It has been reported [1] that the interfacial energy at Au electrode surface in aqueous solution becomes larger when metals such as Sn, Pb, and Sb are electrodeposited. 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. It was also reported [1] that a droplet of oil, e.g., hexane and nitrobenzene, put on the Au electrode moved spontaneously during the electrodeposition. This motion was of interest from the viewpoint of energy conversion because the mechanical motion was driven by simple electrochemical reactions.

The driving force inducing the droplet motion was an imbalance of the interfacial tension acting on the droplet, as illustrated in Figure 1a. The droplet spontaneously moved toward the area where the interfacial tension is relatively high, i.e., where the electrodeposition occurred relatively efficiently. The electrodeposition occurred less efficiently at the rear side because it was suppressed by adsorbed oil molecules remaining on the metal surface. This mechanism seems highly plausible. However, it cannot explain how the droplet starts to move.

Then, in order to investigate other factors creating the imbalance, we studied the spontaneous motion of nitrobenzene droplets during the Sn electrodeposition (Sn²⁺ + 2e^- → Sn) in H₂SO₄ solution containing SnSO₄ [2]. Figures 1b and 1c show the experimental setup for observing the motion and the photo images of the moving droplet during the Sn electrodeposition, respectively. The study demonstrated that the imbalance was caused by the occurrence of a side reaction, namely, hydrogen evolution reaction (HER), near the rear side of nitrobenzene droplet. 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. The HER occurred on the rear side probably because the surface of the Au electrode used was not uniform. In other words, the non-uniform surface could be one of the factors that create the interfacial tension imbalance.

Recently, we found that the droplet put on an Au film started to move spontaneously without the occurrence of the HER at the rear side. The Au film was produced on a Si wafer by electroless plating, and thus it was more uniform than the surface of the Au electrode used in the previous study [2], namely, the Au surface polished with alumina powders. This clearly indicates that an interfacial tension imbalance was induced by other factors rather than the occurrence of the HER. In this presentation, the factors that create the imbalance will be reported and the mechanism of the droplet motion will be reconsidered.

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 0.5 M H₂SO₄ + 0.01M SnSO₄. (c) Snapshots of a nitrobenzene droplet taken at -0.66 V vs.SHE.