Study On The Two-Step Synthesis Of ZnO And Ag-doped ZnO Nanorods

Zinc oxide is an n-type direct band gap semiconductor with a band gap of3.37eV at room temperature and an exciton binding energy of60meV. Due to excellent optical and electrical properties, ZnO has important and promising applications in the ultraviolet light-emitting diodes, lasers, blue-green variety of light-emitting devices, solar cells, piezoelectric devices, displays and lighting etc. Hydrothermal growth of ZnO nanorods not only has advantages of conventional preparation methods such as sol-gel, RF magnetron sputtering (RF-sputtering), but also has advantages in low cost, simple process, easy doping. In addition, Ag doping of ZnO is a potential and promising method to obtain p-type ZnO. In the present paper, the two-step preparation method was adopted to prepare pure and Ag-doped ZnO nanorods with different morphologies and structures, and the influences of various experimental parameters on the morphology and optical properties of nano-scaled ZnO were studied by scanning electron microscopy, X-ray diffraction, spectrophotometer and PL spectra. The detailed results are listed as follows:(1) Variation of the mass and heat of ZnO with temperature in the process of zinc acetate thermal decomposition was studied by using TG method. The results show that the thermal decomposition of Zn (CH3COO)2.2H2O can be divided into the two processes:Zn (CH3COO)2.2H2O first lose crystal water and form into anhydrous salt; anhydrous salt is thermally decomposited into ZnO.(2) Using hydrothermal method on the ZnO seed layers synthesize ZnO nanorods and Ag-doped ZnO nanorods, and ZnO seed layer was prepared by the thermal decomposition of zinc acetate. Effect of the pyrolysis temperature (T) of zinc acetate and the molar ratio of Ag ions to Zn ions (RAg/zn) was studied on the optical and structure properties of ZnO nanorods. The results show that all samples have hexagonal polycrystalline structure, indicating ZnO-(002),(100),(101),(102),(103) as well as (004) diffraction peaks. The ZnO nanorods show a clear<002> c-axis preferred orientation and the crystallinity obtains the maximum value at350℃. With increasing the T value, the residual strain relaxation is enhanced in the growth process, resulting into the reduced macroscopic stress. At T≥350℃, the stress changes from tensile stress to compressive stress. Average transmittance in the visible region of ZnO nanorods is lower. With the pyrolysis temperature increasing, the average transmissivity first increases to maximum value at T=350℃and then decreases. The trends of average head-face dimension and the average transmissivity of the ZnO nanorods with the pyrolysis temperature are similar.2) Ag ions enter the crystal lattice by substitution of Zn ions. The c-axis-preferred orientation of the ZnO nanorods decreased as RAg/zn increased. At RAg/Zn>1-0%, ZnO nanorods lost their c-axis-preferred orientation. The average transmissivity in the visible region first increased and then decreased as RAg/zn increased. The absorption edge also initially blue shifted and then red shifted. The influence of Ag doping on the average head face, and axial dimensions of the ZnO nanorods may be optimized to improve the average transmissivity at RAg/zn<1.0%.(3) Using hydrothermal method on the DC magnetron-sputtered Al-doped ZnO (AZO) seed layers synthesize ZnO nanorods and Ag-doped ZnO nanorods with different morphologies and optical properties, and the influences of the molar ratio of Ag ions to Zn ions (RAg/Zn), the AZO seed layer and the annealing temperature (Ta) before and after hydrothermal synthesis on the structural and optical properties of the ZnO nanorods and Ag-doped ZnO nanorods were studied. The results show that1) the changes in the microstructure and optical property of Ag-doped ZnO nanorods are closely related to the change in the average head-face dimension which is induced by Ag doping as RAg/zn increases. The photoluminescence intensity in the visible region for the ZnO nanorods growing on the15min-sputtered AZO is stronger than that of the ZnO nanorods growing on the10min-sputtered AZO seed layer at the same RAg/zn-More point defects caused by Ag doping are produced as RAg/zn increases, resulting in the broadening of PL envelope in the visible region.2) The full-width at half maximum (FWHM) and macrostress, average head face dimensions, average transmissivity and intensity of luminescence are found to be dependent on the pre-annealing and post-annealing temperature. XRD results show that all the annealed ZnO nanorods are <002> c-axis-preferred orientation and have the same crystal structure but the intensities of the (002) peak vary with the annealing temperature, reaching a maximum at Ta=300℃and200℃, respectively, for pre-annealing and post-annealing. The average transmissivity in the visible region first increased and then decreased as pre-annealing and post-annealing temperature increased, reaching a maximum both at Ta=300℃. The intensity of luminescence in visible region first decreases and then increases as temperature increases.(4) The structural and optical properties of the hydrothermally synthesized ZnO nanorods on different seed layers prepared by the thermal decomposition of zinc acetate and by the DC magnetron sputtering were compared. The results show that the ZnO nanorods synthesized on the seed layer prepared by DC magnetron sputtering have a more uniform and compact surface and have a better c-axis preferred orientation than that on the seed layer by the thermal decomposition of zinc acetate. These are attributed to the better crystalline quality of the seed layer by DC magnetron sputtering, resulting into the larger average tansmissivity of the ZnO nanorods.