Role of Bilayer Graphene Microstructure on the Nucleation of WSe2 Overlayers.

Over the past few years, graphene grown by chemical vapor deposition (CVD) has gained prominence as a template to grow transition metal dichalcogenide (TMD) overlayers. The resulting two-dimensional (2D) TMD/graphene vertical heterostructures are attractive for optoelectronic and energy applications. However, the effects of the microstructural heterogeneities of graphene grown by CVD on the growth of the TMD overlayers are relatively unknown. Here, we present a detailed investigation of how the stacking order and twist angle of CVD graphene influence the nucleation of WSe2 triangular crystals. Through the combination of experiments and theory, we correlate the presence of interlayer dislocations in bilayer graphene with how WSe2 nucleates, in agreement with the observation of a higher nucleation density of WSe2 on top of Bernal-stacked bilayer graphene versus twisted bilayer graphene. Scanning/transmission electron microscopy (S/TEM) data show that interlayer dislocations are present only in Bernal-stacked bilayer graphene but not in twisted bilayer graphene. Atomistic ReaxFF reactive force field molecular dynamics simulations reveal that strain relaxation promotes the formation of these interlayer dislocations with localized buckling in Bernal-stacked bilayer graphene, whereas the strain becomes distributed in twisted bilayer graphene. Furthermore, these localized buckles in graphene are predicted to serve as thermodynamically favorable sites for binding WSex molecules, leading to the higher nucleation density of WSe2 on Bernal-stacked graphene. Overall, this study explores synthesis-structure correlations in the WSe2/graphene vertical heterostructure system toward the site-selective synthesis of TMDs by controlling the structural attributes of the graphene substrate.

Aqueous phase conversion of CO2 into acetic acid over thermally transformed MIL-88B catalyst.

Sustainable production of acetic acid is a high priority due to its high global manufacturing capacity and numerous applications. Currently, it is predominantly synthesized via carbonylation of methanol, in which both the reactants are fossil-derived. Carbon dioxide transformation into acetic acid is highly desirable to achieve net zero carbon emissions, but significant challenges remain to achieve this efficiently. Herein, we report a heterogeneous catalyst, thermally transformed MIL-88B with Fe0 and Fe3O4 dual active sites, for highly selective acetic acid formation via methanol hydrocarboxylation. ReaxFF molecular simulation, and X-ray characterisation results show a thermally transformed MIL-88B catalyst consisting of highly dispersed Fe0/Fe(II)-oxide nanoparticles in a carbonaceous matrix. This efficient catalyst showed a high acetic acid yield (590.1 mmol/gcat.L) with 81.7% selectivity at 150 °C in the aqueous phase using LiI as a co-catalyst. Here we present a plausible reaction pathway for acetic acid formation reaction via a formic acid intermediate. No significant difference in acetic acid yield and selectivity were noticed during the catalyst recycling study up to five cycles. This work is scalable and industrially relevant for carbon dioxide utilisation to reduce carbon emissions, especially when green methanol and green hydrogen are readily available in future.

Atomistic Mechanisms of Thermal Transformation in a Zr-Metal Organic Framework, MIL-140C.

To understand the mechanisms responsible for thermal decomposition of a Zr-MOF (MIL-140C), we perform atomistic-scale molecular dynamics (MD) simulations and discuss the simulation data in comparison with the TEM images obtained for the decomposed Zr-MOF. First, we introduce the ReaxFF parameters suitable for the Zr/C/H/O chemistry and then apply them to investigate the thermal stability and morphological changes in the MIL-140C during heating. Based on the performed simulations we propose an atomic mechanism for the collapse of the MIL-140C and the molecular pathways for carbon monoxide formation, the main product of the MIL-140C thermal degradation. We also determine that the oxidation state of the ZrOx clusters, evolved due to the thermal degradation, approximates the tetragonal phase of ZrO2. Both simulations and experiments show a distribution of very small ZrOx clusters embedded in the disrupted organic sheet that could contribute to the unusual high catalytic activity of the decomposed MIL-140C.

Molecular Clustering in Formaldehyde-Methanol-Water Mixtures Revealed by High-Intensity, High-q Small-Angle Neutron Scattering.

Methanol-Water (mw) mixtures, with or without a solute, display a nonideal thermodynamic behavior, typically attributed to the structure of the microphase. However, experimental observation of the microphase structures at the molecular length scale has been a challenge. We report the presence of molecular clusters in mw and formaldehyde-methanol-water (fmw) mixtures using small-angle neutron scattering (SANS) experiments and molecular dynamics (MD) simulations. Hydrophobic clusters of methanol in mw and formaldehyde-methanol in fmw mixtures were observed at low methanol compositions (xm ≤ 0.3). A three-dimensional hydrogen-bonded network of water with the solute is observed at xm = 0.5. Linear chains of methanol surrounding the formaldehyde and water molecules were observed at high methanol compositions (xm ≥ 0.7). The calculated size of the molecular clusters (r ≈ 0.5 nm, spherical) from the SANS data and their volume fraction closely matched the MD simulation results.

Controlled Nanoscale Electrohydrodynamic Patterning Using Mesopatterned Template.

We report the path for a possible fabrication of an array of nanogrooves, by electro-hydrodynamic instability-mediated patterning of a thin polymer film using a patterned stamp with much larger features. Using a predictive computational model based on finite element method, we find the route to control the coalescence of initial instabilities that arise with the onset of spatially varying DC electric field generated through topographical patterns in the top electrode. These quasi-steady structures are shown to evolve with the electrostatic and geometric nature of the two-electrode system and are of a stable intermediate during the process of feature replication, under each electrode feature. We identify conditions to obtain nanogrooves for a range of operating conditions. Such simulations are likely to guide experiments, where simultaneous optimization of multiple parameters to fabricate features with lateral dimension smaller than that of the electrode patterns is challenging.