Phase Transformation of GeO₂ Glass to Nanocrystals under Ambient Conditions

Theoretically, the accomplishment of phase transformation requires sufficient energy to overcome the barriers of structure rearrangements. The transition of an amorphous structure to a crystalline structure is implemented traditionally by heating at high temperatures. However, phase transformation under ambient condition without involving external energy has not been reported. Here, we demonstrate that the phase transformation of GeO₂ glass to nanocrystals can be triggered at ambient conditions when subjected to aqueous environments. In this case, continuous chemical reactions between amorphous GeO₂ and water are responsible for the amorphous-to-crystalline transition. The dynamic evolution process is monitored by using in situ liquid-cell transmission electron microscopy, clearly revealing this phase transformation. It is the hydrolysis of amorphous GeO₂ that leads to the formation of clusters with a size of ∼0.4 nm, followed by the development of dense liquid clusters, which subsequently aggregate to facilitate the nucleation and growth of GeO₂ nanocrystals. Our finding breaks the traditional understanding of phase transformation and will bring about a significant revolution and contribution to the classical glass-crystallization theories.