Adsorption of NO\({}_{2}\) and H\({}_{2}\)S on ZnGa\({}_{2}\)O\({}_{4}\)(111) Thin Films: A First-Principles Density Functional Theory Study  
[PERSON]  
1 Center for General Education, China Medical University, Taichung 404, Taiwan; [EMAIL_ADDRESS]  
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doped with palladium at approximately 320 \({}^{\circ}\)C have high selectivity to LPG, but poor sensitivity to CH\({}_{4}\) and CO [6]. Furthermore, [PERSON] et al., reported the use of ZnGa\({}_{2}\)O\({}_{4}\) prepared by spraying coprecipitation for sensing LPG, CO, ethanol (C\({}_{2}\)H\({}_{5}\)OH), and methane [7]. A novel approach described in [PERSON] et al., (2010) [8] is based on the use of ZnGa\({}_{2}\)O\({}_{4}\)/ZnO core-shell nanowires. Sensitivity to nitrogen dioxide (NO\({}_{2}\)) gas can be considerably improved by using ZnGa\({}_{2}\)O\({}_{4}\)/ZnO core-shell nanowires rather than pure ZnO nanowires. Despite the introduction of new approaches to developing gas sensors, the fundamental interaction between gas molecules and metal oxide compounds remains unclear. Density functional theory (DFT) calculations have been widely used in the fields of atomic and molecular adsorption on surfaces and the gas-sensing mechanism [9; 10]. For example, water adsorption on the carbon nanotube field-effect transistor (CNTFET) explains the humidity-induced hysteresis, and it can be reasoned that water adsorption on the CNTFET leads to an increased chemical activity, the modification of the Schottky barrier, and the adsorption of some ionic substances in the vicinity of the carbon nanotube [9]. The gas-sensing mechanism of ZnO applied to H\({}_{2}\), NH\({}_{3}\), CO, and C\({}_{2}\)H\({}_{5}\)OH was established by conducting first-principles DFT calculations [10]. When the gas molecules, i.e., H\({}_{2}\), NH\({}_{3}\), CO, and C\({}_{2}\)H\({}_{5}\)OH molecules, become incident upon the ZnO(10-10) surface, the adsorption-induced reconstruction of the ZnO surface and charge transfer from the gas molecules to the ZnO surface control the sensing process evaluated by the change of electronic conductance of ZnO. [PERSON] et al., reported that the resistance response of ZnO at operating temperature 450 \({}^{\circ}\)C increased (decreased) in the presence of NO\({}_{2}\) (H\({}_{2}\)S) [11]. Moreover, the sensor responses of ZnO and ZnO with gallium contents of 0.5% and 4.0 at% show that an increase of gallium caused a monotonous decrease of the sensor response to H\({}_{2}\)S due to the enhanced electron-donor ability of surface oxygen anions, the H-S bond-breaking in the H\({}_{2}\)S molecule, as well as the decrease of the H\({}_{2}\)S adsorption. Recently, we demonstrated epitaxial growth of ZnGa\({}_{2}\)O\({}_{4}\) thin film grown on the sapphire substrate using a metalorganic chemical vapor deposition (MOCVD) technique that yields a high-selectivity gas sensor [12]. The ZnGa\({}_{2}\)O\({}_{4}\) gas sensor has superior selectivity to NO at the operating temperature of 300 \({}^{\circ}\)C. Reactions of NO molecules on Ga-Zn-O-terminated ZnGa\({}_{2}\)O\({}_{4}\)(111) surfaces were modeled and carried out using a first-principles density functional theory method. The NO molecules combine with the gallium atoms on the ZnGa\({}_{2}\)O\({}_{4}\)(111) surface to produce N-Ga bonds, leading to the work function changes. The sensor response can be gained from the changes in the work functions, indicating that the N-Ga bonding exhibits a very sensitive adsorption response.