An MoSxStructure with High Affinity for Adsorbate Interaction

Methanol carbonylation to acetaldehyde on Au particles supported by single-layer MoS2grown on silica

Homogenous single-layer MoS₂films coated with sub-single layer amounts of gold are found to isolate the reaction of methanol with carbon monoxide, the fundamental step toward higher alcohols, from an array of possible surface reactions. Active surfaces were prepared from homogenous single-layer MoS₂films coated with sub-single layer amounts of gold. These gold atoms formed clusters on the MoS₂surface. A gas mixture of carbon monoxide (CO) and methanol (CH₃OH) was partially converted to acetaldehyde (CH₃CHO) under mild process conditions (308 kPa and 393 K). This carbonylation of methanol to a C₂species is a critical step toward the formation of higher alcohols. Density functional theory modeling of critical steps of the catalytic process identify a viable reaction pathway. Imaging and spectroscopic methods revealed that the single layer of MoS₂facilitated formation of nanoscale gold islands, which appear to sinter through Ostwald ripening. The formation of acetaldehyde by the catalytic carbonylation of methanol over supported gold clusters is an important step toward realizing controlled production of useful molecules from low carbon-count precursors.

Atomic-Scale in Situ Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS2 Films

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize in situ and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS₂) films. To this end, we deposit a model heterostructure of thin amorphous MoS₂ films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS₂ into crystalline MoS₂ domains, thereby emulating widely used elevated temperature MoS₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS₂ films in situ and at the atomic scale. Our observations suggest that during MoS₂ processing, various MoS₂ polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS₂ crystallization can be influenced by a weak MoS₂/graphene support epitaxy. Our atomic-scale in situ approach thereby visualizes multiple fundamental processes that underlie the varied MoS₂ morphologies observed in previous ex situ growth and processing work. Our work introduces a general approach to in situ visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials.

Controlled argon beam-induced desulfurization of monolayer molybdenum disulfide

Sputtering of MoS2 films of single-layer thickness by low-energy argon ions selectively reduces the sulfur content of the material without significant depletion of molybdenum. X-ray photoelectron spectroscopy shows little modification of the Mo 3d states during this process, suggesting the absence of significant reorganization or damage to the overall structure of the MoS2 film. Accompanying ab initio molecular dynamics simulations find clusters of sulfur vacancies in the top plane of single-layer MoS2 to be structurally stable. Measurements of the photoluminescence at temperatures between 175 and 300 K show quenching of almost 80% for an ~10% decrease in sulfur content.