Facet-Engineered BiVO4 Photocatalysts for Water Oxidation: Lifetime Gain Versus Energetic Loss

A limiting factor to the efficiency of water Oxygen Evolution Reaction (OER) in metal oxide nanoparticle photocatalysts is the rapid recombination of holes and electrons. Facet-engineering can effectively improve charge separation and, consequently, OER efficiency. However, the kinetics behind this improvement remain poorly understood. This study utilizes photoinduced absorption spectroscopy to investigate the charge yield and kinetics in facet-engineered BiVO4 (F-BiVO4) compared to a non-faceted sample (NF-BiVO4) under operando conditions. A significant influence of preillumination on hole accumulation is observed, linked to the saturation and, thus, passivation of deep and inactive hole traps on the BiVO4 surface. In DI-water, F-BiVO4 shows a 10-fold increase in charge accumulation (∼5 mΔOD) compared to NF-BiVO4 (∼0.5 mΔOD), indicating improved charge separation and stabilization. With the addition of Fe(NO3)3, an efficient electron acceptor, F-BiVO4 demonstrates a 30-fold increase in the accumulation of long-lived holes (∼45 mΔOD), compared to NF-BiVO4 (∼1.5 mΔOD) and an increased half-time, from 2 to 10 s. Based on a simple kinetic model, this increase in hole accumulation suggests that facet-engineering causes at least a 50-100 meV increase in band bending in BiVO4 particles, thereby stabilizing surface holes. This energetic stabilization/loss results in a retardation of OER relative to NF-BiVO4. This slower catalysis is, however, offset by the observed increase in density and lifetime of photoaccumulated holes. Overall, this work quantifies how surface faceting can impact the kinetics of long-lived charge accumulation on metal oxide photocatalysts, highlighting the trade-off between lifetime gain and energetic loss critical to optimizing photocatalytic efficiency.

Manipulating the surface states of BiVO4 through electrochemical reduction for enhanced PEC water oxidation

Bismuth vanadate (BiVO₄) is a prospective candidate for photoelectrochemical (PEC) water oxidation, but its commercial application is limited due to the serious surface charge recombination. In this work, we propose a novel and effective electrochemical reduction strategy combined with co-catalyst modification to manipulate the surface states of the BiVO₄ photoanode. Specifically, an ultrathin amorphous structure is formed on the surface of BiVO₄ after electrochemical reduction ascribed to the breaking of the surface metal-O bonds. Photoelectrochemical measurements and first-principles calculation show that the electrochemical reduction treatment can effectively reduce the surface energy, thereby passivating the recombined surface states (r-ss) and increasing the mobility of photogenerated holes. In addition, the FeOOH co-catalyst layer further increases the intermediate surface states (i-ss) of BiVO₄, stabilizes the surface structure and enhances its PEC performance. Benefiting from the superior charge transfer efficiency and the excellent water oxidation kinetics, the -0.8/BVO/Fe photoanode achieves 2.02 mA cm^-2 photocurrent at 1.23 V_RHE (2.4 times that of the original BiVO₄); meanwhile, the onset potential shifts 90 mV to the cathode. These results provide a new surface engineering tactic to modify the surface states of semiconductor photoanodes for high-efficiency PEC water oxidation.

Laser-Reduced BiVO4 for Enhanced Photoelectrochemical Water Splitting

The present study proposes a laser irradiation method to superficially reduce BiVO4 photoelectrodes and boost their water oxidation reaction performance. The origin of this enhanced performance toward oxygen evolution reaction (OER) was studied using a combination of a suite of structural, chemical, and mechanistic advanced characterization techniques including X-ray photoelectron (XPS), X-ray absorption spectroscopy (XAS), electrochemical impedance spectroscopy (EIS), and transient absorption spectroscopy (TAS), among others. We found that the reduction of the material is localized at the surface of the sample and that this effect creates effective n-type doping and a shift to more favorable energy band positions toward water oxidation. This thermodynamic effect, together with the change in sample morphology to larger and denser domains, results in an extended lifetime of the photogenerated carriers and improved charge extraction. In addition, the stability of the reduced sample in water was also confirmed. All of these effects result in a two-fold increase in the photocurrent density of the laser-treated samples.

Interpretation of Mott-Schottky plots of photoanodes for water splitting

A large body of literature reports that both bismuth vanadate and haematite photoanodes are semiconductors with an extremely high doping density between 1018 and 1021 cm-3. Such values are obtained from Mott-Schottky plots by assuming that the measured capacitance is dominated by the capacitance of the depletion layer formed by the doping density within the photoanode. In this work, we show that such an assumption is erroneous in many cases because the injection of electrons from the collecting contact creates a ubiquitous capacitance step that is very difficult to distinguish from that of the depletion layer. Based on this reasoning, we derive an analytical resolution limit that is independent of the assumed active area and surface roughness of the photoanode, below which doping densities cannot be measured in a capacitance measurement. We find that the reported doping densities in the literature lie very close to this value and therefore conclude that there is no credible evidence from capacitance measurements that confirms that bismuth vanadate and haematite photoanodes contain high doping densities.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO₂ reduction, and N₂ reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

Controlling Charge Carrier Trapping and Recombination in BiVO4 with the Oxygen Vacancy Oxidation State

The lack of an in-depth understanding of the intrinsic oxygen vacancy (O_V) defect properties in the photoanode BiVO₄ limits the further improvement of its photoelectrochemical water splitting performance. To address this issue, nonadiabatic molecular dynamics simulations are performed to study the impact of O_V on charge carrier lifetimes in BiVO₄. The simulations show that a neutral O_V gives rise to local structural distortions due to the formation of V-O-V bonds, forcing the electrons trapped on the nearer of the two V atoms to form two deep polaron-like V⁴⁺ hole traps. These localized midgap states greatly accelerate nonradiative electron-hole recombination compared to that of pristine BiVO₄, reaching a time scale of several nanoseconds in good agreement with experiments. The ionized O_V state restores the bandgap to its value in pristine BiVO₄ and restores the charge carrier lifetimes due to the fast loss of coherence time. Our study reveals the mechanism of the detrimental role of O_V in BiVO₄ and provides valuable insights for improving the performance of the BiVO₄ photoanode by ionizing the oxygen vacancy.

The Self-Passivation Mechanism in Degradation of BiVO4 Photoanode

BiVO₄ is a promising photoanode material for solar-assisted water splitting in a photoelectrochemical cell but has a propensity to degrade. Investigations carried out here in 0.1 M Na₂SO₄ electrolyte showed that degradation is by dissolution of V in the electrolyte while Bi is retained on the anode probably in the form of solid Bi oxide (Bi₂O₃, Bi₄O₇). Accumulation of Bi oxide on the anode surface leads to passivation from further degradation. Thermodynamic modeling of possible degradation reactions has provided theoretical support to this mechanism. This self-passivation is accompanied by a decrease in photocurrent density, but it protects the anode against extensive photocorrosion and contributes to long-term stability. This is a more definitive understanding of degradation of BiVO₄ during water splitting in a photoelectrochemical cell. This understanding is imperative for both fundamental and applied research.

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A mechanism of degradation is developed for BiVO₄ photoanode during photolysis

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Degradation occurs by V dissolution and Bi accumulation on the anode as oxide

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Accumulating Bi oxide passivates the anode

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Thermodynamic modeling supports this mechanism

Oxygen-deficient bismuth tungstate and bismuth oxide composite photoanode with improved photostability

A homogeneous layer of Bi₂O₃-Bi₁₄WO₂₄ composite (BWO/Bi₂O₃) thin film was fabricated using a combination of electrodeposition and thermal treatment. The evenly distributed Bi₁₄WO₂₄ component within the Bi₂O₃ layer was found to be important in stabilising the photoelectrochemical performances of Bi₂O₃ photoanode by promoting the photoelectron transport. The unmodified Bi₂O₃ suffered from severe photocorrosion as proven by X-ray diffraction (XRD) and inductively coupled plasma (ICP) analyses while the composite thin film was active without noticeable activity decay for at least 3 h of illumination. This strategy might be applicable to other photocatalysts with stability issues.

Rapid Formation of a Disordered Layer on Monoclinic BiVO4 : Co-Catalyst-Free Photoelectrochemical Solar Water Splitting

A surface disordered layer is a plausible approach to improve the photoelectrochemical performance of TiO₂ . However, the formation of a crystalline disordered layer in BiVO₄ and its effectiveness towards photoelectrochemical water splitting has remained a big challenge. Here, we report a rapid solution process (within 5 s) that is able to form a disordered layer of a few nanometers thick on the surface of BiVO₄ nanoparticles using a specific solution with a controllable reducing power. The disordered layer on BiVO₄ alleviates charge recombination at the electrode-electrolyte interface and reduces the onset potential greatly, which in turn results in a photocurrent density of approximately 2.3 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode (RHE). This value is 2.1 times higher than that of bare BiVO₄ . The enhanced photoactivity is attributed to the increased charge separation and transfer efficiencies, which resolve the intrinsic drawbacks of bare BiVO₄ such as the short hole diffusion length of around 100 nm and poor surface oxygen evolution reactivity.

Assembly of g-C3N4-based type II and Z-scheme heterojunction anodes with improved charge separation for photoelectrojunction water oxidation

g-C₃N₄, studied as a metal-free photocatalyst, can lead to excellent results but the recombination of photogenerated charge carriers can substantially limit its performance.

Photoelectrochemical Water Splitting System--A Study of Interfacial Charge Transfer with Scanning Electrochemical Microscopy

Fast charge transfer kinetics at the photoelectrode/electrolyte interface is critical for efficient photoelectrochemical (PEC) water splitting system. Thus, far, a measurement of kinetics constants for such processes is limited. In this study, scanning electrochemical microscopy (SECM) is employed to investigate the charge transfer kinetics at the photoelectrode/electrolyte interface in the feedback mode in order to simulate the oxygen evolution process in PEC system. The popular photocatalysts BiVO4 and Mo doped BiVO4 (labeled as Mo:BiVO4) are selected as photoanodes and the common redox couple [Fe(CN)6](3-)/[Fe(CN)6](4-) as molecular probe. SECM characterization can directly reveal the surface catalytic reaction kinetics constant of 9.30 × 10(7) mol(-1) cm(3) s(-1) for the BiVO4. Furthermore, we find that after excitation, the ratio of rate constant for photogenerated hole to electron via Mo:BiVO4 reacting with mediator at the electrode/electrolyte interface is about 30 times larger than that of BiVO4. This suggests that introduction of Mo(6+) ion into BiVO4 can possibly facilitate solar to oxygen evolution (hole involved process) and suppress the interfacial back reaction (electron involved process) at photoanode/electrolyte interface. Therefore, the SECM measurement allows us to make a comprehensive analysis of interfacial charge transfer kinetics in PEC system.

Understanding the synergistic effect of WO3–BiVO4 heterostructures by impedance spectroscopy

The dc resistance allows assessment of the influence of bulk recombination and charge transfer kinetics on the oxygen evolution performance of heterostructured materials.

New insights into Ag-doped BiVO4 microspheres as visible light photocatalysts

This study describes the synthesis of Ag–bismuth vanadate (Ag–BiVO₄) microspheres, a highly efficient visible light photocatalyst for the degradation of methylene blue, via a one-step hydrothermal method.

Enhanced photoelectrochemical water oxidation of bismuth vanadate via a combined strategy of W doping and surface RGO modification

Combined modifications of tungsten doping and surface reduced graphene oxide modification enhance PEC water oxidation synergistically.

Wireless Solar Water Splitting Device with Robust Cobalt-Catalyzed, Dual-Doped BiVO4 Photoanode and Perovskite Solar Cell in Tandem: A Dual Absorber Artificial Leaf

A stand-alone, wireless solar water splitting device without external energy supply has been realized by combining in tandem a CH3NH3PbI3 perovskite single junction solar cell with a cobalt carbonate (Co-Ci)-catalyzed, extrinsic/intrinsic dual-doped BiVO4 (hydrogen-treated and 3 at% Mo-doped). The photoanode recorded one of the highest photoelectrochemical water oxidation activity (4.8 mA/cm(2) at 1.23 VRHE) under simulated 1 sun illumination. The oxygen evolution Co-Ci co-catalyst showed similar performance to best known cobalt phosphate (Co-Pi) (5.0 mA/cm(2) at 1.23 VRHE) on the same dual-doped BiVO4 photoanode, but with significantly better stability. A tandem artificial-leaf-type device produced stoichiometric hydrogen and oxygen with an average solar-to-hydrogen efficiency of 4.3% (wired), 3.0% (wireless) under simulated 1 sun illumination. Hence, our device based on a D4 tandem photoelectrochemical cell represents a meaningful advancement in performance and cost over the device based on a triple-junction solar cell-electrocatalyst combination.

Morphology-controllable synthesis of novel Bi25VO40 microcubes: optical properties and catalytic activities for the reduction of aromatic nitro compounds

Novel Bi₂₅VO₄₀ microcubes were successfully prepared via a facile hydrothermal synthesis route and they exhibited excellent optical and catalytic properties.

Facile synthesis of V4+self-doped, [010] oriented BiVO4nanorods with highly efficient visible light-induced photocatalytic activity

Self-doped, [010] oriented BiVO₄nanorods with plentiful oxygen vacancies on the surface exhibit strong adsorbability and highly efficient photocatalytic activity.