Preparation And Application Of Novel Nano-titania Photocatalyst For Chemical Oxygen Demand Determination In Water Research

Since humanity entered the 21st century, with the increasing severe environmental pollution, great attention has been paid to water pollution and its treatment in China. The water pollution is a large aspect of the environmental pollution, water pollution is serious in China, therefore, research on water is one of the most important subjects in environmental research and have attracted great attention of the world. 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 method 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 and its accuracy depends on operators' skill. Therefore, an alternative method for COD determination, if it is convenient, time-saving and low-cost, is most desirable.In recent years, nano-materials has attracted great attention, especially nano-TiO₂, which.has been recognized as the most potential photocatalyst. With its special photoelectrical characteristics, nano-TiO₂ can produce strong oxidizing agents such as hydroxide radicals, which play an important role in degradation of organic compounds, by reacting with hydroxyl and/or water on their surface. Therefore, nano-TiO₂ have shown promising prospect in environmental detection. However, its properties, not only the photo-efficiency or activity (the utilizing efficiency of the sunlight is less than 10%(only 3%-5%)), but also the photoresponse, are not sufficient. Meanwhile, the high recombination ratio of photogenerated electron/hole pairs also reduces its catalytic efficiency. Therefore various modifications have been performed on nano-TiO₂ to promote its catalytic ability and develop new photocatalytic functions.In this article, some novel nano-TiO₂ photocatalysts were prepared and characterized, the photoelectrocatalytic mechanism of nano-TiO₂ is studied, a new method for COD determination based on the mechanism of photocatalysis of nano-TiO₂ is described. The established methods have good behaviors and their results are well correlated with those using conventional methods. Thus, they display a great potential in COD on-site monitoring.Chapter 1 OverviewA detail outline and reviews of the current theories and methods for COD determination were described. The photocatalytic mechanism and recent application progresses of nano-TiO2 photocatalyst used in the environmental area were also presented. Developing trends of nano-Ti02 photocatalytic materials were further prospected.Chapter 2 Preparation of TiO₂ nanotubes photoelectrochemical sensor and its application in the detection of chemical oxygen demandTiO₂ nanotubes have high photocatalytic activity because of their large specific surface area. They were always used for photoelectrocatalytic degration of organic compounds, to our knowledge, there is no paper published about the COD detection with TiO₂ nanotubes. Here, a novel photoelectrocatalytic sensor through the fabrication of TiO₂ nanotubes by electrochemical anodic oxidation on a pure sheet to detect COD was proposed. Compared with the conventional dichromate method, this method that we developed in this paper requires no hypertoxic reagents and less analysis time, which suggested that it would be another appropriate method for COD determination in water assessment.Chapter 3 Preparation of the nano-ZnO/TiO₂ film sensor and the application in the detection of chemical oxygen demandA new method for determination of low COD value in surface water using the photocatalytic oxidation technology was described. The nano-ZnO/TiO₂ film promoted the separate efficiency of the charge and extended the range of spectrum, then showed a higher photocatalytic efficiency than the single nano-TiO₂ film. This method was originated from the direct determination of the Mn(VII) concentration change resulting from photocatalytic oxidation of organic compounds on the nano-ZnO/TiO₂ film, and the COD values were calculated from the absorbance of Mn(VII). Under the optimal operation conditions, the detection limit is 0.1mg/L, COD values with the linear range of 0.3～10.0 mg/L were achieved. The results were in good agreement with those from the conventional COD methods.Chapter 4 Preparation of QD-CdS/TiO₂ composite electrodes and their high photoelectrocatalytic activity for the degradation of methyl orangeIn order to enhance the photocatalytic efficiency of TiO₂, QDs-CdS were coupled with TiO₂ and QDs-CdS/TiO₂ modified electrode was prepared. The characteristic of charge transfer and the light-converting efficiency were also studied. Methyl orange was chosen as a model compound and the experiments demonstrated that the degradation efficiency of methyl orange and the photocatalytic efficiency of QDs-CdS/TiO₂ were increased. The QDs-CdS/TiO₂ composite modified electrode has long lifetime and can improve the photogenerated holes and electrons separation, so it may have promising application in the areas of photocatalysis, the storage of solar energy and the solar cell.Chapter 5 Preparation and application of QD-CdS/TiO₂ photoelectrochemical sensor for chemical oxygen demand determination in water researchA photoelectrochemical sensor for chemical oxygen demand (COD) determination based on photosensitized TiO₂ with QDs-CdS has been developed. Compared with bare TiO₂ electrode, the QDs-CdS/TiO₂ composite films electrode exhibited higher photoelectrocatalytic oxidation to organic pollutes. Under the optimizing conditions, the sensor responded linearly to the COD of D-glucose solution in the range of 15-600 mg/L with a linear correlation coefficient of 0.9998. Real sample analyses were also carried out. The results demonstrated that the measured COD values using the sensor and the standard methods were in an excellent agreement.