Co—Fe Prussian Blue Analogue Intercalated into Diamagnetic Mg—Al Layered Double Hydroxides

Abstract

A heterostructure of diamagnetic magnesium–aluminium layered double hydroxides (Mg–Al LDHs) and photomagnetic cobalt–iron Prussian Blue analogue (Co–Fe PBA) was designed, synthesized and then designated as LDH–PB. The cyanide-bridged Co–Fe PBA was two-dimensionally intercalated into the Mg–Al LDH template by the stepwise anion exchange method. LDH–PB showed ferrimagnetic properties with in-plane antiferromagnetic exchange interactions, as well as small photo-induced magnetization by visible light illumination due to the low dimensional structures and the characteristic photo-induced electronic states of the mixed valence of Fe^III(low spin, S = 1/2)–CN–Co^II(high spin, S = 3/2)–NC–Fe^II (low spin, S = 0).

1. Introduction

As the most promising groups of molecule-based magnets, PBAs are cyano-bridged three-dimensional face-centred cubic structures, which have received much attention due to their various interesting properties. For example, high T_C value magnets of V[Cr(CN)₆]₀₈₆ 2.8H₂O (T_C = 315K), [1] electrochemically switchable magnets of [Cr₂₁₂(CN)₆] 2.8H₂O [2] and magnetic pole inversion in (Ni_x^IIMn_1-x^II)₁₅[Cr^III(CN)₆] [3, 4] have been reported. Furthermore, photoswitchable magnetic compounds have especially been developed after first discovering photo-induced magnetization in the cobalt–iron PBA (Co–Fe PBA), [5] which is derived from the electron transfer of Fe^II to Co^III ions upon visible light illumination at low temperature. [6]

Moreover, magnetism on low-dimensional cyanide-bridged compounds has also attracted much attention due to limited magnetic interaction compared to the three-dimensional network. For example, a cyanide-bridged Co–Fe chain displaying magnetic and electric bistabilities arising from electron-transfer-coupled spin-transition, [7] a series of cyanide-bridged Co–Fe squares exhibiting two-step charge-transfer-induced spin-transition in the solid state, [8] and a layered Ni–Cr bimetallic compound expressing metamagnetic behaviour from the antiferromagnetic to the ferromagnetic phase under an applied field of 1,200 Oe [9] have been reported. As seen, various approaches, such as capping stronger ligands [10] or using the Langmuir–Blodgett technique, [11] have been applied in order to explore the properties of low-dimensional cyanide-bridged magnetic compounds.

On the other hand, layered double hydroxides (LDHs) are inorganic layered compounds consisting of positively-charged brucite-like host layers and charge-balancing interlayer anions. The general formula is [M^II_1-xM^III_x(OH)₂, (Aⁿ⁻)_x/n.mH₂O], where M^II and M^III are respectively divalent and trivalent metal ions, while Aⁿ⁻ are interlayer anions with water molecules located interlayer. LDHs have been widely studied owing to their application in catalysts, anion exchangers, separation, electrochemistry, biotechnology, photochemistry and so on. [12-16] Recently, the magnetic properties of LDHs have been reported. [17-19] For example, the magnetism of Ni–Mn LDH has shown competing interactions: in-plane (ferromagnetic) and out-of-plane (antiferromagnetic). The spontaneous magnetization at low temperature was observed in Ni–Cr and Ni–Fe LDHs. [17-20] Ni–Mn LDH has shown that the magnetism was driven by the competition between the ferromagnetic in-plane (due to Ni ions spin state) and the antiferromagnetic inter-plane. [21]

Intercalation of molecule-based magnetic materials into LDH layers might realize novel magnetic materials with a low-dimensional structure. So far, the insertion of Mn–Cr oxalate-bridged molecule-based magnets into Zn–Al LDH hosts has exhibited ferrimagnetic ordering due to the presence of dominant antiferromagnetic interactions between metallic centres. [21] Moreover, the growth of two-dimensional ferromagnetic Ni–Cr PBA into diamagnetic Zn–Al LDH has exhibited long-range magnetic ordering. [21] In addition, the controlled growth of M–Fe PBA (M = Ni, Co) into diamagnetic Mg–Al LDH and magnetic Ni–Al LDH has exhibited spin-glass behavior. [22]

Therefore, in order to understand the photomagnetic properties of low-dimensional Co–Fe PBA, it was two-dimensionally intercalated into the MgAl LDH template by using the stepwise anion exchange method. The obtained compounds were characterized by X-ray diffraction (XRD), infrared (IR) spectroscopy, ⁵⁷Fe Mössbauer spectroscopy, scanning electron microscopy (SEM) and magnetic measurement.

2. Experimental Section

2.1 Synthesis

All reagents used were of analytical grade and not further purified. Potassium hexacyanoferrate (III), magnesium, aluminium, ammonium and cobalt nitrate, urea and ethylene glycol were purchased from Wako. In order to minimize the contamination of carbonate ions, Milli-Q water and ethylene glycol were bubbled with nitrogen gas before the experimental process. The nitrogen flow was continued for 30 minutes at room temperature. In order to prepare the LDH–PB, the intercalation process was applied by stepwise anion exchange (Scheme 1). The detailed procedures are given in the following section.

Scheme 1.

The intercalation process of CoFe PB into LDH. The first step was a synthesis of LDH containing nitrate anions by the homogeneous precipitation method. The second step was the replacement of the nitrate anions (NO₃⁻) with hexacyanoferrate (III) anions ([Fe(CN)₆]³⁻) by the anion exchange method. The last step was the formation of interlayer CoFe PB in a two-dimensional network.

2.1.1 MgAl–NO₃ LDH (LDH–NO₃)

The white product MgAl–NO₃ LDH was synthesized in line with the homogeneous precipitation method. [18] Typically, 0.33 M of magnesium nitrate hexahydrate (Mg(NO₃)₂.6H₂O), 0.165 M of aluminium nitrate (Al(NO₃)₃9H₂O), 1.65 M of urea and 1 M of ammonium nitrate (NH₄NO₃) were dissolved in 100 ml of Milli-Q water. The mixed solution was refluxed for one day under continuous magnetic stirring and nitrogen protection. The resulting precipitate was centrifuged, washed with Milli-Q water and anhydrous ethanol a few times, and finally dried in a vacuum at room temperature (LDH–NO₃).

2.1.2 Intercalation of the hexacyanoferrate (III) anion (LDH–Fe(CN)₆)

The intercalation process was carried out by the anion exchange method. [19] 0.25 g of LDH–NO₃ was suspended in 25 mL of Milli-Q water. Then, 0.5 mL of K₃[Fe(CN)₆] (1 mmol) aqueous solution was added dropwise into the above solution. The mixed solution was magnetically stirred for six hours at room temperature under nitrogen atmosphere. The yellow solid MgAl–Fe^III(CN)₆ LDH (LDH–Fe(CN)₆) was obtained by centrifugation, washed with Milli-Q water and dried in a vacuum at room temperature.

2.1.3 Formation of Co–Fe PBA in the LD Hinterlayer (LDH–PB)

113 mg of LDH–Fe(CN)₆ was dispersed into 16 mL of a mixed Milli-Q water/ethylene glycol (volume=1:1) solution. 1.0 g of cobalt nitrate was added to the above solution, with the mixture stirred for two days at room temperature under nitrogen atmosphere. Finally, the red-brown solid Mg–Al–Co^IIFe^III PBA LDH (LDH–PB) was centrifuged, washed with Milli-Q water and dried in a vacuum at room temperature.

2.2 Physical measurements

Structural information was characterized by an X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy. XRD patterns were collected on a D8 Discover (Bruker) using Ni-filtered Cu Kα radiation (1.5418 Å, 40 kV and 40 mA). The data were collected with a step size of 0.01° in the 5°< 2θ< 75° range. The counting time was 0.42 °/min. Infrared spectra were recorded with an FT-IR 660 Plus spectrometer (JASCO, Japan) in the 4,000–400 cm^{− 1} range using powdered samples diluted in KBr pellets. The ⁵⁷Fe Mössbauer spectra were measured using a Wissel MVT-1000 Mössbauer spectrometer with a ⁵⁷Co/Rh source in the transmission mode. Surface morphology was examined with a field emission scanning electron microscope (FE-SEM) using SIRION(FEI). The homogeneous nature of the hybrid materials and their morphology were studied at a voltage of 5 keV. Elemental maps were obtained using an energy-dispersive X-ray spectrometer (EDX). Magnetic measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL-5). The susceptibility data were recorded in the temperature range of 2–20 K at a constant field of 100 Oe for zero-field-cooled (ZFC) and field-cooled (FC) curves, as well as under the magnetic field range of −5 T – 5 T at 2 K. The Xe lamp was used as the light source. The samples were put onto transparent tape, which was then surrounded with light fiber. The samples were illuminated at 5 K with the filtered visible light for one day.

3. Results and Discussion

3.1 Characterization of LDH–NO₃ and LDH–Fe(CN)₆ during the intercalation process

Figure 1A showed XRD patterns of LDH–NO₃ and LDH–Fe(CN)₆. Diffraction peaks at 10.02° (003), 19.93° (006) and 60.87° (110) were observed in LDH–NO₃, which are typical for LDHs (Figure 1Aa). The d₀₀₃ = 8.8 Å and a = 3.04 Å were consistent with well-known LDHs containing the nitrate anion (Table 1). [23, 24, 28-31] After intercalation of the hexacyanoferrate (III) anion, XRD showed diffraction peaks at 8.16° (003), 16.02° (006), 24.11° (009) and 60.82° (110) in LDH–Fe(CN)₆ (Figure 1Ab). The d₀₀₃ value increased to 10.8 Å due to the intercalation of the Fe(CN)₆ anion (Table 1). The gallery height (6.0 Å) showed that the hexacyanoferrate (III) anions were perpendicular to the LDHs' host layers with the C₃ axis. [27, 32-34]

Table 1.

The powder XRD data and calculated parameters of the compounds (a) LDH–NO₃, (b) LDH–Fe (CN)₆, and (c) LDH–PB. BS is basal spacing.

	hkl=003		hkl=006		hkl=009		hkl=110		Calc'd param.
	2θ°	d Å	2θ°	d Å	2θ°	d Å	2θ°	d Å	a Å	c Å	BS Å
LDH-NO3	10.02	8.82	19.93	4.45			60.87	1.52	3.04
LDH–Fe(CN)6	8.16	10.83	16.02	5.53	24.11	3.69	60.82	1.52	3.04	32.96	10.99
LDH-PB	8.21	10.76	16.47	5.38	24.80	3.59	60.81	1.52	3.04	32.29	10.76
a = 2 d 110/ c = (d003+2d006+3d009)/BS = c/3.

Figure 1.

(A) XRD patterns of (a) LDH–NO₃ and (b) LDH–Fe(CN)₆ from 5° to 75°. (B) The FT-IR spectra of (a) LDH–NO₃ and (b) LDH–Fe(CN)₆ in the region from 4,000 to 400 cm⁻¹ using powdered samples diluted in KBr pellets at room temperature. (C) SEM images of (a) LDH–NO₃ and (b) LDH–Fe(CN)₆.

Figure 1B shows the IR spectra of LDH–NO₃ and LDH–Fe(CN)₆. The absorption peak at 1,380 cm⁻¹ was observed in LDH–NO₃, which is attributable to the symmetric vibration of the interlayer nitrate anion (Figure 1Ba). [28-31, 36] LDH–Fe(CN)₆ showed that an absorption peak at 1,380 cm⁻¹ decreased, whereas other absorption peaks at 2,118 and around 2,040 cm⁻¹ appeared, which can be assigned to the free CN stretching of the Fe^III–CN and Fe^II–CN structures, respectively (Figure 1Bb). The partial reduction of Fe^III to Fe^II occurred during the intercalation of the hexacyanoferrate (III) anion. [25, 37, 38]

SEM images of the LDH–NO₃ and LDH–Fe(CN)₆ are shown in Figure 1C. As can be seen, the rosette aggregation for LDH–NO₃ was observed (Figure 1Ca). These layers were not perfect hexagonal platelet layers due to the rapid increase rate of nucleation as a result of higher reagent concentration. [23] Figure 1Cb shows that the shape and crystallinity remained as a layered structure in LDH–Fe(CN)₆.

3.1.1 Characterization of LDH–PB

XRD patterns showed that the diffraction peaks at 8.21° (003), 16.47° (006), 24.80° (009) and 60.81° (110) in LDH–PB (Figure 2A), similar to LDH–Fe(CN)₆. The LDH layered structure was maintained. The d₀₀₃ (10.76 Å) and gallery height (5.96 Å) values suggested the confined growth of PBAs. [21, 22] Other peaks at 17.5° (200), 24.8° (220) and 35.2° (400) were also observed, accounting for the formation of a three-dimensional face-centred cubic structured PBA. [4] In the IR spectra of LDH–PB, wide absorption bands around 2,090 cm⁻¹ and 2,035 cm⁻¹ were observed (Figure 2B), which can be assigned to bridged CN stretching in the Fe^II–CN–Co^II and free CN stretching in Fe^II–CN structures, respectively. A shoulder peak at 2,118 cm⁻¹ was also observed, which can be assigned to free CN stretching in the Fe^III–CN structure. Furthermore, another absorption peak at 2,125 cm⁻¹ was observed, which can be assigned to the structure with the mixed valence state of Fe^II(low spin, S = 0)–CN–Co^II(high spin, S = 3/2)–NC–Fe^III(low spin, S = 1/2). [39] The ⁵⁷Fe Mössbauer spectrum of LDH–PB at room temperature supports this statement, as well as shows the existence of two species (Figure 2C). The singlet absorption peak (Isomer Shift (I.S.) = −0.141 ± 0.004 mm/s) was assigned to Fe^II(low spin) and the other doublet absorption peak (I.S. = −0.346 ± 0.004 mm/s, Quadrupole Splitting (Q.S.) = 0.293 ± 0.007 mm/s) was assigned to Fe^III (low spin).[40] The existence ratio of Fe^III/Fe^II was 42/58, whose assignment was also consistent with the IR spectra. Although free [Fe^II(CN)₆]⁴⁻ and [Fe^III(CN)₆]³⁻ species might exist in the layer of LDH, their amount is too small because it is well-known that [Fe^II(CN)₆]⁴⁻ shows a single peak (I.S.= −0.05 mm/s, Q.S.= 0.00 mm/s), [41] while [Fe^III(CN)₆]³⁻ shows a doublet peak with small Q.S. (I.S.= −0.12 mm/s, Q.S.= 0.30 mm/s) [42] at room temperature. The EPMA (Electron Probe Micro Analyzer) measurement subsequently showed the mole ratio Fe/Co to be around 2.5, which also supports the view that the possibilities of the existence of amorphous cobalt hydroxide or simply adsorbed cobalt at the surface were very small.

Furthermore, an SEM image of LDH–PB showed a rosette aggregation structure, which means that the layered structure remained after the intercalation of Co–Fe PBA into LDH, although small particles were also observed on the surface of LDH (Figure 2D). It is suggested that three-dimensional Co–Fe PBA particles were also formed. Moreover, EDX analysis on the LDH–PB confirmed the presence of Co–Fe PBA and the homogeneous distribution of the existing elements (Figure 2E).

Figure 2.

Characterizations for LDH–PB. (A) XRD patterns of LDH–PB. (B) The FT-IR spectrum of LDH–PB in the region from 2,200 to 1,950 cm⁻¹ using powdered samples diluted in KBr pellets. (C) The ⁵⁷Fe Mössbauer spectra of LDH–PB. (D) An SEM image of LDH–PB. (E) Elemental maps of LDH–PB by EDX analysis. All were obtained at room temperature.

In order to compare the properties, LDH–PB(Fe^IICo^II) was also prepared by intercalating [Fe^II(CN)₆]⁴⁻ ions in the LDH-Fe(CN)₆ formation process. The characterization and magnetic properties of the LDH–PB(Fe^IICo^II) are shown in Figures S1 and S2 (Supporting information), respectively.

3.1.2 Magnetic properties of LDH–PB

Figure 3A shows the ZFC and FC curves of LDH–PB under 100 Oe. The FC curve increased with a decreasing temperature until it reached saturation at low temperature (Figure 3A, red square). The ZFC and FC curves coincided at high temperature, with separation starting at 13 K. The ZFC curves exhibited a maximum at T_max = 11.5 K, which corresponds to the blocking temperature (T_B) on the basis of the superparamagnetic behaviour of the samples (Figure 3A, black circle). The trends suggest the presence of short-range magnetic ordering due to the two-dimensional interlayer cyanide-bridged structures, which is different from the properties of bulk Co–Fe PBA. [5,43,44] Magnetic field dependence of magnetization at 2 K showed a hysteresis loop with the coercive field of 1,500 Oe (Figure 3B), whose value is also consistent with the short-range magnetic ordering due to the two-dimensional structures. It is suggested that there are two types of exchange interaction between Fe^III and Co^II in Co–Fe PBA. The first one is a ferromagnetic interaction between the t_2g orbital of Fe^III and the e_g orbital of Co^II(high spin). The second one is an antiferromagnetic interaction between the t_2g orbital of Fe^III and the t_2g orbital of Co^II(high spin). When the two exchange interactions both existed, the domination belong to the antiferromagnetic interaction. [6, 45] In the present system, a mixed valence state of Fe^II(low spin)–CN–Co^II(high spin)–NC–Fe^III(low spin) in two-dimensional Co–Fe PBA may also have contributed to the antiferromagnetic interaction, which induced the ferrimagnetic properties at low temperature.

3.1.3 Photomagnetic properties of LDH–PB

LDH–PB showed little response upon visible light illumination. The ZFC and FC curves were measured as a function of temperature before and after the light illumination, respectively. The magnetization values increased a little in both the ZFC and FC curves, although the transition temperature did not change after the illumination. Furthermore, annealing at room temperature gave back to the original values in both the ZFC and FC curves (Figures 4A, 4B). In the case of bulk Co–Fe PBA, it is well-known that photo-induced magnetization is observable, including a change in transition temperature (Tc). The photo-induced change of the electronic states of bulk Co–Fe PBA is considered as an internal electron transfer from Fe^II(low spin, S = 0)–CN–Co^III(low spin, S = 0) to Fe^III(low spin, S = 1/2)–CN–Co^II(high spin, S = 3/2). [5] Here, it is suggested that the small increase in the magnetization for LDH–PB is due to the change in the mixed valence state from Fe^II(low spin, S = 0)–CN–Co^III(low spin, S = 0)–NC–Fe^II(low spin, S = 0) to Fe^III(low spin, S = 1/2)–CN–Co^II(high spin, S = 3/2)–NC–Fe^II(low spin, S = 0), which we attribute to the presumably two-dimensional Co–Fe PBA in Mg–Al LDH.

Figure 4.

(A) ZFC (black circle) and FC (red square) magnetization curves versus temperature in the region from 2 K to 20 K under a magnetic field of 100 Oe for LDH–PB. (B) A magnetic hysteresis loop at 2 K for LDH–PB. Inset: (Top) Field dependence of magnetization in the range of 0 – 5 T; (bottom) magnification of the region from −2 k to 2 kOe for LDH–PB.

Figure 5.

Photomagnetic properties for LDH–PB. (A) ZFC and (B) FC magnetization curves under a magnetic field of 100 Oe. The data were obtained before illumination (black square), after visible light illumination (red circle) and after annealing at room temperature (blue triangle). The sample was illuminated at low temperature (5 K) under 0 Oe.

4. Conclusion

Co–Fe PBA was intercalated into what seems to be a two-dimensional framework of the Mg–Al LDH template by the stepwise anion exchange method. The obtained LDH–PB showed ferrimagnetic properties with in-plane antiferromagnetic exchange interactions, as well as small photo-induced magnetization by visible light illumination due to the localization of electron transfer in the characteristic Fe^III–CN–Co^II–NC–Fe^II structure.