Cationic charge influence on the magnetic response of the Fe₃O₄–[Me²⁺ _1−y Me³⁺ _y (OH₂)] ^y+(Co₃ ²⁻) _y/2·mH₂O hydrotalcite system

2.1 General information

All of the chemicals (such as Mg(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O, Sr(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, FeCl₂·6H₂O, FeCl₃·6H₂O, NaOH, and HNO₃) were purchased in analytical purity from Sinopharm Chemical Reagent (Beijing, China), and used without any further purification. The synthesis process of the materials is illustrated in Figure 1. It begins with the preparation of Fe₃O₄ magnetic nanoparticles via the coprecipitation method, using ferric and ferrous iron salts in a basic aqueous solution. Once Fe₃O₄ is obtained, it is combined with the precursor salts of divalent and trivalent cations for the synthesis of hydrotalcite, also using the coprecipitation method, as described below.

Figure 1

Schematic synthesis of the magnetic hydrotalcite samples with Me²⁺/Me³⁺ molar ratios using the coprecipitation method.

2.2 Synthesis of Fe₃O₄ particles

Magnetic NPs were synthesized by the co-precipitation method using the methodology proposed by Kang et al. [24]. A molar ratio of Fe(ii)/Fe(iii) = 0.5 and pH = 11–12 were used. Briefly, 0.85 mL of 12.1 N HCl and 25 mL of purified, deoxygenated water (by nitrogen gas bubbling for 30 min) were combined, and 5.2 g of FeCl₃ and 2.0 g of FeCl₂ were successively dissolved in the solution with stirring. The resulting solution was added dropwise to 250 mL of 1.5 M NaOH solution under vigorous stirring, generating a black precipitate, which was then centrifuged at 400 rpm and washed with deoxygenated water. Subsequently, 500 mL of 0.01 M HCl was added to neutralize the anionic charges on the NPs. The solid was obtained by centrifugation, washed with distilled water, and then dried at 353 K.

The mechanism for the synthesis of Fe₃O₄ involves three key steps: (1) the dissociation of iron salts in aqueous solution (1), (2) the precipitation of iron hydroxides upon the addition of NaOH, and (3) a subsequent dehydration and redox process that leads to the formation of Fe₃O₄ [25,26]:

2.3 Synthesis of magnetic ternary hydrotalcites

The synthesis of magnetic ternary hydrotalcites was carried out following the methodology proposed by Zhang et al. [22], using Fe₃O₄ as a magnetic source. For this, 1 g of Fe₃O₄ was dispersed in 100 mL of a water/methanol solution (V. water/V. methanol = 1/1, v/v), which was subjected to ultrasonification for 20 min to obtain a uniform suspension. Then, the pH was adjusted to 10 by adding a 2.0 M alkaline solution of Na₂CO₃ and NaOH. A mixture solution containing Mg(NO₃)₂·6H₂O, Me(NO₃)₂·6H₂O, and Al(NO₃)₃·9H₂O with a Me²⁺/Al³⁺ = 2 and 3 ratio (Me = Ni, Co, Sr) with y = 0.33 and 0.25, respectively, was added dropwise to the suspension and aged at 140°C for 24 h. The resulting solid was separated magnetically, washed with deionized water, and dried at 80°C overnight. The materials are denoted as Fe₃O₄–MgCoAl x = 3, Fe₃O₄–MgNiAl x = 3, Fe₃O₄–MgSrAl x = 3, Fe₃O₄–MgCoAl x = 2, Fe₃O₄–MgNiAl x = 2, and Fe₃O₄–MgSrAl x = 2. In addition, Fe₃O₄–MgAl x = 3 and Fe₃O₄–MgAl x = 2 were synthesized as control samples, where x represents the Me²⁺/Me³⁺ ratio, 2 and 3.

Hydrotalcite is synthesized with a [urea]/[ NO 3 − ] molar ratio of 3. The carbonate generated from urea decomposition acts as a compensating anion in the amine phase of hydrotalcite [27], with the following reaction mechanism:

A 2M alkaline solution of NaOH and Na₂CO₃ (1:1 molar ratio) was prepared by dissolving NaOH in distilled water, followed by the gradual addition of Na₂CO₃ until the final volume was reached. The mixture was stirred continuously until a homogeneous, translucent solution was obtained.

2.4 Characterization

The XRD patterns and crystalline phases were recorded with a Panalytical X´pert PRO-MPD equipment with an ultrafast X´Celerator detector in Bragg–Brentano geometry, using Copper Cu-K_α radiation (λ = 1.54056 Å), 2θ = 10–90° with a step of 0.0263°, and a capture time of 100 s. The patterns were analyzed with the General Structure Analysis System (GSAS II) software. Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet iS50 spectrometer in the range of 4,500–600 cm⁻¹ using pressed KBr pellets. Morphology properties of the compounds were evaluated by SEM (JSM 6610-LV JEOL equipment). The Brunauer–Emmett–Teller (BET) isotherm and Barrett, Joyner, and Halenda (BJH) method were used to calculate the specific surface area and pore volume, respectively, and N₂ adsorption–desorption isotherms of the solids were measured at 77 K (Micromeritics ASAP 2020 equipment). The samples were previously degassed at 100°C under vacuum for 18 h. The magnetic measurements were developed using a VSM Quantum Design. The measurements as a function of temperature were carried out in the temperature range of 50–300 K using the zero-field cooled–field cooled (ZFC-FC) mode.

3.1 Structural analysis

The XRD patterns of the synthesized samples (Figure 2) showed characteristic peaks corresponding to the crystal structure of hydrotalcite. The most prominent peak was observed around 11.6° 2θ, which is assigned to the (003) plane diffraction, confirming the formation of the hydrotalcite phase [3]. All of the samples were contrasted with the hexagonal phase of hydrotalcite with space group P 6 ̅ 2 m (189) and JCPDS card No. 22-0452, and thus being able to determine the presence of this phase, which by the shape of the signals, tends to have a nanometric nature with a limit to amorphousness [28,29]. When the diffraction patterns of the different samples were compared, a slight variation in the peak positions and their relative intensities was observed (Table 1). This variation suggests that the molar ratio of metal cations influences the interlayer distance and crystalline ordering. The experimental patterns confirmed the absence of secondary phases in the samples, except for the Sr²⁺ inclusion sample (Figure 2a and b), where a significant presence of SrCO₃ was detected [30].

Figure 2

XRD patterns of the Fe₃O₄MgAl magnetic hydrotalcite with molar ratios of (a) x = 2 and (b) x = 3. Rietveld refinements for (c) Fe₃O₄MgCoAl x = 2, (d) Fe₃O₄MgCoAl x = 3, and (e) Fe₃O₄MgNiAl x = 3 samples. The experimental pattern, calculated profile, and Bragg peak positions are indicated by circles, a black curve, and tick marks, respectively. The bottom curve shows the difference between the observed and calculated intensities.

Additionally, the presence of high-intensity peaks around 30°, 35°, 43°, 53°, 57° and 62° 2θ in all samples indicates the coexistence of Fe₃O₄ particles embedded in the hydrotalcite matrix, corresponding to the (220), (311), (440), (422), (511), and (440) diffraction planes, respectively. This phase was indexed to the cubic Fe₃O₄ phase with space group Fd 3 ̅ m (JCPDS 19-0629). The presence of these signals, considering the synthesis process followed, allows corroborating the successful integration of the magnetic particles on the hydrotalcite structure or the other way around. This observation is supported by the identification of both crystalline phases by XRD, which allows confirming the homogeneous and effective integration of Fe₃O₄ nanoparticles within the hydrotalcite matrix [31,32].

The crystallite size (CS) of the present phases in the materials was also calculated using the Debye–Scherrer equation (5) [33,34],

where K = 0.94 is the Scherrer form factor, λ = 0.1542 nm is the Cu cathode radiation, β is the full width at half-maximum of the peaks, and θ is the Bragg diffraction angle. The diffraction planes used to determine CS were (003) for the hydrotalcite phase and (311) for the Fe₃O₄ phase. Table 1 summarizes the findings. The CS for the Fe₃O₄ phase showed an increase when it coexisted with hydrotalcites with an Me²⁺/Me³⁺ ratio of 3. These hydrotalcites showed variable CS, with a tendency to increase as the molar ratio increases to x = 3.

Rietveld refinement was carried out (using GSAS-I software [35,36,37]) to determine complementary structural properties of the acquired samples and their present phases. The lattice parameters and unit cell volume may be obtained using the established refinement procedure. These, along with other important factors, are provided in Table 1. Figure 2c and d shows the plots created for the refinement of some samples. This research demonstrates that the materials crystallized in a hexagonal and cubic form, for hydrotalcite and Fe₃O₄ phases, respectively, which is consistent with earlier semi-quantitative investigations [31,32], as evidenced by the overlap of theoretical and experimental patterns. All the samples have lattice parameters close to 8.386 Å, which is consistent with a spinel-type structure. Quantitative analysis of the phase percentage in the synthesized samples revealed significant variation in the distribution of Fe₃O₄ and hydrotalcite. The percentages of Fe₃O₄ ranged from 41.35 to 89.39%, while those of hydrotalcite ranged from 5.34 to 35.59%. This variability suggests that the synthesis conditions strongly influence the phase structure and ratio, thereby affecting the physical and chemical properties of the composite material. The sample Fe₃O₄MgNiAl (x = 2) exhibits the highest content of hydrotalcite (35.59%), followed by Fe₃O₄MgNiAl (x = 3) with 25.26%. On the other hand, the samples Fe₃O₄MgSrAl (x = 2) and Fe₃O₄MgCoAl (x = 3) show the lowest contents of hydrotalcite (5.34 and 10.61%, respectively). This suggests that the addition of Sr and Co particularly stabilizes the spinel phase, significantly reducing the amount of hydrotalcite formed. In contrast, Ni appears to promote the formation of hydrotalcite due to its influence on the stability of lamellar phases, while Sr and Co seem to better stabilize the spinel-type structure.

A higher Fe₃O₄ content could favor magnetic properties, while a higher proportion of hydrotalcite could enhance catalytic and adsorption properties. The ability to adjust these ratios is crucial for the optimization of the material for specific applications. Furthermore, Rietveld crystallite sizes were also estimated in both perpendicular L(⊥) and parallel L(∥) directions with equations (6) and (7), based on GSAS revised Lorentzian component parameters (LX and ptec), and K and λ correspond to Scherrer parameters [38,39]. The samples reveal significant variation in L(∥) and L(⊥) parameters, suggesting a direct effect of composition on structural anisotropy. In the pure Fe₃O₄ sample, L(∥) and L(⊥) have values of 65.55 and 59.69 nm, respectively, indicating slight anisotropy in crystal growth. With the addition of the basal hydrotalcite, Fe₃O₄MgAl sample, both parameters decrease, reaching values of 55.92 and 55.07 nm, suggesting that the addition of the hydrotalcites limits the growth of the crystalline domains [40]. In general, a decrease of these parameters was observed in the Fe₃O₄ phase, reaching a minimum in the Fe₃O₄MgSrAl sample. The samples Fe₃O₄MgCoAl x = 3 and Fe₃O₄MgNiAl x = 3 exhibit the largest crystallite sizes, suggesting better crystallization or lower stresses in the lattice. For the hydrotalcite phase, these parameters presented a notable decrease with the addition of the substituent cations, reflecting a more restricted structure in terms of crystal growth and a limitation in the crystalline development with these compositional modifications. In general, the incorporation of different cations influences the size and preferential orientation of crystallite growth, which can directly impact their structural and functional properties.

In general, all systems exhibited a spinel structure obtained through various synthesis methods. Most of the methods reported in the literature produce crystallites with sizes similar to those obtained in this study. For instance, the compounds Zn_1−x Ni _x Fe₂O₄, synthesized by sol–gel, sol–gel auto-combustion, microwave combustion, auto-combustion, thermal decomposition, and chemical co-precipitation methods, exhibited crystallite sizes ranging from 18 to 70 nm, depending on the synthesis technique used [41].

Magnesium plays a significant role in determining the crystal size of the synthesized materials (Table 1). The incorporation of Mg²⁺ into the spinel structure, either alone or in combination with other cations (such as Ni²⁺, Co²⁺, Sr²⁺, and Al³⁺), has a marked effect on the crystallite size and stability of the resulting phase. The incorporation of magnesium generally leads to a slight reduction in the crystal size compared to pure Fe₃O₄. This slight reduction in the crystal size is probably due to the substitution of Fe²⁺ and Fe³⁺ ions by Mg²⁺, which has a smaller ionic radius (0.72 Å) compared to Fe²⁺ (0.78 Å). This substitution can induce local distortions in the crystal lattice, which hinders crystal growth and favors the formation of smaller crystallites.

The effect of the (Mg + Me)/Al ratio on the crystallite size of hydrotalcite could be explained in terms of the presence of cations. The presence of a larger number of trivalent cations (Al³⁺) in the layers enhances the rate of stacking of the layers [42]. Samples containing only magnesium exhibit an (Mg/Al) = 1 ratio, while those combining magnesium with another divalent cation (Ni, Co, Sr) show a higher ratio of 2. This implies that samples with a higher (Mg + Me)/Al ratio have a greater proportion of divalent cations relative to trivalent cations, which may affect the crystallite size and the stability of the lamellar phase due to the lower positive charge density in the layers.

On the other hand, there is a noticeable variation in the hydrotalcite lattice parameters, which is associated with the crystalline development due to the chemical modification. Finally, regarding the parameter Chi (χ ²), it can be concluded that the Rietveld refinements made for the samples obtained were adequate, given the structural conditions exhibited by both phases in the samples. In addition, the R ² parameter, as another reliability indicator, also makes it possible to corroborate the accuracy and validity of the refinement process.

The IR spectra of hydrotalcites show variable vibrational bands assigned to high (greater than 2,000 cm⁻¹), middle (2,000–1,000 cm⁻¹), and low (below 1,000 cm⁻¹) frequency ranges. The infrared spectra reveal distinctive features in the materials synthesized with Me²⁺/Me³⁺ molar ratios of 2 and 3. These spectra resemble those of hydrotalcite and magnetic Fe₃O₄ nanoparticles. In Figure 3 and Table 2, a broad adsorption band with a maximum at 549–574 cm⁻¹ (ν ₁) corresponds to Fe–O bond stretching vibrations [31,32]. The stretching of Al–OH may be responsible for the bands at 667 and 733 cm⁻¹ (ν ₂). The band at 795 cm⁻¹ (ν ₂) is attributed to symmetric and antisymmetric vibrations of the Fe–O–H bond, characteristic of magnetic nanoparticles [32,43,44]. The weak band at 870 cm⁻¹ (ν ₃) was due to the characteristic O–C–O bond stretching vibrations of bidentate carbonate [45]. These signals are less intense in the synthesized ternary magnetic hydrotalcites, suggesting a potential core–shell structure, with the external layer of the hydrotalcite shielding the absorption of the Fe–O bond in the Fe₃O₄ phase [46]. The bands at 855 and 856 cm⁻¹ (ν ₃) are assigned to SrCO₃ bending vibrations in Fe₃O₄-MgSrAl sample [47,48]. These results indicate that the incorporation of magnetic nanoparticles into the hydrotalcite does not alter the laminar structure, whereas changes in the Me²⁺/Me³⁺ molar ratio influence the formation of this structure. The band around 1,000 cm⁻¹ (ν ₄) represents the Me–O–Me skeletal vibrations and is pronounced in materials with Ni²⁺, Co²⁺, and Sr²⁺ [46]. However, this intensity is influenced by the Me²⁺/Me³⁺ molar ratio in brucite-like lamellar layers, being lower in ternary magnetic hydrotalcite due to the presence of another divalent cation. In addition, the absorption band around 1,369 cm⁻¹ (ν ₅) was considered to be caused by the asymmetric stretching bond of the intercalated NO₃ [46,49] and by asymmetric vibrations of the CO 3 2 − anion in the interlayer space of the hydrotalcite [50].

Figure 3

(a) and (b) FTIR spectra of the magnetic hydrotalcite samples with Me²⁺/Me³⁺ molar ratios of 2 and 3; (c) Me²⁺/Me³⁺ comparisons on the IR spectra of hydrotalcites. The dotted lines indicate the main bond vibrations.

The presence of SrCO₃ in the Sr-modified samples generated a vibration band at 1,463 cm⁻¹ (ν ₆) due to the CO₃ symmetric and antisymmetric stretching [51]. In addition, an intense absorption peak at 1,460 cm⁻¹ (ν ₆) was attributed to the bending mode of the –OH groups of the adsorbed water interlayer [45]. The absorption band around 1,626 and 1,632 cm⁻¹ (ν ₇) was caused by the bending oscillation peaks of water molecules between layers [52,53]. The bending vibration of the interlayer water occurs at 1,700 cm⁻¹ (ν ₈) [54]. The band at 2,352 cm⁻¹ (ν ₉) was observed at high-frequency range and corresponded to NH 3 − [55].

The presence of vibrations at 3,403–3,467 cm⁻¹ (ν ₁₀), which indicate the presence of OH stretching vibration of the hydroxyl group in Mg–Al hydrotalcite [44], was caused by the water molecules between the layers of the laminar structure [56]. This shift is attributed to the ratio and not to the presence of magnetic nanoparticles. Additionally, characteristic vibrations of the hydrotalcite exhibit a slight shift to higher frequencies 3,467 (x = 3) – 3,414 (x = 2) cm⁻¹ (ν ₁₀), attributed to the symmetric stretching of the hydroxyl group linked to the Me–OH bond on the surface of the hydrotalcite [53].

3.2 Morphological characterization

SEM images of the synthesized samples were obtained at magnifications of 15 kx and 30 kx (Figure 4). The Fe₃O₄ sample (Figure 4a) exhibits a granular morphology with spherical particles of uniform size, approximately 220 nm in diameter, indicating a controlled and homogeneous synthesis. A more complex structure with aggregated particles is observed in the Fe₃O₄MgCoAl x = 2 (Figure 4b) and x = 3 (Figure 4c) samples. The particles form larger agglomerates, increasing the surface roughness and porosity due to the presence of hydrotalcite, possibly attributed to the semi-amorphous matrix observed. The Fe₃O₄MgAl x = 2 (Figure 4d) and x = 3 (Figure 4g) images show a heterogeneous agglomerated structure, characteristic of hydrotalcites. The sample with x = 2 exhibits higher porosity and a more heterogeneous distribution of nanoparticles, suggesting a more intense interaction between the particles and the hydrotalcite matrix. The Fe₃O₄MgNiAl x = 2 (Figure 4e) and x = 3 (Figure 4f) micrographs demonstrate a more compact morphology for the sample x = 3. The Fe₃O₄ nanoparticles are uniformly distributed in both samples, although a slight tendency to agglomeration is observed in x = 3, causing larger agglomerates and difficult to perceive edges. Finally, the Fe₃O₄MgSrAl x = 2 (Figure 4h) and x = 3 (Figure 4i) images reveal a granular morphology with larger and less defined particles compared to the other samples. The presence of SrCO₃ seems to significantly affect the structure, increasing the particle size and surface roughness. The distribution of Fe₃O₄ nanoparticles is less uniform, with a tendency to agglomerate formation for the sample x = 3. SEM analysis shows that the composition and molar ratio of metal cations significantly influence the morphology and distribution of Fe₃O₄ nanoparticles. These variations allow control of the morphological and structural properties of the material, which is crucial for its optimization of its potential surface applications. In general, the synthesized hydrotalcite samples show zones with irregular plate-like morphology agglomerations, which highlights the inherent characteristics of the layered materials, such as a high surface-to-volume ratio and a structural arrangement that favors the intercalation of ions and molecules. These results are comparable with previous reports [31,57].

Figure 4

SEM images at 15 kx and 30 kx (insets): (a) Fe₃O₄, (b) Fe₃O₄MgCoAl x = 2, (c) Fe₃O₄MgCoAl x = 3, (d) Fe₃O₄MgAl x = 2, (e) Fe₃O₄MgNiAl x = 2, (f) Fe₃O₄MgNiAl x = 3, (g) Fe₃O₄MgAl x = 3, (h) Fe₃O₄MgSrAl x = 2, and (i) Fe₃O₄MgSrAl x = 3.

The calculated grain sizes for all systems are shown in Table 3. The Me²⁺/Me³⁺ ratio has a remarkable relationship with the particle size of the samples [58]. For the sample Fe₃O₄ (without metal addition), the particle size is 220.5 ± 72.3 nm, indicating a relatively large and less controlled structure, without the intervention of divalent or trivalent metals. In the case of Fe₃O₄MgAl (x = 2, Me²⁺/Me³⁺ = 2), the particle size decreases to 119.7 ± 59.2 nm, suggesting that the addition of Mg²⁺ and Al³⁺ favors further nucleation, reducing agglomeration and promoting smaller particles. However, when the ratio of Me²⁺/Me³⁺ in Fe₃O₄MgAl is increased (x = 3, Me²⁺/Me³⁺ = 3), the particle size increases to 224.3 ± 86.1 nm, suggesting that a higher Me²⁺ content could favor the formation of larger particles due to a lower crystallization rate or higher agglomeration. In Fe₃O₄MgCoAl (x = 2, Me²⁺/Me³⁺ = 2), the particle size is 180.7 ± 87.9 nm, showing an intermediate value between the Fe₃O₄ and Fe₃O₄MgAl (x = 2) samples. This size could indicate a moderate control on particle nucleation and growth, favored by the addition of Co²⁺. As the Me²⁺/Me³⁺ ratio is increased to 3 in Fe₃O₄MgCoAl (x = 3), the particle size increases to 206.1 ± 76.5 nm, suggesting increased particle agglomeration due to the higher Me²⁺ content. In Fe₃O₄MgNiAl (x = 2, Me²⁺/Me³⁺ = 2), the particle size is 235.8 ± 167.2 nm, which is considerably larger than in Fe₃O₄MgAl (x = 2), which might reflect the influence of Ni²⁺ on the formation of larger particles. Fe₃O₄MgNiAl (x = 3, Me²⁺/Me³⁺ = 3) exhibits the largest particle size of 785.9 ± 554.9 nm, suggesting that the higher proportion of Me²⁺ favors higher agglomeration or weaker crystallinity, resulting in larger and less homogeneous particles. Finally, the Fe₃O₄MgSrAl samples present an interesting behavior: in Fe₃O₄MgSrAl (x = 2, Me²⁺/Me³⁺ = 2), the particle size is 763.6 ± 260.3 nm, indicating larger agglomeration, while at Fe₃O₄MgSrAl (x = 3, Me²⁺/Me³⁺ = 3), the particle size is reduced to 258.9 ± 50.8 nm, suggesting that the addition of Sr²⁺ at this ratio may promote better formation of smaller and less agglomerated particles.

The textural properties of the synthesized materials were assessed through N₂ adsorption–desorption isotherms at 77 K, using the BET method, as shown in Figure 5. The adsorption isotherms of type III can be observed in the Sr samples. This classification is indicative of materials with limited microporosity or low porosity. This result suggests the potential partial formation of a laminar structure in these materials, possibly due to the presence of SrCO₃, which was identified through FTIR and XRD analysis. Conversely, for the remaining materials, type IV isotherms were observed, characteristic of mesoporous materials, with a H3 type hysteresis loop. This pattern is typical in materials with plate-like particles and slit-shaped mesopores, which is a common feature in hydrotalcite-like materials [46,50].

Figure 5

N₂ adsorption–desorption isotherms of Fe₃O₄MgAl magnetic hydrotalcite with molar ratios of Me²⁺/Me³⁺.

Table 3 presents the parameters of the pore structure of the synthesized materials, including the specific surface area (S _BET), pore volume, and pore size. The results reveal that different Me²⁺/Me³⁺ molar ratios lead to significant changes in textural properties. Materials with a Me²⁺/Me³⁺ molar ratio of 3 exhibit higher surface areas (S _BET) compared to those with an Me²⁺/Me³⁺ ratio of 2. Thus, adjusting the ratio of metallic cations increases the pore size, leading to a decrease in the surface area. The pore size in all materials ranges from 3 to 36 nm, confirming the presence of mesopores in the laminar structure. These results indicate that the presence of magnetic nanoparticles does not impact the textural properties of the materials, and Fe₃O₄ acts as a magnetic separation agent. Furthermore, the potential core–shell structure seems to be favored at an Me²⁺/Me³⁺ ratio of 3, as these materials present a larger surface area, suggesting better coverage of Fe₃O₄ with the hydrotalcite. The Fe₃O₄MgNiAl and Fe₃O₄MgCoAl samples with Me²⁺/Me³⁺ of 3 showed a greater surface area, which is possibly associated with the nature of the synthesis that involves a heat of the suspension, which favors the simultaneous nucleation of the crystals [59].

The samples with the largest surface area are Fe₃O₄MgCoAl (x = 3) (160 m²/g) and Fe₃O₄MgAl (x = 3) (96 m²/g), which is consistent with a larger pore volume. This higher S _BET could be related to higher particle dispersion and greater accessibility to active sites. On the contrary, the samples with lower surface area are Fe₃O₄ (17 m²/g) and Fe₃O₄MgSrAl (x = 3) (2 m²/g), which also present a reduced pore volume, suggesting that the incorporation of certain metals favors particle compaction and reduces the surface area available for interactions.

A trend is observed in which samples with a larger particle size (such as Fe₃O₄MgNiAl (x = 3) with 785.9 ± 554.9 nm) also have a reduced surface area (145 m²/g) and a smaller pore volume. This suggests that larger particle size may be related to lower surface accessibility and less porous structure. On the other hand, samples with a smaller particle size, such as Fe₃O₄MgAl (x = 2), show a larger surface area (52 m²/g) and significant pore volume (0.42 cm³/g), indicating greater exposure to active sites and greater potential for applications where high surface area availability is required.

3.3 Magnetic characterization

The magnetic behavior of the synthesized hydrotalcite-type materials was analyzed by magnetization as a function of magnetic field curves at 50 K (low temperature) and 300 K (room temperature). All samples showed narrow S‐shape type loops (Figure 6), indicating a superparamagnetic behavior with no significant changes observed in the magnetic curves across 50 K. The magnetic saturation (M _s), remanence magnetization (M _r), and coercivity field (H _c) values calculated from magnetically recorded data are listed in Table 4. The M _s values exhibit notable variations depending on the Me²⁺/Me³⁺ ratio and the analysis temperature. Notably, Table 4 shows that M _s values are consistently lower in all magnetic hydrotalcite-type materials when compared to Fe₃O₄. Furthermore, it is worth noting that M _s values increase at lower temperatures across all cases.

Figure 6

Temperature dependence of magnetization ZFC-FC measured at a 1 kOe applied field of the Fe₃O₄ magnetic hydrotalcite with molar ratios of (a) x = 2 and (b) x = 3 measured at room temperature (T = 300 K).

At 300 K, the M _s values display minimal changes in the synthesized materials and do not exhibit significant differences concerning the Me²⁺/Me³⁺ ratio. These findings agree with the results reported by Chen et al. [21] for the Fe₃O₄@CuNiAl-LDH composite. Conversely, M _s values obtained at 50 K show notable variations in materials containing double divalent cations, as well as in relation to the Me²⁺/Me³⁺ ratio at this analysis temperature.

The alteration of Fe₃O₄ magnetic properties when combined with hydrotalcite-type materials can be attributed to the coating of LDH sheets on the Fe₃O₄ core. This phenomenon is a consequence of the decorated structure, where the sheets may adopt a vertical orientation. This orientation is predominantly associated with the synthesis method, particularly influenced by the choice of solvent used for dispersing the Fe₃O₄ nanoparticles in the water/methanol solution (V water/V methanol = 1/1, v/v), which leads to this specific orientation [4]. The lower M _s values observed in magnetic hydrotalcites can be attributed to the high concentration of sheets covering the Fe₃O₄ nanoparticles. The M _r/M _s ratio, which indicates magnetic stability, is generally low (<0.2), suggesting superparamagnetic behavior in samples at 300 K. It was observed to decrease in the following order: Fe₃O₄–MgCoAl > Fe₃O₄-MgNiAl > Fe₃O₄–MgSrAl > Fe₃O₄–MgAl > Fe₃O₄ for Me²⁺/Me³⁺ 2 and 3. This is related to the inter‐ and intragrain exchange interactions, sub‐lattice magnetization, magnetic anisotropy, and morphology of the tested sample [60].

These results indicate that the magnetic, crystalline, and textural properties of the synthesized materials are strongly influenced by the ratio and nature of the divalent cations. In general, a smaller crystallite size with a larger surface area was observed in the materials, which indicates greater surface interactions, except for the Fe₃O₄MgSrAl system (Me²⁺/M³⁺ = 3). This is because smaller particles have a higher surface/volume ratio, which increases the active area of the material. It was noted that the substitution of Mg²⁺ (0.72 Å), Co²⁺ (0.74 Å), and Ni²⁺ (0.69 Å) promotes the formation of layered structures due to their ionic radius similar to Fe³⁺ (0.78 Å). On the other hand, Sr²⁺ (1.18 Å) has a much larger ionic radius, which generates a structural disorder in the hydrotalcite. Thus, while Mg²⁺ stabilizes the structure and enhances the dispersion of Fe₃O₄, Co²⁺ and Ni²⁺ affect the magnetic properties by modifying anisotropy and coercivity. Sr²⁺, due to its large size, tends to destabilize the network, decreasing the active surface and the magnetization, as seen in the small M _s values. The reduction in M _s in these samples may be due to surface effects, where the magnetic moments at the interface do not fully contribute to the magnetic order [61]. This structural modification improves the interaction with other molecules or reagents, which is particularly relevant for catalytic and adsorption applications [62].

These modifications directly influence the magnetic properties since the decrease in saturation magnetization (M _s) in the combined materials is associated with the presence of lamellar layers coating the magnetic nanoparticles. Furthermore, the remanence (M _r) and coercivity (H _c) show variations as a function of the type and proportion of the divalent cation, indicating that the structure affects the interaction between the magnetic domains. It is observed that samples with Ni²⁺ and Co²⁺ maintain relatively high M _s values compared to other substitutions, suggesting that these species favor the preservation of the magnetic ordering of Fe₃O₄. Nevertheless, the slight decrease in M _s with respect to the pure Fe₃O₄ sample could be related to the dilution of the Fe content in the crystal lattice and the possible formation of spinel phases with lower magnetic moment. Furthermore, the increase in the M _r/M _s ratio in samples with Co²⁺ indicates a higher magnetic anisotropy, suggesting a modification in the stability of the magnetic domains [63]. On the other hand, the introduction of Sr²⁺ generates a greater decrease in M _s, possibly due to its large ionic radius, which induces structural distortions and reduces the amount of effective ferromagnetic interactions [36]. Furthermore, the increase in H _c in some samples with Sr²⁺ indicates greater structural disorder, which affects the inversion dynamics of the magnetic domains and their stability. In this context, the adjustment in the molar composition of cations during the synthesis of hydrotalcites allows modulating the crystalline structure, as well as their textural and magnetic properties, offering precise control over the behavior of these materials in various applications.