A comparative study in CCl4 reaction on Ag/Si(111) surfaces: PEEM and PES investigations

A comparative study in CCl4 reaction on Ag/Si(111) surfaces: PEEM and PES investigations Yunxi Yao, Qiang Fu, Xinhe Bao State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China We report a comparative study in the reactivity of bulk Ag(111), monolayer Ag film on Si(111), and Si(111)-77 surfaces via X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), photoemission electron microscopy (PEEM), and scanning tunneling microscopy (STM). The monolayer Ag film grown on Si(111), which is known as √3×√3-Ag-Si surface structure, was obtained by depositing one monolayer Ag on the Si(111)-77 surface at 550 K. The most simple halogen methane, CCl4, was chosen as the probe molecule to study the surface reactivity of the bulk Ag(111) surface, the monolayer Ag film, and the Si(111) surface. XPS and UPS data indicate that the monolayer Ag film presents unique reactivity to CCl4 in comparison to the other two surfaces. For the PEEM study, a dedicated sample consisting of bulk Ag particles supported on the monolayer Ag film was prepared as shown in the schematics in Fig. 1. In situ PEEM imaging of the surface reaction in presence of CCl4 shows a gradual change in the grey intensity from bright to totally dark on the Ag particles but little change on the √3×√3-Ag-Si surface. The grey intensity decrease is due to local work function increase from the dissociated Cl atoms. The experiments suggest that monolayer Ag is inert toward dissociation of CCl4 compared to the Ag(111) and Si(111) surface. It has proposed that the confinement of 5sp electron of Ag atoms in the √3×√3-Ag-Si surface, which is delocalized in the bulk Ag(111) surface, is decisive to the different reactivity. Figure 1: (a) Schematic representation of the √3×√3-Ag-Si surface supported Ag islands; (b) PEEM image of the Ag islands/√3×√3-Ag-Si sample before CCl4 exposure. Field of view (FoV) is 27 μm; (c) PEEM image shows the same area in (b) but after 24 L (5.2×10-9 mbar × 6000 s) CCl4 exposure; (d) the work function change (ΔΦ) of the Ag(111) and √3×√3-Ag-Si surfaces when exposed to different amount of CCl4 at RT, measured from PES results. References: [1] Y.X. Yao, X. Liu, Q. Fu, W.X. Li, D.L. Tan, X.H. Bao, ChemPhysChem 2008, 9: 975-979. *Corresponding author: Fax: +86-411-84694447, E-mail: xhbao@dicp.ac.cn, qfu@dicp.ac.cn