Effects of Different Hydrogenation Regimes on Mechanical Properties

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Effects of Different Hydrogenation Regimes on Mechanical Properties of h-BN: A Reactive Force Field Study Rajesh Kumar, Pierre Mertiny, and Avinash Parashar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05812 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 8, 2016

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The Journal of Physical Chemistry

Effects of Different Hydrogenation Regimes on Mechanical Properties of h-BN: A Reactive Force Field Study Rajesh Kumar1, Pierre Mertiny2 and Avinash Parashar1* 1

Department of Mechanical and Industrial Engineering Indian Institute of Technology, Roorkee - 247667, India 2

Department of Mechanical Engineering University of Alberta, Edmonton, T6G2R3, Canada * Corresponding author: E-Mail: [email protected], Ph.: +91-1332-284801

Abstract This article describes molecular dynamics based simulations, which were performed to investigate the effects of different hydrogenation regimes on the mechanical properties of boron nitride nanosheets (h-BN). The reaction force field (ReaxFF) was used as the interatomic potential to capture atomistic interactions. Separate atomistic models were developed for pristine, semi-hydrogenated (hydrogen is attached either to boron or nitrogen) and fully hydrogenated h-BN (hydrogen is attached to both boron and nitrogen). The radial distribution function was used to study the structural integrity and stability of both pristine and hydrogenated structures. The simulations predicted an improvement in stability and integrity of the atomistic structures under the influence of hydrogenation compared to pristine h-BN. The semi-hydrogenated structure in which hydrogen was attached only to nitrogen was found to be the least stable configuration, while the fully hydrogenated structure was the most stable. Furthermore, the selective hydrogenation of h-BN nanosheets was studied with respect to tailoring the mechanical behaviour of h-BN nanosheets. With applied strain the hydrogen atom shifts its role from hydrogen bond acceptor to donor, which increases the toughness of semi-hydrogenated h-BN nanosheets.

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1.0

Introduction

Hexagonal boron nitride (h-BN) nanosheets have attracted considerable attention from the research community due to their exceptional properties1-3. The hexagonal space frame structure of h-BN along with their excellent mechanical and thermal properties closely resemble that of graphene4-5. h-BN exhibits a high Young’s modulus of about 1.0 TPa and excellent thermal conductivity6-8. The electrical insulating properties1 and its high thermal stability of h-BN at higher temperatures9 provide additional advantages over graphene in applications such as electronic packaging10-13 or nano-devices14-21.

Hydrogen functionalisation of graphene or h-BN has already been explored by researchers for tailoring selective material properties22-27. Hydrogen is a promising fuel for a clean and sustainable economy, yet, providing safe and economical means for hydrogen storage at room temperature has been a challenging task. Two-dimensional nanomaterials have been under intense research in order to explore possibilities of utilising them for hydrogen storage applications28-29. Moreover, hydrogenated BN nanofillers such as BN nanotubes (BNNTs) and nanosheets (h-BN) have an excellent potential as catalysts in hydrogen generation30 and radiation shielding in space programmes31.

Atomistic modelling techniques such as quantum mechanics based density functional theory (DFT), finite element analysis, and molecular dynamics (MD) based simulations are promising tools for capturing the material behaviour at the nanoscale level32. Among these techniques, MD based simulations have frequently been used by researchers for predicting the mechanical and fracture behaviour of nanofillers32. The accuracy of MD based simulations is highly dependent on the interatomic potential employed to capture atomistic interactions. The Tersoff potential is widely used for estimating interatomic interactions in


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nanofillers33-34 due to its straightforward and accurate parameterisation. However, the application of the Tersoff potential is still limited to certain types of atomistic structures. Other approaches such as the DREIDING force field35, the universal force field (UFF)36 and the reaction force field (ReaxFF)37 have also been used by researchers for atomistic modelling, including predicting the behaviour of BN nanofillers38-39. The DREIDING35 and UFF36 are more general than the ReaxFF, and hence, are considered to be not as accurate as the ReaxFF. The ReaxFF employs a distance based bond order criterion to define interatomic interactions and is capable of considering bond breaking and bond formation during system evolution. Quantum mechanics based simulations are more accurate compared to the ReaxFF, however, limitations with respect to their scalability are often an issue. Even though the ReaxFF was initially developed for complex reactions in hydrocarbons, its application has been extended to the modelling of nanofillers such as h-BN systems. Han et al. extended the ReaxFF to capture the interactions of hydrogen in BNNTs40-41, and Weismiller et al.42 used the ReaxFF potential parameters for simulating dehydrogenation and combustion of ammonia borane.

To authors’ knowledge the technical literature is still lacking conclusive studies on the mechanical behaviour of hydrogenated BN nanofillers. Ghazizadeh et al.43 used the UFF to model semi-hydrogenated BNNTs and reported the Young’s modulus to decrease for partial hydrogenation. In their theoretical work, Ghazizadeh et al. considered a BNNT configuration in which hydrogen was attached only to nitrogen. Hence, the study by Ghazizadeh et al. has limitations as far as the mechanical properties of h-BN are concerned, given recent studies 30, 44

which showed that h-BN nanosheets are more stable when hydrogen is attached to boron

instead of nitrogen. With the present paper, the authors augment the technical literature by providing a comprehensive study on the mechanical properties of hydrogenated h-BN



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nanosheets by considering more stable partially and fully hydrogenated h-BN nanosheet configurations.

2.0

Computational Method and Modelling Details

Even though the Tersoff potential is computationally more efficient compared to the ReaxFF, the inability of the Tersoff potential to account for a charge distribution over the atoms in hydrogenated h-BN nanosheets motivated to the selection of the ReaxFF for this study. Since the charge transfer plays an important role in B-N bond dynamics, the Tersoff potential is not sufficient to capture the physics involved in the mechanical behaviour of hydrogenated h-BN. In the present work, the large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code developed by the Sandia National Laboratories

45

was used in conjunction

with the ReaxFF parameters developed by Han et al.40. In the ReaxFF the total energy, Esystem, is expressed mathematically as given by equation1: = + + + + + + + + +

(1)

where Ebond is the bond order energy that depends upon the instantaneous interatomic distance; Eover and Eunder are the energies contributed by over and under-coordination of atoms; Elp is the energy contribution by lone pairs on nitrogen atoms; Eval is the energy contributed by the valence angle distortion; Epen is the penalty energy used for two double bonds sharing an atom in the valence angle like allene, which is equal to zero for BN systems; Etors is the energy associated with the torsion angle, which is equal to zero for linear and uniaxial tension scenarios; Econj is for the conjugation energy; and EvdWaals and ECoulomb


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are the energy potentials for van der Waals and the Coulomb interactions, respectively. Further details regarding these terms and their expressions can be found in37, 46.

For the present simulations the h-BN nanosheet dimensions were kept constant at 5 nm by 5.2 nm along the armchair and zigzag directions, respectively. Four different atomistic configurations were considered for the simulations, which have previously been reported in the technical literature

30, 47

: (i) pristine h-BN nanosheets (without hydrogenation), (ii) fully

hydrogenated with hydrogen attached to both boron and nitrogen (HBN, chair conformation), (iii) partially hydrogenated with hydrogen attached only to boron (H-BN), and (iv) partially hydrogenated with hydrogen attached only to nitrogen (H-NB). Herein, all simulations were performed for two different temperatures, i.e., 100K and 300K. In the first phase of the simulations, h-BN nanosheets were relaxed under NPT ensemble for 12.5 ps with a time step of 0.00025 ps. The simulation temperature and pressure were controlled using Nose-Hoover thermostat and barostat were deployed to control temperature and pressure, respectively. In molecular dynamics based simulations performed to study h-BN and hydrogenated h-BN structures, the equations of motions were solved via Velocity-Verlet scheme. Relaxed structures with respective bond lengths are shown in Fig. 1. The higher B-N bond length in hydrogenated systems is attributed to the shift in bond characteristics from the sp2 to the sp3 hybridisation state.



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Fig. 1. Relaxed structures of (a) pristine h-BN (b) fully hydrogenated h-BN (HBN), and semi-hydrogenated h-BN with (c) H atom on nitrogen only (H-NB) (d) H atom on nitrogen only (H-BN).

In order to avoid edge/size effects, in-plane periodic boundary conditions were employed in all the simulations. For the ReaxFF parameters, hydrogen bonding and near neighbour cut-off distances were kept at 0.6 nm and 4.5 nm, respectively. A charge equilibration was performed after every 10 integration steps. After achieving a relaxed atomic configuration, h-BN nanosheets were subjected to tensile loading at a strain rate of 10-3 ps-1 under NVT ensemble. Stress calculations were performed using a virial formulation as defined by Eq. (2).

=

1 1 Φ 2

+ ! "# $# ( #%&,


(2)

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where i and j denote indices in Cartesian coordinates; α and β are the atomic indices; mα and

vα are the mass and velocity of atom α; rαβ is the distance between atoms α and β; and Φ is the atomistic volume of the system.

3.0

Results and Discussion

The radial distribution function (RDF) is a measure that can be used to predict the stability of the atomistic structure. In order to evaluate the stability of different h-BN nanosheet configurations, i.e. pristine, fully and partially hydrogenated, the RDF for the different configurations was evaluated and compared. After relaxation, the semi-hydrogenated configuration (H-NB) was found to be highly distorted and unstable at 300K, and hence, this configuration was excluded from further studies. The stability and structural integrity of hBN nanosheets in the partially and fully hydrogenated regimes were studied with the help of the full width half maximum (FWHM) values estimated using the RDF. The calculated FWHM values for the various structures at 100K temperature are shown in Table 1. A comparatively lower FWHM value corresponds to a more stable atomistic configuration with a better structural integrity. It can be inferred from the FWHM values in Table 1 that a fully hydrogenated h-BN structure (HBN) is the most stable atomic configuration, whereas the HNB configuration is the least stable. A similar trend was also reported by Wu et al.48 for hydrogenated h-BN nanotubes. In another study, Wu et al.49 predicted that the adsorption of hydrogen on boron atom is an exothermic reaction, whereas the hydrogen adsorption on nitrogen is endothermic. This characteristic indicates that the H-BN configuration has a better structural stability as compared to H-NB, which is corroborated by the structural stability trends of H-BN and H-NB obtained in the present study.



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Table 1. FWHM for various configurations of hydrogenated BNNS.

BN

HBN

H-BN

H-NB

FWHM 0.00542 nm 0.00226 nm 0.00464 nm 0.00522 nm

The tensile response of partially and fully hydrogenated h-BN nanosheets at 100K is given in Table 2. The Young’s modulus of a pristine h-BN was determined as ~1.12 TPa, which is in good agreement with the results available in literature34. Additionally, the stress-strain response for all the h-BN configurations was plotted as shown in Fig. 2. Since the results obtained for the different temperatures differ only slightly quantitatively, but not qualitatively, further discussions were reduced to the results obtained at 100K. It can be inferred from both Fig. 2 and Table 2 that all hydrogenated as well as partially hydrogenated configurations exhibit a decrease in Young’s modulus compared to the pristine h-BN nanosheets. Similar observations were reported in the work by Ghazizadeh et al.43. The stress-strain responses shown in Fig. 2 also indicate that fully hydrogenated h-BN nanosheets offer minimum toughness, while the partially hydrogenated structures (H-BN and H-NB) exhibit a significant toughness improvement compared to pristine h-BN nanosheets. This trend is independent from chirality and temperature. For all four h-BN nanosheet configurations the B-N bond inflection point was found to be near a strain value of ~20%. It can further be observed from Fig. 2 that the semi-hydrogenated H-BN structures exhibit a perfectly plastic behaviour in the strain range from 6% to 12.5%. This behaviour is attributed to an interesting behaviour demonstrated by the hydrogen atoms at higher strain values, which is illustrated in Fig. 3 where images of the simulated h-BN structures at different strain values are shown. These images revealed a gradual tilting of H atoms in H-BN towards nitrogen with applied strain. As a B-N bond is stretched and the strain value in a respective bond reaches a threshold strain of approximately 6% the H atom tilts toward the nitrogen.


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The B-N bonds do not reach the threshold strain simultaneously, and hence, the process of tilting is gradual in nature, that is, the H atoms tilt one by one with the increase in strain. Tilting then stops at about 12.5% strain in the nanosheet. This behaviour of hydrogen tilting towards nitrogen provides an energy dissipation mechanism to the h-BN nanosheets, leading to the higher toughness and plasticity of partially hydrogenated H-BN. Table 2. Mechanical properties of pristine and hydrogenated-configurations of an h-BN nanosheet. Acronyms AC and ZZ refer to the h-BN lattice in armchair and zigzag configuration, respectively.

Temperature (K)

Configuration

Fracture Stress (GPa)

Fracture Strain

AC

ZZ

AC

ZZ

AC

ZZ

BN

1.128

1.123

154

136

0.11

0.10

HBN

0.864

0.865

97

104

0.12

0.13

H-BN

0.894

0.895

110

108

0.18

0.16

H-NB

0.964

0.972

130

132

0.20

0.19

BN

1.122

1.077

143

131

0.11

0.10

HBN

0.888

0.914

90

90

0.11

0.11

H-BN

0.974

0.99

108

111

0.18

0.17

100

300

Young's Modulus (TPa)



160

(a) 120 100

(b)

80 60

120 100 80 60

40

40

20

20

0 0.00

0.05

0.10

0.15 0.20 Strain [ / ]

0.25

160

(c)

0 0.00

0.30

140 120

80 60

(d)

0.25

0.30

BN HBN H-BN

T=300K

60

20 0.15 0.20 Strain [ / ]

0.30

80

20 0.10

0.25

100

40

0.05

0.15 0.20 Strain [ / ]

120

40

0 0.00

0.10

140

Stress (GPa)

100

0.05

160

BN HBN H-BN H-NB

T=100K

BN HBN H-BN

T=300K

140

Stress (GPa)

Stress (GPa)

160

BN HBN H-BN H-NB

T=100K

140

Stress (GPa)

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0 0.00

0.05

0.10

0.15 0.20 Strain [ / ]

0.25

0.30

Fig. 2 Stress-strain plots for different h-BN hydrogenation regimes. Graphs (a) and (b) depict data for the armchair lattice direction, and (c) and (d) for the zigzag direction.


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Fig. 3 Illustration of H atoms tilting toward N atoms at increasing strain levels. Pictures (a), (b) and (c) depict the armchair lattice direction, and (d), (e) and (f) the zigzag direction.

In order to better understand the physics underlying the observed perfectly plastic response, three adjacently bonded H, B and N atoms were chosen and their electrostatic charges were estimated. These charges were then plotted against applied strain, as shown in Fig. 4. It can be observed from this figure that as the strain value in h-BN nanosheet approaches 0.11, corresponding to ~6% in case of the B-N bond, the H atom transfers its charge and a charge redistribution takes place. After this charge redistribution the H atom changes its role from hydrogen bond acceptor to donor by transferring its negative charge to the B and N atoms. As soon as this charge transfer occurs the H atom, now carrying a positive charge, is tilted toward the negatively charged N atom along the loading direction, as was indicated in Fig. 3.



0.3

QB QN

0.2

QH

0.1 Charge (e)

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0.0 -0.1 -0.2 -0.3 0.00

0.04

0.08

0.12

0.16

0.20

0.24

Strain [ / ]

Fig. 4 Variation of the charges of B, N and H atoms with increasing strain.

It can also be inferred from Fig. 4 that with increasing strain in the B-N bond, more and more charge separation takes place between the B and N atoms. This kind of charge separation in B-N bonds has also been reported by Ju et al.50 in a pristine BNNT. The tilting behaviour of hydrogen can also be attributed to the site-selection preference of the H atom depending upon the strain values in the B-N bond as reported by Wu et al.49, who reported in their work that the site of adsorption for H atoms in a BNNT depends upon the amount of deformation in the BNNT. According to Wu et al.49, in an undeformed BNNT the lowest unoccupied molecular orbitals (LUMO) are mainly contributed by B atoms, causing H atoms to be preferentially adsorbed on B sites. But as a BNNT is deformed the N atoms contribute more to the LUMO, and hence, the preference for adsorption of H atom shifts from B to N atoms. While the energy of adsorption for both B and N sites decreases with the increase in deformation, N


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atoms have a lower energy of adsorption for H atoms as compared to B atoms at high strain values. Once all H atoms have tilted towards the N atoms a strain hardening effect was observed in the material, which is reflected in the stress strain response above 12.5% strain as shown in Fig. 2. This may be due the reduced ability for further bond stretching caused by the electrostatic interaction between H and N atom after the H atom tilted towards N atom, and hence, a higher force is required to further stretch the respective bond.

The variation of the FWHM for H-BN and H-NB for increasing strain is plotted in Fig. 5. The depicted decrease in the FWHM values for the H-BN structure along the zig-zag and as well as the armchair direction further emphasises that tilting of H atoms toward nitrogen gradually pushed the structure toward a more stable atomistic configuration. FWHM values for the H-NB configuration decrease with applied strain as well, which implies that the H-NB is also achieving higher stability.



(a)

0.06

H-BN

H-NB

0.05 FWHM

0.04 0.03 0.02 0.01 0 0

0.05

0.1 Strain [ / ]

0.15

0.2

(b)

0.06

H-BN

H-NB

0.05 0.04 FWHM

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0.03 0.02 0.01 0 0

0.05

0.1 Strain [ / ]

0.15

0.2

Fig. 5. Variation of FWHM values the for H-BN and H-NB configurations of BNNS with increasing strain. Graph (a) depicts the armchair lattice direction and (b) the zigzag direction. Failure morphologies for pristine, partially and fully hydrogenated h-BN nanosheets subjected to tensile loading at 100K are shown in Fig. 6. It can be observed from this figure that in both the armchair and zigzag direction, molecular chains are formed upon rupture in the pristine h-BN. Still, the failure of pristine h-BN nanosheets is brittle as indicated in Fig. 3 since these chains do not contribute sufficient support to promote ductile failure. The partially and fully hydrogenated h-BN nanosheets also exhibit brittle failure. In case of HBN, the failure is a neat and perfect cleavage while H-BN and H-NB both show some homoelemental bond formations. The neat cleavage of HBN may be attributed to the fact that all B


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and N atoms are sp3 hybridised in HBN, which restricts any further bond formation during failure.

Fig. 6. Failure morphologies of BN, HBN, H-BN and H-NB at 100K. Pictures (a), (b), (c) and (d) depict the armchair lattice direction, and (e), (f), (g) and (h) the zigzag direction.

4.0

Conclusion

In this study the ReaxFF was successfully employed to predict the mechanical behaviour of partially and fully hydrogenated systems of h-BN nanosheets. The hydrogenation of h-BN nanosheets changes the hybridisation state of B and N from sp2 to sp3, which resulted in an increase in the associated B-N bond length. FWHM value of RDF was employed to evaluate the structural integrity and stability of the different hydrogenation regimes. The simulations showed that hydrogenation improves the structural integrity as well as the stability of h-BN nanosheets. The fully hydrogenated h-BN configuration was found to possess the highest stability, while the configuration with hydrogen on nitrogen (H-NB) was predicted to have the lowest stability. The present work provided new insights into the mechanical behaviour of h-BN nanosheets under the influence of hydrogenation. Even though the Young’s modulus of hydrogenated h-BN nanosheets is reduced, the results obtained in this study confirmed a



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substantial improvement in toughness of the partially hydrogenated h-BN configurations. These improvements in toughness may make partially hydrogenated h-BN nanosheets a promising candidate for hydrogen storage and space applications under extreme conditions.

Acknowledgement This work is supported by the Indian Institute of Technology Roorkee, (Grant No. MID/FIG/100667) and Department of Science and Technology (SR/NM/NS-1469/2014), India.

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Tensile response of semi-hydrogenated h-BN 591x525mm (96 x 96 DPI)


Effects of Different Hydrogenation Regimes on Mechanical Properties

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