Plasma electrolytic oxidized coatings on aluminum and titanium: composition, structure, properties


Elena A. Koblova a,b, Oleg L. Shcheka b, Alexander Yu. Ustinov a,b


(a) Institute of Chemistry, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia,
(b) Far Eastern Federal University, Vladivostok, Russia



Abstract:

A wide range of metal oxide coatings, formed by the method of plasma electrolytic oxidation (PEO) on aluminum and titanium, has been studied using X-ray photoelectron spectroscopy. Besides the base metal, these coatings contain various amounts of nickel and/or copper oxides, and other components depending on the composition of the electrolyte and the features of formation. The atomic composition of the surface and subsurface layers, chemical states of the elements and some structural features have been determined. As follows from the obtained results, the basis of the PEO-coatings is aluminum (titanium) oxide and various oxides of nickel and/or copper. The appropriate investigations showed that PEO-coatings possess a big variety of physical and chemical properties, including catalytic activity in the conversion of CO to CO2. Obviously, these properties are related to the structural features of the coatings. In order to clarify this, quantum chemical calculations of model systems have been performed using the density functional theory, hybrid functional B3LYP and split valence basis sets 6-31+G*, 6-311+G*. According to the obtained by XPS results, the structural fragments of the coatings have been studied in cluster approach, energetic and electronic characteristics have been determined. First of all clusters of (Al2O3)n, n = 1 3 were calculated and it was found that the energetically preferable are linear structure with the bond length of Al-O equal to 1.610 and 1.688 Å, and for n = 2 the dimer structure with Td symmetry and the bond length of 1.743 Å. In the continuation, these clusters have been used to study the adsorption of CO in order to ascertain some aspects of such interaction. Also, calculations of other components of PEO-coatings, such as nickel and copper oxides have been performed by means of (NiO)n, n = 1 12 and (CuO)n, n = 1 4, 6, 8, 16 clusters. Energetic and electronic characteristics of these clusters have been determined as well as the optimal geometric features and some aspects of their interaction with CO. Finally, the calculations of the multicomponent metal oxide systems have been carried out. The optimal geometric parameters of the minimum clusters NiAl2O4 and CuAl2O4 and their interaction with CO have been also studied. As a result, it was found that the more preferential adsorption of CO occurs on 3d-metals rather than on aluminum. For modeling the surface of PEO-coatings and studying the adsorption on it, metal oxide clusters doped with hydrogen, such as MAl3O6H, where M = Ni or Cu, have been modeled.