Hello Zhourui,

I happened to read one paper related to in-situ observation of bubble migration in nanoparticle from Dr. Haimei Zheng. Probably it may help you.

doi: 10.1073/pnas.1510342112

"Bubble nucleation and migration in a lead-iron hydr(oxide) core–shell nanoparticle"

Maybe your sample is not well dried and/or there is some remaining water which boiling under the beam and evaporates from your particles and possibly creates porosity. It could also mean that your sample might be sensitive for the acceleration voltage you are using, although remaining humidity in your sample is more possible since you use water in your synthesis.

Daer Jinming and Antiope,

Thanks for your answers. I think the paper cited by Jinming may explain the phenomenon i encountered. Maybe the composition of bubble-contained nanoparticles is different from what i considered. Maybe some OH occur in the nanoparticles and generate O2 and H2O under electron irradiation.

Best regards,

Ray

I think your sample was not well dried.You can try  with washing with ethanol using centrifuge and make suspension in the ethanol. if Still bubbling persist,you can go through the paper referred by Mr. Jinming Guo,due to some core-shell interaction.