Formation of anatase TiO2 coatings by plasma electrolytic oxidation for photocatalytic applications

This work focuses on the production of TiO2 coatings with a pure anatase phase and relatively large specific surface areas for photocatalytic evaluations. Plasma electrolytic oxidation (PEO) with a pulsed power mode in the range of 200 V-400 V was employed to produce the TiO2 coatings over flexible Ti foils in the environmentally friendly (NaPO3)6-added alkaline electrolytes. When adding (NaPO3)6 in the NaOH electrolytes, crystalline structures of the coatings would transform from mixed rutile and anatase phases into a pure anatase phase while pore sizes and specific surface areas were also enhanced. This is likely due to that the dissociation of (NaPO3)6 in the electrolytes would concurrently retard the transformation into the thermodynamic stable rutile TiO2 and thus, yield only metastable anatase coatings. Nevertheless, more (NaPO3)6 in the electrolytes would also yield amorphous titanium phosphate in the coatings and hence, reduce the relative anatase contents as well as specific surface areas of the TiO2 coatings. With the optimized (NaPO3)6 concentration in the alkaline electrolytes, the highest photocatalytic ability for the TiO2 coatings was achieved. This results from the high anatase contents and large specific surface area of the coatings. This facile controlled formation of anatase TiO2 coatings with relatively large surface specific areas has great potentials in photocatalytic applications.