Unveiling the Growth Mechanism of MoS₂ with Chemical Vapor Deposition: From Two-Dimensional Planar Nucleation to Self-Seeding Nucleation

The deep understanding of nucleation and growth mechanisms is fundamental for the precise control of the size, layer number, and crystal quality of two-dimensional (2D) transition-metal dichalcogenides (TMDs) with the chemical vapor deposition (CVD) method. In this work, we present a systematic spectroscopic study of CVD-grown MoS₂, and two types of MoS₂ flakes have been identified: one type of flake contains a central nanoparticle with the multilayer MoS₂ structure, and the other is dominated by triangular flakes with monolayer or bilayer structures. Our results demonstrate that two types of flakes can be tuned by changing the growth temperature and carrier-gas flux, which originates from their different nucleation mechanisms that essentially depends on the concentration of MoO_3–<i>x</i> and S vapor precursors: a lower reactant concentration facilitates the 2D planar nucleation that leads to the monolayer/bilayer MoS₂ and a higher reactant concentration induces the self-seeding nucleation which easily produces few-layer and multilayer MoS₂. The reactant-concentration dependence of nucleation can be used to control the growth of MoS₂ and understand the growth mechanism of other TMDs

Atomic Mechanism of Electrocatalytically Active Co–N Complexes in Graphene Basal Plane for Oxygen Reduction Reaction

Superior catalytic activity and high chemical stability of inexpensive electrocatalysts for the oxygen reduction reaction (ORR) are crucial to the large-scale practical application of fuel cells. The nonprecious metal/N modified graphene electrocatalysts are regarded as one of potential candidates, and the further enhancement of their catalytic activity depends on improving active reaction sites at not only graphene edges but also its basal plane. Herein, the ORR mechanism and reaction pathways of Co–N co-doping onto the graphene basal plane have been studied by using first-principles calculations and <i>ab initio</i> molecular dynamics simulations. Compared to singly N-doped and Co-doped graphenes, the Co–N co-doped graphene surface exhibits superior ORR activity and the selectivity toward a four-electron reduction pathway. The result originates from catalytic sites of the graphene surface being modified by the hybridization between Co 3d states and N 2p states, resulting in the catalyst with a moderate binding ability to oxygenated intermediates. Hence, introducing the Co–N₄ complex onto the graphene basal plane facilitates the activation of O₂ dissociation and the desorption of H₂O during the ORR, which is responsible for the electrocatalyst with a smaller ORR overpotential (∼1.0 eV) that is lower than that of Co-doped graphene by 0.93 eV. Our results suggest that the Co–N co-doped graphene is able to compete against platinum-based electrocatalysts, and the greater efficient electrocatalysts can be realized by carefully optimizing the coupling between transition metal and nonmetallic dopants in the graphene basal plane