Preparation And Photocatalytic Performance Of BiOCl Nanosheet-TiO₂ Nanotube Arrays Composites

The water pollution problem caused by the indiscriminate discharge of various organic pollutants continues to affect the development of human ecological civilization.It is considered to be one of the most effective ways to solve this problem that the high efficient photocatalyst is prepared by convenient anodizing process and impregnation method to oxidize and degrade wastewater.In this dissertation,the secondary anodization technology is adopted,and the titanium sheet is used as the substrate to provide atomic titanium,the ammonium fluoride glycol solution is as the electrolyte,and oxygen atoms are provided by the solvent water.Through the action of two external currents,the nanotubes and the metal titanium conductive substrate are firmly combined with the Schottky barrier,thus TiO₂ nanotube arrays are prepared.The differences of secondary oxidation technology and traditional oxidation technology are compared,and the optimal process conditions,including temperature,voltage,electrod gap,NH₄F concentrations and the areas of titanium-based anodes,are explored to obtain better microstructure,appearance,photocatalytic and hydrophilic performance.In order to broaden the photoresponse range of TiO₂,reduce the recombination rate of photo-generated electrons and photo-generated holes,and further improve the photocatalytic performance,a semiconductor composite experiment of BiOCl nanosheets-TiO₂ nanotube arrays is carried out,we want to construct a p-n heterojunction TiO₂-based composite photocatalyst.The photocatalytic mechanism of the composites is explored and the phase and morphology,the element and valence state,specific surface area and pore volume,stability are analyzed.At the same time,the photodegradation performance of the composites with different Bi³⁺concentrations on methyl orange and acid red solutions are studied,the effect of p H on the photocatalytic efficiency and reaction kinetic equations are explored.The specific research results are as follows.(1)High-density polycrystalline anatase phase TiO₂ nanotube arrays are successfully prepared on a titanium substrate.XRD and electron microscope photos show that the crystal phase content is highest along the[101]crystal plane direction,the tube length is about 7?m,and the diameter is about 150nm.The arrangement is regular and the structure is dense,it has great arrangement rules and order.(2)Compared with the traditional anodic oxidation method,the secondary can obtain TiO₂ nanotube arrays with higher purity,better crystallinity,and a more regular arrangement through the process of cleaning the oxide film in the reaction stage,and the reaction time can be directly shortened from 10 h to 2 h.(3)The field-enhanced dissolution theory is used to explain the formation and growth mechanism of TiO₂ nanotube arrays.The reaction process is divided into three stages:oxidation reaction stage,nanotube formation and growth stage,and formation-dissolution equilibrium stage.(4)The optimal preparation process conditions of TiO₂ nanotube arrays are explored.Using the method of controlling variables,it is concluded that under the conditions of 25°C,55V,2cm electrod gap,0.1mol/L NH₄F concentrations and5cm×2cm areas of titanium-based anodes,TiO₂ nanotube arrays exhibit the best crystal phase structure,microscopic morphology,photocatalytic activity and hydrophilic properties.The degradation rates of methyl orange and acid red solutions reach 88.7%and 79.3%,respectively,and the lowest optical contact angle is 6.7°.(5)The BiOCl nanosheet-TiO₂ nanotube arrays composite is successfully prepared on the titanium substrate by impregnation method.XRD patterns and electron microscopy pictures show that tetragonal BiOCl nanosheets are generated,and the nanosheets are about 50nm in diameter,grow along the(110)crystal plane,and they are in clusters that contact directly with the surface of the nanotubes.XPS results show the existence of Ti⁴⁺and Bi³⁺in the composite and prove the coexistence state of BiOCl and TiO₂.The nitrogen absorption and desorption test show that the specific surface area and pore volume of the composite have been greatly improved,reaching87.774m²/g and 0.559cm³/g.(6)The energy band theory is used to explain the photocatalytic mechanism of composite.The composite produces p-n heterojunction which makes the depletion zone,and internal electric field formed at the interface where BiOCl and TiO₂ contact each other,which changes the energy band at the interface and provides a driving force for the separation of photogenerated electrons and photogenerated holes.In this way,the negatively charged photogenerated electrons will transfer from the more negative BiOCl conduction band to the conduction band of TiO₂,and the positive charged photo-generated holes will transfer from the valence band of TiO₂ with more positive potential to the valence band of BiOCl,thus the recombinations of photo-generated carriers are avoided and the photocatalytic performance is greatly improved.(7)The influence of BiOCl content on composite is explored.When the Bi³⁺concentration is 0.03mol/L,the tetragonal BiOCl has the highest crystallinity,and the nanosheets are in contact with the nanotube array while leaving gaps,which provides conditions for building p-n heterojunction and receiving sufficient light sources.Under this condition,the composite meets the first-order reaction kinetics equation and has the maximum degradation rate while maintaining the best degradation rate.(8)The degradation effect of composite under different p H environments is explored,the conclusion is that when the p H value is 6,the composite can exert the best photocatalytic performance.