BiVO4 Thin Films Research Articles

Nanostructured bismuth vanadate (BiVO4) thin films for efficient visible light photocatalysis

Successful strategies on combining the versatility of mechanochemical synthesis with rf-sputtering technique were developed for the controllable deposition of non-toxic and stable monoclinic BiVO4 thin films, which has a relatively narrow band gap (≈2.4 eV) that allows a direct photoactivation under visible light. The coupled deposition approach enables the stabilization of single phase, high-quality monoclinic BiVO4 thin films, deposited using sintered target made with nano-powders produced from mechanochemical process. Investigations of structural, vibrational, morphological and optical properties were carried out using HT-XRD, XRD, Raman, AFM, FE-SEM and UV-Visible absorption techniques. Monoclinic scheelite phase of BiVO4 was studied with different morphologies and textures of thin film surfaces including the occurrence of organized nano-islands (20-50 nm) under defined deposition parameters. The sputtered BiVO4 films display an electronic band gap of 2.5 eV, which is suitable for harvesting visible light radiation. Photocatalytic (PC) experiments were performed with selected textured BiVO4 thin films for the degradation of rhodamine 6G (Rh6G) dyes under visible light irradiation. Different PC efficiencies were obtained and the results were interpreted based on the thin film morphologies and surface characteristics.

Solvent-engineering assisted synthesis and characterization of BiVO4 photoanode for boosting the efficiency of photoelectrochemical water splitting

A simple direct, scalable and reproducible approach for preparing photoelectrochemically active BiVO4 films on fluorine-doped tin oxide (FTO) substrates by hydrothermal method using a mixing solvent of ethylene glycol/water was presented for the first time. As-prepared BiVO4 thin films were found to have pure monoclinic scheelite structure with a compact, irregular microstructured morphology and strong absorption in the visible-light region with estimated bandgap of 2.4eV. When applied as an electrode material for water splitting in borate electrolyte solution, a significantly enhanced stability, coupled with a 15-fold increase in photocurrent density (ca. 1.04mAcm−2) are observed compared to those of conventional hydrothermal synthesized films. The BiVO4 prepared with mixed solvent gave a substantial enhancement of the incident photon to current efficiency (IPCE) and absorbed photon to current efficiency (APCE) during light driven oxygen evolution reaction.

Mo-doped BiVO4 thin films – high photoelectrochemical water splitting performance achieved by a tailored structure and morphology

Facile sol–gel synthesis of Mo:BiVO4 thin films with optimized morphology results in reduced surface recombination and enhanced hole transfer efficiency.

Enhanced Photocatalytic Performance Depending on Morphology of Bismuth Vanadate Thin Film Synthesized by Pulsed Laser Deposition.

We have fabricated high quality bismuth vanadate (BiVO4) polycrystalline thin films as photoanodes by pulsed laser deposition (PLD) without a postannealing process. The structure of the grown films is the photocatalytically active phase of scheelite-monoclinic BiVO4 which was obtained by X-ray diffraction (XRD) analysis. The change of surface morphology for the BIVO4 thin films depending on growth temperature during synthesis has been observed by scanning electron microscopy (SEM), and its influence on water splitting performance was investigated. The current density of the BiVO4 film grown on a glass substrate covered with fluorine-doped tin oxide (FTO) at 230 °C was as high as 3.0 mA/cm2 at 1.23 V versus the potential of the reversible hydrogen electrode (VRHE) under AM 1.5G illumination, which is the highest value so far in previously reported BiVO4 films grown by physical vapor deposition (PVD) methods. We expect that doping of transition metal or decoration of oxygen evolution catalyst (OEC) in our BiVO4 film might further enhance the performance.

Direct Time-Resolved Observation of Carrier Trapping and Polaron Conductivity in BiVO4

BiVO4 is a promising photoanode candidate for water splitting applications, but its microscopic charge carrier transport properties are not yet fully understood. We investigated the photoinduced carrier mobility for undoped and 1% tungsten-doped BiVO4 thin films in an early time window from 1 ps to 1 ns using THz spectroscopy. The combined electron–hole effective mobility gradually decreases with time by 1 order of magnitude starting at an upper limit of ∼0.4 cm2 V–1 s–1. The loss is attributed to carrier localization. We provide for the first time direct time-resolved evidence of hole polaron formation accompanied by the temporal buildup of a polaron population in parallel to initial carrier trapping. A mobility of 0.02 cm2 V–1 s–1 is found for the self-trapped carriers, which leads to a thermal hopping activation energy of ∼90 meV.

(Invited) Engineering Bismuth-Based Ternary Oxide Thin Films for Photoelectrochemical Water Oxidation

Bismuth based ternary oxide powders (such as BiVO4, Bi2WO6 and Bi2MoO6) have been reported to be active for oxidation of organic substances and water under visible light. In general, these materials have sufficient absorption within the solar spectrum and stability against photocorrosion. The preparation of bismuth based ternary oxide is inexpensive, environmentally benign and can be made using a number of different facile methods. In this presentation, we investigate the performance of these bismuth based ternary oxides in photoelectrochemical water splitting. A few aspects will be highlighted to substantiate the differences when they were employed in photocatalytic water splitting or through photoelectrochemical means. It is generally acknowledged that surface area and crystallinity of photocatalysts are critical factors regulating performances in powder-type suspension reactions. When they were made into thin film, quality of the contact between bismuth based ternary oxides and the charge collecting substrate becomes another crucial factor. In this work, we also discuss a few strategies formulated to directly synthesise these ternary oxide thin films without having the powder oxide as the intermediate. BiVO4, Bi2WO6 and Bi2MoO6 thin films are synthesised in this work. BiVO4 with controlled exposed facet is engineered to study the dynamic of charge interactions between their respective facets. It is revealed that a balance of charge transfer and charge trapping are occurring to govern the overall performance of BiVO4. From that point of view, specific interface is engineered to boost the higher charge transfer efficiency, for example, by using graphene as electron mediator. Bi2WO6 and Bi2MoO6 thin films are synthesised by a direct conversion of anodised WO3 and MoO3 films. The adhesion of the resultant Bi2WO6 and Bi2MoO6 on the substrate is highly stable. With the control of synthesis parameter, a concentration gradient can be introduced to form the composite of Bi2WO6-WO3 and Bi2MoO6-MoO3 with desired composition. With the favourable band alignment in this composite material, an improved activity in photoelectrochemical water oxidation is observed.

Photoelectrocatalytic oxidation of phenol for water treatment using a BiVO4 thin-film photoanode

Abstract

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.

Effect of the KOH chemical treatment on the optical and photocatalytic properties of BiVO4 thin films

In this work, we present the structural, optical and photocatalytic properties of BiVO4 thin films produced by a dual-magnetron sputtering process using both Bi2O3 (α-phase, 99.98 % purity) and V (99.9 % purity) targets under Ar/O2 atmosphere with a ratio of 18:2. The films were deposited varying the power applied to the targets to obtain stoichiometric films, and the monoclinic structure was achieved by post-deposition annealing. The dual process was chosen to better control the Bi/V ratio since Bi and V have very different sputtering yields. In particular, the influence of a chemical treatment using potassium hydroxide (KOH) on the optical properties and different dye discolorations (acid blue 113 and methyl orange) is discussed. The optical properties were studied by reflectance and transmittance spectroscopy, where the spectra were fitted to obtain the refractive index dispersion and the optical band gap of the BiVO4 as a function of the film structure, as determined by X-ray diffraction and Raman spectroscopy.

Evaluation of the charge transfer kinetics of spin-coated BiVO4 thin films for sun-driven water photoelectrolysis

The present research work focuses on bismuth vanadate (BiVO4) thin films deposited on FTO-coated glass electrodes through the spin-coating technique, and discusses the influence of different film morphologies (dense and porous) on the physicochemical properties and photoelectrochemical (PEC) performance of the as-prepared photoanodes, for the water splitting reaction. The surface recombination phenomenon, which is one of the main issues of BiVO4, has been quantified by means of two distinct approaches: transient photocurrent measurements and electrochemical impedance spectroscopy (EIS). This phenomenon has resulted to be higher in the porous material, thus a poorer performance has been observed than in the dense material. In order to increase the BiVO4 efficiency, a cobalt phosphate (CoPi) catalyst has been photo-electrodeposited onto the best BiVO4 electrode, employing an optimized technique and a photocurrent of up to 3mA/cm2 at 1.23V vs. RHE under neutral pH and 1 sun irradiation (100mW/cm2) has been achieved. The charge transfer kinetics of the BiVO4 photoanodes, with and without CoPi, has also been quantified. The beneficial effect of this water oxidation catalyst, as well as the influence of the preparation method on the uniformity of the film and on its actual performance, is discussed in view of its prospective application in a real PEC device.

Dip-coating deposition of BiVO4/NiO p–n heterojunction thin film and efficiency for methylene blue degradation

This paper deals with a simple FTO/p–n heterojunction electrode assembly formed by BiVO4 and NiO thin films, where the precursor solution is obtained by solution combustion synthesis and precipitation in aqueous media techniques, respectively, whereas the thin films were deposited by the dip-coating deposition process. The FTO/p–n electrodes are characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and UV–Vis spectroscopy. The performance analysis based on the methylene blue degradation reaction have shown that the p–n electrodes, FTO/p-NiO/n-BiVO4 and FTO/n-BiVO4/p-NiO have higher electroactivity under visible light irradiation condition when compared to single BiVO4 thin film, deposited separately, with estimated k obs value of 340 × 10−4 min−1, 270 × 10−4 min−1 and 150 × 10−4 min−1, respectively.

Carbon quantum dots coated BiVO4 inverse opals for enhanced photoelectrochemical hydrogen generation

Carbon quantum dots (CQDs) coated BiVO4 inverse opal (io-BiVO4) structure that shows dramatic improvement of photoelectrochemical hydrogen generation has been fabricated using electrodeposition with a template. The io-BiVO4 maximizes photon trapping through slow light effect, while maintaining adequate surface area for effective redox reactions. CQDs are then incorporated to the io-BiVO4 to further improve the photoconversion efficiency. Due to the strong visible light absorption property of CQDs and enhanced separation of the photoexcited electrons, the CQDs coated io-BiVO4 exhibit a maximum photo-to-hydrogen conversion efficiency of 0.35%, which is 6 times higher than that of the pure BiVO4 thin films. This work is a good example of designing composite photoelectrode by combining quantum dots and photonic crystal.

Mo-doped BiVO4 photoanodes synthesized by reactive sputtering.

We report a scalable and reproducible method for reactive co-sputtering of Mo-doped BiVO4 thin films with broad compositional control. Optimal photoanode performance is achieved at a Mo concentration of 3 at. %. Incorporation of Mo promotes growth of large grains and reduces majority carrier transport limitations, resulting in maximum AM1.5G photocurrent densities of 3.5 mA cm(-2) at 1.23 V vs. RHE in pH 6.8 buffer solution containing 0.1 M Na2 SO3 as a hole scavenger. Operation as a front-illuminated water oxidation photoanode is achieved by balancing the operational stability, catalytic activity, and parasitic optical absorption of a FeOOH oxygen evolution catalyst. FeOOH/Mo:BiVO4 thin film photoanodes enable water oxidation under the front-side illumination conditions used in integrated tandem water splitting devices.

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.

Optical constants, dispersion energy parameters and dielectric properties of ultra-smooth nanocrystalline BiVO4 thin films prepared by rf-magnetron sputtering

BiVO4 thin films have been prepared through radio frequency (rf) magnetron sputtering of a pre-fabricated BiVO4 target on ITO coated glass (ITO-glass) substrate and bare glass substrates. BiVO4 target material was prepared through solid-state reaction method by heating Bi2O3 and V2O5 mixture at 800 °C for 8 h. The films were characterized by X-ray diffraction, UV–Vis spectroscopy, LCR meter, field emission scanning electron microscopy, transmission electron microscopy and atomic force microscopy. BiVO4 thin films deposited on the ITO-glass substrate are much smoother compared to the thin films prepared on bare glass substrate. The rms surface roughness calculated from the AFM images comes out to be 0.74 nm and 4.2 nm for the films deposited on the ITO-glass substrate and bare glass substrate for the deposition time 150 min respectively. Optical constants and energy dispersion parameters of these extra-smooth BiVO4 thin films have been investigated in detail. Dielectric properties of the BiVO4 thin films on ITO-glass substrate were also investigated. The frequency dependence of dielectric constant of the BiVO4 thin films has been measured in the frequency range from 20 Hz to 2 MHz. It was found that the dielectric constant increased from 145 to 343 at 20 Hz as the film thickness increased from 90 nm to 145 nm (deposition time increased from 60 min to 150 min). It shows higher dielectric constant compared to the literature value of BiVO4.

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.

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.

Reactive Sputtering of Bismuth Vanadate Photoanodes for Solar Water Splitting

Bismuth vanadate (BiVO4) has attracted increasing attention as a photoanode for photoelectrochemical (PEC) water splitting. It has a band gap in the visible light range (2.4–2.5 eV) and a valence band position suitable for driving water oxidation under illumination. While a number of methods have been used to make BiVO4 photoanodes, scalable thin film deposition has remained relatively underexplored. Here, we report the synthesis of BiVO4 thin films by reactive sputtering. The use of separate Bi and V sputtering targets allows control of the Bi/V ratio in the film. Under optimized, slightly V-rich conditions, monoclinic phase BiVO4 with photoactivity for water oxidation is obtained. The highest photocurrents, ca. 1 mA cm–2 at the reversible O2/H2O potential with simulated AM 1.5G illumination, are obtained with bilayer WO3/BiVO4, where the WO3 serves as a hole-blocking layer.

WO3/BiVO4 composite photoelectrode prepared by improved auto-combustion method for highly efficient water splitting

We report on the improvement in the water splitting efficiency of a WO3/BiVO4 composite photoelectrode by the application of an improved auto-combustion method to the preparation of porous BiVO4 thin films. The unique feature of this preparation method is the addition of both NH4NO3, as a strong oxidizing agent, and an organic additive into BiVO4 precursor solution. The local decomposition heat of the organic additive and oxidizing agent created a porous film with small, highly crystalline BiVO4 particles. The photoelectrode has many advantages over existing ones, such as the high light-harvesting efficiency (LHE), a single BiVO4 phase, the facile access of the holes to the photoelectrode/electrolyte interface, and the ease of water and oxygen diffusion. The maximum incident photon-to-current efficiency (IPCE) was estimated to be 64% (at 440 nm, 1.23 V vs. RHE) and the applied bias photon-tocurrent efficiency (ABPE) reached as high as 1.28%. This ABPE value is highest among all oxide semiconductor photoelectrodes reported previously, except for the case of a stacking photoelectrode system.

Photoelectrochemical properties of FTO/m-BiVO4 electrode in different electrolytes solutions under visible light irradiation

Photoelectrochemical properties of FTO/BiVO4 electrode were investigated in different electrolytic solutions, potassium chloride (KCl) and sodium sulphate (Na2SO4), and under visible light irradiation condition. In order to accomplish that, an FTO/BiVO4 electrode was built by combining the solution combustion synthesis technique with the dip-coating deposition process. The morphology and structure of the BiVO4 electrode were investigated through X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. Photoelectrochemical properties were analyzed through chronoamperometry measurements. Results have shown that the FTO/BiVO4 electrode presents higher electroactivity in the electrolyte Na2SO4, leading to better current stabilization, response time, and photoinduced current density, when compared to KCl electrolyte. Besides, this electrode shows excellent performance for methylene blue degradation under visible light irradiation condition. In Na2SO4, the electrode has shown higher degradation rate, 51 %, in contrast to 44 % in KCl, plus higher rate constant, 174 × 10−4 min−1 compared to 150 × 10−4 min−1 in KCl. Results presented in this communication leads to the indication of BiVO4 thin films as alternate materials to use in heterogeneous photoelectrocatalysis, more specifically in decontamination of surface water.