Investigation of High-Sensitivity NO₂ Gas Sensors with Ga₂O₃ Nanorod Sensing Membrane Grown by Hydrothermal Synthesis Method

In this work, Ga₂O₃ nanorods were converted from GaOOH nanorods grown using the hydrothermal synthesis method as the sensing membranes of NO₂ gas sensors. Since a sensing membrane with a high surface-to-volume ratio is a very important issue for gas sensors, the thickness of the seed layer and the concentrations of the hydrothermal precursor gallium nitrate nonahydrate (Ga(NO₃)₃·9H₂O) and hexamethylenetetramine (HMT) were optimized to achieve a high surface-to-volume ratio in the GaOOH nanorods. The results showed that the largest surface-to-volume ratio of the GaOOH nanorods could be obtained using the 50-nm-thick SnO₂ seed layer and the Ga(NO₃)₃·9H₂O/HMT concentration of 12 mM/10 mM. In addition, the GaOOH nanorods were converted to Ga₂O₃ nanorods by thermal annealing in a pure N₂ ambient atmosphere for 2 h at various temperatures of 300 °C, 400 °C, and 500 °C, respectively. Compared with the Ga₂O₃ nanorod sensing membranes annealed at 300 °C and 500 °C, the NO₂ gas sensors using the 400 °C-annealed Ga₂O₃ nanorod sensing membrane exhibited optimal responsivity of 1184.6%, a response time of 63.6 s, and a recovery time of 135.7 s at a NO₂ concentration of 10 ppm. The low NO₂ concentration of 100 ppb could be detected by the Ga₂O₃ nanorod-structured NO₂ gas sensors and the achieved responsivity was 34.2%.