Pores in nanostructured TiO2 films. Size distribution and pore permeability

摘要

Nanoporous films of crystalline anatase with intended application in dye-sensitized photovoltaic cells were investigated by NMR cryoporometry, NMR diffusiometry, electron microscopy, and X-ray diffraction. The nanoparticles from which the films were subsequently sintered were prepared in two ways, one with an acidic and one with a basic aqueous process environment and along different temperature regimes. The average morphology was similar in all films as indicated by the roughly identical < 2 kappa V/S > values where kappa is the mean curvature of the pore surface and S/V denotes the surface-to-volume ratio. Self-diffusion of water in the pores is strongly reduced with respect to that of bulk and is influenced both by micrometer-scale obstructions to molecular displacement and by pore-size effect in pore interconnectivity. The investigated samples exhibit different transport regimes as concerning those phenomena. In this initial study performed on a limited set of samples, we 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 observed pore size distributions for the different samples. The rich variation of structural effects and transport properties among the few prepared films call for further studies in order to find an optimal film structure.