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Preparation and Application of ZnO and Silver-based Electronic Materials

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Tutor: HeGaoHong; Yulin Deng
School: Dalian University of Technology
Course: Chemical Engineering and Technology
Keywords: Emulsion liquid membrane,ZnO,AgI,Diode,Photoelectric device
CLC: TN313.5
Type: PhD thesis
Year:  2013
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Abstract:
Design and preparation of novel electronic materials have attracted great interest to meet the demands of flexibility and miniaturization of rectifier diodes and photoelectric devices. It can be achieved through designing the structure of devices based on the physical and chemical properties of materials. This thesis was separated into two parts for fabricating novel rectifier and photoelectric devices. In the first part, ZnO particles with different band gap were prepared via two kinds of liquid phase methods (LP) and further fabricated into heterojunction diode. In the second part, silver-based electrolyte materials were prepared and fabricated into non-water ionic diode. Based on the electrochemical properties and precipitation reaction of Ag-I system, rectification behavior was achieved in the PEG electrolyte. At last, the electrochemical properties of Ag-I system were employed to design AgI photoelectric devices with a long-time running.ZnO is a kind of important semiconductor for preparing electric components, of which the electric property strongly depends on the band gap. Emulsion liquid membrane (ELM) method and direct precipitation method (DP) are similar LP methods based on precipitation reaction but some difference due to the influence of W/O interface of the emulsion. Herein, ZnO particles with band gap of2.93eV were prepared by ELM method where the nucleation process and the morphology and property of the particles are controlled by the nature of W/O interface of the emulsion. The effect of W/O interface properties on the morphology and size of the ZnO particles were proved by investigating the growth mechanism of the particles with SEM and XRD. The morphology of the particles varied from rod-like to sheet-like when the size of internal water droplets was increased from3to5¦Ìm. It was further demonstrated that the surfactant tended to be absorpted on the particles by XRD and interfacial tensiometer studies, which could induce the impurities and defects into the material. The ZnO particles prepared by ELM method exhibit a similar crystal structure to those synthesized by DP method but a different band gap of0.13eV, which fits the requirement of heterojunction formation. Therefore, a ZnO heterojunction diode with a sandwich structure was fabricated with the ZnO prepared by the two methods and exhibited rectifying behavior.The performance of inorganic semiconductor diode is approaching the limits of the physical peroperty of materials. Ionic diodes as the next generation rectifier are expected to replace or extend the function of inorganic semiconductor diode, and have attracted a great deal of attention. Ionic diodes strongly depend on water-based electrolyte which is volatile and instable at high temperature. Therefore, non-water ionic diodes need to be studied for application. In this work, PEG400was chosen as electrolyte host due to its stable property and the ability of ionic dissociation and transport. But its high viscosity caused all reported methods based on nanofluidic channels etc. can not be employed to prepare ionic diode. Therefore, a novel method was demonstrated using Ag+-I-ion pair which had a low electrolysis voltage and could precipitate with each other. The ionic diode was fabricated into sandwich structure, in which AgNO3/PEG and LiI/PEG electrolyte were separated by porous membrane. Under forward bias voltage, Ag+and I-moved to the electrode on their side respectively, and electrolytic reaction occurred to generate current. In this case, the device was conductive. Under backward bias voltage, Ag+and I-moved to their counter electrode and were blocked due to precipitation. Meanwhile, the electrolytic reaction of NO3-and Li+counter ions pair could not happen because they needed a very high applied voltage. The device was unconductive at that time. Therefore, the ionic diode exhibited recitification function. The open-circuit voltage is0.3V. Breakdown voltage is-2.7V. And recitification ratio is40under AC voltage between¡À1.5V, which is higher than most of reported ionic diode. The device could work more than300cycles. The principle of this study is general and can be extended to the fabrication of ionic diodes with different inorganic or organic salt.The appearance of PEG from liquid to solid state could be achieved through increasing their molecular weight. That could be employed to prepare solid state ionic diode. Herein, flexible solid state electrolyte material was prepared using PEG20000as electrolyte host and paper fibers as supporter because PEG20000had poor strength for self-supporting. The flexible paper-based solid state ionic diode was fabricated by AgNO3and KI electrolyte materials. Its recitification ratio is46under AC voltage between¡À1V. The devices exhibited good flexibility and showed stable rectifying behavior after more than800bending cycles. Its standing stabilitytime was32days. Adding10wt%PC as plasticizer could increase the forward current by3times but decreased the standing stabilitytime to less than4days. The results showed that there was a trade-off between rectifying performance and stability.The electrochemical properties of Ag-I system could be extended to prepare AgI photoelectric device for improving its stability (several minutes reported). A novel method was demonstrated to prepare long-time running AgI photoelectric device. The structure of dye-sensitived solar cell and Ag-I2chemical battery enlightened us to fabricate the device to be sandwich structure using ITO glass deposited with AgI film as photoelectrode, Pt as counter electrode and I/I3-/PC solution as electrolyte. A light shield was added between two electrodes for avoiding incident light arriving at the counter electrode. Photochemical reaction could decomposite Agl to be element Ag and I2on the photoelectrode under illumination. Oxidation of element Ag with I and reduction of I2occurred to make the device to be Ag-I chemical battery and outputed current. Agl regenerated due to the oxidation reaction in this process. Theoretically, the device could be operated for a long time without reactants consumption. The prepared device could output0.7¦ÌA/cm2current density with a rapid photoelectric response (1.2s) and kept the performance after47hours continuous illumination. Light shield avoided short-circuit caused by photochemical reaction of AgI at the counter electrode. Comparing the device without light shield, the current was higher by17times. Pt counter electrode could increase the current by7times and accelerate the photoelectric response rate due to its catalytic property. The device could generate different current density under various wavelength or intensity of incident light. Therefore, it exhibits the potential to prepare photodetector.
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