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Langmuir 1997, 13, 5356-5361
Coadsorption of Gold and Oxygen on Ruthenium(100) Stephen Poulston,*,† Mintcho Tikhov, and Richard M. Lambert Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. Received April 1, 1997. In Final Form: July 25, 1997X We have studied the interaction of oxygen with Au overlayers on Ru(100) by examining Au deposition on oxygen-precovered Ru and comparing this with oxygen adsorption on Au-precovered Ru(100). Au deposition on oxygen-precovered Ru(100) led to an increase in the local atomic density of the oxygen phase and enhanced 3-dimensional growth in the Au overlayer relative to Au deposition on the clean substrate, though with increasing Au coverage the preadsorbed oxygen was covered with Au. Annealing of these overlayers resulted in an extensive agglomeration of the Au with the oxygen in intimate contact with the Ru such that part of the surface was covered only with oxygen. The Au rewets the Ru on desorption of the oxygen, which is completed, in the presence of >3 ML Au, before the completion of Au desorption, i.e., at some 300 K lower temperature than in the absence of Au.
1. Introduction A UHV study of metal deposition on an oxygenprecovered surface is useful in understanding both the interaction of the evaporated metal with coadsorbed oxygen and the parameters that determine the growth mode of the metal films. For the Au-oxygen system in particular, the adsorption of oxygen onto a different metal substrate allows investigation of the interaction between atomic oxygen and Au under mild conditions. Using bulk Au substrates, a strong interaction/oxide formation only occurs in UHV with energetically excited oxygen or with highly reactive oxygencontaining molecules. For example, atomic oxygen can adsorb on bulk Au samples (a) when the sample is exposed to oxygen in a microwave discharge1 or (b) when a hot Pt filament is placed close to the sample during exposure to oxygen.2 Alternatively, formation of an atomic oxygen overlayer on Au(111) has been reported from O3 adsorption at 300 K.3 Several authors have studied the deposition of Au on oxygenated Ru(001).4-8 Using a variety of techniques, these authors have concluded that preadsorption of oxygen leads to a much more 3-dimensional, Volmer-Weber, growth mode of the Au overlayer. Another interesting feature of these experiments was a significant reduction of the oxygen desorption temperature in the presence of Au. This study deals principally with the deposition of Au at 300 K onto oxygenated Ru(100) surfaces. In addition, we present results for the reverse sequence of adsorption, oxygen adsorption on Au-precovered Ru, to allow comparison of the two adsorption sequences. We have previously studied the deposition of Au on clean, unoxy-
genated Ru(100) at 300 K, which resulted in a simultaneous multilayer (SM) type growth mode. The growth mode was confirmed by Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy, angle-resolved ultraviolet photoelectron spectroscopy, and low-energy electron diffraction (LEED).9 We have previously reported on the adsorption and thermal properties of oxygen overlayers on Ru(100) using LEED, AES, TPD, and work function measurements.10 The saturation oxygen coverage at 300 K is 1 ML (1 ML ) 1 monolayer ) 8.6 × 1014atoms‚cm-2). At this coverage the oxygen TPD profile exhibits two peaks, β1 and β2, at approximately 1360 and 1180 K, respectively. 2. Experimental Section Experiments were performed in an ultrahigh vacuum chamber that has been described in detail elsewhere.11 This chamber, operated at a base pressure of about 1 × 10-10 Torr, was equipped with an ion gun, capillary array gas doser, and a retarding field analyzer for LEED measurements and, in conjunction with a separate glancing incidence electron gun, AES. A quadrupole mass spectrometer was used for residual gas analysis and TPD. A homemade Knudsen type source was used for Au deposition. The sample was cleaned using a well-established procedure12 involving cycles of argon bombardment, annealing, and oxygen adsorption/desorption. The sample was considered clean when no detectable CO desorption occurred during TPD from a surface subjected to a 10 Langmuir (1 Langmuir ) 1 L ) 1 × 10-6 Torr‚s) oxygen exposure. Abbreviations of the form O/Aux/Ru(100) indicate overlayers formed by oxygen adsorption on Au-precovered Ru(100) where x is the Au coverage in monolayers. Similarly, Aux/Oy/Ru(100) refers to Au deposition on oxygen-precovered Ru(100), x and y being the Au and oxygen coverages, respectively, in monolayers. In both cases Au deposition was carried out with the sample at room temperature (∼300 K).
3. Results †
Present address: University of Reading, Department of Chemistry, Whiteknights, Reading RG6 6AD, U.K. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Evans, S.; Evans, E. L.; Parry, D. E.; Tricker, J. J.; Walters, M. J.; Thomas, J.M. Faraday Discuss. Chem. Soc. 1974, 58, 97. (2) Canning, N. D. S.; Outka, D.; Madix, R. J. Surf. Sci. 1984, 141, 240. (3) Holmes, D.; Koel, B. E. J. Vac. Sci. Technol. 1990, A8, 2585. (4) Kalki, K.; Schick, M.; Ceballos, G.; Wandelt, K. Thin Solid Films 1993, 228, 36. (5) Schroder, J.; Gunther, C.; Hwang, R. Q.; Behm, R. J. Ultramicroscopy 1992, 42-44, 475. (6) Bludau, H.; Skottke, M.; Pennemann, B.; Mrozek, P.; Wandelt, K. Vacuum 1990, 41, 1106. (7) Niemantsverdriet, J. W.; Dolle, P.; Markert, K.; Wandelt, K. J. Vac. Sci. Technol. 1987, A5, 875. (8) Malik, I. J.; Hrbek, J. J. Phys. Chem. 1991, 95, 2466.
S0743-7463(97)00339-9 CCC: $14.00
O/Au/Ru(100). We have previously demonstrated13 using AES and LEED that Au coverages in excess of ∼2.5 ML prevented subsequent oxygen adsorption, as at this coverage all the underlying Ru was covered by Au. For Au precoverages, 1 ML, two distinguishable, new oxygen peaks were observed at ∼1110 K (R1) and 1150 K (R2), which subsequently became unresolved for Au coverages >6 ML. For Au coverages >3 ML, a third, less intense, new oxygen peak (R3) was observed at 1025 K. After the addition of ∼3 ML Au, all the oxygen was desorbed, by ∼1200 K, from the Rx states, whereas below θAu ) 3 ML, oxygen desorption persisted up to 1550 K from the Ox/Ru β2 state. When the oxygen precoverage was reduced to ∼75% of the saturation value, a similar pattern was observed. Oxygen thermal desorption spectra for an oxygen precoverage of 0.7 ML are shown in Figure 9. Note that in this case no R3 peak was observed, only the R1 and R2 peaks, which were observed in the same temperature interval as for Aux/O1/Ru(100). Initially, the R1 and R2 states formed two distinct peaks, though only one broad peak was observed at >6 ML Au coverage. A further reduction in the oxygen precoverage to 40% of the saturation amount resulted in only a single new peak for Au coverages >3 ML. Au desorption spectra for coverages 3 ML, there is sufficient Au to maintain compression of the oxygen overlayer at the Au-O interface until all the oxygen has desorbed. For Au coverages
