Material Design for Metal Oxide Chemiresistive Gas Sensors

Abstract

(Us) and SnO 2 conductivity (G) during CO detection can be presented as the pattern where R and O are the coefficients of R (reducing gas) and O 2 adsorption; R and RO are the coefficients of R and RO desorption; 3 and 4 are the coefficients of charging and neutralization of RO; and N* and N ss are the total number of adsorption sites and sites originating from native (biographic) surface charge, indicates an increase and indicates a decrease. Indicated scheme shows the directions of adsorption/desorption parameters changes, which are necessary to improve sensor response, i.e. increase both the changes of surface potential, and film resistance during interaction with target gas. Theoretically there are no limitations for using any materials for chemiresistive gas sensor design independently of their either physical, chemical, structural or electrical properties [1, 8, 9]. At present, gas sensor's prototypes on the base of covalent semiconductors, semiconducting metal oxides, solid electrolytes, polymers, ionic membranes, organic semiconductors, and ionic salts have been already tested [10]. However, there are no evidences for assertion that all materials are equally effective for gas sensor applications. Therefore, the selection of optimal sensing material becomes key problem in both design and manufacturing of gas sensor with required operation parameters. At present polymers and metal oxides are the most applied materials for manufacturing chemiresistive gas sensors [10]. However, only metal oxides possess required thermal and temporal stability [9, 10]. Therefore, in this paper only metal oxides will be discussed. 2. BINARY METAL OXIDES Early works on chemisorption-type sensors (chemiresistors) focused mostly on studying SnO 2 and ZnO. Sensors based on these oxides have high sensitivity and rather low operating temperatures, 250-400°C. These oxides have negligible concentration of electron states in the band gap and high reactivity to many gaseous species. More recently, other oxide materials, such as WO 3 , TiO 2 , In 2 O 3 , Ga 2 O 3 etc. have been used successfully to design conductometric gas sensors as well [9, 11-15] (see Table 1). The study of other MOXs is a subject of interest to advance various parameters of conductometric sensors. For example, WO 3-based gas sensors have excellent selective response to NH 4 , H 2 S, and NO 2. Moreover, the larger range of sensing materials is required to tailor sensitivity and cross-sensitivity profiles to…