Fabrication Of Nano-Film Photocatalysts And Electrochemical Determination Of Photocatalytic Degradation Productions Or Intermediates Of Pesticides

The pesticide wastewater is more difficult to remove from wastewater due to its hugeamount, complex components, and high toxicity. In view of energy shortage, the investigationof an efficient, low energy consumped pesticide wastewater treatment technology issignificantly important. In recent years, photocatalytic technology has been studied widely inthe field of organic wastewater treatment. However, owing to its low quantum efficiency andphotocatalytic activity, this technology has not been applied in practice. Therefore, we havedeveloped an approach to enhanced the photocatalytic efficiency of TiO₂nanotube membraneby depositing noble metal on the surface of the TiO₂nanotube membrane. Besides, we haveinvestigated the fabrication of nano-porous Ta₂O₅films which can be used as a precursor forpreparation of nitrogen oxide tantalum.It is well known that the degradation products or intermediates are also important asmuch as the parent compound of pesticide due to their high toxicity or high risk. For example,phosphate is one of the products when organic phosphorus pesticides degrade. Phosphate willbe harmful if the content in water reaches a certain level. Hydrazine is another hazardouschemical that can be used as a raw material for the herbicide or fungicide production. Atpresent, there are some methods to detect phosphate and hydrazine such as spectrophotometryand luminescence measurement. Although these methods are normally accurate, they arevulnerable to water color, turbidity, and other factors. Electrochemical analysis possessesmany advantages including simple operation, sensitive test, fast speed, and so on. However, inthe previous works, there were still some problems in practic application, such as bad stability,short service life, harsh testing conditions and complex processes for preparing electrodes.Therefore, the other purpose of this work is to explore a more sensitive and stable electrode inorder to determine hydrazine and phosphate in aqueous solution. The main content is asfollows:（1） We have prepared a TiO₂nanotube film by anodic oxidation, and then modified itwith Au-Pd bimetal. The effect of the bimetal deposition on photocatalytic activity wasinvestigated using malathion, an organic phosphorus pesticide, as a probe. The experimentalresults suggest that the morphology of TiO₂nanotube film modified with Au-Pd （gold richalloy） by the photoreduction method is similar with that of original TiO₂nanotube film. Thephotocatalytic experiments indicate that the catalytic activity of the Au-Pd modified TiO₂nanotube membrane （Au-Pd-TiO₂） is significantly higher than that of the naked TiO₂nanotube, and the reaction rate increases by1.72times. The energy consumption per order isreduced remarkably. The improvement of catalytic activity is mainly attributed to the deposition of Au-Pd alloy. On one hand, the photogenerated electrons can be rapidlytransferred to the Au-Pd, thereby inhibiting the photogenerated charge carriers recombination;on the other hand, the electrons on Au-Pd can be trapped readily because molecular oxygencan form superoxide radical anions （O₂） and hydrogen peroxide （H₂O₂）. Thus, the life ofphotogenerated hole is prolonged, which is of great benefit to the oxidation reaction. Thereduction reaction can also be improved due to the fact that the yield of H₂O₂increased by1.89times after depositing Au-Pd bimetal.（2） We have also prepared nano-porous Ta₂O₅films by anodic oxidation inorganic-inorganic hybrid electrolytes. The influences of preparation conditions on the filmgrowth were investigated using SEM, XRD, etc. The results demonstrate that water/phosphatecontent and fluorine ion content are two essential parameters in anodic oxidation processes.Ta₂O₅film exhibites a dense, lamellar morphology at a higher water or phosphate content.When the water content is decreased, i.e.10%water with the rest of ethylene glycol as theelectrolyte solution （3%ammonium fluoride）, nano-porous Ta₂O₅films appeare uniformly.The pores interlace each other with an average diameter of30-50nm and a depth of150-200nm. The film is contacted with the substrate closely and firmly. Undoubtedly, owing its highsurface area, the nano-porous Ta₂O₅film will be far superior to a dense, lamellar film. Aftercalcination, amorphous Ta₂O₅can be crystallized orthorhombic crystal system. The absorptionin the short wave ultraviolet light region is improved greatly.（3） Poly-malachite green （PMG） was electrochemically polymerized on a glassy carbonelectrode. The results indicate that PMG is well contacted with the glassy carbon electrode,and the reaction between them is controlled by electron transfer kinetics. On the surface ofPMG modified electrode, the redox of phosphomolybdic acid, a electroactive substanceformed by phosphate and ammonium molybdate, is mainly controlled by mass transfer, whichis the theoretical basis of the voltammetric detection of phosphate. The linear range for thevoltammetric detection of phosphate is greatly expanded due to PMG modification. The linearrange of the method for differential pulse voltammetry （DPV） is0.05-10.0mg·L-1, which ismore than10times wider than that for the standard photometric method. In addition,compared with that of the bare glassy carbon electrode, the detection limit of PMG modifiedelectrode is lower and reach the national standard photometric level （0.01mg·L-1）. Thevoltammetric detection of phosphate （especially DPV） using PMG modified electrode canprovide a lower detection limit and a wider linear range, which is mainly derived fromexcellent characteristics of the conductive polymerized film PMG, such as charge transfer, iontransport, and catalytic ability.（4） The electrochemical behavior of hydrazine sulfate was investigated on a boron dopeddiamond （BDD） electrode under a high potential. At the same time, the voltammetricdetection of hydrazine sulfate was also carried out. The results demonstrate that hydrazine sulfate can be directly oxidized on the BDD electrode. The reation is controlled by the masstransfer process. The number of electrons involved in the reation is four. Electrochemicalimpedance studies indicate that the charge transfer resistance of hydrazine oxidation issubstantially smaller than that of oxygen evolution. The charge transfer resistance can bedecreased from200kΩ to300Ω （about3orders） by adding hydrazine sulfate into solutionsunder the same conditions. The linear range for DPV detection of hydrazine sulfate is dividedto two sections,0.002-0.1mM and0.1-4.0mM, while the sensitivity in different segments isnot same. The BDD electrode is more sensitive to hydrazine when its concentration is lowerthan0.1mM. One mmol of hydrazine could produce55.75μA signal current. The currentdecreases to33.50μA in the range of0.1-4.0mM. The reponse value deviates from the linearwhen hydrazine sulfate concentration exceed4mM. In addition, the parallel experimentresults confirme that this method could be well repeated.