Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

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.

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Graphene in Photocatalysis: A Review

Hydrogen generation from the direct splitting of water by photocatalysis is regarded as a promising and renewable solution for the energy crisis The key to realize this reaction is to find an efficient and robust photocatalyst that ideally makes use of the energy from sunlight Recently, due to the attractive properties such as appropriate band structure, ultrahigh specific surface area, and more exposed active sites, two-dimensional (2D) photocatalysts have attracted significant attention for photocatalytic water splitting This Review attempts to summarize recent progress in the fabrication and applications of 2D photocatalysts including graphene-based photocatalysts, 2D oxides, 2D chalcogenides, 2D carbon nitride, and some other emerging 2D materials for water splitting The construction strategies and characterization techniques for 2D/2D photocatalysts are summarized Particular attention has been paid to the role of 2D/2D interfaces in these 2D photocatalysts as the interfaces and heterojunctions a

Role of Interfaces in Two-Dimensional Photocatalyst for Water Splitting

The production of solar fuel through photocatalytic water splitting and CO2 reduction using photocatalysts has attracted considerable attention owing to the global energy shortage and growing environmental problems. During the past few years, many studies have demonstrated that graphene can markedly enhance the efficiency of photocatalysts for solar-fuel generation because of its unique 2D conjugated structure and electronic properties. Herein we summarize the recent advances in the application of graphene-based photocatalysts for solar-fuel production, including CO2 reduction to hydrocarbon fuel and water splitting to H2. A brief overview of the fundamental principles for splitting of water and reduction of CO2 is given. The different roles of graphene in these graphene-based photocatalysts for improving photocatalytic performance are discussed. Finally, the perspectives on the challenges and opportunities for future research in this promising area are also presented.

Graphene‐Based Photocatalysts for Solar‐Fuel Generation

We developed a highly efficient photocatalyst for both H2 and O2 generation under visible-light irradiation by attaching Bi2WO6 (BWO) nanocrystals on graphene nanosheets to produce a graphene–Bi2WO6 composite (Gr–BWO-T). The composite was prepared by a sonochemical method where graphene oxide (GO) served as the support on which BWO formed in situ. Bi2WO6 nanoparticles with the size of 30–40 nm were homogeneously dispersed on the surface of graphene sheets, due to their bonding with graphene. When used as a photocatalyst under visible-light irradiation, O2 production rate reached a value up to 20.60 μmol h−1, 4.18 times higher than that of bare BWO, resulting from the strong covalent bonding between graphene and BWO nanoparticles. The chemical bonding facilitated the electron collection and transportation and inhibited the recombination of photo-generated charge carriers, even in this system with a large amount of graphene inside (40 wt%). More interestingly, H2-production by Gr–BWO-T was also observed to be as high as 159.20 μmol h−1. This could be ascribed to the existence of the graphene that led to decrease in conduction band potential and resulted in a more negative reduction potential than H+/H2. This facile sonochemical approach provides a new strategy for engineering ternary compound nanoparticles on graphene sheets, with great potential application in energy conversion.

A high-performance Bi2WO6-graphene photocatalyst for visible light-induced H2 and O2 generation.

Sonochemical fabrication of mesoporous TiO2 inside diatom frustules for photocatalyst.

Graphene (GR) has triggered new research fields in material science, due to its unique monolayer structure, remarkably high conductivity, superior electron mobility, extremely high specific surface area and chemical stability. It is considered to be an ideal matrix and electron mediator of semiconductor nanoparticles for environmental and energy applications. In particular, GR-based nanocomposites have attracted significant attention when used as photocatalysts. This review will focus on oxygen evolution from water using GR-semiconductor photocatalytic systems. Recent achievements of effective strategies in the fabrication of GR-based semiconductor photocatalysts for oxygen evolution from water are summarized. Furthermore, the morphology control and composition design of semiconductors on GR sheets are also reviewed in relation to their photocatalytic oxygen generation properties. This review ends with a summary and some perspectives on the major challenges and opportunities in the future research.

Graphene-based photocatalysts for oxygen evolution from water

A H2O2-mediated hydrothermal method was developed for the fabrication of hydrophilic Ta2O5/graphene composite. The composite shows a superior H2 productivity, up to 30 mmol g−1 h−1 when used as a photocatalyst for water splitting, corresponding to an apparent quantum efficiency of 33.8% at 254 nm. This superior performance is due to the hydrophilic nature of the composite and more importantly due to the ultrafine Ta2O5 nanoparticles (about 4.0 ± 1.5 nm) which are covalently bonded with the conductive graphene. The hydrophilic property of the composite is attributed to the use of H2O2 in the hydrothermal process. The ultrafine size of the Ta2O5 particles which are covalently bonded with the graphene sheets is attributed to the use of sonication in the synthesis process. Furthermore, the hydrophilic Ta2O5/Gr composite is durable, which is beneficial to long term photocatalysis. The strategy reported here provides a new approach to designing photocatalysts with superior performance for H2 production.

http://iopscience.iop.org/article/10.1088/0957-4484/25/21/215401/pdf

Superior H2 production by hydrophilic ultrafine Ta2O5 engineered covalently on graphene

AgBr nanoparticles on boron-doped reduced graphene oxide aerogels (AgBr/B-RGO) are synthesized by a facile hydrothermal method, which shows a superior performance in the photoreduction of toxic hexavalent chromium (CrVI) in aqueous media under visible light irradiation. The composition and structure of the samples have been characterized by using XPS, Raman, XRD, TEM and SEM measurements. As compared with that of AgBr on none-doped reduced graphene oxide aerogels (AgBr/RGO), the improved photocatalytic properties, can be attributed to the introduction of boron atoms in reduced graphene oxide (RGO), bringing in the improvement of electron transfer efficiency, and the depression of the recombination of photo-excited electrons and holes. Further tests in the photoreduction of CrVI reveal that the obtained AgBr/B-RGO presents excellent cycling performance with an interesting increase in the photocatalytic efficiency upon cycling number. This observation can be explained by the fact that the gradual emergence of Ag0 formed from the photo-induced decomposition of AgBr, introduces a Surface Plasmon Resonance (SPR) effect to the system. The approach herein reported could be extended to the design and fabrication of other photocatalysts with high performance that combine the boron-doped graphene and SPR effect.

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Lin Mao

Papers

A high-performance Bi2WO6-graphene photocatalyst for visible light-induced H2 and O2 generation.

Sonochemical fabrication of mesoporous TiO2 inside diatom frustules for photocatalyst.

Graphene-based photocatalysts for oxygen evolution from water

Superior H2 production by hydrophilic ultrafine Ta2O5 engineered covalently on graphene

Fabrication of AgBr/boron-doped reduced graphene oxide aerogels for photocatalytic removal of Cr( vi ) in water