ZnO Electrodeposition on Boron-Doped Diamond: Effects of Zinc Precursor Concentration
The electrodeposition of a metal oxide such as ZnO consists to a redox process at the electrode surface, to raise the local pH and then to precipitate an oxide or hydroxide phase. Three different precursors can be used to produce hydroxide at electrode surface: hydrogen peroxide, nitrite salt or dissolved oxygen. This last one is used in our work. Dissolved oxygen is reduced, raising the local pH on the conductive diamond electrode and causing the precipitation of zinc hydroxide . Then the dehydration of hydroxide leads to zinc oxide according to the following scheme.
O2 + 2 H2O + 4 e- -> 4 OH- (1)
Zn2+ + 2 OH- -> Zn(OH)2 (2)
Zn(OH)2 -> ZnO + H2O (3)
In this work, zinc chloride (ZnCl2) as precursor is used for the first time on BDD and potassium chloride (KCl) as support electrolyte. Our aim was to study the influence of deposition parameters such as zinc precursor concentration, substrate’s surface chemistry or bath temperature, on ZnO morphologies. The resulting ZnO deposits were characterized using X-ray Diffraction (XRD) and scanning electron microscope (SEM).
The effect of [Zn2+] is illustrated on figures 1 and 2. As evidencing by XRD (Fig. 1), deposits obtained at 60°C, on as-grown BDD substrate, with 1 mM and 5 mM of Zn2+ correspond to well-crystallized zinc oxide. In both cases, the intensity of diffraction peak (002) indicates a preferential growth to the c-axis. The diffraction peaks from the substrate are labeled with an asterisk.
SEM images (fig. 2) confirm the preferential orientation along c-axis. However, ZnO morphologies are completely different by changing Zn2+ concentration. Indeed, with a concentration of 5 mM, micrometric hexagonal grains are obtained while with a concentration ≤1 mM, nanometric pyramids are grown.
Finally, our work shows that electrodeposition of ZnO is fully applicable on diamond substrate. And, parameters such as concentration of Zn2+, BDD’s surface chemistry or bath temperature, allow modifying the morphology of deposits.
 Wang, C. X.; Yang, G. W.; Zhang, T. C.; Liu, H. W.; Han, Y. H.; Luo, J. F.; Gao, C. X.; Zou, G. T. Diam. Relat. Mater. 2003, 12, 1548–1552.
 Luo, J. T.; Zeng, F.; Pan, F.; Li, H. F.; Niu, J. B.; Jia, R.; Liu, M. Appl. Surf. Sci. 2010, 256, 3081–3085.
 Zhao, Z.; Lei, W.; Zhang, X.; Wang, B.; Jiang, H. Sensors (Basel). 2010, 10, 1216–1231.
 Sang, D.; Li, H.; Cheng, S. Appl. Surf. Sci. 2011, 258, 333–336.
 Yu, Q.; Li, H. D.; Zou, G. T. Diam. Relat. Mater. 2011, 20, 351–354.
 Hikavyy, a.; Clauws, P.; Vanbesien, K.; De Visschere, P.; Williams, O. a.; Daenen, M.; Haenen, K.; Butler, J. E.; Feygelson, T. Diam. Relat. Mater. 2007, 16, 983–986.
 Lincot, D. Thin Solid Films 2005, 487, 40–48.
 Chatterjee, A.; Foord, J. Diam. Relat. Mater. 2006, 15, 664–667.