Langmuir 2001, 17, 1399-1405
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Speciation and Crystal Chemistry of Fe(III) Chloride Hydrolyzed in the Presence of SiO4 Ligands. 2. Characterization of Si-Fe Aggregates by FTIR and 29Si Solid-State NMR Emmanuel Doelsch,*,† William E. E. Stone,‡ Sabine Petit,§ Armand Masion,† Je´roˆme Rose,† Jean-Yves Bottero,† and Daniel Nahon† CEREGE, UMR 6635 CNRS, Universite´ Aix-Marseille III, Europole Me´ diterrane´ en de l’Arbois, BP 80, 13545 Aix-en-Provence Cedex 04, France; Section de Physico-Chimie Mine´ rale, MRAC, ULB, Campus de la Plaine CPI 232, Boulevard du Triomphe, 1050 Bruxelles, Belgium; and Laboratoire HydrASA, UMR 6532 CNRS, Universite´ de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France. Received September 13, 2000. In Final Form: December 19, 2000 This work aims at a better understanding of the interactions existing between Fe and Si in freshly precipitated Fe(III)/Si phases (prepared at pH 3, 5, 7, and 10 with Si/Fe molar ratios in the range 0.25-4). By coupling the results of two spectroscopic techniques, FTIR and 29Si NMR, interesting structural information emerges. We show that Si and Fe atoms do not form separate silica and FeOOH particles and that the presence of Si-O-Fe bonds hinders the formation of Fe oxyhydroxides. The ratio Si/Fe ) 1 constitutes a transition point between Si-O-Fe and Si-O-Si bond formation at pH ) 3 and 5, where Si-clusters appear once the maximum amount of Si-O-Fe bonds are formed. This is confirmed by 29Si NMR which demonstrates the presence of Si pockets in three of the eight examined samples. The return to equilibrium of the 29Si magnetization leads to a value for both dimensionality of the silica-rich pockets (D ) 2.2) and length over which dimensionality is observed (2 nm). By using both FTIR and 29Si solid-state NMR, we clearly demonstrate how the pH of synthesis determines the structural properties of the formed samples. The results obtained are in good agreement with our previous study conducted by EXAFS.
Introduction The effect of silicate ions on the formation and transformation of ferric oxyhydroxides has been the subject of numerous studies. A wide range of applications are indeed concerned with such interactions: magnetic and catalytic applications,1,2 corrosion and resistance of alloys,3,4 synthesis of clay minerals,5-7 coagulation in water treatment,8 soil sciences.9,10 Fe oxides or oxyhydroxides can be formed by a heating or aging process, and it has been shown that the presence of Si affects the shape, nature, and size of the formed products.11-26 However, very little is known * Telephone: (+33) 442 97 15 43. Fax: (+33) 442 97 15 59. E-mail:
[email protected]. † Universite ´ Aix-Marseille III. ‡ Section de Physico-Chimie Mine ´ rale. § Universite ´ de Poitiers. (1) Bruni, S.; Cariati, F.; Casu, M.; Lai, A.; Musinu, A.; Piccaluga, G.; Solinas, S. Nanostruct. Mater. 1999, 11, 573-586. (2) Corrias, A.; Ennas, G.; Mountjoy, G.; Paschina, G. Phys. Chem. Chem. Phys. 2000, 2, 1045-1050. (3) Calderon, J. P.; D’Granda, S.; Martinez, L. Mater. Perform. 1995, 34, 32-37. (4) Calderon, J. P.; Brito-Figueroa, E.; Gonzalez-Rodriguez, J. G. Mater. Lett. 1999, 38, 45-53. (5) Harder, H. Chem. Geol. 1976, 18, 169-180. (6) Decarreau, A.; Bonin, D. Clays Clay Miner. 1986, 21, 861-877. (7) Decarreau, A.; Bonin, D.; Badaut-Trauth, D.; Couty, R.; Kaiser, P. Clays Clay Miner. 1987, 22, 207-223. (8) Gregory, J.; Duan, J. Water Sci. Technol. 1998, 38, 113-120. (9) Iron in Soils and Clays Minerals; Stucki, J. W., Goodman, B. A., Schwertmann, U., Eds.; NATO ASI Series; D. Reitel Publishing Co.: Dordrecht, 1985; p 893. (10) Cornell, R. M.; Schwertmann, U. The Iron Oxides-Structure, Properties, Reactions, Occurrence and Uses; VCH: New York, 1996. (11) Schwertmann, U.; Thalmann, H. Clay Miner. 1976, 11, 189199. (12) Zahurul, K. Clays Clay Miner. 1984, 32, 181-184. (13) Anderson, M. A.; Palm-Gennen, M. H.; Renard, P. N.; Defosse, C.; Rouxhet, P. G. J. Colloid Interface Sci. 1984, 102, 328-336.
about the initially formed amorphous phases and the physical parameters associated with the transitions toward the homogeneous phases. The techniques usually employed in such studies often relate solely to the crystalline state (such as X-ray diffraction) or to larger scale characterization (such as microscopy). In a previous study, we used Fe K-edge EXAFS27 to study the local structure of freshly prepared precipitates at various Si/ Fe molar ratios (from 0 to 4) and pH values (pH ) 3, 5, 7, and 10). We demonstrated that the growth regime of Fe(III) particles depends on the amount of Si present in the system. At Si/Fe < 1, the growth of Fe species is shown to occur through edge and corner sharing bonds, whereas for Si/Fe > 1 growth occurred mainly by edge sharing. Si/Fe ) 1 ratios are characterized by the formation of (14) Anderson, P. R.; Benjamin, M. M. Environ. Sci. Technol. 1985, 19, 1048-1053. (15) Cornell, R. M.; Giovanolli, R.; Schindler, P. W. Clays Clay Miner. 1987, 35, 21-28. (16) Cornell, R. M.; Giovanoli, R. J. Chem. Soc. 1987, 413-414. (17) Vempati, R. K.; Loeppert, R. H. Clays Clay Miner. 1989, 37, 273-279. (18) Parfitt, R. L.; Gaast, S. J. V. d.; Childs, C. W. Clays Clay Miner. 1992, 40, 675-684. (19) Kandori, K.; Uchida, S.; Kataoka, S.; Ishikawa, T. J. Mater. Sci. 1992, 27, 719-728. (20) Mayer, T. D.; Jarrel, W. M. Water Res. 1996, 30, 1208-1214. (21) Hansen, H. C. B.; Raben-Lange, B.; Raulund-Rasmussen, K.; Borggaard, O. K. Soil Sci. 1994, 158, 40-46. (22) Hansen, H. C. B.; Wetche, T. P.; Raulund-Rasmusen, K.; Borggaard, O. K. Clay Miner. 1994, 29, 341-350. (23) Deng, Y. Water Res. 1997, 31, 1347-1354. (24) Glasauer, S.; Friedl, J.; Schwertmann, U. J. Colloid Interface Sci. 1999, 216, 106-115. (25) Swelund, P. J.; Webster, J. G. Water Res. 1999, 33, 3413-3422. (26) Glasauer, S. M.; Hug, P.; Weidler, P. G.; Gehring, A. U. Clays Clay Miner. 2000, 48, 51-56. (27) Doelsch, E.; Rose, J.; Masion, A.; Bottero, J.-Y.; Nahon, D.; Bertsch, P. M. Langmuir 2000, 16, 4726-4731.
10.1021/la0013188 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/26/2001
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Table 1. Samples Studied by FTIR and
29Si
NMR
pH 3
5
FTIR NMR
x
Si/Fe ) 0 x x
x
FTIR 29Si NMR
x
Si/Fe ) 0.5 x x
FTIR 29Si NMR
x x
Si/Fe ) 0.2 x x x x
29Si
7
10
3
5
7
10
x
Si/Fe ) 0.25 x x
x
x
x
Si/Fe ) 1 x x
x
x x
x x
Si/Fe ) 4 x x x x
x x
small sized clusters. Thus, at low (Si/Fe ) 0-0.5) and high (Si/Fe ) 2-4) Si/Fe ratios, the growth of Fe colloids is, respectively, three- and two-dimensional. Si/Fe ratios around 1 represent a crossover between these two growth regimes. This work allowed us to detect a change in Fe speciation but did not provide any information about the precise localization of Si atoms in the particles. Indeed, the major problem encountered in the fitting of Fe K-edge EXAFS curves is to detect a light element such as Si atoms in the vicinity of a heavy element such as Fe. To overcome this problem, the present work aims at a better understanding of the interactions existing between Fe and Si in freshly precipitated Fe(III)/Si phases (at various Si/Fe molar ratios and pH values) by coupling the results of two spectroscopic techniques which allow to characterize the local environment of Si: FTIR and 29Si solid-state NMR. Materials and Methods Sample Preparation. The samples investigated in the present work were the same as those previously used for an Fe K-edge EXAFS study.27 Briefly, Si/Fe mixtures (Si/F e )0, 0.25, 0.5, 1, 2, and 4; [Fe] ) 0.2 M) were hydrolyzed using 10 M sodium hydroxide until pH)3, 5, 7, and 10 were reached. Precipitates were centrifuged at 40 000 rpm during 2 h. The centrifugates were freeze-dried, outgassed at 10-3 Torr, and then ground to a fine powder. FTIR. Infrared spectra were recorded in the 400-4000 cm-1 range on a Nicolet 510 Fourier transform infrared spectrometer with a 4 cm-1 spectral resolution. The samples were mixed and pressed with an excess of KBr (1% w/w). To remove adsorbed water, pellets were dried overnight at 110 °C. Spectral line fittings were performed using an IGOR software package. In Table 1 are reported the samples studied by FTIR. 29Si NMR. In silicates, silicon sites are usually denoted28 Q n where n (ranging from 0 to 4) stands for the number of directly linked silicon tetrahedra. By solid-state 29Si NMR, Qn sites can be distinguished by their values of chemical shift, provided the corresponding lines are sufficiently sharp. For amorphous samples containing a substantial amount of paramagnetic Fe ions, the width of each 29Si line is such that this is no longer possible. As described elsewhere,29 information concerning the spatial distribution of 29Si nuclei can however be extracted by examining how (after saturation) the nuclear magnetization relaxes back to thermal equilibrium. Such an approach is used here. 29Si MAS NMR were run using a Bruker MSL 300 spectrometer operating at 59.63 MHz for 29Si. Spectra were referenced with respect to TMS (tetramethylsilane). High-speed 5 mm spinning MAS rotors were used up to 15 kHz. The length of the 90° pulses was 4.5 µs. Baseline distortions of the FIDs were removed after correction for deadtime effects. The pulse sequence used to study the recovery of spin magnetization consists of a saturation comb of twenty 90° pulses separated by 400 µs (28) Engelhardt, G., Michel, D., Eds. High-Resolution Solid-State NMR of Silicates and Zeolites; Wiley: Chichester, U.K., 1985; p 122134. (29) Devreux, F.; Boilot, J. P.; Chaput, F.; Sapoval, B. Phys. Rev. Lett. 1990, 65, 614-617.
Figure 1. Infrared spectra of Si/Fe ) 0 series. Table 2. Assignments of Infrared Absorption Bandsa
a
band (cm-1)
attributions
3580 3470 3360 1080 970 930 840 800 680 460 410
Si-OH Fe-OH and H2O (νOH stretch) Si-OH (νOH stretch) Si-O-Si (asymmetric stretch) Si-O (stretch) Si-O-Fe Fe-O (asymmetric stretch) T-O-T (symmetric stretch) Fe-O (asymmetric stretch) O-Si-O (bend) Fe-O (symmetric stretch)
See references 10, 30, 31, and 36.
followed at variable times by an observation pulse. Because the Fe content is high, the return to equilibrium of the 29Si magnetization is very fast; so short recycle delays (0.2 s) and large numbers of accumulations could be used (5000-30 000 for the shortest delays). In Table 1 are reported the samples studied by NMR.
Results Infrared Spectroscopy. Si/Fe ) 0 Series. In Figure 1 are plotted the infrared spectra of samples (pH ) 3, 5, 7, and 10) synthesized without Si. For Si/Fe ) 0 at pH ) 3, absorption bands (410, 680, 840, 3340, and shoulder at 3470 cm-1; see Table 2 for assignments) are characteristic of the iron oxyhydroxide akaganeite (β-FeOOH).10,30 This confirms our previous XRD findings showing moderate amounts of poorly crystallized akaganeite which tend to decrease with increasing pH or Si/Fe.27 The FTIR spectra of the Si/Fe ) 0 samples at pH ) 5, 7, and 10 (Figure 1) show broader lines (450, 580, a shoulder at 700, 840, and 3350 cm-1). A comparison of these spectra with those of known iron oxyhydroxides given in the literature (e.g., goethite, akaganeite, ferrihydrite, and so on)10,30 did not allow unequivocal characterization of samples. Si/Fe ) 1 at pH ) 3 Sample. In Figure 2 are plotted the different IR patterns for the Si/Fe ) 1 series. An analysis of the Si/Fe ) 1 at pH ) 3 spectrum allows one to characterize the totality of the absorption bands present in all the samples. Three main areas can be distinguished. First, the 3200-3800 cm-1 region which corresponds to the νOH stretching mode of the Si-OH and Fe-OH groups and the H2O molecules. Second, the 800-1400 cm-1 region where the asymmetric Si-O-Si stretching vibrations and Si-O stretching vibrations of Si-OH groups are found. Finally, the 400-700 cm-1 region corresponding to asymmetric Fe-O stretching vibrations and O-Si-O bending vibrations (the assignments are given in Table 2). Thus, this spectrum displays bands due to the presence of both akaganeite (β-FeOOH) and silica. (30) Infrared Spectra of Minerals and Related Inorganic Compounds; Gadsden, J., Ed, Butterworth & Co.: London, 1975.
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Table 3. Peaks Used for FTIR Line Fittings of the Spectral Region 800-1300 cm-1 pH ) 3 series Si/Fe ) 0.25; 0.5 Si/Fe ) 1 Si/Fe ) 2; 4 pH ) 5 series Si/Fe ) 0.25-2 Si/Fe ) 4 pH ) 7 and 10 series Si/Fe ) 0.25-4
800 x x 800 x
858 x x 870 x
896
x 896 x
930 x x 930 x
970 x x 970 x
1030 x
1020 x
1080 x x 1080 x
870
930
1010
1080
x
x
x
x
1110
1180
x
x x x
1140
1180
x
x
Figure 3. Evolution of the maximum of 800-1300 cm-1 spectral region.
Influence of pH. As observed in Figure 2 for the Si/Fe ) 1 series, when the pH is increased from 3 to 10 a decrease in intensity of the asymmetric Fe-O stretching band (680 cm-1)30 is observed. This decrease is accompanied by a decrease of the shoulder at 3470 cm-1, which corresponds to νOH stretching mode of Fe-OH groups of akaganeite.10,30 Whatever the Si/Fe molar ratio (Si/Fe ) 0, 0.25, 0.5, 1, 2, or 4), an increase in the value of pH (from 3 to 10) always leads to a decrease of the absorption bands of akaganeite (680 and 3740 cm-1). Other important modifications concern the absorption band whose maximum is located at 1080 cm-1 (see Figure 2) for Si/Fe ) 1 at pH ) 3, and shifts to 970 cm-1 for pH ) 10. This absorption region corresponds to Si-O-Si and Si-O vibrations.31 Influence of Si Concentration. Variations in position of the intense absorption band located in the 800-1300 cm-1 region are also observed when the Si concentration is changed. As shown in Figure 3, whatever the pH of synthesis, the maximum of the band shifts from low to
high wavenumbers with increasing Si concentration (Si/ Fe ) 0.25-4). Spectral Region 800-1300 cm-1. As the main variations observed in the samples concern the spectral region 8001300 cm-1, it was line fitted, keeping the number of components to a minimum (Table 3). To carry out the line fitting procedure, a number of peaks at fixed position are needed (varying according to the values of Si/Fe and pH). Peaks having assignments referenced in the literature were given priority: the 800 cm-1 line corresponds to a T-O-T symmetrical stretching, T being a tetrahedrally coordinated Si,31 930 cm-1 is associated to a Si-O-Fe bond,11,17,18,32 970 cm-1 to δSi-OH,31 and 1080 cm-1 to the asymmetric Si-O-Si stretching vibrations.31 The other peaks (e.g. 858, 870, 1110 cm-1) were used to improve the quality of the fits and are not assigned to a given vibration. Considering only well assigned lines, the fittings (see Figure 4 and Table 3) show that the 930 cm-1 peak corresponding to Si-O-Fe bonds is present in all samples except at pH ) 3 for Si/Fe ) 2 and 4 and at pH ) 5 for Si/Fe ) 4. The evolution of the three peaks characteristic of a Si phase (800, 970, and 1080 cm-1) is highly dependent on the pH. At the lowest pH, the peaks are detected for Si/Fe g 1. At pH ) 5, the peaks are present only for the highest Si content. Finally, at the highest pH values, the fittings included only the 1080 cm-1 line as characteristic Si phase peak. 29Si NMR. Samples Si/Fe ) 4 and 2 prepared at pH ) 3, 5, 7, and 10 were examined by 29Si MAS NMR. Of the eight examined samples, only three gave a 29Si NMR signal: Si/Fe ) 4 at pH ) 3 and 5 and Si/Fe ) 2 at pH ) 3. Indeed, given the very large amounts of Fe(III) ions present in these samples, a 29Si NMR signal is not expected if silicon and iron atoms are intimately mixed throughout the aggregate. This is due to the very strong electronnucleus interaction which widens the 29Si signal beyond
(31) The Infrared Spectra of Minerals; Farmer, V. C., Eds.; Mineralogical Society: London, 1974.
(32) Carlson, L.; Schwertmann, U. Geochim. Cosmochim. Acta 1981, 45, 421-429.
Figure 2. Infrared spectra of Si/Fe ) 1 series.
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Figure 6. 29Si MAS NMR of sample Si/Fe ) 4 at pH ) 3 taken at (A) 7 and (B) 15 kHz.
only two rotational sidebands plus the central line are left. The results of the magnetization recovery experiments are given in Figure 5 as log S(t) versus log t where S(t) is the observed magnetization. The data are normalized so to be directly comparable. The observed recovery of magnetization is in all cases strongly nonexponential but as seen from Figure 5 follows a power law S(t) ≈ tR over nearly 2 orders of magnitude in t until a plateau value is reached. Discussion Figure 4. Spectral line fitting of the 800-1300 cm-1 region for pH ) 3 series.
Figure 5. 29Si magnetization recovery plotted as log S(t) vs log t for samples Si/Fe ) 4 at pH ) 3 and 5 and Si/Fe ) 2 at pH ) 3 (intensities in au are normalized and directly comparable).
detection. The observed normalized signal intensities (see Figure 5) of samples Si/Fe ) 4 at pH ) 3 and 5 and Si/Fe ) 2 at pH ) 3 are in the ratio 1:0.3:1. All signals centered around -100 ppm are very large; the lines extend over a range of about 60 kHz for samples prepared at pH ) 3 and around 75 kHz for the one at pH ) 5. Thanks to highspeed MAS and short relaxation times, these small, wide signals are however relatively easy to detect. As shown in Figure 6, at 7 kHz the manifold of rotation sidebands provides an image of the static absorption line; at 15 kHz
FTIR. As previously shown by X-ray diffraction,27 akaganeite (β-FeOOH) is essentially present at pH ) 3 whatever the Si/Fe molar ratio. Its amount decreases with an increase in pH or Si concentration. This result is confirmed by the FTIR fittings showing the decrease (Figure 1) or disappearance (Figure 2) of the absorption bands of akaganeite. As previously underlined, major modifications are observed in the 1080 cm-1 region (referenced in the Si/Fe ) 1 at pH ) 3 sample, Figure 2). This set of vibrations will thereafter be referred to as peak A. Width of Peak A. The peak A corresponds among other things to the asymmetric Si-O-Si stretching vibrations and Si-O stretching vibrations.31 The width of this set of vibrations depends on the pH and the Si content. Indeed, for the same Si/Fe (e.g., 0.25), it expands from 780 to 1250 cm-1 for pH ) 3 (Figure 4) whereas for pH ) 10 it expands from 800 to 1180 cm-1. For the same pH, the width of peak A increases when the Si content increases (e.g., for Si/Fe ) 0.25 at pH ) 3, the width of peak A ranges from 800 to 1250 cm-1, whereas for Si/Fe ) 4 at pH ) 3 it ranges from 750 to 1350 cm-1(Figure 4)). This behavior is close to the one observed for silicate minerals.31 For tectosilicates, the width of the absorption band (800-1200 cm-1) is larger than that for nesosilicates (820-1000 cm-1)31 and has been attributed to variations in the degree of polymerization of Si. In tectosilicates, each SiO4 tetrahedron is a Q4 site (e.g., quartz), whereas in nesosilicates, SiO4 tetrahedra are isolated (Q0 sites) and bound to each other through ionic bonds by interstitial cations (e.g., olivine).33 By analogy, we can assume that different degrees of Si polymerization also exist in the samples depending on pH and Si content (Figures 2-4):
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Figure 7. Evolution of the area of 930 cm-1 peak for pH ) 5 series (diamond), pH ) 7 series (open circles), and pH ) 10 series (triangle).
(i) for the same Si/Fe molar ratio, the degree of Si polymerization decreases when pH increases, which is correlated with the higher solubility of silica at pH > 734 and (ii) for the same pH, the degree of Si polymerization increases when the Si content increases. Shift of the Maximum of Peak A. A complementary explanation of the shift of this region from high to low wavenumbers has been developed by Wada et al. (1975)35 who studied Al-Si gels and by Schwertmann et al. (1976)11 who studied synthetic Si-ferrihydrite. They suggest that Si-O-Al and Si-O-Fe bonds cause the shift of the Si-O stretching vibrations. Subsequently, numerous studies11,14,15,17,18,22,25,32,36 of synthetic and natural samples have interpreted the shift of Si-O stretching bands by the formation of the Si-O-Fe bonds. In the present case, for the pH ) 3 series (Figure 4), the area of the 930 cm-1 peak (which corresponds to Si-O-Fe bonds) is maximum at Si/Fe ) 0.5 and close to 0 for Si/Fe > 1. This could however be due to the fact that in the fitting procedure the SiO-Fe signal is masked by the presence of a large amount of silica. This large amount of silica is revealed by the 800 cm-1 peak (attributed to symmetrical stretching Si-OSi between two neighboring tetrahedra) which is present and increases from Si/Fe ) 1. This can be interpreted as reflecting an increase in the polymerization of the Si species, which is in good agreement with the evolution of the Si polymerization described previously. To estimate the importance of Si-O-Fe bonds in the samples, the area of the 930 cm-1 absorption band is plotted versus Si/Fe in Figure 7 for pH ) 5, 7, and 10. For low pH (3 and 5), the area of the 930 cm-1 absorption band increases for Si/Fe < 1 and tends toward 0 at higher Si concentrations. However, as mentioned above, this may be due to high amounts of silica masking the Si-O-Fe signal. For the pH ) 7 and 10 samples, the area increases up to Si/Fe ) 1 and then remains constant. Thus, for pH ) 5, 7, and 10, the number of the Si-O-Fe bonds reaches a maximum at Si/Fe ) 1. The dramatic change observed at Si/Fe ) 1 constitutes a crossover point in the transition between Si-O-Fe and Si-O-Si bonds formation. We showed previously,27 by Fe K-edge EXAFS that at all pH values the Fe polymerization indeed reaches a minimum at Si/Fe ) 1. It therefore seems reasonable to argue that (33) Klein, C.; Hurlbut, C. S. J. Manuel of Mineralogy; 21st ed.; John Wiley & Sons: 1993. (34) Iler, R. K. The Chemistry of Silica. Solubility, Polymerisation, Colloid and Surface Properties and Biochemistry, A Wiley-Interscience Publication: John Wiley & Sons: New York, 1979. (35) Wada, K.; Kubo, H. J. Soil. Sci. 1975, 26, 100.
Figure 8. (A) Evolution of the area of 930 cm-1 peak (diamond) and 1080 cm-1 peak for pH ) 10 series (open circles). (B) Partial infrared spectra for Si/Fe ) 0 and 4 at pH ) 10.
the restriction of Fe-O-Fe bond formation is mainly due to the formation of Si-O-Fe bonds. Evolution of the Si-O-Si and Si-O-Fe Absorption Bands. A good fitting of all spectra is obtained by introducing an Si-O-Si absorption band located for most samples at 1080 cm-1 except for Si/Fe ) 0.25 and 0.5 at pH ) 3 and Si/Fe ) 0.25-2 at pH ) 5 where the peak is at a lower wavenumber (see Table 3). As an example, Figure 8a shows how the area of this peak varies as a function of Si/Fe for the pH ) 10 series. The increase up to Si/Fe ) 1 of the 1080 cm-1 peak area is correlated with an increase of the 930 cm-1 peak (Figure 8(i)). Therefore, Si-O-Fe and Si-O-Si bonds are formed simultaneously and Si tetrahedra are never isolated in an Fe matrix, even at the lowest Si/Fe ratios. Si/Fe ) 1 is a crossover point between two behaviors characterized by a sharp increase of the area of 1080 cm-1 peak from Si/Fe ) 0.25 to 1 which levels out at higher Si/Fe ratios (Figure 8a). When Si/Fe > 1, a shoulder at 3580 cm-1 characteristic of a silanol stretching absorption band appears on the spectrum (Figure 8b).37 This means that for samples having a high Si concentration (Si/Fe ) 2 and 4), once a certain maximum amount of Si-O-Fe bonds is formed any excess of Si leads to Si-clusters having silanol groups. At low pH and high Si concentration (Si/ Fe ) 2 at pH ) 3 and Si/Fe ) 4 at pH ) 3 and 5), where a peak at 800 cm-1 clearly shows the presence of highly (36) Manceau, A.; Ildefonse, P.; Hazemann, J. L.; Flank, A. M.; Gallup, D. Clays Clay Miner. 1995, 43, 304-317. (37) Burneau, A.; Barre`s, O.; Gallas, J. P.; Lavalley, J. C. Langmuir 1990, 6, 1364-1372.
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polymerized Si, the characteristic line corresponding to Si-O-Fe bonds at 930 cm-1 could not be detected. This peak is however present for the same Si/Fe ratios at pH ) 7 and 10 where Si is less polymerized. However, because of the low resolution of FTIR, it is very difficult by this technique to extract the fine details of how the two elements finally associate when Si/Fe and pH are varied. 29Si NMR which is very sensitive to the presence of paramagnetic atoms and to the way of which both elements are distributed, can provide some additional structural information. 29Si NMR. A NMR signal is observed for only three of the examined samples: at pH ) 3 and 5 for samples having Si/Fe ) 4 and at pH ) 3 for Si/Fe ) 2. The intensities of the Si/Fe ) 4 signals (Figure 5) are observed to decrease (while the line-widths increase) with increasing pH. The detection of these signals can only be interpreted as resulting from a spatial separation at low pH of certain of the Si and Fe atoms leading to pockets of a silica-type phase relatively free of iron. The number or dimensions of these pockets decreases as the pH increases. As seen in Figure 5, for the two samples Si/Fe ) 4 and 2 prepared at pH ) 3 the amount of observable silicon is the same. At low values of pH, an increase in the amount of Fe does not seem at first sight to alter drastically the Fe/Si distribution in the prepared solid. However, it is interesting to note that for the Si/Fe ) 2 at pH ) 3 sample, a second much wider NMR signal than the one treated above seems to be present (it is much too weak and noisy to be analyzed) pointing to the possible presence of a family of Fe richer silica-like pockets. In silica-related compounds, structural ordering at intermediate ranges (i.e., beyond the SiO4 tetrahedra) can be present depending on how the SiO4 polyhedra interconnect; this may lead to a reduced network dimensionality. In our case, dimensionality can be linked to nuclear magnetic relaxation caused by the presence in the solid of paramagnetic ions. Basically, the relations used to establish this link are the following:29,38 (i) in the absence of spin-diffusion, dipolar coupling between a nucleus located at a distance r from a fixed paramagnetic ion leads to a transition probability P for the nucleus equal to
P ) A/r6
(1)
with A ) (2/5)(γnγep)2I(I + 1)t/(1 + ωI2τ2) where γn and γe are the nuclear and electronic gyromagnetic ratios, ωn is the nuclear Larmor frequency, I is the electron spin (I ) 5/ for Fe(III)), p is the Planck constant divided by 2π, and 2 τ is the correlation time of the z component of the free I electron spin. Spin diffusion is certainly negligible in our case as we are dealing with a nonabundant type nucleus (29Si 4.7%) examined under magic angle spinning. (ii) p(r, t) defines a local nuclear magnetization density which in a nonequilibrium situation varies with both time t and position r; in absence of spin diffusion, the solution of the rate equation of p is
p ) p0{1 - exp(-Ct
∑n (r - rn)-6}
where rn is the position of the paramagnetic ion and p0 is the thermal equilibrium value of p. The total magnetization S(t) measured at time t following saturation is given by the integral of p taken over the entire examined volume. Because nuclei located very close to a paramagnetic ion go undetected, a cutoff (38) Blumberg, W. E. Phys. Rev. 1960, 79-84.
radius b0 around each paramagnetic center is introduced so to exclude all nuclei having their resonance frequency shifted away from the range of observation. The value of S(t) is highly dependent on the exact details of the paramagnetic distribution in the solid (regularly or randomly spaced, clustered). However, for short times after saturation, a nucleus will be primarily influenced by its interaction with the nearest paramagnetic ion. In this case, an approximate solution for p is
p(r, t) ) p0{1 - exp(-Ctr-6)} where r is now the distance from the nearest paramagnetic ion. For each value of t, p then approximately defines a sphere (of radius r) in which all nuclei have recovered their thermal equilibrium value. A characteristic timedistance relation can then be written as
r ) (Ct)1/6
(2)
whatever the case, the experimental recovered magnetization S(t) is proportional to the relaxed thermal equilibrium magnetization So, which depends on the number or mass M of spins present in the sample:
S(t) ≈ So ≈ M ≈ rD
(3)
where the last part of eq 3 is the mass distribution law with D expressing the dimensionality of the spin system; D provides us with an information of how the observed 29Si spins are distributed with respect to the paramagnetic centers (for a homogeneous three-dimensional solid, D would be equal to 3 whatever the examined length scale). With the time to distance relation given by eq 2, then
S(t) ≈ (Ct)D/6 ≈ tR
(4)
Experimentally, the exponent R ) D/6 can be obtained from the slope of the nuclear magnetization recovery curve plotted as log S(t) versus log t. As shown in Figure 5, the slopes of all curves are (given the experimental dispersion of points) quite similar. For all curves, points below 2 ms appear to diverge from the general linear trend probably as a result of field distortions encountered by those nuclei very close to a paramagnetic center. Values for the exponent R were therefore extracted by fitting the curves from t ) 2 ms to half the magnetization value obtained at the saturation plateau at t ) 0.1 s. An average value for R ) 0.375 is obtained giving a value for D ) 2.2. Provided the value of constant C is known, the time to distance relation r ) (Ct)1/6 (eq 2) can be used to transform the time axis of Figure 5 into a distance coordinate corresponding to the size of the relaxed region. The main problem in evaluating C is obtaining a value for τ. EPR spectra obtained for these samples consist of simple Gaussian lines. An estimate for τ can then be deduced from the line widths which are around 550 G giving τ ) 2 × 10-10 s. Constant C is then equal to 6.8 nm6/s. A delay time of 2 ms corresponds to a distance around 0.5 nm which seems to be a good estimate29,39 for b0. The plateau values at 0.1 s which in distance units are approximately 1 nm correspond to the boundary of the relaxed volume (i.e., half the average distance R between Fe(III) ions in a single paramagnetic center model with R ) (3/4πNp)1/3 and Np the number of Fe(III) ions/cm3). The length over which dimensionality is observed is therefore on the order (39) Stone, W. E. E.; Torres-Sanchez, R. M. J. Chem. Soc., Faraday Trans. 1988, I, 117-132.
Speciation and Crystal Chemistry of Fe(III) Chloride
of 2 nm. These values can be compared to what is found29 for Gd3+ doped biphasic Li2Si4O9 glasses (Gd/Si ) 10-3) formed of Q3 and Q4 structural units which yield a dimensionality length of about 4 nm with a value of D ) 2.6. Another study40 was carried out on homogeneously doped aerogels with low paramagnetic ion contents, Cr/Si ) 6 × 10-4 and 5 × 10-3. In that case, D is found to be, respectively, 2.1 and 2.3 with corresponding lengths on the order of 12 and 5 nm. Conclusion The characterization of Si-Fe aggregates by FTIR and solid-state NMR provides interesting structural information. Both sets of data are not only complementary but in good accord with each other; they are also in good agreement with results obtained for the same samples by Fe K-edge EXAFS.27 We show here that Si and Fe atoms do not form separate silica and FeOOH particles and that the presence of Si-O-Fe bonds hinders the formation of Fe oxyhydroxides. The Si/Fe ) 1 ratio always constitutes a crossover point between two behaviors: (a) Si-O-Fe 29Si
(40) Sen, S.; Stebbins, J. F. Phys. Rev. B 1994, 50, 822-830.
Langmuir, Vol. 17, No. 5, 2001 1405
bond formation comes to a halt, and (b) the slower increase of Si-O-Si peak area corresponds to the formation of Si clusters. For samples Si/Fe ) 2 and 4 at pH ) 3 and Si/Fe ) 4 at pH ) 5, no Si-O-Fe bonds are detected by FTIR. Indeed, it is only for these three samples that a NMR signal is observed, confirming the presence of a silicatype phase in areas relatively free of iron. By examining the return to equilibrium of the 29Si magnetization, we were able to calculate the dimensionality of these silicarich pockets (D ) 2.2) together with the length over which this dimensionality is observed (2 nm). By using both FTIR and 29Si solid-state NMR, we clearly demonstrate the influence of the pH of synthesis on the structural properties of the samples. Indeed, the pH value seems a key factor in controlling the proportion of SiO-Fe bonds which affect the speciation of Fe(III) chloride hydrolyzed in the presence of SiO4 ligands. Acknowledgment. This work was partially supported by CNRS-NSF collaboration Agreement Number 7383. LA0013188
