Engineering of graphene/epoxy nanocomposites with ...

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Cite this: DOI: 10.1039/c6tc00607h

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Engineering of graphene/epoxy nanocomposites with improved distribution of graphene nanosheets for advanced piezo-resistive mechanical sensing† Tran Thanh Tung,a Ramesh Karunagaran,a Diana N. H. Tran,a Boshi Gao,a Suvam Nag-Chowdhury,b Isabelle Pillin,b Mickael Castro,b Jean-Francois Fellerb and Dusan Losic*a Conductive nanostructured composites combining an epoxy polymer and graphene have been explored for applications such as electrostatic-dissipative, anti-corrosive, and electromagnetic interference (EMI) shielding, stealth composite coating and specifically for sensors. For many of these applications, the limits of dispersion of graphene nanosheets and the interface between fillers and matrices have affected their electrical, structural and mechanical properties. To address these problems, we present the use of a dimethylbenzamide (DMBA)-based hardener to modify the surface of reduced graphene oxide (RGO) and create a 3D architecture with a micro-porous structure. DMBA is applied to provide two functions: one is to act as a stabilizer to avoid restacking of graphene sheets during the reduction process, and the second is to provide a linkage between RGO and epoxy for the formation of homogeneous nanocomposites. Thin films of conductive polymer graphene composites (CPCs) were prepared using a simple doctor blade method, while piezoresistive sensors were prepared by spraying to demonstrate their application for mechanical strain sensing. The electrical properties of the composites as a function of graphene fillers were shown to significantly increase from 1012 O sq1 for neat epoxy to 106 O sq1 for 2 wt% RGO in epoxy composites, while the modulus calculated using nanoindentation exhibited a 43.3% enhancement from 3.56 GPa for epoxy to 6.28 GPa for the composites containing 2 wt% graphene. The results of piezo-resistive performance for mechanical strain sensing under both static and dynamic strain modes showed good sensitivity with a gauge factor (GF) of 12.8 and a fast response time of 20 milliseconds. A minor loading/unloading hysteresis loop after 1000 cycles indicated good reversibility and reproducibility of the sensors. Excellent reproducibility, long-term stability and reliability of the sensing

Received 10th February 2016, Accepted 13th March 2016 DOI: 10.1039/c6tc00607h

devices are confirmed working without decay of sensitivity after a 6-month exposure to ambient atmosphere. The results obtained suggest that these types of piezo-resistive sensors based on RGO/epoxy CPCs due to their simple, scalable and low cost production could lead to the development of highperformance mechanical strain sensors for a broad range of applications including real-time monitoring,

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wearable electronics, and structural health monitoring (SHM).

Introduction Graphene, a two dimensional (2D) one-atom thick carbon nanosheet, has attracted enormous research interest in the past ten years due to its extraordinary chemical, structural, electronic, mechanical, and thermal properties relevant1–3 for unlimited

a

School of Chemical engineering, the University of Adelaide, 5005 North Terrace, Adelaide, South Australia, Australia. E-mail: [email protected] b Smart Plastics Group, European University of Brittany (UEB), LIMATB-UBS, rue de Saint-Maude´, 56321 Lorient, France † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6tc00607h

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applications for sensors, flexible electronics, and construction materials.4–8 In particular, composite materials made of 2D materials including graphene and transition metal dichalcogenides with polymers represent an emerging class of nanoscale multifunctional materials having a combination of unique properties of individual components enhanced with a synergistic effect.9–14 Among them, the combination of graphene with a thermosetting polymer (e.g. epoxy resin) for the formation of conductive polymer composites (CPCs) has been widely explored offering considerable applications in many technological fields such as electrostatic-dissipative,15 anti-corrosive,16 and electromagnetic interference (EMI) shielding,17 stealth

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composite coating18 and sensors.19,20 However, the technological development of CPC-based graphene on a large scale has been hampered because of the challenges in the synthesis and processing of bulk-quality graphene sheets.15,21 One of the limitations in designing and making these graphene polymer composites is the fact that graphene sheets tend to form irreversible agglomerates or restack into graphite-like structures through van der Waals interaction, resulting in poor dispersion, non- uniform distribution and reduced interaction between polymer matrices and dispersed graphene.12,22 To address these issues, the electrostatic attachment of organic molecules mainly to aromatic rings and surfactant properties on the surface of graphene were explored to prevent graphene sheets from aggregating.23–26 However, the presence of these foreign stabilizers is undesirable for most applications, especially in nanocomposites where excellent interface adhesion for efficient stress transfer from graphene into polymer matrices is of importance.15,27,28 Considerable effort has been made in recent years toward the synthesis of advanced graphene/epoxy CPCs either by improving synthetic protocols or by applying different composite formation techniques.29–32 However, to produce relatively clean graphene sheets in bulk quantity while keeping them individually dispersed into polymer matrices still remains unsolved. To address these limitations here we present a new concept for the preparation of advanced composite formulations of uniform graphene nanosheet dispersions in an epoxy matrix designed for piezo-resistive sensing applications. The functionalization of reduced graphene (RGO) nanosheets with 2,6-dimethylbenzamide (DMBA) is introduced specifically for two reasons: one is to act as a surfactant to prevent them from aggregating during the reduction stage and the second is to provide crosslinking between RGO and the polymer matrix. The proposed reinforcement through excellent dispersion of graphene sheets and strong graphene cross-linking with epoxy

Journal of Materials Chemistry C

resin will provide significant improvements in their electromechanical performance, which was evaluated in this work. The concept of proposed synthetic protocols and the design of piezo-resistive electromechanical sensing devices are presented in Fig. 1. The objective of this work is to demonstrate this synthetic concept and show its applicability to design very simple, scalable and highly sensitive electromechanical piezo-resistive sensors. Comprehensive structural and chemical characterizations using SEM, TEM, XPS, AFM, UV-vis, FTIR and Raman spectroscopy were performed during all preparation steps. In order to optimize the performance, several formulations with different ratios of RGO were explored. The piezo-resistive sensing performances of the prepared sensing devices were determined using a uniaxial dynamic pressure test system consisting of a universal testing machine and a voltage–current meter. These results indicate that these types of piezo-resistive sensors have considerable potential for the development of low-cost sensors and systems for structural health monitoring (SHM).

Experimental section Materials and Chemicals Graphite flakes (497%) were supplied by Valence Industries, South Australia. Potassium permanganate (KMnO4) was purchased from Sigma-Aldrich, Australia. Sulphuric acid (H2SO4, 98%), phosphoric acid (H3PO4, 85% w/w), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, 35%) were purchased from Chem-Supply, Australia. All chemicals were used directly without further processing. The epoxy resin and hardener were purchased from West System, Australia. Preparation of the RGO/dimethylbenzamide (DMBA) aerogel Graphene oxide was prepared by a modified Hummer’s method using graphite as detailed in our previous publications.33,34 To prepare the DMBA/RGO aerogel, 1000 mg of GO was dispersed in DI water (conc. 2 mg mL1) and kept in an ultrasonic bath for 30 min. Then, DMBA-modified GO was prepared at a DMBA/GO mass ratio of 3/1 by adding DMBA in GO suspension and stirred at room temperature for 30 min. The temperature was increased to 95 1C over 6 h for the covalent reaction of DMBA with the GO surface; at the same time it also partly reduced GO. Subsequently, 500 mL of hydrazine was slowly added to the mixture and the reduction was continued for an additional 6 h. After cooling, the solid RGO/DMBA product was isolated by vacuum filtration using a membrane (JTTP, 0.2 mm pore size, Millipore). The expanded RGO/DMBA product was kept in a wet state and subsequently frozen by placing on the shelf of a heat exchanger pre-cooled at 20 1C. The sample was left under a pressure of 10 Pa for 2 days to obtain a light weight, porous structure aerogel. The porous structure of the aerogel was proposed to prevent RGO from stacking during its drying process. Preparation of modified RGO/epoxy nanocomposite films

Fig. 1 Surface modification of graphene oxide with a DMBA-based hardener, its chemical reduction process and blending with epoxy resin; the as-prepared nanocomposite based-sensor device and the electromechanical sensing response towards strain stimuli.

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The as-prepared RGO/DMBA was added to epoxy resin at different concentrations ranging from 0.3 to 2 wt%. The complex was well-mixed in a mortar for 30 min, and then an additional


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DMBA hardener was added (epoxy/DMBA = 3/1) with further grinding for 15 min. The composite paste was then laminated onto Teflon films using a doctor blade method and curing at 60 1C for 12 h to make CPC films, which are then cut into small pieces for determining mechanical properties, as well as electrical conductivity. Published on 14 March 2016. Downloaded by UNIVERSITY OF ADELAIDE on 12/04/2016 06:12:39.

Fabrication of nanocomposite-based skin sensor devices In order to fabricate micro-thickness sensor devices, the prepared nanocomposites paste was suspended in acetone using ultrasonication for 30 min and sprayed layer-by-layer on the epoxy laminated substrate with the position and size of sensor arrays designed by a mask. The spraying conditions and the number of sprayed layers were well-controlled in order to achieve a surface resistance of a few MO per square. The sensor arrays were further annealed at 160 1C for 3 h under vacuum. In this case, RGO was further thermal reduced so that the resistances of micro-thickness sensor arrays decreased and were in the range of 0.8–1 MO per square. The skin sensors were connected with electrical wires with the assistance of silver pain for electromechanical tests. Characterization Scanning electron microscopy (SEM) images were recorded using a Quanta 450 instrument operated at an accelerating voltage of 10 kV and an emission current of 10 mA. Transmission electron microscopy (TEM) images were recorded on a TECNAI 20 microscope operated at 120 kV. The AFM images of nanocomposites were obtained on a Veeco multimode scanning probe microscope equipped with a Nanoscope IIIa controller. The IR spectra were measured using a FT-IR spectrometer (Nicolet 6700 FTIR). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCA2000 (VG Microtech) system using a monochromatized aluminum Ka anode (hn = 1486.6 eV), with 10 min of acquisition. The XPS instrument is calibrated to give an Au 4f7/2 metallic gold binding energy of 83.95 eV and the spectrometer dispersion is adjusted to give a binding energy of 932.63 eV for metallic Cu 2p3/2. Peak deconvolution of the obtained spectra has been processed using the ‘‘Avantage’’ program from Thermoelectron Company. XPS spectra have been calibrated to the main line of the carbon 1s spectrum (adventitious carbon) set to 284.8 eV avoiding any shift during measurements.14 Shirley backgrounds were subtracted from the raw data to obtain the areas of the C1s peak. Thermal gravimetric analysis (TGA) was carried out on a TGA/DSC2 Star system (Mettler Toledo) at a heating rate of 10 1C min1 under air. The surface resistivity was obtained using a CMT series JANDEL four-point probe at room temperature in which a current of 10 mA was applied. The Raman mapping spectra were recorded using a DXRtxi Raman imaging microscope. Spectra were recorded over the range 500–4000 cm1 and the excitation wavelength was 532 nm.

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and stress was transmitted to the multimeter, while the resistance changes were monitored simultaneously under external stimuli. The piezo-resistive properties were measured using a uniaxial dynamic pressure test system consisting of a universal testing machine (INSTRON 5543) and a Keithley 2400 voltage– current meter. The applied strain on the specimens was provided by a testing machine forced on the epoxy beam yielding relative differential resistance responses. The resistance relative amplitude or relative differential resistance responses (AR) of sensors were recorded with a multimeter using data acquisition software Lab view and defined by Ar ¼

DR R R0 ¼ R0 R0

(1)

where Ar is the resistance relative amplitude, R is the resistance and R0 is the initial resistance of the sensor. This parameter is used in the following sections to describe the piezo-resistive mechanism in CPC strain sensors and investigate the parameters having an influence on the sensing response of CPC-based strain sensors such as the network architecture, stress, deformation, and oscillation frequency.

Results and Discussion Characterization of the morphology and chemical composition of the DMBA-modified RGO aerogel The porous structure of the DMBA–RGO aerogel prepared using a freeze-drying technique was studied by SEM, as depicted in Fig. 2(a). The images show that macroporous irregular structures with an average size of hundreds of nanometers were formed inside a 3D complex. It has been known that freeze-drying a suspension results in porous materials, where the pore structures are replicas of the ice crystals formed during freeze-drying.35 By enlarging the matrix of the pore wall (Fig. 2b), it was found that RGO sheets were covered by a thin layer of polymer that helps

Electromechanical sensing properties of the nanocomposites Thin composite film-based skin sensors were subjected to study of their electrochemical behaviour by using a universal testing machine. Deformation was obtained using an extensometer,


Fig. 2 (a and b) SEM images of the DMBA–RGO aerogel before grinding and (c and d) after grinding using a mortar and pestle.

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Fig. 3 TEM images of DMBA-modified RGO showing RGO sheets covered by a thin layer of DMBA. A N peak appeared in the EDX spectra corresponding to the TEM images (data not shown).

RGO to disperse in the initial suspension. Fig. 2(c and d) show the SEM images of DMBA–RGO after grinding using a mortar and pestle; The pores are deformed or rearranged; however their surfaces are exposed with wrinkles and folded regions of the interconnected 3D architecture. It would therefore be reasonable to expect that DMBA grafted on graphene sheets will play the role of a cross-linker between graphene and epoxy forming coherent composites in the following step. The TEM images (Fig. 3) reveal that RGO sheets were effectively exfoliated to form separated sheets in which DMBA molecules bound the graphene sheets, making them well-dispersed in water. The images also show that DMBA– RGO sheets are folded and stacked with each other. Herein, the driving forces for the interaction of DMBA and RGO include hydrogen bonding between functional groups,36,37 p–p stacking of the aromatic ring on the basal plane of RGO,32,38 and covalent bonding of the amide group with the hydroxyl group on GO. In contrast, the images of dried powders of both RGO only and DMBAmodified RGO without freeze-drying show the aggregated form of the non-porous structure (Fig. S1, ESI†). Their dispersions due to extensive aggregation were not found to be effective when used as conductive fillers for preparing the composite with epoxy resin. The comparative Fourier transform infrared (FT-IR) spectra of RGO and DMBA-modified RGO presented in Fig. 4a give the characteristic absorption of functional groups and organic modified molecules on the graphene sheets. The FT-IR spectra

Fig. 4 Comparative characterization of RGO and DMBA-modified RGO using (a) FT-IR spectra, (b) UV-vis spectra, and (c) DLS measurements; a digital photo and optical micrographs (d–f) showing the DMBA-assisted dispersion level of RGO.

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show carbonyl CQO, aromatic C–C, C–N, epoxy/ether C–O, and alkoxy/alkoxide C–O stretches. Their appearance in DMBAmodified RGO compared with stacked RGO indicates the presence of DMBA in the product. The UV-vis spectra of colloidal dispersions of GO, RGO and DMBA–RGO in water are summarised in Fig. 4b. The spectra show an absorption peak at 230 nm for GO, while upon reduction it is red-shifted to 262 and 270 nm for RGO and DMBA–RGO, respectively, along with an increase in the background absorption. These results indicate the restoration of the p-conjugated network of graphene sheets, and also the covalent bonding between the stabilizer (DMBA) and the graphene surface.32,36 As DMBA plays an important role in stabilizing graphene sheets during chemical reduction, the measurements of the particles size were performed and are presented in Fig. 4c. The size of RGO exhibited two separated peaks at 144 nm and 777 nm, respectively. The average size was determined as 1894 nm and the zeta potential as 25 6.2 mV. The size of DMBA–RGO shows a single peak at 321.7, an average particle size within 598.2 nm and an opposite zeta potential with a value of 20 4.6 mV. Fig. 4d shows a digital photo of compared dispersion abilities of RGO and DMBA–RGO in aqueous solution; while RGO is aggregated and precipitated after a couple of hours, DMBA–RGO was found to be a dispersed homogeneous colloidal system without visible agglomerates for a week of free-standing. The optical microspore of their corresponding sonicated suspension sandwiched in glass slides shows agglomeration of 10–15 mm for RGO (Fig. 4e), whereas no aggregated appearance is observed for the DMBA–RGO complex (Fig. 4f). This suggested the chemical interaction of DMBA with the surface of RGO sheets prevented the aggregation of platelets during their reduction. The amount of DMBA in the product was estimated to be about 12 wt% using TGA measurement as shown in Fig. S2 (ESI†). Fig. 5a shows the Raman spectra of DMBA-modified RGO with the mapping image inserted. The red colour is the RGO area with clear typical D-band (1348 cm1) and G-band (1580 cm1), whereas the blue colour denotes the DMBA area in which no Raman characteristic bands are found. The spectra also suggested that GO has been reduced which increased the ID/IG from 0.80 for pure GO to 1.08.39 The chemical composition of DMBA–RGO in comparison with RGO was monitored by XPS using both survey spectra and high C1s resolution spectra. The comparative survey spectra are presented in Fig. 5b and Fig. S3 (ESI†). The C1s XPS spectrum is deconvoluted into five main components corresponding to carbon atoms in different functional groups: the non-oxygenated ring C (C–C, CQC), the C in C–O, the carbonyl C (CQO), and carboxylate C (O–CQO). The C ring intensity increased and oxygen functional groups diminished in both RGO (Fig. 5c) and DMBA–RGO (Fig. 5d) as compared to GO (Fig. S3b, ESI†). It can also be observed that CQO and C–O in the DMBA–RGO sample are higher than RGO only. The O1s spectra (Fig. S3c and d, ESI†) also indicated higher C–O and CQO components for the DMBA–RGO sample. In addition, the calculation of the C/O atomic ratio by XPS is increased from 2 for GO to 9.5 and 7.9 for RGO and DMBA– RGO, respectively, which reflected the removal of oxygen-containing


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Fig. 5 (a) Raman imaging spectra of DMBA–RGO showing the spatial distribution of DMBA and RGO in the complex; in the mapping image (inset), red colour indicates RGO and blue colour refers to as DMBA. (b) Comparative XPS survey spectra of RGO and DMBA-modified RGO showing elemental analysis in which B9.09 at% of N is associated with DMBA; C1s XPS spectra of (c) RGO and (d) DMBA-modified RGO.

groups from GO by chemical reduction,40 and DMBA organic molecules had been modified on the surface of RGO. Characterization of graphene/epoxy nanocomposites Fig. 6a and b present the AFM topography images of RGO– DMBA/epoxy nanocomposites using a tapping mode in air. The samples were prepared by spin coating of diluted suspensions in acetone on the silicon substrate. AFM imaging data suggest that an ultrathin layer of epoxy resin is covered on the surface of RGO sheets, and the corresponding line-scan (red) indicates an approximately 6.2–6.8 nm-thick shell model on both sides of RGO sheets. However, some RGO sheets that were not wellcoated by epoxy with a corresponding thickness of B2.5–3 nm (blue line-scan) were observed, suggesting the DMBA–RGO sheets and epoxy have been spat out of their surface at high speed (e.g. 2000 rpm) of spin-coating. This was avoided using lower speed coating, but graphene sheets in that case were embedded inside the composites and were not observed by AFM imaging the top surface (Fig. S4, ESI†). Here, DMBA acts as linkers between the RGO sheets and epoxy, where RGO and epoxy are strongly bonded and stacked. The inner microstructure of the prepared RGO–DMBA/epoxy composites was studied by cross-sectional top view SEM imaging as presented in Fig. 6c, and the epoxy-coated graphene can be seen from the side view (Fig. 6d). The images suggest dispersion of RGO sheets without aggregation confirming that RGO sheets are well embedded in epoxy resin, whereas controlled samples of RGO/epoxy composites (without modification) show inhomogeneous distribution of stacked RGO in the matrix, with the presence of separated clusters (Fig. S5, ESI†). These results suggested that by using DMBA as a surface functionalization the sheet/matrix interface is effectively improved which is critical for the sensing performance of the RGO/epoxy composites.


Fig. 6 (a and b) AFM image of the RGO–DMBA/epoxy nanocomposite suspension deposited on the surface of SiO2 in tapping mode in air showing the morphology of single graphene sheets covered by epoxy resin on both sides; (c) the top view SEM image of a cross-sectional composite, and (d) the cross-sectional side view.

The conductivity of the prepared graphene/epoxy composites with different conductive fillers sprayed on glass slides, room temperature curing, and curing at 160 1C under vacuum for 3 h was measured and their surface resistance is presented in Fig. 7a. The initial sheet resistance decreased while the concentration of RGO–DMBA increased from 0.3 to 2 wt%. In addition, the temperature curing gives lower resistance due to complete

Fig. 7 (a) Sheet resistance of composites sprayed on glass slides for just curing (black) and further thermal treatment at 160 1C (red). (b) Load– displacement curves, (c) elastic modulus and hardness of neat epoxy and composites containing 1 and 2 wt% DMBA modified RGO in epoxy.

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removal of the solvent trapped in the composites and/or further reduction of RGO by removing more oxygen content and shrinkage of RGO sheets.41 The mechanical properties and deformation behaviour of the prepared composites were characterized by nanoindentation, where Young’s modulus (E) and hardness (H) are defined as elastic behaviour and resistance against the deformation of materials, respectively. For enhancing the sensing performance, the dispersion of graphene in the polymer matrix and strong bonding between RGO and epoxy are required since their poor bonding limits load transfer from the matrix to graphene.42–44 Fig. 7b shows typical load-displacement curves and comparison of E and H values of neat epoxy containing 1 wt and 2 wt% of RGO–DMBA in epoxy (Fig. 7c). It can be clearly seen that the modulus and hardness of the composites increased with the increase in the RGO–DMBA content. In fact, the mean modulus increases from 3.56 GPa for neat epoxy to 4.66 and 6.28 GPa corresponding to 23.6 and 43.3% enhancement for the samples containing 1 and 2 wt% RGO–DMBA in epoxy, respectively. Besides, the average hardness value increased 25 and 41.2% in relation to the neat epoxy sample. These results strongly supported the proposed concept that the presence of DMBA-modified RGO in epoxy not only makes composites conductive, but also effectively improves the mechanical properties which is important for their piezomechanical sensing properties. Characterization of the piezo-resistive sensing behaviour The performance of the optimised 2 wt% RGO–DMBA/epoxy composite as a piezo-resistive sensor was explored by testing the reversibility and the damage detection as a function of an increasing mechanical strain. This was monitored by the resistance variation under cyclic loading with the progressively increasing strain peak, as presented in Fig. 8a. The graph confirms the peak of strain in the first four cycles when strain is less than 1%, corresponding to the linear range; the variation of resistance closely follows the strain, which returns to the initial point after unloading demonstrating the good coupling between mechanical and electrical properties. This also indicates that no damage has been initiated either in the sensor or in the composite. However, when the strain exceeds the value of the linear range (1% strain) during the fifth cycle, the value of resistance starts to deviate the curve of strain, suggesting that the elastic domain is crossed and damage of the graphene

Fig. 8 Piezo-resistive behaviour of the 2 wt% RGO–DMBA/epoxy composite, a synchronism of relative resistance and strain versus time under (a) increasing cyclic tensile tests and (b) repeating cycles.

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composite sensor is caused by micro-cracks of the interface.45–48 The results are explained by the correlation of sensitivity versus stress as plotted and shown in the inset in Fig. 8a, which shows the linear behaviour in the range 0.5–5 MPa of the tested stress values. However, at higher stress values, the sensitivity went out of linear fitting. Fig. 8b shows tensile loading (stress)–unloading (relax) cycles of the sensor as a function of time. In this test, the material was strained up to 1% deformation, which is the elastic domain boundary of the epoxy substrate, and the specimen was then unloaded back to the initial state (strain = 0). Four cycles in strain were set on the tensile test machine to investigate the sensitivity and reproducibility of the sensor. The graph clearly shows that the normalized resistance (DR/R0, %) characteristics of the composite sensor repeated several times, accurately follows stress and strain given the linear relationship. A comparative test of RGO–DMBA/epoxy composite vs. pristine RGO/epoxy composite had been performed and is presented in Fig. S6 (ESI†). It was observed that in contrast with excellent response towards external stimuli of RGO–DMBA/epoxy, the RGO/epoxy composite-based sensor was unstable, exhibited high noise, and a drifted signal between successive cycles. This again reflects that our modification of RGO with DMBA which is a critical point for enhancing the interface resulted in a very stable sensor for sensing strain in repeated measurements. These results, which are similar to those of CNT composites,49–51 silver nanowires,52 nano-platelets,53 suggest that the strain sensing mechanism can be explained by a network resistor calculated by using Kirchhoff’s current law and Ohm’s law. Fig. 9 presents a model of the graphene sheet network in the epoxy matrix, in which the junction between graphene sheets can be classified into three main categories: complete connection, tunnelling junctions and disconnection. Based on the model, the total resistance of the composite-based film sensor can be determined as the following equation: Rtotal = Rconn + Rturn + Rdisconn

(2)

where Rtotal is the network resistance of the composite film. Rconn is the resistance of connected/overlapped sheets, position A (connected): Rpair = R1 + R2. Rtunnel is the tunneling resistance between neighbouring graphene sheets, position B (tunneling): Rpair = R1 + R2 + Rtunn. Rdisconn is the disconnected resistance (no current can pass through two adjacent sheets), position C (disconnected): Rpair = N. When external stimuli were applied to the composite-based film sensor, the position and orientation of graphene sheets

Fig. 9 A model of strain-induced resistance change of a graphene/epoxy composite thin film presenting the piezo-resistive sensing mechanism. Position A: connected/overlapped pair; position B: tunnelling pair; position C: disconnected pair.


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changed. Physically, under low strain (o0.5%), the number of completed contact networks may be slightly decreased. However, the number of tunnelling junctions increased and significantly affected by applied strain. Thus, Rconn { Rtunn, and the tunneling effects among graphene sheets (electrical charges can transfer between sheets when they are close to each other, i.e., within 1.0 nm) are dominant in the increased total resistance.54–56 The tunneling resistance between two neighbouring graphene sheets is related to the shortest distance d of two graphene sheets defined as: V h2 d 4pd pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi exp Rtunnel ¼ ¼ 2ml (3) AJ Ae2 2ml h where J is the tunnelling current density; V is the electrical potential difference; A is the cross sectional area of tunnel; e is the quantum of electricity; m is the mass of electron; h is Plank’s constant; d is the distance between conductive particles; l is the height of energy barrier (for epoxy, 0.5–2.5 eV). As continuously increased external stimuli induced higher strain (e.g., e Z 1%), the loss of contact among graphene sheets or breakup of conductive paths of graphene will be increased.57 Thus Rconn { Rtunn { Rdisconn, and disconnected resistance is significant and also plays a role in the sensing mechanism. In this case, the relative different resistance change (sensitivity) can be higher. However, a drift, irreversibility and irreproducibility of the sensor should be taken into account in the sensing performance. The sensor response under dynamic conditions of loading with a frequency, typically, of 1 Hz has been investigated as shown in Fig. 10. The sensor showed a good response towards the applied load, with a high degree of repeatable sinusoidal electrical signals obtained following the load (Fig. 10a). In addition, the sensor worked well under an oscillation of 1 Hz without any time lag or breakage (Fig. 10b). The response time was determined by plotting the resistance change vs. real time measurements under a sudden loading; as indicated in Fig. 10c, an immediate jump of the relative different resistance within 20 milliseconds (ms) was observed. The stabilization of sensors was also studied by exposing them to ambient conditions for 6 months to test the effect of oxygen, humidity, light and aging after massive cycles (1000 cycles per each test), which affected the sensor performance and lifetime. A re-testing was performed each month under the same conditions. The decay of sensitivity vs. aging time during a period of 6 months is shown in Fig. 10d; as it can be seen, the sensors are working excellently without any change in sensitivity. These data clearly demonstrated a fast response, long-term stability and reliability of the sensors. Fig. 10e shows the evolution of DR/R0 (%) under loading–unloading cycles when the composite material was strained up to 0.068% deformation. A minor loading/unloading hysteresis loop after 1000 cycles again confirmed good reversibility and reproducibility of the sensor responses. From this curve, the gauge factor (GF) can be calculated as the ratio of normalized resistance and deformation (e), GF = (DR/R0)/e. A GF of 12.8 was determined under an applied stress of 0.3 MPa.


Fig. 10 (a) Dynamic sensing response of a graphene/epoxy-based sensor at a frequency of 1 Hz, (b) extracted evolution of normalized resistance and stress vs. time, (c) a highly distinct response toward an external stimulus by the resistance change pulse within 20 milliseconds, (d) stabilization of the sensor after 6-month aging showing no decay, (e) normalized resistance– deformation relationship in the linear elastic domain, and (f) benchmark sensitivity of different carbon nanocomposites.

However the GF will be higher when the composite sensor layer is thinner and the applied stress is increased up to 0.6 MPa (can be reached 24 typically). This GF sensing factor is several times higher than commercial metallic gauges.58 In comparison with the other graphene/polymer composite sensors, the gauge factor of the present sensors (Fig. 10f) is higher than those of carbon nanotube/epoxy composites (GF = 5–8 or 11),59,60 graphene ribbons (GF = 1.9),61 CVD graphene on PDMS (GF = 6.1),62 graphene/TPU (GF = 5.2),63 graphene-nanocellulose/PDMS (GF = 7.1),64 and graphene-CNT/epoxy composites (GF B 10).65 However, it is lower than graphene films (GF B 37)66 and graphene platelet/epoxy composite (GF B 56.7),67 and graphene woven fabric/PDMS (GF B 1000).68

Conclusion The fabrication of graphene/epoxy composites with advanced mechanical and electrical properties for piezo-resistive sensing is demonstrated. A hardener (DMBA) which acts as a surfactant against the aggregation of RGO during the chemical reduction process and a cross-linker to interconnect graphene and epoxy is used for providing a better dispersion and a higher interface between RGO and epoxy which are critical points for the fabrication of 3D graphene polymer composites. The homogeneous composites of graphene filled epoxy exhibited simultaneously remarkable improvement in mechanical and electrical properties as compared to neat epoxy. The prepared sensors responded

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well to both static and dynamic modes, in which the sensitivity with a GF of 12.8 is comparable with or better than recent CPCbased sensors. The sensing signals were highly repeatable and reproducible (thousands of cycles) with a fast response time of 20 milliseconds and an excellent stability without degradation of sensitivity after 6-month aging. However, we expect that the sensitivity can be improved by optimising the thickness of sensors, embedded in the substrate as well as by optimizing the applied loads. We believe that the present graphene/epoxy composite-based sensors have considerable potential to develop artificial biometrics that can monitor the safety conditions of SHM.

Acknowledgements The authors thank the support of the Australian Research Council (FT110100711) and The University of Adelaide, School of Chemical Engineering, for this work. The authors acknowledge the support of this graphene research by Valence Industries Ltd. We thank Adam Wanan from Skiffs Australia for providing facilities of composite making. Shervin Karibi is acknowledged for supporting this work. Dr Animesh at Adelaide Microscopy Centre, School of Medicine, is gratefully acknowledged for TEM training and nanoindentation tests.

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