It is important to note that the divalent cation Cd2+ has an ionic radius of 0.097 nm [22], which leads to a number of speculative points; (1) the diameter of the cation is smaller than the internal cavity of the amylose helix (double or single; ca. 0.4–1.0 nm), which itself is presumably flexible under the synthesis conditions to accommodate introduction of the Cd2+ into its hydrophobic cavity (perhaps occupying micropores (Table 1); (2) the diameter is also approximately the same length as the (OH) H-bonding distances (i.e. 0.19–0.21 nm) [23] between adjacent glucose residues that form the amylose helix (believed to be key to the formation of the porous polysaccharide phase); [15,24] consequently coordination of Cd2+ may stabilise an alternative metastable polysaccharide confirmation(s), gel state and in turn expanded surface properties. It is thought that at low loadings (e.g. CdS3), Cd2+ is adsorbed initially within the (smaller) pores of MS, leading to a nanoconfinement of the forming CdS phase. CdS will nucleate here presumably according to a pseudo-epitaxial growth, generating confined, amorphous nanoparticles or “seeds” (as indicated from XRD data). As CdS nanoparticles grow and fill the porous domains as loading increases, the polysaccharide structure remains flexible, particularly in the presence of water (or water/alcohol mixtures), and the hydrogen bond network may then rearrange accordingly to accommodate the inorganic phase. As the CdS clusters grow in size (and becoming increasingly crystalline) in the mesopores, the electrostatic repulsion increases the average pore size but with no reduction in surface area presumably as a consequence of an increased primary particle separation within the gel phase. This is an interesting observation and may lead to the synthesis of a variety of polysaccharide/inorganic hybrids and templated materials, as well as the potential to utilise ionic potential to direct textural properties of polysaccharide gels and associated porous xero- and aerogels.
divalent cation Cd2+
ionic radius
0.097 nm
nm
H-bonding
distances
0.19–0.21 nm
nm