Nanostructure Engineering and Modulation of (Oxy)Nitrides for Application in Visible-Light-Driven Water Splitting

A Theoretical Investigation on the Structural, Electronic and Photocatalytic Properties of BaTaO2N Adsorbed with Metal Cocatalysts

We have performed DFT calculations to study the adsorption of single metal atoms (M=Ti, V, Cr, Mn, Fe, Co, Ni, Cu) on both BaO- and TaON-terminated surfaces of cis-BaTaO2N (001). We have identified the most stable adsorption configuration of each case and explored the relative stability, structural deformations, charge transfer, work function, density of states and mechanism of photocatalytic HER. For BaO termination, all of the adatoms bind covalently on top of the surface oxygens. For TaON termination, the metal atoms are located at the fourfold hollow site. The single metal atoms tend to exist on TaON-termination while they are apt to aggregate on BaO-termination. The formation of impurity states in the band gap is mostly originated from the adatom. When electrons are transferred from the adatom to the surface, the conduction band of semiconductor becomes partially occupied. The charge gained from the BaO termination or transferred to the TaON termination reduces with the increase in electronegativity of metal adatoms. The attachment of metal atoms on the BaO termination is favorable to the improvement of HER activity. While the TaON termination adsorbed with Ti, V and Cr may have better or comparable performance of HER compared with the pure surface.

Covalent organic framework without cocatalyst loading for efficient photocatalytic sacrificial hydrogen production from water

Metals are typically essential as either integral components within photocatalysts or as cocatalyst modifiers to enable efficient artificial photosynthesis, such as water splitting and carbon dioxide reduction. However, developing photocatalysts that function effectively without metal cocatalysts remains challenging due to their cost and scarcity. Here we show a nonstoichiometric β-ketoenamine-linked covalent organic framework that operates without cocatalysts, achieving hydrogen production rates of 15.48 mmol·g⁻¹·h⁻¹ from seawater and 22.45 mmol·g⁻¹·h⁻¹ from water with an ascorbic acid scavenger under visible light. It outperforms many reported platinum-modified covalent organic frameworks and metal-containing inorganic photocatalysts. The enhanced performance is attributed to its broad light absorption edge extending to approximately 660 nm, efficient charge separation, and the presence of abundant active oxygen sites derived from carbonyl groups, which exhibit a low hydrogen-binding Gibbs free energy change. This work lays the groundwork for designing cost-effective photocatalytic systems suitable for large-scale hydrogen production.

Lithiation: Advancing Material Synthesis and Structural Engineering for Emerging Applications

Lithiation, a process of inserting lithium ions into a host material, is revolutionizing nanomaterials synthesis and structural engineering as well as enhancing their performance across emerging applications, particularly valuable for large-scale synthesis of high-quality low-dimensional nanomaterials. Through a systematic investigation of the synthetic strategies and structural changes induced by lithiation, this review aims to offer a comprehensive understanding of the development, potential, and challenges associated with this promising approach. First, the basic principles of lithiation/delithiation processes will be introduced. Then, the recent advancements in the lithiation-induced structure changes of nanomaterials, such as morphology tuning, phase transition, defect generation, etc., will be stressed, emphasizing the importance of lithiation in structural modulation of nanomaterials. With the tunable structures induced by the lithiation, the properties and performance in electrochemical, photochemical, electronic devices, bioapplications, etc. will be discussed, followed by outlining the current challenges and perspectives in this research area.

Efficient Visible Light Photocatalytic Hydrogen Evolution by Boosting the Interfacial Electron Transfer in Mesoporous Mott-Schottky Heterojunctions of Co2P-Modified CdIn2S4 Nanocrystals

Photocatalytic water splitting for hydrogen generation is an appealing means of sustainable solar energy storage. In the past few years, mesoporous semiconductors have been at the forefront of investigations in low-cost chemical fuel production and energy conversion technologies. Mesoporosity combined with the tunable electronic properties of semiconducting nanocrystals offers the desired large accessible surface and electronic connectivity throughout the framework, thus enhancing photocatalytic activity. In this work, we present the construction of rationally designed 3D mesoporous networks of Co2P-modified CdIn2S4 nanoscale crystals (ca. 5-6 nm in size) through an effective soft-templating synthetic route and demonstrate their impressive performance for visible-light-irradiated catalytic hydrogen production. Spectroscopic characterizations combined with electrochemical studies unravel the multipathway electron transfer dynamics across the interface of Co2P/CdIn2S4 Mott-Schottky nanoheterojunctions and shed light on their impact on the photocatalytic hydrogen evolution chemistry. The strong Mott-Schottky interaction occurring at the heterointerface can regulate the charge transport toward greatly improved hydrogen evolution performance. The hybrid catalyst with 10 wt % Co2P content unveils a H2 evolution rate of 20.9 mmol gcat -1 h-1 under visible light irradiation with an apparent quantum efficiency (AQE) up to 56.1% at 420 nm, which is among the highest reported activities. The understanding of interfacial charge-transfer mechanism could provide valuable insights into the rational development of highly efficient catalysts for clean energy applications.

Charge redistribution of a spatially differentiated ferroelectric Bi4Ti3O12 single crystal for photocatalytic overall water splitting

Artificial photosynthesis is a promising approach to produce clean fuels via renewable solar energy. However, it is practically constrained by two issues of slow photogenerated carrier migration and rapid electron/hole recombination. It is also a challenge to achieve a 2:1 ratio of H2 and O2 for overall water splitting. Here we report a rational design of spatially differentiated two-dimensional Bi4Ti3O12 nanosheets to enhance overall water splitting. Such a spatially differentiated structure overcomes the limitation of charge transfer across different crystal planes in a single crystal semiconductor. The experimental results show a redistribution of charge within a crystal plane. The resulting photocatalyst produces 40.3 μmol h-1 of hydrogen and 20.1 μmol h-1 of oxygen at a near stoichiometric ratio of 2:1 and a solar-to-hydrogen efficiency of 0.1% under simulated solar light.

Constructing Bipolar Dual-Active Sites through High-Entropy-Induced Electric Dipole Transition for Decoupling Oxygen Redox

It remains a significant challenge to construct active sites to break the trade-off between oxidation and reduction processes occurring in battery cathodes with conversion mechanism, especially for the oxygen reduction and evolution reactions (ORR/OER) involved in the zinc-air batteries (ZABs). Here, using a high-entropy-driven electric dipole transition strategy to activate and stabilize the tetrahedral sites is proposed, while enhancing the activity of octahedral sites through orbital hybridization in a FeCoNiMnCrO spinel oxide, thus constructing bipolar dual-active sites with high-low valence states, which can effectively decouple ORR/OER. The FeCoNiMnCrO high-entropy spinel oxide with severe lattice distortion, exhibits a strong 1s→4s electric dipole transition and intense t2g(Co)/eg(Ni)-2p(OL) orbital hybridization that regulates the electronic descriptors, eg and t2g, which leads to the formation of low-valence Co tetrahedral sites (Coth) and high-valence Ni octahedral sites (Nioh), resulting in a higher half-wave potential of 0.87 V on Coth sites and a lower overpotential of 0.26 V at 10 mA cm-2 on Nioh sites as well as a superior performance of ZABs compared to low/mild entropy spinel oxides. Therefore, entropy engineering presents a distinctive approach for designing catalytic sites by inducing novel electromagnetic properties in materials across various electrocatalytic reactions, particularly for decoupling systems.

Fully Conjugated 2D sp2 Carbon-Linked Covalent Organic Frameworks for Photocatalytic Overall Water Splitting

Covalent organic frameworks (COFs) hold great promise for solar-driven hydrogen production. However, metal-free COFs for photocatalytic overall water splitting remain elusive, primarily due to challenges in simultaneously regulating their band structures and catalytic sites to enable concurrent half-reactions. Herein, two types of π-conjugated COFs containing the same donor-acceptor structure are constructed via Knoevenagel condensation and Schiff base reaction to afford cyanovinylene- and imine-bridged COFs, respectively. The difference in the linkage leads to a remarkable difference in their photocatalytic activity toward water splitting. The 2D sp2 carbon-linked COF exhibits notable activity for photocatalytic overall water splitting, which can reach an apparent quantum efficiency of 2.53% at 420 nm. In contrast, the 2D imine-linked COF cannot catalyze the overall water-splitting reaction. Mechanistic investigations reveal that the cyanovinylene linkage is essential in modulating the band structure and promoting charge separation in COFs, thereby enabling overall water splitting. Moreover, it is further shown that crystallinity substantially impacts the photocatalytic performance of COFs. This study represents the first successful example of developing metal-free COFs with high crystallinity for photocatalytic overall water splitting.

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

Perovskite BaTaO2 N: From Materials Synthesis to Solar Water Splitting

Barium tantalum oxynitride (BaTaO2 N), as a member of an emerging class of perovskite oxynitrides, is regarded as a promising inorganic material for solar water splitting because of its small band gap, visible light absorption, and suitable band edge potentials for overall water splitting in the absence of an external bias. However, BaTaO2 N still exhibits poor water-splitting performance that is susceptible to its synthetic history, surface states, recombination process, and instability. This review provides a comprehensive summary of previous progress, current advances, existing challenges, and future perspectives of BaTaO2 N for solar water splitting. A particular emphasis is given to highlighting the principles of photoelectrochemical (PEC) water splitting, classic and emerging photocatalysts for oxygen evolution reactions, and the crystal and electronic structures, dielectric, ferroelectric, and piezoelectric properties, synthesis routes, and thin-film fabrication of BaTaO2 N. Various strategies to achieve enhanced water-splitting performance of BaTaO2 N, such as reducing the surface and bulk defect density, engineering the crystal facets, tailoring the particle morphology, size, and porosity, cation doping, creating the solid solutions, forming the heterostructures and heterojunctions, designing the photoelectrochemical cells, and loading suitable cocatalysts are discussed. Also, the avenues for further investigation and the prospects of using BaTaO2 N in solar water splitting are presented.

Transition-metal (oxy)nitride photocatalysts for water splitting

Solar-driven water splitting based on particulate semiconductor materials is studied as a technology for green hydrogen production. Transition-metal (oxy)nitride photocatalysts are promising materials for overall water splitting (OWS) via a one- or two-step excitation process because their band structure is suitable for water splitting under visible light. Yet, these materials suffer from low solar-to-hydrogen energy conversion efficiency (STH), mainly because of their high defect density, low charge separation and migration efficiency, sluggish surface redox reactions, and/or side reactions. Their poor thermal stability in air and under the harsh nitridation conditions required to synthesize these materials makes further material improvements difficult. Here, we review key challenges in the two different OWS systems and highlight some strategies recently identified as promising for improving photocatalytic activity. Finally, we discuss opportunities and challenges facing the future development of transition-metal (oxy)nitride-based OWS systems.

Synthesis of Layered Lead-Free Perovskite Nanocrystals with Precise Size and Shape Control and Their Photocatalytic Activity

Halide perovskites have attracted enormous attention due to their potential applications in optoelectronics and photocatalysis. However, concerns over their instability, toxicity, and unsatisfactory efficiency have necessitated the development of lead-free all-inorganic halide perovskites. A major challenge in designing efficient halide perovskites for practical applications is the lack of effective methods for producing nanocrystals with precise size and shape control. In this work, a layered perovskite, Cs4ZnSb2Cl12 (CZS), is found from calculations to exhibit size- and facet-dependent optoelectronic properties in the nanoscale, and thus, a colloidal method is used to synthesize the CZS nanoparticles with size-tunable morphologies: zero- (nanodots), one- (nanowires and nanorods), two- (nanoplates), and three-dimensional (nanopolyhedra). The growth kinetics of the CZS nanostructures, along with the effects of surface ligands, reaction temperature, and time were investigated. The optoelectronic properties of the nanocrystals varied with size due to quantum confinement effects and with shape due to anisotropy within the crystals and the exposure of specific facets. These properties could be modulated to enhance the visible-light photocatalytic performance for toluene oxidation. In particular, the 9.7 nm CZS nanoplates displayed a toluene to benzaldehyde conversion rate of 1893 μmol g-1 h-1 (95% selectivity), 500 times higher than the bulk synthesized CZS, and comparable with the reported photocatalysts. This study demonstrates the integration of theoretical calculations and synthesis, revealing an approach to the design and fabrication of novel, high-performance colloidal perovskite nanocrystals for optoelectronic and photocatalytic applications.

Efficient full solar spectrum-driven photocatalytic hydrogen production on low bandgap TiO2/conjugated polymer nanostructures

The development of photocatalysts that can utilize the entire solar spectrum is crucial to achieving efficient solar energy conversion. The utility of the benchmark photocatalyst, TiO2, is limited only to the UV region due to its large bandgap. Extending the light harvesting properties across the entire spectrum is paramount to enhancing solar photocatalytic performance. In this work, we developed low bandgap TiO2/conjugated polymer nanostructures which exhibit full spectrum activity for efficient H2 production. The highly mesoporous structure of the nanostructures together with the photosensitizing properties of the conjugated polymer enabled efficient solar light activity. The mesoporous TiO2 nanostructures calcined at 550 °C exhibited a defect-free anatase crystalline phase with traces of brookite and high surface area, resulting in the best performance in hydrogen production (5.34 mmol g-1 h-1) under sunlight simulation. This value is higher not only in comparison to other TiO2-based catalysts but also to other semiconductor materials reported in the literature. Thus, this work provides an effective strategy for the construction of full spectrum active nanostructured catalysts for enhanced solar photocatalytic hydrogen production.

Achieving Record-High Photoelectrochemical Photoresponse Characteristics by Employing Co3O4 Nanoclusters as Hole Charging Layer for Underwater Optical Communication

The physicochemical properties of a semiconductor surface, especially in low-dimensional nanostructures, determine the electrical and optical behavior of the devices. Thereby, the precise control of surface properties is a prerequisite for not only preserving the intrinsic material quality but also manipulating carrier transport behavior for promoting device characteristics. Here, we report a facile approach to suppress the photocorrosion effect while boosting the photoresponse performance of n-GaN nanowires in a constructed photoelectrochemical-type photodetector by employing Co₃O₄ nanoclusters as a hole charging layer. Essentially, the Co₃O₄ nanoclusters not only alleviate nanowires from corrosion by optimizing the oxygen evolution reaction kinetics at the nanowire/electrolyte interface but also facilitate an efficient photogenerated carrier separation, migration, and collection process, leading to a significant ease of photocurrent attenuation (improved by nearly 867% after Co₃O₄ decoration). Strikingly, a record-high responsivity of 217.2 mA W^-1 with an ultrafast response/recovery time of 0.03/0.02 ms can also be achieved, demonstrating one of the best performances among the reported photoelectrochemical-type photodetectors, that ultimately allowed us to build an underwater optical communication system based on the proposed nanowire array for practical applications. This work provides a perspective for the rational design of stable nanostructures for various applications in photo- and biosensing or energy-harvesting nanosystems.

Flux-assisted synthesis of tungsten-doped layered perovskite oxychloride with promoted visible-light-responsive O2 evolution performance

Here, we successfully prepared Ba₂Bi₃Ta₂O₁₁Cl via a simple one-step molten salt method and adjusted its crystal morphology and structure, based on which the O₂-evolving activity was significantly improved. In addition, W doping promotes the charge separation efficiency, lowers the energy barrier for water oxidation reaction, and thus improves the activity. Finally, the optimized W-doped sample after molten salt treatment shows the best O₂ production activity (55 μmol h^-1) without loading any cocatalyst, which is 6 times higher than that of pristine Ba₂Bi₃Ta₂O₁₁Cl and 2 times higher than that of the undoped Ba₂Bi₃Ta₂O₁₁Cl treated with molten salt, respectively.

Accelerating Electron-Transfer Dynamics by TiO2 -Immobilized Reversible Single-Atom Copper for Enhanced Artificial Photosynthesis of Urea

Photocatalysis as a sustainable technology is expected to provide a novel sight for the green synthesis of urea directly using N₂ , CO₂ , and H₂ O under mild conditions. However, the fundamental issue of inefficient electron transfer in photocatalysis strongly hinders its feasibility, especially for the above multi-electron-demanding urea synthesis. Herein, an effective strategy of accelerating electron-transfer dynamics is reported by TiO₂ -immobilized reversible single-atom copper (denoted as Cu SA-TiO₂ ) to enhance the performance for photosynthesis of urea from N₂ , CO₂ , and H₂ O. As revealed by a series of quasi-in-situ characterizations (e.g., electron paramagnetic resonance, and wavelength-resolved and femtosecond time-resolved spectroscopies), the expedited dynamics behaviors originating from reversible single-atom copper in as-designed Cu SA-TiO₂ (electron extraction rate: over 30 times faster than the reference photocatalysts) allow the assurance of abundant and continual photogenerated electrons for multi-electron-demanding co-photoactivation of N₂ and CO₂ , resulting in considerable rates of urea production. The strategy above for improving the photoelectron-extraction ability of photocatalysts will offer a high-efficiency and promising route for artificial urea photosynthesis and other multi-electron-demanding photocatalytic reactions.

Sandwich-Polarized Heterojunction: Efficient Charge Separation and Redox Capability Protection for Photocatalytic Overall Water Splitting

Photocatalytic overall water splitting is a potential strategy to address energy crisis and environmental issues. However, it remains a great challenge to design an efficient photocatalyst, which not only possesses large spatial separation of photogenerated electrons and holes (PEH) to suppress recombination, but also can preserve the redox capability to drive the reaction. Herein, we design a new type of sandwich-polarized heterojunction by inserting a polarized semiconductor into the interlayer of the conventional photocatalyst. The inserted sublayer with out-of-plane polarization can induce a large electrostatic potential difference between the top and bottom photocatalytic sublayers. Then, the band edges of the top and bottom sublayers can be shifted to form the type II band alignment. Also, the valence band maximum and conduction band minimum will be located on different photocatalytic sublayers to facilitate the spatial separation of PEH. Simultaneously, different from the conventional type II heterojunction that reduces the redox capability, the electrostatic potential difference also acts as an auxiliary booster to offset the reduced redox potential of PEH. Taking the C₂N/In₂Se₃/C₂N heterojunction as an example, the polarized In₂Se₃ effectively promotes the interface transfer of PEH in 1-5 ps and extends the lifetime of PEH to ∼186 ns, which is about six times that of bilayer C₂N. Simultaneously, the redox power of C₂N is well preserved. Our work offers a promising scheme to advance the photocatalytic overall water splitting.

First-principles calculations of the structural, energetic, electronic, optical, and photocatalytic properties of BaTaO2N low-index surfaces

We present here the influence of different surface terminations on the electronic, optical, and photocatalytic properties of trans and cis BaTaO2N using density functional theory calculations.