Recent advances in earth-abundant photocatalyst materials for solar H2 production

Harvesting solar energy attracts great attention due to its abundant, clean, and permanent characteristics. Thus, photocatalysts have emerged as promising candidates for converting the solar energy to practically useful hydrogen molecules. Tremendous efforts have been devoted in developments of efficient photocatalysts for water splitting, but most of photocatalysts utilize noble metals to improve photocatalytic performance. Progress in photocatalyst materials for the hydrogen production coupled with a better understanding of the basic catalytic mechanisms has enabled better selection of catalytic nanomaterials with improved performance. In this review, we analyze the current state of the art in photocatalyst materials for photochemical hydrogen production through water splitting using earth-abundant materials. We also explore two main factors involved in both material morphology and sacrificial agent to further improve the activity, efficiency and stability of photocatalysts.     

Recent progress in oxynitride photocatalysts for visible-light-driven water splitting

Photocatalytic water splitting into hydrogen and oxygen is a method to directly convert light energy into storable chemical energy, and has received considerable attention for use in large-scale solar energy utilization. Particulate semiconductors are generally used as photocatalysts, and semiconductor properties such as bandgap, band positions, and photocarrier mobility can heavily impact photocatalytic performance. The design of active photocatalysts has been performed with the consideration of such semiconductor properties. Photocatalysts have a catalytic aspect in addition to a semiconductor one. The ability to control surface redox reactions in order to efficiently produce targeted reactants is also important for photocatalysts. Over the past few decades, various photocatalysts for water splitting have been developed, and a recent main concern has been the development of visible-light sensitive photocatalysts for water splitting. This review introduces the study of water-splitting photocatalysts, with a focus on recent progress in visible-light induced overall water splitting on oxynitride photocatalysts. Various strategies for designing efficient photocatalysts for water splitting are also discussed herein.     

Recent progress in oxynitride photocatalysts for visible-light-driven water splitting

Abstract

Photocatalytic water splitting into hydrogen and oxygen is a method to directly convert light energy into storable chemical energy, and has received considerable attention for use in large-scale solar energy utilization. Particulate semiconductors are generally used as photocatalysts, and semiconductor properties such as bandgap, band positions, and photocarrier mobility can heavily impact photocatalytic performance. The design of active photocatalysts has been performed with the consideration of such semiconductor properties. Photocatalysts have a catalytic aspect in addition to a semiconductor one. The ability to control surface redox reactions in order to efficiently produce targeted reactants is also important for photocatalysts. Over the past few decades, various photocatalysts for water splitting have been developed, and a recent main concern has been the development of visible-light sensitive photocatalysts for water splitting. This review introduces the study of water-splitting photocatalysts, with a focus on recent progress in visible-light induced overall water splitting on oxynitride photocatalysts. Various strategies for designing efficient photocatalysts for water splitting are also discussed herein.     

Design of Heterostructured Hollow Photocatalysts for Solar‐to‐Chemical Energy Conversion

Direct conversion of solar energy into chemical energy in a sustainable manner is one of the most promising solutions to the energy crisis and environmental issues. Fabrication of highly active photocatalysts is of great significance for the practical applications of efficient solar‐to‐chemical energy conversion systems. Among various photocatalytic materials, semiconductor‐based heterostructured photocatalysts with hollow features show distinct advantages. Recent research efforts on rational design of heterostructured hollow photocatalysts toward photocatalytic water splitting and CO₂ reduction are presented. First, both single‐shelled and multishelled heterostructured photocatalysts are surveyed. Then, heterostructured hollow photocatalysts with tube‐like and frame‐like morphologies are discussed. It is intended that further innovative works on the material design of high‐performance photocatalysts for solar energy utilization can be inspired.     

Recent advances in visible light-driven water oxidation and reduction in suspension systems

In order to solve the shortage of sustainable energy and the related concern about combustion of fossil fuels, converting the most abundant solar energy into chemical fuels becomes one of the most promising choices to provide the everlasting and environmentally friendly energy vector along with the minimum impact on environment. Among the methods of converting solar energy into chemical fuels, there is a significant interest in the renewable hydrogen production by photocatalysts from abundant water under visible light irradiation. Therefore, the development of efficient photocatalysts for water reduction and oxidation in a suspension system is the footstone for the development of solar energy conversion. In this review, the fundamental theory of photocatalysis and key factors affecting photocatalysis will be introduced first. Then the new materials development covering inorganic materials (oxides, nitrides and sulfides), carbon-based photocatalysts, and semiconductor-coordination compound photocatalysts developed over the past 10?years will be addressed with discussion about dominating factors in the photochemical process. This review would provide a comprehensive reference to exploring the efficient and novel materials working for the solar energy conversion to affordable and sustainable fuels. Finally, the perspective of the technology is also discussed.     

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.     

Aerosol spray pyrolysis synthesis of water-splitting ferrites for solar hydrogen production

Aerosol spray pyrolysis (ASP) was employed for the synthesis of oxygen-deficient doped ferrite systems to be used as redox materials for the production of solar Hydrogen from water via a two-step thermochemical water-splitting cycle. In the first step (water splitting) the reduced state of a metal oxide is oxidized by taking oxygen from water and producing hydrogen; in the second step (regeneration) it is reduced again by delivering some of its lattice oxygen. Redox materials of the iron spinel family doped with other bivalent metals (Zn, Ni, Mn) were synthesized via ASP, characterized and evaluated with respect to their water-splitting activity. Organic additives, like citric acid, in the precursor solutions were found to result in products with finer particle size and to enhance the water-splitting activity of the synthesized materials. Material performance (water splitting activity, hydrogen yield, regeneration capability) was found to depend on the dopants’ kind and stoichiometry; in particular high percentages of Zn dopant seem to enhance the overall materials’ performance. ASP synthesized materials have demonstrated higher water conversion and hydrogen yields than materials of the same composition synthesized through solid-state routes. The ASP synthesis process was scaled-up successfully maintaining the favorable characteristics of the synthesized materials.     

利用太阳能制氢的方法及发展现状

发展清洁可再生能源是人类面临的巨大技术挑战,氢气作为一种理想的清洁能源,其制取及储运技术近年来都取得了很大进展。综述了利用太阳能分解水制氢的基本途径及发展现状,主要包括电解水制氢及人工模拟光合作用制氢、半导体光解水及其催化剂以及最有希望实现的高温热化学循环分解水制氢技术。     

纳米异质结光催化材料制取太阳能燃料研究进展

光催化制取太阳能燃料主要包括光催化分解H₂O制取H₂及光催化还原CO₂制取碳氢化合物, 是应对能源危机最具前景的方法之一。目前, 太阳能燃料的最高转化效率为5%, 无法满足商业化要求(≥10%)。纳米异质结由于能展现出单组分纳米材料或体相异质结所不具备的独特性质, 更能促进光生电子和空穴快速转移, 提供更多的光生电子或使光生电子具有更强的还原性, 因而能显著提高光催化活性。本文主要综述了几种纳米异质结(I-型、II-型、p-n型及Z-型)的光催化原理及其在制取太阳能燃料方面的研究进展, 并展望了研究发展方向。     

Carbon nitride photocatalyst with internal electric field induced photogenerated carriers spatial enrichment for enhanced photocatalytic water splitting

Photocatalytic water splitting is a cost-effective way to convert sustainable solar energy into chemical energy. Among various photocatalytic systems, coupling the H₂- and O₂- evolving photocatalysts has been widely used in photocatalytic water splitting. However, due to the close spatial distance between surface electrons and surface holes, this heterogeneous material easily catalyzes the unwanted reverse reaction, limiting the solar energy conversion efficiency. Here we present a carbon nitride nanosheet (CNN) homojunction which possesses electrons-enriched region and holes-enriched region induced by the interfacial internal electric field. The reverse reactions are significantly suppressed by benefiting from the spatial separation of the oxidation (+2.21 V) and reduction (-1.19 V) regions. The homojunction exhibits efficient photocatalytic activity for H₂ and O₂ evolution (1270.5 and 36.0 μmol h⁻¹) with the scavenger. Meanwhile, the solar-to-hydrogen efficiency of overall water splitting was improved to 0.14%. This research provides a new way for semiconductor design in solar energy conversion applications.     

Conjugated Microporous Polymer Nanosheets for Overall Water Splitting Using Visible Light

Direct water splitting into H₂ and O₂ using photocatalysts by harnessing sunlight is very appealing to produce storable chemical fuels. Conjugated polymers, which have tunable molecular structures and optoelectronic properties, are promising alternatives to inorganic semiconductors for water splitting. Unfortunately, conjugated polymers that are able to efficiently split pure water under visible light (400 nm) via a four‐electron pathway have not been previously reported. This study demonstrates that 1,3‐diyne‐linked conjugated microporous polymer nanosheets (CMPNs) prepared by oxidative coupling of terminal alkynes such as 1,3,5‐tris‐(4‐ethynylphenyl)‐benzene (TEPB) and 1,3,5‐triethynylbenzene (TEB) can act as highly efficient photocatalysts for splitting pure water (pH ≈ 7) into stoichiometric amounts of H₂ and O₂ under visible light. The apparent quantum efficiencies at 420 nm are 10.3% and 7.6% for CMPNs synthesized from TEPB and TEB, respectively; the measured solar‐to‐hydrogen conversion efficiency using the full solar spectrum can reach 0.6%, surpassing photosynthetic plants in converting solar energy to biomass (globally average ≈0.10%). First‐principles calculations reveal that photocatalytic H₂ and O₂ evolution reactions are energetically feasible for CMPNs under visible light irradiation. The findings suggest that organic polymers hold great potential for stable and scalable solar‐fuel generation.     

Dye-sensitized photocatalyst for effective water splitting catalyst

Abstract

Renewable hydrogen production is a sustainable method for the development of next-generation energy technologies. Utilising solar energy and photocatalysts to split water is an ideal method to produce hydrogen. In this review, the fundamental principles and recent progress of hydrogen production by artificial photosynthesis are reviewed, focusing on hydrogen production from photocatalytic water splitting using organic–inorganic composite-based photocatalysts.     

All‐Solid‐State Z‐Scheme Photocatalytic Systems

The current rapid industrial development causes the serious energy and environmental crises. Photocatalyts provide a potential strategy to solve these problems because these materials not only can directly convert solar energy into usable or storable energy resources but also can decompose organic pollutants under solar‐light irradiation. However, the aforementioned applications require photocatalysts with a wide absorption range, long‐term stability, high charge‐separation efficiency and strong redox ability. Unfortunately, it is often difficult for a single‐component photocatalyst to simultaneously fulfill all these requirements. The artificial heterogeneous Z‐scheme photocatalytic systems, mimicking the natural photosynthesis process, overcome the drawbacks of single‐component photocatalysts and satisfy those aforementioned requirements. Such multi‐task systems have been extensively investigated in the past decade. Especially, the all‐solid‐state Z‐scheme photocatalytic systems without redox pair have been widely used in the water splitting, solar cells, degradation of pollutants and CO₂ conversion, which have a huge potential to solve the current energy and environmental crises facing the modern industrial development. Thus, this review gives a concise overview of the all‐solid‐state Z‐scheme photocatalytic systems, including their composition, construction, optimization and applications.     

Transition‐Metal‐Based Electrocatalysts as Cocatalysts for Photoelectrochemical Water Splitting: A Mini Review

Converting solar energy into hydrogen via photoelectrochemical (PEC) water splitting is one of the most promising approaches for a sustainable energy supply. Highly active, cost‐effective, and robust photoelectrodes are undoubtedly crucial for the PEC technology. To achieve this goal, transition‐metal‐based electrocatalysts have been widely used as cocatalysts to improve the performance of PEC cells for water splitting. Herein, this Review summarizes the recent progresses of the design, synthesis, and application of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting. Mo, Ni, Co‐based electrocatalysts for the hydrogen evolution reaction (HER) and Co, Ni, Fe‐based electrocatalysts for the oxygen evolution reaction (OER) are emphasized as cocatalysts for efficient PEC HER and OER, respectively. Particularly, some most efficient and robust photoelectrode systems with record photocurrent density or durability for the half reactions of HER and OER are highlighted and discussed. In addition, the self‐biased PEC devices with high solar‐to‐hydrogen efficiency based on earth‐abundant materials are also addressed. Finally, this Review is concluded with a summary and remarks on some challenges and opportunities for the further development of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting.     

(Ga₁-xZnx)(N₁-xOx) nanocrystals: visible absorbers with tunable composition and absorption spectra

Bulk oxy(nitride) (Ga(1-x)Zn(x))(N(1-x)O(x)) is a promising photocatalyst for water splitting under visible illumination. To realize its solar harvesting potential, it is desirable to minimize its band gap through synthetic control of the value of x. Furthermore, improved photochemical quantum yields may be achievable with nanocrystalline forms of this material. We report the synthesis, structural, and optical characterization of nanocrystals of (Ga(1-x)Zn(x))(N(1-x)O(x)) with the values of x tunable from 0.30 to 0.87. Band gaps decreased from 2.7 to 2.2 eV over this composition range, which corresponded to a 260% increase in the fraction of solar photons that could be absorbed by the material. We achieved nanoscale morphology and compositional control by employing mixtures of ZnGa(2)O(4) and ZnO nanocrystals as synthetic precursors that could be converted to (Ga(1-x)Zn(x))(N(1-x)O(x)) under NH(3). The high quality of the resulting nanocrystals is encouraging for achieving photochemical water-splitting rates that are competitive with internal carrier recombination pathways.     

Recent Advances in Conjugated Polymers for Visible-Light-Driven Water Splitting

With the ambition of solving the challenges of the shortage of fossil fuels and their associated environmental pollution, visible-light-driven splitting of water into hydrogen and oxygen using semiconductor photocatalysts has emerged as a promising technology to provide environmentally friendly energy vectors. Among the current library of developed photocatalysts, organic conjugated polymers present unique advantages of sufficient light-absorption efficiency, excellent stability, tunable electronic properties, and economic applicability. As a class of rising photocatalysts, organic conjugated polymers offer high flexibility in tuning the framework of the backbone and porosity to fulfill the requirements for photocatalytic applications. In the past decade, significant progress has been made in visible-light-driven water splitting employing organic conjugated polymers. The recent development of the structural design principles of organic conjugated polymers (including linear, crosslinked, and supramolecular self-assembled polymers) toward efficient photocatalytic hydrogen evolution, oxygen evolution, and overall water splitting is described, thus providing a comprehensive reference for the field. Finally, current challenges and perspectives are also discussed.     

Recent Progress of Single-Atom Photocatalysts Applied in Energy Conversion and Environmental Protection

Photocatalysis driven by solar energy is a feasible strategy to alleviate energy crises and environmental problems. In recent years, significant progress has been made in developing advanced photocatalysts for efficient solar-to-chemical energy conversion. Single-atom catalysts have the advantages of highly dispersed active sites, maximum atomic utilization, unique coordination environment, and electronic structure, which have become a research hotspot in heterogeneous photocatalysis. This paper introduces the potential supports, preparation, and characterization methods of single-atom photocatalysts in detail. Subsequently, the fascinating effects of single-atom photocatalysts on three critical steps of photocatalysis (the absorption of incident light to produce electron-hole pairs, carrier separation and migration, and interface reactions) are analyzed. At the same time, the applications of single-atom photocatalysts in energy conversion and environmental protection (CO₂ reduction, water splitting, N₂ fixation, organic macromolecule reforming, air pollutant removal, and water pollutant degradation) are systematically summarized. Finally, the opportunities and challenges of single-atom catalysts in heterogeneous photocatalysis are discussed. It is hoped that this work can provide insights into the design, synthesis, and application of single-atom photocatalysts and promote the development of high-performance photocatalytic systems.     

Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Photoelectrochemical (PEC) cells for solar‐energy conversion have received immense interest as a promising technology for renewable hydrogen production. Their similarity to natural photosynthesis, utilizing sunlight and water, has provoked intense research for over half a century. Among many potential photocatalysts, BiVO₄, with a bandgap of 2.4–2.5 eV, has emerged as a highly promising photoanode material with a good chemical stability, environmental inertness, and low cost. Unfortunately, its charge transport properties are modest, at most a hole diffusion length (L_p) of ≈70 nm. However, recent rapid developments in multiple modification strategies have elevated it to a position as the most promising metal oxide photoanode material. This review summarizes developments in BiVO₄ photoanodes in the past 10 years, in which time it has continuously broken its own performance records for PEC water oxidation. Effective modification techniques are discussed, including synthesis of nanostructures/nanopores, external/internal doping, heterojunction fabrication, surface passivation, and cocatalysts. Tandem systems for unassisted solar water splitting and PEC production of value‐added chemicals are also discussed.     

磷烯光催化分解水研究进展

半导体光催化分解水被认为是解决全球能源短缺和环境污染问题的潜在途径之一。近年来, 磷烯(BP)由于具有带隙可调、空穴迁移率高、吸收光谱宽等特性而在光催化分解水方面得到了广泛关注。本文综述了国内外近年来在磷烯光催化分解水领域所取得的重要研究进展, 总结了磷烯基光催化剂的合成方法、表面修饰和异质结构构建等改性策略, 阐述了磷烯基光催化剂的构-效关系和电荷转移机制, 并展望了磷烯基光催化剂所面临的机遇和挑战, 揭示了磷烯基材料在太阳能利用和转化方面的重要应用潜力。     

Carbon Nitride Loaded with an Ultrafine,Monodisperse, Metallic Platinum-Cluster Cocatalyst for the Photocatalytic Hydrogen-Evolution Reaction

For the realization of a next-generation energy society, further improvement in the activity of water-splitting photocatalysts is essential. Platinum (Pt) is predicted to be the most effective cocatalyst for hydrogen evolution from water. However, when the number of active sites is increased by decreasing the particle size, the Pt cocatalyst is easily oxidized and thereby loses its activity. In this study, a method to load ultrafine, monodisperse, metallic Pt nanoclusters (NCs) on graphitic carbon nitride is developed, which is a promising visible-light-driven photocatalyst. In this photocatalyst, a part of the surface of the Pt NCs is protected by sulfur atoms, preventing oxidation. Consequently, the hydrogen-evolution activity per loading weight of Pt cocatalyst is significantly improved, 53 times, compared with that of a Pt-cocatalyst loaded photocatalyst by the conventional method. The developed method is also effective to enhance the overall water-splitting activity of other advanced photocatalysts such as SrTiO₃ and BaLa₄Ti₄O₁₅.