Sol–Gel Synthesis and Photocatalytic Activity of CeO 2 /TiO 2 Nanocomposites Huaming Yang, w Ke Zhang, and Rongrong Shi Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China Aidong Tang School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Uniform CeO 2 /TiO 2 composite nanoparticles with different Ce/ Ti molar ratios have been successfully synthesized via the sol–gel method. The samples were characterized using differential ther- mal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The surface state analysis by means of X-ray photo- electron spectroscopy (XPS) shows that the Ti element mainly exists as a chemical state of Ti 41 , while the Ce element exists as a mixture of Ce 31 and Ce 41 oxidation states. The photo- catalytic degradation of methyl orange (MeO) in CeO 2 /TiO 2 suspension was investigated. The results indicate that the CeO 2 / TiO 2 nanocomposites show higher photocatalytic activity than pure TiO 2 . Photodegradation of MeO can be improved by increasing the Ce/Ti molar ratio in the initial 15 min. I. Introduction I T is well-known that TiO 2 is an excellent catalyst in the pho- todegradation of organic pollutants because it is an effective, photostable, reusable, inexpensive, non-toxic, and easily availa- ble catalyst. 1–3 The low-photocatalytic degradation efficiency on the surface of TiO 2 particles is partly due to the fast recom- bination rate of photogenerated electron–hole pairs. There is always a high level of interest in improving the photocatalytic acitivity of TiO 2 . However, the photocatalytic property of TiO 2 is highly affected by the particle size, because nanostructured materials exhibit unusual physical and chemical properties, sig- nificantly different from conventional bulk materials and single atoms due to their extremely small size or large specific surface area. It has been indicated that nanocrystalline TiO 2 is especially beneficial in promoting its useful properties. An ionic liquid- assisted route and sol–gel method have been developed to pre- pare TiO 2 nanoparticles with a high surface area or mesoporous films for environmental applications. 4–6 Numerous efforts have also been attempted to improve the photocatalytic activity by modifying the surface or bulk prop- erties of TiO 2 , such as doping, codeposition of metals, surface chelation, mixing of two semiconductors, etc. 7,8 Photocatalytic efficiency can be remarkably enhanced by the introduction of rare-earth metal elements. Lin and Yu 9 investigated the photo- catalytic activity of mixed TiO 2 -rare-earth oxide for the oxida- tion of acetone in air. The results revealed that TiO 2 doped with 0.5 wt% La 2 O 3 or 0.5 wt% Y 2 O 3 after calcined at 7001 or 6501C exhibited higher photoactivity than pure TiO 2 . Baiju et al. 10 re- ported the sol–gel synthesis of La 2 O 3 -doped TiO 2 nanoparticles. Doping of lanthanum oxide has led to a considerable increase in specific surface area. The doped samples, after being calcined at 8001C, showed enhanced photocatalytic activity in comparison with the undoped sample. Lanthanide oxides exhibit unique properties to enhance the photocatalytic performance of TiO 2 . Cerium is a particular lan- thanide element and it has two stable oxidation states: Ce 41 and Ce 31 . As a result of an apparent difference in the ionic radius between Ce and Ti ions, CeO 2 and TiO 2 phases can coexist in the mixed oxides. Several researchers have reported the structure and redox properties of the Ce x Ti 1x O 2 catalyst and thought that this kind of mixed oxide showed a potential interest in the field of photodegradation of organic pollutes. 11–13 This work reports the sol–gel preparation, characterization, and photocatalytic activity of CeO 2 /TiO 2 nanocomposites. II. Experimental Procedure (1) Preparation of CeO 2 /TiO 2 Nanocomposites CeO 2 /TiO 2 composite nanoparticles were prepared by the sol–gel route using Ti(OBu) 4 and the rare-earth metal salt Ce(NO 3 ) 3  6H 2 O as titanium and cerium sources, respectively. 10 mL Ti(OBu) 4 was dissolved in 10 mL anhydrous alcohol, and ultrasonically dispersed. Ce(NO 3 ) 3  6H 2 O solution in the required stoichiometry (the molar ratio of Ce/Ti in CeO 2 /TiO 2 was 1/00– 50/100) was added dropwise into Ti(OBu) 4 alcohol solution. The mixture was stirred for 2 h at room temperature, while ammonia water was used to adjust the pH value of 10.0. A yellow sol was then obtained. The hydrogel was prepared by aging the sol for 24 h at room temperature without stirring, followed by filtering, washing several times with deionized water and anhydrous alco- hol, and drying at 1001C for 12 h to produce a precursor. Sub- sequent calcination of the precursor at 6001C for 4 h in air resulted in the formation of nanocrystalline CeO 2 /TiO 2 powder. (2) Characterization Thermal analysis of thermogravimetry-differential thermal anal- ysis (TG-DTA) of the precursor was performed using an STA 449C thermal analyzer (NETZSCH Co., Germany). The tem- perature was scanned from 201 to 7801C at a heating rate of 101C/min. The structure of the sample was examined using a D/ max 2550 VB 1 18FW diffractometer (Rigaku Co., Japan; CuKa radiation, l 5 0.154056 nm). The average crystal size (D) of the nanocrystal was calculated according to Scherrer equation: D 5 0.89l/b cos y, where l is the X-ray wavelength, b is the line with at half maximum height, and y is the diffraction angle. X-ray photoelectron spectroscopy (XPS) analysis of the samples was carried out with an Escalab MK II analyzer (VG Company, U.K.). Spectra were recorded using Al Ka radiation (1486.6 eV). P. Gouma—contributing editor This work was financially supported by Program for New Century Excellent Talents in University (NCET-05-0695) and the Program for New Century 121 Excellent Talents of Hunan Province (05-030119). w Author to whom correspondence should be addressed. e-mail: hmyang@mail.csu. edu.cn Manuscript No. 21966. Received July 1, 2006; approved November 21, 2006. J ournal J. Am. Ceram. Soc., 90 [5] 1370–1374 (2007) DOI: 10.1111/j.1551-2916.2007.01540.x r 2007 The American Ceramic Society 1370