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 TiO 2 electrodes deposited over a transparent conducting substrate of SnO 2(F). It is shown that at those potentials where the TiO 2 nanostructure is in the dielectric state, the capacitance is controlled by the contact SnO 2(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 TiO 2 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.