Mo-doped BiVO4 Research Articles

Ultrastable low-bias water splitting photoanodes via photocorrosion inhibition and in situ catalyst regeneration

Photoelectrochemical (PEC) water splitting offers a means for distributed solar hydrogen production. However, the lack of stable and cost-effective photoanodes remains a bottleneck that hampers their practical applications. Here we show that particulate Mo-doped BiVO4 water oxidation photoanodes, without costly and complex surface modifications, can possess comparable stability to that of solar cells. The photoanode exhibits enhanced intrinsic photocorrosion inhibition and self-generation and regeneration of oxygen evolution catalysts, which allows stable oxygen evolution for >1,000 h at potentials as low as 0.4 V versus the reversible hydrogen electrode. The significantly improved photocorrosion resistance and charge separation are attributed to the unusual high-temperature treatment. In situ catalyst regeneration is found to be a site-specific and oxygen evolution rate change-induced process. Our findings indicate the potential of PEC water splitting to compete with other solar hydrogen production solutions, and should open new opportunities for the development of feasible PEC water splitting systems. Using photoelectrodes to split water is a promising approach to convert solar energy to fuel, but photoanode stability is often an issue. Now, a Mo-doped BiVO4 photoanode is shown to stably evolve oxygen for 1,000 h due to in situ regeneration of the catalyst, and inhibition of photocorrosion.

Photocatalyst Sheets Composed of Particulate LaMg1/3Ta2/3O2N and Mo-Doped BiVO4 for Z-Scheme Water Splitting under Visible Light

Z-Scheme water splitting by photocatalyst sheets is a promising approach for efficient and scalable H2 production. One of the most important challenges in developing such photocatalyst sheets is the application of photocatalysts with long absorption edges. In this study, RhCrOx-loaded LaMg1/3Ta2/3O2N (RhCrOx/LaMg1/3Ta2/3O2N) and Mo-doped BiVO4 (BiVO4:Mo), with absorption edges at 600 and 540 nm, are investigated as the hydrogen evolution photocatalyst and oxygen evolution photocatalyst in photocatalyst sheets, respectively. The (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet exhibits water splitting activity that is ∼5 times higher than that of corresponding powder suspensions. Stable water splitting is achieved under visible light using the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet coated with an amorphous TiO2 protective layer. However, the results show the low H2 evolution activity of the RhCrOx/LaMg1/3Ta2/3O2N imposes a ceiling on the activity of the photocatalyst sheet. The surface modification of LaMg1/...

BiVO4 photoanodes for water splitting with high injection efficiency, deposited by reactive magnetron co-sputtering

Photoactive bismuth vanadate (BiVO4) thin films were deposited by reactive co-magnetron sputtering from metallic Bi and V targets. The effects of the V-to-Bi ratio, molybdenum doping and post-annealing on the crystallographic and photoelectrochemical (PEC) properties of the BiVO4 films were investigated. Phase-pure monoclinic BiVO4 films, which are more photoactive than the tetragonal BiVO4 phase, were obtained under slightly vanadium-rich conditions. After annealing of the Mo-doped BiVO4 films, the photocurrent increased 2.6 times compared to undoped films. After optimization of the BiVO4 film thickness, the photocurrent densities (without a catalyst or a blocking layer or a hole scavenger) exceeded 1.2 mA/cm2 at a potential of 1.23 VRHE under solar AM1.5 irradiation. The surprisingly high injection efficiency of holes into the electrolyte is attributed to the highly porous film morphology. This co-magnetron sputtering preparation route for photoactive BiVO4 films opens new possibilities for the fabrication of large-scale devices for water splitting.

Solar-driven BiVO4 Photoanodes Prepared by a Facile Screen Printing Method

Mo-doped BiVO4 powder was successfully prepared by a facile liquid-solid-state reaction and was applied to a photoanode for water splitting. Pulverization of the Mo-doped BiVO4 particles by ball-mi...

Electrospun Mo-doped BiVO4 photoanode on a transparent conductive substrate for solar water oxidation

Molybdenum-doped porous bismuth vanadate (BiVO4) nanostructures were successfully synthesized on a fluorine-doped tin oxide (FTO) substrate by a simple electrospinning technique. These nanostructures were used as the photoanode for solar water oxidation. Post hot-pressing treatment was crucial in improving the photocurrents of electrospun BiVO4 by contributing to their adhesion on the substrate and for charge separation.

Green-synthesized W- and Mo-doped BiVO4 oriented along the {0 4 0} facet with enhanced activity for the sun-driven water oxidation

A green hydrothermal synthesis approach, which employs almost neutral pH conditions and ammonium carbonate as a structural directing agent, has been used for the preparation of W- and Mo-doped BiVO4 powders with a monoclinic scheelite-like structure and preferential growth along the {040} facet. The crystal structure and morphology of the samples have been studied by means of XRD and FESEM analyses. The proper introduction of the W and Mo dopants in the lattice of BiVO4 crystals has been confirmed from XRD and XPS analyses. Moreover, it has been shown that the superficial amount of W and Mo dopants (obtained by means of XPS) plays a more important role than the overall (bulk and surface) dopant quantities in the BiVO4 (estimated by means of ICPMS analysis). The best of the so prepared materials demonstrated an extraordinary enhancement of the water oxidation rate under simulated sunlight (i.e. 942μmol/g.h O2), which is about two-fold and 100 times higher than those of a state-of-art Mo-doped and the non-doped BiVO4 reference materials, respectively. The here reported BiVO4 photocatalysts are highly promising for different applications such as renewable energy production systems (i.e. solar fuels production form water and sunlight) and to resolve environmental issues (i.e. photocatalytic degradation of organic pollutants, water remediation, among others).

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 device based on Mo-doped BiVO4 enables smart analysis of the global antioxidant capacity in food.

For a healthy diet, which is an extension of a high quality lifestyle, tremendous attention has been focused on using antioxidant capacity indicators for food inspections and health guides. Although photoelectrochemical transducers have broadened our horizons for global antioxidant activity analysis, a growing body of foods and beverages needs to be quantified in the visible region and the necessary photoelectrochemical instrumentalization is still in its infancy. Generally, BiVO4 is considered as an ideal starting material for antioxidant surveillance under visible light irradiation. However, it is subjected to unsatisfied charge collection and utilization in practical applications. Herein, we studied the effects of successive molybdenum substitution of vanadium on the photocatalytic behavior of BiMo x V(1-x)O4 under visible light illumination. A superior photocurrent density was obtained for BiMo0.015V0.985O4 due to the flower-like architecture and favorable crystalline form. At the same time, this superhybrid BiMo0.015V0.985O4 composite successfully acted as a sensing unit in a photoelectrochemical platform for antioxidant capacity evaluation in foodstuffs. The related mechanism was further unearthed and discussed in-depth. Such a straightforward yet cogent principle was also applied to our integrated device for the "smart" analysis of the global antioxidant capacity, whereby collected data can be treated as a nutritive value index for routine quality control in the food industry. On the basis of this achievement, it is anticipated that mobile app-based quantitative antioxidant capacity detection will soon be realized.

Why the photocatalytic activity of Mo-doped BiVO4 is enhanced: a comprehensive density functional study.

To explore the origin of the enhanced photocatalytic activity of Mo-doped monoclinic BiVO4, variations of the structures and the electronic properties, as well as the adsorption behavior of water on the (010) surface, introduced by the Mo dopant have been investigated by means of density functional theory computations. For the bulk phase, Mo atoms prefer to substitute the V atoms, which can effectively accelerate the separation of carriers. For the (010) surface, Mo atoms prefer to substitute the Bi atoms at the outermost layer. Mo doping on the surface can result in surface oxygen quasi-vacancies and enhance the exposure of surface Bi atoms, which is confirmed to improve the adsorption of water molecules. Our results demonstrate that the enhanced photocatalytic activity of Mo-doped monoclinic BiVO4 is derived from the facilitated separation of photoinduced carriers and introduced surface oxygen quasi-vacancies.

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.

Marked enhancement in electron–hole separation achieved in the low bias region using electrochemically prepared Mo-doped BiVO4photoanodes

Mo-doped BiVO4 electrodes were prepared by an electrochemical route for use as photoanodes in a photoelectrochemical cell. The purpose of Mo-doping was to improve the electron transport properties, which in turn can increase the electron-hole separation yield. The poor electron-hole separation yield was known to be one of the main limiting factors for BiVO4-based photoanodes. The electrochemical route provided an effective way of doping BiVO4, and the optimally doped sample, BiV(0.97)Mo(0.03)O4, increased the electron-hole separation yield from 0.23 to 0.57 at 0.6 V vs. RHE, which is a record high separation yield achieved for BiVO4-based photoanodes. As a result, BiV(0.97)Mo(0.03)O4 generated impressive photocurrents, for example, 2 mA cm(-2) at a potential as low as 0.4 V vs. RHE for sulfite oxidation, which has fast oxidation kinetics and, therefore, the loss of holes by surface recombination is negligible. For photooxidation of water, BiV(0.97)Mo(0.03)O4 was paired with FeOOH as an oxygen evolution catalyst (OEC) to improve the poor catalytic ability of BiV(0.97)Mo(0.03)O4 for water oxidation. The resulting BiV(0.97)Mo(0.03)O4/FeOOH photoanodes generated a significantly improved photocurrent for water oxidation compared to previous reported results, but the photocurrent of BiV(0.97)Mo(0.03)O4/FeOOH for water oxidation could not reach the photocurrent of BiV(0.97)Mo(0.03)O4 for sulfite oxidation. In order to examine the cause, the effects of Mo-doping on the interaction between BiVO4 and FeOOH and the effects of FeOOH on the electron-hole separation yield of BiV(0.97)Mo(0.03)O4 were investigated in detail, which provided new insights into semiconductor-OEC interactions.

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.

On the improvement of photoelectrochemical performance and finite element analysis of reduced graphene oxide–BiVO4 composite electrodes

Incorporation of thermally reduced graphene oxide (RG-O) into the metal oxide semiconductor BiVO4 improves its photoactivity by about three times for sulfite oxidation. The enhancement of photoactivity is attributed to reduced electron–hole recombination of BiVO4 using the RG-O as a conductive matrix in the composite photocatalyst. Photoelectrochemical behavior of the BiVO4 and RG-O/BiVO4 composite electrodes were simulated using finite element analysis to obtain the carrier mobility and its lifetime in the photoelectrodes. In contrast with sulfite oxidation, the improvement of photocurrent for water oxidation of RG-O/BiVO4 composite electrode was not significant because of the slower kinetics for water oxidation. To address the kinetic limitations, platinum (Pt) as an effective electrocatalyst was photodeposited on the RG-O/BiVO4 electrode. Addition of the Pt significantly improved water oxidation photocurrent of the Pt/RG-O/BiVO4 electrode. RG-O/W–Mo-doped BiVO4 composite electrodes were also prepared to investigate a further enhancement of photoactivity of W–Mo-doped BiVO4. The behavior obtained from RG-O/W–Mo-BiVO4 and RG-O/BiVO4 electrodes provides a valuable insight into the role of RG-O as a conducting additive and the role of W and Mo as dopants into BiVO4.

Effects of Surface Electrochemical Pretreatment on the Photoelectrochemical Performance of Mo-Doped BiVO4

The surface pretreatment by electrochemical cyclic voltammetry (CV) in the dark was found to remarkably enhance the photocurrent of Mo-doped BiVO4 from the front side illumination. The variation of the samples before and after the surface pretreatment was investigated by scanning electron microscopy, X-ray photoelectron spectroscopy, and Mott–Schottky methods. The results showed that the photocurrent enhancement came from both the removal of the surface recombination center, including Mo6+ ions, and reoxidation of the reduced species. The part of the reduced ions can be reoxidized in air. However, the photocurrent enhancement from the Mo6+ dissolution can be kept at high potential under illumination. A possible mechanism was also proposed to explain the reason for the photocurrent enhancement.

Factors in the Metal Doping of BiVO4 for Improved Photoelectrocatalytic Activity as Studied by Scanning Electrochemical Microscopy and First-Principles Density-Functional Calculation

Metal doping of the metal oxide photoelectrocatalyst, BiVO4, dramatically increases its activity for water oxidation. Scanning electrochemical microscopy (SECM) was used to screen various dopants for their photoelectrochemical performance and to optimize the used dopant material concentrations with this photocatalyst. For example, adding Mo to W-doped BiVO4 enhanced the performance. The photocatalytic activity was examined on larger electrodes by means of photoelectrochemical and electrochemical measurements. The developed photoelectrocatalyst, W- and Mo-doped BiVO4, shows a photocurrent for water oxidation that is more than 10 times higher than undoped BiVO4. Factors that affect performance are discussed, and enhanced separation of excited electron–hole pairs by doping onto the semiconductor is suggested by first-principles density-functional theory (DFT) calculations. Distortion of the crystal structure of monoclinic scheelite-like BiVO4 by addition of W and Mo doping predicted by DFT is also revealed b...

Cobalt-phosphate (Co-Pi) catalyst modified Mo-doped BiVO4 photoelectrodes for solar water oxidation

A cobalt-phosphate based oxygen evolution catalyst (Co-Pi OEC) was electrochemically deposited onto the surface of a porous bismuth vanadate electrode doped with 2 atom% Mo (BiV0.98Mo0.02O4). The porous BiV0.98Mo0.02O4electrode was prepared using a surfactant assisted metal–organic decomposition technique at 500 °C. The comparison of the photocurrent–voltage characteristics of the BiV0.98Mo0.02O4electrodes with and without the presence of Co-Pi catalyst demonstrated that the Co-Pi catalyst enhanced the anodic photocurrent of the BiV0.98Mo0.02O4electrode with its effect more pronounced at lower potentials. A stable photocurrrent density of 1.0 mA cm−2 at 1.0 V vs.Ag/AgCl was achieved under standard AM 1.5 illumination using 0.5M Na2SO4 aqueous solution in phosphate buffer at pH7. Relative to the BiV0.98Mo0.02O4electrode, a sustained enhancement, nearly doubled photocurrent density was observed at 1.0 V vs.Ag/AgCl for Co-Pi/BiV0.98Mo0.02O4 composite photoelectrode. Significant performance gains are obtained on BiV0.98Mo0.02O4electrodes upon modification with Co-Pi water oxidation catalyst.