Formation and structures of Au–Rh bimetallic nanoclusters supported on a thin film of Al₂O₃/NiAl(100)

Abstract

Self-organized alloying of Au with Rh in nanoclusters on an ordered thin film of AlO/NiAl(100) was investigated via various surface probe techniques under ultrahigh-vacuum conditions and calculations based on density-functional theory. The bimetallic clusters were formed on the sequential deposition of vapors of Au and Rh onto AlO/NiAl(100) at 300 K. The formation was more effective on the oxide seeded with Rh, since all post-deposited Au joined the pregrown Rh clusters; for metal deposition in the reverse order, some separate Rh clusters were formed. The contrasting behavior is rationalized through the easier nucleation of Rh on the oxide surface, due to the stronger Rh-oxide and Rh-Rh bonds. The alloying in the clusters proceeded, regardless of the order of metal deposition, toward a specific structure: an fcc phase, (100) orientation and Rh core-Au shell structure. The orientation, structural ordering and lattice parameters of the Au-Rh bimetallic clusters resembled Rh clusters, rather than Au clusters, on AlO/NiAl(100), even with Rh in a minor proportion. The Rh-predominated core-shell structuring corresponds to the binding energies in the order Rh-Rh > Rh-Au > Au-Au. The core-shell segregation, although active, was somewhat kinetically hindered, since elevating the sample temperature induced further encapsulation of Rh. The bimetallic clusters became thermally unstable above 500 K, for which both Rh and Au atoms began to diffuse into the substrate. Moreover, the electronic structures of surface elements on the bimetallic clusters, controlled by both structural and electronic effects, show a promising reactivity.

Observation of a novel double layer surface oxide phase on Ni3Al(111) at low temperature

“…This preparation route cannot be performed on pure Al as only amorphous Al 2 O 3 can be grown as thin layer, whereas the formation of crystalline Al 2 O 3 on Al requires thicker layers or proceeds in island growth at high temperature. 18 The synthesis of well ordered oxide lms on metallic substrates is used to produce model support surfaces for heterogeneous catalysts that may host metal clusters, 19,20 metal particles [21][22][23][24][25][26][27] and also metal oxide particles. [28][29][30] The oxidation of Ni 3 Al(111) 7,8,[31][32][33][34] and especially the formation of the so called (O67 Â O67)R12.2 surface oxide is of great interest, 33,35 because it has nano-template properties that can be used to stabilize almost monodisperse distributions of metal particles on the thin alumina support.…”

Au–Rh Surface Structures on Rh(111): DFT Insights into the Formation of an Ordered Surface Alloy

“…[18][19][20][21][22][23][24][25][26][27][28][29] In a number of studies Au and Rh were sequentially evaporated by physical vapor deposition (PVD) on oxide single crystalline surfaces. 18,19,21,22,28,29 When first Au nanoparticles were prepared on a rutile TiO 2 (110) surface followed by evaporation of Rh, an efficient place exchange already occurred at room temperature, leading to the incorporation a great part of Rh atoms into the subsurface of Au clusters, while the outermost layer still mostly consisted of gold. 18 The driving force for this process is the smaller surface energy of Au compared to that of Rh (1.283 J/m 2 for Au(111) and 2.472 J/m 2 for Rh(111)).…”

“…While computational analysis of the interaction of Au with Rh(111) is the subject of the present paper as a continuation of our recent experimental studies, 21,31 other relevant surfaces were addressed in previous works. 24,28,[32][33][34] Scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES) investigations showed that Rh forms three dimensional (3D) islands on Au(111) at room temperature according to Volmer -Weber growth.…”

“…28 When a small Rh platelet of 13 atoms was placed on Au(100), DFT structure optimization indicated the distortion and buckling of substrate Au atoms, as well as the shrink and partial immersion of the Rh platelet into the topmost gold plane. 28 Molecular dynamics simulations at 300 K showed aggregation and sinking Rh atoms into Au(100). 28 Other molecular dynamics simulations of Rh growth on Au(111) and Au(100) indicated the formation of 3D rhodium particles on gold up to 500 K without interfacial mixing.…”

“…28 Molecular dynamics simulations at 300 K showed aggregation and sinking Rh atoms into Au(100). 28 Other molecular dynamics simulations of Rh growth on Au(111) and Au(100) indicated the formation of 3D rhodium particles on gold up to 500 K without interfacial mixing. When Au adatoms were also added to the simulation at 500 K, their stabilization at the perimeter/on top of Rh clusters were shown.…”