Synthesis of sulfur films from S2 gas: spectroscopic evidence for the formation of Sn species

I4 March 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 267

(I 997)

65-7 1

Synthesis of sulfur films from S, gas: spectroscopic evidence for the formation of S, species J. Hrbek a,*, S.Y. Li a3’,J.A. Rodriguez a, D.G. van Campen b, H.H. Huang ‘, G.-Q. Xu ’ a Chemistry b Nutionul

Depurtment,

Synchrotron

’ Depurtment

Light

Brookhuven

Nutional

Source. Brookhuven

<$Chemi.stry,

Nutionul

Lubwutory,

Nutionul

University

Upton, NY 11973-5000,

Luborutory,

oj’Sin~upore,

USA

Upton, NY 11973-5000,

Singuporr

Received 2 September 1996; in final form 23 December

USA

119260. Singupore 1996

Abstract Thin films of molecular sulfur were prepared by the deposition of S, gas from an electrochemical doser on single crystal metal surfaces kept at temperatures below 250 K. These films were characterized by XPS, UPS, TDS and IRAS, and were found to be stable up to 390 K even under UHV conditions. Cyclooctasulfur S, was identified as one of the sulfur molecules present.

1. Introduction The interaction of sulfur and its compounds with metal surfaces is of considerable interest to catalysis [l] and corrosion [2]. At room temperature sulfur forms a strongly bonded chemisorbed layer on metal surfaces that has been the subject of many surface science investigations. The maximum coverage of chemisorbed sulfur depends on the sulfur source: a layer prepared by chemisorption of H,S [3] has smaller coverage than that prepared by deposition of sulfur atoms and sulfur dimers [4]. Deposition of S/S, from the gas phase on metal surfaces kept at low temperature (< 250 K) leads to

* Corresponding author.

’Permanent

address: Department of Chemistry, National Uni-

versity of Singapore.

OGO9-2614/97/$17.00

Copyright 0

PII SOOO9-2614(97)00069-9

the formation of a sulfur multilayer. Patterson and Lambert [5] and Gellman et al. [6] were the first to prepare sulfur multilayers on Pd(1 1 1) or Mo(l10) at lower temperatures and suggest that the sulfur multilayer is made of sulfur dimers 161. It is, however, known that solid elemental sulfur exists in a large variety of allotropic forms [7], the most stable of which is cyclooctasulfur @,-ring in the crown conformation) molecule. S, forms an orthorombic molecular crystal a-Ss with all fundamental excitation below 500 cm-’ [S]. In this work we studied sulfur films supported on single crystal metal substrates. Infrared data show clearly that cyclooctasulfur is formed after sulfur deposition on a Cu substrate held at 150 K. This finding is further supported by valence band spectra. We also found that S, and S, were among the species desorbing from the surface.

1997 Elsevier Science B.V. All rights reserved.

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2. Experimental

The experiments with the sulfur films were performed in four UHV chambers with base pressures in the low lo-” torr range. The XPS data were measured on a Mo(1 10) substrate using a hemispherical electron energy analyzer with multichannel detection and a Mg Ka X-ray source [9]. The TDS measurements of S/Ru(OOl) surfaces were carried out in a second chamber using a differentially pumped appertured mass spectrometer [lo]. A 150” electron analyzer and He I photon source were used for the UPS study of S/Ru(OOl) surface in the third chamber [ 111. The IRAS study was carried in the fourth chamber on S/Cu (111) at the NSLS’s U4IR beam line [ 121. Since we are working with thick sulfur films the substrates have no effect on the properties examined in this work. The crystals were cleaned following standard procedures reported in literature [9- 121.The sulfur multilayers were prepared by exposing cooled crystals to a flux of mainly sulfur dimers 2 from a sulfur doser. S, species are generated in situ in a solid-state electrochemical cell Pt/Ag/AgI/AgS/Pt [ 131. The XPS and UPS chambers allowed the closest doser approach to the samples. The sample-doser distances were larger for TDS and IRAS experiments. Thus dosing times indicated for a given experiment provide only an approximate measure of coverage assuming unity sticking coefficients of S/S,. However, there is no direct correlation of deposition times between the experiment in different chambers. In addition, the actual deposition rate is dependent on the doser ‘history’ [4].

3. Results and discussion

In Fig. 1 we present a set of S 2p XPS spectra measured as a function of deposition time on Mo( 110) at 80 K. Starting from the bottom curve we see a single well-defined doublet. A linear background

’ Species detected in the flux of sulfur leaving the collimator of the electrochemical doser arc S, S, and S,, and their uncorrected ratios are 50/100/ 1, respectively. No S, species was detected.

S/Mo(llO) 80 K O,dependence

XPSMg

Ka

S 2P

166

166

164

162

160

BINDING ENERGY (eV) Fig. I. S 2p XPS spectra acquired after dosing sulfur to Md I IO) at 80 K. The parameter is the S, dosing time in minutes. The fitting procedure is described in the text.

was subtracted from the spectrum that was then fitted using a many-body line-shape convoluted with a Gaussian function to account for instrumental resolution [ 141. The results of the fit are shown as three lines, two for each peak of the doublet and the third one as a sum of the two components. The S 2p3,2 level is located at 16 1.65 eV; the separation between the two peaks of the doublet is 1.24 eV and their intensity ratio is 2:l. The binding energy, spin-orbit splitting and intensity ratio are typical of atomic sulfur chemisorbed on MO surfaces [9]. The spinorbit splitting and the peak intensity ratio agree with published values for S on Ru(001) [ 151 and on W( 100) [ 161.This saturated sulfur layer has coverage 0s = 0.91 ML, when 0s is reported relative to the number of Mo(ll0) surface atoms [ 171. A second dose of sulfur leads to the appearance of a new doublet shifted by 1.60 eV to a higher binding

J. Hrhek et ul./ Chcmicul Physics Letters 267 (1997) 65-71

energy of 163.25 eV. At 3 min of deposition, this new doublet already dominates the spectrum. The results of the fitting of this spectrum with two doublets are shown in Fig. 1. The higher binding energy doublet grows with increasing sulfur exposure, while the doublet from chemisorbed S is progressively attenuated and for the highest exposure shown it disappears completely. We estimate that the sulfur film is - 13 ML thick. The experimental spectrum of the thick multilayer was fitted with a single doublet, and the result of the fit is shown as lines. The S 2p,,* core level of the sulfur multilayer is found at 163.25 eV, the 2p,,,splitting is 1.18 eV and the inten2P ,,* spin-orbit sity ratio value is 2.05. The binding energy of the S 2p,,, core level of 163.25 eV is smaller than that tabulated [ 181. Possible reasons for the discrepancy are a poor conductivity of the bulk samples that may lead to peaks shifts and their broadening, and binding energy calibration errors. The problems related to the surface charging effects of insulating materials can be eliminated by using thin films of insulators supported on a metallic substrate [ 193. Fig. 2 displays UPS He I valence band spectra S/Ru(OOl) surface at 80 K as a function of sulfur deposition time. Starting with a clean Ru, we see that the density of states of Ru at the Fermi edge is strongly reduced by the first layer of chemisorbed sulfur (0, = 0.62 ML). The S-derived photoemission features at 2.7 and 6.5 eV dominate the spectrum of the saturated monolayer and can be assigned to the emission from the S 3p orbitals of sulfur adatoms in two different adsorption sites. In a photoemission study of S chemisorbed on Mo(100) [6] authors assigned two peaks observed at 6.8 and 4.6 eV to sulfur adsorbed at two surface sites as did the authors of S/W(OOl> work [ 161. With increasing sulfur exposure the Fermi edge is gradually attenuated and is no longer visible for a 5 min deposition that corresponds to a S coverage of - 5 ML. The complete loss of the Ru emission at @s < 5 ML suggests that the sulfur film grows in a layer-by-layer mode. The film is, however, disordered because only a high intensity uniform background is visible in LEED. The first peak at 3 eV broadens and its onset shifts to higher binding energy with increasing 0,.

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i

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Fi-C’ 2. Valence photocmission spectra of a clean and sulfur covered Ru(001) surface. He I radiation was used for the excitation of electrons. The parameter is the S, dosing time in minutes.

Two more peaks appear between 4 and 9 eV for 1 and 2 min dosing. The sulfur multilayer has a welldeveloped 2 eV wide band gap and two broad bands separated by a deep ridge at 5 eV. Bulk solids with a band gap around 2 eV are usually yellow which is the color of stable cyclooctasulfur allotrope [7]. The data presented in Fig. 3 are results of an annealing experiment. Judging from the loss of the emission at the Fermi edge a large fraction of sulfur film remains on the Ru surface after warming up to 300 K 3. The sulfur multilayer is therefore stable under UHV conditions at room temperature and its

’ Our attempts to prepare sulfur multilayer at room temperature failed. Temperature of 250 K or lower is needed for the multilayer preparation because Sz does not stick to the sulfur monolayer held at 300 K.

J. Hrhek et al./ Chemicul Physics Letters 267 (1997) 65-71

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UP spectra show additional features not seen at lower temperature. The low energy band has broadened and developed a low energy shoulder together with a distinct peak at 4.8 eV. The deeper lying band has a new additional feature at 7 eV. In Fig. 3 multilayer sulfur spectra are compared with gas phase UP spectra of sulfur molecular species. The gas phase spectra were aligned with the spectra of sulfur films. Although we find a reasonably good agreement between the 80 K film and S, gas spectra [20], the match with the 300 K spectrum is not good. The S, gas phase spectrum 1211 with a sharp peak at N 5 eV that was aligned with a new peak on the low energy band, provides better agreement in this case. The data suggest that as deposited S-film is transformed from a phase containing S, dimers to a new phase that contains a fraction of cyclooctasulfur, S,. This species can be also gener-

n

UPS He I

ated by dosing large amounts of S, at 80 K (see Fig. 2). Thermal desorption data confirmed the thermal stability of sulfur multilayers. In Fig. 4 we show a set of desorption experiments, where three masses corresponding to S,, S, and S, were monitored simultaneously as qfunction of temperature for five different sulfur exposures. Chemisorbed sulfur bonded directly to the Ru surface desorbs at temperatures from 1100 to 1600 K [22]. The most interesting feature besides the thermal stability of sulfur multilayer in the UHV environment 4 is the desorption of S, and S, molecules. S, and S, desorb from the second layer but the sulfur multilayer must be 5 4 ML thick to detect S,. The intensity ratios of desorption peaks of three masses are not constant. The S (not shown) and S, signals track each other closely; in contrast, the S, and S, traces do not. We conclude that both S, and S, are present on the surface at the moment of desorption in addition to S,. Their variations with coverage and differences between the low and high temperature suggest that the S, and S, origin is both fragmentation of S, molecule in the ionizer and desorption from the surface. Approximately one third of the sulfur multilayer desorbs around 300 K, and the remaining sulfur desorbs at 400 K. The actual positions of desorption maxima are coverage dependent as are the intensity ratios of the low and high temperature peaks. The low temperature desorption peak shifts from 285 K at low coverage to 325 K, at the highest coverage shown. The high temperature peaks maxima are located between 395 K and 405 K. The activation energy for desorption estimated from the temperature of desorption peaks using the Redhead’s equation [24] for first order desorption kinetics with a preexponential of 10 I3 s- ’, is 18.6 and 24.6 kcal/mol for 305 and 400 K peaks, respectively. These values may be compared with AHsub = 23.8 kcaI/mol of

..i;::’ S, gas phase

GL

4Equilibrium

I”‘~/“‘~1”‘~/““1’~“,~~“I”“,““/‘~”,’~’ 10

9

8

vapor pressure data of elemental

sulfur [8,23]

disagree with our results. A high vapor pressure of sulfur (4X 7

6

BINDING

5

4

3

2

1

0

ENERGY (eV)

lO-6

Torr at 300 K) would limit severely the lifetime of the sulfur sample in vacuum. Other phases than the (Y-Ss phase with much lower vapor pressure (e.g.,

polymeric

sulfur) must be therefore

Fig. 3. Valence photoemission spectra of a sulfur multilayer at 80

present in the sulfur film. Annealing transforms the S-film to S,,

K and at 300 K. Spectra are compared with the gas phase UPS

the most stable form of sulfur, which then desorbs in a reaction-

data for S, 1221 and S, [23]).

limited process.

.I. Hrhek et cd./ Chemid

Physics Letters 267 (1997) 1X-71

SIRu(001)

TDS

r--+ m/e 256

200

250

300

350

Temperature Fig. 4. S, (m/e 64), S, (m/r desorption spectra acquired after for 4, 12, 20, 30 and 60 min. simultaneously, heating rate is 2

400

450

500

(K)

128) and S, (m/e 256) thermal dosing sulfur to Ru(OOl) at 80 K All three masses were recorded K/s.

the cr-S,(s) + cycle-S,(g) phase transition [7]. A Hsub of phase transitions of other sulfur allotropes are not known. The data in Fig. 4 show that S, is by far the most prevalent species in desorption spectra. For example, the S,/S, ratio of the 400 K peak is - 1500 for the highest coverage shown. However, after correction for ionization efficiency, detector sensitivity and quadrupole transmission [25] we see that at least 3% of sulfur is desorbing as S, species and another 2% as S,. The concentration of individual sulfur species cannot be measured because the fragmentation patterns of S, molecules applicable for our experimental system are not known or available. In the mass spectroscopy study of cyclododecasulfur [26] Buchler concluded that S,, is converted to S, at the temperature of measurement and the mass spectrum measured is that of S,. Among the fragments the S,

69

and S, species have 7 and 2 times higher intensity than that of S, parent molecule, respectively. Our estimate of S desorbing as S, therefore represents the lower limit. Both UPS and TDS data indicate the presence of S, molecules in the S-multilayer. UPS data are not conclusive, and thermal desorption as a dynamic tool probes the surface at the moment of desorption and does not reflect the adsorbate phase composition at equilibrium. We used surface infrared spectroscopy [27,28], which can provide direct evidence for the presence of surface species, to measure vibrational spectra of sulfur adsorbed on Cu(ll1) substrate at 100 K. Based on literature data [8,29] we expect to find vibrational losses of sulfur molecular species at frequencies around 500 cm-‘. We therefore used a synchrotron based infrared system 1301 that covers the frequency range not readily accessible with conventional infrared instrumentation. Two sets of far infrared spectra of sulfur multilayers are shown in Fig. 5. Both sets were measured with two different detectors to cover a wider frequency range. Our experimental geometry allowed simultaneous sulfur deposition and data acquisition. Operating parameters of the doser were adjusted to provide a deposition rate of 1 ML in - 5 min as calibrated by AES. The top panel of Fig. 5 shows vibrational spectra in the 175 to 600 cm- ’ region measured with a boron-doped silicon bolometer. Four vibrational modes can be seen in the 588 s spectrum. With increasing sulfur exposure four minima grow in intensity without any shift in position. Three minima at 190, 245 and 470 cm- ’ agree very well with the infrared active vibrations of the a-S, crystal [7,8], listed as two stretching modes vg at 191 cm-’ and V~ at 244 cm-‘, and one bending mode vg at 471 cm-‘. Based on this excellent agreement we conclude that some sulfur deposited on the Cu surface at 100 K is present in the form of o-S, species. The fourth minimum at 400 cm-’ is most probably a mode belonging to other sulfur allotropes [7]. The bottom pane1 of Fig. 5 displays spectra measured with a copper-doped germanium photoconductive detector in 350-800 cm-’ region. The spectrum acquired after 520 s dose has only one minimum at 768 cm-’ that disappeared as the sulfur coverage increased. We know that most of the sulfur is de-

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Physics Letters 267 (1997) 65-71

4. Conclusion

1 000

0.995

0.990

0.985 /

A 300

200

400

500

600

.I

E 1.005~~,~~,

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s,.s,.s,

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“”

“‘4 JI” /

i:.::I-:::_ 1 1600 s

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In this work we have demonstrated that films of elemental sulfur can be prepared under UHV and studied by surface science techniques. UPS, TDS and IRAS suggest the presence of S, species in sulfur films and its conversion to cyclooctasulfur molecules. The combination of TDS and IRAS results provides a clear proof for the existence of S, species in the S multilayer. These sulfur films can be very useful for studying the nature of metal-sulfur interactions, and for the preparation of films of metal sulfides [31]. This opens new possibilities for studying the surface chemistry of metal sulfides under well defined conditions using standard techniques of surface science.

Acknowledgements

0.9901 I

1

400

500

600

700

800

Wavenumber(cm~')

Fig. 5. Far-IRAS spectra of a S film growing on a Cu( I I I) surface at 100 K. The parameter is the sulfur deposition time. Boron-doped Si bolometer (top panel) and copper-doped Ge (bottom panel) detectors were used. Deposition time (in seconds) is indicated on the individual curves. Approximately a 5 minute dose is necessary to grow I ML.

This work was carried out at Brookhaven National Laboratory and supported by the US Department of Energy (DE-AC02-76CHOO016), office of Basic Energy Sciences. SYL thanks the National University of Singapore for a financial support (~~9406751~).

References

posited as dimers and slowly transforms into higher sulfur allotropes. The sulfur dimer vibration in the gas phase, observed in Raman spectra at 718 cm-‘, is forbidden in infrared [7]. The growing multilayer (middle, 1040 s spectrum) showed four new modes. These features seen at 410, 465, 635 and 670 cm-’ dominate the 1600 s spectrum. The 670 and 465 cm-’ minima are close to vibrational frequencies of S, and S,/S,/S,/S,, allotropes, respectively. Because of the complexity of sulfur multilayers the 410 and 635 cm-’ modes can be attributed to sulfur allotropes. The conversion of S, may occur on the time scale of the IRAS experiment because the sample temperature is higher and the deposition rate is lower than in the UPS or TDS experiments. The notion of sulfur transformation within the film is clearly supported by three experimental techniques used in this study.

[II

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