Ultrafast Time-resolved Spectroscopy Of Zinc Oxide Based Semiconductor Photocatalysts

Inspired by photosynthesis of Nature,aim to transform the energy of sun to energy-rich compounds,many types of semiconductor photocatalysts artificial photosynthesis catalytic systems are designed.The associated photo-induced energy and charge transfer process involve lots of active sites spatially and span a multiple time scale temporally.The improvement for photocatalytic efficiency of the semiconductor photocatalysts is directly related to the light absorption efficiency,charge transfer and charge separation efficiency and interfacial reaction efficiency.To further optimize the material design,in this thesis,the time-resolved spectroscopic technology with multi-time scale was utilized to research the recombination kinetics of photogenerated charge carriers in semiconductor photocatalysts after the pulsed laser excitation.ZnO is a semiconductor photocatalyst with hexagonal close-packed structure and a bandgap of 3.37 e V,it is a polar semiconductor with the internal polar electric field.For ZnO,there are lots of property shared with the photocatalytic prototype material of Ti O₂,however,the photocatalytic property of ZnO is great different with Ti O₂.Based on the wide utilization and several unclear catalytic mechanisms for ZnO,in this thesis,by time-resolved spectroscopic technology,the kinetics of photo excited carriers in ZnO are systematically researched to reveal the micro-mechanism.Our main findings are listed as below:(1)Metal-semiconductor heterojunction is thought to improve the charge transfer and charge separation and ultimately the catalytic efficiency,especially for Ti O₂,the lifetime of photogenerated electrons is extended for more than one order of magnitudes in which after loaded with noble metal co-catalyst.However,the selective rule of noble metal co-catalyst for semiconductor ZnO is not yet determined.Hence,we studied the charge transfer and separation between ZnO and noble metal co-catalysts Ag,Au and Pt utilized time-resolved transient absorption spectroscopy.It is found:for Ohmic contact of Ag-ZnO nanoparticles,the photo-excited free-like electrons are transferred to Ag nanoparticles from the semiconductor ZnO nanoparticles and the corresponding transfer rate is 1/14.1 ps^-1.Whilst,the electron transfer process has not been observed for the Schottky contact of Pt-ZnO and Au-ZnO nanoparticles.Moreover,the charge transfer from the shallow-trapped defects to noble metal co-catalyst is much slow from ZnO,at the rate of 1/10 ns^-1.Furthermore,we proposed a configuration of dual-metal co-catalyst for improving the charge separation and the photocatalytic activities.Thus our results provide a novel idea for the selective rule of noble metal co-catalyst.(2)The carrier relaxation and recombination process from the photo-excited polaron determines carrier lifetime and further influence the catalytic efficiency of photocatalytic materials.A polaron refers to the electrons coupled to the distorted surrounding lattices by their charge.For the polaron in ZnO,the research at femtosecond time resolution is absent.To realize the ultrafast time resolution and the excitation and detection with energy states selective at the same time,the femtosecond time-resolution spectroscopy with scanned energy excitation and board-band detection is utilized to research the carrier kinetics of the polaron excited from the oxygen vacancy in ZnO.By exciting the bandgap states with scanned energy,it is found when excitation energy is below bandgap and above 3.18 e V,shallow bound defects are excited.When excitation energy is further decreased,polaron excitation is found,and after about 1 ps,the excited polaron is transformed to free electrons states in conduction band,then the released free electrons undergo trapping and recombination at long timescale.The oscillation signals on the kinetics of excited polaron on the frequency of 50 GHz infers the coupling of electron states with lattice acoustic phonon.It indicates that polaron is useful for improving the photocatalytic efficiency by the much extended lifetime of carriers after polaron excitation comparing with bandgap excitation.