A Research Of Characteristics And COD Detection Method On Interdigitated Microband Electrode Arrays Of Boron-Doped Diamond Film (BDD)

For the controlling of the summation of pollutant emission, it is very significant to monitor organic pollutant emissions by using chemical oxygen demand (COD) fast detection. The electrochemical detecting methods of COD, which can be used to detect different types of pollutant systems, is a simple, rapid and environmental friendly way. The work electrodes used in traditional COD electrochemical detecting methods have some obvious shortages, such as easy contamination, poor stability and slow mass transfer. Usually, the detecting values of COD are different from COD_Cr value by those electrochemical methods. All these factors limit the applications of electrochemical methods. So it is actually significant to establish a new and available COD electrochemical detecting method with better relativity to COD_Cr value.In this thesis, with regard to the advantages of boron-doped diamond film (BDD) electrodes, such as non-contamination, wide potential window, and microelectrode arrays with fast response and high current density, the interdigitated microband electrodes arrays of BDD were prepared by standard photolithography technology and a new fast electrochemical method for detecting COD was proposed on the basis of systematically studying on the electrochemical characteristics of the arrays. In this method, the BDD microelectrode arrays were used to replace macro-electrodes in tranditional methods. Considering the strong oxidation ability of Ce⁴⁺ with its standard electrode potential ( ? Cθe 4 +/Ce³⁺) between that of Cr2O72-/Cr3+ and MnO4-/Mn2+, a small amount of Ce³⁺ ions were added in the samples to produce electrically Ce⁴⁺ ions which can be used to oxidate the organic pollutants in the samples. We researched the relationship between the total electric quantity, QT, and COD values by measuring the current-time curves. The COD detecting results are consistent with those of standard potassium dichromate method well. The main work is summarized as following:(1) Preparation, surface modification and electrochemical characteristics of BDD electrodesThe BDD thin films employed in our work were prepared on silicon (100) wafers substrate by using the hot-filament chemical vapor deposition (HF-CVD) technique. In the preparation, acetone was used as carbon source, hydrogen as carrier gas and B2O3 as boron doping source. The BDD films exhibit micron-sized polycrystalline surfaces with moderately boron-doped level.The BDD electrode surface was modified by electrochemical oxidation methods. After anodic oxidation treatment, the BDD surface was transformed from hydrophobic C-H groups to hydrophilic C-O groups. Then a modified BDD electrode was prepared with stable electrochemical characteristics and hydrophilic surface. The experimented conditions of BDD electrochemical oxidation modification were optimized. With regard to Ce⁴⁺/Ce³⁺ couples as positive ions, the classic Fe³⁺/Fe²⁺ and Fe(CN)₆^3-/Fe(CN)₆^4- couples were choosed to analyze and evaluate the characteristics of the prepared and electrochemically modified BDD electrodes. By analyzing the data from the cyclic voltammetric (CV) curves in the two redox systems respectively, a conclusion can be drawn that the prepared BDD and electrochemically modified BDD electrodes exhibit typical quasi-reversibility in the two redox couples. The rate constants of electron transfer reaction, k, were calculated by using the nΔEp data.(2) The research of electrochemical characteristics of interdigitated BDD microband electrode arraysThe interdigitated BDD microband electrodes arrays were prepared by using microelectronic lithography technology. Electrochemical characteristics of single microband electrode arrays (S-MBEA) and interdigitated microband electrodes arrays (I-MBEA) of BDD film are strongly different in CV analysis. The Fe(CN)₆^3-/Fe(CN)₆^4- redox reaction in S-MBEAs is controlled by linear diffusion-limited transport for the overlapping of diffusion layer along the adjacent microband electrodes. But the redox process in I-MBEA working style approached a quasi-steady-state non-linear diffusion to gain S-shape for the intense edge effect of every microband of BDD electrode film. The relationship between current limit, ilim, and sweep rate, v, was obtained by defining ilim as a sum of the current limit from hemispherical diffusion, the peak current from linear diffusion and the other factors. And, the value of current from hemispherical diffusion was the primary one in I-MBEA working style. The current limit in I-MBEA working style is about 1000 times higher than the peak current in S-MBEAs. The charge transfer characteristics at the electrode surface in S-MBEA and I-MBEA working style have been studied by electrochemical impedance spectra (EIS) analysis.(3) The theoretical research of the new electrochemical detecting methode of COD The current equation on interdigitated BDD microband electrode arrays in I-MBEA working style was described as the expression by adding nonlinear diffusion faction in Cottrell equation. The relationship between electric quantity, Q, and time,t, was obtained by integrating the current to t. Comparing the coefficients of t and t1/2 item (defined as A and B respectively) in the Q- t equation, we found that the ratio of B2/A shows a positive correlation to the COD value of the system. So the COD value can be gained from the ratio of B2/A by using linear second-order polynomial fitting for Q-t1/2. Additionally, We calculated the total electric quantity, QT, from I-t curves by integrating along the whole detecting time range. QT shows a positive correlation to the COD value of the system theoretically. So QT can be used to give the COD values too.The I-t curves in the electrochemical cells containing different amounts of glucose and potassium hydrogen phthalate (KHP) were measured by using interdigitated BDD microband electrode arrays. With linear second-order polynomial fitting for Q-t1/2, we ploted a curve of ThOD to B2/A. We studied a standard curve of ThOD～B2/A and QT ～ThOD in the two redox sample systems, the obtained two relationship expressions had a great difference in the two redox system. This can be explained that the electrochemical oxidation of organic pollutants was not only relevant with the diffusion coefficient of the system, but also determined by the efficiency of the working electrode. So, the standard curves are used to detect COD in a single component or fixed multi-component waste water.(4) The research of the new electrochemical detecting methode of COD with electrocatalytic oxidation of Ce⁴⁺/Ce³⁺ couple on BDD microelectrode arraysBy adding a small amount of Ce³⁺ in the solution, the organic matters were oxidated by electrochemically produced Ce⁴⁺ ions and this way was the dominant oxidation in the measure process. The most appropriate concentration of Ce³⁺ ion was 0.8mmol?dm-3. The linear relationship expressions of QT and ThOD from the two sample systems were very close. Combining the linear relationship of QT and ThOD in the two COD standard samples, a complete QT-ThOD standard curve with linear correlation coefficient as 0.975 was gained. We detected the COD values of the actual waste water samples by the new QT method. The detecting results were consistent with those of standard potassium dichromate method. The average recovery rates of 6 times'COD detecting results were falling in 98.5%～106.4%.