Langmuir 1998, 14, 6113-6120
6113
Self-Assembly of Anthraquinone-2-carboxylic Acid on Silver: Fourier Transform Infrared Spectroscopy, Ellipsometry, Quartz Crystal Microbalance, and Atomic Force Microscopy Study Sang Woo Han, Tai Hwan Ha, Chang Hwan Kim, and Kwan Kim* Department of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received March 3, 1998. In Final Form: May 18, 1998 The adsorption of anthraquinone-2-carboxylic acid (AQ-2-COOH) in ethanol on a silver surface has been investigated by reflection-absorption infrared (RAIR) spectroscopy, ellipsometry, quartz crystal microbalance (QCM), and atomic force microscopy (AFM). The RAIR spectral data were found to be consistent with those gathered by other experimental means. Specifically, from the RAIR spectral data it was concluded that the molecule chemisorbs on silver as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the metal substrate. The molecular plane was determined to be tilted by 40° from the surface normal. The ellipsometric thickness of the monolayer, 10.6 ( 1.2 Å, agreed well with that predicted from the RAIR data, 10.4 Å. QCM data suggested that the self-assembling process could be described in terms of the Langmuir adsorption model, providing the value of the free energy of adsorption at -21.8 kJ/mol. The limiting surface coverage was determined from the QCM data to be 2.5 × 10-10 mol/cm2, corresponding to the area per adsorbate to be 65 Å2. The latter value was quite close to that predicted from the RAIR spectral data, 66 Å2/molecule. Neither a periodic nor a molecularly resolved image could be obtained under a contact AFM measurement, but the lateral force microscopy (LFM) images revealed that the anthraquinone moieties were close-packed on the silver surface, with the rings assembled parallel to one another to form a brick like architecture. The area per adsorbate estimated from the LFM images, 62 ( 5 Å2, was also consistent with that estimated from the QCM and RAIR data.
Introduction Adsorption of molecular monolayers on metal surfaces has been a focus of tremendous research interest. In addition to the fundamental interest in such metaladsorbate systems, practical considerations such as modification of metal surfaces and preparation of organic thin films have increased research activity in this area.1,2 In particular, redox molecules adsorbed on metal substrates have been regarded to be model systems to understand electron transfer in biological systems.3,4 Adsorption of long-chain aliphatic acids on Ag or Al surfaces has been heavily investigated in relation to the applications mentioned above.5 In contrast, adsorption of aromatic acids and their analogues on metal substrates has rarely been studied. In principle, aromatic acids can be bound to a metal surface via either the ring π orbitals or the oxygen lone pair electrons and/or the π orbitals of the carboxylate group. It has been found, however, that simple aromatic acids such as benzoic acid,6 p-nitrobenzoic acid,7 and 4-cyanobenzoic acid8 adsorb on a silver surface * To whom all correspondence should be addressed. Fax: 822-8743704 and 82-2-8891568. E-mail:
[email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) Murray, R. W. Molecular Design of Electrode Surface; Techniques of Chemistry Series; John Wiley & Sons: New York, 1992; Vol. XXII. (4) Pierrat, O.; Lechat, N.; Bourdillon, C.; Laval, J.-M. Langmuir 1997, 13, 4112. (5) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. Sondag, A. H. M.; Rass, M. C. J. Chem. Phys. 1989, 91, 4926. Ahn, S. J.; Son, D. H.; Kim, K. J. Mol. Struct. 1994, 324, 223 and cited references therein. (6) Kwon, Y. J.; Son, D. H.; Ahn, S. J.; Kim, M. S.; Kim, K. J. Phys. Chem. 1994, 98, 8481. (7) Osawa, M.; Ataka, K.-I.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497.
exclusively via oxygen lone pair electrons of the carboxylate group, after deprotonation, with the benzene rings assuming a perpendicular orientation with respect to the metal substrate. As for simple aromatic acids, anthraquinone-2-carboxylic acid (AQ-2-COOH), known to be useful in the relief or prophylaxis of allergic conditions,9 was concluded by Osawa et al. to adsorb on silver through the carboxylate group. Interestingly, the adsorbed molecule was claimed to change its orientation from a near perpendicular stance to a flat one as the quinone-hydroquinone type redox took place on the Ag electrode.10 In general, quinone derivatives have been known to play very important roles in biological systems. Their redox properties as well as their adsorption behavior on various electrode surfaces have thus been studied using electrochemical and spectroscopic techniques.11-16 On Pt and Au surfaces, some quinone compounds such as benzoquinone and its derivatives were reported to interact with the metal surface via their quinone rings. Certain (8) Kim, S. H.; Ahn, S. J.; Kim, K. J. Phys. Chem. 1996, 100, 7174. (9) Patai, S., Rappoport, Z., Eds. The Chemistry of the Quinonoid Compounds; John Wiley & Sons: New York, 1988; Vol. 2. (10) Osawa, M.; Ataka, K.-I.; Yoshii, K.; Yotsuyanagi, T. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 371. (11) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3813. (12) Stern, D. A.; Wellner, E.; Salaita, G. N.; Davidson, L. L.; Lu, F.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885. (13) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (14) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 2735. (15) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 3937. (16) Soriaga, M. P.; Soriaga, E. B.; Hubbard, A. T.; Benziger, J. B.; Pang, K. W. P. Inorg. Chem. 1985, 24, 65.
S0743-7463(98)00259-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/12/1998
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quinone compounds that were physisorbed on a Pt surface were known to exhibit orientational change as a function of solution concentration.13-16 The orientation of thiol derivatives of quinoid compounds that were bound to noble metal electrodes by forming covalent metal-sulfur bonds was reported, however, to be barely affected even by a change in the electrode potential.11-13 On the other hand, the detailed molecular packing in the multilayers of anthraquinone derivatives that had long alkyl chains and were assembled by a Langmuir-Blodgett method was reported to depend strongly on the length as well as the position of the alkyl chain attached at the anthraquinone moiety.17 The most popular naturally occurring quinoid compounds are anthraquinone derivatives.18 Considering that such quinone derivatives play very important roles in biological systems, we have undertaken more detailed study to elucidate the adsorption behavior of AQ-2-COOH on silver by infrared spectroscopy, ellipsometry, quartz crystal microbalance, and atomic force microscopy. In conjunction with the aforementioned reasoning, elucidation of the adsorption behavior of aromatic acids on silver is of course the primary purpose of the present work, but more significantly it is intended to discover whether different surface probing techniques will in fact provide consistent and unified results regarding the characteristics of a self-assembly process in general. Experimental Section The silver substrates used for self-assembly of AQ-2-COOH were prepared by the resistive evaporation of titanium (Aldrich, >99.99%) and silver (Aldrich, >99.99%) at 1 × 10-6 Torr on batches of glass slides, cleaned previously by sequentially sonicating in isopropyl alcohol, hot 1:3 H2O2 (30%)/H2SO4, and distilled deionized H2O. Deposition of titanium prior to that of silver was performed to enhance adhesion to the substrate. After the deposition of approximately 200 nm of silver, the evaporator was back-filled with nitrogen. The silver substrates were immersed subsequently into a 1 mM solution of AQ-2-COOH (Aldrich, 98%) in ethanol for a predetermined period of time. After the substrates were removed, they were rinsed with excess ethanol to remove any trace of physisorbed adsorbates and then subjected to a strong nitrogen gas jet to blow off any remaining liquid droplets on the surface or the edges of the substrates. Thereafter, the reflection-absorption infrared (RAIR) spectra and the ellipsometry thicknesses were recorded. The infrared spectra were obtained with a Bruker IFS 113v Fourier transform spectrometer equipped with a globar light source and a liquid-N2-cooled mercury cadmium telluride detector. The method for obtaining the RAIR spectra has been reported previously.8,19,20 Each spectrum was obtained by averaging 512 or 1024 interferograms at 4 cm-1 resolution, with p-polarized light incident on the silver substrate at 80°. To reduce the effect of water vapor rotational lines, the sample and reference interferograms were recorded alternately after every 32 scans. The Happ-Genzel apodization function was used in Fourier transforming all the interferograms. The RAIR spectra are reported as -log(R/R0), where R and R0 are the reflectivities of the sample and the bare clean metal substrates, respectively. The ellipsometric thickness of self-assembled AQ-2-COOH was estimated using a Rudolph Auto EL II optical ellipsometer. The measurement was performed with a 632.8 nm line of He/Ne laser incident upon the sample at 70°. The ellipsometric parameters, ∆ and Ψ, were determined for both the bare clean substrates and (17) Nakahara, H.; Fukuda, K. J. Colloid Interface Sci. 1979, 69, 24. (18) Thomson, R. H. Naturally Occurring Quinones, 2nd ed.; Academic Press: New York, 1971. (19) Son, D. H.; Ahn, S. J.; Lee, Y. J.; Kim, K. J. Phys. Chem. 1994, 98, 8488. (20) Lee, Y. J.; Jeon, I. C.; Paik, W.-K.; Kim, K. Langmuir 1996, 12, 5830.
Han et al. the self-assembled films. As usual, the refractive index of the AQ-2-COOH film was assumed to be 1.45.21 The so-called DafIBM program supplied by Rudolph Technolologies was employed to determine the thickness values. At least five different sampling points were considered to get the averaged thickness value. Separately, the self-adsorption of AQ-2-COOH on silver was examined in situ with a quartz crystal microbalance (QCM). For this purpose, titanium and silver were evaporated consecutively onto an AT-cut quartz crystal (International Crystal, fundamental resonance frequency, fo ) 10 MHz). The area of silver was 0.20 cm2. The QCM apparatus consisted of a thus obtained electrode, a house customized oscillator, and a frequency counter (Fluke PM6681) that was interfaced with a personal computer. A selfassembly cell, made of Teflon, was designed such that only the silver-coated side of the quartz was in contact with the solution. The other side was exposed to air to reduce the shunt effect, which might be significant when both sides were in contact with a polar medium.22 A more detailed experimental setup was described elsewhere.23 To perform the QCM measurement, ethanol was initially added to the cell, and then the system was allowed to stabilize. In a few minutes, the measured frequency was stabilized within a range of 1-2 Hz. Thereafter, an aliquot of stock solutions of AQ-2-COOH in ethanol was added to the cell with a microsyringe, and subsequently frequency changes of the crystal were monitored as a function of time. Not only to obtain the roughness factor of the silver coated quartz surface but also to see the packing structure of AQ-2COOH monolayer on silver at a molecular level, an atomic force microscopy (AFM) measurement was also performed. For the latter purpose, titanium and silver were thermally evaporated onto the freshly cleaved mica sheets, and they were used as substrates for the self-assembly of AQ-2-COOH (all the conditions were same as those used for the RAIR measurement). AFM images were obtained in air at room temperature by using a Digital Instruments Nanoscope IIIa scanning probe microscope. Using a V-shaped and 200 µm long Si3N4 cantilever with a nominal spring constant of 0.12 N/m (Nanoprobe, Digital Instruments), topographic images were recorded in the conventional height mode (contact mode, normal AFM). Friction maps were also recorded in an LFM (lateral force microscopy) mode. Imaging force was about 5 nN. Images obtained at a scan rate of 20-30 Hz were flattened and Fourier transform (FT) filtered when necessary. To determine the roughness factor of the quartz electrode, the corresponding topography images were analyzed with a “Fractal Analysis” program supplied by Digital Instruments. Unless otherwise specified, all chemicals and gases were reagent grade and used as received. Triply distilled water, of resistivity greater than 18.0 MΩ‚cm, was used in the preparation of aqueous solutions.
Results and Discussion Vibrational Assignment of AQ-2-COOH. To obtain information on the surface adsorption mechanism from the RAIR spectrum, it is necessary to analyze spectral changes caused by the surface adsorption. In this respect, a correct vibrational assignment is required. Unfortunately, the vibrational assignment of AQ-2-COOH is not available in the literature. We have thus made the vibrational assignment of AQ-2-COOH referring to that of 9,10-anthraquinone (AQ). This is based on the fact that the vibrational property of carboxyl (or carboxylate) group is in general very localized. In addition, the vibrational coupling between the carboxyl group and the carbonyl groups of the central ring in AQ-2-COOH should not be significant since they are separated by at least three CC bonds. (21) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (22) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (23) Ha, T. H. M.S. Dissertation, Seoul National University, Seoul, Korea, 1998.
Self-Assembly on Silver
Langmuir, Vol. 14, No. 21, 1998 6115 Table 1. Infrared Spectral Data and Vibrational Assignment of AQ-2-COOH in Various Conditionsa
Figure 1. TRI spectra of (a) AQ and (b) AQ-2-COOH dispersed in KBr matrixes.
In the last 3 decades, the assignment of the fundamental vibrational frequencies of AQ itself has attracted much attention. The infrared, Raman, and phosphorescence spectra of AQ have been reported and force field calculations have been performed.24-27 Being planar with D2h symmetry, the 66 fundamentals of AQ have been resolved to the following symmetry species: Γ ) 12Ag + 4B1g + 6B2g + 11B3g +5Au + 11B1u + 11B2u + 6B3u. Among them, only the B1u, B2u, and B3u vibrations are infrared active in the free molecule. To see whether the vibrational assignment made on AQ can be transferred to that on AQ-2-COOH, the transmission infrared (TRI) spectra are recorded. Parts a and b of Figure 1 show, respectively, the TRI spectra of AQ and AQ-2-COOH in KBr matrixes. Although the relative peak intensities are somewhat variable between the two spectra, the major ring vibrational bands appear at nearly the same positions as summarized in Table 1. For instance, one of the CC stretching modes of the central ring of AQ, i.e., ν25(b1u), appears at 1591 cm-1, and the corresponding mode appears at 1592 cm-1 for AQ-2-COOH (we adopt the numbering and symmetry labeling for AQ used by Spanget-Larsen et al.27). Two other CC stretching modes of AQ, i.e., ν43(b2u) and ν44(b2u), appear at 1335 and 1284 cm-1, and their counterparts appear at 1331 and 1281 cm-1 for AQ-2-COOH. The outof-plane vibrations, ν62(b3u)and ν63(b3u), of AQ appear at 808 and 692 cm-1, respectively, and the corresponding modes appear at 795 and 702 cm-1 for AQ-2-COOH. In addition, the stretching vibration of the carbonyl groups of the central ring of AQ-2-COOH appearing at 1678 cm-1 is comparable to that of AQ appearing at 1676 cm-1. These features suggest that the directions of the transition dipoles of AQ-2-COOH ring modes are comparable to those of AQ. Namely, the transition dipoles corresponding to the peaks at 1331 and 1281 cm-1 for AQ-2-COOH should be directed along the long axis of the anthraquinone moiety. The transition dipole related with the carbonyl stretching peak at 1678 cm-1 for AQ-2-COOH can be thought to be aligned toward the short axis of the anthraquinone moiety. RAIR Spectral Feature. Parts a and b of Figure 2 show the RAIR spectra of AQ-2-COOH self-assembled on the silver surface in 1 mM AQ-2-COOH in ethanol for 5 (24) Pecile, C.; Lunelli, B. J. Chem. Phys. 1967, 46, 2109. (25) Singh, S. N.; Singh, R. S. Spectrochim. Acta 1968, 24A, 1591. (26) Girlando, A.; Ragazzon, D.; Pecile, C. Spectrochim. Acta 1980, 36A, 1053. (27) Spanget-Larsen, J.; Christensen, D. H.; Thulstrup, E. W. Spectrochim. Acta 1987, 43A, 331 and cited references therein.
AQ-2-COOH
AQ neatb
neatb
Ag saltb,c
621 692 808 936 969 1170 1284 1335
633 702 795 935 972 1173 1281 1331
1451 1473 1579 1591
1433 1487 1569 1592
814 935 976 1171 1294 1333 1394 1456 1479 1558 1593
1676
1678 1699 3057 3068 3092
635 704 800 935 976 1171 1290 1331 1390 1458 1475 1556 1593 1606 1678 3049 3078 3097
3076
3066 3073 3093
RAIRd
1680
assignmente ν31(b1u) skeletal def (ip) ν63(b3u) C-H bend (op) ν62(b3u) skeletal def (op) ν47(b2u) skeletal def (ip) ν61(b3u) C-H bend (op) ν28(b1u) C-H bend (ip) ν44(b2u) C-C str ν43(b2u) C-C str νs(COO-) ν26(b1u) C-C str ν42(b2u) C-C str ν41(b2u) C-C str ν25(b1u) C-C str νas(COO-) ν24(b1u) CdO str ν(CdO) for COOH ν23(b1u) C-H str ν39(b2u) C-H str ν22(b1u) C-H str
a Wavenumbers in cm-1. b Taken in KBr matrix. c Silver salt formed by mixing AgNO3 and AQ-2-COOH solutions in a 2:1 mole ratio. d Reflection-absorption infrared spectrum on silver film. Peak positions of 5 min and 2 h SA films are exactly the same. Due to a very noisy background, the RAIR peak positions below 750 cm-1 could not be determined precisely. e Assigned based on refs 24-27.
Figure 2. RAIR spectra of AQ-2-COOH self-assembled on silver for (a) 5 min and (b) 2 h. (c) TRI spectrum of silver salt of AQ-2-COOH dispersed in KBr matrix. In the RAIR spectra, the background below 750 cm-1 is so noisy that the region is not reproduced in the corresponding figures.
min and 2 h, respectively. For comparative purposes, a TRI spectrum of the silver salt of AQ-2-COOH is shown in Figure 2c. The major peaks shown in Figure 2 are also summarized in Table 1. It is evident from Figures 1b and 2c that the CdO stretching vibration of the COOH group,
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Han et al.
appearing at 1699 cm-1 in the neat state, becomes absent upon forming the silver salt. Instead, the symmetric and asymmetric stretching vibrations of the COO- group appear distinctly at 1390 and 1606 cm-1, respectively, in the salt spectrum. In the RAIR spectra, the CdO stretching band of the COOH group is not identified. The asymmetric stretching band of the COO- group is also missing, but the symmetric stretching band of the COOgroup is clearly seen at 1394 cm-1 in both parts a and b of Figure 2. The presence of the νs(COO-) band in the RAIR spectra indicates that AQ-2-COOH is chemisorbed on the silver surface through the carboxylate group after deprotonation, in agreement with Osawa et al.10 Recalling the RAIR surface selection rule that only the vibrational modes whose dipole moment derivatives have components normal to the metal surface are exclusively infrared active, the absence of the νas(COO-) band suggests that the two oxygen atoms of the COO- group are bonded to the silver surface symmetrically. It can be noticed from Figure 2a that AQ-2-COOH is adsorbed on the silver surface very favorably. The RAIR peak intensities of the adsorbed species for 5 min selfassembly (SA) are comparable to those for 2 h SA. Moreover, the peak positions observed for the two films are exactly same. Comparing the spectra in Figure 2a (or Figure 2b) and Figure 2c, the RAIR spectrum is substantially different from the salt spectrum. This indicates that the adsorbed AQ-2-COO- on silver possesses an anisotropic orientation on the surface. We have already mentioned that the asymmetric stretching mode of the COO- group is missing in the RAIR spectra. The intensity of the CdO stretching mode of the central ring becomes reduced by a fair amount upon the surface adsorption while the ring CC stretching vibration (ν44(b2u)) whose transition dipole is directed along the long axis of the anthraquinone moiety exhibits strong intensity. The CH stretching mode aligned along the long axis is also clearly seen at 3076 cm-1 in the RAIR spectra. On the other hand, in the RAIR spectra the out-of-plane modes such as ν61 and ν62 are also clearly identified at 976 and 814 cm-1, respectively. All of these RAIR spectra indicate that the molecular plane of the adsorbed AQ-2-COO- is neither perpendicular nor flat with respect to the silver surface. Namely, the molecular plane seems to have a tilted orientation on the surface. Orientation of AQ-2-COOH on Silver. To estimate the orientation of adsorbed AQ-2-COO- on silver, one can first consider only the anthraquinone moiety. Figure 3a defines the laboratory coordinates so that Z is the direction perpendicular to the surface, while X and Y represent directions at the surface. The angle between the long axis of the anthraquinone moiety and the Z coordinate is defined as θ (i.e., the tilt angle of the long axis away from the surface normal). Similarly, the angle between the plane of anthraquinone moiety and the X-Z plane is defined as φ, i.e., the rotation angle of the plane of AQ moiety. Recalling the theory of RAIR spectroscopy, θ and φ can be related to the intensity ratios of selected bands in RAIR and TRI spectra using the following equations28
A|(R)/Ai⊥(R) ) [A|(T)/Ai⊥(T)][cot2 θ/cos2 φ]
(1)
A|(R)/Ao⊥(R) ) [A|(T)/Ao⊥(T)][cot2 θ/sin2 φ]
(2)
where A|(R) and A|(T) are the absorbance of a band in the RAIR and TRI spectra, respectively, having dipole mo(28) Young, J. T.; Boerio, F. J.; Zhang, Z.; Beck, T. L. Langmuir 1996, 12, 1219.
Figure 3. (a) Definition of tilt angle θ and rotation angle φ for the AQ moiety of AQ-2-COOH adsorbed on silver. (b) Definition of transition dipole vectors, µs and µas, related with the COOsymmetry and asymmetry stretching vibrations, respectively.
ments parallel to the long axis; Ai⊥(R) and Ai⊥(T) are the absorbances of a band in the RAIR and TRI spectra, respectively, having dipole moments perpendicular to the long axis and in the plane of the anthraquinone moiety; and Ao⊥(R) and Ao⊥(T) are the absorbances of a band in the RAIR and TRI spectra, respectively, having the dipole moment perpendicular to the long axis but out of the molecular plane. It is not certain whether COO- is coplanar to the anthraquinone moiety. An ab initio calculation at the UHF/6-31G and UB3-LYP/6-31G levels resulted in the planar structure as a global minimum configuration.29 In the crystalline state, the COOH plane of 2-naphthoic acid is known to be twisted by at best 2° with respect to the aromatic ring.32,33 Thus, it may not be unreasonable to (29) Ab initio UHF (unrestricted Hartree-Fock) and UB3-LYP (unrestricted B3-LYP, a kind of density functional theory30) calculations were performed with the Gaussian 94 program31 running on a CrayC90 XMP/Unicos. (30) Han, S. W.; Kim, K. J. Phys. Chem. 1996, 100, 17124. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Peng, C. Y.; Ayala, P, Y.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94; Gaussian, Inc.: Pittsburgh, PA, 1995. (32) Trotter, J. Acta Crystallogr. 1961, 14, 101.
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assume that AQ-2-COO- has a planar structure. On the other hand, we have previously concluded that AQ-2-COOis bonded to the silver surface through the two oxygen atoms of the COO- group symmetrically. In this sense, the orientation of AQ-2-COO- on silver is better represented by the tilt angle of the COO- plane. To obtain the latter angle, the angle θ should be determined in advance (vide infra). Rather than the use of two separate equations, (1) and (2) above, the angle θ itself can be readily determined by using the combined equation
2 h, respectively. These values correspond, respectively, to the rotation angles (φ) of 62° and 60°. To figure out the orientation of the COO- plane, the following equations were derived using a simple vector analysis;
tan2 θ )[A|(T)/Ai⊥(T)]/[A|(R)/Ai⊥(R)] +
in which R and β represent, respectively, the angles between the transition dipole vector of either the COOsymmetric or the asymmetric stretching modes (µs and µas) and the surface normal; i.e., R is the tilt angle of the COO- plane from the surface normal (see Figure 3b). Substituting the θ and φ values determined above into eq 4, the tilt angle of the COO- plane, R, is estimated to be 43° and 40° for AQ-2-COO- self-assembled on silver for 5 min and 2 h, respectively. A similar calculation using eq 5 results in the β values to be 90° for the two cases. The values of R suggest that the molecular plane of AQ-2COO- should take a slightly more perpendicular stance upon increasing the dipping time. On the other hand, the β values lead us to confirm that AQ-2-COO- is symmetrically bound to silver through the two oxygen atoms of the COO- group. By use of the known bond lengths, bond angles, van der Waals atomic radii, and the approximate distance between the oxygen atoms of the carboxylate group and the silver surface (1.45 Å34) the thickness of the AQ-2-COOH monolayer on silver is estimated to be 10.4 Å when the tilt angle of 40° is invoked. It would be desirable to compare this value with the ellipsometric measurement, albeit a direct comparison may be controversial due to the various uncertainties in the ellipsometry measurement (i.e., oxide formation and ambient contamination). Since the solution of the ellipsometry equations, i.e., refractive index and thickness, could not be obtained simultaneously, a refractive index of 1.45 was assumed for the estimation of thickness. From a three-phase optical model, the ellipsometric parameters corresponded to the thickness of 10.6 ( 1.2 Å for AQ-2-COOH self-assembled for 2 h on silver in 1 mM ethanolic solution. Although we could not rule out the growth of the oxide overlayer and the other ambient contamination, the ellipsometric thickness agrees surprisingly well with the predicted value within a 10% error limit. Considering that the thickness determined by ellipsometry is, in principle, the spatially averaged value and thus does not reflect the real molecular height, the present agreement would be accidental. Nonetheless, referring to the QCM and LFM observation that the adsorbate is close-packed on the silver substrate (vide infra), the close match between the measured and predicted thicknesses may not be unreasonable. QCM Measurements. Figure 4 shows typical QCM data observed after the silver-coated quartz was placed in contact with a 0.5 mM AQ-2-COOH ethanol solution (the concentration attained after equilibrium). Upon injection of the AQ-2-COOH solution, the resonannt frequency of quartz abruptly decreased and stabilized after ∼2 min. This implies that the adsorption of AQ-2-COOH on silver occurs very rapidly, in agreement with the RAIR observation. The time interval for the frequency decrease to reach the plateau value was dependent on the concentration of AQ-2-COOH, but in the concentration range employed in this work (0.02-2 mM), the time interval did
o
o
[A|(T)/A ⊥(T)]/[A|(R)/A ⊥(R)] (3) To apply eq 3, one has to select three different kinds of bands: the transition dipole of the first kind should be aligned along the long axis of the anthraquinone moiety, the transition dipole of the second kind should be perpendicular to the long axis in the plane of the anthraquinone moiety, and the transition dipole of the third kind should be perpendicular to the long axis but out of the molecular plane. With assignment of the band appearing near 1680 cm-1 in Figure 2 to the CdO stretching mode of the central ring of AQ-2-COO-, it can be classified as one of the second kind. Similarly, the band appearing at 976 cm-1 in Figure 2 can be classified as one of the third kind. (One can notice from the TRI spectrum of silver salt that the out-of-plane ring mode at 704 cm-1 is very intense. Quite an intense band seemed also present near 710 cm-1 even in the RAIR spectra. The background was very noisy in that region, however, so that the peak could not be used for a quantitative analysis.) On selection of the first kind of band, a certain difficulty arises. It has been reported that the two intense bands near 1290 and 1330 cm-1 in the TRI spectrum of AQ are all due to the b2u type fundamental vibrations.27 If the transition dipoles of the corresponding bands in AQ-2COO- are aligned along the long axis of the anthraquinone moiety and if the two modes are not coupled strongly with each other and/or with other modes, their relative intensities should be rather invariant not only by the salt formation but also by the orientation change on the silver surface. However, it is seen from Figure 2 that the intensity of the band at ∼1330 cm-1 is variable with respect to that of the νs(COO-) band near 1390 cm-1 although the intensity variation for the band at ∼1290 cm-1 is somewhat insignificant. Spanget-Larsen et al.27 reported that the fundamental b2u modes of AQ, i.e., ν43 at ∼1330 cm-1 and ν44 at ∼1280 cm-1, and the combination modes such as ν10(ag) + ν48(b2u) ) ∼1310 cm-1 and ν12(ag) + ν47(b2u) ) ∼1301 cm-1 could appear simultaneously in the region of 1280-1330 cm-1. Furthermore, it is claimed that the latter two combination bands gain considerable intensity by Fermi resonance with the nearby b2u fundamentals at ∼1330 and ∼1280 cm-1. Thus, the subtle intensity variation in the 1280-1330 cm-1 region in Figure 2 can be understood by supposing that the extent of Fermi resonance is very sensitive to a small structural change occurring in the packing of the AQ moiety. Accordingly, we used the absorbance of the peak at 935 cm-1 to represent a transition dipole aligned along the long axis of the AQ moiety. In fact, the relative intensity of the peak was barely subjected to change in all spectra. By use of the TRI and RAIR spectra in Figure 2, the angle θ is then estimated to be 51° and 48° for AQ-2COO- self-assembled on the silver surface for 5 min and (33) Fitzgerald, L. J.; Gerkin, R. E. Acta Crystallogr. 1993, C49, 1952.
cos R )
x3 1 sin θ cos φ + cos θ 2 2
cos β ) -
x3 1 sin θ cos φ + cos θ 2 2
(34) Upton, T. H. J. Chem. Phys. 1985, 83, 5084.
(4) (5)
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c c 1 ) + ∆f ∆fmax Kads ∆fmax
Figure 4. Frequency change of QCM in a 0.5 mM AQ-2-COOH solution in ethanol. Arrow indicates the time at which an AQ2-COOH stock solution was injected into the cell.
in which c is a solution concentration of the adsorbate, ∆f is the measured frequency change at the plateau region, ∆fmax is its possible maximum value, and Kads is the thermodynamic adsorption constant. Figure 5b represents a plot drawn for the values of c/∆f measured versus c. The plot exhibits a linear relationship between the two values, implying that the Langmuir adsorption isotherm is applicable to the present system. In accordance with eq 6, the values of ∆fmax and Kads were thus estimated to be 15.7 Hz and 9.1 × 103 M-1, respectively, from the slope and intercept. By use of these values, a hypothetical Langmuir isotherm was drawn in Figure 5a (see the solid curve). It is remarkable that the hypothetical isotherm matches well with the experimental isotherm. Since the Langmuir adsorption model is applicable to the present system, the free energy of adsorption at infinite dilution (∆Goads) is estimated from the value of Kads by invoking a relation, ∆Goads ) -RT ln Kads, to hold. The value of Kads determined above gives the free energy of adsorption to be -21.8 kJ/mol. It is worth to note that the ∆Goads value of AQ-2-COOH on silver is quite comparable to that of n-octadecanethiol adsorbing on a gold surface.22 This indicates that the adsorption of AQ-2-COOH on a silver surface is in fact very favorable. In the above analysis, we have referred to only the frequency changes of the quartz substrate. However, on conversion of the amount of frequency change into a mass increase, the degree of surface coverage of AQ-2-COOH on silver may also be estimated. For this purpose, we assumed the Sauerbrey equation35 to hold in our system
∆f ) -
Figure 5. (a) Adsorption isotherm of AQ-2-COOH on silver measured by QCM. Solid curve represents a theoretical Langmuir isotherm under the conditions of ∆fmax ) 15.7 Hz and Kads ) 9.1 × 103 M-1. See text. (b) Langmuir plot for the adsorption data of AQ-2-COOH on silver.
not exceed 5 min. Figure 5a represents the amount of frequency difference measured at the plateau region as a function of AQ-2-COOH concentration in bulk (see the filled squares). It indicates that the silver surface is nearly saturated with AQ-2-COOH as long as its bulk concentration is above 0.5 mM. We have previously confirmed that the self-assembly of small organic molecules such as 4-cyanobenzoic acid on silver could be described in terms of the Langmuir adsorption model.8 On these grounds, a similar analysis has been made in this work by using the following equation;
(6)
2fo2 ∆m µq1/2Fq1/2A
(7)
in which ∆f is the amount of frequency change resulting from a change in mass (∆m) by adsorption or desorption of molecules on a quartz substrate, µq is the shear modulus of quartz (2.947 × 1011 dyn/cm2 for an AT-cut quartz), Fq is the density of quartz (2.648 g/cm3), and A is the area of the metal coated quartz. Substituting the previously determined ∆fmax (15.7 Hz) into eq 7, the maximum mass increase should be 71 ng/cm2. Recalling the molecular mass of AQ-2-COOH and the apparent surface area of the silver-coated quartz (0.20 cm2), the latter value should correspond to a surface coverage of 2.8 × 10-10 mol/cm2, and this in turn implies the mean area per adsorbate species to be 59 Å2. Considering the surface roughness factor (1.1, vide infra), these values are corrected to be 2.5 × 10-10 mol/cm2 and 65 Å2, respectively. In the liquid phase experiment, it is usually claimed that the QCM data do not follow the Sauerbrey equation.36 However, several examples are available in the literature that the equation obeys in a solution phase.20,37,38 In this respect, it is intriguing that the mean area estimated above is surprisingly consistent with the RAIR spectral data. Namely, referring to the known bond lengths and bond angles as well as the standard van der Waals atomic radii, (35) Sauerbrey, G. Z. Phys. 1959, 155, 206. (36) Yang, M.; Thompson, M.; Duncan-Hewitt, W. C. Langmuir 1993, 9, 802. Yang, M.; Thompson, M. Langmuir 1993, 9, 1990. Urbakh, M.; Daikhin, L. Langmuir 1994, 10, 2836. Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546. (37) Okahata, Y.; Kimura, K.; Ariga, K. J. Am. Chem. Soc. 1989, 111, 9190. (38) Ebara, Y.; Okahata, Y. Langmuir 1993, 9, 574.
Self-Assembly on Silver
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Figure 6. (a) AFM image (1.0 µm × 1.0 µm) of silver-coated QCM electrode. (b) Output of “fractal analysis” performed for the image of (a).
the molecular area of AQ-2-COO- is calculated to be 102 Å2. Supposing that the adsorbate assumes a tilted orientation on silver, as determined from the analysis of the RAIR spectral data by 40°, the area projected on silver by one adsorbate is then estimated to be 66 Å2. In fact, this value is almost same as that obtained from the analysis of QCM data. The present observation may imply that the AQ-2-COOH molecules are packed very closely when forming a monolayer on the silver surface. AFM Measurements. Figure 6a shows a typical topographic image of a silver-coated quartz substrate used in the QCM measurement (the AFM image was obtained with the contact mode). The AFM image is given in a topview presentation with the lighter areas denoting higher regions and the darker areas representing lower regions. As mentioned in the Experimental Section, the image was used in obtaining the surface roughness factor of the substrate through a fractal analysis. To perform the fractal analysis, the image was subdivided into smaller sized triangular cells. Every pixel in the image was eventually modeled to correspond to a vertical point of the triangular cell.39 Figure 6b represents the result of the fractal analysis for the image shown in Figure 6a, drawn in terms of the surface area versus the cell size. Upon a decrease of the cell size, the surface area converges to a value of 1.1 µm2. Since the apparent size of the AFM image in Figure 6a is 1.0 µm × 1.0 µm, the surface roughness factor is thus calculated to be 1.1 (vide supra). The same value was obtained independent of the sampling points and sizes of the AFM image. Furthermore, the value was reproducible for different batches of silver coated quartz substrates. As can be seen in Figure 6a, the evaporated silver film on a quartz substrate exhibits a granular structure, with the grain sizes ranging in a region of 100-150 nm. Smooth terraces at the top of the grains are usually larger than 20 nm. In fact, we observed the same images for silver films deposited on freshly cleaved mica sheets. This can be expected since titanium is deposited initially on both substrates, quartz and mica, prior to depositing silver. (39) Command Reference Manual of NanoScope IIIa; Digital Instruments: Santa Barbara, CA, 1996.
Figure 7. LFM image (3.0 nm × 3.0 nm) of self-assembled monolayer of AQ-2-COOH on silver; raw image was flattened and FT-filtered to remove noise.
Although we have not taken any AFM images for the silver films used in the RAIR measurement, the surface structures are believed to be similar to those of the other silver films mentioned above. On the basis of the above argument, we have attempted to record the AFM image of AQ-2-COOH self-assembled on a mica-supported silver film. However, neither a periodic pattern nor a molecularly resolved image could be obtained under the contact mode AFM measurement. Nonetheless, the LFM measurement resolved molecular images. Figure 7 shows a typical LFM image, in a size of 3.0 nm × 3.0 nm, of the self-assembled monolayer of AQ-2-COOH on silver (the raw image was previously flattened and FT-filtered to remove noise). In this image, the lighter areas denote higher friction regions and the darker areas represent lower friction regions. The image does not exhibit a well-ordered structure, but one can
6120 Langmuir, Vol. 14, No. 21, 1998
notice that the adsorbates are more or less close-packed on the silver surface. The adsorbed species are arranged to be parallel to one another and exhibit a brick like structure. The present observation may imply that molecularly resolved images could be obtained by an LFM technique even for ill-ordered monolayers, provided that the adsorbed species were close-packed to some degree on the underlying substrate. Referring to the LFM image in Figure 7, the area occupied by one adsorbate of AQ-2COOH is estimated to be 62 ( 5 Å2. It is remarkable that the latter value is very close to the estimation by the QCM data of 65 Å2. Accordingly, it appears that all the information obtained in this work via RAIR spectroscopy, ellipsometry, QCM, and LFM are consistent with one another. Conclusions The adsorption behavior of AQ-2-COOH on a silver surface revealed by infrared spectroscopy was found to be consistent with ellipsometry, quartz crystal microbalance, and scanning probe microscopy data. Self-adsorption of AQ-2-COOH in ethanol onto silver occurred very favorably; in the concentration range of 0.02-2 mM, the QCM frequency reached a plateau value within 5 min, and the RAIR peak positions and intensities observed after 5 min of self-assembly (SA) were barely different from those observed after 2 h of SA. The QCM data suggested further that the self-assembling process could be described in terms of the Langmuir adsorption model; the free energy of adsorption of AQ-2-COOH on silver appeared quite close to that of n-octadecanethiol adsorbing on a gold surface. On the basis of the infrared surface selection rule, AQ-
Han et al.
2-COOH was concluded to chemisorb on silver as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the metal substrate. In addition, the RAIR spectral pattern revealed the molecular plane to be tilted by ∼ 40° from the surface normal; referring to this angle, the thickness of the AQ-2-COOH monolayer on silver was estimated, and it was in good agreement with the ellipsometry measurement. The LFM image revealed that the anthraquinone moieties were closepacked on the silver surface and the rings were assembled parallel to one another to form an overall brick like architecture. The area per adsorbate estimated from the LFM image was also consonant with that estimated from the QCM and RAIR data. All of these findings clearly illustrate that information obtained from different experimental techniques can be consistent, and we expect such consistent data to be piled up for other systems to understand the SA process more precisely. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation through the Center for Molecular Catalysis at Seoul National University (SNU) and by the Ministry of Education, Republic of Korea, through the Basic Science Research Fund. We appreciate several helpful comments from Professor YeonTaik Kim in Yonsei University. K.K. acknowledges the Research Institute of Molecular Science in SNU for providing an instrument fund to purchase a scanning probe microscope. S.W.H. acknowledges the System Engineering Research Institute for allocating the time to use the Cray-C90 computer. LA980259J
