Solid State Sciences 13 (2011) 1676e1686

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Experimental (X-Ray Photoelectron Spectroscopy) and theoretical studies of benzene based organics intercalated into layered double hydroxide S. Fleutot a, *, H. Martinez a, J.C. Dupin a, I. Baraille a, C. Forano b, c, G. Renaudin b, c, D. Gonbeau a a

Université de Pau et des Pays de l’Adour, IPREM CNRS UMR 5254, Helioparc Pau Pyrénées, 2 avenue du président Angot, 64053 Pau cedex 9, France Laboratoire des Matériaux Inorganiques, CNRS UMR 6002, Université Blaise Pascal, France c Ecole Nationale Supérieure de Chimie de Clermont-Ferrand, Clermont Université, 63177 Aubière cedex, France b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 October 2010 Received in revised form 8 April 2011 Accepted 10 May 2011 Available online 18 May 2011

The present paper deals with a fine physico-chemical analysis of some hybrid materials combining an inorganic layered double hydroxide phase (LDH) with an organic benzene derivative entity R0 -C6H4-R with R ¼ eSO3, eCO2 and R0 ¼ eH, eOH. The main topic of this work is, in a nanoscale, to propose a way to approach the understanding of the interactions between inorganic and organic sub-systems. The role of the anionic headgroup R is discussed in term of reactivity with the mineral support. The classical expertise of materials via the PXRD technique puts into light an effective LDH interlayer space enlargement with the organics incorporation and a minimum angle of inclination for every organics within the LDH matrix. The originality of this study is to use the X-Ray Photoelectron spectroscopy (XPS) as a local probe of the chemical environments of the headgroup of the organic entities. In a parallel way, some quantum calculations (by using molecular and periodical codes) are carried out to best appreciate the electronic and structural evolutions before and after the intercalation step. A specific reactivity of the eSO3 group compared with the eCO2 one is evidenced. Moreover, a correlation at the first order is then set up between the net charges of atoms and the XPS binding energies of their core levels. Ó 2011 Published by Elsevier Masson SAS.

Keywords: Hybrid materials Benzene derivatives Layered double hydroxides Intercalation XPS Quantum calculations

1. Introduction In this paper, four benzene derivatives salts, the 4-hydroxybenzene sulfonate salt (shortly called HBSNaþ salt), the benzene sulfonate salt (BSNaþ salt), the 4-hydroxybenzoate acid salt (HBCNaþ salt) and the benzoate acid salt (BCNaþ salt), were intercalated into a hydrotalcite-type layered double hydroxide lattice [Zn1xAlx(OH)2]xþ[Xx/mm.nH2O]x-, represented as ZnRAl LDH (R ¼ (1x)/x for more clarity) with X ¼ HBS, BS, HBC or BC. Benzene sulfonate derivatives are important in some industrial processes and are used for example as intermediates in the production of chemicals such as azo dyes, optical brighteners, detergents [1] or as corrosion inhibitors [2]. The presence of these derivatives may lead to perturbations of the ecosystems. The use of anion exchange properties of LDHs and the adsorption capacity of the positively charged layers has been extended to the removal of organic pollutants as acid organic species for example. In the same scheme, benzoic acid derivatives are found in food and dye

* Corresponding author. Tel.: þ33 5 59407599. E-mail address: solenne.fl[email protected] (S. Fleutot). 1293-2558/$ e see front matter Ó 2011 Published by Elsevier Masson SAS. doi:10.1016/j.solidstatesciences.2011.05.007

processing effluents [3,4] but can also be considered as a natural agent playing a major role in plant defense responses against pathogen attack [5]. For example, formation of 4-hydroxybenzoate can be elicited by treatment with pathogen fungal elicitors, as observed in various cultures [6]. In a hybrid materials context, the properties and applications of the LDH/acid organics systems have been the subject of a number of papers [7e10] but few papers report a study for the understanding of the interactions nature between the mineral support and the incorporated organic molecule. A comparison between two systems LDH/sulfonate derivatives prepared by a coprecipitation technique has been reported [11] and an estimation of the molecular orientation is obtained based on XRD data. The aim of this work was then to investigate the sub-lattices interactions within four hybrid model systems, LDH-HBS, LDH-BS, LDH-HBC and LDH-BC hybrid materials through an experimental/ theoretical coupled approach. In addition, the specific reactivity of each anionic group (eSO3 for HBS and BS; eCO2 for HBC and BC) with the host matrix has been investigated. Note that BS and BC present a unique functional group and their respective study would allow to estimate the role of the OH group for the two other bi-substituted benzene derivatives (HBS and HBC). Beside the

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use of the powder X-Ray Diffraction (PXRD), to monitor the structural evolution during the intercalation process, an original survey was achieved via the X-Ray Photoelectron Spectroscopy (XPS) to probe the electronic and chemical evolutions between the two subsystems (the mineral support and the organic entities). The comparison of some XPS data with quantum calculations was afterwards carried out to better understand the nature of the interactions. The different precursors of the hybrid materials, ie the layered inorganic matrix and the salts, have been first investigated to make an inventory of their physico-chemical specificities; the composite materials and the nature of resulting interactions were then studied on the basis of these initial experimental and theoretical data. 2. Experimental section and computational details 2.1. Materials: LDH and LDH-derivatives materials The hybrid materials, with a general formula [Zn1xAlxþ [Xx/mm.nH2O]x with x ¼ MIII/(MII þ MIII) (x ¼ 0.33) and X ¼ HBS, BS, HBC or BC, have been synthesized by the standard coprecipitation method [12,13]. The choice of the composition, x ¼ 0.33, among different prepared materials was done in view of the better crystalline degree recorded for this type of sample by comparison with the other matrix compositions, x ¼ 0.25 and x ¼ 0.2 [14]. These samples were obtained by the coprecipitation of zinc chloride hexahydrate (Acros organics reagent, 98% pure), aluminium chloride hexahydrate (Acros organics reagent, 99þ% pure) and sodium salts. The sodium salts HBSNaþ [sodium 4-hydroxybenzene sulfonate dehydrate, (Aldrich, CAS: 10580-195)], BSNaþ [sodium benzene sulfonate, (Aldrich, CAS: 515-42-4)], HBCNaþ [sodium 4-hydroxybenzoate, (Acros organics, CAS: 11463-6)] and BCNaþ [sodium benzoate, (Acros organics, CAS: 53232-1)] have been used for intercalation. A mixture, of ZnCl2.6H2O and AlCl3.6H2O was then dissolved in 200 ml of deionised water to obtain a Zn2Al-Cl solution (1 M). The aqueous solution was slowly added (0.013 mL/min) under magnetic stirring and nitrogen flux to 100 mL of X solution (0.1 M) at room temperature with X the sodium salt. Experimental conditions of pH were continuously maintained to an 8.5  0.2 value for all mixtures with a controlled addition of NaOH solution (1 M) to ensure the correct building of the host LDH phase. The resulting suspensions were stirring for 24 h under nitrogen flux at 70  C for LDH-HBS hybrid phase and for 22h at 20  C for the other samples. The final products were afterwards filtered, washed with distilled water and kept in a drier at 80  C during 48 h. For more convenience and clarity, inorganic matrix and hybrid phases have been so labeled, according their chemical composition: Zn2Al-Cl, Zn2Al-HBS, Zn2Al-BS, Zn2Al-HBC and Zn2Al-BC. x(OH)2]

2.2. Analytical techniques 2.2.1. Powder X-Ray diffraction (PXRD) After their preparation, all the samples have been systematically analysed by the PXRD to monitor the qualitative phase composition, the crystalline state and possible structural changes after organics incorporation. PXRD patterns were obtained with an X-Pert Pro X-Ray diffractometer using a CuKa radiation (1.5418 Å) equipped with a graphite back-end monochromator and an Argon-filled proportional counter. During data collection, the sample holder was rotating at the speed of 30 revolutions per min. Measurements conditions were: diffraction interval 2 < 2q < 70 , step size D(2q) ¼ 0.04 and counting time per step 20 s.

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All the powder patterns were analysed by the Rietveld method by using the Fullprof.2k Program [15] to extract structural and microstructural parameters. The hybrid LDH phase was refined by using the Profile Matching mode (because no structural model were available for these hybrid compounds) with the hydrotalcite R3m symmetry usually used to describe the Zn2Al LDH phases in agreement with powder patterns recorded. 2.2.2. X-Ray Photoelectron Spectroscopy (XPS) XPS analyses were performed on a Kratos Axis Ultra 165 spectrometer which employed a magnetic immersion lens to increase the solid angle of photoelectrons collection from small analysis areas to minimise the aberrations of the electron optics. A monochromatic and focused (spot dimensions of 700 mm by 300 mm) Al Ka radiation (1486.6 eV) was operated at 450 W under a residual pressure of 2  109 mbar. The spectrometer was calibrated using the photoemission lines of Au (Au4f7/2 ¼ 83.9 eV, with reference to the Fermi level) and Cu (Cu2p3/2 ¼ 932.5 eV); for the Au4f7/2 line, the full width at half maximum (FWHM) was 0.86 eV in the recording conditions. Charge effects were compensated by the use of a charge neutralisation system (low energy electrons [typically 1.85 eV]) which had the unique ability to provide consistent charge compensation. All the neutraliser parameters remained constant during analysis. Peaks were then shifted to align adventitious hydrocarbon C1s photoemission to a 284.6 eV binding energy. High-resolution regions were acquired at constant pass energy of 40 eV. All the samples were grinded prior to analysis to avoid effects due to the surface texture. Short acquisition time spectra were recorded before and after each normal experiment to check that the samples did not suffer from degradation during the measurements. The XPS signals were analysed by using a least squares algorithm and a non-linear baseline. The fitting peaks of the experimental curves were defined by a combination of Gaussian (70%) and Lorentzian (30%) distributions. Quantification was performed on the basis of Scofield’s relative sensitivity factors [16]. 2.3. Computational details The electronic structures of the HBSNaþ and HBCNaþ salts were performed using the periodic LCAO-B3LYP approach developed in the periodic ab initio CRYSTAL06 code [17], and by referring to the crystallographic data from the literature [18,19]. The crystalline orbitals are expanded in terms of localized atomic Gaussian basis set, in a way close to the LCAO (linear combination of atomic orbitals) method currently adopted for molecules. The eigenvalues equations are solved for the B3LYP functional using the Becke’s exchange [20] and Lee-Yang-Parr’s correlation functional [21]. All electrons basis set have been adopted: 8-41G* for oxygen [22], 6-21G* for carbon [23], 86-311G* for sulphur [24], 8-511G for sodium [25], 2-11G* for hydrogen [26], 8-31G for aluminium [27] and 86-411d4G for zinc [28] In optimizing the geometry, we allowed the relaxation of all atoms, cell parameters and the fractional atomic coordinates. For studying the organics geometry and the sub-lattices interactions, a molecular model approach using the GAUSSIAN03 package [29] was considered (basis set: 6-311G* [30]) by interfering a mineral cluster Al(OH)3 (representing the active part of the LDH layers and which geometry was deducted from a periodical approach -see Section 4-) with the organic molecule. Remind that the LDH layers are positively charged due to the partial substitution of some zinc (II) atoms by some aluminium (III) ones. This allows to consider the aluminium atoms as positive centers which will be in direct interaction with the organic fragment (HBS, BS, HBC and BC).

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3. Results 3.1. Precursor salts characterization 3.1.1. XPS results Considering the structure of organic entities, the anionic headgroups (eSO3 for HBS and BS and eCO2 for HBC and BC) are supposed to be directly into interaction with the positive charged LDH mineral layers. Then, the core levels S2p and O1s are, in the case of HBSNaþ, the pertinent probes to monitor electronic changes in the molecule surrounding. Fig. 1a presents the S2p doublet core peak located at 168.0e169.2 eV, associated with a sulfonate type environment [31]. For the oxygen atoms, the O1s peak (Fig. 1b) presents two components. The one located at low binding energy 531.6 eV (75.0%) corresponds to oxygen atoms from a sulfonate function [32]. This result reports the identical influence of the Naþ ion on the three oxygen atoms from the eSO3 function. At higher binding energy (532.8 eV (25.0%)), the component could be associated to phenol type oxygen atoms [33]. Note that the relative proportions of the components (1:3) are found to be in agreement with the salt formula (Table 1). For HBCNaþ, the carboxylate group is the interfering function and the C1s and O1s peak are then recorded. The C1s spectrum (Fig. 2a) can be fitted into different components: at 284.6 eV (69.7%), the first component is characteristic of CeC and CeH of the aromatic ring. The two other components, located at 286.3 eV (13.2%) and 288.2 eV (12.1%) are respectively associated with CeOH and eCO2 [34] environments. The component observed around 291.9 eV (5.0%) is assigned to a pep* satellite shake-up (un-localized electrons of the aromatic ring) [35]. In the case of the O1s peak, two components at 530.9 eV (66.7%) and 533.3 eV (33.3%) are considered to be respectively characteristic of the carboxylate and phenol environments. The quantitative survey matches with the expected atomic proportions (Table 1). The examination of the two salts revealed differences in the O1s binding energies for the para-hydroxyl function (532.8 eV for HBSNaþ, 533.3 eV for HBCNaþ); this observation underlines a possible effect in a long range of distances (X-Ophenol ¼ 5.929 Å for HBSNaþ and 5.623 Å for HBCNaþ) of the XNaþ headgroup (X ¼ eSO3, eCO2) via the aromatic ring, as suggested by the Na1s binding energies difference (DBE ¼ 0.4 eV) recorded between both salts. The two smaller systems examined (BSNaþ and BCNaþ (Table 1, Fig. 3) respectively revealed very weak differences with HBSNaþ and HBCNaþ with similar general observations: identical influence of the sodium ion on all oxygen atom from the anionic function (eSO3 and eCO2) and relative proportions of the carbon components in agreement with the BCNaþ salt formula. At this stage, the influence of the hydroxyl group over both sulfonate or carboxylate groups could not be identified.

3.1.2. Quantum calculations The geometry of the dihydrate sodium HBSNaþ salt was calculated by referring to the crystallographic data from the literature [18] (space group P1 and lattice parameters: a ¼ 7.986 Å, b ¼ 12.798 Å, c ¼ 5.481 Å, a ¼ 94.08 , b ¼ 95.45 , g ¼ 95.81 ). All the calculations have been achieved by considering two 4-phenol sulfonate entities, each of them interfering with a sodium atom and being encircled with three water molecules (Fig. 4a). The optimization of the lattice parameters with the CRYSTAL06 code gives the following results: a ¼ 8.737 Å, b ¼ 12.480 Å, c ¼ 6.149 Å, a ¼ 90.00 , b ¼ 106.12 , g ¼ 101.19 . The sodium salt HBCNaþ crystallizes into a monoclinic system arrangement in the P21 space group [19] with the lattice parameters: a ¼ 16.0608 Å, b ¼ 5.3829 Å, c ¼ 3.6383 Å, a ¼ 90.00 , b ¼ 92.87, g ¼ 90.00 . The optimization of the lattice parameters with the CRYSTAL06 code gives the following results: a ¼ 15.8102 Å, b ¼ 5.2636 Å, c ¼ 3.6204 Å, a ¼ 90.00 , b ¼ 88.89 , g ¼ 90.00 . Each sodium atom is coordinated to six oxygen atoms in the form of a distorted trigonal prism (Fig. 4b). Once this initial step done, further calculations have been planned to get an overview of the atomic charges (Table 2) of each salt. The evolution of the net charges in relation with the chemical surrounding around an atom can be associated with the observed shifts of XPS binding energies. Generally, as the positive character of an atom increases, so does its core levels binding energies. Different relations between chemical shifts and real charge variations have been used successfully [36]. At the simplest level of approximation, the correlation between binding energy and charge can be written as follows: DBE ¼ k.Dq, where DBE is the experimental shift of binding energy, Dq is the charge variation, and k is a constant characteristic of the element. For both salts, a good correlation was obtained between the XPS data and the Mulliken charge analysis. For HBSNaþ and HBCNaþ salts, the XPS analysis revealed as expected that the oxygen atoms from the anionic function have a more negative character than oxygen atoms in a phenol environment. This observation agrees with the quantum calculations for each of both salts (Table 2) (0.86 e for eSO3 compared with 0.42 e for V-OH for HBSNaþ; 0.74 e for eCO2 compared with 0.69 e for V-OH for HBCNaþ). The three oxygen atoms of the sulfonate function of HBSNaþ have the same electronic charge (0.86 e), in agreement with an identical influence of the Naþ ion previously revealed by XPS. The same trend is observed concerning the oxygen atoms of the carboxylate function of HBCNaþ (0.74 e). The characteristic C1s spectrum of the HBCNaþ salt reported three different environments. The component relative to carbon atoms from the benzene cycle is located at the lowest binding energy (284.6 eV), in agreement with the corresponding charges calculated (0.07 e to 0.23 e for C1 bonded to the functional

Fig. 1. S2p (a) and O1s (b) XPS core peaks of the HBSNaþ salt and a schematic representation of organic.

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Table 1 XPS data for organic salts: binding energies (eV), FWHM (eV) in parentheses and atomic percentage (%). Compound BE region Na1s O1s C1s

S2p3/2-1/2

HBSNaþ 1071.2 531.6 532.8 284.6 286.0

BSNaþ

HBCNaþ

(1.5) (1.3) (2.2) (1.1) (1.1)

8.2% 22.1% 7.4% 41.5% 8.4%

1070.9 (1.5) 531.4 (1.3)

6.6% 25.8%

284.6 (1.0)

52.9%

289.6 (2.5) 168.0 (0.9) 169.2 (0.9)

2.6% 6.5% 3.3%

290.9 (2.5) 167.9 (0.9) 169.1 (0.9)

5.4% 6.2% 3.1%

group, C3 and C5 located in meta positions and C2 and C6 located in ortho positions with regard to the functional group) which are the lowest net charges for the carbon atoms. The charges obtained for the carbon atoms C4 bounded to the hydroxyl group and that of the carboxylate function (X ¼ C) are respectively 0.32 e and 0.80 e, in agreement with components located at 286.3 and 288.2 eV. We also observed an influence of the anionic functional group on the binding energy characteristic of a phenol environment around the oxygen atom. Indeed, the O1s binding energies (BE) are respectively observed at 532.8 and 533.3 eV for HBSNaþ and HBCNaþ salts. This result implies that oxygen atoms V-OH of HBSNaþ have a more negative character than those of HBCNaþ. The binding energy distinction reflects a difference between the electronic structures for both salts, confirmed by the different positions of the Na1s core peaks (DBE ¼ 0.4 eV) (Table 1). On the whole, these results on precursor salts show a rather good correlation between XPS chemical shifts observed for heteroatoms and calculated charges deduced from periodic quantum calculations. 3.2. Intercalation process 3.2.1. PXRD results The intercalation process of organics between layers of the LDH host system is usually monitored in regards of PXRD patterns. Different lines corresponding to reflections of mineral planes perpendicular to the stacking direction clearly move to the low angles side consecutive to the interlamellar space enlargement. For the present study, powder pattern analyses were performed by using the Rietveld method with the program Fullprof.2k [15]. The refined cell parameters of the Zn2Al-Cl precursor and hybrid materials are given in Table 3. The XRD patterns are shown in Fig. 5. The refinement of the parent LDH Zn2Al-Cl diffractogram was calculated by taking into account the whole structural parameters. Zn2Al-HBS, Zn2Al-BS, Zn2Al-HBC and Zn2Al-BC diffractograms were

1070.8 530.9 533.3 284.6 286.3 288.2 291.9

(1.3) (1.3) (1.3) (1.0) (1.0) (1.0) (2.3)

BCNaþ 7.8% 18.2% 9.7% 44.9% 8.5% 7.7% 3.2%

1071.3 (1.5) 531.1 (1.1)

11.4% 19.1%

284.6 (1.0)

58.1%

288.3 (0.9) 291.2 (1.6)

8.7% 2.7%

performed using the Profil Matching procedure. Refinement convergences were satisfactory by using a hexagonal unit cell with the space group R3m, resulting from a three layers polytype. XRD pattern of the Zn2Al-Cl host matrix indicates a single phase (coprecipitation method) which corresponds to the expected hydrotalcite-like mineral (JPSCD file n 38-0487). The calculated lattice parameter a ¼ 3.0713(2) Å is related to the metalemetal distance through a ¼ 2  d100 relation and is in good agreement with the experimental Zn2þ/Al3þmolar ratio of 1.99. The calculated basal spacing (d003 ¼ c/3 ¼ 7.7223(3) Å) corresponds to the presence of the chloride anions between the sheets [37]. The height of the interlayer space has been estimated to be ca. 2.942 Å considering the thickness of the brucite-like octahedral hydroxide layer made with Zn and Al (about 4.780 Å [12,38]). For the four hybrid materials, powder patterns clearly indicate the efficiency of the intercalation process via the shift of the (00l) reflection lines to lower angles (e.g., a 5.742 , 5.676 , 5.873 and 5.784 displacement for the (003) peak respectively for Zn2Al-HBS, Zn2Al-BS, Zn2Al-HBC and Zn2Al-BC). The basal spacing of the layered structure of LDHs is mainly determined by the orientation of the inserted anions in the interlayer space. To better interpret the intercalation process, a first simulation of the organics orientation within the host systems has been performed considering the recorded XRD data. Classically, the height of the gallery (7.912 Å, 8.090 Å, 7.567 Å and 7.800 Å respectively for Zn2Al-HBS, Zn2Al-BS, Zn2Al-HBC and Zn2Al-BC) can be deduced by subtracting to the basal spacing (15.392 Å, 15.570 Å, 15.047 Å and 15.280 Å respectively for Zn2Al-HBS, Zn2Al-BS, Zn2Al-HBC and Zn2Al-BC) the value of a single mineral sheet thickness (composed by the oxide octahedral thickness (2.080 Å) and a hydrogen bond domain (2.700 Å)) increased by a hydrogen bond domain (ca. 2.700 Å thick in the hydrotalcite case [39]) on one side of the inorganic layer (7.480 Å) (Fig. 6). Finally, the layer thickness includes the hydrogen bond domains on the both side (2.080 þ 2  2.700 ¼ 7.480 Å) [40]. The resulting value gives the occupied space by the organics and the

Fig. 2. C1s (a) and O1s (b) XPS core peaks of the HBCNaþ salt and schematic representation of organic.

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Fig. 3. S2p (a) and O1s (b) XPS core peaks of the BSNaþ salt and schematic representation of organic. C1s (c) and O1s (d) XPS core peaks of the BCNaþ salt and schematic representation of organic.

hydrogen bonds domains (Table 3). The (110) plane presents very weak evolutions which are consistent with a constant distance value between Zn and Al atoms. In order to determine the anion orientation between the sheets, we have optimized the geometry of each of them by using a molecular approach (GAUSSIAN03 code, B3LYP 6-311G* basis set). These calculations lead to determining the size of the systems from the oxygen atom of the anionic group (eSO3 or eCO2) to the hydrogen atom of the hydroxyl group for HBS and HBC and to the hydrogen atom bonded to carbon atom in para position for BS and BC.

The structures optimizations of the organic anions give four stabilized geometries with a 6.710 Å size for HBS, 6.090 Å for BS, 6.610 Å for HBC and 5.990 Å for BC. By considering the oxygen ionic radius (rO- ¼ 1.40 Å) and the hydrogen atomic radius (rH ¼ 0.53 Å), these values are respectively extended to 8.640 Å, 8.020 Å, 8.540 Å and 7.930 Å [41]. Except for Zn2Al-BS, the interlamellar space is lower than the anion size, which implies that HBS, HBC et BC could not be perpendicular oriented to the sheets. The minimum inclination angle is given in Table 3. A schematic representation is represented in Fig. 6 for the HBS anion.

Fig. 4. (a) Sodium hydrated HBSNaþ salt structure considered for the calculation, (b) Sodium HBCNaþ salt structure considered for the calculation.

S. Fleutot et al. / Solid State Sciences 13 (2011) 1676e1686 Table 2 Calculated atomic net charges in periodical approach for hydrated HBSNaþ and HBCNaþ salts. C1 is the carbon atom bonded to the functional group eSO3 or eCO2 and C4 the carbon atom bonded to the OH group. C3 and C5 are the carbon atoms located in meta positions with regards to the functional group and C2 and C6 the carbon atoms located in ortho positions. Compound atoms X (S or C) O (SO3 or CO2) C1 C2 C3 C4 C5 C6 O (O-H) Na

Hydrated HBS salt

HBC salt

1.35 0.86 0.17 0.15 0.18 0.11 0.24 0.20 0.42 0.83

0.80 0.74 0.07 0.18 0.22 0.32 0.23 0.20 0.69 0.82

3.2.2. XPS results For a whole XPS description of the hybrid material, a first examination of the mineral layers atoms-binding energies is necessary. Zn3p and Al2p surveys give a set of data representative of the LDH in order to state of some possible changes with the insertion. Expected data are obtained for the hydroxide environments with the following binding energy values: (Zn3p3/21/ 2) ¼ 89.2 eVe92.2 eV and (Al2p) ¼ 74.6 eV as often reported [42,43]. The Zn/Al atomic ratio was found around 2.1 which is in good agreement with the expected value of 2 (Table 4). In the case of hybrids, these data do not change and the associated peaks do not show any clear widening (DFWHM ¼ þ0.1 eV) (Table 5). All these observations traduce a chemical and electronic stability of the mineral layers during the insertion of organics. Moreover, the absence of chlorine and sodium signals attest of a quite effective anionic exchange for a single phase as previously suggested with the PXRD conclusions. The synthesis reaction was controlled under nitrogen flux. For any hybrid material, the N1s signal was void. In addition to the host mineral matrix XPS observation, special care was also given to the organic sub-system with the objective of describing inorganic/organic interactions. For Zn2Al-HBS, the sulphur oxidation state was then recorded through S2p peak (Fig. 7a) to probe possible electronic changes. The associated binding energies noticed for the doublet (168.0 eVe169.2 eV) are similar to those of the salt but the consequent increase of the peaks FWHMs (þ0.5 eV) attests chemical changes in the sulphur atoms surrounding. The same modifications are observed for Zn2Al-BS (Table 5). The 2p3/2-1/2 binding energies (168.0e169.1 eV) for the sulphur doublet of the Zn2Al-BS are similar to binding energies of the BS salt. The peaks FWHMs increase for the hybrid (1.4 eV instead of 0.9 eV for the salt). For Zn2Al-HBC and Zn2Al-BC, same survey can be led via the evolution of the carboxylate (eCO2) component of the C1s spectrum. With a value of 288.2 eV (Fig. 8, Table 5) for Zn2Al-HBC, corresponding to a well-defined carboxylate environment, no modifications are observed in comparison with the salt. Similarly,

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no significant changes are observed for Zn2Al-BC. The binding energy (288.5 eV) corresponds to a carboxylate environment and is similar to the salt (þ0.2 eV). The FWHMs are unchanged (DFWHM ¼ þ0.2 eV) in comparison to the HBCNaþ and BCNaþ salts. Beside these specific S2p and C1s spectra analyses, the simultaneous examination of the O1s energetic area has shown a sole component (531.6  0.2 eV) for each hybrid material. This component correspond to a hydroxide phase whatever the hybrid system studied (Fig. 7b). This response in the oxygen signal traduces the difficulty to distinguish the large population of oxygen surroundings in the whole material. Relate to the parent host matrix, for a quite same binding energy, it can be nevertheless noticed a þ0.3 eV widening which characterizes a new trend. The weak electronic modifications could not be reported as clear differences on the O1s spectrum. By considering a single organic molecule (anionic entity) incorporated for an aluminium atom (positive center) of an LDH layer, it could be estimated the Oorganics/OLDH layers ratio. The previous results deduced form the PXRD experiments have precise a c/3 value (¼d003 in Å) of 15.392 Å, 15.570 Å, 15.047 Å and 15.280 Å for Zn2Al-HBS, Zn2Al-BS, Zn2Al-HBC and Zn2Al-BC respectively. In these conditions and in support of the fact that XPS can reach approximatively a 50 Å depth signal, one can consider that only 3 layers and 3 interlamellar spaces of the host LDH are probed. Then, in the primitive unit cell, a 1/8 ratio is valued between oxygen atoms in the hydroxyl function of the HBSNaþ and HBCNaþ salts and those of the LDH matrix. These relative proportions associated with an important FWHM and small differences of chemical shifts do not allow observing the presence of the hydroxyl function of the organics via the O1s peak. 4. Discussion The previous XPS data on hybrid intercalated compounds highlighted different results depending on the functional headgroup eSO3 and eCO2 which can be interpreted as a more important disorder (different chemical environments) in the first case. A theoretical approach is then achieved for a better understanding. If the previous molecular calculations of HBS and HBC were used to calculate the size of anions incorporated into the LDH matrix, new comments can be done in regard with XPS results: Important geometrical constraints are observed for the HBC anion which presents a strong rigidity because of its plane geometry. This rigidity is stressed by an sp2 carbon atom from the carboxylate group, associated with a strong p conjugation on the aromatic cycle (Fig. 9a). By not deforming, the carboxylate group allows only one type of interaction with the LDH sheet. This could explain the same XPS signal between the salt and the hybrid material. The geometrical constraints are less important for the HBS anion which presents a sp3 type sulphur atom. This sp3 hybridization is associated with a p*SO3 type orbital leading to a “hyperconjugation” phenomenon which corresponds to a conjugation of

Table 3 Refined lattice parameters of the synthesized products and comparison between height of gallery and anion size (Å); tilt angle ( ). a (Å) Zn2Al-Cl Zn2Al-HBS Zn2Al-BS Zn2Al-HBC Zn2Al-BC a

3.0713 3.0640 3.0658 3.0575 3.0624

c (Å) (2) (1) (5) (2) (2)

23.1670 46.1760 46.7100 45.1410 45.8400

Symmetry (3) (6) (9) (5) (5)

Oxygen ionic radius and hydrogen atomic radius [40].

R3m R3m R3m R3m R3m

Height of gallery (Å)

Anion size without ri (Å)

Anion size with ri (Å)a

Tilt angle q ( )

7.912 8.090 7.567 7.800

6.710 6.090 6.610 5.990

8.640 8.020 8.540 7.930

23.7 0.0 27.6 10.4

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Fig. 5. XRD powder patterns (l CuKa ¼ 1.5418 Å) of Zn2Al-Cl, the Zn2Al-BS hybrid, the Zn2Al-HBS hybrid, the Zn2Al-BC hybrid and the Zn2Al-HBC hybrid.

the p aromatic cycle with the antibonding molecular orbital on the SeO bond in a plane perpendicular to the cycle plane (Fig. 9b). This “hyperconjugation” phenomenon is more or less pronounced according to the variation of the q angle formed by oxygen and sulphur atoms engaged in this bonding and the aromatic cycle. To determine if the sulphur net charge is sensitive to an evolution of the q angle, we imposed on this angle determined from the geometrical optimization (104.1 ) an evolution of 5 . This evolution is made possible by the capacity of the polyhedral deformation of eSO3. This study reveals a net charge qS which decreases from 1.11 to 1.08 when the q angle increases from 99.1 to 109.1. This situation corresponds to a less pronounced phenomenon of “hyperconjugation” when the q angle increases and the net charge qS decreases. The same study was led by varying the position of the triedre eSO3 as explained farther. For these variations, the sulphur net charge does not evolve.

For BC, the p conjugation phenomenon over the aromatic cycle associated with the carbon (from the carboxylate group) sp2 hybridization observed for HBC is also evidenced. Also, the geometrical structure of the anion BS is subjected to a hyperconjugation phenomenon which depends on the q angle, similar to that observed for the HBS anion. As previously, if we change the q angle determined by geometry optimization (103.8 ) of 5 , we observe a decrease of the hyperconjugation phenomenon and the sulphur net charge (from 1.11 e to 1.08 e). These observations are in agreement with the evolution of the XPS S2p data. The perturbed chemical environment around the sulphur atoms lead to an increase of the FWHM of the XPS S2p core peak which can be explained as resulting from several types of possible interactions between the triedre eSO3 and the LDH sheet (associated with different q angles). The possible competition between several interactions leads to a higher disorder during the HBS and BS intercalation compared with HBC and BC. To formalize the highest capacity of deformation of HBS with regard to HBC, each entities containing the eCO2 group or the eSO3 group is subjected to a rotation with regard to the aromatic cycle plane (Fig. 10). This rotation is achieved from the optimized geometries by 30 steps (from 0 to 360 ). This approach allows the determination of the rotation barrier, defined as the difference between an energy minimum and maximum during a deformation resulting from a rotation around a bond. Then, we can estimate the strength of the interaction between the functional group and the aromatic cycle, previously revealed (phenomenon of p conjugation for HBC and BC and p*SO3 hyperconjugation for HBS and BS). The rotation barrier is respectively equal to 0.16 eV (15.44 kJ mol1) for HBC [44] (Fig. 10a) and 0.0018 eV (0.18 kJ mol1) for HBS [45] (Fig. 10b). This result is consistent with a highest ease of rotation of the eSO3 group toward the aromatic cycle compared with the eCO2 group. The hyperconjugation phenomenon is more important when the SeO bond (from the eSO3 group) is in a perpendicular plane in that of the benzene cycle (each 60 ). The hyperconjugation phenomenon tends to decrease when none of the SeO bonds is located in a perpendicular plane of the cycle. The minimum is obtained for a rotation of 30 of the dihedral (for this angle, one of the SeO bond is situated in the same plane as the cycle (Fig. 10b). For carboxylate, the phenomenon of p conjugation associated with the plane geometry of molecules is not continuous during the rotation of the eCO2 functional group. Indeed, for this type of organic entities, the p orbitals can overlap only if the molecule is plane (for a 180 rotation). The p orbitals of oxygen and carbon atoms overlap by giving a character of double bond to the C (CO2)-C bond. The rotation of the functional group with regard to the cycle plane is then more difficult and led to a break of the conjugation phenomenon for a rotation of 90 . For this angle, the eCO2 group is situated in a perpendicular plane in that of the cycle plane (Fig. 10a). Table 4 XPS data of the Zn2Al-Cl matrix: binding energies (eV), FWHM (eV) in parentheses and atomic percentage (%). Compound BE region O1s C1s

Cl 2p3/21/2 Zn3p3/21/2 

Fig. 6. Scheme of a possible conformation of the intercalated HBS hydrotalcite host matrix.

anion in the

Al2p

Zn2Al-HBS 531.8 284.9 286.6 289.3 198.8 200.5 89.2 92.2 74.6

(1.5) (1.4) (1.5) (1.9) (1.5) (1.9) (2.6) (2.6) (1.5)

61.7% 9.7% 1.8% 1.0% 3.8% 1.9% 9.0% 4.8% 6.3%

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Table 5 XPS data of the hybrid materials: binding energies (eV), FWHM (eV) in parentheses and atomic percentage (%). Compound BE region C1s

O1s S2p3/2-1/2 Zn3p3/2-1/2 Al2p

Zn2Al-HBS

Zn2Al-HBC

284.6 (1.3) 286.1 (1.3)

26.5% 4.3%

289.2 531.8 168.0 169.3 89.2 92.2 74.6

1.3% 50.8% 2.1% 1.0% 5.8% 2.9% 5.3%

(2.1) (1.9) (1.4) (1.3) (2.7) (2.7) (1.5)

284.6 286.3 288.2 291.5 531.8 e e 89.2 92.2 74.6

Zn2Al-BS

(1.3) (1.3) (1.3) (2.7) (1.8)

22.2% 4.2% 3.5% 1.3% 56.0%

(2.5) (2.5) (1.4)

5.3% 2.6% 4.9%

During the rotation, the net charge qS on the sulphur atom remains unchanged when that of the carbon atoms (from eCO2) qC evolves from 0.40 (a ¼ 0 ) to 0.27 e (a ¼ 90 ). Concerning the HBS molecule, this invariance of the sulphur net charge is in agreement with the stability of the binding energy of the S2p peak observed by XPS during the intercalation. Moreover, the continuity of the hyperconjugation phenomenon revealing for various conformations of the molecule allows their coexistence in the interlayer space. These several conformations explain the increase of the S2p full width at half maximum (FWHM) for the intercalated compound. On the contrary, the stability of the HBC molecule limiting its deformation confirms the presence of a single type of interaction for the Zn2Al-HBC system, characterized by a binding energy and an FWHM of the C1s component unchanged. 4.1. Intercalation process In a general way, the intercalation simulation consists in modeling the interaction of the anion with the mineral network reduced to Al(OH)3 aggregate at the first order. The eSO3 and eCO2 groups are then positioned in front of the optimized molecular aggregate Al(OH)3 (Fig. 11) which simulates the mineral framework. To set up the local geometry surrounding the aluminium atoms (positive centers which a priori, play a main role in the interactions with the intercalated anionic entities) in order to simulate the LDH phase, the first step consists in performing a periodic calculation

a

Zn2Al-BC

284.6 (1.1)

31.8%

284.6 (1.3)

43.5%

290.2 531.6 168.0 169.1 89.2 92.2 74.6

2.1% 48.4% 2.8% 1.4% 5.7% 2.8% 5.0%

288.5 291.4 531.6 e e 89.2 92.2 74.6

(1.1) (2.4) (2.0)

3.3% 1.5% 39.6%

(3.1) (3.1) (1.8)

5.2% 2.6% 4.3%

(3.2) (1.6) (1.3) (1.2) (2.5) (2.4) (1.4)

(by using the CRYSTAL06 code) of the Zn(OH)2 phase, (Mg(OH)2 brucite-like structure [46,47]). The structure Zn(OH)2 crystallizes in the CdI2 system (P3m1, space group n 164) and can be readily described as a simple juxtaposition of stacked layers along a crystallographic axis. The slabs of octahedrons are made of two hydroxyl layers (eOH) sandwiching a metallic plane (Zn). The hydroxyl layers (in where each oxygen atom is bonded to one hydrogen atom) describe a hexagonal close packing with the metal occupying half of the octahedral sites. The group forming the basic lattice structure is composed of one zinc atom (localized in site 1a), two oxygen and two hydrogen atoms (both localized in sites 2d). We have defined a unit cell which is periodically repeated in two dimensions and built from the optimized lattice parameters of the bulk as a starting point (a ¼ b ¼ 3.278 Å, c ¼ 4.732 Å). A Zn(OH)2 slab consisting of 5 layers (HeOeZneOeH) is then considered and the relaxation of the surface is taken into account by optimizing the cell parameters and all the atomic positions in order to obtain the equilibrium geometry (a ¼ b ¼ 3.243 Å). Note that these results are in agreement with DFT calculations [48] achieved on brucite-like compounds as Zn(OH)2 or Mg(OH)2. In a second step, the whole Zn2Al(OH)6 hydrotalcite sheets considered in this work were obtained by forming a 3  3 supercell with the partial substitution of some Zn atoms by some Al atoms. The new generated slab geometry was performed using a biperiodical approach within the periodic LCAO-B3LYP approximation developed in the periodic ab initio code CRYSTAL06 [17]. The analysis of the electronic properties in terms of atomic population clearly shows that the electronic density is weaker for the OH

b

Fig. 7. (a) Evolution of the XPS S2p peak between the HBSNaþ salt and the Zn2Al-HBS hybrid material, (b) Evolution of the XPS O1s peak between the precursors (the HBSNaþ salt and Zn2Al-Cl) and the Zn2Al-HBS hybrid material.

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284.6 eV 286.3 eV 288.2 eV

C (C-OH)

HBC salt C (CO2-)

C (C-C)

Zn2Al-HBC 294

292 290 288 286 284 282 Binding energy (eV)

Fig. 8. Evolution of the XPS C1s peak between the HBCNaþ salt and the Zn2Al-HBC hybrid.

groups bonded to Al atoms. As expected, the H atoms connected to Zn are less discharged than those around Al sites. So we can suppose that the HBS-LDH interaction concerns preferentially the Al sites of the hydrotalcite. As reported in a theoretical study which considers three metal cations [49], the 2Zn-Al cluster presents a high electronic stability. Considering these results, in this first approach, the sheet is represented by the Al(OH)3 fragment. The local Al(OH)3 geometry surrounding the aluminium atoms is obtained, i.e. bond lengths AleO, OeH and dihedral angles OeAleO/HeOeAl which should be introduced in order to preserve the structure around the aluminium atoms in agreement with previous NMR and DRX results [50] which indicate an aluminium atom in an octahedral environment and the stability of host matrix revealed by XPS analysis. Note that the systems could not be fulloptimized and some geometrical constraints on the mineral cluster and the organic molecule should be introduced in order to establish a correlation between the net charges on the C (eCO2) and S (eSO3) atoms and the XPS results. The bond lengths and the angles of the eCO2 and eSO3 headgroups can be optimized to determine the interaction nature between inorganic and organic sub-systems. In the following, the different oxygen atoms acting in the intercalation are called, O1 for the sulfonate or carboxylate headgroups, O2 for the hydroxyl headgroup and O3 for the mineral sheet (Fig. 11). For HBS and BS, the oxygen atoms from the eSO3 group are positioned below the hydrogen atoms from the mineral

Fig. 10. Rotation barrier for (a) eCO2 for HBC and to (b) eSO3 for HBS.

framework, d(O1-H) ¼ 2.526 Å corresponding to the interaction domain of hydrogen bonds. In a first step, the geometry of the HBS- Al(OH)3 hybrid system was then optimized (Table 6). A slight increase of the SeO1 bond lengths (1.492 Å) within the organic entity is noted simultaneously to a significant reduction of the distance between O1 and H of Al(OH)3 (1.995 Å) [51].The results obtained after optimization for BS - Al(OH)3 are very closed with a slight increase of the SeO1 bond (1.491 Å compared to 1.488 Å for the anion) and a decrease of the O1eH distance. In the same time, for the two systems, while angles O3eAleO3 tend to increase (þ0.2 ), angles O1eSeO1 close up (0.6 ). The evolutions of the S-O1 bonds and the O1eSeO1 angles are in agreement with a deformation of the triedral e SO3 involving a perturbed chemical order around the sulphur atoms during the insertion compared with salts. The same resulting net charges of sulphur atoms for the hybrid systems [HBS-Al(OH)3] and [BS-Al(OH)3] (qs ¼ þ1.16 e) are obtained. They are close to the HBS and BS anions (qs ¼ þ1.10 e). In a first approximation, if we simply consider an initial state effect, this weak evolution agrees with the non-chemical shift observation of the XPS S2p3/2

Fig. 9. Representation of the molecular orbital (HOMO) concerned by (a) the p “conjugation” over the aromatic cycle for HBC anion and (b) the “hyperconjugation” phenomenon for HBS anion.

S. Fleutot et al. / Solid State Sciences 13 (2011) 1676e1686

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Fig. 11. Representative scheme of the interaction between Al(OH)3 and (a) HBS and (b) HBC during the intercalation process.

core peak. Moreover, it confirms the weak interaction between subsystems via hydrogen bonds. A similar approach is conducted to model the insertion of HBC and BC derivatives. For both systems, the CeO1 bond distances remain unchanged and the O1eCeO1 angle opens slightly (0.2 ) in agreement with the rigid structure of anions. The O1eH distance decreases in a significant way (2.526e1.980 Å) characteristic of a stabilization of the hybrid systems by hydrogen bonds. The net charge calculated for the carbon atom from the carboxyl group (þ0.52 e for both systems) is close to that calculated for anions (0.49 e). These results are consistent with an identical XPS signature of the carboxyl component before and after insertion of HBC and BC. Table 6 Intercalation model e bond lengths (Å), angles after optimization of the two molecular fragments (HBS and HBC) [first column], and the hybrid material (O1 ¼ O of the sulfonate group from HBS and O of the carboxylate group from HBC) [second column]. HBS (molecular approach)

Hybrid system HBS-Al(OH)3 (molecular approach)

d(S-O1) (Å) d(S-C) (Å) O1-S-O1 ( ) O1eS-C ( )

1.488 1.824 114.8 103.7 Al(OH)3 (periodical approach)

1.492 1.824 114.2 103.7

d(Al-O3) (Å) d(O3-H) (Å) O3-Al-O3 ( ) AleO3-H ( )

2.100 0.958 99.3 120.0 HBC (molecular approach)

2.084 0.958 99.5 120.9 Hybrid system HBC-Al(OH)3 (molecular approach)

d(C-O1) (Å) d(CeC) (Å) O1-C-O1 ( ) O1eC-C ( )

1.257 1.553 130.0 115.0 Al(OH)3 (periodical approach)

1.257 1.553 130.2 115.0

d(Al-O3) (Å) d(O3-H) (Å) O3-Al-O3 ( ) AleO3-H ( )

2.100 0.958 99.3 120.0

2.084 0.958 99.4 120.9

The comparison of the net charge evolution for sulfonate and carboxylate systems exhibits a more important difference for the first ones (þ0.06 e for sulfur atoms of eSO3 group compared with þ0.03 e for carbon atoms of eCO2 group). This last result could be correlated to the hyperconjugation phenomenon previously discussed. 5. Conclusion The main objective of this work was to better understand the interactions within model hybrid systems during the insertion of organic anions in a mineral matrix (LDH type). An approach coupling a surface technique (XPS) with some periodic and molecular theoretical calculations was then implemented. A large part of this work was devoted to a preliminary description of organic salts precursors by XPS. This characterization allowed to report chemical and electronic environments of the atoms of the functional groups eSO3 and eCO2(respectively for HBSNaþ/ BSNaþ and HBCNaþ/BCNaþ). At the same time, quantum calculations were led in a periodic approach to optimize the geometrical structure of salts HBSNaþ and HBCNaþ and to calculate the Mulliken net charges. These last data are in agreement with the chemical different environments of salts and could be correlated to the binding energies obtained by XPS. Four model hybrid LDH phases, synthesized by coprecipitation method from these salts and from a Zn2Al LDH matrix, were then characterized by XRD and IR. This classic characterization allowed to confirm the process of insertion and to propose an arrangement of the organic entities between the layers of the mineral network. A minimum slope angle of these was deduced from the experimental data and from theoretical results. After this structural analysis, XPS was used as a local probe of the chemical and electronic environments of the organic and inorganic constituents. The obtained results allowed to conclude to the stability of the mineral host and to the stabilization of anions by hydrogen bonds. The comparison of XPS spectra obtained for salts precursors and for the hybrid materials showed a perturbed chemical environment around the sulphur atom (of eSO3) for Zn2Al-HBS and Zn2Al-BS. This result

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