ICP-AES results showed that the P/Zr of the ZrHEDP samples ranged from 1.8 to 3.0. With the speculation of the schematically represented structure in Fig. )-(e), the ZrHEDP samples are composed of the composite structure of P/Zr = 2 (Fig. )) and 4 (Fig. (e)) with some unreacted zirconia precursor. If phosphonic acid groups in HEDP were not captured in Zr layers, unreacted HEDP should be eliminated in the washing process by water (see methods) because HEDP is water-soluble. Hence, as the P/Zr ratio increases, more phosphonic acid groups should be attached to Zr layers, as shown in Fig. .