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 Co2+ cations or both Co2+ and Co3+ in (CoCoIII)-Co-II-NO3-hydrotalcite-like compound ((CoCoIII)-Co-II-NO3-HT) are replaced with Al3+ or Mg2+ and Al3+. 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 (CoCoIII)-Co-II-NO3-HT, aluminum is not only incorporated into the basal layers but is also inserted into the interlayer space (in the form of H2AlO3-), 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 (CoAl)-Al-II-NO3-HT and MgAl-NO3-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.