| This paper focused on the synthesis and gas sensing properties of zinc oxide (ZnO)nanomaterials via liquid phase and gas phase methods. The influences of the composition ofreactant materials, reaction time, reaction temperature and pH value of the reaction systemon the morphologies and exposed crystal facets of the products were investigated by a seriesof experiments. The side-heated gas sensors were fabricated from as-prepared ZnOnanomaterials. Through the comparative studies on their gas sensing properties, the effectsof morphologies and exposed crystal facets on the gas sensing properties of ZnO weredetermined. In addition, the relationships between the exposed crystal facets of ZnO andtheir gas sensing properties were computationally studied by first-principles calculation.Finally, it is not only achieved the controlled synthesis of ZnO nanomaterials with uniquestructure and high gas sensing performance, but also provides new guidance for the peopleto further understand the sensing mechanism of ZnO material. The results can besummarized as follows:1. ZnO yolk-shell microspheres were successfully synthesized by a template-freeliquid phase method. The characterization results show that zinc citrate microspheres arefirstly formed during the growth process. As the aging time increases from0to24h, thezinc citrate microspheres transform from a solid microsphere through a yolk-shellmicrosphere to a hollow microsphere. After the heat treatment, the corresponding ZnOmicrospheres are obtained. The evolution of the morphology of the zinc citratemicrospheres with aging time can be understood by the Ostwald ripening mechanismassociated with a progressive redistribution of matter from the cores to the shells of themicrospheres, finally causing the cores disappears, the diameters of the microspheres andthe thickness of the shells increase. The gas sensing test results show that ZnO yolk-shellmicrospheres exhibit the best gas sesing performance than ZnO solid microspheres andhollow microspheres. At the working temperature330℃, the response of ZnO yolk-shellmicrospheres toward50ppm ethanol is52.9, while the reponses of ZnO solid microspheres and hollow microspheres are15.4and32.7, respectively.2. A facile and controllable route based on carbothermal reduction reaction wasemployed to prepare biomimetic nest-like ZnO in a muffle furnace under normal airatmosphere condition, without the uses of tube furnace and gas control procedures. Thecharacterization results show that the rare material Zn powders act as the templates in thegrowth process of ZnO nest-like structure. With the heating temperature rising, Zn powdersbegin to be oxidized from outside to inside. Because of the production of gas and thechange of Zn vapor concentration, the final hollow nest-like ZnO microstructure with aprickly porous shell should be formed. In fact, the pricks grown on the shell are ZnOnanorods. The gas sensing test results indicate that the3D nest-like ZnO has an excellentgas sensing behavior toward ethanol than ZnO nanoparticles, nanorods and hollow shperes.The response of the sensor toward50ppm ethanol at the working temperature of330℃is100.6. This is mainly benifited from the unique composite structure of nest-like ZnO, whichcan promote the gas adsorption and diffusion, and make maximum use of the large specificsurface area.3. The evolution of ZnO hexagonal structures was achieved via a simple two-step lowtemperature hydrothermal reaction by only adjusting the pH value of the reactive solution,without the assistances of template or surfactant. Through the characterizations of theproducts, it can be indicated that the small simonkolleite (Zn5(OH)8Cl2·H2O) hexagonaldisks are formed in the first step of reaction, and then they tend to stack together layer bylayer along their C-axis directions to form hexagonal prismoid, prism and pyramid with theincrease of pH value. After the calcination, the corresponding ZnO hexagnolmicrostructures are obtained. The structure evolution results in the weakening dominance of(0001) planes in the total exposed crystal facets. In the gas sensing test, the gas sensingpropertiy of the sensor based on ZnO hexagnol disk is better than other sensors. At theworking temperature330℃, the gas response of ZnO hexagnol disk with the most exposed(0001) planes toward50ppm ethanol is87.4, which is nearly2,3, and6times higher thanthose of prismoid, prism and pyramid, respectively. Considering the similar specific surface areas of these ZnO structures, this superior gas sensing performance of ZnO hexagnol diskstrongly depends on the predominantly exposed facets (0001). The exposed (0001) facets ofZnO crystal structure can provide more active sites for oxygen adsorption and subsequentreaction with the detected gas than other facets, and thus increase the gas sensing response.4. For further investigating the relationship between exposed crystal facet and gassensing property of ZnO, the porous ZnO nanosheets with similar morphology, specificsurface areas and different exposed crystal facets were prepared through two hydrothermalmethods. Their exposed crystal facets are (0001) and (1010), respectively. The gas sensingtest results show that the ZnO nanosheets predomaintly exposed with (0001) facet exibit abetter gas sensing performance than the (1010) one. At the working temperature330℃, theresponse of the former toward50ppm ethanol is83.6, while the later is28.4. When theconcentration of ethanol reduces to1ppm, the response of the former is3.9, however, thereponse of the later is only1.4. Moreover, the long-term stability of the former is also betterthan the later. Based the PL spectrum and the crystal structure of hexagnol wurzite ZnO, itis suggested that there are more unsatured dangling bonds and oxygen vacancies exist in the(0001) facet of ZnO than {1010} facets. The (0001) facet exhibits a high surface activity,which can enhance the gas adsorption and promote the sequential sensing reaction, finallyresulting in the improved gas sensing property.5. Based on first-principles of density functional theory, the adsorption properties ofethanol, methanol, formaldehyde, ammonia, hydrogen, carbon monoxide, oxygen andnitrogen dioxide on ZnO (0001) and (1010) surface were calculated and investigated. Theresults show that the gases tend to absorb on the (0001) surface. The adsorption of the gaschanges the surface density of state of ZnO. New impurity energy level appears in the bandgap and the band gap becomes narrow, resulting in the variation of the surface conductance.The electron populations study indicates that the electrons transfer from ZnO to gasmolecules, which can enhance the adsorptions of the gases and promote the reactionsbetween gas molecules and chemisorbed oxygens on the surface. In this way, more electrons will be released and finally the sensors display response signals. |
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