Shifts in Valence States in Bimetallic MXenes Revealed by Electron Energy-Loss Spectroscopy (EELS)

MXenes are an emergent class of two-dimensional materials with a very wide spectrum of promising applications. The synthesis of multiple MXenes, specifically solid-solution MXenes, allows fine tuning of their properties, expands their range of applications, and leads to enhanced performance. The functionality of solid-solution MXenes is closely related to the valence state of their constituents: transition metals, oxygen, carbon, and nitrogen. However, the impact of changes in the oxidation state of elements in MXenes is not well understood. In this work, three interrelated solid-solution MXene systems (Ti2-yNbyCTx, Nb2-yVyCTx, and Ti2-yVyCTx) were investigated with scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) to determine the localized valence states of metals at the nanoscale. The analysis demonstrates changes in the electronic configuration of V upon modification of the overall composition and within individual MXene flakes. These shifts of oxidation state can explain the nonlinear optical and electronic features of solid-solution MXenes. Vanadium appears to be particularly sensitive to modification of the valence state, while titanium maintains the same oxidation state in Ti-Nb and Ti-V MXenes, regardless of stoichiometry. The study also explains Nb's influential role in the previously observed electronic properties in the Nb-V and Nb-Ti systems.

Hydrogen Permeation Behavior of Zirconium Nitride Film on Zirconium Hydride

Hydrogen permeation barrier plays an important role in reducing hydrogen loss from zirconium hydride matrix when used as neutron moderator. Here, a composite nitride film was prepared on zirconium hydride by in situ reaction method in nitrogen atmosphere. The phase structure, morphology, element distribution, and valence states of the composite film were investigated by XRD, SEM, AES, and XPS analysis. It was found that the composite nitride film was continuous and dense with about 1.6 μm thickness; the major phase of the film was ZrN, with coexistence of ZrO2, ZrO, and ZrN0.36H0.8; and Zr-C, Zr-O, Zr-N, O-H, and N-H bonds were detected in the film. The existence of ZrN0.36H0.8 phase and the bonds of O-H and N-H revealed that the nitrogen and oxygen in the film could capture hydrogen from the zirconium hydride matrix. The hydrogen permeation performance of nitride film was compared with oxide film by permeation reduction factor (PRF), vacuum thermal dehydrogenation (VTD), and hydrogen permeation rate (HPR) methods, and the results showed that the hydrogen permeation barrier effects of nitride film were better than that of oxide film. The zirconium nitride film would be a potential candidate for hydrogen permeation barrier on the surface of zirconium hydride.

Mechanism of hopping conduction in Be–Fe–Al–Te–O semiconducting glasses and glass–ceramics

Electrical properties of beryllium-alumino-tellurite glasses and glass–ceramics doped with iron ions were studied using impedance spectroscopy. The conductivity was measured over a wide frequency range from 10 mHz to 1 MHz and the temperature range from 213 to 473 K. The D.C. conductivity values showed a correlation with the Fe-ion concentration and ratio of iron ions on different valence states in the samples. On the basis of Jonscher universal dielectric response the temperature dependence of conductivity parameters were determined and compared to theoretical models collected by Elliott. In glasses, the conduction process was found to be due to the overlap polaron tunneling while in glass–ceramics the quantum mechanical tunneling between semiconducting crystallites of iron oxides is proposed. The D.C. conductivity was found not to follow Arrhenius relation. The Schnakenberg model was used to analyze the conductivity behavior and the polaron hopping energy and disorder energy were estimated. Additionally, the correlation between alumina dissolution and basicity of the melts was observed.

High Thermoelectric Performance Achieved in Sb-Doped GeTe by Manipulating Carrier Concentration and Nanoscale Twin Grains

Lead-free and eco-friendly GeTe shows promising mid-temperature thermoelectric applications. However, a low Seebeck coefficient due to its intrinsically high hole concentration induced by Ge vacancies, and a relatively high thermal conductivity result in inferior thermoelectric performance in pristine GeTe. Extrinsic dopants such as Sb, Bi, and Y could play a crucial role in regulating the hole concentration of GeTe because of their different valence states as cations and high solubility in GeTe. Here we investigate the thermoelectric performance of GeTe upon Sb doping, and demonstrate a high maximum zT value up to 1.88 in Ge0.90Sb0.10Te as a result of the significant suppression in thermal conductivity while maintaining a high power factor. The maintained high power factor is due to the markable enhancement in the Seebeck coefficient, which could be attributed to the significant suppression of hole concentration and the valence band convergence upon Sb doping, while the low thermal conductivity stems from the suppression of electronic thermal conductivity due to the increase in electrical resistivity and the lowering of lattice thermal conductivity through strengthening the phonon scattering by lattice distortion, dislocations, and twin boundaries. The excellent thermoelectric performance of Ge0.90Sb0.10Te shows good reproducibility and thermal stability. This work confirms that Ge0.90Sb0.10Te is a superior thermoelectric material for practical application.

First principles calculation of electronic properties and effective mass of zinc-blende GaN

Based on the first principle pseudopotential plane wave method, the electronic structure of zinc-blende semiconductor GaN is calculated. Using the relativistic treatment of valence states, the spin orbit splitting energy of valence band top near the center of Brillouin region is calculated. Based on the effective mass approximation theory, the effective mass of electrons near the bottom of the conduction band and the effective mass of light and heavy holes near the Γ point along the directions of [100], [110] and [111] are calculated. These parameters are valuable and important parameters of optoelectronic materials.

Europium modified TiO2 as a high-rate long-cycle life anode material for lithium-ion batteries

Europium modified TiO2 with different valence states was prepared by hydrothermal method and calcined in air. Microscopic analyses revealed that the average particle size of europium-modified TiO2 nanoparticles is approximately...

Unexpected Selective Absorption of Lithium in Thermally Reduced Graphene Oxide Membranes

Lithium plays an increasingly important role in scientific and industrial processes, and it is extremely important to extract lithium from a high Mg2+/Li+ mass ratio brine or to recover lithium from the leachate of spent lithium-ion batteries. Conventional wisdom shows that Li+ with low valence states has a much weaker adsorption (and absorption energy) with graphene than multivalent ions such as Mg2+. Here, we show the selective adsorption of Li+ in thermally reduced graphene oxide (rGO) membranes over other metal ions such as Mg2+, Co2+, Mn2+, Ni2+, or Fe2+. Interestingly, the adsorption strength of Li+ reaches up to 5 times the adsorption strength of Mg2+, and the mass ratio of a mixed Mg2+/Li+ solution at a very high value of 500 : 1 can be effectively reduced to 0.7 : 1 within only six experimental treatment cycles, demonstrating the excellent applicability of the rGO membranes in the Mg2+/Li+ separation. A theoretical analysis indicates that this unexpected selectivity is attributed to the competition between cation–π interaction and steric exclusion when hydrated cations enter the confined space of the rGO membranes.

Theoretical Study of the Rotational Structure of the c4′1Σu + (6)–X1Σg +(0–9) Absorption Bands of N2

We have theoretically determined the absorption oscillator strengths and wavenumbers for rotationally resolved transitions of the c4′1Σu + (6)-X1Σg +(0–9) bands of N2, which are relevant to analyze the spectra of planetary atmospheres. The Molecular Quantum Defect Orbital method has been used in our calculations. The interaction between the c4′1Σu + (6) Rydberg state and the b′1Σu + valence states has been considered using an adequate rovibronic energy matrix. In addition, we have calculated the lifetimes of the rotational levels of the c4′1Σu + (6) state. We hope that the reported data, most of them for the first time, can be useful in the interpretation of planetary atmospheres where N2 is present.