Mechanism for Surface Reaction of ZnO and Application in Gas Sensor

Abstract

Understanding the surface reaction mechanisms of ZnO is essential for various kinds of applications such as molecule synthesis/decomposition, biomolecule detectors, photocatalysts, and gas sensing devices. For this reason, there have been numerous studies of the interactions of molecules with ZnO. However, the surface chemistry of ZnO is no doubt complex and some of the surface reaction mechanisms are still controversial. Although the surface reaction of ZnO is extremely complicated, the surface reaction mechanisms have been studied for long time. They have usually been investigated in two ways: (a) by theoretical calculations such as ab initio and density functional theory (DFT), and (b) by experiments usually done in vacuum. In both ways, they provide some hints for the surface reactions of ZnO with molecules; however, it is difficult to apply the results to real conditions since the surface reactions of ZnO are investigated only for the restricted condition. In the present thesis, surface reactions of ZnO with some molecules and gas sensing mechanisms of ZnO were studied. The experimental analyses of surface reaction were carried out in ultra-high vacuum (UHV) system, whereas the mechanism studies for gas sensors were performed in ambient air condition. The circumstances and conditions of gas sensor system seem very different from those of UHV system. Nevertheless, some efforts were made to connect experimental analyses of surface reactions of ZnO with mechanism studies for ZnO gas sensors. Through the combination of surface reaction and gas sensor mechanism studies, finally, improved ZnO gas sensors with enhanced gas sensing properties were developed and characterized. In chapter 2, reactions of ZnO nanowires with atomic hydrogen were investigated in an ultra-high vacuum (UHV) system. Temperature-programmed desorption (TPD) was used to study the reaction of atomic hydrogen with hydrothermally grown ZnO nanowire arrays. During TPD, molecular H2, H2O, and atomic Zn desorbed from ZnO NWs pretreated with atomic H at 220 K. Three distinct H2 TPD peaks, two from surface H states and one from a bulk H state, were identified: H2 desorption states at 330, 450, and 553 K were attributed to chemisorbed H(a) on Zn and O sites on non-polar side-wall surfaces of ZnO nanowires and H absorbed by the ZnO bulk, respectively. The TPD assignment of the bulk H state was corroborated by significantly suppressed deep-level emission at 564 nm and enhanced near-band-edge emission at 375 nm in PL experiments. Also, etching of ZnO NWs by atomic H was confirmed by desorption of molecular H2O and atomic Zn in TPD and by electron microscopic images of H-treated ZnO NWs. From the results, a mechanistic model for underlying H/ZnO nanowires reactions was proposed and discussed. In chapter 3, reactions of ZnO(10-10) non-polar surface with ethanol were studied in an UHV system. Desorption products of ethanol from ZnO(10-10) were examined as function of temperature and exposure by temperature-programmed desorption (TPD). During TPD, ethanol and acetaldehyde molecules were desorbed as the main desorption products. A desorption peak was observed for chemisorbed ethanol desorbing at 360 and 410 K. Acetaldehyde desorbed at 360 K, which correspond to the temperature of ethanol desorption, and also at 400 and 470 K. From the results, reaction mechanisms were suggested for the decomposition of ethanol on ZnO surface. In chapter 4, ethanol gas sensing properties of ZnO were studied and their gas sensing mechanism was investigated. ZnO films were grown by sputtering, and ZnO nanorod arrays were synthesized on the sputtered ZnO films through a simple hydrothermal method. A furnace-type gas sensing system was used to characterize the sensing properties of ZnO films and ZnO nanorod arrays in the present of dilute ethanol gas at different sensing temperature. The maximum response temperatures were 400°C for both the ZnO film and the ZnO nanorod gas sensors, which revealed maximum sensitivities of 14 and 60, respectively. The sensing mechanism in the ZnO-based sensor was discussed, using models based on well-known surface reaction between ethanol and adsorbed oxygen species on ZnO. The ethanol gas sensing properties of ZnO films and ZnO nanorod arrays that had been pretreated with H2O2 and annealed were also examined. H2O2 pretreatment and annealing generated oxygen vacancies on the ZnO surface. The induced surface oxygen vacancies enhanced the gas sensing responses. The gas sensing mechanism in the ZnO-based gas sensor was discussed in terms of the effects of the oxygen vacancies. In chapter 5, hydrogen sensing properties of ZnO films and ZnO nanorod arrays that had been pretreated with H2O2 and annealed were examined. ZnO films and nanowire arrays were successfully synthesized by sputtering and an ammonia-solution method, and the hydrogen gas sensing properties of a simply fabricated ZnO sensor were evaluated in a furnace-type sensing system. The ZnO nanorod array gas sensors exhibited enhanced gas sensing properties, compared with the ZnO film, in the presence of hydrogen gas. H2O2 pretreatment and annealing processes produced oxygen vacancies on the ZnO surface. The enhanced chemisorbed oxygen coverage on the ZnO nanorods with oxygen vacancies yielded improved sensing properties compared to the untreated ZnO nanorods.