ZnO Based Composites Prepared By Calcining Layered Double Hydroxides And Their Volatile Organic Compounds(VOCs) Sensing Properties

Over the years, gas sensors have been extensively used for detecting toxic, harmful, combustible and explosive gases. Until now, the sensors are widely used in many fields, such as industrial production, environmental monitoring, indoor and the public, food and medical care, which are closely related to human health and safety. Volatile organic compounds (VOCs) are flammable and toxic, and they are seriously damage to human health and the environment. Typical metal oxide gas sensors are belong to a kind of important semiconductor gas sensors, which are convenient to design, cheap to fabricate, easy to detect and possess high sensitivity etc., but there also exist some disadvantages such as poor long term stability and weak selectivity etc. Therefore, it is still necessary to optimize and improve the component of gas sensitive materials. Accordingly, much effort has been paid to improve the sensing properties. Among these, it is proved that to develop multi-components’composite oxides is an important way to improve the gas-sensing properties effectively due to the synergistic effects between different sensing components. Consequently, it has important and practical significance to study the composite metal oxides for detecting volatile organic compounds.This thesis mainly studies the gas sensing properties of ZnO-based composites to volatile organic compounds (VOCs) such as ethanol, triethylamine. The ZnO-based composites have been prepared by calcining Zn-containing LDHs precursors. Firstly, Zn2Al-LDH has been prepared and used as a single source to prepare the composite of ZnO/ZnAl2O4via a calcination method of which the ethanol sensing behavior has been observed as well. The effects of the calcination temperature and the preparation approach on their ethanol sensing properties have been summarized. In addition, new type of Znln-LDH with the intercalated anion of sodium dodecyl sulfate has been prepared by co-precipitation method. Znln-LDH has been used as a single source via calcining at600℃to prepare gas sensing material of ZnO/In2O3. Its gas sensing behavior to triethylamine has been further observed. The factors which influence the performance of triethylamine gas sensing will be discussed thoroughly, such as the contents of indium in LDH precursors, the calcination temperatures, etc. The details and results are shown as follows:1) Zn2Al-LDH precursor has been synthesized and followed by a calcination process at different temperatures of600,800and1000℃,the resulting composite ZnO/ZnAl2O4named as ZSX-600, ZSX-800and ZSX-1000, respectively. The gas sensing measurement indicates that ZSX-1000shows much higher response to ethanol due to its better crystallinity of ZnAl2O4than ZSX-600and ZSX-800, respectively. The response of ZSX-1000to500ppm ethanol is88at its optimal operation temperature (240℃) and is about16times of ZSX-600’s ethanol response and4times of ZSX-800’s ethanol response. The response time to ethanol of was ZSX-1000is1s, which is the shortest one among the three samples (ZSX-600,54s, ZSX-800,5s). In addition, the sample calcined at1000℃exhibits fine repeatability and good selectivity to ethanol in comparison with other gases, such as, acetone, methylbenzene, ethylene glycol, benzene, ammonia etc. In order to illustrate the advantage of preparing method of calcining the LDHs precursors, the composite of ZAH-1000was obtained by physical blending and calcining the corresponding metal salts, which has the same metal contents as ZSX-1000’. It is proved that the composite oxide obtained from Zn2Al-LDH precursor shows well synergistic effect between ZnO and ZnAl2O4. The operation temperature of ZAH-1000is higher than ZSX-1000and the gas sensing sensitivity of ZAH-1000is only1.5to500ppm ethanol until its operation arrives at270℃. Below this temperature, there is almost no response.2) Znln-LDH with sodium dodecyl sulfate (DS) as the interlayer anion has been synthesized by the co-precipitation method. Furthermore, it was calcined at600℃to prepare the gas sensing material ZnO/In2O3, and the triethylamine gas sensing properties were observed. The results illustrate that the gas material possesses fine stability to20ppm triethylamine gas, and it also has high selectivity to triethylamine gas in the range of the following volatile organic compounds (VOCs): ethylenediamine, ammonia, formamide, methanol, ethanol, methyl-benzene, benzene etc. Therefore, it can be drawn a conclusion that the sample calcined ZnIn-LDH at600℃shows fine repeatability and high selectivity to triethylamine gas.Afterwards, we discussed the influence of indium’s content to the performance of triethylamine gas sensing properties. The LDHs has been synthesized with different contents of indium, and were further calcined at600℃, results in ZDM-600, ZIC-2-600, ZIC-3-600, ZIC-4-600, respectively. It can be found that ZIC-4-600shows much higher sensitivity to100ppm triethylamine gas with the sensitivity of60at270℃than that of ZIC-3-600, ZIC-2-600and ZDM-600(45,29,26, respectively) due to the synergistic effect between each component and the larger surface area of ZIC-4-600.Finally, at the effect of calcination temperatures on the performance of triethylamine gas sensing properties was further discussed. Zn4In-LDH was taken for further measurement and was calcined at600,800and1000℃, named as ZIC-4-600, ZIC-4-800, ZIC-4-1000, respectively. It turns out that ZIC-4-600showed much higher sensitivity to100ppm triethylamine gas than that of ZIC-4-600and ZIC-4-800attributing to the decrease of effective gas sensitive materials and the reduction of surface area.