Ultra-sensitive and selective NH₃ room temperature gas sensing induced by manganese-doped titanium dioxide nanoparticles

The study of the fabrication of ultra-high sensitive and selective room temperature ammonia (NH₃) and nitrogen dioxide (NO₂) gas sensors remains an important scientific challenge in the gas sensing field. This is motivated by their harmful impact on the human health and environment. Therefore, herein, we report for the first time on the gas sensing properties of TiO₂ nanoparticles doped with various concentrations of manganese (Mn) (1.0, 1.5, 2.0, 2.5 and 3.0 mol.% presented as S1, S2, S3, S4 and S5, respectively), synthesized using hydrothermal method. Structural analyses showed that both undoped and Mn–doped TiO₂ crystallized in tetragonal phases. Optical studies revealed that the Mn doped TiO₂ nanoparticles have enhanced UV → Vis emission with a broad shoulder at 540 nm, signifying induced defects by substituting Ti⁴⁺ ions with Mn²⁺. The X-ray photoelectron spectroscopy and the electron paramagnetic resonance studies revealed the presence of Ti³⁺ and singly ionized oxygen vacancies in both pure and Mn doped TiO₂ nanoparticles. Additionally, a hyperfine split due to Mn²⁺ ferromagnetic ordering was observed, confirming incorporation of Mn ions into the lattice sites. The sensitivity, selectivity, operating temperature, and response-recovery times were thoroughly evaluated according to the alteration in the materials electrical resistance in the presence of the target gases. Gas sensing studies showed that Mn²⁺ doped on the TiO₂ surface improved the NH₃ sensing performance in terms of response, sensitivity and selectivity. The S1 sensing material revealed higher sensitivity of 127.39 at 20 ppm NH₃ gas. The sensing mechanism towards NH₃ gas is also proposed.