Research Journal of Recent Sciences _________________________________________________ ISSN 2277-2502 Vol. 4(ISC-2014), 197-201 (2015) Res. J. Recent. Sci. International Science Congress Association 197 Effect of pH values on surface Morphology and Particle size variation in ZnO Nanoparticles Synthesised by co-precipitation Method Swaroop K. and H.M. Somashekarappa* Centre for Application of Radioisotopes and Radiation Technology (CARRT), USIC, Mangalore University, Mangalagangotri-574 199, Karnataka, INDIAAvailable online at: www.isca.in, www.isca.me Received 1st November 2014, revised 16th January 2015, accepted 20th February 2015 AbstractZinc oxide (ZnO) nanoparticles of size varying from 16 to 31 nm were synthesised by co-precipitation method using zinc acetate dihydrate (Zn(CHCOO).2HO) and sodium hydroxide (NaOH) as precursor materials. The pH value of solution was varied to study the surface morphology and particle size variations in ZnO. The X-ray diffraction peaks of all the samples corresponds to hexagonal wurtzite structure of ZnO and the data also shows significant variation in particle size as well as the lattice strain of ZnO nanoparticles with respect to pH values of the solution. The Scanning Electron Microscopy (SEM) images show different morphology at different pH values. Hexagonal shaped nanorod structures were observed at pH value of 7 and 9, and plate like structures were observed “at pH values 10.5 and 12.5. Fourier Transform Infrared Spectroscopy (FT-IR) confirms the formation of ZnO at ~450 cm-1. UV-visible Spectroscopy (UV-vis) analysis shows symmetrical shift in the absorption edge towards the lower wavelength or higher energy region with decrease in particle size of the ZnO samples. Keywords: Zinc oxide, wurtzite, X-ray diffraction, Scanning Electron Microscopy, FT-IR. Introduction Zinc oxide (ZnO) is one of the most important II-VI semiconductor material with direct wide band gap (3.37 eV or 375 nm) and large exciton energy of 60 meV. This semiconductor has several favourable properties, including good transparency, high electron mobility, wide band gap, and strong room temperature luminescence1,2. ZnO exhibits the most splendid and abundant configurations of nanostructures that one material can form. ZnO has become an attractive inorganic material, owing to its unique properties and potential applications, electro and photo-luminescence devices, chemical sensors and so on3-7. The first requirement of any novel study of nanoparticles is the synthesis of the materials. The development of systematic studies for the synthesis of ZnO nanoparticles is a current challenge. Different techniques such as sol-gel, spray pyrolysis, thermal evaporation, wet chemical processes etc are used for the synthesis of ZnO nanoparticles. But co precipitation method has proven to be the simple, fast and economic way of synthesising ZnO nanomaterials8-10. Material and Methods Zinc acetate dehydrate (Zn(CHCOO).2HO) was procured from the Central drug house, Delhi and Sodium hydroxide (NaOH) was procured from Merck. All the chemicals used for the experiment were of analytical grade and used without further purification. MilliQ water was used throughout the experiment. ZnO nanoparticles were prepared by simple co-precipitation method. In this typical synthesis procedure four different concentration of NaOH varying from 0.2 to 0.8 M was added drop wise to four replicates of 0.1 M Zn(CHCOO).2HO solution and pH of all four final mixtures was measured. The precipitates was allowed to settle down, washed several times with MilliQ water and dried in hot air oven at 110 C for 6 hours. The powder so obtained was collected and used for further characterization. The chemical reaction for ZnO nanoparticles under precipitation condition at room temperature is described below: Zn(CHCOO).2HO + 2NaOH Zn(OH) + 2CHCOONa + 2HO Zn(OH) +2HO Zn(OH)2- + 2H Zn(OH)2- ZnO + HO +2OH- Crystallite size (D), lattice parameter (‘a’ and ‘c’), and average lattice strain (av) of the sample were estimated by powder X-ray diffraction (Rigaku Miniflex) using Cu-K line (=0.15406 nm) with a scan speed of 3 per min over a 2 range of 25-80. The surface morphologies of all the samples were obtained by Field- Emission Scanning Electron Microscope (FESEM) (ULTRA 55 FESEM, Karl Zeiss). Different functional groups and structural features were analysed using Fourier Transform Infrared Spectrophotometer (FTIR) (Shimadzu, Prestige 21). The UV– visible absorption spectrum (Shimadzu, UV-2600) was recorded for optical characterization. Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 4(ISC-2014), 197-201 (2015) Res. J. Recent. Sci. International Science Congress Association 198 Results and Discussion The pH values of all the solutions with respect to their NaOH concentration were measured with a digital pH meter and values were recorded. The typical powder XRD pattern of the ZnO nanostructures synthesised from precipitation method is shown in figure-1. The sharp intense peaks of ZnO confirms the high purity and good crystalline nature of ZnO and the peaks originated from (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) represents the hexagonal wurtzite structure of ZnO as per JCPDS (card no. 01-79-0206) standards. The lattice parameters ‘a’ and ‘c’ calculated were also in accordance with the reported value. The average grain size of the samples was estimated with the help of Debye Scherrer equation. (1) Where is the wavelength (Cu K), is the full width at the half- maximum (FWHM) of the ZnO (101) peak and is the diffraction angle. It has been observed that the average grain size calculated by equation 1. showed decreasing trend from 31 nm to 16 nm as the pH increases from 7 to 12.5. It has been observed that the FWHM of the peaks were inversely proportional to crystallite size, which indicates better crystalline quality with respect to crystallite size of ZnO. Figure-1 X-ray diffraction patterns with (hkl) values of ZnOnanoparticles synthesised by co-precipitation method with different pH values The lattice strain (av) has been calculated using tangent formula (equation 2). The calculated results show significant increase in the lattice strain associated with the reduction of crystallite size of ZnO. The graph between average lattice strain and particle size is shown in figure 2. The obtained results are in agreement with the work carried out by Muhammad et al11. The variation in the particle size, lattice constants ‘a’ and ‘c’ and average lattice strain corresponding to different pH values were tabulated in table-1. (2) Figure-2 Variation in average crystallite size (D) and average lattice strain (av) with samples of different concentration of NaOH The surface morphology of all the samples were characterised using FESEM. All the images show the formation of different surface morphology with respect to the samples. The figure 3(a) shows typical hexagonal rod like ZnO nanostructures corresponding to the sample Z1. The deformation of hexagonal rod like structures was observed in sample figure 3(b) which corresponds to sample Z2. The formation of plate like structures of sample Z3 and Z4 were observed in figure 2 (c) and 2 (d) respectively. The variation in the surface morphology of the samples is attributed to the lattice mismatch, chemical bonding across the interface and presence of residual oxides12FTIR analysis was done by mixing ZnO sample with potassium bromide (KBr) in open air atmosphere. The significant absorption peaks near 3350, 1550, 1400 and 450 cm-1 can be noticed from the spectrum (figure 4). The strong absorption peak at 450 cm-1represents the Zn-O stretching frequency13,14. The broad peak near 3350 cm-1 suggests the presence of the hydroxyl group (O-H)14. The weak bands near 1550 cm-1 and 1408 cm-1 represent C=O and C-O stretch respectively15,16Table-1 Structural information on ZnO nanoparticles grown with respect to the pH of the solution by co-precipitation method Sample name pH 2 FWHM ' Crystallite size ‘D’ in nm Lattice parameters Lattice strain in % ‘a’ in nm ‘c’ in nm Z1 7 35.95 0.2670 31 0.2884 0.4992 0.3591 Z2 9 35.85 0.2936 28 0.2892 0.5005 0.3960 Z3 10.5 35.92 0.3920 21 0.2886 0.4996 0.5277 Z4 12.5 36.04 0.4975 16 0.2877 0.4980 0.6673 Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 4(ISC-2014), 197-201 (2015) Res. J. Recent. Sci. International Science Congress Association 199 Figure-3 FESEM images of the ZnO nanoparticles based on the pH. (a) at pH 7 (b) at pH 9 (c) at pH 10.5 (d) at pH 12.5. Figure-4 FTIR spectrum of ZnO nanoparticles with respect to the pH values Figure 5 shows the UV-Visible absorption spectra of the nanostructures with a sharp excitonic absorption speak around ~368 nm. It is clear that the absorption edge systematically shifts to the lower wavelength with decreasing size of the nanoparticles17. Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 4(ISC-2014), 197-201 (2015) Res. J. Recent. Sci. International Science Congress Association 200 Figure-5 UV-Vis spectrum of ZnO nanoparticles with respect to the pH values Conclusion Highly crystalline and pure ZnO nanoparticles of different structures were successfully synthesised by simple co-precipitation method. The grown nanostructures were characterised by powder XRD, FESEM, FTIR spectroscopy and UV-Vis absorption spectroscopy. The powder XRD analysis confirms the hexagonal wurtzite structure of the ZnO nanoparticles and the sharp exitonic peaks suggests the high crystallinity of the ZnO nanocrystals. Also the decrease in the particle size is observed with increase in the pH of the solution. The average lattice strain (av) increased with decrease in crystallite size of the ZnO nanoparticles. The FESEM images showed different surface morphology with respect to pH. The FTIR analysis reveals the characterization peaks of Zn-O stretching. The blue shift observed in the UV–Vis spectrum is the typical signature of size connement in ZnO nanocrystals [15]. Variation in pH value has played crucial role in tailoring the crystallite size as well as different morphology of the ZnO nanoparticles. Acknowledgement: Authors are thankful to the Coordinator, DST-PURSE program, Mangalore University for permitting to use FESEM facility. Authors acknowledge the discussion with Dr. B. 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