Claudia Luhrs

In situ Raman spectroscopy and thermal analysis of the formation of nitrogen-doped graphene from urea and graphite oxide

A comprehensive in situ analysis of the formation of graphene during the thermal exfoliation via reduction expansion synthesis (RES) of graphite oxide (GO) in the presence of urea is presented. The addition of urea expedites the exfoliation and reduction process while introducing nitrogen (N) impurities, which serve as dopants within the graphene lattice. The aim of this study is to provide deeper insight into the physical and chemical processes that take place during the synthesis and to gain better understanding of the effect of urea on the reduction, thermal exfoliation and doping of GO. In situ Raman spectroscopy was employed to shed light on the structural changes that occurred during the GO–graphene transformation in the temperature range 25–800 °C. Thermogravimetric analysis, differential scanning calorimetry, and mass spectroscopy, were used as complementary techniques to monitor sample weight, reaction heat, and gas evolution, in order to distinguish between the various chemical reactions. Finally, the above characterization methods were utilized to gather information about the thermal stability and reactivity of the N-doped graphene by exposing the synthesized sample to additional heating–cooling cycles, after the initial transformation was completed.

Microwave plasmas applied for the synthesis of free standing graphene sheets

Self-standing graphene sheets were synthesized using microwave plasmas driven by surface waves at 2.45 GHz stimulating frequency and atmospheric pressure. The method is based on injecting ethanol molecules through a microwave argon plasma environment, where decomposition of ethanol molecules takes place. The evolution of the ethanol decomposition was studied in situ by plasma emission spectroscopy. Free gas-phase carbon atoms created in the plasma diffuse into colder zones, both in radial and axial directions, and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the hot region of the plasma in the outlet plasma stream where nanostructures assemble and grow. Externally forced heating in the assembly zone of the plasma reactor has been applied to engineer the structural qualities of the assembled nanostructures. The synthesized graphene sheets have been analysed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy. The presence of sp3 carbons is reduced by increasing the gas temperature in the assembly zone of the plasma reactor. As a general trend, the number of mono-layers decreases when the wall temperature increases from 60 to 100 °C. The synthesized graphene sheets are stable and highly ordered.

Review: Engineering Particles Using the Aerosol-Through-Plasma Method

For decades, plasma processing of materials on the nanoscale has been an underlying enabling technology for many ldquoplanarrdquo technologies, particularly virtually every aspect of modern electronics from integrated-circuit fabrication with nanoscale elements to the newest generation of photovoltaics. However, it is only recent developments that suggest that plasma processing can be used to make ldquoparticulaterdquo structures of value in fields, including catalysis, drug delivery, imaging, higher energy density batteries, and other forms of energy storage. In this paper, the development of the science and technology of one class of plasma production of particulates, namely, aerosol-through-plasma (A-T-P), is reviewed. Various plasma systems, particularly RF and microwave, have been used to create nanoparticles of metals and ceramics, as well as supported metal catalysts. Gradually, the complexity of the nanoparticles, and concomitantly their potential value, has increased. First, unique two-layer particles were generated. These were postprocessed to create unique three-layer nanoscale particles. Also, the technique has been successfully employed to make other high-value materials, including carbon nanotubes, unsupported graphene, and spherical boron nitride. Some interesting plasma science has also emerged from efforts to characterize and map aerosol-containing plasmas. For example, it is clear that even a very low concentration of particles dramatically changes plasma characteristics. Some have also argued that the local-thermodynamic-equilibrium approach is inappropriate to these systems. Instead, it has been suggested that charged- and neutral-species models must be independently developed and allowed to ldquointeractrdquo only in generation terms.

Microwave plasma based single step method for free standing graphene synthesis at atmospheric conditions

Microwave atmospheric pressure plasmas driven by surface waves were used to synthesize graphene sheets from vaporized ethanol molecules carried through argon plasma. In the plasma, ethanol decomposes creating carbon atoms that form nanostructures in the outlet plasma stream, where external cooling/heating was applied. It was found that the outlet gas stream temperature plays an important role in the nucleation processes and the structural quality of the produced nanostructures. The synthesis of few layers (from one to five) graphene has been confirmed by high-resolution transmission electron microscopy. Raman spectral studies were conducted to determine the ratio of the 2D to G peaks (>2). Disorder D-peak to G-peak intensity ratio decreases when outlet gas stream temperature decreases.

Hybrid Carbon Fibers/Carbon Nanotubes Structures for Next Generation Polymeric Composites

Pitch-based carbon fibers are commonly used to produce polymeric carbon fiber structural composites. Several investigations have reported different methods for dispersing and subsequently aligning carbon nanotubes (CNTs) as a filler to reinforce polymer matrix. The significant difficulty in dispersing CNTs suggested the controlled-growth of CNTs on surfaces where they are needed. Here we compare between two techniques for depositing the catalyst iron used toward growing CNTs on pitch-based carbon fiber surfaces. Electrochemical deposition of iron using pulse voltametry is compared to DC magnetron iron sputtering. Carbon nanostructures growth was performed using a thermal CVD system. Characterization for comparison between both techniques was compared via SEM, TEM, and Raman spectroscopy analysis. It is shown that while both techniques were successful to grow CNTs on the carbon fiber surfaces, iron sputtering technique was capable of producing more uniform distribution of iron catalyst and thus multiwall carbon nanotubes (MWCNTs) compared to MWCNTs grown using the electrochemical deposition of iron.

Hybrid Composites Based on Carbon Fiber/Carbon Nanofilament Reinforcement

Carbon nanofilament and nanotubes (CNTs) have shown promise for enhancing the mechanical properties of fiber-reinforced composites (FRPs) and imparting multi-functionalities to them. While direct mixing of carbon nanofilaments with the polymer matrix in FRPs has several drawbacks, a high volume of uniform nanofilaments can be directly grown on fiber surfaces prior to composite fabrication. This study demonstrates the ability to create carbon nanofilaments on the surface of carbon fibers employing a synthesis method, graphitic structures by design (GSD), in which carbon structures are grown from fuel mixtures using nickel particles as the catalyst. The synthesis technique is proven feasible to grow nanofilament structures—from ethylene mixtures at 550 °C—on commercial polyacrylonitrile (PAN)-based carbon fibers. Raman spectroscopy and electron microscopy were employed to characterize the surface-grown carbon species. For comparison purposes, a catalytic chemical vapor deposition (CCVD) technique was also utilized to grow multiwall CNTs (MWCNTs) on carbon fiber yarns. The mechanical characterization showed that composites using the GSD-grown carbon nanofilaments outperform those using the CCVD-grown CNTs in terms of stiffness and tensile strength. The results suggest that further optimization of the GSD growth time, patterning and thermal shield coating of the carbon fibers is required to fully materialize the potential benefits of the GSD technique.

Fabrication of a low density carbon fiber foam and its characterization as a strain gauge

Samples of carbon nano-fiber foam (CFF), essentially a 3D solid mat of intertwined nanofibers of pure carbon, were grown using the Constrained Formation of Fibrous Nanostructures (CoFFiN) process in a steel mold at 550 °C from a palladium particle catalysts exposed to fuel rich mixtures of ethylene and oxygen. The resulting material was studied using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDX), Surface area analysis (BET), and Thermogravimetric Analysis (TGA). Transient and dynamic mechanical tests clearly demonstrated that the material is viscoelastic. Concomitant mechanical and electrical testing of samples revealed the material to have electrical properties appropriate for application as the sensing element of a strain gauge. The sample resistance versus strain values stabilize after a few compression cycles to show a perfectly linear relationship. Study of microstructure, mechanical and electrical properties of the low density samples confirm the uniqueness of the material: It is formed entirely of independent fibers of diverse diameters that interlock forming a tridimensional body that can be grown into different shapes and sizes at moderate temperatures. It regains its shape after loads are removed, is light weight, presents viscoelastic behavior, thermal stability up to 550 °C, hydrophobicity, and is electrically conductive.

Novel Graphitic Structures by Design

Graphitic Structures by Design (GSD) is a novel technology for growing graphite in precise patterns from the nano to the macroscale, rapidly (>1 layer/sec), at low temperatures (ca. 500°C), and in a single step using ordinary laboratory equipment. The GSD process consists of exposing particular metals (Ni, Pd, Pt, Co), which act as ‘templates’, to a fuel rich combustion environment. As an example, we have thoroughly characterized graphite growth on nickel in a mixture of ethylene and oxygen (O2 /C2 H4 ratio<3), and found that it grows in a geometry remarkably consistent with the shape of the metal template at a rate of the order one graphene layer/second at temperatures between about 500 and 700°C. Graphite structures created with GSD to date include two dimensional ‘screens’ that are inches in extent, yet are composed of micron scale squares graphite foam, hollow nanoparticles, and micron scale particles. All alternative technologies for graphite growth require specialty equipment, such as 2000 °C + ovens, and multiple steps. The alternatives are also not suited for a wide variety of pattern growth in either two or three dimensions. We propose to change focus from demonstrating GSD to determination of the mechanism of graphite growth. GSD could meet a number of recognized technological needs for future generation integrated circuits (IC). Precise patterns of oriented graphite are envisioned as: i) replacements of carbon fibers as structural elements in some aerospace and transport applications, ii) as heat conductive pathways aiding thermal management in ICs iii) as electrical conduits in ICs, iv) as the basic elements of nano-scale logic circuits. GSD graphite is arguably superior to the older and more broadly studied carbon nanotubes technology for all these IC applications for many reasons: only GSD be grown in any pattern on any surface, GSD is far cleaner (no metal residue in the graphite structure, in contrast to nanotubes), GSD structures can be formed consistently and cheaply, at low temperature, and only GSD can be readily grown into large designed macrostructures required for some heat transfer applications.Copyright

Reduction Expansion Synthesis as Strategy to Control Nitrogen Doping Level and Surface Area in Graphene

Graphene sheets doped with nitrogen were produced by the reduction-expansion (RES) method utilizing graphite oxide (GO) and urea as precursor materials. The simultaneous graphene generation and nitrogen insertion reactions are based on the fact that urea decomposes upon heating to release reducing gases. The volatile byproducts perform two primary functions: (i) promoting the reduction of the GO and (ii) providing the nitrogen to be inserted in situ as the graphene structure is created. Samples with diverse urea/GO mass ratios were treated at 800 °C in inert atmosphere to generate graphene with diverse microstructural characteristics and levels of nitrogen doping. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the microstructural features of the products. The effects of doping on the samples structure and surface area were studied by X-ray diffraction (XRD), Raman Spectroscopy, and Brunauer Emmet Teller (BET). The GO and urea decomposition-reduction process as well as nitrogen-doped graphene stability were studied by thermogravimetric analysis (TGA) coupled with mass spectroscopy (MS) analysis of the evolved gases. Results show that the proposed method offers a high level of control over the amount of nitrogen inserted in the graphene and may be used alternatively to control its surface area. To demonstrate the practical relevance of these findings, as-produced samples were used as electrodes in supercapacitor and battery devices and compared with conventional, thermally exfoliated graphene.

Engineering aerosol-through-plasma torch ceramic particulate structures: Influence of precursor composition

This is the second in a series of articles demonstrating the unique character of the aerosol-through-plasma (A-T-P) process for producing nanoparticles. This study is focused on the impact of two parameters, cation ratio (1:3, 1:1, 3:1) and solvent (evaporated prior to generation of aerosol), on the structures of Ce:Al oxides particles. These two simple changes were found to impact virtually every aspect of particle structure, including the fraction of hollow versus solid, fraction of nanoparticles, phase structure, and even the existence of surface phase segregation. CeAl mixed oxides were found only over a limited range of compositions, and that range was a function of the solvent. At all other cation ratios, only ceria was a crystalline phase, and most if not all the alumina is amorphous. It is notable that the fraction of hollow micron-sized particles and nanoparticles is greatly influenced by the cation ratio and solvent identity. Indeed, significant numbers of nanoparticles were only produced using an aqueous precursor with a Ce:Al ratio of 1:1. Another unique finding is that phase segregation exists in individual particles on the length scale of nanometers. This study compliments an earlier study of the influence of operating conditions on particle structure. Taken together, the studies suggest a means to engineer (as well as limits to the engineering possibilities) ceramic particle structures using the A-T-P method.