Heterostructured Inter‐Doped Ruthenium–Cobalt Oxide Hollow Nanosheet Arrays for Highly Efficient Overall Water Splitting

The development of transition‐metal‐oxides (TMOs)‐based bifunctional catalysts toward efficient overall water splitting through delicate control of composition and structure is a challenging task. Herein, the rational design and controllable fabrication of unique heterostructured inter‐doped ruthenium–cobalt oxide [(Ru–Co)O_x] hollow nanosheet arrays on carbon cloth is reported. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru–Co)O_x nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, the catalyst showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm^?2 and a small Tafel slope of 23.5 mV dec^?1, as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm^?2. As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm^?2 for alkaline overall water splitting.     

Surface‐Polarity‐Induced Spatial Charge Separation Boosts Photocatalytic Overall Water Splitting on GaN Nanorod Arrays

Photocatalytic overall water splitting has been recognized as a promising approach to convert solar energy into hydrogen. However, most of the photocatalysts suffer from low efficiencies mainly because of poor charge separation. Herein, taking a model semiconductor gallium nitride (GaN) as an example, we uncovered that photogenerated electrons and holes can be spatially separated to the nonpolar and polar surfaces of GaN nanorod arrays, which is presumably ascribed to the different surface band bending induced by the surface polarity. The photogenerated charge separation efficiency of GaN can be enhanced significantly from about 8 % to more than 80 % via co‐exposing polar and nonpolar surfaces. Furthermore, spatially assembling reduction and oxidation cocatalysts on the nonpolar and polar surfaces remarkably boosts photocatalytic overall water splitting, with the quantum efficiency increased from 0.9 % for the film photocatalyst to 6.9 % for the nanorod arrays photocatalyst.     

Nanocomposites of Tantalum‐Based Pyrochlore and Indium Hydroxide Showing High and Stable Photocatalytic Activities for Overall Water Splitting and Carbon Dioxide Reduction

Nanocomposites of tantalum‐based pyrochlore nanoparticles and indium hydroxide were prepared by a hydrothermal process for UV‐driven photocatalytic reactions including overall water splitting, hydrogen production from photoreforming of methanol, and CO₂ reduction with water to produce CO. The best catalyst was more than 20 times more active than sodium tantalate in overall water splitting and 3 times more active than Degussa P25 TiO₂ in CO₂ reduction. Moreover, the catalyst was very stable while generating stoichiometric products of H₂ (or CO) and O₂ throughout long‐term photocatalytic reactions. After the removal of In(OH)₃, the pyrochlore nanoparticles remained highly active for H₂ production from pure water and aqueous methanol solution. Both experimental studies and density functional theory calculations suggest that the pyrochlore nanoparticles catalyzed the water reduction to produce H₂, whereas In(OH)₃ was the major active component for water oxidation to produce O₂.     

Inorganic Photocatalysts for Overall Water Splitting

Photocatalytic water splitting using semiconductor photocatalysts has been considered as a “green” process for converting solar energy into hydrogen. The pioneering work on electrochemical photolysis of water at TiO₂ electrode, reported by Fujishima and Honda in 1972, ushered in the area of solar fuel. As the real ultimate solution for solar fuel‐generation, overall water splitting has attracted interest from researchers for some time, and a variety of inorganic photocatalysts have been developed to meet the challenge of this dream reaction. To date, high‐efficiency hydrogen production from pure water without the assistance of sacrificial reagents remains an open challenge. In this Focus Review, we aim to provide a whole picture of overall water splitting and give an outlook for future research.     

Quaternary Iron Nickel Cobalt Selenide as an Efficient Electrocatalyst for Both Quasi‐Solid‐State Dye‐Sensitized Solar Cells and Water Splitting

Iron nickel cobalt selenides are synthesized through a one‐step hydrothermal method. Quaternary Fe_0.37Ni_0.17Co_0.36Se demonstrates multifunctionality and shows high electrocatalytic activity for quasi‐solid‐state dye‐sensitized solar cells with a power conversion efficiency of 8.42 %, the hydrogen evolution reaction, the oxygen evolution reaction, and water splitting. The electric power output from tandem quasi‐solid‐state dye‐sensitized solar cells under one‐sun illumination is sufficient to split water and exhibits a solar‐to‐hydrogen conversion efficiency of 5.58 % with Fe_0.37Ni_0.17Co_0.36Se as the electrocatalyst in this integrated system. Owing to a remarkable synergistic effect, quaternary Fe_0.37Ni_0.17Co_0.36Se is proven to be superior to ternary nickel cobalt selenide in terms of conductivity, electrocatalytic activity, and photovoltaic performance.     

Graphene-Based Nanomaterials for Solar-Driven Overall Water Splitting

Water splitting through photocatalysis and photoelectrochemical methods is a promising strategy for solar energy utilization. Graphene is widely used in solar-driven overall water splitting because of its versatile properties. This review summarizes the preparation of graphene-based photocatalysts and photoelectrodes and the functions of graphene, and highlights the challenges and prospects of the future applications of graphene in solar-driven water splitting.     

A Hydrogen-Deficient Nickel–Cobalt Double Hydroxide for Photocatalytic Overall Water Splitting

Developing highly efficient and low-cost photocatalysts for overall water splitting has long been a pursuit for converting solar power into clean hydrogen energy. Herein, we demonstrate that a nonstoichiometric nickel–cobalt double hydroxide can achieve overall water splitting by itself upon solar light irradiation, avoiding the consumption of noble-metal co-catalysts. We employed an intensive laser to ablate a NiCo alloy target immersed in alkaline solution, and produced so-called L-NiCo nanosheets with a nonstoichiometric composition and O²⁻/Co³⁺ ions exposed on the surface. The nonstoichiometric composition broadens the band gap, while O²⁻ and Co³⁺ ions boost hydrogen and oxygen evolution, respectively. As such, the photocatalyst achieves a H₂ evolution rate of 1.7 μmol h⁻¹ under AM 1.5G sunlight irradiation and an apparent quantum yield (AQE) of 1.38 % at 380 nm.     

Intercalation of Highly Dispersed Metal Nanoclusters into a Layered Metal Oxide for Photocatalytic Overall Water Splitting

Metal nanoclusters (involving metals such as platinum) with a diameter smaller than 1 nm were deposited on the interlayer nanospace of KCa₂Nb₃O₁₀ using the electrostatic attraction between a cationic metal complex (e.g., [Pt(NH₃)₄]Cl₂) and a negatively charged two‐dimensional Ca₂Nb₃O₁₀^? sheet, without the aid of any additional reagent. The material obtained possessed eight‐fold greater photocatalytic activity for water splitting into H₂ and O₂ under band‐gap irradiation than the previously reported analog using a RuO₂ promoter. This study highlighted the superior functionality of Pt nanoclusters with diameters smaller than 1 nm for photocatalytic overall water splitting. This material shows the greatest efficiency among nanosheet‐based photocatalysts reported to date.     

Intrinsic Facet‐Dependent Reactivity of Well‐Defined BiOBr Nanosheets on Photocatalytic Water Splitting

Surface atomic arrangement and coordination of photocatalysts highly exposed to different crystal facets significantly affect the photoreactivity. However, controversies on the true photoreactivity of a specific facet in heterogeneous photocatalysis still exits. Herein, we exemplified well‐defined BiOBr nanosheets dominating with respective facets, (001) and (010), to track the reactivity of crystal facets for photocatalytic water splitting. The real photoreactivity of BiOBr‐(001) were evidenced to be significantly higher than BiOBr‐(010) for both hydrogen production and oxygen evolution reactions. Further in situ photochemical probing studies verified the distinct reactivity is not only owing to the highly exposed facets, but dominated by the co‐exposing facets, leading to an efficient spatial separation of photogenerated charges and further making the oxidation and reduction reactions separately occur with different reaction rates, which ordains the fate of the true photoreactivity.     

Black Phosphorus-Based Semiconductor Heterojunctions for Photocatalytic Water Splitting

Solar-to-hydrogen (H₂) conversion has been regarded as a sustainable and renewable technique to address aggravated environmental pollution and global energy crisis. The most critical aspect in this technology is to develop highly efficient and stable photocatalysts, especially metal-free photocatalysts. Recently, black phosphorus (BP), as a rising star 2D nanomaterial, has captured enormous attention in photocatalytic water splitting owing to its widespread optical absorption, adjustable direct band gap, and superior carrier migration characteristics. However, the rapid charge recombination of pristine BP has seriously limited its practical application as photocatalyst. The construction of BP-based semiconductor heterojunctions has been proven to be an effective strategy for enhancing the separation of photogenerated carriers. This Minireview attempts to summarize the recent progress in BP-based semiconductor heterojunctions for photocatalytic water splitting, including type-I and type-II heterojunctions, Z-Scheme systems, and multicomponent heterojunctions. Finally, a brief summary and perspective on the challenges and future directions in this field are also provided.     

Spatially Separated Photosystem II and a Silicon Photoelectrochemical Cell for Overall Water Splitting: A Natural–Artificial Photosynthetic Hybrid

Integrating natural and artificial photosynthetic platforms is an important approach to developing solar‐driven hybrid systems with exceptional function over the individual components. A natural–artificial photosynthetic hybrid platform is formed by wiring photosystem II (PSII) and a platinum‐decorated silicon photoelectrochemical (PEC) cell in a tandem manner based on a photocatalytic‐PEC Z‐scheme design. Although the individual components cannot achieve overall water splitting, the hybrid platform demonstrated the capability of unassisted solar‐driven overall water splitting. Moreover, H₂ and O₂ evolution can be separated in this system, which is ascribed to the functionality afforded by the unconventional Z‐scheme design. Furthermore, the tandem configuration and the spatial separation between PSII and artificial components provide more opportunities to develop efficient natural–artificial hybrid photosynthesis systems.     

Water Splitting via Decoupled Photocatalytic Water Oxidation and Electrochemical Proton Reduction Mediated by Electron‐Coupled‐Proton Buffer

Water splitting mediated by electron‐coupled‐proton buffer (ECPB) provides an efficient way to avoid gas mixing by separating oxygen evolution from hydrogen evolution in space and time. Though electrochemical and photoelectrochemcial water oxidation have been incorporated in such a two‐step water splitting system, alternative ways to reduce the cost and energy input for decoupling two half‐reactions are desired. Herein, we show the feasibility of photocatalytic oxygen evolution in a powder system with BiVO₄ as a photocatalyst and polyoxometalate H₃PMo₁₂O₄₀ as an electron and proton acceptor. The resulting reaction mixture was allowed to be directly used for the subsequent hydrogen evolution with the reduced H₃PMo₁₂O₄₀ as electron and proton donors. Our system exhibits excellent stability in repeated oxygen and hydrogen evolution, which brings considerable convenience to decoupled water splitting.     

Considerations for a More Accurate Evaluation Method for Photocatalytic Water Splitting

Over the past decades, various photocatalysts have been developed and great progress has been achieved in the field of solar-driven photocatalytic water splitting. However, the lack of an accurate and comprehensive evaluation method greatly hinders the meaningful comparison between different systems and becomes a serious impediment for the development of photocatalysts. Although many researchers are aware of this, there has been little work in this area. In this Viewpoint, we first analyze the insufficiencies of the existing evaluation methods and then make preliminary suggestions, aiming to stimulate discussion in the research community and hopefully lead to a widely accepted and authoritative evaluation system to assess photocatalyst performance.     

Hollow Bimetallic Zinc Cobalt Phosphosulfides for Efficient Overall Water Splitting

Bimetallic sulfides with earth-abundant transition-metal elements are proposed to enhance the electrocatalytic activities. Further replacement of S atom by less electronegative P atom improves the electrocatalytic performance of OER and HER. Herein, hollow bimetallic zinc cobalt phosphosulfides (Zn_0.3Co_2.7S₃P) are synthesized by a two-step process. The optimal catalyst of Zn_0.3Co_2.7S₃P with particle size of 50 nm displays an excellent electroactivity and long-term durability toward efficient overall water splitting process in alkaline medium. The excellent bifunctional electrocatalytic performance may be ascribed to the synergistic effect of hollow structure, anion substitution tuning and unique size control.     

Bifunctional Electrocatalysts for Overall Water Splitting from an Iron/Nickel‐Based Bimetallic Metal–Organic Framework/Dicyandiamide Composite

Pyrolysis of a bimetallic metal–organic framework (MIL‐88‐Fe/Ni)‐dicyandiamide composite yield a Fe and Ni containing carbonaceous material, which is an efficient bifunctional electrocatalyst for overall water splitting. FeNi₃ and NiFe₂O₄ are found as metallic and metal oxide compounds closely embedded in an N‐doped carbon–carbon nanotube matrix. This hybrid catalyst (Fe‐Ni@NC‐CNTs) significantly promotes the charge transfer efficiency and restrains the corrosion of the metallic catalysts, which is shown in a high OER and HER activity with an overpotential of 274 and 202 mV, respectively at 10 mA cm^?2 in alkaline solution. When this bifunctional catalyst was further used for H₂ and O₂ production in an electrochemical water‐splitting unit, it can operate in ambient conditions with a competitive gas production rate of 1.15 and 0.57 μL s^?1 for hydrogen and oxygen, respectively, showing its potential for practical applications.     

Van der Waals Heterostructures Comprised of Ultrathin Polymer Nanosheets for Efficient Z‐Scheme Overall Water Splitting

Inspired by natural photosynthesis, Z‐scheme photocatalytic systems are very appealing for achieving efficient overall water splitting. Developing metal‐free Z‐scheme photocatalysts for overall water splitting, however, still remains challenging. The construction of polymer‐based van der Waals heterostructures as metal‐free Z‐scheme photocatalytic systems for overall water splitting is described using aza‐fused microporous polymers (CMP) and C₂N ultrathin nanosheets as O₂‐ and H₂‐evolving catalysts, respectively. Although neither polymer is able to split pure water using visible light, a 2:1 stoichiometric ratio of H₂ and O₂ was observed when aza‐CMP/C₂N heterostructures were used. A solar‐to‐hydrogen conversion efficiency of 0.23 % was determined, which could be further enhanced to 0.40 % by using graphene as the solid electron mediator to promote the interfacial charge‐transfer process. This study highlights the potential of polymer photocatalysts for overall water splitting.     

Highly Dispersed and Small‐Sized Nickel(II) Hydroxide Co‐Catalyst Prepared by Photodeposition for Hydrogen Production

Photodeposition has been widely used as a mild and efficient synthetic method to deposit co‐catalysts. It is also worth studying how to synthesize non‐noble metal photocatalysts with uniform dispersion. Different synthetic conditions in photodeposition have a certain influence on particle size distribution and photocatalytic activity. Therefore, we designed experiments to prepare the inexpensive composite photocatalyst Ni(OH)₂/g‐C₃N₄ by photodeposition. The Ni(OH)₂ co‐catalysts disperse uniformly with particle sizes of about 10 nm. The photocatalytic hydrogen production rate of Ni(OH)₂/g‐C₃N₄ reached about 19 mmol g^?1 h^?1, with the Ni(OH)₂ deposition amount about 1.57 %. During 16 h stability testing, the rate of hydrogen production did not decrease significantly. The composite catalyst also revealed a good hydrogen production performance under sunlight. The Ni(OH)₂ co‐catalyst enhanced the separation ability of photogenerated carriers, which was proved by surface photovoltage and fluorescence analysis.