Comment on the stability of decorated C48B12 heterofullerene

Deb Sankar De, Santanu Saha, and Stefan GoedeckerDepartment of Physics, Universitat Basel, Klingelbergstr. 82, 4056 Basel, Switzerland

Luigi GenoveseUniv. Grenoble Alpes, INAC-MEM, L Sim, F-38000 Grenoble, France

(Dated: September 18, 2018)

A good hydrogen storage material should adsorb hydrogen in high concentrations and with optimalbinding energies. Numerous mixed carbon boron fullerenes which are decorated with metal atomswere previously constructed by hand and proposed as a promising material in this context. Wepresent a fully ab-initio, unbiased structure search in the configurational space of decorated C48B12

and find that most of the hitherto postulated ground state structures are not ground states. Wedetermine the energetically lowest configurations for Be, Ca, Li and Sc decorated C48B12 clusters.

I. INTRODUCTION

The emergence of green energies and new technologiesin the past decades has led to a strong growth in re-search activities in material discovery. Amidst the theo-retically and experimentally studied materials, fullereneand fullerene based structures [1–4] play a prominentrole due to their applicability in various fields. Recentexperimental observation of borospherene [5] has gen-erated renewed interest in application of fullerene basedstructures in the field of green energy.

Among the fullerene based structures, the boron sub-stituted fullerene C48B12 was predicted theoretically byManna et. al [6]. This structure was constructed by sub-stituting 12 C atoms by B atoms homogeneously in thefullerene such that there are no B-B bonds. This struc-ture will be referred to as ”diluted” C48B12 in the fol-lowing. However, in an unbiased minima hopping (MH)structure prediction Mohr. et. al. found a patched con-figuration, where all the B atoms are close together ina two-dimensional patch, that is considerably lower inenergy than the homogeneously distributed arrangement[7]. This structure will be referred to as a ”patched” con-figuration. Similar studies have been conducted for dec-orated Si20 [8] and exohedrally decorated borospherene(B40) [9].

Based on DFT calculations, metal decorated C48B12

has recently been predicted to have superior H2 adsorp-tion capabilities compared to pure C48B12. Through asystematic search, Sun et. al. found that that 12 Liatoms that are homogeneously distributed on the surfaceof the fullerene form the most stable configuration [10].Gao et al. constructed two microporous frameworks con-sisting of organic linkers and exohedral metallofullerenenodes. They showed through the calculation that theboth frameworks can store H2 with a gravimetric den-sity up to 8—9.2 wt % [11]. The patched configurationswas not considered as a basic unit in this study. Zhaoet. al. reported that a structure, that is homogeneouslycoated with 12 Sc atoms, is stable and also suitable forH2 adsorption but no structural stability search was per-formed [12]. Er et. al. found that homogeneously dis-

tributing 6 Ca atoms on the fullerene is stable with re-spect to decomposition into the fullerene molecule andCa bulk metal [13]. Most recently a similar study wasconducted by Qi et al. on the same 6 Ca atom deco-rated diluted configuration to study the hydrogen uptakemechanism [14]. Lee et. al constructed a few hand madestructures by adding 6 diluted Be atoms on the surfaceand they concluded that homogeneously distributed Beatoms on C48B12 give rise to the most stable configura-tion [15].

In these studies of metal decorated C48B12, two majorfactors were overlooked: (a) only diluted structures weredecorated with different elements whereas the patchedstructure, that is lower in energy by 1.8 eV, was not con-sidered and (b) the investigated structures were obtainedthrough chemical intuition and local relaxation. With-out knowing the true ground state of the these systems,it has been conjectured that they are good for hydrogenadsorption. A thorough unbiased search of the PES isnecessary to obtain reliable conclusions on their stabilityand their resulting capacity for hydrogen storage.

Using the Minima Hopping Method (MHM) [16], weperformed a comprehensive structural search, decoratingboth diluted and patched C48B12 fullerenes. Almost inevery case (except for Li), the ground state structure wasmassively distorted by the decoration and the metal dec-orated patched configurations are much lower in energythan their diluted counter part. This is in contrast toC60 whose structure remains stable under decoration aswas previously shown [17].

II. COMPUTATIONAL METHODOLOGY:

All the geometry optimizations and the energy calcula-tions have been performed on the density functional the-ory (DFT) level as implemented in BIGDFT [18]. Thiscode uses Daubechies wavelets as a basis set and the ex-change correlation was described by the Perdew- Burke-Ernzerhof (PBE) functional [19, 20]. A grid spacing of0.4 Bohr was used. Convergence parameters of BIGDFTwere set such that total energy differences were converged

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TABLE I. Largest adsorption energies (Eads) for differenttypes of decorating atoms

Type ofStructure

Eads in eVLi Be Ca Sc

Patch 3.20 3.41 3.62 5.28Dilute 2.88 2.66 2.89 4.22

up to 10−4 eV and all configurations were relaxed untilthe maximal force component of any atom reached thenoise level of the calculation, which was of the order of1 meV/ A. Soft Goedecker-type pseudopotentials withnon-linear core corrections were used [21]. The minimahopping method (MHM) was used for an unbiased searchof new configurations [16, 22]. It efficiently finds low-energy structures by exploiting the Bell-Evans-Polanyiprinciple for molecular dynamics [23]. The potential en-ergy surface (PES) is explored by performing consecutiveshort molecular dynamics escape steps followed by localgeometry relaxations.

III. RESULTS:

We first adsorbed single atoms on both diluted andpatched configurations. There are 8 possible sites to ad-sorb a single atom on the diluted configuration and 10 onthe patched configuration. We have considered all of ofthem and report the largest adsorption energy in Table I.The adsorption energy of n decorating atoms on C48B12

is calculated by the following equation:

Eads = EC48B12 + n× Eatoms − EnatomC48B12 (1)

For the diluted configuration, single Be, Ca and Sc atomsprefer the hexagonal site which has two neighboringboron atoms. A single Li atom prefers the hexagonalsite where the hexagon has a single boron atom. For thepatched configuration, Be atoms prefer the pentagon sitewith 3 neighboring borons, Li prefers the hexagon sitewith one boron, Ca prefers the hexagon site with twoborons and Sc prefers to be on top of boron when boronis in the middle of a hexagonal site with four neighboringboron atoms.

We next present our results for the cases where theC48B12 fullerene is decorated with several metal atoms,namely 12 for Li and Sc and 6 for Ca and Be. Fig. 1summarizes the results.

A. Be

A structure consisting of six Be atoms which are homo-geneously distributed on C48B12 was proposed as a goodH absorber [15]. MH runs revealed that even for thediluted fulerene cage there are many different structureswhich are lower in energy than this previously proposedone. We observed that the most of the Be atoms form

lusters and that a single Be atom prefers to stay on anhexagonal-site far from the Be-cluster. The underlyingdiluted fullerne structure is however not destroyed by thedecoration.

Even lower energy configurations can be obtained ifone decorates the patched fullerene structure. Thepatched configuration is broken during the MH runs anda few of the Be atoms go inside the C48B12. This sug-gests that though the patched configuration is more sta-ble than the diluted configuration, it is more reactive toBe atoms.

B. Ca

The hitherto known lowest energy structure consists of6 homogeneously distributed Ca atoms on the H-sites ofthe diluted configuration [13] [14]. We found a new struc-ture based on a diluted fullerene that is 0.73 eV lower

FIG. 1. The Lowest energy structures for diluted and patchedconfigurations for Be, Ca, Li and Sc decorated C48B12. Thecolumn in the center shows the previously predicted struc-tures. The column on the right the lowest energy structuresfor decorations of the diluted fullerene and the left columnthe lowest energy structures for the patched fullerene. Theenergy difference is given with respect to the middle struc-ture for each type of decoration. It shows that for all typesof atoms except Li the patched configurations are the moststable ones.

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than the previously predicted configuration. In contrastto C60, where the Ca atoms form a patch [17] they re-main homogeneously distributed on the dilute C48B12.The patched configuration with clustered Ca atoms ishowever also in this case by far the lowest in energy. Inthese configuration some Ca atoms again enter into thecage of the patched fullerene.

C. Sc

Zao et al proposed a configuration where 12 Sc atomsare homogeneously distributed over surface of the diluteC48B12 as the most stable configuration [12]. For the Cadecorated dilute C48B12 configuration, we have again ob-served several lower energy configurations than the previ-ously predicted configuration. In the lowest energy con-figuration the diluted C48B12 the cage structure is brokenand a few Sc atom went inside the cage, resulting in adrastic energy lowering.

Similarly, in the patched C48B12 configuration deco-rated with Sc, the cage gets partially destroyed. Thisconfiguration is again the lowest one.

D. Li

12 Li atoms regularly distributed on the P sites of thediluted C48B12 structure were proposed to be most sta-ble configuration [10]. The energy can however be low-ered by distributing the Li atoms in a more random way,resulting in some Li-Li bonds. This was the lowest en-ergy configuration found in our study. Decorations of thepatched C48B12 fullerene resulted in this case in higherenergy configurations. So decoration with Li atoms sta-bilizes the diluted configuration.

IV. CONCLUSION:

In conclusion, we have shown that the ground state ofdifferent metal decorated boron-carbon heterofullerenesis fundamentally different from previously publishedstructures [10, 12, 13, 15]. The metals considered inour study are not able to homogeneously coat the diluteC48B12 cage. With the exception of Li, the decorationactually destroys the cage. The bonding character in allthese configurations is quite complicated and there is nosimple rule available to predict the type of distributionof different elements on the C48B12 surface. Our resultsgive also valuable information for experimental synthesisefforts of such heterofullerenes. Our investigation revealsthat most previous computational studies on metal atomdecorated fullerenes for hydrogen storage are based onunrealistic structures.

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