Comparative study between pristine and Nb-modified BiVO 4 films employed for photoelectrocatalytic production of H 2 by water splitting and for photocatalytic degradation of organic pollutants under simulated solar light

Enhanced photoactivity towards bismuth vanadate water splitting through tantalum doping: An experimental and density functional theory study

The activation of hole trap states in bismuth vanadate (BiVO4) is considered an effective strategy to enhance the photoelectrochemical (PEC) water-splitting activity. Herein, we propose a theoretical and experimental study of tantalum (Ta) doping to BiVO4 leading to the introduction of hole trap states for the enhanced PEC activity. The doping of Ta is found to alter the structural and chemical surroundings via displacement of vanadium (V) atoms that cause distortions in the lattice via the formation of hole trap states. A significant enhancement of photocurrent to ∼4.2 mA cm-2 was recorded attributing to the effective charge separation of efficiency of ∼96.7 %. Furthermore, the doping of Ta in the BiVO4 lattice offers improved charge transport in bulk and decreased charge transfer resistance at the electrolyte interface. The Ta-doped BiVO4 displays the effective production of hydrogen (H2) and oxygen (O2) under AM 1.5 G illumination with a faradaic efficiency of 90 %. Moreover, the density functional theory (DFT) study confirms the decrease in optical band gap and the activation of hole trap states below the conduction band (CB) with a contribution of Ta towards both valence and CB that increases the charge separation and majority charge carrier density, respectively. The findings of this work propose that the displacement of V sites with Ta atoms in BiVO4 photoanodes is an efficient approach for enhanced PEC activity.

Facile Synthesis of BiVO4@ZIF-8 Composite with Heterojunction Structure for Photocatalytic Wastewater Treatment

Water pollution has always been a serious problem across the world; therefore, facile pollutant degradation via light irradiation has been an attractive issue in the field of environmental protection. In this study, a type of Zn-based metal–organic framework (ZIF−8)-wrapped BiVO₄ nanorod (BiVO₄@ZIF−8) with high efficiency for photocatalytic wastewater treatment was synthesized through a two-step hydrothermal method. The heterojunction structure of BiVO₄@ZIF−8 was confirmed by morphology characterization. Due to the introduction of mesoporous ZIF−8, the specific surface area reached up to 304.5 m²/g, which was hundreds of times larger than that of pure BiVO₄ nanorods. Furthermore, the band gap of BiVO₄@ZIF−8 was narrowed down to 2.35 eV, which enabled its more efficient utilization of visible light. After irradiation under visible light for about 40 min, about 80% of rhodamine B (RhB) was degraded, which was much faster than using pure BiVO₄ or other BiVO₄-based photocatalysts. The synergistic photocatalysis mechanism of BiVO₄@ZIF−8 is also discussed. This study might offer new pathways for effective degradation of wastewater through facile design of novel photocatalysts.

Binary and Ternary Vanadium Oxides: General Overview, Physical Properties, and Photochemical Processes for Environmental Applications

This review article is a comprehensive report on vanadium oxides which are interesting materials for environmental applications. Therefore, a general overview of vanadium and its related oxides are presented in the first two parts. Afterwards, the physical properties of binary and ternary vanadium oxides in single and mixed valence states are described such as their structural, optical, and electronic properties. Finally, the use of these vanadium oxides in photochemical processes for environmental applications is detailed, especially for the production of hydrogen by water splitting and the degradation of organic pollutants in water using photocatalytic and photo-Fenton processes. The scientific aim of such a review is to bring a comprehensive tool to understand the photochemical processes triggered by vanadium oxide based materials where the photo-induced properties are thoroughly discussed based on the detailed description of their intrinsic properties.

Benefits on photocarrier transfer from the transition of 3D to a 2D morphology

In the transition from three-dimensional to a two-dimensional morphology, the area of the (010) facet (electron surface) increases, and the conduction band bottom becomes more negative.

Visible-light-driven photoelectrocatalytic activation of chloride by nanoporous MoS2@BiVO4 photoanode for enhanced degradation of bisphenol A

The massive emission of bisphenol A (BPA) has imposed adverse effects on both ecosystems and human health. Herein, nanoporous MoS₂@BiVO₄ photoanodes were fabricated on fluorine-doped tin oxide (FTO) substrates for photoelectrocatalytic degradation of BPA. The photocurrent density of the optimized photoanode (MoS₂-3@BiVO₄) was 5.4 times as that of BiVO₄ photoanode at 1.5 V vs. Ag/AgCl under visible light illumination, which was ascribed to the reduced recombination of photogenerated charge carriers of the well-designed hybrid structure. 10 ppm of BPA could be completely degraded in 75 min by MoS₂-3@BiVO₄ photoanode, with a bias of 1.5 V vs. Ag/AgCl and 100 mM of NaCl as the supporting electrolyte. The electron paramagnetic resonance (EPR) and free radicals scavenging experiments confirmed that chlorine oxide radical (•ClO) played a dominant role in the degradation of BPA. 14 intermediates were detected and identified during photoelectrocatalytic degradation of BPA by MoS₂-3@BiVO₄ photoanode and 3 pathways were proposed based on the above intermediates. The hybrid film exhibited high stability and reusability, and promising application potential in photoelectrocatalytic degradation of organic pollutants in aqueous solution.

Solar-Light-Responsive Titanium-Sheet-Based Carbon Nanoparticles/B-BiVO4/WO3 Photoanode for the Photoelectrocatalytic Degradation of Orange II Dye Water Pollutant

We report the preparation and application of a heterostructured photoelectrocatalyst comprising carbon nanoparticles (CNPs) and boron codoped BiVO₄ and WO₃ for the removal of an organic dye pollutant in water. The materials, synthesized by hydrothermal method, were characterized by X-ray diffraction, diffuse reflectance UV-visible spectroscopy, energy-dispersive X-ray spectroscopy, and electron microscopy. The catalysts were immobilized on treated titanium sheets by drop-casting. The fabricated electrodes were characterized by linear sweep voltammetry (LSV) and chronoamperometry. Diffuse reflectance spectroscopy of the catalysts reveals that the incorporation of CNPs and B into the structure of monoclinic BiVO₄ enhanced its optical absorption in both UV and visible regions. The LSV measurements carried out in 0.1 M Na₂SO₄ showed that the BiVO₄- and WO₃-based photoelectrode demonstrated significant photoactivity. CNP/B-BiVO₄ and CNP/B-BiVO4/WO₃ photoanodes gave photocurrent densities of approximately 0.83 and 1.79 mA/cm², respectively, at 1.2 V (vs 3 M Ag/AgCl). The performance of the electrodes toward degradation of orange II dye was in the order BiVO₄ < B-BiVO₄ < WO₃ < CNP-BiVO₄ < CNP/B-BiVO₄ < CNP/B-BiVO₄/WO₃, and the apparent rate constants obtained by fitting the experimental data into the Langmuir Hinshelwood kinetic model are 0.0924, 0.1812, 0.254, and 0.845 h^-1 for BiVO₄, WO₃, CNP/B-BiVO₄, and CNP/B-BiVO₄/WO₃, respectively. The chemical oxygen demand abatement after 3 h of electrolysis at the best performing photoanode was 58%. The study showed that BiVO₄ and WO₃ are promising anodic materials for photoelectrocatalytic water treatment plant.

Recent Advances in Niobium-Based Materials for Photocatalytic Solar Fuel Production

The search for renewable and clean energy sources is a key aspect for sustainable development as energy consumption has continuously increased over the years concomitantly with environmental concerns caused by the use of fossil fuels. Semiconductor materials have great potential for acting as photocatalysts for solar fuel production, a potential energy source able to solve both energy and environmental concerns. Among the studied semiconductor materials, those based on niobium pentacation are still shallowly explored, although the number of publications and patents on Nb(V)-based photocatalysts has increased in the last years. A large variety of Nb(V)-based materials exhibit suitable electronic/morphological properties for light-driving reactions. Not only the extensive group of Nb2O5 polymorphs is explored, but also many types of layered niobates, mixed oxides, and Nb(V)-doped semiconductors. Therefore, the aim of this manuscript is to provide a review of the latest developments of niobium based photocatalysts for energy conversion into fuels, more specifically, CO2 reduction to hydrocarbons or H2 evolution from water. Additionally, the main strategies for improving the photocatalytic performance of niobium-based materials are discussed.

Niobium Doping in BiVO4 : Interplay Between Effective Mass, Stability, and Pressure

We have applied density functional theory to study the electronic structure changes caused by Nb incorporation in BiVO₄ and the application of external pressure. The overall solubility of Nb in BiVO₄ is usually high, and the presence of oxygen vacancies affect the dopability of Nb in BiVO₄ . Through the analyses of the chemical-potential landscape, we have determined the single-phase stability zone of BiVO₄ with the Nb doping. The most favorable Nb doping is simultaneous substitutions at both V- and Bi-sites. Even though Nb substitution at only V-site is next favorable, the band gap change is not very significant which agrees with an earlier experiment. However, it does change the electron effective mass by 20 % owing to the presence of Nb 4d bands in the conduction bands, which explains better catalytic activity by Nb-doped BiVO₄ . In addition, application of external pressure the single-phase stability zone in the chemical-potential landscape. We have also focused on the local structural distortions near the Nb doping site, especially on the BiO₈ octahedra. We have shown here that pressure-induced symmetrization of BiO₈ dodecahedron lowers the electron's effective mass further and therefore can help to improve the photoconduction property of BiVO₄ .

Enhancement of visible-light photocatalytic activities of BiVO4 coupled with g-C3N4 prepared using different precursors

Graphitic-like carbon nitride (g-C₃N₄) photocatalyst was synthesized by a facile chemical pyrolysis method, which was built on the self-condensation of different precursors to generate g-C₃N₄, e.g., melamine, urea, and thiocarbamide. And the different precursors produced a great influence on the photocatalytic activities of g-C₃N₄. Heterojunctions of g-C₃N₄ and BiVO₄ were synthesized using a facile solvent evaporation method. The formation of BiVO₄/g-C₃N₄ composites were confirmed by XRD, FT-IR, SEM, XPS, and UV-Vis DRS. The photocatalytic activities for RhB degradation were evaluated under visible-light irradiation. The photocatalytic activity of g-C₃N₄ prepared by urea was higher than that of g-C₃N₄ prepared by melamine and thiocarbamide, which was attributed to its favorable dispersibility, larger specific surface area, and higher oxidation capacity. The heterojunction composites exhibited higher photocatalytic activity than pure g-C₃N₄ or BiVO₄. The results showed obvious removal efficiency for RhB, and the optimal sample with a BiVO₄ content of 10% exhibited higher efficiency than pure g-C₃N₄ and BiVO₄, and 10 wt%BiVO₄/CN-U showed the highest photocatalytic activity. The enhanced photocatalytic activity of BiVO₄/g-C₃N₄ composite can be attributed to the intimate coupling between the two host substrates, resulting in an efficient charge separation.

Bismuth vanadate-based semiconductor photocatalysts: a short critical review on the efficiency and the mechanism of photodegradation of organic pollutants

The number of publications on photocatalytic bismuth vanadate-based materials is constantly increasing. Indeed, bismuth vanadate is gaining stronger interest in the photochemical community since it is a solar-driven photocatalyst. However, the efficiency of BiVO₄-based photocatalyst under sunlight is questionable: in most of the studies investigating the photodegradation of organic pollutants, only few works identify the by-products and evaluate the real efficiency of BiVO₄-based materials. This short review aims to (i) present briefly the principles of photocatalysis and define the photocatalytic efficiency and (ii) discuss the formation of reactive species involved in the photocatalytic degradation process of pollutants and thus the corresponding photodegradation mechanism could be determined. All these points are developed in a comprehensive discussion by focusing especially on pure, doped, and composite BiVO₄. Therefore, this review exhibits a critical overview on different BiVO₄-based photocatalytic systems with their real efficiency. This is a necessary knowledge for potential implementation of BiVO₄ materials in environmental applications at larger scale than laboratory conditions.

An efficient Nb-modified BiVO4 film for photo-induced bacterial inactivation and photocatalytic removal of organic pollutants

For the first time, Methicillin-resistant Staphylococcus aureus (MRSA) was inactivated using a Nb-modified BiVO₄ photocatalyst.

Integrated BiPO4 nanocrystal/BiOBr heterojunction for sensitive photoelectrochemical sensing of 4-chlorophenol

A sensor platform was constructed by using a BiPO₄ nanocrystal/BiOBr heterojunction, which displayed superior performance for monitoring 4-chlorophenol.

Synthesis of α-Bi2Mo3O12/TiO2 Nanotube Arrays for Photoelectrochemical COD Detection Application

One-dimensional anodic TiO₂ nanotube arrays hold great potential as a photoelectrochemical sensor for the determination of chemical oxygen demand (COD). In this work, we report a warm synthesis of modified TiO₂ nanotube arrays with enhanced photoelectrochemical determination performance. Herein, a bismuth-based semiconductor (α-Bi₂Mo₃O₁₂) was introduced into TiO₂ nanotube arrays by sequential chemical bath deposition (CBD) at room temperature. Field-emission scanning electron microscopy, X-ray diffraction, Raman, and X-ray photoelectron spectroscopy were used to investigate the morphologies, structures, and elemental analysis of the products. The photoelectrochemical properties of TiO₂ and α-Bi₂Mo₃O₁₂/TiO₂ NTAs were measured by amperometry and cyclic votammetry methods. The α-Bi₂Mo₃O₁₂/TiO₂ nanotube arrays decrease the background photocurrent and increase the current response to organics at the same time, both of which are beneficial to enhancing the photoelectrochemical detection performance. The optimized α-Bi₂Mo₃O₁₂/TiO₂ NTAs with enhanced photoelectrochemical detection performance can achieve a detection sensitivity of 2.05 μA·cm^-2/(mg·L^-1) and a COD detection range of 0.366-208.9 mg/L respectively. With the α-Bi₂Mo₃O₁₂ modification, the surface electrochemical reactions of TiO₂ NTAs were regulated, the mechanisms of which were also further studied.