Plasma-Synthesized Combined Nitrogen and Cationic Species Doped-MnO2: Impact on Texture, Optical Properties, and Photocatalytic Activity

Abstract

This work explored the possibility of doping MnO₂ structure simultaneously by cationic (Na⁺, Mg²⁺ or K⁺) and nitrogen species during its synthesis through gliding arc plasma route. Therefore, NaMnO₄, Mg(MnO₄)₂ or KMnO₄ precursor has been precipitated via plasmachemical reduction thanks to NO⋅ and NO₂⁻ respectively being short and long-lived species generated in plasma plume (gas phase) and plasma post-discharge (liquid phase). Physicochemical characterizations revealed nanostructured NaN–MnO₂, MgN–MnO₂ and KN–MnO₂ respectively with specific surface areas of 36, 110 and 116 m²/g, nitrogen atomic loading at surface of 0.6, 1.0 and 1.5%, and band gap values of 1.20, 1.30 and 1.45 eV. The three precursors with different cationic species allowed different nitrogen loading for their respective plasma-synthesized MnO₂, which led to the different values of band gap energy. An increase of the N-loading induced an increase of band gap energy and enlarged the absorption capability of MnO₂ from visible light to the UV region. Solar photocatalytic removal of TY revealed bleaching degrees of 53, 97 and 94% respectively for NaN–MnO₂, MgN–MnO₂ and KN–MnO₂ materials. This enlargement, together with the increased specific surface area of the plasma-synthesized N–MnO₂, led synergistically to an enhancement of its photocatalytic activity. This work highlights the usefulness of the synthesis via glidarc plasma, without any additional reagent, of MnO₂, as allowing cationic species insertion in their structure, and simultaneously their doping with different N-loading, so leading to different crystalline structures, and photocatalytic activities.

Graphical Abstract