Pores in Nanostructured TiO2 Films. Size Distribution and Pore Permeability

Nanoporous films of cryst. anatase with intended application in dye-sensitized photovoltaic cells were studied by NMR cryoporometry, NMR diffusiometry, electron microscopy, and x-ray diffraction. The nanoparticles from which the films were subsequently sintered were prepd. in 2 ways, one with an acidic and one with a basic aq. process environment and along different temp. regimes. The av. morphol. was similar in all films as indicated by the roughly identical 〈2κV/S〉 values where κ is the mean curvature of the pore surface and S/V denotes the surface-to-vol. ratio. Self-diffusion of H2O in the pores is strongly reduced with respect to that of bulk and is influenced both by micrometer-scale obstructions to mol. displacement and by pore-size effect in pore interconnectivity. The studied samples exhibit different transport regimes as concerning those phenomena. In this initial study performed on a limited set of samples, the authors found no linear correlation between particle and pore sizes. Instead, total porosity is controlled by particle-particle jamming which, together with particle size polydispersity, may also dominate the effects that lead to the obsd. pore size distributions for the different samples. The rich variation of structural effects and transport properties among the few prepd. films call for further studies to find an optimal film structure.