J. Phys. Chem. B 2000, 104, 11987-11995
11987
Adsorption Characteristics of Anthraquinone-2-carboxylic Acid on Gold Sang Woo Han,† Sang Woo Joo,† Tai Hwan Ha,† Yunsoo Kim,‡ and Kwan Kim*,† School of Chemistry and Molecular Engineering and Center for Molecular Catalysis, Seoul National UniVersity, Seoul 151-742, Korea, and AdVanced Materials DiVision, Korea Research Institute of Chemical Technology, Taejon 305-343, Korea ReceiVed: July 24, 2000
The adsorption of anthraquinone-2-carboxylic acid (AQ-2-COOH) on a gold surface has been investigated by reflection-absorption infrared (RAIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), surface-enhanced Raman scattering (SERS), quartz crystal microbalance (QCM), and ab initio quantum mechanical calculation. The RAIR spectral data clearly showed that the molecule was chemisorbed on gold as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the gold substrate; the molecular plane was determined to be tilted by 30° from the surface normal. The XPS and SERS spectral features were also consonant with the RAIR data. On the other hand, in CV, symmetrical redox peaks arising from a surface-confined species were reversibly identified at -647.5 mV, and the surface coverage determined from CV, that is, 1.8 × 10-10 mol/cm2, was consistent with that estimated from QCM. The adsorption rate of AQ-2-COOH on gold was, however, much slower than that on silver, as well as the adsorption strength to gold being considerably weaker than to silver. Control experiments showed that ordinary aromatic and aliphatic carboxylic acids hardly adsorb by self-assembly on the gold surface, in contrast to AQ-2-COOH. Theoretical considerations based on a frontier orbital approach, however, failed in the elucidation of the higher adsorptivity of AQ-2-COOH to gold over ordinary aromatic and aliphatic carboxylic acids.
Introduction From the aspect of the application of self-assembled monolayers (SAMs), redox molecules adsorbed on metal substrates have been regarded as model systems for understanding electron transfer in biological systems.1 With the acknowledgment that quinone derivatives play very important roles in biological systems, their redox properties as well as their adsorption behavior on various electrode surfaces have been studied using electrochemical and spectroscopic techniques.2 The most popular naturally occurring quinoid compounds are anthraquinone derivatives.3 On these grounds, we recently performed thorough analyses on the adsorption behavior of anthraquinone-2-carboxylic acid (AQ-2-COOH) on silver by reflection-absorption infrared (RAIR) spectroscopy, ellipsometry, quartz crystal microbalance (QCM), and atomic force microscopy (AFM).4 We revealed that AQ-2-COOH should chemisorb on silver as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the metal substrate. On the other hand, the molecular plane of AQ-2-COO- was determined by RAIR spectroscopy to be tilted by 40° from the surface normal. The area per adsorbate estimated from the RAIR spectroscopy was consonant with that estimated from the QCM data and the AFM image. In particular, the QCM data indicated that the adsorption kinetics of AQ-2-COOH on silver is very fast and its adsorption free energy is comparable to that of n-octadecanethiol adsorbing on a gold surface. In fact, the self-assembly of carboxylic acids has been reported to occur quite favorably on the surfaces of silver, copper, * To whom all correspondence should be addressed. Fax: 82-2-8743704 and 82-2-8891568; e-mail:
[email protected]. † Seoul National University. ‡ Korea Research Institute of Chemical Technology.
aluminum, platinum, and mercury.5 Although the adsorption characteristics of carboxylic acids on the gold surface have also been studied extensively,6-15 most of the studies have been carried out at the gold electrode because the molecules are hardly chemisorbed on gold without applying positive potentials.6-13 We report herein that AQ-2-COOH can be chemisorbed even on gold simply via a self-assembly process. These are revealed by the combined measurements of RAIR spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), surface-enhanced Raman spectroscopy (SERS), and QCM. To understand the adsorptivity of AQ-2-COOH on gold, we carried out ab initio molecular orbital calculations for a few selected anion species. To the best of our knowledge, this is the first report that shows the chemisorption of a carboxylic acid by a self-assembly technique onto a gold substrate. Experimental Section 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. Preparation of SAMs. The gold substrates used for selfassembly of AQ-2-COOH were prepared by the resistive evaporation of titanium (Aldrich, >99.99%) and gold (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 (piranha solution), and distilled deionized H2O. Deposition of titanium before that of gold was performed to enhance adhesion to the substrate. After the deposition of approximately 200 nm of gold, the evaporator was back-filled with nitrogen. The gold substrates were immersed subsequently into a 1 mM solution of AQ-2-COOH (Aldrich, 98%) in ethanol
10.1021/jp002630t CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000
11988 J. Phys. Chem. B, Vol. 104, No. 50, 2000 for 48 h. After the substrates were removed, they were rinsed thoroughly with excess ethanol, followed by drying in a N2 gas stream. IR Spectroscopy. The IR spectra were obtained with a Bruker IFS 113v Fourier transform spectrometer equipped with a globar light source and a liquid N2-cooled mercury-cadmiumtelluride detector. The transmission spectrum of solid AQ-2COOH was taken after making a pellet with KBr powders. The method for obtaining the RAIR spectra has been reported previously.4,16 The RAIR spectra are reported as -log(R/Ro), where R and Ro are the reflectances of the sample and the bare clean metal substrates, respectively. Voltammetry Measurements. All voltammetric measurements were carried out in a three-electrode cell using a CH Instruments model 600A potentiostat using CHI 600A Electrochemical Analyzer software (v. 2.03) running on an IBMcompatible Pentium computer. A gold-coated glass substrate onto which AQ-2-COOH was adsorbed served as a working electrode. The electrode was clamped against an O-ring in a joint on the side of the electrochemical cell. The O-ring provided a liquid-tight seal and also defined the area of the working electrode, which was estimated to be about 0.28 cm2. The reference electrode was a saturated calomel electrode (SCE). All potentials reported in this paper were relative to that of SCE. The counter electrode was a platinum spiral wire immersed in a solution of the supporting electrolyte. The supporting electrolyte was NaClO4, and all experiments were carried out at room temperature. The electrolyte solution was deaerated with high-purity N2 gas before initiating any electrochemical measurement. Preparation of Au Sol. The gold sol was prepared by following the recipes in the literature.17 Namely, 133.5 mg of KAuCl4 (Aldrich) was initially dissolved in 250 mL of water, and the solution was brought to boiling. A solution of 1% sodium citrate (25 mL) was then added to the KAuCl4 solution under vigorous stirring, and boiling was continued for ca. 20 min. The resulting Au sol solutions were stable for several weeks. To 1 mL of Au sol solution 10-3 M ethanol solution of acids was added dropwise to a final concentration of 5 × 10-5 M by using a micropipette. Raman Spectral Measurement. Raman spectra were obtained using a Renishaw Raman system model 2000 spectrometer equipped with an integral microscope (Olympus BH2UMA). The 632.8-nm radiation from a 17-mW air-cooled He/ Ne laser (Spectra Physics model 127) was used as an excitation source for the Au sol SERS experiments. A holographic notch filter was set in the spectrometer and Raman scattering was detected with 180° geometry using a Peltier-cooled (-70 °C) CCD camera (400 × 600 pixels). A glass capillary (KIMAX51) with an outer diameter of 1.5-1.8 mm was used as a sampling device. The holographic grating (1800 grooves/mm) and the slit allowed the spectral resolution to be 1 cm-1. The Raman band of a silicon wafer at 520 cm-1 was used to calibrate the spectrometer, and the accuracy of the spectral measurement was estimated to be better than 1 cm-1. The Raman spectrometer was interfaced with an IBM-compatible personal computer, and the spectral data were analyzed using Renishaw WiRE software v. 1.2 based on the GRAMS/32C suite program (Galatic). QCM Measurement. The adsorption of AQ-2-COOH on gold was examined in situ with a QCM. The gold-coated ATcut quartz crystals were purchased from International Crystal Co. The fundamental resonance frequency (f0) of the crystal was 10 MHz and the area of the gold electrode was 0.20 cm2. Before use, the crystal was soaked in a piranha solution for 5
Han et al.
Figure 1. RAIR spectra of AQ-2-COOH self-assembled on (a) gold and (b) silver, and (c) transmission IR spectrum of solid AQ-2-COOH in KBr matrix.
min and sonicated in distilled water for 1 min. The detailed experimental setup and procedure of the in situ QCM measurement were described previously.4,18 For an ex situ QCM measurement, a gold-coated quartz substrate was immersed into a 1 mM solution of AQ-2-COOH in ethanol for a predetermined period of time, removed from the solution, rinsed thoroughly with excess ethanol, and dried by a N2 gas stream; then the frequency change was measured. XPS Measurement. XPS measurements were carried out using a VG Scientific ESCALAB MK II spectrometer. A Mg KR X-ray (1253.6 eV) was used as the light source, and peak positions were internally referenced either to the Au 4f peak at 84.0 eV or to the C 1s peak at 284.6 eV. This C 1s binding energy corresponds to the most intense carbon signal coming from the aromatic C-C carbons (excluding the carbons bonded to oxygens). The base pressure of the chamber was ∼2 × 10-10 Torr (1.5 × 10-10 mbar, 1.5 × 10-8 Pa) and the electron takeoff angle was 90°. Computations. Ab initio restricted Hartree-Fock (RHF) calculations were performed for some anion species with the Gaussian 94 program19 running on an IBM SP2 computer. Geometry optimization and orbital energy calculations were carried out with the 6-31G basis set. Electrostatic charges were calculated by natural population analysis20 at the same level. Results and Discussion RAIR Spectral Features. In our previous study,4 AQ-2COOH was found to form a full-covered monolayer on silver within 5 min of self-assembly in ethanol. In the present work, we find that far longer time is needed for AQ-2-COOH to form a near-saturated monolayer on the gold surface (vide infra). This indicates that the kinetics of the self-assembly of AQ-2-COOH on gold is much slower than that on silver. Nonetheless, the SAMs on gold finally obtained resemble in many respects those on a silver surface. This is depicted by the RAIR spectra shown in Figure 1; Figure 1a and b show the RAIR spectra of AQ-2COOH self-assembled, respectively, on gold and silver in 1 mM ethanol solution for 48 h and 5 min. The overall RAIR spectral features are in fact little different from each other, suggesting that the adsorbate structure on gold is comparable to that on silver. The RAIR spectrum of AQ-2-COOH on silver was thoroughly analyzed in a previous publication.4 The adsorption characteristics of AQ-2-COOH on gold can be understood by a similar spectral analysis. First of all, one can notice that the
Anthraquinone-2-carboxylic Acid CdO stretching band of the carboxyl group, which appears at 1699 cm-1 in the transmission IR spectrum of solid AQ-2COOH (Figure 1c), is no longer identified in the RAIR spectrum on gold (Figure 1a). Instead, a band attributable to the symmetric stretching vibration of the carboxylate group (COO-) is clearly observed at 1382 cm-1 in Figure 1a, although the antisymmetric stretching band of the COO- group is hardly identified. The presence of the νs(COO-) band in the RAIR spectrum indicates that AQ-2-COOH is chemisorbed on gold as carboxylate after deprotonation, as is the case on silver. Recalling the IR surface selection rule that only the vibrational modes whose dipole moment derivatives have components normal to the metal surface are exclusively IR active,21 the absence of the νas(COO-) band suggests, on the other hand, that the two oxygen atoms of the COO- group are bonded to the gold surface symmetrically.4,22 [We have to mention that the RAIR spectrum in Figure 1a was not subjected to change even after the repeated solvent rinsing. We also have to mention that the position of the νs(COO-) band observed at 1398 cm-1 in the ionic salt, i.e., AQ-2-COO-Na+, is quite different from that in the RAIR spectrum on gold. Although we have failed in synthesizing the Au salt, these support the view that the species responsible for Figure 1a is indeed AQ-2-COOH chemisorbed on gold. The more clear evidence on chemisorption will be provided in the sections on XPS and SERS measurements.] It is remarkable that the peak position of the νs(COO-) mode in the RAIR spectrum on gold is lower by as much as 12 cm-1 than that on silver. Considering that the oxygen lone-pair electrons of the carboxylate group have antibonding characteristics, the electron donation from adsorbate to metal surface will lead to the strengthening of the C-O bonds, resulting in a blueshift of the νs(COO-) band.22 On these grounds, the lower frequency on gold supposedly indicates that the carboxylateto-gold interaction is weaker than the carboxylate-to-silver interaction. As one certainly expects, we find separately that the adsorbed AQ-2-COOH species on gold is readily displaced by organic thiols. For instance, AQ-2-COOH was completely displaced by benzenethiol upon immersing the former SAMs on gold in 1 mM benzenethiol in ethanol for 5 min. The subsequent immersion of the thiol-modified gold in an AQ-2COOH solution did not give rise to any displacement of thiol with AQ-2-COOH. These reveal that the adsorptivity of AQ2-COOH on gold is much weaker than that of thiols. We can conjecture on the adsorbate structure of AQ-2-COOH on gold by comparing the relative peak intensities of the RAIR and transmission IR spectra in Figure 1a and c. It is seen from Figure 1a and c that the intensity of the CdO stretching mode of the central ring at 1680 cm-1 becomes reduced by a fair amount upon surface adsorption while the ring CC stretching modes at 1333 and 1293 cm-1, whose transition dipoles are directed along the long axis of the anthraquinone moiety, exhibit strong intensity. The CH stretching mode aligned along the long axis is also clearly seen at 3076 cm-1 in the RAIR spectrum. In addition, the out-of-plane modes such as ν61 and ν62 are also clearly identified at 975 and 820 cm-1, respectively, in the RAIR spectrum. All of these RAIR spectral features indicate that the molecular plane of the adsorbed AQ-2-COO- is neither perpendicular nor flat with respect to the gold surface. That is, the molecular plane seems to have a tilted orientation on the surface. In fact, a detailed analysis of the RAIR peak intensities led to a conclusion that the tilt angle of the molecular plane should be 30° with respect to the gold substrate; the analysis was made following the scheme reported earlier for AQ-2COOH on silver.4 Recalling that the tilt angle on silver was
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11989
Figure 2. High-resolution XP spectra of C 1s photoelectrons of AQ2-COOH (a) in solid state and (b) in adsorbed state on gold.
∼40°, AQ-2-COO- should take intriguingly a slightly more perpendicular stance on gold than on silver. Given that the surface coverage is lower on Au than on Ag, this may indicate that the carboxylate-gold interaction is more important than the interadsorbate interaction when AQ-2-COOH is adsorbed on gold by self-assembly. Separately, we attempted to prepare SAMs on gold from other simple aromatic and aliphatic acids such as benzoic acid, 4-cyanobenzoic acid, 4-nitrobenzoic acid, 4-aminobenzoic acid, 1-naphthoic acid, 2-naphthoic acid, and stearic acid. No noticeable peaks were observed in their RAIR spectra, implying that usual carboxylic acids could hardly form SAMs on Au. The difficulty of the formation of SAMs on gold from those acids was also demonstrated by the SERS and QCM measurements (vide infra). XPS Measurement. To provide further evidence for the chemisorption of AQ-2-COOH on gold, we obtained the XP spectra of AQ-2-COOH in pure solid state as well as in adsorbed state on Au. The expected peaks from C 1s and O 1s core levels were clearly detected in all XP spectra; no trace of contamination was found even for the adsorbed film. In the case of adsorbed film, the Au 4f7/2 peak was identified at 84.0 eV; the latter value compares well with those of pure gold23 and n-alkanethioladsorbed Au surfaces.24 Figures 2 and 3 show the highresolution XP spectra of (a) solid and (b) adsorbed film of AQ2-COOH in the C 1s and O 1s spectral regions, respectively; the observed peaks are collectively listed in Table 1. In fact, for the solid AQ-2-COOH, three C 1s peaks were identified at 284.6, 287.2, and 289.3 eV. Consulting the literature data,5f,23,25 these peaks can be attributed to the carbon atoms in the aromatic (C-C), ring carbonyl (>CdO), and carboxylic (-COOH) moieties, respectively. Upon adsorbing on the gold surface, the C 1s peak due to the carboxylic carbon disappeared. Instead, a peak attributable to the carboxylate carbon was observed at 288.7 eV (see Figure 2b and Table 1). This clearly indicates that AQ-2-COOH is adsorbed on gold as carboxylate. Regarding the O 1s peaks, two peaks were identified at 531.5 and 533.2 eV for the solid AQ-2-COOH. These two peaks can be attributed, respectively, to the oxygen atoms in the ring carbonyl (>CdO) and the carboxylic (-COOH) moieties.23,25c,26 However, for the adsorbed film, only a single peak was identified at 532.1 eV in the O 1s spectral region. Considering that AQ-2-COOH is adsorbed on gold as carboxylate, the peak is supposed to arise from both the ring carbonyl oxygen (>CdO) and the carboxylate oxygen (-COO-) atoms. In fact,
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Figure 3. High-resolution XP spectra of O 1s photoelectrons of AQ2-COOH (a) in solid state and (b) in adsorbed state on gold.
TABLE 1: XPS Binding Energies of AQ-2-COOH C 1s
O 1s
solid (eV)
SAM (eV)
C-C >CdO -COOH -COO-
284.6 287.2 289.3
284.6 287.3
>CdO -COOH -COO-
531.5 533.2
288.7 532.1 532.1
the XPS binding energy of the O 1s core level in a carboxylate group is generally known to be smaller than that in a carbonyl group.26 The creation of half a negative charge unit on oxygen has been attributed to a shift of about -3 eV in a free ion.27 We nonetheless could not distinguish such two different oxygen atoms from the XP spectrum on the gold surface. The observation of singlet rather than a doublet in this work may be ascribed to the interaction of carboxylate with the Au surface. Voltammetry Measurement. Figure 4a shows a typical cyclic voltammogram (CV) of AQ-2-COOH on an Au electrode in an aqueous 0.2 M NaClO4 solution. The Au electrode was the same substrate used for obtaining the RAIR spectrum shown in Figure 1a. Symmetrical peaks attributable to a quinonehydroquinone-type redox reaction2 are reversibly identified at -647.5 mV. The separation of the anodic and cathodic peak potentials, ∆Ep, is negligibly small. The small ∆Ep value reflects a fast electron-transfer rate. On the other hand, as shown in Figure 4b, the peak height varies linearly with the potential scan rate, V, in the range V ) 0.05 V/s to V ) 0.7 V/s. This illustrates that the CV peak is in fact derived from surface-confined species.28 Using the faradic charges in CV, the surface coverage of AQ2-COOH on gold is estimated to be 1.8 × 10-10 mol/cm2 after accounting for surface roughness (roughness factor 1.129). This measured value of surface coverage on gold is about 30% smaller than that of AQ-2-COOH on silver, that is, 2.5 × 10-10 mol/cm2. The lower surface coverage on gold than on silver can be attributed to the rather loosely packed monolayer structure of AQ-2-COOH on gold. In any event, we have to mention that the anodic as well as cathodic currents are decreased upon repeated potential cycling. This obviously contrasts with the case of the SAMs of AQ derivatives that possess a thiol moiety as a tethering group to metal electrodes. In the latter case, repeated scanning between -1.0 and -0.3 V
Figure 4. (a) Cyclic voltammogram of AQ-2-COOH monolayer on a gold electrode, recorded at a scan rate of 100 mV s-1 in 0.2 M NaClO4. (b) Anodic peak current (ip,a) drawn with respect to the scan rate.
versus SSCE does not affect the CV (within the potential range, thiols should not desorb from the electrode surface).30 The instability of the SAMs of AQ-2-COOH on gold with respect to electrochemical treatment can then be attributed to the relatively weak adsorbate-surface interaction as mentioned in the discussion of the RAIR spectral data. SER Spectral Features of AQ-2-COOH in Au Sol. SERS has been most frequently used for obtaining information on the structural details of adsorbates not only at full-covered limits but also at submonolayer coverages.31 On this basis, we have attempted by SERS to examine the feasibility of self-assembly of carboxylic acids on the gold surface. Indeed, we could not obtain good-quality SER spectra when aromatic and aliphatic acids such as benzoic acid, 4-cyanobenzoic acid, 4-nitrobenzoic acid, 4-aminibenzoic acid, 1-naphthoic acid, 2-naphthoic acid, and stearic acid were dissolved in aqueous gold sol; neither the para substitution of the benzene ring nor the presence of the polyaromatic ring allowed the carboxylic acids to adsorb favorably on the gold surface. However, in agreement with the RAIR spectroscopy study, we could readily obtain high-quality SER spectra when AQ-2-COOH was dissolved in gold sol. This indicates that the adsorptivity of AQ-2-COOH on gold is much stronger than that of other carboxylic acids. As mentioned in the Introduction, numerous studies have been reported regarding the characteristics of carboxylic acids on the gold surface.6-15 In particular, the system of benzoic acid has been extensively investigated in aqueous solutions by means of differential capacity8 and conductivity measurements,9 SERS,10 potential difference IR spectroscopy,11 and radiotracer technique.13 Differently from the present study, the carboxylic acids were allowed in the earlier studies to adsorb on gold by applying
Anthraquinone-2-carboxylic Acid
Figure 5. (a) SER spectrum of 5 × 10-5 M AQ-2-COOH in gold sol. (b) OR spectrum of solid AQ-2-COOH.
positive potentials. Moreover, except for the work of Corrigan and Weaver,11 chemically transformed species such as benzoate were never presumed to present on the gold electrode. Even in the study of Corrigan and Weaver, direct evidence was not provided at all, however, for the chemical interaction between carboxylate and gold. Figure 5a shows the Raman spectrum of AQ-2-COOH taken at 5 × 10-5 M in gold sol. In the absence of gold sol, no Raman peak is identified at this concentration so that the spectrum in Figure 5a must be a SER spectrum. Considering the average diameter of gold particles, 17 nm,32 the concentration of AQ2-COOH, 5 × 10-5 M, is close to that needed to cover all gold particles. To help analyze the SER spectrum, we also show the ordinary Raman (OR) spectrum of neat AQ-2-COOH in Figure 5b. One can notice that the CdO stretching band of the COOH group appearing at 1682 cm-1 in Figure 5b is completely absent in Figure 5a. Instead, the νs(COO-) and δ(COO-) bands are seen at 1383 and 856 cm-1, respectively, in Figure 5a. This indicates that AQ-2-COOH is chemisorbed on gold as carboxylate as revealed by RAIR spectroscopy. In this sense, the broad band near 180 cm-1 in Figure 5a can be attributed to the AuCOO- stretching vibration. The adsorption mechanism of an adsorbate can be deduced from its SER spectrum through a detailed analysis of the peak shift and band broadening caused by the surface adsorption.22,33 It is worthwhile on this basis to note that the peak positions as well as the bandwidths of the ring modes of AQ-2-COOH, such as at 1602, 1179, 1088, and 688 cm-1, in Figure 5a are barely different from those in Figure 5b. According to the SERS investigation of benzene derivatives by Gao and Weaver,10 the ring modes have to red-shift by around 10 cm-1 along with an increase in their widths insofar as the surface-ring π orbital interaction is the driving force of the surface adsorption; the red shift can be attributed to the bond weakening caused by the electron back-donation from metal to the antibonding π* orbital of the benzene moiety.10,34 The close match of the SER spectrum to the OR spectrum then indicates that the molecular plane of AQ-2-COO- never lies flat on the gold sol surface, even though it is still undetermined whether the adsorbate takes a perpendicular stance or a tilted orientation with respect to the gold surface. According to the electromagnetic (EM) selection rule,35 vibrational modes whose polarizability tensor elements are perpendicular to a metal surface should be strongly enhanced
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11991 in a SER spectrum, namely those corresponding to R′zz where z is along the surface normal. Vibrations derived from R′xz and R′yz should be the next most intense modes and those corresponding to R′xx, R′yy, and R′xy should be least enhanced. On the basis of the EM theory, Creighton36 reported that for aromatic C2V molecules, relative enhancement factors for different modes should be a1:a2:b1:b2 ) 1-16:4:4:1 for faceon adsorption of benzene ring and a1:a2:b1:b2 ) 1-16:1:4:4 for perpendicular adsorption. Recalling our previous observation that the directions of transition dipoles of AQ-2-COOH ring modes are comparable to those of 9,10-anthraquinone, the modes classified as ag, b1g, b2g, and b3g in AQ-2-COO- can be correlated, respectively, with the a1, a2, b1, and b2 modes in aromatic C2V molecules. On these grounds, it would be informative that the most distinct bands in Figure 5a arise from the ag and b3g modes. Although the b2g bands were also identified clearly, for instance, at 786 and 270 cm-1, the b1g band was hardly observed in the SER spectrum in Figure 5a. These suggest that AQ-2-COO- should take at least a tilted orientation on the gold surface. The noticeable appearance of the C-H stretching band at 3074 cm-1 in Figure 5a is also indicative of such an orientation on gold.36,37 The SER spectral features of AQ-2COOH in Au sol seem thus to be consistent with the RAIR spectral features observed on a vacuum-evaporated gold film. Analyzing the SER spectra of aromatic carboxylic acids in aqueous Ag sol, Suh and Kim38 proposed recently that the νs(COO-) band should be much stronger than the δ(COO-) band when the COO- group was adsorbed flat on silver through π-electrons. When the COO- group was tilted on the surface by adsorption through both of its oxygen atoms, the νs(COO-) and δ(COO-) bands were claimed to have similar intensities. When the carboxylate group was adsorbed through only one of its oxygen atoms, the δ(COO-) was proposed to be stronger than the νs(COO-) band. It is seen in Figure 5a that the δ(COO-) band at 856 cm-1 is more intense than the νs(COO-) band at 1383 cm-1. On the grounds of the proposition of Suh and Kim,38 these SER spectral data have to be interpreted as reflecting a tilted orientation of AQ-2-COO- on gold with the carboxylate group being bound to gold through only one of the two oxygen atoms in the carboxylate group; namely, a unidentate state has to be presumed. However, in a unidentate coordination, the COO- group should also have shown the νas(COO-) and ν(CdO) bands in the SER spectrum.39 The absence of these bands suggests that AQ-2-COO- is adsorbed on gold resuming a bidentate or a bridging state rather than a unidentate state. When monocarboxylic acids are adsorbed even on silver surfaces, a unidentate state is rarely observed. Usually, the two oxygen atoms of a carboxylate ion are symmetrically bonded to silver, resulting in either a bridging or a bidentate state. There are many cases22,40 in which the finally concluded carboxylate structure on silver is different from that predicted on the basis of the proposition of Suh and Kim. In conjunction with the RAIR spectral features, we also have to mention that the peak position of the νs(COO-) band in Figure 5a is nearly the same as that in the OR spectrum of AQ-2COO- in basic aqueous solution (not shown). If the σ donation of electrons from carboxylate to gold occurs effectively, the bonds in carboxylate will become stronger, resulting in a blueshift of the νs(COO-) band.22 A negligible peak shift can be understood to indicate that the carboxylate-to-gold interaction is not remarkable, as presumed in the discussion of the RAIR spectral data. On the other hand, consulting the IR frequencies of various carboxylate complexes, the similar peak positions of the νs(COO-) bands in adsorbed and free anion states may
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Figure 6. Frequency changes of QCM in (a) 1 mM AQ-2-COOH and (b) 2 mM benzoic acid in ethanol. Arrow indicates the time at which the stock solutions of acids were injected into the QCM cell.
imply that AQ-2-COO- is adsorbed on gold, resuming a bridging state rather than a bidentate state. QCM Measurements. In our previous QCM experiment,4 the resonant frequency of silver-coated quartz abruptly decreased upon coming into contact with an AQ-2-COOH ethanol solution, and stabilized within 5 min in the concentration range 0.02 to 2 mM. Such an abrupt frequency change is not observed when gold-coated quartz is in contact with an AQ-2-COOH solution. This can be seen from Figure 6a, which shows a typical QCM profile recorded after the gold-coated quartz has been brought into contact with a 1 mM AQ-2-COOH ethanol solution. The frequency of quartz reaches a plateau after ca. 1.5 h, illustrating that the adsorption rate of AQ-2-COOH on gold is substantially lower than that on silver. Owing to the slow adsorption kinetics of AQ-2-COOH on gold, a concentration-dependent QCM experiment could not be carried out reliably. Hence, we have not attempted to see whether the Langmuir adsorption model is also applicable to the AQ-2-COOH/Au system, as likely in the AQ-2-COOH/Ag system.4 As a control experiment, we separately performed the QCM measurement in another carboxylic acid medium. Figure 6b shows a typical QCM profile recorded after the gold-coated quartz has been placed in contact with 2 mM benzoic acid in ethanol. The frequency change is very small in the present case. Much the same observation is made with other carboxylic acids, that is, 4-cyanobenzoic acid, 4-nitrobenzoic acid, 4-aminobenzoic acid, 1-naphthoic acid, 2-naphthoic acid, and stearic acid. These observations are consistent with the RAIR and SER spectral observations. The amount of molecular species adsorbed on the metalcoated quartz substrate is in principle directly related to the frequency change of the quartz substrate. Usually, the Sauerbrey equation41 is assumed to hold in various systems:
∆f ) -
2 f02∆m 1/2 µ1/2 q Fq A
(1)
where ∆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 gold-coated quartz (0.20 cm2). However, in the liquid-phase experiment, the QCM data should be interpreted carefully to include the fluid effect on
the piezoelectric movement of quartz. When a metal part is in contact with a solution, the piezoelectric movement is usually damped, resulting in a decreased frequency.42 Such a frequency change may be ignored, however, if both the initial and final states are under nearly the same environmental conditions with respect to the density, viscosity, and temperature of the solution phase. In fact, several examples are available in the literature showing that the Sauerbrey equation is obeyed in the solution phase.16b,43 In our previous work,4 the area per AQ-2-COOon silver estimated from the QCM data using eq 1 was surprisingly consonant with that estimated from RAIR spectroscopy, ellipsometry, and lateral-force microscopy studies. Referring to the electrochemically determined surface coverage of AQ-2-COOH on gold, that is, 1.8 × 10-10 mol/cm2, the eventual QCM frequency change is expected to be at best about 13 Hz on the basis of eq 1.44 However, it is seen in Figure 6a that the actual measured frequency change is as large as 44 Hz. It is not clear whether the discrepancy has to be solely attributed to the failure of the Sauerbrey equation in the present liquid system. Owing to a slow kinetics of adsorption of AQ-2-COOH on gold, a substantial amount of solvent molecules may have been entrapped within the interfacial region to exhibit a larger frequency change. In any event, to obtain a more reasonable surface coverage value of AQ-2-COOH on gold, we have performed an ex situ QCM measurement in which the frequency change is recorded in the ambient conditions after the exposure of gold-coated quartz to a 1 mM ethanol solution of AQ-2COOH for 48 h. It is noteworthy that the frequency change measured in this way was 15 ( 3 Hz, which was in fact comparable to that estimated from the electrochemical experiment, that is, 13 Hz. Theoretical Consideration of Adsorption of AQ-2-COOH on Gold. The RAIR, SERS, and QCM measurements discussed so far clearly demonstrate that AQ-2-COOH has very high chemisorbing ability on gold through the Au-carboxylate interaction. It is desirable to provide a theoretical basis to explain the higher adsorption ability of AQ-2-COOH than ordinary carboxylic acids on gold. In principle, a different adsorptivity originates from different adsorption free energies. The adsorption free energy is determined by several factors including the solvation energies of the adsorbate and the surface as well as the adsorbate-surface and the adsorbate-adsorbate interaction energies. Considering that stearic acid barely adsorbs on gold, whereas it does so on silver, the adsorbate-adsorbate interaction appears not to be an essential factor for a carboxylic acid to assemble a monolayer on a metal surface. The fact that the SAMs of AQ-2-COOH on gold possess a somewhat loosely packed structure inherently suggests that the intermolecular adsorbate interaction is not responsible for the feasible adsorption of AQ-2-COOH on gold. On these grounds, we will hereafter consider the strength of the adsorbate-surface interaction and the change of the solvation free energy upon adsorption as major possible factors controlling the relative adsorptivities of various acids on gold. Previously, for the systems of sulfur-containing molecules on gold, Garrell et al.45 evaluated theoretically the adsorbatesurface Coulombic and charge-transfer interactions and the relative solvation energies of the molecules in solution. Using a similar theoretical analysis scheme, we have examined the relative adsorptivities of various carboxylic acids and thiols onto the gold surface. The basis of the theory Garrell et al.45 invoked is that, when two species interact with each other, the energy change of the total system, ∆E, is given by the energy gained or lost upon the overlap of the reactant orbitals.46 The energy
Anthraquinone-2-carboxylic Acid change can further be partitioned into four contributing terms, namely, the first-order closed shell repulsion, the Coulomb repulsion or attraction, the charge transfer, and the solvation free energy change. For surface adsorption system, however, the first term would not contribute to ∆E because it involves only the interaction between closed shells. We will thus consider the other three terms to explore the relatively higher adsorptivity of AQ-2-COOH on gold. To this end, ab initio quantum mechanical calculations have been performed not only for AQ2-COO- but also for benzoate (C6H5COO-) and propanoate (CH3CH2COO-). (Benzoic acid and propanoic acid are taken as two representative aromatic and aliphatic acids. Considering that AQ-2-COOH is present on gold as carboxylate, the adsorbate species interacting with the gold surface are all assumed to be anions, however.) To assess the relative importance of the Coulombic interaction, it would be worthwhile to recall that the potential of zero charge of gold electrode and of colloids such as the one used in this work is the negative of the open circuit potential in air, ethanol, or water.45,47 Consulting the data in the literature, the effective charges of the gold surfaces used in this work may then all be presumed to be positive. On the other hand, according to our natural population analysis at the RHF/6-31G level, the net charges of the carboxylate oxygen atoms were -0.85, -0.86, and -0.88 electron charge for AQ-2-COO-, C6H5COO-, and CH3CH2COO-, respectively. If the Coulombic interaction is in fact the dominant factor, this calculation suggests that both the benzoic and propanoic acids must adsorb on gold as favorably as AQ-2COOH, in contrast to the actual observations. Hence, the adsorbate-surface Coulombic interaction seems not to be the crucial factor responsible for the higher adsorptivity of AQ-2COOH on gold. Invoking the fact that the net charges of the sulfur atoms in C6H5S- and CH3CH2CH2S- were at best -0.51 and -0.76 electron charge, respectively, at the RHF/6-31G level, the Coulombic interaction might not play a crucial role even in the self-assembly of thiols on the gold surface. In principle, a metal can accept electrons from an electron donor into its lowest unoccupied or partially occupied molecular orbital (LUMO or conduction band); the closer the adsorbate HOMO (highest occupied molecular orbital) and the substrate LUMO are in energy, the larger the magnitude of the chargetransfer interaction. In fact, Barclay and Caja´48 reported that the relative adsorptivities of simple anions on electrode surfaces could be explained in terms of charge-transfer interactions. Anions with higher HOMO energies adsorbed more readily on metals such as mercury and gold. These considerations suggest that the charge-transfer interaction may be a dominant factor in controlling the relative adsorptivities of carboxylic acids on gold. In this light, we compare in Figure 7 the HOMO energies of AQ-2-COO-, C6H5COO-, and CH3CH2COO- that have been computed at the RHF/6-31G level; Figure 7 also includes the LUMO energies of each anion. For comparative purposes, the HOMO energies of other relevant anions such as benzenethiolate (C6H5S-), propanethiolate (CH3CH2CH2S-), benzenesulfinate (C6H5SO2-), benzenesulfonate (C6H5SO3-), and benzeneselenolate (C6H5Se-) are also presented in Figure 7. Recalling the report of Barclay and Caja´,48 organic thiols and selenols can be supposed to adsorb on gold very favorably49,50 since C6H5S-, CH3CH2CH2S-, and C6H5Se- all possess high HOMO energies in Figure 7. The lower HOMO energy of C6H5SO3- may also reflect the previous experimental observation that benzenesulfonate does not form SAMs on gold, whereas benznesulfinate does on gold.45,51 The HOMO energies in Figure 7 may similarly indicate that the adsorptivity of AQ-2-COOH on gold is much
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11993
Figure 7. HOMO and LUMO energies in eV’s. Herein, orbital energy of C6H5Se- was obtained at the RHF/LANL2DZ level, whereas others were obtained at the RHF/6-31G level.
lower than those of thiols and selenols. However, using these guidelines, the adsorptivities of CH3CH2COO- and C6H5COOon gold must be higher than that of AQ-2-COO-, in contrast with the actual experimental observations. Hence, the relative HOMO energies seem not to be the major factor discriminating the relative adsorptivities of carboxylic acids on the gold surface. Recalling the fact that benzenesulfinate can still form SAMs on gold,45,51 the threshold HOMO energy needed for a feasible adsorption to occur on gold via the charge-transfer interaction may be located near that of C6H5SO2-. When assessing the relative adsorptivities of molecules or ions onto the metal surface, the HOMO-LUMO energy gap of the adsorbates may also be an important parameter. In general, the smaller the HOMO-LUMO energy gap is, the softer the molecule or ion is.52 Considering then that gold metal can be classified as a soft acid,53 a soft base with a smaller HOMOLUMO energy gap will adsorb more readily on gold. That is, with a smaller energy gap, not only the adsorbate but also the substrate are easily polarized, and thus the adsorbate-surface interaction becomes more favorable through the added mutual polarization effect.52 The data in Figure 7 dictate that the relative adsorptivity cannot be correlated with the magnitude of the HOMO-LUMO gap, however. Regarding the relative adsorptivity, one may also need to consider the changes in the solvation energies involved in surface adsorption. In fact, a more strongly solvated adsorbate has to lose more stabilization energy upon surface adsorption. Assuming that the solvation free energy of a molecule is proportional to its solubility, the adsorbate with a lower solubility will adsorb more favorably on the metal substrate. On this basis, we have to mention that the solubility of AQ-2COOH in various organic solvents is indeed much lower than those of other acids considered in this work. For instance, the solubility of AQ-2-COOH in ethanol is at best 1.962 g/L at room temperature,54 whereas benzoic acid dissolves in alcohol at 434.8 g/L.55 However, the effect of relative solubility on adsorption must be judiciously considered because the concept will be valid only when the headgroup-substrate interactions are nearly equivalent for different adsorbates. We are aware from experiment that the adsorbate-to-gold interactions are quite different for C6H5COOH and AQ-2-COOH; therefore the favorable adsorptivity of AQ-2-COOH on gold may not be attributed simply to its lower solubility. It was unfortunate that our frontier orbital approach did not clarify the origin of the favorable adsorption of AQ-2-COOH on gold. More extensive theoretical works seem thus necessary to understand the adsorption of AQ-2-COOH on gold. Regarding
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Han et al.
Figure 8. Electrostatic potential surface of the HOMO of AQ-2-COO- in front, side, and top views.
metal surface as revealed by the RAIR spectroscopy. With a perpendicular orientation, the metal-to-adsorbate, that is, from gold to the LUMO of AQ-2-COO-, electron donation will be infeasible. Summary and Conclusions
Figure 9. Electrostatic potential surface of the LUMO of AQ-2-COOdrawn together with the HOMO in meshed lobes.
these matters, one may have to take account of all the orbital interactions between the donor (adsorbate) and acceptor (surface). It is certainly needed to examine the second-highest occupied molecular orbital as well as the lower-lying orbitals because they may contribute significantly to the charge-transfer energetics. Although the origin of the favorable adsorption could not be resolved from our theoretical consideration, the bonding site(s) of AQ-2-COOH on gold would be revealed from the electrostatic potential surfaces of the HOMO and LUMO of AQ-2COO- drawn schematically in Figures 8 and 9, respectively. It can be noticed that the HOMO is quite localized at the oxygen atoms of the carboxylate group, whereas the LUMO is localized at the anthraquinone moiety. These indicate that AQ-2-COOhas to be bound to gold via the oxygen atoms of the carboxylate group symmetrically. Considering the signs of the HOMO lobes, a flat adsorbate structure is not feasible; the adsorbate has to take a perpendicular (or a tilted) orientation with respect to the
We were able to demonstrate that AQ-2-COOH could adsorb favorably on a gold surface. Control experiments showed clearly that ordinary aromatic and aliphatic carboxylic acids hardly formed SAMs on the gold surface. Although the adsorption rate of AQ-2-COOH on gold was much slower than that on silver and the adsorption strength of AQ-2-COOH to gold was also much weaker than that to silver, the RAIR spectral data revealed that AQ-2-COOH was chemisorbed on gold as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the metal substrate. The molecular plane was determined to be tilted by 30° from the surface normal. The RAIR spectral features observed on a flat gold surface were also consistent with the XPS and SER spectral features. The adsorption of AQ2-COOH on gold could be confirmed, on the other hand, by CV and QCM. The surface coverage value determined from CV was in good agreement with that estimated from QCM, suggesting, however, that the adsorbate, AQ-2-COO-, had a rather loosely packed structure on the gold surface. Using perturbation theory, we attempted to explain the favorable adsorption characteristics of AQ-2-COOH on gold, but more extensive calculations seemed necessary, taking account of all the orbital interactions between the donor (adsorbate) and acceptor (surface). Acknowledgment. This work was supported in part by the Korea Research Foundation (KRF) grant (KRF-042-D00073) and by the Korea Science and Engineering Foundation (KOSEF) grant (KOSEF-1999-2-121-001-5). K.K. also acknowledges the KOSEF for providing an instrument purchasing fund through the Center for Molecular Catalysis at Seoul National University. S.W.H., S.W.J., and T.H.H. acknowledge the KRF for providing a BK21 fellowship. References and Notes (1) (a) Murray, R. W. Molecular Design of Electrode Surface; Techniques of Chemistry Series 22; John Wiley & Sons: New York, 1992. (b) Pierrat, O.; Lechat, N.; Bourdillon, C.; Laval, J.-M. Langmuir 1997, 13, 4112. (2) (a) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3813. (b) Stern, D. A.; Wellner, E.; Salaita, G. N.;
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