Ionic Interactions in Crystallite Growth of CoMgAl-hydrotalcite-like Compounds

With XRD, FTIR, CHN, ICP, and DTA/TGA techniques, ionic interactions in the forms of cation replacement/incorporation in basal layers and anion exchange/insertion in the interlayer space have been investigated. The crystallite size is increased significantly, whereas basal spacing is reduced when Co 2+ cations or both Co 2+ and Co 3+ in Co II Co III -NO 3 -hydrotalcite-like compound (Co II Co III -NO 3 -HT) are replaced with Al 3+ or Mg 2+ and Al 3+ . IR vibrational modes of hydroxyl groups and intercalated anions have been utilized in identifying exchanged interlayer species, and thermal events revealed by DTA/TGA have been correlated in verifying compositional changes and ionic interactions. In exchange experiments of Co II Co III -NO 3 -HT, aluminum is not only incorporated into the basal layers but is also inserted into the interlayer space (in the form of H 2 AlO 3 - ), which causes an increase in planar growth of basal layers (to about 3 times) and a similar increase (to 2−3 times) in the basal layer stacking along the c -axis. Other experiments for Co II Al-NO 3 -HT and MgAl-NO 3 -HT also indicate similar increases in the planar growth (to 2−3 times) when nitrate anions are replaced by aluminate anions, although the extent of increase in layer number in the c -axis is not as high. It appears that stronger cation-to-anion attraction promotes the inter-basal-layer stacking in the c -axis, while smaller anion-to-anion lateral repulsion is beneficial to the intra-basal-layer growth; these can be practically achieved by incorporation of more trivalent cations and/or replacement of low-charged anions with higher charged ones.