Photocatalytic and electrochemical properties of single- and multi-layer sub-stoichiometric titanium oxide coatings prepared by atmospheric plasma spraying

We studied the photocatalytic and electrochemical impedance properties of two different sub-stoichiometric titania powders deposited using plasma spraying. Two different commercial powders with markedly diverse mean size as well as size distribution were chosen. Thermal oxidation of these as-received powders was carried out to restore the O/Ti ratio to be 2.0. By this way, another two kinds of feedstock powders were obtained. Total of four kinds of feedstock powders were used in the experiments. All powders were sprayed using identical plasma spraying parameters. For some spraying runs, carbon steel served as the substrate, and for others, the previously sprayed titania coating from a different kind of feedstock. Combinations of single- and double-layer coatings were studied. Porosity, microstructure, phase composition, chemical composition, band gap based on reflectance measurement, and photocatalytic activity were examined. Electrochemical impedance spectroscopy demonstrated substantial differences between samples sprayed from the fine and coarse powders. Coatings from oxidized powders were slightly more photocatalytic. High spraying distance used for several of them was good for obtaining low band gap, whereas surface roughness and phase composition were not substantially different compared to standard spraying distance coatings.