Chapter 5
Infrared Reflection—Absorption Spectroscopy
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New Technique for In Situ Determination of Local Surface Structure W. Kevin Kuhn, Jian-Wei He, and D. Wayne Goodman Department of Chemistry, Texas A & M University, College Station, TX 77843-3255
Ultra-thin films of Cu, Ni and Co on Rh(100) and Mo(110) substrates have been studied using carbon monoxide adsorption in conjunction with infrared reflection absorption spectroscopy (IRAS). The CO vibrational frequency on Cu/Rh(100) and Cu/Mo(110) at low Cu coverage (~0.1 ML) shows a blue-shift relative to its gas phase stretching frequency (2143 cm ). This blue-shift is explained as arising from CO adsorbed on well -dispersed Cu adatoms which are slightly positively charged due to polarization arising from the CO-Cu interaction. Three-dimen sional Cu clusters, well-ordered pseudomorphic two dimensional Cu islands and single Cu atoms are distinctively characterized by their CO IR peaks. In addition, it is found that IR spectra of adsorbed CO show a remarkable sensitivity to surface structural phase transitions. Both order-order and disorder-order transitions are observed for Ni and Co overlayers on a Mo(110) substrate. It is further shown that localized segregation and ordering of C and Ο on Mo(110) and S on Co/Mo(110) are observable. -1
Considerable scientific attention has been directed toward the understanding of ultra-thin metal films supported on single crystal metal substrates (1-11). The interest in using these systems as well-characterized models for the more complex supported bimetallic systems relates to the importance of mixed metal catalysts. In a typical study, a metal is deposited onto a single crystal surface at ultrahigh vacuum conditions. The morphology and structural properties of the ultra-thin metal films are then characterized using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and temperature programmed desorption spectroscopy (TPD). Recently, we have found that CO combined with infrared reflection absorption spectroscopy (IRAS) can be used as a molecular probe to study the
0097-6156/92/0482-0071$06.00/0 © 1992 American Chemical Society In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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electronic properties, morphology and local surface structure of these ultra-thin metal films. The information obtained will be qualitative rather than quantitative in nature since the integrated peak areas obtained from IRAS spectra may not correlate directly with the CO surface concentration. An additional feature of IRAS is the inherent capability of being able to study surface structure at relatively high pressures. Thus, potentially the morphology of the bimetallic surface can be studied at reaction temperatures and pressures. Experimental data on Cu/Rh(100), Cu/Mo(110), Ni/Mo(110) and Co/Mo(110) will be presented and discussed in this paper. Experimental The experiments were carried out in an ultrahigh vacuum chamber equipped for IRAS, AES, and LEED. This apparatus has been discussed in detail elsewhere (12). The samples were spot-welded to two Ta wires which allowed resistive heating of the sample to 1500K and cooling to 80K. The Rh(100) and Mo(llO) surfaces were cleaned using an oxidation and heating procedure described in reference 13. After this treatment, AES indicated a clean surface, with C, Ο and S impurities less than 1 atom %, and L E E D exhibited sharp substrate patterns. The infrared spectra were obtained using a Mattson Fourier transform infrared spectrometer (Cygnus 100) in the single reflection mode at an 85° incident angle. The spectra in this work were obtained using 4 cm" resolution. Cu, Co and Ni were evaporated from a copper, cobalt or nickel wire wrapped around a W filament. Prior to each deposition, the source was degassed extensively. AES showed that no impurities accumulated on the surface during the metal deposition. The copper, cobalt and nickel coverages were determined using the relationship of the AES ratio versus the correspond ing TPD area from references 8, 9 and 11, respectively. One monolayer (ML) was defined to be one overlayer atom per substrate atom, i.e., the atomic densities of the Rh(100) and Mo(110) surfaces. 1
Results and Discussion Ultra-Thin Metal Overlayers. Figure 1 shows the IR spectra of CO adsorbed on Cu/Rh(100) at the indicated Cu coverages ( 0 ) . The Rh(100) surface was dosed with Cu at 85K, flashed to -900K, and exposed to 10 L of CO at 85K. It is noteworthy that the CO adsorbed on 0.1 M L of Cu on Rh(100) shows a peak at 2155 cm' which is higher than that of gas phase CO (2143 cm' ). It is well known that the bonding of CO to metals is due to backdonation of electronic charge from the metal to the CO 2n* orbital in conjunction with donation of electronic charge from the CO 5σ orbital to the metal (14). On metal surfaces, the 2π* backdonation is usually the predominant contribution. Because of the antibonding nature of the 27r* orbital, this predominant backdonation consequently results in a red-shift of the CO stretching frequency relative to its gas phase value (15). Figure 1, however, shows a blue-shifted frequency for CO adsorbed onto a low coverage of Cu supported on a Rh(100) surface. An unusually high CO stretching frequency (2138 cm' ), although not Cu
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In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: January 23, 1992 | doi: 10.1021/bk-1992-0482.ch005
5.
Infrared Reflection—Absorption Spectroscopy 73
K U H N ET AL.
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