DOI: 10.1002/chem.200601092 One-Step Fabrication and High Photocatalytic Activity of Porous TiO 2 Hollow Aggregates by Using a Low-Temperature Hydrothermal Method Without Templates Zhaoyang Liu,* [a] Darren D. Sun,* [a] Peng Guo, [b] and James O. Leckie [c] Introduction Nanosized TiO 2 is the most popular photocatalyst for the elimination of pollutants. [1] However, application of these nanosized TiO 2 materials is challenging, in particular, when removing contaminants from water. For commercial applica- tions as a photocatalyst, a material must have superior pho- tocatalytic activity, and be easily synthesized and recovered. TiO 2 nanoparticles might re-pollute the water owing to the tremendous difficulties in separation and recovery. In addi- tion, TiO 2 nanoparticles often show lower efficiencies owing to aggregation problems. [2] Immobilizing TiO 2 nanofilms on supporting materials has been frequently adopted to im- prove reclamation procedures. [3] Unfortunately, a significant loss in the surface area by using this technique limits the photocatalytic efficiency. Nanostructured TiO 2 exhibits superior photocatalytic effi- ciency relative to conventional bulk materials as a result of its large surface area and is one of the most intensively re- searched substances in recent years. [4,5] To enhance the pho- tocatalytic ability, various morphologies of nanostructured TiO 2 , including porous particles, fibers, tubes, and spheres, have been prepared by means of chemical or physical meth- ods. [6–11] Micro- and nanostructures with hollow interiors have attracted significant interest owing to their many at- tractive characteristics, such as low density, economical use of materials, and high surface-to-volume ratios. Recently, Li and co-workers described a sol–spraying–calcination method to fabricate a new type of TiO 2 microsphere photocatalyst with a particle size of 30–160 mm. [12] Xie and co-workers re- ported the fabrication of crystallized rutile phase TiO 2 hollow spheres by using potassium titanium oxalate as the precursor and by employing a simple hydrothermal method, although the photocatalytic performance was lower owing to the rutile phase. [13] More recently, Chen and co-workers fab- ricated long TiO 2 hollow fibers with mesoporous walls, Abstract: Porous TiO 2 hollow aggre- gates have been synthesized on a large scale by means of a simple hydrother- mal method without using any tem- plates. The as-prepared products were characterized by means of field emis- sion scanning electron microscopy, XRD, TEM, nitrogen adsorption, UV/Vis diffuse reflectance spectrosco- py, and FTIR spectroscopy. The photo- catalytic activity of the aggregates was demonstrated through the photocata- lytic degradation of Rhodamine B. Structural characterization indicates that the porous TiO 2 aggregates are 500–800 nm in diameter and display mesoporous structure. The average pore sizes and BET surface areas of the aggregates are 12nm and 168m 2 g 1 , respectively. Optical adsorp- tion investigations show that the aggre- gates possess an optical band-gap energy of 3.36eV. The as-prepared products were substantially more effec- tive photocatalysts than the commer- cially available photocatalyst P25. The dye degradation rate of the porous TiO 2 hollow aggregates is more than twice that of P25. The high photoactivi- ties of the aggregates can be attributed to the combined effects of several fac- tors, namely, large surface areas, the existence of mesopores, and the high band-gap energy. In addition, the as- prepared products can be easily recy- cled. Keywords: aggregation · micro- ACHTUNGTRENNUNGporous materials · nanostructures · photocatalysis · titanium dioxide [a] Dr. Z. Liu, Prof. Dr. D. D. Sun School of Civil and Environmental Engineering Nanyang Technological University, Block N1, Nanyang Avenue Singapore 639798 (Singapore) Fax:(+ 65)679-10-676 E-mail: Liu_zhaoyang2003@yahoo.com DDSun@ntu.edu.sg [b] Dr. P. Guo Institute of Chemistry, Chinese Academy of Sciences Zhongguancun, Beijing, 100080 (China) [c] Prof. Dr. J. O. Leckie Department of Civil and Environmental Engineering Stanford University, Stanford, California 94305-4020 (USA) Chem. Eur. J. 2007, 13,1851–1855 # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1851 FULL PAPER