Harnessing plasma-generated reactive species for the synthesis of different phases of molybdenum oxide to study adsorption and photocatalytic activity

Abstract

This study employs plasma–liquid interaction technique to synthesize different phases of molybdenum oxide using air and argon as plasma-forming gases. In situ plasma-generated nitrogen species primarily NO₃⁻/NO₂⁻ and hydrogen species (H⁺) facilitate the reduction of the molybdenum precursor anion (Mo₇O₂₄⁻). The reduced Mo species subsequently reacts with reactive oxygen species, forming MoO₆ octahedra, which is the building block of a molybdenum oxide crystal. Varied concentrations of NO₃⁻/NO₂⁻ and H⁺ species in air and argon plasma treatment significantly influence the growth process. Air plasma synthesis yields hexagonal molybdenum oxide microrods, which upon calcination changes its phase to orthorhombic 2D layered structure. Moreover, the argon plasma synthesized sample exhibits a mixed phase of hexagonal and orthorhombic molybdenum oxide due to the heavy argon ion bombardment, inducing material porosity and surface oxygen vacancies. The mixed-phase material exhibits superior adsorption and photo-degradation towards cationic dye compared to the other two phases. The higher photocatalytic performance may be responsible for the extended lifetime of the photo-generated charge carriers possessed by the mixed-phase material. Radical scavenging tests have identified holes and hydroxyl radicals as the key reactive species that take part in the photo-degradation process.