Bulk synthesis of tungsten-oxide nanomaterials by a novel, plasma chemical reactor configuration, studies on their performance for waste-water treatment and hydrogen evolution reactions

Abstract

We report a novel, yet simple, DC non-transferred thermal plasma torch assisted, green, plasma-chemical reactor configuration, for high-rate synthesis of tungsten-oxide nanomaterials, up to two hundred grams per hour. An expanded argon/oxygen plasma beam is produced with a collimated, frozen structure, which interacts uniformly and in a controlled manner, over a large area of the remotely placed, tungsten target, to produce superfine particles in bulk amounts and without mixed with large aggregates. The target surface gets oxidized to form volatile tungsten-oxide, from which nanoparticles are nucleated by gas phase condensation process. Optical emission spectroscopic studies reveal that a massive argon plasma ion density leads to the production of reactive atomic oxygen radicals that were responsible for the rapid oxidation kinetics. Non-stoichiometric tungsten-oxide in pale blue colour and nanometer sizes were deposited on the water-cooled walls of the vacuum chamber. This sample was annealed to produce nanocrystalline, tungsten-oxide in yellow colour, which was characterized with narrow size distribution and fine material crystallinity. Functional properties of the synthesized nanomaterials were also measured, their photocatalytic characteristics for the degradation of the dye Rhodamine-B and antibacterial properties against E. coli bacteria. The annealed powder sample could remove the dye almost completely in about three hours. The same nanopowders demonstrated promising antibacterial properties, which may therefore lead to multi-functionality during the treatment of wastewater. Their hydrogen evolution reaction activity is also investigated that demonstrates moderate performance.