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Preparation and Structural Properties of Tin Oxide-Montmorillonite Nanocomposites J. Ne´meth,† I. De´ka´ny,*,†,‡ K. Su¨vegh,§ T. Marek,| Z. Klencsa´r,| A. Ve´rtes,§ and J. H. Fendler⊥ Department of Colloid Chemistry, University of Szeged, and Nanostructured Materials Research Group of the Hungarian Academy of Science, Aradi V. t. 1., H-6720 Szeged, Hungary; Department of Nuclear Chemistry and Laboratory of Nuclear Methods in Structural Chemistry, Eo¨ tvo¨ s Lora´ nd University, P. O. Box 32, H-1518 Budapest 112, Hungary; and Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5814 Received October 4, 2002. In Final Form: February 14, 2003 Tin oxide intercalated montmorillonite nanocomposites were prepared by an in situ preadsorption method in aqueous solution in the absence of surfactants and organic solvents. The tin oxide particle size was controlled by adjusting the relative supersaturation ratio, S (S ) (Q - L)/L), where Q is the amount of the dissolved material, the Sn(OH)4 precipitate, and L is its solubility). Increasing the relative supersaturation results in a progressive increase of the size of the SnO2 nanoparticles up to a maximum, after which a further increase of S results in a decrease of the size of the nanoparticles, since their growth is inhibited. Calcination of the 2-3 nm diameter SnO2/Sn(OH)4 nanoparticles at 400 °C for 3 h resulted in the formation of an oxide lattice structure from a tin oxide/hydroxide structure. XRD measurements indicated the sizes of tin oxide nanocrystals to be in the 1-2 nm diameter range between the layers of montmorillonite. The specific surface area of montmorillonite significantly changed from 30 to 112.5 m2/g upon SnO2 intercalation, as evidenced by BET. The thermoanalytical investigations revealed the optimal calcination temperature of the nanocomposites (400 °C) and proved the presence of the SnO2 nanoparticles by the heat effect of their crystallization process. The Mo¨ssbauer studies of the samples indicated the particle size dependence on the effective vibrating mass (Meff) and the Debye temperature (ΘM) of the tin oxide nanocrystals. It was confirmed by all of the measurement methods that smaller particle size can be attained by increasing the relative supersaturation ratio of the Sn(OH)4 precipitate.
Introduction The recognized importance of size quantized semiconductor particles has prompted their extensive investigations and the appearance of an ever increasing number of primary publications,1-5 reviews,6,7 and books8,9 in this area. Tin dioxide, a material with versatile applicability in a large number of physicochemical procedures, is one of the most intensively studied semiconductors. As an n-type semiconductor, it is extremely sensitive to reducing gases (CO, H2, ethanol, hydrocarbons) and may therefore * Corresponding author. E-mail:
[email protected]. † University of Szeged. ‡ Nanostructured Materials Research Group of the Hungarian Academy of Science. § Department of Nuclear Chemistry, Eo ¨ tvo¨s Lora´nd University. | Laboratory of Nuclear Methods in Structural Chemistry, Eo ¨ tvo¨s Lora´nd University. ⊥ Clarkson University. (1) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789-3798. (2) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 28332838. (3) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41-53. (4) De´ka´ny, I.; Turi, L.; Tomba´cz, E.; Fendler, J. H. Langmuir 1995, 11, 2285-2292. (5) De´ka´ny, I.; Turi, L.; Galba´cs, G.; Fendler, J. H. J.Colloid Interface Sci. 1999, 213, 584-591. (6) Collins, R. W.; Fauchet, P. M.; Shimizu, I.; Vial, J. C.; Shimada, T.; Alivisatos, A. P. Advances in Microcrystalline and Nanocrystalline Semiconductors; Materials Research Society, Pittsburgh, PA, 1997. (7) Dushkin, C. D.; Saita, S.; Yoshie, K.; Yamaguchi, Y. Adv. Colloid Interface Sci. 2000, 88, 37-78. (8) Fendler, J. H.; De´ka´ny, I. Nanoparticles in Solids and Solutions; NATO ASI Series; Kluwer Acad. Publ.: 1996; High Technology Vol. 18. (9) Pelizetti, E. Fine Particles Science and Technology, From Micro to Nanoparticles; NATO ASI Series; Kluwer Acad. Publ.: 1996.
be made use of as a gas sensor.10,11 When utilized as a sensor, the size of the tin dioxide particles plays an important role, since reduction in size results in enhanced sensitivity.12 It is advantageous in many applications to introduce SnO2 nanocrystals immobilized in a film; therefore, the preparation of layered nanofilms containing tin dioxide has recently gained importance.13,14 The thin films composed of SnO2 nanoparticles obtained by the solgel method can be applied as nanofiltration membranes15 and anticorrosion coatings.16 Due to its photocatalytic properties, it may be put to use in the degradation of organic molecules in an aqueous medium,17 a problem often encountered in environmental protection.18 In addition, it is also widely used as a substrate for the construction of electrodes.19 A prerequisite of all these applications is the preparation of nanocrystalline tin dioxide with large specific surface (10) Yamazoe, N.; Kurokawa, Y.; Seiyama, T. Chem. Lett. 1982, 1899. (11) Nitta, N.; Otani, S.; Haradome, M. J. Electron. Mater. 1980, 9, 727. (12) Ansari, S. G.; Boroojerdian, B.; Sainkar, S. R.; Karekar, R. N.; Aiyer, R. C.; Kulkarni, S. K. Thin Solid Films 1997, 295, 271-276. (13) Xundao, Y.; Lixin, C.; Haibao, W.; Guangfu, Z.; Shiquan, X. Thin Solid Films 1998, 327-329, 33-36. (14) Lixin, C.; Lihua, H.; Guichen, P.; Dongmei, W.; Guangfu, Z.; Shiquan, X. Thin Solid Films 1999, 347, 258-262. (15) Santos, L. R. B.; Larbot, A.; Persin, M.; Santilli, C. V.; Pulcinelli, S. H. J. Sol-Gel Technol. 1998, 13, 806. (16) Rizzato, A. P.; Santilli, C. V.; Pulcinelli, S. H.; Messaddeq, Y. J. Non-Cryst. Solids 1999, 154, 256-257. (17) Vinodgopal, K.; Kamat, P. V. Environ. Sci. Technol. 1995, 841845. (18) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49-68. (19) Lee, J. H.; Parks, S. J.; Hiroto, K. J. Am. Chem. Soc. 1990, 73, 2772.
10.1021/la0266536 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/29/2003
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area and small particle size. One of the methods to achieve this goal is gas-phase condensation.20,21 The majority of the methods presently in use to produce tin dioxide nanoparticles includes the use of reversed micelles (or microemulsions).22,23 Consumption of expensive and environmentally unfriendly organic solvents and surfactants is the disadvantage of using reversed micelles. Therefore, efforts have been made to develop a hydrothermal procedure for the synthesis of tin dioxide nanoparticles24,25 using inexpensive and environmentally friendly stabilizing materials, for example, layer silicates.26-28 Intercalated semiconductors (nanocomposites) containing layered structural elements are gaining increasing importance due to their versatility in terms of host and guest materials, methods of preparation, and utilization.29 In the present work a new synthesis of tin oxide, in situ between the lamellae of montmorillonite, is reported. The preparation employed an aqueous medium and no surfactants. The size of the nanoparticles was controlled by the relative supersaturation ratio of the precursor ions. Experimental Section Materials. Tin(IV) chloride pentahydrate (Riedel deHae¨n) and sodium hydroxide (Reanal) were used as received. Sodium montmorillonite (Wyoming montmorillonite, USA) was purified by sedimentation prior to use, resulting in the removal of particles with a diameter larger than 2 µm. The purified clay mineral was used as a 1% (w/v) aqueous suspension. The basal spacing of the air-dried material was dL ) 1.1 nm, and its cation exchange capacity was found to be 85 mequiv/100 g, determined by the ammonium acetate method.30 Water with a resistance of 18 MΩ‚ cm was obtained by purification in a MilliQ system. Sample Preparation. SnO2-montmorillonite nanocomposites were prepared in the following way: to 3 × 200 mL of a 1% Na-montmorillonite suspension were added 0.7012, 2.1036, and 4.2072 g of SnCl4‚5H2O with constant stirring (magnetic stirrer); stirring was continued for 1 day for ion exchange processes to be completed. 24, 72, and 144 mL of 0.5 M NaOH solution were next added dropwise, ensuring a 50% stoichiometric excess of hydroxyl ions. The mixture was stirred for 5 min and allowed to stand for 1 h. After centrifugation the sediment was washed with absolute ethanol and dried for 2 h at 80 °C. The product was finally calcinated in a Carbolite type tube furnace for 3 h at 400 °C which had been heated to this temperature with a heating rate of 5 °C/min. Instruments. X-ray diffraction measurements were taken on a Philips PW 1820 diffractometer, with Cu KR radiation (λ ) 0.154 nm) being used at 40 kV and 35 mA in the 2θ ranges 1° e 2θ e 15° and 20° e 2θ e 70°. The basal distances dL were calculated from the first Bragg reflections by using the PW 1877 automated powder diffraction software. The accuracy of the dL values was (0.01 nm. The adsorption capacities of N2, specific surface areas, and porosities were determined with a Gemini 2735 (Micromeritics) automated sorptometer at 77.4 ( 0.5 K. Prior to measurements, (20) Jime´nez, V. M.; Gonza´lez-Elipe, A. R.; Espino´s, J. P.; Justo, A.; Fernandez, A. Sens. Actuators, B 1996, 31, 29. (21) Jime´nez, V. M.; Caballero, A.; Fernandez, A.; Ocan˜a, M.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. Solid State Ionics 1999, 116, 117-127. (22) Kim, D.-W.; Oh, S.-G.; Lee, J.-D. Langmuir 1999, 15, 15991603. (23) Song, K.-C.; Kim, J.-H. J. Colloid Interface Sci. 1999, 212, 193196. (24) Nu¨tz, T.; Haase, M. J. Phys. Chem. B 2000, 104, 8430-8437. (25) Richards, R.; Li, W.; Decker, S.; Davidson, C.; Koper, O.; Zaikowski, V.; Volodin, A.; Rieker, T.; Klabunde, K. J. J. Am. Chem. Soc. 2000, 122, 4921-4925. (26) De´ka´ny, I.; Szu¨cs, A.; Mogyoro´si, K.; Kira´ly, Z. Mol. Cryst. Liq. Cryst. 2000, 341, 361-368. (27) De´ka´ny, I.; Turi, L.; Kira´ly, Z. Appl. Clay Sci. 1999, 15, 221239. (28) Kiricsi, I.; Pa´linko´, I.; Tasi, G.; Hannus, I. Mol. Cryst. Liq. Cryst. 1994, 244, 149-154. (29) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399-438. (30) Fraser, A. R.; Russel, J. D. Clay Miner. 1969, 8, 229-230.
Langmuir, Vol. 19, No. 9, 2003 3763 the samples were pretreated in a vacuum at 393 K for 2 h. Stepby-step (cumulative) measurements were made for 4 h in the scan mode, controlled by the Gemini software. The thermal behavior of SnO2-montmorillonite nanocomposites was investigated using a MOM Q-1500 D type derivatograph, which is capable of recording TG, DTG, and DTA curves simultaneously. 100 mg quantities of samples were heated in a ceramic crucible in the temperature range 298-1273 K. The measurements were performed in air, with alumina as reference material. The evaluation of the results was carried out by the computer program of the instrument. The TEM images were taken using a Philips CM-10 electron microscope, using an accelerating voltage of 100 kV. Powder samples were suspended in ethanol (at a concentration of approximately 0.01%), and the aliquots were dropped on 2-mm diameter Formvar-coated copper grids. The particle size distributions were determined by using the UTHSCSA Image Tool program. The 119Sn Mo¨ssbauer measurements were carried out in transmission geometry. γ rays were provided by a 119Sn(CaSnO3) radioactive source with 3 × 108 Bq activity. The samples were measured at three different temperatures: 78, 173, and 290 K. The 119Sn isomer shift values are given relative to the source material (CaSnO3).
Results and Discussion Formation of SnO2 Nanoparticles. SnO2-montmorillonite nanocomposites were prepared by in situ (preadsorption) synthesis. Preadsorption of 2, 6, and 12 mmol of SnCl4 in a 1% suspension of 2 g of clay mineral resulted in the exchange of sodium ions bound to montmorillonite for Sn4+ ions and in the introduction of additional Sn4+ ions between the expanded silicate layers. The formation of tin oxide nanoparticles on the surface of the montmorillonite lamellae, after ion exchange, can be represented by the chemical equations
Sn-montm + NaOH f Sn(OH)x-montm + NaCl 3 h, 400 °C
and Sn(OH)x-montm 98 SnO2-montm SnO2 nanocrystals also can be formed in the interlamellar space of the layered silicate from adsorbed ions present there in excess over the cationic exchange capacity:
SnCl4 + 4NaOH f Sn(OH)4 + 4NaCl 3 h, 400 °C
Sn(OH)4 98 SnO2 Since the exchange capacity of montmorillonite (cec) for Sn4+ ions is 0.21 mmol/g, the excess over the cec value was 0.79, 2.79, and 5.79 mmol/g, respectively. This means that the majority of the SnO2 particles were formed from Sn4+ ions in the interlamellar space of montmorillonite. Intercalated Structure of Nanocomposites. The X-ray diffraction measurements revealed the changes brought about by the incorporation of nanocrystalline tin dioxide into the montmorillonite layers. The XRD patterns of the dry powder of calcinated SnO2-montmorillonite nanocomposites are shown in Figure 1. (Sample notation. (SnO2/M/P/13): SnO2, type of the semiconductor (tin oxide); M, type of the support (montmorillonite); P, preparation method (preadsorption); 13, semiconductor content of the samples (13 wt %)). It can be established that the parallel lamellar structure of the clay mineral is altered by the formation of SnO2 particles, since they are incorporated into the interlamellar space of the layered silicate, resulting in an intercalation peak in the lower angle range beside the original montmorillonite reflection at 2θ ) 9.2° (basal distance, dL) 0.96 nm). Increasing the amount of SnO2 resulted in a
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Figure 2. Structure of montmorillonite intercalated with tin dioxide nanoparticles (dL(1) ) basal distance of SnO2 intercalated montmorillonite; dL(2) ) basal distance of montmorillonite; 0.96 nm is the thickness of the montmorillonite sheet).
Figure 1. XRD patterns of powdered calcinated montmorillonite and SnO2-montmorillonite nanocomposites in the low angle range (1°-15°). When the Sn4+ concentration is increased during the preparation process, the basal distances decrease, indicating the smaller size of the formed SnO2 nanocrystallites. The original peak of montmorillonite gradually disappears as the result of the more complete intercalation at higher concentrations.
to the expected tendency, however, the larger the amount of the reagent SnO2 particles are formed of, the smaller the basal spacing of the montmorillonite became (Table 1). This fact may imply that with increasing concentrations the diameters of the pillaring SnO2 particles formed decrease rather than increase. These results are explicable in terms of von Weimarn’s law, which states that a plot of the particle size in disperse systems as a function of relative supersaturation has a maximum. The relative supersaturation ratio is defined by
Table 1. Basal Spacing Values of SnO2-Montmorillonite Nanocompositesa
S ) (Q - L)/L
SnO2 content sample
wt %
mmol/g
basal distance (nm)
calcinated montm SnO2/M/P/13 SnO2/M/P/31 SnO2/M/P/47
13 31 47
1 3 6
0.96 3.7 3.1 2.4
a The basal distances calculated from the intercalation peaks show that the size of the pillaring SnO2 nanoparticles decreases by increasing the Sn4+ concentration during the preparation process.
decrease in the intensities of Bragg reflections and in an increase in half-width. The reduced intensity of the peak (at 2θ ) 9.2°) corresponding to the basal spacing of the original montmorillonite may be attributed to the circumstance that, as the amount of SnO2 is increased, an increasingly larger proportion of the clay mineral lamellae is pillared by the nanoparticles formed; thus, in the sample containing 47% SnO2 the original montmorillonite structure is practically nonexistent, indicating complete intercalation. Another remarkable phenomenon was the observed change in basal spacing of the intercalated clay mineral with increasing amount of SnO2 (Table 1.). These values fall within the range 2-3, nm which necessarily means that the size of the particles pillaring the lamellae is also in the nanometer range according to eq 1:
dL(1) ) 0.96 + dSnO2
(1)
where dL(1) is the basal distance of montmorillonite intercalated by tin oxide, 0.96 nm is the thickness of a montmorillonite layer, and dSnO2 is the size of the SnO2 nanoparticles (Figure 2). In the figure dL(2) indicates the basal distance of the not pillared original montmorillonite structure. In contrast
(2)
where Q is the amount of the dissolved material (the Sn(OH)4 precipitate) and L is its solubility. Increasing the relative supersaturation results in a progressive increase of the size of the SnO2 nanoparticles up to a maximum, after which a further increase of S results in a decrease of the size of the nanoparticles, since their growth is inhibited. Due to the low solubility of Sn(OH)4 (L ) 10-56 g/100 mL at room temperature), the applied relative supersaturation ratio is very high (S is in the range 1.5 × 1055 to 9 × 1055). This is the reason for the observed small particle size (diameters of only a few nanometers). When the concentration of Sn4+ precursor ions is increased in the course of synthesis, the higher particle number density (the material is rapidly distributed to a large number of nuclei) results in the particles inhibiting each other’s growth and the size of the pillaring SnO2 nanoparticles becoming smaller and smaller in the interlamellar space of the montmorillonite, which manifests itself in a decreased basal distance. This can be the explanation of the setback of the basal distances, increasing the precursor ion concentration during the preparation procedure. By the evidence of XRD measurements, clay mineral lamellae act as a stabilizing agent in the systems studied by effectively inhibiting the aggregation of primary particles. On the basis of the XRD measurements, a “twophase” nanostructured disperse system shown in Figure 2 is likely to develop, where dL(1) ) 2.4-3.7 nm and dL(2) is 0.96 nm in calcinated state. The tin oxide crystallite sizes (dXRD) calculated from the data of the intercalation peaks are listed in Table 2:
dXRD ) dL(1) - 0.96 nm
(3)
The diameters of the SnO2 nanoparticles estimated by XRD (taking advantage of the intercalation reflection and
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Table 2. Diameter of the SnO2 Nanocrystallites Obtained by Different Methods: Calculated from the Intercalation Reflections (dXRD), Calculated from the Broadening of the Diffraction Lines (dScherrer), and Determined by Transmission Electron Microscopy (dTEM) sample
dXRD (nm)
dScherrer (nm)
dTEM (nm)
SnO2/M/P/13 SnO2/M/P/31 SnO2/M/P/47
2.7 2.1 1.4
4.5 3.8 2.0
9.1 7.4 6.6
Figure 3. XRD patterns of the SnO2-montmorillonite nanocomposites in the wide angle range (20°-70°). In the presence of the smaller SnO2 nanocrystals, the half-width of the characteristic SnO2 reflections gradually increases. The crystallite size was calculated from the half-width of the (101) diffraction line by the Scherrer equation.
the Scherrer equation) are markedly smaller than those determined by TEM. Considering the assumptions involved in the treatment of the XRD data, we place a greater reliability on the SnO2 diameters directly determined by TEM. When the Sn4+ concentration is increased, the diameters decrease according to the principle of von Weimarn’s law. Figure 3 shows the XRD patterns of the SnO2 intercalated montmorillonites in the 2θ ) 20°-70° angle range. From the broadening of the (101) tin oxide reflection at 2θ ) 33,98° the crystallite size (dScherrer) of the nanoparticles was calculated (Table 2) according to the Scherrer equation:31
dScherrer ) λ/(B cos θ)
(4)
where λ is the X-ray wavelength, B is the width of the diffraction line at the half-maximum, and θ is the Bragg angle. The macrocrystal SnO2 (Aldrich) was used as the standard material with a half-width 2θ ) 0.233° of the (101) diffraction line. The particle size calculated by the Scherrer method shows the same tendency: the particle size decreases with increasing relative supersaturation of the Sn4+ ion in the preparation process. Particle Size and Morphology. The TEM micrographs clearly show the presence of the SnO2 nanoparticles (Figure 4). The size distribution functions of the samples take place in the 5-15 nm diameter range, but more than 70% of the particles are in the 5-10 nm size range. These data indicate that the nanocrystallite populations have a fair monodispersity, ensured by the stabilization effect of (31) Scherrer, P. Go¨ ttinger Nachrichten 1918, 2, 98.
the montmorillonite sheets. Increasing the semiconductor content, the average diameter of nanocrystals weakly decreases according to the TEM micrographs (Table 2). This observation also confirms that, by applying a higher precursor concentration, smaller crystallite size can be achieved according to the principle of von Weimarn’s law. Another important observation is that the calcination of the samples at 400 °C for 3 h results in the partial aggregation of the nanoparticles even if the stabilization effect of the clay mineral sheets inhibits this process. To avoid this partial aggregation, a shorter calcination time or the use of a costabilizator seems to be desirable. The larger particle size (Table 2) measured by TEM is likely to be the result of distortion from spherical geometry. This distortion is confirmed by comparing the top view of the nanoparticles, as seen by TEM (spherical shapes with diameters larger than those observed by other methods are obtained), with their cross sections, determined by XRD (basal distances). Since the diameters of the nanoparticles obtained by the Scherrer method are between the values of those obtained by TEM and XRD (Table 2), it can be concluded that this method provides average values for the randomly oriented cylindrical nanoparticles. Specific Surface Area and Porosity. Values of specific surface area (aSBET) for the samples prepared were determined from N2 adsorption measurements using the linearized BET equation (Table 3). It is unambiguously demonstrated by the nitrogen adsorption isotherms presented in Figure 5 that the structure and the adsorption characteristics of montmorillonite are significantly altered by the incorporation of SnO2 nanoparticles. The amounts of adsorbed nitrogen measured at identical relative pressures are considerably higher in both calcinated and noncalcinated nanocomposites than in the bare montmorillonite support. In the SnO2 pillared samples, N2 molecules have access to a much larger surface (the nanoparticles’ own surface also contributes to the overall increase in surface area) than is the case in the unmodified support, where unpillared surfaces can adhere to each other unhindered (Table 3). At the same time, calcination reduces the value of the specific surface area to some extent, owing to the coarsening of the system by heat. It is clearly shown by the isotherms and aSBET values, however, that it is not worth increasing the amount of added SnO2 excessively: while the values of the 13% and 31% samples are similarly high, in the case of the 47% sample a significant reduction is observed. The reason for this is that in the latter case the interlamellar space of the clay mineral is filled up to such an extent that the large number of SnO2 particles block the route of N2 gas, forming a kind of stopper in the pores of the material; in addition, stronger aggregation of the particles may also play a role in the decrease of specific surface area. When, according to the de Boer’s t-method,32 the volume of the adsorbed N2 is plotted against the thickness of the adsorbed layer, and a straight line is fitted to the points in the range of t ) 0.35-0.5 nm, the surface area of the micropores (aSmp, m2/g) can be calculated from the slope of the straight line and their volume (VSmp, cm3/g) from the position of the intersection (Figure 6). The data listed in Table 3 allow the conclusion to be drawn that, in calcinated samples, increasing the amount of SnO2 incorporated is concomitant with a marked increase in the proportion of micropore surface within the total specific surface area, reaching a maximum in the 47% sample. This fact indicates that, in this highly (32) de Boer, J. H.; Linsen, B. G.; van der Plas, T.; Zondervan, G. J. J. Catal. 1965, 4, 319.
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Figure 4. TEM images and size distributions of tin oxide nanoparticles on montmorillonite support; the average particle diameters are (a) d ) 9.1 ( 2.8 nm, (b) d ) 7.4 ( 2.6 nm, and (c) d ) 6.6 ( 2.3 nm.
concentrated system, nanoparticles are arranged among the lamellae of montmorillonite in larger or smaller aggregates, reducing the accessibility of the particle surface and thereby the specific surface area but at the same time creating favorable conditions for the formation of micropores.
The pore size distribution functions shown in Figure 7 were calculated from the desorption isotherm, section pr ) 0.99-0.20, applying the BJH method.33 It is clearly demonstrated by Figure 7 that nanocomposites contain a considerably larger number of pores than the unmodified montmorillonite. The pores in the 3-4
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Table 3. Parameters of Calcinated and Non-calcinated SnO2-montmorillonite Nanocomposites Obtained by N2 Gas Adsorption Measurements sample
aSBETa (m2/g)
aSmpb (m2/g)
VSmpc (cm3/g)
calcinated montm SnO2/M/P/13 SnO2/M/P/31 SnO2/M/P/47 Sn(OH)4/M/P/13 Sn(OH)4/M/P/31 Sn(OH)4/M/P/47
30.0 98.5 98.1 39.9 112.5 111.3 65.9
2.95 6.34 7.72 10.75 8.65 21.72 3.80
0.001 25 0.002 46 0.003 00 0.005 22 0.003 74 0.010 29 0.001 41
a
Specific surface area.
b
Micropore area. c Micropore volume.
Figure 6. de Boer curves of calcinated samples and montmorillonite with the fitted straight lines. The higher slope of the fitted lines indicates the higher number of micropores present in the intercalated structure of the SnO2-montmorillonite nanocomposites.
Figure 5. N2 adsorption isotherms of calcinated nanocomposites and montmorillonite at 77 K. After the incorporation of the SnO2 nanoparticles into the montmorillonite, the adsorbed N2 volume significantly increases as the result of the higher specific surface area of the pillared structure than that of the empty montmorillonite.
nm diameter range can be attributed to the original montmorillonite structure while the increasing number of the wider pores (above 5 nm) is the result of the nanoparticle incorporation. Thermal Behavior of the Nanocomposites. During the preparation the tin hydroxide particles are converted to tin dioxide by heat treatment. Calcination at too low a temperature leads to incomplete conversion, whereas too high temperature brings about aggregation of the particles, that is, coarsening of the system. For this reason as well as for considerations of energy saving, it is highly important to determine the optimal calcination temperature. Thermoanalytical studies not only help such determinations but also shed light, through changes in heat and mass, on structural differences between the individual preparations. The TG (thermogravimetric) curves presented in Figure 8 show the changes in the mass of the individual samples as a function of temperature. The decrease in the mass of the samples occurring during the time from the onset of heating to the end of the measurement is due to the evaporation of the water content of the samples. According to the evidence of the TG curves, this is a three-step process. In the first step, up to 100 °C, the water physically bound within the samples is removed. The subsequent (33) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380.
Figure 7. Pore size distribution functions of calcinated samples. The incorporation of the SnO2 nanocrystals has a strong effect on the pore structure of the montmorillonite. The change is the most significant in the case of the pores in the 5-10 nm size range.
steady decrease in mass between 200 and 600 °C is the manifestation of the dehydration process concomitant with the conversion of tin hydroxide to tin dioxide: -2H2O
Sn(OH)4 98 SnO2 The continuous decrease is much more pronounced in the case of the SnO2/M/P/47 sample than in the case of the other two nanocomposites. This phenomenon is the consequence of the higher initial tin hydroxide (and water) content of the sample. Finally, at 600-700 °C the water content bound to the montmorillonite lamellae starts to evaporate, resulting in the structural collapse of the clay mineral.
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Figure 8. Thermogravimetric (TG) curves of the samples. During the first step the physically adsorbed water content is lost, the second step between 200 and 600 °C is the dehydration of tin hydroxide, and finally, at 600 °C the evaporation of the structural water of the montmorillonite lamellae (dehydroxylation) takes place. Table 4. Loss of Mass of SnO2-Montmorillonite Nanocomposites after Heating to 1000 °Ca sample
weight loss (%)
montmorillonite Sn(OH)4/M/P/13 Sn(OH)4/M/P/31 Sn(OH)4/M/P/47
7.9 9.7 10.0 17.8
Figure 9. Differential thermal analysis (DTA) curves of the preparations. Compared to the “empty” montmorillonite, the intercalated samples show the effect of the heat-producing SnO2 crystallization process in the 200-450 °C temperature range. Table 5. Characteristic Properties of Samples Investigated by 119Sn Mo1 ssbauer Spectroscopy sample no. 1 2 3 4
sample
SnO2 content (wt %)
SnO2/M/P/13 SnO2/M/P/31 SnO2/M/P/47 dialyzed Sn(OH)4 sol, r ) 620 nm
13 31 47 100
sample wt (mg) 35 46 37 46
a The increasing loss as the result of the water evaporation indicates the higher Sn(OH)4 content in the nanocomposites.
It is clearly seen in Figure 8 that the final loss of mass increases parallel with tin dioxide content, since a higher tin dioxide content means a higher initial water content in the form of tin hydroxide. The exact weight loss values are presented in Table 4. Each of the DTA curves shown in Figure 9 exhibits a minimum at 100 °C, indicating heat absorption correlated with the evaporation of physisorbed water. As temperature is increased, the curves of the samples containing nanoparticles start to rise in the exothermic direction, accounted for by the heat liberated in the course of the crystallization of the gradually forming SnO2 particles, partially compensating heat absorption due to the continuing evaporation of water. Thus, the DTA curve of the 31% sample is entirely exothermic in the range 200-450 °C. The curves turn endothermic again above 500 °C, indicating the enthalpy requirement of the dehydroxylation processes taking place in montmorillonite. The temperature of the “empty” montmorillonite sample lacking nanoparticles, however, steadily decreases as compared to that of the reference sample over the entire temperature range, indicating that in this sample heatproducing crystallization processes similar to those occurring in the systems containing tin dioxide do not take place. The conclusion to be drawn from the data presented here is that the temperature ideal for calcination is around 400 °C because at this temperature the conversion of Sn(OH)4 to SnO2 is complete, whereas dehydroxylation of the support promoted by temperatures above 600 °C has not started yet.
Figure 10. 119Sn Mo¨ssbauer spectrum of SnO2/M/P/13 taken at T ) 290 K. The Mo¨ssbauer spectra were fitted with a single Lorentzian quadrupole doublet.
Particle Size Effect on Mo1 ssbauer Parameters. The characteristic features of the nanocomposites investigated by 119Sn Mo¨ssbauer spectroscopy are given in Table 5. A characteristic 119Sn Mo¨ssbauer spectrum of the SnO2/M/ P/13 sample taken at T ) 290 K is shown in Figure 10. The Mo¨ssbauer spectra were fitted with a single Lorentzian quadrupole doublet representing an electric quadrupole interaction between the 119Sn nucleus and the neighboring electrical charges. On the basis of the temperature dependence of the 119Sn isomer shift and the relative spectrum area, one can calculate the effective vibrating mass, Μeff, which can be considered as the molar mass of those atoms that
SnO2-Montmorillonite Nanocomposites
Langmuir, Vol. 19, No. 9, 2003 3769
Table 6. Debye-Equivalent Properties Calculated on the Basis of the Temperature Dependence of the Mo1 ssbauer Parameters: Meff [g/mol] (Effective Vibrating Mass) and ΘΜ [Κ] (Debye Temperature) sample
dL(nm)
Meff (g/mol)
ΘM (K)
SnO2/M/P/13 SnO2/M/P/31 SnO2/M/P/47
3.7 3.1 2.4
219 195 163
190 234 260
apparently vibrate together with the Sn atom, and the Debye temperature (ΘΜ), which corresponds to the ideal Debye solid for which the temperature dependence of the Mo¨ssbauer parameters would agree with that obtained for the nanocomposite in question.34 We have found the effective vibrating mass and the Debye temperature to have a correlation with the size of nanoparticles. The calculated Meff and ΘΜ parameters are listed in Table 6 for each of the investigated nanocomposites. The Meff effective vibrating mass increases and the ΘΜ Debye equivalent temperature decreases with increasing size of the pillaring particles. This correlation shows that the crystallite size (indicated by the basal distances of the nanocomposites: dSnO2 ) dL(1) -0.96) has an impact on the vibrational state of 119Sn nuclei in the SnO2 nanoparticles. The smaller Meff effective vibrating mass can be explained by the smaller particle size, because, according to the Weimarn’s law, when increasing the reagent contrentration smaller nanocrystals are formed. To explain the dependence of the ΘΜ Debye temperature on the size of the nanocrystals, the next theory seems to be worth considering. The Debye temperature is constant for a given substance under given circumstances and defines the maximum frequency in the spectrum of particle vibration of a solid:
ΘΜ )
hνmax k
(5)
where h is Planck’s constant, k is the Boltzman constant, and νmax is the maximal frequency of vibration of the solid. As the equation shows, the Debye temperature is directly proportional to the maximal frequency of vibration of the oscillating atom. In our case a 119Sn atom is taking its place in a small nanoparticle. In the case of such a small (34) Herber, R. H. Chemical Mo¨ ssbauer Spectroscopy; Plenum Press: New York, 1984; p 199.
nanoparticle (which sometimes contains only several tens of atoms), we can assume that there is a close relationship between the vibration of the individual atom and that of the complete nanoparticle. Thus, if a sample has a higher Debye temperature value, it means that the 119Sn atom has a higher maximum vibration frequency. Since this vibration is closely attached to the vibration of the complete nanoparticle of small size, we can assume that the nanoparticle also has a higher maximum vibration frequency on its own. This means that the particle has a smaller mass and size (because the maximum vibration frequency is inversely proportional to the mass). In this way, a correlation can be determined between the Debye temperature and nanoparticle size. Conclusion Tin dioxide nanoparticles were prepared on montmorillonite support of a layered structure by means of synthesis in aqueous medium. The prepared materials obtained were studied by several independent methods. Of these, XRD and Mo¨ssbauer studies, and positron annihilation studies unequivocally demonstrated the intercalation of SnO2 nanoparticles in montmorillonite. The above methods and TEM investigations also helped establish that particle size can be conveniently manipulated according to the principles of von Weimarn’s law, becuse only the precursor concentrations were to be changed and the other conditions were the same. It was shown by N2 gas adsorption measurements that the incorporation of SnO2 nanoparticles brings about singificant changes in the specific surface area and porosity of montmorillonite, causing a considerable increase in both parameters. Changes in heat and mass accompanying the heating of the products were monitored by thermoanalysis. It was established that the ideal calcination temperature of the samples is 400 °C. It was also observed that the presence of SnO2 nanoparticles within the samples markedly affects the course of TG and DTA curves; this is the reason the thermoanalytical measurements can be applied to reveal the presence of the SnO2 nanocrystals. Acknowledgment. The authors are very thankful for the financial support of the Hungarian National Scientific Fund (OTKA) T034430 and for the NATO Science for Peace Program SfP 972652. LA0266536
