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Electronic coupling and catalytic effect on H2 evolution of MoS2/graphene nanocatalyst.

Liao T, Sun Z, Sun C, Dou SX, Searles DJ - Sci Rep (2014)

Bottom Line: Inorganic nano-graphene hybrid materials that are strongly coupled via chemical bonding usually present superior electrochemical performance.An obvious reduction of the metallic state of the MoS2 nanolayer is noticed as electrons are transferred to form a strong contact with the reduced graphene support.The easiest evolution path is from the Mo edge sites, with the presence of the graphene resulting in a decrease in the energy barrier from 0.17 to 0.11 eV.

View Article: PubMed Central - PubMed

Affiliation: 1] AIBN Centre for Theoretical and Computational Molecular Science, University of Queensland, Brisbane, QLD 4072, Australia [2] Institute for Superconducting &Electronic Materials, University of Wollongong, Wollongong, NSW 2500, Australia.

ABSTRACT
Inorganic nano-graphene hybrid materials that are strongly coupled via chemical bonding usually present superior electrochemical performance. However, how the chemical bond forms and the synergistic catalytic mechanism remain fundamental questions. In this study, the chemical bonding of the MoS2 nanolayer supported on vacancy mediated graphene and the hydrogen evolution reaction of this nanocatalyst system were investigated. An obvious reduction of the metallic state of the MoS2 nanolayer is noticed as electrons are transferred to form a strong contact with the reduced graphene support. The missing metallic state associated with the unsaturated atoms at the peripheral sites in turn modifies the hydrogen evolution activity. The easiest evolution path is from the Mo edge sites, with the presence of the graphene resulting in a decrease in the energy barrier from 0.17 to 0.11 eV. Evolution of H2 from the S edge becomes more difficult due to an increase in the energy barrier from 0.43 to 0.84 eV. The clarification of the chemical bonding and catalytic mechanisms for hydrogen evolution using this strongly coupled MoS2/graphene nanocatalyst provide a valuable source of reference and motivation for further investigation for improved hydrogen evolution using chemically active nanocoupled systems.

No MeSH data available.


(a) Two views of the free-standing 3 × 3 × 1 MoS2 nanolayer before structural optimization. (b) The top image shows the 7 × 7 × 1 graphene supercell with MoS2 nanolayer sitting area highlighted as a blue shadow, and on the bottom image the atomic structures of the reduced graphene in the shadow area before and after structural relaxation. (c) The optimized structure of the MoS2/graphene nanocontact. The cyan, grey and yellow spheres represent Mo, S, and C atoms, respectively. The corner sites are highlighted by dashed rectangles in (a). The missing carbon atoms are highlighted as yellow circles and the neighboring carbon atoms with dangling bonds are denoted with numbers in (b).
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f1: (a) Two views of the free-standing 3 × 3 × 1 MoS2 nanolayer before structural optimization. (b) The top image shows the 7 × 7 × 1 graphene supercell with MoS2 nanolayer sitting area highlighted as a blue shadow, and on the bottom image the atomic structures of the reduced graphene in the shadow area before and after structural relaxation. (c) The optimized structure of the MoS2/graphene nanocontact. The cyan, grey and yellow spheres represent Mo, S, and C atoms, respectively. The corner sites are highlighted by dashed rectangles in (a). The missing carbon atoms are highlighted as yellow circles and the neighboring carbon atoms with dangling bonds are denoted with numbers in (b).

Mentions: MoS2 consists of layers of S-Mo-S sandwiches. The stoichiometry and coordination numbers of the edge atoms can differ from the bulk. The most active sites are Mo edges passivated by S atoms that have low coordination numbers, providing a sulfide rich environment19. Although the S-saturation of the Mo edge observed in experiments is mostly S-rich, by altering the external environment, such as exposure to atomic hydrogen, the S-saturated Mo edge can be reduced to pure Mo edge again with missing sulphur atoms21. The binding energies of sulphur atoms to the Mo edges of MoS2/graphene nanocontact and free-standing MoS2 nanolayer were compared, and as shown in Supplementary Table 1, the binding energies of sulphur atoms at the Mo edge decrease almost 30% when the reduced graphene support is present. Therefore the defective graphene support stabilises a MoS2 nanolayer which is deficient in S. Therefore, in this manuscript we focus on the case where a MoS2 nanolayer with pristine edges is allowed to relax to its minimum energy structure since this is what occurs under standard reaction conditions and is therefore an environment that often occurs experimentally. Figure 1a displays the two kinds of bare edge of the MoS2 nanolayer, the sulphur-terminated edge and the molybdenum-terminated edge, as well as the active S- and Mo-corner sites which are highlighted in the dashed boxes. This is motivated by the frequent observation of steps and corners in nanosized MoS2 using high-resolution TEM telescope20. To produce a vacancy mediated graphene sheet, two carbon atoms were removed from the graphene sheet, directly beneath the MoS2 nanolayer, and the structure was optimised. This leads to the presence of a number of dangling bonds on the neighboring carbon sites, as denoted with numbers in Figure 1b, which can efficiently chemically interact with the MoS2 nanolayer. Figure 1c displays the optimized structure of MoS2 on the vacancy mediated graphene surface showing the covalent bond that is formed between them, i.e. between the dangling bond of the graphene surface (C2 and C4 carbon atoms) and the edge sulphur atom of the MoS2 nanolayer. These S-C bonding distances are ~(1.76–1.78) Å, consistent with typical bond lengths in most sulphur compounds. The real applications of nanocatalysts supported on graphene are limited as weak bonding is usually found, involving the π orbital perpendicular to the graphene plane22. On one hand, the underlying graphene can hardly modify the reactivity of supported nanoparticles due to this weak interaction. On the other hand, the anchored nanoparticles are highly mobile on graphene due to the weak interaction and in some cases they easily agglomerate into large particles. A common practice in heterogeneous catalysis, controlling the catalytic performance via tuning the interaction between the reactive nanoparticles and the underlying support, is to introduce sp2 dangling bonds on the graphene surface as a result of the formation of carbon vacancies, resulting in vacancy-mediated graphene2324. Most importantly, strongly coupled inorganic nano-graphene hybrid materials stabilized via this sort of chemical bonding usually present superior electrochemical performance than the traditional weakly contacted counterparts.


Electronic coupling and catalytic effect on H2 evolution of MoS2/graphene nanocatalyst.

Liao T, Sun Z, Sun C, Dou SX, Searles DJ - Sci Rep (2014)

(a) Two views of the free-standing 3 × 3 × 1 MoS2 nanolayer before structural optimization. (b) The top image shows the 7 × 7 × 1 graphene supercell with MoS2 nanolayer sitting area highlighted as a blue shadow, and on the bottom image the atomic structures of the reduced graphene in the shadow area before and after structural relaxation. (c) The optimized structure of the MoS2/graphene nanocontact. The cyan, grey and yellow spheres represent Mo, S, and C atoms, respectively. The corner sites are highlighted by dashed rectangles in (a). The missing carbon atoms are highlighted as yellow circles and the neighboring carbon atoms with dangling bonds are denoted with numbers in (b).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4150123&req=5

f1: (a) Two views of the free-standing 3 × 3 × 1 MoS2 nanolayer before structural optimization. (b) The top image shows the 7 × 7 × 1 graphene supercell with MoS2 nanolayer sitting area highlighted as a blue shadow, and on the bottom image the atomic structures of the reduced graphene in the shadow area before and after structural relaxation. (c) The optimized structure of the MoS2/graphene nanocontact. The cyan, grey and yellow spheres represent Mo, S, and C atoms, respectively. The corner sites are highlighted by dashed rectangles in (a). The missing carbon atoms are highlighted as yellow circles and the neighboring carbon atoms with dangling bonds are denoted with numbers in (b).
Mentions: MoS2 consists of layers of S-Mo-S sandwiches. The stoichiometry and coordination numbers of the edge atoms can differ from the bulk. The most active sites are Mo edges passivated by S atoms that have low coordination numbers, providing a sulfide rich environment19. Although the S-saturation of the Mo edge observed in experiments is mostly S-rich, by altering the external environment, such as exposure to atomic hydrogen, the S-saturated Mo edge can be reduced to pure Mo edge again with missing sulphur atoms21. The binding energies of sulphur atoms to the Mo edges of MoS2/graphene nanocontact and free-standing MoS2 nanolayer were compared, and as shown in Supplementary Table 1, the binding energies of sulphur atoms at the Mo edge decrease almost 30% when the reduced graphene support is present. Therefore the defective graphene support stabilises a MoS2 nanolayer which is deficient in S. Therefore, in this manuscript we focus on the case where a MoS2 nanolayer with pristine edges is allowed to relax to its minimum energy structure since this is what occurs under standard reaction conditions and is therefore an environment that often occurs experimentally. Figure 1a displays the two kinds of bare edge of the MoS2 nanolayer, the sulphur-terminated edge and the molybdenum-terminated edge, as well as the active S- and Mo-corner sites which are highlighted in the dashed boxes. This is motivated by the frequent observation of steps and corners in nanosized MoS2 using high-resolution TEM telescope20. To produce a vacancy mediated graphene sheet, two carbon atoms were removed from the graphene sheet, directly beneath the MoS2 nanolayer, and the structure was optimised. This leads to the presence of a number of dangling bonds on the neighboring carbon sites, as denoted with numbers in Figure 1b, which can efficiently chemically interact with the MoS2 nanolayer. Figure 1c displays the optimized structure of MoS2 on the vacancy mediated graphene surface showing the covalent bond that is formed between them, i.e. between the dangling bond of the graphene surface (C2 and C4 carbon atoms) and the edge sulphur atom of the MoS2 nanolayer. These S-C bonding distances are ~(1.76–1.78) Å, consistent with typical bond lengths in most sulphur compounds. The real applications of nanocatalysts supported on graphene are limited as weak bonding is usually found, involving the π orbital perpendicular to the graphene plane22. On one hand, the underlying graphene can hardly modify the reactivity of supported nanoparticles due to this weak interaction. On the other hand, the anchored nanoparticles are highly mobile on graphene due to the weak interaction and in some cases they easily agglomerate into large particles. A common practice in heterogeneous catalysis, controlling the catalytic performance via tuning the interaction between the reactive nanoparticles and the underlying support, is to introduce sp2 dangling bonds on the graphene surface as a result of the formation of carbon vacancies, resulting in vacancy-mediated graphene2324. Most importantly, strongly coupled inorganic nano-graphene hybrid materials stabilized via this sort of chemical bonding usually present superior electrochemical performance than the traditional weakly contacted counterparts.

Bottom Line: Inorganic nano-graphene hybrid materials that are strongly coupled via chemical bonding usually present superior electrochemical performance.An obvious reduction of the metallic state of the MoS2 nanolayer is noticed as electrons are transferred to form a strong contact with the reduced graphene support.The easiest evolution path is from the Mo edge sites, with the presence of the graphene resulting in a decrease in the energy barrier from 0.17 to 0.11 eV.

View Article: PubMed Central - PubMed

Affiliation: 1] AIBN Centre for Theoretical and Computational Molecular Science, University of Queensland, Brisbane, QLD 4072, Australia [2] Institute for Superconducting &Electronic Materials, University of Wollongong, Wollongong, NSW 2500, Australia.

ABSTRACT
Inorganic nano-graphene hybrid materials that are strongly coupled via chemical bonding usually present superior electrochemical performance. However, how the chemical bond forms and the synergistic catalytic mechanism remain fundamental questions. In this study, the chemical bonding of the MoS2 nanolayer supported on vacancy mediated graphene and the hydrogen evolution reaction of this nanocatalyst system were investigated. An obvious reduction of the metallic state of the MoS2 nanolayer is noticed as electrons are transferred to form a strong contact with the reduced graphene support. The missing metallic state associated with the unsaturated atoms at the peripheral sites in turn modifies the hydrogen evolution activity. The easiest evolution path is from the Mo edge sites, with the presence of the graphene resulting in a decrease in the energy barrier from 0.17 to 0.11 eV. Evolution of H2 from the S edge becomes more difficult due to an increase in the energy barrier from 0.43 to 0.84 eV. The clarification of the chemical bonding and catalytic mechanisms for hydrogen evolution using this strongly coupled MoS2/graphene nanocatalyst provide a valuable source of reference and motivation for further investigation for improved hydrogen evolution using chemically active nanocoupled systems.

No MeSH data available.