Subsurface Engineering Induced Fermi Level De-pinning in Metal Oxide Semiconductors for Photoelectrochemical Water Splitting

Modulating built-in electric field via Bi-VO4-Fe interfacial bridges to enhance charge separation for efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting on semiconductor electrodes is considered to be one of the important ways to produce clean and sustainable hydrogen fuel, which is a great help in solving energy and environmental problems. Bismuth vanadate (BiVO4) as a promising photoanode for photoelectrochemical water splitting still suffers from poor charge separation efficiency and photo-induced self-corrosion. Herein, we develop heterojunction-rich photoanodes composed of BiVO4 and iron vanadate (FeVO4), coated with nickel iron oxide (NiFeOx/FeVO4/BiVO4). The formation of the interface between BiVO4 and FeVO4 (Bi-VO4-Fe bridges) enhances the interfacial interaction, resulting in improved performance. Meanwhile, high-conductivity FeVO4 and NiFeOx oxygen evolution co-catalysts effectively enhance bulk electron/hole separation, interface water's kinetics and photostability. Concurrently, the optimized NiFeOx/FeVO4/BiVO4 possesses a remarkable photocurrent density of 5.59 mA/cm2 at 1.23 V versus reversible hydrogen electrode (vs RHE) under AM 1.5G (Air Mass 1.5 Global) simulated sunlight, accompanied by superior stability without any decreased of its photocurrent density after 14 h. This work not only reveals the crucial role of built-in electric field in BiVO4-based photoanode during PEC water splitting, but also provides a new guide to the design of efficient photoanode for PEC.

Amorphous FeSnOx Nanosheets with Hierarchical Vacancies for Room-Temperature Sodium-Sulfur Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries, noted for their low material costs and high energy density, are emerging as a promising alternative to lithium-ion batteries (LIBs) in various applications including power grids and standalone renewable energy systems. These batteries are commonly assembled with glass fiber membranes, which face significant challenges like the dissolution of polysulfides, sluggish sulfur conversion kinetics, and the growth of Na dendrites. Here, we develop an amorphous two-dimensional (2D) iron tin oxide (A-FeSnOx) nanosheet with hierarchical vacancies, including abundant oxygen vacancies (Ovs) and nano-sized perforations, that can be assembled into a multifunctional layer overlaying commercial separators for RT Na-S batteries. The Ovs offer strong adsorption and abundant catalytic sites for polysulfides, while the defect concentration is finely tuned to elucidate the polysulfides conversion mechanisms. The nano-sized perforations aid in regulating Na ions transport, resulting in uniform Na deposition. Moreover, the strategic addition of trace amounts of Ti3C2 (MXene) forms an amorphous/crystalline (A/C) interface that significantly improves the mechanical properties of the separator and suppresses dendrite growth. As a result, the task-specific layer achieves ultra-light (~0.1 mg cm-2), ultra-thin (~200 nm), and ultra-robust (modulus=4.9 GPa) characteristics. Consequently, the RT Na-S battery maintained a high capacity of 610.3 mAh g-1 and an average Coulombic efficiency of 99.9 % after 400 cycles at 0.5 C.

Molecular interactions of photosystem I and ZIF-8 in bio-nanohybrid materials

Bio-nanohybrid devices featuring natural photocatalysts bound to a nanostructure hold great promise in the search for sustainable energy conversion. One of the major challenges of integrating biological systems is protecting them against harsh environmental conditions while retaining, or ideally enhancing their photophysical properties. In this mainly computational work we investigate an assembly of cyanobacterial photosystem I (PS I) embedded in a metal-organic framework (MOF), namely the zeolitic imidazolate framework ZIF-8. This complex has been reported experimentally [Bennett et al., Nanoscale Adv., 2019, 1, 94] but so far the molecular interactions between PS I and the MOF remained elusive. We show via absorption spectroscopy that PS I remains intact throughout the encapsulation-release cycle. Molecular dynamics (MD) simulations further confirm that the encapsulation has no noticeable structural impact on the photosystem. However, the MOF building blocks frequently coordinate to the Mg2+ ions of chlorophylls in the periphery of the antenna complex. High-level quantum mechanical calculations reveal charge-transfer interactions, which affect the excitonic network and thereby may reversibly change the fluorescence properties of PS I. Nevertheless, our results highlight the stability of PS I in the MOF, as the reaction center remains unimpeded by the heterogeneous environment, paving the way for applications in the foreseeable future.

Particle-Based Photoelectrodes for PEC Water Splitting: Concepts and Perspectives

This comprehensive review delves into the intricacies of the photoelectrochemical (PEC) water splitting process, specifically focusing on the design, fabrication, and optimization of particle-based photoelectrodes for efficient green hydrogen production. These photoelectrodes, composed of semiconductor materials, potentially harness light energy and generate charge carriers, driving water oxidation and reduction reactions. The versatility of particle-based photoelectrodes as a platform for investigating and enhancing various semiconductor candidates is explored, particularly the emerging complex oxides with compelling charge transfer properties. However, the challenges presented by many factors influencing the performance and stability of these photoelectrodes, including particle size, shape, composition, morphology, surface modification, and electrode configuration, are highlighted. The review introduces the fundamental principles of semiconductor photoelectrodes for PEC water splitting, presents an exhaustive overview of different synthesis methods for semiconductor powders and their assembly into photoelectrodes, and discusses recent advances and challenges in photoelectrode material development. It concludes by offering promising strategies for improving photoelectrode performance and stability, such as the adoption of novel architectures and heterojunctions.

Treasure-bowl style bifunctional site in cerium-tungsten hetero-clusters for superior solar-driven hydrogen production

Electrochemical water splitting powered by renewable energy sources hold potential for clean hydrogen production. However, there is still persistent challenges such as low solar-to-hydrogen conversion efficiency and sluggish oxygen evolution reactions. Here, we address the poor kinetics by studying and strengthening the coupling between Ce and W, and concurrently establishing Ce-W bi-atomic clusters on P,N-doped carbon (WN/WC-CeO2-x@PNC) with a "treasure-bowl" style. The bifunctional active sites are established using a novel and effective self-sacrificial strategy involving in situ induced defect formation. In addition, by altering the coupling of the W(d)-N(p) and W(d)-Ce(f) orbitals in the WN/WC-CeO2-x supramolecular clusters, we are able to disrupt the linear relationship between the binding energies of reaction intermediates, a key to obtain high catalytic performance for transition metals. Through the confinement of the WN/WC-CeO2-x composite hetero-clusters within the sub-nanometre spaces of hollow nano-bowl-shaped carbon reactors, a stable and efficient hydrogen production via water electrolysis could be achieved. When assembled together with a solar GaAs triple junction solar cell, a solar-to-hydrogen conversion efficiency of 18.92% in alkaline media could be realized. We show that the key to establish noble metal free catalysts with high efficiency lies in the fine-tuning of the metal-metal interface, forming regions with near optimal adsorption energies for the reaction intermediates participating in water electrolysis.

Fermi level pinning in metal oxides: influence on photocatalysis and photoelectrochemistry

Photocatalysis and photoelectrochemical (PEC) reactions are complex processes involving both the physical properties and surface chemistry of the semiconductor photocatalyst. Their interplay applies specific limitations on the performance of different materials in light-driven reactions, often despite their optimal band structure and optical absorption. One of the ways to properly characterize the photocatalytic and PEC properties of semiconductors remains the measurement of the photopotential, which characterizes a driving force of photoinduced processes in the material. In this work, we give a general scope on the photopotential in PEC reactions that finds its origin in semiconductor physics. It is shown that the photopotential does not always play an interchangeable role with the photocurrent in comparative analysis of the photocatalytic performance of different materials. Furthermore, a correlation between the photopotential and the kinetics of methylene blue dye photocatalysis is shown for anatase-TiO2, CeO2and WO3as photocatalysts. Fermi level pinning (FLP) in the bandgap of CeO2is observed limiting the photoactivity of the compound, which is attributed to the high defectivity of CeO2. A short review is given on the possible origins of FLP in metal oxides and ways to overcome it. It is pointed out that the shift of the Fermi level after illumination of CeO2can trigger the chemical instability of the material accompanied by the FLP process.

Rapid Surface Reconstruction of In2S3 Photoanode via Flame Treatment for Enhanced Photoelectrochemical Performance

Surface reconstruction, reorganizing the surface atoms or structure, is a promising strategy to manipulate materials' electrical, electrochemical, and surface catalytic properties. Herein, a rapid surface reconstruction of indium sulfide (In2S3) is demonstrated via a high-temperature flame treatment to improve its charge collection properties. The flame process selectively transforms the In2S3 surface into a diffusionless In2O3 layer with high crystallinity. Additionally, it controllably generates bulk sulfur vacancies within a few seconds, leading to surface-reconstructed In2S3 (sr-In2S3). When using those sr-In2S3 as photoanode for photoelectrochemical water splitting devices, these dual functions of surface In2O3/bulk In2S3 reduce the charge recombination in the surface and bulk region, thus improving photocurrent density and stability. With optimized surface reconstruction, the sr-In2S3 photoanode demonstrates a significant photocurrent density of 8.5 mA cm-2 at 1.23 V versus a reversible hydrogen electrode (RHE), marking a 2.5-fold increase compared to pristine In2S3 (3.5 mA cm-2). More importantly, the sr-In2S3 photoanode exhibits an impressive photocurrent density of 7.3 mA cm-2 at 0.6 V versus RHE for iodide oxidation reaction. A practical and scalable surface reconstruction is also showcased via flame treatment. This work provides new insights for surface reconstruction engineering in sulfide-based semiconductors, making a breakthrough in developing efficient solar-fuel energy devices.

Recent advanced strategies for bimetallenes toward electrocatalytic energy conversion reactions

Designing low-dimensional nanomaterials is vital to address the energy and environmental crisis by means of electrocatalytic conversion reactions. Bimetallenes, as an emerging class of 2D materials, present promise for electrocatalytic conversion reactions. By leveraging atomically thin layers, bimetallenes present unsaturated surface coordination, high specific surface area and high conductivity, which are all indispensable features for heterogeneous electrochemical reactions. However, the intrinsic activity and stability of bimetallenes needs to be improved further for bimetallene electrocatalysts, due to the higher demands of practical applications. Recently, many strategies have been developed to optimize the chemical or electronic structure to accommodate transfer of reactants, adsorption or desorption of intermediates, and dissociation of products. Considering that most such work focuses on adjusting the structure, this review offers in-depth insight into recent representative strategies for optimizing bimetallene electrocatalysts, mainly including alloying, strain effects, ligand effects, defects and heteroatom doping. Moreover, by summarizing the performance of bimetallenes optimized using various strategies, we provide a means to understand structure-property relationships. In addition, future prospects and challenges are discussed for further development of bimetallene electrocatalysts.

Superhydrophilic CoFe Dispersion of Hydrogel Electrocatalysts for Quasi-Solid-State Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is an attractive strategy to convert solar energy to hydrogen. However, the lifetime of PEC devices is restricted by the photocorrosion of semiconductors and the instability of co-catalysts. Herein, we report a feasible in situ inherent cross-linking method for stabilizing semiconductors that uses a CoFe-dispersed polyacrylamide (PAM) hydrogel as a transparent protector. The CoFe-PAM hydrogel protected BiVO4 (BVO) photoanode reached a photocurrent density of 5.7 mA cm-2 at 1.23 VRHE under AM 1.5G illumination with good stability. The PAM hydrogel network improved the loading of Fe sites while enabling the retention of more CoFe co-catalysts and increasing the electron density of the reaction active sites, further improving the PEC performance and stability. More importantly, by tuning the polymerization network, we pioneer the use of quasi-solid-state electrolytes in photoelectrochemistry, where the high concentration of ionic solvent in the PAM hydrogel ensures effective charge transport and good water storage owing to the hydrophilic and porous structure of the hydrogel. This work expands the scope of PEC research by providing a class of three-dimensional hydrogel electrocatalysts and quasi-solid-state electrolytes with huge extension potential, and the versatility of these quasi-solid-state electrolytes can be employed for other semiconductors.

Surface Oxygen Species in Metal Oxide Photoanodes for Solar Energy Conversion

Converting and storing solar energy directly as chemical energy through photoelectrochemical devices are promising strategies to replace fossil fuels. Metal oxides are commonly used as photoanode materials, but they still encounter challenges such as limited light absorption, inefficient charge separation, sluggish surface reactions, and insufficient stability. The regulation of surface oxygen species on metal oxide photoanodes has emerged as a critical strategy to modulate molecular and charge dynamics at the reaction interface. However, the precise role of surface oxygen species in metal oxide photoanodes remains ambiguous. The review focuses on elucidating the formation and regulation mechanisms of various surface oxygen species in metal oxides, their advantages and disadvantages in photoelectrochemical reactions, and the characterization methods employed to investigate them. Additionally, the article discusses emerging opportunities and potential hurdles in the regulation of surface oxygen species. By shedding light on the significance of surface oxygen species, this review aims to advance our understanding of their impact on metal oxide photoanodes, paving the way for the design of more efficient and stable photoelectrochemical devices.

Optimizing Hot Electron Harvesting at Planar Metal-Semiconductor Interfaces with Titanium Oxynitride Thin Films

Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO₂ and titanium oxynitride (TiON)/TiO_2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO_2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO₂, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO_0.5N_0.5) exhibits the highest carrier extraction efficiency (N_FC ≈ 2.8 × 10¹⁹ m^-3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (N_HE ≈ 1.6 × 10¹⁸ m^-3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride.

Nearly 100% selective and visible-light-driven methane conversion to formaldehyde via. single-atom Cu and Wδ

Direct solar-driven methane (CH₄) reforming is highly desirable but challenging, particularly to achieve a value-added product with high selectivity. Here, we identify a synergistic ensemble effect of atomically dispersed copper (Cu) species and partially reduced tungsten (W^δ+), stabilised over an oxygen-vacancy-rich WO₃, which enables exceptional photocatalytic CH₄ conversion to formaldehyde (HCHO) under visible light, leading to nearly 100% selectivity, a very high yield of 4979.0 μmol·g^-1 within 2 h, and the normalised mass activity of 8.5 × 10⁶ μmol·g^-1_Cu·h^-1 of HCHO at ambient temperature. In-situ EPR and XPS analyses indicate that the Cu species serve as the electron acceptor, promoting the photo-induced electron transfer from the conduction band to O₂, generating reactive •OOH radicals. In parallel, the adjacent W^δ+ species act as the hole acceptor and the preferred adsorption and activation site of H₂O to produce hydroxyl radicals (•OH), and thus activate CH₄ to methyl radicals (•CH₃). The synergy of the adjacent dual active sites boosts the overall efficiency and selectivity of the conversion process.