Carlos E. Castano

A fundamental analysis of enhanced cross-coupling catalytic activity for palladium clusters on graphene supports

Combining the recyclability of heterogeneous catalysts with the high activity of ligated homogeneous catalysts for the production of complex organic molecules is a cardinal goal of catalyst development. We have investigated the activity of ultra-fine Pd clusters bound to vacancy defective sites in graphene and found that the defective graphene both serves as a support to stabilize the recyclable catalyst, and also functions as a ligand enhancing the catalytic activity. In this paper, we report computational and experimental results that provide insights into the nature of the interfacial interactions between metal nanoparticles and defect sites on the graphene surface. Theoretical investigations reveal that while the vacancy/void sites on the graphene surface strongly bind to the metal clusters providing enhanced stability against leaching, graphene also serves as a reservoir of electron density that effectively reduces the activation energy of specific steps within the catalytic cycle. Furthermore, multiple experimental methods were used to unambiguously demonstrate that these cross-coupling reactions are occurring at the Pd/G catalyst surface.

Synthesis of FeCo alloy magnetically aligned linear chains by the polyol process: structural and magnetic characterization

FeCo alloy magnetically aligned linear chains (MALCs) were synthesized by the polyol process using a unique reaction format departing from conventional bench-top reactions. FeCo MALC morphology was obtained in the presence of an external dynamic magnetic field. XRD analysis confirmed the body-centered cubic (bcc) FeCo alloy. Transverse and tangential to the incident X-ray path, XRD analyses of as-synthesized FeCo MALCs were performed further substantiating the linear chain morphology. The average chain diameter was ≈206 ± 52 nm while Scherrer analysis of (110) gave an average crystallite size of 29 nm. Chemical attachment of each aligned FeCo alloy segment was confirmed by tangential cross-sectioning by a focused ion beam (FIB). Electron diffraction confirmed the bcc FeCo phase with d-spacings of (110), (200) and (211) in good agreement with XRD. The as-synthesized FeCo alloy MALCs possessed a saturation magnetization value, Ms, of 205 emu g−1 and a coercivity, Hc, of 150 Oe at 300 K. Magnetization was recorded from 300 to 1000 K with increasing temperatures correlating with transitions from hcp Co/α1 to fcc Co/α1 at 500 K and to α1 at 600 K for Co rich portions of the MALCs (Fe32Co68). Annealed (1000 K) FeCo MALCs possessed an Ms of 212 emu g−1 with a Hc value of 120 Oe. Morphological changes of the 1000 K annealed sample were microwire formation and the secondary phase of cubic FeO. As-synthesized FeCo alloy MALCs as highly magnetic, air stable, non-tethered and high aspect ratio structures are candidates for radar absorbent materials (RAMs) when magnetically oriented in coatings or for magnetic sensing devices.

Single-step flame synthesis of carbon nanoparticles with tunable structure and chemical reactivity

A novel method for the flame synthesis of carbon nanoparticles with controllable fraction of amorphous, graphitic-like and diamond-like phases is reported. The structure of nanoparticles was tailored using a conical chimney with an adjustable air-inlet opening. The opening was used to manipulate the combustion of an inflamed wick soaked in rapeseed oil, establishing three distinct combustion regimes at fully-open, half-open and fully-closed opening. Each regime led to the formation of carbon coatings with diverse structure and chemical reactivity through a facile, single-step process. In particular, the fully-closed opening suppressed most of the inlet air, causing an increased fuel/oxygen ratio and decreased flame temperature. In turn, the nucleation rate of soot nanoparticles was enhanced, triggering the precipitation of some of them as diamond-like carbon (DLC). Surface characterization analyses using Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron spectroscopy confirmed this hypothesis, indicating a short-range ordered nanocrystalline structure and ∼80% sp3 bonds in the coatings deposited at fully-closed opening. Furthermore, three groups of 5 MHz Quartz Crystal Microbalances (QCMs) coated with soot and DLC, corresponding to each of the three combustion regimes, showed different frequency responses to aqueous ethanol and isopropanol solutions in the concentration range of 0–12.5 wt%. The DLC coated QCMs exhibited relatively constant frequency shift of ∼2250 Hz regardless of the chemical, while the response of soot coated counterparts was influenced by the quantity of heteroatoms in the film. Our method can be applied in chemical sensing for the development of piezoresonance liquid sensors with tunable sensitivity.

Polyaniline-Functionalized Nanofibers for Colorimetric Detection of HCl Vapor

Hydrogen chloride (HCl) gas is a hazardous byproduct of industrial processes. Colorimetric approaches to facilitate portable and remote detection are especially desirable. We graft polyaniline to the surface of electrospun nylon nanofibers to minimize mass transfer. Using the resulting nanofibers, we demonstrate colorimetric detection of HCl at sub-ppm levels. We investigated the reusability of the fibers and observed a twofold increase in the limit of detection with multiple uses because of dedoping of the PANi indicated by elemental analysis. The limit of detection using visual detection was compared to spectrophotometric analysis. The ΔE from CIE LAB color space analysis via diffuse reflectance spectroscopy enhances the limit of detection by ∼fivefold when compared to visual detection. This analysis is a promising approach for remote detection using simple commercial digital cameras to achieve low limits of detection.