Fractal Analysis of Porous Polycrystalline Titania by Adsorption

Abstract

Porous titania gel monoliths with pure anatase, rutile, and their coexistent phases are prepared by the sol−gel method and subsequent heat treatment, respectively. Their apparent surface nanoscale topography and average roughness are studied using atomic force microscopy (AFM). The AFM images show that the porous titania gel monoliths are composed of aggregates of spherical primary particles. These aggregates also form interparticle pores. The pores are interconnected, open to outer ambient conditions, and are the main contributor to their specific surface area. The surface fractal dimensions (dSF) evaluated with the modified form of the Frenkel−Hasley−Hill (FHH) equation instead of the average roughness from AFM are used to characterize the surface irregularity of these porous crystal titania. The results show that the dSF of titania depends on its crystal structure (CS), and for the same CS, the dSF is independent of heat treatment. The surface dimensions measured are approximately 2.441, 2.500, and 2.413 for pure anatase, pure rutile, and their coexistent titania, respectively, thereby indicating that the fractal surfaces of the rutile are slightly more irregular than that of the other titania phases.