Electrodeposition of band gap reduced p-type ZnO nanorods on FTO; effect of Cu ion doping on its structural and electronic properties

Zinc oxide (ZnO) as a n-type semiconductor (3.37 eV) considered a promising alternative to TiO2 for photocatalytic and photovoltaic applications. However, similar to TiO2, its major drawback is the large band gap that allows it to absorb only uv light. P-type doping of ZnO has attracted great attentions,because of its potential application for next-generation short-wavelength optoelectronic devices. Like many other transparent metal oxides, p-type doping of ZnO is difficult. Among group IB elements those known as ideal candidate for p-type ZnO doping, Cu is the best choice to convert n-type to p-type ZnO,because substitutional Cu behave as an acceptor. Among several methods, electrochemical deposition (ECD) has been reported an appropriate technique for Cu-doped ZnO preparation, owing to its low cost, simplicity, low temperature process, controlling size and shape and p-type doping[1]. In this study, we have electrodeposited Cu-doped ZnO nanorods (NRs) on FTO. The effect of Cu doping on structure, optical and electrical properties of ZnO investigated Experimental ZnO NRs were electrodeposited potentiostatically at -1 V vs. Ag/AgCl in three-electrode system, for 60min at 80 ºC. After electrodeposition, prepared samples were annealed at 300 ºC for 30 min in the air atmosphere. The undoped and Cu doped ZnO NRs were electrodosited from aqueous solution containing 1 mM Zn(NO3)2.6H2O, 1 mM hexamethylenetetramine, 0.1 M NaNO3 and 0-1.5 μM CuSO4.Results and discussion Fig. 1 shows XRD patterns of pure and Cu-doped ZnO prepared at different Cu doping concentration(0.5, 1 and 1.5 %). The observed patterns reveal a hexagonal wurtzite phase in agreement with JCPDScard no. 36-1451. All of the samples shows the strong (002) peaks, indicating preferential growth along c-axis. In addition to decrease in intensity, the position of the (002) peak shifted slightly to higher anglesas Cu contents in ZnO:Cu increased, which is attributed to the substitution of Zn2+ ions by the smallerCu2+ ions and changes in the atomic environment due to the extrinsic doping of ZnO. Uv-vis absorption spectroscopy shows the optical band gap of un-doped ZnO NRs is 3.41 eV, while this value decreased to 3.21 eV, 3.10 eV and 3.04 eV when dopant concentration was 0.5 %, 1 % and 1.5 %,respectively. Mott-Schottky analysis indicates the undoped ZnO NRs conductivity is n-type, however by doping withCu ions, conductivity convert to p-type. Conclusion In summary, the influence of Cu doping in ZnO NRs was investigated. XRD patterns showed thesubstitution of Cu2+ in ZnO NRs. Uv-vis spectra revealed the optical band gap decreased by Cu doping. Also Mott-Schottky analysis indicate the conversion of n-type to p-type semiconductor by Cu doping.

کلیدواژه ها:

ZnO nanorods ، Cu doping ، Mott-Schottky analysis ، Optical band gap