Jianxing Shen

Nano-p–n junctions on surface-coarsened TiO2 nanobelts with enhanced photocatalytic activity

Nano-p–n junction heterostructure TiO2 nanobelts have been produced by assembling p-type semiconductor NiO nanoparticles on n-type TiO2 nanobelts for enhancement of the photocatalytic properties of TiO2 nanobelts. NiO/TiO2 nano-p–n junctions were synthesized on the surfaces of TiO2 nanobelts and surface-coarsened TiO2 nanobelts. The nanobelts were obtained using alkaline- and acid-assisted hydrothermal processes. The chemical-solution-deposition–decomposition process was used to form NiO nanoparticle/TiO2 heterostructure composite nanobelts (NiO-NP/TiO2 NBs), and NiO nanoparticle/surface-coarsened TiO2 heterostructure composite nanobelts (NiO-NP/TiO2 coarsened NBs). The uniform assembly of p-type NiO nanoparticles produces a large number of nano-p–n junction heterostructures on the surface of the TiO2 nanobelts, where NiO and TiO2 form p- and n-type semiconductors, respectively. Compared with both pure NiO nanoparticles and TiO2 nanobelts, NiO-NP/TiO2 NBs exhibit much enhanced photocatalytic activity. Interestingly, the optimized composite NiO-NP/TiO2 coarsened NBs exhibit an enhanced photocatalytic activity in the decomposition of a model dye compound, methyl orange (MO), under both ultraviolet and visible light irradiation. It is argued that the nano-p–n junctions effectively reduce the recombination of electrons and holes, thus leading to the enhancement of the photocatalytic properties of the heterostructure composites. The larger number of abundant photocatalytic-active surfaces in the surface-coarsened nanobelts increases photo-absorption and the production of charge carriers, which gives the composites an enhanced photocatalytic performance. The established approach allows for controlling the nano-p–n junction heterostructure of the nanobelts, and hence, their photocatalytic effect. The NiO/TiO2 nanobelt-based nano-p–n junction heterostructure TiO2 can provide a practical way to design and prepare nano-composites for applications as solar-cell electrodes, in solar photocatalysis, solar photolysis of water and other related fields.

Bio-synthesis participated mechanism of mesoporous LiFePO4/C nanocomposite microspheres for lithium ion battery

In this paper we report a bio-synthesis route towards controllable mesoporous LiFePO4/C nanocomposite microspheres (MP-LFP/C-NC-MS). During the synthesis bakers yeast cells were used as both structure templates and a carbon source. Then we clarify the bio-deposited and biomolecular self-assembly mechanisms of iron phosphate by means of the Langmuir biosorption isotherms of the yeast biomass in iron ion solution and by applying the model of heterogeneous nucleation of iron phosphate in a yeast cell. The MP-LFP/C-NC-MS show a uniform size distribution (4.76 μm), high tap density (1.74 g cm−3) and a large specific surface area (203 m2 g−1). The microsphere is composed of densely aggregated nanoparticles and interconnected nanopores. The open mesoporous structure allows lithium ions to easily penetrate into the spheres, while a thorough coating of the biocarbon network on the surface of the LiFePO4 nanoparticles facilitates lithium ion and electron diffusion. The MP-LFP/C-NC-MS have a high discharge capacity of about 158.5 mA h g−1 at a current density of 0.1 C, discharge capacity of 122 mA h g−1 at 10 C, and high capacity retention rate. Therefore the mesoporous microspheres are an ideal type of cathode-active materials for making high-power Li-ion batteries.

LiFePO4/NaFe3V9O19/porous glass nanocomposite cathodes for Li+/Na+ mixed-ion batteries

The discovery and optimisation of high performance cathode materials are critical to future breakthroughs for next-generation rechargeable batteries. LiFePO4 (LFP)/NaFe3V9O19 (NFV)/Na2O–FeO–V2O3–P2O5–biocarbon (NFVPB) porous glass nanocomposites (LFP/NFV/NFVPB) offer new possibilities for Li+/Na+ mixed-ion batteries with high-rate capability and high discharge voltage plateaus. Here we have successfully synthesized these nanocomposites via self-assembly of adenosine disodium triphosphate (Na2ATP) biotemplates and a carbon thermal reduction method. As a novel cathode material, LFP/NFV/NFVPB delivers a reversible capacity of 202.8 mA h g−1 at 0.1C in the Li+/Na+ mixed-ion cell with the electrochemically active redox reactions of Fe2+/Fe3+ and V3+/V4+, which is far higher than the theoretical capacity of LiFePO4 (170 mA h g−1). The cell exhibits two high voltage plateaus with well-defined discharge voltage near 3.4 and 3.7 volts, and a coulombic efficiency of approximately 90 percent. Because the low-energy nonequilibrium paths of the fast phase transformation process in LFP/NFV composite nanoparticles can improve the high-rate performance, the cell still exhibits a higher specific capacity of about 100.4 mA h g−1 at 10C. These results are attributed to the nanocomposite structure of LiFePO4 and NaFe3V9O19 and better percolation of electrochemically active glass with a hierarchical pore structure. This work will contribute to the development of Li+/Na+ mixed-ion batteries.

Synthesis of BiOI nanosheet/coarsened TiO2 nanobelt heterostructures for enhancing visible light photocatalytic activity

For high photocatalytic activity, BiOI nanosheets were deposited on acid-corroded TiO2 nanobelts, where a large number of BiOI nanosheet/TiO2 nanobelt heterojunctions could be built. The structure, morphology and optical properties of the BiOI nanosheet/coarsened TiO2 nanobelt heterostructure composites (BiOI/TiO2 CNHs) were characterized by XRD, XPS, SEM, HRTEM, DRS, PL, TOC and nitrogen sorption. The XRD results show that only two phases of TiO2 and BiOI were found in the composites. The HRTEM image shows clear lattice fringes, which prove the formation of heterojunctions at the interface between BiOI and coarsened TiO2. The photocatalytic activity of the BiOI/TiO2 CNHs for the degradation of methyl orange under visible-light irradiation was evaluated. The results reveal that the BiOI/TiO2 CNHs exhibited much higher photocatalytic activity compared with coarsened TiO2 nanobelts and pure BiOI nanosheets due to the introduction of BiOI onto the surface of the coarsened TiO2 nanobelts and the formation of heterojunctions. BiOI acts as a visible light photosensitizer in the BiOI/TiO2 CNHs. In addition, the large surface area and matched energy bands of the BiOI/TiO2 CNHs both contribute to enhancing the photocatalytic activity. In addition, the possible mechanism of promotion of the photocatalytic performance of the new heterostructure system is explained.

Recent progress in hybrid cathode materials for lithium ion batteries

Rechargeable lithium ion batteries (LIBs) are considered as one of the most promising power sources for electric vehicle (EV) and hybrid electric vehicle (HEV) applications due to their high energy density, durable cycle life, higher output power and safety issues. Previous research has mainly focused on a single electrode. However, it is difficult for a single cathode material to meet the requirements for EVs and HEVs, and so the concept of a “hybrid cathode material” was proposed. A so-called hybrid cathode material combines a number of excellent electrochemical properties of each cathode material and has become an alternative to conventional single cathode materials. Present study of hybrid cathode materials is mainly focused on composites of two kinds of cathode materials. In this review, we summarize recent progress in hybrid cathode materials, such as LiFePO4–Li3V2(PO4)3, LiFePO4–LiCoO2, LiFePO4–LiMn2O4, LiFePO4–LiVPO4F, LiFePO4–LiMnPO4, Li3V2(PO4)3–LiMnPO4, Li3V2(PO4)3–LiVPO4F, Li3V2(PO4)3–LiVOPO4 and LiCoO2–LiMn2O4. A perspective of hybrid cathode materials is also discussed.

TiO2-B Nanoribbons Anchored with NiO Nanosheets as Hybrid Anode Materials for Rechargeable Lithium ion Batteries

A new type of TiO2-B nanoribbon anchored with NiO nanosheets (TiO2@NiO) is synthesized via a hydrothermal process and a subsequent homogeneous precipitation method. XRD analysis indicates that TiO2-B and cubic NiO phases exist in the composites. According to SEM images, the morphology of the TiO2@NiO hybrid material is unique, similar to that of leaf mosaic in biological systems. According to electrochemical investigations, the nanostructured hybrid material as an anode exhibits superior initial charge/discharge capacity and capacity retentions. The initial discharge capacity of the TiO2@NiO hybrid nanostructure is 395 mA h g−1, and the capacity remains 380 mA h g−1 after 50 charge/discharge cycles, which is about 96.2% capacity retention and 7.8% higher than that of pristine TiO2-B nanoribbons.

Wafer-scale porous GaN single crystal substrates and their application in energy storage

Porous GaN has recently attracted much interest due to its high surface area, shift of bandgap and efficient luminescence. However, the porous GaN materials obtained so far are mainly fabricated via an electrochemical etching or a photoelectrochemical etching method. Here, we report the fabrication of wafer-scale (2 inch) porous GaN by a novel and simple high temperature annealing process. A model is proposed, based on scanning electron microscopy as well as the stabilities of the GaN crystallographic plane, to explain the formation mechanism of the porous GaN. Improvement of the crystal quality, enhancement of the optical quality and strain relaxation of porous GaN are confirmed by the HRXRD, PL and Raman spectroscopy results. A three-electrode system based on the porous GaN electrodes is assembled. The good capacitive behavior and superior electrochemical reversibility validate the concept of porous GaN-based supercapacitors and point to their potential in energy storage applications.

Effects of ball milling on the crystal face of spinel LiMn2O4

Lithium manganese oxide (LiMn2O4) cathode materials are synthesized by a facile solid state reaction method using planetary ball-milled mixtures of Li2CO3 and MnO2 as materials. The as-prepared LiMn2O4 is analyzed by XRD, SEM and AAS to investigate the effects of ball milling on the grain size and morphology of LiMn2O4. It is found that ball milling treatment can reduce the (111) surfaces of LiMn2O4 and improve the corresponding electrochemical performance. The LiMn2O4 prepared after 6 h ball-milling shows higher first discharge capacity (129.3 mA h g−1 at 0.5 C) and excellent cycling stability (94.95% after 30 cycles).