Controlled Growth of Cyano-Bridged Coordination Polymers into

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Controlled Growth of Cyano-Bridged Coordination Polymers into Layered Double Hydroxides Geraldine Layrac,† Didier Tichit,*,† Joulia Larionova,*,‡ Yannick Guari,‡ and Christian Guerin‡ †

Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, MACS, 8 rue de l'Ecole Normale, 34296 Montpellier cedex 5, France ‡ Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, CMOS, Universite Montpellier II, Place E. Bataillon, 34095 Montpellier cedex 5, France

bS Supporting Information ABSTRACT: The controlled growth of cyano-bridged coordination polymers was developed by using layered double hydroxides (LDH) as bidimensional host structures. A series of nanocomposites [B0.66Al0.33(OH)2]0.33þ/[MFe(CN)6](B = Mg, Ni; M = Ni, Co) were obtained by step-by-step coordination of hexacyanoferrate building blocks and bivalent metal ions into the interlayer domain of the matrix. The obtained nanocomposites were studied by infrared (IR), UV-vis spectroscopy, X-ray diffraction, and magnetic measurements, which reveal the presence of cyano-bridged coordination polymers [MFe(CN)6]- intercalated into the LDH. [MFe(CN)6]- confined into diamagnetic [Mg0.66Al0.33(OH)2]0.33þ and magnetic [Ni0.66Al0.33(OH)2]0.33þ LDH layers show the presence of a spin-glass behavior in which the magnetic parameters depend on the nature of both the confined coordination polymer and the LDH host.

I. INTRODUCTION Cyano-bridged coordination polymers, also called Prussian Blue analogues, belong to an important family of molecule-based materials presenting interesting magnetic, optic, photoswitchable, and intercalation properties.1 These compounds may be used as molecular sieves, as materials for hydrogen storage, or as radioactive poison antidotes.2 Consequently, during the last 20 years, numerous compounds of this family with various structures have been synthesized and extensively studied due to their fundamental interest as well as their technological applications. In recent years, the research activity was not only devoted to investigations of various cyano-bridged coordination polymers with one-, two-, and three-dimensional structures at the macroscopic level, but also to their synthesis, organization, and studies at the nanosized level regime.3 Coordination polymer nanoobjects have been prepared a decade ago by S. Mann and co-workers4 with the first work on the synthesis of Prussian Blue nanocrystals of 12-50 nm. Since then, the number of articles dealing with these nano-objects is in constant expansion. The reason for the present interest in coordination polymer nanoobjects is due to their specific nature, which is different in comparison to other inorganic nanoparticles and to their intrinsic interesting physical properties. On one hand, these objects present all advantages of bulk molecule-based materials, such as determined and flexible molecular structures, determined and r 2011 American Chemical Society

adjustable physical and chemical properties, porosity, low density, and the possibility to combine several properties. Furthermore, they may be obtained from molecular building blocks by combining “soft” chemistry self-assembling reactions with conventional nanochemistry approaches. On the other hand, it is possible to design the nano-objects in such a way that they possess dimensionality at the nanoscale level, as well as controlled size, shape, and properties. In this connection, the syntheses of numerous cyano-bridged metallic nanoparticles (0D),5 nanowires (1D),6 and nanofilms or nanolayers (2D)7-10 have been reported. Concerning nanolayers (2D), various approaches affording the positioning of the cyano-bridged metallic networks at surfaces were explored using spin coating,7 grafting on surfaces,8 and Langmuir-Blodgett9 layer by layer techniques.10 In the present work, we choose layered double hydroxides (LDH) as matrix to achieve a cyano-bridged coordination polymer nanolayer. LDHs, well-known as anionic clays or hydrotalcite-like compounds, are a large class of natural and synthetic materials, which have the ideal formula [MII1-xMIIIx(OH)2][(An-)x/n 3 mH2O] in which x ranges between 0.2 and 0.33, MII = Mg, Zn, Ni, Co, and MIII = Al, Fe, Ga.11 These structures are made of Received: October 20, 2010 Revised: December 8, 2010 Published: February 07, 2011 3263

dx.doi.org/10.1021/jp1100335 | J. Phys. Chem. C 2011, 115, 3263–3271

The Journal of Physical Chemistry C positively charged mixed metal hydroxides layers [MII1-xMIIIx(OH)2]xþ whose charge is counterbalanced by exchangeable anions An- located in the interlayer domains, such as CO32-, NO3-, Cl-, and others along with water molecules. The interlayer distance can be tailored to allow confinement of a nanolayer of cyano-bridged coordination polymers. Recently, a pioneering work on the confinement of [NiCr(CN)6]- into diamagnetic Zn/Al LDH has been reported by Coronado et al.12 In this work, a continuous nanolayer of the coordination polymer inserted into LDH exhibiting a long-range magnetic ordering has been observed. In the present Article, we describe the synthesis and study of new nanocomposite materials in which magnetic cyano-bridged coordination polymers [MFe(CN)6]- (M = Ni, Co) were confined between diamagnetic [Mg 0.66Al0.33(OH)2 ]0.33þ and magnetic [Ni0.66Al0.33(OH)2]0.33þ LDH layers by using step-bystep coordination of hexacyanoferrate and metal ions building blocks into the matrix. We give a special emphasis on the investigation of the magnetic behavior of cyano-bridged coordination polymers into both diamagnetic and magnetic hosts.

II. EXPERIMENTAL SECTION II.1. Synthesis. All of the chemical reagents used in these experiments were analytical grade. Potassium hexacyanoferrate(III) (K3[Fe(CN)6]), cobalt, nickel, and aluminum nitrates (Co(NO3)2 3 6H2O, Ni(NO3)2 3 6H2O, Al(NO3)3 3 9H2O) were purchased from Sigma Aldrich and used as received. II.1.1. Pristine [B0.66Al0.33(OH)2][(NO3)0.33 3 mH2O] LDHs. The pristine LDHs (B = Mg (A), Ni (B)) were synthesized by a conventional coprecipitation method under ambient atmosphere.13 0.06 mol of Mg(NO3)2 3 6H2O (or Ni(NO3)2 3 6H2O) and 0.03 mol of Al(NO3)3 3 9H2O (B2þ/Al3þ molar ratio of 2) were dissolved in 200 mL of deionized water. This solution was delivered by a peristaltic pump (0.8 mL min-1) into a beaker. Simultaneously, a NaOH solution (2 M) was added to the same beaker at a controlled rate to maintain the pH close to 10 (Mg/Al LDH) or 8 (Ni/Al LDH) with a pH-STAT Titrino (718 Stat Titrino, Metrohm) apparatus. After complete precipitation, the gel obtained was refluxed at 353 K for 12 h. It was then repeatedly washed at 298 K (cycles of suspension in distilled water (4 L) followed by centrifugation) and finally was dried overnight at 353 K. II.1.2. Intercalation of [Fe(CN)6]3- between the Layers and Formation of [B0.66Al0.33(OH)2]0.33þ/[Fe(CN)6]3- (B = Mg, Ni) LDHs. The intercalation of [Fe(CN)6]3- into the pristine LDHs A and B was performed by an anionic exchange reaction in water. Three grams of the host LDH (A, B) was dispersed in 100 mL of water containing the required amounts of K3Fe(CN)6 corresponding to the theoretical anionic exchange capacity (AEC) equal to ca. 3.75 and ca. 3.00 mequiv g-1 for A and B, respectively. The exchange process was performed by stirring the aqueous suspension in air at room temperature for 15 h. The 1A and 1B solids were recovered and washed by dispersion and centrifugation in deionized water and finally were dried at 353 K for 15 h. II.1.3. Nanocomposites LDHs [Mg0.66Al0.33(OH)2]0.33þ/ [MFe(CN)6]- (Where M=Ni (2A) and Co (3A)) and [Ni0.66Al0.33(OH)2]0.33þ/[NiFe(CN)6]-(2B). The LDH-based nanocomposite materials containing [MFe(CN)6]- (M = Ni, Co) cyano-bridged coordination polymer were obtained by the successive treatments of the [B0.66Al0.33(OH)2]0.33þ/[Fe(CN)6]3-

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LDHs 1A, 1B with an aqueous solution of M(NO3)2 3 6H2O (M = Ni, Co). Two grams of hexacyanoferrate intercalated LDHs 1A or 1B were dispersed in 100 mL of water containing an amount of M(NO3)2 3 6H2O corresponding to the M2þ/[Fe(CN)6]3molar ratio of 1. After being recovered and washed by dispersion and centrifugation in deionized water, the 2A (B = Mg; M = Ni), 3A (B = Mg; M = Co), and 2B (B = Ni; M = Ni) solids were dried at 353 K for 12 h. II.2. Physical Measurements. IR spectra were recorded on a Perkin-Elmer 1600 spectrometer with 4 cm-1 resolution. UV-vis spectra were recorded in KBr disks on a Perkin-Elmer Lambda 14 spectrometer. Elemental analyses were performed by the Service Central d'Analyze (CNRS, Vernaison, France). The samples were heated at 3000 °C under He. Oxygen was transformed in CO and detected by using an IR detector. X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer using Cu KR1 radiation (λR = 1.54184 Å, 40 kV, and 50 mA). Data were collected between 2° and 70° 2Θ with a step size of 0.02° and a counting time of 0.2 s/step. Magnetic susceptibility data were collected with a Quantum Design MPMS-XL SQUID magnetometer working in the temperature range of 1.8-300 K and the magnetic field range of 0-50 kOe. The data were corrected for the sample holder, and the diamagnetism contributions were calculated from Pascal's constants.14

III. RESULTS AND DISCUSSION III.1. Synthesis and Characterizations. Two types of LDHs were selected for the confinement of cyano-bridged metallic coordination polymers: the diamagnetic A and the magnetic B LDHs whose chemical formulas were reported in Table 1. The BII/Al (with B = Mg or Ni) molar ratio in the layers was close to the expected value of 2 existing in the synthesis solutions. Charge equilibrium was reached in both samples with nitrates representing 83-86% of the anionic charge and cointercalated carbonates. The intercalation of cyano-bridged metallic layers into these LDHs was performed in two steps (Scheme 1). The first step consists of the intercalation of [Fe(CN)6]3- between the layers of the LDH by anionic exchange of NO3- to form [B0.66Al0.33(OH)2]/[Fe(CN)6] (B=Mg (1A), Ni (1B)) LDHs. The second step is the formation of the cyano-bridged metallic [MFe(CN)6]- (M = Ni, Co) species between the LDHs sheets by successive treatment of 1A and 1B with the corresponding metal ions. III.1.1. Intercalation of [Fe(CN)6]3- between the Layers and Formation of [B0.66Al0.33 (OH)2]0.33þ/[Fe(CN)6]3-/4LDHs. The elemental analyses of 1A and 1B allow one to suggest the formulas reported in Table 1. For the two host LDHs, the exchange of the initial NO3- is complete, and the amount of intercalated hexacyanoferrate species represents 66% and 75% of the anionic exchange capacity (AEC) in 1A and 1B, respectively. Charge equilibrium is satisfied due to the presence of CO32- anions and also assuming that [Fe(CN)6]4- species are formed by reduction of a part of the Fe3þ to Fe2þ during the intercalation as previously claimed by several authors.15b,16 The presence of carbonates anions may be reduced by the synthesis in the inert atmosphere.16b The amount of [Fe(CN)6]4- represents ca. 16% and 5% of the hexacyanoferrate content in 1A and 1B, respectively. It is noteworthy that K was found in trace amounts in 1A and 1B. 3264

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The Journal of Physical Chemistry C

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Table 1. Some Characteristics of LDHs Before (A, B) and After Intercalation of [Fe(CN)6]3-/4- (1A, 1B) chemical composition (%) sample

B(II)

Al

A

17.94

10.16

Fe

C

N

0.30

4.50

d003 (nm)

suggested formula

IR (ν CtN) (cm-1)

0.88

Mg0.66Al0.33(OH)2(NO3)0.285 (CO3)0.022 3 0.59H2O

1A

19.44

11.00

4.68

Mg0.66Al0.33(OH)2([Fe(CN)6]3-0.058

6.84

1.08

2119(s), 2090(w sh), 2044(m)

[Fe(CN)6]4-0.011(CO3)0.056) 3 0.26H2O B

33.48

7.96

0.3

3.39

Ni0.66Al0.34(OH)2(NO3)0.282

0.87

(CO3)0.029 3 0.83H2O

1B

31.94

7.78

4.01

5.55

5.96

Ni0.65Al0.35(OH)2([Fe(CN)6]3-0.082

1.05

[Fe(CN)6]4-0.004(CO3)0.044) 3 0.97H2O

2116(s), 2091(sh), 2056(s), 2043(m)

Scheme 1. Schematic Representation of an Elaboration of Nanocomposites Containing Cyano-Bridged Coordination Polymers into LDHs

To prove the presence of hexacyanoferrate anions between the layers, the IR spectra were recorded for the as-obtained LDHs 1A, 1B, especially in the spectral window 2000-2300 cm-1, that is, in the vicinity of the CN stretching mode, which is a fingerprint of the structural and electronic changes occurring into the cyano-group. The CN stretching frequency of a free CN- ion is 2080 cm-1, whereas upon coordination to a metal ion, it shifts to higher frequencies.17 The IR spectra of 1A, 1B show four bands at 2116, 2090, 2044, and 2035 cm-1 (Table 1 and Supporting Infomation Figure 1). The high frequency band corresponds to the stretching vibration of [Fe(CN)6]3- species, and the three other ones correspond to the presence of [Fe(CN)6]4- formed during the intercalation as previously suggested.15a,16b,16c In the other spectral domains, a medium band at 1625 cm-1 is due to the deformation mode of water molecules, and a band at 1370 cm-1 is due to the mode ν3 of the carbonate anions. Comparatively to the spectra of 1A, the broadening of the peaks with the appearance of a defined band at 2055 cm-1 and of several shoulders observed in 1B reveals a decrease in symmetry of the hexacyanoferrate species, suggesting a strong interaction or coordination to the brucite-like layers.16b,16c The XRD patterns of the pristine LDHs A and B are typical of LDH materials whose reflections can be indexed to a hexagonal lattice with a R3 rhomboedral symmetry (Figure 1). They display the characteristic peaks at ∼10 and 20° (2θ) assignable to the (003) and (006) reflections. The d003 basal spacing of 0.88 nm is in agreement with the intercalation of NO3- as the main compensating anion. Sharper (00l) diffraction peaks in A than in B account for the higher crystallinity of the former LDH material. A noticeable evolution is observed in the XRD patterns of 1A and 1B after exchange with [Fe(CN)6]3-. A shifting of the (00l) diffraction peaks to lower 2θ angle is observed corresponding to an increase of the basal spacing from 0.88 nm in A and B to 1.08 and 1.04 nm in 1A and 1B, respectively. These values are close to

Figure 1. XRD powder patterns of (a) A, (b) 1A, (c) B, and (d) 1B.

those reported in the literature for hexacyanoferrate-containing LDHs.15 The presence of three high order harmonics for (00l) reflections accounts for a good stacking order of the layers in these hybrid materials whose crystallinity is similar to that of the host LDHs. The slightly smaller d003 value obtained in 1B in comparison to 1A is in agreement with the strong interaction of the hexacyanoferrate species with the brucite-like layers of B previously suggested by IR spectroscopy. III.1.2. Growing of Cyano-Bridged Coordination Polymers into the LDHs. In the second step of our approach, hexacyanoferrate-intercalated LDHs 1A, 1B were reacted with aqueous solutions of M(NO 3 )2 3 6H 2 O (M = Ni or Co) (Scheme 1) corresponding to the molar ratio MII/Fe = 1, giving the nanocomposite LDHs [Mg0.66Al0.33(OH)2]0.33þ/[MFe(CN)6]- (where M = Ni (2A), Co (3A)) and [Ni0.66Al0.33(OH)2]0.33þ/[NiFe(CN)6]- (2B). The suggested formulas for the nanocomposite 3265



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Table 2. Some Relevant Characteristics of the Nanocomposite Samples 2A, 3A, and 2B: Elemental Analysis, Suggested Formula, Interlayer Distance, Infrared, and UV-Vis Spectroscopy Data chemical composition (%) sample

B(II)

Al

Fe

Ni/Co

C

N

2A

16.07

9.96

4.91

4.13

7.20

7.05

suggested formula Mg0.64Al0.36(OH)2([NiFe(CN)6]-0.067

d003, nm 1.08

[NiFe(CN)6]2-0.013(CO3)0.12) 3 0.16H2O

3A

14.50

9.43

5.61

2B

31.92

7.50

3.89

3.69

8.05

8.30

5.41

6.03

Mg0.63Al0.37(OH)2([CoFe(CN)6]-0.062 [Fe(CN)6]3-0.026[Fe(CN)6]4-0.017 (CO3)0.08) 3 0.78H2O Ni0.65Al0.35(OH)2([NiFe(CN)6]-0.025 [Fe(CN)6]3-0.057[Fe(CN)6]4-0.004

IR (ν CtN), cm-1 2182(w), 2095(s),

UV-vis λ, nm 410

2051(m), 2035(m) 1.08

2182(vw), 2117(s), 2095(sh), 2049(m), 2034(m)

585

0.99

2167(w), 2098(s),

405

2043(m)

(CO3)0.046(NO3)0.045) 3 0.73H2O

materials based on the results of elemental analyses and assuming charge equilibrium are reported in Table 2. They indicate that anionic [MFe(CN)6] species with the expected MII/Fe value of 1 are formed by reaction of MII with 100% and 60% of the intercalated hexacyanoferrate species in 2A and 3A, respectively, and ca. 30% of them in 2B.17 This fact suggests a diffusional limitation in the accessibility of M2þ to the intercalated hexacyanoferrate species in 2B and 3A, with the coordination polymer network being likely formed at the entrance of the galleries. The carbonate content in 2A and 3A and both the carbonate and the nitrate contents in 2B increase after intercalation of Ni2þ or Co2þ, comparatively to the parent 1A and 1B samples to ensure charge equilibrium in the materials. After coordination of M2þ, the nanocomposite LDHs change their color from greenish to green-yellow for 2A and 2B and brownish for 3A. The electronic spectra of these nanocomposites show adsorption bands in the visible region corresponding to intermetal charge-transfer bands from M to Fe, which can also be found in the UV-vis spectra of the bulk counterparts (Table 2).17 The IR spectra of the obtained nanocomposites 2A and 2B in the spectral window 2000-2200 cm-1 present four bands listed in Table 2. These nanocomposites materials show, in addition to the previously observed band at ca. 2116 cm-1 characteristic of terminal cyano groups of [Fe(CN)6]3-, a high frequency band in the region 2167-2182 cm-1, which can be attributed to the stretching vibrations of the CN ligand bridged between M2þ and Fe3þ, as it was reported for the bulk cyano-bridged coordination polymers (Table 2 and Supporting Information).17 In addition, the previously observed bands at 2090 and 2043 cm-1 attributed to the presence of [Fe(CN)6]4- were also shifted toward higher frequency in 2A and observed at ca. 2096 and 2051 cm-1, suggesting the coordination of M2þ to [Fe(CN)6]4-. The XRD patterns of LDH nanocomposites are greatly modified after coordination of M2þ to the intercalated hexacyanoferrate species (Figure 2). The intensity of the peaks decreases comparatively to the parent LDHs. It must be pointed out that this likely concerns the (00l) peaks situated in the 2-30° (2Θ) range, rather than the peaks in the 30-70° (2Θ) range. Remarkably, the d003 value of 1.08 nm is similar in comparison with the values observed for the composite LDHs before coordination of the M2þ cations. Besides, a series of weakly intense peaks at 17.6, 24.8, and 35.2° (2Θ) are present, which are respectively assigned to the (200), (220), and (400) reflections in the cubic space group Fm3m of the Prussian blue analogues.18 This feature accounts for the formation of a network of cyano-bridged coordination polymers [MFe(CN)6]-/[MFe(CN)6]2- in the

Figure 2. XRD powder patterns of (a) 2A, (b) 3A, and (c) 2B (b, cyano-bridged coordination polymer; [, LDH).

interlayer space of host LDHs by consecutive coordination of M2þ to previously intercalated [Fe(CN)6]3- and [Fe(CN)6]4species. The structure of the brucite-like layers is maintained in these different composites, as the peaks situated in the 30-70° (2Θ) range are almost unaffected. In contrast, a turbostratic or stacking disorder explains the vanishing of the (00l) peaks due to the intercalation of the coordination polymers, which is more important in 2A than in 3A according to its larger content of coordination polymer.

IV. MAGNETIC PROPERTIES OF THE NANOCOMPOSITES The magnetic measurements of the nanocomposite materials were performed by using dc and ac modes on the SQUID magnetometer working in the temperature range between 1.8 and 300 K to understand the magnetic behavior of the inserted coordination polymers. In this study, we will describe and compare the magnetic behavior of M2þ/[Fe(CN)6]3- (M2þ = Ni2þ, Co2þ) coordination polymers into the diamagnetic Mg/Al LDH matrix A and into the magnetic Ni/Al LDH one, B. Note that the previously published study performed for magnetic LDH with [Ni1.86Al1.14(OH)6]1.14þ layers shows the presence of ferromagnetic exchange interactions between Ni2þ ions inducing an appearance of long-range magnetic ordering below 3.8 K.20 However, the Ni/Al stoichiometry in this LDH is different in comparison to our LDH host B. For this reason, the magnetic 3266



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Figure 3. (a) Temperature dependence of χT for 2A performed with an applied magnetic field of 1000 Oe. Inset: Hysteresis loop for 2A performed at 1.8 K. (b) Temperature dependence of χT performed with an applied magnetic field of 1000 Oe for samples B (4) and 2B (O). Inset: Hysteresis loop performed at 1.8 K for B (b) and 2B (O).

Table 3. Magnetic Data for the Nanocomposites Arrhenius lawb χT at 300 K

power lawd

Hc

Ms

sample

(emu K mol-1)

Ta (K)

(Oe)

(emu mol-1)

Ea/kB (K)

τ0 (s)

Ea/kB (K)

τ0 (s)

zν

Tg (K)

τ0 (s)

zν

2A 3A

0.10 0.15

12.5 9.9

1000 950

1122 1832

957(3) 1107(7)

8.76 10-35 5.90 10-42

86 062(11) 90 138(9)

6.91 10-13 7.81 10-15

1.8 2.3

11.7 9.5

1.99 10-10 2.80 10-11

7.8 7.1

B

0.66

2.9

75

6681

192(2)

3.22 10-27

307(2)

2.31 10-11

4.5

2.0

0.043

0.4

2B

0.70

2.9

131

7618

194(3)

1.19 10-27

352(2)

3.35 10-12

5.0

1.9

0.065

0.3

900(9)

1.85 10-45

129 000(13)

7.65 10-22

2.2

8.4

1.45 10-11

7.8

∼8 a

scaling lawc

Maximum value on the ZFC curve performed with 100 Oe. bτ = τ0 exp(Ea/kBT). c τ = τ0 exp(Ea/kBTzν). d τ = τ0[Tg/(T - Tg)]zν.

properties of B were also investigated and used for a comparison with the properties of the nanocomposite 2B. IV.1. DC Magnetic Measurements on Nanocomposites [B0.66Al0.33(OH)2]0.33þ/[MFe(CN)6]- (B = Mg, Ni; M = Ni, Co). First, the temperature dependence of the magnetic susceptibility was performed on coordination polymers inserted into the diamagnetic matrix A. Figure 3a shows the temperature dependence of the χT product for sample 2A performed with an applied magnetic field of 1000 Oe. The χT value at 300 K is equal to 0.10 emu K mol-1 that corresponds well to 6.7% of 1.375 emu K mol-1 calculated for [NiFe(CN)6]- with one Ni2þ (S = 1) and one Fe3þ (S = 1/2) without interactions. This value is in good agreement with the elemental analysis taking into account the presence of paramagnetic [NiFe(CN)6]2- (calculated value 0.013 emu K mol-1) (see Table 2). The χT value increases as the temperature decreases first slowly then abruptly after 50 K, reaches a maximum value at 3.47 K (χT = 3.48 emu K mol-1), and then decreases. The temperature dependence of 1/χ was fitted above 80 K with the Curie-Weiss law giving the Curie constant C = 0.09 emu mol-1 and the Weiss constant Θ = 61 K. The positive Weiss constant corresponds to the presence of predominant ferromagnetic interactions in the nanocomposite. It should be noted that ferromagnetic Ni2þ-Fe3þ interactions through the cyano bridge have been observed in the analogous bulk compound18 and in the respective nano-objects.5k,5m,9a The temperature dependence of χT performed for 3A presents a different shape: χT decreases as the temperature decreases, presents a minimum value at 67 K, and then increases and present a maximum value at 11.25 K. The 1/χ = f(T) curve was fitted with Curie-Weiss law giving the Curie constant

C = 0.14 emu mol-1 and the Weiss constant Θ = -55 K. The negative sign of the latter indicates both the presence of predominant antiferromagnetic interactions and the presence of the spin-orbit coupling of Co2þ as it was also found in the bulk analogous.18 The χT values at 300 K are equal to 0.15 emu K mol-1, which is slightly lower than the value expected for 6.2% of [CoFe(CN)6]- into the LDH A with the calculated χT value (3.54 emu K mol-1) for one Co2þ (3.16 emu K mol-1)17 and one Fe3þ (0.375 emu K mol-1) without interactions (Table 3), taking also into account the presence of a slight amount of paramagnetic [Fe(CN)6]3- into LDH (0.009 emu K mol-1). Figure 3b shows the temperature dependences of the χT products both for the pristine LDH [Ni0.66Al0.33(OH)2]0.33þ B and for the nanocomposite 2B where [NiFe(CN)6]- was inserted into B performed with an applied field of 1000 Oe. The χT values at 300 K are equal to 0.66 and 0.70 emu K mol-1 for B and 2B, respectively. For B, this value corresponds well with the calculated one for the spins of Ni2þ (S = 1) without interactions (Table 3). In the case of 2B, this room temperature χT value corresponds to what is expected for a sum of the values obtained for B (0.66 emu K mol-1), 2.5% of 1.375 emu K mol-1 calculated for [NiFe(CN)6]- with Ni2þ (S = 1) and Fe3þ (S = 1/2) without interactions (0.033 emu K mol-1), and 5.7% of the paramagnetic [Fe(CN)6]3- (0.021 emu K mol-1). For both curves, χT increases as the temperature decreases and reaches a maximum value at 5.83 K, and then decreases. The temperature dependences of 1/χ fitted with the Curie-Weiss law give the Curie constants C = 0.68 and 0.69 emu mol-1 and the Weiss constants Θ = 10.3 and 11.1 K for B and 2B, respectively. The positive Weiss constants correspond to the presence of predominant 3267



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Figure 4. (a) Field-cooled (0) and zero field-cooled (b) magnetization (FC/ZFC) curves versus temperature for the nanocomposite 2A. Inset: Fieldcooled and zero field-cooled magnetization (FC/ZFC) curves versus temperature for the nanocomposite 3A. (b) FC (b) and ZFC (O) magnetization curves versus temperature for the nanocomposite 2B. Inset: FC and ZFC magnetization curves versus temperature for the pristine LDH B. The applied magnetic fields are 100 Oe.

ferromagnetic interactions in the pristine LDH and in the nanocomposite. The field dependences of the magnetization were performed at low temperature (1.8 K) on these samples to confirm the observed M2þ-Fe3þ interactions. The magnetization for 2A increases as the field increases and at 50 kOe tends to saturation (inset of Figure 3a). The value of the magnetization at 50 kOe is equal to 1122 emu mol-1 (0.2 μB), which corresponds well to the expected values for 6.7% of 16 755 emu mol-1 (3.0 μB) calculated for {NiFe} unit with ferromagnetic Ni2þ-Fe3þ interactions through the cyano bridge. This curve shows the presence of the hysteresis effect with a coercive field of 1000 Oe (Table 3). Sample 3A presents hysteretic behavior similar to 2A with the value of the magnetization at 50 kOe of 1385 emu mol-1 (0.25 μB), corresponding to 6.2% of inserted [CoFe(CN)6]-, taking into account an antiferromagnetic Co2þ - Fe3þ interactions (4 μB). The value of the coercive field is 950 Oe (Table 3). The M = f(H) curve for 2B presents also an open hysteresis loop, but the value of the coercive field of 131 Oe is smaller than for 2A (inset of Figure 3b, Table 3). This fact may be explained by the small amount of inserted coordination polymer and the influence of the magnetic host. Note that the pristine LDH B exhibits a very similar curve (inset of Figure 3b). For both samples, the magnetization at 50 kOe does not reach the saturation. For B, the value of the magnetization at 50 kOe is equal to 6681 emu mol-1, and for 2B it is of 7099 emu mol-1 that corresponds to the sum of the magnetization of B and the expected value for 2.5% of [NiFe(CN)6]- inserted into the matrix (Table 3). To determine the temperature of the magnetic transitions, the zero field-cooled (ZFC)/field-cooled (FC) magnetization curves were performed on our samples. The ZFC/FC curves for the nanocomposites 2A, 3A, and 2B performed in the range of 2-35 K are shown as examples in Figure 4a,b. The ZFC curves were obtained by recording the magnetization when the sample is heated under a field of 100 Oe after being cooled in zero magnetic field. The FC data were obtained by cooling the sample under the same magnetic field after the ZFC experiment and recording the change in sample magnetization with temperature. The ZFC curve obtained for 2A exhibits a maximum at Tmax = 12.5 K, and the FC curve increases as the temperature decreases and never reaches the saturation. The FC and ZFC curves for this sample coincide at high temperatures and start to separate at ca. 16 K. Note that as the applied magnetic field increases, the peak

on the ZFC curve is shifted toward low temperature and occurs at 3.5 K at 1000 Oe. The ZFC/FC curve of 3A, where [CoFe(CN)6]- was inserted into the diamagnetic host, presents behavior similar to the maximum on the ZFC curve at 9.94 K (inset of Figure 4a). These curves suggest the presence of a short-range rather than a long-range magnetic ordering usually observed for the cyano-bridged coordination polymer nanoparticles. The ZFC curve of the nanocomposite 2B shows a peak at 2.9 K and a small shoulder around 8 K (Figure 4b). The FC curve coincides with ZFC at high temperature and at ca. 10 K starts to separate from the ZFC curve and increases as the temperature decreases. Regarding the ZFC/FC curves of the pristine LDH B (inset of Figure 4b), it clearly appears that the low temperature transition in 2B corresponds to the one of the host. Indeed, the ZFC curve for the pristine matrix B shows a narrow peak at Tmax = 2.9 K. The shoulder on the ZFC curve of 2B may signify the second magnetic transition caused by the presence of the [NiFe(CN)6]- species into the host LDH. The presence of this second transition is confirmed by dynamic (ac) measurements (vide infra). IV.2. Dynamic Magnetic Measurements of Nanocomposites [B0.66Al0.33(OH)2]0.33þ/[MFe(CN)6]- (B = Mg, Ni; M = Ni, Co). The irreversibility of the ZFC/FC curves was investigated in detail by using alternating current (ac) susceptibility measurements. First, the temperature dependences of the in-phase, χ0 (absorptive), and out-of-phase, χ00 (dispersive), components of the ac susceptibility were measured with no static field applied for frequencies from 1 to 1498 Hz for 2A (Figure 5). At 1 Hz, both, χ0 and χ00 responses present a peak at 14.0 and 12.4 K, respectively, which shifted toward higher temperatures as the frequency increases. Sample 3A containing [CoFe(CN)6]- species shows frequency-dependent behavior similar to that of the χ0 and χ00 peaks at 12.6 and 11.9 K at 1 Hz. The temperature dependences of χ0 and χ00 measured for the nanocomposite 2B are shown in Figure 6. At 1 Hz, both χ0 and χ00 responses present a frequency-dependent peak at 3.1 and 2.8 K, respectively, similar to what is observed in the corresponding pristine LDH B (inset of Figure 6). Note that such frequencydependent behavior has been previously observed in the case of several magnetic LDHs and attributed to the presence of spinglass behavior due to the low dimensional character of structures and/or random spin distribution in the networks.20 The temperature dependence of χ0 and χ00 for nanocomposite 2B presents 3268



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Figure 7. Relaxation time dependence on frequency fitted with the critical scaling law model τ = τ0[Tg/(Tmax - Tg)]zν for 2A.

Figure 5. Temperature dependence of in-phase, χ0 (top), and outphase, χ00 (bottom), components of the ac susceptibility of 2A performed with zero dc magnetic field. Frequencies: 1 (O), 125 (b), 499 (0), 998 (9), and 1498 Hz (4).

Figure 6. Temperature dependences of in-phase, χ0 (top), and out-phase, χ00 (bottom), components of the ac susceptibility of 2B performed with zero dc magnetic field. Insets: Temperature dependences of in-phase, χ0 (top), and out-phase, χ00 (bottom), components of the ac susceptibility of 2. Frequencies: 1 (O), 125 (b), 499 (0), 998 (9), and 1498 Hz (4).

also a second peak at 9.7 and 9.1 K, respectively, which is visibly due to the presence of [NiFe(CN)6]- between the layers. It

should be noted that analogous bulk cyano-bridged coordination polymers exhibiting long-range magnetic ordering at 23 K does not present a frequency-dependent behavior,18 but the nanoparticles of these coordination polymers exhibiting a short-range magnetic ordering usually show such frequency dependence.3,5 Generally, the frequency-dependent behavior of the ac susceptibility occurs: (i) in superparamagnetic isolated clusters or nanoparticles21 or (ii) in spin-glass or spin-glass-like systems.22 Different models were considered to determine the magnetic regime of the obtained composites. First, the temperature dependence of the relaxation time extracted from the maximum of the χ00 component of the ac susceptibility was fitted with an Arrhenius law, τ = τ0 exp(Ea/kBT), where Ea is the average energy barrier for the reversal of the magnetization, τ0 is the attempt time, and kB is the Boltzmann constant. According to the Neel model, this law governs the temperature dependence of the relaxation of the magnetization of noninteracting superparamagnetic systems.21 The values of the energy barrier, Ea/kB, and of the pre-exponential factor τ0 are equal to 957(3) K and 8.76 10-35 s for 2A (Table 3). The obtained values of τ0 for this sample are much smaller than those observed for pure superparamagnetic systems (10-9-10-12 s) and have no physical meaning.21,22 Closed parameters were obtained for the samples 3A and 2B considering both the first and the second magnetic transitions of the latter (Table 3). We further verified if the dynamics of these samples would exhibit critical slowing down, as observed in canonical spin-glass systems. Two relevant characteristics of the spin-glass dynamics are: (i) a continuous closely logarithmic time relaxation and (ii) the appearance of aging phenomena at low temperature. In threedimensional (3D) spin glasses, the relaxation time diverges at a finite temperature (Tg 6¼ 0 K), whereas in two-dimensional (2D) systems it diverges at T = 0 K.23 Accordingly, the dynamics of the samples were analyzed with two different models for the critical slowing down of relaxation time, one for 3D and the other for 2D spin glasses. Considering that an equilibrium ordered phase occurs at a finite critical temperature, Tg 6¼ 0 K, the relaxation time dependence on frequency can be fitted by the conventional critical scaling law of the spin dynamics, τ = τ0[Tg/(Tmax Tg)]zν, where Tg is the glass temperature and zν is a critical exponent.23,24 The best fits give Tg = 11.7 K, τ0 = 1.99 10-10 s, and zν = 7.8 for 2A, and Tg = 9.5 K, τ0 = 2.80 10-11 s, and zν = 7.1 for 3A (Figure 7, Table 3). The obtained zν values are in the range 4-12 expected for classical spin-glass 3269



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Figure 8. (a) Relaxation time dependence on frequency fitted with the critical scaling law model τ = τ0[Tg/(Tmax - Tg)]zν for 2B; (b) relaxation time dependence on frequency fitted with the 2D critical scaling law model τ = τ0 exp(Ea/kBTzν) for B (b) and 2B (O).

systems, as well as the τ0 values are in the expected range of 10-7-10-12 s.22 In the second (2D) model, where the transition temperature occurs at Tg = 0 K, the frequency dependence of the relaxation time can be described by a generalized Arrhenius law τ = τ0 exp(Ea/kBTmaxzν).23 However, no satisfactory fits (τ0 value in the range 10-7-10-12 s) were obtained in the case of 2A and 3A, indicating that this model is not appropriate (Table 3). For the nanocomposite 2B where [NiFe(CN)6]- species was inserted into magnetic LDH, the frequency-dependent behavior was considered for two magnetic transitions. The temperature dependence of the relaxation time for the high temperature transition was fitted with a conventional critical scaling law of the spin dynamics, τ=τ0[Tg/(Tmax-Tg)]zν (Tg 6¼ 0 K), giving satisfactory parameters Tg = 8.4 K, τ0 = 1.45 10-11 s, and zν = 7.8 (Figure 8a, Table 3) as it was obtained in the case of [NiFe(CN)6]- inserted into diamagnetic LDH 2A.23 On the other hand, reasonable parameters were obtained for the low temperature transition by using a generalized Arrhenius law τ = τ0 exp(Ea/kBTmaxzν) (Tg = 0)23 with zν = 5.0, τ0 = 3.35 10-12 s, and Ea/kB = 352(2) K (Figure 8b, Table 3). Note that closed parameters were also obtained for the pristine host B (Table 3). Thus, the investigations of the relaxation dynamics of the magnetization show that all inserted coordination-polymer nanoparticles behaves mostly as cluster-glass systems and may be described by using a classical scalling law of the spin dynamics with Tg 6¼ 0. This shows that the observed transitions for 2A, 3A, and the second transition for 2B have to be attributed to a glasslike transition more than to the long-range magnetic ordering or to the sum of independent single particle blocking-unblocking processes perturbed by interparticle interactions.

V. CONCLUSIONS The concept of using the interlayer space of LDHs as a wellorganized bidimensional domain for the growth of coordination polymers appears very fruitful. Moreover, in this approach, the nanocomposites formed can associate the magnetic properties of the host and the guest entities. In the present work, cyano-bridged coordination polymers [MFe(CN)6]- (where M = Ni, Co) were included in both diamagnetic Mg/Al LDH and magnetic Ni/Al LDH hosts. The two-step procedure in the confined interlayer space of the LDH that we have adopted for the synthesis of the coordination polymers accounts for the growth of the cyano-bridged 2D

network. At high hexacyanoferrate loadings, steric hindrance can limit the access of Ni2þ or Co2þ to the hexacyanoferrate species. Therefore, the coordination polymers are formed with some remaining free hexacyanoferrate anions. The magnetic measurements performed on these nanocomposites LDHs by using both dc and ac modes confirm the formation of the coordination polymers between the layers. The amount of magnetic species [MFe(CN)6]- into LDHs found from magnetic measurements corresponds well to what is found from chemical analysis taking into account the presence of a small amount of [Fe(CN)6]4-. A different magnetic behavior in comparison to the ones for the bulk analogues presenting long-range magnetic ordering was observed. In particular, a decrease of the maximum on the ZFC curves (Tmax) in comparison with the critical temperatures of the bulk analogous was pointed out. Similar behavior has been previously mentioned for the cyano-bridged metallic thin films obtained by the Langmuir-Blodgett techniques9 and for [NiCr(CN)6]- layers in Zn/Al LDH.12 The dynamic analysis in these systems reveals a frequencydependent behavior of the relaxation time of magnetic transitions. Different models, such as Arrhenius law describing superparamagnetic behavior of isolated entities, and critical scaling down models describing spin glass or spin-glass-like behavior were used to determine the nature of the observed low-temperature transitions. The fits of the experimental data with a critical scaling down law appropriated for classical spin-glass systems well describe the case of the composites. Such difference in the magnetic behavior in comparison with the [NiCr(CN)6]obtained into Zn/Al LDH12 presenting long-range magnetic ordering may be explained by the presence of diamagnetic entities [Fe(CN)6]4-, which has not been observed in the case of hexacyanochromate and which induces an appearance of a short-range ordering. In the case of the cyano-bridged [NiFe(CN)6]- layer confined into the magnetic Ni/Al LDH, the observed low temperature magnetic transitions present the same nature as the continuous layers confined into diamagnetic LDH. However, an obvious decrease of the coercive field was observed that is probably due to the presence of the magnetic host.

’ ASSOCIATED CONTENT

bS

Supporting Information. FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*(D.T.) Fax: (33) 4 67 16 34 70. E-mail: [email protected]. (J.L.) Fax: (33) 4 67 14 38 52. E-mail: joulia.larionova@ univ-montp2.fr.

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Controlled Growth of Cyano-Bridged Coordination Polymers into

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