Manganese Oxide and Oxide-Hydroxide Composites With Halloysite Nanotubes as Cationic & Anionic Dye Degradation Photocatalysts

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A series of manganese oxide and manganese oxide-hydroxide samples and their composites with halloysite nanotubes (HNTs) was chemically synthesized using NH₄⁺-containing solutions at pH = 5–7 and 10 to study their use as degradation photocatalysts for cationic (methylene blue) and anionic (Congo red) dyes. It was shown that the template role of NH₄⁺ ions in the formation of open-structure manganese dioxide polymorphs is limited to acidic media and electrodeposition. With band gaps in the range of 2.0–2.7 eV, the Mn₃O₄, Mn₂O₃, α-MnOOH, β-MnOOH, γ-MnO₂, and β-MnO₂ phases are able to decorate the surface of HNTs or occupy their lumens depending on the charge of Mn species in the reaction medium. The most effective strategy for enhancing photocatalytic activity of methylene blue was the alkaline-induced surface modification of HNTs, which generated silanol-rich defects identified by the FTIR bands at 3506 and 3560 cm⁻¹. It promotes the formation and anchoring of Mn₃O₄ nanoparticles onto HNTs as an important condition for enhanced photocatalytic degradation of a cationic dye by HNT@Mn₃O₄. α-MnOOH-based composites were the best photocatalysts for anionic dye degradation. The distinctive features of their energy band diagrams were discussed.