Qiang Tao

Highly Active, Nonprecious Electrocatalyst Comprising Borophene Subunits for the Hydrogen Evolution Reaction

Developing nonprecious hydrogen evolution electrocatalysts that can work well at large current densities (e.g., at 1000 mA/cm2: a value that is relevant for practical, large-scale applications) is of great importance for realizing a viable water-splitting technology. Herein we present a combined theoretical and experimental study that leads to the identification of α-phase molybdenum diboride (α-MoB2) comprising borophene subunits as a noble metal-free, superefficient electrocatalyst for the hydrogen evolution reaction (HER). Our theoretical finding indicates, unlike the surfaces of Pt- and MoS2-based catalysts, those of α-MoB2 can maintain high catalytic activity for HER even at very high hydrogen coverage and attain a high density of efficient catalytic active sites. Experiments confirm α-MoB2 can deliver large current densities in the order of 1000 mA/cm2, and also has excellent catalytic stability during HER. The theoretical and experimental results show α-MoB2s catalytic activity, especially at large current densities, is due to its high conductivity, large density of efficient catalytic active sites and good mass transport property.

High catalytic activity of a PbS counter electrode prepared via chemical bath deposition for quantum dots-sensitized solar cells

A PbS counter electrode (CE) has been fabricated by a chemical bath deposition method, and can function as a counter electrode with high catalytic activity for quantum dots-sensitized solar cells (QDSSCs). The PbS nanoparticles can act as an excellent electrical tunnel for fast electron transport from an external circuit to the CE. Electrochemical impedance spectroscopy reveals a low charge transfer resistance (Rct1) between polysulfide and PbS, the optimized PbS CE shows an Rct1 as low as 15.42 Ω cm2. The current density–voltage curves of the QDSSCs were investigated under AM 1.5 light at 100 mW cm−2. CdS quantum dots-sensitized solar cells with the PbS CE achieved a power energy conversion efficiency of 2.91% and showed no obvious degradation of current density over 72 h under ambient conditions.

Enhanced Vickers hardness by quasi-3D boron network in MoB2

Molybdenum borides including α-MoB2 and β-MoB2 have been successfully synthesized from boron and molybdenum at high pressure and high temperature (HPHT). The crystalline structures are confirmed by Rietveld refinements in the hexagonal (P6/mmm) and rhombohedral (R-3m) crystal systems for α- and β-MoB2, respectively. The values of Vickers hardness (HV) are 15.2 GPa for α-MoB2, which is firstly obtained, and 22.0 GPa for β-MoB2. The hardness results for α- and β-MoB2 are in good agreement with theoretical values calculated by first-principle calculations. The difference in hardness between α- and β-MoB2 is attributed to the puckered quasi-3D (three dimensional) boron layers in β-MoB2 which is confirmed by the calculated results of the Electron Localization Function (ELF) and elastic constants. These results are helpful to understand the hardness mechanism and to design superhard transition-metal borides (TMBs).

Manganese borides synthesized at high pressure and high temperature

We synthesized various kinds of manganese borides by liquid-solid reaction from manganese and amorphous boron powders at high pressure and high temperature. The mixed powders with various atomic ratio of boron to manganese, B/Mn = 0.6, 1.2, 2, 3, 4, 8, were treated at temperature 1100–1350 °C and pressure 4.8-5.5 GPa, for 10–285 min. Typical manganese borides such as Mn4B, Mn2B, MnB, Mn3B4, MnB2, MnB4, and MnBx were synthesized. MnB2 plays an important role in the reaction, which is the middle product and it changes to Mn3B4 and boron with increasing holding time at a mixed atomic ratio of B/Mn = 2 and MnB2 reacted with boron to MnB4 with increasing holding time at a mixed atomic ratio of B/Mn = 8. The preferred orientation of Mn3B4 was also obtained at a mixed atomic ratio B/Mn = 2 and whose growth is highly oriented. The crystalline phases MnB2, Mn3B4, MnB, and Mn2B were prepared with large excess boron content.

In situ measurement of electrical resistivity and Seebeck coefficient simultaneously at high temperature and high pressure.

A method for performing simultaneous measurements of the electrical resistivity and the Seebeck coefficient at high pressure and high temperature (HPHT) in cubic multi-anvil apparatus is described. For high pressure and high temperature measurements, a four-probe arrangement is used to measure the electrical resistivity and two pairs of chromel-alumel type thermocouples are employed to determine the Seebeck coefficient, respectively. Results of an expected temperature-induced phase transition, pressure-induced metallization and enhancement of the thermoelectric properties were obtained in Ag2Te. This method can provide the necessary data of thermoelectric materials at HPHT.

Manganese mono-boride, an inexpensive room temperature ferromagnetic hard material

We synthesized orthorhombic FeB-type MnB (space group: Pnma) with high pressure and high temperature method. MnB is a promising soft magnetic material, which is ferromagnetic with Curie temperature as high as 546.3 K, and high magnetization value up to 155.5 emu/g, and comparatively low coercive field. The strong room temperature ferromagnetic properties stem from the positive exchange-correlation between manganese atoms and the large number of unpaired Mn 3d electrons. The asymptotic Vickers hardness (AVH) is 15.7 GPa which is far higher than that of traditional ferromagnetic materials. The high hardness is ascribed to the zigzag boron chains running through manganese lattice, as unraveled by X-ray photoelectron spectroscopy result and first principle calculations. This exploration opens a new class of materials with the integration of superior mechanical properties, lower cost, electrical conductivity, and fantastic soft magnetic properties which will be significant for scientific research and industrial application as advanced structural and functional materials.

Synthesis and Mechanical Character of Hexagonal Phase δ−WN

In this work, high-quality bulk WC-structured WN (δ-WN) was synthesized via an untraditional method and the structure was accurately determined by X-ray diffraction and Rietveld refinement. In the process of synthesizing δ-WN, W2N3 and melamine were used as tungsten source and nitrogen source, respectively. The result of successfully synthesized high-quality δ-WN indicates that our method is an effective route for synthesizing high-quality bulk δ-WN and melamine is a pure nitrogen source for introducing the nitrogen to the metal precursor. The mechanical properties, bulk modulus, and Vickers hardness (HV) were first investigated by in situ high-pressure X-ray diffraction and Vickers microhardness tests, respectively. It is worth noting that the bulk modulus of δ-WN is 373 ± 8.3 GPa, which is comparable to that of c-BN. The Vickers hardness is 13.8 GPa under an applied load of 4.9 N. It is worth noting that W-W metallic bond and W-N ionic bond are mainly chemical bond in δ-WN based on the analysis of electron localization function (ELF), density of states (DOS), and Mulliken population. This result can well clarify that δ-WN is only a hard material for the lack of strong W-N covalent bonds to form 3D network structure. Our results are helpful to understand the hardness mechanism and design superhard materials in transition-metal nitrides.

Pressure induced structural transition of small carbon nano-onions

Small carbon nano-onions (S-CNOs) were prepared by annealing nanodiamonds (ND) in an argon atmosphere. The structure and morphology of S-CNOs were determined by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) and the average grain size of the S-CNOs was about 8 nm. In situ high pressure Raman spectra of S-CNOs were investigated by diamond anvil cell experiments at pressures up to 22.5 GPa. A reversible structural transition occurred at about 7.4 GPa, resulting from the polygonization of S-CNOs. The structural transition pressure of S-CNOs is lower than that of large CNOs (L-CNOs) and onion-like carbon (OLC) nanospheres. S-CNOs derived from the annealing of ND have a high defect density, a large number of sp3 bonds and high free energy. In addition, high pressure can be generated in the interior of S-CNOs at high temperatures. The results indicated that nanotwinned diamond (nt-diamond) may be prepared using S-CNOs derived from the annealing of NDs as a raw material below 10 GPa, which is much lower than the pressure needed for synthesizing nano-polycrystalline diamond (NPD) and nt-diamond with other carbon resources (usually more than 15 GPa).

An ultra-incompressible ternary transition metal carbide

The ternary transition metal carbide Mo0.5W0.5C was synthesized under high pressure and high temperature and the crystalline structure was confirmed by Rietveld refinements as being hexagonal (Pm2). The mechanical properties, bulk modulus and Vickers hardness were also investigated by in situ high-pressure X-ray diffraction and Vickers microhardness testing, respectively. The fitting bulk modulus of the ternary transition metal carbide is 399.9 ± 9.3 GPa, which is as compressible as diamond, and its asymptotic Vickers hardness is 15.3 GPa, nearly 60% harder than molybdenum carbide. The high bulk modulus is attributed to the high valence electron density and the greater hardness compared with γ-MoC is due to the strong bond between tungsten and carbon atoms.

Pressure-induced disordering and phase transformations in Eu2Zr2O7 pyrochlore

ABSTRACT The structural properties of pyrochlore Eu2Zr2O7 under high pressure have been studied by using Raman spectroscopy and in situ angle-dispersive X-ray diffraction (ADXRD). The results of Raman spectra indicate that Eu2Zr2O7 undergoes a reversible structural change around 21.2 GPa. The results of Rietveld refinements from in situ ADXRD data indicate that the ordered pyrochlore structure (Fd-3m) transforms to the defect-cotunnite structure (Pnma) at 26.5 GPa. The phase transition is irreversible and the transformation process is mainly induced by the accumulations of anti-site defects of the cation sublattice and Frenkel defects on the anion sublattice. Besides, the bonds should play a more important role than the bonds in the process of the phase transformation.