Visible-Light-Activated Type II Heterojunction in Cu₃(hexahydroxytriphenylene)₂/Fe₂O₃ Hybrids for Reversible NO₂ Sensing: Critical Role of π–π* Transition

Metal-organic frameworks (MOFs) with high surface area, tunable porosity, and diverse structures are promising platforms for chemiresistors; however, they often exhibit low sensitivity, poor selectivity, and irreversibility in gas sensing, hindering their practical applications. Herein, we report that hybrids of Cu₃(HHTP)₂ (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) nanoflakes and Fe₂O₃ nanoparticles exhibit highly sensitive, selective, and reversible detection of NO₂ at 20 °C. The key parameters to determine their response, selectivity, and recovery are discussed in terms of the size of the Cu₃(HHTP)₂ nanoflakes, the interaction between the MOFs and NO₂, and an increase in the concentration and lifetime of holes facilitated by visible-light photoactivation and charge-separating energy band alignment of the hybrids. These photoactivated MOF-oxide hybrids suggest a new strategy for designing high-performance MOF-based gas sensors.