Synthesis and Characterization of Nano titania Particles Embedded in Mesoporous Silica with Both High Photocatalytic Activity and Adsorption Capability Yuanzhi Li* ,†,‡ and Sun-Jae Kim* ,‡ Department of Chemistry, China Three Gorges UniVersity, Yichang, Hubei, 443002, People’s Republic of China, and Sejong AdVanced Institute of Nano Technologies, #98 Gunja-Dong, Gwangjin-Gu, Sejong UniVersity, Seoul, 143-747, Korea ReceiVed: March 12, 2005; In Final Form: April 22, 2005 TiO 2 -xSiO 2 composites with a high specific surface area (up to 645 m 2 /g), large pore volume, and narrow distribution with average pore sizes ranging from 15 to 20 Å have been synthesized by the sol-gel method. The results of characterization by XRD, BET, TEM, FTIR, and DRUV reveal that these TiO 2 -xSiO 2 composites exhibit a core/shell structure of a nano titania/Ti-O-Si species modified titania embedded in mesoporous silica. As compared to pure anatase, the embedding of nano titania particles into the mesoporous silica matrix results in a substantial blue shift of absorption edge from 3.2 to 3.54 eV and higher UV absorption intensity, which are attributed to the formation of the Ti-O-Si species modified titania in the interface between titania and silica. The as-synthesized TiO 2 -xSiO 2 composites exhibit both much higher absorption capability of organic pollutants and better photocatalytic activity for the photooxidation of benzene than pure titania. The better photocatalytic activity of as-synthesized TiO 2 -xSiO 2 composites than pure titania is attributed to their high surface area, higher UV absorption intensity, and easy diffusion of absorbed pollutants on the absorption sites to photogenerated oxidizing radicals on the photoactive sites. Introduction Nanostructured titania as a cheap, nontoxic, efficient photo- catalyst for the detoxication of air and water pollutants has received much research attention during the past two decades. 1 For its practical application, it is very important to improve the efficiency of titania-based photocatalysts because titania with high photocatalytic activity usually has a relatively lower surface area and low pore volume, which leads to its low adsorption capability of organic pollutants. This limits their practical application because in the dark or under the irradiation of weak light, these titania photocatalysts could not efficiently decrease the concentration of pollutants in air or water for later irradiation. Therefore, a photocatalyst with both high photocatalytic activity and adsorption capability is desirable. To overcome this problem, two strategies have been developed: one is the synthesis of mesoporous titania with high specific surface area 2 and another is the combination of titania materials with absorbent. 3,4 For the former strategy, although many cases of the synthesis of mesoporous titania have been reported, in most of them titania exists in an amorphous or semicrystalline phase that has low photocatalytic activity. There are few reports on the synthesis of stable mesoporous titania with a crystalline wall that has a high photocatalytic activity. 2h-j It has been proven that the later strategy is relatively easy to accomplish. As an absorbent, silica is the best candidate as it is easy to synthesize silica with a large specific surface area and pore volume, and also, silica has no absorption in the range of UV. There have been many reports on titania-silica mixed oxides as photo- catalysts. The methods of synthesizing titania-silica mixed oxides include grafting titania on silica support 5 and fabricating titania-silica composites. 6-13 By choosing silica with a high surface area and large volume, it is easy to obtain a photocatalyst with high adsorption capability by grafting titania on silica support. But its photocatalytic activity is no good because of the longer transport distance from adsorption sites to photoactive sites. It is hoped that a photocatalyst with both high photocata- lytic activity and adsorption capability by fabricating titania- silica composites would be obtained. But the reported titania- silica composites with high photocatalytic activity have a relatively low specific surface area. 10-13 The ideal titania-silica composite photocatalyst design would maximize the absorption behavior and the proximity of the SiO 2 adsorption zones to TiO 2 photocatalytic active ones. 4 Here, we provide an approach to synthesize TiO 2 -SiO 2 composites with nano titania/Ti-O-Si species modified titania core/shell structure embedded in mesoporous silica by the sol-gel method. These TiO 2 -xSiO 2 composites have high specific surface areas (up to 645 m 2 /g), large pore volumes, a narrow distribution of pore sizes ranging from 15 to 20 Å, and exhibit both much higher absorption capability of organic pollutants and better photocatalytic activity for photocatalytic oxidation of benzene than pure titania. In addition, it is well-known that it is easy to synthesize micro- porous material with a pore size <10 Å and mesoporous materials with a pore size between 20 and 100 Å. The expansion of the micropore size from less than 10 to 10-20 Å is an important goal of the current research on microporous materials and remains a significant synthetic challenge today. 14,15 Our method provides an facile approach to synthesize titania-silica composites with a narrow distribution of pore sizes ranging from 15 to 20 Å. * Corresponding authors. E-mail: (S.-J.K.) sjkim1@sejong.ac.kr; (Y.L.) liyuanzhi66@263.net. † China Three Gorges University. ‡ Sejong Advanced Institute of Nano Technologies. 12309 J. Phys. Chem. B 2005, 109, 12309-12315 10.1021/jp0512917 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/02/2005