Ultrasound-assisted synthesis of mesoporous β-Ni(OH)2 and NiO nano-sheets using ionic liquids

Abstract

Via a facile ultrasound synthesis from nickel acetate and sodium hydroxide with ionic liquids as the solvent and template it is possible to obtain nano-β-Ni(OH)₂ of various dimensionalities depending on the reaction conditions with the ionic liquid (IL) being the most important factor. Scanning electron microscopy (SEM) imaging showed β-Ni(OH)₂ to form as nanosheets, nanorods and nanospheres depending on the IL. ILs with strong to moderate hydrogen bonding capability like [C₃mimOH][Tf₂N] (1-(3-hydroxypropyl)-3-methylimidazolium bis(trifluoromethanesulfonylamide)), [C₄mim][Tf₂N] (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylamide)) and [Edimim][Tf₂N] (1-ethyl-2,3-diemethylimidazolium bis(trifluoromethanesulfonylamide)) lead to the formation of nanosheets whilst [Py₄][Tf₂N] (butyl-pyridinium bis(trifluoromethanesulfonylamide)) leads to nanoparticles and [N₁₈₈₈][Tf₂N] (methyltrioctylammonium bis(trifluoromethanesulfonylamide)) to nanorods. Subsequent calcination of the materials at elevated temperatures (285–425 °C) leads to the conversion of β-Ni(OH)₂ to NiO under preservation of the nanostructure. Scanning electron microscopy (SEM), X-ray diffraction (XRD), TG-DTA, X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray spectroscopy (EDX) were used to observe the morphology, crystallinity, and chemical composition in more detail. Mesoporous NiO nanosheets obtained in [C₄mim][Tf₂N] possess an exceptionally high surface area of 141.28 m² g⁻¹ and a pore volume of 0.2 cm³ g⁻¹ at 285 °C. As a result of calcination at 425 °C the surface area decreased to 92.84 m² g⁻¹, but the pore volume increased to 0.48 cm³ g⁻¹. In addition, the product has an extraordinarily high saturation magnetization of 1.38 emu g⁻¹, a coercivity of 117 Oe and an excellent specific capacitance of 199.4 F g⁻¹ which renders the material highly interesting for application in supercapacitors.