Highly Crystalline Ni–Co Layered Double Hydroxide Fabricated via Topochemical Transformation with a High Adsorption Capacity for Nitrate Ions

Layered double hydroxide (LDH) has emerged as promising candidates for removing harmful oxoanions (i.e., SO4(2-), HPO4(2-), and NO3(-) ions) from wastewater because of their intrinsic ability to accommodate anionic species in the interlayer space. Highly crystalline [Ni0.67Co0.33(OH)2]Cl0.29.0.53H2O (Ni-Co LDH) particles with an exceptionally high anion-exchange capacity of 58.8 mg g(-1) and a distribution coefficient (Kd) of 2396 mL g(-1) for NO3(-) ions were successfully prepared by the flux method and a topochemical strategy. Layered Na0.97Ni0.67Co0.33O2 (NNCO) was prepared using a high-temperature flux and used as a starting material for topotactic transformation consisting of oxidative hydrolysis with KOH and NaClO and subsequent reduction with H2O2 and NaCl. During the transformation from NNCO to Ni-Co LDH, a drastic change in the valences of the Ni and Co belonging to the host layer and in the cationic and anionic species occurs in interlayer space; the valences of the Ni and Co in NNCO were increased from Ni(2+,3+) and Co(2+,3+) to Ni(3+) and Co(2+,3+,4+) by an oxidative hydrolysis reaction with simultaneous intercalation of K(+) ions and deintercalation of Na(+) ions, and subsequently decreased from Ni(3+) and Co(2+,3+,4+) to Ni(2+) and Co(2+,3+) by a reduction reaction with simultaneous intercalation of anionic species such as CO3(2-) and Cl(-) ions and deintercalation of Na(+) and K(+) ions. Through synchrotron powder X-ray diffraction analysis and Rietveld refinement, the resultant Ni-Co LDH was clearly shown to exhibit high crystallinity with less compositional deviation even after topochemical transformation in comparison with the one prepared by traditional coprecipitation and solid-state methods. Furthermore, the adsorption isotherm for NO3(-) ions elucidated that homogeneous adsorption sites are consistently constructed in the crystal structure, which could be found from the fitting to a Langmuir curve, with the R(2) value being 0.98. This work opens up a new route for the fabrication of excellent not only ion-exchangeable but also ion-conductive inorganic materials for direct utilization in environmental and energy-storage processes.