Assembly of thin mesoporous titania films and their effects on the voltammetry of weakly adsorbing redox systems

Abstract

Titania (anatase) films composed of nanoparticulate building blocks are formed in a layer-by-layer deposition procedure followed by calcination. Field emission gun scanning electron microscopy (FEGSEM) and conducting probe atomic force microscopy (C-AFM) techniques are used to assess the topography and electrical conductivity of the resulting titania films in the dry state. Two types of molecular binder, cyclohexane hexacarboxylic acid and phytic acid, are employed in the deposition process. The surface charge of the resulting titania films is dramatically affected by the presence of phosphate (from phytic acid) after pyrolysis of the molecular binder. Voltammetric responses for the Ru ( NH 3 ) 6 3 + / 2 + redox system obtained at mesoporous titania coated electrodes immersed in aqueous KCl are strongly affected by the adsorption of Ru ( NH 3 ) 6 3 + in the presence of phosphate. In order to further investigate the effects of the mesoporous membrane on voltammetric features, an approximate adsorption and transport model supported by a simple numerical simulation (of explicit finite-difference type) with reversible adsorption/desorption equilibria is developed. Based on the comparison of experimental and numerical data, it is proposed that the electrical conductivity (electron diffusion) within the wet titania film in the potential range where the reduction of Ru ( NH 3 ) 6 3 + occurs is important in addition to ion transport.