Journal of Catalysis 253 (2008) 105–110 www.elsevier.com/locate/jcat Synthesizing mixed-phase TiO 2 nanocomposites using a hydrothermal method for photo-oxidation and photoreduction applications Gonghu Li a , Shannon Ciston b , Zoran V. Saponjic c , Le Chen a , Nada M. Dimitrijevic c , Tijana Rajh c , Kimberly A. Gray a,b,∗ a Institute for Catalysis in Energy Processes and Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA b Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA c Chemical Sciences and Engineering Division and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA Received 6 July 2007; revised 12 October 2007; accepted 20 October 2007 Available online 26 November 2007 Abstract Mixed-phase titanium dioxide (TiO 2 ) materials, such as Degussa P25, show high photocatalytic activity due largely to the synergistic effect between anatase and rutile phases, in which spatial charge separation hinders charge recombination. Our previous studies indicate that a particular nanostructured assembly of anatase and rutile crystallites is necessary for the synergy. In this paper, we apply this structure-function understanding to the synthesis of highly active TiO 2 nanocomposite photocatalysts. Using simple synthetic procedures, we demonstrate an ability to design a highly active nanocomposite that shows enhanced photoactivity in both oxidative and reductive chemistry. Studies by electron paramagnetic reso- nance spectroscopy indicate the existence of the critical nanostructured assembly and thus the optimization of charge transfer between anatase and rutile phases in the synthesized nanocomposite. These results illustrate the potential of rationally designing photocatalysts for energy applications.  2007 Elsevier Inc. All rights reserved. Keywords: Titanium dioxide; Photocatalysis; Mixed-phase; Nanocomposite; CO 2 reduction 1. Introduction Among the many wide-band semiconductors, titanium diox- ide (TiO 2 ) is an ideal photocatalyst which is stable, inexpen- sive, safe, and highly reactive [1,2]. Mixed-phase TiO 2 materi- als consisting of anatase and rutile phases are generally better photocatalysts than pure-phase materials [3–6]. Similar to that of coupled semiconductors, reasoning based on an enhance- ment of spatial charge separation is usually invoked to explain the high photoactivity of mixed-phase TiO 2 . Because the po- sition of the conduction band edge of anatase is higher than that of rutile, photogenerated electrons were assumed to flow from anatase to rutile, while the holes transferred from rutile to anatase [7–9]. Ohno and co-workers observed that oxidation and reduction reactions occurred on rutile and anatase particles, respectively, * Corresponding author at: Department of Chemical and Biological Engineer- ing, Northwestern University, Evanston, IL 60208, USA. E-mail address: k-gray@northwestern.edu (K.A. Gray). when mixed-phase TiO 2 was used in photocatalytic oxidation of naphthalene [10]. Electron paramagnetic resonance (EPR) studies by Gray and co-workers indicated that photogenerated electrons actually migrated from rutile to lower energy anatase trapping sites [11–14]. In addition, these authors observed that significantly greater amounts of 2,4,5-trichlorophenol were ad- sorbed on Degussa P25, a commercial mixed-phase TiO 2 mate- rial, than either pure anatase or pure rutile and that charge trans- fer complexes formed only on Degussa P25 [15,16]. They pro- posed that the difference between surface reactions on Degussa P25 and those on pure-phase TiO 2 is related to the morphol- ogy of Degussa P25, wherein anatase crystallites are interwo- ven with rutile crystallites and the anatase–rutile interfaces give rise to unique adsorption sites and photocatalytic “hot spots” [16,17]. Mixed-phase TiO 2 materials have been recently fabricated in our lab by chemical and physical methods, including a solvothermal method and reactive DC magnetron sputter- ing, and have demonstrated excellent photocatalytic activity [18–20]. Whereas most of the studies in the literature report the 0021-9517/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2007.10.014