The growth mechanism, thermal stability, and reactivity of palladium mono-and multilayers on Cu(110)

The room-temperature growth of palladium (Pd) on Cu(110) has been studied b y X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (ST M), low-energy electron diffraction (LEED), and temperature-programmed deso rption (TPD). XPS signal versus deposition time plots rule out a simple lay er-by-layer growth mechanism. STM/LEED indicates formation of regions of (2 x 1) overlayer at low Pd coverages (theta(Pd) < 1 ML), with considerable d isorder in the form of monolayer deep pits and islands. Higher Pd coverages lead to the formation of a granular film consisting of densely packed, fla t-topped Pd clusters of, average size 75 x 150 Angstrom and with largely a rectangular shape. The favored growth mechanism is of multilayered Pd islan ds above a mixed (2 x 1) CuPd interface of two to three atomic layers thick ness. The thermal stability of the Pd/Cu(110) system was investigated with XPS peak intensity versus annealing temperature plots that indicate that bu lk intermixing takes place rapidly between 500 and 600 K. The Pd 3d(5/2) XP S peak widths narrow, suggesting the formation of a largely homogeneous CuP d surface alloy. STM indicates that heating to 500 K leaves the Pd clusters in a largely unaltered morphology with no sign of Ostwald ripening, wherea s annealing to 600 K leads to considerable changes in topography. The granu lar structure of the Pd film is disrupted, leading to a surface with irregu larly shaped flat domains separated by mono-atomic steps. High temperature (720 K) annealing leads to further flattening and appearance of regular par allel lines in STM images. The spacing of these Lines varies with Pd loadin g, and they are assigned to strain due to lattice mismatch between the "cap ping" copper monolayer and the underlying mixed CuPd alloy. The reactivity of the Pd/Cu(110) surface has been probed by dosing formic acid and monitor ing formate decomposition. High Pd coverages lead to a substantial destabil ization of the formate relative to clean Cu(110), which is assigned to form ate adsorption on mixed CuPd sites.