The Designing And Studying Of Photoelectric Gas Sensing Prototype Device On ZnO Nanoparticles

With the development and requirement of science and technology, many new types of functional materials appear including nanostructure semiconductors. In comparison with bulk, the photic, superior magnetic, catalytic and transport properties of materials are greatly changed as the size and dimension are minished to nanoscale, which exhibits widely applications. Recently nanostructured ZnO materials have received broad attention due to their distinguished performance; one of the research focuses is in the field of gas sensors.According the researches of recent progress on semiconductor gas sensors, we find most commercial sensor based on particulate or thin-film operate at 200-700℃to make the concentration of carriers attain a certain value to gain sufficient sensitivity. High temperature also can help to enhance the surface molecular desorption kinetics and continuously "clean" the sensor. The high temperature operation of these oxide sensors is not favorable in many cases, particularly in an explosive environment. New approaches must be explored in order to overcome this drawback and fulfill the requirement of the market. Therefore, a new type of energy should be considered instead of heating. In this thesis, ZnO nano-materials are used to explore a new photoelectric gas sensing original device owing to its distinguished photoelectric performance. The surface photocurrent-gas sensing system is established, which can be used at room temperature to detect different gas by the change of photocurrent signals. As we all know, ZnO is a wide band-gap compound semiconductor. Electrons can be excited to generate photoelectric effect only the energy of light is higher than 3.35 eV (λ< 370 nm). It is also not suitable for practical application. One of our research emphases is how to extend the photoelectric effect into visible light region. Microscopic mechanisms of the photoelectric gas sensor based on ZnO nanoparticles have also been discussed in detail in this thesis. These investigations supply the necessary experimental evidence and theory foundation for the application of ZnO-based functional materials.The main results are illuminated as follows:1. ITO comb-like electrode was designed firstly and used in the photoelectric gas sensing detecting system. We introduced the setup in detailed and gave the possible factors, i.e. the concentration of carriers and the height of potential barriers, which could affect surface photocurrent.2. ZnO nanoparticles and nanoribbons were synthesized by sol-gel method and muticomponent precursor solution method, respectively. The photoelectric properties were studied by surface photovoltage/photocurrent spectrum. The results showed the size and dimension of grain affect on transfer speed of photoinduced carriers and intensity of photoresponse. Under the illumination of 370 nm, the surface photocurrent was increased by adsorption of both CO and oxygen.3. Based on the comprehension of Dye Sensitized Solar Cell (DSSC), ZnO nanoribbon was sensitized by azo pigment (4, 4'-2-{N-[β-hydroxyl-γ-(ρ- nitroanilino) naphthyl]}azodiphenyl) to extend the photoelectric response into visible light range. One of significant factors for the generation of photocurrent is that the energy levels of ZnO and dye must match. The energy levels can be measured by means of electrochemical method. Under the illumination photoinduced charges transfer at the interface between ZnO and azo particles, resulting the generation of surface photocurrent. The experiment of the gas probe shows that different gas adsorbates have different effects on photogenerated charge carriers. The adsorption of ethylenediamine takes effect on the producing course of photogenerated carriers under the illumination of 370 nm, whereas affects on the transfer process of carriers with the light of 545 nm. We firstly found that one adsorption gave different effects on photoinduced charges under the illumination of different light. Meanwhile the light of 370 nm will cause azo-ethylenediamine compound dissociating, inducing desorption of ethylenediamine from the surface of ZnO/As-Bs system. These results will provide theoretical foundation for designing a new type of photoelectric gas sensor; exhibit the latent capacity of modified ZnO nanoparticles in the field of photoelectric devices.4. In Chapter 5 RuN3 and CuPc were used to sensitize ZnO nanoparticls to detecting gas at room temperature with visible light. The result indicated that two factors affect the surface photoconductance: one is the concentration of carriers in the conductance band and the other is the height of barriers between grains. In the system of ZnO/RuN3, CO molecule may share its lone pair electrons with the oxygen vacancy of the ZnO surface and has an effect of"donor", causing the increasing of photocurrent. O2 as an acceptor can directly capture the electrons on the surface. The concentration of electrons in the conductance band of ZnO decreases and grain-boundary potential barriers increases. These two factors lead to a diminishment of photocurrent intensity after adsorption of oxygen. Surface photocurrent ZnO/CuPc also showed repeated and reliable response to oxygen gas under visible light (545 nm) at room temperature. Photocurrent signals of ZnO/RuN3 and ZnO/CuPc decreases and increases, respectively, when exposed to oxygen, owing to the different microscopic mechanism for the surface charges between oxygen molecules and dyes. Charge-transfer interaction takes place between CuPc and oxygen molecules, inducing the change in the ZnO/CuPc conductance. Such a phenomenon does not occur for ZnO/RuN3. Therefore, from these experiments, we can conclude that the photocurrent response of dye-sensitized ZnO is highly dependent on the properties of the dyes. These results have important implications in the design of ZnO photoelectric sensors for use at room temperature, given the extremely simple fabrication method and a high sensitivity of the potential device.5. Co doped ZnO nanoparticles and nanofibres were successfully synthesized by muticomponent precursor solution method and electricspinning technology, respectively. The chemical and photoelectric properties were characterized. The results showed that Co2+ ion was doped inside the lattice of ZnO and photoelectric response was extended into visible range. Under the illumination of visible light, photocurrent responses were influenced by the properties of adsorbate. Lots of photogenerated electron-hole pairs were created under the excitation of 370 nm with high activity. From our experiment it was proved that only in the light of 370 nm the adsorbate could leave from the surface completely. Both Co doped ZnO nanofibres and nanoparticles showed a very short response and recovery time to oxygen under the illumination of Xe lamp.The properties of metal oxide semiconductor can be modified by doping. In this thesis Co doped ZnO nanoparticles were explored. The possible effect of co-doping that is simultaneous doping of ZnO with two or more transition metal ions, are interesting. Combined with the high conductivity, ferromagnetic exchange and photoelectric properties that can be achieved by doping, this leads to applications in many fields. In our group some singnficent work is still undergoing.