Formation mechanism of (001) oriented perovskite SrTiO3 microplatelets synthesized by topochemical microcrystal conversion.

To develop a better understanding of the mechanism responsible for topochemical microcrystal conversion (TMC) from Aurivillius SrBi4Ti4O15 precursors to perovskite SrTiO3 microplatelets, compositional/structural evolutions, morphological development, and reaction interface evolution of the (001) oriented SrBi4Ti4O15 microplatelets were investigated during the conversion process. The results show that multiple topotactic nucleation events of SrTiO3 occurred directly on the surfaces of SrBi4Ti4O15 above 700 °C, while reacting zones of intermediate phase(s) with less Bi(3+) contents were observed to form in the interior of SrBi4Ti4O15. Extensive exfoliation of the precursors occurred generally parallel to the (001) surfaces above 775 °C. At 950 °C, the original single-crystal SrBi4Ti4O15 platelet was replaced by a polycrystalline aggregate consisting of (001) aligned SrTiO3 crystallites and poorly crystallized intermediate phase(s). With further increasing the temperature or holding time, the SrTiO3 phase formed from related intermediate phase(s), and the aligned crystallites were sintered to form dense SrTiO3 with strong (001) orientation. The obtained SrTiO3 microplatelets preserved the shape of SrBi4Ti4O15 and show high chemical and phase purity. This TMC mechanism has general applicability to a variety of compounds and will be very useful for the design and synthesis of novel anisotropic perovskite crystals with high quality in the future.

Photovoltaic Effect of La and Mn Co-Doped BiFeO3 Heterostructure with Charge Transport Layers.

Bismuth ferrite BiFeO3 (BFO)-based ferroelectrics have great potential as inorganic perovskite-like oxides for future solar cells applications due to their unique physical properties. In this work, La and Mn co-doped BFO thin films with compositions Bi0.9La0.1(Fe1-xMnx)O3 (x = 0, 0.05, 0.1, 0.15) (denoted as BLF, BLFM5, BLFM10, BLFM15, respectively) were prepared via a sol-gel technique on indium tin oxide (ITO) glass. All the films are monophasic, showing good crystallinity. The optical bandgap Eg was found to decrease monotonously with an increase in the Mn doping amount. Compared with other compositions, the BLFM5 sample exhibits a better crystallinity and less oxygen vacancies as indicated by XRD and XPS measurements, thereby achieving a better J-V performance. Based on BLFM5 as the light absorbing layer, the ITO/ZnO/BLFM5/Pt and ITO/ZnO/BLFM5/NiO/Pt heterostructure devices were designed and characterized. It was found that the introduction of the ZnO layer increases both the open circuit voltage (Voc) and the short circuit current density (Jsc) with Voc = 90.2 mV and Jsc = 6.90 µA/cm2 for the Pt/ BLFM5/ZnO/ITO device. However, the insertion of the NiO layer reduces both Voc and Jsc, which is attributed to the weakened built-in electric field at the NiO/BLFM5 interface.

Needle-like CoO nanowire composites with NiO nanosheets on carbon cloth for hybrid flexible supercapacitors and overall water splitting electrodes.

A nanoscale core-shell NiO@CoO composite is prepared on flexible carbon cloth for electrodes in supercapacitors and overall water splitting. The needle-like CoO nanowires with NiO nanosheets as the active materials improve the elemental constituents as well as surface area. The NiO@CoO electrode boasts a capacity of 2.87 F cm-2 (1024.05 F g-1) at 1 A g-1 current density, and even at a large current density of 20 A g-1 the retention ratio is 80.9% after 5000 cycles. The excellent specific capacity with high rate capability can be ascribed to the unique structure which increases the area of the liquid-solid interface and facilitates electron and ion transport, improving the utilization efficiency of active materials. The asymmetric hybrid supercapacitor prepared with the core-shell electrode shows the energy output of 40.3 W h kg-1 at 750 W kg-1 with a better retention (71.7%) of specific capacitance after 15 000 cycles. In addition, linear sweep voltammetry is performed to assess the performance of the electrode in water splitting and the electrode shows excellent activity in the OER as manifested by a Tafel slope of 88.04 mV dec-1. Our results show that the bifunctional structure and design strategy have large potential in energy applications.

The grain size effect on the properties of Aurivillius phase Bi3.15Nd0.85Ti3O12 ferroelectric ceramics.

Aurivillius phase, bismuth layer structured ferroelectric Bi(3.15)Nd(0.85)Ti(3)O(12) (BNdT) ceramics with average grain sizes from 90 nm and high densities (>97%) were fabricated by spark plasma sintering. Decreasing grain size produced a diffuse ferro-paraelectric phase transition and a decrease in the Curie point. Compared with BNdT ceramics with grain sizes of micrometre scale, nanograined BNdT ceramics exhibit a depression of the dielectric maximum at the Curie point, enhanced dielectric constant from room temperature to 350 degrees C and dramatically decreased losses. Although ferroelectric switching was greatly inhibited in nanograined ceramics, both ferroelectric and piezoelectric measurements still clearly showed that BNdT ceramics with 90 nm average grain sizes are ferroelectrically switchable. This is the first reported evidence that nanoscale Aurivillius phase ceramics are ferroelectrically active.

Geometry Design, Principles and Assembly of Micromotors.

Discovery of bio-inspired, self-propelled and externally-powered nano-/micro-motors, rotors and engines (micromachines) is considered a potentially revolutionary paradigm in nanoscience. Nature knows how to combine different elements together in a fluidic state for intelligent design of nano-/micro-machines, which operate by pumping, stirring, and diffusion of their internal components. Taking inspirations from nature, scientists endeavor to develop the best materials, geometries, and conditions for self-propelled motion, and to better understand their mechanisms of motion and interactions. Today, microfluidic technology offers considerable advantages for the next generation of biomimetic particles, droplets and capsules. This review summarizes recent achievements in the field of nano-/micromotors, and methods of their external control and collective behaviors, which may stimulate new ideas for a broad range of applications.

Semiconductor/relaxor 0-3 type composites without thermal depolarization in Bi0.5Na0.5TiO3-based lead-free piezoceramics.

Commercial lead-based piezoelectric materials raised worldwide environmental concerns in the past decade. Bi0.5Na0.5TiO3-based solid solution is among the most promising lead-free piezoelectric candidates; however, depolarization of these solid solutions is a longstanding obstacle for their practical applications. Here we use a strategy to defer the thermal depolarization, even render depolarization-free Bi0.5Na0.5TiO3-based 0-3-type composites. This is achieved by introducing semiconducting ZnO particles into the relaxor ferroelectric 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 matrix. The depolarization temperature increases with increasing ZnO concentration until depolarization disappears at 30 mol% ZnO. The semiconducting nature of ZnO provides charges to partially compensate the ferroelectric depolarization field. These results not only pave the way for applications of Bi0.5Na0.5TiO3-based piezoceramics, but also have great impact on the understanding of the mechanism of depolarization so as to provide a new design to optimize the performance of lead-free piezoelectrics.

Effect of Spark Plasma Sintering on the Structure and Properties of Ti1-xZrxNiSn Half-Heusler Alloys.

XNiSn (X = Ti, Zr and Hf) half-Heusler alloys have promising thermoelectric properties and are attracting enormous interest for use in waste heat recovery. In particular, multiphase behaviour has been linked to reduced lattice thermal conductivities, which enables improved energy conversion efficiencies. This manuscript describes the impact of spark plasma sintering (SPS) on the phase distributions and thermoelectric properties of Ti0.5Zr0.5NiSn based half-Heuslers. Rietveld analysis reveals small changes in composition, while measurement of the Seebeck coefficient and electrical resistivities reveals that all SPS treated samples are electron doped compared to the as-prepared samples. The lattice thermal conductivities fall between 4 W·m-1·K-1 at 350 K and 3 W·m-1·K-1 at 740 K. A maximum ZT = 0.7 at 740 K is observed in a sample with nominal Ti0.5Zr0.5NiSn composition.