ABSTRACT
This chapter describes the application of ZnO in gas sensors. As a well-known n-type metal oxide semiconductor (MOS), ZnO thin films have attracted intensive interests due to their low cost, easy production, and various nanostructures. Many techniques have been employed to synthesize ZnO thin films, such as rf magnetron sputtering, pulsed laser deposition, molecular beam epitaxy, and sol-gel. Sol-gel is considered a powerful alternative for vacuum deposition. ZnO has been used in both thin film and thick film sensors; however, we limit our discussion to thin film sensors. The dependence of sensitivity on different factors, such as film thickness, operating temperature, grain size, and doping is discussed in detail with examples. The sensitivity of ZnO has been tuned by changing the above properties in film fabrication process. Also, ZnO has been mixed with noble metal or transition metal oxides to increase catalytic activity of sensor surfaces and sensor selectivity. Surface modifications of zinc oxide may result in an increase in sensitivity. Surface modification affects grain size, porosity, crystallinity and internal stresses, and hence affect the sensitivity. Thus, there are thousands of research papers that have been published on ZnO-based gas sensors in the past decade. This chapter provides relevant examples for tunability of gas sensing characteristics of ZnO by operation temperature, crystallinity, film thickness, grain size, and doping.
