Insights into\nSelective Sensitivity of In<sub>2</sub>O<sub>3</sub>‑CuO Heterojunction\nNanocrystals to CH<sub>4</sub> over CO and H<sub>2</sub>: Experiments\nand First-Principles Calculations

Metal oxide semiconductor gas sensors have demonstrated\nexceptional\npotential in gas detection due to their high sensitivity, rapid response\ntime, and impressive selectivity for identifying various sorts of\ngases. However, selectively distinguishing CH<sub>4</sub> from those\nof CO and H<sub>2</sub> remains a significant challenge. This difficulty\nprimarily stems from the weakly reducing nature of CH<sub>4</sub>,\nwhich results in a low adsorption response and makes it prone to interference\nfrom stronger reducing gases in the surroundings. Herein, we synthesized\nIn<sub>2</sub>O<sub>3</sub>-<i>x</i>CuO nanocomposites using\na hydrothermal method to explore their gas sensing properties toward\nCH<sub>4</sub>, CO, and H<sub>2</sub>. Characterization tests confirmed\nthe successful preparation of In<sub>2</sub>O<sub>3</sub>-<i>x</i>CuO nanocomposites with different In:Cu molar ratios and\nthe formation of a p-n heterojunction. The gas sensing test results\nindicated that the In<sub>2</sub>O<sub>3</sub>-2.1CuO nanocomposites\ncalcined at 500 °C and measured at 350 °C displayed a p-type\nresponse for CH<sub>4</sub> and an n-type response for CO and H<sub>2</sub>, allowing for accurate differentiation of CH<sub>4</sub> from\nCO and H<sub>2</sub>. Moreover, the In<sub>2</sub>O<sub>3</sub>-2.1CuO\nsensor also showed excellent stability and reproducibility across\nall three gases. First-principles calculations revealed distinct changes\nin the electronic structure of the In<sub>2</sub>O<sub>3</sub>-CuO\nheterojunction upon adsorption of CH<sub>4</sub>, CO, and H<sub>2</sub>, a finding that aligns with empirical evidence. The gas selectivity\nmechanism was effectively explained by variations in the energy band\ngap, driven by electrical behavior during the adsorption process.\nThis work suggests a promising approach for developing selective gas\nsensors capable of detecting weakly reducing gases.