Photocatalytic degradation of NO by MnO2 catalyst: The decisive relationship between crystal phase, morphology and activity

Abstract

This study investigates the critical relationship between the crystal phase, morphology, and photocatalytic activity of MnO₂. The δ-MnO₂ nanosheets, characterized by multiple exposed crystal planes forming junctions, exhibit optimized optical and electrical properties. Oxygen vacancy concentrations were observed in the order δ-MnO₂ > γ-MnO₂ > α-MnO₂, with corresponding increases in band gap width from 1.38 eV (δ-MnO₂) to 1.68 eV (α-MnO₂). The δ-MnO₂ nanosheets achieved over 80% NO removal efficiency and effectively suppressed the production of NO₂ byproducts, outperforming α-MnO₂ nanorods and γ-MnO₂ nanospheres. The adsorption energy of O₂ followed the trend δ-MnO₂ > γ-MnO₂ > α-MnO₂, while the adsorption energy of NO was lowest on δ-MnO₂, facilitating its interaction with reactive species such as •O₂⁻ and •OH. For γ-MnO₂, NO directly reacted with •O₂⁻. The findings highlight the dependence of MnO₂ photocatalytic performance on its crystal phase and morphology, with δ-MnO₂ effectively inhibiting photogenerated electron-hole recombination due to its superior properties. This work presents a straightforward approach to designing high-performance transition metal photocatalysts through crystal phase and morphology control, offering valuable insights for future photocatalyst research.