Effect of phosphorous incorporation on crystallinity, morphology, and photocatalytic activity of anodic oxide layer on titanium

This study investigated the effect of P incorporated into an anodic Ti oxide layer in terms of crystallinity, morphology, and photocatalytic activity. For this purpose, Ti substrates were anodized in H3PO4 aqueous solutions of various concentrations, and some of the substrates were subsequently annealed at 723 K in air. The electrolyte with higher concentrations increased the amount of P in the Ti oxide layer, thereby inhibiting the thermal crystallization induced by the post-annealing treatment. Areas of exfoliation of the oxide layer could be observed on the surface. The size of these areas decreased with increasing applied voltage, whereas the use of higher concentrations of electrolytes reduced the size of the exfoliated areas. In the absence of annealing, the photocatalytic activity increased with increasing applied voltage; however, in the case of an applied voltage over 250 V, the use of high-concentration electrolytes reduced the photocatalytic activity. When anodizing in the H3PO4 electrolytes, the enhancement of the photocatalytic activity by annealing at 723 K was minimal, unlike cases using H2SO4 electrolytes.