Preparation And Application Of Novel Nano-photocatalysts For Chemical Oxygen Demand Determination

Environmental pollution has been a difficult problem in the high development of China economy,especially the water pollution and the water pollution is an important part of it.Whether the sewage can reach the discharge standard is determined by the pollution index of the sewage,among which the most important is the Chemical Oxygen Demand(COD)value of the sewage.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,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 and low-cost,is most desirable Therefore,an alternative method for COD determination,if it is convenient, time-saving,accurate,environment friendly and highly sensitive,is most desirable. With its special photoelectrical characteristics,nano-semiconductor oxides can produce strong oxidizing agents including oxygenic radicals which can realize the total mineralization of the organic pollutants in the water.Based on these characteristics,we can expect a rapid and real time monitoring of organic pollutants, thus providing a brand new concept and perspective for monitoring environmental waters.In this dissertation,some novel nano-TiO₂ photocatalysts were prepared and characterized.In this article,the photoelectrocatalytic mechanism of nano-TiO₂ was studied,a new method for COD determination based on the mechanism of photocatalysis of nano-TiO₂ was described.This dissertation falls into 4 sections:Part 1 Overview(Chapter 1)In Chapter 1,we attempt to give an overview of the development of TiO₂ nanomaterials and to review the current literature in terms of its potential application in determination of chemical oxygen demand. Part 2 Photocatalytic mechanism of nano-TiO₂ and its composite materials and its application in determination of chemical oxygen demand(Chapter 2,3)The reactive properties of electron-hole pairs were generated in the TiO₂ under illumination of light whose energy is greater than the semiconductor band gap. Meanwhile,these charge pairs can easily recombine if no suitable scavengers exist in this system.Hence,inorganic oxidant as electron acceptor is easily to receive the electrons and prohibits the recombination of holes and electrons.In Chapter 2,a new method for determining chemical oxygen demand(COD) value in water using ion chromatography coupled with nano TiO₂-K₂S₂O₈ co-existing system was described.The photocatalytic oxidation system and nano TiO₂-K₂S₂O₈ co-existing system could degrade the organic compounds in water.All sulfur-containing species in the reactive solution were eventually transformed to sulfate which could be determined by conductivity detector in ion chromatography. The change of conductivity of sulfate was proportional to COD value.The optimal experimental conditions and the mechanism of the detection were discussed.The application range was 10.0-300.0 mg l^-1and the lowest limit of detection was 3.5 mg l^-1.It was considered that the value obtained could be reliably correlated with the COD value obtained using the conventional methods.In Chapter 3,a composite nano-ZnO/TiO₂ film as photocatalyst was fabricated with vacuum vaporized and sol-gel methods.The nano-ZnO/TiO₂ film improved the separate efficiency of the charge and extended the range of spectrum,which showed a higher efficiency of photocatalytic than the pure nano-TiO₂ and nano-ZnO film.The photocatalytic mechanism of nano-ZnO/TiO₂ film was discussed,too.A new method for determination of low chemical oxidation demand(COD)value in ground water based on nano-ZnO/TiO₂ film using the photocatalytic oxidation technology was founded.This method was originated from the direct determination of the Mn(â…¦) 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(â…¦).Under the optimal operation conditions,the detection limit of 0.1 mg l^-1, COD values with the linear range of 0.3-10.0 mg l^-1were achieved.The results were in good agreement with those from the conventional COD methods.Part 3 Preparation of TiO₂ nanotube arrays and its composite materials and study their photoelectrochemical properties(Chapter 4,5,6,7,8) In Chapter 4,a highly ordered titanium dioxide nanotube arrays(TiO₂ NTs) electrode was prepared in hydrofluoric acid solution by electrochemical anodization technique on a pure titanium sheet.The TiO₂ NTs electrode was characterized by FE-SEM microscopy,X-ray diffraction,and UV-vis spectra.The linear-sweep photovoltammetry response on the TiO₂ NTs electrode is presented in this work.The photogenerated current of 0.3 mA/cm2 was observed significantly upon illumination with applied potential of 0.5 V in the UV regions at the central wavelength of 253.7 nm.Photoelectrocatalytic(PEC)and photocatalytic(PC)activities of the TiO₂ NTs electrode were evaluated in the degradation of methyl orange(MeO)in aqueous solution.A set of optimized conditions such as anodic potential,calcinations temperature,and MeO concentration on the PEC activity was investigated.The PEC and PC activities of TiO₂ NTs electrode were compared.We concluded that the TiO₂ NTs electrode was an effective photoelectrode for achieving an enhanced MeO degradation.In Chapter 5,the degradation of azo dye,methyl orange(MeO)in aqueous solution with sonophotoelectrocatalytic process was investigated.The TiO₂ nanotubes were used as electrode in photoelectrocatalytic(PEC),sonophotoelectrocatalytic (SPEC)processes or as photocatalyst in photocatalytic(PC),sonophotocatalytic(SPC) processes,respectively.Experimental results showed that the hybrid processes could efficiently enhance the degradation efficiency of MeO,and followed pseudo-first-order kinetics.At the optimized experimental conditions,the rate constants of decolorization of MeO were 0.0732 min^-1for SPEC process;0.0523 min^-1for PEC process,0.0073 min^-1for SPC process and 0.0035 min^-1for PC process. The rate constants obviously indicated that there existed synergistic effect in the ultrasonic,electro-assisted and photocatalytic processes.In Chapter 6,the ZnO nanorods embedded in highly ordered TiO₂ nanotube arrays(ZnO/TiO₂ NR/Ts)electrodes were fabricated through two steps:(1) electrosynthesis of TiO₂ nanotube arrays(TiO₂ NTs)in HF solution by anodization method;(2)followed by cathodic electrodeposition of ZnO embedded in the TiO₂ nanotube arrays.The morphological characteristics and structures of ZnO/TiO₂ NR/Ts electrodes were examined by field-emission scanning electron microscopy(FE-SEM), energy dispersive X-ray(EDX)spectroscopy,X-ray diffraction(XRD)analysis,and UV-vis spectra.The linear-sweep photovoltammetry response on the ZnO/TiO₂ NR/Ts electrode was presented and the photocurrent was dramatically enhanced on the ZnO/TiO₂ NR/Ts electrode,comparing with that on bare TiO₂ NTs electrode.The photocatalytic and photoelectrocatalytic activity of ZnO/TiO₂ NR/Ts electrode was evaluated in the degradation of methyl orange(MeO)in aqueous solution.In Chapter 7,we demonstrated in this paper that polypyrrole encapsulated in highly ordered TiO₂ nanotubes array(PPy/TiO₂ NTs)electrode could be obtained via two steps:(1)electrosynthesis of TiO₂ nanotube array in HF solution by anodization method;(2)followed by electropolymerization of pyrrole encapsulated in the TiO₂ nanotubes array.The morphological characteristics and structures of PPy/TiO₂ NTs electrode were examined by field-emission scanning electron microscopy,X-ray diffraction,and energy dispersive X-ray spectroscopy.The photoelectrochemical properties were studied in a three-electrode cell with 0.5 M Na₂SO₄ solution as supporting electrolyte.The photocurrent was dramatically enhanced on the PPy/TiO₂ NTs electrode,comparing with that on bare TiO₂ NTs electrode.A photocurrent density of 1.52 mA cm^-2and the incident photon to electron conversion efficiency of 73.21%were obtained under an UV illumination of 10 mW cm^-2.In Chapter 8,for the first try,an effective and versatile technique of photoelectrocatalysis(PEC)was developed to encapsulate Au nanoparticles in highly ordered TiO₂ nanotube arrays(Au/TiO₂ NTs),and the honeycomb-like structure has been formed.Compared to the photocatalytic(PC)deposition,the photoelectrocatalytic deposition showed higher deposition efficiency and the electrochemical process played an important role in the formation of Au nanoparticles. The PEC method can be regarded as a versatile way to degrade noble metals on photocatalytic nanomaterials.Part 4 Development of capilary array photocatalytic reactor and its application in determination of chemical oxygen demand(Chapter 9)In Chapter 8,develop a novel capillary array photocatalytic reactor(CAPCR) and apply in determination of chemical oxygen demand.The CAPCR was designed using of capillaries,thus obtained much more surface area for catalyst coating per unit reactor volume and much better contact between target compounds and photocatalyst, consequently for much more specific reactor capacity.UV-vis and SEM techniques were used to characterize the TiO₂ film in capillary.The innovative points of the dissertion1.Ion Chromatography was firstly used in determining chemical oxygen demand (COD)value of water and gained satisfactory results.2.A new photocatalytic system,nano-ZnO/TiO₂-Mn(â…¦)coexisted system was firstly applied to determining chemical oxygen demand(COD)value of water,its COD detection limit is 0.1 mg l^-1,proved to be a rapid,accurate,sensitive method for COD determination,can determine the low chemical oxygen demand(COD)I with satisfactory results.3.TiO₂ nanotubes photoelectrocatalytic sensor was fabricated with the electrochemical anodic oxidation technique and firstly applied to azo dye degradation.TiO₂ nanotubes can get improved photocatalytic efficiency as a result of its large specific surface area.Based on the TiO₂ nanotubes,a series of novel photocatalytic materials such as ZnO-nanorods/TiO₂ nanotubes,PPy/TiO₂ nanotubes,Au/TiO₂ nanotubes were fabricated.4.A new method,a hybrid processes of ultrasonic,electrochemical and photocatalytic oxidation was firstly used in the degradation of organic materials in water.During the method,we used TiO₂ nanotubes as electrode and combined photoelectrocatalytic and ultrasonic technique.Methyl orange was used as model pollutant to test the catalytic efficiency of the method,and then a obvious synergistic effect was found between these technique involved in.5.The Sol-gel method was used to cover the inner wall of the quartz capillaries with TiO₂ nano-film,and then a capillary array photocatalytic reactor was assembled. Methyl orange was used as model pollutant to study its photoelectrocatalytic kinetics.The reactor was used in the COD determination.