Reduced Graphene Oxide/Alumina, A Good Accelerant for Cellulose-Based Artificial Nacre with Excellent Mechanical, Barrier, and Conductive Properties Kiran Shahzadi,† Xueming Zhang,‡ Imran Mohsin,§ Xuesong Ge,† Yijun Jiang,*,† Hui Peng,† Huizhou Liu,† Hui Li,† and Xindong Mu*,† †
Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, China ‡ Beijing Key Lab Lignocellulos Chemistry, Beijing Forestry University, Beijing 100083, P.R. China § Shenzhen Institute of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen 518055, China S Supporting Information *
ABSTRACT: In this article, a simple strategy was employed to fabricate bioinspired hybrid composite with carboxymethyl cellulose (CMC), graphene oxide, and reduced graphene oxide/alumina (rGO/Al) by a facile solution casting method. The tensile strength and toughness of rGO/Al-CMC-GO can reach 586.6 ± 12 MPa, 12.1 ± 0.44 MJm−3, respectively, due to the interface strengthening of alumina, which is 1.43 and 12 times higher than steel and about 4.3 and 6.7 times that of nature nacre. The artificial nacre hybrid composite is conductive due to the introduction of rGO/Al on the surface. Interestingly this structure can also be coated on the surface of cotton thread to give the thread good mechanical performance and conductivity. Additionally, the artificial nacre has better fire shielding and gas barrier properties. The oxygen permeability (OP) for 1% rGO/Al-CMC decreased from 0.0265 to 0.003 mLμm m−2 day−1 kpa−1, the water vapor permeability (WVP) decreased from 0.363 to 0.205 gmmm−2 day−1 kpa−1 when the concentration increased from 1% rGO/Al to 6% rGO/Al. It is believed this work provided a simple and feasible strategy to fabricate ultrastrong and ultratough graphene-based artificial nacre multifunctional materials. KEYWORDS: ultrastrong, ultratough, reduced graphene oxide and alumina, conductivity, barrier properties
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based on inorganic micro/nanoplatelets. The employed micro/ nanoplatelets include clays,9−11,13−16 Al2O3,17−19 and other artificial platelets.20,21 Graphene, a two-dimensional lattice of sp2-bonded carbon that is only one-atom thick, exhibits extraordinary properties as the strongest and stiffest material ever measured22 and is the best-known electrical conductor could have promising applications in many fields.23,24 All of the above exclusive characters makes it an excellent candidate as the “bricks” for fabricating nacre-like composites. In this perspective, many graphene-based artificial nacres (GBAN) have been reported. Very recently, the mechanical performance (tensile strength and toughness) for these GBAN were also well summarized by Cheng Qufeng.25 For example, An Zhi26
s conventional structural materials reach their performance limits, one of the major scientific challenges for the 21st century is the development of new high performance, multifunctional materials to support advances in diverse strategic fields, ranging from building and transportation to energy and biotechnology.1 In the past decade, the structure of nacre has received enormous attention for its extraordinary combination of stiffness, toughness, and strength.2−6 Natural nacre develops a hierarchical microstructure through a bio mineralized process to optimize its mechanical properties. The excellent mechanical performance of this biological material originates from a hierarchically ordered arrangement of two-dimensional (2D) aragonite platelets and soft biopolymer layers, which is alternately stacked into a brick and-mortar structure.7,8 Recently, great attempts have been made to fabricate artificial composites by mimicking the second level of hierarchy in nacre and built layered composites through a range of assembly techniques © 2017 American Chemical Society
Received: February 21, 2017 Accepted: June 6, 2017 Published: June 6, 2017 5717
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Figure 1. Schematic illustration for preparation process of rGO/Al-CMC-GO hybrid film.
Figure 2. Digital image of rGO/Al-CMC-GO hybrid film (a), SEM image for rGO/Al powder (b), SEM-EDS mapping for alumina in rGO/Al (c), SEM image for 1% rGO/Al-CMC-GO (d), EDS spectra for 1% rGO/Al-CMC-GO (e), and SEM-EDS mapping for alumina indicating uniform distribution in hybrid film (f).
± 13.1 MPa and 11.8 ± 0.4 MJm−3, which are 3.55 and 6.55 times that of nature nacre.29 Although lots of achievements for the designing and fabricating of GBAN (graphene-based advance nanomaterial) have been made, GBAN is still in its early stages the mechanical of GBAN would be greatly enhanced by the synergistic effects of designing interface interactions and combining different building blocks.30 But, optimization of the tensile strength often comes at the expense of toughness.30 So it is still a great challenge to develop some simple and facile methods to obtain integrated bioinspired layered materials, which exhibit high tensile strength and toughness simultaneously. Additionally, it is also desirable to give the GBAN added-function. In this article, a facile solution casting method was employed to fabricate bioinspired hybrid composite with carboxymethyl cellulose, graphene oxide, and reduced graphene oxide/alumina. The artificial nacre has the following advantages: (i) Ultrastrong and ultratough. The tensile strength and toughness of CMC-reduced GO/alumina can reach 586.6 ± 12 MPa, 12.1 ± 0.44 MJm−3, respectively,
demonstrated ultrastiff artificial nacre (tensile strength of 160 ± 18 MPa, toughness of 0.14 MJm−3) through borate orthoester covalent bonding between GO nanosheets (GO-borate), Tian and his co-workers also demonstrated high-strength artificial nacre (tensile strength of 209.9 MPa. toughness of 0.23 MJm−3) through covalent bonding between polydopamine (PDA)-modified GO nanosheets and poly(ether imide) (PEI) polymers.27 Similarly, Zhang28 fabricated reduced GO materials with ultrahigh strength and toughness with poly(acrylic acid-co(4-acrylamidophenyl) boronic acid) (PAPB), which interacts extremely well with GO nanosheets. The tensile strength (382 MPa) and toughness (7.5 MJ m−3) reach two and four times higher than those of natural nacre (tensile strength of 80−135 MPa, toughness of 1.8 MJm−3), respectively. Very recently, we developed a simple and facile in situ reduction and cross-linking strategy to fabricate a high strength integrated artificial nacre based on sodium carboxy methyl cellulose (CMC) and borate cross-linked graphene oxide (GO) sheets. The tensile strength and toughness of cellulose based hybrid material reached 480.5 5718
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and oxide groups, which also indicated that the alumina coating, prevented the overlapping of GO sheet.35 From inset of Figure S4 (Supporting Information) it is clear that XRD patterns for rGO/Al-CMC-GO hybrid films have almost no difference due to well dispersion and small quantity. FTIR was employed to give the information about the structural changes of the assynthesized samples (Figure 3a). It can be seen that the GO
due to the interface strengthening of alumina, which is 1.43 and 12 times higher than steel31,32 and about 4.3 and 6.7 times of that of nature nacre.32 (ii) Green and sustainable. Instead of commercial polymer, sustainable biobased polymer CMC was employed to act as organic composition. (iii) Conductive. The artificial nacre hybrid composites are conductive due to the introduction of reduced graphene oxide/alumina on the surface. It is believed this research work provided a simple and feasible strategy to fabricate ultrastrong and ultratough graphene based artificial nacre functional materials.
RESULTS AND DISCUSSION Figure 1 is showing the whole process for fabricating rGO/AlCMC-GO hybrid film. First, the Al nuclei are continuously precipitated on the surface of GO as the water evaporated, and then all Al nuclei are transformed in to Al oxide material and GO is reduced by oxidation and calcination under high temperature.33,35 During this process, agglomeration of GO is prevented because π-electron-stacking between GO planes is weakened by coating Al material. Second, the rGO/Al was dispersed in CMC-GO solution and then casting the solution to form the GBAN. This rGO/Al can be well dispersed in CMC to form a homogeneous solution (Figure S1, Supporting Information) which also indicated the agglomeration of rGO is prevented in our process. At last, in order to get a conductive GBAN, the rGO/Al sol was coated on the surface of GBAN. It can be clearly observed that the cross-section of the blank CMC is smooth with SEM images (Figure S1, Supporting Information), after the introduction of GO, the biobased hybrid materials showed typical nacre-like structure (Figure S1, Supporting Information). SEM images for rGO/Al clearly showing similar morphology like GO. Amorphous alumina deposited on rGO as shown in Figure 2b. SEM show that the alumina was coated on the surface of GO tightly and the average size of alumina ranged from 4 to 14 nm. The average value for particle size found 8 nm with a standard deviation of 2 nm (Figure S2, Supporting Information). To get further information about the surface of rGO/Al, atomic force microscopy (AFM) was employed. It is found the height of rGO/Al found approximately 16 nm with variation of 0.6 nm, which is larger than the size of alumina particle due to the existence of GO layer. (Figure S3, Supporting Information) At last, the thermogravimetric analysis (TG) was conducted to calculate the amount of alumina in rGO/Al. The TG results showed that the amount of alumina and GO in the rGO/Al is 59.8 and 40.2 wt%, respectively (Figure S2d).While X-ray photoelectron spectroscopy (XPS) in Figure 2c demonstrated the existence of alumina in rGO/Al. It should be noticed that the SEM images of rGO/Al-CMC-GO show well-ordered layered structure in Figure 2d. In hybrid film with high concentration of rGO an alumina aligned sheet-like arrangement took place with little aggregation as shown in Figure S1 (Supporting Information). EDS spectra and SEM-EDS mapping further confirmed alumina presence and well distribution in resultant nanocomposite (Figure 2e,f). XRD and FTIR analysis of rGO/ Al composite powder and rGO/Al-CMC-GO further confirmed the reduction of GO with alumina. XRD patterns of GO, rGO/Al-composite powder, and rGO/Al-CMC-GO are shown in Figure S4 (Supporting Information). In GO a sharp peak identified at 10.80° corresponds to an interlayer distance of 7.6 Å (d 002) due to overlapping of GO sheets. While in rGO/Al a broad peak observed at 23.5° after removal of water molecules
Figure 3. FTIR spectrum for GO, CMC, rGO/Al, and 1% rGO/AlCMC-GO (a). Raman spectrum for GO, rGO/Al, and 1% rGO/AlCMC-GO (b). C 1s XPS spectrum for GO (c). C 1s XPS spectrum for rGO/Al (d). O 1s XPS spectrum for rGO/Al (e). Al 2p spectrum (1−3) for alumina, rGO/Al, and 1% rGO/Al-CMC-GO, respectively (f).
shows two peaks at 1620 and 1724 cm−1, which were attributed to C=C and C=O, respectively. When GO was reduced with alumina (1% rGO/Al-CMC-GO), the peaks at 1620 and 1724 cm−1 were absent, indicating the reduction of GO with alumina. Due to removal of some oxygen containing groups, these peaks were also found to be absent in 1% GO-CMC hybrid films after partial reduction.28 Furthermore, in pure CMC and 1% GOCMC a peak at 3370 cm−1 that can be assigned to O−H group almost diminished in 1% rGO/Al-CMC due to reduction of GO. For pure CMC, peaks at 1050 and 1600 cm−1 can be assigned to typical C−O stretching and symmetrical modes of carboxylate ions.36 In rGO/Al and rGO/Al-CMC two peaks at 1400 and 1595 cm−1 detected can be attributed to alumina.37,38 In which peaks at 1400 and 1595 cm−1 can be due to surface species of alumina and O−H bending, respectively. Additionally, compared with 1% CMC-GO being sharper, there was a peak shift from 1050 to 1080 cm−1 in 1% rGO/Al-CMC indicating some interaction between CMC and alumina.39 Raman spectra of GO showed two peaks at 1340 cm−1 (D 5719
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Figure 4. Stress−strain curves for CMC, CMC-Al3+, CMC-Al, 1% GO-CMC, 1% GO-CMC+Al3+, and 1% rGO/Al-CMC-GO (a). Tensile strength and toughness for all hybrid films with different (%) rGO/Al (b).Tensile strength comparison of present work with other reported hybrids and structural steel and (c) fracture morphology of modified film (d).
band) and 1598 cm−1 (G band) that can be assign to first order scattering of the E2g phonon of sp2 carbon atoms and size of in plane sp2 domains, respectively.33 The D/G intensity ratio for GO, rGO/Al, and rGO/Al-CMC-GO was found to be 1.2, 1.6, 1.58, respectively. This increase in D/G intensity ratio is actually due to increase in the number of sp2 domains further confirming GO reduction in CMC at high temperature and with alumina (Figure 3b). Similarly the D/G intensity for 1% GO-CMC found more than GO (Figure S4, Supporting Information) indicating partial reduction in hybrid, this result agreed well with our previous experiments.29 Finally UV−vis spectroscopy was employed to characterize rGO/Al and rGO/ Al-CMC-GO film (Figure S4, Supporting Information). It is found that the peak centered at 233 nm with GO (π → π* transitions of the aromatic C=C bonds) shifted to 270 nm for rGO/Al and rGO/Al-CMC-GO film. That is also further indication of reduction for rGO/Al-CMC-GO. XPS analysis were further conducted to explore the interaction of alumna with GO after reduction as shown in Figure 3c. The high resolution C 1s XPS spectrum of GO
showed diverse functional groups C=C (282.61 eV), C−O (284.6 eV), and C−O−C or C=O bonds (286.12 eV).40,41,42 While increased C=C peak at 284.8 eV and decreased signal intensity of C−O at 286.9 eV in C 1s XPS spectrum of rGO-Al further indicates the reduction of GO (Figure 3d). A change in peak shift of C=C from 284.2 to 284.8 eV can be attributed to reduction of GO with alumina.24,30 UV−vis-spectrum and XPS analysis collectively suggested that the Al on the surface of GO has some interaction with the rGO, and also reacted with CMC. Furthermore, the GO can also be reduced by CMC, and then has some interaction with CMC as our previous report.29 So, these bindings between Al-rGO, Al-CMC, and CMC-rGO would really help to improve film final strength. The mechanical performance of all the films was evaluated. It is found that the strength and toughness of the pure CMC film are 98.8 ± 4.8 MPa and 3.8 ± 0.8 MJm−3, respectively (Figure 4a). Addition of Al3+ ions in to CMC could increase the strength and toughness to reach 112 ± 3.5 MPa and 3.85 ± 0.6 MJm−3. Compared with Al3+, the same concentration of 5720
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CMC-CMC. When the GO was reduced by alumina, then there was an existing strong interaction between rGO and alumina due to the introduction of some covalent bond.35 When the rGO/Al was introduced into the CMC film, there are more interfaces (CMC-CMC, CMC-rGO, CMC-Al, rGO-Al) to be broken during pulling or external force in the film. It can be expected that the energy to break CMC-Al and rGO-Al is much higher than that of CMC-GO. It is well-known that water content has a great effect on mechanical properties of GObased biofilm. For this a series of relative humidities (RHs), such as 25, 50, 75, and 100%, were tuned by environmental conditions. The mechanical properties, i.e., tensile strength, toughness, and tensile strain, of pure CMC, 1%GO-CMC, and 1% rGO/Al-CMC-GO under different RHs were tested. From Figure 5a−c it is clear there was prominent decrease in tensile
alumina can also increase the strength and toughness of CMC film to get 127.8 ± 5.5 MPa and 3.9 ± 0.4 MJm−3, indicating the alumina can increase the strength of the CMC film more due to the interaction between the alumina and CMC. Compared with alumina, when GO was added to the CMC film, the strength and toughness were improved very much due to the m reduction as we previously reported.29 Strength and toughness of GO-CMC film can reach 293.5 ± 4.5 MPa and 8.6 ± 0.5 MJm−3 when 0.4% GO was added and maximum 376.7 ± 5.6 MPa and 9.8 ± 0.4 with 1% GO was found. Increasing the concentration of GO more than 1% can reduce film strength as shown in Figure S5 (Supporting Information), which can be attributed to the agglomeration of GO layers. Interestingly introduction of Al3+ in 1% CMC-GO media can improve strength and toughness up to 478.6 ± 4.5 MPa and 10.2 ± 0.3 MJm−3, respectively. Cross-linking of Al3+ with GO and CMC can be a possible reason for this kind of improvement in mechanical strength.43 For comparison we also prepared rGO/ Al-CMC hybrid films. Like GO increasing concentration of rGO/Al, strength and toughness increased gradually and maximums were found to be 495.2 ± 2.3 MPa and 10.3 ± 0.3MJm−3, respectively, for 1% rGO/Al-CMC. Increasing concentration of rGO/Al more than 1% resulted in reduction of strength like GO (Figures S5c and 4a). To improve the mechanical performance of the bio hybrid film further, the rGO/Al was employed utilizing two step method. In which initially in situ reduction of 1% GO in CMC media, improved the strength to certain extent while introduction of rGO/Al improved the strength further. Surprisingly, introduction of rGO/Al can significantly improve the mechanical properties for hybrid film. This can be due to strong covalent bonding between the alumina and rGO. The tensile strength and toughness for bio hybrid film of 1% rGO/Al-CMC-GO are 586.6 ± 12 MPa and 12.1 ± 0.44MJm −3, respectively, which is about 2 and 1.4 times of GO-CMC film, 6 and 3.3 times of that of pure CMC. It should be noticed that this data of strength for rGO-CMC hybrid film was higher than that of structure steel (400 MPa),31 4.3 times that of natural nacre (80−135 MPa).32 To the best of our knowledge, this rGO-CMC showed the highest strength among the reported GO-CMC films. Compared with some GO-hybrid, i.e., GO/gelatin and GO/ cellulose,44,45 most of hybrids showed less strength than 1% rGO/Al-CMC-GO. For example, 2.85 times that of rGO-PDA (204.9 MPa),33 1.11 times that of rGO−CS (526 MPa),46 2.14 times that of rGO−CNC (273 MPa),47 1.89 times that of rGOPAA (309 MPa),48 and 1.53 times of rGO-PAPB (382 MPa).28 For the toughness, this hybrid film is 6.7 times higher than that of natural nacre (1.8 MJm−3)32 and 12 times than that of steel (1.0 MJm−3).31 At the same time, the mechanical property for the hybrid film is superior to other previous reported artificial nanocomposites, such as MTM-PVA,12 GO-PMMA,49 rGOPAPB,28 rGO-PDA,33 GO-PCDO,52 GO-SL, rGO-SL,51 GOborate,26 GO-PEI,27 GO-GA,50 rGO−DWNT-PCDO,53 GOMoS2-TPU, rGO-MoS2-TPU,54 GO-Mg2+,GO-Ca2+,55 GOPVA, and rGO−PVA,56 shown in Figure 4c. This high strength of hybrid film can be illustrated by strengthening the mechanism of material interface (Figure 4d). For pure CMC, the strength is derived from the interaction of CMC molecules by hydrogen and ionic bonding. When the GO was introduced into the film, except for the interaction among CMC molecules, the interaction between CMC and GO play an important role to increase the strength of the film. The energy to destroy the interface energy of GO-CMC must be much higher than that of
Figure 5. Effect of relative humidity on (a) tensile strength. (b) Young’s modulus. (c) Tensile strain for pure CMC, 1% GO-CMC, and 1% rGO/Al-CMC hybrid films. (d) Water stability (1) for pure CMC after 30 min, (2) 1% GO-CMC after 90 min, (3) 1% GO +CMC after 2 h, (4) and (5) rGO/Al-CMC-GO after 2 and 24 h, respectively.
strength and toughness at high RHs. The tensile strength for rGO/Al-CMC was reduced from 586.6 ± 12.1 MPa to 510.5 ± 4.5 MPa, when the humidity is changed from 25 to 100%. In the same condition, for GO-CMC, the tensile strength was reduce form 376.7 ± 5.6 MPa to 302.5 ± 2.5 MPa. Obviously, the tensile strength of GO-CMC is more sensitive than rGO/ Al-CMC-GO due to the hydrophilic character of GO. While the toughness of GO-CMC is not sensitive to RH due to better extensibility under high RH. To further prove the water stability of rGO/Al-CMC-GO, the film of CMC, GO-CMC, and rGO/ Al-CMC-GO was immerged in water. It was found that the pure CMC film dissolved within 30 min, the film of GO-CMC dissolved within 90 min, while the film of rGO/Al-CMC-GO could not be dissolved even after 24 h. This result strongly suggested the rGO/Al probably produced some bonding net in the film, which was also proved by the FTIR, Raman, and XPS. These results were found to be consistent with theoretical prediction by simulation.57 It is known that the rGO can show excellent conductivity due to its different structure. So, in our hybrid film the alumina coated rGO sheets were coated on the surface of hybrid film to improve film conductivity. Resistivity measurements confirmed successful reduction of GO with alumina and conductivity of rGO/Al-CMC-GO film. While 5721
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Figure 6. Resistivity graph with different rGO/Al content and inset showing (1) pure CMC and (2) GO-CMC are nonconductor while (3) hybrid film of rGO/Al−CMC is conductive (a). Circuit scheme with lightening of LED with 6% rGO/Al-CMC (b). DTG curves with inset of TG curves for (1) for pure CMC, (2) 1% GO-CMC, and (3) 1% rGO/Al-CMC (c). Fire shielding behavior of 1% rGO/Al-CMC which can protect cotton wool more than 1 min (d).
thread good mechanical performance and conductivity. Interestingly, the successfully coated cotton thread not only exhibited improved strength (Figure S7, Supporting Information), but also showed good conductivity as shown in Figure S8 (Supporting Information). It should be noticed that coating of rGO/Al-CMC-GO does not effect fibers flexibility as shown in(Figure S9 (Supporting Information). So this method can be easily applied in large scale by simple coating. This thermally stable hybrid film also showed improved barrier properties which can be due to coating of alumina on rGO sheets. These rGO/Al composites can be excellent filler for improving barrier properties of polymer matrix, which protect CMC film from absorption of water and oxygen molecules from surroundings.58 Figure S10 (Supporting Information) demonstrated the effect of rGO/Al loading on barrier properties on modified films. In our case extremely low oxygen permeability (OP) was observed with increasing concentration of rGO/Al in resultant component. The oxygen permeability (OP) for 1% rGO/AlCMC-GO decreased from 0.0265 to 0.003 mLμm m−2 day−1 kpa−1 when the concentration increased from 1% rGO/Al to 6% rGO/Al (Figure S8, Supporting Information). The hybrid film also showed good water vapor permeability (WVP) (Figure S10, Supporting Information). WVP values decreased from 0.363 to 0.205 gmmm−2 day−1 kpa−1 for 1% rGO/AlCMC and with 6% rGO/Al-CMC-GO it reduced to 0.150 gmmm−2 day−1 kpa−1. In this work, bio hybrid film also showed good fire retardant behavior with alumina and rGO. Such rGO/ Al-CMC-GO films can be used to protect flammable biological materials from burning in short time. A piece of cotton took a few seconds to burn, while placed behind the modified film did not catch fire even after 2 min as shown in Figure S11 and movie provided in Supporting Information. From SEM image of Figure S11 (Supporting Information), it is clear that our multifunctional biofilm did not change its shape even after 1 min when exposed to fire, owing to the alumina coating and rGO sheets with strong interaction of CMC. The limit oxygen index (LOI) values of CMC and other modified films with
pure CMC and GO-CMC were not conductive as shown in Figure 6a. This can be due to electrical insulating properties of CMC and GO.56 So biohybrid films with varying concentration of rGO/Al were analyzed for conductivity. Film resistivity values reduced with increasing the concentration of rGO/Al content in CMC. When concentration of rGO/Al increased from 1 to 6% film resistivity values decreased gradually and found minimum 0.5 KΩ/Sq. This was further demonstrated in Figure S6 (Supporting Information) in which 1% GO-CMC did not flash the LED lamp while for hybrid films with different contents of rGO/Al lightened the LED lamp and its intensity increased with more concentration of rGO/Al. This increase in conductivity can be attributed to the uniformly dispersed rGO/ Al sheets which make conductive networks in insulating matrix. While in case of GO more oxygen containing groups destroy its conductivity. Interestingly all modified films had flexibility. Even film with 3% rGO/Al content could be twisted and folded without any damage and LED lamp flashed well under this condition as shown in Figure S6 (Supporting Information). The thermogravimetric (TGA) and differential thermogravimetric (DTG) analysis were used to characterize the thermal properties of CMC, 1% GO-CMC, and 1% rGO/Al-CMC-GO composite films. It is clear from the Figure 6c that residual weight percent of 1% rGO/Al-CMC was found to be higher than that of pure CMC and 1% GO-CMC, which can be attributed to excellent thermal stability of rGO.41 From (DTG) curves as shown in inset of Figure 6c the initial, maximum, and final decomposition temperatures for CMC were 290, 332, and 370 °C. While for 1% GO-CMC and 1% r GO/Al-CMC-GO hybrid films were 331, 390, and 422 °C and 391, 445, and 495 °C, respectively. Thus, rGO/Al in CMC with excellent mechanical properties also makes it thermally stable, the initial decomposition temperature can be increased to about 100 °C due to the introduction of alumina. Alumina coated rGO sheets homogeneously dispersed in CMC provide extra thermal stability. To improve the applicability of our methods, the method was also employed to coat cotton thread to give the 5722
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then dried in oven at 70 °C for 20 min. Similarly, cotton threads with varying concentration of rGO/Al were synthesized in same manner. Characterization. Scanning electron microscopy (SEM) images were recorded on Hitachi S-4800 Japan with energy dispersive spectroscopy (EDS) mode. Atomic force microscopy (AFM) images were acquired with Agilent 5400 scanning probe microscope with a Nano drive controller in tapping mode with MikroMasch NSC-15 AFM tips with resonant frequencies ∼300 kHz. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB 250Xi photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with Al Kα X-ray radiation as the X-ray source for excitation. Thermogravimetric analysis (TGA) was carried on Rubotherm Dyntherm HP Germany using temperature range from 20 to 700 °C with heating rate 10 °C/min. X-ray diffraction (XRD) profiles were obtained with D8 Advance Bruker diffract meter Germany. Fourier transform infrared spectroscopy (FTIR) was used to get spectra on (Thermo Scientific Nicolet IN10 USA). Mechanical properties were characterized by universal testing machine equipped with 500N load cell at room temperature and an average humidity 20%. While film resistivity was measured on resistivity meter with four point probe. LOI values for fire retardant property were calculated on a JF-3 flame meter (Jiangning in Nanjing City) according to the method of GBT 2406.2-2009: For reproducibility five samples were analyzed with dimensions of 70−100 mm × 6.5 mm × 3 mm.
different concentration of GO and rGO/Al contents are shown in Figure S11 (Supporting Information). The LOI value for pure CMC found 23.8% indicating it is flammable polymeric material. It can be seen clearly that introduction of rGO/Al in the CMC improved its LOI value from 23.8 to 30.4%. The alumina introduction can increase the carbon forming of the film and reduce the permeability of the oxygen. Considering this extra property in modified film, such an effort could contribute to finding variable, environmentally friendly alternatives for future fire-barrier materials.
CONCLUSION In summary, bioinspired artificial nacre was fabricated with carboxy methyl cellulose and reduced graphene oxide/alumina by a facile solution casting method. The artificial nacre is ultrastrong and ultratough. The tensile strength and toughness of CMC-reduced GO/alumina can reach 586.6 ± 12 MPa and 12.1 ± 0.44 MJm−3, respectively, due to the interface strengthening of alumina, which is about 4.3 and 6.7 times of that of nature nacre. The artificial nacre hybrid composite is flexible and conductive due to the introduction of reduced graphene oxide/alumina on surface. This artificial nacre structure can also be coated on the surface of cotton thread to give good mechanical performance. Additionally, the artificial nacre has better fire shielding and gas barrier properties which give the material multifunctions for application. It is believed this work provided a simple and feasible strategy to fabricate ultrastrong and ultratough graphene-based artificial nacre multifunctional material.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01221. SEM images for cross-section of CMC, 1% GO-CMC, 6% rGO/Al-CMC-GO; SEM image of the edge of single rGO flake, showing the measurement of its thickness; high magnification SEM image for alumina on rGO surface; distribution of alumina particles sizes on rGO; TG analysis for rGO/Al; thermal stability of rGO/Al in oxygen atmosphere; AFM image of rGO/Al thickness of which is approximately 15 nm; AFM surface morpholgy of rGO/Al depicting rough surface; XRD patterns for GO and rGO/Al; UV−vis spectra for CMC, GO, rGO/ Al-CMC-GO; stress−strain curves for CMC, CMC-Al, 0.4% GO-CMC, 1% GO-CMC, 3% GO-CMC, 6% GOCMC, and 1% rGO/Al−CMC-GO in which Al concentration varies from, 0.6, 0.8, 1%, respectively; lightening of LED for 1% GO-CMC, 1% rGO/Al-CMCGO, 3% rGO/Al-CMC-GO; resistivity graph for modified cotton threads with different rGO/Al content (%) and inset with LED lightening representing cotton thread conductivity; digital images for flexibility of 1% rGO/Al-CMC and 6% rGO/Al-CMC coated cotton threads, respectively; oxygen permeability and water vapor permeability; and SEM image for hybrid film after fire treatment and LOI (%) with different rGO/Al content (PDF)
METHOD/EXPERIMENTAL Materials. Graphite powder (
