Multimodal Gas Sensor Detecting Hydroxyl Groups with Phase Transition Based on Eco-Friendly Lead-Free Metal Halides

An electrical and optical responsive chemical-semiconductor gas sensor is developed using Cu(I) halides with a phase transition mechanism. Cu(I) materials exhibit reversible phase transitions between the Cs3Cu2I5 and CsCu2I3 with different properties, owing to the formation/destruction of the Cs-I and Cu-Cu bonding when the hydroxyl groups are attached/detached. Among various toxic and atmospheric gases, a Cu(I)-based gas sensor has high selectivity in detecting the hydroxyl group, and the highest sensitivity to water. The reactivity is determined by the polarity, where water with the largest polarity has the strongest attraction for Cu+ and I- ions, letting phase transition occur easily. The multimodal sensor red-shifts emission from 445 to 575 nm when it detects hydroxyl gas, and the moisture-sensitive sensor visually confirms response to breaths in less than 5 s and recovery in the air in less than 30 s. The gas sensor can operate at room temperature while applying a constant bias voltage of 1 V, and it also shows excellent sensitivity in a relative humidity range of 15-75%. Furthermore, the sensor maintained approximate to 90% of its initial responsivity over 1 year.