3.1 Structural analysis
The nanocomposites formation is initially assessed by recording the XRD pattern [Fig. 1]. The diffraction peaks at 2θ = 25.39◦, 37.85◦, 48.11◦, 62.76◦, 69.05◦, and 74.98◦ are the reflections of the (101), (004), (200), (204), (116), and (215) planes, respectively, originated from the anatase phase of TiO2. While, 2Ѳ at 27.50◦, 36.14◦, 41.30◦, 44.16◦, 54.37◦, 56.69◦, 64.12◦ and 69.87◦ corresponds to the (110), (101), (111), (210),(211),(220),(310) and (312) plane of rutile phase of TiO2[9]. The mixed-phase growth of TiO2/ZnO was identified with the wurtzite hexagonal planes at 31.80◦, 34.50◦, 36.20◦, 47.60◦, 68.00◦ , 62.90◦ and 68.10◦ along with the prominent TiO2 phase planes [10]. Apace with TiO2/ZnO planes, reflections at 23.99◦, 30.30◦, 32.87◦, 35.42◦, 40.53◦, 49.05◦, and 61.90◦ are from the hematite phase of Fe2O3 [11]. These peaks confirm the formation of TiO2/ZnO/ Fe2O3 composites and the broad intense peaks are substantiating evidence for the growth of nano-sized grains, this speculation is established with the grain size calculations performed by Scherrer equation [12] [Table. I]. The dislocation density has been calculated through Williamson and Smallman's formula [Table I].
Table.I. Average grain size calculated from the Scherrer equation and dislocation density from Williamson-Smallman’s formula as well as the BET results, regression coefficient and rate constant of TiO2, TiO2/ZnO and TiO2/ZnO/Fe2O3.
Compound
|
Avarage Grain Size (nm)
|
Dislocation Density
x1016m2
|
BET surface area (m2/g)
|
Pore volume (cm3/g)
|
Regression coefficient
(R2)
|
Kapp(min)-1
|
TiO2
|
22.53
|
0.20
|
29.1
|
0.029
|
0.99
|
0.007
|
TiO2/ZnO
|
22.80
|
0.19
|
43.7
|
0.049
|
0.99
|
0.01
|
TiO2/ZnO/Fe2O3
|
24.26
|
0.17
|
78.5
|
0.1335
|
0.93
|
0.02
|
3.2 Surface Morphology: Transmission Electron Microscopy and Brunauer–Emmett–Teller analysis
Figure 2 displays TEM and HRTEM images and SAED ring patterns for the surface topography of TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3. The low magnification images obtained from TEM results are presented in the first and second rows of the figure. The images divulge aggregated growth of particles in semi-globular shape, in the bilayer and triple layer composites. The shapes of particles are unaltered with roughly in the spherical shape. However, the average particle size obtained for TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3 were 19.1, 54.4, and 26 nm respectively. From the high-resolution TEM image the gap between two fringes in TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3 were obtained and are 0.27nm and 0.25 nm, these are indexed as (100) and (111) planes correspond to ZnO and Fe2O3 respectively [Fig. 2h and 2i]. The measurements obtained from the SAED rings for the three samples are consistent with the results of the XRD pattern. The surface-to-volume ratio obtained from BET measurements for TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3 is 29.1, 43.7, and 78.5 m2/g, respectively. The specific surface area has a pronounced effect on photocatalytic activity. The BET results indicate that the presence of Zn2+ and Fe3+ ion has significantly influenced the increase of the surface area of the samples. This is expected to activate more sites on the surface of the catalyst and thus to improve the photocatalytic activity [13].
3.3 UV-Visible Spectroscopy and Photoluminescence spectra
The UV-Visible spectra of TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3 nanosystem is represented in Figure 3a. The spectra of TiO2/ZnO is slightly red-shifted in comparison with the spectra of bare TiO2, this is further redshifted when the tertiary ionic system is formed. This is because of the presence of impurity levels spawn between the conduction band and valence band of TiO2 as the Ti4+ either replaced by Zn2+/Fe3+ ions or the at the interstitials and thus resulted in an altered band structure [13]. The prominent transition observed ≅ at 480 nm is the d→d (T2g →A2g) transition [14-15]. The optical band gap energies are estimated from the Kubelka-Munk re-emission function [15] and the corresponding values are 3.17eV, 3.07eV, and 2.9eV for TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3 respectively [Figure. 3b]. The reduction of the bandgap is expected and it is common for composite/hybrid materials consisting of two or more compounds with different bandgaps. The hybrid structure tune the energy band diagram by altering the valance band and conduction band entirely different from that of individual compounds and this will modulate optical properties with prominent influences the electronic charge exchange. Bandgap reduction is accompanied by enhancing the light absorption capability, which is beneficial for advancing photocatalytic properties of the composite. Figure.3(c) displays the PL spectrum of TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3, which is directly related to the electron-hole recombination and strongly influences the efficiency of photocatalysis. High PL intensity is expected to correspond to low photocatalytic efficiency. The materials exhibited two broad PL emission signals at 485 and 525 nm. The signal at 485 nm is due to the surface oxygen vacancies and emission at 525nm is due to the localized F+ centers on the surfaces of compounds [15-17]. The surface oxygen vacancies will act as charge trapping centers and prevent the electron-hole recombination. The order of emission intensity of the PL spectra is obtained in the following order TiO2> TiO2/ZnO>TiO2/ZnO/Fe2O3. The decrease of PL intensity indicates the lower radiative recombination rate of photogenerated electron-hole in the composite. Because of possible efficient charge transport and separation due to beneficial conduction and valence band alignments.
3.4 Photocatalytic activity
Degradation of 1x10-4M methylene blue (MB) solution has been studied to test the photocatalytic performance of the composite TiO2/ZnO/Fe2O3. Photocatalytic degradation of MB took place within 150 minutes [Fig. 4(a) ]. The sunlight irradiation using the same catalyst the complete mineralization was achieved in 210 minutes, and its corresponding absorption spectra is indicated in Fig. 4(b). Evaluation of the photocatalytic dye degradation experiment was performed using the equation lnC0/C=kt. The photocatalysis followed the first-order kinetics. The estimated rate constant and regression constants are presented in Table. I. The degradation efficiency is calculated using the following equation [18-20].
Degradation percentage = ((A0-At)/A0) ×100%.
Here, A0 and At are the absorbance of methylene blue at the time of 0 and t respectively [18-20]. From this study, we have evaluated the photocatalytic performance of the composites. TiO2-based photocatalyst exhibited the lowest performance. The composite TiO2/ZnO showed larger efficiency whereas the three-layer composite TiO2/ZnO/Fe2O3 exhibited the largest efficiency. The improved photocatalytic activity of TiO2/ZnO/Fe2O3 can be mainly due to the reduced bandgap, enhanced optical absorption, more efficient charge transport, and separation.
3.5 Mechanism
The reason for the advanced photocatalytic process by the double and triple layer composites can be explained in the following way: Photogenerated electrons will migrate from the conduction band of ZnO to the conduction band of TiO2 as the conduction band and valence band edges of ZnO is slightly higher than that of TiO2 [22-24]. In the case of Fe2O3, the conduction band and valence bands are placed between those of TiO2 [22-25]. Hence these electrons continue to transfer to the conduction band of Fe2O3. At the same time, the holes from the valence band of TiO2 are transferred to the valence band of Fe2O3 enhancing the effective spatial charge separation. Thus Fe2O3 acts as a recombination center making available the rest of electrons and holes in the valence band of TiO2 and conduction band of ZnO respectively for dye degradation. Both the electrons and holes play a vital role in the degradation of methylene blue. The holes react with water molecules adsorbed on the surface to produce hydroxyl radicals whereas the photogenerated electrons on reacting with the adsorbed oxygen produce superoxide ions. The hydroxyl radicals and superoxide ions can decompose the dyes. Thus, the ternary system act as an efficient photocatalytic system by inhibiting the electron-hole recombination.