The photocatalytic activity of ZnO prepared by simple thermal decomposition method at various temperatures

The photocatalytic activity of ZnO prepared by simple thermal decomposition method at various temperatures

Journal of Molecular Liquids 177 (2013) 394–401 Contents lists available at SciVerse ScienceDirect Journal of Molecular Liquids journal homepage: ww...

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Journal of Molecular Liquids 177 (2013) 394–401

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

The photocatalytic activity of ZnO prepared by simple thermal decomposition method at various temperatures R. Saravanan a, E. Thirumal a, V.K. Gupta b, c, V. Narayanan d, A. Stephen a,⁎ a

Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai-600 025, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247 667, India King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia d Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai-600 025, India b c

a r t i c l e

i n f o

Article history: Received 18 September 2012 Received in revised form 1 October 2012 Accepted 9 October 2012 Available online 22 October 2012 Keywords: Photocatalytic activity UV-light Nanorod Thermal decomposition method

a b s t r a c t Nanorods of pure ZnO were synthesized by simple thermal decomposition method. The ZnO is prepared by direct calcination of zinc acetate dihydrate. The hexagonal structure of ZnO is confirmed by X-ray diffraction. The nanorods shape and size have been identified through SEM and TEM analyses. The surface component and the oxidation states of ZnO sample were investigated using XPS. The calculated bandgap values of ZnO suggest that the photocatalytic activity may be good under UV light irradiation. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The broad range of chemical contaminants produced by industry and agricultural activities has become an important issue that affects the ecological safety. People are exposed to a spectrum of pollutants present within the environment. The major pollution which affects the environment is water pollution. The harmful effluents from textile and other industries are released directly or indirectly into water sources [1–22]. One of the best ways to reduce contamination of water is by photocatalytic treatment. Photocatalysis is a unique process for rectifying energy and environmental issues. During the last two decades, semiconductor metal oxides and sulphides were mostly used in photocatalytic activity. Zinc oxide (ZnO) is a functional semiconducting material that has interesting properties such as non-hazardous, good chemical and thermal stability. In addition, ZnO has shown a great deal of research interest in the field of catalysts, sensors, DSSCs and antibacterial due to some of its fascinating properties. ZnO has received a considerable attention as a cost effective alternative to other oxides [23–26]. Number of methods has been used to synthesize various nanostructures of ZnO including the hydrothermal process, vapour phase growth, chemical solution route, thermal evaporation, sol–gel process, etc. ZnO has been synthesized with a variety of well defined nanostructures with various morphologies such as nanospheres ⁎ Corresponding author at: Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai-600 025, India. Tel.: +91 44 2220 2802; fax: +91 44 22353309. E-mail address: [email protected] (A. Stephen). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.10.018

[27], nanowires [28], nanorods [29], nanonails [30] and nanobridges [30]. In this paper, ZnO is prepared by simple thermal decomposition method at different temperatures. The prepared samples were characterized by various physical and chemical techniques. Finally ZnO samples were used as a catalyst to degrade the organic pollutants. The degradation efficiency of the ZnO samples prepared at three different temperatures was compared and the results are discussed in detail. 2. Materials and methods Zinc acetate dihydrate (Rankem) used in the present study was of analytical reagent grade without further purification. Methylene blue (MB) and methyl orange (MO) were purchased from Aldrich chemicals. All aqueous solutions were prepared by using double distilled water. 2.1. Thermal decomposition method Thermogravimetric analysis (TGA) was first performed using zinc acetate dihydrate for a better understanding of its thermal stability and decomposition temperature. The samples were analysed with a heating rate of 10 °C per minute. The TGA thermogram of zinc acetate dihydrate at the temperature range of 50–700 °C is shown in Fig. 1. Initial weight loss of 16% at 170 °C is due to dehydration, yielding anhydrous zinc acetate. A weight loss of 52%, in the range of 171–312 °C indicated the decomposition of acetate and the formation of zinc oxide. This helped to determine the residual zinc oxide value

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2.2. Self designed UV light photocatalytic reactor The experiment was carried out in a 600 mL capacity borosilicate cylindrical beaker surrounded by water jacket. The beaker placed on a magnetic stirrer. There was an additional opening in the cylindrical beaker for collection of solution from the reactor from time to time. During the irradiation time, the reaction was maintained at 30 °C to 33 °C temperature with the help of water circulation continuously through the double-walled water jocket (preventing from thermal catalytic effect). The cylindrical beaker completely wrapped by an aluminium foil for reflect UV light irradiation of back into the solution and exerting the outer wall of the reaction. The radiation source was a low-pressure 8 W mercury vapour lamp emitting ultraviolet (UV) radiation with a wave length of 365 nm. The lamp was installed into a quartz glass tube and fitted the inner cylindrical beaker with use of lid. The quartz glass tube acted as sheathes of the lamp and to protect it from direct contact with the aqueous solution. The detailed schematic view of the experimental set-up is illustrated in Fig. 2. The complete reactor setup is maintained in dark box. Power supply to the UV lamp is controlled by a timer circuit so that the lamp will get on and off automatically after every irradiation time intervals.

Fig. 1. Thermogravimetric analyses for zinc acetate dehydrate.

of 32% which coincided with the calculated theoretical residual value of 37.5% when ZnO was the only residue. The 5.5% weight difference is due to the sublimation of zinc acetate species or zinc organic composition such as Zn4O(CH3CO2)6 [31–33]. There was no further decomposition beyond 312 °C. The process resulted in the synthesis of ZnO with the total weight loss being 68%. The decomposition temperature determined in this analysis was used for fixing the temperature for the synthesis of ZnO. 5.0 g of zinc acetate dihydrate was taken in the mortar and ground well for 1 h then the powder was calcined in an alumina crucible at different temperatures 350 °C, 650 °C and 950 °C for 3 h. The ZnO were formed by the following chemical reaction. Δ

ZnðCH3 COOÞ2  2H2 O →ZnðCH3 COOÞ2 þ 2H2 O↑ Δ

4ZnðCH3 COOÞ2 þ 2H2 O → Zn4 OðCH3 COOÞ6 þ 2CH3 COOH↑ Δ

Zn4 OðCH3 COOÞ6 þ 3H2 O → 4ZnO þ 2CH3 COOH↑ Δ

Zn4 OðCH3 COOÞ6 → 4ZnO þ 2CH3 COOH↑ þ 3CO2 ↑

ð1Þ ð2Þ ð3Þ ð4Þ

2.3. Photocatalytic testing In order to show their potential environmental application for the degradation of two water soluble dyes such as methylene blue (MB) and methyl orange (MO) using ZnO catalyst under UV light irradiation. Reaction suspensions were prepared by adding 500 mg catalyst into 500 mL of MB/MO solution with an initial concentration of 3 × 10 −5 mol/l. Before photocatalytic run was started, the aqueous suspension containing MB/MO and the catalyst were stirred under dark conditions without light illumination and found that the aqueous suspension did not change the colour. When the UV light was irradiated in the absence of ZnO, still there was no change in the colour of dye. Therefore, the presence of both irradiation and ZnO was essential for the efficient degradation. During the photocatalytic experiments, the reaction mixtures were stirred for 30 min under dark condition to establish adsorption–desorption equilibrium condition. The samples from the suspension were collected at regular intervals of time, centrifuged and filtered. The concentration of MB and MO in each sample was analysed using UV–visible spectrophotometer at a wavelength of 664 and 464 nm respectively. All UV–visible

Fig. 2. Schematic diagram of the UV-light photocatalytic experimental set-up.

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measurements were carried out at room temperature. The degradation percentage is calculated from the expression η ¼ ð1−C=C0 Þ  100

Table 1 The structural information, crystallite size, bandgap and surface area of ZnO samples.

ð5Þ

where C0 is the concentration of MB or MO before illumination and C is the concentration after a certain irradiation time. 3. Result and discussion 3.1. X-ray diffraction Fig. 3(a) shows the XRD pattern of the starting material zinc acetate dihydrate. All the peaks of zinc acetate dihydrate are well matched with JCPDS no 33-1464. The starting material was calcined for 3 h at different temperatures like 350 °C, 650 °C and 950 °C, and the corresponding XRD patterns are obtained as shown in Fig. 3(b), (c) and (d) respectively. After calcinations, the zinc acetate dihydrate peaks are vanished and these have been visibly observed from the XRD pattern. The peaks of the different calcined samples are indexed and the results showed the formation of hexagonal structure of ZnO without any impurities according to the JCPDS no: 79-0208. Hence the XRD results showed the change of zinc acetate dihydrate to ZnO successfully. The structural parameters and the crystallite size of calcined samples are tabulated in Table 1. The crystallite size of the prepared samples was calculated using Scherrer's formula [34–37]. D ¼ kλ=β cosθ where k λ

is the shape factor, is the wavelength of X-ray,

ð6Þ

β θ

Calcinations temperature

350 °C for 3 h

650 °C for 3 h

950 °C for 3 h

JCPDS no Structure Lattice parameters Crystallite size BET surface area Band gap value

79-0208 Hexagonal a(Å) c(Å) 3.262(8) 5.206(3) 24 nm 8.6 m2g−1 ~3.44 eV

79-0208 Hexagonal a(Å) c(Å) 3.235(9) 5.154(3) 37 nm 5.4 m2g−1 ~3.26 eV

79-0208 Hexagonal a(Å) c(Å) 3.233(9) 5.150(3) 52 nm 1.2 m2g−1 ~3.18 eV

is the line broadening at full width half maximum (FWHM) intensity in radians and is the Bragg's angle.

The crystallite size increases with increasing temperatures and this similar result was already reported by other groups [4,38–41]. The ZnO calcined at 350 °C have broader peaks and weaker intensities compared with other two temperatures. The direct observation of XRD pattern indicates that when the temperature is increased, the diffraction peaks are narrow because of the growth of crystallites and the improvement of crystallisation. In other words, the crystallite size is inversely related to the full width half maximum (FWHM) of an individual peak in the diffraction pattern. If the peaks are narrow, the FWHM decreases and hence the crystallite sizes are larger. The periodicity of the individual crystallite domains reinforces the diffraction of the X-ray beam, resulting in a narrow peak. In contrast; if the peaks are broader, the crystals are randomly arranged or have low degrees of periodicity. This is generally in the case of nanomaterial assemblies. Thus, it is obviously confirmed that the FWHM of the diffraction peak is related to the size of the nanomaterials.

Fig. 3. X-ray diffraction pattern of (a) zinc acetate dihydrate, (b) ZnO calcined at 350 °C, (c) ZnO calcined at 650 °C and (d) ZnO calcined at 950 °C.

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3.2. XPS measurements The surface components and oxidation states of ZnO prepared at 350 °C for 3 h sample was investigated by XPS analysis and the survey spectrum is shown in Fig. 4(a). The results indicate the presence of Zn, O and C. The presence of C comes from the contamination of hydrocarbon which is a common feature of XPS analysis. No peak for other

Fig. 4. (a) XPS Survey spectrum of ZnO prepared at 350 °C, (b) the high resolution XPS spectrum of zinc and (c) the high resolution XPS spectrum of oxygen.

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elements was observed. The binding energies in the XPS spectra were calibrated using C 1s (284.8 eV). The high resolution spectrum shown in Fig. 4(b) indicates the binding energy 1023 eV and 1046 eV for 2p3/2 and 2p1/2 of Zn 2+ states in ZnO nanorods. Fig. 4(c) indicates two types of oxygen species in the samples; first one is around 531 eV which should be associated with lattice oxygen of ZnO and another one around 532.5 eV for the oxygen caused by the surface hydroxyl group [42]. Presence of surface hydroxyl groups facilitates the trapping of photoinduced electrons and holes, thus enhance the photocatalytic degradation process [43]. Hence, the XPS result confirms 2 + oxidation state of Zn.

Fig. 5. SEM image of ZnO calcinated at (a) 350 °C, (b) 650 °C and (d) 950 °C.

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3.3. Scanning electron microscopy The surface morphology of ZnO is viewed through SEM images. All the SEM images were carried out in same magnification at the scale of 1 μm. The SEM micrographs of ZnO powder synthesized at various calcination temperatures are shown in Fig. 6. The ZnO sample calcinated at 350 °C for 3 h denotes that the sample contains large amount of nanorods. This is apparently shown in Fig. 5(a). When the temperature was raised to 650 °C for 3 h, the image (Fig. 5(b)) indicates the nanorods with increase in diameter and decrease in length. Further, the rise in temperature to 950 °C for 3 h represents that the morphology of ZnO is irregular (Fig. 5(c)). As the calcination temperature increases, the particle morphology changes from nanorod to irregular shape. This is because of the higher temperature afforded anisotropic growth on ZnO particles. Thus, the higher temperature promotes the disorder movement of the particles [44]. Therefore, the SEM images revealed that the calcination temperature is one of the essential factors to influence the surface morphology. Fig. 7. UV–vis absorption spectra of ZnO.

3.4. Transmission electron microscopy The TEM image of ZnO synthesized at 350 °C for 3 h by thermal decomposition method is shown in Fig. 6(a). The image indicates the irregular one dimensional ZnO nanorods. The average diameter of the nanorods range approximately 35 nm and the average length is 350 nm. The aspect ratio (the ratio of the length to the diameter) of the nanorods is 0.1. The chemical composition of the prepared sample was analysed by the EDX, and the result showed (Fig. 6(b)) the existence of Zn and O in the rods. 3.5. UV-absorption spectra The absorption spectra of ZnO are shown in Fig. 7. It can be seen that the absorption edge of ZnO sample synthesized at 350 °C for 3 h exhibits blue shift compared with other two temperatures. The blue shift denotes the decrease in size of the particle and increase in band gap energy. The bandgap energy (Ebg) of ZnO is calculated from the following equation [45–47]. Ebg ¼ 1240=λ

ðeVÞ

ð7Þ

where, Ebg λ

is the bandgap energy in eV and is the wavelength in nanometers.

The bandgap (Ebg) values of ZnO are 3.44 eV, 3.26 eV and 3.18 eV for the temperatures 350 °C, 650 °C and 950 °C respectively; and the corresponding wavelength exists in the UV region. Hence the wavelength of light is essential for the effective photocatalytic process. The result of the absorption spectra indicates that the UV light is an excellent source for the photocatalytic activity of ZnO.

3.6. Photocatalytic degradation of MB and MO solution The main aim of the present study is to degrade the organic pollutants. For this purpose, methylene blue (MB) and methyl orange (MO) were used as model organic pollutants. The photocatalytic activity is performed using UV light irradiation because the bandgap lies between

Fig. 6. (a) The TEM image of ZnO prepared at 350 °C and (b) EDX spectrum of ZnO prepared at 350 °C.

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use of ZnO catalyst at 350 °C, 650 °C and 950 °C respectively. In Fig. 8, disappearance of the band at 664 nm indicates that most of the MB has been degraded by ZnO under UV light irradiation within 2 h time. This absorption of MB decreases with increase in irradiation time and hence the degradation efficiency increase. Fig. 9(a), (b) and (c) shows the absorption spectra of MO using ZnO catalyst at 350 °C, 650 °C and 950 °C respectively. The disappearance of the band at 464 nm indicates that MO has been photodegraded by ZnO within 2 h time. The time course degradation curve of MB and MO are shown in Figs. 10 and 11. The degradation efficiency of the catalyst was calculated using Eq. (5) and the values are tabulated in Table 2. From the table, the ZnO sample prepared at 350 °C for 3 h exhibits the highest photocatalytic activity. Both the dyes (MB and MO) were irradiated by UV light for 2 h. But the degradation efficiency of methylene blue is higher than methyl orange. Since methylene blue is acting simultaneously as sensitizer of the photocatalyst, its degradation rate is faster over the other methyl orange. Few research groups have used ZnO for the degradation of methylene blue and methyl orange under UV light irradiation previously [48–51]. Kim et al. reported that the rod-shape ZnO more effectively decomposed 90% of MB in 120 min than the other ZnO powders. The higher activity of the rod shaped ZnO is due to its increased absorption property [52]. Delgado et al. reported the ZnO thin film synthesized by sol–gel method and that the photocatalytic degradation of MB was achieved with 5 h of UV light irradiation [53]. Recently, Lv et al. reported the ZnO thin film prepared by sol–gel method at different temperature (200 °C–900 °C) and the results indicates the photocatalytic activity of ZnO 800 °C samples shows higher degradation (88%) of MB within 180 min, this is due to grain size and oxygen deficiency of sample [54]. This variation in degradation time is due to the difference in synthesis method, the particle size, crystallinity of the catalyst and the surface area. Compared with other reports, ZnO sample was prepared by this thermal decomposition method shows higher degradation efficiency. At the same time compared to the other methods mentioned in previous reports, this thermal decomposition method is simple, fast, cost-effective and also the sample have attractive morphology The photocatalytic reaction mechanism is represented in the schematic diagram as shown in Fig. 12. When the energy of UV light in terms of photon is equal or greater than the bandgap of ZnO, the electrons receive the energy and transfer of electrons takes place from valence band (VB) to conduction band (CB) which results in the formation of a hole (h+) in the VB and an electron (e−) in the CB. The holes react with water and generate OH radical, which can oxidize the organic pollutants. The conduction band electron reacts with oxygen in the reduction process and produces OH radical. These radicals reduce the organic pollutants. This oxidation and reduction processes were capable of degrading the organic pollutants under UV light irradiation. The photocatalytic reaction mechanism is explained by the following equation based on earlier reports [1].

Fig. 8. The change in absorption spectra of MB using ZnO prepared at (a) 350 °C, (b) 650 °C and (c) 950 °C under UV light illumination.

(3.44 eV to 3.17 eV) UV region. The different calcination temperatures of ZnO samples by thermal decomposition were examined to find out the photodegradation of MB and MO solution. The required amount of the catalyst is mixed with MB/MO aqueous solution under stirring was irradiated by UV light for uniform time intervals. The resulting irradiated samples were centrifuged, filtered and the absorbance measurements were carried out using UV–vis spectrophotometer. The change in absorption spectra of MB were shown in Fig. 8(a), (b) and (c) with the

  − þ ZNO þ UV lightð365 nmÞ→ZnO ecb þ hvb

ð8Þ

  þ Dye þ ZnO hvb →Oxidation process

ð9Þ

  þ þ ZnO hvb þ H2 O→ZnO þ H þ OH·

ð10Þ

  þ − ZnO hvb þ OH →ZnO þ OH·

ð11Þ



Dye þ ZnOðecb Þ→Reduction process −



ZnOðecb Þ þ O2 →ZnO þ O·2 −

þ

O·2 þ H →HO·2

ð12Þ ð13Þ ð14Þ

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Fig. 10. The time course degradation curve of MB at 664 nm.

Fig. 11. The time course degradation curve of MO at 464 nm.

Basically for semiconducting materials, the crystallinity could be promoted by high temperature [4,30]. On this basis 950 °C sample should give higher crystallinity as well as good photocatalytic activity compared with other two temperatures. But the present photocatalytic efficiency was in contrary for higher temperatures due to the specific surface area. As the calcinations temperature is increased, the BET specific surface area decrease and this is evidently shown in Table 1. Therefore both the crystallinity and surface area are the essential factors for photocatalytic activity. From this result, it is understood that the ZnO at 350 °C for 3 h sample have good crystallinity and higher surface area. Fig. 9. The change in absorption spectra of MO using ZnO prepared at (a) 350 °C, (b) 650 °C and (c) 950 °C under UV light illumination.

HO·2 þ HO·2 →H2 O2 þ O2

ð15Þ

− − H2 O2 þ O·2 →OH· þ OH þ O2

ð16Þ

Dye þ OH·→Degradation products

ð17Þ

Table 2 The degradation percentage of different calcinations temperature of ZnO samples. Calcinations temperature

Methylene blue

Methyl orange

350 °C 650 °C 950 °C

97% 80% 33%

80% 48% 31%

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Fig. 12. Schematic diagram represents photocatalytic mechanism of ZnO.

4. Conclusion The nanorods of ZnO were successfully prepared by simple thermal decomposition method. This method is commercially gainful, easily producible large quantity of catalyst and the necessity of special equipment is unessential. The ZnO prepared at 350 °C for 3 h shows higher photocatalytic activity due to its crystallinity and specific surface area. Acknowledgements We acknowledge the National Centre for Nanoscience and Nanotechnology, University of Madras, India for XPS, TEM and EDXS characterizations. References [1] I.K. Konstantinou, T.A. Albanis, Applied Catalysis B: Environmental 49 (2004) 1–14. [2] U.G. Akpan, B.H. Hameed, Journal of Hazardous Materials 170 (2009) 520–529. [3] J. Peral, X. Domenech, D.F. Ollis, Journal of Chemical Technology and Biotechnology 70 (1997) 117–140. [4] C. Tian, Q. Zhang, A. Wu, M. Jiang, Z. Liang, B. Jiang, H. Fu, Chemical Communications 48 (2012) 2858–2860. [5] V.K. Gupta, A. Mittal, L. Krishnan, J. Mittal, Journal of Colloid and Interface Science 293 (2006) 16–26. [6] A. Mittal, L. Kurup, Vinod K. Gupta, Journal of Hazardous Materials 117 (2005) 171–178. [7] Vinod Kumar Gupta, R. Jain, S. Varshney, Journal of Hazardous Materials 142 (2007) 443–448. [8] Ajay K. Jain, Vinod K. Gupta, Suhas S. Jain, Environmental Science and Technology 38 (2004) 1195–1200. [9] Vinod K. Gupta, Arshi Rastogi, Arunima Nayak, Journal of Colloid and Interface Science 342 (2010) 135–141. [10] Rajendra N. Goyal, Vinod K. Gupta, M. Oyama, N. Bachheti, Talanta 71 (2007) 1110–1117. [11] Vinod K. Gupta, S.K. Srivastava, R. Tyagi, Water Research 34 (2000) 1543–1550. [12] V.K. Gupta, R. Jain, A. Mittal, M. Mathur, S. Sikarwar, Journal of Colloid and Interface Science 309 (2007) 464–469. [13] Vinod K. Gupta, S. Sharma, Industrial and Engineering Chemistry Research 42 (2003) 6619–6624. [14] Vinod K. Gupta, A. Mittal, L. Kurup, J. Mittal, Journal of Colloid and Interface Science 304 (2006) 52–57. [15] V.K. Gupta, R. Jain, S. Varshney, Journal of Colloid and Interface Science 312 (2007) 292–296. [16] V.K. Gupta, I. Ali, V.K. Saini, Water Research 41 (2007) 3307–3316. [17] V.K. Gupta, A. Rastogi, Journal of Hazardous Materials 163 (2009) 396–402. [18] V.K. Gupta, R.N. Goyal, R.A. Sharma, International Journal of Electrochemical Science 4 (2009) 156–172. [19] A.K. Jain, V.K. Gupta, U. Khurana, L.P. Singh, Electroanalysis 9 (1997) 857–860. [20] S.K. Srivastava, V.K. Gupta, M.K. Dwivedi, S. Jain, Analytical Proceedings including Analytical Communications 32 (1995) 21–23. [21] V.K. Gupta, M. Al Khayat, A.K. Singh, Manoj K. Pal, Analytica Chimica Acta 634 (1) (2009) 36–43.

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