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Reduction-Coagulation Preparation of Hybrid Nanoparticles of Graphene and Halloysite Nanotubes for Use in Anticorrosive Waterborne Polymer Coatings Yanling Jia, Teng Qiu, Longhai Guo, Jun Ye, Lifan He, and Xiaoyu Li ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00044 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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ACS Applied Nano Materials

Reduction-Coagulation Preparation of Hybrid Nanoparticles of Graphene and Halloysite Nanotubes for Use in Anticorrosive Waterborne Polymer Coatings

Yanling Jia,ab Teng Qiu,*ab Longhai Guo,ab Jun Ye,ab Lifan He,a b Xiaoyu Li*ab a

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, PR China

b

Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, PR China

* Corresponding authors: E-mail: [email protected] E-mail: [email protected]

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ABSTRACT Graphene has been extensively concerned as an ideal modifier in the exploration on mordern polymer coatings with high performance. However, the poor dispersibility of graphene in water strongly limits the application in waterborne systems. Here, we demonstrate a simple but effective reduction-coagulation method for the preparation of hybrid nanoparticles of halloysite nanotubes (HNTs) and reduced graphene oxide (rGO). The work was started from the simple mixture of the aqueous dispersion of graphene oxide(GO) and HNTs. As the subtle introduction of the reducing agent, GO was reduced into rGO and co-coagulated with HNTs. Tailored by the residual hydrophilic groups inside the nanotube of HNTs, the phase separation was controlled in colloidal scale, and stable aqueous dispersion of HNTs-rGO hybrid nanoparticles was thereafter obtained conveniently. We directly blended the product into the polymer latex for the formation of coating films. The tests reveal that the composite coating system with the addition of 0.5 wt % of HNTs-rGO can provide excellent corrosion protection for more than 90 days in all the testing mediums except 10 wt % H2SO4, which is much superior to the blank polymer coating.

Keywords: graphene, halloysite nanotubes, hybrid nanoparticles, waterborne coating, anticorrosion

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1. INTRODUCTION Graphene, a legend of two-dimensional materials, has received great attentions in extensive fields.1-2 The chemical reduction of graphene oxide (GO) is mostly used for the preparation of graphene.3 However, sharply different from GO which is compatible with water, reduced GO (rGO) cannot be dispersed in aqueous mediums as the consequence of the elimination of the polar groups on their surfaces.4-5 The poor water comparability is the bottleneck of rGO for the application in the field of waterborne polymer coatings, which has been much concerned in recent days encouraged by the environmental friendliness.6-7 In some literature, graphene is introduced in the form of GO and enhanced mechanical performance is achieved.8 But the post-reduction on the composite of GO-polymer is required when rGO is in need for better electrical or barrier properties.9 In comparison with complex post-reduction process, the direct introduction of rGO into polymer latex will be of more advantages if the poor disperse capability of rGO is modified. Excellent pioneer work has been done solving the problems. Using different surfactants, Guardia et al have dispersed graphene in aqueous media.10-11 However, desorption and emigration of the physically absorbed amphiphilic organic molecules on rGO would happen in the coating film, which would result in the rapid deterioration on the anticorrosion performances. Alternatively, Ye et al presented a surface grafting strategy, which permits the high grafting density to a single graphene face and results in good solubility and process ability.12 But the grafting strategy would be complex in compared with surfactant modification. We suppose, inspired by the recent successes in Pickering emulsions, inorganic nanoparticles with proper surface wetting characteristics can be introduced to modify the surface of rGO instead of organic surfactants. 3



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Recently, halloysite nanotubes (HNTs) have been reported as a new type of polymers additive, which are proved of positive contribution on the thermal, mechanical and anticorrosive properties.13-14 HNTs are a kind of naturally deposited aluminosilicate. Being cheap and environmental friendly,15 HNTs are unique as their predominantly hollow microtubular structures.16 The internal lumen surface and external surface are different: the external surface is composed of siloxane (Si-O-Si) groups and of a small number of aluminol (Al-OH) and silanol (Si-OH) groups on the edges and surface defects; the interlayer surface and the internal lumen surface consist of aluminol (Al-OH) groups.17 Importantly, HNTs are hydrophilic with good water wettability,18-21 which is attractive to be applied as the building block of hybrid nanoparticles with enhanced water dispersibility. The incorporation of HNTs and GO has already been seen in literature,22 but in need of coupling agents, which have to be grafted on HNTs or GO or both. Aiming to a simpler method, we turn our sights on coagulation method. The essence of coagulation method is the induced microphase separation by the subtle

changing

on

the

thermodynamic

and/or

kinetic

parameters

under

well-controlled interfacial conditions. The concept is not new, however, it has been explored more and more in the pioneer fields for the new trigger mechanism as well as interfacial control mechanism aiming to the preparation of aqueous dispersion system with complex structures and functionalities.23-24 Although the phase separation can traditionally be triggered by non-solvent,24 temperature25 and electrolytes,26 there is not report utilizing the reduction reaction to induce the coagulation as far as we know. Considering that the reducing of GO into rGO in aqueous dispersion is actually a process of the step-by-step losing of the hydrophilicity of the nano-sheet, we think it would be possible to use the reaction as the trigger for the coagulation of HNTs and 4


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rGO. The external surface of HNTs can provide the deposition sites of rGO, and the hydroxyl groups on the edges and surface defects can provide hydrogen bonding sites to anchor the deposition. Moreover, the Al-OH groups on the internal surface of the tubular would not be changed by the deposition, which can proper ionized under proper pH to stabilize the hybrid of HNTs and rGO in aqueous dispersion by electrostatic effects. HNTs-rGO hybrid nanoparticles would then be obtained with the characteristic geometric size in colloidal scale by our reduction-coagulation method. Remarkable performances such as good dispersion stability, excellent compatibility with waterborne systems and remarkable barrier properties of the hybrid nanoparticles modified polymer coatings are expected relying on the synergism of the two high-performance nano materials. To verify the assumption, in this paper, we fabricated HNTs-rGO hybrid nanoparticles by our reduction-coagulation method as described above. Hydrazine hydrate was added in the dispersion mixture of GO and HNTs as the reducing agent. The rGO was stabilized by HNTs, which served as the heteronucleation agent to coagulate with rGO for the formation of hybrid nanoparticles in water. The hybrid nanoparticles dispersion can be directly used as the modifier of the latex coating systems based on following considerations: first, the aqueous dispersion of HNTs-rGO is easy to be prepared with good compatibility with polymer latex in water, which means the two dispersion (the hybrid nanoparticles and the polymer latex) could be blended directly; second, both of HNTs and rGO are desirable nano modifiers for polymers.27-28 Therefore, the synergistic effect on the coating films should be obtained. The enhanced compatibility of the rGO-based component with the polymer system would result in dense coating film without the possible defects from serious phase separation. The increased shape complexity in micro scale by the 5



formation of 1D/2D hybrids would further prolong the diffusion path of the corrosion-active small molecules. These, in addition with the prominent barrier capability rGO, all result in the remarkable anticorrosion property of the coatings. The characterization on the hybrid nanoparticles as well as the anticorrosion performance of the modified coating systems are discussed in detail in the following work.

2. MATERIALS AND METHODS 2.1. Materials Graphite powders (325 meshes, 99.9995%) were purchased from Alfa Aesar Co. Ltd. Concentrated sulfuric acid (H2SO4, 98%), concentrated phosphoric acid (H3PO4, 85%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), hydrazine hydrate(H6N2O), ammonium persulfate(APS) was purchased from Beijing Chemicals Co. Ltd. Redox initiators was used in the emulsion polymerization of the polymer latex, which is composed by APS (oxidizing agent) and Bruggolite FF6M (reducing agent, purchased from Inosol company). Emulsifier of Sodium dodecyl sulfate (SDS), monomer of Butyl acrylate(BA), methyl methacrylate(MMA), acrylic acid (AA) and costabilizer of hexadecane (HD) was purchased from Fuchen Chemical Reagent Factory in Tianjin. Bispheneol A epoxy resin (E44, EEW=221.07g/equiv) was purchased from Baling Petrochemical Co. Waterborne curing agent of AB-HGF-100 was purchased from Zhejiang Anbang New Material Development Co.Ltd. Halloysite nanotubes (HNTs) were purchased from Yichang, Hubei, China.

2.2. Characterization Fourier transform infrared (FTIR) spectra were obtained by a Bruker Tensor 37 spectrometer. FTIR data were collected in the transmission mode. A total of 64 scans 6


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were accumulated for signal-averaging of each IR spectral measurement from 400 to 4000 cm-1 to ensure a high signal-to-noise ratio with a 1 cm-1 resolution. The spectra of X-ray photoelectronic spectroscopy (XPS) was on Thermo Fisher Scientific ESCALAB 250. The radiation source was Al Kα. The instrument was operated at a base pressure of 1×10-9 mbar. The structures of nano objects were observed by Hitachi H-800 transmission electron microscope (TEM) with an accelerating voltage of 100kV. Samples for TEM were prepared by drop casting the dispersion onto a holey carbon-coated copper grid and the samples were then dried at room temperature before observation. Ultraviolet-visible (UV–vis) absorption spectra were recorded on Shimadzu UV-3150. Particle sizes and zeta potentials in aqueous dispersion were measured using Malvern Zetasizer Nano ZS. Zeta potential measurements of all the samples are performed at the pH of 7. The fracture surfaces of the coated steel sheets were coated with a thin gold layer using a sputter coater (Polaron, SC502) and observed with a scanning electron microscope (FE-SEM Hitachi S4700) at an accelerating voltage of 20 kV. The electrochemical measurements were carried out in a conventional three-electrode electrochemical cell employing 3.5 wt % NaCl aqueous solution as the electrolyte. A three-electrode system was adopted, in which the reference electrode was the saturated calomel electrode (SCE), the counter electrode was a platinum electrode, and the working electrode was the coated steel sheet. Electrochemical impedance spectroscopy (EIS) were performed on a Zahner electrochemical workstation (Messsysteme, Germany). EIS measurements were carried out at the open circuit potential with 10 mV of sinusoidal perturbation over the frequency range of 0.01 Hz to 10 kHz. EIS data were analyzed by Zsimwin software. The chemical resistance of the coated panels was evaluated by acid & alkali as well as water immersion method according to ASTM D-1308 and ASTM D-870, respectively. 7



The degree of adhesion and visual inspection of blister and cracks were evaluated for coated panels after immersion for 24 h. The salt spray test was carried out on coated steel plates with a 5 wt % NaCl solution at 100% relative humidity at 35 ℃ according to ASTM B117-03. The samples were checked every 12 h. 2.3. Synthesis of GO GO was prepared from the natural graphite powders by improved Hummer’s method.29 Briefly, graphite powders (1.0 g), concentrated H2SO4 (120 mL) and concentrated H3PO4 (13.3 mL) were mixed in the reaction vessel which was placed in an ice-water bath. KMnO4 (6 g) was slowly added to the mixture under magnetic stirring at 0 °C. Then, the mixture was continuously stirred at 50 °C for 12 h. After that, the product was carefully poured into ice water (140 mL) which contained 2 mL of H2O2 (30%). The dispersion was kept at 0 °C for another 2 h. When the color of the dispersion turned from brown to yellow, the reaction was stopped. The produce was washed by deionized water and isolated by high-speed centrifugation at 5000 rpm until the pH value of the supernatant reached 7. The as-prepared GO was re-dispersed in deionized water by tip sonication (Model: 250 Digital Sonifier, American branson; the method was: 30% Amplitude, 3 seconds ON, 2 seconds OFF, 60 minutes total time) for next preparation. 2.4. Fabrication of HNTs-rGO Hybrid Nanoparticles Typically, 0.5 g HNTs was added and dispersed into 500 g of GO dispersion (1 mg/g) under tip sonication (Model: 250 Digital Sonifier, American branson; the method was: 30% Amplitude, 10 seconds ON, 3 seconds OFF, 30 minutes total time). Then, hydrazine hydrate (0.35 g) was dropped in the mixture in the reaction vessel. The vessel was incubated at 96 °C under continuously stirring for 1 h. The hybrid nanoparticles were washed repeatedly to neutral by using deionized water. The final 8


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product was in the aqueous dispersion form with the concentration of 1mg/mL. For comparison, we fabricate the rGO in a similar way except for the addition of HNTs. Figure 1 presents the fabrication process of HNTs-rGO and rGO. The effect of the amount of HNTs was also conducted, and the specific formula of fabrication is in Table S1.

Figure 1. Fabrication process of HNTs-rGO by reduction-coagulation method (a) and rGO by a conventional reduction method (b).

2.5. Preparation of Polymer Latex The polymer latex in this work was the polymer composite of epoxy and (methyl)acrylate resin, which was synthesized via miniemulsion polymerization as described in patent CN 105732881 A. The typical formula is shown in Table S2. The monomers were dispersed in water under magnetic stirring at room temperature assisted by the addition of emulsifiers (SDS) and costabilizer (HD). We employed high-pressure homogenizer (Model: AH-100D, Canada ATS; the method: 80 MPa pressure, three times) to re-disperse the raw emulsion into the miniemulsion. The miniemulsion was transferred into a four-neck round-bottom flask immersed in a water bath. The flask was purged by nitrogen for 15 min before the temperature elevation. After the temperature reached 60°C, redox initiators (APS and Bruggolite 9



FF6M) were added to initiate the polymerization. The system was incubated at 60oC for 4 h under mechanical stirring protected by nitrogen atmosphere. The polymerization was stopped by being cooled to room temperature. The conversion and the solid content is up to 90% and 35%, respectively. The average particle size is 208nm with the PDI of 0.052 as characterized by DLS. 2.6. Preparation of Waterborne Coating Films Certain content of HNTs-rGO was added to the polymer latex. The addition content was varied from 0.25 to 0.5, 0.75 and 1 wt % of the polymer. After being stirred for 10 min, proper amount of waterborne curing agent (AB-HGF-100) was added. The mixture was stirred for another 15 min and then sprayed on the steel substrate. The coating film was dried at 40 °C until its surface was touch dry. After that, it was incubated at 90 °C for 2 h until its surface was completely dry. The thickness of the dry film was 40 ± 2 µm. For the comparison purpose, the blank polymer coating was prepared in a similar way except for the addition of inorganic modifiers. The coating samples with the addition of only rGO or HNTs (0.5 wt % of the polymer) were also prepared using the same polymer latex.

3. RESULTS AND DISCUSSION 3.1. Dispersibility of HNTs-rGO Hybrid Nanoparticles In comparison with rGO reduced under the same condition, the improved water dispersibility of HNTs-rGO prepared by our reduction-coagulation method is illustrated in Figure 1. After seven days of storage, it is obvious that the aqueous dispersion of HNTs-rGO maintains stable and homogeneous as shown in Figure 1a. On the contrary, precipitation has been seen on the bottom of the bottle of rGO in Figure 1b. The dispersion of GO is also presented in Figure 1a, which maintains 10


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stable and homogeneous, similar to the aqueous dispersion of HNTs-rGO. But the color differs significantly between the aqueous dispersion of GO and HNTs-rGO. The color changing from yellow into black is a typical phenomenon indexing the reduction of GO into rGO.30-31 Digital photos of HNTs and GO dispersion are shown in Figure S1. It could be found that the two samples are of similar appearance without occurrence of the agglomeration and precipitation of particles. The effect of HNTs amount on the dispersing stability of HNTs-rGO hybrid nanoparticles is presented as Figure S2. After seven days of storage, we find that the HNTs-rGO dispersion would be unstable when HNTs amount is 0.125 g as indicated by the obvious deposition. However, the other samples are homogeneous when HNTs amount comes up to 0.25 g or more.

3.2. Characterirzation of HNTs-rGO Hybrid Nanoparticles The reduction of GO into rGO as well as its incorporation with HNTs can be illustrated by FTIR as presented in Figure 2. The absorption bands at 3699, 3629, 3453, 1100, 1030, 909 and 541 cm-1 in the spectrum of HNTs are easily found in the spectrum of HNTs-rGO. The bands at 3699 and 3629 cm-1 are ascribed to –OH group on the surface of HNTs. The band at 3453cm-1 is assigned to Si-OH.32 The absorption at 1100, 1030, 909 and 541 cm-1 are assigned to the bonds of Si-O, Si-O-Si, Al-OH, Al-O-Si, respectively.33 We can also observe an additional band of C-OH group at 1230 cm-1 in the spectrum of HNTs-rGO as marked by the dotted box. Except that, the other absorption bands of GO at 1740 cm-1 (C=O), 1625 cm-1(C=C), 1057 cm-1(C-O-C) are not seen in the spectrum of HNTs-rGO. The results demonstrate that most of the oxide groups on GO have been reduced in the hybrid nanoparticles. But in comparison with the spectrum of pure rGO where there is no peak at 1230cm-1, we 11



have to admit the reduction of GO in HNTs-rGO is not complete.

Figure 2. FTIR spectra of HNTs-rGO, HNTs, GO and rGO.

The XPS survey spectra of HNTs-rGO, GO and HNTs are shown in Figure 3a. The Si2p peak of HNTs at 102.9 eV in Figure 3b is attributed to Si–OH and Si–O bonds.34 However, Si2p peak of HNTs-rGO shifts to 103.4 eV, which is possibly contributed by the formation of hydrogen bonds between the residual oxygen containing groups of rGO and Si-OH. Except the peaks of C, O and Si, there is an additional peak of N1s in the spectrum of HNTs-rGO at the binding energy of 400.3 eV, which is ascribed as an indication of the reduction of rGO by hydrazine hydrate.35 Another evidence for the reduction is illustrated by the spectra of C1s, which are provided as Figure 3c and d. The spectrum of GO in Figure 3c is composed by a series of peaks for C1s in different oxidation states appearing typically at 284.4, 285.2, 287.1 and 288.6 eV, which are ascribed to the C-C, C–O, C=O and the carbonyl ester or acid bonds, respectively.36 The peaks also emerged in the spectrum of HNTs-rGO in Figure 3d at similar positions. But if we use the C1s peak for C-C bonds as the reference, we can find that the C1s peaks for oxygen containing groups are much weaker in the spectrum of HNTs-rGO, indicating the partial reduction of GO. The residual oxygen containing groups like hydroxyl and carboxyl on rGO would have positive contribution in the 12


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hydrogen bonding formation with Si-OH, which is in consistent with the above results.

Figure 3. XPS spectra. (a) Survey spectra of HNTs-rGO, GO and HNTs. (b)Si2p spectra of HNTs-rGO and HNTs. (c) C1s spectra of GO. (d) C1s spectra of HNTs-rGO.

The direct evidence for the formation of hybrid nanoparticles in the mixed dispersion can be achieved by TEM as displayed in Figure 4. As shown in Figure 4a, GO has a typical transparent corrugate and lamellar structure. The low contrast from the background implies that it has been sufficiently exfoliated and dispersed well in the medium.37 The typical morphology of HNTs used in this work is presented in Figure 4b, which demonstrates that the length of HNTs is ~700 nm, with an inner diameter of ~20 nm and an external diameter of ~50 nm. In Figure 4c, we can clearly discover the binding of nanotube with nanosheets. The coagulation triggered by the reduction of GO is well controlled in the colloidal scale as what we have anticipated. 13



Referring the FTIR and XPS study above, we can know the hydrogen bonding interactions are responsible for the incorporation of rGO with HNTs. The further coagulation of the hybrid nanoparticles should be suppressed attributed to the screen effect of the residual hydrophilic groups on the internal surface of HNTs. Otherwise, as shown in Figure 4d, the direct reduction of GO gives out large and irregular aggregates without the incorporation of HNTs.

Figure 4. TEM images (a) GO; (b)HNTs; (c)HNTs-rGO; (d)rGO.

The UV-vis spectra of GO, rGO, HNTs, HNTs-GO and HNTs-rGO dispersion are shown in Figure 5. The absorption peak of GO located at 228 nm is assigned to π–π* transition of C=C bond.38 HNTs show no peak in the range from 200 nm to 400 nm. HNTs-GO also shows the typical peak of GO at 228 nm. The intensity is lessened, which is reasonable as the concentration effect. The linear addition of separated spectra of HNTs and GO is displayed in the insert image. It can be seen that there is no obvious difference between the simulated and the experimental results, indicating a simple mixture without additional interactions between HNTs and GO. The isolation 14


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of the two kinds of nano objects in mixed dispersion can be explained by the electrostatic screening effect. The reduction of the oxygen-containing groups on GO changes the electron conjugation on the nanosheets. The characteristic absorption peak of rGO moves to 260 nm. In addition, the spectrum of rGO shows obviously baseline uplift, which implies the formation of large and irregular aggregates as observed in TEM characterization. Comparatively, the baseline of the spectrum of HNTs-rGO dispersion maintains at the same level of HNTs or GO. It provides another evidence for the good dispersion stability of HNTs-rGO in water. The reduction of GO in the hybrid nanoparticles is suggested by the appearance of the low but broad peak at around 260 nm. The peak is not very remarkable because there is a stronger shoulder peak at about 222 nm, which is the index of the residual oxygen containing groups. The spectrum is also compared with the linear addition of separated spectra of HNTs and rGO. The difference implies the additional interactions between HNTs and rGO in the dispersion of HNTs-rGO hybrid nanoparticles.

Figure 5. (a) UV-vis spectra of HNTs, GO and HNTs-GO. The linear addition of separated spectra of HNTs and GO is shown in the insert as the dotted line. (b) UV-vis spectra of HNTs, rGO and HNTs-rGO. The linear addition of separated spectra of HNTs and rGO is shown in the insert as the dotted line. 15



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The contribution of electrostatic effect in the stabilization mechanism is illustrated by the measurement on zeta potential, whose results are shown in Table 1. Zeta potential of GO is -37.2 mV. The high absolute value of zeta potential assures the dispersion stability of GO.39-40 On the contrary, zeta potential of rGO is -17.8 mV, indicating the lose of the dispersing stability in water.41 It is worth noting that the zeta potential of HNTs-rGO is -28.7 mV, whose absolute value is obviously higher than that of rGO, indicating a modified dispersing stability. Since there is a single peak with no sign for the existence of the separated phase of HNTs whose zeta potential is -19mV, we think the enlarged absolute value of zeta potential which is higher than that of pristine rGO and HNTs is the result of the hybrid nanoparticles formation. In the reduction-coagulation process, the incorporation of the separated nano objects in dispersion is permitted by the slight decrease of electrostatic repulsion between them as the consequence of the reducing of GO into rGO. However, once the colloidal hybrid nanoparticles have formed, the charge density of the small aggregates would increase as indicated by the enlarged absolute value of zeta potential. The electrostatic repulsive forces are then dominated to forbid the further coagulation of HNTs-rGO into large depositions. The residual hydrophilic groups inside the HNTs tube and their ionization ensure that there are sufficient electrostatic repulsions to balance the cohesion between different nano particles and guides to a controlled coagulation toward HNTs-rGO hybrid nanoparticles dispersion with considerable kinetic stability.

Table 1. Zeta potential data of different aqueous dispersions Dispersed phase

GO

HNTs

rGO

HNTs-rGO

Zeta potential/mV

-37.2

-19

-17.8

-28.7

16


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3.3. Anticorrosion Performance of Composite Coatings The stable dispersion with moderate negative zeta potential suggests the good compatibility of HNTs-rGO with popular polymer latex stabilized by anionic surfactants like the latex synthesized in this work. Digital image of the coatings and the corresponding latex after six months of storage is displayed in Figure S3. In comparison with the blank polymer latex and the blank polymer coating film (BPc), no obvious phase separation is observed in the blended dispersion or the coating film containing 0.5 wt % HNTs-rGO (0.5HNTs-rGO/Pc). The blending of HNTs-rGO and polymer latex is still stable and uniform after six months of storage at room temperature. The thermal cured films of both BPc and 0.5HNTs-rGO/Pc are flat and uniform. In addition, the cross-section of the two coating films were characterized by SEM, which convinced that the hybrid nanoparticles had been well dispersed in the polymer matrix. The corresponding SEM images are shown in Figure S4. It is well accepted that the introduction of rGO could improve the compactness of the coating film which is dramatically attractive in the design of anticorrosion coatings.42 In order to investigate contribution of HNTs-rGO in the corrosion protection of the coating films, we applied electrochemical impedance spectroscopy (EIS). The impedance modulus in EIS at the low frequency of 10 mHz is generally used to judge the protective capabilities of coatings. The coating film is considered as of acceptable anti-corrosion capability when its impedance modulus at the frequency of is higher than 108 Ω·cm2.43-44 Figure 6a shows the bode plots of BPc and 0.5HNTs-rGO/Pc at the initial and after 90 days of soaking in 3.5 wt % NaCl aqueous medium. At the initial stage of immersion, the impedance moduli of BPc and 0.5HNTs-rGO/Pc at 10 mHz were both higher than 109 Ω·cm2, which means that they were of excellent anti-corrosion capability at that time. After 90 days of soaking, the 17



impedance modulus of BPc at 10 mHz dropped to lower than 108 Ω·cm2. Comparatively, the impedance modulus of 0.5HNTs-rGO/Pc at 10 mHz was still higher than 108 Ω·cm2. It is apparent that 0.5HNTs-rGO/Pc has better anti-corrosion capability than BPc. The dependence of the impedance moduli at 10 mHz on HNTs-rGO content is shown in Figure 6b. All the impedance moduli were higher than 109 Ω·cm2 at the time of beginning, indicating that all the surface coatings provided sufficient protection on the substrate. After 90 days of soaking, the impedance modulus of the coating film containing 0.25 wt % HNTs-rGO (0.25HNTs-rGO/Pc) was 1.38×108 Ω·cm2 and higher than that of BPc. The maximum impedance modulus (3.68×108 Ω·cm2) is observed on the sample of 0.5HNTs-rGO/Pc. The further increase on HNTs-rGO content has no positive contribution in the impedance modulus. As shown in Figure 6b, the impedance modulus is 2.58×108 Ω·cm2 and 9.92×107 Ω·cm2 in corresponding to the HNTs-rGO content of 0.75 wt % and 1 wt %, respectively. The modulus decrease suggests that the superfluous HNTs-rGO might count against the film structure rather than improve the coating compactness.

Figure 6. (a) Representative Bode-Z plots. (b)The trends chart of impedance value at the frequency of 10 mHz. 18


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We also evaluated the chemical resistance of the coatings by immersion the coating system in different mediums of acid, alkali and distilled water as well as the salt spray test. The time duration before the coating has been lapsed is listed in Table S3. We can find that 0.5HNTs-rGO/Pc and 0.75HNTs-rGO/Pc provide the longest protection time among all the tested samples which validates the EIS results. In the immersion test, the substrate surface protected by the coating of 0.5HNTs-rGO/Pc and 0.75HNTs-rGO/Pc can sustain to more than 90 days in the medium of 5 wt % NaOH, 3.5 wt % NaCl and distilled water. In the medium of 10 wt % H2SO4, the above two coating can provide the protection of 50 days. In the environmental of 5 wt % natural salt fog, no change was observed during 90 days of test. Comparatively, the pure polymer coating can only provide the protection of 32, 65, 52, 19, 74 days in the medium of 10 wt % H2SO4, 5 wt % NaOH, 3.5 wt % NaCl, distilled water and 5 wt % natural salt fog, respectively. The coatings with 0.25 wt % and 1 wt % addition of HNTs-rGO show also some improvement on the anticorrosion performance, but not as remarkable as the 0.5 wt % and 0.75 wt % samples. 0.5HNTs-rGO/Pc was further compared to BPc, the coating containing 0.5 wt % of rGO (0.5rGO/Pc) and 0.5 wt % of HNTs (0.5HNTs/Pc). The Tafel plots of different coatings in 0.35 wt % NaCl aqueous solution are presented in Figure 7a. It can be seen that 0.5HNTs-rGO exhibits a more positive corrosion potential than the others, indicating it is the most impermeable. Thus, the trend shown in the Tafel plots provides us a direct evidence to support the excellent anti-corrosion capability of 0.5HNTs-rGO/Pc on steel. However, it should be noted that conductive fillers can increase the conductivity of coating, which means that the coating itself may serve as an extra corrosion-promoting electrode.45 Therefore, the Tafel plots may not be 19



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enough to evaluate the anticorrosion performance of rGO/Pc. Then we employ the EIS results to confirm the improved anticorrosion performance of the experimental coatings. The Bode plots after 60 days of immersion in 0.35 wt % NaCl aqueous solution is shown in Figure 7b and c. The Bode phase plots of 0.5HNTs-rGO/Pc and 0.5HNTs/Pc display only one peak, indicating one time constant (Figure 7c). Such a time constant suggests the classical barrier behavior. However, for rGO/Pc and BPc, an additional time constant is visible at intermediate/low frequencies (Figure 7c). The additional time constant is the responses of metal and reflects the corrosion behavior of metal substrate. The improved anticorrosion capability of HNTs-rGO in comparison with rGO and HNT shown in Bode phase plots and in Tafel plots is mutually corroborated. The equivalent circuits shown in Figure 7d are used to quantitative analyze the EIS data obtained at different immersion time. The equivalent circuit of Model A is for impermeable coatings, while that for permeable coatings is Model B. We obtain the data of the coating resistance (Rc). As shown in eq.1, Rc is a denotation of the deterioration of coatings and in relation to the number of pores or capillary channels in the coating film:46 Rc =

d κNAc

(1)

where κ is the electrolyte conductivity, N is the channel number, Ac is the average cross-section area of the permeability channels, and d is the channel length, i.e., the

d2

thickness of the coating film. The evolution of Rc of different coating systems is shown in Figure 7e as the function of the immersion time. In the initial 1~15 days of immersion, Rc of BPc descended sharply, illustrating the generation of capillary channels or pores. The electroactivity of rGO would also be response for the initial 20


d3

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lowest Rc of rGO/Pc. However, there was a rebound on Rc of rGO/Pc, which was higher than that of BPc during the period from 9 to 30 days of immersion. It could be attributed to the increased tortuosity of diffusion pathways, which is the result of the addition of impermeable graphene sheets as shown in Figure 7f(II).47 However, Rc of rGO/Pc decreased sharply after 30 days. In the long time of immersion, the Rc of rGO/Pc dropped to even lower than the blank polymer sample. Such an undesirable decline on Rc has been explained by the formation of connected circuit of graphene, substrate and corrosive media.45 However, the interfacial defects from the strong phase separation of the waterborne polymer and water incompatible rGO should take some

responsibility.

Comparatively,

HNTs/Pc

shows

better

anti-corrosive

performance than both BPc and rGO/Pc.27-28 It is interesting that HNTs seems more effective in waterborne polymer system than rGO because of the better compatibility. But the best performance is still achieved on the sample of 0.5HNTs-rGO/Pc especially in the long immersion time. The incorporation of HNTs with rGO in nano hybrid form can not only improve interfacial compatibility with waterborne polymers to avoid the defects from phase separation, but also combine the characteristics of HNT fillers of rGO for the good barrier capability in the coating film. The risk for the formation of corrosion-promoting electrode45 also reduce as the incorporated HNTs on rGO. Moreover, as presented in Figure 7f(IV), the micro-complexity of 1D/2D hybrid nanoparticles would further increased the diffusion tortuosity. These all result in the remarkable compactness of the surface coating. Although promising results have been achieved here, it only provides a clue for the design of graphene modified waterborne coating systems. Abundant efforts are still in need to optimize the shape, structure, interfacial conditions of the hybrid nanoparticles as well as their application composition in the real anticorrosion systems. It would be an open topic deserving 21



further explorations.

Figure 7. (a) The Tafel plots of different coatings in 0.35 wt % NaCl aqueous solution after 1 day. (b) Bode plots after 60 days of immersion in 0.35 wt % NaCl aqueous solution(Z-f), (c) Bode plots after 60 days of immersion in 0.35 wt % NaCl aqueous solution (Phase-f). (d) Equivalent electrical circuits used to fit EIS results. (e) The variations of Rc as the function of the immersion time for BPc, 0.5rGO/Pc,

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0.5HNTs/Pc and 0.5HNTs-rGO/Pc. (f)Proposed penetration pathways for BPc(I), 0.5 rGO/Pc(II), 0.5HNTs/Pc(III) and 0.5HNTs-rGO/Pc(IV).

4. CONCLUSIONS In summary, we have fabricated the hybrid nanoparticles of HNTs-rGO in aqueous dispersion by the simple but effective reduction-coagulation method. The reduction of GO into rGO and the formation of hybrid colloidal nanoparticles have been convinced by FTIR, XPS, TEM, UV and zeta potential studies. The limited reduction degree and the incorporation with HNTs both have contribution on the good water compatibility of HNTs-rGO hybrid nanoparticles, which, together with certain amount of waterborne curing agent were added to polymer latex for the building of waterborne coating film of HNTs-rGO/Pc. The films could be applied on the steel substrate through traditional spraying and thermal curing technique. The introduction of HNTs-rGO into polymer latex with an appropriate content (0.5 wt %) can greatly improve the anticorrosion performance of the surface coating, as revealed by the impedance analysis and the immersion tests. The electrochemical analyses show that the property is from the enhanced barrier capability. The intrinsic characteristic of graphene nanosheets, the increased micro-complexity by the formation of 1D/2D hybrid nanoparticles and the modified interfacial condition which largely avoid the defects from phase separation all result in the remarkable compactness of the surface coating. The film could provide effective protection on the substrate from the corrosion in acid, alkali, electrolytes, distill water and salt frog conditions. All these characteristics endow the HNTs-rGO with potential application as high anticorrosion performance waterborne composite coating.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Formula of the fabrication of hybrid nanoparticles; digital photos of dispersions; formula of miniemulsion polymerization; digital images of coating and the corresponding latex; soak test and salt spray test results; SEM characterization (PDF)

ACKNOWLEDGEMENTS This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFC0204402), the National Natural Science Foundation of China (Grant No. 21304007), and the Fundamental Research Funds for the Central Universities at Beijing University of Chemical Technology (Grant No.

ZY1605,

JD1703, BUCTRC201614).

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