Synthesis and characterization of spray pyrolyzed nanocrystalline CeO 2–SiO 2 thin films as passive counter electrodes

Abstract

Nanocrystalline CeO 2–SiO 2 thin films were spray pyrolyzed onto fluorine-doped tin oxide (FTO) coated glass substrates using a blend of equimolar concentrations of cerium nitrate hexahydrate and trimethoxymethylsilane in methanol with appropriate volumetric proportions. CeO 2–SiO 2 films were polycrystalline with cubic fluorite crystal structure and transforms to amorphous with increasing SiO 2 content. The room temperature electrical resistivity of the film varied from 1.05×10 10 to 1.13×10 6 Ω cm with the increase in SiO 2 concentration (0–6 vol% SiO 2). In all cases the resistivity follows an Arrhenius behavior with negative temperature dependence in the range of 298–553 K with a thermal activation energy of 0.84±0.04 eV. Films were transparent ( T∼80%), showing a decrease of the band gap energy ( E g) from 3.45 eV for pristine CeO 2 to 3.02–3.12 eV for CeO 2–SiO 2 films. The ion storage capacity (ISC) and electrochemical stability of the films was affected by different morphological features of the film obtained with different CeO 2–SiO 2 compositions. A CeO 2–SiO 2 film prepared with 3 vol% Si having a thickness of 580 nm showed the highest porosity and the high ISC of 28 mC cm −2 with an electrochemical stability of 3500 cycles in 0.5 M LiClO 4+PC electrolyte. The optically passive behavior of such CeO 2–SiO 2 film is confirmed by its negligible transmission modulation upon Li + ion insertion/extraction, irrespective of the extent of Li + ion intercalation.