Sonochemical edge functionalisation of molybdenum disulfide

Liquid-phase exfoliation (LPE) has been shown to be capable of producing large quantities of high-quality dispersions suitable for processing into subsequent applications. LPE typically requires surfactants for aqueous dispersions or organic solvents with high boiling point. However, they have major drawbacks such as toxicity, aggregation during solvent evaporation or the presence of residues. Here, dispersions of molybdenum disulfide in acetone are prepared and show much higher concentration and stability than predicted by Hansen parameter analysis. Aiming to understand those enhanced properties, the nanosheets were characterised using UV-visible spectroscopy, zeta potential measurements, atomic force microscopy, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and scanning transmission microscopy combined with spatially-resolved electron energy loss spectroscopy. Also, the performance of the MoS2 nanosheets exfoliated in acetone was compared to those exfoliated in isopropanol as a catalyst for the hydrogen evolution reaction. The conclusion from the chemical characterisation was that MoS2 nanosheets exfoliated in acetone have an oxygen edge-functionalisation, in the form of molybdenum oxides, changing its interaction with solvents and explaining the observed high-quality and stability of the resulting dispersion in a low boiling point solvent. Exfoliation in acetone could potentially be applied as a pretreatment to modify the solubility of MoS2 by edge-functionalisation

Hydrothermally Synthesized h‑MoO₃ and α‑MoO₃ Nanocrystals: New Findings on Crystal-Structure-Dependent Charge Transport

The charge transfer characteristics of metastable-phase hexagonal molybdenum oxide (h-MoO₃) and stable-phase orthorhombic MoO₃ (α-MoO₃) nanocrystals have been investigated for the first time using impedance spectroscopy. The results imply that the metastable phase h-MoO₃ displays a 550-fold increase (at 150 °C) in the electrical conductivity relative to the stable phase α-MoO₃. The conductivity also increases as the temperature increases from 130 to 170 °C, whereby analysis shows a thermal activation energy (<i>E</i>_a) of ∼0.42 eV. The investigation clearly identifies that the presence of intercalated ammonium ions (NH₄⁺) and crystal water molecules (H₂O) in the internal structure of h-MoO₃ plays a vital role in enhancing the charge transfer characteristics and showing an ionic conductive nature. Before the impedance investigations, the h-MoO₃ and α-MoO₃ nanocrystals were successfully synthesized through a wet-chemical process. Here, a controlled one-step hydrothermal route was adopted to synthesize stable-phase α-MoO₃ nanocrystals sequentially from metastable-phase h-MoO₃ nanocrystals. The hydrothermal reaction conditions, such as the choice of precipitant, amount of precipitant, reactant solvent medium, reaction time, and reaction temperature, play significant roles in defining the crystal structure, crystallite size, and particle morphology. On the basis of the crystal structure, size, and morphology evolution with respect to the hydrothermal reaction conditions, a possible formation mechanism of MoO₃ nanocrystals is proposed