Xuehua Liu

Spray-Drying-Induced Assembly of Skeleton-Structured SnO2/Graphene Composite Spheres as Superior Anode Materials for High-Performance Lithium-Ion Batteries

Three-dimensional skeleton-structured assemblies of graphene sheets decorated with SnO2 nanocrystals are fabricated via a facile and large-scalable spray-drying-induced assembly process with commercial graphene oxide and SnO2 sol as precursors. The influences of different parameters on the morphology, composition, structure, and electrochemical performances of the skeleton-structured SnO2/graphene composite spheres are studied by XRD, TGA, SEM, TEM, Raman spectroscopy, and N2 adsorption-desorption techniques. Electrochemical properties of the composite spheres as the anode electrode for lithium-ion batteries are evaluated. After 120 cycles under a current density of 100 mA g-1, the skeleton-structured SnO2/graphene spheres still display a specific discharge capacity of 1140 mAh g-1. It is roughly 9.5 times larger than that of bare SnO2 clusters. It could still retain a stable specific capacity of 775 mAh g-1 after 50 cycles under a high current density of 2000 mA g-1, exhibiting extraordinary rate ability. The superconductivity of the graphene skeleton provides the pathway for electron transportation. The large pore volume deduced from the skeleton structure of the SnO2/graphene composite spheres increases the penetration of electrolyte and the diffusion of lithium ions and also significantly enhances the structural integrity by acting as a mechanical buffer.

Synthesis of defect-rich palladium-tin alloy nanochain networks for formic acid oxidation

Unique and novel Pd4Sn nanochain networks were successfully synthesized with an average diameter of 5 nm, rendering a modified Pd electronic structure with rich defects such as atomic corners, steps or ledges as catalytic active sites for great enhancement of charge transfer and electrode kinetics. The prepared Pd4Sn nanochain networks held an electrochemically active surface area as high as 119.40 m2 g-1, and exhibited higher catalytic activity and stability toward formic acid oxidation compared with Pd3Sn nanochain networks, Pd5Sn nanochain networks, Pd4Sn dendrites and Pd/C. The fundamental insight of the enhancement mechanism is discussed, and this work offers a novel, less expensive but highly active catalyst for direct formic acid fuel cells.

Structure versus properties in α-Fe2O3 nanowires and nanoblades

We report structure/property relationships in bicrystalline α-Fe2O3 nanowires (NWs) and nanoblades (NBs), synthesized by thermal oxidation of iron foils with different surface roughness. The electrical properties of individual nanostructures were studied by in situ transmission electron microscopy. Current-voltage (I-V) measurements using gold electrodes showed that a Schottky contact forms between α-Fe2O3 NWs whereas an ohmic contact forms between α-Fe2O3 NBs. The difference in transport properties is attributed to the existence of oxygen vacancies in the coincidence-site-lattice boundary region of α-Fe2O3 NBs. Magnetic measurements indicate that the temperature-dependent zero-field-cooled magnetization rises more rapidly near the Morin transition temperature for α-Fe2O3 NBs than that for NWs. The distinct magnetic properties of the NBs are ascribed to the enhanced magnetic order induced by the structural order in the two-dimensional NBs. These α-Fe2O3 NBs are promising building blocks for electronic and magnetic devices since their 2D geometries facilitate integration into devices with realistic pathways to manufacturing. In addition, our study shows that boundary engineering is an effective approach for tailoring the physical properties of nanomaterials.

Three-dimensional nitrogen-doped porous carbon anchored CeO2 quantum dots as an efficient catalyst for formaldehyde oxidation

Catalytic oxidation of formaldehyde with non-noble metal catalysts is desirable yet still challenging. In this work, we describe a hard template method in combination with an in situ chelating strategy to construct a novel three-dimensionally structured CeO2-based composite which contains CeO2 quantum dots of 1.5–2.5 nm size anchored on nitrogen doped hierarchical porous carbon with both macropores and mesopores. When used as an oxidation catalyst, complete oxidation of formaldehyde could be achieved at a temperature 130 °C lower than that of bulk nano-CeO2 catalyst. The significantly improved catalytic activity could be attributed to the higher amount of oxygen vacancies and enhanced redox properties of CeO2 quantum dots as well as the three-dimensional porous texture.

Carbon Wrapped Ni3S2 Nanocrystals Anchored on Graphene Sheets as Anode Materials for Lithium-Ion Battery and the Study on Their Capacity Evolution

Ni3S2 nanocrystals wrapped by thin carbon layer and anchored on the sheets of reduced graphene oxide (Ni3S2@C/RGO) have been synthesized by a spray-coagulation assisted hydrothermal method and combined with a calcination process. Cellulose, dissolved in Thiourea/NaOH aqueous solution is chosen as carbon sources and mixed with graphene oxide via a spray-coagulation method using graphene suspension as coagulation bath. The resulted cellulose/graphene suspension is utilized as solvent for dissolving of Ni(NO3)2 and then used as raw materials for hydrothermal preparation of the Ni3S2@C/RGO composites. The structure of the composites has been investigated and their electrochemical properties are evaluated as anode material for lithium-ion batteries. The Ni3S2@C/RGO sample exhibits increasing reversible capacities upon cycles and shows a superior rate performance as well. Such kinds of promising performance have been ascribed to the wrapping effect of carbon layer which confines the dislocation of the polycrystals formed upon cycles and the enhanced conductivity as the integration of RGO conductive substrate. Discharge capacities up to 850 and 630 mAh·g−1 at current densities of 200 and 5000 mA·g−1, respectively, are obtained. The evolution of electrochemical performance of the composites with structure variation of the encapsulated Ni3S2 nanocrystals has been revealed by ex-situ TEM and XRD measurements.

Inhibitive formation of nanocavities by introduction of Si atoms in Ge nanocrystals produced by ion implantation

Germanium nanocrystals (Ge-nc) were successfully synthesized by co-implantation of Si and Ge ions into a SiO2 film thermally grown on (100) Si substrate and fused silica (pure SiO2), respectively, followed by subsequent annealing at 1150 °C for 1 h. Transmission electron microscopy (TEM) examinations show that nanocavities only exist in the fused silica sample but not in the SiO2 film on a Si substrate. From the analysis of the high-resolution TEM images and electron energy-loss spectroscopy spectra, it is revealed that the absence of nanocavities in the SiO2 film/Si substrate is attributed to the presence of Si atoms inside the formed Ge-nc. Because the energy of Si-Ge bonds (301 kJ·mol−1) are greater than that of Ge-Ge bonds (264 kJ·mol−1), the introduction of the Si-Ge bonds inside the Ge-nc can inhibit the diffusion of Ge from the Ge-nc during the annealing process. However, for the fused silica sample, no crystalline Si-Ge bonds are detected within the Ge-nc, where strong Ge outdiffusion effects prod...

One-Pot Decoration of Graphene with SnO2 Nanocrystals by an Elevated Hydrothermal Process and Their Application as Anode Materials for Lithium Ion Batteries

Tin dioxide (SnO₂), with a high theoretical storage capacity of 782 mAhg-1, is a potential alternative anode for rechargeable lithium ion batteries (LIBs). However, its low electronic conductivity and poor stability during cycling (due to a change in volume) hinder its practical applications for energy storage. Composite materials of SnO₂-nanocrystal-decorated graphene, which show excellent electrochemical characteristics, were prepared using a one-pot elevated hydrothermal method at 250 °C without subsequent carbonization treatment. The effects of graphene, solvent composition, and temperature on the morphology, structure, and electrochemical properties of the SnO₂/graphene composites were investigated using XRD, SEM, TEM, and N₂ adsorption-desorption techniques. The as-prepared SnO₂/graphene composites deliver a high initial discharge capacity of 1734.1 mAh g-1 at 200 mA g-1 and exhibit a high reversible capacity of 814.7 mAh g-1 even after 70 cycles at a current density of 200 mA g-1. The composites also exhibit a high rate capability of 596 mAh g-1 at 2000 mAg-1, indicating a long cycle life and promising capability when used as anode materials for lithium ion batteries and suggesting that SnO₂/graphene composites have wide application prospects in LIBs.

Microstructural Defects and Their Formation Mechanisms in Ba 0.75 Sr 0.25 TiO 3 Epitaxial Film: Microstructural Defects and Their Formation Mechanisms in Ba 0.75 Sr 0.25 TiO 3 Epitaxial Film

Ba0.75Sr0.25TiO3 film was epitaxially grown on a (001) LaAlO3 substrate using single-target pulsed laser deposition. The microstructure of the epitaxial film was investigated by high-resolution transmission electron microscope (HRTEM), and the formation mechanism of microstructural defects was explored. It was shown that misfit and threading dislocations existed in the epitaxial Ba0.75Sr0.25TiO3 film. Apart from the dislocations, two different kinds of antiphase boundaries, straight and zig-zagged, were observed. For misfit dislocations, they were formed due to the lattice mismatch between LaAlO3 and Ba0.75Sr0.25TiO3 which could dissociate into several partial dislocations. For the threading dislocations, it was found that their dissociation usually coexists with stacking faults. The formation mechanism of the antiphase boundaries is attributed to the terrace or step on the surfaces of LaAlO3 substrate. If the nucleation site is just on the terrace, straight antiphase boundaries will be formed. However, if the nu286 无 机 材 料 学 报 第 25 卷 cleation site is not just on the terrace but a little far away from the terrace, zig-zagged antiphase boundaries will be produced. The results could shed light on the microstructural defects in other perovskite epitaxial films.