Postpressing dependence of the effective electron diffusion coefficient in electrophoretically prepared nanoporous ZnO and TiO₂films

The porosity of electrophoretically prepared nanoporous ZnO and TiO₂films was systematically decreased by postpressing at different pressures. The nanoporous structure of the films was fixed by sintering after the postpressing procedure. The postpressing-induced change of the internal surface area of the nanoporous films was monitored using the dye-removal technique. The effective electron diffusion coefficient (D_eff) of the unpressed nanoporous films depended on the thickness according to Fick’s second law. When pressed, the diffusion coefficient of the films increases significantly. In nanoporous TiO₂, the increase ofD_efffollows the percolation theory where transport rate depends on the particle-coordination number. In contrast to the TiO₂films, the value ofD_effof pressed nanoporous ZnO films changed with the porosity much stronger than one would expect from the percolation theory with hard spheres. This property has been attributed to the strong increase of necking between ZnO nanoparticles with increasing pressure as indicated by a strong decrease of the internal surface area.