A structural and kinetic study of chlorine chemisorption and surface chloride formation on Cr(100)

Surface Science 115f1982) 141-160 North-Holland Pubhshing Company

A STRUCTURAL AND KINETIC STUDY OF CHLORINE CNEMISORPTIUN AND SURFACE CHLORIDE FORMATION crcm,

Received

17 June 1981; accepted

for publication

26 October

ON

1981

Chlorine adsorption on (I x I)-Cr(lO0) has been studied using AES, UPS, XPS, LEED, TDS and A+ measurements. For gas exposures of ~8.0~ IO” molecules m-r, Cl, adsorbs dissociatively with high sticking probability into an atomic overlayer. Continuous compression of an initial ~(2 X 2) phase occurs as coverage increases, until a ~(2x5) structure is formed with a Cl-Cl nearest neigbbour separation of 3.41 k. in this coverage regime a single adsorbate induced band appears at -6.0 eV in UPS and the Cr(2p) and Cl(2p) binding energies remain constant in XPS. Thermal desorption occnrs as CrCl, and the TD spectra exhibit a Gngfe peak which corresponds to an activation energy of 300-340 kJ mot - I. After higher gas exposures, chemical shifts in XPS and the appearance of additional bands in UPS reveal that epitaxial growth of chromium dichloride occurs. This hahde phase has a twinned layer structure; it desorbs as CrCl, with an activation energy of - 240 kJ mol - ‘, The system now exhibits three-dimensional island growth, and the rate of corrosion of the chromium substrate is shown to be - 0.02 that of overlayer formation.

1. Introduction

Low pressure studies of halogen interactions with metal surfaces are of interest since they can be expected to provide information on such processes as catalyst poisoning and promotion, corrosion and epita.xial growth. This paper deals with the surface chemistry of the ~hro~um-~hlo~ne interface, and in particular, we present detailed kinetic and structural data on the formation of chlorine overlayers on Cr(lo0) and the subsequent nucleation and growth of chromium dichloride. The observation of halide growth under the high vacuum conditions employed in this investigation is in marked contrast with most previous halogen adsorption studies (e.g. refs. [l-15]). However, closely related work in progress in this laboratory an a range of other transition metals (Y, V, Co, Rh, Pd [l&17]) would appear to indicate that it is a widespread phenomenon. Gewinner et at have reported f 181 that the clean Cr(lO0) surface reconstructs in a similar fashion to W(lOO) [ 191 and this has been investigated * Department

of Chemistry,

University

0039-6028/82/0000-0#001$02.75

of Southampton.

Q 1982 North-Holland

theoretically reconstruction

[20]. Here, is impurity

we raise induced.

the

question

as to what

extent

the Cr

2. Experimental Experiments involving photoelectron spectroscopy were performed in a standard VG ADES 400 ultrahigh vacuum chamber, equipped with UPS, XPS, LEED and Ar + ion etching facilities. All other experiments were carried out in the stainless-steel ultra-high vacuum chamber which has been described elsewhere [21]. Base pressures of 4 X 10m9 Pa were routinely obtainable in both chambers. The Cr(lOO) single-crystal specimen was spark eroded from a 99.999% purity chromium ingot after orientation to within 0.5” using back-reflection Laue photography. The crystal was in the form of an elliptical disc of dimensions 9 mm X 6 mm X 0.7 mm and its temperature could be raised to 1400 K by conduction from resistively heated Ta support wires. Temperature measurement was by means of a Pt/Pt-10% Rh thermocouple spot-welded to the rear face of the sample. Chlorine was generated in situ by solid state electrolysis of the appropriate silver halide 1221. The gas doses quoted below refer to the flux of chlorine molecules at the centre of the Cr(100) surface in Cl, molecules per square metre. They were calculated by assuming that chlorine was emitted from the anode surface in a cosine distribution and experimentally, it was found that the Cl, flux varied by 15% between the centre of the specimen and its edges.

3. Sample preparation It appears that the ‘degree of surface cleanliness achieved in earlier work on Cr( 100) has not been high. Two studies 123,241 used the presence or absence of fractional order beams in electron diffraction patterns as a monitor of impurity levels, and were thus carried out on nitrogen~conta~nat~ surfaces (231. This impurity was also present in a later investigation [25] and contamination levels in the most modern work [ 18,261 were only reduced to - 0.1 monolayer. Nevertheless, it has been proposed [1X] that clean Cr( 100) undergoes a surface reconstruction which is stable up to temperatures of 800 K. Since such reconstructions are often peculiarly sensitive to the presence of trace contaminants, the Cr(lOO) crystal used in this work was subject to an extremely lengthy cleaning procedure to produce an impurity-free surface, as determined by AES and XPS. After insertion into the vacuum chamber and annealing at 1000 K for 30 mm, the AE spectrum of the Cr(100) surface in fig. la was recorded. Ne+ bomb~dment (10 PA, 300 eV) at 300 K for 1 h removed the cont~nants which are indicated in this spectrum but subsequent speci-

J.S. Foord, A.M.

143

Lumberr / Chlorine chemisorprion on Cr(I0i3)

d

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~ 130

320

510

I

130

320 ELECTRON

Fig. I. Cr(100) Auger nitrogen contaminated; E, = 2500 eV.

Fig. 2. LEED XeR27O)-N,

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510 130 ENERGY

spectra: (a) after insertion (c) clean; (d) after dosing

320

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into vacuum and annealing at ICOOK: (b) with 6.0X IO +I8 molecules mm2 of chlorine.

patterns observed during specimen cleaning: (a) (1 X 1)-N, 145 eV: (b) (v’?R45’ 133 eV; (c) c(2X2)-N, 130 eV; (d) clean (IX I), 143 eV.

men annealing at 1000 K caused large amounts of sulphur and nitrogen to segregate to the surface from the crystal bulk. Ion bombardment at 1000 K for 12 h was sufficient to free the crystal from sulphur but not from nitrogen, as can be seen from the AE spectrum in fig. lb which was monitored at this time. It was necessary to Ne+ bombard the sample at 1000 K for somefouv hundred hours before the bulk nitrogen contaminant was removed. After such treatment, the clean surface AES spectrum in fig. lc was recorded and prolonged annealing at 1200 K led to no detectable impurities segregating to the surface. Several nitrogen LEED patterns were observed during the above cleaning procedure, and these are presented in fig. 2. On surfaces which exhibited an N(399 eV):Cr(483 eV) Auger peak intensity ratio (RN) of 0.5, the (1 X 1) pattern shown in fig. 2a was visible. It is the formation of this (1 X 1)-N structure which makes LEED insensitive to high concentrations of surface nitrogen. As R, decreased to 0.3, the (fiR45” X 6R27”) pattern in fig. 2b appeared, and a c(2 X 2) surface structure (fig. 2c) formed as R, fell to 0.2. Finally a (1 X 1) pattern (fig. 2d) was obtained when all the nitrogen had been removed from the crystal. No impurities could be detected by AES on the final (1 X 1) surface, but the sensitivity of this technique towards oxygen is low because of partial overlap between oxygen and chromium Auger transitions. XPS, however, revealed that the oxygen contamination level was < 0.01 monolayer. We therefore presume the (1 X 1) LEED pattern in fig. 2d is representative of the clean Cr(100) surface and thus conclude that surface reconstruction does not occur at 300 K. In an attempt to reconcile this conclusion with that of earlier work [18], the effect of carbon and oxygen impurities on the structure of the (1 X 1) surface was studied. It was found that surface concentrations of 0.05-0.20 monolayer of either of these contaminants produced an intense c(2 X 2) LEED pattern which faded as impurity levels rose to 0.50 monolayer. This is in excellent agreement with the’previous investigation [ 181. It therefore seems that although clean Cr( 100) possesses a (1 X 1) structure at 300 K, low concentrations of carbon, oxygen and possibly nitrogen may stab&e a Cr(lOO)-c(2 X 2) displacive phase 1191. Alternatively, the fractional order beams could arise from c(2 X 2) adsorbate domains on a patchy surface.

4. Results 4.1. Chlorine adsorption at 300 K Chlorine was found to adsorb rapidly on Cr(100) at 300 K. The absolute intensity of the 181 eV Cl(L,,,M,,,M,,,) Auger transition was measured as a function of chlorine dose to the crystal and an uptake curve is shown in fig. 3. It can be seen that the chlorine coverage increases almost linearly with dose, for gas exposures up to 4.7 X lOI* m-*. After this, the sticking probability

J. S. Foot-d, R. M. Lambert /

1.0 Cl2 exposure

Fig. 3. Chlorine

uptake

2.0

followed

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145

Chlorine chemisorption on CrfIoOl

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using AES (#) and work function

measurements

(0).

appears to fall off rapidly, although it was notable that the Cl Auger signal continued to rise, albeit slowly, as the chlorine dose increased to 5.0 X 102’ rnm2. The sensitivity of AES towards chlorine is high, and Auger spectra recorded after large gas exposures exhibit a Cl(181 eV): Cr(483 eV) peak intensity ratio of - 5.1 (fig. Id). Electron impact effects were looked for, but not observed. Thus the cross-section for electron-stimulated desorption of chlorine by the 2.5 keV primary Auger beam was found to be < 1O-26 m*, over the whole coverage range investigated in this work. The variation in work function (A$) during chlorine adsorption was studied -using the electron beam-stop technique [27], and a A+ uptake curve is also presented in fig. 3. The graph shows a rapid initial increase, but then quickly levels off at a value of A+ = 1.37 * 0.05 eV, after which it remains constant for gas exposures up to 10” me2. LEED observations showed that a series of ordered structures are formed by chlorine on Cr(lO0). Little change in the cleansurface LEED pattern could be seen for chlorine doses of less than 2 X 1Or8mm2 (fig. 4a), but subsequent chlorine exposure caused an uneven background to develop. Weak, diffuse (4, f) spots appeared after a chlorine dose of 2.8 X 10” rnm2, and these sharpened up into quartests of sports centred about the (4, f) positions as the gas exposure increased up to 3.5 X lo’* me2 (fig. 4b). These quartets of spots continuously moved out towards (0, f) positions as the crystal was dosed with more chlorine. Simultaneously, (0, i) spots appeared so that the pattern shown in fig. 4c was visible after a chlorine exposure of 4.8 X lOI m-*. It may be simply indexed as ~(2 X 4). As the surface coverage of chlorine increased, splitting of the (0,;) spots and further movement of the other fractional order spots in this $2 X 4) structure occurred. The diffraction pattern passed through the intermediate stage shown in fig, 4d, before a final pattern, indexing as

Fig.4. LEED patterns produced by chlorine exposures of: (a) 2.0X 10” me2, 140 eV; (b) 3.5X 10’s mm2. 135 eV; (c)4.8X IO’* m-‘, 124 eV; (d) 5.9X 10’s m-*. 108 eV: (e) 7SX IO’” me*. 102 eV: (f) 2.0X IO” me2, I@4 eV.

J.S.

Foord. R. M. Lambert / Chlorine chemisorption

a

147

on Cr(iO0)

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ELECTRON BINDINGENERGY(eVl Fig. 5. AI Ka, He I and He II excited photoelectron speetra. (a) Cr(2p) XPS: (i) clean: (ii) exposed to 6.0X 10” m-’ Cl,: (iii) exposed to 1.0X IO*’ me2 Cl,. (b) Cl(2pf XPS after exposures of: (if 3.0X IO’* rne2: (ii} 8.0X IO” me2; (iii) 1.0X IO” m-*: (iv) 2.0X 10” m-*. (c) UPS: (i) He I. clean; (ii) He I after 6.0X 10iR m-’ Cl,; (iii) He1 after 1.0X 102’ rn-’ Cl,; (iv) He II after 1.0X 102’ mw2 Cl 2

p(2 X 5), formed (fig. 4e) after exposing the crystal to 7.5 X IO’” me2 of chlorine. Thus chlorine adsorption on Cr(lOO) produces a poorly ordered c(2 X 2) structure, which develops continuously into a p(2 X 5) phase, via a c(2 X 4) intermediate, as the surface concentration of chlorine increases. No change was visible in the diffraction properties of the p(2 X 5) adlayer as the chlorine dose rose from 8.0 X IO’* to 1.3 X 1Or9 m-‘_ However higher chlorine exposures caused the p(2 X 5) pattern to slowly fade and be replaced by the one shown in fig. 4f. The substrate beams have been completely extinguished in this latter pattern, which arises from four pseudo-hexagons domains defined by mesh vectors of lengths 3.5 and 3.9A, with an included angle of 117O. This structure could only be observed on the crystal after chlorine exposures of 8.0 X lO*O rns2 or more, and co-existing p(2 X 5) domains did not disappear until the gas dose exceeded 1.6 X 102’ m-*. Photoemission from the 2p core levels of chromium and adsorbed chlorine was investigated by XPS and the recorded spectra are shown in figs. 5a and 5b. Clean chromium is characterised by intense 2p,,, and 2p,,, emission at binding energies of 574.3 and 583.4 eV, measured with respect to E,. Little change occurred in these two peaks during exposure of the crystal to 6.0 X lOi8 mm2 of chlorine. However, subsequent chlorine dosing caused the peaks to broaden and it is apparent from fig. 5a that distinct shoulders with a binding energy shift of + 1.5 eV are present after chlorine exposures of lo*’ mol mW2.

Photoemission from the chlorine 2p level appeared in our spectrometer as a poorly-resolved spin-orbit doublet (fig. 5b). The intensity of the emission from this level smoothly increases with surface coverage and the binding energy remains constant, for chlorine doses of less than 6.0 X 10’” mol rnm2. At higher surfaces coverages, it can be seen that the spectra appear to broaden and eventually the emission shifts by 0.7 eV to greater binding energies. XPS thus indicates that chlorine adsorbs into two states on Cr( 100). The state populated at surface concentrations below that of the p(2 X 5) adlayer produces no change in the Cr(2p) binding energy, while that populated at higher coverages causes positive shifts in binding energy of both the chlorine and chromium 2p levels. The existence of two distinct chlorine chemisorption states on Cr(lOO) is confirmed by the UPS spectra shown in fig. 5c, which were excited by He I and He II radiation. The main feature in the clean surface spectrum is a strong structured peak just below E,, which arises from Cr(3d) levels. After dosing the crystal with 6.0 X lOI mW2 of chlorine, this emission is attenuated and a broad band at -6.0 eV appears in the He I spectra. if the specimen is dosed with more chlorine, new features at -3.5 and -7.5 eV develop and Cr(3d) emission near E, is reduced still further. Essentially the same results are obtained with He II radiation, although the relative intensity of the band at -6.0 eV is somewhat reduced at the higher photon energy. 4.2. Thermal desorption Thermal desorption from the chlorine dosed surface was studied, using a linear heating rate of 60 K s-‘, and desorption products were looked for at 35 amu (Cl’), 52 amu (Cr +), 70 amu (Cl; ), 87 amu (CrCl+), 122 amu (CrCl: ) and 157 amu (CrClT ). No desorption occurred at 70 amu or 157 amu, but strong signals were observed at all of the other masses. Experiments performed with the ionisation source of the mass spectrometer switched off showed that all the desorbing species were electrically neutral. The 35 amu (Cl’) desorption traces are presented in fig. 6a. At low exposures (< 5.3 X 10” mp2), only a single high temperature peak is visible in the spectrum. As more chlorine is dosed onto the surface, the leading edge of the desorption profile shifts rapidly to lower temperatures, and a narrow low temperature, peak begins to develop after gas exposures of 8.0 X 10’” mm2 or more. This peak gives no indication of ever reaching a saturation coverage and eventually it completely dominates the desorption spectrum. For convenience, the two clearly resolved peaks in the spectra will be referred to as arising from the (Y adsorption state (low temperature peak) or /3 adsorption state (high temperature peak), although it should be borne in mind that there is a broad band of desorption between these two peaks which does not clearly belong to either. Desorption traces monitored at 52 amu (Cr ‘) are shown in fig. 6b. It can

J.S. Food.

R.M.

Lumhert

/ Chlorine

chemisorptron

on Cr(lO0)

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Fig. 6. Thermal desorption spectra. (a) At 35 amu (Cl ‘): spectra (i)-(x) refer to exposures of 0.8, 1.6. 2.4, 3.2, 4.0, 5.6, 7.0, 14.0, 37.0 and 70.0X IO’* mm2 respectively. (b) At 52 amu (Cr+): spectra (i)-(k) refer to doses of 4.0, 5.6, 7.0 and 39.0X IO’* mm2 respecively. (c) After a dose of 44.0X IO’* mP2: (i) at 122 amu (CrCI:): (ii) at 87 amu (CrCl+).

be seen that there is some considerable resemblance between these spectra and those recorded at 35 amu. Thus there is a low temperature peak which has a coverage versus temperature dependence identical to that of the (Ystate in the 35 amu traces. This is then followed by a broad tail which merges into another peak at the temperature of the 35 amu p state. A complication arises here, in as much as the bulk chromium lattice begins to evaporate at a significant rate near the end of the desorption sweep. However, it was possible to verify the existence of a true p peak at 52 amu by subtracting the clean surface evaporation profile from the desorption spectra of the chlorine-dosed surface. Strong desorption signals were also detected at 87 amu (CrCl + ) and 122 amu (CrCl,’ ) and spectra taken at these masses are shown in fig. 6c. The (Ystate is a prominent feature of the desorption profiles at both masses, but the j3 state is only present in the spectrum at 87 amu. Although it is clear that chromium chloride(s) are present in the desorption products, their exact composition cannot be straightforwardly deduced without knowledge of the fragmentation patterns of the desorbing species in our mass spectrometer. To obtain this information, the mass spectrum of the evaporation products from a bulk sample of chromium dichloride was recorded. Strong signals were observed at 52 amu (Cr +), 87 amu (CrCl+) and 122 amu (CrCll ), with intensities in the ratio 1: 2 : 3.7. A prominent signal was also observed at 35 amu (Cl’) but its intensity could not be reliably estimated

150

J.S. Foord, R.M. Lumhert / Chlorine chemisorption

on Cr(lO0)

since a large background Cl+ signal built up in the spectrometer due to ESD effects within the ion source. Previous workers [28] have shown that both the Cr + and CrCl + signals in the mass spectrum of gaseous chromium dichloride result from the fragmentation of CrCl,. We can therefore use the above intensity ratio for the Cr + , CrCl+ and CrClc signals as a fingerprint for the detection of pure CrCl *(g). Since this ratio is very close to the 1: 2 : 3.5 ratio observed for the signal intensities of the desorption products from the (Y binding state on Cr(lOO), it may be concluded that CrCl, is the only chloride which desorbs from this state. The Cr + and CrCl + signals thus result solely from the fragmentation of CrCl, in the ion source. Some desorption of chlorine atoms from the OLstate cannot be ruled out, but it seems likely that the Cl + signal also arises from the fragmentation of CrCl,. This is supported by the fact that the CrCl: : Cl+ signal ratio remains constant during the desorption sweep up to 900 K, for all initial chlorine coverages. Above 1000 K, where desorption of the j3 state occurs, the CrCl: desorption trace tails off, whereas Cl+ , Cr + and CrCl+ desorption profiles exhibit the /3 peak. This suggests that desorption from j3 state occurs as CrCl. Estimates of the desorption activation energies for the two binding states were made by constructing Arrhenius plots of the leading edges of the desorption profiles near take-off, where the surface coverage remains relatively constant. The activation energy for desorption from the (Ystate remains essentially constant at 210 ? 15 kJ mol- ‘, whereas the corresponding value for the p state varies from 340 k 20 kJ mol-’ in the zero coverage limit down to 300 k 20 kJ mol - ’ at the coverage produced by a chlorine exposure of 3.1 X lOI* mp2. The behaviour of the chlorine-dosed surface during thermal desorption was also examined using AES, A+ and LEED techniques. Little change occurred in A+ or the Cl Auger signal until the specimen was heated above 700 K, whereupon both quantities started to decrease smoothly down to zero. If surfaces exhibiting the LEED pattern shown in fig. 4f were warmed to the a! state desorption temperature, p(2 X 5) domains began to appear and eventually they completely replaced the higher coverage structure. Further heating caused the p(2 X 5) pattern to revert to (1 X 1) via its c(2 X 4) and c(2 X 2) precursors. The c(2 X 4) and lower coverage structures were found to be stable up to the j3 desorption temperature, but this was not always the case for the p(2 X 5) adlayer. If chlorine chemisorption was halted at the first appearance of the p(2 X 5) pattern, subsequent annealing at 550 K caused a reversion to the c(2 X 4) pattern. At this annealing temperature, no thermal desorption occurs and the chlorine Auger signal remains constant. Further exposure to chlorine then caused the p(2 X 5) structure to reappear. It was possible to carry out the p(2 X 5) c-) c(2 X 4) conversion cycle several times by alternately annealing the specimen at 550 K and exposing it to chlorine. However, the p(2 X 5) structure eventually became dominant at 550 K, and further heating to the thermal desorption temperature was necessary, before it disappeared.

The WPS, XPS and thermal desorption experiments indicate that only one distinct chlorine binding state is populated on Cr(lOO), during gas exposures of less than 84 X fOi8 rn-‘_ fn tine with the c.onclusions of other authors who

have studied halogen adsorption on the hoc metals W, Fe and V f7-9, M- f6] we take the view that adsorption into this state resufrs in the formaticm of a single b~~uge~ adiayer on the metal sttrface. The large increase in work function (A+ = + 1.4 ev) which a~~~rnpa~~es this process is most ~~~s~ste~t with the adsorbate existing as a simple electronegative overgayer, rathf~r than a re~o~~t~~~ted corrosion layer. F~~th~~ore, the dissociative nature of the chlorine present on the surface is clear from the LEED observations, which are discussed below. Thus, at low expclsure~~ chemisorbed chlorine efiibits a surface chemistry which is typical of mast halogen-transition metal adsa.rption systems, LEED patterns similar to those observed in this work during chlorine overlayer formation have also been seen in halogen adsorption studies on Fe( loci] f71 and Wf 100) f 143. They can be thought tu originate from a chlorine overlayer mesh given by b $=

x

I -I

cfI?#ailA 4E1 cotanA I a2f

x 6 ~~.~r 6;. I -2

tanA5a;. tanA i a;_

and +, a2 are vectors which respeetivefy define the ~d~~o~ md substrate &it meshes, and the asterisk superscript denotes the ~orres~o~di~~ vector af the reciprocal mesh. A is & variable parameter which is defined in fig. 7, where the real and reciprocal lattices are illustrated. The surEace coverage, 8, measured relative to the number of atoms in the Cr(lO0) surface (1.19 X 10’” m-“) is equal to OS tan A. As A increases, the LEED pattern co~t~~u~~sl~ changes from ~(2 X 2) (A = 45.0”, 8 = OS) to ~(2 X 4) (A = S6,3Q, 8 = 0,751 and finally to p(2 X 5) (A = 58.0”), @= 0.80). ff the interpretation of the LEED patterns is correct, there should be some correlation between the relative surface coverages deduced by LEED and by AES. The amplitude of the chlorine Auger signal was de~e~~~ for each of the c@ X 21, ~((2X 4) and p(2 X 5) LEED patterns and the results are indicated in the nptake curve in fig, 3. It can be seen that the AES signal ratios are in good a~eerne~~ with the relative coverages fO.50:0,75 : 0.80) of the three structures concerned, The uptake curve in fig. 3 shows that chlorine adsorption follows the usual precursor kinetics and the initially constant sticking probability (So) may be estimated by calibration of the relative surface coverage, as determined by AES with the absolute surface coverage as determined by LEED. S, is found to be Here b,, 6,

0.9 2 02 if the scattering unit in the overlayer is a &for&e atom; if ir is a chlorine m&de, S, is rsf course twice as great. Thus, it may be conch~ded that ebiorfne adsorbs ~~~s~~~a~~~e~~ on Ca(loa) with a sticking pro~a~il~t~ ~Ioss to llriity,

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Fig. 7. (a) Adlayer reciprocal lattice. fb) Adlayer direct lattice; small circles refer to the nverlaycr. shaded circles denote Cl atoms, iarge circles to the substrate. fc) Chlorine overlayer structures; open circles denote underlying chromium atoms. (d) ~(2 X 2) antiphase domains: BR’= antiphase boundary: black circles denote chromium atoms, open circles denote CI atoms. (e) Reciprocal lattice of two domains of C&I,; mm’=twin plane; squares refer to the substrate lattice. open and blackened hexagons to the two halide domains.

The real space c(2 X 2), ~(2 X 4) and p(2 X 5) structures are shown in fig. 7~; it is apparent that compression of the overlayer along a single direction, 010 or 001 depending on domain orientation, occurs as the surface coverage increases.

J.S.

Food,

R. M. Lomhert

/ Chlorine chemisorption

on Cr(lO0)

153

The 3.41 A Cl-Cl spacing which occurs in the saturated adlayer in some 0.20 A less than the Van der Waals diameter of chlorine; this appears to be the smallest nearest neighbour separation reported so far for atomically adsorbed chlorine (see for comparison refs. [ 1,5,7,10,11]. It must be pointed out that a radically different model to the one favoured here, is capable of explaining the observed LEED patterns. It is known that patterns such as the one shown in fig. 4b are generated by c(2 X 2) antiphase domains [29]. If the antiphase boundaries had the structure shown in fig. 7d, increases in the chlorine coverage could be accommodated by concomitant increases in the number of the spacing such boundaries present on the surface. Indeed, by reducing between the antiphase boundaries from a large value to twice the Cr( 100) lattice parameter. the LEED pattern would change from c(2 X 2) to c(2 X 4) and the surface coverage would increase by 50%. This is in good agreement with the experimental results. The c(2 X 4) structure which is generated in such a way consists of ordered vacancies in a (1 X 1) array of Cl atoms and it is clear that the p(2 X 5) pattern could be generated in a similar manner. The proposed structures are essentially those predicted by the “high-symmetry” model of Huber et al. (30,311. We reject them here to the following reasons. Firstly, they require a Cl-Cl spacing of 2.89A, which is far smaller than any nearest neighbour separation of adsorbed chlorine which has been observed previously. Secondly, the absence of a sharp c(2 X 2) pattern most probably implies that the adatoms do not have the strong preference for the four-fold adsorption sites which is necessary for the “high-symmetry” mode to be physically reasonable. In contrast, our preferred interpretation in terms of uniformly compressed overlayers does not suffer from either of these defects. It is interesting to note that the p(2 X 5) adlayer produced by chlorine adsorption at 300 K reconverted to a c(2 X 4) structure when the crystal was heated to temperatures below those at which thermal desorption occurs. Although this would seem to suggest that chlorine was lost from the surface during the annealing procedure, AES showed that it was not. The most likely explanation for this behaviour is as follows. Moleculer chlorine rapidly chemisorbs into the required two adsorption sites only if they are in close juxtaposition. Thus isolated vacancies will be present in the p(2 X 5) overlayer, since they are kinetically inert. Annealing the specimen causes the defected p(2 X 5) overlayer to relax to the less dense c(2 X 4) phase with the elimination of these vacancies. If such a hypothesis is correct, the p(2 X 5) structure should become thermally stable once all the vacancies have been removed. This is exactly what was observed experimentally; after repeating a cycle in which the specimen was heated to produce a c(2 X 4) adlayer, subsequent annealing did not bring about the p(2 X 5) -+ c(2 X 4) conversion. It should be noticed that the antiphase domain model cannot account for these structural transformations in a natural way. Desorption of the chlorine overlayer on Cr(100) was found to occur in the form of CrCl with an activation energy which varied between 300 and 340 kJ

mol - ‘, depending on surface coverage. It is of interest to inquire why CrCl rather than Cl, Cl, or CrCl, is the favoured product. The differences between the desorption energies of the relevant species may be readily deduced from Born-Haber cycles. If E, denotes the desorption energy of species X and L&, denotes the bond energy of gaseous CrCl, consideration of a simple thermochemical cycle shows that (1)

Ec, - Ec,c* = %Cl - EC,* The relative deso~tion &Cd&

=

ratio, R,,

hCdd

of CrCl and Cl can be written in the form:

ev(A&l/RT~~

(2)

where vcFcI, vc, are pre-exponentials to desorption and AE,, is the difference between the desorption activation energies of Cl and CrCl. Provided the adsorption of both Cl and CrCl is non-activated, AE,, = E,, - Ii,,,. The gas phase dissociation energy of ‘CrCl, A.,,,, is 366 kJ mol-’ [32] and if E,, is taken to be the sublimation energy of chromium, E,, (396 kJ mol - ’ [33]), A E,, as calculated from eq. (1) equals - 30 kJ mol -I. The use of this value for AEC, in eq. (2) would suggest that desorption should occur as Cl rather than the observed CrCl, if ~~,-r = v~,. This disagreement with the experimental results implies either that the deduced value of AE,, is incorrect or that vcK, > v~,. There are two possible sources of error in the de~vation of AEC,. Firstly, it was assumed that the desorption activation energies could be equated with the desorption energies. This is likely to be true in the case of Cl, and any discrepancy in the case of CrCl would only favour the desorption of Cl and make the disagreement with the experiment even worse. Secondly it was assumed that E,, could be equated with E,%Jb.E,, is strictly the binding energy of a Cr atom in the chlorided surface; EC, could therefore be less then Ezb and this would favour the desorption of CrCl as opposed to Cl. However we do not believe that this explanation applies here because as f?,, + 0, EC, --, Ecb and Cl desorption should be observed at low coverages. We thus conclude that desorption does not occur as Cl atoms because the production of CrCl is energetically more favourable, but rather because +rCi > z+-.,.Using the derived value of AEC, in eq. (2) together with a conservative estimate for the relative sensitivity towards Cl and CrCl desorption leads to a lower bound for the quantity v~~~/z+,. The result is that undetectable Cl desorption implies that C= magnitude of this quantity suggests that the ‘v 100. The numerical VCKYl /%I enhanced desorption rate of CrCl could be due to the excitation of rotational degrees of freedom in the transition state (in fact the classical partition function for a CrCl rotator confined to a plane is of order 100 at the desorption temperature). Born-Haber cycles can easily be constructed to relate ECrC., ECrCl, and Ec12. These give: E CrCl,

-

43Cl

=

&rC,

+

=b-C,

Ecr, - EC,CI = Ec,, + 2&i

-

L)crC,,

-

Ee,,

- %, - 2Ec,.

(3) (4)

J.S. Foord, R.M.

Lrrmbert /

Chlorine chemisorption on Cr(IO0)

155

In these equations, Dcro2 and Dclz, the atomisation energies of CrCl,(g) and Cl,(g), respectively, equal 710 and 242 kJ mol - ’ [33]. In the zero coverage limit, where EC, approximately equals the sublimation energy of chromium, E CrClz - JfLCl and &I, - J%C, have values of -40 and +40 kJ mol- ‘. Thus, the failure to detect any Cl, desorption is understandable in view of the latter result. The absence of CrCl,’ in the desorption products could originate in either or both of two possible causes. Firstly, it seems likely that the necessary formation of adsorbed CrCl, units from the chlorine overlayer, prior to CrCl, desorption, will result in C&l,(g) having a desorption acfj~afion energy greater than its desorption energy. Support for this proposal comes from a closely related study of bromine adsorption on V(lO0) [16], where it was found that conversion of the bromine overlayer to a layer of vanadium dibromide occurred with a significant activation energy. Secondly, any mobility in the chlorine overlayer at the desorption temperature would result in the preexponential for CrCI, desorption being smaller than that for CrCl desorption. Mobility in chlorine overlayers has been reported on germanium [34], silver [35] and rhodium [17]. The desorption of CrCl in preference to CrCl, is therefore consistent with the known properties of haiogen overlayers as deduced from previous studies. 5.2. Halide growth

Saturation of a single overlayer binding state occurs, as discussed above, during chlorine exposures of 8.0 X lOI m-*. After higher gas exposures, the thermal desorption measurements showed that chlorine is also present in a more weakly bound (a) state. This second state in the desorption profiles could arise either from further adsorption into the chlorine overlayer or from the nucleation and growth of a bulk chromium chloride at the metal-halogen interface. The results firmly indicate that it is the latter process which occurs in this case. Firstly, the (Y state could never be saturated with chlorine and it completely dominated the desorption profile after high gas doses. This behaviour is clearly ~consistent with chlorine adso~tion into a single adlayer but it follows naturally if a bulk halide corrosion phase forms at the surface. Secondly, the LEED pattern formed at high chlorine coverages (fig. 4f) shows that the elastically back-scattered electrons do not sample the Cr( 100) periodicity at such coverages. This implies that a phase with a thickness considerably in excess of that of a single adlayer must form on top of the chromium substrate. Thirdly, the XPS results indicate that a second bulk phase forms over the chromium substrate during high chlorine exposures. Thus the various experimental techniques are in good agreement that corrosion of the chromium substrate occurs after saturation of the chlorine overlayer. It was notable that the electron probes which measure area-averaged properties of the exposed surface suggested that little change occurred at the metal-gas interface when the chlorine exposure increased from 8.0 X lOI m-* to more than an order of

156

J.S. Foord. R. M. I.omhert

/ Chlorine

chentmwpkm

ON Cr(lO0)

magnitude higher, whereas the thermal desorption profiles, which measure the total amount of chlorine adsorbed, changed rapidly in this exposure regime. This implies that halide growth follows the Volmer-Weber mechanism, whereby the halide forms in three-dimensional crystallites and large areas of the surface are left unperturbed. Such a situation could be observed directly by LEED after gas exposures between 8.0 X 102’ and 1.6 X 102’ rnp2, where co-existing domains of bulk halide and the p(2 X 5) chlorine overlayer were seen. The stoichiometry of the halide phase is of particular interest since two stable chlorides, CrCl, and CrCl,, are known to exist [33]; it may be recalled that desorption from the (Ystate occurred solely as CrCl,. Now, both the dichloride and the trichloride sublime to yield vapours which predominantly consist of the parent molecular species, i.e. CrCl,(g) and CrCl,(g) respectively [33]. Thus the observation that CrCl, is the sole desorption product of the (II state provides a good indication that it is the dichloride which forms on the chromium surface. This conclusion is supported by the XPS measurements which showed that the 2p binding energy of chromium species in the halide phase was 1.5 eV higher than that of Cr atoms in the underlying substrate. On the other hand, the corresponding chemical shift between chromium and a number of Cr”’ compounds is some l- 1.5 eV greater than this [36,37]. Thus, we adopt the view that our observed binding energy shift arises from the epitaxial growth of CrCl, rather than CrCl,. This might seem unexpected since it is known from bulk chemistry that the direct interaction of the elements normally forms CrCl,. The reason for the production of the dichloride here would seem to lie in the low chlorine pressures that were employed in this work (typically 10M5 Pa in the chlorine beam). Although an examination of the thermodynamic data [33] shows that CrCl, is unstable with respect to CrCl, at such chlorine pressures, it is also the case that the reaction of the higher chloride with excess metal is even more strongly favoured under the same conditions 2 Cr(s) + 2 CrCl,(s)

-f 3 CrCl,(s),

hG,,,

= - 109 kJ mall’,

Thus, while CrCl, does not spontaneously decompose into CrCl, under our reaction conditions, it does react exoergically with free chromium to form the dihalide. Two kinetic factors can control which halide is actually formed: (i) the impinging flux of chlorine onto the surface, (ii) the diffusion rate of species between the halide and the underlying chromium substrate. If (i) is rapid (high PC,,) and (ii) is slow, the nichloride will be kinetically favoured. Alternatively if (i) is slow (low PC,,) and (ii) is rapid, CrCl, will be the preferred product. Hence a rapid diffusion of chromium and/or chlorine species between the halide and the bulk metal, when compared to the mass transfer rate of chlorine from the gas phase onto the surface, can explain the formation of the observed product.

J.S. Food,

R. M. Lambert

/

Chlorine

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157

5.3. Photoemission data: further considerations Although UPS and Cl(2p) core-level photoemission measurements are most directly useful in showing the existence of two distinct chlorine adsorption states, it is worthwhile to consider these results in more detail in order to establish whether these observations are consistent with bulk halide formation. The 2p binding energy of chlorine adsorbed in the (Ystate is 0.7 eV higher than the corresponding value for the p binding state. Calculations (e.g. ref. [38]) reveal that the transition Cl (free atom) + Cl (ionic lattice) typically induces a chemical shift of -4.5 eV in the Cl(2p) binding energy. Three principal factors will cause our chemical shift to differ from this value: (i) extra-atomic final state relaxation energies in the Cl overlayer will serve to increase the -4.5 eV shift by - +3 eV [39], (ii) the difference between the electrostatic potential of the Cl overlayer adsorption site and the vacuum level could add uncertainties of the same order as the dipole part of the work function (a few ev> [40], (iii) metal + adsorbate change transfer in the initial state of adsorbed chlorine (as revealed by A+ measurements) will cause the chemical shift to be even more positive. It is clear that the magnitudes of these quantities are sufficiently large as to make our chemical shift observation consistent with the transition from overlayer to halide. We now consider the UPS data (fig. 5~). The strong clean surface Cr(3d) emission just below E, is extremely sensitive to the presence of electronegative adsorbates and it thus undergoes considerable attenuation during the formation of the chlorine overlayer. Simultaneously a prominent peak develops at -6.0 eV which may be simply interpreted as arising from Cl(3p) levels in the chlorine adlayer. Interesting changes occur when the (Y state populates on the surface: the 3d emission just below E, undergoes further attenuation and peaks at -3.5 and -7.5 eV appear in the spectra. Remarkably, the observed spectra bear a close resemblance to the corresponding gas phase spectra of CrCl, [41] both in the relative positioning of the three adsorbate ii&&d bands and, especially in the relative intensity variation which occurs in moving from He I to He II excitation. It thus seems likely that the peaks at - 3.5 and - 7.5 eV can be thought to arise from emission from the ligand field split Cr ‘+ d orbitals and the -6.0 eV peak from ionisation of the chlorine 3p orbitals, as in the gas phase spectra. We may thus conclude that the UPS measurements are in agreement with the proposed overall reaction scheme. 5.4. LEED data: further considerations The structure of the halide layer and its epitaxial relations with the underlying chromium substrate may be deduced from the LEED observations. The LEED pattern observed when the surface is covered with chloride indexes, as we have stated earlier, in terms of four domains of a pseudo-hexagonal mesh with lattice parameters equal to 3.5 and 3.9 A and an included angle of 117”. A

158

J.S.

Foord, R.M.

Lwnhert

/ Chlorine chenkwptiorz

OS CrfioO)

computerised fitting routine failed to match this unit mesh to a plane of the normal rhombohedral form [33] of bulk chromium dichloride. However, much more common crystal structures for the halides of the first row transition metals are the CdCl, and Cd!, layer structures in which planes of cations are located between alternate layers of cubic or hexagonal close-packed anions [42]. Indeed, the dibromides and dichlorides of V, Mn, Fe, Co all have such a structure with the corresponding hexagonal unit cells having a lattice parameter 4 in the range 3.5-3.7 A 1421.The growth of CrCl, in such a structure with the layers lying parallel to the (100) surface seems to be the only sensible way of explaining the LEED pattern which we observe. Good support for such a model comes from the work of Vigner et al. [43] in which a thin film hexagonal form of CrCl, was also reported. The orientations of the four domains that form on the surface bear an interesting relationship to one another (fig. 7e). The two domains that we have illustrated in fig. 7e generate the two remaining meshes, when reflected in the (010) mirror plane as expected. However, the symmetry related domains that would be generated by reflection in the (001) mirror plane do not appear in the LEED pattern. This implies that the actual crystal surface does not possess the full four-fold symmetry of a bee (100) plane. Most obviously this would arise if steps arranged symmet~cally with respect to 010 were present on the surface. Such a lowering of the four-fold sy~et~ was nor apparent in LEED observations of the clean surface nor of the chlorine overlayer so it seems unlikely that the step density is particularly high. This in turn suggests that the steps that are present have a long range influence on the orientation of the epitaxial layer. We tentatively propose that this arises because the halide nucleates at the steps and then grows out onto the terraces. Although reflection in the (010) mirror plane transforms one pair of the four observed domains (the pair shown in fig. 7e) into the second pair, no element of symmetry is present in the crystal by which the domains in each pair can be related. Densely-packed planes in the pseudo-hexagonal layers lie along the reciprocal lattice vectors shown in fig. 7e. It can be seen from this figure that the two domains have in common a vector and hence a lattice plane along 023. In addition, reflection in this close-packed lattice plane transforms the two domains into one another, even though it is not a plane of mirror symmetry in the Cr crystal. Such a situation is well known in crystallography: the two domains are said to be reflection twins. This most probably arises because of small distortions in the halide film which removes the hexagonal symmetry. Because of these distortions, the (032) plane is no longer a mirror plane in the halide structure and thus an (032) reflection generates a slightly misaligned second domain. The important remaining question concerns the rate of growth of the halide layer. This can best be estimated by XPS, using the relative areas of the Cr(2p) emission peaks from the substrate and the chromium dichloride film as a measure of the amount of halide present on the surface. This ratio, measured at

J.S. Foord, R. M. Lumbert / Chlorine chemisorpkm

on Cr(IOO)

159

an emission angle of 60”, was approximately equal to 1.4 after a chlorine exposure of 2.0 X 102’ m -2. We assume that the halide film is of a uniform thickness at such coverages and indeed LEED showed that the entire surface was covered with halide at this stage. Using the halide structure that we have proposed, the ratio of the densities of metal atoms in the metal and the halide is equal to 2.9. The mean free path of Cr(2p) electrons equals 12 A in Cr and Cr,O, f37J and we may approximate the mean free path in CrCl, to this value. Within these appro~mations and using formulae developed previously [44], the thickness of the hahde film after a chlorine exposure of 2.0 X 10zl mm2 can be shown to be 16A. This implies that the average sticking probability during halide growth up to this film thickness is - 0.02. Thus the corrosion of the substrate proceeds much more slowly than the formation of the chemisorbed overlayer.

Acknowledgements We are grateful to Johnson Matthey Limited for the loan of precious metals. J.S.F. thanks the SRC and Selwyn College, Cambridge for financial support.

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on CrflOO)

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JCS