graphene

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Materials and Interfaces

Controlled preparation of MgAl-layered double hydroxide/ graphene hybrids and its applications for metal protection Wen Sun, Tingting Wu, Lida Wang, Chuang Dong, and Guichang Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01742 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

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Industrial & Engineering Chemistry Research

Controlled preparation of MgAl-layered double hydroxide/graphene hybrids and its applications for metal protection Wen Sun,†,‡,// Tingting Wu,†,§ Lida Wang,† Chuang Dong,‡ and Guichang Liu*,† †School

of Chemical Engineering, and ‡Key Lab for Materials Modification by Laser, Ion and

Electron Beams of Education Ministry, Dalian University of Technology, Dalian 116024, China §State

Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy

of Sciences, Dalian National Laboratory for Clean Energy, Dalian, 116023, China //Material

Corrosion and Protection Key Laboratory of Sichuan Province, Sichuan University of

Science & Engineering, Zigong, 643099, China Keywords: Graphene; Layered double hydroxide; Nanocomposites; Metal protection

Abstract: The ion-conductive nature of layered double hydroxides (LDHs) and poor dispersibility of graphene are considered to be vital problems that limit their applications in the field of metal protection. This work aims to simultaneously solve the two problems by tuneable assembly of LDHs and graphene. MgAl-LDHs/graphene hybrids (LGHs) with different micro-structures were prepared by controlled hydrothermal method. The experiments reveal that LGHs prepared after 6 h of hydrothermal reaction are less permeable to water than LDHs and more dispersible than graphene, which makes that LGHs6/polymer coating possess a more excellent metal protection performance than pristine LDH/polymer coating and graphene/polymer coating. 1 ACS Paragon Plus Environment

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1. Introduction Graphene is a one-atom-thick two-dimensional carbon material with marvellous mechanical, electrical, thermal and optical properties.1-4 Since its discovery, graphene has shown great potentials for applications in energy, environmental, biomedical and electronic related areas.5-7 Up till now, one of the most promising strategies of graphene application is to embed graphene materials into a polymeric matrix so that graphene/polymer nanocomposites with excellent performance and versatile function can be prepared.8,9 Though it has been reported that graphene can be prepared by micromechanical cleavage, chemical vapor deposition, liquid-phase exfoliation of graphite and reduction of graphene oxide (GO),10 only reducing graphene oxide by chemical reducing agent and thermal treatment has been proved to be a large-scale method which is suitable for production of graphene nano-filler for polymer nanocomposites.11 However, it should be noted that graphene is particularly prone to aggregate due to strong π–π interactions, hydrophobic interactions, and van der Waals forces.12-15 Therefore, the poor dispersibility of chemically converted graphene in organic solvents is a key issue in the preparation of graphene/polymer nanocomposites by solution blending method since most of engineering polymers are soluble in organic solvents. Surface functionalization has been demonstrated to be an effective method that can improve the dispersibility of graphene.12,16 Generally, covalent grafting of organic molecular chains onto graphene surface functionalization is able to endow graphene/polymer nanocomposites with enhanced performances, including barrier properties, mechanical properties, thermal stability, thermal conductivity, electrical conductivity, etc.9,16 Especially, the major effect of functionalized graphene in the graphene/polymer nanocomposites used for corrosion protection is to increase the barrier properties so that the metal substrates can be physically isolated from aggressive environments.17-19 However, failure in protection performance is inevitable for coatings because almost all the protective

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coatings are susceptible to defects, which may be induced by improper coating, mechanical damage and environmental degradation. Therefore, developing functional graphene fillers would be of great significance for the long-term protection of metallic facilities.20-22 Among the existing fillers that have been used for metal protection, layered double hydroxides (LDHs) have been demonstrated to be one of the most promising multi-functional materials due to its special structure, facile preparation and low costs.23,24 Up till now, the most conspicuous application of LDHs has been demonstrated to be in “smart” protective coating because the possibility of ion intercalation into the unique layered structure of LDHs makes LDH materials be able to act as nanocontainers carrying corrosion inhibitors, which can be slowly released into electrolyte when the local environment undergoes slight changes that caused by corrosion at coating defects.25,26 Another function of LDHs is to act as nano-traps that can entrap corrosive chloride anions by ion-exchange and drastically reduce the permeability of chloride anions through the protective coatings.27 Furthermore, it is generally believed that LDHs are two-dimensional layered materials, incorporating LDHs nanosheets into the matrix of polymer coatings can lengthen the diffusion pathway of aggressive species in the coating matrix, of which the barrier function is similar to that of graphene.28,29 Unfortunately, to the best of our knowledge, reports on the barrier properties or corrosion protection of LDHs-reinforced polymer nanocomposite is very few. An important cause of this phenomenon is that LDHs are hydrophilic ionic conductor.30,31 The presence of LDHs would provide a shortcut for the penetration of water and corrosive ions through the coating matrix, which accelerates the coating failure.32,33 Therefore, in order to enhance the corrosion protection performance of LDHs/polymer composite coatings, inhibiting the hydrophility and ionic conductivity of LDHs is vital important. Graphene has a hydrophobic nature and is impermeable to almost all the ions.34,35 Combining the advantages of graphene and LDH materials, multifunctional fillers with excellent performance are



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expected to be prepared. In this work, MgAl-layered double hydroxide/graphene hybrids (LGHs) with different micro-structure were prepared by hydrothermal reaction and embedded into the matrix of acrylic resin. The corrosion protection performances of LGNs/acrylic resin nanocomposite coating (LGNs/AR) were detailedly investigated by electrochemical impedance spectroscopy (EIS) and Tafel polarization. The results reveal that only properly compounding graphene with LDHs is able to improve the dispersibility of graphene and decrease ion conductivity in LDHs at the same time. 2. Experimental 2.1 Preparation of LGHs LGHs was synthesized by hydrothermal method. Typically, 15.6 g of GO dispersion (8 mg/mL) was added into 50 mL deionized water. After magnetically stirring for 10 min, the dispersion was ultrasonically treated for 60 min. Subsequently, 20 mL of aqueous solution containing 5.12 g of Mg(NO3)2•6H2O and 3.75 g of Al(NO3)3•9H2O was slowly added into the dispersion with continuously stirring. Then, 3.64 g of hexamethylenetetramine (HMTA) was dissolved into another 20 mL of aqueous solution and added to the above-mentioned mixture. After fully mixed, the mixture was transferred into a 200 mL Teflon-lined stainless autoclave and heated at 140 °C for 2~12 h. The LGHs was collected by centrifugation (7500 rpm for 5 min) and washed with deionized water for several times. Finally, LGHs powder was obtained after freeze-drying. For comparison, pristine MgAl-layered double hydroxide (MgAl-LDH) was prepared by a similar method without using GO and reduced GO (rGO) was also prepared by hydrothermally reducing 90 mL1.38 mg/mL GO with 3.64 g of HMTA at 140 °C for 12 h. 2.2 Preparation of LGNs/ARs In order to investigate how the micro-structure of MgAl-LDH growing on the surface of graphene influences the dispersion of graphene in coating matrix and the resulted enhancement of corrosion


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protection performance of the LGNs/ARs, neat graphene loading in the matrix of acrylic resin was controlled to be ~1 wt.% by adjusting the LGHs loading. Detailed experimental procedure is presented as following. Typically, a certain amounts of the as-obtained LGHs were dispersed in 6.0 mL of acetic ether with the assistance of an 80 W sonic bath for 1 h. Then, 4.0 g of acrylic resin and 1.0 g of curing agent (Bense Paint, Shanghai Kangheng Co., Ltd.) was subsequently added into the dispersion with vigorous stirring. After being sonicated for another 30 min, the LGHs/acrylic resin dispersion was coated on a round copper substrate, followed by horizontally placing at room temperature for 4 h and further curing at 80 °C for 1 h. After coating twice, the final coating thickness was measured to be ~8.6±0.7 μm (TV110, Beijing Time Sun, China). Similarly, for comparison, neat acrylic resin was also prepared. 2.4 Characterization The micro-structure of MgAl-LDHs and LGHs samples were characterized by scanning electron microscopy (SEM, FEI Nova NanoSEM 450, USA), transmission electron microscope (TEM, FEI Tecnai G2 F30, USA), X-Ray Diffraction (XRD, XRD-7000S, Japan), X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi, USA), Flourier transformation infrared spectroscopy (FTIR, EQUINOX55, Germany) and Raman spectra (DXR Microscope, USA). The thickness of filler were measured via a field emission scanning electron microscopy (FE-SEM, Zeiss SUPRA 55, Germany) or atomic force microscopy (AFM, Dimension ICON, Germany). The dispersion of fillers in the coating matrix was observed via an optical microscope (YYS-120E, Yiyuan Optical Instrument, China). Electrochemical impedance spectroscopy (EIS) and Tafel polarization were performed on SP-300 electrochemical workstation (Bio-logic, France) using a three-electrode cell with a saturated calomel reference electrode (SCE), a platinum foil counter electrode and the coated copper sample as the



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working electrode. The corrosive electrolyte was 3.5 wt.% NaCl aqueous solution. EIS were performed at open circuit potential (Eocp) using a potential amplitude of 10 mV and a frequency range of 100 kHz ~ 10 mHz. Tafel polarization was also recorded on the SP-300 electrochemical workstation in potential range of -250 ~ +250 mV vs. Eocp at a scanning rate of 10 mV/min. 3. Results and Discussion Figure 1 shows the SEM images of MgAl-LDHs, GO, rGO and LGHs prepared at different reaction times by hydrothermal method. It can be seen from Figure 1a that pure MgAl-LDHs were mainly composited of hexagonal nano-platelets with 1~3 μm in plane and ~20-80 nm in thickness. Some of the MgAl-LDH nano-platelets are even transparent to electron (Figure S1a and S2c). The SEM images also showed that GO and rGO exhibited a typically wrinkled structure of smooth nanoplatelets (Figure 1b and c). For LGHs, the morphology depended on the reaction time. Generally, when immersed in the solution of Mg(NO3)2 and Al(NO3)3, negatively charged GO would adsorb Mg2+ and Al3+ ions onto its surface. After hydrothermal reaction, hexagonal MgAl-LDH nano-platelets tended to grow gradually on the surface of graphene with the prolonging of reaction time. Figure 1d revealed that there were very few MgAl-LDH nano-platelets that grew on the graphene surface after 2 h of hydrothermal reaction. Under higher magnification, numerous needle-like nano-particles with a length of ~150 nm and a diameter of ~10 nm were observed on the surface of rGO (Figure S1d). In addition, the obtained LGHs-2 was black and its apparent feature was similar to that of rGO. The presence of rGO in the hybrids was due to the fact that hydrothermal treatment of GO in alkaline solution can remove most of the oxygen-containing functional groups on GO surfaces.36 When implemented the hydrothermal reaction for 4 h, noticeable MgAl-LDH nano-platelets and nano-needles were observed on the surface of rGO (Figure S1d). The in-plane size of MgAl-LDH nano-platelets was 1~2 μm and their thickness was ~12-22 nm (Figure 1e and Figure S2e). The size of nano-needles was comparable


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to that in LGHs-2. Additionally, these MgAl-LDH nano-platelets were also electron transparent (Figure S1d inset). The LGHs-4 was also black due to the fact that the MgAl-LDH nano-platelets in LGHs-4 was still restricted. Further prolonged the reaction time to 6~12 h, the colour of LGHs gradually faded into greyish white, indicating there were more and more MgAl-LDH nano-platelets formed in the LGHs. SEM images of LGHs revealed that the in-plane size of MgAl-LDH nanoplatelets on graphene surface gradually increased to about 6 μm (Figure 1g-i) and the nano-needles on the rGO surface decreased obviously. Contrarily, the mass of LGHs increased gradually. The initial mass of GO was ~0.125 g (36.4 wt.% O and 63.6 wt.% C). After 2~12 h of hydrothermal reaction, the mass of LGHs gradually increases to 1.67, 1.73, 2.06, 2.27, 2.37 and 2.43 g, which indicated that there were more and more MgAl-LDH nano-needles or nano-platelets growing on graphene surface.

Figure 1. SEM images of (a) MgAl-LDH nano-platelets, (b) GO, (c) rGO, (d) LGHs-2, (e) LGHs-4, (f) LGHs-6, (g) LGHs-8, (h) LGHs-10, and (i) LGHs-12. (bar=10 μm) TEM was also employed to analyse the micro-structure of the LGHs. Figure 2a showed the TEM image of MgAl-LDHs. It revealed a single plate of MgAl-LDH nano-platelets with well-defined



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hexagonal morphology. The TEM images presented in Figure 2b and c demonstrated that GO and rGO was flexible and electron transparent structure. In addition, GO and rGO had a clear surface. After 2 h of hydrothermal reaction, MgAl-LDH nano-needles were observed on the surface of graphene. Further prolonged the reaction time to 4, 6 and 8 h, MgAl-LDH nano-platelets with more and more distinguishable hexagonal structure appeared on the graphene surface, whereas the MgAl-LDH nanoneedles disappeared obviously. Figure 2h and i revealed that the hexagonal morphology of MgAl-LDH nano-platelets gradually changed into regular polygon or even close to circle. Besides MgAl-LDH nano-platelets, electron transparent graphene nanosheets were observed in the TEM images of LGHs.

Figure 2. TEM images of (a) MgAl-LDH nano-platelets, (b) GO, (c) rGO, (d) LGHs-2, (e) LGHs-4, (f) LGHs-6, (g) LGHs-8, (h) LGHs-10, and (i) LGHs-12. (bar=500 nm) 8 ACS Paragon Plus Environment

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The XRD patterns of the GO, rGO, MgAl-LDH and LGHs were shown in Figure 3. The sharp and symmetric reflections in the XRD pattern of MgAl-LDH at 11.6°, 23.5°, 34.9°, 39.4°, 47.0°, 61.0°, and 62.1° correspond to the typical characteristics of the layered crystalline MgAl-CO3-LDH phase: basal spacing of (003), (006), (012), (015), (018), (110) and (113), respectively. The d003 spacing value of the MgAl-LDH was calculated to be 0.75 nm, which was the same as that reported in the literature for MgAl-LDH.37,38 Graphene oxide shows a significant (002) peak at 10.5°, which corresponds to an interlayer spacing of 0.84 nm. Compared to graphite, the enlarged interlayer spacing of GO can be attributed to the presence of hydroxyl, carboxyl and epoxy groups. After the GO was hydrothermally treated in the aqueous solution of Mg(NO3)2, Al(NO3)3 and hexamethylenetetramine at 140 °C for 2~6 h, the XRD pattern exhibited several peaks, especially two featured diffraction peaks at 10.0° and 19.8°. According to the observation of SEM and TEM, these peaks could be ascribed to the existence of MgAl-LDH nano-needles on graphene surface. The XRD patterns also revealed that the characteristic peak of GO at 10.5° gradually disappeared with the prolonging of reaction time, which might indicate an increased in-plane spacing of GO. In addition, no obvious characteristic peaks of MgAl-LDH nanoplatelets can be observed in the XRD patterns of LGHs-2, which indicates that there is little MgAlLDH nano-platelets. For LGHs-4 and LGHs-6, obvious characteristic peaks of MgAl-LDH nanoneedles were also observed at 10.0° and 19.8°, and weak characteristic peaks of MgAl-LDH nanoplatelets were observed at 11.8°, 23.6° and 34.9°, indicating the co-existence of MgAl-LDH nanoneedles (dominant component) and nano-platelets on the graphene surface. The XRD patterns of LGHs-8 revealed that the characteristic peaks of MgAl-LDH nano-needles became very weak, while the characteristic peaks of MgAl-LDH nano-platelets were strong, which indicated the MgAl-LDH nano-platelets were dominant components in the LGHs. For LGHs-10 and LGHs-12, the XRD patterns only showed the existence of MgAl-LDH nano-platelets in the hybrids. XRD analysis confirmed that



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the LGHs with different micro-structure were successfully synthesized. FT-IR spectra and Raman spectra were also used to demonstrate the successful preparation of LGHs. Both the FT-IR spectra of MgAl-LDH and LGHs showed that there were bands recorded below 800 cm−1, especially a sharp and strong characteristic band around 452 cm−1 appeared due to the vibration of metal-oxygen bond in the brucite-like lattice,39 indicating the presence of MgAl-LDH in LGHs. (Figure S3a). Furthermore, the Raman spectra of LGHs showed D bond and G bond similar to graphene materials, demonstrating the presence of graphene in LGHs. (Figure S3b)

Figure 3. XRD patterns of GO, MgAl-LDHs and LGHs prepared with different hydrothermal reaction time. Figure 4 showed the XPS survey spectra of GO, MgAl-LDH and LGHs prepared with different reaction time. Compared to that of GO, the XPS spectra of LGHs not only exhibited O1s peak (532.5 eV) and C1s peak (284.8 eV), but also showed two featured peaks of MgAl-LDH at 75 and 1304 eV, which was attributed to Al2p and Mg1s, respectively. The XPS results verified the formation of LDHs in the hybrids. The XPS C1s spectra of the LGHs shown in Figure 5 were further analysed to verify the reduction of GO in alkaline solution during hydrothermal reaction. The XPS C1s spectrum of GO


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showed two featured peaks at 284.6 and 286.7 eV, which corresponds to C=C/C-C components of carbon skeleton and C–O components of hydroxyl and epoxide groups, respectively. A broad small shoulder at approximately 288.5 eV was also observed in the spectrum, which was attributed to OC=O/C=O components of carboxyl groups (Figure S4). After 2~4 h of reaction, the peak corresponding to the C–O components of hydroxyl and epoxide groups at 285.7~286.6 eV in the spectra of LGHs were significantly decreased by hydrothermal reduction in alkali solution. However, the spectra still showed small peaks in that region. Extending hydrothermal reaction to 6~12 h, the change of C–O peak did not decrease further, which indicated a maximum reduction degree of GO. Furthermore, the O-C=O/C=O peak around at 288.6 eV stuck to the end during 2~12 h of hydrothermal reactions. As –COOH on GO surface could be removed easily in alkali conditions, the O-C=O/C=O peak, which was also observed in the XPS C1s spectra of pristine MgAl-LDHs, could be ascribed to the presence of carbonates in the LGHs. The elemental and chemical bonding analysis of XPS spectra suggested that the growth of MgAl-LDH on graphene surface and the reduction of GO occurred simultaneously during the hydrothermal reactions.

Figure 4. XPS survey spectra of GO and LGHs prepared with different hydrothermal reaction time.



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Figure 5. XPS C1s spectra of LGHs prepared with different hydrothermal reaction time. (a) 2 h; (b) 4 h; (c) 6 h; (d) 8 h; (e) 10 h; (f) 12 h. The evolution of the corrosion protection performance of LGHs-reinforced acrylic resin coatings which were immersion in 3.5 wt.% NaCl aqueous solution for 60 days was investigated by EIS. Figure 12 ACS Paragon Plus Environment

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6 showed the EIS results of acrylic resin reinforced with 1 wt.% of different fillers. The Bode-modulus plots revealed that the acrylic resin loading with LGHs-4, LGHs-6, LGHs-8 and LGHs-10 had an impedance modulus which was higher than 1010 Ω•cm2 at 0.01 Hz, pristine acrylic resin and acrylic resin loading with rGO exhibited an impedance modulus around 1.6×108 Ω•cm2, while the acrylic resin loading with LGHs-2, LGHs-12 and pristine MgAl-LDH had an impedance modulus of was 2.0×105, 3.2×106 and 8.0×108 Ω•cm2. (Figure 6a) For the Bode-phase plots of all the samples, two peaks could be distinguished from the profiles. (Figure 6b) The peak appeared at high frequencies could be attributed to the responses of coatings (>103 Hz) and the peak appeared at medium/low frequencies could be ascribed to the corrosion responses of metal substrates (

graphene

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