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Langmuir 1996,11, 1252-1256
Tricyclohexylphosphine Adsorbed on Gold Kajsa Uvdal," Ingmar Persson,? and Bo Liedberg Laboratory of Applied Physics, Department of Physics and Measurement Technology, Linkoping University S-581 83 Linkoping, Sweden Received April 22, 1994. In Final Form: January 13, 1 9 9 P Tricyclohexylphosphine adsorbates on gold, as well as multilayer films of tricyclohexylphosphine,are investigated by X-ray photoelectron spectroscopy (XPS)and infrared reflection-absorption spectroscopy (IRAS). Molecular orientation and molecular surface interaction for adsorbates prepared from solution are studied, using the multilayer films as references. Since phosphines are easily oxidized in contact with air, the reference, made from a pressed pellet of the starting material, consists of a considerable amount of oxidized tricyclohexylphosphine. Therefore an oxygen-free reference would also be desirable. Such a reference of a tricyclohexylphosphine multilayer was obtained by fractionating tricyclohexylphosphine from tricyclohexylphosphineoxide by a sublimation process under ultrahigh vacuum. The results from both a pressed pellet of the starting material and an evaporated tricyclohexylphosphine multilayer are compared with the results from the adsorbate. When tricyclohexylphosphine is adsorbed from solution onto gold, there is a preferential adsorption of the unoxidized tricyclohexylphosphine. The molecular structure is intact during adsorption and the molecules are oriented with the phosphine atoms close to the surface. The chemical shift of the P(2py2) binding energy during adsorption indicates an electron donation from the adsorbate to the metal.
Introduction Monolayers of thiols and sulfides are promising candidates for sophisticated surface modification where surfaces with specific properties are required. Ligands such as thiols and sulfides bind stronglyto metal surfaces, and the carbon chain of such organic compounds can be easily modified. It is therefore possible to design surfaces with well-defined physical and chemical properties. The properties of these modified surfaces are not only correlated to the tail group's functionality and the ligandmetal interaction but also to the molecular coverage of the surface, as well as to hydrocarbon chain orientation and conformation. Recently, long tail disulfides and thiols which form ordered, densely packed, organic monolayers on copper, silver, and gold surfaces have been characterized and the wetting properties of these self-assembledfilmshave been investigated.l+ Furthermore, coadsorption of alkenethi01s with different chain lengths as well as coadsorption of disulfides and thiols has been investigated in detaiL6e7 Monolayers of thiols and sulfideshave also been studied as potential systems for corrosion inhibition of metal surfaces and for improvement of adhesion between metal surfaces and organic material^.^.^ The surface chemistry of sulfur-containingadsorbates such as mercaptopropionic acid and L-cysteine have been reported.loJ1 These small
* Author to whom correspondence should be sent at Linktiping University. T Department of Chemistry Swedish University of Agricultural Sciences, 5-750 07 Uppsala, Sweden Abstract published in Advance A C S Abstracts, March 1,1995. (1)Throughton, E.B.; Bain, C. D.; Whitesides, G. M.Langmuir 1988, 4,365. (21Nuzzo, R.G.;Fusco, F. A,; Allara, D. L. J.Am. Chem. Soc. 1987, 109,2358. (3)Bain, C.D.; Troughton, E. B., Tao, Y.-T.; Evall, J.; Whitesides, G.M.; Nuzzo, R. G. J.Am. Chem. SOC.1989,111,321. (4)Nuzzo, R. G.;Dubois, L. H.; Allara, D. L. J.Am. Chem. Soc. 1990, 112,558. (5)Laibinis, P.E.;Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991,95,7017. (6)Bain, C . D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5,723. (7)Laibinis, P. E.;Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992,96,5097. (8)Labinis, P . E.; Whitesides, G. M. J.Am. Chem. Soc. 1992,114, 9022. (9)Swalen, J. D. Langmuir 1987,2, 932. @
molecules also form stable overlayers suitable for applications in rough chemical environments. These adsorbates are currently used for biological interaction studies and the results seem to be very promising.12 In this study, a trialkylphosphine system is tested with respect to the ligand-metal interaction. A preliminary calorimetricstudy has shown that phosphines have strong interactions not only with the coinage metals but also with palladium, platinum, and rhodium. This and other physical-chemical studies13J4have indicated that tricyclohexylphosphineis one of the strongest electron-pair donors among the trialkylphosphines. Several articles about different phosphine complexes has been p ~ b l i s h e d . l ~ -However, ~l very little has been written about phosphines adsorbed to metal surfaces, let alone about one specific phosphine, namely tiphenylphosphine.22 In current studies of phosphines on different metal surfaces we have used XPS and IFUS. Core level XPS is used to get the chemical composition of the monolayer as well as information about how strongly adsorbates are bound to the surface. Angle dependent XPS is used to estimate how the molecules are oriented on the surfaces. For basic concepts of XPS and for further details regarding angle dependent XPS used for molecular orientation studies, see refs 23-25. Data (10)Uvdal, K.; Bodo, P.; Liedberg, B. J. Colloid Interface Sci. 1992, 149 (l), 162. (11)Ihs, A.;Liedberg, B. J. Colloid Interface Sci. 1991, 144,282. (12)(a) Tengvall, P.;Lestelius, M.; Liedberg, B.; Lundstrtim, I. Langmuir 1992,8, 1236. (b) Lestelius, M.; Liedberg, B.; Lundstrtim, I.; Tengvall, P. J. Biomed. Mater. Res. 1994,28,871. (13)Buergi, H.B.; Kunz, R. W.; Pregosin, P. S. Inorg. Chem. 1980, 19,3707. (14)Colton, R.;Dakternieks, D. Aust. J. Chem. 1981,34,323. (15)van Attekum, P. M. Th. M.; van der Velden; Trooster, J. M. Inorg. Chem. 1980,19,701. (16)Nefedov, V. I.; Salyn, YA. V.; Moiseev, I. I. Inorg. Chim. Acta 1979,35,L343. (17)Hoste, S.;Willemen, H.; van de Vondel, D.; van der Kelen, G. P. J . Electron Spectrosc. Relat. Phenom. 1976,5,227. (18)Johnson, 0.Chem. Scr. 1975,8, 166. (19)McNeillie, A.; Brown, D. H.; Smith, W. E. J. Chem. SOC. Dalton 1980,5,767. (20)Morgan, W. E.;Stec, W. J.; Abridge, R. G.; van Wazer, J. R. Inorg. Chem. 1971,10,926. (21)Leigh, G.J.;Bremser, W. J. Chem. SOC.Dalton Trans. 1972, 1216. (22)Steiner,U.B.; Neuenschwander, P.; Caseri, W. R.; Suter, U. W.; Stucki, F. Langmuir 1992,8, 90.
0743-746319512411-1252$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11,No. 4,1995 1253
Tricyclohexylphosphine Adsorbed on Gold
containingdetailed informationabout molecular structure and the molecular orientation on the surface is obtained by IRAS. For further details regarding IRAS, see refs 26 and 27. Thus, XPS and IRAS provide a combination that is very powerful in studies of the orientation of adsorbed molecules on metal surfaces. In the following we will present IRAS and XPS data for tricyclohexylphosphine adsorbed onto a gold surface.
Cls
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9
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Experimental Section The tricyclohexylphosphinewas obtained from Strem Chemicals Inc. The gold substrates were prepared by thermal evaporation onto clean silicon single-crystal (111)wafers. The silicon wafers had been primed with an adhesion layer of 5-10 Achromium before the evaporation ofgold. The gold layers were about 2000 A thick. The base pressure was 2 x Torr during evaporation and the evaporation rate was 5 &s. These films have a preferred (111)texture, and a grain size of 70-300 A. Further details and characterization of these gold films have been described elsewhere.10 The tricyclohexylphosphine adsorbates were made from cyclohexane and toluene solutions with a concentration of 1mM. No apparent differenceswere observed between the two solvents. The gold films were immediately immersed into solution after they had been taken out of the evaporation chamber. After an incubation time of 20 min the substrates were removed from the solution, gently rinsed in the solvent and then dried by letting a stream of gaseous nitrogen pass over the surface. The samples were then immediately put into the XPS spectrometer. The XPS spectra were collected on a commercial VG spectrometer. The measurements were carried out with unmonochromatized Mg Ka photons (1253.6 eV). The resolution was determined from the full width at half-maximum (FWHM)of the Au(4f712) line which was 1.5 eV. The pressure during the measurements was approximately Torr. A pressed pellet of the starting material was prepared as a reference to the adsorbate. When measuring a pressed pellet of cyclohexylphosphine,there was a small charging of the sample. The spectrum was lined up through the C(ls) peak (285.3 eV) in the monolayer. The starting material seemed to contain a large amount of oxidized tricyclohexylphosphine. We tried to obtain the unoxidized reference by chemically purifying the starting material, but since the material easily oxidizes,the bcst result was obtained by a sublimation process in ultrahigh vacuum. Since the unoxidized tricyclohexylphosphine fraction turned out to evaporate at lower temperature than the oxidized one, it was possible to separate the two fractions and collect an unoxidized multilayer. The substrate was slightly cooled t o facilitate the multilayer formation. The XPS results from the pressed pellet and the evaporated multilayer were then used for comparison with the studies ofthe adsorbates made from solution. Angle dependent XPS was used to estimate how the molecules in the adsorbates were oriented on the surfaces. The surface sensitivity is correlated to the angle of the analyzed electrons and this is due to the relatively short electron mean free path in organic materials.28 Thus, in the surface sensitive mode, the electron intensity from outermost atoms is enhanced relative to that of deeper lying atoms. In this work, the bulk sensitive mode and surface sensitive mode correspond to electron take off angles of 30" and 70" (relative to the surface normal), respectively. The IR unit was a Bruker IFS 113 v Fourier transform spectrometer equipped with a DTGS detector. The R-A spectra were obtained with a Bruker GIR (grazing angle incidence reflection) accessory29aligned at 83" and with 4 cm-l resolution. (23) Fadely, C. S. Basic Concepts ofx-ray Photoelectron Spectroscopy; Univ. Hawaii: HI, 1978; Vol. 2, p 1. (24) Fadely, C. S. J.EZectron Spectrosc. Relat. Phenom. 1974,5,895. (25) Salaneck, E. W.; Uvdal, K.; Elving, H.; Askendal, A.; Salaneck, W. R. Colloid Interface Sci. 1990,136 (2), 440. (26) Greenler, R. G. J. Chem. Phys. 1969,50,1963. (27)Debe, M. K. J.Appl. Phys. 1984,55,3354. (28) Clark, D. T.; Thomas, H. R. J. Polym. Sci. Polym. Chem. Ed. 1977,15,2843. (29)Ihs, A.; Liedberg, B.; Uvdal, K; Tornkvist, C.; Bodo, P.; Lundstrom, I. J. Colloid Interfuce Sci. 1990,140,192.
4
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Binding energy eV Figure 1. The C(1s) XPS core level spectra for tricyclohexylphosphine: (a)pressed pellet of the starting material, (b) multilayer prepared by physical vapor deposition, (c) adsorbate on gold prepared from solution. 1.2
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.+
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0.8
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.+
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0.2 0.0
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Binding Energy eV Figure 2. The P(2p) XPS core level spectra for tricyclohexylphosphine: (a)pressed pellet of the starting material, (b) multilayer prepared by physical vapor deposition, (c) adsorbate on gold prepared from solution. The FTIR samples were made as described above and gently rinsed in the solvent used. Without the rinsing procedure the peak in the IR spectrum becomes strong, broad, and indistinct, indicating some kind of aggregation on the surface. As a comparisonto the adsorbates, a pressed KBr pellet of the starting material of cyclohexylphosphinewas analyzed in the transmission mode.
Results and Discussion X-rayPhotoelectron Spectroscopy. Pressed Pellet. The core level XPS spectra of tricyclohexylphosphine are shown in Figures 1-3. The carbon binding energy spectrum for the pressed pellet of tricyclohexylphosphine shows one sharp, symmetric peak at 285.3 eV(Figure la). Furthermore, one state of phosphorus is observed in the tricyclohexylphosphine pressed pellet (Figure 2a). The
1254 Langmuir, Vol. 11, No. 4, 1995
Uvdal et al.
"
-550
-540
-530
-520
Binding energy eV Figure 3. The O(ls) XPS core level spectra for tricyclohexylphosphine: (a)pressed pellet of the startingmaterial,(b) multilayer prepared by physical vapor deposition, (c) adsorbate on gold prepared from solution. Table 1. XPS Binding Energies in Electron Volts and Relative Intensities for Tricyclohexylphoephine binding energy (eV) relative intensity value Cls P2p 01s C/P c/o pressed pellet 285.3 132.5 533.1 17.5 3.3 17.3 50.6 evaporated 285.3 130.2 533.3 adsorbate 285.3 130.7 533.9 27.0 13.5
phosphorus peak consists of a spin-split doublet, with the P(2p312)and the P(2Pm) binding energies at 132.5 and 134.0 eV, respectively. The relative intensity ratio between the C(1s)and P(2p) peaks, with the cross section taken into account, is 17.5 (see Table l),which is close to the stoichiometric value, 18. This indicates a starting material free from hydrocarbon contamination. However, there is a large contribution from oxygen in the XPS spectrum due to the fact that tricyclohexylphosphine is easily oxidized in contact with air. Also, some tightly bounded water could contribute to the oxygen signal. The relative intensity ratio between the C(ls)and O(1s)peaks with the cross section taken into account is 3.3. Evaporated Multilayer. We tried a physical vapor deposition process (see the Experimental Section) to decrease the oxygen content. The core level XPS spectra for these UHV-prepared multilayers are shown in Figures lb, 2b, and 3b. The carbon binding energy peak, with a position at 285.3 eV, is still sharp and symmetric. The P(2p312)binding energy peak position is 130.2 eV, which is much lower than for the pressed pellet. The relative C/P ratio is 17.3 (see Table 11, which is still close to the stoichiometric value, indicating that the evaporated multilayer is free from hydrocarbon contamination. The relative C/O ratio has increased to 50.6, indicating a very low oxygen content compared to the pressed pellet. The P ( 2 p d binding energy peak position is in agreement with the low oxygen content. Thus, the evaporated multilayer seemed to be a better reference to the adsorbate than the pressed pellet, especially when studying chemical shifts and molecular binding strength to the substrate. The P ( 2 p d binding energy peak position found in the literature differs from article to article and this may have to do with the degree of oxidationof the material. The P(2pm) binding energy peak position found for the starting
material, 132.5eV, is very close to what was reported for triphenylphosphine oxide and trioctylphosphine oxide, with the binding energy peak position at 132.8 and 132.6 eV, respectively.20 Adsorbate. The C(1s)spectrum for adsorbate prepared from solutionis shown in Figure IC.There are no changes in the line shape of the C(1s) spectrum for the adsorbate comparedwith the evaporated multilayer and the pressed pellet, indicating that the hexyl rings are intact during adsorption. The P(2pm) binding energy peak position is 130.7eV. The chemical shiR to lower binding energy when compared to the starting material, 132.5 eV, indicates a selectiveadsorption of nonoxidized tricyclohexylphosphine to the gold surface. This significant selectivityis further supported by IRAS results (see below). The relative intensity ratio between the C(ls) and P(2p) peaks, with the cross section taken into account,is 27, which is higher than for the pressed pellet and evaporated multilayer. There are several effects correlated to the relatively high C/P ratio. Some hydrocarbon contamination for monolayers prepared in this way is expected since gold has a high affinity for hydrocarbons. The freshly evaporated gold samples were briefly exposed to air before immersion into the adsorption solution. Even a small amount of hydrocarbon contamination on the gold surface will show up in the stoichiometricvalue. Effects correlatedto escape depth for an oriented monolayer will result in suppression ofthe P(2p) signal. Thus, the molecular orientationcaused by P-Au bonding is expected to reduce the P(2p) signal. The molecular binding to the metal surface may also induce cross sectional changes of the P(2p)peak. Similar effects have been observed for thiol monolayers. Although a drastic decrease in the oxygen contentoccurs during the adsorption process, there is still some oxygen present in the system. Since P(2pm)binding energy peak position as well as the line shape of the C(1s) spectrum indicate that the adsorbed phosphine molecules are free from oxygen, this indicates coexistence of oxygen and phosphines on the surface. The oxygen content may be correlated to a low concentration of dissociated oxygen on the gold surface and thus the tricyclohexylphosphine molecules seem to preferentially adsorb on empty sites on the gold surface. Coadsorption (coexistence)of oxygen on the surface may slightly affect the binding energy peak position of the P(2p) on the order of a few tenths of an electron volt.18 In the case of adsorption of phosphines onto more catalytic metals, the dissociation of oxygen on the surface will affect the adsorption of phosphines much more. This will be discussed in more detail in a forthcoming paper. In the following section we will compare the adsorbate with the evaporated mutilayer. There is a chemical shift of 0.5 eVto higher binding energy in the P(2p3n)spectrum for the monolayer when compared to the evaporated multilayer. This indicates an adsorption through the phosphorus atoms to the metal surface and that the adsorbate is an electron donor; i.e., there is an electron transfer from the phosphine to the substrate. Angle dependent XPS measurements show an increase of C/P ratio in the surface sensitive mode that supports a molecular orientation with the phosphorus atoms close to the surface. Different palladium-triphenylphosphine compounds have shown chemical shifts of 0.4-0.8 eV to higher binding energy, depending on the exact composition of the compound, when compared to the free ligand.16 Infrared Refiction-AdsoTption Spectroscopy. A transmission spectrum of a KBr pellet of tricyclohexylphosphine is shown in Figure 4. This spectrum shows all the IRactive modes of vibration in the molecule, without any metal-surface interaction, and the spectrum is taken as
Langmuir, Vol. 11, NO. 4, 1995 1255
Tricyclohexylphosphine Adsorbed on Gold 1.0
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WAVENUMBER cm” Figure 4. Transmission IR spectrum of KBr pellet of tricyclohexylphosphine. a reference to the reflection spectra of the monolayers. The CHZstretching vibrations in the transmission spectrum of tricyclohexylphosphine can be seen at 2927 and 2851 cm-l, respectively. The band at 1447 cm-’ can be assigned to a CHZ scissoring vibration and the bands around 1000cm-’ correspond to different ring vibrations. The band at 1160 cm-l corresponds to P-0 stretching vibration, showing that the starting material is partly oxidized. A reflection spectrum of multilayer tricyclohexylphosphine prepared by physical vapor deposition is shown in Figure 5b. The strong CHZstretching vibrations can be seen at 2929 and 2852 cm-’, respectively, and the CHZ scissoring vibration is observed a t 1442 cm-’. The P-0 stretching mode, initially seen at 1160 cm-l in the spectrum of the starting material (Figure 4), is no longer present in the spectrum of the evaporated multilayer. This observation is in agreement with the XPS results which show that the evaporated multilayer is free from oxygen. A reflection spectrum of tricyclohexylphosphine adsorbed from cyclohexane solution followed by rinsing in cyclohexane and ethanol is shown in Figure 5a. The CH2 stretching vibrations are strong even for the adsorbate and the CHZ scissoring vibration can still be seen, indicating that the hexyl rings are intact in the adsorbate. The P-0 stretching vibration for the adsorbate is gone, supporting a selective adsorption of nonoxidized tricyclohexylphosphine. Care must be taken, though, when analyzing IRAS intensity data from organized assemblies on metals. This is due to the surface dipole selection rule which only allows modes with transition dipole moments aligned perpendicular to the surface to be active and appear in the IRAS spectrum. Thus, it would not be possible to see the P=O mode in a spectrum of highly organized monolayer where all of the P-0 bonds are aligned parallel to the surface. However, we do not believe that the absence of the P-0 mode in the monolayer spectrum is due to an orientation effect. The XPS measurements do not show a P(2p) peak characteristic for P=O units, nor do they show evidence for organic phosphate P-0-C units in the C(1s) spectrum. The total intensity of the monolayer spectrum is very weak, indicating a low packing density of the phosphine adsorbates. The packing ofphosphineson gold is expected
c t
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Figure 6. IFUS spectrum of tricyclohexylphosphine: (a) adsorbate on gold prepared from solution, (b) multilayer prepared by physical vapor deposition.
to be less dense than the ( 4 3 x d3)R3Oo overlayer structure observed for alkyl thiols, as the phosphines occupy a much larger surface area due to its “umbrella shape”the C-P-C angle in about 105°.30~31 Nevertheless, the chemisorbed tricyclohexylphosphine complex seems to be stable. A recent temperature-programmed desorption (TPD) study shows that a monolayer of dimethylphenylphosphine is stable up to about 500 Kin ultrahigh vacuum.32 The chemisorbed tricyclohexylphosphinecomplex seems to be very stable; consequently, the Au-P bond is strong, and gold surfaces can certainly be modified by trialkylphosphines. More extensive studies on the chemisorption of trialkyl- and triarylphosphines on the coinage and platinum group metals are under way. The trialkylphosphines have the strongest eledron-pair donor properties of all ligands,while oxygen donor ligands, e.g. trialkylphosphine oxide, have significantly weaker electron-pair donor abilities.33 It is expected that the ligand with the strongest electron-pair donor properties will be chemisorbed to the gold surface; thus the goldligand bond has mainly covalent character. The significant chemisorption observed also indicates that the gold atoms in the surface are good electron acceptors. (30) Sjbgren, B. Unpublished results. (31)Haberlen, 0. D.; Riisch, N. J.Phys. Chem: 1993,97, 4970. (32) Kariis,H.;Persson,I.; Westemark, G.;Liedberg, B. Manuscript in preparation. (33) Sandstrbm, M.;Persson,I.; Persson, P.Acta Chem. Scand. 1990, 44, 653.
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1256 Langmuir, Vol. 11,No. 4, 1995
Conclusion We have studied the molecular orientation and the binding strength for tricyclohexylphosphine adsorbed on to gold. Detailed XPS and IRAS data are presented fora pressed pellet, a physically evaporated multilayer of tricyclohexylphosphine, and tricyclohexylphosphine adsorbed from solution on to gold. The starting material contained a huge fraction of oxidized tricyclohexylphosphine. However, the drastic decrease of the oxygencontent after adsorption,the P(2p312)binding energy peak position for the adsorbate, and the line shape of the C(ls) spectrum, as well as the lack of P=O stretching vibration in IRAS spectrum, indicate a selective adsorption of nonoxidized tricyclohexylphosphine on to the gold surface.
The evaporated multilayer of tricyclohexylphosphine showed a very low oxygen content and was therefore a useful reference for the adsorbate. Accordingto bothXPS and IRASresults, the molecular structure is intact during adsorption. The shift in the P(2p) level during adsorption strongly indicates a chemisorption through the phosphorus atoms where the adsorbates donate electrons to the substrate.
Acknowledgment. The financial support of this work by the SwedishResearch Councilfor EngineeringSciences (TFR)is gratefully acknowledged. LA9403385
