J. Phys. Chem. C 2008, 112, 19963–19968
19963
Ti-Decorated Doped Silicon Fullerene: A Possible Hydrogen-Storage Material Sonali Barman,† Prasenjit Sen,‡ and G. P. Das*,† Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700032, India, and Harish Chandra Research Institute, Allahabad 211019, India ReceiVed: May 26, 2008; ReVised Manuscript ReceiVed: September 18, 2008
From first-principles density functional electronic structure calculations, we show that Si60H60 forms a highly symmetric (meta)stable fullerene structure on which Ti atoms can be capped exohedrally in cationic form, enabling storage of hydrogen in the molecular state. Whereas the H atoms on a Si60H60 cage saturate the Si dangling bonds, the Ti+ ion sitting on the energetically favorable hexagonal face of the Si60H60 fullerene allows a maximum of four H2 molecules to be attached, with a binding energy intermediate between those of physisorption and chemisorption. However, two Ti atoms added to Si fullerene tend to dimerize, thereby reducing the hydrogen-storage efficiency. This clustering tendency of the Ti atoms on the fullerene surface can be avoided by doping the fullerene with P atoms. Our first-principles results show that Ti-decorated P10Si50H50 clusters can serve as a potential hydrogen-storage material, with a storage capacity of up to 5.23 wt % corresponding to full Ti coverage. 1. Introduction The search for an ideal material for hydrogen storage is dictated by the need to simultaneously satisfy several criteria, such as high gravimetric density, near-ambient temperature and pressure of operation, and fast recharge/discharge kinetics.1-5 To meet the first target, i.e., high gravimetric density (∼6 wt % or more), only elements with low atomic numbers can be strictly entertained, so that any efficient hydrogen-storage medium can be built only with light elements, such as Li, Be, B, C, Na, Mg, Al, Si, and P. Although complex hydrides of many of these light elements are being extensively studied,6-11 C- and Si-based nanostructures, in particular, have attracted much attention in the recent past. This is because nanostructures of C and Si with large surface areas can be formed and can also be decorated and encapsulated with suitable metal atoms. Such hybrid structures show a high degree of structural stability and can be functionalized for efficient hydrogen storage. For optimum performance as a storage material, the ideal form of binding between the host material and adsorbed hydrogen molecules should be intermediate between physisorption (binding energies on the order of millielectronvolts) and chemisorption (binding energies on the order of electronvolts), such that the molecular bond of hydrogen does not break but rather elongates slightly as a result of charge polarization or orbital overlap. Studies on various kinds of doped fullerenes have been reported in the literature. For example, the laser-induced photofragmentation behavior of substitutionally doped metal fullerene clusters C60-xMx (M ) Ti, V, Fe, Nb, Ta) and Sidoped fullerene clusters C60-xSix were studied by Branz et al.12 and Billas et al.,13 respectively. Also, DFT studies of C60-xSix heterofullerenes14 such as C40Si20, C36Si24, and C30Si30 have shown most stable isomers to be those in which Si atoms and C atoms are segregated to form two distinct homogeneous subnetworks. However, specifically in a search for candidate * To whom correspondence should be addressed. E-mail:
[email protected]. † Indian Association for the Cultivation of Science. ‡ Harish Chandra Research Institute.
materials for hydrogen storage, various porous structures with high surface areas and/or large cavities, such as Si-C semiconductor nanocomposites,15 clathrates,16 carbon nanotubes,17-22 and metal-organic framework compounds,23 have been investigated. Experimental and theoretical studies have revealed that pure carbon24-29 and boron nitride30 nanostructures are not suitable for practical application as hydrogen-storage materials because of their low gravimetric density and inefficient desorption kinetics. Quite interestingly, a new possibility has opened up with the doping of transition metal (TM) atoms on C60 fullerene.31-34 In this class of materials, the TM-fullerene binding takes place through a charge-transfer mechanism, and the TM remains in the cationic state, thereby leading to molecular absorption of incoming hydrogen. Later, it was shown by a density functional study33 that, on a C60 cage, exohedral Ti atoms prefer to form clusters on the surface rather than decorating the 12 pentagonal faces. Therefore, Ti12C60 might not be suitable for storing more than 3 wt % hydrogen. Thus, it is imperative to find a nanocluster on which Ti atoms do not cluster together, so that a Ti-decorated nanoscale material can be used as a hydrogen-storage medium. It was natural that, after the discovery of carbon-based nanostructures (fullerenes, nanotubes) and their properties, questions would arise about Si-based nanostructures, Si being the next group IV element in the periodic table after carbon. Unlike C, it is known that Si does not form stable fullereneand nanotube-like structures, mainly because of its unfavorable sp2 hybridization. Because of its sp3 hybridization in the bulk, Si, when used in fullerenes or nanotubes, prefers to form dangling bonds rather than single/double bond chains across the pentagon and hexagon rings. In recent years, there has been renewed interest in Si-based nanostructures because of their expected potential applications in nanotechnology. For example, cluster-assembled solids designed from C5Si24H36 nanoparticles15 have been studied as promising materials for hydrogen storage with an estimated efficiency of ∼3.2 wt % H2. These Si-C nanocomposites have shorter C-C bond lengths in the core that pull the Si atoms from the surface, thereby reducing their separation and hence increasing the probability of formation of
10.1021/jp804637x CCC: $40.75 2008 American Chemical Society Published on Web 11/24/2008
19964 J. Phys. Chem. C, Vol. 112, No. 50, 2008 TABLE 1: Ground-State Properties of Si60H60 Fullerene property
value
BE (eV) HOMO-LUMO gap (eV) bond angles (deg) bond length(s) (Å) Si-Si Si-H Mulliken population Si H
400.077 3.56 (i) 101, (ii) 102 2.37, 2.36 1.5 +0.0335 -0.0335
SiH2 dihydrides. Among hollow cagelike structures, Si nanotubes (SiNTs)35-40 and Si clathrates41 have been successfully synthesized, but Si fullerenes have not yet been prepared in the laboratory. Nevertheless, both theoretical and experimental attempts have been going on to stabilize Si cages by adding endohedral metal atoms42-47 and by coating silica48 on Si clusters. The stability of SinHn fullerenes (n ) 16 and 20) was first shown by Kumar and Kawazoe.49 Hydrogenation as a route to stabilize Si60 fullerene has also been explored by Wang et al.50 and Zhang et al.51 In particular, Si60H60 turns out to be a perfectly icosahedral fullerene cage, just like C60. It then becomes interesting to investigate whether these Si fullerenes can be used as hydrogen-storage materials. It has already been theoretically proposed by Zhang et al. that a Si60H60 fullerene can accommodate 58 H2 molecules endohedrally.51 They estimated that, including the 60 hydrogen atoms on the exterior surface, formation of 58H2@Si60H60 leads to a hydrogen storage density of 9.48 wt %. However, their estimate also shows that the H2 molecules have to overcome a barrier in order to enter or escape the cage through the hexagonal face, which can be a hindrance in practical applications. In this work, we have investigated Si60H60 fullerene, decorated exohedrally with Ti, as a possible hydrogen-storage material. There are two main motivations behind this effort: (a) to ensure that H2 can be adsorbed on the surface of the Si60H60 cage with a binding energy in the acceptable range and (b) to avoid the clustering tendency of the Ti atoms on the fullerene surface by doping Si60H60 with P. The rest of this article is organized as follows: The computational methods employed in the work is presented in section 2, results are presented and discussed in section 3, and our major conclusions are given in section 4. 2. Computational Details All calculations were carried out using density functional theory (DFT) as implemented in the DMol3 software.52 To solve the Kohn-Sham equations, DMol3 uses numerical orbitals for the basis functions where each basis function corresponds to an atomic orbital (AO). Among the different kinds of numerical basis sets in DMol3, we used the double-numeric basis sets (DNP) for the expansion of molecular orbitals (MOs) to obtain the best accuracy. We used the Perdew-Wang (PW91)53 functional for the exchange-correlation energy. We relaxed the geometries for each cluster without any symmetry constraints. 3. Results and Discussion 3.1. TiSi60H60 Complex. We started with the relaxed structure of Si60H60 fullerene that turns out to be a perfectly icosahedral cage, as reported earlier.50,51 The structural, energetic, and chemical details of this cluster are given in Table 1, which match exactly with the data in the work Wang et al.40 We have defined the binding energy (BE) as the energy gained
Barman et al. in assembling the cluster from its isolated constituent atoms. Hence, the BE of Si60H60 is
EB ) 60E(Si) + 60E(H) - E(Si60H60)
(1)
where E represents the total energy of the respective cluster/ atom. To ensure that the fullerene structure is indeed a (meta)stable structure, we also calculated the harmonic vibrational frequencies of Si60H60. All of the vibrational frequencies were found to be real and positive, lying in the range from 2180 to 53 cm-1, i.e., from the infrared to the far-infrared region. In this range, the higher frequencies correspond to Si-H vibrational modes, and the lower ones correspond to Si-Si modes. To ascertain the thermodynamic stability of the hollow Si60H60 fullerene cage, we chose a competitive structure, viz., a 60-atom sp3-bonded Si nanocrystal with an approximately spherical shape (similar to the C5Si24H36-based cluster-assembled solid used in ref 15) whose surface was truncated and saturated with hydrogen atoms. The BE of this Si60H60 nanocrystal was found to be 405 eV, i.e., ∼0.12 eV per Si-H unit higher compared to that of the Si60H60 fullerene cage. This shows that the fullerene structure is competitive in energy with other possible sp3-bonded nanocrystals. In summary, Si60H60 fullerene cage is a metastable cluster that can possibly be synthesized experimentally. Because our aim was to explore Ti-decorated Si60H60 fullerene for hydrogen storage, it was crucial to know the favorable binding sites of a single Ti atom on the surface of this cluster. We considered four high-symmetry sites on which a Ti atom can attach to a Si60H60 fullerene: (i) on top of a hexagonal face, (ii) on top of a pentagonal face, (iii) on bridge sites connecting two neighboring hexagons, and (iv) on bridge sites connecting a pentagon and a hexagon. We placed a Ti atom at each of these sites and relaxed the resulting structure without any symmetry constraints. The energy gain in attaching a Ti atom to a Si60H60 fullerene is called the adsorption energy (∆E) and is defined as follows
∆E ) EB(TiSi60H60) - EB(Si60H60)
(2)
From our definition of BE, a positive value of ∆E would indicate an energy gain in attaching a Ti atom to a Si60H60 fullerene. The ground-state chemical properties of TiSi60H60 fullerenes are given in Table 2. The positive values of ∆E indicate that there is, in fact, an energy gain in attaching a Ti atom to a Si60H60 fullerene. Moreover, it is also seen that the hexagonal site is energetically the most favorable site for a Ti atom. It has been argued in the literature that unfilled d orbitals53 and/or a cationic charge state54-57 on TM atoms are essential for hydrogen to be attached to them in molecular form. Figure 1 shows isosurface plots for the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the TiSi60H60 fullerene when the Ti atom is on a hexagonal site. These are both d orbitals on the Ti atom. The Ti d character of the LUMO ensures that there are unfilled d orbitals available for interaction with incoming hydrogen molecules. Table 2 also shows that the Ti atom is in a cationic state with a Mulliken charge of +0.039. Later, we will argue that the cationic state of the TM atom is not essential for hydrogen attachment in molecular form. 3.2. Hydrogen Adsorption. Once we found the stable structures for the Si60H60 and TiSi60H60 fullerenes, we then studied their suitability for hydrogen storage. The two most crucial questions in relation to hydrogen storage are (i) whether the storage material has the right energetics and kinetics and
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TABLE 2: Properties of TiSi60H60 for a Ti Atom at the Four High-Symmetry Sites on a Si60H60 Fullerene binding site of Ti on Si60H60 hexagonal site pentagonal site bridge site connecting two hexagons bridge site connecting one pentagon one hexagon
bond length(s) (Å)
HOMO-LUMO Mulliken ∆E (eV) gap (eV) charge on Ti
Ti-Si
Si-Si
Si-H
1.662 1.223 1.500
0.701 0.760 2.058
0.039 0.056 0.311
2.51(4), 2.77, 2.78 2.620, 2.406(2), 2.360(2), 2.719 1.51(4), 1.50(2) 2.42(2), 2.59(2), 2.67 3.56, 2.48(2), 2.42, 2.41 1.52(3), 1.51(2) 2.44, 2.45 2.37 2.31, 2.41
0.842
0.388
0.433
2.60, 2.70
(ii) what maximum gravimetric density of hydrogen can be achieved. Pure Si60H60, however, does not saisfy the required energetics, as our calculations showed that the BE values for H2 on the pentagonal and hexagonal faces of bare Si60H60 are 0.01 and 0.02 eV, respectively, which are too low and hence unsuitable for hydrogen storage. To address the above questions, we therefore focussed on Ti-decorated Si60H60 fullerene and allowed H2 molecules to approach Ti ions from different directions. The possibility of dissociation of the hydrogen molecules was also taken into account by relaxing all of the atoms without any constraints. We found that the added hydrogen molecules did not dissociate and were absorbed in molecular form, which is encouraging from the point of view of hydrogen storage. The average energy gain per H2 molecule in attaching the nth H2 molecule was defined as
∆En ) {EB[TiSi60H60(H2)n] - EB(TiSi60H60) - nEB(H2)}/n
(3) where EB(H2) is the BE of a hydrogen molecule. The ∆En values and other chemical details (HOMO-LUMO gap, Mulliken charge on Ti site) of these clusters are reported in Table 3. It is seen that there is energy gain in attaching up to four H2 molecules to the Ti atom on the hexagonal face of Si
Figure 1. Isosurface plots of the (a) HOMO and (b) LUMO of a TiSi60H60 cluster. The HOMO and LUMO have dz2 and dxy characters, respectively, on the Ti atom.
2.26
1.69, 1.74
TABLE 3: Properties of TiSi60H60 + nH2 n
∆En (eV)
HOMO-LUMO gap (eV)
Mulliken charge on Ti
1 2 3 4
0.47 0.48 0.34 0.26
0.92 1.29 1.20 1.39
-0.04 -0.45 -0.37 -0.44
fullerene. The energy of the system increased when we tried to add a fifth H2 molecule. Positive ∆En values and large HOMO-LUMO gaps (Table 3) in all clusters up to n ) 4 indicate their thermodynamic and kinetic stability. The atomic charges on all of the atoms were calculated by Mulliken population analysis and are included in Table 3. The Ti atom in TiSi60H60 is in a cationic state, as already discussed. After adsorption of the first H2 molecule, the charge on the Ti atom becomes negative, and there is a cationic-to-anionic transition. The charge on the Si60H60 complex, however, remains almost unaffected. The charge variation of Ti reflects the consequence of the overlap of the orbitals of the Ti atom and the H2 molecule. What is interesting is that, even in the anionic state, the Ti atom can adsorb three more H2 molecules.
Figure 2. (a) Geometry of four H2 molecules attached to a Ti atom on a hexagonal face of Si60H60. (b) HOMO of the cluster in a, which is a combination of Ti d and H2 molecular states.
19966 J. Phys. Chem. C, Vol. 112, No. 50, 2008
Barman et al.
Figure 3. Geometry after H2 molecules react with a Ti-Ti dimer on Si60H60. One of the H2 molecules dissociates and is adsorbed as two H atoms (dark atoms).
Figure 2a shows the geometry of four H2 molecules attached to the Ti atom on a hexagonal face of a Si60H60 fullerene. Figure 2b shows an isosurface plot of the HOMO level of the resulting complex. As can be clearly seen, this is essentially a combination of Ti dz2 and hydrogen states. The H-H bonds are slightly elongated (0.81-0.83 Å) compared to that in a H2 molecule, but they are not broken. As there are 20 hexagonal faces on a Si60H60 fullerene cage, we expect that an exohedrally Tidecorated cage, i.e., a Ti20Si60H60 complex, can adsorb a maximum of 80 H2 molecules. This corresponds to 6 wt % of stored hydrogen. Although a theoretical possibility of storing 6 wt % hydrogen on Ti-decorated Si60H60 fullerene is quite encouraging, one has to worry about the possibility of the Ti adatoms clustering on the surface of the fullerene. Sun et al.33 have shown that it is energetically favorable for Ti atoms to cluster together on the surface of C60 fullerene. The same possibility exists on the surface of Si60H60 fullerene as well. As an indication of this clustering tendency, it was observed that a Ti-Ti dimer attached to a C60 fullerene has a lower energy than two Ti atoms attached to two opposite pentagonal faces. The energy difference between the two structures was found to be 1.29 eV. Following this approach, we calculated the energies of the following two structures: (i) two Ti atoms attached to two opposite hexagonal faces of a Si60H60 fullerene and (ii) a Ti-Ti dimer attached to a hexagonal face of a Si60H60 fullerene. Even on a Si60H60 fullerene, the Ti-Ti dimer is found to have an energy that was 0.32 eV lower than that of two isolated Ti atoms. This indicates that Ti atoms would tend to cluster together on this surface. To understand how this tendency affects the hydrogen-storage capacity of the Ti atoms, we allowed eight H2 molecules to react with a Ti-Ti dimer attached to a hexagonal face of a Si60H60 fullerene (as two Ti atoms on opposite hexagonal faces can accommodate eight H2 molecules). It was observed (Figure 3) that one of the H2 molecules dissociates and is absorbed in atomic form. Hydrogen attached to a storage material in atomic form generally has too high of an adsorption energy to be useful for mobile applications. Therefore, we conclude that clustering of Ti atoms degrades their hydrogen-storage capacity. As discussed, on both C60 and Si60H60 fullerene surfaces, Ti atoms tend to cluster together, which degrades their hydrogenstorage capacity. However, the energy gain in dimerization on
Figure 4. (a) Geometry of a P10Si50H50 fullerene. The lightly shaded spheres represent Si atoms, and the dark spheres represent P atoms. (b) Geometry of four H2 molecules attached to a Ti atom adsorbed on a hexagonal face of the same fullerene. The local distortion around the Ti atom can be clearly seen.
a Si60H60 fullerene surface is much lower than that on a C60 surface. This gives the former an advantage as a possible hydrogen-storage material. As the clustering tendency of Ti atoms on the Si60H60 fullerene surface is already quite weak, we next attempted to make it energetically unfavorable by means of chemical doping as discussed in detail in the next section. 3.3. P-Doped Si Fullerene. We note that whether a Ti-Ti dimer or isolated Ti atoms have lower energy is a competition between the adsorption energy (∆E) of a Ti atom on the fullerene surface and the Ti-Ti binding energy. If, by some means, one can increase the former, the dimerization might become energetically unfavorable. One way of increasing adsorption energy is to change the chemical nature of the host fullerene. From our studies on Ti adsorption on Si60H60 fullerene, we found that charge is transferred from the Ti atom to the fullerene. Thus, replacing a few of the Si atoms in the fullerene by another atom that is more electronegative might increase the electron affinity of the host fullerene, leading to a larger ∆E. For chemical substitution, an atom must be chosen that has an atomic radius very similar to that of Si, as otherwise, the fullerene structure would be distorted. (In fact, it became distorted when we replaced a few Si atoms by N). The clear choice is P. There is no recipe as to how many Si atoms are to be replaced by P atoms. In the past, Sc-decorated B12C48 fullerene, in which 12 C atoms in C60 fullerene were replaced by B atoms, was explored for hydrogen storage.31 While attempting to replace 12 Si atoms by P, we realized that this gives rise to many
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Figure 5. Geometry of a Ti-Ti dimer on a hexagonal face of (a) pure Si fullerene and (b) P-doped Si fullerene. Note that only the geometries of the dimer and the hexagonal face to which it binds are shown and that of the rest of the fullerene has been omitted for reasons of clarity.
(chemically) inequivalent pentagonal and hexagonal sites on the resulting fullerene. To keep the number of inequivalent sites to a minimum and yet to change the chemical nature of the host fullerene sufficiently, we replaced 10 Si atoms by P atoms with the constraint that all hexagonal faces had one Si replaced by P. This gives 10 pentagonal faces having P atom at one corner and leaves two pentagonal faces with Si atoms at all corners. Figure 4a shows this particular structure. Clearly, not all pentagonal faces are equivalent. Moreover, although all hexagons have one P each, considering their nearest-neighbor faces, not all of them are equivalent either. As we discuss later, the composition of the nearest-neighbor faces does affect the chemical environment at the center of a given face. It turns out that there are two chemically inequivalent types of hexagonal faces in our way of replacing 10 Si atoms by P atoms, much less than obtained to when 12 Si atoms are replaced by P atoms. In the resulting cluster, only the Si atoms are capped by H, so the host fullerene now has the chemical composition P10Si50H50. We found that this cluster also has a perfectly icosahedral fullerene structure with a BE of 366.23 eV and a HOMO-LUMO gap of 2.23 eV, indicating its stability. We next studied the adsorption of Ti atoms on the two inequivalent hexagonal faces of P10Si50H50 fullerene. On one hexagonal face, the Ti adsorption energy, ∆E, was found to be 1.95 eV, and the Mulliken charge on the Ti atom was +0.014. On an inequivalent hexagonal face, ∆E was found to be 1.91 eV, and the Ti atom was in a charge state of -0.013. Thus, ∆E was larger on both of these hexagonal faces than on the hexagonal face of a Si60H60 fullerene, as we had anticipated. The next big question was whether Ti atoms prefer to cluster on the surface of a P10Si50H50 fullerene. To address this issue, as before, we studied the behavior of two Ti atoms on the surface of this fullerene. In one case, the two Ti atoms were attached to two hexagonal faces of a P10Si50H50 fullerene: one on a hexagonal face on which the Ti was in a cationic state and the second on a hexagon diametrically opposite to it. In the other case, a Ti-Ti dimer was placed on the first hexagonal face. It turns out that the Ti dimer had an energy that was ∼0.05 eV higher than that of two Ti atoms on opposite faces. For comparison, we show the geometries of a Ti-Ti dimer on a hexagonal face of each a Si60H60 fullerene and a P10Si50H50 fullerene in Figure 5. Clearly, the Ti-Ti distance is much larger and the Ti-fullerene distance is smaller on the P-doped fullerene surface, leading to a weaker binding of the dimer, which shows up in the ∆E values. This leads to the very important conclusion that Ti atoms do not cluster on the surface of P10Si50H50 fullerene and that this P-doped Si fullerene could be practically useful in hydrogen storage. As discussed above, a Ti atom on one of the hexagonal faces is in a cationic state. From our calculations on TiSi60H60
fullerenes, we claim that these Ti atoms can store up to four H2 molecules each. The last question to be addressed was whether the Ti atoms in the anionic state bind hydrogen in molecular form. To test the hydrogen-storage capacity of the anionic Ti atoms, we allowed four H2 molecules to interact with one such Ti atom. All four of the H2 molecules attached to the Ti atom in molecular form, and the corresponding adsorption energy was found to be 0.84 eV/H2. Figure 4b shows the geometry of four H2 molecules attached to the Ti atom on a P10Si50H50 fullerene. The H-H bonds in these H2 molecules range between 0.8 and 0.85 Å. Thus, the H-H bonds are stretched as a result of orbital overlap, but they are not broken. The slightly higher adsorption energy obtained in this case is the result of a slight local distortion of the host fullerene around the Ti atoms, as can be seen in Figure 4b. In the absence of a clustering tendency of the Ti atoms on the surface of P10Si50H50 fullerene, the latter holds great promise for ways of developing materials for mobile hydrogen storage. Using first-principles molecular dynamics calculations up to 5-ps time scale, we found that, at room temperature (300 K), the cage structure of P10Si50H50 fullerene is retained and hydrogen molecules remain attached to the system. Ideally, we should have performed a calculation with 20 Ti atoms on all 20 hexagonal faces with four H2 molecules on each of the Ti atoms to estimate the maximum hydrogen-storage capacity of a fully Ti-decorated P10Si50H50 cluster. A fully selfconsistent calculation on this system, clearly, is a very large one that is, unfortunately, beyond the scope of our present computational resources. Thus, instead of performing such large calculations, we executed one additional calculation in which we placed two Ti atoms on two neighboring hexagonal faces of a P10Si50H50 cluster and investigated whether these two Ti atoms could also adsorb four H2 molecules each. In fact, we found that eight H2 molecules could be attached to these two Ti atoms, with an adsorption energy of 0.2 eV/H2. Therefore, although the binding of the H2 molecules becomes slightly weaker, it is expected that Ti atoms on all 20 hexagonal faces of a P10Si50H50 cluster would be able to adsorb four H2 molecules each, corresponding to a gravimetric density of ∼5.23 wt %. 4. Conclusion In conclusion, we have explored P-doped Si-based fullerenes for their potential to store hydrogen for mobile applications through first-principles DFT methods. We first confirmed the stability of an exohydrogenated Si60H60 icosahedral fullerene. A Ti atom adsorbed on a hexagonal face of this fullerene can store up to four H2 molecules, giving a maximum gravimetric density of 6 wt % when all 20 hexagonal faces of the fullerene are decorated with one Ti atom each. However, it is energetically
19968 J. Phys. Chem. C, Vol. 112, No. 50, 2008 favorable for these Ti atoms to cluster together, adversely affecting their hydrogen-storage capacity. It was also found that the energy gain due to dimerization of two Ti atoms on Si60H60 is much lower than that gained upon dimerization on a C60 fullerene, giving the former an advantage in terms of energetics. Most importantly, we found that chemical modification of the Si60H60 fullerene in which 10 Si atoms are replaced by P atoms increases the adsorption energy of a single Ti atom on the surface by a sufficient amount to make dimerization of Ti atoms energetically unfavorable. We believe that this is a crucial finding that promises a means of circumventing one of the major disadvantages of Ti-decorated C-based nanomaterials for hydrogen storage. We hope that this will spawn further theoretical and experimental work on the possible use of Si-based nanomaterials for hydrogen storage. In fact, the recent work of Zhang et al.51 enforces our conviction that Si-based fullerenes are serious candidates for nanoscale hydrogen-storage media. We further found that Ti atoms doped on the hexagonal faces of P10Si50H50 fullerene can also adsorb up to four H2 molecules each, leading to a possible hydrogen storage density of ∼5 wt %. Acknowledgment. The authors would like to thank Professor P. Jena and Professor G. Galli for helpful discussions. Note Added after ASAP Publication. This paper was published ASAP on November 24, 2008. Text was updated in the third paragraph of the Introduction. The revised paper was reposted on December 11, 2008. References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature (London) 2001, 414, 354. (2) Steele, B. C. H.; Heinzel, A. Nature (London) 2001, 414, 345. (3) Cortright, R. D.; Davada, R. R.; Dumesic, J. A. Nature (London) 2002, 418, 964. (4) Alper, J. Science 2003, 299, 1686. (5) Coontz, R.; Hanson, B. Science 2004, 305, 957. (6) Bogdanovic, B.; Schwickardi, M. J. Alloys Compd. 1997, 253254, 1–9. (7) Zaluska, A.; Zaluski, L.; Stro¨m, J. O. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 157–165. (8) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Nature (London) 2002, 420, 302. (9) Miwa, K.; Ohba, N.; Towata, S.; Nakamori, Y.; Orimo, S. Phys. ReV. B 2005, 71, 195109. (10) Araujo, C. M.; Li, S.; Ahuja, R.; Jena, P. Phys. ReV. B 2005, 72, 165101. (11) Magyari-Ko¨pe, B.; Ozolins, V.; Wolverton, C. Phys. ReV. B 2006, 73, 220101. (12) Branz, W.; Billas, I. M. L.; Malinowski, N.; Tast, F.; Heinebrodt, M.; Martin, T. P. J. Chem. Phys. 1998, 109, 3425–3430. (13) Billas, I. M. L.; Tast, F.; Branz, W.; Malinowski, N.; Heinebrodt, M.; Martin, T. P.; Boero, M.; Massobrio, C. Eur. Phys J D. 1999, 9, 337– 340. (14) Matsubara, M.; Massobrio, C. J. Phys. Chem. A 2005, 109, 4415– 4418. Matsubara, M.; Massobrio, C. J. Chem. Phys. 2005, 122, 084304. (15) Williamson, A. J.; Reboredo, F. A.; Galli, G. Appl. Phys. Lett. 2004, 85, 2917–2919. (16) Struzhkin, V. V.; Militzer, B.; Mao, W. L.; Mao, H.-K.; Hemley, R. J. Chem. ReV. 2007, 107, 4133–4151.
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