Study On Photoelectrocatalytic Activities Of Modified Nano-TiO₂ Photocatalysts And Novel Method For COD Determination

With the high development of human society and economy, environmental pollution in the world has been getting more and more serious. Environmental problems not only restrict the development of humankind but also threaten the human existence. The governments of different coutries are becoming aware of the close relationship between enviroment and human development, attaching more and more importance to pollution control and enviroment protection. Water pollution stands out in the environmental pollution, thus research on water environment is an important part of environment research and has aroused extensive attention. Whether the sewage can reach the discharge standard is determined by the pollution index of the sewage, among which the most important is the COD value of the sewage. Chemical oxygen demand (COD), which displays great significance on the evaluation of water environment, has preferentially been used as a parameter to represent the degree of organic pollution of wastewater. The conventional methods for COD determination is manual and is based on titration of the samples with a toxic reagent such as potassium permanganate or potassium dichromate. This type of standard manual method is time consuming ,involves complicated procedure,has secondary pollution and its accuracy depends on operators' skill. Therefore, an alternative method for COD determination, if it is convenient, time-saving ,accurate,environment friendly and highly sensitive, is most desirable.Since the 1980s, nano-materials has attracted great attention, especially nano-TiO₂, which.has been recognized as the most potential photocatalyst. Under the excitation of light or electricity , nano-TiO₂ can produce strong oxidizing agents such as hydroxide radicals, which can achieve the complete degradation and mineralization of organic compounds and can directly transform the light energy to chemical energy to motivate chemical reaction, by reacting with hydroxyl and/or water on their surface. Thus , the photocatalytic oxidation of TiO₂ exhibits unique characteristics in the degradation of various organic pollutants and reductive inorganic pollutants especially the toxic and harmful substances which are hard to be degraded by organism, and is regarded as an extremely promising technique in deeply purifying environmental pollution. However, TiO₂ exists some disadvantages, such as the high recombination ratio of photogenerated electron/hole pairs, low photocatalytic efficiency , showing photochemical activity only in ultraviolet region because of the high band gap up to 3.2 eV (the utilizing efficiency of the sunlight is less than 10%) . All of these have restricted its application in environment analysis. To broaden the light absorption spectrum of TiO₂, lower the recombination ratio of photogenerated electron/hole pairs and make its Photocatalysis come into practical application , various modifications have been performed on nano-TiO₂ by researchers .In this dissertation, some novel nano-TiO₂ photocatalysts were prepared and characterized, their mechanisms and behaviors in photoelectrocatalyzing organic substances were discussed and a new method for COD determination involving a new reactor for Photocatalysis i.e.a capillary array photocatalytic reactor was developed.Chapter 1 OverviewIn this part, a detail outline and reviews mainly on the preparation, application and development of novel nano-TiO₂ photocatalysts and the current situation and development of the COD determination were described.Chapter 2 Photoelectrocatalytic activity of highly ordered TiO₂ nanotube arrays electrode for azo dye degradationA highly ordered titanium dioxide nanotube arrays (HOTDNA) electrode was prepared by electrochemical anodization technique and applied in photoelectrocatalytic (PEC) degradation of a azo dye i.e.methyl orange (MeO). The linear-sweep photovoltammetry response on the HOTDNA electrode was presented. The photoelectrocatalytic and photocatalytic behaviors of the electrode were evaluated and compared. A set of optimized conditions such as anodic potential,calcinations temperature, and MeO concentration on the PEC activity was investigated.Chapter 3 Preparation and photoelectrocatalytic activity of ZnO nanorods embedded in highly ordered TiO₂ nanotube arrays electrode The ZnO nanorods embedded in highly ordered TiO₂ nanotube arrays electrodes were prepared and used in PEC degradation of azo dye. The linear-sweep photovoltammetry response on the electrode was presented. The photocurrent was dramatically enhanced on the ZnO/ TiO₂ NR/Ts electrode, compared with that on bare TiO₂ NTs electrode. The ZnO nanorods embedding modification can improve the photoelectrocatalytic activity of TiO₂ NTs.Chapter 4 Photoelectrocatalytic fabrication of Au nanoparticles encapsulated in highly ordered TiO₂ nanotube for photoelectrocatalytic applicationWe have demonstrated for the first time the effective photoelectrocatalytic fabrication of Au nanoparticles encapsulated into the TiO₂ nanotube channels that lead to enhancement of photocatalytic and photoelectrocatalytic activity of TiO₂ NTs. Compared to the photocatalytic deposition method, the photoelectrocatalytic deposition method were much more effective and more easily controlled. Considering the fact that the size of diameters of channels can be controlled in the anodic process, the Au nanoparticles of different sizes could be obtained easily. A conclusion can be made that the present method provided the versatile way in fabrication of noble metals on the surface of photocatalytic materials, and the employment of electrochemistry showed promising prospect in the fabrication of nanomaterials.Chapter 5 Development of a capillary array photocatalytic reactorBased on the Photocatalysis of nano semiconductor, a capillary array photocatalytic reactor was developed and successfully applied in COD determination. The inner wall of the quartz capillaries was covered with TiO₂ nano film, thus reaction solution can have a complete contact with the nano photocatalyst in the capillaries. The reactor possessed several advantages such as high Photocatalysis efficiency, high stability, no photocatalyst consumption, low cost, achieving COD determination in water in the room temperature.