Boundary States in Graphene Heterojunctions

A new type of states in graphene-based planar heterojunctions has been studied in the envelope wave function approximation. The condition for the formation of these states is the intersection between the dispersion curves of graphene and its gap modification. This type of states can also occur in smooth graphene-based heterojunctions.Comment: 5 pages, 3 figure

The thermodynamic and kinetic properties of hydrogen dimers on graphene

The thermodynamic and kinetic properties of hydrogen adatoms on graphene are important to the materials and devices based on hydrogenated graphene. Hydrogen dimers on graphene with coverages varying from 0.040 to 0.111 ML (1.0 ML $= 3.8\times10^{15}$cm$^{-2}$) were considered in this report. The thermodynamic and kinetic properties of H, D and T dimers were studied by ab initio simulations. The vibrational zero-point energy corrections were found to be not negligible in kinetics, varying from 0.038 (0.028, 0.017) to 0.257 (0.187, 0.157) eV for H (D, T) dimers. The isotope effect exhibits as that the kinetic mobility of a hydrogen dimer decreases with increasing the hydrogen mass. The simulated thermal desorption spectra with the heating rate $\alpha = 1.0$ K/s were quite close to experimental measurements. The effect of the interaction between hydrogen dimers on their thermodynamic and kinetic properties were analyzed in detail.Comment: submitted to Surface Scienc

Ab initio simulations of the kinetic properties of the hydrogen monomer on graphene

The understanding of the kinetic properties of hydrogen (isotopes) adatoms on graphene is important in many fields. The kinetic properties of hydrogen-isotope (H, D and T) monomers were simulated using a composite method consisting of density functional theory, density functional perturbation theory and harmonic transition state theory. The kinetic changes of the magnetic property and the aromatic $\pi$ bond of the hydrogenated graphene during the desorption and diffusion of the hydrogen monomer was discussed. The vibrational zero-point energy corrections in the activation energies were found to be significant, ranging from 0.072 to 0.205 eV. The results obtained from quantum-mechanically modified harmonic transition state theory were compared with the ones obtained from classical-limit harmonic transition state theory over a wide temperature range. The phonon spectra of hydrogenated graphene were used to closely explain the (reversed) isotope effects in the prefactor, activation energy and jump frequency of the hydrogen monomer. The kinetic properties of the hydrogen-isotope monomers were simulated under conditions of annealing for 10 minutes and of heating at a constant rate (1.0 K/s). The isotope effect was observed; that is, a hydrogen monomer of lower mass is desorbed and diffuses more easily (with lower activation energies). The results presented herein are very similar to other reported experimental observations. This study of the kinetic properties of the hydrogen monomer and many other involved implicit mechanisms provides a better understanding of the interaction between hydrogen and graphene.Comment: Accepted by J. Phys. Chem.

Effect of external pressure on the magnetic properties of LnFeAsO (Ln = La, Ce, Pr, Sm)

We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln = La, Ce, Pr, La) by using muon-spin relaxation measurements and ab-initio calculations. Both magnetic transition temperature $T_m$ and Fe magnetic moment decrease with external pressure. The effect is observed to be lanthanide dependent with the strongest response for Ln = La and the weakest for Ln = Sm. The trend is qualitatively in agreement with our DFT calculations. The same calculations allow us to assign a value of 0.68(2) $\mu_B$ to the Fe moment, obtained from an accurate determination of the muon sites. Our data further show that the magnetic lanthanide order transitions do not follow the simple trend of Fe, possibly as a consequence of the different $f$-electron overlap.Comment: 16 pages, 11 figure

Methanol electrosynthesis from CO2 at Cu2O/ZnO prompted by pyridine-based aqueous solutions

In this study, we examine the electrochemical-driven reduction of CO2 to methanol at Cu2O/ZnO gas diffusion electrodes in soluble pyridine-based electrolytes at different concentrations. The process is evaluated first by cyclic voltammetric analyses and then, for the continuous reduction of CO2 in a filter-press electrochemical cell. The results showed that the use of pyridine-based soluble co-catalysts lowered the overpotential for the electrochemical reduction of CO2, enhancing also reaction performance (i.e. reaction rate and Faradaic efficiency). Reaction outcome is discussed on the basis of the role that N-ligands play on the mechanism and the inductive effect caused by the electron-releasing or electron-withdrawing substituents of the aromatic ring. In particular, the maximum methanol formation rate and Faradaic efficiency reached at the 2-methylpyridine (with electron-releasing substituents)-based system with a pH of 7.6 and an applied current density of j = 1 mA cm−2 were r = 2.91 μmol m−2 s−1 and FE = 16.86%, respectively. These values significantly enhance those obtained in the absence of any molecular catalyst (r = 0.21 μmol m−2 s−1 and FE = 1.2%). The performance was further enhanced when lowering the electrolyte pH by adding HCl (r = 4.42 μmol m−2 s−1 and FE = 25.6% at pH = 5), although the system showed deactivation in the long run (5 h) which appears largely to be due to a change in product selectivity of the reaction (i.e. formation of ethylene).The authors gratefully acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness (MINECO), under the projects CTQ2013-48280-C3-1-R, CTQ2014-55716-REDT and Juan de la Cierva program (JCI-2012-12073)