Time-Resolved Spectroscopy And Dynamics Of Photocatalytic Titanium Dioxide-Based Photocatalyst

The conversion of solar energy into chemical energy driven by the semiconductor photocatalysis has drawn great attention.It provides a promising method to solve the energy and environmental issue we faced today.During the photocatalytic process,the generation and separation of the charge carriers play an essential role in linking the light-harvesting and the surface catalytic reaction.While,this process always occurs in the timescale from femtosecond to nanosecond.Therefore,time-resolved ultrafast spectroscopy is always employed to reveal the underlying kinetic mechanism.In this dissertation,we investigated two typical Ti O₂-based photocatalysts by using our home-made time-resolved transient infrared absorption spectroscopy,focusing on the key components:the kinetics of photogenerated carriers,which is usually affected by defects in the semiconductors.The main results are as follow:（1）By using nanosecond time-resolved transient infrared absorption-excitation energy scanning spectroscopy（TRIRA-EESS）and femtosecond time-resolved transient mid-IR absorption spectroscopy,we investigated a boron-doped anatase Ti O₂microspheres（i.e.,B-free surface Ti O₂ and B-containing surface Ti O₂）.B-free surface Ti O₂ shows excellent hydrogen evolution reaction（HER）in photocatalytic water splitting reaction,but less effective in oxygen evolution reaction（OER）.While B-containing surface Ti O₂ gives high activity in OER and poor production in HER.We characterized these two boron-doped Ti O₂ microspheres and find that surface boron doping can eliminate the trap states above the valence band significantly.The photogenerated holes can migrate to the surface freely,which can cause effective OER.However,the surface boron dopant can also act as a recombination center for the photogenerated electrons.It also leads to inefficient electron transfer to the co-catalyst Pt.Both of these effects give rise to their inferior photocatalytic capability in HER.（2）Reduced Ti O₂（110）single crystal was obtained by annealed the pristine stoichiometric rutile single crystal in the vacuum.The exitance of the excess electron has been demonstrated by Raman spectroscopy,X-ray absorption spectroscopy,etc.By employing femtosecond time-resolved transient IR absorption-excitation energy scanning spectroscopy,we studied the distribution and the kinetics of the excess electrons in reduced rutile Ti O₂（110）single crystal.By tuning the excitation wavelength from UV to visible,to infrared region,we find three different kinds of electron transition.（1）VB electrons transition to the CB under UV excitation;（2）trapped electrons transition from E_Fs(E_Fs denotes the Fermi level of trapped electrons in the bandgap)to the unoccupied states below the CB under visible and near-IR excitation;（3）polaron transition under mid-IR excitation.Additionally,our study also presents the midgap energy levels of the trapped electrons together with their relative densities in the reduced Ti O₂（110）.This dissertation reiterates the importance of defects and midgap states in the Ti O₂-based photocatalysis and the detailed knowledge of electronic structure will be useful guidance for constructing more efficient photocatalysts in the years ahead.