The paper linked below deals with similar experimental results and should be helpful. It appears that irradiation can cause doping of the material which causes the colour change.

http://pubs.rsc.org/en/content/articlepdf/2016/pp/c6pp00167j?page=search

Dear Keith Fraser,

Thanks for your kind assistance. But please notice that my experient was carried out in vacuum, so there was not any other element except Ti and O in my system. From my point of view, it must arise from the electronic structure change of P25 itself (Ti4+ to Ti3+ or generation of oxygen vacancies?). But what's the mechanism behind it？

Dear Xue:

Cause generation of oxygen vacancies.

See

J. Appl Spectroscopy, 2012. V.79 N5.

Best regards.

Dear Tihomir Nikolaev Tsanov and Yuri Mirgorod

Thank both of you very much for positive responses. I have carried out a comparison experiment in which I set the wavelength of the light source > 420 nm and interestingly, the color remained white without any change. In other words, only the UV light contributes to the color change. So I guess because the energy of UV light is greater than the bandgap of TiO2, it may excite the transition of electrons from VB to CB of TiO2 and then such photo-generated electrons can interact with Ti4+ in TiO2 to form Ti3+. This is my personal thought and needs further experimental evidences to support. But at last, if just as Mr. Yuri Mirgorod said, it is due to the generation of oxygen vacancies, how are they formed? Any suggestions from others are also welcome.

I think I remember that this effect known in literature. The reaction leading to excess electrons as mentioned in your response normally leads to a blue coloration of the powder. You have to look for tio2 and esr measurements.

If you do not solve the problem, it will inform the formation of oxygen vacancies in personal correspondence.

Regards.

I am going to characterize my sample carefully. Thank all of you for help.