Zno nanoparticles via moringa oleifera green synthesis physical properties mechanism of formation

Accepted Manuscript Title: ZnO NANOPARTICLES via Moringa oleifera GREEN SYNTHESIS: physical properties and mechanism of formation Authors: N Matinise, X.G Fuku, K Kaviyarasu, N Mayedwa, M Maaza PII: DOI: Reference: S0169-4332(17)30242-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.219 APSUSC 35014 To appear in: APSUSC Received date: Revised date: Accepted date: 15-10-2016 12-1-2017 21-1-2017 Please cite this article as: N.Matinise, X.G.Fuku, K.Kaviyarasu, N.Mayedwa, M.Maaza, ZnO NANOPARTICLES via Moringa oleifera GREEN SYNTHESIS: physical properties and mechanism of formation, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.219 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 proof before it is published in its final 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 ZnO NANOPARTICLES VIA MORINGA OLEIFERA GREEN SYNTHESIS: PHYSICAL PROPERTIES & MECHANISM OF FORMATION N Matinise1-2, a*, X G Fuku1-2, b, K Kaviyarasu1-2, c, N Mayedwa1-2, d, M Maaza1-2, e 1UNESCO-UNISA Africa Chair in Nanoscience-Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk ridge, PO Box 392, Pretoria-South Africa 2Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, Old Faure road, Somerset West 7129, PO Box 722, Somerset West, Western Cape-South Africa a nmatinise@tlabs.ac.za, bfuku@tlabs.ac.za, ckasinathankariyarasu@gmail.com, nmyedi@gmail.com, emaaza@tlabs.ac.za d HIGHLIGHTS  Biosynthesis of ZnO nanoparticlesl by green process using Moringa Oliefera extract  Electrochemical studies were confirmed by cyclic and Square wave voltammetry  XRD, HRTEM, TGA/DSC, FTIR were used to characterized the nanoparticles Abstract The research work involves the development of better and reliable method for the bio-fabrication of Zinc oxide nanoparticles through green method using Moringa Oleifera extract as an effective chelating agent The electrochemical activity, crystalline structure, morphology, isothermal behavior, chemical composition and optical properties of ZnO nanoparticles were studied using various characterization techniques i.e Cyclic voltammetry (CV), X-ray powder diffraction (XRD), High resolution transmission electron microscopy (HRTEM), Selected area electron diffraction (SEAD), Differential scanning calorimetry/thermogravimetric analysis (DSC/TGA), Fourier Transform Infrared analysis (FTIR) and Ultraviolet spectroscopy studies (UV–vis) The electrochemical analysis proved that the ZnO nano has high electrochemical activity without any modifications and therefore are considered as a potential candidate in electrochemical applications The XRD pattern confirmed the crystallinity and pure phase of the sample DSC/TGA analysis of ZnO sample (before anneal) revealed three endothermic peaks around 140.8 ⁰C, 223.7 ⁰C and 389.5 ⁰C These endothermic peaks are attributed to the loss of volatile surfactant, conversion of zinc hydroxide to zinc oxide nanoparticles and transformation of zinc oxide into zinc nanoparticles Mechanisms of formation of the ZnO nanoparticles via the chemical reaction of the Zinc nitrate precursor with the bioactive compounds of the Moringa oleifera is proposed for each of the major family compounds: Vitamins, Flavonoids, and Phenolic acids Keywords: ZnO, nanoparticles, Green chemistry, electrochemical, Moringa oleifera INTRODUCTION ZnO is a member of the group II-VI semiconductors family, whose covalence is on the boundary between ionic and covalent semiconductors Being a wide and direct band-gap semiconductor (3.2 eV) at room temperature, it crystallizes in the wurtzite structure [1] Relatively to its competitor GaN, ZnO being sufficiently stable with a melting temperature of the order of 2248 K, making it withstanding high temperature treatments associated with doping and forming ohmic contacts The intrinsic properties of ZnO such as broad range of radiation absorption, high photostability, and large electrochemical coupling coefficients, makes it a candidate of choice for short wavelength optoelectronic and photonic devices [2-8] Most notable among these properties is the singular high excitonic binding energy of 60 meV, which allows for high efficiency operation of devices at and above room temperature However, on the most fundamental application of ZnO nanomaterials are extended further its technological applications such as sensor, energy generator, optoelectronics, bio-medicine and drug delivery design [1, 5-6] Because of its hardness and rigidity, it is an important material in the ceramics industry, while its low toxicity, biocompatibility and biodegradability make it a material of interest for biomedicine and in pro-ecological systems such as cosmetics [7-8] Moreover, papers such as ZnO have been studied and widely produced by different methods is based on the reaction between stoichiometric metallic salts in a mixed solution without using any organic dispersant or capping agent often attributed to band tail effect related to intrinsic defects [9-13] In Fig TEM images of Moringa Oleifera/ZnO composites are the bio-active materials are very important for the improvement of biomedicine and energy storage technologies From its various synthesis parameters which are used for ZnO materials is a versatile application The synthetic methods include sol-gel combustion, chemical vapor deposition, sonochemical, hydrothermal, wet polymerization, solvothermal, thermal decomposition, microwave assisted, precipitation, micro-emulsion, lyophilization and laser ablation [14-28] In addition to these standard synthesis methodologies, the green synthetic method employing biological plant extracts is one of the more extensively acknowledged routine due to its several advantages, such as require no additional chemicals, simple, environmental friendly, inexpensive and reliable method [22-39] In this article, we report the synthesis of ZnO nanoparticles using the Moringa oleifera’s natural extract as an effective chelating agent Their morphological, structural, optical, and electrochemical activity are presented Likewise, mechanisms of formation of the ZnO nanoparticles via the reaction of the Zinc nitrate and the phyotochemical bioactive compounds of the natural extract are presented EXPERIMENTS DETAILS Zinc nitrate hexahydrate (Zn(NO3)2 6H2O) was purchased from Sigma Aldrich, and Moringa oleifera leaves was from Burkina Faso (West Africa) Moringa Oleifera plant contains several phytochemical compounds in its natural extract as summarized in Fig [40] It is well established that such bioactive compounds exhibit effective antibacterial, antifungal activity, antitumor, antiepileptic, anti-inflammatory, antidiabetic, antiulcer, antioxidant and cholesterol lowering properties [31-33, 38] Preparation of Moringa Oleifera extract: Immersed 30 g of cleaned Moringa Oleifera leaves (dried) in 300 ml of boiled deionized water (DI-H2O) under magnetic stirrer for 1h45 at 50 ⁰C The mixture was cooled to room temperature and filtered through nylon mesh, followed by Millipore filter The filtered Moringa Olefeira extract was stored in refrigerator at ⁰C for further studies Synthesis of the targeted Zinc oxide nanoparticles: 50 ml of filtered Moringa oleifera extract was used to dissolve a different masses of 0.3444 to 10.333 g Zn(NO3)2 6H2O to produce concentrations (C1 to C6) The Zinc salts were completely dissolved in Moringa oleifera extract without any additional heat treatment The mixtures (pH 5) were covered with foil to avoid any photoinduced phenomenon and kept to room temperature Yet there was an obvious color change after 18 h, but no precipitate suggesting that any formation of a Zinc complex was in the form of a suspension Accordingly, and for each concentration was dried at 100 ⁰C in a standard oven and the powder was collected and washed several times with DI-H2O to remove any residue of the extract The obtained powders were subjected to an additional heat treatment in air at 500 ⁰C for h Characterization of the synthesized Zinc based nano particles Various techniques were used to characterize the physical, optical, thermal, chemical and electrochemical properties of Zinc Oxide nanoparticles A simultaneous differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) was used to characterize the decomposition and thermal stability of nanoparticles measured from 50 to 600 ⁰C at the heating rate of 10 ⁰C /min High Resolution Transmission Electron Microscopy (HRTEM) analysis was carried out on a Philips Technai TEM instrument operated at an accelerating voltage of 120 kV The Energy Dispersive Xray Spectroscopy (EDS) was performed on an EDS Oxford instrument for elemental analysis The phase identification of the annealed powders ZnO (C1 to C6) were performed on an X-Ray diffraction Model Bruker AXS D8 advance with radiation Cuk =1.5406 Å A Fourier transforminfrared (FT-IR) absorption spectrometer (Shimadzu 8400s spectrophotometer, 400-4000cm-1) was used to verify the surface coating and the chemical bonding The UV-VIS-NIR experiments were performed on a Nicolette Evolution 100 Spectrometer (Thermo Electron Co-operation, UK) to analyze the optical properties The spectra were recorded in the wavelength range of 350–700 nm Electrochemical properties The electrochemical analysis of ZnO electrode was performed by cyclic and square wave, conducted on BAS 100W integrated automated electrochemical workstation from Bio Analytical Systems (BAS, West Lafayette, IN) The electrochemical measurements were carried out in a threeelectrode system, in which glassy carbon (GC) electrode as working, platinum wire as counter and Ag/AgCl as reference electrodes with a M NaCl salt brigde solution and 0.1 M NaOH as a supporting electrolyte The voltammetry (CV) measurements were carried out at a potential window of -1000 mV to 1000 mV at various scan rates Preparation of GC/ ZnO electrode: The amount of ZnO nanoparticles was dissolved in ethanol and added ul of % nafion solution The solution was ultra-sonicated using warm water bath for 15 minutes Alumina micro-polish (1.0, 0.3 and 0.05 mm alumina slurries) and polishing pads (Buehler, IL, USA) were used for polishing the electrode (GC) surface area before and after measurements The solution was drop-coated on the surface area of a glassy carbon electrode and dried in the oven at 35 °C for 1h All experimental solutions were de-oxygenated by bubbling with high purity argon gas for 15 and blanketed with argon during all measurement RESULTS AND DISCUSSION Thermogravimetric & calorimetry investigations Fig reports typical DSC/TGA curves of the biosynthesized ZnO based product at a heating of 10 ⁰C /min for the sample of concentration C6 TGA profile exhibits a continuous weight loss with quasi sharp changes occurring at 139, 223, and 392 ⁰C followed by nearly a constant plateau Hence, annealing above 400 ⁰C seems to guarantee the formation of stable ZnO nanoparticles The DSC curve of the biosynthesized Zn based product reveals endothermic peaks centered at about 139, 223 and 392 ⁰C Based on similar TGA/DSC results reported in the literature [26], the peak around 139 ⁰C might be attributed to the loss of volatile surfactant molecules adsorbed on the surface of Zinc based complexes during synthesis conditions whereas the one around 223 ⁰C is ascribed to the conversion of Zinc complex to Zinc hydroxide The 3rd endothermic peak at 392 ⁰C could be assigned to the formation of Zinc oxide nanoparticles and decomposition of organic materials Morphology& elemental analysis of the synthesized particles Fig reports typical HRTEM images of the biosynthesized ZnO based products which was dried at 100 ⁰C and annealed at 500 ⁰C (Concentration C6) In the different cases, the particles are nanoscaled with a certain degree of shape anisotropy in the case of the 500 ⁰C annealed sample For the 100 ⁰C dried sample, one observes a series of small particles with size ranging from to 10 nm approximately with a certain degree of short range atomic ordering The annealed sample at 500 ⁰C consists of highly crystalline nanoparticles with edged interfaces Their average spherical equivalent diameter ranges from 16 to 31.9 nm As one can notice in Fig 3, this later sample contains a population of small rods with the basal and longitudinal sizes ranging within 13-28 nm and 32-61 nm respectively This shape anisotropy could be related to the preferential and fast growth mechanism of ZnO in the c-direction Fig which reports the selected areas electron diffraction patterns indicates that the biosynthesized Zn based products which was dried at 100 ⁰C and annealed at 500C (Concentration C6) are amorphous and crystalline respectively More accurately, the annealed sample at 500 ⁰C exhibits a polycrystalline structure mainly as per the observed (110), (102), (101), (002) and (100) diffraction rings It is noteworthy to observe that the (002) diffraction ring seems to be the most intense This could be due to the contribution of the nanorods component Fig shows the corresponding energy dispersive X-ray spectroscopy spectra of the biosynthesized Zn based products which was dried at 100 ⁰C and annealed at 500 ⁰C We distinguish as elements C, O, Zn, and Cu in addition to K peak at mid energy channels This latter element K which quasidisappears after annealing at 500 ⁰C, is likely to originate from the bio-compounds of the Moringa oleifera natural extract Likewise, the C and O could partially originate from the natural extract too The rest would originate from the C coated Cu grid and the ZnO formed particles respectively The elemental ratio Zn/O is of the order of 1.010.05 for the sample annealed at 500 ⁰C, suggesting, a priori, the formation of stoichiometric ZnO Structural analysis and X-rays diffraction Fig reports the XRD spectra of the annealed Zinc oxide (500 ⁰C) powdered samples corresponding to the various concentrations Cj in the angular range 2Θ =30-70 deg As one can notice, all XRD profiles exhibit distinguishable Bragg diffractions Yet there is a slight angular shift, these latter peaks fit with the Bragg angular positions of the pure hexagonal wurtzite structure of ZnO as per the JCPDF file nº00 -036-1451 denoting the single phase nature of the ZnO nanoparticles and their non-textured nature The Miller indexation of the various Bragg diffractions is (100), (002), (101), (102), (110), (103), (200), (112) and (201) Using the corresponding reticular plane distance’s relation dhkl = (4/3(h2+hk+k2)/a2+l2/c2)-1/2, the average values of the lattice parameters aexp and cexp have been derived and summarized in Table As shown in Fig 7, excluding the values corresponding to the lowest concentration i.e C1, both aexp and cexp decrease with the initial Zinc precursor’s concentration reaching the bulk values at the highest concentration C6: abulk= 0.32495 nm and cbulk=0.52069 nm Such a trend exhibited by aexp and cexp suggest that the nanoparticles are under elongation (negative compression) for smaller sizes and that such a strain/stress dissipates as the nanoparticles’ size increases This can be quantified via the ratio Δd/dbulk (Δd = [dexp - dbulk]) which is positive in general sustaining the fact that both the a and c directions are under stretching conditions Likewise, one can notice that all the Bragg peaks are relatively broad with a width at half maximum 1/2 >0.5 deg sustaining the nanoscale aspect of the formed ZnO even after the thermal annealing Yet the ZnO nanoparticles exhibit a shape anisotropy, the Debye-Scherrer approximation (Ø∼0.9λ/ 1/2 cos(B)) was used here to estimate the variation of their average size with the concentration Ø = f(Cj) (Table1) Accordingly to Fig 8, the spherical equivalent diameter Ø varies in a quadratic way with Cj Optical analysis In UV Fig 9, reports the absorbance profile which starts decreasing 350 nm approximately with an inflexion point about 380 nm is equivalent of 3.2 eV [41] This value is in good agreement with the bandgap of ZnO and hence the purity and the single phase nature of the biosynthesized ZnO nanoparticles obtained after annealing at 500 ⁰C Vibrational analysis and potential mechanism of formation In view of confirming the composition, purity, the nature of the functional groups on their surfaces as well as identifying the potential mechanism (s) of ZnO formation, Fourier Transform Infrared (FTIR) spectroscopy was used Such a study was carried out on the Moringa oleifera natural extract and the biosynthesized Zn based product which was dried at 100 ⁰C as well as the ZnO nanoparticles obtained following the annealing at 500 ⁰C (both at concentration C6) Fig 10 reports the corresponding FTIR spectra The Moringa Oleifera extract displays a rich set of broad IR absorption bands located at 2909-36098 cm-1, 2063-2182 cm-1, 942-1714 cm-1 and 473-703 cm-1 spectral regions The intense broad band region at 2909 – 3698 cm-1 can be assigned to O-H, NH2, H3CO, HO-C=O and C–H aromatic stretching bands of the various bioactive compounds (Fig 1) The absorption band at 2063-2182 cm-1 corresponds to N=C=S stretching vibrations which could involve the thiamine based bioactive compounds The absorption region at 942-1714 cm-1 is due to C=O, C=N, NH, C=C aromatic stretching vibrations Stretching vibrations located at around 473703 cm-1 represent C-H, C=C, N-H In generally, metal oxides are characterized by intrinsic absorption bands below 1000 cm-1 (the so called fingerprint region) arising from inter-atomic vibrations [18-26] The FTIR spectrum of the biosynthesized Zinc nanoparticles showed an absorption peak exhibited in lower wavenumbers (481 cm-1) which is associated to the stretching mode of Zn-O and in higher wavenumbers which can be assigned to O-H group [27] As in the case of the photoluminescence, this could be an indication of the formation of ZnO clusters during the early stage of the biosynthesis The plant extract showed a broad peak range at 2909-3698 cm-1 which indicates the presence of biological compounds mentioned above, further after the synthesis of ZnO NPs the broad peak became small and shifted towards lower wavenumbers 3400 cm-1 (NH, O-H stretching) Excluding the intrinsic Zn-O band centered at 481 cm-1, the FTIR spectrum of ZnO annealed at 500 ⁰C reveal no significant absorption peak at higher wavenumbers, indicating the nature of the formed ZnO nanoparticles Considering reference (Gurib-Fakim 2006) and the various included database, and in view of understanding the transformation of Zn(NO3)2 6H2O salt to ZnO nanoparticles by the action of different biological compounds which would act as both chelating and capping agents, the proposed mechanism of biosynthesis of nano-scaled ZnO is proposed in Fig 11 Three chemical reactions of the solvated Zn2+ ions are considered with the phytochemicals of the Moringa oleifera i.e with a phenolic acid, a flavonoid and vitamin based compounds An altered chemical behavior of L- ascorbic acid and Zinc nitrate, probable oxidation of biological compound i.e L-ascorbic acid to L-dehydro ascorbic acid via free radical, followed by electrostatic attraction between free radical and cation the of precursors Electrochemical properties Voltammetric techniques (CV and SWV) were used to evaluate the electro-catalytic properties of ZnO nanoparticles films on glassy carbon electrode by studying the reversibility of electron transfer Typical voltammograms of bare glassy carbon electrode (unmodified) and modified glassy carbon with ZnO nanoparticles are shown in Fig 12 (a & b.) Based on the results, there is no peak observed for the unmodified electrode (bare GCE); the peaks were observed after modified electrode with ZnO nanoparticles In the positive-going sweep, a strong reduction peak in Fig 12a observed at around ≈ -500 mV which can be attributed to the reduction of Zn oxide into elemental Zn during the cathodic sweep: Zn – O + 2H+ +2e-  Zn +H2O The shape of the CV curves indicates the pseudo-capacitive characteristics of the loaded ZnO nanoparticles Further characterization was carried out using SWV as shown in Fig 13b, for the confirmation of CV analysis However SWV is a very sensitive technique compared to CV According to the observed results, the ZnO nanoparticles show good electrochemical behavior and therefore are considered as a promising electrocatalyst for electrochemical applications The effect of scan rate (v) on the electrochemistry of ZnO NPs was studied as shown in Fig 12c; where the chosen scan rates (v) were 20, 40, 60, 80 and 100 mV/s The electrochemical performance of the material can be evaluated by observing the peak current in a cathodic and anodic scan The reduction peak current increases linearly as a function of scan rate, suggesting a fast diffusion controlled electrolyte ion transport kinetic at the interface and the stability of nanoparticles on the electrode The reduction peak potentials shift slightly to more negative values, which may attributed to the fast faradaic redox reaction because of the good interaction between the conductive Glassy carbon electrode and the electroactive ZnO in the alkaline electrolyte 3 CONCLUSION Zinc oxide nanoparticles with particle size varying from 12.27 and 30.51 nm have been successfully synthesized natural by Moringa oleifera extract and characterized using different techniques The XRD and EDS studies have shown that an annealing at about 500 ⁰C in air is required for the synthesis of pure wurtzite ZnO phase This was confirmed via the XRD investigations shed-lighting on the polycrystalline nature of the nanoparticles Likewise, the trend exhibited by the ZnO lattice parameters suggest that the nanoparticles are under negative compression for smaller sizes and that such a strain/stress dissipates as the nanoparticles’ size increases In addition, it was found that the spherical equivalent diameter Ø of the ZnO nanoparticles varies in a quadratic way with the Zn precursor Cj Considering the various database in literature, and the FTIR observations, proposed mechanism of biosynthesis of nano-scaled ZnO from Zn(NO3)2 6H2O salt and its reaction with different biological compound are proposed Three chemical reactions of the solvated Zn2+ ions are considered with the phytochemicals of the Moringa oleifera i.e with a phenolic acid, a flavonoid and vitamin based compounds The voltammetric analysis of the CV studies indicated the likely reduction of Zn oxide into elemental Zn The CV results exhibited that the ZnO nanoparticles have a good electrochemical activity and are therefore considered as a potential electrocatalyst ACKNOWLEDGMENTS This research was generously supported by Grant 98144 of the National Research Foundation of South Africa, iThemba LABS, the UNESCO-UNISA Africa Chair in Nanosciences and Nanotechnology, to whom we are all grateful 5 REFERENCES J Kennedy, P P Murmu, E Manikandan and S.Y Lee, Investigation of structural and photoluminescence properties of gas and metal ions doped zinc oxide single crystals J Alloy Cmpnd 616: 614–617 (2014) C, Jagadish and S Pearton, Zinc Oxide Bulk, Thin Films and Nanostructures Elsevier Limited 1–20 (2006) D Segets, J Gradl, R.K Taylor, V Vassilev and W Peukert Analysis of optical absorbance spectra for the determination of ZnO nanoparticle size distribution, solubility, and surface energy ACS Nano 3: 1703–1710 (2009) J Wang, J Cao, B Fan, P Lu, S Deng and H Wang, Synthesis and characterization of multipod, flower-like, and shuttle-like ZnO frameworks in ionic liquids Mater Lett 59: 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DSC/TGA of synthesized ZnO NPs (before anneal) Figure HRTEM of ZnO NPs, 100 ⁰C dried and Annealed at 500 ⁰C Figure Selected electron diffraction patterns of ZnO NPs, 100 ⁰C dried and Annealed at 500 ⁰C Figure EDX spectra of ZnO NPs, Before anneal and Annealed at 500 ⁰C Figure XRD pattern of ZnO NPs annealed at 500 0C Figure Lattice parameters (a and c) of ZnO NPs annealed at 500 0C Figure Crystallite average of ZnO NPs annealed at 500 0C Figure UV-vis spectra of synthesized ZnO NPs (before annealing and annealed at 500 0C Figure 10 FT-IR spectra of Moringa Oleifera extract and synthesized ZnO Nps (Before anneal and annealed at 500 ⁰C) Figure 11 Proposed mechanism of synthesized ZnO Nps via different biological compounds Figure 12 Voltammetric curves of synthesized ZnO NPs: A Cyclic voltammetry, B Square wave voltammetry and C different scan rates Table1 The structure and Lattice parameters of ZnO nanoparticles annealed at 500 ⁰C Samples C6 C5 C4 C3 C2 C1 Spherical equivalent diameter Ø(nm) 30.51 23.70 17.10 14.78 13.95 12.27 aexp (nm) 0.32495 0.32536 0.32537 0.32565 0.3259 0.32561 cexp (nm) 0.52069 0.52136 0.52137 0.52183 0.52225 0.52177 ... conversion of zinc hydroxide to zinc oxide nanoparticles and transformation of zinc oxide into zinc nanoparticles Mechanisms of formation of the ZnO nanoparticles via the chemical reaction of the.. .ZnO NANOPARTICLES VIA MORINGA OLEIFERA GREEN SYNTHESIS: PHYSICAL PROPERTIES & MECHANISM OF FORMATION N Matinise1-2, a*, X G Fuku1-2, b, K Kaviyarasu1-2,... Maaza, Green synthesis of ZnO nanoparticles via Agathosma betulina natural extract, Mater Lett 161: 124-127 (2015) 40 D Raoufi, Synthesis and photoluminescence characterization of ZnO nanoparticles,