Crystal Shape Tailoring in Perovskite Structure Rare-Earth Ferrites REFeO₃ (RE = La, Pr, Sm, Dy, Er, and Y) and Shape-Dependent Magnetic Properties of YFeO₃

Controllable growth of perovskite oxide with tailored shapes is challenging but promising for shape-dependent physical and chemical property studies and probable applications. In this article, we report a general method for tailoring the crystal shape of perovskite structure rare-earth ferrite (REFeO₃) crystals in hydrothermal conditions. By adjusting the ratio of KOH to urea, various shapes of REFeO₃ crystals can be prepared, such as LaFeO₃ truncated cubes, PrFeO₃ perpendicular cross prisms, SmFeO₃ crossed bars with trustum, DyFeO₃ double pyramids on cubes, ErFeO₃ distorted octahedrons, and YFeO₃ long bars and thick hexagonal elongated plates. Detailed shape tailoring conditions for each phase of the crystals have been discussed clearly. The structure-dependent shape growing mechanism for each REFeO₃ is generally discussed by consideration of the variance of reduced unit cell parameters in reference to the ideal cubic ABO₃ perovskite structure. DyFeO₃ was taken as an example to elucidate the crystal shape formation mechanism based on the Bravais–Friedel–Donnay–Harker theory. The magnetic property of the YFeO₃ crystal shows shape dependence: elongated bars have the highest saturated magnetization, while the lowest coercive field, while the tailored polyhedrons are vice versa. This paper not only builds a general technique for tailoring the crystal shape to various shapes of REFeO₃ crystals but also provides many crystals for further study and application of anisotropy either in physical or in chemical properties

Programmable Structure Control in Cigarlike TiO₂ Nanofibers and UV-Light Photocatalysis Performance of Resultant Fabrics

Novel cigarlike nanofibers with an outer-shell and inner-continuous-pore structure and resultant fabrics have been fabricated by coupling the self-assembly of polystyrene-<i>block</i>-poly­(ethylene oxide) (PS-<i>b</i>-PEO) containing titanium precursors with the electrospinning technique in our previous work [You et al. ACS Appl. Mater. Interfaces 2013, 5, 2278]. In the current work, the structure control in these nanofibers has been investigated in detail using scanning electron microscopy, focused ion beam, and small angle X-ray scattering. Our results indicate that electrospinning conditions, the adopted solvent, the volume fraction of PS-<i>b</i>-PEO block copolymer, and the amount of titanium tetraisopropoxide in the mixture produce significant effects on both outer-shell and inner-continuous structures in the nanofibers. The parameters discussed above make it possible to achieve programmable structure control in the aspect of the diameter, thickness of the outer shell, and inner continuous pore. As a result, both micropores among fibers and nanopores in certain fibers are under their control. Furthermore, the photocatalytic activity of resultant TiO₂ fabrics was investigated by taking the photodegradation of Rhodamine B as an example. The results suggest that the degradation efficiency and rate constant exhibit sensitivity on the structure of nanofibers

Novel Cigarlike TiO₂ Nanofibers: Fabrication, Improved Mechanical, and Electrochemical Performances

By coupling the self-assembly of polystyrene-block-poly­(ethylene oxide) (PS-b-PEO) containing titanium precursors with the electrospinning technique, novel cigarlike nanofibers with an outer-shell and inner-continuous-pore structure and resultant fabrics were fabricated. Different from typical porous metal oxides, the prepared high-surface-area nonwoven fabrics show excellent mechanical properties. Not only are these fabrics self-supporting over a large area, but they can also be cut using scissors, which is important for large-scale applications. Furthermore, as electrode materials in Li-ion batteries, these fabrics exhibit much higher charge/discharge capacity and cycle stability compared with the commercially available nanosized TiO₂ (P25). The improved mechanical and electrochemical performances are attributed to the presence of an outer-shell, inner-bicontinuous structures (including continuous TiO₂ frame and continuous nanopores) and hierarchical pores from the cigarlike nanofibers