NO Gas Sensor Based on Ti-Doped ZnO Thin Films Prepared by SILAR Method

Abstract

In this study, Ti-doped ZnO films with flower-rod-like nanostructures were synthesized by the SILAR method for enhanced NO gas sensing applications. The stoichiometric ratio of Ti in the host ZnO lattice was confirmed by atomic absorption and energy-dispersive X-ray spectroscopies. All the synthesized films exhibited a pure wurtzite hexagonal structure that seemed to deteriorate at high Ti doping contents as it was manifested by the measured X-ray diffraction patterns. Scanning electron microscopy images of ZnO revealed the coexistence of porous flower- and rod-like structures, and became finer, denser and more compact with Ti doping. By UV-Vis measurements, the transmittance of the synthesized pure ZnO thin film in the visible region (~75%) increased by about 10% with Ti doping, and the energy bandgap seemed to decrease up to a some limit of Ti content. Among the fabricated sensors (based on pure ZnO, 1% Ti-doped, 3% Ti-doped and 5% Ti-doped ZnO films), the best sensing performance was observed for the 1% Ti-doped ZnO film. At first, this was associated with its high density of oxygen vacancies present on the surface of the film and ionized oxygen vacancies present in the ZnO lattice (confirmed respectively by X-ray photoelectron and photoluminescence spectroscopies). Nonetheless, this may be also due to its increased crystallinity (confirmed by X-ray diffraction and photoluminescence spectroscopy), high area-to-volume ratio (confirmed by scanning electron microscopy images), high specific surface area (confirmed by Brunauer-Emmett-Teller measurements) as well as high mobility and carrier concentration (confirmed by Hall measurements). The sensor was highly selective to NO gas and showed notable stability as well as very short response and recovery times which makes it eligible for the early detection of any indoor or outdoor NO gas leakages. Keywords : SILAR, Ti-doped ZnO, Structural properties, Optical properties, Electrical properties, Gas sensor. Acknowledgement This research was supported by the research grant 021220FD2201 “Development of highly sensitive MOS based nano-film gas sensors” from Nazarbayev University. Figure 1