Structural, optical, photocatalytic and antibacterial activity of zinc oxide and manganese doped zinc oxide nanoparticles
Introduction
Semiconductor materials, especially II–VI semiconductors have attracted great interest due to their unique properties and potential applications [1]. ZnO is a semiconductor with a wide band gap (3.3 eV), large exciton binding energy, abundant in nature and environmentally friendly and these characteristics make this material attractive for many applications such as solar cells, optical coatings, photocatalysts, antibacterial activities, electrical devices, active medium in UV semiconductor lasers and in gas sensors [2].
Various techniques have been used to synthesize ZnO and doped ZnO nanoparticles and can be categorized into either chemical or physical methods. The former methods are for example, thermal hydrolysis techniques, hydrothermal processing, and sol–gel method while the latter methods are vapour condensation method, spray pyrolysis and thermochemical/flame decomposition of metal organic precursors. ZnO can absorb UV light with the wavelength equal or less than 385 nm. However, for higher photocatalytic efficiency and many practical applications, it is desirable that photocatalysts such as ZnO should absorb not only UV but also visible light due to the fact that visible light accounts for 45% of energy in the solar radiation while UV light less than 10%. In order to absorb visible light, band gap of ZnO has to be narrowed or split into several sub-gaps, which can be achieved by implanting transition metal ions, or by doping ‘nitrogen’ [3]. Surface area and surface defects play an important role in the photocatalytic activities of metal oxides. The reason is that doping of metal oxide and/or transition metals [like Mn] increases the surface defects [4]. In addition it affects the optical and electronic properties and can presumably shift the optical absorption towards the visible region.
Microbial contamination is a serious issue in healthcare and food industry so that development of antimicrobial agents and surface coatings has been attracting increasing attention in recent years. It has been recognized that toxicity of nanoparticles is generally larger than in the case of larger particles of the same materials, even for materials with relatively low toxicity [5]. Therefore, developments of nanoparticles with antimicrobial properties are of considerable interest.
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Experiment
Co-precipitation is one of the most successful techniques for synthesizing ZnO powders having narrow particle size distribution. This method is an efficient pre-concentration technique for tracing heavy metal ions. This process can avoid complex steps such as refluxing of alkoxides, resulting in less time consumption compared to other techniques. The only drawback of this method is that all the cations should have similar solubility product.
Mn doped ZnO were prepared by the reaction of Zn2+, Mn
Results and discussion
X-ray diffractogram of the powder, taken from diffractometer was analyzed to obtain information about various crystalline aspects. Fig. 1a shows that XRD patterns of synthesized undoped ZnO nanopowders. The sharp and intense peaks indicate that the samples are highly crystalline and ZnO nanoparticles have polycrystalline structure. The XRD peaks for (1 0 0), (0 0 2) and (1 0 1) planes indicates the formation of phase pure Wurtzite structure of ZnO. The high intensity of (1 0 1) peak suggests that the
Conclusions
In this present work undoped and Mn doped ZnO nanoparticles are prepared by the co-precipitation method. From XRD analysis it is seen that Mn2+ ions exactly replaced the Zn2+ ions without affecting the crystal structure. From optical spectra it is seen that incorporation of dopant material does not enhance the absorption in visible region. Because of Burstein–Moss effect, dopant material failed to narrow or split the band gap of ZnO into several sub-gaps. So, the photocatalytic activity of Mn
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