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The Theoretical Study on Interaction of Hydrogen with Single-Walled Boron Nitride Nanotubes

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The Theoretical Study on Interaction of Hydrogen with Single-Walled Boron Nitride Nanotubes THE JOURNAL OF CHEMICAL PHYSICS 123, 114703 ͑2005͒ The theoretical study on interaction of hydrogen with single-walled boron nitride nanotubes. I. The reactive force field ReaxFFHBN development ͒ Sang Soo Han, Jeung Ku Kang, and Hyuck Mo Leea Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea Adri C. T. van Duin and William A. Goddard III Materials and Process Simulation Center, California Institute of Technology, California 91125 ͑Received 2 November 2004; accepted 21 June 2005; published online 19 September 2005͒ We present a new reactive force field ReaxFFHBN derived to accurately model large molecular and condensed phase systems of H, B, and N atoms. ReaxFFHBN has been tested against quantum calculation data for B–H, B–B, and B–N bond dissociations and for H–B–H, B–N–B, and N–B–N bond angle strain energies of various molecular clusters. The accuracy of the developed ReaxFFHBN for B–N–H systems is also tested for ͑i͒ H–B and H–B bond energies as a function of out of plane ͑ ͒ ͑ ͒ ͑ ͒ in H–B NH2 3 and H–N BH2 3, respectively, ii the reaction energy for the B3N3H6 +H2 → ͑ ͒ B3N3H8, and iii crystal properties such as lattice parameters and equations of states for the hexagonal type ͑h-BN͒ with a graphite structure and for the cubic type ͑c-BN͒ with a zinc-blende structure. For all these systems, ReaxFFHBN gives reliable results consistent with those from quantum calculations as it describes well bond breaking and formation in chemical processes and physical properties. Consequently, the molecular-dynamics simulation based on ReaxFFHBN is expected to give a good description of large systems ͑Ͼ2000 atoms even on the one-CPU machine͒ with hydrogen, boron, and nitrogen atoms. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1999628͔ I. INTRODUCTION lotropes such as tubular and conical forms, and other poly- morphs of a zinc-blende type ͑c-BN͒, a wurtzite type The aim of the work described here is to develop the ͑w-BN͒, a graphite type ͑h-BN͒, and a rhombohedra type multiscale computational frame usable to describe chemical ͑r-BN͒. The h-BN crystal has a stacking sequence of processes and physical properties for large systems of H, B, AaAa,..., while the r-BN crystal has a three-layer sequence and N atoms with the accuracy of the first-principles meth- of ABCABC,... .3–7 Since the discovery of the boron nitride ods, but with the efficiency of a force field-based molecular- nanotube ͑BNNT͒ in 1995,3 there has been great interest in dynamics simulation method. For example, a quadratic the BNNT because it has about 5.5-eV band gap irrespective configuration-interaction ͑QCISD͑T͒͒ ͑Ref. 1͒ method, in- of its helicity. This is a very promising property in that the cluding both single and double excitations and connected BNNT can be more cheaply developed for nanometer scale triples, have proven remarkably accurate. On the other hand, size transistors than carbon nanotubes ͑CNTs͒. This is be- the computational time for this ab initio method is propor- cause the band gap of CNTs varies significantly depending tional to O3V4 where O and V are the number of occupied on its chirality.8 In addition, modification of the x, y, and z ͑ ͒ and virtual orbitals, respectively. Consequently, QCISD T is stoichiometries of BxCyNz nanotubes also presents a means limited in application to small chemical systems due to its to tailor their electronic properties.9 Also it was recently re- computational complexity and expense. Also, the develop- ported from theoretical studies that BN fullerene materials ment of density-functional theory ͑DFT͒ techniques, for ex- have hydrogen storage capacities larger than carbon fullerene 2 ample, B3LYP, has resulted in methods that are significantly materials.10,11 Ma et al.12 showed that their multiwalled bam- more efficient than ab initio methods of comparable accu- boolike BNNTs had the hydrogen storage capacity of about racy. However, any current DFT method is still not fast 2.6 wt % at room temperature. On the other hand, Tang et enough to describe chemical and physical properties accu- al.13 found that their BNNTs with collapsed structures ab- rately. Consequently, there is currently no computational sorbed 4.2-wt % hydrogen at room temperature. These re- method that is both chemically accurate and computationally sults show that BNNTs have great potentials to be used as efficient enough to be applied to large, dynamic systems with hydrogen storage media. Moreover, the synthesis of a c-BN a few thousand to million atoms of boron, nitrogen, and hy- crystal has recently gained an increased interest in a wide drogen elements. range of areas stemming from its extreme hardness, wide The boron nitrogen systems have several condensed al- band gap, high thermal conductivity, and chemical inertness.14 There have been some reports on the synthesis of ͒ a Electronic mail: [email protected] boron nitride films by employing physical and chemical- 0021-9606/2005/123͑11͒/114703/8/$22.50123, 114703-1 © 2005 American Institute of Physics Downloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp 114703-2 Han et al. J. Chem. Phys. 123, 114703 ͑2005͒ TABLE I. Atom parameters ͑energies are in units of kcal/mol͒. TABLE III. van der Waals and bond radius parameters. a a a ␴ ͑ ͒ ␲ ͑ ͒ ͑ ͒ ͑ ͒ ␥ ͑ ͒ Atom pov/un punder pv,3 pv,5 pv,6 Atom r Å r Å rvdW Å ee kcal/mol aa w Å B −2.50 31.91 3.96 3.00 2.84 B 1.30 1.00 1.65 0.08 13.00 2.00 aUsed for all the valence angles when the B atom is in a central position. ͑Ref. 25͒ interfaces. As described in Refs. 23 and 24, the 15–17 vapor deposition techniques. However, the boron nitride total energy in the ReaxFF is partitioned into several energy thin films synthesized so far exhibit multilayered structures terms as given in Eq. ͑1͒, with different phases.18,19 The interfacial region next to the substrate was usually an amorphous layer of a few nanom- Esystem = Ebond + Eover + Eunder + Elp + Eval + Epen + Etors ͑ ͒ eters, on the top of which was a textured h-BN layer, and a + Econj + EvdWaals + ECoulomb. 1 c-BN layer was observed upon the h-BN layer. Harris et al.20 used hydrogen for etching and found that hydrogen could The detailed description for each term was described in the remove other phases except the c-BN phase. Consequently, Refs. 23 and 24. On the other hand, ReaxFFHBN is derived the selectivity of hydrogen in etching BN phases is consid- for the systems of hydrogen, boron, and nitrogen atoms using ͑ ered to be the key factor for success in synthesizing of high- the above framework. Parameters for ReaxFFHBN see Tables ͒ quality c-BN films. I–VII are determined through optimization against DFT The molecular-dynamics simulation studies using data. The bond, angle, and torsion parameters for nitrogen 22 DREIDING ͑Ref. 21͒ and universal force field ͑UFF͒ have and hydrogen atoms are taken from the force field used in the ͑ ͒ 25 been reported on large systems of boron, nitrogen, and hy- nitramine hexahydro-trinitro-triazine RDX study. How- drogen elements, but the results based on conventional force ever, we optimize parameters for B atom, including B–B, fields21,22 are not considered to give a reliable and accurate B–H, and B–N bonds; H–B–H, B–N–B, N–B–N, H–B–N, description of chemical reactivity. In this respect, the ReaxFF and H–N–B angles; and all torsion angles with a central B–B ͑Refs. 23–26͒ has been recently proposed to describe well and B–N. For the cluster systems, DFT calculations are per- 27 ͑ ͒ bond breaking and bond formation in chemical processes, formed on the JAGUAR program using the B3LYP Ref. 2 ** 28 from which the conventional force fields are suffering, as method with a triple-zeta Pople basis set of 6-311G . For well as the crystal and mechanical properties for condensed boron nitride crystals, we fit two phases of the h-BN crystal ͑ ͒ ͑ ͒ phases accurately.23,24,26 3 and the c-BN crystal 4 , where the numbers in parenthe- In this study we develop and present the new reactive ses mean the coordination numbers of B or N atom. The ReaxFF was shown to describe well the variation of the co- force field, called “ReaxFFHBN,” to allow accurate descrip- 23–26 tion of chemical processes and physical properties for large ordination numbers of one atom in the crystal. For DFT systems with hydrogen, boron, and nitrogen atoms. The calculations on condensed periodic systems we employ the generalized gradient approximation ͑GGA͒ based on the ReaxFFHBN parameters are derived against the data from 29 quantum simulations of various molecules, clusters, and con- Perdew-Wang functional for the exchange-correlation po- tential with ultrasoft pseudopotentials30 to replace the core densed phases. The accuracy of the new parameter set is also 31 tested in several cases, including ͑i͒ H–B and H–N bond electrons, which is implemented in the CASTEP program. ͑ ͒ We use a kinetic-energy cutoff of 480.0 eV and the energies as a function of out of plane in H–B NH2 3 and 32 ͑ ͒ ͑ ͒ Monkhorst-Pack scheme to generate the k-space grid.
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