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Facile synthesis and characterization of zinc oxide nanoparticles and studies of their catalytic activity towards ultrasound-assisted degradation of metronidazole
Author’s Accepted Manuscript Facile synthesis and characterization of zinc oxide nanoparticles and studies of their catalytic activity towards ultrasound-assisted degradation of metronidazole. Bishal Bhuyan, Bappi Paul, Debraj Dhar Purkayastha, Siddhartha Sankar Dhar, Satyananda Behera
PII: DOI: Reference:
www.elsevier.com
S0167-577X(16)30020-9 http://dx.doi.org/10.1016/j.matlet.2016.01.024 MLBLUE20152
To appear in: Materials Letters Received date: 9 November 2015 Revised date: 23 December 2015 Accepted date: 6 January 2016 Cite this article as: Bishal Bhuyan, Bappi Paul, Debraj Dhar Purkayastha, Siddhartha Sankar Dhar and Satyananda Behera, Facile synthesis and characterization of zinc oxide nanoparticles and studies of their catalytic activity towards ultrasound-assisted degradation of metronidazole., Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.01.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 Facile synthesis and characterization of zinc oxide nanoparticles and studies of their catalytic activity towards ultrasound-assisted degradation of metronidazole. Bishal Bhuyana, Bappi Paula, Debraj Dhar Purkayasthaa, Siddhartha Sankar Dhara*, Satyananda Beherab a
Department of Chemistry, National Institute of Technology, Silchar, Silchar-788010, Assam, India
b
Department of Ceramic Engineering, National Institute of Technology Rourkela, Rourkela, 769008, Odisha, India
*Corresponding authors. Tel: +91-03842-242915; fax: +91-03842-224797 Email: [email protected] (S.S.Dhar) Abstract A novel and facile approach for synthesis of zinc oxide (ZnO) nanoparticles (NPs) employing homogeneous chemical precipitation followed by hydrothermal heating technique is reported. The present method of synthesis of ZnO NPs is very efficient and cost effective. As-synthesized ZnO NPs were characterized by XRD, FT-IR, EDX, TEM, and N2 adsorption-desorption (BET) studies. The powder XRD pattern furnished evidence for the formation of hexagonal close packing structure of ZnO NPs having average crystallite size 21.93 nm. The shapes of synthesized ZnO NPs are mostly quasi-cylindrical with sizes 20-50 nm. These ZnO NPs were used as catalysts for the degradation of a pharmaceuticals waste, metronidazole under ultrasound irradiation. Keywords: Zinc Oxide; Nanoparticles; X-ray techniques; Electron microscopy; Pharmaceuticals waste. 1. Introduction In the recent years, metal-oxide nanoparticles have been the subject of much interest because of their unusual properties, which often differ from the bulk. These materials have received significant attention as efficient catalysts in many organic reactions due to their high surface area to volume ratio and coordination centers which provide a larger number of active sites per unit area in comparison with their heterogeneous counter sites [1-4]. Several methods have been used to prepare ZnO NPs. Some of the important ones are laser ablation, combustion method, electrochemical depositions, sol–gel method, hydrothermal methods, thermal decomposition, chemical vapor deposition, ultrasound, microwave-assisted combustion method, co-precipitation, and mechanical milling [5]. Although each of these methods has its own merits, some of them suffer from drawbacks such as use of expensive chemicals, surfactants, calcination at high temperature etc. Among these various methods, hydrothermal procedure has several advantages as this method does not require high temperature and organic solvents [6]. Moreover, any
2 extra work-up such as grinding and calcination can be avoided and the products can be obtained in high purity and crystallinity. Attention of the readers may be drawn to the fact that ultrasound irradiation in degrading the organic pollutant has been proved to be an useful tool in accelerating dissolution, enhancing reaction rates, renewing the surface of solid catalysts or reactants etc [7-9]. Thus combination of nanocatalysts with ultrasound irradiation in degradation reactions will certainly open up a new avenue for highly efficient environmentally friendly synthetic protocols. The presence of pharmaceuticals waste in surface water from industries and personal care products are considered to an emerging environmental issue of concern due to their impact on human health and aquatic life in recent years [7,10]. Therefore, development of newer technologies from removal of pharmaceuticals waste before their release into the environment through degradation will always be important in the present day context. A comprehensive literature study reveals that zinc oxide NPs catalyzed degradation of metronidazole by ultrasound assisted technique does not appear to have reported previously. As a sequel to our current endeavour [11-14] on the synthesis and application of nanocatalysts, we report herein a new facile synthesis of ZnO NPs and their application as catalyst for ultrasound assisted degradation of metronidazole. 2. Experimental 2.1 Materials and physical measurements Zinc chloride (ZnCl2) and tributylamine (C12H27N) were purchased from Merck India Ltd. FT-IR spectrum was recorded on KBr matrix with Bruker 3000 Hyperion Microscope with Vertex 80 FT-IR system. XRD measurements were carried out on a Bruker AXS D8-Advance powder X-ray diffractometer with Cu-Kα radiation (λ=1.5418Å) with a scan speed of 2°/min. Transmission electron microscopy images were obtained on a JEOL, JEM2100 equipment. The sample powders were dispersed in ethanol under sonication and TEM grids were prepared using a few drops of the dispersion followed by drying in air. Sonication was performed in Qsonica Q700 sonicator with a frequency 20 kHz and at a nominal power of 250 W. 2.2. Synthesis of ZnO NPs Zinc chloride (0.68 g, 5 mmol) was dissolved in minimum volume of distilled water. To this, tributylamine (TBA) (0.93 ml, 5 mmol) was added and the resultant solution was then stirred magnetically at room temperature for 1 h.
3 The white suspension formed after stirring for 1 h was transferred to 150 ml Teflon-line autoclave and heated at 180°C in a hot air oven for 3 h. The solid thus obtained was cooled to room temperature, washed with deionized water and ethanol, and dried in vacuum. 2.3 Catalytic performance test The degradation of metronidazole (MTZ) was performed in presence of ZnO catalyst under ultrasound irradiation. In the typical run, 2 ml of freshly prepared SB solution (0.2 M) was mixed with 50 ml (10 mg L -1) aqueous solution of MTZ. In this solution 1 mg of catalyst was added and irradiated under US. At a regular interval of time, 4 ml of the suspension was withdrawn and centrifuged immediately. The absorbance of the supernatant was then measured using UV-visible spectrophotometer. The reaction was also monitored without catalyst. The reactions were carried out at room temperature (30 ± 1oC). 3. Results and Discussion 3.1 Preparation of ZnO nanocatalyst In the present method, ZnO nanoparticles have been successfully synthesized by hydrothermal heating of a zinc hydroxide (Zn(OH)2) precursor obtained by a homogeneous chemical precipitation method. Here, hydroxide anions are produced by hydration of TBA, which cause a uniform rise in pH of the solution till the solubility limit. The main advantage in this process is that uniform rise in pH prevents the occurrence of high local super saturation, allowing nucleation to occur homogeneously throughout the solution. The reactions involved in the formation of ZnO NPs are believed to be as follows
(C4H9)3N + H2O Zn2+ +2OHZn(OH)2
(C4H9)3NH+ + OH-
(1)
Zn(OH)2
(2)
ZnO + H2O
(3)
3.2 Characterization of the ZnO nanocatalyst The powder XRD patterns (Fig.1(a)) were recorded for the identification of phases exhibited by the synthesized materials. The diffraction peaks match well with the reported data of ZnO (JCPDS File no. 89-1397). Zinc oxide nanostructures are in crystalline form with the hexagonal wurtzite phase and high purity. Zn atom is arranged in hexagonal close packing and each Zn2+ ion is surrounded by four oxygen atoms to form [Zn-O4]6- tetrahedron. Every tetrahedron is connected through the corners to form the 3-D structure [15, 16]. The average crystallite size of ZnO
4 NPs was calculated by the Debye-Scherrer formula using a Gaussian fit and was found to be 22 nm. The average crystallite size of ZnO NPs after 5 catalytic cycles increased to 29 nm.
Fig.1. The FT-IR spectrum (Fig.1(b)) of ZnO NPs showed an intense peak at 549 cm-1 which corresponds to Zn-O stretching mode. The broad band at 3529 cm-1 appeared due to O-H stretching mode of the hydroxyl group present in the surface of the nanoparticles. The band 1619 cm-1 arose due to the bending vibrational mode of surface adsorbed water molecules. The TEM image of synthesized ZnO (Fig.2) showed quasi-cylindrical particles of diameters 20-50 nm. The lattice fringes in HRTEM image are separated by 0.24 nm, which corresponds to the (101) plane of ZnO. The electron diffraction (ED) pattern indicated polycrystalline nature. The TEM image of spent catalyst (Fig.3) after five cycles of experiment reveals that the particles shape remain almost same but their diameters increased slightly due to agglomeration. The lattice fringes are separated by 0.25 nm, which corresponds to the (101) plane of ZnO. The ED pattern indicated polycrystalline nature. The EDX patterns (Fig.S1 in ESI) also indicated presence of Zn and O in the synthesized materials.
5
Fig.2.
6
Fig.3. ZnO NPs synthesized with the present synthetic methods has low BET surface area and pore volume of 15.65 (m2/g) and 0.093 (cm3/g) (Fig.4(a)). This follows the characteristics of a type IV isotherm with a type H3 hysteresis loop associated with aggregates of plate-like particles forming slit-like pores [17]. The Barrett Joyner Halenda (BJH) pore size distribution indicated that most of the pores range from 2 to 11 nm (Fig. 4(a)) Inset) and a pore size of 23.87 nm. The catalytic efficiency of ZnO NPs in degradation of metronidazole under ultrasound irradiation was investigated in presence of SB and the reaction was monitored by UV-visible spectroscopy (Fig.4(b)). The catalytic reaction was monitored spectrophotometrically by following the decrease of absorbance at λ max 318nm (characteristic absorption peak of MTZ) with time. The intensity of this peak decreases gradually during the irradiation and finally disappears within 27 min of the degradation processes. The absorbance of MTZ decreases only slightly without the catalyst indicating very slow reaction rate, whereas those with catalyst decreases abruptly, suggesting the faster reaction rate. The degradation of MTZ fitted well with the pseudo first order equation [11] and the value of ‘k’ was obtained from the slope of the graph between ln(Ao/A) and time (Fig.4(c)) and was found to be 0.072 min-1.
7
Fig.4. 3.4 Reclyability of the catalyst Another intriguing facet of this protocol is the easy recyclability of the catalyst. After the catalytic run, the catalyst was separated out from the solution, washed with methanol and vacuum dried to ensure purity of recovered catalyst. The recovered catalyst was further used to check the catalytic efficiency. The results of the recyclability test are shown in (Fig.4(d)). The ZnO NPs catalyst exhibits an excellent activity even after five cycles. 4. Conclusion In summary, a new facile approach to synthesis of ZnO NPs has been demonstrated. The procedure adopted herein is simple and may be readily utilized for large-scale synthesis. As synthesized ZnO NPs were found to be highly efficient heterogeneous catalysts for ultrasound-assisted degradation of metronidazole. Moreover, this catalytic protocol is simple and efficient with advantages of easy recovery and recyclability of the catalyst. Thus this new protocol may serve as viable alternative to numerous existing procedures for degradation of pharmaceuticals waste. Acknowledgement The authors thank SAIF, IIT Madras, SAIF, IIT Bombay and SAIF, North Eastern Hill University for analysis. The Director NIT Silchar is acknowledged for financial support.
8 References [1] Bing Z, Scott H, Raja R, Somorjai GA. Nanotechnology in Catalysis. Ottawa: Springer; 2007. [2] Lu H, Liao L, Li J, Wang D, He H, Fu Q, Xu L, Tian Y. J. Phys. Chem. B 2006;110:23211-4. [3] Lee SH, Kim J, Hong KH, Shin J, Kim S, Kim K. ACS Appl. Mater. Interfaces. 2012; 4:1365-70. [4] Tripathy N, Ahmad R, Song JE, Ko HA, HahnYB, Khang G, Mater Lett. 2014;136:171–4. [5] Kołodziejczak-Radzimska A, Jesionowski T. Materials. 2014;7:2833-81. [6] ChenY, Zhang C, Huang W, Situ Y, Huang H, Mater. Lett. 2015;141:294–7 [7] Yuan N, Zhang G, Guo S, Wan Z. Ultrason. Sonochem. 2016; 28:62–8. [8] Liu HN, Li GT, Qu JH, Liu HJ. J. Hazard. Mater. 2007;144:180–6. [9] Elshafei GMS, Yehia FZ, Dimitry OIH, Badawi AM, Eshaq G. Ultrason. Sonochem. 2014; 21:1358– 65. [10] Hu X, Fan J, Zhang K, Yu N, Wang J. Ind. Eng. Chem. Res. 2014;53:14623−32. [11] Paul B, Bhuyan B, Purkayastha DD, Dey M, Dhar SS. Mater. Lett. 2015;148:37-40. [12] Paul B, Bhuyan B, Purkayastha DD, Dhar S.S. Catal. Commun. 2015;69:48-54. [13] Paul B, Bhuyan B, Purkayastha DD, Dhar SS, Behera S. J. Alloys Compd. 2015;648:629-35. [14] Paul B, Bhuyan B, Purkayastha DD, Dhar SS. J. Mol. Liq. 2015; 212:813-817. [15] Liu Y, Zhoua J, Larbot A, Persin M. J. Mater. Process. Technol. 2007;189:379-83. [16] Al-Gaashani R, Radiman S, Tabet N, Daud AR. Mater. Chem. Phys. 2011;125:846-52. [17] Mou FZ, Guan JG, Xiao ZD, Sun ZG, Shi WD, Fan XA. J. Mater. Chem. 2011;21:5414-21.
Figure and table captions Fig.1. (a) Powder XRD patterns of ZnO nanocatalysts. (b) FT-IR Spectrum of ZnO NPs. Fig.2. (a, b) TEM images (c) HRTEM image and (d) ED pattern of ZnO nanocatalyst. Fig.3. (a, b) TEM images (c) HRTEM image and (d) ED pattern of ZnO nanocatalyst after five cycles. Fig.4. (a) N2 adsorption-desorption isotherms of ZnO nanocatalyst. (b) UV-visible spectra of degradation of metronidazole in presence of ZnO NPs as catalyst (c) plot of ln(Ao/A) vs. time (min) and (d) Recyclability tests of ZnO nanocatalyst.
9
Highlights Synthesis of ZnO nanoparticles by chemical precipitation cum hydrothermal heating. The XRD pattern shows hexagonal ZnO with average crystallite size 22 nm. TEM image showed quasi-cylindrical shape having diameters 20-50 nm. Particles showed significant catalytic activity in degradation of metronidazole.
PII: DOI: Reference:
www.elsevier.com
S0167-577X(16)30020-9 http://dx.doi.org/10.1016/j.matlet.2016.01.024 MLBLUE20152
To appear in: Materials Letters Received date: 9 November 2015 Revised date: 23 December 2015 Accepted date: 6 January 2016 Cite this article as: Bishal Bhuyan, Bappi Paul, Debraj Dhar Purkayastha, Siddhartha Sankar Dhar and Satyananda Behera, Facile synthesis and characterization of zinc oxide nanoparticles and studies of their catalytic activity towards ultrasound-assisted degradation of metronidazole., Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.01.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 Facile synthesis and characterization of zinc oxide nanoparticles and studies of their catalytic activity towards ultrasound-assisted degradation of metronidazole. Bishal Bhuyana, Bappi Paula, Debraj Dhar Purkayasthaa, Siddhartha Sankar Dhara*, Satyananda Beherab a
Department of Chemistry, National Institute of Technology, Silchar, Silchar-788010, Assam, India
b
Department of Ceramic Engineering, National Institute of Technology Rourkela, Rourkela, 769008, Odisha, India
*Corresponding authors. Tel: +91-03842-242915; fax: +91-03842-224797 Email: [email protected] (S.S.Dhar) Abstract A novel and facile approach for synthesis of zinc oxide (ZnO) nanoparticles (NPs) employing homogeneous chemical precipitation followed by hydrothermal heating technique is reported. The present method of synthesis of ZnO NPs is very efficient and cost effective. As-synthesized ZnO NPs were characterized by XRD, FT-IR, EDX, TEM, and N2 adsorption-desorption (BET) studies. The powder XRD pattern furnished evidence for the formation of hexagonal close packing structure of ZnO NPs having average crystallite size 21.93 nm. The shapes of synthesized ZnO NPs are mostly quasi-cylindrical with sizes 20-50 nm. These ZnO NPs were used as catalysts for the degradation of a pharmaceuticals waste, metronidazole under ultrasound irradiation. Keywords: Zinc Oxide; Nanoparticles; X-ray techniques; Electron microscopy; Pharmaceuticals waste. 1. Introduction In the recent years, metal-oxide nanoparticles have been the subject of much interest because of their unusual properties, which often differ from the bulk. These materials have received significant attention as efficient catalysts in many organic reactions due to their high surface area to volume ratio and coordination centers which provide a larger number of active sites per unit area in comparison with their heterogeneous counter sites [1-4]. Several methods have been used to prepare ZnO NPs. Some of the important ones are laser ablation, combustion method, electrochemical depositions, sol–gel method, hydrothermal methods, thermal decomposition, chemical vapor deposition, ultrasound, microwave-assisted combustion method, co-precipitation, and mechanical milling [5]. Although each of these methods has its own merits, some of them suffer from drawbacks such as use of expensive chemicals, surfactants, calcination at high temperature etc. Among these various methods, hydrothermal procedure has several advantages as this method does not require high temperature and organic solvents [6]. Moreover, any
2 extra work-up such as grinding and calcination can be avoided and the products can be obtained in high purity and crystallinity. Attention of the readers may be drawn to the fact that ultrasound irradiation in degrading the organic pollutant has been proved to be an useful tool in accelerating dissolution, enhancing reaction rates, renewing the surface of solid catalysts or reactants etc [7-9]. Thus combination of nanocatalysts with ultrasound irradiation in degradation reactions will certainly open up a new avenue for highly efficient environmentally friendly synthetic protocols. The presence of pharmaceuticals waste in surface water from industries and personal care products are considered to an emerging environmental issue of concern due to their impact on human health and aquatic life in recent years [7,10]. Therefore, development of newer technologies from removal of pharmaceuticals waste before their release into the environment through degradation will always be important in the present day context. A comprehensive literature study reveals that zinc oxide NPs catalyzed degradation of metronidazole by ultrasound assisted technique does not appear to have reported previously. As a sequel to our current endeavour [11-14] on the synthesis and application of nanocatalysts, we report herein a new facile synthesis of ZnO NPs and their application as catalyst for ultrasound assisted degradation of metronidazole. 2. Experimental 2.1 Materials and physical measurements Zinc chloride (ZnCl2) and tributylamine (C12H27N) were purchased from Merck India Ltd. FT-IR spectrum was recorded on KBr matrix with Bruker 3000 Hyperion Microscope with Vertex 80 FT-IR system. XRD measurements were carried out on a Bruker AXS D8-Advance powder X-ray diffractometer with Cu-Kα radiation (λ=1.5418Å) with a scan speed of 2°/min. Transmission electron microscopy images were obtained on a JEOL, JEM2100 equipment. The sample powders were dispersed in ethanol under sonication and TEM grids were prepared using a few drops of the dispersion followed by drying in air. Sonication was performed in Qsonica Q700 sonicator with a frequency 20 kHz and at a nominal power of 250 W. 2.2. Synthesis of ZnO NPs Zinc chloride (0.68 g, 5 mmol) was dissolved in minimum volume of distilled water. To this, tributylamine (TBA) (0.93 ml, 5 mmol) was added and the resultant solution was then stirred magnetically at room temperature for 1 h.
3 The white suspension formed after stirring for 1 h was transferred to 150 ml Teflon-line autoclave and heated at 180°C in a hot air oven for 3 h. The solid thus obtained was cooled to room temperature, washed with deionized water and ethanol, and dried in vacuum. 2.3 Catalytic performance test The degradation of metronidazole (MTZ) was performed in presence of ZnO catalyst under ultrasound irradiation. In the typical run, 2 ml of freshly prepared SB solution (0.2 M) was mixed with 50 ml (10 mg L -1) aqueous solution of MTZ. In this solution 1 mg of catalyst was added and irradiated under US. At a regular interval of time, 4 ml of the suspension was withdrawn and centrifuged immediately. The absorbance of the supernatant was then measured using UV-visible spectrophotometer. The reaction was also monitored without catalyst. The reactions were carried out at room temperature (30 ± 1oC). 3. Results and Discussion 3.1 Preparation of ZnO nanocatalyst In the present method, ZnO nanoparticles have been successfully synthesized by hydrothermal heating of a zinc hydroxide (Zn(OH)2) precursor obtained by a homogeneous chemical precipitation method. Here, hydroxide anions are produced by hydration of TBA, which cause a uniform rise in pH of the solution till the solubility limit. The main advantage in this process is that uniform rise in pH prevents the occurrence of high local super saturation, allowing nucleation to occur homogeneously throughout the solution. The reactions involved in the formation of ZnO NPs are believed to be as follows
(C4H9)3N + H2O Zn2+ +2OHZn(OH)2
(C4H9)3NH+ + OH-
(1)
Zn(OH)2
(2)
ZnO + H2O
(3)
3.2 Characterization of the ZnO nanocatalyst The powder XRD patterns (Fig.1(a)) were recorded for the identification of phases exhibited by the synthesized materials. The diffraction peaks match well with the reported data of ZnO (JCPDS File no. 89-1397). Zinc oxide nanostructures are in crystalline form with the hexagonal wurtzite phase and high purity. Zn atom is arranged in hexagonal close packing and each Zn2+ ion is surrounded by four oxygen atoms to form [Zn-O4]6- tetrahedron. Every tetrahedron is connected through the corners to form the 3-D structure [15, 16]. The average crystallite size of ZnO
4 NPs was calculated by the Debye-Scherrer formula using a Gaussian fit and was found to be 22 nm. The average crystallite size of ZnO NPs after 5 catalytic cycles increased to 29 nm.
Fig.1. The FT-IR spectrum (Fig.1(b)) of ZnO NPs showed an intense peak at 549 cm-1 which corresponds to Zn-O stretching mode. The broad band at 3529 cm-1 appeared due to O-H stretching mode of the hydroxyl group present in the surface of the nanoparticles. The band 1619 cm-1 arose due to the bending vibrational mode of surface adsorbed water molecules. The TEM image of synthesized ZnO (Fig.2) showed quasi-cylindrical particles of diameters 20-50 nm. The lattice fringes in HRTEM image are separated by 0.24 nm, which corresponds to the (101) plane of ZnO. The electron diffraction (ED) pattern indicated polycrystalline nature. The TEM image of spent catalyst (Fig.3) after five cycles of experiment reveals that the particles shape remain almost same but their diameters increased slightly due to agglomeration. The lattice fringes are separated by 0.25 nm, which corresponds to the (101) plane of ZnO. The ED pattern indicated polycrystalline nature. The EDX patterns (Fig.S1 in ESI) also indicated presence of Zn and O in the synthesized materials.
5
Fig.2.
6
Fig.3. ZnO NPs synthesized with the present synthetic methods has low BET surface area and pore volume of 15.65 (m2/g) and 0.093 (cm3/g) (Fig.4(a)). This follows the characteristics of a type IV isotherm with a type H3 hysteresis loop associated with aggregates of plate-like particles forming slit-like pores [17]. The Barrett Joyner Halenda (BJH) pore size distribution indicated that most of the pores range from 2 to 11 nm (Fig. 4(a)) Inset) and a pore size of 23.87 nm. The catalytic efficiency of ZnO NPs in degradation of metronidazole under ultrasound irradiation was investigated in presence of SB and the reaction was monitored by UV-visible spectroscopy (Fig.4(b)). The catalytic reaction was monitored spectrophotometrically by following the decrease of absorbance at λ max 318nm (characteristic absorption peak of MTZ) with time. The intensity of this peak decreases gradually during the irradiation and finally disappears within 27 min of the degradation processes. The absorbance of MTZ decreases only slightly without the catalyst indicating very slow reaction rate, whereas those with catalyst decreases abruptly, suggesting the faster reaction rate. The degradation of MTZ fitted well with the pseudo first order equation [11] and the value of ‘k’ was obtained from the slope of the graph between ln(Ao/A) and time (Fig.4(c)) and was found to be 0.072 min-1.
7
Fig.4. 3.4 Reclyability of the catalyst Another intriguing facet of this protocol is the easy recyclability of the catalyst. After the catalytic run, the catalyst was separated out from the solution, washed with methanol and vacuum dried to ensure purity of recovered catalyst. The recovered catalyst was further used to check the catalytic efficiency. The results of the recyclability test are shown in (Fig.4(d)). The ZnO NPs catalyst exhibits an excellent activity even after five cycles. 4. Conclusion In summary, a new facile approach to synthesis of ZnO NPs has been demonstrated. The procedure adopted herein is simple and may be readily utilized for large-scale synthesis. As synthesized ZnO NPs were found to be highly efficient heterogeneous catalysts for ultrasound-assisted degradation of metronidazole. Moreover, this catalytic protocol is simple and efficient with advantages of easy recovery and recyclability of the catalyst. Thus this new protocol may serve as viable alternative to numerous existing procedures for degradation of pharmaceuticals waste. Acknowledgement The authors thank SAIF, IIT Madras, SAIF, IIT Bombay and SAIF, North Eastern Hill University for analysis. The Director NIT Silchar is acknowledged for financial support.
8 References [1] Bing Z, Scott H, Raja R, Somorjai GA. Nanotechnology in Catalysis. Ottawa: Springer; 2007. [2] Lu H, Liao L, Li J, Wang D, He H, Fu Q, Xu L, Tian Y. J. Phys. Chem. B 2006;110:23211-4. [3] Lee SH, Kim J, Hong KH, Shin J, Kim S, Kim K. ACS Appl. Mater. Interfaces. 2012; 4:1365-70. [4] Tripathy N, Ahmad R, Song JE, Ko HA, HahnYB, Khang G, Mater Lett. 2014;136:171–4. [5] Kołodziejczak-Radzimska A, Jesionowski T. Materials. 2014;7:2833-81. [6] ChenY, Zhang C, Huang W, Situ Y, Huang H, Mater. Lett. 2015;141:294–7 [7] Yuan N, Zhang G, Guo S, Wan Z. Ultrason. Sonochem. 2016; 28:62–8. [8] Liu HN, Li GT, Qu JH, Liu HJ. J. Hazard. Mater. 2007;144:180–6. [9] Elshafei GMS, Yehia FZ, Dimitry OIH, Badawi AM, Eshaq G. Ultrason. Sonochem. 2014; 21:1358– 65. [10] Hu X, Fan J, Zhang K, Yu N, Wang J. Ind. Eng. Chem. Res. 2014;53:14623−32. [11] Paul B, Bhuyan B, Purkayastha DD, Dey M, Dhar SS. Mater. Lett. 2015;148:37-40. [12] Paul B, Bhuyan B, Purkayastha DD, Dhar S.S. Catal. Commun. 2015;69:48-54. [13] Paul B, Bhuyan B, Purkayastha DD, Dhar SS, Behera S. J. Alloys Compd. 2015;648:629-35. [14] Paul B, Bhuyan B, Purkayastha DD, Dhar SS. J. Mol. Liq. 2015; 212:813-817. [15] Liu Y, Zhoua J, Larbot A, Persin M. J. Mater. Process. Technol. 2007;189:379-83. [16] Al-Gaashani R, Radiman S, Tabet N, Daud AR. Mater. Chem. Phys. 2011;125:846-52. [17] Mou FZ, Guan JG, Xiao ZD, Sun ZG, Shi WD, Fan XA. J. Mater. Chem. 2011;21:5414-21.
Figure and table captions Fig.1. (a) Powder XRD patterns of ZnO nanocatalysts. (b) FT-IR Spectrum of ZnO NPs. Fig.2. (a, b) TEM images (c) HRTEM image and (d) ED pattern of ZnO nanocatalyst. Fig.3. (a, b) TEM images (c) HRTEM image and (d) ED pattern of ZnO nanocatalyst after five cycles. Fig.4. (a) N2 adsorption-desorption isotherms of ZnO nanocatalyst. (b) UV-visible spectra of degradation of metronidazole in presence of ZnO NPs as catalyst (c) plot of ln(Ao/A) vs. time (min) and (d) Recyclability tests of ZnO nanocatalyst.
9
Highlights Synthesis of ZnO nanoparticles by chemical precipitation cum hydrothermal heating. The XRD pattern shows hexagonal ZnO with average crystallite size 22 nm. TEM image showed quasi-cylindrical shape having diameters 20-50 nm. Particles showed significant catalytic activity in degradation of metronidazole.