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

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, tetramethylammonium hydroxide (TMAH). Ni-doped hematite (α-Fe2O3) nanorods were obtained by calcination of Ni-doped goethite nanorods at 400 °C. The Ni 2+-for-Fe 3+ substitution in goethite and hematite was confirmed by determination of the unit cell expansion (due to the difference in the ionic radii of Fe 3+ and Ni 2+) using XRPD and determination of the reduction of a hyperfine magnetic field (due to the difference in magnetic moments of Fe 3+ and Ni 2+) 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 Ni 2+-for-Fe 3+ substitution in goethite, and larger Nidoped 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.

Photocatalytic and Cathode Active Abilities of Ni-Substituted α-FeOOH Nanoparticles

“…The transmission band showed approximately 3454 cm −1 ascribed to -OH [ 21 ] in Ni0 and shifted to a lower wave number, and the peak intensity increased with the increasing Ni content. The small transmission peak observed at 3130 cm −1 corresponds to the O-H stretching vibration of the hydroxyl group [ 18 , 22 ]. The band centre of the hydroxyl group gradually shifts to a higher wave number, and the peak intensity decreases with the increasing Ni content, as shown in Figure 3 .…”

“…The band centre of the hydroxyl group gradually shifts to a higher wave number, and the peak intensity decreases with the increasing Ni content, as shown in Figure 3 . The intense two transmission peaks were observed at 797 cm −1 and 895 cm −1 related to the Fe-O-H bending vibrations out of plane and in plane [ 17 , 18 , 19 , 22 ], respectively. The intensity of both transmission peaks gradually decreased with the Ni content.…”

“…These two bands slightly shifted towards higher wave numbers with up to 30 mol% Ni content, and with high Ni content (40 and 50 mol%), the centre of these bands changed to lower wave numbers. It is known that the bending bands’ positions depend on the crystallinity degree and the transition metal substituted with Fe [ 17 , 22 ]. In this study, Ni substituted with Fe caused a shift toward lower wave numbers with the increasing Ni content from Ni0 to Ni30, while in Ni40 and Ni50, an increase in particle size caused a change to a higher wave number.…”

“…These results agree with XRD and DTA (see Figure 1 and Figure 2 ). The transmission band was observed at 645 cm −1 in pure-FeOOH due to Fe-O symmetric stretching [ 22 ], and the band centre gradually shifted to a lower wave number with the increasing Ni content from 0 to 0.5. A new transmission peak appeared in higher Ni concentration samples (Ni40 and Ni50) at 472 and 476 cm −1 , respectively.…”

Photocatalytic and Cathode Active Abilities of Ni-Substituted α-FeOOH Nanoparticles

“…These results agree with XRD and DTA (see Figures 1 and 2). The transmission band was observed at 645 cm −1 in pure-FeOOH due to Fe-O symmetric stretching [22], and the band centre gradually shifted to a lower wave number with the increasing Ni content from 0 to 0.5. A new transmission peak appeared in higher Ni concentration samples (Ni40 and Ni50) at 472 and 476 cm −1 , respectively.…”