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Tuning the Magnetic Properties of MoS Single Nanolayers by 3d Metals Edge Doping Mohamad Saab, and Pascal Raybaud J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02865 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on May 2, 2016

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The Journal of Physical Chemistry

Tuning the Magnetic Properties of MoS2 Single Nanolayers by 3d Metals Edge Doping

Mohamad Saab, Pascal Raybaud* IFP Energies nouvelles, Rond-point de l’échangeur de Solaize BP 3, 69360 Solaize, France

Corresponding author: [email protected], +33.4.37.70.23.20

1 Environment ACS Paragon Plus


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Abstract MoS2 nanolayers are versatile systems for new energy technologies such as spintronics, optoelectronic or (electro)catalytic materials and for current industrial catalytic processes. Using spin polarized density functional theory (DFT) calculations, we show that the magnetic moment value at edges of MoS2 single nanolayers follows a periodic trend as a function of the 3d metal (Me) dopants (Me=V, Cr, Mn, Fe, Co). The magnetic moment and ordering depend on the Me dopant and also on its location at the M- or S-edge with higher values at M-edge. In the case of CoMoS2 single nanolayers, Co atoms located on the M-edge exhibits a weak magnetic moment (0.6-0.7 µB) which may also be considered as a finger print of the catalytically active sites located on this given edge. A detailed electronic and structural analysis reveals that a superexchange interaction involves the Me-dopant, S-bridging ligands and Mo atoms at sub-edge, particularly on the M-edge. In perspectives, we propose to combine these magnetic edge effects with the two-dimensional morphology of Me-MoS2 single nanolayers for optimizing 2D-MoS2 based nanomagnets.


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1. Introduction Nanolayered transition metal dicalchogenides such as MoS2 exhibit numerous potential applications in nanotechnology as optoelectronics, spintronics,1 electrocatalysis2 and are industrially used as heterogeneous catalysts3. For catalytic applications, the targeted properties of the MoS2 nanolayers depend closely on atomic scale features: the concentration and type of dopants - also called “promoters” -4,5 impacting the number and nature of sites of the twodimensional (2D) nanolayers.3,6 For magnetic application, the doping process corresponds mainly to the substitution of a small amount of Mo atoms inside the bulk part of MoS2 nanolayers4,5,7 or MoS2 nanotubes.8 An alternative way would be to benefit from the formation of ternary Me-MoS2 nanolayers of which edges can be selectively doped by 3d transition metals (Me). This appealing process leads to a 2D morphology of the single nanolayers where the dopant exhibits two different edge structures (Figure 1). The 2D morphology may vary from an hexagon to a truncated triangle as of function of the dopant itself and/or the preparation conditions.3,9 In particular, it has been shown that Me-MoS2 single nanolayers with various 2D morphologies can be formed with Me=Fe, Co, Ni and Cu9,10 on various supports such as gold, graphene and oxides.3,9 Mössbauer spectroscopy revealed the existence of a ternary “CoMoS” phase for the first time in the early eighties,11 and this phase is known today to be at the origin of the high catalytic activities in hydrodesulfurization of fuels12 and in hydrodeoxygenation of biomass feedstocks.13 The origin of this catalytic behavior is also attributed to the edge decoration of the MoS2 nanolayers, modifying their 2D morphology and simultaneously the electronic properties of the edge active sites.7,9 For optoelectronic and spintronic applications, the



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impact of such a magnetic edge doping of MoS2 single nanolayers remains far less explored to the best of our knowledge. Hence, the present work explores by density functional theory (DFT) calculations how the edge doping of Me-MoS2 single layers with Me=V, Cr, Mn, Fe and Co could be susceptible to provide tunable 2D magneto-structural effects. It also aims at furnishing magnetic features of Me-MoS2 edges which may help for a better understanding of their catalytic properties. We first show a clear dependency of the magnetic moment with the type of the 3d transition metal (Me) and its edge location. Through an electronic analysis, we explain the origin of these magnetic properties. We then consider the case of Me=Co with relevant implications for “CoMoS” catalysts. We finally discuss the possible impact of two-dimensional (2D) morphology Me-MoS2 single nanolayers.

Figure 1. Example of an hexagonal Me-MoS2 single nanolayer and local structures of the Me doped S-edge (tetrahedral) and M-edge (quasi square planar). (Yellow balls: sulfur, green balls: molybdenum, brown and blue balls: Me at the S-edge and M-edge respectively).


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2. Computational Methods For total energy calculations, we used the Vienna ab-initio simulation package (VASP),14 based on periodic plane wave density functional theory (DFT), within the projected augmented wave formalism.15 The generalized gradient approximation (GGA) was used with the Perdew-Wang functional (PW91),16 and the interpolation scheme of Vosko, Wilk and Nusair17 to include properly spin polarization corrections. The cut-off energy of the plane wave basis set is fixed at 337.0 eV. A full geometry optimization is completed when the convergence criteria on ionic forces becomes smaller than 0.03 eV/Å. To simulate the edges of the two dimensional Me-MoS2 particles represented in Figure 1, periodic slabs are used for the calculations related to the Me-doped edges as illustrated in Figure 2. Due to the non-symmetrical slabs, all calculations include dipolar corrections. Note that for these calculations no assumption on the morphology: electronic and magnetic properties of the edges are investigated independently one from each other. In addition, our calculations is made one single Me-MoS2 layer and we do not consider multiple layers’ stacking.



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Figure 2. Periodic supercells (12.29 Å, 12.80 Å, 27.00 Å) used for the simulation of the MeMoS2 slab : a) Me located on the M-edge, b) Me located on the S-edge. Color legend: blue balls: Mo atoms, brown balls: Me atoms, Yellow balls: S atoms.

For total energy calculations, we used a (3x3x1) k-point mesh as it was previously shown by Krebs et al. that this is sufficient to ensure the convergence according to the large supercells reported in Figure 2.18 For the density of states (DOS) calculation, we have extended the k-point mesh to (5x5x1). Due to the metallic character of our systems, we preferentially used the method of Methfessel-Paxton for smearing with a sigma value of 0.1. Complementary data on the magnetic properties of reference bulk sulfides are given in supplementary information (S1).


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3. Results and discussion 3.1 Energetic considerations Previous DFT investigations have shown that the substitution of Mo by Co in the bulk is thermodynamically less favorable than the substitution of Mo at the edge,19 so we focus only on the edge substitution in what follows. Two relevant edges of the Me-MoS2 single nanolayers are generally considered : the M-edge and the S-edge (Figure 1).9,10 On both edges, Me atom exhibits the same Me-S coordination number but its structural environment differs significantly. On the M-edge, Me has a square planar environment, while it has a tetrahedral environment on the S-edge. Me(II) complexes in square planar environment have been also reported in organometallic chemistry.20,21 Using the two existing Co-MoS2 edge structures as reference ones,9,10 we calculated substitution energies for Me at both edges (Table 1), according to the following chemical equation: CoMoSedge + MeSbulk → MeMoSedge + CoSbulk

(1)

where MeMoSedge are the slab structures illustrated in Figure 2, and MeSbulk correspond to the bulk MeS monosulfides (Table S1).

Table 1. Substitution energy (including the spin polarization correction) of Co by Me=Cr, Mn, Fe on Me-MoS2 edges, according to equation (1). Energies are expressed in eV per Me atom. V

Cr

Mn

Fe

M-edge

+0.433

-0.059

-0.288

-0.111

S-edge

-0.168

-0.178

-0.151

-0.206



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We find exothermic substitution energies for all Me atoms, with one exception : V at the Medge. Hence, Cr, Mn and Fe atoms are more stabilized than Co at the edges of the Me-MoS2 nanolayers. This result thus justifies the further investigation of the electronic and magnetic properties of these ternary phases.

3.2 Periodic trend of the magnetic moment Figure 3 reports the variation of magnetic moments as a function of Me and for the two edges (numerical values are given in Table S2 and S3). The calculated magnetic moments projected on the Me atoms are always larger on the M-edge than on the S-edge. For both edges, the highest magnetic moments are observed for Mn with 3.13 µ B on the M-edge, and 2.24 µ B on the S-edge. The lowest magnetic moment is observed for Co on the M-edge (~0.7), while magnetization vanishes for Co and Fe on the S-edge. For bulk Me monosulfides, magnetic moments calculated with the same DFT method are generally higher (Table S1 and Ref. 22) which reveals that the Me-MoS2 phases exhibit a magnetic behavior distinct from reference bulk Me monosulfides and also from bulk doped MoS2 nanolayers4,5,7 or nanotubes.8 In addition, the energy difference between magnetic and non-magnetic (NM) states follows a similar periodic trend: the energy correction due to spin polarization effect is the largest on the M-edge and for Mn, while it becomes negligible for Co and V.


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Figure 3: a) Magnetic moment projected on the Me atoms for the two edges, b) edge energy difference between the most stable magnetic state and the non-magnetic reference state. FM = ferromagnetic, AFM = antiferromagnetic, NM = Non-magnetic. FM/AFM and NM/FM means that the two states are very close in energy (less than 10 meV/Me atom).

3.3 Electronic analysis This trend can be explained by the analysis of the density of states projected (PDOS) on Mo, S and Me dopant atoms. Considering first the non-magnetic (NM) state (Figures S3 and S4), the PDOS on Me atoms at Fermi level reveals a metallic character for all systems. So according to the Stoner-Wohlfarth criterion, PDOS values at Fermi level (all greater than 1 at M-edge) are expected to induce the instability of the paramagnetic state. So a magnetic ordering is expected for the M-edge structure with all Me atoms (including Co). On the S-edge, only Cr and Mn are suspected to be more concerned by a magnetic ordering, whereas for Fe and Co on the S-edge,



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pseudo-gaps are observed on the NM PDOS which explains the stabilization of the paramagnetic state or eventually a very weak FM ordering for Fe. For the most stable magnetic states, the spin polarized PDOS reveals a significant reduction of the PDOS at the Fermi level associated to d-band splitting for all Me atoms : it is illustrated for FM and AFM Cr on both edges in Figure 4 and for Mn in Figures S5 and S6. The systematic study of magnetic ordering shows that for Me=V, Cr, Fe, and Co the one dimensional (1D) ferromagnetic (FM) ordering of the Me row is generally preferred on M-edge, whereas for Mn, AFM is preferred (Figure 3a and Table S2). On the S-edge, AFM and FM orderings are very close in energy for V, Mn and Cr (Figure 3a and Table S3). Note that the AFM ordering is preferred in bulk CrS(NiAs), MnS(NiAs) and FeS(troilite), while bulk FeS(NiAs) and CoS(NiAs) are paramagnetic (Table S1 and Ref. 22). As previously said, this confirms the distinct behavior of the doped edges. Interestingly, the sub-edge Mo rows (located below the Me-atom at the edge), and the S-bridging atoms are involved in the magnetization process (Figure 4) which gives rise to ferrimagnetic ordering when considering the edge and sub-edge together. Note also that for the AFM state of the M-edge, the sub-edge Mo rows is not observed in the case of Cr (Figure 4c), Mn (Figure S4a) and Fe. This observation can also be interpreted as a super-exchange interaction involving cationic Cr, Mo atoms and the anionic S-bridging ligands. Considering the local environment of Me atoms at the M-edge (quasi square planar) or at the S-edge (tetrahedral), the Cr-S-Cr or Cr-SMo angles are close to 90° (even if not equal) meaning that according to Goodenough-KanamoriAnderson rules, the stabilization of FM ordering is possible as found for V, Cr, Fe and Co at Medge. This seems also coherent with the fact that the sub-edge Mo atom is magnetized in FM state, whereas it is not in AFM state.


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Figure 4. Projected density of states (PDOS) on Cr, Mo and S for (a) FM M-edge, (b) FM Sedge, (c) AFM M-edge, (d) AFM S-edge. The local structures in insets illustrate the FM and AFM ordering of Cr edge atoms associated to the magnetization of Mo sub-edge atoms and/or S bridging atoms. The dotted vertical line represents the Fermi level. Same color legend as in figure 2.



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On the M-edge, one single type of S bridging atom exists : S is connected to one Mo atom and two Cr atoms (structural insets of Figure 4a). On the S-edge, two types of S-bridging atoms exist: S1 is connected to two Mo atoms, and to one Cr atom, while S2 is connected to two Cr atoms only (structural insets of Figure 4b). As a consequence, 3p-orbitals of S1 atoms hybridize predominantly with the t2g states of the 4d band of the Mo-atoms (Figure 4b) whereas 3p-orbitals of S-atom are shared between the Mo 4d band and Cr 3d band (Figure 4a). By contrast, 3porbitals of S2 predominantly hybridize with the the eg states of 3d states of Cr-atoms in the tetrahedral environment of the S-edge (Figure 4b). Due to this significant 3p-3d hybridization in the lower part of the valence, the magnetic moments calculated for Cr, Mn or Fe are smaller than the Hund’s rule limit on the M-edge. On the S-edge, they are even more quenched. This electronic analysis is also consistent with the local structures: Me-S2 (and Me-S1) bond lengths (Tables S2 and S3) are smaller than those of Me-S on the M-edge, confirming the stronger ligand fields induced by S2 atoms and S1 (to a less extend) in the tetrahedral environment. As a consequence, a weaker magnetic moment is found on the S-edge for all Me atoms. Simultaneously, a 4d Mo-3d Cr hybridization occurs at the top of the valence band, also illustrated by the Mo-Cr distance depending closely on the magnetic state (Tables S2 and S3). Such a cooperative effect of 4d-3d metals was also previously reported in CoRh nanoparticles.23

3.4 Case of Co-MoS2 single nanolayers used in catalysis We now focus on the case of Co-MoS2 nanolayers, because of their wide interest in industrial catalytic applications.3 Previous experimental investigations of magnetic properties have reported various values for the effective moment per Co atom : 0.73 µ B,24 1.44 µ B,25 and 1.73


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µ B.26,27 One first important and rather new insight from the present theoretical results, is that the Co doped M-edge (and not the S-edge) explains this experimental magnetization. This result does not exclude the presence on the S-edge but implies that Co active sites are also present on the M-edge. The calculated values (0.65-0.73 µ B) are in reasonable agreement with the reported lowest spin states and more particularly with experimental data obtained on unsupported CoMoS2.24 In addition, the Co-S distances (2.20 Å, 2.10 Å and 2.15 Å) and short Co-Mo distances (2.77 Å and 2.97 Å) reported in Tables S2 and S3 are fully compatible with EXAFS distances: Co-S=2.18-2.24 Å and Co-Mo=2.80-2.87 Å.28

Figure 5. Projected density of states (PDOS) on Co for the M-edge FM (a) and AFM (b). The local structures illustrate the FM ordering of Cr edge atoms associated to opposite magnetization of Mo sub-edge atoms and S bridging atoms. Magnetic measurements revealed that the AFM ordering should be favored for alumina supported Co-MoS2 nanoparticles27 and FM ordering for Co-MoS2 nanosheets.29 The AFM ordering (Table S2) is only 3 meV/Co atom less stable than FM, so both orderings are possible within DFT accuracy. As discussed for Cr, FM ordering is stabilized by involving S- and Mo-



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atoms in a super-exchange interaction (Figure 5a), while AFM involves an interaction with the Mo-atoms only (Figure 5b). Such an interaction of Co with Mo-atom was previously invoked in previous experimental studies30. The present work seems to demonstrate unambiguously this contribution of Mo atoms. As for the other dopants, this super-exchange interaction implies a rather large delocalization of the charge which may also explain why the magnetic moment (0.60.7 µ B) is lower than for the spin-only magnetic moment (1.73 µ B) of one unpaired electron on an ideal Co(II) site.

4. Conclusions Using DFT calculations, we have undertaken a systematic investigation of the magnetic moments and orderings of Me-MoS2 single nanolayers, which can be clearly differentiated from the reference bulk Me-sulfides. It has been found that the magnetic properties not only depend on the nature of the 3d metal, but also on its location at the edge of the single nanolayers in a tetrahedral or quasi square planar environment. To a certain extent, this MoS2-edge effect can be qualitatively compared to graphene where the electronic and magnetic properties are known to depend on the local structure of the edges.31 The maximum magnetic moment (3.13 µB) is calculated for Mn when located on the M-edge. There is a subtle interplay between the 3d metallic dopant, the S-ligands and the Mo atoms in the sub-edge within a super-exchange interaction which governs the FM or AFM magnetic ordering at the edges. Co implies a weak magnetization at the M-edge (0.6-0.7 µB) which seems to be compatible with earlier experimental measurements reported in the literature. This result may have consequences for the refined identification of CoMoS edge sites known to be catalytically active in hydrodesulfurization.


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For spintronic application, this magnetism-morphology relationship may open new routes to generate original 2D-magnets based on the edge doping of the individual dichalcogenide nanolayers with controlled morphologies (Figure 6).

M-edge

a)

b)

M-edge

c)

M-edge

S-edge S-edge Figure 6. Three types of magnetic ordering as a function of the 2D-morphology for Cr-MoS2 nanolayers : a) nanoribond exhibing M-edge at the top and S-edge at the bottom, b) triangle exhibiting M-edge only, c) hexagon exhibiting M-edge and S-edge alternatively. Blue line: row of Me edge atoms with magnetic moment represented by blue arrows, dotted red line: row of Mo sub-edge atoms with magnetic moments (red arrows).

If we extend our results to various 2D-morphologies of Me-MoS2 single nanolayers such as nanoribbons, nanowires, triangular or hexagonal shapes as revealed by various high resolution microscopy techniques,3,7 such morphologies may simultaneously induce non-collinear magnetism distributed along the 1D edge (Figure 6). This would imply that the magnetic properties (effective moment and ordering) could be tuned by the two-dimensional (2D) shape of the nanolayers (on the hand) and by the nature/number of Me atoms present at the edges (on the other hand). For instance, Me-MoS2 triangular shapes exposing M-edge only will exhibit the



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highest magnetic moments particularly if Mn is used as a dopant (Figure 6b). According to quantitative XPS characterization of Co-MoS2 nanocatalysts, Co/Mo atomic ratio inside the CoMoS2 layers can be as high as 0.5, with an a usual ratio of 0.3 for catalysis applications of supported CoMoS nanolayers.32 This high ratio is obtained typically for nanolayers exhibiting edge size of 3-4 nm, as measured by HRTEM.3,32 Since such rather high atomic amount of Me dopants can be reached, high magnetism is expected to be measured in MoS2 single nanolayers (particularly for Me=Mn) when they are properly dispersed on a well-defined support. Moreover, it is known that the stacking of the Me-MoS2 nanolayers may be controlled by the preparation method or by the nature of the support.33 Although it is beyond the scope of the present work, we expect that the interlayer magnetic coupling will also impact the resulting magnetic properties. In particular, the magnetic ordering depends on the number of stacked nanolayers. So, controlling the 2D morphology and the stacking number simultaneously may provide an original route to tune the properties of the nanomagnets. Finally, we think that this trend can certainly be extended to other nanolayered dichalcogenides which are known to exhibit similar 2D-morphology effects in the presence of a dopant or promoter at edges, such as Me-WS2, Me-NbS2 or Me-TaS2 systems.34

ASSOCIATED CONTENT Supporting Information. S1. Complementary DFT data on magnetic properties of bulk sulfides S2. Structural and magnetic properties of the Me-MoS2 edges S3. Projected Density of States (PDOS) for the non-magnetic edges S4. Projected Density of States (PDOS) of the magnetic edges for Mn-MoS2.


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This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements M.S. thanks IFPEN for the funding of his post-doctoral research work.

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