Layered Zinc Hydroxide Dihydrate, Zn ₅(OH) ₁₀·2H ₂O, from Hydrothermal Conversion of ε-Zn(OH) ₂ at Gigapascal Pressures and its Transformation to Nanocrystalline ZnO

Abstract

Layered zinc hydroxides (LZHs) with the general formula (Zn ²⁺) _x (OH ^–) _{2 x− my} (A ^m– ) _y · nH ₂O (A ^m– = Cl ^–, NO ₃ ^–, ac ^–, SO ₄ ^2–, etc) are considered as useful precursors for the fabrication of functional ZnO nanostructures. Here, we report the synthesis and structure characterization of the hitherto unknown “binary” representative of the LZH compound family, Zn ₅(OH) ₁₀·2H ₂O, with A ^m– = OH ^–, x = 5, y = 2, and n = 2. Zn ₅(OH) ₁₀·2H ₂O was afforded quantitatively by pressurizing mixtures of ε-Zn(OH) ₂ (wulfingite) and water to 1–2 GPa and applying slightly elevated temperatures, 100–200 °C. The monoclinic crystal structure was characterized from powder X-ray diffraction data (space group C2/ c, a = 15.342(7) Å, b = 6.244(6) Å, c = 10.989(7) Å, β = 100.86(1)°). It features neutral zinc hydroxide layers, composed of octahedrally and tetrahedrally coordinated Zn ions with a 3:2 ratio, in which H ₂O is intercalated. The interlayer d(200) distance is 7.53 Å. The H-bond structure of Zn ₅(OH) ₁₀·2H ₂O was analyzed by a combination of infrared/Raman spectroscopy, computational modeling, and neutron powder diffraction. Interlayer H ₂O molecules are strongly H-bonded to five surrounding OH groups and appear orientationally disordered. The decomposition of Zn ₅(OH) ₁₀·2H ₂O, which occurs thermally between 70 and 100 °C, was followed in an in situ transmission electron microscopy study and ex situ annealing experiments. It yields initially 5–15 nm sized hexagonal w-ZnO crystals, which, depending on the conditions, may intergrow to several hundred nm-large two-dimensional, flakelike crystals within the boundary of original Zn ₅(OH) ₁₀·2H ₂O particles.