Synthesis,Surface Morphology, Optical Properties and Photocatalyst Activities of TiO2/ZnO/Fe2O3 Nanocomposites.

The photocatalytic degradation of methylene blue in aqueous solutions is enhanced signicantly by formulating multiphase TiO 2 /ZnO/Fe 2 O 3 nanocomposites. The photocatalytic activity of unary TiO 2 , binary TiO 2 /ZnO, and ternary TiO 2 /ZnO/Fe 2 O 3 compounds are compared and reported. Using TiO2/ZnO/Fe2O3, methylene blue degradation became rapid and the reaction followed rst-order kinetics. The consequences of the phase transition, surface features, and optical properties are compared and elucidated. The reduced photoluminescence intensity and decreased optical band gap energy in tertiary compounds impose higher degradation of methylene blue under irradiation.


Introduction
In the last few decades, the wide ecological imbalances are created by the usage of toxic materials that are accumulated in the environment to a large extent, in this context, there needs a destructive mineralization process that roots out the pollutants from the ecosystem [1] . Among the various toxic materials that are being discharged into nature, the dyes released from the textile industry occupies the forefront position [2] . Methylene blue, a heterocyclic aromatic byproduct compound of the aforesaid can interact with the aquatic environment and thus harms the aquatic organisms by depleting the content of oxygen [3][4] . The heterogeneous photocatalysis method is the competent method for the degradation of hazardous materials in the environment through the production of excitons upon illumination [5] . Various photocatalysts have been designed for the past years. Despite their high photocatalytic performance, the high electron-hole recombination and absorption of light near the ultraviolet region observed in such systems limit their practical applicability. One of the strategies to overcome this shortcoming is to use coupled semiconductors. Several materials are reported in the binary versions metal oxides with degradation properties and recently the efforts have been taken to the development of ternary photocatalytic systems. Because in ternary systems, the probability of multi-excitons generation and suppression of the electron-hole recombination can be higher [6] . The potential gradient developed at the interface results in the higher photocatalytic activity. Scientists have conducted various studies on the binary system TiO 2 /ZnO.The studies prove that coupling enhances the charge separation and modify the electronic properties of the catalyst material [7][8] . .Herein, we report the synthesis and effectiveness of a single composition of TiO 2 , TiO 2 /ZnO, and TiO 2 /ZnO/Fe 2 O 3 system for the photocatalytic degradation of methylene blue. To elucidate the properties we report the structural, morphological, and optical properties of the compounds.

Materials And Methods
High purity tetra butyl titanate (TBOT), zinc nitrate Zn(NO ) .6H 2 O, and ferric nitrate Fe(NO) 3 .9H 2 O and the analytic grade reagents ethanol and acetic acid were used for the sol-gel synthesis of the composite of TiO 2 /ZnO/Fe 2 O 3 . 12.9 ml TBOT initially dissolved in 51.1 ml absolute ethanol under vigorous stirring and 8.7 ml acetic acid, 0.66 ml water, and 12.9 ml ethanol are added slowly until a yellow transparent solution is formed. Secondly, in a mixed solution of 20 ml, absolute ethanol, 6.5 ml acetic acid, and 2.5 ml deionized water; Zn(NO 3 ).6H 2 O and Fe(NO) 3 .9H 2 O is added in required amounts. This second solution mixed in the rst solution drop-wise with vigorous stirring at least for two hours. After aging this for 48 hours, the prepared sol was dried at 100 o C for 12 hours and then calcined at 500 o C for 5 hours in a microwave mu e furnace to obtain the end product. The same method was repeated for the synthesis of TiO 2 and TiO 2 /ZnO in the absence of corresponding metal oxide raw agents. 50 ml of prepared standard methylene blue with a concentration of 10 − 4 M and 0.2 g of calcined photocatalyst (TiO 2 , TiO 2 /ZnO, TiO 2 /ZnO/Fe 2 O 3 ) was added to the dye solution and stirred in the dark for 30 minutes to maintain the adsorption-desorption equilibrium. This is placed in, LZC-4X-Luzchem photo reactor provided with a bead and exposed to UV light for photocatalytic reaction measurements. The reaction was monitored by withdrawing 3 ml aliquots at an interval of 20 minutes. After degradation, the solution was centrifuged to eliminate the effects of scattering before the evaluation of the photocatalytic activity. The same experiment was conducted in direct sunlight with the most active catalyst. The intensity of sunlight at an interval of ve minutes was measured using LUXMETER (LX-103). To characterize the materials optically UV-Vis DRS measurement was done using JASCO V-750 spectrophotometer and photoluminescence by JASCO Spectro uorometer (FP-8300). BET surface area, pore-volume, and pore diameter were analyzed by BELSORP-max, automatic gas adsorption measuring unit for gas adsorption, vapor adsorption, and chemisorption. The diffraction patterns for X-ray diffraction analysis were recorded on Bruker AXS D8 advance diffractometer using Cu-Kα radiation (λ = 1.5406A0). The TEM analysis of the catalyst was performed on a JEOL/JEM 2100 transmission electron microscope.

Structural analysis
The nanocomposites formation is initially assessed by recording the XRD pattern [ Fig . The mixed-phase growth of TiO 2 [11] . These peaks con rm the formation of TiO 2 /ZnO/ Fe 2 O 3 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] [    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, TiO 2 /ZnO, and TiO 2 /ZnO/Fe 2 O 3 is 29.1, 43.7, and 78.5 m 2 /g, respectively. The speci c surface area has a pronounced effect on photocatalytic activity. The BET results indicate that the presence of Zn 2+ and Fe 3+ ion has signi cantly in uenced 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] .
The spectra of TiO 2 /ZnO is slightly red-shifted in comparison with the spectra of bare TiO 2 , 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 TiO 2 as the Ti 4+ either replaced by Zn 2+ /Fe 3+ 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 TiO 2 /ZnO/Fe 2 O 3 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 in uences the electronic charge exchange. Bandgap reduction is accompanied by enhancing the light absorption capability, which is bene cial for advancing photocatalytic properties of the composite. Figure.3(c) displays the PL spectrum of TiO 2 , TiO 2 /ZnO, and TiO 2 /ZnO/Fe 2 O 3 , which is directly related to the electron-hole recombination and strongly in uences the e ciency of photocatalysis. High PL intensity is expected to correspond to low photocatalytic e ciency.
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][16][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 TiO 2 > TiO 2 /ZnO>TiO 2 /ZnO/Fe 2 O 3 . The decrease of PL intensity indicates the lower radiative recombination rate of photogenerated electron-hole in the composite. Because of possible e cient charge transport and separation due to bene cial conduction and valence band alignments.

Photocatalytic activity
Degradation of 1x10 -4 M methylene blue (MB) solution has been studied to test the photocatalytic performance of the composite TiO 2 /ZnO/Fe 2 O 3 . 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 rst-order kinetics. The estimated rate constant and regression constants are presented in Table. I. The degradation e ciency is calculated using the following equation [18][19][20] .
Here, A0 and At are the absorbance of methylene blue at the time of 0 and t respectively [18][19][20] . From this study, we have evaluated the photocatalytic performance of the composites. TiO 2 -based photocatalyst exhibited the lowest performance. The composite TiO 2 /ZnO showed larger e ciency whereas the threelayer composite TiO 2 /ZnO/Fe 2 O 3 exhibited the largest e ciency. The improved photocatalytic activity of TiO 2 /ZnO/Fe 2 O 3 can be mainly due to the reduced bandgap, enhanced optical absorption, more e cient charge transport, and separation.

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 TiO 2 as the conduction band and valence band edges of ZnO is slightly higher than that of TiO 2 [22][23][24] . In the case of Fe 2 O 3 , the conduction band and valence bands are placed between those of TiO 2 [22][23][24][25] . Hence these electrons continue to transfer to the conduction band of  TEM images of (a) TiO2 (b) TiO2/ZnO (c) TiO2/ZnO/Fe2O3, inset shows seed images of corresponding particles. In second raw, low magni cation TEM images of (d) TiO2 (e) TiO2/ZnO (f) TiO2/ZnO/Fe2O3. Third raw high magni cation TEM images of (g) TiO2 (h) TiO2/ZnO (i) TiO2/ZnO/Fe2O3.