Graphite and Hexagonal Boron-Nitride Possess the Same Interlayer Distance. Why?

Graphite and hexagonal boron nitride (h-BN) are two prominent members of the family of layered materials possessing a hexagonal lattice. While graphite has non-polar homo-nuclear C-C intra-layer bonds, h-BN presents highly polar B-N bonds resulting in different optimal stacking modes of the two materials in bulk form. Furthermore, the static polarizabilities of the constituent atoms considerably differ from each other suggesting large differences in the dispersive component of the interlayer bonding. Despite these major differences both materials present practically identical interlayer distances. To understand this finding, a comparative study of the nature of the interlayer bonding in both materials is presented. A full lattice sum of the interactions between the partially charged atomic centers in h-BN results in vanishingly small monopolar electrostatic contributions to the interlayer binding energy. Higher order electrostatic multipoles, exchange, and short-range correlation contributions are found to be very similar in both materials and to almost completely cancel out by the Pauli repulsions at physically relevant interlayer distances resulting in a marginal effective contribution to the interlayer binding. Further analysis of the dispersive energy term reveals that despite the large differences in the individual atomic polarizabilities the hetero-atomic B-N C6 coefficient is very similar to the homo-atomic C-C coefficient in the hexagonal bulk form resulting in very similar dispersive contribution to the interlayer binding. The overall binding energy curves of both materials are thus very similar predicting practically the same interlayer distance and very similar binding energies.Comment: 18 pages, 5 figures, 2 table

Ammonia gas sensing characteristics of V2O5 nanostructures: A combined experimental and ab initio density functional theory approach

A combined experimental and density functional theory of α-V2O5 for ammonia gas sensing have been investigated. The material was synthesized from hydrated NH4VO3 in CVD at 400 °C in N2 atmosphere for different time (12 h and 24 h). Highly crystalline orthorhombic α-V2O5 nano-rods with dominant (001) and (110) planes/facets nano-rods were observed from XRD, SEM and TEM characterizations. Using VSM technique, para-to ferro-magnetic transition was observed in the α-V2O5 nanoparticles synthesized at 24 h. Improved gas sensing was observed in case of the paramagnetic α-V2O5 nano-rods (nanoparticles synthesized at 12 h) compared with the one synthesized at 24 h. Additionally, significant rise in gas sensing response was observed around the metal to insulator transition temperature. Calculation of adsorption of NH3 molecule(s) on (001), (110), (200) and (400) facets showed that (001), (200) and (400) possessed more active sites than (110) surface. However, at higher concentration of NH3 molecules, the number of adsorbed molecules was found to be limited by the available adsorption sites in the case of (001) thereby causing the surface to be unstable. DFT calculations were also used to investigate NH3 adsorption on (110) surface of α-V2O5 with the analysis showing exponential decrease in the electronic band gap of the material's surface with the increasing numbers of NH3 loadings.</p