The Synthesis And Gas Sensing Properties Of Zinc Oxide And Tin Oxlde

Semiconductor metal oxides as gas sensing materials have received considerableattention due to their advantageous features, such as good sensitivity to the ambientconditions and simplicity in fabrication. Zinc oxide and tin oxide, as the earliestdiscovered and most widely applied oxide gas sensing materials, have beenextensively studied for the synthesis and sensing features. It is well known that thesize and morphology of the nanomaterials are crucial issues for their sensingproperties. Therefore, controlling the shape and structure of the sensing materials is ofgreat importance for the development of gas sensor.In this paper, flowerlike ZnO microstructures constructed from nanorods ofdifferent sizes were prepared by a hydrothermal/microwave hydrothermal (MH)process in the presence of aromatic organic compound. SnO₂ nanoparticles withdifferent sizes were prepared by a microwave-assisted hydrothermal method.Furthermore, we raised a new microwave two-phase method for the synthesis of SnO₂nanocrystals. The samples are characterized by X-ray diffraction (XRD), scanningelectron microscopy (SEM) and transmission electron microscopy (TEM). The effectsof the factors such as additive agent, space steric hindrance effects of additive agent,concentration of raw materials and solution basicity on the morphology and structureof the samples were researched. The sensitivity of the sensors made from theas-prepared ZnO and SnO₂ samples were investigated. The relationships between thestructure and morphology of the sensing materials and their sensing properties werealso studied primarily. The results can be summarized as follows:1. P-nitrobenzoic acid is used in the synthesis of flower-like 3D ZnOmicrostructures through a facile hydrothermal method. In the experiment,p-nitrobenzoic acid is thought to play an important role in the formation ofthe flower-like ZnO microstructures. Only microrods with hexagonalmorphology were obtained in the experiment of hydrothermal process without p-nitrobenzoic acid. The resulting flower-like ZnO microstructureshave been used as gas sensors towards several organic vapors, which exhibitgood performance to ethanol due to the large surface-to-volume ratio;2. O- (m- or p-) nitrobenzoic acid was used as the assistant agent to synthesizethe flower-like 3D ZnO microstructures. When o- (m- or p-) nitrobenzoicacid was used in the reaction system, the diameter of the sphere organizationof ZnO microstructures was about 4μm, 6μm and 8μm, respectively.Subsequently, the corresponding size-dependent ethanol sensing properties ofthe flower-like 3D ZnO microstructures were investigated. We find that thesensitivity of the sensors is influenced by the size and the amount ofnanorods, the sensor made from larger flower with crowded nanorods hashigh sensor response;3. Benzoic acid was used as a structure-directing agent for the synthesis offlower-like 3D ZnO nanostructures under MH conditions. The morphologicalevolution of the products with different solution basicity was investigated.With the increase of the solution basicity, the diameter of ZnO flowers wasincreased from 2μm to 20μm and the morphology of the product changedfrom flower bud to large fireworks. The gas sensing results indicated that thesensitivity of the sensor decreased with the increasing of the particle size.4. SnO₂ nanoparticles with a pure tetragonal phase having the rutile structurehave been prepared by a microwave-assisted hydrothermal method. Theeffects of the factors such as additive agent (p-nitrobenzoic acid) and solutionbasicity on the morphology and structure of the samples were researched.The gas sensing results indicated that all the products had higher sensitivityto ethanol compared to other tested organic vapors. The sample preparedfrom the reaction system consisted of 3 mmol SnCl4 and 10 mmol NaOH asstarting materials had the highest sensor response to ethanol as the concentration varied from 10 to 1000 ppm and the response can reached 85 at800 ppm;5. SnO₂ nanoparticles with a pure tetragonal phase having the rutile structurecan also be prepared by a microwave-assisted two-phase method. The effectof the parameters such as concentration of starting materials and solutionbasicity on the size of the products has been studied. Keeping the gas sensingexperimental parameters unchanged, it was found that the SnO₂ nanoparticlesprepared from the system consisted of 1 mmol SnCl4, 4 mmol NaOA and 8mmol KOH have obviously better gas-sensing properties than those of theother samples. The highest ethanol response was 33.4 as the concentrationvaried from 10 to 1000 ppm, which was lower compared to that of SnO₂nanoparticles prepared from hydrothermal method. The reason for thisprobably lay in the fact that SnO₂ measured here may had its surface cappedwith abundant oleic acid, which inevitably hinder its gas sensingperformance.