Photoinduced Charge-Transfer Dynamics in WO3/BiVO4 Photoanodes Probed through Midinfrared Transient Absorption Spectroscopy

Breaking the Activity-Selectivity Trade-off for CH4-to-C2H6 Photoconversion

Photocatalytic conversion of methane (CH4) to ethane (C2H6) has attracted extensive attention from academia and industry. Typically, the traditional oxidative coupling of CH4 (OCM) reaches a high C2H6 productivity, yet the inevitable overoxidation limits the target product selectivity. Although the traditional nonoxidative coupling of CH4 (NOCM) can improve the product selectivity, it still encounters unsatisfied activity, arising from being thermodynamically unfavorable. To break the activity-selectivity trade-off, we propose a conceptually new mechanism of H2O2-triggered CH4 coupling, where the H2O2-derived ·OH radicals are rapidly consumed for activating CH4 into ·CH3 radicals exothermically, which bypasses the endothermic steps of the direct CH4 activation by photoholes and the interaction between ·CH3 and ·OH radicals, affirmed by in situ characterization techniques, femtosecond transient absorption spectroscopy, and density-functional theory calculation. By this pathway, the designed Au-WO3 nanosheets achieve unprecedented C2H6 productivity of 76.3 mol molAu-1 h-1 with 95.2% selectivity, and TON of 1542.7 (TOF = 77.1 h-1) in a self-designed flow reactor, outperforming previously reported photocatalysts regardless of OCM and NOCM pathways. Also, under outdoor natural sunlight irradiation, the Au-WO3 nanosheets exhibit similar activity and selectivity toward C2H6 production, showing the possibility for practical applications. Interestingly, this strategy can be applied to other various photocatalysts (Au-WO3, Au-TiO2, Au-CeO2, Pd-WO3, and Ag-WO3), showing a certain universality. It is expected that the proposed mechanism adds another layer to our understanding of CH4-to-C2H6 conversion.

Improved Photoelectrochemical Performance of WO3/BiVO4 Heterojunction Photoanodes via WO3 Nanostructuring

WO3/BiVO4 heterojunction photoanodes can be efficiently employed in photoelectrochemical (PEC) cells for the conversion of water into molecular oxygen, the kinetic bottleneck of water splitting. Composite WO3/BiVO4 photoelectrodes possessing a nanoflake-like morphology have been synthesized through a multistep process and their PEC performance was investigated in comparison to that of WO3/BiVO4 photoelectrodes displaying a planar surface morphology and similar absorption properties and thickness. PEC tests, also in the presence of a sacrificial hole scavenger, electrochemical impedance analysis under simulated solar irradiation, and incident photon to current efficiency measurements highlighted that charge transport and charge recombination issues affecting the performance of the planar composite can be successfully overcome by nanostructuring the WO3 underlayer in nanoflake-like WO3/BiVO4 heterojunction electrodes.

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

CuWO4 is a ternary semiconductor oxide with excellent visible light harvesting properties up to 550 nm and stability at high pH values, which make it a suitable material to build photoanodes for solar light conversion to hydrogen via water splitting. In this work, we studied the photoelectrochemical (PEC) performance of transparent CuWO4 electrodes with tunable light absorption and thickness, aiming at identifying the intrinsic bottlenecks of photogenerated charge carriers in this semiconductor. We found that electrodes with optimal CuWO4 thickness exhibit visible light activity due to the absorption of long-wavelength photons and a balanced electron and hole extraction from the oxide. The PEC performance of CuWO4 is light-intensity-dependent, with charge recombination increasing with light intensity and most photogenerated charge carriers recombining in bulk sites, as demonstrated by PEC tests performed in the presence of sacrificial agents or cocatalysts. The best-performing 580 nm thick CuWO4 electrode delivers a photocurrent of 0.37 mA cm-2 at 1.23 VSHE, with a 7% absorbed photon to current efficiency over the CuWO4 absorption spectrum.

Band Edges Engineering of 2D/2D Heterostructures: The C3 N4 /Phosphorene Interface

We investigate the interface between carbon nitride (C₃ N₄ ) and phosphorene nanosheets (P-ene) by means of Density Functional Theory (DFT) calculations. C₃ N₄ /P-ene composites have been recently obtained experimentally showing excellent photoactivity. Our results indicate that the formation of the interface is a favorable process driven by Van der Waals forces. The thickness of P-ene nanosheets determines the band edges offsets and the charge carriers' separation. The system is predicted to pass from a nearly type-II to a type-I junction when the thickness of P-ene increases, and the conduction band offset is particularly sensitive. Last, we apply the Transfer Matrix Method to estimate the efficiency for charge carriers' migration as a function of the P-ene thickness.

Enhanced Charge Carrier Separation in WO3/BiVO4 Photoanodes Achieved via Light Absorption in the BiVO4 Layer

Photoelectrochemical (PEC) water splitting converts solar light and water into oxygen and energy-rich hydrogen. WO₃/BiVO₄ heterojunction photoanodes perform much better than the separate oxide components, though internal charge recombination undermines their PEC performance when both oxides absorb light. Here we exploit the BiVO₄ layer to sensitize WO₃ to visible light and shield it from direct photoexcitation to overcome this efficiency loss. PEC experiments and ultrafast transient absorption spectroscopy performed by frontside (through BiVO₄) or backside (through WO₃) irradiating photoanodes with different BiVO₄ layer thickness demonstrate that irradiation through BiVO₄ is beneficial for charge separation. Optimized electrodes irradiated through BiVO₄ show 40% higher photocurrent density compared to backside irradiation.

Crystal Facet-Modulated WO3 Nanoplate Photoanode for Photoelectrochemical Glyoxal Semi-oxidation into Glyoxylic Acid

Transforming glyoxal to value-added glyoxylic acid (GA) is highly desirable but challenging due to the uncontrollable over-oxidation. In this work, we report on a first demonstration of semi-oxidation of glyoxal with high selectivity (86.5%) and activity on WO₃ nanoplate photoanode through the photoelectrochemical strategy. The optimization of reactivity was achieved via crystal facet regulation, showing a satisfactory GA production rate of 308.4 mmol m^-2 h^-2, 84.0% faradaic efficiency, and 4.3% total solar-to-glyoxylic acid efficiency on WO₃ with enriched {200} facets at 1.6 V versus RHE. WO₃ with a high {200} facet ratio exhibits more efficient electron-hole transfer kinetics, resulting in the facilitated formation of hydroxyl radicals (^•OH) and glyoxal radicals. Meanwhile, the theoretical calculation results indicate that the high selectivity and activity come from the strong adsorption ability for glyoxal and the low reaction energy for glyoxal radical generation on the (200) facets of WO₃. Moreover, the high energy demand toward oxalic acid production on WO₃ leads to the exciting semi-oxidation process.

Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping

Metal oxides are promising for photoelectrochemical (PEC) water splitting due to their robustness and low cost. However, poor charge carrier transport impedes their activity, particularly at low-bias voltage. Here we demonstrate the unusual effectiveness of phosphorus doping into bismuth vanadate (BiVO₄) photoanode for efficient low-bias PEC water splitting. The resulting BiVO₄ photoanode shows a separation efficiency of 80% and 99% at potentials as low as 0.6 and 1.0 V_RHE, respectively. Theoretical simulation and experimental analysis collectively verify that the record performance originates from the unique phosphorus-doped BiVO₄ configuration with concurrently mediated carrier density, trap states, and small polaron hopping. With NiFeO_x cocatalyst, the BiVO₄ photoanode achieves an applied bias photon-to-current efficiency of 2.21% at 0.6 V_RHE. The mechanistic understanding of the enhancement of BiVO₄ properties provides key insights in trap state passivation and polaron hopping for most photoactive metal oxides.

Construction and excellent photoelectric synergistic anticorrosion performance of Z-scheme carbon nitride/tungsten oxide heterojunctions

The use of heterojunctions for metal corrosion protection is a highly innovative and challenging task. Based on the composition and structure of tungsten oxide-based heterojunctions, Z-scheme heterojunctions were designed and synthesized by the electrostatic self-assembly method using energy band-matched g-C₃N₄ and WO₃ materials. Applied in the field of anticorrosion, they overcame the problems of poor reduction ability and transmission inefficiency of traditional materials. The Z-scheme heterojunctions ensured unidirectional electron transfer, while the aggregation of the retained strongly reduced electrons on the surface of the iron substrates provided a strong driving force for retarding corrosion occurrence. Meanwhile, the inherent shielding properties of the two-dimensional material g-C₃N₄ and the confinement of chloride ions as an electroactive layer hindered the penetration of the corrosive solution. After being corroded for 72 h, the corrosion impedance of the g-C₃N₄/WO₃ heterojunction system was improved by 640.11% compared with the epoxy resin coating. In addition, the g-C₃N₄/W₁₈O₄₉ heterojunction was synthesized by using mixed valence tungsten oxide, which overcame the problems of photogenerated electron yield and lifetime, and enhanced the anticorrosion performance compared with a single g-C₃N₄ phase. This research provided ideas for designing efficient and environmentally friendly heterojunction anticorrosion materials.

Rational Design of Semiconductor Heterojunctions for Photocatalysis

Electronic structure calculations provide a useful complement to experimental characterization tools in the atomic-scale design of semiconductor heterojunctions for photocatalysis. The band alignment of the heterojunction is of fundamental importance to achieve an efficient charge carrier separation, so as to reduce electron/hole recombination and improve photoactivity. The accurate prediction of the offsets of valence and conduction bands in the constituent units is thus of key importance but poses several methodological and practical problems. In this Minireview we address some of these problems by considering selected examples of binary and ternary semiconductor heterojunctions and how these are determined at the level of density functional theory (DFT). The atomically precise description of the interface, the consequent charge polarization, the role of quantum confinement, the possibility to use facet engineering to determine a specific band alignment, are among the effects discussed, with particular attention to pros and cons of each one of these aspects. This analysis shows the increasingly important role of accurate electronic structure calculations to drive the design and the preparation of new interfaces with desired properties.

BiVO4 Ceramic Photoanode with Enhanced Photoelectrochemical Stability

Monoclinic bismuth vanadate (BiVO₄) is an attractive material with which to fabricate photoanodes due to its suitable band structure and excellent photoelectrochemical (PEC) performance. However, the poor PEC stability originating from its severe photo-corrosion greatly restricts its practical applications. In this paper, pristine and Mo doped BiVO₄ ceramics were prepared using the spark plasma sintering (SPS) method, and their photoelectrochemical properties as photoanodes were investigated. The as-prepared 1% Mo doped BiVO₄ ceramic (Mo-BVO (C)) photoanode exhibited enhanced PEC stability compared to 1% Mo doped BiVO₄ films on fluorine doped Tin Oxide (FTO) coated glass substrates (Mo-BVO). Mo-BVO (C) exhibited a photocurrent density of 0.54 mA/cm² and remained stable for 10 h at 1.23 V vs. reversible hydrogen electrode (RHE), while the photocurrent density of the Mo-BVO decreased from 0.66 mA/cm² to 0.11 mA/cm² at 1.23 V vs. RHE in 4 h. The experimental results indicated that the enhanced PEC stability of the Mo-BVO (C) could be attributed to its higher crystallinity, which could effectively inhibit the dissociation of vanadium in BiVO₄ during the PEC process. This work may illustrate a novel ceramic design for the improvement of the stability of BiVO₄ photoanodes, and might provide a general strategy for the improvement of the PEC stability of metal oxide photoanodes.

WO3/BiVO4 Photoanodes: Facets Matching at the Heterojunction and BiVO4 Layer Thickness Effects

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO₃/BiVO₄ heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO₃/BiVO₄ interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO₄ layer over a WO₃ scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO₄ growth over the WO₃ layer along the BiVO₄ {010} face, driven by the formation of a stable interface with new covalent bonds, with a favorable band alignment and band bending between the two oxides. This crystal facet phase matching allows a smooth transition between the electronic states of the two oxides and may be a key factor ensuring the high efficiency attained with this heterojunction. The photoelectrochemical activity of the WO₃/BiVO₄ photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-absorbing BiVO₄ layer, a 75 nm thick BiVO₄ layer on WO₃ being best performing.

Surface Modifications of (ZnSe)0.5(CuGa2.5Se4.25)0.5 to Promote Photocatalytic Z-Scheme Overall Water Splitting

Charge separation is crucial for an efficient artificial photosynthetic process, especially for narrow-bandgap metal sulfides/selenides. The present study demonstrates the application of a p-n junction to particulate metal selenides to enhance photocatalytic Z-scheme overall water splitting (OWS). The constructed p-n junction of CdS-(ZnSe)_0.5(CuGa_2.5Se_4.25)_0.5 significantly boosted charge separation. A thin TiO₂ coating layer also was introduced to inhibit photocorrosion of CdS and suppress the backward reaction of water formation from hydrogen and oxygen. By employing Pt-loaded TiO₂/CdS-(ZnSe)_0.5(CuGa_2.5Se_4.25)_0.5 as a hydrogen evolution photocatalyst (HEP), we assembled a Z-scheme OWS system, together with BiVO₄:Mo and Au as an oxygen evolution photocatalyst and electron mediator, respectively. An apparent quantum yield of 1.5% at 420 nm was achieved, which is by far the highest among reported particulate photocatalytic Z-scheme OWS systems with metal sulfides/selenides as HEPs. The present work demonstrates that a well-tailored p-n junction structure is effective for promoting charge separation in photocatalysis and opens new pathways for the development of efficient artificial photosynthesis systems involving narrow bandgap photocatalysts.

Self-Powered Cathodic Photoelectrochemical Aptasensor Comprising a Photocathode and a Photoanode in Microfluidic Analysis Systems

An intriguing self-powered cathodic photoelectrochemical (PEC) microfluidic aptasensor with enhanced cathodic photocurrent response is proposed for sensitive detection of prostate-specific antigen (PSA). The self-powered system is constructed by a cadmium sulfide-sensitized zinc oxide nanorod array (CdS/ZnO NA) as a photoanode with an iodide-doped bismuth oxychloride flower-array (I_0.2:BiOCI_0.8) as a photocathode, which can generate the electrical output under visible light irradiation with no external power supply. In addition, the p-type semiconductor I_0.2:BiOCI_0.8 with a special internal electric field between the iodide ion layer and the [Bi₂O₂]²⁺ layer could increase the cathodic photocurrent response by facilitating the separation of electron/hole pairs under visible light excitation. It is worth noting that dissolved oxygen as an electron acceptor can be reduced by the photogenerated electron to form a superoxide radical (^•O₂^-) in the self-powered cathodic PEC system. The further enhanced cathodic photocurrent response can be achieved by eliminating ^•O₂^- that reacts with the luminol anion radical (L^•-) to produce chemiluminescence emission, which serves as an inner excitation light source. What is more exciting is that the integration of the photoanode and the photocathode into a microfluidic chip could realize automatic sample injection and detection. On this basis, the proposed aptasensor presents excellent reproducibility and high sensitivity for detecting PSA and exhibits a good linearity range (50 fg·mL^-1 to 50 ng·mL^-1) with a low detection limit (25.8 fg·mL^-1), which opens up a new horizon of potential for sensitively detecting other kinds of disease markers.

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.

Direct Z-Scheme Heterojunction of Semicoherent FAPbBr3/Bi2WO6 Interface for Photoredox Reaction with Large Driving Force

Metal halide perovskites with direct band gap and strong light absorption are promising materials for harvesting solar energy; however, their relatively narrow band gap limits their redox ability when used as a photocatalyst. Adding a second semiconductor component with the appropriate band structure offsets can generate a Z-scheme photocatalytic system, taking full advantage of the perovskite's intrinsic properties. In this work, we develop a direct Z-scheme photocatalyst based on formamidinium lead bromide and bismuth tungstate (FAPbBr₃/Bi₂WO₆) with strong redox ability for artificial solar-to-chemical energy conversion. With desirable band offsets and strong joint redox potential, the dual photocatalyst is shown to form a semicoherent heterointerface. Ultrafast transient infrared absorption studies employing selective excitation reveal synergetic photocarrier dynamics and demonstrate Z-scheme charge transfer mechanisms. Under simulated solar irradiation, a large driving force photoredox reaction (∼2.57 eV) of CO₂ reduction coupled with benzyl alcohol oxidation to benzaldehyde is achieved on the Z-scheme FAPbBr₃/Bi₂WO₆ photocatalyst, harnessing the full synergetic potential of the combined system.

Nanocarbon-Enhanced 2D Photoelectrodes: A New Paradigm in Photoelectrochemical Water Splitting

Solar-driven photoelectrochemical (PEC) water splitting systems are highly promising for converting solar energy into clean and sustainable chemical energy. In such PEC systems, an integrated photoelectrode incorporates a light harvester for absorbing solar energy, an interlayer for transporting photogenerated charge carriers, and a co-catalyst for triggering redox reactions. Thus, understanding the correlations between the intrinsic structural properties and functions of the photoelectrodes is crucial. Here we critically examine various 2D layered photoanodes/photocathodes, including graphitic carbon nitrides, transition metal dichalcogenides, layered double hydroxides, layered bismuth oxyhalide nanosheets, and MXenes, combined with advanced nanocarbons (carbon dots, carbon nanotubes, graphene, and graphdiyne) as co-catalysts to assemble integrated photoelectrodes for oxygen evolution/hydrogen evolution reactions. The fundamental principles of PEC water splitting and physicochemical properties of photoelectrodes and the associated catalytic reactions are analyzed. Elaborate strategies for the assembly of 2D photoelectrodes with nanocarbons to enhance the PEC performances are introduced. The mechanisms of interplay of 2D photoelectrodes and nanocarbon co-catalysts are further discussed. The challenges and opportunities in the field are identified to guide future research for maximizing the conversion efficiency of PEC water splitting.

Multilayer WO3/BiVO4 Photoanodes for Solar-Driven Water Splitting Prepared by RF-Plasma Sputtering

A series of WO3, BiVO4 and WO3/BiVO4 heterojunction coatings were deposited on fluorine-doped tin oxide (FTO), by means of reactive radio frequency (RF) plasma (co)sputtering, and tested as photoanodes for water splitting under simulated AM 1.5 G solar light in a three-electrode photoelectrochemical (PEC) cell in a 0.5 M NaSO4 electrolyte solution. The PEC performance and time stability of the heterojunction increases with an increase of the WO3 innermost layer up to 1000 nm. A two-step calcination treatment (600 °C after WO3 deposition followed by 400 °C after BiVO4 deposition) led to a most performing photoanode under back-side irradiation, generating a photocurrent density of 1.7 mA cm−2 at 1.4 V vs. SCE (i.e., two-fold and five-fold higher than that generated by individual WO3 and BiVO4 photoanodes, respectively). The incident photon to current efficiency (IPCE) measurements reveal the presence of two activity regions over the heterojunction with respect to WO3 alone: The PEC efficiency increases due to improved charge carrier separation above 450 nm (i.e., below the WO3 excitation energy), while it decreases below 450 nm (i.e., when both semiconductors are excited) due to electron–hole recombination at the interface of the two semiconductors.

Hydrogenation of ZnFe2O4 Flat Films: Effects of the Pre-Annealing Temperature on the Photoanodes Efficiency for Water Oxidation

The effects induced by post-synthesis hydrogenation on ZnFe2O4 flat films in terms of photoelectrochemical (PEC) performance of photoanodes for water oxidation have been deeply investigated as a function of the pre-annealing temperature of the materials. The structure and morphology of the films greatly affect the efficacy of the post synthesis treatment. In fact, highly compact films are obtained upon pre-annealing at high temperatures, and this limits the exposure of the material bulk to the reductive H2 atmosphere, making the treatment largely ineffective. On the other hand, a mild hydrogen treatment greatly enhances the separation of photoproduced charges in films pre-annealed at lower temperatures, as a result of the introduction of oxygen vacancies with n-type character. A comparison between present results and those obtained with ZnFe2O4 nanorods clearly demonstrates that specific structural and/or surface properties, together with the initial film morphology, differently affect the overall contribution of post-synthesis hydrogenation on the efficiency of zinc ferrite-based photoanodes.

Nanoporous BiVO4 nanoflake array photoanode for efficient photoelectrochemical water splitting

A high-quality nanoporous BiVO₄ nanoflake array photoanode was prepared by using an in situ transformation approach, which exhibited an excellent photoelectrochemical activity.

Advanced space- and time-resolved techniques for photocatalyst studies

Nanoparticle photocatalysts present the obvious characteristic of heterogeneity in structure, energy, and function at spatial and temporal scales.

Photoinduced electron transfer in WO3/BiVO4 heterojunction photoanodes: effects of the WO3 layer thickness

The PEC performance of WO₃/BiVO₄ heterojunction photoanodes with a fixed BiVO₄ thick top layer and different WO₃ layer thicknesses was investigated under backside irradiation, in comparison with the performance of the same electrodes without a top BiVO₄ layer. While the performance of these latter increase with increasing WO₃ thickness, the presence of a BiVO₄ layer, besides leading to an effective sensitization up to 520 nm, leads to a decrease of incident photon to current efficiency in the short wavelength's range. After having excluded major WO₃ filter effects, this has been attributed to charge carrier recombination effects occurring when both oxides get excited and becoming more relevant with increasing WO₃ thickness and decreasing excitation wavelength.

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.

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.

Efficient Photocatalytic Fuel Cell via Simultaneous Visible-Photoelectrocatalytic Degradation and Electricity Generation on a Porous Coral-like WO3/W Photoelectrode

Photocatalytic fuel cells (PFCs) have proven to be effective for generating electricity and degrading pollutants with a goal to resolve environmental and energy problems. However, the degradation of persistent organic pollutants (POPs), such as perfluorooctanoic acid (PFOA), remains challenging. In the present work, a porous coral-like WO₃/W (PCW) photoelectrode with a well-designed energy band structure was used for the photoelectrocatalytic degradation of POPs and the simultaneous generation of electricity. The as-constructed bionic porous coral-like nanostructure greatly improved the light-harvesting capacity of the PCW photoelectrode. A maximum photocurrent density (0.31 mA/cm²) under visible light (λ > 420 nm) irradiation and a high incident photon conversion efficiency (IPCE) value (5.72% at 420 nm) were achieved. Because of the unique porous coral-like structure, the suitable energy band position, and the strong oxidation ability, this PCW photoelectrode-based PFC system exhibited a strong ability for simultaneous photoelectrocatalytic degradation of PFOA and electricity generation under visible-light irradiation, with a power output of 0.0013 mV/cm² using PFOA as the fuel. This work provides a promising way to construct a reliable PFC using highly toxic POPs to generate electricity.

WO3/BiVO4: impact of charge separation at the timescale of water oxidation

Unveiling the role of applied bias on the charge carrier dynamics in the WO₃/BiVO₄ junction during water oxidation.

The four hole oxidation of water has long been considered the kinetic bottleneck for overall solar-driven water splitting, and thus requires the formation of long-lived photogenerated holes to overcome this kinetic barrier. However, photogenerated charges are prone to recombination unless they can be spatially separated. This can be achieved by coupling materials with staggered conduction and valence band positions, providing a thermodynamic driving force for charge separation. This has most aptly been demonstrated in the WO₃/BiVO₄ junction, in which quantum efficiencies for the water oxidation reaction can approach near unity. However, the charge carrier dynamics in this system remain elusive over timescales relevant to water oxidation (μs–s). In this work, the effect of charge separation on carrier lifetime, and the voltage dependence of this process, is probed using transient absorption spectroscopy and transient photocurrent measurements, revealing sub-μs electron transfer from BiVO₄ to WO₃. The interface formed between BiVO₄ and WO₃ is shown to overcome the “dead-layer effect” encountered in BiVO₄ alone. Moreover, our study sheds light on the role of the WO₃/BiVO₄ junction in enhancing the efficiency of the water oxidation reaction, where charge separation across the WO₃/BiVO₄ junction improves both the yield and lifetime of holes present in the BiVO₄ layer over timescales relevant to water oxidation.

Photoelectrocatalytic degradation of emerging contaminants at WO3/BiVO4 photoanodes in aqueous solution

Advantages and limitations of WO₃/BiVO₄ heterojunctions applied to the photoelectrochemical treatment of some environmental Contaminants of Emerging Concern (CEC).