Unbiased ZnFe ₂ O ₄ /CeO ₂ Nanostructured Heterojunctions for Light-Assisted Room Temperature Ammonia Sensing

The continuous advancements in smart electronic systems and the expanding landscape of the Internet of Things (IoT) have substantially increased demand for self-powered, room-temperature gas-sensing devices with high selectivity and operational reliability. In this context, nanostructured metal-oxide heterojunctions have emerged as a promising platform owing to their tunable interfacial electronic properties arising from nanoscale engineering. Herein, we report a ZnFe 2 O 4 /CeO 2 (ZFO/CeO 2 ) nanostructured heterojunction by anchoring CeO 2 nanoparticles onto ZFO nanorods via a hydrothermal route, which senses ammonia (NH 3 ) at room temperature under illumination without requiring any bias voltage. Due to the synergistic effects of nanoscale heterojunction formation, enhanced charge-carrier transport, and the intrinsic photovoltaic nature, the device demonstrates room-temperature gas sensing with rapid response/recovery times, a low limit of detection (∼77 ppb), superior selectivity, and repeatability toward NH 3 even under zero-bias conditions. Moreover, the device shows consistent response across a broad NH 3 concentration range (5 to 500 ppm) and long-term performance (≥95% response retention) over 30 days, with minimal humidity impact on its response. The underlying nanoscale sensing mechanism is also investigated using comprehensive band alignment analysis via ultraviolet photoelectron spectroscopy (UPS) in conjunction with optical spectroscopy, along with the depletion-layer modulations at the nanostructured ZFO/CeO 2 heterointerface under varying operational conditions. This study offers a promising approach to realizing self-powered gas-sensing technologies in future smart, portable electronic devices.