Platinum electrodeposition on graphite: Electrochemical study and STM imaging

The electrodeposition of Pt on a thermally oxidized HOPG surface was performed by single potential perturbation in dilute chloroplatinic acid solutions. The quantitative analysis of the current versus time transient responses, on the time scale of seconds, indicated a low saturation density of nuclei of 2×10⁶ cm^-2. The characterization by STM revealed a heterogeneous distribution of deposited Pt on the substrate surface. Most of the deposits were composed of agglomerates of spherical nanoparticles. Local particle densities exceeding 10¹⁰ cm^-2 were observed, which is several orders of magnitude higher than the saturation coverage of nuclei evaluated from the analysis of the transient at t>0.3 s. From the electrochemical and the structural investigations, it appears that Pt electrodeposition on graphite involves several faradaic steps on the time scale. Firstly, at t<0.3 s, a large number of nanosized clusters are possibly formed. In the course of the electrodeposition process, these very small clusters may be mobile and assemble in an accidental distribution of Pt agglomerates. Secondly, at t>0.3 s, it is very likely that the more rapid growth of a small number (2×10⁶ cm^-2) of Pt particles gives a transient response, which is adequately described by the model of Scharifker and Hills for a diffusion controlled growth. As a result, it seems that cluster diffusion contributes significantly to the structural evolution of platinum electrodeposits on graphite.