Effect of humidity on the ac conductivity of nanoporous TiO2

Impedance spectroscopy measurements are performed on a sintered network of TiO2 nanoparticles at different humidity. A transition from nearly normal to dispersive transport is observed at a certain frequency depending on the humidity. The low-frequency conductivity depends on the humidity by power law with an exponent of 1.73, while the low-frequency dielectric constant remains constant. The results can be explained by hopping proton conduction over the percolation threshold. We propose that chemisorbed water molecules mainly supply the protons and the hopping sites are provided by physisorbed water molecules.