Synthesis and characterization of spray pyrolyzed nanocrystalline CeO2–SiO2 thin films as passive counter electrodes

Nanocrystalline CeO2–SiO2 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. CeO2–SiO2 films were polycrystalline with cubic fluorite crystal structure and transforms to amorphous with increasing SiO2 content. The room temperature electrical resistivity of the film varied from 1.05×1010 to 1.13×106 Ω cm with the increase in SiO2 concentration (0–6 vol% SiO2). 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 (Eg) from 3.45 eV for pristine CeO2 to 3.02–3.12 eV for CeO2–SiO2 films. The ion storage capacity (ISC) and electrochemical stability of the films was affected by different morphological features of the film obtained with different CeO2–SiO2 compositions. A CeO2–SiO2 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 LiClO4+PC electrolyte. The optically passive behavior of such CeO2–SiO2 film is confirmed by its negligible transmission modulation upon Li+ ion insertion/extraction, irrespective of the extent of Li+ ion intercalation.