Competitive chemisorption of bifunctional carboxylic acids on H:Si(100): A first-principles study

We investigate competitive chemisorption processes of bifunctional α-carboxy ω-alkenes and ω-alkynes on fully hydrogenated H:Si(100), using first-principles density functional theory, in extended surface simulations. We study the structural properties and quantify the energetics and activation barriers, analyzing the reaction paths. Our results reveal that, if the plain, unactivated chemisorption reaction is always achieved through high barriers, once realized the configurations are very stable, ensuring robustness and reliability of the functionalized interface. We identify the conditions where disordered configurations are more likely to arise, with both functionalities offered at the free surface. For all stable configurations, a thorough analysis of the electronic properties and the extent of hybridization in the functionalized interface allows us to identify promising candidates for applications in molecular electronics

An in situ and ex situ TEM study into the oxidation of titanium (IV) sulphide

AbstractTitanium (IV) sulphide (TiS2) is a layered transition metal dichalcogenide, which we exfoliate using liquid phase exfoliation. TiS2 is a candidate for being part of a range of future technologies. These applications are varied, and include supercapacitor and battery energy storage devices, catalytic substrates and the splitting of water. The driving force behind our interest was as a material for energy storage devices. Here we investigate a potential failure mechanism for such devices, namely oxidation and subsequent loss of sulphur. This degradation is important to understand, since these applications are highly property-dependent, and changes to the chemistry will result in changes in desired properties. Two approaches to study oxidisation were taken: ex situ oxidation by water and oxygen at room temperature and in situ oxidation by a 5% O2/Ar gas at elevated temperatures. Both sources of oxygen resulted in oxidation of the starting TiS2 flakes, with differing morphologies. Water produced amorphous oxide slowly growing in from the edge of the flakes. Oxygen gas at ≥375 °C produced crystalline oxide, with a range of structures due to oxidation initiating from various regions of the observed flakes.</jats:p

Doping carbon electrodes with sulfur achieves reversible sodium ion storage

We present a combination of experiments and theory to study the effect of sulfur doping in hard carbons anodes for sodium-ion batteries. Hard carbons are synthesised through a two step process: hydrothermal carbonisation followed by pyrolysis of a biomass-derived carbon precursor. Subsequent sulfur doping is introduced via chemical-vapour deposition. The resulting sulfur-doped hard carbon shows enhanced sodium storage capacity with respect to the pristine material, with significantly improved cycling reversibility. Atomistic first principles simulations give insight into this behaviour, revealing that sulfur chemisorbed onto the hard carbon increases the sodium adsorption energies and facilitates sodium desorption. This mechanism would increase reversible Na storage, confirming our experimental observations and opening a pathway towards more efficient Na-ion batteries

Solvent Exfoliation of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds

We have studied the dispersion and exfoliation of four inorganic layered compounds, WS₂, MoS₂, MoSe₂, and MoTe₂, in a range of organic solvents. The aim was to explore the relationship between the chemical structure of the exfoliated nanosheets and their dispersibility. Sonication of the layered compounds in solvents generally gave few-layer nanosheets with lateral dimensions of a few hundred nanometers. However, the dispersed concentration varied greatly from solvent to solvent. For all four materials, the concentration peaked for solvents with surface energy close to 70 mJ/m², implying that all four have surface energy close to this value. Inverse gas chromatography measurements showed MoS₂ and MoSe₂ to have surface energies of ∼75 mJ/m², in good agreement with dispersibility measurements. However, this method suggested MoTe₂ to have a considerably larger surface energy (∼120 mJ/m²). While surface-energy-based solubility parameters are perhaps more intuitive for two-dimensional materials, Hansen solubility parameters are probably more useful. Our analysis shows the dispersed concentration of all four layered materials to show well-defined peaks when plotted as a function of Hansen’s dispersive, polar, and H-bonding solubility parameters. This suggests that we can associate Hansen solubility parameters of δ_D ∼ 18 MPa^1/2, δ_P ∼ 8.5 MPa^1/2, and δ_H ∼ 7 MPa^1/2 with all four types of layered material. Knowledge of these properties allows the estimation of the Flory–Huggins parameter, χ, for each combination of nanosheet and solvent. We found that the dispersed concentration of each material falls exponentially with χ as predicted by solution thermodynamics. This work shows that solution thermodynamics and specifically solubility parameter analysis can be used as a framework to understand the dispersion of two-dimensional materials. Finally, we note that in good solvents, such as cyclohexylpyrrolidone, the dispersions are temporally stable with >90% of material remaining dispersed after 100 h

Electronic Properties and Chemical Reactivity of TiS₂ Nanoflakes

Transition metal dichalcogenides have a laminar structure, with strongly covalently bonded layers weakly interacting through van der Waals forces. They are of special interest also because of their unique properties once exfoliated in nanoflakes. We analyze the microstructure of oxidized TiS₂ nanoflakes with atomically resolved scanning transmission electron microscopy and propose a comprehensive model for their reactivity by means of first-principles simulations. In particular we find that reaction to water proceeds from the edges of the flake, while it is thermodynamically possible but kinetically hindered in the middle, unless it is initiated by the presence of a surface vacancy. Importantly O substitution for S allows fine-tuning control of the flake bandgap, paving the way for the use of TiS_2‑<i>x</i>O_<i>x</i> alloys as surface catalysts and photovoltaic materials