Comparative Study of Electrochemically Directed Assembly versus

Roar Søndergaard , Sebastian Strobel , Eva Bundgaard , Kion Norrman , Allan G. Hansen , Edgar Albert , Gyorgy Csaba , Paolo Lugli , Marc Tornow , Fre...
0 downloads 0 Views 126KB Size
Download PDF
Langmuir 2004, 20, 1335-1341

1335

Comparative Study of Electrochemically Directed Assembly versus Conventional Self-Assembly of Thioacetyl-Terminated Oligo(phenylene ethynylene)s on Gold and Platinum Surfaces Long Cheng, Jiping Yang, Yuxing Yao, David W. Price, Jr., Shawn M. Dirk, and James M. Tour* Department of Chemistry and Center for Nanoscale Science & Technology, Rice University, MS-222, 6100 Main Street, Houston, Texas 77005 Received December 1, 2003 The assembly of thioacetyl-terminated oligo(phenylene ethynylene)s (OPEs) on Au and Pt surfaces under an electric potential (electrochemical assembly, EA) was compared to assembly at an open circuit (conventional self-assembly, CSA). Cyclic voltammetry and ellipsometry were used to characterize the adsorption kinetics of self-assembled monolayers formed by these two techniques. The adsorption rate of the EA was remarkably faster at positive potentials but slower at negative potentials than that of the CSA. The EA at 400 mV proceeded about 800 times faster than the CSA when exposed to the same solution concentrations. The adsorption rates of both EA and CSA were found to be dependent on the molecular structures of OPEs. OPEs containing electron-donating groups assemble faster than those with electronwithdrawing groups. The amount of time that the thioacetyl-terminated OPE is in the presence of the base, for removal of the acetyl group to generate the thiolate, is called the deprotection time. Deprotection times play a critical role in achieving the maximum difference in adsorption rates between the EA and the CSA. The assembly must be initiated no later than 5 min after the basic deprotection is commenced so that the thiolate concentration remains low. The difference in the adsorption rates between EA and CSA might enable selective deposition of certain OPEs onto specific electrodes.

Introduction Self-assembled monolayers (SAMs) of conjugated thiols on Au have drawn considerable attention as a result of their potential use in molecular electronics.1 The authors have previously reported the synthesis,1,2 self-assembly,3 and electrical properties4,5 of numerous oligo(phenylene ethynylene)s (OPEs). These compounds show greater conductivity than alkanethiols and have been shown to serve as molecular device components.4-6 The success of some molecular electronic architectures will depend on the precise placement of molecular device components on a patterned substrate. Thus, it becomes important to accurately direct the assembly of the components onto * Corresponding author. Tel.: (713) 348-6246. Fax: (713) 3486250. E-mail: [email protected]. (1) (a) Tour, J. M. Molecular Electronics: Commercial Insights, Chemistry, Devices, Architecture and Programming; World Scientific: Teaneck, NJ, 2003. (b) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (c) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378. (2) (a) Dirk, S. M.; Mickelson, E. T.; Henderson, J. C.; Tour, J. M. Org. Lett. 2000, 2, 3405. (b) Dirk, S. M.; Tour, J. M. Tetrahedron 2003, 59, 287. (c) Hwang, J.-J.; Tour, J. M. Tetrahedron 2002, 58, 10387. (d) Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y.; Jagessar, R. C.; Dirk, S. M.; Price, D. W.; Reed, M. A.; Zhou, C.-W.; Chen, J.; Wang, W.; Campbell, I. Chem.sEur. J. 2001, 7, 5118. (e) Dirk, S. M.; Price, D. W., Jr.; Chanteau, S.; Kosynkin, D. V.; Tour, J. M. Tetrahedron 2001, 57, 5109. (3) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. J. J. Am. Chem. Soc. 1995, 117, 9529. (4) (a) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (b) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (5) Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224. (6) (a) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (b) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303.

specific electrodes. Described here is an electrochemical assembly (EA) technique that might address this need for discrete molecular placement. Several groups have reported successful electrochemical oxidative adsorption of alkanethiols on various surfaces, such as Au,7-10,14 Ag,11,12 and Hg.13 Recently, Hsueh and co-workers reported the electrochemical oxidation of alkyl thiosulfate (R-S2O3-) on Au electrodes at 1.2 V (versus Ag/AgNO3).7 Monolayer formation took place preferentially on the biased Au electrodes, while the electrodes that were not biased experienced little or no deposition. Other groups have reported electrochemically controlled self-assemblies on gold;14 however, the molecules assembled were alkanethiols or alkylthiolates but not conjugated molecular wires. Most recently, we have shown that some OPEs formed SAMs on gold surfaces more rapidly with the aid of an electrical potential (EA) than using conventional self-assembly (CSA).15 To our knowledge, no detailed investigation on the EA and CSA of OPEs (7) Hsueh, C. C.; Lee, M. T.; Freund, M. S.; Ferguson, G. S. Angew. Chem., Int. Ed. 2000, 39, 1227. (8) (a) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (b)Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (9) Byloos, M.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1999, 103, 6554. (10) Ron, H.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13444. (11) Hatchett, D.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1997, 119, 1062. (12) Hatchett, D.; Uibel, R. H.; Stevenson, K. J.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1998, 120, 1062. (13) Stevenson, K. J.; Mitchell, M.; White, H. S. J. Phys. Chem. B 1998, 102, 1235. (14) (a) Badia, A.; Arnold, S.; Scheumann, V.; Zizlsperger, M.; Mack, J.; Jung, G.; Knoll, W. Sens. Actuators, B 1999, 54, 145. (b) Dijksma, M.; Kamp, B.; Hoogvliet, J. C.; Bennekom, W. P. Langmuir 2000, 16, 3852. (c) Qu, D.; Morin, M. J. Electroanal. Chem. 2001, 517, 45. (15) Cai, L.; Yao, Y.; Yang, J.; Price, D. W.; Tour, J. M. Chem. Mater. 2002, 14, 2905.

10.1021/la036254q CCC: $27.50 © 2004 American Chemical Society Published on Web 01/20/2004

1336

Langmuir, Vol. 20, No. 4, 2004

Cheng et al.

on Au or Pt electrodes has been previously reported, and this paper presents an interesting method of directed EA for potential device construction on the basis of careful comparison in adsorption kinetics between CSA and EA. Experimental Section Materials. Ethanol (Pharmco Products, Inc., 200 proof, USP grade) was degassed with nitrogen prior to use. Tetrahydrofuran (THF; Aldrich) was freshly distilled from Na/benzophenone under an atmosphere of nitrogen and used immediately. Tetrabutylammonium tetrafluoroborate (TBABF4) was purchased from Aldrich and used without purification. The syntheses of the OPEs are described elsewhere.1,2,5,16 Au substrates were prepared by the sequential vapor deposition of Cr (50 nm) and Au (120 nm) onto a clean single-crystal Si wafer. Metal depositions were carried out using an Auto 306 vacuum coater (Edwards High Vacuum International) at an evaporation rate of ∼1 Å/s and a pressure of ∼4 × 10-6 mmHg. Pt substrates were prepared by sputtering a ∼50-nm layer of chromium (CrC-100 sputtering systems from Plasma Sciences, Inc.) followed by a ∼120-nm layer of Pt on clean surfaces of the single-crystal Si wafer. Au substrates were cleaned immediately prior to use by placing them in an aqueous solution of H2O2/NH4OH (H2O2/NH4OH/H2O ) 1:1:5) for 15 min, followed by a thorough washing with deionized water and ethanol. Pt substrates were used without further cleaning. For electrochemical experiments, Au and Pt disk electrodes were polished sequentially with aqueous slurries of 1.0- and 0.3-µm alumina (Mark V laboratory). Preparation of Thioacetyl-Terminated OPE Solutions. A total of 1.0 mg of the OPE and 0.33 g TBABF4 was dissolved under sonication in 20 mL of degassed ethanol. A total of 20 µL of 0.27 M NaOH was then added to deprotect the thiol (removal of the acetyl moiety) and form the free thiol or thiolate in solution. Assembly on the metallic substrate was initiated 5 min after adding the NaOH to the thioacetyl OPE. That is, the deprotection time was 5 min, unless otherwise stated, before adding the metal substrate. It is critical that this timing be maintained. EA and CSA. In EA, a CV-50W voltammetric analyzer (BAS, Bioanalytical Systems, Inc.) was used to control the electrical potential applied to the electrodes. The auxiliary electrode was a twisted Pt wire of large surface area and a Ag|AgNO3 electrode was the nonaqueous reference electrode. The potential of the nonaqueous reference electrode (Ag|AgNO3) depends on the solvent, the electrolyte, and the concentration of silver nitrate. Because the potential of a nonaqueous reference electrode can vary among different electrodes, redox potentials measured using such a reference electrode should be calibrated relative to an internal reference compound (e.g., ferrocene). In this experiment, the reference electrode, Ag|AgNO3, was calibrated with a freshly prepared 4.2 mM ferrocene/0.1 M TBABF4/acetonitrile solution. The cyclic voltammetry (CV) of ferrocene exhibits an oxidation peak at 99 mV and a reduction peak at 13 mV versus the reference electrode Ag|AgNO3 used in this experiment. The oxidation and reduction peaks are 512 and 430 mV versus the reference electrode Ag|AgCl, respectively. There is about a 415-mV difference between the two reference electrodes Ag|AgNO3 and Ag|AgCl. Therefore, the interconversion of potential scales can be easily accomplished. One of the following working electrodes was used: evaporated Au or Pt plates, Au disk electrodes, or Pt disk electrodes. An electrical potential of 400 mV (vs Ag|AgNO3) was applied to the working electrode at the same time that it was immersed in the OPE solution for EA. CSA was conducted using the same OPE solution at the same time as the EA, to keep all the experimental conditions the same, except that no electrical potential was applied on the electrode. Unless otherwise stated, EA and CSA deprotection times using NaOH were 5 min, as discussed previously, while the assembly times (the total time that the metallic substrate was immersed in the OPE/NaOH solution) were 2 min. Precise adherence to this protocol is essential. After assembly, the electrodes were rinsed with ethanol and sonicated in acetone for about 10 s to remove any physisorbed compounds. (16) Jones, L., II; Schumm, J. S.; Tour, J. M. J. Org. Chem. 1997, 62, 1388.

Figure 1. Plot of the thickness versus assembly time for SAMs of 1 on Au using CSA. Ellipsometry was used to determine the thickness of SAMs on thermally evaporated Au substrates that were exposed to fresh solutions of 1 for different time periods. 1 was deprotected (treated with NaOH) for precisely 5 min prior to immersion of the Au substrate in the solution, as described in the Experimental Section. Unless otherwise stated, all deprotection times were 5 min, and precise adherence to this deprotection time must be maintained. Measurements. The thicknesses of the SAMs prepared by the EA and CSA methods were measured using ellipsometry (Rudolph Instruments model 431A31WL633 or a Gaertner LSE Stokes ellipsometer). The He-Ne laser (632.8 nm) was incident at 70° to the sample surface. Measurements were carried out before and immediately after monolayer adsorption. A refractive index (nf) of 1.55 was used for the thickness calculation of the OPE films.3,17,18 At least three spots were measured on each sample, and their average thickness is reported here. The length of an OPE molecule was calculated from a sulfur atom to the furthest proton for the minimum energy extended forms by molecular mechanics. The theoretical thickness was then obtained with the assumed linear Au-S-C bond angle and 0.24 nm for the Au-S bond length. Cyclic voltammograms were recorded by a CV-50W voltammetric analyzer (Bioanalytical Systems, Inc.) with a three-electrode configuration. A twisted Pt wire was used as the auxiliary electrode. Two types of aqueous reference electrodes were used in this experiment, either a saturated calomel electrode (SCE) or a silver|silver chloride electrode (Ag|AgCl). The working electrodes were either Au disk electrodes or Pt disk electrodes with or without an OPE film. Two kinds of Au working electrodes were used in this experiment with disk diameters being either 1.0 mm (EE046, Cypress Systems, Inc.) or 1.6 mm (MF-2014, Bioanalytical Systems, Inc.). The Pt working electrode was of 1.6 mm in diameter (MF-2013, Bioanalytical Systems, Inc.). CV was performed in an aqueous solution of KCl/K3[Fe(CN)6] (0.1 M/1.0 mM) at a scan rate of 100 mV/s.

Results and Discussion Thiols are readily oxidized to disulfides in the presence of atmospheric oxygen, and this process occurs far more readily on aromatic thiols than on alkanethiols. We have, therefore, typically used acetyl-protected thiols on the OPEs.3 Exposure of the protected thiols to acidic or basic conditions permits the rapid in situ formation of free thiols upon deprotection, although only basic deprotection was studied here.3,15 The thioacetyl-terminated OPEs studied in this paper are listed in Chart 1. Assembly of OPEs without Potential (CSA). Ellipsometric measurements provide important information on the average thickness of the collection of adsorbed molecules. Compound 1, featuring two protected thiol termini and two electron-donating ethyl groups on the central phenyl ring,3 was assembled under CSA conditions on Au, and the change in thickness over time was measured with ellipsometry (Figure 1). Rapid assembly took place within the first 20 min. For example, the film

Thioacetyl-Terminated Oligo(phenylene ethynylene)s

Langmuir, Vol. 20, No. 4, 2004 1337 Chart 1

thickness was about 0.5 and 1.0 nm after 5 and 12 min of adsorption, respectively. The rate quickly tapered off, with the thickness reaching 1.88 nm after 133 min. In contrast to 1, compounds containing electron-withdrawing groups assemble more slowly under CSA. Compound 2, for example, with a nitro group on the central phenyl ring, took 15 min to reach a film thickness of 0.4 nm. This result agrees with the kinetic results obtained by Liao and coworkers that electron-donating substituents accelerate the adsorption process while the electron withdrawing groups retard the process.19 Electrochemical measurements of heterogeneous electron transfer are a highly sensitive tool to examine the average compactness and extent of structural defects of overall SAMs formed on electrodes. The rate of heterogeneous electron transfer is greatly increased as the degree of structural integrity of SAMs decreases as a result of film incompleteness due to insufficient assembly or existence of pinholes and grain boundaries. Ferricyanide [Fe(CN)63-] is a good electroactive probe owing to its electrochemically reversible outer-sphere one-electron redox reaction. Decreases in the current of Fe(CN)63reduction to Fe(CN)64-, due to the passivation effects of the molecular layer, give a good indication of the surface coverage of the SAMs. CV at 100 mV/s was used to check the passivation effects on Au electrodes before and after being immersed in solutions of 1 under CSA conditions for different time periods. It is worth noting that freshly polished and verified Au electrodes, and new solutions of 1, deprotected for the same time period (5 min), were used for each electrochemical measurement to get accurate and comparable results. As shown in Figure 2A, the redox current decreases while the redox peak separation increases with increasing assembly time. The surface (17) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (18) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721. (19) Liao, S.; Shnidman, Y.; Ulman, A. J. Am. Chem. Soc. 2000, 122, 3688.

coverage (θ) of SAMs can be defined as θ(t) ) 1 - It/I0, where θ(t) is the surface coverage at any instant of time t, while I0 and It are the reduction currents on a bare Au electrode and the same Au electrode after being assembled in solutions of 1 for a time period of t, respectively. The θ values correspond to ∼10, 30, 47, and 91% coverages after 1, 20, 120, and 3600 min, respectively. Figure 2B shows a nonlinear dependence of the surface coverage on the assembly time and a fit of the experimental data with the first-order Langmuir isotherm model, that is, θ(t) ) 1 - exp(-kt), and a rate constant of the adsorption kinetics of k ) ∼1 × 10-4 s-1. Assembly of OPEs with an Applied Voltage (EA). The absorption kinetics of alkanethiols on Au has been intensively studied, and a two-step process has been proposed by Bain et al.20 and Ellis et al.21 They found that the main process of film formation is quite rapid (∼1-2 min). Compared to this, films formed from our thiolterminated OPEs grow more slowly. It occurred to us that variation of molecular structure, in conjunction with the use of applied electric potential, might produce selective deposition of the OPEs on the biased Au electrodes. We, therefore, synthesized molecules with different substituents and measured the assembly rate under EA conditions. To determine a suitable potential for EA, CV was conducted in a 20-mL degassed ethanol solution containing 0.33 g of TBABF4 and 20 µL of 0.27 M NaOH on a bare Au electrode. Note that this solution has all the components except the OPEs. As shown in Figure 3, an oxidation starts at potentials larger than 400 mV and peaks at 764 mV with a small cathodic peak at about 450 mV in the reversing sweep. The anodic peak current is mostly attributed to oxidation of the solvent, ethanol, with a partial contribution from Au oxidation that results in the small cathodic peak in the reverse scan.10 Therefore, 400 mV or less was selected for the EA in the following (20) Bain, C. D.; Troughton, E. B.; Tao, Y. Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (21) Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H. Langmuir 1997, 13, 5335.

1338

Langmuir, Vol. 20, No. 4, 2004

Cheng et al. Table 1. Thicknesses of SAMs Prepared by the EA Process on Au and Pt Surfaces time thicknessa potential entry compound surface (mV vs Ag/AgNO3) (min) (nm) 1 2 3 4 5 6 7 8 9 10 11 12b 13b 14 15 16 17 18 19 20

1 1 1 1 1 1 1 2 2 2 2 3 3 4 4 4 5 5 6 6

Au Au Au Au Au Pt Pt Au Au Pt Pt Au Au Au Au Au Au Au Au Au

+400 +400 +400 -800 -1000 +400 +400 +400 +400 +400 +400 +400 +400 +400 +400 +400 +400 +400 +400 +400

2 6 10 10 10 2 10 2 10 2 10 2 20 2 10 20 2 10 2 10

2.2 3.2 4.3 1.0 0.5 2.0 3.6 2.0 2.2 0.7 2.1 0.4 1.5 0.3 0.8 1.8 3.3 3.9 2.2 6.1

a Determined by ellipsometry with a precision value of (0.2 nm. The base used here was concentrated ammonium hydroxide (20 µL) instead of 0.27 M NaOH.

b

Figure 2. (A) Cyclic voltammograms in a solution of KCl/ K3[Fe(CN)6] (0.1 M/1 mM) on Au electrodes before (solid line) and after being immersed in 1/NaOH for 1 min (long dashed line), 20 min (dotted line), 60 min (dashed-dotted line), 120 min (dashed-dotted-dotted line), and 2470 min (short dashed line) under CSA conditions. Au electrodes of 1.0-mm diameter were used here. (B) The growth rate of SAMs of 1 on a Au electrode using the CSA conditions. The surface coverage, θ, is defined in the text and determined by data from the CV in part A.

Figure 3. Cyclic voltammogram in the 20-mL degassed ethanol solution containing 0.33 g of TBABF4 and 20 µL of 0.27 M NaOH on a bare Au electrode of 1.0-mm diameter.

experiments to avoid complication from oxidation of ethanol and Au. At such mild potentials below 400 mV, no redox reaction of the OPE molecules should occur either. Table 1 summarizes the thickness results of SAMs of these OPEs assembled on Au or Pt substrates for different time periods using EA at different potentials. Several points should be emphasized here. First, these compounds assemble much more quickly under positive potentials. For example, compound 1 already formed a 2.2-nm SAM with an assembly time of 2 min using EA at 400 mV (entry 1), which is in significant contrast to the SAMs formed under CSA conditions (Figure 1). Second, comparison of

the thickness results of SAMs on both Au and Pt substrates using EA at 400 mV for 2 min revealed that compounds 1, 5, and 6 (entries 1, 6, 17, and 19) formed thicker films than the compounds 2, 3, and 4 (entries 8, 10, 12, and 14). It is suggested that molecules containing electron-donating groups assembled much faster than those containing strong electron-withdrawing groups under the EA conditions, similar to the trend found in SAMs prepared under the CSA conditions (vide supra). Third, the thickness of the assembled layers is roughly equivalent to their calculated molecular length (∼2.1 nm)3 for monothioOPEs. One exception is compound 5, for which the layer is thicker than the length of the molecule (entries 17 and 18). Others have reported a similar phenomenon; CSA of alkylthiols on Au from ethanol gave layers 20% thicker than the length of the molecule.21 In contrast, dithio-OPEs tended to form thicker layers than their molecular lengths upon extended assembly time (entries 2, 3, 7, and 20), presumably as a result of multilayer formation via disulfide as promoted by trace oxygen or the applied electric potential.3 One might argue that the excess molecules were physisorbed on top of the monolayer. However, sonication of the SAMs in THF did not significantly change their thicknesses, indicating that the excess molecules were chemically bonded to the underlayers. Therefore, a short assembly time in an inert atmosphere excluding oxygen is required to get a monolayer of these molecular systems. Finally, molecular adsorption on Au seems to proceed somewhat faster than on Pt under the EA conditions, as suggested by comparison of entries 1 and 8 with 6 and 10 for compounds 1 and 2, respectively. This point was also verified by the CV results (vide infra). In addition to these ellipsometric results, electrochemical results more clearly revealed the significant difference in the adsorption rates using the two different assembly techniques EA and CSA. Figure 4A shows a direct comparison between CSA and EA of compound 1 on Au electrodes for the same assembly time. The heterogeneous electron transfer of Fe(CN)63- was almost completely blocked on the Au electrode after EA at 400 mV for only 2 min. Conversely, the Au electrode was only a little bit passivated after CSA for 2 min. Clearly, EA was significantly faster than CSA. Similar electrochemical results were observed on Pt electrodes. Figure 4B shows cyclic voltammograms of Pt electrodes before (solid line) and

Thioacetyl-Terminated Oligo(phenylene ethynylene)s

Langmuir, Vol. 20, No. 4, 2004 1339

Figure 4. (A) Cyclic voltammograms in a solution of KCl/ K3[Fe(CN)6] (0.1 M/1 mM) on Au electrodes before (solid line) and after being immersed in 1/NaOH for 2 min using the CSA (dotted line) and EA at 400 mV (dashed line). Au electrodes of 1.6-mm diameter were used here. (B) Cyclic voltammograms in a solution of KCl/K3[Fe(CN)6] (0.1 M/1 mM) on Pt electrodes before (solid line) and after being immersed in 1/NaOH for 10 min using CSA (dotted line) and EA at 400 mV (dashed line).

after being immersed in 1/NaOH for 10 min using the CSA (dotted line) and EA at 400 mV (dashed line). For the EA, the surface coverage was very high, as indicated by the enormous passivation effect on the redox reaction of Fe(CN)63-. In contrast, CSA of 1 on Pt for the same assembly time was minimal. A longer assembly time (10 min) was required to achieve the passivation effect on Pt than that (2 min) on Au. Furthermore, the relatively larger capacitance current also suggests somewhat less densely packed molecular monolayers on Pt after EA than on Au. Accordingly, the electrochemical results indicate that layers of OPEs grow relatively slower on Pt than on Au, which is consistent with the ellipsometric results. The adsorption kinetics of EA was further investigated on Au electrodes with compound 1 as a representative, which can be directly compared with the kinetic results of the CSA (vide supra). Figure 5A depicts cyclic voltammograms of Au electrodes before and after EA at 400 mV for different assembly times. The surface coverage, θ, corresponds to ∼26, 55, 74, and 98% after 3, 10, 30, and 60 s, respectively (Figure 5B). Recall that only ∼10% surface coverage was obtained after CSA for 60 s (Figure 2). As shown in Figure 5B, θ values of two deprotection time periods (5 and 27 min), calculated with the same equation [θ(t) ) 1 - It/I0] as that used for CSA, were drawn as a function of the assembly time and fitted according to the first-order Langmuir isotherm model. Two rate constants of the adsorption kinetics, k ) ∼0.08 and 0.17 s-1, were obtained for the deprotection time periods of 5 and 27 min, respectively. The fact that the rate constant is larger for the 27-min deprotection than for the 5-min deprotection is not surprising because longer deprotection is expected to give a higher concentration of free thiolates and, therefore, faster adsorption kinetics than the shorter deprotection. With the same deprotection time of 5 min, the rate of adsorption kinetics is ∼800 times higher for the EA than the CSA.

Figure 5. (A) Cyclic voltammograms in a solution of KCl/ K3[Fe(CN)6] (0.1 M/1 mM) on Au electrodes before (solid line) and after being immersed in 1/NaOH for 3 s (dashed line), 10 s (dotted line), 30 s (dashed-dotted line), and 60 s (dasheddotted-dotted line) while applying 400 mV (EA conditions). Au electrodes of 1.0-mm diameter were used here. (B) The growth rates of SAMs of 1 on Au electrodes using EA determined by CV with deprotection times (the time that the acetyl-protected OPE was in the presence of NaOH prior the Au electrode being immersed in the solution) being 5 min (squares, determined by data from the CV in part A) and 27 min (circles, CV not shown here), respectively.

Effects of the Deprotection Time on the Adsorption Kinetics of OPEs. We have found that the time allowed for the basic deprotection of thiol is crucial in differentiating the EA and CSA processes (Figure 6). Thioacetyl-terminated OPEs only slowly bond on Au or Pt surfaces if no deprotection is conducted.3 The concentration of free thiolate groups (basic deprotection) is expected to increase with time until a thermodynamic equilibrium is reached. Investigated here were the EA and CSA on Au electrodes in compound 1 solutions after different deprotection time periods. In all cases, the assembly time was 2 min and the surface coverage of the thus-formed SAMs was checked with CV in Fe(CN)63-. For EA, 300 mV was used instead of 400 mV to slow the adsorption rate, therefore allowing us to clearly determine surface coverage changes. Typical results are shown in Figure 6. The surface coverage values are 80, 99, and 100% for SAMs formed by EA but 17, 39, and 45% for SAMs formed by CSA at deprotection time periods of 5, 83, and 353 min, respectively. The difference in comparative adsorption rates between the EA and the CSA is largest in the early stages of the deprotection process where the concentration of the deprotected molecules is low. We surmise that the negatively charged thiolates are driven to the working electrode surface by the positive electric field. On the other hand, under the CSA conditions, there is no extra driving force except the passive self-assembly;

1340

Langmuir, Vol. 20, No. 4, 2004

Cheng et al.

Figure 7. Cyclic voltammograms in a solution of KCl/K3[Fe(CN)6] (0.1 M/1 mM) on a bare Au electrode (solid line), a Au electrode that was immersed in a solution of 1/NaOH while applying -500 mV (curve 2, dashed line) for 2 min, and a Au electrode that was immersed in 1/NaOH while applying -1000 mV (curve 3, dotted line) for 2 min. The deprotection time was 170 min. Au electrodes of 1.0-mm diameter were used here.

Figure 6. Cyclic voltammograms in a solution of KCl/ K3[Fe(CN)6] (0.1 M/1 mM). In panel A, EA (300 mV) was used to assemble 1 on Au electrodes; in panel B, CSA was used to assemble 1 on Au electrodes. In both panels A and B, the curves 1 (solid line) were recorded on bare Au electrodes, while the curves 2-4 were recorded on Au electrodes exposed to 1/NaOH for 2 min. However, each curve 2-4 represents the use of 1/NaOH solutions that were deprotected for different time periods: 5 min (curves 2, dashed line), 83 min (curves 3, dotted line), and 353 min (curves 4, dashed-dotted line). Au electrodes of 1.0-mm diameter were used here.

the local concentration of negatively charged thiolate OPEs is too low for significant coverage to take place on the metal. Therefore, to take advantage of the faster adsorption rate under EA conditions, the assembly process has to be initiated quickly (but not immediately), for example, 5 min or less, after adding the base into the OPE solutions. Control Experiments. To test if the TBABF4 electrolyte or ethanol solvent makes any contributions to the passivation effect seen under EA conditions, to a freshly polished bare Au electrode was applied a potential of 400 mV for 2 min in an OPE-free electrolyte solution of TBABF4 (0.33 g, 1 mmol) and NaOH (20 µL, 0.27 M) in degassed ethanol (20 mL). No significant change in the CV was found, indicating that the electrolyte and solvent make negligible contributions to the passivation effect. Formation of OPE molecular layers on the working electrodes are, therefore, the likely reasons for the passivation effects. We have demonstrated that monolayers of OPEs are formed faster on Au or Pt substrates at positive electric potentials than at open circuit. Conversely, negative

electric potentials can impede the formation of the monolayers. Shown in entries 4 and 5 of Table 1 are some ellipsometric results of monolayers of compound 1 formed on Au at negative potentials. The layers were only 1.0and 0.5-nm thick at -800 and -1000 mV, respectively, much thinner than 4.3 nm at 400 mV (entry 3 of Table 1) for the same assembly time. In addition, CV results are consistent with the ellipsometric results. Figure 7 shows cyclic voltammograms in Fe(CN)63- on a bare Au electrode, a Au electrode that was immersed in a solution of 1/NaOH while applying -500 mV, and a Au electrode that was immersed in the same solution while applying -1000 mV. According to the levels of passivation, the θ values correspond to 36% and 18% for SAMs formed at -500 and -1000 mV, respectively. Compared with the surface coverage of nearly 100% for SAMs formed at 400 and 300 mV (Figures 4A and 6A), the formation of OPE SAMs on Au electrodes is significantly slowed when negative potentials are applied. Note that the deprotection time was 170 min here, much longer than the 5 min that we normally used. We did this to be able to see the differences that negative electric potentials (-500 and -1000 mV) had on the formation of molecular layers. Considering the fact that the surface coverage values are 39 and 45% for molecular layers formed by CSA at deprotection times of 83 and 353 min (Figure 6B), the surface coverage values (18 and 36%) obtained here suggest that even slower adsorption occurs for EA at negative electrical potentials than under the CSA conditions. These results strengthen our explanation that deprotected OPEs with negatively charged thiolate groups are rapidly driven to the Au or Pt working electrodes under a positive electrical field and accumulated to give a higher local concentration of thiolates. Logically, a higher local concentration of thiolates should reduce the assembly time because the reaction is not mass-transport-limited by diffusion of thiolates into the reaction zone and, therefore, achieves faster formation of SAMs with EA at positive potentials. Conclusions We have demonstrated that the adsorption rate of thioacetyl-terminated OPEs on Au or Pt substrates is greatly enhanced at positive electric potentials but retarded at negative electric potentials. Comparative kinetic investigation proves that the adsorption rate of EA at positive potentials is remarkably faster than the CSA, by a factor of ∼800 in the case of EA at 400 mV and 5-min deprotection times. The EA process may be driven

Thioacetyl-Terminated Oligo(phenylene ethynylene)s

by an imposed electric field and not directly by electrochemistry. The electrostatic potentials could increase the local thiolate concentration near the working electrodes, which results in a faster equilibrium being established with the assembled film. The adsorption rate of both EA and CSA is dependent on the molecular structures of OPEs. OPEs containing electron-donating groups assemble faster than those with electron-withdrawing groups. Conversely, the EA at negative potentials significantly impedes the formation of SAMs such that the EA process becomes even slower than the CSA process. Deprotection time plays a critical role in achieving the maximum difference in adsorption rates between the EA and the CSA. To take advantage of the difference in

Langmuir, Vol. 20, No. 4, 2004 1341

adsorption, the assembly must be initiated e5 min after the basic deprotection of thiol moieties has begun. On the basis of the differences in assembly rates between the EA and CSA processes, it may be possible to selectively assemble OPEs on specific electrodes by either accelerating or decelerating the formation of molecular layers on different electrodes using EA at positive or negative potentials, respectively. Acknowledgment. The Defense Advanced Research Project Agency and the Office of Naval Research supported this work. We thank D. Allara and P. Harder (Pennsylvania State University) for helpful suggestions. LA036254Q