Effect of Bath Composition on Titanium Anodization Using the Constant-Current Approach: A Crystallographic and Morphological Study

Porous TiO2 thin films were prepared via electrochemical anodization of commercial-grade titanium foils in baths containing variable amounts of ethylene glycol. X-Ray diffraction, scanning electron microscopy, and UV/visible spectroscopy were employed to assess the effect of ethylene glycol on the nature of TiO2 layers. Emphasis is given to the modification of pore size and anatase-to-rutile ratio since these characteristics strongly affect the catalytic performance of TiO2. To simplify the scaling up of the process, a single-step anodization process was employed on a commercial grade 2 titanium foil in constant-current mode without the use of fluorides-conditions that are easily replicable on an industrial scale. We point out some interesting relationships among operating parameters, such as bath composition and current densities, and the characteristics of the anodization layers evidence that the pore size and anatase-to-rutile ratio can be strictly controlled. Increasing the amount of ethylene glycol stimulated the formation of a thinner and less porous TiO2 layer, richer in rutile phase, and characterized by reduced-diameter pores. These results demonstrate the effectiveness and, to some extent, the tunability of the morphology and mineralogic composition of titanium anodization in fluoride-free and ethylene-glycol-bearing acidic solutions.