Ashi Ofir

Porosity dependence of electron percolation in nanoporous TiO2 layers

The electron diffusion coefficient at varying porosity has been determined in a series of nanostructured TiO(2) films of different initial thicknesses. The porosity was changed by applying different pressures prior to sintering, thereby modifying the internal morphology of the films though not their chemical and surface conditions. A systematic increase of the effective diffusion coefficient was observed as the porosity was decreased, indicating the improvement of the internal connectivity of the network of nanoparticles. The experimental results have been rationalized using percolation theory. First of all, applying a power law dependence, the diffusion coefficient as a function of porosity from different films collapsed in a single master curve. In addition, application of the models of effective medium approximation (EMA) allows us to compare the experimental results with previous data from Monte Carlo simulation. The different data show a similar dependence in agreement with the EMA predictions, indicating that the geometrical effect of electron transport due to variation of porous morphology in TiO(2) nanoparticulate networks is well described by the percolation concept.

Influence of the porosity on diffusion and lifetime in porous TiO2 layers

Photocurrent transients were investigated on pressed and sintered porous TiO2 layers which were immersed in electrolyte during the measurements. Unpressed porous TiO2 layers were prepared by electrophoretic deposition. The porosity of the layers was changed systematically by pressing. The surfaces of the TiO2 nanoparticles and the intimate contact between them were identically conditioned by sintering in air at 450°C after pressing. With decreasing porosity, the diffusion coefficient increased while the electron lifetime decreased. The results are discussed on the base of the change of the mean coordination number between TiO2 nanoparticles.

Influence of sintering temperature, pressing, and conformal coatings on electron diffusion in electrophoretically deposited porous TiO2

The electrophoretic deposition of nanoporous TiO2 layers allows us to investigate separately the influence of sintering temperature, porosity, and conformal surface coatings on the effective diffusion coefficient (Deff) of excess electrons in porous layers. Photocurrent transients were measured to obtain Deff in nanoporous TiO2 layers immersed in aqueous electrolyte. The applied treatments control parameters such as the contact between interconnected nanoparticles, the coordination of nanoparticles in the porous network, and the surface passivation of TiO2 nanoparticles. The hierarchy of the different factors for transport optimization in porous TiO2 is discussed. Under fixed geometry of the nanoporous network, trapping on surface states can strongly limit electron diffusion in porous TiO2.