Self-assembly of and charge transfer at N-docosyl-N′-methyl viologen on electrode surfaces

Journal of Electroanalytical Chemistry 468 (1999) 70 – 75

Self-assembly of and charge transfer at N-docosyl-N%-methyl viologen on electrode surfaces Yeon-Su Park, Jai-Man Jang, Chi-Woo Lee * Department of Chemistry, Korea Uni6ersity, Jochiwon, Choongnam 339 -700, South Korea Received 29 September 1998; received in revised form 18 January 1999; accepted 9 March 1999

Abstract Self-assembled molecular films were formed on the electrode surfaces of glassy carbon (GC), platinum (Pt), gold, silver and indium–tin oxide in aqueous electrolyte solutions of N-docosyl-N%-methyl viologen (C22VC1). The temperature dependence of voltammetric responses showed a higher stability with higher surface coverage and with a Pt than a GC surface. Charge transfer reactions of the solution redox species of Ru(NH3)36 + and Fe(CN)46 − at the irreversibly self-assembled C22VC1 GC interface were found to take place by an interplay of direct penetration of solution species through the self-assembled molecular layer of C22VC1 and of cross reaction between solution and surface bound redox agents. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Self-assembly; Viologen; Charge transfer

1. Introduction Molecular self-assemblies of surfactant viologens are of recent interest because they can form functional electrodes as well as micellar assemblies, which can be profitably utilized for display devices, photoelectrochemical studies and electrocatalysis [1–8]. Despite the wide usefulness of this class of redox components, the electrochemical processes involved are not fully understood. Lee and Bard [3,4] were the first to observe the well-resolved two peaks of the first single-electron transfer process (2+/ + ) with Langmuir–Blodgett (LB) films of N-hexadecyl-N%methyl viologen (C16VC1) and N-docosyl-N%-methyl



Poster presented at the International Symposium on New Trends in Electroanalytical Chemistry, Seoul, South Korea, 10–12 September, 1998. * Corresponding author. Fax: +82-415-867-6823. E-mail address: [email protected] (C.-W. Lee)

viologen (C22VC1) at indium–tin oxide surfaces, which were attributed to the regular arrangement of the redox sites and the stronger interaction between 2+ and + than between 2 + /2+ or + /+ pairs based on the statistical mechanical description of a surface-bound monomolecular redox film by Tokuda et al. [9]. Buttry et al. performed extensive electrochemical and vibrational spectroscopic studies on the double-peaked voltammetric waves by employing the chemisorbed self-assembled monolayers of CH3(CH2)m V(CH2)n SH at gold electrode surfaces and proposed that the sharply peaked response was due to the formation of p complex dimers of the cation radicals within the monolayer [6]. In this report, we wish to describe the experimental conditions of multiple voltammetric peaks of the first redox process of viologens observed at C22VC1 molecular films irreversibly self-assembled on electrode surfaces [10]. In addition, we describe charge transfer dynamics of the solution redox species of Ru(NH3)36 + and Fe(CN)46 − at the glassy carbon electrode with physically self-assembled molecular films of N-docosyl-N%-methyl viologen.

0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 1 2 7 - 8

Y.-S. Park et al. / Journal of Electroanalytical Chemistry 468 (1999) 70–75

2. Experimental The C22VC1Br2 was prepared as before [4]. Other reagents were of the best commercial quality available. Solutions were prepared from laboratory-deionized water that was passed through a purification train (Millipore Continental Water System). Solutions for electrochemical experiments were deoxygenated with prepurified argon. All experiments were conducted at ambient laboratory conditions. Samsung indium–tin oxide electrodes were cleaned with soap, potassium hydroxide+alcohol solutions and distilled water, and dried in an oven at 120°C before each trial. The other electrodes were polished successively with 3 and 0.3 mm alumina water slurry, washed with copious amounts of water, and sonicated before each trial. Additional experiments were performed in an identical solution by employing platinum and gold electrodes which were electrochemically cleaned in sulfuric acid solutions [11,12] to determine whether or not the multiple-peaked voltammetric curve originates from the presence of a contaminant film produced during the cleaning procedure described in the above for the electrode materials used. Transfer of the electrodes with viologen assembly was made in less than several minutes. The temperature dependence of the self-assembled molecular films of C22VC1 at GC or Pt was investigated in a water-jacked electrochemical cell where the solution contained 0.1 M NaCl and 10 mM C22VC1. A control experiment was done to confirm the equilibrium temperature of the electrochemical cell by inserting the temperature probe at the interface between the electrode and electrolyte solution and comparing the probe temperature with the bath temperature of the circulating water. In our temperature variation experiments, 30 min of water circulation was sufficient to reach the desired temperature but usually 1 h of circulation was allowed before a voltammetric measurement was made at a given temperature. Conventional electrochemical instrumentation and cells were employed for cyclic voltammetric measurements. Quoted potentials are given with respect to a saturated sodium chloride calomel electrode (SSCE).

3. Results and discussion Fig. 1(A) shows the cyclic voltammogram obtained at a glassy carbon electrode in the aqueous solution of 10 mM C22VC1 and 0.1 M NaCl. The peak current (Ip) versus scan rate (n) yielded a linear line with zero intercept, as expected for a surface-bound species [11]. The voltammetric signal observed with the self-assembled C22VC1 molecular film at electrode surfaces changed little for at least one hour after transfer to the sample solution containing 0.1 M NaCl without the viologen. The total width at half-height of the voltam-

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metric peak is larger than 90 mV for the ideal Nernstian reaction under Langmuir isotherm conditions, suggesting that charge transfer reactions of the viologen self-assembly are not simple in the present case. The facts that the anodic curve is a ‘spiked’ shape structure and that a larger background current is observed for V + than for V2 + may indicate that the structure of the self-assembled layer of the viologen depends strongly on the oxidation state of the viologen site and that it is less compact at the V + state than at the V2 + state. The extent of physically self-assembled molecular films of C22VC1 was estimated by integration of the voltammogram recorded at 500 mV s − 1 over the concentration range 3 to 20 mM (Fig. 2). The error bars in Fig. 2 indicate the range of the results from five independent measurements at each concentration. The relatively constant molecular self-assembly over the concentration range studied suggests that full coverage is apparently reached at 3 mM. The self-assembled molecular film 7.0×10 − 10 mol cm − 2 does not correspond to a monolayer of C22VC1 parallel to the electrode surface but is reasonably close to that with molecules perpendicular to it when the microscopic area of the GC electrode is taken as ca. 1.6 times larger than its geometric area with the molecular area of 39 A, 2 [4]. The exact surface roughness of the electrode was not determined in the present study and will be investigated in future work. When the C22VC1 GC electrode was transferred to the solution containing 0.1 M NaClO4 with or without C22VC1, cyclic voltammograms of multiple peaks were observed as are shown in Fig. 1(B). This may not be related to an ageing effect observed with thick films at electrodes [13], because (i) a multiply peaked voltammogram was observed at the first cycle immediately after transfer to the sample solution of perchlorates (B 1 min); (ii) the singly peaked voltammogram was quickly recovered when the C22VC1 GC electrode was transferred to the sample solution of 0.1 M NaCl with or without C22VC1, and (iii) similar multiple voltammetric peaks were observed with the self-assembled molecular films of C22VC1 at the surfaces of indium–tin oxide (ITO), gold (Au), platinum (Pt) and silver (Ag) in the presence of perchlorate electrolytes, as was previously reported [10]. Figs. 1(C) and (D) show a series of cyclic voltammograms for the surface C22V2 + / + C1 redox process obtained in 0.1 M NaClO4 solution by using the Pt and Au electrodes which were electrochemically cleaned in sulfuric acid solutions [11,12]. Thus the multiple voltammetric peaks observed in this study are dependent on neither the substrates employed nor the way a given electrode was pretreated. The voltammograms with ITO, Au, Pt and Ag became singly peaked when the perchlorate was replaced with chloride electrolytes. In addition, the multiple peaks of the redox process were observed only when the adsorption reached a full surface coverage for each electrode inves-

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tigated. Experiments in the presence of sodium paratoluene sulfonate produced less well-developed multiple peaks. The strongly multiple voltammetric peaks observed with the first redox process in the present studies appear to be caused by the perchlorate anion used. We note that perchlorate electrolytes were employed in the electrochemical studies of the LB films of C22VC1 and C16VC1 at ITO surfaces [3,4]. Perchlorate ions are hydrophobic and may cause positively charged C22VC1 hydrophobic films to become highly compact at the full surface coverage, as was the case with LB films, to produce thermodynamically distinguishable voltammetric peaks. Strong ionic interactions between anions and viologen moieties embedded in chemisorbed self-assembled monolayers of CH3(CH2)m V(CH2)n SH have been reported to be responsible for the wave shape and the apparent redox potential in the voltammogram [6,14].

The present simple procedures to obtain one-electron transfer multiple voltammetric peaks may be significant in that they can be easily applicable for in situ electrochemical spectroscopic and atomic force microscopic (AFM) studies to investigate the origin of the unusual voltammograms observed at self-assembled viologens on electrode surfaces including which end of the C22VC1 is interacting with the electrode, how ClO4− partitions into the film, etc. Currently electrochemical AFM is being employed to this end in this lab. Fig. 3 shows the relati6e temperature dependence of C22VC1 GC or Pt structures after the formation of self-assembled molecular films of C22VC1 on solid electrode surfaces at 25°C. When the molecular film at the coverage of 5.9× 10 − 10 mol cm − 1 on GC underwent temperature variation, rapid decay in surface coverage was observed as the temperature of electrochemical cell was raised and the coverage decreased to 1.2 ×10 − 10

Fig. 1. Cyclic voltammetric responses at a C22VC1 GC electrode immersed in an aqueous solution of 0.1 M NaCl with or without 10 mM C22VC1 (A) and after transfer to the solution containing 0.1 M NaClO4 (B), and at C22VC1 Pt (C) and at C22VC1 Au (D) electrodes under the conditions of (B); scan rate (mV s − 1): 500, 400, 300, 200, 100, 50 for A and B, and 1000, 900, 800, 700, 600, 500, 400, 300, 200, 100 for C and D. Pt and Au electrodes were electrochemically cleaned.

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Fig. 2. Extent of self-assembled C22VC1 GC as a function of bulk concentration in 0.1 M NaCl solution.

mol cm − 2 at 70°C, 20% of that at 25°C (Fig. 3(A)). On the other hand in the case of the surface coverage of 8.3 × 10 − 10 mol cm − 2, 67%, 5.6 ×10 − 10 mol cm − 2, of the molecular film formed at 25°C survived at 70°C (Fig. 3(B)). Thus the self-assembled molecular films at the higher coverage are more resistant to temperature perturbation. When the same experiments were performed with C22VC1 Pt structures at the surface coverage of 6.7× 10 − 10 mol cm − 2, 70%, 4.7× 10 − 10 mol cm − 2, of the surface films remained at 70°C (Fig. 3(C)).Apparently the self-assembled structure is more stable on Pt than on GC. The results of temperature

Fig. 3. Relative temperature dependence of C22VC1 GC (A, B) and C22VC1 Pt (C); surface coverage at 25°C: 5.9×10 − 10 mol cm − 2 (A), 8.3 ×10 − 10 mol cm − 2 (B), 6.7× 10 − 10 mol cm − 2 (C).

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Fig. 4. Cyclic voltammetric responses at C22VC1 GC in an aqueous solution of 0.1 mM Ru(NH3)36 + , 10 mM C22VC1 and 0.1 M NaClO4; scan rate (mV s − 1): 500, 400, 300, 200, 100 and 50.

dependence of C22VC1 molecular assembly on the solid electrode surfaces suggest that the interfacial self-assembly exhibits a higher stability with higher surface coverage and with a Pt than a GC surface. Charge transfer dynamics at the C22VC1 GC interface were investigated by employing the simple redox species of multiply charged ions Ru(NH3)36 + and Fe(CN)46 − , which were chosen because the former bears a relatively large positive charge while the latter is negative. Because of the large charges of the redox agents, strong electrostatic interactions between the redox agents selected and the viologen moieties of C22VC1 self-assembly were expected to play an important role in the electron transfer reaction at the C22VC1 GC interface. Fig. 4 shows a set of voltammetric scans obtained at different scan rates with C22VC1 GC in an aqueous solution of 0.1 mM Ru(NH3)36 + , 10 mM C22VC1 and 0.1 M NaClO4. The potential regions for the redox processes of Ru(NH3)36 + /2+ and V2 + / + are sufficiently separated to be differentiated. It is interesting to note that the voltammetric features of V2 + / + are little changed from those in the absence of Ru(NH3)36 + (Fig. 1(B)) and that the electrochemical behavior of Ru(NH3)36 + /2 + at C22VC1 GC is almost the same as that at a bare electrode, even though strong repulsive interaction between the positively charged ruthenium complex and the viologen dication with a hydrophobic alkyl chain would be expected to prohibit the direct electroreduction of the ruthenium complex at the C22VC1 GC interface. The fact that the Ru(NH3)36 + waves do not differ significantly from those at bare electrodes indicates that the outer-sphere redox couple is able to exchange electrons

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with the electrode through the self-assembled layer of C22VC1 present on GC at the potentials where Ru(NH3)36 + is reduced. Apparently the molecular selfassembly has little repulsive interaction with Ru(NH3)36 + /2 + and a sufficient number of microholes may exist for the solution redox agents to penetrate it. A direct voltammetric observation of solution phase species at the electrode with irreversibly adsorbed films was previously reported by Anson et al. with a 9,10phenanthrene quinone pyrolytic graphite interface for Ru(NH3)36 + /2 + [15]. The identical experiments in 0.1 M NaCl instead of NaClO4 showed a quite different voltammetric signal, one cathodic and a corresponding anodic peak with the former larger than the latter. This may be correlated to the facts that the first redox process of the viologen in the self-assembled state has a more positive formal potential in NaCl than that in NaClO4 and that the viologen is more soluble in NaCl solution. Thus the C22VC1 GC structure is dynamic in NaCl electrolyte solution but is not in NaClO4. The important message here may be that the electrochemical behavior of the interfacial self-assembled C22VC1 GC depends strongly on the electrolytes employed and that multiple voltammetric peaks are observed in the presence of perchlorate electrolytes. In addition, when a C22VC1 GC self-assembled electrode was rotated in 0.1 mM Ru(NH3)36 + , 10 mM C22VC1 and 0.1 M NaCl while the potential was scanned towards the negative direction, the cathodic current increased sharply to the limiting plateau near the formal potential of C22V2 + / + C1 with a shoulder near the formal potential of Ru(NH3)36 + /2 + , which suggests that the electroreduction of Ru(NH3)36 + at the C22VC1 GC interface takes place via direct penetration of Ru(NH3)36 + in the solution as well as cross reaction between C22V + C1 and solution phase Ru(NH3)36 + . Namely charge transfer reactions at the irreversibly self-assembled C22VC1 GC interface take place by an interplay of direct penetration of solution species through the self-assembled layer of C22VC1 and cross reaction between solution and surface bound redox agents. Cyclic voltammograms observed at a C22VC1 GC self-assembly in an aqueous solution of 0.1 mM Fe(CN)46 − , 10 mM C22VC1 and 0.1 M NaCl are shown in Fig. 5. The peak currents of Fe(CN)46 − /3 − were much larger than those expected from diffusion of the redox species dissolved in the solution phase. Plots of the cathodic peak currents versus square root of scan rates showed a positive deviation from that expected for diffusing redox species (Fig. 6). In Fig. 6, dotted lines were drawn from the origin through the experimental data points at 100 mV s − 1 at different bulk concentrations. At lower bulk concentration and at higher scan rate the positive deviation became more severe, indicating that a significant portion of the current was contributed from the surface species. The

Fig. 5. Cyclic voltammetric responses at C22VC1 GC in an aqueous solution of 0.1 mM Fe(CN)46 − , 10 mM C22VC1 and 0.1 M NaCl; scan rate (mV s − 1): 500, 400, 300, 200 and 100.

results were expected because the highly negative anion Fe(CN)46 − would be electrostatically bound to the dications at the C22VC1 GC interface, forming the Fe(CN)6 C22VC1 GC interface. Table 1 summarizes the voltammetric data obtained at a scan rate of 100 mV s − 1 with the C22VC1 GC electrode of the surface coverage of 8.6×10 − 10 mol cm − 2 in an aqueous solution of 10 mM C22VC1 and 0.1 M NaCl when the bulk concentration of Fe(CN)46 − was varied. Four meaningful features can be noted from the table. Firstly the peak current or surface coverage of the self-assembled viologen remains unchanged over the concentration change of Fe(CN)46 − from 0.1 to 0.91 mM, implying that the viologen molecular film is sufficiently stable under the present experimental variations. Secondly the peak separation of Fe(CN)46 − /3 − at the bulk concentra-

Fig. 6. Plots of cathodic peak currents versus (scan rate)1/2. The dotted lines were drawn from the origin through the data points at the slowest scan rate.

Y.-S. Park et al. / Journal of Electroanalytical Chemistry 468 (1999) 70–75

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Table 1 Voltammetric data with change in Fe(CN)4− bulk concentration at the C22VC1 GC electrode in aqueous solutions of 10 mM [C22VC1]+0.1 M 6 a [NaCl]+X mM [Fe(CN)4− 6 ] [Fe(CN)4− 6 ]/mM V2+/+

Epc/V Epa/V Ipc/mA

Fe(CN)4−/3− 6

Epc/V Epa/V Ipc/mA Ipa/mA

a b

0

0.10

0.20

0.38

0.57

0.74

0.91

−0.45 −0.41 12

−0.53 −0.48 12

−0.53 −0.49 12

−0.53 −0.50 12

−0.53 −0.50 12

−0.54 −0.50 12

−0.53 −0.51 12

0.12 0.16 8 8

0.12 0.17 15 14

0.13 0.19 27 26

0.13 0.19 35 33

0.12 0.19 38 36

0.12 0.19 48 46

0.16b 0.22b

Scan rate =100 mV s−1. Surface coverage = 8.6×10−10 mol cm−2. Peak potentials at bare GC for Fe(CN)4−/3− in 0.1 M NaCl. 6

tion of 0.1 mM is only 40 mV although it increases to 70 mV at the bulk concentration of 0.91 mM, which may reflect the possible resistive nature of electrostatically accumulated Fe(CN)6 at the C22VC1 GC interface. Thirdly the peak currents are not proportional to the bulk concentration but become relatively larger at lower concentrations. Fourthly the formal potential of the viologen (2+ /+) shifts to the negative direction by ca. 80 mV while that of Fe(CN)46 − /3 − does so by 30–40 mV, when the voltammetrically measured formal potentials in the presence of both species are compared to those in the absence of either. All these four may be taken as experimental evidence of interfacial charge transfer complex formation [16,17] between Fe(CN)6 and viologen at the surface of glassy carbon, resulting in Fe(CN)6 C22VC1 GC structures. At higher bulk concentration and at lower scan rate the contribution from the surface bound species to the current was less important, which suggests that the electrochemical reaction of solution phase species at the Fe(CN)6 C22VC1 GC interface takes place via direct penetration of solution redox species through Fe(CN)6 C22VC1 layers on the GC electrode surface or by self-exchange reaction Fe(CN)36 − /4 − between the surface bound and solution phase species or both.

4. Conclusions

ture Fe(CN)6 C22VC1 GC was found to form at the electrode solution interface. Both pathways of direct penetration and cross reaction were operative during charge transfer reactions at the irreversibly self-assembled C22VC1 GC interface.

Acknowledgements This work was supported financially by the Korean Ministry of Education (BSRI-97-3404). Helpful discussions with Professors F.C. Anson and A.J. Bard are gratefully acknowledged.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Perhaps the most important experimental findings in this work are that N-docosyl-N%-methyl viologen formed self-assembled molecular films at the electrode surfaces of GC, Pt, Au, Ag and ITO, and that multiple voltammetric peaks were observed for the first redox process of viologen (2+/ + ) in perchlorate electrolyte solutions. The self-assembled structure exhibited a higher stability with higher surface coverage and with a Pt than a GC surface. Further, a new interfacial struc-

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