Sylvain Coulombe1
McGill University1
The plasma medium enables the direct coupling of renewable (possibly intermittent) electricity sources with chemical reactors providing processing conditions not achievable with traditional approaches. The full gamut of gas phase chemistry, from nonequilibrium to equilibrium is achievable through an almost unlimited number of combinations of reactor and power supply designs, volume and surface interactions, reactants and energy delivery schemes, and materials combinations. Multiphase and multi-stage reactors add even more options. In the context of the energy transition, processing plasmas play multiple roles, from the direct conversion and synthesis of gases (e.g., CO<sub>2</sub> methanation, CH<sub>4</sub> pyrolysis, NH<sub>3</sub> synthesis from and cracking to H<sub>2</sub>) and liquids to the synthesis of advanced materials acting as catalysts, selective (ab)adsorbers, energy storage and transport vectors, to name a few applications. Of particular interest is the synthesis and functionalization of nanomaterials and nanostructures with optimal performances and minimal uses of resources.<br/>We developed a dual-plasma source approach for the synthesis, deposition and functionalization of fine nanoparticles (NPs) onto nanostructured substrates with minimal use of resources. Pulsed laser ablation (PLA) of a metal target under an inert low-pressure environment leads to the formation of fine (3-5 nm) nanoparticles from the metal vapor plasma plume expansion and supersaturation. Surface charging by the plasma electrons ensures electrostatic repulsion of the as-produced nanoparticles and uniform dispersion over the collecting substrate. The background pressure and ablation time are varied to adjust the granularity and open porosity of the coatings. An alternating system of two pure metal targets allows the synthesis, deposition and formation of triple-interface nanomaterials (Metal A + Metal B) NPs@substrate for optimal adsorption (immobilization) of dissimilar gaseous molecules and catalytic activity. Pre- or post-PLA radiofrequency (RF) plasma-enhanced chemical vapor deposition (PECVD) of the substrate enables surface activation, tuning of the adsorption properties and compatibility with host materials. Simultaneous PLA and RF plasma in reactive gas atmospheres enables the formation of compound nanoparticles (metal oxide, nitride) and thermodynamically unfavorable phases. Several examples of successful nanoparticle decoration (low NP loading) and granular coating formation (high NP loading) will be presented, including platinum nanoparticles on multiwalled carbon nanotubes (Pt NP@MWCNT) as electrocatalysts, ruthenium oxynitride on MWCNT (RuN NP@MWCNT) as supercapacitor electrodes, and cobalt-molybdenum and ruthenium-iron on boron nitride nanotubes as gas conversion catalysts (CoMo NP@BNNT and RuFe NP@BNNT). Preliminary results on the deposition of dual function materials for optimal gas adsorption and catalytic activity will be presented.<br/>Though the PLA-PECVD approach enables the synthesis of nanostructures with extraordinary performances and with high energy and material efficiencies, the sustainability of the process and produced nanomaterials are questionable. Looking ahead, truly sustainable plasma-enabled technologies and materials will need to be circular. Material structure upgradability or at the very least, recyclability, needs to be added as a hard-coded process and material design constraint. Life cycle and circularity assessments shall become as important as the performance and economics in the decision-making processes. This requires a paradigm shift in our usual linear way of thinking which will undoubtedly lead to innovation in plasma processing and material selection and assembly.