Effect Of Defects On Photoelectrochemical Performance For G-C₃N₄/TiO₂ Nanocomposite

Titanium oxide（TiO₂）,compared with other semiconductor catalysts,has received much attention due to its strong oxidizing ability,better ultraviolet absorptivity,good dispersion,nontoxicity,high chemical stability and other advantages.In various forms of TiO₂,the well-organized array structure of TiO₂ nanotubes exhibits unique photoelectrochemical properties due to larger specific surface area,better adsorption capacity and convenient for recycling.However,the photocatalytic efficiency of TiO₂ is significantly limited due to its wide bandgap.The main approach to improve photocatalytic performance of semiconductor is to design and construct composite semiconductor materials.Graphitic carbon nitride（g-C₃N₄）,as a new semiconductor,can be combined with TiO₂ to increase the transfer of photogenerated carriers and the utilization rate of light.However,the type-II composite semiconductors have relatively weak reduction and oxidation capacity.The subject is worth investigating on heterostructure construction with both fast separation efficiency of electron-hole and strong redox ability by the effective charge transport at the interface of composite semiconductors.The g-C₃N₄/TiO₂ nanocomposite with interfacial vacancy were constructed using TiO₂ nanotube arrays using TiO₂ nanotube arrays as precursor.The effect of different defects and the thickness of vacancy layer at the interfacial of g-C₃N₄/TiO₂ on the optical absorption,photoelectrochemical performance and visible photocatalytic degradation of pollutants was thoroughly studied.The main work was divided into three parts:（1）The g-C₃N₄/OV-TiO₂ heterostructures containing an interfacial oxygen vacancy layer were sequentially constructed by anodic oxidation,NaBH₄ reduction,and vapor deposition methods.The effect of oxygen vacancy layer thickness on the photoelectrochemical properties of g-C₃N₄/OV-TiO₂ nanocomposite was also studied.SEM images indicate that the structure of TiO₂ nanotube arrays kept stable after NaBH₄reduction.The UV-vis DRS results shows that the g-C₃N₄/OV-TiO₂ nanocomposite exhibits enhanced visible light-harvesting ability compared with that of single TiO₂ or g-C₃N₄/TiO₂ samples.XPS and ESR results show that oxygen vacancy were formed on the surface of TiO₂ by NaBH₄ reduction.When the concentration of NaBH₄ aqueous solution is 3 M,the photocurrent of g-C₃N₄/OV-TiO₂ reached a maximum（0.7 mA/cm²）at 1.23 V vs.RHE,8 times higher than that of g-C₃N₄/TiO₂ without interfacial oxygen vacancy layer.Compared with single TiO₂ and g-C₃N₄/TiO₂,g-C₃N₄/OV-TiO₂ exhibits the highest photocatalytic activity for methyl orange degradation under visible light,and the photocatalytic degradation rate reached almost100%within 60 minutes.The time-resolved fluorescence decay spectra further indicate that the existence of interfacial oxygen vacancy is beneficial to the transfer of photogenerated carriers between g-C₃N₄ and TiO₂,thus contributing to the improvement of photochemical properties of composite semiconductors.The mechanism of g-C₃N₄/OV-TiO₂ nanocomposite was analyzed by fluorescence capture experiment and density functional theory（DFT）calculation.（2）The particle size of g-C₃N₄ has significant effect on the photochemical properties of g-C₃N₄/OV-TiO₂ nanocomposite.Therefore,the protonated g-C₃N₄/OV-TiO₂ nanocomposite（p-g-C₃N₄/OV-TiO₂）can be obtained by protonating g-C₃N₄ with HCl and then coupled with OV-TiO₂.The SEM images show that the g-C₃N₄ nanosheets become thinner and smaller in size after protonation treatment.The XRD images indicates that the characteristic peak of g-C₃N₄ shifts slightly to a low angle at about 27.4°.This is due to the loosening of the layered structure caused by H⁺intercalation between g-C₃N₄ layers in HCl after protonation treatment of g-C₃N₄.The results of photoelectrochemical properties show that the photocurrent density of g-C₃N₄/OV-TiO₂ is higher than that of g-C₃N₄/OV-TiO₂ when the concentration of hydrochloric acid is 6M,which is about 1.4 times higher than that of g-C₃N₄/OV-TiO₂,p-g-C₃N₄（6 M）/OV-TiO₂ exhibits the highest photocatalytic activity of methyl orange degradation under visible light irradiation.（3）Nitrogen-vacancy g-C₃N₄/TiO₂ nanocomposite（TiO₂/g-C₃N_X-Y）was obtained by deposition the nitrogen-vacancy g-C₃N₄ on the surface of TiO₂ nanotube arrays by chemical vapor deposition.The nitrogen-deficient g-C₃N₄ was prepared by treating urea with potassium hydroxide solution-assisted route.Form XRD spectra it can be seen that the characteristic peak of（100）crystal plane of g-C₃N₄ is weakened after alkali solution treatment compared with pure g-C₃N₄,which show the structure of triazine unit in the plane of g-C₃N₄ changed and the g-C₃N₄ containing nitrogen defect was successfully obtained.From the N 1s XPS spectra of TiO₂/g-C₃N_X-Y,it can be seen that the peak of C-N=C decreases gradually with the increase of KOH content,which indicates that nitrogen defects are formed on the surface of g-C₃N₄.The photocurrent density of TiO₂/g-C₃N_x-0.006 is the highest,which is about twice as high as that of g-C₃N₄/TiO₂.TiO₂/g-C₃N_x-0.006 exhibits the highest photocatalytic activity under visible light irradiation among all samples.Nitrogen vacancy concentration has an important effect on the photochemical performance of all samples.The appropriate nitrogen defect concentration as the interfacial vacancy could promote the transfer of photoconductive carriers in TiO₂/g-C₃N_X-Y nanocomposite,which is more conducive to the improvement of photocatalytic performance.