New acetate-intercalated CoNiFe, ZnNiFe and CoZnFe layered double hydroxides: Synthesis by forced hydrolysis in polyol medium and characterization

New layered double hydroxides (LDHs) CoNiFe, ZnNiFe and CoZnFe with M II /M III molar ratio of 3, with acetate ions in the interlayer region have been prepared by forced hydrolysis in polyol medium. The physico-chemical properties of all products were characterized by the methods of powder X-ray Diffraction, Infrared Spectroscopy, UV-Spectroscopy, Elemental analysis, Thermal analysis, Transmission Electron Microscopy and Scanning Electron Microscopy. It was found that all compounds present the typical features of hydrotalcite-like structure and exhibit a turbostratic character. The insertion of acetate anions into LDHs was confirmed and the interlayer spacing values were respectively 12.47, 13.64 and 14.69 Å for CoNiFe, ZnNiFe and CoZnFe. We notice a difference between the interlayer spacing for all synthesized phases with and without zinc. This can be explained by a structural modification on the level of the layers and/ or different arrangement from the inserted species (anions + water) in the interlayer space for zinc materials.


INTRODUCTION
Hydrotalcite-like compounds, also called anionic clays or layered double hydroxides (LDHs) are very attractive materials for a broad range of applications [1,2].They can be presented by the general formula [M II 1-x M III x (OH) 2] +x/n (A -n ) x/n.mH2O. The positive layer charge is created by substituting a divalent cation with a trivalent one in the octahedral hydroxide layer, with anions (A -n ) and water in the interlayer space [3].
LDHs are elaborated by several synthesis methods: coprecipitation at fixed pH and exchange reactions [4], reconstruction [5], mechanochemical process [6] and reverse microemulsion method [7]. However the coprecipitation is the most commonly used method with carbonate as the intercalated anion in the most cases. Despite its widespread use, this method presents two main disadvantages. First, it needs a precise control of several parameters mainly the pH. Second, the carbonate anion has a great affinity toward the inorganic layer charged positively. This limits exchange reactions and thus the obtention of LDHs with desired intercalated anions.
Recently an alternative synthesis method has been proposed. It takes advantages from the various properties of polyols. Indeed it has been shown that two main reactions can be easily conducted in such medium: namely reduction and forced hydrolysis along with inorganic polymerization [8] [9]. The competition between these two reactions can be easily controled by the hydrolysis ratio defined as h=nH2O/ nM. This led to the synthesis of various inorganic materials: metal, oxides, layered hydroxide salts (LHS) and more recently LDH compounds: LDH-Ni-Al [10] and LDH-Ni-Fe [11] with acetate as the intercalated anion.
In this paper, we want to put this novel route, efficiency for synthesis of a series of layered double hydroxides containing three transition metals CoNiFe, ZnCoFe, and ZnNiFe with acetate anions intercalated in the interlayer space. The obtained LDH are characterised by several complementary analyses (X--ray diffraction, TEM, DTA-TGA, IR, UV and Mossbauer spectroscopy). Such LDHs based on transition metals are of great interest as precursors for metal-spinel oxides composites with potential magnetic and catalytic properties [12].

Synthesis of LDHs Samples
The samples were synthesized with a molar ratio (M/Fe) of 3 by forced hydrolysis in polyol medium according to the method described by Poul et al. for elaboration of layered hydroxysalts [8]. It consists in the hydrolysis at 130°C of mixture of acetate salts dissolved in DEG with a total molar concentration of 0.1 mole/L.
The precipitation of the corresponding LDHs occurred when the hydrolysis and alkalinity ratios h and b were fixed at 100 and 2 respectively, where h=nH2O/n (M+Fe) and b= nNaOH/n (M+Fe).
The suspensions were kept under continuous stirring for 6 hours. After separation by centrifugation, the solids CoNiFe-Ac, ZnNiFe and ZnCoFe-Ac were dried under air at 60°C.

Characterization
Powder X-ray diffraction (PXRD) patterns were obtained at room temperature using a Inel diffractometer with Co-Kα1 radiation (λCoα1 = 1.7889 Å). The crystallite size was calculated using the Scherrer equation. Scanning electron microscopy study was performed using LEICA STEREOSCAN 440 instrument. Electron microscopy and diffraction studies were performed on a JEOL-100 CX II microscope. Infrared spectra were recorded by transmission on PERKIN ELMER 1750 spectrometer on pressed KBr pellets with 4cm -1 resolution between 400 and 4000cm -1 . Elemental analysis (metal, carbon and hydrogen) was performed by ICP method at the centre national de la recherche scientifique, service central d'analyse à SOLAIZE (France). Differential thermal and thermogravimetric analyses (DTA and TGA) were carried out on a Setaram TG 92-12 thermal analyzer with heating rate 1°C/mn under argon, in alumina crucible. UV-Visible spectra were recorded between 200 and 2500 nm, with Cary 5/Varian spectrometer. Polytetrafluoroethylene (PTFE) was used as a reference. The 57 Fe Mössbauer spectra were recorded in a transmission geometry using a 57 Co/ Rh -ray source. They were analyzed by a least-squares fitting method to lorentzian function. The isomer shifts () were referred to that of -Fe at 300 K. The sample (area: 3 cm 2 ) has been prepared by dispersion of the compound (40 mg) in a specific resin.

X-ray diffraction study
The X-ray diffraction patterns of all the synthesized compounds ( Fig. 1) have typical features of turbostratic lamellar compounds [13]. Indeed, three (00l) sharp and symmetric peaks at low diffraction angles, and two broad, less intense and asymmetric peaks can be clearly identified. The patterns were indexed in a hexagonal lattice with rhombohedral symmetry (R-3m), commonly used as a description of the layered double hydroxide structure [14]. The value of the crystallographic parameter a which is the average metal-metal distance in the brucite-like layers of the LDH compounds, has been N o v e m b e r 1 6 , 2 0 1 4 calculated from d110 (a=2d110), whilst, the value of parameter c which is related to the thickness of the brucite-like layers, the nature of the interlayer anion and water content, has been also determined.
The insertion of acetate anions into LDHs was confirmed and the interlayer spacing values were respectively 12.47, 13.64 and 14. 69 Å for CoNiFe, ZnNiFe and CoZnFe (Table 1)  We notice a difference between the interlayer spacing for all synthesized phases. This can be explained by a structural modification on the level of the layers and/ or different arrangement for the inserted species (anions + water) in the interlayer space for zinc materials. The correlation length along c deduced from the XRD analysis (Table 1) reveals a better cristallinity for ZnCoFe-Ac in comparison with CoNiFe-Ac and ZnNiFe-Ac compounds. In this case, it corresponds to the stacking of approximately 33 layers by crystallite.

2. Morphology
SEM images of all synthesized materials show the low level of cristallinity and confirm the turbostratic character of these compounds. Fig. 2(a) and (b) depict the SEM images of CoNiFe-Ac and ZnNiFe-Ac systems respectively, it is clearly seen that these particles are precipitated as small, pseudo-spherical aggregates of platelets; they are also very similar to the morphology of other LDHs [15].
The particles of ZnCoFe-Ac (Fig. 2(c)) sample are formed by the aggregation of small platy crystallites without any definite shape. They appear to be denser compared to those of the CoNiFe-Ac and ZnNiFe-Ac. For TEM images, the particles of all compounds appear as aggregates of thin crumpled sheets with irregular size and shape (Fig. 3). This aggregation is similar to that described by Taibi et al. for NiFe-Ac LDH [11]. Electron diffraction pattern of these samples confirm the hexagonal symmetry of the brucite structure, to which all the hydrotalcite-like materials belong. The preferential orientation of the crystallites along the (00l) heaxogal plane is clearly established for CoNiFe sample (Fig. 4).

Infrared Spectroscopy
IR spectra for all samples are shown in Figure 5. The broad absorption bands at high frequency (3600-3400 cm -1 ) indicate the presence of OH groups and water molecules in the brucite-like layer. The absorption band around 1640 cm -1 for CoNiFe-Ac and ZnCoFe-Ac is assigned to δH2O vibration of the water molecules [16,17]. In the case of ZnNiFe-Ac, this band is obscured by the strong band due to the νas(-COO -) of the acetate anion [18]. for CoNiFe-Ac and ZnCoFe-Ac and 1579 cm -1 for ZnNiFe-Ac. νs (-COO -) = 1410 cm -1 for CoNiFe-Ac and ZnNiFe-Ac and 1386 cm -1 for ZnCoFe-Ac [19].
The absorption bands around 1344 cm -1 are attributed to CH3 vibrations [19]. The existence of a low-intensity bands between1300 and 1000 cm -1 indicate the presence of DEG molecules as adsorbed species [11]. Bands at lower wavenumber (ν < 800 cm The band close to 1360 cm -1 is absent in all spectra, confirming the intercalation of all compounds with acetate anions without any trace impurity of carbonate anions.

4. Thermal analysis
The TG/DTA analysis for all samples is shown in Figure 6. Three weight losses are observed for each sample. The first weight loss corresponds to the removal of adsorbed and interlayer water molecules, the second weight loss is due to dehydroxylation of the brucite-like layers, while the third one is due to the removal of interlayer anions [22].
For all compounds, three endothermic effects can be distinguished in the DTA curve. The first endothermic peak due to the loss of surface and interlayer water was observed at 116°C with loss of mass of 6.3% for CoNiFe-Ac, at 83°C (7.3%) for ZnNiFe-Ac and at 82 and 127°C (9.4%) for ZnCoFe-Ac.

5. Chemical analysis
First it should be noted that under synthesis conditions, Fe 2+ was oxidized to Fe 3+ as confirmed by Mössbauer analysis (as shown hereafter). This is due to the presence of a sufficient amount of water able to oxidize ferrous ion. In Table 2, are summarized the results of elemental chemical analysis of the compounds and their corresponding formulae. It can be observed that in CoNiFe-Ac compound, the calculated M II /Fe III ratio is in a good agreement with the experiment data, but in ZnNiFe-Ac and ZnCoFe-Ac this ratio is larger than in the starting solution.
For all samples, the results show that the observed carbon amount is higher than that expected in the acetate anions; this is due to the adsorption of polyol (DEG) on the surface of compounds as has been seen in the IR study.
Similar results have been observed for Ni-Co hydroxyacetates prepared in polyol medium [23], where the amount of adsorbed polyol has not been taken into account in the chemical formula. N o v e m b e r 1 6 , 2 0 1 4  Table 2 for exact formula).

6. UV-Visible Spectroscopy
For CoNiFe-Ac and ZnCoFe-Ac, the absorption bands at around 1200 and 500 nm ( Fig. 7(a) and (c)) are attributed to the d-d transitions of octahedral coordinated Co 2+ [24].They are assigned as ν1(Co  [23,25,26]. The UV-Visible spectra of the CoNiFe-Ac and ZnNiFe-Ac (Fig. 7(a) and (b)) show that Ni 2+ ions are also located in octahedral sites with the presence of absorption bands at around 380, 665 and 1115 nm due to ν2 (Ni  Table 3).   Each spectrum shows only one doublet with isomer shift (δ) ranging from 0.33 to 0.34 mm/s and quadrupole splitting (Δ) 0.45 -0.56 mm/s indicating Fe 3+ nature of the Fe atom, and no evidence of Fe 2+ signal was found [32]. The small value of quadrupole splitting (Δ) corresponds to high spin Fe 3+ ions in octahedral site.

CONCLUSION
In the current study, a novel series of ternary layered double hydroxides with acetate anion intercalated in the interlayer region has been prepared by forced hydrolysis in polyol medium, in the ratio range 3.17 ≤ M II Fe III ≤ 4.24.
X-Ray diffraction investigations show that these materials derive from the brucite structure with variable basal spacing, the Zinc based compounds having the more higher interlamellar distances.
IR study shows that acetate anions are located in between the brucite sheets as free species. No trace of carbonate CO3 anion was detected.
Because of the turbostratic character of CoNiFe-Ac, ZnNiFe-Ac and ZnCoFe-Ac, it was hard to get more insight into their intralayer structural organization by X-ray diffraction analysis. Thus two complementary analysis techniques namely UV-Visible and Mössbauer Spectroscopy have been used.
For all samples, UV-Visible spectra present characteristic bands of Co 2+ and Ni 2+ in octahedral sites, and 57 Fe Mössbauer measurement reveals that Fe atoms are present in the LDHs lattice in Fe 3+ state and no such Fe 2+ ions are present.
Owing to their chemical compositions, these based 3 d-transition LDHs are belevied to be very versatile precursors for a wide range of metal-spinel nanocomposites.