Image of the Month

January 2018

Surface structures and compositions of Au–Rh bimetallic nanoclusters supported on thin-film Al2O3/NiAl(100) probed with CO

FIG. 1. STM images of (a) pure Au (1.0 MLe), (b) Au (1.0 MLe) and subsequently Rh (1.0 MLe), (c) pure Rh (1.0 MLe), (d) Rh (1.0 MLe) and subsequently Au (0.2 MLe) deposited on thin-film Al2O3/NiAl(100) at 300 K. The insets of each figure show histograms of diameters and heights of the clusters. The grey bars in the histograms of (b) and (d) correspond to the STM images and the red bars are from those of (a) and (c), respectively, shown side by side to demonstrate the variation of the size distribution.

Abstract
The surface structures and compositions of Au–Rh bimetallic nanoclusters on an ordered thin film of Al2O3/NiAl(100) were investigated, primarily with infrared reflection absorption spectra and temperature-programmed desorption of CO as a probe molecule under ultrahigh-vacuum conditions and calculations based on density-functional theory. The bimetallic clusters were formed by sequen- tial deposition of vapors of Au and Rh onto Al2O3/NiAl(100) at 300 K. Alloying in the clusters was active and proceeded toward a specific structure—a fcc phase, (100) orientation, and Rh core-Au shell structure, regardless of the order of metal deposition. For Au clusters incorporating deposited Rh, the Au atoms remained at the cluster surface through position exchange and became less coordinated; for deposition in reverse order, deposited Au simply decorated the surfaces of Rh clusters. Both adsorption energy and infrared absorption intensity were enhanced for CO on Au sites of the bimetallic clusters; both of them are associated with the bonding to Rh and also a decreased coordination number of CO- binding Au. These enhancements can thus serve as a fingerprint for alloying and atomic inter-diffusion in similar bimetallic systems.

Reference:
The Journal of Chemical Physics 147, 044704 (2017); doi: 10.1063/1.4995598

Credits:
Hsuan Lee,1 Zhen-He Liao,1 Po-Wei Hsu,1 Ting-Chieh Hung,1 Yu-Cheng Wu,1 Yuwei Lin,2 Jeng-Han Wang,2,a) and Meng-Fan Luo1,a)

1 Department of Physics, National Central University, 300 Jhongda Road, Taoyuan 32001, Taiwan

2 Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan 

a) Authors to whom correspondence should be addressed: [email protected] edu.tw and [email protected]

Microscope:
Beetle UHV 300