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Underpotentially Deposited Copper Promotes Self-Assembly of Alkanephosphonate Monolayers on Gold Substrates Murray V. Baker,† G. Kane Jennings,‡ and Paul E. Laibinis*,‡ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Chemistry, University of Western Australia, Nedlands, Western Australia 6907, Australia Received September 17, 1999. In Final Form: December 31, 1999 Gold surfaces that had been previously modified electrochemically by the underpotential deposition (upd) of a monolayer of copper adsorb alkanephosphonic acids from solution to form oriented monolayers. While bare gold surfaces do not adsorb alkanephosphonic acids, a layer of copper at less than a monolayer is sufficient to alter the chemisorptive properties of the gold substrate. Octadecanephosphonic acid forms oriented oleophobic monolayers on the Au/Cu(upd) surface that are stable in both polar and nonpolar solvents. The alkanephosphonate films on Au/Cu(upd) are less densely packed than those that form on copper, but the Au/Cu(upd) substrate offers greater tolerance against oxidation and greater convenience of use. Electroactive monolayers can be formed on Au/Cu(upd) using 11-ferrocenylundecanephosphonic acid, where the assembled films exhibit the reversible electrochemical signature of the ferrocenyl group and a surface coverage of ∼4.0 × 10-10 mol/cm2.
Introduction Gold is often used in the construction of electrochemical sensors and other devices because it is highly conductive, it resists oxidation by air, and it can be easily evaporated to produce platforms such as thin films, interdigitated electrodes, and metal interconnects. Gold can be functionalized by adsorption to produce organic monolayers on its surface that can expose a variety of chemical groups; however, the organic compounds that can be used to functionalize gold surfaces are limited to those that contain a “soft” atom or functional group, such as various sulfurbased compounds (e.g., thiols,1,2 disulfides,3 sulfides4), phosphines,2 and selenols.5 Organic functionalities such as carboxylic acids and hydroxyl groups, which are dominated by “hard” atoms and have a strong affinity for oxide surfaces such as ZrO2, TiO2, and Al2O3, show no (or weak) affinity for gold.1 For example, the oxidation of adsorbed thiols on gold produces sulfonates that interact less strongly with gold and desorb readily from the surface.6 This oxidation process limits the lifetime and usefulness of self-assembled monolayers (SAMs) on gold (and other metals), and methods to form robust adherent SAMs using adsorbates containing “hard” headgroups that would be stable against oxidation might offer advantages for practical applications that require extended exposure to the atmosphere. † ‡
University of Western Australia. Massachusetts Institute of Technology.
(1) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (2) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (3) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (4) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (5) Samant, M. G.; Brown, C. A.; Gordon, J. G. Langmuir 1992, 8, 1615. (6) Li, Y.; Huang, J.; McIver, R. T., Jr.; Hemminger, J. C. J. Am. Chem. Soc. 1992, 114, 2428. Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305.
Alkanephosphonates interact strongly with many divalent transition-metal ions (including Mn,7,8 Fe,7 Co,7 Ni,7 Cu,7,9-11 and Zn7,8,12) to form layered crystalline materials and have attracted interest as agents for the formation of Langmuir-Blodgett films.13 Alkanephosphonates adsorb from solution onto the surfaces of mica14 and various metal oxides7,15-17 to form self-assembled monolayers. For the development of electrochemical sensors, the insulating nature of these substrates limits their use. For such purposes, gold would be a preferred substrate because of the ease of fabricating structures with this metal by a variety of deposition techniques. However, the phosphonate group is a “hard” group, and alkanephosphonates do not interact strongly with gold to form stable self-assembled monolayers on this substrate. Alkanebisphosphonate multilayers have been adsorbed onto gold surfaces modified with functionalized alkanethiolate anchoring layers,10,11,18 and Cu/alkanebisphospho(7) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (8) Cao, G.; Lee, H.; Lynch, V. M.; Mallouk, T. E. Inorg. Chem. 1988, 27, 2781. (9) Poojary, D. M.; Zhang, B.; Bellinghausen, P.; Clearfield, A. Inorg. Chem. 1996, 35, 4942. (10) Brousseau, L. C., III; Mallouk, T. E. Anal. Chem. 1997, 69, 679. (11) Brousseau, L. C., III; Aurentz, D. J.; Benesi, A. J.; Mallouk, T. E. Anal. Chem. 1997, 69, 688. (12) Ortiz-Avila, Y.; Rudolf, P. R.; Clearfield, A. Inorg. Chem. 1989, 28, 2137. (13) Byrd, H.; Pike, J. K.; Talham, D. R. Chem. Mater. 1993, 5, 709. Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 295. (14) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626. (15) Hong, H.-G.; Mallouk, T. E. Langmuir 1991, 7, 2362. Katz, H. E.; Schilling, M. L. Chem. Mater. 1993, 5, 1162. Hanken, D. G.; Corn, R. M. Anal. Chem. 1995, 67, 3767. (16) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. Horne, J. C.; Huang, Y.; Liu, G.-Y.; Blanchard, G. J. J. Am. Chem. Soc. 1999, 121, 4419. Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182. (17) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (18) Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S.-L.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855.
10.1021/la991247g CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000
Self-Assembly of Alkanephosphonate Monolayers
Figure 1. Schematic illustration of a Au/Cu(upd)/SAM assembly.
nate multilayer structures have been used as sensors for NH310 and CO2.11 Alkanephosphonate SAMs that exhibit reversible electrochemical behavior have recently been formed onto indium-tin oxide.19 Various metals, including copper and silver, can be deposited at submonolayer coverages onto gold substrates by the underpotential deposition (upd) process.20 Previous work in this laboratory has shown that self-assembled monolayers of alkanethiolates form on gold modified with upd layers of copper21 or silver21,22 and that the resulting SAMs (Figure 1) are more robust than similar assemblies on gold that lacked the upd layer.21,22 In this paper, we show that the affinity of alkanephosphonic acids for the gold surface can be readily modified by the upd of copper. A single atomic layer of copper is sufficient to allow formation of robust alkanephosphonate SAMs on gold and to retain the ability of the gold substrate to function as an electrode. Experimental Section Octane- and octadecanephosphonic acids were obtained from Alfa Chemicals and used as received. 11-Ferrocenylundecanephosphonic acid was synthesized by Dr. Bentley J. Palmer following the procedure of Lee.23 Gold films bearing upd copper adlayers were prepared on silicon substrates as described previously.21 The surface coverage of copper (φCu) on these samples was ∼0.7, as determined by X-ray photoelectron spectroscopy (XPS).21 Immediately after deposition of the copper adlayer on a sample in 1 mM CuSO4/0.l M H2SO4(aq),21 the sample was rinsed with deionized water, blown dry under a stream of N2, and placed in a 2.5 mM solution of an alkanephosphonic acid in absolute ethanol. After a period of 30 min to 48 h, the sample was removed from the solution, rinsed with ethanol, and blown dry under a stream of N2. Samples were characterized by contact angle goniometry, XPS, and reflectance infrared spectroscopy as described previously.21,24 When samples were to be characterized by ellipsometry, the gold substrates were cleaned with an argon plasma (Harrick PDC-23G plasma cleaner, low power, 0.3 Torr, 2 min) immediately prior to upd of Cu, and ellipsometric constants for the resulting Au/Cu(upd) substrate were measured within 3 min of completion of the upd procedure. The freshly plasmacleaned gold substrates were wet by water both before and after the upd of copper. Ellipsometry of the derivatized substrate and calculation of the thickness were as described previously.21 Cyclic (19) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927. (20) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1978; Vol. 11; pp 125-271. (21) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208. (22) Jennings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6173. (23) Lee, E. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1993. (24) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 113, 7152.
Langmuir, Vol. 16, No. 7, 2000 3289 voltammetry of ferrocene-terminated monolayers was performed inside a N2-filled drybox. The electrolyte (freshly prepared 0.1 M tetrabutylammonium hexafluorophosphate in O2-free acetonitrile) was contained in a glass cell. A gold substrate without a copper adlayer was used as the counter electrode, and the reference electrode was silver wire in contact with 0.01 M AgNO3 in O2-free acetonitrile (+450 mV vs SCE). Coverages are based on geometric areas and do not take in account effects due to surface roughness. Other details for the electrochemical studies were as described previously.21 SAMs were also adsorbed from ethanolic solutions of octadecanephosphonic acid onto evaporated Cu films (∼1000 Å) following procedures outlined above for upd Cu films. Because of the former’s susceptibility to oxidation in the laboratory atmosphere, SAMs on copper were formed immediately after removal of films from the evaporator and were not characterized by ellipsometry.
Results and Discussion Au/Cu(upd)/SAM Formation. When gold substrates bearing upd adlayers of Cu were immersed in solutions of octane- or octadecanephosphonic acid in ethanol for 12 h, an alkanephosphonate25 monolayer formed on the Au/ Cu(upd) surfaces (see below). The resulting SAMs were not removed by rinsing the sample with various solvents (ethanol, isooctane, and water). Both before and after adsorption of the SAMs, the coverage of copper on the gold substrates, as determined by XPS,21 was 0.70 ( 0.06,26 even after exposure of samples to an alkanephosphonic acid solution for up to 4 days. The XPS data suggest that the copper adlayer is not removed from the gold substrate by exposure to the alkanephosphonic acid and that it does not readily diffuse into the gold film. The presence of the upd Cu adlayer is, however, essential for formation of a robust monolayer. For example, when gold substrates were immersed in a solution of octadecanephosphonic acid in ethanol and then rinsed with ethanol, no monolayer could be detected by wettability or ellipsometry studies. These gold surfaces exhibited water contact angles of 85° (advancing) and 50° (receding), were wet by hexadecane, and had thicknesses of less than 5 Å by ellipsometry, characteristics that are typical of gold films bearing adventitious surface contaminants. The highest quality alkanephosphonate SAMs, as indicated by contact angle measurements and infrared spectroscopy, were obtained using evaporated gold substrates within 2 days of preparation and adsorption times of 12-48 h from 2.5 mM solutions of the alkanephosphonic acid in ethanol. The data presented are for SAMs formed under these conditions. Contact Angle and Ellipsometry Measurements. Octadecanephosphonate SAMs on Au/Cu(upd) showed advancing contact angles (θa) for water and hexadecane of ∼111° and 28°, respectively (Table 1). These values are compatible with the formation of a densely packed oriented monolayer on the Au/Cu(upd) surface. While both liquids exhibited lower contact angles and greater contact angle hystereses (∼30°) than those observed for thiol-based monolayers on the gold and Au/Cu(upd) surfaces,21 θa(H2O) was similar to that reported for Y-type bilayer films derived from octadecanephosphonic acid and ZrOCl2;13 contact angles for water (receding) and for other (25) For convenience, monolayers derived from alkanephosphonic acids are referred to as alkanephosphonate monolayers throughout this paper. The ionization state of the phosphonic acid groups in these monolayers (i.e., RPO3H2, RPO3H-, RPO32-, or some mixture of these forms) is not known and could not be established. (26) We note that the silver coverage of 0.7 is less than that of 0.9 previously reported by us in ref 21. The value of 0.7 reflects a more accurate estimate of the coverage as the value of 0.9 was determined using an incorrect sensitivity factor for Au in XPS.
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Table 1. Wetting Properties and Ellipsometric Thicknesses of Various SAMs on Au/Cu(upd) and Cu Substrates
adsorbate/substrate CH3(CH2)7PO3H2/Au/Cu(upd) CH3(CH2)17PO3H2/Au/Cu(upd) CH3(CH2)17PO3H2/Cu CH3(CH2)17SH/Au/Cu(upd)d
contact angles (advancing, receding; in deg)a H2O hexadecane 106, 81 111, 80 116, 93 112, 98
22, 28, 36, 27 45, 35
ellipsometric thickness (Å)b 8 20 c 23
a A dash (-) indicates a receding contact angle where the liquid could not be removed from the surface. For these systems, θ ) 0°. r Substrates were cleaned in an Ar plasma immediately prior to upd of Cu, and base ellipsometric constants were measured for Au/Cu(upd) samples within 3 min of completing the upd procedure. c The need to minimize the oxidation of this substrate prevented determination of thickness by ellipsometry. d Data taken from ref 21.
b
probe liquids (advancing and receding) on such films have not been reported. The lower contact angles and higher contact angle hystereses for octadecanephosphonate SAMs on Au/Cu(upd) compared to those for octadecanethiolate SAMs on Au/Cu(upd) and of octadecanephosphonate SAMs on Cu films (Table 1) may reflect a lower density of alkyl chains within the octadecanephosphonate-Au/Cu(upd) system. Table 1 lists the ellipsometric thicknesses for alkanephosphonate monolayers adsorbed on samples that had been cleaned in an argon plasma immediately before upd of the copper adlayer. When the plasma cleaning step was omitted, the ellipsometric thicknesses of films were typically 5-7 Å less than those indicated in Table 1 and exhibited much lower reproducibility. These effects are presumably a consequence of contaminants1 that are adsorbed on the gold surface, remain through the upd process, and are displaced during formation of the SAM.27 Because the plasma cleaning step had no detectable effect on the wetting properties or infrared spectra of the monolayers, we consider the ellipsometric thicknesses determined for monolayers on substrates that were plasma cleaned prior to the upd step more reliable estimates of the monolayer thicknesses. These values for the alkanephosphonate SAMs are lower than those observed for SAMs of the corresponding alkanethiolates on Au (Table 1),1 consistent with the suggestion that the alkanephosphonate SAMs have a lower density of adsorbed groups. Reflectance Infrared Spectroscopy. Figure 2 shows reflectance infrared spectra for octane- and octadecanephosphonate SAMs adsorbed onto Au/Cu(upd) substrates as well as spectra obtained on evaporated Au and Cu substrates after their exposure to octadecanephosphonic acid; a spectrum for octadecanethiol on Au is also included for comparison. The spectrum for the octadecanephosphonate SAM on Au/Cu(upd) shows CH2 absorption bands at ∼2921 and 2852 cm-1 for the asymmetric and symmetric modes, respectively. These values are between those observed for liquid n-alkanes (2928 and 2856 cm-1)28 and octadecanethiolate monolayers on Au or Au/Cu(upd) (2918 and 2850 cm-1).21 The positions of the CH2 modes for the octadecanephosphonate SAM on Au/Cu(upd) indicate a higher density of gauche conformers28 than that for (27) The thickness of a monolayer by ellipsometry was determined by comparing measurements made on the original “clean” substrate (e.g., an evaporated Au film or a Au/Cu(upd) sample) before and after formation of a monolayer.1 Because any adventitious contaminants on the surface are typically displaced by the monolayer, ellipsometry can underestimate its thickness. Because the Au substrates were plasma cleaned immediately before a sequence of Cu upd and ellipsometric characterization, the Au/Cu(upd) samples used here likely had a lower level of adsorbed contaminants than substrates used in other studies without plasma cleaning. Consequently, the extent that substrate contamination would cause ellipsometry to underestimate the thickness of a monolayer should be less than that in other work. (28) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.
Figure 2. Grazing incidence polarized infrared spectra for octane- and octadecanephosphonate SAMs on Au/Cu(upd) and related substrates. For the octadecanephosphonate SAM on Au/Cu(upd), band positions are 2921 (CH2 asym), 2852 (CH2 sym), 2967 (CH3 asym), and 2880 (CH3 sym) cm-1. For the octanephosphonate SAM on Au/Cu(upd), band positions are 2927 (CH2 asym), 2856 (CH2 sym), 2967 (CH3 asym), and 2881 (CH3 sym) cm-1. The spectra obtained for evaporated Au (no upd Cu adlayer) and Cu films that had been exposed to octadecanephosphonic acid and for a Au film exposed to octadecanethiol are included for comparison. The spectra have been offset vertically for clarity.
octadecanethiolate SAMs on this substrate but much fewer than that for a purely liquidlike film. This less crystalline structure is consistent with a lower density of octadecyl chains within this SAM compared to those formed from octadecanethiol on Au and Au/Cu(upd). The presence of gauche conformers in the octadecanephosphonate SAM on Au/Cu(upd) prevented calculation of an average tilt for the film, as its structure does not conform to the rigid single-chain model used in such calculations. For both the octane- and octadecanephosphonate SAMs on Au/Cu(upd), the symmetric and asymmetric CH3 modes are clearly evident and have similar intensities in the two spectra. The CH2 modes are less intense in the spectrum of the former SAM, as expected given the fewer number of CH2 groups in the octyl system. The CH2 modes for the octanephosphonate SAM occur at higher frequencies than those for the octadecanephosphonate SAM, indicative of a more liquidlike character for the shorter alkyl chains. Similar effects of chain length have been observed for alkyl-based SAMs on other substrates.29,30 (29) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
Self-Assembly of Alkanephosphonate Monolayers
In the spectrum of the octadecanephosphonate SAM on Cu, the intensities of the CH2 bands are significantly less than those in the spectrum of the corresponding SAM on Au/Cu(upd). This observation suggests that the octadecyl chains are oriented closer to the surface normal and are present in a higher density in the SAM on the Cu substrate. The less dense structure on the Au/Cu(upd) surface may be a result of the lower density of copper sites (∼0.7 of a monolayer) for adsorption. In the spectrum for the film adsorbed from an octadecanephosphonic acid solution onto a bare Au substrate, the absence of identifiable CH3 modes and the positions of the CH2 modes and their low intensities indicate that a well-defined octadecanephosphonate SAM did not form on the gold surface. The spectrum is more compatible with the adsorption of adventitious organics rather than formation of a structured layer of the adsorbate as occurs when an atomic layer of copper atoms is present on the gold surface. In comparing the spectra of the octadecanethiolate SAM on Au and the octadecanephosphonate SAM on Au/Cu(upd), we note that the spectrum for the latter SAM exhibits broader CH2 peaks, albeit with integrated intensities similar to those of the peaks for the former sample. The data suggest that the alkyl chains within the two SAMs share some similarity in their canted structures; however, the former sample has a structure that is more homogeneous and better defined. A notable difference between the two spectra is the relative intensities of the methyl modes, where the intensities of the asymmetric and symmetric methyl modes are almost equal for the octadecanethiolate SAM on Au and are different for the octadecanephosphonate SAM on Au/Cu(upd). The intensity distribution for the latter is similar to that for n-alkanethiolate SAMs on gold that contain an odd number of carbons in the alkyl chain.30 The different spectral characteristics reflect different average orientations for the terminal methyl groups and can result from a preferred geometry for coordination of the headgroup with the surface. This inference is compatible with the structure proposed later in this paper. In principle, the phosphonate groups can also be characterized by examining the P-O absorption bands in the region 1000-1150 cm-1. Alkanebisphosphonatecopper multilayer films have been prepared by Mallouk and co-workers,10,11,18 and these films exhibited a strong band in this region. For our alkanephosphonate SAMs on Au/Cu(upd), this region of the infrared spectra was obscured by other absorbances and we could not detect distinguishable infrared bands due to P-O groups. Alkanephosphonic acids also show OH stretching vibrations in the ranges 2700-2500 and 2350-2100 cm-1,18 but we observed no signals in these regions of the infrared spectra for our alkanephosphonate SAMs. This observation may reflect a deprotonated state for the adsorbed phosphonate groups. XPS. Table 2 summarizes the XPS data for an octanephosphonate SAM on Au/Cu(upd). We characterized the shorter-chained SAM in order to minimize attenuation of signals from the substrate and headgroup. The Cu(2p3/2) region of the XPS shows signals at 931.3 and 934.0 eV (in a 10:1 ratio) that we attribute to the upd Cu layer and some more oxidized Cu2+ species, respectively. Underpotentially deposited Cu on Au is susceptible to oxidation, and the Cu binding energies are similar to those we have observed previously.21 The Cu(2p3/2) binding energy of (30) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.
Langmuir, Vol. 16, No. 7, 2000 3291 Table 2. XPS Data for Octanephosphonate SAM on Au/Cu(upd) element
binding energy (eV)a
atomic ratiob
Cu
931.3 934.0 530.0 531.7 189.1 284.2
2.2 ( 0.4 0.3 ( 0.1 3.0 ( 0.1 0.6 ( 0.2 1.0
O P C
a Binding energies referenced to Au(4f ) at 84.0 eV. b Atomic 7/2 ratios are corrected for attenuation effects24 and normalized to phosphorus content.
931.3 eV for the Cu adlayer is the same as that observed for unfunctionalized Au/Cu(upd) samples.21 In contrast, when Au/Cu(upd) samples are modified by an alkanethiol, the Cu(2p3/2) signal shifts to 931.8 eV.21 We suggest that these binding energies reflect the nature of the interaction between the upd Cu layer and the binding group of the adsorbed species, with the Cu(upd)/phosphonate interaction being essentially an ionic interaction, whereas the Cu(upd)/thiolate interaction likely results from a formal oxidative process. The O(1s) region of the XPS also showed two signals, at 530.0 and 531.7 eV (in a 5:1 ratio), that we attribute to adsorbed phosphonate groups and present oxides, respectively. Because the Cu, O, and P atoms are located at approximately the same depth within the sample, the XPS intensities of these elements can be used to determine their stoichiometric ratio. By assuming that the photoelectrons from these elements are attenuated only by a hydrocarbon film with a thickness of 7-11 Å, we calculate the atomic ratio of Cu:O:P to be approximately 2:3:1. This ratio suggests coordination of one phosphonate headgroup for every two copper atoms (i.e., “2Cu+” “RPO32-”). Electrochemistry. In addition to SAMs prepared from simple alkanephosphonic acids, we examined SAMs prepared from 11-ferrocenylundecanephosphonic acid. In these SAMs, the ferrocenyl group serves as an electroactive tag that can be used to quantify monolayer stability and surface coverage. These SAMs also provide a demonstration of the suitability of using upd substrates for generating electroactive systems. The SAMs were prepared by the above methods and showed the expected signals in the infrared spectrum (CH2 bands centered at 2928 and 2853 cm-1 and a weak band, attributed to the ferrocenyl C-H vibrations, at 3103 cm-1). The larger size of the ferrocenyl tail group prevents dense packing of the alkyl chain and results in its more liquidlike state. The electrochemical properties of the ferrocene-terminated SAM on Au/Cu(upd) were explored by cyclic voltammetry. The Au/Cu(upd)/ferrocenylundecanephosphonate SAMs proved sufficiently robust in O2-free anhydrous CH3CN to allow exploration of their electroactivity (Figure 3).31 The cyclic voltammograms for the SAMs exhibited the reversible chemistry expected for the Fc+/0 redox couple and showed peaks centered at ca. +150 mV with respect to Ag+/0. A linear increase in peak current density with scan rate was observed and confirmed the attachment of the ferrocenyl groups to the electrode surface. The coverages obtained by coulometry for the adsorbed ferrocenyl species were 4.0 × 10-10 mol/cm2. This value is lower than the values of (4.3-5.6) × 10-10 mol/ cm2 reported for ferrocenylalkanethiolates on Au21,32 and Au/Ag(upd)21 on evaporated substrates, indicating a lower (31) After 20 continuous cyclic voltammograms, the integrated intensity for the ferrocene signal for an adsorbed ferrocenylundecanephosphonate SAM on the Au/Cu(upd) substrate was ∼85% of its initial value.
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Figure 3. Cyclic voltammograms of a monolayer prepared from Fc(CH2)11PO3H2 supported on Au/Cu(upd). The electrolyte was 0.1 M [Bu4N]PF6 in anhydrous O2-free CH3CN.
density of adsorbates in the Au/Cu(upd)/phosphonate system than in the alkanethiolate-based SAMs. Comparisons to Other Systems. The above studies collectively suggest that the alkanephosphonic acids form oriented monolayers with a less densely packed and less well-defined structure than those for alkanethiolate SAMs on gold. Detailed electrochemical studies of the underpotential deposition of Cu from aqueous CuSO4 solutions onto Au(111) substrates indicate that copper and sulfate coadsorb on the gold surface in a 2:1 ratio for Cu coverages between 0.25 and 0.7533 and that the adsorbed Cu has an oxidation state close to +1.34 For upd copper films on Au(111) from sulfate solutions, the Cu atoms deposit into the 3-fold hollow sites of a gold surface and form a honeycomb lattice, with the three oxygen atoms from each sulfate group bonding to Cu atoms.35,36 For the SAM, the substitution of the adsorbed sulfate groups (O-SO32-) by alkanephosphonate groups (R-PO32-) could produce an isostructural adlayer such as that illustrated in Figure 4. The packing density and positions of the adsorbed groups in Figure 4 are similar to the reported (x3 × x3)R30° structure for alkanethiolate SAMs on Au(111).37 For the alkylphosphonate SAMs, the binding energy of Cu and the Cu:O:P atomic ratios from XPS are consistent with such a structure for the monolayer (Figure 4). We note that the infrared data in Figure 2 are incompatible with this well-defined structure and suggest a somewhat lower density of adsorbed groups for our prepared alkanephosphonate SAMs. The lower density of adsorbed groups in the present system may be a consequence of imperfections in the upd process (e.g., that result from the polycrystalline nature of the Au substrate or the adsorption of contaminants to the substrate that disrupt the upd process), the partial conversion of Cu(upd) atoms into Cu oxide, or the adsorption of contaminants to the Cu(upd) surface that resist displacement by the alkanephosphonate groups. The configuration of the phosphonate headgroupswith its three oxygen atoms localized on the Cu upd adlayer (32) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128-1132. Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (33) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 365, 303. (34) Tadjeddine, A.; Guay, D.; Ladouceur, M.; Tourillon, G. Phys. Rev. Lett. 1991, 66, 2235. (35) Huckaby, D. A.; Blum, L. J. Electroanal. Chem. 1991, 315, 255. Blum, L.; Huckaby, D. A. J. Electroanal. Chem. 1994, 375, 69. (36) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Yee, D.; Sorensen, L. B. Phys. Rev. Lett. 1995, 75, 4472.
Figure 4. Possible structure for alkanephosphonate SAMs on Au/Cu(upd). The adlayer structure is based on one proposed by Huckaby and Blum35 and confirmed by Toney et al.36 for the underpotentially deposited Cu/sulfate layer formed on Au(111). The orientation of the hydrocarbon chains was selected only for drawing ease and is likely more canted (see text). White circles ) Au, gray ) Cu, lightly shaded ) P, black ) S, striped ) hydrocarbon chain.
(Figure 4)sis implied by the relative intensities of the C-H stretching modes from the terminal CH3 groups for the adsorbed octane- and octadecanephosphonic acids. For this configuration of the phosphonate headgroup, the C-P bond would be oriented along the surface normal, and an extended hydrocarbon chain tilted ∼30° from the surface normal with an even chain length would terminate with its methyl group tilted away from the surface normal. Such a configuration would yield infrared spectra in the C-H stretching region with intensities in the CH2 region similar to those for alkanethiolate SAMs on gold of equivalent chain length but intensities in the CH3 region similar to those for alkanethiolate SAMs on gold with chain lengths of one additional or one fewer CH2 unit.38 This difference in spectral intensity suggests different orientations for the C-X bond (X ) PO32-, S-), where the C-S bond has been suggested to orient away from the surface normal30,39 and the C-P bond appears to orient more along the surface normal. Our use of an upd layer of copper to enhance the adsorption of alkanephosphonic acids onto gold can be compared with the results of Folkers et al.,17 who examined the ability of octadecanephosphonic acid to form SAMs on various metal oxides. They concluded that SAMs formed best onto the native oxides of Ti, Al, Zr, and Cu and less well onto the native oxides of Ag and Fe. Of these metals, adlayers of silver and copper on gold can be readily formed by upd, making them candidates for use as atomic-scale (37) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 709. (38) See ref 30 for examples of odd-even spectral variations in alkanethiolate SAMs on gold. (39) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.
Self-Assembly of Alkanephosphonate Monolayers
adhesion promoters between gold and phosphonic acids. As for the bulk metals, we observed the formation of superior quality SAMs onto Au/Cu(upd) substrates than onto Au/Ag(upd) surfaces using the procedures detailed above. This similarity in reactivity suggests a parallel nature between the adsorption characteristics of bulk metals and their single-layer counterparts, as prepared by upd. The upd substrates, however, offer a greater ease of use because they are less susceptible to oxidation (in terms of both redox potential and amount of material available for oxidation) and can serve as electrodes at potentials not accessible to the bulk metals (as in Figure 3). Conclusions Underpotentially deposited Cu adlayers promote binding of alkanephosphonates to gold substrates. The presence of this single atomic layer of Cu markedly changes the adsorptive properties of gold because the native gold surface shows little affinity for alkanephosphonates. The alkanephosphonate SAMs on modified polycrystalline gold
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supports have a lower packing density than SAMs of alkanephosphonates on Cu or alkanethiols on Au or Au/ Cu(upd) substrates. Cu(upd)-promoted binding of phosphonates to gold provides an alternative to the use of thiolbased anchoring agents such as 4-mercaptobutanephosphonic acid for constructing organized assemblies from these adsorbates.10,11,15 The Au/Cu(upd) substrates offer superior air stability to Cu substrates and make Au/Cu(upd) the preferred substrate for these alkanephosphonate SAMs. Acknowledgment. We gratefully thank the Office of Naval Research for financial support, the Australian Department of Industry, Science, and Tourism for a Research Travelling Fellowship (to M.V.B.), Dr. Bentley J. Palmer for synthesis of the 11-ferrocenylundecanephosphonic acid and assistance with electrochemistry, Ivan H. Lee for assistance with XPS, and Namyong Kim for helpful discussions. LA991247G
