The Growth Mechanism, Thermal Stability, and Reactivity of Palladium Mono- and Multilayers on Cu(110)

Abstract

The room-temperature growth of palladium (Pd) on Cu(110) has been studied by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD). XPS signal versus deposition time plots rule out a simple layer-by-layer growth mechanism. STM/LEED indicates formation of regions of (2 × 1) overlayer at low Pd coverages (θPd < 1 ML), with considerable disorder in the form of monolayer deep pits and islands. Higher Pd coverages lead to the formation of a granular film consisting of densely packed, flat-topped Pd clusters of average size 75 × 150 Å and with largely a rectangular shape. The favored growth mechanism is of multilayered Pd islands above a mixed (2 × 1) CuPd interface of two to three atomic layers thickness. The thermal stability of the Pd/Cu(110) system was investigated with XPS peak intensity versus annealing temperature plots that indicate that bulk intermixing takes place rapidly between 500 and 600 K. The Pd 3d5/2 XPS peak widths narrow, suggesting the formation of a largely homogeneous CuPd surface alloy. STM indicates that heating to 500 K leaves the Pd clusters in a largely unaltered morphology with no sign of Ostwald ripening, whereas annealing to 600 K leads to considerable changes in topography. The granular structure of the Pd film is disrupted, leading to a surface with irregularly shaped flat domains separated by mono-atomic steps. High temperature (720 K) annealing leads to further flattening and appearance of regular parallel lines in STM images. The spacing of these lines varies with Pd loading, and they are assigned to strain due to lattice mismatch between the “capping” copper monolayer and the underlying mixed CuPd alloy. The reactivity of the Pd/Cu(110) surface has been probed by dosing formic acid and monitoring formate decomposition. High Pd coverages lead to a substantial destabilization of the formate relative to clean Cu(110), which is assigned to formate adsorption on mixed CuPd sites.