© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 12 REVIEW wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de Sachin Rawalekar and Taleb Mokari* Rational Design of Hybrid Nanostructures for Advanced Photocatalysis Dr. S. Rawalekar, Dr. T. Mokari Ilse Katz Institute for Nanoscale Science and Technology Department of Chemistry Ben-Gurion University of the Negev Beer Sheva 84105, Israel E-mail: mokari@bgu.ac.il DOI: 10.1002/aenm.201200511 1. Introduction A catalyst is a material which increases the rate of chemical reaction without itself being consumed. It decreases the free energy of activation which actually controls the rate of chem- ical reaction. Catalysts have generated an increasing interest in many applications including energy processing, selective chemical transformation, production of bioactive chemicals, etc. Furthermore, significant improvement has been made in catalytic performance with development of nanotechnology, which offers the use of nano-sized particles for the catalysis of a variety of reactions. More importantly, the requirement of less amount of expensive catalyst makes “nanocatalyst” economi- cally favourable over the conventional catalyst. The word “nano- catalyst” means a catalyst having a particle size in nanometers, which not only increases the rate of reaction, but can also lead the specific products. The nanocatalysts are favourable over tra- ditional catalysts due to the enhanced surface to volume ratio, consisting of more catalytic active sites. The catalytic activity of the materials can be sensitive to particle size due to the mod- ified electronic structure at smaller sizes. With modern synthetic methods for nano- materials, the size and shape of metal [1–9] and semiconductor [10–15] nanoparticles can be altered in a controllable manner which increases their use as catalysts in different reactions. Catalysts are distinguished into two types based on their phase difference in reaction: homogeneous and heteroge- neous catalysts. In homogenous catalysis, catalysts are generally present in solution (i.e., in similar phase); whereas, in hetero- geneous catalysis, catalysts are supported on solid porous surfaces like Al 2 O 3 , SiO 2 , TiO 2 etc. In multipath chemical reactions, the selective formation of desired product remains imperative. Numerous reports are published on selective catalysis using nano- catalysts, for example Pt, [16–24] Pd, [25–35] Co [27,36–38] and Rh, [39,40] indicating that nanocatalysts can answer the demand for potential catalysts. The selective product formation was demonstrated previously through the systematic variation of the particle size and shape of the noble and transition metal elements. The catalytic per- formance of the catalysts is highly governed by their surface characteristics, the reactant species adsorb and desorb on the surface of the catalyst during the chemical reaction. The devel- opment of in situ surface characterization techniques such as X-ray photoelectron spectroscopy (XPS), [41,42] scanning tun- neling microscopy (STM) [43,44] and sum frequency generation vibrational spectroscopy (SFG), [45,46] supported by density func- tional theory calculation on surface-occurring processes for dif- ferent catalysts, [47–53] has provided new tools to design modified catalysts with improved activity. Apart from size and shape induced catalysis by metal nano- particles, catalytic activity can be tuned by changing nanocatalyst composition. With proper choice of pair of metals, desired com- position of alloy (for example: Ag-Au, [54] Ag-Pd, [55] Ag-Pt, [56] PtX, X:Bi, Pb, Pd, Ru, [57] Fe-Pt, [58] Pt-Cu, [59] Pt-Ni, [60] and others [61–65] ) or core-shell (Au-Sn, [66] Pd-Pt, [67] Ag-Pt, [68] Rh-Pd and Pt-Pd, [42] etc.) bimetallic nanoparticles can be engineered with a specific crystal facet in different morphology. The structure-dependent catalytic activity of bimetallic alloy and core-shell nanoparticle demonstrates that addition of transition metal to noble metal can influence their bond distance, coordination numbers, elec- tronic properties and stability. Bimetallic catalysts composed of Nanocatalysis has been a growing field over the past few decades with signifi- cant developments in understanding the surface properties of nanocatalysts. With recent advances in synthetic methods, size, shape and composition of the nanoparticles can be controlled in a well defined manner which facilitates achieving selective reaction products in multipath reactions. Nanoparticles with specific exposed crystal facets can have different reactivity than other facets for reaction intermediates, which favours selective pathways during the course of reaction. Heterogeneous catalysts have been studied extensively; nano-sized metal particles are absorbed on mesoporus supports, facilitating access to the large surface area of the nanoparticles and hence exposure of more catalytic sites. Photocatalysis is attractive area of catalysis, in which photoinduced charge carriers are used for a variety of catalytic applications. More interestingly, clean and renewable liquid fuels energy sources such as hydrogen and methyl alcohol can be generated using photocatalysts through water splitting and CO 2 reduction, respectively. Herein, we highlight the progress of nanocatalysis through metal, bimetallic nanoparticle, metal- semiconductor hybrid nanostructures and oxide nanoparticles for various reactions. Adv. Energy Mater. 2013, 3, 12–27