Letter pubs.acs.org/JPCL
Porous Boron Nitride with Tunable Pore Size Jun Dai,† Xiaojun Wu,‡ Jinlong Yang,§ and Xiao Cheng Zeng*,†,§ †
Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, 536 Hamilton Hall, Lincoln, Nebraska 68588, United States ‡ CAS Key Lab of Materials for Energy Conversion, Department of Materials Science and Engineering and Hefei National Lab for Physical Science at Microscale, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, China § Department of Chemical Physics and Hefei National Lab for Physical Science at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *
ABSTRACT: On the basis of a global structural search and first-principles calculations, we predict two types of porous boron-nitride (BN) networks that can be built up with zigzag BN nanoribbons (BNNRs). The BNNRs are either directly connected with puckered B (N) atoms at the edge (type I) or connected with sp3-bonded BN chains (type II). Besides mechanical stability, these materials are predicted to be thermally stable at 1000 K. The porous BN materials entail large surface areas, ranging from 2800 to 4800 m2/g. In particular, type-II BN material with relatively large pores is highly favorable for hydrogen storage because the computed hydrogen adsorption energy (−0.18 eV) is very close to the optimal adsorption energy (−0.15 eV) suggested for reversible hydrogen storage at room temperature. Moreover, the type-II materials are semiconductors with width-dependent direct bandgaps, rendering the type-II BN materials promising not only for hydrogen storage but also for optoelectronic and photonic applications. SECTION: Molecular Structure, Quantum Chemistry, and General Theory
B
materials via either template-assistant or template-free methods.19−30 Besides the application for hydrogen storage, microporous BN materials are also shown to exhibit favorable adsorption toward organic pollutants and metal ions and thereby to have great potential for water cleaning treatment.28,31,32 A number of computational designs of microporous BN materials have been reported in the literature for seeking new ways of synthesizing porous BN materials with controllable pore sizes. Wu et al. designed both honeycomb and triangular BN foam structures with a mass density of 0.78 and 1.63 g/cm3, respectively.33 Hao et al. simulated a phase transition of BNNT bundles under transverse pressure and predicted a sp2-sp3-sp2 transition that yields a porous sheet-stacking form of BN.34 Metallicity was also recently found in certain microporous BN materials.35,36 Note that a major obstacle in the synthesis of highly ordered BN nanomaterials is the limitation in B−N source. In other words, the likelihood of synthesis of highquality precursor BN nanomaterials is a very important factor for designing highly ordered microporous BN nanostructures. Today, BNNRs can be synthesized with high-quality edges via longitudinally unzipping BN nanotubes (BNNTs).37−39 The thickness of the synthesized BNNRs is dependent on the wall number of the unzipped nanotubes and can be reduced to
oron nitride (BN) is isoelectronic to carbon. Like carbon, BN materials can possess a variety of low-dimensional morphologies such as 1D nanotubes and 2D nanosheets; they have high thermal conductivity, thermal stability; and they are also mechanically very robust. However, BN nanostructures also possess distinct properties dramatically different from those of carbon. For example, BN nanostructures are insulators or wide-gap semiconductors and are chemically inert and not prone to oxidation.1−8 BN nanostructures are also good adsorbent materials because both B and N are light elements and thus can achieve a high gas uptake in gravimetric capacity. In addition, the high chemical inertness and oxidization resistance warrant sustainable recycling of the adsorbent, an important prerequisite for practical applications. Both experimental9−11 and theoretical studies12−18 have shown that BN nanostructures can exhibit high H2 uptake capacity due to strong interaction between H2 and heteropolar B−N bonds or partial H2 chemisorption. As an example, the multiwalled and bamboo-like BNNTs can adsorb 1.8 and 2.6 wt % H2, respectively, at 10 MPa pressure and room temperature,11 and this uptake capacity may be enhanced up to 4.6 wt % by defect structures.9 The partial H2 chemisorption tends to be irreversible, and typically ∼50−70% of the adsorbed H2 is retained even after lowering the pressure. Moreover, it has been suggested that achieving high H2 uptake capacity requires a high surface area per unit mass, high total pore volume, and high ultramicroporosity.19 Indeed, many experimental efforts have been devoted to the fabrication of microporous BN © 2014 American Chemical Society
Received: December 6, 2013 Accepted: January 5, 2014 Published: January 6, 2014 393
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Figure 1. Predicted highly stable porous BN structures: (a) dz2-BN, (b) dz4-BN, (c) lz1-BN, (d) lz1z2-BN, and (e) lz2-BN. The green and silver spheres denote B and N atoms, respectively.
Table 1. Computed Equilibrium Volume (V) (Å3 per BN unit), Ratio (R) of sp2 and sp3 B (N) Atoms to the Number of B (N) Atoms in the Unit Cell, and Bulk Modulus B (GPa), Shear Modulus G (GPa), and Young modulus Y (GPa) at the LDA Level for the Five Predicted BN Materials Shown in Figure 1 BN material
V
Rsp3
Rsp2
B
G
B/G
Y
dz2-BN dz4-BN lz1-BN lz1z2-BN lz2-BN
15.28 16.48 15.61 17.45 20.83
0 0 33.33 25 20
100 100 66.67 75 80
212.4 177.4 269.1 213.5 175.5
131.3 78.5 179.1 128.6 97.3
1.62 2.26 1.50 1.66 1.80
326.6 205.3 439.8 321.3 246.4
double-layer using a simple sonication treatment.39 Very recently, synthesis of high-quality single-walled BNNTs with nearly uniform diameter distribution has been reported,40 removing a major obstacle for synthesizing high quality singlelayer BNNRs. Interestingly, most BNNRs produced from cutting the BNNTs are zigzag type, whereas BNNTs are either mostly armchair or mostly zigzag.38 From the zigzag BNNRs, one can easily conceive a porous structure simply by fusing BNNR together. However, whether these porous BN materials are energetically favorable, how the BNNRs are fused together, and what new properties they would entail have not been thoroughly investigated. In this study, using a global optimization method based on the particle-swarm optimization (PSO) techniques,41,42 we perform a comprehensive structure search of porous BN materials built upon zigzag BNBRs. Two types of energetically favorable porous BN materials are revealed. Figure 1 displays two types of zigzag BNNR-fused porous BN materials, both with high stabilities. The first type (type-I) materials can be viewed as a direct connection of edge-atompuckered zigzag BNNRs, where the adjacent BNNRs are connected to each other via B−N bonding (see Figure S1a in the Supporting Information for details). The entire network structure is sp2 hybridized. On the basis of the width of zigzag nanoribbons, these type-I porous BN materials can be named directly fused zigzag BNNRs with width of n (dzn-BN). Following this naming rule, two structures in Figure 1a,b can be named dz2-BN and dz4-BN, respectively. The second type (type-II) of BN materials are composed of zigzag BNNRs and B−N chains, where the adjacent BNNRs are linked by B−N chains with sp3 B−N bonding (see Figure S1b in the
Supporting Information). On the basis of the width of the zigzag BNNRs, these type-II BN materials are named sp3 linked zigzag BNNRs with width of n (lzn-BN). Following this rule, structures in Figure 1c−e can be named lz1-BN, lz1z2-BN, and lz2-BN, respectively. Detailed structural parameters of these newly predicted porous BN materials are summarized in Table 1. Taking dz2-BN as an example, its space group is IMA2, its B atoms are located at 4a (0.0, 0.0, 0.33) and 4b (0.25, 0.87, 0.59), and the N atoms are located at 4a (0.0, 0.0, 0.51) and 4b (0.25, 0.63, 0.74), respectively. The 4b B (N) atoms are connected to each other, giving rise to a 3D network containing 1D pores. Detailed structural information of other predicted porous BN materials is summarized in the Supporting Information (SI) Table S1. The computed equilibrium volume of these BN materials is listed in Table 1. Owing to the microporosities, these materials possess larger equilibrium volume than the known bulk BN materials, for example, cBN, w-BN, and some predicted high pressure phases, such as bct-BN and Z-BN42−44 (see Table S2 in the Supporting Information for comparison). Although the structure search is limited to the maximal 10 BN units in a unit cell, one can easily construct larger type-I and type-II porous BN materials by using BNNRs with larger widths. To examine stabilities of the predicted porous BN materials, we first compute their cohesive energies and then compare them with those of popular BN materials. The cohesive energy is defined by the formula: Ecoh = (nEB + nEN − E(BN)n)/n, in which EB, EN, and E(BN)n are the total energies of a single B atom, a single N atom, and the (BN)n compound, respectively. The cohesive energies of the predicted porous BN materials, 394
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(BOMD) simulations with a constant-pressure and constanttemperature (NPT) ensemble. The time step is 3 fs, and the total simulation time is 9 ps. The structural integrity is wellretained during the simulation for both the dz2-BN and lz1-BN structures (see Figure 2c,d), suggesting good thermal stability for both porous structures. We have also computed elastic constants of the predicted porous BN materials (see Table S4 in the Supporting Information). The elastic tensor is calculated by applying finite distortions onto the lattice from which the elastic constants are obtained from the strain−stress relationship.49 The computed elastic constants for bulk h-BN and c-BN are also listed in Table S4 in the Supporting Information for comparison. One can see that the calculated data are in very good agreement with the experimental values, which validates the computational methods used in this work. All independent elastic constants meet the known Born stability criteria,50 again confirming mechanical stability of the predicted BN materials. The calculated elastic constants clearly show an anisotropic behavior of the BN materials. Along the axial direction, C33 is close to that of the h-BN along its axial direction, while along the radial directions, it is much smaller. By using the Voigt−Reuss−Hill approximation,51 the bulk, shear, and Young’s moduli are also calculated, as listed in Table 1. One can see that for each type of material the bulk, shear, and Young’s moduli decrease with the increase in the pore size. For type-II materials, the bulk, shear, and Young’s moduli also decrease with the increase in the ratio of sp2-bonding atoms. In addition, the calculated ratio B/G indicates that dz4- and lz2-BN are ductile while the other three are brittle. This is because a high (low) B/G ratio is associated with ductility (brittleness), and the critical B/G value that divides the brittle and ductile material is ∼1.75.50 The computed electronic band structures of lz1-, lz2-, dz2-, and dz4-BN are plotted in Figure 3. Type-I porous BN
including BN nanotubes, BN cages, and several recently predicted BN allotropes, are summarized in Table S3 in the Supporting Information. From the computed cohesive energies, the predicted BN materials are found to be metastable compared with the h-BN, w-BN, and c-BN, as the latter have greater cohesive energy than those of NiAs-type BN and some BN nanostructures, such as B12N12 cage, and the predicted stable small-radius (3,0) and (2,2) BN nanotubes45 at the local density approximation (LDA) level. Because the LDA is not so accurate for computing atomization energy, we also perform cohesive energy calculations using the HSE0646 functional. It is worth mentioning that the computed bandgap of monolayer BN and h-BN using HSE06 is 5.69 and 5.57 eV, respectively, which is in very good agreement with experimental results, indicating the reliability of the HSE06 functional for the BN systems. The computed cohesive energies based on HSE06 also show the energetic preference of the predicted BN structures over some known BN nanostructures (see Table S3 in the Supporting Information). Dynamic stability of the type-I and type-II porous BN materials is confirmed by phonon spectrum calculation using the density functional perturbation theory method implemented in the Quantum-Espresso package.47 Because of structural similarity in the predicted type-I or type-II BN materials, we only consider two representative structures, namely, dz2-BN for type-I and lz1-BN for type-II for the phonon calculations. As shown in Figure 2a,b, no imaginary
Figure 2. Computed phonon band structures of (a) dz2-BN and (b) lz1-BN and snapshots of (c) dz2-BN and (d) lz1-BN at 9 ps of the Born−Oppenheimer molecular dynamics simulation in the NPT ensemble (T = 1000 K). G (0.0, 0.0, 0.0), Y (0.5, 0.5, 0.0), S (0.5, 0.0, 0.0), Z (0.0, 0.0, 0.5), and T (0.5, 0.5, 0.5) refer to special points in the first Brillouin zone.
Figure 3. Electronic band structures of (a) dz2-BN, (b) dz4-BN, (c) lz1-BN, and (d) lz2-BN at the LDA level. The Fermi level is set to zero.
phonon frequencies are observed in the Brillouin zone for the dz2-BN and lz1-BN, indicating inherent dynamic stability of the type-I and type-II bonding of BNNRs. The highest frequency at the zone-center G point is 1421 and 1372 cm−1 for the dz2-BN and lz1-BN, respectively, close to that of h-BN.48 This vibrational mode is due to the in-plane B−N stretching. Besides the dynamic stability (a necessary and sufficient condition for mechanical stability of a crystal at low temperature) we also examine thermal stability of the new BN materials at an elevated temperature (1000 K) and zero pressure. We perform Born−Oppenheimer molecular dynamics
materials are all indirect bandgap semiconductors with the valence band maximum (VBM) being located at G point and the conduction band minimum (CBM) being located at Y point. The predicted bandgaps for dz2-BN and dz4-BN are 3.42 and 3.03 eV, respectively. Interestingly, type-II porous materials exhibit direct bandgap feature with both VBM and CBM being located near the Z point (see Figure 3c,d). Our LDA calculations suggest that the bandgap increases with the width 395
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which is about 3.3 to 3.5 Å in type-I materials and ∼4.4 Å in lz1- and lz1z2-BN. For type-II BN materials with relatively larger pore size, the adsorption of hydrogen molecule becomes highly favorable for hydrogen storage.55 Specifically, the computed hydrogen adsorption energies for lz2-BN and lz3BN are −0.24 and −0.15 eV, respectively. To address the effect of hydrogen ratio on the adsorption energies, we computed the adsorption energies for the first four hydrogen molecules within lz2-BN and lz3-BN, respectively (see Table 3). Although the
of BNNRs for lz1-BN, lz1z2-BN, and lz2-BN, and their bandgaps are 0.45, 1.30, and 1.53 eV, respectively. The behavior differs from the case of BNNRs in which the band gap decreases with the width of BNNRs.52 More detailed analysis shows that the VBM is contributed from the sp2 N atoms in the BNNRs, and these N atoms are also the nearest neighbors of the sp3 B atoms in the link, while the CBM is contributed from the sp2 B atoms in the BNNRs, which are also the nearest neighbors of the sp3 N atoms in the link (see Figure S2 in the Supporting Information). With increasing the width of the BNNRs, the interactions between the VBM (CBM) electrons decrease, resulting in a wider band gap. Because the LDA tends to underestimate the band gaps for semiconductors, a more accurate hybrid HSE06 functional is also used to confirm the trend of bandgaps of the type-II BN materials as a function of the width of BNNRs. The HSE06 calculation gives bandgap values of 1.85, 2.85, and 3.0 eV for lz1-BN, lz1z2-BN, and lz2BN, respectively, showing the same trend as the LDA calculation (see Figure S3 in the Supporting Information). The direct bandgap feature in conjunction with tunable bandgap via controlling the width of BNNRs for the type-II porous BN materials may be exploited for applications in optoelectronics and photonics. Because of the porosity of the predicted BN materials, the predicted BN materials all possess sizable surface area per unit mass (>2800 m2/g, see Table 2). To assess their potential
Table 3. Computed Adsorption Energies (eV) of the First Four Hydrogen Molecules Adsorbed within a Unit Cell of lz2-BN and lz3-BN lz2-BN lz3-BN
dz2-BN dz4-BN dz6-BN lz1-BN lz1z2-BN lz2-BN lz3-BN a
S (Å2)
Ead (eV)
∼4800 ∼3800 ∼3600 ∼3700 ∼3400 ∼3300 ∼2800
∼12 ∼27 ∼41 ∼19 ∼28 ∼43 ∼74
1.63 1.69 1.68 0.56 0.30 −0.24 −0.15
2nd-H2
3rd-H2
4th-H2
−0.24 −0.15
−0.24 −0.14
0.20 −0.22
0.07 −0.21
adsorption energy of the third hydrogen molecule in lz2-BN becomes positive, the adsorption of the third and fourth hydrogen molecules in lz3-BN is even stronger than the first two hydrogen molecules, and the average adsorption energy for the first four hydrogen molecules is about −0.18 eV, very close to the optimal adsorption energy (∼−0.15 eV) suggested for reversible adsorptive hydrogen storage at room temperature.55 In conclusion, on the basis of a global structural search and the first-principles calculations, two types of highly stable porous BN materials are revealed. Both types of BN materials can be viewed as a network of fused zig-zag BNNRs. Mechanical and thermal stabilities of these materials are examined via phonon spectrum calculations, Born stability criteria, and BOMD simulation with temperature controlled at 1000 K. All porous BN materials possess sizable surface area per unit mass, ranging from 2800 to 4800 m2/g. In particular, type-II BN material lz3-BN with a relatively large pore size appears to be highly favorable for hydrogen adsorption as the computed average hydrogen adsorption energy is very close to the optimal adsorption energy suggested for reversible adsorptive hydrogen storage at room temperature. Type-II porous BN materials are also direct bandgap semiconductors, whose bandgap can be engineered via controlling the width of constituent BNNRs. These desirable properties render type-II BN materials promising for either optoelectronic or hydrogenstorage applications.
Table 2. Computed Surface Area (SA) Per Unit Mass (m2/ g), Pore Sizes (S) (Å2), and Adsorption Energies (eV) of one Hydrogen Molecule in a Unit Cell of the Five Predicted BN Materials shown in Figure 1 and Two Other Constructed dz6-BN and lz3-BNa SA (m2/g)
1st-H2
Negative adsorption energy represents attractive interaction.
■
applications for hydrogen storage, we compute the adsorption energy of a hydrogen molecule in a pore of these porous materials. The adsorption energy is defined as Ead = Ea+b − Ea − Eb, where Ea+b is the total energy of the porous BN material with adsorbate and Ea and Eb are the total energies of porous BN material and hydrogen molecule, respectively. The van der Waals interaction between the hydrogen molecule and BN is accounted for using a dispersion-corrected DFT method, optB88-vdW (implemented in VASP 5.3 software package).53 This method has been previously tested for layered materials, such as graphite and h-BN.54 The computed hydrogen adsorption energies are summarized in Table 2. In addition, the hydrogen adsorption energies of two constructed larger porous BN materials (dz6-BN and lz3-BN) are listed for comparison. Our calculations show that the hydrogen adsorption is energetically unfavorable in type-I BN materials and type-II BN materials with relatively small pore sizes (e.g., lz1-BN and lz1z2-BN). The repulsion between the wall of BNNRs and the hydrogen molecule is likely due to the relatively short distance between two neighboring BNNRs,
COMPUTATIONAL METHODS The structure search is performed through a global minimization of the free-energy surface based on the PSO algorithm as implemented in CALYPSO code.41,42 The algorithm can predict stable structures with only the knowledge of the chemical composition. The code adopts a particularly devised geometrical structure parameter that allows elimination of similar structures during the structural evolution and thus improves the structure search efficiency. The code has been proven through its successful predictions of novel structures of many new materials,56−61 including porous carbon nanotube polymers and porous metallic BN.35,62 Here, for the search of porous BN materials, simulation cell size ranging from 1 to 10 formula units is considered. The local structure optimization and electronic property calculations are performed using the density functional theory method within the local density approximation (LDA) as implemented in the Vienna ab initio package (VASP 5.3).63,64 The ion−electron interaction is 396
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(9) Tang, C.; Bando, Y.; Ding, X.; Qi, S.; Golberg, D. Catalyzed Collapse and Enhanced Hydrogen Storage of BN Nanotubes. J. Am. Chem. Soc. 2002, 124, 14550−14551. (10) Wang, P.; Orimo, S.; Matsushima, T.; Fujii, H.; Majer, G. Hydrogen in Mechanically Prepared Nanostructured h-BN: a Critical Comparison with that in Nanostructured Graphite. Appl. Phys. Lett. 2002, 80, 318−320. (11) Ma, R.; Bando, Y.; Zhu, H.; Sato, T.; Xu, C.; Wu, D. Hydrogen Uptake in Boron Nitride Nanotubes at Room Temperature. J. Am. Chem. Soc. 2002, 124, 7672−7673. (12) Jhi, S.-H.; Kwon, Y.-K. Hydrogen Adsorption on Boron Nitride Nanotubes: a Path to Room-temperature Hydrogen Storage. Phys. Rev. B 2004, 69, 245407. (13) Wu, X.; Yang, J.; Hou, J.; Zhu, Q. Deformation-induced Site Selectivity for Hydrogen Adsorption on Boron Nitride Nanotubes. Phys. Rev. B 2004, 69, 153411. (14) Wu, X.; Yang, J.; Hou, J.; Zhu, Q. Hydrogen Adsorption on Zigzag (8, 0) Boron Nitride Nanotubes. J. Chem. Phys. 2004, 121, 8481. (15) Wu, M.; Gao, Y.; Zhang, Z.; Zeng, X. C. Edge-decorated Graphene Nanoribbons by Scandium as Hydrogen Storage Media. Nanoscale 2012, 4, 915−920. (16) Wu, X.; Gao, Y.; Zeng, X. C. Hydrogen Storage in Pillared Lidispersed Boron Carbide Nanotubes. J. Phys. Chem. C 2008, 112, 8458−8463. (17) Zhou, J.; Wang, Q.; Sun, Q.; Jena, P.; Chen, X. Electric Field Enhanced Hydrogen Storage on Polarizable Materials Substrates. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 2801−2806. (18) Jena, P. Materials for Hydrogen Storage: Past, Present, and Future. J. Phys. Chem. Lett 2011, 2, 206−211. (19) Yang, S. J.; Kim, T.; Im, J. H.; Kim, Y. S.; Lee, K.; Jung, H.; Park, C. R. MOF-derived Hierarchically Porous Carbon with Exceptional Porosity and Hydrogen Storage Capacity. Chem. Mater. 2012, 24, 464−470. (20) Schlienger, S.; Alauzun, J.; Michaux, F.; Vidal, L.; Parmentier, J.; Gervais, C.; Babonneau, F.; Bernard, S.; Miele, P.; Parra, J. Micro-, Mesoporous Boron Nitride-based Materials Templated from Zeolites. Chem. Mater. 2011, 24, 88−96. (21) Dibandjo, P.; Bois, L.; Chassagneux, F.; Cornu, D.; Letoffe, J. M.; Toury, B.; Babonneau, F.; Miele, P. Synthesis of Boron Nitride with Ordered Mesostructure. Adv. Mater. 2005, 17, 571−574. (22) Alauzun, J. G.; Ungureanu, S.; Brun, N.; Bernard, S.; Miele, P.; Backov, R.; Sanchez, C. Novel Monolith-type Boron Nitride Hierarchical Foams Obtained Through Integrative Chemistry. J. Mater. Chem. 2011, 21, 14025−14030. (23) Vinu, A.; Terrones, M.; Golberg, D.; Hishita, S.; Ariga, K.; Mori, T. Synthesis of Mesoporous BN and BCN Exhibiting Large Surface Areas via Templating Methods. Chem. Mater. 2005, 17, 5887−5890. (24) Rousseas, M.; Goldstein, A. P.; Mickelson, W.; Worsley, M. A.; Woo, L.; Zettl, A. Synthesis of Highly Crystalline sp2-Bonded Boron Nitride Aerogels. ACS Nano 2013, 7, 8540−8546. (25) Yin, J.; Li, X.; Zhou, J.; Guo, W. Ultralight Three-Dimensional Boron Nitride Foam with Ultralow Permittivity and Superelasticity. Nano Lett. 2013, 13, 3232−3236. (26) Rushton, B.; Mokaya, R. Mesoporous Boron Nitride and Boronnitride-carbon Materials from Mesoporous Silica Templates. J. Mater. Chem. 2008, 18, 235−241. (27) Weng, Q.; Wang, X.; Zhi, C.; Bando, Y.; Golberg, D. Boron Nitride Porous Microbelts for Hydrogen Storage. ACS Nano 2013, 7, 1558−1565. (28) Lei, W.; Portehault, D.; Liu, D.; Qin, S.; Chen, Y. Porous Boron Nitride Nanosheets for Effective Water Cleaning. Nat. Commun. 2013, 4, 1777. (29) Han, W.-Q.; Brutchey, R.; Tilley, T. D.; Zettl, A. Activated Boron Nitride Derived from Activated Carbon. Nano Lett. 2004, 4, 173−176. (30) Dibandjo, P.; Bois, L.; Chassagneux, F.; Miele, P. Thermal Stability of Mesoporous Boron Nitride Templated with a Cationic Surfactant. J. Eur. Ceram. Soc. 2007, 27, 313−317.
treated using the projector augmented wave (PAW) technique.65 A cutoff energy of 500 eV is used for the planewave basis set. The Brillouin zone is sampled using k-points with 0.03 Å−1 spacing in the Monkhorst-Pack scheme,66 while for density of states (DOS) calculation, denser k-point grids with 0.015 Å−1 spacing are adopted. For the geometric optimization, both lattice constants and atomic positions are relaxed until the forces on atoms are
