Junction performance in sensing NO2 at room temperature with a Mn-doped TiO2/TiO2 bilayer structure

Constructing of hetero/homojunctions is widely recognized as the most promising approach for enhancing the gas sensing performance of semiconductor materials. While direct evidence of the junction performance is rarely reported. Here, we fabricated Mn-doped TiO2/TiO2 bilayer homojunction thin films using a two-step hydro-thermal method and applied them to the room-temperature detection of nitrogen dioxide (NO2). The homojunction not only demonstrated excellent sensing performance, with a high response value (23.3) and a fast response time (5 s), but also good repeatability and stability. The sensing performance of the homojunction under back-to-back bias, reverse bias, and forward bias was measured, enabled by the specially designed bilayer structure. The back-to-back bias provided the optimal performance, followed by the reverse bias, and then forward bias. This suggests that all the junctions within the structure contribute to the sensing performance, with the reverse bias being the primary contributor. The barrier heights in air and in the presence of NO2 were calculated, and the sensing mechanism was accordingly elucidated.