The Surface Chemistry of Dimethyl Disulfide on Copper - Langmuir

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The Surface Chemistry of Dimethyl Disulfide on Copper† Octavio J. Furlong, Brendan P. Miller, Zhenjun Li, Joshua Walker, Luke Burkholder, and Wilfred T. Tysoe* Department of Chemistry and Biochemistry, and Laboratory for Surface Studies, University of Wisconsin;Milwaukee, Milwaukee, Wisconsin 53211 Received May 3, 2010. Revised Manuscript Received June 22, 2010 The surface chemistry of dimethyl disulfide (DMDS) is studied on a Cu(111) single crystal and a polished copper foil in ultrahigh vacuum as a basis for understanding its tribological chemistry using a combination of temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). Low-energy electron diffraction reveals that the polished foil becomes ordered on heating in vacuo and displays identical surface chemistry to that found on the Cu(111) surface. Dimethyl disulfide reacts with the copper surface at 80 K to form thiolate species. Heating the surface to ∼230 K causes a small portion of the thiolate species to decompose to form methyl groups adsorbed on the surface. Further heating results in methane and C2 hydrocarbon desorption at ∼426 K, due to a reaction of adsorbed methyl species, to completely remove carbon from the surface and to deposit atomic sulfur.

Introduction Sulfur-containing molecules, in particular those with sulfursulfur linkages, are used as lubricant additives for ferrous surfaces1-14 so that dialkyl disulfides have been used as simple model compounds to explore the surface and tribological chemistry on iron15,16 where they react at the high temperatures attained at the interface during rubbing to deposit a ferrous sulfide film. However, the tribological chemistry can depend critically on the nature of the substrate so that a good lubricant additive for one type of surface may not be applicable to another. In particular, the lubrication of sliding copper-copper interfaces in electrical motors17-20 provides a particular challenge. Gas† Part of the Molecular Surface Chemistry and Its Applications special issue. *Author to whom correspondence should be addressed. Telephone: (414) 229-5222. Fax: (414) 229-5036. E-mail: [email protected].

(1) Davey, E. D.; Edwards, E. D. Wear 1957, 1, 291. (2) Forbes, E. S.; Reid, A. J. D. ASLE Trans. 1972, 16, 50. (3) Forbes, E. S. Wear 1970, 15, 87. (4) Allum, K. G.; Forbes, E. S. J. Inst. Pet. 1967, 53, 173. (5) Prutton, C. F.; Turnbull, D.; Dlouhy, G. J. J. Inst. Pet. 1946, 32, 96. (6) Davey, W. J. J. Inst. Pet. 1946, 32, 575. (7) Spikes, H. A.; Cameron, A. ASLE Trans. 1973, 17, 283. (8) Bovington, V; Dacre, B. ALSE Trans. 1982, 25, 44. (9) Dacre, B.; Bovington, C. H. ASLE Trans. 1982, 25, 272. (10) Tomaru, M.; Hironaka, S.; Sakurai, T. Wear 1977, 41, 141. (11) Murakami, T.; Sakai, T.; Yamamoto, Y.; Hirano, F. ASLE Trans. 1985, 28, 363. (12) Plaza, S.; Mazurkiewicz, B.; Gruzinski, R. Wear 1994, 174, 209. (13) Plaza, S. ASLE Trans. 1987, 30, 493. (14) Kajdas, C. ASLE Trans 1985, 28, 21. (15) Kaltchev, M.; Kotvis, P. V.; Blunt, T. J.; Lara, J.; Tysoe, W. T. Tribol. Lett. 2001, 10, 45. (16) Lara, J.; Blunt, T. J.; Kotvis, P.; Riga, A.; Tysoe, W. T. J. Phys. Chem. B 1998, 102, 1703. (17) Appleton, A. D. IEEE Trans. Magn. 1983, 19, 1047. (18) Walters, J. D.; Sondergaard, N. A.; Levedahl, J.; Waltman, D.; Golda, E. M.; Fikse, T. H. Nav. Eng. J. 1998, 110, 107. (19) Superczynski, M. J.; Waltman, D. J. IEEE Trans. Appl. Supercond. 1997, 7, 513. (20) Hazelton, D. W.; Gardner, M. T.; Rice, J. A.; Walker, M. S.; Trautwein, C. M.; Haldar, P.; Gubser, D. W.; Superczynski, M.; Waltman, D. IEEE T. Appl. Supercond. 1997, 7, 664. (21) Slade, P. G. Electrical Contacts: Principles and Applications; Marcel Dekker: New York, 1999. (22) Holm, R. Electrical Contacts: Theory and Applications; Springer-Verlag: Berlin, 1976.

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phase lubricants based on water vapor have been used to reduce friction and wear,21-25 but they tend to lead to asymmetric wear rates and failure at higher temperatures.26,27 The following explores the chemistry of dimethyl disulfide (DMDS) on copper surfaces to establish whether it is sufficiently reactive to potentially form a tribofilm near room temperature as required for lubrication of the sliding copper-copper contact in an electric motor. These experiments will provide the background information for ultimately examining the frictional properties of dialkyl disulfides in ultrahigh vacuum (UHV)28-32 and are thus carried out on both a Cu(111) single crystal substrate and copper foils, since the latter type of sample will eventually be examined in the UHV tribometer rather than single crystal samples. Based on the chemistry on iron15,16 and on other surfaces,33-35 it is anticipated that thiolate species will form on the copper surfaces. These have received considerable attention, since they form the basis of longer-chain alkyl thiolate self-assembled monolayers (SAMs), generally formed from alkane thiols. Thus, the surface chemistry of the simplest of these, methane thiol, has been extensively studied on Cu(111)36-38 and Cu(100)39-43 surfaces, (23) Gao, C.; Kuhlmann-Wilsdorf, D. IEEE Trans. Compon., Hybrids, Manuf. Technol. 1991, 14, 37. (24) Gao, C.; Kuhlmann-Wilsdorf, D. Wear 1991, 149, 297. (25) Gao, C.; Kuhlmann-Wilsdorf, D. Trans. ASME J. Tribol. 1992, 114, 174. (26) Furlong, O.; Li, Z.; Gao, F.; Tysoe, W. T. Tribol. Lett. 2008, 31, 167. (27) Boyer, L.; Noel, S.; Chabriere, J. P. Wear 1987, 116, 43. (28) Wu, G.; Gao, F.; Kaltchev, M.; Gutow, J.; Mowlem, J.; Schramm, W. C.; Kotvis, P. V.; Tysoe, W. T. Wear 2002, 252, 595. (29) Gao, F.; Kotvis., P. V.; Tysoe, W. T. Wear 2004, 256, 1005. (30) Gao, F.; Erdemir, A.; Tysoe, W. T. Tribol. Lett. 2005, 20, 221. (31) Gao, F.; Furlong, O.; Kotvis, P. V.; Tysoe, W. T. Tribol. Lett. 2005, 20, 171. (32) Gao, F.; Furlong, O.; Kotvis, P. V.; Tysoe, W. T. Tribol. Lett. 2008, 31, 99. (33) Kang, D. H.; Friend, C. M. Langmuir 2004, 20, 11443. (34) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. B. 1998, 102, 3431. (35) Maksymovych, P.; Sorescu, D. C.; Yates, J. T., Jr. J. Phys. Chem. B. 2006, 110, 21161. (36) Jackson, G. J.; Woodruff, D. P.; Jones, R. G.; Sing, N. K.; Chan, A. S. Y.; Cowie, B. C. C.; Formoso, V. Phys. Rev. Lett. 2000, 84, 119. (37) Driver, S. M.; Woodruff, D. P. Surf. Sci. 2000, 457, 11. (38) Ferral, A.; Patrito, E. M.; Paredes-Olivera, P. J. Phys. Chem. B 2006, 110, 17050. (39) Kariapper, M. S.; Fisher, C.; Woodruff, D. P.; Cowie, B. C. C.; Jones, R. G. J. Phys.: Condens. Matter 2000, 12, 2153. (40) Driver, S. M.; Woodruff, D. P. Surf. Sci. 2001, 488, 207.

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where the C-S bond of the thiolate species is found to be tilted on the Cu(111) surface but oriented perpendicularly to the surface on Cu(100). The following examines the surface chemistry and decomposition pathways of DMDS on copper surfaces using temperatureprogrammed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS).

Experimental Section The equipment used for RAIRS and TPD has been described in detail elsewhere.44 Briefly, RAIR spectra are collected in a UHV system coupled with a Bruker Equinox spectrometer; typically, each spectrum is acquired for 1000 scans at a resolution of 4 cm-1. TPD data were collected in another UHV chamber equipped with a Dycor quadrupole mass spectrometer interfaced to a computer that allowed up to five masses to be sequentially monitored in a single experiment. The sample could be cooled to 80 K in both chambers by thermal contact to a liquid-nitrogen-filled reservoir, and resistively heated to ∼1000 K. X-ray photoelectron spectra were collected in a chamber operating at a base pressure of 1  10-10 Torr. Spectra were collected with a Mg KR X-ray power of 250 W using a double-pass, cylindrical-mirror, electron energy analyzer operating at a pass energy of 50 eV. Temperature-dependent XPS spectra were collected by heating the sample to the indicated temperature for 5 s and then allowing the sample to cool to ∼80 K, following which the spectrum was recorded. The spectra were fit using XPSPEAK 4.1. After performing a Shirley background subtraction, peaks were fit by allowing the position and area to vary to optimize the fit, while keeping the full widths at half maxima and the Gaussianto-Lorentzian ratios of each of the components fixed. Low-energy electron diffraction (LEED) experiments were carried out in a chamber operating at a base pressure of 8  10-11 Torr. The sample in this chamber could be cooled to 80 K by contact with a liquid-nitrogen-filled reservoir and resistively heated to 1000 K. The copper foil samples were polished to a mirror finish using 1 μm diamond paste in a random orbit polisher. Once in UHV, the Cu(111) single crystal and copper foils were cleaned using a standard procedure which consisted of argon ion bombardment (∼1 kV, ∼ 2 μA/cm2) and annealing cycles up to ∼850 K, and the cleanliness of the samples was monitored using Auger spectroscopy. The DMDS (Aldrich, 99.0% purity) was transferred to glass bottles and attached to the gas-handling systems of the vacuum chambers. The purity was monitored using mass spectroscopy.

Results The surface chemistry of DMDS ((CH3S)2) was studied on copper samples consisting of a polished, high-purity copper foil and a Cu(111) single crystal, both of which had been cleaned in UHV using the procedures indicated above. Initial LEED experiments were performed on a polished copper foil that had been cleaned by argon ion bombardment and heating. After several repetitions of this cleaning procedure, a faint, square (1  1) LEED pattern was found at 187 eV electron beam energy. After repeating the cleaning/annealing cycle four more times, the square (1  1) LEED pattern became significantly sharper and was visible over a wide range of electron beam energies from 70 to 235 eV. (41) Kondoh, H.; Saito, N.; Matsui, F.; Yokoyama, T.; Otha, T.; Huroda, H. J. Phys. Chem. B 2001, 105, 12870. (42) Bussolotti, F.; Corradini, V.; Di Castro, V.; Betti, M. G.; Mariani, C. Surf. Sci. 2004, 566-568, 591. (43) Schach von Wittenau, A. E.; Hussain, Z.; Wang, L. Q.; Huang, Z. Q.; Ji, Z. G.; Shirley, D. A. Phys. Rev. B 1992, 45, 13614. (44) Kaltchev, M. G.; Thompson, A.; Tysoe, W. T. Surf. Sci. 1977, 391, 145.

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Figure 1. TPD profiles of various exposures of dimethyl disulfide to a clean copper foil at 80 K collected at 64 amu at a heating rate of 4.2 K/s.

TPD results are shown in Figure 1, in this case for the copper foil, and they were collected by monitoring 64 amu (one of the most intense fragments of molecular DMDS in our mass spectrometer) following various exposure of DMDS. Exposures are given in Langmuirs (L) (1 L = 1  10-6 Torr s), where the dosing pressure was not corrected for ionization gauge sensitivity. The desorption profiles show no intensity at a DMDS exposure of ∼0.5 L and only a small intensity at an exposure of 1.25 L, displaying a sharp feature centered at ∼159 K which grows indefinitely with increasing exposure. This suggests that the copper foil surface is sufficiently reactive that all of the DMDS that adsorbs directly on the surface decomposes. Molecular desorption then only occurs when the second and subsequent layers have been populated. Figure 2 displays the corresponding 16 amu (methane) desorption profile as a function of exposure. This also shows a sharp feature at ∼159 K due to the mass spectrometer ionizer fragmentation of the DMDS multilayer, as well as a feature at ∼426 K. Measuring the desorption profile at other masses, and comparing with the mass spectrometer fragmentation patterns of possible products, indicates that the 426 K feature is due to methane formation. However, the integrated intensity of the signal at ∼426 K is smaller at saturation coverage than that for the 0.5 L exposure. Signals were also detected at 30 amu, and the resulting desorption profiles are shown in Figure 3. The sharp peak at ∼159 K is due to mass spectrometer ionizer fragmentation of DMDS multilayers, and an additional feature is detected at ∼436 K. Measuring the signal at other masses indicates that the feature at ∼436 K is due to ethane formation. Finally, ethylene was found to desorb at ∼426 K, coincident with methane formation (data not shown). Identical results were obtained for a Cu(111) single crystal (data not shown). The corresponding temperature-dependent XPS results for DMDS adsorbed on a copper foil are displayed in Figure 4. Following DMDS adsorption at 80 K, the spectra show C 1s features at ∼285.6 and ∼284.5 eV binding energies (BE) (Figure 4a) and also S 2p features at ∼164.1 and ∼162.1 eV BE (Figure 4b). Peaks at 285.6 eV (C 1s) and at 164.1 eV BE (S 2p) are assigned to the adsorption of molecular DMDS. On heating to ∼200 K, the Langmuir 2010, 26(21), 16375–16380

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Figure 2. TPD profiles of various exposures of dimethyl disulfide to a clean copper foil at 80 K collected at 16 amu at a heating rate of 4.2 K/s.

Figure 4. X-ray photoelectron spectra of 2 L of dimethyl disulfide adsorbed onto a copper foil at 80 K and heated for a period of 10 s to various temperatures. Annealing temperatures are indicated adjacent to the corresponding spectrum where the spectra where recorded after the sample had been allowed to cool to 80 K once again, showing (a) the C 1s and (b) the S 2p regions of the spectrum.

Figure 3. TPD profiles of various exposures of dimethyl disulfide to a clean copper foil at 80 K collected at 30 amu at a heating rate of 4.2 K/s.

multilayer peaks disappear (compare with Figures 2 and 3), while the C 1s feature at ∼284.5 eV BE and the S 2p feature at ∼162.1 eV BE show a small increase in intensity. The intensity ratio of the C 1s and S 2p features on heating to ∼200 K is identical to that for molecular DMDS, indicating a C/S ratio of unity on heating the surface to this temperature. The C 1s signal disappears on heating to ∼500 K due to adsorbate decomposition (see Figures 2 and 3), and the S 2p peak shifts to a lower binding energy (161.1 eV BE) due to sulfur adsorbed on the copper surface. Langmuir 2010, 26(21), 16375–16380

Figure 5 shows XPS data taken as a function of DMDS exposure at 80 K; in this case, only the S 2p region is shown. After dosing 0.5 L of DMDS, only the feature at ∼162.1 eV BE is detected, while the peak at ∼164.1 eV BE appears at a 1 L exposure and grows with increasing exposure. The corresponding infrared spectra of DMDS adsorbed on Cu(111) are displayed in Figure 6 as a function of exposure. Figure 6a shows the evolution in the spectra up to relatively large DMDS doses (up to 8 L), and Figure 6b displays the corresponding variation at lower exposures. The final spectrum after an exposure of 8 L yields a series of peaks at 692, 960, and 1301 cm-1 and a doublet at 1411 and 1429 cm-1 in the lower-frequency region, and peaks at 2825 and 2912 cm-1 and a doublet at 2980 and 2991 cm-1 in the C-H stretching region. The majority of these modes can be assigned by comparison with liquid DMDS,45 and the peaks are summarized and assigned in Table 1. The additional weak feature at ∼2825 cm-1 is likely to be an overtone of the methyl deformation mode. The spectra at lower exposures (Figure 6b) differ significantly from the multilayer spectrum at high exposures. In particular, at an exposure of ∼0.24 L, the 1415 cm-1 mode is detected while the ∼960 cm-1 mode is absent. (45) Frankiss, S. G. J. Mol. Struct. 1969, 3, 89.

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Furlong et al. Table 1. Vibrational Frequencies of Multilayers of Dimethyl Disulfide (DMDS) on Cu(111) at 80 K Compared with Those of DMDSa vibrational frequency/cm-1 multilayer DMDS/Cu(111), 80 K 2991 2980 2912 2825 1429 1411 1301 960 692 a δ, deformation; F, rock; ν, stretch.

DMDS45

assignment

2991 2986 291

νa(C-H) νa(C-H) νs(C-H) overtone δa(CH3) δa(CH3) δs(CH3) F(CH3) ν(C-S)

1430 1415 1303 955 691

Figure 5. X-ray photoelectron spectra of dimethyl disulfide adsorbed at 80 K at various exposures, which are indicated adjacent to the corresponding spectrum.

Figure 6. Reflection absorption infrared spectra of various exposures of dimethyl disulfide adsorbed onto a Cu(111) single crystal at 80 K for (a) exposures up to 8 L and (b) exposures up to 1 L.

Figure 7. Reflection absorption infrared spectra of 1 L of dimethyl disulfide adsorbed onto a Cu(111) single crystal at 80 K and heated for period of 10 s to various temperatures. Annealing temperatures are indicated adjacent to the corresponding spectrum where the spectrum was recorded after the sample had been allowed to cool to 80 K once again, showing (a) the whole spectral region and (b) the C-H stretching region.

Above this coverage, features grow once again with approximately the same relative intensities as the multilayer.

Figure 7 shows the thermal evolution of the infrared spectra for a surface dosed with 1 L of DMDS at 80 K where the

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annealing temperatures are displayed adjacent to the corresponding spectrum. Heating to ∼160 K causes a substantial change in the nature of the spectrum and the disappearance of the peaks at ∼960, 1302, and 2990 cm-1, so that the spectrum displays features at ∼693, 1415, and 2917 cm-1. Note that the DMDS multilayer has completely desorbed by ∼160 K (Figures 1 and 2). Further heating to ∼230 K results in additional changes, where no features are detected between ∼550 and 2200 cm-1, while significant intensity is still found in the C-H stretching region (Figure 7b). This indicates that hydrocarbon species are still present on the surface and exhibit a feature at ∼2827 cm-1 and an intense peak at 2917 cm-1. These peaks persist on heating to ∼290 K, attenuate at ∼390 K, and have disappeared on heating to ∼480 K, implying that the adsorbates react to form methane, ethylene, and ethane at ∼430 K (Figures 2 and 3).

Discussion The surface chemistry found on a Cu(111) single crystal surface is identical to that observed on foils that have been cleaned and annealed in UHV. This assertion is supported by the fact that, after ion bombarding and annealing a copper foil several times in UHV, a square (1  1) LEED pattern is observed indicating the formation of ordered (100) facets on the surface. This is particularly important for tribological experiments in UHV where it is not feasible to use a single-crystal substrate due to the significant surface damage produced during rubbing. The detection of all of the infrared features of DMDS in the multilayer (Figure 6 and Table 1) indicates that it is relatively randomly oriented on the surface. The DMDS multilayer adsorbed on the copper surface is weakly bound and desorbs at ∼159 K. In particular, the presence of molecular DMDS is indicated by the splitting of the intense methyl deformation mode into two components at 1429 and 1411 cm-1 due to a coupling between the two methyl modes. This result is also in accord with the XPS data (Figure 4) which show a S 2p feature at ∼164.1 eV BE and a C 1s peak at ∼285.6 eV BE due to the sulfur and methyl carbon atoms in molecular DMDS. However, at lower exposures, there is only a S 2p feature at ∼162.1 eV BE (Figure 5), indicating the formation of another surface species after adsorption at 80 K. This is confirmed by the infrared spectrum found at lower exposures at 80 K, which is different from the multilayer spectrum (Figure 6b). Here the spectrum shows a broad feature at ∼1302 cm-1, a peak at 1415 cm-1, a very weak feature at ∼693 cm-1, but only relatively weak intensity at ∼960 cm-1. A S 2p peak at 162.4 eV BE has been ascribed to the formation of methyl thiolate species on copper surfaces,46 and it is distinct from the binding energy of adsorbed atomic sulfur at 161.1 eV BE. This suggests that some methyl thiolate species are initially formed on the copper surface following the adsorption of DMDS, even at 80 K. It has been found that methyl thiolate species have a C-S bond that is tilted with respect to the surface on gold,47 exhibiting a spectrum with weak ν(S-C) (680 cm-1) and F(CH3) (949 cm-1) modes and more intense δS(CH3) (1298 cm-1) and δa(CH3) (1415 cm-1) modes. This suggests a similar assignment of the peak at ∼693 cm-1 to ν(S-C), the peak at 1302 cm-1 to δS(CH3), and the 1415 cm-1 feature to δa(CH3). The presence of the ν(S-C) mode indicates that the C-S bond remains intact. The cleavage of the S-S bond is not evident from the infrared data, since the S-S stretching mode lies outside the spectral region that is accessible using our equipment (between 500 and 550 cm-1 45). (46) Lai, Y.-H.; Yeh, C.-T.; Cheng, S.-H.; Liao, P.; Hung, W.-H. J. Phys. Chem. B 2002, 106, 5438. (47) Yourdshahyan, Y.; Rappe, A. M. J. Chem. Phys. 2002, 117, 825.

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The combined XPS and infrared data suggest that initially a portion of the dimethyl disulfide dissociates on the surface to form methyl thiolate species, while the remainder adsorbs molecularly to form DMDS multilayer. Heating the sample above ∼160 K desorbs any multilayer (Figures 1, 4, and 7) and leads to the formation of a surface structure with vibrational frequencies at ∼693, 1415, and 2917 cm-1, a S 2p binding energy of ∼162.1 eV (Figures 4a and 5), and a C 1s binding energy of ∼284.5 eV (Figure 4b). The XPS chemical shifts strongly suggest the presence of thiolate species on the surface,46 and the slight increase in intensity of the methyl thiolate S 2p feature formed on heating to ∼200 K compared with that initially formed during adsorption at ∼80 K suggests that some additional thiolate species are formed as the DMDS-covered surface is heated. Further heating to ∼230 K results in the disappearance of the asymmetric methyl bending mode (at 1415 cm-1), the appearance of a weak peak at ∼2827 cm-1, and an intensification of the 2917 cm-1 mode. Infrared spectra have been obtained for methyl iodide adsorption on copper at ∼100 K,48 but only highresolution electron energy loss spectra (HREELS) of the methyl species formed by heating this surface to ∼150 K to dissociate the C-I bond are available for comparison.49 The methyl group orientation is found to vary with coverage, and at the highest coverage, where the methyl group is oriented perpendicular to the surface, it exhibits two weak vibrational modes at 1200 cm-1 (δS(CH3)) and 2835 cm-1 (νS(CH3)). Thus, the weak C-H stretching mode at ∼2827 cm-1 is consistent with the formation of methyl groups adsorbed on a copper surface. Methyl thiolate species formed from alkanethiols on Cu(111) have been proposed to decompose to form sulfur between 320 and 370 K.46 Further evidence for the ultimate formation of methyl groups adsorbed on the copper surface comes from the TPD data (Figures 2 and 3) where methane and C2 products are formed at close to the temperature at which they are found for the decomposition of adsorbed methyl species on copper.49-51 While this does not prove that methyl groups have formed by C-S bond cleavage, it is consistent with this proposal. The existence of the peak at ∼2917 cm-1 suggests the continued presence of methyl thiolate species on the surface on heating to ∼230 K. The 2917 cm-1 mode intensifies slightly on heating to ∼290 K, and the 2827 cm-1 mode remains (Figure 7b). The symmetric methyl stretching modes have dynamic dipole moments that are oriented along the C-S bond of the thiolate. Thus, according to the surface selection rules,52 changes in methyl stretching mode intensity could imply either a change in coverage or orientation, and indeed, as discussed above, orientation changes as a function of coverage have been noted for methyl species adsorbed on copper. The XPS data (Figure 4) indicate that thiolate formation is complete by ∼200 K, so that the increase in intensity of the 2917 cm-1, thiolate C-H stretching mode on heating to ∼290 K (Figure 7) cannot be due to the formation of additional thiolate species on the surface but must be due to a reorientation, so that the C-S bond is closer to perpendicular to the surface. Density functional theory calculations suggest that the thiolate species alone are tilted on the Cu(111) surface.53 One possible explanation for the proposed orientation change is that the adsorbed (48) Jenks, C. J.; Bent, B. E.; Bernstein, N.; Zaera, F. J. Phys. Chem. B 2000, 104, 3008. (49) Lin, J.-L.; Bent, B. E. J. Vac. Sci. Technol., A 1992, 10, 2202. (50) Lin, J.-L.; Bent, B. E. J. Am. Chem. Soc. 1993, 115, 2849. (51) Chiang, C.-M.; Wentzlaff, T. H.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836. (52) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (53) Ferral, A.; Patrito, E. M.; Paredes-Olivera, P. J. Phys. Chem. B 2006, 110, 17050.

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methyl species formed on the surface (as indicated by the feature at 2827 cm-1) cause the surface to become crowded, resulting in the C-S bond of the thiolate being oriented so that it is close to perpendicular to the surface. As the surface is heated to ∼390 K, where some methane and C2 hydrocarbons desorb (Figures 2 and 3), the intensity of the thiolate modes decrease as it decomposes to form additional adsorbed methyl species. A weak feature at ∼2827 cm-1 indicates the continued presence of some methyl species on the surface. This process is essentially complete by ∼480 K where only sulfur remains on the surface (Figure 4b). The S 2p binding energy (at 161.1 eV) has also been assigned previously to chemisorbed sulfur on copper.46 The attenuation of the asymmetric methyl bending mode (at 1415 cm-1) on heating to ∼230 K and higher is in accord with the observation that the methyl thiolate changes orientation as the temperature increases as proposed above. However, the symmetric methyl bending mode (at 1301 cm-1 in DMDS and 1298 cm-1 for thiolate species on gold47), where the dynamic dipole moment is aligned similarly to the symmetric methyl stretching mode, is not detected. However, the symmetric bending mode for methyl species adsorbed on copper (at 1200 cm-1 49) was not detected, although an associated, weak stretching mode (at 2827 cm-1) was found. The temperature at which methane forms from the decomposition of the thiolate species formed from DMDS (∼430 K) is slightly lower than that for methyl species formed from iodomethane (∼450 K). This may be due to either the effect of

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different coadsorbates (sulfur versus iodine) or the surface crowding of the adsorbed methyl species.

Conclusions Dimethyl disulfide reacts with both Cu(111) single crystals and vacuum-annealed copper foils to desorb methane and C2 hydrocarbons at ∼430 K, similar to the reaction temperature of methyl species formed on copper by the dissociative adsorption of methyl iodide.48 The similarity of the chemistry on single crystals and foils allows tribological experiments to be carried out on welldefined surface films rather than having to carry these out on single crystal surfaces. The S-S bond in DMDS starts to cleave at 80 K, with some additional cleavage occurring on heating to ∼200 K, to form adsorbed thiolate species. The resulting surface crowding causes the thiolate group to reorient, and steric hindrance prevents further thiolate decomposition. Further heating to ∼390 K causes the adsorbed methyl species to react to form methane and C2 hydrocarbons leaving chemisorbed sulfur on the copper surface. Finally, these results suggest that dialkyl disulfides are sufficiently reactive that they could form the basis of lubricants for the sliding copper-copper interface at room temperature. Acknowledgment. We gratefully acknowledge the Chemistry Division of the National Science Foundation under Grant Number CHE-0654276 and the Office of Naval Research for support of this work.

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