Thermodynamic and microstructural analyses of photocatalytic TiO2 from the anodization of biomedical-grade Ti6Al4V in phosphoric acid or sulfuric acid

Abstract

TiO2 coatings were fabricated by anodization of Ti6Al4V in 1 M H3PO4 or H2SO4, at room temperature at 120 V for 10 min, and followed by annealing at 300° or 500 °C for 8 h. Analyses include mineralogy (GAXRD, Raman), chemistry (XPS), morphology and microstructure (FESEM, FIB, 3D confocal microscopy), thermodynamic, optical (UV–Vis), and photocatalytic performance (MB degradation). The present work highlights factors that govern the nature of the materials and their performance. The influence of the oxidation strength of the acid is pervasive in that it impacts on the crystallinity, microstructural homogeneity, coating thickness, Ti3+ concentration, gas generation during arcing to form pores, and resultant pore size and distribution density. A key observation is that the pores form a subsurface network of variable continuity, which has a significant impact on the surface area and associated density of photocatalytically active sites, access by liquids and gases inside the coating, penetration depth of incident radiation, gas condensation, and residual liquid trapping. These data and the related thermodynamic analyses of the acids, anodization processes, and oxidation processes facilitate the generation of schematic models for the anodization mechanisms and the resultant surface, bulk, and microstructural effects that dominate the photocatalytic performance.