Influence of Fe(III) doping on the crystal structure and properties of hydrothermally prepared β-Ni(OH)2 nanostructures

This paper systematically examines the influence of the level of Fe(III) doping on the crystal structure and other properties of Ni(OH)2. Reference β-Ni(OH)2 and Fe-doped Ni(OH)2 samples were synthesized by hydrothermal precipitation of mixed Ni(II) and Fe(III) nitrate aqueous solutions in a highly alkaline medium. The samples were investigated using X-ray powder diffraction (XRPD), scanning and transmission electron microscopy (FE-SEM and TEM), energy dispersive X-ray spectroscopy (EDS), Mössbauer spectroscopy, magnetic measurements, Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy, thermogravimetric analysis (TGA) and electrochemical measurements. Incorporation of Fe in β-Ni(OH)2 by cation substitution was confirmed from the shifts in position of XRPD lines due to the difference in the ionic radius of Fe3+ and Ni2+. The Fe3+-for-Ni2+ substitution in β-Ni(OH)2 caused formation of an interstratified structure with β-Ni(OH)2 and α-Ni(OH)2 structural units interconnected within the same structural layers and crystallites. Mössbauer spectra revealed the presence of Fe3+ ions in highly distorted octahedral sites, presumably at the boundary between the α-Ni(OH)2 and β-Ni(OH)2 structural units within the same structural layer. Electrochemical measurements showed significant increase in oxygen evolution reaction (OER) catalytic activity of Fe-doped Ni(OH)2 compared to pure phase.