Fig. 1: Structural characterization of NBFYE HNPs synthesized by adding 1.5 g PAA and 5 mL H2O to the reaction system.

a Schematic diagram of the structural change from solid to hollow. b TEM image. c Elemental mapping images. d EDS spectra. e SAED pattern. f HRTEM image. XPS spectra: (g) survey spectrum, and (h) high-resolution spectra of Na 1s, Bi 4f, F 1s, Yb 4d, and Er 4d