Journal of Pure Applied and Industrial Physics, Vol.7(6), 239-245, June 2017 (An International Research Journal), IF = 4.715, www.physics-journal.org 239 ISSN 0976-5727 (Print) ISSN 2319-8133 (Online Improving the Optical Properties of ZnO Nanopowders Synthesized using Combustion Method through Ag + Co Doping T. Yesudoss 1 Post Graduate, Department of Physics, Annai Vailankanni Arts & Science college, Thanjavur - 613 503, Tamil Nadu, INDIA. email: tyesudossphy@gmail.com. (Received on: May 20, Accepted: June 7, 2017) ABSTRACT The present study reports the optical activities of undoped and Ag (2 at. %) + Co(5 at.%) doped ZnO nanopowders synthesized using combustion method. The synthesized particles have been tested to find the effect of doping on the antibacterial activities against Bacillus Cereus. Further structural, morphological, and optical properties were studied using X ray diffractometer (XRD), Field emission scanning electron microscope (FESEM), Transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), UV-vis spectroscopy and Photoluminescence spectroscopy, respectively. Keywords: ZnO:Ag:Co nanopowders; Combustion method ; XRD; FESEM; UV. 1. INTRODUCTION Metal-oxide semiconductor nanoparticles have been extensively investigated because of their tunable electronic, magnetic, antibacterial, photocatalytic and optical properties which pave the way for variety of applications in various fields. ZnO is one of such semiconductors which is non-toxic and low-cost material exhibiting greater photosensitivity, heat resistance, chemical stability and UV blocking capability 1,2. It is Generally Recognized as Safe material (GRAS) by the U.S. Food and Drug Administration 3. All these properties of ZnO make it a good antibacterial agent for biomedical applications such as drug delivery, bio-sensing and imaging etc 4. Investigations of the antibacterial activity of ZnO based nanoparticles yield interesting results. The antibacterial efficiency of ZnO can be enhanced by two main factors, viz. (i) enhanced generation of ROS and (ii) release of Zn 2+ ions from the surface of nanoparticles 5. Various researchers proved that doping of ZnO with suitable transition metals like Fe, Mn, Co, Ag is considered to be an effective way to enhance its antibacterial efficacy 6.
Among these elements, silver has been known to be a bactericide since ancient times 7. Recently, nanosized silver nanoparticles are widely used in several applications like photocatalytic, antibacterial activity, chemical and biological sensing, due to their surfaceenhanced Raman scattering (SERS), localized surface plasmon resonance (SPR), and metalenhanced fluorescence effects 8. Thus silver is selected as one the dopants in this study. Materials capable of performing multiple functions simultaneously or even sequentially in time are of significance to improve performance of products. As Co is a material of this kind, which can enhance simultaneously both the magnetic and antibacterial properties of ZnO, in the present work, Co is added as the second dopant along with Ag. Co is specifically selected as it has similar ionic radius (0.72 Å) as that of Zn (0.74 Å) 9, the abundant energy states of electron and more importantly, it has strong magnetic moment compared to other 3d metals which can be used to tune the magnetic properties of ZnO 10. In addition, Co has stability as a dopant within the ZnO host lattice, according to the lattice compatibility theory (LCT). Several techniques have been used for the synthesis of ZnO nanopowders such as sonochemical synthesis 11, hydrothermal method 12, sol-gel synthesis 13, polyol method 14 and combustion method 15. Of these methods, combustion method is one of the inexpensive and fascinating methods. Based on the Food borne Diseases Active Surveillance Network (Food Net) data published for the years 2004 and 2007, it is reported that the estimated occurrence of infections caused by various bacteria like Shigella did not diminished significantly 16. Moreover, foodborne illness-outbreaks create socio-economic burdens bringing the fear for the reemergence of infectious diseases. In addition, the increased antibiotic resistance ability of various pathogenic bacteria continues to remain as a great challenge to material scientists so that they should search for newer antibacterial materials with enhanced efficiencies. Keeping the above perception in mind, in the present study, the antibacterial activity of ZnO:Ag:Co against Shigella flexneri (gram negative) along with Bacillus cereus (Gram positive) is investigated. In the present work, undoped, and Ag (2 at.%) +Co (5 at.%) doped ZnO nanopowders have been synthesized using the combustion method and their antibacterial activity along with certain physical properties have been investigated and reported. 2. METHOD AND MATERIALS 2.1 Synthesis process Undoped, and Ag+Co doped ZnO nanopowders were synthesized using combustion method. The host precursor zinc nitrate hexahydrate [Zn(NO 3) 2.6H 2O] (0.2 M) is dissolved in 200 ml of de-ionized water to get an aqueous starting solution. Silver nitrate [AgNO 3], and cobalt acetate tetrahydrate Co(C₂H₃O₂)₂(H₂O)₄ are used as dopant precursors. Required amount of ammonia solution is added to maintain the ph value of the starting solution at 9. Polyethylene glycol is added as surfactant which can be used to get the gel formation and to 240
achieve the homogeneity in the final product. The resultant mixture is heated at 50 C and magnetically stirred for 20 min. After the completion of the stirring process, the mixture is shifted to heating mantle, which is maintained at 90 C for 2 h till the mixture gets ignited. The synthesized powder was finally calcined at 550 C for 2 h and allowed to attain room temperature to get the final product. 2.2 Characterization of ZnO:Ag:Co nanopowders The crystalline structure of the synthesized powders was studied using x-ray powder diffraction technique (PANalytical-PW 340/60 X pert PRO) using Cu-K α radiation ( = 1.5406 Å). The surface morphology of synthesized ZnO nanopowders was observed using scanning electron microscope (Carl Zeiss Ultra 55 FE-SEM). 2.3 Evaluation of antibacterial activity The antibacterial activity of the synthesized ZnO nanopowders was tested against Bacillus cereus (Gram positive) bacteria using well diffusion method. Mueller Hinton Broth is used as nutrient agar medium for bacterial growth. This medium was sterilized in an autoclave at 121 C for 15 min and then poured into sterile Petri plate and allowed to solidify in a laminar air flow chamber. After solidification, using a sterile cotton swab, fresh bacterial culture was spread over the plate using spread plate technique. Three wells each of 5 mm in diameter were made in the agar plates with the help of sterile cork borer. The wells were inoculated with 10, 15 and 20 μg/ml of stock solution of the product. All the plates were incubated at 37 C for 24 h. After incubation, the plates were observed for the formation of clear inhibition zone around the well. The zone of inhibition was noted by measuring the diameter of the inhibition zone around the well. 3. RESULTS AND DISCUSSION 3.1 Structural studies Fig.1 shows the XRD patterns of undoped and Ag+Co doped ZnO nanopowders. In the case of undoped ZnO, all the observed peaks are concordance well with JCPDS data card no. 36-1451 which belongs to hexagonal wurtzite structure of ZnO. The diffraction peaks appear at 2θ = 31.76, 34.43, 36.27, 47.53, 56.54, 62.82, 66.43, 67.97, 69.12 and 77.03 correspond to the lattice planes (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202), respectively. From the XRD pattern of undoped ZnO, it is obvious that the planes (101), (100) and (002) are the first three strongest peaks. All the other peaks are also agreed well in order with the standard JCPDS card (36-1451) indicating the purity of the ZnO powder. No peaks correspond to the various possible phases of Co are observed. This result affirms that Co ions are substituted properly into the ZnO lattice. This is because of the better compatibility of the lattice structure of Co with that of ZnO. Based on their lattice compatibility theory; they clearly established that the structure of cobalt is compatible with ZnO lattice. So that Co 2+ ion can be easily settled in the Zn 2+ regular site of ZnO matrix. 241
However, in the case of Ag+Co doped ZnO samples, a new peak arises at 2θ = 38.065 (111). The crystallite size (D) of undoped and doped ZnO nanopowders is estimated using the Debye- Scherrer s formula 17. D= 0.9λ / cos (1) where λ (1.5406 Å) is the wavelength of the X-ray used, is the broadening of the diffraction peak at half of its maximum intensity (FWHM) and is the Bragg s angle. The lattice constants a and c are calculated using the following formulae 18. 1 d 2 = 4 3 (h 2 +hk+k 2 ) + l2 a 2 c 2 The calculated lattice constants for undoped and doped ZnO are a = 3.246 nm: c = 5.201 nm and a = 3.244 nm: c = 5.196, respectively. The lattice constant of undoped ZnO is consistent with standard data available in JCPDS card no 36-1451. 3.2 Optical studies Fig. 1. XRD Patterns of doped and undoped ZnO nanopowders. The optical absorption spectra of undoped, ZnO:Ag:Co nanopowders are shown in fig. 1.1. The absorbance is mainly based on several factors such as impurity centres, oxygen deficiency and band gap 19. In general, the absorption edge depends on the band gap of the material. From the figure, the absorption edge of the undoped ZnO is observed at 376 nm which can be owing to the excitation of electrons from the valance band to the conduction band. The absorption edge of the doped ZnO powder shifts towards the higher wavelength side (382 nm) indicating decrease in the band gap caused by doping 20. The band gap values of all samples were calculated using the formula E g hc. The calculated values found to decrease with doping. It is clear that Ag + Co doping leads to the narrowing of band gap. 242
Fig.1.1. UV absorption spectra of doped and undoped ZnO nanopowders 3.3 Surface Morphology Studies The FE-SEM images of undoped and (Ag + Co) doped ZnO nanopowders are shown in fig. 1.2. FE-SEM. From the SEM images, it can be seen that the undoped ZnO powder shows grains of different shapes like closely packed leaves and flowers, whereas (Ag+Co) doped ZnO show agglomerated spherical particles with a size of 34 nm. This reduction in grain size may be due to the Zener pinning 21. As a consequence, there is an increase in the surface to volume ratio that helps to provide more contact area to interact with the micro organisms which is one of the reasons for the enhancement in the antibacterial activity of synthesized ZnO nanopowders. Fig.1.2. SEM images of (a) ZnO (b) Ag+Co doped ZnO nanopowders 243
3.4 Antibacterial activities The zone of inhibition observed for the undoped, and Ag+Co doped ZnO nanopowders against Bacillus cereus (Gram positive) bacteria are shown in Fig. 1.3. The inhibition zone is measured for three different concentrations of samples viz. 10, 15 and 20 μg. The antibacterial activity of the ZnO is mainly based on the three factors. (i) generation of reactive oxygen species (ROS) (ii) release of Zn 2+ ions for the ZnO system and (iii) reduced size of the grains 22. The mechanism proposed for the generation of reactive oxygen species viz. superoxide anion (O 2 ), singlet oxygen ( 1 O 2) and hydroxyl radicals (OH ) in the system containing ZnO and Ag. CONCLUSION Fig. 1.3 Zone of inhibition of ZnO:Ag:Co nanopowders against (a) B. cereus bacteria The ZnO and Ag+Co doped ZnO nanopowders synthesized using the combustion method found to exhibit good antibacterial efficiency against tested bacterium viz. B. cereus. When compared with undoped ZnO, Ag+Co doped ZnO, fair better in antibacterial activity against the bacteria. In addition to that, the observed red shift may be due to the presence of metallic Ag on the surface of the ZnO matrix which provides the impurity band in the energy gap resulting in the formation of p type in the host material. Moreover, the observed higher absorption intensity in doped samples indicates that simultaneous doping enhances the carrier concentration in ZnO nanostructure. REFERENCES 1. R. Wahab, S. G. Ansari, Y. S. Kim, H. K. Seo, H. S. Skin, Appl. Surf. Sci., 253, 7622 (2007). 244
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