Investigation of High-Sensitivity NO 2 Gas Sensors with Ga 2 O 3 Nanorod Sensing Membrane Grown by Hydrothermal Synthesis Method.

In this work, Ga 2 O 3 nanorods were converted from GaOOH nanorods grown using the hydrothermal synthesis method as the sensing membranes of NO 2 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(NO 3 ) 3 ·9H 2 O) 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 SnO 2 seed layer and the Ga(NO 3 ) 3 ·9H 2 O/HMT concentration of 12 mM/10 mM. In addition, the GaOOH nanorods were converted to Ga 2 O 3 nanorods by thermal annealing in a pure N 2 ambient atmosphere for 2 h at various temperatures of 300 °C, 400 °C, and 500 °C, respectively. Compared with the Ga 2 O 3 nanorod sensing membranes annealed at 300 °C and 500 °C, the NO 2 gas sensors using the 400 °C-annealed Ga 2 O 3 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 NO 2 concentration of 10 ppm. The low NO 2 concentration of 100 ppb could be detected by the Ga 2 O 3 nanorod-structured NO 2 gas sensors and the achieved responsivity was 34.2%.