In this study, we pursued the gas-sensing mechanism of the adsorption of CO-reducing molecules on a clean ZnGa2O\({}_{4}\)(111) and Pd-decorated ZnGa\({}_{2}\)O\({}_{4}\)(111) surfaces. Human inhalation of carbon monoxide will combine with hemoglobin in the human body and diminish the hemoglobin's oxygen-carrying capacity, resulting in tissue hypoxia [6]. The lack of oxygen in the human body will cause dizziness, headache, nausea, weakness, and other symptoms, and in severe cases, coma, convulsions, and even death. The development of CO sensors has been an important and popular research topic for a long time [1; 2; 5; 7; 8; 9; 10; 11]. For example, [PERSON] et al. reported the sensitivity of CO gas detection of the CuO- and ZnO-doped SnO\({}_{2}\) gas sensor [7]. The maximum sensitivity of SnO\({}_{2}\) to 200 ppm CO is 7 at 350 \({}^{\circ}\)C, while the addition of 1 mol% CuO and 3 mol% ZnO increases the sensitivity of SnO\({}_{2}\) sensor to 8 at 200 \({}^{\circ}\)C. [PERSON] et al. prepared Cu-doped ZnO (CZO) thin films on glass substrates by cosputtering using ZnO and Cu targets [8]. The CZO films with the columnar structures consisted of small crystals with an average grain size of about 5 nm. The CO sensing properties of the CZO films exhibit the highest sensitivity to 40 ppm CO at 350 \({}^{\circ}\)C, while the resistance values of the CZO films are also observed when the sensor is exposed to 6 ppm CO at 150 \({}^{\circ}\)C. [PERSON] et al. reported that ZnO sensing films deposited on gold-coated prisms exhibit high sensitivity with very fast response to CO gas in a wide concentration range (0.5-100 ppm) at room temperature [9]. [PERSON] et al. proposed a micromachined twin sensor, which consists of two sensing layers, a sensing layer made of Cr\({}_{x}\)Ti\({}_{y}\)O\({}_{2}\), and a SnO\({}_{2}\) metal oxide gas sensing layer [10]. When the reducing gas CO interacts with the sensing surface, the \(n\)-type SnO\({}_{2}\) gas sensor materials can obtain more electrons, thereby reducing the resistance value of the SnO\({}_{2}\) material. [PERSON] et al. studied a palladium-doped ZnO gas sensor to detect CO gas [11]. The sensitivity of the palladium-doped ZnO (Pd/ZnO) was 4.5 times higher than that of ZnO when the CO concentration was 100 ppm. The Pd/ZnO injected with CO concentrations of 10-600 ppm shows a decrease in the resistance responses, whereas ZnO shows no significant change under the CO concentrations of 10-600 ppm. The response time of Pd/ZnO at a CO concentration of 100 ppm is 200 s, compared with 750 s for ZnO, which indicates that the change of the response time of Pd/ZnO is three times shorter than that of ZnO. The experimental results show that doping palladium helps to improve the sensitivity and response time of the ZnO sensor. Gallium(III) oxide is another material of interest due to its large bandgap and physical and chemical stability. Tin-doped Ga\({}_{2}\)O\({}_{3}\) improves material conductivity, which makes the higher Lewis acidity of Sn\({}^{4+}\) cations than Ga\({}^{3+}\) ones, which leads to a significant sharp increase in sensor signal for detecting CO and NH\({}_{3}\) at high temperature of 500 \({}^{\circ}\)C [12]. On the other hand, carbon dioxide adsorption on the nonpolar (10\(\overline{1}\)0) surface of ZnO shows the tridentate binding mode to be the most energetically favorable, which the internal C=O bonds of CO\({}_{2}\) lengthened upon adsorption from the initial value from 1.16 A to 1.26-1.24 A [13]. If defective surface with one oxygen vacancy on the nonpolar (10\(\overline{1}\)0) surface of ZnO exists, the adsorption of carbon dioxide on the surface causes the oxygen atom in CO\({}_{2}\) to be trapped, acting as a CO molecule remaining on the surface.