Xiao-Li Fan

Nanopolygons of Monolayer MS2: Best Morphology and Size for HER Catalysis

With first-principles calculations, we find a new strategy for developing high-performance catalysts for hydrogen evolution reaction (HER) via controlling the morphology and size of nanopolygons of monolayer transition-metal dichalcogenides (npm-MS2, with M = Mo, W, or V). Particularly, through devising a quantitative method to measure HER-active sites per unit mass and using such HER site density to comparatively gauge npm-MS2 performance, we identify three keys in making npm-MS2 with optimal HER performance: (a) npm-MS2 should be triangular with each edge being M-terminated and each edge-M atom passivated by one S atom; (b) each edge of npm-MoS2 and WS2 should have 5-6 metal atoms as HER site density drops below/above these sizes optimal both for HER and practical npm growth; and (c) npm-VS2 is immune to this overly fastidious size dependence. Known experimental data on npm-MoS2 indeed support the plausibility of practicing these design rules. We expect that raising the nucleation density and controlling the growth time to favor the production of our proposed ultrasmall npm-MS2 are critical but practical. Research on npm-VS2 would bear the highest impact because of its size-forgiving HER performance and relatively high abundance and low cost.

Stability and flexibility of self-assembled monolayers of thiols consisting of a horizontal large π-system and a vertical spacer

Self-assembled monolayers (SAMs) of (4-mercaptophenyl) (10-nitro-9-anthryl) acetylene (MPNAA) on Au(111) are studied with scanning tunneling microscopy (STM). Through careful analysis of the bias-dependent sub-molecular features observed in the high-resolution STM images, important structural details of the MPNAA SAM are disclosed, in addition to its structural character common for the anthracene-based thiol SAMs studied in a recent paper (Dou et al 2006 Langmuir 22 3049). With these experimental results, a new model is proposed for the SAM structure to explain, particularly, the molecule–substrate, molecule–molecule and intramolecular interactions, as well as their competitions and compromises in the formation of the SAM structure. Flexibilities and application potentials of the SAMs of thiols based on a long π-system (like that of anthracene or even pentacene) arranged horizontally and a vertical spacer (like the phenyl–acetylene group) are also discussed.

Effects of intrinsic defects on methanthiol monolayers on Cu(111): A density functional theory study

Density functional theory calculations were used to examine the effects of intrinsic surface defects of Cu(111) on the adsorption of methylthiol (CH3SH). The examination covers both the initial non-dissociative adsorption and the subsequent dissociation reaction pathways to form intermediate and final reaction products. By comparing the most probable adsorption structures likely formed after the adsorption of CH3SH on Cu(111) with and without the presence of adatoms (Cu(ad)) and vacancies, this computational work offers new insights about the geometry and thermodynamic stability of these structures. Particularly, it reveals a new type of surface complexes having two CH3S bonding to one Cu(ad) (referred therein as CH3S-Cu(ad)-CH3S). In addition, this work also yields new reaction dynamics results on transition states and activation barriers. The results reveal that the presence of Cu(ad) indeed significantly changes the kinetics of adsorption and dissociation of CH3SH on Cu(111). The most kinetically favorable reaction pathway turns out to be that involving the formation of a special surface complex formed by one Cu(ad) plus two CH3S fragments from the dissociation of CH3SH, with the two S atoms located at the bridge sites of Cu(111). Finally, this work also gives simulated scanning tunneling microscopic images for the most important adsorption species in the course of the transition from CH3SH∕Cu(111) to CH3S∕Cu(111), which may stimulate future experimental studies of self-assembled monolayers on practical metal substrates such as thiols on copper.