Bismuth Tantalum Oxyhalogen: A Promising Candidate Photocatalyst for Solar Water Splitting

As wide range of light absorption and suitable redox potentials are prerequisites for photocatalytic water splitting, exploring new semiconductor‐based materials with proper band structures for water splitting still calls for longstanding efforts. In this work, a series of photocatalysts, bismuth tantalum oxyhalide, Bi₄TaO₈X (X = Cl, Br), with valence band and conduction band positions at ≈?0.70 and ≈1.80 eV versus the reversible hydrogen electrode (RHE), respectively, are found to be capable for both water oxidation and reduction under visible light irradiation. Using flux synthetic methods, Bi₄TaO₈X (X = Cl, Br) with microplatelet morphology can be successfully prepared. The photocatalyst based on these materials shows an apparent quantum efficiency as high as 20% at 420 nm for water oxidation. In addition, a Z‐scheme system coupling Bi₄TaO₈Br with Ru/SrTiO₃:Rh is successfully achieved for overall water splitting with a stoichiometric ratio of H₂ and O₂ evolutions. This work demonstrates a new series of semiconductors Bi₄TaO₈X (X = Cl, Br) with the promising application in the field of solar energy utilization.     

A Floating Sheet for Efficient Photocatalytic Water Splitting

Solar photocatalytic water splitting has been a promising way to provide clean hydrogen energy. There are two weaknesses in the typical photocatalytic process in which photocatalysts are generally dispersing in water under stir. One is the inadequate utilization of light energy and the other one is the cumbersome operation in the recycling procedure. This study demonstrates an efficient solar photocatalytic water splitting using a floating sheet with a novel WSe₂ cocatalyst. The sheet is fabricated by laser‐depositing WSe₂ film on a carbon foam (CF) substrate and drop‐casting of the synthesized nanodiamond‐embedded Cu₂O (NEC) photocatalysts. This is a new‐type artificial photocatalytic system that overcomes the above‐mentioned weaknesses of a powder‐dispersing system. The WSe₂ cocatalyst acts as an electron sink to promote electron–hole separation, resulting in the further improvement of photocatalytic performance. This floating NEC/WSe₂/CF structure achieves efficient water splitting upon simulated solar irradiation with an increased H₂ evolution rate, which is 13.2 times that of the powder‐dispersing system. This study offers a strategy for the design of new‐type photocatalytic system and discovery of alternative noble metal‐free cocatalysts.     

High Efficiency Photocatalytic Water Splitting Using 2D α‐Fe2O3/g‐C3N4 Z‐Scheme Catalysts

Photocatalysis is the most promising method for achieving artificial photosynthesis, but a bottleneck is encountered in finding materials that could efficiently promote the water splitting reaction. The nontoxicity, low cost, and versatility of photocatalysts make them especially attractive for this application. This study demonstrates that small amounts of α‐Fe₂O₃ nanosheets can actively promote exfoliation of g‐C₃N₄, producing 2D hybrid that exhibits tight interfaces and an all‐solid‐state Z‐scheme junction. These nanostructured hybrids present a high H₂ evolution rate >3 × 10⁴ µmol g^‐1 h^‐1 and external quantum efficiency of 44.35% at λ = 420 nm, the highest value so far reported among the family of g‐C₃N₄ photocatalysts. Besides effectively suppressing the recombination of electron–hole pairs, this Z‐scheme junction also exhibits activity toward overall water splitting without any sacrificial donor. The proposed synthetic route for controlled production of 2D g‐C₃N₄‐based structures provides a scalable alternative toward the development of highly efficient and active photocatalysts.     

Metal Halide Perovskites for Solar‐to‐Chemical Fuel Conversion

This review article presents and discusses the recent progress made in the stabilization, protection, improvement, and design of halide perovskite‐based photocatalysts, photoelectrodes, and devices for solar‐to‐chemical fuel conversion. With the target of water splitting, hydrogen iodide splitting, and CO₂ reduction reactions, the strategies established for halide perovskites used in photocatalytic particle‐suspension systems, photoelectrode thin‐film systems, and photovoltaic‐(photo)electrocatalysis tandem systems are organized and introduced. Moreover, recent achievements in discovering new and stable halide perovskite materials, developing protective and functional shells and layers, designing proper reaction solution systems, and tandem device configurations are emphasized and discussed. Perspectives on the future design of halide perovskite materials and devices for solar‐to‐chemical fuel conversion are provided. This review may serve as a guide for researchers interested in utilizing halide perovskite materials for solar‐to‐chemical fuel conversion.     

Nitrogen‐Doped Carbon‐Coated CuO‐In2O3 p–n Heterojunction for Remarkable Photocatalytic Hydrogen Evolution

CuO as a catalyst has shown promising application prospects in photocatalytic splitting of water into hydrogen (H₂). However, the instability of CuO in amine aqueous solution limits the applications of CuO‐based photocatalysts in the photocatalytic H₂ evolution. In this work, a novel dodecahedral nitrogen (N)‐doped carbon (C) coated CuO‐In₂O₃ p–n heterojunction (DNCPH) is designed and synthesized by directly pyrolyzing benzimidazole‐modified dodecahedral Cu/In‐based metal‐organic frameworks, showing long‐term stability in triethanolamine (TEOA) aqueous solution and excellent photocatalytic H₂ production efficiency. The improved stability of DNCPH in TEOA solution is ascribed to the alleviation of electron deficiency in CuO by forming the p–n heterojunction and the protection with coated N‐doped C layer. Based on detailed theoretical calculations and experimental studies, it is found that the improved separation efficiency of photogenerated electron/hole pairs and the mediated adsorption behavior (|?G_H*|→0) by coupling N‐doped C layer with CuO‐In₂O₃ p–n heterojunction lead to the excellent photocatalytic H₂ production efficiency of DNCPH. This work provides a feasible strategy for effectively applying CuO‐based photocatalysts in photocatalytic H₂ production.     

In Situ Phase‐Induced Spatial Charge Separation in Core–Shell Oxynitride Nanocube Heterojunctions Realizing Robust Solar Water Splitting

Efficient spatial charge separation is critical for solar energy conversion over solid photocatalysts. The development of efficient visible‐light photocatalysts has been of immense interest, but with limited success. Here, multiband core–shell oxynitride nanocube heterojunctions composed of a tantalum nitride (Ta₃N₅) core and nitrogen‐doped sodium tantalate (NaTaON) shell have been constructed via an in situ phase‐induced etching chemical strategy. The photocatalytic water splitting performance of sub‐20‐nm Ta₃N₅@NaTaON junctions exhibits an extraordinarily high photocatalytic activity toward oxygen and hydrogen evolution. Most importantly, the combined experimental results and theoretical calculations reveal that the strong interfacial Ta? O? N bonding connection as a touchstone among Ta₃N₅@NaTaON junctions provides a continuous charge transport pathway rather than a random charge accumulation. The prolonged photoexcited charge carrier lifetime and suitable band matching between the Ta₃N₅ core and NaTaON shell facilitate the separation of photoinduced electron–hole pairs, accounting for the highly efficient photocatalytic performance. This work establishes the use of (oxy)nitride heterojunctions as viable photocatalysts for the conversion of solar energy into fuels.     

Layered Double Hydroxide Nanostructured Photocatalysts for Renewable Energy Production

An enormous research effort is currently being directed towards the development of efficient visible‐light‐driven photocatalysts for renewable energy applications including water splitting, CO₂ reduction and alcohol photoreforming. Layered double hydroxide (LDH)‐based photocatalysts have emerged as one of the most promising candidates to replace TiO₂‐based photocatalysts for these reactions_, owing to their unique layered structure, compositional flexibility, controllable particle size, low manufacturing cost and ease of synthesis. By introducing defects into LDH materials through the control of their size to the nanoscale, the atomic structure, surface defect concentration, and electronic and optical characteristics of LDH materials can be strategically engineered for particular applications. Furthermore, through the use of advanced characterization techniques such as X‐ray absorption fine structure, positron annihilation spectrometry, X‐ray photoelectron spectroscopy, electron spin resonance, density‐functional theory calculations, and photocatalytic tests, structure‐activity relationships can be established and used in the rational design of high‐performance LDH‐based photocatalysts for efficient solar energy capture. LDHs thus represent a versatile platform for semiconductor photocatalyst development with application potential across the energy sector.     

Atomic‐Level Reactive Sites for Semiconductor‐Based Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction is an effective means to generate renewable energy. It involves redox reactions, reduction of CO₂ and oxidation of water, that leads to the production of solar fuel. Significant research effort has therefore been made to develop inexpensive and practically sustainable semiconductor‐based photocatalysts. The exploration of atomic‐level active sites on the surface of semiconductors can result in an improved understanding of the mechanism of CO₂ photoreduction. This can be applied to the design and synthesis of efficient photocatalysts. In this review, atomic‐level reactive sites are classified into four types: vacancies, single atoms, surface functional groups, and frustrated Lewis pairs (FLPs). These different photocatalytic reactive sites are shown to have varied affinity to reactants, intermediates, and products. This changes pathways for CO₂ reduction and significantly impacts catalytic activity and selectivity. The design of a photocatalyst from an atomic‐level perspective can therefore be used to maximize atomic utilization efficiency and lead to a high selectivity. The prospects for fabrication of effective photocatalysts based on an in‐depth understanding are highlighted.     

g‐C3N4‐Based Heterostructured Photocatalysts

Photocatalysis is considered as one of the promising routes to solve the energy and environmental crises by utilizing solar energy. Graphitic carbon nitride (g‐C₃N₄) has attracted worldwide attention due to its visible‐light activity, facile synthesis from low‐cost materials, chemical stability, and unique layered structure. However, the pure g‐C₃N₄ photocatalyst still suffers from its low separation efficiency of photogenerated charge carriers, which results in unsatisfactory photocatalytic activity. Recently, g‐C₃N₄‐based heterostructures have become research hotspots for their greatly enhanced charge carrier separation efficiency and photocatalytic performance. According to the different transfer mechanisms of photogenerated charge carriers between g‐C₃N₄ and the coupled components, the g‐C₃N₄‐based heterostructured photocatalysts can be divided into the following categories: g‐C₃N₄‐based conventional type II heterojunction, g‐C₃N₄‐based Z‐scheme heterojunction, g‐C₃N₄‐based p–n heterojunction, g‐C₃N₄/metal heterostructure, and g‐C₃N₄/carbon heterostructure. This review summarizes the recent significant progress on the design of g‐C₃N₄‐based heterostructured photocatalysts and their special separation/transfer mechanisms of photogenerated charge carriers. Moreover, their applications in environmental and energy fields, e.g., water splitting, carbon dioxide reduction, and degradation of pollutants, are also reviewed. Finally, some concluding remarks and perspectives on the challenges and opportunities for exploring advanced g‐C₃N₄‐based heterostructured photocatalysts are presented.     

Alkali Etching of Layered Double Hydroxide Nanosheets for Enhanced Photocatalytic N2 Reduction to NH3

Layered double hydroxide (LDH) nanosheets show good activity in a wide range of photoreactions, with this activity being generally attributable to an abundance of surface oxygen vacancies or coordinatively unsaturated metal cations in the nanosheets which serve as active sites for reactant adsorption and activation. Recently, LDH nanosheets have been shown to be very effective for photocatalytic N₂ reduction to NH₃ using water as the reducing agent. Herein, it is demonstrated that a simple pretreatment of ZnCr‐LDH, ZnAl‐LDH, and NiAl‐LDH nanosheets with aqueous NaOH can greatly enhance the concentration of oxygen vacancies and low coordination metal centers in the nanosheets, thus significantly enhancing their photocatalytic activity for N₂ reduction to NH₃ under UV–vis irradiation (without the need for added sacrificial agents or cocatalysts). The facile alkali etching strategy introduced here is expected to be widely adopted in the future development of high‐performance LDH photocatalysts for ammonia production and other challenging chemical transformations (e.g., CO₂ reduction and water splitting).     

Photosystem II: Structure and mechanism of the water:plastoquinone oxidoreductase

This mini-review briefly summarizes our current knowledge on the reaction pattern of light-driven water splitting and the structure of Photosystem II that acts as a water:plastoquinone oxidoreductase. The overall process comprises three types of reaction sequences: (a) light-induced charge separation leading to formation of the radical ion pair P680^+•Q_A^−•; (b) reduction of plastoquinone to plastoquinol at the Q_B site via a two-step reaction sequence with Q_A^−• as reductant and (c) oxidative water splitting into O₂ and four protons at a manganese-containing catalytic site via a four-step sequence driven by P680^+• as oxidant and a redox active tyrosine Y_Z acting as mediator. Based on recent progress in X-ray diffraction crystallographic structure analysis the array of the cofactors within the protein matrix is discussed in relation to the functional pattern. Special emphasis is paid on the structure of the catalytic sites of PQH₂ formation (Q_B-site) and oxidative water splitting (Mn₄O _x Ca cluster). The energetics and kinetics of the reactions taking place at these sites are presented only in a very concise manner with reference to recent up-to-date reviews. It is illustrated that several questions on the mechanism of oxidative water splitting and the structure of the catalytic sites are far from being satisfactorily answered.     

Awakening Solar Water‐Splitting Activity of ZnFe2O4 Nanorods by Hybrid Microwave Annealing

1D zinc ferrite (ZnFe₂O₄) photoanode is fabricated on an F‐doped tin oxide substrate at a low temperature by an all‐solution method. To activate the material for photoelectrochemical water splitting, the hybrid microwave annealing (HMA) is applied with graphite powder as a susceptor. Thus, HMA treatment of ZnFe₂O₄ photoanode synthesized at 550 °C increases the photocurrent density of water oxidation by 15 times. In contrast, the conventional thermal annealing at 800 °C brings only 1.7‐fold increase. The various physical characterizations and hole scavenger photooxidation experiments with H₂O₂ reveal that the post‐HMA treatment anneals the surface defect states and enhances the crystallinity. Hence, HMA treatment is effective to suppress charge‐carrier recombination both on the surface and in the bulk of ZnFe₂O₄ to bring out its latent solar water‐splitting activity.     

An Advanced Separator for Li–O2 Batteries: Maximizing the Effect of Redox Mediators

Although Li–O₂ batteries are promising next‐generation energy storage systems with superior theoretical capacities, they have a serious limitation regarding the large overpotential upon charging that results from the low conductivity of the discharge product. Thus, various redox mediators (RMs) have been widely studied to reduce the overpotential in the charging process, which should promote the oxidation of Li₂O₂. However, RMs degrade the Li metal anode through a parasitic reaction between the RM and the Li metal, and a solution for this phenomenon is necessary. In this study, an effective method is proposed to prevent the migration of the RM toward the anode side of the lithium using a separator that is modified with a negatively charged polymer. When DMPZ (5,10‐dihydro‐5,10‐dimethylphenazine) is used as an RM, it is found that the modified separator suppresses the migration of DMPZ toward the counter electrode of the Li metal anode. This is investigated by a visual redox couple diffusion test, a morphological investigation, and an X‐ray diffraction study. This advanced separator effectively maximizes the catalytic activity of the redox mediator. Li–O₂ batteries using both a highly concentrated DMPZ and the modified separator exhibit improved performance and maintained 90% round‐trip efficiency up to the 20th cycle.     

Boosting Oxygen Evolution Kinetics by Mn–N–C Motifs with Tunable Spin State for Highly Efficient Solar‐Driven Water Splitting

Solar‐driven water splitting is in urgent need for sustainable energy research, for which accelerating oxygen evolution kinetics along with charge migration is the key issue. Herein, Mn³⁺ within π‐conjugated carbon nitride (C₃N₄) in form of Mn–N–C motifs is coordinated. The spin state (e_g orbital filling) of Mn centers is regulated by controlling the bond strength of Mn–N. It is demonstrated that Mn serves as intrinsic oxygen evolution reaction (OER) site and the kinetics is dependent on its spin state with an optimized e_g occupancy of ≈0.95. Specifically, the governing role of e_g occupancy originates from the varied binding strength between Mn and OER intermediates. Benefiting from the rapid spin state‐mediated OER kinetics, as well as extended optical absorption (to 600 nm) and accelerated charge separation by intercalated metal‐to‐ligand state, Mn–C₃N₄ stoichiometrically splits pure water with H₂ production rate up to 695.1 µmol g^?1 h^?1 under simulated sunlight irradiation (AM1.5), and achieves an apparent quantum efficiency of 4.0% at 420 nm, superior to most solid‐state based photocatalysts to date. This work for the first time correlates photocatalytic redox kinetics with the spin state of active sites, and suggests a nexus between photocatalysis and spin theory.     

Novel Black BiVO4/TiO2−x Photoanode with Enhanced Photon Absorption and Charge Separation for Efficient and Stable Solar Water Splitting

Recent advances in solar water splitting by using BiVO₄ as a photoanode have greatly optimized charge carrier and reaction dynamics, but relatively wide bandgap and poor photostability are still bottlenecks. Here, an excellent photoanode of black BiVO₄@amorphous TiO_2?x to tackle both problems is reported. Its applied bias photon‐to‐current efficiency for solar water splitting is up to 2.5%, which is a new record for a single oxide photon absorber. This unique core–shell structure is fabricated by coating amorphous TiO₂ on nanoporous BiVO₄ with the aid of atomic layer deposition and further hydrogen plasma treatment at room temperature. The black BiVO₄ with moderate oxygen vacancies reveals a bandgap reduction of ≈0.3 eV and significantly enhances solar utilization, charge transport and separation simultaneously, compared with conventional BiVO₄. The amorphous layer of TiO_2?x acts as both oxygen‐evolution catalyst and protection layer, which suppresses anodic photocorrosion to stabilize black BiVO₄. This configuration of black BiVO₄@amorphous TiO_2?x may provide an effective strategy to prompt solar water splitting toward practical applications.     

CdS/Graphene Nanocomposite Photocatalysts

Heterogeneous photocatalysis using semiconductors and renewable solar energy has been regarded as one of the most promising processes to alleviate, and even solve, both the world crises of energy supply and environmental pollution. In the past few years, many encouraging achievements have been made in the research area of graphene‐based semiconductor photocatalysts. Among them, CdS/graphene nanocomposites have attracted extensive attention as an important kind of photocatalyst in chemical and material science, due to its superior photocatalytic activity and photostability under visible‐light irradiation. The aim here is to address the enhancement mechanism of the photocatalytic performance of CdS/graphene composite photocatalysts, and systematically summarize recent progress regarding the design and synthesis of CdS/graphene nanocomposites. These nanocomposites are promising for a great diversity of applications in visible‐light photocatalytic fields, including artificial photosynthetic systems (photocatalytic hydrogen production and CO₂ reduction), environmental remediation, and organic photosynthesis. Special attention is given to the photocatalytic hydrogen production and pollutant photodegradation over CdS/graphene nanocomposite photocatalysts. Furthermore, perspectives on CdS/graphene‐based materials are discussed, including the various remaining challenges for large‐scale applications, identifying prospective areas for related research in this field.     

Enhanced Photocatalytic Activity for H2 Evolution under Irradiation of UV–Vis Light by Au-Modified Nitrogen-Doped TiO2

Background Purpose

Photocatalytic water splitting for hydrogen evolution is a potential way to solve many energy and environmental issues. Developing visible-light-active photocatalysts to efficiently utilize sunlight and finding proper ways to improve photocatalytic activity for H₂ evolution have always been hot topics for research. This study attempts to expand the use of sunlight and to enhance the photocatalytic activity of TiO₂ by N doping and Au loading.

Methods

Au/N-doped TiO₂ photocatalysts were synthesized and successfully used for photocatalytic water splitting for H₂ evolution under irradiation of UV and UV–vis light, respectively. The samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), and photoelectrochemical characterizations.

Results

DRS displayed an extension of light absorption into the visible region by doping of N and depositing with Au, respectively. PL analysis indicated electron-hole recombination due to N doping and an efficient inhibition of electron-hole recombination due to the loaded Au particles. Under the irradiation of UV light, the photocatalytic hydrogen production rate of the as-synthesized samples followed the order Au/TiO₂ > Au/N-doped TiO₂ > TiO₂ > N-doped TiO₂. While under irradiation of UV–vis light, the N-TiO₂ and Au/N-TiO₂ samples show higher H₂ evolution than their corresponding nitrogen-free samples (TiO₂ and Au/TiO₂). This inconsistent result could be attributed to the doping of N and the surface plasmonic resonance (SPR) effect of Au particles extending the visible light absorption. The photoelectrochemical characterizations further indicated the enhancement of the visible light response of Au/N-doped TiO₂.

Conclusion

Comparative studies have shown that a combination of nitrogen doping and Au loading enhanced the visible light response of TiO₂ and increased the utilization of solar energy, greatly boosting the photocatalytic activity for hydrogen production under UV–vis light.     

Advances in Piezo‐Phototronic Effect Enhanced Photocatalysis and Photoelectrocatalysis

Direct conversion of solar light into chemical energy by means of photocatalysis or photoelectrocatalysis is currently a point of focus for sustainable energy development and environmental remediation. However, its current efficiency is still far from satisfying, suffering especially from severe charge recombination. To solve this problem, the piezo‐phototronic effect has emerged as one of the most effective strategies for photo(electro)catalysis. Through the integration of piezoelectricity, photoexcitation, and semiconductor properties, the built‐in electric field by mechanical stimulation induced polarization can serve as a flexible autovalve to modulate the charge‐transfer pathway and facilitate carrier separation both in the bulk phase and at the surfaces of semiconductors. This review focuses on illustrating the trends and impacts of research based on piezo‐enhanced photocatalytic reactions. The fundamental mechanisms of piezo‐phototronics modulated band bending and charge migration are highlighted. Through comparing and classifying different categories of piezo‐photocatalysts (like the typical ZnO, MoS₂, and BaTiO₃), the recent advances in polarization‐promoted photo(electro)catalytic processes involving water splitting and pollutant degradation are overviewed. Meanwhile the optimization methods to promote their catalytic activities are described. Finally, the outlook for future development of polarization‐enhanced strategies is presented.     

Multifunctional Coatings from Scalable Single Source Precursor Chemistry in Tandem Photoelectrochemical Water Splitting

The straightforward and inexpensive fabrication of stabilized and activated photoelectrodes for application to tandem photoelectrochemical (PEC) water splitting is reported. Semiconductors such as Si, WO₃, and BiVO₄ can be coated with a composite layer formed upon hydrolytic decomposition of hetero­bimetallic single source precursors (SSPs) based on Ti and Ni, or Ti and Co in a simple single‐step process under ambient conditions. The resulting 3d‐transition metal oxide composite films are multifunctional, as they protect the semiconductor electrode from corrosion with an amorphous TiO₂ coating and act as bifunctional electrocatalysts for H₂ and O₂ evolution based on catalytic Ni or Co species. Thus, this approach enables the use of the same precursors for both photoelectrodes in tandem PEC water splitting, and SSP chemistry is thereby established as a highly versatile low‐cost approach to protect and activate photoelectrodes. In an optimized system, SSP coating of a Si photocathode and a BiVO₄ photoanode resulted in a benchmark noble metal‐free dual‐photoelectrode tandem PEC cell for overall solar water splitting with an applied bias solar‐to‐hydrogen conversion efficiency of 0.59% and a half‐life photostability of 5 h.     

Modifying La0.6Sr0.4MnO3 Perovskites with Cr Incorporation for Fast Isothermal CO2‐Splitting Kinetics in Solar‐Driven Thermochemical Cycles

Perovskites are promising oxygen carriers for solar‐driven thermochemical fuel production due to higher oxygen exchange capacity. Despite their higher fuel yield capacity, La_0.6Sr_0.4MnO₃ perovskite materials present slow CO₂‐splitting kinetics compared with state‐of‐the‐art CeO₂. In order to improve the CO production rates, the incorporation of Cr in La_0.6Sr_0.4MnO₃ is explored based on thermodynamic calculations that suggest an enhanced driving force toward CO₂ splitting at high temperatures for La_0.6Sr_0.4Cr_xMn_1?xO₃ perovskites. Here, reported is a threefold faster CO fuel production for La_0.6Sr_0.4Cr_0.85Mn_0.15O₃ compared to conventional La_0.6Sr_0.4MnO₃, and twofold faster than CeO₂ under isothermal redox cycling at 1400 °C, and high stability upon long‐term cycling without any evidence of microstructural degradation. The findings suggest that with the proper design in terms of transition metal ion doping, it is possible to adjust perovskite compositions and reactor conditions for improved solar‐to‐fuel thermochemical production under nonconventional solar‐driven thermochemical cycling schemes such as the here presented near isothermal operation.