Study On Wet-chemical Preparation And Gas-Sensing Property Of Metal Oxide Nano-materials

Metal oxide gas sensors are one of the most widely researched and used metal oxide semiconductor sensors due to their advantageous features, such as high sensitivity under ambient conditions, low power consumption, low price, prompt response and simple structure. At the same time, fathomable gas types have been increasing by metal oxide gas sensors. At present, metal oxide gas sensors have been widely used in some fields, such as gas detectors, annunciators, food discriminators, etc. However, metal oxide gas sensors have the same defects and deficiency as other metal oxide semiconductor sensors, for example, poor thermal stability, reliability, selectivity, anti-interference, high operating temperature, which limit the further development and widely application of metal oxide gas sensors. In allusion to these defects and deficiency, the effect of rare earth-dopant, the fabrication of nano-composite materials, shape controls of nano-materials on wet-fabricated metal oxide gas sensors were discussed in this paper, on the basis of ZnO and WO₃ systems.Firstly, defects or deficiency of metal oxide semiconductor sensors and development trend of gas-sensing materials and sensors were introduced in this paper. Gas-sensing mechanism metal oxide semiconductor sensors was expatiated, and factors to affect gas-sensing property were analyzed and researched. In addition, ways, reported in the world, to improve gas-sensing property were reviewed. At last, all kinds of wet-chemical ways to prepare ZnO nano-materials and films were introduced in detail, moreover, the current research evolvements on the controllable fabrication and gas-sensing property improvements of ZnO nano-materials were summarized across-the-board.Secondly, Re-doped ZnO nano-materials and films were prepared by sol-gel method, and the effect of the dopant concentration, operating temperature, UV- irritation, etc. on gas-sening property were discussed. The results show that the appropriate concentration of La and Ce dopant could improve gas-sensing property. Sensitivities, to VOCs, have been increasing with the increase of La and Ce dopant concentration, but the superabundant dopant concentration will worsen gas-sensing property. At the same time, sensitivities, to VOCs, will increase and then decrease with the increase of the operating temperature. UV-irritation could enhance sensitivity and reduce the operating temperature. XRD results show that the addition of La and Ce lead to new La₂O₂CO₃与CeO₂ phase. FESEM results show almost uniform spherical grains are about 20–75 nm in diameter, and the grain size tends to decrease with the increase of the concentration of the additives. The cross-section SEM image show the surface of the films, with the thickness of about 5 um, is rough. A new physical model of the CeO₂ dopant influence on the gas-sensing properties of ZnO thin films is proposed.Thirdly, two kinds of special-shape ZnO nano-materials were prepared by the hydrothermal method and evaporation-induced self-assembly (EISA), respectively, and their gas-sensing property were also discussed. In view of hydrothermal fabrication of nano-ZnO, hexagonal pillar nano-ZnO was formed at lower temperature, but hexagonal pyramidal nano-ZnO was formed at higher temperature. Higher temperature results in stronger alkalescent in solution, which induces the solution produce [Zn(OH)4]2--groups with electronegative characteristic. These [Zn(OH)₄]^2--groups as growth units could easily superpose on anodic plane of ZnO single crystals, but difficultly grow on cathodal plane of ZnO single crystals. Therefore, nano-ZnO formed finally is hexagonal and pyramidal shape. In EISA, due to the amphipathic property and the superfluous addition of F₁₂₇, the chloro-alkoxide Zn(Cl)_2-x(OEt)_x nano-entities connected each other via a hydroxy-bonding interaction to form arrays of amorphous nano-entities surrounded by the F₁₂₇ molecules. After heattreatment, the surfactant removal causes the formation of ZnO nanorod-bundle structures. In addition, the addition of F₁₂₇ could improve their gas-sensing property.Fourthly, Zn-W-O nano-composite materials were fabricated by sol-gel and surface-coated methods, respectively, and their gas-sensing property to VOCs was discussed. XRD, FESEM and gas-sensing test results show that ZnWO₄ phase with a monoclinic structure appears in the whole concentration range (1–99 at. %) by two methods. The ZnWO₄ phase with high resistance almost did not respond to PVOCS. The ZnWO₄ phase with a scheelite structure can increase the number of Lewis acid centers on the surface of ZnO and WO₃. Therefore, their ability of oxidizing dehydrogenation can be strengthened, and thus the gas-sensing properties can be improved further. But the superabundant ZnWO₄ phase can congregate on the limited Lewis acid centers on the surface of ZnO and WO₃ and decrease the number of available acid sites, and thus worsen the gas-sensing properties.Finally, Rectangular WO₃ nanosheets were fabricated by a facile hydrothermal process employing PEG400 as the structure-directing agent, and their gas-sensing property to NO₂ was discussed. XRD, FESEM, TEM and HRTEM results reveal the whole production process of WO₃ phase, namely, WO₃·H2O phase forms quickly and is subsequently dehydrated to yield WO₃ phase under the hydrothermal condition. Rectangular WO₃ nanosheets continue to grow along 2-dimension with the increase of reaction time. Gas sensing tests show that rectangular WO₃ nanosheets could promptly response to NO₂ at ppb level.