Pristine BiVO4 Research Articles

Methanol, ethylene glycol, and glycerol photoelectrochemical oxidation reactions on BiVO4: Zr,Mo/Pt thin films: A comparative study

The green H2 production plays a key role in the energy transition, however it still needs improvements to be applied in a large scale. The water oxidation reaction, as an anodic process to provide both protons and electrons for H2 production, possesses kinetic limitations. Therefore, the use of an alternative process like the oxidation of biomass-derivative species is extremely desired to replace water oxidation during H2 production. In this work, we provide a comparative study of different small organic molecules (methanol, ethylene glycol (EG), and glycerol) as alternatives to water oxidation at pristine BiVO4, Mo-Zr dopped BiVO4, and Pt co-catalyst BiVO4 photoanodes in neutral media. While the band energy diagram and surface morphology are similar for the distinct materials, the presence of Zr-Mo increases the charge carriers density. The presence of Pt acts as co-catalyst for the organic molecules. Regardless of the material employed, the activity order for each organic molecule reaction was: jwater< jmethanol < jEG < jglycerol affeered by both linear potential sweep and chronoamperometry at 1.23 V vs RHE. The presence of both Zr-Mo and Pt increases the photoactivity for the alcohol's oxidation, however the presence of Pt is more advantageous for glycerol oxidation than for other species, maintaining 80 % of activity after 24 h electrolysis. The reactivity of alcohols can be tentatively explained by the greater stability of radicals formed from larger molecules.

Revealing Asymmetric Phase Transformation of the BiVO4 Anodes for Potassium-Ion Batteries by In Situ Transmission Electron Microscopy.

Potassium-ion batteries (PIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs), thanks to the cost-effectiveness of potassium resources and a favorable redox potential of approximately -2.936 V. The monoclinic BiVO4, known for its layered structure, shows noteworthy electrochemical properties when utilized as an anode material for both LIBs and sodium-ion batteries. However, the fundamental electrochemical reaction mechanisms of the BiVO4 anode during the potassium insertion/extraction processes remain unclear. Here, we constructed a BiVO4 anode PIB inside the transmission electron microscope (TEM) to explore the real-time potassiation/depotassiation behaviors of BiVO4 during electrochemical cycling. Utilizing the state-of-art in situ TEM technique, the BiVO4 nanorods are found to undergo an asymmetric phase transformation for the first time, where the pristine BiVO4 material is transformed into an amorphous KxBiVO4 phase after the first cycle. More interestingly, the anode materials near and far from the potassium source exhibit opposite volume-changing trends under the same voltage potential. Also, this phenomenon should be attributed to the mass flow of the unstable K-Bi alloy under the electric field. Our findings provide significant insights into the electrochemical mechanism of BiVO4 nanorods during the potassiation/depotassiation process, with the hope of assistance in designing anodes for high-performance PIBs.

2D porous hexaniobate-bismuth vanadate hybrid photocatalyst for photodegradation of aquatic refractory pollutants

Metal oxide semiconductors are highly promising due to their excellent photocatalytic performance in the photodegradation of industrial waste containing refractory chemical compounds. A hybrid structure with other semiconductors provides improved photocatalytic performance. In this work, porous and two-dimensional (2D) hexaniobate-bismuth vanadate (Nb6-BiVO4) Z-scheme hybrid photocatalysts are synthesized by chemical solution growth (CSG) of BiVO4 over electrophoretically deposited Nb6 thin films. The structural and morphological analysis of Nb6-BiVO4 hybrid thin films evidenced the well-crystalline uniform growth of monoclinic scheelite BiVO4 over lamellar Nb6 nanosheets. The Nb6-BiVO4 hybrid thin films exhibit a highly porous randomly aggregated nanosheet network, creating the house-of-cards type morphology. The Nb6-BiVO4 hybrid thin films display a strong visible light absorption with band gap energy of 2.29 eV and highly quenched photoluminescence signal, indicating their visible light harvesting nature and intimate electronic coupling between hybridized species beneficial for photocatalytic applications. The visible-light-driven photodegradation performance of methylene blue (MB), rhodamine-B (Rh-B) dyes, and tetracycline hydrochloride (TC) antibiotic over Nb6-BiVO4 hybrid are studied. The best optimized Nb6-BiVO4 thin film shows superior photocatalytic activity for photodegradation of MB, Rh-B dyes, and TC antibiotic with photodegradation rates of 87.3, 92.8, and 64.7 %, respectively, exceptionally higher than that of pristine BiVO4. Furthermore, the mineralization study of Nb6-BiVO4 thin film is conducted using chemical oxygen demand (COD) analysis. The optimized Nb6-BiVO4 thin film shows superior percentage COD removal of 83.33, 85.42, and 61.36 % for MB, Rh-B dyes and TC antibiotic, respectively. The present results highlight the expediency of hybridization in enhancing the photocatalytic activity of pristine BiVO4 by minimizing its charge recombination rate and improving chemical stability.

Photoelectrocatalytic CO2 Reduction to Formate Using a BiVO4/ZIF-8 Heterojunction.

Converting CO2 into high-value chemical fuels through green photoelectrocatalytic reaction path is considered as a potential strategy to solve energy and environmental problems. In this work, BiVO4/ZIF-8 heterojunctions are prepared by in-situ synthesis of ZIF-8 nanocrystals with unique pore structure on the surface of BiVO4. The experimental results show that the silkworm pupa-like BiVO4 is successfully combined with porous ZIF-8, and the introduction of ZIF-8 can provide more sites for CO2 capture. The optimal composite ratio of 4 : 1-BiVO4/ZIF-8 exhibits excellent CO2 reduction activity and the lowest electrochemical transport resistance. In the electrocatalytic system, the formate Faraday efficiency of 4 : 1-BiVO4/ZIF-8 at -1.0 V vs. RHE is 82.60 %. Furthermore, in the photoelectrocatalytic system, the Faraday efficiency increases to 91.24 % at -0.9 V vs. RHE, which is 10.8 times higher than the pristine BiVO4. The results show that photoelectric synergism can not only reduce energy consumption, but also improve the Faraday efficiency of formate. In addition, the current density did not decrease during 34 h electrolysis, showing long-term stability. This work highlights the importance of the construction of heterojunction to improve the performance of photoelectrocatalytic CO2 reduction.

Ultrasensitive xylene sensor based on RuO2-modified BiVO4 nanosheets

Two-dimensional (2D) BiVO4 nanosheets (NSs), featuring distinctive chemical properties and dangling-bond-rich surfaces, are promising for developing high-performance gas sensors. However, the previously reported 2D BiVO4 NSs-based devices suffer from a low responsivity and poor selectivity. In this work, we introduce RuO2 nanoparticles to boost the gas sensing properties of 2D BiVO4 NSs-based sensor. Intriguingly, the RuO2-decoration changes the selectivity of the corresponding gas sensor from acetone to xylene. Compared to the pristine BiVO4 sensor, the RuO2-BiVO4 sensor displays a reduced optimal operating temperature of 260℃ and exhibits a response of 19.0–5 ppm xylene, reaching a five-fold enhancement. Simultaneously, the RuO2-BiVO4 sensor also exhibits a rapid response and recovery time of 10 and 25 s respectively, as well as robust long-term stability with less than 5 % variation in response after a storage of 60 days. The improvement in sensing performance can be attributed to the RuO2-decoration from three aspects: heterojunction construction, surface state modulation and gas oxidation catalysis on the surface. This work not only provides a high-performance xylene sensing material but also provides insightful comprehension regarding the modification effects of RuO2.

Tethering Cobalt Ions to BiVO4 Surface via Robust Organic Bifunctional Linker for Efficient Photoelectrochemical Water Splitting.

In the quest for efficient and stable oxygen evolution catalysts (OECs) for photoelectrochemical water splitting, the surface modification of BiVO4 is a crucial step. In this study, a novel and robust OEC, based on 3-(bis(pyridin-2-ylmethyl) amino) propanoic acid bifunctional linker known as dipicolyl alanine acid (DPAA) and cobalt ions, is prepared and fully characterized. The DPAA is anchored to the surface of BiVO4 and utilized to tether cobalt ions. The Co-DPAA/BiVO4 photoanode exhibits remarkable stability and efficiency toward photoelectrochemical water oxidation. Specifically, it showed anodic photocurrent increase of 7.1, 5.0, 3.0, and 1.3-fold at 1.23VRHE as compared to pristine BiVO4, DPAA/BiVO4, Co-BiVO4, and Co-Pi/BiVO4 photoanodes, respectively. The photoelectrochemical and IMPS studies revealed that the Co-DPAA/BiVO4 photoanode exhibits a longer transient decay time for surface-trapped holes, higher charge transfer kinetics, and charge separation efficiency compared to Co-Pi/BiVO4 and pristine BiVO4 photoelectrodes. This indicates that the Co-DPAA effectively reduces surface recombination and facilitates charge transfer. Moreover, at 1.23VRHE, the Co-DPAA/BiVO4 photoanode achieved a faradic efficiency of 92% for oxygen evolution reaction and could retain a turnover frequency of 3.65s-1. The- exhibited effeciency is higher than most of the efficient molecular oxygen evolution catalyst based on Ru.

Highly enhanced photoelectrocatalytic activity of NiFe/Ni/BiVO4 photoanode by a facile photoelectron-activation process in neutral solution

Photoelectrocatalytic water splitting is a promising approach to convert solar energy to hydorgen energy. Delicate design of photoanode is crucial for the excellent catalytic activity. Here, a NiFe-LDH/Ni/BiVO4 composite photoanode was prepared by magnetron sputtering of Ni followed by electrochemical deposition of NiFe-LDH on BiVO4 photoelectrode. Interestingly, after a concise photoelectron-activation process, a photocurrent of 4.9 mA/cm2 (1.23 V vs. RHE) was obtained in neutral solution, which is 4 times of the pristine BiVO4 photoanode. The photoelectro-activation process not only enhances the photocurrent, but also significantly supresses the positive spiking phenomenon of the photocurrent. A series of characterizaitons including XRD, SEM, EDX, HRTEM, and XPS ect. were performed, which revealed that the photoelectron-activation process led to the reconstruction of the surface structure of NiFe-LDH/Ni/BiVO4. The dynamic characterizaitons including stepped potential chronoamperometry, steady-state photoluminescence (PL) spectra, open-circuit potential diagrams, and electrochemical impedance spectroscopy (EIS) ect. were performed. It indicates that the activated samples can provide an enhanced internal electric field and enable the charge carriers being efficiently injected into the electrode/electrolyte interface to promote the water oxidation reaction. This investigation provided a facile activation method for BiVO4-based composite photoanode, and highly increase the PEC performance in neutral condition.

Improved glyceraldehyde generation through FeOOH co-catalysts-modified BiVO4 featuring Bi-O-Fe active sites for photoelectrocatalytic glycerol oxidation

Heterojunction engineering is crucial for converting glycerol into valuable organic compounds via photoelectrochemistry, but controlling the structure precisely to enhance conversion efficiency remains challenging. In this study, we effectively created a compound photoanode with unique active regions by growing amorphous FeOOH co-catalysts on BiVO4. The results revealed that the Fe2+ ions located on the surface of the co-catalyst FeOOH facilitate the glycerol transformation process, whereas the electron-rich Bi-O-Fe sites contribute significantly to the preferential transformation of glycerol into glyceraldehyde. These sites not only serve as primary reactive centers, catalyzing the adsorption and conversion of glycerol hydroxyl clusters, but also act as repositories for trapping holes. This dual function accelerates the breaking of CH bonds to generate glycerol aldehyde and exhibits excellent desorption capacity, ensuring exceptional product selectivity. This led to exceptional glyceraldehyde production (709mmol m-2 h−1) compared to pristine BiVO4, showcasing a 14.98-fold increase.

Polypyrrole as photo-thermal-assisted modifier for BiVO4 photoanode enables high-performance photoelectrochemical water splitting

Herein, iron nickel oxide (FeNiOx) and conductive polymer polypyrrole (PPy) with excellent hole transport property and catalytic oxidation ability combine with bismuth vanadate (BiVO4) based photoanode to obtain a remarkable improvement of photocurrent density. More importantly, due to the photo-thermal effect of PPy, the operating temperature of the photoanode reaches 323.15 K under AM 1.5G illumination without infrared light source, thus activating the photoanode surface and accelerating the water splitting reaction. As a result, when the temperature of electrolyte reaches 323.15 K, the photocurrent density of BiVO4/FeNiOx/PPy photoanode achieves 6.91 mA cm−2 at 1.23 V versus reversible hydrogen electrode (vs. RHE), which is 3.04 folds of pristine BiVO4 (2.27 mA cm−2). Such a photo-thermal-assisted enhancement strategy based on the temperature sensitivity of BiVO4 provides a new way for the breakthrough of the performance bottleneck of BiVO4 photoanode and the design of the next generation PEC catalyst materials.

Promoting the Photoelectrochemical Properties of BiVO4 Photoanode via Dual Modification with CdS Nanoparticles and NiFe-LDH Nanosheets.

Bismuth vanadate (BiVO4) has long been considered a promising photoanode material for photoelectrochemical (PEC) water splitting. Despite its potential, significant challenges such as slow surface water evolution reaction (OER) kinetics, poor carrier mobility, and rapid charge recombination limit its application. To address these issues, a triadic photoanode has been fabricated by sequentially depositing CdS nanoparticles and NiFe-layered double hydroxide (NiFe-LDH) nanosheets onto BiVO4, creating a NiFe-LDH/CdS/BiVO4 composite. This newly engineered photoanode demonstrates a photocurrent density of 3.1 mA cm-2 at 1.23 V vs. RHE in 0.1 M KOH under AM 1.5 G illumination, outperforming the singular BiVO4 photoanode by a factor of 5.8 and the binary CdS/BiVO4 and NiFe-LDH/BiVO4 photoanodes by factors of 4.9 and 4.3, respectively. Furthermore, it exhibits significantly higher applied bias photon-to-current efficiency (ABPE) and incident photon-to-current efficiency (ICPE) compared to pristine BiVO4 and its binary counterparts. This enhancement in PEC performance is ascribed to the formation of a CdS/BiVO4 heterojunction and the presence of a NiFe-LDH OER co-catalyst, which synergistically facilitate charge separation and transfer efficiencies. The findings suggest that dual modification of BiVO4 with CdS and NiFe-LDH is a promising approach to enhance the efficiency of photoanodes for PEC water splitting.

Isotype BiVO4 heterostructure and the effect of photo-sono induced electron-hole pair

Owing to its suitable energy band and strong catalytic capacity, BiVO4 has received extensive attention in photocatalysis. Here, we suggest a straightforward approach to address the challenges of insufficient compatibility, poor charge transport characteristics, and limited surface adsorption properties commonly found in traditional BiVO4 photocatalysts. A heterojunction BiVO4 structure is developed by incorporating two distinct crystal phases within a single semiconducting material. A facile hydrothermal procedure is used to synthesize distinct crystalline phases of BiVO4 photocatalysts, viz., tetragonal, monoclinic, and monoclinic/tetragonal heterophase. The physicochemical characteristics of the pristine and isotype BiVO4 heterojunctions were characterized using various techniques. The photocatalytic activity of BiVO4 samples was examined by monitoring the degradation of rhodamine B (RhB). In order to boost the degradation reaction, ultrasonic sound waves are employed within the reaction medium. The present study examined the photocatalytic, sonocatalytic, and sonophotocatalytic activity of BiVO4 microcrystals in relation to the degradation of RhB dye. The results show that the crystalline phases of BiVO4 samples significantly influence the behaviour of photo-sono-induced charges. An interface in the monoclinic/tetragonal heterophase creates a spatial environment that facilitates charge transfer and enhances the separation of photo-sono-induced electron-hole pairs. The paper extensively examines the correlation between the behaviour of photo-sono-induced charge carriers and the level of sonophotocatalytic activity. This would provide greater insight into the intrinsic reasons for the enhancement in sonophotocatalytic activity.

Unlocking the unique catalysts of CoTiO3/BiVO4@MIL-Fe(53) for improving Cr(VI) reduction and tetracycline degradation

Environmental contamination by hexavalent chromium (Cr(VI)) and antibiotic drug residues pose significant challenges to public health and ecosystems. This study investigates the application of CoTiO3/BiVO4@MIL-Fe(53) (CT/BV@Fe-MOF) for the reduction of Cr(VI) and degradation of tetracycline (TCL) under visible light. After grafting an iron-based metal–organic framework MIL-Fe(53) on a modified CoTiO3/BiVO4 composite, the photogenerated electrons could easily be transferred from CoTiO3 to BiVO4/Fe-MOF species via interfacial charge transfer. UV–vis diffuse reflectance spectroscopy showed that charge carriers were formed in response to visible light absorption. The effect of different operating parameters, including catalyst load, pH, initial Cr(VI), and TCL concentration, was systematically evaluated during the photocatalytic process. The CT/BV@Fe-MOF composite exhibited 98.7% reduction efficiency in Cr(VI) (50 ppm) and 97.5% degradation efficiency towards TCL (30 ppm) within 90 min, resulting in a greater efficiency than the pristine CoTiO3, BiVO4, and Fe-MOF materials. The CT/BV@Fe-MOF composite displayed excellent stability over six cycles, highlighting its potential for practical applications. In addition, the plausible degradation pathway of TCL was evaluated using LC-ESI/MS analysis, while the TEST program was utilized to investigate the toxicity of the products generated during the degradation process.

An investigation on the visible light-driven Z-scheme BiVO4/g-C3N4 heterostructures: Performance, evaluation, and mechanism for dye and antibiotics degradation

Herein, the novel and efficient yellow BiVO4/g-C3N4 (BCN) binary composite photocatalyst was successfully fabricated via a simple ultrasonicated-assisted approach for the enhanced photocatalytic removal of methylene blue, tetracycline hydrochloride and ciprofloxacin under simulated visible irradiation. The crystal structure, composition, morphology, textural, optical, and electrochemical properties of the catalysts were comprehensively analyzed. The fishbone-liked BiVO4 were uniformly dispersed on the nano-layered g-C3N4, which not only contributed to the red-shift in the adsorption range but also suppressed the recombination of generated electron-holes. The stable photocurrent of BCN21 with a BiVO4/g-C3N4 mass ratio of 2:1 is higher than that of pristine BiVO4 and g-C3N4. The degradation experiments indicated that the BCN21 exhibited an outstanding photocatalytic removal rate for TC, which was 2.4 and 1.7 times as much as that of the pristine g-C3N4 and BiVO4, respectively. BCN21 also performed an excellent MB discoloration ability (up to 94 % within 180 min). It also showed superior ciprofloxacin degradation rates (3 and 4 times higher than pristine g-C3N4 and BiVO4). Both •O2, •OH, and h+ are involved in the photocatalytic degradation of TC, and that •O2 plays the most crucial role. Moreover, the cyclic experiments proved that the BCN21 possesses good stability after five consecutive runs with no obvious deactivation observed. Overall, this present study provides an insightful strategy for the development of heterojunction photocatalysts towards practical environmental purification.

BiVO4 Photoanode with NiV2O6 Back Contact Interfacial Layer for Improved Hole-Diffusion Length and Photoelectrochemical Water Oxidation Activity.

The short hole diffusion length (HDL) and high interfacial recombination are among the main drawbacks of semiconductor-based solar energy systems. Surface passivation and introducing an interfacial layer are recognized for enhancing HDL and charge carrier separation. Herein, we introduced a facile recipe to design a pinholes-free BiVO4 photoanode with a NiV2O6 back contact interfacial (BCI) layer, marking a significant advancement in the HDL and photoelectrochemical activity. The fabricated BiVO4 photoanode with NiV2O6 BCI layer exhibits a 2-fold increase in the HDL compared to pristine BiVO4. Despite this improvement, we found that the front surface recombination still hinders the water oxidation process, as revealed by photoelectrochemical (PEC) studies employing Na2SO3 electron donors and by intensity-modulated photocurrent spectroscopy measurements. To address this limitation, the surface of the NiV2O6/BiVO4 photoanode was passivated with a cobalt phosphate electrocatalyst, resulting in a dramatic enhancement in the PEC performance. The optimized photoanode achieved a stable photocurrent density of 4.8 mA cm-2 at 1.23 VRHE, which is 12-fold higher than that of the pristine BiVO4 photoanode. Density Functional Theory (DFT) simulations revealed an abrupt electrostatic potential transition at the NiV2O6/BiVO4 interface with BiVO4 being more negative than NiV2O6. A strong built-in electric field is thus generated at the interface and drifts photogenerated electrons toward the NiV2O6 BCI layer and photogenerated holes toward the BiVO4 top layer. As a result, the back-surface recombination is minimized, and ultimately, the HDL is extended in agreement with the experimental findings.

Nanocone-substrated BiVO4-Mo/MOFs photoanodes for highly efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) performance of photoanode is limited by the cascade-dynamic processes, mainly including light absorption, charge separation, and surface reaction. This work adopted a strategy of nanocones modification on FTO substrate (FTO-NC) to enhance light utilization and photoelectrons collection for improved PEC water splitting. Comparative studies showed that FTO-NC enhanced the light scattering, promoting the conversion process of photon to electron/hole and enlarging the effective surface area of BiVO4 for surface reaction. Meanwhile, cooperative Mo-doping and Co-MOF decoration improved the bulk electron transportation in BiVO4, hole migration towards the electrode surface, and accelerated the water oxidation reaction of photogenerated holes. The photocurrent density of FTO-NC/BiVO4-Mo/MOFs reached 3.56 mA cm−2, 6.14 times that of pristine BiVO4. The IPCE of FTO-NC/BiVO4-Mo/MOFs photoanode achieve 60% at 1.23 V vs. RHE, demonstrating a significantly enhanced light utilization performance. This work presents new insight into boosting cascade-dynamic processes through collaborative strategies for highly efficient PEC water-splitting.

An Efficient Photocatalytic Oxygen Evolution System with the Coupling of Polyoxometalates with Bismuth Vanadate

In this work, a coupling system consisting of bismuth vanadate (BiVO4) and cobalt-based polyoxometalates (Co-POMs) was developed to enhance the oxygen evolution reaction. Crystallization-driven self-assembly and the wet chemical synthesis method were deployed in synthesizing Co-POMs and monoclinic–tetragonal mixed–phase BiVO4, respectively. The introduction of Co-POMs into a BiVO4-containing mixture significantly enhanced the water oxidation reaction, with a more than twofold increment in the total amount of oxygen evolved. For instance, 461.2 µmol of oxygen was evolved from the system containing 20 mg of Co-POMs compared to 195 µmol of oxygen produced from a pristine BiVO4 system. This extraordinary improvement in the oxygen evolution reaction indicates the existence of a positive synergic effect between BiVO4 and Co-POMs, in which Co-POMs could act as effective cocatalysts to extract photogenerated charge carriers generated by BiVO4 and improve the charge transfer process. However, the amount of oxygen produced was slightly reduced to 440.7 µmol with an increase in AgNO3 loading from 30 mg to 60 mg. This unforeseen phenomenon could be elucidated by the shielding effect of silver particles, in which a higher AgNO3 loading led to a more prominent shielding effect. The presence of silver nanoparticles on post-reaction BiVO4 was confirmed by TEM and XPS analysis. This newly established process scheme provides an insight into the development of an efficient photocatalytic oxygen evolution system in realizing future commercial applications toward green energy production.

Boosting photoelectrochemical water oxidation by self-assembled Co@β-cyclodextrin supramolecules modified on BiVO4 photoanode

Severe charge recombination and poor surface water oxidation kinetics are key restrictions of BiVO4 photoanode for efficient photoelectrochemical (PEC) water oxidation. In order to overcome this issue, we introduce Co@β-cyclodextrin (Co@β-CD) supramolecule as oxidation evolution catalyst (OEC) into BiVO4 photoanode for the first time via a facile two-step immersion method. The optimal Co@β-CD/BiVO4 photoanode displays a remarkable photocurrent density of 5.3 mA cm−2 at 1.23 VRHE, which is ca. 3.5 times as high as that of pristine BiVO4 photoanode. Further analyses indicate that photogenerated holes could quickly transfer from BiVO4 to Co sites through β-CD, thereby suppressing charge recombination. Moreover, due to the spatial confinement effect of β-CD, the dispersed Co sites in Co@β-CD are considered to possess the high catalytic activity in similar with single-atom catalyst, which accelerates the surface water oxidation kinetics. This work not only demonstrates the superiority of supramolecule/semiconductor composite photoanode, but also provides a facile preparation route for the large-scale preparation of high-performance photoanodes.

Gradient‐Tuned VO4 Vacancies in BiVO4 Photoanode for Boosting Bulk Hole Transport and Oxygen Evolution Reaction Performance

AbstractAlthough bismuth vanadate (BiVO4) photoanode has been widely used, the solar‐driven oxygen evolution reaction (OER) performance is constrained due to the substantial bulk recombination and poor mobility of charge carriers. Herein, a facile solvothermal post‐treatment approach that employs N,N‐dimethylformamide (DMF) is developed to induce a VO4 vacancy gradient from bulk to surface of BiVO4 film, which not only improves the charge diffusion but also establishes an advantageous upward band gradient to promote the bulk hole transport. Under Air Mass 1.5 Global (AM1.5G) simulated solar illumination, the optimized BiVO4 photoanode with a VO4 vacancy gradient (denoted as B(VO)1‐δ) exhibits excellent OER performance with a charge separation efficiency of 74.4% at 1.23 V versus reversible hydrogen electrode (RHE) and incident‐photon‐to‐current conversion efficiency of 30.7% at 445 nm, 1.2 and 1.4 times higher than that of the pristine BiVO4, respectively. After loading nickel‐iron hydroxyl oxide (NiFeOOH) as a cocatalyst, the photocurrent density of B(VO)1‐δ escalates to 5.92 mA cm−2 in a hole scavenger (Na2SO3) solution and 5.07 mA cm−2 in a potassium borate buffer solution at 1.23 V versus RHE, far superior to the pristine BiVO4. This work highlights that the gradient‐tuned VO4 vacancies can effectively modulate the bulk band structure and charge transfer in BiVO4 photoanode, providing a new strategy for boosting solar water splitting performance.

Investigating the interfacial charge transfer between electrodeposited BiVO4 and pulsed laser-deposited Co3O4 p-n junction photoanode in photoelectrocatalytic water splitting

The formation of a p-n junction consisting small-sized, and homogeneous material over the porous semiconductor is an effective strategy for improved photoelectrocatalytic (PEC) water oxidation. Here, the facile pulsed laser deposition (PLD) method was used to load small-sized Co3O4 on BiVO4. The BiVO4/Co3O4 (30 s) photoanode exhibits a higher photocurrent density of 4.66 mA cm−2 @ +1.23 VRHE which is ∼ 4-fold higher than pristine BiVO4. This improved PEC performance is due to the availability of more active sites, improved charge separation, rapid interfacial charge transfer process, higher injection ability, and higher water oxidation kinetics. The higher open circuit voltage (VOC) of BiVO4/Co3O4 offers the greater driving force for charge separation. On the other hand, the density functional theory (DFT) investigations were carried out for further understanding of the facile interfacial charge transfer process. The electrostatic plots from the DFT studies reveals that Co3O4 has a deeper potential than BiVO4 and this potential difference can induce a strong internal electric field across the junction which promotes facile charge transfer from BiVO4 to Co3O4 until equilibration. Under illumination, BiVO4 and Co3O4 act electron acceptor and hole acceptor, respectively. Hence, the hole accumulated water oxidation is taken place in the valence band maximum (VBM) of Co3O4 for an enhanced PEC performance of the p-n junction photoanode.

Enhanced bulk and interfacial charge transfer in Fe:VOPO4 modified Mo:BiVO4 photoanodes for photoelectrochemical water splitting

Bismuth vanadate (BiVO4) is a promising photoanode material for photoelectrochemical (PEC) water oxidation. However, its performance is greatly hindered by poor bulk and interfacial charge transfer. Herein, to address this issue, iron doped vanadyl phosphate (Fe:VOPO4) was grafted on molybdenum doped BiVO4 (Mo:BiVO4) for significantly enhancing charge transfer and oxygen evolution kinetics simultaneously. Consequently, the resultant Fe:VOPO4/Mo:BVO4 photoanode exhibits a remarkable photocurrent density of 6.59 mA cm−2 at 1.23 V versus the reversible hydrogen electrode (VRHE) under AM 1.5G illumination, over approximately 5.5 times as high as that of pristine BiVO4. Systematic studies have demonstrated that the hopping activation energy of small polarons is significantly reduced due to the Mo doping, resulting in accelerated bulk charge transfer. More importantly, the deposition of Fe:VOPO4 promotes the interfacial charge transfer between Mo:BiVO4 and Fe:VOPO4 via the construction of V−O−V and P−O bonds, in addition to facilitating water splitting kinetics. This work provides a general strategy for optimizing charge transfer process, especially at the interface between photoanodes and cocatalysts.