Xingang Kong

Ferroelectric mesocrystals of bismuth sodium titanate: formation mechanism, nanostructure, and application to piezoelectric materials.

Ferroelectric mesocrystals of Bi0.5Na0.5TiO3 (BNT) with [100]-crystal-axis orientation were successfully prepared using a topotactic structural transformation process from a layered titanate H1.07Ti1.73O4·nH2O (HTO). The formation reactions of BNT mesocrystals in HTO-Bi2O3-Na2CO3 and HTO-TiO2-Bi2O3-Na2CO3 reaction systems and their nanostructures were studied by XRD, FE-SEM, TEM, SAED, and EDS, and the reaction mechanisms were given. The BNT mesocrystals are formed by a topotactic structural transformation mechanism in the HTO-Bi2O3-Na2CO3 reaction system and by a combination mechanism of the topotactic structural transformation and epitaxial crystal growth in the HTO-TiO2-Bi2O3-Na2CO3 reaction system, respectively. The BNT mesocrystals prepared by these methods are constructed from [100]-oriented BNT nanocrystals. Furthermore, these reaction systems were successfully applied to the fabrication of [100]-oriented BNT ferroelectric ceramic materials. A BNT ceramic material with a high degree of orientation, high relative density, and small grain size was achieved.

High Pseudocapacitance in FeOOH/rGO Composites with Superior Performance for High Rate Anode in Li-Ion Battery

Capacitive storage has been considered as one type of Li-ion storage with fast faradaic surface redox reactions to offer high power density for electrochemical applications. However, it is often limited by low extent of energy contribution during the charge/discharge process, providing insufficient influences to total capacity of Li-ion storage in electrodes. In this work, we demonstrate a pseudocapacitance predominated storage (contributes 82% of the total capacity) from an in-situ pulverization process of FeOOH rods on rGO (reduced graphene oxide) sheets for the first time. Such high extent of pseudocapacitive storage in the FeOOH/rGO electrode achieves high energy density with superior cycling performance over 200 cycles at different current densities (1135 mAh/g at 1 A/g and 783 mAh/g at 5 A/g). It is further revealed that the in-situ pulverization process is essential for the high pseudocapacitance in this electrode, because it not only produces a porous structure for high exposure of tiny FeOOH crystallites to electrolyte but also maintains stable electrochemical contact during ultrahigh rate charge transfer with high energy density in the battery. The utilization of in-situ pulverization in an Fe-based anode to realize high surface pseudocapacitance with superior performance may inspire future design of electrode structures in Li-ion batteries.

Soft chemical in situ synthesis, formation mechanism and electrochemical performances of 1D bead-like AgVO3 nanoarchitectures

The soft chemical process is a useful and unique method for the preparation and design of one-dimensional (1D) nanoarchitectures. 1D bead-like AgVO3 nanoarchitectures are prepared via a soft chemical in situ reaction using the layered structure K2V6O16·2.7H2O platelike particles as the precursor. The formation mechanism is investigated through tracing the evolution of the structure and morphology of intermediate products during the reaction, and it consists of two processes. One is the in situ reaction of the Ag+ ions with the V3O8 layers of K2V6O16. The other is the fragmentation of the 2D platelike composite into a 1D fiber composite. Moreover, the electrochemical investigation shows that after 50 cycles at a current density of 100 mA g−1, the 1D bead-like nanostructure cathode exhibits a higher discharge capacity (127 mA h g−1) than that of 1D nanowires (75 mA h g−1), mainly due to the larger specific surface area and fine particles-constructed architecture.

Lightweight NiFe 2 O 4 with controllable 3D network structure and enhanced microwave absorbing properties

3D network structure NiFe2O4 was successfully synthesized by a templated salt precipitation method using PMMA colloid crystal as templates. The morphology, phase composition and microwave absorbing properties of as-prepared samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), vector network analyzer (VNA), and so on. The results revealed that the 3D network structure was configurated with smooth spherical walls composed of NiFe2O4 nanocrystals and their pore diameters being in the range of 80–250 nm. The microwave absorption properties of the 3D network structure NiFe2O4 were crucially determined by the special structure. The synergy of intrinsic magnetic loss of magnetic NiFe2O4 and the interfacial polarization enhanced by 3D network structure and the interaction of multiple mechanisms endowed the sample with the feature of strong absorption, broad bandwidth and lightweight. There is more than one valley in the reflection loss curves and the maximum reflection loss is 27.5 dB with a bandwidth of 4 GHz. Moreover, the 3D network structure NiFe2O4 show a greater reflection loss with the same thickness comparing to the ordinary NiFe2O4 nanoparticles, which could achieve the feature of lightweight of the microwave absorbing materials.

Topotactic synthesis and photocatalytic performance of one-dimensional ZnNb2O6 nanostructures and one-dimensional ZnNb2O6/KNbO3 hetero-nanostructures

This paper introduces one-dimensional ZnNb2O6/KNbO3 hetero-nanostructures and one-dimensional ZnNb2O6 nanostructures. These nanostructures are synthesized via in situ topotactic structural transformation reaction using the tunnel structure K2Nb2O6 filiform crystal as precursor. Firstly, Zn2+ ions intercalate into K2Nb2O6 crystal by exchanging K+ ions from the K2Nb2O6 crystal with Zn2+ from Zn(NO3)2 or Zn(CH3COO)2 aqueous solution, to form two different Zn2+-exchanged samples, and then these Zn2+-exchanged samples topotacticly transform into one-dimensional ZnNb2O6/KNbO3 hetero-nanostructures and ZnNb2O6 nanostructures during heat-treatment. The formation reaction and structure of these samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and energy-dispersive spectroscopy (EDS). Photocatalytic experiments showed that one-dimensional ZnNb2O6/KNbO3 hetero-nanostructures and ZnNb2O6 nanostructures have excellent photocatalytic performance for the degradation of methylene blue (MB), rhodamine B (RhB), and methyl orange (MO).

pH-regulated template-free assembly of Sb4O5Cl2 hollow microsphere crystallites with self-narrowed bandgap and optimized photocatalytic performance.

Sb4O5Cl2 hollow microspheres with self-narrowed bandgap and optimized photocatalytic performances are synthesized via a facile template-free method. It is found that the crystal structure and morphology of Sb4O5Cl2 crystallites are strongly dependent on the pH values of precursors. Nano-sized irregular-cuboids assembled Sb4O5Cl2 micro-particles and hollow microspheres can be synthesized at pH 1 and 2, whereas individual Sb4O5Cl2 micro-belts become to form when the pH is higher than 3. The irregular-cuboids assembled Sb4O5Cl2 micro-particles and hollow microspheres exhibit self-narrowed bandgap and higher light absorption ability compared with individual Sb4O5Cl2 micro-belts. The photoelectrochemical measurements show that the assembled Sb4O5Cl2 hollow microsphere crystallites prepared at pH 2 exhibit enhanced carrier density, improved separation efficiency of electron-hole pairs and decreased electron-transfer resistance. As a result, the irregular-cuboids assembled Sb4O5Cl2 hollow microspheres prepared at pH = 2 exhibit the highest photocatalytic activity for the degradation of gaseous iso-propanol (IPA) and Rhodamine B (RhB) aqueous solution. The good photocatalytic activity of Sb4O5Cl2 sample prepared at pH = 2 may be caused by the synergistic effect of its higher light absorption, the decreased electron-transfer resistance, the suppressed recombination of photogenerated electrons and holes, and the increased surface area.

Ti-O-O coordination bond caused visible light photocatalytic property of layered titanium oxide.

The layered titanium oxide is a useful and unique precursor for the facile and rapid preparation of the peroxide layered titanium oxide H1.07Ti1.73O4·nH2O (HTO) crystal with enhanced visible light photoactivity. The H2O2 molecules as peroxide chemicals rapidly enter into the interlayers of HTO crystal, and coordinate with Ti within TiO6 octahedron to form a mass of Ti-O-O coordination bond in the interlayers. The introduction of these Ti-O-O coordination bonds result in lowering the band gap of HTO, and promoting the separation efficiency of the photo induced electron–hole pairs. Meanwhile, the photocatalytic investigation indicates that such peroxide HTO crystal has the enhanced photocatalytic performance for RhB degradation and water splitting to generate oxygen under visible light irradiating.

In situ synthesis and photocatalytic performance of WO3/ZnWO4 composite powders

The WO3/ZnWO4 composite powders were synthesized through an in situ reaction process with tunnel structure K10W12O41·11H2O filiform crystallites used as a precursor. At first, Zn2+ ions was intercalated into the K10W12O41·11H2O crystal by exchanging K+ ions, then these Zn2+-exchanged samples were transformed into WO3/ZnWO4 composite powders during heat-treatment. The formation reaction and structure of these samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometer (EDS). The results showed that the WO3/ZnWO4 composite powders consisted of WO3 nanoparticles and ZnWO4 nanorods. Photocatalytic experiments exhibited an excellent photocatalytic performance for the degradation of methylene blue (MB) and the degradation efficiency was about 95% after 70 min under simulated sunlight.

Topochemical conversion of protonated titanate single crystals into platelike Ba0.5Sr0.5TiO3 mesocrystals with controllable microstructures

This report introduces a platelike Ba0.5Sr0.5TiO3 (BST) mesocrystal synthesized by a two-step solvothermal soft chemical process. In the first step, a platelike layered titanate H1.07Ti1.73□0.27O4·xH2O (□: vacancy of Ti) (HTO) single crystal is treated in a Ba(OH)2 solution to obtain a platelike BaTiO3 (BT)/HTO/anatase nanocomposite. In the second step, the generated BT/HTO/anatase nanocomposite is treated in a Sr(OH)2 solution to obtain the platelike BST mesocrystal. The formation mechanism and nanostructure of the mesocrystal are investigated by XRD, TEM, SAED, and FESEM. The platelike BST mesocrystal is constructed from [110]-oriented BST nanocrystals and shows a set of single-crystal-like electron diffraction spots. The mesocrystal is formed via an in situ topochemical mesocrystal conversion mechanism. There is a definite relationship between the crystal-axis directions of the HTO precursor and the BST mesocrystal. The platelike BST mesocrystals with uniform microstructure and high crystallinity can be achieved by controlling the ethanol content in the reaction solvent.

Controllable synthesis and morphology evolution from two-dimensions to one-dimension of layered K2V6O16·nH2O

A two-dimensional (2D) layered K2V6O16·2.7H2O platelike single crystal was successfully synthesized in a short time of 3 h via a simple hydrothermal method. The number of H2O molecules and the morphology of the layered K2V6O16·nH2O hexavanadate crystal can be easily controlled by adjusting the pH value of the system and the reaction time used for the hydrothermal process. By studying the structure and morphology of hexavanadate samples obtained at different reaction times under acid conditions using an X-ray diffractometer (XRD), a field emission scanning electron microscope (FE-SEM) and a transmission electron microscope (TEM), it can be concluded that the 2D layered K2V6O16·2.7H2O platelike single crystal hexavanadate gradually evolves into a one-dimensional (1D) layered K2V6O16·1.5H2O fiberlike single crystal hexavanadate during the hydrothermal process as the reaction time is prolonged. In addition, the photocatalytic performance of the as-prepared products was explored.