Mott-Schottky analysis of nanoporous semiconductor electrodes in dielectric state deposited on SnO₂(F) conducting substrates

This paper analyzes the dark capacitance of nanostructured electrodes in the dielectric state, with particular emphasis on TiO₂ electrodes deposited over a transparent conducting substrate of SnO₂(F). It is shown that at those potentials where the TiO₂ nanostructure is in the dielectric state, the capacitance is controlled by the contact SnO₂(F)/(electrolyte, TiO₂). The partial or total covering of the substrate by a dielectric medium causes a modification of the Mott-Schottky plot of the bare substrate. We provide a mapping of the various Mott-Schottky curves that will appear depending on the film characteristics. If the dielectric nanoparticles completely block part of the substrate surface, the slope of the Mott-Schottky plot increases (with the same apparent flatband potential) as an effect of area reduction. The covering of a significant fraction of the surface by a thin dielectric layer shifts the apparent flatband negatively. Measurements on several TiO₂ nanostructured electrodes show that the capacitance contribution of the semiconductor network in the dielectric state is very low, indicating that the field lines penetrate little into the TiO₂ network, not much further than the first particle. The different surface covering observed for rutile-anatase and pure anatase colloids is explained by lattice matching rules with the substrate. By comparing different electrodes, the Helmholtz capacitance at the SnO₂(F)/solution interface was calculated and the apparent flatband potential was corrected for the effect of band unpinning.