Synthesis and Properties of Ni-doped Goethite and Ni-doped Hematite Nanorods

Iron oxides and oxyhydroxides are common and widespread compounds in nature. Goethite (α- FeOOH) and hematite (α-Fe2O3) are the most stable and the most ubiquitous iron oxyhydroxide and iron oxide, respectively. A number of favourable properties (nontoxicity, stability, low cost, band gap in the visible range, etc.) make these compounds promising candidates for different applications (catalysts, photocatalysts, gas sensors, battery electrodes, supercapacitors, etc.). Properties of goethite and hematite can be enhanced by modification of particle size and shape, as well as by incorporation of different metal cations into their crystal structure. For example, elongated (1D) hematite nanoparticles (nanorods) showed higher visible-light photocatalytic activity compared to hematite nanoplates (2D) or nanocubes (3D). Substitution of Fe3+ ions with other metal ions can also significantly modify different properties. Ni-doped goethite (α-FeOOH) nanorods were synthesized from mixed Fe(III)-Ni(II) nitrate aqueous solutions by a hydrothermal treatment in a highly alkaline medium using a strong organic alkali tetramethylammonium hydroxide (TMAH). Small amounts of Ni-containing ferrihydrite and Ni ferrite were present as minor phases in samples with 10 mol% Ni and more. Ni-doped hematite (α- Fe2O3) nanorods were obtained by calcination of Ni-doped goethite nanorods at 400 °C. Ni-for-Fe substitution in the structure of goethite and hematite was confirmed by the determination of the unit cell expansion using XRPD and by the measurement of the hyperfine magnetic field (HMF) reduction using Mössbauer spectroscopy. Ni-doped goethite and hematite nanorods were smaller compared to reference samples for 5 mol % Ni but significantly larger for higher Ni contents. Electrochemical measurements showed significant increase in oxygen evolution reaction (OER) catalytic activity of Ni-doped goethite and Ni-doped hematite nanorods compared to pure phase.