Study On Cathodic Electrodeposition Of Nano-ZnO And La-doped ZnO/La(OH)₃ Mixed Films

Zinc oxide (ZnO), an impotantⅡ-Ⅵsemiconductor material with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature, has been widely used in luminescence, solar cells, gas sensors, piezoelectric transducers and photocatalysts due to its excellent optical, electrical and magnetic properties. Recently years, rare earth ions (e.g. Tb, Er, Eu, Dy and Ce)-doped ZnO has been widely researched because of its unique luminescent properties. However, among all the researches, most of them concentrated on investigation of ZnO substituted doping, the studies of rare earth hydroxide/ZnO co-electrodeposited doping are rare reported.In this paper, the temperature effected on potential-pH, species repartition diagrams and solubility of ZnO/Zn(OH)2 were studied by thermodynamic method via a three-electrode potentiostatic electrodeposition technique during ZnO films was electrodeposited, electrochemical fabrication of ZnO nanorod array films with high c-axis preferred orientation and La doped ZnO/La(OH)3 co-precipitation films on the indium tin oxide(ITO) conducting electrode surface.A thermodynamic study of the temperature effects on the Zn(NO3)2 system by means of potential-pH, species repartition diagrams and solubility of ZnO/Zn(OH)2 is presented with the view to guide electrodeposited conditions and predict ultimate product. These thermodynamic analysis has been testified by the curve of the cathodic current density as the function of the time and cyclic voltammetry(CV), X-ray diffraction(XRD), scanning electron microscopy(SEM), thermal gravimetric-differential scanning calorimeter(TG-DSC), ultraviolet-visible(UV-Vis) spectra experiments. Optical studies revealed that the ZnO films exhibited high transmittance in the visible region (>80%) and steep absorption edge. At last, the reaction mechanism was proposed for ZnO electrodeposition in zinc nitrate aqueous solution.Transparent and compact ZnO nanorod array films were successfully prepared on ITO substrate using a potentiostatic electrodeposition method in simple zinc nitrate solution by adding hexamethylenetetramine(HMT) as shape control agents. The crystallinity, micro structure of surface, composition and optical properties of the obtained ZnO were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, UV-vis transmittance spectrum and photoluminescence spectrum. The results indicated that the as-synthesized ZnO nanorod with high c-axis preferred orientation by the HMT assistance possessed a high-quality single-crystal wurtzite structure; the average diameter was about 300 nm, the surface smoothness obviously increased. The UV-vis transmittance spectra showed that the ZnO films exhibited a high transmittance in the visible region(>80%) and a steep absorption edge, the strong and sharp ultraviolet emission peak and weak green emission peak in the room temperature photoluminescence spectrum indicated that such ZnO array films were highly crystallized and of excellent luminescent quality.The preparation of lanthanum doped ZnO/La(OH)3 co-precipitation films in 0.01M Zn(NO3)2+0.1M KNO3 solution containing different concentration of La3+ ion using a potentiostatic electrodeposition method is investigated. First, a thermodynamic study of the La-NO3-H2O system has been carried out by means of potential-pH, solubility of ZnO/La(OH)3 and species repartition diagrams at 25℃and 70℃in order to predict the electrochemical behavior of La3+ dissolved in aqueous solution containing NO3- ions. A comparison of the La-NO3-H2O and Zn-NO3-H2O systems indicates that the first deposition species is ZnO, La(OH)3 would precipitate and form ZnO/La(OH)3 mixed co-deposition films with former deposited ZnO on ITO conducting electrode with the interfacial pH increasing because of the NO3- ions reduction in solution. The decomposition temperature of La(OH)3 in co-deposition films is calculated about 662K. The thermodynamic predictions have been confirmed experimentally by the electrochemical co-deposition of ZnO/La(OH)3 thin films on ITO conducting electrode and thermal gravimetric-differential scanning calorimeter (TG-DSC) experiment. Increasing La3+ concentration leads to the rapid appearance of two phases:dispersed flower-like zinc oxide and, at the bottom of the La-doped ZnO, a covering layer containing La(OH)3 and ZnO crystallites. The optical transparency of ZnO/La(OH)3 co-precipitation films decreases with increasing La3+ concentration because surface regularity decreases.