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.