| Possessing high exciton binding energy (60 meV) and wide band gap (3.37 eV), zinc oxide (ZnO) exhibits great potentials in the application of piezoelectric and photoelectric devices. Incorporating other elements into ZnO can effectively adjust its structure, morphology and property, thus, expand its application fields. Therefore, the scientific researches on doped ZnO develop rapidly in recent years. As a typical transition metal elecment, Co has rich surface states with a similar ionic radius to Zn2+, and easily incorporates into ZnO lattice. Co-doping can effectively adjust the band gap of ZnO, and thus improve the optical properties and applications. Meanwhile, Co has magnetic performance. It incorporates into ZnO lattice and forms ZnO-based diluted magnetic semiconductor (DMS), which can be applied in electron spin devices and all kinds of detectors. Hence, it is great significant to prepare ZnO-based materials with optical and magnetic performance together. In this paper, we successfully synthesized ZnO:Co nanoparticles via a simple boiling reflux method, using ZnCl2, CoCl2, NaOH and NH3·H2O as raw materials. The structure and morphology of the products were characterized by XRD, FESEM, EDS and XPS. The optical and magnetic properties of the samples were studied by UV-vis, PL and PPMS.The main contents of this paper are as follows:(1) ZnO:Co nanoparticles were prepared via a simple boiling reflux method at normal pressure, using ZnCl2, CoCl2, NaOH and NH3·H2O as raw materials. The optimum conditions for the synthesis of ZnO:Co, including the adding sequence of reagents, the solution basicity, the initial Zn2+ concentration, and the refluxing time, were probed in detail. With NaOH as complexant, ZnO:Co nanowhiskers with a length of 3μm and an average diameter of 200 nm (the aspect ratio is 15) were prepared in the alkaline solution with molar ratios of Zn2+/OH- = 1:3.61:4.0. ZnO:Co nanorods with a large aspect ratio were synthesized in a higher alkaline solution with molar ratio of Zn2+/OH- = 1:9.0 with an additional pre-stirring treatment at 40℃for the precursor. Using NH3·H2O as complexant, ZnO:Co nanotube clusters with a novel morphology were prepared with molar ratio of Zn2+/OH- = 1:4.0.(2) ZnO:Co nanoparticles with different doping levels were prepared under the optimum conditions, using ZnCl2, CoCl2, NaOH and NH3·H2O as raw materials. The influences of Co-doping levels on the structure and morphology of the samples were investigated. The optical and magnetic properties dependent on Co-doping level were studied in detail. Results show that Co incorporates into ZnO lattice in the valence of +2. Co2+ ions replace part of Zn2+ ions without changing the wurtzite structure of ZnO. However, the morphology of the samples change obviously after Co-doping. The doping leads to a band gap narrowing of ZnO, and thus the UV absorption and UV emission peaks of the samples shift to longer wavelength. The yellow-green luminescence of the doped ZnO samples induced by oxygen vacancy is quenched due to the energy transferring which occurs when the energy of the defect emission is similar to that of the 4A2(F)→2A1(G) transition of Co2+. With proper Co-doping levels, the samples exhibit room temperature ferromagnetism originated from the Co2+-Co2+ ferromagnetic coupling within the impurity band induced by oxygen vacancies. With higher Co-doping levels, the Co2+-Co2+ pairs occupying the next-nearest neighbouring lattice sites exhibit antiferromagnetic couping, which makes the samples behave paramagnetism under the external applied filed. |
