TiO2 photocatalytic films on stainless steel: The role of Degussa P-25 in modified sol–gel methods

Abstract

The role of Degussa P-25 loading (0–100g/L) in the alkoxide sol was investigated for the synthesis of immobilized TiO2 photocatalytic films on 304 stainless steel using the P-25 powder-modified sol–gel method (PPMSGM). The structural properties of the films (PPMSGFs) obtained after gel drying and calcination at 600°C were examined using different materials characterization techniques including X-day diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of films with good adherence to the stainless steel support were evaluated using 4-chlorobenzoic acid (4-CBA) as a model organic contaminant and UV-A radiation. The P-25 loading did not have a significant effect on the size of the crystallites in the films. However, increasing the P-25 loading in the sol resulted in an increase in (i) the amount of crystalline material retained on the support (i.e., for both anatase (101) and rutile (110) crystal phases); (ii) the number of grains (aggregates of P-25 particles and crystallites formed from the alkoxide sol); (iii) the number of pores in the film (in the range of 0–50g/L); and (iv) the number of microcracks on the surface of PPMSGFs. On the other hand, increasing the P-25 loading in the sol resulted in a decrease in the size of grains on the surface of PPMSGFs. XPS analysis revealed the presence of Cr3+, Mn3+ and Fe3+ on the surface of PPMSGFs as a result of diffusion of these species from the stainless steel support during film calcination at 600°C. The concentration of these foreign species on the film surface decreased with an increase in the P-25 loading in the sol. Increasing P-25 loading in the sol yielded films with higher photocatalytic activity but a concentration of 50g/L P-25 in the sol was found as the maximum for obtaining films with good adherence on the stainless steel support. Increase in the photocatalytic activity of the films with increasing P-25 loading in the sol was mainly attributed to the enhancement of the number of P-25 active sites exposed to the solution due to film morphology and surface characteristics and to the reduction in Cr3+ and Fe3+ concentrations on the surface of the films.