•SnO2 nanocrystals with abundant oxygen vacancies were prepared by vacuum heat treating.•The preparation process is simple and no additives were used.•The SnO2 nanocrystals can be prepared in mass production.•The resulted SnO2 nanocrystals exhibited enhanced room temperature NO2 sensing performance.SnO2 oxide nanocrystals were synthesized by annealing the precursor powders at 550 °C in vacuum and air environment, respectively. The nanocrystals were characterized by using techniques including X-ray diffraction, transmission electron microscopy, UV-Vis diffuse reflectance spectra, and X-ray photoelectron spectroscopy. It was found that the SnO2 nanocrystals obtained in vacuum contained more oxygen vacancies than the SnO2 nanocrystals prepared in air. When used as gas sensors, the SnO2 nanocrystals prepared in vacuum showed much enhanced room temperature sensing performance to NO2 gas relative to the SnO2 nanocrystals prepared in air. This result confirms the important role of oxygen vacancies in improving gas response of the oxide nanocrystals. The oxygen vacancies make the grain surface possess special chemistry state thereby improving the NO2 adsorption at low operating temperatures and enhancing the charge transfer from the surface to the adsorbate. It suggests that the vacuum annealing is a valid method to generate oxygen vacancies in SnO2 nanocrystals. Such synthetic method with the merits of simplicity and no using any surfactants or additives may pave the way to acquire other oxides with oxygen vacancies thereby being used as advanced materials.Download high-res image (279KB)Download full-size image

Indium oxide nanocrystals with size of 8–20 nm have been synthesized by annealing the precursor particles at ambient pressure. No surfactants or capping agents were used in the synthesis. Depending on the ripening time of the precursor particles in their mother solution, rough control of the crystal size of the annealed indium oxide was achieved. It is interesting that the size of the annealed indium oxide crystals decreases with prolonging the ripening time of the precursor particles, which is the opposite as expected. We proposed a possible mechanism, that is the pre-disintegrating of the precursor particles happened during the ripening process, to explain the rough control of the crystal size. Promoted by attributes of the crystals such as small size, free of surfactant, and abundant defects, we fabricated indium oxide gas sensors and found that these sensors had good response to NO2 gas and can achieve a detection limit as low as 20 ppb.