Nanoparticles (NPs) of noble metals such as gold and platinum are utilized in a wide range of applications in various fields because of their unique catalytic, electronic, and optical properties. Furthermore, NPs have large surface area to volume ratios, which leads to an increase in catalytic efficiency and also to a cost-reduction. A variety of methods using chemical reactions are currently available for synthesizing NPs in solution phase and also for controlling their shapes and surface properties. In the field of electrochemistry, platinum NPs (PtNPs) deposited on carbon materials such as graphite, glassy carbon, and carbon blacks have been extensively studied because they are useful as efficient electrocatalysts for fuel cells, water electrolysis, and so on.

Nishimura et al. have developed a variable method for preparing shape-controlled PtNPs on graphite surface [1]. In the method, a constant current is applied between a Pt anode and a graphite cathode. This galvanostatic electrolysis was performed under the conditions that the water electrolysis (2H₂O → 2H₂ + O₂) occurred in strong acid solutions such as H₂SO₄ and HNO₃. During the electrolysis, the Pt anode dissolved to form Pt ions and the graphite cathode reduced them. The method was cost friendly because PtNPs were obtained without any reducing and stabilizing agents, and also because 72 % of dissolved Pt ions were electrodeposited as PtNPs. This simple method produced not only tetragonal nanoparticles which were enclosed by (111) planes, that is, thermodynamically stable nanoparticles, but also cubic nanoparticles enclosed by (100) planes. Interestingly, the cubic nanoparticles were preferentially obtained by controlling current densities of the cathode and anode.

In our previous report [2], we have shown that gold NPs (AuNPs) can be produced by galvanostatic electrolysis using an Au anode and a graphite cathode. This electrolysis was essentially the same as the simple method that has been used to prepare PtNPs [1]. The Au anode, where oxygen evolution occurred, dissolved to form Au ions (Au → Au³⁺ + 3e^-) and the graphite cathode, where hydrogen evolution occurred, reduced them (Au³⁺ + 3e^- → Au). The number density of AuNPs depended on the kind of electrolyte solution: it was higher when the electrolysis was conducted in 0.5 M H₂SO₄ solution than in 0.5 M HNO₃ solution. This was because the rate of Au dissolution (or the formation rate of Au³⁺), which was directly measured using quartz crystal microbalance, was higher in the H₂SO₄ solution than in the HNO₃ solution. It is interesting that this dependence was opposite to the one for PtNPs: PtNPs were effectively obtained in the HNO₃ solution compared with in the H₂SO₄ solution [1].

In this present work, we show AuNPs can be produced on glassy carbon (GC) by the galvanostatic electrolysis. The use of GC as cathodes facilitates us to observe AuNPs by scanning electron microscope (SEM) because the surface of GC is smoother than that of graphite. Figure 1 show SEM images of GC cathodes taken after the electrolysis. Uniformly seized AuNPs are clearly observed after the electrolysis is conducted for a long time, e.g., 120 minutes (compare panel a1 with a2), which will be discussed in the presentation. On the other hand, AuNPs can be produced without using Au anode when the electrolysis solution contains HAuCl₄ (panels b1 and b2). A graphite plate is used as the anode for this electrolysis to avoid the formation of Au ions from the anode. The size of AuNPs is larger than that produced in the electrolysis without HAuCl₄. The difference in the size of AuNPs will be also discussed in the presentation.

REFERENCES

[1] T. Nishimura, T. Nakade, T. Morikawa, H. Inoue, Electrochimi. Acta, 129 (2014) 152–159.

[2] Y. Mukouyama, Y. Fukuda, T. Nishimura, ECS Trans., 80 (2017) 1425-1431.

FIGURE CAPTION

Figure 1. SEM images of GC cathodes taken after galvanostatic electrolysis at a cathodic current density of 10 mA cm^-2. The electrolysis was conducted at 30℃ for (left) 30 minutes and (right) 120 minutes. (a1 and a2) Electrolysis solution: 0.5 M H₂SO₄. Anode: Au wire. Anodic current density: 2.5 mA cm^-2. (b1 and b2) Electrolysis solution: 0.5 M H₂SO₄ + 0.1 mM HAuCl₄. Anode: graphite plate. Anodic current density: 0.49 mA cm^-2.