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Article

Synthesis and Structural Analysis of Ternary Ca–Al–Fe Layered Double Hydroxides with Different Iron Contents

1
Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620, Korea
2
Department of Chemistry and Medical Chemistry, College of Science and Technology, Yonsei University, Wonju 220710, Korea
3
Korea Radioactive Waste Agency, 19, Chunghyochun-gil, Gyeongju 38033, Korea
4
Department of Chemistry, Kyungpook National University, Daegu 41566, Korea
*
Author to whom correspondence should be addressed.
Submission received: 20 September 2021 / Revised: 16 October 2021 / Accepted: 22 October 2021 / Published: 26 October 2021
(This article belongs to the Section Inorganic Crystalline Materials)

Abstract

:
Hydrocalumite structured layered double hydroxides (LDHs) with various Fe3+ ratios were prepared through a coprecipitation method. In order to control the Fe3+ content in LDH, binary Ca–Fe LDHs were first synthesized with various Ca/Fe ratios. The X-ray diffraction pattern showed that only a limited Ca/Fe ratio resulted in LDH formation. The Fe3+ content in LDH was controlled by applying Al3+ while the divalent and trivalent metal ratio was set to 2. Through X-ray diffraction patterns, ternary LDHs with Ca–Al–Fe composition were successfully synthesized without significant impurities, with the Al increasing crystallinity. Quantification showed that Al moiety participated in the formation of the LDH framework more than Ca and Fe, implying a structural stabilization in the presence of Al. In order to investigate the global and local structure of Fe moiety in the LDH, both solid state UV-vis and X-ray absorption spectroscopies were carried out. Both spectroscopies revealed that the existence of Al induced slight local distortion in coordination but global crystal stabilization.

1. Introduction

Layered double hydroxide (LDH) is a family of 2-dimensional layered inorganics having positively charged nanolayers and charge compensating anions [1,2,3,4]. LDHs are attracting increasing interest as drug delivery systems [5,6,7,8], anionic reservoir/adsorbents [9,10,11,12], precursors for catalysts [13,14,15,16,17], catalytic supports [18,19,20], etc., due to their unique layer-by-layer structure, positive surface charge, anisotropic morphology, and various metal compositions. The general chemical formula of LDH is M2+1−xM3+x(OH)2(An−)x/n⋅mH2O, where M2+, M3+, and An−⋅mH2O stand for divalent metal, trivalent metal, and hydrated interlayer anion, respectively. The ratio of M2+/M3+ is known to be in the range of 1.5–4.0 with crystallinity lowering being observed at both margins [21]. A layer is composed of M(OH)6 octahedrons having either divalent or trivalent metal ions, and the metal hydroxide units are linked in the crystallographic ab-plane direction by sharing each other’s edges [22]. Usually, the sizes of divalent and trivalent cations are similar to each other and thus the crystal structure of LDH is similar to that of brucite (Mg(OH)2) [23]. Meanwhile, when Ca2+ is involved in the layer structure, there is a slight distortion in the lattice; relatively large Ca2+ ions are stabilized in a capped octahedron geometry with a coordination number of 7 [24]. This specific phase of LDH is often referred to as hydrocalumite, in which Ca2+/M3+ ratios are set to ~2/1 for structural balance [25,26,27].
Due to the high biocompatibility, eco-friendliness, and water-soluble properties of Ca–hydroxide, hydrocalumite, i.e., Ca–Al LDH, has been widely studied for biomedical and environmental applications. The Ca–Al LDH has been studied as a Ca-supplement [28], for nutritional molecule stabilization [29], and as a drug delivery system [30]. On the other hand, wastewater treatment using Ca–Al LDHs has also been reported on consistently; the removal of various water pollutants like B, Mo, Se [31], Cr(VI) [32], and arsenate [33] by Ca–Al LDHs has been studied extensively. Recently, the biomedical and environmental application study on hydrocalumite structure LDH shifted to a Fe-containing material. Various bio-functional moieties, including naproxen [34], antibacterial [35], and even natural extracts [36] were incorporated into Ca–Fe LDHs for potential drug delivery systems. Similarly, in environmental applications, the utilization of Ca–Fe LDHs for water treatment has recently been reported [37,38].
Although there has been much research on the application of Ca–Al or Ca–Fe LDHs, there has been only limited research on the structural properties of Fe-containing hydrocalumite LDH. As far as we know, the structures of Ca–Fe LDHs are less disclosed but different from the composition of Ca–Al LDHs. Kim et al. investigated different coprecipitation behavior of Ca–Fe solution compared with Ca–Al solution and suggested an optimized pH condition to stabilize Ca–Fe LDH [24]. Meanwhile, Sipiczki et al. prepared Ca–Fe LDHs with various ratios and found a difference in Ca(OH)2 impurity and distortion of Fe-hydroxide structure [39]. We hypothesize, in the current research, that the different ionic radius between Al3+ (67 pm) and Fe3+ (78 pm) influences the structural stabilization of hydrocalumite when Fe3+ is substituted for Al3+. In this regard, we aimed to synthesize hydrocalumite-type LDHs with various Fe ratios. First, the Ca/Fe ratio was controlled in a precursor solution to investigate Ca(OH)2 impurity formation and the optimum M2+/M3+ ratio for the pure phase. For the next step, the two kinds of trivalent cations, Al3+ and Fe3+, were used at various ratios to monitor the structural evolution and local structures. Various spectroscopic analyses, including X-ray diffraction, solid UV-vis spectroscopy and X-ray absorption spectroscopy, were applied to analyze the local and global stabilization of Fe in Ca–Al–Fe LDHs.

2. Materials and Methods

2.1. Synthesis of Binary Ca–Fe LDHs with Various Ca/Fe Ratios

Hydrocalumite-type LDHs were coprecipitated by titrating mixed metal solutions with alkaline solutions. Both calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O), purchased from Sigma-Aldrich LLC (Saint-Louis, LO, USA), were dissolved in deionized water with various Ca/Fe ratios of 1/1, 2/1, 3/1, 4/1, and 5/1. Alkaline titration with 0.5 mol/L NaOH (Daejung Chemical Co. Ltd. Siheung, Korea) was carried out until the pH reached ~13 for thorough precipitation of metal hydroxide. The suspension was aged for 24 h under vigorous stirring. Then, the precipitate was collected by centrifugation, washed with deionized water, and lyophilized for further characterization. The synthesized LDH with a Ca/Fe ratio of 2/1 was named CAF1 for further comparison.

2.2. Synthesis of Ternary Ca–Al–Fe LDHs with Various Ratios

Synthesis of ternary LDHs with Ca–Al–Fe composition was carried out by precipitation method with various Fe3+ contents while the Ca2+/M3+ ratio was set to 2/1. Mixed metal solutions containing Ca2+, Al3+ (Al(NO3)3·9H2O was purchased from Sigma-Aldrich LLC, Saint-Louis, LO, USA), and Fe3+ were prepared with different Ca/Al/Fe ratios. The final product with Ca/Al/Fe ratios of 3.33/0.67/1, 4.67/1.33/1, and 5.33/1.67/1, respectively, were named CAF2, CAF3, and CAF4. Each metal solution was titrated by alkaline solution until pH~13. After the reaction for 24 h, the precipitates were centrifugated, washed, and lyophilized.

2.3. Characterization

The crystal structure of the synthesized samples was first analyzed by powder X-ray diffractometer (XRD, 2D Phaser, Bruker AXS GmbH, Kalsruhe, Germany). The local structure around the Ca and Fe was investigated by X-ray absorption fine structure (XAFS) at 8C NanoXAFS beamline in Pohang Accelerator Laboratory (PAL), Pohang, Korea. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of Ca K-edge and Fe K-edge were obtained in transmission mode. XANES spectra were normalized with default parameters by Athena software. Fourier transformed EXAFS spectra data were not phase shift corrected. The electronic spectra were examined by measuring solid state UV-vis spectroscopy (EVOLUTION 220, ThermoFisher Scientific, Waltham, MA, USA). The chemical composition of obtained LDHs was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 8300, PerkinElmer, Waltham, MA, USA) after digestion in hydrochloric acid. The microstructures of the Ca–Fe LDHs and Ca–Al–Fe LDHs were visualized by scanning electron microscopy (SEM) with JSM-6700F (JEOL). The measured sample was prepared by tenderly spreading the LDH powder on carbon tapes.

3. Results and Discussion

3.1. X-ray Diffraction (XRD) Analysis

According to the XRD patterns of Ca–Fe LDHs with nominal Ca/Fe ratios of 1, 2, 3, 4, and 5 (Figure 1), it was confirmed that the pure hydrocalumite phase could be obtained with a certain Ca/Fe ratio range. As shown in Figure 1a,b, the LDHs with nominal Ca/Fe ratios of 1/1 and 2/1 showed a well-developed hydrocalumite structure intercalated with NO3 (JCPDS No. 48-0065) [40]. With a higher Ca/Fe ratio, we could observe dual phases of hydrocalumite and Ca(OH)2. In order to quantitatively analyse the lattice dimensions of Ca–Fe LDHs depending on the nominal Ca/Fe ratios, the lattice parameters of the LDHs were calculated utilizing all the diffraction peaks and are summarized in Table 1. It was notable that the lattice parameter values were the same as each other regardless of the nominal Ca/Fe ratio. The result suggested that the Ca–Fe hydrocalumites were obtained from a common crystal structure, although there was a Ca(OH)2 impurity with an increasing Ca/Fe nominal ratio. The discrepancy in c values and basal spacings with respect to Ca/Fe ratio did not mean a difference in crystal structure. It is rather attributed to the water content in the interlayer space according to previous reports that describes the relationship between basal spacing and water content in clay materials [41,42]. Upon increasing Ca content in the starting metal solution, impurity phases appeared obviously (Figure 1c,d). The intense impurity peaks observed at 2~18° and ~34° are attributed to the reflection along (001) and (101), respectively, of the Ca(OH)2. The intensity of the diffraction peak for Ca(OH)2 gradually increased upon increasing Ca content in the starting solution. This result indicated that only a limited amount of Ca2+ was incorporated into the Ca–Fe LDH lattice and that the remaining Ca2+ moiety precipitated as the Ca(OH)2 phase. This result is consistent with previous research [39], which reported the formation of Ca(OH)2 to stabilize hydrocalumite structure within the optimized Ca2+/Fe3+ ratio of ~2 when an excessive amount of Ca2+ was present in the starting solution. Through the XRD analyses, we could suggest that the M2+/M3+ ratio should be maintained between 1 and 2 in the precursor solution to gain pure hydrocalumite phased LDH.
In order to vary the Fe content in the LDH structure, mixed metal solutions containing diverse ratios of Ca/Al/Fe were utilized as precursors for Ca–Al–Fe LDH, while the Ca2+/M3+ ratio was set to 2/1. Lattice constants a and c of Ca–Al–Fe LDHs were calculated with all the observed diffraction peaks and listed in Table 2. A slight decreasing pattern in lattice parameter a in CAFs were observed from 6.0 Å to 5.8 Å with increasing Al, which was attributed to both the difference in ionic radius between Al3+ and Fe3+ and total metal ratio (Table 3). The d-values along the (002) plane of CAF1, CAF2, CAF3, and CAF4 were 8.4 Å, 8.8 Å, 8.7 Å, and 8.7 Å, respectively. Even though all of the prepared LDHs had the same intercalated anion of NO3, there was a discrepancy in parameter c and basal spacing of the prepared LDHs, which was attributed to water content in the inter-layer space [41,42]. As shown in Figure 2, all the samples showed a well-developed hydrocalumite phase without significant impurities regardless of Ca/Al/Fe ratios in the starting solution. The crystallinity became higher with increased Al content. For a semi-quantitative analysis of the crystallinity change, the crystallinity (ordered domain) along the crystallographic c-axis and ab-plane direction were calculated by the Scherrer’s equation (t = (0.9λ)/(Bcosθ), t: ordered domain, λ: X-ray wavelength, B: full-width at half-maximum (FWHM)). The FWHM values of (002) peaks were 0.7°, 0.5°, 0.4°, and 0.4° for CAF1, CAF2, CAF3, and CAF4, respectively, matching the range of ordered domain 11 nm, 17 nm, 20 nm, and 20 nm, respectively, in the c-axis direction. The FWHM values of (110) were 0.3°, 0.3 °, 0.2°, and 0.3° for CAF1, CAF2, CAF3, and CAF4, respectively, resulting in ordered domain sizes of 26 nm, 33 nm, 36 nm, and 32 nm, respectively. This result clearly showed that the crystallinity of Ca–Al–Fe LDH increased in the presence of Al. In other words, the lattice structure became more stable with both Al3+ and Fe3+ in the hydrocalumite layer than with Fe3+ alone. Additionally, it is recommended that a ternary composition is prepared in order to stabilize Fe3+ in the Ca–Al–Fe LDH structure.

3.2. Quantification of Ca/Al/Fe Ratios in CAF Samples

The actual Ca/Al/Fe ratios in CAF samples were evaluated with ICP-OES and compared with the nominal metal ratios (Table 3). We could clearly observe a discrepancy between the nominal and actual ratios. First of all, the CAF1, in which only Ca2+ and Fe3+ were utilized, showed slightly higher Ca2+ and lower Fe3+ content in LDH than in the precursor solution (see Ca2+/Mtot and Fe3+/Mtot value). This result showed that Fe3+ was preferentially precipitated compared with Ca2+ at a coprecipitation pH~13. We also investigated the difference between nominal and actual metal ratios upon the addition of Al3+. The content of a certain metal species over total (Ca2+/Mtot, Al3+/Mtot, and Fe3+/Mtot) exhibited that both Ca and Fe content in LDH decreased compared with the value in the precursor metal solution. On the other hand, the Al content was larger in LDH products than in the metal solution. More quantitatively, the Fe/Mtot and Ca/Mtot ratio decreased in the range of 85–93% and 79–87%, respectively, while the Al/Mtot ratio increased within the 171–192% range. We could see two interesting points in the quantification data. First, metal cations participated in a precipitation reaction with different efficiencies when several cations were mixed together, and the tendency of participation was in the order Al>Fe>Ca. Second, the degree of participation in the precipitation reaction did not change significantly at different metal ratios. The former finding suggested that Al3+ was the best stabilized in the metal hydroxide framework, and the latter implied that the Fe3+ was well distributed in the LDH lattice without serious waste during the reaction. Although we could not preserve the metal ratios of the starting solution in the final CAF samples, the data proposed how to control Fe content in the LDH structure (Fe wt% values in Table 3).

3.3. Solid State UV/Vis Spectroscopy of CAF Samples

In order to estimate the electronic structure of Fe3+ ions in the LDH lattice, solid state UV-vis spectra were measured (Figure 3a). The absorption of Fe(III)(OH)6 was strongly found in the UV region and widely found in the visible regions with a wavelength range of 400–600 nm. As the Fe3+ in hydroxide lattice has a t2g3eg2 electronic structure, the d–d transition is spin-forbidden, and only ligand to metal electron transfer (LMCT) accounts for the visible light absorption [43]. The intensity in the wavelength 400–600 nm region was the largest in binary Ca–Fe LDH (CAF1), while the peak intensities of ternary Ca–Al–Fe LDHs (CAF2, 3, and 4) were fairly comparable to each other. The result is different from the general expectation that the visible light absorption was dependent on Fe contents. As shown in Table 3, the weight% values for Fe3+ in CAF1, 2, 3, and 4 were 23.1wt%, 14.1wt%, 10.4wt%, and 8.8wt%, respectively. The Fe3+ content and absorption (400–600 nm) intensity were not linear in ternary Ca–Al–Fe LDH. In spite of lower Fe3+ content in CAF3 than in CAF2, it showed slightly higher absorption at around 500 nm. Similarly, CAF4, which has the lowest Fe3+ content, showed comparable absorption intensity with CAF2 and CAF3. This result indicated that LMCT around Fe3+ in ternary Ca–Al–Fe LDH was facilitated with low Fe3+ content. It was previously reported that the local distortion or ligand vacancy in metal hydroxide increased the number of photons involved in charge transfers [44,45]. Although the XRD data (Figure 2) showed higher crystallinity with lower Fe content in CAFs, the UV-vis spectroscopy suggested that the local structure around Fe could be distorted. It should be noted, however, that the distorted local structure does not stand for the instability of Fe within the LDH layer. Instead, it can be concluded that the Fe3+ was evenly distributed throughout the globally crystallized Ca–Al–Fe LDH lattice with slight local distortions.
The energy gap between the valence band and conduction band (band-gap) was calculated using the Tauc function, with values of 3.01, 2.76, 2.50, and 2.35 eV for CAF1, CAF2, CAF3, and CAF4, respectively (Figure 3b). The band-gap values are highly related to the macroscopic structure such as overall ordering, particle size, degree of unsaturated coordination, etc. In the previous study on perovskite materials, the global distortion in the crystal structure increased the band-gap [46,47]. Furthermore, small size and impurity tended to increase the band-gap [48,49,50]. The smaller band-gap with increasing Al3+ (decreasing Fe3+) indicated that the lattice structure of LDH near Fe was stable and pure in the presence of Al. Taking into account the XRD and UV-vis spectroscopy, we can conclude that the Al3+ is a crucial element to stabilize Fe-containing hydrocalumites.

3.4. X-ray Absorption Fine Structure (XAFS)

Local structure of CAFs around Ca and Fe cations was evaluated by X-ray absorption spectroscopy (XAS). First, the XANES spectra of Ca K-edge were investigated as Ca is the most abundant metal cation in the CAF framework. As shown in Figure 4, all four XANES spectra showed two-step absorption: pre-edge peak and main edge attributed to 1s→3d and 1s→4p transition, respectively [24]. The main edge position of Ca K-edge spectra is known to be strongly related to the coordination number of Ca; the main edge energy increases with an increasing coordination number around Ca [51,52]. The main edge of the Ca K-edge was similar in all the CAF samples, at 4040.6, 4040.9, 4040.9, and 4040.7 eV for CAF1, CAF2, CAF3, and CAF4, respectively. This result implied that the local structure of CAFs around Ca2+ ions was not significantly affected by the distribution of Al3+ and Fe3+ cation. It is parallel to the result that the Ca/Mtot ratios were not notably different from each other in CAFs regardless of Ca/Al/Fe ratios.
The Fe K-edge XANES was also checked to confirm the local structure of CAFs around Fe3+ cations. As shown in the left panel of Figure 5, the XANES spectra of all the CAFs were fairly similar in terms of peak shape and intensity, suggesting the chemical environments around Fe3+ were fairly similar to each other in CAFs. However, a closer look at the pre-edge region (right panel of Figure 5) revealed that the increasing Al3+ content resulted in an enhancement of pre-edge. As is well known, the pre-edge and main edge of Fe K-edge XANES are attributed to 1s→3d and 1s→4p transitions, respectively [53]. The La Porte forbidden 1s→3d transition is allowed when the symmetry around metal ions is distorted. It is known that Fe in LDH is stabilized in the octahedral center; in fact, however, the octahedrons have rather D4h than Oh symmetry [21,39]. Due to the size difference between Al3+ (67 pm) and Fe3+ (78 pm in high spin configuration), the local structure around Fe3+ could be distorted. This finding is in parallel with the facilitated LMCT phenomena in ternary LDH, as discussed with regard to the solid UV-vis spectroscopy (Figure 3). In addition, it is also interesting that the main edge position of Fe K-edge was slightly different with different Al3+ content. The calculated main edge positions were 7129.5, 7128.5, 7127.4, and 7127.4 eV for CAF1, CAF2, CAF3, and CAF4, respectively. Decreasing main edge position according to increasing Al3+ would be interpreted by the shielding theory. As Al3+ is small and hard acid, it can attract electron density; consequently, electron density of OH- coordinated to Fe3+ would be reduced by the neighboring Al3+. As Al3+ content increases, electrons in Fe3+ would experience less shielding and less energy is required for the 1s→4p transition.
As we found a slight difference in the chemical environment around Fe, we further analyzed EXAFS spectra for all CAFs shown in Figure 6. Obviously, the first shell Fe–O distance appeared at 1.5Å (non-phase-shift-corrected) for all the CAFs. However, the position of Fe–O in CAF4 shifted to the shorter region, while the second shell of ternary LDHs (CAF2, CAF3, and CAF4) was located in a shorter position than binary LDH (CAF1). There was no significant difference in the structure of the second shell, where the position of calcium ions is modulated by the intercalated anions and contributes to the NO3 anions. The notable difference can be found in the third shell. The calcium ions of CAF1 lay the furthest from the Fe(III) ion (3.11–3.13 Å) and the other three had a smaller distance to Fe(III), attributed to calcium content.

3.5. Electron Microscopy Study of the As-Prepared Ca–Fe LDH and Ca–Al–Fe LDH Samples

The morphology of the as-prepared Ca–Fe LDHs with Ca/Fe ratios of 2, 3, and 4, and Ca–Al–Fe LDHs with Ca/Al/Fe ratios of 8/1/3 and 5/1/1.5 were evaluated by SEM, as shown in Figure 7 and Figure 8, respectively. The Ca–Fe LDH with a Ca/Fe ratio of 2 performed randomly assembled platelet-like particles (Figure 7a), suggesting that it was hydrocalumite. The average lateral dimension and thickness of that were 400 nm and 30 nm, respectively. However, the platelet-like and granular particles of the Ca–Fe LDH with a Ca/Fe ratio of 3 and 4 were observed (Figure 7b,c), and we suspected that the hydrocalumite and other impurities coexisted, suggesting that Ca(OH)2 was one of the impurity candidates, which was confirmed by the results from XRD. Moreover, the morphologies of granular agglomeration and platelets were evident in Ca–Al–Fe LDHs with Ca/Al/Fe ratios of 8/1/3 and 5/1/1.5, respectively (Figure 8). Notably, there was only a single-phase generated in Ca–Al–Fe LDHs with Ca/Al/Fe ratios of 8/1/3 and 5/1/1.5, consistent with the XRD data shown in Figure 2.

4. Conclusions

The structure and composition of hydrocalumite type LDH with Ca–Al–Fe composition were investigated in this study. It was confirmed that pure binary LDH with Ca–Fe composition could only be obtained with a limited metal ratio of 2/1. XRD analyses confirmed that excessive Ca moiety in precursor solution resulted in a mixed phase with Ca(OH)2. In order to introduce Fe3+ in hydrocalumite type LDH at various ratios, ternary Ca–Al–Fe LDH with various metal ratios were prepared. The XRD analyses showed that the crystallinity became higher with increasing Al moiety while there existed Ca moiety, which did not participate in precipitation reactions in the presence of Al. The band-gap calculation from solid state UV-vis spectroscopy suggested that the global crystallinity and ordering around Fe were higher in the presence of Al in the LDH framework. On the other hand, the LMCT phenomena in solid state UV-vis spectra and X-ray absorption spectra suggested that local the structure around Fe could be slightly distorted in the presence of Al in the LDH framework. This local distortion was attributed to the size mismatch between Fe and Al, and it did not seriously affect the stable incorporation of Fe in the LDH framework. It can be concluded that the Fe moiety could be stably incorporated in a well-crystallized hydrocalumite type LDH framework when Al coexisted with Fe and Ca.

Author Contributions

Conceptualization, J.X. and J.-M.O.; methodology, G.-H.G., M.L. and S.-M.P.; software, J.X. and G.-H.G.; validation, G.-H.G., M.L. and S.-M.P.; formal analysis, J.X.; investigation, J.X., G.-H.G., M.L. and S.-M.P.; resources, M.L. and S.-M.P.; data curation, J.X. and G.-H.G.; writing—original draft preparation, J.X.; writing—review and editing, J.-M.O.; visualization, J.X. and G.-H.G.; supervision, J.-M.O.; project administration, J.-M.O.; funding acquisition, J.-M.O. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by a grant from the International Research & Development Program (2020K1A3A1A21039816) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning, Korea.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

Acknowledgments

The fabrication processes in this work were supported by the Nano-Bio Materials Lab.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns for Ca–Fe LDHs with various nominal Ca/Fe ratios of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5. * stands for the Ca(OH)2 impurity.
Figure 1. XRD patterns for Ca–Fe LDHs with various nominal Ca/Fe ratios of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5. * stands for the Ca(OH)2 impurity.
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Figure 2. XRD patterns of Ca–Al–Fe LDHs with various Ca/Al/Fe ratios (a) CAF1, (b) CAF2, (c) CAF3, and (d) CAF4.
Figure 2. XRD patterns of Ca–Al–Fe LDHs with various Ca/Al/Fe ratios (a) CAF1, (b) CAF2, (c) CAF3, and (d) CAF4.
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Figure 3. (a) Solid state UV-vis spectra and (b) Tauc plot of Ca–Al–Fe LDHs with various Ca/Al/Fe ratios.
Figure 3. (a) Solid state UV-vis spectra and (b) Tauc plot of Ca–Al–Fe LDHs with various Ca/Al/Fe ratios.
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Figure 4. Ca K-edge XANES spectra for Ca–Al–Fe LDHs with various Ca/Al/Fe ratios.
Figure 4. Ca K-edge XANES spectra for Ca–Al–Fe LDHs with various Ca/Al/Fe ratios.
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Figure 5. Fe K-edge XANES spectra for Ca–Al–Fe LDHs with various Ca/Al/Fe ratios (right panel shows magnified spectra around pre-edge region).
Figure 5. Fe K-edge XANES spectra for Ca–Al–Fe LDHs with various Ca/Al/Fe ratios (right panel shows magnified spectra around pre-edge region).
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Figure 6. Fe K-edge EXAFS spectra for Ca–Al–Fe LDHs with various Ca/Al/Fe ratios.
Figure 6. Fe K-edge EXAFS spectra for Ca–Al–Fe LDHs with various Ca/Al/Fe ratios.
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Figure 7. SEM images for CaFe LDHs with Ca/Fe ratios of (a) 2, (b) 3, (c) 4.
Figure 7. SEM images for CaFe LDHs with Ca/Fe ratios of (a) 2, (b) 3, (c) 4.
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Figure 8. SEM images of Ca–Al–Fe LDH with Ca/Al/Fe ratios of (a) 8:1:3 and (b) 5:1:1.5.
Figure 8. SEM images of Ca–Al–Fe LDH with Ca/Al/Fe ratios of (a) 8:1:3 and (b) 5:1:1.5.
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Table 1. Structural parameters of the synthesized materials.
Table 1. Structural parameters of the synthesized materials.
Nominal
Ca/Fe Ratio
Lattice ParametersBasal Spacing (Å)
a (Å)c (Å)
16.017.48.7
26.017.08.4
36.017.38.6
46.017.18.4
56.017.08.4
Table 2. Structural parameters of the synthesized materials.
Table 2. Structural parameters of the synthesized materials.
SamplesLattice ParametersBasal Spacing (Å)
a (Å)c (Å)
CAF16.016.98.4
CAF25.917.28.8
CAF35.816.98.7
CAF45.816.98.7
Table 3. Quantitative data on Ca–Al–Fe LDHs. Nominal (nom.) and actual (act.) ratios stand for molar metal ratios in starting solution and the values calculated from ICP-OES, respectively. Mtot indicates the total metal contents including Ca, Al, and Fe.
Table 3. Quantitative data on Ca–Al–Fe LDHs. Nominal (nom.) and actual (act.) ratios stand for molar metal ratios in starting solution and the values calculated from ICP-OES, respectively. Mtot indicates the total metal contents including Ca, Al, and Fe.
SampleNominal RatioActual RatioCa2+/MtotAl3+/MtotFe3+/MtotCa2+/(Al3++Fe3+)Fe wt%
CaAlFeCaAlFenom.act.nom.act.nom.act.nom.act.
CAF12.000.001.002.440.001.000.670.710.000.000.330.292.002.4423.07
CAF23.330.671.003.301.411.000.670.580.130.250.200.182.001.3714.14
CAF34.671.331.004.112.681.000.670.530.190.340.140.132.001.1210.43
CAF45.331.671.004.973.341.000.670.530.210.360.130.112.001.158.82
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Xie, J.; Gwak, G.-H.; Lee, M.; Paek, S.-M.; Oh, J.-M. Synthesis and Structural Analysis of Ternary Ca–Al–Fe Layered Double Hydroxides with Different Iron Contents. Crystals 2021, 11, 1296. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11111296

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Xie J, Gwak G-H, Lee M, Paek S-M, Oh J-M. Synthesis and Structural Analysis of Ternary Ca–Al–Fe Layered Double Hydroxides with Different Iron Contents. Crystals. 2021; 11(11):1296. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11111296

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Xie, Jing, Gyeong-Hyeon Gwak, Minseop Lee, Seung-Min Paek, and Jae-Min Oh. 2021. "Synthesis and Structural Analysis of Ternary Ca–Al–Fe Layered Double Hydroxides with Different Iron Contents" Crystals 11, no. 11: 1296. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11111296

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