Synthesis of black cobalt and tin oxide films by the sol-gel process: surface and optical properties

Thin films of cobalt oxide (CO) and tin oxide (TO) were prepared by the dipping sol-gel method upon glass, stainless-steel (SS) and nickeled SS substrates. The aim of this study is to prepare selective photothermal coatings for medium temperature applications by superimposing TO films upon those of CO. Spectral reflectance measurements in both the UV-vis and the infrared ranges were used to characterize the relevant properties (absorptance and emittance). Also, atomic force microscopy and X-ray photoelectron spectroscopy (XPS) measurements were made on the films. The XPS data tells us that several phases of CO coexist (Co3O4, Co2O3) in the films under the preparation conditions used. Besides, large amounts of carbon were detected in several configurations: both as graphite particles and carbon bonded to metallic and oxygen atoms. Regarding the TO films, both SnO and SnO2 phases were detected besides carbon particles as in CO. AFM studies made on samples upon glass substrates enable us to conclude that the dipping method yields surface morphologies with relatively low roughness, i.e., smooth film surfaces. By observing several tandem structures, it was found that placing a TO coating upon a CO film slightly improves the whole coating selectivity, but adding a nickel coating on the SS substrate improves even more such selectivity value.