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.

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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...

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 the material. Based on the first-principles calculation, in this study the repairing of the S vacancy in the MoS 2 monolayer has been investigated by using CO, NO and NO 2 molecules. For CO and NO, the repairing process consists of the filling of the first molecule into the S vacancy and the removing of the extra O atom by the second molecule. However, for NO 2 , when the molecule is filled into the S vacancy, it is dissociated directly to form a O-doped MoS 2 monolayer. After the repairing, the C, N and O-doped MoS 2 monolayers can be obtained by the adsorption of CO, NO, and NO 2 molecules, respectively. And especially, the electronic properties of the material can be significantly improved by the N and O doping. Furthermore, according to the calculated energy, the process of the S vacancy repairing with CO, NO and NO 2 should be easily achieved at the room temperature. This study presents an promising strategy for repairing MoS 2 nanosheets and improving electronic properties of the material, which may also apply to other transition metal dichalcogenides.its implementation in a wide range of applications is hindered by its material quality and comparatively low conductivity and photoconductivity. 19,22 The phonon-limited mobility was theoretically predicted to be ~ 410 cm 2 V -1 s -1 for the MoS 2 monolayer at the room temperature. 23 However, the experimentally attainable mobility for the untreated samples is still one or two order-of-magnitude lower than the theoretical value. [24][25][26] 2D semiconductors have an extremely high surface-to-mass ratio owing to their

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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.