EXAFS study on disorder in microstructure of amorphous mesoporous titania with large surface areas

Abstract

The mesoporous titania prepared by templating with hexadecylamine has an amorphous nature with a narrow pore size distribution centered at 2.8 nm and a Brunauer–Emmett–Teller (BET) surface area of 1230 m2 g−1. The extremely large surface area implies that most of the atoms are exposed at the surface. XAFS spectroscopy was applied to investigate the microstructure of mesoporous titania. The EXAFS function, k3χ(k), was considerably different from that of anatase. The amplitude decreased significantly in the large k region, suggesting large static and thermal disorders of the Ti-Ti bond. The third peak in the radial distribution function, which appeared at 3.4 Å in anatase and was attributed to the Ti atoms at the next nearest neighbouring site, was hardly observed in the Fourier transform of mesoporous titania. A comparison of the spectra at different temperatures revealed that the temperature dependence of the Debye–Waller factor for the Ti-Ti bond was significant. σ2 was plotted against temperature and the effective surface Debye temperature of mesoporous titania was calculated to be 365 K. This is significantly lower than for the bulk titania crystals, 520 and 530 K for anatase and rutile, respectively.