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.