Construction Of TiO₂ Nanofiber-based S-scheme Heterojunction And The Performance For Photocatalytic H₂-production

With the arrival of 2020,due to the environmental damage caused by extensive use of fossil fuels,the trend of global warming is becoming more and more serious accompaning with that extreme weather and natural disasters occur frequently.There is an urgent need to alleviate the energy crisis and environmental pollution challenges,and one of the most promising ways is photocatalysis.First of all,hydrogen energy is a green energy with high calorific value and no pollution.In addition,in the photocatalytic system,solar energy is a healthy,environmentally friendly and renewable energy.However,sunlight is mainly composed of visible light and infrared light（accounting for 43%and 52%respectively）.For the star material named TiO₂,because of its large band gap（about 3.2 e V）,it is difficult to use infrared light and visible light,which accounts for the most solar energy so that the quantum efficiency（QE）is very low.However,TiO₂ itself has many excellent properties;it is urgent to modify TiO₂ materials.In the photocatalytic system,in order to make better use of visible light,it is necessary to reduce the band gap to increase absorbance,but the reduction of band gap will inevitably lead to quick photogenerated carriers’recombination.For this reason,on the one hand,a step-scheme（S-scheme）photocatalytic heterojunction is designed to increase the light absorption and reduce the recombination of electron-hole pairs.It is necessary to find a suitable semiconductor to recombine with it.On the other hand,through interface engineering and morphology control,the preparation of one-dimensional（1D）nanofibers（NFs）from traditional TiO₂ nanoparticles not only increases the active area of reaction and enhances the transport capacity of carriers,but also enhances the liquid adsorption and light absorbance capacity because of the formation of hierarchical structure,in which electrospinning is an excellent preparation method.The main contents of this thesis are as follows:Firstly,based on the basic idea of S-scheme heterojunction recently proposed,CdS was first determined as a reduction photocatalyst compounded with TiO₂.A large number of TiO₂ NFs with good morphology were obtained by electrospinning,and then TiO₂/CdS step-scheme heterojunction nanofibers were successfully prepared by in-situ thermal solid-state deposition on TiO₂ NFs.The in-situ X-ray photoelectron spectroscopy（XPS）and the tracking of 5,5-dimethyl-1-pyrroline-N-oxide（DMPO）with hydroxyl radical（·OH）were used to confirm the existence of the built-in electric field after semiconductor contact and the carriers’moving orientation after illumination,which indicated that the heterojunction is a step-scheme mechanism rather than the traditional type-Ⅱmechanism.In the performance test,the fact that the composite sample is not loaded with precious metals（such as Pt,etc.）implies higher photocatalytic hydrogen production performance.The QE of hydrogen production of the composite sample is nearly 20 times higher than that of the pristine TiO₂ NFs.Secendly,the morphology of nanofibers has a great influence on the performance of photocatalytic reaction,and the adjustment of its morphology involves many complex synthesis strategies such as the compound of 1D amd 2D.Thus,we using tetrabutyl titanate as electrospinning solution.A number of pure TiO₂NFs were synthesized by electrospinning,and then 1D/2D TiO₂/Cd_0.98In_0.04S_1.04step-scheme heterojunction nanofibers were synthesized by ion deposition in oil bath.Cd_0.98In_0.04S_1.04 has strong light absorption capacity and good photocorrosion resistance,with the hierarchical structure formed by 1D nanofiber and 2D nanosheet great advantages in photocatalytic hydrogen production reaction.The performance is better and the activity is more stable.The formation mechanism of the built-in electric field was determined by XPS and calculating their work functions.The high charge carriers’transfer of nanofibers and the high carrier separation ability of step-scheme heterojunctions were confirmed by electrochemical impedance spectroscopy.Finally,the concentrations of·OH and superoxide radical were detected by DMPO,and the related structures were in good agreement with the step-scheme heterojunction mechanism.