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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) The calculated ΔEbond (H and 2H) at the peripheral sites of MoS2/graphene nanocontact and free-standing MoS2 nanolayer, and (b) the optimized structures of the reactants 2H. The H atoms are red.
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f3: (a) The calculated ΔEbond (H and 2H) at the peripheral sites of MoS2/graphene nanocontact and free-standing MoS2 nanolayer, and (b) the optimized structures of the reactants 2H. The H atoms are red.

Mentions: In Figure 3a, we have plotted the calculated values of ΔEbond when hydrogen is bound to the peripheral atoms of the isolated MoS2 nanolayer and MoS2 in contact with the graphene substrate and show structural configurations for the latter (H bonded isolated MoS2 layers show similar structures to the attached one so are not displayed). We found that for the free-standing MoS2 nanolayer, H can bind to the S- or Mo-edge and Mo-corner, resulting in negative ΔEbond, as shown in Figure 3a. The H atoms bound to the S-atom exposed Mo-edge systems yield very stable structures in which the two H atoms are bound to different S atoms at different levels of the MoS2 distorted sandwich structure. The S-edge less strongly binds the H atoms with a /ΔEbond/ which is closer to zero, suggesting for isolated MoS2 particles, the S edge could play a major role on its catalytic performance for hydrogen evolution. A stable complex of H atoms bound on the Mo corner is formed in which the two H atoms are bound to the same corner Mo atom. The main difference between the isolated and attached MoS2 nanolayers is that the atomic sites at the Mo-edge have a positive ΔEbond in the graphene attached MoS2, with a much lower magnitude than that obtained for the isolated MoS2 nanolayer. For free-standing MoS2, the S-edge, with its low H binding energy, is the predominant factor which would lead to a high activity for the HER of the whole layer. The reactivity of the graphene supported MoS2 nanolayer might be a better catalyst for the HER because the Mo-edge can contribute to its catalytic activity with a much reduced /ΔEbond/. Although in our study the support does not play any direct role in bonding or activating the H atoms, except for the role of stabilizing and modifying the MoS2 nanolayer, the S atoms on the Mo-edge which have low coordination numbers will be less trapping of the H atoms. This could be the result of the fact that some metallic states associated with the unsaturated peripheral sites are missing after the MoS2 is bound to the reduced graphene support, and therefore the MoS2 will more easily release H in the next step of electrochemical hydrogen evolution.


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) The calculated ΔEbond (H and 2H) at the peripheral sites of MoS2/graphene nanocontact and free-standing MoS2 nanolayer, and (b) the optimized structures of the reactants 2H. The H atoms are red.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: (a) The calculated ΔEbond (H and 2H) at the peripheral sites of MoS2/graphene nanocontact and free-standing MoS2 nanolayer, and (b) the optimized structures of the reactants 2H. The H atoms are red.
Mentions: In Figure 3a, we have plotted the calculated values of ΔEbond when hydrogen is bound to the peripheral atoms of the isolated MoS2 nanolayer and MoS2 in contact with the graphene substrate and show structural configurations for the latter (H bonded isolated MoS2 layers show similar structures to the attached one so are not displayed). We found that for the free-standing MoS2 nanolayer, H can bind to the S- or Mo-edge and Mo-corner, resulting in negative ΔEbond, as shown in Figure 3a. The H atoms bound to the S-atom exposed Mo-edge systems yield very stable structures in which the two H atoms are bound to different S atoms at different levels of the MoS2 distorted sandwich structure. The S-edge less strongly binds the H atoms with a /ΔEbond/ which is closer to zero, suggesting for isolated MoS2 particles, the S edge could play a major role on its catalytic performance for hydrogen evolution. A stable complex of H atoms bound on the Mo corner is formed in which the two H atoms are bound to the same corner Mo atom. The main difference between the isolated and attached MoS2 nanolayers is that the atomic sites at the Mo-edge have a positive ΔEbond in the graphene attached MoS2, with a much lower magnitude than that obtained for the isolated MoS2 nanolayer. For free-standing MoS2, the S-edge, with its low H binding energy, is the predominant factor which would lead to a high activity for the HER of the whole layer. The reactivity of the graphene supported MoS2 nanolayer might be a better catalyst for the HER because the Mo-edge can contribute to its catalytic activity with a much reduced /ΔEbond/. Although in our study the support does not play any direct role in bonding or activating the H atoms, except for the role of stabilizing and modifying the MoS2 nanolayer, the S atoms on the Mo-edge which have low coordination numbers will be less trapping of the H atoms. This could be the result of the fact that some metallic states associated with the unsaturated peripheral sites are missing after the MoS2 is bound to the reduced graphene support, and therefore the MoS2 will more easily release H in the next step of electrochemical hydrogen evolution.

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.