Synthesis of mesoporous TiO2 (anatase) in the absence of templates

Abstract

Since the discovery of mesoporous silica of the MCM41 type [1], increasing attention has been focused on surfactant-templated synthesis of non-siliceous mesoporous materials based on metal oxides [2–6]. Specifically, many efforts have been devoted to the synthesis of mesoporous TiO2 due to its excellent performance in photocatalytic reactions. An initial synthesis of ordered mesoporous TiO2 was achieved by a modified sol-gel route using phosphate surfactants as templates [2]. Subsequently, amine surfactants were used as templates for the synthesis of mesoporous TiO2 with disordered, small mesopores ( 4 nm) [6, 10, 11]. However, these methods usually gave mesoporous TiO2 with amorphous or semicrystallized channel walls, which impaired their photocatalytic activities because amorphous titania generally affords low quantum yield for photocatalytic reactions. Recently, an interesting report showed the first synthesis of mesoporous TiO2 with a crystalline framework using a triblock copolymer as a template and CeCl3 as a stabilizer followed by a hydrothermal process [12]. Here we present a novel synthesis of mesoporous TiO2 with a crystalline framework and uniformly sized, large mesopores (5–10 nm) by a hydrothermal sol-gel process in the absence of templates. Such a facile synthesis of mesoporous, crystalline TiO2 may represent a novel route to mesoporous, crystalline metal oxides that have potentially wide applications. The synthesis of mesoporous TiO2 with a crystalline anatase framework was achieved in a facile manner by a hydrothermal sol-gel process at room temperature. In a typical synthesis, 0.01 mol of titanium butoxide (Ti(OBu)4) was dissolved in 20 mL of ethanol. To this solution, 16 mL of water was added dropwise under vigorous stirring. The mixture was stirred for 24 h at ambient temperature. The resulting gel suspension was transferred to an autoclave for hydrothermal treatment at 120–220 ◦C for 6 h. The product thus obtained was centrifuged, washed with deionized water, dried at 90 ◦C overnight, and finally calcined in air at 400 ◦C for 5 h (the samples were denoted as TiO2-x , where x represents the temperatures of hydrothermal aging). For comparison, two samples untreated hydrothermally were also prepared with one dried at 90 ◦C (denoted as TiO2-90) and the other dried at room temperature under vacuum (denoted as TiO2). Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Dmax-2000 diffractometer using Cu Kα radiation. Conventional transmission electron microscopy (TEM) images were obtained on a Jeol JEM 200CX microscope operated at 200 kV, and highresolution TEM (HRTEM) images were taken with a Hitachi H-9000HAR microscope operated at 300 kV. The nitrogen adsorption-desorption isotherm was measured at 77 K on a Micromeritics ASAP 2010 system. The sorption data were analyzed using the BarrettJoyner-Halenda (BJH) method. Fig. 1 presents wide-angle X-ray diffraction patterns of the obtained mesoporous TiO2 samples before and after calcination at 400 ◦C. Distinct anatase peaks were observed in the XRD pattern for the sample TiO2-90 even before calcination, which was not subjected to hydrothermal treatment but dried at 90 ◦C. After calcination, the anatase peaks became slightly sharper and the crystallite size increased from 5.4 to 6.4 nm according to the line width analysis of the anatase (101) diffraction peak. In addition a brookite peak around 31◦ (2θ ) became evident after calcination. For both the asdried and the calcined samples hydrothermally treated at 120–220 ◦C, the anatase diffraction peaks were gradually sharpened with increasing hydrothermal treatment temperature, corresponding to a gradual increase in the nanocrystal size. Calcination always resulted in a slightly larger crystallite size. Notably, the brookite peak became weaker with increasing hydrothermal treatment temperature and disappeared for the sample hydrothermally treated at 220 ◦C, suggesting the