Spontaneous Motion of Oil Droplets on Au Electrode during Sn Electrodeposition in Nitric Acid Solutions
Authors: Ishibashi Yutaka , Mukouyama Yoshiharu 
Published: February 2020

It has been reported [1] that the interfacial energy at Au electrode surface in aqueous solutions becomes larger during the electrodeposition of metals such as Sn, Pb, and Sb than that in the absence of the electrodeposition. This was because the reaction intermediates of the electrodeposition, i.e., the active adatoms on the electrode surface, increased the interfacial energy, or the interfacial tension at electrode-electrolyte interface. The previous work also reported [1] that droplets of oil, e.g., hexane and nitrobenzene, put on Au electrodes moved spontaneously during the electrodeposition when the interfacial tension at the front side of the droplets was higher than that at the rear side (Figure 1a). Furthermore, it mentioned [1] that the velocity of the motion increased with the negative shift in the electrode potential. The motion was interesting from the viewpoint of energy conversion because the mechanical motion was driven by simple electrochemical reactions. However, the mechanism of the motion was not fully elucidated because the previous work was not focused on studying the difference in the interfacial tension between the front and rear sides of the droplets.
Then, in order to clarify the factors that caused the interfacial tension difference, we studied the spontaneous lateral motion of nitrobenzene droplets during the Sn electrodeposition (Sn2+ + 2e- → Sn) in H2SO4 + SnSO4 solutions [2]. The main factor was found to be the occurrence of a side reaction, i.e., hydrogen evolution reaction (HER), near the rear side of the nitrobenzene droplets. The rate of Sn electrodeposition decreased as that of HER increased, and therefore the interfacial tension became lower at the rear side than that at the front side where the Sn electrodeposition occurred efficiently without the occurrence of the HER.
Recently, we have studied if the spontaneous lateral motion occurs when the Sn electrodeposition is performed in HNO3 + SnSO4 solutions. Figure 1b shows the experimental setup for observing the motion. In the nitric acid solutions, a nitrobenzene droplet started to move immediately after the electrode potential was set at the potential at which the Sn electrodeposition took place, e.g., -0.66 V vs. SHE (Figure 1c), though both the Sn electrodeposition and the HER were less favorable than in the sulfuric acid solutions. Therefore, it was possible that the interfacial tension difference was induced by other factor rather than the occurrence of the HER; hence the reduction reaction of nitrate ions occurring at Sn atoms deposited on Au surface probably played a major role in the factor. In this presentation, the droplet motion in the nitric acid solutions will be reported and discussed.

REFERENCES

[1] S. Nakanishi, T. Nagai, D. Ihara, Y. Nakato, Chemphyschem, 9 (2008) 2302-2304.
[2] Y. Mukouyama, T. Shiono, J. Electrochem. Soc., 163 (2016) H36-H41.

FIGURE CAPTION

Figure 1. (a) A schematic cross section of an oil droplet placed on an electrode. Spontaneous lateral motion of the droplet induced by an imbalance of electrode-electrolyte interfacial tension between the front and rear sides of the droplet. (b) Schematic of the experimental setup. A nitrobenzene droplet, the volume of which is about 1.0 mL, is put on an Au disc electrode in 1.1 M HNO3 + 0.01M SnSO4. (c) Snapshots of a nitrobenzene droplet taken at -0.66 V vs. SHE.

Figure 1