Quanjun Li

Synthesis of High-Density Nanocavities inside TiO2−B Nanoribbons and Their Enhanced Electrochemical Lithium Storage Properties

Single crystalline TiO2-B nanoribbons with high-density nanocavities were successfully synthesized via a simple hydrothermal route. The as-prepared TiO2-B nanoribbons exhibited a large Brunauer, Emmett, and Teller (BET) surface area of about 305 m(2)/g because of the high-density nanocavities inside the thin nanoribbons. Electrochemical measurements indicated that the TiO2-B nanoribbons with dense nanocavities showed discharge specific capacity higher than those of TiO2-B nanotubes and nanowires. It was found that the dense nanocavities have an important influence on the electrochemical lithium intercalation properties.

Synthesis of ZnS nanocrystals with controllable structure and morphology and their photoluminescence property

The controllable synthesis of ZnS nanocrystals with desirable morphology and correlative structure has been carried out via the solvothermal method by simply changing the molar ratio of the reactants. The hexagonal-shaped ZnS nanosheets with a zinc-blende structure were synthesized in one step for the first time. ZnS nanorods with wurtzite structure and large ratio of length to diameter were also fabricated. We found that phase transformation is easily induced and there is a strong correlation between the morphology and structure of the ZnS nanocrystals by changing the ratio of the reactants. The photoluminescence spectra of the ZnS nanosheets and nanorods exhibit different emission bands. ZnS nanosheets show a strong emission at 534 nm while the nanorods have two emissions located at 520 and 578 nm.

Predicting new superhard phases

The search for new superhard materials is of great importance in view of their major roles played for the fundamental science and the industrial applications. Recent experimental synthesis has made several great successes, but the difficulties associated with synthesis in general remain. Materials design technique is greatly desirable as a request to assist experiment. In this paper, two rational theoretical methods of design of superhard materials have been reviewed: (i) substitutional method, which is successful in some cases, but limited to the known chemically related phases, and (ii) global free energy minimization method, which can be applied to large scale of materials with the only information of chemical compositions. The successful applications have been described and the main principles are summarized.

Rotational dynamics of confined C60 from near-infrared Raman studies under high pressure.

Peapods present a model system for studying the properties of dimensionally constrained crystal structures, whose dynamical properties are very important. We have recently studied the rotational dynamics of C60 molecules confined inside single walled carbon nanotube (SWNT) by analyzing the intermediate frequency mode lattice vibrations using near-infrared Raman spectroscopy. The rotation of C60 was tuned to a known state by applying high pressure, at which condition C60 first forms dimers at low pressure and then forms a single-chain, nonrotating, polymer structure at high pressure. In the latter state the molecules form chains with a 2-fold symmetry. We propose that the C60 molecules in SWNT exhibit an unusual type of ratcheted rotation due to the interaction between C60 and SWNT in the “hexagon orientation,” and the characteristic vibrations of ratcheted rotation becomes more obvious with decreasing temperature.

Reversible Polymerization in Doped Fullerides Under Pressure: The Case Of C60(Fe(C5H5)2)2

High-pressure Raman studies have been carried out on single crystalline C(60)(Fc)(2) nanosheets up to 25.4 GPa. Our results show that the charge transfer between Fc (ferrocene) and C(60) increases in the low-pressure range. Above 5 GPa, C(60) molecules start to form a chainlike polymer structure, and this polymerization is reversible upon decompression, in contrast to that of pristine C(60). The special layered structure of C(60)(Fc)(2) restricts the polymerization of C(60) molecules in some directions and explains the formation of the linear chainlike polymeric structure of the C(60) lattice under pressure. We suggest that the reversible polymerization is related to the increased charge transfer and the overridden steric repulsion of counterions.

Green synthesis of 3D SnO2/graphene aerogels and their application in lithium-ion batteries

Three-dimensional (3D) tin oxide/graphene aerogels (SnO2/GAs) were constructed by a simple, facile and environmentally friendly process. The small-sized SnO2 nanoparticles (6 nm) are encapsulated within graphene-based aerogels with interconnected 3D networks for the SnO2/GAs nanocomposite. When used as an anode material in lithium ion batteries, it delivers a high reversible capacity that is close to the theoretical capacities of SnO2 and graphene after 50 cycles. TEM observations of the samples before and after 50 cycles illustrate that the structures of the graphene network and SnO2 NPs are preserved, which explains well the good cyclic stability of the electrode. The excellent electrochemical performance of the nanocomposites can be explained by their unique 3D porous architecture and the combination of the advantages of both SnO2 and graphene in Li ion storage and transport.

One-step synthesis, growth mechanism and photoluminescence properties of hollow GeO2 walnuts

Hollow GeO2 walnuts were synthesized via a simple one-step process in an emulsion system. We investigated the growth mechanism and optical properties of the products and found that the reactive temperature and the addition of ethanol were crucial factors in controlling the morphology of GeO2 crystals. Above the ethanol boiling temperature, hollow walnuts were formed, whereas well-dispersed solid GeO2 polyhedrons and dimers were obtained below the critical point. A possible formation mechanism of the hollow interior of GeO2 walnuts is proposed suggesting that it was formed from the gas bubble released by the boiling ethanol and followed by the Ostwald ripening process of the encapsulating crystals. Photoluminescence measurement shows an enhanced emission peak at 538 nm for hollow GeO2 walnuts with blue shift compared with that of the solid structure. Our results indicate hollow GeO2 walnuts may have potential applications in light-emitting nanodevices. This method also suggests a new approach for fabricating other particles with hollow structure.

A New Carbon Phase Constructed by Long-Range Ordered Carbon Clusters from Compressing C70 Solvates

An ordered amorphous carbon cluster (OACC) structure with building blocks of highly deformed/collapsed C70 is synthesized by compressing C70 *m-xylene, which exhibits an exceptionally high hardness. Different from compressing C60 *m-xylene, a new structure transition is observed in C70*m-xylene at above 30 GPa, indicating the formation of a new OACC structure under pressure.

Shape-selective synthesis and optical performance of ceria nanocrystal/graphene hybrid composites

The morphology of CeO2 nanomaterials determines their properties and synthesis of well dispersed CeO2 nanocrystals with desired morphology is thus crucial for their applications. In this paper, we report the successful synthesis of CeO2 nanocrystals with different morphologies anchored on graphene hybrid composites by a hydrothermal method. Nanorods/graphene (NRs-G), nanoparticles/graphene (NPs-G) and nanocubes/graphene (NCs-G) have been selectively synthesized by optimizing experimental conditions. These nanocrystals show different exposed crystal planes and the size or morphology of the loaded CeO2 can be controlled by the alkali concentration and reactive temperature, as well as the adding of GO or not. In addition, significant changes in the photoluminescence (PL) spectra intensity and position (energy) have been observed in the synthesized samples, which depend on the morphology and size of the loaded CeO2. These interesting phenomena can be easily rationalized by the difference of oxygen storage capacity (OSC) of samples.

Two-dimensional Penta-BP 5 Sheets: High-stability, Strain-tunable Electronic Structure and Excellent Mechanical Properties

Two-dimensional (2D) crystals exhibit unique and exceptional properties and show promise for various applications. In this work, we systematically studied the structures of a 2D boronphosphide (BP) monolayer with different stoichiometric ratios (BPx, x = 1, 2, 3, 4, 5, 6 and 7) and observed that each compound had a stable 2D structure with metallic or semiconducting electronic properties. Surprisingly, for the BP5 compounds, we discovered a rare penta-graphene-like 2D structure with a tetragonal lattice. This monolayer was a semiconductor with a quasi-direct band gap of 2.68 eV. More importantly, investigation of the strain effect revealed that small uniaxial strain can trigger the band gap of the penta-BP5 monolayer to transition from a quasi-direct to direct band gap, whereas moderate biaxial strain can cause the penta-BP5 to transform from a semiconductor into a metal, indicating the great potential of this material for nanoelectronic device applications based on strain-engineering techniques. The wide and tuneable band gap of monolayer penta-BP5 makes it more advantageous for high-frequency-response optoelectronic materials than the currently popular 2D systems, such as transition metal dichalcogenides and black phosphorus. These unique structural and electronic properties of 2D BP sheets make them promising for many potential applications in future nanodevices.