Metal Semiconductor Heterostructures for Photocatalytic Conversion of Light Energy Sumit Kumar Dutta, † Shyamal Kumar Mehetor, † and Narayan Pradhan* Department of Materials Science and Center for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata 700032, India ABSTRACT: For fast separation of the photogenerated charge carriers, metal semiconductor heterostructures have emerged as one of the leading materials in recent years. Among these, metal Au coupled with low bandgap semiconductors remain as ideal materials where both can absorb the solar light in the visible region. It is also established that on excitation, the plasmonic state of gold interacts with excited state of semiconductor and helps for the delocalization of the photogenerated electrons. Focusing these materials where electron transfer preferably occurs from semiconductor to metal Au on excitation, in this Perspective, we report the latest developments in the synthetic chemistry in designing such nano heterostructures and discuss their photocatalytic activities in organic dye degradation/reduction and/or photocatalytic water splitting for generation of hydrogen. Among these, materials such as Au-CZTS, Au-SnS, Au-Bi 2 S 3 , Au-ZnSe, and so forth are emphasized, and their formation chemistry as well as their photocatalytic activities are discussed in this Perspective. P hotocatalysis, where photons are used for catalytically activating chemical reactions on the surface of photo- sensitized catalysts, remains one of the leading hubs of research for harvesting the solar light. 1-12 Typically, photocatalysts generate the charge carriers on excitation and under suitable conditions, these are transferred from the catalysts to the reaction medium, which in turn initiate the chemical reaction. The efficiency of a typical photocatalytic process mostly depends on (1) the nature of photosensitized catalyst, (2) appropriate photon source for excitation, (3) the substrate which can promptly accept the photogenerated charge carriers, and importantly, (4) the spatial distance between the catalyst and substrate. In recent progress, different size/shape tunable plasmonic noble metals 12-20 and semiconducting nanomateri- als 7,21-27 remain the leading inorganic catalytic materials, which can have tunable absorption in solar spectrum and can generate photoelectrons for utilizations in various chemical reactions. In addition, these catalysts have also been used for bacterial detoxifications, organic pollutant removal and generation of hydrogen via catalytic water splitting. 1-5,21,27-34 However, recent developments suggest that the combination of both plasmonic metal and semiconductor can be an even better catalyst for harvesting the solar energy compared with those individual components. 28,33-46 These combined materi- als, widely known as heterostructured materials, can retain the properties of the individual entities or generate new properties when placed together within a close proximity. For ideal combination, this can help in quick transfer of the photo- generated charge carriers from one to other; it can delocalize the photoelectrons over the excited states of both metal and semiconductor, which in turn hinders carrier recombination providing a better opportunity for their utilization in activating the chemical reactions. 28,33,38,40,43,47 Further, these materials can also provide various combinations of facets on their surfaces, which can give more opportunity for the substrate molecules getting adsorbed. 40,48,49 All these advantages make these metal-semiconductor heterostructures more efficient photocatalysts than only the metal or semiconductor catalysts. From the literature reports, it is revealed that these metal- semiconductor heterostructured photocatalysts can be broadly classified in two categories. In the first case, either of the materials is photoactive and, on excitation, the excited charge carriers are transferred to other part of the material, which then induces the catalytic process. With plasmonic gold, the combination of high bandgap semiconductors such as TiO 2 falls in this category, where on photoexcitation the plasmon electrons are transferred from Au to the semiconductor for initiating the catalytic reaction. 12,35,37,48,50-53 These are known as plasmonic photocatalysts and these have been widely studied and reported in literature. A possible electron transfer from the surface plasmon state of Au to the high bandgap semiconductor has been schematically shown in Figure 1a. 54,55 The other possibility is that the metal and semiconductor both are photosensitized and absorb the solar light. Materials where the plasmonic gold is coupled with low bandgap semiconductors, such as Au-CdSe, Au-CdS, Au-PbS, and so forth 40,49,56,57 remain in this category. When analyzed, it is observed that there can have possibility of the electron transfer from either Au to semiconductor or vice versa depending on the band alignment as well as the excitation source. For a particular case, with exclusive excitation of the semiconductor can facilitate the electron transfer from the semiconductor conduction band Received: January 19, 2015 Accepted: February 26, 2015 Published: February 26, 2015 Perspective pubs.acs.org/JPCL © 2015 American Chemical Society 936 DOI: 10.1021/acs.jpclett.5b00113 J. Phys. Chem. Lett. 2015, 6, 936-944