Cr-doped TiO₂ thin films for highly effective NO₂ sensing

Abstract Nitrogen dioxide (NO 2 ) is a hazardous atmospheric pollutant with severe health and environmental consequences, necessitating ultrasensitive detection systems. This study presents a breakthrough in NO 2 sensing through chromium-doped titanium dioxide (Cr-doped TiO 2 ) thin films fabricated via RF magnetron sputtering. By optimizing Cr doping levels (7%, 8%, and 13% atomic percent) and leveraging advanced characterization (SEM-EDS, XPS, AFM), we demonstrate a sensor achieving an ultra-low detection limit of 2.017 parts per trillion (ppt), the lowest reported for Cr-doped TiO 2 systems. The sensor exhibits rapid response/recovery dynamics (55/182 s under dry conditions, 76/172 s at 50% RH) and exceptional stability over 400 cycles, overcoming critical limitations in existing gas-sensing technologies. XPS analysis reveals a systematic link between Cr oxidation states (Cr 3+ ↔ Cr 4+ ) and performance, validating the emergence of p-type conductivity through lattice charge imbalance and hole generation. Crucially, the sensor maintains reliability under humidity, a major challenge in real-world applications, due to Cr-induced charge modulation and hydrophilic oxide states. These advancements position Cr-doped TiO 2 as a transformative platform for environmental monitoring, offering unprecedented sensitivity and robustness for air quality control and industrial safety.