Electronic Structure and Initial Dehydrogenation Mechanism of M(BH4

Article pubs.acs.org/JPCC

Electronic Structure and Initial Dehydrogenation Mechanism of M(BH4)2·2NH3 (M = Mg, Ca, and Zn): A First-Principles Investigation Xiaowei Chen and Xuebin Yu* Department of Materials Science, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: The electronic structure and initial dehydrogenation mechanism of M(BH4)2·2NH3 (M = Mg, Ca, and Zn) have been systematically studied using first-principles calculations. A detailed study of the electronic structure reveals that the metal cations in M(BH4)2·2NH3 play a crucial role in both suppressing ammonia emission and destabilizing the N−H/B− H bonds. The calculation results of hydrogen removal energies are in agreement with the tendency of dehydrogenation temperatures of these ammoniates, i.e., Zn(BH4)2·2NH3 < Mg(BH4)2·2NH3 < Ca(BH4)2·2NH3. The initial dehydrogenation of M(BH4)2·2NH3 is achieved by the dissociation of (N)Hδ+ from NH3 and (B)Hδ− atoms from BH4 groups, resulting in the formation of N−B dative bonds and the reduction of the neighboring (N)Hδ+···(B)Hδ− dihydrogen bonds, which accelerate the subsequent dehydrogenation.

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INTRODUCTION As an ideal candidate for energy carrier, hydrogen exhibits a series of advantages such as high heating value per mass, regenerative, and environmentally friendly. Hydrogen storage is one of the key challenges in developing hydrogen as fuel for mobile and stationary applications.1−4 Recently, many efforts have been paid to chemical hydrides due to their high gravimetric energy density.5−7 Ammonia borane (AB) is an ideal candidate for chemical hydrogen storage owing to its high H-capacity of 19.6 wt %.8,9 However, upon dehydrogenation of AB, accompanied volatile compounds, i.e., ammonia, diborane, and borazine, are also evolved, which lead to a reduction of hydrogen capacity and are fatal for fuel cell application. During the past 10 years, many different methods have been adopted10−18 to promote the efficient hydrogen generation from AB. One such approach is to substitute one H atom in the NH3 group by an alkali metal to form single or double metal amidoborane (MAB), e.g., LiNH2BH3,7,13,14 NaNH2BH3,7,14 Ca(NH 2 BH 3 ) 2 , 1 5 , 1 6 Na 2 Mg(NH 2 BH 3 ) 4 , 1 7 and NaLi(NH2BH3)2.18 These metal amidoborane compounds have been reported to improve the dehydrogenation properties of AB in terms of the reduced dehydrogenation temperatures, accelerated H2 release kinetics, and/or minimized borazine release. Ammine metal borohydrides (AMBs), which combine the properties of metal hydrides and ammonia borane, thus exhibiting high hydrogen capacity and favorable dehydrogenation properties, have been developed recently as promising materials for hydrogen storage.19−30 However, it has been shown that the dehydrogenation properties of AMBs were affected by their metal cations strongly. For example, Ca(BH4)2·2NH3 and LiBH4·NH3 mainly released ammonia © 2012 American Chemical Society

rather than hydrogen during decomposition under the dynamic flow mode (i.e., TG-DSC);20,21 Mg(BH4)2·2NH3 and Al(BH4)3·6NH3 were found to dehydrogenate with only a small amount of ammonia,19 and Zn(BH4)2·2NH3 presented a pure hydrogen release upon decomposition.28 It suggests that the dehydrogenation properties of AMBs could be tuned extensively using different metal cations, probably resulting from their various electonegativity, which may account for the electronic structure of AMBs.23,29 Therefore, a detailed investigation of electronic structure and decomposition pathway will be helpful for understanding the dehydrogenation mechanism of AMBs so that to further improve and modify their hydrogen storage properties. Nevertheless, the calculations study on the AMBs is rare. In this article, the electronic structure and initial dehydrogenation mechanism of M(BH4)2·2NH3 (M = Mg, Ca, and Zn) were studied by firstprinciples calculations. It is expected that this work could provide some insights into the understanding of the bonding characters and decomposition properties of AMBs and thus be helpful for seeking valuable clues to improve their dehydrogenation and regeneration performances.

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COMPUTATIONAL METHOD The crystal structures of Mg(BH4)2·2NH3, Ca(BH4)2·2NH3, and Zn(BH4)2·2NH3 are illustrated in Figure 1, where B−H and N−H bonds are drawn for clarity. Both the Mg(BH4)2·2NH3 and Ca(BH4)2·2NH3 crystallize in the orthorhombic structures with space groups Pcab and Pbcn, Received: February 29, 2012 Revised: May 15, 2012 Published: May 15, 2012 11900

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Figure 1. Conventional unit cell of (a) Mg(BH4)2·2NH3, (b) Ca(BH4)2·2NH3, and (c) Zn(BH4)2·2NH3.

respectively,19,20 while Zn(BH4)2·2NH3 has a monoclinic structure with space group P21. The geometry of M(BH4)2·2NH3 (M = Mg, Ca, and Zn) were optimized using density functional theory (DFT) as implemented in the ABINIT DFT code.31,32 Exchange and correlation were treated in the generalized gradient approximation (GGA) of Perdew− Burke−Ernzerhof (PBE).33 The ultrasoft pseudopotentials with valence states 2p63s2 for Mg, 3s23p64s2 for Ca, 3d104s2 for Zn, 2s22p1 for B, 2s22p3 for N, and 1s for H were used to describe the core electrons. Plane waves with kinetic energy cutoff of 650 eV were used. The Brillouin-zone integration was done by using a 3 × 3 × 3 Monkhorst−Pack k-point meshes.34 Structural relaxations of atomic positions, cell shapes, and cell volumes were carried out by BFGS method35 until the residual forces and stresses were less than 0.03 eV·Å−1 and 0.05 GPa, respectively. The energy of the hydrogen molecule was calculated by placing the H2 molecule in a 10× 10 × 10 Å cubic cell. We first calculated the equilibrium lattice parameters of unit cells and compared with the available experimental information (as shown in Tables S1−S4, Supporting Information). In general, the calculation results for the lattice parameters are in reasonable agreement with the available experimental data. The calculated electronic structure and hydrogen removal energies of Ca(BH4)2·2NH3 and Zn(BH4)2·2NH3 were performed in 2 × 1 × 1 and 2 × 1 × 2 supercells, respectively, to avoid unphysical interactions between adjacent cells and make the computational results more comparable.

approximation.36 The valence band of Mg(BH4)2·2NH3 and Ca(BH4)2·2NH3 are split into four and three groups, respectively. The DOS of the two compounds show some similar features: the lowest energy region is dominated by B 2s and (B)H 1s states; the next band is primarily composed of N 2p and (N)H 1s hybridized states; the top of the valence band is composed of B 2p and (B)H 1s hybridized states. It should be noted that the sharp peak around −1.9 eV in Mg(BH4)2·2NH3 is arisen from Mg 2s and N 2p states. In comparison, Ca cations have hardly any projection in the valence band, indicating that they play the role of electron donors in the compound. It can be seen in Figure 2c that the valence band of Zn(BH4)2·2NH3 is split into four groups. Zn orbitals have some p state characteristics, which may be arisen from the hybridization between the Zn 4s and 3d, N and/or B 2p states. The bottom of the valence band is contributed by Zn 3d, B 2s, N 2p, and H 1s states. The energy region located between −4.6 to −3.6 eV is mainly composed of Zn 3d states, accompanied with a small contribution from the N 2p and B 2s states. The next area located between −3.5 and −3.0 eV is composed of Zn 3d, N 2p, and (B)H 1s states. The top of the valence band is dominated by B 2p and (B)H 1s states, and a small contribution from Zn p, Zn 3d and N 2p states. The distribution of Zn 4s, 3d and p states in valence band may result in strong association between Zn and NH3/BH4 groups and further destabilize the N−H and B−H bonds. Similar bonding features can be found in Zn(BH4)2 and Cu(BH4)2.37 The electronic structure was further analyzed by examining the charge distribution around metal cations. Figure 3 shows the contour map of charge density of M(BH4)2·2NH3. In general, the charge density is strongly localized around BH4 and NH3 groups, suggesting the predominant covalent nature of the N−H and B−H bonds in these three compounds. The electron distribution between Ca and neighboring BH4/NH3 is considerably low, indicating the highly ionized Ca cations. In the case of Mg(BH4)2·2NH3, the charge densities surrounding NH3 groups is slightly distorted toward Mg, and a certain amount of overlapping between them were found, suggesting the partial covalence bonding feature of Mg−NH3. The interactions between the coordinated metal cations Zn and NH3/BH4 groups in the Zn(BH4)2·2NH3 are relatively stronger than that in the Mg(BH4)2·2NH3 and Ca(BH4)2·2NH3, as the

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RESULTS AND DISCUSSION Electronic Properties. In order to understand the bonding nature of M(BH4)2·2NH3, the charge density states and the charge density distribution were investigated. The calculated total and partial electronic density of states (DOS) for M(BH4)2·2NH3 (M = Mg, Ca and Zn) are shown in Figure 2, which exhibits rather large energy gap of about 6.2, 6.0, and 5.5 eV for Mg(BH4)2·2NH3, Ca(BH4)2·2NH3, and Zn(BH4)2·2NH3, respectively, indicating that these three compounds are wide-gap insulators. However, the band gaps of semiconductors and insulators are usually underestimated since density functional theory has difficulties in treating the excited states. The actual band gaps of the M(BH4)2·2NH3 composites could be even larger. It has been reported that the excited states and band gaps can be further corrected by using the GW 11901

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Figure 2. Total (first row) and partial (second to last rows) electronic density of states for (a) Mg(BH4)2·2NH3, (b) Ca(BH4)2·2NH3, and (c) Zn(BH4)2·2NH3. The Fermi level is set to 0 eV and marked by the vertical dashed line.

weak interaction between Ca and NH3 groups. Only a small amount of ammonia was released19 during the decomposition of Mg(BH4)2·2NH3, resulting from the partial covalence bonding feature of Mg-NH3. Given the Zn 4s and 3d states distributed in valence bands, Zn cations are strongly associated with NH3 groups, which is consistent with the observation that there was no ammonia emission during the decomposition of Zn(BH4)2·2NH3. Dehydrogenation Mechanism. Two groups of hydrogen atoms can be clearly identified in the M(BH4 ) 2 ·2NH 3 compounds: the hydrogen atoms bonded to the nitrogen

higher charge density between Zn−NH3 and Zn−BH4 was observed in Figure 3d,e. The above analysis of electronic structure suggests that these three compounds show similar bonding features of B−H and N−H bonds due to the strong sp hybridization: the covalent interaction of B−H bonds are contributed by B 2s, 2p and H 1s states; the N−H covalent bonds are composed of N 2p and H 1s states. The emission of ammonia during decomposition is attributed to the weak interplay of metal cations and NH3 groups. It was observed that Ca(BH4)2·2NH3 mainly released ammonia rather than hydrogen,20 which may be due to the 11902

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Figure 3. Charge density contour map of metal cation environment in (a,b) Mg(BH4)2·2NH3, (c) Ca(BH4)2·2NH3, and (d,e) Zn(BH4)2·2NH3.

Table 1. Removal Energies for the First Hydrogen Moleculea ΔE1 (eV) shortest (N)Hδ+···(B)Hδ− bond longest (N)Hδ+···(B)Hδ− bond

Mg(BH4)2·2NH3

Ca(BH4)2·2NH3

Zn(BH4)2·2NH3

2.24 2.25 0.37 2.24 2.23 0.60

2.31 2.29 0.75 2.27 2.27 1.04

1.89 1.90 0.32 1.91 1.92 0.33

(N)H (B)H (N)Hδ+···(B)Hδ− (N)H (B)H (N)Hδ+···(B)Hδ−

Hydrogen removal energies were calculated as ΔE1 = E(M8B16N16H112−n) + (n/2)E(H2) − Etotal. Where Etotal is the total energy of the M(BH4)2·2NH3 supercells; E(H2) is the energy of isolate hydrogen molecular; E(M8B16N16H112−n) represents the total energy of the M(BH4)2·2NH3 supercells after H atoms are removed; and n denotes the number of removed hydrogen atoms. a

Figure 4. Local atomic structure of Mg(BH4)2·2NH3 (a) before and (b) after the removal of (N)Hδ+···(B)Hδ− bond. All bond lengths are given in angstroms.

atom, which have lost a significant part of their electronic charge, and the hydrogen atoms bonded to the boron atom, which possess a significant negative charge. These differences in the charge state may result in different strength of hydrogen− host bonds. As a result, the initial step of dehydrogenation could be attributed to the break of (i) N−H bonds, (ii) B−H bonds, and (iii) the combination of Hδ+ from the NH3 groups and Hδ− from the BH4 groups. The proposed three dehydrogenation routes were investigated by removing one or two hydrogen atoms from the M(BH4)2·2NH3 supercells and reoptimizing the atomic coordinates of the structures. Both the experimental and calculated results suggest that the existence of (N)Hδ+···(B)Hδ− dihydrogen bonds in AB and AMBs plays a key role in decreasing the dehydrogenation temperature;9,19,21,24 therefore, the (N)H and/or (B)H atoms with shortest or longest (N)Hδ+···(B)Hδ− dihydrogen bond were removed. The bonding strength of hydrogen−host bonds can

be quantified by the hydrogen removal energies using the following equation:38−41 ΔE = E(M8B16N16H112 − n) +

n E(H 2) − Etotal 2

Where Etotal is the total energy of the M(BH4) 2·2NH3 supercells; E(H2) is the energy of isolate hydrogen molecular; E(M8B16N16H112−n) represents the total energy of the M(BH4)2·2NH3 supercells after H atoms are removed; and n denotes the number of removed hydrogen atoms. The calculated hydrogen removal energies are listed in Table 1. Generally, it shows almost the same energies for removing a (N)H/(B)H atom from the same compound, indicating the similar bonding strength of N−H and B−H bonds. However, the studies in LiNH2BH3 suggest that the N−H bonds are stronger than the B−H bonds.40,42 This difference could be attributed to the various structures and bonding characters of AMBs and MAB. By comparing the hydrogen removal energies 11903

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Figure 5. Local atomic structure of Ca(BH4)2·2NH3 (a) before and (b) after the removal of the (N)Hδ+···(B)Hδ− bond. All bond lengths are given in angstroms.

of these three compounds, we can find that it costs almost the same energy of removing an H atom from Mg(BH4)2·2NH3 and Ca(BH4)2·2NH3. However, the H atom removal energies for Zn(BH4)2·2NH3 are about 0.3−0.4 eV less than that for Mg(BH4)2·2NH3 and Ca(BH4)2·2NH3, implying the weaker bonding strength of B−H/N−H in Zn(BH4)2·2NH3. These results are consistent with the electronic structure analysis, indicating that the Zn cations destabilize the N−H/B−H bonds. It should be noted that the removal energies of (N)Hδ+···(B)Hδ− dihydrogen bonds are significantly lower than the energies cost of removing an H atom, suggesting that the initial dehydrogenation via a combination of (N)Hδ+···(B)Hδ− is energetically more favorable. In addition, the (N)Hδ+···(B)Hδ− removal energies vary with the coordinated metal cations, which may account for the different hydrogen desorption temperature in AMBs. Ca(BH4)2·2NH3 possesses substantially greater (N)Hδ+···(B)Hδ− removal energies than that for the Mg(BH4)2·2NH3 and Zn(BH4)2·2NH3; thus, the dehydrogenation of Ca(BH4)2·2NH3 would occur at higher temperature, which is consistent with the experimental observations.20 Furthermore, the energy for removing the shortest (N)Hδ+···(B)Hδ− bond from the Mg(BH4)2·2NH3 is 0.37 eV, which is slightly higher than that from the Zn(BH4)2·2NH3. This is in agreement with the experimental trend, where the dehydrogenation temperature of Mg(BH4)2·2NH3 is higher than that of Zn(BH4)2·2NH3.19 It can be explained by the fact that, compared with Ca and Mg, the Zn 3d delocalized states weaken the bonding strength of B−H and N−H bonds, thus reducing the energy cost of removing hydrogen atoms from Zn(BH4)2·2NH3. The relaxation structures before and after the removal of the first (N)Hδ+···(B)Hδ− bond were examined in order to shed light on the initial dehydrogenation process. As shown in Figure 4, the removal of the (N)Hδ+···(B)Hδ− dihydrogen bond in Mg(BH4)2·2NH3 leads to a significant rearrangement of the surrounding lattice. The NH2 and BH3 groups reorient and move toward each other with the formation of a NH2−BH3 complex. The N−B distance reduces to 1.567 Å, which is comparable to that in NH3BH3 solid state,9 indicating the formation of N−B dative bond during dehydrogenation. Some of the (N)Hδ+···(B)Hδ− dihydrogen bonds are broken due to the movement of NH2 and BH3 groups. For example, one of the (N)Hδ+···(B)Hδ− distances increased from 2.233 Å to 3.565 Å. In addition, the structure is further stabilized by orienting the surrounding BH4 and NH3 groups to shorten their neighboring dihydrogen bonds. For instance, the BH4 and NH3 groups located in the left side of the H2N−BH3 complex orient themselves to reduce the dihydrogen bond from 2.103 Å to

1.966 Å. However, the lengths of B−H and N−H bonds keep almost the same during structural rearrangements. The structural rearrangements around the removal of (N)H δ+ ···(B)H δ− bonds in Ca(BH 4 ) 2 ·2NH 3 and Zn(BH4)2·2NH3 (Figures 5 and 6) show some similar characters

Figure 6. Local atomic structure of Zn(BH4)2·2NH3 (a) before and (b) after the removal of the (N)Hδ+···(B)Hδ− bond. All bond lengths are given in angstroms.

as that in Mg(BH4)2·2NH3. The removal of (N)Hδ+···(B)Hδ− bonds also leads to reorientation and movement of NH2 and BH3 groups toward each other to form the N−B dative bonds. Besides, the structural relaxation of BH4 and NH3 groups lead to the reduction of the neighboring (N)Hδ+···(B)Hδ− bonds, which may enhance the interaction between (N)Hδ+ and (B)Hδ−, and further reduce the energy cost of dehydrogenation. The previous report suggests that the formation of the intermolecular N−B bond in LiNH2BH3 could lead to new pathways for N−B polymerization during dehydrogenation.43 It arises the question that whether the formation of N−B dative bonds in M(BH4)2·2NH3 compounds may also induce new decomposition pathways and further accelerate the decomposition process. On the basis of the above discussion of hydrogen removal energies and structural rearrangements of the M(BH4)2·2NH3 compounds, the release of a subsequent hydrogen molecule could be attributed to two different mechanisms: (1) combination of (N)Hδ+ and (B)Hδ− atoms from NH3 and BH4 groups, respectively; (2) dissociation of (N)Hδ+ from NH3 and (B)Hδ− from the adjacent H2N−BH3 complex, which may result in the formation of H2N−BH3− NH2 complex. The proposed two dehydrogenation mecha11904

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initial dehydrogenation mechanism could be summarized as the following equation:

nisms were investigated by removing the shortest (N)Hδ+···(B)Hδ− bond in the M(BH4)2·2NH3 compounds and further optimizing the atomic coordinates of the structure. Table 2 shows the calculation results for the removal energies of the second hydrogen molecule, where ΔE12 and ΔE22

n M(BH4)2 ·2NH3 → [M(BH3NH 2)2 ]n + 2nH 2

where [M(BH3NH2)2]n represents M−B−N−H amorphous phases. The above decomposition mechanism is also supported by the experimental observation in which both FTIR and 11B NMR have confirmed the formation of N−B bonds during dehydrogenation.21,28

Table 2. Removal Energies for the Second Hydrogen Moleculea Mg(BH4)2·2NH3 Ca(BH4)2·2NH3 Zn(BH4)2·2NH3

ΔE12 (eV)

ΔE22 (eV)

0.31 0.49 0.28

0.54 1.87 1.14 δ+

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CONCLUSIONS The present study systematically investigated the electronic structure and initial dehydrogenation mechanism of M(BH4)2·2NH3 (M = Mg, Ca, and Zn), which provides useful information for further understanding and improving the hydrogen storage properties of AMBs. A detailed study of the electronic structure reveals a highly ionic character of Ca cations in Ca(BH4)2·2NH3, whereas partial covalence bonding features of Mg−NH3 and Zn−NH3 were found in Mg(BH4)2·2NH3 and Zn(BH4)2·2NH3, respectively. It suggests that the metal cations in AMBs play a crucial role in both suppressing ammonia emission and destabilizing the N−H/B− H bonds. The calculated hydrogen removal energies are in agreement with the experimental tendency of dehydrogenation temperatures, i.e., Zn(BH4)2·2NH3 < Mg(BH4)2·2NH3 < Ca(BH4)2·2NH3. Structural and energetic analysis indicate that dehydrogenation is achieved by that combination of (N)Hδ+ and (B)Hδ− atoms, resulting in the formation of N−B dative bonds and the reduction of (N)Hδ+···(B)Hδ− bonds, which further promotes the subsequent dehydrogenation. The initial dehydrogenation mechanism of M(BH4)2·2NH3 could be summarized as nM(BH4)2·2NH3 → [M(BH3NH2)2]n + 2nH2.

δ−

ΔE1 represents the removal energies of (N)H and (B)H atoms from NH3 and BH4 groups, respectively; ΔE22 represents the removal energies of (N)Hδ+ from NH3 and (B)Hδ− from the adjacent H2N− BH3 group. a

2

(1)

represent the removal energies in conditions (1) and (2), respectively. Generally, ΔE12 < ΔE22, indicates that the formation of the H2N−BH3−NH2 complex is not energetically favorable in the initial dehydrogenation; the emission of a second hydrogen molecule may ascribe to the dissociation of (N)Hδ+ and (B)Hδ− atoms from NH3 and BH4 groups, respectively. The removal energies ΔE12 of Ca(BH4)2·2NH3 are about 0.2 eV higher than that of Mg(BH4)2·2NH3 and Zn(BH4)2·2NH3, indicating the poor dehydrogenation properties of Ca(BH4)2·2NH3. Besides, by comparing the (N)Hδ+···(B)Hδ− removal energies of ΔE1 (Table 1) and ΔE12 (Table 2), it can be found that the removal energies for the second hydrogen molecule from Mg(BH4)2·2NH3 and Zn(BH4)2·2NH3 are slightly lower than the first one, while the removal energy for Ca(BH4)2·2NH3 is about 0.26 eV lower compared with the first one. These results suggest that the second hydrogen molecule is easier to release than the first one, particularly for Ca(BH4)2·2NH3. The relaxation structures after desorption of the second hydrogen molecule are shown in Figure 7. In general, the structural rearrangements show similar tendency as the previous one. Overall, the initial dehydrogenation mechanism can be understood as follows: the combination of (N)Hδ+ and (B)Hδ− result in the formation of the N−B bonds and reduction of the neigboring (N)Hδ+···(B)Hδ− distances, which further promote the following dehydrogenation. This process may continue until all the initial intermolecular dihydrogen bonds, from NH3 and BH4 groups, were consumed, resulting in the formation of amorphous [M(BH3NH2)2]n. As a result, the

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ASSOCIATED CONTENT

S Supporting Information *

Calculated and experimental lattice parameters of Mg(BH4)2·2NH3, Ca(BH4)2·2NH3, and Zn(BH4)2·2NH3. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-21-5566 4581. E-mail: [email protected]. cn.

Figure 7. Structural rearrangements around the removal of the second (N)Hδ+···(B)Hδ− bond; (a) Mg(BH4)2·2NH3, (b) Ca(BH4)2·2NH3, and (c) Zn(BH4)2·2NH3. All bond lengths are given in angstroms. 11905

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Notes

(27) Sun, W. W; Chen, X. W; Gu, Q. F; Wallwork, K. S.; Tan, Y. B.; Tang, Z. W; Yu, X. B. Chem.Eur. J. 2012, DOI: 10.1002/ chem.201102651. (28) Gu, Q. F.; Gao, L.; Guo, Y. H.; Tan, Y. B.; Tang, Z. W.; Wallwork, K.; Zhang, F. W.; Yu, X. B. Energy Environ. Sci 2012, DOI: 10.1039/C2EE02485C. (29) Guo, Y. H.; Wu, H.; Zhou, W.; Yu, X. B. J. Am. Chem. Soc. 2011, 133, 4690−4693. (30) Ramzan, M.; Silvearv, F.; Lebègue, S.; Ahuja, R. J. Phys. Chem. C 2011, 115, 20036−20042. (31) Gonze, X.; Amadon, B.; Anglade, P. M.; Beuken, J. M.; Bottin, F.; Boulanger, P.; Bruneval, F.; Caliste, D.; Caracas, R.; Cote, M.; et al. Comput. Phys. Commun. 2009, 180, 2582−2615. (32) Gonze, X.; Rignanese, G.; Verstraete, M.; Beuken, J.; Pouillon, Y.; Caracas, R.; Jollet, F.; Torrent, M.; Zerah, G.; Mikami, M.; et al. Z. Kristallogr. 2005, 220, 558−562. (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (34) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188−5192. (35) Pfrommer, B. G.; Côté, M.; Louie, S. G.; Cohen, M. L. J. Comput. Phys. 1997, 131, 233−240. (36) Bheema Lingam, C.; Ramesh Babu, K.; Tewari, S. P.; Vaitheeswaran, G.; Lebegue, S. J. Phys. Chem. C 2011, 115, 18795− 18801. (37) Nakamori, Y.; Miwa, K.; Ninomiya, A.; Li, H.; Ohba, N.; Towata, S.; Zuttel, A.; Orimo, S. Phys. Rev. B 2006, 74, 045126. (38) Li, S.; Jena, P.; Ahuja, R. Phys. Rev. B 2006, 73, 214107. (39) Araujo, C. M.; Li, S.; Ahuja, R.; Jena, P. Phys. Rev. B 2005, 72, 165101. (40) Li, W.; Scheicher, R. H.; Araujo, C. M.; Wu, G. T.; Blomqvist, A.; Wu, C. Z.; Ahuja, R.; Feng, Y. P.; Chen, P. J. Phys. Chem. C 2010, 114, 19089−19095. (41) Løvvik, O.; Opalka, S. Phys. Rev. B 2005, 71, 054103. (42) Ramzan, M.; Silvearv, F.; Blomqvist, A.; Scheicher, R.; Lebegue, S.; Ahuja, R. Phys. Rev. B 2009, 79, 132102. (43) Kim, D. Y.; Singh, N.; Lee, H. M.; Kim, K. S. Chem.Eur. J. 2009, 15, 5598−5604.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Ministry of Science and Technology of China (2010CB631302), the National Natural Science Foundation of China (Grant No. 51071047), the Ph.D. Programs Foundation of Ministry of Education of China (20110071110009), and Science and Technology Commission of Shanghai Municipality (11JC1400700 and 11520701100).

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REFERENCES

(1) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670−4679. (2) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; OKeeffe, M.; Yaghi, O. M. Science 2003, 300, 1127−1129. (3) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353−358. (4) Zuttel, A. Mater. Today 2003, 6, 24−33. (5) Grochala, W.; Edwards, P. P. Chem. Rev. 2004, 104, 1283−1315. (6) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zuttel, A.; Jensen, C. M. Chem. Rev. 2007, 107, 4111−4132. (7) Xiong, Z. T.; Yong, C. K.; Wu, G. T.; Chen, P.; Shaw, W.; Karkamkar, A.; Autrey, T.; Jones, M. O.; Johnson, S. R.; Edwards, P. P. Nat. Mater. 2008, 7, 138−141. (8) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46, 8116−8118. (9) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010, 110, 4079−4124. (10) Gutowska, A.; Li, L.; Shin, Y.; Wang, C. M.; Li, X. S.; Linehan, J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw, W. Angew. Chem., Int. Ed. 2005, 44, 3578−3582. (11) Bluhm, M. E.; Bradley, M. G.; Butterick, R., III; Kusari, U.; Sneddon, L. G. J. Am. Chem. Soc. 2006, 128, 7748−7749. (12) Feaver, A.; Sepehri, S.; Shamberger, P.; Stowe, A.; Autrey, T.; Cao, G. J. Phys. Chem. B 2007, 111, 7469−7472. (13) Kang, X. D.; Fang, Z. Z.; Kong, L. Y.; Cheng, H. M.; Yao, X. D.; Lu, G. Q.; Wang, P. Adv. Mater. 2008, 20, 2756−2759. (14) Luedtke, A. T.; Autrey, T. Inorg. Chem. 2010, 49, 3905−3910. (15) Wu, H.; Zhou, W.; Yildirim, T. J. Am. Chem. Soc. 2008, 130, 14834−14839. (16) Diyabalanage, H. V. K.; Shrestha, R. P.; Semelsberger, T. A.; Scott, B. L.; Bowden, M. E.; Davis, B. L.; Burrell, A. K. Angew. Chem., Int. Ed. 2007, 46, 8995−8997. (17) Wu, H.; Zhou, W.; Pinkerton, F. E.; Meyer, M. S.; Yao, Q.; Gadipelli, S.; Udovic, T. J.; Yildirim, T.; Rush, J. J. Chem. Commun. 2011, 47, 4102−4104. (18) Fijalkowski, K. J.; Genova, R. V.; Filinchuk, Y.; Budzianowski, A.; Derzsi, M.; Jaroń, T.; Leszczyński, P. J.; Grochala, W. Dalton Trans. 2011, 40, 4407−4413. (19) Soloveichik, G.; Her, J. H.; Stephens, P. W.; Gao, Y.; Rijssenbeek, J.; Andrus, M.; Zhao, J. C. Inorg. Chem. 2008, 47, 4290−4298. (20) Chu, H. L.; Wu, G. T.; Xiong, Z. T.; Guo, J. P.; He, T.; Chen, P. Chem. Mater. 2010, 22, 6021−6028. (21) Guo, Y. H.; Xia, G. L.; Zhu, Y. H.; Gao, L.; Yu, X. B. Chem. Commun. 2010, 46, 2599−2601. (22) Johnson, S. R.; David, W. I. F.; Royse, D. M.; Sommariva, M.; Tang, C. Y.; Fabbiani, F.; Jones, M. O.; Edwards, P. P. Chem. Asian J. 2009, 4, 849−854. (23) Yuan, F.; Gu, Q. F.; Guo, Y. H.; Sun, W. W.; Chen, X. W.; Yu, X. B. J. Mater. Chem. 2012, 22, 1061−1068. (24) Guo, Y. H.; Yu, X. B.; Sun, W. W.; Sun, D. L; Yang, W. N. Angew. Chem. 2011, 123, 1119−1123. (25) Tang, Z. W.; Tan, Y. B.; Gu, Q. F.; Yu, X. B. J. Mater. Chem. 2012, 22, 5312−5318. (26) Xia, G. L.; Gu, Q. F.; Guo, Y. H.; Yu, X. B. J. Mater. Chem. 2012, 22, 7300−7307. 11906

dx.doi.org/10.1021/jp301986k | J. Phys. Chem. C 2012, 116, 11900−11906