Langmuir 1985,1,663-669
663
Influence of Compositional Heterogeneity on the Chemisorption and Reactivity of Small Molecules on Copper/Copper Silicide Surfaces Lawrence H. Dubois* and Ralph G. Nuzzo* AT&T Bell Laboratories, Murray Hill, New Jersey 07974 Received February 4, 1985 We have examined the chemisorption of CO on Cu(100) in the presence of ordered, copper silicide islands. Our results show an important role for surface diffusion and domain heterogeneity in this system. It is found that, even though CO does not dissociate on either clean Cu(100) or on a uniform silicide overlayer, island morphologies result in a facile dissociation reaction. We show that this arises from the selective adsorption of molecular CO on the exposed copper substrate (CuSi, does not bind CO under these conditions) followed by diffusion to the edges of islands where dissociation can and does occur. Thus, unlike the direct adsorption of a reactive molecule with a low sticking probability (where only a limited number of attempts to cross the dissociation barrier are possible), disparate domains can enhance reactivity by effectively increasing the attempt frequency. This simple model is used to explain our results for 02,C02, CH,OH, and H 2 0 chemisorption as well.
Introduction Until recently, most fundamental surface chemistry studies have been performed on clean metal surfaces under ultrahigh vacuum (UHV) conditions. Surface heterogeneity, when present, has been introduced largely through the use of vicinal surfaces and, in many instances, it has been found that terrace and step reactivities can differ dramatically. l s 2 More recently, the influence of surface poisons (S, 0, Cl),394promoters (Na, K),4+ and alloy and intermetallic compound formation (as in Nisi2,’ PhTi: for example) have been explored. Although these latter surfaces are compositionally heterogeneous on a microscopic scale (1-5 A), they are still uniform when examined from a more macroscopic viewpoint (20-100 A). The reactive chemistry of bifunctional surfaces-materials in which one portion of the surface performs a certain task, while a second, physically isolated and chemically distinct portion performs a different task-has not been explored in detail. This omission is significant as the properties of such materials form the basis of several technologies which are of immense economic importance. For example, the efficient production of high-performance fuels centrally involves catalysts with this structural characteristic. The traditional focus of studies of bifunctional surface reactivity has been the influence of metal oxides on the catalytic properties of supported metal^.^ These studies, which have led to many descriptive acronyms, attest to the (1) See, for example: Davis, S. M.; Somorjai, G. A. In “The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis”; King, D. A., Woodruff, D. P., Eds.; Elsevier: New York, 1982; Vol. 4, pp 217-364 and references cited therein. (2) See, for example: Serri, J. A.; Tully, J. C.; Cardillo, M. J. J. Chem. Phys. 1983, 79,1530-1540 and references therein. (3) See, for example: Kiskinova, M.; Goodman, D. W. Surf. Sci. 1981, 108,64-76. (4) See, for example: Stair, P. C. J. Am. Chem. SOC.1982, 104, 4044-4052 and references cited therein. (5) See, for example: Garfunkel, E. L.; Crowell, J. E.; Somorjai, G. A. J. Phys. Chem. 1982,86, 310-313 and references cited therein. (6) Ertl, G.;We&, M.; Lee, S. B. Chem. Phys. Lett. 1979,60,391-394. (7) Dubois, L. H.; Nuzzo,R. H.J.Am. Chem. SOC. 1983,105,365-369. (8) Bardi, U.;Somorjai, G. A.; Ross,P. N. J . CataE. 1984,85,272-276. (9) For a listing of classic references and a general discussion of this area as it relates to reforming catalysis, see: Bond, G. C. “Catalysis by Metals”; Academic Press: London, 1962; pp 437-466. Immiscible bimetallic clusters present an additional example, one which may more closely resemble the type of model proposed herein. For an overview of these latter materials, see: Sinfelt, J. H. ‘Bimetallic Catalysts”; Wiley: New York. 1983.
0743-74631 8512401-0663$01.50/0
importance of these effects on the scale of the discrete “domainsn appropriate to catalysis by supported metals. The insights gained and the techniques developed, while important, are highly specific and a broad understanding of domain-dependent reactivity has yet to emerge. We have taken a very different and, hopefully, more general approach to this problem. The central feature of our approach involves the preparation and study of single-crystal surfaces presenting islands of a well-defined and chemically distinct overlayer. If, on examination, the reactivity of the “island surface” is different from either that of the clean substrate or of a uniform overlayer, we can reasonably infer that we are exploring a perturbed reactivity that reflects the bifunctional nature of the material. For our first study in this area, we decided to explore a “simple” reaction, the dissociation of carbon monoxide. In order to develop and examine the subtle effects of ultrahigh vacuum, bifunctional surface chemistry without the added complexitiesof either surface defects or impurities, we have chosen overlayers of copper silicide on Cu(100) for these experiments. It is well-known that copper, in many physical forms (foils, powders, crystals, etc.), exhibits comparatively low reactivity toward most small molecules (we will show that the highly reactive silicide overlayer can break the C-0 triple bond in carbon monoxide). Thus, even though a single-crystal copper substrate may contain 1-2 % defects and/or impurities, these will not and do not dominate its reactive surface chemistry under the conditions which typify most UHV surface studies. In our experiments, we form the silicide overlayer in situ via the surface-mediated decomposition of silane (SiH4),a technique that we have previously used to grow intermetallic thin films, as well as both uniform and island overlayers.’O Since the chemisorption of small, oxygen-containing molecules on copper has been extensively studied by both high-resolution electron energy loss spectroscopy (EELS)11-14and X-ray photoelectron spectroscopy (XPS),’“’’ we use these (10) Dubois, L. H.;Nuzzo, R. G. J. Vac. Sci. Technol., A 1984, 2, 441-445. (11) Sexton, B. A. Chem. Phys. Lett. 1979,63,451-454. Andersson, S. Surf. Sci. 1979, 89,471-485. (12) Sexton, B. A. Surf. Sci. 1979,88, 299-318. (13) Sexton, B. A. J . Vac. Sci. Technol. 1979,113, 1033-1036. (14) Andersson, S.;Nyberg, C.; Tengstal, C. G. Chem. Phys. Lett. 1984, 104, 305-310.
0 1985 American Chemical Society
664 Langmuir, Vol. 1, No. 6, 1985
Dubois and Nuzzo
Table I. Adsorption of Simple, Oxygen-Containing Molecules on Cu(100). CuSi,, and CuSi,/Cu(lOO) Surfaces adsorbate Cu(100) CuSi, CuSi,/Cu(lOO) 0 2 0 SiO," SiO," co co SiO," + C'
co2
CHBOH HZO
c u (100)+ 0, x 1000
1080
I
CH,OHb HzOb
"Si02or "SiOz-likenspecies. b A t low temperatures either methanol ice or water ice may be formed on Cu(100). In addition, these molecules will chemisorb in the presence of preadsorbed oxygen atoms. Carbon observed by both XPS and AES.
x 1000
techniques in a "fingerprint" fashion t o form a qualitative picture of the chemistry of our surfaces.
Experimental Section Experiments were performed in two UHV chambers. One was a diffusion and titanium sublimation pumped system equipped with four-grid low-energy electron diffraction optics (Varian), a single-pass cylindrical mirror analyzer (Phi) for Auger electron spectroscopy (AES),a quadrupole residual gas analyzer (Inficon), and a high-resolution electron energy loss spectrometer. The design of the high-resolution EELS spectrometer is similar to that of Sexton.I4 For these experiments, the angle of the incident electron beam (60" to the surface normal) and its impact energy (-4.5 eV) were held constant, and electrons were collected only in the specular direction. Typical incident beam currents were (1-2) X A, and the elastic scattering peak from a clean copper surface was (1-2) X lo5 counts s-l, with a full width a t halfmaximum (fwhm) between 50 and 70 cm-'. The XPS experiments were run in a second ion-pumped UHV chamber containing a modified Kratos X-SAM 800 X-ray photoelectron spectrometer. The hemispherical electron energy analyzer was operated in a fixed analyzer transmission mode with an instrumental resolution of -1.1 eV. A Mg K a X-ray source was used throughout. All core levels have been referenced to the gold 4fTI2core level (binding energy = 84.0 eV). AES could also be performed in this system. Both UHV chambers contained ion sputtering guns for sample cleaning and simple effusive molecular beam sources for gas dosing. This method of gas dosing was found to be extremely important at the low concentration of silane used (1% in Ar) since silane readily reacts with (i.e., is gettered by) oxygen and water on the walls of the chamber to form nonvolatile products.10 All other gases were of research purity (Matheson) and used without further purification. Both the H 2 0 (doubly distilled and deionized) and CH30H (spectroscopic grade) were degassed by several freeze-pump-thaw cycles prior to dosing. Two Cu(100) single crystals (>99.999% pure) were oriented (i1/2"),cut, and polished by standard metallographic techniques. The samples were cleaned of trace carbon, oxygen, and sulfur impurities by repeated cycles of argon ion bombardment (1000 eV, 12 PA a t both 25 and 700-750 "C)followed by annealing in vacuum at 700-750 "C to restore surface order. Sample cleanliness was carefully monitored by both AES and XPS. The sample could be cooled to -140 "C with liquid N2for low temperature adsorption studies.
290
I
I
0
I
I
I
I
000
I600 ENERGY LOSS ( c m - l )
2400
Figure 1. Lower trace shows a high-resolution EELS spectrum of the 0 (~'%2V'/2)R45~overlayer structure on Cu(100). In the middle trace, the clean surface was reacted with SiH4to form the "complex" LEED pattern. This overlayer was then reacted with oxygen, and the resulting high-resolution EELS spectrum is shown in the upper trace. All spectra were recorded a t --110 "C.
j
cu 1100) +co
I
XI000
1070
3L S l H 4 @ 130'
Results
Clean Cu( 100)
+ Adsorbates.
Table I summarizes our
high-resolution EELS results for the chemisorption of 02, CO, COz7CH,OH, and H20 on clean Cu(100). In agreement with previous studies, we find that the dissociative adsorption of oxygen occurs only at elevated temperatures.12 Both (.\/2xd$R45O and (v'/8X2~'5)R45O overlayer structures are observed and are characterized b y vibrational modes at 331 and 291 cm-l, respectively.12 The lower trace of Figure 1presents a representative spectrum
F i g u r e 2. Chemisorption of carbon monoxide studied on clean Cu(100) (lower trace), a surface partially covered with CuSi, islands (middle trace), and after slowly warming this surface to room temperature (upper trace). While CO chemisorption is molecular a t low temperatures, dissociation a t elevated temperatures can occur in the presence of CuSi, islands.
(15) See: Scheffler,M.; Bradshaw, A. M. In ref 1, Vol. 2, pp 234-235. (16) Wandelt, K. Surf. Sci. Rep. 1982,2 and references cited therein. (17) Hofmann, P.; Mariani, C.; Horn, K.; Bradshaw, A. M. Proc. Int. Conf. Surf. Sci., 4th 1980, 541-544.
for the chemisorption of 1500 langmuir (1langmuir = lo4 torr s) of O2 o n Cu(100) at 200 "C. The (v'/2X2.\//2)R45" overlayer formed under these conditions has a substrateoxygen stretching vibration at 290 cm-l, a value i n good
0
800
1600 ENERGY LOSS
2400
(cm-ll
Influence of Heterogeneity on Chemisorption and Reactivity
-
Langmuir, Vol. 1. No.6, 1985 665
C
0
A
Figure 3. (A) Complex LEED pattern resulting from the reaction of clean Cu(100) with 5 langmuir of silane at 130 "C. This pattern can be reasonably well indexed as two domains of a (3x5) surface structure. The origin of the splitting is unknown at present. (B and C)After flashingthe surfacein (A) to 350 O C in vacuum. This incommensurate hexagonal overlayer can be indexed as
'"':I I:; Incident beam energies me 50 eV in (A) and (B)and 72 eV in (C). agreement with that found in previous studies.lZ In contrast, CO chemisorption is molecular and is observed only a t low temperature. Single metal-carbon (340 em-') and carbon-oqgen (2090 cm-') vibrational modes are ob~erved (see,for example, the lower trace of Figure 2, showing data for clean Cu(100) exposed to 10 langmuir of CO at -130 "C)." The intensity of both modes increases monotonically with increasing surface coverage. There is no evidence for C-O bond scission on this surface. Complementary XPS data for the chemisorption of CO and oxygen on Cu(100) are summarized in Table 11. We find no evidence for the chemisorption of COO CH,OH, or HzO on clean Cu(100) under our reaction conditions (substrate temperatures as low as -130 OC and gas exposures as high as several hundred langmuirs). Both CH,OH and H,O can be physisorbed on this surface, however, a t low temperatures'"'' or chemisorbed in the presence of surfacebound oxygen.'2 For example, exposing an oxygen-covered Cu(100) surface to -10 langmuir of CH,OH a t -130 'C and wanning to -30 'C results in the formation of a stable surface methoxide.12 These EELS results are in g o d agreement with the conclusions of earlier studies and will not be discussed further. Cu(100) + SiH,. In contrast to our previous studies on the reaction of silane with the Ni(ll1) surface,18we.find no evidence, a t low temperature, for adsorbed silyl fragments when clean Cu(100) is exposed to >10 langmuir of SiH, at -130 OC. The LEED pattern shows no increased background or extra spots,and AES shows no evidence for trace amounts of surface silicon. These are not conclusive tests, however, as we believe these adsorbed species are sensitive to electron beam stimulated desorption. Highresolution EELS experiments also proved inconclusive due to the similarity between the weak Si-H stretching vibration (21W2200 cm-' in substituted silanes) and the intense mode observed from CO adsorbed from the background (uco = 2090 cm-'). The weight of the evidence, including that obtained by XF'S, indicates that Little or no surface silicon is present, however. The reaction of Cu(100) with more than 3 langmuir of silane at elevated substrate temperatures (100 "C < T < 200 "C), on the other hand, results in the formation of the complex LEED pattern shown in Figure 3A. Neglecting (18) Duboii. L. H.; Nuno. R C. Surf. Sei. 1986, 149, 136145.
Table 11. XPS Data for the Adsorption of Osand CO on Cu(lO0) and CnSi.fCu(lO0) Svrfaees binding energy (fwhmY
surface treatment Cu2pa12 none 932.6 (1.3) 50 langmuir of 02. 932.6 (1.4) 9 M *c -"" 1500 langmuir of 02, 932.5 (1.4)
Si2p
CIS
530.4 (1.9)
-
200 "C 20 langmuir of CO, -140 'C 20 langmuir of SiH,, 140 "C ~~~
530.4 (1.9)
932.6 (1.4) 932.6 (1.6)
01s
286.3'
534.2
99.8 (2.0)
~
20 langmuir of SiH,. 140 "C, loo0 langmuir of 02,25
-
+
932.6 (1.4) 103.1'
532.1 (2.0)
or
6 langmuir SiH,d 140 932.6 (1.4) 'C. + 20 lanamuir of CO. -140 6 langmuir of SiH,, 932.6 (1.4) 140 "C. 20
+
99.8 (2.0) 286.3b 5 3 4 2 99.9 (2.1) 285Bb 532.3c
Ggmiir GCO, -140 "C. warm to -90 oc
a Values in eV. bComplexline shape;fwhm not determined (See Figure 5b. for example). cComplex background: fwhm not determined (see Figure 5a, for example). d A -6-langmuir SiH, exposure in the XPS spedrometer is equivalent to a -4-langmuir gas e x p u r e in the EELS system.
the small splitting of spots apparent in thia LEED pattern, the surface is reasonably indexed as two coexistent (3x5) domains; the origin of the splitting is unknown. A t 3-4 langmuir gas exposures, the spots are slightly broader than those shown here, and a higher background intensity is observed. The pattern is fully developed, with low background intensity a t 5-6-langmuir silane exposures (gas doses required in the XPS chamber for comparable coverage were approximately 50% higher). Since a well-developed LEED pattern, with higher background intensity, is observed at the lower gas exposures used, we conclude that ordered islands of the overlayer must be forming under these latter conditions. From the coherence length of LEED, we estimate the dimensions of these islands to be of the order of tens of angstroms. As we will show below, the formation of such island films has profound consequences for the reactivity of this surface.
Dubois and Nuzzo
666 Langmuir, Vol. 1, No. 6, 1985
of Figure 1. As is clearly evident, no peaks are observed; weak phonon modes (similar to those observed on Nisi2(111))7320 may be hidden in the tail of the elastic peak. Modes characteristic of either surface silyl groups or adsorbed hydrogen (vsx between 1800and 2200 cm-') are also absent. What is important to note here is that, using this procedure, clean films can be grown simply and reproducibly. If the surface structure characterized by the LEED pattern shown in Figure 3A is heated to 300 "C in vacuum, a new LEED pattern (shown in Figure 3B,C) is observed. Although these patterns have many similarities, the approximately 2 / 5 order spots which are doublets in the initially formed material become triplets after heating. This pattern originates from an incommensurate hexagonal overlayer (most likely containing both silicon and copper; see below) and can be indexed as follows:
dN (E) dE
7(3"*)
lo
I ,
l
,
l
20 40
l
l
{PURE SI l
l
l
l
l
l
l
l
l
l
l
l
60 80 100 120 140 160 IS( ENERGY
(ev)
Figure 4. Auger electron spectrum of clean Cu(100)(upper trace), after reacting with SiH, to form the complex LEED pattem shown in Figure 3A (middle trace), and after subsequent exposure to oxygen (lower trace). The latter two spectra are characteristic of copper silicidelgand SiO, formation, respectively.
An Auger analysis of the overlayer described above is shown in the middle trace of Figure 4. The doublet structure in the Si LMM region of the spectrum (two peaks centered at 90 and 94 eV) is characteristic of the formation of copper silicide and has been observed previously in the AES spectra of bulk C y S i as well as in materials grown by thin film interdiffusi~n.'~No changes in the Cu LMM peaks were seen. This feature is seemingly unique to copper as the AES spectra of nickel and other transitionmetal silicides show a single, well-resolved peak in the Si LMM region (for the nickel silicides, perturbations, in the form of three, small, additional features, are seen in the Ni LMM region).'JoJ8 Due to this tremendous perturbation of the silicon line shape (pure silicon is characterized by a single peak a t 92 eV-see arrow in Figure 4, lower trace), absolute surface coverages could not be estimated. The Si (94 eV) to Cu (60 eV) peak-to-peak height ratio increases monotonically with increasing silane exposure, however. Saturation is reached a t -6 langmuir in the EELS chamber (- 10 langmuir in the XPS spectrometer) for substrate temperatures near 150 "C. The observed silicon 2p core level binding energy (99.8 eV) is consistent with surface silicide formation (shifts in the C U ~ P , core ,~ level are negligible, see Table 11). A high-resolution EELS spectrum of a copper silicide thin film grown on Cu(100) is shown in the middle trace (19) Grant, J. T.;Haas, T. W. Surf. Sci. 1970,23,347-362. Hiraki,A.; Shimmu, A.; Iwami, M.; N m a w a , T.;Komiya, S. Appl. Phys. Lett. 1975, 26, 57-60. Frank, T. C.; Falconer, J. L. Appl. Surf. Sci. 1982-1983, 14, 359-374.
-7 141
Such an assignment indicates a unit cell of approximately 14 substrate lattice spacings in length. Both X P S and AES analyses indicate that silicon has not diffused into the bulk of the sample; the line shapes observed initially, and their intensities, remain unchanged. Heating to higher temperatures, however, does result in the dissolution of silicon and, thus, was avoided in order to minimize the contamination of the bulk sample. The reactivity of this overlayer is similar to that discussed above. Cu(100) Silane + Adsorbates. Table I summarizes our results for the chemisorption of small, oxygen-containing molecules on copper silicide surfaces grown on Cu(100). Of the five adsorbate molecules studied, only oxygen chemisorbs on a uniform thin film. We find no EELS or XPS evidence for either molecular or dissociative adsorption of CO, Cog,CH,OH, or H20 under our reaction conditions (exposures 550 langmuir and surface temperatures between -130 and 30 "C). The observed EELS spectra are all similar to that shown in the middle trace of Figure 1. The upper trace of Figure 1 shows a representative spectrum which results from the dissociative chemisorption of O2 (1500 langmuir) at -110 "C on a Cu(100) surface previously treated with silane (complementary XPS data are summarized in Table 11). The three broad peaks found a t -450, 770, and 1080 cm-' are characteristic of the initial stages of the formation of SOz. Qualitatively similar spectra have been observed also for the chemisorption of oxygen on both NiSi2(111)' and on silicon single-crystal surfaces a t room temperature.21 Previous XPS and UPS studies have shown that such reactions result in the preferred formation of oxidized silicon.10,22 No changes in either the complex LEED pattern (shown in Figure 3A) or in the Si LMM doublet are observed for low oxygen exposures (C50 langmuir). This presumably indicates that, initially, reaction takes place a t defects in the silicide overlayer. A t higher gas exposures (- 1000 langmuir) and, more generally, for exposure a t elevated temperatures, the background in the LEED pattern increases dramatically. Changes in the Auger spectrum (lower trace of Figure 4), the LEED pattern, and the XPS core level binding energies (Table
+
(20) Dubois, L. H.; Rowe, J. E. J . Vuc. Sci. Technol., A 1983, I , 1232-1235. (21) Ibach, H.;Horn, K.; Dorn, R.; Luth, H. Surf. Sci. 1973, 38, 433-454. Ibach, H.; Bruchmann, H.D.; Wagmer. H.Appl. Phys. A 1982, A29, 113-124. Serri, J. A,; Cardillo, M.J.; Becker, G. E. J . Chem. Phys. 1982, 77, 2175-2189. (22) Abbati, I.; Roasi, G.; Calliari, L.; Braicovich, L.; Lindau, I.; Spicer, W. E. J . Vuc. Sci. Technol. 1982,21, 409-412.
Influence of Heterogeneity on Chemisorption and Reactivity 01
( o ) C ~ ( 1 0 0 ) + 6 LS I H ~ @ 1400~ *2OL co
@ -140.C
I
i W A R M TO -90-C
1
540
1
Langmuir, Vol. I, No. 6, 1985 667 ( b ) CU(100)+6L SlH4@ 14OoC + 2 0 L C O @ -14OOC
Lil:.
>
k v)
z
W
+ z
-
> a a a
c_ m a a
I
3 00
I
295
I
I
290 285 B I N D I N G E N E R G Y (ev)
I
280
,
535 530 B I N D I N G ENERGY ( e v )
Figure 5. The 0 1 s (a) and Cls (b) core level spectra for the chemisorption of 20 langmuir of CO on a Cu(100) surface partially covered with CuSi, islands. In the upper traces, the substrate was held at -140 "C, and in the lower traces, the sample was allowed to warm slowly to -90 "C. The interpretation of these peaks is discussed in the text. Background subtractions have been made in the 01s
data,
11) are consistent with the formation of disordered "Si02-like" islands. Both the EELS and XPS results for the chemisorption of 02,C02, CH,OH, and H 2 0 on a Cu(100) surface partially covered with silicide islands are not dramatically different from that discussed above. In these studies, island-covered surfaces were formed by first exposing the clean copper substrate to -3 langmuir (-6 langmuir in the XPS chamber) of silane a t 130 "C and then cooling. Irrespective of whether the adsorption was conducted at low (-130 "C) or high (25 "C) temperature, we observe the dissociation of O2 (and the concomitant oxidation of silicon). For gas exposures and surface temperatures comparable to those described above, no reaction was observed with either C02, CH,OH, or H20. The EELS data obtained are similar to the comparable data shown in the upper two traces of Figure 1. The chemisorption of carbon monoxide on the islandcovered surface is quite different from that discussed above. When 10 langmuir of CO is adsorbed on this surface a t -110 to -130 "C, only the molecular species is osberved. EELS data show substrate-carbon and carbonoxygen stretching vibrations a t 340 and 2090 cm-l, respectively (Figure 2, middle trace). Both the 01s (Figure 5a, upper trace) and Cls (Figure 5b, upper trace) binding energies and peak shapes are similar (though lower in total integrated intensity) to those observed for the adsorption of carbon monoxide on clean Cu(100) (see Table 11). We believe that the CO is bonded to open copper areas for four reasons. First, a coincidence exists between the M-C and C-0 stretching vibrations seen on both the clean and island-covered surfaces. Second, the intensities of these peaks vary inversely with the silane exposure. Third, the XPS data show the same diagnostic 0 1 s and Cls line shapes and binding energies (although differing in intensity) for the two surfaces. Fourth, we find no evidence for
molecular CO adsorption on a completely silicided surface. On the basis of both the relative EELS and XPS peak intensities, we estimate that -75% of the surface is covered by copper silicide islands using the above-mentioned preparative procedure. Slowly (-0.5 "C/min) warming a carbon monoxide coordinated island film from low temperature to 30 "C produces an overlayer characterized by the EELS spectrum shown in the upper trace of Figure 2. In analaogy to the interpretation of the data for the chemisorption of oxygen on this surface (see above), we assign the peaks observed in this spectrum at -450,780, and 1070 cm-I to materials arising from the selective oxidation of silicon (the oxygen being derived from the dissociation of CO). The 01s core level shift from 534.2 to 532.3 eV is consistent with this interpretation (Figure 5a, lower trace). Although loss features attributable to the carbon cannot be detected in the EELS spectrum (its intensity may be weak and hidden by the 450-cm-' loss, as is the case for CO dissociation and subsequent silicon oxide formation on Nisi2(111)y it is clearly observed by both AES and XPS (Figure 5b, lower trace, and Table 11);the Cls line shape determined by the latter spectroscopy is radically different from that seen for molecular CO adsorption. Integrated peak intensities indicate that a significant fraction of the initially adsorbed carbon monoxide has desorbed. More rapid heating of the sample increases this fraction appreciably.
Discussion The data presented above clearly and compellingly demonstrate that island domains of copper silicide exhibit "enhanced reactivity" toward carbon monoxide. On clean Cu(lOO),CO adsorbs as the molecular species at low temperatures and, under these conditions, is characterized by a high sticking coefficient; the only thermal chemistry observed is simple desorption. Copper silicide surfaces,
668 Langmuir, Vol. 1, No. 6,1985
Dubois and Nuzzo
in the form of continuous, ordered overlayers on Cu(lOO), do not adsorb molecular CO very strongly, if at all, under these same conditions. Our experiments suggest that the sticking coefficient of CO on this surface at -130 “C must be at least 3 orders of magnitude less than that of the clean Cu(100) surface. The observations made in the presence of island overlayer morphologies is a much more complicated and, to us, somewhat surprising result. First, CO chemisorption still is observed and is characterized by a high sticking coefficient on the open copper areas at -130 “C. Second, our data (XPS and EELS) show no suggestion of “spillover” of CO from the discrete Cu(100) areas onto the intermetallic. Given the low barrier to lateral diffusion which is expected to pertain (2-3 k c a l / m ~ l ) ,this ~ , ~result ~ strongly suggests a thermodynamic constraint (rather than a low sticking coefficient) to molecular CO adsorption on the latter surface. Third, the barrier to CO dissociation on the islands at -130 “C must be appreciably greater than k T. On warming a CO coordinated island film, two processes are observed (eq 1). In the first, we see a significant loss CO(ads)
CO(g)
A C(ads) + O(ads) (1)
of CO to the gas phase. In the second, a substantial amount of the adsorbed CO dissociates, the exact quantity being sensitive to sample preparation and heating history. The combined weight of the data suggests that this dissociation occurs initially a t the edges of the intermetallic domains. Since both and dissociation (Figure 5) readily occur at -90 “C, we conclude that the activation barrier for the latter process must be very close to the binding energy of CO on Cu(100) (12-16 kcal/mol, depending on coverage). No other interpretation apparent to us would seemingly explain the closely competitive desorption /dissociation processes we observe, given the slow (-0.5 “C/min) heating rates employed. The description of these processes in simple kinetic language is reasonably given by eq 2, where d(CO)/dt is d(CO)/dt = (ak1 + bkZ(1 - t9))cO (2) the rate of disappearance of molecular CO on the surface, 8 and (1- 0) are the fractional surface areas of the copper and copper silicide phases, respectively, kl and k2 are the rate constants for desorption and dissociation, respectively, and a, b, and c are constants. In this description, the term ct9 reflects the scaled fraction of the “clean” copper surface area which coordinates CO and is thus representative of its coverage. This general formulation, justified by the conditions cited above (high sticking coefficient on Cu, low on Cu,Si; all thermal processes, except diffusion, closely competitive and irreversible, i.e., k, = k,; kl = k 2= 0), illustrates that the “yield” in any reaction channel is proportional to the total surface area of ”clean” copper. The enhanced reactivity results simply, then, as a consequence of the higher concentration and residence time of CO on this surface. Thus, even though the barrier to dissociation is expected to be low on the continuous intermetallic phase,25the weak binding of the molecular adsorbate, as reflected in a low and offsetting attempt (23) Viswanathan, R.; Burgess, D. R., Jr.; Stair, P. C.; Weitz, E. J. Vac. Sci. Technol. 1982,20,605-606. (24) Tracy, J. C. J. Chem. Phys. 1972,56, 2748-2754. (25) Rapid CO dissociation has been observed previously on the surfaces of numerous nickel, cobalt, and iron silicides. Reference 7 and: Imamura, H.; Wallace, W. E. J. Phys. Chem. 1979,83. 2009-2012.
frequency (i.e., a negligibly small residence time), makes this process less kinetically competent. By providing a mechanism for the facile adsorption and rapid diffusion of CO (the diffusion coefficient for CO on clean Cu(100) at -130 “C is (3.5 f 1.5) X lo4 cm2),23we also modify the attempt frequency which describes this reaction channel. The net result is the observance of “increased” reactivity. Warming the sample will increase both the rate of diffusion of molecules across the surface (i.e., increase the attempt frequency at the perimeter of an island) and raise their energy so that traversing the dissociation barrier becomes more facile. Raising the substrate temperature too rapidly can also lead to desorption rather than dissociation, which is precisely what we observe. During the course of the reaction, the silicide islands most likely are breaking up, thus allowing conversions of CO greater than that predicted from consideration of the dimensions of the island perimeters. This inference is consistent with the relative integrated peak intensities observed in both the EELS spectrum (Figure 2, upper trace) and in the XPS spectra (Figure 5b). It should be noted that the data we present above do not allow definitive conclusions to be drawn as to the physical nature of the sites effective in the dissociation of CO. Several general inferences can be made, however. On the basis of an analogy to CO dissociation on NiSi2(111),7 a system for which complete structural data exist,26it is likely that the dissociation and initial bonding of the carbon and oxygen fragments occurs a t bridged Cu-Si sites.27 Unlike Nisi2(1111,however, oxygen migration and the nacient segregation of a “silicon oxide” appear to occur rapidly. This latter conclusion is compelled by the fact that both EELS and X P S strongly suggest a dominant role for the bonding of atomic oxygen exclusively to silicon. In the absence of clearer structural assignments and higher spectral resolution, however, these interpretations should be considered cautiously. The simple kinetic model given above can also be used to explain the lack of reactivity observed with C02, CH30H, and H20. In all three of these adsorbates, the C-0 or 0-H bond strengths are significantly weaker than that of the C-0 triple bond in carbon monoxide and, therefore, might be expected to dissociate more readily. As shown in Table I, however, none of these molecules chemisorbs molecularly on either clean or partially or even completely silicided Cu(100). Thus, even though the respective enthalpic barriers to dissociation may be lower than that of CO, the attempt frequency will also be low, and no dissociation is expected under these conditions of low surface temperature and low gas exposure. For catalytic reactions, in which both the adsorbate and the surface “turn over”, the development of an exact kinetic model becomes much more complex.28 The general mechanistic implications suggested above remain valid, however. That is, mixed though discrete phases of dif(26) Yang, W. S.; Jona, F.; Marcus, P. M. Phys. Reu. B 1983, 12, 7377-7380. (27) We would note that the LEED data presented above does not compel the assignment of the island domains as discrete intermetallic phases (involving in-plane copper-silicon bonding) as opposed to a pure silicon island overlayer. Copper silicides of the general stoichiometry CusSi are known to exist in both cubic and hexagonal (twinned rhombohedral) structures, the latter preferred for this exact stoichiometry. This analogy to the LEED patterns we observe, in conjunction with the profound effects exhibited in the electron spectroscopies (XPS, Auger), strongly suggests the formation of intermetallic domains. For a clear discussion of the structural characteristics of Cu3Si, see: Solberg, S. K. Acta Crystallogr., Sect. A 1978, A34, 684-698 and references cited therein. (28) See the formalism developed in: Grinstein, F. F.; Rabitz, H.; Askar, A. J.Chem. Phys. 1985,82,3430-3441 and references cited therein.
Langmuir 1985,1, 669-672 fering chemical activity and reactivity may perturb the chemical kinetics of complex reactions for reasons unrelated simply to changes in the electronic or detailed microscopic structure of its constituents (the latter points being the typical expression of structure-reactivity effects in alloy and multimetallic catalysts). It is only a recent advent that the important mechanistic subtleties arising from competitive adsorption have come to be considered in fundamental surface science studies of reactions on metal surfaces.2 When such reasoning is extended to chemically heterogeneous surfaces-a feature that characterizes most technologically important catalysts and
669
catalytic processes-it would seem prudent to invoke an additional mechanistic consideration, namely, the effect that domain-dependent reactivity may be having on the observed kinetics and product balance. We will report further on this topic, especially as it relates to catalytic reaction mechanisms, in subsequent publications.
Acknowledgment. We thank E. G. McRae for his help in analyzing the complex LEED patterns. Registry No. CO, 630-08-0; CuSi,, 12643-20-8;02,7782-44-7; COZ, 124-38-9; CHSOH, 67-56-1;HzO, 7732-18-5; CU, 7440-50-8.
Carbon-13 NMR Study of the Effects of pH on Dodecyldimethylamine Oxide Solutions David L. Chang, Henri L. Rosano," and Arthur E. Woodward Department of Chemistry, The City College of The City University of New York, New York, New York 10031 Received February 22, 1985. In Final Form: June 24, 1985 Carbon-13 NMR chemical shifts were measured on solutions of a nonionic-cationic surfactant, dodecyldimethylamine oxide. Downfield chemical shifts were found for all carbons upon micellization,and the magnitude is most pronounced for carbon atoms in the central part of the chain. These shifts are attributed to rotational isomerization about the carbon-carbon bonds. It was calculated that the percent change of rotamers upon micellization (gauche to trans) is in the order of 25% for the central carbons and that the amount of gauche rotamers in the nonionized micelle is about 34%. Protonation of the amine oxide monomer resulted in upfield shift of carbon atoms near the head group, caused by the enhanced hydration in the presence of the charge. Ionization of the amine oxide group at the micelle surface resulted in a significantupfield chemical shift of the thee carbons near the ionized head group region when compared with the respective carbons in the nonionized form, suggesting that these adjacent carbons near the hydrophilic group are now surrounded by water molecules. This property was attributed to the higher solubility of the protonated species, resulting in an increased degree of water penetration between the head groups as the micelle becomes more cationic in character. These results were contrasted with monolayer studies of the same class of compounds.
Introduction Surfactant molecules in aqueous medium form micelles if the concentration exceeds a certain value, the critical micelle concentration (cmc). Many physical properties of surfactant solutions, such as surface tension, conductivity, and turbidity, undergo drastic alterations upon micellization. All of these properties indicate the existence of entities due to self-assembly. The size and shape of these micelles are usually determined by scattering techniques, while the characteristics a t the water-micelle interface can be simulated by way of monolayer studies. It has been demonstrated in recent years that 13CNMR studies of surfactant solutions can provide a wealth of information on the conformations of surfactant molecules,'+ due to the high resolution of 13CNMR and a large (1) Williams,E.; Sears, B.; Allerhand, A.; Cordes, E. H. J. Am. Chem. SOC.1973, 95, 4871. (2) Drakenberg, T., Lindman, B. J. Colloid Interface Sci. 1973,44,184. Drakenberg,T.;Lindman, B. J.Phys. Chem. 1976, (3) Persson, B. 0.; 80. 2124. _. (4) Ulmus, J.; Lindman, B.; Lindblom, G.; Drakenberg, T. J. Colloid Interface Sci. 1978, 65, 88.
chemical shift range. For example, detailed studies of the concentration dependence of the chemical shifts of the carbons in an alkyl chain can be used to deduce quantitative information on the micelle aggregation n ~ m b e r , ~ , ~ and, the shift change on passage from the intermicellar solution to a micelle gives qualitative information on any accompanying conformational change of the alkyl In the present study, the average conformation of the aliphatic chain, as well as the effect of charging the head group on the chemical shifts of a nonionic-cationic surfactant, dodecyldimethylamine oxide (C12DAO),are investigated with this technique. Depending on the pH of the solution, an equilibrium exists between the two forms of amine oxide, viz.,
+
CH,(CH2),,(CH3)2N~0 H++ CH3(CH2)1i(CH3)2N+OH in neutral or alkaline medium, the nonionic form domin a t e ~ . Below ~ ~ ~ pH 7, the fraction of the ionic species increases and, a t low pH the amine oxide molecules are
I
(5) Maeda, H.; Ozeki, S.;Ikeda, S.; Okabayashi, H.; Matsushita, K. J . Colloid Interface Sci. 1980, 76, 532. (6) Chevalier, Y.; Chachaty, C. Colloid Polym. Sci. 1982, 262, 489.
0743-7463/85/2401-0669$01.50/0
(7) Kolp, D. G.; Laughlin, R. G.; Krauss, R. P.; Zimmerer, R. E. J. Phys. Chem. 1963,67, 51. (8) Tokiwa, F.; Ohki, K. J . Phys. Chem. 1966, 70, 3437.
0 1985 American Chemical Society
