Nucleation and growth of catalytically active Pd islands on Au(111)-22x√3 studied by scanning tunnelling microscopy

The nucleation and growth of Pd films on Au(111)-(22 x root 3) substrates from 0.07 ML coverage has been characterized using an ultra high vacuum scanning tunnelling microscope. Initially, polygonal islands nucleate and grow at sites near the surface edge dislocations in the elbows of the herringbone reconstruction. At low coverage, the herringbone reconstruction remains relatively undisturbed and most Pd islands are found on fcc regions of the Au(111) substrate. Increasing coverage leads to distortion of the underlying reconstruction which in turn affects the surface morphology of the Pd islands. Atomic resolution images of Pd island surfaces show that they are well ordered and close packed. Second-layer growth is also in the form of polygonal islands on the underlying layers. The morphological evolution of the system with increasing Pd coverage provides a good explanation for its catalytic behaviour. (C) 1998 Elsevier Science B.V.