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Langmuir 2005, 21, 11684-11689
Direct Observation of Thiolate Displacement Reactions on Au(111): the Role of Physisorbed Disulfides Mark G. Roper and Robert G. Jones* Department of Physical Chemistry, School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K. Received March 8, 2005. In Final Form: September 14, 2005 Line-of-sight mass spectroscopy (LOSMS) has been used to study the displacement reaction of (x3×x3)R30° methylthiolate on Au(111) by butylthiolate. The reaction was carried out at room temperature and constant saturation coverage, by exposing the methylthiolate-covered surface to dibutyl disulfide gas. The adsorbed methylthiolate was desorbed as dimethyl disulfide and the cross product methylbutyl disulfide, both identified by LOSMS. This shows that reaction occurs between adsorbed thiolates of different chain length at room temperature, while the kinetics indicate that a rapid equilibrium is established between immobile, chemisorbed thiolates, and highly mobile, physisorbed disulfides.
Introduction In this work, we follow at fixed temperature (300 K) and fixed saturation coverage the displacement of dimethyl disulfide (CH3S-SCH3, DMDS) and the formation of methylbutyl disulfide (CH3S-SCH2(CH2)2CH3, MBDS) when dibutyl disulfide (CH3(CH2)2CH2S-SCH2(CH2)2CH3, DBDS) reacts with a saturation coverage of methylthiolate on a Au(111) surface. The work demonstrates that densely packed, immobile, chemisorbed thiolate phases on Au(111) at room temperature are in rapid equilibrium with a mobile physisorbed disulfide phase above the chemisorbed phase. This provides a mechanism for scrambling the positions of chemisorbed thiolates on Au(111) which are otherwise immobile and, hence, is of relevance to the general field of thiolate self-assembled monolayers (SAMs) on gold. Dissociative adsorption of dimethyl disulfide vapor on Au(111), in which the disulfide bond is cleaved to produce two chemisorbed methylthiolate species (CH3S-, MT), was first studied by Nuzzo.1 Since then, many studies have been carried out into the anchoring of organic species to gold surfaces via a Au-S linkage.2 The majority of these involve adsorption of thiols (RSH) or disulfides (R1SSR2) from solution onto predominantly (111)-orientated thin films of gold to form SAMs; a minority involve adsorption from the vapor in ultrahigh vacuum. The goal of such studies is to manipulate the surface properties, both physical and chemical, by varying the functionality of the organic R group. Grunze et al.3 have already shown that solution-phase adsorption of an asymmetric dialkyl disulfide, R1SSR2, does not lead to a 1:1 stoichimetry of R1S-:R2S- on the surface, but rather, the long-chain thiolates replace the short chain thiolates. In solutionphase deposition, it is difficult to study the details of adsorption and surface reaction due to the monolayer of interest being sandwiched between two condensed phases. For gas-phase deposition, however, the full power of mass spectroscopy can be used to identify reactants arriving at, * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: (0044) 115 9513468. Fax: (0044) 115 9513562. (1) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740. (2) Schreiber F. Prog. Surf. Sci. 2000, 65, 151-256. (3) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze M. Langmuir 1999, 15, 5440-5443.
and products leaving from, the surface and, hence, allow measurements to be made which are not possible in solution. Here we use line-of-sight mass spectroscopy (LOSMS), to study the reactions occurring between chemisorbed thiolates on Au(111). In the LOSMS technique, only those molecules emanating from a specific patch on the sample surface and traveling by line of sight to the ionization region of a mass spectrometer are detected; all other sources of gas fail to reach the mass spectrometer. The technique was first proposed in ref 4 and has been used to study a range of systems.5-12 Figure 1 shows the experimental geometry. The mass spectrometer is surrounded by a liquid-nitrogencooled shroud having two small apertures between the sample surface and the mass spectrometer ionization volume. The size and position of these apertures are chosen such that a circular patch is defined on the sample surface which has line of sight to the ionization volume. Species emanating from within the circular patch on the sample and flying directly to the ionization region, indicated by flux “Fexp” in Figure 1, are detected by the mass spectrometer. However, all other trajectories from all other surfaces in the vacuum chamber encounter the liquidnitrogen-cooled shroud and are terminated by condensation to a low-vapor-pressure solid before they can reach the ionization volume. The mass spectrometer is therefore blind to all but those molecules which fly by line of sight from the circular patch on the sample. The boiling point of liquid nitrogen (77 K) is sufficient to trap all species with the exception of H2, He, Ar, CO, N2, and a few other low-boiling-point gases, for which a lower temperature would be required; the differential pumping in Figure 1 (4) Jones, R. G.; Fisher, C. J. Surf. Sci. 1999, 424, 127-138. (5) Chan, A. S. Y.; Turton, S.; Jones, R. G Surf. Sci. 1999, 433-435, 234-238. (6) Jones, R. G.; Clifford, C. A. Phys. Chem. Chem. Phys. 1999, 1, 5223-5228. (7) Turton, S.; Kadodwala, M.; Jones, R. G. Surf. Sci. 1999, 442, 517-530. (8) Chan, A. S. Y.; Jones, R. G. Surf. Sci. 2000, 451, 232-237. (9) Turton, S.; Jones, R. G. Surf. Sci. 2000, 468, 165-175. (10) Chan, A. S. Y.; Skegg, M. P.; Jones, R. G. J. Vac. Sci. Technol. A 2001, 19, 2007-2012. (11) Chan, A. S. Y.; Jones, R. G. J. Vac. Sci. Technol. A 2001, 19, 1474-1480. (12) Jones, R. G.; Chan, A. S. Y.; Turton, S.; Jackson, G. J.; Singh, N. K.; Woodruff, D. P.; Cowie B. C. C. J. Phys. Chem. B 2001, 105, 10600-10609.
10.1021/la050624o CCC: $30.25 © 2005 American Chemical Society Published on Web 11/02/2005
Thiolate Displacement Reactions on Au(111)
Figure 1. Schematic of the sample, apertures 1 and 2, and the ionization region of the quadrupole mass spectrometer showing how a circular patch on the sample surface is defined by the two apertures and the line of sight to the ionization region. The diagram is not to scale.
is to remove these gases. Flux, Fexp, may be some or all of the following: species originating from the surface in a temperature programmed desorption (TPD) experiment; product species originating from the surface due to chemical reaction with an applied pressure (flux “FIn” in Figure 1) of gas, under any temperature or pressure program one may wish to apply; and incident species (FIn) which have been reflected and hence exhibit a sticking probability of S < 1. Disulfides, rather than thiols, were used in this study to form the high-coverage “standing up” phases of lowmolecular-weight thiolates (CnH2n+1S-Au, n < 8) because, whereas the sticking probability of thiols on gold drops rapidly (S < 10-3) after forming the lying-down phase of the thiolate,13 the disulfide sticking probability remains high (0.1 < S DMDS, leading to dissociative adsorption of the molecules in the same order. This may explain why most of the MT desorbs as DMDS, with only some desorbing as MBDS. To check whether the above description is consistent with the overall shape of the measured LOS data, we have incorporated it into a simple simulation, the results of which are shown in Figure 3B. Appendix 1 describes how the simulation was carried out, how the various kinetic parameters were estimated, and the choice of the four fitting parameters. The rather good fit between the simulation and the experimental data illustrates that the proposed mechanism is consistent with the results, but further experiments over a range of temperatures are needed to make the parameters used chemically meaningful. From the above, we see that chemical reaction between different-chain-length thiolates does occur at room temperature and that equilibrium is rapidly established between chemisorbed MT and BT and physisorbed DMDS and MBDS at room temperature. This means that even at room-temperature MT and BT can move from site to site via a mobile physisorbed disulfide. We would suggest that a similar equilibrium could exist between BT and physisorbed DBDS. The consequence of such equilibria is to allowing mixing of small chemisorbed thiolates at room temperature. Conclusion We have shown the following. Dibutyl disulfide reacts with a saturation coverage, (x3×x3)R30°, methylthiolate surface on Au(111) at 300 K to form a (x3×x3)R30° butylthiolate-covered surface containing g0.25 ML of butylthiolate and e0.083 ML of methylthiolate. The displacement reaction leads to the desorption of dimethyl disulfide and the cross product methylbutyl disulfide. The results can only be explained if there is a rapid equilibrium between immobile, chemisorbed methylthiolate and butylthiolate species and highly mobile, physisorbed dimethyl disulfide and methylbutyl disulfide species. Appendix 1. Kinetic Simulation of the Displacement Reaction at 300 K and TPD at 350-450 K Reaction steps 2-8 and 10-12 for the displacement reaction have been incorporated into a simple computer simulation of the surface kinetics at 300 K. For simplicity, we have assumed that the displacement reaction goes to completion and that all rate constants are independent of the adlayer composition. The chemisorbed surface is assumed to consist of a random mixture of methyl thiolate (MT), butyl thiolate (BT), and clean surface with concentrations NMT, NBT, and Nclean molecules m-2. As methylthiolate and butylthiolate both form the same (x3×x3)R30° “standing up” surface structure of 1/3 ML coverage on Au(111), we have assumed a constant total of sites of 4.62x1018 m-2 ()NMT + NBT + Nclean) on the surface. Formation of physisorbed DMDS, MBDS, and DBDS from chemisorbed MT and BT, reactions 4, 8, and 10, are assumed to be first order, with rate constants KMTMT, KMTBT and KBTBT and to be proportional to the concentrations of MT/MT (NMTMT), MT/BT (NMTBT),
Thiolate Displacement Reactions on Au(111)
and BT/BT (NBTBT) pairs within the chemisorbed layer; i.e., the two relevant species must be adsorbed adjacent to each other if they are to react. For an hexagonal surface with random population of sites, the concentrations of these pairs are given by NMTMT ) 3N2MT/(NMT + NBT + Nclean), NMTBT ) 6NMTNBT/(NMT + NBT + Nclean), and NBTBT ) 3N2BT/(NMT + NBT + Nclean).25 Cracking of the three physisorbed species back to chemisorbed species, reactions 3, 7, and 12, were assumed to be second order with rate constants KDMDS, KMBDS, and KDBDS. Using a simple two-dimensional collision theory model, in which the physisorbed molecules behave as an ideal two-dimensional gas with an average speed of cav (m s-1), having a collision cross section σ (m) with the clean surface (note in 2D the cross section is a length), the rate constant K ) σcv (if we assume that the activation energy is zero). For DMDS, MBDS, and DBDS, cav is ∼200 ms-1, and we can approximately estimate the value of σ as follows. Geometrically, a clean (x3×x3)R30° site is a 60° parallelogram of side 5 Å, while the all-trans forms of the DMDS, MBDS, and DBDS molecules are ∼5-6 Å wide by ∼7.5, ∼10, and ∼13.5 Å long, respectively. Assuming the effective diameter of a clean site is 5 Å and taking the biggest molecule, DBDS, and assuming that its effective diameter is the average of its length and width (∼10 Å), we get σ e 15 Å, giving K e 3 × 10-7 m2 s-1, for these molecules. K will be a lot less than this if the physisorbed molecules cannot move as a 2D gas, but undergo diffusive hops with an activation barrier, or if there is an activation energy for cracking the disulfides. As we have assumed zero activation energy for cracking onto clean surface, the 2D gas of physisorbed disulfides only exists above the chemisorbed MT and BT species and not above the clean surface. The activation energies for desorption of the physisorbed molecules from the surface, reactions 2, 6, and 11, were approximated by the enthalpy of vaporization, ∆Hvap, of the bulk material: 35.5,26 49.5, and 62.2726 kJ mol-1 for DMDS, MBDS, and DBDS, respectively. ∆Hvap for MBDS was estimated using the boiling points of all three compounds and assuming ∆Svap for MBDS is the average of the DMDS and DBDS values. Pre-exponentials of 1 × 1013 s-1 were used for reactions 2, 6, and 11. The sticking probability for reaction 5 was set at 1.0 (the initial, measured, overall sticking probability was 1.0, so the sticking probability for this mechanistic step must also be 1.0). An experimental DBDS pressure of 1 × 10-7 Torr was used, measured with an ion gauge calibrated for nitrogen (sensitivity SN2). If we assume the relative ion gauge sensitivity to DBDS, SDBDS/SN2, is approximately the same as that for octane (n ) 8), given by the following equation for long chain alkanes,27 SDBDS/SN2 ) 1.1n + 0.4 ) 9.2 for n ) 8, then the true pressure of DBDS was ∼1 × 10-8 Torr, which corresponds to ∼1.5 × 1016 molecules m-2 s-1. Figure 3B shows an approximate fit of the simulated desorption fluxes of DMDS, MBDS, and DBDS to the experimental data. The four adjustable parameters (and their best fit values) were incident DBDS flux (2.5 × 1016 (25) Thanks to P. M. W. Gill, School of Chemistry, University of Nottingham, UK, for deriving these equations. (26) NIST Webbook; http://webbook.nist.gov/cgi/cbook.cgi. (27) Wetterer, S. M.; Lavrich, D. J.; Cummings, T.; Bernasec, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 9266-9275.
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molecules m-2 s-1), KMTMT (0.1 s-1), KMTBT (6 s-1), and KBTBT (360 s-1). KDMDS, KMBDS, and KDBDS were all fixed at 3 × 10-7 m2 s-1, though one would not expect them to all be the same, and the parameters used for molecular desorption were as described above. We should also point out that we do not know the relative sensitivity factors of the mass spectrometer signals, but we would expect, using the sensitivity formula above, that they would be within a factor of 2 of each other. KMTMT is required to be of the order of 0.1 s-1 to ensure that DMDS desorption does not occur at 300 K when there is no incident flux of DBDS (in agreement with experiment). To get a fit with the experimental DMDS and DBDS LOS curves in Figure 3B, the values of KMTBT and KBTBT have to be approximately 60× and 3600× the value of KMTMT, respectively. In other words, the rate constants for forming dibutyl disulfide from chemisorbed butylthiolate, as well as methybutyl disulfide from methylthiolate and butlythiolate, must both be substantially larger than the rate constant for forming dimethyl disulfide. This is the opposite of what one might have expected. It should be pointed out that the absolute values of KMTMT, KMTBT, and KBTBT can take a range of values, provided they remain approximately in the ratio 1:60:3600. Because of the approximations used, we have not gone beyond a reasonable visual fit; further study is required to determine these parameters more accurately. If we now consider temperature programmed desorption and apply the steady-state approximation to the physisorbed DMDS in eqs 2-4, we find that the rate of desorption is NMTMTK2K4/(K3Nclean + K2). As soon as the coverage drops below saturation, K3Nclean will exceed the rate constant for desorption of the disulfide (K2), as Nclean increases rapidly. Hence, the overall rate constant becomes K2K4/K3Nclean, with an activation energy of E2 + E4 - E3, which becomes E2 + E4 because E3 ≈ 0 (nonactivated). If we assume that the first-order rate constant KMTMT (0.1 s-1) has a pre-exponential factor of 1 × 1013 s-1, then the activation energy for reactions 4 is E4 ) 80 kJ mol-1 to form physisorbed dimethyl disulfide from the chemisorbed MT. As noted above, E2 ) 35.5 kJ mol-1, so E2 + E4 ) 115.5 kJ mol-1. The pre-exponential is given by ν2ν4/ (ν3Nclean) ≈ 3 × 1014 s-1 given that ν3 ≈ ν4 ≈ 1013 s-1, ν3 ≈ 3 × 10-7 m2 s-1, and Nclean ≈ 1018 m-2 as soon as significant desorption has started. Figure 4 shows a simulated first-order desorption curve using an activation energy of 116 kJ mol-1, a preexponential of 3 × 1014 s-1, and an MT coverage of 0.042 ML (i.e., 0.021 ML of DMDS). Considering the approximations involved, the match to experiment is remarkably good, the experimental width of the DMDS peak being slightly larger than the calculated width. The same analysis can be done for MBDS and DBDS, indicating that all three have similar overall activation energies and will desorb at about the same temperature in a TPD experiment, as observed in Figure 4. (Note that for DBDS, and probably MBDS, other cracking reactions become active at this temperature, reducing the desorbed quantity of these two molecules). Acknowledgment. M.G.R. thanks the EPSRC of Great Britain and the University of Nottingham. LA050624O
