The catalytic oxidation of CO on Pt/X-graphene (X = "pri" for pristine- or "SV" for defective-graphene with a single vacancy) is investigated using the first-principles method based on density functional theory. In contrast to a Pt atom on pristine graphene, a vacancy defect in graphene strongly stabilizes a single Pt adatom and makes the Pt adatom more positively charged, which helps to weaken the CO adsorption and facilitates the O(2) adsorption, thus enhancing the activity for CO oxidation and alleviating the CO poisoning of the platinum catalysts. The CO oxidation reaction on Pt/SV-graphene has a low energy barrier (0.58 eV) by the Langmuir-Hinshelwood (LH) reaction (CO + O(2)→ OOCO → CO(2) + O(ads)) which is followed by the Eley-Rideal (ER) reaction with an energy barrier of 0.59 eV (CO + O(ads)→ CO(2)). The results validate the reactivity of catalysts on the atomic-scale and initiate a clue for fabricating carbon-based catalysts with low cost and high activity.
Monolayer tellurium (Te) or tellurene has been suggested by a recent theory as a new two-dimensional (2D) system with great electronic and optoelectronic promises. Here we present an experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy. Scanning tunneling microscopy of ultrathin layers of Te reveals rectangular surface cells with the cell size consistent with the theoretically predicted β-tellurene, whereas for thicker films, the cell size is more consistent with that of the [101[combining macron]0] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements show that the films are semiconductors with the energy band gaps decreasing with increasing film thickness, and the gap narrowing occurs predominantly at the valence-band maximum (VBM). The latter is understood by strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory calculations.
Toxic gaseous SO2 adsorption on O-terminated M2CO2 (M = Sc, Hf, Zr, and Ti) monolayers has been studied by means of first-principle calculations. It is demonstrated that monolayer Sc2CO2 is most preferred for the SO2 molecule with suitable adsorption strength and apparent charge transfer. Moreover, the electronic conductivity of Sc2CO2 displays a sharp increase after the adsorption of SO2. In particular, the adsorption strength of SO2 on Sc2CO2 can be further enhanced or weakened by applying tensile strains or controlling external electronic fields, which is greatly desirable to realize the capture or reversible release of toxic SO2 gas. These distinctive features endow the Sc2CO2 monolayer with high selectivity and sensitivity as a potential candidate for SO2 gas sensor, as well as efficient control for gaseous SO2 capture.
Monolayer (ML) transition metal dichalcogenides (TMDs) are of great research interest due to their potential use in ultrathin electronic and optoelectronic applications. They show promise in new concept devices in spintronics and valleytronics. Here we present a growth study by molecular-beam epitaxy of ML and sub-ML MoSe 2 , an important member of TMDs, revealing its unique growth characteristics as well as the formation processes of domain boundary (DB) defects. A dramatic effect of growth temperature and post-growth annealing on DB formation is uncovered.
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.
Transition-metal dichalcogenides (TMDs) with the common formula MX 2 (M = Mo, W; X=S, Se, Te) exist in different phases such as the hexagonal (2H), octahedral (1T), monoclinic (1T') and orthorhombic (Td) structures. [1,2,[3][4][5][6][7] The 2H phase is most common including, for example, metal disulfides (MS 2 ) and diselenides (MSe 2 ), which are direct gap semiconductors for monolayer (ML) thin films. [1,8] They have attracted extensive research attention in recent years due to their appeals in microelectronic, optoelectronic, spin and valley electronic applications. [1,9,10,11,12] The 1T' or Td phase MX 2 are of the distorted 1T structure, which usually show semi-metallic behavior. [3,[13][14][15] Examples of the latter include WTe 2 and -MoTe 2 , which have drawn special interests lately following some recent revelations of, e.g., the large and unsaturated magnetoresistance, [4,15,16,17,18] pressure-driven superconductivity, [7,19] novel optical properties and characteristics, [11,12,17,18,20] and the topological insulator [14] and Weyl semimetal states. [6,7,21] Metallic TMDs are also good catalysts for hydrodesulfurization and hydrogen evolution reactions. [22] The large difference in electrical properties between 2H and 1T' (Td) phases of MX 2 further makes them promising for phase-change electronics. [23,24] Therefore, tuning and stabilizing the different phases of MX 2 can be of great scientific and application relevance.Among the various TMDs, MoTe 2 takes a special place as there is a small energy difference between its 2H and 1T' phases (~ 43meV per formula unit [5] ). The hexagonal phase of MoTe 2 is slightly more stable than 1T' MoTe 2 under ambient conditions, while the latter becomes more favorable at high temperature and/or under 3 tensile strain. [5,24] In any case, due to the small energy difference between the two structures, there is a high chance for one to obtain samples containing coexisting phases or to purposely tune the structure of MoTe 2 crystal by applying external constraints. This would lead to many new applications of the TMD thin films. [18] In this work, we report growths of both 2H and 1T' MoTe 2 ML by molecular beam epitaxy (MBE). We reveal a dramatic effect of Te adsorption on 1T' phase MoTe 2 formation and growth. By changing the conditions of MBE and by annealing, we can achieve effective tuning of the structural phase of MoTe 2 . Employing scanning tunneling microscopy and spectroscopy (STM/S), we establish unambiguously the structures and electronic characteristics of 2H and 1T' MoTe 2 ML such as the energy bandgap and the density of states (DOS). By consulting with the first principles calculations, we provide an explanation for the stabilization of the otherwise metastable 1T' phase MoTe 2 at the temperature and pressure condition, which is associated with Te adsorption on surface. Figure 2d. From the experimental STS data, we further derive an electronic energy gap of ~ 1.4 eV (refer to Supplementary), a value that is in qualitative agreement with that reported in li...
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.
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