Synthesis and Optical Properties of Transition Metal Doped ZnO ...

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of ZnO can be enhanced by creating more defects on its surface. R. Wang et al. [9] has demonstrated that silver ion doped ZnO has improved photocatalytic activities owing to the increased surface defects caused by the enhanced oxygen vacancies. Doping of ZnO with metal and transitional metals might shift the optical absorption of ZnO to the visible region i.e. to the longer wavelength. This shifting of optical absorption of doped ZnO will make this material capable to operate at lower excitation energy and can generate electron hole pair upon visible light irradiation from solar spectrum. In a technical study by K. Vanheusden et al. [13] it has been observed that lead (Pb) doping in ZnO narrows the effective band gap of ZnO nanopowders and decreases both the photoluminescence and the free-carrier concentration. Doping of ZnO films with Cobalt (Co) [12] has been reported to significantly decrease the bang gap of ZnO up to 2.75 ev. This decrease in the band gap of cobalt-doped ZnO films resultantly causes hyperchromic shift in its optical absorption. Incorporating cupper and manganese ions in ZnO thin films separately, bring opposite effect on the grain size of ZnO, cupper doping increases while the manganese doping decreases the grain size [14]. It has been reported that both the dopant Cu and Mn resulted a slight decrease in the optical band gap of ZnO films. Similarly [5] band gap tailing effect has been observed for aluminum doped ZnO, which caused reduction in the band gap of ZnO, likely due to the doping of donors. The doping of ZnO nanocrystals with various ions was accomplished by Y. S. Wang et al. [15] using a family of dopants such as Cd, Mg, Mn, and Fe ions. Shift in the band gap was attributed to the effect of dopant, which causes decrease in the band gap with Cd doping while increase in the band gap for other dopants. An enhancement in the optical absorption has been found for various dopants level in case of Fe and Mn doped ZnO nanocrystals. It is well known from the various studies that doping of ZnO with metal and transition metals could decrease the effective band gap and can subsequently increase optical absorption of this band gap tunable material. We therefore, doped ZnO with manganese using the co-precipitation techniques. This material can be used as a better photocatalyst and might have applications in photoelectrochemical hydrogen production. This study focused on the optical characteristic of manganese doped ZnO nanoparticles, synthesized by coprecipitation techniques. 3. EXPERIMENT The synthesis method described earlier in our previous work [16] was pursued with a little modification for preparation of doped ZnO nanoparticles. Two types of dopants, manganese and cupper were investigated at the earlier stages. In a typical process, 4 mili moles of Zinc acetate dehydrate were dissolved in 40 ml of ethanol and heated at 50C along with vigorous stirring for half an hour, thus making precursor solution A. Since then 4 mili moles of Sodium hydroxide were dissolved in 40 ml of ethanol and heated at 50 C along with vigorous stirring for one hour, making precursor solution B. The dopant solutions were also prepared by dissolving 0.02 mili moles of manganese acetate and cupper acetate each in 20 ml of ethanol separately. The two solutions were heated at 50 o C along with vigorous stirring for half an hour. In order to make ZnO:Mn 2+ / ZnO:Cu 2+ colloids, a complex of 20 ml precursor solution A and 20 ml dopant (manganese acetate and cupper acetate) solutions each were complexed and heated at 80C for half an hour along with vigorous stirring. After cooling to room temperature, 20 ml of precursor solution B (NaOH solution) was mixed with the two complex solutions (for hydrolysis), in order to 307
transform zinc hydroxide to ZnO. The solutions were kept in water bath at 60~65C for 2 hours. It was observed that solutions started precipitating after one hour in water bath. Subsequently to the 2 hours water bath, the solutions were cold down to room temperature followed by 4 hours aging. The colloidal solutions were centrifuged for 20 minutes at 4 k rpm to remove the large sized agglomerates. It was observed that nanoparticles of almost uniform size were suspended in the solution. ZnO:Mn 2+ / ZnO:Cu 2+ nanoparticles thus synthesized were then used for further experimental analysis. Optical characteristics of doped ZnO (ZnO:Mn 2+ , ZnO:Cu 2+ ) were determined with double beam UV/VIS spectrophotometer (Model SL 164 from ELICO). Manganese doped ZnO nanoparticles were further studied based on the enhanced optical absorption when compared with ZnO:Cu 2+ . Therefore structural characterizations of only ZnO:Mn 2+ were carried out with Transmission Electron Microscope (JEOL/JEM-2100F version) operated at 200KV, Fourier Transform Infrared Spectroscope (System 2000 FTIR, Perkin– Elmer).We used PCS machine from MALVERN Instrument Zetasizer Nano Model ZS Zen3600 fitted with a red laser (633nm) which can measure particle size within a range of 0.6nm to 600nm. Folded Capillary Cell (DTS1060) was used for zeta potential measurements and Disposable low volume polystyrene (DST0112) cuvette was used for size measurement 4. Results and discussion Synthesis of doped ZnO was performed in alcoholic solution consecutively to avoid formation of ZnOH [17]. Therefore zinc acetate, manganese acetate, cupper acetate and NaOH all were dissolved in ethanol. The nucleation and aggregation of nanoparticles are strongly solvent dependent, and are increasing with decreasing the dielectric constant of solvent [ 1 8]. Water has a dielectric constant of about 80 while for ethanol it is 24.3. The nucleation and growth of ZnO is faster in ethanol than in water and hence ZnO doped colloids were synthesized in ethanol in order to avoid oxidation of dopant ions. UV/VISspectroscopy of both the cupper doped ZnO (ZnO: Cu 2+ ) and manganese doped ZnO (ZnO:Mn 2+ ) as well as undoped [16] newly prepared nanoparticles showed evidence of a significant divergence in the absorption intensity in the blue region, as shown in figure 1. This enhancement in the absorption intensity within the visible region is attributed to the doping of ZnO with Cu, and Mn. Figure 1 further illustrates that Mn ions affect the absorption characteristic of the nanoparticles more markedly than Cu ions. This increase in the absorption intensity in the blue region can be attributed to the more pronounced doping of ZnO with manganese ion [12, 19]. It demonstrates that manganese doping in ZnO creates more defects sites as compared to Cu doping [20]. Figure 1. UV Visible spectroscopy of cupper doped, manganese doped and undoped ZnO Fourier Transform Infrared Spectroscopy of the hydrolysed particles (figure 2) shows strong peaks at 1562 cm -1 indicating the formation of ZnO [ 21 ] and peak at 1404 cm -1 that may be assigned to the symmetric stretching of carboxylate group (COO - ) probably from the un-reacted acetates. We assume here that solubility of Cu in ZnO is less than that of the 308
Page 1: Synthesis and Optical Properties of
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Synthesis and Optical Properties of Transition Metal Doped ZnO ...

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