Photocatalytic properties of TiO2/WO3 coatings formed by plasma electrolytic oxidation of titanium in 12-tungstosilicic acid

In this paper, we have investigated photocatalytic properties of TiO2/WO3 coatings formed by plasma electrolytic oxidation (PEO) of titanium in 12-tungstosilicic acid water solution and compared with photocatalytic activity of pure TiO2 coatings. The photocatalytic activity of the coatings is evaluated by measuring the degradation of methyl orange under simulated sunlight conditions. Photocatalytic activity of TiO2/WO3 coatings varied with duration of PEO process and higher photoactivity is obtained for a shorter process time. Photocatalytic activity of TiO2/WO3 coatings is related to morphology and phase and elemental composition of coatings. The oxide coatings morphology is strongly dependent on PEO time. The elemental components of the coatings are Ti, W and O. The oxide coatings are partly crystallized and mainly composed of WO3 and anatase. Decrease in the number of microdischarge channels and agglomeration of particles on the surface leads to a decrease in photocatalytic activity. The reduction of the photocatalytic activity with increased time of PEO process is accompanied with the increase of WO3 concentration on the coatings' surface. Diffuse reflectance spectroscopy has shown that coatings enriched with tungsten oxide exhibit significant red shift with respect to the pure TiO2 coatings. TiO2/WO3 coatings with improved catalytic activity, compared to pure TiO2 coatings, are grouped around energy band gap of 2.6 eV. The results of PL measurements of TiO2/WO3 coatings are in agreement with photocatalytic activities. The increase of PL intensity corresponds to decrease of photocatalytic activity of the coatings, indicating fast recombination of electron-hole pairs