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

The role of Degussa P-25 loading (0-100 g/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 degrees 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 (1 0 1) and rutile (1 1 0) 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 0-50 g/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 degrees 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 50 g/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. (c) 2005 Elsevier B.V. All rights reserved.