W.S. Abdel Halim

The stability of peroxide ion O22− at (110), (210) and (001) surfaces of MgO, CaO and SrO periodic ab initio calculations

The stability of peroxide ion O22− resulting from the interaction between an adsorbed oxygen O and surface oxygen O2− at the (110), (210) and (001) five layer surface films of the alkaline earth oxides MgO, CaO and SrO has been investigated using periodic density functional theory calculations. The 1:1 supercell model in which two incoming oxygens were added per each supercell, one at each side of the film, was employed in the calculations. The incoming oxygen/solid interaction energies exhibit exothermic character at the three considered films. The interaction energies at the fully relaxed surfaces increase monotonically from MgO to CaO to SrO, mainly from (110) to (210) to (001) planes, and are explainable in terms of the basicities and ionization potentials of the cations as well as the acceptor property of the incoming oxygen. Based on charge electron density maps, electrostatic potential contours and Mulliken population analysis the results confirm: (i) a genuine charge transfer from the surface to the adsorbed oxygen, (ii) the overlap population between the adsorbed oxygen and the surface oxygen is much larger than that between two oxygens in the crystal bulk, (iii) the overlap population between adsorbed oxygen and surface oxygen increases from MgO to CaO to SrO, being consistent with predicted order of stability of the peroxy bond.

Theoretical characterisation of highly efficient dye-sensitised solar cells

Molecular electronic structure calculations, employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methodologies, have been carried out to improve the performance of the synthesised dye YD2-o-C8 which is characterised by 11.9%–12.7% efficiencies. We aimed to narrow the band gap of YD2-o-C8 to extend the light-harvesting region to near-infrared (NIR). This was done by incorporating Cd instead of Zn onto the porphyrin ring and elongating the length of π-conjugation by adding ethynylene link and anthracene unit, so that the performances of the suggested cells could be expected to exceed the 11.9%–12.7% efficiencies with TiO2, ZnO2, and WO3 oxide electrodes. The effects of modifying the central metal and elongating the length of π-conjugation on cell performance are confirmed in terms of frontier molecular orbital (FMO) energy gaps, density of states (DOS), molecular electrostatic potentials (MEPs), non-linear optical (NLO) properties, ultraviolet–visible (UV–vis) electronic absorption, and 1H nuclear magnetic resonance chemical shifts. Increasing the length of π-conjugation of the D–π–A dyes leads to increasing the DOS near Fermi levels, more active NLO performance, strong response to the external electric field, delocalisation of the negative charges near the anchoring groups, deep electron injection, suppressing macrocycle aggregation, active dye regeneration, and inhibited dye recombination. The calculated band gap/eV of the present DMP-Zn is correlated with the experimental (E1/2(oxidation)–E1/2(reduction)/V) potentials of the identical YD2-o-C8. A co-sensitiser is suggested for NIR sensitisation (550–950 nm) to increase the power-to-conversion efficiency beyond 14%.

Tuning hydrogen storage of carbon nanotubes by mechanical bending: theoretical study

Abstract The effects of mechanical bending on tuning the hydrogen storage of titanium functionalised (4,0) carbon nanotube have been assessed using density functional theory calculations with reference to the ultimate targets of the US Department of Energy (DOE). The assessment has been carried out in terms of physisorption, gravimetric capacity, projected densities of states, statistical thermodynamic stability and reaction kinetics. The Ti atom binds at the hollow site of the hexagonal ring. The average adsorption energies (−0.54 eV) per hydrogen molecule meet the DOE target for physisorption (−0.20 to −0.60 eV). The curvature attributed to the bending angle has no effect on the average adsorption energies per H2 molecule. With no metal clustering, the system gravimetric capacities are expected to be as large as 9.0 wt%. The reactions of the deformed (bent) carbon nanotube have higher probabilities of occurring than those of the un-deformed carbon nanotube. The Gibbs free energies, enthalpies and entropies meet the ultimate targets of the DOE for all temperatures and pressures. The closest reactions to zero free energy occur at (378.15 K/2.961 atm.) and reverse at (340 and 360 K/1 atm.). The translational component is found to exact a dominant effect on the total entropy change with temperature. Favourable kinetics of the reactions at the temperatures targeted by DOE are reported regardless of the applied pressure. The more preferable thermodynamic properties assigned to the bending nanotube imply that hydrogen storage can be improved compared to the nonbending nanotube.

BOND FUNCTIONS AND CORE CORRELATION ENERGY CONTRIBUTIONS TO HeBe POTENTIAL

An empirical scheme for implementation of bond functions in heteronuclear diatomics is suggested and applied to HeBe using universal even-tempered functions. The effects of bond functions and core-correlation energy on the interaction potential of HeBe calculated at the uncorrelated (SCF) and correlated (MBPT and CC) levels are examined. The results confirm that an accuracy of sub μ Hartree level can be obtained using even-tempered functions with s-, p-, and d- symmetry and that bond functions of size {4s2p} for He and {6s3p} for Be recovers 100% of energy lowering obtained from the addition of 10d atom-centered functions to He and 13d atom centred functions to Be. The various treatments of the electron correlation, conclude that the system is interacting weakly with a well depth from 14.5–24.7 μEh at a separation near 9.1a0 compared with 20.7–25.5 μEh previously reported with a rather limited basis set. The most reliable well depth corrected for BSSE (19.0 μEh) was obtained at the CC-SD(T)level at separation of 8.71a0 taking into account the effects of bond functions and core correlation energy. Potential energy curves at the CC-SD(T) valence and CC-SD(T) valence + core correlation levels are analyzed in analytical forms in terms of exchange repulsion, induction and dispersion components.