Qianku Hu

MXene: A New Family of Promising Hydrogen Storage Medium

Searching for reversible hydrogen storage materials operated under ambient conditions is a big challenge for material scientists and chemists. In this work, using density functional calculations, we systematically investigated the hydrogen storage properties of the two-dimensional (2D) Ti2C phase, which is a representative of the recently synthesized MXene materials ( ACS Nano 2012 , 6 , 1322 ). As a constituent element of 2D Ti2C phase, the Ti atoms are fastened tightly by the strong Ti-C covalent bonds, and thus the long-standing clustering problem of transition metal does not exist. Combining with the calculated binding energy of 0.272 eV, ab initio molecular dynamic simulations confirmed the hydrogen molecules (3.4 wt % hydrogen storage capacity) bound by Kubas-type interaction can be adsorbed and released reversibly under ambient conditions. Meanwhile, the hydrogen storage properties of the other two MXene phases (Sc2C and V2C) were also evaluated, and the results were similar to those of Ti2C. Therefore, the MXene family including more than 20 members was expected to be a good candidate for reversible hydrogen storage materials under ambient conditions.

Synthesis and electrochemical performance of Ti3C2Tx with hydrothermal process

In this study, a simple hydrothermal method has been developed to prepare Ti3C2Tx from Ti3AlC2 as a high-performance electrode material for supercapacitors. This method is environmentally friendly and has a low level of danger. The morphology and structure of the Ti3C2Tx can be controlled by hydrothermal reaction time, temperature and NH4F amounts. The prepared Ti3C2Tx was characterized by X-ray diffraction, field emission scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and Brunauer-Emmet-Teller. The results show that the prepared Ti3C2Tx is terminated by O, OH, and F groups. The electrochemical properties of the Ti3C2Tx sample exhibit specific capacitance up to 141 Fcm−3 in 3 M KOH aqueous electrolyte, and even after 1000 cycles, no significant degradation of the volumetric capacitance was observed. These results indicate that the Ti3C2Tx material prepared by this hydrothermal method can be used in high performance supercapacitors.

Structural Transformation of MXene (V2C, Cr2C, and Ta2C) with O Groups during Lithiation: A First-Principles Investigation

For high capacities and extremely fast charging rates, two-dimensional (2D) crystals exhibit a significant promising application on lithium-ion batteries. With density functional calculations, this paper systematically investigated the Li storage properties of eight 2D M2CO2 (M = V, Cr, Ta, Sc, Ti, Zr, Nb, and Hf), which are the recently synthesized transition-metal carbides (called MXenes) with O groups. According to whether the structural transformation occurs or not during the adsorption of the first Li layer, the adsorption of Li can be grouped into two types: V-type (V2CO2, Cr2CO2, and Ta2CO2) and Sc-type (Sc2CO2, Ti2CO2, Zr2CO2, Nb2CO2, and Hf2CO2). The structural transformation behaviors of V-type are reversible during lithiation/delithiation and are confirmed by ab initio molecular dynamic simulations. Except for Nb-MXene, the V-type prefers the sandwich H2H1T-M2CO2Li4 structure and the Sc-type prefers the TH1H2-M2CO2Li4 structure during the adsorption of the second Li layer. The H2H1T-M2CO2Li4 structure of O layer sandwiched by two Li layers preferred by V-type can prevent forming Li dendrite and therefore stabilize the lithiated system. The tendency of O bonding to Li rather than M in V-type is bigger than that in Sc-type, which causes that the sandwich structure of H2H1T-M2CO2Li4 is more suitable for V-type than Sc-type.

The thermal expansion of a highly crystalline hexagonal BC2N compound synthesized under high temperature and pressure

The thermal expansion has been investigated for a highly crystalline hexagonal BC2N compound synthesized by the compression of a turbostratic B–C–N precursor with iron catalyst at the high temperature of 1500 °C and the high pressure of 5.5 GPa. The thermal expansion in the c direction is large and linear with an expansion coefficient of 35.86 × 10−6 K−1 up to 1000 °C, while in the basal plane, the a dimension displays a slight linear contraction up to 750 °C with a contraction coefficient of −8.76 × 10−7 K−1, but above 750 °C a linear expansion is observed with a larger expansion coefficient of 1.52 × 10−6 K−1.

Effects of 2-D transition metal carbide Ti2CTx on properties of epoxy composites

Here, 2-D nanostructured Ti2CTx was successfully synthesized by selectively exfoliating the Ti2AlC MAX phase via a liquid etching technique. Investigated as a promising and functional reinforcement additive, 2-D Ti2CTx nanosheets were dispersed into an epoxy matrix evenly at different mass percentages to obtain self-lubricating and anti-friction Ti2CTx/epoxy resin (EP) nanocomposites with high toughness and low creep strain by in situ intercalative polymerization. The phase structure and morphology of Ti2CTx and Ti2CTx/EP nanocomposites were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Subsequently, this work studied the effect of Ti2CTx on the EP matrix in terms of fracture surface morphology, glass transition temperature (Tg), storage modulus (Es), thermal stability, mechanical properties and dry sliding anti-friction properties. The results show that Ti2CTx interlayers were intercalated and exfoliated with EP molecular chains and intertwined with each other, which produced a greater interfacial area between the inorganic additive and the polymer matrix, resulting in the formation of Ti2CTx/EP nanocomposites without adverse structural defects. With the addition of an appropriate mass fraction of Ti2CTx, the fracture toughness and flexural strength of EP composites reached 17.8 kJ m−2 and 98 MPa, which represented increases of 76% and 66%, respectively, compared with those of pure EP. In addition, increases in Tg and Es of 15% and 38% were achieved, respectively, relative to those of pure EP when the concentration of Ti2CTx was 2.0 wt%. Moreover, tribological analysis indicated that there was a great improvement in anti-friction properties and the morphology of worn surfaces of the nanocomposites appeared to be smoother than that of pure EP, which implied the transformation of fatigue and abrasion wear into adhesion wear. This work is of great significance in that novel 2-D transition metal carbide crystals were employed to fill and modify a thermoset polymer resin and enhance its toughness and strength and, as a self-lubricating additive, enabled a great breakthrough in the anti-friction properties of polymer-matrix composites.

Novel Hierarchical TiO2/C Nanocomposite with Enhanced Photocatalytic Performance

Hierarchical TiO2/carbon nanocomposites were synthesized by oxidation of two-dimensional (2D) Ti3C2 nanosheets at different temperatures. Crystal structures and morphologies of the obtained samples were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Raman spectroscopy. The results show that 2D Ti3C2 nanosheets are partially oxidized to form a novel hierarchical nanostructure which is composed of carbon nanosheets and TiO2 nanoparticles. With the calcination temperature increasing, the crystal structure of TiO2 nanoparticles changes from anatase to rutile and the hierarchical structure was gradually destroyed. The photodegradation results reveal that the samples obtained at 200°C and 285°C show much better photocatalytic properties than P25. And meanwhile the photocatalytic property will become worse with the increase in calcinations temperature.

Synthesis and Electrochemical Properties of Two-Dimensional RGO/Ti3C2Tx Nanocomposites

MXene is a new type of two-dimensional layered material. Herein, a GO/Ti3C2Tx nanocomposite was prepared by a simple liquid phase method, and the obtained GO/Ti3C2Tx was transformed into RGO/Ti3C2Tx under high temperature with Ar/H2. The prepared samples were characterized using X-ray diffraction (XRD), Raman measurement, scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). As an electrode material in lithium-ion batteries, the RGO/Ti3C2Tx nanocomposite exhibited an excellent electrochemical performance and an excellent rate performance. Compared to pure Ti3C2Tx, the nanocomposite had a better reversible capacity at different current densities and had no attenuation after 200 cycles, which is one time higher than pure Ti3C2Tx. The improvement in the specific capacity was due to the excellent electrical conductivity and the unique structure of RGO, in which a charge transfer bridge was built among the Ti3C2Tx flakes. Such a bridge shortened the transmission distance of the electrons and ions and effectively controlled the restacking of the laminated materials.

The Synthesis Process and Thermal Stability of V2C MXene

The effect of etching solution on the synthesis process of two-dimensional vanadium carbide (V2C MXene) was researched. Three etching solutions were used to etch ternary carbide V2AlC at 90 °C. The three solutions were: lithium fluoride + hydrochloric acid (LiF + HCl), sodium fluoride + hydrochloric acid (LiF + HCl), and potassium fluoride + hydrochloric acid (KF + HCl). It was found that only NaF + HCl solution was effective for synthesizing highly pure V2C MXene. The existence of sodium (Na+) and chloridion (Cl−) in etching solution was essential for the synthesis. The thermal stability of the as-prepared V2C MXene in argon or air was studied. From thermogravimetry and differential thermal analysis, V2C MXene was found to be stable in argon atmosphere at a temperature of up to 375 °C. As the temperature increased, V2C MXene was gradually oxidized to form nanoparticles composed of vanadium trioxide (V2O3) and a part of V2C MXene was broken and transformed to vanadium carbide (V8C7) at 1000 °C. In air atmosphere, V2C MXene was stable at 150 °C. At 1000 °C, V2C MXene was oxidized to form vanadium pentoxide (V2O5).

The influence of carbon spheres on thermal and mechanical properties of epoxy composites

In order to improve the properties of epoxy resion, a reinforcement addictive of carbon spheres (CSs) was successfully synthesized by hydrothermal methods and CSs/Epoxy composite was prepared using in-situ polymerization technique. The morphology and structure of CSs and CSs/Epoxy composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that CSs distributed homogeneously in epoxy matrix and the integrating state of interface between CSs and epoxy is good. TGA and DMA analysis showed that the thermal stability of CSs/Epoxy composite in air and the glass transition temperature (Tg) increased with the addition of CSs. DMA results show that the creep recovery property also improved. Additionally. The impact fracture strength of EP composites increased after the addition of CSs compared to pure EP, it means the toughness of epoxy improved effectively, which attribute to the effective dissipation of the crash energy. Finally, the mechanism for the improvement of mechanical properties by the CSs is also discussed.

Carbon dioxide adsorption of two-dimensional carbide MXenes

Two-dimensional carbide MXenes (Ti3C2Tx and V2CTx) were prepared by exfoliating MAX phases (Ti3AlC2 and V2AlC) powders in the solution of sodium fluoride (NaF) and hydrochloric acid (HCl). The specific surface area (SSA) of as-prepared Ti3C2Tx was 21 m2/g, and that of V2CTx was 9 m2/g. After intercalation with dimethylsulfoxide, the SSA of Ti3C2Tx was increased to 66 m2/g; that of V2CTx was increased to 19 m2/g. Their adsorption properties on carbon dioxide (CO2) were investigated under 0–4 MPa at room temperature (298 K). Intercalated Ti3C2Tx had the adsorption capacity of 5.79 mmol/g, which is close to the capacity of many common sorbents. The theoretical capacity of Ti3C2Tx with the SSA of 496 m2/g was up to 44.2 mmol/g. Additionally, due to high pack density, MXenes had very high volume-uptake capacity. The capacity of intercalated Ti3C2Tx measured in this paper was 502 V·v–1. This value is already higher than volume capacity of most known sorbents. These results suggest that MXenes have some advantage features to be researched as novel CO2 capture materials.