Brindaban Modak

Exploring the Role of La Codoping beyond Charge Compensation for Enhanced Hydrogen Evolution by Rh-SrTiO3.

In this theoretical study, we investigate recent observation of enhancement of hydrogen evolution efficiency of Rh-doped SrTiO3 due to codoping with La at the Sr lattice site. Using hybrid density functional theory, we have systematically studied the electronic structure of (Rh, La)-codoped SrTiO3 and compared with that of Rh-doped SrTiO3, La-doped SrTiO3, and undoped SrTiO3. The aim of the present study has been to explore the role of different factors toward the observed enhanced photoactivity of (Rh, La)-codoped SrTiO3. Doping with only Rh significantly reduces the photoabsorption energy by introducing localized acceptor states between the valence band and conduction band. Unfortunately, these states act as efficient sources for charge carrier trapping. Besides, the oxygen vacancy found to be present in the Rh-doped SrTiO3 as a charge compensating defect also accelerates the electron-hole recombination rate. We have shown that codoping with La and Rh leads to the formation of clean band structure without encountering any midgap states. Introduction of La into the Rh-doped SrTiO3 not only reduces the quantity of Rh(4+) species but also suppresses the oxygen vacancy due to formation of a charge-compensated system. The presence of La favors Rh doping into the crystal structure of SrTiO3 by reducing the formation energy. Moreover, the conduction band minima are found to be shifted in the upward direction significantly due to codoping with Rh and La, thereby increasing the reducing behavior at the conduction band. This leads to enhancement of hydrogen evolution activity of SrTiO3 during photocatalytic water splitting under visible light.

Effect of ionic size on the structure of spherical double layers: a Monte Carlo simulation and density functional theory study

The effect of ionic size on the diffuse layer characteristics of a spherical double layer is studied using Monte Carlo simulation and density functional theory within the restricted primitive model. The macroion is modelled as an impenetrable charged hard sphere carrying a uniform surface charge density, surrounded by the small ions represented as charged hard spheres and the solvent is taken as a dielectric continuum. The density functional theory uses a partially perturbative scheme, where the hard sphere contribution to the one particle correlation function is evaluated using weighted density approximation and the ionic interactions are calculated using a second-order functional Taylor expansion with respect to a bulk electrolyte. The Monte Carlo simulations have been performed in the canonical ensemble. The detailed comparison is made in terms of zeta potentials for a wide range of physical conditions including different ionic diameters. The zeta potentials show a maximum or a minimum with respect to the polyion surface charge density for a divalent counterion. The ionic distribution profiles show considerable variations with the concentration of the electrolyte, the valency of the ions constituting the electrolyte, and the ionic size. This model study shows clear manipulations of ionic size and charge correlations in dictating the overall structure of the diffuse layer.

Structure of colloidal solution in presence of mixed electrolytes: a solvent restricted primitive model study.

The structure of colloidal solution in presence of mixed electrolytes is studied using Monte Carlo simulation and density functional theory, based on a four-component model of the spherical double layer. In this model the ions and solvent molecules are treated as charged and neutral hard spheres, respectively, having equal diameter, and in addition the mixture of mono- and multivalent co-ions are also considered. The macroion is considered as a uniformly charged hard sphere surrounded by the electrolyte and the solvent. The density functional theory is based on a partially perturbative scheme, where the electrical part is calculated through perturbation with respect to uniform density and the hard sphere contribution is approximated using a weighted density approach. The theory is found to be in quantitative agreement with the Monte Carlo simulation results, for singlet density as well as the mean electrostatic potential profiles. The system is studied over a wide range of parametric conditions, viz. with different ionic valences as well as size, at varying electrolyte concentration ratio of mono- and multivalent co-ions of mixed electrolytes, at different surface charge densities, and radius of the macroion. The present work reflects that even a simple primitive model for the solvent is able to manipulate the hard-sphere and electrostatic correlations of the diffuse double layer in the ionic density as well as mean electrostatic potential profiles.

Improving visible light photocatalytic activity of NaNbO3: a DFT based investigation

Depleting sources of fossil fuels and their adverse environmental impact drive global interest to find efficient materials for hydrogen generation through solar water splitting. Although, NaNbO3 has several key features as an efficient photocatalyst, its large band gap restricts its photoactivity only in the UV-region of the solar spectrum. In this theoretical study, we investigate the effect of doping on the electronic structure of NaNbO3 aiming at improving its visible light photocatalytic activity. For this purpose, we employ hybrid density functional theory (DFT), which successfully reproduces the experimental band gap of NaNbO3. Doping with N has been found to reduce the effective band gap significantly by introducing localized acceptor states which however are known to promote the electron hole recombination rate. To overcome this, we propose codoping with W at the Nb lattice site. Interestingly this completely passivates those localized acceptor states. The band gap is found to be sufficiently reduced to enhance the visible light activity. The present strategy reduces the band gap in such a controlled way that (W, N)-NaNbO3 satisfies the thermodynamic criteria to execute the overall decomposition of H2O, indicated in the relative position of its band edges with respect to water redox levels. Moreover, doping of N is found to be facilitated in the presence of W due to reduction in formation energy. Additionally, spontaneous formation of charge compensated valence defects is expected to be reduced in the presence of the (W, N) pair in comparison to individual dopant elements due to maintaining the total electrical charge neutrality.

Zeta potential of colloidal particle in solvent primitive model electrolyte solution: a density functional theory study

A systematic study of zeta potential for a spherical double layer (SDL) around a colloidal particle in electrolyte solutions, is performed using density functional theory and Monte Carlo simulation. The usual recipe under the solvent primitive model is employed to model the system, where macroion, counterions, and coions are represented by charged hard spheres of uniform charge density and the presence of solvent is taken into account by modelling it as neutral hard spheres. All the components of the system are embedded in a dielectric continuum in order to consider the electrostatic effect of the solvent. The density functional theory employs a suitable weighted density approximation to calculate the hard-sphere contribution, whereas the residual electrostatic interactions are calculated as a small perturbation around the uniform fluid. The zeta potential profiles of a SDL in the presence of a number of electrolytes have been calculated and are found to be considerably influenced in the presence of solvent with an increase in the concentration of the electrolyte. The theory successfully predicts the maxima and sign reversal of the zeta potential profiles at high macroion surface charge density and in the presence of multivalent counterions, as obtained from the Monte Carlo simulation.

Solvent primitive model study of structure of colloidal solution in highly charge asymmetric electrolytes

Monte Carlo simulation and density functional theory have been applied to study the structure of colloidal solution in presence of highly charge-asymmetric electrolytes. The calculations have been performed based on solvent primitive model for a spherical double layer. The macroion is considered as a uniformly charged large spherical hard sphere. Small ions and solvent molecules are represented by equally sized charged and neutral hard spheres, respectively. The density profiles of ionic components and solvent molecules and mean electrostatic potential profile of the system for different electrolytes under a wide variety of conditions are found to satisfactorily reproduce a number of enriched structural features.