C. Chapon

Structural characterization of MgO(100) surfaces

Abstract The structure, the microtopography and the cleanliness of MgO(100) surfaces have been investigated by a combination of thermal helium scattering, surface decoration and Auger spectroscopy. The microstructure and cleanliness are strongly dependent on the preparation of the surface. MgO surfaces obtained by cleavage under UHV are clean and present a high coefficient for coherent He reflection. MgO surfaces prepared by cleavage in air are contaminated and irreversibly damaged by water vapour. Such surfaces can be cleaned by vacuum annealing but the coefficient of coherent He reflection remains low because of the presence of a high number of defects.

Growth and morphology of palladium particles epitaxially deposited on a MgO(100) surface

Abstract Palladium particles (1.5–12 nm) have been epitaxially grown on UHV-cleaved MgO(100) surfaces. Their crystalline structure and their morphology have been studied by various TEM techniques. At high deposition temperatures, homogeneous distributions of particles, displaying the single shape of an half octahedron truncated on the top by a (100) plane, have been obtained. At low deposition temperature particles were flatter with mainly (100) facets. At high temperature, on clean air-cleaved substrates, homogeneous distributions of very small particles (1.5–2 nm) with a high number density and sharp size distributions were obtained. These homogeneous distributions of well defined particles can be used in surface studies as model catalysts.

Size effect in the CO chemisorption on palladium clusters supported on magnesium oxide

Abstract In a previous paper [C. Duriez et al., Surf. Sci. 253 (1991) 190], molecular beam measurements were made of the adsorption-desorption kinetics of CO on Pd clusters, epitaxially grown on MgO(100). A large increase (∼8 kcal/mol) in the adsorption energy for clusters smaller than 3 nm grown on clean air-cleaved MgO was observed relative to clusters larger than 5 nm grown on UHV-cleaved MgO. New experimental measuremnts have shown that clusters smaller than 5 nm, grown on UHV-cleaved MgO or mica, give the same enhancement of reactivity as on a clean air-cleaved MgO surface, ruling out a possible substrate effect. The origin of this size effect is explained by the presence of a strong adsorption energy on some sites, the proportion of which increasing when the size of the clusters is decreased. Checks for carbon contamination by CO dissociation or CO disproportionation were negative.

Structure and morphology of small palladium particles (2–6 nm) supported on MgO micro-cubes

Abstract Small MgO cubes were prepared in a controlled atmosphere and protected against air contamination. Their clean surfaces were used as supports for Pd particles (2–6 nm) vacuum-deposited from a Knudsen cell in the same chamber. The Pd particles epitaxially oriented on the substrate were observed by HRTEM, microdiffraction and convergent-beam electron diffraction. An isotropic lattice expansion was determined by microdiffraction on isolated particles, increasing between 2% and 8% for particle sizes decreasing from 6 to 2 nm. From the HRTEM images of particles observed in two directions, parallel and perpendicular to the interface with the MgO substrate, the particles were characterized as single crystals with fcc structure and octahedral shape. The octahedra are truncated by (001) planes at the top and at the interface.

Atomic resolution on small three-dimensional metal clusters by STM

Abstract Small, three-dimensional Pd clusters ( d 2 ) are imaged with atomic resolution, both in air and in UHV, using scanning tunneling microscopy (STM). We show that a complete characterization of the morphology of the cluster can be achieved: three-dimensional shape; azimuthal orientation on the substrate; and arrangement of atoms on lateral facets. These results are in good agreement with those obtained from TEM observations on large particles ( d > 10 nm). Some limitations of STM for such studies are discussed.

Molecular beam study of the chemisorption of CO on well shaped palladium particles epitaxially oriented on MgO(100)

Abstract The CO adsorption-desorption transients have been measured on supported Pd particles by a molecular beam technique. The Pd particles were epitaxially grown, in UHV, on clean MgO(100) surfaces. The particles, fully characterized by TEM in the preceding paper, have a top-truncated half octahedron shape. For particles larger than 4 nm the desorption is first order and the zero coverage desorption energy is nearly the same as for a continuous film ( ~ 30 kcal mol ) . At high coverage the transients display two desorption regimes explained by a fast and a slow desorption from the (100) and (111) facets, respectively. For particles smaller than 3 nm, the desorption kinetics does not follow a simple first-order law, even at low coverage, and the desorption rate is considerably reduced in comparison with larger particles. The origin of this size effect, corresponding to an increase of the desorption energy, is discussed in terms of the modification of the electronic structure of the particles and of the presence of additional high-energy adsorption sites.

High-resolution transmission electron microscopy study of gold particles (greater than 1 nm), epitaxially grown on clean MgO microcubes

Abstract Gold particles (greater than 1 nm) were vacuum deposited on clean surfaces of MgO microcubes in situ synthesized in a controlled atmosphere. The particles were observed simultaneously with the MgO substrate in plan view and in cross-section by high-resolution transmission electron microscopy, microdiffraction and convergent-beam electron diffraction. The gold particles (less than 4 nm) had a f.c.c. structure and were perfectly accommodated with the substrate, leading to a lattice expansion of 2.9%. In this size range (1-4 nm), all the particles were single crystals and had an half-octahedron shape with (001) truncations on the edges. The interface between the particles and the MgO substrate was flat; no defects were imaged. In contrast larger (greater than 4 nm) particles have the lattice parameter of the bulk material. The high stability of the particles in the electron beam was certainly due to their strong interaction with the clean MgO surface.

Adsorption — desorption kinetics on epitaxially oriented palladium clusters

Palladium clusters with a low size dispersion and a single crystallographic shape have been obtained by epitaxial growth on (100) MgO. The isothermal adsorption — desorption kinetics of CO is obtained by a molecular beam method allowing the direct determination of the global adsorption probability and of the mean life time of CO molecules on the clusters. Three main results are deduced from these measurements: — an increase of the adsorption rate of CO on the clusters by the capture of CO molecules physisorbed on the substrate, — a faster desorption rate from (100) than from (111) facets at high coverage, — an increase of the adsorption energy for clusters smaller than 3 nm.

High-resolution transmission electron microscopy studies of structural deformations at the interface between Pd particles and MgO surfaces

Abstract Pd clusters were condensed on the surfaces of MgO microcubes, prepared in situ and observed by high-resolution transmission electron microscopy (HRTEM) in order to see directly both the profiles of the particles and the Pd-MgO interfaces. The HRTEM images in cross-sectional view were studied by numerical analysis. Small (1–2 nm) particles, with homogeneously expanded lattice distances, are perfectly accommodated to the MgO substrate and the dilatations are isotropic. In large (5–6 nm) particles, some dilatations of the d 200 distances were measured only at the interface, the top and the edges. Theoretical images of these particles (5 nm) were simulated by the multislice method. They provide evidence that the large Pd particles keep the bulk parameter, except in the first three deposited layers, which are expanded and progressively accommodated to the substrate. The deformations observed at the top and the edges are only artefacts of microscopy.

Electron-beam-induced transformations of gold particles epitaxially grown on MgO microcubes

Abstract Different epitaxial orientations (100), (110) and (111) of gold particles crystallized on clean MgO cubes were observed in HRTEM. During the electron irradiation, terraces (presumably MgO) are formed on the surfaces of the cubes, preferentially near the gold particles and at their interface with the substrate. During the growth of the terraces, the particles in (110) or (111) orientations rotate on the substrate and stay in the so-called “quasimolten state” until they get the stable (100) orientation. The origin of the growth of the terraces is attributed to the formation of Mg(OH) 2 on the surface of the MgO during the air exposure, which dehydrates in MgO in the electron beam.