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Register a new userOn the mechanism of gas adsorption for pristine, defective and functionalized graphene
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Defect is no longer deemed an adverse aspect of graphene. Contrarily, it can pave ways of extending applicability of graphene. Here, we discuss the effects of three types of defects on graphene: carbon deficiency, adatom (single Fe) dopant and introduction of functional groups (carboxyl, pyran group) on NO2 gas adsorption via density functional theory method. We have observed that the unsaturated carbon in defected graphene is highly active to attract NO2 molecules. Our study suggests that introducing Fe on graphene can enhance the NO2 adsorption process. Adsorption energy calculations suggest the enhancement in NO2 adsorption is more profound for Fe-doped mono and tetra vacant graphene than Fe doped bi- and tri-vacant graphene. This study could potentially be useful in developing adsorption-based applications of graphene.
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The adsorption and diffusion of F2 molecules on pristine graphene have been studied using first-principles calculations. For the diffusion of F2 from molecular state in gas phase to the dissociative adsorption state on graphene surface, a kinetic barrier is identified, which explains the inertness of graphene in molecular F2 at room temperature, and its reactivity with F2 at higher temperatures. Studies on the diffusion of F2 molecules on graphene surface determine the energy barriers along the optimal diffusion pathways, which help to understand the high stability of fluorographene.
We have studied the adsorption of gas molecules (CO, NO, NO2, O2, N2, CO2, and NH3) on graphene nanoribbons (GNRs) using first principles methods. The adsorption geometries, adsorption energies, charge transfer, and electronic band structures are obtained. We find that the electronic and transport properties of the GNR with armchair-shaped edges are sensitive to the adsorption of NH3 and the system exhibits n type semiconducting behavior after NH3 adsorption. Other gas molecules have little effect on modifying the conductance of GNRs. Quantum transport calculations further indicate that NH3 molecules can be detected out of these gas molecules by GNR based sensor.
The adsorption energies and orientation of methanol on graphene are determined from first-principles density functional calculations. We employ the well-tested vdW-DF method that seamlessly includes dispersion interactions with all of the more close-ranged interactions that result in bonds like the covalent and hydrogen bonds. The adsorption of a single methanol molecule and small methanol clusters on graphene are studied at various coverages. Adsorption in clusters or at high coverages (less than a monolayer) is found to be preferable, with the methanol C-O axis approximately parallel to the plane of graphene. The adsorption energies calculated with vdW-DF are compared with previous DFT-D and MP2-based calculations for single methanol adsorption on flakes of graphene (polycyclic aromatic hydrocarbons). For the high coverage adsorption energies we also find reasonably good agreement with previous desorption measurements.
To elucidate the physics underling magnetism observed in nominally nonmagnetic materials with only $sp$-electrons, we built an extreme model to simulate H-adsorption (in a straight-line form) on graphene. Our first principles calculations for the model produce a ferromagnetic ground state with a magnetic moment of one Bohr magneton per H atom and an estimated Curie temperature above 250~K. The removal of the $p_z$-orbitals from sublattice B of graphene introduces $p_z$-vacancies. The $p_z$-vacancy-induced states are not created from changes in interatomic interactions but are created because of a $p_z$-orbital imbalance between two sublattices (A and B) of a conjugated $p_z$-orbital network. Therefore, there are critical requirements for the creation of these states (denoted as $p_z^{rm imbalance}$) to avoid further imbalances and minimize the effects on the conjugated $p_z$-orbital network. The requirements on the creation of $p_z^{rm imbalance}$ are as follows: 1) $p_z^{rm imbalance}$ consists of $p_z$-orbitals of only the atoms in sublattice A, 2) the spatial wavefunction of $p_z^{rm imbalance}$ is antisymmetric, and 3) in principle, $p_z^{rm imbalance}$ extends over the entire crystal without decaying, unless other $p_z$-vacancies are crossed. Both the origin of spin polarization and the magnetic ordering of the model arise from the aforementioned requirements.
In this paper, we investigate the adsorption of water monomer on fluorinated graphene using state-of-the-art first principles methods within the framework of density functional theory (DFT). Four different methods are employed to describe the interactions between water and the carbon surface: The traditional DFT calculations within the generalized gradient approximation (GGA), and three types of calculations using respectively the semi-empirical DFT-D2method, the original van der Waals density functional (vdW-DF) method, and one of its variants. Compared with the adsorption on pristine graphene, the adsorption energies of water on fluorinated graphene are significantly increased, and the orientations of water diploe moment are notably changed. The most stable configuration is found to stay right above the top site of the C atom which is bonded with F, and the dipole moment of water molecule aligns spontaneously along the surface normal.
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