Xianhu Zha

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

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.

The thermal and electrical properties of the promising semiconductor MXene Hf2CO2.

With the growing interest in low dimensional materials, MXenes have also attracted considerable attention recently. In this work, the thermal and electrical properties of oxygen-functionalized M2CO2 (M = Ti, Zr, Hf) MXenes are investigated using first-principles calculations. Hf2CO2 is determined to exhibit a thermal conductivity better than MoS2 and phosphorene. The room-temperature thermal conductivity along the armchair direction is determined to be 86.25~131.2 Wm−1 K−1 with a flake length of 5~100 μm. The room temperature thermal expansion coefficient of Hf2CO2 is 6.094 × 10−6 K−1, which is lower than that of most metals. Moreover, Hf2CO2 is determined to be a semiconductor with a band gap of 1.657 eV and to have high and anisotropic carrier mobility. At room temperature, the Hf2CO2 hole mobility in the armchair direction (in the zigzag direction) is determined to be as high as 13.5 × 103 cm2V−1s−1 (17.6 × 103 cm2V−1s−1). Thus, broader utilization of Hf2CO2, such as the material for nanoelectronics, is likely. The corresponding thermal and electrical properties of Ti2CO2 and Zr2CO2 are also provided. Notably, Ti2CO2 presents relatively lower thermal conductivity but much higher carrier mobility than Hf2CO2. According to the present results, the design and application of MXene based devices are expected to be promising.

Electronic structures and mechanical properties of Al(111)/ZrB2(0001) heterojunctions from first-principles calculation

Employing first-principles density functional theory (DFT), the structures and electronic and mechanical properties of Al(111)/ZrB2(0001) heterojunctions are investigated. It is found that both B-terminated ZrB2(0001) and Zr-terminated ZrB2(0001) can form heterojunction interfaces with Al(111) surface. The heterojunction with B-terminated ZrB2(0001) is demonstrated to be most stable by comparing the surface adhesion energies of six different heterojunction models. In the stable configurations, the Al atom is found projecting to the hexagonal hollow site of neighbouring boron layer for the B-terminated ZrB2(001), and locating at the top site of the boron atoms for Zr-terminated ZrB2(001) interface. The mechanisms of interface interaction are investigated by density of states, charge density difference and band structure calculations. It is found that covalent bonds between surface Al atoms and B atoms are formed in the B-terminated heterojunction, whereas the Al atoms and Zr atoms are stabilised by interface metallic bonds for the Zr-terminated case. Mechanical properties of Al/ZrB2 heterojunctions are also predicted in the current work. The values of moduli of Al/ZrB2 heterojunctions are determined to be between those of single crystal Al and ZrB2, which exhibit the transition of mechanical strength between two bulk phases. DFT calculations with the current models provide the mechanical properties for each heterojunction and the corresponding contributions by each type of interface in the composite materials. This work paves the way for industrial applications of Al(111)/ZrB2(0001) heterojunctions.

New insight into the helium-induced damage in MAX phase Ti3AlC2 by first-principles studies

In the present work, the behavior of He in the MAX phase Ti3AlC2 material is investigated using first-principle methods. It is found that, according to the predicted formation energies, a single He atom favors residing near the Al plane in Ti3AlC2. The results also show that Al vacancies are better able to trap He atoms than either Ti or C vacancies. The formation energies for the secondary vacancy defects near an Al vacancy or a C vacancy are strongly influenced by He impurity content. According to the present results, the existence of trapped He atoms in primary Al vacancy can promote secondary vacancy formation and the He bubble trapped by Al vacancies has a higher tendency to grow in the Al plane of Ti3AlC2. The diffusion of He in Ti3AlC2 is also investigated. The energy barriers are approximately 2.980 eV and 0.294 eV along the c-axis and in the ab plane, respectively, which means that He atoms exhibit faster migration parallel to the Al plane. Hence, the formation of platelet-like bubbles nucleated from the Al vacancies is favored both energetically and kinetically. Our calculations also show that the conventional spherical bubbles may be originated from He atoms trapped by C vacancies. Taken together, these results are able to explain the observed formation of bubbles in various shapes in recent experiments. This study is expected to provide new insight into the behaviors of MAX phases under irradiation from electronic structure level in order to improve the design of MAX phase based materials.

Controllable magnitude and anisotropy of the electrical conductivity of Hf3C2O2 MXene

Hf3C2O2, a new MXene member synthesized recently, was predicted to be a semi-metal with high mechanical strength. Based on the unique electronic structure, the energy bands and electrical conductivities of the MXene under various strains are comprehensively investigated in this paper. Biaxial and two orthogonal uniaxial strains in both compressive and tensile manners are studied. Results from this study suggest that Hf3C2O2 shows a transition between semi-metal and semi-conductor under both biaxial and uniaxial strains. A compressive strain generally induces a larger energy overlap between the conduction band minimum and the valance band maximum, while a tensile strain reduces the energy band overlap and even opens a band gap. As a consequence, the magnitude of electrical conductivity decreases drastically from compressive to tensile strains applied. Moreover, the uniaxial strains are determined to be efficient in manipulating the anisotropy of the electrical conductivity. These data imply that the Hf3C2O2 MXene is a promising candidate material for devices such as strain sensors.

Designing a reductive hybrid membrane to selectively capture noble metallic ions during oil/water emulsion separation with further function enhancement

Owing to the ever-increasing demand for noble metals in modern industry, the extraction of noble metals from ores and electronic wastes is a significant topic. Conventional extraction means involving surfactants, organic solvents, and toxic extracting agents suffer from the limitation of complex heterogeneous separation and extraction operation as well as environmental pollution. Herein, a new carbon nanotube (CNT) hybrid membrane modified with eco-friendly and reductive poly acryloyl hydrazide (PAH) is reported, integrating the extraction of noble metal ions with heterogeneous emulsion separation. The hybrid membrane with underwater superoleophobic surface can achieve one-step preferential extraction of noble metal ions during oil/water emulsion separation, greatly simplifying the extraction operation. The reductive extraction yields nanoparticles loaded in situ on CNTs, which allows precise evaluation of the recovery performances by monitoring resistance variation of the hybrid membrane. Furthermore, the extracted hybrid membrane can be recycled for the catalytic conversion of organic contaminants as well as emulsion separation. The multi-functional hybrid membrane realizes comprehensive recovery of noble metal ions and heterogeneous separation for further recycling utilization, showing great potential for practical application towards the simple and integrated recycling of noble metals.

First-principles study on the electrical and thermal properties of the semiconducting Sc3(CN)F2 MXene

The two-dimensional materials MXenes have recently attracted interest for their excellent performance from diverse perspectives indicated by experiments and theoretical calculations. For the application of MXenes in electronic devices, the exploration of semiconducting MXenes arouses particular interest. In this work, despite the metallic properties of Sc3C2F2 and Sc3N2F2, we find that Sc3(CN)F2 is a semiconductor with an indirect band gap of 1.18 eV, which is an expansion of the semiconducting family members of MXene. Using first-principles calculations, the electrical and thermal properties of the semiconducting Sc3(CN)F2 MXene are studied. The electron mobilities are determined to possess strong anisotropy, while the hole mobilities show isotropy, i.e. 1.348 × 103 cm2 V−1 s−1 along x, 0.319 × 103 cm2 V−1 s−1 along the y directions for electron mobilities, and 0.517 × 103 cm2 V−1 s−1 along x, 0.540 × 103 cm2 V−1 s−1 along the y directions for hole mobilities. The room-temperature thermal conductivity along the Γ → M direction is determined to be 123–283 W m−1 K−1 with a flake length of 1–100 μm. Besides, Sc3(CN)F2 presents a relatively high specific heat of 547 J kg−1 K−1 and a low thermal expansion coefficient of 8.703 × 10−6 K−1. Our findings suggest that the Sc3(CN)F2 MXene might be a candidate material in the design and application of 2D nanoelectronic devices.