Xinfeng Zhang

High performance supercapacitors based on highly conductive nitrogen-doped graphene sheets

Thermal nitridation of reduced graphene oxide sheets yields highly conductive (∼1000-3000 S m(-1)) N-doped graphene sheets, as a result of the restoration of the graphene network by the formation of C-N bonded groups and N-doping. Even without carbon additives, supercapacitors made of the N-doped graphene electrodes can deliver remarkable energy and power when operated at higher voltages, in the range of 0-4 V.

A composite material of uniformly dispersed sulfur on reduced graphene oxide: Aqueous one-pot synthesis, characterization and excellent performance as the cathode in rechargeable lithium-sulfur batteries

AbstractSulfur-reduced graphene oxide composite (SGC) materials with uniformly dispersed sulfur on reduced graphene oxide sheets have been prepared by a simple aqueous one-pot synthesis method, in which the formation of the composite is achieved through the simultaneous oxidation of sulfide and reduction of graphene oxide. The synthesis process has been tracked ex situ by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy, which both confirm that the majority of graphene oxide has been reduced during the synthesis reaction. The sulfur contents in the SGC, determined by thermogravimetry and elementary analysis, have been adjusted in the range from 20.9 to 72.5 wt.%. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images reveal that most of the sulfur is uniformly dispersed on the reduced graphene oxide sheets, for which no sulfur in particulate form could be observed. The SGC materials have been tested as the cathode of rechargeable lithium-sulfur (Li-S) batteries, and demonstrated a high reversible capacity and good cycleability. The SGC-63.6%S can deliver a reversible capacity as high as 804 mA·h/g after 80 cycles of charge/discharge at a current density of 312 mA/g (ca. 0.186 C), and 440 mA·h/g after 500 cycles at 1250 mA/g (ca. 0.75 C).n

Nonspecific adsorption of charged quantum dots on supported zwitterionic lipid bilayers: real-time monitoring by quartz crystal microbalance with dissipation.

Understanding how the composition and environmental conditions of membranes influence their interactions with guest species is central to cell biology and biomedicine. We herein study the nonspecific adsorption of charged quantum dots (QDs) onto a supported zwitterionic lipid bilayer by using quartz crystal microbalance with dissipation (QCM-D). It is demonstrated that (1) the adsorption of charged QDs is charge-dependent in a way similar to but much stronger than that of the capping molecules by reason of size effect; (2) the adsorption behavior of charged QDs is dominated by electrostatic interaction, which can be well described by an adsorption window; (3) the adsorption window can be broadened by exploiting the bridge role of Ca(2+) ions; and (4) by introducing a cationic lipid into the zwitterionic lipid bilayer, one can achieve preferential adsorption of anionic QDs but suppression of the cationic QD adsorption. Our QCM-D data also indicates that these different adsorption traits effect different changes in the dissipation of supported lipid bilayers (SLBs) after adsorption of the charged QDs. The different adsorption propensities of cationic and anionic QDs on SLBs have reinforced the picture of electrostatic interactions. We believe that these findings provide important information on QD-lipid membrane interactions, which will help to develop new drug molecules and efficient drug delivery systems, and to predict and unravel their potential toxicities if any.

Enhanced cycle life of lead-acid battery using graphene as a sulfation suppression additive in negative active material

In this article, we report the addition of graphene (Gr) to negative active materials (NAM) of lead-acid batteries (LABs) for sulfation suppression and cycle-life extension. Our experimental results show that with an addition of only a fraction of a percent of Gr, the partial state of charge (PSoC) cycle life is significantly improved by more than 140% from 7078 to 17157 cycles. The particle size on a charged Pb-graphene (PbG) plate after the PSoC test is also found to be reduced by around 25% when compare with a Pb plate. Charge and discharge densities measurements from the cyclic voltammetry (CV) test show an enhancement with the addition of Gr, indicating an improvement in the reversibility reaction of PbSO4. An electrochemical model, which takes into account of reduced interfacial resistance, improved charge transfer and enhanced electroactive surface area, is proposed to elucidate the role of Gr throughout the course of a PSoC cycle test. It is demonstrated that experimental results are aligned with our proposed model with enhanced cycle life performance, where PbG plate maintains a higher electroactive surface area for adsorption and desorption of Pb2+ ions at the interface between active material and electrolyte occurs in parallel to reduced charge transfer.

Highly conductive polymer composites from room-temperature ionic liquid cured epoxy resin: effect of interphase layer on percolation conductance

An ionic liquid type imidazolium catalyst enhanced the conductivity of epoxy-anhydride resin based electrical conductive adhesives (ECAs) by more than two orders of magnitude compared with common imidazole compounds. Percolation growth dynamics study combining with epoxy curing kinetics results indicates that the enhancement of conductivity was attributed to the higher percolation efficiency within ionic liquid catalyzed ECAs. An “interphase layer” model was proposed to explain the percolation mechanism in the thermoset based conductive composite. The present work highlights the importance of cross-linking the polymer matrix with filler interconnections, and provides practical guidance on the control over percolation dynamics in conductive composites in order to achieve enhanced percolation conductivity.

Two-dimensional self-assemblies of silica nanoparticles formed using the "bubble deposition technique".

Two-dimensional silica nanoparticle assemblies were obtained by deposition of bubble made from a surfactant solution containing nanoparticles onto hydrophobic silicon substrate. The morphologies of the nanoparticle assemblies can be finely controlled by several experimental parameters, including surfactant concentration, nanoparticle concentration, and deposition time. Monolayer of nanoparticles with surface coverage of about 100% can be obtained under appropriate conditions. The method can also be applied to another hydrophobic substrate, HMDS (hexamethyldisilazane)-modified silicon substrate. Furthermore, it can be applied directly to lithography patterned substrates, meaning a high compatibility with the well-developed conventional top-down approaches to nanodevices. This bubble deposition technique is expected to be a promising method in the field of nano-object assembly and organization and has great application potentials.

Conductive, transparent, flexible electrode from silver nanowire thin film with double layer structure

Herein we report a double layer structured transparent/flexible transparent electrode from silver nanowire (Ag NW) network film with graphene oxide (GO) coating. The electrode consists of two layers on top of a transparent substrate successively: the first layer is a thin film composed of Ag NW networks, and a GO coating on top. The top encapsulation layer not only greatly decreases the film resistivity while keeping its transparency, but also enhances the thermal and chemical stability. It is expected that the proposed GO/Ag NW double layered electrode might serve as an ultimate solution to the practical applications of Ag NW based electrode.

High performance electrical conductive composites with ultralow percolation threshold

Particle-laiden polymer composites are a broad range of functional materials. Most of the polymer composite properties are strongly dependent on the filler loading level, which typically have an abrupt change near the critical filler loading. This phenomenon has been widely investigated and theoretically modeled by percolation theory. Generally, controlling the percolation threshold became a dominating approach to adjust the percolation properties of a composite and have attracted intensive interests. Here we report a generized strategy to control over the percolative properties in polymer composite. The percolation threshold of the composite <;4 v% was demonstrated in polymer composite by using the commercially available microparticles, comparing to a threshold value of ~18 v% in a conventional composite. For the electrically conductive composite, a reduction of 40 wt% of filler loading was achieved while keeping its conductivity. The percolation threshold controls not only increase the percolation properties of the electrical conductivity, but also the thermal conductivity and dielectric properties of the composites. The strategy described can be implemented in most of the current polymer composite manufacturing processes. The findings reported here are expected to revolutionize the conventional fields of composite materials.

Highly conductive die attach adhesive from percolation control and its applications in light-emitting device thermal management

Herein, we reported on the study of percolation dynamics in thermoset-based die attach (DA) materials and its effect on percolation conductivity. Two types of percolation mechanism in thermoset based DA were discovered, i.e., the curing reaction-induced percolation and the physical aging-induced percolation. The former features in a fast percolation network growth rate, which is one order of magnitude higher than the latter. It is demonstrated that the percolation kinetics largely affects the apparent percolation conductivity under the traditional packaging conditions; and reaction-induced percolation allows ultrahigh efficiency in reaching the volume fraction-limiting percolation conductance, resulting in enhanced thermal performance of DA.

Highly thermal conductive transparent die attach material for LEDs

Transparent die attach materials (DA) with moderate thermal conductivity without yellowing are preferred for mid-power LEDs, which dominate the LED backlighting and general solid-state lighting markets. A novel DA was developed based on a new kind of silicone base material specifically designed and synthesized. Compared with widely used transparent DA of KER-3000-M2 from Shin-Etsu Chemical Co., Ltd., the newly developed DA has high thermal conductivity of 0.53 W/m-K, low viscosity of 7 Pa-s at 1 s-1, and a transmittance of 97% at wavelength of 450 nm without trading off the good adhesion. Packaged in 0.3 W blue LEDs, the new DA helps achieve around 26% thermal resistance reduction and 7.7% radiometric power improvement.