二氧化钛微晶结构相变与光致发光

以钛酸四丁脂为前驱体制备了二氧化钛(TiO₂)胶体,将其粉末在不同温度下作热处理,采用差热分析、X射线衍射、荧光光谱等手段对样品进行测试。结果表明,随着热处理温度升高,TiO₂由板钛矿相经锐钛矿相向金红石相转变,在808℃左右出现一级相变。原始粉末样品以及在600℃以下热处理的样品,在400nm处可观察到TiO₂纳米晶的带边发光,在470nm处可观察到表面态发光;当热处理温度达到850℃时,主晶相转变为金红石相,400nm处的带边发光峰消失,位于470nm处发光峰成为最强峰,但强度减弱,并且样品发光的波长范围明显变窄。其原因是,随着热处理温度升高,晶粒不断长大,量子限域效应减弱乃至消失,晶粒的表面状况发生变化,导致样品的荧光发射行为发生变化。

Structural Phase Transition and Photoluminescence of TiO2 Nano-crystals

Colloidal TiO₂ was prepared by hydrolyzation of tetra-n-butyl titanate.The prepared TiO₂ powder was heat-treated at 400,600 and 850℃ respectively.The original and the heat-treated TiO₂ powder samples were characterized through thermoanalyses(DTA-TG),X-ray diffraction(XRD) and photoluminescence(PL) measurements.DTA-TG and XRD data were used to investigate the structural phase transition and check the crystal types.The XRD data indicated that the original sample and the sample heat-treated at 400℃ were mainly brookite,the sample treated at 600℃ was mainly anatase,and the sample treated at 850℃ was mainly rutile.The DTA-TG data and the XRD data suggested that,as the heat-treatment temperature was elevated,the TiO₂ crystals transformed from brookite to anatase and,finally,to rutile and that a first order structural phase transition occurred at 808℃.On the PL spectra of the original powder and the powder samples heat-treated below 600℃,two emission peaks can be observed at 400 nm and 470 nm respectively.The emission at 400 nm could be assigned to the band to band transition.The weaker emission at 470 nm was associated with the surface states of the crystals.With increasing heat-treatment temperature,the size of the TiO₂ particles increased,and the interband emission peak at 400 nm shifted to red.In other words,as the size of the TiO₂ particles decreased,and the interband emission peak at 470 nm shifted to blue,which arose from the quantum-size effect.As the heat-treatment temperature was elevated to 850℃,the emission peak at 400 nm disappeared,the emission peak at 470 nm became the highest peak,and the emission wavelength range(became) narrower obviously.These phenomena were resulted from the changes of TiO₂ crystalline structures.With the increasing heat-treatment temperature,the size of TiO₂ crystals increased.As a result,the quantum-size effect became weaker and weaker,and the number of the structural defects became less.Therefore,the photoluminescence of the samples exhibited markedly changes.