Synergistic effects in a Mn3O4@Bi5O7/BiOI photocatalyst: Probing charge transfer pathways and reactive species for efficient NO removal

A novel self-assembled Mn 3 O 4 @Bi 5 O 7 I/BiOI (Mn 3 O 4 -BBOI) bi-heterojunction photocatalyst was synthesized via a facile one-step hydrothermal method and systematically characterized to investigate its optoelectronic properties. The photocatalytic performance was evaluated through the degradation of NO in a continuous-flow microreactor. Results indicate the successful construction of two distinct interfacial structures within the Mn 3 O 4 -BBOI system: an interface between Mn 3 O 4 and the Bi 5 O 7 I/BiOI heterostructure, and an internal interface between Bi 5 O 7 I and BiOI. These interfaces exhibit lattice distortions and vacancies, which are critical for promoting charge separation. Specifically, Mn 3 O 4 acts as an electron acceptor, facilitating the transfer of electrons from I⁻ in the bismuth-based substrate to Mn 3 + in Mn 3 O 4 , thereby optimizing the generation and recombination dynamics of charge carriers. The Mn 3 O 4 -BBOI photocatalyst demonstrates stable performance, achieving 56–60 % NO removal efficiency under light irradiation, along with excellent suppression of toxic NO₂ byproducts. Introduction of a small amount of water vapor (5 vol%) increase the NO removal efficiency to the instantaneous maximum value of 65 %, although the inhibitory effect on NO₂ is slightly attenuated. Mechanistic studies reveal that superoxide radicals (•O 2 ⁻) serve as the primary active species responsible for the oxidation of NO to nitrite/nitrate (NO 2 ⁻/NO 3 ⁻), while hydroxyl radicals (•OH), generated via the reaction between •O 2 ⁻ and adsorbed H 2 O, contribute secondarily to the oxidation process.