Deep Eutectic Solvent Functionalized Graphene Composite as an

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The Deep Eutectic Solvent Functionalized Graphene Composite as an Extremely High-Potent Flame Retardant Dattatray Appasha Pethsangave, Rahul Vijay Khose, Pravin Harishchandra Wadekar, and Surajit Some ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09587 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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ACS Applied Materials & Interfaces

The Deep Eutectic Solvent Functionalized Graphene Composite as an Extremely High-Potent Flame Retardant Dattatray A. Pethsangavea, Rahul V. Khosea, Pravin H. Wadekar a, Surajit Somea* a. Department of Dyestuff Technology, Institute of Chemical Technology, Matunga, Mumbai‐400 019, India. KEYWORDS: Choline chloride, flame retardant, graphene oxide, monosodium dihydrogen orthophosphate and phos‐ phorous. ABSTRACT: We report a simple and green approach to develop the deep eutectic solvent functionalized graphene deriva‐ tive, as an effective flame retardant. The deep eutectic solvent functionalized graphene oxide (DESGO) was synthesized by introducing nitrogen supported phosphorus functional groups on the surface of graphene derivative via a deep eutectic solvent, which is prepared by the treatment of monosodium dihydrogen orthophosphate and choline chloride. Subse‐ quently, the resultant DESGO material is characterized by X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FT‐IR), Raman spectroscopy, thermo gravimetric analysis (TGA) and scanning electron microscopy (SEM). The as prepared DESGO solution coated cloth piece was sustaining its initial shape and size by releasing little amount of smoke at the early stage without catching fire for more than 540 s (9 minutes), whereas the pristine cloth is totally burned out within 10 s by leaving small amounts of black mass. This simple method of direct functionalized of deep eutectic solvent on the graphene oxide surface can be a common process for the cost effec‐ tive bulk production of nano carbon template for extremely high‐potential, nontoxic flame retardant applications.

Introduction: Fire is the major revolutionary discovery that has led the entire human life into a new era of comfort zone. Fire has killed more people than all other natural calamities. Every coin has two sides; likewise, fire has also got both positive and negative impact on human life. Therefore, fire can also lead to major calamities if not controlled properly. Fire related deaths and property loss are key problem. The main role of fire related research is the de‐ velopment of highly effective material that protects hu‐ man life and society. The establishment of new materials as flame retardant that can reduce the fire risk should meet various safety merits to reduce the effect on the en‐ vironment, and healthy human life is very challenging. Generally, cotton is known as the most important natural textile fibers used to manufacture clothes, home decora‐ tions and industrial products; how‐ever, this cellulosic material is highly flammable. Numerous approaches have been used to improve the combustion characteristics of textiles, including cotton.1 The halogenated flame retard‐ ants and boron‐based flame retardant have been elimi‐ nated as they produce toxic gases due to burning that can be harmful to human life and the entire environment.2 Durable phosphorus‐containing materials have secured attention due to their effectiveness such as resisting re‐ peated wash cycles,3 decreasing the volatile of fuels, re‐ ducing the pyrolysis temperature, enlarging the carbona‐ ceous char, and reducing the afterglow.4 In recent years, graphene, a two dimensional structure consisting of sp2

carbon atoms arranged in a hexagonal lattice, has a great deal of attention from researchers from varied fields of physics, chemistry, materials science, energy, and bio‐ chemistry due to its high surface area, good electronic, mechanical, and thermal properties.5‐10 Furthermore, it is the most thermodynamically stable allotrope of carbon; therefore, it is difficult to integrate heteroatoms into car‐ bon back bone via different approaches.11 However recent‐ ly, many researchers12 have revealed the possible approach of fabricating heteroatom on the carbon back bone of graphene from graphene oxides (GOs).13 Recently, re‐ searchers have reported the synthesis of phosphorus doped graphene oxide (PGO)14 and graphene phosphonic acid (GPA)15 as efficient fire retardant materials. The re‐ quirement remains for highly efficient materials, which will be easy to fabricate, environmentally friendly, mild, and cost effective. In the recent year also, ionic liquids (ILs) are continuing as new “green” solvents because of their unique physicochemical properties.16,17 An advance type of IL analogues is interpreted as deep eutectic sol‐ vent (DES). It has been observed that DESs have compa‐ rable behavior with traditional ILs.18 DESs perform as a replacement to standard fluids and regular ILs due to the nonreactivity with water, biocompatible, and biodegrada‐ bility properties.18 Therefore, it is worthwhile to use DES to prepare nanomaterial to serve as an extremely high‐ potent flame retardant. For the first time, we have de‐ signed a simple and green approach to develop the deep eutectic solvent functionalized graphene oxide (DESGO)

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as an extremely high‐potent flame retardant. Therefore, our as made graphene‐supported DES composite is easy to synthesize, cost‐effective and environment friendly. It is studied that GO has oxygen rich functional active sites with high surface area, which is easy to synthesize and has low toxicity potential.19 The presence of large number of oxygen containing functional groups such as epoxy, carbonyl, carboxylic acid and hydroxyl groups of gra‐ phene oxide are capable to activate the formation of intra and inter hydrogen bonding with active counterpart.20 On the account of such Properties, we have applied a DES on the surface of as made GO to introduce active Phosphorus functional groups. We have synthesized this novel DES by applying choline Chloride and monosodium dihy‐ drogengen orthophosphate (Scheme 1). DESGO was pre‐ pared by the treatment of as made DES in addition of GO followed by heating at 80 °C for 8 h. The as prepared ma‐ terial DESGO showed high amount of phosphorous func‐ tionalization. The as prepared material is used in flame retardant experiment.

Scheme 1 Synthesis of Highly Efficient Flame Retardant (DESGO). Experimental Section Material and Method: Graphite powder, potassium permanganate, sulphuric acid, sodium nitrite, hydrogen peroxide, choline chloride and monosodium dihydrogen orthophosphate are all pur‐ chase from Sigma Aldrich without further purification. Preparation of graphene oxides: GO were prepared from natural graphite powder by us‐ ing a modified Hummer’s method.22 Synthesis of deep eutectic solvents functionalized gra‐ phene oxide (DESGO): Monosodium dihydrogen orthophosphate of 5.5 gm. were mixed with 5 gm. choline chloride and was stirred overnight at 80°C. The resulting DES solution is filtered out (~ 5.5g). Then 1g of filtrate DES and 200 mg of GO was mixed (weight ratio 5:1); this mixture was kept at 80°C for 8 h to get the resulting DESGO nanocomposite. The as‐

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made DESGO nanocomposite was washed with DI water, and the resultant was dried under vacuum (~ 0.980g). Preparation of DESGO and GO coated with cloths: The cloth was cut into 5.5 × 9.5 cm pieces. The cloth samples were coated with DESGO solution and GO solu‐ tions dried in air for 2 h and kept in vacuum oven at 60°C overnight, followed by a flame‐retardant experiment. Result and Discussion: The as prepared materials are characterized by X‐ray photo electronic spectroscopy (XPS), Fourier transform (FT‐IR), X‐ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The as prepared materials are ana‐ lyzed by XPS spectroscopy to investigate that chemical composition of materials. The as prepared GO contained a low C/O ratio of 2.02 (Figure 1a, Table S1). The subse‐ quent DESGO contained a C/O ratio of 0.96, which demonstrated that it also has a high amount of oxygen‐ containing functional groups. In comparison with the initial material GO (Figure S1), which gives a rise to no‐ ticeable C1s and O1s peaks at 282.6 and 530.5 eV, respec‐ tively (Figure 1b and c),21,22 DESGO provided a rise to characteristic P2s (Figure S2) and P2p peaks at nearby 191.1 and 134.0 eV, respectively (Figure 1e), besides the strong C1s and O1s peaks. The C1s XPS spectrum of the DESGO showed a sharp peak at 284.8eV, which corresponds to the C‐C bonds of carbon atoms in the structure. The peak at 286.2 eV could be recognized to different C‐O, C‐P and C‐N bonds, whereas the peaks at 288.8 eV could be indicated to different car‐ bonyls C=O configurations due to the harsh oxidation and the destruction of the sp2 atomic structure of original graphite (Figure 1b). 15, 22‐23 Peaks at 531.1 and 532.7 eV in the O1s XPS spectrum of the DESGO correspond to C=O, P=O and C‐O, P‐O, respectively (Figure 1c). The peak at 400.0 eV and in 402.5 eV in the N1s XPS spectrum of DESGO could be recognized to C‐N and quaternary ni‐ trogen atom (Figure 1d).24 XPS spectra with curve fittings for the C1s (Figure 1b), O1s (Figure 1c), N1s (Figure 1d), P2p (Figure 1e), and P2s (Figure S2) peaks determine that the aromatic C‐P, P‐O, C‐N and P=O bonds are the noticeable P and N components in DESGO, and it confirmed the presence of the high amount of phosphorus functional groups. The FT‐IR spectra of GO and DESGO are shown in Figure 1f. The following peaks observed in the FT‐IR spectrum of GO. The peak at 3424 cm−1 indicates the broad vibration of O‐H band. The peak appears at 1728 and 1623 cm−1 confirmed the presence of C=O stretching of acid and C=C stretching vibration of aromatic carbon atom, respectively. Peaks appeared at 1412 and 1053 cm−1, indicating for C‐H bends and C‐O stretching, respectively. 23 The FT‐IR spectrum of DESGO confirmed the presence of strong aromatic P=O, C‐P, P‐O, P‐OH, and P‐O (bond rocking), stretching peak, which appeared at 1166, 1147, 1017, 941 and 491 cm‐1 respectively. 15 Peaks at 3500 cm −1 indicates the O‐H stretching, 1728 and 1623 cm−1 indicate the presence C=O group and C=C stretching vibration.


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The FT‐IR spectra of choline chloride and DES are shown in (Figure S3). The peak appears at 3230, 1482, 1056 and 951 cm‐1 confirmed the presence of O‐H stretching, CH3 rocking vibration, CH2 stretching and the C‐C‐O stretch‐ ing of cholinium ion, repectively. 25 The FT‐IR spectra of DES confirmed the presence of P=O, P‐O, P‐OH and P‐O (rocking) which appeared at 1159, 1046, 942 and 501 cm‐1 respectively.14, 15 After flame test of GO, DESGO, DES coated cloths and blank cloth, char residue of each was obtained and their FT‐IR spectra were recorded (Figure S8). Raman spectroscopy was applied to evaluate the quality of the as prepared material to analyze the struc‐ ture and quality of carbon nanomaterial, mainly to de‐ termine defects and the structural orientation of nano‐ material. As shown in Figure 2a, the G‐band of GO peak at 1605 cm−1 shifted from the same of graphite at 1585 cm−1, which is suggested that conjugated double bonds that resonates the frequencies greater than that of G‐band of graphite.22 The as prepared DESGO material showed that the G‐band at 1621 cm−1, which is suggesting that the insertion of nitrogen based phosphorous and oxygen functionalized groups into the graphite structure of car‐ bon atoms with disorder (Figure 2a).

Figure 1. (a) XPS survey spectra of GO and DESGO; (b) XPS spectra of DESGO C 1s ; (c) XPS spectra of DESGO O 1s ; (d) XPS spectra of DESGO N 1s ; (e) XPS spectra of P 2p; (f) IR spectra of GO and DESGO.

The ID/IG ratio of DESGO (1.02:1) increased in compar‐ ison to that of only GO (0.90:1), which represents that more structural defects occurred in contrast to GO struc‐ ture,22 suggesting the successive functionalization of ni‐ trogen‐based phosphorous and oxygen groups. Whereas no D‐band was observed in the case of graphite.14 Ther‐ mogravimetric analysis (TGA) of the GO and DESGO was used to establish the weight loss of them. Both the sam‐ ples were heated under an air atmosphere from room temperature to 600°C at a heating rate of 20°C min−1 The as‐made GO sample showed a low thermal strength under the air atmosphere as shown in the TGA curve. The major weight loss was in between 100 and 200°C, identifying the release of CO2 and vapor from the most labile functional groups during the pyrolysis.

Figure 2. (a) Raman spectra of GO and DESGO; (b) TGA of GO and DESGO; and (c) XRD spectra of GO, DESGO and graphite.

At temperature 600°C, the total weight loss was 80% which was due to the thermal decomposition of carbon skeleton.26 In contrast, TGA analysis of DESGO also show stepwise weight loss at close temperatures 100°C, 220– 260°C, and 580°C, respectively (Figure 2b). The initial weight loss ~100°C is due to thermally induced loss of bound water, which indicated the hygroscopic nature of the as‐made material. The pyrolysis of the wide range of labile hetero atom (N, P, and O) containing functional



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groups was attributed to the weight loss at ~ 220–260°C. 23 Third step weight loss at ~ 580°C was assigned to the thermal condensation of phosphonic acid (Figure 2b) and the degradation of the graphitic structure in air.22 Ther‐ mal stability of GO, DES, DESGO coated cloth and blank cloth have also measured (Figure S9). The interlayer dis‐ tance in GO and DESGO were confirmed by using XRD pattern. The 2θ peak of graphite powder was observed at 26.71°, which indicates that the interlayer distance of graphite is 3.34 Å (Figure 2c).23 The as prepared GO that produced a 2θ peak at 11.42° represents the interlayer dis‐ tance as 7.25 Å, which interprets that the interlayer space increased via the full oxidation of graphite layers into GO (Figure 2c).23 The XRD pattern of the as prepared DESGO showed only single peak at 7.26°, which demonstrates an interlayer distance of ~12 Å (Figure 2c).The change of 2θ peak position of GO (11.42°) in comparison with DESGO (7.26°) demonstrates that the interlayer distance of DESGO increased due to DES functionalized into GO.

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showed a distinct morphology in comparison to GO (Fig‐ ure 3a‐d). SEM images of DESGO coated cloths were ob‐ served, before and after flame retarding test (Figure 3a– 3d). The image of the DESGO coated cloth before the flame test reveals the presence of DESGO cloths surface (Figure 3a, 3c). After the flame test, DESGO‐coated cloth preserve the textile fiber morphology and structure, which was prevented by the formation of shielding layer of DESGO on the cloth surface (Figure 3b, 3d). Elemental mapping was performed to exhibit the distribution of phosphorus atom on the DESGO surface. Follow‐ing Car‐ bon (Green), Nitrogen (Blue), Oxygen (Grey), and phos‐ phorous (Red) atoms, uniform distribution on the cloth surface was confirmed from the SEM. SEM images of GO coated cloth before and after flame test have shown in Figure S7.

Figure 4. Snapshots of the flame retardant test of the GO coated cloth (left) with and blank cloth (right) with respect to time from (a‐c) 0‐10 s. Snapshots of the flame retardant test of the DES coated cloth (left) with and blank cloth (right) with respect to time (d‐f) 0‐35 s. Snap‐ shot of the flame retardant test of DESGO coated cloth (left) with and only cloth (right) with respect to time from (g‐i) 0‐540 s.

Figure 3. SEM Images of DESGO/Cloth (a) before fire and (b) after fire retardant experiment respectively. Mag‐ nified SEM Images of DESGO on cloth (c) before and (d) after. Corresponding EDX element mapping (e) before (f) after of carbon (green), oxygen (grey), phosphorous (red) and nitrogen (blue). Scanning electron microscopy (SEM) was applied to study the overall surface morphology of the as‐prepared materials. In case of GO image, we observed the thin and wrinkled sheet (Figure S4).22 The as prepared DESGO

The possible mechanism of insertion of phosphorus supported functional groups on graphene surface is demonstrated in Figure S5.14 After the structural confir‐ mation of DESGO, the material was dispersed in DI‐water (1 mg/ml).(Figure S6) The as‐made DESGO ink was used for further experiment of flame retardant test. For the preparation of the flame‐retardation sample, DESGO ink was used to coat a piece of cloth and dried under vacuum oven at 60°C overnight, while control cloth sample re‐ main was not coated (right image in Figure 4). The load‐ ing of DESGO with respect to the mass of the cloth was approximately 5.54 wt. %. (See Table S2). We have meas‐ ured the tensile strength of DESGO coated cloth with loading (1‐6%) in comparison to blank cloth (Figure S 10) The control cloth sample and DESGO‐coated cloth (DESGO/cloth) were tested with same time in air with ethanol flame (Figure 4 g‐i and video in supporting in‐ formation). The control cloth sample captured fire with in 5 s and was completely burned within 10 s to convert into small black mass (right images in Figure 4 a‐i). Subse‐ quently, the DESGO coated cloth initial emitted little


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smoke without catching fire up to >540 s (9 minutes) and maintained almost in its initial form (Figure 4 g‐i). Whereas in case of DES coated cloth has caught fire with‐ in ~35 s and has developed little self‐extinguishing prop‐ erty and on the other hand GO coated cloth experiment caught fire within 5 s (Figure 4 a‐f) as control experi‐ ments. Therefore, DESGO is a better application in com‐ parison to only GO and other reported materials till date as a flame retardant on the basis of lighting time (See supporting Information Table S3). The DESGO coated cloth confirms the excellent flame retardation property in comparison to previously reported methods on the basis of lighting time. We have carried out washing fastness of DESGO/cloth and DESGO cloth/Binder before and after washing % weight loss as shown in Table S5. The probable retardation mechanism was suggested under the impres‐ sion of the TGA and other data (Scheme 1). As per the TGA data, the initial little smoke was emitted from the steam by the physically trapped moisture and the by‐ products formation by the condensation of phosphorus functional groups and also the thermal decomposition of DESGO. The temperature range from ~220 to 260°C could be as‐signed to hetero atom (N, P, and O) containing functional derivatives. 15 The graphitic part was a major ingredient of above 500°C. Hence, physical grounds for this retardation are associated to the cooling of the exter‐ nal burning due to endothermic reactions and vaporiza‐ tion. The % weight loss at different temperature for blank cloth, GO/cloth, DES/cloth and DESGO/cloth were rec‐ orded from TGA data as shown in Table .S7 and S8. Chemical ground retardation is associated to the thermal condensation of phosphorus functional groups into char formation on the external surface of the cloth. Before the flame treatment, the DESGO/cloth sample displayed thread morphology of the cloth with DESGO coating (see the Figure 3a). We have calculated DTA data from TGA data to explain the behavior our materials in detail Figure S11. The DESGO/cloth also maintained the thread mor‐ phology after the flame treatment by the formation of a preventive layer on the outside of the cloth (see Figure 3b). The DSC data of blank cloth, GO/cloth, DES/cloth and DESGO/cloth have indicated that these are exother‐ mic reactions as shown in Figure S12 and Table S9.In con‐ trast, the SEM images of the DESGO/cloth (Figure 3a, and b) before and after flame treatment and the flame‐ treated DESGO/cloth still maintained thread textures. Neverthe‐ less, in the case of the GO/cloth, no thread morphology was noticed after the flame treatment (See Figure S7. Standard method limiting oxygen index (LOI) test is also used to evaluate the thermal stability of GO, DES, DESGO coated cloths and blank cloth. The LOI values of GO, DES, DESGO coated cloths and blank cloth have listed in Table S4.34, 35 The vertical flammability test with excellent result and details flammability data have been reported in Table S6. Conclusions: In conclusion, we have developed a simple, easy and a green approach to functionalized novel effective DES into

the GO surface. The as made DES is functioned into the GO surface via hydrogen bonding. The as made DES is acting as a resource of phosphorus fictionalization, which helps to introduced phosphorus functional group on the back bone as well as on the surface of graphene compo‐ site. The as prepared DESGO coated with cloth has shown utmost efficiency in the flame‐retardant experiment. At the initial stage, DESGO coated cloth emitted little smoke at the starting point without grabbing fire for more than >540 s (9 minutes) maintaining its primary form with slight contraction, whereas the only cloth caught fire within 5 s and completely burned out within 10 s, fol‐ lowed by producing small amounts of black mass. The sample of cloth coated with solution of DESGO showed excellent flame retardation properties. The DESGO coat‐ ed cloth confirms the excellent flame retardation property in comparison to previously reported methods on the basis of lighting time. The direct functionalization of DES into GO to produce graphene supported nano composite by using a simple and green approach, which is a high‐ potent for flame retardation experiment. ASSOCIATED CONTENT Supporting Information AUTHOR INFORMATION Corresponding Author * Dr. S. Some E‐mail: [email protected]. Present Addresses Department of Dyestuff Technology, Institute of Chemical Technology Matunga, Mumbai‐400 019, India. Author Contributions D.A.P.and S.S. wrote the manuscript, D.A.P. performed experiment, D.A.P., R.V.K. and P.H.W. prepared graphene oxide and nanocompsite.D.A.P. and R.V.K. analysed all data, S.S.supervised all work. Funding Sources UGC Start‐Up‐ Grant (No.F.4‐5/2006 (BSR)), BRNS fund (34/14/14/2015/BRNS) and DST fund (YSS/2015/00078). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by BRNS fund (34/14/14/2015/BRNS), DST fund (YSS/2015/00078) and UGC Start‐Up‐ Grant (No.F.4‐5/2006 (BSR)). D.A.P. is thankful to UGC‐SAP for providing fellowship. ABBREVIATIONS GO, graphene oxide; DESGO, deep eutectic solvent functionalized graphene oxide; DES, deep eutectic sol‐ vent.



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SYNOPSIS TOC (Word Style “SN Synopsis TOC”). In this work, we have designed a simple and green approach to develop the deep eutectic solvent functionalized graphene oxide (DESGO) by introducing nitrogen supported phosphorous functional groups on the surface of gra‐ phene derivative as a high‐potent flame retardant.


Deep Eutectic Solvent Functionalized Graphene Composite as an

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