Birabar Ranjit Kumar Nanda

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.

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.