Fig. 1 shows the influence of CdS particlesconcentration in supporting electrolyte on the surface morphology of thecoatings formed after 2 min of PEO processing. Surface morphology is notsignificantly influenced by the concentration of CdS particles in supportingelectrolyte. Numerous microdischarge channels of varying diameter as well asregions resulting from the rapid cooling of molten material, decorate thesurface of the coatings.
Since the concentration of CdS particles in PEOcoatings was either close to or below the detection limit of EDS system we usedwavelength dispersive XRF measurement to obtain Ti/Cd ratio. XRF measurementsconfirmed that content of CdS in coatings increases with increasing CdSparticle concentration in supporting electrolyte (Table 1).XRD pattern of pure CdS powder and XRD patterns ofcoatings formed in supporting electrolyte with addition of variousconcentrations of CdS particles are shown in Fig. 2. The peaks observed in XRDpatterns of CdS particles at 2? values of 26.5, 43.8, and 51.9, unambiguously matched(111), (220), and (311) crystalline planes of the face centered cubic structureof CdS (PDXL DB Card No.
9008839). Fig. 2 shows that obtained coatings are wellcrystallized with clearly pronounced diffraction peaks corresponding to anatasephase of TiO2 (PDXL DB Card No. 9008213), which is photocatalyticallyactive phase. Elemental Ti originates from the substrate due to penetration ofX-rays through the porous surface layer and reaching the substrate.
The absenceof visible peaks of CdS in XRD patterns could be a consequence of the lowconcentration of uniformly dispersed CdS particles all over the TiO2surface coatings. In order to investigate whether CdS particles are present inTiO2 coatings, we performed Raman measurements (Fig. 3). Raman spectrum of CdS powder(Fig. 3a) is characterized by a strong band at about 296 cm?1 assignedto the first-order longitudinal optical phonon (1LO) and the peak at about 592cm?1 corresponding second-order (2LO) optical phonons 20.
The dominantmodes in the Raman spectra of pure TiO2 coating at about 144 cm?1(Eg(1)), 197 cm?1 (Eg(2)), 395 cm?1 (B1g(1)), 514 cm?1(A1g, B1g(2)), and 637 cm?1 (Eg(3)) can be assigned to the Ramanactive modes of the anatase crystal phase 21. Bands originating from TiO2coating and CdS particles can be identified on Raman spectra of coatings formedin supporting electrolyte with addition of CdS particles, thus confirming thepresence of CdS particles in TiO2 coatings. This also suggests that CdSparticles are inertly incorporated into the PEO coatings 15.