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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Understanding Aqueous Dispersibility of Boron Nitride Nanosheets from 1H Solid State NMR and Reactive Molecular Dynamics Vaishali Arunachalam and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Stable aqueous dispersions of BN nanosheets can be obtained by the sonication assisted exfoliation of hexagonal BN powders in water without the need of stabilizers: a phenomenon not observed for the isoelectronic graphite, of comparable hydrophobicity. We show here that the aqueous dispersions are stabilized by electrostatic repulsive interactions and establish from ζ potential measurements that the BN nanosheets are positively charged on delamination with the medium turning increasingly basic as sonication proceeds. We have investigated how charge develops on the sheets by reactive force-field (ReaxFF) molecular dynamics simulations of the interaction of water with BN nanosheets and independently identified the major chemical species present on the nanosheets from 1H and 11B solid-state NMR measurements. Charges develop on the sheets as a consequence of the dissociation of water molecules at the edges of the sheet that leave the nitrogen edge atoms protonated and the release of hydroxyl groups into the bulk leading to an increased basicity of the medium.

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INTRODUCTION The exfoliation of layered materials into their two-dimensional nanosheet counterpart can result in new materials that are uniquely different from the bulk. The best example of this is the exfoliation of graphite to graphene and the wealth of new properties as well as potential applications that have emerged from this discovery.1,2 This, in turn, has catalyzed the search for new 2D nanosheets that can be produced by exfoliation. Today a wide choice of layered materials that can be delaminated to produce nanosheets of widely varying properties are known, and their properties well-documented.3 Among the layered structures that can be exfoliated to give nanosheets is hexagonal boron nitride (BN) with lattice structure of a single monolayer similar to graphene, but with an isoelectric boron−nitride pair replacing a pair of carbon atoms in the hexagonal lattice. Although isoelectronic to graphene, the properties of BN nanosheets are very different. The material exhibits high temperature stability with resistance to oxidation, chemical inertness and is intrinsically an electrical insulator. Within each layer, boron and nitrogen atoms are bound together by strong sp2 covalent bond but due to the electro-negativity differences between the boron and the nitrogen atoms, the π electrons tend to localize around the nitrogen atomic centers. This results in a much larger electrostatic contribution to the interlayer interactions as compared to graphite and consequently exfoliation of BN nanosheets from the bulk is more difficult.4 Nevertheless, monolayer and few-layer BN boron nitride nanosheets (BNNS) have been obtained by the “scotch tape method” as well as by sonication in polar organic solvents like © XXXX American Chemical Society

N,N-dimethylformamide (DMF) or 1,2-dichloroethane, using procedures very similar to that used for obtaining graphene.5,6 One of the surprising observations are two recent reports of the sonication-assisted exfoliation of BN in water to give stable dispersions of single or few-layer BN nanosheets at reasonable concentrations, without the need for stabilizers.7,8 What is unusual, apart from the fact that graphene, which is considered easier to exfoliate, shows no such behavior, is the fact that bulk hexagonal BN is traditionally considered to be hydrophobic; the water contact angles of BN (50°−70°) are comparable with graphite (60°−90°).9−12 In addition, BN exhibits limited susceptibility toward hydrolysis.7,13 The two reports, however, differed on the origin of the stability of the dispersion. While the earlier report proposed that it was the sonication-assisted hydrolysis that promoted the cutting of the pristine BN sheets to yield smaller and exfoliated BN nanosheets, the later report found no evidence, based on electron microscopy, for edge functionalization and discounted that possibility, for the stability of the BN aqueous dispersion. Here we have investigated the origin of the aqueous dispersibility of BN nanosheets obtained by the sonication assisted exfoliation of bulk BN in water. We find that electrostatic interactions play a dominant role in the stability of the aqueous dispersions. We have tried to address the question of how charge develops on the BN sheets during Received: December 14, 2017 Revised: February 1, 2018 Published: February 6, 2018 A

DOI: 10.1021/acs.jpcc.7b12288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

expensive and can, at best, be used for a smaller scaled-down representative simulation. A simulation method that can efficiently handle large systems in a manner similar to empirical force-fields while at the same time probe dissociative and reactive behavior of the species involved is the reactive forcefield, ReaxFF: a bond order dependent empirical reactive forcefield. The force-field has been used with considerable success to probe the dissociation and reactions of water at the surfaces of Si, SiO2, and TiO2 and is ideally suited for the present study.12−15 The ReaxFF formalism develops a general relationship between bond order and bond distance as well as between bond energy and bond order to model bond formation and dissociation during a MD simulations.20 The interaction between atoms is described in terms of bond order and the forces acting on each atom is obtained from the energy expression

sonication by performing molecular dynamics simulations using reactive force-fields (ReaxFF) to model the interaction of water with the BN nanosheet. ReaxFF is a bond order dependent empirical reactive force-field developed to describe chemical reactivity through a bond order formalism, where bond order is empirically calculated from interatomic distances while electronic interactions are considered implicitly.14 The ReaxFF method has had considerable success in describing the reaction and dissociation of water on the surfaces of TiO2, SiO2, and Si as well as at interfaces.15−18 The simulations are able to reproduce the changes in pH that we observed during sonication. Using11B and 1H solid-state NMR measurements we are able to identify the species predicted by the MD simulations. The ReaxFF MD simulations in conjunction with the NMR measurements are thus able to provide a comprehensive molecular perspective of the stability of dispersions of BN nanosheets in aqueous media.

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Esystem = Ebond + Eover + Evalencyangle + Etorsion + EvdWaals

EXPERIMENTAL DETAILS Aqueous boron nitride dispersions were prepared by sonicating 2 mg/mL of hexagonal-BN powder (Sigma-Aldrich, 1 μm > 98%) in 500 mL of Milli-Q water for 6 h using a tip sonicator (Ultrasonics, 250W). The suspension was left to stand overnight and subsequently centrifuged for 15 min at 5000 rpm to remove the larger aggregates. The concentration of boron nitride in the dispersion was estimated by UV−visible absorption spectroscopy from the reported value of the absorption coefficient at 300 nm (α300 = 2367 L/g·m).19 The concentration of exfoliated BN nanosheets in the as-prepared dispersion was estimated to be 0.05 mg/mL. The BN nanosheets in the dispersion were characterized using SEM (Carl-Ziess Ultra 55 Gemini microscope), tappingmode AFM (Veeco MultiMode IV microscope) and TEM (JEOL JEM 2100F electron microscope) measurements. Samples were prepared by drop coating the diluted dispersion onto a appropriate substrate for SEM and AFM measurements and carbon coated grids for recording TEM images. 1H and 11B MAS solid-state NMR spectra were recorded on a Bruker AVANCE III 500 MHz solid state spectrometer with a double resonance 1.3 mm DVT MAS probe. The 1H solid state MAS NMR spectra were recorded using 1.3 mm zirconia rotors with Kel-F caps, spinning at 60 kHz. Details of the procedure for the suppression of the background signals are described in the Supporting Information, section S1. Spectral data were acquired in 64 scans with a relaxation delay of 5 s between subsequent scans. For the solid-state NMR experiments the as-prepared dispersions were further centrifuged (Beckman Coulter Ultracentrifuge) at 30 000 rpm for 2 h and the centrifugate dried in air at 60 °C.

+ ECoulomb + ESpecific

where Ebond, Evalency angle, and Etorsion represent the bond energy, the valence angle energy (three body) and the torsion angle energy (four body), respectively. Eover is an energy penalty term that prevents over coordination of the atoms with respect to their valency while Especif ic captures system specific properties like lone pair, conjugations or hydrogen bonding.20 These two terms are the additional energy contributions than those found in the empirical force fields. The non bonded interactions are described by ECoulomb and EvdWaals, which accounts for electrostatic and dispersive interactions, respectively, and are calculated between all the atom pairs, irrespective of their connectivity. The ReaxFF force-fields are parametrized so as to yield bond strengths and interatomic distances that agree with quantum mechanically predicted values. In the present study the ReaxFF B/N/H/O parameters were obtained by combining the interlayer potentials developed for hexagonal BN (B/N, B/H and N/H; Supporting Information of ref 21) with that of water.21,22 The B/O and N/O parameters were taken from those developed for aminoborane.23 The present simulation assumes that these parameters are transferable and appropriate for the BN-water system. The transferability of B/O parameters obtained from aminoborane to the present BN−H2O system was validated by comparison with quantum chemical DFT-B3LYP calculations performed for a single water molecule interacting with a BN sheet. The results from the two calculations were found to be in good agreement (Supporting Information, section S2). The B/ N/H/O ReaxFF parameters used in the present study are included in the Supporting Information, section S3.

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COMPUTATIONAL DETAILS The Reactive Force-Field (ReaxFF) Method. The interaction of BN nanosheets with water was investigated by molecular dynamics simulations. Conventional classical molecular dynamics based on empirical force-fields have predefined interatomic potentials to calculate forces acting on individual atoms and hence cannot be used to probe bond formation and bond breaking. Ab initio molecular dynamics, on the other hand, generate potentials from quantum chemical calculations that allow computation of forces from concurrent electronic structure and thus can monitor and simulate chemically reactive systems. Unfortunately, the method is computationally

SIMULATION METHODOLOGY Simulations were performed on BN sheets of hexagonal shape with zigzag terminated edges (alternating B and N terminated edges) as well as with the armchair edge configuration (edges with alternating B and N atoms) with the former containing a total of 150 boron and nitrogen atoms and the latter with 114 atoms, but both having the same number (30) of edge atoms (Figure S3, Supporting Information, section S4). The BN sheets were immersed in a simulation cell containing 3840 water molecules. The system was equilibrated by performing classical nonreactive force-field MD simulations using the B

DOI: 10.1021/acs.jpcc.7b12288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Digital image of BN nanosheet dispersions prepared by sonication of BN powder in water. (b) SEM (c) AFM and (d) TEM images. The inset in part b shows the lateral size distribution and that in part c the distribution of the number of BN layers per flake in the deposited dispersion. (e) HRTEM image of a single flake in part d. The inset shows the diffraction pattern.

Figure 2. (a) Digital photograph of the precipitation of BN from the dispersion on addition of NaCl. (b) ζ potential of the BN aqueous dispersion. (c) pH of the BN dispersion as a function of sonication time.

LAMMPS software.24 The simulations used an NPT (1 atm, 300 K) ensemble with a time integration step of 1 fs. The temperature of the cell was maintained by a Nosé−Hoover thermostat and pressure by a Nosé−Hoover barostat. The MD equilibration step was performed keeping the sheets uncharged and rigid. The atomic charges and force field parameters for water used in the MD simulations, were derived from TIP3P while the previously reported force-field parameters were used for the boron and nitrogen atoms in the BN sheet.25,26 Nonbonded Columbic interactions were treated using the longrange particle-particle-mesh integration implementation provided by LAMMPS. For van der Waals interactions a LennardJones potential with a cutoff distance of 8 Å was used. Bond and angle constraints were applied to water molecules using the SHAKE algorithm in LAMMPS. The final equilibrium configuration was used as the input for the ReaxFF simulations (Figure S4, Supporting Information, section S5).

ReaxFF simulations were carried out for 50 ps using a NPT ensemble (1 atm, 300 K) with a time step of 0.5 fs. The sheet was held rigid throughout the simulation and the charges on individual atoms calculated on-the-fly by the charge equilibration method. Convergence of the total energy and the volume of the cell was attained within the first 5 ps. The MD simulation and ReaxFF were performed using the LAMMPS software running on the Intel-Xeon PHI coprocessor nodes of a Cray XC-40 at the HPC facility at SERC, IISc. The LAMMPS output files containing the bond, species and trajectory information were analyzed using in-house MATLAB and VMD TCL scripts to monitor bond formation and determine the number of species formed during the ReaxFF simulations.

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RESULTS AND DISCUSSION Aqueous Dispersions of BN. Sonication of hexagonal-BN in water gives stable dispersions of exfoliated BN nanosheets (Figure 1a). SEM and AFM images of the drop-coated images C

DOI: 10.1021/acs.jpcc.7b12288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. Chemical events captured during the course of a ReaxFF MD simulation of the interaction of water molecules with the zigzag edges of a BN nanosheet. (a) 0.2 ps, the chemisorption of H2O water molecules at the B and N terminated edges. (b) 1.1 ps, the dissociation of water molecules. On the N edge dissociation occurs with the release of the OH group into the bulk, leaving the proton attached to the N atom. On the B edge dissociation leads to the H atom attaching to an adjacent B atom while the OH remains attached to the original site of water chemisorption. (c) 1.3 ps, water molecule chemisorbed on a site adjacent to the BH group forms a loosely bound complex with the B−H. (d) 1.5 ps, the loosely bound complex dissociates with water released to the bulk leaving a H bound to the B atom at the site where water had chemisorbed. (e) Snapshot at the end of the simulation (50 ps). All atoms on the N terminated edge are protonated. The B edge atoms have −OH groups, chemisorbed H2O, and labile H attached.

The sequence of events along the simulation trajectory for the zigzag-terminated BN sheet are shown in Figure 3a−e. For clarity only a part, of two of the edges, of the BN nanosheet and a reduced number of water molecules are shown. A complete movie file along the trajectory is included as part of the Supporting Information, Movie S1. It was observed that during the entire 50 ps of the simulation water molecules reacted exclusively at the edges of the BN sheet and never on the basal plane. This observation is in agreement with the HRTEM results (Figure 1e) that showed the basal plane of the BN nanosheets to be intact and free of defects. The first event observed in the simulation, and which occurs within the first, 0.2 ps from the start, is the chemisorption of water molecules at the edges of the nanosheet. Adsorption at both the boron and nitrogen edges of the BN nanosheet are seen (Figure 3a). On the nitrogen edge it is the hydrogen atom of the water molecule that attaches to the nitrogen atom while on the boron terminated edges water binds through oxygen. The water molecules subsequently dissociate, but the dissociation patterns on the boron and nitrogen edges are quite different (Figure 3b). On the nitrogen edge the water molecules dissociate releasing hydroxyl species into the bulk while the proton remains attached to the nitrogen edge atom. In contrast, on the boron edge water binds through oxygen to the boron atom and dissociation occurs with the hydrogen of the water attaching itself to an adjacent boron atom. On dissociation a B−OH bond is formed on the boron atom that adsorbed the water with the H atom attached to the neighboring boron atom (Figure 3b). The BH bond tends to be labile and is subsequently abstracted by a water molecule chemisorbed on an adjacent boron atom. This complex then breaks up releasing water but now with the H atom attached to the adjacent boron (Figures 3c,d). On the N terminated edge the reactions are simpler, chemisorbed water molecules dissociate releasing OH groups into the bulk and by the end

showed sheets that have, typically, lateral dimensions of 150 nm (Figure 1b) and are about 8 nm thick (about 8 BN layers) (Figure 1c). The TEM images showed single or few-layer BN nanosheets that have a hexagonal honeycomb structure (Figures 1d,e). The HRTEM images of the basal planes showed no evidence of defects. The aqueous dispersions of BN nanosheets have stabilities in excess of six months. Addition of salt (>0.05 M NaCl) to the dispersion, however, results in flocculation, with BN settling at the bottom of the container (Figure 2a). The observation clearly suggest that the stability of the dispersion is electrostatic in origin, a fact confirmed by ζ−potential measurements that indicated a positive value of 28 mV for the BN dispersion (Figure 2b). The results indicate that the BN nanosheets in the dispersion are positively charged. It was also observed that during sonication the dispersions turned increasingly basic as sonication proceeded (Figure 2c) (Supporting Information, section S6). These results imply that a dissociative reaction, involving the BN sheet or water or both, must be operative that would impart an effective positive charge to the sheet while the solution turns basic. ReaxFF Molecular Dynamics Simulations. In order to confirm the above inference, and decipher the mechanism of the dissociation reactions on the BN nanosheets, molecular dynamics simulations were performed using the ReaxFF forcefield. Simulations were performed for BN sheets with zigzag terminated edges (alternating B and N terminated edges), as well as for sheets terminating in the armchair edge configuration, immersed in a simulation cell containing water molecules. The progress of the simulation was analyzed from snapshots at different stages along the MD trajectory. In the following sections we follow the progress of the simulation for the zigzag terminated BN nanosheets and subsequently on the armchair terminated sheets. D

DOI: 10.1021/acs.jpcc.7b12288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Variation of the concentration of different groups as the ReaxFF MD simulation of the zigzag-BN−water system progresses. (a) Concentration of H atoms and −OH groups bound to the edges of the BN nanosheet. (b) Concentration of H and −OH groups in the bulk.

Figure 5. Chemical events captured during the course of the reax simulations of water molecules interacting with armchair terminated BN nanosheet (a) 0.1 ps; chemisorption of water on a N atom. (b) 0.15 ps; the free H atom of the chemisorbed water attaches to an adjacent B atom. (c) 0.2 ps; the complex flips leaving the O atom attached to the B atom. Chemisorption of water on a B atom (d) 0.25 ps; the complex breaks-up leaving the − OH attached to the B atom and the H on the N atom. (e) Snapshot at the end of the simulation (35 ps). All edge nitrogen atoms either have a proton attached or an adsorbing-desorbing water molecules, while the boron edge atoms have either a stable hydroxyl group or an adsorbed water molecule. The atom color codes are the same as in Figure 3.

of the simulation all the edge N atoms of the BN nanosheet are protonated (Figure 3e). On the boron atom terminated edge, however, multiple species, B−OH, chemisorbed H2O as well as the labile B−H, are observed (Figure 3e). The change in concentration of bound and free, protons and hydroxyl groups as the simulation proceeds are shown in Figures 4, parts a and b. The plots clearly show that the dissociation of water on the BN sheet results in an excess of hydroxyl ions in the solution accompanied by an increase of proton terminating edge atoms on the sheet. The latter are predominantly stable NH groups. The results would explain the experimentally observed increase in the pH of the medium with increasing sonication time (Figure 2c) and why the BN sheets are positively charged. The interaction of water with BN nanosheets with edges terminating in the armchair configuration are shown in Figure 5. A complete movie file along the trajectory is included as part of the Supporting Information, Movie S2. Similar to the zigzag

terminated sheets no reactions are observed on the basal plane, but the dissociation pattern of water molecules at the edges are quite different. Within 0.1 ps from the start of the simulation water molecules are observed to chemisorb on the both the B and N edge atoms. On both atoms, water molecules are attached through their hydrogen atoms (Figure 5, parts a and b) . In the second step the free proton of the adsorbed water attaches to an adjacent atom. In Figure 5b, it may be seen that the water molecules that were initially chemisorbed on the N atom (Figure 5a) forms a complex with the free hydrogen of the chemisorbed water attached to the adjacent B atom. This complex is unstable. The bridging proton on the boron atom flips with the oxygen atom resulting in a complex wherein the adjacent boron and nitrogen atoms are linked through the water oxygen and proton, respectively (Figure 5c). The complex immediately dissociates resulting in the formation of an edge B−OH with the proton attached to the adjacent nitrogen atom forming a NH bond (Figure 5d). Both the B− E

DOI: 10.1021/acs.jpcc.7b12288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 6. (a) 1H solid-state NMR spectrum of BN nanosheets. The fit to a sum of three Lorentzians and the individual components is also shown. (b) 11B solid-state NMR spectra of BN nanosheets, hexagonal BN powder, and ammonia borane.

recorded at a spinning speed of 60 kHz is shown in Figure 6a. The spectra shows an intense signal at 1.6 ppm and in addition weak broad features that appear downfield. The experimental spectra may be decomposed as three Lorentzians centered at 8.3, 4.6, and 1.6 ppm. The peak at 4.6 ppm is assigned to water and would correspond to water adsorbed on the nanosheets.29 The downfield peak at 8.3 ppm would correspond to an acidic proton such as B(OH)3, which is known to have an average chemical shift around 8.6 ppm.30 The upfield signal at 1.6 ppm could correspond to either a -BH or -NH functional group. The chemical shift of a proton attached to either a nitrogen or boron atom appear around the same region and hence difficult to assign this peak to either of these groups.31 If, the intense peak at 1.6 ppm in 1H the solid state spectra is due to a BH group it should show up in the 11B NMR spectra of the BN nanosheets. The 11B NMR spectra of the nanosheets along with the spectra of bulk hexagonal BN is shown in Figure 6b. Also shown in Figure 6b is the 11B MAS NMR spectrum of ammonia borane that is known to possess a N−B−H linkage. It may be seen that the spectra of the BN nanosheets is comparable with the spectra of bulk BN and the intense peak at −20 ppm seen in the spectra of ammonia borane is absent, suggesting that B−H groups are not present on the BN nanosheets. This, in turn, would imply that the intense peak at 1.6 ppm observed in the 1H solid state NMR spectra (Figure 6a) is due to N−H groups on the BN nanosheets. The results of the solid state NMR spectra, especially the 1H MAS NMR spectra, are in broad agreement of the ReaxFF simulations of the interaction of water molecules with a BN sheets, that too had indicated the presence N−H and B−OH groups along with chemisorbed water.

OH and N−H bonds are stable and undergo no further reactions. The entire process from adsorption to complex formation to dissociation is fast and is over within three to four consecutive frames. A similar sequence of events occurs when a water molecule is initially chemisorbed on a boron atom. The free hydrogen atom attaches to an adjacent N atom followed by a flip so that the oxygen of the water is attached to the B atom. The complex breaks up leaving the OH attached to the B atom and a NH bond. The dissociation of water on the BN nanosheets that terminate in the armchair configuration is a cooperative event and requires adjacent B and N atoms that are available. If, for example, water molecules chemisorb on an edge N atom that has both the adjacent boron atoms with hydroxyl groups attached, the only event that is observed is the adsorption and desorption of water. Similarly, if a water molecule chemisorbs on a B atom that has both the adjacent N atoms protonated, no dissociation is observed and only the adsorption−desorption of water molecules occurs. At the end of simulation all edge nitrogen atoms either have a proton attached or an adsorbingdesorbing water molecules, while the boron edge either has a stable hydroxyl group or an adsorbed water molecule that is continuously undergoing adsorption−desorption (Figure 5e). A notable difference between the interaction of water with the BN nanosheets that terminate in armchair and zigzag conformations is that on the former the B−OH and N−H bonds are stable and consequently if the number of edge B and N atoms are identical there is no change in the H and OH concentrations in the bulk solvent as the simulation proceeds. The results on the armchair terminated BN edges are in contradiction with the experimental observations, which showed the dispersions turning increasingly basic as sonication proceeds, and would suggest, within the limitations of the present ReaxFF simulations, that the BN nanosheets produced by sonication assisted exfoliation have predominantly zigzag terminated edges. The conclusions are in agreement with earlier experimental studies that showed a predominance of evidence for the presence of more number of zigzag terminated edges as compared to armchair edges in exfoliated BN nanosheets.27,28 1 H and 11B MAS NMR Measurements. The ReaxFF simulations provide a hint as to what to search for in experimental measurements. Here we have used 1H and 11B MAS solid-state NMR spectroscopy to identify the possible species present on the sonication assisted exfoliated BN nanosheets. The 1H solid state spectra of the BN nanosheets

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CONCLUSIONS We have investigated the stability of aqueous dispersions of BN nanosheets obtained by the sonication assisted exfoliation of bulk hexagonal BN powders in water. The production of aqueous dispersions of BN nansoheets without the use of stabilizers is rather unusual as a similar phenomenon is not observed for the isoelectronic graphite. BN and graphite have comparable hydrophobicities, and in fact, the latter is more difficult to exfoliate because of stronger interlayer electrostatic interactions. We show here that the dispersions are stabilized by electrostatic repulsive interactions with the delaminated BN nanosheets positively charged and the medium turning increasingly basic as sonication proceeds. F

DOI: 10.1021/acs.jpcc.7b12288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 7. Comparison of the results of the ReaxFF molecular dynamics simulations of the interaction of BN nanosheets with water and the 1H MAS solid-state NMR of BN nanosheets. The panel on the left shows the species present at the end of the simulation on BN nanosheets terminating in zigzag edges while the panel on the right the experimental 1H solid-state NMR spectra.

The interaction of water with BN nanosheets terminating in both the zigzag and armchair edge configurations were investigated by ReaxFF: a bond order dependent empirical reactive force-field−molecular dynamics simulation. The simulations indicate that on BN nanosheets that terminate in the armchair conformation stable B−OH and N−H bonds are observed in equal concentration with water molecules adsorbed on the vacant edge sites. The results of the simulation of the interaction of water with the zigzag terminated edges of BN are closer to experiment; all N atoms are protonated but only part of the atoms of the B edge are bonded to OH, the others having either chemisorbed water or a labile proton. An interesting consequence of the dissociation of water at the N terminated edges is the release of OH groups into the bulk, which mirrors the experimentally observed increase in the pH of the medium, as sonication proceeds. 1H and 11B MAS solidstate NMR measurements have been able to identify the major species present on the exfoliated BN nanosheets. These include, in addition to BN, NH, B−OH and chemisorbed water. These species had also been identified in the MD simulations. The results from the MD simulations and NMR spectroscopy are summarized in Figure 7. In conclusion, our results, both from experiment and from simulations, are able to establish the origin of the stability of aqueous dispersions of BN nanosheets obtained by sonication assisted exfoliation. The dispersions are stabilized by electrostatic interactions between positively charged BN sheets. Charges develop on the sheets as a consequence of the dissociation of water molecules at the edges of the sheet that leave the nitrogen edge atoms protonated and the release of hydroxyl groups into the bulk leading to an increased basicity of the medium. These dissociation reactions occur exclusively at the edges of the BN nanosheet leaving the basal plane intact and defect free. The dissociation of water molecules at the edges is crucial to the stability of the aqueous dispersions of BN and our results would explain why graphite, which is generally considered easier to exfoliate as compared to BN, does not exhibit similar behavior.

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Section S1, 1H solid state MAS NMR pulse sequence, section S2, validation of the ReaxFF parameters, section S3, ReaxFF parameters, section S4, BN sheet configurations, section S5, BN−water simulation cell, and section S6, pH measurements (PDF) Movie S1, movie file along the trajectory of ReaxFF MD simulation for the zigzag BN−water system (AVI) Movie S2, movie file along the trajectory of ReaxFF MD simulation for the armchair BN−water system (AVI)

AUTHOR INFORMATION

Corresponding Author

*(S.V.) E-mail: [email protected]. Telephone: +91-80-2293-2661. Fax: +91-80-2360-1552/0683. ORCID

Sukumaran Vasudevan: 0000-0002-5059-6098 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of the NMR Research Centre for use of the AV500 solid state NMR spectrometer. The authors thank the Supercomputer Education and Research Centre at the Indian Institute of Science, Bangalore, India, for use of the HPC Cray-XC40 facility. S.V. thanks the Department of Science and Technology, Government of India, for the J. C. Bose national fellowship.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12288. G

DOI: 10.1021/acs.jpcc.7b12288 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b12288 J. Phys. Chem. C XXXX, XXX, XXX−XXX