Van der Waals hetero-structures of 1H-MoS2 and N-substituted graphene for catalysis of hydrogen evolution reaction

First-principles theoretical analysis of the catalytic activity of van der Waals hetero-structures of 1H-MoS2 and graphene substituted with three chemical types of nitrogen species (i) Graphitic (G), (ii) Pyridinic (Pn) and (iii) Pyrrolic (Pr), for application in catalysis of hydrogen evolution reaction (HER) has been presented. Graphitic and pyrrolic N substituents result in n-type electronic structure, whereas substitution of pyridinic N imparts p-type electronic character to the hetero-structure. Work functions (φ) of the hetero-structures suggest that graphitic N-graphene:MoS2 hetero-structure ( φ = 3.8 eV ) is expected to be effective in catalysing the reduction of H+ to evolve H2. 1H-MoS2 monolayer in the hetero-structure contributes by enabling increased H2O adsorption and offsetting the band edge energies optimal for the catalytic activity. Near optimum Gibbs free energy of H-adsorption ( Δ G H ) were obtained for graphitic ( Δ G H ∼ 0.29 eV) and pyrrolic ( Δ G H ∼ −0.2 eV) N-graphene:MoS2 hetero-structures. Our work showcases how catalytic and electronic properties of the N-doped graphene:MoS2 hetero-structure depends on the chemical identity of N-sites and uncovers a route to 2D hetero-structures with high catalytic activity.


Introduction
Graphene [1], a 2-dimensional sheet of sp 2 hybridized carbon atoms, initiated intense research activity in the field of 2D materials. In addition to exhibiting exceptional in-plane tensile strength [2] and a large specific surface area [3], graphene also exhibits properties such as high electrical and thermal conductivities [4,5], good transparency [6], due to its unique electronic structure and high in-plane elastic stiffness. Being atomically thick, graphene serves as a perfect candidate to form hetero-structure with other 2D materials. Another well studied 2D-material is molybdenum disulfide (MoS 2 ) [7], a member of the family of Transition Metal Dichalcogenides (TMDCs). MoS 2 is widely used in solid-state lubrication, photovoltaic devices, and rechargeable batteries [8]. Bulk 2H-MoS 2 comprises of S-Mo-S monolayers which are held together by van der Waals interactions [9].
Recently, exciting research has emerged in exploration of hetero-structures formed by stacking or adjoining different 2D materials together [10]. These 2D crystals can be coupled in a horizontal fashion creating an inplane interface or by stacking them on top of one another forming a vertical hetero-structure. An attractive feature of these hetero-structures is that each layer acts as a bulk 2D material and an interface simultaneously [11]. These vertical hetero-structures are held together by van der Waals interactions and are known to showcase novel interface-induced physical and chemical properties [12]. A particularly well-studied hetero-structure is graphene:MoS 2 hetero-structure [12][13][14][15] which was first fabricated by Chang et al [16]. This stacked graphene:MoS 2 hetero-structure has recently been exploited for its catalytic activity towards the HER. Hydrogen evolution reaction (HER) is the reduction involved in the water splitting reaction and gives hydrogen (H 2 ) as the product. H 2 , is an attractive fuel for storage of energy in the form of chemical energy. One of the challenges in use of hydrogen is the scarcity of earth abundant materials to catalyse the conversion of protons (H + ) to H 2 .
Li et al showed that graphene layer as a support couples electrically with MoS 2 hence affecting the charge density distribution in MoS 2 [17]. The stacking leads to an in-built electric field in the hetero-structure resulting in excess negative charge on MoS 2 monolayer which improves its electrocatalytic activity towards HER. Similarly, a significant increase in the activity of inert MoS 2 surface was reported when graphene oxide (GO) was used as a support [18]. The catalytic activity of MoS 2 basal plane was influenced by the oxygen concentration in the GO. N-doping in the GO substrates further enhanced the catalytic activity giving a D~-G 0.014 eV H [18]. A recent report by Biroju et al [12] discussed the p-type doping effect of MoS 2 which promotes the hydrogen adsorption on the graphene side of the graphene:MoS 2 hetero-structures. Substitutional N-doping in graphene [19] is known to tune the electronic spectrum and enhancing its properties for various applications [20]. While there have been many theoretical [12,21,22] and experimental [12,23] works on graphene:MoS 2 hetero-structures, effects of N substitution in graphene on the catalytic and electronic properties of the hetero-structure have not been investigated yet.
Here, a detailed theoretical analysis of nitrogen (N)-doped graphene:MoS 2 bilayered hetero-structures, considering three different chemical types of N substituent: (i) Graphitic (G), (ii) Pyridinic (Pn), and (iii) Pyrrolic (Pr) is presented. Our focus is on the effects of chemical nature of N on the catalytic activity of graphene:MoS 2 hetero-structure. It is observed that the chemical type of substitutional N greatly influences the frontier electronic states of a hetero-structure whereas the interlayer binding and distance are about the same. Work function (j) of such a hetero-structure is thus dependent on the chemical type of N substituent; G N-atoms lower the j (n-type doping) while Pn N-atoms as well as Pr N-atoms increase the j (p-type doping) of the hetero-structure. Pr N-doped graphene:MoS 2 hetero-structure is a direct narrow gap semiconductor with a band gap value (E g ) of~266 meV. Catalytic activity of graphene:MoS 2 towards HER enhances with N-substitution as the Gibbs free energy for hydrogen adsorption reduces from DG H~1 .27 eV for pristine hetero-structure to DG H~-0.2 eV and DG H~0 .295 eV for pyrrolic and graphitic N-doped graphene:MoS 2 hetero-structures.

Results and discussion
Lattices of MoS 2 and graphene monolayers have a nontrivial size mismatch. To construct a model heterostructure, we place a 5x5 supercell of graphene on a 4x4 supercell of 1H-MoS 2 , with a C atom of graphene placed on top of one of the Mo atom of the MoS 2 sheet. It was previously shown that relative in-plane translation of graphene over the MoS 2 monolayer neither affects the energetics, nor the electronic properties of the system [21]. This is because local chemical environment of atoms averages out due to use of large supercell containing 5x5 units of graphene and 4x4 units of MoS 2 . To determine the properties of N-graphene:MoS 2 hetero-structure we considered three configurations: the 'graphitic' N corresponds to simply replacing a C atom by N atom in the graphene layer. In G N-graphene:MoS 2 (figure 1(b)) we replace 6% C atoms of the graphene sheet with N atoms and place them far from each other to keep the interaction between the N atoms weak. The 'pyridinic' and 'pyrrolic' N, atoms which contribute one and two electrons each respectively to the π conjugated system [24]. Pn N-graphene:MoS 2 (figure 1(c)) comprises of three pyridine rings with 6% N-substitution and 2% vacancies in the graphene sheet creating a pyridine-like local chemical environment for N atoms. Similarly, the Pr N-graphene:MoS 2 (figure 1(d)) structure is obtained with two N atoms inducing pyridine rings and one N atom forming a 5-membered pyrrole ring. A hydrogen (H) atom bonding with N atom is introduced to satisfy the valency of pyrrolic N substituent.

Interlayer spacing
The optimum interlayer spacing between the two monolayers is obtained by minimizing binding energy (E b ) between the N-doped/pristine graphene and MoS 2 monolayers: are the energies of the hetero-structure, isolated 1H-MoS 2 (4×4), and isolated N-doped/pristine graphene monolayer (5x5), respectively (see table 1 and figure S2 is available online at stacks.iop.org/MRX/6/124006/mmedia). It is evident that the optimum separation is about 3.4 Å which is in good agreement with earlier reports [15,21]. The vdW forces are almost solely responsible for the adhesion between the monolayers (table S1). Regardless of the chemical nature of substituted N, E b changes by only about~10% (table 1) showing that N-doping does not strongly affect the strength of adhesion of graphene sheet on MoS 2 .

Electronic properties
To understand the effect of chemical nature of N on the electronic properties of these graphene:MoS 2 heterostructures, we determine the electronic structure and projected density of states (PDOS) (see figure 2). Electronic structure of pristine graphene:MoS 2 hetero-structure is a simple superposition of electronic structures of each component ( figure 2(a)). Graphene retains the linear dispersion of its bands and the Dirac point falls in the energy gap of 1H-MoS 2 . Interestingly, the Dirac point of graphene shows a tiny gap (~2.8 meV), consistent with the report by Ma et al [21]. The orbitals of the C atoms of graphene contribute to states at energies between those of Mo-4d and S-3p orbitals as seen in the PDOS of the hetero-structure (figure 2(a)). Electronic structure of the graphene:MoS 2 hetero-structure changes with the chemical nature of N-substituents remarkably. Replacing three C atoms of 5x5 supercell of graphene with N atoms (graphitic N-substitution) ( figure 1(a)) results in donation of electrons causing the E F to shift up, imparting n-type character to the hetero-structure. States arising from N-substituents can be seen just above the Fermi level (E F ) in the PDOS (figure 2(b) inset). With a small gap (~0.1 eV) opening up at the Dirac point, dispersion of bands at the Dirac point is parabolic in nature. This is evident in all the N-doped hetero-structures studied here irrespective of the chemical identity of N-atom.
Our configuration of graphitic N-substitution differs from that with pyridinic (Pn) N in the sense that in the former, N atoms are intentionally kept away from each other so as to lessen the N-N interaction ( figure 1(b)). In Pn-N substitution, three N atoms are placed in close vicinity to each other and a vacancy defect is introduced at the centre of these dopants breaking the π conjugation and creating a pyridine ring like environment ( figure 1(c)). Here, two of the five valence electrons of N participate in σ bonding with the neighboring C atoms while one contributes to π state within the pyridine ring. The remaining two electrons form a lone pair as the π conjugation is absent due to the vacancy adjacent to the dopants, and this lone pair gives rise to two new bands just below the E F (see figure 2(c)). The Pn-N doping shifts the E F down relative to the Dirac point, imparting   1(d)). Pyrrolic-N atom uses three of its five valence electrons in σ bonding, two with C atoms and one with H atom, the remaining two electrons are delocalized, imparting the pyrrole ring its aromatic nature. Therefore, the pyrrolic-N atom lends two electrons to the π conjugated system, imparting an n-type character to the system. Whereas two Pn-N atoms present in the sheet promote p-doping with downward shift in E F as explained earlier. The opposite effects of the two types of N atoms (figure 2(d)) cancel each other out resulting in a small band gap opening at the Dirac point, and thus Pr N-graphene:MoS 2 hetero-structure is a direct narrow band gap semiconductor (figure 2(d)) with a band gap (E g ) of~266 meV.
Further the work function (j) which is defined as the minimum amount of energy required to remove an electron from a material to vacuum, and was recently shown to be relevant to the reactivity of the a catalyst [25] is calculated. To estimate work functions of pristine graphene:MoS 2 , graphitic N-graphene:MoS 2 , and pyridinic N-graphene:MoS 2 hetero-structures the following expression is used and for pyrrolic N-graphene: where E VBM and V vac is the energy of valence band maximum and vacuum energy respectively. j is used to align the energies of band edges with respect to vacuum and hydrogen evolution potential (HER) (figure 3) to assess the suitablity of these graphene:MoS 2 hetero-structures as catalysts for hydrogen evolution. Doping graphene with graphitic N-atoms reduces the j of the hetero-structure, while Pn and Pr N-substitutions increase the j (see table 1). The hydrogen evolution potential (HER) is just below E F (figure 3) of graphitic N-graphene:MoS 2 , suggesting it to be a viable electrocatalyst for HER. Pristine, Pn N-substituted and Pr N-substituted graphene:MoS 2 hetero-structures, due to appropriate band edge energies, are predicted to be photocatalysts for HER ( figure 3). To check the viability of the pristine and the N-substituted graphene:MoS 2 hetero-structures as photocatalysts, we obtained their optical conductivities. All the four hetero-structures show absorbance in the visible part of the spectrum, mainly blue-violet light (see figure S5), showing their suitability to use solar energy.
To understand the role of MoS 2 in the hetero-structures, we align the projected band edge energies of pristine and N-doped graphene monolayers on the SHE potential scale (see figure 3). As is evident here, MoS 2 support shifts E F down and increases the work function of the pristine as well as N-substituted graphene monolayers (see table S2). To understand electronic properties at the graphene:MoS 2 interface of the hetero-structures, we examine the planar average charge density difference. Charge transfer across the interface between two monolayers (pristine/N-graphene and 1H-MoS 2 ) is evident as an excess negative charge accumulates on the 1H-MoS 2 monolayer ( figure S3). This leads to built-in electric field at the interface, consistent with the observation made by Li et al [17]. Interestingly, the trend in charge transfer characteristics correlates with the work function of four hetero-structures studied here, i.e., higher the charge accumulation on the MoS 2 monolayer, lower is the work function. For graphitic N-graphene:MoS 2 , a relatively larger charge transfers from N-graphene to the MoS 2 monolayer, reflected in charge accumulation on the MoS 2 lattice, correlating with its low work function.

Simulations of adsorption
To quantify the catalytic performance of these hetero-structures, the strength of adsorption of H-atom, H 2 O molecule and OH at various adsorption sites on the N:graphene side of four hetero-structures (see figure 1) is obtained. The interaction strength between the adsorbate and the surface is evaluated using: / are taken as the chemical potential of an isolated H-atom (m H ) and OH (m OH ), respectively. Doping graphene with N strengthens the hydrogen adsorption and the effect varies with the chemical nature of N dopant (table S3). Analysis of the projected density of states (PDOS) of H-atom adsorbed (site 1, figure 1) on pristine and N-doped graphene:MoS 2 hetero-structure (figure 4) reveal the nature of interaction between the highest occupied molecular orbital (HOMO) of the adsorbate (H) with the bands of the catalyst. The HOMO peak of adsorbate on pristine graphene:MoS 2 hetero-structure lies at E F (see figure 4(a)) showing its weak interaction with the C-2p orbital of the substrate. In graphitic N-graphene:MoS 2 , the HOMO of adsorbed H-atom is fully occupied and lies below the E F (see figure 4(b)). Also, the associated peak (HOMO) in PDOS splits and becomes broad showing its covalent interaction with the graphitic N-substituents. The HOMO peak of H-atom adsorbed on pyridinic N-graphene:MoS 2 hetero-structure is sharp and lies just above E F (see figure 4(c)), resonant with the N states, resulting in weaker adsorption. Relatively high DE ads H at sites 2, 3 and 4 of pyridinic N graphene:MoS 2 hetero-structure are due to the vacancy defect present in the graphene ring. H-atom adsorbs strongly at the N-atom vacancy site, and increases (in magnitude) the energy of adsorption (see figure S3). The HOMO peak of H-atom attached to the pyrrolic N (dashed dark green peak) and the HOMO of adsorbed H, both lie much deeper in energy and resonate with the pyrrolic N-atom (see figure 4(d)). The adsorbed H-atom, prefers to bind to the electron rich N site which leads to the H-atom attached to the pyrrolic N being pushed out of plane (see figure 4(d) inset). This is also shown in the iso-surfaces of wave functions where the HOMO of the adsorbed H-atom is interacting with the N-2p orbital (See figure S7) which enhances the strength of adsorption (see figure S4). The binding of H 2 O to the graphene:MoS 2 hetero-structures remains relatively unaltered on introduction of N-substituents. Poisoning of the catalytically active sites by OH (in alkaline media) is unlikely for both pristine and N-substituted hetero-structures since DE ads OH is positive for all cases considered here.   Plot recreated using data from [28]. The blue circle and red square/green diamond correspond to G H ∆ for graphitic and pyrrolic N-graphene:MoS 2 respectively.

Conclusions
It is concluded that the electronic properties of graphene:MoS 2 hetero-structures can be tuned with different chemical types of N-doping. The graphitic N-doped hetero-structure exhibits optimal band edge positions for reduction of H + to evolve H 2 . Calculated Gibbs free energies of H adsorption (DG H ) show that N dopants strengthen the adsorption of H-atom on the hetero-structure, and nearly optimum interaction energies are obtained for graphitic and pyrrolic N-graphene:MoS 2 hetero-structures, DG 0.295 eV H and D~-G 0.2 eV, H respectively. It is also show that using 1H-MoS 2 as a support strengthens the adhesion between the adsorbate (H 2 O) and hetero-structure, and also imparts a p-type character to the hetero-structure. The graphene:MoS 2 hetero-structures are shown to be photocatalysts for HER due to suitable band edge energies and absorbance in the visible part of solar spectrum. Another subtle result is that a tiny gap (~2.8 meV) opens up at the Dirac point of pristine graphene over 1H-MoS 2 , consistent with the reported value [21]. These results offer useful insights into deciphering and enhancing catalytic activity of vdW hetero-structures by substitutional doping.