Research & Reviews: Journal of Material Sciences
MenuISSN:2321-6212
ISSN:2321-6212
Juliya A.P1,2, Jayaram P3*,Abdul Mujeeb V.M2, Muraleedharan.K2
1 Department of Chemistry, University of Calicut Approved Research Centre, Malappuram, India
2 Department of Chemistry, University of Calicut, Malappuram, India
3 Department of Physics, Materials and Condensed Matter Physics Laboratory, Ponnani, India
Received: 25-Nov-2022, Manuscript No. JOMS-22-81261; Editor assigned: 28-Nov-2022, Pre QC No. JOMS-22-81261 (PQ); Reviewed: 12-Dec-2022, QC No. JOMS-22-81261; Revised: 19-Dec-2022, Manuscript No. JOMS-22-81261(R); Published: 26-Dec-2022, DOI: 10.4172/2321-6212.11.1.001.
Citation: Jayaram P, et al. Enhanced Photocatalytic Degradation of Methylene Blue Using TiO2/ZnO/Fe2O3 Ternary Nanocomposites. RRJ Mater Sci. 2023;11:001.
Copyright: © 2023 Juliya AP, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Herein we report the synthesis and enhanced photocatalytic activity of ternary nanocomposite prepared via a single-step sol-gel process. Various characterization techniques were used for the analysis of the photocatalyst. XRD analysis clearly indicated the multiphase growth of nanocomposite. The average particle size obtained from Transmission Electron Microscopy (TEM) analysis of the photocatalysts TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3 were 19.1, 54.4, and 26 nm respectively. From BET analysis, it follows that there was a pronounced increase in the surface area of the ternary nanocomposite. The UV Visible DRS analysis indicated a considerable red shift of TiO2/ZnO/Fe2O3 compared to that of TiO2 and TiO2/ZnO nanocomposite.
A comparative study on the photocatalytic performance of the prepared samples was performed using methylene blue as the model dye. The photocatalytic degradation of methylene blue in aqueous solutions was enhanced significantly when TiO2/ZnO/Fe2O3 ternary nanocomposite was used as a photocatalyst. The consequences of the phase transition, surface features, and optical properties were compared and reported. The reduced photoluminescence intensity, decreased optical band gap energy and high surface area in ternary nanocomposite resulted in higher degradation of methylene blue under UV irradiation.
Methylene blue; Degradation; Photoluminescence; Nanocomposites; Transmission Electron Microscopy
For the last few decades, we are under the threat of serious ecological imbalance that is being created by environmental pollution [1]. Toxic materials have accumulated in the environment to such a large extent that there needs a destructive mineralization process that roots out the pollutants from the ecosystem. Among the various toxic materials that have been discharged into nature, the dyes released from the textile industries occupy the forefront position [2]. Methylene blue is a heterocyclic aromatic compound which when released to the aquatic environment harms the aquatic organisms by depleting the content of oxygen and releasing toxic substances [3,4]. Scientists have reported various methods for the remediation of methylene blue from the ecosystem [5-7].
The heterogeneous photocatalytic method is the competent method for the degradation of hazardous materials in the environment through the production of excitons upon illumination [8]. TiO2 serves to be an efficient photocatalyst for the degradation of methylene blue [9]. Though there are many factors like cost effectiveness, nontoxic nature, resistance to corrosion, stability, etc. that attract TiO2 as a good photocatalyst, the wide band gap and visible light inactivity render the scientist to modify the structural properties of TiO2 [10-11]. Scientists use various approaches like doping with metals and non-metals, photosensitization, coupling with organic and inorganic material, etc. to increase the performance of TiO2 as a photocatalyst [12, 13].
ZnO is a prominent photocatalyst with unique properties like nontoxicity, stability, low price, electro-optical, catalytic properties etc., [14, 15]. The attractive features of ZnO make it suitable for various applications like solar cells, diodes, antireflection coatings, sensors, etc., [16-18]. One way to improve the property of TiO2 is to couple it with a suitable semiconducting metal oxide. Scientists have conducted various studies on the binary system TiO2/ZnO. The studies prove that coupling enhances charge separation and modifies the electronic properties of the catalyst material [19-20]. For the past years efforts have been taken by researchers to develop the ternary photocatalytic system. The photocatalytic activity of the binary system can be improved by the introduction of a third system to the binary material. This further suppresses the electron-hole recombination and improves the photocatalytic efficiency of the system. Studies have reported that the doping of TiO2 with Fe increases the visible light sensitivity of TiO2 [21-23].
There are different preparative methods for the synthesis of metal oxides i.e., a dip coating method, ball milling process, electrochemical method, sol-gel method, etc. The sol-gel method is a low-cost method for the synthesis of nanomaterials with high purity and homogeneity [24-26]. In the present study, we report the synthesis and effectiveness of TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3nanocomposite for the photocatalytic degradation of methylene blue. As there is a lack of studies on the photocatalytic degradation of methylene blue by TiO2/ZnO/Fe2O3we have focused our studies on this area. To elucidate the properties we report the structural, morphological, and optical properties of the compounds.
High purity Tetrabutyl Titanate (TBOT), Zinc Nitrate (Zn (NO3)2.6H2O) and Ferric Nitrate (Fe (NO)3.9H2O) and the analytic grade reagents ethanol and acetic acid were used for the sol-gel synthesis of the composite of TiO2/ZnO/Fe2O3.12.9 ml TBOT was 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 were added slowly until a yellow transparent solution was formed. Secondly, in a mixed solution of 20 ml absolute ethanol, 6.5 ml acetic acid, and 2.5 ml deionized water; Zn (NO3).6H2O and Fe (NO)3.9H2O was added in the required amounts.
This second solution was mixed with the first solution drop wise with vigorous stirring at least for two hours. After aging this for 48 hours, the prepared sol was dried at 100ºC for 12 hours and then calcined at 500ºC for 5 hours in a microwave muffle furnace to obtain the end product. The same method was repeated for the synthesis of TiO2 and TiO2/ZnO in the absence of Zn(NO3).6H2O, Fe(NO)3.9H2O, and Fe (NO)3.9H2O respectively. 50 ml of prepared standard methylene blue with a concentration of 10 M-4 M and 0.2 g of calcined photocatalyst (TiO2, TiO2/ZnO, TiO2/ZnO/Fe2O3) was added to the dye solution and stirred in the dark in UV reactor for 30 minutes in order to maintain the adsorption-desorption equilibrium.
This was followed by the performance of the reaction in the presence of UV light. The photocatalytic reaction was carried out in LZC-4X-Luzchem photo reactor provided with a bead. 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 five minutes was measured using LUXMETER (LX-103). 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. BET surface area was analyzed by BELSORP-max, an automatic gas adsorption measuring unit for gas adsorption, vapor adsorption, and chemisorption. To characterize the materials optically UV-Vis DRS measurement was done using a JASCO V-750 spectrophotometer and photoluminescence by JASCO Spectrofluorometer (FP-8300).
XRD analysis
The nanocomposites formation was initially assessed by recording the XRD pattern indicated in Figure 1. The diffraction peaks 2θ at 25.39º, 37.85º, 48.11º, 62.76º, 69.05º, and 74.98º, respectively are the reflections of the (101), (004), (200), (204), (116), and (215) planes originated from the anatase phase of TiO2. While, 2Ãâò at 27.5º, 36.14º, 41.3º, 44.16º, 54.37º, 56.69º, 64.12º, and 69.87º corresponds to the (110), (101), (111), (210), (211), (220), (310) and (312) plane of rutile phase of TiO2 [27].
Figure 1: X-Ray Diffraction pattern of the compounds indicating composite formation as the number of cations increases.
The mixed phase growth of TiO2/ZnO was identified with the wurtzite hexagonal planes originating from points 31.8º, 34.5º, 36.2º, 47.6º, 6.8º, 62.9º and 68.1º along with the prominent TiO2 phase [28]. Apace from TiO2/ZnO planes, reflections at 23.99º, 30.3º, 32.87º, 35.42º, 40.53º, 49.05º, and 61.9º originated due to the hematite phase of Fe2O3 [29-31]. These peaks confirmed the formation of TiO2/ZnO/Fe2O3nanocomposites and the broad intense peaks are substantiating evidence of the growth of nano-sized grains.
This speculation is established with the grain size calculations performed by Scherrer equation and are displayed in Table 1. By means of Williamson and Smallman's formula, we were able to calculate the Dislocation density and presented it in table 1.
Surface morphology: Transmission Electron Microscopy and Brunauer-Emmett-Teller analysis. The surface topography of TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3are displayed in Figures 2a-2j, comprising TEM, HRTEM images, and SAED ring patterns. The images divulge aggregated growth of particles in semi-globular shapes. In binary and ternary composites, the shapes of particles were unaltered roughly spherical in shape.
However, the average particle size obtained for TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3was 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/Fe2O3were 0.32, 0.27 nm, and 0.25 nm, which can be indexed to (110), (100) and (111) planes corresponding to TiO2, ZnO and Fe2O3 respectively (Figures 2g-2i).
Figure 2: (a) SAED patterns of TiO2, (b) SAED patterns of TiO2/ZnO (c) SAED patterns of TiO2/ZnO/Fe2O3 (d) TEM images of TiO2 (e) TEM images of TiO2/ZnO (f) TEM images of TiO2/ZnO/Fe2O3 (g) HRTEM images of TiO2 (h) HRTEM images of TiO2/ZnO (i) HRTEM images of TiO2/ZnO/Fe2O3 (j) SAED patterns of TiO2, TiO2/ZnO and TiO2/ZnO/Fe2O3.
The measurements obtained from the SAED rings for the three samples were consistent with the results of the XRD pattern. From the TEM image, it is evident that interfacial contact exists in the ternary nanocomposite. The BET results of the catalysts are shown in Table 1. As the results presented, the BET surface area of TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3were 29.1, 43.7, and 78.5 m2/g respectively. Specific surface areas have a pronounced effect on photocatalytic activity.
| Compound | Avarage Grain Size (nm) | Dislocation Density δ × 1016m2 | BET surface area (m2/g) | Pore volume (cm3/g) | Regression coefficient(R2) | Kapp(min)-1 |
|---|---|---|---|---|---|---|
| TiO2 | 22.53 | 0.2 | 29.1 | 0.029 | 0.99 | 0.007 |
| TiO2/ZnO | 22.8 | 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 |
Table 1. 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.
The BET results revealed the presence of Zn2+ and Fe 3+ ions significantly increased the surface area of the samples. The increase in the surface area of the ternary system can be attributed to the replacement of the Ti4+ (0.605 Å) which is having lower ionic radii by Zn+2 (0.74 Å) and Fe3+ ions (0.645Å) that created imperfection in the crystal making the sample rough and disordered. This is expected to activate more sites on the surface of the catalyst and thus improved the photocatalytic activity [32].
UV-visible DRS analysis and photoluminescence studies
The UV-visible DRS of TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3nanosystems was represented in Figures 3a-3c. The UV-visible spectra of TiO2/ZnO were slightly red-shifted in comparison to the spectra of bare TiO2. This was further red-shifted in the ternary nanocomposite. The red shift of the ternary nanocomposite can be attributed to the promotion of electrons from the valence band of Fe2O3 to that of TiO2 [33]. The optical band gap energies were estimated from the Kubelka-Munk plot and the corresponding values were 3.17 eV, 3.07 eV, and 2.9 eV for TiO2, TiO2/ZnO, and TiO2/ZnO/Fe2O3respectively. The reduction of the band gap may be explained by relating the structural deformation of the compounds, as the novel composite structure was formed as depicted by XRD. However, the incorporation of iron enhanced the visible light absorption power of the catalyst. The narrowing of the band gap of TiO2/ZnO/Fe2O3was expected to bolster the photocatalytic activity.
Figure 3: (a) UV-visible spectra of the samples recorded in diffuse reflectance mode (b) The optical band gap energy determined by using Kubelka-Munk theory (c) The photoluminescence spectra of the samples. Note:
Figure 3c the PL spectrum of TiO2, TiO2/ZnO and TiO2/ZnO/Fe2O3are displayed. PL emission intensity is directly related to the electron-hole recombination; the higher the PL intensity photocatalytic rate will be lower. The materials exhibited two broad PL emission signals at 485 nm and 525 nm, the signal at 485 was due to the surface oxygen vacancies and the emission at 525 nm was due to the localized F+ centres on the surfaces of compounds [34, 35]. The surface oxygen vacancies will act as charge-trapping centres and prevent electron-hole recombination. The emission intensity of the PL spectra was obtained in the following order TiO2>TiO2/ZnO>TiO2/ZnO/Fe2O3. The decrease in PL intensity indicated the lower recombination rate of photogenerated electron-hole pairs. The presence of ZnO and Fe2O3 in the crystal symmetry of TiO2 has obviously resulted in the lowering of PL intensity. The results propound the photocatalytic activity can be higher in TiO2/ZnO/Fe2O3 as it can suppress the combination between the photogenerated electrons and holes.
Photocatalytic activity
Ultraviolet and sunlight irradiation on a 10 M-4 M methylene blue solution was used to test the photocatalytic dye degradation activity. The complete degradation of methylene blue under UV irradiation using TiO2/ZnO/Fe2O3catalyst occurred within a period of 130 minutes (Figures 4a-4f). However, on sunlight irradiation using the same catalyst complete mineralization was achieved within 170 minutes, and its corresponding absorption spectra are indicated in Figure 4b. Evaluation of the photocatalytic dye degradation experiment was performed using the equation in C0/C=kt. The photocatalysis followed first-order kinetics. The values of the rate constant and regression constant are presented in Table 1. The degradation percentage was calculated using the following equation.
Figure 4: UV absorption Spectra of photocatalytic degradation of methylene blue in the presence of (a) UV light irradiation (b) Sunlight irradiation (c) Percentage of degradation of methylene blue under UV irradiation and (d) Percentage of degradation of methylene blue under sunlight irradiation (e) ln C0/C versus time of methylene blue degradation under UV irradiation (f) lnC0/C ver\sus time of Methylene blue degradation under sunlight irradiation. Note: 
Degradation percentage=((A0-At)/A0) × 100%
Where A0 and At represents the absorbance of methylene blue at the time o and t respectively. The order of photocatalytic activity of the catalyst was as follows, TiO2<TiO2/ZnO<TiO2/ZnO/Fe2O3. This affirms the formation of ternary composites has resulted in a significant enhancement in photocatalytic activities. The improved photocatalytic activity of TiO2/ZnO/Fe2O3nanocomposite can be mainly attributed to the reduction in the band gap of the ternary nanocomposite. Alongside, as observed from the surface studies, the higher surface area of the ternary composite is a contributing factor to the high photocatalytic efficiency. PL intensity of TiO2/ZnO/Fe2O3was lowered in comparison to the other two samples indicating the slow rate of electron-hole recombination taking place in the samples in this category [36]. All these facts account to the highest photocatalytic activity of TiO2/ZnO/Fe2O3nanocomposites.
Mechanism
The matching band structure of TiO2, ZnO, and Fe2O3 is a favourable condition for the effective charge transfer [37]. The coupling of semiconductors helped in improving the visible light sensitivity and suppressing the electron-hole recombination and promoting the interfacial charge transfer [38]. In order to describe the mechanism of photocatalytic dye degradation prior knowledge of the band edge position of the conduction band and valence band of TiO2, ZnO and Fe2O3 should be acquired. Before formation of ternary system, both the conduction band edge and valence band edge level of Fe2O3 were placed in between that of ZnO [39]. As Fe2O3 comes in contact with ZnO, there occurs a shift in the energy bands of ZnO and Fe2O3 till the Fermi level attains equilibrium [40-42]. So, on irradiation, the electrons continue to transfer from the conduction band of Fe2O3 to that of ZnO and TiO2. The conduction band edge and valence band edge of ZnO are slightly higher than that of TiO2 [43]. At the same time, the holes from the valence band of TiO2 are transferred to the valence band of Fe2O3 and ZnO enhancing the effective spatial charge separation. Both the electrons and holes play a vital role in methylene blue degradation. The holes react with water molecules adsorbed on the surface to produce hydroxyl radicals whereas the photogenerated electrons reacting with the adsorbed oxygen produce superoxide anions. The hydroxyl radicals and superoxide anions have the ability to decompose the dyes. Thus, the ternary system acts as an efficient photocatalytic system by inhibiting electron-hole recombination.
The study can be summarized as follows, binary and ternary metal oxide nanocomposites have been successfully synthesized incorporating TiO2, ZnO, and Fe2O3 metal oxides through a facile sol-gel method. The synthesized samples were subjected to various studies in order to depict their photocatalytic performance in methylene blue degradation. X-Ray Diffraction study confirmed the mixed growth of metal oxide powders in the multiphase composite category. The measurements obtained from SAED patterns were in consistent with planes calculated from XRD. Brunauer-Emmett-Teller analysis revealed a predominant increase of surface area in TiO2/ZnO/Fe2O3composite in comparison to that of TiO2 and TiO2/ZnO. It was inferred from the study that TiO2/ZnO/Fe2O3nanocomposite possesses visible light sensitivity, enhanced charge separation, and high surface area than the other two compound systems. The aforementioned factors shaped TiO2/ZnO/Fe2O3nanocomposite, a far superior candidate than the bare TiO2 and TiO2/ZnO in the photocatalytic performance of methylene blue degradation. The complete mineralization of the dye in the presence of UV light was achieved within a period of 130 minutes is one of the major findings of the study alongside with the degradation of the same concentration of the dye in direct sunlight within a period of 170 minutes.