confirm a thermally activated hopping barrier of ∼570 meV, consistent with small electron polaron conduction. This simple approach for synthesis of high-quality epitaxial BiVO₄, without the need for complex deposition equipment, enables a broadly accessible materials base to accelerate research aimed at understanding and optimizing photoelectrochemical energy conversion mechanisms.

Recent advances and perspectives for solar-driven water splitting using particulate photocatalysts

The conversion and storage of solar energy to chemical energy via artificial photosynthesis holds significant potential for optimizing the energy situation and mitigating the global warming effect. Photocatalytic water splitting utilizing particulate semiconductors offers great potential for the production of renewable hydrogen, while this cross-road among biology, chemistry, and physics features a topic with fascinating interdisciplinary challenges. Progress in photocatalytic water splitting has been achieved in recent years, ranging from fundamental scientific research to pioneering scalable practical applications. In this review, we focus mainly on the recent advancements in terms of the development of new light-absorption materials, insights and strategies for photogenerated charge separation, and studies towards surface catalytic reactions and mechanisms. In particular, we emphasize several efficient charge separation strategies such as surface-phase junction, spatial charge separation between facets, and polarity-induced charge separation, and also discuss their unique properties including ferroelectric and photo-Dember effects on spatial charge separation. By integrating time- and space-resolved characterization techniques, critical issues in photocatalytic water splitting including photoinduced charge generation, separation and transfer, and catalytic reactions are analyzed and reviewed. In addition, photocatalysts with state-of-art efficiencies in the laboratory stage and pioneering scalable solar water splitting systems for hydrogen production using particulate photocatalysts are presented. Finally, some perspectives and outlooks on the future development of photocatalytic water splitting using particulate photocatalysts are proposed.

Theoretical calculations of the effect of nitrogen substitution on the structural, vibrational, and electronic properties of wolframite-type ScTaO4 at ambient conditions

In this study, the effect of nitrogen substitution in wolframite-type ScTaO₄ was investigated using density-functional theory calculations. First, structural and mechanical properties, as well as the dynamical stability of ScTaO₄ were examined deeply for the ambient-pressure structure. Subsequently, we studied how lattice vibrations are affected by hydrostatic pressure and determined the elastic moduli of ScTaO₄. The results of our study show that the monoclinic structure of ScTaO₄ is rigid and non-compressible. In addition, band-structure calculations show that ScTaO₄ has a wide direct band-gap of 4.04 eV, which in turn leads to a possible tuning of electronic properties. We have found that this task can be conducted by partially substituting oxygen atoms in the unit cell with nitrogen atoms. Both band-structure calculations and charge-density analyses revealed a narrowing in the band gap caused by the presence of nitrogen atoms, which act as a shallow acceptor state, resulting in weak repulsive interactions and structural distortions in both Sc and Ta coordination polyhedra; reducing the crystal symmetry from monoclinic to triclinic.

Spatial Separation of Photogenerated Charges on Well-Defined Bismuth Vanadate Square Nanocrystals

Crystal facet engineering has been recognized as a powerful strategy to finely modulate the charge separation behavior in semiconductor photocatalysis; however, disclosing the intrinsic roles that the morphologies and crystal facets play on photogenerated charge separation of semiconductor nanocrystals remains elusive. Herein, exemplified on the typical visible-light-responsive photocatalyst bismuth vanadate (BiVO₄ ), for the first time, the successful fabrication is reported of well-defined BiVO₄ square nanocrystals with precisely controllable (040)/(200) facet proportion, which undergo a dissolution-recrystallization-facet growth process accompanied with tetragonal to monoclinic phase transition. Spatial separation of photogenerated electrons and holes has been evidently demonstrated to take place between (040) and (200) facets of BiVO₄ nanocrystals, on which the charge separation efficiency is verified to definitely depend on the facet proportion of (040)/(200). Further theoretical simulation reveals that the matching degree of charge collection length and crystal configuration is considered to be the major factor determining charge separation efficiency of BiVO₄ nanocrystals. This study presents a strategy to fabricate morphology-tailored semiconductors, which will be favorable to advance the understanding of spatial charge separation in semiconductor photocatalysis.

Lessons learned from first-principles calculations of transition metal oxides

Transition metal oxides (TMOs) are an important class of materials with diverse applications, ranging from memristors to photoelectrochemical cells. First-principles calculations are critical for understanding these complex materials at an atomic level and establishing relationships between atomic and electronic structures, particularly for probing quantities difficult or inaccessible to experiment. Here, we discuss computational strategies used to understand TMOs by focusing on two examples, a photoanode material, BiVO₄, and an oxide for low-power electronics, La_1-xSr_xCoO₃. We highlight key aspects required for the modeling of TMOs, namely, the descriptions of how oxygen vacancies, extrinsic doping, the magnetic state, and polaron formation impact their electronic and atomic structures and, consequently, many of the observed properties.

Role of surface termination in forming type-II photocatalyst heterojunctions: the case of TiO2/BiVO4

In this work we investigate TiO₂ and BiVO₄ nanostructures by means of density functional theory (DFT) calculations, to provide an estimate of the band alignment in TiO₂/BiVO₄ interfaces, highly active in photo-electrochemistry and photocatalytic water splitting. Calculations were carried out with both DFT range separated and self-consistent dielectric dependent hybrid functionals (HSE06 and PBE0_DD). The impact of systems' size has been investigated. The converged electronic levels of TiO₂ and BiVO₄ surfaces have been used to predict the band alignment in TiO₂/BiVO₄ heterostructures. Results indicated that when TiO₂ (101) surface is matched with BiVO₄ (110), a type-II alignment is obtained, where the band edges of BiVO₄ are higher in energy that those of TiO₂. This picture is favorable for charge-carriers separation upon photoexcitation, where electrons move toward TiO₂ and holes toward BiVO₄. On the contrary, if TiO₂ (101) is interfaced to BiVO₄ (010) the offset between the band edges is negligible, thus reducing the driving force toward separation of charge carriers. These results rationalize the dependence on the facet's exposure of the observed photocatalytic performances of TiO₂/BiVO₄ composites, where the TiO₂ (101)/BiVO₄ (110) interface outperforms the TiO₂ (101)/BiVO₄ (010) one.

Sunlight Selective Photodeposition of CoOx(OH)y and NiOx(OH)y on Truncated Bipyramidal BiVO4 for Highly Efficient Photocatalysis

Facet-engineered monoclinic scheelite BiVO₄ particles decorated with various cocatalysts were successfully synthesized by selective sunlight photodeposition of metal or metal oxy(hydroxide) nanoparticles onto the facets of truncated bipyramidal BiVO₄ monoclinic crystals coexposing {010} and {110} facets. X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning Auger microscopy revealed that metallic silver (Ag) and cobalt (oxy)hydroxide (CoO_x(OH)_y) particles were selectively deposited onto the {010} and {110} facets, respectively, regardless of the cocatalyst amount. By contrast, the nickel (oxy)hydroxide (NiO_x(OH)_y) photodeposition depends on the nickel precursor amount with an unprecedented selectivity for 0.1 wt % NiO_x(OH)_y/BiVO₄ with a preferential deposition onto the {010} facets and the edges between the {110} facets. Moreover, these noble metal-free heterostructures led to remarkable photocatalytic properties for rhodamine B photodecomposition and sacrificial water oxidation reactions. For instance, 0.2 wt % CoO_x(OH)_y/BiVO₄ led to one of the highest oxygen evolution rates, i.e., 1538 μmol h^-1 g^-1, ever described which is ten times higher than that found for bare BiVO₄. The selective deposition of cobalt (oxy)hydroxide species onto the more electron-deficient facet of truncated bipyramidal monoclinic BiVO₄ particles favors photogenerated charge carrier separation and therefore plays a key role for efficient photochemical oxygen evolution.

Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo-BiVO4 Thin Films

The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO₄ . By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V₂ O₅ at grain boundaries of Mo-BiVO₄ thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO₄ . After photo-electrochemical operation, the degraded Mo-BiVO₄ films show similar grain center and grain boundary spectra indicating V₂ O₅ dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.

Significant Impact of Exposed Facets on the BiVO4 Material Performance for Photocatalytic Water Splitting Reactions

The impact of the four predominant (010), (110), (001), and (121) exposed facets obtained experimentally for monoclinic BiVO₄ on its photocatalytic performance for water splitting reactions is investigated on the basis of the hybrid density functional theory including the spin-orbit coupling. Although their electronic structure is similar, their transport and redox properties reveal anisotropic characters based on the crystal orientation and termination. The particular role of each facet in proton reduction was correlated with the surface Bi coordination number and their geometrical distribution. Our work predicts the (001) facet as the only good candidate for both HER and OER, while the (010) facet is a fitting candidate for OER only. The (110) and (121) surfaces are acceptable candidates only for OER but less potential than (001) and (010). These outcomes will efficiently conduct experimentalists for an attentive design of facet-oriented BiVO₄ samples toward improving water oxidation and proton reduction.

Carbon Monoxide Oxidation Promoted by Surface Polarization Charges in a CuO/Ag Hybrid Catalyst

Composite structures have been widely utilized to improve material performance. Here we report a semiconductor-metal hybrid structure (CuO/Ag) for CO oxidation that possesses very promising activity. Our first-principles calculations demonstrate that the significant improvement in this system's catalytic performance mainly comes from the polarized charge injection that results from the Schottky barrier formed at the CuO/Ag interface due to the work function differential there. Moreover, we propose a synergistic mechanism underlying the recovery process of this catalyst, which could significantly promote the recovery of oxygen vacancy created via the M-vK mechanism. These findings provide a new strategy for designing high performance heterogeneous catalysts.

Photocatalytic Oxygen Evolution from Water Splitting

Photocatalytic water splitting has attracted a lot of attention in recent years, and O2 evolution is the decisive step owing to the complex four-electrons reaction process. Though many studies have been conducted, it is necessary to systematically summarize and introduce the research on photocatalytic O2 evolution, and thus a systematic review is needed. First, the corresponding principles about O2 evolution and some urgently encountered issues based on the fundamentals of photocatalytic water splitting are introduced. Then, several types of classical water oxidation photocatalysts, including TiO2, BiVO4, WO3, α-Fe2O3, and some newly developed ones, such as Sillén-Aurivillius perovskites, porphyrins, metal-organic frameworks, etc., are highlighted in detail, in terms of their crystal structures, synthetic approaches, and morphologies. Third, diverse strategies for O2 evolution activity improvement via enhancing photoabsorption and charge separation are presented, including the cocatalysts loading, heterojunction construction, doping and vacancy formation, and other strategies. Finally, the key challenges and future prospects with regard to photocatalytic O2 evolution are proposed. The purpose of this review is to provide a timely summary and guideline for the future research works for O2 evolution.

Transient IR spectroscopy as a tool for studying photocatalytic materials

Over the years, a considerable amount of attention has been given to the thermodynamics of photocatalysts, i.e. to the location of their valence and conduction bands on the energy scale. The kinetics of the photoinduced charge carriers at short times (i.e. prior to their surface redox reactions) is no less important. While significant work on the transient electronic spectra of photocatalysts has been performed, the transient vibrational spectra of this class of materials was hardly studied. This manuscript aims to increase the scientific awareness to the potential of transient IR spectroscopy (TRIR) as a complementary tool for understanding the first, crucial, steps of photocatalytic processes in solid photocatalysts. This was done herein first by describing the various techniques currently in use for measuring transient IR signals of photo-excited systems and discussing their pros and cons. Then, a variety of examples is given, representing different types of photocatalysts such as oxides (TiO₂, NaTaO₃, BiOCl, BiVO₄), photosensitized oxides (dye-sensitized TiO₂), organic polymers (graphitic carbon nitride) and organo-metalic photocatalysts (rhenium bipyridyl complexes). These examples span from materials with no IR fingerprint signals (TiO₂) to materials having a distinct spectrum showing well-defined, localized, relatively narrow, vibrational bands (carbon nitride). In choosing the given-above examples, care was made to represent the several pump & probe techniques that are applied when studying transient IR spectroscopy, namely dispersive, transient 2D-IR spectroscopy and step-scan IR spectroscopy. It is hoped that this short review will contribute to expanding the use of TRIR as a viable and important technique among the arsenal of tools struggling to solve the mysteries behind photocatalysis.

Strong Hole Trapping Due to Oxygen Dimers in BiVO4: Effect on the Water Oxidation Reaction

We present a study of hole bipolarons in BiVO₄. We show that in the presence of two holes O-O dimers are formed, leading to strong charge trapping. While the formation of bipolarons in bulk BiVO₄ requires overcoming a kinetic barrier, we find that these defects should be spontaneously formed at the surface of the material and its interface with water. Through molecular dynamics simulations, we study the effect of bipolarons on the water-splitting reaction and show that their presence may be especially beneficial in alkaline conditions.

Theoretical treatment of semiconductor heterojunctions for photocatalysis: the WO3/BiVO4 interface

The valence and conduction band (CB) alignments and the nature of the WO₃/BiVO₄ heterojunction have been analysed in detail based on hybrid functionals density functional theory calculations. The WO₃/BiVO₄ junction is widely studied in photocatalysis for its capability to reduce electron-hole recombination and to improve efficiency. This is assumed to be due to a favourable band alignment of the junction's components, which generates a flow of negative charge carriers towards WO₃, and positive ones towards BiVO₄. This conclusion is often based on the properties of the two isolated, non-interacting units. Here, we propose an explicit interface model where the (0 0 1) surface of WO₃ is put in contact with the (0 1 0) surface of BiVO₄ rotated by about 45°, which leads to a small strain and a favourable cation-anion matching. The interface displays a moderate charge transfer and a small interface dipole. This leads to only moderate effects on the band alignment, which remains qualitatively similar to that obtained from the two independent oxides. We also considered in detail the role of the amount of exact exchange used in the description of the heterojunction, and in particular of the BiVO₄ component, for which rather different hybrid functional approaches have been proposed in the literature.

Template-Free Nanostructured Fluorine-Doped Tin Oxide Scaffolds for Photoelectrochemical Water Splitting

The synthesis and characterization of highly stable and conductive F:SnO₂ (FTO) nanopyramid arrays are investigated, and their use as scaffolds for water splitting is demonstrated. Current densities during the oxygen evolution reaction with a NiFeO_x catalyst at 2 V vs reversible hydrogen electrode were increased 5-fold when substituting commercial FTO (TEC 15) by nanostructured FTO scaffolds. In addition, thin α-Fe₂O₃ films (∼50 nm thick) were employed as a proof of concept to show the effect of our nanostructured scaffolds during photoelectrochemical water splitting. Double-layer capacitance measurements showed a drastic increase of the relative electrochemically active surface area for the nanostructured samples, in agreement with the observed photocurrent enhancement, whereas UV-vis spectroscopy indicates full absorption of visible light at wavelengths below 600 nm.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Photoelectrochemical (PEC) cells for solar-energy conversion have received immense interest as a promising technology for renewable hydrogen production. Their similarity to natural photosynthesis, utilizing sunlight and water, has provoked intense research for over half a century. Among many potential photocatalysts, BiVO₄ , with a bandgap of 2.4-2.5 eV, has emerged as a highly promising photoanode material with a good chemical stability, environmental inertness, and low cost. Unfortunately, its charge transport properties are modest, at most a hole diffusion length (L_p ) of ≈70 nm. However, recent rapid developments in multiple modification strategies have elevated it to a position as the most promising metal oxide photoanode material. This review summarizes developments in BiVO₄ photoanodes in the past 10 years, in which time it has continuously broken its own performance records for PEC water oxidation. Effective modification techniques are discussed, including synthesis of nanostructures/nanopores, external/internal doping, heterojunction fabrication, surface passivation, and cocatalysts. Tandem systems for unassisted solar water splitting and PEC production of value-added chemicals are also discussed.

One-Pot Synthesis of BiCuSO Nanosheets under Ambient Atmosphere as Broadband Spectrum Photocatalyst

Cuprous based chalcogenides have attracted intensive research interest due to the potential applications in solar energy conversion. However, typical fabrications of these compounds are often carried out under severe conditions, such as inert gas protection, high vacuum, and/or extreme high temperature. Here we reported a one-pot process for cuprous based chalcogenides synthesis in aqueous solution. A strategy for BiCuSO nanosheets fabrication without toxic chemicals or rigorous reagents at pretty low temperatures under an ambient atmosphere was established, with the practicality of morphology controlling and the compatibility of multifarious precursors. Platelike BiCuSO with a thickness range from several to hundreds nanometers are fabricated by adjusting the alkali concentration, reaction time, and temperature. The positive effect of alkali hydroxide concentration is proposed cautiously based on the experimental results. The photocatalytic activities of BiCuSO nanosheet under UV, visible, and near-infrared irradiation were also investigated. BiCuSO obtained at room temperature with a thickness of 4.5 nm showed the most impressive efficiency to decompose organic contaminants. Our research presented a new way for cuprous sulfides fabrication, and might open up a new vista for large-scale synthesis of cuprous based materials as promising broadband spectrum light-absorbing materials.

Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.

Photoelectrochemical Device Designs toward Practical Solar Water Splitting: A Review on the Recent Progress of BiVO4 and BiFeO3 Photoanodes

Solar-driven water splitting technology is considered to be a promising solution for the global energy challenge as it is capable of generating clean chemical fuel from solar energy. Various strategies and catalytic materials have been explored in order to improve the efficiency of the water splitting reaction. Although significant progress has been made, there are many intriguing fundamental phenomena that need to be understood. Herein, we review recent experimental efforts to demonstrate enhancement strategies for efficient solar water splitting, especially for the light absorption, charge carrier separation, and water oxidation kinetics. We also focus on the state of the art of photoelectrochemical (PEC) device designs such as application of facet engineering and the development of a ferroelectric-coupled PEC device. Based on these experimental achievements, future challenges, and directions in solar water splitting technology will be discussed.

Nanostructured shuriken-like BiVO4 with preferentially exposed {010} facets: preparation, formation mechanism, and enhanced photocatalytic performance

The synthesis, formation mechanism, and application of nanostructured shuriken-like BiVO₄ with preferentially exposed {010} facets have been reported.

Strategies for Efficient Charge Separation and Transfer in Artificial Photosynthesis of Solar Fuels

Converting sunlight to solar fuels by artificial photosynthesis is an innovative science and technology for renewable energy. Light harvesting, photogenerated charge separation and transfer (CST), and catalytic reactions are the three primary steps in the processes involved in the conversion of solar energy to chemical energy (SE-CE). Among the processes, CST is the key "energy pump and delivery" step in determining the overall solar-energy conversion efficiency. Efficient CST is always high priority in designing and assembling artificial photosynthesis systems for solar-fuel production. This Review not only introduces the fundamental strategies for CST but also the combinatory application of these strategies to five types of the most-investigated semiconductor-based artificial photosynthesis systems: particulate, Z-scheme, hybrid, photoelectrochemical, and photovoltaics-assisted systems. We show that artificial photosynthesis systems with high SE-CE efficiency can be rationally designed and constructed through combinatory application of these strategies, setting a promising blueprint for the future of solar fuels.

Implicating the contributions of surface and bulk states on carrier trapping and photocurrent performance of BiVO4 photoanodes

We report the correlation of the surface and bulk structure with carrier dynamics and photocurrent performance of bare BiVO₄ photoanodes.

Capturing local structure modulations of photoexcited BiVO4 by ultrafast transient XAFS

Femto- and picosecond transient XAFS revealed the photoexcitation dynamics of BiVO₄.

Facet-Engineered Surface and Interface Design of Photocatalytic Materials

The facet-engineered surface and interface design for photocatalytic materials has been proven as a versatile approach to enhance their photocatalytic performance. This review article encompasses some recent advances in the facet engineering that has been performed to control the surface of mono-component semiconductor systems and to design the surface and interface structures of multi-component heterostructures toward photocatalytic applications. The review begins with some key points which should receive attention in the facet engineering on photocatalytic materials. We then discuss the synthetic approaches to achieve the facet control associated with the surface and interface design. In the following section, the facet-engineered surface design on mono-component photocatalytic materials is introduced, which forms a basis for the discussion on more complex systems. Subsequently, we elucidate the facet-engineered surface and interface design of multi-component photocatalytic materials. Finally, the existing challenges and future prospects are discussed.

Unravelling Small-Polaron Transport in Metal Oxide Photoelectrodes

Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10(-4) and 10(-2) cm(2) V(-1) s(-1), respectively. Future directions to build a full picture of charge transport in this family of materials are discussed.

A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals

Polarization dependent V L-edge XAS spectra showing anisotropy in the electronic band structure of a W:BiVO₄ single crystal.