Supramolecular chiral self-assemblies of Gly-Pro dipeptides on metallic fcc(110) surfaces.

Adsorption of the Glycine-Proline (Gly-Pro) dipeptide has been investigated using surface science complementary techniques on Au(110) and Ag(110), showing some interesting differences both in the chemical form and surface organization of the adsorbed peptide. On Au(110), Gly-Pro mainly adsorbs in neutral form (COOH/NH2), at low coverage or for a short interaction time; the surface species become zwitterionic at a higher coverage or longer interaction time. These changes are accompanied by a complete reorganization of the molecules at the surface. On Ag(110), only anionic molecules (COO-/NH2) were detected on the surface and only one type of arrangement was observed. These results will be compared to some previously obtained on Cu(110), thus providing a unique comparison of the adsorption of the same di-peptide on three different metal surfaces; the great influence of the substrate on both the chemical form and the arrangement of adsorbed di-peptides was made clear.

Critical Au Concentration for the Stabilization of Au-Cu Nanoparticles on Rutile against Dissociation under Oxygen.

Controlling aging of catalysts is of crucial importance to preserve their properties, in particular for bimetallic nanoparticles (NPs) where reaction can modify the composition. Herein, we have studied the stability upon oxygen exposure of gold-copper NPs supported on rutile. We have used in situ scanning tunneling microscopy to follow the evolution of individual Au, Cu and Au-Cu NPs with various compositions grown on the TiO2(110) surface, during each step from their nucleation to their modification with oxygen. We demonstrated a direct relation between the stability of the nanoparticles and their Au concentration. Whereas pure Cu nanoparticles dissociate under O2, Au-Cu NPs containing at least 20% Au are stable. This is explained by a modification of the local density of states of Cu atoms upon alloying.

Stoichiometry-dependent chemical activity of supported MgO(100) films.

Here, we show that the stoichiometry and, consequently, the chemical activity toward hydroxylation of MgO(100) films grown by reactive deposition on Ag(100) strongly depend on the O(2) partial pressure during film growth. Oxygen-deficient films undergo dramatic relative oxygen uptake either by exposure to a partial pressure of water vapor or by aging in vacuum for a sufficiently long time. Conversely, on stoichiometric monolayer MgO islands, photoemission analysis of the O 1s level and scanning tunneling microscopy images are consistent with the prediction that dissociative adsorption of water occurs only at the borders of the islands.