Influence of an external magnetic field on the formation of self-assembled monolayers of dodecanethiol on polycrystalline gold electrode

Thin Solid Films 517 (2009) 3661–3666

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f

Influence of an external magnetic field on the formation of self-assembled monolayers of dodecanethiol on polycrystalline gold electrode Jiongjia Cheng a, Jianyuan Dai a, Jing Jin a, Li Qiu b, Guiyin Fang b, Zhiguo Li a, Yongbo Sun a, Shuping Bi a,⁎ a School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry of China and Key Laboratory of MOE for Life Science, Nanjing University, Nanjing 210093, China b Department of Physics, Nanjing University, Nanjing 210093, China

a r t i c l e

i n f o

Article history: Received 30 January 2008 Received in revised form 6 January 2009 Accepted 15 January 2009 Available online 23 January 2009 Keywords: Dodecanethiol Polycrystalline gold electrode Self-assembled monolayers Magnetic field

a b s t r a c t This paper reports the preparation of dodecanethiol self-assembled monolayers (C12SH-SAMs) on polycrystalline gold electrodes in a magnetic field. The qualities of C12SH-SAMs were characterized by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that highly dense and well-ordered SAMs could form in a relatively short time (3 h), indicating that the rate of SAMs formation increased under an external magnetic field compared with the natural self-assembly process. Moreover, the results of CV and EIS measurements also suggested that the presence of a magnetic field had influenced the qualities of the SAMs; the stronger magnetic intensity can help to obtain much denser and well-ordered SAMs. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The technology of self-assembled monolayers (SAMs) has received great attention in the past 20 years due to their easy preparation, good stability and versatility [1–3]. Nowadays, this technology has been used to study a variety of interface reaction phenomena, such as lubrication, adhesion, corrosion, polymerization, adsorption [4,5], which have great significance both in fundamental research and in technology development [1–5]. Of the numerous self-assembly approaches, the one based on chemisorption of organothiols onto gold surface to form Au–S bond is the most widely used [6–9]. However, this spontaneously assembling process is slow (usually more than 24 h) [6,8], which has limited realtime monitoring and commercial application development. Thus, the improvement of self-assembly process still needs further exploration. It is well known that adding energy to a chemical reaction process will impact its yield or rate [10–13]. Among other types of energies, magnetic field has been applied and has been shown to influence on electrochemical behavior of hemoglobin [14] and the systems Cu2+/ Cu, Ni2+/Ni, [IrCl6]2−/[IrCl6]3− [15], charge transfer rate [16,17] and other reactions [18,19]. In the area of chemical synthesis, the possibility to control nanoparticle assembling process by applying a magnetic field has been used to fabricate electronic devices [20], while enhancement of the orientation of polymer chains under a magnetic field was also used in electrochemical polymerization [21,22].

⁎ Corresponding author. Tel.: +86 25 86205840; fax: +86 25 83317761. E-mail address: [email protected] (S. Bi). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.01.039

In the process of self-assembling, it has been reported that the SAMs formed considerably faster under an electric potential [11,23] or in ultrasonic condition [24] than under usual procedure and showed exceptional properties. The orientation and optimized insulation effect of magnetic field on SAMs were also studied [25,26]. Thus, it is worthwhile to systematically characterize the influence of a magnetic field on the properties of SAMs considering the known effect of magnetic field. In this work, we chose dedecanethiol (C12SH), a common used organic substance to study the self-assembly process of it onto polycrystalline gold electrode under an external magnetic field. The specific parameters measured by electrochemical experiments showed that magnetic field can help to form highly dense and wellordered C12SH-SAMs on the gold electrode in a relatively short period of time. 2. Experimental details 2.1. Reagents and apparatus A C12SH solution (10 mM) was prepared by dissolving 1dodecanethiol (98+%, Aldrich) in ethanol (99.9%, Merck). A Na2SO4 solution (0.1 M) was prepared by dissolving sodium sulfate anhydrous (99%, Alfa) in twice-distilled water. K3Fe(CN)6 and K4Fe(CN)6 stock solution (0.05 mM) was prepared by dissolving potassium hexacynoferrate (III) (99%, Sigma) or potassium hexacynoferrate (II) trihydrate (99+%, Acros) in twice-distilled water. All the chemicals were of analytical grades and were used without further purification. Water was twice-distilled with a quartz apparatus.

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2.2. Electrode pretreatment and thiol adsorption Polycrystalline gold electrode (2.0 mm diameter, CH Instruments) was polished on microcloth pads with 1.0 µm, 0.3 µm and 0.05 µm alumina slurries and then sonicated in twice-distilled water for 15 min. The gold surface was cleaned with freshly prepared aqua regia solution (HCl: HNO3: H2O = 3: 1: 6, v/v) for 5 min. After that the electrode was rinsed with double-distilled water and cycled from − 0.4 to + 1.5 V in 0.5 M H2SO4 at 0.1 V s− 1 until a stable a reproducible cyclic voltammogram was obtained. The real surface area of working electrode was determined by integrating the charge of reduction peak of gold, assuming a value of 400 µC cm− 2 for a monolayer of chemisorbed oxygen on polycrystalline gold [27]. The roughness factor of the polycrystalline gold was about 1.1. The electrode was then rinsed with twice-distilled water and finally dried by 99.999% highly pure N2. C12SH-SAMs were prepared by keeping the electrode in 10 mM ethanol solution of C12SH in a magnetic field for different self-assembly time. The device was located at the same position with the surface perpendicular to the magnetic field direction for every experiment to maintain the stability and the consistency, as shown in Fig. 1. For comparison, we also prepared the C12SH-SAMs by immersing the electrode in the same solution without magnetic field. Upon removal, the SAM-coated electrodes were rinsed with a copious amount of absolute ethanol followed by double-distilled water, and immediately used for the electrochemical characterization.

Fig. 1. Assemblies of the cell for the self-assembled adsorption in magnetic field.

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed using a CHI-660B electrochemical workstation (CH Instruments, USA) and an Autolab system (Eco Chemie, Netherlands). The different magnetic field with the intensity (B0) of 0.15 T, 0.19 T and 0.22 T measured at 5200 Hall effect magnetometer (Oxford Instruments, England) was severed by three permanent magnets (Honglun Electronic Ltd., Nanjing, China). All electrochemical characterizations (CV and EIS) were conducted under a nitrogen atmosphere at 25 ± 1 °C which is kept by the CS501SP Super digital thermostat bath (Huida Experimental Equipment Ltd., Chongqing, China).

2.3. Electrochemical characterization For electrochemical characterization, a conventional three-electrode electrochemical cell was used. The SAM-coated gold electrode was used as working electrode, a saturated calomel electrode (SCE) and a platinum electrode served as the reference and counter electrodes, respectively. Supporting electrolyte, 0.1 M Na2SO4, was purged with 99.999% highly pure N2 for 10 min prior to measurements, and kept under a nitrogen atmosphere during the course of the experiments. All experiments were carried out at 25 ± 1 °C. CV

Table 1 Interfacial parameters of C12SH-SAMs (values calculated in the magnetic field B0 of 0.22 T were the averages obtained by triplicate measurements and added with standard deviation). Time

CV

EIS

Cd (µF cm− 2)a

θ (%)c

0.32 0.76 0.92 1.01 0.95 1.14

99.985 99.994 99.995 99.996 99.995 99.996

Q (µF cm− 2)c

nc

2.05 1.68 1.44 1.58 1.78 2.02

0.963 0.987 0.983 0.977 0.980 0.967

50 V s− 1

1.84 1.75 1.54 1.56 1.64 2.04

± 0.09 ± 0.16 ± 0.13 ± 0.06 ± 0.20 ± 0.53

1.73 1.72 1.51 1.51 1.59 1.92

1.68 1.70 1.50 1.50 1.58 1.85

5.67 2.40 2.00 1.33 1.38 1.74

Different magnetic field B0 with 3 h self-assembly time 0.19 T 2.63 2.09 1.94 0.15 T 3.94 2.42 1.93

1.86 1.78

1.82 1.72

2.55 4.40

83° 76°

0.67 0.09

99.993 99.952

2.00 2.00

0.973 0.969

Common 15 min 3h 6h 10 h 16 h 24 h

2.15 2.35 2.12 2.16 1.30 1.19

2.06 2.28 2.03 2.06 1.30 1.17

8.34 5.92 5.69 5.44 3.54 0.97

82° 82° 83° 84° 86° 88°

0.07 0.06 0.07 0.06 0.36 1.04

99.937 99.928 99.937 99.929 99.987 99.996

2.28 2.67 2.49 2.47 1.40 1.23

0.971 0.960 0.959 0.972 0.956 0.980

–

88° [29]

–

99.99 [29]

1.1–1.2 [31]

0.98 [31]

Magnetic 15 min 30 min 1h 3h 6h 10 h

field (B0 = 0.22 2.92 ± 0.17 2.16 ± 0.42 2.02 ± 0.36 1.92 ± 0.36 1.97 ± 0.06 2.66 ± 1.03

b c

T) 2.05 1.82 1.59 1.66 1.70 2.19

immersing protocol 3.76 2.73 3.83 2.91 3.55 2.66 3.85 2.59 1.90 1.60 1.71 1.36

Literature value 24 h a

1 V s− 1 ± 0.14 ± 0.20 ± 0.14 ± 0.10 ± 0.16 ± 0.70

2.32 2.54 2.34 2.32 1.40 1.25

1.1–1.2 [31]

± 0.04 ± 0.16 ± 0.12 ± 0.04 ± 0.18 ± 0.38

± 0.07 ± 0.16 ± 0.11 ± 0.04 ± 0.18 ± 0.29

± 4.88 ± 1.06 ± 0.97 ± 0.42 ± 1.08 ± 0.97

81° 85° 86° 87° 87° 87°

Rct (MΩ cm2)c

20 V s− 1

0.1 V s− 1

Φ1

a

5 V s− 1

Δip (µA cm− 2)b

Hz

± 1° ± 2° ± 3° ± 2° ± 1° ± 2°

Cd and Φ1 Hz values were obtained in 0.1 M Na2SO4 solution. Δip values were obtained in 0.1 M Na2SO4 solution containing 2 mM K3Fe(CN)6. Rct, θ, Q and n values were obtained in 0.1 M Na2SO4 solution containing 2 mM K3Fe(CN)6 and 2 mM K4Fe(CN)6.

± 0.09 ± 0.24 ± 0.16 ± 0.24 ± 0.12 ± 0.45

± 0.004 ± 0.002 ± 0.001 ± 0.001 ± 0.001 ± 0.002

± 0.26 ± 0.12 ± 0.21 ± 0.06 ± 0.27 ± 0.54

± 0.026 ± 0.003 ± 0.006 ± 0.009 ± 0.003 ± 0.029

J. Cheng et al. / Thin Solid Films 517 (2009) 3661–3666

Fig. 2. Cd vs. scan rate plots of C12SH-SAMs in 0.1 M Na2SO4. a–f: different self-assembly time in magnetic field (B0 = 0.22 T). a: 15 min ( ); b: 30 min ( ); c: 1 h (........Δ........); d: 3 h (__■__); e: 6 h ( ); f: 10 h (_ _☆_ _). Inset: Cyclic voltammograms of C12SH-SAMs in 0.1 M Na2SO4.. a–f: different self-assembly time in magnetic field. a: 15 min ( ); b: 30 min ( ); c: 1 h (...............); d: 3 h (___); e: 6h( ); f: 10 h ( ). Scan rate: 20 V s− 1.

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Fig. 4. Φ1 Hz vs. time plots of C12SH-SAMs formed by (a) magnetic field (__■__); (b) common immersing protocol ( ). Inset: Bode phase angle plots of C12SHSAMs in 0.1 M Na2SO4. a–f: different self-assembly time in magnetic field (B0 = 0.22 T). a: 15 min (□); b: 30 min (○); c: 1 h (Δ); d: 3 h (■); e: 6 h (◊); f: 10 h (☆).

average and standard deviation of values obtained from three independent sample preparations. measurements contain two parts: (1) differential capacitance (Cd) was calculated from cyclic voltammograms performed from − 0.2 V to 0.2 V at different scan rates in 0.1 M Na2SO4 [28]; (2) difference value of the current density at − 0.2 V and 0.5 V (Δip = (ip,− 0.2V − ip,0.5 V) / A) was obtained from cyclic voltammograms performed in 0.1 M Na2SO4 containing 2 mM K3Fe(CN)6 at 0.1 V s− 1, where A is the real electrode area (cm2). EIS measurements also contains two parts: (1) the phase angle at 1 Hz (Φ1Hz) was obtained by impedance spectroscopy performed in 0.1 M Na2SO4 by applying an AC impedance in a frequency range between 0.1 Hz to 100 kHz with a perturbation signal of 5 mV at 0 V (vs. SCE) [28,29]; (2) the charge transfer resistance (Rct) was measured using impedance spectroscopy performed in 0.1 M Na2SO4 containing 2 mM K3Fe(CN)6 and 2 mM K4Fe(CN)6 by applying an AC impedance with the same perturbation signal and frequency range at the formal potential of the redox couple (0.18 V). Reported values and ranges represent the

Fig. 3. Cyclic voltammograms of C12SH-SAMs in 2 mM K3[Fe(CN)6]–0.1 M Na2SO4. a–f: different self-assembly time in magnetic field (B0 = 0.22 T). a: 15 min ( ); b: 30 min ( ); c: 1 h (...............); d: 3 h (____); e: 6 h ( ); f: 10 h ( ). Inset: Cyclic voltammograms of C12SH-SAMs formed on different conditions. a: in magnetic field for 3 h (____); b–c: by common immersing protocol. b: 3 h ( ); c: 24 h (...............). Scan rate: 0.1 V s− 1.

3. Results and discussion 3.1. The CV characterization of C12SH-SAMs formed in fixed magnetic field Cd measurements can be used to evaluate the ionic permeability of the SAMs. In this work, the Cd values of the C12SH-SAMs formed in the magnetic field with different assembled time (15 min, 30 min, 1 h, 3 h, 6 h and 24 h) were calculated by CVs performed from − 0.2 to 0.2 V at different scan rates in 0.1 M Na2SO4 using Cd = Δi/2vA [30] (Table 1), where Δi is the sum of the cathodic and anodic capacitive current measured at 0 V (μA), and v is the scan rate (V s− 1). The relationship between Cd and v is shown in Fig. 2. Difference value of the Cd (ΔCd) measured at 0.1 V s− 1 and 50 V s− 1 for C12SH-SAMs formed in magnetic field at 0.22 T (ΔCd = 0.42 µF cm− 2 for 3 h) is similar to that of the C12SHSAMs formed by common immersing protocol (ΔCd = 0.54 µF cm− 2 for 24 h), indicating that the C12SH-SAMs are dense and well-ordered and

Fig. 5. Charge transfer resistance Rct vs. time plot of C12SH-SAMs formed by magnetic field. Inset: Nyquist plots of C12SH-SAMs formed in 2 mM Fe(CN)3−/4− –0.1 M Na2SO4. 6 a–f: different self-assembly time in magnetic field (B0 = 0.22 T). a: 15 min (□); b: 30 min (○); c: 1 h (Δ); d: 3 h (■); e: 6 h (◊); f: 10 h (☆).

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SAMs (Table 1). The smaller values of Δip suggest the stronger blocking effect. All these results indicate that a high-quality monolayer can form in a short period of time (3 h) in the magnetic field, which completely blocked the electrochemical reaction of Fe(CN)3− 6 [29]. 3.2. The EIS characterization of C12SH-SAMs formed in fixed magnetic field

Fig. 6. Surface coverage θ vs. time plots of C12SH-SAMs formed by (a) magnetic field (B0 = 0.22 T) (__■__); (b) common immersing protocol ( ).

can be considered to resist ionic permeation [31]. However, the values of Cd (1.51 ± 0.04 µF cm− 2 for 3 h) calculated at 20 V s− 1 are larger than those previously reported in the literatures (1.1–1.2 µF cm− 2) [28], which might be ascribed to the effect of the magnetic field. In order to investigate the blocking ability of C12SH-SAMs by studying the electron transfer reaction of a redox probe molecule on the SAM-modified surfaces, the SAM-modified electrodes were placed in 2 mM K3[Fe(CN)6]–0.1 M Na2SO4 solution for CV measurements (as shown in Fig. 3). C12SH-SAMs modified electrodes do not show any redox peak for Fe(CN)3− 6 , indicating that the redox reaction is completely blocked by the monolayers. With the increase of selfassembly time in the magnetic field, the blocking effect of C12SH-SAMs on the electron transfer between Fe(CN)3− 6 and electrode surface was strengthened. Inset of Fig. 3 shows the CVs of C12SH-SAMs formed in magnetic field and by common immersing protocol. The blocking ability of the C12SH-SAMs formed in the magnetic field at 0.22 T for 3 h is similar to the C12SH-SAMs formed by common procedure for 24 h and much stronger than that formed by common procedure for 3 h. Δip values provided specific evidence for the blocking ability of C12SH-

In order to further assess the insulating properties of SAMs, the ionic permeation in an inert electrolyte without any redox species was studied by EIS. The impedance measurement was carried out in 0.1 M Na2SO4 aqueous solutions as shown in inset of Fig. 4. By measuring the phase angle at an ion-diffusion-associated frequency (1 Hz), the C12SH-SAMs can be identified as a pure capacitor (Φ1 Hz ≥ 88°) or a leaky capacitor (Φ1 Hz b 87°) [29]. The phase angle Φ1 Hz vs. time plots is shown in Fig. 4 (detailed see Table 1). The Φ1 Hz values of the SAMs formed in magnetic field for more than 3 h are all close to 88° and bigger than that for the same assembly time by common immersing protocol, which suggests that the ionic permeation is lower and the SAMs possess better insulating properties [28,31]. The charge transfer resistances of C12SH-SAMs modified electrodes against the diffusion of the redox probe were measured using EIS to further evaluate the structural integrity of the monolayer. The measurement was carried out in 0.1 M Na2SO4 aqueous solutions containing 2 mM Fe(CN)3−/4− . The impedance values were fitted to a 6 standard Randle's equivalent circuit (the abbreviation is R(Q(RW)), which means that the circuit comprises a parallel combination of constant phase element (CPE) represented by Q and a faradaic impedance (Zf) in series with the uncompensated solution resistance (Rs), where Zf is a series combination of charge transfer resistance (Rct) and the Warburg impedance (W) [29]. The fitting results were shown in Table 1. The CPE is given by ZCPE = 1 / Q(jω)n, where Q is the amplitude of the CPE and ω is the angular frequency and n is the exponent. When n = 1, purely capacitive behavior is expected and Q = Cd [32]. The values of n in the case of C12SH-SAMs formed in magnetic field for all the assembly time are almost equal to 1, indicating that SAMs can be considered as pure capacitors. The interface differential capacitance of C12SH-SAMs obtained from EIS corresponds with the results from CV (Table 1).

Fig. 7. The comparison of C12SH-SAMs formed in the magnetic with different intensity. (A) Cyclic voltammograms of C12SH-SAMs in 0.1 M Na2SO4. (B) Cyclic voltammograms of C12SH-SAMs in 2 mM K3[Fe(CN)6]–0.1 M Na2SO4. (C) Bode phase angle plots of C12SH-SAMs in 0.1 M Na2SO4. (D) Nyquist plots of C12SH-SAMs formed in 2 mM Fe(CN)3−/4− –0.1 M 6 Na2SO4. a–c: different magnetic intensity B0. a: 0.22 T (___ or ■); b: 0.19 T ( or ○); c: 0.15 T (............... or Δ).

J. Cheng et al. / Thin Solid Films 517 (2009) 3661–3666

Nyquist plots of C12SH-SAMs are shown in inset of Fig. 5, and the relationship between the Rct values of C12SH-SAMs and the assembly time can be seen in Fig. 5. The Rct values are keeping stable when the assembly time is beyond 3 h (Rct = (1.01 ± 0.24) × 106 Ω cm2 for 3 h), which are similar to the result of C12SH-SAMs formed by common immersing protocol for 24 h (1.04 × 106 Ω cm2) and much larger than that formed by common immersing protocol for the same time (6.16 × 104 Ω cm2 for 3 h). This again indicates that the C12SH-SAMs are well-ordered and well-organized with defect-free structure. In addition, the impedance data were also used to calculate the surface coverage (θ) of the monolayer-coated electrodes. The value of θ was calculated based on the Rct value using θ = 1 − R0ct/Rct [29], where R0ct represents the charge transfer resistance of bare Au electrode (R0ct = 45.3 Ω cm2) (details see Table 1). The θ values vs. time plots of C12SH-SAMs formed in magnetic field and by common immersing protocol are shown in Fig. 6. The θ value of C12SH-SAMs formed in magnetic field for 3 h (99.996%) is in agreement with that reported in the literature (99.99%) [29] and the SAMs formed by common procedure for 24 h (99.996%), and larger than that formed by common procedure for 3 h (99.928%). From these results, we can conclude that the rate of formation of high-quality C12SH-SAMs is remarkably improved by magnetic field compared to the common procedure. 3.3. The characterization of C12SH-SAMs formed with different magnetic intensity The C12SH-SAMs formed in the magnetic field with different intensity were also characterized by CV and EIS. The assembly time was chosen as 3 h since the highly dense and well-ordered C12SHSAMs could form during this time in the magnetic filed (B = 0.22 T). From the CV and EIS results (Fig. 7 and Table 1), it can be seen that C12SH-SAMs formed in the magnetic filed of 0.22 T possess better insulating properties, higher surface coverage and lower ionic permeation compared to those of formed in 0.15 T and 0.19 T. These results indicate that the strong magnetic field can accelerate the formation of high-quality C12SH-SAMs. 3.4. Exploration of the mechanism of magnetic field effect on SAMs The study of Cd can be used to further evaluate the properties of the SAMs such as pinhole density and film thickness [30]. According to the Helmholtz capacitor model, the thickness d (Å) of the SAMs can be

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calculated using d = εε0/Cd [30], where ε is the dielectric constant of the monolayer and ε0 is the permittivity of free space (8.85 × 10− 14 F cm− 1), and Cd is measured at 20 V s− 1. In addition, the tilt angle φ value of C12SH molecules in the monolayer can be calculated using φ = acos(l/d) based on the C12SH molecular length l of ~ 20.3 Å [33,34]. As can be seen in Table 2, the values of d (d = 13.4 ± 0.4 Å for 3 h) and φ (φ = 48.8° ± 1.5° for 3 h) is different from the results reported in the literature (d: 17 ± 1 Å; φ: ~ 30°) [33,34]. This result might be due to the effect of magnetic field on the chemical and physical properties of substance, such as ε [35,36]. In order to testify this explanation, ε was calculated by assuming that C12SH was well-ordered assembled on the gold electrode surface with the φ value 30° (Table 2). Under the natural assembly condition, ε value decreases with increasing assembly time, and reach to the similar value to the literature reports after 24 h. However, when the magnetic field was applied, ε value is always larger than that reported in the literature (ε = 2.3) [34], which decreases gradually when the apply time less than 1 h, and then increases gradually when the apply time beyond 1 h. Finally, ε value reaches 3.8 ± 0.8 when the magnetic field was applied for 10 h. It can be concluded that there is evident effect of magnetic field on ε value. As for the mechanism of magnetic field effect on the acceleration of self-assembly process, it may be due to the magnetohydrodynamic (MHD) effect [14,15,17,19] and the orientation function of magnetic field on molecules [21,22]. Considering the MHD effect, it will lead to an increase of the limiting current density and improve the mass transport rate of electrochemical reactions owing to magnetically stimulated convection [14,15,17]. Consequently, the formation rate of SAMs will be accelerated by the MHD effect. On the other hand, the SAMs formed on gold surface will be well-ordered and well-organized due to the orientation function of magnetic field [21,22]. 4. Conclusion We prepared the C12SH-SAMs on the polycrystalline gold electrode in the magnetic field and studied the properties of the C12SH-SAMs formed with different assembly time by CV and EIS. The results showed that the SAMs could form in a relatively short time (3 h) in magnetic field, which had similar properties as the SAMs formed by common immersing protocol (24 h). It is reasonable to ascribe this result to the MHD effect and the orientation function of magnetic field. The mechanism for the formation of SAMs in magnetic field needs some further studies. Acknowledgements

Table 2 Interfacial Parameters of C12SH-SAMs calculated by CV at the scan rate of 20 V s− 1. Cd (µF cm− 2)

Da (Å)

Φa

εb

Magnetic field (B0 = 0.22 T) 15 min 1.73 ± 0.04 30 min 1.72 ± 0.16 1h 1.51 ± 0.12 3h 1.51 ± 0.04 6h 1.59 ± 0.18 10 h 1.92 ± 0.38

11.8 ± 0.4 11.8 ± 1.2 13.5 ± 1.1 13.4 ± 0.4 12.9 ± 1.4 10.8 ± 2.1

54.3° ± 1.4° 54.3° ± 4.0° 48.3° ± 4.2° 48.8° ± 1.5° 50.6° ± 5.2° 57.6° ± 7.0°

3.4 ± 0.1 3.4 ± 0.3 3.0 ± 0.2 3.0 ± 0.1 3.2 ± 0.4 3.8 ± 0.8

Common protocol 15 min 2.15 3h 2.35 6h 2.12 10 h 2.16 16 h 1.30 24 h 1.14

9.50 8.68 9.61 9.41 15.6 17.9

62.1° 64.7° 61.8° 62.4° 39.7° 28.2°

4.27 4.67 4.22 4.29 2.58 2.26

Literature value 24 h 1.1–1.2 [31]

17 ± 1 [33,34]

~ 30° [33,34]

2.3 [33]

Time

a

Values were calculated by assuming that ε was 2.3, using these formulas: d = εε0/ Cd, φ = arccos(l/d); b Values were calculated by assuming that the title angle φ was 30°, using this formula: ε = Cdd /ε0.

This project is supported by Grant from MOE for PhD Program (20050284030), National Natural Science Foundation of China (Nos. 20575025 & NFFTBS-J0630425), Research Funding from State Key Laboratory of Electrochemistry of China at Changchung Applied Chemistry Institute (2008008) and Analytical Measurement Founding of Nanjing University. References [1] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 105 (2005) 1103. [2] D. Chen, J.H. Li, Surf. Sci. Rep. 61 (2006) 445. [3] A. Badia, R.B. Lennox, L. Reven, Acc. Chem. Res. 33 (2000) 475. [4] P. Gupta, A. Ulman, S. Fanfan, A. Korniakov, K. Loos, J. Am. Chem. Soc. 127 (2005) 4. [5] S. Nitahara, N. Terasaki, T. Akiyama, S. Yamada, Thin Solid Films 499 (2006) 354. [6] T. Arakawa, D. Hobara, M. Yamamoto, Electrochem. Commun. 7 (2005) 848. [7] M.G. Badin, A. Bashir, S. Krakert, T. Strunskus, A. Terfort, C. Woll, Angew. Chem. (Int. Ed.) 46 (2007) 3762. [8] Y.J. Yang, K.S. Beng, Electrochem. Commun. 6 (2004) 87. [9] D. Losic, J.G. Shapter, J.J. Gooding, Electrochem. Commun. 4 (2002) 953. [10] B.M. Rosen, V. Percec, Nature 446 (2007) 381. [11] C.R. Hickenboth, J.S. Moore, S.R. White, N.R. Sottos, J. Baudry, S. Wilson, Nature 446 (2007) 423. [12] C.E. Banks, R.G. Compton, ChemPhysChem 4 (2003) 169.

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[13] D.W. Kim, C.H. Jeong, K.N. Kim, H.Y. Lee, H.S. Kim, Y.J. Sung, G.Y. Yeom, Thin Solid Films 435 (2003) 242. [14] J.D. Zhang, W.Z. Wei, Electroanalysis 13 (2001) 888. [15] A. Bund, S. Koehler, H.H. Kuehnlein, W. Plieth, Electrochim. Acta 49 (2003) 147. [16] O. Lioubashevski, E. Katz, I. Willner, J. Phys. Chem. B 108 (2004) 5778. [17] O. Aaboubi, P. Los, J. Amblard, J.P. Chopart, A. Olivier, J. Electrochem. Soc. 150 (2003) E125. [18] T. Kaneko, H. Matsuoka, T. Hirata, R. Hatakeyama, K. Tohji, Thin Solid Films 506 (2006) 259. [19] E. Katz, O. Lioubashevski, I. Willner, J. Am. Chem. Soc. 126 (2004) 11088. [20] M. Nawa, R. Baba, S. Nakabayashi, C. Dushkin, Nano Lett. 3 (2003) 293. [21] R.G. Lv, S.L. Zhang, Q.F. Shi, J.Q. Kan, Synth. Met. 150 (2005) 115. [22] S. Osawa, T. Ogawa, M. Ito, Synth. Met. 90 (1997) 109. [23] L. Cheng, J.P. Yang, Y.X. Yao, D.W. Price, S.M. Dirk, J.M. Tour, Langmuir 20 (2004) 1335. [24] M. Atobe, T. Yamada, T. Fuchigami, T. Nonaka, Electrochim. Acta 48 (2003) 1759.

[25] N. Yoshimoto, K. Ogawa, S. Ogawa, IEEE Trans. Appl. Supercond. 14 (2004) 1600. [26] W.J. Guo, S.H. Chen, B.D. Huang, H.Y. Ma, X.G. Yang, Electrochim. Acta 52 (2006) 108. [27] J.C. Hoogvliet, M. Dijksma, B. Kamp, W.P. van Bennekom, Anal. Chem. 72 (2000) 2016. [28] E. Boubour, R.B. Lennox, Langmuir 16 (2000) 4222. [29] V. Ganesh, V. Lakshminarayanan, Langmuir 22 (2006) 1561. [30] Q. Cheng, A. Brajter-Toth, Anal. Chem. 67 (1995) 2767. [31] C. Naud, P. Calas, A. Commeyras, Langmuir 17 (2001) 4851. [32] U.K. Sur, V. Lakshminarayanan, J. Electroanal. Chem. 565 (2004) 343. [33] M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559. [34] T. Kondo, M. Yanagida, K. Shimazu, K. Uosaki, Langmuir 14 (1998) 5656. [35] J.T. Wang, C. Zhang, Ferroelectrics 301 (2004) 211. [36] P.M. Botta, J. Mira, A. Fondado, J. Rivas, Mater. Lett. 61 (2007) 2990.