[Show abstract][Hide abstract] ABSTRACT: We report a facile hydrothermal route for the synthesis of Fe2O3 nanoparticles and their composites with conductive additives, reduced graphite oxide (rGO) and conductive polymer PEDOT:PSS, as anode materials for lithium ion batteries (LIBs). The addition of conductive polymer layer on Fe2O3 nanoparticles may facilitate the electron transport but hinders the Li+ ion insertion at higher current density. On the other hand, Fe2O3@rGO prepared by growing Fe2O3 nanoparticles directly on rGO sheets shows the best electrochemical behavior due to a beneficial combination of the rGO nanosheets partially wrapped around Fe2O3 which facilitates both the electron transport and the Li+ ion insertion. The enhanced conductivity of the composites was proved. The high specific capacity and stable rate performance of Fe2O3@rGO composites encourages their further study to be potential candidate for the anode materials in LIBs. These results will be helpful in further elucidation of the role of conductive additives in improving the electrochemical performance of Fe2O3 based composite anodes and this simple synthetic strategy can be applied for the large scale production of metal oxides with conductive additives for LIBs.
Journal of Power Sources 03/2014; 259:227-232. · 5.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Three-dimensional (3D) nanoporous architectures, possessing high surface area, massive pores, and excellent structural stability, are highly desirable for many applications including catalysts and electrode materials in lithium ion batteries. However, the preparation of such materials remains a major challenge. Here, we introduce a novel method, instant gelation, for the synthesis of such materials. The as-prepared porous 3D MoS2@C nanocomposites, with layered MoS2 clusters or strips ingrained in porous and conductive 3D carbon matrix, indeed showed excellent electrochemical performance when applied as anode materials for lithium ion batteries. Its interconnected carbon network ensures good conductivity and fast electron transport; the micro-, and mesoporous nature effectively shortens the lithium ion diffusion path and provides room necessary for volume expansion. The large specific surface area is beneficial for a better contact between electrode materials and electrolyte.
[Show abstract][Hide abstract] ABSTRACT: Direct growth of mesoporous TiO2 on Ni foam via a one-step soft template synthesis was directly used for a binder-free anode for lithium-ion batteries. The mesoporous TiO2 with a high specific surface area of 158.8 m2 g-1 and average pore size of 5.4 nm formed network-like sheets on the surface of the Ni foam. The binder-free TiO2/Ni anode shows improved electrochemical performance with a capacity as high as 341 mA h g-1 at a current density of 100 mA g-1 after 10 cycles and 82.4 mA h g-1 at a current density of 1000 mA g-1 after 30 cycles. The enhanced electrochemical performance is attributed to the mesoporous structure that shortens the lithium ion diffusion path and facilitates the transport of lithium ions.
[Show abstract][Hide abstract] ABSTRACT: Studies of DNA translocation through graphene nanopores have revealed their potential for DNA sequencing. Here we report a study of protein translocation through chemically modified graphene nanopores. A transmission electron microscope (TEM) was used to cut nanopores with diameters between 5 and 20 nm in multilayer graphene prepared by chemical vapor deposition (CVD). After oxygen plasma treatment, the dependence of the measured ionic current on salt concentration and pH was consistent with a small surface charge induced by the formation of carboxyl groups. While translocation of gold nanoparticles (10 nm) was readily detected through such treated pores of a larger diameter, translocation of the protein ferritin was not observed either for oxygen plasma treated pores, or for pores modified with mercaptohexadecanoic acid. Ferritin translocation events were reliably observed after the pores were modified with the phospholipid-PEG (DPPE-PEG750) amphiphile. The ion current signature of translocation events was complex, suggesting that a series of interactions between the protein and pores occurs during the process.
[Show abstract][Hide abstract] ABSTRACT: Bismuth oxide directly grown on nickel foam (p-Bi2O3/Ni) was prepared by a facile polymer-assisted solution approach and was used directly as a lithium-ion battery anode for the first time. The Bi2O3 particles were covered with thin carbon layers, forming network-like sheets on the surface of the Ni foam. The binder-free p-Bi2O3/Ni shows superior electrochemical properties with a capacity of 668 mAh/g at a current density of 800 mA/g, which is much higher than that of commercial Bi2O3 powder (c-Bi2O3) and Bi2O3 powder prepared by the polymer-assisted solution method (p-Bi2O3). The good performance of p-Bi2O3/Ni can be attributed to higher volumetric utilization efficiency, better connection of active materials to the current collector, and shorter lithium ion diffusion path.
Journal of Materials Chemistry A: Materials for Energy and Sustainability 10/2013; 1(39).
[Show abstract][Hide abstract] ABSTRACT: Unlike most conventional anode materials, the newly developed TiNb2O7 (TNO) does not form a solid electrolyte interface (SEI) layer, which makes it safe for high power requiring lithium-ion batteries. In this paper, we demonstrated an SBA-15 confined synthetic approach to prepare TNO nanoparticles (S-TNO) with a small particle size around 10 nm and a large BET surface area of 79.5 m(2) g(-1). It is worth mentioning that this is the smallest size reported so far for TNO. In contrast, the TNO (L-TNO) synthesized without SBA-15 has a particle size above 100 nm and a BET surface area of only 4.3 m(2) g(-1). The S-TNO shows better lithium-ion storage properties than L-TNO. The excellent electrochemical performance of S-TNO is attributed to its small crystalline size, which not only provides a larger effective area for better contact between the electrode material and the electrolyte, but also reduces the rate-limiting Li diffusion path. Moreover, S-TNO shows a high Coulombic efficiency (above 98% over 300 cycles) and negligible increase of impedance after cycling, which confirms no SEI layer formation in the operational voltage (1-3 V) of TNO.
[Show abstract][Hide abstract] ABSTRACT: In this paper we discuss the effect of background pressure and synthesis temperature on the graphene crystal sizes in chemical vapor deposition (CVD) on copper catalyst. For the first time, we quantitatively demonstrate a fundamental role of the background pressure and provide the activation energy for graphene nucleation in atmospheric pressure CVD (9 eV), which is substantially higher than for the low pressure CVD (4 eV). We attribute the difference to a greater importance of copper sublimation in the low pressure CVD, where severe copper evaporation likely dictates the desorption rate of active carbon from the surface. At atmospheric pressure, where copper evaporation is suppressed, the activation energy is assigned to the desorption energy of carbon clusters instead. The highest possible temperature, close to the melting point of copper, should be used for large single crystal graphene synthesis. Using these conditions, we have synthesized graphene single crystals with sizes over 0.5 mm. Single cry
The Journal of Physical Chemistry C 08/2013; · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: New nanocomposite formulation of the FeS based anode for lithium-ion batteries is proposed, where FeS nanoparticles wrapped in reduced graphene oxide (RGO) are produced via a facile direct-precipitation approach. The resulting nanocomposite FeS@RGO structure has better lithium ion storage properties exceeding those of FeS prepared without RGO sheets. The enhanced electrochemical performance is attributed to the robust sheet-wrapped structure with smaller FeS nanoparticles and synergetic effects between FeS and RGO sheets, such as increased conductivity, shortened lithium ion diffusion path, and the effective prevention of polysulfide dissolution.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that large scale high quality graphene synthesis can be performed using atmospheric pressure chemical vapor deposition (CVD) on Cu and illustrate how this procedure eliminates major difficulties associated with the low pressure CVD approach while allowing straightforward expansion of this technology to the roll-to-roll industrial scale graphene production. The detailed recipes evaluating the effects of copper foil thicknesses, purity, morphology and crystallographic orientation on the graphene growth rates and the number of graphene layers were investigated and optimized. Various foil cleaning protocols and growth conditions were evaluated and optimized to be suitable for production of large scale single layer graphene that was subsequently transferred on transparent flexible polyethylene terephthalate (PET) polymer substrates. Such "ready to use" graphene-PET sandwich structures were as large as 40 '' in diagonal and >98% single layer, sufficient for many commercial and research applications. Synthesized large graphene film consists of domains exceeding 100 mu m. Some curious behavior of high temperature graphene etching by oxygen is described that allows convenient visualization of interdomain boundaries and internal stresses. Published by Elsevier Ltd.
[Show abstract][Hide abstract] ABSTRACT: Hydrophobicity is a fundamental property that is responsible for numerous physical and biophysical aspects of molecular interactions in water. Peculiar behavior is expected for water in the vicinity of hydrophobic structures, such as nanopores. Indeed, hydrophobic nanopores can be found in two distinct states, dry and wet, even though the latter is thermodynamically unstable. Transitions between these two states are kinetically hindered in long pores but can be much faster in shorter pores. As it is demonstrated for the first time in this paper, these transitions can be induced by applying a voltage across a membrane with a single hydrophobic nanopore. Such voltage-induced gating in single nanopores can be realized in a reversible manner through electrowetting of inner walls of the nanopores. The resulting I-V curves of such artificial hydrophobic nanopores mimic biological voltage-gated channels.
[Show abstract][Hide abstract] ABSTRACT: pH switchable valves were constructed using nanoporous membranes, the surface of which was modified by mixtures of aminopropyl trimethoxy silane and butyl trimethoxy silane. The modified membranes are dry at neutral and basic conditions because of their hydrophobicity but open to flux of aqueous solutions at slightly acidic pH because of protonation of amino groups. The resulting high contrast between the open and the closed states and a high flux in the open state because of large pore size make the approach attractive in applications where pH switching is employed, for example, in drug delivery applications.Keywords: pH sensitive valve; hydrophobicity
Chemistry of Materials 07/2011; 23(16). · 8.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We show that graphene chemical vapor deposition growth on copper foil using methane as a carbon source is strongly affected by hydrogen, which appears to serve a dual role: an activator of the surface bound carbon that is necessary for monolayer growth and an etching reagent that controls the size and morphology of the graphene domains. The resulting growth rate for a fixed methane partial pressure has a maximum at hydrogen partial pressures 200-400 times that of methane. The morphology and size of the graphene domains, as well as the number of layers, change with hydrogen pressure from irregularly shaped incomplete bilayers to well-defined perfect single layer hexagons. Raman spectra suggest the zigzag termination in the hexagons as more stable than the armchair edges.
[Show abstract][Hide abstract] ABSTRACT: In this paper we present a study of graphene produced by chemical vapor deposition (CVD) under different conditions with the main emphasis on correlating the thermal and electrical properties with the degree of disorder. Graphene grown by CVD on Cu and Ni catalysts demonstrates the increasing extent of disorder at low deposition temperatures as revealed by the Raman peak ratio, IG/ID. We relate this ratio to the characteristic domain size, La, and investigate the electrical and thermal conductivity of graphene as a function of La. The electrical resistivity, ρ, measured on graphene samples transferred onto SiO2/Si substrates shows linear correlation with La − 1. The thermal conductivity, K, measured on the same graphene samples suspended on silicon pillars, on the other hand, appears to have a much weaker dependence on La, close to K ~ La1/3. It results in an apparent ρ ~ K3 correlation between them. Despite the progressively increasing structural disorder in graphene grown at lower temperatures, it shows remarkably high thermal conductivity (102–103 W K − 1 m − 1) and low electrical (103–3 × 105 Ω) resistivities suitable for various applications.
[Show abstract][Hide abstract] ABSTRACT: Wetting and drying of hydrophobic pores with diameters lower than 0.2 μm by aqueous solutions at different hydrostatic pressures is investigated by measuring the ionic conductance variation through the nanopores. The critical pressure for water intrusion into the nanopores increases with lowering the pore diameter and the surface tension of the hydrophobic modification, in agreement with the Laplace equation. Nevertheless, restoring the pressure to the atmospheric one does not result in spontaneous pore dewetting unless bubbles are left inside the pores. Such bubbles can appear at the regions of narrowing cross section and/or varying quality of the hydrophobic modification and thus can be engineered to control water expulsion.
[Show abstract][Hide abstract] ABSTRACT: Nanoporous membranes and single nanopores have found a broad range of applications in material science and sensor design. We demonstrate here such applications in biochemical and chemical sensors, where surface of the nanopores is modified to achieve their specificity to physical stimuli and/or binding desired biochemical and chemical analytes. Various detection methods can be utilized but we will focus on measuring ionic conductance through the pores as the most convenient; different mechanisms affecting the condcutance will be reviewed.
64th American Chemical Society Southwest Regional Meeting; 11/2009
[Show abstract][Hide abstract] ABSTRACT: Monolayer assemblies of stearic, elaidic and oleic acids on alumina (flat surfaces and nanoporous membranes) and aminated glass were characterized by IR and the contact angle with water and organic solvents. The saturated (stearic) acid has the highest density of the surface bound molecules due to its good packing while the unsaturated elaidic (the trans isomer) and the oleic (the cis isomer) acids have their packing densities lower because of a kink due to the double bond. As a result, such modified surfaces have the contact angles decreasing in the same order not only with water but also with long hydrocarbons such as hexadecane. The effect is less pronounced with short hydrocarbons. We propose to employ these experiments in undergraduate laboratory as illustration of the molecular nature of the effect of unsaturated fatty acids on the density/fluidity of bilipid membranes.
64th American Chemical Society Southwest Regional Meeting; 11/2009
[Show abstract][Hide abstract] ABSTRACT: The surface charge effect in controlling ionic conductance through a nanoporous alumina membrane is investigated for its application in a convenient detection method of unlabeled DNA. To this goal, surface modification with mixtures of neutral silanes and morpholinos (neutral analogues of DNA) was optimized to yield a strong effect on ionic conductance change upon DNA binding, which can exceed an order of magnitude. The effect can be employed in fabrication of inexpensive DNA sensors.
[Show abstract][Hide abstract] ABSTRACT: A design of smart surfaces responsive to biochemical analytes is demonstrated in the example of mixed monolayers of biotin/fluorocarbon. The contact angle of aqueous solutions on such surfaces decreases upon streptavidin binding and can be used in detecting this protein. The specificity of the effect is confirmed by the lack of a contact angle change by streptavidin blocked with biotin and by bovine serum albumin.
[Show abstract][Hide abstract] ABSTRACT: Recently reported experimental and theoretical studies of nanofluidic nonlinear devices, such as bipolar and unipolar ionic diodes, have yet to answer the question about the possibility of their further miniaturization. In this Article, we theoretically investigate the effects of size reduction, applied bias, and solution ionic strength in such devices. We compare the numerical solutions of the Poisson, Nernst-Planck (PNP), and Navier-Stokes (NS) equations with their one-dimensional, analytical approximations. We demonstrate that the contribution of electroosmosis is insignificant and find analytical approximations to PNP for bipolar and unipolar diodes that are in good agreement with numerical 3D solutions. We identify the minimal dimensions for such diodes that demonstrate ion current rectification behavior and demonstrate the importance of the edge effect in very short diodes.