Langmuir 1986,2, 147-150
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Chlorine Chemisorption and Surface Chloride Formation on Iron: Adsorption/Desorption and Photoelectron Spectroscopy S. Hino* Department of Image Science and Technology, Faculty of Engineering, Chiba University, Yayoi-Cho, Chiba 260, Japan
R. M. Lambert Department of Physical Chemistry, University of Cambridge, Cambridge CB2 1EP, England Received February 4, 1985. I n Final Form: July 10, 1985 The low-pressure interaction of chlorine with iron has been studied by Auger spectroscopy, work function measurements, thermal desorption, XPS, and UPS. It is shown that at 300 K nucleation and growth of a chloride of iron follow the completion of a chemisorbed overlayer. All the data indicate that the chloride in question is FeCl,. Desorption of FeCl,(g) and its dimer are observed from the multilayer halide phase, while the valence band and core-level photoelectron spectra are consistent with the eventual formation of FeCl, at the surface. A thermochemical analysis of the system readily rationalizes the observation that the first adsorbate layer also desorbs preferentially as FeC1, rather than FeCl or Cl,. The behavior of iron toward chlorine is therefore not anomalous, as previously supposed, but is seen to be consistent with that of many other transition metals.
1. Introduction The high chemical reactivity of iron has led t o many studies of its surface chemistry, in relation to oxidation, chemisorption properties, and catalytic behavior.'-l0 This high reactivity makes the removal of impurities difficult, and in the case of single-crystal specimens, the phase transition at 1200 K can further complicate matters. In their investigation of the low-pressure interaction of chlorine with Fe(100) a t 300 K, Dowben and JonesgJo concluded that adsorption ceased with the completion of a monolayer of C1 adatoms and without the formation of an iron chloride. Because they failed to observe any desorbing species, they further concluded that thermally induced C1 loss was the result of diffusion into the bulk or desorption of C1 atoms.g They later modified this conclusionlo to suggest that iron monochloride desorption may have been occurring (undetectably) while retaining the view that chloride formation did not occur under their conditions. In this article we present adsorption/desorption and spectroscopic data which show that a t 300 K and low chlorine partial pressures, chloridation of the surface to form FeC1, does occur after completion of the chemisorbed layer. It is also shown that at all coverages, heating results in the preferential desorption of the dichloride rather than FeCl or C1 atoms. It thus emerges that the behavior of Fe surfaces toward Clz is not anomalous but entirely in line with that of the other bcc metals of the first transition series-and indeed with that of the great majority of transition metals that have been studied (ref 11-15 and references therein). In the present work, an Fe(100) specimen was used for the photoemission measurements, while it proved much more feasible to use a well-annealed polycrystalline foil specimen for the desorption experiments. This avoided problems caused by repeated traversing of the phase transition with the single crystal; the relatively large volume change associated with the bcc fcc phase transition almost inevitably leads to specimen fracture. On the other hand, well-annealed polycrystalline foil specimens of bcc metals are known to preferentially expose (100)planes a t the surface.
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2. Experimental Section Experiments involving photoelectron spectroscopy were performed on the Fe(100) single crystal in a standard VG ADES 400 ultrahigh vacuum chamber equipped with UPS, XPS, AES, and ion etching facilities. All other experiments were carried out on an iron foil in a stainless-steel ultrahigh vacuum chamber which has been described elsewhere.16 This latter apparatus incorporated a retarding field analyzer, a cylindrical mirror Auger spectrometer, and a quadrupole mass spectrometer. Base pressure in these chambers was routinely less than 5 X 10-lo torr. The Fe(100) single-crystalspecimen was spark eroded from a 99.99% purity iron ingot after orienting it to within 0.5' accuracy using Laue photography. The crystal had an elliptical shape with dimensions 9 mm X 6 mm X 0.8 mm; the polycrystalline specimen was a 7-mm square 0.1 mm thick which was cut from a 99.99% purity iron foil. Temperature measurement was by means of Pt/Pt-13% Rh thermocouples spot-weldedonto the back faces of each specimen. Chlorine gas was generated in each apparatus by in situ solid-state electrolysis of silver chloride." Calculation of the effusive properties of these sources suggested that about 6% of the chlorine from the source used for the photoelectron measurements and about 30% from the source used for the other measurements was incident on the iron surface. The chlorine doses quoted below refer to the estimated number of chlorine molecules arriving a t (1) Brucker, C. F.; Rhodin, T. N. J . Catal. 1977,47, 214.
(2) Rhodin, T. N.; Brucker, C. F.; Anderson, A. B. J . Phys. Chem. 1978, 82, 894. (3) Yoshida, K.; Somorjai, G. A. Surf. Sci. 1978, 75, 46. (4) Yu, K. Y.: Spicer, W. E.; Lindau, I.: Pianetta, P.: Lin, S. F. Surf. Sci. 1976, 57, 157.' (5) Brundle, C. R. Surf. Sci. 1977, 66, 581. (6) Brundle, C. R.; Chaung, T. J.; Wandelt, K. Surf. Sci. 1977,68,498. ( 7 ) Mason, R.; Textor, M. Proc. R. SOC.London, Ser. A 1977, A356, 47. (8) Broden, G.; Gafner, G.; B o n d , H. P. Appl. Phys. 1977, 13, 333. (9) Dowben, P. A.; Jones, R. G. Surf. Sci. 1979,84, 449. (10) Dowben, P. A.; Jones, R. G. Surf. Sci. 1979, 88, 348. (11) Khan, I. H. Surf. Sci. 1975, 48, 537. (12) Davies, P. W.; Lambert, R. M. Surf. Sci. 1980, 95, 571. (13) Foord, J. S.; Lambert, R. M. Surf. Sci. 1982, 115, 141. (14) Reed, A. P. C.; Lambert, R. M.; Foord, J. S.Surf. Sci. 1983,134, 689. (15) Cox, M. P.; Lambert, R. M. Surf. Sci. 1981, 107, 547. (16) Foord, J. S.; Goddard, P. J.; Lambert, R. M. Surf. Sci. 1980,94, 339. (17) Spencer, N. D.; Goddard, P. J.; Davies, P. W.; Kitson, M.; Lambert, R. M. J . Vac. Sci. Technol., A 1983, 1, 1554.
0 1986 American Chemical Society
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148 Langmuir, Vol. 2, No. 2, 1986
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Figure 1. Chlorine uptake on iron. (Top) Cl/Fe Auger intensity ratio and (bottom) work function shift (A$) as a function of Clp dose. the iron surface per square meter. Thus although absolute chlorine dosages may be in error by a factor of about 2, relative exposures can be taken as being accurate to within 10%. The main impurities in the foil specimen were a large amount of carbon and a little sulfur; sulfur was the dominant impurity in the single-crystal specimen, followed by carbon and oxygen. Surface segregation of S occurred a t around 900 K a t which temperature C diffusion into the bulk became significant. Different cleaning methods were therefore used for the two specimens. The foil specimen was Ne+ bombarded (500 eV, 3 X A m-’, 1200 K) until the 150-eV sulfur Auger signal was barely detectable. This was followed by a further 40 h of Ne+ bombardment a t 600 K, at which temperature carbon segregated to and was removed from the surface. Auger spectroscopy then showed that the total impurity level remained below 0.05 monolayers even after repeated flashing to 1300 K. The single-crystal specimen was heated at 1000 K for 3 days under 1 atm of hydrogen. This was followed by 50 cycles of heating (1100K/2 h) and Ar+ bombardment (3 KV, lo-’ A m-?. After this procedure, flashing the specimen to 1100 K gave a surface impurity level comparable with that of the purified foil. In this case, however, the presence of residual bulk sulfur could lead to significant surface segregation after prolonged heating to -1100 K. Care was therefore taken not to exceed 900 K once an acceptably clean surface had been produced.
3. Results 3.1. Polycrystalline Specimen: Auger Spectroscopy, Work Function Measurements, Thermal Desorption Data. Chlorine was found to adsorb rapidly on the Fe foil a t 300 K. The relative intensity of the 181-eV C1 (L23M23M23) Auger transition to that of 654-eV Fe (L23M23M45) transition was measured as a function of chlorine dosage and an uptake curve is shown in Figure 1. Chlorine coverage increased linearly with dosage up to a gas exposure of 1.1x lot9molecules m-2. After this the sticking probability appeared to fall quickly; the signal ratio did not change further for doses of 2.1 X 1019m-2 or more. Electron-stimulated desorption of chlorine was looked for but was not detected. The work function change during chlorine adsorption was studied using the electron beam-retarding potential technique,18 and these results are also shown in Figure 1. A rapid initial increase of work function is followed by a somewhat slower rate, the curve finally leveling off a t a (18) Ertl, G.; Kuppers, J. ‘Low Energy Electrons and Surface Chemistry”; Verlag Chemie: Weinheim, 1974.
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F i g u r e 2. 126-amu (FeC12+)thermal desorption spectra as a function of Clz dose; heating rate, 100 K s-’.
value of Aq5 = 1.0 f 0.05 eV. When the chlorine-saturated foil was heated to 420 K, A$ decreased by 0.2 eV suggesting either rearrangement or partial evaporation of the adsorbate. Thermal desorption data from the chlorine dosed surface were measured a t 35 (el+), 70 (Cl,’), 91 (FeCP), 108.5 (FezC12+),126 (FeC12+),161 (FeCl,+), 182 (Fe2ClZ+),217 (Fe2C13+),and 252 amu (Fe2C1,+). The strongest signals were at 91 and 126 amu and the corresponding desorption spectra closely resembled each other. This suggests that both signals (FeCl+, Feelz+) arise respectively from the mass spectrometric fragmentation and ionization of FeCl,(g). There were no detectable signals at 70 (C12+)and 161 amu (FeC13+)even after very large doses of chlorine ( lozomolecules m-2). The 35 amu ((21’) spectra could not be reliably obtained a t low chlorine coverages because of interference from the spurious background signal resulting from electron stimulated desorption of C1+ in the mass spectrometer ionizer. However, following large doses of chlorine (1.4 X 1020 m-2) the C1+ desorption spectra exhibited a single peak at 550 K. Figure 2 shows desorption spectra monitored a t 126 amu (FeClZ+).The low coverage spectra exhibit a single peak a t -1220 K; with increasing coverage, this feature progressively shifts to lower temperatures, eventually settling at -1100 K. The dip in the curve at 1180 K is an artifact which arises because of the interruption to the temperature sweep resulting from the bcc fcc phase transition at this point. Continued exposure to chlorine results in the leading edge of the desorption profile moving rapidly to lower temperatures and the eventual appearance of a narrow peak a t 550 K for doses of 23.4 X 1019 m-2. As already noted, the 91-amu (Feel+) spectra mirrored those obtained a t 126 amu (FeC12+).Allowing for the change in instrumental transmission function between these two masses indicated that the intensity of the FeCP fragment ion was of the same order as that of the FeC12+parent ion. At the highest coverages, identical desorption spectra were observed at 182 (Fe2C12+),217 (Fe2C13+),and 252 amu (FezC1,+). These consisted of a single peak a t -550 K whose intensity was about an order of magnitude smaller than the corresponding feature in the FeC1+/FeCl2+
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Langmuir, Vol. 2, No. 2, 1986 149
Chlorine Chemisorption and Surface Chloride Formation A
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Figure 3. He(1) photoelectron of Clzdosed Fe(100)showing full spectral scans (A) and the region immediately below EFonly (B). Gas doses are 0,1.7,3.4, and 1500 X 1019molecules/m2for spectra a-d in each half of the figure.
spectra. It seems likely that these higher mass species arise from the evaporation, ionization, and fragmentation of FeC1, dimers.lg 3.2. Single-Crystal Specimen: Ultraviolet and Xray Photoelectron Spectroscopy. He(1) UP spectra from the clean (100) surface were essentially the same as those reported e l ~ e w h e r e . ' ~Chlorine ~ ~ ~ ~ dosing led to the appearance of a new peak a t 5.7 eV below EF-a feature that is derived from the Cl(3p) levels. This feature grew with increased exposure to C1, (Figure 3), and at the highest doses, additional features on either side of it became apparent between 4 and 9 eV. Figure 3 also shows the eventual appearance of a peak a t 16.5 eV below E F . This is probably derived from the Cl(3s) level, because its binding energy relative to the 3p-derived adsorbate feature (+10.7 eV) is in good agreement with the corresponding value for the gaseous C1 atom.21 The limiting work function shift after extended dosing was +1.0 eV, in good agreement with the result obtained for the polycrystalline specimen. XP spectra of the Fe(2p) and Cl(2p) levels are shown in Figure 4. Clean iron is characterized by strong 2p312 and 2pIl2 peaks with binding energies of 706.5 and 720.4 eV, respectively (relative to EF). Little change in these peaks was detectable for gas doses up to -4 X 1021m-,. However, increased dosing led to the gradual emergence of a new feature on the high binding energy side of the 2p3/, peak; after a dose of 4 X loz2m-, this appeared as a peak lying 3.2 f 0.5 eV in binding energy above the 2p3j2 peak associated with the clean surface. The association of this higher binding energy peak with a surface species was confirmed by examining the angular dependence of the Fe(2p) spectra. It can be seen from Figure 4c,d that (19)Schoonmaker, R. C.; Porter, R. F. J. Chem. Phys. 1958,29,116. (20)Pessa, M.;Heimann, P.; Neddermeyer, H. Phys. Rev. B 1976,14, 3488. Heimann, P.; Neddermeyer, H. Phys. Rev. B 1978, 18, 3537. Schulz, A.; Courths, R.; Schulz, H.; Hufner, S. J. Phys. F 1979,9,L41. (21)'CRC handbook of Chemistry and Physics"; CRC Press: Boca Raton, FL, 1983.
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Figure 4. Mg K a X-ray photoelectron spectra of C1 -dosed Fe(100). (A) shows Cl(2p) level after a dose of 1.2 X loz7,molecules/m*. (B) shows Fe(2p) spectra. (a) and (b) refer to the clean surface with electron emission angles of 40" and 65" relative to the surface normal. (c) and (d) taken after a Clz dose of 4 X loz2 molecules/m2with collection angles of 40" and 65", respectively.
the relative intensity of the high binding energy component of the 2psl2 emission is enhanced a t more grazing angles of electron emission (for constant photon angle of incidence). The Fe(2pIj2)emission also shows some evidence of similar behavior as a function of chlorine exposure and electron emission angle; however, the data suffer from a significantly poorer signal/noise ratio. 4. Discussion Dowben and Jones have concludedgJOthat the lowpressure interaction of chlorine with Fe( 100) terminates upon completion of a chemisorbed monolayer. In part, they were led to this view by their observations on the variations with exposure of the work function and the C1 Auger intensity. We have observed similar behavior (Figure l),but the limiting values attained by both these properties do not necessarily imply a cessation of chemical reaction at the interface. They are equally explicable in terms of the growth of a film of surface compound with constant stoichiometry. It will be shown below that this compound is FeC1,. We begin with an examination of the nature of the desorbing species as revealed by the thermal desorption spectra. The main issues here are (a) the nature of the processes leading to the low- and high-temperature peaks and (b) why there is no desorption of molecular Cl,. The close resemblance between the FeCP and FeC12+desorption spectra indicates that for all initial coverages,the only desorbing species of importance is FeC1,. (The -550 K C1+ peak observed after extensive C12 dosing is ascribed to a fragment ion signal derived from FeC12(g).) Inspection of the leading edges of the 550 K peak for spectra obtained for different initial doses of chlorine suggests that the corresponding desorption process is kinetically of zero or fractional order. This would be consistent with evaporation of FeC1, from a dispersed surface phase of FeC12 in dynamic equilibrium with microcrystallites of the halide.
150 Langmuir, Vol. 2, No. 2, 1986
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An estimate of the desorption energy associated with this low-temperature peak may be obtained from an Arrhenius plot of the low-temperature/ high-coverage data. Such a procedure leads to a value of 100 f 15 kJ/mol for the activation energy to desorption of FeC1,. This may be compared with the accurate value for the bulk sublimation enthalpy of FeCl,(s) FeClJg) which is 82 kJ/mol.,, Given the relatively crude nature of our estimate, and the fact that it refers to conditions of evaporation which may differ somewhat from those under which the thermodynamic data were obtained, it does not seem unreasonable to associate the low-temperature peak with the evaporation of crystallites of FeC1,. This view is strengthened by the indication that concurrent evaporation of FeC1, dimers occurs in this same temperature regime; dimer evaporation is a knownlg feature of the vaporization of bulk FeC1,. These phenomena occur in a dosage regime in which the growth of multilayers is possible; they therefore strongly suggest that chloride growth commences on Fe during the dosing procedure at 300 K: i.e., subsequent heating during the desorption sweep is not needed to induce chloridation. The low-coverage high-temperature desorption peak may then be identified with FeCl, desorption from the first adsorbed monolayer with a desorption energy of 250 kJ/mol. The marked coverage-dependent shift of this peak to lower temperature resembles the behavior reported for AgCl evaporation from C1 overlayers on Ag(ll1). In that case the authors ascribed the effect to the influence on the desorption kinetics of a mobile precursor state to desorption;26 it seems likely that a similar explanation holds in the present case. It is of some interest to inquire why this first layer desorbs as the dichloride rather than as some other chloride species13 or by reevaporation of molecular Clz. The difference between the desorption energies (E,) of various species (X) can readily be deduced from a consideration of appropriate thermochemical cycles. From a knowledge of the atomization energies of Clz(g), FeCl(g),23FeCl,(g)z2and the sublimation enthalpyz1of Fe it is easily shown that ECI2 - EFeCI2 = +134 kJ/mol
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i.e., dichloride desorption is strongly preferred over Cl, desorption-in accord with experiment. Another interesting question arises by comparison with the closely related Cr(100)-C12 system.13 In that case it was found that the monolayer evaporated preferentially as the monochloride (CrC1) although growth and evaporation of di(22) Sime, J. R.; Gregory, N. W. J. Phys. Chem. 1960, 64, 86. (23) ‘Bond Energies, Ionization Potentials and Electron Affinities”; Vedenev, V. I., Gurvich, L. V., Kondrat’yev, V. N., Medevev, V. A., Frankevich, Ye. L., Eds.; St. Martin’s: New York, 1966. (24) Grunze, M.; Dowben, P. A. Appl. Surf. Sci. 1982, 10, 209. (25) Lee, E. P. F.; Potts, A. W.; Doran, M.; Hillier, I. H.; Delaney, J. F.; Hankoworth, R. W. J. Chem. Soc., Faraday Trans. 2 1980, 76, 506. (26) Bowker, M.; Waugh, K. C. Surf. Sci. 1983, 134, 639.
chloride did eventually occur at high chlorine loadings. Once again we can use the observed low coverage desorption energy of FeCl, from the first chemisorbed layer (EFeCl2 256 kJ/mol) in conjunction with the published thermochemical data21-23to obtain an estimate for EFeC1. The result is EFecl 386 kJ/mol-confirming that in this case the dichloride is indeed the preferred evaporation product from the chemisorbed overlayer. Thus it appears that the thermal properties of this system can be understood simply in terms of energetics without having to invoke effects of the entropy. In the particular case of FeC1, vs. FeCl evaporation from the monolayer, an inspection of the various quantities involved reveals that the pivotal feature is the very strong Fe-C1 bond energy in FeClz(g). The low-coverage UP data are in agreement with spectra obtained from chlorocarbon chemisorptionz4on Fe where it is believed that initial dissociation is followed by diffusion of carbon into the bulk. The 6-eV Cl(3p) derived peak is a common feature in metal-C1 chemisorption systems as is the decrease in metal d-emission near EF which accompanies it. The more complex emission features between 4 and 9 eV which arise on either side of the Cl(3p) level a t higher chlorine doses are noteworthy: they correlate quite well with the reported spectraz5of gaseous FeC1,. The latter are characterized by a central band of ‘ Cl(3p) emission flanked on either side (at 1.5-2.0-eV displacement) by emission of comparable intensity from Fez+ d-levels. In this respect, our observation qualitatively resembles the results obtained for Cr( 10O)-Cl2. However, in that case, thanks to the less complex appearance of the valance level emission from CrCl,(g), it was possible to assign the three adsorbate bands to Cl(3p) and ligand-field split Cr2+d-orbitals. Such an unambiguous assignment is not possible in the present case, but the data are certainly consistent with the presence of FeC1, at the surface. The presence of a higher oxidation state of iron a t the surface is borne out by the Fe(2p) binding energy shift of 3.2 f 0.5 eV. The reported shifts for FeO and Fes03 are 2.7 and 4.2 eV, respectively, providing further evidence that the identity of the surface chloride is FeC1,. Our results do not permit us to draw any detailed conclusions about the way in which the overlayer halide transformation actually takes place. It may be of relevance that in the case of V(lOO)-Brz, for which detailed structural information was available,12 it appeared that above a certain critical halogen concentration, all the surface halogen atoms were incorporated into the halide phase.
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Acknowledgment. S.H. thanks the Ramsay Memorial Fellowship Trust for financial support. We are grateful to Johnson Matthey Ltd for the loan of precious metals. We acknowledge with thanks the help of N. D. Spencer and G. A. Somorjai in arranging for hydrogen treatment of the single-crystal specimen. Registry No. C12, 7782-50-5; Fe, 7439-89-6.
