The effect of F--doping and temperature on the structural and textural evolution of mesoporous TiO2 powders

Mesoporous F--doped TiO2 powders were prepared by hydrolysis of titanium tetraisopropoxide (TTIP) in a mixed NH4F-H2O solution. Effects of F- ion content and calcination temperatures on the phase composition and porosity of mesoporous titania were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and BET surface areas. The results showed the BET surface area (S-BET) of the pure and doped powders dried at 100degreesC ranged from 260 to 310m(2)/g as determined by nitrogen adsorption. With increasing calcination temperatures, the SBET values of the calcined titania powders decreased due to the increase in crystalline size. The pore size distribution was bimodal with fine intra-particle pore and larger inter-particle pore as determined by nitrogen adsorption isotherms. The peak pore diameter of intra-particle pore increases with increasing F- ion content. At 700degreesC, all the titania powders exhibit monomodal pore size distributions due to the complete collapse of the intra-particle pores. The crystallization of anatase was obviously enhanced due to F(-)doping at 400degreesC and 500degreesC. Moreover, with increasing F- ion concent, F- ions not only suppressed the formation of brookite phase at low temperature, but also prevented phase transition of anatase to rutile at high temperature. (C) 2003 Elsevier Inc. All rights reserved.