Swapan K. Ghosh

Improving photocatalytic properties of SrTiO3 through (Sb, N) codoping: a hybrid density functional study

A systematic study using hybrid density functional theory has been carried out to investigate the synergistic effect of Sb and N doping on the photocatalytic properties of SrTiO3 under visible light. The calculated band gap (3.19 eV) for SrTiO3 with the Heyd, Scuseria, and Ernzerhof hybrid functional is found to be very close to the experimentally observed value of 3.2 eV. Although doping with N is able to enhance the visible light activity by reducing the effective band gap to 2.31 eV, the localized occupied and unoccupied states in the forbidden region may affect the photocatalytic activity. However, the presence of Sb not only passivates those unoccupied states completely, but also shifts the localized occupied states near the valence band to form a continuum band structure. The introduction of N into the SrTiO3 crystal structure is favored by the presence of Sb. In the codoped system charge compensation is established, thereby unwanted vacancy formation will be minimized. The absorption curve for the (Sb, N)-codoped SrTiO3 is found to shift towards the visible region due to reduction in band gap to 2.66 eV. Moreover, the band alignment shows that the (Sb, N)-codoping makes SrTiO3 thermodynamically more suitable for hydrogen production as compared to the undoped system. Based on the present study, we can propose (Sb, N)-codoping as one of the effective approaches to improve the photocatalytic activity of SrTiO3 for water splitting under visible light irradiation.

Improving KNbO3 photocatalytic activity under visible light

An increasing number of photocatalytic applications of KNbO3 in different fields motivated us to find an efficient strategy to reduce its band gap so that it can utilize the solar spectrum. Using density functional theory (DFT) with a hybrid functional proposed by Heyd, Scuseria, and Ernzerhof, the experimental band gap (3.24 eV) of KNbO3 was successfully reproduced (3.23 eV). In the present study, we systematically investigated the effect of doping with N and W on the geometry and electronic structure of KNbO3. Because of the closeness of the ionic radius small changes in the parent crystal structure occurs. However, the electronic structure showed major changes in both the cases. N introduces impurity states adjacent to the top of the valence band and the bottom of the conduction band (CB), thus reducing the band gap significantly. Doping with W results in an n-type semiconductor, and introduces occupied states adjacent to the CB. Although both the dopant elements can improve the visible light absorption, it may accelerate electron–hole recombination. Therefore, individually they may not be able to improve the photocatalytic activity of KNbO3. Interestingly, a highly favourable band structure was produced with a reduced band gap when both N and W are simultaneously doped into the crystal structure of KNbO3. The calculated formation energy indicates that the doping of N becomes more feasible in the presence of W. This may be due to the formation of a charge compensated system, which also reduces the vacancy defect formation. More importantly, the band edge shifting in the presence of both N and W occurs in such a controlled fashion that KNbO3 still remains suitable for overall water splitting. Therefore, one can justify the choice of the (N, W) pair for improving the visible light driven photoactivity of KNbO3.

Improving the photocatalytic activity of s-triazine based graphitic carbon nitride through metal decoration: an ab initio investigation

Graphitic carbon nitride based semiconductor materials are found to be potential photocatalysts for generating hydrogen through solar water splitting. Through more accurate hybrid density functional theory calculations, we attempted to tune the electronic band structure of poly s-triazine based graphitic carbon nitride by decorating it with different metal atoms and clusters for improving its visible light absorption efficiency. For deposition on the two-dimensional carbon nitride surface, a range of metals have been considered which include all the 3d transition metals and the noble metals (Ag, Au, Pt and Pd). Our study reveals that though the band gaps of all the metal decorated systems were less than that of pristine carbon nitride, in most of the cases, metal decoration leads to the formation of mid gap impurity states, which can hinder the mobility of charge carriers. However, in the case of Ag and its four atom cluster deposited systems, no mid gap states were observed. In all the metal decorated systems, the measured band edge potentials were also found to satisfy the thermodynamic criterion for overall water splitting. The calculated optical absorption spectra show a shift in the absorption band towards the visible region upon metal decoration. Our results indicate that among all the considered metal atoms silver is the preferred candidate for deposition on the carbon nitride surface for improved photocatalytic activity.

DNA and protein binding, double-strand DNA cleavage and cytotoxicity of mixed ligand copper(II) complexes of the antibacterial drug nalidixic acid

The water soluble mixed ligand complexes [Cu(nal)(diimine)(H2O)](ClO4) 1-4, where H(nal) is nalidixic acid and diimine is 2,2-bipyridine (1), 1,10-phenanthroline (2), 5,6-dimethyl-1,10-phenanthroline (3), and 3,4,7,8-tetramethyl-1,10-phenanthroline (4), have been isolated. The coordination geometry around Cu(II) in 1 and that in the Density Functional Theory optimized structures of 1-4 has been assessed as square pyramidal. The trend in DNA binding constants (Kb) determined using absorption spectral titration (Kb: 1, 0.79±0.1<2, 1.06±0.1<3, 1.79±0.2<4, 1.84±0.2×105M-1) is in line with that (Kapp) determined by competitive ethidium bromide binding studies. The large red-shift (10nm) observed for 2 suggests that the phen co-ligand is stacked with a frayed DNA base pair. In contrast, 3 and 4 are involved in intimate hydrophobic interaction with DNA through the methyl substituents on phen ring, which is supported by viscosity and protein binding studies. DNA docking studies imply that 4 is involved preferentially in DNA major groove binding while 1-3 in minor groove binding and that all the complexes, upon removing the axially coordinated water molecule, bind in the major groove. Interestingly, 3 and 4 display prominent double-strand DNA cleavage while 1 and 2 effect only single-strand DNA cleavage in the absence of an activator. The complexes 3 and 4 show cytotoxicity higher than 1 and 2 against human breast cancer cell lines (MCF-7). The complex 4 induces apoptotic mode of cell death in cancer cells.

Hydrogen adsorption in lithium decorated conjugated microporous polymers: a DFT investigation

Two-dimensional carbon materials like graphene, porous graphene, covalent organic frameworks, graphyne and graphdiyne etc. decorated with light metals are found to be one of the prospective candidates as effective hydrogen storage materials. Here, we report an investigation of expanded porous two-dimensional models of the conjugated microporous polymers based on benzene, 1,3,5-triethynyl (CMP-1) and benzene, 1,3,5-tributadiyne (HCMP-1) decorated with lithium metal as efficient hydrogen adsorption materials. The modelled CMP-1 and HCMP-1 based networks are found to have band gaps of 3.5 and 4.0 eV respectively. These materials are found to hold lithium atoms above and below the C6 rings with a binding energy of around −44.9 kcal mol−1, which is more than the cohesive energy of lithium. Each lithium site is found to carry partial positive charge which can bind molecular hydrogen through ion-induced dipole interactions. The calculated adsorption energy per molecular hydrogen is found to be around −3.0 kcal mol−1 which is reasonably good. With three hydrogen molecules adsorbed per metal site, the CMP-1 network can adsorb hydrogen with a gravimetric density of 8.76 wt% and the corresponding density in HCMP-1 based material is found to be 7.06 wt% at 0 K.

Computational investigation of hydrogen adsorption in silicon-lithium binary clusters #

AbstractTheoretical studies on hydrogen adsorption properties of silicon-lithium binary clusters are carried out. We have considered three different clusters viz., Si5Li

Towards an exact factorization of the molecular wave function

_{5}^{-}

Density functional theory of vapor to liquid heterogeneous nucleation: Lennard–Jones fluid on solid substrate

, Si5Li6 and Si5Li

Density functional theory of surface tension of real fluids using a double well type Helmholtz free energy functional: application to water and heavy water

_{7}^{+}

Exploring triazine and heptazine based self assembled molecular materials through first principles investigations

and for each cluster, the geometries of different possible isomers are optimized. In all the minimum energy isomers of the three clusters considered, two of the lithium atoms are found to be situated in the axial positions and the remaining lithium atoms are in the equatorial position in the Si5 plane. The lithium atoms which are in Si5 plane are bonded to the Si-Si edge through a bridged bond instead of a corner in the Si5 ring. From the calculated atomic charges, it is found that there is a charge transfer from lithium to silicon leaving a partial positive charge on the Li atoms and the axial lithium atoms are more charged as compared to the remaining lithium atoms. In the case of Si5Li6 and Si5Li