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

S. Krehula a, *, M. Risti_c a, C. Wu b, e, X. Li b, d, e, L. Jiang b, c, J.Wang b, d, G. Sun b, T. Zhang d, e, M. Perovi_c f, M. Bo_skovi_c f, B. Anti_c f, L. Kratofil Krehula g, B. Kobzi h, S. Kubuki h, S. Musi_c a

a Division of Materials Chemistry and Center of Excellence for Advanced Materials and Sensing Devices, RuCer Bo_skovi_c Institute, P.O. Box 180, HR-10002, Zagreb, Croatia

b Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China

c College of Material Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China

d Mossbauer Effect Data Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China

e University of the Chinese Academy of Sciences, Beijing, 100039, China

f Condensed Matter Physics Laboratory, Vin_ca Institute of Nuclear Sciences, University of Belgrade, P. O. Box 522, 11001 Belgrade, Serbia

g Faculty of Chemical Engineering and Technology, University of Zagreb, Maruli_cev Trg 19, P. O. Box 177, HR-10000, Zagreb, Croatia

h Department of Chemistry Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo, 192-0397, Japan

Abstract

Ni-doped goethite (α-FeOOH) nanorods were synthesized from mixed Fe(III)-Ni(II) nitrate solutions with various Ni/(Ni+Fe) ratios (0, 5, 10, 20, 33 and 50 mol % Ni) by hydrothermal precipitation in a highly alkaline medium using the strong organic alkali, tetramethyl-ammonium hydroxide (TMAH). Ni-doped hematite (α-Fe2O3) nanorods were obtained by calcination of Ni-doped goethite nanorods at 400 °C. The Ni2+-for-Fe3+ substitution in goethite and hematite was confirmed by determination of the unit cell expansion (due to the difference in the ionic radii of Fe3+ and Ni2+) using XRPD and determination of the reduction of a hyperfine magnetic field (due to the difference in magnetic moments of Fe3+ and Ni2+) using Mössbauer spectroscopy. Single-phase goethite nanorods were found in samples containing 0 or 5 mol % Ni. A higher Ni content in the precipitation system (10 mol % or more) resulted in a higher Ni2+-for-Fe3+ substitution in goethite, and larger Ni-doped goethite nanorods, though with the presence of low crystalline Ni-containing ferrihydrite and Ni ferrite (NiFe2O4) as additional phases. Significant changes in FT-IR and UV-Vis-NIR spectra of prepared samples were observed with increasing Ni content. Electrochemical measurements of samples showed a strong increase in oxygen evolution reaction (OER) electrocatalytic activity with increasing Ni content.

Keywords: α-FeOOH, α-Fe2O3, Ni doping, XRPD, Mössbauer spectroscopy, FE-SEM, OER.