An STM study of structural transitions during the nucleation and growth of Pd and Cu cluster catalysts on HOPG

We report a scanning tunneling microscopy investigation of the morphology of model metal cluster catalysts deposited on highly oriented pyrolytic graphite. Coverages have been determined from a quartz crystal oscillator which has been calibrated in absolute terms using anodic stripping voltammetry. Both metals (Pd and Cu) show marked structural transitions as a function of the coverage. At high coverages (> 50 monolayers) an essentially continuous metal film is formed. However, the film is granular in nature consisting of tightly packed spherical clusters which have a surprisingly narrow size distribution. Cu clusters have a most probable diameter of 7.5 nm with a distribution of ±2.5 nm and Pd clusters are smaller having a most probable diameter of 4.2 nm with a distribution ranging from 3 to 6.4 nm. The cluster size is coverage independent down to the lowest coverages imaged which are of monolayer thickness. At intermediate coverages (4 to 8 monolayers) large areas of unadorned graphite are present, the clusters are nucleated in rafts in which a large number of clusters are aggregated both on the atomically flat terraces and at defect sites such as atomic and cleavage steps. At low coverages (≤ 3 monolayers) the clusters are located almost entirely at defect sites. No Ostwald-Ripening has been observed at 300 K. Time dependent morphological changes have been observed, indicating Cu aggregates appear to be highly mobile at room temperature. Aggregates are in continuous motion involving fission and re-attachment with a tendency for concentration of clusters along steps. This study illustrates that the exact morphology observed is a function of when the imaging occurs after growth.