EFFECTS OF ANNEALING AND GAS TREATMENT ON THE MORPHOLOGY OF PLATINUM CLUSTER-SIZE ON HIGHLY ORIENTED PYROLYTIC-GRAPHITE BY SCANNING-TUNNELING-MICROSCOPY

We have used STM to examine the effects of annealing time in the prese nce of different gaseous species (Ar, H-2 and N2) on the morphological transformation of vacuum-vapor deposited Pt clusters on highly orient ed pyrolytic graphite (HOPG). This study shows that the morphological transformation of the Pt clusters on HOPG is a function of both gaseou s species and annealing time. Without annealing, the as-deposited plat inum consists of an aggregation of smaller clusters. These clusters be come more uniform and spherical after annealing for 4 h in the presenc e of Ar and H-2. Further annealing shows a morphological transformatio n from spherical to elongated shape. Small amounts of hydrocarbon impu rities in Ar or hydrogen might possibly explain the shape transformati on of the Pt clusters. However, the morphological transformation of th e Pt clusters is totally different when the sample is annealed in low sticking coefficient gas, N2, suggesting adsorbed gases (hydrocarbon o r hydrogen) have profound effects on the shape transformation. These r esults indicate that pretreatment conditions are important factors in the shape transformation, which may have significant influence on cata lytic activity and selectivity.