Porous titanium dioxide coatings synthesized by anodic oxidation

Titanium dioxide is one of the most used materials in heterogeneous photocatalysis due to its commercial availability, low cost, chemical stability, low toxicity and high photocatalytic activity. One of the most efficient and economical methods to obtain TiO2 coatings is the plasma electrolytic oxidation. In this anodization process, a direct current is circulated between a cathode and an anode immersed in an electrolyte solution, at a high voltage that induces the formation of spark discharges on the surface of the anode. In the present work, porous TiO2 coatings were obtained by plasma electrolytic oxidation, using 1 M sulfuric acid as the electrolyte, a Pt cathode, titanium Grade 2 as the anode and a maximum current density of 3250 A m(-2). The influence of voltage (90-180 V) and surface finishing (with and without metallographic polishing) on the morphology and crystalline structure was studied. The temporal evolution of voltage and current density were recorded. The morphology of the oxides was observed by scanning electron microscopy, and the crystalline phases were determined by grazing incidence X-ray diffraction. The curves of current density and voltage showed behaviors characteristic of galvanostatic-potenciostatic grown of anodic oxides, with fluctuations as the result of the spark discharge condition. Micrographs of the oxides showed pores with an average diameter between 60 and 160 nm, which increased at higher voltages. The diffractograms showed the presence of the anatase and rutile phases of TiO2, with a decrease in the anatase fraction at the higher voltages. No influence of surface finishing was observed on the morphology and crystallinity of the coatings. The synthesized oxides show favorable characteristics as potential materials for heterogeneous photocatalytic processes.