Microwave-Assisted Generation of Secondary Nanoparticles and Flame-Assisted Generation of an Amorphous Layer for Improving NO₂ Gas Sensing Behaviors: A Mini Review

This mini review is comprised of two approaches of the enhancement of gas sensor response: microwave-assisted and flame-assisted strategies. To achieve enhancement, defects and surface modifications must be addressed. When a SnO₂-graphene mixture is irradiated with microwaves, SnO₂ acquires oxygen vacancies because carbon takes oxygen away from its surroundings. An oxygen vacancy, a type of defect, creates free electrons, increasing the response [1]. In addition, decoration with amorphous carbon, which is a type of surface modification, establishes a heterojunction in the main substance [2]. The heterojunction leads to rectification; hence, electrons flow in one direction to balance the electron concentration. A change in the concentration of electrons affects electron mobility. The gas sensor response is affected by the mobility and concentration of electrons. Oxygen vacancies create electrons according to the Kröger-Vink equation, and heterojunctions accelerate electrons. The sensor response changes when the metal oxide semiconductor gas sensor is exposed to oxidizing and reducing gases. When a substance is oxidized, surface functional groups lose their electrons to remain in equilibrium. Oxygen, a surface functional group, loses electrons and traps them as ions on the surface, resulting in band-bending [3]. Therefore, defects and decorations increase gas sensor response.