Microwave-Assisted Hydrothermal Synthesis of Co Doped ZnO Nanocrystals

Abstract

Spintronic devices employing both semiconducting and ferromagnetic properties of materials are to be realized by doping of high spin transition metal ions to semiconductors such as ZnO. Transition metal oxides are also interesting as electrocatalysts for electrochemical conversion of renewable electricity to chemical energy for its storage. In this study, we have attempted microwave (MW)-assisted hydrothermal synthesis of Co, Mn-doped ZnO nanoparticles.Aqueous precursor solutions containing ZnCl2 and CoCl2 or MnCl2 at various ratios in a total concentration of 0.2 M were basified with NaOH to ca. pH 13, and subjected to a 2.45 GHz MW[1,2] to promote reaction at 160°C for 30 min. The products were characterized by FE-SEM, XRD and UV-Vis. Mesoporous electrodes of ZnO and Zn0.92Co0.08O were fabricated by doctor blading method. The green-colored Co-doped samples exhibit characteristic absorption peaks at around 560, 610 and 650 nm due to d-d transition of Co(II) and a red shift of the bandgap absorption onset. Phase separation of Co(OH)2 has also been suggested by XRD and FE-SEM images for x > 0.1, so that doping up to about 10% was apparently possible. Systematic shift of the three major XRD peaks of Wurtzite ZnO towards low angles was recognized upon increasing x, suggesting substitution of the lattice Zn2+ ions with Co2+. Lattice constants were calculated as a = b = 3.252 Å, c = 5.204 Å for ZnO, whereas they were enlarged to 3.277, 5.244 Å, respectively, for x = 0.20. CVs of ZnO and Zn0.92Co0.08O electrodes in a neutral electrolyte are compared in Fig. 1. Besides the clear decrease of cathodic charging and discharging currents, Co-doping results in an anodic wave at around + 1 V due to Co2+ → Co3+, followed by a large increase of irreversible anodic current, as compared to the non-doped sample. Chronoamperograms for a few minutes confirmed this current enhancement and gas bubble evolution from the anode due to catalytic oxidation of water (2H2O → O2 + 4H+ + 4e-). [1] Y. Hirai, K. Furukawa, H. Sun, Y. Matsushima, K. Shito, A. Masuhara, R. Ono, Y. Shimbori, H. Shiroishi, M. S. White & T. Yoshida, “Microwave-assisted hydrothermal synthesis of ZnO and Zn-terephthalate hybrid nanoparticles employing benzene dicarboxylic acids” Microsyst Technol (2017). doi: 10.1007/s00542-017-3392-y[2] H. Sun, L. Sun, T. Sugiura, M. S. White, P. Stadler, N. S. Sariciftci, A. Masuhara & T. Yoshida, “Microwave-assisted hydrothermal synthesis of struture-controlled ZnO nanocrystals and their properties in dye-sensitized solar cells” Electrochemistry, in press. Figure 1