The distribution of reactive Ni2+ in 2D Mg2−xNixAl-LDH nanohybrid materials determined by solid state 27Al MAS NMR spectroscopy

Abstract

Layered double hydroxides (LDHs), especially (doped) with transition metals, as well as nanohybrid and 2D materials derived from these structures, are interesting materials due to their catalytic and electrochemical properties. Their reactivity is determined by the atomic level distribution of the transition metal in the LDH cation layer, which is essential to control the design of LDHs with optimized properties. However, low crystallinity, absence of long range order, and/or isoelectronic ions often prevent atomic level structural characterization. A series of poorly crystalline Mg_2−xNi_xAl-NO₃ LDH materials were investigated by ultrafast ²⁷Al MAS NMR spectroscopy to determine the distribution of Ni²⁺ in these as well as possible superstructures and their miscibility gaps. Four Ni₂Al-LDH samples with interlayer distances ranging from 7.6 to 17.5 Å were prepared to assess the contribution of inter- and intralayer magnetic interactions. The effects of the Ni²⁺ content and the atomic level distribution of Ni²⁺ were probed by ultrafast ²⁷Al MAS NMR spectroscopy: the Al distribution can be modeled using a binomial distribution and neither a superstructure was identified for the MgNiAl-LDH sample nor a miscibility gap. The ²⁷Al isotropic shift, δ_iso(²⁷Al), is a very sensitive probe for a number of neighboring Ni²⁺ in the first metal ion sphere, but to a smaller degree it is also affected by the intercalated anion (interlayer distance). These results were used for detailed characterization of an exfoliated (2D)–restacked Mg_1.83Ni_0.17Al-LDH nanohybrid material and a Mg_1.83Ni_0.17Al-LDH-alginate nanohybrid material, in which ²⁷Al MAS NMR showed how the structure and partial dissolution of the LDHs were retained. In contrast, both powder X-ray diffraction and vibrational spectroscopies (IR and Raman) reflected only the overall change in sample composition.