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Langmuir 1992,8, 90-94
Adsorption of NPh3, PPh3, AsPh3, SbPh3, and BiPh3 on Gold and Copper Ulrich B. Steiner, Peter Neuenschwander, Walter R. Caseri, and Ulrich W. Suter* Eidgenossische Technische Hochschule, Institut fur Polymere, ETH-Zentrum, CH-8092 Zurich, Switzerland
Fredi Stucki Asea Brown Boveri Limited, Research Center, CH-5405 Baden-Dattwil, Switzerland Received July 8,1991.I n Final Form: September 17,1991 The triphenyl compounds of the elements of the 15th group (N, P, As, Sb, Bi) adsorb from solution onto gold and copper. The adsorption was confirmed by reflection infrared spectroscopy at grazing incidence, ellipsometry, contact angle measurements,and in two cases,X-rayphotoelectron spectroscopy. The results suggest that at least three types of surface layers can occur: monolayer formation (typical examples NPhs, PPh3, and AsPh3 on gold), reaction of XPh3 with metal atoms forming a multilayer (typical example PPh3 on copper), and oxidation to oligomeric or polymeric products (typical example BiPh3 on gold). The wetting properties are characteristic for the type of metal rather than the adsorbate, apart from BiPh3 on gold. With the exception of SbPh3 and BiPh3 on gold, XPh3 desorbs at reduced pressure. Desorption in pure ethanol takes place within several minutes for the adsorbates on copper and within hours or days on gold.
Introduction A number of publications report on the adsorption of sulfur compounds, e.g., alkanethiols or dialkyl disulfides, from solution onto solid gold,l-17copper,lJ* and with widely varying chemical behavior. For instance, the adsorption of alkanethiols and dialkyl disulfides results in decomposition reactions of the disulfide group in contact with g ~ l d ,copper,' ~ - ~ or ~ i l v e r ,and ~ . ~similar reactions of the thiol group in contact with g ~ l dare ~ -likely. ~ On gold, the formation of gold thiolates is suggested after adsorption of alkanethiols as well as dialkyl disulfides.6 Our goal was to investigate whether adsorption from solution could also take place with some compounds that do not contain sulfur atoms and that do not adsorb by decomposition of the head group, Le., with typical coordinating agents. Principally there are compounds of many different elements that can act as potential ligands to transition metals, at least in solution; e.g., the trialkyl or triaryl compounds of the elements of the 15th group (1)Nuzzo, R. G.;Fusco, F. A.; Allara, D. L. J.Am. Chem. SOC.1987, 109,2358. (2)Nuzzo, R. G.;Allara, D. L. J.Am. Chem. SOC.1983,105,4481. (3)Allara, D. L.; Nuzzo, R. G. J. Electron Spectrosc. Relat. Phenom. 1983,30,11. (4)Arndt, T.; Schupp, H.; Schrepp, W. Thin Solid Films 1989,178, 319. ..
(5)Whitesides, G.M.;Laibinis, P. E. Langmuir 1990,6, 87. (6) Bain, C. D.; Whitesides, G. M. Angew. Chem. 1989,101,522. (7) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. .; M Nuzzo, R. G. J. Am. Chem. SOC.1989,11,321. ( 8 )Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (9)Bain, C. D.; Whitesides, G. M. J,Am. Chem. SOC.1989,111,7164. (10)Troughton, E.B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4,365. (11) Bain, C. D.; Whitesides, G. M.J. Am. Chem. SOC.1988,110,3665. (12) Finklea, H.0.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3.409. (13)Bain, C.D.; Whitesides, G. M. J. Am. Chem. SOC.1988,110,5897. (14)Nuzzo,R. G.;Dubois, L. H.;Allara, D. L. J.Am. Chem. SOC.1990, 112. 558. (15) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC.1990,112,4301. (16) Stouffer, J. M.; McCarthy, T. J. Macromolecules 1988,21,1204. (17)Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5,723. (18)Mielczarski, J.; Souninen, E.; Johansson, L.-S.;Laajalehto,K. Int. J.Miner. Process. 1989,26, 181.
(nitrogen, phosphorus, arsine, antimony, and bismuth) could coordinate to solid gold and copper. Amine and phosphine ligands are particularly widely used in organometallic chemistry. While amine ligands are usually stable in air, ligands with other elements of the 15th group are often sensitive to air. We selected the triphenyl compounds of N, P, As, Sb, and Bi because they all are quite stable in air whereas the correspondingtrialkyl compounds of P, As, Sb, and Bi are generally decomposed fast. In the following,these compounds are abbreviated as NPh3, PPh3, AsPh3, SbPh3, and BiPh3 and generically as XPh3.
Experimental Section Gold and copper slides were prepared by evaporation of ca. 2000 8, of metal onto silicon wafers, previously cleaned by immersing the slides in ethanol for 1min under the influence of ultrasound and then covered with a layer of ca. 50 8,of chromium for adhesion promotion. The slides were instantly immersed, under a nitrogen atmosphere, into a 1 mM XPh3 solution in ethanol or hexane, previously deoxygenatedby nitrogen bubbling. After 6 h, the slides were removed from solution,rinsed with ca. 2 mL of ethanol, and dried in an argon stream. The triphenyl compounds of the group 15 elements were obtained from Fluka AG, Buchs (Switzerland). Ethanol was purchased from Fluka AG and hexadecane from Sigma,St. Louis, MO. IR reflection absorption spectra were recorded on a Nicolet 5SXC FTIR spectrometer equipped with a mercury-cadmiumtelluride detector. The angle of incidence was 83O. The measurements were carried out under atmospheric conditions. To eliminate bands due to atmospheric water and COz and to reduce the baseline noise, sample and blank measurements were taken in alternatingcycles using a step-motor-driventranslation stage. Each cycle consisted typically of 10 single measurements, and 30 cycles, corresponding to 300 scans, were accumulated. After the last cycle, the blank spectrum was subtracted from the sample spectrum. Because of the large peak width, the spectra were subjected to a "five-point" smoothingprocess that reduced noise while all the significant peaks still were resolved. Ellipsometric measurements were performed using a PLASMOS SD 2300 ellipsometer, equipped with a He-Ne laser (A = 632.8 nm), at an angle of incidence of 70'. The beam diameter was ca. 1mm. For layer thickness measurements, the slides were
0743-7463/92/2408-0090$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8, No. 1,1992 91
Adsorption of NPh3, PPh3, AsPh3, SbPh3, and BiPh3 immersed into the adsorption solutions only to the middle. The slides were then removed from solution, rinsed with ca. 2 mL of ethanol, and dried in an argon stream. Then, the refractive index and the absorption coefficient of each single metal substrate was measured immediately on the not immersed part of the substrate (this procedure for the measurement of the optical constants of the substrate leads to better reproducible results than using a separate, freshly prepared, blank metal slide, probably due to the “aging” of optical constants’g). Immediately after the determination of the optical constants of the substrate, the layer thickness was measured on at least 5 separate spots, and 10 measurements were performed a t each spot. For the thickness calculation, refractive indices of 1.353 for NPhslzo 1.5718 for PPh3,Z1 1.6139 for A~Ph3,2~ 1.6948 for S b p h ~and , ~ ~1.7040 for BiPhsZ4were used. Refractive index differences of 0.2 lead to variation in the computed thickness in the range of 20-25%, corresponding to ca. 1-1.5 A for a thickness in the order of a XPh3 monolayer. Contact angle measurements were performed on a Ram6-Hart 100-00 goniometer at room temperature and ambient humidity. Under these conditions the contact angles were stable for at least several minutes. For the measurements, 3 pL of water or hexadecane was put on the surface, followed by adding another 3 pL to the first drop. By this procedure, the front of the final drop advances across the surface and the contact angle values measured are ‘advancing contact angle^".^^* The experimental error of this method is estimated to be So.Analogous to other systems described in the literature,7*9J0we found that the values for receding contact angles are lower than those for the corresponding advancing contact angles. XPS spectra were obtained on a KRATOS ES 300 spectrometer using Mg K a radiation. The Au(4f7,~)signal was referred to 84.0 eV. The pressure during the measurement was approximately mbar. Surface profiles were recorded on an Alphastep 200 (Tencor Instruments) surface profilometer using a stylus with a radius of 1.5-2.5 fim. The stylus force was adjusted to 14 mg. The horizontal resolution was 400 A for the 16-fim horizontal range and 1 fim for the 1-mm horizontal range. A vertical resolution of 5 A was selected.
Results The following experiments relate to adsorption from 1 mM ethanolic solution for 6 h, if not otherwise indicated. The adsorption, however, is not limited to ethanol; e.g., hexane can also be used as a solvent. Ethanol was selected for the studies reported because it is most widely used in the literature on the adsorption of thiols or dithiols on gold. IR spectra at grazing incidence were taken for all of the adsorbed species on gold and copper. The full widths at half-maximum of the signals of the adsorbed species are broad (see Figure 11, typically ca. 50 cm-’, while this value is in the range of 7 cm-’ for the bulk spectra recorded in KBr. Such a broadening of peaks upon adsorption on metal surfaces is not unc0mmon.~5 The broadness of the signals causes a decrease in resolution and intensity, maybe exacerbated by a tendency for parallel orientation to the surface of transition dipole moments.’ As a consequence, the intensity of the C-H stretching vibration was found to be at the limit of detectability while vibrations in the C-C ring stretching and C-H out-of-plane deformation vibration region are still (19) Komi, S.;Fukui, M.; Shintani, Y. Surf. Sci. 1990, 237, 321. (20) Handbookof ChemistryandPhysics;R. C. Weast, Ed.;CRCPress: Boca Raton FL, 1988. (21) Beilsteins Handbuch der organischen Chemie;Springer Verlag: Berlin, 1980; Vol. 16/I, p 420. (22) Reference 21, p 431. (23) Reference 21, p 513. (24) Reference 21, p 523. (25) Bradshaw, A. M.; Schweizer, E. In Spectroscopy of Surfaces (Adouncesin Spectroscopy Vol. 16); Clark, R. J. H., Heater, R. E., Eds.; John Wiley & Sons: Chichester, 1988.
1600
1400
1200
Wave numb e r
1000
800
,
600
(cm-1 )
Figure 1. (a) Transmission IR spectrum of SbPh3 in KBr and (b) reflection IR spectrum at grazing incidence for SbPh3 adsorbed on gold.
clearly visible. Comparison of peak positions in the region of phenyl ring C-C stretching and C-H out-of-plane deformation vibrations is difficult, but some typical vibrations of bulk XPh3 can always be found also in the adsorbed state, indicating that an adsorption indeed takes place. The substitution pattern in the region of C-H outof-plane deformation vibrations shows that intact phenyl rings are always present. Additional peaks appear at ca. 1555,1540, and 1530 cm-l after adsorption; these peaks could be due to decomposition reactions or coadsorption of impurities, and also to surface IR effectsz5 or phenyl rings in different environments rather than in the bulk, and cannot by themselves be taken as a proof of reaction after adsorption. There are additional broad peaks in the region of 13751400,1100-1300, and 1000 cm-’. At least some of these peaks are composed of several signals. It is possible that vibrations of adsorbed hydrocarbon or ether impurities disturb the spectra in this region; however, there is no evidence for coadsorbed ethanol since some key signals, especially a strong band at ca. 1070 cm-l, are absent. This signal is clearly detected when an IR spectrum of a slide treated with XPh3 is measured immediately after immersion in ethanol (this measurement must be carried out quickly, since the adsorbed ethanol evaporates fast). Using standard values for bond lengths and assuming a coordination of N, P, As, Sb, or Bi to the surface atoms, a straightforward estimation of the thickness of a monolayer on copper varies from ca. 5 A for NPh3 to ca. 6 A for BiPh3. This assumption agrees with crystal structure data of Cu-PPh3 fragments in complexes26(Figure 2). The Au-X distances are expected to be ca. 0.2 A larger than the Cu-X distances; i.e., the thickness of a monolayer is in the same order for both metals. We estimate the accuracy of the thickness measurements from ellipsometry as being somewhat less than these estimates of XPh3 monolayers (see Discussion). Therefore, an approximate determination of the thickness of the surface layer after XPh3 adsorption is possible; the approximate surface layer (26) Karlin, D. K.; Ghoah, P.; Cruse, R. W.; Farooq, A.; Yilma, G.; Jacobson, R. R.; Blackburn, N. J.; Strange, R. W.; Zubieta, J. J. Am. Chem. SOC.1988, 110,6769.
Steiner et al.
92 Langmuir, Vol. 8, No. 1, 1992 __.. --... ... _..
Table 11. Advancing Contact Angle of Water on Copper and Gold Slides Modified by XPh3, after Exposure to Air for 5 h Followed by Immersing into Ethanol and Brief Drying with Argon advancing contact angle, deg, of water after immersion in ethanol for metal Cu
Au
Figure 2. Cu-PPha fragment of an organometallic complex26as a possible model for PPh3 coordinated t o a copper surface. Table I. Surface Layer Thickness after Immersion of Gold and Copper Slides in 1 mM XPh3 Ethanolic Solution for 6 h, Expressed as Number of Monolayer Equivalents of XPh$ number of monolayer equivalents X on Cu on Au N 1-2 1 6-8 P 1 1 1 As 1 Sb 1 Bi 1-3 2-3 a
Estimated precision *0.5 monolayer equivalent.
thicknesses after an immersion time of 6 h, expressed in units of one XPh3 monolayer, are shown in Table I. On gold the data indicate that, with the exception of BiPh3, the surface layer thickness is on the order of one monolayer. The data on copper are less uniform. The results for AsPh3 and SbPh3are consistent with one monolayer; those for NPh3 and BiPh3 indicate that the thickness of the surface layer corresponds to up to three monolayers. Adsorption of PPh3 for 6 h, however, leads to a thicker surface layer, on the order of 6-8 PPh3 monolayer units. This surface layer also shows further growth upon further immersion. The thickness of the surface layers of AsPh3, SbPh3, and BiPh3 on copper also increases if immersed for another 54 h, but only slightly (albeit significantly), and does not exceed two monolayer units. On gold, however, no difference in surface layer thickness was found for AsPh3, SbPh3, and BiPh3 after 6- and 60-h immersion time. The contact angle of hexadecane after XPh3 deposition on copper or gold is below lo”, as on the “pure” metal surfaces, and remains so even after prolonged contact to air. The contact angle of water shows values of 60-68’ after exposing the “modified” slides to air for 5 h (Table 11),again in the same range as the contact angle of a pure copper (65’) or gold substrate (71’) after exposure to air for 5 h. Immersing the XPh3-treated copper slides into ethanol leads to a decrease in contact angle from 61-63’ to 31-36’ which is close to the contact angle of the copper substrate (32’). The decrease takes place mainly within the first 2 min, and immersion for another 2 min does not lead to significant further changes. When the contact angles are measured immediately after removing the slides from the adsorption solution and drying, the initial contact angle is as low as 38’. This value increases only slowly with exposure to air. It drops to 32-34’ after immersing the slides into pure ethanol. Since there is no indication that significant desorption of XPh3 took place after immersing the slides in pure ethanol (see below), it seems that
a
X N P As Sb Bi a N P As Sb Bi a
Omin 61 62 63 63 62 65 68 60 62 64 63 71
lmin 64 35 36 37 32 44 73 56 59 62 35 71
2min 35 35 34 33 33 31 66 59 61 61 40
65
3min 33 34 32 34 34 31 66 59 59 60 39 66
4min 34 36 34 33 31 32 64 61 60 60 38 66
Pure copper or gold surface.
atmospheric contaminants adsorb within 5 h. Some atmospheric impurities can obviously be removed by washing with ethanol. This phenomenon is also known for pure gold slides.1° On gold, after an immersion into pure ethanol for 4 min, the contact angles of NPh3, PPh3, AsPh3, and SbPh3 (6064’) are comparable to those before immersion and to that on pure gold after immersion (66’). In contrast, the contact angle of BiPh3-covered gold slides decreases to 38’. It is noteworthy that the ellipsometric surface layer thickness measurements also indicate an unusual behavior for BiPh3: Only treatment with BiPh3 solution seems to lead to a surface layer that exceeds one monolayer equivalent on gold. The desorption process of adsorbed XPh3 in pure ethanol was followed qualitatively by IR spectroscopy and ellipsometry. After 4 min of immersion, no desorption could be found from gold or copper. After 1h, XPh3 had completely desorbed from copper but not from gold. Within some days, however, all the adsorbates fully desorb also from gold. Therefore, the adsorption process is reversible as expected, e.g., for coordination compounds. The reversibility is also true for BiPh3 on gold, whatever the surface may be in this case. With the exception of SbPh3 and BiPh3 on gold, all adsorbates on gold and copper completely desorb within 30 min at a reduced pressure sufficient for X-ray photoelectron spectroscopy (XPS) (ca. mbar). From gold, thin layers of SbPh3 and BiPh3 (one or a few monolayers) do not desorb, but layers of a thickness of ca. 1pm, prepared by evaporation of a solution of the triphenyl compounds, do get thinner. While infrared measurements show indirect evidence for the presence of the ligand atoms themselves, XPS shows directly the presence of antimony and bismuth on gold. For SbPh3 the Sb(3dSp) signal was found at 528.9 eV (the same value is reported for bulk SbPh327);for BiPh3 the Bi(4fYp) signal appears at 158.8 eV. As mentioned in the Introduction, phosphorus, arsine, antimony, and bismuth compounds can decompose in air. Although bulk PPh3, AsPh3, SbPh3, and BiPh3 are stable in air for the intervals necessary in this work for adsorption and measuring periods, their surface reactivity toward oxygen could be higher than the bulk reactivity. Even during adsorption experiments carried out under nitrogen (27) Wagner, C. D. In Practical Surface Analysis,2nd ed.; Briggs, D., Seah,M. P., Eds.; John Wiley & Sons: Chichester, 1990;Vol. 1,Appendix 5.
Langmuir, Vol. 8, No. 1, 1992 93
Adsorption of NPh3, PPh3, AsPh3, SbPh3, and BiPh3
atmosphere, compounds of the formula O=XPh3 could i ! !I. . . . . . . . . . . . ..1. . . . . . . . . . . . . . . . . . . . . . . . . : . . . be formed by traces of oxygen. Moreover, for all the physical measurements the samples have had air contact and oxidation could take place during this time. SbPh3 and BiPh3 could also form oxidation products other than 0=XPh3,2“32 e.g. oligomeric or polymeric p r o d u ~ t s . ~ ~ - ~ l 1”................................................. . . . . . . . . :! O=XPh3, however, is known not to be suitable as a ligand j i :_ -. i ... -: ... for copper and gold ions or atoms in homogeneous solution, and O=BiPh3 is reduced in the adsorption medium (eth4 Pm anol) to BiPh3.29 Nevertheless, it was attempted to find evidence for or against formation of oxides by IR spectroscopy and, if possible, XPS. Because of the large peak ... ...... . widths in IR spectroscopy,aclear statement is not possible . . .il except for adsorbed AsPh3 on gold and copper. Here, no il signal is observed at 886 cm-l, the frequency for the As=O bond.31 BiPh3 on gold shows an oxygen signal at 532.6 eV which i! is not present on a gold slide before adsorption or after immersing a gold slide into ethanol for 6 h. This oxygen .... _._ . ._ .......... _....... ........... signal is also present after adsorption from hexane, and it is unlikely that the corresponding oxygen atoms stem 250 p m from ethanol or from a reaction product of ethanol. Figure 3. Surface profiles of copper evaporated onto silicon Further, there is no evidence from IR spectroscopy for wafers, recorded on a surface profilometer. The upper profile coadsorbed ethanol, as mentioned above. The X-ray phospans a much smaller horizontal distance than the lower one and toelectron spectroscopic oxygen signal after BiPh3 adhas a better horizontal resolution. sorption from hexane, however, is shifted to 528.1-529.6 eV (at least two signals). O=BiPh3 appears at 530.3 eV.33 simultaneously with the removal of the surface oxide layer, This signal is not present after BiPh3 absorption from suggesting that copper xanthate complexes are formed. ethanol nor from hexane. It is most plausible that illOn the basis of known chemistry, substitution of oxide or defined oligomeric or polymeric oxidation products of hydroxide ions by the electricallyneutral XPh3 compounds BiPh3 have been formed. The existence of oligomeric or is hardly possible unless unexpected redox reactions would polymeric oxidation products could explain the lower take place. However, surface complexes of Au(0) or Au(1) contact angle and higher surface layer thickness compared with BiPh3, of Cu(1) or Cu(I1) with NPh3, and of Cu(O), to the other adsorbates on gold. Since the Sb(3d512)signal Cu(I), or Cu(I1)with PPh3, AsPh3, SbPh3,and BiPh3 would appears in the region of the O(1s) signal, no statement is not be unlikely. If the ligands were able to penetrate the possible about the presence of oxygen in the case of SbPh3 surface, multilayers of complexed metal atoms, ions, or on gold. charged or uncharged clusters could be built up. According to data for low molecular Cu(1) complexes,34the highest Discussion stability for a Cu-XPh3 complex is expected for X = P. After immersion of copper in PPh3 solution for 6 h, a It has been shown by IR spectroscopy,and in some cases surface layer thickness of 6-8 monolayer equivalents was by XPS, that compounds with the elements of the 15th measured. This value is significantly higher than in the group (N, P, As, Sb, Bi) adsorb from solution on gold and other cases investigated here, in which for adsorption on copper surfaces. According to ellipsometric measuregold and copper a monolayer coverage or a coverage with ments, it seems that a coverage in the order of one monomaximally three monolayer equivalents is indicated. layer occurs on gold after 6-h immersion time. On copper Consideringthe ellipsometricresults, one must be aware the experiments yield less uniform results. On gold (upon that the experimental accuracy is limited. However, the prolonged immersion) only BiPh3 leads to a surface layer reproducibility is suitable for the determination of the significantly thicker than was expected for a monolayer; number of monolayer equivalents with an estimated error on copper this is true for PPh3 and maybe NPh3 and BiPh3, off 0.5 monolayer equivalents (an error of a few angstroms as far as this can be concluded from ellipsometric data. could be due to adsorbed atmospheric contamination7J0). After an immersion time of 60 h, however, the surface Surface roughness must also be considered. The surface layer thickness of AsPh3 and SbPh3on copper has increased roughness of copper and gold evaporated onto silicon (significantly)to 1-2 monolayer units; i.e., monolayers had wafers is 5-15 A when measured with a surface profilombeen defective (incomplete) before and were now more eter over a horizontal range of 16pm or 1mm. A roughness perfect, packing of the monolayers has become denser, or in the same order of magnitude is obtained for silicon multilayers have been formed. wafers, in agreement with the literature (50 In Figure Multilayer formation is reported after adsorption of xan3, typical surface profiles of copper are displayed over a thates on copper.ls In this case, multilayer growth occurs vertical range of 16 pm and 1mm (corresponding to the diameter of the laser beam of the ellipsometer). Surface (28) Wieber, M.; Hettwer, U. Organoantimony Compounds. Gmelin profiles of gold are comparable to those of copper. Thus, Handbuch der anorganischen Chemie;Bitterer, H., Ed.;SpringerVerlag: Berlin, 1981; Part 1, Chapter 1.1.1.1.6. the surface roughness is, on a large horizontal scale, larger (29) Samann, S. In Houben Weyl Methoden der organischen Chemie, than a XPh3 monolayer. However,we estimate the surface As, Sb, Bi; Kropf, H., Ed.; Georg Thieme: Stuttgart, 1978. roughness of evaporated slides to have a minor influence (30) Goel, R. G.; Prasad, H. S. J . Organomet. Chem. 1972, 36, 323. (31) Venezky, D. L.; Sink, C. W. J . Organomet. Chem. 1972,35,131. on the accuracy of the measurements since the roughness (32) Wieber, M.; Hettwer,U. Bismut-organischeVerbindungen. Gmeis small compared to the wavelength of the laser beam. lin Handbuch der anorganischen Chemie; Bitterer, H., Ed.; Springer I
-
:
~.
~
I
I.
Verlag: Berlin, 1977; Vol. 47, Chapter 1.3.3.2.4. (33) Hoste, S.; van de Vondel, D. F.; van der Kelen, G. P. J . Electron Spectrosc. Relat. Phenom. 1979, 17, 191.
;
I
I
.,-L.
(34) Ahrland, S.; Balzamo, S. Inorg. Chim. Acta 1988, 142, 285. (35) Allara, D. L. ACS Symp. Ser. 1980,137, 37.
94 Langmuir, Vol. 8, No. 1, 1992 Consideringthe adsorption series on gold, one finds that the surface layer after adsorption of BiPhs is distinguished from the other adsorbates in thickness as well as in wetting properties. The ellipsometric values indicate that BiPh3 does not adsorb in a monolayer. The contact angle of water on the “purified” adsorption layer (immersed into ethanol for 2-4 min) is significantly lower than that on the purified gold substrate. After the adsorption process, oxygen compounds are detected by XPS. The surface layer thickness does not agree with the assumption that a monolayer was oxidized by air after adsorption. It is therefore likely that oligomeric or polymeric compounds have been formed by reaction of BiPh3 with traces of dissolved oxygen during the adsorption process. As far as can be concluded from IR spectroscopy and ellipsometry, an immersion for 1 h into pure ethanol leads to “complete” desorption from copper for all compounds investigated, while “no” desorption takes place on gold. This is not due to higher adsorption equilibrium constants on gold but to a kinetic effect since the surface layers also desorb from gold over a period of several days. In contrast, monolayers of alkanethiols7 or imida~ole-2-thiones~~ do not desorb even over longer periods. The better vacuum stability of SbPhs and BiPh3 on gold compared with copper could be due not only to kinetic effects but also to the formation of nonvolatile oligomericor polymeric products on gold, at least for BiPh3. The different behavior of gold and copper surfaces toward adsorbates could, among other factors, be connected with a surface oxide layer. According to the literature, a gold surface in contact with air is free from oxide2t7while copper is covered rapidly with an oxide layer when exposed to the atmosphere’s (up to a thickness in the order of 40 ~ 4 ~On ~ )all. the copper surfaces, copper oxides were detected by XPS after vacuum desorption of XPh3. The presence of metal atoms in higher oxidation states than zero could lead to stronger complex formation with XPh3. On the other hand, a less densly packed surface oxide layer could favor a penetration of XPh3 into the interior. The wetting properties of water on the “pure” metal surfaces are similar to those of most XPh3-modified surfaces. After immersing the air-exposed slides into ethanol for 2-4 min, only the contact angle of BiPh3 (-28’) on gold differs from that of the “pure” metal slides. Thus, it seems that the wetting properties of water with most modified surfaces remain almost unchanged after adsorp(36) Arduengo, A. J., 111; Moran, J. R.; Rodriguez-Parada, J.; Ward, M. D.J. Am. Chem. SOC.1990,112, 6153. (37) Stewart, W. C.; Leu, J.; Jensen, K. F. Mater. Res. SOC.Symp. h o c . 1989,153, 285.
Steiner et al.
tion of XPh3. Upon adsorption of long-chain alkanethiols on gold, for instance, the wetting properties change drastically and the surface becomes very hydrophobic.7J0-l2 However, attention has to be paid to the fact that atoms burried under the surface, e.g., oxygen atoms, can influence the contact angle of water even when they are 5 A below the surface.13 Thus, in the case of a 6-8, XPh3 monolayer, the metal substrate or the oxide layer on it could still influence the contact angle of water. In addition, it is possible that even a dense packing of XPhs does not cover the surface completely, e.g., if XPh3 was adsorbed in a disklike conformation forming a hexagonal structure, at least for the monolayers. The decrease in contact angle on the copper samples after washing with ethanol is probably due to removal of hydrocarbons previously adsorbed from the atmosphere. This assumption is in agreement with the observation that the contact angle on a freshly prepared surface layer is similar to an old, washed layer. Moreover, there is no evidence for a reduction in intensity of the adsorbate vibrations in the IR spectra after washing; i.e., the decrease in contact angle is probably not due to XPh3 desorption but to the removal of adsorbed atmospheric species, as described for surfaces of alkanethiols or dialkyl disulfides on gold7J0 (however, xanthates on copperl8 are reported to be mainly free from atmospheric contaminants). On the gold samples, the influence of ethanol immersion is usually not pronounced, if there is one at all. Only the slide with BiPh3 shows a marked decrease in contact angle after immersion. This could be due to a different type of adsorption layer, since BiPh3 also leads to a larger thickness of the surface layer on gold compared to the other adsorbates.
Conclusions We conclude that it is most likely that (1)all group 15 element triphenyl compounds except BiPh3 adsorb as monolayers on gold, (2) BiPh3 tends to form oxygencontaining oligomericor polymeric species on gold, (3) on copper, the thickest surface layers (corresponding to several monolayers) are obtained with PPh3, due to formation of a layer of Cu-PPh3 complexes, and (4) at least the adsorbates on copper are covered with atmospheric hydrocarbon impurities when exposed to air for some time. Acknowledgment. We are grateful for support of this work by the Swiss National ScienceFoundation. We thank Roland Rapold for help in creating Figure 2. Registry No. Au, 7440-57-5; Cu, 7440-50-8;NPh3,603-34-9; PPh3, 603-35-0; AsPh3, 603-32-7; SbPh3, 603-36-1; BiPh3, 60333-8.
