S. Abdel Aal

The effect of ruthenium on the performance of porphyrin dye and porphyrin–fullerene dyad solar cells predicted by DFT and TD-DFT calculations

The effect of ruthenium on the performance of porphyrin dye and porphyrin–fullerene (PF) dyad solar cells is investigated by using density functional theory and time-dependant density functional theory calculations. The results reveal that ruthenium facilitates rapid electron injection from porphyrin to fullerene, narrows the band gaps of porphyrin dye and PF dyad and alters the density of states near the corresponding Fermi levels. The HOMOs are localised on the donor moieties and the LUMOs on the acceptor moieties. The donor and acceptor dyads form good donor–acceptor pairs for photo-to-current conversion under the effect of ruthenium. HOMOs of porphyrin and ruthenium metalloporphyrin dyes fall within the (TiO2)60 and Ti38O76 gaps, and support the issue of typical interfacial electron transfer reaction. The calculated transition energies of porphyrin are almost insensitive to ethanol solvent effects. The introduction of ruthenium to the porphyrin ring leads to more active nonlinear optical performance, stronger response to the external electric field and induces higher photo-to-current conversion efficiency. Moreover, ruthenium shifts the absorption bands of porphyrin and makes it a potential candidate for harvesting light for photovoltaic applications.

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.

Spin transition energies of Cr in complexes and CO binding with Cr deposited on S2− and Se2− anion impurities of MgO (001) surface density functional theory calculations

An attempt has been made to analyze the spin transition energies of Cr, a representative example of transition metals, in a variety of complexes formed at regular (001) surfaces of MgO, as well as the adsorption of CO by means of hybrid density functional theory calculations and embedded cluster models. Clusters of moderate sizes are embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. While the spin states of Cr are reduced and preserved in all complexes, be they defect-free or-defect containing, the combined effects of the adsorbate and the substrate in the defect-free OC.Cr.Mg9O13O2− complex were strong enough to favor the low-spin state and to quench the spin. The deposited Cr atoms enhance the adsorption of CO. The significant weakening of bond strength between OC and Cr in complexes supports the concept of bond order conservation. The natural bond orbital (NBO) analysis reveals that the electronic structure of the adsorbed metal represents a qualitative change with respect to that of the free metal. The effects of spin contamination on the geometry, Mulliken charges and adsorption energy are examined. The binding of CO precursor is dominated by the E (i)Cr.CO pairwise additive components, and the role of the support was not restricted to supporting the metal. Relations are established between the process of spin transition and the energy gaps between frontier orbitals. The results show that the spin state of adsorbed metal atoms on oxide supports and the role of precursor molecules in the properties of spin transition energies of Cr in complexes need to be explicitly taken into account.