BiVO4 Film Research Articles

Chemically induced porosity on BiVO4 films produced by double magnetron sputtering to enhance the photo-electrochemical response

Double magnetron sputtering (DMS) is an efficient system that is well known because of its precise control of the thin film synthesizing process over any kind of substrate. Here, DMS has been adopted to synthesize BiVO4 films over a conducting substrate (FTO), using metallic vanadium and ceramic Bi2O3 targets simultaneously. The films were characterized using different techniques, such as X-ray diffraction (XRD), UV-Vis spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and profilometry. The photo-electrochemical analysis was performed using linear scan voltammetry, chronoamperometry and electrochemical impedance spectroscopy (EIS) under the illumination of simulated solar light at 1 Sun. The photocurrent density of the sputtered BiVO4 thin films could be improved from 0.01 mA cm(-2) to 1.19 mA cm(-2) at 1.23 V vs. RHE by chemical treatment using potassium hydroxide (KOH). The effect of KOH was the removal of impurities from the grain boundaries, leading to a more porous structure and more pure crystalline monoclinic BiVO4 particles. Such variations in the microstructure as well as the improvement of the charge transfer properties of the BiVO4 film after the KOH treatment were confirmed and studied in depth by EIS analysis.

Photoelectrochemical performance of W-doped BiVO4 thin films deposited by spray pyrolysis

The effects of tungsten doping and hydrogen annealing on the photoelectrochemical (PEC) performance of bismuth vanadate (BiVO4) photoanodes for solar water splitting were studied. Thin films of BiVO4 were deposited on indium tin oxide-coated glass slides by ultrasonic spray pyrolysis of an aqueous solution containing bismuth nitrate and vanadium oxysulfate. Tungsten doping was achieved by adding either silicotungstic acid (STA) or ammonium metatungstate (AMT) to the precursor. The 1.7- to 2.2-μm-thick films exhibited a highly porous microstructure. Undoped films that were reduced at 375°C in 3% H2 exhibited the largest photocurrent densities under 0.1 W cm−2 AM1.5 illumination, where photocurrent densities of up to 1.3 mA cm−2 at 0.5 V with respect to Ag/AgCl were achieved. Films doped with 1% or 5% (atomic percent) tungsten from either STA or AMT exhibited reduced PEC performance and greater sample-to-sample performance variations. Powder x-ray diffraction data indicated that the films continue to crystallize in the monoclinic polymorph at low doping levels but crystallize in the tetragonal scheelite structure at higher doping. It is surmised that the phase and morphology differences promoted by the addition of W during the deposition process reduced the PEC performance as measured by photovoltammetry.

One-step preparation of the BiVO4 film photoelectrode

A one-step method of preparing photoelectrochemically active nanostructured BiVO4 films is reported based on thermolysis (500 °C in air) of a polyethylene glycol (PEG300) “paint-on” precursor solution containing Bi3+ (as nitrate) and VO43− (as the metavanadate ammonium salt). Films are formed directly on tin-doped indium oxide (ITO) substrates and characterised by electron microscopy (scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS)), X-ray diffraction, Raman spectroscopy, and photoelectrochemistry. The nanocrystalline film exhibited typically up to 52 % incident photon to current efficiency (IPCE) at 1.0 V vs. saturated calomel electrode (SCE) in aqueous 0.5 M Na2SO4 with oxalate, strongly enhancing photocurrents.

Photoelectric properties of BiVO4 thin films deposited on fluorine doped tin oxide substrates by a modified chemical solution deposition process

BiVO4 films deposited on Fluorine doped tin oxide glass substrates were successfully prepared by a modified chemical solution deposition process. Structure and optical spectrum analysis show that the resultant BiVO4 films consist entirely of monoclinic scheelite structure and have a narrow band gap of ～2.66 eV. The films were investigated by photoelectrochemical and photovoltaic measurements with regard to hydrogen production and solar energy conversion under visible light. The BiVO4 photoanodes show significantly higher visible light induced photoelectrochemical performance (～1.1 mA/cm2 at 1.0 V vs. Ag/AgCl) than those reported ones, which is very promising for splitting water to H2 and O2. A Schottky BiVO4 solar cell was also investigated for comparison with photoelectrochemical measurements. The correlation between the photoelectrochemical and photovoltaic behavior for BiVO4 was explained. Our research should provide important support for the applications of BiVO4 films or its modified forms such as doping and nanocomposite in heterojunction photoelectrochemical cells and solar cells with suitable energy level alignment at the interface.

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.

Efficient solar photoelectrolysis by nanoporous Mo:BiVO4through controlled electron transport

A detailed understanding of doping level, electron diffusion length and coefficient, as well as light capture and charge separation efficiencies in nanoporous Mo-doped BiVO4 (Mo:BiVO4) photoanodes is obtained using photoelectrochemical techniques. Efficient water oxidation is achieved by doping with 1.8% Mo, resulting in a several-fold enhancement in photooxidation rate versus non-doped BiVO4. Two techniques are used to study the effect of Mo doping on the electron transport: (1) an analysis of the front/back illumination ratio of incident photon-to-current efficiency and (2) intensity modulated photocurrent spectroscopy. These techniques show that Mo doping improves the diffusion coefficient four-fold and increases the diffusion length to ca. 300 nm (from 10 nm for the non-doped material), which is also the empirically-determined optimal Mo:BiVO4 film thickness for photoelectrolysis. These films are found to have a 90% charge separation efficiency and an 80% absorbed photon-to-current efficiency, excellent values for metal oxide photoabsorbers. Among the many oxygen evolution catalysts studied, surface modification with iron oxyhydroxide (FeOOH), a simple earth abundant catalyst, dramatically enhances the water oxidation performance of Mo:BiVO4 to an integrated IPCE of 2.41 mA cm(-2) and a photocurrent density of 2.77 mA cm(-2) in neutral phosphate at 1.23 V vs. RHE.

Reduced monoclinic BiVO4 for improved photoelectrochemical oxidation of water under visible light

A monoclinic BiVO4 film was grown on a transparent conducting substrate for photoelectrochemical oxidation of water. A photocurrent up to 2.3 mA cm(-2) under visible light (λ > 420 nm) was achieved after treating the sample simply by electrochemical reduction followed by NaBH4. The high photocurrent is believed to be due to the improved carrier separation and transportation as a result of increased donor density.

Adsorption-controlled growth of BiVO4 by molecular-beam epitaxy

Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 ∥ (001)YSZ with [100]BiVO4 ∥ [100]YSZ. Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 ± 0.1 eV.

Computational and Photoelectrochemical Study of Hydrogenated Bismuth Vanadate

We demonstrate hydrogenation as a facile method to significantly enhance the performance of BiVO4 films for photoelectrochemical water oxidation. Hydrogenation was performed for BiVO4 films by annealing them in hydrogen atmosphere at elevated temperatures between 200 and 400 °C. Hydrogen gas can reduce BiVO4 to form oxygen vacancies as well as hydrogen impurities. DFT calculation predicted that both oxygen vacancies and hydrogen impurities are shallow donors for BiVO4 with low formation energies. These defects could increase the donor densities of BiVO4 without introducing deep trap states. Electrochemical impedance measurements showed that the donor densities of BiVO4 films were significantly enhanced upon hydrogenation. Hydrogen-treated BiVO4 (H-BiVO4) photoanodes achieved a maximum photocurrent density of 3.5 mA/cm2 at 1.0 V vs Ag/AgCl, which is 1 order of magnitude higher than that of air-annealed BiVO4 obtained at the same potential. The enhanced photoactivities were attributed to increased donor densities of H-BiVO4, which facilitates the charge transport and collection.

Fabricating CdS/BiVO4 and BiVO4/CdS heterostructured film photoelectrodes for photoelectrochemical applications

The photoelectrochemical (PEC) properties of heterostructured CdS/BiVO4 and BiVO4/CdS film electrodes on conducting glass for hydrogen production under visible light were investigated. These two types heterostructured film electrodes were prepared using spin coating method and ultrasonic spray pyrolysis method. The structural analyses of the prepared films were determined by using XRD, SEM, EDX and UV–vis. Photoelectrochemical measurements were carried out in a convenient three electrodes cell with 0.5 M Na2SO3 aqueous solution. In order to investigate band gap influence of electrode PEC property, a series ITO/Cd1−xZnxS/BiVO4 and ITO/BiVO4/Cd1−xZnxS (x = 0 ∼ 1) film electrodes were also synthesized. After PEC test, a maximum photocurrent density from ITO/CdS/BiVO4 film electrode was confirmed. The maximum photocurrent density, 3 times and 113 times as that of single CdS film electrode and single BiVO4 film electrode, respectively. Incident photon to current conversion (IPCE) of as prepared film electrodes were measured and the value were 65% (ITO/CdS/BiVO4), 22% (single CdS film) and 10% (ITO/BiVO4/CdS) at 480 nm with 0.3 V external bias. Comparison with ITO/BiVO4/CdS electrode and single Cd1−xZnxS electrodes, the heterostructured ITO/CdS/BiVO4 electrode can effectively suppress photogenerated electron-hole recombination and enhance light harvesting. Therefore, the ITO/CdS/BiVO4 electrode gave the maximum photocurrent density and IPCE value.

Structuring porous “sponge-like” BiVO4 film for efficient photocatalysis under visible light illumination

The porous “sponge-like” BiVO4 films were prepared with the polystyrene (PS) as pore forming material and F-doped SnO2 (FTO) glass as substrate. SEM observation displayed that “sponge-like” BiVO4 film with interconnect pore structure was successfully obtained. DRS analysis indicated the light absorption ability of BiVO4 film was enhanced by constructing porous structure. The measurement of surface area showed that porosity could elevate the surface area of the BiVO4 film. The experiment of PEC degradation of phenol showed that the degradation rate on the porous BiVO4 film (with 200μL) was 2.68times as much as that on the BiVO4 film. The enhanced PEC performance was attributed to the increased photo absorption ability, elevated surface area, and more efficient reactant transfer.

Fabrication Porous Ag/BiVO<sub>4</sub> Film for Efficient Degradation of Phenol under Visible Light

Porous BiVO4 film was fabricated by templating procedure with polystyrene (PS) balls as the template. Ag particles were loaded on the porous BiVO4 film by photoreduction technique. SEM revealed there were many pores in the film and Ag particles were successfully loaded on the porous BiVO4 film. XRD analysis indicated that the chemical state of Ag particles was metallic Ag. The experiment of photocatalytic degradation of phenol showed that the degradation rate on porous Ag/BiVO4 film was 4.74 times as much as that on the BiVO4 film. The enhanced photocatalytic performance was firstly attributed to the rapid transfer of the photogenerated electrons from BiVO4 to Ag, which could increase the charge carrier separation, limit their recombination, and enhance the photocatalytic efficiency. Furthermore, porosity of the photocatalyst may enhance diffusion process and decrease diffusion resistance of the reactants throughout the channels in the photocatalyst, and thus improves the surface reactions.

Nature and Light Dependence of Bulk Recombination in Co-Pi-Catalyzed BiVO4 Photoanodes

BiVO4 is considered to be a promising photoanode material for solar water splitting applications. Its performance is limited by two main factors: slow water oxidation kinetics and poor charge separation. We confirm recent reports that cobalt phosphate (Co-Pi) is an efficient water oxidation catalyst for BiVO4 and report an AM1.5 photocurrent of 1.7 mA/cm2 at 1.23 V vs RHE for 100 nm spray-deposited, compact, and undoped BiVO4 films with an optimized Co-Pi film thickness of 30 nm. The charge separation of these films depends strongly on light intensity, ranging from 90% at low light intensities to less than 20% at intensities corresponding to 1 sun. These observations indicate that the charge separation efficiency in BiVO4 is limited by poor electron transport and not by the presence of bulk defect states, interface traps, or the presence of a Schottky junction at the back-contact.

Efficient and Stable Photo-Oxidation of Water by a Bismuth Vanadate Photoanode Coupled with an Iron Oxyhydroxide Oxygen Evolution Catalyst

BiVO(4) films were prepared by a simple electrodeposition and annealing procedure and studied as oxygen evolving photoanodes for application in a water splitting photoelectrochemical cell. The resulting BiVO(4) electrodes maintained considerable photocurrent for photo-oxidation of sulfite, but generated significantly reduced photocurrent for photo-oxidation of water to oxygen, also decaying over time, suggesting that the photoelectrochemical performance of BiVO(4) for water oxidation is mainly limited by its poor catalytic ablity to oxidize water. In order to improve the water oxidation kinetics of the BiVO(4) electrode, a layer of FeOOH was placed on the BiVO(4) surface as an oxygen evolution catalyst using a new photodeposition route. The resulting BiVO(4)/FeOOH photoanode exhibitied significantly improved photocurrent and stability for photo-oxidation of water, which is one of the best among all oxide-based phoatoanode systems reported to date. In particular, the BiVO(4)/FeOOH photoanode showed an outstanding performance in the low bias region (i.e., E < 0.8 V vs RHE), which is critical in determining the overall operating current density when assembling a complete p-n photoelectrochemical diode cell. The photocurrent-to-O(2) conversion efficiency of the BiVO(4)/FeOOH photoanode is ca. 96%, confirming that the photogenerated holes in the BiVO(4)/FeOOH photoanode are indeed excusively used for O(2) evolution.

Incorporation of Mo and W into nanostructured BiVO4 films for efficient photoelectrochemical water oxidation

Porous, nanostructured BiVO4 films are incorporated with Mo and W by simultaneous evaporation of Bi, V, Mo, and W in vacuum followed by oxidation in air. Synthesis parameters such as the Bi : V : Mo : W atomic ratio and deposition angle are adjusted to optimize the films for photoelectrochemical (PEC) water oxidation. Films synthesized with a Bi : V : Mo : W atomic ratio of 46 : 46 : 6 : 2 (6% Mo, 2% W) demonstrate the best PEC performance with photocurrent densities 10 times higher than for pure BiVO4 and greater than previously reported for Mo and W containing BiVO4. The films consist of a directional, nanocolumnar layer beneath an irregular surface structure. Backside illumination utilizes light scattering off the irregular surface structure resulting in 30-45% higher photocurrent densities than for frontside illumination. To improve the kinetics for water oxidation Pt is photo-deposited onto the surface of the 6% Mo, 2% W BiVO4 films as an electrocatalyst. These films achieve quantum efficiencies of 37% at 1.1 V vs. RHE and 50% at 1.6 V vs. RHE for 450 nm light.

Highly Improved Quantum Efficiencies for Thin Film BiVO4 Photoanodes

BiVO4 has received much recent interest as a promising photocatalyst for oxygen evolution from water, but little is known about the factors that limit its performance as a photoanode. In this article, we report on highly efficient and reproducible BiVO4 photoanodes prepared by a new spray pyrolysis recipe. For undoped films deposited on a transparent conducting substrate (F-doped SnO2, FTO), electron transport and charge collection at the back-contact were found to limit the photoresponse. Electron transport could be greatly enhanced by donor-doping with 1% W, while the charge collection problem has been solved by introducing a thin (∼10 nm) interfacial layer of SnO2 in between the FTO and the BiVO4. This layer presumably acts as a hole mirror that prevents recombination via FTO-related defect states at the FTO/BiVO4 interface. By addressing these two issues, the external quantum efficiency (IPCE) of spray-deposited BiVO4 films was improved by a factor of ∼7, leading to an unprecedented IPCE of 46% (at 450 nm) at 1.63 VRHE for 1% W-doped BiVO4 films deposited on FTO/SnO2.

Photocatalytic Microreactor Using Monochromatic Visible Light

A photocatalytic planar reactor with an immobilized nanoporous BiVO4 film has been integrated with a blue-light panel LED, which acts as the light source. The microreactor utilizes two BiVO4-coated glasses as the top and the bottom layer and a UV curable resin layer as the spacer. The UV spacer is patterned with tree-shaped microchannels and a reaction chamber (10 ´ 10 ´ 0.1 mm3). It was found that under blue light of 140 mW/cm2 (the LED is powered at 11 V, 402 nm ) and the flow rate of 37.5 µl/min, the microreactor has the optimum performance with a degradation rate of about 3%/s and a photonic utilization efficiency ³ 0.02%. However, the photonic efficiency can be improved to 0.3% when LED is powered at 9 V.

Tailoring Interfaces for Electrochemical Synthesis of Semiconductor Films: BiVO4, Bi2O3, or Composites

The mechanistic aspects of a two-step method for the electrodeposition of a BiVO4 semiconductor (previously developed in the Rajeshwar/Tacconi laboratory) were elaborated by the combined application of voltammetry and EQCM. The electrosynthesized films were also characterized ex situ using SEM, EDX, XRD, and XPS. Stripping of pre-electrodeposited bismuth films, followed by reaction either with VO43− (formed by hydrolysis from the initially added VO3− species) or with hydroxide ions, produced BiVO4 or Bi2O3 thin films in situ on the Pt electrode. The deposition potential, pH of the electrolyte, and choice of vanadium precursor were shown to be crucial variables in the composition of the electrodeposited film. When a more positive potential than 0.5 V (vs Ag/AgCl reference) was applied to the Bi-modified electrode in VO3−-containing electrolyte, the content of Bi2O3 in the film increased instead of BiVO4. Stripping efficiency of the predeposited bismuth layer was increased at acidic electrolytes and resulte...

Photoelectrochemical Oxidation of Water Using Nanostructured BiVO4 Films

Nanostructured BiVO4 films were synthesized by coevaporation of bismuth and vanadium in an oxygen ambient, a process referred to as reactive ballistic deposition (RBD). The films were tested in various electrolyte solutions to assess their activity for photoelectrochemical water oxidation. Deposition parameters, including the V/Bi atomic flux ratio and the incident angle of deposition, were adjusted. Films deposited with excess vanadium (V/Bi = 2) and incident angles of deposition at 65° showed the highest initial photocurrents with IPCE values above 21% for light wavelengths of 340−460 nm (in 0.5 M Na2SO4 at 1.0 V vs Ag/AgCl). With continued illumination the excess vanadium in these films dissolved into the electrolyte and the photocurrents dropped by 60−75% before reaching steady state. The steady-state photocurrent and IPCE values (above 14% for 340−460 nm light) were higher than the initial values for films synthesized with stoichiometric amounts of vanadium and bismuth (V/Bi = 1) and incident angles ...

Cobalt–phosphate complexes catalyze the photoelectrochemical water oxidation of BiVO4 electrodes

BiVO(4) semiconductor electrodes were coupled with cobalt-phosphate complexes (CoPi) to enhance the photoelectrochemical (PEC) performance for water oxidation reaction. CoPi was deposited on a 550 nm-thick BiVO(4) film via electrodeposition (ED) and photodeposition (PD) methods for comparison of their effects. The CoPi on BiVO(4) exhibited Co : P atomic ratios of approximately 1 : 7 for the electrodeposited sample and approximately 1 : 18 for the photodeposited sample, and Co(2+) and Co(3+) co-existed in both samples. Optimized CoPi ED resulted in a CoPi overlayer of approximately 850 nm thick, which showed an electrochromic-like behavior that was likely due to limited access of phosphate into BiVO(4) across the CoPi layer. Optimized CoPi PD, however, had very thin and rather uniform CoPi dispersion and did not show electrochromic-like behavior. Despite the lesser amount of CoPi, the PEC performance of BiVO(4)/CoPi (PD) was comparable to that of BiVO(4)/CoPi (ED). Real-time measurements of the headspace molecular oxygen that evolved from water oxidation indicated that CoPi enhances O(2) production and photocurrent generation at BiVO(4) by a factor of around 15 and a maximum of 20, respectively, at 0.576 V(SCE) (equivalent to 1.23 V(RHE)) under air mass 1.5 irradiation (400 mW cm(-2)). Prolonged irradiation of BiVO(4)/CoPi (ED) resulted in a reduced Co : P ratio to 1 : 1.77 without changing the mixed valency of Co(II/III). This finding indicates that incorporation of phosphate into the CoPi was kinetically slower than water oxidation. The primary role of CoPi has been suggested as a hole-conducting electrocatalyst making the photogenerated electrons more mobile and, consequently, increasing conductivity and boosting the PEC water oxidation performance of BiVO(4).