Dual selective sensing of CH4 and ultra-low NO gas utilizing Ag-decorated CeO2-CuO nanorods: Role of humidity in p-n conductivity transition

Abstract

Detecting hazardous gases like methane (CH4) and nitric oxide (NO) under real-world conditions is a significant challenge for gas sensors. Herein, pure and (0.5–2 wt%) Ag-decorated CeO2-CuO nanorods were prepared using a hydrothermal approach and tested as dual-gas sensors for NO and CH4, with controlled relative humidity (RH). The crystal structures, optical properties, surface adsorption states, and chemical states of the materials were probed using various analytical techniques. The sensors were tested at different temperatures for multiple gases, including benzene, acetone, xylene, carbon monoxide, and CH4. At 175 °C, a 2 wt% Ag-decorated CeO2-CuO nanorods demonstrated a superior response and selectivity towards 10,000 ppm CH4 gas. In comparison, at 200 °C, the 0.5 wt% Ag-decorated CeO2-CuO nanorods showed a remarkable selectivity towards a trace level of (5–100 ppb) NO gas. The sensor showed a notable p-n transition in its electrical response based on the gas and humidity levels. However, an opposite response emerged under humid conditions (RH >50 %), indicating a switch to n-type conductivity. This shift is due to humidity-driven surface hydroxylation, electron donation from Ag nanoparticles, and charge effects at the CeO2-CuO interface. Water molecules on the surface change band bending and increase electron accumulation, promoting n-type behaviour. The sensing mechanism associated with humidity-controlled conduction reversal is discussed in detail