Langmuir 1999, 15, 8549-8551
Spontaneous Adsorption of 4-Ferrocenylphenyl Isocyanide and 11-Mercaptoundecanoyl Ferrocene on Chromium Olivier Clot and Michael O. Wolf* Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 Received May 7, 1999. In Final Form: July 8, 1999
Introduction Over the past two decades, self-assembled monolayers (SAMs) have received intense interest. These welldefined organic films have numerous applications, for example, in sensing technology,1 the controlled reactivity of surfaces,2,3 and high-resolution microlithographic patterning.4-6 Many organic molecules, such as thiols, silanes, carboxylic acids, and isocyanides, are known to undergo spontaneous organization in a film when adsorbed on a large variety of metal or metal oxide surfaces.3,7-9 Isocyanides have been reported to adsorb on Pt as well as on several other transition metal surfaces, and several binding modes have been observed:10-20 terminal upright bonding, in which the isocyanide carbon binds to one surface metal atom (i.e., Au, Pt);10-15 bridging, with the carbon binding to several surface metal atoms simultaneously (i.e., Rh);17 and tilted or lying flat, with the CtN parallel to the surface (i.e., Ni).18-20 The most commonly studied substrates for the formation of SAMs of thiols are Au and Pt, but other surfaces have also been modified.8,21 On Au, thiols typically form tilted layers, in which the sulfur binds to one or more surface atoms.8 * To whom correspondence should be address. Tel.: (604) 8221702. Fax: (604) 822-2847. E-mail:
[email protected]. (1) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219-227. (2) Buess-Herman, C. Prog. Surf. Sci. 1994, 46, 335-375. (3) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (4) Calvert, J. M. J. Vac. Sci. Technol., B 1993, 11, 2155-2163. (5) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (6) Kumar, A.; Wilbur, J. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. Nanotechnology 1996, 7, 452-457. (7) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128-1132. (8) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (9) Grunze, M.; Hild, R.; David, C.; Mueller, H. U.; Voelkel, B.; Kayser, D. R. Langmuir 1998, 14, 342-346. (10) Angelici, R. J.; Robertson, M. J. Langmuir 1994, 10, 14881492. (11) Angelici, R. J.; Shih, K.-C. Langmuir 1995, 11, 2539-2546. (12) Angelici, R. J.; Ontko, A. C. Langmuir 1998, 14, 1684-1691. (13) Henderson, J. I.; Feng, S.; Ferrence, G. M.; Bein, T.; Kubiak, C. P. Inorg. Chim. Acta 1996, 242, 115-124. (14) Wrighton, M. S.; Whitesides, G. M.; Hickman, J. J.; Zou, C.; Ofer, D.; Harvey, P. D.; Laibinis, P. E.; Bain, C. D. J. Am. Chem. Soc. 1989, 111, 7271-7272. (15) Wrighton, M. S.; Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M. Langmuir 1992, 8, 357359. (16) Poilblanc, R.; Queau, R. J. Catal. 1972, 27, 200-206. (17) Semancik, S.; Haller, G. L.; Yates, J. T., Jr. J. Chem. Phys. 1983, 78, 6970-6981. (18) Friends, C. M.; Stein, J.; Muetterties, E. L. J. Am. Chem. Soc. 1981, 103, 767-772. (19) Friends, C. M.; Muetterties, E. L.; Gland, J. L. J. Phys. Chem. 1981, 85, 3256-3262. (20) Hemminger, J. C.; Muetterties, E. L.; Somorjai, G. A. J. Am. Chem. Soc. 1979, 101, 62-67. (21) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96.
8549
To expand the utility of surface modification with SAMs, it is important to find new combinations of surfaces and modifying groups. Toward this end, we chose to investigate the adsorption of electroactive species containing isonitrile and thiol functional groups on Cr surfaces. Only a few studies of the formation of organic thin-films on Cr have been reported; these include the assembly of surfactants, such as myristic acid22 and dodecyl sulfate, on Cr23 and silanes on Cr oxide.9 Langmuir-Blodgett films of organic species, such as phthalocyanines24 and polyimides,25 have also been deposited onto Cr electrodes. It is possible to predict which functional groups are prone to adsorb on a given surface by surveying the known complexes of the metal that contains ligands bearing the functional group of interest. Thiols26 and isocyanides27 are both known to form complexes with Cr(0), suggesting that these classes of compounds are likely to adsorb on Cr(0) surfaces. In this paper, we report the adsorption of 4-ferrocenylphenyl isocyanide (1) and 11-mercaptoundecanoyl ferrocene (2) on freshly prepared Cr surfaces.
Experimental Section General. 4-Ferrocenylphenyl isocyanide28 and 11-mercaptoundecanoyl ferrocene7 were prepared according to literature procedures. Solvents (Fisher) were dried when necessary using conventional methods or used as received. Other chemicals (Aldrich, Fisher) were used as received. Electrochemical measurements were conducted on a Pine AFCBP1 bipotentiostat using a Pt wire coil counter electrode and a silver wire reference electrode. For nonaqueous solutions, an internal reference (decamethylferrocene) was added to correct the measured potentials to the saturated calomel electrode (SCE). In the blocking experiment, the K4[Fe2(CN)6] redox potential was measured at a Pt electrode to correct the measured potentials to the SCE.29 The supporting electrolyte was 0.1 M [(n-Bu)4N]PF6, which was recrystallized three times from ethanol and dried at 110 °C in a vacuum prior to use. X-ray photoelectron spectroscopy (XPS) was performed on a Leybold MAX200 equipped with an Al KR source with a pass energy of 192 eV; the sampling area was 2 × 4 mm. Electrode Preparation. Electrodes (1-2 cm2) were prepared by thermal evaporation of Cr onto glass microscope slides or silicon wafers using a JEE-4B Japan Electron Optics Laboratory evaporator. Prior to deposition of Cr, the surfaces were cleaned by sonication for 30 min in a 1% aqueous solution of FL-70 detergent (Fisher), rinsed with distilled water and acetone, sonicated in methanol for 30 min, then rinsed with acetone and dried. The electrodes were stored under N2 after Cr deposition. (22) Boerio, F. J.; Boerio, J. P.; Bozian, R. C. Appl. Surf. Sci. 1988, 31, 42-58. (23) Arnebrant, T.; Baeckstoerm, K.; Joensson, B.; Nylander, T. J. Colloid Interface Sci. 1989, 128, 303-312. (24) Itoh, E.; Kokubo, H.; Shouriki, S.; Iwamoto, M. J. Appl. Phys. 1998, 83, 372-376. (25) Iwamoto, M.; Yoneda, Y.; Fukuda, A. Jpn. J. Appl. Phys. 1992, 31, 3671-3674. (26) Darensbourg, M. Y.; Longridge, E. M.; Payne, V.; Reibenspies, J.; Riordan, C. G.; Springs, J. J.; Calabrese, J. C. Inorg. Chem. 1990, 29, 2721-2726. (27) Yamamoto, Y. Coord. Chem. Rev. 1980, 32, 193-233. (28) Herrmann, R.; Siglmueller, F.; El-Shihi, T.; Carvalho, M. F. N. N.; Pombiero, A. J. L. J. Organomet. Chem. 1987, 335, 239-247. (29) Meites, L.; Zuman, P. CRC Handbook Series in Inorganic Electrochemistry; CRC Press: Boca Raton, Fl, 1983; Vol. III.
10.1021/la990547v CCC: $18.00 © 1999 American Chemical Society Published on Web 09/24/1999
8550
Langmuir, Vol. 15, No. 24, 1999
Figure 1. Cyclic voltammograms of a 1-modified Cr electrode in methylene chloride containing 0.1 M [(n-Bu)4N]PF6. Scan rates ) 50-600 mV/s. Inset: plot of oxidation peak current (ip(ox)) vs scan rate. The line is intended to guide the eye. Formation of Monolayers. The electrodes were first cleaned by sonication for 30 min in methylene chloride and then dried under a stream of N2. They were exposed to a ∼1 mM solution of 1 (30 min) or 2 (1 h) in dry methylene chloride under N2. After removal from solution, the electrodes were carefully rinsed with methylene chloride, dried with a stream of N2, and characterized immediately.
Results and Discussion Immersion of a freshly prepared Cr electrode in a methylene chloride solution of 1 results in the adsorption of 1 on the surface. Complex 1 contains a redox-active ferrocenyl group, which may be used to characterize the modified electrode using cyclic voltammetry. The cyclic voltammogram of the 1-modified Cr surface contains a wave at 0.52 V versus SCE attributed to the ferrocenyl groups (Figure 1). This wave appears 20 mV higher than the corresponding wave for 1 in methylene chloride solution at a Pt electrode. The peak splitting (∆Ep ) Ep(ox) - Ep(red)) increases with increasing scan rate and is larger than that obtained for 1 on a Pt electrode at a comparable scan rate (∆Ep ) 100 mV at 400 mV/s for 1 on Cr; ∆Ep is 60 mV at 500 mV/s for 1 on Pt).14 In other surface-confined layers that show a similar dependence of ∆Ep on scan rate, this behavior has been attributed to a slow electron-transfer rate on the time scale of the experiment.30,31 The plot of peak current versus scan rate is nearly linear between 50 and 200 mV/s, confirming that the ferrocenyl groups are confined to the surface (Figure 1, inset).32 At faster scan rates, the plot deviates from linearity, as observed in other systems featuring slow electron-transfer kinetics.30,31 The full width at halfmaximum (∆Efwhm) measured for 1-modified Cr electrodes is 154 ( 6 mV (compared to ∼200 mV for 1 on Pt).14 The surface coverage, determined by integration of the cyclic voltammogram of 1 on several electrodes, is (5 ( 1) × 10-11 mol/cm2. These surface coverage values are calculated using the geometrical area of the electrodes, assuming a roughness factor of 1 and assuming that all the surface-confined molecules are redox active. This coverage corresponds to less than one monolayer of 1 on the Cr surface, assuming a perpendicular orientation of the molecules with respect to the surface. Monolayers of (30) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173-3181. (31) Hong, H.-G.; Mallouk, T. E. Langmuir 1991, 7, 2362-2369. (32) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley & Sons: New York, 1980.
Notes
Figure 2. Cyclic voltammogram of a 2-modified Cr electrode in methylene chloride containing 0.1 M [(n-Bu)4N]PF6. Scan rate ) 200 mV/s.
ferrocenyl alkanethiols on Au33 typically have a surface coverage of ∼ 4 × 10-10 mol/cm2, and SAMs of 1 have been reported on Pt with a surface coverage of (2.4 ( 0.2) × 10-10 mol/cm2.14 Thorough washing of the 1-modified electrode with methylene chloride does not result in any changes in the cyclic voltammogram. A Cr electrode exposed for 2 h to a solution of ferrocene in methylene chloride also shows no redox waves in the cyclic voltammogram after rinsing, which demonstrates that the isocyanide group is necessary for adsorption. Spontaneous adsorption of 2 occurs when freshly prepared Cr electrodes are immersed for 1 h in a solution of 2 in methylene chloride. The cyclic voltammogram of a 2-modified electrode contains one redox wave due to the ferrocenyl group at 0.71 V versus SCE with a peak splitting of 50 mV (Figure 2). This wave appears 10 mV below the wave observed for 2 in methylene chloride solution at a Pt electrode. The peak splitting is smaller than that observed for a 1-modified Cr electrode but comparable to the splitting observed for SAMs of 2 on Au.34 The ∆Efwhm measured for a 2-modified Cr electrode is 90 ( 4 mV, which is much smaller than that observed for 1-modified electrodes. The surface coverage, determined by integration of the cyclic voltammograms, is (6 ( 1) × 10-11 mol/ cm2; this corresponds to less than one monolayer, assuming a perpendicular orientation of the molecules. It is important that the Cr surfaces are kept under N2 both before and after modification. If freshly prepared Cr substrates are left in air for more than 6-7 h, adsorption of 1 and 2 no longer occurs, as evidenced by the lack of a redox wave in the cyclic voltammograms of such electrodes. This is likely due to the formation of an oxide layer on the Cr surface. Surface oxidation of Cr has been extensively studied35,36 and proceeds in a stepwise fashion. In this process, oxygen physisorption is followed by the appearance and growth of Cr2O3 islands. With prolonged oxygen exposure, these islands coalesce, creating a compact oxide layer which slowly thickens.35 We have examined a Cr electrode cleaned using the same procedure used for surface modification by XPS, and the spectrum contains two sets of two peaks. The set at 571.1 and 583.5 eV is assigned to the Cr 2p3/2 and 2p1/2 peaks for Cr(0) and (33) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306. (34) We find that the peak splitting for 2 on Au under the same experimental conditions used for 2 on Cr is identical to that obtained in ref 7. (35) Landolt, D.; Palacio, C.; Mathieu, H. J. Surf. Sci. 1987, 182, 41-55. (36) Bard, A. J.; Fan, F. F.; Yang, H.; Moffat, T. P. J. Electrochem. Soc. 1992, 139, 3158-3167.
Notes
Langmuir, Vol. 15, No. 24, 1999 8551
Figure 3. Cyclic voltammograms of K4[Fe2(CN)6] at a bare Cr electrode (solid line) and a 1-modified Cr electrode (dashed line) in water containing 0.1 M KCl. Scan rate ) 200 mV/s.
the other set, at 576.6 and 586.0 eV, is assigned to the corresponding Cr peaks of Cr2O3.37 After exposure of the Cr surface to air for several days, the XPS spectrum of the surface contains only the set of peaks assigned to Cr2O3. When the oxide film completely covers the surface, 1 and 2 no longer adsorb. The stability of adsorbed 1 and 2 on Cr was examined in vacuum, in air, and in methylene chloride solution. After an electrode was derivatized with 1 or 2 and the surface coverage was determined by cyclic voltammetry, it was left for 30 h in air at room temperature, and the surface coverage was determined again. After this time the redox wave due to the surface-bound ferrocenyl groups of 1 or 2 was entirely gone. It is possible that upon immersion into the electrolyte solution, the layers desorb due to the air oxidation of the underlying Cr substrate. For comparison, identical experiments were carried out with electrodes left under N2 or in vacuum (ca. 0.1 mmHg) and in both cases, the adsorbed layers were intact after 30 h, as assessed by cyclic voltammetry. When a 1-modified electrode was left immersed in methylene chloride containing 0.1 M [(n-Bu)4N]PF6 under N2, slow desorption of the isocyanide was observed by cyclic voltammetry. The surface coverage decreased by 10% after 38 h and by 75% after 96 h. Repeated scans (up to 100) between 0 and 0.9 V versus SCE on a freshly prepared 1-modified electrode resulted only in a slight decrease in the area of the redox waves. When a 2-modified Cr electrode was left in an identical solution under N2, desorption occurred much faster, and cyclic voltammetry showed that the layer desorbed completely in 20 min. The blocking behavior of the modified Cr electrodes toward a solution redox couple was examined. SAMs of isocyanides and thiols on Au show such behavior.13 The Fe(II)/(III) redox wave of K4[Fe2(CN)6] measured in water containing 0.1 M KCl on a bare Cr electrode is shown in Figure 3. The distortion of the wave is attributed to the presence of some chromium oxide on the surface of the electrode.36 When 1- or 2-modified Cr electrodes were characterized by cyclic voltammetry in the same aqueous solution, the Fe redox wave was blocked; this indicates that the surface layer efficiently prevents electron transfer from the solution (Figure 3). The unmodified surface consists of both Cr2O3 islands and bare Cr, and current should only pass at exposed Cr. When 1 and 2 are adsorbed on the bare Cr, these regions are blocked, while the Cr2O3 (37) Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: New York, 1979.
Figure 4. Cyclic voltammograms of modified Cr electrodes in methylene chloride containing 0.1 M [(n-Bu)4N]PF6. Scan rate ) 200 mV/s. (a) Electrode exposed to an equimolar solution of 1 and 2 for 1 h; (b) 1-modified electrode exposed for 1 h to a solution of 2 in methylene chloride; (c) 2-modified electrode exposed for 30 min to a solution of 1 in methylene chloride.
prevents electron transfer from solution on the remainder of the surface. The surface coverage determined by cyclic voltammetry is calculated assuming a uniform distribution of molecules over the electrode; however, the blocking results are more consistent with a nonuniform distribution consisting of both Cr2O3 islands and 1 or 2 packed relatively densely on Cr. When a Cr electrode is exposed for 1 h to an equimolar mixture of 1 and 2 in methylene chloride solution, the cyclic voltammogram of the modified electrode contains two waves of similar area corresponding to the two adsorbates (Figure 4a). Exposure of a 1-modified Cr electrode to a solution of 2 for 1 h leads to an almost total displacement of 1 (Figure 4b). Similarly, exposing an electrode modified with 2 for 30 min to a solution of 1 completely displaces 2 from the electrode (Figure 4c) to produce a layer with a surface coverage comparable to that obtained for an electrode directly modified with 1. Surface-confined 1 and 2 on Cr are in equilibrium with the solution, as demonstrated by the loss of both 1 and 2 from the surface in solvent and by the displacement of one surface-bound species by the other. The air stability does not improve with a mixed layer of 1 and 2, compared to layers containing only 1 or 2. In summary, we have demonstrated that 4-ferrocenylphenyl isocyanide and 11-mercaptoundecanoyl ferrocene adsorb on Cr surfaces. The adsorbed layers are stable under N2 and in a vacuum, but not in air, and desorb when soaked in methylene chloride. Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada for funding this work. We also thank Dr. Kin Chung Wong and Prof. Keith Mitchell for carrying out the XPS measurements. LA990547V
