Fig. 3

i XRD patterns of the prepared 2%Er³⁺, 20%Yb³⁺-codoped BaYF₅ synthesized at 200 °C for 24 h, (a) solvent is ethanol, (b) solvent consists of 95% ethanol and 5% PEI, (c)–(e) citric acid as surfactant was added, the ratio of CIT to Y is 1:1, 2:1, and 4:1, respectively. The standard XRD pattern of BaYF₅ (JCPDS no.46-0039) is also given for conducting the comparison. ii UC emission spectra of BaYF₅ prepared. (a) Solvent is ethanol, (b) solvent consists of 95% ethanol and 5% PEI, (c) CIT/Y = 1:1, (d) CIT/Y = 2:1, (e) CIT/Y = 4:1. iii SEM images of the samples synthesized via adding different surfactants with different concentrations. (A) 5% PEI, (B) CIT/Y = 1:1, (C) CIT/Y = 4:1