Surface Science Letters The 2-D growth of gold on single-layer graphene/Ru(0001): Enhancement of CO adsorption Li Liu a , Zihao Zhou a , Qinlin Guo b , Zhen Yan a , Yunxi Yao a , D. Wayne Goodman a, ⁎ a Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012, United States b Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People's Republic of China abstract article info Article history: Received 8 March 2011 Accepted 29 April 2011 Available online 7 May 2011 Keywords: Graphene Ru(0001) STM 2-D Au CO adsorption IRAS The growth and morphology of two-dimensional (2-D) gold islands on a single-layer graphene supported on Ru(0001) have been studied by scanning tunneling microscopy (STM). Our findings show that gold exhibits 2-D structures up to a gold dosage of 0.75 equivalent monolayers, and that these 2-D gold islands are thermally stable at room temperature. Parallel polarization modulation infrared reflection absorption spectroscopic (PM-IRAS) and high resolution electron energy loss spectroscopic (HREELS) studies indicate that carbon monoxide (CO) adsorbs on these 2-D gold islands at 85 K, showing a characteristic CO stretching feature at 2095 cm -1 for a saturation coverage of CO. The red shift of the CO stretching frequency compared to that on charge neutral gold is consistent with electron transfer from graphene to gold, i.e., an electron-rich gold overlayer. Preliminary data obtained by dosing molecular oxygen onto this CO pre-covered surface suggest that the 2-D gold islands catalyze the oxidation of CO. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Several studies have addressed the growth of metal clusters on graphene supported on transition metals [1–5]. For example, N'Diaye et al. [1] investigated the deposition of Ir onto a moiré-patterned graphene/Ir(111) surface and found that Ir forms monodispersed clusters when deposited on graphene at low coverages. The metal clusters so formed grow exclusively in hcp hollow regions. At higher coverages, the Ir clusters exhibit two-dimensional (2-D) structures, transforming to three-dimensional (3-D) structures while maintain- ing high metal dispersity. Other metals, namely Pt, W, Re, Fe, and Au have also been studied on graphene/Ir(111) [2], with Pt, W, and Re forming epitaxial cluster superlattices whereas Fe and Au do not. In more recent investigations, metal cluster deposition was studied on the graphene/Ru(0001) surface [3–5]. Although similarities in cluster morphologies were observed between graphene/Ru(0001) and graphene/Ir(111), distinct differences were found with respect to cluster decoration sites (fcc versus hcp), cluster dispersion, and the metal coverages at which 2-D to 3-D transitions occur. Particularly noteworthy was the case of Au deposition, where the formation of 2-D Au layers was observed on graphene/Ru(0001) [5]. This is intriguing not only because Au nanoparticles exhibit exceptional catalytic activity for several reactions [6–15], but also Au particles with a bilayer structure are the most active due to their unique electronic and chemical properties compared to bulk Au [9,10]. In the present study, we have focused on the decoration of Au on graphene/Ru(0001) to investigate the detailed morphology of the 2-D Au islands as a function of Au coverage and their ability to adsorb carbon monoxide (CO). 2. Experimental These experiments were carried out in three ultrahigh vacuum (UHV) chambers (base pressure ~1×10 -10 Torr), each with a cylin- drical mirror analyzer for Auger electron spectroscopy (AES) and a low energy electron diffraction (LEED) optics. The three chambers are equipped with an Omicron STM-1 [16], a LK-2000 high resolution electron energy loss spectrometer (HREELS) [17], and a polarization modulation infrared reflection absorption spectrometer (PM-IRAS) [18], respectively. Each system has been described in detail in the noted reference. The Ru(0001) substrate was cleaned using a standard approach, which includes cycles of Ar ion sputtering, annealing in 1.0 × 10 -7 Torr of O 2 at ~1100 K, and flashing to 1800 K in vacuum. The surface cleanness was confirmed by AES, LEED, and, when available, STM. Single-layer graphene was formed by first exposing a clean Ru(0001) sample to ethylene or propylene with a pressure of 1×10 -7 Torr at room temperature, followed by annealing the sample to 1300 K, and then slowly cooling to 1000 K. This process can be repeated several times to generate graphene that fully covers the Ru substrate. Au was deposited by evaporation from a high-purity Au wire wrapped around a resistively-heated tantalum wire. The deposition amount is defined in terms of monolayer equivalents (ML) where 1 ML corresponds to the number density of Ru atoms in the Ru(0001) Surface Science 605 (2011) L47–L50 ⁎ Corresponding author. Tel.: + 1 979 845 0214; fax: + 1 979 845 6822. E-mail address: goodman@mail.chem.tamu.edu (D.W. Goodman). 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.04.040 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/ locate/susc