| Currently, the world is facing serious energy and environmental problems, so gas sensing, photo-assisted gas sensing, photocatalysis, and photoelectric conversion have caused widespread concern. Metal oxide semiconductor materials have attracted much attention of researchers, due to their wide applications in the above mentioned areas, which are mostly based the conductive behaviors under different external fields. However, there are a lot of controversies on the mechanisms of metal oxides, which seriously affected the further development of their performance in application. For these controversies, one reason is the structures of the materials are different in each report, and another reason is the lack of acknowledge on the conductive mechanism of metal oxide, which is caused by the the lack of method on studying the conductive behaviors under multi-variable external fileds. Therefore, in this work, we chose the porous nanocrystalline film, which has a simple preparation process and an easily controlled microstructure, as our research object. By coupling light, heat, bias, and atmosphere on the conductivity test, we established a method to evaluate the coupling effects of multi-variable external fileds on the conductivities of metal oxides, and clarified the mechanisms of porous nanocrystalline metal oxide films.Firstly, to solve the problems existing in the photoelectric conversion application of TiO2, the potential application of gas sensitized TiO2 porous film was proposed. The conductive behavior of porous nanocrystalline TiO2 film under different bias and formaldehyde atmospheres with different concentrations and different flow rates were systematicly studied. By extracting parameters including carrier life times under different conditions, the coupling effects of UV light and formaldehyde gas on each dynamic process of carriers were analized in detail. The results showed that the substantial increasment of photoconductivity was due to the reduced surface recombination and lower interfacial barrier, which caused by the photocatalytic reaction of formaldehyde. Compared to formaldehyde, the effect of bias on photogenerated electron-hole separation was much less.For the wide use of metal oxide in gas sensing, we introduced the temperature field to explore the effect of heat. On the base of the defect ionization and electron transport theory, we proposed a new and simple method named temperature-programmed-dependent conductivity-based synchronous derivation(TPDCBSD) method, to extract electron concentration, mobility, and a range of other parameters related to the mechanism of gas sensing under different temperature and oxygen conditions. These parameters showed that the major effect of oxygen gas was to change the grain boundary barrier to significantly alter the mobility, while the major effect of heat was to ionize oxygen vacancies and influence the electrons cattering at the interfacial barrier. The thermal desorption of oxygen made mobility increase as elevating temperature, and led the positive coupling effect of oxygen gas and heat. With higher temperature and larger oxygen partial pressure, the coupling effect is more significant, indicating the stronger interactions between oxygen molecule and ZnO, which may bean important reason for the higher working temperature of metal oxide gas sensor.Besides, we studied the coupling effect of UV light/heat/oxygen gas on the conductive behavior of porous zinc oxide nanocrystalline film. A method to access the coupling effect of multivariable fields was conducted. Through a rational experiment design and theoretical analysis based on the dynamics, the influences of UV light, temperature, and oxygen gas on the electron concentration and mobility were isolated, and the important roles of oxygen adsorption-induced interface barriers and photo-assisted thermal ionization were proposed to help illustrate the coupling effect of UV light / heat / oxygen gas. At low temperature, UV light and oxygen gas showed a positive coupling effect. As temperature increasing, the coupling effect of UV light and oxygen gas were gradually replaced by the coupling effect of heat and oxygen gas, due to the enhancement of thermal ionization and thermal oxygen desorption.Finally, with methanol as the typical atmosphere, a systematic study on the coupling effect of UV light / heat / atmosphere containing oxygen and methanol gas on the conductivity behaviors of porous zinc oxide nanocrystalline film was conducted. In the dark, the adsorption of methanol molecule lowered the interfacial barrier of zinc oxide. As the temperature increasing, reaction of the methanol moleculeand the adsorbed oxygen ions made the conductivity increase more rapidly. At high temperature, methanol molecule react with lattice oxygen, which led the rapid increase of electron concentration until the concentrations of electrons and ionized oxygen vacancies were large enough for oxygen atoms to enter into the crystal lattice fast. After introducing UV light, the conductivity increased for several orders of magnitude, due to the reactions of superoxide anion with the holes and the methanol molecular. The contribution of electron concentration was less than that of mobility, which revealed the mechanism of photoconductive and the gas sensitization at room temperature. Under UV illumination, the mobility varied little as temperature increasing since the interfacial barrier was very low in methanol gas, while the electron concentration increased much due to the photo-assisted thermal ionization and the reduced enthalpy of reaction between methanol molecular and lattice oxygen atoms. This explained why the UV light assistant gas sensor had lower operating temperature and higher sensitivity. Furthermore, oxygen molecule was important for the coupling effect of UV light, heat, and methanol gas to play.Our work cleared the mechanisms of metal oxidesfor different applications, and provided guidance for studying the electrical properties of metal oxides and developing performances of porous metal oxide nanocrystaline films. |
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