Effect of porous structural properties on lithium-ion and sodium-ion storage: illustrated by the example of a micro-mesoporous graphene1-x (MoS2) x anode.

In this work, we synthesized micro-mesoporous graphene1-x (MoS2) x with different compositional ratios via co-reduction of graphite oxide and exfoliated MoS2 platelets. We systematically studied the performance of the micro-mesoporous graphene1-x (MoS2) x as anodes in lithium-ion batteries and sodium-ion batteries. The results show that the specific surface areas of the composites decrease with introducing MoS2. The irreversible capacitance, which is related to the formation of solid electrolyte interphases, also decreases. Besides specific surface area, we found that micropores can benefit the lithiation and sodiation. We demonstrated that a specific charge capacity of 1319.02 mA h g-1 can be achieved at the 50th cycle for the graphene½(MoS2)½ anode in lithium-ion batteries. Possible relationships between such a high specific capacity and the micro-mesoporous structure of the graphene1-x (MoS2) x anode are discussed. This work may shed light on a general strategy for the structural design of electrode materials in lithium-ion batteries and sodium-ion batteries.

Modified Sauerbrey equation: a facile method to quantitatively probe the conformation of isolated molecules at solid-liquid interfaces.

Despite the increasingly popular application of the quartz crystal microbalance (QCM) technique in monitoring phenomena taking place at solid-liquid interfaces, ranging from changes in mass to changes in conformation, a simple, direct relationship between QCM signal and surface mass remains elusive. In this paper, we report that the proportional relationship between the QCM signal and the surface mass arises from the linear relationship between the viscosity of the layer adsorbed at the solid-liquid interface and the surface coverage, as well as a small viscosity shift. The proportionality coefficient depends on the intrinsic viscosity of adsorbates, solvent density, and quartz crystal thickness. The intrinsic viscosity is dominated by the conformation of the entire molecular chain and the adsorption blob for end-grafted and physisorbed molecules, respectively. Using this modified Sauerbrey equation, the phenomena relating to the conformation of discrete chains at the solid-liquid interfaces can be semi-quantitatively described.

Unique interconnected graphene/SnO2 nanoparticle spherical multilayers for lithium-ion battery applications.

We have designed and synthesized a unique structured graphene/SnO2 composite, where SnO2 nanoparticles are inserted in between interconnected graphene sheets which form hollow spherical multilayers. The hollow spherical multilayered structure provides much flexibility to accommodate the configuration and volume changes of SnO2 in the material. When it is used as an anode material for lithium-ion batteries, such a novel nanostructure can not only provide a stable conductive matrix and suppress the mechanical stress, but also eliminate the need of any binders for constructing electrodes. Electrochemical tests show that the unique graphene/SnO2 composite electrode as designed could exhibit a large reversible capacity over 1000 mA h g-1 and long cycling life with 88% retention after 100 cycles. These results indicate the great potential of the composite for being used as a high performance anode material for lithium-ion batteries.

Comparison of reduction products from graphite oxide and graphene oxide for anode applications in lithium-ion batteries and sodium-ion batteries.

Hydrazine-reduced graphite oxide and graphene oxide were synthesized to compare their performances as anode materials in lithium-ion batteries and sodium-ion batteries. Reduced graphite oxide inherits the layer structure of graphite, with an average spacing between neighboring layers (d-spacing) of 0.374 nm; this exceeds the d-spacing of graphite (0.335 nm). The larger d-spacing provides wider channels for transporting lithium ions and sodium ions in the material. We showed that reduced graphite oxide as an anode in lithium-ion batteries can reach a specific capacity of 917 mA h g-1, which is about three times of 372 mA h g-1, the value expected for the LiC6 structures on the electrode. This increase is consistent with the wider d-spacing, which enhances lithium intercalation and de-intercalation on the electrodes. The electrochemical performance of the lithium-ion batteries and sodium-ion batteries with reduced graphite oxide anodes show a noticeable improvement compared to those with reduced graphene oxide anodes. This improvement indicates that reduced graphite oxide, with larger interlayer spacing, has fewer defects and is thus more stable. In summary, we found that reduced graphite oxide may be a more favorable form of graphene for the fabrication of electrodes for lithium-ion and sodium-ion batteries and other energy storage devices.

Electrical and optoelectrical modification of cadmium sulfide nanobelts by low-energy electron beam irradiation.

In this report, we describe a method for modifying electrical and optoelectrical properties of CdS nanobelts using low-energy (lower than 10 keV) e-beam irradiation in a scanning electron microscope. The electrical conductivity of the nanobelts was dramatically improved via the irradiation of e-beams. The modified conductivity of the nanobelts depends on the energy of the e-beam; it exhibits a larger photocurrent and higher external quantum efficiency but slower time-response than that before the modification. A possible mechanism about the modification is the increase of electron accumulation (injected electrons) in the nanobelts due to e-beam irradiation. In addition, the optoelectrical modification could be caused by the trapped electrons in the nanobelts and the decrease of contact resistance between the nanobelts and metal electrodes induced by e-beam irradiation. The results of this work are significant for the in situ study of semiconductor nanostructures in the electron microscope. Besides, the method of electrical and optoelectrical modification presented here has potential application in electronics and optoelectronics.

Conjugated foldamers with unusually high space-charge-limited current hole mobilities.

Charge carrier mobility and its optimization play a critical role in the development of cutting-edge organic electronic and optoelectronic devices. Even though space-charge-limited current (SCLC) hole mobilities as high as 1.4 cm(2) V(-1) s(-1) have been reported for microscopically sized highly ordered liquid-crystalline conjugated small molecules, the SCLC hole mobility of device-sized thin films of conjugated polymers is still much lower, ranging from 10(-6) to 10(-3) cm(2) V(-1) s(-1). Herein, we report the synthesis, characterizations, and thin-film SCLC mobility of three discotic conjugated polymers, INDT-TT, INDT-BT, and INDT-NDT. Optical studies indicate that polymer INDT-NDT adopts a folded conformation in solutions of good or poor solvents, whereas polymer INDT-TT stays as random monomeric chains in good solvents and interchain aggregates in poor solvents. INDT-BT polymer chains, however, stay as foldamers in dilute solutions of good solvents but interchain aggregates in concentrated solutions or poor solvents. Circular dichroism spectroscopy provides clear evidence for the helical folding of INDT-NDT in solutions. Thin films spin-coated from 1,2-dichlorobenzene solutions of the polymers show SCLC hole mobility of 2.20 × 10(-6), 8.79 × 10(-5), and 2.77 × 10(-2) cm(2) V(-1) s(-1) for INDT-TT, INDT-BT, and INDT-NDT, respectively. HRTEM and powder XRD measurements show that INDT-NDT pristine thin films contain nanocrystalline domains, whereas the INDT-TT and INDT-BT films are amorphous. Thin films of INDT-NDT:PC71BM blends show increased crystallinity and further improved SCLC hole mobility up to 1.29 × 10(-1) cm(2) V(-1) s(-1), one of the highest SCLC mobility values ever recorded on solution-processed organic semiconducting thin films. The persistent folding conformation of INDT-NDT is believed to be responsible for the high crystallinity of its thin films and its high SCLC mobilities.

Viscoelastic signature of physisorbed macromolecules at the solid-liquid interface.

Viscoelastic behavior of a solution boundary layer at a solid-liquid interface could differ from that of bulk solution due to molecular adsorption at the interface. Such a property can be used as a characteristic signature to indicate the molecular adsorption at the interface. In this work, we systematically measured the viscoelastic properties of polyethylene glycol (PEG) solution boundary layers in contact with a gold surface using a quartz crystal resonator technique. The results show that viscosity and shear modulus of the PEG boundary layer increase with the PEG concentration in the solution; the increasing rate depends on the molecular weight. For relatively small PEG molecules, the viscoelastic property of the PEG solution boundary layer is almost indistinguishable from that of the bulk solution of the same concentration, indicating no adsorption at the interface. For larger PEG polymers (with molecular weights above a few thousands grams per mole), the viscoelastic property of the solution boundary layer differs distinctively from that of the corresponding bulk solution. The difference can be attributed to physisorption of PEG molecules on the Au surface, which alters the viscoelastic behaviors of the boundary layer. The results suggest that adsorption behaviors of macromolecules at a solid-liquid interface might be inferred from the changes of the viscoelastic properties of a solution boundary layer.

Probing local surface conductance using current sensing atomic force microscopy.

We have analyzed correlations between surface morphology and current sensing images obtained using a current sensing atomic force microscope (CSAFM) and the implication of surface conductivity derived from the current sensing images. We found that in cases where the diameter of a CSAFM probe tip is much smaller than the correlation length of the surface morphological features, the current detected using the probe should have little correlation with the surface features imaged by the same probe. If the sample thickness is much larger than the tip size, the surface conductivity distribution of a sample can be derived from a current sensing image using the Holm resistance relation, and the current probed using a CSAFM reflects the conductance variations in a layer on the surface with the thickness comparable to the probe diameter. However, if the thickness of a sample is comparable to or smaller than the tip diameter, CSAFM measures the conductance across the entire portion of the sample sandwiched between the tip and the electrode.

Configuration changes of conducting channel network in Nafion membranes due to thermal annealing.

We have investigated changes of proton channel network in Nafion membranes annealed at elevated temperatures using current sensing atomic force microscopy aimed at understanding the aging process of the membranes. The results reveal that the changes of proton channel network undergo two steps: First, the configuration of the ionic domains on the membrane surface changes from cluster-like to chain-like structure, accompanied by an increase of the proton conductivity of the membrane. As the annealing continues, the chain-like configuration for the proton channels persists but the conductance of the membranes decreases. The time constant of the conductivity decay decreases with the annealing temperature. The observed changes can be explained in terms of reorientation of proton channels near the membrane surface from perpendicular to parallel to the surface as the annealing temperature approaches the glass transition of the membranes.

Characteristics of microscopic proton current flow distributions in fresh and aged Nafion membranes.

Proton conductance in fresh and aged Nafion ionomer membranes has been studied using current sensing atomic force microscopy which measures the proton current flow from a nanometer-sized tip in contact with the membrane surface to an electrode attached to the other side of the membrane. By scanning the tip across the membrane, the measured conductance maps out the local proton conductance variations, revealing the distributions of ionic domains on the membrane surface. For fresh Nafion membranes, the local proton conductance always displays a single-peaked distribution, while, for membranes after being aged for about three years in an ambient environment, the local proton conductance shows a distribution with two peaks, one at high current corresponding to the variation of contact and the conductive path of proton current and the other at near zero current which corresponds to the appearance of large nonconductive domains on the membranes. A detailed analysis reveals that the aging causes significant changes in the ionic domains or channels exposed on the membranes' surfaces.

Conductance mapping of proton exchange membranes by current sensing atomic force microscopy.

We show that proton conductance variation in Nafion membranes can be mapped using current sensing atomic force microscopy (CSAFM) to reveal and to characterize the heterogeneity in proton transport properties of the membranes. The distribution obtained by using a conventional CSAFM probe tip coated with a catalyst layer on a fresh Nafion membrane under a humid condition always displays a Gaussian-like shape. Such a feature can be explained in terms of the size of the contact area between a CSAFM tip and the membrane in comparison to the size of the tip apex. The conductance of individual proton channels and the density of the channels can be derived from the distribution. The averaged conductance and the density of proton channels derived are found to be consistent with that obtained in a recent simulation work based on the small-angle X-ray scattering (SAXS) data.

Resonant frequency and dissipation factor isotherms of adsorbed molecules on solid-liquid interfaces probed by quartz crystal microbalance.

Changes in the resonant frequency Deltaf and the dissipation factor DeltaD of a quartz crystal microbalance due to adsorption of molecules onto the electrode surface in solutions at different concentrations have been numerically analyzed. It has been found that the contribution from the solution due to the variation of the concentration can mask the overall behaviors of the Deltaf and DeltaD isotherms. However, if the solution is sufficiently dilute or the contribution of the solution can be characterized accurately, the corrected Deltaf and DeltaD , as a function of the solution concentration, will reveal characteristic features of the adsorption processes for the adsorbed films. Specifically, steplike behaviors will be displayed in Deltaf isotherms, which correspond to layer formation and layer-by-layer growth of the films, but the onsets of the steps usually do not coincide with the layer completion. Oscillatory behaviors will be revealed in DeltaD isotherms if the adsorbed molecular layer is soft at low concentration and becomes rigid near layer completion.

Viscoelastic response and profile of adsorbed molecules probed by quartz crystal microbalance.

The acoustic responses of molecular films with continuous viscoelastic profiles adsorbed on solid-liquid interfaces probed by quartz crystal microbalance (QCM) are analyzed using a continuum mechanics model. The numerically calculated results show that the shift of resonant frequency and the change of dissipation factor of a QCM are determined mostly by the change of viscoelastic profile of the layers in solution adjacent to the quartz-solution interface due to the adsorbed molecular film. For films with the same amount of absorbed mass, the changes in resonant frequency and the dissipation factor vary approximately linearly with the width of the film-solution interface in the profiles. Other viscoelastic properties of the adsorbed films are also affected by the profiles.

Adsorption isotherms and dissipation of adsorbed poly(N-isopropylacrylamide) in its swelling and collapsed states.

Poly(N-isopropylacrylamide) (PNIPAM) physisorbed on gold surfaces in aqueous solutions has been studied using a quartz crystal microbalance with dissipation monitoring (QCM-D). The adsorption isotherms of the polymer, that is, the adsorbed mass versus the concentration of PNIPAM in solution, show distinctly different behaviors at temperatures below and above a lower critical solution temperature (LCST). Below the LCST, PNIPAM forms a single compact layer in solutions with concentrations up to 100 ppm in weight; above the LCST, much thicker films of PNIPAM form in the same concentration range. Changes in the dissipation factor versus solvent concentration show a behavior similar to those in the isotherms. The difference in the adsorption behavior below and above the LCST can be qualitatively explained in terms of the conformation difference of the polymer in its swelling and collapsed states.

Local probe and conduction distribution of proton exchange membranes.

Proton exchange membranes (Nafion) have been studied using current sensing atomic force microscopy to examine the correlation between the surface morphology and the ionic domains, and to probe the local ionic conduction distribution in the membranes. It is found that the local ionic conduction generated from the current sensing images follows a Gaussian-like distribution, with the peak value and the width of the distribution increasing with the relative humidity in the sample chamber and, thus, the water content in the membranes. Two types of Nafion membranes, Nafion 112 and Nafion 117, were studied using the method. The implications of the distribution in relation to the ionic conducting channels in the membranes are discussed.

Probing the viscoelastic response of glassy polymer films using atomic force microscopy.

The mechanical properties of glassy films and glass surfaces have been studied using an atomic force microscope (AFM) through various imaging modes and measuring methods. In this paper, we discuss the viscoelastic response of a glassy surface probed using an AFM. We analyzed the force-distance curves measured on a glassy film or a glassy surface at temperatures near the glass transition temperature, Tg, using a Burgers model. We found that the material's characteristics of reversible anelastic response and viscous creep can be extracted from a force-distance curve. Anelastic response shifts the repulsive force-distance curve while viscous creep strongly affects the slope of the repulsive force-distance curve. When coupled with capillary force, due to the condensation of a thin layer of liquid film at the tip-surface joint, the anelasticity and viscous creep can alter the curve significantly in the attractive region.

Cloning and high expression of hbFGF with a new strategy.

Computer program DNASIS v2.5 was used to help designing the site-directed mutations for optimizing the expression of hbFGF in E. coli. The secondary structure of the translation initiation region (TIR) is a determinant factor for translation initiation rate, meanwhile, codon preference plays an important role, too. According to the two principles, 4 sites in 5' end of hbFGF cDNA were definitely changed, and another 4 sites randomly changed. These mutations will lead to potential variation in the secondary structure of TIR. Then computer program DNASIS v2.5 was utilized to analyse the total 32 TIR sequences resulted from the combination of the 4 randomly mutated sites. Ten sequences with highest free formation energy (delta G0) were chosen for subsequent cloning. By PCR using synthetic primers containing the 8 changed sites described above, ten hbFGF cDNA were amplified and cloned to pET-3c respectively. E. coli strain BL21 (DE3) was transformed and induced to express recombinant hbFGF. Two high-expression clones were obtained by SDS-PAGE and MTT assay, indicating that computer program-aided design for optimizing expression of foreign genes in E. coli is useful.