Noble Metal-modified TiO₂ Nanotube Arrays For Photoelectrocatalysis And Degradation Of Methyl Orange

Recently,water pollution control has been considered as one of the focus issue for improving people's livelihood and environment.Due to the advantages of simple operation,high efficiency and no secondary pollution,photoelectrocatalytic technology has shown a good prospect in the field of water pollution control.As a classical photoelectric catalyst,the composite of noble metal/TiO₂ has attracted extensive attention of researchers due to its good response to visible light.It is of great significance to improve the catalytic performance of the material and reveal its mechanism of action for promoting the development of photoelectric technology.In this thesis,we mainly focus on the composite material of noble metal/TiO2nanotubes?NTs?array,and the main work are as follows.Firstly,the preparationof TiO₂ NTs substrate was systematically studied.Taken Ti sheets as raw materials,two kinds of TiO₂ NTs with different morphological characteristics were successfully prepared by selecting appropriate electrolyte,regulating reaction conditions,and optimizing the process parameters.The photoelectric properties of two kinds of TiO₂ NTs were systematically characterized.The results showed that when TiO₂ NTs was prepared in inorganic medium,its light absorption capability was much lower than that of TiO₂ NTs synthesized in organic medium.Meanwhile,compared with TiO₂ NTs prepared in inorganic medium,TiO₂ NTs prepared in organic medium has excellent photoelectrocatalytic performance,and its photocurrent density is about 10 times of the former.Further analysis shows that the use of organic media can significantly reduce the charge transfer resistance of TiO₂ NTs,so that the material has excellent photogenerated electron-hole pair separation ability.The Au/TiO₂ composite was then prepared by thermal evaporation after the Au was supported on the surface of TiO₂ NTs and annealed.A series of composite materials with different Au loads were prepared by adjusting the evaporation time.Modified by this method,Au exists mainly in the form of nanoparticles?NPs?.Au NPs was uniformly distributed on the surface of TiO₂ NTs with narrow particle size distribution.No surfactant,block copolymer and other connecting reagents are used in the modification process.Therefore,this composite material provides a good research platform for revealing the intrinsic action principle of Au/TiO₂ system.The photoelectrocatalytic degradation of methyl orange by the composite was studied.When the Au evaporation thickness is 5 nm,the material has the best degradation effect,and the degradation rate was 75%better than 36.7%of pure TiO₂ NTs with good stability.By characterizing the physical and chemical properties of each sample,it can be seen that when the Au load is low,the composite material has a poor ability to absorb visible light,but photogenerated electrons are more likely to migrate from Au to TiO₂.On the contrary,although the samples with high Au load have strong visible light response,the photoelectron utilization rate is low.5-Au NPs/TiO₂ NTs has both light absorption capacity and quantum efficiency,so it has the optimal photoelectrocatalysis activity.The effects of quenching agents on the degradation rate of methyl orange were investigated,and it was speculated that hydroxyl radical,superoxide anion and photogenic hole all played important roles in the catalytic degradation of methyl orange.Compared with Au,Ag usually has stronger surface plasmon resonance effect and is more cheap and easy to obtain.In this thesis,Ag NPs is modified on the surface of TiO₂ NTs by thermal evaporation technology.The basic properties showed that the composition of the composite was anatase TiO₂ NTs and zero-valent Ag NPs.The Ag load and the size of Ag NPs increased with the time of evaporation.Photoelectrocatalytic degradation experiments of methyl orange showed that the photoelectrocatalytic performance of Ag NPs/TiO₂ NTs composite was closely related to the thickness of Ag evaporation.Among them,5-Ag NPs/TiO₂ NTs had the best degradation performance,and the degradation rate of pollutants increased from 36.7%to 81%at 210 min,which was better than 75%of 5-Au NPs/TiO₂ NTs.The samples of Ag NPs/TiO₂ NTs were analyzed by impedance spectrum,ultraviolet visible diffuse reflection absorption spectrum and other photoelectric chemical characterization methods.The results showed that 5-Ag NPs/TiO₂ NTs not only had outstanding visible light absorption performance,but also showed good electron transmission capacity,making it an ideal photoelectrocatalytic material.Finally,different proportions of Au and Ag NPs were successfully modified on the surface of TiO₂ NTs uniformly by the co-evaporation technology of thermal evaporation.Au/Ag NPs modified TiO₂ NTs can better improve the photoelectric performance of TiO₂ NTs,and 3-Au NPs/3-Ag NPs/TiO₂ NTs presents the best photoelectric enhancement characteristics,which is about 4 times of pure TiO₂ NTs,better than 2.67 times of 5-Au/TiO₂NTs and 3 times of 5-Ag NPs/TiO₂ NTs.The results showed that there was a synergistic effect between Au NPs and Ag NPs.In conclusion,a variety of precious metals NPs were modified on the surface of TiO₂NTs by thermal evaporation in this thesis.The process is simple to operate,and the noble metal particles are uniformly distributed with controllable size.The composite exhibited good photoelectrocatalytic degradation performance,and the degradation rate was closely related to the time of evaporation.This is mainly due to that the coverage and size of the noble metal particle can be changed by adjusting the evaporation time,and then the light absorption and electron transfer properties of composite materials are affected.