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.     

Progressive Design of Plasmonic Metal–Semiconductor Ensemble toward Regulated Charge Flow and Improved Vis–NIR‐Driven Solar‐to‐Chemical Conversion

Surface plasmon resonance (SPR)‐mediated photocatalysis without the bandgap limitations of traditional semiconductor has aroused significant attention in solar‐to‐chemical energy conversion. However, the photocatalytic efficiency barely initiated by the SPR effects is still challenged by the low concentration and ineffective extraction of energetic hot electrons, slow charge migration rates, random charge diffusion directions, and the lack of highly active sites for redox reactions. Here, the tunable, progressive harvesting of visible‐to‐near infrared light (vis–NIR, λ > 570 nm) by designing plasmonic Au nanorods and metal (Au, Ag, or Pt) nanoparticle codecorated 1D CdS nanowire (1D CdS NW) ensemble is reported. The intimate integration of these metal nanostructures with 1D CdS NWs promotes the extraction and manipulated directional separation and migration of hot charge carriers in a more effective manner. Such cooperative synergy with tunable control of interfacial interaction, morphology optimization, and cocatalyst strategy results in the distinctly boosted performance for vis–NIR‐driven plasmonic photocatalysis. This work highlights the significance of rationally progressive design of plasmonic metal–semiconductor‐based composite system for boosting the regulated directional flow of hot charge carrier and thus the more efficient use of broad‐spectrum solar energy conversion.     

The Development of Cocatalysts for Photoelectrochemical CO2 Reduction

The ever‐increasing anthropogenic consumption of fossil fuels and the resulting large emission of CO₂ have led to a severe energy crisis and climate change. Photocatalytic reduction of CO₂ into fuels using solar energy is considered as a promising way to address these two problems. In particular, photoelectrochemical (PEC) reduction of CO₂ can integrate and optimize the advantages of both photocatalysis and electrocatalysis for improved conversion efficiency and selectivity. In addition to the charge generation and separation, the efficient reduction of CO₂ on the surface of a semiconductor‐based photoelectrode remains a scientifically critical challenge, which can be greatly enhanced by the surface modification of cocatalysts. Herein, the recent developments of cocatalysts in PEC CO₂ reduction over semiconductor‐based photoelectrodes are described, and the basic principles of PEC CO₂ reduction and the function of the cocatalyst in photoelectrocatalysis are discussed. The structure optimization between the photoelectrodes and the cocatalysts is also summarized since the loading of cocatalyst may shield the incident light and hinder charge transfer between them. Furthermore, the challenges and perspectives for PEC reduction of CO₂ are also presented.     

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.     

Ultrathin 2D Photocatalysts: Electronic‐Structure Tailoring,Hybridization, and Applications

As a sustainable technology, semiconductor photocatalysis has attracted considerable interest in the past several decades owing to the potential to relieve or resolve energy and environmental‐pollution issues. By virtue of their unique structural and electronic properties, emerging ultrathin 2D materials with appropriate band structure show enormous potential to achieve efficient photocatalytic performance. Here, the state‐of‐the‐art progress on ultrathin 2D photocatalysts is reviewed and a critical appraisal of the classification, controllable synthesis, and formation mechanism of ultrathin 2D photocatalysts is presented. Then, different strategies to tailor the electronic structure of ultrathin 2D photocatalysts are summarized, including component tuning, thickness tuning, doping, and defect engineering. Hybridization with the introduction of a foreign component and maintaining the ultrathin 2D structure is presented to further boost the photocatalytic performance, such as quantum dots/2D materials, single atoms/2D materials, molecular/2D materials, and 2D–2D stacking materials. More importantly, the advancement of versatile photocatalytic applications of ultrathin 2D photocatalysts in the fields of water oxidation, hydrogen evolution, CO₂ reduction, nitrogen fixation, organic syntheses, and removal pollutants is discussed. Finally, the future opportunities and challenges regarding ultrathin 2D photocatalysts to bring about new opportunities for future research in the field of photocatalysis are also presented.     

Hollow Nanostructures for Photocatalysis: Advantages and Challenges

Photocatalysis for solar‐driven reactions promises a bright future in addressing energy and environmental challenges. The performance of photocatalysis is highly dependent on the design of photocatalysts, which can be rationally tailored to achieve efficient light harvesting, promoted charge separation and transport, and accelerated surface reactions. Due to its unique feature, semiconductors with hollow structure offer many advantages in photocatalyst design including improved light scattering and harvesting, reduced distance for charge migration and directed charge separation, and abundant surface reactive sites of the shells. Herein, the relationship between hollow nanostructures and their photocatalytic performance are discussed. The advantages of hollow nanostructures are summarized as: 1) enhancement in the light harvesting through light scattering and slow photon effects; 2) suppression of charge recombination by reducing charge transfer distance and directing separation of charge carriers; and 3) acceleration of the surface reactions by increasing accessible surface areas for separating the redox reactions spatially. Toward the end of the review, some insights into the key challenges and perspectives of hollow structured photocatalysts are also discussed, with a good hope to shed light on further promoting the rapid progress of this dynamic research field.     

Nanometals for Solar‐to‐Chemical Energy Conversion: From Semiconductor‐Based Photocatalysis to Plasmon‐Mediated Photocatalysis and Photo‐Thermocatalysis

Nanometal materials play very important roles in solar‐to‐chemical energy conversion due to their unique catalytic and optical characteristics. They have found wide applications from semiconductor photocatalysis to rapidly growing surface plasmon‐mediated heterogeneous catalysis. The recent research achievements of nanometals are reviewed here, with regard to applications in semiconductor photocatalysis, plasmonic photocatalysis, and plasmonic photo‐thermocatalysis. As the first important topic discussed here, the latest progress in the design of nanometal cocatalysts and their applications in semiconductor photocatalysis are introduced. Then, plasmonic photocatalysis and plasmonic photo‐thermocatalysis are discussed. A better understanding of electron‐driven and temperature‐driven catalytic behaviors over plasmonic nanometals is helpful to bridge the present gap between the communities of photocatalysis and conventional catalysis controlled by temperature. The objective here is to provide instructive information on how to take the advantages of the unique functions of nanometals in different types of catalytic processes to improve the efficiency of solar‐energy utilization for more practical artificial photosynthesis.     

Black Phosphorus,a Rising Star 2D Nanomaterial in the Post‐Graphene Era: Synthesis,Properties, Modifications,and Photocatalysis Applications

Semiconductor photocatalysis, a sustainable and renewable technology, is deemed to be a new path to resolve environmental pollution and energy shortage. The development of effective photocatalysts, especially the metal‐free photocatalysts, is a critical determinant of this technique. The recently emerged 2D material of black phosphorus with distinctive properties of tunable direct bandgap, ultrahigh charge mobility, fortified optical absorption, large specific surface area, and anisotropic structure has captured enormous attention since the first exfoliation of bulk black phosphorus into mono‐ or few layered phosphorene in 2014. In this article, the state‐of‐the‐art preparation methods are first summarized for bulk black phosphorus, phosphorene, and black phosphorus quantum dot and then the fundamental structure and electronic and optical properties are analyzed to evaluate its feasibility as a metal‐free photocatalyst. Various modifications on black phosphorus are also summarized to enhance its photocatalytic performance. Furthermore, the multifarious applications such as solar to energy conversion, organic removal, disinfection, nitrogen fixation, and photodynamic therapy are discussed and some of the future challenges and opportunities for black phosphorus research are proposed. This review reveals that the rising star of black phosphorus will be a multifunctional material in the postgraphene era.     

Graphene in Photocatalysis: A Review

In recent years, heterogeneous photocatalysis has received much research interest because of its powerful potential applications in tackling many important energy and environmental challenges at a global level in an economically sustainable manner. Due to their unique optical, electrical, and physicochemical properties, various 2D graphene nanosheets‐supported semiconductor composite photocatalysts have been widely constructed and applied in different photocatalytic fields. In this review, fundamental mechanisms of heterogeneous photocatalysis, including thermodynamic and kinetics requirements, are first systematically summarized. Then, the photocatalysis‐related properties of graphene and its derivatives, and design rules and synthesis methods of graphene‐based composites are highlighted. Importantly, different design strategies, including doping and sensitization of semiconductors by graphene, improving electrical conductivity of graphene, increasing eloectrocatalytic active sites on graphene, strengthening interface coupling between semiconductors and graphene, fabricating micro/nano architectures, constructing multi‐junction nanocomposites, enhancing photostability of semiconductors, and utilizing the synergistic effect of various modification strategies, are thoroughly summarized. The important applications including photocatalytic pollutant degradation, H₂ production, and CO₂ reduction are also addressed. Through reviewing the significant advances on this topic, it may provide new opportunities for designing highly efficient 2D graphene‐based photocatalysts for various applications in photocatalysis and other fields, such as solar cells, thermal catalysis, separation, and purification.     

Coupling Solar Energy into Reactions: Materials Design for Surface Plasmon‐Mediated Catalysis

Enabled by surface plasmons, noble metal nanostructures can interact with and harvest incident light. As such, they may serve as unique media to generate heat, supply energetic electrons, and provide strong local electromagnetic fields for chemical reactions through different mechanisms. This solar‐to‐chemical pathway provides a new approach to solar energy utilization, alternative to conventional semiconductor‐based photocatalysis. To provide readers with a clear picture of this newly recognized process, this review presents coupling solar energy into chemical reactions through plasmonic nanostructures. It starts with a brief introduction of surface plasmons in metallic nanostructures, followed by a demonstration of tuning plasmonic features by tailoring their physical parameters. Owing to their tunable plasmonic properties, metallic materials offer a platform to trigger and drive chemical reactions at the nanoscale, as systematically overviewed in this article. The design rules for plasmonic materials for catalytic applications are further outlined based on existing examples. At the end of this article, the challenges and opportunities for further development of plasmonic‐mediated catalysis toward energy and environmental applications are discussed.     

Heavy‐Metal‐Free Colloidal Semiconductor Nanorods: Recent Advances and Future Perspectives

Quasi‐1D colloidal semiconductor nanorods (NRs) are at the forefront of nanoparticle (NP) research owing to their intriguing size‐dependent and shape‐dependent optical and electronic properties. The past decade has witnessed significant advances in both fundamental understanding of the growth mechanisms and applications of these stimulating materials. Herein, the state‐of‐the‐art of colloidal semiconductor NRs is reviewed, with special emphasis on heavy‐metal‐free materials. The main growth mechanisms of heavy‐metal‐free colloidal semiconductor NRs are first elaborated, including anisotropic‐controlled growth, oriented attachment, solution–liquid–solid method, and cation exchange. Then, structural engineering and properties of semiconductor NRs are discussed, with a comprehensive overview of core/shell structures, alloying, and doping, as well as semiconductor–metal hybrid nanostructures, followed by highlighted practical applications in terms of photocatalysis, photodetectors, solar cells, and biomedicine. Finally, challenges and future opportunities in this fascinating research area are proposed.     

Polymeric Photocatalysts Based on Graphitic Carbon Nitride

Semiconductor‐based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g‐C₃N₄) for visible‐light photocatalytic water splitting, g‐C₃N₄‐based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g‐C₃N₄‐based photocatalysts, including the fabrication and nanostructure design of pristine g‐C₃N₄, bandgap engineering through atomic‐level doping and molecular‐level modification, and the preparation of g‐C₃N₄‐based semiconductor composites. Also, the photo­catalytic applications of g‐C₃N₄‐based photocatalysts in the fields of water splitting, CO₂ reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non‐noble‐metal cocatalysts, and Z‐scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g‐C₃N₄‐based photocatalysts are highlighted.     

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.     

Structure,Synthesis, and Applications of TiO2 Nanobelts

TiO₂ semiconductor nanobelts have unique structural and functional properties, which lead to great potential in many fields, including photovoltaics, photocatalysis, energy storage, gas sensors, biosensors, and even biomaterials. A review of synthetic methods, properties, surface modification, and applications of TiO₂ nanobelts is presented here. The structural features and basic properties of TiO₂ nanobelts are systematically discussed, with the many applications of TiO₂ nanobelts in the fields of photocatalysis, solar cells, gas sensors, biosensors, and lithium‐ion batteries then introduced. Research efforts that aim to overcome the intrinsic drawbacks of TiO₂ nanobelts are also highlighted. These efforts are focused on the rational design and modification of TiO₂ nanobelts by doping with heteroatoms and/or forming surface heterostructures, to improve their desirable properties. Subsequently, the various types of surface heterostructures obtained by coupling TiO₂ nanobelts with metal and metal oxide nanoparticles, chalcogenides, and conducting polymers are described. Further, the charge separation and electron transfer at the interfaces of these heterostructures are also discussed. These properties are related to improved sensitivity and selectivity for specific gases and biomolecules, as well as enhanced UV and visible light photocatalytic properties. The progress in developments of near‐infrared‐active photocatalysts based on TiO₂ nanobelts is also highlighted. Finally, an outline of important directions of future research into the synthesis, modification, and applications of this unique material is given.     

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.     

A Nonmetal Plasmonic Z‐Scheme Photocatalyst with UV‐ to NIR‐Driven Photocatalytic Protons Reduction

Ultrabroad‐spectrum absorption and highly efficient generation of available charge carriers are two essential requirements for promising semiconductor‐based photocatalysts, towards achieving the ultimate goal of solar‐to‐fuel conversion. Here, a fascinating nonmetal plasmonic Z‐scheme photocatalyst with the W₁₈O₄₉/g‐C₃N₄ heterostructure is reported, which can effectively harvest photon energies spanning from the UV to the nearinfrared region and simultaneously possesses improved charge‐carrier dynamics to boost the generation of long‐lived active electrons for the photocatalytic reduction of protons into H₂. By combining with theoretical simulations, a unique synergistic photocatalysis effect between the semiconductive Z‐scheme charge‐carrier separation and metal‐like localized‐surface‐plasmon‐resonance‐induced “hot electrons” injection process is demonstrated within this binary heterostructure.     

Multifunctional Metal–Organic Frameworks for Photocatalysis

Metal–organic frameworks (MOFs) have attracted significant research attention in diverse areas due to their unique physical and chemical characteristics that allow their innovative application in various research fields. Recently, the application of MOFs in heterogeneous photocatalysis for water splitting, CO₂ reduction, and organic transformation have emerged, aiming at providing alternative solutions to address the world‐wide energy and environmental problems by taking advantage of the unique porous structure together with ample physicochemical properties of the metal centers and organic ligands in MOFs. In this review, the latest progress in MOF‐involved solar‐to‐chemical energy conversion reactions are summarized according to their different roles in the photoredox chemical systems, e.g., photocatalysts, co‐catalysts, and hosts. The achieved progress and existing problems are evaluated and proposed, and the opportunities and challenges of MOFs and their related materials for their advanced development in photocatalysis are discussed and anticipated.     

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.     

Multichannel Charge Transfer and Mechanistic Insight in Metal Decorated 2D–2D Bi2WO6–TiO2 Cascade with Enhanced Photocatalytic Performance

Promising semiconductor‐based photocatalysis toward achieving efficient solar‐to‐chemical energy conversion is an ideal strategy in response to the growing worldwide energy crisis, which however is often practically limited by the insufficient photoinduced charge‐carrier separation. Here, a rational cascade engineering of Au nanoparticles (NPs) decorated 2D/2D Bi₂WO₆–TiO₂ (B–T) binanosheets to foster the photocatalytic efficiency through the manipulated flow of multichannel‐enhanced charge‐carrier separation and transfer is reported. Mechanistic characterizations and control experiments, in combination with comparative studies over plasmonic Au/Ag NPs and nonplasmonic Pt NPs decorated 2D/2D B–T composites, together demonstrate the cooperative synergy effect of multiple charge‐carrier transfer channels in such binanosheets‐based ternary composites, including Z‐scheme charge transfer, “electron sink,” and surface plasmon resonance effect, which integratively leads to the boosted photocatalytic performance.