Pornjuk Srepusharawoot

Li-decorated metal-organic framework 5 : A route to achieving a suitable hydrogen storage medium

A significant improvement in molecular hydrogen uptake properties is revealed by our ab initio calculations for Li-decorated metal–organic framework 5. We have found that two Li atoms are strongly adsorbed on the surfaces of the six-carbon rings, one on each side, carrying a charge of +0.9e per Li atom. Each Li can cluster three H2 molecules around itself with a binding energy of 12 kJ (mol H2)−1. Furthermore, we show from ab initio molecular dynamics simulations with a hydrogen loading of 18 H2 per formula unit that a hydrogen uptake of 2.9 wt % at 200 K and 2.0 wt % at 300 K is achievable. To our knowledge, this is the highest hydrogen storage capacity reported for metal–organic framework 5 under such thermodynamic conditions.

A comparative investigation of H2 adsorption strength in Cd- and Zn-based metal organic framework-5

Hydrogen binding energies for the primary and secondary adsorption sites in the Cd- and Zn-based metal organic framework-5 (MOF-5) were studied using density functional theory. Out of the three exchange-correlation functionals employed in our study, we find that the local density approximation yields a qualitatively correct description of the interaction strengths of H(2) in MOF-5 systems. The H(2) adsorption energies for all trapping sites in Zn- and Cd-based MOF-5 are seen to be of the same order of magnitude but with a generally stronger binding in Cd-based MOF-5 as compared to Zn-based MOF-5. In particular, the H(2) binding energy at the secondary adsorption sites in Cd-based MOF-5 is increased by around 25% compared to Zn-based MOF-5. This result suggests that Cd-based MOF-5 would be better suited to store hydrogen at higher temperatures than Zn-based MOF-5.

Origin(s) of the apparent colossal permittivity in (In1/2Nb1/2)xTi1−xO2: clarification on the strongly induced Maxwell–Wagner polarization relaxation by DC bias

The effects of DC bias on the dielectric and electrical properties of co-doped (In1/2Nb1/2)xTi1−xO2 (IN-T), where x = 0.05 and 0.1, and single-doped Ti0.975Nb0.025O2 ceramics are investigated. The low-frequency dielectric permittivity (e′) and loss tangent of IN-T ceramics with x = 0.05 and 0.1 are greatly enhanced by applying a DC bias at 40 and 20 V, respectively, whereas the relatively high-frequency e′ remains unchanged. The induced low-frequency Maxwell–Wagner polarization completely vanishes by immediately applying no DC bias. After overload limited measurement, this polarization permanently emerges without DC bias, whereas the primary polarization remains unchanged. Using combined Z′′ and M′′ spectroscopic plots, it is found that the strongly induced-polarizations are contributed from the combination effects of the sample–electrode contact and resistive outer surface. Very high performance of the colossal permittivity in IN-T ceramics is attributed to the formation of a resistive outer-surface layer and insulating grain boundaries. These results not only provide important insights into the origins of the colossal dielectric response in the IN-T ceramic system, but are also important for deciding the doping conditions of TiO2-based materials for practical applications.

Effects of Ge substitution on the structural and physical properties of CuFeO2 delafossite oxide

Delafossite CuFe1?xGexO2 (0.0 ? x ? 0.1) semiconductors were synthesized by solid-state reaction. The effects of Ge concentration on microstructural, optical, magnetic and electrical properties were investigated. X-ray diffraction (XRD) analysis results reveal the delafossite structure of all the samples. The lattice spacing of CuFe1?xGexO2 decreased with increasing substitution of Ge at the Fe site. The optical properties measured at room temperature by UV?visible spectroscopy showed an absorption peak at 283 nm (4.38 eV). The corresponding direct optical band gap was found to decrease with increasing Ge content (from 3.69 eV for x = 0 to 3.61 eV for x = 0.10), exhibiting transparency in the visible region. The magnetic hysteresis loops measured at room temperature showed that the Ge-doped CuFeO2 samples exhibit ferromagnetic behavior. The Curie temperature suggests that ferromagnetism originates from CuFe1?xGexO2 matrices. The substitution of Fe3+ by Ge4+ produces a mixed effect on the magnetic properties of CuFeO2 delafossite oxide. The resistivity of CuFe0.99Ge0.01O2 was observed to be ?0.1 ??cm at room temperature.

Density Functional Theory Study of Hydrogen Adsorption in a Ti-Decorated Mg-Based Metal-Organic Framework-74

The Ti-binding energy and hydrogen adsorption energy of a Ti-decorated Mg-based metal-organic framework-74 (Mg-MOF-74) were evaluated by using first-principles calculations. Our results revealed that only three Ti adsorption sites were found to be stable. The adsorption site near the metal oxide unit is the most stable. To investigate the hydrogen-adsorption properties of Ti-functionalized Mg-MOF-74, the hydrogen-binding energy was determined. For the most stable Ti adsorption site, we found that the hydrogen adsorption energy ranged from 0.26 to 0.48 eV H2 (-1) . This is within the desirable range for practical hydrogen-storage applications. Moreover, the hydrogen capacity was determined by using ab initio molecular dynamics simulations. Our results revealed that the hydrogen uptake by Ti-decorated Mg-MOF-74 at temperatures of 77, 150, and 298 K and ambient pressure were 1.81, 1.74, and 1.29 H2  wt %, respectively.

Electronic Properties of h-BCN/Blue Phosphorene van der Waals Heterostructures

Van der Waals heterostructures, a new class of materials made of a vertically selective assembly of various 2D monolayers held together by van der Waals forces, have attracted a great deal of attention due to their promise to deliver novel electronic and optoelectronic properties that are not achievable by using individual 2D crystals. Using density functional theory (DFT), it is revealed that van der Waals heterostructures composed of monolayers of hexagonal boron nitride (h-BN) and the latest P allotrope blue phosphorus (blue phosphorene, BlueP) forms a straddling type I band offset for which the band edges exclusively belong to BlueP. This feature enables h-BN to act as a protective coating material to resolve the air instability of BlueP. Furthermore, substitutional doping of C into h-BN (h-BCN) at a suitable concentration induces h-BCN-BlueP into staggered type II band offset. The type II band alignment triggered by the intensified built-in electric field across the sheets implies improved carrier mobility and the suppressed recombination of photogenerated hole pairs. These major benefits can pave the way for the potential functionality of h-BCN-BlueP to be exploited for efficient photovoltaic devices.

Phase stability and superconductivity of strontium under pressure

We have used the ab initio random structure searching method together with density functional theory calculations to find stable structures of strontium under pressures up to 50 GPa. We predict a sequence of structural phase transitions and the stability of an orthorhombic structure of Cmcm symmetry above 25 GPa. Our energy, lattice dynamics, and molecular dynamics calculations confirm the stability of the Cmcm structure. The electron-phonon coupling calculations show that superconductivity arises in the bcc structure of compressed Sr and that it continues to exist in the Cmcm structure. The calculated superconducting transition temperatures are in good agreement with experiment. Our study gives an excellent account of the experimental observations.

Hydrogen adsorption of Be-, Zn-, and Cd-zeolitic imidazolate framework-23: A comparative study

Hydrogen adsorption energies of Be-, Zn-, and Cd-zeolitic imidazolate framework-23 were investigated using the Van der Waals density functional theory implemented in the Quantum ESPRESSO program. From the structural parameters of these systems, we found that the imidazole ligand is unchanged and only the tetrahedral metal nitride cluster is varied. Moreover, our results revealed that the Cd-zeolitic imidazolate framework-23 has the highest electric dipole moment followed by Zn- and Be-zeolitic imidazolate framework-23. On the basis of hydrogen adsorption energy calculations, we found that hydrogen molecules cannot bind with either Be- or Zn-zeolitic imidazolate framework-23 at any of the considered adsorption sites, whereas hydrogen molecules can be trapped on the Cd-zeolitic imidazolate framework-23 with the hydrogen binding energy in the range from 60 to 130 meV. This result suggested that replacing Zn in the metal nitride cluster with a larger metal can enhance the hydrogen adsorption energy of zeolitic imidazolate framework-23.