Preparation and Characterization of Clay Mineral Intercalated

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Preparation and Characterization of Clay Mineral Intercalated Titanium Dioxide Nanoparticles K. Mogyoro´si,† I. De´ka´ny,† 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, and Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5814 Received May 20, 2002. In Final Form: October 2, 2002 Titanium dioxide nanoparticles with a diameter in the 5 nm range were prepared by the sol-gel method, in an acidic environment, under several finely tuned experimental conditions. The titanium dioxide nanoparticles prepared were subjected to hydrothermal treatment and calcination. Using absorption spectrophotometry, X-ray diffraction, dynamic light scattering, and transmission electron microscopy permitted the assessments of the diameters of the nanoparticles and their composition (in terms of percentages of anatase, rutile, and amorphous phases). One of the methods of preparation led to samples that contained 72.4 wt % anatase and 27.6 wt % amorphous phase. Using montmorillonite and hectorite clay mineral platelets and titanium dioxide nanoparticles, nanocomposites were also prepared and calcinated under a variety of experimental conditions. Two major experimental conditions were employed. Method A involved the preadsorption of the titanium alkoxides into/onto the clay mineral platelets and their subsequent hydrolysis and calcination. In method B previously crystallized titanium dioxide nanoparticles and clay mineral platelets were heterocoagulated and calcinated. All the samples were characterized by absorption spectrophotometry (to assess their optical band gaps), by X-ray diffractometry (to determine the basal spacing of the clay mineral platelets in the nanocomposite as well as the diameters of the titanium dioxide nanoparticles), by transmission electron microscopy (to image the structures of the nanocomposites), by thermoanalytical measurements (to observe the structural changes that accompanied calcination), and by BET gas adsorption analysis (to determine the specific surface areas). Calcination was found not to be necessary for the preparation of titanium dioxide-clay mineral nanocomposites that had high specific surface areas and well-crystallized anatase contents and thus could be used as an efficient photocatalyst.

Introduction Titanium dioxide nanoparticles are increasingly employed as photocatalysts.1 The crystalline titanium dioxide has three polymorphs: rutile (chains of TiO6 octahedra sharing two edges), anatase (chains of TiO6 octahedra sharing four edges), and brookite (chains of TiO6 distorted octahedra sharing three edges).2 Of these polymorphs, the rutile modification has been investigated most extensively since it is the phase that is most often formed in crystallizations and colloid chemical preparations. Anatase, however, has a wider optical band gap (3.2 eV vs 3.0 eV for rutile), a smaller electron effective mass, a higher Fermi level, and a higher mobility of charge carriers than the thermodynamically stable rutile phase.3 For these reasons the anatase modification is the preferred form for photocatalysis. Despite the large number of publications, there is a scarcity of well-documented conditions for the preparation of TiO2 nanoparticles that are predominantly in the anatase form.4 Evidently, careful and stringent adjust* Corresponding author. E-mail: [email protected]. † University of Szeged and Nanostructured Materials Research Group of the Hungarian Academy of Science. ‡ Clarkson University. (1) Fox, M. A. Acc. Chem. Res. 1983, 16, 314-321. Yoneyama, H. Crit. Rev. Solid State Mater. Sci. 1993, 18, 69-111. Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49-68. Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269-277. Gratzel, M. Nature 2001, 414, 338-344. (2) Baur, V. W. H. Acta Crystallogr. 1961, 14, 214. (3) Mo, S. D.; Ching, W. Y. Phys. Rev. B: Condens. Matter 1995, 51, 13023-13032. (4) Tsevis, A.; Spanos, N.; Koutsoukos, P. G.; van der Linde, A. J.; Lyklema, J. J. Chem. Soc., Faraday Trans. 1998, 94, 295-300.

ment of the experimental conditions is a must for the selective production of anatase. These experimental conditions involve both the type and concentrations of the precursors, their ratios, order, and rate of additions; the polarity of the solvent, the pH, the temperature, and the absence or presence of small amounts of dopants; and many other and possibly not yet recognized parameters. Exclusive formations of anatase were reported, for example, in the continuous precipitation of TiO2 from a supersaturated solution of TiOSO4 and potassium hydroxide (pH 1.97) in the presence of Li+ or Nb5+ or W6+ dopant ions (in ca. 10 µM concentration) at 25 °C.4 The hydrothermal processing of TiO2 in the presence of HF and HCl led to anatase, whereas in the presence of citric acid and nitric acid rutile was formed.5 Calcination generally resulted in rutile formation and sintering, and also caused a loss of surface area. As part of the work reported here, we developed methodologies for the preparation of crystalline anatase nanoparticles that did not involve hydrothermal synthesis or required calcination. Preparation and characterization of clay mineral intercalated titanium dioxide nanocomposites (i.e., TiO2 pillared clays) for photocatalytic purposes has been the main goal of the work described here. Pillared clay minerals have been extensively used as adsorbents and catalysts.6-11 The reported preparations of TiO2-mont(5) Yin, H. B.; Wada, Y.; Kitamura, T.; Kambe, S.; Murasawa, S.; Mori, H.; Sakata, T.; Yanagida, S. J. Mater. Chem. 2001, 11, 16941703. (6) Pinnavaia, T. J. Advances in Chemistry Series; American Chemical Society: Washington, DC, 1993; No. 245. (7) Rozengart, M. I.; V’yunova, G. M.; Isagulyants, G. V. Russ. Chem. Rev. 1988, 57, 115-128.

10.1021/la025969a CCC: $25.00 © 2003 American Chemical Society Published on Web 03/08/2003

Clay Mineral Intercalated TiO2 Nanoparticles

morillonite samples were limited to the simple mixing of titania and clay mineral dispersions.8-17 We report here details of two approaches for the formation of clay mineral intercalated titanium dioxide nanoparticles. The first approach involved the adsorption of titanium alkoxides onto/into the clay mineral platelets and subsequent hydrolysis thereon/therein. Advantage was taken of heterocoagulation of TiO2 nanoparticles and montmorillonite platelets in the second approach. Selecting carefully the experimental conditions, we were able to produce size quantized anatase nanoparticle-clay mineral composites with high surface areas. The TiO2 nanoparticles and the different TiO2-clay mineral nanocomposites have been characterized by absorption spectrophotometry, dynamic light scattering (DLS), X-ray diffraction (XRD), transmission electron microscopy (TEM), gas adsorption analysis (BET), and thermoanalytical measurements. Experimental Section Materials and Methods. Titanium(IV) ethoxide (tetraethyl orthotitanate, Merck, pro anal.), titanium(IV) isopropoxide (tetraisopropyl orthotitanate, Fluka Chemika, pract.), hydrochloric acid (Reanal, pro anal.), sodium hydroxide (Reanal, puriss), 2-propanol (Reanal, puriss), and nitrogen gas (Linde, 99.95 v/v %) were used as received. Similarly, commercial samples of P25 Degussa TiO2 (25 wt % rutile and 75 wt % anatase), Aldrich anatase (puriss), and Optigel SH (Su¨d-Chemie AG, synthetic hectorite) were used as received. Sodium montmorillonite, M (Wyoming montmorillonite, USA, and Volclay montmorillonite, Su¨d-Chemie AG), was purified by sedimentation. Ten grams of M per liter was suspended in water. The suspension was allowed to settle in a 20 cm high jar for 24 h. The supernatant (containing 100 29.3

rutile,c wt %

rutile diam,d nm

20.4 24.0 27.8 35.4 0 0 0 100 0 25

a

8.8 12.2 15.4 17.0

>100 44.4

abs spectra diam,e nm

amorphous + brookite phase,f wt %

1.4 1.5 2.5 4.0 1.3

15.6 8.2 3.1 4.8 27.6 0 0 0 0 0

b

Determined by the integration of the area of the 101 anatase XRD peak. Determined by the use of the Scherrer equation for the 101 anatase XRD peak. c Determined by the integration of the area of the 110 rutile XRD peak. d Determined by the use of the Scherrer equation for the 110 rutile XRD peak. e Determined from the absorption spectra as described in ref 25. f Determined by subtracting the sums of anatase + rutile phase wt % from the wt % of the total.

Figure 3. X-ray diffraction (XRD) patterns of sol-gel prepared TiO2 samples prior (TiO2/0/S2) and subsequent to calcination at 400 °C for 4 h (TiO2/0/S2C-400/4) as well as prior (TiO2/0/S1) and subsequent to 12 h of heat treatment at 100 °C (TiO2/0/ S1HT-100/12), 125 °C (TiO2/0/S1HT-125/12), and 150 °C (TiO2/ 0/S1HT-150/12). Indicated are the positions for anatase (101) and rutile (110). The insert illustrates the determination of the peak areas and the presence of the trace amount of the brookite (121) phase at 30.8 2Θ°. Calculations of the anatase and rutile contents were made by calibration to the anatase (TiO2/0/AC400/4) and rutile (TiO2/0/AC-800) standards.

9, 161 ( 38, and 13.2 ( 0.2 nm at 1, 2, 4, 6, and 12 h. (See the size distribution of the nanoparticle aggregates of TiO2/ 0/S2 sample in the insert of Figure 2.)32 X-ray diffraction (XRD) plots of the powders, obtained from aqueous TiO2 dispersions prior (TiO2/0/S1) and subsequent to 12 h of heat treatment at 100 °C (TiO2/0/ S1HT-100/12), 125 °C (TiO2/0/S1HT-125/12), and 150 °C (TiO2/0/S1HT-150/12) as well as prior (TiO2/0/S2) and subsequent to calcination at 400 °C for 4 h (TiO2/0/S2C400/4), are illustrated in Figure 3. The TiO2/0/S1 sample is characterized by peaks at 25.3 2Θ° and 27.5 2Θ° which correspond to the anatase (101) and rutile (110) phases of the TiO2 (as indicated by the letters “A” and “R”, respectively, in Figure 3). These assignments are based on literature values33 for TiO2 and on comparisons to authentic pure anatase (TiO2/0/AC-400/4) and rutile (TiO2/ (32) Pelizzetti, E.; Minero, C.; Borgarello, E.; Tinucci, L.; Serpone, N. Langmuir 1993, 9, 2995-3001. (33) Brindley, G. W.; Brown, G. Crystal Structures of clay minerals and their X-ray identification; Mineralogical Society Monograph 5: London, 1980; p 400.

0/AC-800) samples. The XRD data for these standards are collected in Table 2. A small amount of brookite phase was also observed in the XRD patterns of the TiO2/0/S1HT samples (area ratio of the 101 anatase to 121 brookite reflection ) 100:1, see insert in Figure 3). The preparation conditions profoundly influenced the structure of the TiO2 formed in the sol-gel preparation. Changing the alkoxide:propanol:water:HCl molar ratio from 1:3:250:0.35 (as in the TiO2/0/S1 sample) to 1:70: 2500:3.5 (as in the TiO2/0/S2 sample) leads to the disappearance of the rutile phase. In the TiO2/0/S2 sample only the anatase phase could be found (Figure 3 and Table 1). The TiO2/0/S2 sol was neutralized by 1.0 M aqueous NaOH solution, washed with deionized Milli-Q water, centrifuged (three times), and calcinated at 400 °C to yield the TiO2/0/S2C-400/4 sample in which the rutile phase is also entirely absent (Figure 3). Increasing the temperature of the hydrothermal heat treatment (i.e., from room temperature to 100 and 125 °C for samples TiO2/0/S1, TiO2/0/S1HT-100/12, and TiO2/0/ S1HT-125/12, respectively) increased the anatase and rutile content of TiO2 at the expense of the amorphous phase (see Table 1). Heat treatment also provided a means for controlling the growth of the TiO2 particles. Transmission electron micrographic images indicated that the diameters of the TiO2 nanoparticles prepared by the sol-gel method (TiO2/0/S1C-400/4) ranged between 2 and 8 nm (mean diameter ) 5.4 nm) and those obtained by the hydrolysis of the alkoxide in 2-propanol (TiO2/0/ AC-400/4) ranged between 4 and 11 nm (mean diameter ) 7.3 nm) (Figure 4). TiO2-Montmorillonite Nanocomposites, Prepared by Method A (Adsorption of Titanium Alkoxides onto/into the Clay Mineral Platelets and Subsequent Hydrolysis thereon/therein). The structure of the hydrated titanium dioxide is best discussed in terms of a cross-linked polymeric titanium dioxide structure with a general formula of TiO2‚YH2O.19 Based on 31% weight loss of the TiO2/0/A sample (see Figure 5), we assessed Y ) 2.0, which means that the formula is TiO2‚2H2O. Performing analogous thermoanalytical measurements leads to Y values of 0.81, 0.18, and 0.09 for TiO2/0/S2, TiO2/0/AC-400/4, and P25 Degussa TiO2. Calcination of TiO2‚2H2O is a multistep polycondensation process. Thermogravimetric measurements established that the transformation of TiO2‚2H2O into TiO2 involved the endothermic loss of water up to 200-250 °C and a subsequent exothermic formation of the anatase crystals between 300 and 400 °C, and finally the formation of the rutile phase between 500 and 750 °C occurs (Figure 5, TiO2/0/A).34 The first weight loss below 250 °C is

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Table 2. XRD Powder Data for the Standards Used for Calibrating the Anatase Contents of the TiO2 Samples Prepared by the Sol-Gel Method dL,c nm Miller index 101 (A) 110 (R) 101 (R) 103 (A) 004 (A) 112 (A) 111 (R) 210 (R) 200 (A) 105 (A) 211 (R) 211 (A) c

[hkl]a

TiO2/0/AC-400/4b

TiO2/AC-800b

0.352

TiO2/0/S2C-400/4b

Aldrich anatase

P25 Degussa TiO2

0.351

0.351

0.237 0.234d

0.243 0.238 0.234d

0.353 0.327 0.248 0.243 0.238

0.324 0.248 0.243 0.238 0.234d 0.219 0.205 0.189 0.170

0.189

0.189 0.170

0.168

0.167

0.169 0.167

0.210 0.205 0.189 0.168 0.166

a Taken from ref 33. In parentheses A ) anatase and R ) rutile. b See Notes for definitions of the symbols used for the different samples. Basal spacing. d This reflection is associated with the sample holder (Al).

Figure 4. TEM images of TiO2 nanoparticles, in TiO2/0/S1C400/4 and TiO2/0/AC-400/4 samples and their size distributions.

Figure 5. Thermoanalytical measurements of TiO2/0/A sample (TiO2‚2H2O) and Na-montmorillonite. The endothermic peak of dehydration at 114 °C and the two exothermic peaks of anatase and rutile phase evolution at 313.5 °C and near 600 °C are seen in the DTA curve of TiO2‚2H2O. The TG curve of Na-montmorillonite shows the dehydration and dehydroxylation of Na-montmorillonite.

attributed to the desorption of water on internal and external surfaces of Na-montmorillonite, and the second (34) Ha, P. S.; Youn, H.-J.; Jung, H. S.; Hong, K. S.; Park, Y. H.; Ko, K. H. J. Colloid Interface Sci. 2000, 223, 16-20.

Figure 6. Broadening of the XRD of anatase (101) reflection peaks for Aldrich anatase, Degussa P25 TiO2, TiO2 synthesized by the hydrolysis of the alkoxide (TiO2/0/AC-400/4), and TiO2montmorillonite nanocomposites (TiO2/M/P4C-400/4), prepared by method A (adsorption of titanium alkoxides onto/into the clay mineral platelets and subsequent hydrolysis thereon/ therein). The samples were scanned in 0.02 2Θ° steps per second in the 20-30 2Θ° range.

weight loss corresponds to the dehydroxylation of the silicates above 550-700 °C (TG Na-montmorillonite).16 We carried out, therefore, the calcinations at 400 °C, in accord with the reported calcination temperatures (300500 °C for 2-12 h).15,16,19 The calcination-induced crystalline anatase formation manifested itself in the broadening of the XRD peaks at 2Θ ) 25.3° (Figure 6). Taking advantage of this broadening, we estimated the diameters of the crystalline anatase in Aldrich anatase, Degussa P25 TiO2, and TiO2 synthesized by the hydrolysis of the alkoxide (TiO2/0/AC-400/4) and the TiO2 preadsorbed onto montmorillonite and calcinated at 400 °C for 4 h (TiO2/ M/P4C-400/4) to be >100 nm, 29.3, 12.0, and 7.6 nm, respectively by the Scherrer equation.12,18,27 Evidently, intercalation into (onto) the clay mineral platelets decreases the sizes of the TiO2 nanoparticles, because the growing of hydrolysated TiO2‚2H2O particles is rather limited by the interlamellar space between the sheets in alcoholic dispersion liquid. Introduction of increasing amounts of TiO2‚2H2O between the montmorillonite layers increased the basal spacing from 1.20 to 3.00 nm (subsequent to hydrolysis and drying; Figure 7 and Table 3). Calcination also increased the collapsed state of the clay mineral lamellae with decreasing amounts of TiO2 (Figure 7). It should be

Clay Mineral Intercalated TiO2 Nanoparticles

Figure 7. Effects of TiO2‚2H2O concentration on the XRD pattern. The TiO2/M/P1, TiO2/M/P2, and TiO2/M/P3 samples were prepared by using 0.9, 6.6, and 13.3 mmol of Ti(OEt)4/1 g of montmorillonite. Samples were hydrolyzed, precipitated, and dried at 70 °C (15 h) prior to the XRD measurements. Effects of 4 h of calcination at 320 °C are also illustrated in samples TiO2/M/P1C-320/4, TiO2/M/P2C-320/4, and TiO2/M/ P3C-320/4.

noted, however, that there is a reflection of TiO2‚2H2O which indicates the lamellar structure of our precipitate with a basal spacing of 1.5 nm (Table 3). Subsequent to heating above 200 °C the unsupported clay mineral gave an XRD reflection which corresponded to dL ) 0.96 nm of the collapsed montmorillonite sheet basal distance (dTOT). Nitrogen adsorption (BET) measurements indicated the penetration of nitrogen molecules well into the interior of the clay mineral pillars. As expected, increasing the amount of the titanium ethoxide added to the montmorillonite samples increased the amount of TiO2 in the nanocomposites and hence the specific surface area. Thus, the specific surface area was found to increase from 55.0 to 142.8 m2/g from TiO2/M/P1C-320/4 to TiO2/M/P3C-320/4 (Table 3), in which the millimoles of Ti(OEt)4 per gram of montmorillonite added initially increased from 0.9 to 13.3. Changing the calcination temperature from 320 to 400 °C (using 13.3 mmol of alkoxide/g of montmorillonite in the TiO2/M/P4C-400/4 sample) affected only slightly the surface area, but the diameter of the TiO2 particles increased from 6.1 to 7.6 nm (Table 3). Integration of the areas of the 2Θ ) 25.3° peaks (using montmorillonite matrix and TiO2/0/AC-400/4 anatase as standard) permitted the assessment of the anatase contents to be in the 25-50 wt % range (Table 3). The extent of anatase formation was found to depend on the calcination conditions. In general, increasing the temperature or the amount of titanium alkoxide increased the percentage of anatase formed. For example, calcination at 320 °C (TiO2/M/P3C-320/4 sample) resulted in 28.3 wt % anatase while calcination at 400 °C (TiO2/M/P4C-400/4 sample) produced 47 wt % anatase, close to the theoretical maximum of 51.6 wt % (1.063 g of TiO2 + 1.000 g of montmorillonite ) 2.063 g of nanocomposite, which contains a maximum of 1.063 g of TiO2, i.e., 51.6 wt %).

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It was necessary to calcinate the hectorite samples for a longer time and to raise the temperature to affect complete crystallization to the anatase. Calcination at 450 °C for 8 h (TiO2/Hec/PC-450/8) resulted only in 27.2 wt % anatase formation, for example. TEM images of the TiO2-montmorillonite nanocomposites (TiO2/M/P4C-400/4) indicated two populations of differently sized TiO2 particles, one in the 10.8 ( 2.6 nm range and one in the 4.7 ( 1.2 nm range (Figure 8). It is tempting to speculate that smaller particles are acting as pillars while the larger particles are located at the exterior of the clay mineral lamellae (Figure 8). Analysis of the XRD data supports this assumption. The small angle XRD peak of the TiO2/M/P4C-400/4 peak corresponds to a basal spacing of dL ) 4.07 nm (Figure 7). The preadsorption method resulted in larger particle diameters calculated by the Scherrer equation than those calculated from the basal distance values (DTiO2 ) dL - dTOT, around 3.1 nm). Using the Scherrer equation leads, of course, to average diameters of the TiO2 particles: 6.1 nm for the TiO2/M/ P3C-320/4 sample, 7.6 nm for the TiO2/M/P4C-400/4 sample, and 11.7 nm for the TiO2/Hec/PC-450/8 sample (Table 3). Differences between these sets of X-ray data must correspond to the nanoparticles on the exterior of the clay mineral lamellae. TiO2-Montmorillonite Nanocomposites, Prepared by Method B (Heterocoagulation). The optimal ratios of clay mineral platelets and TiO2 nanoparticles (TiO2/ 0/S2, only anatase) to form heterocoagulated nanocomposites were determined by electrokinetic titrations. In the titrations of 10 mL volumes of 0.005 w/v %, 0.01 w/v %, and 0.05 w/v % montmorillonite suspensions, additions of 3.3, 4.8, and 15 mL volumes of 0.1 w/v % TiO2 were required to reach the zero point surface potentials of the heterocoagulated nanocomposites (Figure 9). These corresponded to montmorillonite (M):TiO2 weight % ratios of 1:6, 1:5, and 1:3, respectively. In the scaled-up preparations (using 1.0 w/v % M suspensions) we employed M:TiO2 ratios of 2:1 (33 wt % TiO2), 1:1 (50 wt % TiO2), 1:2 (66 wt % TiO2), and 1:3 (75 wt % TiO2) and refer to them as TiO2/M/H33, TiO2/M/H50, TiO2/M/H66, and TiO2/M/H75, respectively (see Notes). Increasing the amount of TiO2 led, of course, to increased specific surface areas (from 167.3 m2/g in TiO2/M/H33 to 205.2 m2/g in TiO2/M/H50 and to 248.6 m2/g in TiO2/M/ H66) up to a maximum beyond which the surface area decreased (to 211.3 m2/g in TiO2/M/H75); thus the optimal surface area was reached in the TiO2/M/H66 sample (Table 4). Calcination decreased the specific surface areas to 138.7, 142.4, 161.1, and 122.9 m2/g for TiO2/M/H33C-400/ 4, TiO2/M/H50C-400/4, TiO2/M/H66C-400/4, and TiO2/M/ H75C-400/4 but only slightly affected the basal spacings of the nanocomposites (dL ) 3.92-4.13 nm), respectively (Table 4, Figure 10). Apparently, the greater the TiO2 content the greater the percentage of decrease of the specific surface area (17.1%, 30.6%, 35.2%, and 41.8% for the TiO2/M/H33C400/4, TiO2/M/H50C-400/4, TiO2/M/H66C-400/4, and TiO2/ M/H75C-400/4 nanocomposites). This behavior is the consequence of saturating the available sites for TiO2 incorporation into (and onto) the clay mineral platelets. Markedly smaller basal spacings were previously reported for TiO2-clay mineral nanoclusters (dL ) 2.12.7 nm).9-11 This discrepancy is likely to originate in the incomplete crystallization of the nanoclusters which were in their amorphous and hence photocatalytically inactive phase. More recent publications report XRD patterns for these nanocomposites that do not contain reflections in the 1-10° (2Θ) range; only the presence of a small shoulder

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Table 3. Structural Parameters of Nanocompositesa Prepared by the Preadsorption Method sample

basal spacing, dL, nm

M/C-320/4 TiO2/M/P1C-320/4 TiO2/M/P2C-320/4 TiO2/M/P3C-320/4 TiO2/M/P4C-400/4 TiO2/0/AC-400/4 P25 Degussa TiO2

(1.21) (1.39) 3.89, 0.96 (1.43) 4.20, 0.96 (3.00) 4.07 (2.92) 4.36 (1.58)

TiO2 content,b wt %

27.6 28.3 47.0 100 100

diam of TiO2,c D, nm

sp surf. area, aSBET, m2/g

5.6 6.1 7.6 12.0 29.3,d 44.4e

(6.9) 4.9 (50.7) 55.0 (100.6) 97.8 (198) 142.8 130.2 (576.9) 105.8 42.9

a Data in parentheses were obtained prior to calcination while those without parentheses refer to samples that had been calcinated. Using TiO2/0/AC-400/4 as standard. c Determined by the Scherrer equation. d Determined for anatase particles. e Determined for rutile particles.

b

Figure 9. Electrokinetic measurements of heterocoagulation. Shown are the titration curves of 10 mL of 0.005 w/v % (squares), 0.01 w/v % (diamonds and triangles, to demonstrate reproducibility), and 0.05 w/v % (circles) montmorillonite suspensions with 0.1 w/v % TiO2 dispersions in the Mutek PCD 02 particle charge detector. Figure 8. TEM images of different portions of the TiO2montmorillonite nanocomposites (TiO2/M/P4C-400/4), prepared by method A (adsorption of titanium alkoxides onto/into montmorillonite clay platelets and subsequent hydrolysis thereon/therein).

indicates the possibility of the presence of larger (>5 nm) pillars.12-14 Titanium dioxide pillared montmorillonite with basal spacings (3.54, 4.74, and 6.00 nm for samples treated in ion-exchange reaction at 45, 60, and 75 °C, respectively, and calcinated at 400 °C for 2 h) similar to those reported here were also prepared by mixing silica and titania sols.16 It is important to point out, however, that these preparations were carried out in acidic media. The previously reported high specific surface areas of titanium dioxide pillared clay minerals (aS ) 250-400 m2/g) prepared in acidic media are likely to be the result of the partial destruction of the clay mineral platelets10 or the result of the amorphous phase of titania (dL ) 2.02.5 nm).8-11,17 Surface areas of 202 and 254 m2/g were reported for TiO2 pillared clay mineral nanocomposites that had been extracted by absolute ethanol or supercritical carbon dioxide and calcinated at 500 °C for 12 h (see TiO2-PILC1-500/12 and TiO2-PILC2-500/12 in Table 4).12,13 They have found that the photocatalytic reaction rate increases as the particle size decreases (in the 6.512.5 nm diameter range). It is possibly important because our samples were prepared with very similar parameters (TiO2 content, aSBET), containing nanoparticles in the 4-4.5 nm diameter range using a much simpler preparation process. Ooka and co-workers produced catalysts with similar specific surface areas which were prepared by

using two different supplementary heat treatments (aSBET ) 210-259 m2/g, Table 4).14 Transmission electron microscopic images of the TiO2/ M/H33C-400/4 and TiO2/M/H50C-400/4 nanocomposites indicate the mean diameters of the TiO2 nanoparticles to be 4.3 ( 1.2 and 3.4 ( 0.8 nm (Figures 11 and 12). These values are quite similar to those reported previously.14 It should be noted that the calcination did not increase the size of the intercalated TiO2 nanoparticles (Table 4). In contrast, sizes of naked TiO2 nanoparticles, prepared by the sol-gel method (TiO2/S2C-400/4), increased to 10.6 nm from 4.7 nm (Table 1). It needs to be reemphasized that the Scherrer equation and the assessments of the basal distance values reflect different measurements and, therefore, it is entirely expected that they report different values for the diameters of the TiO2 nanoparticles. The Scherrer equation, derived from XRD measurements, is related to the mean diameters of the particles functioning either as pillars or being located on the external surfaces of the clay mineral shapes. In contrast, the assessments of the changes of the basal distances of the incorporated TiO2 particles considers only those that function as pillars. Increasing TiO2 contents in the nanocomposites results in the distribution of a progressively greater proportion of the particles to the external surfaces of the clay mineral. Sintering, as expected, influences the TiO2 particles that are attached to the clay mineral sheets to a greater extent than those that function as pillars. TEM images support with this postulate. The height of the TiO2 pillars estimated from the changes in the basal distances (3.1 nm) is almost the same as that obtained from the TEM images (3.5 nm).

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Langmuir, Vol. 19, No. 7, 2003 2945

Table 4. Structural Parameters of Nanocompositesa Prepared by the Heterocoagulation Method sample

basal spacing, dL, nm

TiO2 content,b wt %

diam of TiO2,c D, nm

sp surf. area, aSBET, m2/g

TiO2/0/S2C-400/4 TiO2/M/H33C-400/4 TiO2/M/H50C-400/4 TiO2/M/H66C-400/4 TiO2/M/H75C-400/4 TiO2-PILC1-500/12e TiO2-PILC2-500/12e TiO2-UT-1-500f TiO2-HT-200/6f

(3.92, 1.38) 4.13, 0.97 (4.08) 4.02 (4.13) 4.13 (4.13) 4.19 not detected not detected not detected not detected

100 33d 50d 66d 75d 56 56 47.7 43.1

(4.7) 10.6 (4.1) 6.3 (4.2) 7.0 (4.1) 6.9 (4.6) 8.6 8.5 6.6 not detected 5.0

(236.7) (167.3) 138.7 (205.2) 142.4 (248.6) 161.1 (211.3) 122.9 202 254 256 210

a Data in parentheses were obtained prior to calcination while those without parentheses refer to samples that had been calcinated. Using TiO2/0/AC-400/4 as standard. c Determined by the Scherrer equation. d Theoretical TiO2 content (calculated data). e Prepared under acidic conditions; TiO2 pillared montmorillonite nanocomposites that had been extracted by absolute ethanol or supercritical carbon dioxide and calcinated at 500 °C for 12 h (TiO2-PILC1-500/12 and TiO2-PILC2-500/12). Taken from ref 12. f Prepared under acidic conditions; untreated (UT) or hydrothermally treated (HT) and finally both calcinated at 500 °C. Taken from ref 14.

b

Figure 12. TEM image of a calcinated TiO2-montmorillonite nanocomposite (TiO2/M/H50C-400/4).

Figure 10. XRD patterns of calcinated TiO2-montmorillonite nanocomposites, prepared by method B (heterocoagulation).

Figure 11. TEM image of a calcinated TiO2-montmorillonite nanocomposite (TiO2/M/H33C-400/4).

Conclusions Several significant results are reported in the present work. First, titanium dioxide nanoparticles, prepared by the sol-gel method with diameters less than 5 nm, have been shown to crystallize in the anatase form even in the absence of any hydrothermal heat treatment or calcina-

tion. Calcination of the samples has increased the size of the particles and decreased their specific surface areas. Clay mineral intercalated titanium dioxide nanoparticles have been prepared by two different approaches: the adsorption of titanium alkoxides onto/into the clay mineral platelets followed by hydrolysis, and the heterocoagulation of TiO2 nanoparticles and clay mineral nanoplatelets. Of these methods, heterocoagulation is preferred since it has yielded small photocatalytically highly active nanoparticles and large surface areas even in the absence of calcination. The successful use of these nanocomposites for pollutant photodegradation will be reported in a subsequent communication. Notes. In shorthand notation: TiO2/support/preparation method and heat treatment, °C/time. The following abbreviations are used for support: 0 ) no support; M ) montmorillonite; Hec ) hectorite. The following abbreviations are used for preparation method: S1 ) sol-gel sample 1; S2 ) sol-gel sample 2; P1, P2, P3 ) preadsorption samples 1, 2, 3 prepared from different amounts of Ti(OEt)4; P4 ) preadsorption sample 4 prepared from Ti(OiPr)4; H ) heterocoagulation including wt % TiO2. Abbreviations used for heat treatment: HT ) hydrothermal, C ) calcinated. The following samples were used for TiO2 Nanoparticles Prepared by the Sol-Gel Method: TiO2/0/S1 ) sol-gel prepared TiO2 sample 1 (sometimes referred to the “as-prepared aqueous TiO2 nanoparticle dispersion”); TiO2/0/S1HT-100/12 ) sol-gel prepared TiO2 sample 1 in a hydrothermal reaction, heated to 100 °C for 12 h; TiO2/0/S1HT-125/12 ) sol-gel prepared TiO2 sample 1 in a hydrothermal reaction, heated to 125 °C for 12 h; TiO2/0/S1HT-150/12 ) sol-gel prepared TiO2 sample 1 in

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Langmuir, Vol. 19, No. 7, 2003

a hydrothermal reaction, heated to 150 °C for 12 h; TiO2/ 0/S2 ) sol-gel prepared TiO2 sample 2; TiO2/0/S2C-400/4 ) sol-gel prepared TiO2 sample 2, calcinated at 400 °C for 4 h. The following samples were used for the Preparation of TiO2-Montmorillonite Nanocomposites by Method A (Adsorption of Titanium Alkoxides onto/into the Clay Mineral Platelets and Subsequent Hydrolysis thereon/ therein): TiO2/M/P1 ) preadsorbed Ti(OEt)4 was hydrolyzed onto montmorillonite; TiO2/M/P1C-320/4 ) preadsorbed Ti(OEt)4 was hydrolyzed onto montmorillonite and then calcinated at 320 °C for 4 h; TiO2/M/P4 ) preadsorbed Ti(OiPr)4 was hydrolyzed onto montmorillonite; TiO2/M/ P4C-400/4 ) preadsorbed Ti(OiPr)4 was hydrolyzed onto montmorillonite and then calcinated at 400 °C for 4 h; TiO2/Hec/P1 ) preadsorbed Ti(OiPr)4 was hydrolyzed onto hectorite; TiO2/Hec/PC-450/8 ) preadsorbed Ti(OiPr)4 was hydrolyzed onto hectorite and then calcinated at 450 °C for 8 h. The following samples were used in the Preparation of TiO2-Montmorillonite Nanocomposites, by Method B (Heterocoagulation): TiO2/M/H33 ) heterocoagulated

Mogyoro´ si et al.

TiO2(33 wt %) and montmorillonite; TiO2/M/H33C-400/4 ) heterocoagulated TiO2(33 wt %) and montmorillonite, calcinated at 400 °C for 4 h; TiO2/M/H50 ) heterocoagulated TiO2(50 wt %) and montmorillonite; TiO2/M/H50C400/4 ) heterocoagulated TiO2(50 wt %) and montmorillonite, calcinated at 400 °C for 4 h; TiO2/M/H66 ) heterocoagulated TiO2(66 wt %) and montmorillonite; TiO2/M/H66C-400/4 ) heterocoagulated TiO2(66 wt %) and montmorillonite, calcinated at 400 °C for 4 h; TiO2/ M/H75 ) heterocoagulated TiO2(75 wt %) and montmorillonite; TiO2/M/H75C-400/4 ) heterocoagulated TiO2(75 wt %) and montmorillonite, calcinated at 400 °C for 4 h. Acknowledgment. We are grateful to NATO Science for Peace Program, Project No. SfP-972652, and Hungarian National Research Fund OTKA T 13 034430 for financial support. LA025969A