Nanoparticles 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, nanoparticles have large surface area to volume ratios, which lead to an increase in catalytic efficiency and also to a cost-reduction. A variety of methods using chemical reactions are currently available for synthesizing nanoparticles in solution phase and also for controlling their shapes and surface properties. In the field of electrochemistry, platinum nanoparticles (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.

Recently, 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 graphite cathode and a Pt anode, the latter of which dissolves to form Pt ions which are reduced at the former. The method is cost friendly because it can be performed in acidic solutions without any reducing and stabilizing agents, i.e., in H₂SO₄ and HNO₃solutions, and also because 72 % of dissolved Pt ions are electrodeposited as PtNPs. This simple method can produce not only tetragonal nanoparticles which are 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 this present work, we studied if gold nanoparticles (AuNPs) could be produced by the same simple method except for employing a gold wire as the anode. Figure 1 shows SEM images of graphite cathodes taken after galvanostatic electrolysis in 0.5 M HNO₃ solution for 8 hours. The current densities at the graphite cathodes were (a) -10 and (b) -100 mA cm^-2. Note that the main products at the cathode and anode were hydrogen and oxygen due to the electrolysis of water. The images clearly show that AuNPs were electrodeposited on the graphite surface and also that the number of AuNPs obtained at -100 mA cm^-2 was larger than that obtained at -10 mA cm^-2. Furthermore, the number of AuNPs produced in a 0.5 M H₂SO₄ solution was found to be larger than that produced in the 0.5 M HNO₃solution. In the presentation, we will show the dependence of the number on electrolysis conditions.

REFERENCES

[1] T. Nishimura, T. Nakade, T. Morikawa, H. Inoue, Electrochimica Acta129 (2014) 152–159.

FIGURE CAPTION

Figure 1. SEM images of graphite cathodes taken after galvanostatic electrolysis.