Periodic stacking of 2D charged sheets: Self-assembled superlattice of Ni-Al layered double hydroxide (LDH) and reduced graphene oxide

Vertically stacked artificial 2D materials, such as van der Waals heterostructures, hold great scientific and technological promise. Stacking 2D atomic layers with stronger electrostatic forces in a controlled fashion could be more challenging. Positively charged atomic sheets of layered double hydroxide (LDH) such as hydrotalcite mineral with weakly bound anions have known intercalation and anion exchange properties, while reduced graphene oxide (rGO) are known to be negatively charged. So self-assembly of periodic (LDH/rGO)_n superlattice is possible, although true periodicity at atomic scale has never been demonstrated for Ni-Al LDH. This work introduces a new protocol for the synthesis of true Ni-Al LDH/GO superlattice and the corresponding reduced product Ni-Al LDH/rGO, by systematically optimizing various key experimental parameters in chemical exfoliation, dispersion and self-assembly by co-feeding. This method is further applied to the successful synthesis of more complex Ni-Co-Al/GO superlattice. The Ni-Al LDH/rGO superlattice is then tested as cathode in alkaline hybrid supercapacitor, with 129Ah/kg capacity at 8-min discharge, two times that of pristine Ni-Al LDH, and maintains 72.7% of its initial capacity after 10,000 charge/discharge cycles. Our superlattice synthesis strategy and its energy applications demonstrate the potential to design artificial 2D materials.