Tea-Carbon Dots-Reduced Graphene Oxide: An Efficient Conducting

Oct 13, 2017 - Subsequently, the tea-CDs reduced graphene oxide (TCD-rGO) was used for fabrication of a cotton-based conducting fabric with anticipate...
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Research Article pubs.acs.org/journal/ascecg

Tea-Carbon Dots-Reduced Graphene Oxide: An Efficient Conducting Coating Material for Fabrication of an E‑Textile Achyut Konwar,† Upama Baruah,† Manash J. Deka,† Amreen A. Hussain,‡ Sultana R. Haque,† Arup R. Pal,‡ and Devasish Chowdhury*,† †

Material Nanochemistry Laboratory, ‡Plasma Nanotech Laboratory, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Garchuk, Guwahati 781035, India ABSTRACT: The present study reports a facile and green method for reduction of graphene oxide using carbon dots (CDs) derived from “Assam CTC (crush tear curl) Tea”. The efficient reduction of graphene oxide by tea-CDs was monitored using UV− visible spectroscopy. The reduced graphene oxide (rGO) prepared by this method adsorbs some amount of tea-CDs on its surface and forms a very good dispersion in aqueous medium without the use of any other capping or stabilizing agents. Subsequently, the tea-CDs reduced graphene oxide (TCD-rGO) was used for fabrication of a cotton-based conducting fabric with anticipated applicability in different electronic gadgets where high flexibility of the conducting material is required. Coating of cotton with TCD-rGO improved the thermostability of the fabric. The initial degradation temperature for the TCD-rGO coated fabric was found to increase by 30 °C compared to that of the bare cotton fabric. The electrical property of the coated fabric was evaluated. TCD-rGO coated fabric possessed a sheet resistance of 229 ± 20 Ω/sq and electrical conductivity 623 ± 54 S/m, which was comparable to that of the other graphitic conducting textile materials reported so far. The ohmic behavior and the electrical stability of the material was also studied. This particular approach eliminates the use of any toxic chemicals and other high cost synthetic products for fabrication of conducting textiles. Finally, to test the practical viability of the material developed, a stripe from the fabricated conducting fabric was used in a circuit to light up a series of light-emitting diode bulbs. KEYWORDS: Carbon dots, Reduced graphene oxide, Conductivity, Conducting fabric

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INTRODUCTION Graphene has now emerged as one of the most interesting conducting materials of this present era. This new allotrope of carbon is able to compete with the metallic conductors with its multiple advantages like sustainability, noncytotoxicity, twodimensional (2D) structure, good electrical and thermal conductivity, high Young’s modulus, etc.1 Fabrication of graphene based electronic devices is now a very hot topic of research around the globe. But, large-scale production of graphene is still a challenge. Nondispersibility of graphene in aqueous medium further limits its application. Graphene is oxidized to graphene oxide (GO) to make it dispersible in aqueous medium. But when oxidized, graphene loses its electronic conjugation and hence conductivity value also reduces by many fold. This electrical conductivity is regained to some extent by restoring the electronic conjugation of graphene oxide using various reduction methods. The extent of restoration of the electronic conjugation depends upon the type of reduction process. Chemical reduction is the most popularly used reduction method for GO. However, in some cases, the low dispersibility of the chemically reduced graphene oxide sheets in water and most organic solvents seems to occur as the strong π−π stacking tendency between chemically reduced GO sheets lead to the formation of irreversible agglomerates unless the capping reagents (polymers or surfactants) are used. On the © 2017 American Chemical Society

other hand, the capping reagents may have the chance to affect the properties of the graphene sheets, limiting their practical applications. Moreover, most widely used reducing agents, such as hydrazine, dimethylhydrazine, HI, and NaBH4 are highly toxic; trace amount of these poisonous agents may restrict its application. Handling of these hazardous chemicals can lead to industrial cost and risk for human health.2,3 Researchers are now finding solutions for the replacement of these toxic chemical reducing agents for GO. For example, Zang et al. showed the possibility of reduction of GO using L-ascorbic acid.3 Wang et al. demonstrated the reduction of GO by polyphenolic groups extracted from green tea leaves.4 Different strategies, methods, and techniques have been used for fabrication of self-standing products from graphene based nanomaterials for various electronics and other applications. This graphene based nanomaterials have also been proven to be efficient nanofillers for improvement of thermal and mechanical as well as surface properties of various polymeric composite materials. Nowadays, with the increase in demand of wearable electronics worldwide, research is also going on development of low cost and effective electronic textile (E-textile) materials. Received: August 30, 2017 Revised: September 25, 2017 Published: October 13, 2017 11645

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Figure 1. (a) Schematic representation of synthesis of carbon-dots from tea. (b) PL emission obtained with progressively longer excitation wavelengths from 380−500 nm of TCDs obtained from tea dispersed in 0.1 M acetic acid. (c) TEM image of TCDs. (inset) HRTEM of TCD. (d) DLS histogram of size for TCDs dispersed in 0.1 M acetic acid.

Conductive textile materials find application in a large number of electronic devices. They can be polymer based (using different conducting polymers), carbon based (graphene or carbon nanotube), or metal based. But to satisfy commercial demands, an E-textile must be conductive as well as strong, highly elastic, mechanically flexible, wearable, and lightweight.5 So, low cost, noncytotoxic, conductive graphene based materials have the highest possibility to become suitable candidates for fabrication of E-textile materials. Yun et al. reported the fabrication of a textile material based on nylon and cotton where the textile material was first coated with a protein layer; then it was again coated with GO followed by reduction with HI vapor.6 In this context we aimed for the fabrication of a self-standing flexible conducting textile material using reduced graphene oxide. We also report herein a very efficient reduction method for GO using nontoxic, low cost, and easy to synthesize carbon dots (CDs) prepared from “Assam CTC Tea” dispersed in 0.1 M acetic acid. CDs are a new age carbon based fluorescent nanoprobe. Their advantages like easy availability of the source material, easy synthetic techniques, high photoluminescence properties, noncytotoxicity, etc., make these CDs superior over semiconductor quantum dots and also a promising advanced nanomaterial with broad applicability. Properties of CDs depend entirely upon the source material. Moreover, the properties of CDs can also vary with size, surface functionalization, pH, etc. Demonstration on the application of CDs have been found in various fields like sensing, bioimaging, nanomedicine, photocatalysis, electro-catalysis, and so on.7−11 Tea is one of the major agro product of India and abundantly available in the northeastern part of the country. Reduction of GO can be accomplished within a very short interval of time using teaCDs (TCDs). Unlike other chemically reduced graphene oxides, the TCD-reduced graphene oxide (TCD-rGO) has very

good dispersibility in aqueous medium. The TCD-rGO was subsequently used for coating a compactly woven cotton fabric. This TCD-rGO coated cotton fabric material shows good electrical conductivity. Since the whole process avoids the use of any expensive, hazardous, or toxic chemicals, it is safe to use such textile materials even in direct contact with the human skin making them potential candidates for wearable electronic devices.

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EXPERIMENTAL SECTION

Materials Used. The Assam CTC Tea used here was purchased from local market with the brand name “Golaghat Tea”. Graphite nanopowder used in this work was purchased from SRL India. Other chemicals like H2SO4, HNO3, and acetic acid were purchased from Merck India and used as obtained. The 100% pure cotton fabric was obtained from Crystal Indus and Logistic Park, Ahmedabad, India. Preparation of Graphene Oxide (GO). Graphene oxide was synthesized by oxidation and exfoliation of commercially available graphite nanopowder following the method already reported by our group.1 In this simple and single-step method, 0.1 g of graphite nanopowder was dispersed in a 3:1 acid mixture (total 40 mL) of concentrated H2SO4 (98 wt %) and HNO3 (16 M). This mixture was then sonicated in a bath sonicator for 24 h and subsequently diluted. The graphene oxides formed were collected by centrifugation and washed with Milli-Q water several times to remove all the acids. Then, these graphene oxide nanoplatelets were again dispersed in the required amount of Milli-Q water and sonicated with an ultraprobe sonicator. Synthesis of Tea Carbon Dots. The commercially available Assam Tea (in the crush tear curl (CTC) form) was first heated at 100 °C for about 2 h, followed by grinding to powder form. This tea powder was again heated at 200 °C for about 8 h. The so formed black carbonized powder of tea was cooled to room temperature and stored in a glass vial. A quantity of 300 mg of the carbonized tea powder was dispersed in 10 mL of 0.1 M acetic acid and kept for 40 h. The dispersed medium was then centrifuged (at 10 000 rpm for 0.5 h), and the 11646

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Figure 2. (a) SEM image of the calcinated tea powder. (b) EDX spectra. (c) Histogram plot showing the percentage of different elements present in tea powder. supernatant liquid containing tea carbon dots was collected and preserved.7 Reduction of GO using Tea Carbon Dots. Reduction of graphene oxide was done by using TCDs solution. In a 250 mL round-bottom flask, 20 mL (2 mg/mL) GO dispersion was refluxed with 40 mL of TCDs in 0.1 M acetic acid solution at 90 °C using an oil bath for 2 h in the presence of N2 environment. The product was then collected by centrifugation and washed with Milli-Q water for several times to remove all the unused TCDs solution. Then this tea carbon dots (TCD) reduced graphene oxide (TCD-rGO) was redispersed in MilliQ water. Fabrication of rGO Coated Textile. A very compactly woven 100% cotton cloth was chosen for coating with TCD-rGO. The coating was done with the help of a sintered glass filter (used for membrane of diameter ≈47 mm). The cloth was fitted tightly and a 10 mL (1 mg/ mL) TCD-rGO dispersion was poured on it. A very smooth coating of TCD-rGOs formed on the surface of the textile. It is noteworthy to mention here that almost all the TCD-rGOs were retained by the compactly woven cotton cloth of our choice on its surface. When coating on one side was completed, the cloth was dried at 70 °C and then opposite side of the coated portion was coated repeating the same procedure. The coated fabric was then compressed using a lab scale manual compression molding machine. Characterization. Synthesized graphene oxides and reduced graphene oxides were characterized by Fourier transform infrared (FTIR) spectrometry (Nicolet 6700 FT-IR instrument) to investigate different types of interactions and functional group present. The size and zeta potential of tea carbon dots in 0.1 M acetic acid solution were evaluated using a Malvern Zetasizer NanoZS 90. Scanning electron microscopy (SEM) images and EDX datas were collected on a Carl Zeiss Sigma VP microscope. Transmission electron microscopic images were collected by a TEM (JEOL, Model: JEM 2100) instrument. Samples for TEM images were prepared on copper grid by drop casting a TCD dispersion in 0.1 M acetic acid solution. Powder X-ray diffraction (XRD) spectra were collected on a Bruker D8 Advance diffractometer. To analyze the thermostability of the coated textile material, thermogravimetric analysis (TGA) thermograms of fabrics were recorded with a PerkinElmer 4000 instrument, and the analysis was performed in the range of 35−800 °C at a heating rate of 10 °C/min and with a nitrogen flow rate of 20 mL/min. The fabrics were cut into small pieces and dried overnight in a vacuum oven before TGA was performed. The electrical measurements were recorded with a Keithley 6517B electrometer and with a Kaivo fourpoint probe resistance tester (dc).

TEM, FTIR, and dynamic light scattering (DLS) techniques. PL spectra show the fluorescence behavior of the TCDs. With the increase in the excitation wavelength, emission wavelength also increases which is a characteristic behavior of TCDs in some cases. From the TEM and DLS, the size of the TCDs was confirmed to be below 10 nm. DLS also gave the zeta potential of the TCDs to be −229 mV. Tea is a rich source of different types of polyphenolic compounds with high aromatic structures along with extensive carboxylate, hydroxyl, amine groups, etc.12−14 So, heating the CTC tea powders at 200 °C for 8 h during synthesis of TCDs may lead to some reduction of these pendant groups leaving a high content of aromatic groups in the final product. This heating step turns the color of the CTC tea powders from dark brown to black. EDX analysis shows the highest percentage of carbon in this calcinated tea powder followed by oxygen. EDX analysis also shows the presence of a small amount of nitrogen and traces of calcium and potassium in the calcined tea powder (Figure 2). The presence of CC groups in the TCDs is confirmed by the peak appearing at 1642 cm−1 in its FTIR spectra (Figure 3).12 It should be noted that

Figure 3. FTIR spectra of TCDs.

carbon dots consist of both sp2 and sp3 carbons.15 The FTIR spectra (peak at 3418 cm−1) also gives evidence for presence of hydroxyl groups in the CDs obtained from tea. Reduction of graphene oxide by TCDs was monitored with the help of UV−vis spectroscopy (Figure 4). Liao et al.16 suggested one SN2 mechanism by which polyphenolic compounds may reduce GO. The oxygen atom of the hydroxide group present in polyphenolic compounds attacks one carbon atom of the epoxide groups extensively present on the surface of GO (Scheme 1). Ultimately, the reaction leads to the formation of a double bond leaving a water molecule. It was observed in the UV−vis spectra that the peak arising at 298 nm in the graphene oxide which probably arises due to an nπ*

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RESULTS AND DISCUSSION Tea-CDs (TCDs) from commercially available Assam CTC Tea dispersed in 0.1 M acetic acid was synthesized by following a simple and standard protocol as described in the Experimental Section.12 Figure 1 is a pictorial representation of the protocol followed for synthesis of the TCDs. The TCDs were characterized using photoluminescent (PL) spectroscopy, 11647

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Figure 4. UV−visible spectroscopic study for monitoring the reduction reaction of graphene oxide (GO) by tea carbon dots.

transition seemed to disappear and a new peak appeared at 270 nm within 30 min from the starting point of the reaction. Disappearance of the peak at 298 nm and appearance of a new peak at 270 nm with higher intensity in the UV-spectra indicates a reduction of the polar functional groups initially linked on the surface of the graphene oxide. The peak arising at 270 nm is the characteristic of reduced graphene oxide already established by other reduction methods indicating the restoration of the electronic conjugation.1,3,4 Thus, UV−vis spectroscopy provides the direct evidence of completion of the reduction process by TCDs within a shorter interval of time compared to that of using green tea leaf extracts as reported earlier.4 The size effect of the reducing agent may be responsible for this faster reaction kinetics. We studied the reduction reaction up to a total of 2 h, but there was no observed change of the peak at 270 nm. Thus, we can ensure that the reaction for reduction of GO by TCDs goes to completion within 30 min. FTIR spectra (Figure 5a) of TCD-rGO confirms the reduction of GO showing the decrease in relative intensity of the peaks at 1383 and 1085 cm−1 ascribed to the carboxylate and epoxy functional groups present in GO. Moreover, the peak appearing at 1085 cm−1, corresponding to the C−O−C stretching frequency in GO, was not only reduced in intensity to a great extent but also shifted to 1051 cm−1 in the case of TCD-rGO. On the other hand, the intensity of the peak at 1640 cm−1 corresponding to the CC bonding frequency seemed to increase in TCD-rGO which also confirms the partial restoration of electronic conjugation. SEM images of the TCD-rGO shows adsorption of TCDs on the surface of TCD-

Figure 5. (a) FTIR and (b) XRD spectra of GO and TCD-rGO.

rGO which could not be removed even after washing with Milli-Q water for more than 5 times. Therefore, the peaks with reduced intensity, corresponding to the hydroxyl as well as for ether (C−O−C) groups in the FTIR spectra of TCD-rGO might be attributed to the attached TCDs.12,17 Powder XRD spectra (Figure 5b) serves as more convincing evidence for the reduction of GO by the TCDs. While GO has its characteristic peak at 2θ ≈ 7.4° with high intensity, in the case of TCD-rGO, the peak at 2θ ≈ 26.3° appears with higher intensity which complies with the previous reports.17 It is noteworthy to mention here that unlike the other chemically reduced graphene oxides (rGOs), the TCD reduced

Scheme 1. Reduction of Graphene Oxide by TCDs

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ACS Sustainable Chemistry & Engineering Scheme 2. Fabrication of TCD-rGO Coated Conducting Fabric

GO (TCD-rGOs) has very good dispersibility in aqueous medium. It is already mentioned that the chemically reduced graphene oxides without any stabilizing agents generally have low dispersibility in aqueous medium as they have the tendency to get agglomerated due to increased hydrophobicity in the absence of any stabilizing or capping agent. The adsorbed TCDs on the surface of the TCD-rGOs may be responsible for stabilization of TCD-rGOs dispersion preventing the agglomeration of reduced graphene oxide sheets.2,3 A compactly woven cotton cloth was coated by TCD-rGOs using a sintered glass filter. Scheme 2 presents the pictorial representation of fabrication of TCD-rGO coated conducting fabric. This method of coating not only facilitates to get a uniform coating over the textile material but also helps to have control over the coating thickness. This particular technique unlike the dip-coating technique also prevents the wastage of rGOs.5,6 As evident from the FTIR spectra of TCDs, the residual polar groups present in the TCDs might also be contributing in the interaction between the TCD-rGOs and the cotton fabric and hence helps in the formation of a stable coating without using any other costly compatibilizing agent. Literature also reveals that tea extract has the ability to dye cotton and other fabric materials.18−20 We thus obtained a stable and compact coating on the surface of the cotton fabric. The compactness of the coating was further increased by compressing the coated portion of the fabric with the help of a lab scale polymer press. Thermostability of the rGO Coated Cotton Cloth. The thermostability of the TCD-rGO coated fabric was evaluated with the help of a thermogravimetric analyzer (TGA). Figure 6 represents the TGA thermograms of the cotton fabric and the TCD-rGO coated fabric. The thermograms showed that the bare cotton fabric which is entirely of cellulosic structure starts to degrade at 296 °C. But in the case of the TCD-rGO coated fabric, the degradation started at around 326 °C, much higher than the uncoated cotton fabric. Hence, the thermostability of the TCD-rGO coated textile was found to be higher compared to that of the bare cotton fabric. This improvement in thermostabilty of the TCD-rGO coated fabric is obvious as the TCDs-rGO contains high amount of thermostable aromatic moieties which work as a thermal barrier for the cotton fabric.12,17 This increases the initial degradation temperature for the TCD-rGO coated fabric compared to that of the exposed uncoated cotton fabric.

Figure 6. TGA thermograms of bare and TCD-rGO coated cotton fabric. Using TCD-rGOs, we could successfully coat a cotton based fabric.

Electrical Properties of the TCD-rGO Coated Cotton Fabric. The electrical properties of the TCD-rGO coated cotton fabric were investigated, and the measurements were carried out at room temperature and at relative humidity of 60%. The sheet resistance (RS with units of Ohms per square) was measured directly with the help of a four-probe electrical resistance meter and found to be 229 ± 20 Ω/sq. Using the measured value of sheet resistance, the electrical conductivity of the material was determined using the formula: 1/(RSt), where t (μm) is the thickness of the rGO coating. The conductivity of rGO coated fabric was found to be 623 ± 54 S/m. This conductivity value is comparable with the already reported conductivity values for similar type of carbon or graphitic based textile and fabric materials reported earlier. Table 1 represents the electrical conductivity values of some advanced conducting textile material reported earlier. Here it is to be noted that as the cotton fabric is nonconducting, so the conductivity of the conducting textile material is entirely of TCD-rGO coating. To validate the electrical conductivity of the TCD-rGO coating on the surface of cotton fabric, we separately fabricated a selfstanding TCD-rGO film using a sintered glass filter and cellulose nitrate filter paper.21 The conductivity of this selfstanding film was found to be 578 ± 50 S/m. The little higher conductivity of the fabricated conducting textile material is may be because of the compression during its fabrication process leading to increase in the compactness of the coating. Coating a cotton fabric with TCD-rGO and thus making a conducting 11649

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agent for GO until date. Moreover, earlier reports show that for fabrication of conducting papers or textiles, they are first loaded or coated with graphene oxide (GO) to avoid the dispersibility problem of reduced graphene oxides and then the materials were made conducting by reducing with different chemical reducing agents. But use of such toxic chemicals is not preferable to apply directly on wearable textile. Considering all these issues, we have tried to provide a method for fabrication of a reduced graphene oxide based conducting wearable fabric without use of any hazardous chemicals. Figure 7a presents the current−voltage (I−V) characteristics of the TCD-rGO coated conducting cotton fabric. A linear I−V characteristic graph was observed indicating ohmic behavior. The corresponding resistance was calculated to be 68.80 Ω (at 0.6 V). The low resistance value obtained for the TCD-rGO coated cotton fabric evidenced the enhanced conductivity of the material which is well consistent from the four-probe measurements as discussed above. Additionally, in order to be fit for practical applications, it is important to check the electrical stability of the TCD-rGO coated cotton fabric. Figure 7b shows the plot of current versus time, to check the electrical stability of the material at ambient conditions. The current values of the TCD-rGO coated cotton fabric remains almost the same after 1000 s which shows a good electrical stability of the material. It is worth mentioning that a small variation in the current values were observed which is mainly due to the heating effect generated on application of a fixed bias of 0.5 V for longer duration. A stripe from the conducting fabric was used in a circuit to test the practical viability of the material. As seen from the images (Figure 7c, d), a series of LEDs could be illuminated while connecting the TCD-rGO coated fabric in a closed circuit.

Table 1. Description and Electrical Properties of Some Advanced Conducting Textile Materials Reported so Far ref. no.

component responsible for conductivity

22

poly(3,4ethylenedioxythiophene) (PEDOT) SWNT

23

24

SWNT

6 25

rGO (reduced with HI) rGO (reduced with HI)

our material

description of the material electrically conducting textiles (PET and nylon based polymeric material) carbon nanotube/fabric composites (sprayed technique) (incubation technique) (quasi-Langmuir−Blodgett transfer technique) stretchable, porous, and conductive energy textiles rGO coated fabric rGO-chitosan-Au nanoparticle-cellulose composite paper TCD-rGO coated fabric

maximum conductivity (S/m) 200

533 800 1380 12500 1232 853.4

623

textile not only reduces the cost of the ultimate product but also enables good flexibility. It is relevant to mention here that chemically reduced GO sheets lead to the formation of irreversible agglomerates unless the capping agents (polymers or surfactants etc.) are used, which can greatly hamper the electrical as well as other properties of reduced graphene oxides. But on the other hand, it has been reported that molecules possessing the π-staking ability when interact with GO, it leads to improvement of conductivity of GO.26 TCDs on the surface of TCD-rGOs possessing CC groups (evident from FTIR of TCDs) may also have the possibility to contribute to its electrical property or they may have less chance to hamper the conductivity of reduced GO. Importantly, the conductivity value of the TCDrGOs coated fabric is comparable to that of the HI reduced GO coated fabric. HI is reported to be the most efficient reducing

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CONCLUSIONS In conclusion, we demonstrated the successful reduction of graphene oxide in to reduced graphene oxide with good

Figure 7. (a) I−V characteristics and (b) stability determination of the TCD-rGO coated conducting fabric. Demonstration of successful working of the conducting fabric in an electronic circuit both in normal (a) and twisted (b) state. 11650

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(11) Liu, S.; Tian, J.; Wang, L.; Zhang, Y.; Qin, X.; Luo, Y.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Hydrothermal Treatment of Grass: A lowcost, green route to nitrogen-doped, carbon-rich, photoluminescent polymer nanodots as an effective fluorescent sensing platform for label-free detection of Cu(II) ions. Adv. Mater. 2012, 24, 2037−2041. (12) Konwar, A.; Gogoi, N.; Majumdar, G.; Chowdhury, D. Green chitosan−carbon dots nanocomposite hydrogel film with superior properties. Carbohydr. Polym. 2015, 115, 238−245. (13) Wheeler, D. S.; Wheeler, W. J. The medicinal chemistry of tea. Drug Dev. Res. 2004, 61, 45−65. (14) Balentine, D. A.; Wiseman, S. A.; Bouwens, L. C. M. The chemistry of tea flavonoids. Crit. Rev. Food Sci. Nutr. 1997, 37, 693− 704. (15) Georgakilas, V.; Perman, J. A.; Tucek, J.; Zboril, R. Broad family of carbon nanoallotropes: classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene, nanodiamonds, and combined superstructures. Chem. Rev. 2015, 115, 4744−4822. (16) Liao, R.; Tang, Z.; Lei, Y.; Guo, B. Polyphenol-reduced graphene oxide: mechanism and derivatization. J. Phys. Chem. C 2011, 115, 20740−20746. (17) Konwar, A.; Kalita, S.; Kotoky, J.; Chowdhury, D. Chitosan− iron oxide coated graphene oxide nanocomposite hydrogel: a robust and soft antimicrobial biofilm. ACS Appl. Mater. Interfaces 2016, 8, 20625−20634. (18) Kim, S. Dyeing characteristics and uv protection property of green tea dyed cotton fabrics-focusing on the effect of chitosan mordanting condition. Fibers Polym. 2006, 7, 255−261. (19) Deo, H. T.; Desai, B. K. Dyeing of cotton and jute with tea as a natural dye. Color. Technol. 1999, 115, 224−227. (20) Bechtold, T.; Turcanu, A.; Ganglberger, E.; Geissler, S. Natural dyes in modern textile dyehouses -how to combine experiences of two centuries to meet the demands of the future? J. Cleaner Prod. 2003, 11, 499−509. (21) Bi, H.; Chen, J.; Zhao, W.; Sun, S.; Tang, Y.; Lin, T.; Huang, F.; Zhou, X.; Xie, X.; Jiang, M. Highly conductive, free-standing and flexible graphene papers for energy conversion and storage devices. RSC Adv. 2013, 3, 8454−8460. (22) Hong, K. H.; Oh, K. W.; Kang, T. J. preparation and properties of electrically conducting textiles by in situ polymerization of Poly(3,4ethylenedioxythiophene). J. Appl. Polym. Sci. 2005, 97, 1326−1332. (23) Hecht, D. S.; Hu, L.; Grüner, G. Electronic properties of carbon nanotube/fabric composites. Curr. Appl. Phys. 2007, 7, 60−63. (24) Hu, L.; Pasta, M.; La Mantia, F. L.; Cui, L.; Jeong, S. Stretchable, Porous, and Conductive Energy Textiles. Nano Lett. 2010, 10, 708− 714. (25) Ling, Y.; Li, X.; Zhou, S.; Wang, X.; Sun, R. Multifunctional cellulosic paper based on quaternized chitosan and gold nanoparticle− reduced graphene oxide via electrostatic self-assembly. J. Mater. Chem. A 2015, 3, 7422−7428. (26) Zhang, Z.; Huang, H.; Yang, X.; Zang, L. Tailoring electronic properties of graphene by π―π stacking with aromatic molecules. J. Phys. Chem. Lett. 2011, 2, 2897−2905.

dispersibility following a simple and green approach. The reduction was achieved within a very short interval of time and confirmed from the UV−vis and FTIR spectroscopy. SEM images and the FTIR spectra evidenced the attachment of carbon dots on the surface of reduced graphene oxide. This TCD-rGO was then utilized for successfully coating the surface of a cotton fabric without using any other compatibilizing agent. The coating by tea carbon dots reduced graphene oxide also improved the thermostability of the fabric significantly. The electrical conductivity of this fabric material is found to be comparable to that of the other reported advanced conducting textiles. Finally, the practical use of the conducting fabric was shown by using it in a close circuit to light a series of LEDs. This approach eliminates the conventional use of toxic chemicals and other high cost synthetic materials for fabrication of conducting textiles. Thus, such TCD-rGO is found to be a suitable and effective coating material for fabrication of flexible, wearable, E-textile materials with diverse potential applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91 361 2279909. Tel.: +91 361 2912073. ORCID

Devasish Chowdhury: 0000-0003-4829-6210 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.C. thanks SERB, New Delhi, India (Grant SB/S1/PC-69/ 2012), and BRNS, Mumbai, India (Grant 34/14/20/2014BRNS), for funding. A.K. acknowledges CSIR for a fellowship.

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REFERENCES

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