Pore Size Distribution Analysis of Mesoporous TiO2 Spheres by ~1H Nuclear Magnetic Resonance (NMR) Cryoporometry

摘要

Mesoporous TiO2 spheres with various pore sizes were prepared by varying the calcination temperature in the range of 300-700 °C. Increasing calcination temperature was found to increase the crystal size, decrease the surface area, and increase the pore size. The morphologies of mesoporous TiO2 spheres consist of well-defined spherical shapes of monodisperse sizes near 0.8 [an. To determine the pore size distributions (PSDs) of these mesoporous TiO2 spheres, ~1H nuclear magnetic resonance (NMR) cryoporometry and Barrett-Joyner-Halenda (BJH) analysis were conducted. NMR cryoporometry is based on the theory of the melting point depression (MPD) of a probe molecule confined within a pore, which is dependent on the pore diameter. MPD was determined by analyzing the variation of the NMR spin-echo intensity with temperature. From the resulting spin-echo intensity versus temperature (I-T) curves, it was found that the maximum MPD of a probe molecule confined within the pores of mesoporous TiO2 decreases with increasing calcination temperature; that is, the pore size increases with increasing calcination temperature. Because mesoporous TiO2 spheres consist of aggregates of nanocrystallite TiO2 and mesopores located at intercrystallites, an increase in the calcination temperature induces an increase in the crystallite size and, thus, in the pore size because the small pores collapse and the large pores increase in size. We also confirmed by BJH analysis that the pore size of mesoporous TiO2 increases with increasing calcination temperature. This trend is in agreement with our 'H NMR cryoporometry results. Overall, these findings indicate that NMR cryoporometry is a very effective method for determining the PSDs of mesoporous TiO2 spheres.