Phantom Electrons in Mesoporous Nanocrystalline SnO2 Thin Films with Cation-Dependent Reduction Onsets

Mesoporous thin films comprised of ∼15 nm diameter SnO2 nanocrystallites were synthesized and characterized in acetonitrile electrolytes by electrochemical and spectroscopic techniques. Spectroelectrochemical reduction of the thin films resulted in broad, non-superimposable UV/vis absorption changes. Simultaneous analysis of potential-dependent spectra, by a process termed “potential associated spectra”, resulted in the identification of three unique absorption spectra for reduced SnO2, while only one spectrum was identified for TiO2. Reduction of SnO2 resulted in the appearance of (1) a broad absorption that spans across the visible and near-IR regions, (2) a blue-shifted fundamental absorption, and (3) an absorption band in the blue region. The absorption onsets were dependent on the electrolyte cation, present as the perchlorate salt of Li+, Na+, Mg2+, Ca2+, and TBA+, where TBA+ is tetrabutylammonium. Correlations between the charge within the thin film and the absorbance intensity revealed that significant charge was transferred to SnO2 films before significant visible color changes were observed. This suggested the presence of electrons within the SnO2 thin films that did not absorb visible light and were termed “phantom electrons”.