Figure 2.

The gas response of the synthesized NiO nanoflakes sensor toward 30 ppm of CH₄ at different working temperatures (100–350°C) (A), gas response of the synthesized ultrathin NiO nanoflakes sensor to various concentration CH₄ in the range from 0.2 to 50 ppm at 225°C (B), the response and recovery curves of the synthesized ultrathin NiO nanoflakes sensor to 20 and 30 ppm CH₄ at 225°C (C), the response of NiO nanoflakes sensor toward 30 ppm of different testing gases at 225°C (D), and the long-term stability of NiO nanoflakes sensor to 20 and 30 ppm CH₄ at 225°C (E).