| The increasingly serious energy crisis and environmental pollution are the biggest challenges facing mankind to realize the vision of sustainable development.The development and utilization of clean,environmentally friendly and sustainable solar energy is one of the effective ways to solve energy and environmental problems.The design and development of high-efficiency semiconductor photoelectric conversion materials is the key to transform solar energy into electric energy and chemical energy.The most notable disadvantage of wide band gap Ti O2semiconductor photocatalyst is that it has no response to visible light,which seriously affects its utilization rate of solar energy.The results of doping,recombination and other methods make the photogenerated charge behavior and catalytic mechanism of Ti O2-based semiconductor more complex,but the current research is not in-depth.Therefore,this paper focuses on the photogenerated charge behavior and catalytic mechanism of Ti O2-based semiconductor.Firstly,the behavior of photogenerated charge at the interface of Ti O2/CdS heterostructure was investigated by using surface photovoltage technology,which verified the generation mechanism of Z-scheme in Ti O2/CdS heterostructure for the first time.Furthermore,based on the first-principle study of Co-Ti O2band structure and Di-（2-ethylhexyl）phthalate（DEHP）molecular orbital,the theoretical mechanism of Co-Ti O2photogenerated charge excitation and photocatalytic degradation of DEHP were obtained,which provided a theoretical basis for the design and development of Co-Ti O2with good visible light photocatalytic performance.Finally,the theoretical mechanism was verified by the experimental study of Co-Ti O2visible light photocatalytic degradation of DEHP,which provided the theoretical and technical basis for the development of semiconductor photocatalyst for the degradation of phthalate esters plasticizer.The research content,results and conclusions of this paper are as follows:1.Ti O2/CdS heterostructure was constructed by simple method,which could effectively avoid the influence of the special morphology or special surface on the interface of Ti O2/CdS heterostructure.The excitation,separation,migration and recombination behavior of photogenerated charges of Ti O2/CdS heterostructures were investigated by using surface photovoltage（SPV）,photoacoustic（PA）and transient photovoltage（TPV）.The results show that due to the difference in the photogenerated electrons diffusion rates of Ti O2and CdS,the photogenerated electrons of Ti O2with faster diffusion rate migrate to the interface of Ti O2/CdS heterostructure firstly,and accumulate over time,the charge balance in the space charge region on one side of Ti O2is broken,and the two phases of Ti O2and CdS form a built-in electric field.Under the action of the built-in electric field,the photogenerated electrons accumulated in the Ti O2conduction band recombine with the photogenerated holes in the CdS valence band and radiate phonons.The photogenerated electrons of CdS with slower diffusion rate migrate to the interface of the Ti O2/CdS heterostructure and accumulate continuously over time.The charge balance in the space charge region on the CdS side is broken again.The two phases of Ti O2and CdS form a built-in electric field again.Under the action of the built-in electric field,the photogenerated electrons accumulated in the CdS conduction band can migrate to the Ti O2conduction band.The mechanism of Z-scheme in Ti O2/CdS heterostructure were first verified from dynamics and thermodynamics,and it was proposed for the first time that Z-scheme in Ti O2/CdS heterostructure competes with the action of the built-in electric field in conventional semiconductor heterostructure for the transfer of photogenerated charge at the interface of the semiconductor heterostructure.This study provides a theoretical basis for the design and development of semiconductor heterostructure materials with Z-scheme,which has guiding significance for the practical application of photocatalysis of semiconductor heterostructure materials.2.Firstly,the band structure and state density of Co-Ti O2were calculated by first principles.The results show that the doping of transition metal Co element introduces impurity level between the conduction band and valence band of Ti O2,which makes the band gap of Co-Ti O2narrow obviously.The impurity level is composed of the 3d electron orbital of Co atom,the 3d electron orbital of Ti atom and the 2p electron orbital of O atom.The electrons occupying the impurity level above the Co-Ti O2valence band can absorb visible light and directly transition to the Co-Ti O2conduction band,and retain the photogenerated holes in the impurity level.The impurity level below the Co-Ti O2conduction band can act as the intermediate energy level.The electrons in the Co-Ti O2valence band absorb visible light and transition to the intermediate energy level.Then the photogenerated electrons occupying the impurity level can transition to the Co-Ti O2conduction band,and retain the photogenerated holes in the Co-Ti O2valence band.The first-principles study based on Co-Ti O2band structure verified the influence of impurity level on the photogenerated charge excitation behavior of Co-Ti O2,which provides a theoretical basis for the design and development of Co-Ti O2materials with good visible light catalytic performance.Furthermore,the highest occupied molecular orbital（HOMO）,lowest unoccupied molecular orbital（LUMO）and electrostatic potential surface（EPS）of DEHP molecules were calculated by first principles.Through the maximum value of FEDHOMO2+FEDLUMO2,it is determined that 1C atom in DEHP molecule is vulnerable to·OH radical attack,and the branch chain connected by 1C atom in DEHP molecule is prone to fracture.EPS of DEHP molecule also determines that 1C atom of DEHP molecule is vulnerable to·OH radical attack.The first-principles study of photocatalytic DEHP mechanism based on DEHP molecular orbital provides theoretical basis for photocatalytic degradation of phthalate plasticizers.3.A series of mesoporous Co-Ti O2microspheres were prepared by sol-gel method and hydrothermal method,which the doping amount of transition metal Co element was 0.25～1.0 at.%,and the photocatalytic degradation of DEHP was studied.The results of structural characterization and spectral analysis show that the mesoporous Co-Ti O2microspheres have visible light response ability,and the band gap of the mesoporous Co-Ti O2microspheres becomes narrower with the increase of the doping amount of Co element.The band gap of the 1.0%-Co-Ti O2microspheres is the narrowest,and the band gap is 2.71 e V.The surface of mesoporous 1.0%-Co-Ti O2microspheres had mesoporous channels with pore size of 6 nm,pore volume of 0.63cm3·g-1and specific surface area of 170 m2·g-1.Mesoporous 1.0%-Co-Ti O2microspheres have oxygen vacancies,which can effectively inhibit the recombination of photogenerated charges as trapping traps.The results of visible light photocatalytic degradation of DEHP showed that mesoporous 0.25%-Co-Ti O2microspheres had the worst degradation effect,and the photocatalytic degradation rate was only 22.8%.The mesoporous 1.0%-Co-Ti O2microspheres had the best degradation effect,with a photocatalytic degradation rate of 79.8%,which had good stability and could be recycled in photocatalytic reaction.Through gas chromatography-mass spectrometry analysis,the product of DEHP degradation by Co-Ti O2microspheres under visible light was 2-ethylhexyl benzoate（EHB）.The mechanism of photocatalytic degradation of DEHP is as follows:Firstly,·OH radical attacks 1C atom in DEHP molecule,and the branched chain connected by 1C atom breaks,generating acyl carbocation and phenyl radical.Then,OH-ion reacts with acyl carbocation to form 2-ethylhexanol and CO2.Finally,·O2-radical reacts with phenyl radical to form photocatalytic product EHB.In this paper,the first-principle study of photocatalytic degradation DEHP mechanism based on Co-Ti O2band structure and DEHP molecular orbital was verified by visible light photocatalytic degradation DEHP experiment,which has guiding significance for the development of Ti O2-based semiconductor visible light catalyst for the photocatalytic degradation of phthalate esters plasticizer. |
PDF Full Text Request