Paraotwayite-type α-Ni(OH)₂ Nanowires: Structural, Optical, and Electrochemical Properties

Abstract

Paraotwayite-type nickel hydroxide [Ni(OH)2] nanowires with typical diameters of 20–30 nm and lengths up to several micrometers were prepared via a simple hydrothermal synthesis. The as-prepared nanowires had a mean composition of Ni(OH)1.64(SO4)0.18·0.3H2O and crystallized in a layered monoclinic structure (unit cell parameters: a = 0.78867(1) nm, b = 0.29661(3) nm, c = 1.35164(2) nm, and β = 91.1°), which is isostructural to α-Ni(OH)2 and has sulfate anions and water molecules sandwiched in the two-dimensional Ni(OH)2 principle layers. The as-prepared Paraotwayite-type α-Ni(OH)2 nanowires showed an indirect-allowed electron transition with a band gap energy Eg of about 3.8 eV, whereas the corresponding NiO nanowires obtained from a topotactic transformation of Paraotwayite-type α-Ni(OH)2 nanowire precursors exhibited a direct-allowed electron transition with a similar band gap energy of Eg ∼ 3.8 eV. Both Paraotwayite-type α-Ni(OH)2 nanowires and NiO nanowires performed similar electrochemical redox reactions in aqueous alkaline solutions, whereas the OH– ion insertion/extraction reactions were found to be greatly enhanced in Paraotwayite-type α-Ni(OH)2 nanowires due to their intrinsic layered structure. The as-prepared Paraotwayite-type α-Ni(OH)2 nanowires exhibited an anodic electrochromism related to the redox couple of Ni2+ → Ni3+, which corresponds to the coloration from light green to dark brown. Paraotwayite-type α-Ni(OH)2 nanowires are an interesting material system for photoelectrochemical devices and energy-storage applications.