Superior Performance of Artificial Nacre Based on Graphene Oxide

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Superior Performance of Artificial Nacre Based on Graphene Oxide Nanosheets Yang Wang, Hao Yuan, Piming Ma, Huiyu Bai, Mingqing Chen, Weifu Dong, Yi Xie, and Yogesh S Deshmukh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13834 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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ACS Applied Materials & Interfaces

Superior Performance of Artificial Nacre Based on Graphene Oxide Nanosheets Yang Wang1, Hao Yuan1, Piming Ma1, Huiyu Bai1, Mingqing Chen1, Weifu Dong1*, Yi Xie1, Yogesh S. Deshmukh2#*

*E-mail: [email protected], [email protected] Tel.: +86-510-8532-6290. 1. The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China; 2. Department of Biobased Materials, Faculty of Humanities and Sciences, Maastricht University, P.O. Box 616, 6200MD, Maastricht, The Netherlands. # Current address: Wavin Innovation and Technology, Rollepaal 20, 7701BS, Dedemsvaart, the Netherlands.

Abstract Natural nacre is well-known by its unique properties due to the well-recognized “bricks-and-mortar” structure. Inspired by the natural nacre, graphene oxide (GO) was reduced by dopamine with simultaneous coating by polydopamine (PDA) in aqueous solution to yield polydopamine-capped reduce GO (PDG). The artificial nacre nanocomposite materials of poly(vinyl alcohol) (PVA) and PDG presenting layered structure had been successfully 1 ACS Paragon Plus Environment


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constructed via vacuum-assisted assembly process, in which PDG and PVA served as “bricks” and “mortar”, respectively. A combination of hydrogen bonding, strong adhesion and friction between PDG nanosheets and PVA chains resulted in enhancements for mechanical properties. The tensile strength, elongation at break, and toughness of PDG–PVA nanocomposite reached to 327±19.3 MPa, 8±0.2% and 13.0±0.7 MJ m-3, which is simultaneously 2.4, 8 and 7 times higher than that of nature nacre with 80-135 MPa, ~1% and ~1.8 MJ m-3, respectively. More interestingly, the obtained nanocomposites demonstrated a high anisotropy of thermal conductivity (k∥/k⊥~380). Combined with superior mechanical properties and high anisotropy of thermal conductivity make these biomimetic materials promising candidates in aerospace, tissue engineering and thermal management applications.

Keywords: nacre, graphene oxide, interfacial adhesion, integrated superior performance, thermal conductivity

INTRODUCTION Nacre, a natural optimized material consisting of two-dimensional aragonite and a small amount of biopolymer fillers, exhibits a remarkable strength and hardness characteristics, which draws considerable interest of researchers in the design of novel biomimetic materials.1 On the basis of the nacre’s model, different types of nacre-like layered bioinspired nanomaterials with significant enhanced properties have been successfully fabricated, including clay–poly(vinyl alcohol) (PVA) nanocomposites by layer–by–layer (LBL) assembly, double hydroxide(LDHs)–

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PVA by spin coating, alumina–chitosan via the hydrogen bonding and Al2O3–poly(methyl methacrylate) by ice template crystallization method.2,

3

Among these layered materials,

graphene oxide (GO) is one of the best candidates for fabricating layered nanomaterials owing to its outstanding strength (about 63 GPa), modulus (200-500 GPa), optoelectronic devices and electrical properties.4 Dikin et al5 constructed the stiffness and strength nacre-like layered nanocomposites based on GO nanosheets via filtration of aqueous, but their mechanical properties are still much less than individual GO due to the lack of strong interfacial interaction between adjacent GO layers. Actually, GO-based artificial nacre will first be destroyed at interlayer contacts within GO layers, rather than GO layers itself under application of the tensile-stress mode.6, 7 Therefore, much attention has been dedicated in building strong interfacial interaction between adjacent GO layers by introducing different types of adhesives. For instance, Putz K et al.8 cross-linked GO layers with hydrophobic poly(methyl methacrylate) (PMMA) and hydrophilic PVA, respectively. The GO–PVA composites exhibited greatly improved modulus values in comparison to films of either PVA or GO film. Modulus values for GO–PMMA composites were intermediate to those of pure components. Tian et al.9 prepared high stiffiness GO films by crosslinking of multifunctional polyetherimide (PEI). Song et al.10 also prepared GO-SSEBS (sulfonated styreneethylene/butylene-styrene) composite films with super high toughness of 15.3 MJ m-3 via synergistic hydrogen bonds and π–π conjugated interactions, preventing the slippage ability of GO nanosheets. However, their strength is 158 MPa and still far below the theoretically estimates of GO. Thereby, further efforts on preparation of superior mechanical GO-based nanomaterials in large-scale arrays must be addressed. 3 ACS Paragon Plus Environment


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Dopamine (DA), a well-known hormone and neurotransmitter which contains many functional groups (-NH2 and -OH), and thus is chosen as a specialized molecule mimic for the adhesive proteins.11, 12 At a weak alkaline pH (8.5), DA will self-polymerize into an adherent polydopamine (PDA), adhere to various substrates, with the accompanied oxidation of catechol groups to the quinone form.13-15 Moreover, DA have recently been used as reducing agent for GO.16 PVA, a water-soluble polymer with excellent chemical resistance, high performance and extensively used in many areas.17 It also has been widely selected as an idea matrix for researching the mechanism of the reinforcement between matrix and the reinforcing agents due to the high efficiency to form strong hydrogen bonding and superior capability to transfer load between matrix and fillers. Herein, we successfully fabricated the strong integrated nacre-like layered material based on PDG nanosheets and PVA through vacuum-assisted filtration self-assembly. The resultant artificial nacre materials exhibited significant enhancements in mechanical properties as a result of increased interfacial interactions produced by the combined mechanisms of hydrogen bonding between PDA and PVA. Meanwhile, it also had a high anisotropy of the thermal conductivity. This novel artificial nacre materials opened up a new path for preparing composites with multifunctional properties, which have potential applications in aerospace, tissue engineering and thermal management.

MATERIALS AND METHODS Materials. Graphite flakes (99.95%) was obtained from Qingdao Jinrilai Graphite Co., Ltd. 4 ACS Paragon Plus Environment

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Dopamine (DA, 98%) was purchased from Aladdin. Sulfuric acid H2SO4 (98%), Phosphoric acid (H3PO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%) were provided by Sinopharm Chemical Reagent Co., Ltd. Poly(vinyl alcohol) (PVA-1799) was supplied from Sinopec Sichuan Vinylon Works. Synthesis of GO. Synthesis of GO was based on the “improved” methods.18 The purified natural graphite (2.0 g) was put into a solution of concentrated H2SO4/H3PO4 (360:40 mL), and the mixture was heated to 40 ℃. Under vigorous stirring, KMnO4 (9.0 g) was added slowly to the mixture. Then the mixture was heated to 50℃ and stirred constantly for 12 h. The mixture was cooled to room temperature using a water bath and poured onto ice (about 400 mL) with H2O2 (30% 3mL). The mixture was filtered and washed with warm diluted HCl solution (3% 200 ml) in order to remove metal ions then dried under reduced pressure overnight at room temperature. Synthesis of PDG. In a typical procedure,16 GO (0.1 g) was dispersed in deionized water (20 mL) with stirring and ultrasonicating into the GO dispersion. Then, the GO dispersion and dopamine (0.05 g) were added into the Tris buffer solution (200 mL pH=8.5), followed by sonication for 10 min in an ice bath. The mixing solution was stirred vigorously at 60 ℃. After aging for 24 h, the PDG was filtered with a 0.2 µm membrane filter and washed with deionized several times. Synthesis of PDG–PVA artificial nacre. The obtained PDG was added to the PVA solution. The mixtures were stirred and sonicated to form a uniform suspension. The nanocomposite films were prepared using the method of vacuum-assisted self-assembly of a 5 ACS Paragon Plus Environment


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homogeneous dispersion of PDG–PVA through a cellulose membrane followed by air-dried until they could be peeled off from the membrane. After vacuum-drying at 60 ℃ for overnight, the nanocomposites were obtained. The elementary steps of PDG–PVA nanocomposites were illustrated in Figure 1.

Figure 1. Scheme of the formation of the PDG–PVA artificial nacre via vacuum-assisted filtration self-assembly. Insert: Photographs of natural nacre.

Characterization. The XRD patters of samples were recorded on a Bruker-D8 X-ray diffractometer (Cu Kα radiation, Germany). ATR-FTIR examinations were conducted on a Nicolet 6700 FTIR spectrometer. The tensile behavior of films were tested using an Instron 5967 tester at a loading rate of 1 mm min-1. Five specimens of each sample were tested and the averaged results were presented. Thermal gravimetric analysis (TGA) of sample was performed 6 ACS Paragon Plus Environment

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on a Meteler-Toledo 1100SFinstrument from 20 to 800℃ at a heat rate of 20℃ min-1 under nitrogen atmosphere. Scanning electron microscopy (SEM) images were obtained by S-4800. Atomic force microscope (AFM MuLtimode 8, Bruker Nano, U.S.A.) was used to observe the microstructure of the samples. The thermal of samples were performed by a hot disk thermal constants analyzer.

RESULTS AND DISCUSSION The GO nanosheets were produced from natural graphite flakes with the improved Hummer’s method. The characteristics of GO were further confirmed by XRD and ATR-FTIR measurements. XRD patterns of graphite, GO, and PDG were shown in Figure 2. Standard diffraction peak for graphite at 2θ=26.5° with a corresponding d-spacing of 0.34 nm. After oxidation, there was a narrow and strong diffraction peak at around 2θ=10.6° (d-spacing of 0.83 nm), which was caused by different oxygenic functional groups.5 It indicated that graphite had been fully oxidized. With reduction of DA, the typical sharp peak almost disappeared. Instead, a weak diffraction peak was observed around 23.4° (d-spacing of 0.38 nm), indicating the successful reduction of GO.16 ATR-FTIR spectra of GO, PDG and PDA were shown in Figure 2b. The typical FTIR spectrum of GO displayed O-H stretching vibration around 3420 cm-1, C–H asymmetric stretching at 2928 cm-1, C=O stretching vibrations at 1725 cm-1, C=C bond at 1620 cm-1, C–O–C stretching vibrations at 1175 cm-1 respectively.19 After GO was functionalized with DA, the characteristic peak of carboxyl C=O and epoxy C–O–C almost disappears, demonstrating that 7 ACS Paragon Plus Environment


GO was reduced.

a

b

1725 2925

2852 GO

1398 1175

Transmittance (a.u)

GO 0.83 nm

Intensity (a.u)

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PDG 0.38 nm

0.34 nm

1620 1580

PDG

PDA

pristine graphite

5

10

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25 30 35 2θ (degree)

40

45

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4000

3500

3000

2500

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-1

Wavenumber (cm )

Figure 2. (a) XRD patterns of graphite, GO, and PDG. (b) ATR-FTIR spectra of GO, PDG, and PDA.

The morphology of GO and PDG was examined by SEM. GO showed the sheet structure with some small wrinkles (Figure 3a). Figure 3 b, c were the SEM images of PDG. A lot of PDA nanoparticles could be clearly observed, indicating that DA produced conformal coating on surfaces of nanosheets during its self-polymerization.

Figure 3. SEM images of the (a) GO nanosheets and (b, c) PDG nanosheets. 8 ACS Paragon Plus Environment

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AFM gived further evidence to support the conclusion. As shown in Figure 4, GO was fairly ultrathin, the thickness of GO sheets was about 0.94 nm which was typical for a one-aton-thick GO nanolayer. Whereas for PDG nanosheets, the average thickness was 1.37 nm, obviously thicker than that of GO, which indicated that PDA coating layer was indeed formed on the surface of GO.

Figure 4. AFM images of (a) GO and (b) PDG sheets; (c) and (d) were the height profiles of (a) and (b), respectively. Digital images of aqueous of GO (e) and PDG (f).

Figure 5 showed optical photographs of GO and PDG–PVA films prepared by



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vacuum-assisted assembly process. The original GO film showed brown color. However, the color of PDG–PVA nanocomposite film turned black, and the surface of the film was flat and glossy. Moreover, it was flexible and could be shaped into various desired structures with a knife or be bent into a large angle. As shown in Figure 5c, d, the GO film was re-dispersed into solution after a mild shake. In contrast, the PDG–PVA film couldn’t be destroyed by shaking or stirring, not even by ultrasonication. A plausible explanation was that reason was that PVA chains consolidate the PDG nanosheets together similar to the mortar, hence exhibiting a well water-resistance behavior.

Figure 5. Digital images of GO (a) and PDG–PVA nanocomposite (b). The stability comparison of GO and PDG–PVA films, before (c) and after (d) mild shake.


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The content of PVA was estimated by TGA measurement. As shown in Figure 6, the mass loss of GO (100~300 ℃) corresponded CO, CO2, and steam release from the labile functional group. Between 300 and 800℃, the mass loss of GO arise from the removal of stable oxygen functional group.18 As for PVA, we could find that the thermal decomposition of PVA could be regarded as two steps. The first step, the mass loss was associated with the removal of absorbed H2O and side-groups at lower temperature. The second step, a breakdown of PVA backbone began at higher temperature.20 The content of PVA could been calculated by the mass loss between 215 and 490 ℃. PVA content in films was 2.0, 7.3 and 10.5 wt% determined by TGA. The experimental GO content in PDG–PVA composites was about 66.6, 63.0 and 60.9 wt%, respectively. After deduction the content of GO, the PDG content in PDG–PVA composites was about 31.4, 29.7 and 28.6 wt%, respectively.

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GO PDG PVA PDG-PVA(2.0 wt%) PDG-PVA(7.3 wt%) PDG-PVA(10.5 wt%)

Deriv.Weight (%℃ )

a 100

weight (%)

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-0.4 -0.6 GO PDG PVA PDG-PVA(2.0 wt%) PDG-PVA(7.3 wt%) PDG-PVA(10.5 wt%)

-0.8 -1.0

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-1.2 100

200

Temperature (℃ )

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Temperature (℃)

Figure 6. (a) TGA and (b) DTG comparison of PDG, PVA, and PDG–PVA composite films.

The relevant nacre-like SEM images for the films were shown in Figure 7. Pristine GO film



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showed a lamellar structure with prominent interlayer gaps and withdrawn GO nanosheets (Figure 7a, a’). In Figure 7b, b’, PDA was observed on the surface of nanosheets. The adhesion of PDA decreased the gaps of nanosheets and improved the interlayer interfacial interaction. What’s more, with integration of PVA, PDG nanosheets were visibly piled close together and PVA interpenetrated into the interlayer. PVA served as “mortar” to make close the sheets, wrapped up sheets and filled in their air pockets to form a compact lamellar microstructure (Figure 7c). Similar to nacre, PDG–PVA films were well identified as the “brick”-“mortar” microstructure due to the aligned PDG nanosheets.


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Figure 7. Side-view SEM images of films with different magnification: (a,a’) GO, (b,b’) PDG, and (c, c’) PDG–PVA nanocomposites.

Mechanical Properties of PDG–PVA nacre

Typical stress-strain curves of pristine GO, PDG, GO–PVA and PDG–PVA were shown in Figure 8. The detailed data was illustrated in Table 1. The tensile strength, elongation at break, and toughness of pristine GO was 90.7±1.5 MPa, 2.1±0.2% and 1.1±0.1 MJ m-3, respectively. As



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a contrast, GO–PVA were prepared. Integration of PVA into GO matrix leaded to a slight increase of the mechanical behavior. The tensile strength, elongation at break, and toughness of GO–PVA composite was 119.8±4.3 MPa, 2.6±0.1%, and 1.2±0.3 MJ m-3, respectively. After DA reduction, tensile strength, elongation at break, and toughness of GO reached to 119.2±5.2 MPa, 3.2±0.6% and 2.6±0.1 MJ m-3, respectively. The enhancement was ascribed to the improved load-bearing capabilities of PDA as a consequence of the adhesion effect. It was found that integration of PVA into the PDG matrix result in notable improvement of the mechanical properties. For example, 2.0 wt% of PVA could increase the tensile strength, elongation at break, and toughness to 210±14.5 MPa, 3.8±0.4% and 3.7±0.5 MJ m-3. The tensile strength, elongation at break, and toughness of PDG–PVA nanocomposite material further reached to 327±19.3 MPa, 8±0.2% and 13.0±0.7 MJ m-3 with 7.3 wt% PVA loading, which was simultaneously 2.4, 8 and 7 times higher than that of nature nacre with 80-135 MPa, 1% and 1.8 MJ m-3, respectively.21, 22 Compared that of pristine GO, increased of 260%, 280% and 1200%, respectively. The stress in PDG–PVA materials could be effectively transferred due to the strong interface interaction. Most importantly, the ductility which was characterized by the elongation at break increased remarkably with the increase of PVA content. Among the composite films with different PVA content, the PDG–PVA(7.3 wt%) owned an optimum ratio, which leaded to the batter “bricks”-“mortar” interface interaction. The advantage of PDG–PVA was displayed in Figure 8b. The pink arrow in Figure 8b illustrated the trend in integrated high performance. Many processes to designing GO-based layered films often result in the improved in just one type of mechanical properties (strength or toughness). For instance, the GO materials with PEI cross-linking possess 14 ACS Paragon Plus Environment

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tensile strength reach up 209.9 MPa, the elongation dramatically decrease into only 0.2%, and the toughness is only 0.23 MJ m-3, which is far lower than that of natural nacre.9 The rGO–SL23 shows tensile strength of 300 MPa, however, the toughness is only 2.8 MJ m-3. The low toughness could also be observed in treated GO film24, and GO/CMC.25 The toughness of PDG– PVA is still 4.6 and 56 times higher than rGO-SL and GO-PEI, respectively. Compared with rGO–MoS2–TPU26 composites through synergistic interface interactions, the tensile strength and toughness of PDG–PVA nanocomposite is 1.4 and 1.9 times higher, respectively. The tensile strength of PDG–PVA nanocomposite is comparable to rGO–PAPB27 (382 MPa) and the toughness is 1.7 times higher than that of rGO/PAPB (7.5 MJ m-3). Notably, PDG–PVA composites exhibited a good compromise between high strength and excellent toughness, outperforming other GO-based films.

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PDG-PVA(10.5 wt%) PDG-PVA(2.0 wt%)

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PGO-PEI[9]

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GO-PMMA[8]

Nacre[21,22] rGO-PCDO[29] GO-PVA[32] GO-GA[35]

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This work

rGO-PAPB[27]

GO-Al2O3-PVA[28]

GO-PS[31]

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Figure 8. (a) Tensile testing curves of GO, PDG, GO–PVA and PDG–PVA films. (b) Comparison of tensile strength and toughness of PDG–PVA composite films and other layered GO-based 15 ACS Paragon Plus Environment


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materials.

Table 1. The mechanical properties of the GO, GO–PVA, PDG and PDG–PVA composite films Stress

Strain

Toughness

(MPa)

(%)

(MJ m-3)

GO

90.7±1.5

2.1±0.2

1.1±0.1

PDG

119.2±5.2

3.2±0.6

2.6±0.1

GO–PVA(10 wt%)

119.8±4.3

2.6±0.1

1.2±0.3

PDG–PVA(2.0 wt%)

210±14.5

3.8±0.4

3.7±0.5

PDG–PVA(7.3 wt%)

327±19.3

8.0±0.2

13.0±0.7

PDG–PVA(10.5 wt%)

250±17.0

9.5±1.1

13.8±0.7

Sample

To explore the synergistic effect from PDA and PVA, a model was proposed and it was akin to the deformation of natural nacre,36-38 as shown in Figure 9. As we know, when tensile strain was gradually applied to GO film, relative slippage began to occur between the adjacent building sheets until crack emerged under a large applied strain, which leaded to the tensile failure.26, 39 Actually, GO nanosheets can be reduced by DA simultaneous coating by PDA due to self-polymerization of DA. The adhesion of PDA and strong covalent cross-linking between PDA and GO increased more friction between adjacent PDG sheets and enhance interlayer contact and interactions of sheets. In addition, the PVA interpenetrated into the interlayer and act as “mortar”, which could supply enough interactions between adjacent PDG sheets. At the onset of plastic deformation, friction produced by asperities may play an important role in resisting shear. Subsequently, the tensile strength was the result of the stretching of PVA chains. PDG 16 ACS Paragon Plus Environment

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nanosheets extensively slide against each other and the coiled long-chain molecules of PVA stretch along the sliding direction. As a result, the process with large deformation was accompanied by dissipation of a large amount of energy. When loading were further increased the hydrogen bonding between PVA and PDG sheets were broken, and the PDG sheets began to slide over each other. Meanwhile, the broke of covalent bonds could lead to further absorb energy during loading. This unique intrinsic processing allowed the PDG–PVA composite films to endure stronger load and greater shape deformation.

Figure 9. Schematic illustration of the stress-transfer for PDG–PVA films.

Thermal Conductivities of films.

Thermal management of modern electronics requires material with strongly thermal conductivity anisotropy, where the in-plane thermal conductivity (k∥) is substantially higher than cross-plane thermal (k⊥). The thermal conductivity (k) of films were measured and shown in Figure 10. It could be seen that the in-plane thermal conductivity and cross-plane thermal of GO were both lower than those of PDG. As we know, GO sheet itself has lower thermal conductivity than reduced GO in composites. On the other hand, Phonons are considered as the dominant 17 ACS Paragon Plus Environment


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carriers in heat conduction. Generally, phonons scattering, which is mainly due to defects and interface mismatch, played a key role in the thermal conductivity.40 The GO sheets interacted with each other via weak van der Waals forces, only low frequency phonon vibration modes were available to carry a small amount of heat energy.41 Moreover, plentiful air pockets among sheets were observed (shown in Figure 7a and b), which resulted in increasing phonon scattering, and further limiting the heat transport. The increased adhesion between nanosheets was another contribution for the high thermal conductivity. In addition, with integration of PVA could increase affinity through hydrogen bond and make PDG nonosheets orient further, which reducing the interfacial thermal resistance. However, the excess polymer would isolate nanosheets, thereby resulting a slight decrease in thermal conductivity. Interestingly, the thermal conductivity anisotropy of PDG–PVA (7.3 wt%) reached a high value of k∥/k⊥~380. According to SEM images (Figure 7), PDG–PDG linkages existed in the aligned structure. Those aligned structure could efficiently form thermal conductive pathways along the in-plane direction. On the other hand, the transfer channel in cross-plane direction was PDG-polymer-PDG path. The different heat transfer channel endowed PGA–PVA anisotropic thermal conductivity.


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a 0.4

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-1

Thermal conductivity (Wm K )

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Thermal conductivity (Wm K )

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PDG-PVA PDG-PVA PDG-PVA 7.3 wt% 10.5 wt% 2.0 wt%

PVA

GO reference PDG

PDG-PVA PDG-PVA PDG-PVA 2.0 wt% 7.3 wt% 10.5 wt%

PVA

Figure 10. Cross-plane (a) and In-plane (b) thermal conductivity of films.

Conclusion

Inspired by natural nacre, we had succeeded in fabricating the superior nacre-like layered material based on PDG nanosheets and PVA –OH (or –NH–) of PDA and –OH were in favor of the formation of hydrogen-bonds, enhancing the interfacial interaction between PDG nanosheets and PVA chain. The morphology of composites demonstrated nacre-like layered structures with superior mechanical properties. The tensile strength, failure strain, and toughness of PDG– PVA(7.3 wt%) were 327±19.3 MPa, ~8±0.2% and ~13.0±0.7 MJ m-3, respectively. The obtained composite films also demonstrated an exceptionally strong anisotropy of the thermal conductivity (k∥/k⊥~380). Combined with superior mechanical properties and high anisotropy of thermal conductivity make these biomimetic materials promising candidates in aerospace, tissue engineering and thermal management applications.



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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (51373070), the Research Project of Chinese Ministry of Education (No.113034A), the Fundamental Research Funds for the Central Universities (JUSRP51624A), and the Innovation Project for College Graduates of Jiangsu Province (KYLX16_0784).

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Table of Contents Graphic


Superior Performance of Artificial Nacre Based on Graphene Oxide

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