Electrochemical Nucleation and Growth of Gold Nanoparticles on Single-Walled Carbon Nanotubes: New Mechanistic Insights

The electrodeposition mechanism of gold nanoparticles (NPs) on pristine single walled carbon nanotubes (SWNTs) at high driving forces has been elucidated using the microcapillary electrochemical method. Here, a small capillary (internal diameter ∼50−100 μm) filled with a gold plating solution, and positioned so that the capillary meniscus makes contact with a two-dimensional SWNT random network, was used to record current−time transients. Nucleation and growth transients were observed in which the current increased with time to a maximum value beyond which the current decreased (planar diffusion regime). With increased driving force, the current maximum shifted dramatically to increasingly shorter times. Atomic force microscopy (AFM) analysis indicated that this was not due to significant differences in NP growth rates, but rather to increased densities of NPs formed at more cathodic potentials. Detailed microscopic analysis showed that the size of the NPs initially increased with deposition time and the particle surface coverage was constant. However, at the highest driving forces the NP density decreased with deposition time and AFM revealed the presence of both larger and smaller particles at long times. This was attributed to electrochemically induced Ostwald ripening, whereby larger particles grow at the expense of smaller ones. As NP nucleation and growth on SWNT two dimensional network electrodes is highly directional and enforced in particular locations, it is inappropriate to analyze electrochemical data using conventional models. There is thus a need to complement chronoamperometric measurements with high resolution microscopy to fully interpret nucleation on complex electrode surfaces.