Chemical Exfoliation of Layered Magnesium Diboride To Yield

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Chemical Exfoliation of Layered Magnesium Diboride to Yield Functionalized Nanosheets and Nanoaccordions for Potential Flame Retardant Applications Saroj Kumar Das, and Kabeer Jasuja ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00101 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Applied Nano Materials

Chemical Exfoliation of Layered Magnesium Diboride to Yield Functionalized Nanosheets and Nanoaccordions for Potential Flame Retardant Applications Saroj Kumar Das and Kabeer Jasuja* Discipline of Chemical Engineering, Indian Institute of Technology Gandhinagar, Gujarat 382355, India *Corresponding author: [email protected] Abstract: Metal borides are known for their extraordinarily rich chemistry and extensive range of properties. Although their diversity and chemically tunable properties create abundant possibilities for several applications, their potential has not been fully realized. This is because metal borides have been primarily investigated in their bulk form. In this work, we present a chemical method to nanoscale MgB2, a representative from the family of layered metal diborides that comprise metal atoms sandwiched in between boron honeycomb planes. Their lattice structure offers a unique opportunity to obtain access to graphenic planes of boron upon exfoliation. We show that a treatment of MgB2 with acid, followed by intercalation with organoammonium ions swells the crystals, and partly delaminates these to multi-layer-thick (~300-400 nm) lamellas, which resemble the shape of accordions. These nanoaccordions can be sonicated in water to yield few-layer-thick (~3-5 nm) nanosheets. These nanostructures are found to be Mg-deficient and functionalized with oxygen-based moieties. We also present a preliminary study to demonstrate that these oxyfunctionalized nanostructures have the potential to be utilized as flame retardant nanofillers. The thermogravimetric analysis reveals that a composite of epoxy resin with sonicated nanoaccordions exhibit an effective char residue gain of 6.0%. Furthermore, by adding only 2% sonicated nanoaccordions, the LOI value of epoxy was found to increase from 20.1 to 22.5, while the burning rate (determined by UL-94 horizontal burning test) decreased significantly from 31.7 mm/min to 15.1 mm/min. These flame retardant enhancement metrics are not only superior when compared with graphene and its analogs, but are also at the extremes of other flame retardant nanofillers at similar loadings. The ability to exfoliate a layered metal boride to obtain chemically functionalized nanoaccordions with flame retardant properties presents an unprecedented perspective to utilize MgB2 and showcases the rich prospects offered by the family of layered metal diborides. Keywords: Magnesium diboride, nanosheets, nanoaccordions, polymer nanocomposite, flame retardancy, Exfoliation

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1. Introduction Layered inorganic materials have generated renewed interest in the past decade on account of their ability to yield single and few-layer-thick planar nanostructures, which exhibit a rich palette of useful properties.1 Most of these layered materials are amenable to exfoliation because the layers are weakly held together by Van der Waals forces.1-4 It is gradually being evidenced that several inorganic layered materials, in which layers are held together by stronger forces, can also be exfoliated.5,6 For example, in Ti3AlC2 crystals, the forces holding together the Ti3C2 layers are a combination of covalent, ionic, and metallic bonds.7-9 Gogotsi and co-workers have shown that such a complex layered material can also be exfoliated by selectively etching the inter-layer Al atoms using hydrofluoric acid to yield functionalized Ti3C2 nanosheets.7 The authors have also successfully extended this selective etching to exfoliate other layered transition metal carbides and nitrides.9 Similarly, layered metal silicides10 (such as CaSi2), metal oxides (such as Cs0.7Ti1.825O4, K0.45MnO2, KCa2Nb3O10), and double hydroxides have also been exfoliated to a variety of quasi-2D structures by harnessing their ability to exchange their interlayer cations with bulky organic counter ions.11 Recently, we have demonstrated that it is possible to exfoliate a similar ionic layered material – magnesium diboride (MgB2). It represents a large family of layered metal diborides (LMDBs) having metal atoms sandwiched in between boron honeycomb planes (Figure 1).12 MgB2 has been primarily known for its superconducting properties, and until our previous study, MgB2 had not been probed as a potential candidate for yielding quasi-2D nanostructures. In that earlier study, our research group had demonstrated that MgB2 crystals could be exfoliated to few-layer-thick functionalized nanosheets by physical forces generated during ultrasonication in an aqueous medium. Following that study, we had also showcased a chemical exfoliation of MgB2 by selectively extracting the interlayer Mg atoms using chelation.13 Recently, a few more reports have appeared which demonstrate that layered MgB2 can be exfoliated by ion-exchange14-16 or ultrasonication in ionic liquids.17 This capability of delaminating MgB2 encouraged us to investigate the prospects of exfoliation by creating a possible synergy between the chemical and physical aides. Such a synergy can be realized in a methodology which facilitates the expansion of the MgB2 crystals before exposing these to ultrasonication. Intercalation of ionic species within the layered crystals is a commonly used chemical method to facilitate the expansion of layered crystals and is frequently used to promote delamination because it significantly reduces the inter-layer energy barrier for exfoliation.1,18-23 In this article, we show that a treatment of MgB2 with sulphuric acid (H2SO4) followed by tetramethylammonium hydroxide (TMAOH), causes simultaneous swelling and partial delamination 2 ACS Paragon Plus Environment

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of parent crystals to yield multi-layer thick (in the range of 300-400 nm) lamellas resembling “nanoaccordions.” We find that a significant fraction of these nanoaccordions can be subsequently exfoliated by ultrasonication in water to yield boron based nanosheets (with a thickness in the range of 3-5 nm) (Figure 1). The nanoaccordions and nanosheets obtained from the chemical exfoliation of MgB2 are found to be functionalized with oxygen based groups. We also demonstrate that these nanostructures can be used as flame retardant nanofillers. This work constitutes the first report establishing the ability to obtain nanoaccordions from a layered metal boride. This study also forms the maiden effort demonstrating the potential of nanoscaled metal borides to be utilized as fillers for enhancing the flame retardancy of a polymer matrix.

Figure 1. Illustration of layered MgB2 parent crystals undergoing exfoliation. (a) In MgB2 crystals, the Mg atoms are sandwiched in between the 2-D boron honeycomb planes. (b) A step by step chemical exfoliation recipe of MgB2 crystals comprises of etching in presence of H2SO4, intercalation by TMAOH (organoammonium ions), followed by sonication in water. This weakens the interlayer forces resulting in nanoaccordions and nanosheets. 2. Experimental Section 2.1. Synthesis of nanoaccordions and nanosheets from layered MgB2 Exfoliation of MgB2 crystals was carried out by first adding 2 g of MgB2 powder (–100 mesh size, Sigma-Aldrich, purity ≥99%) to 25 ml of sulfuric acid (H2SO4, 20% in water) in a conical flask placed in an ice bath. The addition of MgB2 powder was carried out gradually, over a time span of 30 minutes to minimize any overheating during the reaction. The reaction mixture was stirred at 150 rpm for 72 hours using Teflon coated magnetic bar, at the end of which, the mixture exhibits a pastelike appearance. Deionized water (DI water, resistivity of 18 MΩ.cm at 25ᵒC, 75 ml) was then added to the paste, followed by thorough mixing to obtain a homogeneous suspension. The suspension was centrifuged at 7800 rpm for 20 minutes (Eppendorf-5430R) to recover the sediments by decanting 3 ACS Paragon Plus Environment


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the supernatant. Following this, the sediments were thoroughly mixed in a fresh batch of DI water and again recovered through the steps of centrifugation and decantation. This procedure was repeated for five cycles until the supernatant attains a pH value close to neutrality (in the range of 67). The sediment recovered after the last washing cycle was mixed with 30 ml of tetramethylammonium hydroxide (TMAOH, 25 wt % in H2O). The mixture was allowed to stir at 200 rpm (using a Teflon coated magnetic bar) for seven days at ambient temperature (at this point the pH of the mixture was >12), after which the residual TMAOH was separated from the mixture. To achieve this, the reaction mixture was diluted with DI water, mixed thoroughly, and centrifuged at 7800 rpm for 20 minutes to obtain the desired sediments by decanting the supernatant. The above procedure was repeated for six cycles until the supernatant attains a pH value close to neutrality (in the range of 7-8). As explained ahead, the sediment recovered after the last washing cycle was found to contain accordion-like nanostructures, which are termed as “nanoaccordions (NA).”

These

nanoaccordions were then ultrasonicated using a 1-inch Sonic flat head tip (Sonic Vibracell-VC505, 500 watts, 20 kHz) by dispersing in DI water for 1 hour (amplitude: 60%, 6 sec on/2 sec off). Ice cooling was used to control the heat generated during ultrasonication. As shown ahead, the resultant dark suspension was found to contain a mixture of nanoaccordions and exfoliated nanosheets. The above mixture was termed as sonicated nanoaccordions (sNA). The powder form of the sNA sample was obtained by vacuum filtration of the colloidal suspension through a 0.2 μm polyvinylidene difluoride (PVDF) membrane, which was then dried in a desiccator. The synthesized NA and sNA were used for characterization and as nanofillers in epoxy-based composites. 2.2. Preparation of Epoxy/NA, Epoxy/sNA composites A composite of epoxy with nanoaccordions (EP/NA) was prepared using in situ bulk polymerization method. Briefly, 4.3 g of epoxy resin (Araldite LY556, diglycidyl ether of bisphenolA) was measured in a glass vial. To this vial, 43 mg (1 wt% of epoxy) of the powder form of nanoaccordions was added. The mixture was stirred at 400 rpm (using a Teflon coated magnetic bar) for 3 hours at 80ᵒC. Because the epoxy resin was highly viscous at room temperature, the mixing temperature was maintained at 80ᵒC to enable an efficient mixing. Subsequently, 430 mg of hardener (Araldite HY951, triethylene tetraamine) was added to the mixture containing epoxy and nanoaccordions to ensure that the ratio of hardener to epoxy is maintained at 1:10 (wt/wt). The final mixture was stirred for 5 minutes and poured into a silicone mould for being cured at room temperature for 24 hours. The obtained EP/NA nanocomposite was then post-cured at 160ᵒC for another 2 hours. After curing, the nanocomposite was allowed to cool till it attained room temperature. The as-prepared samples were designated as EP/NA X, where X% indicates the


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weight percentage of nanofillers. Nanocomposites of epoxy with sonicated nanoaccordions (designated as EP/sNA) were also prepared using the same procedure.

Figure 2. Schematic showing the synthesis process used to produce few-layered-thick boron based nanosheets from parent MgB2 crystals. (a) Layered MgB2 is reacted with aqueous solution of 20% H2SO4. (b) The reaction results in the removal of a fraction of Mg atoms from the interlayers to yield etched crystals. (c)The etched crystals are then placed in aqueous solution of TMAOH, which results in an expansion of layered structure. These expanded crystals exhibit an accordion-like morphology. (d) Furthermore, these expanded crystals are exfoliated using bath sonication in water to produce mixture of nanosheets and nanoaccordions. (e) Photograph of parent MgB2 crystals. (f) FESEM image of etched crystal showing crevices on its surface. (g) FESEM image of expanded crystal showing accordion-like morphology. (h) TEM image of an exfoliated nanosheet. 3. Results and discussion Layered metal diborides hold exclusive merit in view of the on-going studies aimed at realizing 2D materials beyond graphene. By virtue of their native structure, which comprises boron honeycomb planes alternated with Mg atoms (Figure 1a and Figure 2a), these present an excellent prospect to furnish boron based planar nanostructures. Extraction of the interlayer Mg atoms is expected to delaminate the crystals and yield nanosheets, which can enable access to planar forms of boron. In this study, we present a step-by-step chemical recipe which enables a controlled exfoliation of MgB2 via an accordion-like swelling of the parent crystals (Figure 2).



3.1 Synthesis and characterization of nanostructures derived from MgB2

Figure 3. Electron Microscopy images of MgB2 crystals during different stages of the chemical exfoliation. (a) MgB2 parent crystals before H2SO4 treatment exhibit an intact appearance. (b) MgB2 crystals after H2SO4 treatment exhibit several crevices on their surface. There are some incidences of layers peeling off from the parent crystals. (c) Accordion-like swelling of MgB2 crystals, as observed after keeping acid-treated MgB2 crystals in an aqueous solution of TMAOH for 7 days, suggesting an increased inter gallery spacing. (d) TEM image of an expanded crystal appearing as stacked lamellar slabs and exhibiting better-defined edges. (e) HRTEM images of a nanoaccordion showing expansion of crystal layers which are non-uniformly distributed, (f) A magnified section of (e) showing expansion sites. Inset shows selected-area electron diffraction pattern obtained from the image in (d) and (f). The sharp dot patterns of SAED indicate that the nanoaccordions are crystalline in nature. The first step of the chemical recipe involves the gradual addition of parent MgB2 crystals to an aqueous solution of 20% H2SO4; this addition was accompanied by a rapid effervescence. Upon allowing the mixture to be stirred continuously for 72 hours, a dark black colored paste was obtained. To obtain insights into the effect of H2SO4 treatment on MgB2 crystals, we recovered residues from the paste by several cycles of aqueous washing and centrifugation (see Methods for details), and studied these under FESEM. These acid treated crystals exhibit distinct crevices on their surfaces; such openings are not present in the parent MgB2 crystals (Figure 3a-b, see SI section S2 and S3 for more representative images of parent and acid treated MgB2 crystals). There are a few instances where these crevices are prominent enough causing some layers to be peeled off from the acid-treated crystals (Figure 3b). These crevices are likely the sites from where the inter-gallery Mg atoms have been etched out by reaction with acid. Such a possibility is supported by the chemical 6 ACS Paragon Plus Environment

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composition of acid treated crystals, obtained by energy dispersive X-ray (EDX) spectroscopy carried out under FESEM. A significant drop in the Mg signal is observed in the MgB2 crystals after H2SO4 treatment (see EDX analysis in SI section S4). This observation implies that during acid treatment, a substantial fraction of interlayer Mg atoms is being removed. Owing to their Mg deficiency, we refer these acid treated crystals as “etched crystals.” This observation is in agreement with other similar reports, in which selective extraction of inter-layer metal atoms from layered crystal by acids leads to metal deficient etched crystals with similar features (such as CaSi2→ Ca1-xSi2, Ti3AlC2→ Ti3C2, KxMnO2→Hx MnO2).9-11,20 We made another supporting observation that, when the supernatant collected from acid treated MgB2 crystals were stored for ~2 months at room temperature, boric acid crystals were formed (we have explained more about this observation in SI section S3). It is gradually being established that intercalation of charged molecules within layered structures weaken the interlayer forces and thereby reduce the energy barrier for exfoliation1. This led us to investigate if, after the acid treatment, the etched crystals can be expanded by intercalation of organoammonium ions, which are widely known to induce expansion in other layered materials like metal oxides, hydroxides, and nitrides.19,20,24 Thus, the second step of the chemical recipe involves placing the etched MgB2 crystals in an aqueous solution of TMAOH. After this step, a large number of these crystals were found to exhibit a distinct expansion and a characteristic “accordion-like morphology” (Figure 3c and SI, Figure S5). Similar morphologies have also been observed upon swelling of various other layered materials in the presence of quaternary ammonium hydroxides.9,21-25 To the best of our knowledge, this is the first report, where such accordion-like nanostructures have been derived from any layered metal boride. Under TEM, the parent MgB2 crystals exhibit an extremely dark contrast indicating a compact structure (SI Figure S2), whereas the nanoaccordions exhibit better-defined edges and appear as stacked lamellar slabs indicating the swelling in crystals (Figure 3c and SI Figure S6). Such an accordion-like swelling has also been reported for layered perovskite oxides (e.g., Ca2Nb3O10-)21 and transition metal carbides (e.g., layered Ti3C2).25 HRTEM images of these nanoaccordions were found to exhibit a striated pattern, along with a few areas that appear as relatively bright spots (Figure 3e). These spots are likely the localized sites of expansion. A magnified view of a selected section from Figure 3e is shown in Figure 3f, which depicts that these expansion sites are nonuniformly distributed (see red arrows in Figure 3e-f) within the stack of layers. An example of a more magnified image obtained from a nanoaccordion is shown in Figure 3f. Selected Area Electron Diffraction (SAED) obtained from nanoaccordions resulted in spot patterns (inset in Figure 3d & 3f), indicating that the nanoaccordions are crystalline, which is in agreement with the XRD analysis (SI



Figure S7 and S9). We also validated this by obtaining the FFT, IFFT, and line profiling of the HRTEM image; these are presented in the Supporting Information (Figure S6). This retention of crystallinity is also in line with the observation by Mashtalir et al., who observed that removal of Al from layered Ti3AlC2 followed by intercalation results in crystalline expanded functionalized-TiC2.26 We quantified this expansion by comparing WAXD patterns of parent MgB2 crystals, nanoaccordions, and sonicated nanoaccordions as shown in Figure 4e. The first peak observed at 2θ=25ᵒ in the diffraction pattern of parent MgB2 crystals corresponds to the (001) plane and c-lattice parameter (c-LP) of 3.5 Å. For the nanoaccordions, this peak was found at a lower value of 2θ=18.5ᵒ, which corresponds to the c-LP of 4.7 Å (PDF01-071-5972, for details, see SI Section S7 and Figure S9). A similar comparison between the parent and expanded crystals have also been reported by Naguib et al.,27 Urbankowski et al.,24 and Anasori et al.18 We have also presented some preliminary insights on the expansion induced in the MgB2 crystals in SI sections S7-S8. In the near future, we hope to obtain more insights on this aspect by detailed theoretical studies.

Figure 4. Ultrasonication of nanoaccordions to yield few-layer-thick nanosheets. a(i) TEM image of a nanosheet, a(ii) High-resolution TEM image of the selected edge of a nanosheet. Inset shows the selected-area electron diffraction pattern; (b) TEM image of a crystal captured during the process of delamination. The solid arrow is pointed towards the region where the delamination is initiated; (c) FESEM image of exfoliated nanosheet drop-casted on SiO2/Si substrate showing flower-like morphology. (d) AFM image of nanosheet and corresponding height profile indicates its thickness in 35 nm range. This suggests that the nanosheets are few-layer-thick. (e) WAXD analysis obtained for parent MgB2 crystals, nanoaccordions, and sonicated nanoaccordions in the 2θ range of 5-30ᵒ. 8 ACS Paragon Plus Environment

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Nanoaccordions are expected to be less tightly bound compared with parent MgB2 crystals owing to their expanded morphology, and hence can be delaminated by exposing these expanded crystals to shear forces. With this view, the third and final step of the chemical recipe involves ultrasonication of the nanoaccordions; we chose an aqueous medium for this step. TEM analysis of the sonicated samples indicates the presence of nanosheets along with nanoaccordions, suggesting that some of the nanoaccordions have undergone a further exfoliation. Figure 4a(i) represents the TEM image of a nanostructure, which resembles exfoliated graphite and exhibits a high degree of transparency suggesting its ultra-thin nature (see SI Figure S10 for more images). These nanosheets exhibit lateral dimensions of up to few micrometers. In some cases, the nanosheets also exhibit folds near the edges, which is expected on account of their ultra-thin nature. Figure 4a (ii) shows an HRTEM image of a selected region from the nanosheet depicted in Figure 4a (i). The corresponding SAED pattern collected from this region shows a diffused ring-like pattern indicating the nanosheet to be amorphous. The exfoliated nanosheets are expected to be amorphous, as these are few-layerthick (as evidenced ahead by AFM) and carry oxy-functional groups; this observation is similar to our earlier studies.12,13,28 While imaging under TEM, we also came across a rare instance of partial exfoliation(Figure 4b) where a nascent sheet-like structure appears to be in the process of delaminating from a parent crystal, supporting that some nanoaccordions undergo further exfoliation due to ultrasonication. We have also presented FESEM images of such instances in SI, Figure S12. FESEM images of the ultrasonicated sample also indicated the presence of both nanoaccordions as well as nanosheets which are observed in the form of aggregates (Figure 4c, and S11). Owing to their varying orientation, some of these nanosheets protrude from the aggregates (Figure 4c) and appear to exhibit flower-like morphologies. These nanostructures are distinctly different from the parent MgB2 crystals which consist of relatively thicker and intact structures (Figure 3a, and SI Section S2). The apparent thickness of nanosheets was determined by AFM analysis as shown in Figure 4d (See Figure S13 for more images). The corresponding height profiles (as shown in the inset of Figure 4d) indicate the thickness of nanosheets to be in the range of 3-5 nm.



Figure 5. Preparation of epoxy and its nanocomposites. (a-c) Photographs of neat epoxy (EP), (df) a composite of epoxy with 1% nanoaccordions (EP/NA), (g-i) and a composite of epoxy with 1% sonicated nanoaccordions (EP/sNA) during different stages of fabrication – after being poured in a silicone mould (left panels), after being cured for 24 hours (central panels), and after being postcured at 160°C for 2h (right panels). To obtain a quantitative insight into the chemical composition of these nanostructures, we analyzed these by ICP-AES (as shown in SI Table S2). We found the nanoaccordions to exhibit a stoichiometry of Mg0.6B2, suggesting that these are deficient in Mg when compared with parent MgB2 crystals. A weaker signal of Mg in the nanoaccordions is anticipated on account of the etching step in the recipe. As explained earlier, EDX analysis of these nanostructures suggested the presence of oxygen and hence the possibility of functionalization. The nature of chemical groups functionalizing the nanostructures was probed by X-ray photoelectron spectroscopy (XPS). The Mg2p and B1s spectra obtained from the nanoaccordions exhibit newer daughter peaks at higher binding energies (52.4 eV and 195.0 eV respectively) when compared with the spectra obtained from parent MgB2 crystals (see SI Figure S14). These new daughter peaks suggest that these nanoaccordions carry oxy-functional groups.13,29-31 This can be explained as one of the outcomes of a chemical reaction between MgB2 and acid, which results in a fraction of Mg atoms being etched out from 10 ACS Paragon Plus Environment

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MgB2. The loss of etched Mg atoms is likely being compensated by a subsequent gain of oxyfunctional groups from the aqueous solution. This observation is similar to our earlier studies, where magnesium deficient hydroxyl-functionalized nanosheets are obtained by exfoliation of MgB2 in water.12,13,28 Similar attainments of oxy and hydroxyl groups have also been reported for accordionlike nanostructures derived from Ti4N3, Mo2TiAlC2, and Cr2TiAlC2.18,24

Figure 6. Epoxy and its nanocomposites (a) FESEM image of a fractured surface in the EP/NA nanocomposite. Inset shows the elemental color mapping obtained from the marked region by EDX analysis for carbon, oxygen, magnesium, and boron. The presence of B and Mg is attributed to the contribution by nanoaccordions from MgB2, whereas signals of C and O are assigned to the epoxy polymer. These indicate that the nanoaccordions are distributed uniformly within the polymer matrix. (b) TGA curves of epoxy and its nanocomposites under nitrogen. The mass loss of all the samples from temperature range 700-800°C is magnified and shown as an inset in (b). From TGA curves, it can be seen that incorporation of NA and sNA produces higher char residues in comparison with neat EP, indicating a better thermal stability of nanocomposites at high temperature. (c-e) Photographs of TG product obtained in an alumina crucible for EP, EP/NA 1%, and EP/sNA 1%. (f) Mg2p and B1s spectra obtained from the XPS analysis of char obtained as a result of the thermal decomposition of EP/NA 1% nanocomposite.



3.2. Flame retardant properties of epoxy nanocomposite: The presence of oxy-functional groups in these MgB2 derived nanostructures motivated us to investigate if these can be utilized as nanofillers for enhancing the flame retardancy of a flammable matrix. The rationale to assess such a candidacy originated from the fact that inorganic compounds comprising oxy-functionalized Mg and B (magnesium hydroxide and borates) are widely used as flame retardants owing to their endothermic decomposition and ability to form insulating layers.32,33 Furthermore, because the accessibility to oxy-functionalized Mg and B would be remarkably increased due to the enhanced surface area of nanostructures, we conceptualized that the functionalized nanostructures derived from MgB2 would exhibit a substantial enhancement in flame retardancy. To study this potentiality, we incorporated these nanostructures into a flammable matrix at low loadings to obtain polymer nanocomposites and tested their flame retardancy. In this work, we used epoxy (EP) as the flammable polymer matrix, and added the nanoaccordions (NA) or sonicated nanoaccordions (sNA) as fillers (at a loading of 1% by weight) to obtain two types of composites - epoxy/nanoaccordion (EP/NA) and epoxy/sonicated nanoaccordion (EP/sNA) (as shown in Figure 5). To understand the distribution of nanoaccordions in epoxy resin, we examined the fractured surface of nanocomposites under FESEM. As shown in previous reports, the roughness observed in the fractured surface is an indicator of the dispersion level and interfacial interaction of nanofillers within the polymer matrix.34-37 We observed that the fractured surface of neat EP (SI Figure S15) was very smooth, whereas the fractured surface of the nanocomposite was quite rough (Figure 6a). Moreover, the roughness was found to increase with an increase in the content of NAs within the matrix (Figure S15). This indicates that the nanoaccordions have interfaced and dispersed well in the surrounding matrix.34,37 We did not observe any pulled-out aggregates on the fractured surface, suggesting that there is a uniform dispersion of NAs within the polymer matrix. This is further supported by the evenly distributed Mg, and B signals in the EDX color maps (inset of Figure 6a, SI Figure S15) obtained during FESEM imaging. We have also obtained the TEM images of the nanocomposite as shown in Figure S21. The presence of exfoliated nanostructures is manifested as sharp lines (indicated by arrows). The nanostructures were found to be well dispersed within the matrix without any incidences of aggregation. Subsequent to this, we obtained insights on the thermal degradation behaviors of these nanocomposites by performing thermogravimetric analysis (TGA) under nitrogen (Figure 6b). Figure 6b shows that the neat EP, EP/NA, and EP/sNA exhibit a major mass loss in the temperature range of 300-450ᵒC; this is ascribed to the degradation of principal macromolecular EP network.38,39 The


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thermal stability of nanocomposites is observed to be significantly improved at high temperatures, as indicated by the decreased mass loss or increased char residues (denoted by the inset in Figure 6b) as compared with neat EP. The char residue values for EP, EP/NA, and EP/sNA at 700ᵒC and 800ᵒC are shown in Table 1, along with the corresponding char gain values for the nanocomposites. We observed that both types of nanocomposites exhibit significant char residue gains — 4.0% (composite with nanoaccordions), and 6.0% (composite with sonicated nanoaccordions) with respect to neat EP. An increase in the char residues indicates a lesser mass loss and hence more flame retardancy.40 This suggests that the nanomaterial-derived from MgB2 can indeed impart flame retardancy. Importantly, not only are these nanostructures flame retardant, but we also found these to be superior to their counterparts. As shown in Table 2, the char gain values exhibited by these nanostructures are prominently higher when compared with other analogous flame retardant nanofillers (such as graphene, graphene oxide, hydroxylated boron nitride, and layered double hydroxides) at similar loadings. This observation on superior flame retardant properties showcases an unprecedented perspective to the potential of layered metal diborides. Table 1: Char residue data of epoxy and its nanocomposites obtained by TGA.

Char (800°C, wt%)

In N2

Gain in char at 700ᵒC with respect to neat EP (%)

In N2

Gain in char at 800ᵒC with respect to neat EP (%)

EP

8.2

_

7.8

_

EP/NA 1%

12.2

4.0

11.9

4.0

EP/sNA 1%

14.2

6.0

13.9

6.0

Sample

Char (700ᵒC, wt%)

The enhancement in thermal stability upon incorporation of the nanostructures derived from MgB2 is also evidenced by the nature of pyrolysis products obtained at the end of TG experiments. The EP/NA (Figure 6d for the optical image, Figure S16 for FESEM image) and EP/sNA (Figure 6e) composites are found to maintain a distinctly stable structure at the end of a TG run. In contrast, no such stable structure remained in the case of neat epoxy, indicating that they have been combusted in entirety in the absence of any nanofiller (Figure 6c). The thermal degradation of a sample comprising solely of nanoaccordions was also studied under nitrogen. We found the char residue to be ~97% (SI, Figure S17), implying that nanoaccordions synthesized from MgB2 have superior thermal stability at high temperatures. This further indicates the potential of these nanostructures for enhancing the flame retardancy. To quantify this flame retardant potential of the



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nanocomposites, we measured their limiting oxygen index (LOI) values and burning rates by the UL94 horizontal burning tests. Table 2: A summary of char gain of various EP/nanofiller nanocomposite obtained by TG analysis.

No.

Types of nanofiller in epoxy based composites

Nanofiller Content (%)

Char gain (%)

References

(1-5) char obtained at 700°C 1

Sonicated nanoaccordions (sNA) from MgB2

1%

6.0

Present work

2

nanoaccordions (NA) from MgB2

1%

4.0

Present work

3

Graphene oxide (GO)

1%

2.0

Liao et al.41

4

Reduced graphene oxide (RGO)

1%

2.0

Yu et al.38

5

Mesoporous silica (mSiO2)

2%

1.1

Jiang et al.42

(6-11) char obtained at 800°C Sonicated nanoaccordions (sNA) from MgB2 nanoaccordions (NA) from MgB2

1%

6.0

Present work

1%

4.0

Present work

8

hydroxylated hexagonal boron nitride (BNO)

1%

1.6

Yu et al.39

9

Taurine modified layered double hydroxide (T-LDH)

6%

0.8

Kalali et al.43

10

Expanded LDH by betacyclodextrin (sCD-LDH)

6%

5.8

Kalali et al.43

11

Nano Polyanilines (PANI)

5%

1.4

Zhang et al.44

6 7

Note: Higher char gain is considered as more efficient fire resistant nanofiller. Violet color(1-5) represents char obtained at 700°C; Orange color (6-11) represents char obtained at 800°C.

Table 3: LOI and UL-94 data of neat EP and its nanocomposites LOI (%)

UL-94 HB (Burning Rate in mm/min)

Observation

EP

20.1

31.7

Continued burning

EP/sNA 1%

21.8

23.6

Self-extinguished

EP/sNA 2%

22.5

15.1

Self-extinguished

Sample


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We observed that neat epoxy exhibited an LOI value of 20.1; whereas epoxy containing 1% and 2% sonicated nanoaccordions exhibited an LOI value of 21.8, and 22.5 respectively (as summarized in Table 3). An increase in the LOI value upon an increase in the fraction of nanoaccordions validates their flame retardant capability. This is further supported by the UL-94 test where the burning rate of neat EP (31.7 mm/min) was found to be higher compared with the burning rate of EP/sNA 1% (23.6 mm/min) and EP/sNA 2% (15.1 mm/min). It can be noted that the neat EP was found to burn vigorously during the test and left no unburnt mass. In contrast to this, the nanocomposites self-extinguished. A reduction in the burning rates along with the phenomenon of extinguishing by itself reaffirms the flame retardant properties of the nanoaccordions. This is evident from the digital photographs of burnt specimen shown in Figure S19.

Figure 7. Snapshots of burning test with respect to time in seconds. Top panel shows the burning behaviour of neat epoxy (0-173 seconds), central panel shows the burning behaviour of EP/NA 1% nanocomposite (0-75 seconds), and bottom panel shows the burning behaviour of EP/sNA 1% nanocomposite (0-47 seconds). The images are captured from the movie in the Supporting Information.

Following up the characterization by UL-94 horizontal burning test, we also studied the response of nanocomposites upon exposing these to a flame in entirety. Briefly, a small piece of polymer (or the nanocomposite) was placed inside a ceramic crucible and drenched with 250 μl of acetone for initiating the flame. A series of photographs were captured during different stages of the burning test as displayed in Figure 7 (see SI for a movie comparing the burning behaviors of neat EP, EP/NA, and EP/sNA). We observed that the neat EP started burning vigorously before selfextinguishing at 173 s; the vigorous burning is attributed to the highly flammable property native to



Page 16 of 21

EP (Top panel in Figure 7). Contrastingly, the EP/NA (central panel) and EP/sNA (bottom panel) nanocomposite were found to self-extinguish at 75 s, and 47 s respectively without undergoing a vigorous burning as shown in Figure 7. This significant reduction in burning time (from 173 s in neat EP to 75 s in EP/NA and 47 s in EP/sNA) indicates that the MgB2 derived nanostructures enable the polymer to sustain the flame. Moreover, the burnt products of epoxy resin and the nanocomposite exhibit a distinct difference in their structural integrity. Whereas the EP burnt product is fracturable, the EP/NA and EP/sNA composites retained their integrity and appeared to be unaffected. The extraordinarily high enhancement observed in the flame retardancy of epoxy upon addition of MgB2 derived nanostructures may be attributed to a concurrence of their peculiar morphology and unique chemistry, explained as follows: Effect of Morphology: The expansion sites characteristic to nanoaccordions make these structures thermally insulating, as shown by Luo et al. and Camino et al.

45-47

Thus, in the event of a polymer

matrix burning, the nanoaccordions derived from MgB2 are expected to not only act as physical barriers to mass transfer but also inhibit the exchange of heat between flame zone and matrix, suffocating the flame. The quasi-planar nature exhibited by nanosheets is also expected to play an important role. The presence of nanosheets within the polymer matrix significantly alters the diffusion path of pyrolysis products during burning (tortuous path effect), thereby slowing down their escape, and resulting in the formation of additional char residues.39,47,48 This is further supported by our observation that EP/sNA nanocomposite, in which nanoaccordions, as well as nanosheets, are present, exhibits a slightly higher char gain (6.0%) when compared with EP/NA nanocomposite (4.0%), which contains only nanoaccordions as fillers (as shown in Table 1). Effect of Chemistry: We obtained some insights into the role played by the functional groups in the flame retardant ability by comparing the XPS of the nanoaccordions and the char of nanocomposite. The B1s spectrum of nanoaccordions exhibits two broad peaks, one in the range of ~187-189 eV, and the other in the range of ~192-195 eV (Figure S14). The former peak is characteristic of the Mg-B bond, whereas the latter is ascribed to the oxides and hydroxides of boron.14,49 However, the B1s spectrum from the char exhibits only one strong peak in the range of ~192-194 eV, which is assigned to the oxides of boron (Figure 6f).50,51 We inferred a similar formation of Mg oxides by comparing the Mg2p spectra of char and nanoaccordions by the appearance of a prominent peak at 50.3 eV.52 This observation suggests the formation of oxides of B and Mg during combustion. These oxides form dense char layers on the surface of burning polymer, which are expected to insulate the heat transformation and further propagation of flammable gas into the underlying polymer matrix, thereby improving the flame retardancy.53 It is also pertinent to state that while oxy-functionalized 16 ACS Paragon Plus Environment

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forms of B and Mg are flame retardants in their rights, it is well known that when used in conjunction with each other, these are also known to exhibit a remarkable synergy rather than a simple additive effect.33,53 The co-existence of oxy-functionalized B and Mg in the chemically modified nanostructures derived from MgB2 is expected to facilitate a similar synergistic effect to enhance the flame retardancy. It would be promising to study if the flame retardancy of these nanoaccordions can be further improved by tuning the surface functionality, e.g., by varying the parent layered metal borides or by introducing phosphorous and nitrogen-containing functional groups.54,55 We plan to comprehensively investigate these aspects in the near future. 4. Conclusion This work presents maiden efforts on the synthesis of accordion-like nanostructures from a layered metal diboride. This also forms the first study demonstrating that nanoscaled metal borides have the potential to enhance the flame retardancy of a polymer matrix. The ability to exfoliate MgB2 to yield nanoaccordions and nanosheets paves the way to explore if other layered metal borides, (such as TiB2, TaB2, HfB2; around 20 metal borides analogous to MgB2 are known), can also yield such nanostructures. Furthermore, it would be promising to see if the nanoscaled metal borides can be used to capitalize the excellent physicochemical properties (high chemical stabilities, high melting points, ultra-mechanical hardness, high electrical and thermal conductivities) offered by this family of layered materials. We have initiated research in this pursuit, and we are currently exploring if the nanoaccordions derived from MgB2 can be utilized for enhancing the mechanical property of a matrix. We anticipate that the findings presented in this study will motivate the scientific community to explore the rich potential offered by metal borides. 5. Acknowledgment This research work has been supported by seed funding from IIT Gandhinagar, Fast Track Research Grant for Young Scientists (SB/FTP/ETA-114/2013), Extramural Research (EMR) fund by Science and Engineering Research Board, and INSPIRE Faculty Award Research Grant (DST/INSPIRE/04/2014/001601) by Department of Science and Technology, India. The authors thank Vikas Patel (SICART, Anand, India), Bharati Patro (SAIF, IIT Bombay), Anu A S (M G University) for help with the TEM and HRTEM imaging; Chintan Chavda (ATIRA, Ahmedabad) for LOI and UL-94 test; ESCA lab IIT Bombay for XPS analysis. We deeply acknowledge the help extended by Awaneesh Upadhyay and Vikram Karde (with FESEM), Sophia Varghese and Narendra Bandaru (with XRD analysis), Chetan Singh (with AFM analysis and nanocomposite preparation), Sanat Maiti (with TGA and DSC analysis), Ramchandra Gawas (for reviewing the manuscript), and Asha Liza James (for her



observation on formation of boric acid crystal). We thank the Central Research Facility of IIT Gandhinagar for access to the equipment during research. Associated Content Supporting Information Twenty-one additional figures including additional FESEM images of parent MgB2 crystal, H2SO4 treated crystal, TMAOH treated sample showing nanoaccordions, ultrasonicated sample showing nanosheets; Detailed sample characterization procedure; boric acid crystal formation; EDX analysis under FESEM; Additional TEM images of parent MgB2 crystal, nanoaccordions, and nanosheets; XRD analysis of samples collected at different stages of chemical treatment with possible explanations for expansion mechanism; FESEM and TEM images showing partial exfoliation; XPS analysis indicating the oxygen functionalization; Additional AFM image showing thickness of nanosheets; TGA curve of nanoaccordions obtained under nitrogen; FESEM image of char residues; TEM images of nanocomposites; Table listing ICP-AES values for the nanoaccordions and the nanocomposite char; Table listing comparative information on the LOI values of various epoxy based nanocomposites; Supporting Movie S1 showing the burning test of epoxy and its nanocomposites. References: 1. Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, DOI: 10.1126/science.1226419. 2. Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.-Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568-571. 3. Zheng, M.; Dong, H.; Xiao, Y.; Liu, S.; Hu, H.; Liang, Y.; Sun, L.; Liu, Y. Facile One-step and High-yield Synthesis of Few-Layered and Hierarchically Porous Boron Nitride Nanosheets. RSC Adv. 2016, 6, 45402-45409. 4. Xiao, F.; Chen, Z.; Casillas, G.; Richardson, C.; Li, H.; Huang, Z. Controllable Synthesis of FewLayered and Hierarchically Porous Boron Nitride Nanosheets. Chem. Comm. 2016, 52, 3911-3914. 5. Gupta, A.; Sakthivel, T.; Seal, S. Recent Development in 2D Materials Beyond Graphene. Prog. Mater. Sci. 2015, 73, 44-126. 6. Naguib, M.; Gogotsi, Y. Synthesis of Two-Dimensional Materials by Selective Extraction. Acc. Chem. Res. 2015, 48, 128-135. 7. Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248-4253. 8. Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y. 25th Anniversary Article: MXenes: a New Family of Two-Dimensional Materials. Adv. Mater. 2014, 26, 992-1005. 9. Naguib, M.; Mashtalir, O.; Carle, J.; Presser, V.; Lu, J.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional Transition Metal Carbides. ACS Nano 2012, 6, 1322-1331.


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Chemical Exfoliation of Layered Magnesium Diboride To Yield

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