Jiupeng Zhao

Layered polyaniline/graphene film from sandwich-structured polyaniline/graphene/polyaniline nanosheets for high-performance pseudosupercapacitors

Binder-free layered graphene/polyaniline composite film was prepared by an environmentally friendly and facile two-step route for the first time. Firstly, a sandwich-structured PANI/graphene/PANI nanosheet was prepared in situ from aqueous solution, followed an electrophoretic deposition process. By observations of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM), it conforms that the graphene sheet is uniformly covered by an ultralthin PANI film (3.7 nm). Raman spectra, Fourier transform infrared (FT-IR) spectra and X-ray photoelectron spectroscopy (XPS) are jointly used to confirm strong π–π electron and hydrogen bond interaction in the nanosheets. The layered PANI/graphene composite film exhibits an excellent gravimetric capacitance of 384 F g−1 at 0.5 A g−1 which is much higher than many other hybrid supercapacitors reported to date. It maintains its capacity up to 84% over 1000 cycles at a current density of 2 A g−1. This preparation method may provide a promising strategy for preparation of graphene-based composites with other conducting polymers and binder-free film electrodes.

Structural evolution and characteristics of the phase transformations between α-Fe2O3, Fe3O4 and γ-Fe2O3 nanoparticles under reducing and oxidizing atmospheres

The mechanism and conditions for the phase transformations from α-Fe2O3 to Fe3O4 and Fe3O4 to γ-Fe2O3 nanoparticles have been investigated by the thermal analysis method. The morphologies and structures of various nanoparticles after annealing in H2 : Ar at 294 °C and in an O2 atmosphere at 302 °C have been examined and characterized. Finally, we confirmed that the monodisperse, porous and magnetic γ-Fe2O3 nanoparticles could be obtained by annealing the α-Fe2O3 nanoparticles synthesized by the hydrothermal route.

Electrodeposition of 3D ordered macroporous germanium from ionic liquids: a feasible method to make photonic crystals with a high dielectric constant.

A promising method for the production of germanium photonic crystals consists of electrodeposition of Ge from GeCl(4)-containing ionic liquids inside templates of polystyrene colloidal crystals and subsequent removal of the template. This room-temperature method gives rise to the fabrication of a three-dimensional highly ordered macroporous germanium nanostructure (see picture; scale: 2 microm) as a prototype of a photonic crystal.

Improved electrochromic performance and lithium diffusion coefficient in three-dimensionally ordered macroporous V2O5 films

Three-dimensionally ordered macroporous (3DOM) vanadium pentoxide (V2O5) films with various pore diameters were prepared by anodic deposition into colloidal crystal templates. The influence of the 3DOM structure on lithium ion (Li+) diffusion coefficient was investigated for the first time. X-ray diffraction analysis and HRTEM measurements show that the skeleton walls are composed of crystallites and amorphous V2O5. The study of electrochromic properties indicates that the pore size has a significant impact on the electrochromic performance. Small pores in the film lead to higher optical contrast and faster switching response. A high transmittance modulation in the visible and near-infrared spectral regions (50% at λ = 650 nm and 47% at λ = 900 nm) with fast response time (1.7 s for coloration and 3.2 s for bleaching) is observed in the 3DOM V2O5 film with a pore size of 210 nm. Because of the fully interconnected macroporous network in the 3DOM structure, the transport and reaction of lithium ions and electrons both behave in an effective 3D model throughout the whole nanostructure. Additionally, due to their influence on the polarization of the electrode and surface defects, sharp nanoscale edges around pores and rough surfaces can further promote Li+ diffusion and intercalation/de-intercalation. The 3DOM V2O5 film with a pore size of 210 nm exhibits a very high Li+ diffusion coefficient of 3.78 × 10−9 cm2 s−1, which is higher than any coefficient ever reported for a V2O5 film.

Semiconductor nanostructures via electrodeposition from ionic liquids

The fascinating properties of ionic liquids make it possible to synthesize semiconductor nanostructures via a simple and low-cost electrochemical pathway. The present paper summarizes our recent work on the synthesis of Si, Ge, and SixGe1–x nanostructures from ionic liquids: thin films, nanowires and photonic crystals. We also introduce our first results on the template-assisted electrodeposition of SixGe1–x photonic crystals from 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]Tf2N) ionic liquid, and some optical measurements on the previously prepared Ge photonic crystals. Our results confirm that electrochemistry in ionic liquids is excellently suited to the synthesis of high-quality semiconductor nanostructures.

Enhanced photon absorption of single nanowire α-Si solar cells modulated by silver core

Single nanowire solar cells are a promising candidate as nanoelectronic power sources. Metallic cores were integrated in single nanowire solar cells, and the influence of the silver core on the absorption efficiency and the short circuit current was studied in this work. A Full-wave Vectorial Finite Element Method approach was used to rigorously solve Maxwells equations in two dimensions. The photon absorption in solar cells was modulated delicately to achieve higher absorption efficiencies and short circuit currents, by tuning the core size and radius of nanowire solar cells. The light trapping stemmed mainly from the localized surface plasmons and also from Mie scattering and leaky mode resonances. The results showed that an enhancement of 16.6% in the photocurrent could be achieved by α-Si nanowire solar cells with the proper core size and filling-ratio compared to that without silver core.

Large area orientation films based on graphene oxide self-assembly and low-temperature thermal reduction

Graphene oxide (GO) and reduced graphene oxide (RGO) have many outstanding physical and mechanical properties. Uniform and thickness controllable RGO films with large area were prepared by evaporation-induced self-assembly at a liquid/air interface on glass substrates in combination with low temperature thermal reduction at 200 °C. This process has the advantage of good compatibility with flexible and non-flexible substrates. The films are of centimeter scale and their thickness can be controlled. The structural evolution was characterized. The obtained thermal RGO films exhibit excellent optical properties, a high elastic modulus of 76.18 GPa, and a hardness of 6.89 GPa.

The roles of lithium-philic giant nitrogen-doped graphene in protecting micron-sized silicon anode from fading

A stable Si-based anode with a high initial coulombic efficiency (ICE) for lithium-ion batteries (LIB) is critical for energy storage. In the present paper, a new scalable method is adopted in combination with giant nitrogen-doped graphene and micron-size electrode materials. We first synthesize a new type of freestanding LIB anode composed of micron-sized Si (mSi) particles wrapped by giant nitrogen-doped graphene (mSi@GNG) film. High ICE (>85%) and long cycle life (more than 80 cycles) are obtained. In the mSi@GNG composite, preferential formation of a stable solid electrolyte interphase (SEI) on the surface of graphene sheets is achieved. The formation and components of SEI are identified for the first time by using UV-resonance Raman spectroscopy and Raman mapping, which will revive the study of formation and evolution of SEI by Raman. New mechanism is proposed that the giant graphene sheets protect the mSi particles from over-lithiation and fracture. Such a simple and scalable method may also be applied to other anode systems to boost their energy and power densities for LIB.

Versatile displays based on a 3-dimensionally ordered macroporous vanadium oxide film for advanced electrochromic devices

Three-dimensionally ordered macroporous (3DOM) vanadium oxide film was fabricated by anodic deposition of vanadium oxide into colloidal crystal template. The as-prepared films were investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). The electrochemical and electrochromic behaviors of the films were studied in 1.0 M LiClO4–propylene carbonate solution. Because of the larger surface area and shorter diffusion distance, the 3DOM film exhibited better electrochromic performance than the dense film prepared without the template. The 3DOM film exhibited multicolor changes (yellow, green and black) in the voltage window from +1 to −1 V with a high transmittance modulation of ca. 34% at 460 nm and high switching speed (1.5 s for coloration and 2.1 s for bleaching). Cyclic voltammetry (CV) and chronoamperometry measurements showed that the 3DOM structure is beneficial for Li+ diffusion in vanadium oxide films. An ITO/vanadia/gel/PEDOT:PSS/ITO electrochromic device was assembled, and the device showed multicolor changes with fast switching speed (3.5 s for anodic coloration and 3.4 s for cathodic coloration) and good cycling stability.

Novel morphology changes from 3D ordered macroporous structure to V2O5 nanofiber grassland and its application in electrochromism

Because vanadium pentoxide (V2O5) is the only oxide that shows both anodic and cathodic coloration electrochromism, the reversible lithium ion insertion/extraction processes in V2O5 lead to not only reversible optical parameter changes but also multicolor changes for esthetics. Because of the outstanding electrochemical properties of V2O5 nanofibers, they show great potential to enhance V2O5 electrochromism. However, the development and practical application of V2O5 nanofibers are still lacking, because traditional preparation approaches have several drawbacks, such as multiple processing steps, unsatisfactory electrical contact with the substrate, expensive equipment, and rigorous experimental conditions. Herein, we first report a novel and convenient strategy to prepare grass-like nanofiber-stacked V2O5 films by a simple annealing treatment of an amorphous, three-dimensionally ordered macroporous vanadia film. The V2O5 nanofiber grassland exhibits promising transmittance modulation, fast switching responses, and high color contrast because of the outstanding electrochemical properties of V2O5 nanofibers as well as the high Li-ion diffusion coefficients and good electrical contact with the substrate. Moreover, the morphology transformation mechanism is investigated in detail.