Effect of the electrolyte composition on the anatase fraction of photocatalytic active TiO2 coatings prepared by plasma assisted anodic oxidation

Titanium dioxide is one of the most commonly used semiconductors for photocatalytic applications. In the majority of experimental studies, titanium dioxide is used in a powder form. In the present investigations titanium dioxide layers were formed by the plasma assisted anodic oxidation process SOLECTRO®. In this process a titanium substrate (anode) is dipped into an electrolyte bath containing a titanium precursor. Impressing a voltage greater than 100 V, a titanium dioxide layer is deposited on the substrate in several minutes. The titanium dioxide is formed from the constituents of the electrolyte. For this reason, the composition of the electrolyte influences the deposition process and also the phase fractions and the layer morphology. It is known that the photocatalytic activity of titanium dioxide layers depends strongly on the anatase volume fraction, the layer morphology and therefore the specific surface area. The standard SOLECTRO® process generates titanium dioxide layers consisting of nearly 25 vol.% anatase and 75 vol.% rutile. In the present study, the electrolyte's constituents, especially the chelating agent, were varied to increase the anatase percentage. The results show that the anatase fraction and the specific surface area can be increased by substitution of ethylendiammintetraacetic acid by dieethylenetriaminepentaacetic acid. The anatase fraction amounts to 44 vol.% using the new electrolyte composition.