How the Method of Synthesis Governs the Local and Global Structure of Zinc Aluminum Layered Double Hydroxides

Seven zinc aluminum layered double hydroxides (ZnAl LDHs), [Zn1–xAlx(OH)2Ax·nH2O], A = NO3–, Cl–, or CO32–, prepared by the urea and coprecipitation synthesis methods were investigated to determine how the synthesis parameters (pH, metal ion concentration, and postsynthesis treatment) affect the local (atomic) and long (global) range structure of the LDH product. Sample composition, purity, crystal defects, and other structural aspects of the LDH products were obtained from powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), micro-Raman, elemental analysis, and solid state 1H, 27Al, and 67Zn NMR spectroscopy. The urea method results in LDHs, which on the global scale are highly crystalline LDHs but disordered on the local scale. The disorder is correlated with the presence of Al-rich phases, which are undetected by bulk techniques (TEM, PXRD), as they are either defects within the LDH particles or separate phase(s) associated with LDHs. In contrast, samples prepared by coprecipitation with careful pH control and hydrothermal treatment have high local order and good crystallinity (large particle size). Our results show that both local (NMR) and bulk techniques are needed to assess the composition of LDHs, as the conventional PXRD and TEM analysis of LDHs failed to identify the many structural defects and/or amorphous phases present.