Surface Enhanced Infrared Absorption Spectroscopy Studies of DMAP

Jan 5, 2009 - Scott M. Rosendahl, Brook R. Danger, J. P. Vivek, and Ian J. Burgess*. Department of Chemistry, UniVersity of Saskatchewan, Saskatoon, ...
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Langmuir 2009, 25, 2241-2247

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Surface Enhanced Infrared Absorption Spectroscopy Studies of DMAP Adsorption on Gold Surfaces Scott M. Rosendahl, Brook R. Danger, J. P. Vivek, and Ian J. Burgess* Department of Chemistry, UniVersity of Saskatchewan, Saskatoon, Saskatchewan, S7N 5C9 Canada ReceiVed October 14, 2008. ReVised Manuscript ReceiVed December 3, 2008 Attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements have been employed to study the adsorption of dimethylaminopyridine (DMAP) and its conjugate acid (DMAPH+) on gold surfaces as a function of applied potential and solution pH. Based on our transmission measurements, we have been able to demonstrate that the acid/base forms of this pyridine derivative can be readily differentiated due to their distinct IR signals. When the solution pH is equal to the pKa of DMAPH+, we demonstrate that the adsorbing species is DMAP, oriented with its heterocyclic ring perpendicular to the electrode surface. In acidic electrolytes, our SEIRAS data provide direct spectroscopic evidence of DMAP monolayer formation even though the pH is 5 units below the pKa of the conjugate acid. Our data support a potential induced deprotonation of the endocyclic nitrogen and resulting coordination of the nitrogen lone pair to the gold surface. Both of these results confirm our existing model of DMAP adsorption previously based solely on electrochemical measurements. However, the present SEIRAS study also indicates that, at low pH, DMAPH+ can electrostatically coordinate to very negatively charged surfaces. This mode of adsorption was previously unobserved, illustrating the ability of in situ spectroscopic techniques to reveal new information that is not apparent from traditional electrochemical techniques such as differential capacity and chronocoulometry.

1. Introduction The adsorption of dimethylaminopyridine (DMAP) on gold surfaces has been the subject of interest in recent years.1-4 The attention given to this system stems largely from the reports of water-soluble, DMAP stabilized Au nanoparticles (DMAP-AuNP) first described by Gittins and Caruso.5 Unlike thiol stabilized analogues, the absence of a chemical bond between molecule and metal allows for facile ligand exchange on DMAP-AuNP surfaces.6,7 Additionally, DMAP-AuNP have been shown to be positively charged,3-5,8 providing strong electrostatic interactions with oppositely charged molecules or surfaces. There are now numerous examples in the literature utilizing these unique properties of DMAP-AuNP for polyelectrolyte adsorption (layerby-layer assembly),9-13 electrochemical sensing,14,15 and DNA templating.16 Lennox and Gandubert have demonstrated that the basicity of the pyridine derivative is integral for phase-transfer and stabilization of metal nanoparticles,4 explaining why DMAP (pKb ) 4.3) and not pyridine (pKb ) 8.7) stabilized MNPs have * Corresponding author. E-mail: [email protected]. (1) Barlow, B. C.; Burgess, I. J. Langmuir 2007, 23, 1555–1563. (2) Larson, I.; Chan, D. Y. C.; Drummond, C. J.; Grieser, F. Langmuir 1997, 13, 2429–2431. (3) Vivek, J. P.; Burgess, I. J. J. Phys. Chem. C 2008, 112, 2872–2880. (4) Gandubert, V. J.; Lennox, R. B. Langmuir 2005, 21, 6532–6539. (5) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001–3004. (6) Rucareanu, S.; Gandubert, V. J.; Lennox, R. B. Chem. Mater. 2006, 18, 4674–4680. (7) Raguse, B.; Chow, E.; Barton, C. S.; Wieczorek, L. Anal. Chem. 2007, 79, 7333–7339. (8) Gittins, D. I.; Susha, A. S.; Schoeler, B.; Caruso, F. AdV. Mater. 2002, 14, 508–512. (9) Cho, J.; Caruso, F. Chem. Mater. 2005, 17, 4547–4553. (10) Dong, W. F.; Sukhorukov, G. B.; Moehwald, H. Phys. Chem. Chem. Phys. 2003, 5, 3003–3012. (11) Skirtach, A. G.; Dejugnat, C.; Braun, D.; Susha, A. S.; Rogach, A. L.; Sukhorukov, G. B. J. Phys. Chem. C 2007, 111, 555–564. (12) Dorris, A.; Rucareanu, S.; Reven, L.; Barrett, C. J.; Lennox, R. B. Langmuir 2008, 24, 2532–2538. (13) Gandubert, V. J.; Lennox, R. B. Langmuir 2006, 22, 4589–4593. (14) Yu, A.; Liang, Z.; Cho, J.; Caruso, F. Nano Lett. 2003, 3, 1203–1207. (15) Zhang, L.; Yuan, R.; Chai, Y.; Li, X. Anal. Chim. Acta 2007, 596, 99– 105. (16) Stanca, S. E.; Eritja, R.; Fitzmaurice, D. Faraday Discuss. 2006, 131, 155–165.

been reported. As well as increasing the basicity of the molecule, the presence of the para-substituent raises interesting questions about possible differences between the adsorption behavior of pyridine and DMAP on metal surfaces. On polycrystalline gold, it is well-known that pyridine undergoes a potential-dependent phase transition. Arising from the interaction between its π electrons and the metal, pyridine adsorbs in a flat-lying, low density monolayer at negatively charged surfaces. If the surface is charged positively, the molecules reorient to form a more compact, vertical layer involving a strong physisorption bond between the nitrogen atom and the gold.17 At low pH (ca. 4-5), our previous chronocoulometric studies1 have shown that DMAP follows a similar horizontal-to-vertical transition at potentials close to the potential of zero charge (Epzc) with a corresponding near doubling of the surface coverage. However, our results also indicated that the horizontally adsorbed species was likely the protonated dimethylaminopyridinium (DMAPH+) ion rather than the neutral molecule and that the transition to the vertical phase was accompanied by a deprotonation process. This deprotonation is somewhat remarkable given that the solution pH is nearly 5 orders of magnitude below the pKa of DMAPH+. A second important difference between DMAP and pyridine adsorption is the greater propensity of the former to adsorb in the higher density, vertical layer at pH near the pKa. At pH g 9.7, we observed no evidence of the horizontal adsorption state for DMAP on polycrystalline gold. This is in contrast to pyridine adsorption under analogous conditions where both phases are observed depending on the electrode’s surface charge.18,19 The DMAP(H+) adsorption model summarized above is the result of electrochemical studies alone. It is based on electrostatic models and the thermodynamics of adsorption on ideally polarized (17) Lipkowski, J.; Stolberg, L. Molecular adsorption at gold and silver electrodes. In Adsorption of Molecules at Metal Electrodes; Jacek Lipkowski, P. N. R., Ed.; VCH: New York, 1992; pp 171-238. (18) Stolberg, L.; Richer, J.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1986, 207, 213–234. (19) Stolberg, L.; Morin, S.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1991, 307, 241–242.

10.1021/la803404u CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

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electrodes. Corroborating evidence from extra-thermodynamic techniques such as scanning probe microscopy or spectroscopic methods can often offer new information concerning molecular adsorption. For example, in situ vibrational spectroscopy has emerged as a tremendously powerful tool for characterizing thin molecular films at electrode surfaces.20 While surface enhanced Raman scattering (SERS) provides great sensitivity to adsorbed molecules, it is usually restricted to roughened surfaces of coinage metals which precludes extraction of information concerning molecular orientation. In contrast, single-crystal electrodes can be used in reflectance absorption spectroscopy (RAS)21 and sum frequency generation22 experiments at the cost of lower sensitivity relative to SERS. In the past decade, the surface enhanced infrared absorption spectroscopy (SEIRAS) technique has been pioneered by Osawa23 and has been shown to have a surface sensitivity over 10 times greater than that of IR-reflectance absorption spectroscopy (IR-RAS) techniques. Both SEIRAS and SERS require some degree of surface roughness to achieve large electric field enhancements. Very rough, but ill-defined, metal surfaces can be created by oxidation-reduction cycles.24 Alternatively, much better defined metal films consisting of ∼100 nm sized, elliptical islands can be prepared by sputtering or electroless metal deposition and provide excellent enhancement for both SEIRAS and SERS measurements.25,26 Therefore, even though the gold surface used in SEIRAS is not a polished single-crystal such as those commonly employed in IR-RAS measurements, qualitative and even quantitative information on molecular orientation can be extracted from SEIRAS experiments. In a study particularly relevant to our current work, Cai et al. employed SEIRAS to study pyridine adsorption on a Au(111) textured electrode.27 Their results confirmed the horizontal-to-vertical transition that had not been forthcoming from previous infrared28 and SERS29 studies. In this paper, we report the results of our attempts to provide additional understanding on the adsorption behavior of DMAP and its conjugate acid. Herein, we report SEIRAS studies of DMAP adsorbed on gold films under variable pH conditions. While our IR results in basic solutions are complementary to our previous electrochemical results, our low pH measurements provide new details on the adsorption behavior of the conjugate acid.

2. Experimental Section Reagents, Solutions, and Electrode Materials. 4-(Dimethylamino)pyridine (DMAP) (99%), sodium fluoride (99.998%), potassium perchlorate monohydrate (+99%), potassium hydroxide (20) Osawa, M. In-Situ Surface-enhanced Infrared Spectroscopy of the Electrode/Solution Interface. In Diffraction and Spectroscopic Methods in Electrochemistry; Alkire, R. C., Kolb, D., Lipkowski, J., Ross, P. N., Eds.; WileyVCH: Weinheim, 2006; Vol. 9, pp 269-314. (21) Zamlynny, V.; Lipkowski, J. Quantitative SNIFTIRS and PM IRRAS of Organic Molecules at Electrode Surfaces. In Diffraction and Spectroscopic Methods in Electrochemistry; Alkire, R. C., Kolb, D., Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: Weinheim, 2006; Vol. 9, pp 315-376. (22) Tadjeddine, A.; Le Rille, A. Sum and Difference Frequency Generation at Electrode Surfaces. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; pp 317-344. (23) Osawa, M. Top. Appl. Phys. 2001, 81, 163–187. (24) Wu, D. Y.; Li, J. F.; Ren, B.; Tian, Z. Q. Chem. Soc. ReV. 2008, 37, 1025–1041. (25) Delgado, J. M.; Orts, J. M.; Perez, J. M.; Rodes, A. J. Electroanal. Chem. 2008, 617, 130–140. (26) Posey, K. L.; Viegas, M. G.; Boucher, A. J.; Wang, C.; Stambaugh, K. R.; Smith, M. M.; Carpenter, B. G.; Bridges, B. L.; Baker, S. E.; Perry, D. A. J. Phys. Chem. C 2007, 111, 12352–12360. (27) Cai, W. B.; Wan, L. J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992–6998. (28) Ikezawa, Y.; Sawatari, T.; Toriba, K. Electrochim. Acta 1998, 43, 3297– 3301. (29) Stolberg, L.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1991, 300, 563–584.

Rosendahl et al. (Semiconductor grade, +98%), perchloric acid (70%, double distilled), ammonium fluoride (+98%), ammonium chloride (+98%), sodium sulfite (+98%), sodium thiosulfate (99%), and hydrogen tetrachloroaurate III (99.9%) were purchased from Aldrich and were used as received. All aqueous solutions were prepared from Milli-Q (>18.2 MΩ cm-1) water or deuterium oxide (D, 99.9%) (Cambridge Isotope Laboratories, Inc., Andover, MA), as indicated in the text. Ethanol (95%), used for cleaning the silicon hemisphere, was purchased from Commercial Alcohols Inc. (Brampton, ON, Canada). 4-(Dimethylamino)pyridinium perchlorate ((DMAP)HClO4) was prepared as per the literature.1 Caution! Although we are unsure of the reactiVity of (DMAP)HClO4, organic perchlorate salts in general are potentially explosiVe and should be handled with due care! Electroless Deposition of Gold onto Silicon Hemispherical Prism. The general procedure we followed for deposition of gold onto the silicon hemispherical prism is reported elsewhere by Osawa et al.30 We also adopted the slight modifications described by Delgado et al.25 The reflecting plane of a 25 mm diameter, nondoped, silicon hemispherical prism (Harrick Scientific Products, Pleasantville, NY) was successively polished with finer grade diamond suspensions (Leco Corporation, St. Joseph, MI) down to 0.5 µm. The prism was then degreased by sonication in ethanol and finally rinsed in Milli-Q (>18.2 MΩ cm-1) water before deposition. To remove the oxide layer and to terminate the silicon surface with hydrogen, the reflecting surface was left in contact with a 40% (w/w) solution of NH4F for 5 min. The gold deposition was done at 55 °C by dropping a solution containing 5 mg of HAuCl4, 0.3 M Na2SO4, 0.1 M Na2S2O3, 0.1 M NH4Cl, and 2% HF (2:1 in volume) directly onto the polished face of the silicon hemispherical prism. After 80 s, the gold deposition was quenched by rinsing the prism with copious amounts of Milli-Q (>18.2 MΩ cm-1) water. ATR-SEIRAS Measurements. Our ATR-SEIRAS measurements were performed using an experimental setup adapted from previous reports.31,32 Briefly, we used a vertical spectroelectrochemical cell in the Kretschmann configuration. The cell was constructed from Teflon, and a reference electrode (Ag/AgCl, saturated KCl) was connected to the working cell via a glass salt bridge. Electrical contact was made to the working electrode by pressing a coiled loop of gold wire against the Au-plated surface of the Si hemisphere. The counter electrode was a coil of gold wire, flame annealed before immersing into the working cell and electrolyte. Potential control was maintained using a PAR 173 potentiostat and a custom program written in LabVIEW (National Instruments). The electrolyte was deaerated by purging with argon for 30 min, and a continual blanket of argon was maintained over the electrolyte throughout the experiment. The pH of the electrolyte was adjusted by using either potassium hydroxide or perchloric acid. All SEIRAS spectra were measured using p-polarized incident radiation at 70° with a resolution of 4 cm-1 using a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer equipped with a mercury cadmium telluride (MCT) liquid nitrogen cooled detector. All transmission IR spectra were measured similarly to the SEIRAS spectra except at a spectral resolution of 2 cm-1. The sample chamber of the spectrometer was purged throughout the experiment using CO2 and H2O free air supplied by a Parker Balston FT-IR purge gas generator 75-62 (Parker Hannifin Corporation, Haverhill, MA). Differential Capacity Measurements. Differential capacity (DC) measurements were performed in an all-glass cell using the working electrode positioned on the electrolyte surface in a hanging meniscus configuration or with the electrode immersed in the electrolyte. The working electrodes used were gold coated silicon, polished polycrystalline gold, and a Au (111) single crystal. A coiled gold wire served as the counter electrode, and a saturated Ag/AgCl electrode was used as the reference electrode. The pH of the electrolyte (50 mM KClO4) was adjusted to 9.7 using KOH. The electrolyte solution (30) Miyake, H.; Ye, S.; Osawa, M. Electrochem. Commun. 2002, 4, 973–977. (31) Ataka, K. i.; Yotsuyanagi, T.; Osawa, M J. Phys. Chem. 1996, 100, 10664– 10672. (32) Wandlowski, T.; Ataka, K.; Pronkin, S.; Diesing, D. Electrochim. Acta 2004, 49, 1233–1247.

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Figure 1. Differential capacity curves for a polished polycrystalline gold electrode (black, solid line), gold (111) crystal (blue, dotted line), and gold coated silicon (red, dashed line) in 50 mM KClO4 (pH adjusted to 9.7) in the presence of 0.1 mM formal concentration DMAP.

was deaerated by purging with argon for at least 30 min prior to the experiments, and an argon blanket was maintained over the solution throughout the experiment. The DC was calculated by measuring the in-phase and out-of-phase currents arising from the superposition of a 5 mV/s DC sweep and an AC perturbation (5 mV rms, 25 Hz) assuming an RC equivalent circuit. Measurements were carried out at room temperature (20 ( 2 °C) using a SRS730 lock-in amplifier (Standford Research Systems, Sunnyvale, CA) and a HEKA potentiostat PG590 (HEKA, Mahone Bay, NS, Canada).

3. Results and Discussion Electrochemistry. Below, we describe our SEIRAS data for DMAP adsorption on gold films electrolessly deposited on a Si ATR element. In order to compare our previous electrochemical data concerning DMAP adsorption and the results of our current study, we first needed to determine the crystallography of our deposited gold films. Previous reports in the literature have claimed that Au films thermally evaporated on Si provide (111) textured surfaces,25,27,31 and we wished to determine if such an orientation was present on our substrates. To do so, we ran differential capacity measurements of various gold substrates in the presence of 0.1 mM DMAP (formal concentration) at pH 9.7. Figure 1 compares the positive-going differential capacity curves for a polished polycrystalline gold electrode (solid line), a Au(111) single crystal (dotted line), and a Si wafer coated by a thin Au layer by chemical deposition as described above (dashed line). The Au(111) crystal shows several differences in its adsorption behavior compared to the smooth polycrystalline electrode. It can be observed that the pseudocapacitive peak corresponding to the onset of molecular adsorption occurs at E ≈ -0.6 V for DMAP on Au(111). On polycrystalline gold, this peak is superimposed on a much broader pseudocapacitive feature centered at E ≈ -0.6 V. The DC curve for polycrystalline gold shows a potential-independent capacitance after the initial adsorption and agrees with our previous model of a single state of DMAP adsorption for polycrystalline gold at high electrolyte pH.1 In contrast, the corresponding curve for Au(111) shows a distinct phase transition peak at E ≈ 0 V. The presence of this peak is somewhat remarkable, as it implies that, at high pH, multiple states of adsorption occur for DMAP on (111) but not on polycrystalline surfaces. The adsorption behavior of pyridine has been extensively studied using gold single crystals and has been shown to be dependent on which low-index surface is exposed to the electrolyte.17 The differential capacity results in Figure 1 imply that the adsorption behavior of DMAP also depends on the surface crystallography of the gold substrate. We can use these two distinct behaviors of DMAP adsorption on

Figure 2. Top panel: Absorbance IR spectra measured through KBr pellets for DMAP (a) and DMAP-HClO4 (b). Bottom panel: Absorbance spectra of concentrated DMAP solutions in water measured using an uncoated Si ATR element. The acidity of the solutions was adjusted to pH 11 to observe DMAP modes (c) and pH 3.5 to see DMAPH+ modes (d). Instrument resolution was 2 cm-1 for all four spectra.

polycrystalline gold and Au(111) to explain DMAP adsorption on our Au coated silicon substrates. The adsorption pseudocapacitive peak of DMAP on gold coated silicon occurs at E ≈ -0.6 V which registers nearly identically with the position of the adsorption peak for Au(111) and implies that our surface is perhaps preferentially (111) oriented. On the other hand, the phase transition peak at E ≈ 0 V which is very pronounced for the single crystal is only weakly apparent for our deposited film. The low intensity of the peak at E ≈ 0 V on Au coated silicon in contrast to Au(111) can be rationalized by the fact that discontinuous Au(111) domains on the Au coated silicon makes the phase transfer less distinctive compared to a perfect Au(111) crystal. Infrared Spectroscopy: Transmission. Figure 2 shows the transmission spectra of DMAP and (DMAP)HClO4 measured in KBr pellets (top panel) and in solution measured with a bare Si ATR element (bottom panel). Table 1 summarizes the assignment of the observed bands for DMAP and its conjugate acid in both the solid state and in solution. Based on the experimental and theoretical work of Kozhevina et al.,33,34 the point group of DMAP is Cs and the modes of vibration can be generally assigned to in-plane distortions of the pyridine ring and vibrations in the dimethylamino group. The normal modes for DMAP can be divided into A′ and A′′ symmetry classes whose overall transition (33) Kozhevina, L. I.; Chotii, K. Y.; Goncharova, L. D.; Rybachenko, V. I.; Titov, E. V. Ukr. Khim. Zh. (Russ. Ed.) 1989, 55, 83–88. (34) Kozhevina, L. I.; Rybachenko, V. I. J. Appl. Spectrosc. 1999, 66, 165– 170.

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Table 1. Assignment of Vibrational Bands for DMAP and Its Conjugate Acid Observed in the Solid State and in Solutiona Dimethylaminopyridine Vibrations description of mode

symmetry

ring distortion + C-N stretch ring distortion ring distortion methyl group bending C-N single bond stretch

A′ A′′ A′ A′ A′

DMAP (solid state)

DMAP (solution, pH 11)

-1

1609cm-1 (vs) n/a 1534cm-1(m,b) 1447cm-1 (w) 1385cm-1 (w)

(vs) 1605cm 1540cm-1(sharp) 1520cm-1 (b) 1447cm-1 (s) 1385cm-1 (s)

Dimethylaminopyridinium Vibrations

a

description of mode

symmetry

DMAPH+ · ClO4- (solid state)

DMAP (solution, pH 3.5)

ring distortion + C-N stretch ring distortion ring distortion methyl group bending C-N single bond stretch

A′ A′′ A′ A′ A′

1647cm-1(vs) 1590cm-1(sh) 1564cm-1 (b) 1446cm-1 (m) 1404cm-1 (m)

1652cm-1(vs) n/a 1569cm-1 (s) 1445cm-1 (w) 1405cm-1 (w)

vs, very strong; s, strong; m, medium; w, weak; b, broad; sh, shoulder; n/a, band not observed.

dipole moments are oriented collinear with the mirror plane (A′) and orthogonal to the mirror plane (A′′), respectively. Kozhevina and Rybachenko’s computational treatment reveals that, upon protonation, a positive charge delocalizes in the heterocycle which leads to an increase in the vibrational frequencies involving the stretching vibrations of the C-C and C-N bonds in the pyridinium ring.33,34 Similar behavior is known to occur in the case of pyridine.35,36 In solution, at pH 11, the ATR spectrum (Figure 2c) reveals principal peaks corresponding to the four A′ bands seen in the solid state, although the weaker A′′ mode is lost in the solution phase spectrum. We note that the position and relative intensities of the bands are slightly different for the ATRsolution measurements compared to the solid-state transmission spectra. The relative intensities differ, as we have not corrected our ATR data for the differences in path length and depth of penetration, dp, of the evanescent wave. There are also slight shifts in the position of the bands in the two sets of spectra which arise from (1) solvation effects and (2) the fact that the frequency of ATR absorption bands depends on the refractive index of the sample as well as dp.37 At pH 3.5 (Figure 2d), all the bands shift to higher frequencies except for the peak at 1447 cm-1, as the bending modes of the methyl group are not influenced by protonation of the endocyclic nitrogen. To further evidence the pH dependence of the peak positions, we performed a titration of the solution phase DMAP. In Figure 3, we present the magnitude of the integrated intensities of the bands appearing at 1652 and 1609 cm-1 as a function of pH. During the titration, we observed that the band intensities change but that there was no shifting in the peak positions. The plots in Figure 3 clearly exhibit the expected sigmoidal shape of a titration curve and provide a pKa value of 10. This is in very good agreement with the reported value of 9.7 for DMAP’s conjugate acid.13,38 Infrared Spectroscopy: SEIRAS. We performed electrochemical ATR-SEIRAS measurements at two pH values, namely, pH 10 and pH 4.5. In these experiments, we acquired 128 single beam scans at a reference potential and then immediately stepped to a variable potential of interest. After waiting for 2 min for the system to reach adsorption equilibrium, 128 scans were acquired (35) Cook, D. Can. J. Chem. 1961, 39, 2009–2024. (36) Travert, A.; Vimont, A.; Lavalley, J. C. Appl. Catal., A 2006, 302, 333– 334. (37) Mirabella, F. M. Principles, Theory, and Practise of Internal Reflection Spectroscopy. In Internal Reflection Spectroscopy: Theory and Applications; Mirabella, F. M., Ed.; Marcel Dekker: New York, 1993; pp 17-52. (38) Lange’s Handbook of Chemistry, 15th ed.; Dean, J. A., Ed.;McGrawHill: New York, 1999.

Figure 3. Integrated intensities of the 1652 and 1609 cm-1 bands in the ATR-IR spectra of aqueous DMAP solutions as a function of pH.

at the variable potential. A data set consisted of a family of variable potentials (e.g., -0.8 V e E e 0.5 V) and their corresponding reference potential measurements. In a typical experiment, we collected four data sets to provide a total of 512 single beam measurements and an average was calculated for each variable and reference potential. We then calculated a relative change in the IR signal:

∆S (SjE - Sjref) ) S Sjref

(1)

where SjE and Sjref are the average of 512 individual single beam signals at the variable and reference potentials, respectively. In this format, a negative-going peak in the processed data indicates an increase in that vibrational mode, due to either a greater number density or an increasing alignment of the mode’s transition dipole moment with the electrode’s surface normal. A positive-going peak indicates a diminishment of that vibration. In Figure 4, we present the results of the relative IR signal for the 0.1 mM DMAP in 50 mM supporting electrolyte (pH 10) experiment. In this case, the reference potential was -0.80 V where DMAP species are completely desorbed from the electrode surface.1 At potentials less than -0.40 V, the difference spectra are featureless, indicating that the surface remains surfactant free. Starting at E ) -0.40 V, the data reveal four downward-going bands appearing at 1389, 1446, 1538, and 1623 cm-1. The most pronounced signal is from the 1623 cm-1 vibration which is superimposed upon a very broad, positive absorption. The bimodal feature arises from the displacement of water molecules on the surface by the adsorbing

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Figure 5. Scaled integrated intensities (absolute values) of the 1538 cm-1 (O) and 1623 cm-1 (0) bands versus electrode potential. These plots are superimposed on the measured differential capacity curve for 0.1 mM DMAP (formal concentration) on Au coated Si at pH 10 (solid line).

DMAP species. The position of the 1389, 1446, and 1538 cm-1 bands are in excellent agreement with the A′ modes observed in transmission at pH 11 and indicate that the absorbed species is the vertically aligned deprotonated DMAP molecule as previously inferred from our electrochemical measurements. Determining if the highest frequency absorption band corresponds to DMAP or DMAPH+ is initially somewhat problematic, as it lies close to halfway between the protonated and deprotonated forms observed from our transmission experiments. Recalling that this vibration is very strongly coupled with a symmetric ring deformation, it would be expected that this mode will be blueshifted if the pyridine ring’s lone pair of electrons strongly coordinate to the metal surface. Similar shifts to higher frequencies have been reported for pyridine upon N-bonded adsorption on Au(111).27,28,39,40 We therefore interpret all four bands in our spectroelectrochemical data at pH 10 as corresponding to the A′ modes of the deprotonated form of the DMAP molecule. The intensities of these signals are found to increase with increasingly positive potential. In Figure 5, we plot the scaled integrated intensities of the 1538 and 1623 cm-1 bands versus electrode potential as well as the measured differential capacity curve for 0.1 mM DMAP on Au coated Si. We chose these two bands, as they provided the strongest signals, although the integration of the 1623 cm-1 band is somewhat complicated by its superimposition upon the water deformation absorption. Integration of the 1389 and 1446 cm-1 peaks provides qualitatively identical results but with larger scatter (data not shown). Recalling that a higher DMAP surface coverage corresponds to a lower measured capacity, Figure 5 provides good correlation between our electrochemical and spectroscopic signals for E > -0.2 V. The increase in the IR signal intensity is congruent with the observation of a limiting capacity value of ∼10-11 µF cm-2. A maximum IR signal is measured at E ) 0.2 V following which the signal begins to decrease. It is interesting to note that the DC curve reflects this as the capacity begins to increase at potentials greater than 0.2 V. We believe this arises from the onset of competitive

hydroxide adsorption in high pH solutions which displaces some of the DMAP molecules from the electrode’s surface. The consistency between the DC curve and our IR measurements for E > -0.2 V provides excellent corroborating evidence of our model of vertically oriented DMAP adsorbing on the gold surface at this pH. However, there is some discrepancy between the onset of the very broad pseudocapacitive peak in the differential capacity data and the initial appearance of IR intensity. The lack of IR signal in the potential range -0.7 V e E e -0.5 V may imply that, even at pH 10, the DMAP species may initially adsorb horizontally on the surface. In our earlier electrochemical study, we observed that the pseudocapacity peak associated with the phase transition shifts cathodically with increasing pH by approximately 60 mV/pH unit. At pH 10, this peak would correspond to the shoulder clearly visible on the anodic side of the adsorption pseudocapacity peak. As horizontally adsorbed DMAP species are invisible to our IR measurements (Vide infra), the narrow potential window between the initial adsorption and phase transition would explain why the IR intensity does not begin to rise until approximately -0.4 V. Further evidence of an end on, vertical orientation of the DMAP molecule can be deduced from the change in the peak position as a function of potential. Although the band appearing at 1445 cm-1 is potential invariant, the three other peaks all shift to higher frequencies with increasing potential. Potential induced shifts in IR vibrations have been observed for many adsorbates, particularly CO adsorption on Pt.41 Two general mechanisms are cited for this observation: the Stark effect and the chargetransfer model.42 The Stark effect involves a coupling between the potential induced electric field and the polarizable electrons in the adsorbate, whereas the charge-transfer model is based on changes in the bonding structure of the adsorbate with changes in the electrode’s charge density. Figure 6 provides plots of peak position versus electrode potential for the 1623, 1538, and 1390 cm-1 bands. The slopes of the 1623 and 1538 cm-1 bands are similar at 7-8 cm-1/V, whereas the 1391 cm-1 has a smaller potential dependence (slope ) 4.5 cm-1/V). For comparison, the A1 ring vibration found in pyridine has a 5-6 cm-1/V dependence28 and has been cited as arising from either a weak Stark effect or a charge-transfer effect resulting from electron donation via the nonbonding orbital located on the nitrogen atom. These two vibrations are strongly coupled with the ring’s nitrogen,

(39) Hoon-Khosla, M.; Fawcett, W. R.; Chen, A.; Lipkowski, J.; Pettinger, B. Electrochim. Acta 1999, 45, 611–621. (40) Nanbu, N.; Kitamura, F.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 1999, 470, 136–143.

(41) Mehandru, S. P.; Anderson, A. B. J. Phys. Chem. 1989, 93, 2044–2047. (42) Nichols, R. J. IR Spectroscopy of Molecules at the Solid-Solution Interface. In Adsorption of Molecules at Metal Electrodes; Jacek Lipkowski, P. N. R., Ed.; VCH: New York, 1992; pp 347-390.

Figure 4. Subtractively normalized SEIRAS data for 0.1 mM DMAP (formal concentration) as a function of potential. The electrolyte was 50 mM NaF, pH adjusted to 10 with KOH. The reference potential for these experiments was -0.8 V vs Ag/AgCl. 512 scans were signal averaged for each potential.

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Figure 7. Subtractively normalized SEIRAS data for 0.1 mM (DMAP)HClO4 as a function of potential. The electrolyte was 50 mM KClO4, pH adjusted to 4.5 with HClO4. The reference potential for these experiments was -0.8 V vs Ag/AgCl. 128 scans were signal averaged for each potential.

Figure 6. Peak position as a function of potential for the three strongest A′ modes of DMAP observed in Figure 4 (pH 10): (a) 1623 cm-1 band, (b) 1538 cm-1 band, and (c) 1390 cm-1 band.

whereas the 1391 cm-1 vibration primarily involves the C-N stretch. Using the Stark effect model, the rapid drop of the electric field across the inner Helmholtz plane of the electrical double layer should provide a greater Stark shift for the end of the molecule closest to the electrode’s surface. The larger potentialinduced shifts in the ring-dependent modes (1623 and 1538 cm-1) compared to those of the C-N vibration (1391 cm-1) is further evidence that DMAP adsorbs vertically on the Au surface through its endocyclic nitrogen at high pH. We also used the ATR-SEIRAS technique to follow the adsorption behavior of DMAP/DMAPH+ in more acidic electrolytes. We performed SEIRAS experiments in 50 mM KClO4 + 0.1 mM (DMAP)HClO4 (pH adjusted to 4.5). As the possibility exists that both DMAP and DMAPH+ signals are present in these conditions, we used D2O rather than H2O as a solvent. This avoids convolution of the H-O-H deformation signal with the 1653 cm-1 signal of DMAPH+ and the 1623 cm-1 signal of DMAP. In Figure 7, we present the results of our measurements

using a reference potential of -0.8 V vs Ag/AgCl. This reference potential was chosen because our previous differential capacity measurements indicated that the electrode is surfactant free at these potentials for low formal DMAP concentrations. Using this reference potential was slightly problematic because there is a significant amount of hydrogen evolution at -0.8 V in pH 4.5 electrolytes. We found that the hydrogen evolution reaction (HER) led to deterioration of our gold films. Similar effects have also been commented upon by Osawa et al.27 As a consequence of repeatedly stepping into the HER, we observed slight changes in the optical response of our film from one data set to the next. Although replicate measurements were qualitatively identical, coaddition of multiple data sets had the deleterious effect of actually decreasing our signal-to-noise ratio, and consequently, the spectra in Figure 7 correspond to 128 scans rather than 512 scans. Our previous electrochemical studies led us to propose a model of horizontally adsorbed DMAPH+ at potentials negative of the pzc and vertically oriented DMAP when E > Epzc. As the surface selection rules for SEIRAS render the horizontally adsorbed DMAPH+ invisible to our IR measurements, we predicted that our spectra for pH 4.5 experiments would be featureless for E < ∼0.2 V and show negative-going bands for E > ∼0.2 V. To our surprise, the subtractively normalized data in Figure 7 show the appearance of two positiVe-going bands starting at E ) -0.7 V. These bands increase in intensity with increasingly positive potential up to E ∼ -0.3 V. The frequency of the peak centers (1643 and 1559 cm-1) indicates that these signals arise from the protonated DMAPH+ ion. The potential-dependent intensities and the shift in frequencies compared to Figure 2d indicate that these bands arise from adsorbed species rather than DMAPH+ ions in solution. The fact that the bands are positive in direction indicates a decrease in either the concentration or the molecular alignment in the direction of the surface normal. Starting at about E ) 0.3 V, new, negatiVe bands appear in the subtractively normalized spectra at 1622, 1538, and 1390 cm-1, producing pronounced bimodal features for the high frequency A′ modes. The frequency of the negative-going bands strongly indicates

ATR-SEIRAS Studies of DMAP Adsorption

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pseudocapacity peak represents the vertical ion to horizontal ion transition rather than the onset of molecular adsorption as we had originally surmised from our electrochemical studies. The fact that the area under the 1643 cm-1 band does not plateau until -0.3 V indicates that between -0.7 V < E < -0.3 V the electrode is covered by a mixed phase of both DMAPH+ ion orientations. However, at more positive potentials, the vertical ion is completely displaced from the surface.

4. Summary and Conclusions

Figure 8. Scaled integrated intensities (absolute values) of the positivegoing 1643 cm-1 (0) and the negative-going 1623 cm-1 (O) bands versus electrode potential. Plots are superimposed on the measured differential capacity curve for 0.1 mM DMAP (formal concentration) on Au coated Si at pH 4.5 (solid line).

that they correspond to the neutral DMAP molecule and are nearly identical in position to the bands seen in our high pH experiment. Although only visible over a short range of potentials, the negative-going signals show a clear Stark effect, shifting to higher frequencies with increasing positive potentials. In contrast, the peak center positions of the positive modes in Figure 7 do not shift with potential. To explain the spectral behavior of our pH 4.5 experiment, we start with the realization that the positivegoing bands can only be rationalized if the reference potential does not correspond to a state of complete surfactant desorption. Considering that the electrode surface is highly negatively charged at -0.8 V, we propose that the DMAPH+ ion is electrostatically held to the electrode surface and adopts a vertical orientation to maximize charge compensation. When the potential is stepped to more positive values, the magnitude of the negative surface charge density decreases and the DMAPH+ ions adopt a low coverage, horizontally oriented state of adsorption. In this model, the charged molecule switches from being IR active at the reference potential (E ) -0.8 V) to being IR inactive in its adsorbed state (ca. -0.7 V < E < -0.2 V), which explains both the appearance of the positive-going bands and their absence of a Stark shift in Figure 7. At further positive potentials, the sign of the surface charge density switches from negative to positive, causing the DMAPH+ ions to deprotonate and reorient vertically on the surface. Compared to the reference potential state of electrostatically bound, vertical ions, this would equate to a loss of DMAPH+ signal and the appearance of negative-going bands corresponding to the adsorbed DMAP molecule. This conversion of adsorbed DMAPH+ to DMAP explains the bimodal features seen in Figure 7 for E > 0.2 V. In Figure 8, we provide the absolute integrated band intensities as a function of electrode potential for the positive-going 1643 cm-1 band and the negative-going 1623 cm-1 band. Superimposed on these plots is the corresponding differential capacity curve for this system. The 1643 cm-1 band plateaus in intensity at E ∼ -0.3 V, and the onset of the 1623 cm-1 signal correlates very well with the region of potentials with the lowest capacity in the DC curve. The latter point is convincing evidence that, even in acidic electrolytes, deprotonated DMAP molecules are the adsorbing species on Au surfaces at positive potentials. The intensity of the 1643 cm-1 band begins to increase at potentials corresponding to the first pseudocapacity peak in the DC curve (ca. -0.65 V) and plateaus at the onset of the second pseudocapacity peak (ca. -0.25 V). This implies that the first

We have used ATR-SEIRAS to obtain IR spectra of DMAP and its conjugate acid, DMAPH+, adsorbed on a gold surface. The high quality of our spectra is evidenced by excellent signalto-noise ratios and the large magnitude peaks observed in the subtractively normalized data. Electrochemical studies of our Au films indicate that the surface is preferentially (111) oriented but the size of these domains is probably quite small, and the result is a surface that shows intermediate behavior between a well-ordered (111) single crystal and a polished polycrystalline electrode. At high pH, our IR data show potential-dependent absorption signals corresponding to the various A′ ring deformation modes of the DMAP molecule but no evidence of modes corresponding to the conjugate acid. These results confirm the existence of a monolayer of vertically oriented DMAP molecules over a very wide range of double-layer potentials. Experiments performed at pH 4.5 show IR signals that arise from the base form of DMAP despite the fact that in the bulk of solution this species exists exclusively in the form of the conjugate acid. Although no positive-going IR features are observed until E > 0.2 V, the differential capacity data clearly indicate that DMAP species are adsorbing on the surface at -0.6 V < E < -0.1 V. This is consistent with the model we had previously inferred from our electrochemical measurements of a horizontal-to-vertical reorientation of DMAP species. As the surface selection rules prevent direct IR measurement of horizontally adsorbed DMAP (or DMAPH+), we cannot conclusively verify this phase transition from IR measurements alone. However, as the IR data offers no contradicting evidence, it tacitly supports our model of a potentialdependent horizontal-to-vertical transition at pH 4.5. Our IR measurements also confirm that at relatively positive potentials (and correspondingly positive surface charge densities) the adsorption of DMAPH+ results in a deprotonation of the acid. This result implies that the pKa of DMAPH+ is considerably different at the Au-solution interface compared to the bulk of solution. Although the perturbation of the acidity of a molecule at a surface is well-known, we feel that this system is an excellent example of how the electrical state of the interface may further perturb acid/base behavior. Our IR data also provided an unexpected result for the low pH experiments. The presence of positive-going bands from the A′ modes of dimethylaminopyridinium reveal that the DMAPH+ ions are still adsorbed on the electrode surface at very negative potentials presumably due to an electrostatic attraction. This is a result that was entirely not forthcoming from our previous electrochemical studies (differential capacity and chronocoulometry), and it evidences the power of in situ spectroscopic techniques in revealing new information concerning interfacial adsorption at electrified interfaces. Acknowledgment. This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. LA803404U