Transition Metal Oxides: Controllable Synthesis In Polyol System, Gas Sensing And Catalytic Properties

As we all know, materials would play a crucial role in human survival anddevelopment. Functional materials have been the subjects of intense research for a longtime, along with the rapid development of social productivity and the emergence ofincreasingly serious environmental and energy issues. Transition metal oxides have beenwidely used as gas sensor, opoelecric device, catalysis and electrochemical enery storage,etc, owing to their excellent physical and chemical properties, expecially thefourth-period transition metal oxides. In recent years, the rapid development ofnanotechnology has injected new elements for the further study of functional materialsand the controllable synthesis of the microstructures. At the same time, people becomemore aware of the importance of the relationship between structure and applications offunctional materials. Therefore, it is of great significance to improve the functions bydoping or composite for composition-regulating and porous structure, hierarchicalstructure, active facets exposing or nanostructure for structre-regulating. In this work, wefocuse on the controllable preparation of the fourth-period transition metal oxides (forexample Co, Ni, Cu and Zn) in the polyol system, and we expect an improvement in gassensor and water oxidation.The main parts of this thesis including:1. Porous Cu2O/CuO cubes were successfully prepared by a novel precursor-mediatedsynthetic method. In the synthesis process of the precursor, ethylene glycol functioned asnot only a solvent, but also a reductive agent so as to obtain Cu2O/CuO composite withporous structure through a simple thermal theatment. We can obtain porous Cu2O/CuOcubes with different ratio of Cu2O to CuO by modulating calcination temperature, and theporous Cu2O/CuO calcined at350oC was shown to serve as the most efficient gas sensorfor acetone detection. Moreover, its response was more than fourth that of thecommercial CuO at the acetone concentration of500ppm. Furthermore，the selectivity towards acetone of the sensor has been explained on the basis of the result of temperatureprogrammed desorption (TPD).2. Hierarchical CdO/ZnO microspheres composed of porous nanoplates are preparedby simple thermal treatment of a “pre-synthesized” cadmium-zinc bimetallic carbonateprecursor. Moreover, all the elements (Cd, Zn, and O) were homogeneously distributedover the entire CdO/ZnO microparticle. The as-obtained hierarchical CdO/ZnOnanomaterial can be used as an efficient gas-sensing material for ethanol detection withhigh selectivity and fast response/recovery. And it is demonstrated that CdO cansignificantly improve the ZnO’s response for ethanol detection, with the one with CdO:ZnO=7.5:100exhibiting the highest sensitivity. The sensor has proved to reduce thedetection limit of ethanol detection to0.5ppm, which is much lower than that of thecommercial ZnO sensor (2ppm). Possible reasons are proposed to explain why CdO canimprove the sensing performance of ZnO.(1) CdO is an n-type semiconductor with alarge amount of native oxygen vacancies. This property would be helpful to theadsorption of oxygen molecules on the surface of the CdO/ZnO composite.(2) CdO is alow electrical resistivity material, and thereby it would decrease the resistance of theCdO/ZnO composite material. This would enable the rapid electron transportationbetween the electrodes of the sensor3. we report a new simple hydrothermal method to prepare high surface area Ni-Felayered double hydroxide (LDH) nanosheets with controllable Ni/Fe ratio by using nickelalkoxide and FeSO4as the only starting materials. Moreover, the as-obtained LDHmaterial can serve as an active and stable electrocatalytic water oxidation catalyst inalkaline media. To be more precise, the catalyst with Fe: Ni=0.52:1can afford a currentdensity of10mA/cm2(a value related to practical relevance) at the overpotential of344mV. This outstanding performance can be attributed to the porous structures and largeBET surface area. And more importantly, Fe specie in Ni-Fe0.52has higher bondingenergy value by comparison of the Fe2p XPS spectra of Ni-Fe0.52and Fe2O3, indicatingthe difference of local microstructure of Fe species in the two materials. The higher chemical bonding energy of iron in LDH structure means the stronger electron-acceptingability, which is considered to be beneficial for the electrocatalytic water oxidationreaction.4. A porous Co3O4material with unique octahedron-in-octahedron core-shell-typemorphology is prepared through simple thermal treatment of an octahedron-shaped cobaltalkoxide precursor. The as-obtained material possesses large surface area of~190m2/gand good catalytic activity towards water oxidation reaction with an O2evolution rate of~211.5μmol g-1min-1in the sensitizer [Ru(bpy)3]2+(bpy=2,2’-bipyridine)-Na2S2O8system under visible light irradiation (>420nm). Furthermore, the catalytic activity startsto decrease after a long time and the possible reason is the formation of silica which maybe caused by the hydrolysis of Na2SiF6in the buffer during photolysis.