Oxalic Acid-Assisted Hydrothermal Synthesis and Luminescent of Hexagonal NaYF₄:Ln³⁺ (Ln = Sm, Eu, Yb/Er) Micro/Nanoplates

Figure 7

SEM images of the NaYF₄ products at different pH value (24 h, 200°C): (a) pH = 3; (b) pH = 5; (c) pH = 7; and (d) pH = 9; insert is the local magnified image of a single microsphere.

(a)

(b)

(c)

(d)