Heterojuntion Construction And Visible-light-driven Photocatalytic Activity Of Titanium Dioxide-based Photocatalysts

Recently,TiO₂ has been widely used in governing various environmental pollutants and developing new energy due to its many advantages,such as low-cost preparation and nontoxicity.But some defects still exist in TiO₂,including weak visible light absorption property,large band gap and high recombination of photoinduced electron-hole pairs.These problems seriously restrain the photocatalytic activity and applications for TiO₂.Up to now,many methods have been used aiming to solve these problems,for example metallic or non-metallic doping,surface modification and coupling with other semiconductors.Among these methods,the last one has attracted more attentions because of the synergetic effect between TiO₂ and other semiconductors,which is beneficial for the separation of photoinduced electron-hole pairs and improving the photocatalytic performance of the composites.In this paper,according to the structure designing and heterojunction construction strategies,these novel TiO₂-based photocatalysts with different heterojunction structure had been prepared.We then carefully studied the affects of heterojunction structure on enhancing the photocatalytic activity of composites under visible light irradiation.The main contents are shown as following.?1?The TiO₂ nanobelts was synthesized as matrix materials,then the Sb₂WO₆nanoparticles were decorated onto the surface of TiO₂ nanobelts through another hydrothermal method.Finally,we prepared a series of Sb₂WO₆/TiO₂ composites with different content of Sb₂WO₆.The SEM and TEM images show that Sb₂WO₆nanoparticles are uniformly distributed on the surface of TiO₂ nanobelts compactly,which facilitates the construction of heterojunction between Sb₂WO₆ and TiO₂.The heterojunction could promote the separation of photoinduced carriers.The results of UV-vis.DRS show that the visible light absorption performance of composites is much higher than that of TiO₂,and the light absorption edge of composites also shifts to longer wavelength.The PL results illustrate that the recombination of photoinduced electrons and holes is restrained obviously,and the 30%Sb₂WO₆/TiO₂ has the lowest fluorescence intensity.The experiments for the degradation of RhB solution under visible light irradiation show that the 30%Sb₂WO₆/TiO₂ sample has the best photocatalytic activity.Its kinetics constant is 0.476 h^-1,which is 22 times higher than that of pure TiO₂.The recycling experiments prove that the Sb₂WO₆/TiO₂ composites have high stability in photocatalytic process.?2?The spherical morphology r-TiO₂ was prepared by a solvothermal method in ethanol solution for the first time,and ascorbic acid was used as reducing agent.We used g-C₃N₄ as modified materials to combine with r-TiO₂,and then prepared g-C₃N₄/r-TiO₂ composites.The EPR spectrum of composite shows the characteristic peak of oxygen vacancy,demonstrating the existence of r-TiO₂ in the composites.The SEM images show that the r-TiO₂ microspheres are distributed on the surface of g-C₃N₄ sheets.The UV-vis.DRS illustrate that the 15%g-C₃N₄/r-TiO₂ has the highest visible light absorption performance,and its band gap is 1.71 eV.The N₂absorption-desorption analyses show that the composites belong to mesoporous materials,and have large surface area.The EIS analyses indicate the g-C₃N₄/r-TiO₂has lower resistance,which promotes the transfer of photoinduced electrons and holes.The RhB degradation experiments show that the 15%g-C₃N₄/r-TiO₂ sample has highest photocatalytic activity from which 88.6%of the RhB could be degraded under visible light irradiation within 90 min.Its kinetics constant is 1.339 h^-1,which is higher than those of g-C₃N₄ and r-TiO₂.The fourth time recycling experiments prove that the composite has a good stability.Through the results of trapping experiments,it can be found that the main active species are h⁺and?OH in the photocatalytic process.Based on these results,the novel Z-scheme carriers transfer mechanism has been confirmed.?3?We used 15%g-C₃N₄/r-TiO₂?denoted as S1?obtained from the above experiment as matrix materials to composite with Ag₃VO₄,and finally prepared a ternary Ag₃VO₄/g-C₃N₄/r-TiO₂ composite?denoted as Ag₃VO₄/S1?by precipitation method.Then SEM and TEM images show that Ag₃VO₄ nanoparticles are well distributed on the surface of S1 without conglomeration.The UV-vis.DRS show that all Ag₃VO₄/S1 composites have strong visible light absorption performance due to the introduction of Ag₃VO₄ in composites,and the 30%Ag₃VO₄/S1 has the highest visible light absorption intensity.The degradation experiments of MB solution reveal that the photocatalytic performance of composite has been enhanced.30%Ag₃VO₄/S1 has the optimal photocatalytic activity from which 97.6%of the MB could be degraded under visible light irradiation within 90 min,and its kinetics constant is 0.0339 min^-1.The recycling experiments prove that the Ag₃VO₄/S1 has a good reusability and stability.Studying the photocatalytic mechanism,we find the Ag₃VO₄ could act as the bridge for electron transfer,which promotes the separation of electrons and holes during photocatalytic process.In these experiments,some of novel semiconductor materials,including Sb₂WO₆,g-C₃N₄ and Ag₃VO₄ with high chemical stability and suitable bang gap,have been chosen as the modified agents to couple with TiO₂ catalyst,respectively.Using the structure designing and heterojunction construction strategies to overcome these defects of TiO₂-based photocatalysts.These results powerfully prove that both structured microstructure and heterojunction not only can extend the light response range of TiO₂ to enhance its visible light absorption,but also could promote the transfer and separation of photoinduced carriers to improve its photon utilization.Therefore,this paper could provide some references on structure designing,materials selection and heterojunction construction for studying high-efficiency photocatalysts,as well as it promotes the practical applications of photocatalytic technique in the future.