Aiping Fu

Preparation of Porous Hollow SiO2 Spheres by a Modified Stöber Process Using MF Microspheres as Templates

Using the surface charged and acid dissolvable melamine formaldehyde (MF) microspheres as sacrificial hard templates, silica coated MF core–shell composite microspheres, denoted as MF@SiO2, were synthesized via a surfactant-assisted sol–gel process by using tetraethyl orthosilicate (TEOS) as silica source. Hollow SiO2 spheres with mesoporous shells were then obtained after selective removal of the MF cores and the pore directing surfactant by hydrochloric acid etching or calcinations in air. Interesting shrinkage phenomena were observed in both the hollow products derived from hydrochloric acid etching and calcinations. The influence of the ratio of MF sphere to TEOS and the removal method of the MF core on the size of the hollow spheres, the shell thickness and the shell surface roughness have been studied. The composition, the thermal stability, the morphology, the surface area and pore size distribution, the wall thickness and adsorption properties of the hollow spheres derived from hydrochloric acid etching and calcinations were also investigated and compared based on the FTIR, SEM, TEM, TGA, Nitrogen adsorption–desorption and spectrophotometer techniques or measurements.

One-step solvothermal preparation of Fe3O4/graphene composites at elevated temperature and their application as anode materials for lithium-ion batteries

A one-step high-temperature solvothermal process (can be used up to 400 °C) has been explored for the preparation of Fe3O4/graphene composites. The influence of high temperature (>230 °C) on the structure, morphology and electrochemical properties of the resulting Fe3O4/graphene composites was investigated by XRD, SEM, TEM and N2 adsorption–desorption measurements. Electrochemical performances of the as-prepared Fe3O4/graphene composites at different temperatures were evaluated in coin-type cells as anode materials for lithium-ion batteries. In comparison with the traditional solvothermal method (<240 °C), the high-temperature method does not require an additional calcination process yet it still could result in Fe3O4/graphene composites with pure phase and excellent electrochemical properties. A preferred solvothermal temperature of 280 °C has been deduced based on a series of control experiments. The Fe3O4/graphene composite derived at 280 °C exhibited a high reversible capacity of 907 mA h g−1 at 0.1 C (92.6 mA g−1) even after 65 cycles, showing outstanding cycle stability. It also exhibited a high rate capability of 410 mA h g−1 at 2 C (1852 mA g−1). The role of the graphene substrates in improving the electrochemical properties of the composite is discussed based on the morphology, structure, phase and electrochemical property studies.

An RAPET approach to in situ synthesis of carbon modified Li4Ti5O12 anode nanocrystals with improved conductivity

Carbon modified lithium titanate (Li4Ti5O12) anode nanocrystals for Li-ion batteries were synthesized by directly treating the titanium alkoxide and lithium acetate ethanol solution via the Reaction under Autogenic Pressure at Elevated Temperature (abbreviated to RAPET). The mixture of the liquid precursors decomposed during the RAPET process and then reacted in situ and transformed into carbon-modified Li4Ti5O12 anode nanocrystals. The organic moieties in the titanium alkoxide and the lithium salt provided both the oxygen and carbon for the synthesis. The resulting products were characterized by X-ray diffraction (XRD), elemental analysis, scanning electronic microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), nitrogen adsorption–desorption measurements, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge testing. The influences of the titanium alkoxide precursors, i.e. the length of the alkoxy group, on the properties of the final products and the presence of the in situ resulting carbon on the electrochemical performance have been investigated.

Preparation of cellulose based microspheres by combining spray coagulating with spray drying

Porous microspheres of regenerated cellulose with size in range of 1-2 μm and composite microspheres of chitosan coated cellulose with size of 1-3 μm were obtained through a two-step spray-assisted approach. The spray coagulating process must combine with a spray drying step to guarantee the formation of stable microspheres of cellulose. This approach exhibits the following two main virtues. First, the preparation was performed using aqueous solution of cellulose as precursor in the absence of organic solvent and surfactant; Second, neither crosslinking agent nor separated crosslinking process was required for formation of stable microspheres. Moreover, the spray drying step also provided us with the chance to encapsulate guests into the resultant cellulose microspheres. The potential application of the cellulose microspheres acting as drug delivery vector has been studied in two PBS (phosphate-buffered saline) solution with pH values at 4.0 and 7.4 to mimic the environments of stomach and intestine, respectively.

Preparation of magnetically separable mesoporous Co@carbon/silica composites by the RAPET method

Air-stable metallic Co nanoparticles were incorporated into the pores or anchored onto the external surface of an ordered mesporous carbon/SBA-15 composite by simply treating the cobalt precursor-impregnated surfactant-containing SBA-15 via the Reaction under Autogenic Pressure at Elevated Temperature (abbreviated to RAPET) process, resulting in bifunctional Co@carbon/SBA-15 composites. Surfactant-containing mesoporous SBA-15 (denoted as surf-SBA-15 composite) was firstly impregnated with a cobalt precursor solution and then calcined under autogenic pressure at elevated temperature using a Swagelok-like high-temperature autoclave in an N2 atmosphere. During the RAPET process, the cobalt precursor impregnated into the channel of the surf-SBA-15 composites decomposed and was reduced by the surfactant species, meanwhile, the surfactant, which was used as a soft template for the formation of mesoporous silica, was carbonized and coated onto the internal and the external surface of mesoporous silica and partially onto the incorporated metallic Co nanoparticles, forming Co nanoparticles encapsulated by a mesoporous carbon/SBA-15 composite (mesoporous Co@carbon/SBA-15) in situ. The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption–desorption measurements and X-ray diffraction (XRD). The influences of the concentration of cobalt precursors on the morphologies and the magnetic properties of the mesoporous Co@carbon/silica composites were studied. The adsorption/separation ability of the magnetic mesoporous composite to organic guest molecules and the stability of the nanosized metallic Co decorations in air or in an acid environment were also investigated.

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 and Characterization of N-Doped Porous TiO2 Hollow Spheres and Their Photocatalytic and Optical Properties

Three kinds of N-doped mesoporous TiO2 hollow spheres with different N-doping contents, surface area, and pore size distributions were prepared based on a sol–gel synthesis and combined with a calcination process. Melamine formaldehyde (MF) microspheres have been used as sacrificial template and cetyltrimethyl ammonium bromide (CTAB) or polyvinylpyrrolidone (PVP) was selected as pore-directing agent. Core–shell intermediate spheres of titania-coated MF with diameters of 1.2–1.6 μm were fabricated by varying the volume concentration of TiO2 precursor from 1 to 3 vol %. By calcining the core–shell composite spheres at 500 °C for 3 h in air, an in situ N-doping process occurred upon the decomposition of the MF template and CTAB or PVP pore-directing surfactant. N-doped mesoporous TiO2 hollow spheres with sizes in the range of 0.4–1.2 μm and shell thickness from 40 to 110 nm were obtained. The composition and N-doping content, thermal stability, morphology, surface area and pore size distribution, wall thickness, photocatalytic activities, and optical properties of the mesoporous TiO2 hollow spheres derived from different conditions were investigated and compared based on Fourier-transformation infrared (FTIR), SEM, TEM, thermogravimetric analysis (TGA), nitrogen adsorption–desorption, and UV–vis spectrophotoscopy techniques. The influences of particle size, N-doping, porous, and hollow characteristics of the TiO2 hollow spheres on their photocatalytic activities and optical properties have been studied and discussed based on the composition analysis, structure characterization, and optical property investigation of these hollow spherical TiO2 matrices.

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.

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.