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The relationship between the morphology of nanostructured anatase TiO 2 films and electron migration in these electrodes has been investigated. Toward this end, three different TiO 2 powders have been studied, all of which have a unique microstructure. Photocurrent dynamics is used to elucidate electron transport in nanostructured TiO 2 networks. In particular, intensity-modulated photocurrent spectroscopy proves to be a powerful characterization technique. In all cases the transport of electrons through a TiO 2 network can be described with a macroscopic diffusion model. By fitting the data to this model it is possible to determine the absorption coefficient, the diffusion coefficient, and the lifetime of the electrons. Upon illumination, electrons accumulate in the nanostructured film. A fraction of these electrons are stored in deep surface traps. Another fraction of the electrons reside in the conduction band and are free to move. The average concentration of these excess mobile conduction band electrons amounts to one electron per nanoparticle, irrespective of the type of electrode, the film thickness, or the irradiation intensity. Equilibration of the quasi-Fermi level combined with infinite fast extraction of electrons at the substrate explains this saturation effect.
Zanden et al. (2000) studied this question.