Growth And Morphology Control Of ZnO Films By Aqueous Solution Method

Zinc Oxide (ZnO) is an importantâ…¡-â…¥group semiconducting material which can be widely used in photoelectric field. In this paper, well-aligned and orienting hexagonal wurtzite ZnO nano/micro-rod arrays were prepared by two-step aqueous solution growth method using zinc nitrate and ammonia as precursors. The growth mechanism of rod crystal films was investigated, and effects of the processing parameters, such as seed surface of substrate, growth time and primary zinc concentration were also studied. Moreover, role of Cd(NO₃)₂ addition into growth solution were analyzed by adjusting Cd(NO₃)₂ concentration, pH value of growth solution, added manner of Cd(NO₃)₂ and growth time. Crystal structure, chemical composition and surface morphology of ZnO films were characterized by XRD, SEM, EDS, FESEM, TEM and HRTEM.Investigation into ZnO seed layer, growth time and primary zinc concentration on growth of rod crystals indicated that ZnO seed layer was crucial for growth of well-aligned ZnO nanorod array. Rod size of films were increased with prolong growth time. Primary zinc concentration mainly affect growth rate in c axis direction, and the array density and rod size increase with primary zinc concentration. The rod density of ZnO films decreased by chemical corrosion to the small ZnO rods as pH value of growth solution was increased to 10.0.When Cd(NO₃)₂ was added into growth solution, a part of positive cadmium complexes [Cd(NH₃)₆]²⁺ instead of [Zn(NH₃)₄]²⁺ were adsorbed on the negative (0001) plane by electrostatic force. As a result, a great deal of [Zn(NH₃)4]²⁺ species was forced to be adsorbed on (1000) and (1010) planes, leading to fast growth in rod diameter direction. The maximum change of rod diameter was from about 180nm to about 700nm. Cd(NO₃)₂ concentration in original growth solution could alter average rod diameter from～180nm to～400nm. The average rod diameter could increase to～700nm by first growing in 0.06M Zn-concentration solution for 1h and sequential growing in other 0.06M Zn-concentration solution with Cd concentration of 0.018 M for 3h.The fast diameter growth resulted in that ZnO rods embeded each other due to limited space among ZnO rods so that rod array films became well-oriented multicrystal and compact ZnO films. In addition, the ZnO films consisted of Sheet-like shaped and Needle-like shaped grains could be prepared by adding C6H5Na3O7·2 H2O and NH4F.