The production of anatase-rich photoactive coatings by plasma electrolytic oxidation

A promising approach to producing photoactive surfaces is to apply the plasma electrolytic oxidation (PEO) process to commercially pure titanium substrates, creating a highly porous surface with a high anatase content (> 90% by volume). In this paper, work is outlined in which such PEO coatings have been produced under a variety of conditions, and their photo-catalytic efficiency assessed. Coating phase constitutions and surface microstructures have been investigated. Both treatment time and electrolyte concentration were varied, in an attempt to optimise the process for this application. Microstructures did, as expected, exhibit high anatase contents and porosity levels, but there were changes as the coatings became thicker and also a dependence on electrolyte composition. As an example of a photo-catalysed reaction, a study was made of the degradation of methylene blue dye under UV irradiation, in the presence of both untreated titanium substrates and PEO-treated surfaces. There are indications that both high anatase content and thicker coatings have beneficial effects on the catalytic efficiency, although a tendency was observed for the anatase content to fall as coating thickness was increased. There were also variations in the phosphorus contents of these coatings, although this did not appear to affect the photo-catalytic efficiency.