Hydrothermal Synthesis Of Nanometer ZnO And Their Gas Sensing Properties

ZnO is a kind of direct wide band-gap semiconductor material with uniquephotoelectric characteristic and gas sensing performance. A variety ofmorphologies and structures of ZnO nanomaterials lead to their outstandingphysical properties and potential applications. In this thesis, nanometer ZnO withvarious morphologies have been synthesized by ultrasonic assisted hydrothermaltechnique, which including with flower-like ZnO nanorods, ZnO nanotubes,castellated micro/nano ZnO, turriform micro/nano ZnO and ZnO nanoshuttles.SEM, XRD, TEM, and TG/DSC had been used to characterize the morphologiesand the structures of synthesized ZnO. The growth behavior of each ZnO withspecial morphology and the gas sensing properties of ZnO to C2H5OH, CO, H2and C6H5CH3gas are also investigated.The morphology transformation procedures of ZnO nanorods synthesized byultrasonic assisted hydrothermal process are investigated in detail. Especially, theformation procedure of flower-like ZnO nanorods synthesized by alone ultrasonicprocess is revealed for the first time. Experimental results indicate that there arefour successive conversion stages for ZnO nanorods flowers prepared byultrasonic treatment, including with block Zn(OH)2, block Zn(OH)2coexistedwith ZnO nanoflakes, ZnO nanoflakes coexisted with flower-like ZnO nanorodsand complete flower-like ZnO nanorods, respectively. Moreover, flower-like ZnOnanorods are successfully synthesized when ultrasonic time reaches60min,which has the diameter of30to45nm and the length of50-70nm. Ultrasonictreatment after hydrothermal process can effectively disperse ZnO nanorods. ZnOnanorods with different scattered morphologies can be obtained by controllingultrasonic time. The results of sensitivity detection demonstrate that the gassensor based on flower-like ZnO nanorods shows good gas sensitive performance to C2H5OH gas at425℃and the sensitivityproperty can reach71.4%when it isexposed upon5ppm C2H5OH gas.The effects of different hydrothermal factors on the growth behavior of ZnOnanorods are also studied. The research focus is mainly concentrated onfollowing aspects involving with hydrothermal temperature, hydrothermal time,initial NaOH concentrations and reaction precursor concentrations. Experimentalresults show that hydrothermal factors have great effects on the morphologicaltransformation of ZnO nanorods. With increasing hydrothermal temperature andhydrothermal time, the lengths and diameters of ZnO nanorods all increase, butwhen hydrothermal time is continuously prolonged, ZnO nanorods will notmaintain radial growth habit. When the ratio of Zn2+:OH-is1:5,1:10,1:20and1:40, respectively, ZnO nanocrystals furfill the morphology conversion from ZnOparticles to medium nanorod morphology and then to final ZnO with slicehierarchical structure.The effect of annealing treatment on the morphologies of ZnO nanorods isalso studied. When annealing temperature is lower than400℃, ZnO maintainprevious nanorods morphology. When annealing temperature is higher than600℃, the ends of nanorods turn round. When annealing temperature is over800℃, ZnO nanorods exhibit a kind of aggregated morphology. Annealingprocess greatly influences the sensing properties of ZnO nanorods to C2H5OHgas. Comparing with ZnO nanorods without undergoing annealing treatment, gassensitivity of ZnO nanorods to25ppm C2H5OH gas can increase from55%to74%after400℃annealing treatment.ZnO nanotubes were also fabricated by two procedures of hydrothermaltreatment combining with alkaline etching process. The effects of alkalineconcentration and etching time on the morphologies of ZnO nanotubes aremainly investigated. The results of sensitivity examination to C2H5OH gas withdifferent concentrations indicate that with increasing gas concentration, gassensitivity of ZnO nanotubes sensor gradually increases. The sensing value canreach87.4%to5ppm C2H5OH gas. Comparing with the sensor based on ZnOnanorods, the sensing value of ZnO nanotubes sensor increases by23.9%.Moreover, a novel gas sensor based on circular arrays of ZnO nanorods issuccessfully fabricated by one-step hydrothermal process. The length and diameter of ZnO nanorods are general2μm and50nm, respectively. Comparingwith the sensor based on scattered ZnO nanorods, the sensing properties ofcircular arrays sensor based on ZnO nanorods to C2H5OH, CO, H2and C6H5CH3gas are common increased by5%.The growth behaviors of ZnO nanocrystals in the strong alkaline solutionare finally studied. Novel micro/nanometer castellated ZnO, turriform ZnO andshuttle-like ZnO had been successfully fabricated. Castellated ZnO has six equalsymmetric features with the length of3μm and the diameter of~1.2μm.Turriform ZnO has a radical branch feature with the diameter of500-700nm.Shuttle-like ZnO has the length of2μm and the maximum diameter of0.5μm.The sensitivity results indicate that castellated ZnO has the sensing value of76.5%, but turriform ZnO has the sensing value of81.3%. Moreover, the sensorbased on shuttle-like ZnO has only40%gas sensitivity to5ppm C2H5OH gas.