K. A. Soliman

Theoretical characterisation of irreversible and reversible hydrogen storage reactions on Ni-doped C60 fullerene

An attempt has been made to characterise the irreversible and reversible hydrogen storage reactions on Ni-doped C60 fullerene by using the state of the art density functional theory calculations. The single Ni atom prefers to bind at the bridge site between two hexagonal rings of C60 fullerene, and can bind up to four hydrogen molecules with average adsorption energies of −0.85, −0.83, −0.58, and −0.31 eV per hydrogen molecule. No evidence for metal clustering in the ideal circumstances and the hydrogen storage capacity is expected to be as large as 8.9 wt%. While the desorption activation barriers of the complexes nH2NiC60 (n = 1, 2) are outside the desirable energy window recommended by the department of energy for practical applications (–0.2 to –0.6 eV), the desorption activation barriers of the complexes nH2NiC60 (n = 3, 4) are inside this window. The irreversible 2H2 + NiC60 and reversible 3H2 + NiC60 interactions are characterised in terms of several theoretical parameters such as: (1) densities of states and projected densities of states, (2) pairwise and non-pairwise additivity, (3) infrared, Raman, and proton magnetic resonance spectra, (4) electrophilicity, and (5) statistical thermodynamic stability.

Adsorption and spin state properties of Cr, Ni, Mo, and Pt deposited on Li + and Na + monovalent cation impurities of MgO (001) surface: DFT calculations

We have analyzed, by means of density functional theory calculations and the embedded cluster model, the adsorption and spin-state properties of Cr, Ni, Mo, and Pt deposited on a MgO crystal. We considered deposition at the Mg2+ site of a defect-free surface and at Li+ and Na+ sites of impurity-containing surfaces. To avoid artificial polarization effects, clusters of moderate sizes with no border anions were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The interaction between a transition metal atom and a surface results from a competition between Hunds rule for the adsorbed atom and the formation of a chemical bond at the interface. We found that the adsorption energies of the metal atoms are significantly enhanced by the cation impurities, and the adsorption energies of the low-spin states of spin-quenched complexes are always more favorable than those of the high-spin states. Spin polarization effects tend to preserve the spin states of the adsorbed atoms relative to those of the isolated atoms. The metal–support interactions stabilize the low-spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. Spin quenching occurs for Cr and Mo complexes at the Mg2+ site of the pure surface and at Li+ and Na+ sites of the impurity-containing surfaces. Variations of the spin-state properties of free metals and of the adsorption and spin-state properties of metal complexes are correlated with the energies of the frontier orbitals. The electrostatic potential energy curves provide further understanding of the nature of the examined properties.

Magnetic and binding properties of Co-doped single-walled carbon nanotubes: a first principles study

An attempt has been made to analyze the magnetic-spin quenching property of Co, as a representative of transition metals, in Co-doped single-walled carbon nanotubes (SWCNTs) as well as the binding property of CO with the side walls of the Co-doped SWCNTs by means of hybrid density functional theory (DFT) calculations. Four different types of SWCNTs are considered: semi-conducting (5,0) zigzag, metallic (5,2) and semi conducting (5,3) chirals, and metallic (5,5) armchair. The results show that while the spin states of Co in the whole of the present Co-doped SWCNTs were preserved, the combined effects of adsorbate (CO) and substrate (Co-doped SWCNT) were strong enough to favor the low-spin states, and to quench the spins in the Co-doped SWCNTs (5,0) and (5,2). The doped Co atom converts the endothermic reactions of CO molecules on the outer surfaces of the pure SWCNTs into exothermic reactions. The nature of charge transfer between the d-orbitals of Co, and the π* orbital of the nearby C of CO is clarified. Natural bond orbital (NBO) analysis reveals that the electronic configuration of the doped Co metal represents a qualitative change with respect to that of the free-metal. The binding of CO precursor is mostly dominated by the metal E(i)Co..CO pairwise additive contributions, and the role of the SWCNTs is not restricted to supporting the metal. The spin quenched SWCNTs are characterized in terms of isodensity contours of frontier orbitals. Molecular electrostatic potentials (MEPs) indicate that SWCNTs can act as effective gas sensors for nucleophiles. The results show that Co-doped SWCNTs can be useful in spintronics applications and sensor technology.

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.

Theoretical design of highly efficient porphyrazine solar cells

Density functional theory and time-dependent density functional theory methodologies have been applied to design porphyrazine (tetraazaporphyrin) sensitizers. This was done by replacing the porphyrin macrocycle cavity with a porphyrazine macrocycle cavity and increasing the number of p-carboxyphenyl moieties at the macrocycle periphery so that the performances of the suggested cells could be expected to exceed the efficiency of the reference YD2-o-C8 porphyrin sensitizer with Ti38O76, (TiO2)60, SiC, ZrO2, and GaP semiconductor electrodes. Macrocycle replacement assists in promoting the efficiency in the red shoulder of the spectrum more effectively than increasing the number of anchoring groups. The expected effects of the former structural modifications on cell performances are confirmed in terms of natural transition orbitals, energy gaps, semiconductor valence bands and conduction band edges, density of states, UV–visible absorption, and lifetimes of the excited states ΦLHE, Φinject., and ΔGregen. The structural modifications resulted in charge-separated states, unidirectional charge transfer, narrower band gaps, increase of density of states nearby Fermi levels, delocalization of the negative charges near the anchoring groups, efficient light harvesting and electron injection, suppressing macrocycle aggregation, active dye regeneration, and inhibited dye recombination. Co-sensitizers are suggested for near-infrared sensitization.

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.