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.     

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.     

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.     

Tantalum (Oxy)Nitride: Narrow Bandgap Photocatalysts for Solar Hydrogen Generation

Photocatalytic water splitting, which directly converts solar energy into hydrogen, is one of the most desirable solar-energy-conversion approaches. The ultimate target of photocatalysis is to explore efficient and stable photocatalysts for solar water splitting. Tantalum (oxy)nitride-based materials are a class of the most promising photocatalysts for solar water splitting because of their narrow bandgaps and sufficient band energy potentials for water splitting. Tantalum (oxy)nitride-based photocatalysts have experienced intensive exploration, and encouraging progress has been achieved over the past years. However, the solar-to-hydrogen (STH) conversion efficiency is still very far from its theoretical value. The question of how to better design these materials in order to further improve their water-splitting capability is of interest and importance. This review summarizes the development of tantalum (oxy)nitride-based photocatalysts for solar water spitting. Special interest is paid to important strategies for improving photocatalytic water-splitting efficiency. This paper also proposes future trends to explore in the research area of tantalum-based narrow bandgap photocatalysts for solar water splitting.     

Visible-light responsive organic nano-heterostructured photocatalysts for environmental remediation and H_2 generation

The ease of molecular design and functionalization make organic semiconductors(OSCs) unit the electronic, chemical and mechanical benefits with a material structure. The easily tunable optoelectronic properties of OSCs also make it promising building blocks and thereby provide more possibilities in photocatalytic applications. So far, organic nanostructures have gained great impetus and found wide applications in photocatalytic organic synthesis, remediation of water and air, as well as water splitting into hydrogen. But they still suffer from low charge separation and sunlight absorption efficiencies.Accordingly, many strategies have been explored to address these issues, and one of the most effective solutions is to develop nano-heterostructures. To give an impulse for the developments of this field, this review attempts to make a systematic introduction on the recent progress over the rational design and fabrication of organic nano-heterostructured photocatalysts, including the types of organic semiconductor/semiconductor(OSC/SC), organic semiconductor/metal(OSC/M), organic semiconductor/carbon(OSC/C), and OSC-based multinary nano-heterostructures. The emphasis is placed on the structure/property relationships, and their photocatalytic purposes in environmental and energy fields.At last, future challenges and perspectives for the ongoing development of OSC materials and their use in high-quality optoelectronic devices are also covered.     

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。     

Two-dimensional photocatalyst design: A critical review of recent experimental and computational advances

In recent years, two-dimensional (2D) semiconductor photocatalysts have been widely applied in water splitting, CO₂ reduction, N₂ fixation, as well as many other important photoreactions. Photocatalysts in the form of 2D nanosheet possess many inherent advantages over traditional 3D nanopowder photocatalysts, including improved light absorption characteristics, shorter electron and hole migration paths to the photocatalysts’ surface (thus minimizing undesirable electron-hole pair recombination), and abundant surface defects which allow band gap modulation and facilitate charge transfer from the semiconductor to adsorbates. When synergistically exploited and optimized, these advantages can impart 2D photocatalysts with remarkable activities relative to their 3D counterparts. Accordingly, a wide range of experimental approaches is now being explored for the synthesis of 2D photocatalysts, with computational methods increasingly being used for identification of promising new 2D photocatalytic materials. Herein, we critically review recent literatures related to 2D photocatalyst development and design. Particular emphasis is placed on 2D photocatalyst synthesis and the importance of computational studies for the fundamental understanding of 2D photocatalyst electronic structure, band gap structure, charge carrier mobility and reaction pathways. We also explore the practical challenges of using 2D photocatalysts, such as their difficulty to synthesize in large quantity and also their characterization. The overarching aim of this review is to provide a snapshot of recent work targeting high-performance 2D photocatalysts for efficient solar energy conversion, thus laying a firm base for future advancements in this rapidly expanding area of photocatalysis research.     

Smart Utilization of Carbon Dots in Semiconductor Photocatalysis

Efficient capture of solar energy will be critical to meeting the energy needs of the future. Semiconductor photocatalysis is expected to make an important contribution in this regard, delivering both energy carriers (especially H₂) and valuable chemical feedstocks under direct sunlight. Over the past few years, carbon dots (CDs) have emerged as a promising new class of metal‐free photocatalyst, displaying semiconductor‐like photoelectric properties and showing excellent performance in a wide variety of photoelectrochemical and photocatalytic applications owing to their ease of synthesis, unique structure, adjustable composition, ease of surface functionalization, outstanding electron‐transfer efficiency and tunable light‐harvesting range (from deep UV to the near‐infrared). Here, recent advances in the rational design of CDs‐based photocatalysts are highlighted and their applications in photocatalytic environmental remediation, water splitting into hydrogen, CO₂ reduction, and organic synthesis are discussed.     

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 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 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.     

Direct Z-scheme photocatalysts: Principles,synthesis, and applications

The properly designed semiconductor photocatalysts are promising materials for solving the current serious energy and environmental issues because of their ability of using sunlight to stimulate various photocatalytic reactions. Especially, the constructed direct Z-scheme photocatalysts, mimicking the natural photosynthesis system, possess many merits, including increased light harvesting, spatially separated reductive and oxidative active sites, and well-preserved strong redox ability, which benefit the photocatalytic performance. This review concisely compiles the recent progress in the fabrication, modification, and major applications of the direct Z-scheme photocatalysts; the latter include water splitting, carbon dioxide reduction, degradation of pollutants, and biohazard disinfection. It finishes with a brief presentation of future challenges and prospects in the development of direct Z-scheme photocatalytic systems.     

Recent Progress in Interface Engineering of Nanostructures for Photoelectrochemical Energy Harvesting Applications

With rapid and continuous consumption of nonrenewable energy, solar energy can be utilized to meet the energy requirement and mitigate environmental issues in the future. To attain a sustainable society with an energy mix predominately dependent on solar energy, photoelectrochemical (PEC) device, in which semiconductor nanostructure-based photocatalysts play important roles, is considered to be one of the most promising candidates to realize the sufficient utilization of solar energy in a low-cost, green, and environmentally friendly manner. Interface engineering of semiconductor nanostructures has been qualified in the efficient improvement of PEC performances including three basic steps, i.e., light absorption, charge transfer/separation, and surface catalytic reaction. In this review, recently developed interface engineering of semiconductor nanostructures for direct and high-efficiency conversion of sunlight into available forms (e.g., chemical fuels and electric power) are summarized in terms of their atomic constitution and morphology, electronic structure and promising potential for PEC applications. Extensive efforts toward the development of high-performance PEC applications (e.g., PEC water splitting, PEC photodetection, PEC catalysis, PEC degradation and PEC biosensors) are also presented and appraised. Last but not least, a brief summary and personal insights on the challenges and future directions in the community of next-generation PEC devices are also provided.     

Phosphorene-Based Heterostructured Photocatalysts

Semiconductor photocatalysis is a potential pathway to solve the problems of global energy shortage and environmental pollution. Black phosphorus (BP) has been widely used in the field of photocatalysis owing to its features of high hole mobility, adjustable bandgap, and wide optical absorption range. Nevertheless, pristine BP still exhibits unsatisfactory photocatalytic activity due to the low separation efficiency of photoinduced charge carriers. In recent years, the construction of heterostructured photocatalysts based on BP has become a research hotspot in photocatalysis with the remarkable improvement of photoexcited charge-separation efficiency. Herein, progress on the design, synthesis, properties, and applications of BP and its corresponding heterostructured photocatalysts is summarized. Furthermore, the photocatalytic applications of BP-based heterostructured photocatalysts in water splitting, pollutant degradation, carbon dioxide reduction, nitrogen fixation, bacterial disinfection, and organic synthesis are reviewed. Opportunities and challenges for the exploration of advanced BP-based heterostructured photocatalysts are presented. This review will promote the development and applications of BP-based heterostructured photocatalysts in energy conversion and environmental remediation.     

光解水制氢半导体光催化材料的研究进展

自从Fujishima-Honda效应发现以来,科学研究者一直努力试图利用半导体光催化剂光分解水来获得既可储存而又清洁的化学能--氢能.近一二十年来,光催化材料的研究经历了从简单氧化物、复合氧化物、层状化合物到能响应可见光的光催化材料.本文重点描述了这些光催化材料的结构和光催化特性,阐述了该课题的意义和今后的研究方向.     

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.     

State-of-the-art advancements of crystal facet-exposed photocatalysts beyond TiO2: Design and dependent performance for solar energy conversion and environment applications

Tailoring semiconductor crystals with optimized reactive facets is considered one of effective strategies to improve photocatalytic activity and selectivity for energy conversion and environmental remediation. The arrangement of surface atom structure through crystal facet engineering could tune surface free energy, electronic band structure, charge transfer and separation, the reactant adsorption and product desorption, and surface redox sites. This progress report aims to concisely highlight recent state-of-the-art progress of crystal facet-dependent performance of promising photocatalysts beyond TiO₂. It includes (1) design of crystal-facet exposed photocatalysts with various routes through altering the relative order of the surface energy; (2) crystal facet-based surface junctions to promote the charge transfer and separation; (3) in situ techniques to detection of charge accumulation on crystal-faceted surfaces; (4) exposed face-determined photocatalytic application in water splitting, photoreduction of CO₂ into renewable fuels, degradation of organic contaminants from the point of the reactant adsorption and activation. The challenges and prospects for future development are also presented.     

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.     

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.     

Modulation of Bi2MoO6‐Based Materials for Photocatalytic Water Splitting and Environmental Application: a Critical Review

Highly active photocatalysts driving chemical reactions are of paramount importance toward renewable energy substitutes and environmental protection. As a fascinating Aurivillius phase material, Bi₂MoO₆ has been the hotspot in photocatalytic applications due to its visible light absorption, nontoxicity, low cost, and high chemical durability. However, pure Bi₂MoO₆ suffers from low efficiency in separating photogenerated carriers, small surface area, and poor quantum yield, resulting in low photocatalytic activity. Various strategies, such as morphology control, doping/defect‐introduction, metal deposition, semiconductor combination, and surface modification with conjugative π structures, have been systematically explored to improve the photocatalytic activity of Bi₂MoO₆. To accelerate further developments of Bi₂MoO₆ in the field of photocatalysis, this comprehensive Review endeavors to summarize recent research progress for the construction of highly efficient Bi₂MoO₆‐based photocatalysts. Furthermore, benefiting from the enhanced photocatalytic activity of Bi₂MoO₆‐based materials, various photocatalytic applications including water splitting, pollutant removal, and disinfection of bacteria, were introduced and critically reviewed. Finally, the current challenges and prospects of Bi₂MoO₆ are pointed out. This comprehensive Review is expected to consolidate the existing fundamental theories of photocatalysis and pave a novel avenue to rationally design highly efficient Bi₂MoO₆‐based photocatalysts for environmental pollution control and green energy development.