Tuning the physio-chemical properties of ZnO nanoparticles through annealing process and their photocatalytic activity for dye degradation

Hexagonal phase zinc oxide nanostructures were synthesized via hydrothermal method and annealed at different temperatures viz., 200, 400, 600, 800 and 1000 °C. The prepared nanostructures were characterized by XRD, FTIR, FESEM, HR-TEM and UV-DRS techniques. The annealing seems to have a profound effect on the crystallinity, morphology and properties of nanostructures. The lattice strain (ε) is calculated from Williamson-Hall (W- H) plot ; for nanostructure with small crystallite size the strain is caused by compressive force (intercept is negative) whereas for the large crystallite size tensile force (intercept is positive) determines the lattice strain. The photocatalytic performances were studied by analyzing the degradation of methylene blue (MB) in an aqueous solution. ZnO annealed at 1000 °C exhibited better photocatalytic activity due to enhanced crystallinity, comparatively lower bandgap and high oxygen vacancy. Thus, ZnO nanostructures prepared here provide a potential solution to general wastewater treatment and drinking water purification.


Introduction
Development of efficient photocatalyst is the need of the hour. Owing to energy crisis and environmental pollution caused by rapid industrialization, the quest for better photocatalysts is getting intensified. Researchers around the globe are working on several photocatalysts, especially on various metal oxide compounds. Intervening the above research, ZnO is found to be a vital photocatalyst, despite its low chemical stability. Being an amphoteric oxide, ZnO possess good stability near neutral pH, whereas at extreme conditions (Low and high pH), its stability diminishes. ZnO exhibits key scientific and technological advantages such as the efficient charge separation, high photocatalytic activity, simple cost-effective synthetic methods resulting in varied morphological nanostructures, kindled the research towards ZnO nanostructures. However, to make it a commercially viable catalyst, its stability, suppression of charge carrier recombination and light absorption range to be improved [1].
Annealing is a heat treatment pathway often determines the crystal structure, morphology and properties of materials [2]. Numerous reports emphasized the importance of annealing. Kim et al., synthesized ZnO/TiO2 nanocomposites, found that the photocatalytic activity of the nanocomposites enhanced with increasing annealing temperature of the seed layer from 300°C to 500°C. The high activity of nanocomposites obtained from annealing the seed layer at 500°C was attributed to its high UV absorption ability, large surface area and copious oxygen defects which promoted separation of electron-hole pairs reducing electron recombination [3]. ZnO nanoparticles synthesized by Umar et al., via solution combustion method were subjected to annealing at different temperatures. It was found that the particle size increases from 40 nm to 120 nm with increase in annealing temperature. The ZnO nanoparticles annealed at 600 °C exhibited almost complete photodegradation of the DR-23 dye following a pseudo first order reaction kinetics [4]. Though the particle size increases with annealing temperature, the activity of photocatalyst is boosted because of its increased native defects, enhanced lifetime of charge carriers and higher quantum yield.
Herein, ZnO nanostructure were synthesized by hydrothermal method, then subjected to annealing at various temperatures, followed by characterization using XRD, UV-DRS, FTIR, FE-SEM and TEM. The photocatalytic activity of ZnO is evaluated by taking methylene blue (MB) as a model pollutant, under UV light.

Materials
All the chemicals were of analytical grade and used without any further purification. All solutions were prepared by deionized water. Zinc acetate (Zn(CH3COO)2.2H2O) and Methylene Blue (C16H18ClN3S) were purchased from Merck, Dodecylamine (C12H27N) and Diethylenetriamine (C4H13N3) were purchased from Hi Media Laboratories Pvt. Ltd.

Synthesis of ZnO nanoparticles
In the typical experiment, 0.1 mol of zinc acetate dihydrate was dispersed in 80ml of deionized water. The alkaline pH (pH 9-pH 11) is brought down by the addition of surfactants,

Characterization
The crystal structure of the samples was characterized using Rigaku X-ray powder diffraction (XRD) with Cu-Kα radiation of wavelength (λ) 0.15418nm. The optical studies were performed using JASCO V-670 double beam spectrophotometer and Bruker NEXUS 470 spectrometer for UV-DRS and FTIR, respectively. The morphology and particle size of the samples were visualized using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6390LV) and transmission electron microscopy (TEM, Hitachi H-600). The photocatalytic activity of nanoparticles was tested using lab-made photocatalytic reactor, having xenon lamp as UV light source.

Photocatalytic activity
10 ppm of methylene blue (MB) dye was prepared by dissolving 10 mg of MB in one litre of water. 50 mg of catalyst powder was added to 100 mL of 10 ppm MB solution taken in a specially designed 500 ml photo-reactor with a water circulating unit for cooling. The contents were stirred in dark for 30 min to reach the adsorption and desorption equilibrium using a constant magnetic stirring. After equilibrium, the MB dye with catalyst was irradiated with 400 W xenon lamp with UV filter and the samples were taken at regular interval of 10 min. Nearly, 2 mL of dye solution was pipette-out from the reactor and catalyst particles were separated from the solution by centrifugation at 5000 rpm for 5 min. The concentration of the dye solution was determined by measuring absorbance at 664 nm using a UV-Vis spectrophotometer (JASCO V-670). From the absorbance values, degradation efficiency of the catalyst was determined. The degradation efficiency of the dye i.e. % adsorption has been calculated using equation [17].
Where is the initial concentration of dye before photo irradiation and is the concentration of dye after photo irradiation.

x-ray diffraction
The XRD patterns of ZnO nanostructures annealed at 200 °C , 400 °C , 600 °C , 800 °C , 1000 °C was as shown in Fig.1. All the annealed nanostructures exhibited wurtzite structure with P63 mc space group, matches with JCPDS card No. 36-1451 [5][6][7][8]. It was found that the position of diffraction pattern varies a little due to change in strain with annealing. With increase in annealing temperature, peak broadening decreases, whereas peak intensity increases [9]. These changes in diffraction pattern are attributed to increased particle size and crystallinity of nanostructures.   Table 1 that the crystallite size of ZnO nanostructures increases with increase in annealing temperature. Apparently, the sign of the lattice strain (ε) is negative for small sized particles whereas the comparatively larger sized particles the sign changes. It is because the crystal lattice will be under the influence of compressive forces, when the lattice strain value is negative and tensile forces will act on the crystal lattice when the strain value is positive. According to Fig. 3 lattice strain changes the sign to positive, when the annealing temperature exceeds 400 °C. Our results are in concurrent with the report of Zak et al., [11] where crystallite size increases with annealing temperature and crystal is found to under compressive forces, in case of small crystallite size. Fig. 3. Plot of lattice strain vs crystallite size and annealing temperature. and bending vibration of results from moisture adsorbed at the surfaces [12]. The peaks at the range of 415-520 cm -1 are originated to the stretching vibration modes of Zn-O-Zn indicating to the complete transformation from zinc acetate to zinc oxide [13], [14]. The bands at 1300 and 1600 cm -1 is assigned to the C-O and C=O stretching modes, whereas, weak absorption bands at 2900 cm -1 is attributed to C-H stretching vibration. These vibrations (C-O, C=O, C-H stretching) might have aroused due to the presence of residues from raw materials.  shaped nanostructure and the size is calculated to be 300 nm (Fig. 5 (e-f)). On comparing all the FESEM images, it is found that the particle size increased rapidly with increase in annealing temperature. Further, it is noted that hierarchical cauliflower-like nanostructures consist of numerous small sized nanoparticles whereas, quasi spherical shaped nanostructure doesn't seem to have any small particles. Quasi spherical nanostructures might have resulted by the Ostwald ripening mechanism happened during annealing process. In the typical mechanism, bigger particle grown at the expense of smaller particles to form quasi spherical nanostructures. The thermal annealing process proves to be effective in tailoring the morphology and property of ZnO nanostructures.

TEM and HR-TEM analysis
The internal morphology of the 1000 o C ZnO samples were studied using TEM and HR-TEM analysis are shown in Fig. 4. The TEM images exhibited quasi spherical ZnO, which is good agreement with the FE-SEM images. The average particles size found to be 300nm for quasi sphere like ZnO. The measured lattice spacing value of 0.27 nm corresponds to the (0001) plane of the ZnO crystal ( Fig. 4(b)).

UV-Vis DRS analysis
The absorbance spectra as synthesized and annealed nanostructures were shown in Fig.7. The UV absorption peak of ZnO ranges from 363 nm to 380 nm. For as-synthesized nanostructures, the absorption peak is at 363 nm and it gradually increases with increasing temperature. The absorption peak is 380 nm for nanostructures annealed to 1000 °C. This shift in absorption peak is attributed to the formation of oxygen vacancies at higher temperature [15]. These oxygen vacancies will aid photocatalytic process for faster degradation organic dye. Based upon the absorbance spectra the optical band gap of the prepared nanoparticles can be calculated from energy and wavelength relation. The decrease in band gap may be due to the influence of various factors such as grain size, structural parameter and carrier concentration [16,17]. The calculated band gaps were 3.18eV, 3.16eV, 3.15eV, 3.13eV, 3.11eV and 3.10eV for as prepared ZnO, 200 ⁰ C, 400 ⁰ C, 600 ⁰ C, 800 ⁰ C and 1000 ⁰ C as shown in Fig. 7(b), respectively.

Photocatalytic activities
The photocatalytic performance of the ZnO and annealed ZnO were assessed against the photodegradation of MB dye under UV light irradiation. Fig. 8 shows the MB degradation with as-synthesized ZnO and annealed ZnO nanostructures. It shows the decrease in absorbance intensity of MB dye at regular time intervals. Fig. 9 shows the comparative study of degradation efficiency of as synthesized and annealed nanostructures. From the Fig.7, it is obvious that the photocatalytic degradation efficiency of nanostructures increases with annealing temperature and ZnO annealed at 1000 °C exhibited better degradation efficiency of 94% in 60 min. From XRD, FESEM and TEM measurements, it is found that the particle size of ZnO increases with increasing annealing temperature. In general, the surface-volume ratio of the larger particles will decrease thereby leading to low photocatalytic efficiency. But, in the present case, though the size of the

Recycle test
The stability and reusability properties of the prepared catalyst were investigated by cyclic test (photocatalytic degradation) experiments for ZnO annealed at 1000 o C against MB dye degradation. From the Fig. 11(A), it is evident that the degradation efficiency of catalyst against MB dye were good upto three cycles [23,24]. Further, the catalyst was subjected to XRD studies after each cycle and is shown in Fig. 9(B). There is no appreciable change in the diffraction pattern of catalyst. It shows that the catalyst possesses good stability.

Conclusion
ZnO nanostructures synthesized by hydrothermal method were subjected to annealing at various temperatures. The crystallite size of the nanostructures was calculated using Debye-Scherrer and it was compared with W-H plot. For samples with lower annealing temperature, crystallite size from Debye-Scherrer formula is higher, whereas, for samples with higher annealing temperature crystallite size from W-H plot is greater. This is because at lower annealing temperature the lattice strain is caused by compressive force and at higher annealing temperature the lattice stain is due to tensile force. The band gap of ZnO decreases with annealing temperature, and the ZnO nanostructure annealed at 1000 °C exhibits maximum red shift with a band gap of about 3.10 eV. The photodegradation efficiency of ZnO annealed at 1000 °C was approximately