BiVO4 Layer Research Articles

Solar‐Water‐Splitting BiVO4 Thin‐Film Photoanodes Prepared By Using a Sol–Gel Dip‐Coating Technique

AbstractA facile and low cost method to construct a bismuth vanadate thin film photoanode was implemented with the aim of integrating it in a tandem dual water splitting photoelectrochemical cell. Multilayer semi‐transparent thin films of BiVO4 were fabricated by a sol–gel process and deposited by dip‐coating onto transparent conducting oxide substrates with intermediate annealing treatment between layers and final calcination at a low temperature of 450 °C in air. The effect of the intermediate annealing temperature has a great impact on the porosity, and therefore density, of thin layers of BiVO4 when fabricated by sol–gel dip‐coating methods; thus, for optimal activity, the annealing temperature should be kept at 400 °C for thinner layers and 450 °C for thicker layers. The annealing temperature has a direct effect on the size of the crystallites which determines the microstructural density and porosity. In contrast, the final calcination temperature must be 450 °C in order to achieve good electrochemical performances. Optimized BiVO4 photoanodes exhibit a photocurrent of up to 2.1 mA cm−2 with an average Faradic efficiency of 85 % for oxygen evolution in neutral pH potassium phosphate buffer at 1.23 V vs. RHE under 350 mW cm−2 light irradiation.

Doping strategy to promote the charge separation in BiVO4 photoanodes

We fabricated Mo:BiVO4/Co:BiVO4 photoanodes by depositing Co:BiVO4 layers on top of Mo:BiVO4 layers, and demonstrated the enhanced PEC performances with improved charge separation both in the bulk and at the interface by doping strategy in this work. Co:BiVO4 layers in Mo:BiVO4/Co:BiVO4 photoanodes were found to be important for the enhanced charge separation efficiencies. The surface exposed Co2+ ions in Co:BiVO4 layers can act as reactive sites for water oxidation to promote the interfacial charge separation. While, the Co2+ doping inside Co:BiVO4 cannot only tune the built in electric fields in Mo:BiVO4/Co:BiVO4 homojunctions, but also be able to optimize the charge transport in Co:BiVO4 layers to facilitate the bulk charge separation. By optimizing the Co contents and the number of Co:BiVO4 layers in Mo:BiVO4/Co:BiVO4 photoanodes, 3Mo-1Co-6% consisted of 3 layers of 3% Mo:BiVO4 and 1 layer 6% Co:BiVO4 yields the highest photocurrent of 2.09mA/cm2 at 1.23V vs RHE, with a ηb and ηi value of 77.8% and 86.5%, respectively. This work provides a new thread for the design and fabrication of photoanode with high charge separation efficiencies by doping strategy, which could be helpful for the further improvement of PEC water splitting performances.

Three-Dimensional FTO/TiO2/BiVO4 Composite Inverse Opals Photoanode with Excellent Photoelectrochemical Performance

A poor electron transport property, short charge carrier diffusion lengths, and slow water oxidation kinetics severely limit the photoelectrochemical (PEC) performance of the BiVO4 photoelectrodes. To address these problems, we report the design and fabrication of a three-dimensional FTO/TiO2/BiVO4 core–shell inverse opals photoanode for PEC hydrogen production by combining atomic layer deposition and electrodeposition routes for TiO2 and BiVO4 layer deposition on F:SnO2 (FTO) inverse opal skeletons, respectively. Benefiting from the highly conductive transparent FTO invese opal networks providing fast electron pathways and TiO2/BiVO4 heterojunctions, the as-fabricated 3D FTO/TiO2/BiVO4 inverse opals photoanode delivers excellent PEC performance with a maximum photocurrent density of 4.11 mA/cm2 at 1.23 V vs a reversible hydrogen electrode in the presence of a hole scavenger in contrast to that of the counterparts FTO/TiO2 and FTO/BiVO4 inverse opals electrodes, respectively, which could be attributed to ...

Low-potential driven fully-depleted BiVO4/ZnO heterojunction nanodendrite array photoanodes for photoelectrochemical water splitting

To enhance photoelectrochemical water splitting performance of the intrinsic BiVO4, a low-potential driven fully-depleted intrinsic BiVO4-based photoanode is realized in this work by the conformal formation of thin BiVO4 layers (<15nm) on the 3-μm-thick ZnO nanodendrite (ND) array followed by the deposition of co-catalyst cobalt phosphate (Co-Pi). The Co-Pi/BiVO4/ZnO ND photoanode is fully-depleted at 0.8V vs. the reversible hydrogen electrode (RHE) by the electric fields developed in radial directions of the nanorods and branches. Driven by the electric fields, the photogenerated electron-hole pairs in the whole electrode are efficiently separated and then the holes swiftly drift to the photoanode/electrolyte interface for oxygen evolution. Rather than diffusion, charge transport mechanism is governed by drift in the fully-depleted ND heterojunction array photoanode. As a result, in the high-light-harvesting BiVO4/ZnO ND array photoanode, the obstacle of slow charge transport in BiVO4 can be surmounted due to the construction of the light absorption and hole drifting paths in different directions. The photocurrent density of the Co-Pi/BiVO4/ZnO ND photoanode is optimized to be 3.5mAcm−2 at 1.23V vs. RHE.

Cascading Alignment of Multilayered SnO2/WO3/BiVO4 Inverse Opal Skeletons in Photoelectrochemical Water Splitting

Tin dioxide (SnO2) inverse opals (IOs) having a pore size of approximately 260 nm in the 370 nm sized polystyrene bead (PS) templates were developed by a spin-coating-assisted sol−gel process. Upon this template, the SnO2/WO3 core-shell IOs were developed by the facile electrodeposition under a constant potential (−0.47 V vs. sat. Ag/AgCl), where the thickness of WO3 layer depended on the applied charge amount for WO3 electrodeposition (200−800 mC/cm2). As a control sample, a pure WO3 IO film with the same thickness of ∼3.1 μm and a band gap (Eg ) of 2.6 eV was also prepared in the same method. The pore diameter of the SnO2 IO structure declined noticeably as the charge amount of the deposited WO3 layer increased from 200 to 800 mC/cm2, leading to eventual coverage of the SnO2 IO structure in the WO3 (800 mC/cm2) layer. The optimum photoelectrochemical (PEC) response was achieved with the SnO2/WO3 (600 mC/cm2) IO electrode, which exhibited the highest photocurrent density (Jsc) of 2.8 mA/cm2 (0.5 VAg/AgCl) under full-sun conditions and 0.91 mA/cm2 (0.5 VAg/AgCl) under visible light, indicating that the enhancement of the Jsc under visible light contributed significantly to the improvement of the total Jsc, compared with the values for the pure SnO2, SnO2/WO3 (200, 400, and 800 mC/cm2), and WO3 IO electrodes. Furthermore, considering that the favorable cascading band alignment can boost the fast charge separation and transport through the conductive SnO2 IO skeleton, we try to develop the multilayered SnO2/WO3/BiVO4 IOs structure showing the well-matched band alignment. And then, the dual doped (W and Mo) BiVO4 layer was adapted in the surface layer due to the more narrowing Eg of ~ 2.4 eV. Herein, the PEC performance and other results would be presented. Figure 1

An Effective Way to Improve the Structural Stability and Photoelectrochemical Performance of BiVO4 Photoanodes in Basic Media: Surface Passivation with Zinc Ferrite

A photoelectrochemical water splitting technology (PEC) using sustainable solar energy has been considered as one of the most promising methods to produce directly renewable energy (i.e. H2) from water. This system has still lots of challenges in improving water-splitting PEC efficiency. In particularly, development of electrode materials to covert efficiently solar energy to hydrogen is one of the most challenges facing many scientists and engineers in this field. Among the electrode materials for use in a water-splitting PEC cell, n-type bismuth vanadate, or BiVO4, has recently been identified as a promising metal oxide photoanode for O2 evolution, because of a narrow band gap (2.4-2.5 eV) for absorbing substantial position of visible spectrum and a favourable conduction band edge position which is very near the thermodynamic hydrogen reduction potential. Most of the BiVO4 studies for water-splitting PECs have mainly been investigated for use under neutral conditions (pH ∼7) because BiVO4 is chemically unstable and gradually dissolves in strong basic and acidic solutions. When the operating conditions of BiVO4 can be extended to basic or acidic media, BiVO4 can be coupled to more diverse catalysts or photocathodes, which perform optimally only under basic or acidic conditions. Additionally, using basic or acidic media may offer an advantage of achieving higher solution conductivities without using additional supporting electrolytes or buffers for PEC operation. In order to solve such weakness, we tried to add spinel zinc ferrite (ZnFe2O4) as a protection layer to use BiVO4 photoanode in basic condition. A thin layer of ZnFe2O4 was placed on the surface of a nanoporous BiVO4 electrode using following process: (1) photodeposition of iron oxyhydroxide (FeOOH), (2) a mild chemical and thermal treatment of FeOOH with Zn precursor. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that a ZnFe2O4 layer formed a uniform and conformal coating layer on a BiVO4 particles, and the thickness of the layer was 10 -15 nm. The ZnFe2O4 coating layer was very thin and was x-ray amorphous structure as evidenced by a conventional powder x-ray diffractometer. But, the selected area electron diffraction (SAED) clearly indicated that ZnFe2O4 was spinel structure. The effect of the ZnFe2O4 layer on the prevention of chemical dissolution of BiVO4 in basic media in the dark was first tested by immersing BiVO4 and BiVO4/ZnFe2O4 electrode in a 0.1 M KOH solution (pH 13) for 72 h. The SEM images taken after 72 h of immersion showed that the ZnFe2O4-free BiVO4 electrode was considerably dissolved, whereas the ZnFe2O4-coated BiVO4 electrode did not show any detectable sign of dissolution. The effect of the ZnFe2O4 layer on the photoelectrochemical properties and photostabilities of BiVO4 was tested by measuring J−V and J-t plots in 0.1 M KOH (pH 13) under simulated AM 1.5G irradiation (100 mW/cm2), using a three-electrode configuration. The obtained BiVO4/ZnFe2O4 electrode generated a photocurrent density of 2.76 mA/cm2 at 1.23 V vs. RHE with a significantly improved stability compared to the pristine BiVO4 electrode (ca. 1.04 mA/cm2 at 1.23 V vs. RHE). The incident and absorbed photon-to-current conversion efficiencies along with absorption spectra suggested that the ZnFe2O4 protection layer also contributes to photocurrent generation by increasing photon absorption and electron-hole separation of the BiVO4 layer. In addition, when the surface of the ZnFe2O4 layer was modified with Co2+ ions as oxygen evolution reaction catalyst, the resulting BiVO4/ZnFe2O4/Co2+ electrode generates a more improved photocurrent density (ca. 2.83 mA/cm2 at 1.23 V vs. RHE) with a more significantly improved stability. These results suggest that further investigation of protection and catalyst layers can enable more stable and efficient operation of BiVO4-based photoanodes in basic media.

Three-Dimensional ZnO-BiVO4 Core-Shell Nanosturctured Array for Photoelectrochemical Water Oxidation

In this work, three-dimensional (3D) ZnO-BiVO4 core-shell nanostructured arrays were constructed on fluorine-doped tin oxide (FTO) substrates for use in photoelectrochemical (PEC) water splitting. The ZnO nanodendrite (ND) arrays were grown on FTO substrates using a wet chemical route. A photocurrent density of 0.6 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) under AM 1.5 at 100 mW cm-2 is monitored in the ZnO ND array photoanode with a thickness of 3 mm. To construct the 3D ZnO-BiVO4 core-shell nanostructured arrays, the BiVO4 nanoparticle (NP) layers were subsequently formed on the surface of ND arrays using metal organic decomposition method. The absorption spectrum of the 3D ZnO-BiVO4 core-shell nanostructured array shows an absorption edge at the wavelength of ~510 nm. An optimized photocurrent density of 2.5 mA/cm2 at 1.23 V vs. RHE is obtained in the 3D ZnO-BiVO4 core-shell nanostructured array photoanode. The 3D ZnO ND array provides a large surface area for the deposition of the thin BiVO4 layer. Moreover, due to the formation of type II heterojunction in the ZnO-BiVO4 core-shell nanostructured array, the photocarriers generated in the ZnO ND core and BiVO4 shell can be effectively separated. Therefore, photoholes generated and collected in the thin BiVO4 layer can transport to the surface for water oxidation. On the other hand, the photoelectron generated and collected in the ZnO ND core can transport to the FTO substrate due to the quasi-single crystal structure of the ZnO ND array. To further improve the PEC performance, Co–Pi was further electrodeposited on the surface of the 3D ZnO-BiVO4 core-shell nanostructured array photoanode. With the Co-Pi deposition, the onset potential in the J-V curve of the 3D ZnO-BiVO4 core-shell nanostructured array photoanode shows a cathodic shift from 0.7 V to 0.2 V vs. RHE. Moreover, the photocurrent density of the 3D ZnO-BiVO4 core-shell nanostructured array photoanode is significantly improved. An optimized photocurrent density of 3.5 mAcm-2 are monitored in the Co-Pi deposited photoanode at 1.23 V vs. RHE.

Fabrication of an Efficient BiVO4-TiO2 Heterojunction Photoanode for Photoelectrochemical Water Oxidation.

In this work, a simple planar BiVO4/TiO2 heterojunction photoanode was prepared on a fluorine-doped tin oxide (FTO) substrate for photoelectrochemical (PEC) water oxidation. The measurements of surface photovoltage, photocurrent transient behavior, and hole-scavenger-assisted PEC performance indicate that charge separation efficiency is improved compared to that of the BiVO4/FTO photoanode. This improvement is caused by the formation of the staggered BiVO4/TiO2 heterojunction. However, the photocurrent densities of the BiVO4/TiO2/FTO photoanode are higher than those of the BiVO4/FTO one only at potentials >1.2 V vs reversible hydrogen electrode, although the two BiVO4 layers with comparable light harvesting efficiencies were prepared by the same method. The hole-scavenger-assisted PEC measurements reveal that the hole injection efficiency of the BiVO4/TiO2/FTO photoanode is inferior to that of the bare BiVO4/FTO anode for oxygen evolution. It shows that the surface property of the BiVO4 layers is altered as they are deposited on different substrates. On the basis of these characterizations, the cocatalyst cobalt phosphate (Co-Pi) was further deposited on the surface of BiVO4/TiO2/FTO photoanode to improve the hole injection efficiency. Subsequently, the photocurrent density and stability of the Co-Pi/BiVO4/TiO2/FTO photoanode were significantly improved compared to those of the bare BiVO4/FTO photoanode.

Enhanced charge separation in copper incorporated BiVO4 with gradient doping concentration profile for photoelectrochemical water splitting

BiVO4 is one of promising materials that are stable, abundant, non-toxic and inexpensive for hydrogen production by water splitting. High recombination rate inside BiVO4 was reported as a limiting factor for photoelectrochemical (PEC) performance of BiVO4 photoanode. In this work, Cu incorporated BiVO4 with gradient doping concentration profile was synthesized by depositing a CuO layer between BiVO4 and FTO followed with annealing. Cu-doped BiVO4 films with homogeneous concentration profile were prepared as a counter part to show the different behaviors between the gradient doping and homogeneously doping on charge transport and separation. It is found that the PEC performance of BiVO4 electrode is significantly improved by gradient doping concentration profile in BiVO4 layer, especially at the high applied bias range. While homogeneously doped BiVO4 showed a decreased PEC performance as Cu element acts as a recombination center. Mott–Schottky test showed that the flat band level of CuO-BiVO4 shifts negative slightly, which is beneficial for hydrogen production. While a positive shift is observed for homogeneously Cu-doped BiVO4 which could be a result from the recombination centre nature of Cu in BiVO4. This work provide an efficient and inexpensive means to reduce charge recombination in BiVO4 and improve its of solar water splitting efficiency.

Atomic Layer Deposition of Bismuth Vanadates for Solar Energy Materials.

The fabrication of porous nanocomposites is key to the advancement of energy conversion and storage devices that interface with electrolytes. Bismuth vanadate, BiVO4 , is a promising oxide for solar water splitting where the controlled fabrication of BiVO4 layers within porous, conducting scaffolds has remained a challenge. Here, the atomic layer deposition of bismuth vanadates is reported from BiPh3 , vanadium(V) oxytriisopropoxide, and water. The resulting films have tunable stoichiometry and may be crystallized to form the photoactive scheelite structure of BiVO4 . A selective etching process was used with vanadium-rich depositions to enable the synthesis of phase-pure BiVO4 after spinodal decomposition. BiVO4 thin films were measured for photoelectrochemical performance under AM 1.5 illumination. The average photocurrents were 1.17 mA cm(-2) at 1.23 V versus the reversible hydrogen electrode using a hole-scavenging sulfite electrolyte. The capability to deposit conformal bismuth vanadates will enable a new generation of nanocomposite architectures for solar water splitting.

High Light Absorption and Charge Separation Efficiency at Low Applied Voltage from Sb-Doped SnO2/BiVO4 Core/Shell Nanorod-Array Photoanodes.

BiVO4 has become the top-performing semiconductor among photoanodes for photoelectrochemical water oxidation. However, BiVO4 photoanodes are still limited to a fraction of the theoretically possible photocurrent at low applied voltages because of modest charge transport properties and a trade-off between light absorption and charge separation efficiencies. Here, we investigate photoanodes composed of thin layers of BiVO4 coated onto Sb-doped SnO2 (Sb:SnO2) nanorod-arrays (Sb:SnO2/BiVO4 NRAs) and demonstrate a high value for the product of light absorption and charge separation efficiencies (ηabs × ηsep) of ∼51% at an applied voltage of 0.6 V versus the reversible hydrogen electrode, as determined by integration of the quantum efficiency over the standard AM 1.5G spectrum. To the best of our knowledge, this is one of the highest ηabs × ηsep efficiencies achieved to date at this voltage for nanowire-core/BiVO4-shell photoanodes. Moreover, although WO3 has recently been extensively studied as a core nanowire material for core/shell BiVO4 photoanodes, the Sb:SnO2/BiVO4 NRAs generate larger photocurrents, especially at low applied voltages. In addition, we present control experiments on planar Sb:SnO2/BiVO4 and WO3/BiVO4 heterojunctions, which indicate that Sb:SnO2 is more favorable as a core material. These results indicate that integration of Sb:SnO2 nanorod cores with other successful strategies such as doping and coating with oxygen evolution catalysts can move the performance of BiVO4 and related semiconductors closer to their theoretical potential.

Morphology engineering of WO3/BiVO4heterojunctions for efficient photocatalytic water oxidation

Four types of nanostructural WO3/BiVO4 heterojunctions were designed by depositing a BiVO4 layer onto WO3 scaffolds with different morphologies and investigated to understand the influence of heterojunction morphology on their photoelectrochemical performances. The architecture of WO3 scaffolds was controlled by adjusting the solvents used in their solvothermal synthesis. The solvent was found to exert a great influence on the architecture of the obtained WO3 scaffolds. Methanol leads to the formation of block-like nanostructures and ethanol facilitates the formation of nanorods, while isopropyl alcohol and ethylene glycol give rise to the formation of triangular nanowalls and 3D hierarchical nanostructures, respectively. Monoclinic WO3 nanostructures were obtained via removal of ammonium or structural water by annealing the samples obtained at 510 °C in air. A BiVO4 layer was then deposited on the monoclinic WO3 scaffolds with different morphologies by spin-coating to form WO3/BiVO4 heterojunction films, which show significantly enhanced photocurrents compared to their corresponding bare WO3 scaffolds. Among these WO3/BiVO4 nanostructure heterojunctions, the one based on a rod-like WO3 nanostructure shows the best photoelectrochemical (PEC) performance, which can be ascribed to its optimal rod-like morphology, for the construction of WO3/BiVO4 heterojunction regarding efficient charge carrier transfer and collection, while the other one based on WO3 nanowalls exhibits better stability and a higher photocurrent in a long-run test, indicating that morphology engineering significantly influences the activity and stability of WO3-based heterojunctions.

Micropatterning of BiVO4 Thin Films Using Laser‐Induced Crystallization

Relatively high temperatures even up to 500 °C are required to obtain bismuth vanadate (BiVO4) films with the scheelite monoclinic (s‐m) structure that shows the highest photocatalytic activity. This requirement limits the possible choice of substrates. Moreover, high quality thin layers of crystalline BiVO4 cannot be prepared with current methods. In this study a light‐induced crystallization approach is presented, which is a step toward preparation and patterning of BiVO4 (s‐m) films for applications on plastic substrates. Thin films of amorphous BiVO4 are prepared by pulsed laser deposition. The possibility of using green (514.7 nm) laser illumination for crystallization of BiVO4 is investigated. The laser‐induced phase transition is tracked using Raman spectroscopy. The results are compared with those obtained from thermally annealed samples, crystalline structure of which is confirmed by measuring X‐ray diffraction. The homogeneity and quality of crystallization are verified using micro‐Raman spectroscopy imaging, while time‐dependent experiments reveal the crystallization rate. The conductivity of the crystallized region is investigated using conductive atomic force microscopy. A strong increase in the conductivity is found in the patterned regions. Experimental results demonstrate the possibility of using the laser‐induced crystallization of BiVO4 to prepare patterns of improved conductivity and semiconducting properties in comparison to amorphous surroundings.

Overall Photoelectrochemical Water Splitting using Tandem Cell under Simulated Sunlight.

A stand-alone photoelectrochemical (PEC) water-splitting system driven only by sunlight was demonstrated with a tandem-scheme of Pt/CdS/CuGa3 Se5 /(Ag,Cu)GaSe2 photocathode and NiOOH/FeOOH/Mo:BiVO4 photoanode in a neutral phosphate buffer solution as an electrolyte. The as-prepared semi-transparent Mo:BiVO4 layer allows sunlight to pass through the top photoanode and reach the bottom photocathode. Consequently, the tandem cell showed stoichiometric hydrogen and oxygen evolution with a solar-to-hydrogen (STH) conversion efficiency of 0.67 % over 2 h without degradation. The stability and STH efficiency are the highest among similar configuration of PEC tandem cells.

Unassisted photoelectrochemical water splitting beyond 5.7% solar-to-hydrogen conversion efficiency by a wireless monolithic photoanode/dye-sensitised solar cell tandem device

Achieving the spontaneous evolution of hydrogen from photoelectrochemical (PEC) cells in water using solar light is a desirable but difficult goal. Here, we report a highly efficient wireless monolithic tandem device composed of bipolar highly transparent BiVO4-sensitised mesoporous WO3 films/Pt and a porphyrin-dye-based photoelectrode achieving 5.7% without any external bias. A sandwich infiltration process was used to produce a thin BiVO4 layer coated onto mesoporous WO3 films while preserving high transparency, enabling high photonic flux into the second dye-sensitised photoanode. In addition, the porphyrin-dye-sensitised photoanode with a cobalt electrolyte generated sufficient bias, realising highly efficient unassisted solar water splitting in the tandem cells. By combining the highly transparent BiVO4-sensitised mesoporous WO3 films with the state-of-the-art water oxidation catalyst and a single dye-sensitised solar cell with a high open circuit potential in a monolithic tandem configuration, an extraordinarily high solar-to-hydrogen (STH) conversion efficiency with spontaneous hydrogen evolution was obtained.

Enhanced photoelectrochemical water oxidation on bismuth vanadate by electrodeposition of amorphous titanium dioxide.

n-BiVO4 is a promising semiconductor material for photoelectrochemical water oxidation. Although most thin-film syntheses yield discontinuous BiVO4 layers, back reduction of photo-oxidized products on the conductive substrate has never been considered as a possible energy loss mechanism in the material. We report that a 15 s electrodeposition of amorphous TiO2 (a-TiO2) on W:BiVO4/F:SnO2 blocks this undesired back reduction and dramatically improves the photoelectrochemical performance of the electrode. Water oxidation photocurrent increases by up to 5.5 times, and its onset potential shifts negatively by ∼500 mV. In addition to blocking solution-mediated recombination at the substrate, the a-TiO2 film-which is found to lack any photocatalytic activity in itself-is hypothesized to react with surface defects and deactivate them toward surface recombination. The proposed treatment is simple and effective, and it may easily be extended to a wide variety of thin-film photoelectrodes.

Nanostructured WO3 /BiVO4 photoanodes for efficient photoelectrochemical water splitting.

Nanostructured photoanodes based on well-separated and vertically oriented WO3 nanorods capped with extremely thin BiVO4 absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO3 -NRs/BiVO4 photoanode modified with Co-Pi oxygen evolution co-catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm(2) at 1.23 V versus a reversible hydrogen electrode in a stable Na2 SO4 electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO4 layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO3 -NRs/BiVO4 photoanode effectively increases the optical thickness of the BiVO4 layer and results in efficient absorption of the incident light.

Morphology control of one-dimensional heterojunctions for highly efficient photoanodes used for solar water splitting

In a dual bandgap system such as WO3/BiVO4, the morphology of each component should be controlled by understanding its properties, in particular with respect to the charge flow in the system. For WO3/BiVO4 photoanodes, a porous BiVO4 film allows contact of an electrolyte to the bottom layer with enhanced surface area, thereby promoting the oxidation reaction, while one-dimensional (1-D) WO3 nanorods, directly grown on F-doped tin oxide, are advantageous for transporting electrons to the back contact. The morphology of the BiVO4 film covered by 1-D WO3 nanorods varies with the addition of organic additives such as ethylcellulose in the metal precursor solution. The cross-sectional images from scanning electron microscopy show that 1-D WO3 nanorods is coated with the BiVO4 layer, which forms a porous top layer that can effectively absorb visible light and enhance charge transfer resulting in enhanced photocurrents. We report on the highest photocurrent at a potential of 1.23 V versus a reversible hydrogen electrode (RHE) by means of a 1-D WO3/BiVO4/Co-Pi photoanode. The strategies for constructing such kind of heterojunctions are well applicable to other dual bandgap photoanodes.

BiVO4thin film photoanodes grown by chemical vapor deposition

BiVO4 thin film photoanodes were grown by vapor transport chemical deposition on FTO/glass substrates. By controlling the flow rate, the temperatures of the Bi and V sources (Bi metal and V2O5 powder, respectively), and the temperature of the deposition zone in a two-zone furnace, single-phase monoclinic BiVO4 thin films can be obtained. The CVD-grown films produce global AM1.5 photocurrent densities up to 1 mA cm(-2) in aqueous conditions in the presence of a sacrificial reagent. Front illuminated photocatalytic performance can be improved by inserting either a SnO2 hole blocking layer and/or a thin, extrinsically Mo doped BiVO4 layer between the FTO and the CVD-grown layer. The incident photon to current efficiency (IPCE), measured under front illumination, for BiVO4 grown directly on FTO/glass is about 10% for wavelengths below 450 nm at a bias of +0.6 V vs. Ag/AgCl. For BiVO4 grown on a 40 nm SnO2/20 nm Mo-doped BiVO4 back contact, the IPCE is increased to over 40% at wavelengths below 420 nm.

Ethanol Gas Sensor Based on Pure and La-Doped Bismuth Vanadate

Bismuth vanadate (BiVO4) and lanthanum-doped bismuth vanadate (La-doped BiVO4) were prepared via the precipitation method. Their films were produced by simple drop-coating of the initial solutions over gold electrodes, which were coated over a glass substrate. The structural properties of BiVO4 and La-doped BiVO4 samples were studied using x-ray diffractometer, diffuse reflectance spectroscopy, scanning electron microscopy, atomic force microscopy, and compositional analysis. A chamber was designed to install the sensing device and also controllable tools for gas flow rate and temperature. Changes in the resistance of the prepared layers were recorded during exposure to various amounts of ethanol vapor at different temperatures. Both BiVO4 and La-doped BiVO4 layers showed measurable responses in the form of resistance drop (increased conductivity). The higher temperatures up to 450 °C led to stronger signals. The layer containing lanthanum showed signals with shorter recovery times. Introduction of lanthanum caused smaller crystallite sizes in addition to the formation of tetragonal phase of BiVO4. Presence of lanthanum increased the amounts of grain boundaries, magnitude of the response, and sensitivity. Sensitivity of La-doped BiVO4 was almost twice that of the BiVO4 at concentrations of 150–500 ppm of ethanol. Also, the correlation of the response as a function of concentration of ethanol in gas phase was exploited, and two different linear ranges were observed for the lower and higher concentrations.