Langmuir 1987,3, 1015-1025 method is preferable because it consistently fits the data better. The values of Y~obtained by either method are in good agreement with those obtained by other independent methods, whereas the critical surface tension yCis always lower.
1015
Acknowledgment. I acknowledge valuable discussions with Dr. I. D. Morrison. Registry No. PE, 9002-88-4; PTFE, 9002-84-0; P3FE, 24980-67-4;PVDF, 24937-79-9;PVF, 24981-14-4; PVDC, 900285-1;PVC, 9002-86-2;PS, 9003-53-6;PMMA, 9011-14-7;PCTFE, 9002-83-9; PA66, 32131-17-2; PET, 25038-59-9.
Adsorption of Boron on Molybdenum(100) and Its Effect on Chemisorption of Carbon Monoxide, Ethene, Propene, and 3,3,3-Trifluoropropene T. B. Fryberger,? J. L. Grant,i and P. C. Stair*l The Ipatieff Laboratory and Department of Chemistry, Northwestern University, Evanston, Illinois 60201 Received February 10,1987. In Final Form: May 11, 1987 The interaction of boron with Mo(100) and its effect on surface reactivity have been investigated for the first time. Boron-covered surfaces can be prepared by adsorption and decomposition of diborane (BZH6) at 300 K. The B(1s) binding energy (BE) measured by XPS increases approximately linearly with boron coverage from 186.9 eV at 0.2 monolayer to 187.6 eV at 1.1monolayers (saturation coverage),indicative of coverage-dependentB-B interactions. The absence of multiple B(1s) peaks or peak broadening suggests the overlayer grows by uniformly filling the available surface sites. After annealing to 1073 K and above, two distinct phases are formed: a low-coveragephase (-0.2 monolayer) with B(ls) BE = 186.8 eV and a three-dimensional, B-rich, surface "boride" phase (MOB,)with B(1s) BE = 188.2 eV. The assignment of the latter phase is established by comparison of the B(ls) BE with the literature value for MoBz. The B(1s) signal disappears upon heating above 1700 K, indicating loss of surface boron. Adsorption of carbon monoxide, ethene, propene, and 3,3,34rifluoropropene on B-covered surfaces was studied. Comparison of the results with data for adsorption of the same molecules on carbon- and oxygen-modified Mo(100) suggests that the B adatoms are not located in 4-fold hollow sites on the surface. 1. Introduction There is considerable interest in understanding the chemical modification of transition-metal surfaces by foreign adsorbate atoms. Surface modifiers receiving attention have primarily been either highly electronegative (e.g., C, 0, S ) or electropositive (alkali metals) elements compared to transition metals. Boron, however, has a Pauling electronegativity (2.0) which is comparable to that of transition metals. In addition, boron displays an unusual tendency toward forming B-B bonds. For example, in transition-metal borides, one-, two-, and three-dimensional boron networks are progressively formed with increasing boron content.' These properties suggest that the interaction of boron with a transition-metal surface may be quite distinct from the other chemical modifiers alluded to above and that the chemical interaction of the resulting surface with adsorbate molecules may be unusual. The present paper presents the results of X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements of the interaction of boron with a Mo(100) surface and of the adsorption of CO and olefins on Bmodified Mo(100). Thermal desorption spectroscopy (TDS) measurements were also performed to confirm some of the conclusions from the electron spectroscopy data. Section 3.1 describes the preparation of B-modified Mot Present address: National Bureau of Standards, Gaithersburg, Maryland 20899. t Present address: 3M Corporation, St. Paul, Minnesota 55144-
1000.
Aldred P. Sloan Foundation Fellow 1984-1988.
0743-7463/87/2403-1015$01.50/0
(100) surfaces by adsorption and decomposition of diborane @&I6). From XPS, UPS, and TDS data on diborane adsorption at low temperatures, it is shown that B2H6completely decomposes on the surface below room temperature, and Hzdesorbs in the temperature range 280-400 K. In section 3.2, the behavior of adsorbed boron as a function of coverage at room temperature is discussed. Measured B(ls) and B(2p) binding energy shifts suggest that interactions between boron adatoms become important with increasing coverage up to a saturation value of 1.1monolayers (1.0 monolayer is taken to be 1.0 X 1015/ cm2,which is equal to the Mo surface atom density). The effects of stepwise heating of B/Mo(100) surfaces are described in section 3.3. At boron coverages above 0.5 monolayer a phase transition occurs upon annealing to 1073K. At this temperature, the coexistence of two phases is observed: a three-dimensional "boride" phase with an approximate stoichiometry MoBz and the low-coverage (0.2 monolayer) chemisorbed phase. The two phases persist for annealing temperatures up to about 1600 K, where the boride phase is no longer detectable by XPS. Finally, after annealing above 2000 K no boron remains on the surface. Results are presented in section 4 for the adsorption of carbon monoxide, ethene, propene, and 3,3,3-trifluoropropene on the boron-modified Mo(100) surface. These molecules were chosen primarily because they have been studied on C- and 0-modified M O ( ~ O Oand, ) ~ ~ hence, ~ (1)Kiessling, R. Acta Chem. Scand. 1950, 4, 209. (2) Deffeyes, J. E.; Horlacher Smith, A.; Stair, P. C. Appl. Surf. Sci. 1986, 26, 517.
0 1987 American Chemical Society
1016 Langmuir, Vol. 3, No. 6, 1987
Fryberger et al.
Table I. B(1s) Binding Energies and Widths (in eV) and Boron Atomic Coverages (0,) for Diborane on Mo(100) at Low TemDerature' low coverage saturation coverage T. K EaF fwhm OR. monolavers EaF fwhm OR. monolavers 100 187.79 (06) 1.95 (06) 0.21 187.71 (05) 1.71 (04) 0.61 189.65 (02) 1.73 (02) 1.43 150 187.45 (29) 1.76 (14) 0.46 189.15 (37) 2.23 (37) 0.70 200 187.13 (06) 1.69 (06) 0.20 187.09 (07) 1.34 (05) 0.37 188.32 (23) 2.32 (17) 0.60 250 187.48 (03) 0.96 1.64 (03) 300 186.93 (03) 1.29 (03) 0.20 187.45 (02) 1.55 (02) 1.13 Numbers in parentheses denote uncertainties in Gaussian fit to data. Coverages were calculated from B(ls)/Mo(3d5,J peak area ratios.
provide a basis for comparing boron with more electronegative modifiers. All of these molecules dissociate on clean Mo(100)at room temperature or below. Preadsorbed boron inhibits molecular CO adsorption, and the amount of adsorption decreases with increasing boron coverage. On the other hand, adsorption and dissociation of ethene, propene, and trifluoropropene are not inhibited by boron. This contrasts with adsorption of these molecules on Cand 0-modified surfaces where (with the exception of a small amount of 3,3,3-trifluoropropeneon C-modified Mo) no dissociation is observed.
I
I
I
2. Experimental Section A modified AEI ES200 photoelectron spectrometer pumped by a turbomolecular pump with an operating base pressure of 2 X Torr was used for XPS and UPS measurements. The UHV chamber is equipped with a Varian quadrupole mass spectrometer, a Physical Electronics 1-kV ion gun for sputter cleaning, an ion and cryogenic sorption pumped gas introduction facility, and a hemispherical eledron energy analyzer. The sample holder4 is capable of precise positioning in UHV as well as rapid heating to 2000 K by electron bombardment and cooling to 85 K with liquid nitrogen. The sample temperature was monitored by a 5-2670 tungsten-rhenium thermocouple. Unmonochromatized radiation, provided by either an Al(1486.6 eV) or M g (1253.6 eV) target, was used for X P S . He I radiation (21.2 eV), provided by a differentially pumped helium discharge lamp (Vactronic), was used for UPS. During lamp operation, the helium pressure rose to about lo4 Torr. Data were collected on a multichannel analyzer (Tracor Northern) and subsequently transferred to an Apple 11+ computer for storage and analysis. XPS spectra were fit to a Gaussian form with a linear background by an iterative least-squarea p r o ~ e d u r e . ~Uncertainties reported for peak positions, widths (fwhm), and heights are calculated by the fitting program and characterize the goodness of fit or deviation (by 1standard deviation) of the experimental distribution from the assumed distribution. The experimental uncertainties found by repeating measurements are somewhat larger than the calculated values: about i0.05 eV for adsorbate peak energies and widths and *lo% for heights. Binding energies were referenced to the M0(3d,/~)core level at 227.5 eV measured from the clean sample. The Man grade molybdenum single crystal (MaterialsResearch Corp.) was oriented in the (100) direction and cut and polished by standard X-ray and metallographic techniques. Bulk contaminants, mainly carbon, were removed from the polished crystal by heating a t 1300 K in 5 X lo-' Torr of oxygen for 48 h. The sample was then mounted on the sample manipulator and placed inside the UHV chamber. Once in UHV, final cleaning of the surface was performed by heating in 5 X lo4 Torr of oxygen for several hours to remove residual carbon, followed by flashing the sample to 2000 K several times to remove oxygen. This procedure (3) KO,E. I.; Madix, R. J. Surf. Sci. 1981, 109, 221. (4) Fryberger, T. B. Ph.D. Dissertation, Northwestern University,
1986.
( 5 ) Burgess, D. R. F. Ph.D. Dissertation, Northweatern Unversity,
1985.
191 189 187 185 BINDING ENERGY (eV)
Figure 1. AI K a excited B(1s) spectra as a function of annealing temperature following saturation adsorption of diborane at 100 K. The vertical bar next to each spectrum corresponds to a count rate of 50 Hz at 100 K, 15 Hz at 200 K, 35 Hz at 250 K, and 40 Hz at 300 K. was repeated until no contaminants could be detected by XPS (detection limit 0.05 monolayer). The B/Mo(100) surfaces were prepared by exposing the clean sample to a dopant mixture containing 1TOdiborane (BzHe) in Argon (Matheson). Diborane exposures, reported as langmuirs (1langmuir = lo4 Torr-s), take account of this dilution factor. A total pressure of 3 or 6 X lo-' Torr was typically used during exposures. Adsorbate fractional coverages were determined with C(ls), O(ls), or B(1s) peak areas (from Gaussian parameters) by calibration against a Mo(100) surface saturated with dissociated carbon monoxide. This surface consists of 0.5 monolayer each of adsorbed C and 0 atoms? The ratio of the experimental cross sections1 for adsorbed boron and oxygen was determined by measuring the B(ls)/O(ls) peak area ratio for B(OCH)3condensed on the surface a t 100 K.* The relative coverages are accurate to within 0.02 monolayer, and the absolute coverages are good to f0.15 monolayer.
3. Adsorption and Decomposition of Diborane 3.1. Low-Temperature Adsorption and Decomposition. Results. The adsorption and decomposition of (6) Felter, T. E.; Eatrup, P. J. Surf. Sci. 1976, 54, 197; 1978, 76, 464. (7) Brundle, C. R. J.Electron Spectrosc. Relat. Phenom. 1974,5, 291.
Langmuir, Vol. 3, No. 6, 1987 1017
Adsorption of Boron on Mo(100)
1
a . W 11
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100
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300
500
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Figure 2. UPS spectra for saturation adsorption of diborane at 100 K followed by annealing at 150,250, and 300 K. diborane below room temperature were investigated by
XPS,UPS, and TDS.s For photoemission measurements the clean sample was exposed to diborane at 100 K, followed by stepwise heating of the surface. After the sample was held a t the desired temperature for 60 s the B(1s) or valence level spectrum was recorded as the surface cooled back to 100 K. The XPS data for both a low boron coverage (OB = 0.2 monolayer) and a high-coverage (saturation) surface are summarized in Table I. In Figure 1,B(1s) spectra are shown for the surface satyated with BzH6at 100 K and after annealing to 200, 250, and 300 K. The spectrum a t 100 K is clearly composed of two peaks whose total area corresponds to 2.0 monolayers of boron: 0.6 monolayer at 187.7 eV and 1.4 monolayer at 189.6 eV. Upon annealing of this surface to 200 K, the high binding energy peak is attenuated while the area under the low binding energy peak has not changed appreciably. In addition, both peaks have shifted to lower binding energy (see Table I). After annealing to 250 K, the high binding energy peak is completely gone, and the area under the low binding energy peak has increased to a value correspondingto 1.0 monolayer of boron. Finally, between 250 and 300 K only small changes are observed in the B(1s) binding energy and fwhm. UPS spectra for the surface saturated with diborane at 100 K and after subsequent annealing at 150,250, and 300 K are shown in Figure 2. At 100 K, broad, adsorbateinduced peaks are seen at 5.0, 6.5, and 8.5 eV below the Fermi level accompanied by attenuation of the Mo(4d) band a t 2.5 eV. As the sample is heated, the intensity of the feature at 5.0-eV binding energy increases while the other peaks decrease. After annealing to 250 or 300 K,the spectra exhibit only the peak at 5.0 eV and a broad band of emission at lower binding energy. The latter emission is attributed to adsorption of residual gas during the long time required for data acquisition. In addition, the small peak near 11.0 eV in the 250 and 300 K spectra is due to -
(8)The TDS experiments were performed in a second U H V chamber by Anne H. Smith and Joan Deffeyes. This chamber is equipped for line-of-sight, computer-controlled TDS experiments. Details of this chamber can be found in reference 2. Although these experiments were performed on a different Mo(100) crystal, the cutting, polishing, and cleaning techniques were identical with those used in the photoemission experiments.
(r, 100
, , , , , , , , , , , , , 300
500
7 00
,
,
, , , , , 900 1lo(
Tup.raturs (I()
Figure 3. Thermal desorption spectra for saturation exposure to diborane at 100 K: (a) 11m u (boron);(b) 2 amu (HZ)H .eating rate 12 K/s. increased secondary electron emission caused by adsorption. In order to further clarify the low-temperature behavior of BzHBon Mo( 100),TDS experimentss were carried out on the saturated surface. Figure 3a shows the mass 11 (boron) spectrum with a peak temperature (TJat 160 K, indicating that some boron-containing species (most likely B&) desorbs a t this temperature. Figure 3b shows the mass 2 (hydrogen) TDS spectrum with T = 280 K and a second small peak centered at 450 K. T i e evolution of molecular hydrogen indicates that diborane has decomposed at temperatures below 280 K. Discussion. The XPS data show that roughly half of the surface boron which resulta from a saturation coverage of diborane adsorbed a t 100 K is removed below 250 K. The boron remaining on the surface is derived from diborane decomposition in the same temperature range. The coexistence of both molecular and decomposed boroncontaining species on the saturated surface is indicated by the appearance of two B(ls) peaks below 150 K in the X P S spectra of Figure 1. The most likely mechanism for removal of boron is desorption of a boron-containing species. This is verified by the TDS spectrum of Figure 3a, which showb a mass 11 (boron) peak in the same temperature range where the high binding energy B(1s) peak was observed to disappear. Low-coverage B(1s) spectra (not shown; see Table I) exhibit only one peak, whose intensity remains constant in the temperature range 100-300 K. This suggests that at low coverage all of the diborane decomposes upon annealing and that all of the boron remains on the surface. The evidence for decomposition at low coverage is confirmed by high-temperature annealing studies discussed in section 3.3. The decomposition of adsorbed diborane with increasing anneal temperature is responsible for a reduction in the
1018 Langmuir, Vol. 3, No. 6, 1987 B(1s) binding energy and fwhm. These changes occur for both the low binding energy peak on the saturated surface and for the single peak on the low OB surface (see Table I). Similar adsorbate (1s) shifts with temperature have been reported for NH3 on W(l10),9 CO on Fe(lOO),1° and CO on M0(100).~The shifts were attributed to dissociation of the molecules. UPS and TDS results for saturation diborane coverage confirm that the B(1s) shifts are due to decomposition of the adsorbed molecule. At 100 K, it is possible that diborane either adsorbs molecularly or, since the molecule is very reactive, dissociates to BH3 on the surface." However, as seen in Figure 2, the UPS peak positions cannot be directly correlated with either the experimental BzH6gas-phase spectral2 or with the energy levels calculated for BH3,11both of which are shown at the top of the figure. This suggests that even at 100 K the adsorbed layer is a mixture of molecular fragments. When the surface is heated, all structure which could be attributed to molecular species is lost. By 250 K, only a broad peak at 5.0 eV is observed consistent with emission from atomic boron 2p derived states. Emission from adsorbed hydrogen atoms most likely occurs in the region of the Mo(4d) band emission and, therefore, cannot be di~tinguished.'~ The Hz desorption spectrum (Figure 3b) is further evidence for the decomposition of B2HP Since XPS and UPS spectra indicate that adsorbate decomposition occurs below 250 K (Figures 1 and 2), the H2 desorption peaks centered at 280 and 450 K appear to be desorption rate limited. In fact, the H2desorption spectrum is essentially identical with that observed for hydrogen adsorbed on clean Mo(~OO),~ which provides further evidence that diborane is completely dissociated to adsorbed B and H atoms at 250 K. 3.2 Room Temperature Adsorption and Decomposition. Results. Adsorption of diborane was studied as a function of exposure at room temperature. The growth of the B(1s) core level with increasing exposure is shown in Figure 4a. The peaks are symmetric a t all exposures and do not broaden significantly with increasing exposure. In addition, the B(1s) peak shifts to higher binding energy with increasing exposure. As seen in Figure 4b, where the B(1s) binding energy is plotted against boron coverage, the binding energy increases by 0.7 eV from 0.2- to 1.1-monolayer coverage. Boron coverage is plotted against exposure in Figure 4c, where it is seen that OB increases linearly up to about 4 langmuir and then levels off to a saturation coverage of 1.1 monolayer. Because of the arrangement of the leak valve, sample holder, and ionization gauge in the UHV chamber, the pressure at the surface during exposures is about 10 times greater than indicated by the ion gauge. Thus, only an approximate sticking probability for B2H6 on Mo(100) can be calculated. After multiplying the exposures shown in Figure 4c by 10, assuming that each diborane molecule occupies two adsorption sites, and using a value of 1 X 1015sites/cm2 for the Mo(100) surface, we obtain an estimated sticking probability of 0.032 for adsorption at 300 K. Figure 5 shows the UV spectra for 0.0,0.3, 0.6, and 1.1 monolayer of adsorbed boron. At low coverages of boron (0.3 monolayer), adsorbate-induced peaks appear a t (9) Grunze, M.; Brundle, C. R.; Tomanek, D. Surf. Sci. 1982,119, 133. (IO) Benziger, J.; Madix, R. J. Surf. Sci. 1980, 94, 119. (11) Muetterties, E. L. Boron Hydride Chemistry; Academic: New
York, 1975. (12) Brundle, C.R.;Robin, M. B.; Basch, H.; Pinsky, M.; Bond, A. J. A m . Chem. SOC.1970,92, 3863. (13) Feuerbacher, B.; Adriaens, M. R. Surf. Sci. 1974,45, 553.
Fryberger et al. a
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Figure 4. (a) Al Ka excited B(1s) core level spectra aa a function of diborane exposure at 300 K. 1 L = 1 langmur = lo* Toms. Background count level 400 counta/s. (b) B(1s) binding energy as a function of coverage (1 ML = 1 monolayer) for adsorption of diborane at 300 K. (c) Boron coverage (monolayers) vs exposure for adsorption of diborane at 300 K.
binding energies of 4.3 and 7.0 eV. As OB is increased, these two peaks shift to higher binding energies, 5.0 and 8.0 eV at OB = 1.1monolayer, and the Mo(4d) band at 2.5 eV is increasingly attenuated. From the change in the width of the valence bandI4 upon adsorption to 1.1 monolayer, a (14) Evans, S. In Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy; Briggs, D., Ed.; Heyden & Son: London, 1977; p 131.
Langmuir, Vol. 3, No. 6, 1987 1019
Adsorption of Boron on Mo(100)
ELECTRON ENERGY ( e V )
Figure 5. W spectra for diborane adsorbed at 300 K: - = 0.3, 0.6, and 1.1 monolayer; hv = 21.2 eV. ~
work function change (A4) of +0.30 (5) eV is found. Work function changes at low coverage were too small to be resolved with this technique. Discussion. The position and shapes of the XPS B(ls) and UPS valence peaks for adsorption of B2H6at 300 K are identical at saturation coverage with those shown in the previous section for adsorption at 100 K followed by annealing to 300 K. Thus, it is assumed that adsorption of B2H, at room temperature results in complete dissociation of the molecule and desorption of hydrogen as discussed above. The binding energy shift of the B(1s) level over the boron coverage range (0.2-1.1 monolayer) provides an empirical measure of the boron coverage. The shift arises from a combination of three factors.15 First, changes in the geometry of the adsorbate complex and in the density of neighboring adatoms alter the chemical environment. Second, the extra-atomic relaxation energy could change as a result of (1)an increase in the number of neighboring adatoms that contribute to relaxation or (2) a decrease in the surface polarizability with increasing adsorbate density as the surface region transforms from metal to metal boride. Third, increasing interactions between the negative ends of the adsorbate dipoles can lead to depolarization of adjacent dipoles and a lower effective charge per adsorbate atom. Unfortunately, separation of these three factors is generally not feasible. However, we believe that the shifts are primarily chemical in origin. Significant depolarization effects would be expected for adsorbate/substrate systems that exhibit a high degree of charge transfer (i.e., a large dipole moment) and hence a large coverage-dependent work function change. For example, with K on Fe(llO), coverage-dependent work function changes correlate with potassium core level shifts.16 However, the small work function change measured at saturation boron coverage indicates that the dipole moment produced by adsorbed boron is small. This is consistent with findings for transition-metal borides17and (15) Fuggb,J. C.;M e n d , D. Surf. Sci. 1975,53, 21. (16) Broden, G.;Bonzel, H. P. Surf. Sci. 1979,85, 106. (17) Joyner, D.J.; Willis, R. F. Philos. Mag., [Part] A 1981, 43, 815.
with the similar Pauling electronegativities of B (2.0) and Mo (1.8),'* which suggest that charge transfer between these elements is minimal. Hence, it is unlikely that depolarization effects are an important contribution to the B(1s) shift. A decrease in relaxation energy due to a decrease in the surface polarizability would produce an upward shift in the B(1s) binding energy. It is difficult to predict the magnitude of this effect since the electronic structure of metal borides is poorly underst00d.l~Strong chemical interactions between adsorbed boron atoms are a likely explanation for the large B(1s) shift with coverage. It is consistent with the tendency of boron to form boron-boron bonds in transition-metal borides as shown by the progressive formation of one-, two-, and three-dimensional boron networks with increasing boron concentration.' In spite of our inability to experimentally separate the factors responsible for the binding energy shifts, some conclusions can be drawn from the results without understanding the mechanism of the shifts in detail. For example, the data are not consistent with an island growth mechanism of monolayer formation. For an island growth mechanism the B(1s) peak would be composed of two overlapping peaks due to boron atoms located at the edges and in the interior of the i~1ands.l~At low coverage the measured peak would be centered at the binding energy characteristic of island edge atoms while at high coverage the binding energy would be characteristic of boron atoms at sites located in the interior of the islands. The difference in binding energies associated with the two sites would correspond to the experimentally measured shift of 0.6 eV. The inconsistency between this model and the data becomes evident when one examines the widths of the measured B(1s) peaks. To be consistent with the island growth mechanism, the envelope of the two overlapping B(1s) peaks should exhibit splitting or at least broadening at intermediate coverages compared to data measured at low and high coverages. For example, assuming equal concentrations of island edge and interior atoms and using the peak positions and widths at low and high coverage indicated in Figure 4a, we calculate the B(1s) envelope should be 2.0-eV fwhm. In fact, the B(1s) peak broadens monotonically and only very slightly from 1.4-eV fwhm at 0 = 0.35 monolayer to 1.5-eV fwhm at 8 = 1.1monolayer. An alternative growth mechanism consistent with the X P S data involves random filling of the surface by isolated boron monomers or dimers. The observed B(1s) binding energy shift is then attributed to interactions between these isolated units with increasing coverage. The UV spectra in Figure 5 show two adsorbate-induced peaks which shift from 4.0- and 7.0-eV binding energy at low coverage (0.3 monolayer) to 5.0- and 8.0-eV binding energy at saturation coverage (1.1monolayer). The peak at 4.0 to 5.0 eV below the Fermi level is assigned to emission from boron 2p states. As expected, the intensity of this peak increases, while the Mo(4d) band is attenuated with increasing boron coverage. The intensity of the peak at 7.0-8.0-eV binding energy remains approximately constant with coverage and may be due to adsorption of residual gases. However, XPS showed no detectable surface carbon and less than 0.1 monolayer of oxygen on the boron-covered surfaces. Another possibility is that this peak is due to emission from boron 2sp2hybrid states. In a UPS study of iron borides, peaks due to emission from boron (18) Healop, R. B.; Jones, K. In Inorganic Chemistry, a Guide to Aduanced Study; Elsevier: Amsterdam, 1976; p 106. (19) Salmeron, M.; Ferrer, S.;Jazzar, M.; Somorjai, G. A. Phys. Rev. B Condens. Matter 1983, B28,1158.
1020 Langmuir, Vol. 3, No. 6, 1987
Fryberger et al. a. 1473
WlS) BlNDlNQ ENERQY (aV)
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Figure 7. (a) K a excited B(1s)core level spectra for OB = 1.1 monolayers at 300 K followed by annealing to various temperatures. Background 400 counts/s. (b) Total boron coverage (monolayers) vs annelation temperature. Electron Energy ( e v )
Figure 6. (a) Al Ka excited B(1s) core level spectra for = 0.2 monolayer at 300 K followed by annealing at 1073 K. Background 400 counts/s. (b) He I spectra of 0.2-monolayer B/Mo(100) surface at 300 K followed by annealing at 1073 and 2000 K. 2p, 2sp2, and 2s states were identified.17 The bonding picture proposed there was one of 2sp2hybridization, which increases with the B/Fe ratio due to increasing B-B bonding. The B(2p) and B(2sp2)peaks were located at 4.0and 7.0-eV binding energy for Fe2B and 6.0- and 10.0-eV binding energy for FeB, respectively. The locations and separation (3.0 eV) between the two peaks in Figure 5 are very similar to the published iron boride data. In addition, in a manner similar to the iron borides, these peaks shift to higher binding energies with increasing boron concentration. However, according to ref 17, the 2sp2 emission is due to B-B interactions. Therefore, one would expect that emission from this state would increase with boron coverage, whereas our findings show no change in intensity of the 7.0-8.0-eV peak with coverage. Thus, this peak cannot be unequivocally assigned to the 2sp2 state. 3.3 High-TemperatureAnnealing Studies. Results. The high-temperature behavior of adsorbed boron was investigated by monitoring the boron 1s and valence levels as the surface was heated in a stepwise fashion. In these
experiments, the clean surface was exposed to B2H6at 300 K, annealed for 60 s, and then rapidly cooled ta below room temperature before the spectra were recorded. Coverages were calculated from the B(ls) peak areas before annealing. The B(1s) peak positions for various initial coverages are summarized in Table 11. Figure 6a shows the spectra taken before and after heating a low-coverage (0.2-monolayer) surface to 1073 K. Except for a slight decrease in binding energy of 0.1 eV, these spectra are virtually identical. Likewise, the UPS spectrum of a 0.2-monolayer boron surface (Figure 6b) is not significantly changed by annealing at 1073 K. Thus, it would appear that annealing a low-coverage surface at 1073 K causes little or no change in the B/Mo interaction. Both XPS (not shown) and UPS spectra (Figure 6b) indicate that no boron remains on the surface above 2000 K. XPS results for heating the saturated surface (1.1 monolayer) are shown in Figure 7a. Below 1073 K the B(1s) peak shape remains nearly constant but shifts to higher binding energy (see Table II) with increasing anneal temperature. Between 1073 and 1473 K two distinct B(1s) peaks are observed. Within this temperature range the low binding energy peak area remains constant while the high binding energy peak decreases in area. By 1673 K the high binding energy peak is no longer detectable.
Langmuir, Vol. 3, No. 6, 1987 1021
Adsorption of Boron on Mo(100) Table 11. B(1s) Binding Energies (in eV) with Respect to the Fermi Level as Functions of Boron Atomic Coverage (Determined at 300 K) and Anneal Temperaturea monolayers T.(K) 0.2 0.4 0.8 1.1 300-- 186.83 (02) 187.10 (04) 187.43 (01) 187.45 (02) 500 186.73 (03) 187.76 (01) 187.78 (02) 673 187.79 (01) 873 187.93 (01) 1023 187.92 (01) 1073 186.74 (03) 186.95 (05) 188.00 (03) 188.17 (05) 186.77 (04) 186.79 (06) 1273 188.25 (09) 186.86 (08) 1473 188.41 (16) 186.95 (08) 1673 187.11 (08)
Numbers in parentheses denote uncertainty in Gaussian fit to data.
21.2 e V
-
300K
-.-12
-6
ELECTRON ENERGY ( e V )
O=EF
Figure 8. He I spectra of 1.1-monolayers B/Mo(100) surface at 300 K followed by annealing at 1073,1473, and 2000 K.
Finally, as for the low-coverage surface, no B(1s) signal is found after heating to 2000 K. Figure 7b is a plot of the total peak area as a function of annealing temperature. The B(1s) signal decreases slightly (20%) between 300 and 1400 K and then drops off sharply above 1400 K. The valence band spectra as a function of annealing temperature up to 2000 K are shown in Figure 8. A large increase in the Mo(4d) band at 2.5 eV and a large decrease in the B(2p) level at 5.0 eV occur upon heating to 1073 K. In addition, the work function decreases by 0.3 eV between 300 and 1073 K. Discussion. Both UPS and XPS results for low boron coverages indicate that the B/Mo(100) surface is stable up to about 1700 K. Above this temperature surface boron disappears, as is evident from the loss of boron core and valence electron emission intensity. Since there was no observable pressure rise above 1700 K, we believe the disappearance is due to dissolution of boron into the bulk of the crystal. On the other hand, for high boron coverages (greater than 0.5 monolayer), the B(1s) and valence band spectra change substantially when the surface is heated. These changes are interpreted as the formation of a three-dimensional “boride” phase and a low-coverage “chemisorbed” phase. The XPS data in Figure 7 and Table I1 clearly show that two distinct boron states with
binding energies of 188.2 and 186.8 eV are formed by annealing at 1073 K. By use of the empirically derived relationship between boron coverage and B(1s) binding energy plotted in Figure 4b, the boron/molybdenum stoichiometry for each state can be determined. The state at 186.8 eV corresponds to a B/Mo stoichiometry of about 1:5 (the peak corresponds to about 0.3 monolayer). By extrapolating the OB vs binding energy relationship obtained from our measurements at OB 5 1.1to higher coverages, the peak at 188.2 eV would correspond to about 2.0 monolayers of boron, i.e., a B/Mo stoichiometry of 2:l. This extrapolation is semiquantitative at best, since the B(1s) binding energy increases with (anneal) temperature as well as with boron coverage. However, it is reasonable that the two boron states formed by annealing to temperatures above 1073 K are due to the formation of phases with high and low boron concentration. Since the B/Mo stoichiometry is >1for the high-coverage phase, it is not a purely two-dimensional surface phase. Indeed this phase can be identified as a threedimensional boride corresponding to a stoichiometric compound. First, a decrease (15%) in B(ls) intensity signals a loss of surface boron (Figure 7b) upon heating the surface between 300 and 1073 K. Yet, the TDS spectrum of Figure 3a does not show any boron desorption between 300 and 1100 K. This implies that boron must be going below the surface. Second, the increase in B(1s) binding energy with increasing anneal temperature is consistent with formation of a nonmetallic compound whose electronic polarizability is reduced compared to molybdenum metal. Analogous increases in adsorbate (1s) binding energies have also been reported for “nitride”: ”oxide”,Zoand “silicide”21formation on transition-metal surfaces. Finally, the B/Mo stoichiometry established above for the 188.2-eV binding energy peak identifies this peak with the compound MoB2. In fact, the published B(1s) binding energy for MOB, is 188.2 eV.2Z The UPS data in Figure 8 are also consistent with the formation of a three-dimensional boride phase. The XPS data (Figure 7b) show only a small decrease in boron signal upon annealing the surface up to 1400 K. Yet the UV spectra in Figure 8 exhibit a large decrease in B(2p) emission after heating to 1073 K and look very similar to the low-coverage spectra of Figure 6b. The likely explanation for this disparity between the core and valence level results is that some fraction of the boron atoms in the boride phase is situated below the top surface layer. Since the angle between the surface plane and the collection slit was considerably smaller for UPS ( 1 2 O ) than for XPS (45O), making the UPS even more surface sensitive, attenuation of B(2p) emission relative to B(1s) emission for subsurface boron is expected. Finally, the work function change due to annealing the saturated surface at 1073 K (-0.3 eV) is also consistent with penetration of boron adatoms below the surface. The work function change upon adsorption of 1.1 ML of boron at 300 K is +0.3 eV; annealing the surface to 1073 K causes the work function to return to the clean surface (or low 6,) value. Penetration of boron atoms below the surface would cause a decrease in the surface dipole, thereby producing a change in the sign of A ~ J . ~ ~ * ~ ~ (20) Fuggle, J. C. In Handbook of X-ray and Ultrauiolet Photoelectron Spectroscopy; Briggs, D., Ed.; Heyden & Son: London, 1977; p 273. (21) Duboie, L. H.; Nuzzo, R. G.Surf.Sci. 1985, 149,133. (22) Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corp., 1979; p 369 (23) Sargood, A. J.; JoWett, C. W.; Hopkins, B. J. Surf.Sci. 1970,22, 343.
(24) Bauer, E.; Poppa, H. Surf.Sci. 1979, 88,31.
1022 Langmuir, Vol. 3, No. 6, 1987
Fryberger et al.
Table 111. O(1s) and C(1s) Binding Energies and Widths (in eV) for Carbon Monoxide Adsorbed on B/Mo(lOO)n 00s) C(lS) OC9 Mo(100) surface EnF fwhm EBF fwhm monolayers cleanb 531.32 (12) 1.76 (03) 284.39 (04) 1.17 (04) 1.o ( Tads= 200 K) clean 530.41 (02) 2.06 (04) 282.70 (02) 1.57 (05) 0.5 ( Tads= 400 K) 1.88 (03) 0.5 monolayer of B b 532.72 (02) 286.26 (03) 1.36 (03) 0.5 (Tads = 150 K) 3.30 (30) 282.60 (05) 1.69 0.45 0.2 monolayer of Be 531.40 (10) 0.5 monolayer of B 532.20 (11) 3.04 (13) 282.70 (05) 1.45 (05) 0.2 0.75 monolayer of B 531.80 (12) 3.53 (13) 282.8
