Yong Pei

Ab initio study of graphene-like monolayer molybdenum disulfide as a promising anode material for rechargeable sodium ion batteries

Using first-principles study based on density functional theory (DFT), the adsorption sites, diffusion kinetics, theoretical capacity and average voltage of Na atoms in graphene-like monolayer MoS2 are systematically investigated in comparison with bulk MoS2. It is found that for the graphene-like monolayer MoS2, a maximum theoretical capacity of 335 mA h g−1 could be achieved by double-side Na adsorption. Upon sodiation process, the graphene-like monolayer MoS2 can maintain a low voltage platform at about 1.0 V. A Na diffusion pathway on the graphene-like monolayer MoS2 is identified as from two adjacent T-sites passing through the nearest-neighbor H site in a zigzag manner. The activation barrier of this process is only 0.11 eV, a considerable decrease compared to that of the bulk MoS2 interlayer migration (0.70 eV), which indicates that Na can diffuse faster in the graphene-like monolayer MoS2 than in bulk MoS2. The present results suggest that the graphene-like monolayer MoS2 can provide excellent battery performance as the anode material of a sodium ion battery.

Novel solution-processible small molecules based on benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene for effective organic photovoltaics with high open-circuit voltage

Two novel A–D–A small molecules D1 and D2, containing benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene as the central electron-donating unit, 3-ethylrhodanine as end-capped electron-withdrawing units, and two thiophenes or three thiophenes as conjugated π-bridges, were designed and synthesized. The effects of the conjugated π-bridges on the photophysical, electrochemical and photovoltaic properties as well as the aggregation structure, were fully investigated. Compared with D2, D1 shows stronger packing capability, deeper HOMO energy levels, higher hole mobility, and more appropriate microphase separation with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), which lead to better photovoltaic performance with higher short-circuit current density (Jsc) and open-circuit voltages (Voc). Bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices were fabricated using a blend of the as-synthesized small molecules and PC61BM in different solvents. The D1-CF device prepared from chloroform solution with a weight ratio of 1 : 0.5 exhibited a power conversion efficiency (PCE) of 2.10% and an amazingly high Voc of 1.10 V under AM 1.5G (100 mW cm−2) illumination.

First-principles investigation on the structural, electronic properties and diffusion barriers of Mg/Al doped NaCoO2 as the cathode material of rechargeable sodium batteries

Mg/Al doped NaCoO2 layered transition-metal oxides as potential cathode materials for sodium ion batteries have been investigated by first-principle calculations. The effects of divalent Mg ion or trivalent Al ion doping on the crystal structure, electron transfer, the changes of valence, average intercalation voltage and diffusion barriers of NaCoO2 are studied. The DFT calculations indicate NaCoO2 with Mg or Al ions doping will lead to a higher average intercalation voltage, which is beneficial for obtaining high energy density. Charge disproportionation induced by divalent Mg ion doping results in the appearance of electronic holes in Na(Co0.92Mg0.08)O2, which may enhance its conductivity significantly. The nudged elastic band calculation results indicate that trivalent Al ion doping has a slight effect on the diffusion barriers of NaCoO2, but divalent Mg ion doping can significantly decrease the diffusion barriers and enhance the Na ion diffusion rate, which is beneficial to the improvement of the rate capability.

Density functional theory (DFT) studies of CO oxidation reaction on M13 and Au18M clusters (M = Au, Ag, Cu, Pt and Pd): the role of co-adsorbed CO molecule

Using the icosahedra M13 (M = Au, Ag, Cu, Pt, Pd) and hetero-atom doped Au18M (M = Ag, Cu, Pt, Pd) clusters as model systems, we have systematically investigated the role of the co-adsorbed CO molecule played in the CO oxidation reaction on the basis of density functional theory (DFT) calculations. The results indicate that the co-adsorbed CO molecule at a triangular active site can induce the dissociation of the OCOO* intermediate via a tri-molecular reaction route. This mechanism is also validated on other larger single doped gold alloy clusters such as AunAg and AunCu (n = 32–34, 54). The underlying reason for promoting the oxidation effect of a co-adsorbed CO molecule is unraveled. It is found that the relatively weaker d–π* back bonding of CO on group 11 elements like Au, Ag and Cu may increase its electrophilic activity, which can facilitate the dissociation of nearby OCOO* intermediates. For the CO molecule that is bounded to the Pd and Pt atoms, it can also induce the dissociation of OCOO* intermediate, but shows weaker electrophilic activity. By explicitly considering the elementary reaction steps in a Kinetic Monte Carlo (KMC) simulation, we have shown that the tri-molecular reaction route is an alternative reaction channel of CO oxidation, which is competitive to the conventional bi-molecular route on a doped Au18M cluster.

Molecular dynamics simulations of the self-organization of side-chain decorated polyaromatic conjugation molecules: phase separated lamellar and columnar structures and dispersion behaviors in toluene solvent

The self-organization of five model side-chain decorated polyaromatic asphaltene molecules with or without toluene solvent was investigated by means of atomistic molecular dynamic (MD) simulations. It was found that the organizational structure of polycyclic asphaltene molecules is significantly affected by the position and length of side chains. In the present study, two types of phase-separated stacking configurations, including the phase separated lamellar structure (PSLS) and the phase separated columnar structure (PSCS), were found. The PSLS and PSCS were also maintained in the presence of a small amount of toluene additive (30% wt fraction). When adding excess toluene molecules, the asphaltene molecules formed highly dispersed nanoaggregates. The dynamic properties of the π–π stacking structures in the PSLS and PSCS, as well as the nanoaggregates, were probed. It was found that the number and size of alkyl side chains significantly impacted the size and number of π–π stacking structures in the aggregates. Through tracking the structural evolution of the nanoaggregates, a possible dissociation mechanism of nanoaggregates is also suggested.

Molecular Dynamics Simulations of the Oil-Detachment from the Hydroxylated Silica Surface: Effects of Surfactants, Electrostatic Interactions, and Water Flows on the Water Molecular Channel Formation

The detachment process of an oil molecular layer situated above a horizontal substrate was often described by a three-stage process. In this mechanism, the penetration and diffusion of water molecules between the oil phase and the substrate was proposed to be a crucial step to aid in removal of oil layer/drops from substrate. In this work, the detachment process of a two-dimensional alkane molecule layer from a silica surface in aqueous surfactant solutions is studied by means of molecular dynamics (MD) simulations. By tuning the polarity of model silica surfaces, as well as considering the different types of surfactant molecules and the water flow effects, more details about the formation of water molecular channel and the expansion processes are elucidated. It is found that for both ionic and nonionic type surfactant solutions, the perturbation of surfactant molecules on the two-dimensional oil molecule layer facilitates the injection and diffusion of water molecules between the oil layer and silica substrate. However, the water channel formation and expansion speed is strongly affected by the substrate polarity and properties of surfactant molecules. First, only for the silica surface with relative stronger polarity, the formation of water molecular channel is observed. Second, the expansion speed of the water molecular channel upon the ionic surfactant (dodecyl trimethylammonium bromide, DTAB and sodium dodecyl benzenesulfonate, SDBS) flooding is more rapidly than the nonionic surfactant system (octylphenol polyoxyethylene(10) ether, OP-10). Third, the water flow speed may also affect the injection and diffusion of water molecules. These simulation results indicate that the water molecular channel formation process is affected by multiple factors. The synergistic effects of perturbation of surfactant molecules and the electrostatic interactions between silica substrate and water molecules are two key factors aiding in the injection and diffusion of water molecules and helpful for the oil detachment from silica substrate.