Cryogenic defect and microstructure engineering for ultrafast room-temperature ethanol sensing in TiO₂ nanoparticles

Titanium dioxide (TiO 2 ) is a commercially available and widely used gas sensing material. Here, TiO 2 nanoparticles (TNPs) are first synthesized using the sol-gel method (as-synthesized, T1). The samples are heated in an air environment at 400°C and then allowed to cool (to room temperature, T2) or quenched to different final temperatures (ice at -4°C, T3 and liquid nitrogen i.e. LN2 at -196°C, T4) to initiate thermally induced microstructural changes. These thermally treated TNPs are then evaluated for their gas sensing performance by testing them against ethanol vapours (1-6 ppm). The response and recovery times are measured to be ∼ 5.7 s and 7 s, respectively, for 6 ppm ethanol under ambient conditions for the LN2-quenched sample. However, the best response and recovery times are measured as 1.3 s and 3.7 s, respectively for 4 ppm ethanol. Interestingly, while T1, T2, and T3 samples exhibited p-type behaviour upon ethanol exposure, T4 showed n-type behaviour. Crystallite size, porosity, oxygen vacancies, band gap changes and the presence of functional groups are understood to be important attributes influencing the sensor performance. This work demonstrates that cryogenic quenching can be a simple, effective strategy to control conduction type, charge carrier dynamics, and gas sensing performance, offering a route for high-performance, selective and tunable ethanol sensors. A switch from p- to n–type conduction in the same material, demonstrated here, opens various avenues like dual-mode sensing, homojunction sensing and highly tunable sensor design.