Hydrogen Adsorption on Rh, Ni, and Pd Functionalized Single-Walled

Feb 23, 2011 - (1, 2) However, it is very difficult to store hydrogen under ambient ..... takes place between H-s orbital and Rh-d orbital near Fermi ...
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Hydrogen Adsorption on Rh, Ni, and Pd Functionalized Single-Walled Boron Nitride Nanotubes L. P. Zhang,* P. Wu,* and M. B. Sullivan Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632 ABSTRACT: Rhodium, nickel, and palladium functionalized single-walled boron nitride nanotubes (SWBNNTs) and their applications to hydrogen storage have been investigated using density functional theory (DFT). Single Rh, Ni, and Pd atoms prefer to bind strongly at the axial bridge site of BN nanotube, and each Rh, Ni and Pd atom bound on BNNT may adsorb up to four, three, and two H2 molecules, respectively, with the H-H bonds of H2 molecules significantly elongated. More H2 molecules would bind with metal atoms and tubes when four metal atoms are dispersed at the bridge sites per cell, the presence of Rh, Ni, and Pd metal atoms leads to high hydrogen storage capacity on BNNTs. In addition, our calculation results also show that the nature of interaction between hydrogen and metal-doped BNNT is due to the hybridization of the metal d orbital with the hydrogen s orbital. Our work not only predicts hydrogen capacities and their binding energies for metal-doped BNNTs but also advances the understanding of the nature of hydrogen adsorption for efficient hydrogen storage.

1. INTRODUCTION Hydrogen is recognized as the ideal fuel due to its high utilization efficiency and environmental friendliness.1,2 However, it is very difficult to store hydrogen under ambient conditions due to very weak intermolecular interactions among hydrogen molecules. Hence, the development of a safe, effective, stable, and cheap hydrogen storage medium has attracted increasing attention in the scientific community. An efficient hydrogen storage material must possess fast sorption kinetics, high volumetric/ gravimetric density, relatively low dehydrogenation temperatures for chemical hydrides,3-6 and the hydrogen adsorption energy should be in the range of -0.2 to -0.7 eV at room temperature.6 To date, extensive theoretical and experimental studies have been reported to obtain highly efficient hydrogen storage. Various nanotubes,2,7-9 metal organic frameworks,10 and other nanostructure materials,11,12 especially carbon nanotubes (CNTs) have attracted considerable interest for promising a high capacity hydrogen storage medium owing to their high surface to volume ratios, light mass density, and good interaction between carbon and hydrogen molecules.8 While most experimental and theoretical studies have focused on the CNT/ transition-metal systems,13-15 experimental results show that the maximum hydrogen uptake capacity of pristine CNT is only 1.1 wt % at room temperature without any specific treatment. Theoretically, Yildirim and Ciraci predicted that a single Ti atom on CNT can bind up to four H2 molecules resulting in as much as 8 wt % of hydrogen absorbed on high coverage Ti-coated CNT.14 Xiao et al.13 reported that Pd-coated CNT can absorb 2.88 wt % of H2 molecules. Boron nitride nanotubes (BNNTs) exhibit a similar structure of CNTs, BNNTs are always semiconductors,16-18 in contrast to CNTs’ metallic or semiconducting properties. In addition, BNNTs possess unique electronic and mechanical r 2011 American Chemical Society

properties, high thermal stability and outstanding chemical inertness, which provide great potential for applications on energy storage, nanotube electronic devices, and new composite materials through functionalization of BNNTs by various materials.19-22 Recent experiments showed that pristine BNNTs could uptake 2.6 wt % hydrogen under 10 MPa at room temperature8 and Pt-treated BNNTs could reach an even higher storage capacity (4.2 wt %) at room temperature.23 Theoretically, Han et al.24 showed the possibility of covering the outer surface of single-walled BNNTs by up to 50% with hydrogen. However, both experimental and theoretical results demonstrate that pristine BNNTs cannot satisfy the requirement of hydrogen storage capacity for practical applications due to the weak interaction between the tubes and hydrogen molecules at ambient temperature and pressure. Recently, some efforts have been made to improve the hydrogen storage capacity of BNNT. Experimental results have indicated an improvement of electrocatalytic activity by surface modification with metal or alloy coatings would enhance the hydrogen storage capacity of BNNTs.25 Theoretical studies also suggested that functionalization of BNNTs with suitable metal atoms or coating BNNTs with transition metal atoms can significantly enhance the capacity of hydrogen storage23,26-29 since H2 molecules are directly bonded to the metal centers by the Kubas interaction.30 Durgun et al. showed that Ti atom adsorbed on BNNT can bind up to four H2 molecules,28 and Li et al. found that one Pt atom in functionalized BNNT can bind two hydrogen molecules.27 All of these studies show the metalBNNT system to be a possible hydrogen storage media. Received: August 19, 2010 Revised: December 23, 2010 Published: February 23, 2011 4289

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Figure 1. The most preferred optimized structures of (8, 0) BNNT doped with (a) Rh, (b) Ni, (c) Pd metal atoms.

Elemental palladium (Pd), nickel (Ni), and rhodium (Rh) are well-known to have a high affinity for hydrogen and form hydrides in reaction with hydrogen. They are used in hydrogen storage technology and are used as catalysts for hydrogenation due to their high hydrogen solubility, diffusivity, and corrosion resistance.31-35 In this work, the hydrogen adsorption and maximum number of H2 molecules on single Rh, Ni, and Pd functionalized BNNTs are investigated by first-principles density functional theory calculations so as to understand the nature of hydrogen interaction with metal functionalized BNNTs. Our results show that functionalization of BNNT with Rh, Ni, and Pd can optimize the hydrogen adsorption energy and enhance the hydrogen storage capacity by playing the roles of mediators for the interaction of hydrogen with BNNTs.

2. COMPUTATIONAL METHODS AND DETAILS Density functional theory calculations are carried out using the Vienna ab initio simulation package (VASP).36,37 The exchange correlation potential is approximated by the generalized gradient approximation (GGA) functional of Perdew and Wang38 and projector augmented wave (PAW) potentials are used to represent the electron-ion interaction.39 Since the electronic properties of BNNTs are weakly dependent on the tube diameter and helicity, calculations are performed on prototypical single-walled zigzag (8, 0) BNNTs without any loss of generality. The calculations are carried out in a supercell with dimensions 25  25  8.68 Å using periodic boundary conditions, 8.68 Å taken to be twice the relaxed nanotube lattice constant along the tube axis direction. A cutoff energy of 520 eV is used for the plane-wave basis and the total energy is converged to within 10-5 eV for each atom. The positions of all of the atoms in the supercell are fully relaxed and the Brillouin zone is sampled using a 1  1  3 k-point grid within the Monkhorst-Pack scheme.40 For the density of states (DOS) calculations, the k-point grid is increased to 1  1  10. 3. RESULTS AND DISCUSSIONS 3.1. Rh, Ni, and Pd Functionalized BNNTs. Before studying hydrogen adsorption on metal (M = Rh, Ni, and Pd) functionalized BNNTs (M-BNNT), pristine and metal functionalized (8, 0) BNNTs are investigated. The relaxed B-N bond length in a bare (8,0) BNNT is calculated to be 1.45 Å, similar to the bulk value for h-BN. Five possible high symmetry sites for metal atom adsorption on BNNT are considered as initial positions. Figure 1(a) shows different adsorption configurations, including the sites on top of the boron (B site) and nitrogen atoms (N site), the site at the center of a hexagon (H site), the bridge site over an axial B-N bond (BP site), and the bridge site over a zigzag B-N bond (BV site). The metal adsorption energy of the functionalized BNNT is defined as Ea = E(BNNTþM) - (E(BNNT) þ E(M)),

Figure 2. The binding energy of a metal atom as a function of its distance from the outer wall of a (8, 0) BNNT.

where E(BNNTþM) is the total energy of the fully relaxed M-BNNT, and E(BNNT) and E(M) are the energies of the isolated systems. By definition, a negative value of Ea corresponds to exothermic adsorption. For these sites, our calculations show that the adsorption of Rh, Ni, and Pd are all exothermic and the most stable sites are BP sites. Here, the Rh, Ni, and Pd atoms shift slightly toward the nitrogen atom (Figure 1) without encountering any energy barriers (Figure 2), and this is consistent with the results of previous theoretical studies.41-43 For metal atoms that start at the B and N sites, they eventually move to the BP site after relaxation. For the case of a Rh-BNNT system, the adsorption energy at the BP site is -1.90 eV and the Rh-B and Rh-N bond lengths are 2.16 and 2.10 Å, respectively. The B-N length nearest to the Rh atom elongates from 1.45 to 1.50 Å. For Rh atoms starting at the BV site, they move to the BP site after geometry optimization. Although Rh atoms starting at the H site remains there after geometry optimization, the adsorption energy is only -1.44 eV, much smaller than that at the BP site. This indicates that the H site is energetically less favorable than the BP site. For the Ni-BNNT system, the adsorption energy at the BP site is -1.72 eV with bond lengths of Ni-B and Ni-N being 2.10 and 1.84 Å respectively. This is consistent with the results suggested by Zhao and Wu41-43 and the value predicted by Auwarter et al.44 Ni atom at BV site spontaneously moves toward BP sites upon relaxation while Ni atom at the H site is still there with adsorption energies of -0.78 eV. Again, the most stable site for Ni atom adsorption is BP sites. The adsorption energy of the Pd-BNNT is -1.19 eV at BP site with the bond lengths of Pd-B and Pd-N being 2.25 and 2.11 Å respectively, in agreement with previous theoretical studies (Table 1). Unlike the adsorption of Rh and Ni on BNNTs, Pd atoms at the BV and the H site stay at the same sites with adsorption energies of -1.00 and -0.85 eV, respectively, after geometry optimization. However, the energy differences between the BP and BV, as well as the BP and H sites, are much larger than the thermal energy at room temperature (about 0.04 eV), indicating that adsorbed Pd atoms prefer to adhere to BP site. According to the Mulliken and Bader charge analysis of single Rh, Ni, and Pd atoms adsorbed on BNNTs,45,46 metal atoms donate electrons to the neighboring boron and nitrogen atoms on the BNNT, and this charge transfer decreases for boron and nitrogen atoms far away from the metal atoms. The d orbitals of Rh, Ni, and Pd atoms overlap with the sp2 orbitals of the M-B and M-N bonds (M = Rh, Ni, and Pd) to form a mixed sp2-d hybridization. This charge transfer behavior leads to metal atoms in cationic form and renders extensive heteropolar bonding between the metal atoms and the nearest boron and nitrogen 4290

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atoms. As a consequence, extra dipole moments are formed, thus resulting in an increase in the H2 molecule uptake. 3.2. Adsorption of H2 Molecule on Metal (M = Rh, Ni, and Pd) Functionalized BNNT. 3.2.1. H2 Adsorption on Rh-BNNT. The interaction between H2 molecules and the outer surface of Rh-BNNTs is investigated by first optimizing the geometry of the Rh atom at the BP site. Our calculation results show that a single Rh atom can chemically bind up to four H2 molecules. The relaxed structures of Rh-BNNT with adsorbed H2 molecules are shown in Figure 3(a). The average binding energy for hydrogen over the M-BNNT is defined as Eb(H2) = (E(BNNTþMþnH2) E[(BNNTþM] - nE(H2))/n, where E(BNNTþMþnH2) is the total energy of the fully relaxed M-BNNT with hydrogen, E(BNNTþM) is the total energy of fully relaxed M-BNNT system, E(H2) is the energy of an isolated hydrogen molecule, and n is the number of hydrogen molecules. By definition, negative adsorption energy corresponds to exothermic adsorption. When the first H2 molecule is adsorbed on Rh-BNNT, it adsorbs on top of the Rh atom with a binding energy of -1.15 eV and the direction of the H-H bond is perpendicular to the axial B-N bond. The H2 molecule is still intact with a significantly increased bond length of 0.91 Å and the bond length of Rh-H is 1.69 Å (Table 2). The Table 1. Adsorption Energy Ea, Bond Length between Metal Atom (M = Rh, Ni, Pd) and BNNT Surface, and Charge Transferred from Metal Atom to BNNT structure

Ea (eV)

Mulliken

Bader

M-B (Å)

M-N(Å)

charge (e)

charge (e)

Rh-doped

-1.90

2.16

2.10

0.55

-0.17

Ni-doped

-1.72

2.10

1.84

0.44

-0.15

Pd-doped

-1.19

2.25

2.11

0.33

-0.08

corresponding distances of Rh to the nearest boron and nitrogen atoms increase to 2.30 and 2.12 Å, respectively. Compared with the Rh-BNNT in the absence of H2 molecule, the adsorption of the H2 molecule weakens the interaction between Rh and BNNT due to a hybridization of hydrogen with the d orbitals of the Rh atom. This Rh atom donates electrons to the H2 molecule and these electrons are forced to occupy the antibonding orbital of the H2 molecule due to the Pauli exclusion principle. The antibonding electron weakens the H2 bond and leads to its elongation. In addition, the charge transfer from Rh to BNNT decreases upon adsorption of the H2 molecule, which weakens the interaction between Rh and the nanotube. In the case of two H2 molecules adsorbed on Rh-BNNT, it is found that the H2 molecules present a symmetric configuration on top of the Rh atom around the axial B-N bond. The average binding energy of each hydrogen molecule decreases to -0.81 eV accompanied by an elongation of H-H bond lengths of 0.91 Å (Table 2). To examine the maximum hydrogen storage capacity, more hydrogen molecules are added to the Rh-BNNT system. The binding energy decreases to -0.49 eV for the fourth H2 molecule, and the H-H bond lengths range from 0.74 Å to 0.77-0.87 Å. It can be seen from Table 2 that the distance of Rh-H2 increases with the number of adsorbed H2 molecules while elongation of H-H bond length decreases. Further study indicates that a fifth H2 molecule does not bind to the Rh atom, indicating that the capacity limitation is four H2 molecules per Rh atom on the BNNT. The binding energy per H2 molecule and binding strength between the metal-hydrogen complex and the BNNT decreases as the number of adsorbed H2 molecules is increased. To check the stability of the model RhH8 complex on the BNNT, the binding energy between the RhH8 complex and BNNT (Eb(MHx)) is calculated to be -1.02 eV, much larger than

Table 2. Average Binding Energies (per H2) (eV) and Bond Length between Metal Atom (M = Rh, Ni, Pd) and (8, 0) BNNT Surface (Å), Distance between Metal Atom and H2 (Å), Bond Length of H-H (Å), Charge Transferred (per H2) M/H2 for MetalDoped BNNT with Adsorbed H2 (e) structure Rh-doped

no. of H2

Pd-doped

M-B/M-N

M-H

H-H

1

-1.15

2.30/2.12

1.69

0.91

2

-0.81

2.46/2.19

1.70

0.91

1.71

0.91

3

-0.63

2.50/2.39

1.73

0.88

1.75

0.88

4

Ni-doped

Eb

-0.49

2.56/2.42

1.75

0.88

1.77

0.87

1.79 1.79

0.87 0.85

2.12

0.77

Mulliken charge M/H2 0.74/-0.28 0.9/-0.28,-0.24

Bader charge M/H2 0.19/-0.06 0.23/-0.06,- 0.05

1.05/-0.28 (2),-0.20

0.25/ -0.05,0.04(2)

1.25/-0.28(2), -0.22,-0.18

0.26/-0.05(2),-0.03,-0.01

1

-1.49

2.20/1.88

1.52

0.90

0.83/-0.42

2

-1.02

2.30/2.00

1.55

0.90

1.01/-0.38,-0.32

0.25/-0.06(2)

1.55

0.90

3

-0.80

2.37/2.06

1.09/-0.28,-0.26(2)

0.27/-0.05(2),-0.04

1.58

0.87

1.59 1.67

0.86 0.83

0.2/-0.06

1

-1.16

2.41/2.11

1.69

0.87

0.49/-0.22

0.10/-0.04

2

-0.77

2.57/2.31

1.74

0.87

0.72/-0.26,-0.22

0.12/-0.04,-0.03

1.74

0.86 4291

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Figure 3. Optimized structures for single (a) Rh, (b) Ni, (c) Pd metal-atom doped BNNTs with adsorbed H2 molecules.

the desorption energy -0.07 eV of the fourth H2 molecule. This implies that the RhH8 complex can stay chemically adsorbed on the BNNT during the process of H2 molecule release. 3.2.2. H2 Adsorption on Ni-BNNT. For Ni-BNNT, the configurations for the adsorption of H2 molecules are shown in Figure 3(b). Our calculations show that the first H2 molecule adsorption on Ni-BNNT is similar to that in Rh-BNNT. The binding energy is -1.49 eV with Ni-H bond lengths of 1.52 Å. The H2 molecule is partially dissociated with H-H bond lengths of 0.90 Å and the Ni-B and Ni-N distances are 2.20 and 1.88 Å, respectively (Table 2). Our calculations show that electrons are transferred from the Ni atom to both BNNT and H2 molecule. Compared with the Ni-BNNT in the absence of the H2 molecule, fewer electrons are transferred from Ni to the BNNT, and this leads to increased Ni-B and Ni-N lengths. When more H2 molecules are adsorbed on Ni-BNNT, the average binding energies for each H2 molecule decrease to -1.02 eV upon adsorption of the second H2 molecule and -0.80 eV for the third hydrogen molecule. The binding energies and electron transferred from Ni to each H2 molecule is a decreasing function of the H2 coordination number, while the Ni-B and Ni-N lengths are increasing. Our calculations show that only three H2 molecules can be chemisorbed onto the Ni atom and attempts to add a fourth H2 molecule at various positions failed. Similar to the case of Rh-BNNT, the adsorption energy between the NiH6 complex and the BNNT is -1.38 eV, much larger than the H2 molecule desorption energy of -0.35 eV, ensuring the stability of the NiH6 complex on BNNT in the event of H2 molecule release. 3.2.3. H2 Adsorption on Pd-BNNT. Figure 3(c) shows the relaxed structures of Pd-BNNT with H2 molecules. When the

first H2 molecule is adsorbed on the optimized Pd-BNNT, the adsorption energy is -1.16 eV. This displays a higher binding energy than that on Pd-CNT.13 The distances of Pd-H2 and H-H bond lengths are 1.69 and 0.87 Å, respectively, while the distances of Pd to the nearest boron Pd-B and nitrogen Pd-N become 2.41 Å and 2.11 Å respectively (Table 2). Compared with the case of Pd-BNNT in the absence of a H2 molecule, the amount of charge transferred from Pd to BNNT is reduced, leading to a weakened interaction between the Pd atom and BNNT. When the second H2 molecule approaches the PdBNNT, the average binding energy per H2 molecule is -0.77 eV with a Pd-H2 distance of 1.74 Å, while the H-H bond lengths elongate from 0.74 Å to 0.86 and 0.87 Å. The charge transfer from Pd to BNNT decreases upon adsorption of the second H2 molecule; this second H2 molecule further weakens the interaction between Pd and the BNNT. Additional studies show that only two H2 molecules can bind with the Pd atom. As before, the adsorption energy between the PdH4 complex and the BNNT is calculated to be -0.88 eV, while the second H2 molecule desorption energy is -0.23 eV, indicating that the interaction between the PdH4 complex and BNNT is still binding in the event of H2 release. From the above discussion, it can be seen that the adsorbed H2 molecules carry a negative charge in metal hydrides and H2 molecules are trapped by Rh, Ni, and Pd cation via the charge polarization mechanism.47 The presence of the H2 molecule induces a charge density redistribution between the metal and the nanotube, causing a weakening of the metal-nanotube interaction. Similarly, the metal-nanotube interaction reduces the number of electrons in the metal that can hybridize with the H2 4292

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Figure 4. Optimized structures for eight (a) Rh, (b) Ni, (c) Pd metal atoms placed uniformly on unit cell of BNNT.

molecule. The amount of charge transferred from Rh, Ni, and Pd atoms to the antibonding orbital of H2 molecule is a decreasing function of the coordination number of H2 molecules, leading to the weakening of the M-H bond. When a single H2 molecule is introduced to the metal-BNNT, the adsorption energy of the H2 molecule decreases in the order of Ni > Pd > Rh and monotonically with the atomic radii of metal atoms. However, when more H2 molecules are introduced, the above sequence is changed due to their different d states, but for the same d8 configuration of Rh and Ni, the sequence is the same (Ni > Rh). Comparing the hydrogen storage capacity in these cases, Rh-BNNT can bind to the largest number of H2 molecules. In the electronic configuration of Rh and Ni, the d orbitals have more vacancies than that of Pd. In addition, more charge is transferred from Rh and Ni to the nanotube than that from Pd. In the cases of Rh and Ni, eight electrons occupy the d orbitals and two more electrons are needed to fill up the d configuration, while in the case of Pd, 10 electrons fully occupy the d orbitals. Hence, Rh and Ni can bind to more H2 molecules, yielding a higher storage capacity than Pd. Between Rh and Ni, the number of electrons in the d orbitals is the same, however, there is only one electron in the 5s orbital of Rh while there are two in Ni. This leads to more vacancies in Rh to be filled by electrons from H2 molecules than in Ni. In addition, Rh transfers more electrons to the BNNT than Ni. Hence, Rh-BNNT complexes can bind to more H2 molecules than Ni-BNNT. The above shows that the electronic configuration of the metal atom plays a more important role than the metal size for hydrogen adsorption. To improve the hydrogen storage capacity, systems containing four and eight Rh, Ni and Pd atoms per cell placed uniformly on the tube surface are also investigated. The metal atoms are placed using the energetically most favorable location obtained from the optimization of a single atom adsorbed on BNNT. For eight adatoms per cell, results show that Rh, Ni, and Pd atoms dimerized with the binding energy of -1.01, -0.98, and -1.02 eV per atom. As shown in Figure 4, the interaction between two metal atoms leads to dimerization. Compared with the bulk metals (Rh: d = 2.72, Ni: d = 2.49, and Pd: d = 2.80 Å), the bond lengths of dimer atoms are contracted to 2.52, 2.26, and 2.63 Å upon full relaxation, but are elongated relative to stand-alone dimer distances (2.21, 2.09, and 2.49 Å). In addition, all distances between metal atoms and the tube surfaces are increased. These changes indicate that the formation of metal Rh2, Ni2, and Pd2 dimerization weakens the interaction between the tube surface and metal atoms while the interaction between the metal atom and the tube surface also weakens the metal dimer bonds. In the case of four metal atoms per cell, the metal atoms are uniformly distributed on the surface of BNNT after full relaxation, indicating that no dimerization has occurred. This is due to the presence of an energy barrier between the metal atoms preventing

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dimerization. The adsorption energies per atom for Rh, Ni, and Pd are -1.82, -1.70, and -1.18 eV, respectively. The nearest M-B and M-N distances are 2.20, 2.09, 2.11, 1.84, 2.26, and 2.11 Å for Rh-B, Rh-N, Ni-B, Ni-N, Pd-B, and Pd-N, respectively. Metal atoms chemically adsorbed on BNNTs are similar to a single metal atom adsorption on BNNT. With these configurations, the ratio of M:B:N = 1: 4: 4 (M = Rh, Ni, and Pd) is quite moderate and the length between two metal atoms at the nearest bridge sites along the nanotube axis is 4.344 Å. This is sufficiently large to avoid clustering of two chemisorbed metal atoms on BNNTs while obeying doping rules for high coverage metals. Table 3 lists the calculated hydrogen storage capacities, binding energies, and geometric parameters for (8, 0) BNNT functionalized with Rh, Ni, and Pd and Figure 5 shows the corresponding configurations of the relaxed geometries. The region of adsorbed H2 molecules is either close to or far from the metal atoms. For the 8Rh-BNNT-40H2 system, the average binding energy of H2 is -0.45 eV with a H-H bond length of 0.75 Å, the binding energy of four H2 molecules far from Rh atoms is -0.18 eV, which is close to the lowest binding requirement of -0.20 eV/H2. With this configuration, the hydrogen storage capacity is approximately 4.9 wt %. When 48 H2 molecules are adsorbed on Rh-BNNT, the calculated binding energies of close to Rh atom are -0.36 eV. However, that of H2 molecules far from the Rh atom is only -0.12 eV, which is much smaller than the lowest requirement but much larger than that on pristine BNNT (-0.05 eV). The calculated results and geometries of 8Ni-BNNT and 8Pd-BNNT with different number of H2 molecules are listed in Table 3 and shown in Figure 5. For the 8Ni-BNNT system, it is interesting to find that the H2 molecules between two Ni atoms keep an upright position on the tube. This geometry is robust to different starting configurations after geometry optimization. In the case of 8Pd-BNNT, the average binding energy is -0.35 eV with the hydrogen storage capacity of 3.9 wt %. It must be mentioned here that metal atoms tend to aggregate into clusters when their concentration is large. Hence, the hydrogen storage capacities achieved in these ideal circumstances might be reduced once metal clustering occurs. 3.3. Nature of Interaction between Hydrogen and MetalDoped BNNTs. In order to understand the nature of hydrogen adsorption on metal-doped BNNTs, the projected densities of states (PDOS) for the s-orbitals of the H2 molecule and the s, d orbitals of the metal atom are calculated. As shown in Figure 6, the Fermi energy is set to zero in the DOS plots. It is found that significant orbital interactions arise between the hydrogen s-orbital and metal atom d-orbitals, resulting in split-off d orbitals as explained by ligand field theory: H2 molecules approaching the positively charged metal cation leads to loss of d orbital degeneracy as the electrons of the ligand will be closer to some of d orbitals while farther away from others. Thus, the d electrons closer to the ligand will have a higher energy than those farther away. This results in the d orbitals splitting in energy to reduce the total energy and stabilize the system. The PDOS of RhBNNT with H2 molecules are shown in Figure 6(a), where the number of H2 molecules bound with Rh is from one to four. It is evident that the significant orbital interactions take place between the H2-σ* antibonding orbital of the ligand and Rh-d orbitals with the hybridization of H-s orbital and Rh-d orbital and the formation of Rh-H bond. For Rh-BNNT-1H2, the interaction takes place between H-s orbital and Rh-d orbital near Fermi level. There is a pronounced Rh-dxy orbital peak and a small Rhdx2-y2 orbital at hydrogen energy state -1.18 eV, which means 4293

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Table 3. Number of Adsorbed H2, Average Binding Energies Eb (per H2) and Binding Energies of H2 Molecules Far Away from Atoms Eb-TH (per H2) (eV), Bond Length between Metal Atoms (M = Rh, Ni, Pd) and (8, 0) BNNT Surface (Å), the Distance between Metal Atoms and H2, Bond Length of H-H (Å) and Weight Percentage for H2 Adsorbed on Metal-Doped BNNT structure

no. of H2

Eb

Eb-TH

M-B/M-N

8Rh-doped

40

-0.45

-0.18

2.51/2.17

48

8Ni-doped

24

32

40

8Pd-doped

32

40

-0.36

-0.12

2.54/2.17

-0.70

-0.57

-0.45

2.27/2.04

-0.16

2.27/2.04

-0.10

2.34/2.07

-0.35

-0.28

2.48/2.29

-0.06

2.49/2.29

M-H

H-H

wt% 4.9

1.65

0.93

1.71

0.91

2.68

0.76

2.81

0.76

1.69 1.71

0.91 0.89

2.81

0.76

3.18

0.75

3.42

0.75

1.59

0.87

1.60

0.87

1.66 1.60

0.83 0.86

1.61

0.86

1.67

0.83

1.56

0.89

1.62

0.85

1.63

0.85

1.72 1.78

0.87 0.84

2.87

0.76

3.13

0.75

1.74

0.87

1.81

0.84

2.78

0.76

2.92

0.75

5.9

3.8

5.1

6.3

3.9

4.9

Figure 5. Optimized structures for eight (a) Rh, (b) Ni, (c) Pd metal atoms doped BNNTs with high density hydrogen coverage.

the protrusion of the dxy and dx2-y2 closer to the hydrogen electrons of the ligand makes them the most responsive d level to the approaching H2 molecule. In addition, the overlap of density of states between the H-s orbital and Pd-s appears in the range from -8.00 to -7.00 eV, indicating that H2-σ forms a band. The

adsorption states near the Fermi level are mainly attributed to the hybridization between Rh-d orbital and H-s orbital after the hydrogen adsorption. In Rh-BNNT-2H2, major hybridization also takes place between H-s orbital and Rh-dxy and Rh-dxz orbitals. Upon adsorption of the third and fourth H2 molecule, it 4294

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Figure 6. Projected density of states (DOS) plots for single (a) Rh, (b) Ni, (c) Pd metal-atom doped BNNTs with adsorbed H2 molecules.

can be seen that there are three and four overlaps of DOS between H-s orbital and Rh-d orbitals. It shows that there is more splitting of orbitals after more H2 molecules are adsorbed, resulting in several small peaks and a larger orbital splitting in energy. As a comparison, the PDOS for Ni-BNNT and Pd-BNNT with H2 molecules are also shown in Figure 6(b),(c). Similar results are found when H2 molecules are brought close to these

two systems. The significant orbital interactions occur between the H-s orbital and M-d orbital near Fermi level, and electron hybridization leads to charge redistributions. Meanwhile, the overlap of PDOS of H-s orbitals and metal-s orbitals are in the range from -8.50∼-6.00 eV, and it can be seen that the σ bonding orbital of the H2 molecule forms a bond with the metal atoms. For the Ni-BNNT complex, when three H2 molecules are adsorbed, an annular-like field is created, and the electron 4295

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The Journal of Physical Chemistry C hybridization between the H-s and Ni-d orbitals are also uniform, which results in splitting orbitals without priority toward the σ of the H2 molecule. The nature of the interaction between the hydrogen molecule and metal (M = Rh, Ni and Pd) functionalized BNNTs can be explained using hybridization between metal-d electrons, antibonding orbitals of H2 molecules, and sp2 orbitals of the nearest boron and nitrogen atoms. Metal atoms donate electrons to sp2 orbitals of the nearest boron and nitrogen to bind the metal atoms to BNNT. Upon H2 molecules adsorbed on BNNT, H2-σ orbitals of the ligand donate electron to vacant d orbital of metal atoms. Meanwhile, the filled d orbitals reverse the process by back-donating electrons into unoccupied H2-σ* antibonding orbitals of the ligand, resulting in the formation of metal-H2 complex and H-H bond elongation. When the interaction reaches a saturation level, no H2 molecule can be absorbed. For the splitting of d-orbitals, it is affected by the nature of the metal atom and the arrangement of the H2 molecules around the metal cation. In addition, the positions of the most responsive d level toward the adsorbed H2 molecules are determined by the ligand field splitting of the d orbitals based on the local symmetry.

4. CONCLUSIONS The interaction of hydrogen with Rh, Ni, and Pd functionalized BNNTs is investigated by density functional theory calculation. The calculation results show that Rh, Ni, and Pd atoms can make chemisorption bonds with the outer surface of the BNNT, and single Rh, Ni, and Pd atoms energetically prefer the axial bridge site of BNNT. Also, it has been shown that single Rh, Ni, and Pd adsorbed on BNNT can bind up to four, three, and two H2 molecules accompanied by a significant elongation of H-H bonds. The binding energy per H2 and the binding strength of the metal-hydrogen complex decrease with an increase in the number of H2 molecules. Hydrogen adsorptions are energetically favorable and they simultaneously weaken the interaction between the metal atom and tube surface. The presence of Rh, Ni, and Pd atoms can optimize the hydrogen adsorption energy and enhance high hydrogen storage capacity on BNNTs. Our calculations suggest a novel approach to engineer new nanostructured materials for hydrogen storage and catalysis. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (L.P.Z.); [email protected]. edu.sg (P.W.).

’ ACKNOWLEDGMENT This work was supported by the Institute of High Performance Computing and the Agency of Science, Technology, and Research (A*STAR), Singapore. ’ REFERENCES (1) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353–358. (2) Wang, Q. K.; Zhu, C. C.; Liu, W. H.; Wu, T. Int. J. Hydrogen Energy 2002, 27, 497–500. (3) Durgun, E.; Ciraci, S.; Yildirim, T. Phys. Rev. B 2008, 77, 085405. (4) Coontz, R.; Hanson, B. Science 2004, 305, 957. (5) Chan, S. P.; Chen, G.; Gong, X. G.; Liu, Z. F. Phys. Rev. Lett. 2001, 87, 205502.

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