Xiaohu Ren

Novel sintering and band gap engineering of ZnTiO3 ceramics with excellent microwave dielectric properties

Pure phase ZnTiO3 ceramics were successfully synthesized for the first time by a solid state reaction method. The synthesis temperature was higher than the phase transition temperature, wherein the ZnO nanoparticles acted as inhibitors to prevent the formation of the secondary phase, Zn2TiO4, which was inevitable by conventional preparation methods. As the small nano-ZnO regions dispersed in the ceramic grains, the bulk diffusion of Ti ions, formation of nucleation centers and migration of phase boundaries were largely suppressed, indicating that nano-ZnO was desirable for stabilizing the ZnTiO3 phase above the phase transition temperature. The R (no. 148) space groups of the single phase were determined by X-ray diffraction Rietveld analysis. X-ray photoelectron spectroscopy and photoluminescence emission spectroscopy were also carried out to investigate the electronic microstructure of the obtained ZnTiO3 phase. Finally, excellent microwave dielectric properties were achieved (er ∼ 31.5, Q × f ∼ 59 800 GHz and τf ∼ 1.2 ppm °C−1) with a high sintering temperature (900–950 °C). Moreover, given its good chemical compatibility with the Ag electrode and the merits of easy scale-up, high-efficiency, low-cost and environmentally benign synthesis, ZnTiO3 is a promising candidate for LTCC applications. This work paves a great way towards practical applications.

Lead-free Bi5−xLaxTi3FeO15 (x = 0, 1) nanofibers toward wool keratin-based biocompatible piezoelectric nanogenerators

Biocompatible nanogenerators (NGs) are of vital importance for in vivo applications. In this work, Bi5−xLaxTi3FeO15 (x = 0, 1) nanofibers (NFs) toward wool keratin-based biocompatible piezoelectric NGs are demonstrated. The refined structure of Bi4LaTi3FeO15 (BLTF) NFs by the Rietveld method was confirmed to be a four-layer Aurivillius oxide with an orthorhombic space group A21am. Wool keratin was extracted using the oxidizing and subsequent deoxidizing reaction process. BLTF NFs are biocompatible in a bio-environment shown here using mice pre-osteoblasts MC3T3-E1. The BLTF NF NG can generate an output voltage of 0.14 V and output current of 41 nA (with a current density of 20.5 nA cm−2), with quite reproducible power generation. The prominent power generation of the BLTF NF NG partly originated from the La3+ substitution for the A-site Bi3+, which decreases the number of oxygen vacancies in the perovskite layers. Furthermore, piezoelectric NFs as exceptional fillers coupled sufficient connectivity, a larger degree of crystallinity and Youngs modulus of the system together in the keratin matrix also contributed to enhanced piezoelectric power generation. In addition, one mechanism responsible for the optimal BLTF NF NG was proposed. This study offers new insights into the design and application of promising lead-free biocompatible devices with better fillers.

Hydrothermally Induced Oxygen Doping of Graphitic Carbon Nitride with a Highly Ordered Architecture and Enhanced Photocatalytic Activity

As an amorphous or semicrystalline material, graphitic carbon nitride (g-C3 N4 ) displays poor photocatalytic activity owing to rapid recombination of the photogenerated charge carriers, which is mainly caused by a high density of defects in the graphitic structure. In this work, a porous O-doped g-C3 N4 (P-CNO) nanosheet with a highly ordered architecture is fabricated by introducing a novel hydrothermal treatment to the precursor before the final thermal condensation. The photocatalytic hydrogen evolution rate (HER) and HER per surface area of P-CNO are 13.9 and 1.7 times higher than that of bulk g-C3 N4 . The improved photocatalytic activity is ascribed to a synergistic effect of O doping, a porous sheet-like morphology, and increased crystallinity. This work also provides a new approach for the synthesis of other polymer-based photocatalysts with high crystallinity and excellent performance.

Unusual devisable high-performance perovskite materials obtained by engineering in twins, domains, and antiphase boundaries

With the widespread application, engineering of microstructures, domains, twins, and antiphase boundaries (APBs) is attracting significant attention. However, the origin of the domains, especially in the paraelectric phase, as well as the mechanism of variation in twins or domains and their relationship are still not clear. Generally, these structures are recognized as one of the key origins of intrinsic loss. Our studies, however, reveal that the formation of twins is closely related to the asymmetry of the crystal structure. With the introduction of a lattice blockage-trigonal NdAlO3 phase, the increase in the symmetry of the CaTiO3 tetragonal phase results in a transformation from the (110)-oriented twins to the (111)-oriented twins. Then, it forms a new ordered structure. With the help of peak-differentiation and imitation of the Raman spectra, the A-site in the perovskite structure is found to be a dominant factor in lattice energy and performance. By designing an A-site displacement, i.e., Sr2+ or Ba2+ substitution into Ca2+, we created a controllable structure of twins by symmetry regulation and APBs by introducing ferroelectric spontaneous polarization. Via selected area electron diffraction patterns (SAED) and variable temperature electric field piezoresponse force microscopy (PFM) images, we found that 180° and 90° domains could coexist in the grains of xCaTiO3–(1 − x)NdAlO3 ceramics. Interestingly, the 90° domains and twin boundaries (TB) play more important roles in the anisotropic resistance to the electron/hole transfer. Our studies prove that the defect engineering can realize a controllable enhanced dielectric performance by defect regulation within the host lattice. These may pave a possible way for the design of the microstructure of the defects to achieve better predictable performances of the materials.