Controlling surface structure at the atomic scale is paramount to developing effective catalysts. For example, the edge sites of MoS(2) are highly catalytically active and are thus preferred at the catalyst surface over MoS(2) basal planes, which are inert. However, thermodynamics favours the presence of the basal plane, limiting the number of active sites at the surface. Herein, we engineer the surface structure of MoS(2) to preferentially expose edge sites to effect improved catalysis by successfully synthesizing contiguous large-area thin films of a highly ordered double-gyroid MoS(2) bicontinuous network with nanoscaled pores. The high surface curvature of this catalyst mesostructure exposes a large fraction of edge sites, which, along with its high surface area, leads to excellent activity for electrocatalytic hydrogen evolution. This work elucidates how morphological control of materials at the nanoscale can significantly impact the surface structure at the atomic scale, enabling new opportunities for enhancing surface properties for catalysis and other important technological applications.

Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis

Defect-rich MoS2 ultrathin nanosheets are synthesized on a gram scale for electrocatalytic hydrogen evolution. The novel defect-rich structure introduces additional active edge sites into the MoS2 ultrathin nanosheets, which significantly improves their electrocatalytic performance. Low onset overpotential and small Tafel slope, along with large cathodic current density and excellent durability, are all achieved for the novel hydrogen-evolution-reaction electrocatalyst.

Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution

Despite recent progress in the synthesis and characterization of molybdenum disulphide, little is yet known about its microstructure. Using refined chemical vapour deposition synthesis, high-quality crystals of monolayer molybdenum disulphide have now been grown. Single-crystal islands and polycrystals containing tilt and mirror twin grain boundaries are characterized, and the influence of the grain boundaries on the material properties of molybdenum disulphide is assessed.

Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide

Atomic-layered MoS(2) is synthesized directly on SiO(2) substrates by a scalable chemical vapor deposition method. The large-scale synthesis of an atomic-layered semiconductor directly on a dielectric layer paves the way for many facile device fabrication possibilities, expanding the important family of useful mono- or few-layer materials that possess exceptional properties, such as graphene and hexagonal boron nitride (h-BN).

Large-area vapor-phase growth and characterization of MoS(2) atomic layers on a SiO(2) substrate.

Recent progress in large-area synthesis of monolayer molybdenum disulfide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapor deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulfide up to 120 um in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology, and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror boundaries cause strong photoluminescence quenching while tilt boundaries cause strong enhancement. In contrast, the boundaries only slightly increase the measured in-plane electrical conductivity.

Grains and grain boundaries in highly crystalline monolayer molybdenum disulfide

A method for the formation of monodisperse polystyrene (PS) and polymethylmethacrylate (PMMA) spheres, in the micron diameter range, was developed. The polymerisation reaction was carried out in a series of alcohols, using homo- and co-polymers as steric stabilisers in combination with a quaternary ammonium salt which probably acts as an electrostatic co-stabiliser. Monodisperse PS spheres were formed in the particle size range of 1–6 μm, by a single step process. The size range could be extended by further addition of monomer to the reaction mixture during the polymerisation. The solubility parameter of the alcohol used and the nature of the surfactant determine the diameter of the resulting spheres. However, the concentration of surfactant did not seem to affect strongly the size of the spheres but only the dispersity.

Monodisperse polymeric spheres in the micron size range by a single step process

Shape-selective alkylation of naphthalene and methylnaphthalene with methanol over H-ZSM-5 zeolite catalysts

Solar energy storage via a closed-loop chemical heat pipe

Comparative study of shape-selective toluene alkylations over HZSM-5

MoS2, a layered compound with tribological and catalytic applications, is known to form a range of hollow closed nanostructures and nanoparticles, including graphene-like structures. These have been demonstrated experimentally through high-temperature synthesis and pulsed laser ablation (PLA), and theoretically with quantum chemical calculations. The smallest allowed structures are nanooctahedra of 3 to 8 nm size. Nicknamed the “true inorganic fullerene” in analogy to carbon fullerenes, they differ from larger multiwalled MoS2 fullerene-like nanoparticles both in their morphology and predicted electronic properties. The larger fullerene-like particles are quasi-spherical (polyhedral) or nanotubular, typically with diameters of 20 to 150 nm. Above a few hundred nm in size, these nanoparticles transform into 2H-MoS2 platelets. Fullerene-like particles have been recognized as superior solid lubricants with numerous commercial applications, and MoS2 nanooctahedra may have catalytic applications. Understanding the fundamental commonality of these two morphologies might prove essential in the development of new materials. The research on hollow MoS2 nanostructures of minimal size (< 10 nm in diameter) was initiated in 1993 upon the first independent proposal of the formation of nanooctahedra of MoS2 [3,5] (and BN) with six rhombi in their corners. In 1999, it was demonstrated that twoto four-walled MoS2 nanooctahedra, 3–5 nm in size and up to ca. 10 atoms, could be obtained by PLA. Similar results were subsequently reported in Ref. [1, 2] as illustrated in Figure 1a. Recent studies of high energy density methods such as laser ablation and arc– discharge resulted in small structures with only a limited number of atoms: Mo–S clusters or double-walled nanooctahedra.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/anie.201006719

Moshe Levy

Papers

Monodisperse polymeric spheres in the micron size range by a single step process

Shape-selective alkylation of naphthalene and methylnaphthalene with methanol over H-ZSM-5 zeolite catalysts

Solar energy storage via a closed-loop chemical heat pipe

Comparative study of shape-selective toluene alkylations over HZSM-5

MoS2 Hybrid Nanostructures: From Octahedral to Quasi‐Spherical Shells within Individual Nanoparticles