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Electrochemical and anti-corrosion properties of octadecanethiol and benzotriazole binary self-assembled monolayers on copper
Electrochimica Acta 220 (2016) 245–251
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical and anti-corrosion properties of octadecanethiol and benzotriazole binary self-assembled monolayers on copper Wenji Yanga,b , Tianqi Lia,b , Haihui Zhoua,b,* , Zheng Huanga,b , Chaopeng Fua,b , Liang Chena,b , Mengbo Lia,b , Yafei Kuanga,b,* a b
State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha, 410082, PR China College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China
A R T I C L E I N F O
Article history: Received 17 June 2016 Received in revised form 14 October 2016 Accepted 18 October 2016 Available online 18 October 2016 Keywords: Self-assembled monolayers(SAMs) Corrosion inhibition Octadecanethiol Benzotriazole Synergetic effect
A B S T R A C T
Self-assembled monolayers based on octadecanethiol (C18H37-SH) and benzotriazole (BTA) (C18H37SH&BTA SAMs) were fabricated on copper surface, showing effective corrosion protection for the substrate. The self-assembled monolayers (containing the composite SAMs and the SAMs based on single C18H37-SH or BTA) were characterized by surface enhanced Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and the results show that both C18H37-SH and BTA are successfully self-assembled on copper surface, forming compact SAMs. The anti-corrosion performance of the SAMs was studied by potentiodynamic polarization and electrochemical impedance spectroscopy. It is revealed that the anti-corrosion performance of the binary (C18H37-SH and BTA) composite SAMs is influenced by the assembling sequence. The C18H37-SH&BTA SAMs display the best anti-corrosion performance for copper with the highest charge transfer resistance of 5.0105 V cm2. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Copper (Cu) and its alloys, as a kind of structural and functional materials, are widely used in industrial and artistic manufacturing due to their excellent electrical and thermal conductivities, mechanical workability and shining appearance [1]. However, these Cu-based materials are often prone to corrosion in some media, such as atmosphere or aqueous solutions with high ionic concentration, especially chlorine [2]. When being exposed to these severe circumstances, copper will be corroded, which will deteriorate the performance and appearance of copper-based materials, it is therefore of great significance to seek for effective, low-cost and environmentally friendly techniques to prevent copper corrosion. With regard to the protection of copper and its alloys, the generally employed method is to isolate the materials from the surrounding environment, including electroplating [3], coating [4–6], conversion films and employment of adsorptive corrosion inhibitor [7–12]. Among them, adsorptive corrosion inhibitor, namely self-assembled monolayers (SAMs) inhibitor [13–18], can effectively protect copper from corrosion by forming compact
* Corresponding author. E-mail addresses: [email protected] (H. Zhou), [email protected] (Y. Kuang). http://dx.doi.org/10.1016/j.electacta.2016.10.123 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
layers on copper surface. This method has many advantages, such as fast film-forming and high efficiency. Some organics such as alkylthiol [19], sulfoure, phenylamine, aldehyde and some heterocyclic compounds containing S, N, P atoms have been developed as adsorptive corrosion inhibitors [20–25]. Laibinis et al. firstly reported the good performance of alkylthiol for copper protection under atmosphere condition [26]. Petrovi c et al. also demonstrated the protective ability of alkylthiol SAMs for copper in a sodium acetate solution [27]. Meanwhile, Wang et al. studied carbazole and N-vinylcarbazole adsorption films for protecting copper in a NaCl solution [28]. On the other hand, benzotriazole (BTA), a representative heterocyclic compound, possesses outstanding corrosion inhibition for copper, because it can not only inhibit the corrosion of copper, but also stabilize the copper ion in the media to prevent its deposition on copper [29]. Recently, Qin et al. reported effective protection for fresh copper by 2,5-Dimercapto-1,3,4-thiadiazole (DMTD) SAMs [30]. Further, Chen et al. found that 5-Mercapto-3-phenyl-1,3,4-thiadiazole-2thione potassium (MPTT) could be adsorbed on copper surface and formed a hydrophobic film to protect the substrate [31]. Liao et al. reported that ammonium pyrrolidine dithiocarbamate SAMs could be formed on copper surface to achieve a good corrosion inhibition performance [32]. Previous studies demonstrated that the corrosion inhibition ability is enhanced when heterocycle and sulfydryl groups coexist in a molecule due to the synergistic effect on
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corrosion inhibition. Therefore, most of current studies focus on using single compound containing both heterocycle and sulfydryl groups for copper corrosion protection. However, on one hand, those compounds containing both heterocycle and sulfydryl groups normally involve in complex synthesis and high cost, on the other hand, organic solvents are needed to dissolve the compounds for the self-assembling process, which leads to higher cost and is more toxic to environment than using aqueous media. To the best of our knowledge, there are few reports investigating on using separate heterocycle and sulfydryl compounds to form binary SAMs on copper for protecting it from corrosion. In this work, we choose octadecanethiol (C18H37-SH) containing sulfydryl group and benzotriazole (BTA) containing heterocycle group to form binary SAMs to protect copper from corrosion. Aqueous media are used for the self-assembling process to avoid usage of organic solvents. The results show that the binary SAMs are superior to the single SAMs based on C18H37-SH or BTA for corrosion protection of copper due to the synergistic effect between sulfydryl and heterocyclic compounds. 2. Experimental 2.1. Chemicals All the chemicals are analytical grade and purchased from Adamas Reagent Co. Ltd and Alfa Aesar Co. Ltd. Deionized water was used as the solvent of all the aqueous solutions. 2.2. Preparation of the working solutions The formula of various working solutions (A: C18H37-SH working solution, B: BTA working solution, C: C18H37-SH&BTA working solution) are listed in Table 1. A surfactant was added to assist the dispersion of C18H37-SH due to the limited solubility of C18H37-SH in water.
2.4. Structure and composition characterization of the SAMs Surface-enhanced Raman spectroscopy (SERS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structure and composition of the SAMs formed on copper specimens. FTIR spectrometer was in reflection mode with a spectral resolution of 2 cm1, and the angle between the horizontally polarized incident light and the normal direction of specimen was 80 . Raman measurement was performed by using the laser confocal microRaman spectrometer system RamLab-010 (Jobin Yvon) with a HeNe Lasers beam of 632 nm operated at 5 mW and exposing time of 20 s. XPS analysis was conducted on an ESCALAB250 XPS spectrometer with an Mg Ka X-ray source (1350 eV). 2.5. Electrochemical performance of the SAMs Electrochemical measurements were performed in a 3.5 wt% NaCl solution with a three-electrode system, which consisted of an exposed working electrode of 1 cm2, a saturated calomel reference electrode (SCE) and a platinum counter electrode, on an electrochemical workstation (Model: CHI 660C). The tip of the reference electrode was positioned at a very close proximity to the working electrode surface in order to minimize ohmic potential drop. The electrochemical impedance spectroscopy (EIS) measurement was carried out at the open circuit potential over a frequency range from 0.01 Hz to 100 kHz with a sinusoidal potential perturbation of 5 mV in amplitude. The impedance data were analyzed with the ZSimpWin software. All the potential values in this paper are relative to SCE. The polarization curves were obtained independently for the anodic and cathodic curves from the corrosion potential with a scan rate of 1 mVs1 (a minimum of six separate scans was performed to avoid occasionality). 3. Results and discussion
2.3. Preparation of the specimens
3.1. Structure and composition of the various SAMs
Firstly, copper (99.9%) sheets were ultrasonically washed in deionized water for three minutes to clean the surface. Then the specimens were immersed in a mixed alkali (NaOH 60 g L1, Na2CO3 30 g L1, Na3PO4 20 g L1) solution at 65 C for 3 5 minutes to remove the oil stains on the surface. The specimens were then dipped in a polishing solution (H3PO4 540 mL L1, CH3COOH 300 mL L1, HNO3 100 mL L1) at 85 C for 30 seconds. Lastly, the specimens were activated in a dilute sulfuric acid solution, then thoroughly rinsed in deionized water and dried in a stream of cold air. The SAMs was obtained by dipping copper specimens into the working solutions at 25 C for approximately five minutes. The BTA/C18H37-SH SAMs were acquired by dipping copper in working solution B firstly, and then in working solution A. The C18H37-SH/ BTA SAMs were obtained by dipping copper in working solution A first and then in working solution B. The C18H37-SH&BTA SAMs were obtained by dipping copper into working solution C. The BTA SAMs and C18H37-SH SAMs were obtained by immersing the copper specimens in B and A working solutions, respectively.
The chemical structure of the various SAMs on copper was investigated by SERS and FTIR. Fig. 1 shows the SERS spectra of three SAMs on copper substrate, and Table S1 summarizes the SERS data. Four main peaks located at 632/782 cm1, 1391 cm1 and 1596 cm1 are observed in BTA SAMs (Fig. 1), which are assigned to the triazole ring vibrating, triazole ring and benzene stretching
Table 1 The formula of various working solution.
A: C18H37-SH working solution B: BTA working solution C: C18H37-SH&BTA working solution
C18H37-SH/g L1
BTA/g L1
2 / 2
/ 2 2
Fig. 1. SERS spectra of the C18H37-SH&BTA SAMs, C18H37-SH SAMs and BTA SAMs on copper surface.
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modes [33], respectively, indicating BTA molecules were adsorbed on the copper substrate. Additionally, the intensity emerged at 1099 cm1 for BTA SAMs is assigned to the N-H stretching mode. The decreased intensity at 1099 cm1 compared with that of the BTA powder (Fig. S1a) is due to the break of some N-H bond, indicating that the adsorption of BTA on copper is achieved by bonding the N atom from triazol with copper atom. Likewise, three main peaks located at 734 cm1 and 1131/1297 cm1 observed in C18H37-SH SAMs are assigned to the C18H37-SH triazole ring vibrating and CH2 rocking modes [34,35], respectively, indicating C18H37-SH molecules were also adsorbed on copper substrate. Furthermore, the peak located at 1063 cm1 for C18H37-SH SAMs, with decreased intensity compared with that of the C18H37-SH powder (Fig. S2b) is assigned to the S-H stretching mode. This indicates that the adsorption of C18H37-SH on copper is induced by deprotonation covalent bond adsorption of SH group. Importantly, the main characteristic peaks of the C18H37-SH SAMs and BTA SAMs are observed in C18H37-SH&BTA binary composite SAMs, manifesting both C18H37-SH and BTA molecules were selfassembled on the copper substrate simultaneously. The FTIR spectra of three SAMs on copper substrate are shown in Fig. 2. In the FTIR spectrum of C18H37-SH SAMs, two weak broad peaks appear at 2964 cm1 and 2875 cm1, corresponding to the asymmetric and symmetric stretching modes of CH3. Other two main peaks are also observed at 2919 cm1 and 2850 cm1, corresponding to the asymmetric and symmetric stretching modes of CH2 [36]. The results show that C18H37-SH molecules have successfully self-assembled on the copper surface. For the BTA SAMs, a broad peak is observed in region of 2800 3000 cm1, corresponding to the stretching vibration of triazol [37], displaying BTA molecules were also assembled on copper substrate. Likewise, the FTIR spectrum of C18H37-SH&BTA SAMs exhibits not only the characteristic peaks of C18H37-SH but also that of BTA, in good agreement with the SERS result, further confirming that C18H37-SH and BTA can be assembled together on the copper surface. Fig. S2a shows the XPS survey spectrum of the BTA SAM, and C, O and N are observed. The corresponding C, O and N contents in the SAMs are 72.1 at%, 8.9 at% and 19.0 at%, respectively, demonstrating the presence of BTA molecules on the copper surface. The details scan of N 1s (shown in Fig. S2b) is resolved into four peaks with binding energies of 399.3 eV, 399.8 eV, 400.2 eV and 400.8 eV, which are assigned to the aromatic N, NH, N-H and C-N-Cu bonds respectively [12,38,39]. The presence of C-N-Cu bond reveals that the BTA SAMs was generated by the covalent bond and coordination bond formed between aromatic N atoms with copper.
Accordingly, the C, O and S peaks are observed in the full-range XPS survey spectrum of the C18H37-SH SAMs, and their corresponding contents (shown in Fig. S2c) are 82.6 at%, 13.9 at% and 3.5 at%, respectively, demonstrating the presence of C18H37-SH molecules on the copper surface. Fig. S2d shows the high resolution S 2p spectrum, which is resolved to four peaks associated with Cu2-S (162.25 eV), Cu-S (162.5 eV), C-S (163.7 eV) and S-H (163.8 eV) species, respectively [40–42]. Therefore, the self-assembly mechanism of the C18H37-SH SAMs on the copper surface is proposed as follow: Cu + C18H37-SH ! Cu-S-C18H37 + 1/2 H2
(1)
2Cu + C18H37-SH ! Cu2-S-C18H37 + 1/2 H2
(2)
The C18H37-SH molecules are self-assembled onto the copper surface due to the strong coordination between S and Cu, resulting in forming Cu-S and Cu2-S species, as shown in equation (1) and (2). Furthermore, we speculate the self-assembled process of C18H37-SH SAMs via two steps, firstly the C18H37-SH molecules are adsorbed on the copper surface by Langmuir adsorption explained in the SI, and then the tiled molecules realign with the proceeding of the adsorption process, thus the SAMs with a high density are obtained along the normal direction of the surface [26]. Fig. 3 shows the XPS result of the C18H37-SH&BTA SAMs. The XPS survey spectrum shows the presence of C, O, S and N elements, and the elemental analysis reveals that the contents of the corresponding four elements are 83.3 at %, 8.0 at % 3.9 at %, and 4.8 at % (Fig. 3a), respectively. Therefore, the BTA molecules and C18H37-SH molecules exist simultaneously in the C18H37-SH&BTA SAMs. Interestingly, every BTA molecule contains three N atoms, while C18H37-SH molecule contains one S atom, but the S content is similar to that of N content in the C18H37-SH&BTA SAMs, suggesting more C18H37-SH molecules are assembled on copper surface due to the stronger adsorption capacity of C18H37-SH. The high resolution S 2p spectrum and N 1s spectrum are also resolved into their corresponding species peaks (Fig. 3b and c), respectively. Particularly, the peaks assign to S-C and S-Cu bonds shift slightly, which may be caused by the interaction between C18H37-SH and BTA molecules. 3.2. Electrochemical performance 3.2.1. Potentiodynamic polarization The anti-corrosion performances of various SAMs were firstly investigated by potentiodynamic polarization curves. As seen in Fig. 4, the self-corrosion potentials of the copper specimens covered with three types of SAMs (BTA SAMs, C18H37-SH SAMs and C18H37-SH&BTA SAMs) are more negative than that of the bare copper, indicating these three kinds of SAMs were providing protection to the underlying copper surface. Additionally, the anodic and cathodic current densities of the copper covered with C18H37-SH&BTA SAMs obviously are the smallest. Table S2 shows self-corrosion potential (Ecorr) and self-corrosion current density (jcorr) of each system measured by potentiodynamic polarization curves. Obviously, the copper covered with binary C18H37-SH&BTA SAMs revealed the sharp decrease in current density, manifesting the best anti-corrosion performance. This trend is typified by Fig. 4c. Synergistic parameter (S) used to further investigate the synergistic inhibition of the two inhibitors can be calculated using the following equation [43,44]: S¼
Fig. 2. FTIR spectra of the C18H37-SH&BTA SAMs, C18H37-SH SAMs and BTA SAMs on copper surface.
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jcorr1 jcorr2 jcorr1;2 jo
ð3Þ
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Fig. 3. XPS survey spectrum (a), high resolution S 2p spectrum (b), and high resolution N 1s spectrum (c) of C18H37-SH&BTA SAMs.
Fig. 4. Cathodic polarization curves (a) and anodic polarization curves (b) of the copper electrode covered with C18H37-SH&BTA SAMs, C18H37-SH SAMs, BTA SAMs and the bare copper electrode tested in a 3.5 wt.% NaCl solution, and Ecorr versus jcorr for them (c) with respective shifts.
Where jcorr1 is the corrosion current density of the copper with BTA SAMs, jcorr2 is the corrosion current density of the copper with C18H37-SH SAMs, jcorr1,2 is the corrosion current density of the copper with C18H37-SH&BTA SAMs, and j0 is the corrosion current density of the bare copper. As known that S > 1 indicates synergistic interaction between the two inhibitors, while S < 1 indicates competitive adsorption between the two inhibitors, and S 1 indicates no interaction between inhibitors. In our case, the synergistic parameter of the binary SAMs is 12, suggesting very good synergistic effect of the inhibition performance between BTA and C18H37-SH. 3.2.2. Electrochemical Impedance Spectroscopy EIS was employed to further investigate kinetics characteristics of the three types of SAMs. Fig. 5 shows the Nyquist plots of the
Fig. 5. Nyquist plots of the bare copper, the BTA SAMs, the C18H37-SH SAMs and the C18H37-SH&BTA binary composite SAMs covered copper electrodes in a 3.5 wt.% NaCl solution.
bare copper and the copper specimens modified with C18H37-SH SAMs, BTA SAMs and C18H37-SH&BTA SAMs. Clearly, all the Nyquist plots show a semicircle at high frequency region, corresponding to the charge transfer resistance at the electrode/solution interface. It is noticed that the Nyquist plot of the bare copper contains a sloping line at low frequency region, which is related to the Warburg impedance. All the EIS data are simulated by equivalent circuits for the bare copper electrode and copper electrode modified with SAMs (shown in Fig. S3). In the equivalent circuits, Rs, Rt and Rsam represent solution resistance, charge transfer resistance at the copper interface, and the transfer resistance when electrons pass through SAMs, respectively, whereas CPE (constant phase element) stands for the double layer capacitance, and W refers to the Warburg impedance. Because of the larger value of Rsam than Rt, the equivalent circuit in Fig. S3b is simplified to that in Fig. S3c. The calculated parameter values from the EIS are listed in Table S3. Compared with the resistance of the bare copper, there is a sharp increase in Rt for the copper specimens modified with SAMs, indicating the copper assembled with SAMs can effectively provide obstacles for the corrosion of copper. The C18H37-SH&BTA binary composite SAMs shows the largest charge transfer resistance value of 5.0105 V cm2, confirming the anti-corrosion performance of the binary SAMs is much better than that of the single SAMs, and indicating the excellent synergetic anti-corrosion effect between C18H37-SH and BTA. 3.2.3. Effect of assembling sequence on corrosion inhibition performance The effect of assembled sequence on the binary SAMs was also investigated. Fig. S4 shows the XPS results of three types of the binary SAMs. Obviously all XPS survey spectra show the presence of C, O, S and N. In addition, the high resolution S 2p and N 1s spectra of the SAMs also display the existence of Cu-S and Cu-N bonds, demonstrating that the molecules can be adsorbed on copper regardless of the assembling sequence. Interestingly, the atom ratio of S to N in C18H37-SH/BTA is 2.6, which is larger than that in BTA/C18H37-SH (1.2), certifying that the former molecules assembled on the copper surface can act as a barrier for the later molecules due to the competitive adsorption between them.
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Fig. 6. Nyquist plots of the C18H37-SH&BTA, C18H37-SH/BTA and BTA/C18H37-SH SAMs covered on copper electrodes in a 3.5 wt.% NaCl solution.
Furthermore, the total contents of S and N atoms in C18H37-SH&BTA was 8.7 at %, which is larger than that in C18H37-SH/BTA (4.7 at %) and BTA/C18H37-SH (3.5 at %), demonstrating that C18H37-SH&BTA SAMs possessed the highest coverage on copper because of synergistic adsorption between the BTA and C18H37-SH molecules. Therefore, both competitive effect and synergistic effect exist between C18H37-SH and BTA molecules. Fig. 6 shows the EIS of the bare copper and copper modified with BTA/C18H37-SH SAMs, C18H37-SH/BTA SAMs and C18H37-SH&BTA SAMs. All the Nyquist plots only show a semicircle at high frequency region, corresponding to the charge transfer resistance at the
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electrode/solution interface, which may be attributed to the presence of SAMs. Similarly, all EIS data were also simulated by using the equivalent circuits (shown in Fig. S3), and the calculated parameter values from the EIS data are listed inTable S4. Compared to the values of the bare copper, there is a sharp increase in Rt for the copper covered with three types of binary SAMs, indicating the three types of binary SAMs possess desirable anti-corrosion performance for copper. Interestingly, the charge transfer resistance for BTA/C18H37-SH SAMs is 2.5104 V Cm2, which is much smaller than 2.0105 V Cm2 for C18H37-SH SAMs (Table S3). This can be explained that C18H37-SH molecules are hindered to be adsorbed onto the copper substrate by the adsorbed BTA molecules. Therefore, the inhibition performance follow the order of C18H37SH&BTA > C18H37-SH/BTA > BTA/C18H37-SH. The structure analysis results (Fig. S3) show that there is not only competition but also synergistic effect between C18H37-SH and BTA molecules during the self-assembling process. Therefore, the former assembled molecules can impede the subsequent assembling in a two-step assembling process, and one-step assembling process can improve the coverage of binary SAMs. As a result, the C18H37-SH&BTA shows the best corrosion inhibition performance. 3.2.4. Stability test The stability of the various SAMs was investigated by immersing the copper covered with SAMs in a NaCl solution with a concentration of 3.5 wt% for 15 days. Fig. 7 shows the SEM images of copper with various SAMs after the stability testing. The SEM image of bare copper shows a plenty of non-uniform etching pits on the surface (Fig. 7a), while the copper covered with single SAMs shows a few shallow etching pits on the surface (Fig. 7b and c). In contrast, the copper assembled with binary C18H37-SH&BTA SAMs exhibits sheeny and smooth surface without etching pits (Fig. 7a). These results indicate the good corrosion inhibition stability of C18H37-SH&BTA SAMs.
Fig. 7. SEM images of the bare copper (a), the BTA SAMs (b), the C18H37-SH SAMs (c), the C18H37-SH&BTA SAMs (d) immersed respectively in a 3.5 wt.% NaCl solution for 15 days at 25 C.
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Fig. 8. FTIR spectra of the C18H37-SH&BTA compounded SAMs before and after corrosion.
To further explain the good corrosion inhibition stability of C18H37-SH&BTA SAMs, Fig. 8 shows the FTIR spectrum of the copper with C18H37-SH&BTA SAMs after being immersed in the 3.5 wt. % NaCl solution for 15 days. The symmetrical stretching vibration of CH2 turns weaker while the asymmetric stretching vibration becomes stronger, displaying that the inclination of C18H37-SH molecules becomes lager, but the structure of alkyl chain do not change. Additionally, there is little change in the vibration peaks of CH3, which is attributed to the improved tightness structure of C18H37-SH molecules under the cooperation of BTA molecules. Therefore, the C18H37-SH&BTA SAMs shows good long-term stability in the corrosive medium. 3.3. Adsorption mechanism of the binary SAMs of C18H37-SH and BTA Previously, Brusic et al. demonstrated that BTA molecules were self-assembled to form amorphous films on copper by covalent bond and coordination bond [45]. In addition, C18H37-SH molecules interact with copper substrate to form SAMs, which is connected by Cu-S covalent bond. Because of the individual characters of different molecules, they are assembled along the path with the least resistance when they are adsorbed on substrate surface. The XPS result shows that C18H37-SH and BTA molecules interact with copper by Cu-S and C-N-Cu bond, respectively (Fig. S2). Because C18H37-SH molecules bond with copper by sole S atom, and the long-linked structure is erect on the copper surface, while BTA
molecules lie on the substrate. Through the discussion above, it is deduced that the C18H37-SH SAMs shows better inhibition performance than the BTA SAMs, and the binary SAMs display the improved inhibition performance. Moreover, when they are adsorbed together, the amount of the adsorbed C18H37-SH molecules is larger. It indicates that there are competitive and synergistic effects between the C18H37-SH and BTA molecules during the self-assembling process, and the C18H37-SH molecules possess high binding rate. Therefore, they are self-assembled in their own ways due to the different active centers and diverse adsorption sites of the two molecules. The interspaces between the assembled C18H37-SH molecules are filled by the BTA, and meanwhile the interspaces between the assembled BTA molecules are also filled by the C18H37-SH, resulting in the formation of more compact and integrated SAMs. If arranging the adsorbing order of the two types of molecules, the former molecules will impede the path of the later ones, and in this way, they cannot reach the appropriate active sites. It also demonstrates that the molecules connect with copper surface by deprotonation covalence adsorption. Therefore, we speculate that the two kinds of molecules will make up for each other when they work on the copper together. Fig. 9 shows the scheme diagram of the self-assembled process of BTA SAMs (a), C18H37-SH SAMs (b), C18H37-SH&BTA SAMs (c) onto the copper substrates. 4. Conclusions In summary, a simple and facile synthesis approach to generate organic binary composite SAMs with high anticorrosion performance on the copper surface has been developed. The C18H37-SH&BTA binary composite SAMs exhibit the best performance for corrosion protection of copper. SERS, FTIR and XPS data indicate that C18H37-SH and BTA molecules are assembled on copper surface. The potentiodynamic polarization and EIS manifest that the binary C18H37-SH&BTA SAMs provide more effective corrosion protection of copper compared to sole molecule SAMs. Further, the performance of the binary composite SAMs is influenced by the assembling sequence. Both C18H37SH and BTA molecules are adsorbed on the copper surface to form binary composite SAMs with high corrosion inhibition performance and high stability. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 51672075, 21271069, J1210040, 51238002), Science and Technology Program of Hunan Province (Grant No. 2015JC3049), and the Fundamental Research Funds for the Central Universities (No. 531107040898).
Fig. 9. The scheme diagram of the self-assembling process of BTA SAMs (a), C18H37-SH SAMs (b), C18H37-SH&BTA SAMs (c) onto the copper substrates.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.123.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical and anti-corrosion properties of octadecanethiol and benzotriazole binary self-assembled monolayers on copper Wenji Yanga,b , Tianqi Lia,b , Haihui Zhoua,b,* , Zheng Huanga,b , Chaopeng Fua,b , Liang Chena,b , Mengbo Lia,b , Yafei Kuanga,b,* a b
State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha, 410082, PR China College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China
A R T I C L E I N F O
Article history: Received 17 June 2016 Received in revised form 14 October 2016 Accepted 18 October 2016 Available online 18 October 2016 Keywords: Self-assembled monolayers(SAMs) Corrosion inhibition Octadecanethiol Benzotriazole Synergetic effect
A B S T R A C T
Self-assembled monolayers based on octadecanethiol (C18H37-SH) and benzotriazole (BTA) (C18H37SH&BTA SAMs) were fabricated on copper surface, showing effective corrosion protection for the substrate. The self-assembled monolayers (containing the composite SAMs and the SAMs based on single C18H37-SH or BTA) were characterized by surface enhanced Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and the results show that both C18H37-SH and BTA are successfully self-assembled on copper surface, forming compact SAMs. The anti-corrosion performance of the SAMs was studied by potentiodynamic polarization and electrochemical impedance spectroscopy. It is revealed that the anti-corrosion performance of the binary (C18H37-SH and BTA) composite SAMs is influenced by the assembling sequence. The C18H37-SH&BTA SAMs display the best anti-corrosion performance for copper with the highest charge transfer resistance of 5.0105 V cm2. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Copper (Cu) and its alloys, as a kind of structural and functional materials, are widely used in industrial and artistic manufacturing due to their excellent electrical and thermal conductivities, mechanical workability and shining appearance [1]. However, these Cu-based materials are often prone to corrosion in some media, such as atmosphere or aqueous solutions with high ionic concentration, especially chlorine [2]. When being exposed to these severe circumstances, copper will be corroded, which will deteriorate the performance and appearance of copper-based materials, it is therefore of great significance to seek for effective, low-cost and environmentally friendly techniques to prevent copper corrosion. With regard to the protection of copper and its alloys, the generally employed method is to isolate the materials from the surrounding environment, including electroplating [3], coating [4–6], conversion films and employment of adsorptive corrosion inhibitor [7–12]. Among them, adsorptive corrosion inhibitor, namely self-assembled monolayers (SAMs) inhibitor [13–18], can effectively protect copper from corrosion by forming compact
* Corresponding author. E-mail addresses: [email protected] (H. Zhou), [email protected] (Y. Kuang). http://dx.doi.org/10.1016/j.electacta.2016.10.123 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
layers on copper surface. This method has many advantages, such as fast film-forming and high efficiency. Some organics such as alkylthiol [19], sulfoure, phenylamine, aldehyde and some heterocyclic compounds containing S, N, P atoms have been developed as adsorptive corrosion inhibitors [20–25]. Laibinis et al. firstly reported the good performance of alkylthiol for copper protection under atmosphere condition [26]. Petrovi c et al. also demonstrated the protective ability of alkylthiol SAMs for copper in a sodium acetate solution [27]. Meanwhile, Wang et al. studied carbazole and N-vinylcarbazole adsorption films for protecting copper in a NaCl solution [28]. On the other hand, benzotriazole (BTA), a representative heterocyclic compound, possesses outstanding corrosion inhibition for copper, because it can not only inhibit the corrosion of copper, but also stabilize the copper ion in the media to prevent its deposition on copper [29]. Recently, Qin et al. reported effective protection for fresh copper by 2,5-Dimercapto-1,3,4-thiadiazole (DMTD) SAMs [30]. Further, Chen et al. found that 5-Mercapto-3-phenyl-1,3,4-thiadiazole-2thione potassium (MPTT) could be adsorbed on copper surface and formed a hydrophobic film to protect the substrate [31]. Liao et al. reported that ammonium pyrrolidine dithiocarbamate SAMs could be formed on copper surface to achieve a good corrosion inhibition performance [32]. Previous studies demonstrated that the corrosion inhibition ability is enhanced when heterocycle and sulfydryl groups coexist in a molecule due to the synergistic effect on
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corrosion inhibition. Therefore, most of current studies focus on using single compound containing both heterocycle and sulfydryl groups for copper corrosion protection. However, on one hand, those compounds containing both heterocycle and sulfydryl groups normally involve in complex synthesis and high cost, on the other hand, organic solvents are needed to dissolve the compounds for the self-assembling process, which leads to higher cost and is more toxic to environment than using aqueous media. To the best of our knowledge, there are few reports investigating on using separate heterocycle and sulfydryl compounds to form binary SAMs on copper for protecting it from corrosion. In this work, we choose octadecanethiol (C18H37-SH) containing sulfydryl group and benzotriazole (BTA) containing heterocycle group to form binary SAMs to protect copper from corrosion. Aqueous media are used for the self-assembling process to avoid usage of organic solvents. The results show that the binary SAMs are superior to the single SAMs based on C18H37-SH or BTA for corrosion protection of copper due to the synergistic effect between sulfydryl and heterocyclic compounds. 2. Experimental 2.1. Chemicals All the chemicals are analytical grade and purchased from Adamas Reagent Co. Ltd and Alfa Aesar Co. Ltd. Deionized water was used as the solvent of all the aqueous solutions. 2.2. Preparation of the working solutions The formula of various working solutions (A: C18H37-SH working solution, B: BTA working solution, C: C18H37-SH&BTA working solution) are listed in Table 1. A surfactant was added to assist the dispersion of C18H37-SH due to the limited solubility of C18H37-SH in water.
2.4. Structure and composition characterization of the SAMs Surface-enhanced Raman spectroscopy (SERS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structure and composition of the SAMs formed on copper specimens. FTIR spectrometer was in reflection mode with a spectral resolution of 2 cm1, and the angle between the horizontally polarized incident light and the normal direction of specimen was 80 . Raman measurement was performed by using the laser confocal microRaman spectrometer system RamLab-010 (Jobin Yvon) with a HeNe Lasers beam of 632 nm operated at 5 mW and exposing time of 20 s. XPS analysis was conducted on an ESCALAB250 XPS spectrometer with an Mg Ka X-ray source (1350 eV). 2.5. Electrochemical performance of the SAMs Electrochemical measurements were performed in a 3.5 wt% NaCl solution with a three-electrode system, which consisted of an exposed working electrode of 1 cm2, a saturated calomel reference electrode (SCE) and a platinum counter electrode, on an electrochemical workstation (Model: CHI 660C). The tip of the reference electrode was positioned at a very close proximity to the working electrode surface in order to minimize ohmic potential drop. The electrochemical impedance spectroscopy (EIS) measurement was carried out at the open circuit potential over a frequency range from 0.01 Hz to 100 kHz with a sinusoidal potential perturbation of 5 mV in amplitude. The impedance data were analyzed with the ZSimpWin software. All the potential values in this paper are relative to SCE. The polarization curves were obtained independently for the anodic and cathodic curves from the corrosion potential with a scan rate of 1 mVs1 (a minimum of six separate scans was performed to avoid occasionality). 3. Results and discussion
2.3. Preparation of the specimens
3.1. Structure and composition of the various SAMs
Firstly, copper (99.9%) sheets were ultrasonically washed in deionized water for three minutes to clean the surface. Then the specimens were immersed in a mixed alkali (NaOH 60 g L1, Na2CO3 30 g L1, Na3PO4 20 g L1) solution at 65 C for 3 5 minutes to remove the oil stains on the surface. The specimens were then dipped in a polishing solution (H3PO4 540 mL L1, CH3COOH 300 mL L1, HNO3 100 mL L1) at 85 C for 30 seconds. Lastly, the specimens were activated in a dilute sulfuric acid solution, then thoroughly rinsed in deionized water and dried in a stream of cold air. The SAMs was obtained by dipping copper specimens into the working solutions at 25 C for approximately five minutes. The BTA/C18H37-SH SAMs were acquired by dipping copper in working solution B firstly, and then in working solution A. The C18H37-SH/ BTA SAMs were obtained by dipping copper in working solution A first and then in working solution B. The C18H37-SH&BTA SAMs were obtained by dipping copper into working solution C. The BTA SAMs and C18H37-SH SAMs were obtained by immersing the copper specimens in B and A working solutions, respectively.
The chemical structure of the various SAMs on copper was investigated by SERS and FTIR. Fig. 1 shows the SERS spectra of three SAMs on copper substrate, and Table S1 summarizes the SERS data. Four main peaks located at 632/782 cm1, 1391 cm1 and 1596 cm1 are observed in BTA SAMs (Fig. 1), which are assigned to the triazole ring vibrating, triazole ring and benzene stretching
Table 1 The formula of various working solution.
A: C18H37-SH working solution B: BTA working solution C: C18H37-SH&BTA working solution
C18H37-SH/g L1
BTA/g L1
2 / 2
/ 2 2
Fig. 1. SERS spectra of the C18H37-SH&BTA SAMs, C18H37-SH SAMs and BTA SAMs on copper surface.
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modes [33], respectively, indicating BTA molecules were adsorbed on the copper substrate. Additionally, the intensity emerged at 1099 cm1 for BTA SAMs is assigned to the N-H stretching mode. The decreased intensity at 1099 cm1 compared with that of the BTA powder (Fig. S1a) is due to the break of some N-H bond, indicating that the adsorption of BTA on copper is achieved by bonding the N atom from triazol with copper atom. Likewise, three main peaks located at 734 cm1 and 1131/1297 cm1 observed in C18H37-SH SAMs are assigned to the C18H37-SH triazole ring vibrating and CH2 rocking modes [34,35], respectively, indicating C18H37-SH molecules were also adsorbed on copper substrate. Furthermore, the peak located at 1063 cm1 for C18H37-SH SAMs, with decreased intensity compared with that of the C18H37-SH powder (Fig. S2b) is assigned to the S-H stretching mode. This indicates that the adsorption of C18H37-SH on copper is induced by deprotonation covalent bond adsorption of SH group. Importantly, the main characteristic peaks of the C18H37-SH SAMs and BTA SAMs are observed in C18H37-SH&BTA binary composite SAMs, manifesting both C18H37-SH and BTA molecules were selfassembled on the copper substrate simultaneously. The FTIR spectra of three SAMs on copper substrate are shown in Fig. 2. In the FTIR spectrum of C18H37-SH SAMs, two weak broad peaks appear at 2964 cm1 and 2875 cm1, corresponding to the asymmetric and symmetric stretching modes of CH3. Other two main peaks are also observed at 2919 cm1 and 2850 cm1, corresponding to the asymmetric and symmetric stretching modes of CH2 [36]. The results show that C18H37-SH molecules have successfully self-assembled on the copper surface. For the BTA SAMs, a broad peak is observed in region of 2800 3000 cm1, corresponding to the stretching vibration of triazol [37], displaying BTA molecules were also assembled on copper substrate. Likewise, the FTIR spectrum of C18H37-SH&BTA SAMs exhibits not only the characteristic peaks of C18H37-SH but also that of BTA, in good agreement with the SERS result, further confirming that C18H37-SH and BTA can be assembled together on the copper surface. Fig. S2a shows the XPS survey spectrum of the BTA SAM, and C, O and N are observed. The corresponding C, O and N contents in the SAMs are 72.1 at%, 8.9 at% and 19.0 at%, respectively, demonstrating the presence of BTA molecules on the copper surface. The details scan of N 1s (shown in Fig. S2b) is resolved into four peaks with binding energies of 399.3 eV, 399.8 eV, 400.2 eV and 400.8 eV, which are assigned to the aromatic N, NH, N-H and C-N-Cu bonds respectively [12,38,39]. The presence of C-N-Cu bond reveals that the BTA SAMs was generated by the covalent bond and coordination bond formed between aromatic N atoms with copper.
Accordingly, the C, O and S peaks are observed in the full-range XPS survey spectrum of the C18H37-SH SAMs, and their corresponding contents (shown in Fig. S2c) are 82.6 at%, 13.9 at% and 3.5 at%, respectively, demonstrating the presence of C18H37-SH molecules on the copper surface. Fig. S2d shows the high resolution S 2p spectrum, which is resolved to four peaks associated with Cu2-S (162.25 eV), Cu-S (162.5 eV), C-S (163.7 eV) and S-H (163.8 eV) species, respectively [40–42]. Therefore, the self-assembly mechanism of the C18H37-SH SAMs on the copper surface is proposed as follow: Cu + C18H37-SH ! Cu-S-C18H37 + 1/2 H2
(1)
2Cu + C18H37-SH ! Cu2-S-C18H37 + 1/2 H2
(2)
The C18H37-SH molecules are self-assembled onto the copper surface due to the strong coordination between S and Cu, resulting in forming Cu-S and Cu2-S species, as shown in equation (1) and (2). Furthermore, we speculate the self-assembled process of C18H37-SH SAMs via two steps, firstly the C18H37-SH molecules are adsorbed on the copper surface by Langmuir adsorption explained in the SI, and then the tiled molecules realign with the proceeding of the adsorption process, thus the SAMs with a high density are obtained along the normal direction of the surface [26]. Fig. 3 shows the XPS result of the C18H37-SH&BTA SAMs. The XPS survey spectrum shows the presence of C, O, S and N elements, and the elemental analysis reveals that the contents of the corresponding four elements are 83.3 at %, 8.0 at % 3.9 at %, and 4.8 at % (Fig. 3a), respectively. Therefore, the BTA molecules and C18H37-SH molecules exist simultaneously in the C18H37-SH&BTA SAMs. Interestingly, every BTA molecule contains three N atoms, while C18H37-SH molecule contains one S atom, but the S content is similar to that of N content in the C18H37-SH&BTA SAMs, suggesting more C18H37-SH molecules are assembled on copper surface due to the stronger adsorption capacity of C18H37-SH. The high resolution S 2p spectrum and N 1s spectrum are also resolved into their corresponding species peaks (Fig. 3b and c), respectively. Particularly, the peaks assign to S-C and S-Cu bonds shift slightly, which may be caused by the interaction between C18H37-SH and BTA molecules. 3.2. Electrochemical performance 3.2.1. Potentiodynamic polarization The anti-corrosion performances of various SAMs were firstly investigated by potentiodynamic polarization curves. As seen in Fig. 4, the self-corrosion potentials of the copper specimens covered with three types of SAMs (BTA SAMs, C18H37-SH SAMs and C18H37-SH&BTA SAMs) are more negative than that of the bare copper, indicating these three kinds of SAMs were providing protection to the underlying copper surface. Additionally, the anodic and cathodic current densities of the copper covered with C18H37-SH&BTA SAMs obviously are the smallest. Table S2 shows self-corrosion potential (Ecorr) and self-corrosion current density (jcorr) of each system measured by potentiodynamic polarization curves. Obviously, the copper covered with binary C18H37-SH&BTA SAMs revealed the sharp decrease in current density, manifesting the best anti-corrosion performance. This trend is typified by Fig. 4c. Synergistic parameter (S) used to further investigate the synergistic inhibition of the two inhibitors can be calculated using the following equation [43,44]: S¼
Fig. 2. FTIR spectra of the C18H37-SH&BTA SAMs, C18H37-SH SAMs and BTA SAMs on copper surface.
247
jcorr1 jcorr2 jcorr1;2 jo
ð3Þ
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Fig. 3. XPS survey spectrum (a), high resolution S 2p spectrum (b), and high resolution N 1s spectrum (c) of C18H37-SH&BTA SAMs.
Fig. 4. Cathodic polarization curves (a) and anodic polarization curves (b) of the copper electrode covered with C18H37-SH&BTA SAMs, C18H37-SH SAMs, BTA SAMs and the bare copper electrode tested in a 3.5 wt.% NaCl solution, and Ecorr versus jcorr for them (c) with respective shifts.
Where jcorr1 is the corrosion current density of the copper with BTA SAMs, jcorr2 is the corrosion current density of the copper with C18H37-SH SAMs, jcorr1,2 is the corrosion current density of the copper with C18H37-SH&BTA SAMs, and j0 is the corrosion current density of the bare copper. As known that S > 1 indicates synergistic interaction between the two inhibitors, while S < 1 indicates competitive adsorption between the two inhibitors, and S 1 indicates no interaction between inhibitors. In our case, the synergistic parameter of the binary SAMs is 12, suggesting very good synergistic effect of the inhibition performance between BTA and C18H37-SH. 3.2.2. Electrochemical Impedance Spectroscopy EIS was employed to further investigate kinetics characteristics of the three types of SAMs. Fig. 5 shows the Nyquist plots of the
Fig. 5. Nyquist plots of the bare copper, the BTA SAMs, the C18H37-SH SAMs and the C18H37-SH&BTA binary composite SAMs covered copper electrodes in a 3.5 wt.% NaCl solution.
bare copper and the copper specimens modified with C18H37-SH SAMs, BTA SAMs and C18H37-SH&BTA SAMs. Clearly, all the Nyquist plots show a semicircle at high frequency region, corresponding to the charge transfer resistance at the electrode/solution interface. It is noticed that the Nyquist plot of the bare copper contains a sloping line at low frequency region, which is related to the Warburg impedance. All the EIS data are simulated by equivalent circuits for the bare copper electrode and copper electrode modified with SAMs (shown in Fig. S3). In the equivalent circuits, Rs, Rt and Rsam represent solution resistance, charge transfer resistance at the copper interface, and the transfer resistance when electrons pass through SAMs, respectively, whereas CPE (constant phase element) stands for the double layer capacitance, and W refers to the Warburg impedance. Because of the larger value of Rsam than Rt, the equivalent circuit in Fig. S3b is simplified to that in Fig. S3c. The calculated parameter values from the EIS are listed in Table S3. Compared with the resistance of the bare copper, there is a sharp increase in Rt for the copper specimens modified with SAMs, indicating the copper assembled with SAMs can effectively provide obstacles for the corrosion of copper. The C18H37-SH&BTA binary composite SAMs shows the largest charge transfer resistance value of 5.0105 V cm2, confirming the anti-corrosion performance of the binary SAMs is much better than that of the single SAMs, and indicating the excellent synergetic anti-corrosion effect between C18H37-SH and BTA. 3.2.3. Effect of assembling sequence on corrosion inhibition performance The effect of assembled sequence on the binary SAMs was also investigated. Fig. S4 shows the XPS results of three types of the binary SAMs. Obviously all XPS survey spectra show the presence of C, O, S and N. In addition, the high resolution S 2p and N 1s spectra of the SAMs also display the existence of Cu-S and Cu-N bonds, demonstrating that the molecules can be adsorbed on copper regardless of the assembling sequence. Interestingly, the atom ratio of S to N in C18H37-SH/BTA is 2.6, which is larger than that in BTA/C18H37-SH (1.2), certifying that the former molecules assembled on the copper surface can act as a barrier for the later molecules due to the competitive adsorption between them.
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Fig. 6. Nyquist plots of the C18H37-SH&BTA, C18H37-SH/BTA and BTA/C18H37-SH SAMs covered on copper electrodes in a 3.5 wt.% NaCl solution.
Furthermore, the total contents of S and N atoms in C18H37-SH&BTA was 8.7 at %, which is larger than that in C18H37-SH/BTA (4.7 at %) and BTA/C18H37-SH (3.5 at %), demonstrating that C18H37-SH&BTA SAMs possessed the highest coverage on copper because of synergistic adsorption between the BTA and C18H37-SH molecules. Therefore, both competitive effect and synergistic effect exist between C18H37-SH and BTA molecules. Fig. 6 shows the EIS of the bare copper and copper modified with BTA/C18H37-SH SAMs, C18H37-SH/BTA SAMs and C18H37-SH&BTA SAMs. All the Nyquist plots only show a semicircle at high frequency region, corresponding to the charge transfer resistance at the
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electrode/solution interface, which may be attributed to the presence of SAMs. Similarly, all EIS data were also simulated by using the equivalent circuits (shown in Fig. S3), and the calculated parameter values from the EIS data are listed inTable S4. Compared to the values of the bare copper, there is a sharp increase in Rt for the copper covered with three types of binary SAMs, indicating the three types of binary SAMs possess desirable anti-corrosion performance for copper. Interestingly, the charge transfer resistance for BTA/C18H37-SH SAMs is 2.5104 V Cm2, which is much smaller than 2.0105 V Cm2 for C18H37-SH SAMs (Table S3). This can be explained that C18H37-SH molecules are hindered to be adsorbed onto the copper substrate by the adsorbed BTA molecules. Therefore, the inhibition performance follow the order of C18H37SH&BTA > C18H37-SH/BTA > BTA/C18H37-SH. The structure analysis results (Fig. S3) show that there is not only competition but also synergistic effect between C18H37-SH and BTA molecules during the self-assembling process. Therefore, the former assembled molecules can impede the subsequent assembling in a two-step assembling process, and one-step assembling process can improve the coverage of binary SAMs. As a result, the C18H37-SH&BTA shows the best corrosion inhibition performance. 3.2.4. Stability test The stability of the various SAMs was investigated by immersing the copper covered with SAMs in a NaCl solution with a concentration of 3.5 wt% for 15 days. Fig. 7 shows the SEM images of copper with various SAMs after the stability testing. The SEM image of bare copper shows a plenty of non-uniform etching pits on the surface (Fig. 7a), while the copper covered with single SAMs shows a few shallow etching pits on the surface (Fig. 7b and c). In contrast, the copper assembled with binary C18H37-SH&BTA SAMs exhibits sheeny and smooth surface without etching pits (Fig. 7a). These results indicate the good corrosion inhibition stability of C18H37-SH&BTA SAMs.
Fig. 7. SEM images of the bare copper (a), the BTA SAMs (b), the C18H37-SH SAMs (c), the C18H37-SH&BTA SAMs (d) immersed respectively in a 3.5 wt.% NaCl solution for 15 days at 25 C.
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Fig. 8. FTIR spectra of the C18H37-SH&BTA compounded SAMs before and after corrosion.
To further explain the good corrosion inhibition stability of C18H37-SH&BTA SAMs, Fig. 8 shows the FTIR spectrum of the copper with C18H37-SH&BTA SAMs after being immersed in the 3.5 wt. % NaCl solution for 15 days. The symmetrical stretching vibration of CH2 turns weaker while the asymmetric stretching vibration becomes stronger, displaying that the inclination of C18H37-SH molecules becomes lager, but the structure of alkyl chain do not change. Additionally, there is little change in the vibration peaks of CH3, which is attributed to the improved tightness structure of C18H37-SH molecules under the cooperation of BTA molecules. Therefore, the C18H37-SH&BTA SAMs shows good long-term stability in the corrosive medium. 3.3. Adsorption mechanism of the binary SAMs of C18H37-SH and BTA Previously, Brusic et al. demonstrated that BTA molecules were self-assembled to form amorphous films on copper by covalent bond and coordination bond [45]. In addition, C18H37-SH molecules interact with copper substrate to form SAMs, which is connected by Cu-S covalent bond. Because of the individual characters of different molecules, they are assembled along the path with the least resistance when they are adsorbed on substrate surface. The XPS result shows that C18H37-SH and BTA molecules interact with copper by Cu-S and C-N-Cu bond, respectively (Fig. S2). Because C18H37-SH molecules bond with copper by sole S atom, and the long-linked structure is erect on the copper surface, while BTA
molecules lie on the substrate. Through the discussion above, it is deduced that the C18H37-SH SAMs shows better inhibition performance than the BTA SAMs, and the binary SAMs display the improved inhibition performance. Moreover, when they are adsorbed together, the amount of the adsorbed C18H37-SH molecules is larger. It indicates that there are competitive and synergistic effects between the C18H37-SH and BTA molecules during the self-assembling process, and the C18H37-SH molecules possess high binding rate. Therefore, they are self-assembled in their own ways due to the different active centers and diverse adsorption sites of the two molecules. The interspaces between the assembled C18H37-SH molecules are filled by the BTA, and meanwhile the interspaces between the assembled BTA molecules are also filled by the C18H37-SH, resulting in the formation of more compact and integrated SAMs. If arranging the adsorbing order of the two types of molecules, the former molecules will impede the path of the later ones, and in this way, they cannot reach the appropriate active sites. It also demonstrates that the molecules connect with copper surface by deprotonation covalence adsorption. Therefore, we speculate that the two kinds of molecules will make up for each other when they work on the copper together. Fig. 9 shows the scheme diagram of the self-assembled process of BTA SAMs (a), C18H37-SH SAMs (b), C18H37-SH&BTA SAMs (c) onto the copper substrates. 4. Conclusions In summary, a simple and facile synthesis approach to generate organic binary composite SAMs with high anticorrosion performance on the copper surface has been developed. The C18H37-SH&BTA binary composite SAMs exhibit the best performance for corrosion protection of copper. SERS, FTIR and XPS data indicate that C18H37-SH and BTA molecules are assembled on copper surface. The potentiodynamic polarization and EIS manifest that the binary C18H37-SH&BTA SAMs provide more effective corrosion protection of copper compared to sole molecule SAMs. Further, the performance of the binary composite SAMs is influenced by the assembling sequence. Both C18H37SH and BTA molecules are adsorbed on the copper surface to form binary composite SAMs with high corrosion inhibition performance and high stability. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 51672075, 21271069, J1210040, 51238002), Science and Technology Program of Hunan Province (Grant No. 2015JC3049), and the Fundamental Research Funds for the Central Universities (No. 531107040898).
Fig. 9. The scheme diagram of the self-assembling process of BTA SAMs (a), C18H37-SH SAMs (b), C18H37-SH&BTA SAMs (c) onto the copper substrates.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.123.
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