Sol–hydrothermal preparation and photocatalysis of titanium dioxide C. Su a, * , C.-M. Tseng a , L.-F. Chen a , B.-H. You a , B.-C. Hsu b , S.-S. Chen b, * a Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan b Graduate Institute of Environmental Planning and Management, National Taipei University of Technology, Taipei 106, Taiwan Available online 28 September 2005 Abstract Nanosized TiO 2 with controlled crystal phase/structure was prepared by a sol – hydrothermal process using titanium (IV) n -butoxide as a precursor of TiO 2 . After hydrothermal processing at 200 -C for 1 h, the granulous anatase phase with average crystal size of 5 nm was formed. Upon increasing the heat treatment period to more than 5 h, the rod-like rutile phase with particle size of 19 Â 70 nm became predominant. The structure evolution from X-ray diffraction (XRD), particle size/morphology from transmission electron microscopy (TEM), surface area from Brunauer –Emmitt –Teller (BET) analysis, and photocatalytic activity of TiO 2 products show strong correlation with the hydrothermal conditions. D 2005 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Sol – hydrothermal; Nanostructures 1. Introduction Photochemical reactions catalyzed by semiconductors have been extensively investigated. The photogenerated electrons and holes migrate to the semiconductor surfaces where they can induce reduction and oxidation of adsorbed molecules. Titanium dioxide (TiO 2 ) is one of the most popular and promising materials in photocatalytic applica- tion [1]. TiO 2 is commercially available and easy to prepare in the laboratory. The crystal of TiO 2 exists in different forms, such as rutile, anatase, and brookite [2]. Anatase and brookite are thermodynamically metastable and can be transformed irreversibly to most stable and condense rutile phase at high temperatures [3]. A number of methods have been used to prepare TiO 2 nanoparticles, such as chemical precipitation [4], microemulsion-mediated hydrothermal [5], hydrothermal crystallization [6–8], and sol–gel [9]. Sol– gel is one of the most successful techniques for preparing nano-sized metallic oxide materials with high photocatalytic activities. By tailoring the chemical structure of primary precursor and controlling the processing variables, nano- crystalline products with very high level of chemical purity can be achieved. In sol–gel processes, TiO 2 is usually prepared by the reactions of hydrolysis and polycondensa- tion of titanium alkoxides, Ti(OR) n to form oxopolymers which are then transformed into an oxide network. To control the hydrolysis process in order to obtain homoge- neous titanium oxide networks, some of the chelating reagents such as diol, carboxylic acid, or diketonate compounds are added [6]. The condensation is usually accomplished by gelization and calcination. Condensation pulls together the constitute particles of the gel into a compact mass, thus building up the metal oxide crystal. Calcination, on the other hand, is especially important for removing the organic molecules from the final products and completing the crystallization. However, high temperature calcination results in aggregation and/or phase transforma- tion and affects the microstructures as well as the properties of TiO 2 nanoparticles [10]. Hydrothermal synthesis is a ‘‘soft solution chemical processing’’ which provides an easy route to prepare a well- crystalline oxide under the moderate reaction condition, i.e. low temperature and short reaction time [11]. Due to the potential to yield high-purity and homogeneous fine 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.07.123 * Corresponding authors. Tel.: +886 2 2771 2171; fax: +886 2 2731 7174. E-mail addresses: f10913@ntut.edu.tw (C. Su), f10919@ntut.edu.tw (S.-S. Chen). Thin Solid Films 498 (2006) 259 – 265 www.elsevier.com/locate/tsf