BiVO4 Film Research Articles

Insight into the Improvement Mechanism of Copper Oxide/BiVO4 Heterojunction Photoanodes for Solar Water Oxidation

N-type BiVO4 films coupled with p-type copper oxide nanoparticles were investigated as photoanodes for water oxidation. Compared with the BiVO4 film, the Cu2O/BiVO4, CuO/BiVO4 and CuxO/BiVO4 films all showed significantly improved photoelectrochemical water oxidation activity and stability. As expected, the radiative recombination of charge on the three kinds of copper oxide/BiVO4 films was decreased due to the p-n junction effect between copper oxide and BiVO4. However, our investigations indicated that the electrocatalytic water oxidation on the copper oxide could be initiated when the copper oxide/BiVO4 photoanodes were irradiated by solar light; therefore, the copper oxide-coupling resulted in a more obvious improvement in the water oxidation kinetics of BiVO4 than the thermodynamic improvement in the charge separation of BiVO4. Of the three kinds of copper oxide/BiVO4 films, the CuxO/BiVO4 film comprising BiVO4 film coupled with a combination of Cu2O/CuO nanoparticles showed the best solar water oxidation performance due to the better electrocatalytic activity for water oxidation of CuxO compared with Cu2O or CuO alone. The kinetic investigations and analyses in the present work provide comprehensive insight into the mechanism of the copper oxide/BiVO4 photoanodes for solar water oxidation.

Multilayered Fluorine Doped SnO2 Inverse Opal/WO3/BiVO4 Film for Solar Water Oxidation: Systematic Development and Defined Role of Each Layer

We developed the multilayered fluorine doped SnO2 (abbreviated as FTO) inverse opal (IO)/WO3/BiVO4 film by sol-gel based spin coating, subsequently followed by facile electrodeposition. Above FTO IO template, both WO3 and BiVO4 layers were prepared by the electrodeposition methods in the variation of charge amount of 200–1,000 mC cm−2 and of 300–1,500 mC cm−2, respectively. On increasing the loaded charge amount of WO3 under the constant loaded amount of BiVO4, the light absorption in the near 400 nm is increased to steadily enhance PEC performance. In contrast, the further loaded WO3 layer of 800–1,000 mC cm−2 on FTO IO film degrades PEC activity, due to the increase of the intrinsic material and interfacial resistances. Meanwhile, on increasing the electrodeposited charge amount of BiVO4 under the constant charge amount of WO3, the light absorption toward the visible wavelength is sharply increased, and gives rise to the enhancement of PEC performance; and conversely, the more loaded BiVO4 of 1,200–1,500 mC cm−2 induced the decay of PEC performance. The optimum condition in FTO IO/WO3 (600 mC cm−2)/BiVO4 (600 mC cm−2) film was well established, and the function or role of each layer was confirmed using photoluminescence, charge separation/transfer efficiency, and photovoltage-time spectra.

Plasmonic Au Nanopraticles Modified Nanopyramid-Arrays BiVO4 with Enhanced Photoelectrochemical Activity

Gold nanoparticles (∼24 nm) were successfully loaded on the surface of BiVO4 nanopyramidal arrays by simple electrostatic self-assembly approach. By decorating gold nanoparticles on the surface of BiVO4 films, The photocurrent of as-prepared Au/BiVO4 photoanode increased from around 0.42 mA/cm2 for pristine BiVO4 to nearly 0.93 mA/cm2 at 1.23 vs. RHE under AM 1.5 illumination. The improved photoresponse of the Au/BiVO4 was found as a result from higher carrier generation and enhanced charge separation with the decoration of Au nanoparticles. The successful deposition provide a new route for desigining plasmonic photoanode and offers better understanding the photochemistry of noble metal-complex oxide heterostructures for photoelectrichemical water splitting.

A BiVO4 film photoanode with re-annealing treatment and 2D thin Ti3C2TX flakes decoration for enhanced photoelectrochemical water oxidation

Bismuth vanadate (BiVO4) has been envisioned as a promising photoanode material for photoelectrochemical (PEC) water oxidation. However, the PEC performance of BiVO4 based photoanodes is usually dragged down by the poor charge transport and slow kinetics for an oxygen evolution reaction (OER). Here, a novel composite photoanode of Ti3C2TX/BiVO4 was fabricated for the first time by spin coating thin Ti3C2TX (MXene) flakes onto the surface of a BiVO4 film grown on the FTO substrate. A facile re-annealing treatment of bare BiVO4/FTO photoanode in argon can impressively boost the photocurrent density from 2.1 mA cm−2 up to 2.95 mA cm−2. Experimentally, we clearly elucidated that by re-annealing, the interfacial (BiVO4/FTO) charge recombination could be effectively suppressed due to the better contact between the BiVO4 film and FTO. The coating of thin Ti3C2TX flakes onto the BiVO4 film would further increase the photocurrent density to 3.45 mA cm−2 (measured at 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination), exhibiting a photoconversion efficiency of 0.78% and a surface charge separation efficiency of 73%. The outstanding PEC performance can be mainly ascribed to the coating of the thin Ti3C2TX (MXene) flakes which function as co-catalysts, facilitating the charge transfer of the photogenerated carriers and reducing the charge recombination on the BiVO4 surface. Our findings provide a facile re-annealing strategy and a novel two-dimensional (2D) material of Ti3C2TX (MXene) as co-catalysts to improve the performance of the BiVO4 photoanode for PEC water oxidation.

Electrospun nanostructured Co3O4/BiVO4 composite films for photoelectrochemical applications

Co3O4/BiVO4 (Co/BiV) based nanostructured photoanode films were fabricated by simple facile electrospinning method and were characterized by a variety of techniques. It was found that the films have web-linked structure, composed of BiVO4 and Co3O4 nanostructures. The photoanode was used for the photoelectrochemical (PEC) degradation of bisphenol-A (BPA) by illumination at 0.25 V vs. SCE assisted by 1 mM peroxymonosulfate (PMS). The removal percentage of BPA was enhanced to 96% for the Co/BiV film that was much better than that of BiVO4 film, which was 48%. The pseudo-first kinetic constants were raised from 0.1126 min−1 to 0.4714 min−1, respectively. The enhanced PEC performance of Co/BiV films can be attributed its p-n heterojunction setup and synergetic contribution of PMS, which efficiently inhibits the recombination of photogenerated electron-hole pairs. The free radicals quenching experiment and electron spin resonance suggested that the major reactive oxygen species were photogenerated holes, superoxide radicals and sulfate radicals. These findings demonstrate that PMS assisted Co/BiV films, are good candidates for PEC application in environmental purification.

Boosted Water Oxidation Activity and Kinetics on BiVO4 Photoanodes with Multihigh-Index Crystal Facets.

The crystal facet of the BiVO4 photoanode has potential influence on its charge-transfer and separation properties as well as water oxidation kinetics. In the present work, a BiVO4 polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was synthesized by a facile Bi2O3 template-induced method and investigated as a photoanode for water oxidation. In comparison with the normal BiVO4 film with a {121} monohigh-index facet, the BiVO4 film with multihigh-index crystal facets shows higher activity and faster kinetics for photoelectrochemical water oxidation. Specifically, a higher photocurrent density of 1.21 mA/cm2 was achieved on the multihigh-index facet BiVO4 photoanode at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 M Na2SO4, which is about 200% improved over the normal BiVO4 photoanode (0.61 mA/cm2 at 1.23 V vs RHE). In addition, a negative shift of 300 mV onset potential for water oxidation was observed on the as-prepared BiVO4 photoanode (0.22 V vs RHE) relative to the normal BiVO4 photoanode (0.52 V vs RHE) in 0.1 M Na2SO4. Although the UV-vis absorbance property and water oxidation pathway not be changed, the charge-transfer and separation properties as well as the overall water oxidation kinetics on the multihigh-index facet BiVO4 film were boosted obviously. Theory calculations reveal that the adsorption of H2O molecules on BiVO4{121} and {132} high-index facets is energetically favorable for subsequent dissociation and oxidation relative to that on {010} and {110} low-index facets. Furthermore, the water oxidation limiting step on {121} and {132} high-index facets of BiVO4 is changed to the step of two protons reacting with •O to form •OOH species (•O + H2O(l) + 2H+ + 2e- → •OOH + 3H+ + 3e-), which is different from the limiting step on {010} and {110} low-index facets that corresponds to the dissociation of H2O to •OH (2H2O(l) + • → •OH + H2O(l) + H+ + e-). In addition, the overpotential of water oxidation limiting step on BiVO4{121} and {132} high-index facets is lower than that on {010} and {110} low-index facets.

Single-Source Bismuth (Transition Metal) Polyoxovanadate Precursors for the Scalable Synthesis of Doped BiVO4 Photoanodes.

Single-source precursors are used to produce nanostructured BiVO4 photoanodes for water oxidation in a straightforward and scalable drop-casting synthetic process. Polyoxometallate precursors, which contain both Bi and V, are produced in a one-step reaction from commercially available starting materials. Simple annealing of the molecular precursor produces nanocrystalline BiVO4 films. The precursor can be designed to incorporate a third metal (Co, Ni, Cu, or Zn), enabling the direct formation of doped BiVO4 films. In particular, the Co- and Zn-doped photoanodes show promise for photoelectrochemical water oxidation, with photocurrent densities >1 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE). Using this simple synthetic process, a 300 cm2 Co-BiVO4 photoanode is produced, which generates a photocurrent of up to 67 mA at 1.23 V vs RHE and demonstrates the scalability of this approach.

Solar-Driven One-Compartment Hydrogen Peroxide-Photofuel Cell Using Bismuth Vanadate Photoanode

One-compartment H2O2-photofuel cells using monoclinic scheelite BiVO4 film deposited on fluorine-doped tin oxide (ms-BiVO4/FTO) as the photoanode, Prussian blue film-coated FTO cathode, and deaerated aqueous electrolyte solution of 0.1 M NaClO4 and 0.1 M H2O2 were constructed. Mesoporous TiO2 photoanode cells with the same cathode and electrolyte solution were also prepared for comparison. The ms-BiVO4/FTO photoanode was prepared by a two-step route consisting of spin coating of a precursor solution on FTO and subsequent heating at 500 °C in the air. The thickness of the ms-BiVO4 film was controlled in the range from 50 to 500 nm by the number of the spin-coating times. There is an optimum thickness of the ms-BiVO4 film in the cell performances under illumination of simulated sunlight (AM 1.5, 100 mW cm–2, 1 sun). Under the optimum conditions, the ms-BiVO4/FTO photoanode cell provides a short-circuit current (Jsc) = 0.81 mA cm–2 and an open-circuit voltage (Voc) = 0.61 V, far surpassing the values of Jsc = 0.01 mA cm–2 and Voc = 0.31 V for the conventional mesoporous TiO2 photoanode cell. The striking cell performance is ascribable to the high visible-light activity of ms-BiVO4 for H2O2 oxidation and its low thermocatalytic activity for the decomposition.

Potential-Sensing Electrochemical AFM Shows CoPi as a Hole Collector and Oxygen Evolution Catalyst on BiVO4 Water-Splitting Photoanodes

Oxide/(oxy)hydroxide overlayers such as cobalt (oxy)hydroxide phosphate (CoPi) enhance the performance of BiVO4 water-spitting photoanodes, but the mechanism of this enhancement remains unclear. We show that if the BiVO4 layer is thin and incompletely covers an underlying conductive glass, the performance dramatically decreases as CoPi loading is increased. This is consistent with direct contact between the CoPi and conducting glass that leads to “shunt” recombination of photogenerated holes accumulated in the CoPi. For thicker BiVO4 layers that completely cover the conducting glass, these shunt pathways are blocked. We then use a nanoelectrode atomic force microscopy probe to measure, in operando, the electrochemical potential of CoPi on thick BiVO4 films under illumination. We find that CoPi is charged to a potential necessary to drive water oxidation at a rate consistent with the measured photocurrent. CoPi acts as a hole collector and is the principal driver of water oxidation on BiVO4.

Unconventional Relation between Charge Transport and Photocurrent via Boosting Small Polaron Hopping for Photoelectrochemical Water Splitting

Doping in semiconductor photoelectrodes controls defect formation and carrier transport that critically determine the device performance. Here we report an unconventional carrier transport relation that is tuned by extrinsic molybdenum (Mo) doping in BiVO4 photoanodes. Using the single-crystalline thin film approach, we identify that Mo doping significantly condenses the optimization regime between carrier transport and photon collection. For Mo-doped BiVO4 films, an unprecedentedly thin layer (50 nm), less than one-third of the pristine BiVO4 thickness, delivers larger photocurrents by overcoming the charge transport limitation, representing a regime not covered in conventional models. We provide direct evidence that Mo doping improves electron transport by boosting not only the donor density but also the electron mobility in the form of a small polaron, with the latter applying substantial impact on the photoelectrochemical performance. Density functional theory calculations reveal that fully ionized Mo...

Photoelectrochemical properties of BiVO4 thin film photoanodes prepared by aerosol pyrolysis

This study describes the preparation by aerosol pyrolysis and characterization of pure and Mo containing semiconducting BiVO4 thin films for application as a photoanode in photoassisted water electrolysis and photoelectrocatalysis.The optimum deposition temperature was 650 °C. Pure BiVO4 films had a monoclinic distorted Scheelite structure. This polymorph has a lower bandgap (2.4 eV) than the tetragonal zircon group phase (2.9 eV), which makes it more desirable for solar irradiation based photo(electro)chemical applications. Addition of Mo (14%) led to the formation of the tetragonal Scheelite type phase and significantly increased the photocurrent (from 20 to 400 μA/cm2 and from 400 to 1100 μA/cm2 at 0.6 V and 1.2 V vs. Ag/AgCl, respectively, under AM1.5 simulated solar illumination). The IPCE (incident photon to current conversion efficiency) at 405 nm was 0.24 for backside illumination at 2 V vs. RHE.

Turnip-inspired BiVO4/CuSCN nanostructure with close to 100% suppression of surface recombination for solar water splitting

Inspired by the morphologies and function of the root and the stalk of a turnip, which has evolved with the unique pattern of growth for enhancing transport phenomenon, a novel architecture of BiVO4/CuSCN architecture is fabricated on fluorine-doped tin oxide coated glass by initially fabricating bulky BiVO4 film (the root) and subsequently depositing p-type CuSCN nanorods (the stalk). With the creation of BiVO4/CuSCN nanorod heterojunction photoanodes, the photocurrent density increases to 1.78 mA cm−2 compared to 1.22 mA cm−2 of bare BiVO4 at 1.23 V vs. reversible hydrogen electrode. More importantly, the onset potential for oxygen evolution reaction exhibits a dramatic cathodic shift (~ 230 mV). The heterojunction also possesses internal quantum efficiency of approximately 50% in the range from 350 to 450 nm with relatively high solar energy conversion efficiency (0.5%) and much higher oxidation efficiency (~ 90%). The unique electrode architecture design favoring the simple water splitting process over conventionally fabricated electrode by providing more active sites and facilitates transportation and consumption of photoinduced holes could open up a new route for the high-efficiency photoanodes.

One-dimensional WO3/BiVO4 heterojunction photoanodes for efficient photoelectrochemical water splitting

In the present work, we reported the exploration of WO3/BiVO4 heterojunction photoanodes, which were consisted of one-dimensional WO3 nanofiber (1D-WO3) cores and BiVO4 film shells. The 1D-WO3 nanofibers were prepared by electrospinning, and the BiVO4 outlayers were subsequently deposited around the surface of 1D-WO3 fibers via a solution immersion process, leading to the construction of 1D-WO3/BiVO4 heterojunctions. They had a maximum photocurrent density of 2.8 mA cm−2 at 1.23 V vs. RHE under AM 1.5 simulated solar light illumination, which was ∼20 times to that of pure 1D-WO3 counterparts (i.e., 0.15 mA cm−2), and comparable to the best one of photoanodes based on WO3/BiVO4 hybrids ever reported. Furthermore, the onset potential of FTO/1D-WO3/BiVO4 photoanode was subsequently reduced, and the maximum incident photon-to-current efficiency (IPCE) was enhanced for ∼4 times in comparison to that of 1D-WO3 counterparts. The mechanism for the enhanced PEC behaviors of WO3/BiVO4 heterojunction photoanodes was proposed.

Electrodeposition of CdS onto BiVO4 films with high photoelectrochemical performance

In this study, CdS/BiVO4 heterojunction was successfully synthesized on the FTO-coated glass substrate through electrodeposition annealing followed by the second electrodeposition. First, the BiVO4 films were prepared by electrodepositing nanoporous BiOI, and then, it was converted to BiVO4 via annealing. Then, CdS was electrodeposited onto the BiVO4/FTO electrode. The obtained CdS/BiVO4 heterojunction shows higher photoelectrochemical (PEC) performance and higher stability for PEC water splitting than the blank BiVO4 films. The maximum photocurrent density of 6.6 mA cm−2 at 0.4 V vs Ag/AgCl was obtained for CdS/BiVO4/FTO, which is 4.2 times of that of the blank BiVO4/FTO (1.6 mA cm−2). The enhancement mechanism of the CdS/BiVO4/FTO heterojunction was investigated. The CdS achieves the efficient visible light absorption and forms heterostructure with BiVO4, which promotes the separation efficiency of photogenerated electron-hole pairs in BiVO4 to enhance PEC performance of CdS/BiVO4 heterojunction.

Enhanced photoelectrochemical water splitting of BiVO4 photonic crystal photoanode by decorating with MoS2 nanosheets

Bismuth vanadate (BiVO4) has been considered as one of the promising Photoelectrochemical (PEC) photoanode materials. However, the performances remain poorly rated due to inefficient carrier separation, short carrier diffusion length, and sluggish water oxidation kinetics. Herein, a photoanode consisting of MoS2 nanosheet coating on the three-dimensional ordered BiVO4 inverse opal is fabricated by a facile combination of nanosphere lithography and hydrothermal methods. By taking advantage of the photonic crystal and two-dimensional material, the optimized MoS2/BiVO4 inverse opal photoanode exhibits a 560% improvement of the photocurrent density and threefold enhancement of the incident photon-to-current efficiency than that of the pristine BiVO4 film photoanode. Systematic studies reveal that the excellent PEC activity should be attributed to enhanced light harvesting and charge separation efficiency.

(Invited) Bismuth-Based Ternary Oxide Thin Film for Solar Water Oxidation

Bismuth based ternary oxide such as BiVO4, Bi2WO6 and Bi2MoO6 have been reported to oxidise water into oxygen under visible light. These materials have sufficient absorption within the solar spectrum (UV and visible region) and stability against photocorrosion. The hybridized valence band of these ternary oxides made them active towards water oxidation. In this presentation, we investigate the performance of these bismuth based ternary oxides in photoelectrochemical water oxidation. A few aspects will be discussed to highlight 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. 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 photoelectrochemical performance is observed.

Turnip-Inspired BiVO4/CuSCN Heterojunction Photoanode for Highly Efficient Photoelectrochemical Water Splitting

Inspired by the functions and structures of the root and stalk of a turnip, a novel BiVO4/CuSCN heterojunction was constructed for photoelectrochemical water splitting, by initially fabricating bulky BiVO4 film (the root) and subsequently depositing p-type CuSCN nanorods (the stalk) on top. This BiVO4/CuSCN photoanode produced an enhanced photocurrent density of 1.78 mAcm-2 and hole injection efficiency of 82% at the potential 1.23 V vs. RHE. About 45% increase in photocurrent density coupled with a dramatic cathodic shift (~220 mV) in onset potential compared with bare BiVO4. The heterojunction also possesses external quantum efficiency of approximately 33% in the range from 350-450 nm with fairly high solar energy conversion efficiency (0.5%). The unique electrode architecture design favors the facile water splitting process over conventionally fabricated electrode by providing the more active sites and facilitates transportation and consumption of photoinduced holes, open a new route for the high-efficiency photoanodes. Figure 1

New BiVO4 Dual Photoanodes with Enriched Oxygen Vacancies for Efficient Solar-Driven Water Splitting.

Bismuth vanadate (BiVO4 ) is a promising photoanode material for photoelectrochemical (PEC) water splitting. However, owing to the short carrier diffusion length, the trade-off between sufficient light absorption and efficient charge separation often leads to poor PEC performance. Herein, a new electrodeposition process is developed to prepare bismuth oxide precursor films, which can be converted to transparent BiVO4 films with well-controlled oxygen vacancies via a mild thermal treatment process. The optimized BiVO4 film exhibits an excellent back illumination charge separation efficiency mainly due to the presence of enriched oxygen vacancies which act as shallow donors. By loading FeOOH/NiOOH as the cocatalysts, the BiVO4 dual photoanodes exhibit a remarkable and highly stable photocurrent density of 5.87 mA cm-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination. An artificial leaf composed of the BiVO4 /FeOOH/NiOOH dual photoanodes and a single sealed perovskite solar cell delivers a solar-to-hydrogen conversion efficiency as high as 6.5% for unbiased water splitting.

Hydrogen treatment and FeOOH overlayer: Effective approaches for enhancing the photoelectrochemical water oxidation performance of bismuth vanadate thin films

The water oxidation capability of the promising photoanode bismuth vanadate (BiVO4) is hampered by poor bulk electron transport and by high rates of charge recombination at the semiconductor/electrolyte interface. Here, we demonstrate that a dual modification of BiVO4 by: (i) annealing in a hydrogen-containing environment and (ii) coating with FeOOH overlayer substantially enhances the water oxidation ability of BiVO4 photoanodes. Hydrogen treated, FeOOH coated BiVO4 photoanodes exhibit a water oxidation photocurrent density of 2.16 mA cm−2 at 1.23 VRHE, which is 5 times higher than for untreated BiVO4 films. Moreover, they showed an impressive low photocurrent onset potential of −0.11 VRHE. A stable photocurrent was observed for 1 h of water oxidation measurement at 1.23 VRHE under 1 Sun illumination. The enhanced photocurrent of FeOOH/H:BiVO4 photoanode is ascribed to an improved bulk charge transport, as confirmed by impedance spectroscopy measurements and Mott-Schottky analysis. The cathodic shift of the onset potential originates from a lowering of the flat band potential and from an improvement of the charge transport at the semiconductor/electrolyte interface. The dual modification strategy used here offers a simple but effective approach of improving the water oxidation performance of BiVO4.

Plasmonic Silver Nanoparticle-Impregnated Nanocomposite BiVO4 Photoanode for Plasmon-Enhanced Photocatalytic Water Splitting

Herein, we developed a fully solution-deposited nanocomposite photoanode based on silver nanoparticle (NP)-impregnated bismuth vanadate (BiVO4) films. The synthesized Ag NPs exhibit diameters of few nanometers and uniform matrix dispersion, which were confirmed by high-resolution transmission electron microscopy. The photoanode composed of the Ag NP-incorporated nanocomposite BiVO4 showed a remarkable enhancement in both low potential and the saturated photocatalytic current densities in comparison with the pristine BiVO4 film. The observed experimental results are attributed to the improved carrier generation and enhanced charge separation by the localized surface plasmon resonance-mediated effect as suggested by electrochemical impedance spectroscopy and a numerical simulation.