Table 3
The most important XPS results reside in the shifts of the binding energy (BE) of the key elements (oxygen, silicon and aluminum)(Table 3) providing evidence of the occurrence of metal interaction with the host surface. For oxygen, the BE value decreased from 532.55 eV down to 531.00 eV and 527.00 eV upon Cu and Ag incorporation in NaMt samples, respectively. Similar sequence was noticed in H20@CT, but with much weaker BE decrease from 533.00 eV down to 531.65 eV and 531.74 eV after Cu and Ag incorporation. This suggests weaker metal retention in this organic composite. Such a BE weakening indicates lower electron binding to their parent-atom due to an attraction by next-neighboring species, and can be explained in terms of Lewis-Acid-Base (LAB) interaction between lattice oxygen and MNPs. This indicates weaker interaction with CuNPs as compared to AgNPs in both materials, and weaker retention strength of both metals in H20@CT.
BE decay was also noticed for Si and Al atoms from 1383.20 to 103 and 1411.29 to 75, respectively, suggesting additional MNP interactions with the aluminosilicate surface (Fig. 2.b) . The presence of a C1s signal in clay samples may be due to CaCO3, as supported by the XRD pattern of bentonite with 113, 202 and 018 plane families between 40-45 degrees. XPS data, more particularly for cellulose-based samples (Fig. S12 ) showed no significant BE shift for C atom, indicating that carbon interaction with MNP, if any, is negligible.