Yanan Tang

Trapping of metal atoms in the defects on graphene

The binding of a single metal atom (Pt, Pd, Au, and Sn) nearby a single-vacancy (SV) on the graphene is investigated using the first-principles density-functional theory. On the pristine graphene (pri-graphene), the Pt, Pd, and Sn prefer to be adsorbed at the bridge site, while Au prefers the top site. On the graphene with a single-vacancy (SV-graphene), all the metal atoms prefer to be trapped at the vacancy site and appear as dopants. However, the trapping abilities of the SV-graphene are varied for different metal atoms, i.e., the Pt and Pd have the larger trapping zones than do the others. The diffusion barrier of a metal atom on the SV-graphene is much higher than that on the pri-graphene, and the Pt atom has the largest diffusion barrier from the SV site to the neighboring bridge sites. On the SV-graphene, more electrons are transferred from the adatoms (or dopants) to the carbon atoms at the defect site, which induces changes in the electronic structures and magnetic properties of the systems. This work provides valuable information on the selectivity of lattice vacancy in trapping metal atoms, which would be vital for the atomic-scale design of new metal-carbon nanostructures and graphene-based catalysts.

First-principles studies of BN sheets with absorbed transition metal single atoms or dimers: stabilities, electronic structures, and magnetic properties

BN sheets with absorbed transition metal (TM) single atoms, including Fe, Co, and Ni, and their dimers have been investigated by using a first-principles method within the generalized gradient approximation. All of the TM atoms studied are found to be chemically adsorbed on BN sheets. Upon adsorption, the binding energies of the Fe and Co single atoms are modest and almost independent of the adsorption sites, indicating the high mobility of the adatoms and isolated particles to be easily formed on the surface. However, Ni atoms are found to bind tightly to BN sheets and may adopt a layer-by-layer growth mode. The Fe, Co, and Ni dimers tend to lie (nearly) perpendicular to the BN plane. Due to the wide band gap of the pure BN sheet, the electronic structures of the BN sheets with TM adatoms are determined primarily by the distribution of TM electronic states around the Fermi level. Very interesting spin gapless semiconductors or half-metals can be obtained in the studied systems. The magnetism of the TM atoms is preserved well on the BN sheet, very close to that of the corresponding free atoms and often weakly dependent on the adsorption sites. The present results indicate that BN sheets with adsorbed TM atoms have potential applications in fields such as spintronics and magnetic data storage due to the special spin-polarized electronic structures and magnetic properties they possess.

Absorption of Pt clusters and the induced magnetic properties of graphene

First-principles total energy calculations are performed to investigate the formation and structures of Pt clusters on graphene. It is found that the formation energy of Pt on graphene increases with increasing Pt coverage. The structures of the absorbed Pt are that it is at the bridge site for a single Pt atom absorption, but form a dimerized cluster when two atoms are absorbed on graphene. For three- and four-Pt-atom absorption, linear and tetrahedral structures form, respectively, and the three-dimensional tetrahedral Pt(4) cluster is most stable in all the configurations investigated. There is a strong interatomic interaction among Pt atoms and so they tend to form clusters. While no magnetic behavior is expected after a single Pt atom is absorbed on graphene, the absorption of tetrahedral Pt(4) leads to Fermi level shifting to the valence band and the spin waves of C atoms in graphene become asymmetric and so they exhibit magnetism. The magnetic properties can thus be tuned by Pt absorption on graphene. The ultimate aim is to apply it in catalytic activity and electronic devices.

Preventing the CO poisoning on Pt nanocatalyst using appropriate substrate: a first-principles study

Adsorption energies and stable configurations of CO on the Pt clusters are investigated using the first-principles density-functional theory method. It is found that the adsorption of CO on the top site of the Pt4 cluster is more stable than that on the bridge site. The atomic charges are unevenly distributed within the charged Pt4 cluster, and the structural positions of the Pt atoms determine their charge states and therefore their activity. A systematic study on the effects of electrons and holes doping in the Pt4 clusters suggest an effective method to prevent the CO poisoning through regulating the total charge in Pt4 clusters. The graphene-based substrate is an ideal catalyst support, which makes the Pt catalyst lose electron and weakens the CO adsorption. The results would be of great importance for designing high active nanoscale Pt catalysts used for fuel cells.

High catalytic activity for CO oxidation on single Fe atom stabilized in graphene vacancies

Inspired by the recently discovered dynamics of single Fe atoms in graphene vacancies, we systemically examined the stable configurations, electronic structures, and catalytic activities of Fe-atom-embedded graphene substrates (including monovacancy graphene (MG) and divacancy graphene (DG)) by using first-principles calculations. We found that the doped Fe on the MG sheet (Fe/MG) is more stable than that on the DG sheet (Fe/DG). Doping with Fe atoms provides more transferred electrons to fill the vacancy defects of graphene and allows it to exhibit a more positive charge, which effectively regulates O2 and CO adsorption. Also, the degree of interactions between the reactants and substrates are connected to the reaction pathways and energy barriers. For the Fe/MG sheet, the low coadsorption energy of gas molecules can promote the catalytic reaction through the Langmuir–Hinshelwood (LH) mechanism. In comparison, the initial step for CO oxidation on the Fe/DG sheet is through the Eley–Rideal (ER) mechanism, which is an energetically more favorable process. Moreover, the more stable Fe/MG sheet is a much more efficient catalyst for CO oxidation at low temperature, because the sequential reaction processes (LH and ER) have low enough energy barriers. These results provide valuable guidance on selecting the metal dopant in graphene materials to design effective atomic-scale catalysts.

Repairing sulfur vacancies in the MoS2 monolayer by using CO, NO and NO2 molecules

As-grown transition metal dichalcogenides are usually chalcogen deficient and contain a high density of chalcogen vacancies, which are harmful to the electronic properties of these materials. Based on the first-principles calculation, in this study the repairing of the S vacancy in the MoS2 monolayer has been investigated by using CO, NO and NO2 molecules. For CO and NO, the repairing process consists of the first molecule filling the S vacancy and the removing of the extra O atom by the second molecule. However, for NO2, when the molecule fills the S vacancy, it is dissociated directly to form an O-doped MoS2 monolayer. After the repair, the C, N and O-doped MoS2 monolayers can be obtained by the adsorption of CO, NO, and NO2 molecules, respectively. And in particular, the electronic properties of these materials can be significantly improved by N and O doping. Furthermore, according to the calculated energy, the process of S vacancy repairing with CO, NO and NO2 should be easily achieved at room temperature. This study presents a promising strategy for repairing MoS2 nanosheets and improving their electronic properties, which may also apply to other transition metal dichalcogenides.

Depletion NOx Made Easy by Nitrogen Doped Graphene

The integrated mechanism of the catalytic oxidation of NO by N2O on the metal-free support of nitrogen doped graphene (NG) is investigated using density function theory calculations. The results indicate that the N2O can be intensively adsorbed on NG support, while the NO, N2, NO2 are all weakly adsorbed. In the oxidation process, a two-step mechanism is identified: the dissociation of N2O followed by the oxidation of NO with the dissociative O-atom. The present work suggests that the NG support, as a high-efficient and metal-free catalyst, is one of the promising candidates for removing the nitrogen oxides gases exhaust.Graphical Abstract

Structural, electronic and catalytic performances of single-atom Fe stabilized by divacancy-nitrogen-doped graphene

Inspired by the experimental discovery of the configuration of a one central transition metal and four surrounding N atom doped graphene sheet (M–GN4), we systemically study the geometry and electronic and catalytic properties of a single-atom Fe embedded GN4 sheet (Fe–GN4) using first-principles calculations. It is found that the neighboring N atoms in graphene strongly stabilize a single Fe atom and make the doped Fe atom more positively charged, which helps to regulate the stability of reactive gases. Besides, the adsorption of gas molecules can modulate the electronic structure and magnetic property of the Fe–GN4 system. Moreover, the catalytic reactions of CO oxidation on the Fe–GN4 substrate are comparably investigated in terms of the Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanism. The results show that the LH reaction as the starting state is energetically more favorable than the ER reaction, since the catalytic process has much smaller energy barriers (0.13 eV) and then promotes the CO oxidation reaction. Therefore, the stable configuration of Fe–GN4 would be a highly efficient catalyst for CO oxidation, which provides a clue for designing atomic-scale catalysts in energy-related devices.

Robust surface state of intrinsic topological insulator Bi2Te2Se thin films: a first-principles study

Bi(2)Te(2)Se, a ternary tetradymite compound, has recently been identified to be a three-dimensional topological insulator. In this paper, we theoretically study the electronic structures of bulk and thin films of Bi(2)Te(2)Se employing spin-orbit coupling (SOC) self-consistently with density-functional theory. It is found that SOC plays an important role in determining the electronic properties of Bi(2)Te(2)Se. A finite bandgap opens up in the surface states of Bi(2)Te(2)Se thin films due to the hybridization of the top and bottom surface states of films. The intrinsic Bi(2)Te(2)Se thin films of three or more quintuple layers exhibit a robust topological nature of electronic structure with the Fermi energy intersecting the Dirac cone of the surface states only once between time-reversal-invariant momenta. These characteristics of Bi(2)Te(2)Se are similar to the topological behavior of Bi(2)Te(3), promising a variety of potential applications in nanoelectronics and spintronics.

Tuning the Schottky contacts at the graphene/WS2 interface by electric field

Using the first-principle calculations, we study the electronic structures of graphene/WS2 van der Waals (vdW) heterostructures by applying an external electric field (Eext) perpendicular to the heterobilayers. It is demonstrated that the intrinsic electronic properties of graphene and WS2 are quite well preserved due to the weak vdW contact. We find that n-type Schottky contacts with a significantly small Schottky barrier are formed at the graphene/WS2 interface and p-type (hole) doping in graphene occurs during the formation of graphene/WS2 heterostructures. Moreover, the Eext is effective to tune the Schottky contacts, which can transform the n-type into p-type and ohmic contact. Meanwhile, p-type (hole) doping in graphene is enhanced under negative Eext and a large positive Eext is required to achieve n-type (electron) doping in graphene. The Eext can control not only the amount of charge transfer but also the direction of charge transfer at the graphene/WS2 interface. The present study would open a new avenue for application of ultrathin graphene/WS2 heterostructures in future nano- and optoelectronics.