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Enhanced mechanical property and flame resistance of graphene oxide nanocomposite paper modified with functionalized silica nanoparticles
Composites Part B 177 (2019) 107347
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Enhanced mechanical property and flame resistance of graphene oxide nanocomposite paper modified with functionalized silica nanoparticles Zhi-Ran Yu a, 1, Shi-Neng Li a, b, 1, Jing Zang a, Ming Zhang c, Li-Xiu Gong a, Pingan Song d, Li Zhao a, Guo-Dong Zhang a, Long-Cheng Tang a, e, * a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, 311121, PR China Institute for Advanced Ceramics, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150001, PR China China National Institute of Standardization, National Library of Standards, Beijing, 100191, PR China d Department of Materials, Zhejiang A&F University, Hangzhou, 311300, PR China e Key Laboratory of Silicone Materials Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou, 311121, PR China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide paper Silica nanoparticles Mechanical properties Flame resistance
Graphene oxide (GO) paper with outstanding integrated multiple functionalities (good mechanical performance, thermal stability and flame resistance, etc.) is strongly needed for numerous potential applications in cuttingedge fields. In this work, we report a facile and green process to fabricate GO-based nanocomposite papers via introducing functionalized silica (f-SiO2) nanoparticles. The results reveal that addition of f-SiO2 produces simultaneous improvements in mechanical strength, stiffness and flame resistance for GO paper. With incor poration of 10 wt% f-SiO2, the tensile strength, elastic modulus and toughness of the GO/f-SiO2 nanocomposite papers can be increased by about 51%, 317%, and 69%, respectively. Various characterizations disclose that hydrogen bonds and covalent interactions between GO sheets and f-SiO2 mainly contribute to the effective load transfer and energy dissipation between them, and thus leading to the improvements of mechanical properties. Based on the structural observation and analysis, the improved flame resistance of the GO/f-SiO2 papers should be attributed to the formation of rGO/SiO2 protective char, which are derived from the decomposition and redeposition of the grafted silane molecules and inorganic SiO2 and thermal reduction of GO into rGO. Our results suggest that the mechanical and thermal properties of GO papers can be tuned by introducing inorganic/ organic f-SiO2, providing a new route for the rational designing and development of mechanically flexible and flame-retardant GO-based nanocomposite paper materials.
1. Introduction Graphene, a unique material with abundant fascinating properties (e.g. high stiffness and strength, high electrical and thermal conductivity and high specific surface area) due to unique mono-layer carbon atomic lattice, has been widespread applied in a variety of fields [1–10]. Among various graphene-based composite materials, highly aligned graphene-based papers (GBPs) show exceptional integrated mechanical and functional properties, with promise for potential applications [11]. Many previous studies have focused on the improvement of the poor mechanical and thermal performance of the GBPs to promote their practical use, and considerable efforts have been devoted by assembling
organic compounds [12,13] or polymer [14] with graphene sheets, which has been proved to be good reinforcing and toughening strategies for fabrication of the high-performance GBPs. During the past several years, with the rapidly increasing develop ment in potential applications, the flame resistance of GBPs is also crucial for potential application in many practical industries, e.g. firealarm materials which are critically required to be tolerant when exposed to the flame attack [15–17]. Thus far, strategies for enhancing the flame resistance properties of GBPs, including use of flame-retardant organic molecules [18], polydopamine [19] and inorganic nano-clay [20], are developed. For instance, Dong et al. demonstrated that the flame resistance and thermal stability of GBPs composite materials could
* Corresponding author. Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, 311121, PR China. E-mail address: [email protected] (L.-C. Tang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.compositesb.2019.107347 Received 10 June 2019; Received in revised form 13 August 2019; Accepted 13 August 2019 Available online 13 August 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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be markedly enhanced by the functionalization of hexa chlorocyclotriphosphazene for graphene oxide (GO) [21]. Chen and co-workers [22] also revealed that the synergistic flame retardant effect of phosphorus and nitrogen atoms could effectively improve the flame resistance of GO via the formation of protective char upon flaming. Luo et al. [19] utilized the self-polymerization of dopamine as a green reduction for GO and thus prepared superior flame retardant and high thermal conductive graphene/polydopamine composites film. However, the tensile strength of obtained composites materials is still relatively low (only ~5 MPa). Moreover, harsh and expensive processing methods, hazardous production (e.g. phosphorus, chlorine) and poor mechanical properties also set a great restriction. Therefore, fabricating the GBPs with a combination of outstanding flame resistance and enhanced mechanical performance by a facile, eco-friendly and low-cost method still remains a great challenge. On account of their attractive integrated properties including good environmental friendliness, excellent thermal stability [23] and flame retardancy, easily preparation and low-cost [24], silicone-based mate rial as one type of flame retardants (halogen-free) has been applied in many fields [25], due to its unique organic/inorganic molecualr struc ture [26,27]. In our previous work, silicone molecules-based coatings have been proved to be an ideal flame retardant candidate for protecting various combustible materials [16,28]. Owing to the existence of highly cross-linked Si-O-Si network in the protective coating, the combustible substrate materials exhibited unexpected thermal stability and flame-retardant property. Inspired by the above results, inorganic nano-silica would be ideal mechanical reinforcement and flame re tardants to prepare mechanically flexible and flame-retardant GBPs. However, the nano-silica dispersion and the interfacial interaction be tween silica nanoparticles and GO sheets are still poor, limiting the effective stress transferring and efficient protective role in the GBPs. Herein, we introduce a simple and green method to prepare me chanically flexible and flame-retardant GBPs by introducing amino propyltriethoxysilane (APTS) functionalized silica (f-SiO2). The APTS molecules can react with the silica nanoparticles and thus promote the interfacial interactions between the silica and the GO sheets. The morphology and micro-structure of GO/f-SiO2 nanocomposite papers with various filler contents of f-SiO2 were investigated to optimize the mechanical strength, thermal stability and flame-resistant properties of the GO/f-SiO2 nanocomposite papers. Furthermore, based on structural observation and analysis, the reinforcing and flame-retardant mecha nisms of f-SiO2 were discussed and clarified to understand the structureproperty relationship of such GBPs.
(5.0 g) was drop-wise added into the flask to avoid the severe agglom eration of APTS cross-linked SiO2, and the mixture was refluxed at 85 � C with continuous stirring for 24 h under nitrogen flow. Subsequently, the modified SiO2 nanoparticles were separated by centrifugation and thoroughly washed with ethanol and deionized water for three times, then was dilated in water to obtain highly dispersed GO aqueous solution. 2.3. Preparation of GO/f-SiO2 nanocomposite paper The fabrication process of the f-SiO2/GO nanocomposite papers is shown in Fig. 1. GO solution was synthesized by a modified Hummers’ method based on our previous work [16,30–32]. The f-SiO2 was ultra sonically dispersed in deionized water to prepare a homogeneous dispersion (10 mg/mL). Then, the f-SiO2 aqueous solution was slowly added into GO solution (0.5 mg/mL) and under continuous stirring to form a well-dispersed GO/f-SiO2 aqueous solution with various f-SiO2 contents. The GO/f-SiO2 aqueous solution was filtered (using an Anodisc membrane filter with 0.2 μm pore size and ~50 mm diameter) under vacuum and GO/f-SiO2 nanocomposite paper could be obtained by hot-pressing at 80 � C for 2 h. A series of nanocomposite papers were denoted as GO/f-SiO2-x, x represents the weight percent of f-SiO2 to GO. For example, GO/f-SiO2-10% means the weight percent of f-SiO2 to GO is 10 wt%. 2.4. Characterizations The morphology of microstructures of the GO/f-SiO2 nanocomposite papers with different f-SiO2 contents was investigated by optical mi croscopy (Nikon Eclipse LV100 POL) and scanning electron microscopy (SEM, Sigma-500, ZEISS). Fourier transform infrared (FTIR) spectra were performed using a Fourier transform infrared (Bruker Alpha-T) in the scan ranging from 4000 cm -1 to 400 cm -1 using the KBr pellet technique. X-ray photoelectron spectra (XPS) were obtained with an ESCALab 220I-XL electron spectrometer from VG Scientific using MgKa radiation. X-ray diffraction (XRD) analysis was carried out on a D/Max 2550 V X ray diffractor (Rigaku, Japan) and the diffraction patterns were recorded in the 2θ range from 5 to 40� with a scan rate of 5� /min. Raman spectra were recorded using a SENTERRA Micro Raman Spec trometer (Bruker Instruments) with a 514 nm laser. Thermo-gravimetric analysis (TGA) was performed on a TA Instrument, Q500 analyzer, under nitrogen gas at a temperature increasing rate of 10 � C/min from room temperature to 800 � C. Tensile measurements tests were performed on rectangle samples (length*width ¼ 40 mm*3 mm) using a mechanical testing machine (Ametek, Ls100plus) with a 100 N load cell and a velocity of 1 mm⋅min-1at room temperature. The strain (ε) under stress was calcu lated on the basis of the change in length relative to the initial length of the specimen (ε¼Δl/l0*100%). The strength was calculated on the basis of the initial cross section (σ ¼F/A0), where “F” is the load force and “A0” is the original specimen cross-sectional area. The toughness of the GObased paper was calculated by integrating the area under the tensile stress-strain curve. The tensile modulus of all samples was determined by the slope of the linear region of the stress-strain curves. The me chanical properties for each sample are based on the average value of five specimens.
2. Experimental section 2.1. Materials Graphite powder was purchased from Shanghai Yi Fan Graphite Co., Ltd. Silica nanoparticles was purchased from XFNANO Materials Tech nology Co., Ltd. APTS was supplied by Shanghai Aladdin Bio-Chem Techonology Co., Ltd. Other reagents, including concentrated sulfuric acid (H2SO4, �98 wt%), hydrochloric acid (HCl, 35 wt%), phosphorus pentoxide (P2O5), hydrogen peroxide (H2O2), potassium persulfate (K2S2O8), potassium permanganate (KMnO4), etc. obtained from Sino pharm Chemical Reagent Co., Ltd., China, and no further purification before use.
3. Results and discussion
2.2. Synthetic procedure for the modification of SiO2
3.1. Fabrication and morphology of GO/f-SiO2 nanocomposite paper
The SiO2 nanoparticles were modified with APTS molecules via hy drolysis and condensation reaction according to the literature [29]. Typically, SiO2 (5.0 g) was added to a 250 mL three-necked flask with the mixture of deionized water (5.0 mL) and ethanol (95 mL), adjusted the pH of the mixture to 9–10 by adding ammonium hydroxide solution and then ultrasonically dispersed for 0.5 h. A certain amount of APTS
As shown in Fig. 1, during the fabrication of the f-SiO2/GO nano composite papers, SiO2 nanoparticles were first modified with APTS molecules via simple hydrolysis and condensation reactions [33] (Fig. 1a). During the above reactions, the alkoxy groups of APTS mole cules form silanols groups, which are expected to covalently react with 2
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Fig. 1. Schematic illustration of fabricating of functionalized-SiO2 and GO/f-SiO2 paper. (a) Surface functionlizaiton process of SiO2 with APTS molecules; (b) Filtration and hot-press fabricating processes of GO/f-SiO2 nanocomposite paper.
the hydroxyl groups of other APTS molecules or nano-silica particles [34,35]. Next, the APTS modified SiO2 (f-SiO2) aqueous solution was mixed with GO aqueous solution to achieve a homogeneous GO/f-SiO2 aqueous solution by stirring and sonication. Afterward, the uniform hybrid aqueous solution was assembled into GO-based nanocomposite papers via a vacuum-assisted filtration at low temerpature condiiton. Note that the residual hydroxyl and amine groups of f-SiO2 would react with the GO sheets during the hot-press process, and thus forming an interconnected network. Thus, after the hot-pressing treatment, the
resulting GO/f-SiO2 papers with a little bit of metallic luster could be obtained and bent or folded without rupture (Fig. 1b), demonstrating a remarkable flexibility. In order to understand the influence of silica nanoparticle on the structure and properties, a series of GO/f-SiO2 nanocomposite papers as a function of the weight ratio of f-SiO2 to GO were fabricated. The morphology and hierarchical structure of the GO/f-SiO2 nanocomposite papers in Fig. 2(a–d) show typical four levels of hierarchy: whole sample (1 mm–5 cm), interconnect network (10 μm–1 mm), surface morphology
Fig. 2. Morphology and hierarchical structure of GO/f-SiO2 nanocomposite papers: (i) pure GO paper, (ii) GO/f-SiO2-5%, (iii) GO/f-SiO2-10%, (iv) GO/fSiO2-15% and (v) GO/f-SiO2-20%. (a) Macro-scale digital and (b) micro-scale optical microscopy images of nanocomposite papers with different f-SiO2 contents; (c) Surface and (d) cross-section of SEM images of the GO/f-SiO2 nanocomposite papers. 3
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(1–10 μm), and aligned structure (100 nm–1 μm). It can be seen that incorporation of f-SiO2 into the black pure GO paper induces the color into apparent metallic luster and the macro-size of the GO/f-SiO2 paper samples has little change at the f-SiO2 content investigated (Fig. 2a). Careful observation suggested that some cracks are visible at a high fSiO2 content of ~20 wt% due to the formation of f-SiO2 cluster at high content. Optical microscopy images under the transmission mode in Fig. 2b display different structures and morphology. For pure GO paper, single brown color distribution under the transmission mode implies the uniform structure parallel to paper surface. Interestingly, the presence of f-SiO2 produces obvious network in the paper, and the network becomes more and more dense with increasing the f-SiO2 content from 5 to 20 wt % (see ii, iii and iv in Fig. 2). SEM images offer some solid evidences to explain the above phe nomena. Obvious wrinkled or rippled surface can be observed for the pure GO paper (see i in Fig. 2c), and the GO sheets are highly aligned to form free-standing paper due to the Van Der Waals force and π-π in teractions [36,37]. Addition of low content (5–10 wt%) of f-SiO2 has little influence on the surface morphology, showing typical smooth feature [11]; while some silica nanoparticles that are highly dispersed on the surface are well attached onto the sheet surface, suggesting the good interactions between the f-SiO2 and GO sheets. However, it is noted that at a relatively high content of f-SiO2 (20 wt%) nano-silica aggre gation is visible, which may be important factors to induce the formation of cracks on the paper surface. On the other hand, the alignment degree of GO sheets would be strongly dependent on the silica content. The f-SiO2 nanoparticles can act as cross-linking points to interconnect the sheets around them, thus inducing the reduced alignment degree of GO sheets. This phenomenon is well consistent with the above network structure observed by the optical microscopy (Fig. 2b). When the f-SiO2 content increases to be 20 wt%, the formation of f-SiO2 cluster can be clearly seen between the sheets and the nanoparticles, as indicated by the dotted circles in Fig. 2d. This would be not favorable for interfacial interactions between the sheets and the f-SiO2, which will be discussed
later. 3.2. Structural analysis and thermal stability FTIR and XPS were employed clarify the chemical compositions and differences of the GO/f-SiO2 nanocomposite papers. The covalent functionalization of GO sheets by APTS molecules can be verified by FTIR result. Compared with pure SiO2, the FTIR spectrum of f-SiO2 (Fig. 3a) indicates a new peak at 2970 cm-1 corresponding to the stretching vibration of C-H of APTS molecules. Moreover, the peak corresponding to the stretching vibration of Si-OH is shifted from 957 cm-1 to 949 cm 1, indicating the existence of interaction between SiOH on the surface of SiO2 and -OH of APTS after the hydrolysis reaction. Meanwhile, an additional peak assigned to the bending vibration peak of Si-OH from SiO2, confirming the successful implantation of f-SiO2. This is further verified by the appearance of Si and N elements in the results of XPS spectra (Fig. 3b). Furthermore, the results of C1s and N1s can offer much more valuable information. The C1s core level spectrum of GO sheets with peak-fitting curves is composed of six main peaks, i.e. 284.3, 284.6, 285.0, 286.8, 287.3 and 289.4 eV, which is attributed to the C¼C, C-C,C-OH, C-O-C, C¼O and C(¼O)O bonding, respectively, as shown in Fig. 3c. While the C1s spectrum of f-SiO2 shows three main peaks, i.e. 283.9, 284.6 and 285.4 eV, which is attributed to the C-Si, C-C and C-N bonding. As expected, the same peaks appear in the GO/f-SiO210% (discussed in the later section), indicating the introduction of f-SiO2 into GO interlayers, which is well consistent with the FT-IR results. Compared with the N1s spectra of f-SiO2 (Fig. 3e and f), the N1s spectra of the GO/f-SiO2-10% shows a dramatically increase for bonded NH2 (~402.1eV) and significant decrease of free NH2, indicating the for mation of strong bonding between f-SiO2 and GO sheets via amine group and oxygen-containing group during the fabricating process. XRD analysis of pure GO and GO/f-SiO2 papers was performed and shown in Fig. 4a. Typically, the XRD spectrum of pure GO paper shows a sharp diffraction peak at ~11.23� , suggesting an interlayer spacing of
Fig. 3. Structural characterizations and analysis. (a) FTIR spectra and (b) XPS survey results of GO, SiO2, f-SiO2 and GO/f-SiO2-10% paper; C1s spectra of (c) GO and (d) f-SiO2, N1s spectra of (e) f-SiO2 and (f) GO/f-SiO2-10% paper. 4
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~7.88 Å. Comparatively, for the GO/f-SiO2 composite, the d-spacing value of the GO sheets increases from approximately 8.83 Å (2θ ¼ 10.08� ) for 5 wt% of f-SiO2 content to 8.96 Å (2θ ¼ 9.87� ) for 20 wt% of f-SiO2 content. This confirms that the introduction of the fSiO2 nanoparticles can enhance the d-spacing between GO sheets. This may be attributed to the grafted APTS molecules on silica nanoparticles, and the interlayer spacing of GO sheets can be tailored through changing the f-SiO2 content, which is consistent with the structure evolution and interlayer distance change of GO/f-SiO2 nanocomposite papers with increasing the f-SiO2 contents (see Fig. 2d). The chemical compositions of GO, SiO2, f-SiO2, GO/f-SiO2-10% and GO/f-SiO2-20% can also be characterized by TGA analysis and the related results are given in Fig. 4b. It can be seen that the SiO2 nano particles show excellent thermal stability and low weight loss of ~2.8% at 800 � C, while the f-SiO2 nanoparticles present similar thermal sta bility and ~10.6% weight loss at 800 � C. This suggests that the grafting ratio of APTS molecules for f-SiO2 is more than 7.8 wt% due to the re sidual mass of APTS molecules after thermal composition. In compari son, the GO paper displays an obvious decomposition at ~220 � C under nitrogen atmosphere because of the pyrolysis of unstable oxygen groups [38]. As expected, the GO/f-SiO2 nanocomposite papers show initial mass loss below 100 � C due to the volatilization of stored water, and a maximum decomposition at ~213 � C, which would be attributed to thermal degradation of Si-OH or -CH3 of APTS molecules. Notably, our previous work indicated that the thermal decomposition of highly cross-linked silicone molecules can form nano-silica structure at high temperature, resulting in enhanced thermal stability of composite ma terials [16]. Benefit from the synergistic effect of grafted APTS mole cules and nano-SiO2, the GO/f-SiO2 nanocomposite papers display higher thermal stability ranging from 250 to 600 � C and a higher residue yield (e.g. 60.4% and 66.4% at 800 � C for GO/f-SiO2-10% and GO/f- SiO2-20%, respectively) than that of GO paper (~55.8% at 800 � C). Based on the above results, it is reasonable to deduce that the intro duction of f-SiO2 nanoparticles produces improved thermal stability of GO paper.
0.6 GPa and 81.2 MPa, respectively [39]. At 10 wt% of f-SiO2, the Young’s modulus and tensile strength values of the nanocomposite pa pers are 2.5 GPa and 122.4 MPa, corresponding to the increases of ~317% and ~51%, respectively. In comparison, the GO/f-SiO2-20% composites show only 22% increase in strength which can be explain by the aggregation of SiO2 in Fig. 2d. At the same loading, the elastic moduli of the nanocomposite papers show similar increases compared with the value of neat GO, i.e. 7.3% (2.4 GPa), 6.9% (2.5 GPa) and 5.5% (2.2 GPa) for GO/f-SiO2-5%, GO/f-SiO2-10% and GO/f-SiO2-15%, respectively. The toughness (area under stress-strain curve) of neat GO paper is 2.8 MJ m-3. The nanocomposite papers with 5 wt% and 10 wt% of f-SiO2 exhibit toughening effects as high as 71% (4.8 MJ m-3) and 69% (4.7 MJ m-3), as illustrated in Fig. 5b and Table 1. With an appropriate f-SiO2 content, the f-SiO2 nanoparticles can act as cross-linking points to interconnect the sheets along GO, which can promote the interactions between the GO sheet that give rise to the enhancement of mechanical property greatly. However, when the content of f-SiO2 is above 10 wt%, the aggregation following, which generates the concentration of stress that leads to the decreased mechanical property including Young’s modulus, tensile strength and toughness. The abovementioned results show significant and simultaneous im provements in stiffness, strength and toughness at low f-SiO2 loading, indicating the effective and tunable interphases obtained between the GO and f-SiO2. The enhanced mechanical properties of GO/f-SiO2 nanocomposite paper should be attributed to the hydrogen bond and covalent interactions between graphene oxide sheets and f-SiO2 and the lubrication of f-SiO2, producing effective load transfer and high energy dissipation during the deformation process [40,41]. The relative in teractions have been proved by the above structural analysis (Section 3.2). At this stage, how the different functional f-SiO2 molecules affect the interfacial characteristics and the surrounding GO network forma tion remains an open question. Nevertheless, the surface functionaliza tion of GO with functional f-SiO2 molecules appears to be a promising approach for obtaining high-performance engineering GO/f-SiO2 nanocomposite papers with improved mechanical performance.
3.3. Mechanical properties of GO/f-SiO2 nanocomposite paper
3.4. Flame retardant property and mechanism analysis
Representative tensile stress-strain curves of pure GO and GO/f-SiO2 nanocomposite papers are shown in Fig. 5a, and the detailed mechanical properties (i.e. Young’s modulus, tensile strength, elongation at break and toughness) are summarized in Table 1. Upon comparison with GO/fSiO2-10% and GO, it can be seen that the former produces an improved ductility, strength and toughness, along with a similar increase in stiff ness. For the neat GO, the Young’s modulus, and tensile strength are
To evaluate the effect of f-SiO2 on the flame resistance of GO-based nanocomposite paper, the nanocomposite papers were directly encountered the flame of the alcohol lamp (a flame temperature of 400–600 � C). Due to the abundant of unstable oxygen groups, e.g. the hydroxyl, carbonyl and carboxylic groups, the pure GO paper exhibits a rapid and vigorous combustion once being ignited (Fig. 6a). The GO paper can be completely burned in only about 16 s and generates gases
Fig. 4. (a) XRD spectra of GO and GO/f-SiO2 nanocomposite papers with different f-SiO2 contents; (b) TGA curves of GO, SiO2, f-SiO2 and GO/f-SiO2 nano composite papers. 5
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Fig. 5. Mechanical properties of GO/f-SiO2 nanocomposite papers. (a) Typical stress-strain curves and (b) tensile strength, Young’ modulus and toughness as a function of f-SiO2 content.
paper can keep the structural integrity after the burning test when compared to the complete thermal degradation of pure GO paper [42]. It should be noted that the square shape of GO/f-SiO2-10% paper still presents slight damage at the edges, as indicated by the red arcs in Fig. 7a. During the burning test, the rectangle edge of the modified paper is completely exposed by the flame, and this results in the rapid thermal decomposition in the edge zone, which is supported by the obvious red hot phenomenon observed in Fig. 6b [15]. Consequently, the formation of smooth arc angle is visible (see ii in Fig. 7a) for the GO/f-SiO2-10% paper after the first burning test. And such arc structure can be kept even exposing it for the second burning test, suggesting the highly improved flame resistance after introducing the f-SiO2. The related structural feature was examined by SEM, which will be discussed in the following section. The structural feature of the GO/f-SiO2 nanocomposite paper after the burning test was determined by SEM analysis and shown in Fig. 7b and c. Typically, compared with the relatively smooth surface of the modified before the burning test (see iii in Fig. 2c), much more nanosilica nanoparticles can be clearly observed to be well attached on the sheet surface (Fig. 7b). This suggests that the formation of nano-silica protective char as an effective barrier layer prevent oxygen from attacking the inside GO sheets but promote the graphitization of the GO sheets via thermal reduction, which thus improve the flame resistance of the GO paper. In fact, our previous work demonstrates that the porous nano-silica/rGO structures with several micrometer was also generated from silicone resin/GO coating during the burning testing [15], which can prevent the heat transportation from outside zone to the inside zone and thus achieve the excellent flame-retardant effect. The thick and porous nano-silica/rGO structure is different from the thin and compact protective layer of the GO/f-SiO2 after the burning test observed in this work. Fig. 7c discloses that a compact nano-silica nanoparticle layer is
Table 1 Tensile properties of GO/f-SiO2 paper as a function of f-SiO2 content. Sample codes
Tensile Strength (MPa)
Elongation at break (%)
Young’s modulus (GPa)
Toughnessa (MJ⋅m 3)
GO GO/fSiO2-5 GO/fSiO210 GO/fSiO215 GO/fSiO220
81.2 � 8.1 108.7 � 6.2
6.7 � 0.6 7.3 � 0.5
0.6 � 0.1 2.4 � 0.2
2.8 � 0.5 4.8 � 0.6
122.4 � 6.0
6.9 � 0.7
2.5 � 0.3
4.7 � 0.7
105.8 � 4.2
5.5 � 0.3
2.2 � 0.2
3.3 � 0.4
99.2 � 7.3
5.1 � 0.7
2.1 � 0.1
2.6 � 0.3
a
Calculated by integrating stress-strain curves.
including CO, CO2, and steam. Comparatively, the presence of f-SiO2 improves the flame resistance of the GO paper significantly. As indicated in Fig. 6b, it can be seen that the modified papers can keep its original shape and size after exposing it in the flame for about 10 s, showing significant improvement in flame resistance compared with the corre sponding combustion behavior of pure GO paper. After exposing the paper in the flame for further combustion, the GO/f-SiO2-10% can almost remain the size and structure, demonstrating the outstanding thermal stability, which is well consistent with the improved thermal stability shown in Fig. 4b. Fig. 7a shows the digital images of pure GO and GO/f-SiO2-10% nanocomposite paper before/after the combustion process. For the same paper size, it is clearly seen that the GO/f-SiO2-10% nanocomposite
Fig. 6. Flame burning tests of (a) pure GO and (b) GO/f-SiO2-10% nanocomposite papers, showing improved flame resistance after introducing f-SiO2. 6
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Fig. 7. Flame-retardant mechanism analysis of GO/f-SiO2 nanocomposite papers. (a) Digital images of GO and GO/f-SiO2 paper before and after burning tests for 10s, and their SEM images of (b) surface-section and (c) cross-section of GO/f-SiO2 papers after the burning test, showing a formation of compact and solid silica protective layer on GO sheets; (d) XPS survey results and (e) the related C1s spectra of GO/f-SiO2 -10% paper before/after the burning test.
eV). The C1s core level spectrum of the GO/f-SiO2 paper before the burning test shows eight typical chemically shifted components: C-C (284.6 eV), C¼C (284.73eV), C-OH (285.0 eV), C-N (285.4 eV), C-Si (283.9 eV), C-O-C (286.8eV), C¼O (287.3 eV), and C(¼O)O (289.4 eV) [44], implying a chemical reaction between the oxygen functional group
well formed on the cross-section surface, which should inhibit the thermal degradation of GO sheets greatly. XPS survey results can offer more evidences [43]. As shown in Fig. 7d, the GO/f-SiO2 paper before/after the burning test are mainly composed of four typical peaks, i.e. C1s (ca. 283 eV), O1s (ca. 529eV), Si2p (ca. 103 eV), Si2s (ca. 153
Fig. 8. Schematic illustration of proposed flame-retardant mechanism and thermal reduction of GO/f-SiO2 paper during the flame attack process. (a) The decomposition course of f-SiO2 (b) the flame treating procedure of GO/f-SiO2 paper. At a high temperature, thermal composition of the modified APTS molecules combined with inorganic nano-silica can produce an effective protective layer, which inhibit the thermal degradation of GO sheets effectively and promote the graphitization to form multi-scale rGO/silica protective char. 7
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of GO and amine group of f-SiO2 molecules. During the burning test, these oxygen groups will thermally degrade and thus induce the sig nificant decrease in the intensity of the C-O-C and C(¼O)O peak. It should be noted that the peaks of oxidized components including C-OH and C¼O are not obvious after the second burning test (see Fig. 7f), indicating that thermal reduction of GO to RGO in the flame and char formation [15]. Based on the abovementioned structural analysis, the introduced functionalized nano-silica particles among the sheets not only promote the stress transferring efficiency but greatly improve the flame resis tance of GO based papers, thus producing mechanically flexible and flame-retardant GO based nanocomposite papers. The proposed flameretardant mechanism is present in Fig. 8. In this GO/f-SiO2 system, the amine groups of APTS molecules on inorganic silica nanoparticles can produce chemical bonding with the hydroxyl and carboxyl groups of GO sheets. During the combustion process, the f-SiO2 particles with high inorganic content make a significant contribution to the improved flame-retardant effect of the modified GO papers. During the combus tion process, thermal depolymerizaiton of cross-linked APTS molecules would generate silicone oligomers and then cross-link or oxide into silicone “soot” to redeposit onto the sheet surface [45]. Considering the much higher inorganic composition in the f-SiO2, the degradation of APTS molecules redeposit and promote the formation of compact nano-silica structure, releasing small amount of gases such as H2O and NO/NO2 due to the thermal degradation of amine group and C-C chains (Fig. 8a). Meanwhile, the oxygen groups of GO sheets or their chemical bonds with the APTS molecules will break and react with the silicone “soot” to form a silica coated sheet structure, as shown in Fig. 7b and c, thus promoting the thermal reduction of GO sheets during the burning test. This would produce the formation of outside compact nano-silica coated RGO layer, which can act as an effective barrier to restrict the thermal composition of inside GO sheets (Fig. 8b), thus leading to improved flame resistance in the GO/f-SiO2 nanocomposite papers.
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4. Conclusions In summary, this work reports a facile and effective strategy to prepare mechanically flexible and flame-retardant GO-based nano composite papers by a combination use of APTS functionalized SiO2 and a filtrating and hot-pressing process. The APTS functionalized SiO2 nanoparticles in the GO-based nanocomposite papers not only acts as effective mechanical reinforcement but also provides efficient barrier role for flame attack. The optimized GO/f-SiO2 nanocomposite papers with 10 wt% f-SiO2 showed improved mechanical performances compared with the pure GO paper, e.g. tensile strength of 122.4 � 6.0 MPa (~51% increase), Young’s modulus of 2.5 � 0.3 GPa (~276% increase) and toughness of 4.7 � 0.7 MJ m-3 (~68% increase), respectively. The formation of strong interfacial interactions between GO sheets and f-SiO2 mainly contributes to promote the stress trans ferring and thus consume energy dissipation during the deformation/ failure process. Furthermore, the presence of f-SiO2 also endows GO papers with improved thermal stability and flame-retardant property, which is attributed to the formation of a compact SiO2 protective layer on the GO sheets when exposed to the flame. Our results suggest that the interfacial interactions between the GO sheets can be tuned by intro ducing surface functionalized silica nanoparticles, thereby providing a promising route for designing and fabricating mechanically strong and flame-retardant GO-based nanocomposite papers for many potential applications. Acknowledgements We acknowledge the funding support from the Natural Science Foundation of China (51973047), the Natural Science Foundation of Zhejiang Province (LY18E030005 and LY15E030015), the Project for the Science and Technology Program of Hangzhou (20191203B16 and 8
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Enhanced mechanical property and flame resistance of graphene oxide nanocomposite paper modified with functionalized silica nanoparticles Zhi-Ran Yu a, 1, Shi-Neng Li a, b, 1, Jing Zang a, Ming Zhang c, Li-Xiu Gong a, Pingan Song d, Li Zhao a, Guo-Dong Zhang a, Long-Cheng Tang a, e, * a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, 311121, PR China Institute for Advanced Ceramics, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150001, PR China China National Institute of Standardization, National Library of Standards, Beijing, 100191, PR China d Department of Materials, Zhejiang A&F University, Hangzhou, 311300, PR China e Key Laboratory of Silicone Materials Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou, 311121, PR China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide paper Silica nanoparticles Mechanical properties Flame resistance
Graphene oxide (GO) paper with outstanding integrated multiple functionalities (good mechanical performance, thermal stability and flame resistance, etc.) is strongly needed for numerous potential applications in cuttingedge fields. In this work, we report a facile and green process to fabricate GO-based nanocomposite papers via introducing functionalized silica (f-SiO2) nanoparticles. The results reveal that addition of f-SiO2 produces simultaneous improvements in mechanical strength, stiffness and flame resistance for GO paper. With incor poration of 10 wt% f-SiO2, the tensile strength, elastic modulus and toughness of the GO/f-SiO2 nanocomposite papers can be increased by about 51%, 317%, and 69%, respectively. Various characterizations disclose that hydrogen bonds and covalent interactions between GO sheets and f-SiO2 mainly contribute to the effective load transfer and energy dissipation between them, and thus leading to the improvements of mechanical properties. Based on the structural observation and analysis, the improved flame resistance of the GO/f-SiO2 papers should be attributed to the formation of rGO/SiO2 protective char, which are derived from the decomposition and redeposition of the grafted silane molecules and inorganic SiO2 and thermal reduction of GO into rGO. Our results suggest that the mechanical and thermal properties of GO papers can be tuned by introducing inorganic/ organic f-SiO2, providing a new route for the rational designing and development of mechanically flexible and flame-retardant GO-based nanocomposite paper materials.
1. Introduction Graphene, a unique material with abundant fascinating properties (e.g. high stiffness and strength, high electrical and thermal conductivity and high specific surface area) due to unique mono-layer carbon atomic lattice, has been widespread applied in a variety of fields [1–10]. Among various graphene-based composite materials, highly aligned graphene-based papers (GBPs) show exceptional integrated mechanical and functional properties, with promise for potential applications [11]. Many previous studies have focused on the improvement of the poor mechanical and thermal performance of the GBPs to promote their practical use, and considerable efforts have been devoted by assembling
organic compounds [12,13] or polymer [14] with graphene sheets, which has been proved to be good reinforcing and toughening strategies for fabrication of the high-performance GBPs. During the past several years, with the rapidly increasing develop ment in potential applications, the flame resistance of GBPs is also crucial for potential application in many practical industries, e.g. firealarm materials which are critically required to be tolerant when exposed to the flame attack [15–17]. Thus far, strategies for enhancing the flame resistance properties of GBPs, including use of flame-retardant organic molecules [18], polydopamine [19] and inorganic nano-clay [20], are developed. For instance, Dong et al. demonstrated that the flame resistance and thermal stability of GBPs composite materials could
* Corresponding author. Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, 311121, PR China. E-mail address: [email protected] (L.-C. Tang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.compositesb.2019.107347 Received 10 June 2019; Received in revised form 13 August 2019; Accepted 13 August 2019 Available online 13 August 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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be markedly enhanced by the functionalization of hexa chlorocyclotriphosphazene for graphene oxide (GO) [21]. Chen and co-workers [22] also revealed that the synergistic flame retardant effect of phosphorus and nitrogen atoms could effectively improve the flame resistance of GO via the formation of protective char upon flaming. Luo et al. [19] utilized the self-polymerization of dopamine as a green reduction for GO and thus prepared superior flame retardant and high thermal conductive graphene/polydopamine composites film. However, the tensile strength of obtained composites materials is still relatively low (only ~5 MPa). Moreover, harsh and expensive processing methods, hazardous production (e.g. phosphorus, chlorine) and poor mechanical properties also set a great restriction. Therefore, fabricating the GBPs with a combination of outstanding flame resistance and enhanced mechanical performance by a facile, eco-friendly and low-cost method still remains a great challenge. On account of their attractive integrated properties including good environmental friendliness, excellent thermal stability [23] and flame retardancy, easily preparation and low-cost [24], silicone-based mate rial as one type of flame retardants (halogen-free) has been applied in many fields [25], due to its unique organic/inorganic molecualr struc ture [26,27]. In our previous work, silicone molecules-based coatings have been proved to be an ideal flame retardant candidate for protecting various combustible materials [16,28]. Owing to the existence of highly cross-linked Si-O-Si network in the protective coating, the combustible substrate materials exhibited unexpected thermal stability and flame-retardant property. Inspired by the above results, inorganic nano-silica would be ideal mechanical reinforcement and flame re tardants to prepare mechanically flexible and flame-retardant GBPs. However, the nano-silica dispersion and the interfacial interaction be tween silica nanoparticles and GO sheets are still poor, limiting the effective stress transferring and efficient protective role in the GBPs. Herein, we introduce a simple and green method to prepare me chanically flexible and flame-retardant GBPs by introducing amino propyltriethoxysilane (APTS) functionalized silica (f-SiO2). The APTS molecules can react with the silica nanoparticles and thus promote the interfacial interactions between the silica and the GO sheets. The morphology and micro-structure of GO/f-SiO2 nanocomposite papers with various filler contents of f-SiO2 were investigated to optimize the mechanical strength, thermal stability and flame-resistant properties of the GO/f-SiO2 nanocomposite papers. Furthermore, based on structural observation and analysis, the reinforcing and flame-retardant mecha nisms of f-SiO2 were discussed and clarified to understand the structureproperty relationship of such GBPs.
(5.0 g) was drop-wise added into the flask to avoid the severe agglom eration of APTS cross-linked SiO2, and the mixture was refluxed at 85 � C with continuous stirring for 24 h under nitrogen flow. Subsequently, the modified SiO2 nanoparticles were separated by centrifugation and thoroughly washed with ethanol and deionized water for three times, then was dilated in water to obtain highly dispersed GO aqueous solution. 2.3. Preparation of GO/f-SiO2 nanocomposite paper The fabrication process of the f-SiO2/GO nanocomposite papers is shown in Fig. 1. GO solution was synthesized by a modified Hummers’ method based on our previous work [16,30–32]. The f-SiO2 was ultra sonically dispersed in deionized water to prepare a homogeneous dispersion (10 mg/mL). Then, the f-SiO2 aqueous solution was slowly added into GO solution (0.5 mg/mL) and under continuous stirring to form a well-dispersed GO/f-SiO2 aqueous solution with various f-SiO2 contents. The GO/f-SiO2 aqueous solution was filtered (using an Anodisc membrane filter with 0.2 μm pore size and ~50 mm diameter) under vacuum and GO/f-SiO2 nanocomposite paper could be obtained by hot-pressing at 80 � C for 2 h. A series of nanocomposite papers were denoted as GO/f-SiO2-x, x represents the weight percent of f-SiO2 to GO. For example, GO/f-SiO2-10% means the weight percent of f-SiO2 to GO is 10 wt%. 2.4. Characterizations The morphology of microstructures of the GO/f-SiO2 nanocomposite papers with different f-SiO2 contents was investigated by optical mi croscopy (Nikon Eclipse LV100 POL) and scanning electron microscopy (SEM, Sigma-500, ZEISS). Fourier transform infrared (FTIR) spectra were performed using a Fourier transform infrared (Bruker Alpha-T) in the scan ranging from 4000 cm -1 to 400 cm -1 using the KBr pellet technique. X-ray photoelectron spectra (XPS) were obtained with an ESCALab 220I-XL electron spectrometer from VG Scientific using MgKa radiation. X-ray diffraction (XRD) analysis was carried out on a D/Max 2550 V X ray diffractor (Rigaku, Japan) and the diffraction patterns were recorded in the 2θ range from 5 to 40� with a scan rate of 5� /min. Raman spectra were recorded using a SENTERRA Micro Raman Spec trometer (Bruker Instruments) with a 514 nm laser. Thermo-gravimetric analysis (TGA) was performed on a TA Instrument, Q500 analyzer, under nitrogen gas at a temperature increasing rate of 10 � C/min from room temperature to 800 � C. Tensile measurements tests were performed on rectangle samples (length*width ¼ 40 mm*3 mm) using a mechanical testing machine (Ametek, Ls100plus) with a 100 N load cell and a velocity of 1 mm⋅min-1at room temperature. The strain (ε) under stress was calcu lated on the basis of the change in length relative to the initial length of the specimen (ε¼Δl/l0*100%). The strength was calculated on the basis of the initial cross section (σ ¼F/A0), where “F” is the load force and “A0” is the original specimen cross-sectional area. The toughness of the GObased paper was calculated by integrating the area under the tensile stress-strain curve. The tensile modulus of all samples was determined by the slope of the linear region of the stress-strain curves. The me chanical properties for each sample are based on the average value of five specimens.
2. Experimental section 2.1. Materials Graphite powder was purchased from Shanghai Yi Fan Graphite Co., Ltd. Silica nanoparticles was purchased from XFNANO Materials Tech nology Co., Ltd. APTS was supplied by Shanghai Aladdin Bio-Chem Techonology Co., Ltd. Other reagents, including concentrated sulfuric acid (H2SO4, �98 wt%), hydrochloric acid (HCl, 35 wt%), phosphorus pentoxide (P2O5), hydrogen peroxide (H2O2), potassium persulfate (K2S2O8), potassium permanganate (KMnO4), etc. obtained from Sino pharm Chemical Reagent Co., Ltd., China, and no further purification before use.
3. Results and discussion
2.2. Synthetic procedure for the modification of SiO2
3.1. Fabrication and morphology of GO/f-SiO2 nanocomposite paper
The SiO2 nanoparticles were modified with APTS molecules via hy drolysis and condensation reaction according to the literature [29]. Typically, SiO2 (5.0 g) was added to a 250 mL three-necked flask with the mixture of deionized water (5.0 mL) and ethanol (95 mL), adjusted the pH of the mixture to 9–10 by adding ammonium hydroxide solution and then ultrasonically dispersed for 0.5 h. A certain amount of APTS
As shown in Fig. 1, during the fabrication of the f-SiO2/GO nano composite papers, SiO2 nanoparticles were first modified with APTS molecules via simple hydrolysis and condensation reactions [33] (Fig. 1a). During the above reactions, the alkoxy groups of APTS mole cules form silanols groups, which are expected to covalently react with 2
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Fig. 1. Schematic illustration of fabricating of functionalized-SiO2 and GO/f-SiO2 paper. (a) Surface functionlizaiton process of SiO2 with APTS molecules; (b) Filtration and hot-press fabricating processes of GO/f-SiO2 nanocomposite paper.
the hydroxyl groups of other APTS molecules or nano-silica particles [34,35]. Next, the APTS modified SiO2 (f-SiO2) aqueous solution was mixed with GO aqueous solution to achieve a homogeneous GO/f-SiO2 aqueous solution by stirring and sonication. Afterward, the uniform hybrid aqueous solution was assembled into GO-based nanocomposite papers via a vacuum-assisted filtration at low temerpature condiiton. Note that the residual hydroxyl and amine groups of f-SiO2 would react with the GO sheets during the hot-press process, and thus forming an interconnected network. Thus, after the hot-pressing treatment, the
resulting GO/f-SiO2 papers with a little bit of metallic luster could be obtained and bent or folded without rupture (Fig. 1b), demonstrating a remarkable flexibility. In order to understand the influence of silica nanoparticle on the structure and properties, a series of GO/f-SiO2 nanocomposite papers as a function of the weight ratio of f-SiO2 to GO were fabricated. The morphology and hierarchical structure of the GO/f-SiO2 nanocomposite papers in Fig. 2(a–d) show typical four levels of hierarchy: whole sample (1 mm–5 cm), interconnect network (10 μm–1 mm), surface morphology
Fig. 2. Morphology and hierarchical structure of GO/f-SiO2 nanocomposite papers: (i) pure GO paper, (ii) GO/f-SiO2-5%, (iii) GO/f-SiO2-10%, (iv) GO/fSiO2-15% and (v) GO/f-SiO2-20%. (a) Macro-scale digital and (b) micro-scale optical microscopy images of nanocomposite papers with different f-SiO2 contents; (c) Surface and (d) cross-section of SEM images of the GO/f-SiO2 nanocomposite papers. 3
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(1–10 μm), and aligned structure (100 nm–1 μm). It can be seen that incorporation of f-SiO2 into the black pure GO paper induces the color into apparent metallic luster and the macro-size of the GO/f-SiO2 paper samples has little change at the f-SiO2 content investigated (Fig. 2a). Careful observation suggested that some cracks are visible at a high fSiO2 content of ~20 wt% due to the formation of f-SiO2 cluster at high content. Optical microscopy images under the transmission mode in Fig. 2b display different structures and morphology. For pure GO paper, single brown color distribution under the transmission mode implies the uniform structure parallel to paper surface. Interestingly, the presence of f-SiO2 produces obvious network in the paper, and the network becomes more and more dense with increasing the f-SiO2 content from 5 to 20 wt % (see ii, iii and iv in Fig. 2). SEM images offer some solid evidences to explain the above phe nomena. Obvious wrinkled or rippled surface can be observed for the pure GO paper (see i in Fig. 2c), and the GO sheets are highly aligned to form free-standing paper due to the Van Der Waals force and π-π in teractions [36,37]. Addition of low content (5–10 wt%) of f-SiO2 has little influence on the surface morphology, showing typical smooth feature [11]; while some silica nanoparticles that are highly dispersed on the surface are well attached onto the sheet surface, suggesting the good interactions between the f-SiO2 and GO sheets. However, it is noted that at a relatively high content of f-SiO2 (20 wt%) nano-silica aggre gation is visible, which may be important factors to induce the formation of cracks on the paper surface. On the other hand, the alignment degree of GO sheets would be strongly dependent on the silica content. The f-SiO2 nanoparticles can act as cross-linking points to interconnect the sheets around them, thus inducing the reduced alignment degree of GO sheets. This phenomenon is well consistent with the above network structure observed by the optical microscopy (Fig. 2b). When the f-SiO2 content increases to be 20 wt%, the formation of f-SiO2 cluster can be clearly seen between the sheets and the nanoparticles, as indicated by the dotted circles in Fig. 2d. This would be not favorable for interfacial interactions between the sheets and the f-SiO2, which will be discussed
later. 3.2. Structural analysis and thermal stability FTIR and XPS were employed clarify the chemical compositions and differences of the GO/f-SiO2 nanocomposite papers. The covalent functionalization of GO sheets by APTS molecules can be verified by FTIR result. Compared with pure SiO2, the FTIR spectrum of f-SiO2 (Fig. 3a) indicates a new peak at 2970 cm-1 corresponding to the stretching vibration of C-H of APTS molecules. Moreover, the peak corresponding to the stretching vibration of Si-OH is shifted from 957 cm-1 to 949 cm 1, indicating the existence of interaction between SiOH on the surface of SiO2 and -OH of APTS after the hydrolysis reaction. Meanwhile, an additional peak assigned to the bending vibration peak of Si-OH from SiO2, confirming the successful implantation of f-SiO2. This is further verified by the appearance of Si and N elements in the results of XPS spectra (Fig. 3b). Furthermore, the results of C1s and N1s can offer much more valuable information. The C1s core level spectrum of GO sheets with peak-fitting curves is composed of six main peaks, i.e. 284.3, 284.6, 285.0, 286.8, 287.3 and 289.4 eV, which is attributed to the C¼C, C-C,C-OH, C-O-C, C¼O and C(¼O)O bonding, respectively, as shown in Fig. 3c. While the C1s spectrum of f-SiO2 shows three main peaks, i.e. 283.9, 284.6 and 285.4 eV, which is attributed to the C-Si, C-C and C-N bonding. As expected, the same peaks appear in the GO/f-SiO210% (discussed in the later section), indicating the introduction of f-SiO2 into GO interlayers, which is well consistent with the FT-IR results. Compared with the N1s spectra of f-SiO2 (Fig. 3e and f), the N1s spectra of the GO/f-SiO2-10% shows a dramatically increase for bonded NH2 (~402.1eV) and significant decrease of free NH2, indicating the for mation of strong bonding between f-SiO2 and GO sheets via amine group and oxygen-containing group during the fabricating process. XRD analysis of pure GO and GO/f-SiO2 papers was performed and shown in Fig. 4a. Typically, the XRD spectrum of pure GO paper shows a sharp diffraction peak at ~11.23� , suggesting an interlayer spacing of
Fig. 3. Structural characterizations and analysis. (a) FTIR spectra and (b) XPS survey results of GO, SiO2, f-SiO2 and GO/f-SiO2-10% paper; C1s spectra of (c) GO and (d) f-SiO2, N1s spectra of (e) f-SiO2 and (f) GO/f-SiO2-10% paper. 4
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~7.88 Å. Comparatively, for the GO/f-SiO2 composite, the d-spacing value of the GO sheets increases from approximately 8.83 Å (2θ ¼ 10.08� ) for 5 wt% of f-SiO2 content to 8.96 Å (2θ ¼ 9.87� ) for 20 wt% of f-SiO2 content. This confirms that the introduction of the fSiO2 nanoparticles can enhance the d-spacing between GO sheets. This may be attributed to the grafted APTS molecules on silica nanoparticles, and the interlayer spacing of GO sheets can be tailored through changing the f-SiO2 content, which is consistent with the structure evolution and interlayer distance change of GO/f-SiO2 nanocomposite papers with increasing the f-SiO2 contents (see Fig. 2d). The chemical compositions of GO, SiO2, f-SiO2, GO/f-SiO2-10% and GO/f-SiO2-20% can also be characterized by TGA analysis and the related results are given in Fig. 4b. It can be seen that the SiO2 nano particles show excellent thermal stability and low weight loss of ~2.8% at 800 � C, while the f-SiO2 nanoparticles present similar thermal sta bility and ~10.6% weight loss at 800 � C. This suggests that the grafting ratio of APTS molecules for f-SiO2 is more than 7.8 wt% due to the re sidual mass of APTS molecules after thermal composition. In compari son, the GO paper displays an obvious decomposition at ~220 � C under nitrogen atmosphere because of the pyrolysis of unstable oxygen groups [38]. As expected, the GO/f-SiO2 nanocomposite papers show initial mass loss below 100 � C due to the volatilization of stored water, and a maximum decomposition at ~213 � C, which would be attributed to thermal degradation of Si-OH or -CH3 of APTS molecules. Notably, our previous work indicated that the thermal decomposition of highly cross-linked silicone molecules can form nano-silica structure at high temperature, resulting in enhanced thermal stability of composite ma terials [16]. Benefit from the synergistic effect of grafted APTS mole cules and nano-SiO2, the GO/f-SiO2 nanocomposite papers display higher thermal stability ranging from 250 to 600 � C and a higher residue yield (e.g. 60.4% and 66.4% at 800 � C for GO/f-SiO2-10% and GO/f- SiO2-20%, respectively) than that of GO paper (~55.8% at 800 � C). Based on the above results, it is reasonable to deduce that the intro duction of f-SiO2 nanoparticles produces improved thermal stability of GO paper.
0.6 GPa and 81.2 MPa, respectively [39]. At 10 wt% of f-SiO2, the Young’s modulus and tensile strength values of the nanocomposite pa pers are 2.5 GPa and 122.4 MPa, corresponding to the increases of ~317% and ~51%, respectively. In comparison, the GO/f-SiO2-20% composites show only 22% increase in strength which can be explain by the aggregation of SiO2 in Fig. 2d. At the same loading, the elastic moduli of the nanocomposite papers show similar increases compared with the value of neat GO, i.e. 7.3% (2.4 GPa), 6.9% (2.5 GPa) and 5.5% (2.2 GPa) for GO/f-SiO2-5%, GO/f-SiO2-10% and GO/f-SiO2-15%, respectively. The toughness (area under stress-strain curve) of neat GO paper is 2.8 MJ m-3. The nanocomposite papers with 5 wt% and 10 wt% of f-SiO2 exhibit toughening effects as high as 71% (4.8 MJ m-3) and 69% (4.7 MJ m-3), as illustrated in Fig. 5b and Table 1. With an appropriate f-SiO2 content, the f-SiO2 nanoparticles can act as cross-linking points to interconnect the sheets along GO, which can promote the interactions between the GO sheet that give rise to the enhancement of mechanical property greatly. However, when the content of f-SiO2 is above 10 wt%, the aggregation following, which generates the concentration of stress that leads to the decreased mechanical property including Young’s modulus, tensile strength and toughness. The abovementioned results show significant and simultaneous im provements in stiffness, strength and toughness at low f-SiO2 loading, indicating the effective and tunable interphases obtained between the GO and f-SiO2. The enhanced mechanical properties of GO/f-SiO2 nanocomposite paper should be attributed to the hydrogen bond and covalent interactions between graphene oxide sheets and f-SiO2 and the lubrication of f-SiO2, producing effective load transfer and high energy dissipation during the deformation process [40,41]. The relative in teractions have been proved by the above structural analysis (Section 3.2). At this stage, how the different functional f-SiO2 molecules affect the interfacial characteristics and the surrounding GO network forma tion remains an open question. Nevertheless, the surface functionaliza tion of GO with functional f-SiO2 molecules appears to be a promising approach for obtaining high-performance engineering GO/f-SiO2 nanocomposite papers with improved mechanical performance.
3.3. Mechanical properties of GO/f-SiO2 nanocomposite paper
3.4. Flame retardant property and mechanism analysis
Representative tensile stress-strain curves of pure GO and GO/f-SiO2 nanocomposite papers are shown in Fig. 5a, and the detailed mechanical properties (i.e. Young’s modulus, tensile strength, elongation at break and toughness) are summarized in Table 1. Upon comparison with GO/fSiO2-10% and GO, it can be seen that the former produces an improved ductility, strength and toughness, along with a similar increase in stiff ness. For the neat GO, the Young’s modulus, and tensile strength are
To evaluate the effect of f-SiO2 on the flame resistance of GO-based nanocomposite paper, the nanocomposite papers were directly encountered the flame of the alcohol lamp (a flame temperature of 400–600 � C). Due to the abundant of unstable oxygen groups, e.g. the hydroxyl, carbonyl and carboxylic groups, the pure GO paper exhibits a rapid and vigorous combustion once being ignited (Fig. 6a). The GO paper can be completely burned in only about 16 s and generates gases
Fig. 4. (a) XRD spectra of GO and GO/f-SiO2 nanocomposite papers with different f-SiO2 contents; (b) TGA curves of GO, SiO2, f-SiO2 and GO/f-SiO2 nano composite papers. 5
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Fig. 5. Mechanical properties of GO/f-SiO2 nanocomposite papers. (a) Typical stress-strain curves and (b) tensile strength, Young’ modulus and toughness as a function of f-SiO2 content.
paper can keep the structural integrity after the burning test when compared to the complete thermal degradation of pure GO paper [42]. It should be noted that the square shape of GO/f-SiO2-10% paper still presents slight damage at the edges, as indicated by the red arcs in Fig. 7a. During the burning test, the rectangle edge of the modified paper is completely exposed by the flame, and this results in the rapid thermal decomposition in the edge zone, which is supported by the obvious red hot phenomenon observed in Fig. 6b [15]. Consequently, the formation of smooth arc angle is visible (see ii in Fig. 7a) for the GO/f-SiO2-10% paper after the first burning test. And such arc structure can be kept even exposing it for the second burning test, suggesting the highly improved flame resistance after introducing the f-SiO2. The related structural feature was examined by SEM, which will be discussed in the following section. The structural feature of the GO/f-SiO2 nanocomposite paper after the burning test was determined by SEM analysis and shown in Fig. 7b and c. Typically, compared with the relatively smooth surface of the modified before the burning test (see iii in Fig. 2c), much more nanosilica nanoparticles can be clearly observed to be well attached on the sheet surface (Fig. 7b). This suggests that the formation of nano-silica protective char as an effective barrier layer prevent oxygen from attacking the inside GO sheets but promote the graphitization of the GO sheets via thermal reduction, which thus improve the flame resistance of the GO paper. In fact, our previous work demonstrates that the porous nano-silica/rGO structures with several micrometer was also generated from silicone resin/GO coating during the burning testing [15], which can prevent the heat transportation from outside zone to the inside zone and thus achieve the excellent flame-retardant effect. The thick and porous nano-silica/rGO structure is different from the thin and compact protective layer of the GO/f-SiO2 after the burning test observed in this work. Fig. 7c discloses that a compact nano-silica nanoparticle layer is
Table 1 Tensile properties of GO/f-SiO2 paper as a function of f-SiO2 content. Sample codes
Tensile Strength (MPa)
Elongation at break (%)
Young’s modulus (GPa)
Toughnessa (MJ⋅m 3)
GO GO/fSiO2-5 GO/fSiO210 GO/fSiO215 GO/fSiO220
81.2 � 8.1 108.7 � 6.2
6.7 � 0.6 7.3 � 0.5
0.6 � 0.1 2.4 � 0.2
2.8 � 0.5 4.8 � 0.6
122.4 � 6.0
6.9 � 0.7
2.5 � 0.3
4.7 � 0.7
105.8 � 4.2
5.5 � 0.3
2.2 � 0.2
3.3 � 0.4
99.2 � 7.3
5.1 � 0.7
2.1 � 0.1
2.6 � 0.3
a
Calculated by integrating stress-strain curves.
including CO, CO2, and steam. Comparatively, the presence of f-SiO2 improves the flame resistance of the GO paper significantly. As indicated in Fig. 6b, it can be seen that the modified papers can keep its original shape and size after exposing it in the flame for about 10 s, showing significant improvement in flame resistance compared with the corre sponding combustion behavior of pure GO paper. After exposing the paper in the flame for further combustion, the GO/f-SiO2-10% can almost remain the size and structure, demonstrating the outstanding thermal stability, which is well consistent with the improved thermal stability shown in Fig. 4b. Fig. 7a shows the digital images of pure GO and GO/f-SiO2-10% nanocomposite paper before/after the combustion process. For the same paper size, it is clearly seen that the GO/f-SiO2-10% nanocomposite
Fig. 6. Flame burning tests of (a) pure GO and (b) GO/f-SiO2-10% nanocomposite papers, showing improved flame resistance after introducing f-SiO2. 6
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Fig. 7. Flame-retardant mechanism analysis of GO/f-SiO2 nanocomposite papers. (a) Digital images of GO and GO/f-SiO2 paper before and after burning tests for 10s, and their SEM images of (b) surface-section and (c) cross-section of GO/f-SiO2 papers after the burning test, showing a formation of compact and solid silica protective layer on GO sheets; (d) XPS survey results and (e) the related C1s spectra of GO/f-SiO2 -10% paper before/after the burning test.
eV). The C1s core level spectrum of the GO/f-SiO2 paper before the burning test shows eight typical chemically shifted components: C-C (284.6 eV), C¼C (284.73eV), C-OH (285.0 eV), C-N (285.4 eV), C-Si (283.9 eV), C-O-C (286.8eV), C¼O (287.3 eV), and C(¼O)O (289.4 eV) [44], implying a chemical reaction between the oxygen functional group
well formed on the cross-section surface, which should inhibit the thermal degradation of GO sheets greatly. XPS survey results can offer more evidences [43]. As shown in Fig. 7d, the GO/f-SiO2 paper before/after the burning test are mainly composed of four typical peaks, i.e. C1s (ca. 283 eV), O1s (ca. 529eV), Si2p (ca. 103 eV), Si2s (ca. 153
Fig. 8. Schematic illustration of proposed flame-retardant mechanism and thermal reduction of GO/f-SiO2 paper during the flame attack process. (a) The decomposition course of f-SiO2 (b) the flame treating procedure of GO/f-SiO2 paper. At a high temperature, thermal composition of the modified APTS molecules combined with inorganic nano-silica can produce an effective protective layer, which inhibit the thermal degradation of GO sheets effectively and promote the graphitization to form multi-scale rGO/silica protective char. 7
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of GO and amine group of f-SiO2 molecules. During the burning test, these oxygen groups will thermally degrade and thus induce the sig nificant decrease in the intensity of the C-O-C and C(¼O)O peak. It should be noted that the peaks of oxidized components including C-OH and C¼O are not obvious after the second burning test (see Fig. 7f), indicating that thermal reduction of GO to RGO in the flame and char formation [15]. Based on the abovementioned structural analysis, the introduced functionalized nano-silica particles among the sheets not only promote the stress transferring efficiency but greatly improve the flame resis tance of GO based papers, thus producing mechanically flexible and flame-retardant GO based nanocomposite papers. The proposed flameretardant mechanism is present in Fig. 8. In this GO/f-SiO2 system, the amine groups of APTS molecules on inorganic silica nanoparticles can produce chemical bonding with the hydroxyl and carboxyl groups of GO sheets. During the combustion process, the f-SiO2 particles with high inorganic content make a significant contribution to the improved flame-retardant effect of the modified GO papers. During the combus tion process, thermal depolymerizaiton of cross-linked APTS molecules would generate silicone oligomers and then cross-link or oxide into silicone “soot” to redeposit onto the sheet surface [45]. Considering the much higher inorganic composition in the f-SiO2, the degradation of APTS molecules redeposit and promote the formation of compact nano-silica structure, releasing small amount of gases such as H2O and NO/NO2 due to the thermal degradation of amine group and C-C chains (Fig. 8a). Meanwhile, the oxygen groups of GO sheets or their chemical bonds with the APTS molecules will break and react with the silicone “soot” to form a silica coated sheet structure, as shown in Fig. 7b and c, thus promoting the thermal reduction of GO sheets during the burning test. This would produce the formation of outside compact nano-silica coated RGO layer, which can act as an effective barrier to restrict the thermal composition of inside GO sheets (Fig. 8b), thus leading to improved flame resistance in the GO/f-SiO2 nanocomposite papers.
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4. Conclusions In summary, this work reports a facile and effective strategy to prepare mechanically flexible and flame-retardant GO-based nano composite papers by a combination use of APTS functionalized SiO2 and a filtrating and hot-pressing process. The APTS functionalized SiO2 nanoparticles in the GO-based nanocomposite papers not only acts as effective mechanical reinforcement but also provides efficient barrier role for flame attack. The optimized GO/f-SiO2 nanocomposite papers with 10 wt% f-SiO2 showed improved mechanical performances compared with the pure GO paper, e.g. tensile strength of 122.4 � 6.0 MPa (~51% increase), Young’s modulus of 2.5 � 0.3 GPa (~276% increase) and toughness of 4.7 � 0.7 MJ m-3 (~68% increase), respectively. The formation of strong interfacial interactions between GO sheets and f-SiO2 mainly contributes to promote the stress trans ferring and thus consume energy dissipation during the deformation/ failure process. Furthermore, the presence of f-SiO2 also endows GO papers with improved thermal stability and flame-retardant property, which is attributed to the formation of a compact SiO2 protective layer on the GO sheets when exposed to the flame. Our results suggest that the interfacial interactions between the GO sheets can be tuned by intro ducing surface functionalized silica nanoparticles, thereby providing a promising route for designing and fabricating mechanically strong and flame-retardant GO-based nanocomposite papers for many potential applications. Acknowledgements We acknowledge the funding support from the Natural Science Foundation of China (51973047), the Natural Science Foundation of Zhejiang Province (LY18E030005 and LY15E030015), the Project for the Science and Technology Program of Hangzhou (20191203B16 and 8
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