Hydrogen storage in La intercalated graphene/MoS2 layers

REN Juan, SHI Wen-Ting, JIA Ruo-Lan, WU Han, LIU Ping-Ping

(1. School of Sciences, Xi’an Technological University, Xi’an 710032,China;2. College of Materials and Engineering, Yangtze Normal University, Chongqing 408000, China)

Abstract：Hydrogen storage in La intercalated graphene/MoS2 layers was investigated via density functional theory (DFT) calculations. The interlayer distance of the graphene/MoS2 heterostructure expands due to the insertion of La. We systematically investigate the hydrogen adsorption behaviors on La intercalated graphene/MoS2 system. The results show that a maximum of six hydrogen molecules can bind to single La atom. The calculated average adsorption energy of each H2 molecule is approximately 0.198 eV with GGA/PBE functional. Appropriate hydrogen adsorption energy can achieve the reversible storage of hydrogen under ambient conditions. What’s more, La atoms can be dispersedly embedded to the graphene/MoS2 heterstructure. This means that more and more adsorption sites will be provided to hydrogen storage. This study shows that La intercalated graphene/MoS2 layers can be used as a promising hydrogen material by theoretical predictions.

Key words：Hydrogen storage; Graphene/MoS2 heterostructure; DFT

1 Introduction

With the increasing serious energy problems，there is an urgent need to find a real clean energy source[1]. Among various renewable energy sources，hydrogen is considered as a better alternative to gradually replace fossil fuels because of its high calorific values，abundance in nature and combustion creating neither air pollutants nor greenhouse[2]. However，it has not been used for commercialized due to some of obstacles to be overcome[3，4]. The biggest challenge is that the current on-board hydrogen storage mainly relies on high-pressure compression strategies which is costly and potential unsafe[5，6]. The key is to find solid materials that can store hydrogen with high gravimetric and volumetric density，as well as favorable thermodynamic and kinetics conditions. In recent years，the adsorption storage has been drawn much attention because of its safety and efficiency.

Since the mechanical stripping of graphene by Novoselov et al. in 2004，two-dimensional materials have attracted much attention due to their particular physicochemical properties[7]. They have the unique two-dimensional layered structures，showing excellent physical，chemical，electronic and optical properties，which have potential applications in many fields[8-12]. For example，two-dimensional materials attracted particular attention as gas storage due to their specific surface area and large porosity[13-18]. They adsorb hydrogen molecules through weak interactions between the materials and hydrogen molecules. More and more hydrogen molecules can be bound to the solid materials only at low temperatures and high pressures. Researchers adopt plenty of scientific ways to improve the adsorption capacity. Among all the modification methods，inserted of two-dimensional materials has emerged as a particularly powerful tool. Metal atomic interpolation can not only stretch the layer spacing of two-dimensional materials，but also provide more molecular hydrogen adsorption sites. For example，the hydrogen gravimetric capacity can reach up to 11.6 wt% at 100 bar of interconnected fullerenes and nanotubes with lithium decoration and boron-doping designed by Bietal.[19]. Cheng et al. theoretically designed a series of Li-doped C20-，C28-，C36-，C50-，C60-，C70-intercalatedh-BN frameworks . The results show that the hydrogen gravimetric and volumetric densities of Li-doped C50-BN framework can reach 3.72 wt% and 30.08 g/L at 193 K and 100 bar[20].The incorporation of Si16nanocluster into the pores of pillared graphene on the base of single-walled carbon nanotubes significantly improved its properties as anode material of Li-ion batteries by quantum-chemical calculation[21].Recently，the porous graphene frameworks were constructed by using Benzene-1，3，5-tricarboxylic acid (BTC)，Benzene-1，3，5-triyltribenzoate (BTB)and Benzene-4，1-diyl tribenzoate (BBC)organic linkers. The results show that lithium dispersed graphene-BBC structure has the highest hydrogen density with 4.26% at 298 K and 100 bar[22]. Therefore，intercalated nanoporous materials exhibit promising applications in the field of hydrogen storage.

Graphene with excellent electronic behavior，large specific surface area，outstanding flexibility，as well as high chemical tolerance has exhibited great natural ability in energy storage[23-26]. In the same way，layered two-dimensional 2H MoS2can be exfoliated just as the situation of graphite[27]. Based on the above thoughts，it is considered that combining graphene with MoS2monolayer should be an intriguing idea to optimize respective advantages for high-density hydrogen storage. Recently，attentions have been now focused lots of two-dimensional systems，for example silicene，transition metal dichalcogenides (TMDs)，Ⅲ-Ⅴbinary compounds and so on. It has been demonstrated that there are various metal atoms/ions at the interface between blue-phosphorus/MoS2，MoSxSe2-x/h-BN，h-BN/MoS2，MoS2/SnS[28-31].

To our knowledge，the rare-earth atoms intercalated layers have been rarely studied. Rare-earth metals have large atomic radii，making it possible to have larger layer spacing. In this work，we investigate the La atom intercalated graphene/MoS2layers for hydrogen storage via density functional theory (DFT)calculations. .

2 Computational details

In this paper，all the geometry optimizations and property calculations are performed based on the density functional theory by the DMol3[32]code. The generalized gradient approximation (GGA)for exchange and correlation potential described by the Perdew-Burke-Ernzerhof (PBE)form.[33]Plenty of results show that the GGA treats the nonlocality of exchange-correction potential better than the local density approximation (LDA). The DFT semicore pseudopotential (DSPP)is used for relativistic effective. Double numerical polarized (DNP)basis set is used. The convergence standard of energy and force were set to 1×10-5Hartree and 0.002 Hartree/?，and a maximum displacement of 0.005 ? for all considered calculations. The orbital cutoff was set to be global with a value of 5.0 ?，smearing was 0.005 Hartree.

The graphene/MoS2layers were modelled as periodic slabs by setting the lattice parameters a=b=12.660 ?，where a 5×5×1 supercell of the graphene and 4×4×1 supercell of the 2H MoS2monolayer were included，which induces the 1.9% lattice mismatch. A vacuum layer of 20 ? was used along the vertical direction of the heterostructure plane，to eliminate the interaction between periodic images. The k-point grids of 5×5×1 and 9×9×1 are used to sample the Brillouin zones geometry optimizations and electronic properties for all systems，respectively.

The La atom adsorption energy of the G/MoS2layers is defined as the following equation

E=E(G/MoS2)+E(La)-E(La@G/MoS2)

(1)

whereE(La@G/MoS2)is the total energy of La intercalated G/MoS2system，andE(G/MoS2)andE(La)are the energies of G/MoS2configuration and single La atom，respectively.

The adsorption energyEbby successive additions of H2to the La@G/MoS2configurations is calculated by the following formula

Eb=E(nH2-La@G/MoS2)+E(H2)-

E[(n-1)H2-La@G/MoS2]

(2)

wheren=1，2，3，4，5，6，E(nH2-La@G/MoS2)，E(H2)，andE[(n-1)H2-La@G/MoS2]，are the total energies ofnH2-La@G/MoS2configuration，free-standing H2molecule，and (n-1)H2-La@G/MoS2configuration，respectively.

(3)

wherenmaxis the maximum number of H2moelcules adsorbed to La@G/MoS2systems.

3 Results and discussions

The lattice constants of graphene (2.456 ?)and MoS2(3.169 ?)are quite different. So a 5×5 supercell of the graphene and 4×4 supercell of the MoS2monolayer were built the layers. The obtained lattice parameters after relaxed bilayer heterostructure area=b=12.660 ?. The bond lengths of Mo-S and C-C are found to be 2.423 and 1.460 ?，respectively. At equilibrium ground state，the distance between graphene and MoS2is 3.320 ?. As shown in Fig.1(c)，the graphene/MoS2(G/MoS2)heterostructure has a small direct bandgap of 3.29 meV，which both valence band maximum (VBM)and conduction band minimum (CBM)are located at the K point. These results are agreement with the reported data[34，35].

Fig. 2 Top and side views of H2 molecules are adsorbed on La@G/MoS2：(a)H2；(b)2H2；(c)3H2；(d)4H2；(e)5H2；(f)6H2.

Fig. 3 The projected DOSs ofLa@G/MoS2 with (a)one，(b)two，(c)three，(d)four，(e)five，(f)six hydrogen molecules adsorbed on La atom.

Fig. 4 The optimized geometric structures of (a)the side view and (b)top view of hydrogen adsorbed on the two La atoms intercalated G/MoS2 layers.

To investigate the heterostructure stable location of La atom in the interlayer of G/MoS2layer，we constructed three different configurations：(i)the BMosite，in which the La atom is placed atop an Mo atom of MoS2，(ii)the BSsite，in which the La atom is located above one S atom，(iii)the H site，in which the La atom is situated directly on the hollow site of MoS2，as shown in Fig.1(a). Finally，by comparing the adsorption energies of different adsorption positions，the BMosite is the most stable adsorption site. The La atom adsorption energy at the BMois 3.926 eV. Since the La atom is intercalated the G/MoS2，the S-Mo bond length of the MoS2layer is almost increased slightly because of the stronger interaction between the adjacnet S atom and the La atom. We employ the most stable structure in the ground state of G/MoS2heterostructure to investigate the hydrogen adsorption. In Fig. 1(b)，d1denotes the distance between graphene layer and top S-layer of MoS2monolayer，andd2is the distance between La and top S-layer of MoS2monolayer. Thed1is extended to 4.602 ? in the La@G/MoS2compared to 3.320 ? in G/MoS2，and thed2is 2.457 ?. The Mulliken charge indicated that there are 0.744 au electrons transferred from the La atom to the G/MoS2heterostructure. In Fig.1(d)，the band structure of La@G/MoS2shows that the Fermi level shifts to valence band minimum (VBM). It is clearly that there are several energy bands located near the Fermi level in the intercalary La system. The La@G/MoS2system is metallic.

Next，we analyse hydrogen adsorption behaviors systematically on La intercalated G/MoS2configurations based on DFT/GGA approach. We firstly optimize the geometries of one hydrogen molecule adsorption on La@G/MoS2. Results after relaxation show that the first H2molecule is bind to La atom and the distances of H2to La are 2.651 ? and 2.608 ?. The distances ofd1andd2are 4.791 ? and 2.214 ?，respectively. The bond lengths of the adsorbed first hydrogen molecule is 0.765 ? compared to 0.751 ? of the free H2molecule. According to the Mulliken charge analysis of La in 1H2-La@G/MoS2，La carrys 0.547 au electrons. The charges of the hydrogen molecule are 0.087 au and 0.094 au，showing charges transfer between the molecular hydrogen and the La metal. When the second hydrogen molecule is bound to La@G/MoS2also in molecular form. Thed2becomes 2.298 ?，and the distances of H2molecules to La are 2.656，2.654，2.621，and 2.557 ?. Despite there is enough space，trying to add more H2molecules in La@G/MoS2is successed. Finally there are a maximum of six hydrogen molecules bounded to La@G/MoS2. The optimized geometries ofnH2-La@G/MoS2(n=1-4)are shown in Figs.2(a)-(d). It can be seen that the hydrogen molecules are almost in the same plane as the La atom，also the d2increase as the number of adsorbed H2molecules is increased as seen in Table 1. The fifth and sixth hydrogen molecules are located above the La atom as shown in Figs. 2(e)and 2(f). It is worth noting that one La atom can capture six H2molecules with an average adsorption energy are greater than 0.18 eV by GGA. Previous studies have indicated that GGA functional always underestimates the H2adsorption energy. Therefore，the adsorption energy in present considered systems is larger than 0.18 eV. The most obvious change in the Mulliken charge of intercalary La，as shown in Table 1，it is 0.547，0.339，0.283，-0.163，-0.294，-0.479 a.u.，respectively. The results indicate that charge transfer between hydrogen molecules and La atom.

Table 1 The distances d1(?)between graphene layer and top S-layer of MoS2 monolayer，the distances d2 (?)between La and top S-layer of MoS2 monolayer，the adsorption energy Eb(eV)by successive additions of H2 molecules to the considered systems，as well as the Mulliken charge (a.u.)of La are shown.

In Fig. 3，we depicted the projected density of states (PDOSs)of hydrogen adsorption on La@G/MoS2configurations. The Fermi energy is set to zero. It is observed that the density of states of La-5p orbital mainly appear in the range from -15 eV to -20 eV，while H-1s orbital mainly in the range from -5 eV to -10 eV. The overlap of density of states between La-5d and H-1s orbitals emerges near the Fermi energy level，where the hybridization takes place. It is worth noting that，H 1s peaks are widen due to the interaction of H2molecules. Especially，H 1s peaks are split when more H2molecules are adsorbed on La metal，indicating the H2-H2interaction is significant.

In the following，to verify the possibility of clustering intercalary La，we will test by doping the second La atom near the metal site and optimize the system without any geometrical restrictions. The La-La distance in the optimized structure is 3.582 ?.To know the hydrogen adsorption on cluster dimmers，we place six hydrogen molecules around each La atom to relax without any geometrical constrains. The optimized structures are shown in Fig. 4(a)and 4(b). It can be seen that there are eleven hydrogen molecules around the metal due to the space，while one hydrogen molecule is far away.

4 Conclusions

In summary，the hydrogen adsorption behaviors onLa@G/MoS2layers were investigated using first-principles based on the density functional theory in this paper. The results indicate that isolate La atom can adsorb a maximum number of six hydrogen molecules. There are eleven hydrogen molecules around the metal due to the clustering and space when the two La atoms intercalate La@G/MoS2. These results may be used to offer a theoretical guiding for the experimental study of hydrogen storage of La@G/MoS2.