Creating oxygen vacancies on porous indium oxide nanospheres via metallic aluminum reduction for enhanced nitrogen dioxide detection at low temperature

•The porous In2O3-x nanospheres were synthesized via metallic aluminum reduction for the first time.•The correlation between the electronic structure of In2O3 and the gas-sensing performance to NO2 was further investigated.•The In2O3-x nanospheres obtained at 300 °C exhibits improved sensitivity and superior selectivity at 80 °C.•The gas mechanism was further studied via the calculated density of states (DOS) of In2O3 and In2O3-x. Investigating the correlation between the electronic structure of indium oxide materials with oxygen vacancy defects and their sensing property to nitrogen dioxide is important for future development of high-efficient sensing materials. For the first time, the defective structure of porous In2O3 nanospheres was introduced in a two-zone tube furnace using metallic Al as reducing agent. Texture characterizations show that In2O3 and In2O3-x are porous nanospheres assembled by small particles with the size of ca. 10–20 nm. The surface oxygen vacancy defects and electronic structure of samples were characterized by X-ray power diffraction, X-ray photoelectron spectroscopy, electron paramagnetic resonance, photoluminescence, ultraviolet-visible spectrophotometer and Hall analysis, and the sensing performance of samples to NO2 gas was evaluated. The results display that the porous In2O3-x nanospheres prepared at the reducing temperature of 300 °C (Vo-In2O3-300) exhibits improved response of 130 to 3 ppm NO2 at 80 °C, which is almost 2.5-folds as high as the response (55) of the porous In2O3 nanospheres. The sensing mechanism study carried out using DFT indicates that oxygen vacancies on the surface of In2O3 as donor level optimize the electron structure and electrical properties of materials, such as supplying more free-electrons in the conduction band, narrowing the band gap and improving the mobility of electrons, which result in the low energy barrier for the adsorption of NO2 molecules on the surface and in turn enhance the gas-sensing performance of In2O3. Therefore, this work can provide some instructive thoughts for constructing In2O3-based sensing materials for high-efficient nitrogen dioxide detection.