Dongwei Ma

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.

Pd1/BN as a promising single atom catalyst of CO oxidation: a dispersion-corrected density functional theory study

Single metal atom catalysts exhibit extraordinary activity in a large number of reactions, and some two-dimensional materials (such as graphene and h-BN) are found to be prominent supports to stabilize single metal atoms. The CO oxidation reaction on single Pd atoms supported by two-dimensional h-BN is investigated systematically by using dispersion-corrected density functional theory study. The great stability of the h-BN supported single Pd atoms is revealed, and the single Pd atom prefers to reside at boron vacancies. Three proposed mechanisms (Eley–Rideal, Langmuir–Hinshelwood, and a “new” termolecular Eley–Rideal) of the CO oxidation were investigated, and two of them (the traditional Langmuir–Hinshelwood mechanism and the new termolecular Eley–Rideal mechanism) are found to have rather small reaction barriers of 0.66 and 0.39 eV for their rate-limiting steps, respectively, which suggests that the CO oxidation could proceed at low temperature on single Pd atom doped h-BN. The current study will help to understand the various mechanisms of the CO oxidation and shed light on the design of CO oxidation catalysts, especially based on the concept of single metal atoms.

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.

CO catalytic oxidation on Al-doped graphene-like ZnO monolayer sheets: a first-principles study

The graphene-like ZnO (g-ZnO) monolayer sheet is a new class of two-dimensional materials with unique properties, which are still largely unexplored. This work studied the modulation of electronic structures and chemical activities of the g-ZnO monolayer sheet by substituting Al for host Zn atoms. It is found that replacing Zn with Al atoms is both energetically and dynamically highly favorable. Al doping introduces electrons into the conduction band of the g-ZnO monolayer sheet, which should significantly enhance the conductance and the chemical activity of the sheet. The CO oxidation by the lattice O atoms via the Mars–van Krevelen mechanism, and by the adsorbed O2via the Langmuir–Hinshelwood and Eley–Rideal mechanisms were comparably studied. The Al-doped g-ZnO monolayer sheet shows good catalytic activity for the CO oxidation via the more favorable Eley–Rideal mechanism with a two-step route. The study presents an effective strategy to tune the electronic structure and the chemical activity of the g-ZnO nanosheets, and gives us an insight into the mechanism of Al-doped ZnO nanostructures sensing reducing gases.

The mechanism of oxygen activation on single Pt-atom doped SnO2(110) surface

SnO2, as a highly stable and carbon-free catalyst, is widely used in proton exchange membrane fuel cell as support. In order to shed light on the mechanism of oxygen reduction reaction on SnO2, the activation of the oxygen atom and molecule on undoped and Pt-doped SnO2 was investigated using the first-principles method in this study. We found that: (a) the Pt dopants in the SnO2(110) surfaces are positively charged; (b) the O-vacancy sites are the most preferable for the adsorption of molecular oxygen; (c) O− species, the most reactive form of oxygen, are easy to be formed through the activation of molecular oxygen on the O-defect sites of the Pt-doped SnO2 surface; and (d) the synergetic effects of Pt dopant and O-vacancy would be the key to the O2 activation. The current results shed light on the activation mechanism of oxygen species on undoped and Pt-doped SnO2 supports.

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

Sulfur doped graphene as a promising metal-free electrocatalyst for oxygen reduction reaction: a DFT-D study

As an efficient metal-free catalyst, graphene doped with heteroatoms is highly active in promoting electrochemical oxygen reduction reaction (ORR). The detailed kinetic and thermodynamic behaviors of the entire ORR process on sulfur doped monovacancy graphene (SGV), as well as the original mechanism are investigated by the dispersion-corrected density function theory (DFT-D) calculations. It is found that the SGV is rather stable and the sulfur dopant is probably the active center. There are two proposed ORR pathways by kinetic process: the dissociation of OOH and the hydrogenation of OOH with the rate-determining steps of 0.75xa0eV and 0.62 eV, respectively. And the Gibbs free energy diagram of the entire ORR indicates that the dissociation of OOH is precluded, because the process of reduction step of O into OH is endothermic, while the hydrogenation of HOOH is the most favorable pathway even at high potential of 0.86 V. Our DFT-D simulation suggests that the SGV would be an efficient electrocatalyst for ORR.

Dependence of memory characteristics on the (ZrO2)x(SiO2)1−x elemental composition for charge trap flash memory applications

Charge trap flash memory capacitors incorporating various (ZrO2)x(SiO2)1−x films (x = 1.0, 0.92, 0.79, 0.63, 0.46, 0.28, 0.17 and 0.08) as the charge trapping layer were fabricated, and the dependence of the memory window, data retention and program/erase speed on the mole fraction value x were investigated. It was observed that changing the elemental composition affects the dielectric constant and equivalent oxide thickness of (ZrO2)x(SiO2)1−x films, and the memory capacitor (x = 0.63) exhibits a memory window as large as 10.7 V for a ±10 V sweeping voltage range, and a low extrapolated charge loss of 9.0% over a period of 10 years. A faster program/erase speed can be obtained for memory capacitors when x = 0.79 and 0.63. The results should be attributed to the different (ZrO2)x(SiO2)1−x microstructures, defect state densities and energy band alignments resulting from the change of compositions. In order to achieve the trade-off between the memory window, data retention, and program/erase speed, the optimal x values of the (ZrO2)x(SiO2)1−x trapping layer are in the range of 0.63 to 0.79 for charge trap flash memory applications. The present study provides useful insights for the composition selection for a complex oxide-based charge trap flash memory.

Tuning metal cluster catalytic activity with morphology and composition: a DFT study of O2 dissociation at the global minimum of PtmPdn (m + n = 5) clusters

The Pt-based alloyed clusters are the important catalysts in the chemical industry. The global minimum structures of PtmPdn (m + n = 5) clusters were searched for based on the CALYPSO algorithm, and the various properties of the lowest-energy PtmPdn clusters have been studied. The results show that introducing the Pd element can modify the morphology and composition of the pure Pt5 cluster, i.e., from a 2D planar to a 3D trigonal bipyramid structure, except for the Pt4Pd1 (square pyramid). According to the average cohesive energy, the Pt4Pd1 cluster possesses the highest stability among these Pt–Pd alloyed clusters. The O2 molecule prefers to anchor on the clusters by the Yeager mode. The catalytic property for O2 dissociation of the pure Pt5 cluster can be further improved by introducing the Pd atoms. Moreover, the Pt4Pd1 cluster with the high Pt composition and the structure of square pyramid shows the highest catalytic activity. More importantly, the Bronsted–Evans–Polanyi relationship is found to be applicable to the current mini alloyed metal clusters. This study will help to understand the prediction of the global minimum structure of metal cluster by CALYPSO and shed light on the design of the less expensive and more effective alloyed cluster catalysts by controlling the morphology and composition in the fuel cells.

The elastic and thermoelectric properties of the Zintl compound Ca5Al2Sb6 under high pressure

The elastic and thermoelectric properties of Ca5Al2Sb6 under pressure are studied using ab initio calculation and semiclassical Boltzmann theory. The calculated elastic constants and minimum thermal conductivity indicate that Ca5Al2Sb6 exhibits an anisotropic structure and high thermoelectric performance. The size of the band gap shows a nonlinear change with increasing pressure. Based on the electronic structure, the calculated thermoelectric parameters show that the Seebeck coefficient has no obvious change under pressure, whereas the electrical conductivity improves with increasing pressure. Therefore, the power factor increases at an appropriate pressure of Pu2009=u20092.6u2009GPa. P-type doping of Ca5Al2Sb6 may achieve better thermoelectric performance than n-type doping, in agreement with experiment. The anisotropic thermoelectric properties of Ca5Al2Sb6 indicate that the thermoelectric performance along the z-direction is superior to other directions. This is attributed to the combination of the large dispersi...