71 Mater. Res. Soc. Symp. Proc. Vol. 1446 © 2012 Materials Research Society DOI: 10.1557/opl.2012. Role of Pt Nanoparticles in Photoreactions on TiO 2 Photoelectrodes Woo-Jin An 1 , Wei-Ning Wang 1 , Balavinayagam Ramalingam 2 , Somik Mukherjee 2 , Dariusz M. Niedzwiedzki 3 , Shubhra Gangopadhyay 2 , and Pratim Biswas 1 1 Aerosol and Air Quality Research Laboratory, Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, One Brookings Drive, Campus Box 1180, St. Louis, MO 63130, U.S.A. 2 Center for Micro/Nano Systems & Nanotechnology, University of Missouri, Columbia, MO 65211, U.S.A. 3 Photosynthetic Antenna Research Center (PARC), Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, U.S.A. ABSTRACT Highly efficient Pt-TiO 2 composite photoelectrodes were synthesized by combining two novel deposition methods: ACVD and a room temperature RF (radio frequency) magnetron sputtering method. A room temperature RF magnetron sputtering method allowed uniform deposition of Pt nanoparticles (NPs) onto the as-synthesized nanostructured columnar TiO 2 films by ACVD. Pt NP sizes from 0.5 to 3 nm demonstrating a high particle density (>10 12 cm í2 ) could be achieved by varying deposition time with constant pressure and power intensity. As- synthesized Pt-TiO 2 films were used as photoanodes for water photolysis. Pt nanoparticles deposited onto the TiO 2 film for 20s produced the highest photocurrent (7.92 mA/cm 2 to 9.49 mA/cm 2 ) and maximized the energy conversion efficiency (16.2 % to 21.2 %) under UV illumination. However, as the size of Pt particles increased, more trapping sites for photogenerated electron-hole pairs decreased photoreaction. INTRODUCTION Ever since Fujishima and Honda developed an innovative method to produce hydrogen (H 2 ) by water photolysis [1], much effort has been made to increase the energy conversion efficiency. Titanium dioxide (TiO 2 ) has been widely used as a photocatalytic material for solar energy applications. Along with the wide bandgap of TiO 2 , its short electron-hole pair lifetime [2] is a limiting factor for decomposing water into oxygen (O 2 ) and hydrogen (H 2 ). One dimensional single crystal TiO 2 films provide favorable electron transport pathways, resulting in enhanced photoreaction [3]. In addition to the film morphology, surface modification by noble metal contacts lead to efficient electron-hole separation, improving photoelectrochemical properties of metal oxide films [4]. Although the size of noble metal particles and the distance between noble metal particles play a significant role in determining the efficiency of solar energy applications [5], existing methods can not precisely control these two factors. In this study, the room temperature RF magnetron sputtering method was employed to deposit monodispersed nano-sized platinum (Pt) metal on the columnar TiO 2 films in a controlled manner. 1222