Microstructure and Composition of Anodic Tin Oxide Grown in NaOH Electrolyte

The work reports on the structure of anodic tin oxide (ATO) films, synthesized in 0.5 M NaOH electrolyte at anodizing voltage of 4–12 V and temperature of 5 and 25°C. The microstructure and composition of the films were characterized with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman and infrared (IR) spectroscopy, and thermogravimetric analysis (TG). Pore diameter of ATO linearly increases with anodization voltage with a slope coefficient of 3.0 nm/V at either 5 or 25°C, providing average pore diameters in the range of 20–45 nm. Temperature does not affect significantly pores diameter, while determining anodization current and the growth rate. Film thickness is limited to ~150 µm due to dissolution of oxide layer at the oxide/electrolyte interface. ATO dissolution results in the diminishing of current efficiency from >95% for the film thickness of ~60 µm (spent charge 44.2 C/cm2) to ~25% for the film thickness of ~120 µm (spent charge 176.8 C/cm2). The films are amorphous, composed of non-stoichiometric SnOx·nH2O with x = 1.5–1.9. It is suggested, that the porous structure forms due to preferential dissolution of Sn4+ oxide, contacting with electrolyte, while Sn2+ at the pores interstitials reveals much higher stability towards basic environment. Most homogeneous films with minimal number of cracking defects, are formed at 8 V, 5°C, and spent charge of >176.8 C/cm2. The films reveal high conductivity as shown with impedance spectroscopy.