Hugo Zea

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.

Aerosol Synthesis of Nano and Micro-Scale Zero Valent Nickel Particles From Oxide Precursors

In this work a novel aerosol method, derived form the batch Reduction/Expansion Synthesis (RES) method, for production of nano / micro-scale metal particles from oxides and hydroxides is presented. In the Aerosol-RES (A-RES) method, an aerosol, consisting of a physical mixture of urea and metal oxide or hydroxides, is passed through a heated oven (1000 C) with a residence time of the order of 1 second, producing pure (zero valent) metal particles. It appears that the process is flexible regarding metal or alloy identity, allows control of particle size and can be readily scaled to very large throughput. Current work is focused on creating nanoparticles of metal and metal alloy using this method. Although this is primarily a report on observations, some key elements of the chemistry are clear. In particular, the reducing species produced by urea decomposition are the primary agents responsible for reduction of oxides and hydroxides to metal. It is also likely that the rapid expansion that takes place when solid/liquid urea decomposes to form gas species influences the final morphology of the particles.

Plasma Torch Production of Ti-Al Nanoparticles

Using the Aerosol-through-Plasma (A-T-P) technique high surface area bi-cationic (Ti-Al) oxide particles of a range of stoichiometries were produced that showed remarkable resistance to sintering. Specifically, we found that homogeneous nanoparticles with surface areas greater than 150 m{sup 2}/gm were produced at all stoichiometries. In particular, for particles with a Ti:Al ratio of 1:3 a surface area of just over 200 m{sup 2}/gm was measured using the BET method. The most significant characteristic of these particles was that their sinter resistance was far superior to that of TiAl particles produced using any other method. For example, A-T-P generated particles retained >70% of their surface area even after sintering at 1000 C for five hours. In contrast, particles made using all other methods lost virtually all of their surface area after an 800 C treatment.

Novel Chemical Process for Producing Chrome Coated Metal

This work demonstrates that a version of the Reduction Expansion Synthesis (RES) process, Cr-RES, can create a micron scale Cr coating on an iron wire. The process involves three steps. I. A paste consisting of a physical mix of urea, chrome nitrate or chrome oxide, and water is prepared. II. An iron wire is coated by dipping. III. The coated, and dried, wire is heated to ~800 °C for 10 min in a tube furnace under a slow flow of nitrogen gas. The processed wires were then polished and characterized, primarily with scanning electron microscopy (SEM). SEM indicates the chrome layer is uneven, but only on the scale of a fraction of a micron. The evidence of porosity is ambiguous. Elemental mapping using SEM electron microprobe that confirmed the process led to the formation of a chrome metal layer, with no evidence of alloy formation. Additionally, it was found that thickness of the final Cr layer correlated with the thickness of the precursor layer that was applied prior to the heating step. Potentially, this technique could replace electrolytic processing, a process that generates carcinogenic hexavalent chrome, but further study and development is needed.

Iron on Carbon Catalaysts for the Photocatalytic Degradation Orange II

Abstract Orange II decomposition was studied on a variety of iron/carbon supported catalysts and control studies of the supports alone (carbon), and iron/alumina (non-active support). Variables tested included the impact of UV radiation, inclusion of hydrogen peroxide, catalyst treatment methods (oven treated and plasma torch treated) and type of the support. Results obtained for Orange II degradation indicated that active sites on carbon are more active for the catalytic decomposition of Orange II molecules, than metal sites. Oven-treated iron catalysts showed higher OII removal than catalysts prepared by plasma torch due to the fact that iron blocks carbon catalytic sites. XRD experiment on the non-active support allowed concluding that the oxidation state of Fe on the catalyst is not the main factor in the photocatalytic degradation of Orange II.