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Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Q1 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation Rizwan Khan a, M. Shamshi Hassan b, Periyayya Uthirakumar a,c, Jin Hyeon Yun a, Myung-Sheb Khil b, In-Hwan Lee a,n a School of Advanced Materials Engineering and Research Center of Advanced Materials Development, Chonbuk National University, Jeonju 561-756, South Korea b Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, South Korea c Nanoscience Center for Optoelectronic and Energy Devices, Department of Science, Sona College of Technology, Salem 636 005, Tamilnadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 5 February 2015 Accepted 23 March 2015
ZnO nanoglobules were synthesized by a facile hydrothermal process. The detailed structural characterizations revealed that the prepared ZnO nanostructures were uniform, crystalline and with the controlled morphology of nanoglobule-like structures. The photocatalytic activity of the nanoglobules was evaluated on degrading methyl orange dye under UV irradiation. A fast decomposition of the MO dye was observed with a degradation rate of 94.3% within 150 min, which was attributed to the high crystalline quality of ZnO nanostructures that produced reactive sites over the catalyst surface to decompose the MO dye. & 2015 Published by Elsevier B.V.
Keywords: ZnO Nanostructures Methyl orange dye Photocatalytic activity
1. Introduction The accumulation of large amount of toxic and waste materials into water from textile industries has caused the severe healthcare and environmental problems [1,2]. Photocatalytic purification of wastewater using semiconducting metal oxides shows great promise. Photocatalysis of organic dyes such as methyl orange (MO) in water is receiving much attention due to the severe ecological impact of various industrial pollutants [3]. MO was chosen as model pollutant because it is one of the most widely used dyes in the textile, cosmetic and photographic industries and has become a common organic pollutant [4]. Semiconductor metal oxides nanostructures with various morphologies have been receiving much attention in these photocatalytic activity studies [5,6]. Nanostructures of metal oxides (ZnO, CuO, TiO2, etc.) have been widely studied in terms of synthesis and photocatalytic activity [7,8]. Among them, ZnO has been extensively used as an excellent material for enhancement of photocatalytic processes owing to its nontoxic nature, high photosensitivity, excellent chemical stability and large band gap (3.37 eV) [9]. The ability to reproducibly generate ZnO nanostructure with well-defined morphologies is of great interest for determining the photocatalytic performance, and thus the
n
Corresponding author. Tel.: þ 82 63 270 2292. E-mail address: [email protected] (I.-H. Lee).
synthesis of ZnO nanostructures with different morphologies and their potential applications to decomposition of organic pollutants have been extensively studied recently [10]. For example, Xu et al. synthesized the leaf-like ZnO nanostructure and used them as photocatalyst for the photocatalytic activity [11]. Ali et al. investigated the fabrication of ZnO nanoparticles by solution combustion method for the degradation of organic dye [12]. It is evident from the previous studies that the ZnO nanostructures with various morphologies exhibited good photocatalytic performance and were suitable for the decomposition of harmful organic pollutants. In this work, we have synthesized high crystalline ZnO nanoglobules (NGls) by a facile hydrothermal process and employed them as an efficient photocatalyst for the degradation of MO under UV irradiation. The ZnO NGls exhibited 94.3% MO decomposition within 150 min, which could be attributed to the high crystalline quality of ZnO NGls.
2. Experimental details ZnO NGls were synthesized as reported previously with some modifications [13]. Typically, zinc chloride (ZnCl2, 0.02 M) and glucose (C6H12O6, 12 g) were dissolved in 20 and 60 mL of deionized water (DI), respectively, under constant stirring. The above two solutions were mixed with constant stirring at room temperature. Then, polyvinylpyrrolidone (PVP, 0.02 g) solutions
http://dx.doi.org/10.1016/j.matlet.2015.03.109 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: Khan R, et al. Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.03.109i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
R. Khan et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
were added into the mixture and were kept for further stirring. Subsequently, the resultant solution was transferred into a Teflonlined stainless steel autoclave with a volume of 100 mL, and then sealed and heated at 160 1C for 5 h. After the growth, it was allowed to cool down to room temperature. Black precipitate was collected and washed 2–3 times in ethanol and deionized water and dried in an oven at 80 1C for 2 h. Finally, the product was calcined at 500 1C for 2 h in air. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to investigate the morphology of ZnO NGls. X-ray diffraction (XRD, λ¼1.54178 Å) was used to investigate the crystal phase and crystallinity of ZnO NGls. The photocatalytic activities of the ZnO NGls were carried out by decomposing MO in aqueous solution under UV light with a xenon arc lamp. Briefly, ZnO NGls (0.2 g) was added in aqueous solution of MO (10 ppm) under constant stirring. Before irradiation, the above solution was vigorously stirred in the dark for 1 h to undergo adsorption– desorption equilibrium. The corresponding decomposed MO dye aqueous solution was taken out at specific time intervals and centrifuged at 5000 rpm to separate the catalyst particles. UV–visible spectroscopy was applied to examine the absorption spectrum of the decomposed dye. Mass spectrometry (Agilent 1100 series LC system) was used to confirm the degraded product.
Fig. 2. XRD pattern of ZnO NGls.
3. Results and discussion FESEM and TEM images of ZnO nanostructures were shown in Fig. 1. It is seen that the prepared nanostructures are densely packed uniform nanoglobules in surface morphology with characteristic dimensions of the ZnO NGls of 60–80 nm. The corresponding selected area electron diffraction (SAED) pattern in the inset of Fig. 1(b) shows the high crystalline nature of the ZnO NGls. The crystalline structure and phase purity of the ZnO NGls were Fig. 3. Room temperature UV–vis absorption spectra of ZnO NGls
Fig. 1. Morphologies of ZnO NGls obtained by FESEM (a), and TEM (b). The inset in (b) corresponds to SAED pattern.
examined by XRD. Fig. 2 shows a typical XRD pattern of the ZnO NGls. All diffraction peaks can be clearly indexed to the cubic phase of ZnO (JCPDS File no. 36-1451). Except for the characteristic ZnO peaks, no impurity-related signals are observed, revealing that the ZnO NGls have a high crystalline purity. Fig. 3 shows the room temperature UV–vis absorption spectra of ZnO NGls. A broad absorption band at 367 nm was observed in the spectrum, exhibiting a characteristic band for the wurtzite hexagonal ZnO [14]. The photocatalytic efficiency of ZnO NGls in the degradation of MO was studied by monitoring the changes in the MO-specific absorbance spectra after exposing the solution containing MO and ZnO NGls to the UV light for different time periods. The photocatalytic properties of ZnO NGls were analyzed by constantly monitoring the photocatalytic degradation of MO in aqueous solution under UV irradiation. Fig. 4(a) shows the UV–vis spectra of the decomposed MO within the time interval of 0 150 min in presence of ZnO NGls photocatalyst. It was observed that the intensity of the absorption peak of MO decreases as the irradiation time increases, and ultimately vanished after 150 min. This result indicates that MO was almost completely decomposed under the photocatalytic action of the ZnO NGls. Fig. 4(b) shows the rate of MO degradation as a function of time in the presence of photocatalyst ZnO NGls under UV irradiation. The degradation rate (%) was calculated as (C0 C/C0) 100, where C0 is the initial concentration and C is the concentration at different time t. A fast decomposition of MO dye with a rate of 94.3% has been observed within 150 min. The obtained degradation rate is higher compare with the previously reported values based on other ZnO nanostructures [15,16]. For instance, the degradation rate was 92%
Please cite this article as: Khan R, et al. Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.03.109i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
R. Khan et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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3
superoxide radicals (O2 ), which can finally be reduced to OH radicals. These radicals are highly reactive and could effectively degrade organic pollutants like MO [4]. The possible reaction mechanism is shown below. ZnO NGlsþhv-e þh þ þ
∙
h þH2O-OH þ H þ
h þOH -OH
þ
∙
OH∙ þMO dye-oxidation of MO molecule (product)
(1) (2) (3) (4)
From the above reactions it is evident that the photocatalytic process mostly depends on the efficiency to form oxygenated radicals via capture of electrons and holes that are generated in ZnO NGls. Thus, the synthesized ZnO NGls with high crystalline quality produces the large amount of these reactive species over catalysts surface, which significantly initiates the fast degradation of MO under UV irradiation. It is contemplated that being of nanoporous structure, the ZnO NGls strongly absorb UV light, which allows the reactant molecules to get into the active sites on the framework walls, hence enhancing the efficiency of photocatalysis and making them more efficient than other nanostructures [11,12].
4. Conclusions ZnO NGls were synthesized by a hydrothermal process and employed for the photocatalytic degradation of MO dye under UV irradiation. MO was degraded by 94.3% within 150 min over the surface of ZnO NGls. A fast degradation of MO was attributed to the rapid formation of oxygenated radicals via capture of electrons and holes generated by UV irradiation in the ZnO NGls with a high crystalline quality.
Acknowledgment This research was supported by National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT and Future Planning (2013R1A2A2A07067688, 2010-0019626). Fig. 4. UV–vis spectra of MO recorded at different time intervals during degradation in the presence of ZnO NGls (a), degradation of MO measured as a function of the time intervals (b), and positive-ion mass spectra for MO dye solution with ZnO NGls after UV irradiation for 150 min (c)
in 210 min in [15], and 65% in 90 min in [16]. Mass spectroscopy for MO dye after 150 min UV irradiation is shown in Fig. 4(c). A variety of signals without mass-to-charge (m/z) signal at 350.6 (specific to MO) indicate the absence of MO dye in the solution being measured. The m/z signal at 350.6 becomes vanished after 150 min and splits into various mass signals due to the photocatalytic effects of ZnO NGls under UV irradiation. The degradation of MO solution by ZnO NGls could be explained by photogeneration of electron–hole (e –h þ ) pairs between the conduction and valence band due to excitation of ZnO under UV illumination. This phenomenon creates an e –h þ pair separation, which might be helpful for the generation of active radicals for the degradation of organic pollutant. Holes could be trapped by hydroxyl group (OH ) to form highly reactive hydroxyl radical (OH∙), while electrons could be easily trapped by the adsorbed O2 to produce
References [1] Tong H, Ouyang S, Bi Y, Umezawa N, Oshikiri M, Ye J. Adv Mater 2012;24:229–51. [2] Shi R, Lin J, Wang Y, Xu J, Zhu YJ. J Phys Chem C 2010;114:6472–7. [3] Lei A, Qu B, Zhou W, Wang Y, Zhang Q, Zou B. Mater Lett 2009;66:72–5. [4] Khan MM, Lee J, Cho MH. J Ind Eng Chem 2014;20:1584–90. [5] Talebian N, Jafarinezhad F. Ceram Int 2013;39:8311–7. [6] Singh S, Barick KC, Bahadur D. CrystEngComm 2013;15:4631–9. [7] Gupta K, Jain R, Agarwal S, Nayak A, Shrivastava M. J Colloid Interface Sci 2012;366:135–40. [8] Vaseem M, Umar A, Hahn YB, Kim DH, Lee KS, Jang JS, et al. Catal Commun 2008;10:11–6. [9] Khan R, Hassan MS, Cho HS, Polyakov AY, Khil MS, Lee IH. Mater Lett 2014;133:224–7. [10] Jang ES, Won JH, Hwang SJ, Choi JH. Adv Mater 2006;18:3309–12. [11] Xu L, Zheng G, Wang J, Lai M, Miao J, Xian F, et al. Mater Lett 2014;122:1–4. [12] Ali MA, Idris MR, Quayum MD. J Nanostruct Chem 2013;3:36 2. [13] Yu J, Yu X. Environ Sci Technol 2008;42:4902–7. [14] Al Hajry A, Umar A, Hahn YB, Kim DH. Superlattices Microstruct 2009;45:529–34. [15] Kumar R, Kumar G, Umar A. Mater Lett 2013;97:100–3. [16] Ahmad M, Ahmed E, Hong ZL, Xu JF, Khalid NR, Elhissi A, et al. Appl Surf Sci 2013;274:273–81.
Please cite this article as: Khan R, et al. Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.03.109i
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 Q2100 Q3 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Q1 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation Rizwan Khan a, M. Shamshi Hassan b, Periyayya Uthirakumar a,c, Jin Hyeon Yun a, Myung-Sheb Khil b, In-Hwan Lee a,n a School of Advanced Materials Engineering and Research Center of Advanced Materials Development, Chonbuk National University, Jeonju 561-756, South Korea b Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, South Korea c Nanoscience Center for Optoelectronic and Energy Devices, Department of Science, Sona College of Technology, Salem 636 005, Tamilnadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 5 February 2015 Accepted 23 March 2015
ZnO nanoglobules were synthesized by a facile hydrothermal process. The detailed structural characterizations revealed that the prepared ZnO nanostructures were uniform, crystalline and with the controlled morphology of nanoglobule-like structures. The photocatalytic activity of the nanoglobules was evaluated on degrading methyl orange dye under UV irradiation. A fast decomposition of the MO dye was observed with a degradation rate of 94.3% within 150 min, which was attributed to the high crystalline quality of ZnO nanostructures that produced reactive sites over the catalyst surface to decompose the MO dye. & 2015 Published by Elsevier B.V.
Keywords: ZnO Nanostructures Methyl orange dye Photocatalytic activity
1. Introduction The accumulation of large amount of toxic and waste materials into water from textile industries has caused the severe healthcare and environmental problems [1,2]. Photocatalytic purification of wastewater using semiconducting metal oxides shows great promise. Photocatalysis of organic dyes such as methyl orange (MO) in water is receiving much attention due to the severe ecological impact of various industrial pollutants [3]. MO was chosen as model pollutant because it is one of the most widely used dyes in the textile, cosmetic and photographic industries and has become a common organic pollutant [4]. Semiconductor metal oxides nanostructures with various morphologies have been receiving much attention in these photocatalytic activity studies [5,6]. Nanostructures of metal oxides (ZnO, CuO, TiO2, etc.) have been widely studied in terms of synthesis and photocatalytic activity [7,8]. Among them, ZnO has been extensively used as an excellent material for enhancement of photocatalytic processes owing to its nontoxic nature, high photosensitivity, excellent chemical stability and large band gap (3.37 eV) [9]. The ability to reproducibly generate ZnO nanostructure with well-defined morphologies is of great interest for determining the photocatalytic performance, and thus the
n
Corresponding author. Tel.: þ 82 63 270 2292. E-mail address: [email protected] (I.-H. Lee).
synthesis of ZnO nanostructures with different morphologies and their potential applications to decomposition of organic pollutants have been extensively studied recently [10]. For example, Xu et al. synthesized the leaf-like ZnO nanostructure and used them as photocatalyst for the photocatalytic activity [11]. Ali et al. investigated the fabrication of ZnO nanoparticles by solution combustion method for the degradation of organic dye [12]. It is evident from the previous studies that the ZnO nanostructures with various morphologies exhibited good photocatalytic performance and were suitable for the decomposition of harmful organic pollutants. In this work, we have synthesized high crystalline ZnO nanoglobules (NGls) by a facile hydrothermal process and employed them as an efficient photocatalyst for the degradation of MO under UV irradiation. The ZnO NGls exhibited 94.3% MO decomposition within 150 min, which could be attributed to the high crystalline quality of ZnO NGls.
2. Experimental details ZnO NGls were synthesized as reported previously with some modifications [13]. Typically, zinc chloride (ZnCl2, 0.02 M) and glucose (C6H12O6, 12 g) were dissolved in 20 and 60 mL of deionized water (DI), respectively, under constant stirring. The above two solutions were mixed with constant stirring at room temperature. Then, polyvinylpyrrolidone (PVP, 0.02 g) solutions
http://dx.doi.org/10.1016/j.matlet.2015.03.109 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: Khan R, et al. Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.03.109i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
R. Khan et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
were added into the mixture and were kept for further stirring. Subsequently, the resultant solution was transferred into a Teflonlined stainless steel autoclave with a volume of 100 mL, and then sealed and heated at 160 1C for 5 h. After the growth, it was allowed to cool down to room temperature. Black precipitate was collected and washed 2–3 times in ethanol and deionized water and dried in an oven at 80 1C for 2 h. Finally, the product was calcined at 500 1C for 2 h in air. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to investigate the morphology of ZnO NGls. X-ray diffraction (XRD, λ¼1.54178 Å) was used to investigate the crystal phase and crystallinity of ZnO NGls. The photocatalytic activities of the ZnO NGls were carried out by decomposing MO in aqueous solution under UV light with a xenon arc lamp. Briefly, ZnO NGls (0.2 g) was added in aqueous solution of MO (10 ppm) under constant stirring. Before irradiation, the above solution was vigorously stirred in the dark for 1 h to undergo adsorption– desorption equilibrium. The corresponding decomposed MO dye aqueous solution was taken out at specific time intervals and centrifuged at 5000 rpm to separate the catalyst particles. UV–visible spectroscopy was applied to examine the absorption spectrum of the decomposed dye. Mass spectrometry (Agilent 1100 series LC system) was used to confirm the degraded product.
Fig. 2. XRD pattern of ZnO NGls.
3. Results and discussion FESEM and TEM images of ZnO nanostructures were shown in Fig. 1. It is seen that the prepared nanostructures are densely packed uniform nanoglobules in surface morphology with characteristic dimensions of the ZnO NGls of 60–80 nm. The corresponding selected area electron diffraction (SAED) pattern in the inset of Fig. 1(b) shows the high crystalline nature of the ZnO NGls. The crystalline structure and phase purity of the ZnO NGls were Fig. 3. Room temperature UV–vis absorption spectra of ZnO NGls
Fig. 1. Morphologies of ZnO NGls obtained by FESEM (a), and TEM (b). The inset in (b) corresponds to SAED pattern.
examined by XRD. Fig. 2 shows a typical XRD pattern of the ZnO NGls. All diffraction peaks can be clearly indexed to the cubic phase of ZnO (JCPDS File no. 36-1451). Except for the characteristic ZnO peaks, no impurity-related signals are observed, revealing that the ZnO NGls have a high crystalline purity. Fig. 3 shows the room temperature UV–vis absorption spectra of ZnO NGls. A broad absorption band at 367 nm was observed in the spectrum, exhibiting a characteristic band for the wurtzite hexagonal ZnO [14]. The photocatalytic efficiency of ZnO NGls in the degradation of MO was studied by monitoring the changes in the MO-specific absorbance spectra after exposing the solution containing MO and ZnO NGls to the UV light for different time periods. The photocatalytic properties of ZnO NGls were analyzed by constantly monitoring the photocatalytic degradation of MO in aqueous solution under UV irradiation. Fig. 4(a) shows the UV–vis spectra of the decomposed MO within the time interval of 0 150 min in presence of ZnO NGls photocatalyst. It was observed that the intensity of the absorption peak of MO decreases as the irradiation time increases, and ultimately vanished after 150 min. This result indicates that MO was almost completely decomposed under the photocatalytic action of the ZnO NGls. Fig. 4(b) shows the rate of MO degradation as a function of time in the presence of photocatalyst ZnO NGls under UV irradiation. The degradation rate (%) was calculated as (C0 C/C0) 100, where C0 is the initial concentration and C is the concentration at different time t. A fast decomposition of MO dye with a rate of 94.3% has been observed within 150 min. The obtained degradation rate is higher compare with the previously reported values based on other ZnO nanostructures [15,16]. For instance, the degradation rate was 92%
Please cite this article as: Khan R, et al. Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.03.109i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
R. Khan et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
3
superoxide radicals (O2 ), which can finally be reduced to OH radicals. These radicals are highly reactive and could effectively degrade organic pollutants like MO [4]. The possible reaction mechanism is shown below. ZnO NGlsþhv-e þh þ þ
∙
h þH2O-OH þ H þ
h þOH -OH
þ
∙
OH∙ þMO dye-oxidation of MO molecule (product)
(1) (2) (3) (4)
From the above reactions it is evident that the photocatalytic process mostly depends on the efficiency to form oxygenated radicals via capture of electrons and holes that are generated in ZnO NGls. Thus, the synthesized ZnO NGls with high crystalline quality produces the large amount of these reactive species over catalysts surface, which significantly initiates the fast degradation of MO under UV irradiation. It is contemplated that being of nanoporous structure, the ZnO NGls strongly absorb UV light, which allows the reactant molecules to get into the active sites on the framework walls, hence enhancing the efficiency of photocatalysis and making them more efficient than other nanostructures [11,12].
4. Conclusions ZnO NGls were synthesized by a hydrothermal process and employed for the photocatalytic degradation of MO dye under UV irradiation. MO was degraded by 94.3% within 150 min over the surface of ZnO NGls. A fast degradation of MO was attributed to the rapid formation of oxygenated radicals via capture of electrons and holes generated by UV irradiation in the ZnO NGls with a high crystalline quality.
Acknowledgment This research was supported by National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT and Future Planning (2013R1A2A2A07067688, 2010-0019626). Fig. 4. UV–vis spectra of MO recorded at different time intervals during degradation in the presence of ZnO NGls (a), degradation of MO measured as a function of the time intervals (b), and positive-ion mass spectra for MO dye solution with ZnO NGls after UV irradiation for 150 min (c)
in 210 min in [15], and 65% in 90 min in [16]. Mass spectroscopy for MO dye after 150 min UV irradiation is shown in Fig. 4(c). A variety of signals without mass-to-charge (m/z) signal at 350.6 (specific to MO) indicate the absence of MO dye in the solution being measured. The m/z signal at 350.6 becomes vanished after 150 min and splits into various mass signals due to the photocatalytic effects of ZnO NGls under UV irradiation. The degradation of MO solution by ZnO NGls could be explained by photogeneration of electron–hole (e –h þ ) pairs between the conduction and valence band due to excitation of ZnO under UV illumination. This phenomenon creates an e –h þ pair separation, which might be helpful for the generation of active radicals for the degradation of organic pollutant. Holes could be trapped by hydroxyl group (OH ) to form highly reactive hydroxyl radical (OH∙), while electrons could be easily trapped by the adsorbed O2 to produce
References [1] Tong H, Ouyang S, Bi Y, Umezawa N, Oshikiri M, Ye J. Adv Mater 2012;24:229–51. [2] Shi R, Lin J, Wang Y, Xu J, Zhu YJ. J Phys Chem C 2010;114:6472–7. [3] Lei A, Qu B, Zhou W, Wang Y, Zhang Q, Zou B. Mater Lett 2009;66:72–5. [4] Khan MM, Lee J, Cho MH. J Ind Eng Chem 2014;20:1584–90. [5] Talebian N, Jafarinezhad F. Ceram Int 2013;39:8311–7. [6] Singh S, Barick KC, Bahadur D. CrystEngComm 2013;15:4631–9. [7] Gupta K, Jain R, Agarwal S, Nayak A, Shrivastava M. J Colloid Interface Sci 2012;366:135–40. [8] Vaseem M, Umar A, Hahn YB, Kim DH, Lee KS, Jang JS, et al. Catal Commun 2008;10:11–6. [9] Khan R, Hassan MS, Cho HS, Polyakov AY, Khil MS, Lee IH. Mater Lett 2014;133:224–7. [10] Jang ES, Won JH, Hwang SJ, Choi JH. Adv Mater 2006;18:3309–12. [11] Xu L, Zheng G, Wang J, Lai M, Miao J, Xian F, et al. Mater Lett 2014;122:1–4. [12] Ali MA, Idris MR, Quayum MD. J Nanostruct Chem 2013;3:36 2. [13] Yu J, Yu X. Environ Sci Technol 2008;42:4902–7. [14] Al Hajry A, Umar A, Hahn YB, Kim DH. Superlattices Microstruct 2009;45:529–34. [15] Kumar R, Kumar G, Umar A. Mater Lett 2013;97:100–3. [16] Ahmad M, Ahmed E, Hong ZL, Xu JF, Khalid NR, Elhissi A, et al. Appl Surf Sci 2013;274:273–81.
Please cite this article as: Khan R, et al. Facile synthesis of ZnO nanoglobules and its photocatalytic activity in the degradation of methyl orange dye under UV irradiation. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.03.109i
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 Q2100 Q3 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123