Phys Chem Minerals (2009) 36:593–602 DOI 10.1007/s00269-009-0304-4
ORIGINAL PAPER
An XPEEM study of structural cation distribution in swelling clays. I. Synthetic trioctahedral smectites Delphine Vantelon · Rachid Belkhou · Isabelle Bihannic · Laurent J. Michot · Emmanuelle Montargès-Pelletier · Jean-Louis Robert
Received: 8 September 2008 / Accepted: 31 March 2009 / Published online: 14 April 2009 © Springer-Verlag 2009
Abstract X-PEEM images and XPS were collected on isolated layers of three synthetic swelling clays, one hectorite and two saponites with various charge, recording the Si(2p), Al(2p) and Mg(2p) core level spectra from the clay sheets. Spectra were Wtted to determine the diVerent components of the core levels. Due to their large full width at half maximum, Si XPS spectra were Wtted using two to three doublets. It appears that, for a given clay mineral, Si, Al and Mg binding energies (BE) were constant, for all the observed layers. However, variations of the Si BE were observed depending on the nature of the mineral investigated. The various components obtained from the Wt of Si spectra could be assigned to diVerent substitution rates; binding energy shifting to lower values with substitution increase in the layer. Furthermore, variations in Si BE according to charge location were assigned to the inXuence of exchangeable cation. Keywords X-PEEM · Swelling clays · Charge distribution · Cation substitution · XPS
D. Vantelon (&) · R. Belkhou Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France e-mail:
[email protected] I. Bihannic · L. J. Michot · E. Montargès-Pelletier Laboratoire Environnement et Minéralurgie, UMR 7569 CNRS INPL, 15 avenue du Charmois, BP 40, 54501 Vandoeuvre-lès-Nancy Cedex, France J.-L. Robert Institut de Minéralogie et de Physique des Milieux Condensés, UMR 7590, Université Paris 6 et 7, 140, av. de Lourmel, Bat. 7, 75015 Paris, France
Introduction Smectites are swelling clay minerals. They are lamellar alumino-silicates formed by two silica sheets sandwiching an aluminum (dioctahedral) or a magnesium (trioctahedral) hydroxide sheet. Isomorphous substitutions by lower charge metal cations in the crystal lattice result in permanent negative charge, which is compensated by electrostatically attracted cations (exchangeable), located in the interlayer space. The nature and valence of these exchangeable cations highly inXuence the hydration and colloidal behavior of smectites. The lateral extension of individual clay layers ranges from 25 to 1,000 nm while they are »10 Å thick. Due to their highly divided status, these materials display high surface area as well as high reactivity. In that regard, these widely encountered minerals on the Earth surface govern the mechanical stability of soils and play a crucial role in plant nutrition (Bergaya et al. 2006). In addition, they control to some extent the fate and transfer of pollutants, which, in addition to their plasticity and low permeability, explains why they are extensively used as barrier materials in waste disposal facilities (Bergaya et al. 2006). A particular case is the management of nuclear wastes where swelling clays are used to prevent the diVusion of radionucleides into the surrounding geological medium. Other applications of swelling clays are related to their ability to form gels at very low solid content. Such gels whose structure remains debated (Bihannic et al. 2001; Michot et al. 2004), are extensively used in industry as thickening and thixotropic agents for drilling Xuids, paints, and cosmetics (Bergaya et al. 2006). Smectites exhibit a wide variation in chemical composition as a result of constituting variable atoms of both the tetrahedral (silica) and octahedral (Al- or Mg-hydroxide) sheets. Furthermore, even for similar compositions, the
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distribution of structural cations and, therefore, of the layer charge can be very variable (Vantelon et al. 2001, 2003). As both parameters must have a signiWcant inXuence on the reactivity and colloidal properties of swelling clays, it is important to fully characterize them. Various techniques have been applied to solve this issue. Layer charge distribution can be approached from the intercalation of n-alkylammonium chains with variable lengths followed by X-ray diVraction (XRD) analysis of the resulting interlayer distances. The evolution of the basal spacing then provides information on the variation of layer charge in a given clay sample (Lagaly et al. 1976). As far as cation distribution inside the layers is concerned, diVerent spectroscopic techniques probing the structural environment of the atoms can be applied. The most commonly used is Infrared spectroscopy (IR). In that case, a detailed analysis of the position of hydroxyl stretching or bending vibrational bands can give insight into the short-range order distribution of octahedral cations in phyllosilicates (Besson and Drits 1997; Besson et al. 1987; Cuadros and Dudek 2006; Farmer and Russel 1964; Vantelon et al. 2001). The repartition of iron atoms inside the layers and their relationships to lighter atoms can be studied in detail using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at the iron K edge (Bonnin et al. 1985; Drits et al. 1997; Manceau 1990; Manceau et al. 1998; Muller et al. 1997; Vantelon et al. 2003). Still in the case of iron, Mössbauer analyses allow discriminating iron oxidation state and coordination (tetrahedral or octahedral), whereas diVerences in quadrupole splitting of Fe3+ cations reveal the distortion of Fe-octahedra due to diVerent local environments around them (Besson et al. 1983; Drits et al. 1997, 2002). Finally, 29Si and 27Al MAS NMR spectroscopy allows studying the distribution of Al and Si in phyllosilicates since Al spectra yield the coordination state of Al (tetrahedral or octahedral) while Si spectra provide information on Al and Si distribution in the tetrahedral layer (Circone et al. 1991; Sanz et al. 2006), this latter information being directly related to charge distribution characterization. In addition, for layers containing iron, the intensity of the Al and Si signals can be used to determine short-range order in Fe-distribution (Schroeder and Pruett 1996). In the case of dioctahedral smectites, these spectroscopic methods can be usefully complemented by electron diVraction to study vacancies distribution (Beermann and Brockamp 2005; Besson et al. 1983; Tsipursky and Drits 1984). Finally, short-range and long-range order of octahedral cations in phyllosilicates have recently been studied by ab initio molecular orbital calculation with Density Functional Theory methods (Timon et al. 2003) or Monte Carlo simulations based on atomistic models (Sainz-Diaz et al. 2004). The results obtained can be compared with FTIR and MAS-NMR experimental data (Cuadros et al. 1999; Sainz-Diaz et al. 2001). As each method provides only
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partial information to determine cation and charge distribution in phyllosilicates, the combination of multiple techniques is required to derive detailed structural information (Cuadros et al. 1999; Dainyak et al. 2006; Drits et al. 1997; Muller et al. 1997). Although accurate, all these techniques remain averaging bulk methods and do not inform on the variability of cations and charge distribution from one clay layer to another. The aim of the present work is to tackle this question by taking advantage of the capabilities of the X-ray Photo-Emission Electron Microscopy (X-PEEM). X-PEEM was originally developed for investigating the chemical and magnetic properties of conducting surfaces (Bauer 1994; Swiech et al. 1997) but is now increasingly applied for other materials such as those of interest to geochemists (Gilbert et al. 2003; Smith et al. 2004). To Wt the PEEM requirements a sample must be Xat, conductor and ultra high vacuum (UHV) compatible. Actually, clays are not altered by UHV and, although they are insulator materials, their lateral extension (25–1,000 nm) combined with their thickness (10 Å) entails that a deposit of isolated layers on a metallic substrate prevents any charging eVects. Then, thanks to a nanometric lateral resolution, high resolution XPEEM imaging using core level spectroscopy performed on clay monolayer deposits enables characterizing the diVerent single clay sheets. From the intensity variations of the photoemission core level spectra of a relevant species (i.e., a structural atom), it is then possible to deduce its local distribution. In parallel, its binding energy (BE) provides information on its electronic and chemical environments. Indeed, substituted cations in the layer inXuence the electronic environment of the neighboring atoms, which results in variations in the core level BE of the atom of interest (Ebina et al. 1997; Seyama et al. 2004; Wagner et al. 1982). In this paper, we present a Wrst application of X-PEEM to study cation distribution in synthetic trioctahedral swelling clays. In a following paper, we will use X-PEEM to study cation distribution in natural dioctahedral smectites.
Materials and methods Samples Three synthetic smectites were studied (Table 1). Synthetic hectorite and saponites were prepared by hydrothermal synthesis in Morey type externally heated pressure vessels, internally coated with a gold tubing, at 400°C, 1 kbar P(H2O), for a run duration of 4 weeks. These synthesis were performed from gels of appropriate compositions, using high-grade Na2CO3, Li2CO3, Mg(NO3)2 and tetraethylorthosilicate (C2H5O)4Si as starting reagents. Full details about synthesis and characterisations are described in
Phys Chem Minerals (2009) 36:593–602 Table 1 Names and structural formulae of the studied clays Clay
Unit cell formula per O20 OH4
Saponite 0.65
(Si6.7Al1.3) (Mg6) Na1.3
Saponite 0.4
(Si7.2Al0.8) (Mg6) Na0.8
Hectorite
(Si8) (Mg5.2Li0.8) Na0.8
Michot et al. (2005). The structural formulae of the clays were determined from chemical analyses measured by ICP-AES using a quantometer Jobin Yvon 70 and their crystalline structure and purity were veriWed by XRD (D8, Bruker) and infrared spectroscopy (IFS 55 Bruker). Samples of isolated layers were prepared depositing a drop of clay suspension (hectorite and saponite 0.65: 6 mg/ L; saponite 0.4: 7.5 mg/L) on a gold wafer to ensure a good electrical conductivity. After air drying, samples were heated at 150°C during 30 min under UHV (base pressure + 10¡10 Torr) in the X-PEEM preparation chamber, to desorb the air contaminant and to ensure high quality XPS signal measurements. Data collection and analysis The experiments were performed using a commercial Elmitec GmbH. LEEM/PEEM III microscope installed on the Nanospectroscopy beamline of the ELETTRA light source. Details on the beamline set up and capacities are described in Locatelli et al. (2003, 2006). A monochromatic X-ray focused beam (20(H) £ 5(V) m2) illuminates the sample. In X-PEEM mode, using a parallel imaging system, the photoelectrons emitted normally from the sample surface are selected in energy (with an energy resolution of 200 meV), forming an image that is magniWed and projected on a phosphor screen. The image is captured by a MCP and a 12-bit CCD camera. For these experiments, we used Field Of View (FOV) of 5 and 10 m with a lateral resolution of »50 nm. These FOV enable on the one hand a high magniWcation power and on the other hand imaging of a suYcient number of layers to perform statistical measurements. The incident beam energy was Wxed at 150 eV, allowing excitation of Mg2p, Al2p, and Si2p core hole transitions from the clay layers and Au4f5/2 and Au4f7/2 core levels from the sample holder. For each element, XPS spectra were collected capturing a photoelectron image using a step of 0.2 eV, scanning the electron Kinetic Energy (KE). Therefore, each image corresponds to a given KE. The absolute BE were calculated taking the substrate Au4f7/2 BE = 84 eV as reference. XPS data were extracted using the dedicated Igor macro software. XPS spectra were Wtted after background removal using a Doniach-Sunjick Wtting procedure (FitXPS2 free
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software). According to physics and experimental conditions, the Gaussian Full Width at Half Maximum (FWHM) was Wxed at 0.45 and the Lorentzian FWHM was maintained below 2.00. The alpha asymmetry parameter was Wxed to 0 due to the non metallic character of the clay material. The 2p3/2 component of the 2p core levels was set twice bigger than the 2p1/2 according to the occupancy selection rules (branching ratio); 20% variations were accepted to take into account the structural dependence of the branching ratio in solids (Photoelectrons scattering and diVraction eVects) (Gota et al. 1993; Shivaprasad et al. 2004; Sieger et al. 1995).
Results and discussion Number of layers per particle Clay suspensions were prepared in order to obtain deposits of isolated layers. This individualization can be veriWed, taking into account the electrons inelastic mean free path (IMFP). Indeed, the mean depth probed by XPS is estimated to be the IMFP of the electrons emitted normally onto the surface (Duc 1998). Its value is dependant on the kinetic energy of the electrons, the composition and the insulating or conducting character of the sample. Then, for a given clay, which is insulating, and electrons with KE = 60 eV, phenomenological calculations give an IMFP ranging from 0.8 to 2.7 nm (Cumspon and Seah 1997; Seah and Dench 1979) which corresponds to a thickness of 1 to 3 clay layers. This large deviation is mainly due to the close correlation of the IMFP with the material density and the electron–phonon interaction. The consequence of such a deviation is that, for a given deposit containing several stacked layers forming a particle, the lowest value implies that only the top layer of the particle is observed, while the highest value implies that the Wrst three layers are observed. Thus, in order to estimate the experimental electron IMFP and the amount of stacked layers in a given particle, we used the attenuation by the clay particles of the gold signal provided by the wafer (Fig. 1). In the Nanospectroscopy beamline conWguration (normal emission and small collecting angle), the attenuation of the Au4f7/2 BE signal intensity by clay particles is equal to exp(¡z/IMPF) with z the particle thickness. In our experiments, the kinetic energy of electrons corresponding to the Au4f7/2 BE is KE(Au4f7/2) +63 eV. Images of samples were collected at this kinetic energy for the three studied clays. Absorption of Au4f7/2 BE signal intensity was calculated as I/I0, with I and I0, the intensity of the signal in and out each particle, minus the background signal, respectively. For all clays, the statistical thickness distributions of particles are reported on Fig. 2.
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(a) 20
(a)
Hectorite
Number of particles
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0
0,5
0,9
1,3
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(b) 1,3 Au 4f Au wafer Clay particle
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Au 4f
Number of particles
Integrated intensity (a.u.)
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7/2
5/2
1
0,9
6
4
2
0,8
0,7 56
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0
66
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Kinetic energy (eV)
Assuming that, at least, part of the clay layers were singled out, the lowest attenuation of the Au signal corresponds to isolated layers. When Wxing IMPF = 1.6 nm for all the studied clays, the lowest attenuation of the Au signal corresponds to particle thickness z = 0.8 nm § 0.2, which is a realistic value for a single layer thickness taking into account the layer thickness itself (0.7 nm) and the exchangeable cations layer considerably reduced by the UHV. In hectorite, particles formed by a stacking of three, four and Wve layers (z = 2.35 nm § 0.25; z = 3.1 nm § 0.3; z = 3.7 nm § 0.05, respectively) are rare while particles with three and four layers are more abundant in saponite 0.4 and saponite 0.65 deposits. For the latter two clays, total dispersion of layers in suspension was more diYcult due to their higher tetrahedral charge. However, for the three
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2,1
2,5
2,9
(c) 20 Saponite 0.65
15
Number of particles
Fig. 1 (a) Image taken at Au7/2 signal at a KE = 63 eV. The Au wafer appears in light gray and clay particles in dark gray. FOV = 10 m. (b) Au XPS spectra extracted from the wafer in red and from a particle in green
1,7
Particle thickness (nm)
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0 0,5
0,9
1,3
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2,1
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Particle thickness (nm)
Fig. 2 Statistical distribution of particles thickness in (a) hectorite, (b) saponite 0.4 and (c) saponite 0.65 deposits
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studied clays, the most abundant particles are formed by one and two layers stacked together (z = 1.6 nm § 0.2).
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(a) 0.4 raw data Si2 2p3/2 Si2 2p1/2 Si1 2p3/2 Si1 2p1/2 fitted data
0.35
Distribution of structural cations in layers Intensity (a.u.)
0.25 0.2 0.15 0.1 0.05 0 98
100 102 104 Bending Energy (eV)
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(b) 0.4 raw data Si2 2p3/2 Si2 2p1/2 Si1 2p3/2 Si1 2p1/2 Si3 2p3/2 Si3 2p1/2 fitted data
0.35 0.3 Intensity (a.u.)
Shift of the core hole electron BE of a given structural atom is the result of variations in its electronic and chemical environment. It then provides information on isomorphic substitutions in the layer. Binding energies of Si2p, Al2p and Mg2p were measured in several layers of the hectorite and saponite clays. Al and Mg XPS spectra were Wtted using only one 2p1/2–2p3/2 component while Si spectra were Wtted using two peaks of both 2p1/2 and 2p3/2 in order to maintain the Lorentzian FWHM value below 2.00. However, for most of the saponite 0.65 particles, Si XPS spectra present an additional shoulder at low BE as illustrated in Fig. 3. As this shoulder occurs in stacked layer particles as well as in isolated ones, it cannot be assigned to a charge eVect artifact. Furthermore, authors have observed a shoulder around 99 eV on Si(2p) XPS spectra (Ebina et al. 1997; He et al. 1995; Herreros et al. 1994) using a conventional Al K X-ray source (non monochromatic). Ebina et al. (1997) assigned such a feature to the parasitic Mg(2s) satellite peak which was due to the Al K3 line. In our experiments, this explanation is not valid since we have used a monochromatic incident beam. Consequently, we have considered it as part of the Si XPS signal. Figures 4, 6 and 7 present the BE values of the 2p3/2 Wtting components for Si, Al, and Mg for the various studied clay layers. Concerning Si, peak 1 is assigned to the 2p3/ 2 component having the highest BE and peak 2 to the 2p3/2 component with the lowest BE. Peak 3 is assigned to the 2p3/2 component used for Wtting the shoulder that can occur for saponite 0.65 clay. For all samples, the spread in BE values is around 0.8–1.0 eV from one particle to another. This data scattering reveals the chemical heterogeneity that exists even in synthetic materials. Si BE median values decrease from hectorite to saponite 0.4, from 103.1 § 0.3 eV to 102.2 § 0.3 eV for peak 1 and from 102.0 § 0.3 eV to 101.1 § 0.3 eV for peak 2. Saponite 0.65 exhibits similar Si BE values as saponite 0.4 with median values at 102.3 § 0.2 eV for peak 1 and 101.2 § 0.3 eV, for peak 2. Si peak 3 arises at 100.1 § 0.2 eV. All the BE values are in agreement with literature data for SiO4 in phyllosilicates (Ebina et al. 1997; Gonzalez-Elipe et al. 1988; Okada et al. 1998; Wagner et al. 1982). The shifts to lower energies are generally related to the increasing negative charge of the layer (Gonzalez-Elipe et al. 1988; Seyama et al. 2004). In our study, all clays exhibit substitutions leading to a negative charge. Although the charge variations are small compared to the variations displayed by the phyllosilicates studied in literature (Ebina et al. 1997; Gonzalez-Elipe et al. 1988;
0.3
0.25 0.2 0.15 0.1 0.05 0 98
100
102
104
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Bending Energy (eV)
Fig. 3 Si XPS raw spectrum, Wt result and Wtting components from a particle of (a) saponite 0.4 and (b) saponite 0.65
Okada et al. 1998; Wagner et al. 1982) variations in Si BE are still signiWcant. In the case of saponite, charge is located in the tetrahedral SiO4 sheet, due to substitution of Si4+ by Al3+, whereas in hectorite it is located in the MgO6 octahedral sheet (substitution of Mg2+ by Li+). In saponite, the eVect of charge on Si BE appears rather obvious. In the case of hectorite, it must be pointed out that the Si tetrahedra sheet is linked to the MgO6 sheet through an out of plane oxygen atom. For this reason, SiO4 sheet (and layer) charge must be sensitive to substitutions in the octahedral layer. It then appears that, for both minerals, substitutions of Si4+ or Mg2+ by a less charged cation induce an increase in the electronegativity of SiO4 which provokes the decrease of Si BE observed experimentally. As a consequence Si peak 1 can be assigned to SiO4 tetrahedra that see no substitution (3 O are linked to Si in the tetrahedral sheet
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(a) 104
Si BE (eV)
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100 Si peak 1 Si peak 2 99 0
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(c) 104 Si peak 1 Si peak 2 Si peak 3
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99 0
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Fig. 4 Si2p3/2 BE value in studied particles of (a) hectorite, (b) saponite 0.4 and (c) saponite 0.65
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and 1 O is linked to Mg atoms from the octahedral sheet), Si peak 2 to SiO4 tetrahedra that see one substitution (1 O is linked to a Li atom from the octahedral sheet in hectorite or 1 O is linked to a Al in the tetrahedral sheet in saponites) and Si peak 3 in saponite 0.65 to SiO4 tetrahedra that see two substitutions (2 O are linked to Al in the tetrahedral sheet). According to the proposed assignment, the relative area of Si peak 1, which is directly linked to the related Si species relative concentration, should be directly correlated to the layer charge. Considering an ordered distribution of Al and Li, the statistical ratio of Si atoms that do not see any substitution should be 60% in hectorite, 67% in saponite 0.4 and 42% in saponite 0.65. In the case of Al, the assumption of an ordered distribution is linked to Loewenstein’s rule (Dove et al. 1996; Palin et al. 2001), which states that Al–O-Al linkages are avoided in tetrahedral sheets when Al content is low enough. High resolution gas adsorption experiments on a series of synthetic saponites (Michot and Villieras 2002) supported such an ordered distribution. In the case of Li, 19F NMR experiments on synthetic Xuorohectorites (Butruille et al. 1993) also revealed an ordered distribution of Li atoms in the octahedral layer. Figure 5 presents the relative areas experimentally obtained from the decomposition of the Si XPS spectra for all particles. Median values for Si peak 1 relative area are 56 § 9% in hectorite, 60 § 9%, in saponite 0.4 and 41 § 11% in saponite 0.65 in close agreement with the theoretical values, which conWrms the proposed assignment for the various Si XPS signals. Such an assignment explains the shift for each clay family but does not account for the diVerences in Si BE between hectorite and saponite. Such a trend could be explained by the position of exchangeable cations that compensate the layer charge, in the interlayer region. In hectorite, for which layer charge is octahedral, exchangeable cations could come closer to the surface, at the top of, or even inside, the ditrigonal cavities formed by the arrangement of SiO4 tetrahedra of the basal faces, whereas in saponite the distance between Si and the dehydrated sodium cation could be slightly larger. The position of exchangeable cations would then have a global impact on SiO4 electronegativity independently of the fact that Si atoms “see” a substitution or not. Indirect support for this hypothesis is provided by the values of the d-spacing (distance between two basal faces of clay) for these two clay minerals under vacuum. Indeed, d-spacing is equal to 9.7 Å for hectorite (Prost 1975) and 10.1Å for saponites whatever their charge value (Michot et al. 2005) thus suggesting that exchangeable Na+ cations are slightly more buried in the ditrigonal cavities of hectorite. In addition, He et al. (2007) have demonstrated that replacement of part of the exchangeable Na+
Phys Chem Minerals (2009) 36:593–602
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75.2
Al BE (eV)
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Number of particles
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Fig. 6 Al2p3/2 BE value in studied particles of (a) saponite 0.4 and (b) saponite 0.65
35 Saponite 0.65 30
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Area Si peak 1 / Total peak area (%)
(c)
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Area Si peak 1 / Total peak area (%)
Fig. 5 Relative area of Si peak 1 (2p1/2 and 2p3/2) for hectorite, saponite 0.4 and saponite 0.65
cations by a surfactant in the interlayer of montmorillonites results in Si BE shifting from 103 to 102 eV accompanied by an increase in basal spacing from 1.24 to 1.78 Å (He et al. 2007). This also suggests that Si BE decreases when compensating charges are located further away from the surface in the interlayer space. According to the proposed assignments, Al BE behavior is rather logical. Indeed, in saponite 0.4 and saponite 0.65, Al BE median values are constant, at 74.8 § 0.1 and 74.9 § 0.2 eV, respectively (Fig. 6). These values are also in agreement with data on phyllosilicates reported in the literature (Ebina et al. 1997, 1999; Gonzalez-Elipe et al. 1988). Fits were performed using only one 2p1/2 2p3/2 doublet which follows the Loewenstein rule (Dove et al. 1996; Palin et al. 2001) avoiding Al–O–Al linkages in tetrahedral sheets. Thus Al atoms are always surrounded by 3 Si and 3 Mg in saponites and are not sensitive to any local variation of layer electronegativity.
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Mg BE (eV)
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At Wrst sight, the case of Mg is a bit puzzling, since in contrast with the behavior of Si, Mg peak positions are similar for hectorite and the two saponites and can be satisfactorily Wtted using a single doublet. The positions thus obtained range from 51.1 to 51.9 eV for all samples with median value at 51.5 § 0.2 eV for hectorite 51.4 § 0.2 eV in saponite 0.4 and 51.5 § 0.2 eV in saponite 0.65 (Fig. 7), whereas one would expect diVerences between hectorite and saponite considering that in hectorite the octahedral Mg sheet is partially substituted by Li atoms. Despite the scarcity of literature data reporting Mg 2p BE in phyllosilicates, the most probable explanation for this lack of variation can be related to the small amplitude of the variations, in agreement with Ebina et al. (1997) results. Taking into account the atomic radii (117 pm for Si and 160 pm for Mg), the diVerences in BE behavior with Si can be explained by the electrostatic potential model classically used to predict XPS displacements and that predicts an increase in XPS shifts for decreasing ionic radius (Duc 1998).
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Fig. 7 Mg2p3/2 BE value in studied particles of (a) saponite 0.4 and (b) saponite 0.65
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We have shown in this work that X-PEEM could be advantageously used to image dispersed swelling clay layers and to obtain at the same time spatially resolved chemical information on the major elements of single clay layers by XPS. Using such a strategy, it was possible to gain information on cation distribution in the layers while analyzing the potential structural heterogeneity of these materials. In this Wrst study, by using synthetic samples for which cation distribution is assumed to be ordered, we could evidence the inXuence of various parameters (charge location, charge amount, exchangeable cation position) on the structural features for Magnesium, Silicon, and Aluminum. Silicon spectroscopy is particularly sensitive to layer chemistry as diVerent populations of Si atoms correlated with substitutions in the layer could be evidenced. We also could show that the variability in such synthetic materials remains limited, which provides sound spectroscopic Wngerprint that can be used for the analysis of naturally swelling clay minerals. In that case, as will be shown in a forthcoming publication, XPEEM is really a crucial tool as it allows unraveling the natural heterogeneity of such materials by analyzing single clay layers. Acknowledgments This work has been supported by the European Community Research Infrastructure Action under the RII3-CT-2004506008 contract. We thank G. Cauchon and S. Brochet for the sample holders preparation. We acknowledge the staV of the nanospectroscopy beamline, A. Locatelli, T. O. Mentes, L. Aballe and M. A. Nino Orti for the experimental assistance during the synchrotron-based investigation.
Phys Chem Minerals (2009) 36:593–602
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