Jawad Nisar

TiO2-Based Gas Sensor: A Possible Application to SO2

Fixation of SO2 molecules on anatase TiO2 surfaces with defects have been investigated by first-principles density functional theory (DFT) calculations and in situ Fourier transform infrared (FTIR) surface spectroscopy on porous TiO2 films. Intrinsic oxygen-vacancy defects, which are formed on TiO2(001) and TiO2(101) surfaces by ultraviolet (UV) light irradiation and at elevated temperatures, are found to be most effective in anchoring the SO2 gas molecules to the TiO2 surfaces. Both TiO2(101) and TiO2(001) surfaces with oxygen vacancies are found to exhibit higher SO2 adsorption energies in the DFT calculations. The adsorption mechanism of SO2 is explained on the basis of electronic structure, charge transfer between the molecule and the surface, and the oxidation state of the adsorbed molecule. The theoretical findings are corroborated by FTIR experiments. Moreover, the (001) surface with oxygen vacancies is found to bind SO2 gas molecules more strongly, as compared to the (101) surface. Higher concentration of oxygen vacancies on the TiO2 surfaces is found to significantly increase the adsorption energy. The results shed new insight into the sensing properties of TiO2-based gas sensors.

Theoretical and experimental evidence of enhanced ferromagnetism in Ba and Mn cosubstituted BiFeO3

Ba and Mn doped BiFeO3 prepared through the pyrolysis of xerogel precursors are characterized by powder x-ray diffraction, high resolution transmission electron microscopy, superconducting quantum interference device magnetometry, and polarization measurements. Structural studies by x-ray diffraction and transmission electron microscopy show a tetragonal lattice for Ba substituted BiFeO3 and a rhombohedral lattice for Mn substituted BiFeO3. A large ferromagnetic hysteresis loop is observed for Ba doped BiFeO3. Coexistence of distorted rhombohedral and tetragonal phases is observed in Ba and Mn codoped BiFeO3, where enhanced ferroelectric and ferromagnetic properties are produced by the internal strain. Density functional calculations are used to substantiate the results.

Mo- and N-doped BiNbO4 for photocatalysis applications

The electronic structure of pure BiNbO4 has been calculated and their electronic band positions have been aligned with respect to the water oxidation/reduction potential. The effect of cationic (Mo), anionic (N), and co-doping (Mo-N) on BiNbO4 has been studied and discussed with respect to the standard redox potential levels. Our results show that co-doping of Mo and N in BiNbO4 reduces the band gap up to 31.8%, thus making it a potential candidate for the photocatalysis of water for hydrogen production. The relative stability between the mono- and co-doped BiNbO4 materials show that co-doped material is more stable and feasible in comparison to the mono-doped materials.

Band gap engineering in BiNbO4 for visible-light photocatalysis

We have investigated the electronic structure of anionic mono- (S, N, and C) and co-doping (N-N, C-N, S-C, and S-N) on BiNbO4 for the visible-light photocatalysis. The maximum band gap reduction of pure BiNbO4 is possible with the (C-S) co-doping and minimum with N mono-doping. The calculated binding energies show that the co-doped systems are more stable than their mono-doped counterparts. Our optical absorption curves indicate that the mono- (C) and co-anionic doped (N-N and C-S) BiNbO4 systems are promising materials for visible light photocatalysis.

Semiconducting allotrope of graphene

From first-principles calculations, we predict a planar stable graphene allotrope composed of a periodic array of tetragonal and octagonal (4, 8) carbon rings. The stability of this sheet is predicted from the room-temperature molecular dynamics study and the electronic structure is studied using state-of-the-art calculations such as the hybrid density functional and the GW approach. Moreover, the mechanical properties of (4, 8) carbon sheet are evaluated from the Youngs modulus and intrinsic strength calculations. We find this is a stable planar semiconducting carbon sheet with a bandgap between 0.43 and 1.01 eV and whose mechanical properties are as good as graphenes.

Cationic–anionic mediated charge compensation on La2Ti2O7 for visible light photocatalysis

The cationic-anionic mediated charge compensation effect was studied in the layered perovskite La2Ti2O7 for the visible light photocatalysis. Our screened hybrid density functional study shows that the electronic structure of La2Ti2O7 can be tuned by the cationic (V, Nb, Ta)/anionic (N) mono- and co-doping. Such mono-doping creates impurity states in the band gap which helps the electron-hole recombination. But if the charge compensation is made by the cationic-anionic mediated co-doping then such impurity states can be removed and can be a promising strategy for visible light photocatalysis. The absolute band edge position of the doped La2Ti2O7 has been aligned with respect to the water oxidation/reduction potential. The calculated defect formation energy shows the stability of the co-doping system is improved due to the coulomb interactions and charge compensations effect.

Screened hybrid density functional study on Sr2Nb2O7 for visible light photocatalysis

The electronic structure of pure Sr2Nb2O7 and its electronic band position are being aligned with respect to the water oxidation/reduction potential level using hybrid functional (HSE06) theory. The experimental band gap (3.90 eV) of pure Sr2Nb2O7 can be reproduced (3.92 eV) using this level of theory. The cationic-anionic co-doping (Mo-N) in layered perovskite Sr2Nb2O7 structure reduces the band gap significantly, and its electronic band position is excellent for the visible-light photocatalysis. The respective cationic and anionic mono-doped systems create an occupied or unoccupied impurity states in the band gap, which can reduce the efficiency of the photocatalysis.

Hole mediated coupling in Sr2Nb2O7 for visible light photocatalysis

The band gap reduction and effective utilization of visible solar light are possible by introducing the anionic hole-hole mediated coupling in Sr(2)Nb(2)O(7). By using the first principles calculations, we have investigated the mono- and co-anionic doping (S, N and C) in layered perovskite Sr(2)Nb(2)O(7) for the visible-light photocatalysis. Our electronic structure and optical absorption study shows that the mono- (N and S) and co-anionic doped (N-N and C-S) Sr(2)Nb(2)O(7) systems are promising materials for the visible light photocatalysis. The calculated binding energies show that if the hole-hole mediated coupling could be introduced, the co-doped systems would be more stable than their respective mono-doped systems. Optical absorption curves indicate that doping S, (N-N) and (C-S) in Sr(2)Nb(2)O(7) can harvest a longer wavelength of the visible light spectrum as compared to the pure Sr(2)Nb(2)O(7) for efficient photocatalysis.

Molecular simulation for gas adsorption at NiO (100) surface.

Density functional theory (DFT) calculations have been employed to explore the gas-sensing mechanisms of NiO (100) surface on the basis of energetic and electronic properties. We have calculated the adsorption energies of NO(2), H(2)S, and NH(3) molecules on NiO (100) surface using GGA+U method. The calculated results suggest that the interaction of NO(2) molecule with NiO surface becomes stronger and contributes more extra peaks within the band gap as the coverage increases. The band gap of H(2)S-adsorbed systems decrease with the increase in coverage up to 0.5 ML and the band gap does not change at 1 ML because H(2)S molecules are repelled from the surface. In case of NH(3) molecular adsorption, the adsorption energy has been increased with the increase in coverage and the band gap is directly related to the adsorption energy. Charge transfer mechanism between the gas molecule and the NiO surface has been illustrated by the Bader analysis and plotting isosurface charge distribution. It is also found that that work function of the surfaces shows different behavior with different adsorbed gases and their coverage. The work function of NO(2) gas adsorption has a hill-shaped behavior, whereas H(2)S adsorption has a valley-shaped behavior. The work function of NH(3) adsorption decreases with the increase in coverage. On the basis of our calculations, we can have a better understanding of the gas-sensing mechanism of NiO (100) surface toward NO(2), H(2)S, and NH(3) gases.

Water Interaction with native defects on rutile TiO2 nanowire: Ab initio calculations

Adsorption of water molecules on stoichiometric and defective surfaces of rutile TiO2 nanowire oriented along the [1¯10] direction is investigated using density function theory calculations. We have investigated, in particular, O and Ti vacancies where energetic, structural, and electronic properties were evaluated. It was found that the water molecules interacting with O-vacancy undergo spontaneous dissociation, forming hydroxyl groups bound to Ti atoms and other OH groups formed by surface O and H-water. The same is not found in the case of perfect and Ti-vacancy containing nanowire. This dissociation of water molecules is due to charge transfer from neighboring Ti atom, which is polarized due to the O-vacancy.