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Morphological influence of ZnO nanostructures and their Cu loaded composites for effective photodegradation of methyl parathion
Solid State Sciences 99 (2020) 106045
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Morphological influence of ZnO nanostructures and their Cu loaded composites for effective photodegradation of methyl parathion Manpreet Kaur Aulakh, Satinder Kaur, Bonamali Pal, Satnam Singh * School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala, Punjab, 147004, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Methyl parathion degradation Morphological effect ZnO nanoparticles Photocatalytic activity Co-catalysis
This report signifies the synthesis of ZnO nanorods, cotton ball like structures, nanospheres and their Cu loaded counterparts for effective photocatalytic degradation of methyl parathion pesticide under UV light. Pesticides accumulation in ecosystem becomes major problem which affects health. Zinc oxide is a promising photocatalyst for degradation of pesticides and dyes because of its higher stability, low cost and non-toxic nature. Herein, the synthesized catalysts were characterised by diffused reflectance spectroscopy, X-ray diffraction, scanning elec tron microscopy techniques and transmission electron microscopy. ZnO nanorods (ZNR) proved to be better catalyst as compare to other synthesized nanostructures because of its elongated morphology. ZNR were nearly 1.4 times more efficient than ZnO cotton ball like structures. Moreover, photocatalytic activity was also enhanced by Cu loading due to low recombination rate of excitons. Cu loaded ZNR were found to degrade whole compound up to 99% in just 80 min whereas bare ZNR took 3 h under similar conditions. Thus, efficiency of CuZNR had been increased nearly 6 folds as that of bare ZNR.
1. Introduction Mostly in developing countries, pesticides are used to increase the crop production by killing unwanted pests. Organophosphorus pesti cides (OPs) are widely applied due to low cost and these are very effective to control weeds [1] and diseases [2]. However, these OPs are frequently detected in surface and ground water [3] due to their highly toxic nature because of inhibition of acetylcholinesterase enzyme which affects nervous system and cause health problems such as carcinoge nicity [4] and neurotoxicity [5]. Photocatalytic degradation is an effective route to lower OPs level in the environment. This specific path was firstly reported by D. F. Ollis and co-workers [6] in which 1,2-dibro moethane and its isomer were fully degraded to HBr and CO2 when treated with TiO2 under UV light. Nowadays, various reports of photo-degradation of many OPs are available in literature with plausible mechanism of reactions [7–9]. This method has many advantages over conventional techniques such as complete oxidation within few hours, no polycyclised product formation, in presence of cheap catalyst, etc. Thus, a better photocatalytic activity is shown with metal oxide (ZnO, TiO2, etc.) semiconductors towards harmful organic compounds oxida tion into non-toxic components under light irradiation. ZnO is highly explored n-type semiconductor [10] due to its green
properties, durability, cheap price, wide band gap energy (3.37 eV), thermally stable, high electron-hole pair energy (60 meV), good piezo electric [11] and optical properties due to its quantum confinement ef fects [12]. These nanostructures found to be excellent material for photocatalysis used in mineralisation of environmental pollutants [13, 14] and various magnetically separable catalysts based on ZnO are highly applicable in pollutants degradation [15]. Moreover, ZnO has also attracted a great deal of attention to handle waste water issues such as degradation of dyes [16,17], pesticides [18], pharmaceutical wastes, etc. In addition, it is well known fact that the photocatalytic activity (PCA) of nanoparticles (NPs) can be enhanced by varying shape and size [19–24]. Various techniques such as hydrothermal procedures, chemical vapor deposition, sputter deposition and microwave synthesis are used to synthesize ZnO NPs different morphologies like flowers and spherical shaped [25], nanowires [25] and many more as reported in literature. These nanostructures proved to be efficient due to excellent delocali sation of charge carriers and presence of larger interface for charge distribution. Moreover, modified ZnO nanocomposites have been investigated to activate these NPs in visible region by fabricating ZnO/CoMoO4 [26], ZnO/Ag/Ag2WO4 [27], ZnO/NiWO4/Ag2CrO4 [28], Fe3O4/ZnO/NiWO4 [29], ZnO/AgBr, and ZnO/Ag2CO3 [30], etc. Furthermore, PCA of ZnO NPs can be notably improved by loading
* Corresponding author. E-mail address: [email protected] (S. Singh). https://doi.org/10.1016/j.solidstatesciences.2019.106045 Received 15 July 2019; Received in revised form 4 October 2019; Accepted 20 October 2019 Available online 23 October 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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suitable co-catalyst which decreases recombination rate of photo generated excitons (e /hþ). Doping or loading of metals or metal oxides [31] introduce new empty energy level and it can accommodate elec trons which are transferred to conduction band (CB) of ZnO and cause electron-hole pair separation and in turn increasing catalyst efficiency. It is evidenced in literature [32] graphitic carbon nitride (g-C3N4) acts as a visible-light-driven photocatalyst with a distinctive 2D structure to enhance the activity of different catalyst support. Furthermore, choice of co-catalyst is also very important because it changes magneto-optical properties, band gap, ferromagnetism, etc. of the support. As reported, Cu is found to be efficient metal to be loaded due to its low cost, similar size as that of Zn which allows Cu to penetrate easily into substitutional sites of ZnO crystal lattice and modify its absorption/emission spectrum in light [33]. In this present work, different shaped ZnO NPs and their Cu loaded counterparts were synthesized and these were found to be effi cient for degradation of methyl parathion (MP) pesticide in the presence of UV light. It revealed that different morphologies and co-catalyst loading was highly influential for effective photocatalytic degradation.
2.3. Synthesis of ZnO cotton ball like structures (ZCB) In this typical procedure [35], 0.27 g of ZnCl2 (2.0 mM) and 0.27 g of NaOH (10.0 mM) were dissolved in 30 mL of DI water and stirred for 5 min. The solution was transferred to Teflon lined stainless sealed autoclave and heated at 80 � C for 24 h and the contents were cooled to the room temperature. The resulting white precipitates were separated by centrifuge, washed with distilled water and ethanol few times and dried at 60 � C for 12 h. 2.4. Synthesis of ZnO nanorods (ZNR)
2. Experimental
As reported in literature [36], 1.68 g of PVP was dissolved in 80 mL of DI water and added 2.3 mM of zinc acetate dihydrate along with stirring for 15 min followed by addition of 0.025 M (1 mL) of NaOH. The resultant solution was transferred to 100 mL of Teflon lined stainless steel autoclave and placed in furnace at 80 � C for 48 h and then cooled to room temperature. The resultant white precipitates were centrifuged and washed with DI water, with ethanol for few time and dried at 60 � C in air.
2.1. Chemicals and reagents
2.5. Photodeposition of copper metal
Zinc nitrite hexahydrate ((Zn (NO3)2⋅6H2O, 98%, loba chemicals), ammonia (NH3 merck specialties private ltd.), zinc chloride (ZnCl2, 97%, loba chemicals), sodium hydroxide (NaOH, 96%, avantor perfor mance material private ltd.), isopropyl alcohol, polyvinylpyrrolidinone (PVP, 90%, spectro chem. ltd), zinc acetate ((Zn(CH3COO)2⋅2H2O, 98%, loba chemicals), cupric nitrite ((Cu(NO3)2⋅3H2O, 99%, loba chemicals), starch, de-ionised (DI) water.
The prepared ZnO catalysts (100 mg) were taken in test tubes con taining 5 mL of aqueous isopropyl alcohol (50 vol%) and 1 wt% of cupric nitrite solution was added. High purity argon gas was purged for 25 min and irradiated by UV light (125 W Hg arc, 10.4 mW/cm2) under constant magnetic stirring for 6 h in a photochemical reactor. The obtained ma terial was separated by centrifugation and washed with DI water and ethanol and dried the precipitates at room temperature.
2.2. Synthesis of ZnO nanospheres (ZNS)
3. Characterization techniques and photocatalytic activity
Zinc nitrite hexahydrate, ammonia and starch were used for the preparation of ZnO nanospheres as reported previously [34]. Starch (2.5 g) was dissolved in 150 mL of boiled DI water to obtain clear starch solution. Then Zn(NO3)2⋅6H2O (0.01 mol/L) was added to the clear starch solution followed by continuously stirring for 15 min at 80 � C. The pH of solution was maintained between 8 and 9 by slow addition of ammonia to get a milky solution. This solution was stirred father for 30 min at 85 � C. The resulted precipitates were centrifuged, washed with DI water and ethanol, and dried at 50 � C. The dried powder was calcined at 500 � C in the presence of air to obtain ZnO spheres and were used for the further application.
The optical absorption spectra of synthesized ZNS, ZCB, ZNR and their Cu loaded composites was recorded with diffuse reflectance spec trophotometer (Avantes) using BaSO4 as standard. The crystallographic studies were carried out with X-ray diffractometer (PANalytical X’Pert PRO) with Cu Kα (λ ¼ 1.54 Ao) radiation. The external morphology, topography and elemental composition of NPs was analyzed by scanning electron microscopy (SEM, JSM-7600 F) and energy dispersive X-ray spectroscopy (EDS) respectively operated from 0.1 to 30 kV. Further, Cu-ZNR were also characterised by transmission electron microscopy (TEM, Hitachi 7500, 2Ao, 120 kV) to confirm the nanorods formation. The photocatalytic activity of prepared ZnO NPs and their Cu loaded
Fig. 1. (a) Diffused reflectance spectra of bare and Cu loaded ZnO nanocatalyts and (b) 2nd order derivative of bare ZnO catalysts.
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respectively. The quantification of CO2 was done with 180 ppm standard. 4. Results and discussion 4.1. Optical and morphological The absorption spectra of bare and Cu deposited ZnO NPs (nanorods, cotton ball like structures and nanospheres) as shown in Fig. 1(a). It depicted a strong absorption edges near 389 nm, 398 nm and 409 nm for ZNR, ZCB and ZNS respectively. The difference in absorption edges is mainly due to variation in the morphologies. Upon Cu loading over ZnO NPs, a new band appeared in 430–480 nm range and the absorption band edge obtained due to ZnO shifted towards the higher wavelength i. e. red shift. This red shift could be attributed to strong interactions be tween the surface of ZnO and Cu [37]. Moreover, second order deriva tive was also plotted so as to confirm λmax values as shown in Fig. 1(b) which clearly shows an appreciable change in the values of wavelengths due to size/shape effect of the synthesized particles. Further, the band gap (Fig. S1) for all bare catalyts was calculated by using Tauc’s relation, αhν ¼ A (hν - Eg)n where hν is energy of photon, α is absorption coeffi cient, A is constant, Eg is band gap of catalyst and n is transition type (n ¼ ½ for direct, 2 for indirect band gap). It can be seen that ZNR, ZCB and ZNS possess band gap energy of 3.14 eV, 3.09 eV and 3.0 eV respectively. It infers that band energy varies by changing the morphology of the particles which in turn shows variation in reaction efficiency of the catalysts. The phases and crystal structures of prepared ZnO catalysts were analyzed by X-Ray diffraction patterns (Fig. 2). The narrow and sharp peaks show the crystalline nature of prepared samples. The main diffraction peaks were found at 2 theta positions 31.98� , 34.33� , 36.48� , 47.57� , 56� , 62.9� , 66.1� , 68.28� , 72.81� and 76.97� with their corresponding planes (100), (002), (101), (102), (110), (103), (200), (112), (201) and (004) respectively. These peaks confirm the hexagonal crystal system of ZnO reported in JCPDS card 36–1451 [35,36,38] with lattice parameters a ¼ 3.251 Å, b ¼ 3.025 Å
Fig. 2. XRD pattern of various nanoparticles.
nanocomposites was evaluated by the degradation of MP. The reaction was carried in a pyrex test tube containing 20 mg of the catalyst and substrate (5 mL, 500 ppm) under constant stirring under UV radiation (125 W Hg arc lamp). The analysis of the products was done using UV spectrometer (Analytic Jena, SPECORD 205) after a fixed time interval. Further, CO2 released during the degradation of pesticides was deter mined by gas chromatography (GC) studies which was done by injecting 1 mL of gaseous mixture from the test tube (tightly sealed) into GC (NUCON-5765) equipped thermal conductivity detector and Porapak-Q column having flow of nitrogen gas as carrier gas. The temperatures of oven, injector and detector were set at 50 � C, 80 � C and 90 � C
Fig. 3. TEM images of synthesized Cu-ZNR. 3
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allowed low recombination of excitons. Further, the visible PL in tensity decreased after Cu loading due to scattering of radiations by the adsorbed Cu atoms [39] on the surface of the catalyst and the added Cu impurity may have resulted into non-radiative recombination which diminish PL emission. Furthermore, the Cu incorporation to prepared ZnO nanostructures create strong interactions between Cu–O as compared to Zn–O interactions which reduces the oxygen vacancies, thus decreasing PL intensity [40]. Nitrogen (N2) adsorption-desorption (Fig. 5 (a)) based Brunner Emmett and Teller (BET) study revealed type IV isotherm which corre sponds to mesoporous nature of the particles which showed similarity with reported literature [34]. ZNR, ZNS, Cu-ZNR and Cu-ZNS exhibited surface area of 31.21 m2 g-1, 19.0 m2 g-1, 23.4 m2 g 1and 10.7 m2 g-1 respectively. The surface area decreased after Cu loading due to pres ence of metal NPs over the surface of the support. The BJH (Barrett- Joyner-Halenda) plot showed the mean pore diameter (Fig. 5 (b)) of bare ZNR and ZNS was found to be 2.06 nm and 2.62 nm which was further increased to 5.2 nm and 13.5 nm respectively with decreased pore volume after Cu deposition due to internal pore strain which is expected after the loading
Fig. 4. Photoluminescence (PL) spectra of bare and Cu loaded ZnO nanostructures.
and c ¼ 5.201 Å. The crystallite size of ZNR, ZCB and ZNS was found to be 40.027 nm, 37.23 nm and 35.46 nm respectively. It was noted that the no separate peak of copper oxide was found in metal loaded XRD spectra and similar pattern was obtained after Cu loading without any observable change except slightly more intense peaks. It shows that there was no influence of Cu ions loading towards the phase and structure of ZnO crystal system. The morphologies of synthesized nanocatalysts were studied by SEM which shows the formation of respective different synthesized shapes (ZNR, ZCB and ZNS) shown in Fig. S2. Moreover, Cu-ZNR were also analyzed by TEM to confirm nanorods formation which is clearly shown in Fig. 3(a) and (b). The prepared nanorods were nearly of 34 � 5 nm dimensions. It was also observed that Cu deposited nanorods formed aggregated particles which in turn took shape of nanoflowers. Further more, the elemental composition of Cu loaded ZNR was further confirmed by EDS analysis (Fig. S3) which confirmed Cu deposition.
4.3. Photocatalytic activity (PCA) The catalytic performance of all prepared nanostructures was eval uated for the degradation of MP pesticide. Figs. 6(a) and 7(a) displayed the changes in UV absorbance spectra in 350–450 nm range for the photo-oxidation of MP (500 ppm) with addition of 20 mg of bare commercially available bulk ZnO powder and prepared ZnO NPs (6 h) and their Cu loaded counterparts (80 min) respectively under UV light. The maximum decrease in peak intensity was observed with ZNR (Fig. 4 (a)) which reflects the morphological effect of NPs for effective pesticide degradation. Moreover, co-catalyst has also attributed to enhance re action efficiency by degrading the whole reactant just in 80 min as shown in Fig. 5(a). This reaction was observed to follow pseudo first order kinetics as depicted from plot of ln Co/C vs time plots (Figs. 6(b) and 7(b)) in which C is the final concentration at specific time and Co is the initial concentration of MP. The reaction rates were calculated (Fig. 9) according to Langmuir Hinshelwood equation, where k is rate constant in pseudo first order reaction:
4.2. Interfacial and BET studies
ln Co = C ¼ kt
The photoluminescence (PL) spectra illustrated in Fig. 4 in the range of 350–600 nm due to surface defects. The band at 399 nm is due to recombination of charged species (electrons and holes). The PL intensity of prepared ZNR was found to be lesser as compared to other bare ZnO NPs which might be due to the morphological effect or band energetics which may
Further, a comparative of MP degradation efficiency is elucidated in Fig. 8 which shows that for 3 h reaction time period under UV irradia tion, ZNR was able to degrade MP up to 95% whereas only 65% and 5% degradation occurred when treated with ZNS and commercially avail able bulk ZnO powder.
Fig. 5. Surface area and porosity study (a) Nitrogen adsorption-desorption isotherm (b) BJH curve of different photocatalysts. 4
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Fig. 6. (a) UV absorbance spectra of degradation of methyl parathion (MP, 500 ppm) and (b) the calculated degradation rates when treated with various bare ZnO NPs (20 mg) under UV light.
Fig. 7. (a) UV absorbance spectra of degradation of methyl parathion (MP, 500 ppm) and (b) the calculated degradation rates when treated with various Cu loaded ZnO NPs (20 mg) under UV light.
Fig. 9. Comparative amount of CO2 produced (shown in bars) with bare and Cu loaded catalysts treated for 3 h and 80 min respectively and values of rate constant (k, shown as line).
treated with Cu-ZNR which shows that metal deposited nanorods offer better charge transportation in one dimensional direction with low recombination rate and thus promotes higher catalytic efficiency [41, 42]. A plausible mechanism for pesticide degradation is shown in Scheme 1, ZnO NPs absorb energy (hν) greater or equal to its band gap energy which results into electronic excitation and transfer of electrons from VB
Fig. 8. Photocatalytic degradation of MP with bare and Cu loaded ZnO composites.
efficiency was greatly influenced by the synthetic method that indirectly controlled size and morphology of the material. Moreover, the PCA was enhanced up to nearly 99% in just 80 min when reactant was 5
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Scheme 1. Plausible mechanism for MP degradation with Cu–ZnO Nanoparticles.
Fig. 10. (a) Recyclability/reusability of Cu-ZNR up to 4 cycles for methyl parathion (MP) degradation and (b) corresponding XRD patterns of reused Cu-ZNR photocatalyst.
to CB and holes remain within VB. The electrons are then captured by metal particles so as to reduce recombination rate of excitons. These electrons and holes results into reduction and oxidation reactions to form superoxide anion radicals (O2�-) and hydroxyl radicals (�OH), respectively. The �OH radicals react with MP and finally oxidize it into CO2, NO3 , SO24 , PO34 and water molecules [43] but herein only CO2 amount was determined. Moreover, H2O2 (2.5 mg/mL) was also used as an electron scavenger while carrying out MP degradation reaction under nitrogen atmosphere so as to present a clear speculation about proposed mechanism. Cu-ZNR nanocomposites used for monitoring this reaction after purging with N2 for 25 min followed by UV irradiations. Further, the reaction analysis was carried out using UV–visible spectrometer. It was found that 90% MP degradation (Fig. S4 (supporting information)) was observed after 80 min which was less as compared to open reaction conditions (99%) using same photocatalyst. It showed that the electrons were scavenged by H2O2 to facilitate oxidation reaction with the help of generated holes in the reaction. Thus, this study evidenced the plausible mechanism as shown in Scheme 1 in which hydroxyl radicals (�OH) are used to decompose MP. Furthermore, GC was used for the quantification of produced CO2 during photodegradation of this pesticide with 180 ppm standard. The pesticide was photo-oxidized to give smaller fragments which eventually degrade into non-toxic fragments like CO2 and water. Fig. 9 shows that 0.88 μmol, 0.65 μmol and 0.57 μmol of
CO2 was produced with ZNR, ZCB and ZNS after 3 h reaction time. Furthermore, the amount of gas produced was different when MP was treated with metal loaded ZnO NPs. Nearly 1.2 μmol of CO2 evolved with Cu-ZNR which is almost double as that of Cu-ZNS. The quantity of gas produced during reaction reveals that much amount of pesticide had been totally degraded to smallest fragments (CO2 and H2O) while the rest of reactant might be converted to other smaller fragments which are not harmful. Moreover, the time interval can be increased to attain final degradation products. Additionally, the most efficient catalyst (Cu-ZNR) was monitored for recycling experiments up to 4 cycles (Fig. 10 (a)) and the reused catalyst was further analyzed using XRD technique (Fig. 10 (b)) after 2nd and 4th cycle. Although, the efficiency was reduced to nearly 93% and 82% after 2nd and 4th cycle but not much notable changes were seen in corre sponding XRD pattern which determines the stability of the catalyst and its crystal structure even after 5–6 h of light irradiation up to 4th cycle reusability. In general, metal particles over ZnO surface prevent photocorrosion of the catalyst by charge carrier separation which is also evi denced by XRD patterns. Moreover, the observed decrease in efficiency might be due to loss of few amount of the catalyst during washing. 5. Conclusion In summary, ZnO nanoparticles of different shapes and sizes have 6
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been synthesized successfully and applied for the photodegradation of MP. It was observed that among all bare catalysts, ZNR exhibited higher efficiency due to its elongated structure. The ZNR showed conversion almost up to 95% of MP to smaller fragments in the presence of UV light for 3 h. Furthermore, these prepared morphologies were decorated with Cu metal which resulted into enhanced activity. The synthesized CuZNR proved to be excellent by degrading the similar pesticide nearly 99% in just 80 min. This increase in the conversion efficiency and rate of photodegradation could be attributed to the low recombination rate of electrons and holes due to Cu loading and elongated nanorods.
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Funding information The authors are highly thankful to the DST-India (Department of Science and Technology) for the financial assistance under (SR/NM/NS1471/2014) Nanomission scheme. Declaration of competing interest The authors declare that they have no conflict of interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solidstatesciences.2019.106045. References [1] F. Longobardi, M. Solfrizzo, D. Compagnone, M. Del Carlo, A. Visconti, J. Agric. Food Chem. 53 (2005) 9389–9394. [2] C. Barata, A. Solayan, C. Porte, Aquat. Toxicol. 66 (2004) 125–139. [3] M. Liess, R. Schulz, M.D. Liess, B. Rother, R. Kreuzig, Water Res. 33 (1999) 239–247. [4] M.C. Alavanja, J.A. Hoppin, F. Kamel, Annu. Rev. Public Health 25 (2004) 155–197. [5] R. Kamanyire, L. Karalliedde, Occup. Med. 54 (2004) 69–75. [6] T. Nguyen, D. Ollis, J. Phys. Chem. 88 (1984) 3386–3388. [7] X. Liu, X. Wu, Z. Long, C. Zhang, Y. Ma, X. Hao, H. Zhang, C. Pan, J. Agric. Food Chem. 63 (2015) 4754–4760. [8] B.K. Avasarala, S.R. Tirukkovalluri, S. Bojja, J. Hazard Mater. 186 (2011) 1234–1240. [9] J. Andersen, M. Pelaez, L. Guay, Z. Zhang, K. O’Shea, D.D. Dionysiou, J. Hazard Mater. 260 (2013) 569–575.
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Morphological influence of ZnO nanostructures and their Cu loaded composites for effective photodegradation of methyl parathion Manpreet Kaur Aulakh, Satinder Kaur, Bonamali Pal, Satnam Singh * School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala, Punjab, 147004, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Methyl parathion degradation Morphological effect ZnO nanoparticles Photocatalytic activity Co-catalysis
This report signifies the synthesis of ZnO nanorods, cotton ball like structures, nanospheres and their Cu loaded counterparts for effective photocatalytic degradation of methyl parathion pesticide under UV light. Pesticides accumulation in ecosystem becomes major problem which affects health. Zinc oxide is a promising photocatalyst for degradation of pesticides and dyes because of its higher stability, low cost and non-toxic nature. Herein, the synthesized catalysts were characterised by diffused reflectance spectroscopy, X-ray diffraction, scanning elec tron microscopy techniques and transmission electron microscopy. ZnO nanorods (ZNR) proved to be better catalyst as compare to other synthesized nanostructures because of its elongated morphology. ZNR were nearly 1.4 times more efficient than ZnO cotton ball like structures. Moreover, photocatalytic activity was also enhanced by Cu loading due to low recombination rate of excitons. Cu loaded ZNR were found to degrade whole compound up to 99% in just 80 min whereas bare ZNR took 3 h under similar conditions. Thus, efficiency of CuZNR had been increased nearly 6 folds as that of bare ZNR.
1. Introduction Mostly in developing countries, pesticides are used to increase the crop production by killing unwanted pests. Organophosphorus pesti cides (OPs) are widely applied due to low cost and these are very effective to control weeds [1] and diseases [2]. However, these OPs are frequently detected in surface and ground water [3] due to their highly toxic nature because of inhibition of acetylcholinesterase enzyme which affects nervous system and cause health problems such as carcinoge nicity [4] and neurotoxicity [5]. Photocatalytic degradation is an effective route to lower OPs level in the environment. This specific path was firstly reported by D. F. Ollis and co-workers [6] in which 1,2-dibro moethane and its isomer were fully degraded to HBr and CO2 when treated with TiO2 under UV light. Nowadays, various reports of photo-degradation of many OPs are available in literature with plausible mechanism of reactions [7–9]. This method has many advantages over conventional techniques such as complete oxidation within few hours, no polycyclised product formation, in presence of cheap catalyst, etc. Thus, a better photocatalytic activity is shown with metal oxide (ZnO, TiO2, etc.) semiconductors towards harmful organic compounds oxida tion into non-toxic components under light irradiation. ZnO is highly explored n-type semiconductor [10] due to its green
properties, durability, cheap price, wide band gap energy (3.37 eV), thermally stable, high electron-hole pair energy (60 meV), good piezo electric [11] and optical properties due to its quantum confinement ef fects [12]. These nanostructures found to be excellent material for photocatalysis used in mineralisation of environmental pollutants [13, 14] and various magnetically separable catalysts based on ZnO are highly applicable in pollutants degradation [15]. Moreover, ZnO has also attracted a great deal of attention to handle waste water issues such as degradation of dyes [16,17], pesticides [18], pharmaceutical wastes, etc. In addition, it is well known fact that the photocatalytic activity (PCA) of nanoparticles (NPs) can be enhanced by varying shape and size [19–24]. Various techniques such as hydrothermal procedures, chemical vapor deposition, sputter deposition and microwave synthesis are used to synthesize ZnO NPs different morphologies like flowers and spherical shaped [25], nanowires [25] and many more as reported in literature. These nanostructures proved to be efficient due to excellent delocali sation of charge carriers and presence of larger interface for charge distribution. Moreover, modified ZnO nanocomposites have been investigated to activate these NPs in visible region by fabricating ZnO/CoMoO4 [26], ZnO/Ag/Ag2WO4 [27], ZnO/NiWO4/Ag2CrO4 [28], Fe3O4/ZnO/NiWO4 [29], ZnO/AgBr, and ZnO/Ag2CO3 [30], etc. Furthermore, PCA of ZnO NPs can be notably improved by loading
* Corresponding author. E-mail address: [email protected] (S. Singh). https://doi.org/10.1016/j.solidstatesciences.2019.106045 Received 15 July 2019; Received in revised form 4 October 2019; Accepted 20 October 2019 Available online 23 October 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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suitable co-catalyst which decreases recombination rate of photo generated excitons (e /hþ). Doping or loading of metals or metal oxides [31] introduce new empty energy level and it can accommodate elec trons which are transferred to conduction band (CB) of ZnO and cause electron-hole pair separation and in turn increasing catalyst efficiency. It is evidenced in literature [32] graphitic carbon nitride (g-C3N4) acts as a visible-light-driven photocatalyst with a distinctive 2D structure to enhance the activity of different catalyst support. Furthermore, choice of co-catalyst is also very important because it changes magneto-optical properties, band gap, ferromagnetism, etc. of the support. As reported, Cu is found to be efficient metal to be loaded due to its low cost, similar size as that of Zn which allows Cu to penetrate easily into substitutional sites of ZnO crystal lattice and modify its absorption/emission spectrum in light [33]. In this present work, different shaped ZnO NPs and their Cu loaded counterparts were synthesized and these were found to be effi cient for degradation of methyl parathion (MP) pesticide in the presence of UV light. It revealed that different morphologies and co-catalyst loading was highly influential for effective photocatalytic degradation.
2.3. Synthesis of ZnO cotton ball like structures (ZCB) In this typical procedure [35], 0.27 g of ZnCl2 (2.0 mM) and 0.27 g of NaOH (10.0 mM) were dissolved in 30 mL of DI water and stirred for 5 min. The solution was transferred to Teflon lined stainless sealed autoclave and heated at 80 � C for 24 h and the contents were cooled to the room temperature. The resulting white precipitates were separated by centrifuge, washed with distilled water and ethanol few times and dried at 60 � C for 12 h. 2.4. Synthesis of ZnO nanorods (ZNR)
2. Experimental
As reported in literature [36], 1.68 g of PVP was dissolved in 80 mL of DI water and added 2.3 mM of zinc acetate dihydrate along with stirring for 15 min followed by addition of 0.025 M (1 mL) of NaOH. The resultant solution was transferred to 100 mL of Teflon lined stainless steel autoclave and placed in furnace at 80 � C for 48 h and then cooled to room temperature. The resultant white precipitates were centrifuged and washed with DI water, with ethanol for few time and dried at 60 � C in air.
2.1. Chemicals and reagents
2.5. Photodeposition of copper metal
Zinc nitrite hexahydrate ((Zn (NO3)2⋅6H2O, 98%, loba chemicals), ammonia (NH3 merck specialties private ltd.), zinc chloride (ZnCl2, 97%, loba chemicals), sodium hydroxide (NaOH, 96%, avantor perfor mance material private ltd.), isopropyl alcohol, polyvinylpyrrolidinone (PVP, 90%, spectro chem. ltd), zinc acetate ((Zn(CH3COO)2⋅2H2O, 98%, loba chemicals), cupric nitrite ((Cu(NO3)2⋅3H2O, 99%, loba chemicals), starch, de-ionised (DI) water.
The prepared ZnO catalysts (100 mg) were taken in test tubes con taining 5 mL of aqueous isopropyl alcohol (50 vol%) and 1 wt% of cupric nitrite solution was added. High purity argon gas was purged for 25 min and irradiated by UV light (125 W Hg arc, 10.4 mW/cm2) under constant magnetic stirring for 6 h in a photochemical reactor. The obtained ma terial was separated by centrifugation and washed with DI water and ethanol and dried the precipitates at room temperature.
2.2. Synthesis of ZnO nanospheres (ZNS)
3. Characterization techniques and photocatalytic activity
Zinc nitrite hexahydrate, ammonia and starch were used for the preparation of ZnO nanospheres as reported previously [34]. Starch (2.5 g) was dissolved in 150 mL of boiled DI water to obtain clear starch solution. Then Zn(NO3)2⋅6H2O (0.01 mol/L) was added to the clear starch solution followed by continuously stirring for 15 min at 80 � C. The pH of solution was maintained between 8 and 9 by slow addition of ammonia to get a milky solution. This solution was stirred father for 30 min at 85 � C. The resulted precipitates were centrifuged, washed with DI water and ethanol, and dried at 50 � C. The dried powder was calcined at 500 � C in the presence of air to obtain ZnO spheres and were used for the further application.
The optical absorption spectra of synthesized ZNS, ZCB, ZNR and their Cu loaded composites was recorded with diffuse reflectance spec trophotometer (Avantes) using BaSO4 as standard. The crystallographic studies were carried out with X-ray diffractometer (PANalytical X’Pert PRO) with Cu Kα (λ ¼ 1.54 Ao) radiation. The external morphology, topography and elemental composition of NPs was analyzed by scanning electron microscopy (SEM, JSM-7600 F) and energy dispersive X-ray spectroscopy (EDS) respectively operated from 0.1 to 30 kV. Further, Cu-ZNR were also characterised by transmission electron microscopy (TEM, Hitachi 7500, 2Ao, 120 kV) to confirm the nanorods formation. The photocatalytic activity of prepared ZnO NPs and their Cu loaded
Fig. 1. (a) Diffused reflectance spectra of bare and Cu loaded ZnO nanocatalyts and (b) 2nd order derivative of bare ZnO catalysts.
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respectively. The quantification of CO2 was done with 180 ppm standard. 4. Results and discussion 4.1. Optical and morphological The absorption spectra of bare and Cu deposited ZnO NPs (nanorods, cotton ball like structures and nanospheres) as shown in Fig. 1(a). It depicted a strong absorption edges near 389 nm, 398 nm and 409 nm for ZNR, ZCB and ZNS respectively. The difference in absorption edges is mainly due to variation in the morphologies. Upon Cu loading over ZnO NPs, a new band appeared in 430–480 nm range and the absorption band edge obtained due to ZnO shifted towards the higher wavelength i. e. red shift. This red shift could be attributed to strong interactions be tween the surface of ZnO and Cu [37]. Moreover, second order deriva tive was also plotted so as to confirm λmax values as shown in Fig. 1(b) which clearly shows an appreciable change in the values of wavelengths due to size/shape effect of the synthesized particles. Further, the band gap (Fig. S1) for all bare catalyts was calculated by using Tauc’s relation, αhν ¼ A (hν - Eg)n where hν is energy of photon, α is absorption coeffi cient, A is constant, Eg is band gap of catalyst and n is transition type (n ¼ ½ for direct, 2 for indirect band gap). It can be seen that ZNR, ZCB and ZNS possess band gap energy of 3.14 eV, 3.09 eV and 3.0 eV respectively. It infers that band energy varies by changing the morphology of the particles which in turn shows variation in reaction efficiency of the catalysts. The phases and crystal structures of prepared ZnO catalysts were analyzed by X-Ray diffraction patterns (Fig. 2). The narrow and sharp peaks show the crystalline nature of prepared samples. The main diffraction peaks were found at 2 theta positions 31.98� , 34.33� , 36.48� , 47.57� , 56� , 62.9� , 66.1� , 68.28� , 72.81� and 76.97� with their corresponding planes (100), (002), (101), (102), (110), (103), (200), (112), (201) and (004) respectively. These peaks confirm the hexagonal crystal system of ZnO reported in JCPDS card 36–1451 [35,36,38] with lattice parameters a ¼ 3.251 Å, b ¼ 3.025 Å
Fig. 2. XRD pattern of various nanoparticles.
nanocomposites was evaluated by the degradation of MP. The reaction was carried in a pyrex test tube containing 20 mg of the catalyst and substrate (5 mL, 500 ppm) under constant stirring under UV radiation (125 W Hg arc lamp). The analysis of the products was done using UV spectrometer (Analytic Jena, SPECORD 205) after a fixed time interval. Further, CO2 released during the degradation of pesticides was deter mined by gas chromatography (GC) studies which was done by injecting 1 mL of gaseous mixture from the test tube (tightly sealed) into GC (NUCON-5765) equipped thermal conductivity detector and Porapak-Q column having flow of nitrogen gas as carrier gas. The temperatures of oven, injector and detector were set at 50 � C, 80 � C and 90 � C
Fig. 3. TEM images of synthesized Cu-ZNR. 3
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allowed low recombination of excitons. Further, the visible PL in tensity decreased after Cu loading due to scattering of radiations by the adsorbed Cu atoms [39] on the surface of the catalyst and the added Cu impurity may have resulted into non-radiative recombination which diminish PL emission. Furthermore, the Cu incorporation to prepared ZnO nanostructures create strong interactions between Cu–O as compared to Zn–O interactions which reduces the oxygen vacancies, thus decreasing PL intensity [40]. Nitrogen (N2) adsorption-desorption (Fig. 5 (a)) based Brunner Emmett and Teller (BET) study revealed type IV isotherm which corre sponds to mesoporous nature of the particles which showed similarity with reported literature [34]. ZNR, ZNS, Cu-ZNR and Cu-ZNS exhibited surface area of 31.21 m2 g-1, 19.0 m2 g-1, 23.4 m2 g 1and 10.7 m2 g-1 respectively. The surface area decreased after Cu loading due to pres ence of metal NPs over the surface of the support. The BJH (Barrett- Joyner-Halenda) plot showed the mean pore diameter (Fig. 5 (b)) of bare ZNR and ZNS was found to be 2.06 nm and 2.62 nm which was further increased to 5.2 nm and 13.5 nm respectively with decreased pore volume after Cu deposition due to internal pore strain which is expected after the loading
Fig. 4. Photoluminescence (PL) spectra of bare and Cu loaded ZnO nanostructures.
and c ¼ 5.201 Å. The crystallite size of ZNR, ZCB and ZNS was found to be 40.027 nm, 37.23 nm and 35.46 nm respectively. It was noted that the no separate peak of copper oxide was found in metal loaded XRD spectra and similar pattern was obtained after Cu loading without any observable change except slightly more intense peaks. It shows that there was no influence of Cu ions loading towards the phase and structure of ZnO crystal system. The morphologies of synthesized nanocatalysts were studied by SEM which shows the formation of respective different synthesized shapes (ZNR, ZCB and ZNS) shown in Fig. S2. Moreover, Cu-ZNR were also analyzed by TEM to confirm nanorods formation which is clearly shown in Fig. 3(a) and (b). The prepared nanorods were nearly of 34 � 5 nm dimensions. It was also observed that Cu deposited nanorods formed aggregated particles which in turn took shape of nanoflowers. Further more, the elemental composition of Cu loaded ZNR was further confirmed by EDS analysis (Fig. S3) which confirmed Cu deposition.
4.3. Photocatalytic activity (PCA) The catalytic performance of all prepared nanostructures was eval uated for the degradation of MP pesticide. Figs. 6(a) and 7(a) displayed the changes in UV absorbance spectra in 350–450 nm range for the photo-oxidation of MP (500 ppm) with addition of 20 mg of bare commercially available bulk ZnO powder and prepared ZnO NPs (6 h) and their Cu loaded counterparts (80 min) respectively under UV light. The maximum decrease in peak intensity was observed with ZNR (Fig. 4 (a)) which reflects the morphological effect of NPs for effective pesticide degradation. Moreover, co-catalyst has also attributed to enhance re action efficiency by degrading the whole reactant just in 80 min as shown in Fig. 5(a). This reaction was observed to follow pseudo first order kinetics as depicted from plot of ln Co/C vs time plots (Figs. 6(b) and 7(b)) in which C is the final concentration at specific time and Co is the initial concentration of MP. The reaction rates were calculated (Fig. 9) according to Langmuir Hinshelwood equation, where k is rate constant in pseudo first order reaction:
4.2. Interfacial and BET studies
ln Co = C ¼ kt
The photoluminescence (PL) spectra illustrated in Fig. 4 in the range of 350–600 nm due to surface defects. The band at 399 nm is due to recombination of charged species (electrons and holes). The PL intensity of prepared ZNR was found to be lesser as compared to other bare ZnO NPs which might be due to the morphological effect or band energetics which may
Further, a comparative of MP degradation efficiency is elucidated in Fig. 8 which shows that for 3 h reaction time period under UV irradia tion, ZNR was able to degrade MP up to 95% whereas only 65% and 5% degradation occurred when treated with ZNS and commercially avail able bulk ZnO powder.
Fig. 5. Surface area and porosity study (a) Nitrogen adsorption-desorption isotherm (b) BJH curve of different photocatalysts. 4
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Fig. 6. (a) UV absorbance spectra of degradation of methyl parathion (MP, 500 ppm) and (b) the calculated degradation rates when treated with various bare ZnO NPs (20 mg) under UV light.
Fig. 7. (a) UV absorbance spectra of degradation of methyl parathion (MP, 500 ppm) and (b) the calculated degradation rates when treated with various Cu loaded ZnO NPs (20 mg) under UV light.
Fig. 9. Comparative amount of CO2 produced (shown in bars) with bare and Cu loaded catalysts treated for 3 h and 80 min respectively and values of rate constant (k, shown as line).
treated with Cu-ZNR which shows that metal deposited nanorods offer better charge transportation in one dimensional direction with low recombination rate and thus promotes higher catalytic efficiency [41, 42]. A plausible mechanism for pesticide degradation is shown in Scheme 1, ZnO NPs absorb energy (hν) greater or equal to its band gap energy which results into electronic excitation and transfer of electrons from VB
Fig. 8. Photocatalytic degradation of MP with bare and Cu loaded ZnO composites.
efficiency was greatly influenced by the synthetic method that indirectly controlled size and morphology of the material. Moreover, the PCA was enhanced up to nearly 99% in just 80 min when reactant was 5
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Scheme 1. Plausible mechanism for MP degradation with Cu–ZnO Nanoparticles.
Fig. 10. (a) Recyclability/reusability of Cu-ZNR up to 4 cycles for methyl parathion (MP) degradation and (b) corresponding XRD patterns of reused Cu-ZNR photocatalyst.
to CB and holes remain within VB. The electrons are then captured by metal particles so as to reduce recombination rate of excitons. These electrons and holes results into reduction and oxidation reactions to form superoxide anion radicals (O2�-) and hydroxyl radicals (�OH), respectively. The �OH radicals react with MP and finally oxidize it into CO2, NO3 , SO24 , PO34 and water molecules [43] but herein only CO2 amount was determined. Moreover, H2O2 (2.5 mg/mL) was also used as an electron scavenger while carrying out MP degradation reaction under nitrogen atmosphere so as to present a clear speculation about proposed mechanism. Cu-ZNR nanocomposites used for monitoring this reaction after purging with N2 for 25 min followed by UV irradiations. Further, the reaction analysis was carried out using UV–visible spectrometer. It was found that 90% MP degradation (Fig. S4 (supporting information)) was observed after 80 min which was less as compared to open reaction conditions (99%) using same photocatalyst. It showed that the electrons were scavenged by H2O2 to facilitate oxidation reaction with the help of generated holes in the reaction. Thus, this study evidenced the plausible mechanism as shown in Scheme 1 in which hydroxyl radicals (�OH) are used to decompose MP. Furthermore, GC was used for the quantification of produced CO2 during photodegradation of this pesticide with 180 ppm standard. The pesticide was photo-oxidized to give smaller fragments which eventually degrade into non-toxic fragments like CO2 and water. Fig. 9 shows that 0.88 μmol, 0.65 μmol and 0.57 μmol of
CO2 was produced with ZNR, ZCB and ZNS after 3 h reaction time. Furthermore, the amount of gas produced was different when MP was treated with metal loaded ZnO NPs. Nearly 1.2 μmol of CO2 evolved with Cu-ZNR which is almost double as that of Cu-ZNS. The quantity of gas produced during reaction reveals that much amount of pesticide had been totally degraded to smallest fragments (CO2 and H2O) while the rest of reactant might be converted to other smaller fragments which are not harmful. Moreover, the time interval can be increased to attain final degradation products. Additionally, the most efficient catalyst (Cu-ZNR) was monitored for recycling experiments up to 4 cycles (Fig. 10 (a)) and the reused catalyst was further analyzed using XRD technique (Fig. 10 (b)) after 2nd and 4th cycle. Although, the efficiency was reduced to nearly 93% and 82% after 2nd and 4th cycle but not much notable changes were seen in corre sponding XRD pattern which determines the stability of the catalyst and its crystal structure even after 5–6 h of light irradiation up to 4th cycle reusability. In general, metal particles over ZnO surface prevent photocorrosion of the catalyst by charge carrier separation which is also evi denced by XRD patterns. Moreover, the observed decrease in efficiency might be due to loss of few amount of the catalyst during washing. 5. Conclusion In summary, ZnO nanoparticles of different shapes and sizes have 6
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been synthesized successfully and applied for the photodegradation of MP. It was observed that among all bare catalysts, ZNR exhibited higher efficiency due to its elongated structure. The ZNR showed conversion almost up to 95% of MP to smaller fragments in the presence of UV light for 3 h. Furthermore, these prepared morphologies were decorated with Cu metal which resulted into enhanced activity. The synthesized CuZNR proved to be excellent by degrading the similar pesticide nearly 99% in just 80 min. This increase in the conversion efficiency and rate of photodegradation could be attributed to the low recombination rate of electrons and holes due to Cu loading and elongated nanorods.
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