Zi-Zhong Zhu

Ru0.01Ti0.99Nb2O7 as an intercalation-type anode material with a large capacity and high rate performance for lithium-ion batteries

RuxTi1−xNb2O7 (x = 0 and 0.01) materials have been synthesized via a solid-state reaction method. X-ray diffraction combined with Rietveld refinements demonstrates that both samples have a Wadsley–Roth shear structure with a C2/m space group without any impurities, and that the unit cell volume increases after the trace Ru4+ doping. Scanning electron microscopy and specific surface area tests reveal that the Ru4+ doping decreases the average particle size. The Li+ ion diffusion coefficient and electronic conductivity of Ru0.01Ti0.99Nb2O7 are respectively 64% and at least two orders of magnitude larger than those of the pristine TiNb2O7. First-principles calculations show that the increased electronic conductivity can result from the formation of impurity bands after the Ru4+ doping. Ru0.01Ti0.99Nb2O7 exhibits a large initial discharge capacity of 351 mA h g−1 at 0.1 C between 3.0 and 0.8 V vs. Li/Li+, approaching its theoretical capacity (388 mA h g−1). At 5 C, unlike the pristine TiNb2O7 with a small charge capacity of 115 mA h g−1, Ru0.01Ti0.99Nb2O7 delivers a large value of 181 mA h g−1, even exceeding the theoretical capacity of the popular spinel Li4Ti5O12 (175 mA h g−1). After 100 cycles, Ru0.01Ti0.99Nb2O7 shows a large capacity retention of 90.1%. These outstanding electrochemical performances can be attributed to its improved Li+ ionic and electronic conductivity as well as smaller particle size.

Structure and stability of platinum nanocrystals: from low-index to high-index facets

High index surfaces are introduced into Pt nanocrystals because they are expected to exhibit higher catalytic activity than low index planes such as {111}, {100}, and even {110}. This article presents a systematic investigation on the structure and stability of polyhedral Pt nanocrystals with both low-index and high-index facets by means of atomistic simulations. It has been found that the stability of Pt nanocrystals depends strongly on the particle shape and surface structures. Those nanocrystals, enclosed by high-index facets of {310}, {311}, and {331}, possess better stability and higher dangling bond density of surface compared with those ones with low-index facets, such as {100} and {110}, suggesting that they should become preferential candidates for nanocatalysts. The octahedral nanocrystals with {111} facets, though they have excellent structural and thermal stabilities, present the lowest dangling bond density of surface.

Direct to indirect band gap transition in ultrathin ZnO nanowires under uniaxial compression

National Natural Science Foundation of China [10702056, 10774124]; Program for New Century Excellent Talents in Fujian Province University, China

Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices.

Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS2 monolayer using the first principles approach. The results are compared with those of graphene/MoS2 superlattice. The distortions of the geometry of germanene, silicene, and MoS2 layers due to the formation of the superlattices are all relatively small, resulting from the relatively weak interactions between the stacking layers. Our results show that both the germanene/MoS2 and silicene/MoS2 superlattices are manifestly metallic, with the linear bands around the Dirac points of the pristine germanene and silicene seem to be preserved. However, small band gaps are opened up at the Dirac points for both the superlattices due to the symmetry breaking in the germanene and silicene layers caused by the introduction of the MoS2 sheets. Moreover, charge transfer happened mainly within the germanene (or silicene) and the MoS2 layers (intra-layer transfer), as well as some part of the intermediate regions between the germanene (or silicene) and the MoS2 layers (inter-layer transfer), suggesting more than just the van der Waals interactions between the stacking sheets in the superlattices.

Cr0.5Nb24.5O62 Nanowires with High Electronic Conductivity for High-Rate and Long-Life Lithium-Ion Storage

Intercalation-type TiNbxO2+2.5x (x = 2, 5, and 24) anode materials have recently become more interesting for lithium-ion batteries (LIBs) due to their large theoretical capacities of 388-402 mAh g-1. However, the Ti4+/Nb5+ ions in TiNbxO2+2.5x with empty 3d/4d orbitals usually lead to extremely low electronic conductivity of <10-9 S cm-1, greatly restricting their practical capacity and rate capability. Herein, we report a class of highly conductive Cr0.5Nb24.5O62 nanowires as an intercalation-type anode material for high-performance LIBs. The as-made Cr0.5Nb24.5O62 nanowires show an open shear ReO3 crystal structure (C2 space group) with 4% tetrahedra and a conducting characteristic with ultrahigh electronic conductivity of 3.6 × 10-2 S cm-1 and a large Li+-ion diffusion coefficient of 2.19 × 10-13 cm2 s-1. These important characteristics make them deliver outstanding electrochemical properties in term of the largest reversible capacity (344 mAh g-1 at 0.1 C) in all the known niobium- and titanium-based anode materials, safe working potential (∼1.65 V vs Li/Li+), high first-cycle Coulombic efficiency (90.8%), superior rate capability (209 mAh g-1 at 30 C), and excellent cycling stability, making them among the best for LIBs in niobium- and titanium-based anode materials.

Zero-Strain Na2FeSiO4 as Novel Cathode Material for Sodium-Ion Batteries

A new cubic polymorph of sodium iron silicate, Na2FeSiO4, is reported for the first time as a cathode material for Na-ion batteries. It adopts an unprecedented cubic rigid tetrahedral open framework structure, i.e., F4̅3m, leading to a polyanion cathode material without apparent cell volume change during the charge/discharge processes. This cathode shows a reversible capacity of 106 mAh g(-1) and a capacity retention of 96% at 5 mA g(-1) after 20 cycles.

Defective Ti2Nb10O27.1: an advanced anode material for lithium-ion batteries.

To explore anode materials with large capacities and high rate performances for the lithium-ion batteries of electric vehicles, defective Ti2Nb10O27.1 has been prepared through a facile solid-state reaction in argon. X-ray diffractions combined with Rietveld refinements indicate that Ti2Nb10O27.1 has the same crystal structure with stoichiometric Ti2Nb10O29 (Wadsley-Roth shear structure with A2/m space group) but larger lattice parameters and 6.6% O2– vacancies (vs. all O2– ions). The electronic conductivity and Li+ion diffusion coefficient of Ti2Nb10O27.1 are at least six orders of magnitude and ~2.5 times larger than those of Ti2Nb10O29, respectively. First-principles calculations reveal that the significantly enhanced electronic conductivity is attributed to the formation of impurity bands in Ti2Nb10O29–x and its conductor characteristic. As a result of the improvements in the electronic and ionic conductivities, Ti2Nb10O27.1 exhibits not only a large initial discharge capacity of 329 mAh g–1 and charge capacity of 286 mAh g–1 at 0.1 C but also an outstanding rate performance and cyclability. At 5 C, its charge capacity remains 180 mAh g–1 with large capacity retention of 91.0% after 100 cycles, whereas those of Ti2Nb10O29 are only 90 mAh g–1 and 74.7%.

Enhanced thermal stability of Au@Pt nanoparticles by tuning shell thickness: Insights from atomistic simulations

Development of core–shell bimetallic nanoparticles with bifunctional catalytic activity and excellent stability is a challenging issue in nanocatalyst synthesis. Here we present a detailed study of thermal stabilities of Au-core/Pt-shell nanoparticles with different core sizes and shell thicknesses. Molecular dynamics simulations are used to provide insights into the melting and diffusive behavior at atomic-level. It is found that the thermal stabilities of core-shell nanoparticles are significantly enhanced with increasing thickness of Pt shell. Meanwhile, the melting mechanism is strongly dependent on the shell thickness. When the core size or shell thickness is very small, the melting is initiated in the shell and gradually spreads into the core, similar to that of monometallic nanoparticles. As the core increases up to moderate size, an inhomogeneous melting has been observed. Due to the relatively weak confinement of thin shell, local lattice instability preferentially takes place in the core, leading to the inhomogeneous premelting of Au core ahead of the overall melting of Pt shell. The diffusion coefficients of both Au and Pt are decreased with the increasing thickness of shell, and the difference in their diffusions favors the formation of inhomogeneous atomic distributions of Au and Pt. The study is of considerable importance for improving the stability of Pt-based nanocatalysts by tuning the shell thickness and core size.

Surface-passivation-induced metallic and magnetic properties of ZnO graphitic sheet

First-principles calculations were used to investigate the electronic and magnetic properties of surface-passivated ZnO graphitic sheets. The results show that ZnO graphitic sheet with hydrogenation on both O and Zn atoms exhibits indirect band gap, while ZnO graphitic sheet is found to be metallic for hydrogenation on only O atoms and magnetic semiconducting for surface passivation by H or NH2 on only Zn atoms. The relative stability of ZnO graphitic sheet passivated by H or NH2 has also been discussed.First-principles calculations were used to investigate the electronic and magnetic properties of surface-passivated ZnO graphitic sheets. The results show that ZnO graphitic sheet with hydrogenation on both O and Zn atoms exhibits indirect band gap, while ZnO graphitic sheet is found to be metallic for hydrogenation on only O atoms and magnetic semiconducting for surface passivation by H or NH2 on only Zn atoms. The relative stability of ZnO graphitic sheet passivated by H or NH2 has also been discussed.

Quantum Spin Hall States in Stanene/Ge(111)

For topological insulators to be implemented in practical applications, it is a prerequisite to select suitable substrates that are required to leave insulators’ nontrivial properties and sizable opened band gaps (due to spin-orbital couplings) unaltered. Using ab initio calculations, we predict that Ge(111) surface qualified as a candidate to support stanene sheets, because the band structure of √3 × √3 stanene/Ge(111) (2 × 2) surface displays a typical Dirac cone at Γ point in the vicinity of the Fermi level. Aided with the result of Z2 invariant calculations, a √3 × √3 stanene/Ge(111) (2 × 2) system has been proved to sustain the nontrivial topological phase, with the prove being confirmed by the edge state calculations of stanene ribbons. This finding can serve as guidance for epitaxial growth of stanene on substrate and render stanene feasible for practical use as a topological insulator.