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Preparation and characterization of triazinedithiol nanofilm on surface of sintered NdFeB permanent magnet.

Abstract In this work, an organic nanofilm was directly prepared on the metal (sintered NdFeB permanent magnet) surface by electrochemical polymerizing of functional triazinedithiol compound 6-(nallyl-l,l,2,2-tetrahydroperfluorodecyl) amino-1,3,5-triazine-2,4-dithiol monosodium. The polymeric nanofilm was characterized by spectroscopic ellipsometry, static water contact angle meter, and atomic force microscopy. The chemical progress of electrochemical polymerization was studied by cyclic voltammetry. The film formation mechanism was investigated via X-ray photoelectron spectroscopy and Fourier transform infrared reflection spectroscopy and the results showed that the thiol groups play an important role in polymerization of the film on the substrate. This research may provide a basis for fabricating a triazinedithiol film on metal surfaces and have a promising future for expanding the application of depositing polymer films on metals.

Keywords Electrochemical polymerization, Triazinedithiol nanofilm, Hydrophobicity, XPS

Introduction

Organic thin films directly adhering to metallic substrates have attracted great attention due to many desirable properties, such as flexibility, no defects, proper thickness, lightweight, and low cost. (1-6) Recently, most studies have been focused on two polymeric films, i.e., Langmuir-Blodgett (LB) films and self-assembled monolayers (SAMs). (7,8) However, LB films attach to the substrate via weak ionic force and van der Waals force; (9) the strength is too low for the adhesion between the film and substrate. On the other hand, most SAMs solutions are poisonous. The solutes such as fluoroalkyl silane, octadecyltrichlorosilane (OTS), and the solvents like toluene, hexane, or hexadecane are all volatile liquids and harmful. (10,11)

As a simple, convenient, and relatively inexpensive method to prepare polymers film, electrochemical polymerization has attracted much attention and has been applied in various research. (12,13) Recently, 6substituted-1,3,5-triazine-2,4-dithiol (triazinedithiols), which has a thermally stable triazine ring and two highly reactive thiol groups, has been receiving enormous attention since the publication of Mori's research on the anticorrosion property of triazinedithiols on copper surface. (14) After that, other applications of triazinedithiols polymeric films have been deeply studied on various metal substrates, (15-19) such as adhesion, lubrication property, dielectric property, and superhydrophobicity. However, previous reports concentrated on the functional performances of the films, while chemical process and formation mechanism of triazinedithiols films have rarely been reported. Therefore, investigating the chemical process and film formation mechanism of triazinedithiols films deposited on metals surface is very important for expanding the application of triazinedithiols films.

In this work, a triazinedithiol compound 6-(n-allyl-1,1,2,2-tetrahydroperfluorodecyl) amino-1,3,5-triazine2,4-dithiol monosodium (ATP) was electrochemically polymerized on the surface of sintered NdFeB permanent magnet by cyclic voltammetry (CV). The NdFeB was used as the metal model to study the reaction between thiols group and metals. The chemical reactions on the surface during the electrochemical process and the formation mechanism for the triazinedithiol film were characterized by the variations on the surface. This research may provide a basis for fabricating a triazinedithiol film on metal surfaces and have a promising future for expanding the application of polymer films on metals.

Experimental

Materials

The composition of NdFeB magnet plate was Fe30.5% Nd-1% B-0.5% Dy-0.4% A1 (wt%, N42, Innuovo) in a demagnetized state (size: 50 mm x 30 mm x 4 mm). The substrates were polished to mirror with a series of emery papers, and followed by degreasing thorough ultrasonic in acetone, and then dried in cold air. The triazinedithiol compound ATP was prepared by reaction between 6-(N-allyl-l,1,2,2tetrahydroperfluorodecyl)-amine-l,3,5-triazine-2,4-dichloride and NaSH, according to the method described in a previous paper, (20) and the chemical structure is shown in Fig. 1. Distilled water and diiodomethane (purity 99%, Alfa Aesar) were used for the measurement of contact angles. All other reagents were analytical grade and used without any further treatment. The electrochemical polymerization solution was prepared by dissolving ATP monomer in sodium carbonate ([Na.sub.2]C[O.sub.3]) aqueous solution. [Na.sub.2]C[O.sub.3] was used as the supporting electrolyte. The concentrations of ATP and [Na.sub.2]C[O.sub.3] were 1 mmol/L and 0.1 mol/L, respectively.

Electrochemical polymerization

Electrochemical polymerization was performed on the electrochemical workstation (IM6ex, Zahner) via CV with three scanning cycles. A typical three-electrode system was used in the electrolytic cell, where NdFeB plate, saturated calomel electrode (SCE), and two stainless steel (SUS304) plates were used as the working electrode, the reference electrode, and the counter electrode, respectively. The initial potential for the first cycle was the open circuit potential and the terminal potential was assigned variously. The potentials for the second and third cycles were performed between zero and the assigned terminal potential. All potentials presented were referenced to SCE and the temperature was controlled at 20 ± 2°C. After polymerization process, the working electrode was immediately removed from the electrolyte and rinsed with distilled water, ethanol, and then dried in cold air. The prepared film was short for ATP film.

Measurements

Static contact angles were measured by an optical contact angle meter (OCA35, Dataphysics) at room temperature. The liquid droplet size used for the measurements was 1.0 µL. The results reported the average values of three different samples. Spectroscopic ellipsometer (UVISEL-NIR, Horiba JobinYvon) was used to measure the film thickness. Surface morphology was observed by atomic force microscope (AFM, Nanoscope III Scanning Probe Microscope, Veeco Metrology Group) in contact mode. X-ray photoelectron spectroscopy (XPS, AXIS Ultra, Kratos) was carried out using a monochromatic Mg Ka. X-ray source (1253.6 eV), and the binding energy was calibrated by the C Is peak energy (284.6 eV). Fourier transform infrared reflection spectroscopy (FTIR, IR Prestige-21, Shimadzu) was used to determine the chemical functional groups.

Results and discussion

Electrochemical reactions during the polymerized process

In order to estimate the effect of the assigned terminal potential of CV on the preparation of ATP film, the variation of contact angles of distilled water and film thickness was measured for evaluating the process of polymerization, and the results are shown in Fig. 2. It can be clearly seen that the contact angles increased from 103.5° for 1.2 V to 135.9° for 1.6 V. However, the film thickness had a different trend, which first increased from 21.11 nm for 1.2 V to 27.56 nm for 1.5 V, and then the film thickness decreased to 23.68 nm for 1.6 V. After polymerization, no matter what the terminal potential was, the contact angles were all over 90°, while the contact angle for the substrate was 43.9°. The results indicated that after electrochemical process, the surfaces were modified by polymerization of ATP. In addition, the variation of contact angles and film thickness revealed that the terminal potential of CV had a great effect on the process of polymerization. When the terminal potential was set at 1.2 V, the polymerization of ATP film was not sufficient, and the contact angle was lower than that under a higher terminal potential. When the terminal potential was assigned to 1.6 V, the film degraded under such a high potential, and the film thickness was decreased compared with that of 1.5 V. Additionally, the anodic oxidation on the working electrode became violent and a little iron rust appeared on the surface under such a high potential.

Based on Wenzel and Cassie law, (21,22) wettability is related to both the roughness structure and the surface free energy (SFE) of the materials. After electrochemical process, the surface was coated by ATP film, which contained the low SFE functional groups -CF3,-CF2, and -C[H.sub.2], leading to the enhancement of the hydrophobicity on the surface. However, when the terminal potential was set at 1.6 V, the high potential led to strongly anodic oxidation, which had an effect on fabricating structures on surface. (23) As a result, the contact angle was the largest. In addition, the high potential led to degradation of the polymerized film, which was faster than the growth rate of the film, and the results led to the decrease of the film thickness. Based upon the analysis of contact angle and film thickness, the terminal potential of CV for polymerization could be neither too low nor too high, and the terminal potential 1.5 V was the best parameter. After determining the terminal potential for polymerization, the electrochemical process was carried out in 0.1 mol/L [Na.sub.2]C[O.sub.3] aqueous solution with or without ATP monomer for further analysis of the electrochemical reaction.

Figure 3a shows the CV curve in [Na.sub.2]C[O.sub.3] aqueous solution without ATP. It is clearly found that point C (0.85 V) divides the curve into two obviously different parts. The part in the lower potential seems flat and the other part in the higher potential rises steeply. It is worth mentioning that the part in the lower potential is not as flat as it looks. In the magnified picture, an oxidation peak between point A and point C can be clearly observed and that is attributed to the oxidation reaction of NdFeB substrate. The main oxidation product is [Fe.su.2+24] and the electrochemical process is displayed in reaction (1). For the steep part of the curve, a further oxidation product [Fe.sup.3+] appears which is associated with the further anodic process, and the oxidation process is displayed in reaction (2). The result is consistent with the findings of Sang et al. (25) that point C is a thermodynamic critical point. If the potential is lower than the critical point, there is not enough energy for the further oxidation. Additionally, when the potential increased to 1.5 V (point B), a few bubbles were observed due to hydrolysis under a high potential. Reaction (3) presents the anodic reactions near point B.

Fe ? [Fe.sup.2]+ + [2.sub.e.sup.-] (1)

Fe ? [Fe.sup.3+] + [3.sub.e.sup.-] (2)

4O[H.sup.-] ? 2[H.sub.2]O + [O.sub.2] + [4.sub.e.sup.-]. (3)

Figure 3b shows the CV curve of NdFeB substrate polarized in 0.1 mol/L [Na.sub.2]C[O.sub.3] aqueous solutions containing 1 mmol/L ATP. Compared to Fig. 3a, the significant difference is the peak marked as point D in Fig. 3b, which is assigned to the polymerization peak of ATP monomer. Moreover, the average current densities are apparently lower than that displayed in Fig. 3a, which is due to the insulation of the deposited ATP film. During the electrochemical process, the phenomenon of bubbles was not observed on the surface demonstrating that the film had been formed and the reaction (3) became weak in the solution containing ATP. The formation of film also can be confirmed by the curve of the second and third cycle. The insulation increased with the growth of the film when the cycle number increased, which led to the current densities gradually decreasing, while the current densities were approximately the same for all cycles shown in Fig. 3a. The obvious differences between Figs. 3a and 3b after point C show that electrochemical polymerization process becomes the major reaction in the solutions containing ATP while the anodic oxidation reaction is the major reaction in the solutions without ATP. The process of polymerization reaction is shown in Fig. 8.

Characterization of ATP film

Figure 4 shows the 3D AFM morphologies of the substrate and the film. The roughness of NdFeB substrate and ATP film over a 2 µm x 2 µm scan area is 0.611 and 3.835 nm, respectively. The surface of NdFeB substrate (Fig. 4a) is nearly flat with a few small particles scattered on it. The small particles may originate from the manufacture of the metal and are difficult to eliminate, which can be clearly seen in the cross-sectional diagram (Fig. 4b) where a few convex parts are distributed on the flat substrate. Figure 4c shows the 3D AFM morphology for the surface of ATP film. Many "rising and falling" nano-clusters are deposited on the surface and distributed in an orderly direction. The ultra-thin polymeric film comprised of these nano-clusters makes the 3D AFM image very different from the flat look of NdFeB substrate. The variation of the surface morphology after electrochemical polymerization process also demonstrates the successful formation of ATP film.

Surface free energy was calculated by OWRK algorithm (26) with two different liquids, distilled water and diiodomethane. Contact angles of distilled water on NdFeB substrate and ATP film surfaces were 43.9° and 122.5°, respectively (Fig. 5). The SFE was composed of dispersion and polar components. The SFE of the substrate was 59.24 mJ [m.sup.-2], while the dispersion and polar components were 37.21 and 22.03, respectively. The SFE of ATP film was 6.91 mJ[ m.sup.-2], while the dispersion and polar components were 6.67 and 0.24, respectively. Generally, the dispersion component corresponds to the structure of the surface and the polar component corresponds to the composition of the surface. After electrochemical polymerization, the relative dispersion component was greatly enhanced from 37.21/59.24 to 6.67/6.91 with the increase in the roughness of the surface. Additionally, with the groups -C[H.sub.2], -[CF.sub.2], and -[CF.sub.3] covered on the surface, the relative polar component was greatly decreased since these groups were nonpolar groups and could lower the SFE greatly. Flerein, the surface was transformed from hydrophilicity to hydrophobicity after modification by ATP film.

Formation mechanism of ATP film

FTIR spectroscopy and XPS were utilized to analyze the chemical structure and the formation mechanism of the deposited ATP film. Figure 6 shows FTIR spectrum of the sample modified by ATP film. Absorption peaks at 1562 and 1485 [cm.sup.-1] are assigned to C=N groups originating from the triazine ring. (27) Those at 1100-1400 [cm.sup.-1] are related to C-F bonds. (28) Peaks at 1253 and 1161 [cm.sup.-1] are attributed to asymmetric and symmetric stretching vibrations of -[CF.sub.3] groups, and the peaks located at 1219 and 1334 [cm.sup.-1] are related to symmetric stretching vibrations and asymmetric stretching vibrations of -[CF.sub.2] groups, respectively. The FTIR spectrum analysis suggests that ATP monomers were successfully deposited on the surface of NdFeB substrate.

The formation of ATP film was further researched by XPS spectra, and the atomic concentration of the relevant elements is summarized in Table 1. The difference in atomic concentrations between the substrate and the film is clearly seen. C, O, Fe, Nd, and B were the main compositions for NdFeB substrate. After polymerization process, the concentration of all the original elements changed to a great extent, and some other elements, such as fluorine (F), sulfur (S), and nitrogen (N) contained in ATP monomers existed. Based on the results, it can be concluded that ATP monomers were deposited onto the NdFeB substrate surface.

Figure 7a shows the high-resolution spectra of S 2p and F Is after polymerization of ATP. Two different atom species (S 2p3/2 and S 2pl/2) are demonstrated in high-resolution XPS spectra of S 2p, with a 2:1 area ratio and a splitting binding energy of 1.2 eV.293n In the present work, S 2p curve was fitted as three doublets structure with the bound thiol groups at the lowest binding energy as illustrated. The binding energy of S 2p3/2 peak located at 162.05 eV (S2pl/2 at 163.25 eV)is assigned to the sulfur atoms bonded to the substrate surface as a thiolate species (S-Metal bonds, mainly refers to S-Fe bonds), and the result is in agreement with the previous works researched by several scholars. (31,32) The S 2p3/2 peak at 162.65 eV (S 2pl/2 at 163.85 eV) is attributed to C-S-C bonds formed during polymerization of ATP monomer. The S 2p3/2 peak at 163.25 eV (S 2pl/2 at 164.45 e V) is related to S-S bonds formed by the mutual reaction of thiol groups. The F Is spectrum shows two main peaks exhibited in Fig. 7b. Referring to the binding energy sequence of -[CF.sub.3] and -[CF.sub.2] groups, the peak with binding energy at 687.07 eV is due to the -[CF.sub.2] groups and the other peak located at 687.90 eV is assigned to -[CF.sub.3] groups.

Based on the chemical composition changes on the surface after the electrochemical process, the film formation mechanism is concluded and is exhibited in Fig. 8. ATP monomers were ionized as dithiolate anions in Na2C[O.sub.3] aqueous solution, and then the dithiolate anions reacted with the substrate to form S-Metal (S-Fe) bonds and caused coupling reaction to form S-S bonds under the effect of electrochemical action. The S-S bonds were linked mutually and formed a reticular structure on the substrate during the process, and the first layer was coated on the NdFeB substrate in this method. As the electrochemical polymerization process continued, the dissociated dithiolate anions reacted with the terminal alkenyl groups on the deposited layer to form C-S-C bonds. The film grew thicker by the continual formation of C-S-C bonds. (27)

Conclusions

In this investigation, a hydrophobic organic nanofilm was successfully deposited on sintered NdFeB substrate surface by electrochemical polymerization of functional triazinedithiol compound ATP. Due to the low SFE of the functional groups and a larger rough structure on the modified surface, the SFE was decreased from 59.24 mJ [m.sup.-2] of NdFeB substrate to 6.67 mJ [m.sup.-2] of ATP film, and the wettability of the sintered NdFeB was transformed from hydrophilic to hydrophobic.

The film formation mechanism was analyzed via CV, FTIR, and XPS. The results show that ATP monomers were combined with the substrate by forming S-Metal bonds, and ATP monomers were linked to each other by the formation of S-S bonds. After the formation of S-Metal bonds and S-S bonds, a reticular structure film was successfully deposited on NdFeB substrate surface, and then the film grew thicker via the formation of CS-C bonds. This research may provide a basis for fabricating a triazinedithiol film on metal surfaces and a new approach to expand the application of depositing polymer films on metals.

DOI 10.1007/s111998-015-9706-z

Q. Liu, T. Zhao, Z. Kang (mail) School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China e-mail: zxkang@scut.edu.cn

Acknowledgments This work is supported by the National Natural Science Foundation of China (No.51075f51), Key Program of Guangdong Natural Science Foundation (No.10251064101000001), and Science and Technology Research Program of Guangzhou (No. 201510010155).

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Table 1: XPS atomic concentrations of
NdFeB substrate and ATP film (at.%)

Sample C 1s 0 1s Fe 2p Nd 3d B 1s N 1s S 2p F 1s

NdFeB substrate 29.61 44.42 17.00 8.25 0.72 -- -- --
ATP film 13.80 2.50 2.87 2.72 0.13 6.98 7.61 63.39


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