Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

Nano‐photocatalytic Materials: Possibilities and Challenges

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

This review highlights recent developments and future perspectives in carbon dioxide usage for the sustainable production of energy and chemicals and to reduce global warming. We discuss the heterogeneously catalysed hydrogenation, as well as the photocatalytic and electrocatalytic conversion of CO2 to hydrocarbons or oxygenates. Various sources of hydrogen are also reviewed in terms of their CO2 neutrality. Technologies have been developed for large-scale CO2 hydrogenation to methanol or methane. Their industrial application is, however, limited by the high price of renewable hydrogen and the availability of large-volume sources of pure CO2. With regard to the direct electrocatalytic reduction of CO2 to value-added chemicals, substantial advances in electrodes, electrolyte, and reactor design are still required to permit the development of commercial processes. Therefore, in this review particular attention is paid to (i) the design of metal electrodes to improve their performance and (ii) recent developments of alternative approaches such as the application of ionic liquids as electrolytes and of microorganisms as co-catalysts. The most significant improvements both in catalyst and reactor design are needed for the photocatalytic functionalisation of CO2 to become a viable technology that can help in the usage of CO2 as a feedstock for the production of energy and chemicals. Apart from technological aspects and catalytic performance, we also discuss fundamental strategies for the rational design of materials for effective transformations of CO2 to value-added chemicals with the help of H2, electricity and/or light.

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Status and perspectives of CO2 conversion into fuels and chemicals by catalytic, photocatalytic and electrocatalytic processes

Let your light shine: the photocatalytic reduction of carbon dioxide to the formate anion under visible light irradiation is for the first time realized over a photoactive Ti-containing metal-organic framework, NH(2)-MIL-125(Ti), which is fabricated by a facile substitution of ligands in the UV-responsive MIL-125(Ti) material.

An amine-functionalized titanium metal-organic framework photocatalyst with visible-light-induced activity for CO2 reduction.

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.

Mesoporous materials are of scientific and technological interest due to their potential applications in various areas. Over the past two decades, significant effort has been devoted to the synthesis of mesoporous materials. For instance, mesoporous silica and phosphate metal oxides have been synthesized and applied widely in many industrial processes. However, little progress has been made in the synthesis of mesoporous metal oxides containing more than one type of metal. To date, a limited number of routes including evaporation-induced self-assembly (EISA) and nonaqueous solvent methods have been developed to synthesize multimetallic mesoporous materials such as Pb3Nb4O13, [3] Bi20TiO32, [4] SrTiO3, MgTa2O6, CoxTi1 xO2 x, [5] and Ce1 xZrxO2. [6] In these routes, introducing surfactant molecules or a template is a general method used to construct the mesostructures. Challenges in using the template method to synthesize multimetallic mesoporous materials are uncontrolled phase separation in the multicomponent reactions and poor thermal and chemical stability of the resulting mesoporous structure. Maintaining the complete mesostructure during removal of the template by heating or chemical treatment is a key process for obtaining the expected mesostructures, and increases the uncertainty in a given synthetic route. In addition, to obtain a crystalline mesoporous material, high-temperature heat treatment is usually required for crystallization of the product. However, this process probably induces collapse of mesostructures. Recently, we developed a synthetic route to mesoporous multimetal oxides that uses the inorganic starting reactants directly as pore makers which aid in building the mesoporous structures of multimetal oxides and improve the thermal stability of the resulting mesostructure. However, in these reported synthetic routes, postcrystallization and introducing or removing an exotemplate are usually needed. In recent years, a route that does not require template removal, which was named “reactive hard templating”, was developed to synthesize porous TiN/carbon composite materials. In this route, the template consists of nanostructures of porous graphitic C3N4, which thermally decomposes completely during formation of porous TiN. This route provides a means to overcome the problems associated with synthesizing multimetal mesoporous materials. A simplified soft-chemistry route based on a reactive template is expected to allow synthesis to proceed at room temperature without requiring the introduction or removal of a template. Here we report a novel direct method for preparing mesoporous ZnGa2O4 with a wormhole framework by an ion-exchange reaction at room temperature involving a mesoporous NaGaO2 colloid precursor. The method does not require any additional processes and can be extended to prepare other porous materials, such as CoGa2O4 and NiGa2O4. The X-ray diffraction (XRD) pattern of NaGaO2 powder, which can be indexed as the orthorhombic phase (JCPDS 762151), is presented in Figure 1. Scanning electron microscopy (SEM) revealed that the powder particles are irregular in shape with little agglomeration, and most of the particles are larger than 500 nm in diameter (see Supporting Information). The as-prepared NaGaO2 powder can be dispersed in water to form a suspension. When the NaGaO2 suspension is illuminated with a 532 nm laser, a Tyndall effect is observed, that is, the suspension behaves as a colloid (see Supporting Information). Multimodal measurements of particle size distribution by dynamic light scattering show that the NaGaO2 colloidal particles exhibit two peak distributions: 20 % of the particles have an average size of 70 nm, and 80 % an average size of 335 nm. Most particulate or macroscopic materials in contact with a liquid acquire an electric charge on their surfaces. The zeta potential is an important and useful indicator of this charge that can be used to predict the stability of colloidal suspensions. The zeta potential of NaGaO2 colloidal particles is 21.57 mV (pH 6). This is lower than the critical zeta potential of 30 mV for maintaining colloid stability in an aqueous system, that is, the colloidal particles are slightly [*] S. C. Yan, J. Gao, M. Yang, X. X. Fan, L. J. Wan, Prof. Y. Zhou, Prof. Z. G. Zou Ecomaterials and Renewable Energy Research Center National Laboratory of Solid State Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093 (China) E-mail: zgzou@nju.edu.cn

Jian Yong Feng

Papers

A Room-Temperature Reactive-Template Route to Mesoporous ZnGa2O4 with Improved Photocatalytic Activity in Reduction of CO2