K. P. S. S. Hembram

Graphene Analogues of BN: Novel Synthesis and Properties

Enthused by the fascinating properties of graphene, we have prepared graphene analogues of BN by a chemical method with a control on the number of layers. The method involves the reaction of boric acid with urea, wherein the relative proportions of the two have been varied over a wide range. Synthesis with a high proportion of urea yields a product with a majority of 1-4 layers. The surface area of BN increases progressively with the decreasing number of layers, and the high surface area BN exhibits high CO(2) adsorption, but negligible H(2) adsorption. Few-layer BN has been solubilized by interaction with Lewis bases. We have used first-principles simulations to determine structure, phonon dispersion, and elastic properties of BN with planar honeycomb lattice-based n-layer forms. We find that the mechanical stability of BN with respect to out-of-plane deformation is quite different from that of graphene, as evident in the dispersion of their flexural modes. BN is softer than graphene and exhibits signatures of long-range ionic interactions in its optical phonons. Finally, structures with different stacking sequences of BN have comparable energies, suggesting relative abundance of slip faults, stacking faults, and structural inhomogeneities in multilayer BN.

Chemical storage of hydrogen in few-layer graphene

Birch reduction of few-layer graphene samples gives rise to hydrogenated samples containing up to 5 wt % of hydrogen. Spectroscopic studies reveal the presence of sp3 C-H bonds in the hydrogenated graphenes. They, however, decompose readily on heating to 500 °C or on irradiation with UV or laser radiation releasing all the hydrogen, thereby demonstrating the possible use of few-layer graphene for chemical storage of hydrogen. First-principles calculations throw light on the mechanism of dehydrogenation that appears to involve a significant reconstruction and relaxation of the lattice.

BCN: A Graphene Analogue with Remarkable Adsorptive Properties

A new analogue of graphene containing boron, carbon and nitrogen (BCN) has been obtained by the reaction of high-surface-area activated charcoal with a mixture of boric acid and urea at 900 degrees C. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy reveal the composition to be close to BCN. The X-ray diffraction pattern, high-resolution electron microscopy images and Raman spectrum indicate the presence of graphite-type layers with low sheet-to-sheet registry. Atomic force microscopy reveals the sample to consist of two to three layers of BCN, as in a few-layer graphene. BCN exhibits more electrical resistivity than graphene, but weaker magnetic features. BCN exhibits a surface area of 2911 m(2) g(-1), which is the highest value known for a B(x)C(y)N(z) composition. It exhibits high propensity for adsorbing CO(2) ( approximately 100 wt %) at 195 K and a hydrogen uptake of 2.6 wt % at 77 K. A first-principles pseudopotential-based DFT study shows the stable structure to consist of BN(3) and NB(3) motifs. The calculations also suggest the strongest CO(2) adsorption to occur with a binding energy of 3.7 kJ mol(-1) compared with 2.0 kJ mol(-1) on graphene.

Remarkable uptake of CO2 and CH4 by graphene-Like borocarbonitrides, BxCyNz.

The surface areas and uptake of CO(2) and CH(4) by four graphene samples are measured and compared with activated charcoal. The surface areas are in the range of 5-640 m(2) g(-1), whereas the CO(2) and CH(4) uptake values are in the range of 18-45 wt % (at 195 K, 0.1 MPa) and 0-2.8 wt % (at 273 K, 5 MPa), respectively. The CO(2) and CH(4) uptake values of the graphene samples vary linearly with the surface area. In contrast, graphene-like B(x)C(y)N(z) samples with compositions close to BC(2)N exhibit surface areas in the range of 1500-1990 m(2) g(-1) and CO(2) and CH(4) uptake values in the ranges 97-128 wt % (at 195 K, 0.1 MPa) and 7.5-17.3 wt %, respectively. The uptake of these gases varies exponentially with the surface area of the B(x)C(y)Z(n) samples, and the uptake of CH(4) varies proportionally with that of CO(2). The uptake of CO(2) for the best BC(2)N sample is 64 wt % at 298 K. The large uptake of both CO(2) and CH(4) gases by BC(2)N betters the performance of graphenes and activated charcoal. First-principles calculations show that the adsorption of CO(2) and CH(4) is more favored on BCN samples compared to graphene.

First‐principles Study of Electronic and Dielectric Properties of Polyoxymethylene

The structure of orthorhombic Polyoxymethylene has been simulated using first‐principles. Its structural parameters have been studied. Calculation of Electronic Density of States in the material shows that the material is an insulator with a band gap of 3.51 eV. Phonon frequencies and dielectric constant have been computed. Effect of change in volume of unit cell on the value of dielectric constant has been studied.

First-principles study of structure and properties of the cyclic pentamer of formaldehyde

Structure of the cyclic pentamer of formaldehylde – Pentaxecane has been studied using first-principles. The structural parameters have been compared with the x-ray data available in the literature. The binding energy per monomer and per atom turn out to be to 2240 kJ/mole/monomer and 560 kJ/mole/atom respectively. Electronic density of states (EDOS) have been calculated which gives a value of 5.64 eV as the band gap. Phonon frequencies have been calculated at the Gamma point. Phonon modes show wave numbers ranging from 22cm−1 to 2995cm−1. Values of dielectric constant along different axes have also been calculated. The dielectric constant has also been determined experimentally and compared.