Cationic Reduced Graphene Oxide as Self-Aligned Nanofiller in the

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Surfaces, Interfaces, and Applications

Cationic Reduced Graphene Oxide as Self-aligned Nanofiller in the Epoxy Nanocomposited Coating with Excellent Anticorrosive Performance and Its High Antibacterial Activity Xiaohu Luo, Jiawen Zhong, Qiulan Zhou, Shuo Du, Song Yuan, and Yali Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01982 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

Cationic Reduced Graphene Oxide as Self-aligned Nanofiller in the Epoxy Nanocomposited Coating with Excellent Anticorrosive Performance and Its High Antibacterial Activity Xiaohu Luo,a,b Jiawen Zhong,a Qiulan Zhou,a Shuo Du,a Song Yuan a and Yali Liu a* a

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and

Chemical Engineering, Hunan University, Changsha, Hunan 410082, People’s Republic of China b

School of Chemistry and Chemical engineering, Qiannan Normal University for

Nationalities, Duyun, Guizhou 558000, People’s Republic of China

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Abstract The design and preparation of an excellent corrosion protection coating is still a grand challenge and is essential for large-scale practical application. Herein, a novel cationic reduced graphene oxide (denoted as RGO-ID+) based epoxy coating was fabricated for corrosion protection. RGO-ID+ was synthesized by in situ synthesis and salification reaction which is stable dispersion in water and epoxy latex, and the self-aligned RGO-ID+ reinforced cathodic electrophoretic epoxy nanocomposited coating (denoted as RGO-ID+ coating) at the surface of metal was prepared by electrodeposition. The self-alignment of RGO-ID+ in the coatings is mainly attributed to the electric field force. The significantly enhanced anticorrosion performance of RGO-ID+ coating is proved by a series of electrochemical measurements in different concentrated NaCl solutions and salt spray tests. This superior anticorrosion property benefits from the self-aligned RGO-ID+ nanosheets and the quaternary-N groups presented in the RGO-ID+ in the RGO-ID+ nanocomposited coating. Interestingly, the RGO-ID+ also exhibits a high antibacterial activity toward Escherichia coli (E. coli) with 83.4 ± 1.3% of the antibacterial efficiency, which is attributed to the synergetic effects of RGO-ID+ and the electrostatic attraction and hydrogen bonding between RGO-ID+ and the E. coli. This work offers new insights for the successful development of effective corrosion protection and self-antibacterial coatings. Keywords: cationic reduced graphene oxide, electrodeposition, self-alignment, corrosion protection, self-antibacterial

1. INTRODUCTION Corrosion of metal surface, especially cars, ships, high-speed rails and other metal equipment has been one of the most imperative problems in metal protection, since it causes the loss of billions of dollars every year.1-4 The corrosion of metal often occurs at the metal-electrolyte solution interface, which represents an electrochemical process. The most effective protection methods include coating technology and surface treatment. Among them, organic coating (i.e., polyurethane and epoxy) has been widely utilized to prevent the metal from corrosion, due to its good physical barrier. However, it shows more or less permeable to


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the corrosive species (i.e., H2O, O2, and Cl-).5-9 Therefore, an enormous amount of research effort have been direct toward the improvement of the anticorrosion performance of the organic coating. Recently, legislative restriction on volatile organic compound (VOC) emission has led to the development for waterborne and ecofriendly barrier coatings.10-11 Cathodic epoxy electrophoretic latex using water as solvent has become the mainstream automotive coating technology due to excellent adhesion, chemical resistance, green, saving time, etc. Addition of the layered impermeable nanofillers (i.e., graphene, clay, and boronnitride) into the organic coating represents an effective approach to improve the impermeability of organic coating to enhance corrosion protection.12-16 The barrier performance of organic coating is mainly determined by the following factors: (1) the layered nanofiller properties, such as the aspect ratio, the inhibition of penetrant diffusion for corrosive electrolytes and the appropriate volume fraction; (2) the good dispersion of layered nanofillers in polymer matrix; (3) the inherent barrier performance of organic coating. For the layered nanofillers, the levels of exfoliation is crux of successful development of the nanocomposited coating.17 As well known, Clay (i.e., organo-montmorillonite, delaminated montmorillonite), as a layered nanofiller, has been widely used in a variety of organic coatings.18-23 But the van de Waals forces at the interface of clays significantly weaken the stability of clays in polymer matrix, leading to its incompletable exfoliation and poor dispersion in the coatings.18-19, 22 Graphene, as a two-dimensional carbon material, exhibits unique physical properties and geometry, such as high aspect ratio, high thermal and electrical conductivities, and mechanical strength.24-27 Moreover, graphene shows the higher aspect ratio than that of clay. The monolayer graphene is around 0.34 nm of thickness, but its diameter is up to several hundred microns. These outstanding properties have attracted research on the possible application in the anticorrosive coating.28-30 The graphene film prepared by chemical vapor deposition shows effective corrosion protection at the copper surface.24 Alex Zettl et al. found that graphene-based films could act as a good phyical barrier to molecules (i.e., water, nitrogen and oxygen).25 The incorporation of the graphene-based materials into the polymer matrix is another efficient method to enhance the anticorrosion performance of the nanocomposited coating, which benefits from the physical barrier property of graphene. However, it is found ACS Paragon Plus Environment


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that graphene or graphene oxide nanosheets incorporated into the coatings are randomly distributed.28-31 Moreover, because of strong van der Waals forces and high specific ratio, grpahene or graphene oxide is easily tended to aggregate each other, greatly limiting their practerial application.32-34 Therefore, how to fabricate dispersible graphene in large scale is a prerequisite for the practical application, particularly, dispersion in aqueous solution. To obtain uniform graphene or graphene oxide dispersion in polymer matrix, ultrasonication, using of surfactant, in situ reduction, and chemical modification have been applied.35-40 Among these approaches, chemical modification of graphene or graphene oxide can provide chemical affinity and a variety of functionalitie.41-46 For example, the introduction of quaternary ammonium salt groups onto the surface of RGO uses their hydrophilicity to improve the dispersion in aqueous solution or aqueous latex. Very recently, some reports show that graphene or GO can inhibit the growth of Esherichia coli (E. coli).47-49 Additionally, some residual oxygen-containing functional groups presented on the surface of RGO are easily modified by other molecules to endow the specific functions, such as antibacterial activity, anticorrosion and so on. Some recent reports show that the quaternary ammonium and amino groups with positive charge can enhance the antibacterial activity by an electrostatic interaction.50-51 However, up to now, the synergetic effects of graphene and the quaternary ammonium and amino groups for antibacterial activity have rarely been reported. Herein, we provide a simple and eco-friendly approach for fabrication of a nanocomposited coating with superior corrosion protection performance by incorporating a cationic reduced graphene

oxide (RGO-ID+).

RGO-ID+ is successfully synthesized

by selectively

functionalizing the surface of reduced graphene oxide with isophorone diisocyanate (IPDI) and N,N-dimethylethanolamine (DMEA) using the two-step reaction (i.e., in situ synthesis and salification reaction). RGO-ID+ nanosheets can be steadily dispersed in aqueous solution and are used as a nanofiller to the aqueous cationic epoxy latex. The epoxy latex

coating

with aligned RGO-ID+ at the metal surface is fabricated by electrodeposition. The anticorrosion property of the nanocomposited coating is significantly improved due to the self-aligned RGO-ID+ nanosheets. Meanwhile, the RGO-ID+ nanosheets exhibits a high self-antibacterial property against Escherichia coli (E. coli), which is attributed to the ACS Paragon Plus Environment

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synergetic effects of RGO-ID+ and quaternary ammonium and amino groups presented at the surface of RGO-ID+.

2. EXPERIMENTAL SECTION 2.1. Raw Material. Concentrated H2SO4 and KMnO4 were purchased from Adamas (Thailand) Co., Ltd. Other used reagents were applied by the Sigma Aldrich Co., Ltd (China). 2.2. Preparation of GO and RGO. GO was obtained from the natural graphite by a modified Hummers method.52-53 3.0 g natural graphite power (500 mesh) was added to 60 °C solution with 8 mL of concentrated H2SO4, 2.5 g of P2O5, and 2.5 g of K2S2O8. The reaction temperature was controlled at 65 °C for 4 h. After that, the mixed solution was cooled to 25 °C, and was separated by vacuum filtration with deionized H2O until the wash solution was a neutral pH value. 3.0 g pre-oxidized graphite powder was slowly added into 0 °C solution of 69 mL concentrated H2SO4, and 9.0 g KMnO4 was gradually added into the above solution with vigorous magnetic stirring while the temperature was controlled below 20 °C for 2 h. After that, the reaction temperature was maintained at 35 °C for 4 h while 69 mL of deionized H2O was gradually added. Successively, 4 ml of H2O2 (30%) was slowly added with vigorous stirring. The supernatant was separated by the ultracentrifugation with 500 mL HCl (2.0 M) solution, and then washed with deionized H2O until the wash solution was a neutral pH value. Finally, the brown GO dispersion was obtained. RGO was prepared from Go by a simple chemical reduction method. In brief, 100 mL brown GO (1.0 g) dispersion was titrated 0.5 M ammonia until the suspension pH reached around 10.0. After that, 2.5 g Vitamin C was gradually added, by followed aging at 80 °C for one day under vigorous stirring. Finally, the black precipitate was washed by centrifugation with deionized H2O, and dried in a vacuum oven at 20 °C for overnight to obtain pure RGO. 2.3. Synthesis of RGO-ID+. RGO-ID+ was synthesized by two-step reaction, including in situ synthesis and salification reaction (Scheme 1). Preparation of RGO-IP. IPDI and DMEA grafted RGO can be successfully synthesized by the in situ synthesis method. Typically, the above pure RGO (0.05 g) was added into 15 ml of acetone (anhydrous, 99.9% pure) and was ultrasonically dispersed, then the mixture



dispersion was placed in an oil bath with the magnetic stirring and 8.83 g of IPDI (anhydrous, 99.5% pure) and 0.001 g dibutyltin dilaurate (anhydrous, 99.5% pure) were added gradually into the flask, and a nitrogen stream was applied, by following refluxing at 85 °C for 24 h. Successively, 2.6 g DMEA (anhydrous, 99.9% pure) and 0.001 g 4-methoxyphenol (anhydrous, 99.9% pure) were added into the mixture, and the temperature was controlled at 60 °C for 3 h. Finally, the crude precipitate was separated by vacuum filtration, following by washing with acetone to dissolve the residual IPDI and DMEA. The finally product is denoted as RGO-IP. Preparation of RGO-ID+. 0.5 g of the synthesized RGO-IP was added into 20 mL of acetone and ultrasonically dispersed for 30 min, and then 1.35 g of acetic acid (99.9% pure) was added slowly into the mixture under mechanical stirring and the temperature was controlled at 28 °C for 3 h. Finally, the cationic graphene was obtained and labeled as RGO-ID+-d2. Other RGO-ID+-d1 and RGO-ID+-d3 were synthesized in the same manner, but with a different reaction time (12 h and 36 h). The RGO-ID+ was dispersed in deionized water under vigorous stirring and ultrasonication to obtain a homogeneous black dispersion (Figure S1) for further use. The dispersion is very stable and no precipitations occur after storing for 60 days (Figure S1). 2.4. Preparation of Coating Systems. Mild steel substrates (5 cm × 15 cm) were used in all experiments, and the composition of the mild steel is (wt.%): C=0.281; Si=0.11; P=0.014; S=0.019; Cr=0.012; Mn=0.03 and balance Fe. Before coating, the SiC papers (800, 1200, 1500, and 2000 grit) were utilized to mechanically grind the surface of the mild steel. After that, the ground mild steels were washed by absolute ethyl alcohol with the ultrasonication, and dried by using the nitrogen gas. On the other hand, pure copper substrates were treated by the same method for further use. Prior to electrodeposition, under vigorous stirring, 10.0 g water dispersion of RGO-ID+ (0.5 g RGO-ID+) was added slowly into 90.0 g of cathodic epoxy electrophoretic emulsion with 25% solid content (Hunan Xiangjiang Kansai Paint Co., Ltd., China), and then ultrasonically dispersed for 2 h, yielding a gray dispersion without settling. The mixed dispersion is very stable and no obvious precipitations occur after storing for 30 days (Figure S2). The mixed dispersion was placed in a 250 mL electrophoretic bath, and the pretreated steel and copper ACS Paragon Plus Environment

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were performed as the cathode and anode, respectively. An electrophoretic coating on the steel surface was formed with 220 V of voltage at the ambient temperature for 2 min. Then, the coated specimens were washed by deionized water for several times to remove residual latex on the surface. Finally, the coated specimens was dried at 150 °C for 20 min using a controlled heating rate (25 °C to 90 °C, 1°C min-1; 90 °C to 150 °C, 2 °C min-1); note that the progressive heating rate was employed to control the evaporation rate of the solvent in the coating. The product is denoted as the RGO-ID+-0.5% coating. Similarly, 0 and 0.8 g of RGO-ID+ were used to prepare the other specimens for comparison, which are denoted as the Pure Epoxy and RGO-ID+-0.8% coatings, respectively. For comparison, the coating with randomly distributed RGO on the steel surface was also synthesized by the spray method.31 100.0 g of cathodic epoxy electrophoretic latex with 0.5 g RGO were sprayed on pretreated steel surface by air spray gun for 45 s. The product is denoted as the RGO-0.5% coating. Characterization. The synthesized samples were characterized by fourier transform infrared (PerkinElmer spectrum equipped with GRTC detector, USA), X-ray diffraction (X'Pert Powder, Holand, Cu Kα radiation λ = 0.15406 nm), Raman spectroscopy (Renishaw Micro-Raman Spectroscopy System), X-ray photoelectron spectroscopy (PHI 5700 ESCA, USA, Al Kα X-ray source hυ = 1486.7 Ev), atomic force microscopy (Bioscope system, USA), field emission scanning electron microscopy (Hitachi S-4800, Japan), high-solution transmission electron microscopy (JEOL JEM-2008, Japan), and themogravimetric analysis (DTG-60, Japan). Electrochemical Measurement. Electrochemical measurements were performed on a electrochemical workstation (CHI-660E, China). The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) curves were measured in 3.5 wt% NaCl and 5.0 wt% NaCl solution at 25 °C. For both measurements, a saturated calomel electrode (SCE), a platinum plate of 4.8 cm2 area and the sample were used as the reference electrode, the counter electrode, and the working electrode, respectively. Before measurement, the working electrode was maintained at its open circuit potential (OCP) for 1.5 h until the OCP reached a steady state. As for the EIS measurement, the AC signal was an amplitude of 20 mV peak to peak with a frequency domain from 0.01 kHz to 100 Hz. The potentiodynamic polarization ACS Paragon Plus Environment


curves were measured with a potential scan rate of 0.5 mV s-1 over the potential range of -0.3 V to 0.3 V vs. OCP. The ZsimpWin 4.0 software was used to fit the electrochemical parameters obtained from EIS data. Antibacterial Testing of RGO-ID+. E. coli has a special UV absorption at 600 nm (OD600), which can be utilized to detect the antibacterial properties of materials. To determine the antibacterial properties of RGO-ID+, we chose GO, RGO and RGO-ID+ for testing. In a brief, 1 mL post-proliferation bacterial solution with an OD600 value of 0.5-0.8 was injected into a sterile tube, and its original OD 600 value was considered as the blank control (OD 0600 ). Additionally, 50 µL of GO, RGO and RGO-ID+ solutions were separately added into the bacterial suspension and then stored for 2 h at 4 °C. The OD600 values were used to assess the antibacterial activity toward E. coli. The antibacterial efficiency is calculated according to the following formula: Antibacterial efficiency (%) = (OD0600 - ODX600) / OD0600 × 100% where the OD060 value represents the initial UV absorption value of the blank control and the ODX600 value corresponds to the measured UV value of each tested materials. Colony-forming units (CFU) of GO, RGO and RGO-ID+ were assayed on PTFE dishes (φ 90 mm,, 10 g NaCl, 10 g peptone, 15 g agar, 5 g yeast extract, pH = 7). At 37 °C, 3 mL of bacterial suspension was poured into a blank PTFE Petri dish and cultivated for 4 h as a blank contral. On the other hand, 3 mL of the bacterial suspension was further added to Petri dishes that coated with the GO, RGO and RGO-ID+ coatings, respectively. Those dishes were also cultured at 37 °C for 4 h. Then, all the bacterial suspension were taken out and diluted to 106 cells mL−1, and 50 µL of the diluted liquid was roll-coated on a Petri dish, the lids were changed and numbered, then cultured at 37 °C in a shaker for 4 h. CFU were calculated, and the reduced rate of colonies can be performed to represent the loss of viability cells for the samples, which was determined by the following equation: Reduced colony ratio (%) = (N0 - NX) / N0× 100% Among them, N0 and NX are the colonies number in the blank control and samples, respectively. Here, all bacterial detection tests were repeated three times.


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3. RESULTS AND DISCUSSION 3.1. Characterization of RGO-ID+ The

treatment

of

RGO

with

isophorone

diisoxcyanate

(IPDI)

and

N,N-dimethylethanolamine (DMEA) can result in the derivatization of residual hydroxyl and carboxyl groups on the surface of RGO nanosheet through formation of amides.54-55 These chemical changes can be determined by FT-IR spectroscopy because of the characteristic IR spectra of the RGO and modified RGO. Figure 1 (a) displays the spectra of RGO, RGO-IP, and RGO-ID+. In the case of RGO, a low absorption band appearing at around 1731 cm-1 is associated with the C=O stretching of residual carboxyl group; the low absorption band appearing at around 1381cm-1 is contributed to C-OH stretching of residual carboxyl group; the absorption peak located at approximately 1063 cm-1 is attributed to C-O stretching vibration.14 A relatively strong and broad adsorption band at about 3426 cm-1 is attributable to the O-H stretching vibration resulting from residual -OH groups on the RGO surface. Compared to the RGO, upon treatment with IPDI, it is found that there are some new absorption bands appearing in the RGO-IP. The C=O stretching at 1731 cm-1 in the RGO is covered by a shaper and stronger absorption band appearing at 1648 cm-1, which is attributable to the C=O stretching for the carbamate esters. A new peak at 1648 cm-1 corresponds to an amide carbonyl-stretching mode which is considered as Amide I vibrational stretch. On the other hand, the new adsorption peak appearing at around 1568 cm-1 is assigned to the coupling of the C-N stretching vibration with HCN deformation vibration which is considered as Amide II vibrational stretch.56 Furthermore, the presence of new absorption peak at 3395cm-1 is assigned as -NH amide stretching. A new strong and sharp stretching at 2795cm-1 is originated from the IPDI, corresponding to the isocyanate group (NCO). The new adsorption peaks at 1236 cm-1 and 1075 cm-1 are attributable to the C-N stretching. These support the successful preparation of RGO-IP. This amide-functionalised RGO with the isocyanate group (NCO) can further undergo with DMEA through in situ reaction. The formation of second-step amide is profitably proved by the disappearance of the isocyanate group (NCO) at 2795 cm-1, since it presents in the precursor RGO-IP (Figure 1 (a)). The -NH amide stretching vibration (3395cm-1), the C=O



stretching vibration of amide vibration (1648 cm-1), and coupling of C-N stretching vibration (1568 cm-1) are relatively intenser than that of RGO-IP. The results suggest that the formation of RGO-ID+. Moreover, the isocyanate group stretching at 2795 cm-1 gradually disappears with reaction time (Figure 1 (b)). This indicates that the residual iscoyanate groups are fully reacted with the hydroxyl groups of DMEA. XPS analysis is often carried out to offer direct evidence for the element composition of surface and near-surface for carbon materials. The XPS spectra of the samples are presented in Figure 2. The C1s and N1s binding energies are about 285 eV and 398 eV, respectively. Compared to RGO, it is evidently found from Figure 2 (a) that the XPS surveys of RGO-IP and RGO-ID+-d2 exhibit a new peak at 398 eV for N. Figure 2 (b-f) display the high resolution XPS spectra of C1s and N1s, respectively. It is seen from Figure 2 (b) that RGO exhibits characteristic binding energies, including C=C/C-C (284.2 eV), C-OH (285.5 eV), C-O-C (286.3 eV), and COOH (288.3 eV), which suggests that there are some residual -COOH and -OH groups presented on the surface of RGO. An amide functional group in RGO-IP compared with the RGO is evidenced by the analysis of the C 1s binding energies: C-N peak locates at 285.7 eV and the peak at 287.8 eV is assigned as HN-C=O. More significantly, the C-OH C1s signal at 285.5 eV is disappeared and COOH C1s peak appearing at 288.3 eV is weak in the C1s spectra for RGO-IP and RGO-ID+-d2, but which are present in the precursor RGO. These changes confirm that IPDI successfully grafted onto RGO to form the amide functional groups. This is further supported by the RGO-IP N1s orbital binding energies (Figure 2 (e)). For example, two peaks appear at 399.6 eV and 400.5 eV are attributable to the C-N and HN-C=O, respectively. Compared to the RGO-IP, it can be seen from RGO-ID+-d2 C1s orbital energies (Figure 2 (d)) that the C-N signal at 285.7 eV and HN-C=O signal at 287.8 eV are relatively intenser than that of RGO-IP, which is in agreement with the RGO-ID+-d2 N1s orbital energies. Most importantly, new peak appearing at 400.4 eV in N1s orbital energies (Figure 2 (f)) is attributed to the quaternary-N (-NH+-). Therefore, we can confirm that the RGO-ID underwent a salification reaction with acetic acid to form the RGO-ID+. Moreover, from Figure 2 (d) and S3, the peak at 285.5 eV (C-OH) in RGO-ID+ C1s orbital energies gradually disappears and peaks at 399.6 eV (C-N), 400.4 eV (-NH+-), and 400.5 eV (HN-C=O) in the N1s orbital energies become relatively stronger with the reaction ACS Paragon Plus Environment

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time, suggesting the amount of amides increase with the reaction time. These results reveal that the IPDI and DMEA monomers were successfully grafted onto RGO and the quaternary-N presented in the RGO-ID+, which is further confirmed by the XRD analysis (Figure S4), TGA measurements (Figure S5), and AFM measurements (Figure S6). The Raman spectra for the samples are presented in Figure S7. Generally, the characteristic peaks appearing at around 1350 cm-1 and about 1570 cm-1 are assigned to the D band and G band, respectively. The D band arises from the first order scattering for E1g photon of sp2 C atom, and the G band resulting from a breathing mode of point photons of A1g symmetry.57-58 The D band, as a Raman-inactive for an infinite graphene nanosheet, can be increased via the reduction in the sp2 domain size, resulting in the increase of the relative ratio of the D band to G band (ID/IG).59-61 The ID/IG values of RGO, RGO-IP, RGO-ID+-d1, RGO-ID+-d2 and RGO-ID+-d3 are 1.368, 1.355, 1.317, 1.309 and 1.307, respectively. This means that the ID/IG values determined from the normalized intensity of RGO, RGO-IP and RGO-ID+-dk are quite close, suggesting that the formation of covalent bonds with IPDI has little impact on the scaffold structure. SEM and TEM were carried out to understand the morphological changes after the growth of IPDI and DMEA on the RGO. SEM images were obtained from RGO and RGO-IP nanocomposites after they were dispersed in NMP solution. The RGO displays smooth surface with a characteristic corrugated and scrolled structure (Figure 3 (a)). After the formation of covalent bonds with IPDI, RGO-IP and RGO-ID+ (Figure 3 (b), (c)) also exhibit wavy and scrolled structure, similar to that of RGO, suggesting that the formation of covalent bonds with IPDI and DMEAs has little impact on the scaffold structure, which is consistent with the Raman results. Furthermore, the detail structures of RGO-ID+-d2 nanocomposites were determined by TEM. As can be seen from Figure 3 (d), the as-obtained RGO-ID+-d2 nanocomposites are electron transparent, indicative of an ultrathin structure. The HRTEM image reveals that the surface of RGO-ID+-d2 nanocomposites is rough with uniformly distributed IPDI and DMEA. Cathodic electrophoretic deposition was performed since RGO-ID+ is positively charged, which was demonstrated by the zeta potential (Figure S8). The RGO dispersed in deionized water is negatively charged with -21.6 mV average zeta potential, owing to the residual ACS Paragon Plus Environment


hydroxyl and carboxyl groups. On the contrary, the RGO-ID+-dk (k=1, 2, 3) are positively charged, with average zeta potential of around 58.7 mV, which is attributed to the -NH+- in the RGO-ID+. Moreover, the electrophoretic deposition behavior of RGO-ID+ nanosheets suspension has been studied. The results show that it follows the Hamaker’s equation (Figure S9). 3.2. Anticorrosion Performance SEM and TEM images obtained from the fracture surfaces of Pure Epoxy, RGO-0.5%, RGO-ID+-0.5% and RGO-ID+-0.8% coatings are shown in Figure 4. As seem from the cross sections exhibited in the SEM images, the coatings are about 29 µm of the thickness. The Pure Epoxy coating shows relative smooth surface in Figure 4 (a). For the RGO-0.5% coating, a random distribution of RGO is exhibited in Figure 4 (c). However, the fracture surfaces of RGO-ID+-0.5% and RGO-ID+-0.8% exhibit relatively rough surfaces in Figure 4 (e) and (g), due to the distribution of RGO-ID+ nanosheet. The TEM images show the nanoscopic dispersion of RGO and RGO-ID+ in the composite films. As seen from the TEM images, the dark lines and bright area represent the individual graphene layers and the epoxy matrix, respectively. The RGO domains exhibit a random orientation with the nanoscopic agglomerations, as shown in Figure 4 (d), indicating that the compatibility between RGO and epoxy matrix is poor. However, for the RGO-ID+-0.5% and RGO-ID+-0.8% coatings, the TEM images show that the RGO-ID+ nanosheets display a dimensional alignment with the lamellar-like morphology and uniform dispersion in the epoxy matrix (Figure 4 (f) and (h)). The aligned lamellar morphologies agree well with recent work.8, 62-63 The self-alignment of lamellar-like nanofillers in polymer composite films has been obtained from RGO/PU, clay/Nafion and RGO/Nafion.64-66 In these cases, the domain driving force for the self-alignment is attributed to the entropic gain with a decrease in the total excluded volume.53 In our case, the main driving force of the self-alignment should be an electric field force. A reasonable explanation is that the RGO-ID+ nanosheets uniformly dispersed in the cationic epoxy latex using the water as the solvent can be rapidly moved to the cathode along a particular direction by the electric field force, which is simultaneously accompanied by the in-plane self-alignment of the RGO-ID+ nanosheets in the epoxy matrix during the electrodeposition process. Therefore, the electric field force is of crucial role for the ACS Paragon Plus Environment

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self-alignment. Additionally, other conditional parameters, such as the elevated temperature and the controllable volatilization rate of solvent, are propitious to improve the self-alignment of RGO-ID+ nanosheets during the coating formation. Potentiodynamic polarization was applied to quantitatively analyze the anticorrosion performance of the samples immersed in a NaCl solution for various immersion times. The polarization curves of Pure Epoxy, RGO-0.5%, RGO-ID+-0.5% and RGO-ID+-0.8% coatingss as a function of immersion time are displayed in Figure 5. The corresponding corrosion kinetic parameters listed in Table 1 were acquired by the nonlinear leas-square fitting method,67 The Ecorr for RGO-0.5% coating after 1 day of immersion time is around - 0.305 ± 0.005 V/SCE, which is higher than that of Pure Epoxy coating (- 0.312 ± 0.009 V/SCE), and the icorr of RGO-0.5% coating is 4.307 ± 0.004 × 10-4 µA/cm2 which is much lower compared to the Pure Epoxy coating (5.632 ± 0.003 × 10-4 µA/cm2). This indicates that the incorporation of RGO into the epoxy matrix can improve the anticorrosion performance of coating. The RGO nanosheets as a physical barrier of the corrosive electrolytes can reduce their diffusion in the coating, improving the anticorrosion property of the coating. As for the RGO-ID+-0.5% and RGO-ID+-0.8% coatings, the values of Ecorr increases and icorr decreases with the RGO-ID+ loading ratio as compared with the RGO-0.5% coating (Figure 5 (c-d) and Table 1). For the RGO-ID+-0.8% coating, the values of Ecorr reach -0.302 ± 0.004 V/SCE with icorr at only 3.141 × 10-4 µA/cm2, which implies that the anticorrosive property of the nanocomposited coating containing the aligned RGO-ID+ nanosheets is obviously enhanced relative to the RGO-0.5% coating. After 30-day immersion, the Ecorr and icorr of RGO-0.5% coating are significant decrease and increase, respectively. The Ecorr is -0.446 ± 0.005 V/SCE and the icorr increases to 1.528 ± 0.004 × 10-1 µA/cm2. The results indicate that the RGO-0.5% coating has been obvious degeneration, and could not effectively protect the metal against corrosion. Also, it is evident that some blisters appear on the RGO-0.5% coating after 30-day immersion (Figure S10 (b)). These indicate that the corrosive electrolytes have reached the underlying metal and activated main corrosion sites on the surface of metal. In principle, corrosion of metal occurs at anodic areas at the surface of metal, and hydroxyl ions are formed at cathodic areas which can seriously affect the adhesion between the coating and metal and builds up penetratively active ACS Paragon Plus Environment


sites.28 Furthermore, these active sites can form the passage of corrosive media through the coating. For RGO-ID+ coatings after 30 days immersion, Ecorr and icorr decreases and increases slightly with the immersion time, respectively. For example, the icorr values of RGO-ID+-0.5% and RGO-ID+-0.8% coatings are approximately 7.193 ± 0.004 × 10-3 µA/cm2 and 1.727 ± 0.005 × 10-3 µA/cm2 after 30 days immersion, respectively. Moreover, no obvious blisters are found on these samples (Figure S10 (c-d)), suggesting that the corrosion protection performance of the RGO-ID+ coatings is evidently enhanced. Figure 6 and Figure S11 display the time-depended Nyquist diagrams and Bode diagrams of the variously coated samples immersed in 3.5 wt% NaCl solution at pH = 7 for various days. In principle, as for an intact coating, the phase angle in Bode plot reaches -90°, which suggests that the coating has pure resistance characteristics. With the diffusion of the corrosive electrolytes into the coating, the phase angle can obviously decreases at a given frequency, owing to the capacitance characteristics of the coating. Thereby, the changes for phase angle can reflect the anticorrosion performance of the coating. The peaks at different frequencies in Bode phase plot exhibit different time constants. Commonly, the time constant appearing at the various frequency ranges can reflect different information for coating. It at high frequency range is seen as the coating layer, and that at low frequency range reflect the corrosion occurring on the metal surface. As for the Pure Epoxy coating, it exhibits two time constants after 1-day immersion (Figure S11 (b)). With the increase of immersion time, the decrease of phase angle is obvious and the time constant vanishes at low frequency range, suggesting that the barrier effect of coating is seriously exacerbated and large area of delamination has been occurred. Moreover, the impedance modulus at 0.01 Hz (|Z|0.01Hz) reflects the ability of a coating to restrain the current between the cathodic and anodic areas. After 30-day immersion, the value of |Z|0.01Hz for the Pure Epoxy coating decreases by more than two orders of magnitudes (Figure S12). Furthermore, the significant reduction of the radius in the Nyquist plot demonstrates the remarkable decrease of the corrosion protection property of the Pure Epoxy coating (Figure 6 (a)). The RGO-0.5% coating shows only one time constant accompanied by a phase angle appearing at around -90° in the Bode phase plots at the early stage of the immersion (i.e. 15 days) (Figure S11 (d)), suggesting the capacitance nature of the coating during the 15 days ACS Paragon Plus Environment

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immersion. After immersion for 22 days, a second time constant is clearly obtained in the Bode plots and Nyquist plots (Figure S11 (c-d) and Figure 6 (b)), indicating that the initiation of corrosion occurs at the coating/metal interface. With prolonged immersion (i.e. 30 days), the peaks at high frequency are further weaken in the Bode phase plots, indicative of the diffusion of the corrosive electrolytes into the coatings. Moreover, after 30-day immersion, the value of |Z|0.01Hz decreases by approximate one order of magnitude, dropping from about 6.5 × 107 Ω cm2 to around 4.0 × 106 Ω cm2 (Figure S12). The corrosion protection performance of RGO-0.5% coating is improved relative to the Pure Epoxy coating, due to the random distribution of RGO in the coating. In the case of the Nyquist plots and Bode plots for RGO-ID+-0.5% and RGO-ID+-0.8% coatings (Figure 6 (c-d) and Figure S11 (e-h)), only one time constant is observed at high frequency range after up to 30 days immersion, indicating that the coatings remain intact and the electrolytes haven’t penetrated the coatings. Although the values of |Z|0.01Hz for RGO-ID+-0.5% and RGO-ID+-0.8% coatings gradually decrease with the increase of immersion time, the values of |Z|0.01Hz maintain above 7.0 × 107 Ω cm2 after 30-day immersion, much higher than that of RGO-0.5% coating (i.e. 4.0 × 106 Ω cm2) (Figure S12). These results suggest that the most substantial improvement for the corrosion protection property of the RGO-ID+ coatings is obtained by loading the self-aligned RGO-ID+ nanosheets. A reasonable explanation for this is that self-aligned RGO-ID+ nanosheets as multi-layer physical barriers were incorporated into the epoxy electrophoretic matrix to efficiently decrease the diffusion of the electrolytes in the coating. The breakpoint frequency (fb) (frequency to -45° phase angle) determined from the Bode plots is performed to probe the anticorrosion property of coating (Figure S13 (a)). As reported, the micro-layered delaminated area of the coating is closely to its anticorrosion property, which could be represented by the following equation:

A  f b = α  d   A0  where Ad and A0 represent the micro-layered delaminated area and the total area for the coating, respectively. Also, the α could be determined according to the following equation:



α=

ρεε 0 2

where ρ is the resistivity of coating, ε shows the dielectric constant of electrolyte in the coating. With the diffusion of the electrolytes in the coating, the ρ and ε value decreases and increases, respectively. ε0 represents the vacuum permittivity. Therefore, fb is approximately proportional to the delaminated area. As shown in Figure S13 (b), the fb value of the Pure Epoxy coating shifts to high frequency significantly compared to the other three coatings. However, the increase of the fb of the RGO-ID+ coatings is negligible throughout the immersion duration, much smaller than RGO-0.5% coating. Therefore, it is evident that the corrosion protective property of RGO-ID+ coating is remarkably improved . The equivalent electric circuits displayed in Figure S14 could be utilized to fit the EIS data. For the intact coatings, Rs represents the solution resistance，and Rc and Qc are the resistance and capacitance of coating respectively. The impedance plots with one time constant is simulated by the equivalent circuit of Figure S14 (a). The equivalent circuit of Figure S14 (b) is used for the simulation of the data for the impedance plots with two time constant. As for the damaged coating, the electrochemical corrosion process occurs on the underlying metal surface. Rct and Cdl are the charge transfer resistance and double layer capacitance, respectively, while Rf is considered as the polarization resistance which can reflect the degree of difficulty of corrosion occurring on the metal surface. The Rf value can be seen as the sum of Rct and Rc. Furthermore, to fit the better result, a constant phase element (CPE) is utilized to replace the Cdl. The impedance of CPE is calculated according to the following equation:

[

Z CPE = f 0 ( jw) n

]

−1

where w represents the the angular frequency, j is seen as the imaginary number, and f0 is a proportionality coefficient. Meanwhile, the Qc can be determined from the CPE parameters through the following equation: QC

(Y × R ) = 0

f

1 n

Rf

where Y0 and n represent the admittance and exponent of CPE, respectively. The corresponding fitted parameters are shown in Table 2. The Rc and Rct would decrease with the diffusion of the electrolyte into the coatings,8,30 thus we can use the Rf to evaluate the


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coating performance in this study. As seen from Table 2, it is found that the Rf of the Pure Epoxy coatings significantly decreases from 5.23 ± 0.02 × 107 Ω cm2 to 7.15 ± 0.02 × 105 Ω cm2 after 30 days’ immersion in 3.5 wt% NaCl solution, suggesting that the anticorrosion performance for Pure Epoxy coating obviously degrades. The Rf of the RGO-ID+ coatings is much higher than that of RGO-0.5% coating throughout the immersion duration. Commonly, Qc increases with penetration of the electrolytes in the coating. The Qc for Pure Epoxy coating exhibits the most remarkable increase with the immersion time (Table 2), and after 30 days of immersion the value of Qc for Pure Epoxy coating is increased by two orders of magnitude as compared to the RGO-ID+ coatings. Furthermore, the Qc value for RGO-0.5% coating is much higher than that of RGO-ID+ coatings after 30 days of immersion. These results imply that RGO-ID+ coatings have incorporated much less electrolyte compared to RGO-0.5% coatings. This phenomenon could be explained by that the penetration mechanism of the electrolyte solution is different in the coatings containing the randomly distributed RGO and aligned RGO-ID+. Because of the aligned orientation of RGO-ID+ nanosheets in the RGO-ID+ coating, the high surface area of self-aligned RGO-ID+ can be fully utilized compared to the randomly distributed RGO, resulting in providing more chances of the electrolytes to interact with the RGO-ID+. Salt spray test has been widely applied to investigate the corrosion protective property of coatings in industry. The optical images of the specimens after salt spray testing for 200 h are presented in Figure S15. The Pure Epoxy coating shows a number of blisters around the scratches and the rusty pots appear (Figure S15 (a)). After incorporation of the RGO and aligned RGO-ID+ into the coatings, the number of blisters and rusty pots decreases obviously (Figure S15 (c), (b) and (d)), suggesting that RGO and RGO-ID+ can enhance the corrosion protective performance of the coating system. In fact, no obvious blisters or rust pots are observed on the RGO-ID+ coatings. These further reveal that the self-aligned RGO-ID+ nanosheets incorporated into the coating can further improve the corrosion protective property. As well known, coatings are often encountered with alkaline environment during the metal protection process. Thus, we further investigate the anticorrosion performances of the coatings immered in 5.0 wt% NaCl solution at pH 12 for 8 days and 30 days. The obtained ACS Paragon Plus Environment


potentiodynamic polarization curves and the corresponding electrochemical polarization parameters are displayed in Figure 7 and Table 3 respectively, and the Bode diagrams are given in Figure S16. The Ecorr and icorr of Pure Epoxy and RGO-0.5% coatings exhibit pronounced changes as the immersion time increases (Figure 7 and Table 3). After 30 days of immersion, the Ecorr and icorr values of the RGO-0.5% coating reach -0.457 ± 0.003 V/SCE and 7.617 ± 0.005 µA/cm2, respectively. However, for the GO-ID+ coating, Ecorr and icorr values exhibit only slight decrease and gradual increase throughout the immersion time, respectively. After 30-day immersion, the icorr value of the RGO-ID+-0.5% coating (i.e. 7.815 ± 0.001 × 10-3 µA/cm2) is approximately 1000 times smaller than that for the RGO-0.5% coating. Also, the changes of Ecorr provide direct evidences that the RGO-ID+ coatings have outstanding corrosion protection property in alkaline environment. For Pure Epoxy coating, the impedance modulus at low frequency range is about 106 Ω cm2 and a second time constant is observed in Bode plots after immersion for 8 days (Figure S16 (a) and (b)), suggesting that the coatings is severely deteriorated, resulting from the alkaline electrolytes attack. In the case of the phase angle for RGO-0.5% coating at high frequency range, it decreases significantly as the immersion time increases, and a second time constant is also clearly evident after immersion for 30 days ( Figure S16 (c) and (d)). The impedance modulus at low frequency range for the RGO-ID+ coatings are higher than 107 Ω cm2 even after 30 days immersion, and there is one time constant with around -90° of phase angle in the Bode plots (Figure S16 (c) and (d)). It is clearly found that the incorporation of RGO and RGO-ID+ into the coatings results in the increase of |Z|0.01Hz compared to the pure epoxy coating, while the |Z|0.01Hz values of the RGO-ID+ coatings are much higher than that of the RGO coating (Figure S17 (a)). As shown in Figure S17 (b), the values of fb for Pure Epoxy coating are much higher than that of RGO and RGO-ID+ coatings, and significantly increase with immersion time. The changes of fb values for the RGO-ID+ coatings are negligible. Therefore, it can be concluded that the coatings with the self-alignement RGO-ID+ nanosheets can protect underlying metal from corrosion even in harsh corrosion environment. The above results show that the anticorrosion performance of the coating containing the aligned RGO-ID+ nanosheets is superior to the most of graphene-based coatings (Table S2). A schematic drawing presented in Figure S18 (a) illustrates an interaction mechanism ACS Paragon Plus Environment

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between the electrolyte solution and the self-aligned distributed RGO-ID+ in the electrophoretic coatings. In the case of a three dimensional randomly distributed RGO-0.5% coatings (Figure S18 (b)), there is a tortuous pathway to the electrolyte solution to diffuse through the epoxy coatings. The corrosion protective performance are enhanced compared to Pure Epoxy coatings. The incorporation of RGO nanosheets effectively increase the tortuous pathway of the electrolytes in the coating, thus the electrolytes should spend more time diffusing through the coating. Additionally, the addition of the RGO to the coating can form some barrier networks, resulting in relatively low incorporation of electrolytes into the coating. As for the electrophoretic coatings with the self-aligned RGO-ID+ nanosheets (Figure S18 (a)), it fully utilizes the surface area of RGO-ID+ as a physical barrier to prevents the corrosive electrolyte from diffusing through the coating. In our study, the RGO-ID+ shows an excellent water-dispersible property due to its cation, resulting in a good nanoscopic dispersion in the latex. Also, the aligned orientation of RGO-ID+ nanosheets in the RGO-ID+ coating by the the electrodeposition compared to the RGO coating with the randomly distributed RGO leads to the higher surface area and further increase of the tortuous path to the electrolyte solution. These factors results in the superior corrosion protective properties of the RGO-ID+ coating system. It is well-known that the coating bonds are easily hydrolytic degradation in alkaline solution, leading to the decrease of the cross-linking density for the coating. Thereby, the damage of epoxy coating can be easily introduced at the high pH. However, the RGO-ID+ coating shows significantly enhanced performance in alkaline (pH = 12) NaCl (i.e. 5.0 wt%) solution compared to RGO-0.5% coating. This may be attributable to the following reasons: (1) the aligned RGO-ID+ nanosheets with a good dispersion in the coatings can form multilayer barrier to decrease the diffusion of electrolytes into the coating; (2) the positive charges on the RGO-ID+ surface. The positively charged RGO-ID+ nanosheets influence the ionic resistance by electrostatic attraction (Figure S18 (a)). In this case, the diffusion of the anions in the epoxy coating can be effectively inhibited. Therefore, the Cl-, OH-, and other anions can be prohibited from the penetration in the coating until the positively charged RGO-ID+ nanosheets become neutral. 3.3. Self-antibacterial Activity ACS Paragon Plus Environment


Gram-negative bacterial E. coli was used to investigate the antibacterial activity of the samples. The antibacterial activity for the as-obtained samples is displayed in Figure 8. The colony counting method was performed to determine the death rate of bacterial cells. Compared to the blank control sample (Figure 8 (a)), GO, RGO and RGO-ID+ show a self-antibacterial activity in a slurry system with fewer colonies formation (Figure 8 (b), (c) and (d)). This well agrees with the death rate of bacterial cell. As shown in Figure 8 (e), the blank control does not affect the cell viability assay. The GO shows a moderate cytotoxicity with the loss of viability at 36.7 ± 1.1%. However, the RGO has the loss of viability at 21.2 ± 1.9%, which is slightly weaker than that of the GO. In the case of RGO-ID+, it exhibits the E. coli inactivation percentage at 83.4 ± 1.3%, which is more than 2-fold of that for GO. This is consistent with the antibacterial efficiency obtained from OD600 values (Figure S19). Thus, RGO-ID+ has significantly enhanced antibacterial activity toward E. coli compared with RGO and GO. Also, the time-dependent antibacterial activities of the three samples were examined, and the results are shown in Figure S20, revealing that the loss of E. coli viability for GO, RGO and RGO-ID+ increases with the increase of incubation time. As for RGO-ID+, the E. coli inactivation percentage increases from 43.6 ± 1.6%, after 1h incubation to 68.6 ± 2.1%, 72.9 ± 2.7%, and 83.4 ± 1.3% after 2, 3, and 4 h incubation, respectively. Comparing the three different materials, RGO-ID+ has much higher antibacterial performance at all tested incubation intervals. The obviously enhanced self-antibacterial activity for RGO-ID+ can be explained by following factors: (1) the growth of E. coli can be inhibited by graphene-based nanomaterials, which owns to membrane stress of graphene.47-48 When the bacterial cells directly contact with graphene-based nanomaterials, the irreversible damages are induced by the membrane stress, leading to the destruction of cell structures. (2) E. coli cells are negatively charged,48, 68-69

while the RGO-ID+ are positively charged (Figure S8), can be stable suspension (Figure

S1), because of the quaternary-N. Additionally, the outer membrane of E. coli cells has the lipopolysaccharide, including phosphates, sugar, and lipids, while RGO-ID+ contains a large number of amide groups (Figure 1). Therefore, the electrostatic attraction and hydrogen bonding can be formed between the E. coli cells and RGO-ID+. Both the hydrogen bonding ACS Paragon Plus Environment

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and electrostatic attraction could boost the RGO-ID+ to adhere to E. coli cells, which prevents the cell from intaking nutrient, and eventually leads to the E. coli cells death. In the case of RGO-ID+, RGO and GO, they are negatively charged, thus RGO or GO and E. coli cells would repel each other, resulting in the decrease of the contact between the E. coli cells and RGO nanosheets.

4. CONCLUSIONS In summary, the cationic reduced graphene oxide RGO-ID+ was synthesized by a facile two-step method and the RGO-ID+ nanosheets were incorporated into the epoxy coating by electrodeposition. The structure and surface morphology for RGO-ID+ were well characterized by FT-IR, Raman, XPS, XRD, TG, AFM, SEM, and TEM. The self-alignment distribution of RGO-ID+ in the coatings was investigated using the SEM and TEM. The RGO-ID+ coating system shows superior anticorrosion performance, assessed by the potentiodynamic polarizationas, EIS and salt spray tests. The superior corrosion protection property should be due to the in-plane aligned RGO-ID+ nanosheets which can fully utilize the high surface area of RGO-ID+ to effectively prohibit water molecule from forming ionic conducting path for the electrolytes. Moreover, the electrostatic attraction between the positively charged quaternary-N groups on the RGO-ID+ and the negatively charged ions can further prevent the negatively charged corrosive ions (such as Cl-, OH-, SO2- 4) from diffusing through the coating. While the antibacterial activities for GO, RGO, and RGO-ID+ toward E. coli are determined. The results obtained from the colony counting method show that the RGO-ID+ exhibits the strongest antibacterial activity with 83.4 ± 1.3% of the antibacterial efficiency. RGO-ID+ nanosheets’ antibacterial activities are time dependent. The high performance of bactericidal effect of RGO-ID+ is attributed to the synergetic effects of the electrostatic attraction and hydrogen bonding between the E. coli cells and RGO-ID+. Supporting Information. Digital photos of water dispersion and cathodic electrophoretic latex dispersion of RGO-ID+, XRD patterns of samples, C 1s and N 1s survey spectra of RGO-ID+-d1 and RGO-ID+-d3, AFM images and Raman spectra of RGO, RGO-IP and



RGO-ID+-d2, the zeta potential of the RGO and RGO-ID+-dk (k=1, 2, 3), digital photos of the samples after 30 days of immersion time in NaCl (3.5 and 5.0 wt%) solution at pH = 7, time-depended Bode and phase diagrams of samples, electrical equivalent circuit, variation of impedance data for samples, Time-dependent antibacterial activities of GO, RGO, and RGO-ID+.

AUTHOR INFORMATION Corresponding Authors Y. Liu ( [email protected]. Tel.:+86 135 0748 8663 ) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research is financially supported by the Major Science and Technology Projects of Hunan Province, China (2015GK1004), the Research Foundation of Education Bureau of Guizhou Province, China (Guizhou [2015]402), and the Joint Foundation of Science and Technology Department of Guizhou Province, China (Guizhou “LH” [2014]7430). The authors thanks to Prof. Xiaorong Zhou in The University of Manchester for his discussions and suggestions.


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Scheme 1. Schematic illustration of the synthesis of the cationic reduced graphene oxide (RGO-ID+) and the electrodeposition process.



RGO

1731 3426

1381 1063 1622

RGO-IP

1075 1648

2893 2841

3395

1468

1236

1568

Absorbance(a.u)

b)

a)

A bsorbance(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

RGO-ID +-d1

RGO-ID +-d2

2795 RGO-ID

+

RGO-ID +-d3

4000

3500

3000 2500 2000 -1 1500 Wavenumber(cm )

1000

500 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1)

Figure 1. FT-IR spectra curves: (a) RGO, RGO-IP and RGO-ID+-d2; (b) FT-IR spectra curves of RGO-ID+-d1, RGO-ID+-d2, and RGO-ID+-d3.


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Page 25 of 42

a)

b)

C1s

284.2, C=C/C-C

O1s

Fitting

Intensity(a.u)

Intensity(a.u)

N1s

RGO-ID+-d2

RGO-IP

285.5, C-OH 286.3, C-O-C 288.3, COOH

RGO 1000

800

600

400

200

0

280

282

c)

284

286

288

292

290

Binding energy(eV)

Binding energy(eV)

d) 284.2, C=C/C-C

284.2, C=C/C-C

Fitting

280

Intensity(a.u)

Intensity(a.u)

285.7, C-N

285.7, C-N 285.5, C-OH 286.3, C-O-C 287.8, HN-C=O 288.3, COOH

282

284

286

288

290

292

280

Binding energy(eV)

e)

286.3, C-O-C 287.8, HN-C=O 288.3, COOH

282

284

286

288

290

292

Binding energy(eV)

f)

Fitting

Fitting 400.5, HN-C=O

400.5, HN-C=O

395

Intensity(a.u)

399.6, C-N

Intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


399.6, C-N

400

Binding energy(eV)

405

395

+

400.4, -N -

400

Binding energy(eV)

405

Figure 2. XPS survey spectra (a) of RGO RGO-IP and RGO-ID+-d2; C 1s spectra of RGO (b), RGO-IP (c) and RGO-ID+-d2 (d); N 1s spectra of RGO-IP (e) and RGO-ID+-d2 (f).



Figure 3. SEM images of RGO (a), RGO-IP (b) and RGO-ID+-d2 (c); TEM image (d) of RGO-ID+-d2 obtained from the RGO-ID+-d2 after dispersion in water. Inset: HRTEM image of RGO-ID+-d2.


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Figure 4. SEM images of the cross-sections of the Pure Epoxy (a), RGO-0.5% (c), RGO-ID+-0.5% (e) and RGO-ID+-0.8% (g) coatings; (b), (d), (f), and (h) are the corresponding TEM images.


(a)

-6

Page 28 of 42

(b)

-8

-9

(5)

(1) 1 day (2) 8 days (3) 15 days (4) 22 days (5) 30 days

(4) (3) (2) (1)

-11

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

Current density log (i/A cm-2)

-7

-10

-7 -8 -9

(1) 1 days -10 (2) 8 days (3) 15 days (4) 22 days -11 (5) 30 days

0.0

-0.7

-0.6

(c) -7


-7

-8

-9

-10

-11

(1) 1 day (2) 8 days (3) 15 days (4) 22 days (5) 30 days -0.6

-0.5

(5) (3) (4)

-0.4

(1)

-0.3

(4) (1)

(3) (2) -0.5

-0.4

-0.3

-0.2

-0.1

(d)

-8

-9

-10

(2)

(5)

Potential (Vvs. SCE)

Potential (Vvs. SCE) Current density log (i/A cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



-0.2

-0.1

-11 -0.6

(1) 1 day (2) 8 days (5) (4) (3) 15 days (3) (4) 22 days (2) (5) 30 days -0.5


-0.4

(1) -0.3

-0.2

-0.1


Figure 5. Time-dependent potentiodynamic polarization curves of Pure Epoxy (a), RGO-0.5% (b), RGO-ID+-0.5% (c), and RGO-ID+-0.8% (d) coatings on the mild steel during 30 days’ immersion time in 3.5 wt% NaCl solution at pH = 7.


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|Z"| (Ω cm2)

30M

70M

(b)

1 days 8 days 15 days 22 days 30 days

1.6M

60M 800.0k

0.0 0.0

800.0k

|Z"| (Ω cm2)

1 day 8 days 15 days 22 days 30 days

(a)

40M

1.6M 21

20M

50M

10.0M

5.0M

0.0 0.0

40M

5.0M

10.0M

30M 20M

10M

10M 0

0 0

80.0M

10M

20M 2 Z' (Ω cm )

30M

0

40M

10M

20M

30M

40M

2

50M

60M

70M

Z' (Ω cm )


(c)

80.0M

(d)


60.0M

|Z"| (Ω cm2)

60.0M

|Z"| (Ω cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40.0M

40.0M

20.0M

20.0M

0.0

0.0 0.0

20.0M

40.0M 2 Z' (Ω cm )

60.0M

80.0M

0.0

20.0M

40.0M

2

Z' (Ω cm )

60.0M

80.0M

Figure 6. Time-depended Nyquist diagrams of Pure Epoxy (a), RGO-0.5% (b), RGO-ID+-0.5% (c), and RGO-ID+-0.8% (d) coatings on the mild steel during 30 days’ immersion in 3.5 wt% NaCl solution at pH = 7.



(a)

-3

-8

-9

-10

-11

Pure Epoxy RGO-0.05% + RGO-ID -0.05% + RGO-ID -0.08%

-12

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Current density log(i/A cm -2)

Current density log(i/A cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 42

(b)

-4 -5 -6 -7 -8

Pure Epoxy RGO-0.5% RGO-ID+-0.5% RGO-ID+-0.8%

-9

-10 -11

Potential(Vvs. SCE)

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

Potential(Vvs. SCE)

Figure 7. Potentiodynamic polarization curves of Pure Epoxy, RGO-0.5%, RGO-ID+-0.5%, and RGO-ID+-0.8% coatings on the mild steel after 8-day immersion (a) and 30-day immersion (b) in 5.0 wt% NaCl solution at pH = 12.


Page 31 of 42

(e) 100 Loss of Viability (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

40

20

0

Control

GO

RGO

+

RGO-ID

Figure 8. Photographs of bacterial colonies formed by the E. coli cell suspension after

incubation (a) blank control, (b) RGO, (c) GO, (d) RGO-ID+ for 4 h; (e) the corresponding cell viability measurement. Loss of cell viability rates was determined by colony counting method. Error bars is the standard deviation (n ≥ 3). **P < 0.03.



Page 32 of 42

Table 1. Time-dependent electrochemical polarization parameters of Pure Epoxy, RGO-0.5%,

RGO-ID+-0.5%, and RGO-ID+-0.8% coatings on the mild steel during 30-days’ immersion time in 3.5 wt% NaCl solution at pH = 7.

3.5 wt% NaCl Solution, pH = 7 Samples Immersion period (days)

Pure Epoxy

RGO-0.5%

RGO-ID+-0.5%

RGO-ID+-0.8%

Ecorr (V/ SCE)

ba (mV

bc (mV

dec-1 )

dec-1 )

icorr (µA/cm2)

1

-0.312 ± 0.009

(5.632 ± 0.003) × 10-4

89 ± 7

-93 ± 2

8

-0.334 ± 0.003

(2.511 ± 0.007) × 10-3

95 ± 3

-102 ± 5

15

-0.371 ± 0.004

(1.585 ± 0.009) × 10-2

112 ± 9

-123 ± 6

22

-0.416 ± 0.001

(4.786 ± 0.003) × 10-2

126 ± 4

-129 ± 9

30

-0.481 ± 0.007

(7.862 ± 0.001)

154 ± 4

-173 ± 11

1

-0.305 ± 0.005

(4.307 ± 0.004) × 10-4

73 ± 2

-86 ± 5

8

-0.318 ± 0.006

(7.349 ± 0.003) × 10-4

78 ± 5

-91 ± 4

15

-0.351 ± 0.008

(2.316 ± 0.001) × 10-3

97 ± 2

-114 ± 7

22

-0.383 ± 0.003

(8.053 ± 0.006) × 10-3

105 ± 2

-127 ± 5

30

-0.446 ± 0.005

(1.528 ± 0.004) × 10-1

117 ± 6

-136 ± 2

1

-0.308 ± 0.002

(3.315 ± 0.001) × 10-4

68 ± 5

-71 ± 4

8

-0.312 ± 0.001

(3.881 ± 0.008) × 10-4

64 ± 2

-75 ± 5

15

-0.332 ± 0.001

(1.013 ± 0.002) × 10-3

72 ± 4

-79 ± 6

22

-0.341 ± 0.006

(2.561 ± 0.005) × 10-3

76 ± 3

-78 ± 6

30

-0.362 ± 0.003

(7.193 ± 0.004) × 10-3

78 ± 5

-83 ± 8

1

-0.302 ± 0.004

(3.141 ± 0.005) × 10-4

69 ± 9

-70 ± 3

8

-0.311 ± 0.004

(3.507 ± 0.007) × 10-4

63 ± 6

-72 ± 7

15

-0.318 ± 0.006

(6.519 ± 0.007) × 10-4

66 ± 6

-77 ± 4

22

-0.322 ± 0.007

(8.752 ± 0.001) × 10-4

74 ± 3

-73 ± 4

30

-0.341 ± 0.003

(1.727 ± 0.005) × 10-3

77 ± 4

-81 ± 3


Page 33 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Time-depended electrochemical parameters extracted from impedance data of Pure

Epoxy, RGO-0.5%, RGO-ID+-0.5%, and RGO-ID+-0.8% coatings on the mild steel during 30 days’ immersion in 3.5 wt% NaCl solution at pH = 7. 2

Samples

2

Rf (Ω cm )

Rs (Ω cm )

period (days)

Pure Epoxy

RGO-0.5%

RGO-ID+-0.5%

RGO-ID+-0.8%

Qc (µF/cm2)

CPE

Immersion

Y0 (µsn/Ωcm2)

n

1

(5.23 ± 0.02) × 107

17.52 ± 0.08

0.37 ± 0.03

0.831

0.36 ± 0.03

8

(1.58 ± 0.01) × 107

18.33 ± 0.04

0.51 ± 0.01

0.639

0.58 ± 0.07

15

(8.34 ± 0.03) × 106

20.91 ± 0.07

15.51 ± 0.02

0.918

1.32 ± 0.07

22

(4.63 ± 0.07) × 106

16.54 ± 0.02

18.41 ± 0.07

0.798

7.46 ± 0.01

30

(7.15 ± 0.02) × 105

15.89 ± 0.01

98.97 ± 0.04

0.919

45.4 ± 0.02

1

(6.73 ± 0.04) × 107

11.12 ± 0.05

0.37 ± 0.08

0.941

0.19 ± 0.06

8

(4.58 ± 0.08) × 107

15.13 ± 0.03

0.47 ± 0.07

0.921

0.37 ± 0.04

15

(3.74 ± 0.04) × 107

21.43 ± 0.01

1.04 ± 0.04

0.518

0.71 ± 0.03

22

(1.63 ± 0.02) × 107

14.14 ± 0.02

1.57 ± 0.03

0.854

1.87 ± 0.02

30

(4.15 ± 0.01) × 106

15.19 ± 0.01

23.97 ± 0.02

0.799

6.89 ± 0.01

1

(8.33 ± 0.05) × 107

15.51 ± 0.05

0.32 ± 0.01

0.891

0.11 ± 0.08

8

(8.08 ± 0.05) × 107

16.43 ± 0.04

0.17 ± 0.04

0.771

0.31 ± 0.06

15

(7.87 ± 0.06) × 107

10.11 ± 0.03

0.23 ± 0.06

0.812

0.48 ± 0.04

22

(7.49 ± 0.01) × 107

19.34 ± 0.02

0.41 ± 0.05

0.738

0.52 ± 0.09

30

(7.14 ± 0.02) × 107

14.89 ± 0.03

1.27 ± 0.05

0.954

0.69 ± 0.07

1

(8.56 ± 0.03) × 107

17.22 ± 0.09

0.28 ± 0.04

0.996

0.13 ± 0.06

8

(8.47 ± 0.04) × 107

15.05 ± 0.07

0.29 ± 0.07

0.867

0.19 ± 0.02

15

(8.26 ± 0.01) × 107

26.74 ± 0.07

0.30 ± 0.07

0.892

0.25 ± 0.03

22

(8.03 ± 0.05) × 107

14.39 ± 0.06

0.31 ± 0.03

0.741

0.39 ± 0.05

30

(7.46 ± 0.02) × 107

28.42 ± 0.08

0.38 ± 0.02

0.967

0.47 ± 0.01



Page 34 of 42

Table 3. Electrochemical polarization parameters of Pure Epoxy, RGO-0.5%, RGO-ID+-0.5%,

and RGO-ID+-0.8% coatings on the mild steel after 6-day immersion and 30-day immersion in 5.0 wt% NaCl solution at pH = 12

5.0 wt% NaCl Solution at pH = 12 Samples

Immersion period Ecorr (V/ SCE)

icorr (µA/cm2)

ba (mV dec-1 )

bc (mV dec-1 )

8

-0.471 ± 0.005

(6.616 ± 0.003) × 10-3

131 ± 6

-143 ± 7

30

-0.607 ± 0.001

(43.318 ± 0.002)

142 ± 9

-182 ± 2

8

-0.381 ± 0.007

(9.583 ± 0.005) × 10-4

119 ± 5

-129 ± 8

30

-0.457 ± 0.003

(7.617 ± 0.005)

127 ± 2

-133 ± 6

8

-0.334 ± 0.003

(4.391 ± 0.009) × 10-4

96 ± 3

-102 ± 4

30

-0.363 ± 0.001

(7.815 ± 0.001) × 10-3

101 ± 8

-108 ± 9

8

-0.323 ± 0.005

(4.161 ± 0.006) × 10-4

94 ± 7

-101 ± 7

30

-0.353 ± 0.009

(3.481 ± 0.001) × 10-3

98 ± 5

-105 ± 5

(days)

Pure Epoxy

RGO-0.5%

+

RGO-ID -0.5%

+

RGO-ID -0.8%


Page 35 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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The graphical abstract shows that the RGO−ID+ nanosheets have a high antibacterial activity toward Escherichia coli and excellent anticorrosive performance.


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Cationic Reduced Graphene Oxide as Self-Aligned Nanofiller in the

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