Shiyu Du

The OH radical-H2O molecular interaction potential

The OH radical is one of the most important oxidants in the atmosphere due to its high reactivity. The study of hydrogen-bonded complexes of OH with the water molecules is a topic of significant current interest. In this work, we present the development of a new analytical functional form for the interaction potential between the rigid OH radical and H(2)O molecules. To do this we fit a selected functional form to a set of high level ab initio data. Since there is a low-lying excited state for the H(2)O.OH complex, the impact of the excited state on the chemical behavior of the OH radical can be very important. We perform a potential energy surface scan using the CCSD(T)/aug-cc-pVTZ level of electronic structure theory for both excited and ground states. To model the physics of the unpaired electron in the OH radical, we develop a tensor polarizability generalization of the Thole-type all-atom polarizable rigid potential for the OH radical, which effectively describes the interaction of OH with H(2)O for both ground and excited states. The stationary points of (H(2)O)(n)OH clusters were identified as a benchmark of the potential.

Photoluminescent Ti3C2 MXene Quantum Dots for Multicolor Cellular Imaging

The fabrication of photoluminescent Ti3 C2 MXene quantum dots (MQDs) by a facile hydrothermal method is reported, which may greatly extend the applications of MXene-based materials. Interestingly, the as-prepared MQDs show excitation-dependent photoluminescence spectra with quantum yields of up to ≈10% due to strong quantum confinement. The applications of MQDs as biocompatible multicolor cellular imaging probes and zinc ion sensors are demonstrated.

A Two-Dimensional Zirconium Carbide by Selective Etching of Al3C3 from Nanolaminated Zr3Al3C5

The room-temperature synthesis of a new two-dimensional (2D) zirconium-containing carbide, Zr3C2T(z) MXene is presented. In contrast to traditional preparation of MXene, the layered ternary Zr3Al3C5 material instead of MAX phases is used as source under hydrofluoric acid treatment. The structural, mechanical, and electronic properties of the synthesized 2D carbide are investigated, combined with first-principles density functional calculations. A comparative study on the structrual stability of our obtained 2D Zr3C2T(z) and Ti3C2T(z) MXenes at elevated temperatures is performed. The obtained 2D Zr3C2T(z) exhibits relatively better ability to maintain 2D nature and strucural integrity compared to Ti-based Mxene. The difference in structural stability under high temperature condition is explained by a theoretical investigation on binding energy.

Role of the surface effect on the structural, electronic and mechanical properties of the carbide MXenes

The two-dimensional material MXene has recently attracted interest for its excellent performance in diverse perspectives. Etched from the parental MAX phase with hydrofluoric acid, the synthesized MXene surface is normally functionalized by oxygen (-O), fluorine (-F) or hydroxyl (-OH) groups. Herein, using first-principles density functional calculations, we investigate the structural, mechanical and electronic properties of the carbide MXene M2CT2 (M=Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; T=-O, -F, -OH). Both the M atom and the surface group T have a significant effect on the MXenes properties. Generally, oxygen functionalized MXenes present smaller lattice parameters and stronger mechanical strength compared to those functionalized by fluorine and hydroxyl groups. Sc2CO2 exhibits the smallest interlayer thickness and W2CO2 shows the strongest mechanical strength. In regard to electronic properties, five oxygen functionalized members M2CO2 (M=Sc, Ti, Zr, Hf, W), two fluorine functionalized members M2CF2 (M=Sc, Mo), and hydroxyl functionalized Sc2C(OH)(2) present semiconducting characteristics, but only Sc2C(OH)(2) exhibits a direct band gap. Copyright (C) EPLA, 2015

Loading Actinides in Multilayered Structures for Nuclear Waste Treatment: The First Case Study of Uranium Capture with Vanadium Carbide MXene

Efficient nuclear waste treatment and environmental management are important hurdles that need to be overcome if nuclear energy is to become more widely used. Herein, we demonstrate the first case of using two-dimensional (2D) multilayered V2CTx nanosheets prepared by HF etching of V2AlC to remove actinides from aqueous solutions. The V2CTx material is found to be a highly efficient uranium (U(VI)) sorbent, evidenced by a high uptake capacity of 174 mg g(-1), fast sorption kinetics, and desirable selectivity. Fitting of the sorption isotherm indicated that the sorption followed a heterogeneous adsorption model, most probably due to the presence of heterogeneous adsorption sites. Density functional theory calculations, in combination with X-ray absorption fine structure characterizations, suggest that the uranyl ions prefer to coordinate with hydroxyl groups bonded to the V-sites of the nanosheets via forming bidentate inner-sphere complexes.

Exceptionally high thermal and electrical conductivity of three-dimensional graphene-foam-based polymer composites

Graphene foams (GF) assembled with one- or few-layered ultrathin two-dimensional crystals have showed huge application potentials owing to their unique three-dimensional (3D) structure and superior properties. Here, we present a polymer-template-assisted assembly strategy for fabricating a novel class of 3D graphene architecture. A free-standing GF architecture has been built to act as thermal and electrical conduction paths in polymer composites. The obtained GF/polymer composites exhibit a high thermal conductivity (1.52 W mK−1) and high electrical conductivity (3.8 × 10−2 S cm−1) at relatively low GF loading (5.0 wt%). The GF/polymer composites are potentially useful in advanced packaging materials of high power LED and microelectronic devices.

Synthesis and Electrochemical Properties of Two-Dimensional Hafnium Carbide

We demonstrate fabrication of a two-dimensional Hf-containing MXene, Hf3C2Tz, by selective etching of a layered parent Hf3[Al(Si)]4C6 compound. A substitutional solution of Si on Al sites effectively weakened the interfacial adhesion between Hf-C and Al(Si)-C sublayers within the unit cell of the parent compound, facilitating the subsequent selective etching. The underlying mechanism of the Si-alloying-facilitated etching process is thoroughly studied by first-principles density functional calculations. The result showed that more valence electrons of Si than Al weaken the adhesive energy of the etching interface. The MXenes were determined to be flexible and conductive. Moreover, this 2D Hf-containing MXene material showed reversible volumetric capacities of 1567 and 504 mAh cm-3 for lithium and sodium ions batteries, respectively, at a current density of 200 mAg-1 after 200 cycles. Thus, Hf3C2Tz MXenes with a 2D structure are candidate anode materials for metal-ion intercalation, especially for applications where size matters.

Direct in situ observation and explanation of lithium dendrite of commercial graphite electrodes

Lithium-ion batteries (LIBs) have some serious safety problems, such as lithium dendrite formation during charging/discharging cycles that may cause internal short-circuiting, fires, and even explosions. A new double-scale in situ experimental setup, which can record all phenomena during the electrochemical testing, was developed. Lithium dendrite growth behavior of commercial LIBs during small-current-density charging at room temperature was observed in situ. The formation, growth, and dissolution of lithium dendrites, and dead lithium residue were all observed and recorded using this new experimental test system. A detailed model of lithium electrodeposition and dissolution processes was proposed. The electrode structures were determined by X-ray diffraction (XRD). The surface morphologies were examined by scanning electron microscopy (SEM). The texture and surface morphology of the graphite active layer affected lithium dendrite initiation as well as its growth processes.

Anisotropic thermal conductivity in uranium dioxide

The thermal conductivity of uranium dioxide has been studied for over half a century, as uranium dioxide is the fuel used in a majority of operating nuclear reactors and thermal conductivity controls the conversion of heat produced by fission events to electricity. Because uranium dioxide is a cubic compound and thermal conductivity is a second-rank tensor, it has always been assumed to be isotropic. We report thermal conductivity measurements on oriented uranium dioxide single crystals that show anisotropy from 4 K to above 300 K. Our results indicate that phonon-spin scattering is important for understanding the general thermal conductivity behaviour, and also explains the anisotropy by coupling to the applied temperature gradient and breaking cubic symmetry.

In situ formation of a cellular graphene framework in thermoplastic composites leading to superior thermal conductivity

Based on the fast growth of the device performance, there has been an increasing demand for handling the issue of thermal management in electronic equipments. Therefore, it is of great significance to improve the thermal conductivity of thermoplastics, which are commonly used in electronic components. However, the difficulty of graphene dispersion and strong interfacial phonon scattering restrict the heat dissipation performance of graphene/thermoplastic composites, especially in the case of polypropylene (PP) or polyethylene (PE). Here, we propose a single-step and versatile approach to fabricate graphene/thermoplastic composites with a remarkable thermal conductivity enhancement. The composites were prepared by coating graphene on polymer powder first, followed by hot pressing. As a result, an interconnected graphene framework can be developed in the thermoplastic matrix, leading to significant heat transfer enhancement of the composites. At a 10 wt% graphene content, the thermal conductivity reaches 1.84, 1.53, 1.43, and 1.47 W m−1 K−1 for PE, PP, PVA (poly(vinyl alcohol)), and PVDF (poly(vinylidene fluoride)) composites, respectively. Our finding provides a path to develop a variety of highly thermally conductive thermoplastic composites for use in heat dissipation and other thermal applications.