Structural Cohesion of MII-MIII ... - ACS Publications

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Structural Cohesion of MII-MIII Layered Double Hydroxides Crystals: Electrostatic Forces and Cationic Polarizing Power Brian Grégoire, Christian Ruby,* and Cédric Carteret Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, Institut Jean Barriol, Université de Lorraine, UMR 7564 CNRS, 405 rue de Vandoeuvre, 54600, Villers-Lès-Nancy, France S Supporting Information *

ABSTRACT: Layered double hydroxides (LDHs), [MII(1−x)MIIIx(OH)2]x+[An−]x/n·zH2O], with various cations and diverse stoichiometries were synthesized by three different coprecipitation routes to highlight the effect of the synthesis method on the composition range of the corresponding materials. While morphology and crystallinity are strongly dependent on the coprecipitation routes, the composition range remains unchanged. Thus, the nature of the cations present in the layer sheet of the material plays an important role in its stability. Further insight into the structural stability of the materials was achieved by two electrostatic models. By computing attractive and repulsive forces involving cationic and anionic punctual charges, it is shown that the most favorable system corresponds to a layer charge of 0.17, slightly lower than naturally occurring minerals for which the layer charge is generally 0.25. This difference may be explained by weaker forces such as hydrogen bonding or crystal cohesion. Thus, a second model is proposed based on the size and the electronic configuration of the cations, that is, the polarizing power, and the effect of the cationic polarizing power on the intensity of the hydrogen bond is described. This model is successful in predicting the lower limit of composition of LDH materials, as a function of the nature of the cations present in the layer sheet.

I. INTRODUCTION The term layered double hydroxides (LDHs) is used to designate synthetic or natural lamellar hydroxides with two (or more) kinds of metallic cations in the main layer and hydrated interlayer domain containing anionic species. They can be described by their general formula [MII(1−x)MIIIx(OH)2]x+[An−]x/n·zH2O, where MII and MIII are di- and trivalent metallic cations, and An− is an anion. They consist of layers of edge-sharing octahedral units, containing the MII and MIII cations that are coordinated to six hydroxyl groups. These positively charged layers are stacked one on the other and intercalated by an interlayer region containing water molecules and charge balancing anions. These materials are of considerable geological relevance because of their anion exchange capacity,1 which can affect the mobility of chemical species in the environment. The positively charged framework is of high interest for numerous applications concerning the removal of toxic anions such as chromates2 and selenates3 from waste waters. LDHs may also have other applications for intercalation chemistry,4,5 storage, and triggered release of functional anions.6−8 They have attracted much attention in recent years as catalysts9,10 and oxide precursors11,12 used in a various range of applications. The structural stability of LDH materials involves a complex combination of electrostatic effects and hydrogen bonding, the intensity of these two interactions being correlated to the layer © 2012 American Chemical Society

charge and the nature of the cations of the brucite layer. For example, significant contraction of the interlayer space induced by the increase of the layer charge x was observed and was generally attributed to the increase of the electrostatic interaction between the cationic layer and the interlayer anions.13−15 The dependence between the cationic composition of the layer and the structural stability is evidenced regarding the possible identity of MII/MIII cations and the range of values of the MII/MIII ratio which can form LDH. Pure LDHs can be found in nature or synthesized within a certain range of x value, typically x ε [0.2−0.33] whereas outside these limits, segregation into other MII or MIII metal (oxy)hydroxides occurs. However, in some cases, the lower limit of composition can be situated outside this range as for NiII-MIII couples where pure phases were obtained for x = 0.09.14 Thus, the flexibility of composition is greatly influenced by the nature of the divalent cation since for both AlIII and FeIII cations, similar ranges of composition were obtained. It is often stated that the range of composition of LDH material obeys geometrical rules, such as the difference of ionic radii between divalent and trivalent cations. Assuming close stacking of OH− ions in an octahedron, there is a hole inside Received: December 5, 2011 Revised: June 22, 2012 Published: July 20, 2012 4324

dx.doi.org/10.1021/cg3006053 | Cryst. Growth Des. 2012, 12, 4324−4333

Crystal Growth & Design

Article

Figure 1. Cationic arrangements within the layer for different x values (a) x = 0.33; (b) x = 0.25; (c) x = 0.14; (d) x = 0.11. been highlighted in our previous publication,14 for materials synthesized by a coprecipitation route at high supersaturation. Further experiments have been run to evidence the effect of the synthesis method on the possibility to tune the layer charge of LDH without observing undesired phases. Variable pH Coprecipitation (VPC1 and VPC2). MII-MIII-LDHCO3 (MIII = AlIII, FeIII, and MII = CoII, NiII, MgII, FeII) samples were prepared by a coprecipitation method at high supersaturation. Solution A containing a mixture of MIICl2 and MIIICl3 salts with MII/MIII molar ratios varying from 2 to 10 were dissolved in 50 mL of distilled water ([MII] + [MIII] = 0.4 M). Solution B containing Na2CO3 (1 M) for NiMIII and Co-MIII couple or a mixture of Na2CO3 (0.25 M) and NaOH (1 M) for MgII-MIII couple was prepared by dissolving the salt in 40 mL of distilled water. Solution B was quickly added (VPC1) or by slow addition (0.3 mL/min) (VPC2) to solution A at room temperature. Constant pH Coprecipitation (CPC). Alternatively, precipitation at low supersaturation was also performed by slow addition of 0.4 M of MIICl2 and MIIICl3 in the desired ratio into the anionic solution containing 0.25 M of Na2CO3. The pH was kept constant at 9 for NiIIMIII and CoII-MIII based mixture or 11 for MgII-MIII by simultaneous addition of NaOH (1 M) solution using an automatic titrator device. For all synthesis and after complete addition of the solution, the slurries were stirred for 2 h at room temperature before hydrothermal treatment at 100 °C for 20 h except for CoII-MIII and FeII-MIII LDHs for which the aging was done at 25 °C. The final solids were washed several times with deionized water and dried under air. II-2. Solid Characterization. II-2-a. Powder X-ray Diffraction (PXRD). Patterns were recorded with a Phillips X’Pert Pro MPD diffractometer in reflection geometry using Cu Kα1 radiation (λ = 1.5406 Å). Data were collected from finely ground samples with a sample holder spinner and continuous rotation of sample to improve statistical representation of the sample. The explored 2θ range was between 5−70° at a speed of 2° /min. II-2-b. Transmission Electron Microscopy (TEM). The morphology and chemical analysis of the samples were determined by TEM (CM20/STEM Philips) coupled with an energy dispersive X-ray system (EDX), using a voltage of 200 kV. One drop of suspension was deposited on a copper grid. Then the grid was introduced in the microscope under a 10−8 Torr vacuum. II-3. Building of the Structural Model. The cohesion of the LDH structure is governed by the electrostatic interaction between the excess of positive charge situated on the MIII cationic sites and the negative charge of the anions present in the interlayer. It was reported

with a radius of about 0.07 nm (M−O bond length minus radius of OH− ions, that is, 0.203 − 0.267/2 = 0.07). Therefore, a cation with a radius not far from 0.07 nm can be incorporated into the brucite like layer.16 Thus, the incorporation of tetravalent cations into the brucitic sheet may be difficult17 and CaII based LDHs have a specific crystallographical structure.18 However, many features of the structure and reactivity of LDH cannot be explained by this model. For example, Frost et al. reported changes in the basal spacing of ternary LDH by varying the cationic composition of the layer while the overall charge was kept constant.19 This study is concerned with the synthesis and characterization of different MII-MIII LDHs with respect to the layer charge, x, and the nature of the MII and MIII cations (MII = CoII, NiII, MgII, FeII, and MIII = FeIII, AlIII). Despite their different electronic configurations and ionic sizes, all these cations can be successfully incorporated into the sheets of the LDHs via various coprecipitation methods.15 Three coprecipitation methods were used to evidence the influence of the synthesis route on the composition range of various LDHs. In the face of the lack of publications devoted to the cationic nature dependence on the structural stability of LDH materials with varying layer charge, two models are developed to rationalize and describe the interactions responsible for their stability. A first model based on the calculations of potential energy considering punctual charges will be proposed to estimate the stability range of LDH with respect to the MII/MIII ratio in terms of electrostatic interactions. The influence of the cationic polarizing power will be discussed in a second model. Although this last notion has been used many times to describe metal−ligand interaction in complex20,21 or surface property in metal hydroxide,22 to the best of our knowledge, it has never been employed to investigate LDH materials.

II. MATERIALS AND METHODS II-1. Synthesis Methods. Probing the composition flexibility of LDH materials requires investigating the effect of the synthesis route on the structural and textural properties of the as-synthesized materials. The large composition range of NiII-FeIII LDH has already 4325



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by several authors that the contraction of the cell parameter c measured for increasing values of MIII molar fraction x was due to an increase of the electrostatic charges both in the cationic layers and in the interlayers.13,23 The following electrostatic model was designed to estimate more quantitatively the electrostatic interactions existing in the LDHs. It is representative of any kind of MII-MIII LDHs containing a divalent anion. The model does not take into account the specific geometry of the anion or the cations that are considered as punctual charges. Since the electrostatic interactions may depend on the position of the cationic charges, the question of cationic ordering must be carefully considered. Short-range or long-range ordering of the cations within the LDH layer was widely discussed in the literature and contradictory conclusions were drawn. Experimental evidence of longrange order is difficult to obtain due to high pseudosymmetry, microcrystallinity, or stacking fault.24,25 Additional XRD reflections assigned to supercell were recorded by Bellotto et al. on MgII-GaIII but not on MgII-AlIII, which was explained as resulting from the larger difference of the metals scattering power of the former.26 Drits et al. reported on simulated XRD patterns that the existence of a small extent of defects dramatically decreases the intensity of these reflections.27 Interestingly, Roussel et al. determined that the supplementary reflection assigned to cation ordering may have an observed intensity close to 0.1% of the maximum intensity obtained for (003) reflections and therefore could not be detected by conventional XRD.28 On the contrary, experimental evidence of short-range ordering is more conclusive by spectroscopic investigations. For example, EXAFS was used to investigate the cationic sequences in the layer, and many authors have shown that trivalent metals cations never occupy two neighboring sites even if no evidence of long-range order was obtained from XRD analyses.23,29,30 The evidence of local ordering has been provided for by the MII/MIII ratio ranging from 2 to 4 and may be extended for higher ratios. Therefore, a satisfactory description of the cations arrangement within the layer may involve ordered domain separated by regular defects. The electrostatic model was designed as an ordered structure since the extent of defect determined by solid state NMR spectroscopy was lower than 10% for MgII/AlIII = 2:1 and negligible for higher ratios (MgII/AlIII = 3:1 and 4:1).31,32 The model was designed by considering the various twodimensional ordered arrangements of the MIII cations in the MII-MIII cationic layer of the LDH (Figure 1). The ordered structure of Figure 1a corresponds to a MIII molar fraction x = 0.33, that is, a ratio MII/ MIII of 2:1; it is often considered as an upper limit of composition.15,23 Indeed for x values higher than 0.33, Figure 1a shows that some of the trivalent cations become first neighbors leading to a sensitive increase of the electrostatic repulsion in the cationic layer. The sublattice of the MIII cations in Figure 1a is characterized by a two-dimensional hexagonal cell of size d(MIII-MIII) = √3 a, where a is the unit cell parameters of the hexagonal pavement. Other hexagonal pavements of the trivalent cations can easily be built for lower values of the MIII molar fraction x, that is, higher values of the distance d(MIII-MIII) (Figure 1b−d). Note that both values are linked by the following relationship: III

III

2

x = [a/d(M ‐ M )]

Figure 2. Schematic representation of electrostatic interactions of the electrostatic model. charges +|e| (Figure 2). It was checked that the final result of the model calculations was not significantly influenced by the following parameters: (i) the global position of the negative charges relatively to the positive charges; for example, all negative charges can also be situated on the top of the positive charges or partially situated on the top of the barycenter of three positive charges, (ii) the increase of the number of unit cell showing that the size of the system is representative of larger ordered structures. The electrostatic potential energy V of this system was calculated by taking into account the charges and the interdistances between the punctual charges according to the classical formula:

V=

∑ qiqj /(4πε0rij) ij

(2)

where qi and qj are electrostatic charges, that is, either +|e| or −2|e|, and rij is the distance between each couple of charges (qi, qj). The summation Σij is performed for all the couple of charges present in the structural model presented in Figure 2a, that is, 153 interactions in this case. The electrostatic potential energy V is the summation of two opposite contributions Vatt and Vrep, where Vatt represents the attraction between the anions and the excess of positive charges (Vatt < 0) and Vrep contains the repulsion between both the positive and the negative charges (Vrep > 0).

III. RESULTS III-1. Probing the Composition Range of LDHs. There is general agreement in the literature that the maximum of x value in a pure LDH phase is x = 0.33 because larger amounts of MIII ions would lead to unfavorable MIII-O-MIII interactions being unavoidable. The minimum value of x is more controversial. At least three indicators can be used to determine the lower limit of composition x of the various LDHs: (i) apparition of new XRD diffraction peaks attributed to secondary minerals that are not due to the LDHs, (ii) deviation in the linearity rule that links the cell parameters a and c of the LDH to the composition x of the initial solution,30,33 (iii) fluctuation of the x values measured by TEM-EDX for different analyses zones.33 The analyses of the metal content of the bulk phase by ICP-AES and that of the platelet surface by TEM-

(1)

III

The ordering of the M cations in all these structures (Figure 1) is characterized by a primitive two-dimensional rhombus cell with a 60° angle. Therefore, the model that is considered here consists of a first sheet of 12 positive charges +|e| situated at the corner of a series of six rhombus cells (Figure 2a). The size of the rhombus cell corresponds to the distance d(MIII-MIII) between the trivalent cations. The punctual charges +|e| represent therefore the excess of positive charges situated on each MIII cationic sites presented in Figure 1. On the top of the cationic layer, a second layer of six negative charges −2|e| is presented (Figure 2a,b). The second parameter of the model is the interlayer spacing h corresponding to the distance between the positively charged and the negatively charged layers (Figure 2b). For simplicity, each row of negative charges −2|e| is parallel to a row of positives charges +|e| and is situated at equidistance of two rows of 4326



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EDX allow the composition of LDH to be determined more precisely. As an illustration, the Figure 3 compares the metal

Figure 4. Correlation between x and structural parameters for various MII-FeIII LDHs: (a) c parameter; (b) a parameter.

The a parameter could also be considered as an indicator of the purity of the LDH phase. Clear break of linearity can be seen in Figure 4b for MgII-FeIII and CoII-FeIII couples. These changes of the curve slopes appear at values of x slightly different of those observed for the variation of the c parameter for MgII-FeIII, whereas a good correlation is found for the CoIIFeIII couple. Since the a parameter is merely influenced by the difference of ionic radii between the divalent and trivalent cations, the slopes of these curves are generally weak. The presence of unexpected phases such as the hydroxycarbonate phase, which may have a similar a parameter, makes analyses of the linearity deviation more difficult to perform. III-2. Influence of the Synthesis Route on the Texture, Structure, and Composition Range of LDHs. TEM micrographs of NiII−FeIII LDHs with x = 0.25 obtained using the three different synthesis methods are illustrated in Figure 5. The expected hexagonal plate-like nature of the crystallite is clearly apparent for the two variable pH methods (Figure 5a,b) while undefined shapes are obtained for the method at constant pH (Figure 5c). The average diameter of the platelet is much smaller for the VPC1 and VPC2 methods, with a lateral size of about 100 nm, and more uniform than the materials obtained using the CPC method, for which the crystallite size ranges between 50 and 300 nm. XRD pattern recorded on NiII-FeIII LDH with a layer charge x = 0.25 prepared by the three proposed routes are displayed in Figure 6. Sharp (00S ) reflections were obtained for VPC1 and

Figure 3. Comparison of the x values determined by TEM-EDX with those obtained by ICP-AES: (a) NiII-AlIII LDHs; (b) NiII-FeIII LDHs.

content determined by ICP-AES and TEM-EDX for NiII-FeIII (Figure 3a) and NiII-AlIII (Figure 3b) LDHs, as both systems exhibit the larger chemical variation. Both analytical methods present very similar results, which strengthen the consistency of the determined x value. Note that ICP analyses are not suitable for determining non-LDH phases since all cations have been neutralized during the coprecipitation. The chemical composition domain of LDH is most often situated in the range x ε [0.2, 0.33] except for the NiII-MIII systems for which a limit of composition x = 0.09 was determined.14 The secondary minerals observed for the syntheses performed at x values lower than 0.2 are either MII hydroxide or MII hydroxycabonate, for example, Ni(OH)2,13 Mg(OH)2, hydromagnesite Mg5(CO3)4(OH)2·5H2O,13 and siderite FeCO3.34 An illustration of linearity deviation of the c parameter is presented for the MII-FeIII systems in Figure 4a. For the NiII-FeIII couple, no deviation is observed because of the high flexibility of this LDH.14 On the contrary, clear breaks of linearity that are concomitant to the formation of secondary minerals are pointed by arrows in Figure 4a for the CoII-FeIII and MgII-FeIII couples. 4327



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Stoichiometry of the cationic layer related to the synthesis methods has been determined by EDX analyses and the results are plotted in Figure 7. Vertical dot lines represent the expected

Figure 7. EDX analyses on NiII-FeIII LDHs prepared by the three coprecipitation routes.

layer charge defined as the molar fraction of trivalent cations added in solution. Five investigated areas covering a variable number of crystallites allow us to determine the average value and the standard deviation for each x value. Independently of the coprecipitation routes, the layer charge of LDH materials can be pushed down to 0.09 with a reasonable standard deviation. These results can also be extended to other couples of cations, for which no differences were observed on the composition range between the constant pH and the variable pH methods. The stability domain upon varying the layer charge is independent of the precipitation routes which leads us to think that the composition range of LDH systems is more intensively governed by structural parameters. III-3. Influence of the Nature of the Cations on the Morphology, Structure, and Composition Range of LDHs. Since the synthesis route does not influence significantly the composition range of the LDHs, morphological investigations were achieved with materials synthesized with the VPC1 method followed by a hydrothermal treatment at 100 °C for 20 h except for CoII-FeIII and FeII-FeIII LDHs for which secondary phases were observed during the aging. Thus, these materials were allowed to age at 20 °C for 20 h. TEM micrographs of the prepared CoII-FeIII, MgII-FeIII, FeIIFeIII, and NiII-FeIII LDHs with a layer charge of 0.33 are illustrated in Figure 8. While well-formed hexagonal-shaped platelet-like crystals can be observed for all LDHs, with some rounded crystallites for the MgII-FeIII couple, their lateral size is greatly dependent on the cations involved in the layer. The crystallites sizes are quite uniform for NiII-FeIII (Figure 8a) and MgII-FeIII LDHs (Figure 8b) with an average diameter of 100 nm. Important polydispersity and bigger particles are clearly visible in the CoII-FeIII and FeII-FeIII samples (Figure 8c,d), with a lateral size ranging between 50 and 200 nm. Table 1 summarizes a compilation of the results obtained by XRD for x values situated in the range 0.05−0.33. Peaks profile analyses (full pattern matching) were used to calculate the standard deviation of the a and c structural parameters from the

Figure 5. TEM micrographs of NiII−FeIII LDHs with x = 0.25 prepared by different coprecipitation routes: (a) VPC1; (b) VPC2; (c) CPC.

Figure 6. Powder XRD patterns of NiII-FeIII LDHs prepared by the three coprecipitation routes.

VPC2, typical of well-crystallized materials, whereas those corresponding to samples prepared by the CPC method exhibit anisotropic peak broadening, consistent with their morphology. Furthermore, large and asymmetric (00S ) reflections, indicating stacking faults were present for VPC1 and CPC methods, while intense and symmetrical peaks would indicate more regular stacking arrangement for the VPC2 method. The position of the diffraction lines is very similar between the three methods indicating similar interlayer organization and water content. 4328



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parameter of the corresponding MII hydroxide (Ni(OH)2, a = 3.12 Å, Mg(OH)2, a = 3.15 Å; Co(OH)2, a = 3.182 Å, Fe(OH)2, a = 3.265 Å). Moreover, for a given divalent cation, the cell parameter a of the MII-AlIII LDH is always slightly lower than the one measured for the MII-FeIII system, in good agreement with the size of these trivalent cations in octahedral coordination (roct (Al3+) = 0.535 Å, roct(Fe3+) = 0.645 Å).36 A slight decrease of the cell parameter a is observed for increasing values of the chemical composition x. This diminution is linked to a small contraction of the hydroxyl octahedra that contains the trivalent cation. The c parameter is significantly influenced by the chemical nature of the MII-MIII couple as can be seen in Table 1. Indeed, the c parameter of MII-AlIII LDHs is slightly lower than the one measured for the MII-FeIII LDHs. A mean value c = 22.7 ± 0.3 Å is measured for an x value of 0.33. In fact, this value is essentially correlated to the nature and the size of the anion situated in the interlayer.37,38 For a carbonate anion, the planar molecule is parallel to the (001) basal plane of the hexagonal structure.39,40 A systematic decrease of the c parameter with increasing values of x is observed for all the MIIMIII couples. This contraction was observed by several authors and is generally correlated to the concomitant increase of the electrostatic charge of both the brucite type sheets, that is, x+, and of the anionic interlayer, that is, x−. This interaction will be estimated in more detail in section IV-1. Morphological analyses by TEM-EDX and XRD have shown that textural and structural properties are closely correlated. While NiII-FeIII LDH exhibits the higher composition range (Table 1) with well-defined and small monodisperse crystallites, CoII-FeIII and FeII-FeIII LDHs have been shown to be restricted to the composition range [0.25; 0.33] or strictly to 0.33, respectively, with a high polydispersity of crystallite size. MgII-FeIII couple exhibits intermediate flexibility range and rounded-hexagonal particles.

Figure 8. TEM micrographs of LDHs with x = 0.33: (a) NiII-FeIII, (b) MgII-FeIII, (c) CoII-FeIII, (d) FeII-FeIII.

Table 1. Variation of the Cell Parameters a and c of Various MII-MIII LDHs as a Function of x = n(MIII)/{n(MIII) + n(MII)} or R = n(MII)/n(MIII) MII-MIII cations II

III

Ni -Al

NiII-FeIII

CoII-AlIII CoII-FeIII FeII-AlIII FeII-FeIII MgII-AlIII

MgII-FeIII

a

x

MII/MIII

a (Å)

c (Å)

0.09 0.14 0.2 0.25 0.33 0.09 0.14 0.2 0.25 0.33 0.25 0.33 0.25 0.33 0.33 0.33 0.2 0.25 0.33 0.2 0.25 0.33

10:1 6:1 4:1 3:1 2:1 10:1 6:1 4:1 3:1 2:1 3:1 2:1 3:1 2:1 2:1 2:1 4:1 3:1 2:1 4:1 3:1 2:1

3.074 3.063 3.052 3.043 3.026 3.09 3.09 3.087 3.086 3.084 3.08 3.07 3.13 3.13 10.805a 3.17588b 3.068 3.057 3.042 3.113 3.109 3.107

24.08 23.76 23.55 23.22 22.66 24.15 23.73 23.56 23.25 22.81 23.05 22.77 23.10 23.06 22.48a 22.7123b 23.55 23.21 22.65 23.70 23.42 23.04

IV. DISCUSSION IV-1. Electrostatic Structural Model. IV-1-a. Influence of the Interlayer Spacing h. In a first step, the variation of the electrostatic potential energy V as a continuous function of the interlayer spacing h is considered. The distance between the trivalent cations, that is, the parameter d(MIII-MIII), is fixed at representative values of 5.4, 6.4, and 9.6 Å. These last values correspond respectively to an MIII molar fraction x of 0.33, 0.25, and 0.11 if eq 1 is used with a value a = 3.087 Å (mean value of the cell parameter a of the NiII-FeIII LDH presented in Table 1). Typical evolutions of the potential energy V as a function of the parameter h are presented (Figure 9). The curves consist of a monotonic increase of V that starts at a minimal value V0 obtained at the origin h = 0. It crosses the abscissa axis V = 0 at a critical distance hcrit and reaches a horizontal asymptotic constant value Vrep(N) for the highest values of h. This trend can easily be understood: the value h = 0 corresponds to a system where positive charges are separated by negatives charges in a unique plane. It corresponds to the most stable system where the distances between opposite charges are minimized. When the distance h increases the potential energy Vatt increases and reaches zero for h → +∞, whereas Vrep does not depend on the value of h because the distance d(MIII-MIII) is fixed (Figure 10a). Therefore, when the values of h diverge at +∞, the potential energy V reaches a constant positive value Vrep(+∞) that corresponds to an unstable system. Electrostatically stable systems are characterized by h distances lower than hcrit with a negative value of

IMA 1992-030 Caresite (space group P3112). bRef 28.

(110) and the (00S ) reflections respectively of the hexagonal cell.35 The values of a are essentially governed by the nature of the divalent cation; they are always slightly lower than the cell 4329



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Table 2. Critical Interplanar Distances hcrit for which V = 0 Given As a Varying Values of the Distance between Trivalent Cations d (MIII-MIII)a d (MIII-MIII) (Å)

x

hcrit (Å)

5.4 6.4 9.6

0.33 0.25 0.11

5.85 6.65 10.1

a

The corresponding MIII molar fraction x values are calculated for the NiII−FeIII LDH characterized by a mean cell parameter a = 3.087 Å.

model predicts an energetically stable configuration (V(hobs) < 0 because |Vatt| > Vrep). The data presented in Figure 9 show that the V curves obtained for the highest value of d(MIII-MIII), that is, d(MIIIMIII) = 9.6 Å, cross the one obtained for d(MIII-MIII) = 5.4 Å at a point called P. It defines two zones denominated I and II situated respectively below and above point P. Therefore, at low interlayer distance h (zone I), the energetically more stable systems are observed for the lowest values of the d(MIII-MIII) distance, while a reverse trend is observed at higher values of h inside zone II. From this point of view, the MII-MIII LDHs synthesized at low x values, for example, the NiIIFeIII-CO3 LDH with x ≈ 0.1 and d(MIII-MIII) = 9.6 Å, could be interesting materials for the intercalation of anionic species larger than CO32−. For instance, several authors41−43 have demonstrated that the LDHs may incorporate long surfactants chains leading to a strong expansion of the interlayer space in the range ∼1 to ∼4 nm. It corresponds to hobs values situated between ∼0.5 and ∼2 nm that are mostly included inside zone II (Figure 9). IV-1-b. Influence of the Distance between the Charges, d(MIII-MIII). The variation of the electrostatic potential energy V is plotted as a function of the distance d(MIII-MIII) for three observed values of the interlayer distance hobs (Figure 11). In

Figure 9. Evolution of the potential energy as a function of the interlayer distance h of the electrostatic model presented in Figure 8.

Figure 10. Evolution of the potential energy as a function of (a) interlayer distance; (b) distance d(MIII-MIII).

Figure 11. Evolution of the potential energy as a function of the distance d(MIII-MIII) for three interlayer spacings h.

the potential energy V. The distances hcrit measured for three values of d(MIII-MIII) are reported in Table 2. From the structural parameters measured for the MII-MIII-CO3 LDHs (Table 1), the observed distances hobs = c/6 can be computed and are situated between ∼3.7 Å and ∼4 Å as indicated in Figure 9. It represents also half of the interlayer distance of the CO32− containing LDHs. The distances hobs are situated well below all the hcrit values presented in Table 2, and therefore the

the low range of d values, the curves decrease strongly and reach a minimum Vmin for a distance d(MIII-MIII) = d0 and increase for the distances d(MIII-MIII) > d0. The asymmetry of the V curve is pronounced and the potential energy V increases only moderately for d > d0. The presence of a minimum value of potential energy Vmin is due to the opposite effect between the variation of Vatt and Vrep as a function of d(MIII-MIII) 4330



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parameter Z/r2, called polarizing power,47 can be interpreted as a measure of the strength of the charge field that affects the neighboring atoms present in the hydroxide matrix. The perturbation of the electronic distribution of an atom may induce a bonding polarization altering the bond length and thus the bond strength. Therefore, the entire geometry of the system undergoes the effect of this electronic anisotropy. The magnitude of the electrical field coming from a spherical density of charge calculated at a distance much longer than the close environment of the atom is independent of the atomic or ionic radius (consequences of the Gauss Law). This allows calculating the potential energy by considering simply punctual charges. However, the charges of the octahedrically coordinated cations are not spherically distributed, and the value of the electric field sensed by the neighboring oxygen atom depends on the cation radius. Since the polarizing power depends also on the radius of the cation, it allows estimating such short-range interactions. IV-2-b. Applications to LDHs. The oxygen atoms of the hydroxyl sheets of layered double hydroxides may have different charge conditions depending on the polarizing power of the cations. The electronic density of the oxygen atoms may be more localized along the M−O bond in such a way that the part of the oxygen which shares the bonding with the hydrogen atom may have a partial default of charge. This anisotropy of charge distribution may affect the stability of LDH systems by promoting both stabilizing and destabilizing interactions. To achieve that, let us decompose the electronic distribution of the oxygen atom along the axis of the hexagonal crystal cell: (i) Along the c axis: A stabilizing interaction is given by the ability of the H atom of the hydroxyl layer to form Hbonding network with interlayer species. The intensity of such interaction is correlated to the polarity of the MO− H bond. The latter is more important if the oxygen atom acquires a more pronounced default of charge, which means a bonding with a metal cation having an important polarizing power. (ii) In the ab plane: A destabilizing interaction is due to the preferential localization of the electrons along the M−O bonds leading to an unequal electronic distribution of the oxygen atom. The stronger the cationic electrical field, the greater the tendency of the electronic density on the oxygen atom to be polarized toward this cation. Therefore, the M−O bond length could be slightly shortened leading to a deformation of the neighboring octahedra. These assumptions have already been observed by EXAFS, and generally, the MII−O bond is slightly longer than the MIII−O bond.28 Since the hydrogen bonds are much lower in energy compared to the electrostatic interaction, the octahedral distortion could be responsible of the limit of composition of the LDH system. Therefore, these assumptions authorize us to think that an optimal electrical field is necessary to have a better compromise between the stabilizing effect (i.e., electrostatic interactions) and the destabilizing effects (i.e., octahedron distortion). Table 3 summarizes the polarizing power of different MIIoct and MIIIoct metal cations octahedrically coordinated. The polarizing power model was designed by considering a linear combination of MIIoct and MIIIoct, weighted by their relative proportion in LDH. It can be expressed as

(Figure 10b). Therefore the distance d0 corresponds to the optimal distance between trivalent cations corresponding to the more stable system. The values of d0 measured are very close to ∼8 Å for all the values of hobs (Figure 11). The relation between the potential energy V and the MIII molar fraction x shown on the top of Figure 11b was computed by using again relation (1) with a = 3.087 Å. One observes from the right to the left of Figure 10 that the potential energy V decreases slowly for x values situated between 0.05 and ∼0.17, this last value corresponding to the most stable system. For values of x higher than 0.17 the potential energy increases progressively. The V values stay relatively close to the minimal value Vmin in a relatively narrow range of MIII molar fraction values situated between x ∼ 0.1 and x ∼ 0.3, in good agreement with the LDHs composition range determined in Table 1 for the various carbonated LDHs. In particular, the instability of LDHs having x values higher than 0.33 is directly linked to the strong increase of potential energy observed in this range of composition. The electrostatic model predicts that LDH with x values higher than ∼0.7 are no more stable (V > 0 due to the strong repulsive forces between MIII cations at a low value of d(MIII-MIII)). It does not correspond to the commonly admitted limit of composition x = 0.33; however the values of potential energy in the range x ε [0.33−0.7] are significantly higher than those found in the range x ε [0.1, 0.3]. One of the drawbacks of the electrostatic models is that it cannot explain the composition range difference between the various MII-MIII LDHs, in particular the various values of the lower limit of the x composition (Table 1). Indeed, the model does not depend on the nature of the divalent and the trivalent cations that are only considered as punctual charges. The following section where the cationic polarization is estimated will give more information about this question. IV-2. Polarizing Power of Metal Cations. IV-2-a. General Considerations and Definition. The relationship between the nature of the implied cations and the composition range of the LDH is not fully understood and was the subject of some discussion. Geometrical considerations based on cationic ionic radii are often relevant to describe the change in the lattice parameter as a function of the trivalent cations content.13,26 Rietveld refinement was generally performed to study the microstructure, but diffraction techniques give the average values of the cell parameters since in most LDHs MII and MIII have often similar scattering power, making them indistinguishable by X-ray techniques.44,45 To bypass this difficulty, computational methods are used sometimes and offer a new insight into the electronic and structural properties of the LDH system. However, making a distinction between the contribution of divalent octahedron and trivalent octahedron on the stability of LDH requires fixing the cationic distribution within the layer. Yan et al. reported that the stability and the composition range of MgII-AlIII LDH is correlated to the electronic interaction between the 2p orbitals of the oxygen atom acting as an electronic donor and the 3s and 3p orbitals of the metallic cations, being the electronic acceptor.46 Thus, the size and the electronic property of the cations should be considered to obtain more insight concerning the interdependence between the nature of the cations and the range of composition of the corresponding LDH. Both parameters can be gathered by considering the polarizing power of the metal cations. One can consider the metal cation as a sphere, its charge is distributed over its surface (or area), and the latter is controlled by the square of the cation radius. Therefore, the 4331



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line MN in Figure 12 refers to the MgII-FeIII couple. The corresponding abscissa gives a lower limit of composition xmin = 0.18. Similarly, the limit of composition of the CoII-FeIII system would be limited to xmin = 0.24. Both results are in very good agreement with the experimental results (Table 1). IV-2-c. Discussion about the Polarizing Power Model. As explained before, the structural stability and the range of composition of the LDH system is assumed to be dependent on two main interactions, acting in two different directions, for which the nature of the cationic layers influences their relative intensities. The interaction described in the c direction was attributed to H bonding and electrostatic interaction between the host layer and the interlayer species. The absolute value of the polarizing power of the LDH system would be an indicator of the polarization of the hydroxyl group, leading to a denser hydrogen bond network. This consideration can explain the c h a n g e o f t h e ba s a l s p a c i n g o f t e r n a r y L D H s [Ni6−yMgyFe2(OH)16]CO3, 4H2O by varying y even though the overall cationic charge is kept constant.19 Similarly, from Table 1, it can be noted that the basal spacing of MII-AlIII LDHs is always lower than MII-FeIII LDHs as a consequence of the denser hydrogen bonding network. The other contribution, and probably the most critical on the range of composition, is the octahedral distortion within the brucite sheet. A greater difference of polarizing power between the divalent and trivalent cations implies an increase of the octahedral distortion. CaII-AlIII LDH illustrates well these assumptions, since the difference of ionic radii between CaII and AlIII gives an important difference of polarizing power between these two cations (P(CaII)oct = 2 and P(AlIII)oct = 10.48). A similar extrapolation as previously detailed gives the lower limit of composition of these materials, that is, xmin = 0.33, in good agreement with results obtained by Rousellot et al.18

Table 3. Polarizing Power of MII and MIII in Octahedral Coordinationa hydroxydes

Z

R (Å)

Z/r2 (Å−2)

II

2 2 2 2 2 3 3 3

0.69 0.72 0.75 0.78 1 0.645 0.535 0.62

4.20 3.86 3.60 3.28 2.00 7.21 10.48 7.80

Ni oct MgIIoct CoIIoct FeIIoct CaIIoct FeIIIoct AlIIIoct GaIIIoct a

Note: Ionic radii presented here has been taken from Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. 1976, A32, 751−767.

P(LDH) = (1 − x)P(MII oct) + xP(MIII oct)

(3)

2

where P is the polarizing power, Z/r and x the molar ratio MIII/(MII + MIII) The polarizing power of different MII-FeIII metals cations as function of the layer charge x is plotted in Figure 12. The

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CONCLUSION LDH materials with different MII, MIII cations, and MII/MIII ratio were synthesized by various methods to evidence the effect of the coprecipitation routes on their composition range. Although their morphology and their structure strongly depend on the synthesis methods, the flexibility of composition does not appear to be influenced. On the other hand, the nature of the cationic layer governs the stability of these systems and their composition range. On the basis of experimental data, two models have been proposed. The electrostatic model allows calculating the potential energy by taking into account attractive and repulsive forces present in the LDH systems, considering metal cations and anions as punctual charges. As expected, a strong increase of potential energy is observed for x values higher than 0.33, which is consistent with the so-called “cation avoidance rules”. The minimum of the potential energy has been found to be lower (x = 0.17) than the commonly admitted one (x = 0.25). These results indicated that the nature of the metal cation within the layer may govern the stability of LDH materials. To this purpose, a second model based on the cationic polarizing power has been developed. Both electronic and geometric properties of the metal cations are crucial parameters which govern the lower limit of composition of LDH materials. It has been found that the smaller the difference of polarizing power between the divalent and the trivalent cations, the larger the composition ranges of the corresponding LDH. Thus, the layer charge of NiII-FeIII systems can be pushed down to 0.09, whereas the composition of CoIIFeIII and FeII-FeIII is limited to [0.25; 0.33], in good agreement

Figure 12. Evolution of the polarizing power as a function of the layer charge.

extrapolation of the polarizing power toward x = 0 or x = 1 corresponds to MIIoct and MIIIoct, respectively. The curves consist of a monotonic linear increase of the polarizing power when the trivalent metal content increases due to its higher polarizing power. For a given value of x, the differences of polarizing power arise solely from the radius of MII since all the other parameters are fixed. Therefore, when the difference of polarizing power between MII and MIII is low, segregation of MII(OH)2 becomes less favorable at a low x value. On the contrary, the important difference of polarizing power promotes the segregation of MII(OH)2 phases at low x value, restricting the composition flexibility of the LDH phase. As previously mentioned, Ni-based LDHs could be formed in a large range of composition with a net charge as low as x = 0.09.14 Considering the corresponding polarizing power of the Ni-based LDHs with a composition x = 0.09 as the minimal electrical field necessary to sustain the lamellar structure (point M in Figure 12), one can extrapolate this value to other LDHs to determine their lower limit of composition. For example, the 4332



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with our experimental results and literature. It has been proposed that the main destabilizing effect which influences the composition range is due to octahedral distortion which depends on the strength of the charge field exerted by the cations.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Additional figures showing XRD patterns for NiII-FeIII, NiII-AlIII, MgII-FeIII, MgII-AlIII, CoII-FeIII, CoII-AlIII from which cell parameters were calculated and given in table 1. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Tel: + 33 3 83 68 52 20. Fax: + 33 3 83 27 54 44. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank J. Ghanbaja for the TEM-EDX analysis (Service Commun de Microscopie Electronique, Nancy Université), E. Wenger and P. Durand for the PXRD analysis (CRM2-Institut Jean Barriol, Université de Lorraine), and John Wall for the correction of this article.

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