Topochemical synthesis of Mn2O3/TiO2 and MnTiO3/TiO2 nanocomposites as lithium-ion battery anodes with fast Li+ migration and giant pseudocapacitance via the mesocrystalline effect.

Transition metal compounds are a promising substitute for graphite as lithium-ion battery (LIB) anodes. In this study, mesocrystalline Mn2O3/TiO2 and MnTiO3/TiO2 nanocomposites were synthesized using a layered titanic acid H1.07Ti1.73O4 (HTO) precursor. The ß-MnOOH layer is intercalated into the interlayer of HTO by Mn2+-exchange treatment of H2O2-intercalated HTO, which includes ion-exchange of Mn2+ with H+ in the interlayer and oxidation of Mn2+ to the ß-MnOOH layer by H2O2 in the interlayer space. Mesocrystalline Mn2O3/TiO2 and MnTiO3/TiO2 nanocomposites with a platelike morphology were obtained by heat treatment of a sandwich layered HTO/ß-MnOOH under air and H2/Ar atmospheres, respectively. The electrochemical results suggest that the mesocrystalline Mn2O3/TiO2 and MnTiO3/TiO2 nanocomposites show a synergistic effect for enhanced cycling stability and a mesocrystalline effect for enhanced discharge-charge specific capacity by improving the Li+ mobility and enhancing the pseudocapacitance of the mesocrystalline nanocomposites as LIB anode materials. The discharge-charge specific capacity of the mesocrystalline Mn2O3/TiO2 nanocomposite is twice as high as that of the polycrystalline one caused by the mesocrystalline effect. Furthermore, the synergistic and mesocrystalline effects led to a stable large discharge-charge specific capacity of 710 mA h g-1 for the mesocrystalline Mn2O3/TiO2 nanocomposite. This work proposes a new concept to enhance the performance of anode materials for LIBs using mesocrystalline materials.

Enhanced Photovoltaic Performance of BiSCl Solar Cells Through Nanorod Array.

BiSCl single-crystalline nanofibers were synthesized by a facile one-pot solvothermal approach for the first time. BiSCl possesses a double chain type structure and grows readily along the c-axis, resulting the fibrous morphology. UV/Vis absorption spectroscopy revealed that BiSCl nanofibers exhibit a strong light absorption in a wavelength range from UV to visible light, corresponding to a bandgap of 1.96 eV. Ultraviolet photoelectron spectroscopy and density functional theory calculations revealed that BiSCl is a direct n-type semiconductor with valence band maximum and conduction band minimum located at 6.04 and 4.08 eV below the vacuum level, respectively. To investigate the photovoltaic performance, the homogeneous thin film of BiSCl-nanorod array was fabricated on a TiO2 porous film by a modified solvothermal process, where the nanorod array is oriented vertically to the surface of the TiO2 porous film. A proper band alignment of BiSCl-based solar cells with an architecture of fluorine-doped tin oxide (FTO)/TiO2 /BiSCl/(I3 - /I- )/Pt gave a PCE of 1.36 % and a relatively large short-circuit photocurrent density of 9.87 mA cm-2 for the first time. The preliminary photovoltaic study result revealed a potential possibility of BiSCl-nanorod array as a light absorber for solar cells that can be fabricated by the low-cost solution process.

Homogeneous dispersion of gallium nitride nanoparticles in a boron nitride matrix by nitridation with urea.

A Gallium Nitride (GaN) dispersed boron nitride (BN) nanocomposite powder was synthesized by heating a mixture of gallium nitrate, boric acid, and urea in a hydrogen atmosphere. Before heat treatment, crystalline phases of urea, boric acid, and gallium nitrate were recognized, but an amorphous material was produced by heat treatment at 400 degrees C, and then was transformed into GaN and turbostratic BN (t-BN) by further heat treatment at 800 degrees C. TEM obsevations of this composite powder revealed that single nanosized GaN particles were homogeneously dispersed in a BN matrix. Homogeneous dispersion of GaN nanoparticles was thought to be attained by simultaneously nitriding gallium nitrate and boric acid to GaN and BN with urea.

Electrochemical growth of vertically-oriented high aspect ratio titania nanotubes by rabid anodization in fluoride-free media.

Vertically-oriented high aspect ratio titania nanotube bundles have been grown by a potentiostatic anodization of titanium sheet in fluoride-free electrolytes. The anodization conditions like the applied voltage were optimized for the synthesis of titania nanotubes in HClO4 and NaCl electrolyte. The resulting nanotubes have a length of about 30 microm, outer diameter about 40 nm, inner pore size of about 10 nm and the aspect ratio was 750:1 by anodization in 0.1 M perchloric acid of pH approximaately 1 at applied voltage of 20 V. While for nanotubes prepared in 0.3 M NaCl of pH 4.3, the length was above 50 microm with the aspect ratio of 1250:1. A method to increase the uniformity of nanotube was demonstrated by pretreatment the titanium sheet by (4 wt% HF + 5 M HNO3) solution prior to anodization. Titania nanotubes were prepared, for the first time, by anodization in aqueous H2SO4 electrolyte alone with tube length above 500 nm. Annealing studies were performed, on high aspect ratio Titania nanotube layers produced in HClO4 electrolyte, in the temperature interval of 300 to 550 degrees C. The XRD patterns and TEM data confirmed the formation of single anatase phase after annealing at 450 degrees C with perfect nanoubular structure. While the rutile titania phase starts to emerege after annealing at about 500 degrees C and the evidence for the appearance of rutile phase due to the oxidation of the underlying Ti metal at the interface between nanotube/Ti-metal was given. On the other hand, the nanotubular structure starts to destroy upon annealing temperature of approximate 550 degrees C by tube flattening and losing of roll-up characteristics as indicated in SEM images. The superior morphology of these high aspect ratio nanotubes and their rapid growth rate foreshadow a bright future in wide applications like dye-sensitized solar cells, water photolysis and nanobiomedical.

Ferroelectric mesocrystalline BaTiO3/BaBi4Ti4O15 nanocomposite: formation mechanism, nanostructure, and anomalous ferroelectric response.

Ferroelectric mesocrystalline nanocomposites are promising materials for the enhancement of ferroelectricity via lattice strain engineering due to their high density of heteroepitaxial interfaces. In the present study, a ferroelectric mesocrystalline BaTiO3/BaBi4Ti4O15 (BT/BBT) nanocomposite was synthesized using the layered titanate H1.07Ti1.73O4via a facile two-step topochemical process. The BT/BBT nanocomposite is constructed from well-aligned BT and BBT nanocrystals oriented along the [110] and [11-1] crystal-axis directions, respectively. Lattice strain is introduced into the nanocomposite through the formation of a BT/BBT heteroepitaxial interface, which results in a greatly elevated Curie temperature for BBT in the range of 400 °C to 700 °C and an improved piezoelectric response with . In addition, the BT/BBT nanocomposite is stable up to a high temperature of 1100 °C; therefore, mesocrystalline ceramics can be fabricated as high-performance ferroelectric materials.

Fabrication and characterization of aluminum nitride/boron nitride nanocomposites by carbothermal reduction and nitridation of aluminum borate powders.

In order to fabricate aluminum nitride/boron nitride (AIN/BN) nanocomposites by pressureless sintering, the present study investigated the synthesis of AIN-BN nanocomposite powders by carbothermal reduction and nitridation of aluminum borate powders. Homogeneous mixtures of alumina (Al2O3), boric acid (H3BO3), and carbon powder were used to synthesize AIN/BN nanocomposite powders containing 10 and 20 vol% BN. Aluminum borate was produced by reacting Al2O3 and B2O3 above 800 degrees C, and AIN and turbostratic BN (t-BN) were produced by reacting aluminum borate with carbon powder and nitrogen gas at 1500 degrees C. Carbothermal reduction followed by nitridation yielded an AIN/BN nanocomposite powder composed of nanosized AIN and t-BN. By pressureless sintering nanocomposite AIN/BN powders containing 5 wt% Y22O3, AIN/BN nanocomposites were obtained without compromising the high thermal conductivity and high hardness.

Anomalous piezoelectric response of ferroelectric mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 nanocomposites designed by strain engineering.

Mesocrystals, a new class of unique materials, not only have potential properties based on the individual nanocrystals but also have a single-crystal-like function. Here, we report a ferroelectric mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT) nanocomposite synthesized from a layered titanate H1.07Ti1.73O4 (HTO) by an ingenious two-step topochemical process for the first time. The BT/BNT nanocomposite is constructed from well-aligned BT and BNT nanocrystals with the same crystal-axis orientation. The BT/BNT heteroepitaxial interface in the nanocomposite is promising for an enhanced piezoelectric performance by using lattice strain engineering, which gives a giant piezoelectric response with a value of 408 pm V-1. The introduced lattice strain at the BT/BNT heteroepitaxial interface causes transitions of pseudo-paraelectric BT and BNT nanocrystals to ferroelectric nanocrystals in the mesocrystalline nanocomposite, which enlarges ferroelectric, piezoelectric and dielectric responses. The lattice strain also results in the elevated Curie temperatures (Tc) of BT and BNT and a new intermediate phase transition.

Synthesis of [111]- and {010}-faceted anatase TiO2 nanocrystals from tri-titanate nanosheets and their photocatalytic and DSSC performances.

[111]- and {010}-faceted anatase nanocrystals with controllable crystal size and morphology were synthesized from tri-titanate H2Ti3O7 nanosheets by hydrothermal reaction. The nanostructures and the formation reaction mechanism of the obtained TiO2 nanocrystals were investigated using XRD, FE-SEM, and TEM. Furthermore, the photocatalytic and dye-sensitized solar cell (DSSC) performances of the synthesized anatase nanocrystals were also characterized. Two types of reactions occur in the formation process of the anatase nanocrystals. One is an in situ topochemical conversion reaction of the layered titanate structure to an anatase structure, and another is the dissolution-deposition reaction on the particle surface, which splits the formed nanosheet-like particles into small TiO2 nanocrystals. The surface photocatalytic activity and the DSSC performance of the anatase nanocrystals are dependent on the crystal facet exposed on the particle surface, which increases in the order of non-facet < [111]-facet < {010}-facet. The increasing order corresponds to the increasing order of the bandgap and energy level of the lowest valence band of the anatase nanocrystals. Furthermore, the facet of the anatase also affects the DSSC performance, which is enhanced in the order of non-facet < [111]-facet < {010}-facet.

Increasing resistivity of electrically conductive ceramics by insulating grain boundary phase.

Increasing resistivity of electrically conductive nonoxide ceramics was investigated by insulating conductive pathways through conductive grains in a sintered body by addition of an insulating grain boundary phase, which was produced by the reaction of sintering additives in liquid phase sintering. When SiC was hot pressed with an additive of 10 vol % of Al2O3 and Y2O3, the resistivity decreased as sintering temperature increased owing to contact between SiC grains during densification. However, by hot pressing at 1750°C, a high resistivity of greater than 1 × 10(11) Ω cm was achieved because of the penetration of an insulating grain boundary phase between the SiC grains. It is possible to fabricate high-resistivity SiC ceramics without losing their excellent mechanical properties by introduction of an insulating grain boundary phase, the volume of which is approximately 1/7 that of the insulating phase incorporated in conventional ceramic composites.

Modification of TiO2 electrode with organic silane interposed layer for high-performance of dye-sensitized solar cells.

Back electron transfer from the TiO2 electrode surface to the electrolyte is the main reason behind the low-open circuit potential (Voc) and the low-fill factor (FF) of the dye-sensitized solar cells (DSSCs). Modifications to the TiO2 electrode, fabricated using {010}-faceted TiO2 nanoparticles with six different kinds of silane, are reported to decrease the back electron transfer on the TiO2 surface. The effect of alkyl chain length of hydrocarbon silanes and fluorocarbon silanes on adsorption parameters of surface coverage and adsorption constant, interfacial resistance, and photovoltaic performances were investigated. Adsorption isotherms, impedance analysis, and photovoltaic measurements were used as the investigation techniques. The reduction of back electron transfer depended on the TiO2 surface coverage by silane, alkyl chain length, and the molecular structure of the silane. Even though Voc and FF were improved, significant reduction in short-circuit photocurrent density (Jsc) was observed after silanization because of desorption of dye during silanization. A new approach, sequential adsorption process of silane and dye, was introduced to enhance Voc and FF without lowering Jsc. Heptadecafluorodecyl trimethoxy-silane showed the highest coverage on the surface of the TiO2 and had the highest effect on the performance improvement of the DSSC, where Voc, FF, and efficiency (η) were improved by 22, 8.0, and 22%, respectively.

Synthesis of SiC/BN nanocomposite powders by carbothermal reduction and nitridation of borosilicate glass, and the properties of their sintered composites.

A new synthetic method for the fabrication of SiC/BN nanocomposites was devised to attain strong machinable ceramics. SiC/BN nanocomposites that contained 10, 20, and 30 vol% hexagonal BN (h-BN) were successfully fabricated by sintering SiC-BN nanocomposite powders by carbothermal reduction and nitridation of borosilicate glass powders. Homogeneous mixtures of silica (SiO(2)), boric acid (H(3)BO(3)), and carbon powder were heated in a nitrogen atmosphere to synthesize SiC-BN nanocomposite powders. Borosilicate glass was obtained by reacting SiO(2) and B(2)O(3) above 800 °C, and SiC and turbostratic BN (t-BN) were obtained by reacting borosilicate glass with carbon powder and nitrogen gas at 1500 °C. Carbothermal reduction followed by nitridation yielded SiC-BN nanocomposite powder composed of nanosized SiC and t-BN. By hot-pressing nanocomposite SiC-BN powders containing 7 wt% Al(2)O(3) and 2 wt% Y(2)O(3), machinable SiC/BN nanocomposites were obtained without a significant decrease in their fracture strength.