Zhenya Liu

Facile synthesis of single-crystalline mesoporous α-Fe2O3 and Fe3O4 nanorods as anode materials for lithium-ion batteries

In this work, single-crystalline α-FeOOH nanorods with a length of 400–700 nm and a diameter of 20–80 nm were successfully synthesized via a facile template-free hydrothermal method. Single-crystalline mesoporous α-Fe2O3 and Fe3O4 nanorods could be obtained from these α-FeOOH precursors after calcining at 350 °C in air and 500 °C in nitrogen, respectively. The as-prepared single-crystalline mesoporous α-Fe2O3 and Fe3O4 nanorods exhibited a large specific surface area and porosity, effectively enhancing the electrochemical reaction area and accommodate the strain during the charge–discharge cycling process.

Self‐Templated Synthesis of Single‐Crystal and Single‐Domain Ferroelectric Nanoplates

Low-dimensional nanomaterials, such as nanowires and nanotubes, 3] have received extensive attention because of their fascinating catalysis, optics, and electronics 7] properties, which offer the opportunity to fabricate nanodevices. Much attention has been paid to materials with two-dimensional (2D) nanostructure because of their unique electronic, magnetic, and storage properties. In particular, the recent development of stable graphene has stimulated great interest in studying free-standing 2D nanomaterials. So far, a variety of 2D free-standing materials with nanostructure, such as PbS nanosheets, WO3 nanoplates, [10] CeO2 nanoplates, [11] MoS2 nanoflakes, 13] and TiO2 nanosheets, [4] have been successfully synthesized. Compared to these simple compounds, 2D free-standing, single-crystal multicomponent oxide nanomaterials, such as ferroelectric oxides, have been rarely reported because of the relatively complex crystal structures and rigid crystalline properties of these oxides. Ferroelectric oxide nanomaterials, such as PbTiO3 (PT), Pb(Zr,Ti)O3, and BaTiO3, have versatile properties for various technical applications ranging from nonvolatile ferroelectric random access memories (NFERAMs) to electromechanical applications. 15] Among these nanomaterials, 2D free-standing, single-crystal ferroelectric materials are highly attractive because of their potential performances. For example, ultrathin single crystals of BaTiO3 [16,17] prepared by focused ion beam (FIB) microscopy have been used as single-crystal capacitors with thicknesses down to about 65 nm. These ferroelectric platelets fabricated by FIB have greatly improved our understanding of the fundamental properties of thin films. However, recent theoretical research predicts that ferroelectric nanodiscs could even favor an ultimate NFERAM density of 60 10 bits per square inch as well as a new toroid moment. In contrast to the conventional properties, the surface chemistry of ferroelectric oxides only gradually became an active field of research. In particular, it has been predicted that CO and NO catalysis could be favored on ultrathin Pt(100) films supported on ferroelectric PbTiO3. [22] Various methods have so far been applied to fabricate 0D and 1D ferroelectric nanomaterials, including templated methods, sol-gel processing, 28] soft-chemistry routes, solvothermal/hydrothermal reactions, 31] and electrospinning techniques. 33] Despite much effort, there is still an absence of a simple wet-chemistry method to prepare 2D single-crystal ferroelectric nanomaterials, such as nanoplates and nanodiscs. Here we report, for the first time, that freestanding, single-crystal PT nanoplates can be synthesized by a facile hydrothermal method. The characterization of the microstructure by X-ray diffraction (XRD) and high-resolution transmussion electron microscopy (HRTEM) demonstrates that these PT nanoplates, with side lengths of 600– 1100 nm and heights of about 150 nm, grow along the ab plane of the tetragonal perovskite structure and that {001} facets at the top and bottom surfaces are exposed. The “self-templated” crystal growth has been employed to discuss the mechanism for the formation of PT nanoplates under hydrothermal conditions. Electrostatic force microscopy in the dynamic contact mode (DC-EFM) of operation was also used to study the ferroelectric properties of the PT nonaplates. Furthermore, the catalytic performance of the nanoplates for the oxidation of carbon monoxide (CO) has been evaluated. In brief, tetragonal-phase PT nanoplates were synthesized by a hydrothermal method at 200 8C by using 6m KOH. Figure 1a shows a typical XRD pattern (JCPDS 70-0746) of PT nanoplates prepared by the hydrothermal process at 200 8C for 12 h with 6m KOH, and indicates that the product has a pure tetragonal perovskite PT structure. More importantly, the intensity of the (001) diffraction peak is dramatically higher than that of (100), which is opposite to the case in conventional perovskite PT (JCPDS 70-0746). This observation reveals that {001} crystal planes are prevailent in the sample. Scanning electron microscopy (SEM) and TEM analysis show that the products consist of well-defined structures with a rectangular outline and a side length of 600–1100 nm (Figure 1b,c). The HRTEM image (Figure 1d) of a PT nanoplate shows a clear crystal lattice with uniform interplanar spacings of 0.390 nm and 0.390 nm, which correspond to the tetragonal (100) and (010) planes, respectively, and indicate that the PT nanoplates grow along the ab plane of the perovskite structure. The effect of the KOH concen[*] C. Y. Chao, Dr. Z. H. Ren, Z. Xiao, Z. Y. Liu, Dr. G. Xu, J. Q. Mai, Dr. X. Li, Dr. G. Shen, Dr. G. R. Han State Key Laboratory of Silicon Materials Department of Materials Science and Engineering Cyrus Tang Center for Sensor Materials and Application Zhejiang University, Hangzhou 310027 (P.R. China) E-mail: renzh@zju.edu.cn hgr@zju.edu.cn

Size-Controlled Single-Crystal Perovskite PbTiO3 Nanofibers from Edge-Shared TiO6 Octahedron Columns

With the minimization of device sizes, great efforts have been devoted to the synthesis and understanding of ferroelectrics at the nanoscale. [ 3 , 4 ] In particular, one-dimensional (1D) perovskite nanostructures have been the focus of numerous research projects for their potential applications in high-density ferroelectric random access memory, piezoelectricity nanogenerators, and nonlinear optics. [ 5–9 ] As a prototypical perovskite-type ferroelectric, lead titanate (PbTiO 3 ) is a desired system to understand ferroelectricity and ferroelectric phase transformation at the nanoscale because of its simple structure and very large spontaneous polarization. [ 10 , 11 ] Recent experimental results indicate that ferroelectricity could be sustainable in a three unit cell (1.2 nm) thick fi lm of PbTiO 3 at room temperature. [ 4 ] For PbTiO 3 nanowires, an unknown phase transition between the vortex polarization and conventional one mediated by strain and surface terminations has been revealed theoretically. [ 12 , 13 ] It also suggests that ferroelectricity could be enhanced within PbO-terminated PbTiO 3 nanowires, thus leading to a critical size of the nanowire down to only one unit cell. As a comparison, the size effect of single-crystal ferroelectric nanowires has rarely been investigated experimentally. Different methods have been exploited in the last decade to synthesize 1D perovskite oxide nanostructures, such as template preparation, [ 14–18 ] hydrothermal methods, [ 19 , 20 ]

Single-crystal nanofibers of Zr-doped new structured PbTiO3: hydrothermal synthesis, characterization and phase transformation

Single-crystal nanofibers of a new structured PbTiO3 with Zr doping concentration of 0–15% have been reproducibly synthesized on a large scale by a polymer-assisted hydrothermal method for the first time. Moreover, the Zr-doped new structured PbTiO3 nanofibers can transform into single-crystalline perovskite Pb(Zr, Ti)O3(PZT) nanofibers by annealing treatment in air.

Pre-perovskite nanofiber: a new direct-band gap semiconductor with green and near infrared photoluminescence

Perovskite-related oxides exhibiting fascinating electric and magnetic properties are important functional materials. We report, for the first time, that pre-perovskite (PP) PbTiO3, characterized by one-dimensional (1D) TiO6 octahedron columns, has a direct-band structure with a gap of 3.20 eV. The faceted single-crystal nanofibers of PP-PbTiO3 demonstrate strong green and near infrared (NIR) photoluminescence (PL) emission at room temperature, which are intrinsic and subjected to quasi-1D crystal structure and electronic band structure. These nanofibers serving as new direct band-gap semiconductors may find applications in novel optical and optoelectronic devices.

Doping and phase transformation of single-crystal pre-perovskite PbTiO3 fibers with TiO6 edge-shared octahedra

Ba-doped (0–8 mol%) pre-perovskite PbTiO3 single-crystal fibers have been synthesized in large-scale reproducibly by a facile polymer-assisted hydrothermal method. X-ray power diffraction, scanning electron microscope and transmission electron microscope were performed to investigate microstructure and the evolution of these single-crystal fibers. The results show that these fibers have a length of 13–120 μm and a diameter of 100–600 nm. In particular, these Ba-doped pre-perovskite PbTiO3 fibers can transform to tetragonal perovskite Ba-doped PbTiO3 (PBT) fibers with single-crystal character. Piezoresponse force microscopy measurements indicate that the perovskite PBT with Ba concentration of 5 mol% has a symmetric “square” phase loop, and the amplitude loop shows a “butterfly shape”, confirming the existence of ferroelectric and piezoelectric properties. Such findings have thus provided a promising route for the nanoscale sensors and generators applications.