We will demonstrate that ultrasonic treatment of a graphite crystal in water can lead to the formation of small graphene-like flakes in solution. The delamination of the graphite can be increased dramatically by intercalation of bromine from a Br2-saturated water solution. After ultrasonic treatment, large amounts of graphene-like flakes with varying thickness are observed in SEM and TEM. They can be adsorbed onto a surface of a suitable substrate by a simple dipping technique. The effect of polar and non-polar solvents as well as adsorption of the graphene on hydrophobic and hydrophilic substrates will be demonstrated and compared. DFT calculations of the intercalation process have been carried out using the SIESTA package and the effect of bromine intercalation on cohesive energy and electronic structure will be discussed and compared with experimental data. Finally, the general approach of using ultrasonic treatment and intercalation as a facile route to graphene synthesis compared to other methods will be discussed.
The excellent properties exhibited by monolayer graphene have spurred the development of exfoliation techniques using bulk graphite to produce large quantities of pristine monolayer sheets. Development of simple chemistry to exfoliate and intercalate graphite and graphite mimics in large quantities is required for numerous applications. To determine the macroscopic behavior of restacked, exfoliated bulk materials, a systematic approach is presented using a simple, redox‐liquid sonication process along to obtain large quantities of 2D and 3D hexagonally layered graphite, molybdenum disulfide, and boron nitride, which are subsequently characterized to observe chemical and structural changes. For MoS 2 sonicated with the antioxidant sodium bisulfite, results from Raman spectroscopy, X‐ray diffraction, and electron microscopy indicate the presence of distorted phases from different polymorphs, and apparent nanotube structures in the bulk, restacked powder. Furthermore, using thermograviemtric analysis, the antioxidant enhances the resistance to oxidative degradation of MoS 2 , upon thermal treatment up to 900 °C. The addition of the ionic antioxidant decreased dispersion stability in non‐polar solvent, suggesting decreased compatibility with non‐polar systems. Using simple chemical methods, the ability to generate tailored multidimensional layered materials with unique macroscopic properties is critical for numerous applications, including electrical devices, reinforced polymer composites, lithium–ion capacitors, and chemical sensing.
X-ray photoelectron spectroscopy (XPS) was used to investigate the surface chemistry of high voltage spinel, LiNi0.5Mn1.5O4 (LNMO) positive electrodes cycled 5 and 10 times in Li-cells with 1 M LiPF6 in (3:7) EC:DMC. The XPS spectra were collected using conventional Mg X-rays with energy of 1253.6 eV as well as synchrotron X-rays with energies of 2493.6 and 3498.4 eV in order to examine the depth distribution of various surface chemical species induced during cycling. The XPS spectra revealed a 5 – 10 nm surface layer of organic and LixPFyOz-type species formed as result of electrolyte decomposition, and a comparatively thinner layer composed of transition metal fluorides and LiF. These results suggest that electrolyte decomposition is a major contributor to parasitic reactions in LNMO battery electrochemistry. Limiting electrolyte decomposition with the use of solvents with wide electrochemical stability windows thus comprises a promising strategy for ensuring the practical feasibility of high voltage spinel materials in future Li-ion systems.
Abstract A six Torr CO 2 glow discharge in combination with a heated W mesh‐reinforced ultrathin Ag membrane is used to generate molecular oxygen. The Ag membrane is a commercially available 25‐μm‐thick Ag foil backed by a 25‐μm‐thick W electroformed mesh. The permeation flux is inversely dependent on the membrane thickness and exponentially dependent on the membrane temperature. Calculations show that a pressure differential of 1 atmosphere can be supported by the W mesh/Ag foil membrane at temperatures up to 350 °C. In this work, a glow discharge for pressures between 2 and 15 Torr CO 2 and temperatures up to 500 °C were reported. The DC glow discharge was produced initially with a solid Ag rod cathode, 2 mm in diameter, and then with a 7‐mm‐diameter Ag rod machined into a hollow cathode, located 5 mm from, and normal to, the Ag membrane anode. The voltage was varied from 440 to 620 VDC with currents up to 41 mA. A stable flux of 1.61 × 10 15 O 2 molecules·cm −2 ·s −1 is observed for a membrane temperature of 450 °C and a CO 2 pressure of 6 Torr. With ~4‐m 2 surface area, this approach is competitive with the present mission qualified Mars Oxygen In‐Situ Resource Utilization Experiment (MOXIE) system planned by National Aeronautics and Space Administration (NASA) for the 2020 Mars rover mission which generates ≈12 g/hr O 2 . The proof of concept technique presented herein can be substantially improved by further reduction of the membrane thickness, refinement of the cathode, and glow discharge plasma.
The need to produce large quantities of graphenic materials displaying excellent conductivity, thermal resistance, and tunable properties for industrial applications has spurred interest in new techniques for exfoliating graphite. In this paper, sonication-assisted exfoliation of graphitic precursors in the presence of chloroform is shown to produce chemically and structurally unique exfoliated graphitic materials in high yields. These exfoliated graphites, referred to as mesographite and mesographene, respectively, exhibit unique properties which depend on the number of layers and exfoliation conditions. Structural characterization of mesographene reveals the presence of nanoscale two-dimensional graphene layers, and three-dimensional carbon nanostructures sandwiched between layers, similar to those found in ball-milled and intercalated graphites. The conductivities of mesographite and mesographene are 2700 and 2000 S/m, respectively, indicating high conductivity despite flake damage. Optical absorption measurements of mesographite sonicated in various solvents showed significant changes in dispersion characteristics, and also indicated significant changes to mesoscopic colloidal behavior. A mechanism for functionalization and formation of capped carbon nanostructures is proposed by integrating the chemical and structural characterization in relation to the various carbon structures observed by electron microscopy. Composites based on common polymers were prepared by solution processing, and changes in thermal properties indicate improved dispersion of mesographite in polar polymers.
A facile and inexpensive method to produce thin films of nanostructured tungsten oxide is described. A nanocrystalline tungstite (WO3·H2O) film is spontaneously formed when a tungsten substrate is immersed in nitric acid at elevated temperatures. The resulting thin film is composed of plate-like tungstite crystals with edges preferentially directed out from the substrate surface. The tungstite can easily be transformed into WO3 by annealing. Patterned WO3·H2O/W structures can be obtained by a combination of lithographic techniques and etching. In this study, the effect of exposure time, acid concentration, and temperature on the microstructure of the films has been investigated. The potential of this inexpensive synthesis method to produce large-area coatings of nanostructured tungsten oxide as well as patterned films makes it interesting for several different applications, such as batteries, gas sensors, and photocatalysts.
