Birabar Ranjit Kumar Nanda

Understanding the atomic-scale interaction mechanism of CO2 and H2O on TiO2 surface is crucial to establish a correlation between the catalytic efficiency with its exposed facet. Here, with the aid of a three-state model, nudged elastic band simulations, and DFT calculations, we examine the chemical restructuring of these molecules during the process of adsorption, coadsorption and conversion on (0 0 1) including (1 × 4)-reconstructed, (0 1 0), and (1 0 1) facets of anatase TiO2 and thereby, evaluate the step selective reactivity order. In addition, the results reveal the unexplored non-trivialities in the reaction mechanisms. For the most stable (1 0 1) facet, we show that the unfavorable carbonate complex formation becomes favorable by switching the reaction from endothermic to exothermic in the presence of water. Further, we find that the small binding energy does not necessarily imply physisorption. It can also give rise to chemisorption, where loss in energy due to repulsive Hartree and Madelung interactions is comparable to the energy gained through the chemical bonding. Such a scenario is demonstrated for the CO2 adsorption on (0 1 0) and (1 0 1) facets. Though (0 0 1) remains the most reactive surface, if it undergoes reconstruction, which happens at ultra high vacuum and high temperature, the number of active sites is reduced by three-fourth.

Frustrated magnets based on oxide double perovskites offer a viable ground wherein competing magnetic interactions, macroscopic ground state degeneracy and complex interplay between emergent degrees of freedom can lead to correlated quantum phenomena with exotic excitations highly relevant for potential technological applications. By local-probe muon spin relaxation (μSR) and complementary thermodynamic measurements accompanied by first-principles calculations, we here demonstrate novel electronic structure and magnetic phases of Ba2MnTeO6, where Mn2 + ions with S = 5/2 spins constitute a perfect triangular lattice. Magnetization results evidence the presence of strong antiferromagnetic interactions between Mn2 + spins and a phase transition at TN = 20 K. Below TN, the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K, which is due to magnetic anisotropy. μSR reveals the presence of static internal fields in the ordered state and short-range spin correlations high above TN. It further unveils critical slowing-down of spin dynamics at TN and the persistence of spin dynamics even in the magnetically ordered state. Theoretical studies infer that Heisenberg interactions govern the inter- and intra-layer spin-frustration in this compound. Our results establish that the combined effect of a weak third-nearest-neighbour ferromagnetic inter-layer interaction (owing to double-exchange) and intra-layer interactions stabilizes a three-dimensional magnetic ordering in this frustrated magnet.

We propose the design of low strained and energetically favourable mono and bilayer graphene overlayer on anatase TiO2 (001) surface and examined the electronic structure of the interface with the aid of first principle calculations. In the absence of hybridization between surface TiO2 and graphene states, dipolar fluctuations govern the minor charge transfer across the interface. As a result, both the substrate and the overlayer retain their pristine electronic structure. The interface with the monolayer graphene retains its gapless linear band dispersion irrespective of the induced epitaxial strain. The potential gradient opens up a few meV bandgap in the case of Bernal stacking and strengthens the interpenetration of the Dirac cones in the case of hexagonal stacking of the bilayer graphene. The difference between the macroscopic average potential of the TiO2 and graphene layer(s) in the heterostructure lies in the range 3–3.13 eV, which is very close to the TiO2 bandgap (~3.2 eV). Therefore, the proposed heterostructure will exhibit enhanced photo-induced charge transfer and the graphene component will serve as a visible light sensitizer.

Adsorption and functional transformation of 2,4,6-trinitrotoluene (TNT) are highly desirable to create a safer environment. Using first principles electronic structure calculations and MD simulations, we have examined the adsorption and catalytic conversion of TNT on the rutile(r) TiO2 (110) surface. TNT is found to remain adsorbed in its molecular form on the pristine r-(110) surface; however, the presence of water and oxygen results in a degradation of TNT to 2,4,6-trinitrobenzoic acid and trinitrobenzaldehyde, while for the latter slightly lower energy barrier is required. Furthermore, the TNT adsorption is dependent on the vacancy concentration. The molecule remains adsorbed on single vacancy reduced surface, while in the presence of double vacancy, the nitro group forms a bond either with the vacant oxygen site or at the five-fold coordinated Ti-site.

The work function (WF) of a material governs the back and forth movement of the charge carriers across the hetero-interface of two materials. Therefore, for optimum device performance, precise knowledge of the WF is prerequisite while employing any new material in electronic devices. In this work, using metal oxide semiconductor capacitors, we experimentally determine the WF of layered van der Waals topological semimetals (TSMs) 1T′-MoTe2, 1T-PtSe2, and Td-WTe2 as 4.87, 5.05, and 4.82 eV, respectively. The experimentally obtained results are corroborated with density functional theory calculations. Furthermore, by analyzing the vertical current transport across the metal oxide semiconductor stack using Fowler-Nordheim tunneling formalism, the barrier height between the TSMs and the gate insulator (SiO2) is experimentally calculated. The obtained barrier heights are also following the same trend as that of WF for three TSMs. These TSMs host unique topological nontrivial phases potentially useful for the development of emerging quantum technologies, and therefore, the findings of this study are significant for designing the future quantum devices.

A suspended layer made up of ferromagnetically ordered spins could be created between two- monolayer or multilayer graphene through intercalation. Stability and electronic structure studies show that, when fluorine molecules are intercalated between two mono/multilayer graphene, their bonds get stretched enough (∼1.9-2.0Å) to weaken their molecular singlet eigenstate. Geometrically, these stretched molecules form a pseudoatomized fluorine layer by maintaining a van der Waals separation of ∼2.6Å from the adjacent carbon layers. As there is a significant charge transfer from the adjacent carbon layers to the fluorine layers, a mixture of triplet and doublet states stabilizes to induce local spin moments at each fluorine site and in turn form a suspended two-dimensional spin lattice. The spins of this lattice align ferromagnetically with nearest-neighbor coupling strength as large as ∼100meV. Our finite-temperature ab initio molecular dynamics study reveals that the intercalated system can be stabilized up to a temperature of 100 K with an average magnetic moment of ∼0.6μB/F. However, if the graphene layers can be held fixed, the room-temperature stability of such a system is feasible.

The unique structural characteristics make the 2D materials potential candidates for designing negative electrodes for rechargeable energy storage devices. Here, by employing density functional theory (DFT) calculations, we study the precise viability of using Si2BN, a graphene-like 2D material, as a high-capacity anode material for Mg-ion battery (MIB) application. The favorable Mg-adsorption sites with maximum possible coverage effect are explored in detail. It is found that the Si2BN sheet can be adsorbed to a configuration of Mg8Si16B8N8, which proposes a theoretical capacity of 647.896 mA h g-1 for divalent Mg2+-ion battery applications. The average open-circuit voltage of 0.6-0.7 V and intercalation migration energy barrier in the range of 0.08-0.35 eV make Si2BN one of the most promising anode materials for MIB applications. The porous Si2BN with high structural stability and metallic electronic structures along with the low Mg2+-ion migration barrier energies predict high electron and Mg-ion conductivity, ensuring fast charge/discharge cyclic performance. The above-mentioned findings validate that the Si2BN sheet can work as an excellent high-performance anode material for MIBs.

Understanding the mechanism of NO2 interaction on semiconductor surfaces such as TiO2 is a key step in designing the catalytic processes for conversion of NO2 to useful products. In the present work, through density functional theory calculations and NEB simulations, we have performed a comprehensive electronic structure study and established the reaction steps for efficient conversion of NO2 to HONO on TiO2 surface in the presence of water vapor. We predict the dimerization of NO2 to form a metastable N2O4. The latter's dissociation to NO+and NO3- complexes occurs in two pathways: (i) direct disproportionation reaction and (ii) through formation of NO2+and NO2- intermediates followed by O transfer. The introduction of H2O on a NO2 chemisorbed surface leads to the formation of nitrous acid through the interaction of NO+ with the water. The reaction pathways leading to formation of nitrous and nitric acids are formulated.

Adsorption of CO 2 on a semiconductor surface is a prerequisite for its photocatalytic reduction. Owing to superior photocorrosion resistance, nontoxicity, and suitable band-edge positions, TiO 2 is considered to be the most efficient photocatalyst for facilitating redox reactions. However, due to the absence of adequate understanding of the mechanism of adsorption, the CO 2 conversion efficiency on TiO 2 surfaces has not been maximized. While anatase TiO 2 (101) is the most stable facet, the (001) surface is more reactive, and it has been experimentally shown that the stability can be reversed and a larger percentage (up to ∼89%) of the (001) facet can be synthesized in the presence fluorine ions. Therefore, through density functional calculations we have investigated the CO 2 adsorption on TiO 2 (001) surfaces. We have developed a three-state quantum-mechanical model that explains the mechanism of chemisorption, leading to the formation of a tridentate carbonate complex. The electronic structure analysis reveals that the CO 2 -TiO 2 interaction at the surface is uniaxial and long ranged, which gives rise to anisotropy in binding energy (BE). It negates the widely perceived one-to-one correspondence between coverage and BE and infers that the spatial distribution of CO 2 primarily determines the BE. A conceptual experiment is devised where the CO 2 concentration and flow direction can be controlled to tune the BE within a large window of ∼1.5eV. The experiment also reveals that a maximum of 50% coverage can be achieved for chemisorption. In the presence of water, the activated carbonate complex forms a bicarbonate complex by overcoming a potential barrier of ∼0.9eV.

Commercialization of aluminium ion battery (AIB) requires limited volume expansion of the host cathode materials after AlCl4 intercalation, lower activation barrier, high theoretical specific capacity (TSC), cyclic durability and thermodynamic stability. Most of the carbon and non-carbon based cathode hosts explored so far failed to address the issue of volume expansion and there is a lack of clarity about thermodynamic stability. In this work, we employed multipronged first principles computational approaches on α- and γ-graphyne (GY) and showed that α-GY as a promising cathode host addresses each of the above concerns. Both α and γ-GYs provide ample space to accommodate more number of AlCl4 molecules leading to a high TSC of 186 mA h g−1 and open circuit voltages of 2.18 and 2.22 V, respectively. The absence of bond dissociation of AlCl4 and deformation of GY sheets at 300 and 600 K, as revealed by ab initio molecular dynamics (AIMD) simulation, indicates the stability of α- and γ-GY with adsorbed AlCl4. α-GY after intercalation shows a volume expansion of 186% which is the lowest among the cathode materials studied so far. The negligible expansion energy per unit surface area (∼0.003 eV Å−2) ensures the reversibility and hence cyclic durability of α-GY. Although the γ-GY shows a volume expansion of 249%, it is still promising. The NEB based diffusion study on monolayer and bilayer GY estimates the activation barriers to be (0.26, 0.06 eV) and (0.42, 0.16 eV) for α and γ phases, respectively. These values are either comparable to or lower than those of earlier reported cathode hosts.