E. Mathan Kumar

Doped h-BN monolayer as efficient noble metal-free catalysts for CO oxidation: the role of dopant and water in activity and catalytic de-poisoning

Using a first principles approach, we investigated the catalytic activity of a noble metal-free n-doped (C → B, O → N) hexagonal boron nitride (h-BN) monolayer for CO oxidation. The CO adsorption ability and hence the preferred Eiley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanism for CO oxidation is dopant-dependent: CO is chemisorbed on O-doped h-BN (OBN) while it physically interacts with the C-doped h-BN (CBN) surface. Even though both C and O doping create similar donor states below the Fermi level (Ef), O doping results in a larger bond length of O–B1 (one of the nearest B atom), out-of-plane displacement of the B1 atom, and less positive charge on the B1 atom, synergistically contributing to higher atomic activity. The presence of a pre-adsorbed O2 molecule on both types of surfaces eliminates any chances of CO poisoning of the surface, and CO oxidation prefers to proceed via the ER mechanism with a small activation barrier. The high values of Sabatier activities suggest that the doped h-BN surface is superior to Au55and Pt55nanoclusters. In case of CO oxidation by means of the LH mechanism, a stable O2⋯CO intermediate is produced, which requires a high barrier energy to break the O–O bond. However, the presence of a H2O molecule increases the activity of the catalyst and helps in catalytic CO de-poisoning.

First principles guide to tune h-BN nanostructures as superior light-element-based hydrogen storage materials: role of the bond exchange spillover mechanism

We investigate the interaction of molecular hydrogen with light-element-based n-doped hexagonal boron nitride (h-BN) nanostructures and moreover explore the bond exchange mechanism for spillover of atomic hydrogen using dispersion-corrected density functional theory (DFT-D) calculations. A number of doped configurations were tested and it has been found that co-doping of C and O on h-BN sheet significantly increases the adsorption energy of molecular H2. The charge transfer from the n-doped h-BN surface to H2 is found to be the reason for the higher interactions that boosted the binding energy. In addition, the doped h-BN surfaces act as catalysts and dissociate the H2 molecule with a very low activation barrier, but the migration of the resulting H atoms on the surface requires high energy. In order to facilitate easy and fast migration of H atoms, we introduce the bond exchange mechanism using external mediators i.e. borane (BH3) and gallane (GaH3) molecules which serve as secondary catalysts and help in lowering the migration barrier, leading to the formation of a hydrogenated surface. The partially hydrogenated surface in turn can also act as a hydrogen storage material, with a higher propensity to adsorb hydrogen molecules when compared to the unhydrogenated surface. Hence the surface proposed in this work can be used to store a substantial quantity of hydrogen as an energy source with easy adsorption and desorption kinetics.

Activation of CO and CO2 on homonuclear boron bonds of fullerene-like BN cages: first principles study.

Using density functional theory we investigate the electronic and atomic structure of fullerene-like boron nitride cage structures. The pentagonal ring leads to the formation of homonuclear bonds. The homonuclear bonds are also found in other BN structures having pentagon line defect. The calculated thermodynamics and vibrational spectra indicated that, among various stable configurations of BN-60 cages, the higher number of homonuclear N-N bonds and lower B:N ratio can result in the more stable structure. The homonuclear bonds bestow the system with salient catalytic properties that can be tuned by modifying the B atom bonding environment. We show that homonuclear B-B (B2) bonds can anchor both oxygen and CO molecules making the cage to be potential candidates as catalyst for CO oxidation via Langmuir–Hinshelwood (LH) mechanism. Moreover, the B-B-B (B3) bonds are reactive enough to capture, activate and hydrogenate CO2 molecules to formic acid. The observed trend in reactivity, viz B3 > B2 > B1 is explained in terms of the position of the boron defect state relative to the Fermi level.

Effect of surface doping on the band structure of graphene: a DFT study

Abstract Various techniques, like doping, vacancy creation, strain engineering are tried to open a gap in the bandstructure of graphene and in some cases the gap has opened up. However, when the gap opens up the Dirac cones disappear. Without Dirac cones, graphene loses all its novelty. So opening a gap in graphene, retaining Dirac cones has become a challenging task. We, through first principles study using Density Functional theory, have done band gap tuning investigations. We have succeeded in opening the band gap, retaining the Dirac cones. Surface doping (adsorption) of various elements are tried and finally surface doping of sulfur is found to induce band gap opening in graphene. The Dirac cones are retained and the graphene is now a semiconductor with fast moving massless Dirac Fermions. We are reporting this type of calculations for the first time.

Antiferro-ferromagnetic transition in ultrathin Ni(OH)2 layer grown on graphene surface and observation of interlayer exchange coupling in Ni(OH)2/graphene/Ni(OH)2 nanostructures

The major limitation of using graphene as a potential spacer element in interlayer exchange coupling (IEC) might be due to destruction of ferromagnetism as a result of the charge transfer effect at the interface if a transition metal based ferromagnetic layer is grown on the graphene surface. To overcome this problem, we have used the antiferromagnetic Ni(OH)2 layer grown on the graphene surface to convert it ferromagnetic due to the charge transfer effect. By growing thin layers of Ni(OH)2 on both sides of the graphene surface, strong antiferromagnetic IEC with ultra-low coercivity (7 Oe) is observed. By lowering the nickel content, an ultrathin layer of Ni(OH)2 is grown on either side of graphene and shows complete ferromagnetism with a giant coercivity of 4154 Oe. Ab initio calculations have been done to substantiate this kind of charge transfer effect at the interface of Ni(OH)2 and graphene. Magnetotransport of the composite material is also investigated to understand the role of IEC in transport pro...

Screening based approach and dehydrogenation kinetics for MgH 2 : Guide to find suitable dopant using first-principles approach

First-principles based calculations are performed to investigate the dehydrogenation kinetics considering doping at various layers of MgH2 (110) surface. Doping at first and second layer of MgH2 (110) has a significant role in lowering the H2 desorption (from surface) barrier energy, whereas the doping at third layer has no impact on the barrier energy. Molecular dynamics calculations are also performed to check the bonding strength, clusterization, and system stability. We study in details about the influence of doping on dehydrogenation, considering the screening factors such as formation enthalpy, bulk modulus, and gravimetric density. Screening based approach assist in finding Al and Sc as the best possible dopant in lowering of desorption temperature, while preserving similar gravimetric density and Bulk modulus as of pure MgH2 system. The electron localization function plot and population analysis illustrate that the bond between Dopant-Hydrogen is mainly covalent, which weaken the Mg-Hydrogen bonds. Overall we observed that Al as dopant is suitable and surface doping can help in lowering the desorption temperature. So layer dependent doping studies can help to find the best possible reversible hydride based hydrogen storage materials.

Si doped T6 carbon structure as an anode material for Li-ion batteries: An ab initio study

First-principles calculations are performed to identify the pristine and Si doped 3D metallic T6 carbon structure (having both sp2 and sp3 type hybridization) as a new carbon based anode material. The π electron of C2 atoms (sp2 bonded) forms an out of plane network that helps to capture the Li atom. The highest Li storage capacity of Si doped T6 structure with conformation Li1.7Si1C5 produces theoretical specific capacity of 632 mAh/g which substantially exceeding than graphite. Also, open-circuit voltage (OCV) with respect to Li metal shows large negative when compared to the pristine T6 structure. This indicates modifications in terms of chemical properties are required in anode materials for practical application. Among various doped (Si, Ge, Sn, B, N) configuration, Si doped T6 structure provides a stable positive OCV for high Li concentrations. Likewise, volume expansion study also shows Si doped T6 structure is more stable with less pulverization and substantial capacity losses in comparison with graphite and silicon as an anode materials. Overall, mixed hybridized (sp2 + sp3) Si doped T6 structure can become a superior anode material than present sp2 hybridized graphite and sp3 hybridized Si structure for modern Lithium ion batteries.

Resonant energy transfer in a van der Waals stacked MoS2 – functionalized graphene quantum dot composite with ab initio validation

Graphene-based van der Waals (vdW) heterostructures can facilitate exciting charge transfer dynamics in between structural layers with the emission of excitonic quasi-particles. However, the chemical formation of such heterostructures has been elusive thus far. In this work, a simple chemical approach is described to form such van der Waals (vdW) heterostructures using few layer MoS2 sheet embedded quantum dots (QDs) and amine-functionalized graphene quantum dots (GQDs) to probe the energy transfer mechanism for tunable photoluminescence (PL). Our findings reveal an interesting non-radiative Förster-type energy transfer with the quenching of functional GQD PL intensity after GQD/MoS2 composite formation, which validates the existing charge transfer dynamics analogous to 0D and 2D systems. The non-radiative type of energy transfer characteristic from GQD into the MoS2 layer through vdW interactions has been confirmed by photoluminescence, time decay analyses and ab initio calculations with the shifting of the Fermi level in the density of states towards the conduction band in the stacked configuration. These results are encouraging for the fundamental exploration of optical properties in other chemically prepared QD/2D based heterostructures to understand the charge transfer mechanism and fingerprint luminescence quenching for future optoelectronic device and optical sensing applications.

Theoretical insights into the minority carrier lifetime of doped Si—A computational study

Using density functional theory, we have analyzed the ways and means of improving the minority carrier lifetime (MCL) by calculating the band structure dependent quantities contributing to the MCL. We have computationally modeled silicon doped with different elements like B, C, N, O, P, Ti, Fe, Ga, Ge, As, In, Sn, Sb, and Pt and looked at the effect of doping on MCL. In co-doping, the systems Si-B-Ga, Si-B-Ge, Si-B-2Ge, Si-B-Pt, Si-Ga-Ge, Si-Ga-2Ge, and Si-Ga-Pt are investigated. From our calculation, it is found that by doping and co-doping of Si with suitable elements having “s” and “p” electrons, there is a decrease in the recombination activity. The predicted effective minority carrier lifetime indicates the possibility of significant improvements. Based on the above studies, it is now maybe possible, with suitable choice of dopant and co-dopant material, to arrive at part of a standard production process for solar grade Si material.Using density functional theory, we have analyzed the ways and means of improving the minority carrier lifetime (MCL) by calculating the band structure dependent quantities contributing to the MCL. We have computationally modeled silicon doped with different elements like B, C, N, O, P, Ti, Fe, Ga, Ge, As, In, Sn, Sb, and Pt and looked at the effect of doping on MCL. In co-doping, the systems Si-B-Ga, Si-B-Ge, Si-B-2Ge, Si-B-Pt, Si-Ga-Ge, Si-Ga-2Ge, and Si-Ga-Pt are investigated. From our calculation, it is found that by doping and co-doping of Si with suitable elements having “s” and “p” electrons, there is a decrease in the recombination activity. The predicted effective minority carrier lifetime indicates the possibility of significant improvements. Based on the above studies, it is now maybe possible, with suitable choice of dopant and co-dopant material, to arrive at part of a standard production process for solar grade Si material.

Hydrogen spillover on DV (555-777) graphene – vanadium cluster system: First principles study

Using dispersion corrected density functional theory (DFT+D), the interaction of Vanadium adatom and cluster with divacancy (555-777) defective graphene sheet has been studied elaborately. We explore the prospect of hydrogen storage on V4 cluster adsorbed divacancy graphene system. It has been observed that V4 cluster (acting as a catalyst) can dissociate the H2 molecule into H atoms with very low barrier energy. We introduce the spillover of the atomic hydrogen throughout the surface via external mediator gallane (GaH3) to form a hydrogenated system.