Metal-semiconductor heterostructures offer a versatile platform for controlling interfacial charge transfer and improving the performance of optoelectronic and photocatalytic devices. In this work, we investigate hybrid gold nanoisland (Au NIs)/cesium bismuth iodide (Cs₃Bi₂I₉) heterostructures for three distinct applications: self-driven full-UV band photodetection, light-activated nitrogen dioxide (NO₂) sensing, and photocatalytic hydrogen evolution. By integrating Au nanoislands with a lead-free halide perovskite thin film, we demonstrate how interfacial charge dynamics and thin-film morphology can be exploited to improve device functionality across diverse applications.

The Au nanoislands serve as catalytic nucleation sites, promoting the formation of vertically oriented, interconnected, petal-like Cs₃Bi₂I₉ structures. This architecture enhances the semiconductor morphology and provides a high density of accessible surface sites for gas adsorption and interfacial photocatalytic reactions. The resulting metal-semiconductor interface facilitates charge separation and transport, while the buried Au nanostructures remain protected beneath the semiconductor layer.

For self-driven UV photodetection, the heterostructure exhibits an open-circuit voltage of 0.6 V, a responsivity of 0.88 A W⁻¹, and a detection threshold of 90 nW cm⁻². Its broad UV response, spanning UV-A, UV-B, and UV-C regions, highlights its potential for on-chip UV optoelectronics. In light-assisted gas sensing, the device demonstrates enhanced NO₂ detection under 365 nm illumination at a low operating bias of 0.3 V. The UV-activated response is 4.5 times higher than that under dark conditions, with response and recovery times of 148 and 151 s, respectively. The lowest experimentally measured NO₂ concentration is 0.54 ppm. Furthermore, photocatalytic hydrogen evolution in a chlorobenzene/HI/H₃PO₂ reaction medium reveals an approximately 384% enhancement relative to the control sample, demonstrating the benefit of interface engineering for light-driven catalytic reactions.

Ultrafast transient absorption spectroscopy and complementary carrier-dynamics measurements support the role of the Au-Cs₃Bi₂I₉ interface in modifying carrier relaxation and facilitating hot-hole transfer from photoexcited Au nanoislands into the valence band of Cs₃Bi₂I₉. The resulting charge-transfer pathways, combined with the vertically interconnected film architecture, contribute to improved device performance across all three applications.

Overall, this work establishes Au nanoisland-Cs₃Bi₂I₉ heterostructures as a multifunctional materials platform for UV optoelectronics, room-temperature gas sensing, and photocatalytic hydrogen generation. The findings emphasize the importance of nanoscale interface engineering and morphology control in developing low-power, stable, and efficient devices based on lead-free halide perovskites.