CHAPTER 4 SYNTHESIS AND CHARACTERIZATION OF ZnO NANOPARTICLES 4.1 INTRODUCTION Because of the very small grain size and large surface area per unit mass, the nanostructured materials exhibit a variety of properties that are different and often considerably improved in comparison with those of conventional coarse grained polycrystalline materials. Ceramic nanoparticles have been under investigation for many years with much success in several areas including synthesis, surface science, texturology, destructive adsorption, catalysis, etc. The oxides of the metals in the middle of the periodic table (Sc to Zn) make up the semiconducting or metallic oxides TiO 2, NiO, Fe2O3, Cr2O3 and ZnO. ZnO has been chosen for the present study because it is an important electronic and photonicmaterial due to its wide direct band gap (3.37 ev) and large exciton binding energy (60 mev). ZnO shows a marked antibacterial activity in the neutral region (ph-7) and important mineral, essential to the human being. This chapter mainly deals with the preparation and study of ZnO nanoparticles. To synthesis ZnO nanoparticles two approaches are possible namely Top-down and Bottom-up approach. For the preparation of ZnO nanoparticles both mechanical milling (i.e., Top-down or physical), sol-gel and autocombustion (i.e., Bottom-up or chemical) methods are applied in the present study. Both the methods have their own importance such as physical method provides the advantage of production of nanoparticles in large quantity, however, chemical method provides control over the size of the particle, increase purity and homogeneity, etc. 4.2 EXPERIMENTAL DETAILS 4.2.1 Synthesis of ZnO nanoparticles ZnO nanoparticles are synthesized by three different methods namely sol-gel and autocombustion and mechanical milling method. All the chemicals used for the preparation of these nanoparticles are of AR grade and are used without further purification. Freshly prepared aqueous solutions of the chemicals are used for the 49
synthesis of nanoparticles. Here the details of the synthesis of ZnO nanoparticles by above stated methods are given below: (a) Sol-gel route: Initially, 1M aqueous solution of zinc-nitrate hexahydrate (Zn(NO3)2 6H2O) was prepared by dissolving it in deionized water. 10 ml of this solution was mixed with 35 ml of methanol (MeOH, as a solvent) with constant stirring. To this 5 ml of oleic acid as capping agent was added with constant stirring under dark conditions, and the stirring was continued further for 1 h. For gelation aqueous solution of NaOH (10 M) solution was added drop wise to the reaction mixture till a milky white solution is observed, the stirring was continued further for 18 h or till the solution turns into a thick gel. After gel is formed the treatment with high energy ultra sonicator for 2 h was carried out. The reactions involved for the process are as follow: Zn (NO3)2 6H2O + CH3OH Zn(OCH3)2+ NaOH Zn(OH)2 Zn(OCH3)2 + 2HNO3+ 6H2O 4.1 Zn(OH)2+ 2CH3ONa ZnO + H2O 4.2 4.3 Thereafter, the collected sample was washed by adding 50 ml of distilled water and centrifugation for 5 minutes a clear cake of precipitate was observed. The supernatant was decanted carefully, and this process was repeated thrice with water. Similar washing was done with the methanol for easy removal of organic residue. The cake of precipitated samples was dried at 110 C overnight for 12 hours, in a heating oven. After cooling, the sample is scraped off, collected and stored for further use. Thermal gravimetric analysis of the prepared samples were carried out (discussed under next head of this chapter) that suggest the calcination temperature. The sample, thus obtained, was calcinated at 600 C for 4 hours in air atmosphere. After cooling, the samples were scraped off, collected and stored for further use. The products thus obtained were analyzed for their purity and morphology using FESEM-EDAX and powder XRD. (b) Autocombustion method: Solutions of Zn(NO3)2 6H2O (0.1 M) and citric acid monohydrate (0.1 M, C6H8O7 H2O) were prepared by dissolving in deionized water [131]. Zinc-nitrate and citric acid solutions were mixed in 1:1 molar ratio and continuously stirred for 30 min at room temperature. Then the solution was heated at 80 C for 5-6 h (or depending on the reaction solution) till the reaction mixture transform into the gel. After gel was formed and dried, it burnt in a self-propagating combustion 50
manner until all gels were completely burnt (see Fig. 4.1) out to form a fluffy, loose powder according to the chemical reaction 4.4. Zn(NO3)2 + 3C6H8O7 + 2O2 ZnO + N2 + 6CO2 + 10H2O 4.4 Fig. 4.1: Different stages of reaction progress in autocombustion method. (c) Mechanical Milling This is a top-down approach, in this, the solid solution of the reactants were mixed together and milled in the high energy ball mill at very high velocity (say 400 to 800 rpm) in which the larger particles of the reactants break-down into smaller particles by the effect of continuously colliding balls into the reaction jar according to the reaction 4.5, a schematic representation of the process is shown here. For the present study ZnCl 2, Na2CO3 and NaCl were mixed together in 1:1:6 molar ratio in the solid form corresponding to the reaction 4.6. The mixture was taken in an agate jar to which agate balls (10 mm in diameter) were added to maintain 3:1 ball to powder weight ratio. The jar was then placed in a planetary ball mill PM 400/2 with a speed of 400 rpm for 50 h. The samples were drawn at every 5 hours interval to determine the size of the particles. 51
A Schematic diagram for process kinetics: Diffusion Reaction Nucleation Growth A (Solid, Bulk) + B (Solid, Bulk) C (Nanoparticles) + D (By-products) 4.5 ZnCl2+Na2CO3+ 6NaCl ZnCO3+ 8NaCl 4.6 (Where, A = ZnCl2; B = Na2CO3; C = ZnCO3; D = NaCl) ZnCO3 ZnO+ CO2 4.7 According to the reaction 4.6, NaCl was used as inert diluent and was added to the reactants so that the volume ratio of the ZnCO3: NaCl in the product phase was 1:8. The precursor was calcined at 400 C in air in a porcelain crucible to prepare the ZnO nanoparticles according to the reaction 4.7. Since the mechanochemical formed, ZnCO3 nanoparticles were isolated in the NaCl matrix. Thereafter, the sample was washed with deionized water to remove NaCl from the reaction mixture. The washed powder was dried in an oven and then calcined at 600 C. 4.3 CHARACTRIZATION OF SYNTHESISED PARTICLES BY VARIOUS TECHNIQUES 52 ZnO NANO-
Synthesized ZnO nanoparticles were characterized for their morphologies, structure, composition and particle size by using, TGA, XRD and FESEM/EDAX techniques. Some specifications and methods employed during characterization are as follows: 4.3.1 Thermal Gravimetric Analysis (TGA) The technique of thermal gravimetric analysis is concerned with an analysis of the variation of the weight of the sample with temperature, in an environment heated or cooled at controlled rate, is recorded as a function of time or temperature. This was done to determine the calcinations temperature for the products obtained by synthetic methods employed herein. In the thermogram weight loss is observed either for evaporation of moisture (which occurs in the range 100-130 C)/ melting of the sample or change in phase of the sample when the temperature is suitable for the reaction. Hence in a particular temperature range, a gradual decrease in weight is observed. A few representative thermograms are shown in Fig. 4.2-4.4. From the TG curve, it is clear that a significant weight loss is observed in the temperature range 400 C to 565 C in the thermograms shown in Fig. 4.2-4.4, which indicates that some structural or phase change takes place within this temperature range. The details of each spectra are given below: In the TGA of the product obtained by sol-gel method (Fig. 4.2) a gradual decrease in the weight is observed in the temperature range 22-141 C (~38.3 %) which suggest the loss of moisture/solvent or oleic acid (used as capping agent). In the DTA curve, an endothermic peak is observed at 126 C. Howevre, only 5.8 % wt. loss is observed in the temperature range 141-400 C. Further, 21.2 % wt. loss is observed in the temperature range 400-555 C, corresponding to this wt. loss step an exothermic peak is observed at 554 C that suggests the change in phase of the product. On further increase of temperature insignificant loss of weight is observed upto 900 C. In the TG curve (Fig.4.3) around ~3% wt. loss is observed 100-170 C, indicating evaporation of moisture content or dehydration. An exothermic peak ~188 C in DTA curve is observed as reported by Roy [133] for autocatalytic anionic oxidation reduction reaction of nitrates with citric acid, to initiate the formation of nano ZnO powder through the solid-state diffusion process. However, in the milled sample a new exothermic peak 53
at 504 C (energy, 5.08 J/mg) in DTA curve and at 507 C (rate of wt loss, 0.63 mg/min) in DTG curve is observed (see Fig. 4.3). Also in the corresponding TG curve a significant weight loss is observed in the temperature range 400 C to 575 C, indicating some structural or phase change. No further weight loss is observed, which demonstrates that the decomposition of the precursor does not happen above this temperature. For the sample milled mechanically for 45 h at 400 rpm an insignificant weight loss (i.e., 1.13%) is observed in the applied temperature range. Therefore, to choose calcinations temperature the wt. loss scale was expended, which suggest that the gradual weight loss is observed in the temperature range 200-500 C. Two peaks at 218 and 280 oc are observed in the DTG curve, however a broad exotherm is seen in the DTA curve. From the thermograms it has been observed around 600 C the stable residues may be ascribed as ZnO nanoparticles (Fig. 4.5-4.8), which is confirmed by XRD (Fig. 4.9-4.11). Hence, 600 C is chosen as calcination temperature for the precursor ZnO powder. The ZnO powder used in Ni-P-ZnO coating was calcined at 600 C for 3h in air atmosphere. The product thus obtained has spherical shape particles as shown in Fig 4.7 (a-b). Fig. 4.2: TG-DTA thermogram of ZnO synthesized by sol-gel method. 54
Fig. 4.3: TG-DTA thermogram of ZnO synthesized by autocombustion method. 200.0 280 Cel 18.3 ug/ min 101.0 218 Cel 10.7 ug/min 20 100.0 0 100.5 0.0 101 Cel 99.96 % -60-300.0 1207 Cel 98.87 % 300 Cel 99.21 % DTG ug/min -200.0-40 100.0 200 Cel 99.79 % TG % DTA uv -100.0-20 25 Cel 100.00 % 99.5-80 -400.0 400 Cel 98.85 % -500.0 500 Cel 98.74 % 700 Cel 98.72 % 899 Cel 98.77 % 1100 Cel 98.86 % -100 99.0-120 -600.0 200 400 600 Temp Cel 800 1000 1200 Fig. 4.4: TG-DTA thermogram of mechanically milled (45 h at 400 rpm) sample. 55
4.3.2 FESEM-EDAX Analysis Morphology and particle size were determined using FESEM at high magnification, i.e., up to 40,000X. To prepare specimen, carbon tape was used as a base that unfortunately, formed the matrix in some of the FESEM pictures. EDAX of the samples were carried out to determine the composition, i.e., elements present in the sample such as Zn, O, C and Na, etc. Peaks of K shell of Zn and O atoms were obtained in most of the cases. In the case of pure ZnO atomic % of Zn and O are in their stoichiometric ratio. Some representative FESEM-EDAX images of different morphology of nanometric ZnO synthesized by different methods are shown in Fig. 4.5-4.7. The product thus obtained was analyzed for their morphology particle size, extent of agglomeration and aspect ratio (for nanorods) using FESEM. FESEM micrographs (Fig. 4.5 a) suggest the formation of nanorods by sol-gel method under experimental conditions and show very less agglomeration in the particles. This sample also showed the best results during the antibacterial testing. The average diameter of nanorods is around 30-40 nm. At calcinations temperature 600 C, FESEM results (Fig. 4.5 b) show that the particles are mostly spherical and globular with less degree of agglomeration. The average particle size is again around 50-75 nm. It shows a significant change in morphology has taken place when calcinedat 600 C. FESEM micrographs of as-synthesized, ball-milled and calcined ZnO powder synthesised by autocombustion method are shown in Fig. 4.6(a-b). SEM micrographs (Fig. 4.6 a) suggest fluppy and porous morphology for as-synthesized ZnO powder was obtained which may be due to the evolution of gases at the time of combustion of reaction mixture. Huge amount of exhaust gases associated with a rapid combustion reaction leads to scattering of irregular sub-nanometric particles to form loose agglomeration morphologies. However, for ball-milled and calcined sample well defined spherical particles are observed (Fig. 4.6b) with less agglomeration. The size observed for the calcined sample by FESEM is in 50-100 nm range (Fig. 4.6 b). FESEM micrographs of samples prepared by 45 hours mechanical milling (Fig. 4.7 a) and calcined at 600 C (Fig. 4.7 b) were taken. FESEM analysis shows that highly agglomerated product is obtained for mechanically milled sample which on calcinations 56
at 600 C shows less agglomeration and has a triangular morphology with average size 50-60 nm. The average crystallite size after 45 hours milling is 140-150 nm. EDAX analysis of these samples are attached with their respective FESEM spectra and the inference drawn from the analysis is that the % of the elements present in the sample i.e., Zn and O along with a peak of Au (gold) should be in their proper stoichiometry. The elemental % corresponding to Zn and O are in good agreement with permissible error. Since gold coating has been carried out on the sample surface to make it conducting therefore, a peak of gold is observed in the EDAX of samples. TEM image of the sample prepared by autocombustion method then milled for 1 h at 400 rpm and then calcined at 600 C was also carried out and shown in Fig. 4.8 (a and b). TEM image of the sample shows the shape of the particles is intermediate between spherical and hexagonal shape with diameter of particles in 30-100 nm size range. SADE pattern Fig. 4.8 (b) of the sample suggests the crystalline nature of the sample. (a) (b) (c) 57
Fig. 4.5: FESEM micrograph and EDAX analysis of ZnO by sol-gel method (a,b) as synthesized ZnO nanorods and(c) calcined at 600 C. (a) (b) Fig.4.6: FESEM/EDAX of ZnO samples synthesised by autocombustion method (a) as-synthesised (b) 6h ball-milling and calcined at 600 C. 58
(a) (b) Fig.4.7: FESEM/EDAX images of mechanically milled samples (a) 45 h milled sample and (b) 45 h milled sample heated at 600 C. 59
(b) Fig. 4.8: TEM image with SADE pattern of ZnO synthesized by autocombustion method. 4.3.3 X-ray Diffraction Study X-ray diffraction pattern of the samples were carried out using Cu Kα target (i.e., 1.54 Å) with 2 /min angle of rotation of goniometry speed between 10-100 on 2θ scale. Plots were drawn between intensity and 2θ angle using the origin software, and the peaks were then compared with standard ZnxOy peaks given in ASTM indent catalogue (JCPDS card No. 00-036-1451). A few representative XRD spectra are shown below in Fig. 4.9-4.11. XRD pattern of ZnO nanoparticles synthesized by sol-gel method in as-synthesized and after calcinations are shown in Fig. 4.9 (a-b). XRD patterns show peaks corresponding to the crystal planes of (100), (002), (101) of ZnO along with less noise which suggest that the synthesized sample belong to the hexagonal crystal lattice of the ZnO. The spectra also suggest that the presence of other phases is negligible. The peaks observed are sharp, which indicate that compounds of only ZnO is present with minimum noise distortion or impurities. XRD spectra of the sample prepared using sol-gel method and calcined at 600 C are shown in Fig. 4.9 b, in which sharp peaks of ZnO indicate that particles are present in highly crystallized form. The XRD spectra of the sample prepared by autocombustion method in as-synthesized, ball-milled (6h), and heat treated (6h ball-milled and then calcined at 600 C for 3h) are shown in Fig. 4.10 (a-d). XRD spectra of as-synthesized and ballmilled ZnO powder are 60
almost same (except intensity of the peaks) and are in good agreement with the JCPDS file of ZnO (JCPDS card No. 00-036-1451, Fig. (4.10 a and b) indicating a single hexagonal ZnO phase with P63mc structure. The sharp and intense peaks of ZnO ballmilled sample indicate that the sample is having crystalline structure. However, for calcined sample, intensity of the peaks are lessen along with broadening of peaks, which clearly indicates the reduction of particle size (JCPDS card No. 01-080-0074, Fig. 4.10 c). The XRD peaks for (100), (002) and (101) planes indicate the formation of phase pure wurtzite structure of ZnO. The size of calcined ZnO particles was determined by TEM (Fig. 4.8 a-b) which is in the range of 30-100 nm range. The crystallite average size of ZnO powder calculated using Scherrer formula by taking FWHM for ((101): highest intensity peak) are 30-70 nm range which is in good agreement with TEM results. In the XRD spectrum of the sample prepared by mechanical milling (45 h and 400 rpm) and calcined at 600 C is shown in Fig. 4.11. The spectrum was analyzed by EXPERT software (NT-MDT SPM software) the presence of ZnO peaks along with some peaks of impurity (e.g., K2Zn6O7) are observed in the spectrum which indicates that complete washing of the sample does not take place. However, XRD spectrum shown in Fig. 4.11 is in good agreement with JCPDS (files No. 361451). 61
(a) (b) Fig. 4.9: XRD pattern of synthesized ZnO by sol-gel process (a) as-synthesised ZnO nanorods and (b) calcined at 600 C. 62
C B A Fig. 4.10: XRD pattern of nanosized ZnO synthesized by autocombustion method (a) as-synthesized (b) milled for 45 h at 400 rpm and (c) calcined at 600 C. (b) Fig. 4.11: XRD pattern of nanosized ZnO synthesized by mechanical milling (milled for 45 h at 600 rpm) andcalcinedat 600 C (Analyzed by EXPERT Software, NTMDT SPM software). 63
4.4 ANTIBACTERIAL ACTIVITY OF SYNTHESIZED NANOPARTICLES The synthesised ZnO powders were tested against both Gram positive (E. coli ATCC 25922) and Gram negative (Micrococcus leuteus) bacterial species. The powders are placed on the lawn of actively growing bacteria (as a spot), and bactericidal activity is determined in the form of zone of inhibition. ZnO nanorods synthesized by using sol-gel route show a clear zone of inhibition against E. coli ATCC 25922 (Fig. 4.12a) and Micrococcus leuteus (Fig. 4.13 a). On the other hand, there was no such zone observed for the sample calcined at 600 C (see Fig. 4.12 b and 4.13 b). Diffusion of the sample prepared by autocombustion method occurred in the LB agar medium and does not occur against the test bacterial species (see Fig. 4.12 a and 4.13 a) but when nanosized ZnO was calcined at 600 C, diffusion occurs upto very short range (Fig. 4.12 b and 4.13 b) when carefully viewed under microscope. Whereas, for the sample prepared by mechanical milling no such zone of inhibition is observed. The results observed here can be explained by considering the fact that the negatively charged capping agent (oleic acid) used for the preparation ZnO in sol-gel route and the distinct shape (rods) and size (30-40 nm) all these factors facilitate the attachment of toxic nanorods to peptidoglycan chains of the bacterial cell surface. For the particles prepared by autocombustion method a highly agglomerated sample may be responsible for no diffusion of the sample in as synthesized form which on calcinations acquires a spherical shape of 30-100 nm (diameter) which is responsible for its diffusion though that is of very short range. 64
(a) (b) Fig. 4.12: Spot-on-Lawn test, depicting the activity of ZnO (a) as-synthesised ZnO powder and (b) calcined at 600 C against E.Coli ATCC 25922 strain (Here: ZnO synthesised by, (1) sol-gel (2) autocombustion (3) mechanical milling method (4) control). (a) (b) Fig. 4.13: Spot-on-Lawn test, depicting the activity of ZnO (a) as-synthesised ZnO powder and (b) calcined at 600 C, against M. leuteus strain (Here: ZnO synthesised by, (1) sol-gel (2) autocombustion (3) mechanical milling method (4) control). 4.4 CONCLUSIONS 65
In this chapter ZnO nanoparticles were synthesized by three different methods namely sol-gel, autocombustion and mechanical milling method. By sol-gel method, nanorods were obtained but the yield of the product is very less and the product formed by mechanical milling method is not as pure as prepared by chemical methods. The product obtained by autocombustion method the purity of the sample and yield is high as well as the method is easiest for the preparation of ZnO nanoparticles for further use in the coatings. From the TG analysis of the samples, the calcinations temperature was chosen as 600 C. XRD analysis of the samples suggests the formation of ZnO particles. The crystallite average size of the sample calculated by using Scherrer equation for highest intensity peak (101) is in the range 30-80 nm range. For the as-synthesized and calcined samples FESEM/EDAX were carried out which shows that the nanorods are formed by sol-gel method with very less agglomeration. However, for the sample prepared by autocombustion method a fluppy powder was obtained which on milling and after calcinations produces somewhat spherical particle which is also confirmed by TEM image. The antibacterial activity of these samples was also carried out against both Gram positive (E. coli ATCC 25922) and Gram negative (Micrococcus leuteus) bacterial species. ZnO nanorods synthesized by using sol-gel route show a clear zone of inhibition against the test bacteria. On the other hand, there was no zone of inhibition is observed for the sample calcined at 600 C. Diffusion of the sample prepared by autocombustion method occurred in the LB agar medium, does not occur against the test bacterial species but when nanosized ZnO was calcined at 600 C, diffusion occur upto very short range. Whereas, for the sample prepared by mechanical milling no such zone of inhibition is observed. 66