Size-Dependent Structural, Energetic, and Spectroscopic Properties of MoS₃ Polymorphs

Abstract

In spite of the interest of a-MoS 3 based nanomaterials, the amorphous nature of the MoS 3 phase makes it challenging to control and understand its chemical reactivity. In particular, the type of the structural building unit such as Mo 3 triangular vs Mo chain is still debated, while the ambivalent interpretation of the nature of sulfur species (S 2− , S 2 2− ) and Mo−Mo bonds leads to ambiguous interpretations of spectroscopic data and reactivity. By density functional theory (DFT), we simulate the energetic, structural, and spectroscopic features of relevant zero-dimensional 0D-, 1D-, and 2D-MoS 3 triangular, chain-like polymorphs, including unprecedented ones (ring, wave, and helix) and revisit the interpretation of EXAFS, IR-RAMAN, and XPS experimental data. We analyze how Mo k S 3k clusters of a few k atoms may grow up to infinite (periodic replica) MoS 3 polymorphs. The evolution of the growth energy and the computed IR spectra within the density functional perturbation theory level suggest the coexistence of various polymorphs in the MoS 3 phase as a function of sizes. Molecular dynamics simulations reveal how the small triangular Mo k S 3k oligomers may transform into condensed MoS 3 patches resembling embryos of the 2D 1T′-MoS 2 phase. Finally, we discuss some plausible transformation pathways from one polymorph to another.

FTIR spectrum analysis to predict the crystalline and amorphous phases of hydroxyapatite: a comparison of vibrational motion to reflection

“…1 The demand for amorphous materials is also increasing due to the important characteristics essential for industrial applications with the addition of emerging elds. 2 Calcium phosphate is an important compound not only for biomedical applications but also for industrial applications, and the main compounds include hydroxyapatite, b-tricalcium phosphate, a-tricalcium phosphate, tetra-calcium phosphate, dicalcium phosphate, dicalcium phosphate monohydrate, triple superphosphate, etc. [3][4][5] Hydroxyapatite (Hap) is a wellknown bio-ceramic that has structural similarities with human hard tissue.…”

Electrochemical Potential-Dependent Stability and Activity of MoS3 during the Hydrogen Evolution Reaction

“…We have simulated Mo 3 S 9−x nanoclusters as a representative active site structure for small nanoparticles of MoS 3 (see Figure 1a) as proposed in our previous study on a-MoS 3 . 21 The Mo 3 S 9−x nanoclusters are positioned on a graphite support model at the location of a Stone−Wales defect, which has been introduced to mimic an anchoring point. Orientations of the nanocluster were explored using DockOnSurf, specifying the Stone−Wales defect as the adsorption site and various sulfur atoms as anchoring points.…”

“…Moreover, all these active sites might interconvert as suggested by in situ Raman measurements: bridging S 2 2– species may convert into top S 2 2– species and can then create unsaturated Mo-sites . DFT simulations of S–S vibrational frequencies for various neutral a -MoS 3 polymorphs suggested that some of these interpretations should be considered with care, as they relied on a questionable assignment of the top and bridging S 2 2– species in a -MoS 3 through infrared (IR) or Raman spectroscopies using charged thiomolybdate clusters as references. Indeed, the complex structure of the a -MoS 3 phase has been debated for many years. − The DFT study by Sahu et al revealed that large sizes of neutral a -MoS 3 polymers favor distorted Mo chain-like structures, while smaller entities with Mo triangular moieties are more stable .…”

“…22−25 The DFT study by Sahu et al revealed that large sizes of neutral a-MoS 3 polymers favor distorted Mo chain-like structures, while smaller entities with Mo triangular moieties are more stable. 21 Such structural features are crucial for identification of the nature of HER active sites.…”

“…Considering this previous knowledge, we will investigate the HER mechanisms of a relevant MoS 3 model and its stability with respect to the release of H 2 S. Since we aim to study MoS 3 entities maximizing the number of potential active sites, we will consider a triangular Mo 3 S 9– x cluster model as determined in our previous work . A conductive support such as glassy carbon, carbon nanotubes, or carbon black is experimentally used to enhance the electron transfer required for HER. , Hence, we will use a model graphite support with a Stone–Wales defect as the anchoring point defect.…”