Cactus shaped FeOOH/Au/BiVO4 photoanodes for efficient photoelectrochemical water splitting

In this work, a FeOOH/Au/BiVO₄ photoanode was fabricated through dual modification with Au nanoparticles (NPs, ∼5 nm) and FeOOH nanoneedles (NNs) on nanoporous BiVO₄ surface. Both the Au NPs and FeOOH NNs were distributed uniformly onto the BiVO₄ by using the electrodeposition and chemisorption routes, respectively. After parameter optimization, the FeOOH/Au/BiVO₄ photoanode displayed a photocurrent density of 4.64 mA cm⁻² at 1.23 V_RHE, which was 3.74 times higher than the pristine BiVO₄ one. Besides, the FeOOH/Au/BiVO₄ photoanodes also displayed a maximum H₂ yield amount of 23.9 μmol cm⁻² h⁻¹. The significantly enhanced performance could be attributed to the following reasons: Introduction of Au NPs enhanced the visible light absorption through localized surface plasmon resonance (LSPR) effect; the photo-excited “hot electrons” of Au NPs were more likely to flow into the conduction band (CB) of BiVO₄ via a direct electron transfer mechanism, leading to an improvement of the charge separation/transfer efficiency; surface modification of FeOOH extracted photogenerated holes, leading to an acceleration of the surface catalytic kinetics. In a word, the present study suggests that co-sensitization of Au NPs and FeOOH might be an effective method for improving the PEC water oxidation kinetics of BiVO₄ photoanodes.     

Refined Z-scheme charge transfer in facet-selective BiVO4/Au/CdS heterostructure for solar overall water splitting

Artificial Z-scheme systems that mimic natural photosynthesis are well applicable to photocatalytic overall water splitting for hydrogen (H₂) production free of electricity. However, it commonly confronts low efficiency with huge challenge of steering charge transfer between H₂ evolution photocatalyst (HEP) and oxygen evolution photocatalyst (OEP). Here we report an all-solid-state Z-scheme system with facet-selective construction that favors charge spatial separation toward HEP and OEP for high efficient solar overall water splitting. Based on the spontaneous separation of photogenerated electrons and holes on the different crystal facets of BiVO₄ decahedra, we successively implemented the selective depositions of Au and CdS nanoparticles (NPs) onto the electron-rich {010} facets, to fortify the Z-scheme charge transfer between BiVO₄ and CdS across Au mediators upon two-step photoexcitation. In-situ photoelectron dynamics ascertains Z-scheme model of resultant BiVO₄/Au/CdS, which enables an impressive overall water splitting with stoichiometric H₂ and O₂ evolution rates of 281 and 138 μmol g^?1 h^?1, respectively, under 1 sun irradiation (100 mW cm^?2, AM 1.5G) without using any sacrificial agents and external bias. This work not only presents a refined Z-scheme overall water splitting system, but also gains insights into photo-induced charge transfer dynamics.     

Nanostructured Ni:BiVO4 photoanode in photoelectrochemical water splitting for hydrogen generation

Photoelectrochemical water splitting using bismuth vanadate (BiVO₄) is drawing attention but on account of presence of high charge recombination and poor water oxidation kinetics its performance is restricted. Present study attempts to understand the role of dopant Ni on BiVO₄ in a) reducing the charge recombination and b) to improve water oxidation kinetics. Ni doped BiVO₄ thin films are prepared via electrodeposition method and photoelectrochemical properties are investigated in 0.1 M phosphate buffer solution with and without sodium sulfite hole scavenger. Photocurrent density of 1.36 mA/cm² at 1.23 V vs. RHE has been obtained using 1.5% Ni doped BiVO₄. This sample also offered lower flat band potential, high open circuit potential and applied bias photon-to-current conversion efficiency. Addition of hole scavenger significantly increases the photoelectrochemical performance. Ni as a dopant therefore can play an important role in not only suppressing the electron-hole pair recombination but also in offering significantly enhanced photoelectrochemical response.     

Enhancement of visible light-driven hydrogen production over zinc cadmium sulfide nanoparticles anchored on BiVO4 nanorods

Photocatalytic water splitting to produce H₂ is a promising technology for clean energy generation. However, the use of expensive noble metals, toxicity, low charge separation efficiency and wide band gap of semiconductors hampering the widespread commercialization. Herein, we showed the potential of combining BiVO₄ nanorods with ZnCdS forming a hetero-structure which extend the spectral responsive range, separate the charge carriers effectively and enhances photocatalytic activity compared to single-component materials. The two components of hetero-structure forms an interface contact which also mitigate the problems of lower conduction band position of BiVO₄ and fast recombination of charge carriers of ZnCdS. The BiVO₄–ZnCdS hetero-structure was studied through surface morphology, crystallization properties, elemental analysis and optical properties. Under visible light irradiation, the BiVO₄–ZnCdS heterostructure produced 152.5 μmol g^?1 h^?1 hydrogen from water splitting, which was much higher than that of the individual components and stability of the hydrogen production was observed in three consecutive cycles. The as-obtained heterostructure showed improved visible light harvesting ability, prolong life of charges carriers and charge separation efficiency and Z-scheme mechanism features which results in enhanced photocatalytic activity for water splitting.     

Charge carrier separation and enhanced PEC properties of BiVO4 based heterojunctions having ultrathin overlayers

The photoelectrochemical performance of a BiVO₄ photoanode is limited by its poor charge transport properties, despite other useful optical absorption properties. Modifying the surface charge transport properties by forming heterojunction of BiVO₄ with other metal oxides layers having ultralow thickness is a promising route, as it may facilitate charge separation/transport without affecting other properties of BiVO₄. In this study, the structural, optical and PEC properties of heterojunction of BiVO₄ having ultrathin overlayers of Fe₂O₃, MoO₃ and ZnO has been investigated. The electrochemical impedance (via electrochemical impedance spectra in PEC cell) and surface photovoltage (using KPFM) measurements indicates improved charge transport owing to staggered band alignment and favourable band bending in case of BiVO₄/MoO₃ heterojunction as compared to pristine BiVO₄, BiVO₄/Fe₂O₃ and BiVO₄/ZnO heterojunctions. Enhanced photocurrent density in BiVO₄/MoO₃ of ~0.22 mA/cm² at 1.23 V_RHE which is 6 times as compared to pristine BiVO₄ layers has been observed. The results of the present study show that by forming heterojunction with a suitable semiconductor material can be used to enhance the PEC response by modifying the surface charge transfer characteristics and there is a large possibility of using other semiconductor materials for further investigations and improvement.     

Plasmonic Bi nanoparticle decorated BiVO4/rGO as an efficient photoanode for photoelectrochemical water splitting

We report the application of plasmonic Bi nanoparticles supported rGO/BiVO₄ anode for photoelectrochemical (PEC) water splitting. Nearly, 2.5 times higher activity was observed for Bi-rGO/BiVO₄ composite than pristine BiVO₄. Typical results indicated that Bi-rGO/BiVO₄ exhibits the highest current density of 6.05 mA/cm² at 1.23 V, whereas Bi–BiVO₄ showed the current density of only 3.56 mA/cm². This enhancement in PEC activity on introduction of Bi-rGO is due to the surface plasmonic behavior of BiNPs, which improves the absorption of radiation thereby reduces the charge recombination. Further, the composite electrode showed good solar to hydrogen conversion efficiency, appreciable incident photon-to-current efficiency and low charge transfer resistance. Hence, Bi-rGO/BiVO₄ provides an opportunity to realize PEC water splitting.     

Photodeposited CoOx as highly active phases to boost water oxidation on BiVO4/WO3 photoanode

Surface decoration of photoanodes with oxygen evolution cocatalysts is an efficient approach to improve the photoelectrochemical water splitting performance. Herein, ultrafine CoO_x was selectively immobilized on the surface of BiVO₄/WO₃ photoanode by using the photogenerated holes to in-situ oxidize Co₄O₄ cubane. The composited photoanode (CoO_x/BiVO₄/WO₃) displayed an enhanced photoelectrochemical (PEC) water oxidation performance, with a photocurrent density of 2.3 mA/cm² at 1.23 V_RHE under the simulated sunlight irradiation, which was 2 times higher than that of bare BiVO₄/WO₃. The characterization results for the morphological, optical and electrochemical properties of the photoelectrodes revealed that, the enhanced PEC performances could be attributed to the improved charge carrier separation/transport behaviors and the promoted water oxidation kinetics when the photoelectrodes were loaded with CoO_x.     

The charge transfer pathway of g-C3N4 decorated Au/Bi(VO4) composites for highly efficient photocatalytic hydrogen evolution

In this report, a novel g-C₃N₄/Au/BiVO₄ photocatalyst has been prepared successfully by assembling gold nanoparticles on the interface of super-thin porous g-C₃N₄ and BiVO₄, which exhibits outstanding photocatalytic performance toward hydrogen evolution and durable stability in the absence of cocatalyst. FESEM micrograph analysis suggested that the intimate contact between Au, BiVO₄, and g-C₃N₄ in the as-developed photocatalyst allows a smooth migration and separation of photogenerated charge carriers. In addition, the XRD, EDX and XPS analysis further confirmed the successful formation of the as-prepared g-C₃N₄/Au/BiVO₄ photocatalyst. The photocatalytic hydrogen production activity of the developed photocatalyst was evaluated under visible-light irradiation (λ > 420 nm) using methanol as a sacrificial reagent. By optimizing the 5-CN/Au/BiVO₄ composite shows the highest H₂ evolution rate (2986 μmolg⁻¹h⁻¹), which is 15 times higher than that of g-C₃N₄ (199 μmolg⁻¹h⁻¹) and 10 time better than bare BiVO₄ (297 μmolg⁻¹h⁻¹). The enhancement in photocatalytic activity is attributed to efficient separation of the photoexcited charges due to the anisotropic junction in the g-C₃N₄/Au/BiVO₄ system. The enhancement in photocatalytic activity is attributed to efficient separation of the photoexcited charges due to the anisotropic junction in the g-C₃N₄/Au/BiVO₄ system.     

Efficient suppression of surface charge recombination by CoP-Modified nanoporous BiVO4 for photoelectrochemical water splitting

BiVO₄ is a promising photoanode material for water splitting due to its substantial absorption of solar light as well as favorable band edge positions. However, the poor water oxidation kinetics of BiVO₄ results in its insufficient photocurrent density. Herein, we demonstrate the use of CoP nanoparticles for facile surface modification of nanoporous BiVO₄ photoanode in potassium borate buffer solution (pH 9.0), which can generate a tremendous cathodic shift of ~430 mV in the onset potential for photoelectrochemical water oxidation. In addition, a remarkable photocurrent density of 4.1 mA cm^?2 is achieved at 1.23 V vs. RHE under AM 1.5G illumination. The photoelectrochemical measurement using sodium sulfite as a hole scavenger clearly shows that the greatly improved performances are attributed to the efficient suppression of interfacial charge recombination through loading of CoP catalyst. Moreover, the maximum surface charge injection yield can reach >81% at 1.23 V vs. RHE and the maximum IPCE of CoP/BiVO₄ can reach 75.8% at 420 nm, suggesting the potential application of CoP-modified BiVO₄ photoanode for overall solar water splitting in cost-effective tandem photoelectrochemical cells.     

Enhanced photoelectrochemical water oxidation on BiVO4 by addition of ZnCo-MOFs as effective hole transfer co-catalyst

Solar-driven water splitting is one of greenways for massive conversion of sustainable and nonpolluting energy applied to meet global energy crisis. Photocatalysts are greatly explored to improve photoelectrochemical (PEC) water oxidation efficiency. Bismuth vanadate (BiVO₄) has been extensively used as photocatalyst for water oxidation, but its passive oxygen evolution kinetics and charge carrier recombination lead to inferior PEC performance under light illumination. Tuning interfacial charge separation and transfer is an eminent way to stimulate water oxidation characteristics of BiVO₄. Herein, a BiVO₄/zinc cobalt metal-organic framework (ZnCoMOF) composite is firstly proposed as photocatalyst for water oxidation. ZnCoMOF nanosheets are loaded on BiVO₄ surface as co-catalyst via solvothermal process. Effects of solvothermal duration and mole ratio of zinc and cobalt are investigated. The optimal BiVO₄/ZnCoMOF electrode shows a photocurrent density of 3.08 mA cm^?2 at 1.23 V vs. reversible hydrogen electrode (RHE), which is 4.21 times greater than that of BiVO₄ electrode. The redox properties of high valence metal ions in ZnCoMOF are used to store photoexcited holes and transfer them to the water oxidation process in the BiVO₄/ZnCoMOF system. This work demonstrates that PEC performance of BiVO₄ can be largely improved via controlling water oxidation kinetics and refining charge recombination and transport properties.     

Optimization of BiVO4 photoelectrodes made by electrodeposition for sun-driven water oxidation

In this work, the synthesis of cheap BiVO₄ photoanodes for the photoelectrochemical water splitting reaction was optimized via the scalable thin film electrodeposition method. Factors affecting the photoelectrochemical activity, such as the electrodeposition time, the ratio of the Bi-KI to benzoquinone-EtOH in the deposition bath, and the calcination temperature, have been investigated by using the Central Composite Design of Experiments. Pristine monoclinic scheelite BiVO₄ photoanodes having a photocurrent density of 0.45 ± 0.05 mA/cm² at 1.23 V vs RHE have been obtained. It was shown that a high photocurrent density is generally dictated by the following physico-chemical properties: a higher crystallite size, optimal thickness and a porous morphology, which give rise to a low charge transfer resistance, low onset potential and a high donor density. Moreover, to the best of our knowledge, this is the first report on the depth profile XPS analysis performed in BiVO₄ photoanodes made by electrodeposition technique, from which it was concluded that the surface V species exist as V⁴⁺ while the bulk V species are V⁵⁺. The V⁴⁺ induces a higher amount of surface oxygen vacancies, which was found to be beneficial for the photoactivity.     

2D/3D WO3/BiVO4 heterostructures for efficient photoelectrocatalytic water splitting

Developing novel photoanodes with high efficiency for photoelectrochemical (PEC) water splitting has become the key to solar energy conversion and storage realm. Herein, 3D worm-like bismuth vanadate (BiVO₄) is grafted on 2D thin tungsten trioxide (WO₃) underlayer by electrodeposition to form mixed–dimensional structured photoanode, resulting in significant improvement of the photocatalytic performance and the charge separation efficiency. Characterization results prove that the mixed–dimensional structured can boost the photocatalytic activity by suppressing back reaction and charge recombination of the bulk BiVO₄. Simultaneously, the electrical conductivity of photoanode can be increased by W⁶⁺ doping. Furthermore, a robust catalyst NiCo₂O_x is coated onto the surface of WO₃/BiVO₄ photoanode, exhibiting a desirable photocurrent of 3.85 mA cm^?2 at 1.23 V vs. RHE and an excellent stability over 3 h. Both the excellent photocurrent density and great operational stability of this 2D/3D WO₃/BiVO₄ photoanode make it a promising material for practical applications.     

Rational design of W-doped BiVO4 photoanode coupled with FeOOH for highly efficient photoelectrochemical catalyzing water oxidation

Developments of promising photocatalyst for PEC water oxidation gain significant interest in the research field of PEC water splitting. The BiVO₄ has been envisioned as suitable photocatalyst material for the PEC water oxidation due to suitable bandgap with favorable band edge positions. Nevertheless, the poor electron-hole separation and low charge transfer efficiency of BiVO₄ yield sluggish surface catalysis reaction. Herein, facile electrodeposition and annealing techniques are proposed to fabricate W-doped BiVO₄ photoanode coupled with FeOOH (W–BiVO₄/FeOOH) for efficient photocatalytic water oxidation. This synthesis is simple, cost-effective and less time consuming. The doping concentration of W and deposition time of FeOOH are optimized to improve photocatalytic ability of BiVO₄. At 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, the W–BiVO₄/FeOOH photoanode exhibits a high photocurrent density of 2.2 mA/cm², which is seven folds higher than that of the pristine BiVO₄ photoanode (0.31 mA/cm² 1.23 V vs. RHE). The enhanced photocatalytic ability of W–BiVO₄/FeOOH photoanode is due to the enhanced charge transport properties and synergistic effects of W doping and FeOOH deposition. The excellent long-term stability with the photocurrent density retention of 90% after continuous light illumination for 1000 s is also achieved for the W–BiVO₄/FeOOH photoanode.     

SnO2 as thin conformal layer over BiVO4 surface for enhanced charge carrier separation towards O2 evolution from water oxidation

Recently, bismuth vanadate (BiVO₄) has attracted substantial attention for photocatalytic applications owing to its many advantages as semiconductor to drive water oxidation. Despite possessing promising characteristics to be applied in photocatalysis, the fast electron-hole pairs recombination is still one of the drawbacks involving this semiconductor. The use of conformal layers to minimize such limitation and to improve charge separation is the aim of this work. Herein, we report on the use of SnO₂ as a conformal coating layer over the BiVO₄ surface for O₂ evolution reaction. The incorporation of SnO₂ over the BiVO₄ surface showed the beneficial effect in O₂ evolution compared to the pristine material, resulting in 490 μmol after 6 h of irradiation. Transient absorption and surface photovoltage spectroscopies indicate enhanced charge carrier separation and transport by passivating BiVO₄ with SnO₂ thin layer. Even though the O₂ evolution tests in this work were carried out without the presence of a co-catalyst, the use of a thin conformal coating of SnO₂ over BiVO₄ showed promising results for further studies.     

Fabrication of BiVO4 photoanode catalyzed with bimetallic sulfide nanoparticles for enhanced photoelectrochemical water oxidation

The photoelectrochemical water oxidation ability of BiVO₄ is usually restrained by its low separation efficiency of the photogenerated charge and slow kinetics for water oxidation. Here, FeCoS₂ bimetallic sulfide nanoparticles are successfully coated on BiVO₄ photoanode to address these problems. The photocurrent density of FeCoS₂/BiVO₄ photoanode is 3.08 times that of BiVO₄ photoanode. Furthermore, the charge separation efficiency can reach 75% and the charge injection efficiency reaches nearly 78%. The onset potential of FeCoS₂/BiVO₄ shifts 300 mV negatively, while the incident photon-to-electron conversion efficiency value of FeCoS₂/BiVO₄ is 2.86 folds of BiVO₄. The enhancement benefits from the loading of FeCoS₂, which promotes the separation of photogenerated electron and hole, and inhibits the recombination of photogenerated charge and hole.     

Facile preparation of BiVO4/FeVO4 heterostructure for efficient water-splitting applications

In this paper, a BiVO₄/FeVO₄ heterostructure photoanode was synthesized by electrospray technique, and its photoelectrochemical water oxidation performance was investigated. The maximum photocurrent density of 0.4 mA cm⁻² at 1.23 V_RHE was 6 times higher than that of pristine BiVO₄ films (0.06 mA cm⁻²). Through the analysis of the electrochemical impedance spectroscopy (EIS) results, the improvement of photoelectrochemical performance could be attributed to the formation of heterostructure at the two-phase interface, which led to the effective separation of electron-hole pairs. This work offers a new effective strategy to construct semiconductor nanocomposites for efficient photoelectrochemical water oxidation.     

Hierarchical mesoporous SnO2/BiVO4 photoanode decorated with Ag nanorods for efficient photoelectrochemical water splitting

Bismuth vanadate has been extensively investigated as a potential visible light photoanode for PEC water splitting. The performance of BiVO₄ is restricted by fast charge recombination and slow oxygen evolution reaction kinetic. To address these issues, hierarchical SnO₂ (HSN) mesoporous support is developed via a novel sol-electrophoretic approach, and BiVO₄ film is decorated with silver nanorods (Ag NRs). The photocurrent density of HSN/BiVO₄ photoanode is 3.98 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE) and onset potential (V_onset) of 0.5 V vs. RHE. The PEC performance is attributed to the appropriate band alignment between SnO₂ and BiVO₄, as well as the hierarchical structure of SnO₂. Ag-HSN/BiVO₄ photoanode shows photocurrent density of 4.30 mA/cm² at 1.23 V vs. RHE and V_onset of 0.28 V vs. RHE. The enhanced photocurrent and negatively shifted V_onset can be attributed to radiative localized surface plasmon resonance decay and catalytic effect of Ag NRs, respectively.     

Bi2S3 entrenched BiVO4/WO3 multidimensional triadic photoanode for enhanced photoelectrochemical hydrogen evolution applications

The catalytic reactivity and photoactivity of WO₃ and BiVO₄ oxide semiconductors have general obstacles as electrodes in emergent photo-electrochemical (PEC) hydrogen evolution applications. The present work comprises the integration of photocatalyst with wide visible photon absorption material which is vital for hydrogen evolution in photo-electrocatalytic water splitting. Herein, the 1D WO₃ NWs have been integrated with stable water oxidation photocatalysts of BiVO₄ and Bi₂S₃ as a photoanode (Bi₂S₃/BiVO₄/WO₃) for photoelectrochemical hydrogen evolution reactions. The morphological variations in the Bi₂S₃/BiVO₄/WO₃ heterostructure manifest catalytic activity and rapid charge transfer characteristics owing to band alignment and a wide range of visible photon absorption. The optimized Bi₂S₃/BiVO₄/WO₃ multidimensional photoanode accomplishes a superior photocurrent density of 1.52 mA/cm², a seven-fold higher than pristine WO₃ photoanode counterpart (0.2 mA/cm²) at 1 V vs. RHE. A prodigious lowest onset potential of ?0.01 V vs. RHE) has been achieved which enables very high solar to hydrogen conversion. The photoelectrode with entangled morphology such as nanosheets, nanocrystals and nanorods expanded their surface to volume ratio having enhanced catalytic performance. The hybrid photoanodes have demonstrated the lowest charge transfer resistance of 360 Ohm/cm² with a 7-fold rise in hydrogen evolution performance. The resultant triadic Bi₂S₃/BiVO₄/WO₃ heterostructure appeared to be an emerging stable photo-electro catalyst for hydrogen evolution applications.     

Irradiation-induced modifications and PEC response - A case study of SrTiO3 thin films irradiated by 120 MeV Ag ions

This paper deals with a study on the effect of 120 MeV Ag⁹⁺ ion irradiation on photoelectrochemical properties of SrTiO₃ thin films deposited on Indium doped Tin Oxide (ITO) coated glass by sol-gel spin-coating technique. The structural evolution in the pristine and irradiated films was determined by X-ray diffraction and X-ray photoelectron spectroscopy. Surface morphology was studied by Atomic Force Microscopy (AFM) and optical measurements were done by UV-visible absorption spectroscopy. Irradiation of SrTiO₃ thin films was found to be effective in improving its photoelectrochemical properties. A noticeable decrease in the average grain diameter from 36 to 26 nm, reduction in bandgap from 3.55 to 3.43 eV and increase in roughness after irradiation contributed in enhancing photoelectrochemical activity of SrTiO₃ thin films. Thin films irradiated at fluence 3 × 10¹² ions cm⁻², when used in PEC cell exhibited enhanced photocurrent of 0.16 mA cm⁻² at zero bias conditions, which was four times higher than that of the unirradiated sample.     

rGO decorated semiconductor heterojunction of BiVO4/NiO to enhance PEC water splitting efficiency

Monoclinic phase of BiVO₄ is a promising photoanode material for photoelectrochemical (PEC) water splitting, but its sluggish water oxidation kinetics and frequent bulk charge recombination greatly reduce its efficiency of PEC water splitting. A novel BiVO₄/NiO/rGO photoanode was very simply prepared by electrodeposition, solution immersion and spin coating methods, in particular, the solution immersion method to loading NiO has never been reported in PEC research. Compared with BiVO₄, the photocurrent density of the ternary photoanode reaches 1.52 mA/cm² at 1.23 V vs RHE, which is 2.41 and 1.39 times higher than that of pure BiVO₄ and binary BiVO₄/NiO photoanode, respectively. The onset potential of the ternary photoanode shows a significant cathodic shift of 130 mV compared with the BiVO₄ photoanode. Moreover, the measured incident photon-to-current efficiency (IPCE) value reaches 50.52% at λ = 420 nm. The improvement is attributed to the type-II heterojunction formation that enhances the separation efficiency of electron/hole and the rGO decoration that accelerates the electron transfer and provides more active sites for gas adsorption.