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Fire-resistant and highly electrically conductive silk fabrics fabricated with reduced graphene oxide via dry-coating
Accepted Manuscript Fire-resistant and highly electrically conductive silk fabrics fabricated with reduced graphene oxide via dry-coating
Yimin Ji, Yuzhou Li, Guoqiang Chen, Tieling Xing PII: DOI: Reference:
S0264-1275(17)30752-9 doi: 10.1016/j.matdes.2017.08.006 JMADE 3265
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
17 May 2017 2 August 2017 3 August 2017
Please cite this article as: Yimin Ji, Yuzhou Li, Guoqiang Chen, Tieling Xing , Fireresistant and highly electrically conductive silk fabrics fabricated with reduced graphene oxide via dry-coating, Materials & Design (2017), doi: 10.1016/j.matdes.2017.08.006
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ACCEPTED MANUSCRIPT Fire-resistant and highly electrically conductive silk fabrics fabricated with reduced graphene oxide via dry-coating
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Yimin Ji, Yuzhou Li, Guoqiang Chen, and Tieling Xing
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National Engineering Laboratory for Modern Silk, College of Textile and Clothing
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Engineering, Soochow University, Suzhou 215123, PR China
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Abstract
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Large-scale and functional silk fabrics were prepared by depositing synthetic graphene oxide (GO) hydrosol onto fabrics via an environmentally friendly
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“dry-coating” method and subsequently reduced in L-ascorbic acid solution. Through
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this modification method, the reduced GO (rGO) sheets deposited uniformly on the silk fabric surface were firmly combined with fibres. Up to 19.5 wt% rGO could be
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deposited relative to the fabric weight. The morphology and structure of prepared
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rGO-coated silk fabric were characterised by scanning electron microscopy, X-ray diffraction, and Raman spectrometry. In comparison with pristine silk fabric, the modified silk fabric exhibited improved fire resistance and smoke suppression properties. The sheet resistance of rGO-coated silk fabric decreased to 0.13 kΩ/sq. Washing test indicated that the rGO-coated silk fabrics prepared had good durability for common use. The functional silk fabric deposited with 19.5 wt% rGO was
Corresponding author. E-mail address: [email protected] (T. Xing).
ACCEPTED MANUSCRIPT designed into a fire-resistant conductor that kept conducting even after 60 s of combustion and can be applied in the fire-fighting field. The silk fabric deposited with 3.9 wt% rGO was successfully assembled into a human motion signal sensor; this easily fabricated, highly sensitive, and flexible sensor has potential for use as
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wearable devices.
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Key words: Silk fabric; Reduced graphene oxide; Fire resistance; Electrical
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conductivity; Conductor; Human motion signal sensor
ACCEPTED MANUSCRIPT 1. Introduction Graphene has recently drawn attention for wide applications based on its unique structure and excellent properties [1-5]. Combining graphene with traditional textiles to develop graphene-based textiles is an emerging research hotspot, and
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graphene-based textiles have proven to be promising in electrical applications such as
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electric conductors [6, 7], strain sensors [8-10], supercapacitors, and energy storage
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[11, 12]. Furthermore, because of excellent properties of textiles such as flexibility, low cost, and easy deformation (similar to the human skin) [13], graphene-based
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textiles are outstanding candidates for fabricating wearable devices. However, textiles
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(depending on their natures) can be easily ignited, quickly release dense smoke, and rapidly induce fires [14, 15]. The inherently flammable and combustible properties of
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textiles could seriously confine their further application. In fact, graphene itself has
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been demonstrated to be an efficient green fire retardant [2]. The 2D-layered structure of graphene can act as a powerful physical barrier, which is favourable for isolating
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oxygen, delaying heat transfer, and preventing escape of pyrolysis products [16, 17].
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Unfortunately, graphene is just regarded as a fabulous fire retarding nano-additive in polymers [18, 19], rather than as a fire retardant that can be directly used to modify fabrics. This is because, as an inorganic carbon material, graphene is not directly absorbed by fabrics. Moreover, the amount of graphene that can be deposited on fabric surface is quite low, which does not effectively endow the fabric with satisfactory fire resistance. Some organo-phosphorous flame retardants have been used to improve the fire resistance of textiles through doping with graphene [6, 20,
ACCEPTED MANUSCRIPT 21]. Dong et al. [6] fabricated a fire-resistant cotton fabric by dipping it in a mixture of graphene oxide (GO) and hexachlorocyclotriphosphazene; after reduction, the modified fabric could hold its shape under combustion for 90 s. Although the flame retardant hexachlorocyclotriphosphazene improved the fire resistance property of
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graphene-based textiles, the electrical conductivity of graphene-based cotton was
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seriously affected. In addition, organo-phosphorous flame retardants are toxic and
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unfriendly to the environment, and are gradually being replaced [22]. Therefore, improving the fire resistance property of graphene-based textiles for fire safety and
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expanding the range of their application while retaining an excellent electrical
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conductivity is still a challenge.
Graphene has both fire resistance and electrical conductivity properties.
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Fabricating fire-resistant and highly electrically conductive graphene-based textiles
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through increasing the amount of graphene deposited on fabric (instead of adding other flame retardants) is a novel approach. GO, a derivative of graphene, has
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functional groups (such as hydroxyl, carboxyl, and epoxide) on the basal planes [23]
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and can be absorbed onto fabric surfaces through the combination of covalent bonds and hydrogen bonds. Therefore, to fabricate graphene-based textiles, depositing GO onto fabrics and subsequently reducing the GO is an efficient approach. However, existing methods cannot deposit a sufficient amount of graphene onto fabric to fabricate fire-resistant graphene-based textiles. Representative modification methods to deposit GO on fabrics involve “dip-coating” [7, 9, 24] and “vacuum filtration” [10, 25, 26]. The “dip-coating” method is limited by the concentration of GO suspension;
ACCEPTED MANUSCRIPT because of the poor solubility of GO sheets in aqueous solution due to a lack of adequate hydrophilic groups in its structure, the method needs fabric to be repeatedly dipped in GO suspension for long times. Even so, the amount of GO deposited on the fabric is still unsatisfactory. Lu et al. [7] fabricated a highly conductive silk fabric
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(sheet resistance: 1.5 kΩ/sq) by dipping silk fabric in GO suspension and reducing for
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7 cycles. Although this method allows the fabric to obtain good electrical conductivity,
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the modified graphene-based silk fabric still did not have fire resistance because of the low amount of graphene deposited on its surface. In the “vacuum filtration”
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method, GO is filtered onto the fabric through vacuum filtration. However, the size
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and shape of the obtained fabric are restricted by the common filter paper, which prevents the approach from being scalable and hence is not suitable for industrial
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production. In addition, this method also could not deposit sufficient graphene on the
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fabric surface for achieving fire resistance. Therefore, developing a new method that can deposit enough graphene on fabric is needed for fabricating graphene-based
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textiles that have both excellent fire resistance and high electrical conductivity.
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In this work, we report an environmentally friendly and time-saving “dry-coating” method to construct functional silk fabrics by coating with synthetic GO hydrosol and subsequent immersion in L-ascorbic acid solution to reduce the GO. This method can deposit highly concentrated GO hydrosol on silk fabric surfaces. Moreover, the amount of reduced GO (rGO) deposited on silk fabric can be easily controlled and calculated. The fire resistance property and electrical conductivity of the modified silk fabrics were measured, and the durabilities of rGO-coated silk fabrics were evaluated
ACCEPTED MANUSCRIPT by washing tests. The feasibilities of modified silk fabric as a fire-resistant conductor and a human motion signal sensor were also investigated.
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2. Experimental
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2.1. Materials
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Silk crepe satin (weight: 60.28 g/m2) was supplied by Suzhou Kasen Silk Garments Co., Ltd. Graphite flakes (~325 mesh) were purchased from Alfa Aesar Co.,
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Ltd. Phosphoric acid (H3PO4, 85%) was obtained from Acros Organics Co., Ltd.
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Potassium permanganate (KMNO4) and L-ascorbic acid were provided by Sinopharm Chemical Reagent Co., Ltd. Sulphuric acid (H2SO4, 95% – 98%), hydrogen peroxide
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(H2O2, 30%), hydrogen chloride (HCL, 36% – 38%), and ethanol (C2H5OH, 99.7%)
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were purchased from Chinasun Specialty Products Co., Ltd. All reagents were used
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without further purification.
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2.2. Preparation of GO hydrosol GO was synthesised from graphite flakes by the improved Hummer’s method [27]. A 9:1 mixture of concentrated H2SO4/H3PO4 (270:30 mL) was added into a 1000 mL flask containing 6 g graphite flakes, then 42 g KMnO4 was added to the mixture within 30 min batch-wise. The reaction system was maintained at 0 °C in an ice water bath for 1 h. Then the reaction was heated to 50 °C and quickly stirred for 8 h. Finally, the reaction was cooled to 0 °C in an ice water bath, and 600 mL of deionised water
ACCEPTED MANUSCRIPT with 12 mL H2O2 (30%) was added to stop the oxidation process. The mixture became golden yellow indicating that highly oxidised GO was obtained. The product was then washed in succession with 200 mL of deionised water and 200 mL of ethanol; for each wash, the product was centrifuged at 8000 rpm for 5 min. GO hydrosol was
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prepared through a simple dialysis process as follows: the GO obtained was added to
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a moderate amount of water and transferred into a dialysis tube (molecular weight
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cutoff: 8000 – 14000 Da) and the GO hydrosol was prepared through dialysis for 3 d
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in deionised water.
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2.3. Fabrication of rGO-coated silk fabrics
The prepared GO hydrosol (~25 mg/mL) was used as a coating agent without
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additives and a sample coating machine was used to deposit GO hydrosol onto silk
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fabric. The pristine silk fabric was cut to 40 cm × 15 cm and fixed in a sample holder. The coating thickness between scraper and silk fabric was adjusted with a feeler
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gauge and GO hydrosol was poured onto the sample holder. Next, an electrical
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scraper was started and the GO hydrosol was coated on the fabric surface mildly and smoothly. The fabric was then dried in an oven at 120 °C for 10 min, through which the wet GO hydrosol could be converted to dry GO sheets on the fabric surface. The fabric was then turned over and the reverse surface was coated by the same coating method. After coating, the GO-coated silk fabric was immersed in L-ascorbic acid (0.25 mol/L) at 90 °C for 1.5 h to reduce the GO sheets. After reduction, the rGO-coated silk fabric obtained was washed with a large amount of deionised water
ACCEPTED MANUSCRIPT to remove the residual reducing agent. Finally, the rGO-coated silk fabric was dried at 80 °C for 30 min.
2.4. Characterisation
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A Hitachi TM3030 desktop scanning electron microscope (SEM) was used
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to observe the morphologies of the samples. The Raman spectra of the samples
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were measured using a Horiba Jobin Yvon HR800 Raman spectrometer. The crystal phases of the samples were identified by X-ray diffraction (XRD) with
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Cu-Kα radiation (X'pert-Pro MRD, Philips, NL). The flammability was
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determined by measuring the limiting oxygen index (LOI) according to ASTM D2863 on an FTT 0002 oxygen index instrument (FTT, UK). Vertical flame
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testing was carried out according to ASTM D6413 using a YG 8158 fabric
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flame-retardance tester (Ningbo textile instrument, China), calculating the damage length, time to self-extinguish, and the ratio (%) of residual char after
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exposing the sample (30 cm × 8 cm) to a 40 ± 2 mm high flame for 12 s. The %
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residual char yield is calculated by Eq (1): × 100
（1）
where W1 is the weight of pristine silk fabric and W2 is the weight of the sample after vertical flame testing [28]. The smoke suppression test was conducted with an FTT 0064 NBS smoke density test chamber (FTT, UK) according to ISO 5659-2. The sheet resistances of the samples were examined by the four-point probe technique (DMR-1C,
ACCEPTED MANUSCRIPT Damin Instruments, China). The electrical signal of the human motion signal sensor during the test was recorded with a RST5000 electrochemical work station (Risetest Electronic Co., Ltd, China). The washing tests for the rGO-coated silk fabrics were conducted following AATCC Test Method 61-2006 in an SWB-12A
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colour fastness test machine without using stainless steel balls. In one washing
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cycle, the sample was immersed in 200 mL of aqueous solution containing
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standard reference detergent (0.37%, w/w) and washed for 45 min. One washing
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cycle under the condition was equal to washing 5 times[14，29].
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3.1. Morphology and structure
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3. Results and discussion
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GO hydrosol was prepared from GO by dialysis. Through the dialysis process, the acid in GO was removed and the pH of the GO hydrosol increased to 5 – 7.
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Along with the exchange of acid and water molecules, strong complex
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cross-linking reactions happened between the GO molecules and water molecules. Hence, the solubility of GO was enhanced to 25 mg/mL (Fig. S1). More importantly, this GO hydrosol possesses high viscosity owing to its high concentration, which enables GO hydrosol to be deposited directly onto the silk fabric surface as a pure coating agent without adhesives. Fourier transform infrared spectrometry and XRD were used to evaluate the quality of the GO hydrosol (Fig. S2).
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Fig.1. Schematic illustration of the fabrication process of rGO-coated silk fabric. (b) Pristine silk fabric
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fixed on a sample holder before coating. (c) The silk fabric after coating with GO hydrosol. (d) Digital
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image showing the as-prepared rGO-coated silk fabric.
Fig. 1a shows a schematic illustration of the key steps in constructing
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graphene-based silk fabrics. The colour of silk fabric changed twice (during coating and reduction). The GO hydrosol was first coated on silk fabric through a
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double faced "dry-coating" method (Fig. 1b & 1c). The fabric colour changed from white to brown indicating that the GO hydrosol was successfully deposited on the fabric surface. After reduction, the fabric colour became black, which suggested that most of the GO on the fabric surface was converted to rGO. Flexible rGO-coated silk fabric was achieved through this two-step method (Fig. 1d).
ACCEPTED MANUSCRIPT The adhesion between GO hydrosol and silk fabric can be ascribed to complex interactions. GO hydrosol has good hydrophilicity and high surface capacity for adsorption, which results in strong adhesion to silk fabric. The functional groups of silk fabric (amide bonds, hydroxyl, and carboxyl groups) and
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GO hydrosol (hydroxyl, carboxyl, and epoxide functional groups) can form
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hydrogen bonds and covalent bonds, which can result in van der Waals forces
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between GO hydrosol and silk fabric [30, 31]. The adhesion of GO on silk fabric
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surface was evaluated by SEM, XRD, and Raman spectroscopy.
Fig. 2 SEM images of pristine silk fabric (a&b). SEM images of rGO-coated silk fabric (c&d).
SEM images of pristine silk fabric show that the untreated fabric had a smooth and clean surface. Each silk fibre could be seen clearly at high magnification (Fig. 2a & 2b). The smooth surface of the horizontally placed fabric with this method provided a favourable condition for the GO hydrosol to directly form GO sheets. Hence, the
ACCEPTED MANUSCRIPT deposited rGO sheets on silk fabric surface could reach a high level. Fig. 2c & 2d show SEM images of rGO-coated silk fabric at low and high magnification, respectively. From the SEM images of the pristine silk fabric, fibres of rGO-coated silk fabric were completely wrapped by rGO sheets and the spaces between fibres
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were filled by rGO sheets. This unique and robust surface can endow silk fabric with
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excellent properties.
XRD and Raman spectra were utilised to characterise the structures of treated
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silk fabrics and to investigate the reduction of GO-coated silk fabric to rGO-coated
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silk fabric. The XRD spectra of pristine silk fabric, GO-coated silk fabric, and rGO-coated silk fabric are shown in Fig. 3a. The spectrum of pristine silk fabric
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shows a strong diffraction peak at 20.7º, which is characteristic of silk with highly
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ordered β-structure [32]. After coating with GO, the spectrum of GO-coated silk fabric displays an additional sharp and strong diffraction peak at 10.4º corresponding
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to the characteristic peak of GO, indicating that GO and silk fabric were integrated
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successfully. However, the diffraction peak at 10.4º disappeared in the spectrum of rGO-coated silk fabric, indicating that GO was converted to rGO. Raman spectroscopy was used to quantify the transformation of sp3-hybridised carbons to sp2 upon the reduction of GO. As shown in Fig. 3b, the Raman spectrum of pristine silk fabric is complex and random. However, after coating with GO and reducing the GO, there were only two prominent peaks at 1345 cm-1 and 1578 cm-1 in the Raman spectra of GO-coated silk fabric and rGO-coated silk fabric, which correspond to the
ACCEPTED MANUSCRIPT D and G bands, respectively. The G band is characteristic of sp2-hybridised C-C bonds in a two-dimensional hexagonal lattice while the D band corresponds to defects and disorder in the planar carbon network [7,33]. The ratio of intensities of the D and G bands (ID/IG) can be used to evaluate the reduction of GO to rGO. The ID/IG of
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GO-coated silk fabric was 0.90; after chemical reduction, the ID/IG of rGO-coated silk
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fabric increased to 1.04, showing that the reduction process removed the majority of
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oxygen-containing functional groups in the GO and partially restored the structure of GO with a high quantity of structural defects. These results confirm that GO was
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successfully converted to rGO and that the rGO sheets were firmly immobilised on
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the silk fabric surface, in agreement with the SEM images and XRD spectra.
Fig. 3 (a) XRD curves of pristine silk fabric, GO-coated silk fabric, and rGO-coated silk fabric. (b) Raman spectra of pristine silk fabric, GO-coated silk fabric, and rGO-coated silk fabric.
3.2. Fire resistance properties of rGO-coated silk fabric In this “dry-coating” method, the amount of rGO deposited on silk fabric is relatively controllable, and different deposited rGO amounts could endow silk fabric
ACCEPTED MANUSCRIPT with different levels of performance. To investigate the influence of deposited rGO amount on the properties of rGO-coated silk fabric, four different coating thicknesses (one side) were selected to coat silk fabrics. The weight percentage of deposited rGO
(
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(2)
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eposited rGO amount
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amount is calculated by Eq (2):
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where Wg is the weight of rGO-coated silk fabric and Wf is the weight of pristine silk fabric. The corresponding relationship between coating thickness and deposited rGO
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amount is presented in Table 1.
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Table 1
Deposited rGO amount on silk fabrics corresponding to different coating thicknesses (one side). The
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LOI value of rGO-coated silk fabrics with different deposited rGO amounts before and after washing
Coating thickness
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10 times.
Deposited rGO
LOI value (%) before
LOI value (%) before
amount (wt %)
washing 10 times
washing 10 times
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24.0 ± 0.3
23.9 ± 0.3
0.05
3.9 ± 0.5
28.8 ± 0.5
26.9 ± 0.5
0.1
7.8 ± 0.5
29.6 ± 0.5
27.8 ± 0.5
0.2
15.6 ± 1.0
37.5 ± 1.0
36.2 ± 1.0
0.25
19.5 ± 1.0
43.5 ± 2.0
42.3 ± 2.0
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0
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(mm) (one side)
Note: The coating thickness (one side) in this Table refers to the technique factor in Experimental 2.3, the thickness between scraper and silk fabric surface.
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Pristine silk fabric naturally has the elements N, P, and S in its composition. Thus, the fire resistance performance of pristine silk fabric is considered better than other natural fabrics such as cotton and flax. This contributes to making silk fabric a
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superior candidate for fabricating fire-resistant devices. After coating with rGO, silk
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fabric was endowed with excellent fire resistance property. The fire resistance
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mechanism of rGO sheets includes the following: 1. rGO sheets can act as a powerful physical barrier to protect the silk fabric from oxygen, fire, or heat, 2. rGO can
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generate large amounts of carbon dioxide when it burns, and the carbon dioxide is
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considered an efficient fire extinguishing agent, and 3. rGO has excellent char-forming property; it can form continuous and dense residual char layers during
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restrain combustion.
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combustion and the residual char layers deposited on the fabric surface can effectively
LOI is an important indicator of the flammability of rGO-coated silk fabrics. LOI
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refers to the minimum concentration of oxygen in a mixture of oxygen and nitrogen
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that can just support flame combustion over a length of 40 mm [34]. There was obvious enhancement in the LOI value of rGO-coated silk fabrics (Table 1) relative to the LOI value of pristine silk fabric (24%), demonstrating that rGO acted as an effective fire retardant. The LOI value of silk fabric with 3.9 wt% deposited rGO increased to 28.8%. Silk fabric after 19.5 wt% rGO coating had an LOI value of 43.5%, which is 81.25% higher than that of pristine silk fabric. The results indicate that LOI values of rGO-coated silk fabrics increased with the amount of rGO
ACCEPTED MANUSCRIPT deposited. After washing 10 times, the LOI value of each rGO-coated silk fabric showed a slight decline but still maintained a high level. Fabric with LOI value over 26% is regarded to be fire resistant; accordingly, the prepared rGO-coated silk fabrics have excellent fire resistance even after washing 10 times.
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Table 2
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Damage length, time to self-extinguish and % of residual char after vertical flame testing of pristine
Damaged length (cm) 27.1 ± 2
3,9 wt% rGO
17 ± 0.5
7.8 wt% rGO 15.6 wt% rGO
35.5% ± 5%
8 ± 0.5
85.3% ± 1%
14.7 ± 0.5
7.5 ± 0.5
86.1% ± 1%
13.2 ± 0.5
6 ± 0.5
88.7% ± 1%
5 ± 0.5
90.1% ± 1%
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12.8 ± 0.5
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19.5 wt% rGO
% residual char
12 ± 1
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Pristine silk fabric
Time to self-extinguish (s)
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Sample
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silk fabric and rGO-coated silk fabrics with different amounts of rGO deposited.
Fig. 4 Post-burn images of (a) pristine silk fabric, (b) rGO-coated silk fabric with 3.9 wt% rGO, and (c) 19.5 wt% rGO.
ACCEPTED MANUSCRIPT Pristine silk fabric and rGO-coated silk fabrics were subjected to vertical flame testing to further evaluate the flame resistance. In this test, samples were cut to dimensions of 30 cm × 8 cm and exposed to a direct flame for 12 s; the results are shown in Table 1 & Fig. 4. Pristine silk fabric was ignited immediately when it was
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exposed to direct flame; it burned for 12 s and the damaged length was 27.1 cm,
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indicating that it is difficult for pristine silk fabric to resist burning in a vertical flame.
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In contrast, rGO-coated silk fabrics showed ability to suppress flame. For example, the direct flames were self-extinguished within 8 s and 5 s in the tests of rGO-coated
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silk fabrics with 3.9 wt% and 19.5 wt% rGO. The corresponding damage lengths were
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only 17 cm and 12.8 cm, respectively. The successful inhibition of flame extension should be due to the larger amounts of rGO sheets and continuous dense layers of
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char formed on the fabric surfaces [35]. The ratio of residual char of all the
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rGO-coated silk fabrics exceeded 85%, larger than that of pristine silk fabric after 12 s burning. Such char forming ability could endow the rGO-coated silk fabric with more
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potential applications.
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Fig. 5 The smoke density versus time curves of pristine silk fabric and rGO-coated silk fabrics with
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different amounts of deposited rGO.
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Smoke density refers to the amount of smoke produced by the material under test
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conditions. It would be more difficult to evacuate personnel and extinguish a fire if the material generates heavy smoke in a fire disaster. The density of smoke (Ds) was
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used to evaluate the smoke suppression performance. The Ds value (unitless) was
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obtained by measuring the transmission of light utilising a photometric system in a smoke density testing chamber [14]. A lower Ds value indicates better smoke suppression performance of the sample. Ds values of the rGO-coated silk fabrics showed significant decline (Fig. 5) relative to the pristine silk fabric. The maximum Ds value of the rGO-coated silk fabric (3.9 wt% rGO) was 10.74, which was 23.2% lower than that of the pristine silk fabric. However, increasing rGO from 3.9 to 7.8 wt% did not distinctly improve the smoke suppression performance of the rGO-coated silk
ACCEPTED MANUSCRIPT fabric, in agreement with the LOI values. When the deposited rGO increased to 19.5 wt%, the maximum Ds value of rGO-coated silk fabric decreased to 6.22, which was 55.5% lower than the Ds value of pristine silk fabric. The results demonstrate that graphene has good shielding effect suppressing smoke release during combustion,
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which is important for decreasing personal casualty in fire.
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3.3. Electrical conductivity of rGO-coated silk fabrics
Fig. 6 Effect of deposited rGO amount on sheet resistance of rGO-coated silk fabrics before and after washing 10 times, and the insert shows a LED integrated with a rGO-coated silk fabric (19.5 wt% rGO) in a circuit and lighted by the 3V battery.
Fire-resistant rGO-coated silk fabrics were successfully obtained by coating with graphene. Therefore, the electrical conductivity of graphene would not be affected. Pristine silk fabric is non-conducting while graphene has high electrical
ACCEPTED MANUSCRIPT conductivity. After coating with rGO, the fabric can be closely integrated with rGO sheets. Although there are many interstices in the surface structure of silk fabric, rGO sheets can fill interstices and connect the fibres like large numbers of 'bridges'. These 'bridges' provide abundant channels for electron transfer, so that
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the silk fabric can gain electrical conductivity. Fig. 6 shows sheet resistance (RS)
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curves of rGO-coated silk fabrics with different rGO amounts deposited before
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and after washing 10 times. The RS of pristine silk fabric could not be detected. The rGO-coated silk fabric (3.9 wt% rGO) had RS of 3.6 kΩ/sq. Furthermore, the
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RS of rGO-coated silk fabric continuously declined with increasing amount of
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rGO deposited. When the deposited amount of rGO reached 19.5 wt%, the RS of rGO-coated silk fabric decreased to 0.13 kΩ/sq, the lowest RS that could be
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obtained. The RS values of the prepared rGO-coated silk fabrics (after washing 10
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times) were used to evaluate the durabilities of these samples. It is seen from Fig. 6 that RS of the prepared rGO-coated silk fabrics increased slightly after washing
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in comparison with RS of the unwashed samples. The results indicated that the
times.
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rGO-coated silk fabrics still had high electrical conductivity after washing 10
To further demonstrate the high electrical conductivity of rGO-silk fabric, a light emitting diode (LED) was connected with an rGO-coated silk fabric (19.5 wt% rGO) in a circuit, then the LED was successfully lit using a 3V button battery (insert of Fig. 6), which further manifested that rGO-coated silk fabrics are ideal materials for fabricating wearable devices.
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3.4. rGO-coated silk fabric as afire-resistant conductor
Fig. 7 LED connected with a burning rGO-coated silk fabric (19.5 wt% rGO).
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Based on the above results, rGO-coated silk fabrics simultaneously had
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excellent fire resistance property and high electrical conductivity. The combination of these two properties can further broaden the application of
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rGO-coated silk fabrics. Moreover, according to the result of vertical flame test,
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this rGO-coated silk fabric has superior char-forming property during combustion. The layers of char formed after combusting can connect to the rest of the fabric and retain the appearance of the fabric as the original. More importantly, the layers of char formed after combustion also had excellent electrical conductivity. These unique properties enable rGO-coated silk fabric to be used as a fire-resistant conductor. As shown in Fig. 7, a piece of rGO-coated
ACCEPTED MANUSCRIPT silk fabric (19.5 wt% rGO) was connected to an LED and placed in an ethanol flame; the LED kept glowing even after combusting for 60 s.
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3.5. rGO-coated silk fabric as a human motion signal sensor
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Fig. 8 The current through the human motion signal sensor shows regular vibration when monitoring finger motion.
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Benefiting from the flexible and recoverable structure of silk fabric, a human motion signal sensor could be fabricated from the fire-resistant and highly
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conductive rGO-coated silk fabric. rGO-coated silk fabric (3.9 wt% rGO) was selected for the sensing test owing to its lighter weight and higher sheet resistance among prepared rGO-coated silk fabrics. To monitor the difference of finger motion while releasing or closing a fist, rGO-coated silk fabric was tailored to be 6 cm long and 1 cm wide with two silver wires immobilised by silver paste on both ends as electrodes. The human motion signal sensor was attached to a finger
ACCEPTED MANUSCRIPT with medical tape. The human motion signal sensor can easily follow the change of finger motion, and the images of releasing and closing a fist are shown in the insert of Fig. 8. The corresponding current is recorded with an electrochemical workstation and shown in Fig. 8. The curve of corresponding current signal
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exhibits regular vibration during testing for 90 s, indicating the repeatability of the
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sensor. The result completely complied with the actual motions, and each peak
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represents the state of motion. Specifically, the negative and positive peaks correspond to the finger while releasing and closing the fist, respectively. For
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example, from releasing the fist to closing the fist, the output current increased
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from 25.6 μA to 97.7 μA, demonstrating that the resistance value of the rGO-coated silk fabric decreased because of the stretching effect. The distribution
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of fibres in the rGO-coated silk fabric changed from the relaxed state to the
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compact state under the influence of force. Consequently, the charge transport distance became shorter, which led to the increase of resistance [9]. The result
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demonstrates that the rGO-coated silk fabric has potential in wearable human
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motion signal sensors. 4. Conclusions
Large-scale and functional graphene-based silk fabrics were successfully constructed by the environmentally friendly "dry-coating" method. GO hydrosol with high concentration was directly coated onto silk fabric followed by reduction with L-ascorbic acid. The rGO sheets deposited on the fabric surface uniformly and firmly simultaneously endowed the fabric with excellent fire resistance
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GO hydrosol can be applied on various fabrics such as cotton, silk, and wool for
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other potential applications including flexible electrodes, antibacterials, and
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hydrophobics. Furthermore, this inexpensive two-step “dry-coating” method is suitable for large-scale industrial production of multifunctional graphene-based
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textiles. We believe that our strategy could be applied for fabricating more kinds
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of functional materials and wearable devices.
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Acknowledgements
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This work was supported by the Six Talent Peaks Project of Jiangsu Province (JNHB-066), the Priority Academic Program Development of Jiangsu Higher
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Education Institutions (PAPD), and the Qing Lan Project.
ACCEPTED MANUSCRIPT References
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Graphical abstract :
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Highlights:
A novel “dry-coating” method was successfully applied to deposit synthetic graphene oxide (GO) hydrosol onto silk fabrics.
The prepared rGO-coated silk fabrics exhibited excellent fire resistance property and high
The rGO-coated silk fabric could be used as a conductor both at ambient temperature and in the
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electrical conductivity simultaneously.
The rGO-coated silk fabric was successfully designed into a wearable and highly sensitive human
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motion signal sensor.
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burning flame.
Yimin Ji, Yuzhou Li, Guoqiang Chen, Tieling Xing PII: DOI: Reference:
S0264-1275(17)30752-9 doi: 10.1016/j.matdes.2017.08.006 JMADE 3265
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
17 May 2017 2 August 2017 3 August 2017
Please cite this article as: Yimin Ji, Yuzhou Li, Guoqiang Chen, Tieling Xing , Fireresistant and highly electrically conductive silk fabrics fabricated with reduced graphene oxide via dry-coating, Materials & Design (2017), doi: 10.1016/j.matdes.2017.08.006
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Fire-resistant and highly electrically conductive silk fabrics fabricated with reduced graphene oxide via dry-coating
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Yimin Ji, Yuzhou Li, Guoqiang Chen, and Tieling Xing
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National Engineering Laboratory for Modern Silk, College of Textile and Clothing
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Engineering, Soochow University, Suzhou 215123, PR China
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Abstract
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Large-scale and functional silk fabrics were prepared by depositing synthetic graphene oxide (GO) hydrosol onto fabrics via an environmentally friendly
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“dry-coating” method and subsequently reduced in L-ascorbic acid solution. Through
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this modification method, the reduced GO (rGO) sheets deposited uniformly on the silk fabric surface were firmly combined with fibres. Up to 19.5 wt% rGO could be
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deposited relative to the fabric weight. The morphology and structure of prepared
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rGO-coated silk fabric were characterised by scanning electron microscopy, X-ray diffraction, and Raman spectrometry. In comparison with pristine silk fabric, the modified silk fabric exhibited improved fire resistance and smoke suppression properties. The sheet resistance of rGO-coated silk fabric decreased to 0.13 kΩ/sq. Washing test indicated that the rGO-coated silk fabrics prepared had good durability for common use. The functional silk fabric deposited with 19.5 wt% rGO was
Corresponding author. E-mail address: [email protected] (T. Xing).
ACCEPTED MANUSCRIPT designed into a fire-resistant conductor that kept conducting even after 60 s of combustion and can be applied in the fire-fighting field. The silk fabric deposited with 3.9 wt% rGO was successfully assembled into a human motion signal sensor; this easily fabricated, highly sensitive, and flexible sensor has potential for use as
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wearable devices.
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Key words: Silk fabric; Reduced graphene oxide; Fire resistance; Electrical
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conductivity; Conductor; Human motion signal sensor
ACCEPTED MANUSCRIPT 1. Introduction Graphene has recently drawn attention for wide applications based on its unique structure and excellent properties [1-5]. Combining graphene with traditional textiles to develop graphene-based textiles is an emerging research hotspot, and
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graphene-based textiles have proven to be promising in electrical applications such as
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electric conductors [6, 7], strain sensors [8-10], supercapacitors, and energy storage
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[11, 12]. Furthermore, because of excellent properties of textiles such as flexibility, low cost, and easy deformation (similar to the human skin) [13], graphene-based
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textiles are outstanding candidates for fabricating wearable devices. However, textiles
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(depending on their natures) can be easily ignited, quickly release dense smoke, and rapidly induce fires [14, 15]. The inherently flammable and combustible properties of
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textiles could seriously confine their further application. In fact, graphene itself has
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been demonstrated to be an efficient green fire retardant [2]. The 2D-layered structure of graphene can act as a powerful physical barrier, which is favourable for isolating
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oxygen, delaying heat transfer, and preventing escape of pyrolysis products [16, 17].
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Unfortunately, graphene is just regarded as a fabulous fire retarding nano-additive in polymers [18, 19], rather than as a fire retardant that can be directly used to modify fabrics. This is because, as an inorganic carbon material, graphene is not directly absorbed by fabrics. Moreover, the amount of graphene that can be deposited on fabric surface is quite low, which does not effectively endow the fabric with satisfactory fire resistance. Some organo-phosphorous flame retardants have been used to improve the fire resistance of textiles through doping with graphene [6, 20,
ACCEPTED MANUSCRIPT 21]. Dong et al. [6] fabricated a fire-resistant cotton fabric by dipping it in a mixture of graphene oxide (GO) and hexachlorocyclotriphosphazene; after reduction, the modified fabric could hold its shape under combustion for 90 s. Although the flame retardant hexachlorocyclotriphosphazene improved the fire resistance property of
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graphene-based textiles, the electrical conductivity of graphene-based cotton was
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seriously affected. In addition, organo-phosphorous flame retardants are toxic and
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unfriendly to the environment, and are gradually being replaced [22]. Therefore, improving the fire resistance property of graphene-based textiles for fire safety and
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expanding the range of their application while retaining an excellent electrical
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conductivity is still a challenge.
Graphene has both fire resistance and electrical conductivity properties.
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Fabricating fire-resistant and highly electrically conductive graphene-based textiles
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through increasing the amount of graphene deposited on fabric (instead of adding other flame retardants) is a novel approach. GO, a derivative of graphene, has
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functional groups (such as hydroxyl, carboxyl, and epoxide) on the basal planes [23]
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and can be absorbed onto fabric surfaces through the combination of covalent bonds and hydrogen bonds. Therefore, to fabricate graphene-based textiles, depositing GO onto fabrics and subsequently reducing the GO is an efficient approach. However, existing methods cannot deposit a sufficient amount of graphene onto fabric to fabricate fire-resistant graphene-based textiles. Representative modification methods to deposit GO on fabrics involve “dip-coating” [7, 9, 24] and “vacuum filtration” [10, 25, 26]. The “dip-coating” method is limited by the concentration of GO suspension;
ACCEPTED MANUSCRIPT because of the poor solubility of GO sheets in aqueous solution due to a lack of adequate hydrophilic groups in its structure, the method needs fabric to be repeatedly dipped in GO suspension for long times. Even so, the amount of GO deposited on the fabric is still unsatisfactory. Lu et al. [7] fabricated a highly conductive silk fabric
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(sheet resistance: 1.5 kΩ/sq) by dipping silk fabric in GO suspension and reducing for
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7 cycles. Although this method allows the fabric to obtain good electrical conductivity,
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the modified graphene-based silk fabric still did not have fire resistance because of the low amount of graphene deposited on its surface. In the “vacuum filtration”
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method, GO is filtered onto the fabric through vacuum filtration. However, the size
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and shape of the obtained fabric are restricted by the common filter paper, which prevents the approach from being scalable and hence is not suitable for industrial
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production. In addition, this method also could not deposit sufficient graphene on the
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fabric surface for achieving fire resistance. Therefore, developing a new method that can deposit enough graphene on fabric is needed for fabricating graphene-based
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textiles that have both excellent fire resistance and high electrical conductivity.
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In this work, we report an environmentally friendly and time-saving “dry-coating” method to construct functional silk fabrics by coating with synthetic GO hydrosol and subsequent immersion in L-ascorbic acid solution to reduce the GO. This method can deposit highly concentrated GO hydrosol on silk fabric surfaces. Moreover, the amount of reduced GO (rGO) deposited on silk fabric can be easily controlled and calculated. The fire resistance property and electrical conductivity of the modified silk fabrics were measured, and the durabilities of rGO-coated silk fabrics were evaluated
ACCEPTED MANUSCRIPT by washing tests. The feasibilities of modified silk fabric as a fire-resistant conductor and a human motion signal sensor were also investigated.
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2. Experimental
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2.1. Materials
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Silk crepe satin (weight: 60.28 g/m2) was supplied by Suzhou Kasen Silk Garments Co., Ltd. Graphite flakes (~325 mesh) were purchased from Alfa Aesar Co.,
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Ltd. Phosphoric acid (H3PO4, 85%) was obtained from Acros Organics Co., Ltd.
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Potassium permanganate (KMNO4) and L-ascorbic acid were provided by Sinopharm Chemical Reagent Co., Ltd. Sulphuric acid (H2SO4, 95% – 98%), hydrogen peroxide
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(H2O2, 30%), hydrogen chloride (HCL, 36% – 38%), and ethanol (C2H5OH, 99.7%)
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were purchased from Chinasun Specialty Products Co., Ltd. All reagents were used
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without further purification.
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2.2. Preparation of GO hydrosol GO was synthesised from graphite flakes by the improved Hummer’s method [27]. A 9:1 mixture of concentrated H2SO4/H3PO4 (270:30 mL) was added into a 1000 mL flask containing 6 g graphite flakes, then 42 g KMnO4 was added to the mixture within 30 min batch-wise. The reaction system was maintained at 0 °C in an ice water bath for 1 h. Then the reaction was heated to 50 °C and quickly stirred for 8 h. Finally, the reaction was cooled to 0 °C in an ice water bath, and 600 mL of deionised water
ACCEPTED MANUSCRIPT with 12 mL H2O2 (30%) was added to stop the oxidation process. The mixture became golden yellow indicating that highly oxidised GO was obtained. The product was then washed in succession with 200 mL of deionised water and 200 mL of ethanol; for each wash, the product was centrifuged at 8000 rpm for 5 min. GO hydrosol was
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prepared through a simple dialysis process as follows: the GO obtained was added to
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a moderate amount of water and transferred into a dialysis tube (molecular weight
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cutoff: 8000 – 14000 Da) and the GO hydrosol was prepared through dialysis for 3 d
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in deionised water.
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2.3. Fabrication of rGO-coated silk fabrics
The prepared GO hydrosol (~25 mg/mL) was used as a coating agent without
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additives and a sample coating machine was used to deposit GO hydrosol onto silk
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fabric. The pristine silk fabric was cut to 40 cm × 15 cm and fixed in a sample holder. The coating thickness between scraper and silk fabric was adjusted with a feeler
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gauge and GO hydrosol was poured onto the sample holder. Next, an electrical
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scraper was started and the GO hydrosol was coated on the fabric surface mildly and smoothly. The fabric was then dried in an oven at 120 °C for 10 min, through which the wet GO hydrosol could be converted to dry GO sheets on the fabric surface. The fabric was then turned over and the reverse surface was coated by the same coating method. After coating, the GO-coated silk fabric was immersed in L-ascorbic acid (0.25 mol/L) at 90 °C for 1.5 h to reduce the GO sheets. After reduction, the rGO-coated silk fabric obtained was washed with a large amount of deionised water
ACCEPTED MANUSCRIPT to remove the residual reducing agent. Finally, the rGO-coated silk fabric was dried at 80 °C for 30 min.
2.4. Characterisation
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A Hitachi TM3030 desktop scanning electron microscope (SEM) was used
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to observe the morphologies of the samples. The Raman spectra of the samples
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were measured using a Horiba Jobin Yvon HR800 Raman spectrometer. The crystal phases of the samples were identified by X-ray diffraction (XRD) with
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Cu-Kα radiation (X'pert-Pro MRD, Philips, NL). The flammability was
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determined by measuring the limiting oxygen index (LOI) according to ASTM D2863 on an FTT 0002 oxygen index instrument (FTT, UK). Vertical flame
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testing was carried out according to ASTM D6413 using a YG 8158 fabric
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flame-retardance tester (Ningbo textile instrument, China), calculating the damage length, time to self-extinguish, and the ratio (%) of residual char after
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exposing the sample (30 cm × 8 cm) to a 40 ± 2 mm high flame for 12 s. The %
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residual char yield is calculated by Eq (1): × 100
（1）
where W1 is the weight of pristine silk fabric and W2 is the weight of the sample after vertical flame testing [28]. The smoke suppression test was conducted with an FTT 0064 NBS smoke density test chamber (FTT, UK) according to ISO 5659-2. The sheet resistances of the samples were examined by the four-point probe technique (DMR-1C,
ACCEPTED MANUSCRIPT Damin Instruments, China). The electrical signal of the human motion signal sensor during the test was recorded with a RST5000 electrochemical work station (Risetest Electronic Co., Ltd, China). The washing tests for the rGO-coated silk fabrics were conducted following AATCC Test Method 61-2006 in an SWB-12A
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colour fastness test machine without using stainless steel balls. In one washing
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cycle, the sample was immersed in 200 mL of aqueous solution containing
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standard reference detergent (0.37%, w/w) and washed for 45 min. One washing
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cycle under the condition was equal to washing 5 times[14，29].
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3.1. Morphology and structure
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3. Results and discussion
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GO hydrosol was prepared from GO by dialysis. Through the dialysis process, the acid in GO was removed and the pH of the GO hydrosol increased to 5 – 7.
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Along with the exchange of acid and water molecules, strong complex
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cross-linking reactions happened between the GO molecules and water molecules. Hence, the solubility of GO was enhanced to 25 mg/mL (Fig. S1). More importantly, this GO hydrosol possesses high viscosity owing to its high concentration, which enables GO hydrosol to be deposited directly onto the silk fabric surface as a pure coating agent without adhesives. Fourier transform infrared spectrometry and XRD were used to evaluate the quality of the GO hydrosol (Fig. S2).
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Fig.1. Schematic illustration of the fabrication process of rGO-coated silk fabric. (b) Pristine silk fabric
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fixed on a sample holder before coating. (c) The silk fabric after coating with GO hydrosol. (d) Digital
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image showing the as-prepared rGO-coated silk fabric.
Fig. 1a shows a schematic illustration of the key steps in constructing
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graphene-based silk fabrics. The colour of silk fabric changed twice (during coating and reduction). The GO hydrosol was first coated on silk fabric through a
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double faced "dry-coating" method (Fig. 1b & 1c). The fabric colour changed from white to brown indicating that the GO hydrosol was successfully deposited on the fabric surface. After reduction, the fabric colour became black, which suggested that most of the GO on the fabric surface was converted to rGO. Flexible rGO-coated silk fabric was achieved through this two-step method (Fig. 1d).
ACCEPTED MANUSCRIPT The adhesion between GO hydrosol and silk fabric can be ascribed to complex interactions. GO hydrosol has good hydrophilicity and high surface capacity for adsorption, which results in strong adhesion to silk fabric. The functional groups of silk fabric (amide bonds, hydroxyl, and carboxyl groups) and
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GO hydrosol (hydroxyl, carboxyl, and epoxide functional groups) can form
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hydrogen bonds and covalent bonds, which can result in van der Waals forces
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between GO hydrosol and silk fabric [30, 31]. The adhesion of GO on silk fabric
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surface was evaluated by SEM, XRD, and Raman spectroscopy.
Fig. 2 SEM images of pristine silk fabric (a&b). SEM images of rGO-coated silk fabric (c&d).
SEM images of pristine silk fabric show that the untreated fabric had a smooth and clean surface. Each silk fibre could be seen clearly at high magnification (Fig. 2a & 2b). The smooth surface of the horizontally placed fabric with this method provided a favourable condition for the GO hydrosol to directly form GO sheets. Hence, the
ACCEPTED MANUSCRIPT deposited rGO sheets on silk fabric surface could reach a high level. Fig. 2c & 2d show SEM images of rGO-coated silk fabric at low and high magnification, respectively. From the SEM images of the pristine silk fabric, fibres of rGO-coated silk fabric were completely wrapped by rGO sheets and the spaces between fibres
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were filled by rGO sheets. This unique and robust surface can endow silk fabric with
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excellent properties.
XRD and Raman spectra were utilised to characterise the structures of treated
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silk fabrics and to investigate the reduction of GO-coated silk fabric to rGO-coated
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silk fabric. The XRD spectra of pristine silk fabric, GO-coated silk fabric, and rGO-coated silk fabric are shown in Fig. 3a. The spectrum of pristine silk fabric
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shows a strong diffraction peak at 20.7º, which is characteristic of silk with highly
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ordered β-structure [32]. After coating with GO, the spectrum of GO-coated silk fabric displays an additional sharp and strong diffraction peak at 10.4º corresponding
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to the characteristic peak of GO, indicating that GO and silk fabric were integrated
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successfully. However, the diffraction peak at 10.4º disappeared in the spectrum of rGO-coated silk fabric, indicating that GO was converted to rGO. Raman spectroscopy was used to quantify the transformation of sp3-hybridised carbons to sp2 upon the reduction of GO. As shown in Fig. 3b, the Raman spectrum of pristine silk fabric is complex and random. However, after coating with GO and reducing the GO, there were only two prominent peaks at 1345 cm-1 and 1578 cm-1 in the Raman spectra of GO-coated silk fabric and rGO-coated silk fabric, which correspond to the
ACCEPTED MANUSCRIPT D and G bands, respectively. The G band is characteristic of sp2-hybridised C-C bonds in a two-dimensional hexagonal lattice while the D band corresponds to defects and disorder in the planar carbon network [7,33]. The ratio of intensities of the D and G bands (ID/IG) can be used to evaluate the reduction of GO to rGO. The ID/IG of
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GO-coated silk fabric was 0.90; after chemical reduction, the ID/IG of rGO-coated silk
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fabric increased to 1.04, showing that the reduction process removed the majority of
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oxygen-containing functional groups in the GO and partially restored the structure of GO with a high quantity of structural defects. These results confirm that GO was
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successfully converted to rGO and that the rGO sheets were firmly immobilised on
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the silk fabric surface, in agreement with the SEM images and XRD spectra.
Fig. 3 (a) XRD curves of pristine silk fabric, GO-coated silk fabric, and rGO-coated silk fabric. (b) Raman spectra of pristine silk fabric, GO-coated silk fabric, and rGO-coated silk fabric.
3.2. Fire resistance properties of rGO-coated silk fabric In this “dry-coating” method, the amount of rGO deposited on silk fabric is relatively controllable, and different deposited rGO amounts could endow silk fabric
ACCEPTED MANUSCRIPT with different levels of performance. To investigate the influence of deposited rGO amount on the properties of rGO-coated silk fabric, four different coating thicknesses (one side) were selected to coat silk fabrics. The weight percentage of deposited rGO
(
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(2)
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eposited rGO amount
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amount is calculated by Eq (2):
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where Wg is the weight of rGO-coated silk fabric and Wf is the weight of pristine silk fabric. The corresponding relationship between coating thickness and deposited rGO
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amount is presented in Table 1.
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Table 1
Deposited rGO amount on silk fabrics corresponding to different coating thicknesses (one side). The
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LOI value of rGO-coated silk fabrics with different deposited rGO amounts before and after washing
Coating thickness
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10 times.
Deposited rGO
LOI value (%) before
LOI value (%) before
amount (wt %)
washing 10 times
washing 10 times
0
24.0 ± 0.3
23.9 ± 0.3
0.05
3.9 ± 0.5
28.8 ± 0.5
26.9 ± 0.5
0.1
7.8 ± 0.5
29.6 ± 0.5
27.8 ± 0.5
0.2
15.6 ± 1.0
37.5 ± 1.0
36.2 ± 1.0
0.25
19.5 ± 1.0
43.5 ± 2.0
42.3 ± 2.0
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(mm) (one side)
Note: The coating thickness (one side) in this Table refers to the technique factor in Experimental 2.3, the thickness between scraper and silk fabric surface.
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Pristine silk fabric naturally has the elements N, P, and S in its composition. Thus, the fire resistance performance of pristine silk fabric is considered better than other natural fabrics such as cotton and flax. This contributes to making silk fabric a
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superior candidate for fabricating fire-resistant devices. After coating with rGO, silk
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fabric was endowed with excellent fire resistance property. The fire resistance
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mechanism of rGO sheets includes the following: 1. rGO sheets can act as a powerful physical barrier to protect the silk fabric from oxygen, fire, or heat, 2. rGO can
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generate large amounts of carbon dioxide when it burns, and the carbon dioxide is
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considered an efficient fire extinguishing agent, and 3. rGO has excellent char-forming property; it can form continuous and dense residual char layers during
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restrain combustion.
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combustion and the residual char layers deposited on the fabric surface can effectively
LOI is an important indicator of the flammability of rGO-coated silk fabrics. LOI
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refers to the minimum concentration of oxygen in a mixture of oxygen and nitrogen
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that can just support flame combustion over a length of 40 mm [34]. There was obvious enhancement in the LOI value of rGO-coated silk fabrics (Table 1) relative to the LOI value of pristine silk fabric (24%), demonstrating that rGO acted as an effective fire retardant. The LOI value of silk fabric with 3.9 wt% deposited rGO increased to 28.8%. Silk fabric after 19.5 wt% rGO coating had an LOI value of 43.5%, which is 81.25% higher than that of pristine silk fabric. The results indicate that LOI values of rGO-coated silk fabrics increased with the amount of rGO
ACCEPTED MANUSCRIPT deposited. After washing 10 times, the LOI value of each rGO-coated silk fabric showed a slight decline but still maintained a high level. Fabric with LOI value over 26% is regarded to be fire resistant; accordingly, the prepared rGO-coated silk fabrics have excellent fire resistance even after washing 10 times.
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Table 2
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Damage length, time to self-extinguish and % of residual char after vertical flame testing of pristine
Damaged length (cm) 27.1 ± 2
3,9 wt% rGO
17 ± 0.5
7.8 wt% rGO 15.6 wt% rGO
35.5% ± 5%
8 ± 0.5
85.3% ± 1%
14.7 ± 0.5
7.5 ± 0.5
86.1% ± 1%
13.2 ± 0.5
6 ± 0.5
88.7% ± 1%
5 ± 0.5
90.1% ± 1%
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12.8 ± 0.5
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19.5 wt% rGO
% residual char
12 ± 1
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Pristine silk fabric
Time to self-extinguish (s)
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Sample
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silk fabric and rGO-coated silk fabrics with different amounts of rGO deposited.
Fig. 4 Post-burn images of (a) pristine silk fabric, (b) rGO-coated silk fabric with 3.9 wt% rGO, and (c) 19.5 wt% rGO.
ACCEPTED MANUSCRIPT Pristine silk fabric and rGO-coated silk fabrics were subjected to vertical flame testing to further evaluate the flame resistance. In this test, samples were cut to dimensions of 30 cm × 8 cm and exposed to a direct flame for 12 s; the results are shown in Table 1 & Fig. 4. Pristine silk fabric was ignited immediately when it was
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exposed to direct flame; it burned for 12 s and the damaged length was 27.1 cm,
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indicating that it is difficult for pristine silk fabric to resist burning in a vertical flame.
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In contrast, rGO-coated silk fabrics showed ability to suppress flame. For example, the direct flames were self-extinguished within 8 s and 5 s in the tests of rGO-coated
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silk fabrics with 3.9 wt% and 19.5 wt% rGO. The corresponding damage lengths were
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only 17 cm and 12.8 cm, respectively. The successful inhibition of flame extension should be due to the larger amounts of rGO sheets and continuous dense layers of
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char formed on the fabric surfaces [35]. The ratio of residual char of all the
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rGO-coated silk fabrics exceeded 85%, larger than that of pristine silk fabric after 12 s burning. Such char forming ability could endow the rGO-coated silk fabric with more
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potential applications.
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Fig. 5 The smoke density versus time curves of pristine silk fabric and rGO-coated silk fabrics with
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different amounts of deposited rGO.
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Smoke density refers to the amount of smoke produced by the material under test
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conditions. It would be more difficult to evacuate personnel and extinguish a fire if the material generates heavy smoke in a fire disaster. The density of smoke (Ds) was
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used to evaluate the smoke suppression performance. The Ds value (unitless) was
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obtained by measuring the transmission of light utilising a photometric system in a smoke density testing chamber [14]. A lower Ds value indicates better smoke suppression performance of the sample. Ds values of the rGO-coated silk fabrics showed significant decline (Fig. 5) relative to the pristine silk fabric. The maximum Ds value of the rGO-coated silk fabric (3.9 wt% rGO) was 10.74, which was 23.2% lower than that of the pristine silk fabric. However, increasing rGO from 3.9 to 7.8 wt% did not distinctly improve the smoke suppression performance of the rGO-coated silk
ACCEPTED MANUSCRIPT fabric, in agreement with the LOI values. When the deposited rGO increased to 19.5 wt%, the maximum Ds value of rGO-coated silk fabric decreased to 6.22, which was 55.5% lower than the Ds value of pristine silk fabric. The results demonstrate that graphene has good shielding effect suppressing smoke release during combustion,
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which is important for decreasing personal casualty in fire.
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3.3. Electrical conductivity of rGO-coated silk fabrics
Fig. 6 Effect of deposited rGO amount on sheet resistance of rGO-coated silk fabrics before and after washing 10 times, and the insert shows a LED integrated with a rGO-coated silk fabric (19.5 wt% rGO) in a circuit and lighted by the 3V battery.
Fire-resistant rGO-coated silk fabrics were successfully obtained by coating with graphene. Therefore, the electrical conductivity of graphene would not be affected. Pristine silk fabric is non-conducting while graphene has high electrical
ACCEPTED MANUSCRIPT conductivity. After coating with rGO, the fabric can be closely integrated with rGO sheets. Although there are many interstices in the surface structure of silk fabric, rGO sheets can fill interstices and connect the fibres like large numbers of 'bridges'. These 'bridges' provide abundant channels for electron transfer, so that
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the silk fabric can gain electrical conductivity. Fig. 6 shows sheet resistance (RS)
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curves of rGO-coated silk fabrics with different rGO amounts deposited before
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and after washing 10 times. The RS of pristine silk fabric could not be detected. The rGO-coated silk fabric (3.9 wt% rGO) had RS of 3.6 kΩ/sq. Furthermore, the
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RS of rGO-coated silk fabric continuously declined with increasing amount of
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rGO deposited. When the deposited amount of rGO reached 19.5 wt%, the RS of rGO-coated silk fabric decreased to 0.13 kΩ/sq, the lowest RS that could be
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obtained. The RS values of the prepared rGO-coated silk fabrics (after washing 10
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times) were used to evaluate the durabilities of these samples. It is seen from Fig. 6 that RS of the prepared rGO-coated silk fabrics increased slightly after washing
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in comparison with RS of the unwashed samples. The results indicated that the
times.
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rGO-coated silk fabrics still had high electrical conductivity after washing 10
To further demonstrate the high electrical conductivity of rGO-silk fabric, a light emitting diode (LED) was connected with an rGO-coated silk fabric (19.5 wt% rGO) in a circuit, then the LED was successfully lit using a 3V button battery (insert of Fig. 6), which further manifested that rGO-coated silk fabrics are ideal materials for fabricating wearable devices.
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3.4. rGO-coated silk fabric as afire-resistant conductor
Fig. 7 LED connected with a burning rGO-coated silk fabric (19.5 wt% rGO).
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Based on the above results, rGO-coated silk fabrics simultaneously had
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excellent fire resistance property and high electrical conductivity. The combination of these two properties can further broaden the application of
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rGO-coated silk fabrics. Moreover, according to the result of vertical flame test,
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this rGO-coated silk fabric has superior char-forming property during combustion. The layers of char formed after combusting can connect to the rest of the fabric and retain the appearance of the fabric as the original. More importantly, the layers of char formed after combustion also had excellent electrical conductivity. These unique properties enable rGO-coated silk fabric to be used as a fire-resistant conductor. As shown in Fig. 7, a piece of rGO-coated
ACCEPTED MANUSCRIPT silk fabric (19.5 wt% rGO) was connected to an LED and placed in an ethanol flame; the LED kept glowing even after combusting for 60 s.
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3.5. rGO-coated silk fabric as a human motion signal sensor
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Fig. 8 The current through the human motion signal sensor shows regular vibration when monitoring finger motion.
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Benefiting from the flexible and recoverable structure of silk fabric, a human motion signal sensor could be fabricated from the fire-resistant and highly
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conductive rGO-coated silk fabric. rGO-coated silk fabric (3.9 wt% rGO) was selected for the sensing test owing to its lighter weight and higher sheet resistance among prepared rGO-coated silk fabrics. To monitor the difference of finger motion while releasing or closing a fist, rGO-coated silk fabric was tailored to be 6 cm long and 1 cm wide with two silver wires immobilised by silver paste on both ends as electrodes. The human motion signal sensor was attached to a finger
ACCEPTED MANUSCRIPT with medical tape. The human motion signal sensor can easily follow the change of finger motion, and the images of releasing and closing a fist are shown in the insert of Fig. 8. The corresponding current is recorded with an electrochemical workstation and shown in Fig. 8. The curve of corresponding current signal
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exhibits regular vibration during testing for 90 s, indicating the repeatability of the
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sensor. The result completely complied with the actual motions, and each peak
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represents the state of motion. Specifically, the negative and positive peaks correspond to the finger while releasing and closing the fist, respectively. For
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example, from releasing the fist to closing the fist, the output current increased
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from 25.6 μA to 97.7 μA, demonstrating that the resistance value of the rGO-coated silk fabric decreased because of the stretching effect. The distribution
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of fibres in the rGO-coated silk fabric changed from the relaxed state to the
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compact state under the influence of force. Consequently, the charge transport distance became shorter, which led to the increase of resistance [9]. The result
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demonstrates that the rGO-coated silk fabric has potential in wearable human
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motion signal sensors. 4. Conclusions
Large-scale and functional graphene-based silk fabrics were successfully constructed by the environmentally friendly "dry-coating" method. GO hydrosol with high concentration was directly coated onto silk fabric followed by reduction with L-ascorbic acid. The rGO sheets deposited on the fabric surface uniformly and firmly simultaneously endowed the fabric with excellent fire resistance
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GO hydrosol can be applied on various fabrics such as cotton, silk, and wool for
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other potential applications including flexible electrodes, antibacterials, and
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hydrophobics. Furthermore, this inexpensive two-step “dry-coating” method is suitable for large-scale industrial production of multifunctional graphene-based
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textiles. We believe that our strategy could be applied for fabricating more kinds
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of functional materials and wearable devices.
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Acknowledgements
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This work was supported by the Six Talent Peaks Project of Jiangsu Province (JNHB-066), the Priority Academic Program Development of Jiangsu Higher
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Education Institutions (PAPD), and the Qing Lan Project.
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Graphical abstract :
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Highlights:
A novel “dry-coating” method was successfully applied to deposit synthetic graphene oxide (GO) hydrosol onto silk fabrics.
The prepared rGO-coated silk fabrics exhibited excellent fire resistance property and high
The rGO-coated silk fabric could be used as a conductor both at ambient temperature and in the
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electrical conductivity simultaneously.
The rGO-coated silk fabric was successfully designed into a wearable and highly sensitive human
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motion signal sensor.
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burning flame.