Libo Wang

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.

Preparation and methane adsorption of two-dimensional carbide Ti2C

Here a novel material for methane adsorption was synthesized and studied, which is a graphene-like two-dimensional (2D) carbide (Ti2C, a member of MXenes), formed by exfoliating Ti2AlC powders in a solution of lithium fluoride (LiF) and hydrochloric acid (HCl) at 40xa0°C for 48xa0h. Based on first-principles calculation, theoretically perfect Ti2C with O termination has a specific surface area (SSA) of 671xa0m2xa0g−1 and methane storage capacity is 22.9xa0wt%. Experimentally, 2.85xa0% exfoliated Ti2C with mesopores shown methane capacity of 11.58xa0cm3 (STP: 0xa0°C, 1xa0bar)xa0g−1 (0.82xa0wt%) under 5xa0MPa and the SSA was 19.1xa0m2xa0g−1. For Ti2C sample intercalated with NH3·H2O, the adsorbed amount was increased to 16.81xa0cm3 (STP) g−1 at same temperature. At the temperature of 323xa0K, the adsorbed amount of as-prepared Ti2C was increased to 52.76xa0cm3 (STP)xa0g−1. For fully exfoliated Ti2C, the methane capacity was supposed to be 28.8xa0wt% or 1148xa0V (STP)v−1. Ti2C theoretically has much larger volume methane capacity than current methane storage materials, though its SSA is not very high.

Electrochemical performance of Ti3C2 supercapacitors in KOH electrolyte

Two-dimensional (2D) carbide Ti3C2 was synthesized by exfoliating Ti3AlC2 in HF solution and used for supercapacitive performance investigation in 3 M KOH electrolyte. The specific surface area (SSA) of as-synthesized Ti3C2 was 22.35 m2/g. Ti3C2-based supercapacitor electrodes exhibited good energy storage ability and had a volumetric capacitance 119.8 F/cm3 at the current density of 2.5 A/g. Moreover, the addition of carbon black into Ti3C2 powders greatly improved the performance of Ti3C2-based capacitors because carbon black restrained the preferred orientation of 2D Ti3C2, providing fast ion transport channels, and in turn, decreasing electrical resistance from 16.7 Ω to 3.5 Ω.

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.

Photoluminescence from Eu ions implanted SiO2 thin films

Abstract Photoluminescence (PL) and PL excitation spectra from Eu 3+ implanted thermal growth SiO 2 thin films have been investigated at room temperature. After annealing at 1000°C in N 2 , the red light emission from Eu ions doped SiO 2 film, corresponding to the 5 D 0 – 7 F J transition of Eu 3+ , was observed. With increasing the annealing temperature ( T a ) to 1200°C, a strong blue light emission band centered at around 450 nm appears. The conversion of Eu 3+ to Eu 2+ is discussed.

Corrosion behavior of Ti3AlC2 in molten KOH at 700 °C

This paper investigated the corrosion behaviors of Ti3AlC2 at 700 °C in molten KOH with various mass ratios. If the mass ratio of KOH:Ti3AlC2⩽2, Ti3AlC2 can resist KOH hot corrosion in 2 h. Ti3AlC2 suffered serious corrosion attack if the mass ratio⩾3. The main compositions of corroded samples were amorphous graphite and potassium titanates (K2O·nTiO2). If the samples were washed by acid and dried, potassium titanates could decompose to K2O and amorphous rutile. Based on the experimental results, a corrosion mechanism of Ti3AlC2 in molten KOH was proposed.

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.