Room temperature ammonia gas sensor based on Ti-doped CeO₂ thin films prepared by nebulizer spray pyrolysis method

In the present study, nebulizer spray pyrolysis was used as the technique to synthesize thin films of CeO₂: Ti (0, 1, 2, 3, 4 and 5 wt%). The analysis of the synthesized thin films was carried out using XRD analysis, FESEM analysis, EDX analysis, UV–Vis spectroscopy, PL spectrometry and gas sensing studies. The XRD studies confirmed that the prepared CeO₂: Ti thin films exhibit fcc fluorite structure. No additional phases were observed, indicating that the Ti dopant atoms occupied substitutional positions in the lattice. Each increment to dopant concentration was found to enhance the grain size while substitution by dopant of smaller radii contributed to a decrease in lattice parameter with increasing dopant concentration. EDX results confirmed the addition of dopants to the host lattice in stoichiometry. The band gap, determined from Tauc plot using transmittance data, decreases with increasing dopant concentration. The lowest band gap of 3.03 eV was observed for CeO₂: Ti (4%) thin film, compared to 3.34 eV for the pristine CeO₂ thin film. The prominent visible lines of the photoluminescence spectra could be attributed to oxygen vacancies created due to the spillover effect of doping with Ti. CeO₂: Ti (4%) sample showed a gas response of 5910 to 250 ppm ammonia gas concentration. The sensor also showed response and recovery time of 14.9 s and 11.2 s respectively. The sensor exhibited significant gas response to ammonia in a near-linear gas response characteristic with quick temporal response, high selectivity towards ammonia over VOCs against which it was tested, and a calibratable linear response to relative humidity. The sensor also showed good repeatability and aging characteristics with a detection limit of 26 ppb making CeO₂:Ti (4%) a good material for ammonia gas sensing applications.