Figure 1. NC-AFM images (3.4 x 3.4 microns (A), and 1.2 x 1.2 microns (B)) of the graphite basal plane surface following the application of a 10 ms platinum pulse. A deposition charge of 4.84 mC cm^-2 was obtained corresponding to 0.02 equivalent platinum atomic layers (assuming an adsorption electrovalency of 4.0). The mean particle height on this surface was 25 + 9 Å.

Figure 2. Same thing except here a larger deposition charge of 37.6 mC cm^-2 was obtained corresponding to 0.15 equivalent platinum atomic layers. The mean particle height on this surface was 52 + 9 Å.

Shown above are NC-AFM images of platinum nanocrystals which we have electrochemically deposited on HOPG(0001) surfaces. Platinum nanocrystallites are indespensible catalysts for a variety of technologically important electrochemical processes such as the oxidation of organic molecules in fuel cells. Many gas phase catalytic processes exhibit a pronounced particle size dependence of the catalytic activity and evidence for a particle size effect in electrochemical reactions - such as the methanol oxidation and oxygen reduction reactions - has recently been documented. Unfortunately, progress in the area of particle size effects in electrocatalysis has been impeded by the absence of good model systems for investigating these effects.

We have recently extended our prior work with metal nanocrystals (silver, copper, and cadmium) to the deposition of platinum nanocrystallites on the graphite basal plane surface. Specifically, platinum nanocrystals were deposited on basal plane oriented graphite surfaces from dilute (1.0 mM) PtCl₆^2--containing electrolytes using a pulsed potentiostatic method. This is basically the same method we used earlier except that we now must suppress electroless Pt deposition on the graphite surface by holding the potential of the graphite surface positive of the platinum plating potential before, and following, the application of the platinum plating pulse. So here's what the resulting plating waveform looks like:

The deposition of platinum nanocrystals occurred via an instantaneous nucleation and diffusion limited growth mechanism which resulted in narrow particle size distributions (35% RSD in diameter) for mean crystallite diameters smaller than 40 Å. The number of particles per unit area on these surfaces was 10⁹ - 10¹⁰ cm^-2. Non-contact atomic force microscopy images like those shown at those top of this page reveal that platinum nanocrystals nucleated both at defect sites - such as step edges - and on apparently defect-free regions of the atomically smooth graphite basal plane.