Fig. 1 shows the influence of CdS particles
concentration in supporting electrolyte on the surface morphology of the
coatings formed after 2 min of PEO processing. Surface morphology is not
significantly influenced by the concentration of CdS particles in supporting
electrolyte. Numerous microdischarge channels of varying diameter as well as
regions resulting from the rapid cooling of molten material, decorate the
surface of the coatings. Since the concentration of CdS particles in PEO
coatings was either close to or below the detection limit of EDS system we used
wavelength dispersive XRF measurement to obtain Ti/Cd ratio. XRF measurements
confirmed that content of CdS in coatings increases with increasing CdS
particle concentration in supporting electrolyte (Table 1).

XRD pattern of pure CdS powder and XRD patterns of
coatings formed in supporting electrolyte with addition of various
concentrations of CdS particles are shown in Fig. 2. The peaks observed in XRD
patterns 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 structure
of CdS (PDXL DB Card No. 9008839). Fig. 2 shows that obtained coatings are well
crystallized with clearly pronounced diffraction peaks corresponding to anatase
phase of TiO2 (PDXL DB Card No. 9008213), which is photocatalytically
active phase. Elemental Ti originates from the substrate due to penetration of
X-rays through the porous surface layer and reaching the substrate. The absence
of visible peaks of CdS in XRD patterns could be a consequence of the low
concentration of uniformly dispersed CdS particles all over the TiO2
surface coatings. In order to investigate whether CdS particles are present in
TiO2 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 assigned
to the first-order longitudinal optical phonon (1LO) and the peak at about 592
cm?1 corresponding second-order (2LO) optical phonons 20. The dominant
modes 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 Raman
active modes of the anatase crystal phase 21. Bands originating from TiO2
coating and CdS particles can be identified on Raman spectra of coatings formed
in supporting electrolyte with addition of CdS particles, thus confirming the
presence of CdS particles in TiO2 coatings. This also suggests that CdS
particles are inertly incorporated into the PEO coatings 15.

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