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Langmuir 2006, 22, 6102-6108
Electrochemical and Surface Characterization of 4-Aminothiophenol Adsorption at Polycrystalline Platinum Electrodes Belinda I. Rosario-Castro,† Estevao R. Fachini,† Jessica Herna´ndez,† Marla E. Pe´rez-Davis,*,‡ and Carlos R. Cabrera*,† Department of Chemistry and Center for Nanoscale Materials, UniVersity of Puerto Rico, Rı´o Piedras Campus, P.O. Box 23346, San Juan, Puerto Rico 00931-3346, and Electrochemistry Branch, NASA John H. Glenn Research Center, 21000 Brookpark Road, CleVeland, Ohio 44135 ReceiVed August 15, 2005. In Final Form: March 2, 2006
The formation of a self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) on polycrystalline platinum electrodes has been characterized by surface analysis and electrochemistry techniques. The 4-ATP monolayer was characterized by cyclic voltammetry (CV), linear sweep voltammetry, Raman spectroscopy, reflection-absorption infrared (RAIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). CV was used to study the dependence of the adsorption time and 4-ATP solution concentration on the relative degree of coverage of 4-ATP monolayers on polycrystalline Pt electrodes. The adsorption time range probed was 24-72 h. The optimal concentration of 4-ATP needed to obtain the highest surface at the lowest adsorption time was 10 mM. RAIR and Raman spectroscopy for 4-ATP-modified platinum electrodes showed the characteristic adsorption bands for 4-ATP, such as νNH, νCHarom, and νCSarom, indicating the adsorption on the platinum surface. The XPS spectra for the modified Pt surface presented the binding energy peaks of sulfur and nitrogen. High energy resolution XPS studies, RAIR, and Raman spectrum for platinum electrodes modified with 4-ATP indicate that the molecules are sulfur-bonded to the platinum surface. The formation of a S-Pt bond suggests that ATP adsorption leads to an amino-terminated electrode surface. The thickness of the monolayer was evaluated via angle-resolved XPS (AR-XPS) analyses, giving a value of 8 Å. As evidence of the terminal amino group on the electrode surface, the chemical derivatization of the 4-ATP SAM was done with 16-Br hexadecanoic acid. This surface reaction was followed by RAIR spectroscopy.
1. Introduction Self-assembled monolayers (SAMs) are formed through a spontaneous adsorption of molecules onto a substrate surface from a precursor solution.1-5 The formation of organic monolayers by self-assembly is directed by a specific interaction between a terminal functional group and the surface. This method has been extensively used for solid surface modification because it is a simple, versatile, and convenient way to define the chemical composition and structure of a surface. SAMs have been used for molecular recognition,6 the altering of wetting properties of the surface,7-9 electron-transfer studies,10 electrocatalysis,11 and corrosion protection.12-14 SAMs have been prepared on a variety * Corresponding author. E-mail:
[email protected] (M.E.P.-D.);
[email protected] (C.R.C.). † University of Puerto Rico. ‡ NASA John H. Glenn Research Center. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (3) Nuzzo R. G.; Allara, D. L. J. Am. Chem. Soc. 1986, 105, 4481. (4) Flinkea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 110. (5) Ulman, A. Chem. ReV. 1996, 96, 1533. (6) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329. (7) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 113, 1990. (8) Laibinies, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (9) Fukishima, F.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. B 2000, 104, 7417. (10) Dong, X. D.; Lu, J.; Cha, C. J. Electranal. Chem. 1995, 388, 195. (11) Cheng, L.; Pacey, G. E.; Cox, J. A. Electrochim. Acta 2001, 46, 4223. (12) Meage, I.; Jaehne, E.; Henke, A.; Adler, H.-J. P.; Bram, C.; Jung, C.; Stratmann, M. Prog. Org. Coat. 1997, 34, 1. (13) Tremont, R.; De Jesus-Cardona, H.; Garcia-Orozco, J.; Castro, R. J.; Cabrera, C. R. J. Appl. Electrochem. 2000, 30, 737. (14) Tremont, R.; Cabrera, C. R. J. Appl. Electrochem. 2002, 32, 783.
of electrode materials, such as silicon,15-16 silver,17 copper,13-14 gold,18-20 and platinum.21-23 Assemblies based on thiols on gold24-27 are the most extensively studied SAMs. SAMs based on aromatic thiols have also been vastly studied.28-31 The strength of the bond formed between the thiol sulfur and the metal32 allows for additional functional groups in the adsorbing molecule with no obstruction of the self-assembly of the molecules through the sulfur. The formation of a densely packed monolayer (15) Brandow, S. L.; Chen, M. S.; Aggarwal, R.; Dulcey, C. S.; Calvert, J. M.; Dressick, W. J. Langmuir 1999, 15, 5429. (16) Wei, Z. Q.; Wang, C. F.; Zhu, C. F.; Zhou, C. Q.; Xu, B.; Bai, C. L. Surf. Sci. 2000, 459, 401. (17) Han, S. W.; Lee, S. J.; Kim, K. Langmuir 2001, 17, 6981. (18) Xiao, X.; Wang, B.; Zhang, C.; Yang, Z.; Loy, M. M. T. Surf. Sci. 2001, 472, 41. (19) Azzaroni, O.; Vela, M. E.; Martin, H.; Hernandez Creus, A.; Andreasen, G.; Salvarezza, R. C. Langmuir 2001, 17, 6647. (20) Ye, H.; Scott, R. W. J.; Crooks, R. M. Langmuir 2004, 20, 2915. (21) Clavilier, J.; Svetlic´ic´, V.; Zutic´, V. J. Electroanal. Chem. 1996, 402, 129. (22) Brito, R.; Rodriguez, V. A.; Figueroa, J.; Cabrera, C. R. J. Electroanal. Chem. 2002, 520, 47. (23) Brito, R.; Tremont, R.; Feliciano, O.; Cabrera, C. R. J. Electroanal. Chem. 2003, 540, 53. (24) (a) Zhong, C.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9. (b) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (25) Kawaguchi, T.; Hiroaki, Y.; Shimazu, K.; Porter, M. D. Langmuir 2000, 16, 9830. (26) Datwani, S. S.; Vijayendran, R. A.; Johnson, E.; Biondi, S. A. Langmuir 2004, 20, 4970. (27) John, S. A.; Kitamura, F.; Tokuda, K.; Ohsaka, T. Langmuir 2000, 16, 876. (28) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (29) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705. (30) Batz, V.; Schneeweiss, M. A.; Kramer, D.; Hagenstrom, H.; Kolb, D. M.; Mandler, D. J. Electroanal. Chem. 2000, 491, 55. (31) Raj, C. R.; Kitamura, F.; Ohsaka, T. Langmuir 2001, 17, 7378. (32) Bard, A. J. Integrated Chemical Systems; John Wiley & Sons: New York, 1994.
10.1021/la0522193 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/01/2006
Characterization of 4-ATP SAMs on Pt Electrodes
is also critical to obtain a single functional group exposed on the external surface.22 An assembly terminated with a desired function provides a supporting structure for further chemical derivatization.21,31 For the attachment of additional molecules or layers to the modified surfaces, coupling agents, such as acid chlorides, anhydrides, and carbodiimides, have been used. Researchers have particular interest in amino-terminated monolayers, since they have been used for several applications, such as the attachment of carbon nanotubes on metal surfaces33-34 and the binding of biomolecules, such as DNA.35-37 Amino-terminated monolayers have also been used for the development of electrochemical sensors for biomolecules. Raj et al. performed the electrochemical transformation of a 4-ATP SAM to obtain a confined redox active diimine for the electrocatalytic sensing of reduced nicotinamide adenine dinucleotide (NADH) on gold substrate.31,38 This work discusses results obtained from spectroscopic and electrochemical techniques used to characterize the self-assembly of 4-aminothiophenol (4-ATP) over polycrystalline platinum electrodes. Electrochemical characterization served to study the packing quality of the 4-ATP assembly. Reflection-absorption infrared (RAIR), Raman, and X-ray photoelectron (XPS) spectroscopies results suggest a successful adsorption of 4-ATP as well as the attachment of the molecules through sulfurplatinum bonds. SAMs fabricated by 4-ATP have significant meaning in terms of the possibility to take advantage of the stability of the S-metal bonding and the reactivity of the terminal groups for attachment of a new layer via chemical derivatization. 2. Experimental Section. 2.1. Materials. 4-ATP (Aldrich, 90%), sulfuric acid (Aldrich, 99.999%), phosphoric acid (Aldrich, 99.999%), potassium ferricyanide (K3Fe(CN)6) (Aldrich, 99%), hexaamineruthenium (III) chloride (Aldrich, 98%), potassium chloride (Aldrich, 99.999%), tetrabutylammonium perchlorate (TBAP; Fluka, 99%), 16-Br hexadecanoic acid, 4-methylmorfoline (Aldrich, 99%), isobutyl chloroformate (Aldrich, 98%), anhydrous acetonitrile (Aldrich, 99.8%), anhydrous ethanol (Aldrich, 200 proof, 99.5+%). The water used for the experiments was previously distilled and pumped through a Nanopure system (Barnstead) to give 18 MΩ cm nanopure water. Platinum disk electrodes with a diameter of 1.6 mm from BioAnalytical Systems (BAS) were used for most of the electrochemical measurements, and MAXTEK, Inc. platinum electrodes (13 mm of diameter) were used for cyclic voltammetry (CV) in H2SO4, XPS, Raman, and RAIR measurements. 2.2. Electrochemical Cleaning Treatment of Platinum Electrodes and Electrochemical Characterization of 4-ATP-Modified Platinum (Pt) Electrodes. Electrochemical measurements were performed using a Bipotentiostat AFCBP1 from Pine Instrument Company. A three-electrode cell was employed in all experiments. The reference electrode used was a Ag|AgCl electrode. A platinum mesh electrode was used as the counter electrode. Nanopure water was used to wash the platinum electrodes before and after every electrochemical treatment in sulfuric acid. The working electrode was a platinum disk or a MAXTEK platinum electrode. CV in 0.5 M H2SO4, between -220 and 1200 mV vs Ag|AgCl, was performed to verify the cleanliness of both types of electrodes before the modification with the 4-ATP SAM. CVs obtained (see Figure 5a) are characteristic of clean polycrystalline platinum surfaces.39,40 To (33) Liu, Z.; Sehn, Z.; Zhu, T.; Hou, S.; Ying, L.; Shi, Z.; Gu, Z. Langmuir 2000, 16, 3569. (34) Nan, X.; Gu, Z.; Liu, Z. J. Colloid Interface Sci. 2002, 245, 311. (35) Brockman, J. M.; Frutos, A. G.; Corn, R. M. J. Am. Chem. Soc. 1999, 121, 8044. (36) Frutos, A. G.; Brockman, J. M.; Corn, R. M. Langmuir 2000, 16, 2192. (37) Smith, E. A.; Wanat, M. J.; Cheng, Y.; Barreira, S. V. P.; Frutos, A. G.; Corn, R. M. Langmuir 2001, 17, 2502. (38) Raj, C. R.; Ohsaka, T. Langmuir 2001, 3, 633. (39) Finklea, H. O. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley & Sons, Ltd: Chichester, U.K., 2000.
Langmuir, Vol. 22, No. 14, 2006 6103 reuse platinum disk electrodes modified with a 4-ATP monolayer, an electrochemical treatment was applied for cleaning. This electrochemical treatment involves repeated cycling in 0.5 M H2SO4 reaching very negative potentials (about -800 mV vs Ag|AgCl). The application of negative potentials helps the desorption of the organic impurities on the electrode surface by hydrogen evolution. CV studies for unmodified and 4-ATP-modified Pt disk electrodes were performed in 2.5 mM K3Fe(CN)6/0.1 M KCl and 2.5 mM Ru(NH3)6/0.1 M KCl electrolyte solutions using a scan rate of 100 mV/s. 2.3. Platinum Surface Modification with 4-ATP: SAMs Method. SAMs were prepared by immersing the Pt electrode in a fresh ethanolic solution of 4-ATP for a period of 24 h. Once the deposition was completed, the Pt electrode was removed, rinsed with ethanol, and dried with a slow stream of Ar. Solutions of 4-ATP of 1, 5, and 10 mM concentration were probed. 2.4. Protonation of the 4-ATP Monolayer Amino Groups. The 4-ATP-modified Pt electrode was immersed in 0.1 M H3PO4 (pH 1) for a few seconds. The sample was removed and dried with a slow stream of Ar. The protonated 4-ATP SAM sample was rinsed several times with a 1 × 10-5 M H3PO4 (pH 5) solution, and then dried as described above. 2.5. Derivatization of the 4-ATP Monolayer. The 16-Br hexadecanoic acid was activated for 1 h in a solution that contained 80 µL of N-methylmorpholine and 100 µL of i-butyl chloroformate in dimethylformamide (DMF). The 4-ATP-modified Pt electrode was immersed overnight in the activating solution. The sample was removed and rinsed with DMF. 2.6. XPS Analysis. XPS data was obtained using a PHI 5600ci spectrometer with an Al KR X-ray source at 15 kV and 350 W. Spectra were recorded at a takeoff angle of 45° and a pass energy of 93.9 eV for the survey analysis and 58.7 eV for the high energy resolution studies. Binding energies were corrected to the aliphatic hydrocarbon C1s signal at 284.5 eV. For the determination of the monolayer thickness, angle-resolved studies using a Pt 4f signal were recorded at different takeoff angles with respect to the surface plane, specifically 20, 25, 30, 35, 40, 45, 50, and 55°. The software package Spartan (version 1.0.1) from Wavefunction, Inc. was used for the 4-ATP molecule length calculation. 2.7. Infrared Spectroscopy Analysis. Transmittance IR spectrum from pure 4-ATP was obtained with a Nicolet Magna IR 750 with a Nic-Plan microscope. Specular reflectance for the 4-ATP SAMmodified Pt electrode was performed using a Smart SAGA accessory at an incidence angle of 80° in a Nicolet Nexus 870 Fourier transform infrared (FT-IR) spectrometer. The spectrum was recorded with 250 scans and a resolution of 8 cm-1. 2.8. Raman Spectroscopy. Raman spectra were obtained with a Nicolet Almega dispersive Raman spectrometer equipped with a visible Raman microscope and a charge-coupled detector. The excitation wavelength was 785 nm. Recording was carried out at a spectral aperture of 25 µm, five scans, and a 3.8-cm-1 resolution.
3. Results and Discussion 3.1. Electrochemical Measurements. CV is one of the tools most frequently used to study SAMs on electrodes because of its high sensitivity, which is attributed to its ability to detect currents from the high rates of mass transfer of redox couples on clean electrodes to small bare pores on a modified electrode.39 In these experiments, Ru(NH3)63+ and Fe(CN)63- were used as the redox probe molecules to study the SAMs on Pt. Figure 1A shows CVs recorded for the redox process of Ru(NH3)63+ at Pt electrodes modified for 24 h using different concentrations of 4-ATP solution to prepare the monolayer. When 1 and 5 mM 4-ATP solutions were used for SAM formation, the peak current was reduced, but the redox process was still observed. This is evidence of a 4-ATP SAM formed with defect sites or areas of poor packing of the molecules. Penetration of the probe molecule (40) Allara, D. L.; Unzo, R. G. Langmuir 1985, 1, 45.
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Figure 1. Cyclic voltammograms in a solution of (A) 2.5 mM Ru(NH3)6 Cl3 and (B) 2.5 mM K3Fe(CN)6 in 0.1 M KCl at a scan rate of 100 mV/s for (a) an unmodified Pt electrode, and 4-ATPmodified Pt electrodes prepared by immersing in (b) 1 mM, (c) 5 mM, (c) and (d) 10 mM 4-ATP solutions for 24 h.
through the pores allows its reduction. The 4-ATP SAM with a higher packing quality must prevent the probe ions from reaching the electrode surface. As can be seen in Figure 1A, a SAM prepared using a 4-ATP solution with a higher concentration (10 mM) presented a substantial decrease in faradaic current, yielding the highest surface coverage. Similar results were obtained when Fe(CN)63- was used as the probe molecule (see Figure 1B), in terms of the current response dependence on 4-ATP solution concentration. However, the current observed for the same modified electrode was higher for Ru(NH3)63+ than for Fe(CN)63because of the extent of pore penetration of Ru(NH3)63+.6,41 Similar experiments were performed for electrodes modified for more than 1 day. CVs of Ru(NH3)63+ and Fe(CN)63- for electrodes modified using the different concentrations of 4-ATP solution (1, 5, and 10 mM) for 48 and 72 h are presented in Figures 2 and 3, respectively. As observed above, the current response for Ru(NH3)63+ was higher than that for Fe(CN)63- for the same modified electrode. From the responses obtained for the Fe(CN)63- probe at the different adsorption times, it could be deduced that a densely packed monolayer was formed from the 5 mM 4-ATP solution, even at the lowest time probe. When the electrode was modified using a 1 mM solution, a complete monolayer was presumably formed at 72 h of adsorption time. However, a different interpretation of the time dependence of the coverage is derived from the results obtained with the probe molecule with the highest level of pore penetration (Ru(NH3)63+). From a 5 mM solution, the monolayer coverage degree was higher at 48 h than it was at 24 h, although the observed current indicated that there were still pinholes. As shown in Figure 3Ac, the highest coverage degree from a 5 mM solution was obtained no earlier than 72 h. On the other hand, for a 1 mM solution, a complete monolayer was not obtained, even at the highest adsorption time explored (see Figure 3Aa). Nevertheless, since the current response decreases with increasing adsorption time, formation of a complete monolayer at longer adsorption time is expected. (41) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884.
Rosario-Castro et al.
Figure 2. Cyclic voltammograms in a solution of (A) 2.5 mM Ru(NH3)6 Cl3 and (B) 2.5 mM K3Fe(CN)6 in 0.1 M KCl at a scan rate of 100 mV/s for (a) an unmodified Pt electrode, and 4-ATPmodified Pt electrodes prepared by immersing in (b) 1 mM, (c) 5 mM, and (d) 10 mM 4-ATP solutions for 46 h.
Figure 3. Cyclic voltammograms in a solution of (A) 2.5 mM Ru(NH3)6 Cl3 and (B) 2.5 mM K3Fe(CN)6 in 0.1 M KCl at a scan rate of 100 mV/s for (a) an unmodified Pt electrode, and 4-ATPmodified Pt electrodes prepared by immersing in (b) 1 mM, (c) 5 mM, and (d) 10 mM 4-ATP solutions for 72 h.
From the results obtained at a given adsorption time (24 h), it can be concluded that the rate of completion of the equilibrium coverage depends on the bulk concentration. Consequently, for higher concentrations, shorter adsorption times are required. Based on these results, the 10 mM solution of 4-ATP was selected for the preparation of 4-ATP monolayers to ensure the formation of a densely packed monolayer at the shorter adsorption time studied (24 h). Comparison between the CV in sulfuric acid of a bare electrode and that of a 4-ATP-modified platinum electrode is also a
Characterization of 4-ATP SAMs on Pt Electrodes
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Figure 4. Cyclic voltammograms of Pt electrodes in a 0.5 M H2SO4 solution for (a) clean Pt electrode, (b) first and (c) tenth cycle of a 4-ATP-modified Pt electrode. The scan rate of 100 mV s-1.
Figure 6. Infrared spectroscopy: (a) Isotropic spectrum for a pure 4-ATP sample and (b) RAIR spectrum for a 4-ATP-modified Pt electrode.
Figure 5. LSV for (a) bare platinum bare and (b) a 4-ATP-modified platinum electrode in a 0.1 M TBAP/CH3CN solution. The scan rate was 100 mV/s, and the geometric area was 0.020 cm2.
significant experiment to determine the packing quality. The CV in 0.5 M H2SO4 was performed for modified electrodes under the same conditions used for unmodified electrodes (Figure 4a). The 1st and 10th cycle obtained from the same 4-ATP-modified electrode (using the 10 mM solution) are presented in Figure 4, CVs b and c, respectively. It can be noticed from the voltammograms that the current at the Pt oxide reduction peak was lower for the modified electrode than the current of the same peak at a clean electrode. Furthermore, the double-layer capacitance around 200 mV vs Ag|AgCl was higher, the peak for Pt oxide formation shifted to higher potentials, and the hydrogen adsorption/desorption peaks were markedly concealed in the voltammogram for the modified electrode. These observations are indicative of 4-ATP adsorption at the surface. The self-assembled 4-ATP monolayer passivates the electrode, substantially inhibiting the electrochemical processes at the platinum surface. In contrast to the 1st cycle, in the 10th cycle for the modified electrode (Figure 4c), the Pt oxide reduction peak is higher, the double-layer capacitance current is diminished, and the hydrogen adsorption/desorption peaks begin to become visible. These observations suggest that the 4-ATP assembly structure is affected under cycling conditions, which possibly promote the formation of defect sites by desorption of 4-ATP molecules. Nevertheless, it is feasible to think that the 4-ATP assembly is stable because there is a shift in the hydrogen adsorption/desorption peaks: the peak for Pt oxidation is still shifted to higher potentials, and the Pt oxide reduction peak current is still lower than that for a bare Pt electrode. Electrochemical reductive desorption from a 4-ATP SAM prepared using a 10 mM solution for 24 h (Figure 5), was performed by linear sweep voltammetry (LSV) in a solution 0.1 M TBAP in CH3CN. The desorption potential determined from this voltammogram was -1.3 V. The cathodic peak indicates the reductive desorption of the monolayer from the surface. This was used to quantify the surface coverage,42 which gave a value of 6.7 × 10-10 mol/cm2, considering a roughness factor of 3.3. This result is in the surface coverage range expected for Au(111)
surfaces (10-10-10-9 mol/cm2),43 which can be compared to the coverage in Pt, since their lattice constants are similar (i.e., 4.08 and 3.92 Å for Au and Pt, respectively).44 3.2. Infrared Spectroscopy Analysis. Infrared spectroscopy has been widely used to verify the formation of SAMs on metal surfaces. This technique was used to look into the composition and structure of the 4-ATP assembly on Pt. Figure 6a,b shows IR spectra for neat 4-ATP and a 4-ATP-modified platinum electrode, respectively. It is noteworthy that the spectrum for the 4-ATP-modified Pt electrode is qualitatively comparable to the IR spectrum for neat 4-ATP, although there are some slight differences. One of the differences between these spectra is the absence of the S-H stretch band at 2550 cm-1 in the modified Pt electrode spectrum, suggesting that the sulfur-hydrogen bond is substituted by a sulfur-platinum bond. This suggests S-H bond breakage and the adsorption of 4-ATP molecules to the Pt electrode through the formation of S-Pt bonds. Chemisorption on the surface through the sulfur atoms of the molecules results in the formation of a self-assembly terminating with amino groups (which is supported by the XPS results, which will be presented below). The second difference is that some of the peak positions for 4-ATP adsorbed on platinum are shifted to higher energies than those for neat 4-ATP. The peak positions for both the 4-ATP self-assembly on Pt and the pure substance spectra are presented in Table 1. The pure 4-ATP peak position for the antisymmetric NH2 stretching mode was at 3432 cm-1. The CH stretching mode was at 3026 cm-1, and the CH bending mode was at 819 cm-1. On the other hand, for the 4-ATP/Pt electrode, these peaks were shifted to 3450, 3030, and 826 cm-1, respectively. Although the shift in peak positions is not well understood, previous reports have attributed this shift to intramolecular lateral interactions within the assembly.45,46 This is indicative of a high level of packing on the monolayer. Another important issue is the selection rules for RAIR spectroscopy. Only vibration modes with a dipole (42) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (43) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Harris, D., Swain. E., Robey, C., Aiello, E., Eds.; John Wiley & Sons: New York, 2001. (44) Sa´nchez, C. G.; Del Po´polo, M. G.; Leiva, E. P. M. Surf. Sci. 1999, 421, 59. (45) Bradshaw, A. M.; Schweizer, E. In AdVances in Spectroscopy. Spectroscopy of Surfaces; Hester, R. E., Clark, R. J. H., Eds.; Wiley: New York, 1988; Vol. 16, pp 413-488. (46) Willis, R. F.; Lucas, A. A.; Mahan, G. D. The Chemical Physiscs of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1981; Vol. 2, pp 67-100.
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Figure 8. XPS spectra for (a) an unmodified Pt electrode immersed in anhydrous ethanol for 24 h and (b) a 4-ATP-modified Pt electrode.
Figure 7. Raman spectra for (a) a pure 4-ATP sample and (b) a 4-ATP-modified Pt electrode. Table 1. Peak Positions of IR Spectra of Neat 4-ATP and a Self-Assembled 4-ATP Monolayer mode assignment
absorbance for pure substance (cm-1)
absorbance for SAM (cm-1)
νaNH2 νsNH2 νNH2 νCHarom νSH νCHovertone νCC νCC νCC νCC νCN νCSarom δCHarom
3432 3358 3213 3026 2554 1888 1620 1595 1496 1447 1284 1090 820
3450 3352 3201 3030 1884 1620 1593 1494 1423 1292 1089 826
moment perpendicular to the metallic surface may be observed. IR peaks produced by vibrations that are parallel to the main molecular axis, such as C-C stretches, are clearly observed for the 4-ATP SAM. This suggests that the molecular axis of the SAM is nearly perpendicular to the Pt surface plane. Preliminary density functional theory (DFT) calculations on the 4-ATP SAM on Pt(111) surfaces show that the molecules are oriented almost perpendicular to the substrate surface and at a 2-fold bridge site.47 3.3. Raman Spectroscopy. Raman spectroscopy technique is similar to IR spectroscopy, since it also produces a unique spectral fingerprint. Platinum is generally accepted to be a nonenhancing metal under conditions of visible wavelength excitation, as in this work. However, normal Raman spectra for monolayers on Pt surfaces have been reported.28 Figure 7a shows the expected spectrum for a pure sample of 4-ATP with its characteristic bands, and the respective values are shown in Table 2. Some of these bands are also present in the spectrum obtained for a 4-ATPmodified Pt electrode, shown in Figure 7b. The νCS frequency for the 4-ATP SAM is downshifted in comparison with the (47) Rosario-Castro, B. I.; Mateo, J. J.; Ishikawa, Y.; Cabrera, C. R. To be submitted for publication.
Figure 9. High-resolution XPS spectra for unmodified and 4-ATPmodified electrodes at (a) carbon region and (b) platinum region. The same XPS experimental conditions were used. Table 2. Peak Positions of Raman Spectra for Neat 4-ATP and for a Self-Assembled 4-ATP Monolayer mode assignment
absorbance for pure substance (cm-1)
absorbance for SAM (cm-1)
νCHarom νSH νCC νCSarom δCHarom
3048 2544 1593 1087 818
3048 1590 1085 820
frequency obtained for the pure 4-ATP sample. A downshift for thiophenol on a Pt surface was observed earlier by Bryant et al.28 This suggests that the 4-ATP molecular adsorption occurs through the formation of sulfur bonds. These spectra serve as additional evidence of the 4-ATP adsorption on the Pt electrode surface. 3.4. XPS Analysis. XPS is another technique used to study the composition of SAM-modified surfaces. Figure 8a,b shows XPS survey spectra for an uncoated and a 4-ATP-modified Pt electrode, respectively. Photoemission peaks attributed to nitrogen
Characterization of 4-ATP SAMs on Pt Electrodes
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Figure 10. Curve-fitted high-resolution XPS spectra for the (a) C1s, (b) N1s, and (c) S2p regions for a 4-ATP-modified platinum electrode. The same XPS experimental conditions were used. Table 3. XPS Atomic Concentrations for a 4-ATP-Modified Platinum Surface species concentration (%)
C1s 74.82
N1s 10.71
O1s 2.28
S2p 10.86
and sulfur atoms are present in the spectrum of the modified electrode, which are not present in the unmodified Pt electrode spectrum. This is indicative of the adsorption of 4-ATP molecules on the electrode. Figure 9a shows high-resolution spectra for the C1s region of an unmodified and a 4-ATP-modified platinum electrode acquired under the same instrumental conditions. The carbon binding energy peak for the modified electrode is higher than the same signal for the unmodified electrode. This increase in peak height is due to the detection of carbon atoms from the aromatic ring of the 4-ATP SAM. Figure 9b shows XPS spectra for the Pt 4f region for unmodified and 4-ATP-modified electrodes (same instrumental conditions). The Pt signal of the 4-ATPmodified electrode was lower than the signal for the unmodified one because 4-ATP adsorption prevented the detection of the Pt 4f electron. From the elemental composition analysis (see Table 3) of the 4-ATP assembly, atomic ratios can be calculated. The experimental atomic ratios C/N ) 6.99:1, C/S ) 6.89:1, and S/N ) 1.01:1 are consistent, within experimental error, with the theoretical values for the 4-ATP molecule. The curve-fitted high-resolution XPS spectrum for the C1s binding energy region (Figure 10a) obtained for the 4-ATPmodified Pt electrode presents two major peaks. The peaks at 284.5 and 285.5 eV are attributable to aromatic C-C (and CaromS) and Carom-N, respectively. These values are comparable with those found in the literature.29 The third peak, at 291.0 eV, is due to a shake-up process that involves the energy of the π f π* transition.48 Although it is well-known that thiol compounds form selfassemblies through S-metal bonding, it is possible that, in the 4-ATP assembly, some molecules are adsorbed through an interaction between N atoms and the Pt surface. The curve fitting for N1s and S2p high-resolution spectra should reveal the chemical nature of the 4-ATP adsorption on platinum electrodes. For the (48) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data; Perkin-Elmer Corporation: Eden Prairie, MN, 1992.
Figure 11. Curve-fitted high-resolution XPS spectra for the (a) S2p and (b) N1s regions for 4-ATP-modified platinum electrode sample treated with H3PO4 solutions. The same XPS experimental conditions were used.
N1s high-resolution spectrum curve fitting (Figure 10b), only one peak was observed at 399.3 eV, which is characteristic of a neutral amino group.29 Moreover, only one sulfur species was observed for the curve fitting of the high-resolution spectrum of the S2p region shown in Figure 10c. The doublet at 163.4 eV can be assigned to thiolate formation on the surface of the platinum surface.49 These results are in accordance with RAIR and Raman spectroscopy, indicating that the adsorption of 4-ATP molecules to the surface occurs through sulfur bonding upon cleavage of the S-H bond. The formation of a 4-ATP assembly composed of just one monolayer is possible, since no more than a single species was found for N and S atoms. There is no evidence of bilayer formation through bonding between two molecules. Protonation of the free amino groups of 4-ATP SAM was performed and followed by XPS. The protonation was performed by use of a 0.1 M H3PO4 (pH 1) solution, since the pKa for the amino group is less than 2. However, peaks for N, S, and Pt were not observed in the XPS survey obtained for this sample (data not shown). After several washes with a 1 × 10-5 M H3PO4 (pH 5) solution, we observed the appearance of the 4-ATP monolayer characteristic peaks by XPS. High-resolution XPS data for the S2p and N1s regions of the washed sample are shown in Figure 11a,b, respectively. Curve fitting performed in the S2p region presents two doublets at 163.1 and 167.5 eV. The appearance of a doublet at 163.1 eV suggests that the 4-ATP molecules are still bonded through S-Pt bonds. However, the doublet of smaller intensity at 167.5 eV could be indicative of some sulfur oxidation or be due to the treatment applied to the 4-ATP SAM. The curvefitted N1s region presents two peaks, at 399.5 and 401.4 eV. The peak at higher binding energy can be assigned to protonated amino groups,29 and the peak at lower binding energy can be assigned to neutral amino groups. Fitting results show a NH3+/ NH2 ratio of 1.1:1.0. The appearance of neutral amino groups could be due to the washes done with a solution with a pH higher than that required for protonation of these groups. XPS results for the protonation experiment serve as additional evidence of (49) Bandyopadhyay, K.; Vijayamohanan, K.; Venkataramanan, M.; Pradeep, T. Langmuir 1999, 15, 5314.
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Figure 12. Plot of ln(Iks/Ios) vs 1/cos θ for the 4-ATP SAM on a Pt surface using AR-XPS data. Table 4. Peak Positions of a RAIR Spectrum for a 4-ATP-Modified Pt Surface Derivatized with 16-Br Hexadecanoic Acid
mode assignment
absorbance for 16-HA/4-ATP (cm-1)
νNH νCH νCH νCO (amide I) νCN + δNH (amide II) νCNH (amide III)
3326 2927 2855 1680 1551 1306
the formation of a 4-ATP monolayer on platinum adsorbed through S-Pt bonds, which contains amino terminal groups. Angle-resolved XPS (AR-XPS) measurements have been used to define the 4-ATP monolayer thickness and molecular orientation. The monolayer thickness, d, can be calculated from a plot of ln[Iks(θ) /I0s(θ)] versus 1/cos θ, where the slope of the linear regression yields (-d/λ). The slope of this attenuation curve for a 4-ATP-modified Pt electrode, shown in Figure 12, is -1.8732, resulting in a thickness of 8 Å, which is reasonable, since the calculated length of the 4-ATP molecule is 7 Å. A preliminary DFT calculation on a 4-ATP SAM on Pt shows that the distance between the hydrogens of the NH2 group and the Pt surface is around 7.8 Å.47 3.5. Derivatization of the 4-ATP Monolayer. To corroborate the reactivity of the amino terminal group of the 4-ATP SAM, 4-ATP-modified Pt surfaces were further derivatized with the compound 16-Br hexadecanoic acid. Characteristic peaks that suggest amide bonding are listed in Table 4. The infrared spectrum for this derivatization is shown in Figure 13. A strong band that appears at 3326 cm-1 could be attributed to the νNH vibration.50 The most important band to support the formation of amide bonds is the ν C-O band, also called the amide I band, which appears at 1680 cm-1.38 The δNH and νC-N frequencies fall close together and therefore interact. That interaction results in the CNH vibrations called amide II and amide III. For a 4-ATPmodified Pt surface, the amide II and amide III bands appear at (50) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: Boston, MA, 1990.
Figure 13. RAIR spectrum for a 4-ATP-modified Pt electrode derivatized with 16-Br hexadecanoic acid.
1551 and 1306 cm-1, respectively.38 These results suggest that a new layer was attached to the existing monolayer via chemical derivatization, opening the possibility of using the 4-ATP/Pt system for the attachment of a variety of biomolecules or even carbon nanotubes. XPS analysis performed for this 4-ATPderivatized sample (data not shown) shows a C/N ratio of 15:1. This is indicative that not all of the free amino groups undergo derivatization, since the theoretical C/N ratio is 22:1 for a full derivatization of the monolayer. The percentage of derivatized amino groups can be calculated from the experimental XPS atomic ratios. It was found that 56% of the 4-ATP molecules on Pt are derivatized.
4. Conclusions The surface analysis of the 4-ATP-modified Pt surface presents evidence of adsorption. RAIR and Raman studies indicate that the 4-ATP molecules are absorbed to the platinum surface through the sulfur, forming S-Pt bonds and giving an amino-terminated SAM. On the other hand, from XPS spectral studies, it is possible to corroborate the S-Pt bonding suggested by the results obtained from RAIR and Raman spectroscopy. In addition, RAIR spectroscopy suggests that the molecules are absorbed almost perpendicular with respect to the metallic surface. The formation of a dense monolayer on Pt by using a 10 mM 4-ATP solution is supported by the hexaamineruthenium CV studies. For 4-ATP solution concentrations of 1 and 5 mM, the adsorption times required for a high coverage degree were more than 72 and 48 h, respectively. The binding of single-walled carbon nanotubes (SWCNTs) on Pt surfaces through the free amino groups of the 4-ATP SAM is currently under investigation in our laboratory. Acknowledgment. We are grateful to the Materials Characterization Center of the University of Puerto Rico for the IR and XPS analyses, to Eunice Wong for her assistance with the Raman analysis at the NASA Glenn Research Center, to Jose´ A. Prieto (UPR) and Yasuyuki Ishikawa (UPR) for their help on the 4-ATP molecular length and DFT calculations. B.I.R.C. would like to acknowledge financial support from NASA Graduate Student Researcher Program fellowship (NGT3-52381). LA0522193
