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Electronic Structure from First-Principles of LiBH4 3 NH3, Sr(NH2BH3)2, and Li2Al(BH4)5 3 6NH3 for Hydrogen Storage Applications Muhammad Ramzan,*,† Fredrik Silvearv,† Sebastien Lebegue,‡ and Rajeev Ahuja†,§ †
Condensed Matter Theory Group, Department of Physics and Astronomy, Box 516, Uppsala University, SE-751 20 Uppsala, Sweden Laboratoire de Crystallographie, Resonance Magnetique et Modelisations (CRM2, UMR CNRS 7036) Institut Jean Barriol, Nancy Universite, BP 239, Boulevard des Aiguillettes 54506 Vandoeuvre-les-Nancy, France § Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden ‡
ABSTRACT: A series of compounds containing boron, nitrogen, and hydrogen atoms, namely, LiBH4 3 NH3, Sr(NH2BH3)2, and Li2Al(BH4)5 3 6NH3, were synthesized recently, and found to be promising for hydrogen storage. We have conducted the corresponding ab initio calculations using density functional theory and analyzed the corresponding electron density as well as derived properties. Also, molecular dynamics simulations were performed to study the diffusion of hydrogen in this series of compounds. We found that despite having widely different crystal structures and difference in their chemical composition, these compounds share common features at the level of their electronic structure. Moreover, diffusion of hydrogen appears to be more favorable in LiBH4 3 NH3 than in Sr(NH2BH3)2 and Li2Al(BH4)5 3 6NH3.
’ INTRODUCTION Hydrogen is an attractive energy source because it is easily renewable and non polluting. However, the problem of storing hydrogen in a safe and efficient way has limited its practical use as fuel for mobile applications. There are several ways of trying to solve this problem, such as stronger container materials, sorbents, and hydrides. Materials such as metal organic framework (MOF)1 4 or silane (SiH4)5 9 and its derivatives (SiH4(H2)2)10 12 have been under focus recently. Also, lightweight element hydrides have attracted special interest because they have been estimated to be the most promising materials to meet the goal of high gravimetric and volumetric storage capacity.13 16 A particular effort is put nowadays on boron nitrogen-hydrogen compounds because they offer high hydrogen storage capacities, together with reasonable dehydrogenation performances. Among many others, ammonia borane,17,18 lithium amidoborane,19 22 ammine magnesium borohydride,23 or LiNH2BH3 3 NH3BH324,25 have been studied experimentally and theoretically. Recently, three new compounds, namely, LiBH4 3 NH3,26 Sr(NH2BH3)2,27 and Li2Al(BH4)5 3 6NH3,28 have been synthesized, with the aim to study their performance for hydrogen storage. However, to our knowledge, a theoretical counterpart is still lacking, for instance to study in detail their electronic structure or to analyze the energetics of the hydrogen desorption. In the present study, we have used density functional theory (DFT) to study these three compounds. After giving the details of our calculations, we analyze their electronic structure using different tools and report on our ab initio molecular dynamics (MD) simulations. Finally, in the last section, we offer our conclusions. r 2011 American Chemical Society
’ COMPUTATIONAL DETAILS Our calculations are performed within the framework of DFT29 based on the Kohn Sham equations. The ab initio self-consistent total energy calculations were carried out with the use of projector augmented wave (PAW) method,30 as implemented in the Vienna ab initio simulation package (VASP).31,32 The exchange-correlation energy was obtained using the PBE33 version of the generalized gradient approximation (GGA).34 The energy cutoff that determines the number of plane waves in the expansion of the wave functions and the number of k points used for the Brillouin zone (BZ) integration are crucial parameters to ensure the proper convergence of these calculations. Therefore, we have tested convergence with respect to the size of the basis set and BZ sampling. A cutoff of 500 eV was found to be sufficient for the three compounds investigated here. Γ-centered k-point meshes for the BZ integrations of 8 8 4, 8 8 8, and 4 4 2 were used for LiBH4 3 NH3, Sr(NH2BH3)2, and Li2Al(BH4)5 3 6NH3, respectively. Whereas the volume and the shape of the cell were taken from experiments, the position of the atoms was relaxed until the forces on every ion became smaller than 0.001 eV/Å. Then, the valence electron charge densities of LiBH4 3 NH3, Sr(NH2BH3)2, and Li2Al(BH4)5 3 6NH3 were obtained and analyzed by using Bader’s theory of atoms in molecules.35 37 Following this scheme, each basin belonging to a particular atom is defined with the use of zero flux surfaces of the charge density. Then, the charge density Received: June 29, 2011 Revised: August 17, 2011 Published: September 07, 2011 20036
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Table 1. Calculated Bader Atomic Charges (in Number of Electrons) for LiBH4 3 NH3 (Only Inequivalent Atoms Are Displayed)a atom
number of electrons
Li
0.13
B
1.44
N
6.20
H1
1.59
H2 H3
0.60 0.59
H4
1.63
H5
1.58
a
Concerning the hydrogen atoms, we have used the same labeling as in the experimental paper (only inequivalent atoms are shown): H2 and H3 are bonded with N, whereas H1, H4, and H5 are bonded with B.
Figure 1. Crystal structure of LiBH4 3 NH3 as obtained from our ab initio calculations.
is integrated over the whole basin to determine the charge assigned to that particular atom. Additionally, we calculated the hydrogen removal energies by taking the difference between the total energy of the given compound and the total energy of the same compound with one hydrogen atom removed (the structure was fully reoptimized after the removal of the hydrogen atom). Finally, ab initio MD simulations were conducted to study the diffusion of hydrogen in LiBH4 3 NH3, Sr(NH2BH3)2, and Li2Al(BH4)5 3 6NH3, again using the PBE version of the GGA for the exchange-correlation functional. The equation of motion was integrated with the use of Verlet algorithm, choosing a time step of 1.0 fs during a time scale of 5.0 ps. We have used the NVT ensemble with a temperature of 500 K to perform our MD simulations. The R.I.N.G.S code38 was used to analyze the corresponding mean-square displacements (MSDs).
’ RESULTS AND DISCUSSION LiBH4 3 NH3. As mentioned in the Introduction, LiBH4 3 NH3 was synthesized and studied by Guo et al.26 using thermogravimetry and mass spectroscopy. They found that it could release hydrogen at low temperature if ammonia is stabilized, opening the path for future studies on related compounds. The same authors have resolved its crystal structure using X-ray diffraction, and found it to be of the Pnma space group (orthorhombic) with lattice parameters a = 5.97213 Å, b = 4.46432 Å, and c = 14.34875 Å. Starting from the experimental structure, we have optimized the atom positions, and obtained a relaxed structure (Figure 1) that is very close to the experimental one. As described in the experimental paper,26 the two most relevant bonds are the N:Li+ bond, which control the release of NH3 upon heating, and the N H 3 3 3 H B bond that is the key for the dehydrogenation process. Our calculated N H 3 3 3 H B bond length of 2.24 Å is
Figure 2. Calculated electron density in the plane of B Li N. The contours are 0.05 e/Å3.
in good agreement with the experimental value (2.25 Å), whereas our calculated N:Li+ bond (2.04 Å) is slightly longer than the one reported by Guo et al.26 (1.89 Å). Then, to quantify the charge distribution in LiBH4 3 NH3, we present in Table 1 our calculated Bader’s charges. As expected, lithium is almost fully ionized, with only 0.13 electrons remaining in the corresponding Bader basin, whereas BH4 carries a charge of 0.87 instead of 1 in a purely ionic picture. The ammonia molecule (NH3) is neutral, with eight electrons in total. An other meaningful way to understand the electron distribution is to look at the electron density in some given planes. In Figure 2, we present our calculated electron density in the plane of the B, Li, and N atoms. The electron distribution between Li+ and the neighboring atoms is very low, which confirms that the bonding is mainly ionic, although an electron density can remain in between the atoms provided that it is below the resolution that we have used for the plots. Then, in Figure 3, we present our calculated total and partial densities of states (DOS). As expected, the compound is an insulator, with a band gap of >5 eV. Note that the experimental value is certainly even larger because of the well-known deficiency of standard approximation to the exchange-correlation potential to predict the correct band gap. The total DOS is made of four well-separated regions in energy: the first one is simply a peak around 15 eV, and it corresponds to the N-2s states. 20037
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Figure 4. Crystal structure of Sr(NH2BH3)2 as obtained from our ab initio calculations.
Figure 3. Total and partial densities of states for LiBH4 3 NH3.
An important point is that a small contribution appears in the PDOS of Li at the same energy, which indicates that the bonding is not totally ionic but contains also a small part of covalency. The second region corresponds to states between 8 and 4 eV. It is made of several peaks coming from N-p and B-s states, together with a contribution from lithium. As for the peak around 15 eV, this contribution from Li is probably induced by a residual covalent bond with the neighboring atoms. Then, from 3 to 0 eV, there are mainly B-p states together with N-p and Li states. Finally, the fourth region above the Fermi level (which is put at 0 eV) is made of conduction bands: the bottom of the conduction band is constituted from B-states with a small contribution from Li and N. Finally, our calculated hydrogen removal energies are 5.23 eV for the B-site and 5.48 eV for the N-site. Therefore, the N H bond is stronger that the B H bond. This is also observed22 in the case of LiNH2BH3, although the difference between the two energies was in this last case larger (0.39 eV for LiNH2BH3 but only 0.25 eV for LiBH4 3 NH3). Sr(NH2BH3)2. Strontium amidoborane Sr(NH2BH3)2 was synthesized recently by Zhang et al.27 This follows recent efforts on ammonia borane NH3BH3 and related compounds, such as the alkali metal amidoboranes LiNH2BH3 and NaNH2BH3.19 Thermal analysis show that it begins to decompose to Sr(NBH)2 and H2 at 60 °C. Then, Sr(NBH)2 continues to decompose, releasing a large amount of NH3 and small amounts of B2H6. The investigation27 of this compound with X-ray diffraction revealed a monoclinic structure (space group C2) with the lattice parameters being a = 8.16604 Å, b = 5.09693 Å, c = 6.7258 Å, and β = 94.3924°. The optimized structure (Figure 4) was obtained with the same procedure as above, although an additional difficulty appeared because the hydrogen positions were not provided in the experimental paper.27 In the structure of Sr(NH2BH3)2, each Sr2+ ion bonded with two (NH2BH3) , the Sr N distance being 2.68 Å. Our calculated Sr N bond length of 2.58 Å is slightly shorter than the experimental value, but our calculated B N bond length of 1.54 Å, agrees almost perfectly with that given by Zhang et al. of 1.53 Å. The Bader’s charges presented in Table 2 show that strontium is ionized to Sr1.59+ instead of Sr2+ in a simple ionic model. Therefore, a charge transfer of 0.8 electron takes place between
Table 2. Calculated Bader Atomic Charges (In Number of Electrons) for Sr(NH2BH3)2 (Only Inequivalent Atoms Are Displayed)s atom
number of electrons
Sr N
8.41 6.45
B
1.17
H1
0.62
H2
0.65
H3
1.63
H4
1.63
s
H1 and H2 are bonded with N while H3 and H4 are bonded with B. In the case of Sr, 4s and 4p states are included as valence electrons in our calculations.
Figure 5. Calculated electron density in the plane of Sr N H. The isocontours are plotted with an increment of 0.05 e/Å3.
Sr and NH2BH3 and mainly benefits the nitrogen atoms, whereas BH3 is only slightly affected with an overall number of electrons of 6.06 instead of 6. The electron density in the plane Sr N H is shown in Figure 5, and we can see a region of low electron density surrounding the Sr atom, an indication of a mainly ionic bond. However, this picture is obscured on this plot by the fact that Sr-4s and Sr-4p states have been included as valence electrons in our calculations. 20038
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Figure 6. Total and partial densities of states for Sr(NH2BH3)2.
The total and partial DOS are shown in Figure 6. The calculated DFT band gap is 4 eV. The Sr-p states and N-s states constitute a region at 16 eV that implies a part of covalency in a mainly ionic bond. The region between 8 and 0 eV corresponds to a mix of N-p, B-s, and B-p states; finally, the conduction band around 6 eV is made by Sr, N, and B p-states. Our calculated hydrogen removal energies are 5.04 eV for the B-site and 5.54 eV for the N-site. It is instructive to compare these values with the ones obtained for LiNH2BH3.22 In this case, the values are 5.63 eV for the B-site and 6.02 eV for the N-site. Therefore, the B H and N H are weaker in Sr(NH2BH3)2 than in LiNH2BH3, which is favorable for the formation of H2, although the replacement of lithium by strontium is defavorable in terms of the percentage of H in the total weight. Li2Al(BH4)5 3 6NH3. Li2Al(BH4)5 3 6NH3 is a double-cation ammine borohydride, which was recently synthesized and studied by Guo et al.28 using thermogravimetry and mass spectroscopy. They28 found >10 wt % of hydrogen release below 120oC, leading toward a promising path to the chemical control of the dehydrogenation properties of the ammine borohydrides. Using X-ray diffraction, they28 have reported it to be crystallized in a hexagonal P3c1 lattice (space group no. 165), having the following lattice parameters: a = 7.79779(5) Å and c = 15.9693 (1) Å. Proceeding in the same way as we previously stated, the experimental crystal structure28 was used as the starting point of our calculations, and we obtained a relaxed structure very close to the experimental structure by optimizing the atom positions, as shown in Figure 7. The Al:N and Li:B bonds in Li2Al(BH4)5 3 6NH3 are very important to understand the electronic structure. We have reproduced very well the Al N bond length, which was found experimentally28 to be 2.05 Å. However, our calculated values of the Li B distances, namely, 2.49 Å, is slightly shorter than 2.52, and 2.69 Å is slightly longer than 2.68 Å, as reported in the experimental paper.28 After having resolved the crystal structure of Li2Al(BH4)5 3 6NH3, we focus on the electronic properties of this material. First, we list the Bader’s charges in Table 3 based on our first-principle calculations. From the Table 3, it is clear that the BH4 molecules are mostly ionic in nature, having charges of 0.86 (with BH4 consisting of B2, H4 and H5) and 0.72 (with BH4 consisting of B1, H6, and H7), respectively, instead of 1, whereas (NH3)
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Figure 7. Crystal structure of Li2Al(BH4)5 3 6NH3 as obtained from our ab initio calculations.
Table 3. Calculated Bader Atomic Charges (In Number of Electrons) for Li2Al(BH4)5 3 6NH3a atom
number of electrons
Li
0.11
Al
0.57
N
6.34
B1
1.36
B2
1.36
H1 H2
0.55 0.55
H3
0.55
H4
1.63
H5
1.62
H6
1.57
H7
1.61
a
Only inequivalent atoms are displayed. We have used the same labeling for the hydrogen atoms as in the experimental paper (only inequivalent atoms are shown): H1, H2, and H3 bonded with N, H4, and H5 are bonded with B2, whereas H6 and H7 are bonded with B1.
molecule is neutral, keeping eight electrons. Moreover, lithium and aluminum are partially ionized, with 0.11 and 0.57 remaining electrons, respectively. Second, we explore the electron distribution in some given planes of Li2Al(BH4)5 3 6NH3 by visualizing the corresponding electron density. Our calculated electron densities in the planes of the N, Al, and H atoms and Li, B, and H atoms are shown in Figures 8 and 9, respectively. A very low distribution of electrons between Al+ and the neighboring atoms, as presented in Figure 8, supports our findings that the bonding is mainly ionic between these atoms. The electron distribution between Li+ and the neighboring atoms is negligible, which confirms that the bonding is ionic. (See Figure 9.) Third, we have calculated the total and partial densities of states of Li2Al(BH4)5 3 6NH3, as plotted in Figure 10. The compound is found to be insulating with a band gap of 6 eV. As we have previously mentioned, the experimental value can be even larger because standard approximations to the exchangecorrelation potential underestimate the values of the band gap. 20039
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Figure 8. Calculated electron density in the plane of Al N H. The contours are 0.05 e/Å3.
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Figure 11. Diffusion of hydrogen atoms in LiBH4 3 NH3.
Figure 12. Diffusion of hydrogen atoms in Sr(NH2BH3)2. Figure 9. Calculated electron density in the plane of Li B H. The contours are 0.05 e/Å3.
Figure 10. Total and partial densities of states for Li2Al(BH4)5 3 6NH3.
The total DOS consists of five well-separated regions in energy: the first peak is around 16.5 eV, and it corresponds to N-2s and some amount of Al-2s states, whereas the second peak is around 16 eV and mainly consists of N-2s states with a small contribution of Al-p states, which shows that the nature of Al bonding with N is not fully ionic, but it also contains a small part of covalent bonding. The third structure is from 7 to 5 eV. It is made of several peaks coming from N-p and B-s states, together with a contribution from aluminum, which also reveal the presence of covalent nature of bonding between Al with B and N bonds. The fourth region is around 0 to 3 eV, mostly consisting of Al-p, B1-p, B2-p, and N-p states, along with a very small contribution of lithium states. Finally, the fifth region above the Fermi level (which is put at 0 eV) is made of conduction bands: the bottom of the conduction band is constituted from B-p, N-p, and lithium states with a small contribution from aluminum. To analyze further the characteristics of Li2Al(BH4)5 3 6NH3, we calculate the binding strength of hydrogen at the B and N sites. We find that the hydrogen removal energies are 5.32 and 5.38 eV for N and B-sites, respectively. This is in contrast with our results for LiBH4 3 NH3 and Sr(NH2BH3)2, for which the N H bond was stronger than the B H bond. Molecular Dynamics Simulations. A sufficient mobility of the hydrogen species is required to have an efficient fuel cell. Therefore, we have performed ab initio MD simulations to get deeper insight concerning the mobility of the hydrogen atoms in the LiBH4 3 NH3, Sr(NH2BH3)2, and Li2Al(BH4)5 3 6NH3 compounds. The corresponding mean square displacements (MSDs) are presented in Figures 11 13, respectively. To have a fair comparison, we have conducted our simulation for the same 20040
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charges from the experimental electron density and to compare them with our calculated values.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel:+46184715850. Fax:+46184715999.
Figure 13. Diffusion of hydrogen atoms in Li2Al(BH4)5 3 6NH3.
period of time (5 ps) and for the same temperature (500 K) for all three materials. Our choice of a temperature of 500 K is made based on the fact that we wish to stay close to the experimental conditions. For instance, in ref 28, the adsorption/desorption of hydrogen is happening in this range of temperature. We observe that the diffusion of hydrogen in LiBH4 3 NH3 appears to be faster than that in Sr(NH2BH3)2 and Li2Al(BH4)5 3 6NH3 because at the end of our simulation the MSD of hydrogens in LiBH4 3 NH3 is clearly larger (close to 100 Å2 at 5 ps) than the corresponding MSD in Sr(NH2BH3)2 and Li2Al(BH4)5 3 6NH3 (between 10 and 20 Å2 at 5 ps). Therefore, with respect solely to diffusion properties, LiBH4 3 NH3 is more interesting than Sr(NH2BH3)2 and Li2Al(BH4)5 3 6NH3 to obtain an efficient fuel cell. This is partially explained by the fact that the crystal structures are different: geometries that are less compact are more favorable for hydrogen transport. Also, as shown by our calculations of the hydrogen removal energies, the environment of the hydrogen atoms is different, and this can play a role on the MSD values, especially at the beginning of the simulations when the hydrogen atoms are still located close to their equilibrium position.
’ CONCLUSIONS The three compounds studied here are all relevant for hydrogen storage. Although they present some significant differences in their crystal structures and chemical composition, they possess some common features. For instance, the metallic ions (Li in LiBH4 3 NH3, Sr in Sr(NH2BH3)2, and Li and Al in Li2Al(BH4)5 3 6NH3) are never fully ionized. Our calculated Bader’s charge for the nitrogen atom is between 6.2 and 6.45 electrons, and the corresponding number of electrons for boron is in the range 1.17 to 1.44. Also, our calculated hydrogen removal energies fall in the range from 5.23 to 5.63 eV for hydrogen atoms bonded with B and in the range from 5.32 to 6.02 eV for hydrogen atoms bonded with N. Although a study on a larger number of compounds is probably needed to draw definitive conclusions, it is expected that these criterion must be fulfilled by compounds relevant for hydrogen storage. In summary, we have studied the electronic structure of LiBH4 3 NH3, Sr(NH2BH3)2, and Li2Al(BH4)5 3 6NH3 using DFT. Moreover, we have calculated the hydrogen removal energies and studied the diffusion of hydrogen using ab initio MD. We hope that our work will stimulate further experiments on the compounds that we have studied here. For instance, if electronic structure factors could be measured at high resolution, then it would be possible to obtain the corresponding Bader’s
’ ACKNOWLEDGMENT We would like to acknowledge STINT, VR, Formas, and Futura for financial support. M.R. is thankful to the Higher Education Commission of Pakistan. SNIC, HPC, and UPPMAX have provided computing time for this project. S.L. acknowledges financial support from ANR PNANO grant no. ANR-06-NANO-053-02. This work was performed using HPC resources from GENCI-CCRT/CINES (grant 2011-085106). ’ REFERENCES (1) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469–472. Z O^aA (2) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127–1129. Z O^aA (3) Rowsell, J. L. C.; Spencer, E. C.; Eckert, J.; Howard, J. A. K.; Yaghi, O. M. Science 2005, 309, 1350–1354. (4) Blomqvist, A.; Araujo, C. M.; Srepusharawoot, P.; Ahuja, R. Proc. Natl. Acad. Sci. U.S.A. 2007, 51, 20173. (5) Ashcroft, N. W. Phys. Rev. Lett. 2004, 92, 187002. (6) Feng, J.; Grochala, W.; Jaron, T.; Hoffmann, R.; Bergara, A.; Ashcroft, N. W. Phys. Rev. Lett. 2006, 96, 017006. (7) Eremets, M. I.; Trojan, I. A.; Medvedev, S. A.; Tse, J. S.; Yao, Y. Science 2008, 319, 1506. (8) Kim, D. Y.; Scheicher, R. H.; Lebegue, S.; Prasongkit, J.; Arnaud, B.; Alouani, M.; Ahuja, R. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 16454. (9) Strobel, T. A.; Goncharov, A. F.; Seagle, C. T.; Liu, Z.; Somayazulu, M.; Struzhkin, V. V.; Hemley, R. J. Phys. Rev. B 2011, 83, 144102. (10) Strobel, T. A.; Somayazulu, M.; Hemley, R. J. Phys. Rev. Lett. 2009, 103, 065701. (11) Wang, S.; Mao, H.; Chen, X.; Mao, W. L. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 14763–14767. (12) Ramzan, M.; Lebegue, S.; Ahuja, R. Phys. Rev. B 2010, 81, 233103. (13) Ramzan, M.; Ahuja, R. Appl. Phys. Lett. 2009, 94, 141903. (14) Ramzan, M.; Hussain, T.; Ahuja, R. Appl. Phys. Lett. 2009, 94, 221910. (15) Ramzan, M.; Ahuja, R. J. Appl. Phys. 2009, 106, 016104. (16) Ramzan, M.; Lebegue, S.; Ahuja, R. Int. J. Hydrogen Energy 2010, 35, 10373–10376. (17) Stowe, A. C.; Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T. Phys. Chem. Chem. Phys. 2007, 9, 1831. (18) Ramzan, M.; Ahuja, R. J. Phys. Chem. Solids 2010, 71, 1137–1139. (19) Xiong, Z.; Yong, C. K.; Wu, G.; Chen, P.; Shaw, W.; Karkamkar, A.; Autrey, T.; Jones, M. O.; Johnson, S. R.; Edwards, P. P.; David, W. F. Nat. Mater. 2008, 7, 138. (20) Wu, H.; Zhou, W.; Yildirim, T. J. Am. Chem. Soc. 2008, 130, 14834. (21) Kang, X.; Fang, Z.; Kong, L.; Cheng, H.; Yao, X.; Lu, G.; Wang, P. Adv. Mater. 2008, 20, 2756. (22) Ramzan, M.; Silvearv, F.; Blomqvist, A.; Scheicher, R. H.; Lebegue, S.; Ahuja, R. Phys. Rev. B 2009, 79, 132102. (23) Soloveichik, G.; Her, J. H.; Stephens, P. W.; Gao, Y.; Rijssenbeek, J.; Andrus, M.; Zhao, J. C. Inorg. Chem. 2008, 47, 4290. (24) Wu, C.; Wu, G.; Xiong, Z.; Han, X.; Chu, H.; He, T.; Chen, P. Chem. Mater. 2009, 3, 22. 20041
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