Essay Example on Shows the influence of CdS particles Concentration

Results and discussion 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 θ 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 The influence of concentration of CdS particles in supporting electrolyte on photocatalytic activity PA of PEO coatings is presented in Fig 4 C0 is the initial concentration of MO and C is the concentration after time t PA varies with the concentration of CdS particles added to supporting electrolyte TiO2 CdS coatings formed in supporting electrolyte with addition of CdS particles up to 0 5 g L show better PA than pure TiO2 coating with the highest PA observed for the coating formed in supporting electrolyte with addition of 0 4 g L of CdS particles On the other hand high concentration of CdS particles in supporting electrolyte significantly reduced PA and even showed significantly lower values than for the pure TiO2 This indicates that CdS particles have considerable influence on the photocatalytic activity of TiO2 CdS coatings i e there is an optimal concentration of CdS in TiO2 coatings that depends on the amount of CdS particles in the supporting electrolyte Taking into account that the influence of CdS particles incorporated into TiO2 coatings on morphology and phase structure is negligible the main contribution of CdS particles to the PA may be in extending the optical absorption range of TiO2 CdS coatings or in preventing fast recombination process of photogenerated electron hole pairs Fig 5 shows UV Vis DRS spectra of pure CdS powder and prepared coatings formed in a supporting electrolyte with various additions of CdS particles Obviously TiO2 CdS coatings do not show the shift of the absorptions into visible light region All spectra show the band edge at about 385 nm 

The absence of adsorption shift might be attributed to low concentration of CdS particles incorporated into coatings indicating that CdS particles are suppressing the recombination of photogenerated electron hole pairs On the other hand if concentration of CdS particles incorporated into TiO2 coatings is too high it increases the concentration of recombination centers for electron hole pairs resulting in lower PA High sensitivity and nondestructive character render PL technique useful in the field of photocatalysis because information such as the efficiency of charge carrier trapping immigration and transfer can be obtained 22 It is generally acknowledged that the increase of PL intensity corresponds to decrease of photocatalytic activity indicating fast recombination of electron hole pairs PL emission spectra of prepared coatings excited at 350 nm are shown in Fig 6a Rise in concentration of CdS particles up to 0 4 g L in electrolyte causes a decrease in PL intensity which then starts to increase up to concentration of 1 0 g L These results are in accordance with photocatalytic measurements Fig 6b i e the decrease of PL intensity corresponds to increase of PA indicating slow recombination of electron hole pairs For higher concentrations of CdS particles in supporting electrolyte 2 g L and more a simultaneous decrease of PL intensity and photocatalytic activity can be related to increased presence of CdS dopants which become capture centers for photoinduced electrons so that the recombination of electron hole pairs can be effectively inhibited