BiVO4 Film Research Articles

Enhancing activity in a nanostructured BiVO4 photoanode with a coating of microporous Al2O3

Nanostructured semiconductor photoanodes play an important role in solar fuel generation, and the design of the semiconductor − aqueous electrolyte interface can be crucial in enhancing the energy conversion efficiency. We have investigated the effects on photoelectrochemical oxygen evolution for monoclinic nanostructured BiVO4 films uncoated and coated with microporous sol-gel Al2O3 “over-layers”. Variation of the thickness of the Al2O3 coating (formed by surface sol-gel deposition and annealing at 435°C) led to a reduction of pseudo-capacitance and allowed optimization of the quantum efficiency. Exploration of the photocurrent enhancement as a function of applied potential reveals two distinct potential domains/mechanisms: (i) a low bias region enhancement effect (assigned to a lowering of the rate of external recombination of electrons with oxygen) and (ii) a high bias region of enhancement (assigned to higher charge carrier mobility due to less trapping in surface states).

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.

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.

CuWO4 Nanoflake Array-Based Single-Junction and Heterojunction Photoanodes for Photoelectrochemical Water Oxidation.

Over recent years, tremendous efforts have been invested in the search and development of active and durable semiconductor materials for photoelectrochemical (PEC) water splitting, particularly for photoanodes operating under a highly oxidizing environment. CuWO4 is an emerging candidate with suitable band gap and high chemical stability. Nevertheless, its overall solar-to-electricity remains low because of the inefficient charge separation process. In this work, we demonstrate that this problem can be partly alleviated through designing three-dimensional hierarchical nanostructures. CuWO4 nanoflake arrays on conducting glass are prepared from the chemical conversion of WO3 templates. Resulting electrode materials possess large surface areas, abundant porosity and small thickness. Under illumination, our CuWO4 nanoflake array photoanodes exhibit an anodic current density of ∼0.4 mA/cm(2) at the thermodynamic potential of water splitting in pH 9.5 potassium borate buffer--the largest value among all available CuWO4-based photoanodes. In addition, we demonstrate that their performance can be further boosted to >2 mA/cm(2) by coupling with a solution-cast BiVO4 film in a heterojunction configuration. Our study unveils the great potential of nanostructured CuWO4 as the photoanode material for PEC water oxidation.

Tailoring Multilayered BiVO4 Photoanodes by Pulsed Laser Deposition for Water Splitting.

Pulsed laser deposition (PLD) is proposed as promising technique for the fabrication of multilayered BiVO4-based photoanodes. For this purpose, bare BiVO4 films and two heterojunctions, BiVO4/SnO2 and BiVO4/WO3/SnO2, have been prepared using consecutive ablation of assorted targets in a single batch. The ease, high versatility and usefulness of this technique in engineering the internal configuration of the photoanode with stoichiometric target-to-substrate transfer are demonstrated. The obtained photocurrent densities are among the highest reported values for undoped BiVO4 without oxygen evolution catalysts (OEC). A detailed analysis of the influence of SnO2 and WO3 layers on the charge transport properties because of the changes at the internal FTO/semiconductor interface is performed through transient photocurrent measurements (TPC), showing that the BiVO4/WO3/SnO2 heterostructure attains a significant decrease in the internal losses and reaches high photocurrent values. This study is expected to open the door to the fabrication of other systems based on ternary (or even more complex) metal oxides as photoanodes for water splitting, which is a promising alternative for obtaining materials able to fulfill the different requierements in the development of more efficient systems for this process.

Atomic Insight into the W-Doping Effect on Carrier Dynamics and Photoelectrochemical Properties of BiVO4 Photoanodes

BiVO4 has become the top candidate for photoanode materials in the light-driven water-splitting reaction. However, its overall efficiency for water splitting still remains well below its theoretical limit. Further development of these materials has been hampered by the lack of knowledge concerning key factors that guide the carrier transport and separation. In this work, we report the effect of W doping on the electronic structure, carrier dynamics, and photocurrent performance of BiVO4 photoanode. Using X-ray absorption spectroscopy, we show that the dopants occupy the V sites, which leads to the reduced Bi-ligand distances and less distorted local structure at Bi center. Using transient absorption spectroscopy, we found that doping altered the hole traps significantly, which inhibits carrier recombination and increases the electron lifetime in the doped film. Compared to the undoped BiVO4 film, the photocurrent of the doped film shows a significant increase while limited difference of photocurrents under back and front illumination was observed. These results suggest that the reduced dimension and less distorted local structure at Bi center favorably extend the electron lifetime by dramatically altering the nature of the trap states, which plays a major role in improving the photoelectrochemical performance of BiVO4 film.

InVO4–BiVO4 composite films with enhanced visible light performance for photodegradation of methylene blue

The InVO4, BiVO4, and InVO4–BiVO4 films were fabricated by a simple doctor blading coating technique. The film thickness of the as-prepared samples was varied to one, two and three layers. The prepared InVO4–BiVO4 composite films were characterized in detail by various analytical techniques that reveal the well crystallinity with orthorhombic structure of InVO4 and monoclinic structure of BiVO4 for the as-prepared material. The as-synthesized photocatalyst films were used as the efficient photocatalysts for the photocatalytic degradation of methylene blue (MB) under visible light illumination. It was found that the three-layered 0.2InVO4–0.8BiVO4 film presented better photocatalytic activity than that of pure phase InVO4 and BiVO4. About 70% of the 1×10−5gL−1 MB was degraded within 120min catalyzed by the three-layered 0.2InVO4–0.8BiVO4 films. Interestingly, an obvious synergy between two catalysts was observed and corresponding mechanism based on facilitated transfer of electrons and holes as a result of a suitable energy level alignment was suggested. Fluorescence method with terephthalic acid (TA) as the probe was employed for the quantification of hydroxyl production.

ChemInform Abstract: Bandgap Tunability in Sb‐Alloyed BiVO4 Quaternary Oxides as Visible Light Absorbers for Solar Fuel Applications.

AbstractSb‐alloyed BiVO4 (with Bi and Sb ranging from 0 to ≈50%) films are prepared in a two‐step synthesis comprising first the preparation of Bi1‐xSbx nanocrystals (x = 0—1) via injecting a 1 M LiEt3BH solution in THF into a 4:1:25 molar mixture of Bi[N(Tms)2]3, Sb(NMe2)3, and Li[N(Tms)2] in toluene under N2 (70 °C, ≈15 sec; quenching to 25 °C).

Solution-processed multilayered BiVO4 photoanodes: influence of intermediate heat treatments on the photoactivity

Intermediate heat treatments to at least 350 °C are needed to obtain fully crystalline monoclinic BiVO4 films with tunable thickness from spin coating of successive layers from a precursor solution.

Textured nanoporous Mo:BiVO4 photoanodes with high charge transport and charge transfer quantum efficiencies for oxygen evolution

Nanoporous, catalyst-free BiVO4 films made by a simple spin coating process show a hole-limited charge transport quantum efficiency of ∼70% and >60% charge transfer efficiency for oxidizing water.

Ultrafast fabrication of highly active BiVO4photoanodes by hybrid microwave annealing for unbiased solar water splitting

Hybrid microwave annealing (HMA) with a silicon susceptor in a household microwave oven produces BiVO4-based photoanodes of much improved performance in photoelectrochemical water oxidation in only 6 min relative to conventional thermal annealing in a traditional muffle furnace (FA) that needs a much longer time, 300 min. This technique can apply equally effectively to bare as well as modified BiVO4 by Mo-doping, heterojunction formation with WO3, and an oxygen evolution co-catalyst. Relative to FA, HMA forms BiVO4 films of smaller feature sizes, higher porosity, and increased three dimensional roughness, which decrease the diffusion distance of holes to the surface and thereby increase mainly the bulk charge separation efficiency (ηbulk) of the photoanodes. Thus, the HMA-treated BiVO4/WO3 film achieves the state-of-the art ηbulk of ∼90% for water oxidation. Combination of a photoanode of NiOOH/FeOOH/BiVO4/WO3 (HMA, 6 min) with a 2p c-Si solar cell allows a solar to hydrogen conversion efficiency of ∼5.0% in unbiased overall water splitting, which is also comparable to the state-of-the-art for a similar material combination.

Bulky crystalline BiVO4 thin films for efficient solar water splitting

Bulky crystalline BiVO4 films on ITO realize a half-cell solar-to-hydrogen energy efficiency of 1.5% after modification with CoOx and NiO.

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.

Size Dependent Plasmonic Effect on BiVO4 Photoanodes for Solar Water Splitting.

Plasmonic nanostructures show great promise in enhancing the solar water splitting efficiency due to their ability to confine light to extremely small volumes inside semiconductors. While size plays a critical role in the plasmonic performance of Au nanoparticles (AuNPs), its influence on plasmon-assisted water splitting is still not fully understood. This holds especially true for low band gap semiconductors, for which interband excitations occur in wavelength regions that overlap with plasmonic resonances. Here, BiVO4 films are modified with AuNPs of diameters varying from 10 to 80 nm to study the size dependence of the plasmonic effect. Plasmon resonance energy transfer (PRET) is found to be the dominant effect in enhancing the water splitting efficiency of BiVO4. “Hot electron” injection effect is weak in the case of BiVO4/AuNP. This is attributed to the interband excitation of BiVO4, which is unfavourable for the hot electrons accumulation in BiVO4 conduction band. The resonant scattering effect also contributes to the enhanced water splitting efficiency for the larger diameter AuNPs. It is also for the first time found that higher PRET effect can be achieved at larger off-normal irradiation angle.

A highly efficient BiVO4/WO3/W heterojunction photoanode for visible-light responsive dual photoelectrode photocatalytic fuel cell

A highly efficient BiVO4/WO3/W heterojunction photoanode was fabricated based on the nanoporous WO3 film, which was prepared on the tungsten plate by anodic oxidation, in order to facilitate the electrons transfer from BiVO4 to WO3 by providing a natural connection between WO3 film and W substrate. Then, a visible-light responsive dual photoelectrode photocatalytic fuel cell (PFC) consisted of BiVO4/WO3/W photoanode and Pt modified commercial buried junction silicon (Pt/BJS) photocathode was constructed. The results showed that the optimized BiVO4/WO3/W photoanode obtained a photocurrent of 2.01mA/cm2 at 0.6V (vs Ag/AgCl) in 0.1M KH2PO4 (pH 7) electrolyte under simulated AM1.5 solar light, which was 180% and 205% higher than that of bare WO3 film and bare BiVO4 film, respectively. The established dual photoelectrode PFC showed high converting performance of organics into electricity. For example, a short-circuit current density of 0.26mAcm−2, which is higher than most of the reported visible-light responsive dual photoelectrode PFC systems, was obtained with the open-circuit voltage of 0.78V and maximum power output of 2.0×10−4 Wcm−2 in 20mgL−1 tetracycline hydrochloride. Furthermore, a removal ratio of 78% after 4h was achieved with a stable output photocurrent in the degradation process.

Origin of the Different Photoelectrochemical Performance of Mesoporous BiVO4 Photoanodes between the BiVO4 and the FTO Side Illumination

Understanding charge separation and charge transport in mesoporous semiconductor films is crucial to designing high efficiency photoelectrochemical water splitting cells. In the present work, we systematically study the origin of the higher photoelectrochemical performance of mesoporous BiVO4 film under FTO-side illumination (F-illumination) than that under BiVO4-side illumination (B-illumination). Via intensity-modulated photocurrent spectroscopy in conjunction with modeling simulation of electron diffusion inside mesoporous BiVO4 films with different thicknesses, we find that the F-illumination is more tolerant to recombination than the B-illumination, leading to a higher charge separation efficiency of the former. Specifically, we have identified a trap-free electron transport region of BiVO4 vicinal to the FTO substrate and a trap-limited transport region farther away under F-illumination, whereas only a trap-limited transport exists under B-illumination. Simulated results accord well with the experim...

Photoanodes on titanium substrates: one-step deposited BiVO4 versus two-step nano-V2O5 films impregnated with Bi3+

One-step deposition of photoelectrochemically active BiVO4 (monoclinic) from a polyethylene glycol (PEG300) precursor paint onto titanium substrates is demonstrated to lead to more complex multioxide phase films. Photocurrents in 0.5 M Na2SO4 remain relatively low when compared to those reported on tin-doped indium oxide (ITO) substrates, possibly due to the effect of detrimental underlying titanium oxide phases or mixed phases. For comparison, the one-step deposition is demonstrated also for the direct formation of V2O5 nanocrystalline films, which also exhibited low photoelectrochemical activity. Impregnation of nanocrystalline V2O5 with aqueous Bi3+ is shown to cause substantial recrystallization with formation of much more photoactive BiVO4. In particular, on low-temperature (high surface area) nano-V2O5 (400 °C) and after further mild annealing at 400 °C much improved photoelectrochemical water oxidation activity (ca. 0.3 mA cm−2 at 1.0 V vs Ag/AgCl (KCl (3 M) at 0.1-W cm−2 xenon lamp radiation) is observed. This opens up a new mild temperature two-step route for BiVO4 photocatalysts on practical titanium substrates.

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.

Photoelectrochemical Phenomena at Nano-V2O5 Films Impregnated with Bi3+

In the last two decades, the development of nanoscience and technology developed several nanostructures at different forms, such as nanoparticles, nanowires, nanotubes and nanoribbons. These materials offer a high potential for improving properties and can be used in many fields such as microelectronics, batteries, photovoltaic and photocatalytic devices. Vanadium oxides belong to the important class of 3d transition metals, with various electric, magnetic and structural properties, making these materials attractive for many industrial applications. This study aimed to obtain and characterize vanadium oxide films on titanium from ammonium metavanadate and then modify it by impregnation with Bi (III). The vanadium pentoxide was synthesized on Ti substrates by painting the surface with a suspension of 250 mg ammonium metavanadate in 5 ml of polyethylene glycol (PEG-300). The coated Ti plate was then heated at 400, 500 or 600 °C for one hour. Subsequently, the film obtained was impregnated with Bi (III) for 120 minutes under constant stirring, using 50 mL of a solution 10 mmol L-1 of bismuth nitrate at nitric acid. Then the samples were again annealed for one hour at the same temperature. The films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Visible spectroscopy and photoelectrochemical. From SEM analysis was possible to observe, after impregnation of Bi(III), the growing of long crystals on V2O5 films at 400 and 500 °C, while for the sample to 600 °C a globular structure was preferentially deposited on the tip of V2O5 crystals. Through EDS analysis it was observed that the sample obtained at 500 °C is that has the highest amount of Bi in the composition. The mapping by EDS showed that the elements were distributed over all surface. From XRD analysis the presence of V2O5 orthorhombic phase and monoclinic BiVO4 was detected. Band-gap calculated for the samples before and after impregnation also proved that V2O5 was converted to BiVO4, since the value before impregnation was 2.20 eV and after impregnation showed a value of 2.42 eV. The photocurrent value obtained for nano-V2O5 films impregnated with Bi3+ at 500 °C was 5 times higher than pure BiVO4 films. The results showed that it has been possible obtain a nanostructured V2O5 / BiVO4film on Ti substrate which is photoactive and could be used both in photoelectrocatalytic water as oxidation of organic molecules. Acknowledgements: FAPESP 2012/23422-5 and CAPES.

Photochemical charge separation at particle interfaces: the n-BiVO4-p-silicon system.

The charge transfer properties of interfaces are central to the function of photovoltaic and photoelectrochemical cells and photocatalysts. Here we employ surface photovoltage spectroscopy (SPS) to study photochemical charge transfer at a p-silicon/n-BiVO4 particle interface. Particle films of BiVO4 on an aluminum-doped p-silicon wafer were obtained by drop-coating particle suspensions followed by thermal annealing at 353 K. Photochemical charge separation of the films was probed as a function of layer thickness and illumination intensity, and in the presence of methanol as a sacrificial electron donor. Electron injection from the BiVO4 into the p-silicon is clearly observed to occur and to result in a maximum photovoltage of 150 mV for a 1650 nm thick film under 0.3 mW cm(-2) illumination at 3.5 eV. This establishes the BiVO4-p-Si interface as a tandem-like junction. Charge separation in the BiVO4 film is limited by light absorption and by slow electron transport to the Si interface, based on time-dependent SPS measurements. These problems need to be overcome in functional tandem devices for photoelectrochemical water oxidation.