A topotactic structural transformation synthetic route for beta-NaYF4 upconversion nanocrystals from YF3 at room temperature

Phase engineering is a crucial issue in nano science and technology, because the physiochemical properties of nanomaterials are first determined by their phase structures besides their size, shape, and surface chemistry. Herein, we described a new topotactic structural transformation synthetic route for hexagonal phase beta-NaYF4 upconversion nanocrystals to address the unavoidable formation of intermediate cubic phase alpha-NaYF4 in the early stage during the aqueous-based nanogrowth process. We found that the high topotactic structural matching of [010]YF3//[0001]beta-NaYF4 induced the in situ topotactic transformation from YF3 to beta-NaYF4, bypassing the formation of the alpha-NaYF4 intermediate phase at room temperature. Besides, we also studied the effects of synthetic parameters on the nanogrowth of beta-NaYF4 nanocrystals, and the upconversion emission properties were also investigated. Our synthetic strategy shows that topotactic structural transformation can act as an efficient tool towards phase engineering for modern nanomaterial science.