Cyclodextrin-Based Nanocontainer: Synthesis and

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 36229−36239

Efficient Graphene/Cyclodextrin-Based Nanocontainer: Synthesis and Host−Guest Inclusion for Self-Healing Anticorrosion Application Chengbao Liu,†,‡ Haichao Zhao,*,† Peimin Hou,§ Bei Qian,∥ Xiao Wang,§ Chunyan Guo,⊥ and Liping Wang*,†

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†

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § State Key Laboratory of Marine Coatings, Marine Chemical Research Institute, Qingdao 266071, China ∥ College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao 266109, China ⊥ Ashine Advanced Carbon Materials, Co. Ltd, Changzhou 213245, China S Supporting Information *

ABSTRACT: Cyclodextrin, with a hydrophobic inner cavity and a hydrophilic exterior, is often used to encapsulate a widest range of guest molecules based on host−guest inclusion interactions. Graphene, an emerging nanobuilding material, exhibits great potential for numerous applications because of its superior characteristics. Herein, we synthesized a novel graphene/βcyclodextrin-based supramolecular nanocontainer with excellent inhibitor encapsulating capacity and high impermeable properties. The benzotriazole (BTA)-loaded nanocontainers were then used to endow coating system with excellent passive and active anticorrosion performance. Local electrochemical impedance spectroscopy (LEIS) was performed to characterize the selfhealing behavior of composite coatings. Results indicated that the protective capability of the scratched coatings can be recovered through BTA release from containers. Furthermore, the long-term corrosion resistance of container-based coating was largely improved as observed from EIS. The effective healing process involves two conditions: (1) the release of BTA from containers and formation of adsorption layers on exposed metal surfaces and (2) the impermeable graphene nanosheets greatly impeded the electrolyte penetration and corrosion extension around the scratch. This novel graphene/β-cyclodextrin-based nanocontainer endows polymer coating with efficient self-healing functionality and durable anticorrosion property. KEYWORDS: nanocontainers, self-healing, β-cyclodextrin, graphene, anticorrosion

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INTRODUCTION Developments of intelligent materials with functionalities have promoted the evaluation of technology and possessed a special niche in scientific research.1 Self-healing materials, inspired by the automatic recovery property of natural organisms, are able to detect the variations of material matrix and to repair the damage.2,3 When imposed by external stimulus such as light,4 thermal,5 moisture,6 and so on, the healing process can be initiated. Self-healing materials exhibit great potential for a wide range of applications, including stretchable devices,7,8 energy,9,10 biomaterials,11 and functional polymer coatings.12−14 Traditional protective coatings can only exert passive protective effect for material substrates. However, micropores and microcracks will occur during coating formation and service, which will cause lifetime reduction, economic losses, and safety accident. Therefore, it is necessary to develop protective coatings with both passive and active self-healing functionalities. © 2018 American Chemical Society

On the basis of whether implanting additional healing agents, self-healing materials can be roughly classified as intrinsic and extrinsic types.15,16 For intrinsic self-healing materials, the healing process can be realized through noncovalent interactions and dynamic covalent chemistry.17−19 Two conditions are necessary for the healing behavior: the contact of composites at defected interfaces and reforming bonds or interactions.20,21 However, these cases require external assistance to facilitate healing reactions, which are difficult to fulfill in the field of marine corrosion. Capsule-based self-healing materials are suitable candidates to fabricate intelligent anticorrosion coatings due to their low cost, simple preparation, and ease of application. Microcapsule-containing healing agents are embedded into the coating matrix. As the Received: July 4, 2018 Accepted: September 27, 2018 Published: September 27, 2018 36229

DOI: 10.1021/acsami.8b11108 ACS Appl. Mater. Interfaces 2018, 10, 36229−36239

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Illustration of the Procedure for Preparing rGO−CD Nanocontainers and Loading the Guest Molecules

impedance spectroscopy (LEIS) test. EIS was employed to evaluate the long-term anticorrosion performance. The prepared coatings exhibit efficient self-healing ability and excellent anticorrosion capability, which are attributed to (1) the release of corrosion inhibitive BTA molecules at exposed metal surfaces and (2) graphene nanosheets serve as barriers significantly impede the penetration of aggressive species.

defects formed in coating, the embedded capsules rupture and release agents to heal cracks through the polymerization reaction. Wang et al. have prepared isophorone diisocyanate22 and hexamethylene diisocyanate trimer23 microcapsules through in situ polymerization and then embedded the prepared capsules in the coating matrix. The protective coatings exhibit robust self-healing property after immersion in NaCl solution. Besides, epoxy (EP),13 linseed oil,24,25 and so forth. have been selected as healing agents to prepare microcapsule-based self-healing anticorrosion coatings. To ensure healing efficiency, large volume capsules with high loading capability are required, which causes difficulty in dispersion and destruction for coating integrity. The embedding of microcontainer/nanocontainer-encapsulating corrosion inhibitor into the coating matrix is a feasible way to design self-healing coatings. The inhibitor can release from containers in a controllable manner and to form a protective film at the scratch interface. Shchukin’s group has conducted extensive studies on smart nanocontainer-based anticorrosion coatings and proposed constructive instructions.26−28 After that, stimulus-responsive nanocontainers were also fabricated for constructing anticorrosion coatings with self-healing property.29−32 An effective anticorrosion coating should possess both passive barrier and active self-healing properties. Among the above self-healing coatings, the introduced containers are generally spherical porous nanoparticles with limited barrier capability. When cracks were generated on the coating, aggressive ions are prone to penetrate into coating matrix before the formation of healing films, which deteriorate the long-term protective function for coatings. Recently, graphene, a two-dimensional nanosheet material, exhibits great potential for developing high-performance protective coatings because of its remarkable mechanical property and extremely high impermeability. Previous studies have reported that evenly dispersed graphene33,34 or graphene oxide (GO)35,36 nanosheets significantly improved the anticorrosion ability of polymer coatings. Herein, we developed a new type of graphene-based containers, which were synthesized through a simple wetchemical strategy between GO and β-CD molecules. On the basis of host−guest inclusion, the corrosion inhibitor benzotriazole (BTA) molecules were successfully loaded into the containers. Release experiment reveals that the inhibitor release process is pH-responsive, which is suitable for preparing anticorrosion coatings. The self-healing property of container-based coatings was determined through the decreased corrosion area as proved from local electrochemical

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MATERIALS AND METHODS

Materials. Graphite, hydrazine hydrate, β-CD, and ammonia solution (25−28 wt %) were purchased from Aladdin Industrial Corporation. Waterborne EP resin (E-51) and curing agent (CU-600) were provided by Shanghai Run Carbon New Material Technology Co., Ltd. The carbon steel electrodes with an area of 1 cm2 were polished by 400 and 800 sand papers and cleaned in acetone and ethanol via ultrasonic vibration. Synthesis of rGO-CD Nanocontainers. GO nanosheets were synthesized from nature graphite by using the modified Hummer’s method.37,38 For this purpose, 120 mL of concentrated H2SO4 (98%) solution containing 2 g of graphite powder was placed in an ice bath with constant stirring for 2 h. To avoid overheating, 10 g of KMnO4 was added slowly to the above suspension under vigorous stirring. Then, the mixture was stirred for 24 h under ambient temperature. The resultant suspension was diluted with slow addition of deionized water, and the oxidation process was terminated through adding H2O2 solution. Finally, the GO was obtained after centrifugation and washing with hydrochloric acid (5%) and deionized water several times and vacuum drying at 50 °C. The reduced GO (rGO)−CD nanomaterial was prepared according to the reported procedure (Scheme 1).39,40 Typically, the GO aqueous solution was obtained with the concentration of 1 mg/ mL. In a separate vial, 500 mg of cyclodextrin was dissolved in 10 mL of water and mixed with 5 mL of GO solution. Then, 200 μL of ammonia solution was dropped into the above solution. After 30 min of stirring, 20 μL of hydrazine solution was added, and the suspension was placed in an oil bath (70−75 °C) for 4 h. The obtained stable black solution was then treated with a dialysis member in deionized water for 48 h (replace the dialysate with deionized water for every 5 h) to purify the product, and it was redispersed in water for the subsequent use. Loading and Releasing of Corrosion Inhibitor of BTA. The BTA molecules, a corrosion inhibitor, were loaded into rGO−CD containers based on the host−guest inclusion interaction. This process was carried out under reduced pressure to ensure the loading efficiency. First, the synthesized containers were mixed with an ethanol solution containing BTA (50 mg/mL) and stirred for 5 h in vacuum. To remove the excess BTA adsorbed on the surface of containers, the suspension was washed with distilled water three times. After centrifugation (5000 rpm) and being dried under 50 °C in a vacuum oven (−0.1 MPa), the BTA-loaded containers were obtained. 36230


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ACS Applied Materials & Interfaces The release behavior of BTA from containers in 3.5 wt % NaCl solution (with pH = 4, 7, and 10, respectively) was evaluated using a UV spectrophotometer. In a specific procedure, 20 mL of prepared NaCl solution containing 100 mg of BTA-loaded containers was introduced into a dialysis tubing (MWCO 10 kDa). Then, the dialysis tubing was placed in 230 mL of 3.5 wt % NaCl solution with continuous stirring at room temperature. To prepare UV testing samples, 0.1 mL of dialysate was taken through a pipette followed by addition of 3 mL water. This process was carried out at different intervals for 24 h. Preparing of Composite Coatings. The composite coatings were prepared according to the following procedures. Specifically, 5 mL of water suspension (10 mg/mL) of BTA-loaded containers was mixed with 3 g of waterborne hardener. This mixture was treated with magnetic stirring for 10 min and ultrasonication for another 1 h. After removing the excess solvent via a rotary evaporator, 2 g of EP resin (E-51) was added followed by magnetic stirring for 30 min. To ensure the denseness of coatings, the mixture was treated with the degassing process in a vacuum oven for 10 min under room temperature. Finally, the composite coatings were applied on the pretreated carbon steel electrodes using a bar coater with a thickness of 40 ± 3 μm. The coated electrodes were cured under room temperature for 48 h and then placed in an oven with 50 °C for 12 h, which was defined as rGO−CD−BTA/EP coating. For comparison, the pure EP coating and the coating with an empty container (rGO−CD/EP) were also prepared in a similar way. Instruments and Characterization. Fourier-transform infrared (FTIR) spectra were recorded on a FTIR spectrometer (Nicolet 6700, USA) with a resolution of 1 cm−1 between 400 and 4000 cm−1. The structural variations were determined through Raman spectroscopy (Renishaw inVia Reflex). UV spectra were examined on a Lambda 18 UV spectrometer. The X-ray diffraction (XRD) patterns were collected using an X-ray powder diffractometer (D8 ADVANCE, Bruker). Thermogravimetric analysis (TGA) measurement was carried out on a Q500 Thermogravimetric Analyzer (TA Instruments, USA) with a heating rate of 10 °C min−1 under a nitrogen atmosphere (100 mL/min). Besides, the differential scanning calorimeter (DSC) test was conducted on METTLER TOLEDO TGA/DSC I under a nitrogen atmosphere (20 mL/min) and at a heating rate of 5 °C min−1 with the range of 80−100 °C. The morphology of prepared containers in water was observed using a transmission electron microscopy (TEM) system (Tecnai F20) and a scanning probe microscopy (SPM) system (Dimension 3100). The self-healing performance of composite coatings was detected by using LEIS, which was conducted at a VersaSCAN microscanning electrochemical workstation (AMETEK, USA). The electrode, with a diameter of 10 μm, was set to vibrate at a speed of 200 μm/s with an amplitude of 10 mV. The impedance values were collected at the frequency of 10 Hz on an area of 5 × 5 mm2 with 26 × 26 scanning points. CHI-660E electrochemical workstation was used to evaluate the electrochemical behavior of coated electrodes using the traditional three-electrode system (a reference electrode: saturated calomel electrode; counter electrode: platinum plate with 2.5 cm2 area; and working electrode: coated mild steel with an exposed area of 1 cm2). The EIS tests were carried out in the frequency range from 105 to 10−2 Hz with a sinusoidal perturbation of 20 mV amplitude, and the recorded data were fitted through ZSimpWin software using the electrical equivalent circuits. In addition, the micromorphology and components of rust layers formed on the steel were identified through optical microscopy and Raman spectroscopy. Furthermore, the barrier properties of composite coatings were also evaluated through measuring the oxygen transmission rates (OTR), which were conducted using a Labthink differential pressure gas permeation analyzer (VAC-V2) under room temperature with 37% relative humidity according to the Chinese standard method of GB1038-2000.

FTIR experiments. Figure 1a depicts the FTIR spectra of GO, CD, and rGO−CD. As for GO, a strong and broad peak was

Figure 1. (a) FTIR and (b) Raman spectra of GO, β-CD, and rGO− CD; (c) UV spectra and (d) XRD patterns of GO and rGO−CD.

presented at 3420 cm−1 attributed to the O−H stretching of C−OH. Besides, the CO stretching of −COOH (1722 cm−1) and the EP vibration (1059 cm−1) were also observed, indicating the abundance of oxygen functional groups on the GO surfaces.41 After functionalization, the CO stretching peak disappeared, which can be ascribed to the reduction process by hydrazine. In addition, the characteristic peaks of CD were detected for the rGO−CD at 2925, 1028, and 1145 cm−1, relating to C−H, C−O−C stretching vibration, and O− H bending vibration, respectively.42 The ring vibrations of typical CD absorption were also observed for rGO−CD at 579, 708, 757, and 944 cm−1 even after extensive dialysis.43 Furthermore, a typical red shift of O−H stretching vibration was also exhibited for rGO−CD, confirming the formation of hydrogen bonds between graphene and CD. 44 These observations clearly illustrated that the CD molecules were successfully attached to the graphene surfaces. The defect and disorder structures of graphene and its derivatives could be characterized by Raman spectroscopy. Two apparent bands centered at 1348 (D band) and 1550 cm−1 (G band) were exhibited in the spectra of GO and rGO− CD (Figure 1b). Usually, the D band is assigned to the vibration of sp3 carbon atoms from the functional groups, whereas the G band is related to the in-plane vibration of sp2 carbon atoms.45 Compared with pristine GO, the rGO−CD presented an increased intensity ratio (ID/IG), which may be attributed to structural distortions resulting from CD.46 This variation indicated a decreased proportion of sp2 domains for rGO−CD and illustrated the interaction between GO and CD. The UV spectrum of GO and rGO−CD is presented in Figure 1c. It can be observed that GO displays two characteristic absorption peaks, corresponding to the π → π* transitions of aromatic C−C bonds (230 nm) and the n → π* transitions of CO bonds (300 nm).47 However, the absorption peak at 230 nm is red-shifted to 259 nm for the rGO−CD, which indicates that the electronic conjugation in GO was restored during the functionalization process. In addition, the color of the dispersion was transformed from yellow to black (inset in Figure 1c), suggesting the successful reduction of GO to rGO.

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RESULTS AND DISCUSSION Characterization of rGO−CD Nanocontainers. The formation of rGO−CD conjugates was determined through 36231


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Figure 2. TEM images of (a) GO and (b) rGO−CD; SPM images (c) GO and (d) rGO−CD; the height for the cross section identified by the line in (c,d): (e) GO and (f) rGO−CD.

The crystalline structures of GO and rGO−CD were studied through wide-angle X-ray scattering analysis, as shown in Figure 1d. A sharp diffraction peak at 10.5° was displayed for GO corresponding to the (001) plane, which reveals a highly ordered structure with interlayer spacing (d) of about 0.84 nm.48 The presence of abundant oxygen groups increased the interlayer spacing of GO. Besides, a narrow (100) peak centered at 43.1° was observed for GO, corresponding to the long range order in the graphitic planes. As for rGO−CD, the characteristic peak of GO disappeared and a new broad diffraction peak at 23.4° appeared. The presence of the broad diffraction peak signifies the disordered structure and the loss of 43° could reflect the loss of planarity for rGO−CD. This can be suggested that the graphene surfaces were partially occupied by β-CD molecules.49,50 Thus, the regular stacking and aggregation of reduced graphene layers could be largely prevented. Morphology of the As-Prepared Containers. The surface morphology of GO and rGO−CD containers was observed through TEM and SPM. As can be seen from the TEM images, the GO nanosheets exhibit smooth and transparent texture with few wrinkles (Figure 2a). Whereas, a more uneven and curled surface, as well as a lower transparency, was presented for rGO−CD containers (Figure 2b), which can be ascribed to the introduction of CD molecules on the graphene surfaces. In addition, the SPM measurement was employed to investigate the thickness and roughness of nanocontainers. Obviously, both GO and rGO− CD present flakelike nanostructures. The corresponding height profiles of the cross sectional are presented in Figure 2e,f. It can be observed that the rGO−CD containers show a rougher and uneven topology as compared with GO. Besides, the thickness of rGO−CD was about 3 nm (Figure 2f), which is thicker than GO (about 1.9 nm). The increased thickness of rGO−CD containers is a typical characteristic of functional molecule-attached graphene layers. These results indicated that the CD molecules were successfully incorporated on the graphene surfaces. Loading of BTA Molecules on rGO−CD Containers. The larger content of CD molecules on the synthesized containers indicates a higher loading capacity. TGA measure-

ment was employed to evaluate the CD content on the surface of graphene. The weight losses for GO, CD, and rGO−CD are presented in Figure 3a. The initial weight losses for three

Figure 3. (a) TGA curves of GO, CD, and rGO−CD; (b) DSC results for BTA, rGO−CD, and rGO−CD−BTA.

compounds in the temperature ranged below 100 °C were observed, which attributed to the adsorbed moisture in powder because of its highly hydrophilic groups. The severe decomposition of oxygen-containing groups of GO appeared at about 200 °C, and the weight percentage of oxygen functional groups on the GO surfaces can be deducted as 28%. Obviously, the CD shows a sharp weight loss region around 300 °C, relating to the thermal decomposition of CD molecules. It should be pointed out that the heat resistance of GO is enhanced after functionalization with CD molecules, which possessed intermolecular hydrogen bonding interactions between them. Therefore, the content of CD molecules on containers can be calculated as approximately 34%. This result reveals that a number of CD molecules “anchored” on the surface of graphene and enriched the loading ability of the asprepared containers. The DSC test was further utilized to determine the loading capacity of containers for BTA molecules. Figure 3b displays the DSC curves of BTA, rGO−CD, and rGO−CD−BTA. For rGO−CD containers, no peaks were observed in the range of test temperature. However, a prominent endothermic peak (about 98 °C) was displayed for BTA and rGO−CD−BTA, revealing the successful loading of BTA. The mass ratio of BTA in containers was calculated to be 24% based on their endothermic enthalpy. These results obtained from the TGA 36232


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pH = 10, followed by pH = 4. An extended release was presented in the second stage, which is beneficial for long-term corrosion inhibition. These results proved that the inhibitorloaded rGO−CD containers can be applied to prepare selfhealing anticorrosion coatings. In order to determine whether the released BTA is adsorbed on the exposed surface of steel, the EIS measurement was conducted for steel electrodes in 3.5 wt % NaCl solution with and without BTA-loaded containers at different pH conditions. The impedance diagrams for steel after different immersion times are presented in Figures 5 and S1. Generally, the impedance modulus at a lower frequency (Zf=0.01Hz) can be regarded as a reference to assess the protective performance of coatings.51,52 Therefore, the linear kinetics for BTA release were obtained through comparing the ratio between the impedance modulus for condition with and without inhibitorloaded containers over time. As can be seen from Figure 6, the

and DSC tests revealed the successful preparation of container and loading of BTA. In other words, the rGO−CD containers introduced in this work exhibit good encapsulating capability. Kinetic Curves of Inhibitor Release from Containers. The release behavior of inhibitor from containers was monitored by detecting the absorbance intensity of BTA in 3.5 wt % NaCl suspensions containing rGO−CD−BTA. The chosen testing conditions (pH = 4, 7 and 10) were weak acid and base, which are typical corrosive environment and can be applied to trigger inhibitor release. It was found that the profiles recorded at different pH values display two stages (an initial fast release and a gradual release) of inhibitor release (Figure 4). A large amount of BTA was released at the first

Figure 4. Profiles of BTA release from containers at different pH values measured with UV spectroscopy.

stage during the initial 2 h. The sample exhibits different release rates at different pH conditions, revealing that the release process is pH-dependent. In addition, the fastest release rate and the largest release amount of BTA were observed at

Figure 6. BTA release kinetics obtained from different conditions.

Figure 5. Bode and Nyquist plots of carbon steel after different immersion times in 3.5 wt % NaCl solution containing BTA-loaded containers at (a) pH = 4, (b) pH = 7, and (c) pH = 10. 36233


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Figure 7. SEM images of the fracture surfaces for (a,b) pure EP, (c,d) rGO−CD/EP, and (e,f) rGO−CD−BTA/EP; (g) typical Raman spectrum of samples with containers; Raman intensity maps for (h) rGO−CD/EP and (i) rGO−CD−BTA/EP.

Figure 8. LEIS maps around the artificial defect for steel electrodes coated with (a) pure EP, (b) rGO−CD/EP, and (c) rGO−CD−BTA/EP coatings immersed in 3.5 wt % NaCl solution.

largest slope (0.278) of kinetic curve was presented at pH = 10, revealing the quickly release and effective adsorption of BTA in this condition. However, the variation of this ratio becomes gradual at pH = 4 (slope of 0.201) and pH = 7 (slope of 0.077). These results are consistent with the release results measured by UV spectroscopy, which affirmed that the prepared container is appropriate to endow coating systems with active anticorrosion function. Fracture Surfaces and Dispersion Stability of Container-Based Coatings. The fractured surfaces for specimens were observed to investigate the interface interaction between

containers and the EP matrix. It can be observed that no significant aggregation was displayed for both rGO−CD/EP (Figure 7c) and rGO−CD−BTA/EP (Figure 7e) coatings, indicating the well dispersion state. For the pure EP coating, its surface is comparatively smooth with many micropores (red circles). After adding graphene sheets in the polymer matrix, the denseness is enhanced, making the surface much rougher because of the strong interface interactions. To further study the distribution of graphene in composite coatings, we utilize Raman spectroscopy to map the relative intensity of the EP band to the G band. Figure 7g shows the typical spectrum in 36234


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ACS Applied Materials & Interfaces the range of 900−1900 cm−1 for container-based coatings. The Raman mapping was performed over the surface of samples, and the scan area was selected as 500 × 500 μm2. The intensity ratios (Iepoxy/IG) are presented in Figure 7h,i for rGO−CD/EP and rGO−CD−BTA/EP coatings, respectively. It is obvious that there are less variations for intensity ratios, revealing the homogenous distribution of graphene sheets. After loading of BTA, the containers also maintained a good dispersion state, which affirmed that the interfacial interaction between graphene and EP resin does not been affected by BTA molecules. Self-Healing Properties of Container-Based Composite Coatings. The self-healing anticorrosion ability of the containers-based coatings was evaluated by using LEIS. Prior to the test, a scratch was made on the coating surface through a scalpel, exposing 4 × 0.2 mm2 steel substrate to the electrolyte. Without the coating protection, the metals are directly exposed to a corrosive environment and prone to induce localized corrosion. Therefore, the impedance values near the scratch are lower than the surrounding area. By detecting the impedance values over the scanned area, the metal oxidation and coating healing process can be investigated. Figure 8 shows the LEIS maps of the coated steel with defect in 3.5 wt % NaCl solution during 20 h immersion. For the pure EP coating, the local impedance values around the scratches decreased greatly just 10 h later (Figure 8a). Because of the poor density for pure EP resin, the electrolytes are apt to penetrate into the inner of coating matrix. This will result in metal deterioration expressed as the increased corrosion range and decreased impedance values. Although the rGO−CD/EP coating exhibits a decreased trend for local impedance (Figure 8b), the diffusion degree of corrosion reaction has been suppressed to some extent compared with pure EP. It is noted that the electrochemical process at the vicinity of scratches for rGO− CD−BTA/EP coating has been largely inhibited with decreasing corrosion area (Figure 8c), revealing the deceleration of mass transfer for redox reactions. The impedance values for the samples coated with different coatings measured around the defect (Y = 2.5 mm) after 20 h of immersion are presented in Figure 9a. It can be observed that the rGO−CD−BTA/EP coating exhibited the highest

impedance modulus compared with pure EP and rGO−CD/ EP coatings. This result indicates the process of metal oxidation and corrosion extension has been lowered. In addition, the difference of impedance values between defect (intermediate area) and intact coating (peripheral area) could reflect the active protection capability of coatings. Apparently, the impedance values at defect are almost the same as the intact coating area, revealing its superior active protection performance. We assumed that the presence of graphene-based containers largely enhanced the impermeable property of composite coatings. Meanwhile, the BTA molecules encapsulated in containers act as a corrosion inhibitor, further reducing the corrosion rate of metal substrate. To confirm the hypothesis and demonstrate the healing process by the BTA inhibition effect, Raman spectroscopy was performed to investigate the chemical components around the scratches after the LEIS test. For rGO−CD−BTA/EP coating, the characteristic peaks for BTA were appeared at 558, 789, 1046, and 1153 cm−1 (Figure 9b).53,54 However, the rust layers for pure EP and rGO−CD/EP coatings mainly consist of γFeOOH (252, 380, 527, and 648 cm−1) and β-FeOOH (384, 480, 676 and 725 cm−1).55−57 The β-FeOOH and γ-FeOOH are in thermodynamic instability, which can act as a cathode to participate in the reduction reaction. In addition, some new peaks were observed in 397, 1001 and 1118 cm −1 , corresponding to α-FeOOH (thermodynamic homeostasis). After incorporating rGO−CD−BTA, the intensity of βFeOOH and γ-FeOOH was lowered and presented an enhanced peak of α-FeOOH. Therefore, we speculate that the released BTA molecules contribute to the coating healing and inhibit the electrochemical corrosion reaction. Besides, the tested samples were observed through optical microscopy after removing coatings. For the steel coated with pure EP (Figure 9c1), its surface is corroded severely with enlarged corrosion area, suggesting the deterioration of coating protective property. The penetration of electrolyte was suppressed for the rGO−CD/EP coating because of its enhanced barrier property (Figure 9c2). It can be observed from Figure 9c3, few corrosive products were presented for steel coated with rGO−CD−BTA/EP coating. We assumed that the diffusion of electrolyte through coating defects could induce steel oxidation. In the coating defect interface, the encapsulated BTA may release from containers and adsorb on the substrate, which will prevent the exposed metal from further corrosion destruction. In addition, the prepared samples with scratch were immersed in 3.5 wt % NaCl solution to study the failure process and self-healing ability of the coatings. Figure S2 exhibits the optical images of the test samples for different immersion times. It can be observed that the pure EP coating (Figure S2a) severely corroded with accumulated corrosive products just for 24 h immersion, indicating its poor protective ability. The extent of corrosion is lowered after incorporating of rGO−CD, while obvious rusts were also displayed after 72 h immersion (Figure S2b). Thus, we concluded that the samples coated with pure EP and rGO−CD−BTA/EP coatings provide only passive protection. Conversely, the scribed area of the rGO−CD−BTA/EP coating is nearly free of corrosion after 200 h immersion (Figure S2c). This is probably due to the barrier property of graphene-based containers and self-healing functionality of BTA molecules. Long-Term Anticorrosion Performance of Composite Coatings. To estimate the durable anticorrosion performance

Figure 9. (a) Impedance values for specimens measured around the defect (Y = 2.5 mm) and (b) Raman spectrum of the scratched coatings after 20 h of immersion; (c) the optical micrographs of steel electrodes beneath the coating after LEIS test: (c1) pure EP, (c2) rGO−CD/EP, and (c3) rGO−CD−BTA/EP coatings. 36235


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Figure 10. Bode and Nyquist plots of (a) pure EP, (b) rGO−CD/EP, and (c) rGO−CD−BTA/EP coatings at different immersion time in 3.5 wt % NaCl solution. The corresponding equivalent electric circuits (d,e).

of nanocomposite coatings, EIS measurements were conducted at different immersion times. The EIS results and the corresponding equivalent electric circuits are presented in Figure 10. For the pure EP coating, the impedance modulus at lower frequency (Zf=0.01Hz) decreased dramatically from 6.29 × 108 to 1.05 × 107 Ω cm2 during the immersion. The gradually reduced capacitive arcs in Figure 10a3 also proved the decreased protection performance. In the equivalent electric circuits, Rs, Rc, and Rct correspond to the solution resistance, coating resistance, and charge transfer resistance, respectively. The constant phase element was used to investigate the electric double layer on the interface of electrode and solution, which denotes capacitance characteristics58 (Qc and Qdl relate to coating and double layered capacitance) when its exponent (n) is close to 1. The deviation from pure capacitance is usually referred as the dispersion effect. Generally, the coating failure process could be roughly divided into two stages according to whether moisture reaches the coating/substrate interface. In stage 1, the corrosive mediums had not yet reached the metal surface, expressed as one time constant. After a period of water penetration, corrosive ions contacted with the substrate and induced metal corrosion, and thus, two time constants were generated in this stage (stage 2).59 Two time constants (two arcs) were reflected in the bode-phase diagram (Figure 10a2) after 25 days immersion, illustrating the degradation of pure EP coatings. The fast failure of pure EP coating was attributed to the existed pores and its poor barrier property.

For the container-based EP coatings, the extent of reduced impedance values was greatly narrowed. Even after 55 days immersion, these values are maintained above 108 Ω cm2 for both rGO−CD/EP and rGO−CD−BTA/EP coatings. In addition, only one time constant was displayed as reflected from Bode-phase and Nyquist diagrams (single arc). The presence of one time constant indicates that the corrosive medium is far from reaching the metal coating interface. The uniformly distributed graphene-based containers effectively reduced the penetration rate for the electrolyte and enhanced the barrier effect of polymer coatings. It is worth noting that BTA-loaded containers/EP coating obtained the highest impedance value during the immersion period, indicating their superior protective performance. The variations of coating resistance (Rc) obtained from fitting results are presented in Figure 11a. It can be observed that all samples

Figure 11. Time-dependent behavior of (a) coating resistance (Rc) and (b) water-uptake (X) for specimens during immersion. 36236


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the penetration of aggressive species into the coating matrix, thus initiating the metal corrosion. Under this condition, we speculate that the microgalvanic couples could be easily formed at the localized area. In anodic zones, the metal is oxidized accompanied by losing electrons. The local pH in anodic zones will decrease because of the hydrolysis of metal ions. However, oxygen near the electrodes is reduced by electrons to generate hydroxide ions, causing local alkaline increase. The BTA molecules could release from containers and gradually adsorbed on the exposed metal surface to exert their corrosive inhibition effect. This process could be accelerated with the variations of local pH values. The schematic representation of the self-healing mechanism is presented in Figure 13. More importantly, this new type of

exhibited a decreased trend with immersion. However, the rGO−CD−BTA/EP coating displayed the highest value (2.40 × 108 Ω cm2) after 55 days immersion compared with rGO− CD/EP (7.79 × 107 Ω cm2) and pure EP (5.71 × 106 Ω cm2) coatings. Furthermore, the barrier effect of coatings was also reflected from the water absorption rate, which was calculated by the Brasher and Kingsbury (BK) equation60−62

( ) × 100

log Xv % =

Cc(t ) Cc(0)

log(80)

(1)

where Xv (%), Cc(t), and Cc(0) represent the volume fraction of water in coating matrix, coating capacitance at time t, and coating capacitance at initial time, respectively. The lowest water absorption rate was observed for rGO−CD−BTA/EP coating, implying its excellent impermeability (Figure 11b). These results proved the excellent long-term protection performance of inhibitor loaded containers/EP coating, attributing to the synergistic effect of impermeability of graphene and corrosion inhibition function of loaded BTA. Oxygen Barrier Properties for Different Composite Coatings. To further examine the impermeability of coating matrix after the introduction of nanocontainers, the OTR was examined, which indicates the volume of oxygen transported through a unit area of the sample during the specified pressure difference and time. According to Fick’s first law, the amount of gas J (m3) transported a unit surface (m2) of coating film in a unit time (s) was obtained by the following equation1 J=P×

Δp d

(2)

where P is the permeability coefficient cm ·cm/(cm ·s·Pa), Δp refers to the pressure difference between oxygen on both sides of the coating, and d is the thickness of coating film. The J divided by Δp gives the corresponding OTR values and is presented in Figure 12. It can be seen that the pure EP coating 3

2

Figure 13. Schematic representation of the self-healing mechanism for graphene-based containers composite coatings.

rGO−CD-based nanocontainer exhibited inhibitor loading and impermeable properties, simultaneously. The penetration process around the defect can be largely inhibited. Thus, the container-based coatings possessed a superior long-term anticorrosion performance.

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CONCLUSIONS In summary, by combination of the inclusion property of βcyclodextrin and the impermeability of graphene nanosheets, we have developed a novel reduced graphene/β-cyclodextrinbased nanocontainer for achieving self-healable waterborne EP coatings. The as-prepared containers exhibit high loading capacity of BTA and impermeability. More importantly, with the incorporation of the BTA-loaded containers, the resulted polymer coating exhibits efficient self-healing function, which may derive from the inhibitive BTA molecules; simultaneously, the excellent barrier property that originates from graphene nanosheets effectively impedes the aggressive species penetration around coating defects. Furthermore, the incorporated containers significantly enhanced the durable anticorrosion performance of polymer coatings. Possessed with dual functions of the self-healing ability and impermeability, the prepared coating described in this study presents a great potential for practical anticorrosion applications.

Figure 12. OTR for different coating systems.

exhibited the largest OTR value 7.08 × 10−3 m3/(m2·d· 0.1MPa), indicating that the oxygen can easily pass through the coating film. After incorporation of rGO−CD containers into the coating matrix, the OTR value was largely reduced to 5.94 × 10−5 m3/(m2·d·0.1MPa), which reveals the high barrier property of container-based coating. In addition, a lower OTR value 6.20 × 10−5 m3/(m2·d·0.1MPa) was also observed for rGO−CD−BTA/EP coating. These can be concluded that the prepared graphene-based containers significantly improved the barrier performance of EP coatings. Schematic of the Self-Healing Mechanism. Coating defects, such as microcracks and micropores, will occur during the process of service and coating preparation, which facilitate 36237


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11108.

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Bode and Nyquist plots of carbon steel after different immersion time in 3.5 wt % NaCl solution without BTA loaded containers and optical images of samples with scratch immersed in 3.5 wt % NaCl solution for different time (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Z.). *E-mail: [email protected] (L.W.). ORCID

Chengbao Liu: 0000-0002-1506-757X Haichao Zhao: 0000-0002-3558-1306 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully appreciate the financial support provided by the “One Hundred Talented People” of the Chinese Academy of Sciences (no. Y60707WR04); Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (no. QYZDY-SSW-JSC009); State Key Laboratory of Marine Coatings Funded Project; Natural Science Foundation of Zhejiang Province (no. Y16B040008); and Zhejiang Province Key Technology Project (2015C01006).

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Cyclodextrin-Based Nanocontainer: Synthesis and

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