Study On The Gas Sensing Properties Of Binary Metal Oxides（AB₂O₄） With Hierarchical Structures

Acetone, as one kind of common volatile organic compounds（VOCs）, is widely applied in many areas. However, acetone is quite inflammable, explosive and irritative. Therefore, the effective detection of acetone leakage would be of great importance for industry safety and human health. Besides, pathological study indicates that acetone could be regarded as expiratory marker of diabetic patients. Consequently, the development of semiconductor oxides based sensor devices with extraordinary sensing performances would have significant importance in painless diagnosis and surveillance of diabetic symptoms. Unfortunately, at present, most semiconductor oxides gas sensors have insufficiencies or defects on detection limit, sensitivity and selectivity, which hugely limit their application in clinical diagnosis. In this regard, current work focus on the construction of high performances acetone sensor devices via accommodating the microstructures, chemical composition, as well as the porosity of sensing materials-binary metal oxides（AB2O4）. The main work is summarized in three parts as follows:A facile and controllable one-step solvothermal approach combined with subsequent annealing process was developed for the synthesis of uniform and well-dispersed porous Zn O/Zn Co2O4 hollow spheres, which were constructed by plenty of nanoparticles. Moreover, EDS-mapping analysis clearly revealed that Zn O and Zn Co2O4 were nested within each other forming a whole sphere rather than creating isolating Zn O and Zn Co2O4 hollow spheres. At last, indirect-heated sensor devices were fabricated via using the as-obtained products and their sensing properties were also evaluated. It was found that novel Zn O/Zn Co2O4 hollow spheres exhibited much higher acetone response than that of Zn O/Zn Co2O4 nanoparticles, quite fast response speed and good repeatability. However, the detection limit to acetone is a little bit high（about 10 ppm）, which can be ascribed to the high sintering temperature.Via using zinc nitrate hexahydrate and ferric nitrate nonahydrate as zinc and iron source, respectively, ethanol and ethylene glycol as reactant agent, porous Zn Fe₂O₄ nanospheres were prepared through the similar solvothermal synthesis process and expected to lower the acetone detection limit. FESEM and TEM measurement results clearly revealed that the spherical structure was built up from numerous nanoparticles with size around 10 nm. When their sensing behaviors were evaluated, it was found that porous Zn Fe₂O₄ nanospheres exhibited extremely high response, very good selectivity, as well as low detection limit to acetone（response to 800 ppb acetone was about 1.4）. These fascinating acetone sensing performances can be attributed to the large surface area derived from the small particle size and the good permeability caused by the porous feature.Yolk-shell Zn Fe₂O₄ microspheres built up from plenty of two-dimensional（2D） nanosheets were first prepared by annealing and etching the precursor, which was prepared through the one-step solvothermal strategy without any templates. The average diameter of yolk-shell Zn Fe₂O₄ microspheres is about 1.6 μm and the thickness of the nanosheets is around 20 nm. TEM measurement results of the products obtained at differernt reaction times showed that Ostwald ripening process played an important role in the formation of the yolk-shell spheres. Furthermore, gas sensing measurements revealed that at the optimum operating temperature of 200℃, the responses of yolk-shell Zn Fe₂O₄ microspheres to 100 ppm was 40.6. Notably, the acetone detection limit of the sensor device could be as low as 500 ppb（response is 1.4） and the sensor device also showed excellent reproducibility and long-term stability during the continuous measurement over 200 cycles and a month. All these excellent outcomes indicate that our gas sensor is close to or even up to the standard for clinical detection of acetone.