Study On Band Structure Regulation And Photocatalytic Performance Of Zirconia Nanotubes

With the increasing environmental pollution and shortage of clean water resources,using new technologies to overcome the shortcomings of traditional wastewater treatment methods has become one of the current research hotspots.Photocatalytic technology is an advanced waste water treatment method with great development potential.It has the advantages of low cost,environmentally friendly and sustainability.The key factor restricting its development lies in the development of photocatalysts.Zirconia is an important structural and functional material.Due to its great chemical stability,unique surface properties,and the strong redox ability of hole-electron pairs given by the band structure,it has received potential applications attention in the field of photocatalytic applications.However,as an oxide semiconductor,zirconia has a large band gap,which severely limits its absorption of sunlight and photocatalytic efficiency.Enhancing the visible light absorption and photocatalytic activity of zirconia by modification treatment or constructing new heterojunction photocatalyst system have important practical significance for expanding the photocatalysis application of zirconia.Particularly,nanostructured zirconia has a larger specific surface area,which can provide more chemical reaction sites in the photocatalytic reaction and is more conducive to enchance the photocatalytic performance.In this paper,zirconia nanotubes were prepared by the electrochemical anodic oxidation and the morphology of the nanotubes were controlled by adjusting anodic oxidation parameters.It is found that the electrolyte composition is a key factor in the formation of nanotubes and significantly affects the size of the nanotubes.Moreover,adjusting the anodizing time and voltage can further obtain the orderly nanotube structure.In order to obtain the nanotubes with larger tube diameter and specific surface area for photocatalysis research,it was finally determined to prepare zirconia nanotubes at 50 V for 2 h in a glycerol-based electrolyte with a composition of 0.35 M NH4F,2.5 vol.%H2O,and 2 vol.%HF.By analyzing the microscopic growth process of zirconia nanotubes,it was found that the growth mechanism is not only related to electric field-assisted dissolution,but may also be related to the plastic flow of oxides.In the zirconia nanotubes,the fluorine-containing compounds were formed during the anodic oxidation process,which can decompose during the heat treatment process.In order to enhance the visible light absorption performance,a defective ZrO2-x nanotubes with rich defect structures can obtained by heat-treated in Ar atmoshpere due to the decomposition characteristics of the fluorine-containing compounds.These defect structures generate new impurity energy levels in the band structure of zirconia,thereby narrowing the band gap and enhancing visible light absorption capacity of the defective ZrO2-x nanotubes.The heat treatment temperature can affect the light absorption performance and photocatalytic activity of the defective ZrO2-x nanotubes.The amount of thoes defects in the defective ZrO2-x nanotubes increased with heat treatment temperature and the fluorine-containing compounds were completely decomposed at high heat treatment temperature.Among them,the defective ZrO2-x nanotubes heat-treated at 500? has the best photocatalytic performance under the dual effects of defect structure and fluorine element doping:the degradation rates of rhodamine B and tetracycline hydrochloride under 2 h visible light were 83%and 57%,respectively.To overcome the high recombination efficiency of photogenerated carriers in a single catalyst and further improve the visible light photocatalytic activity of zirconia nanotubes,a heterojunction photocatalyst composed of defective ZrO2-x nanotubes and g-C3N4 were prepared by vapor deposition method.The g-C3N4 content in the heterojunction photocatalysts was adjusted to optimizing the photocatalytic performance.When the g-C3N4 content is 6.5 wt.%,the g-C3N4/ZrO2-x nanotubes heterojunction possesses the highest photocatalytic activity,and the degradation rate of tetracycline hydrochloride under 1 h visible light irradiation was 90.6%,which was much higher than that of pure g-C3N4 and defective ZrO2-x nanotubes.Through photocurrent testing,energy band structure analysis,and in-situ XPS characterization,the photocatalytic degradation mechanism of tetracycline hydrochloride by g-C3N4/ZrO2-x nanotubes was studied.The heterogeneous interface provides a migration path to photogenerated carriers,which is the main reason for inhibiting the recombination of photogenerated holes and electrons.Due to the bending of the energy band structure and the built-in electric field at the contact interface,the ZrO2-x nanotubes and g-C3N4 formed a direct Z-type heterojunction.The photogenerated active species that play a major role in the photocatalytic degradation of tetracycline hydrochloride are photogenerated holes produced by the defective ZrO2-x nanotubes and ·O2-produced by g-C3N4.These free radicals can degrade tetracycline hydrochloride into small molecules and turn into CO2 and H2O eventually.This work has successfully prepared a zirconia nanomaterial with visible light phoyocatalytic properties,which not only expands the application of zirconia in the field of photocatalysis,but also promotes the development of semiconductor visible light photocatalytic technology.