BiVO4 Layer Research Articles

Elucidating the dynamics and transfer pathways of photogenerated charge carriers in V2O5/BiVO4 heterojunction photoanodes: A transient absorption spectroscopy study

Pairing of bismuth vanadate (BiVO4) with vanadium pentoxide (V2O5) forms a Type II heterojunction photoanode, which has been proven to be a high-performance photoanode architecture for efficient photoelectrochemical (PEC) water oxidation. To further support for advanced rational design and improvement of the heterojunction photoanode, it is quintessential to understand the mechanics and properties of photogenerated charge carriers formed. This study aims to probe the dynamics of photogenerated electron-hole pairs formed in pristine photoanode as well as the heterojunction photoanodes using transient absorption spectroscopy (TAS). Relative to the BiVO4/V2O5 structure, modelling and quantification of the decay constant helps in explaining why the V2O5/BiVO4 heterojunction photoanode exhibited a longer lifetime of the photogenerated charge carriers with a lower decay rate constant that leads to a much-improved overall generation of photocurrent density. An oxygen evolving catalyst of nickel oxyhydroxide (NiOOH) was then anchored on the exterior surfaces of the heterojunction photoanode system to further investigate and modify the transfer pathways of the photogenerated charge carriers. Furthermore, the hole- and scavenger-assisted TAS measurements on the BiVO4/V2O5 heterojunction photoanode revealed that the majority of trapped holes species are accumulated within the BiVO4 layer. The findings suggested that the Type II heterojunction formation in V2O5/BiVO4 could effectively reduce its charge recombination process.

Cadmium doping induced type-II to Z-scheme switching in CdSe/BiVO4 heterojunction for enhancing photocatalytic hydrogen production

Heterojunction type in composite photocatalysts determines their charge carrier transfer path, which is critical to the redox capacity of their charge carriers and needs careful modulation for their designed photocatalytic reactions. In this work, a transit from the type-II (BVO/CdSe) to Z-scheme (CdSe/BVO) in CdSe/BiVO4 heterojunction happened by changing the deposition sequence of CdSe and BiVO4 layers to introduce Cd-doping into the BiVO4 layer, which fundamentally changed their charge carrier separation and transfer behaviors as demonstrated by both experimental and calculation results. So, the charge carrier separation efficiency in CdSe got largely enhanced, more photogenerated electrons with high reduction potential were allowed to participate in the photocatalytic H2 production, and photogenerated hole accumulation in CdSe reduced. As a result, the CdSe/BVO photoelectrode demonstrated the best photocatalytic hydrogen production rate (2.59 μmol cm−2 h−1), which is about 136.3 and 5.9 times higher than that of BVO/CdSe and pure CdSe, and the stability under visible light irradiation is also greatly improved.

Understanding the Internal Conversion Efficiency of BiVO4/SnO2 Photoanodes for Solar Water Splitting: An Experimental and Computational Analysis.

This work aims to understand the spin-coating growth process of BiVO4 photoanodes from a photon absorption and conversion perspective. BiVO4 layers with thicknesses ranging from 7 to 48 nm and the role of a thin (<5 nm) SnO2 hole-blocking layer have been studied. The internal absorbed photon-to-current efficiency (APCE) is found to be nonconstant, following a specific dependence of the internal charge separation and extraction on the increasing thickness. This APCE variation with BiVO4 thickness is key for precise computational simulation of light propagation in BiVO4 based on the transfer matrix method. Results are used for accurate incident photon-to-current efficiency (IPCE) prediction and will help in computational modeling of BiVO4 and other metal oxide photoanodes. This establishes a method to obtain the sample's thickness by knowing its IPCE, accounting for the change in the internal APCE conversion. Moreover, an improvement in fill factor and photogenerated voltage is attributed to the intermediate SnO2 hole-blocking layer, which was shown to have a negligible optical effect but to enhance charge separation and extraction for the lower energetic wavelengths. A Mott-Schottky analysis was used to confirm a photovoltage shift of 90 mV of the flat-band potential.

BiVO4/CuBi2O4 heterojunction photoanodes with enhanced charge separation for efficient photoelectrochemical water splitting

BiVO4/CuBi2O4 photoanodes were fabricated by depositing CuBi2O4 layers on top of BiVO4 layers while employing a solution-based deposition technique. Carrier separation and transfer are facilitated by the heterojunction that forms between BiVO4 and CuBi2O4, which significantly raises carrier density. Time-resolved photoluminescence (TRPL), UV–VIS absorption spectra, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were employed for the characterization of the photoelectrode. The heterojunction photoelectrode demonstrated enhanced photoelectrochemical activity in comparison to BiVO4 alone, with the maximum photocurrent density of 1.15 mA/cm2 at 1.23 V vs. RHE. The improvement is attributed to decreased recombination of photo-generated electrons and holes and improved charge separation and transport in the heterostructure. By the generation of heterojunctions, the current work offers a new avenue for the design and construction of photoanodes with high charge separation efficiencies. This may be useful for the further enhancement of PEC water-splitting performances.

Fabrication and optimization of perovskite-based photoanodes for solar-driven CO2 photoelectroreduction to formate

The photoelectrochemical reduction of CO2 to value-added products represents a promising strategy for mitigating CO2 emissions. However, further research efforts need to be undertaken to enable the technology for scale-up, including the design and fabrication of efficient photoelectrodes capable of yielding substantial photogenerated current densities. In this work, a photoanode combining a commercial calcium titanate perovskite (CaTiO3) and BiVO4 layers coated onto a transparent FTO substrate by automated spray pyrolysis is proposed. Different photoanode configurations are tested, with the most favourable results achieved when BiVO4 is positioned as the top layer with back illumination. The optimization of the catalytic loading is also assessed, finding an optimal at 1 mg cm−2 of CaTiO3 and 3 mg cm−2 of BiVO4, resulting in an impressive current density of –71 mA cm−2 at –1.8 V vs. Ag/AgCl. This optimal photoanode is then integrated into an electrolyzer for continuous visible light-driven CO2 reduction in the gas phase to formate, obtaining a concentration of 63.8 g L−1, with a Faradaic Efficiency of 79.1 %, and solar-to-fuel conversion efficiency of 7.6 %. These results represent a significant advancement in the development of photoanodes, offering promise for the future scalability of photoelectrochemical CO2 reduction processes.

Understanding Performance Limitations in Water Splitting Photoelectrodes at the Nanoscale

Photoelectrochemical (PEC) water splitting is a promising route for efficient conversion of solar energy into chemical fuels. In this context, economically viable photosystems are often based of semiconductor thin films which are polycrystalline or nanostructured with highly complex architectures e.g. with boundaries between differently orientated grains, different crystal facet orientations, as well as locally varying composition or phase. These nano- to micrometer properties often control critical processes, such as efficiency and stability, of the macroscale system. To understand the impact of nanoscale properties on the macroscopic performance, we aim at resolving local structural, chemical, and optoelectronic heterogeneities and at elucidating their effect on light-driven processes.In this work, we use a correlative approach to elucidate the nanoscale properties of polycrystalline BiVO4 thin films – a promising material for solar water splitting. Mapping of the local charge transport properties by photoconductive atomic force microscopy (pc-AFM) reveals the tolerance to grain boundaries.[1] Interestingly, scanning nearfield infrared microscopy shows strongly varying absorption from VO4 stretching modes between grains and at grain boundaries which correlate with the heterogeneities in the local photoconductivity across the polycrystalline thin films. Furthermore, local temperature-dependent current-voltage spectroscopy show that the low intrinsic bulk conductivity of BiVO4 limits the electron transport through the film, and that the transport mechanism can be attributed to space charge limited current (SCLC) in the presence of trap states.[1] Performing the same measurements in-situ in controlled gas-phase environment reveals the influence of chemical interactions of adsorbed oxygen and water on charge transport and interfacial charge transfer.[2] For BiVO4, we demonstrate that adsorbed oxygen acts as a surface trap state for electrons, which enhances the built-in potential and depletes the BiVO4 layer. Overall, combining insights from different nanoscale techniques generates a comprehensive picture of charge separation, transport, and transfer at the nanoscale. The gained nanoscale understanding of energy materials enables the rational design of durable and efficient solar fuel devices.[1] Eichhorn et al., Nat. Commun. 9, 2597(2018).[2] Eichhorn et al., ACS Appl. Mater. Interfaces, 10, 41, 35129 (2018).

Impact of Varying the Photoanode/Catalyst Interfacial Composition on Solar Water Oxidation: The Case of BiVO4(010)/FeOOH Photoanodes.

Photoanodes used in a water-splitting photoelectrochemical cell are almost always paired with an oxygen evolution catalyst (OEC) to efficiently utilize photon-generated holes for water oxidation because the surfaces of photoanodes are typically not catalytic for the water oxidation reaction. Suppressing electron-hole recombination at the photoanode/OEC interface is critical for the OEC to maximally utilize the holes reaching the interface for water oxidation. In order to explicitly demonstrate and investigate how the detailed features of the photoanode/OEC interface affect interfacial charge transfer and photocurrent generation for water oxidation, we prepared two BiVO4(010)/FeOOH photoanodes with different Bi:V ratios at the outermost layer of the BiVO4 interface (close to stoichiometric vs Bi-rich) while keeping all other factors in the bulk BiVO4 and FeOOH layers identical. The resulting two photoanodes show striking differences in the photocurrent onset potential and photocurrent density for water oxidation. The ambient pressure X-ray photoelectron spectroscopy results show that these two BiVO4(010)/FeOOH photoanodes show drastically different Fe2+:Fe3+ ratios in FeOOH both in the dark and under illumination with water, demonstrating the immense impact of the interfacial composition and structure on interfacial charge transfer. Using computational studies, we reveal the effect of the surface Bi:V ratio on the hydration of the BiVO4 surface and bonding with the FeOOH layer, which in turn affect the band alignments between BiVO4 and FeOOH. These results explain the atomic origin of the experimentally observed differences in electron and hole transfer and solar water oxidation performance of the two photoanodes having different interfacial compositions.

Toward preparation of large scale and uniform mesoporous BiVO4 thin films with enhanced photostability for solar water splitting

The preparation methods of photocatalyst thin films play a critical role in increasing their efficiency. Ease of fabrication, the ability to commercialize, production on a large scale, reproducibility, and preparing the layers with stable performance are some of the most important criteria for a synthesis method to prepare a high-performance phot-electrode. A straightforward, cost-effective and reproducible hot spin coating (HSC) method was introduced in this work, to achieve these goals for BiVO4 thin films. The most important HSC parameters, namely substrate revolution speed (ωs), substrate temperature (Ts), and combustion time (Ct), were investigated systematically. The BiVO4 thin films prepared via the HSC method under the optimized parameters of ωs = 1200 rpm, Ts = 200 °C, and Ct = 15 s exhibited a uniform mesoporous surface texture (average pore diameter of 13.4 nm) and high adhesion (5B) to the substrate. Interestingly, the HSC method was capable of preparing super-hydrophilic BiVO4 layers on a large scale (tested up to 8 × 8 cm2) without any surface pinholes. The practical application of the obtained layers was examined inside a photoelectrochemical (PEC) cell. The optimized BiVO4 electrode generated a remarkable and robust photo-current density of ∼500 μA/cm2 without any change after 30 days. The preparation of BiVO4 thin films with such interesting properties via the facile HSC method makes it a promising technique to prepare highly efficient photo-anodes for practical use on a large scale.

Enhancing the photoelectrochemical activity and stability of plate-like WO3 photoanode in neutral electrolyte solution using optimum loading of BiVO4 layer and NiFe–LDH electrodeposition

Tungsten trioxide (WO3) is an attractive candidate that can be used as a photoanode in the photoelectrochemical (PEC) water splitting device. However, WO3 photoanode is known for its high thermodynamic and chemical stability only in strong acidic (low pH) electrolyte solution and its relatively wide bandgap energy (2.8 eV) leads to insufficient absorption of visible light. The current study explored a WO3/BiVO4 heterojunction photoanode, consisting of plate-like WO3 covered with a uniform and highly porous BiVO4 thin layer, with electrodeposition of NiFe–LDH as a co-catalyst for oxygen evolution reaction. This top layer of BiVO4 nanostructure not only exhibited an enhancement in absorption of visible light but also protected the underneath plate-like WO3 from being attacked by the neutral environment of electrolyte solution (pH=∼7), resulting in significant improvement in its stability. The electrodeposition of NiFe–LDH using an acidic aqueous electrolyte solution (pH=3.7) was employed to overcome the sluggish kinetics of water oxidation on the surface of BiVO4. To overcome the weakness of the chemical dissolution of BiVO4 nanostructure in strong acidic solution, the electrodeposition time was carefully optimized for 50 s, leading to a uniform distribution of co-catalyst on the surface of WO3/BiVO4 heterostructure without the dissolving of BiVO4 in the acidic media. The WO3/BVO5/NiFe50 photoanode with the highest PEC performance (photocurrent density of around 1.78 mA/cm2 at 1.23 V vs. RHE) could maintain around 75% of its initial photocurrent after 24 h.

Charge Transport Enhancement in BiVO4 Photoanode for Efficient Solar Water Oxidation.

Photoelectrochemical (PEC) water splitting in a pH-neutral electrolyte has attracted more and more attention in the field of sustainable energy. Bismuth vanadate (BiVO4) is a highly promising photoanode material for PEC water splitting. Additionally, cobaltous phosphate (CoPi) is a material that can be synthesized from Earth's rich materials and operates stably in pH-neutral conditions. Herein, we propose a strategy to enhance the charge transport ability and improve PEC performance by electrodepositing the in situ synthesis of a CoPi layer on the BiVO4. With the CoPi co-catalyst, the water oxidation reaction can be accelerated and charge recombination centers are effectively passivated on BiVO4. The BiVO4/CoPi photoanode shows a significantly enhanced photocurrent density (Jph) and applied bias photon-to-current efficiency (ABPE), which are 1.8 and 3.2 times higher than those of a single BiVO4 layer, respectively. Finally, the FTO/BiVO4/CoPi photoanode displays a photocurrent density of 1.39 mA cm-2 at 1.23 VRHE, an onset potential (Von) of 0.30 VRHE, and an ABPE of 0.45%, paving a potential path for future hydrogen evolution by solar-driven water splitting.

BiVO4-modified anodic nanocoral WO3 structures for enhancement of photoelectrochemical performance

Nanostructuring of metal oxides is a straightforward strategy for enhancing photoelectrochemical (PEC) performance. The present work demonstrates the anodization of W metal to determine the optimum WO3 nanostructure for use in photoanodes. Anodization was performed in a fluoride-containing 1 M Na2SO4 electrolyte. Under optimum conditions, WO3 nanocoral structures were fabricated. The forming mechanism of the nanocoral structures was elucidated by the generation and development of nanobubbles at the metal/oxide interface. To enhance the PEC performance, BiVO4 layers were coated on the anodic WO3 nanocoral structures and annealed at 500 °C in air atmosphere. The prepared BiVO4/WO3 nanocoral layers were characterized by SEM, XRD, EIS and UV–vis spectroscopy. They were then used as photoanodes for PEC and IPCE measurements. Consequently, photocurrent density was improved to 0.42 mA/cm2 at 1.23 V vs. RHE and IPCE at 400 nm (21%) was enhanced by 21 times due to the producing band alignment between BiVO4 and WO3.

Hierarchically arranged γ-NiOOH/β-FeOOH layer supported on modified Molybdenum-BiVO4 thin film for oxygen evolution reaction

Due to the slow kinetics of water oxidation, severe surface charge recombination is a significant energy loss that impedes efficient photoelectrochemical performance. Constructing a scheelite bismuth vanadate (BiVO4) electrode with an oxygen evolution catalyst and doping is an effective way to increase the performance of water oxidation. Importantly, this attempt avoids any sacrificial agents and binders. Using a single-step hydrothermal-based method, a nickel-iron (oxy)hydroxide is deposited unlike conventional two-step photoelectrodeposition method onto the surface of BiVO4 doped with 2% molybdenum dopant. Through this method, the as-formed nickel-iron (oxy)hydroxide is in γ-NiOOH and β-FeOOH forms. The difficulty of transporting charge carriers-hole pairs along the BiVO4 is reduced by incorporating molybdenum. A low-cost but efficient co-catalyst, nickel-iron (oxy)hydroxide is introduced to surpass the charge recombination on BiVO4. A comparison of photocurrent density-voltage and photocurrent density-time curves with and without an oxygen evolution catalyst reveals that nickel-iron (oxy)hydroxide accelerates the anodic photocurrent of the BiVO4 from ∼1 to 5 mA cm−2 (1.23 V vs. RHE). Simultaneously, the molybdenum-doped BiVO4 layer with a small band gap and extremely good photoelectrostability and the (oxy)hydroxide layer with good conductivity provide synergetic enhancement of both stability over 12 hours and photocurrent density.

Photo-electrocatalytic based removal of acetaminophen: Application of visible light driven heterojunction based BiVO4/BiOI photoanode

The presence of organic micro-pollutants (OMPs) in wastewater treatment effluents is becoming a major threat to the water safety for aquatic and human health. Photo-electrocatalytic based advanced oxidation process (AOP) is one of the emerging and effective techniques to degrade OMPs through oxidative mechanism. This study investigated the application of heterojunction based BiVO4/BiOI photoanode for acetaminophen (40 μg L−1) removal in demineralized water. Photoanodes were fabricated by electrodeposition of BiVO4 and BiOI photocatalytic layers. Optical (UV–vis diffusive reflectance spectroscopy), structural (XRD, SEM, EDX) and opto-electronic (IPCE) characterization confirmed the successful formation of heterojunction for enhanced charge separation efficiency. The heterojunction photoanode showed incident photon to current conversion efficiency of 16% (λmax = 390 nm) at an external voltage of 1 V under AM 1.5 standard illumination. The application of the BiVO4/BiOI photoanode in the removal of acetaminophen at 1 V (external bias) vs Ag/AgCl under simulated sunlight showed 87% removal efficiency within the first 120 min compared to 66% removal efficiency of the BiVO4 photoanode. Similarly, combining BiVO4 and BiOI exhibited 57% increase in first order removal rate coefficient compared to BiVO4. The photoanodes also showed moderate stability and reusability by showing 26% decrease in overall degradation efficiency after three cycles of each 5 h experiment. The results obtained in this study can be considered as a stepping stone towards the effective removal of acetaminophen as an OMP present in wastewater.

PEC/Colorimetric Dual-Mode Lab-on-Paper Device via BiVO4/FeOOH Nanocomposite In Situ Modification on Paper Fibers for Sensitive CEA Detection.

A dual-mode lab-on-paper device based on BiVO4/FeOOH nanocomposites as an efficient generating photoelectrochemical (PEC)/colorimetric signal reporter has been successfully constructed by integration of the lab-on-paper sensing platform and PEC/colorimetric detection technologies for sensitive detection of carcinoembryonic antigen (CEA). Concretely, the BiVO4/FeOOH nanocomposites were in situ synthesized onto the paper-working electrode (PWE) through hydrothermal synthesis of the BiVO4 layer on cellulose fibers (paper-based BiVO4) which were initially modified by Au nanoparticles for improving the conductivity of three dimensional PWE, and then the photo-electrodeposition of FeOOH onto the paper-based BiVO4 to construct the paper-based BiVO4/FeOOH for the portable dual-mode lab-on-paper device. The obtained nanocomposites with an FeOOH needle-like structure deposited on the BiVO4 layer exhibits enhanced PEC response activity due to its effective separation of the electron-hole pair which could further accelerate the PEC conversion efficiency during the sensing process. With the introduction of CEA targets onto the surface of nanocomposite-modified PWE assisted by the interaction with the CEA antibody from a specific recognition property, a signal-off PEC signal state with a remarkable photocurrent response decreasing trend can be achieved, realizing the quantitative detection of CEA with the PEC signal readout mode. By means of a smart origami paper folding, the colorimetric signal readout is achieved by catalyzing 3,3',5,5'-tetramethylbenzidine (TMB) to generate blue oxidized TMB in the presence of H2O2 due to the satisfied enzyme-like catalytic activity of the needle-like structure, FeOOH, thereby achieving the dual-mode signal readout system for the proposed lab-on-paper device. Under the optimal conditions, the PEC and colorimetric signals measurement were effectively carried out, and the corresponding linear ranges were 0.001-200 ng·mL-1 and 0.5-100 ng·mL-1 separately, with the limit of detection of 0.0008 and 0.013 ng·mL-1 for each dual-mode. The prepared lab-on-paper device also presented a successful application in serum samples for the detection of CEA, providing a potential pathway for the sensitive detection of target biomarkers in clinical application.

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.

Understanding the Carrier Dynamics of Complex Heterojunctions for Water Splitting through Wavelength-Dependent Intensity Modulated Photocurrent Spectroscopy

Metal oxide semiconductors are promising materials to be employed as photoanodes in photoelectrochemical (PEC) devices to drive the oxygen evolution reaction (OER) for the conversion of solar energy into chemical fuels. These materials need to efficiently absorb light, have good bulk transport properties to separate the photogenerated charges, and induce fast charge transfer from the electrode inside the solution. To this aim, the best selected materials are usually combined to form efficient heterojunction structures, often with the help of surface catalysts.The understanding of charge carrier dynamics in complex heterojunctions is of the utmost importance for the performance optimization of photoelectrochemical cells and “ operando” techniques are most valuable, because they provide information about the charge separation, bulk transport, charge transfer at each junction, and at the semiconductor-electrolyte interface of the system upon external stimuli, such as illumination, applied bias and chemical environment.In the presence of multiple competing processes and complex interfaces, which are often found in photoelectrodes for water splitting, the construction of an electrical model that describe the carrier dynamics under a light stimulus can be difficult, since the frequency response of the PEC system may depend on several interlaced resistive and capacitive contributions, which are sometimes difficult to separate or to interpret in an unambiguous fashion. The characterization of the relevant kinetic processes occurring at junctions and semiconductor/electrolyte interfaces can be effectively carried out by implementing wavelength-dependent Intensity Modulated Photocurrent Spectroscopy (WD-IMPS). This innovative approach allows to selectively probe different layer of the heterojunction and identify the electron transport properties in the bulk and at the interface.We employed this straightforward technique to study the carrier dynamics of a WO3/BiVO4/CoFe-Prussian blue heterojunction in a conventional three electrode cell for water splitting, and we identified interfacial recombination processes affecting the semiconductor heterojunction, as well as the positive contribution of the inorganic catalyst on the charge separation efficiency of the BiVO4 layer.[1] Herein, the proposed methodology is used to separate and address the role of each active component within complex interfaces coupled with different catalysts and in different electrolytic environments and represents a valuable tool for improving the understanding of dynamic processes relevant to PEC water splitting.

Synthesis and characterization of porous BiVO4 thin films: the effect of structural defects on photoelectrochemical properties

BiVO4 thin films with thickness of ~ 1.3 μm were deposited on ITO substrate via pulsed-spray pyrolysis deposition. X-ray diffraction pattern revealed that BiVO4 layers have been crystallized in tetragonal scheelite phase with average crystallite size of ~ 16 nm. According to UV-visible absorption spectra, a band gap energy of ~2.47 eV was determined for the synthesized layers. Scanning electron microscopy observations indicated that a porous BiVO4 structure with average pore diameter of ~ 162 nm and worm-like fine particle diameter of ~ 208 nm has been synthesized. Oxygen vacancies have been induced into the layers via an electrochemical reduction treatment (ET). This employed process increased the surface-related capacitance by about 6 times. A double charge transport resistance and half capacitance for Helmholtz layer was determined after ET, indicating electron transfer from space charge layer to Helmholtz layer upon ET. Using electrochemical impedance spectroscopy, it was found that effective charge carrier life time inside the BiVO4 thin films increased to ~25 ms which is 2-fold longer than the time before electrochemical reduction treatment.

Charge Separation Efficiency in WO3/BiVO4 Photoanodes with CoFe Prussian Blue Catalyst Studied by Wavelength‐Dependent Intensity‐Modulated Photocurrent Spectroscopy

The understanding of charge carrier dynamics in complex heterojunctions is of the utmost importance for the performance optimization of photoelectrochemical cells, especially in operando. Intensity‐modulated photocurrent spectroscopy (IMPS) is a powerful tool to this aim, but the information content provided by this technique can be further enhanced by selectively probing each layer of complex heterojunctions by means of multiple excitation sources. Herein, the charge carrier dynamics of a WO3/BiVO4/CoFe–PB heterojunction, used in a conventional three electrode cell for water splitting, is studied using wavelength‐dependent IMPS (WD‐IMPS). The proposed data analysis allows us to identify the occurrence of interface recombination processes affecting the semiconductor junction, as well as the positive contribution of the inorganic complex catalyst on the charge separation efficiency of the BiVO4 layer. The deep understanding of the fate of charge carriers in the studied photoanode validates WD‐IMPS as a straightforward method to widen the understanding of such structures.

Boosting the Photocurrent of the WO3/BiVO4 Heterojunction by Photoelectrodeposition of the Oxy-Hydroxide-Phosphates Based on Co, Fe, or Ni

Here we investigated the photoelectrodeposition of oxygen-evolving catalysts (OECs) based on oxy-hydroxides-phosphates of Co, Fe, or Ni (CoPi, FePi, and NiPi) on the WO3/BiVO4 heterojunction and their activity in water oxidation. The OECs were deposited by cycles of (i) open-circuit potential (OCP) and (ii) applying a potential positive enough to oxidize the metallic precursor on the WO3/BiVO4. The crystalline and optical properties of the photoanodes were not significantly affected by the OECs deposition. However, there was a remarkable increase in the photocurrent densities (jpc) to water oxidation, where the modification with FePi showed the best result, achieving 2.12 mA cm−2, which corresponds to 2.83 times higher than the heterojunction without the OECs. Furthermore, the OECs deposition changed the morphology of the heterojunction with the deposition of a thin film on its surface. In addition, during the FePi deposition, the BiVO4 layer seems to partially dissolve. Our study shows a facile methodology to boost the activity of photoanodes to the water oxidation by photoelectrodeposition of OECs.

Optimization of photogenerated charge transport using type-II heterojunction structure of CoP/BiVO4:WO3 for high efficient solar-driver water splitting

Bismuth vanadate oxide (BiVO4) is one of the most efficient light-absorber metal oxides for the photoelectrochemical (PEC) water splitting; however, the fast charge recombination and poor kinetics for water oxidation have hindered full utilization of their theoretical performance. The optimization of the band alignment to facilitate charge transport and injection is of paramount importance to achieve the ideal water splitting performance of the photoelectrode. In this study, a type-II heterojunction CoP/BiVO4:WO3 structure has been fabricated for highly efficient PEC water splitting. The WO3 layer in the junction readily collects the photoelectrons harvested from the BiVO4 layer owing to its highly conducting nature, enabling efficient bulk charge transport from the BiVO4:WO3 junction to the fluorine-doped tin oxide (FTO) substrate. In addition, the cobalt phosphide (CoP) nanoparticles (NPs) play the role of the hole conducting layer from the light-absorber layer to the water as well as a catalyst to enhance surface charge injection efficiency at the photoelectrode surface. As a result, the CoP/BiVO4:WO3 photoanode reveals a remarkable photocurrent density of 2.81 mA cm−2 at 1.23 V (vs. RHE) with a negative shift of the onset potential (610 mV) compare to that of bare BiVO4. Moreover, the CoP/BiVO4:WO3 electrode shows a highly stable photocurrent density for at least 5 h.