Insights into Selective Sensitivity of In₂O₃-CuO Heterojunction Nanocrystals to CH₄ over CO and H₂: Experiments and First-Principles Calculations

Metal oxide semiconductor gas sensors have demonstrated exceptional potential in gas detection due to their high sensitivity, rapid response time, and impressive selectivity for identifying various sorts of gases. However, selectively distinguishing CH₄ from those of CO and H₂ remains a significant challenge. This difficulty primarily stems from the weakly reducing nature of CH₄, which results in a low adsorption response and makes it prone to interference from stronger reducing gases in the surroundings. Herein, we synthesized In₂O₃-<i>x</i>CuO nanocomposites using a hydrothermal method to explore their gas sensing properties toward CH₄, CO, and H₂. Characterization tests confirmed the successful preparation of In₂O₃-<i>x</i>CuO nanocomposites with different In:Cu molar ratios and the formation of a p-n heterojunction. The gas sensing test results indicated that the In₂O₃-2.1CuO nanocomposites calcined at 500 °C and measured at 350 °C displayed a p-type response for CH₄ and an n-type response for CO and H₂, allowing for accurate differentiation of CH₄ from CO and H₂. Moreover, the In₂O₃-2.1CuO sensor also showed excellent stability and reproducibility across all three gases. First-principles calculations revealed distinct changes in the electronic structure of the In₂O₃-CuO heterojunction upon adsorption of CH₄, CO, and H₂, a finding that aligns with empirical evidence. The gas selectivity mechanism was effectively explained by variations in the energy band gap, driven by electrical behavior during the adsorption process. This work suggests a promising approach for developing selective gas sensors capable of detecting weakly reducing gases.