Study On The Crystal Phase Transition And Optical Properties Of Brookite TiO₂

TiO₂ exists in nature as three kinds of crystal phases:brookite,anatase,and rutile phase.The anatase and brookite phases will irreversibly transform into rutile structure at high temperature;the brookite and anatase structures will also transform into each other under certain conditions.In recent years,many documents have reported several synthesis methods of the brukite TiO₂ and their optical performances,but there is few reports on the crystal growth mechanism and the phase transition process of brookite phase TiO₂.Studying the microscopic mechanism of crystal growth can provide theoretical guidance for the controllable growth of brookite TiO₂;besides,structural phase transition is the main driving force of TiO₂ crystal growth,which is helpful to understand the optical properties of the mixed-crystal TiO₂.Therefore,this thesis covers the synthesis of brookite TiO₂ by hydrothermal method,the microscopic growth mechanism of the brookite phase,and the phase transformations among the brookite,the anatase,and the rutile phase.The main conclusions are as follows?1?The formation mechanism of brookite phase TiO₂ during hydrothermal process.Atomic pair distribution function indicates that a small amount of TiO6 octahedra have formed after 3 hours' hydrothermal reaction,and packed in an edge sharing way to form a precursor of TiO₂.As the hydrothermal reaction progresses,the TiO₂ precursor will gradually transform into anatase phase and then into brookite phase.?2?The structural phase transformation of brookite phase TiO₂ during the high-temperature calcination.The brookite synthesized in the hydrothermal reaction starts to transform into anatase phase at 750? with the formation of a small amount of Na2Ti6O13 phase;when the temperature is higher than 850?,it will transform into the rutile phase?3?The ultraviolet-visible absorption spectrum shows that the absorption wavelength of the mixed-crystal phase TiO₂?the anatase and brookite?is in the range of 200-400nm,and the band gap width is about 3.0-3.3eV.After the calcination temperature is higher than 800?,the absorbance of TiO₂ in the visible light range increases with the increase of temperature.For the sample calcinated at 950?,the band gap width reduces to about 1.6 eV due to the effect of the Na2Ti6O13phase.?4?Optical performances of the series of TiO₂ samples.Raman scattering experiments show that the characteristic peaks of brookite increase continuously from 322 cm-1 to 366 cm-1,indicating that anatase gradually transforms to the brookite phase as the elapse of the hydrothermal-reaction time;as the increase of the calcination temperature,the characteristic peak of brookite of 322 cm-1 and 366 cm-1 gradually weakens,indicating that the brookite phase gradually decreases;The photocatalysis experiments indicates that the higher the phase content of the brookite phase TiO₂,the better the photodegradation effect.