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

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.

Suppressing Carrier Recombination in BiVO4/PEDOT:PSS Heterojunction for High-Performance Photodetector

The organic-inorganic hybrid heterojunction is introduced for the first time to break through the performance bottleneck of BiVO4-based photodetectors. Through a facile solution process, a p-n heterojunction is established at the BiVO4/PEDOT:PSS interface, and the built-in electric field is designed to separate photogenerated charge carriers. The hybrid heterojunction outputs a significantly increased photocurrent, which is 24 000 times larger than that of the bare BiVO4 thin film. The photodetector shows a satisfactory performance with a responsivity (R) and specific detectivity (D*) of 107.8 mA/W and 4.13 × 1010 Jones at 482 nm illumination. In addition to the fast response speed (100 ms), the device also exhibits an impressive long-term stability with a negligible attenuation in photocurrent after more than 700 cycles. This work provides a novel strategy to suppress carrier recombination of BiVO4, and the coupling of metal oxides and organic semiconductors opens up a new avenue for fabricating high-performance photodetectors.

Time-Domain View of Polaron Dynamics in Metal Oxide Photocatalysts

The polaron is a fundamental physical phenomenon in transition metal oxides (TMOs), and it has been studied extensively for decades. However, the implication of a polaron on photochemistry is still ambiguous. As such, understanding the fundamental properties and controlling the dynamics of polarons at the atomistic level is desired. In this Perspective, we seek to highlight the recent advances in studying small polarons in TMOs, with a particular focus on nonadiabatic molecular dynamics at the ab initio level, and discuss the implications for photocatalysis from the aspects of the structure, intrinsic physical properties, formation, migration, and recombination of small polarons. Finally, various methods were proposed to advance our understanding of manipulating the small-polaron dynamics, and strategies to design high-performance TMO-based photoelectrodes were discussed.

Key Strategies for Enhancing H2 Production in Transition Metal Oxide Based Photocatalysts

Transition metal oxides (TMOs) were one of the first photocatalysts used to produce hydrogen from water using solar energy. Despite the emergence of many other genres of photocatalysts over the years, TMO photocatalysts remain dominant due to their easy synthesis and unique physicochemical properties. Various strategies have been developed to enhance the photocatalytic activity of TMOs, but the solar-to-hydrogen (STH) conversion efficiency of TMO photocatalysts is still very low (<2 %), which is far below the targeted STH of 10 % for commercial viability. This article provides a comprehensive analysis of several widely used strategies, including oxygen defects control, doping, establishing interfacial junctions, and phase-facet-morphology engineering, that have been adopted to improve TMO photocatalysts. By critically evaluating these strategies and providing a roadmap for future research directions, this article serves as a valuable resource for researchers, students, and professionals seeking to develop efficient energy materials for green energy solutions.

Surface Hydroxylation during Water Splitting Promotes the Photoactivity of BiVO4(010) Surface by Suppressing Polaron-Mediated Charge Recombination

Polaron-based electron transport restricts the photoelectrochemical (PEC) water splitting efficiency of BiVO4. However, the location and dynamics of polarons are significantly dependent on the surface hydroxylation. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrated that hydroxylation of BiVO4(010) surface greatly alleviates the detrimental effect of oxygen-vacancy-induced electron polaron (EP). Surface hydroxylation stabilizes the EP at the surface to facilitate water splitting, makes the polaron a shallow localized state, and reduces the intensity of high-frequency V-O bond stretching vibrations. By decreasing the nonadiabatic coupling and decoherence time, the charge carrier lifetimes are extended by 1-3 orders of magnitude depending on the hydroxylation coverage. Our study not only reveals that the surface hydroxylation mitigated detrimental impacts of polarons in metal oxides but also provided valuable insights into the benign effect of intermediate species on the photocatalytic reactivity.

Great Influence of Pressure and Isotope Effects on Nonradiative Charge Loss in Hybrid Organic-Inorganic Perovskites

The intrinsic softness of hybrid organic-inorganic perovskites (HOIPs) allows their lattice and optoelectronic performance to be tunable to external pressure. Using nonadiabatic (NA) molecular dynamics, we demonstrate that a mild pressure accelerates hot electron relaxation and suppresses nonradiative electron-hole recombination in CH3NH3PbI3. Both processes are governed by NA coupling, which is enhanced between the electronic states of the quasi-continuous bands while is decreased between the band-edge states by reducing the electron-hole wave function overlap. Hydrogen/deuterium isotope exchange alleviates the pressure-induced NA coupling by increasing lattice rigidity and decreasing wave function overlap, slowing down both the hot electron relaxation and electron-hole recombination processes. The simulated time scales of sub-3 ps for hot electron relaxation and half nanoseconds for recombination agree well with the experiments. The study suggests that the isotope exchange can mitigate the pressure-caused fast losses of hot electrons and further prolong the charge carrier lifetime in HOIPs.

Operando IR Optical Control of Localized Charge Carriers in BiVO4 Photoanodes

In photoelectrochemical cells (PECs) the photon-to-current conversion efficiency is often governed by carrier transport. Most metal oxides used in PECs exhibit thermally activated transport due to charge localization via the formation of polarons or the interaction with defects. This impacts catalysis by restricting the charge accumulation and extraction. To overcome this transport bottleneck nanostructuring, selective doping and photothermal treatments have been employed. Here we demonstrate an alternative approach capable of directly activating localized carriers in bismuth vanadate (BiVO4). We show that IR photons can optically excite localized charges, modulate their kinetics, and enhance the PEC current. Moreover, we track carriers bound to oxygen vacancies and expose their ∼10 ns charge localization, followed by ∼60 μs transport-assisted trapping. Critically, we demonstrate that localization is strongly dependent on the electric field within the device. While optical modulation has still a limited impact on overall PEC performance, we argue it offers a path to control devices on demand and uncover defect-related photophysics.

Excitation-Wavelength-Dependent Charge-Carrier Lifetime in Hematite: An Insight from Nonadiabatic Molecular Dynamics

Experiments have reported that the photoexcited carrier lifetime in α-Fe₂O₃ has a significant excitation-wavelength dependence but leave the physical mechanism unresolved. In this work, we rationalize the puzzling excitation-wavelength dependence of the photoexcited carrier dynamics in Fe₂O₃ by performing nonadiabatic molecular dynamics simulation based on the strongly constrained and appropriately normed functional, which accurately describes the electronic structure of Fe₂O₃. Photogenerated electrons with lower-energy excitation relax fast in the t_2g conduction band within about 100 fs, while the photogenerated electrons with higher-energy excitation undergo first a slower interband relaxation from the e_g lower state to the t_2g upper state on a time scale of 135 ps, followed by the much faster t_2g intraband relaxation. This study provides insight into the experimentally reported excitation-wavelength dependence of the carrier lifetime in Fe₂O₃ and a reference for regulating photogenerated carrier dynamics in transition-metal oxides through the light excitation wavelength.

Insight into Potassium Vanadates as Visible-Light-Driven Photocatalysts: Synthesis of V(IV)-Rich Nano/Microstructures for the Photodegradation of Methylene Blue

Photocatalysis is regarded as a promising tool for wastewater remediation. In recent years, many studies have focused on investigating novel photocatalysts driven by visible light. In this study, K₂V₆O₁₆·nH₂O nanobelts and KV₃O₈ microplatelets were synthesized and investigated as photocatalysts. Samples were obtained via the facile method based on liquid-phase exfoliation with ion exchange. By changing the synthesis temperature (20–80 °C), different compositions, morphologies, and V⁴⁺/V⁵⁺ ratios were obtained and investigated as photocatalysts for organic dye degradation. Potassium vanadates’ structural, morphological, and optical properties were characterized using X-ray diffraction(XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Physical Property Measurement System (PPMS), thermogravimetric analysis (TGA) with mass spectrometry (MS), N₂ adsorption, scanning electron microscopy (SEM), photoluminescence (PL), and UV–vis diffuse reflectance spectroscopy (DRS). Synthesized K₂V₆O₁₆·nH₂O and KV₃O₈ showed an efficient absorption in the visible wavelength region with a narrow band gap energy of 1.80 and 1.91 eV, respectively. Their photocatalytic activity was evaluated by the degradation of methylene blue (MB) under simulated solar light illumination. The KV₃O₈ microplatelets exhibited the greatest photocatalytic activity, resulting in more than 90% degradation of the dye within the first 30 min. It is suggested that the observed excellent photocatalytic performance is attributed to the high content of V⁴⁺ species. Furthermore, the influence of active species was investigated, and the mechanism responsible for the photodegradation of the MB dye was discussed for the first time for potassium vanadates.

Potassium vanadates were synthesized via the facile method based on liquid-phase exfoliation with ion exchange. Different synthesis temperatures (20−80 °C) resulted in various phase compositions, morphologies, and V⁴⁺/V⁵⁺ ratios. The obtained samples were demonstrated as efficient visible-light driven photocatalysts, resulting in more than 90% degradation of methylene blue within the first 30−60 min. The enhanced photoactivity is attributed to the high content of V⁴⁺ species, which are beneficial for the separation of photogenerated electrons and holes and their lifetime.

Depleted Oxygen Defect State Enhancing Tungsten Trioxide Photocatalysis: A Quantum Dynamics Perspective

Oxygen vacancies generally create midgap states in transition metal oxides, which are expected to decrease the photoelectrochemical water-splitting efficiency. Recent experiments defy this expectation but leave the mechanism unclear. Focusing on the photoanode WO₃ as a prototypical system, we demonstrate using nonadiabatic molecular dynamics that an oxygen vacancy suppresses nonradiative electron-hole recombination, because the defect acts as an electron reservoir instead of a recombination center. The occupied midgap electrons prefer to be populated a priori compared to the band edge transition because of a larger transition dipole moment, converting to depleted/unoccupied trap states that rapidly accept conduction band electrons and then cause trap-assisted recombination by impeding the bandgap recombination regardless of oxygen vacancy configurations. The reported results provide a fundamental understanding of the "realistic" role of the oxygen vacancies and their influence on charge-phonon dynamics and carrier lifetime. The study generates valuable insights into the design of high-performance transition metal oxide photocatalysts.

Correlated organic-inorganic motion enhances stability and charge carrier lifetime in mixed halide perovskites

Organic cations are believed to have little influence on the charge carrier lifetime in hybrid organic-inorganic perovskites. Experiments defy this expectation. We consider formamidinium lead iodide (FAPbI₃) doping with and without Br as two prototypical systems, and perform ab initio time-domain nonadiabatic (NA) molecular dynamics simulations to investigate nonradiative electron-hole recombination. The simulations demonstrate that correlated organic-inorganic motion stabilizes the lattice and inhibits nonradiative charge recombination in FAPbI₃ upon Br doping. Br doping suppresses the rotation of FA and the vibrations of both organic and inorganic components, and leads to hole localization and the extent of localization is enhanced upon thermal impact, notably reducing the NA coupling by decreasing the overlap between the electron and hole wave functions. Doping also slightly increases the bandgap for further decreasing NA coupling and enhances the open-circuit voltage of perovskite solar cells. The small NA coupling and large bandgap beat the slow coherence loss, delaying electron-hole recombination and extending the charge carrier lifetime to 1.5 ns in Br-doped FAPbI₃, which is on the order of 1.1 ns in pristine FAPbI₃. The obtained time scales are in good agreement with experiments. Multiple phonon modes, including those of both the inorganic and organic components, couple to the electronic subsystem and accommodate the excess electronic energy lost during nonradiative charge recombination. The study reveals the unexpected atomistic mechanisms for the reduction of electron-hole recombination upon Br doping, rationalizes the experiments, and advances our understanding of the excited-state dynamics of perovskite solar cells.

CO Adsorbate Promotes Polaron Photoactivity on the Reduced Rutile TiO2(110) Surface

Polarons play a major role in determining the chemical properties of transition-metal oxides. Recent experiments show that adsorbates can attract inner polarons to surface sites. These findings require an atomistic understanding of the adsorbate influence on polaron dynamics and lifetime. We consider reduced rutile TiO₂(110) with an oxygen vacancy as a prototypical surface and a CO molecule as a classic probe and perform ab initio adiabatic molecular dynamics, time-domain density functional theory, and nonadiabatic molecular dynamics simulations. The simulations show that subsurface polarons have little influence on CO adsorption and CO can desorb easily. On the contrary, surface polarons strongly enhance CO adsorption. At the same time, the adsorbed CO attracts polarons to the surface, allowing them to participate in catalytic processes with CO. The CO interaction with polarons changes their orbital origin, suppresses polaron hopping, and stabilizes them at surface sites. Partial delocalization of polarons onto CO decouples them from free holes, decreasing the nonadiabatic coupling and shortening the quantum coherence time, thereby reducing charge recombination. The calculations demonstrate that CO prefers to adsorb at the next-nearest-neighbor five-coordinated Ti³⁺ surface electron polaron sites. The reported results provide a fundamental understanding of the influence of electron polarons on the initial stage of reactant adsorption and the effect of the adsorbate-polaron interaction on the polaron dynamics and lifetime. The study demonstrates how charge and polaron properties can be controlled by adsorbed species, allowing one to design high-performance transition-metal oxide catalysts.

Vacancy defect engineering of BiVO4 photoanodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting has been regarded as a promising technology for sustainable hydrogen production. The development of efficient photoelectrode materials is the key to improve the solar-to-hydrogen (STH) conversion efficiency towards practical application. Bismuth vanadate (BiVO₄) is one of the most promising photoanode materials with the advantages of visible light absorption, good chemical stability, nontoxic feature, and low cost. However, the PEC performance of BiVO₄ photoanodes is limited by the relatively short hole diffusion length and poor electron transport properties. The recent rapid development of vacancy defect engineering has significantly improved the PEC performance of BiVO₄. In this review article, the fundamental properties of BiVO₄ are presented, followed by an overview of the methods for creating different kinds of vacancy defects in BiVO₄ photoanodes. Then, the roles of vacancy defects in tuning the electronic structure, promoting charge separation, and increasing surface photoreaction kinetics of BiVO₄ photoanodes are critically discussed. Finally, the major challenges and some encouraging perspectives for future research on vacancy defect engineering of BiVO₄ photoanodes are presented, providing guidelines for the design of efficient BiVO₄ photoanodes for solar fuel production.

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.