Light activated room temperature NO₂ sensing in Au nanoisland/Cs₃Bi₂I₉ heterojunctions

Light-assisted gas sensing has emerged as a compelling strategy for enhancing sensor performance while eliminating the elevated operating temperatures required by conventional gas sensors. Here, we report a hybrid Au nanoisland (Au NI)/Cs₃Bi₂I₉ heterojunction for selective room-temperature NO₂ detection under optical excitation. Semi-embedded Au NIs promote the growth of vertically oriented, petal-like Cs₃Bi₂I₉ structures, thereby increasing the accessible surface area and the density of adsorption-active sites. High-resolution transmission electron microscopy resolves a sharp crystalline interface, while ultrafast transient absorption spectroscopy reveals altered carrier-relaxation dynamics consistent with hot-hole transfer from Au NIs into Cs₃Bi₂I₉. At a low applied bias of 0.3 V, the device exhibits a markedly stronger NO₂ response under 365 nm illumination than under darkness. The lowest experimentally measured concentration is 0.54 ppm, and the UV-activated response is 4.5-fold higher than that measured in the dark, with response and recovery times of 148 and 151 s, respectively. The sensor also demonstrates pronounced selectivity toward NO₂ and stable operation over repeated exposure-recovery cycles. These findings establish Au NI/lead-free halide-perovskite heterojunctions as a viable platform for low-power, light-activated gas sensing.