Langmuir 1988,4, 1368-1373
1368
Adsorption of Gaseous and Aqueous HCl on the Low-Index Planes of Copper John L. Stickney,* Charles B. Ehlers, and Brian W. Gregory School of Chemical Sciences, University of Georgia, Athens, Georgia 30602 Received April 12, 1988. I n Final Form: July 28, 1988 The reactivity of copper and HC1 has been studied on a copper single crystal with faces oriented to the (ill), (110), and (100) planes. Structures formed on Cu(100) and Cu(ll1) with HCl gas exposures up to
500 langmuirs resembled those formed by Clz: Cu(lOO)c(2X2)-Cland Cu(lll)(d/3xd/3)R30°-C1. Initial exposures of Cu(ll0) to HC1 gas resulted in formation of a 42x2) LEED pattern, at a C1 coverage of 0.5. Further exposures, up to 500 langmuirs, resulted in a series of transitional LEED patterns, culminating in formation of a (3x2) pattern. The transition in the patterns indicated that the C1 overlayer was compressed in the (110) direction, along the troughs in the (110) surface. The final (3x2) LEED pattern resulted from near hexagonally close-packed C1 atoms. Exposure of the copper crystal to the vapor from a 1 mM HC1 solution resulted in formation of disordered corrosion layers containing oxygen and some chlorine. Subsequent immersion in a 1mM HCl solution removed the corrosion products and left ordered adlayers of C1. A 42x2) LEED pattern was observed on Cu(lOO),which was the same pattern observed during exposure to HCl gas. The presence of coadsorbed oxygen resulted in increased diffuse intensity in the LEED pattern of the immersed Cu(100). No oxygen was present on the (111) or (110) surfaces. A (d/3Xd/3)R3Oosplit LEED pattern, with triangles of spots at the (1/3,2/3) positions, was observed on the (111)surface; two possible structures are discussed. Streaks in the (110) direction of the Cu(ll0) LEED pattern indicated some ordering of the adsorbed C1. A structure consonant with these (110) streaks consists of C1 atom strings along the troughs, with a variety of periodicities in the (110) direction and the substrate periodicity in the (100) direction.
Introduction Experimental Section Experiments were conducted with a copper single crystal Studies of HC1 reacting with clean, well-characterized polished on three faces.21 Each face was oriented to a different single crystals of copper have not previously been perlow-index plane: (lll),(110),and (100). Faces were cut so that formed in the gas phase or in solution. These studies were all three were parallel to a vertical axis, allowing examination of performed because of the importance of HC1 in the etching each by rotation around this axis. The crystal was oriented by of copper' and its role as both an electrolyte in electroLauk X-ray diffraction, cut with an electrical discharge machine (EDM),and polished with successively finer grades of emery and chemistry and as a depassivating agent in corrosion. They diamond paste. are intended to reveal similarities and differences between Studies were performed in an ultra-high-vacuum (UHV) surface copper reacting with HC1 in the gas phase and in solution. analysis instrument equipped with optics for LEED (Varian),a Studies of the structures formed by low-pressure expoquadrupole mass analyzer (UTI)for control of sample dosing and sures of C12 to copper single-crystal surfaces have been Most of those C12studies involved Cu(lOO), (1) (a) Winters, H. F. J. Vac. Sci. Technol., A . 1986, 3, 786. (b) and specifically the c(2X2)-Cl, also referred to as the Winters, H. F. J. Vac. Sci. Technol., B. 1986, 3, 9. (d%d/Z)R45O-C1 pattern formed at room tempera(2) Westphal, D.; Goldmann, A. Solid State Commun. 1980,35,437. (3) Citrin, P. H.; Hamann, D. R.; Matteiss, L. F.; Rowe, J. E. Phys. Studies of C1, adsorption on Cu(ll1) center on the Reu. Lett. 1982, 49, 1712. (d/3Xd3)R30°-C1 pattern.6*8i9The techniques of angu(4) Jona, F.; Westphal, D.; Goldmann, A,; Marcus, P. M. J. Phys. C 1983, 16, 3001. lar-dependent photoemission, low-energy electron dif(5) (a) Westphal, D.; Goldmann, A. Surf. Sci. 1983, 126, 253. (b) fraction (LEED),and X-ray absorption spectroscopy Westphal, D.; Goldmann, A. Surf. Sci. 1983, 131, 92. (XAS) have been used to investigate this s t r u c t ~ r e . ~ ~ ~ , (6) ~ Kleinherbers, K. K.; Goldmann, A. Surf. Sci. 1983, 133, 38. (7) Laskowski, B. C.; Bagus, P. S. Surf. Sci. 1984, 138, L142. Goddard and Lambert have also identified several higher (8) Goddard, P. J.; Lambert, R. M. Surf. Sci. 1977, 67, 180. coverage structures on C u ( l l l ) , which formed upon (9) Crapper, M. D.;Riley, C. E.; Sweeney, P. J. J.; McConville, C. F.; heating the Cu(ll1) substrate.8 Woodruff, D. P. Surf. Sci. 1987, 182, 213. (10) (a) Sesselmann, W.; Chung, T. J. Surf. Sci. 1986,176,32; (b) 1986, Studies relevant to the investigations presented here 776, 67. include exposure of polycrystalline Cu to C12gas at pres(11) Baetzold, R. C. J. Am. Chem. SOC.1981, 103, 6116. sures between IO4 and 10 Torr,1° etching of Cu(100) by (12) Moroney, L.;Rassias, S.; Roberts, M. W. Surf. Sci. 1981, 105, L249. Clz in UHV,' chemisorption of chlorine on copper clus(13) Kamath, P.; Prabhakaran, K.; Rao, C. N. R. Surf. Sci. 1984,146, ters," coadsorption of halogens, oxygen,12J3and water14 L551. on copper single-crystal surfaces, and adsorption of other (14) (a) Grider, D. E.; Bange, K.; Sass, J. K. Surf. Sci. 1983, 126, 246. (b) Bange, K.; Grider, D.; Sass, J. K. Surf. Sci. 1983, 126, 437. halogens on copper single ~rystals.'~'' Structures formed (15) Dicenzo, S. B.;Wertheim, G. K.; Buchanan, D. N. E. Surf. Sci. by halogen adsorption on copper surfaces are generally 1982, 121, 411. analogous to each other. Two examples are provided by (16) Richardson, N. V.;Sass, J. K. Surf. Sci. 1981, 103, 496. HCl adsorption on Cu(ll1) and Cu(lOO),described in the (17) Kleinherbers, K. K.; Zimmer, H.-G.; Goldmann, A. Surf. Sci. 1986, 167, 417. present paper, in comparison with the Cu(ll1)(18) Authors' unpublished results. (v'/3Xv'/3)R30°-I s t r u ~ t u r e ' ~and J ~ the Cu(lOO)c(2X2)-Br (19) Stickney, J. L. Metal Deposition on Well-DefinedPt Electrodes; Ph.D. Thesis, University of California, 1984; University Microfilms Ins t r u ~ t u r e . ' ~ JSimilar ~ structures are also found when ternational. halogens are adsorbed on Ag surface^.'^^^^ (20) Westphal, D.; Goldmann, A. Solid State Commun. 1982,38,685.
* Author to whom
correspondence should be addressed.
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(21) Stickney, J. L.; Ehlers, C. B.; Gregory, B. W. In Molecular Phenomena at Electrode Surfaces; American Chemical Society: Washington, D.C., 1988 (in press).
0 1988 American Chemical Society
HCl Adsorption on Cu Low-Index Planes
Langmuir, Vol. 4, No. 6, 1988 1369 E
Figure 1. LEED patterns: (a) CU(~OO)C(~X~)-HCI, 70 eV; (h) Cu(111)(~3x\/3)R30"-HCI,66 eV: (c) Cu(llO)(lxl), 70 eV; (d) Cu(llO)c(2XZ)-HCI, 57 eV; (e) Cu(llO)-HCI, transition pattern, 58 eV: (0 Cu(llO)-HCI, transition pattern, 70 eV; (9) Cu(110)(3X2)-HCI, 79 eV.
Table I. Chlorine Structures and Coverages: Ideal and Experimental normalized' Auger pp ratio initial CI CI/Cu, sticking face surfam atoma/cma C1 structure coverage ideal C1 atoms/cmP atams/cm' mffcient 1.52 X lW6 C(2X2) 'I, 7.6 X 10" 7.6 x 1014 0.8 1.76 X 10" (d%)R30° 5.9 x 1014 6.1 X 10" 0.2 1.08 x 10" 42x2) 5.4 x 1014 5.3 x 10" 0.1 7.2 x 1014 6.9 x 1014 1.08 x 10" (3x2) oRatioa converted to atoms/cmz by best fit with theoretical values. Value of 6.61 X W6,(CI/Cu ratio)/(atoms/cm'). used. thermal desorption studies, and a hemispherical electron energy analyzer (Leybold Heraeus) used for Auger electron spectroscopy (AES). AES was performed with 3000-eV ionizing electrons. Gaskets were gold-plated to prevent copper corrosion products from fouling the instrument. The cryatal was exposed to gaseous HCI (Aldrich) in the UHV analysis chamber at HCI partial pressures below 5.0 X 'OI Torr. Integration of the 36 amu mass spectrometric signal, with a fragmentation and isotope correction, was used to determine total exposure. The sample temperature during dosing was 30 O C . Solution experiments were performed in a stainless steel antechamber attached to the analysis chamber through a gatevalve interlock. The crystal was transferred to the antechamher after ion bombardment, annealing, and examination hy hoth LEKD and AES in the analysis chamber. The antechamber was hackfilled with ultrahigh purity (UHP) nitrogen or argon (Matheson). Immersion of the crystal was performed in an electrochemical H-cell. constructed from Pyrex. This H-cell was located inside a stainless steel bellows-Pealed compartment and interlocked to the antechamber through a gate valve?' Solutions were prepared from pyrolytically distilled wateP and reagent grade chemicals. (22) Conwsy, B. E.; Angoratein-Kozlolusk. H.;Sharp, W. B. A,: Criddle, E. E. Anal. Chem. 1973.45, 1331.
After emersion (removal) of the crystal, the antechamber was pumped down, the sample was transferred hack to the analysis chamber, and the resulting faces were reexamined hy LEED and AES.
Results and Discussion HCl Gas Exposures: Cu(100). Adsorption of HCl on Cu(100) resulted in formation of a sharp ~ ( 2 x 2 LEED ) pattern (Figure la). A similar pattern was described in the literature for adsorption of Cl, on Cu(lOO)?-' Phot o e m i s ~ i o n , 2XAS,S ~ . ~ LEEDP.?" and angle-resolved electron energy loss spectroscopic (AREELS)6 studies, as well as molecular orbital calculations,7 have been used to investigate CI, adsorption. General agreement is found between these techniques; Cl, adsorbs dissociatively with C1 atoms occupying fourfold hollow sites in the unreconstructed (100) surface. Half coverage is optimum for this structure (Table I) where every other fourfold site is occupied. Agreement is also found in the literature for the Cu-CI bond distance, between 0.237 and 0.241 nm, slightly (23) Bullet, D. W. Solid State Commun. 1981,38, 96S972.
1370 Langmuir, Vol. 4, No. 6, 1988
Stickney et al.
1 O!
Figure 3. Proposed structures for HC1 adsorbed on Cu(ll0): c(2X2), transition, and (3x2). structure, the Cu(111)(d/3Xd/3)R30°-C1 structure has been previously studied but was formed by using Clz, not HCL8v9 Woodruff and co-workers investigated the (v‘3Xd3)R30°-C1 pattern using XAS and photoelectron ~pectroscopy.~ Their results agreed with Lambert’s conclusions that the chlorine is present at 1/3 coverage (Table 0 0 I). Woodruff further stated that the C1 atoms occupy the I I I I I 00 250 500 750 1000 1250 fcc sites, sites that would have been occupied by the next HCI EXPOSURE, LANGMUIRS layer of copper atoms in a face-centered cubic l a t t i ~ e . ~ ( L - 1 0 - 6TORR SECl From Figure 2 and Table I it is evident that the initial Figure 2. HC1 adsorption isotherms for Cu(lll), Cu(llO), and sticking coefficient for HCl on Cu(ll1) is a factor of 4 lower Cu(100). One HCl langmuir corresponds to 1.9 X lOI4 atoms/cm2. than that on Cu(100) or Cu(ll0). The simultaneous nature of the present experiments, treating each face equivalently expanded from the 0.235 nm listed in Wyckoff for C U C ~ . ~ ~at all stages, allows quantitation of variations between the The fact that C1, results in the same structures as HC1 is faces after exposures of less than 1 langmuir. The denot surprising since work on Pt has shown that HCl, Cl,, creased adsorptivity of the close-packed (111)plane, as chlorinated hydrocarbons, and aqueous CaC12 all result in compared with the more open (100) and (110) planes, may the same halogen ad layer^.^^,^^ Thus, since equivalent be a reflection of bond number, where the bond number structures were observed, it appears that HCl adsorbs for (111)is 9 out of 12, for (100) is 8 out of 12, and for (110) dissociatively, forming chlorine atoms and probably Hz gas. is 7 out of 12. An alternative explanation of the decreased No other structures were observed with exposures up adsorptivity could be increased ionic character of the to lo3 langmuirs, although a 20% increase in the Cl/Cu chlorine atoms on Cu(ll1) and Coulombic repulsion. Auger peak height ratio over that corresponding to forCharged adsorbed anions have been characterized on Pt.33 mation of the initial sharp ~ ( 2 x 2pattern ) (Figure la) was Exposing Cu(ll1) to Cl,, Goddard and Lambert8 obobserved. The Cl/Cu Auger peak height ratio was deserved several (d/3Xd/3)R30° patterns with the (1/3,2/3) termined by using the C1180-eV peak and the Cu 920-eV spots distinctively split into close-spaced triplets. These peak in the analyzer’s constant retardation factor mode. “split” patterns were reported to represent structures The absence of changes in the LEED pattern geometry containing higher C1 coverages than the simple with this increase in C1 coverage indicates the initial for(d/3Xd/3)R3O0discussed above. They were produced by mation of bulk copper chloride. Recent articles by Sesdosing C1, together with heating the substrate above room selmann and Chung describe the formation of first CuCl temperature. However, they were not observed in the and then CuC12at room temperature as a function of C12 present room temperature investigations with HC1 gas. exposure time and pressure (10-6-10 Torr).lo The present Split spots are indicative of the formation of large unit cells results agree with those of Sesselmann and Chung; a large on the surface. Activation energy, which is not available decrease in the adsorption rate accompanied HCl adwith room temperature adsorption of HC1, may be required sorption past the first half “monolayer”. From Figure 2 to form these large unit cells. Either oxidation of chloride it appears that after formation of the first structures the to adsorbed chlorine atoms or the formation of H, gas may sticking coefficients decreased from approximately 0.5 to be the limiting step for C1 adsorption, preventing the similar to the decreases observed by Sesselmann and formation of the high-coverage split structures. A split Chung on polycrystalline ~ o p p e r . ~ ’ - ~ ~ pattern was observed in the aqueous HCl studies, which Cu(ll1). Exposure of the Cu(ll1) surface to HC1 rewill be discussed below. sulted in formation of a well-ordered (d/3Xd/3)R3Oo 1 2 adsorption on Cu(lll), investigated in our laboratory LEED pattern (Figure lb). As with the Cu(100)c(2x2)-Cl and previously by DiCenzo, Wertheim, and B ~ c h a n a n , ’ ~ , ~ ~ resulted in a clear (v‘/3xd/3)R3O0 pattern at an iodine No split pattern was reported by DiCenzo, coverage of (24) Wyckoff, R. W. Crystal Structures; Interscience: New York, 1953. nor was one observed in our room temperature adsorption (25) Stern, D. A.; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Song, studies up to 500 langmuir~.l~*’~ Halogen adsorption on D.; Lewis, S. K.; Frank, D. G.; Hubbard, A. T. J.Electroanal. Chem. 1987, 217, 101. Ag(ll1) also resulted in formation of a (d/3Xd/3)R30° (26) Garwood, G. A., Jr.; Hubbard, A. T. Surf. Sci. 1982, 112, 281. structure at 1/3 coverage (ref 15 and 27 and ref contained (27) Stickney, J. L.; Rosasco, S. D.; Song, D.; Soriaga, M.; Hubbard, therein). Electrodeposition of copper onto a Pt(ll1) A. T. Surf. Sci. 1983, 130, 326. (28) Stickney, J. L.; Rosasco, S. D.; Hubbard, A. T. J. Electroanal. surface containing a layer of adsorbed iodine atoms28reChem. 1984,131, 260. sulted in formation of split patterns similar to those pro(29) Bertocci, U.; Turner, D. R. Encyclopedia of Electrochemistry of duced by Lambert with C12 on Cu(111).8 the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1976.
,c II ’
I
(30) Spitzer, A.; Luth, H. Surf. Sci. 1982, 118, 121. (31) Salaita, G. B.; Lu, F.; Laguren-Davidson, L.; Hubbard, A. T. J. Electroanal. Chem. 1987, 229, 1. 132) Pauling, L. C . Nature of the Chemical Bond, 3rd ed.; Cornell University: Ithaca, 1980.
(33) Stern, D. A,; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Song, D.; Lewis, S. K.; Frank, D. G.; Hubbard, A. T. J. Electroanal. Chem. 1987, 217, 101.
Langmuir, Vol. 4, No. 6, 1988 1371
HC1 Adsorption on Cu Low-Index Planes
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Figure 4. Auger spectra for copper exposed to aqueous HCl vapor (A-C) and immersed in 1 mM HCl (D-F).
Cu(ll0). A Cu(llO)c(2X2)-Cl pattern was formed after drawn at their van der Waals diameter (0.360 nm).32 The HC1 exposures of less than 1 langmuir. The pattern was 2/3 C1 coverage for the (3x2) structure in Figure 3 is in good optimized a t about 2.5 langmuirs (Figures I d and 2). A agreement with Cl/Cu Auger peak ratios and with C1 proposed structure is shown on the left side of Figure 3. packing densities on the other low-index planes (Table I). This structure is based on the LEED pattern requiring a For a unit cell which contains six substrate atoms, like the coverage of 1/2C1 atom per surface copper atom. The (3X2), there are only a couple of coverages, in this case, tendency for the C1 atoms to adopt high coordinate sites which work: 2 / 3 or s/6. A coverage of 5 / 6 would require has been observed on both Cu(ll1) and C U ( ~ O O ) . No ~ - ~ ~ ~ packing ~~ of the chlorine atoms a t significantly less than reconstruction has been depicted in Figure 2 since one was their van der Waals diameter, and the observed Cl/Cu not reported for the low-coverage C1 structures on Cu(ll1) Auger ratio (Table I) does not support this coverage. This and C U ( ~ O O ) . ~ ~ ~ , ' ~ ~ proposed (3x2) structure (Figure 3) involves a nearly hexagonal packing of the C1 atoms; the same is observed Further exposure of the Cu(llO)c(2X2)-Cl structure to HC1 resulted in a series of LEED patterns (Figure le-g) for adsorption of most halogens on transition-metal surculminating in formation of a (3x2) pattern shown in f a c e ~ . The ~ ~ nearly hexagonal packing in the (3x2) Figure lh. The transition to the (3x2) pattern was disstructure is the result of the similarity in the C1 diameter tinctive; all the spots of the initially present ~(2x2) pattern (0.360 nm) and the long bridge distance in the Cu(ll0) plane, 0.377 nm. split in the (110) direction as the coverage was increased. That is, all changes in the adsorbate structure occurred Immersion in 1 mM HCl. Initially, a blank experiment by variations in the spacing between chlorine atoms in the was performed to investigate the state of the copper direction along the troughs in the (110) surface. The electrode just prior to immersion in solution. The electrode characteristic troughs of the (110) plane allow movement was prepared as in other experiments, transferred into the of the C1 atoms down the troughs but not across the antechamber, and sealed off from the analysis chamber.21 close-packed atoms in the (100) direction that make up The antechamber was bought to ambient pressure with the trough walls. Well-resolved spots were visible UHP Argon, and the electrochemical H-cell was inserted throughout the transition from the 42x2) structure to the into the antechamber from an attached bellows-sealed (3x2) structure, as evidenced by Figure le-f. The clarity compartment. The copper crystal was suspended 1 cm of the patterns, at all coverages, indicates an ordered above the H-cell while the H-cell was filled and drained transition, with uniform spacings between C1 atoms a t a 4 times with 1 mM HC1. The H-cell was then removed, given coverage. As the coverage increased, the spacings the antechamber was evacuated, and the electrode was between neighboring C1 atoms decreased in the (110) reexamined by AES and LEED. direction, resulting in increased splitting of the LEED Examination of the resulting LEED patterns revealed pattern (Figure le-f). A compression structure, proposed only diffuse scattering. No LEED spots were visible on as a typical transition structure, is depicted in the center any of the faces a t energies below 100 eV. Auger spectra of Figure 3. This (16x2) unit cell has a coverage of 7/12. (Figure 4A-C) indicated multiple corrosion layers conThe left side of Figure 3 depicts a 42x2) structure at 1/2 taining oxygen and some chlorine. Evidently, there was coverage (Table I), while the right side depicts a (3x2) enough O2 and HCl in the vapor above the solution to structure a t 2 / 3 coverage (Table I). Chlorine atoms are corrode the copper surfaces. Copper electrodes exposed
1372 Langmuir, Vol. 4, No. 6, 1988
Stickney et al. c
Figure 5. LEED patterns after immersion in 1mM HCI: (a) Cu(lOO)c(2x2)-HCl, 85 eV; (b) C u ( l l l ) ( ~ / 3 X ~ / 3 ) R 3 0 ~ - H63 C leV; , (c) Cu(lll)(~/3X~/3)R3O0-HCI, 15 eV; (d) Cu(ll0) streaked-HCI, 64 eV.
to water vapor or immersed in pyrolytically distilled water also resulted in diffuse layers on the (110) and (111)surfaces, while the (100) surface showed order?' It is felt by us that the oxygen present is due to traces of O2gas and is not the result of a reaction between Cu and water.*l The presence of an ordered structure on Cu(100) after expaure to water vapor was the result of the absence of HC1 in the vapor. Immersion of the Cu crystal into aqueous 1mM HC1 for 30 s resulted in ordered C1 adlattices on all three faces of the copper crystal (Figure 5). The corroded layers were removed. Auger spectra for the emersed (removed) electrode are shown in Figure 4D,F. Differences in these three spectra were dramatic; no oxygen signal WBS evident on the (111)or (110) surfaces, while the (100) face exhibited an oxygen coverage equivalent to lI6 of a monolayer (Figure 4D). Cu(100). The fact that the corroded layers are removed is not surprising from thermodynamic considerations." In water at these pHs, Cu metal, rather than copper oxide, is stable, and in the presence of Cl-, CuCl is formed. The presence of some oxygen on Cu(100) may be a kinetic effect. Auger spectra indicated that significantly more oxidation occurred on the (100) surface during the blank experiment (i.e., when the crystal was not immersed) than on the (111)or (110) surfaces (Figure 4A-C). The 30-s duration of the immersion may have been too short for full removal of the corrosion products from the (100) surface. Despite the presence of both oxygen and chlorine on Cu(lOO), upon emersion from the HCI solution a 42x2) pattern was observed. This pattern (Figure 5a) was similar to that observed after HCl gas dosing (Figure la) but contained a significant increase in diffuse intensity. The chloride coverage observed for the emersed Cu(100) had decreased 25% from the optimum ~ ( 2 x 2 coverage ) (Table I, Figure la). The fact that the mixed C1-0 layer resulted in a pattern at all is probably due to the similarity in bonding sites and coverages adopted by C1 and 0 atoms on Cu(100).2' Oxygen adsorption on Cu(100) also results in a 42x2) structure, with oxygen atoms bonding in fourfold hollow sites.21J0 Cu(ll1). A (v'/3Xd5)R30° split pattem was observed on the Cu(ll1) surface upon emersion from the HC1 solution (Figure 5b,c). Auger spectroscopy indicated only the presence of copper and chlorine on the surface (Figure 4E). This (v'?Xv'/3)R30° split pattern resembled that observed by Goddard and Lambert when they applied Cl, to the surface of a Cu(ll1) crystal, heated to 370 K.8 Lambert suggested that the series of structures they ob-
served could be explained by hexagonally packed lattices of CI adatoms. The pattern in Figure 5b,c was ascribed by Lambert to a Cu(l I ~ ) ( ~ \ / ~ X ~ Y ~ ) Rstructure, ~O~-CI containing 49 CI atoms for a coverage of 0.45. Another explanation for this pattern is that the splitting is the result of antephase boundaries. Hubbard and coworkers have observed a (\/3Xv'3)R30° split pattern on the surface of Ag(ll1) emersed from aqueous iodide solutions?' Hubbard suggested that the Ag surface underwent a reconstruction forming large domains. The top layer of silver and iodine atoms had (\/%v'/.?)RSOo local symmetry at an I atom coverage of The large domains were the result of phase boundaries at regular intervals. These boundaries accounted for the split-spot spacings and orientations while the (\//.?X\/3)R30°local geometry accounted for the localized intensity at the (l/3?/3) positions. These phase boundaries occur due to the presence of two types of threefold symmetric sites in the reconstructed surface of the (\ 3Xv'/3)R30° unit cell. A chlorine atom will occupy only one of these sites in a given unit cell. Where two adjacent domains form from different sites, the intersection will result in a phase boundary and a missing row of halogen This model requires a reconstruction. which explains the need for heating to produce the split pattern in gas-phase HCI experiments? The main problem here is that the resolution of the split spots indicates these large unit cells should be approximately the same size, and it is not clear what determines the unit-cell dimensions. Both of the proposed structures above have good points. This system might lend itself to study by XAS. In Lambert's model, each of the 49 CI atoms in the unit cell would occupy a different site, resulting in a variety of Cu-CI bond lengths. In the antephase domain model:' each chlorine atom would occupy a threefold hollow site in the reconstructed copper surface. All C u C l bonds should be equivalent. XAS above the Cl K-edge should be sensitive to the Cu-CI bond length, and a pronounced feature for the Cu-CI distance would be expected if the antephase model is correct. Cu(ll0). The LEED pattern displayed by the (110) surface upon emersion from 1 mM HCI (Figure 5d) contained streaks in the (110) direction, indicating some ordering of the adsorbed CI. The pattem contained no intensity at the half integral positions in the (100) direction; thin is a significant difference compared to the patterns observed after HCI gas exposures (Figure Id-h). This pattern displays only the substrate periodicity in the "100" direction, while the streaks in the "1 IO" direction indicate
Langmuir 1988, 4 , 1373-1375 that a series of different unit-cell lengths are present simultaneously. The structure responsible for this pattern probably involves strings of C1 atoms in the troughs of the Cu(ll0) surface. The Auger spectrum suggests a coverage close to 0.5, as is similarly observed for the 42x2) structure (Figure 3) produced with HC1 gas. Summary Use of a three-sided single crystal has allowed simultaneous exposure of the low-index planes of copper to HC1. Adsorption of gaseous and aqueous HC1 has been investigated. Exposures of the single crystal to gas-phase HC1 resulted in structures on the Cu(ll1) and Cu(100) faces equivalent to structures observed after exposure to C12. HC1 adsorption on Cu(ll0) evidenced a series of patterns a t increasing HC1 exposures: a 42x2) initially, followed by a series of transition patterns, ending with a (3x2). These patterns indicate that the increase in C1 atom packing density results in tighter packing along the troughs in the (110) direction. This culminates in nearly hexagonal close-packed C1 atoms and formation of a (3x2) pattern.
1373
Immersion of the Cu single crystal into 1mM HC1 also resulted in chlorine adsorption and formation of ordered structures. Close-packed monolayers of C1 atoms characterize these structures, as they characterized the analogous structures formed by exposure to HC1 gas. It appears that chloride adsorption from solution in excess of that observed here probably resulted in soluble copper chlorides, leaving only the monolayer chloride structures. Immersions were performed at open circuit (i.e., no potential control). Under these conditions, the different faces of the crystal contribute to a mixed potential, not indicative of any one Cu crystallographic plane. Experiments are planned to use separate electrodes for each crystal face. These crystals are currently being oriented and polished and are designed so that each face has the same crystallographic orientation. When immersed into solution the potential of these electrodes will be indicative of the intended crystallographic plane. Also planned are potential-dependent experiments to determine surface structure and composition as a function of applied potential. Registry No. HC1, 7647-01-0; Cu, 7440-50-8.
Letters Langmuir-Blodgett Films of an Enzyme-Lipid Complex for Sensor Membraned Yoshio Okahata," Tomoya Tsuruta, Kuniharu Ijiro, and Katsuhiko Ariga Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Merguro-ku, Tokyo 152, Japan Received May 19, 1988. In Final Form: July 13, 1988 A stable monolayer of water-soluble enzyme (glucose oxidase, GOD) could be prepared by spreading a benzene solution of the enzyme-lipid complex on a water subphase. LB films (Y-type, two layers) of the GOD-lipid monolayers could be deposited on a Pt electrode and acted as a glucose-sensing ultrathin membrane with a short response time. Recently it has been of interest to use monolayer and LB films of enzymes and of antibodies as biosensors or biomolecular switches by means of their high selectivity for their substrates and antigens, re~pectively.~-~ However, water-soluble proteins seem to be difficult to spread on the air-water surface and are denaturated easily a t the int e r f a ~ e . Recently ~ we observed that a lipid-coated enzyme could be prepared by mixing aqueous solutions of enzymes and lipids, and the complex is soluble only in organic media without causing denaturation.'S6 For example, the li(1) Enzyme-Lipid Complex 3. For part 2, see: Okahata, Y.; Fujimoto, Y.; Ijiro, K. Tetrahedron Lett., in press. (2) Guilbalt, G. G. In Analytical Uses of Immobilized Enzymes; Marcel Dekker: New York, 1984. ( 3 ) Chang, T. M. S. In Biomedical Application of Immobilized Enzymes and Proteins; Plenum: New York, 1977. (4) Ishii, T.; Muramatsu, M. Bull. Chem. SOC.Jpn. 1971,44, 679. (5) Okahata, Y.; Ijiro, K. J. Chem. SOC.,Chem. Commun., in press.
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pase-lipid complex has been found to vigorously function as a catalyst for esterification in a highly enantioselective manner in homogeneous and nonaqueous organic solvents. In this paper, we report that the enzyme-lipid complex forms a stable monolayer on a subphase and that Langmuir-Blodgett (LB) films can be deposited on a substrate (Pt electrode). When glucose oxidase (GOD) is employed as an enzyme, the GOD-immobilized LB film (two layers, ca. 5 nm thick) on a Pt electrode acts as a sensitive and ultrathin glucose sensor membrane. This is the first report to show that enzymes can be immobilized in ultrathin LB films. The GOD-lipid complex was prepared as follows. A buffer solution (25 mL, 0.01 M acetate, pH 5.6) of 50 mg of GOD and an equal amount of an aqueous dispersion of synthetic cationic dialkyl amphiphile 1 were mixed, and precipitates obtained after incubation a t 4 "C for 1 day were lyophilized. The pale-yellow powder obtained was insoluble in aqueous media but soluble in organic solvents. 0 1988 American Chemical Society
