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Photocatalytic Properties of Microwave-Synthesized TiO2 and ZnO Nanoparticles Using Malachite Green Dye
TiO2 and ZnO nanoparticles (NPs) were synthesized using microwave-assisted method. Synthesized NPs were characterized for their structure, morphology, and elemental composition using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The crystallite size of synthesized NPs of TiO2 and ZnO was about 12.3 and 18.7 nm as obtained from the Scherrer formula from the most intense XRD peak. The synthesized NPs have been found to be in stoichiometric ratio having anatase and hexagonal wurtzite structure for TiO2 and ZnO, respectively, and are spherical in shape. Surface area of TiO2 and ZnO NPs was found to be about 43.52 m2/g and 7.7 m2/g. Photocatalytic (PC) properties of synthesized NPs were studied for malachite green (MG) dye under UV light. TiO2 NPs were found to be highly photocatalytically active among the two, having efficiency and apparent photodegradation rate of 49.35% and , respectively.
Nowadays, organic pollutants like dyes are becoming a serious health problem, which are being discharged by many industries and houses through wastewater in the environment. The discharged wastes containing dyes are toxic to microorganisms and aquatic life . Malachite green (MG) dye is an extensively used biocide in the global aquaculture industry. It is also used as a food colouring agent, food additive, medical disinfectant, and anthelmintic as well as in silk, wool, jute, leather, cotton, paper, and acrylic industries. However, MG has now become a highly controversial compound due to the risks it poses to the consumers of treated fish, including its effects on the immune system and reproductive system and its genotoxic and carcinogenic properties . In the light of the basic and applied researches reviewed, PC oxidation method appears to be a promising route for the treatment of wastewater contaminated with phenols and dyes. In recent years, there has been a wide research interest in developing semiconductor photocatalysts having high PC activities for environmental applications such as water disinfection, hazardous waste remediation, and water purification [3–5]. It is reported that nanosize semiconductors act as an efficient PC agent resulting from either an extremely large-surface-area-to-volume ratio or quantum confinement effects of charge carriers for the degradation of organic pollutants in water under UV irradiation [6–10]. PC function is similar to the chlorophyll in the photosynthesis. In a PC system, photo-induced molecular transformation or reaction takes place at the surface of the catalyst. A basic mechanism of PC reaction on the generation of electron-hole pair and its destination is as follows: when a photocatalyst is illuminated by the light stronger than its bandgap energy, electron-hole pairs diffuse out to the surface of the PCs and participate in the chemical reaction with the electron donor and acceptor. Those free electrons and holes transform the surrounding oxygen or water molecules into OH free radicals with super strong oxidization . One of the major problems in the PC process is the electron-hole recombination. To avoid this problem, UV irradiation source stands up among other sources  because the energy of the UV irradiation is large compared to the bandgap energy of the catalysts. The problem of electron-hole recombination is not fully solved but largely avoided with the use of UV source.
Many photocatalysts like TiO2, ZnO, ZrO2, CeO2, Fe2O3, CdS, and ZnS have been attempted for the PC degradation of a wide variety of environmental contaminants. TiO2  is one of the most widely used semiconductors for PC degradation of waste due to its biological and chemical inertness, high photoreactivity, nontoxicity, and photostability. It is proven that among the three crystalline polymorphs of TiO2, that is, rutile, anatase, and brookite, the anatase phase or the mixed anatase/rutile phase exhibits better PC properties. The anatase form of TiO2 has the desirable properties of being chemically stable, readily available, and active as a catalyst for oxidation processes. Bandgap of about 3.2 eV matches the output of a wide variety of readily available lamps. Recently, ZnO [13–17] has also been reported for PC activity having bandgap close to TiO2. However, only a handful of studies have been attempted which compare the efficiency of different catalysts for a particular organic compound under identical experimental conditions. Serpone and Pelizzetti  have reported that TiO2 and ZnO are the two most active catalysts.
During the recent past, ZnO and TiO2 were the most widely investigated [19, 20] semiconductors. There are several reported methods of NPs synthesis such as sol-gel , organometallic , hydrothermal [23, 24], and microwave methods [25–27]. Due to the intense friction and the collision of molecules created by microwave irradiation, microwave irradiation not only provides the energy for heating but also greatly accelerates the nucleation. With microwave irradiation on the reactant solution, temperature and concentration gradients can be avoided leading to uniform nucleation. Microwave-based synthesis method is one of the easiest, energy-saving, green, and quick methods for large-scale production of nanomaterials. Microwave synthesis is the novel route of synthesis of metal oxide semiconductor NPs which is a clean, cost-effective, energy-efficient, eco-friendly, rapid, and convenient method of heating and results in higher yields in shorter reaction times [26, 27].
In the present work, TiO2 and ZnO NPs were synthesized using the microwave method. These NPs were characterized using XRD, SEM, and EDS. The PC activity of these NPs was studied for MG dye under UV light.
For the synthesis of TiO2, 0.5 M titanium butoxide solution was prepared in 100 mL butanol and stirred for 15 min; further 30 mL deionized water was added dropwise in the above solution to allow hydrolysis. This solution was stirred for 30 min which gave rise to white precipitation. The obtained white precipitate was microwave irradiated for 5 min at 700 W power using Raga’s microwave system. The microwave used for this experiment was having a power range of 140 W to 700 W. This obtained solution was left 24 hr for aging at room temperature and then centrifuged at 2000 rpm for 15 min. Obtained precipitate was dried at 80°C for 12 hrs. After complete drying, powder was crushed and calcinated in air at 500°C for 2 hr to remove hydroxide impurities and recrystallization.
For the synthesis of ZnO, NaOH (0.4 M), and zinc acetate (0.2 M), solutions were mixed slowly with molar ratio of 2 : 1, respectively. The above solution was stirred for 10 min. After that, a small quantity of tea was added, and stirring continued for another 10 min. This solution was put for microwave irradiation at 700 W in two steps, that is, 40°C for 20 min and 60°C for 30 min. Resulting precipitate was washed with DI water 2-3 times before drying at 70°C for 4 hr, then crushed using mortar pestle, and calcinated in air at 500°C for 1 hr.
The prepared samples were characterized for their structure, morphology, and elemental composition using X-ray diffraction (XRD) analysis, scanning electron micrograph (SEM), and energy dispersive spectrometry (EDS), respectively. XRD was performed using Bruker AXS, Germany (Model D8 Advanced), diffractometer in the scanning range of 20–70° (2θ) using Cu Ka radiations of wavelength 1.5406 Å. A JEOL ASM 6360A SEM was used to study the morphology and elemental analysis of the NPs. Measurement of BET surface area was carried out for nitrogen adsorption using a Micromeritrics Ins., USA.
2.3. Photocatalytic Activity
Synthesized NPs were used for the photodegradation of MG dye, chemical formula: [C6H5C(C6H4N(CH3)2)2]Cl-4, IUPAC name: [(4-dimethylaminophenyl)phenyl-methyl]-N,N-dimethylaniline, under UV light. In each experiment, 40 mL of MG solution of 0.025 g mL−1 concentration and 0.005 gr of catalyst were used. This solution was stirred for 10 minutes in the dark for equilibrium of adsorption and desorption process of MG with NPs. After stirring, the solution was irradiated by UV lamp (DYMAX Corporation) under medium intensity spot lamp. The UV lamp was having radiation wavelength range of 200 nm–600 nm with nominal intensity 1000–2000 mW/cm2. The intensity of absorption peaks, and peak absorbance of NPs was examined by an Ocean Optics UV-Vis high-resolution spectrophotometer (Ocean Optics HR 4000). From which concentrations of MG solution initially and after every 4 minutes of irradiation of UV light were measured from the Lambert–Beer Law  that is the absorbance () of MG solution is proportional to its concentration (), which generally followed the following equation: where is the molar absorption coefficient, and is the thickness of the absorption cell. In our experiment, all the testing parameters were kept constant, so that and could be considered constant. Therefore, changes in the concentration of MB aqueous solution can be determined by a UV-Vis spectrophotometer.
3. Results and Discussion
3.1. X-Ray Diffraction (XRD)
XRD spectra of calcinated samples of TiO2 and ZnO are shown in Figures 1(a) and 1(b). The obtained diffraction pattern has been compared with JCPDS datasheet JCPDS-080-0074 and JCPDS-84-1284, respectively. From Figures 1(a) and 1(b), it is confirmed from the XRD spectra that the given phase of TiO2 and ZnO is anatase and hexagonal wurtzite, respectively. The crystallite size, , of NPs has been calculated by XRD line broadening of the most intense peak using the Scherrer formula  where is the wavelength of X-ray, is the full width and half maxima, is Bragg’s angle, and is the shape factor. The dimensionless shape factor has a typical value of about 0.9 but varies with the actual shape of the crystallite. Here, is taken as 0.9. Crystallite sizes for these NPs are about 12.3 nm and 18.7 nm for TiO2 and ZnO, respectively, calculated from the most intense peak and shown in Table 1. The sharpness of peaks shows that TiO2 NPs are highly crystalline in nature.
3.2. Morphology and Elemental Analysis
Morphology of synthesized NPs was studied using scanning electron microscope (SEM) and is shown in Figures 2(a), 2(b), 2(c), and 2(d) at two different magnifications for TiO2 and ZnO, respectively. Synthesized NPs of TiO2 and ZnO were found to be spherical in shape, and ZnO NPs are of lower size than TiO2 with size uniformity. Some agglomeration was also observed in the SEM image of TiO2, which may be due to calcination NPs. The measured BET surface area of TiO2 and ZnO NPs has been found to be, respectively, 43.52 m2/g and 7.7 m2/g. Elemental analysis of NPs was done by using energy dispersive spectrometer (EDS); the plot of spectrum is shown in Figures 3(a) and 3(b), for TiO2 and ZnO, respectively. For TiO2 and ZnO, respectively, the at% of elements detected from EDX is Ti, and O is 54.29 and 45.71; Zn and O are 43.09 and 56.91, respectively, which shows that synthesized NPs are in stoichiometric ratio. The peaks of the plot show the presence of titanium and oxygen elements, zinc and oxygen elements in the corresponding images, that is, Figures 3(a) and 3(b), respectively.
3.3. Photocatalytic Activity
The UV-visible absorbance spectra for photodegradation of MG dye using TiO2 and ZnO are shown in Figures 4(a) and 4(b), respectively, and corresponding UV-visible spectra of TiO2 and ZnO are shown in Figure 5. From the plot, it is observed that as irradiation time increases, the concentration of MG dye decreases, which is shown by the decrease in UV-absorbance spectra. The absorption of photon leads to charge separation due to promotion of an electron (e−) from the valence band of the semiconductor catalyst to the conduction band, thus generating a hole (h+) in the valence band. Light source provides photon energy required to excite the semiconductor electron from the valence band (VB) region to the conduction band (CB) region. With the increase in irradiation having sufficient intensity, photodegradation efficiency increases because electron-hole formation is predominantly increasing, and electron-hole recombination is negligible at sufficiently higher intensity, whereas at lower light intensity, electron-hole pair separation competes with recombination which in turn decreases the formation of free radicals, thereby, causing less effect on the percentage degradation of the dyes.
Mechanism for photodegradation of MG dye can be explained as follows. On illumination of catalyst surface with enough energy (equals or higher than the bandgap energy, , of the catalyst), it leads to the formation of a hole (h+) in the valence band and an electron (e−) in the conduction band. The hole oxidizes either pollutant directly or water to produce radicals, whereas the electron in the conduction band reduces the oxygen adsorbed on the catalyst. The activation of catalyst (TiO2 or ZnO) by UV light can be represented by the following steps: In this reaction, h+ and e− are powerful oxidizing and reductive agents, respectively. The oxidative and reductive reaction steps are expressed as follows: One of the major problems in the photocatalytic process is the electron-hole recombination. To avoid this problem, UV irradiation source stands up among other sources . The energy of UV irradiation is large compared to the bandgap energy of the catalysts. Hence, the problem of electron-hole recombination is not fully but largely avoided with UV source. The percentage efficiency of photodegradation was determined using the following equation : where is the initial solution concentration of MG and is the solution concentration of MG after degradation. The graph of percentage efficiency with irradiation time is shown in Figure 6. It is clear from the figure that as irradiation time increases the efficiency of nanoparticles to degrade MG dye also increases. Here, TiO2 NPs are found to be more efficient catalysts than ZnO NPs. The apparent rate constant for degradation of MG was determined using the following equation : (min−1) is the observed first-order rate constant. In the early stage of the reaction (i.e., up to 30 min), MG degradation is found to follow first-order kinetics according to the equation. This empirical equation has been used from common first-order kinetics and proposed for the simple description of the decay process . The plot of ln () with irradiation time is shown in Figure 7 where the slopes represent the apparent rate constant for degradation of MG dye. The apparent rate constants of TiO2 and ZnO catalysts are given in Table 1. Here, we found that TiO2 NPs have higher apparent rate constant than other ZnO NPs; this is due to TiO2 having wide bandgap and highly sensitivity to UV radiations, as can be seen in Figure 5. The bandgap value of anatase TiO2 is around 3.2 eV, which enables UV light of wavelengths smaller than 400 nm to activate the catalyst .
In this study, synthesis of uniform size TiO2 and ZnO nanoparticles was achieved using microwave synthesis which is considered to be a green, efficient, and cost-effective method having potential for large-scale synthesis. Photocatalytic properties of synthesized TiO2 and ZnO nanoparticles were studied which shows that TiO2 and ZnO nanoparticles have 49.35% and 23.31% photodegradation efficiency, respectively, for MG dye under UV light. TiO2 nanoparticles show more than two-order photodegradation property for MG dye as compared to ZnO.
The authors are thankful to the director of ARDE, Pashan, Pune, for granting permission to publish this work. Thanks are also due to the Department of Physics, University of Pune, for SEM and XRD of samples.
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Copyright © 2013 A. K. Singh and Umesh T. Nakate. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.