Electrochemical Deposition of Zinc Oxide Nanotubes with Countless Small Dimples on the External Surface
Authors: Osamura Keisuke , Matsunaga Mariko 
Published: February 2020

1. Introduction
For the improvement of photoelectric conversion efficiency of dye-sensitized solar cells, oxide semiconductors of different shapes have been developed for utilization as photo-electrode. Then, many studies suggested that the surface of oxide semiconductor play a significant role for loading the dye. Among various oxide semiconductors, titanium oxide (TiO2) is known to be one of the best substrate for loading a large amount of dye stably from the perspective of the strength of chemical interaction, while zinc oxide (ZnO) is known to possess better electron conductivity than TiO2. From this point of view, hybrid material of TiO2 and ZnO has been actively studied. For the both hybrid system and solely use of ZnO, the optimization of surface morphology of ZnO is considered to be required to improve the immobilization condition, including the amount and the stability, of dye without loosing its good conductivity.
Recently, Ellah et al. reports electrochemical one step deposition of ZnO nanotube on indium tin oxide (ITO) substrate [1]. This method is interesting because nanotubes, which is one of the shapes possessing high surface area and good electron conductivity, can be deposited with high density within one hour at moderate temperature (< 100 °C). In this study, we investigate on the influence of several deposition conditions on the morphology of ZnO with much attention to the surface, by using fluorine-doped tin oxide (FTO) as a substrate.
2. Experimental
The FTO glass surface were clarified in a mix solution of deionized water, acetone, and 2-propanol (1: 1: 1), by using ultrasonic cleaner. For the electrochemical deposition, three-electrodes cell was used in which, working electrode is the FTO glass, reference electrode is Ag/AgCl, counter electrode is Pt wire electrode. The electrolyte contained 0.5mM ZnCl2and 0.1 M KCl in deionized water and that bubbled 30 minutes with oxygen beforehand electrochemical deposition. As deposition conditions, 60 minutes, 70℃, -1.0V (vs. Ag/AgCl)- (a) is utilized as standard, and the effect of the time, temperature, and deposition potential on the shape of the ZnO were carefully investigated mainly by scanning electron microscope (SEM).
3. Results and discussion
The deposition of nanotubes, which have hexagonal structure, was confirmed on the FTO substrate by SEM at various preparation conditions. And, it was revealed that the deposition time is proportion to the length of the nanotube, the temperature is proportion to the both length and the diameter as similar to the previous report [1]. In particular, nanotubes with countless small dimples on the external surface, which is expected to increase surface area, was observed by the SEM at the condition of 60 minutes, 70℃, -0.7V. The length and the diameter of this nanotube are 1.0 µm and 0.2 µm, respectively. And, the countless small dimples are about 10 nm (Fig.1). In the presentation, the effect of these surface morphologies on the photoelectric conversion efficiency will be also discussed.
[1] M. A. Ellah et al., “Effect of Electrolyte Conductivity on Controlled Electrochemical Synthesis of Zinc Oxide Nanotubes and Nanorods,” J. Phys. Chem. C 2013, 117 6794-6799.