Alkaline-earth metal-doped In2O3 microtubes: a simple and efficient approach for detection of ppb-level formaldehyde gas

Metal doping has been widely acknowledged as a facile yet impactful approach to optimize the gas-sensing performance of metal oxide semiconductors. In this study, stable-valence alkaline-earth metals (Ca, Sr, and Ba) are incorporated into In₂O₃ microtubes to investigate the influence of ionic radius on formaldehyde gas-sensing performance. The results indicate that heterogeneous doping leads to a reduction in average grain size and bandgap, while concurrently increasing the specific surface area and the concentration of oxygen vacancies. The formaldehyde response values increase from 36.18 for In₂O₃ sensor, to 95.66 for In₂O₃-Ca sensor, to 107.93 for In₂O₃-Sr sensor, and up to 154.32 for In₂O₃-Ba sensor, correlating with the ionic radius of the doped-metals. The In₂O₃-Ba sensor notably demonstrates excellent selectivity toward formaldehyde, long-term stability, and a reduced operating temperature of 190 °C. The increase in ionic radius is associated with greater lattice distortion in the In₂O₃-Ba microtubes, a reduction in average grain size, and an elevated concentration of oxygen vacancies. Larger radius metal-doping significantly elevates oxygen vacancy density and chemisorbed oxygen content. Density functional theory calculations reveal that the adsorption energy for formaldehyde molecules on the In₂O₃-Ba sensor is the lowest at −1.75 eV. The enhanced vacancy and chemisorbed oxygen from acceptor Ba³⁺-doping into the n-type In₂O₃-Ba sensor facilitate the excellent gas-sensing performance, highlighting the contribution of the large-radius acceptor-doping to the high-performance In₂O₃ gas sensor.