Adsorption Behavior of Methimazole Monolayers ... - ACS Publications

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Adsorption Behavior of Methimazole Monolayers on a Copper Surface and Its Corrosion Inhibition Ying-Cheng Pan, Ying Wen,* Lu-Yuan Xue, Xiao-Yu Guo, and Hai-Feng Yang* Department of Chemistry, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, People’s Republic of China ABSTRACT: Improved anticorrosion of a copper surface after being modified with methimazole (MMI) monolayers was studied by electrochemical impedance spectroscopy (EIS), and electrochemical polarization measurement. The EIS mechanism of the copper surface with MMI monolayers was in agreement with the mode of R(Q(RW))(CR), and polarization experiments exhibited the anticorrosive effect of the MMI film on the copper surface with an high efficiency of 91.2%. Surface-enhanced Raman scattering (SERS) results indicated that the MMI molecule was self-assembled on the copper surface through S6 and N2 atoms to form monolayers in a tilted orientation, resulting in a strong interaction between the MMI molecule and the surface. An in situ electrochemical SERS experiment was conducted from 0 to −1.6 V vs SCE for observing the stability of MMI monolayers on the copper surface.

1. INTRODUCTION Copper is widely used in industry due to its excellent electrical and thermal conductivity, good mechanical workability, and its relatively noble properties.1 Copper is resistant to the influence of atmosphere and many chemicals. However, in the presence of complex ions such as Cl−, it may suffer severe corrosion.2 There are many methods to protect copper metal from corrosion. The most effective way is to coat organic inhibitors onto the copper surface for anticorrosion.3−5 Azole compounds, such as benzotriazole (BTA), are known as efficient corrosion inhibitors for copper in different media. BTA and its derivatives are excellent corrosion inhibitors for copper and its alloys in a wide range of media, but BTA is toxic.2−11 Instead, nontoxic imidazole derivatives as an effective inhibitor for copper corrosion are investigated.12,13 As one of the imidazole derivatives, methimazole (MMI) is an important antithyroid drug that is widely used in the clinical treatment of hyperthyroidism.14,15 As shown in Figure 1, the MMI molecule possessing N2, N5 atoms in the ring and the exocyclic S6 atom are available for surface bonding with metals to form an adherent protective film as a barrier to aggressive ions such as chloride. The anticorrosive capability and the adsorption behavior of MMI monolayers at the silver surface in potassium chloride solution have been investigated by Zhang et al. via electrochemistry and Raman observations.16 Larabi et al. have also investigated MMI as corrosion inhibitor for copper in hydrochloric acid.17 So far, little work regarding MMI protection of copper from corrosion has been reported. Observation of the resulting MMI film on the copper surface has been performed by electrochemical impedance spectroscopy (EIS).18 EIS technique, a nondestructive, sensitive, and informative method, has been extensively used for the evaluation of coatings for corrosion inhibition, especially for analyzing the electrode with self-assembled organic monolayers. The electrochemical polarization in the form of a Tafel plot can © 2012 American Chemical Society

provide the corrosion rates from linear polarization and determine the efficiency of the corrosion inhibitor.19−21 Surface-enhanced Raman scattering (SERS) spectroscopy is a highly sensitive technique for probing the adsorption behavior of a molecule on a metal surface. The SERS information could provide abundant molecular vibration information to clarify the configuration of molecular adsorption and the interaction mechanism on the supporting substrate.22−26 The purpose of this work is to investigate the corrosion inhibition behavior of MMI self-assembled monolayers (SAMs) for copper in 0.1 M KCl solution by electrochemical methods. SERS spectroscopy is used to elucidate the structure of MMI coating on the copper surface.

2. EXPERIMENTAL METHODS 2.1. Chemicals and Materials. MMI was purchased from the Sigma-Aldrich Corporation. Sulphuric acid, potassium chloride, and ethanol were of analytical grade and purchased from Sinopharm Chemical Reagents Company. All solutions were prepared with Milli-Q water (18 MΩ·cm). 2.2. Apparatus. Raman spectroscopic measurements were performed by using a confocal microprobe Raman system (LabRam Π, Dilor, France). A liquid nitrogen-cooled 1024 × 800 pixels charge-coupled device was used as a detector, and an exciting line of 632.8 nm was supplied by a He−Ne laser with power of ca. 5 mW. A 50× long-working-length objective was used for focusing the laser spot onto the electrode surface. The slit and pinhole were set at 100 and 1000 μm, respectively. Each spectrum was measured three times, and the acquisition time Received: September 19, 2011 Revised: January 4, 2012 Published: January 9, 2012 3532

dx.doi.org/10.1021/jp2090318 | J. Phys. Chem. C 2012, 116, 3532−3538

The Journal of Physical Chemistry C

Article

Figure 1. Molecular structures of the MMI in (a) thione and (b) thiol forms.

Figure 2. Nyquist impedance plots of (A) blank and (B) MMI monolayers-modified copper electrodes in 0.1 M KCl solution.

was 20 s. Calibration was done referring to the 519 cm−1 line of silicon. The electrochemical measurements were conducted by a CHI 660c electrochemistry workstation (CH Instruments, Inc.). 2.3. Pretreatment for electrode. The copper electrode was made from the polycrystalline copper rod (99.99+%, Sigma-Aldrich) and the geometric area of surface was ca. 0.025 cm2. Before the Raman scattering measurement, the copper electrode was sequentially polished with emery paper and 1.0 and 0.3 μm alumina/water slurries until a shiny, mirror-like surface was obtained, and then was ultrasonically washed with Milli-Q water. For SERS detection, to obtain the necessary roughness of the copper surface, an oxidation reduction cycle treatment was performed in a conventional three-electrode cell.27 The saturated calomel electrode (SCE) and the platinum sheet were used as the reference and the counter electrodes, respectively. All potentials cited in this paper were converted to SCE. 2.4. SAMs. To form the monolayers, the treated copper electrode was etched in 7 M HNO3 solution for 10 s to achieve a fresh and oxide-free surface, then rinsed with Milli-Q water and absolute ethanol as soon as possible, and immersed immediately into the deoxygenated MMI solution for 3 h at room temperature. The modified electrode was taken out of the

solution and rinsed carefully with ethanol and Milli-Q water, and then dried by flowing nitrogen gas prior to further investigations. 2.5. Electrochemical Measurements. EIS measurements were performed at the open circuit potentials with the ac voltage amplitude of 5 mV in the frequency range from 0.01 Hz to 100 kHz. The polarization curves were obtained from −0.4 to 0 V vs SCE with a scan rate of 2 mV s−1.

3. RESULTS AND DISCUSSION 3.1. EIS Studies. In order to illustrate the impedance behavior of MMI monolayers on copper electrode, the EIS data are simulated by ZsimpWin software. The fitting curves of EIS results for the blank and MMImodified copper electrodes are shown in Figure 2. The equivalent circuit R(Q(RW))(QR) in Figure 2A is suitable for the Nyquist plot of the bare copper, while R(Q(RW))(CR) mode in Figure 2B is suitable for the MMI-modified copper electrode. The systems in which the solid/liquid interface are modeled by the simple equivalent circuit involving the parameters Rb, Rct, Rf, W, Qd, Qf, Cf. Rb represents the solution resistance. Rct is the charge transfer resistance corresponding to the corrosion reaction at the copper/solution interface. Rf is the film resistance from the oxide species. W is the Warburg 3533



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Table 1. Values of Electrochemical Parameters for a Blank Copper Surface Obtained by Fitting the Experimental Impedance Data Using the Suggested Equivalent Qf 0.1 M KCl a

Yfilma

Rb (Ω cm2)

solution

−6

1.159 × 10

7120

Qd n

Rf (Ω cm2)

0.6636

1.01 × 10

6

n

Rct (Ω cm2)

W (Ω cm2 s−0.5)

0.9011

5.594 × 10

1.224 × 10−6

Yfilma −8

2.402 × 10

4

The dimensions are S; sn/cm2; if n = 1, they are F cm−2.

Table 2. Values of Electrochemical Parameters for MMI Monolayers on the Copper Surface Obtained by Fitting the Experimental Impedance Data Using the Suggested Equivalent Qd 0.1 M KCl a

Cf (F cm−2)

Rb(Ωcm2)

solution

−8

n

1.078 × 10

5.113 × 10

7868 2

Yfilma

Rf (Ω cm2)

−8

3.44 × 10

5

n

Rct (Ω cm2)

W (Ω cm2 s−0.5)

0.8128

1.707 × 10

1.243 × 10−6

7

−2

The dimensions are S; s /cm ; if n = 1, they are Fcm .

Moreover, after the copper surface is modified with MMI monolayers, the Qf changes from a distorted Warburg impedance (n = 0.6636) to a capacitance Cf corresponding to the MMI film capacity. It indicates that the MMI coating on the copper surface is nonconductive to the electrodes. In all, impedance data suggest that MMI film at the copper electrode exhibits a barrier, and the electron transfer should be refrained through the film. 3.2. Electrochemical Polarization Studies. Figure 3 shows the electrochemical polarization curves in 0.1 M KCl solution for the naked and MMI-covered copper electrodes.

impedance induced by the diffusion of corrosive reactants or corrosion product species. Qd is a constant phase element (CPE), representing a modified double-layer capacitance. The impedance of CPE is defined as

ZCPE = Y 0−1(j ω)−n where Y0 is the modulus, j is the imaginary root, ω is the angular frequency and n is the phase.28 Depending on the value of the exponent n, Q may be a resistance, R (n = 0); a capacitance, C (n = 1); Warburg impedance, W (n = 0.5), or an inductance, L (n = −1).The value range of a real electrode of n is often between 0 and 1. The smaller the value of n, the rougher the electrode surface, and the more serious the corrosion of the electrode.29 The EIS parameters of the blank and MMI-modified copper surfaces are tabulated in Tables 1 and 2, respectively. The inhibition efficiency (ER) can be calculated according to the following equation:30

ER =

R p − R p0 Rp

(1)

Rp0 is the polarization resistance of the naked copper, and Rp is the polarization resistance of the MMI-covered electrode (the polarization resistance is the sum of Rct and Rf). The value of ER is 93.8% in our experiment. It should be mentioned that low values of χ2 and error (%) in parameters are the main criteria while choosing the best fitting model. An acceptable agreement of the χ2 values is around 1 × 10−3. In the present fitting, the χ2 values of 8.7 × 10−4 and 1.09 × 10−3 are for the blank and modified electrodes, respectively, as well as the error of each element in fitting mode below 3%. In Figure 2A and Table 1, Warburg impedance appearing in the equivalent circuit of the blank copper should be created by diffusion of oxide species. Two “time constants” observed in the equivalent circuit might be from the pure copper and its oxide species. Compared with the Rct (5.594 × 104 Ω cm2) of the blank copper given in Table 1, Rct (1.707 × 107 Ω cm2) for the MMI monolayer-modified copper electrode in Table 2 increases remarkbaly, indicating that the MMI monolayers could protect the copper from corrosion at the copper/solution interface effectively.

Figure 3. Tafel plot of the copper electrode in 0.1 M KCl solution: (a) blank and (b) with MMI monolayers, at a scan rate of 2 mV/s.

For the Tafel curve of the blank copper, anodic and cathodic slopes are of 3157 mV·dec−1 and 3363 mV·dec−1, respectively. In turn, in the Tafel curve of the MMI monolayer-modified copper surface, anodic and cathodic Tafel slopes are 16466 mV·dec−1 and 6314 mV·dec−1, respectively. Higher cathodic Tafel slope of copper electrode with MMI monolayers exhibits the anticorrosive feature of the monolayers. Decrease of both anodic and cathodic current densities indicates that the inhibitor suppresses both anodic and cathodic reactions. The dynamic parameters of the corrosion potential (Ecorr) and the corrosion current (Icorr) along with the inhibition efficiency η31 are tabulated in Table 3. A significant change in the corrosion potential (Ecorr) of the copper electrode in the presence of MMI is observed. The Ecorr values at MMI-modified and blank 3534



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the strong interaction between the molecule and the surface. Therefore, further SERS observations on the adsorption behavior of the MMI monolayers are performed. 3.3. SERS Studies. The normal Raman spectrum of powder MMI and the SERS spectrum of MMI monolayers at the copper electrode are shown in Figure 4a,b, respectively. The MMI vibration calculation result is based on B3LYP/6-311G (see Table 5). In the acronym B3LYP/6-311G in density functional theory (DFT), B3LYP is from the Becke−Lee−Yang− Parr nonlocal gradient correction method, while 6-311G is a type of basis set for first-row atoms calculation. In Figure 4a, the prominent peaks at 406, 529, 690, 912, 1092, 1251, 1276, 1461, and 1572 cm−1 come from the thione form, while only three peaks at 261, 599, and 1336 cm−1 are assigned to the thiol form. Thus, it indicates that the MMI molecules could coexist in the thiol form and thione form in the solid state. In Figure 4b, the observed SERS peaks are due to the thione form of MMI. The bands with medium intensities at 504 cm−1 and 622 cm−1 are attributed to the mixed bands of N2−C1−S6 and N5−C1−S6 bending, and the contribution from ring out-ofplane bending and ring CH out-of-plane bending, respectively. The strong peak at 1329 cm−1 is assigned to the ring CN stretching, ring bending, and ring CH (NH) bending contributions. The band at 951 cm−1 is corresponding to the ring bending, ring CH (NH) bending, and H−C1−S6 bending vibrations. The peaks at 700 and 836 cm−1 can be ascribed to the mixed bands of C1−N2−C7 bending and C3−N2−C7 bending, and ring (CH) out-of-plane bending, respectively. According to the surface selection rule for SERS,40,41 the vibrational modes with vertical polarized components with respect to the surface ought to be enhanced, and the vibrational modes with parallel polarized components with respect to the surface will not be enhanced. Considering that the abovementioned dominative bands in SERS spectrum are correlated with the vibrations regarding S6 and N2 atoms, it suggests that at the copper surface MMI molecules adopt S6 and N2 atoms as the direct adsorptions to form compact monolayers with a tilted orientation, and the adsorption fashion is depicted in Figure 6a. Previously, we found that MMI adsorbed naturally on a silver surface via S6, N2, and N5 atoms.16Hence, the adsorption mode makes the azole ring plane much more perpendicular to the copper surface, comparing to the case of MMI on a silver surface. 3.4. In Situ Electrochemical SERS Studies. In situ electrochemical SERS spectra of the MMI monolayers at the copper surface monitored as the potential applied from 0.0 to −1.6 V are shown in Figure 5. The spectral profile in Figure 5a recorded at the open circuit potential identifies with that in Figure 4b, signaling that the MMI molecules remain the adsorption fashion through the S6 and N2 atoms as the direct adsorption sites. In Figure 5a, all SERS bands of MMI increase as the potential is applied at −0.0 V. Such Raman enhancement is possibly related to the hydrogen evolution reaction (HER), during which the reaction intermediate with a matching energy level with the incident light is generated and the effective charge transfer process could occur.42 When the potential is shifted to −0.1 V, a new band appears at 968 cm−1, which could be assigned to the mixture of ring bending, ring CH (NH) bending, and C1−S6−H bending. So there is a medium state of adsorption fashion, in which the azole ring is further approaching the copper surface in a horizontal mode via S6 and N2 adsorption. When the voltage is applied at −0.3 V, the above-mentioned peak disappears, and

Table 3. Corrosion Potential, Corrosion Current Densities, and Inhibition Efficiencies for the Blank Copper Electrode and the Copper Electrode with MMI Monolayers in 0.1 M KCl Solution a. blank b. with MMI monolayers

Ecorr /mV

Icorr /(mA·cm−2)

η /%

−270 −176

4.888 × 10−2 4.288 × 10−3

91.2

electrodes versus SCE are found to be −176 mV and −270 mV, respectively. The η is defined by 1 − Icorr(b)/Icorr(a), where Icorr(a) and Icorr(b) referring to corrosion current densities in the absence and presence of the MMI monolayers at the copper surface, respectively. From Table 3, it can be found that the η of the modified copper electrode reaches 91.2%, which is in good agreement with the value of ER. It indicates that the MMI monolayer has a protection behavior for copper. We have compared our work with other inhibitors studied as the copper anticorrosive reagents such as benzotriazole (BTAH),32−34 2,5-dimercapto-1,3,4-thiadiazole (DMTD),35 bis-(1-benzotriazolymethylene)-(2,5-thiadiazoly)-disulfide (BBTD),36 tetrazole (T) and 5-amino-tetrazole (5NH2-T),37 imidazole (IM), 4-methylimidazole (4-MI), 1-phenyl-4-methylimidazole (PI),38 and MMI17 as well as purine (PU).39 The comparisons are displayed in Table 4. Table 4. Comparison of the Inhibition Efficiencies of Different Copper Inhibitors inhibitor

concentration

BTAH 1 × 10−4 Ma BTAH 5 × 10−4 Ma BTAH 1 × 10−4−1 × 10−2 Ma DMTD 1 × 10−2 Ma BBTD 1 × 10−3 Ma T 1 × 10−3 Ma 5NH2-T 1 × 10−3 Ma IM 1 × 10−4 Ma 4-MI 1 × 10−4 Ma PI 5 × 10−3 Ma MMI 1 × 10−3 Ma PU 1 × 10−2 Ma MMI 1 × 10−3 Mb

medium

η /%

references

0.1 M KCl 70.4 Cao et al.32 0.1 M NaCl 93.0 Törnkvist et al.33 3% NaCl 75.4−89.3 Finšgar et al.34 0.5 M HCl 84.3 Qin et al.35 3% NaCl 87.6 Zhang et al.36 0.1 M NaCl 53.5 Zucchi et al.37 85.3 3% NaCl 49.8 Otmačić et al.38 63.3 94.3 0.5 M HCl 87.5 Larabi et al.17 1 M NaCl 76.0 Scendo40 0.1 M KCl 91.2 this work

a

The concentration of inhibitor in chloride solution. b The concentration of MMI in pretreatment (self-assembled) solution.

The MMI protection film (inhibition efficiency reaching 91.2%) in KCl solution also seems to have a reasonable anticorrosive capability in Table 4. It should be mentioned that, for protecting the copper, the other copper inhibitors are added into salt or acidic corrosion solutions, and in our work, MMI layers are formed at the copper surface by self-assembly, so the MMI solution could be recycled. Simultaneously, the tendency is to replace the toxic compounds (BTAH) with some environmentally friendly inhibitors such as MMI, which could be expected to play an important role in copper protection in the future. Our group also has examined the anticorrosion behavior of MMI monolayers modified on the silver surface.16 Compared with the MMI coating on a silver surface (the value of η is around 82.6%), MMI coating on the copper surface shows a much better anticorrosion behavior. The highly efficient anticorrosive capability of MMI monolayers at the copper electrode should be attributed to 3535



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Figure 4. Normal Raman spectrum of solid MMI (a), and SERS spectrum of MMI monolayers on the copper electrode (b).

Table 5. Experimental and Theoretical Assignments of the SERS Spectra of MMI According to B3LYP/6-311G Calculated Frequenciesa solid (cm‑1)

SERS (cm‑1)

249m 261m 406s 529s 599m 677w 690vs 735w 758vw 838vw 851vw 912s 1014w 1092m 1148w 1243m 1251s 1276m 1336m 1405w 1447w 1461s 1572s a

calculated −SH

calculated S

230 393 441m 504m 622m 679w 700w 741vw

411 614 670 659 734

836vw

839 868 891

951m 968w 1029w 1091m 1148w 1286w 1329s 1371vs 1414w 1463 m 1529 m

501 625 659 666 709

940 1016 1069 1133 1264

895 911 911 1073 1131 1256 1301

1360 1402 1414 1426 1511 1565

approximate assignment C1S6 str. N2−C7 op.bend; H1−S6 op.bend ring rot.; N2−C1−S6 bend; N5−C−S6 bend ring rot.; N2−C1−S6 bend; N5−C1−S6 bend N2−C1−S6 bend; N5−C1−S6 bend ring op bend; ring CH op bend ring op bend; ring CH (NH) op bend C1−N2−C7 bend;C3−N2−C7 bend ring CH op bend ring CH op bend C1−S6−H ip. bend ring bend; ring CH (NH) bend; H−C1−S6 bend ring bend; ring CH (NH) bend; H−C1−S6 bend ring bend; ring CH(NH) bend; C1−S6−H bend ring bend; ring CH bend; CHMe bend; C1−S6-H bend ring CN str.; ring CH(NH) bend C1S6 str.; ring CH bend ring breathing; C7−N2 str.; ring CH(NH) bend ring CN str.; ring bend; ring CH (NH) bend ring CN str.; ring bend; ring CH bend ring CN str., N2−C7 str.; ring CH bend C1S6 str.; ring CN str.; ring NH bend C1S6 str.; ring CN str.; ring NH bend C3−C4 str.; C1−N2 str.; ring CH bend C3−C4 str.; ring CH(NH) bend

Abbreviations used: w, weak; m, medium; s, strong; vs, very strong; str., stretch; ip, in-plane; op, out-of-plane. Wavenumber is given in cm−1. 3536



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Figure 5. In situ SERS spectra of MMI at the copper electrode surface in 0.1 M KCl solution observed at (a) open circuit potential, (b) −0.0, (c) −0.1, (d) −0.2, (e) −0.3, (f) −0.7, (g) −1.0, (h) −1.5, and (i) −1.6 V vs SCE.

4. CONCLUSION In this paper, the relationship between the adsorption mode of MMI monolayers on the copper surface and its anticorrosion is investigated with electrochemical and SERS techniques. Some conclusions are summarized as follows: 1 A Tafel plot shows that the MMI monolayers at the copper surface can improve the anticorrosive capability of the copper (η around 91.2%). 2 The EIS mechanisms of the blank copper and the copper surface modified by MMI monolayers are fitted with the modes of R(Q(RW))(QR) and R(Q(RW))(CR). 3 Stable MMI monolayers at the copper surface could be formed though the S6 and N2 atoms as direct adsorption sites in a tilted orientation. 4 Through in situ SERS observation of the electrochemical desorption process of MMI monolayers from the copper surface, the existence of a medium adsorption state is found around the potential −0.1 V before its complete desorption.

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Figure 6. A proposed desorption procedure for MMI from the copper surface with the applied potentials.

AUTHOR INFORMATION

Corresponding Author

the adsorption fashion is quiet similar to that at open circuit potential. While the potential is applied at −1.6 V, all peaks are vanished in the spectrum. A scheme for the electrochemical desorption process is shown in Figure 6.

*Corresponding authors: Tel: +86 21 64328981. Fax: +86 21 64322511. E-mail: [email protected] (Y.W.); [email protected] (H.-F.Y.). 3537



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(34) Finšgar, M.; Lesar, A.; Kokalj, A.; Milošev, I. Electrochim. Acta 2008, 53, 8287−8297. (35) Qin, T. T.; Li, J.; Luo, H. Q.; Li, M.; Li, N. B. Corros. Sci. 2011, 53, 1072−1078. (36) Zhang, D.-Q.; Gao, L.-X.; Zhou, G.-D. Appl. Surf. Sci. 2004, 225, 287−293. (37) Zucchi, F.; Trabanelli, G.; Fonsati, M. Corros. Sci. 1996, 38, 2019−2029. (38) Otmačić, H.; Stupnišek-Lisac, E. Electrochim. Acta 2003, 48, 985−991. (39) Scendo, M. Corros. Sci. 2007, 49, 373−390. (40) Moskovits, M. J. Chem. Phys. 1982, 77, 4408−4416. (41) Moskovits, M.; Suh, J. S. J. Phys. Chem. 1984, 88, 5526−5530. (42) Liu, F.-m.; Han, Q.; Chen, Y.-x.; Zhong, Q.-l.; Ren, B.; Tian, Z.-q. Dianhuaxue 2001, 7, 74−77.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21073121), Key Laboratory of Resource Chemistry of Ministry of Education, and the Foundation of Shanghai Normal University (Grant No. DYL701).

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