Synthesis, characterization and photocatalytic activity of annealing dependent quasi spherical and capsule like ZnO nanostructures

Applied Surface Science 319 (2014) 221–229

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Synthesis, characterization and photocatalytic activity of annealing dependent quasi spherical and capsule like ZnO nanostructures Manoj Pudukudy a,b,∗ , Ain Hetieqa a , Zahira Yaakob a,∗∗ a Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM, Bangi, 43600 Selangor, Malaysia b Fuel Cell Institute, Universiti Kebangsaan Malaysia, UKM, Bangi, 43600, Selangor, Malaysia

a r t i c l e

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Article history: Received 2 June 2014 Received in revised form 5 July 2014 Accepted 10 July 2014 Available online 17 July 2014 Keywords: ZnO nanoparticles Optical studies Annealing temperature Crystallinity Photocatalysis Hydroxyl radicals

a b s t r a c t Quasi spherical and capsule like ZnO nanostructures have been successfully synthesized via a simple precipitation route without the assistance of external capping agents. The effect of annealing temperature on the properties of ZnO was investigated. In all cases, hexagonal wurtzite crystalline structure of phase pure ZnO was obtained. The crystallinity was found to be gradually increasing with annealing temperature. At low annealing temperatures, more or less spherical ZnO nanoparticles were clearly observed, whereas they tend to grow as nanocapsules with increasing the annealing temperature. The formation of single crystalline nanocapsules was observed at 600 ◦ C. The photoluminescence spectra indicated the annealing dependent emission features, especially in the spectral intensity. The dye pollutant methylene blue was found to be completely degraded under UV light irradiation over the ZnO nano photocatalysts. The highest photoactivity was shown by nanocapsules obtained at 600 ◦ C and was found to be highly reusable. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Dyes are one of the most important hazardous water pollutants with a detrimental effect on the ecosystem [1]. In recent years, many remedial techniques have been investigated. Among the various methods, the adsorption and photodegradation caught a much special attention for the treatment of dye polluted waste water [2,3]. Photocatalytic degradation of dyes using semiconductor metal oxides such as TiO2 , ZnO, Co3 O4 , etc. is an area of current research interest, since adsorption is not a fully validated method to remove the dyes completely [4,5]. The creation of electron–hole pairs by the excitation of electrons from valence band to the conduction band, caused by the light absorption of semiconductor metal oxides constitutes the basis of photocatalysis [6]. The ascreated electron–hole pairs enter the surface of the catalyst and initiate redox reaction with water and oxygen, which results in the

∗ Corresponding author at: Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM, Bangi, 43600 Selangor, Malaysia. Tel.: +60 163851604; fax: +60 389216148. ∗∗ Corresponding author. Tel.: +60 389216422; fax: +60 389216148. E-mail addresses: [email protected], [email protected] (M. Pudukudy), [email protected] (Z. Yaakob). http://dx.doi.org/10.1016/j.apsusc.2014.07.050 0169-4332/© 2014 Elsevier B.V. All rights reserved.

degradation of organic dye molecules adsorbed on the surface of the photocatalysts [7]. ZnO is one of the most extensively studied semiconductor photocatalysts with a versatile application due to its wide band gap energy of 3.37 eV and a binding energy of 60 meV [8]. In addition to the photocatalytic usage, it also have promising applications in various optoelectronic devices, actuators, gas sensors, solar cells, photo detectors, etc. [9–12]. For the synthesis of size and morphology controlled ZnO nanoparticles, much effort have been made by the researchers, since it has a direct influence on the electrical and optical properties of ZnO. Recently many synthetic methods such as hydrothermal, solvothermal, sol gel, chemical vapour deposition, precipitation, solid state mixing, sonochemical, thermal decomposition, etc. have been reported to fabricate ZnO nanoparticles with various morphologies including nanorods, nanospheres, nanocones, nanoflakes nanowires, nanobelts and different types of nano and micro flower-like structures [13–20]. Also been investigated that, a slight variation in the synthetic condition has the impact to alter its structural and morphological properties and also its bulk properties [21]. However the use of additional templates, surfactants and capping agents for the control of size, morphology and properties have been also investigated in detail [22,23]. Among the aforementioned synthetic routes, the precipitation method forms a simple and efficient method for the low cost and industrial scale production of ZnO without the use of special

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precursors and equipments [24,25]. There are numerous reports in literature about the synthesis of ZnO nanostructures by precipitation route using different precursors. However, the properties were found to have an influence on the synthesis conditions such as counter ions, solvents, pH, precipitants, etc. [26–29]. Annealing temperature is one of the important parameters concerning the synthesis and photocatalytic properties of ZnO. Previously Lv et al. [30] studied the effect of annealing temperature on the photocatalytic activity of ZnO thin films prepared by the sol–gel method. As per their report, the ZnO annealed at a temperature of 800 ◦ C provided highest photocatalytic activity for the degradation of methyl orange under UV irradiation, due to the increased surface-to-volume ratio, roughness, grain size and oxygen defects density. Fan et al. [31] also studied the effect of annealing temperature on the photocatalytic activity of ZnO–TiO2 based composite nanotubes. They reported that, among a set of annealed samples, the sample annealed at 600 ◦ C showed highest catalytic activity due to the enhanced formation rate of hydroxyl radicals by the increased crystallinity of the sample. As per the report, the recombination of electron–hole pair is reduced due to the decreased crystalline defects with increase in annealing temperature [32]. Lv et al. [33] also studied the influence of annealing temperature on the photocatalytic activity of TiO2 nanosheets and reported that, with increasing annealing temperature from 200 ◦ C to 600 ◦ C, the photocatalytic activity was decreased due to the reduced surface area by sintering, whereas at an annealing temperature of 700 ◦ C, it showed a maximum photocatalytic activity due to the enhanced crystallization of the sample rather than specific surface area. Yu et al. [34] studied the same with photoactive TiO2 prepared by a liquid phase deposition method and reported that, with increase in the annealing temperature, the photocatalytic activity was gradually increased due to the improved crystallization of the active anatase phase and a highest activity was shown by the sample calcined at 700 ◦ C. In this paper, we report on the synthesis, characterization and photocatalytic activity of ZnO nanoparticles obtained by the conventional precipitation route and the effect of annealing temperature on the structural, morphological and optical properties of ZnO was evaluated. Its influence on the photoactivity was also studied using methylene blue degradation under UV light irradiation over a low catalyst loading. 2. Experimental 2.1. Synthesis of ZnO nanoparticles All of the chemicals used in the experiments were of analytical grade (R&M chemicals, UK) and were used as received without further purification. In the typical procedure, 200 ml of 0.1 M NaOH solution was added drop wise to a beaker containing 1000 ml 0.1 M zinc nitrate solution. After the complete addition of precipitant, the solution was stirred for 1 h. Then the solution was kept for a while and clear solution was decanted. The precipitate was washed several times with distilled water, filtered and dried at 100 ◦ C for an overnight. Finally, it was annealed at different temperatures from 300 ◦ C to 600 ◦ C with an interval of 100 ◦ C to obtain ZnO nanoparticles. 2.2. Characterization of ZnO nanoparticles The synthesized ZnO nanoparticles were characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), Brunauer–Emmet–Teller (BET) and

Barrett–Joyner–Halenda (BJH) analysis. The XRD analysis was carried out in a Bruker D8 Focus powder diffractometer with Cu K␣ radiation wavelength of 0.15406 nm. The samples were scanned from 10◦ to 80◦ with a step size of 0.025◦ . The mean crystalline size of the samples was calculated from the full width at half maximum (FWHM) of the intense diffraction peaks using Scherrer’s equation. The FT-IR spectra of the dried and calcined samples were recorded in transmission mode using a thermoscientific NICOLET 6700 IR spectroscope in the region of 400–4000 cm−1 . The FESEM images for the determination of surface morphology were done on a Zeiss SUPRA 55 scanning electron microscope. The scanning electron micrographs were obtained at an operating voltage of 3 kV. The TEM studies of the calcined samples for the determination of internal morphology and particle size were carried out in a Philips CM-12 instrument at an accelerating voltage of 100 kV. The SAED analysis was performed in a HITACHI-88, HT 7700 TEM instrument, operated at 120 kV. The BET/BJH analyses were carried out in a Micrometrics ASAP 2010 instrument by nitrogen adsorption–desorption analysis at 77 K. The samples were degassed at 300 ◦ C for 6 h prior to the analysis. The specific surface area was assessed by BET method and the pore size and pore volume distributions were determined by the BJH method. The optical properties of the calcined samples were studied using UV–vis absorption and photoluminescence (PL) spectroscopy. The UV–vis absorbance spectra of the samples dispersed in water were recorded in a Perkin-Elmer Lambda-35 UV–vis spectrophotometer using water as the reference solvent. The PL spectra were taken in a PL-FLSP920 Edinburgh instrument, with an excitation wavelength of 300 nm. 2.3. Photocatalytic activities The photocatalytic activity of ZnO nanoparticles was investigated by the photocatalytic degradation of methylene blue in aqueous solution. The degradation experiments were carried out in a photoreactor (Rayonet type Photoreactor, Associate Technica, India) with 16, 8 W UV lamps ( = 352 nm, Hitachi 8 Watt, Hittach, Ltd., Tokyo, Japan). In the typical procedure, 50 ml of methylene blue dye solution (15 mg/L) was taken in a vertical tube of 100 ml capacity and 0.05 g ZnO photocatalyst was dispersed in it using an aerator for 20 min, so that the suspension attain adsorption–desorption equilibrium prior to degradation. Then the solution was irradiated under UV-light with continuous air bubbling as the potential oxidizing agent. After a particular duration of reaction, the suspension was centrifuged and its absorbance was measured to calculate the percentage of photodegradation using a UV–visible spectrophotometer (Varian Cary, Perkin Elmer Lambda35). The absorbance spectra were taken from 200 nm to 800 nm. The photodegradation efficiency was calculated according to the following equation: Degradation (%) = ((A0 − A)/A0 ) × 100, where A0 represents the initial absorbance of the dye solution and A represents the highest absorbance after UV irradiation. 3. Results and discussion 3.1. Characterization of ZnO nanostructures Fig. 1 represents the XRD pattern of the samples at different annealing temperatures. All of the diffraction peaks can be directly indexed to a hexagonal wurtzite crystalline structure of ZnO with the lattice parameters a = 0.3249 nm and c = 0.5206 nm and P63mcz space group (JCPDS: 01-036-1451). No other peaks were detected, indicating the high phase purity of the dried and calcined products. The sharp and strong diffraction peaks confirmed the high crystalline quality of the calcined samples. It is clear that the diffraction

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Fig. 1. XRD patterns of the as-synthesized samples.

peaks become intense and narrower with an increase in the annealing temperature, which indicates the increment in the crystallinity of the ZnO nanoparticles [25]. The line broadening at the base portion of the diffraction peaks indicated that the crystalline size of ZnO particles were very small and in nanorange. The average crystalline size of the ZnO nanoparticles at different annealing temperatures were calculated using Scherrer’s equation and were found to be 18 nm, 31 nm, 34 nm, 37 nm and 42 nm at 100 ◦ C, 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C respectively. The increment in the crystalline size is due to the much higher nucleation rate and growth of ZnO nanocrystallites by the availability of sufficient thermal energy [35]. The FTIR spectra of the samples are shown in Fig. 2. The well resolved wide and sharp band centered at around 485 cm−1 is ascribed to the stretching vibration of zinc–oxygen bond in transmission mode, confirms the formation of ZnO [36]. The weak band centered at 3490 cm−1 in the dried sample (100 ◦ C) is attributed to the stretching vibration of O H bond, which may be due to the presence of surface adsorbed water molecules or due to Zn(OH)2 .

Fig. 2. FTIR spectra of the ZnO nanoparticles at different annealing temperatures.

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Eventhough no characteristic diffraction peaks for zinc hydroxide was detected in the XRD of dried sample, the existence of zinc hydroxide cannot be discarded, because of the detection limit of XRD. Also it is evident that, the surface adsorbed water molecules were eliminated or Zn(OH)2 was completely converted to pure ZnO, when the annealing temperature was increased. The surface morphology of the ZnO nanostructures at different annealing temperatures were studied by FESEM and the results are shown in Fig. 3. The particles were found to be aggregated in a considerable manner. The sample dried at 100 ◦ C and calcined at 300 ◦ C shows the more or less spherical ZnO nanoparticles. However the temperatures higher than 300 ◦ C clearly show the elongation of quasi spherical nanoparticles, resulting in the formation of nanocapsules or nanorods. The size of the ZnO nanoparticles was seen to be increased to a much considerable extent during the annealing process by particle aggregation followed by their growth to capsule like nanostructures. This is mainly due to the tendency of particles to minimize their surface energy as reported by Raoufi [35]. At 400 ◦ C and 500 ◦ C, both quasi spherical or irregular and capsule like nanoparticles were observed in the SEM images. However, the formation of more resolved nanocapsule like structures was observed at 600 ◦ C. The length of the nanocapsules was found to be varied. For the size measurement of ZnO nanoparticles and nanocapsules, the TEM images were taken and the results are shown in Fig. 4. The measured size of ZnO nanoparticles annealed at 300 ◦ C were found to be ranged from 30 nm to 50 nm as shown in Fig. 4(a). However at 500 ◦ C, both nanoparticles and nanocapsules were clearly observed (Fig. 4(b)). The average length of the highly dense nanocapsules obtained at 600 ◦ C was measured to be around 200–600 nm with the width of 50–55 nm as shown in Fig. 4(c). The SAED pattern shown in the inset figure of Fig. 4(d) indicates sharp and well-defined diffraction spots, representing the single crystalline nature of ZnO nanocapsules growing along the c-axis [37]. The growth process can be explained on the basis of Ostwald ripening, arose due to the surface energy variation of ZnO nanoparticles [38,39]. At low temperatures, the kinetically favored small particles would aggregate together and tend to grow with increase in annealing temperature, due to the surface energy difference of particles originated from their size variations [39–41]. Or more clearly it can be said that, the aggregation of ZnO nanoparticles followed by their growth declines the interfacial free energy. However in the case of ZnO, the crystal growth velocities were found to be in the following ¯ > V{0 1 1¯ 0} > V{0 1 1¯ 1} > V{0 0 0 1}. ¯ order, i.e. V{0 0 0 1} > V{0 0 1¯ 1} Because of the higher symmetry of {0 0 1} facet, the particles prefer their growth along the polar +c-axis or in (0 0 0 1) direction. That is why many of the aggregated pseudo spherical particles were converted into single crystalline nanocapsules by their oriented growth in [0 0 0 1] direction with the increase in annealing temperature [42] and it can be found that the growth is more pronounced at 600 ◦ C. The textural properties of some representative samples were studied using nitrogen sorption analysis. The isotherms and pore size distribution curves of ZnO nanoparticles annealed at different temperatures are shown in Fig. 5. The sorption isotherms belong to type IV with H3 hysteresis loop, indicating the presence of a mesoporous texture in the samples [43]. The hysteresis loop is mainly observed at the higher relative pressures, i.e. (P/P0 ) value of around 0.9–1, which is mainly due to the sorption of mesopores by nitrogen via capillary condensation. The BJH plots shown in the inset figures represent the cumulative pore size distribution, which confirms the presence of mesopores (2–50 nm in size) in the samples. The size of pores falls in the range of 10–50 nm and which are attributed to the empty voids created by the close aggregation

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Fig. 3. FESEM images of the ZnO samples at different annealing temperatures.

Fig. 4. TEM images of the ZnO nanoparticles at (a) 300 ◦ C (b) 500 ◦ C and (c, d) 600 ◦ C (the inset figure shows its SAED pattern).

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Fig. 5. BET isotherms and BJH plots of the ZnO samples at (a) 300 ◦ C; (b) 400 ◦ C and (c) 600 ◦ C.

of nanoparticles. However, in the case ZnO annealed at 600 ◦ C, macropores were also observed. The mesopores were found to be formed by the inter-aggregation of nanoparticles as observed in the electron microscopic images. The specific surface area of ZnO nanoparticles measured by the BET method was found to be 12.2, 8.4, and 5.8 m2 /g for the calcination temperatures of 300 ◦ C, 400 ◦ C and 600 ◦ C respectively. The decrease of the specific surface area of the samples with annealing may be due to the increased particle size aroused by the grain growth of particles by sintering [33]. The optical properties of the ZnO nanostructures at different annealing temperatures were studied by UV–vis and photoluminescence spectroscopy. The UV–vis absorbance spectra of the as-synthesized samples are shown in Fig. 6. From the figure, it is clear that the annealing temperature plays a significant role in the optical properties, especially in the absorption characteristics of the samples. A strong absorption band was observed in the UV region of all the samples, which is attributed to the band edge absorption of hexagonal wurtzite crystalline structure of ZnO [44]. The band edge absorption wavelength was found to be increasing with increase in annealing temperature. This is a clear evidence for the decreased band gap energy of the samples with increasing

annealing temperature [45]. Also it is observed that the absorption peak intensity was decreasing with increase in annealing temperature. These results were in good agreement with the previous reports [46]. The band gap energy of the samples was calculated using the following equation, Eg = 1240/, where Eg is the band gap energy in eV and  is the wavelength in nanometers [47]. The calculated values of Eg were found to be 3.36 eV, 3.32 eV, 3.30 eV, 3.27 eV and 3.23 eV for the annealing temperatures of 100 ◦ C, 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C respectively. The decreased band gap energy of ZnO with increase in annealing temperature can be attributed to the increased crystallinity and large particle size of the ZnO [48,49]. Photoluminescence spectroscopy is an important technique to study the efficiency of charge carrier trapping, transfer and electron hole pair recombination rate of semiconductors. Therefore, the optical properties of ZnO nanostructures were also studied using PL spectroscopy and the spectra are shown in Fig. 7. All of the samples shown similar emission features, whereas the intensity of emission bands was found to be annealing dependent. The intense band centered at 403 nm was attributed to the near band-edge emission generated by the free-exciton recombination [50,51]. However, this band was found to be quite shifted from 397 nm, by a

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Fig. 6. UV–vis absorbance spectra of the ZnO nanostructures.

factor of approximately 6 nm, which is due to the radiative defects associated with the boundary traps and the resultant radiative transition to the valence band [52]. With increase in annealing temperature, the near band edge emission (UV) intensity was found to be getting lowered, which is due to the decreased electron-pair recombination of the sample [53]. The other blue–green emission bands centered at ∼439 nm, ∼452 nm, ∼468 nm and ∼486 nm were attributed to the presence of high density of surface defects including oxygen vacancies, zinc vacancies, oxygen interstitials and zinc interstitials in the samples [54,55]. Previously Vishwas et al. [56] reported that the movement of zinc and oxygen atoms in the interstitial and lattice sites can be promoted with increase in annealing temperature and which further drops the electron hole pair recombination rate [57]. 3.2. Annealing dependent photocatalytic activity of ZnO nanostructures The influence of annealing temperature on the photocatalytic activity of ZnO nanostructures was studied using methylene blue

Fig. 7. PL spectra of the ZnO nanostructures.

Fig. 8. (a) Effect annealing temperature on the photodegradation efficiency and (b) change in the absorbance spectra of MB solution with time.

degradation under UV-light irradiation and the results are shown in Fig. 8(a). First of all, a blank experiment without any photocatalysts was conducted under UV light irradiation to validate the potentiality of semiconductor photocatalysis. The results indicated that the self-photolysis of methylene blue was negligible and 9% degradation was noted in a reaction period of 70 min. However, the addition of as-prepared ZnO nano photocatalysts increased the degradation rate considerably. The sample dried at 100 ◦ C showed a maximum degradation of 57% within 70 min. On the other hand, the annealed ZnO nanoparticles provided better photocatalytic activity with slight variation in their efficiency as shown in the degradation plot. From the figure, it is clear that the photocatalytic activity increases with increasing annealing temperature and the order of efficiency was found to be 600 ◦ C > 500 ◦ C > 400 ◦ C > 300 ◦ C > 100 ◦ C. Within 50 min, 100% degradation was achieved by ZnO annealed at 600 ◦ C. This is due to the high crystallization of the sample with increased annealing temperature. Previously it is reported that the surface area and crystallinity plays a crucial role in the photocatalytic activity of the catalysts [58]. But in the present case, it has been observed that the crystallinity plays a predominant role in photocatalytic activity rather than the specific surface area. The crystallinity provides more exposed polar faces for the formation of hydroxyl radicals, which are responsible for the degradation of dye molecules [59–61].

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Fig. 9. Schematic representation of the photocatalytic reaction.

The variations in the photocatalytic activity of calcined samples may be attributed to the variations in their extent of optical characteristics such as crystal defects and band gap energies as shown in the PL and UV results. The photoactivity showed a reverse correlation with the PL spectral intensity [62]. It can be said that the higher the near-band edge emission intensity, and then lower is the activity, which is due to the high rate of electron hole pair recombination [53]. The change in the absorbance spectra of methylene blue solution over the ZnO nanoparticles annealed at 600 ◦ C with time is shown in Fig. 8(b). The lowering of absorbance was due to the decrease of dye concentration by the destruction of dye molecules via photooxidation [63]. The mechanism of photocatalytic reaction over the ZnO nano photocatalysts is represented in Fig. 9. When ZnO nanoparticles are irradiated with UV light of proper energy or higher than its band gap energy, an electron–hole pair was created due to the transfer of electrons from the valence band to the conduction band. The holes and electrons created in the valence and conduction band migrate to the ZnO surface and react with water and oxygen to create the reactive radical species such as hydroxy radicals and superoxide radical anions [64] by the redox reaction. The holes present in the valence band reacts with water and produces hydroxyl (*OH) radicals by oxidation, whereas the electrons present in the conduction band react with the oxygen from air and generates hydroxyl and superoxide anion radicals by reduction. These radicals actively participate in the oxidative degradation of dye molecules by the following equation [65]. (• O2 − ) + H2 O → H2 O2 → 2(• OH)

Fig. 10. Changes in the PL spectra of 0.001 M coumarin solution over 0.05 g ZnO annealed at 600 ◦ C under UV irradiation with time.

Fig. 11. Schematic representation of the formation of 7-hydroxycoumarin.

Fig. 8(a), it exhibited a superior photocatalytic activity compared to the prepared ZnO photocatalysts. For a dye concentration of 15 mg/L, complete degradation was noted within 30 min of UV light irradiation. However, the separation of the TiO2 powder from the final solution is found to be much more difficult compared to the ZnO samples due to the small particle size and low sedimentation of commercial TiO2 powder. In addition, several recycling experiments were conducted to demonstrate the environmental potentiality of the as-prepared samples. One of the significant advantages of heterogeneous photocatalysts is its recycling nature for consecutive runs and which would reduce the quantity of material usage and total cost of the process [68,69]. Therefore, the

(• OH) + Dye → Dyeox → intermediates → CO2 + H2 O The formation of hydroxyl radicals over the ZnO nanocapsules obtained at 600 ◦ C under UV light irradiation was detected by the photoluminescence technique reported by Xiang et al. [66,67], using coumarin as a probe molecule. The procedure for the determination of hydroxyl radicals was based on their report. For the experiment, 0.05 g of ZnO annealed at 600 ◦ C was dispersed in 50 ml of 0.001 M coumarin solution and the same procedure was done to perform the reaction as mentioned in Section 2.3. After reaction, the solution was centrifuged and its PL spectra were recorded with an excitation wavelength of 330 nm. The PL spectra at different irradiation intervals over the ZnO nanocapsules are shown in Fig. 10. It can be seen that by UV-light irradiation, a band was found to be formed at around 455 nm. This is attributed to the formation of 7-hydroxy coumarin by the reaction between photogenerated hydroxyl radicals and coumarin (Fig. 11). With increase in the irradiation time, the peak intensity was found to be increased sharply, indicating the high formation rate of hydroxyl radicals with time. The results are in good agreement with Xiang et al. [66]. The photocatalytic activity of the prepared systems was also compared with the commercial TiO2 (Degussa-P25). As shown in

Fig. 12. Reusability of the ZnO nanocapsules obtained at 600 ◦ C (50 ml 15 mg/L MB, 0.05 g ZnO and continuous air bubbling for 60 min under UV light irradiation).

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used photocatalysts obtained after the degradation experiments were washed several times with distilled water, filtered, dried and further utilized. The ZnO nanocapsules were found to be highly reusable for 5 successive runs as shown in Fig. 12, which depicts the high photostability of catalysts against photocorrosion [70]. A small diminishing effect was observed in each run, which may be due to the loss of catalyst by various steps in the catalyst recovery process, but it can be neglected concerning the overall performance of the photocatalyst [71,72]. 4. Conclusion The quasi spherical and capsule like ZnO nanostructures have been successfully fabricated by the precipitation route at different annealing temperatures. The annealing temperature is found to have an enhanced impact on the properties of ZnO. It increased the crystalline quality significantly. The quasi spherical nanoparticles were changed to nanocapsules with the increase in annealing temperature. The PL spectral intensity was found to be decreased with increase in annealing temperature, indicating the reduced rate of electron–hole pair recombination. The photocatalytic degradation of methylene blue was effectively done over the as-prepared ZnO nanostructures and found that the degradation efficiency was increased with annealing temperature. The ZnO nanocapsules obtained at 600 ◦ C provided superior photoactivity with high reusability. Acknowledgements The authors would like to acknowledge UKM, grant numbers UKM-DIP-2012-04 and UKM-OUP-2012-074 for financial support and FST and CRIM for material analysis. Manoj special thanks to Ms. Hemavathi, Department of Chemical and Process Engineering, UKM for the language support. References [1] G.Z. Kyzas, N.K. Lazaridis, Reactive and basic dyes removal by sorption onto chitosan derivatives, J. Colloid Interface Sci. 331 (2009) 32–39. [2] Y. Qiu, M. Yang, H. Fan, Y. Xu, Y. Shao, X. Yang, S. Yang, Synthesis and characterization of nitrogen doped ZnO tetrapod sand application in photocatalytic degradation of organic pollutants under visible light, Mater. Lett. 99 (2013) 105–107. [3] V.P. Mahida, M.P. Patel, Removal of some most hazardous cationic dyes using novel poly (NIPAAm/AA/N-allylisatin) nano hydrogel, Arab. J. Chem. (2014), http://dx.doi.org/10.1016/j.arabjc.2014.05.016. [4] J. Yu, X. Yu, Hydrothermal synthesis and photocatalytic activity of zinc oxide hollow spheres, Environ. Sci. Technol. 42 (2008) 4902–4907. [5] Z. Liu, W. Xu, J. Fang, X. Xu, S. Wu, X. Zhu, Z. Chen, Decoration of BiOI quantum size nanoparticles with reduced graphene oxide in enhanced visible-lightdriven photocatalytic studies, Appl. Surf. Sci. 259 (2012) 441–447. [6] M. Ahmad, E. Ahmed, Z.L. Hong, J.F. Xu, N.R. Khalid, A. Elhissi, W. Ahmed, A facile one-step approach to synthesizing ZnO/graphene composites for enhanced degradation of methylene blue under visible light, Appl. Surf. Sci. 274 (2013) 273–281. [7] N.R. Khalid, E. Ahmed, Z. Hong, M. Ahmad, Synthesis and photocatalytic properties of visible light responsive La/TiO2 -graphene composites, Appl. Surf. Sci. 263 (2012) 254–259. [8] C. Klingshirn, ZnO: material, physics and applications, ChemPhysChem 12 (2007) 782–803. [9] B. Li, Y. Wang, Hierarchically assembled porous ZnO microstructures and applications in a gas sensor, Superlattices Microstruct. 49 (2011) 433–440. [10] W. Wu, S. Bai, N. Cui, F. Ma, Z. Wei, Y. Qin, et al., Increasing UV photon response of ZnO sensor with nanowires array, Sci. Adv. Mater. 2 (2010) 402–406. [11] U. Periyayya, J.H. Kang, J.H. Ryu, C.H. Hong, Synthesis and improved luminescence properties of OLED/ZnO hybrid materials, Vacuum 86 (2011) 254–260. [12] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, et al., Controlled growth of ZnO nanowires and their optical properties, Adv. Funct. Mater. 12 (2002) 323–331. [13] K. Prabakar, H. Kim, Growth control of ZnO nanorod density by sol–gel method, Thin Solid Films 518 (2010) 136–138. [14] W.D. Zhou, W. Xiao, Y.C. Zhang, M. Zhang, Solvothermal synthesis of hexagonal ZnO nanorods and their photoluminescence properties, Mater. Lett. 61 (2007) 2054–2057.

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