Zongwen Liu

High rate capability caused by surface cubic spinels in Li-rich layer-structured cathodes for Li-ion batteries.

Modified Li-rich layered cathode Li(Li0.2Mn0.54Ni0.13Co0.13)O2 has been synthesized by a simple strategy of using surface treatment with various amounts (0-30 wt.%) of Super P (carbon black). Based on detailed characterizations from X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS), it is suggested that the phase transformation from Li2MnO3-type of structure to spinel-like phase take place at the surface regions of particles during post annealing process at 350 °C, leading to increase in both first coulombic efficiency and rate capability, from 78% and 100 mAh · g(-1) (charge capacity at 2500 mA · g(-1)) of the pristine material to 93.4% and 200 mAh · g(-1). The evidences of spinel formation and the reasons for electrochemical enhancement are systematically investigated.

A combined study by XRD, FTIR, TG and HRTEM on the structure of delaminated Fe-intercalated/pillared clay

Fe-PILC samples were synthesized by the reaction between Na(+)- and/or Ca(2+)-montmorillonite (Mt) and base-hydrolyzed solutions of Fe(III) nitrate. Different from the known usual microporous pillared structure, a meso-microporous delaminated structure containing intercalated or pillared fragments was found in the respective resulting Fe-intercalated or -pillared clays. XRD patterns of Na(+)-Mt-based Fe-intercalated/pillared clays show one large d-spacing above 6.4 nm corresponding to the mesoporous delaminated part, whereas another d-spacing of ca. 1.5 nm was indicative of the microporous pillared part. Fe-intercalated/pillared clays based on Ca(2+)-Mt lead to similar results, but with a d-spacing less than 6 nm and a second low intense d-spacing less than 1.5 nm. In the delaminated Fe-intercalated clays, NO(-)(3) anions were retained even after thorough washing process. They play as counterions to neutralize the positive-charged iron aggregates in the delaminated structure, and can be exchanged by heteropolyanions as [PW(12)O(40)](3-). The delaminated Fe-pillared clays show good thermal stability at 500 degrees C and exhibit at this temperature dramatically higher specific surface area and porosity than the starting montmorillonites. However, calcination at a higher temperature leads to the formation of nanocrystalline hematite. Air-drying after ethanol extraction (EAD) method has an advantage over air-drying (AD) method in preserving the delaminated structure.

Graphene-based surface modification on layered Li-rich cathode for high-performance Li-ion batteries

A Li-rich cathode material Li(Li0.2Mn0.54Ni0.13Co0.13)O2 synthesized by a sol–gel method is further modified by wrapping with graphene oxide (GO) using a simple chemical approach and post thermal annealing. X-ray diffraction (XRD), Raman spectroscopy, high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) characterizations show that annealing of the GO-wrapped material leads to a transformation from the original layered structure to a spinel phase on the surfaces of the powder particles. The electrochemical performance, and in particular the rate capability, is observed to be significantly enhanced as a result of this unique surface-treated Li-rich cathode/graphene composite structure. The modified sample reveals a very high discharge capacity of 313 mA h g−1 at a current density of 12.5 mA g−1, and 201 mA h g−1 charge capacity at an extremely high current density of 2500 mA g−1. Such improvement is ascribed to the co-existence of reduced GO, the locally transformed spinel-like structure and the recrystallized particles on the surfaces of the primary particles.

Ga-filled single-crystalline MgO nanotube: Wide-temperature range nanothermometer

A highly effective one-step approach was developed to synthesize single-crystalline MgO nanotubes and in situ fill nanotubes with Ga. The axes of nanotubes are in the [100] direction of cubic MgO. The prepared nanotube exhibits a square-like cross section both for its interior and exterior. The liquid metal-assisted route is suggested to be a general way to prepare oxide nanotubes. Linear thermal expansion behavior recorded for liquid gallium column confined in the MgO nanotube makes possible creation of a wide-temperature range nanothermometer with superior mechanical properties and environmental structural stability.

Organosilane functionalization of halloysite nanotubes for enhanced loading and controlled release

The surfaces of naturally occurring halloysite nanotubes were functionalized with γ-aminopropyltriethoxysilane (APTES), which was found to have a substantial effect on the loading and subsequent release of a model dye molecule. APTES was mostly anchored at the internal lumen surface of halloysite through covalent grafting, forming a functionalized surface covered by aminopropyl groups. The dye loading of the functionalized halloysite was 32% greater than that of the unmodified sample, and the release from the functionalized halloysite was dramatically prolonged as compared to that from the unmodified one. Dye release was prolonged at low pH and the release at pH 3.5 was approximately three times slower than that at pH 10.0. These results demonstrate that organosilane functionalization makes pH an external trigger for controlling the loading of guest on halloysite and the subsequent controlled release.

Synthesis of tungsten oxide nanowires

Abstract Tungsten oxide nanowires have been prepared by heating a tiny tungsten wire partly wrapped with melted born oxide (B 2 O 3 ) vitreous powder at 1600 °C in nitrogen atmosphere. The nanowires exhibit a well crystallized one-dimensional structure with 10–30 nm in diameter and 2–5 μm in length. A monoclinic crystalline structure could be established by high-resolution transmission electron microscopy study.

Three-dimensional electrodes for dye-sensitized solar cells: synthesis of indium?tin-oxide nanowire arrays and ITO/TiO2 core?shell nanowire arrays by electrophoretic deposition

Dye-sensitized solar cells (DSSCs) show promise as a cheaper alternative to silicon-based photovoltaics for specialized applications, provided conversion efficiency can be maximized and production costs minimized. This study demonstrates that arrays of nanowires can be formed by wet-chemical methods for use as three-dimensional (3D) electrodes in DSSCs, thereby improving photoelectric conversion efficiency. Two approaches were employed to create the arrays of ITO (indium-tin-oxide) nanowires or arrays of ITO/TiO(2) core-shell nanowires; both methods were based on electrophoretic deposition (EPD) within a polycarbonate template. The 3D electrodes for solar cells were constructed by using a doctor-blade for coating TiO(2) layers onto the ITO or ITO/TiO(2) nanowire arrays. A photoelectric conversion efficiency as high as 4.3% was achieved in the DSSCs made from ITO nanowires; this performance was better than that of ITO/TiO(2) core-shell nanowires or pristine TiO(2) films. Cyclic voltammetry confirmed that the reaction current was significantly enhanced when a 3D ITO-nanowire electrode was used. Better separation of charge carriers and improved charge transport, due to the enlarged interfacial area, are thought to be the major advantages of using 3D nanowire electrodes for the optimization of DSSCs.

Structural, optical and magnetic properties of Co-doped ZnO nanorods with hidden secondary phases

Co-doped ZnO nanorods (composition: Zn(0.955)Co(0.045)O) were grown by a simple surfactant-assisted hydrothermal technique. The morphological, structural, optical and magnetic properties of the as-prepared nanorods were investigated by means of scanning electron microscopy, high-resolution transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, micro-Raman spectroscopy, micro-cathodoluminescence, and vibrating sample magnetometry (VSM). The results showed that the sample had rod-like morphology and that the preferential growth direction was along the c axis. While Co was successfully doped into the ZnO wurtzite lattice structure as revealed by several characterization techniques, hidden secondary phases of Zn(y)Co(3-y)O(4) (0≤y≤1) were also clearly detected by the micro-Raman spectroscopic technique. We propose that the predominant diffusion-limited Ostwald ripening crystal growth mechanism under the hydrothermal coarsening yielded such phase segregation. VSM results showed that the nanorods displayed relatively weak room-temperature ferromagnetism. We suggest that the origin of the ferromagnetism is probably due to the presence of the mixed cation valence of Co via a d-d double-exchange mechanism rather than the real doping effect. It is essential to control the crystal growth mechanism and defect states associated with the ferromagnetism in order to realize the intrinsic diluted magnetic semiconductors.

Temperature dependence of the electrical transport properties in few-layer graphene interconnects

We report a systematic investigation of the temperature dependence of electrical resistance behaviours in tri- and four-layer graphene interconnects. Nonlinear current–voltage characteristics were observed at different temperatures, which are attributed to the heating effect. With the resistance curve derivative analysis method, our experimental results suggest that Coulomb interactions play an essential role in our devices. The room temperature measurements further indicate that the graphene layers exhibit the characteristics of semiconductors mainly due to the Coulomb scattering effects. By combining the Coulomb and short-range scattering theory, we derive an analytical model to explain the temperature dependence of the resistance, which agrees well with the experimental results.

Temperature measurement using a gallium-filled carbon nanotube nanothermometer

We report here temperature measurement by means of a Ga-filled C nanotube thermometer with diameter <150 nm and length ∼12 μm. The method relies on the initial identification and calibration of a nanothermometer in a transmission electron microscope (TEM), followed by placing it into an air-filled furnace whose temperature is to be measured, and final TEM reading of a postmeasurement gradation mark visible inside the tubular channel. The mark originates from the fact that, at high temperature, the Ga column tip exposed to the air through the open C nanotube end oxidizes, and a thin Ga oxide layer sticks to the nanotube walls upon cooling. The temperature according to this gradation mark coincides closely with nominal furnace temperature controlled by standard means. The method paves the way for practical temperature measurements using a C nanothermometer in air and within spatially localized regions (e.g., dimensions of tens of micrometers).