J. Phys. Chem. 1994,98, 609-616
609
The Formation and Stability of Sulfhydryl Groups on the Au(ll0) Surface B. Friihberger, M. Grunze,+and D. J. Dwyer' The Department of Chemistry and the Laboratory for Surface Science and Technology, University of Maine, Orono, Maine 04469-5764 Received: September 13, 1993; In Final Form: October 5, 1993'
The resistivity change of thin films of gold upon exposure to H2S has been employed to develop gas sensors that detect H2S. The exact mechanism by which H2S alters the conductivity of these films is not known. However, the adsorption and bonding of H2S on gold surfaces is clearly a critical step in the overall mechanism. In this study, we have explored the interaction of HzS with the clean Au( 110) single-crystal surface. A variety of ultrahigh vacuum surface science techniques were used including X-ray photoelectron spectroscopy (XPS), low electron energy diffraction (LEED), temperature-programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (HREELS). H2S was found to adsorb with a sticking probability of close to unity on the clean surface at 120 K. This adsorption produced a molecularly adsorbed HIS species on the surface that exhibits an SH stretching vibration at 2450 cm-1 and an HIS scissor mode at 1150 cm-1 in the HREEL spectra. TPD results suggest that the HzSdesorbs directly as a molecule without decomposition at temperatures above 200 K. When defects are present in the Au surface or when the adsorbed HIS layer is irradiated with electrons, H2S will dissociate into adsorbed hydrogen and sulfhydryl (SH).
Introduction Relativelylittle work has been done to investigatethe interaction of HzS with well-defined metal surfaces and so far the interest has primarily focused on transition metal surfaces. Surfaces Pt( 1 11),6.7 Mostudied include Ni( Ru( 1 10): Rh( (loo),*w ( 100),9 and Cu( 100).10 Investigations were motivated by the need to gain insight into industrially important processes like hydrodesulfurization and the poisoning of catalytic surfaces by sulfur for which H2S is a common source. Recently, the necessity to monitor environmental pollutants has led to the development of gas sensors utilizing metallic films. Thin gold films exhibit conductivitychanges upon exposure to H2S.I' Gold is also used in metal oxide gas sensors,12in which gold is thought to act as a catalyst for the decomposition of H2S. However, the exact mechanism involved in the gas sensor response is not clearly understood. An understanding of the interaction between gold and H2S is certainly necessary to provide the desirable insight which could lead to optimization of existing technology by improving reliability and overall performance. In addition, the interaction of H2S with gold might serve as a model system for the understanding of the substrate-adsorbate interaction in the self-assembly of alkanethiols on gold s u r f a ~ e s . l ~ - ~ ~ After early work on metal films23 established the ease of decomposition of HIS on metal surfaces, this reaction was primarily used to deposit sulfur and study its effects on altering surface properties.*29 Kostelitz et al. have studied the adsorption of sulfur on various gold surfaces29 using LEED (low electron energy diffraction) and AES (auger electron spectroscopy). A variety of techniques have been employed in investigations on transition metals, namely TPD (temperature-programmed desorption), HREELS (high-resolution electron energy loss spectroscopy), UPS (ultraviolet photoelectron spectroscopy), LEED, AES, and NEXAFS (near-edge X-ray-absorption fine structure).l-I0 At low coverages H2S is generally found to decompose extensively even at temperatures around 100 K. At higher coverages H2S TPD spectra typically show two features: a low-temperaturefeature which is attributed to physisorbed HIS, and a second feature at a somewhat higher temperature (around t Lehrstuhl fiir Angewandte Physikalische Chemie, Institut fiir Physikalische Chemic, Universitit Heidelberg, INF 253, 6900 Heidelberg, Germany. Abstract published in Aduance ACS Abstracts, December 15, 1993.
180 to 250 K, depending on the specific surface). Different interpretations of this second feature have been given on the different surfacesstudied. For Rh( loo), Hedgeand Whitebelieve that adsorption sites modified by coadsorbed sulfur stabilize molecular H2SeS A similar explanation is given by Zhou and White for the Ni( 100) surface,' supported by the NEXAFS data of McGrath et aL3 which suggest that no partial decomposition of H2S occurs. However for the same surface, Baca et a1.2 find evidence in HREELS data that surface sulfhydryl groups (SH) may be present which could combine with coadsorbed hydrogen to result in the higher temperature TPD feature. On the basis of HREELS data, the presence of S H groups has also been suggested by Gland et al. on the Mo( 100) surface,* by Koestner et al. on Pt(l1 l),6v7 and by Leung et al. on Cu(lOO).IO Fisher4 reports the formation of SH on Ru( 110) by comparing his UPS results with gas-phase data. Only one other study has been reported on the interaction of H2S with a single-crystal gold surface. Jaffey and Madix,30 on Au(1 lo), find besides low-temperature features in their H2S desorption spectra a higher temperature feature at -300 K. Solely on the basis of TPD data, this feature was attributed as resulting from the existence of sulfhydryl groups on the surface. Since recombinative hydrogen desorption occurs at -220 K on this surface,3O~~~ no coadsorbedhydrogen is availablefor recombination with S H at 300 K. Therefore, a disproportionation reaction between SH groups was proposed to account for the desorption of molecular H2S at 300 K. The disproportionation reaction leads to deposition of sulfur and desorption of H2S. In this study we present further information on the Au( 110)H2S surface adsorbate system. Our TPD results show that the clean, well-annealed Au( 110) surface is very unreactive toward H2S and that decompositiononly occurs in the presence of surface defects. LEED and HREELS results demonstrate that SH groups are formed on this surface as a result of defects and that the deposition of sulfur can indeed be attributed to a disproportionation of these SH groups. Combining information obtained from both specular and off specular HREEL spectra allow suggestions on possible bonding geometry of H2S and SH on this surface. The reconstructed Au( 110) surface offers a compromise between limiting parameters by using a well-defined surface and offering a variety of potential adsorption sites for surface chemistry to take place.
0022-3654/94/2098-0609%04.50/0 0 1994 American Chemical Society
610 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994
Experimental Section The experiments were carried out in a stainless steel ultrahigh vacuum chamber equipped with a residual gas analyzer, rear view LEED, dual anode Mg/AI X-ray source, Leybold EA10 hemispherical electron energy analyzer, UV source, highresolution EEL spectrometer, and a multichannel capillary array gas doser. The chamber base pressure was 1 X 10-lo Torr. The sample was mounted on an XYZ manipulator with a coldfinger which permitted the sample temperature to be varied between 120 and 1000 K. Sample temperature was measured using a type K thermocouple spot welded to the back of the sample. HIS was obtained from Matheson (99.5% liquid-phase purity) and used without further purification after passivating the stainless steel gas delivery lines by backfilling with H2S and heating. Without such treatment the gas readily decomposes in the gas lines to form sulfur and hydrogen, especially when heated during bakeout. Dfi was obtained from Cambridge Isotope Laboratories (98% D). The molecular beam up-take experiments were carried out by establishinga constant background pressure from the beam doser and then quickly lowering the sample in front of the doser and recording the HIS partial pressure with the residual gas analyzer. Our multichannel array is a 1-mm-thick glass plate, 10 mm in diameter with 10-pm capillaries. The precise distance between sample and multichannel array could be determined by using the sample manipulator and finding the position for which first electrical contact was made between the sample edge and the stainless steel cap supporting the multichannel array at the end of the doser. Using the method described by Campbell and Valone,’2 we could calculate the fraction of the molecular beam emitted by the doser that is intercepted by the sample. Their method has proven to give results that compare well with experiments.31-36 For TPD experiments the sample was exposed to gas from the doser and then ramped with a linear heating rate of 5 K/s. Exposures were estimated by comparison with background dosing experiments and the ion gauge pressure readings were corrected by a factor of 2.2 for H2S relative to nitrogen.’’ XPS spectra were obtained with the Mg anode at 300 W. The analyzer pass energy was 100 eV in the constant pass energy mode. The energy scale was calibrated using the A u 4 f ~ pline at 84.0 eV. Each spectrum was signal averaged for 15 s per data point to obtain a reasonable signal to noise ratio. All HREEL spectra were acquired with a primary beam energy of 5 eV; angles of incidence and reflection were 60° with respect to the surface normal in the specular spectra. The count rate in the elastic peak was typically 1 X lo5 counts per second and the resolution was between 60 and 80 cm-l FWHM. The Au(ll0) crystal was a 3-mm thick disk with a diameter of 10 mm. A 1-mm-deep groove was cut into the sample edge for the Ta support wires, which were also used for resistiveheating. The surface was cleaned by repeated cycles of Ar+ ion sputtering (0.5 kV, 4 PA) and annealing at 750 K. After cleaning, impurity concentrations were below the detection limit of XPS and the sample displayed a sharp p( 1 X 2) LEED pattern characteristic of the missing row reconstructed Au( 1 10) surface.3w3 Sticking Probability Measurement Figure 1 presents results of a typical gas uptake experiment using the molecular beam doser. The H2S partial pressure measured with the residual gas analyzer is plotted as a function of time. At time to the valve was opened to establish a constant background pressure of H2S )@ , with the sample out of the molecular beam. The sample was then quickly lowered into the beam at time 21 and the subsequent change in H2S partial pressure was recorded. There is an initial sharp drop as the sample begins to adsorb gas, followed by a nearly constant region. The pressure then suddenly increases at time t2 to approximately one-half of
Frtihberger et al. I
QMS
signal
m/e=34
[arb. units]
’
f3
t2
tl
to 100
50
150
250
200
300
time [SI
Figure 1. Mass spectral uptake experiment for H2S on Au( 110) at 120 K (see text for details).
+.-
1.4
p
1
I
1.2
0.8
0.5
1
1.5
exposure [L]
Figure 2. Calculated sticking probability for H2S on A u ( l l 0 ) at 120 K as a function of exposure.
the initial pressure indicating an abrupt change in the rate of gas adsorption by the sample. Beyond 22, the pressure continues to drift upward toward pal. At time t3 the same was removed from the beam and the background pressure returned to slightly above itsoriginally establishedvalue. We believe that this slight increase in background is due to a reduction in pumping speed of the coldfinger surface during the experiment. “Blind experiments” with the sample out of the beam and the coldfinger either liquid nitrogen cooled or at room temperature support this assumption. The pressure “spike”immediately after removing the sample from the beam is likely due to supersaturation of the surface under the higher local pressure in the molecular beam. The “spike”occurred only when the sample was removed from the beam after times longer than necessary to reach the initial drop in gas adsorption rate, and its area appeared independent of exposure. The above mentioned “blind experiments” also provided evidence that the pressure “spike” is a not simply a result of desorption of weakly bound molecules caused by stretching or compressing the manipulator bellows. Thegasuptakedatacan beused tocalculate thestickingprobability (S(t)ofH2S onAu(ll0) at 1 2 0 K ~ s i n g ~ ~ 9 ~ ~
wherep, = establishedbackground pressure,p = actual pressure, and f = fraction of the beam intercepted by the sample. The calculated sticking probability is shown in Figure 2, plotted as a function of exposure in Langmuir (L, with 1 L = 1 X 10-6 Torr s). The initial sticking probability is very close to one, remains constant up to about 0.5 L, then drops sharply and keeps slowly decreasing with increasing exposure.
TPD Figure 3 shows TPD results from the clean, well-annealed surface as a function of increasing exposure to H2S. Adsorption
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 611
Sulfhydryl Groups on the Au( 1 10) Surface
n QMS
signal m/e=34
TPD H2Son Au( 110)
QMS
signal m/e=34
[arb.
units]
Annealed Surface
[arb.
units] Exposure
A Annealed Surface After WS
19L 0 95L
0.32L
i 0.064L 0.054L
-
0.016L 0.WL 0.W6L
I
200
400
600
8 I
Temperature [K] Figure 3, HIS temperature programmed desorption spectra for various exposures at 120 K on clean Au( 110). Heating rate was 5 K/s. temperature was 120 K. Only H2S desorption occurred during the experiment and no desorption of sulfur took place as verified by XPS. There are three distinct desorption peaks. The first feature, a3, observed for lowest exposures displays a coverageindependent desorption temperature of about 180 K. For coverages greater than 0.03 L, the second feature, a2, begins to grow in with a desorption temperature of about 160 K which also seems to be independent of coverage. The apparent small shift to lower temperatures with increasing coverage is most likely due to the appearance of the last desorption feature, a],which we observed for exposuresexceeding approximately 0.5L. We were unable to cool the sample below 120 K with our experimental setup and could not clearly resolve this feature. The a1 feature appears to have a desorption temperature of about 130 K, which compares well with data reported by Jaffey and Madi~.~O The TPD spectra did not saturate up to 19 L, the highest exposure used. Assuming first order kinetics and a preexponential factor of 1013 s-1, the simple Redhead modelu gives energies for desorption of --
500
clean
1000 1500 2000 2500 3000
Energy Loss [cm-11 Figure 8. Temperature-dependent HREEL spectra for HzS adsorbedon Au( 110). Each spcctrumwas acquired at 120K. All spectra were scaled to have the same intensity in the elastic peak (FWHM 75 cm-l). Ei = E, = 60'; Eo = 5 eV. therefore simply adsorbs as a molecule on the well-ordered Au(1 10) surface and desorbs molecularly below 200 K. In order to investigate the nature of the 0 TPD feature, we electron irradiated adsorbed H2S at 120 K. An electron dose of about 4 x 10'6 electrons cm-2 at 100 eV was typically used. HREEL spectra after electron irradiation of the surface exposed to 2 L of H2S at 120 K are shown in Figure 9 as a function of temperature. In additionto the features observed for non-electronirradiated HzS, there appears to be a shoulder at -560 cm-' in the spectrum at 120 K. It is, in fact, possible to isolate the molecular species leading to the 0 desorption feature in the TPD spectra. This is shown in the spectrum recorded after the sample was heated to 280 K, past the desorption temperature for molecularly adsorbed HIS, but below the desorption temperature for the fl TPD feature. Only the peaks at 560 and at 2500 cm-l remain. The peak due to the S H stretching vibration is much narrower, and its maximum appears at higher wavenumber than in thespectrumat 120K(namelyat -2500versus -245Ocm-l). The absence of the H2S scissor mode leads us to believe that SH groups are present on the surface with a bending vibration at -560 cm-1 and an SH stretch at 2500 cm-1. The decrease in peak width for the SH stretch is due to the fact that there is a symmetricand antisymmetric SH stretch in molecular HIS while only one SH stretch remains for surface S H groups. Thecomplete absence of the HIS scissor mode was not always as apparent as in the spectrum shown. There was, however, always a very clear change in relative intensities of the'scissor versus stretching mode in favor of the latter, and the change in peak shape for the stretching mode was always observed. No features remained after heating the surface to 340 K past the desorption temperature of the 0 feature. Due to limitations in spectrometer resolution we were only occasionally able to observe a vibration at -275 cm-1 as a shoulder of the elastic peak, which we tentatively assign asan AuSHstretch. The AuSvibrationcouldnever beresolved.
4.zc.l
0
ZW 4m 0 *D ImcI"dc416Lm1*D"Ira~
Energy Loss [cm-1]
F v9. Temperaturdcpcndent HREEL spectraobtained after electron irradiation of H2S adsorbed on Au(ll0) at 120 K. All spectra were acquiredat 120Kafterheatingtotheindicatcdtemperatures. Thespectra were scaled to have the same intensity in the elastic peak. Ei = Er = 60'; Eo = 5 ev. The presenceof sulfur, however, could be verified by XPS. Nuzzo et aL49 report an A u S stretching frequency of 235 cm-I on Au(111). We also camed out HREELS experimentsusing the deuterated compound (D2S) and employed the fact that the analyzer of our HREEL spectrometer is rotatable to measure off specular spectra. Results are shown in Figures 10 and 11. Only the vibrational mode at 430 cm-1 displayed an angular dependence similar to that of the elastic peak, indicating a strong dipole scattering contribution to the total scattering cross section for this mode. The angular dependence for all other modes was small. These modes are therefore believed to be largely due to impact scattering. The spectra were scaled to approximately the same intensity in the scissor and stretching mode to pronouce the relative intensity changes between the vibrational mode at 430 cm-l and the other modes when measured off specular as compared to the specular spectra. Figure 10shows spectra for non-electron-irradiatedH2S and D2S adsorbed at 120 K in specular direction and loo off specular. There appears to be a small shift in the frequency of the low-enernv Deak in the off-swular s w t r u m for adsorbed HIS in Figure 10. Considering the signal-to-noise ratio of our spectra we feel unable to decide whether this is truly a shift or whether the low-energy shoulder of the elastic peak in the specular spectrum actually contains more than one peak with different angular dependence. Both the SD stretch and the D2S scissor mode, involving largely deuterium motion, exhibit the expected frequency shift (see Table 1 for a list of the observed frequencies) when compared to the frequencies for the corresponding modes for HzS. The fact that both of these modes are primarily due to impact scattering might indicate that the molecule is adsorbed in an orientation of C, symmetry with the plane of the SH bonds tilted away from the surface normal. We never observed a peak at 430 cm-* in spectra of adsorbed D2S. This may be a further indication that the vibration giving rise to this peak is a result of
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 615
Sulfhydryl Groups on the Au( 110) Surface
TABLE 1 adsorbed on Au( 110) at 120 K
solid phase5' vibration, cm-I YI (symmetric stretch) u2 (scissor mode) v3 (asymmetric stretch) S-H bend SH2 wag Au-SH stretch" a
H2S 2532 1186 2544
DzS 1843 857 1854
ratio (HzS/DS) 1.37 1.38 1.37
H2S
DS
ratio ( H 9 / D B )
2450 1150
1750 830
1.40 1.39
560 430 275
410
1.37
Tentative assignment.
HREELF1 H,S/Au( 1 10) 2L H2S l2OK specular
Intensity [arb. units]
2L H2S electron irradiated I20K
Intensity [arb. units]
specular
10 deg
specular / L 0
specular /
I
.
fm
200 Ha
I w Iwo 1pw
IHa Ifm
Im
2wo 2200
2w
2Mx)
Energy Loss [cm-11
Energy Loss [cm-11
..
I
'
HREELS DzS/Au( 1 10)
I ::
DzS/Au( 1 10) 4L D2S 120K specular
.....
Intensity [arb. units]
I
4L D2S electron irradiated 120K specular
Intensity [arb. units]
/
I
0
200 IQ) fm
sw
10% 1200
Im
I600 lMxi 2 o x 22w 2m 2600
I
I
a
Energy Loss [cm-11
Figure 10. HREEL spectra for H2S and D2S adsorbed on Au(ll0) at 120 K, collected in specular and in loo off-specular direction. Ei = E, = 60' for the specular spectra; EO = 5 eV. (See text for discussion.)
an SH2 wagging mode (with a corresponding SD2 wag unresolved by our spectrometer). An SH2 wagging mode should also be dipole active for the molecule adsorbed in the above proposed orientation of C, symmetry, which is consistent with our observation. Figure 11 shows a similar set of spectra for H2S and D2S adsorbed at 120 K after electron irradiation in specular direction and loo off specular. The HIS spectra reveal the existence of both molecular H2S and SH groups on the surface at 120 K. The off-specular spectrum for D2S showed a peak at -410 cm-I shifted by a factor of 1.37 compared to the peak observed at 560 cm-I which we assigned as an SH bend. Furthermore, this peak also appears to be due to impact scattering displaying a similar angular behavior as the mode at 560 cm-1. We believe that this demonstrates the correctness of the mode assignment for the peak at 560 cm-I as an SH bending mode which involves mainly hydrogen motion. The species leading to the 6 desorption feature in the TPD spectra therefore seems to indeed be a surface SH species. The dipole selection rule suggests a near-linear orientation for SH with the S H bond along the surface normal, since the SH bending mode appears to be largely due to impact scattering. A strong tilt of the S H bond away from the surface normal should introduce a transition dipole moment perpendicular to the surface for a bending mode allowing dipole
. 2w
4w 600 8W 1000 12w 14w lfm
ISM) 20%
22w 2400 2600
Energy Loss [cm-1]
Figure 11. HREEL spectra obtained after electron irradiating both H2S and D2S adsorbed on Au(ll0) at 120 K, collected in specular and in 10' off-specular direction. Ei = E, = 60' for the specular spectra; EO = 5 eV. (See text for discussion.)
scattering. Sellers50has carried out cluster calculations for SH adsorbed on Au(ll1) and Au(100). He finds that, on both surfaces, S H prefers hollow binding sites. On Au( 100) a linear orientation with the SH bond along the surface normal was reported, while on Au( 111) the SH bond was tilted away from the surface normal.
Conclusions H2S adsorbs with an initial sticking probability of close to unity at 120 K on the clean Au(ll0) surface. No hydrogensulfur bond cleavage was observed with adsorption. As clearly shown by TPD and HREELS, H2S adsorptionoccurs molecularly. This is somewhat surprising since adsorption of long chain alkanethiols on gold surfaces is generally believed to involve SH bond cleavage. No decomposition and no deposition of sulfur takes place during desorption,which is complete for temperatures exceeding 200 K. However, surface defects lead to decomposition of HIS to form surface sulfhydryl(SH) groups as does irradiation withelectrons. This results inanadditional H2Sdesorptionfeature at 300 K and deposition of sulfur during the experiment. HREELS data shows that the S H groups can be isolated and,
616 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994
(7) Koestner, R. J.; Salmerh, M.; Kollin, E. B.; Gland, J. L.Surface Sci. 1986, 172, 668. (8) Gland, J. L.; Kollin, E. B.; Zaera, F. Langmuir 1988, 4, 118. (9) Battacharaya, A. K.; Clarke, L.J.; Morales de la Garza, L.J. Chem. Soc., Faraday Trans. 1 1981, 77,2223. (IO) Leung, K. T.; Zhang, X. S.;Shirley, D. A. J. Phys. Chem. 1989,93,
Clean Au( 1 10) 120K
200K
HIS
HIS
Friihberger et al.
f
6164. ( 1 1) Product literature: Jerome Hydrogen Sulfide Analyzer, Arizona
Instrument Corporation, Tempe, AZ. (12) Falconer, R. S.; Xu, Z.; Vetelino, J. F.; Lee, R. M.; Smith, D. J. 3rd Int. Mtg. on Chemical Sensors, Cleveland, OH, 1990, p 354. (13) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105,4481. (14) Li, T. T.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106,6107. (15) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. J. Am. Chem. SOC.1987,109,3559. (16) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L.J. Am. Chem. Soc. 1987,
Electron Beam Induced Chemistry 120K
120K
220K
300K
Defect Induced Chemistry 120K H2S
-200K ? HIS
-.-)
SH HSH H
220K
+
SHSH
300K
,' H2
~
H2S
',
.--)
Figure 12. Schematic representation of the proposed surface chemistry. On clean, defect-free Au( 1lo), HIS adsorbs and desorbs molecularly without decomposition. Electron irradiation or surface defects lead to the formation of sulfhydryl groups (SH),which subsequently disproportionate upon heating.
together with LEED results, supports the assumption that a disproportionation reaction between surface SH groups forms adsorbed sulfur and gaseous H2S at 300 K. A schematic representation of the proposed surfacechemistry is shown in Figure 12. Acknowledgment. We gratefully acknowledgefinancialsupport by the National Science Foundation, grant no. ECS-901955 1 and travel support from the Deutsche Forschungsgemeinschaft. We would also like to acknowledge John Vetelino and his sensor group at the University of Maine for many meaningful discussions concerning this manuscript. Christoph Wiill is thanked for contributing the Au(ll0) crystal for these studies. References and Notes (1) Zhou, Y.; White, J. M. Surface Sci. 1987, 183, 363. (2) Baca, A. G.; Schulz; M. A.; Shirley, D. A. J. Chem. Phys. 1984,81, 6304. (3) McGrath, R.; MacDowell, A. A.; Hashizume, T.; Sette, F.; Citrin, P. H.Phys. Rev. B 1989, 40, 9457. (4) Fisher, G. B. Surface Sci. 1979, 87, 215. (5) Hedge, R. I.; White, J. M. J. Phys. Chem. 1986, 90, 296. (6) Koestner, R. J.; Salmer6n, M.; Kollin, E. B.; Gland, J. L. Chem. Phys. Lett. 1986, 125, 134.
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