Topochemical Synthesis, Anion Exchange, and Exfoliation of Co-Ni Layered Double Hydroxides: A Route to Positively Charged Co-Ni Hydroxide Nanosheets with Tunable Composition

This paper describes the topochemical synthesis of Co-Ni layered double hydroxides (LDHs) from brucite-like Co-Ni hydroxides through a novel oxidative Intercalation process employing bromine as an oxidizing agent, and their exfoliation into Positively charged unilamellar nanosheets in formamide after anion-exchange treatment. In this protocol, hexagonal microplatelets of brucite-like Co-Ni hydroxides in variable composition were prepared by homogeeous precipitation of a mixed solution of divalent cobalt and nickel ions via hexamethylenetetramine hydrolysis. Subsequent treatment of the brucite-like Co-Ni hydroxides with excessive bromine in acetonitrile promoted partial oxidation of Co2+ into Co3+, producing Br--intercalated Co-Ni LDHs inheriting the hexagonal morphology. This rational topochemical approach was applicable for realizing a pure phase of Co-Ni LDHs with nickel content up to 50% (metal content). Chemical analyses indicated that as-prepared Co-Ni-Br LDHs were unexceptionally characterized by a general chemical formula as [(Co1-3x/22+Ni3x/22+)(2/3)Co-1/3(3+)(OH-)(2)][Br-1/3(-)center dot 0.5H(2)O] (x <= 0.5), a thermodynamically stable LDH structure with a M2+/M3+ ratio of 2:1. We developed an ethanol-assisted anion-exchange approach, which was effective in preventing carbonate contamination in preparing a variety of inorganic and organic anionic forms of Co-Ni LDHs. As-prepared NO3- intercalated Co-Ni LDHs without substantial carbonate contamination were successfully exfoliated into unilamellar nanosheets bearing positive charges upon contact with formamide. The translucent nanosheet suspensions exhibited characteristic colors depending, on the variable Co/Ni ratio.