Preparation And Sensing Properties Of γ-Fe₂O₃ /ZnO Nano Double Layer Gas Sensing Films

In the experiment,firstly, using zinc nitrate and HMTA as zinc source and precipitator, respectively , in addition to different surfactants of dodecyl sulfate, hexadecyl trimethyl ammonium bromide,citric acid,sodium tripolyphosphate,polyvinyl alcohol and polyethylene glycol 2000, we successfully synthesized nano-ZnO with different morphologies on the Al₂O₃ substrates via a hydrothermal method, including ZnO nanorods,ZnO nano-wall,ZnO fine microspheres and flower-like ZnO etc. The obtained nano-ZnO was studied by X-ray diffraction(XRD) and field-emission scanning electron microscopy(FESEM).The results show that the different surfactants influenced the morphologies of nano-ZnO by means of electrostatic interactions or absorption interactions. The amount of surfactants would influence the assemble way of the growth unit of ZnO crystal. And the precursor concentration would influence the size and intensive degree of ZnO crystal.Then,using the results of controlling the mophologies of nano-ZnO via hydrothermal method,a new kind of high gas sensing property nano structure composite films composed ofγ-Fe₂O₃ and ZnO was prepared by screen printing process combined with hydrothermal treatment. By changing different kinds of surfactants and precursor concentration in hydrothermal treatment, composite sensitive films with different microstructures were fabricated and the films'sensitivities to ethanol, formaldehyde, ammonia and methylbenzene were tested .The results indicate that the sensitivities to all four gases ofγ-Fe₂O₃/ZnO composite films were higher compared with single-layerγ-Fe₂O₃ gas sensing film and single-layer ZnO gas sensing film .The sensitivities at the best working temperature of screen printing single-layerγ-Fe₂O₃ film F0 to ethanol, formaldehyde, ammonia and methylbenzene are 15.1,3.9,2.2 and 4.0, respectively; The sensitivities at the best working temperature of hydrothermal single-layer ZnO film Z2 to the four gases are only 5.7,1.4,1.3 and 1.2, respectively; The sensitivity of samble F2 synthesized with 0.01g surfanctant CA to ethanol is 65.4 at 320℃,the sensitivity of samble F2 to formaldehyde is 7.6 at 300℃,the sensitivity of samble F5 synthesized with 0.005 g surfanctant CA and 0.033g SDS to ammonia is 5.9 at 320℃,the sensitivity of samble F5 to methylbenzene is 8.8 at 300℃. The perfect working temperature ofγ-Fe₂O₃/ZnO composite films was about 300℃which was lower than that of single-layer ZnO gas sensing film. Theγ-Fe₂O₃/ZnO composite films with different morphologies displayed very different gas sensing properties. The surface area ,the growth position and the surface energy are the three factors.The gas sensing mechanism ofγ-Fe₂O₃/ZnO composite films was also discussed. We consider that two reasons lead to the improvement of the sensitivities ofγ-Fe₂O₃/ZnO composite films to ethanol, formaldehyde, ammonia and methylbenzene. First,because of different gas sensing mechanism ,ZnO andγ-Fe₂O₃ would react with the testing gases in different ways which lead to greater change of the bulk resistant of composite materials.Thus ,the sensing property of composite film is better than that of single film .Second, both ZnO andγ-Fe₂O₃ are nano scale,the nano effect of sensing materials would also make the composite films display better sensing property. In addition, the different structures of different gases lead to the different sensitivities .