Potential of N2/O2 atmospheric pressure needle-water DC microplasmas for nitrogen fixation: nitrite-free synthesis of nitrates.

A needle-water DC microplasma system working at atmospheric pressure in N2/O2 gas mixtures is used to study the fundamental mechanisms of nitrate/nitrite synthesis in highly complex and yet little-known plasma-water systems. Plasma properties are investigated by means of optical emission spectroscopy while the activated water is analyzed following the treatment using ionic chromatography and UV-Vis absorbance spectroscopy. Experiments highlight that the energy efficiency and selectivity of the process are influenced by the oxygen content and the plasma-induced water heating, with strong differences when the water surface is the anode or the cathode electrode. Nitrates are successfully synthesized without residual nitrites in the solution with a comparatively higher energy efficiency when the water is the cathode. The possible reactions involved in the gas phase and aqueous phase chemistry are presented and future scope for the optimization of the system is discussed.

Atmospheric Pressure Plasma Deposition of Hydrophilic/Phobic Patterns and Thin Film Laminates on Any Surface.

Patterned and layered hydrophilic/phobic coatings were deposited on multiple surfaces using nonfluorinated precursors (AA, acrylic acid; PMA, propargyl methacrylate) with an atmospheric pressure dielectric barrier discharge operating in open air. Water contact angles of the resulting films could be tuned from <5° (superhydrophilic) to >135° (very hydrophobic) by adjusting the AA/PMA feed ratio and/or via postdeposition exposure of films to an Ar/O2 plasma treatment. Coatings could be applied to any surface and were seen to be water stable, due in large part to cross-linking induced from the reactivity of the PMA pendant groups. Hybrid hydrophilic/phobic patterns at submillimeter length scales, and philic/phobic/philic laminates were produced using a shadow mask and sequential deposition, respectively. Chemical heterogeneity of films was assessed using XPS, SIMS, and micro-IR imaging and suggest that localization of COOH and OH groups are primarily responsible for hydrophilicity. Overall, this work demonstrates that atmospheric pressure plasma polymerization is a simple and scalable method for robust and tunable control of wettability of surfaces of all kinds.

Etching processes of polytetrafluoroethylene surfaces exposed to He and He-O2 atmospheric post-discharges.

A comparative study of polytetrafluoroethylene (PTFE) surfaces treated by the post-discharge of He and He-O(2) plasmas at atmospheric pressure is presented. The characterization of treated PTFE surfaces and the species involved in the surface modification are related. In pure He plasmas, no significant change of the surface has been observed by X-ray photoelectron spectroscopy (XPS), dynamic water contact angles (dWCA) and atomic force microscopy (AFM), in spite of important mass losses recorded. According to these observations, a layer-by-layer physical etching without any preferential orientation is proposed, where the highly energetic helium metastables are the main species responsible for the scission of -(CF(2))(n)- chains. In He-O(2) plasmas, as the density of helium metastables decreases as a function of the oxygen flow rate, the treatment leads to fewer species ejected from the PTFE surfaces (in agreement with mass loss measurements and the detection of fluorinated species onto aluminum foil). However, the dWCA and AFM measurements show an increase in the hydrophobicity and the roughness of the surface. The observed alveolar structures are assumed to be caused by an anisotropic etching where the oxygen atoms etch mainly the amorphous phase.

Carbon nanotubes decorated with gold, platinum and rhodium clusters by injection of colloidal solutions into the post-discharge of an RF atmospheric plasma.

In this paper, we present a new, simple, robust and efficient technique to decorate multi-wall carbon nanotubes (MWCNT) with metal nanoparticles. As case studies, Au, Pt and Rh nanoparticles are grafted onto MWCNTs by spraying a colloidal solution into the post-discharge of an atmospheric argon or argon/oxygen RF plasma. The method that we introduce here is different from those usually described in the literature, since the treatment is operated at atmospheric pressure, allowing the realization in only one step of the surface activation and the deposition processes. We demonstrate experimentally that the addition of oxygen gas in the plasma increases significantly the amount of grafted metal nanoparticles. Moreover, TEM pictures clearly show that the grafted nanoparticles are well controlled in size.

Carbon nanotubes randomly decorated with gold clusters: from nano2hybrid atomic structures to gas sensing prototypes.

Carbon nanotube surfaces, activated and randomly decorated with metal nanoclusters, have been studied in uniquely combined theoretical and experimental approaches as prototypes for molecular recognition. The key concept is to shape metallic clusters that donate or accept a fractional charge upon adsorption of a target molecule, and modify the electron transport in the nanotube. The present work focuses on a simple system, carbon nanotubes with gold clusters. The nature of the gold-nanotube interaction is studied using first-principles techniques. The numerical simulations predict the binding and diffusion energies of gold atoms at the tube surface, including realistic atomic models for defects potentially present at the nanotube surface. The atomic structure of the gold nanoclusters and their effect on the intrinsic electronic quantum transport properties of the nanotube are also predicted. Experimentally, multi-wall CNTs are decorated with gold clusters using (1) vacuum evaporation, after activation with an RF oxygen plasma and (2) colloid solution injected into an RF atmospheric plasma; the hybrid systems are accurately characterized using XPS and TEM techniques. The response of gas sensors based on these nano(2)hybrids is quantified for the detection of toxic species like NO(2), CO, C(2)H(5)OH and C(2)H(4).