X-ray Photoelectron Spectroscopy of Cadmium Tin Oxide Ceramics in As-Fired and Electrochemically Reduced Forms

X-ray photoelectron spectroscopy (XPS) has been employed to investigate the chemical nature of samples of dicadmium stannate (Cd₂SnO₄) in the as-fired, electrochemically reduced, and reoxidized states. The reduction of Cd₂SnO₄ was found to be associated with a dramatic color change from bright yellow to dark green, a phenomenon commonly known as the electrochromic effect. Both quantitative XPS results and binding energy measurements proved that, upon exposure of the reduced ceramic bodies to air, the Sn²⁺ to Sn⁴⁺ transition readily took place to produce the intermediate compound, Cd₂SnO₃ with divalent tin. Prolonged exposure to the atmosphere did not result in further progress of reoxidation extending to monovalent cadmium. However, complete reoxidation of the reduced samples was possible by annealing in air at 350°C for a short period of time, e.g., 3 h, by which the original features of the as-fired state such as color and electrical conductivity were restored. The results also showed that reoxidized samples at high temperature assume the same XPS characteristics as those of as-fired ceramics.