Fig. 1. Characterization of PCN-224 and PCN-224-CaO₂-HA. TEM images of (a) PCN-224 and (b) PCN-224-CaO₂-HA. (c) Elemental mapping of Zr, Ca, C, N and O in PCN-224-CaO₂-HA. (d) XPS spectra of CaO₂-HA, PCN-224 and PCN-224-CaO₂-HA. The inset shows the high-resolution XPS spectrum of Ca 2p. (e) Zeta potential of PCN-224 and PCN-224-CaO₂-HA.