J. Phys. Chem. C 2007, 111, 3685-3691
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STM and XPS Study of Growth of Ce on Au(111) S. Ma, X. Zhao,† J. A. Rodriguez, and J. Hrbek* Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: July 11, 2006; In Final Form: December 20, 2006
The growth of Ce on Au(111) was studied with STM and XPS in UHV. Ce islands grew on Au(111) without showing a preference for nucleation at elbows, as reported for some other clusters. XPS data show a shift of up to +0.4 eV for the Au 4f core level with increasing coverage of Ce to 2 ML at room temperature. The Ce 3d core level broadened upon heating, suggesting a strong interaction of Ce atoms with Au atoms. After heating to 470 K, the Au herringbone was distorted into trigonal structures while small Ce clusters with a narrow size distribution decorated the elbows and corners of the trigonal structures. Large islands displaying a moire´ pattern grew by alloying Ce atoms with Au surface in a broad size range. The second alloy phase, embedded into the surface as hexagonal islands with corners located on the herringbone stripes, has a strong resistance to oxidation by O2 and NO2. Star-like coplanar structures seen at the intersection of six dislocation stripes are the preferential nucleation sites for the hexagonal islands.
1. Introduction Supported metal nanoparticles are receiving a lot of attention lately.1 Owing to their small size, nanoparticles differ from bulk metals in their chemical and physical properties and thus have many attractive potential applications. For example, nanoparticles are useful in fabrication of magnetic devices, sensors, and catalysts. A strong dependence of these properties on size calls for preparation of narrowly dispersed particles arranged preferably in ordered arrays for use in fundamental research and applications. Reconstructed single-crystal surfaces provide dislocation patterns of periodic arrays with lattice spacing in ∼5 nm range that can serve as templates for the growth of nanoparticles. The Au(111) surface, known for its unique “herringbone” reconstruction pattern, has been extensively studied and is well understood.2,3 Of the two dislocation stripes bounding the hcp stacking-fault region, one stripe is characterized by elbow sites having 5-fold coordination and the other having 6-fold coordination in the surface.4-6 The undercoordination is responsible for the preferential adsorption of adatoms or clusters at elbow sites.7,8 Here we adopt the classification used in reference4 by calling the 5-fold elbows as “end-on” dislocations to distinguish from the 6-fold ones. The interaction of cerium metal with transition metals is much less studied9-12 compared to ceria that finds use in many practical applications.13 Metallic Ce deposited on Pt(111) forms ordered domains of surface alloys after annealing in vacuum.10 Cerium atoms form a (2 × 2) ordered structure on Rh(111) when grown at room temperature followed by annealing to 523 K in the submonolayer regime.11 Intermixing and formation of a Ce-Rh alloy were proposed. At 3.9 K, Ce atoms have been found to form a hexagonal 2D superlattice on Ag(111) with variable atomic spacing depending on the cerium concentration on the surface.12 In this paper the growth and the reactivity of Ce on the Au(111) surface were studied with STM and XPS. XPS data show * Corresponding author. E-mail:
[email protected]. † Current address: Chemistry Department, The University of Huston, Houston, TX 77004.
that cerium alloys with gold even at room temperature. Upon annealing the Ce-covered Au(111) surface, several different surface structures are seen in STM images including large islands with the moire´ pattern, hexagonal islands bound by the dislocation stripes and small “stars” islands decorating the elbows. The first type of islands grew randomly on terraces or across the step edge in a broad size range. In contrast, the hexagonal islands with a well-resolved (2 × 2) superstructure are imbedded into the surface and have a strong resistance to oxidation by O2 and NO2. The third type of the Ce derived surface features, referred to here as “stars”, originate from the intersection of six dislocation stripes are the nucleation centers for the hexagons. 2. Experiment STM studies were carried out on the Omicron variable temperature STM system that is directly attached to the main chamber equipped with LEED, Auger, surface cleaning facilities and e-beam metal evaporator. The system was pumped to below 5 × 10-11 Torr by an ion pump and periodic operation of a titanium sublimation pump. Chemically etched W tips were used for imaging surfaces with the sample negatively biased. The Au(111) single crystal was cleaned by sputtering with Ne+ at the beam energy of 1 keV and a sample current of 2 µA, and annealing to 700 K for 30 min. Deposition of Ce atoms was made by e-beam heating Ce foils (>99.9% purity) housed in a Ta crucible. Photoemission studies were conducted in a separate UHV chamber with a base pressure less than 4 × 10-10 Torr. The UHV chamber is fitted with a hemispherical electron energy analyzer that has multichannel detection, a quadrupole mass spectrometer, and a twin anode (Mg and Al) X-ray source. An Al KR X-ray source was used in this study. The binding energy values were determined with respect to the Fermi level. The Au(111) single crystal was held by two Ta heating legs of a manipulator.14,15 The sample could be cooled down to 100 K by thermal contact with a liquid nitrogen reservoir, and resistively heated to 1200 K. The temperature was monitored by a type C thermocouple that was fixed on one of the Ta legs.
10.1021/jp064366v CCC: $37.00 © 2007 American Chemical Society Published on Web 02/09/2007
3686 J. Phys. Chem. C, Vol. 111, No. 9, 2007
Ma et al.
Figure 2. (a) Peak intensity of the Ce 3d core levels and (b) Au 4f7/2 binding energy as a function of Ce dose. The accuracy of the core level intensity measurements is ( 10% for 12 min deposition. Figure 1. (a) Ce 3d and (b) Au 4f core-level spectra taken after the deposition of Ce on Au(111) at 300 K for indicated time. After the final dose (12 min), the Ce coverage was in the range of 1.7 ( 0.2 ML.
The Au(111) sample was cleaned by cycles of 1.0 keV Ar+ ion sputtering followed by heating to 900 K until no impurities were detected by XPS. A cerium doser was homemade by filling Ce foils (>99.9% purity) in a Ta basket that can be resistively heated.16 An estimate of cerium coverage was based on the attenuation of the Au 5d valence band14 or on the evaluation of STM images. 3. Results and Discussion 3.1. XPS Study of Ce/Au(111). Figure 1a shows a Ce 3d XPS spectrum collected after depositing different coverages of Ce on the Au(111) surface at 300 K. The final coverage of Ce was in the range of 1.7 ( 0.2 monolayers (ML). It is clear that the shifts in the Ce 3d binding energies with increasing deposition time are very small (
