Al Decoration versus Al Substitution in MnO2 Monolayers: Strategy-Dependent Interfacial Mechanisms for NO2 Capture and Sensing

Abstract Nitrogen dioxide (NO2) is a hazardous atmospheric pollutant, making its efficient adsorption and sensitive detection important for environmental monitoring and pollution control. Pristine MnO2 monolayers, however, generally show weak interfacial interactions and limited charge transfer during NO2 adsorption, which restricts their adsorption and sensing performance. Herein, density functional theory (DFT) calculations were performed to elucidate how two Al modification strategies, namely surface decoration (Al-decorated) and substitutional doping (Al-substituted), regulate NO2 adsorption on MnO2 monolayers. Particular attention was paid to the relationship among modification strategy, interfacial bonding mechanism, and application tendency. H-site Al-decorated MnO2 (H–Al–MnO2) and T2-site Al-substituted MnO2 (T2–Al–MnO2) were identified as the most stable configurations. Although both Al modification strategies substantially strengthen the interaction with NO2, with adsorption energies of −3.689 and −1.133 eV and charge transfers of −0.62 and +0.34 e, respectively, they induce distinct interfacial bonding modes. In H–Al–MnO2, dual-site O–Al interactions promote charge separation and electrostatic attraction, giving rise to a predominantly ionic interface. In contrast, T2–Al–MnO2 favors a single N–O interaction characterized by electron sharing and orbital hybridization, resulting in a more evident covalent component. These strategy-dependent interfacial bonding mechanisms lead to distinct functional tendencies. H–Al–MnO2 is more suitable for NO2 capture owing to strong adsorption, difficult desorption, and preferential NO2 adsorption, whereas T2–Al–MnO2 is more promising for reusable NO2 sensing owing to its moderate adsorption strength, high sensitivity, favorable recovery behavior, and good NO2 selectivity. This work establishes a structure–mechanism–performance relationship in Al-modified MnO2 monolayers, revealing how modification strategy governs interfacial bonding and drives distinct NO2 capture and sensing tendencies.