Ultrasonic Intercalation of Gold Nanoparticles into a Clay Matrix in the

Sonication was performed by using the ultrasonic processor VCX 505 (Sonics & Materials, Newtown, USA). The intercalation progress was monitored for ...
0 downloads 0 Views 4MB Size
Download PDF
J. Phys. Chem. C 2009, 113, 6751–6760

6751

Ultrasonic Intercalation of Gold Nanoparticles into a Clay Matrix in the Presence of Surface-Active Materials. Part II: Negative Sodium Dodecylsulfate and Positive Cetyltrimethylammonium Bromide Valentina Belova,* Helmuth Mo¨hwald, and Dmitry G. Shchukin Max-Planck Institute of Colloids and Interfaces, D14476 Potsdam, Germany ReceiVed: January 15, 2009; ReVised Manuscript ReceiVed: February 25, 2009

The influence of two surfactants with different charges, anionic sodium dodecylsulfate and cationic cetyltrimethylammonium bromide, on the intercalation of preformed Au nanoparticles into a clay matrix under ultrasonic treatment has been investigated. Surfactant addition has been used to modify the active surface area of Na+-montmorillonite and to change the interlamellar space between the clay layers. Then, intercalated surfactant in clay composites has been replaced by Au nanoparticles under sonication. The presence of the anionic surfactant in the clay matrix permits intercalation of Au nanoparticles only at the edge of the clay. The cationic surfactant has a very strong interaction with the negatively charged clay surface subsequently blocking the penetration of large amounts of Au nanoparticles during sonication. Therefore in both cases they do not enable full penetration of the clay layers by Au nanoparticles in contrast to previous findings for nonionic surfactants. 1. Introduction Synthesis of nanoparticle/clay composites is very important in fabrication of different tools as catalysts, adsorbents, optical devices, color filters, sensors, polarizers, magnetic data storage media, and many others.1-8 The most important characteristics of these nanocomposites are the size and shape of the incorporated metal nanoparticles as well as the physical and chemical properties of the basis clay (active surface area, cation exchange capacity, interlamellar space, etc.). Different techniques can be used to obtain metal-clay nanocomposites: (1) synthesis of the metal nanoparticles inside the clay matrix by decomposition of metal precursors; (2) distribution of chemically produced metal nanoparticles in the clay matrix. Ultrasonic treatment can be applied for intercalation of noble nanoparticles into separated silicate layers.9 Sonication enables a drastic decrease of the incorporation time increasing the interlamellar space of clay minerals. Clay minerals such as a Na+-montmorillonite adsorb a large volume of polar molecules on the surface and between layers during the swelling process which, in turn, promotes the expansion of the material.10,11 It has been proved that the clays, which are naturally hydrophilic due to hydration of metal ions, become hydrophobic by ion-exchanging long-chain quaternary amine cations for metal ions.12 Many works studied the effect of cationic surfactants on clay minerals in order to produce efficient sorbent materials for nonionic organic compounds.13-16 Previous research9 indicated that the incorporated poly(ethylene glycol) (PEG) (i.e., neutral polymer) increased the interlamellar space from 1.88 to 7.01 nm. The nanocomposites have been created by replacement of PEG by Au nanoparticles under sonication. Nevertheless, the impact of charged surfactants on the intercalation process has not been investigated yet. In this paper, the study of the systems formed by Na+montmorillonite and two surfactants with different chemical * Author to whom correspondence should be addressed: e-mail, [email protected]; phone, +49 (0) 331-567-9235; fax, +49 (0) 331-567-9202.

structure and charge, sodium dodecylsulfate (anionic) and cetyltrimethylammonium bromide (cationic), are carried out in order to examine the effect on intercalation of Au nanoparticles under ultrasonic treatment. Several analytical techniques have been used: small- and wide-angle X-ray scattering (SAXS and WAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), BET, Fourier transform infrared measurements (FTIR), and thermogravimetric analysis (TGA). The paper is divided in two parts: the methodology and the results presented by different analytical techniques. Finally, summary and major conclusions of the study are given. The aim of this work is to understand the interaction between clay minerals and surfactants, the properties of the surfactant/ clay composites, and the influence of the surfactants on the incorporation of preformed nanoparticles into the clay matrix by ultrasonic treatment. 2. Experimental Section 2.1. Materials. All reagents were used without additional purification. Sodium dodecylsulfate and cetyltrimethylammonium bromide were supplied by Alfa Aesar GmbH&Co. The sodium-montmorillonite,hydrogentetrachloroaurate(III)(99.999%), sodium borohydride (98%), tetraoctylammonium bromide (98%), 4-(dimethylamino)pyridine (99%), potassium perchlorate, and sodium acetate were purchased from Sigma-Aldrich, Germany. All aqueous solutions were made using deionized Millipore Milli-Q water. 2.2. Characterization Methods. The thermogravimetric analysis of the polymer/clay and nanogold/clay composites was performed on NETZSCH TG 209 F1 instruments. Samples (about 7 mg) were heated under nitrogen with a flow rate of 20 mL/min from 20 to 1000 °C at a heating rate of 10 °C/min. X-ray diffraction of the samples was studied on a Bruker AXSD8 ADVANCE X-ray diffractometer and on a Nanostar Bruker AXS diffractometer. Changes in interlamellar spacing of composites have been studied by using SAXS in the angle range of 1° < 2Θ < 10°. The interlamellar space was calculated from

10.1021/jp900431x CCC: $40.75  2009 American Chemical Society Published on Web 03/27/2009

6752 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Belova et al.

Figure 1. Scheme of ultrasonic intercalation of gold nanoparticles into the clay matrix in the presence of the surfactants.

the SAXS peak positions using Bragg’s law. A wavelength (λ) equal to 0.15418 nm has been used for all SAXS measurements. TEM studies were carried out on a JEOL-JEM 100 electron microscope. Scanning electron microscopy was performed with a Gemini Leo 1550 instrument to study the morphology of montmorillonite upon intercalation. Nitrogen adsorption and desorption isotherms were measured at 196 °C using a Micrometrics Gemini 2375 analyzer after degassing of the samples at 120 °C for 1 h. The surface area was calculated from the linear part of the BET plot. The pore size distribution was estimated by the Barrett-Joyner-Halenda (BJH) model involving the use of the Halsey equation.17 The Fourier transform infrared measurements were carried out with a Bruker Hyperion 2000 IR spectrometer equipped with a 158 IR objective and MCT detector at room temperature in KBr pellets. The samples were crushed and blended with potassium bromide. For each sample, 2.5 mg of clay-polymer composites or Au-clay and 508 mg of KBr were weighted and then ground in agate mortar for 10 min before making the pellets. Spectra were taken with 2 cm-1 resolution in a wavenumber range from 4000 to 400 cm-1. 2.3. Synthesis of the Au/Clay Nanocomposite. The Au nanoparticles were synthesized by using the Brust two-phase method18 as described in the previous study.9 The gold nanoparticles, stabilized by DMAP (dimethylamino)pyridine), are very stable over a long time (i.e., 4 months) in aqueous solution without any sign of agglomerates. They are with a diameter of approximately 6 ( 0.5 nm. The synthesis of the Au/clay nanocomposites includes two steps (Figure 1): The first step is the preparation of the surfactant/clay composites for changing the interlamellar space between the clay layers and the charge of the clay surface. Distilled water has been used as solvent for intercalating the surfactant into the clay matrix. A 0.04 g portion of Na+-montmorillonite was mixed with 250 mL of distilled water for 48 h. Different concentrations of anionic and cationic surfactants were used (0.5% solution of SDS and 0.4% solution of CTAB). A 40 mL portion of surfactant solution was added to the swollen clay under ultrasonic treatment (20 kHz, 500 W). Sonication was performed by using the ultrasonic processor VCX 505 (Sonics & Materials, Newtown, USA). The intercalation progress was monitored for different time periods (1, 5, 10, 15, 20, 30, 40 min).

Figure 2. (A) SAXS pattern of (a) initial Na+-montmorillonite, (b) SDS/clay composite at 10 min of sonication, (c) SDS/clay composite at 15 min of sonication, and (d) SDS/clay composite at 20 min of sonication. (B) (a) CTAB/clay composite at 10 min of sonication. (b) CTAB/clay composite at 15 min of sonication. (c) CTAB/clay composite at 20 min of sonication.

The second step is the replacement of the intercalated surfactants by the Au nanoparticles. Three Au colloid-clay mixed solutions have been prepared with different Au content (A1 4.5 wt %; A2 8.5 wt %; A3 12 wt %) by adding defined amounts of Au nanoparticles to swollen Na+-montmorillonite under vigorous stirring. The solutions were then transferred to a sonication flask and sonicated at 20 kHz, 500 W for 40 min. During the sonication process the solution was heated up to 35-45 °C. The precipitated nano-Au intercalated product was separated by centrifugation, washed with water, and dried under vacuum overnight. Afterward, the samples were calcinated in air atmosphere at 800 °C for 4 h.

Systems Formed by Clay and Different Surfactants

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6753

Figure 3. TEM images: (a) initial Na+-montmorillonite; (b) SDS/clay composite at 10 min of sonication; (c) SDS/clay composite at 15 min of sonication; (d) CTAB/clay composite at 10 min of sonication; (e) CTAB/clay composite at 15 min of sonication; (f) CTAB/clay composite at 20 min of sonication.

Figure 4. SEM images: (a) SDS/clay composite at 10 min sonication; (b) SDS/clay composite at 15 min sonication; (c) SDS/clay composite at 20 min sonication; (d) CTAB/clay composite at 10 min sonication; (e) CTAB/clay composite at 15 min sonication; (f) CTAB/clay composite at 20 min sonication.

3. Results and Discussion Part 1. Preparation of the Surfactant/Clay Composites: SDS and CTAB Intercalation. In the first step surfactants are intended to be inserted between the clay layers which may also affect the active surface area.

3.1. Small-Angle X-ray Scattering and Microscopic Characterization. Figure 2 presents the SAXS patterns of the surfactant/clay composites formed at different sonication times. In the previous study, it has been found that the initial clay has an interlayer spacing of about 1.09 nm.9 The basal distance of

6754 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Belova et al.

Figure 6. (A) Pore size distribution of (a) initial Na+-montmorillonite; (b) SDS/clay composite at 10 min of sonication; (c) SDS/clay composite at 15 min of sonication; (d) SDS/clay composite at 20 min of sonication. (B) (a) CTAB/clay composite at 10 min of sonication; (b) CTAB/clay composite at 15 min of sonication; (c) CTAB/clay composite at 20 min of sonication.

+

Figure 5. FTIR spectra of: (A) (a) pure SDS, (b) initial Na montmorillonite, (c) SDS/clay composite at 10 min of sonication, (d) - SDS/clay composite at 15 min of sonication, (e) SDS/clay composite at 20 min of sonication; (B) (a) pure CTAB, (b) CTAB/clay composite at 10 min of sonication, (c) CTAB/clay composite at 15 min of sonication, (d) CTAB/clay composite at 20 min of sonication.

Na+-montmorillonite swollen in distilled water without ultrasonic treatment is 1.54 nm. The structure of SDS/clay composites is profoundly affected by sonication. SAXS patterns show the presence of two different peaks (Figure 2A). Na+-montmorillonite and SDS have assembled in two organized phases: one that corresponds to an intercalated structure of the surfactant into a Na+-montmorillonite interlayer and the other corresponding to the surfactant crystals. After 15 min of sonication the basal spacing of Na+-montmorillonite increased up to 1.74 nm. Furthermore, the small enlargement of the interlamellar space is indicated by the peak appearing at 2θ ) 1.38°. Liquid jets resulting from ultrasonic cavitations are capable of widening the lamellar space, allowing the formation of SDS crystals. According to SAXS patterns of CTAB/clay composites (Figure 2B), the organic cations penetrate into the interlayer space replacing the sodium cations and the peak position corresponding to the interlamellar space shifts to lower angle. The increase of d indicates the successful intercalation of surfactant. The composite formed at 15 min of sonication has a highest d spacing of 2.17 nm. Also, at this time a new peak appears at d ) 4.9 nm. The presence of the peak at 2θ ) 1.8° (d ) 4.9 nm) indicates the opening of the interlamellar space of the clay layers by surfactant expansion between the silicate sheets. Further increase of sonication time decreases the interlamellar space.

TEM analyses (Figure 3) of the composites provides additional information about the intercalated structure. Comparing the TEM micrograph of the ultramicrotomed initial clay with micrographs of the SDS/clay composites reveals an optimal sonication time of 15 min, which does not destroy the inner structure of the clay. Also, the TEM image of the CTAB/clay composite formed after 15 min of sonication shows that the clay sheets have not been exfoliated, preserving the layered structure. Subsequent increase of the sonication time (more than 15 min) does not result in increase of the space between the clay layers. Furthermore, the TEM images of surfactant/clay composites sonicated for more than 15 min show the partial exfoliation of the layers. (Figure 3, see also Figure S1 in Supporting Information) SEM images of the initial clay and surfactant/clay composites are presented in Figure 4. SDS/clay composite formed at 15 min of sonication shows a more porous surface, and the space between surface layers has been enlarged. Longer ultrasound treatment completely degrades the clay while promoting the aggregation of the surfactant on the surface. According to SEM images, the surface of the CTAB/clay composite formed at 15 min of sonication did not undergo morphologic changes after polymer intercalation. Very strong interaction between two oppositely charged materials at 15 min of sonication preserves the structure of the clay surface. 3.2. FTIR and BET Measurements. The FTIR spectra of surfactant/clay composites illustrate the interaction between surfactant and the clay matrix (Figure 5). The assignments of the FTIR bands of the pure SDS, CTAB, initial Na+-montmorillonite, surfactant/clay composites treated at different sonication times are presented in Table S1 (see Supporting Information). The spectrum of the pure SDS (Figure 5A-a) shows two main doublet absorption bands at ∼2850-2924 cm-1 and 1210-1240 cm-1 corresponding to the aliphatic group and sulfate group of

Systems Formed by Clay and Different Surfactants

Figure 7. (A) TGA data of (a) initial Na+-montmorillonite; (b) SDS/ clay composite at 10 min of sonication; (c) SDS/clay composite at 15 min of sonication; (d) SDS/clay composite at 20 min of sonication. (B) TGA data of (a) CTAB/clay composite at 10 min of sonication; (b) CTAB/clay composite at 15 min of sonication; (c) CTAB/clay composite at 20 min of sonication.

SDS, respectively. The peak at ∼1045 cm-1 is assigned to -SdO stretching vibrational modes of the sulfonic acid group present in SDS. The characteristic peaks of the SO4 group correspond to 730-800 cm-1. The spectrum of the initial clay (Figure 5A-b) shows the following characteristic bands: ∼3621 and ∼3447 cm-1 assigned to the -OH stretching vibration of the associated water on the surface of clay; bands at ∼1635 and at ∼1560 cm-1 are assigned to the bending of the HO-H water in the silica matrix; a strong absorption band at ∼1087 cm-1 is assigned to the Si-O-Si stretching vibration. All these peaks are significantly decreased with increasing sonication time. First of all this indicates that the water molecules have been replaced by SDS during ultrasonic irradiation and the surfactant interacts with the clay matrix. The bands corresponding to the surfactant are also found for the SDS/clay composites. New peaks related to the SdO stretch and S-O-C stretching of sulfates were detected at ∼1050 and at ∼770 cm-1, respectively. Furthermore, the stretching mode Al-OH of silicate network (1087 cm-1) shifted to higher wavenumbers (1110 cm-1). This shift could correspond to an interaction between the SDS and the charged octahedral layer of the Na+-montmorillonite. Also, the peaks at ∼2391 and 2383 cm-1 assigned to the H-bonded OH stretch of surfactant appear on the spectra of the SDS/clay composites.

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6755 The spectrum of the pure CTAB (Figure 5B-a) shows the symmetric and asymmetric stretching CH2 vibrations of the alkyl chain corresponding to 2845 and 2917 cm-1, respectively. The peaks at ∼1482 and at ∼1440 cm-1 correspond to asymmetric and symmetric C-H scissoring vibrations of the CH3-N+ moiety. With the intercalation of CTAB, the H2O content is reduced due to the replacement of the hydrated cations by surfactant and the surface of Na+-montmorillonite is changed from hydrophilic to hydrophobic. Therefore the intensity of the adsorption bands at ∼3621 and at ∼3447 cm-1 is decreased in the spectra of all composites. The diffraction peak at ∼1560 cm-1 corresponding to the OH stretching vibration of the silica matrix disappears completely after 15 min of sonication. Also, the intensity of the strong absorption band at ∼1087 cm-1 decreases with increasing sonication time. The same effect is observed also for the band at ∼1635 cm-1 whose intensity is decreased by a factor of 2 and shifted to ∼1630 cm-1. These effects on the silica matrix reveal the interaction of the surfactant molecules with clay matrix. The absorption peak at ∼1475 and at ∼1490 cm-1 appears due to the CH2 bending vibration and NH vibrations in amides. The peak at ∼946 cm-1 corresponds to interaction between the N-containing group of CTAB and the clay surface appears on the spectra. Also, all spectra of CTAB/clay composites demonstrate the vibration at ∼2920 and at ∼2850 cm-1 corresponding to CH2 symmetric and asymmetric stretching vibrations of the alkyl chains of CTAB. The intensities of these bands strongly depend on the sonication time. At 15 min of sonication the quantity of the loaded surfactant reaches a maximum. Adsorption isotherms for the initial clay and the surafctant/ clay composites are presented in Figures S2 and S3 (see Supporting Information). The adsorption efficiency (Figure S2 in Supporting Information) of all composites is lower than that of the initial clay and it decreases with sonication time. Na+montmorillonite adsorbs small amounts of SDS. Anionic surfactants are adsorbed by negative clay surface in a tail-on fashion due to the electrostatic repulsion between the headgroup and the negative charge of the surface. This is also known as “hydrophobic bonding”. For a composite formed at 15 min sonication, the isotherms approximate a plateau at SDS concentration of 2.50 mmol/dm3 with saturation values of 0.32 mmol of SDS/g of clay. Further increasing of sonication time decreases the volume of adsorbed surfactant. Therefore it can be concluded that the sorption of SDS is likely occurring on the surface layers. The pore diameter of clay as a function of pore volume for different sonication times is illustrated in Figure 6A. The pore volume, specific surface area, and pore diameter are collected in Table 1. The higher values of the pore diameter have been found for SDS/clay composites formed after 15 min of sonication. At this time the pore size of the clay surface varies in a range of 6.5 to15 nm. Further increase of sonication decreases the surface area and pore diameter of the composites. For CTAB/clay composites the shapes of isotherms are essentially the same (Figure S3 in Supporting Information). Water molecules leave the interlamellar space of Na+-montmorillonite taken by the adsorbed surfactants because of the interaction between the hydrophobic tails of the cationic surfactant and water. Figure S2b (Supporting Information) shows that the adsorbed amounts of CTAB at 15 min of sonication time on clay are higher than those at shorter sonication time. Specific surface area, pore size, and pore volume of the CTAB/ clay composites, determined by the BET (N2) method, are presented in Table 1. According to the values of the pore characteristic from Table 1, the maximum adsorption of CTAB

6756 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Belova et al.

TABLE 1: Characteristics of SDS/Clay Composites, CTAB/Clay Composites and Au Nanoparticle/Clay Nanocomposites sample name +

Na -montmorillonite

BET surface area (m2/g)

pore volume (cm2/g)

334.3 ( 1

1.58 ( 0.01

7.57 ( 0.01

1.56 ( 0.01 1.53 ( 0.01 1.49 ( 0.01 1.46 ( 0.01 1.50 ( 0.01 1.32 ( 0.01 1.21 ( 0.01 1.49 ( 0.01 1.37 ( 0.01 1.23 ( 0.01

7.13 ( 0.01 6.54. ( 0.01 5.02 ( 0.01 6.45 ( 0.01 5.25 ( 0.01 5.14 ( 0.01 5.09 ( 0.01 7.31 ( 0.01 7.20 ( 0.01 8.35 ( 0.01

1.78 ( 0.01 1.83 ( 0.01 1.93 ( 0.01 2.12 ( 0.01 1.73 ( 0.01 1.71 ( 0.01 1.68 ( 0.01 2.51 ( 0.01 1.22 ( 0.01 1.61 ( 0.01

7.05 ( 0.01 6.48 ( 0.01 5.32 ( 0.01 6.83 ( 0.01 6.07 ( 0.01 5.94 ( 0.01 5.89 ( 0.01 6.95 ( 0.01 7.01 ( 0.01 8.76 ( 0.01

pore diameter (D) (nm)

SDS Surfactant SDS/clay at 1 min of US SDS/clay at 5 min of US SDS/clay at 10 min of US SDS/clay at 15 min of US SDS/clay at 20 min of US SDS/clay at 30 min of US SDS/clay at 40 min of US AuNp/clay A1 AuNp/clay A2 AuNp/clay A3

330.1 ( 1 328.3 ( 1 327.2 ( 1 332.3 ( 1 318.4 ( 1 304.1 ( 1 301.6 ( 1 320.7 ( 1 337.1 ( 1 343.8 ( 1

CTAB/clay at 1 min of US CTAB/clay at 5 min of US CTAB/clay at 10 min of US CTAB/clay at 15 min of US CTAB/clay at 20 min of US CTAB/clay at 30 min of US CTAB/clay at 40 min of US AuNp/clay A1 AuNp/clay A2 AuNp/clay A3

340.1 ( 1 345.8 ( 1 350.7 ( 1 358.3 ( 1 340.5 ( 1 337.4 ( 1 329.8 ( 1 301.3 ( 1 314.2 ( 1 323.9 ( 1

CTAB Surfactant

onto clay occurs at 15 min of sonication; further increasing the sonication time decreases the all adsorption characteristics. The strong sorption is likely to be due to the strong electrostatic attraction between the oppositely charged clay surface and the surfactant headgroup. Figure 6B shows the pore diameter of clay as a function of pore volume for different sonication times. At 15 min of sonication time the pore size of the clay surface varies in a range of 7-11 nm. 3.3. Thermogravimetric Analysis. Figure 7 shows the TGA curves of the initial clay and of surfactant/clay composites formed at different sonication time. The curve for initial Na+montmorillonite indicates an 11.3% weight loss while heating the material to 800 °C. The first part of the weight loss by about 3.5% near 100 °C arises from surface and interlayer water. The second weight loss of about 5% between 100 and 470 °C corresponds to the loss of the OH groups of the polyhydroxyls of aluminum. For SDS/clay composites a first weight loss of 5% occurs around 110 °C (Figure 7A). The second weight loss is around 4% for SDS/clay composite formed at 15 min of sonication observed between 170 and 490 °C. This loss corresponds to the decomposition due to loss of water and OH groups and the degradation of the surfactant in the clay interlayer at temperatures. The higher sonication time does not change this weight loss. In Figure 7B it is seen that the thermal decomposition for the CTAB/clay composites shifts to higher temperatures. All composites have a weight loss below 120 °C, which is due to desorption of water (after 7.9%). The weight loss between 300 and 500 °C (6.2% for CTAB/clay composite at 15 min sonication) is attributed to the decomposition of the intercalated CTAB molecules. When the sample is heated to 800 °C, the residual weight of the clay is 21% for the CTAB/clay composite formed at 15 min of sonication. The composites with intercalated CTAB molecules are thermally more stable than the initial Na+montmorillonite. Part 2. Ultrasonic Intercalation of Gold Nanoparticles. The second step of the current work includes the ultrasonic treatment for replacement of surfactant molecules from the clay matrix by Au nanoparticles from 4.5, 8.5, and 12 wt % Au colloid solution. A sonication time of 40 min for the intercalation

of Au nanoparticles into clay layers has been chosen.9 This value has been found in previous work as optimal for full saturation of clay minerals with Au nanoparticles. 3.4. Small- and Wide-Angle X-ray Scattering and Microscopic Characterization. The SAXS patterns of Au nanoparticle/clay nanocomposites formed in the presence of SDS in the clay matrix (Figure 8 (left)) indicate the formation of the intercalated structure. With increasing concentration of Au colloid solution, the diffraction peaks corresponding to the interlamellar space between clay layers progressively increase. The peaks for nanocomposite loaded from 12 wt % Au colloid solution are located at 2θ ) 4.9° and 2θ ) 1.3° corresponding to d ) 1.81 and 6.7 nm, respectively. The WAXS patterns (Figure 8 (right)) reveal peaks at 2θ ) 38.2° of the Au (111) plane. Other peaks are observed at 2θ ) 44.4°, 64.8°, and 77.9° corresponding to the (200), (220), and (311) planes of Au nanoparticles. All peaks are related to the cubic phase of Au. SAXS patterns of CTAB/clay composites loaded from gold colloid solution are shown in Figure 9(left). The interlamellar spaces of the A1 nanocomposite are equal to 3.17 and 7.3 nm after the CTAB molecules have been replaced by Au nanoparticles. The values of the interlayer space are decreased after the second ultrasonic treatment, since the diffraction peak shifts to a higher 2θ. This can be explained by strong interaction between clay and CTAB during the first intercalation process which preserves the clay matrix from penetration of Au nanoparticles. However, the shoulder of 2θ corresponding to 7.3 nm indicates that some Au nanoparticles have been impregnated into the clay matrix. XRD patterns of nanocomposites loaded with different Au concentration are presented in Figure 9(right). The peaks corresponding to Au are apparent but with lower intensity compared with the results for anionic SDS surfactant. One thus concludes that only a small amount of Au nanoparticles has been intercalated. The TEM images of SDS/clay composites loaded with Au nanoparticles (Figure 10) show a very unusual distribution of Au nanoparticles in the clay matrix. Au nanoparticles did not fully penetrate in the clay. Precisely, they are spread along the edges of the clay layers and near the voids, which were previously created by ultrasonic irradiation. Although Au

Systems Formed by Clay and Different Surfactants

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6757

Figure 8. (Left) SAXS pattern of (a) SDS/clay composite loaded from 4.5 wt % Au colloid solution, (b) SDS/clay composite loaded from 8.5 wt % Au colloid solution, and (c) SDS/clay composite loaded from 12 wt % Au colloid solution. (Right) XRD pattern of (a) initial Na+-montmorillonite without Au nanoparticles, (b) SDS/clay composite loaded from 4.5 wt % Au colloid solution, (c) SDS/clay composite loaded from 8.5 wt % Au colloid solution, and (d) SDS/clay composite loaded from 12 wt % Au colloid solution.

Figure 9. (Left) SAXS pattern of (a) CTAB/clay composite loaded from 4.5 wt % Au colloid solution, (b) CTAB/clay composite loaded from 8.5 wt % Au colloid solution, and (c) CTAB/clay composite loaded from 12 wt % Au colloid solution. (Right) XRD pattern of (a) CTAB/clay composite loaded from 4.5 wt % Au colloid solution, (b) CTAB/clay composite loaded from 8.5 wt % Au colloid solution, and (c) CTAB/clay composite loaded from 12 wt % Au colloid solution.

nanoparticles can interact with SDS, a full intercalation into lamellar space can be obtained only if the repulsion forces between clay and gold are overcome. The TEM images of the nanocomposite loaded from 12 wt % gold colloid solution (Figure 10c) show an enrichment of the Au nanoparticles at the edges of the clay layers. The spherical shape of the gold nanoparticles was preserved with an average diameter of about 6 ( 2 nm. The TEM images of the CTAB/clay composite loaded from 4.5 wt % Au colloid solution demonstrate the presence of Au

nanoparticles in the clay matrix (Figure 10). The gold nanoparticles are nonaggregated, chaotically distributed in the clay matrix near the voids (the white dots on the clay sheets), which were created during ultrasonic irradiation. Increasing the concentration of Au nanoparticles results in higher aggregation. Most of them are placed on the clay surface. Figure 11a shows SEM photomicrographs of the SDS/clay composite loaded from 12 wt % Au colloid solution. The surface is more rough and spongy, without agglomeration of small particles. Therefore the second sonication treatment did not

6758 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Belova et al.

Figure 10. TEM images: (a) SDS/clay composite loaded from 4.5 wt % Au colloid solution; (b) SDS/clay composite loaded from 8.5 wt % Au colloid solution; (c) SDS/clay composite loaded from 12 wt % Au colloid solution; (d) CTAB/clay composite loaded from 4.5 wt % Au colloid solution; (e) CTAB/clay composite loaded from 8.5 wt % Au colloid solution; (f) CTAB/clay composite loaded from 12 wt % Au colloid solution.

Figure 11. SEM images of (a) SDS/clay composite loaded from 12 wt % Au colloid solution and (b) CTAB/clay composite loaded from 4.5 wt % Au colloid solution.

destroy the clay surface which indicates that the internal structure of the clay matrix is also intact. Also, the SEM micrograph of the CTAB/clay composite loaded from 4.5 wt % gold colloid solution is presented (Figure 11b). After second ultrasonic treatment the surface of the nanocomposite is smooth and without aggregated external particles. 3.5. FTIR and BET Measurements. FTIR spectra of the nanocomposites loaded from different concentrations of the Au colloid solution are shown in Figure 12. Compared to the initial spectrum it is clearly seen that all bands corresponding to the silica matrix are found in the spectra (see Table 1). After the gold impregnation, the band at ∼1087 cm-1 attributed to the Si-O-Si is shifted and the intensity is decreased by 30%. Nevertheless, all peaks corresponding to the symmetric and antisymmetric vibrations of the clay matrix are present. Interaction between Au nanoparticles and the clay layers also leads to the appearance of bands at ∼1600, 791, and 404 cm-1 corresponding to the CH ring deformation of pyridines used as ligands to stabilize the Au nanoparticles.

The FTIR spectra of CTAB/clay composites loaded with Au nanoparticles are presented in Figure 12B. The bands of all nanocomposites at ∼1087 cm-1 corresponding to Si-O stretching are shifted to 1100 cm-1 after the second sonication. The intensity of all peaks does not change. The bands at ∼2921 and at ∼ 2851 cm-1 assigned to the asymmetric and symmetric CH2 stretching modes of amine are present in the spectra but with a lower intensity indicating that the CTAB molecules have been partly replaced by Au nanoparticles during intercalation but are still present in the clay matrix. New adsorption peaks at ∼1600, at ∼791, and at ∼404 cm-1 corresponding to the CH out-of-plane ring deformation of pyridines appear in the spectra. The intensity of these peaks has been decreased with increasing concentration of Au colloid solution used for impregnation. That means the further incorporation of positively charged Au nanoparticles is quite difficult to achieve. Only a small amount of Au nanoparticles has been intercalated into the lamellar space between the clay layers; further increase of

Systems Formed by Clay and Different Surfactants

Figure 12. FTIR spectra of (A) (a) SDS/clay composite loaded from 4.5 wt % Au colloid solution, (b) SDS/clay composite loaded from 8.5 wt % Au colloid solution, (c) SDS/clay composite loaded from 12 wt % Au colloid solution without calcination, and (d) - SDS/clay composite loaded from 12 wt % Au colloid solution after calcination; (B) (a) CTAB/clay composite loaded from 4.5 wt % Au colloid solution, (b) CTAB/clay composite loaded from 8.5 wt % Au colloid solution, (c) CTAB/clay composite loaded from 12 wt % Au colloid solution without calcination, and (d) CTAB/clay composite loaded from 4.5 wt % Au colloid solution after calcination. All spectra were shifted along the transmittance axis for clarity but measured with the same sensitivity.

the gold colloid concentration leads to an excess of positively charged particles on the clay surface. The nitrogen adsorption isotherms of Au nanoparticle/clay nanocomposites formed in the presence of SDS are shown in Figure S4 (see Supporting Information). One notices that the shape of the isotherms is not affected by the incorporation of Au nanoparticles. A similar shape of the isotherms is considered as a strong indication that the porous channels on the clay network are not blocked by the Au nanoparticles. The incorporation of the Au nanoparticles of the size obtained in this work leads to significant decrease in the amount of adsorbed nitrogen. It is also found, that the nanocomposites have a narrow pore size distribution which can be clearly seen in Figure 13A. For the A3 nanocomposite the BET surface area is 324.2 m2/g and the pore volume is 1.67 cm2/g. As shown in Table 1, the average value of the pore diameter of the composite is around 7.21 nm which is higher than the size of the Au nanoparticles. Figure S5 (Supporting Information) shows the adsorption isotherms of nanocomposites formed by incorporation of Au nanoparticles into CTAB/clay composites. The N2 adsorptiondesorption isotherms of the CTAB/clay composites loaded with Au nanoparticles do not exhibit distinct abruptness, indicating that the pore size distribution is not uniform. Pore size distribution curves of the composites are illustrated in Figure 13B. These results correlate well with N2 adsorption-desorption

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6759

Figure 13. (A) Pore size distribution of (a) SDS/clay composite loaded from 4.5 wt % Au colloid solution (A1); (b) SDS/clay composite loaded from 8.5 wt % Au colloid solution (A2); (c) SDS/clay composite loaded from 12 wt % Au colloid solution (A3). (B) (a) CTAB/clay composite loaded from 4.5 wt % Au colloid solution (A1); (b) CTAB/clay composite loaded from 8.5 wt % Au colloid solution (A2); (c) CTAB/ clay composite loaded from 12 wt % Au colloid solution (A3).

isotherms. It can be seen that the A1 composite has two types of mesopores with different pore sizes of ca. 7 and 11 nm, the smaller pores being dominant. The BET surface area of the A1 nanocomposite is 20% larger than that of A2 and A3 nanocomposites. The BET surface area, pore diameter, and pore volume of all nanocomposites are listed in Table 1. 3.6. Thermogravimetric Analysis. Figure 14 shows the results of TGA measurements of nanocomposites produced from surfactant/clay composites by ultrasonic impregnation. TGA curves of SDS/clay composites loaded from Au colloid solution (Figure 14A) show a weight loss about 1.2% between room temperature and 150 °C which corresponds to the loss of adsorbed water molecules. By comparing the initial clay (Figure 7A) and the Au nanoparticle/clay composites, one realizes that the nanocomposites contain much less water than the pure clay. The curves corresponding to the nanocomposites reveal a weight loss about 10.5% at a temperature of 510 °C which results from the thermal decomposition of the Au stabilizer. The weight loss between 510 and 800 °C is obtained until complete dehydroxylation of the clay. The incorporation of Au nanoparticles in CTAB/clay composites slightly changes the thermal stability of the clay matrix (Figure 14B). The first weight loss of about 1.8% at 150 °C for all nanocomposites is from adsorbed water in the interlayers. The second weight loss of about 13.4% between 150 and 490 °C corresponds to the loss of the OH groups of the polyhydroxyls of aluminum. Principally, the following weight loss for nanocomposites is very similar to that of CTAB/clay composites. At 800 °C the weight loss approximately is about 21% for Au/ clay composites.

6760 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Belova et al. by sonication shows a completely different distribution of Au nanoparticles in the clay matrix. SAXS demonstrates that the interlamellar space increases up to 3.17 and 7.3 nm. However, the XRD and TEM analyses show the local incorporation of only small amounts of Au nanoparticles in the clay matrix with a partial agglomeration. By comparison of these results with the previous study,9 it can be concluded that the presence of differently charged surface-active materials in the clay matrix affects the ultrasonic intercalation process of Au nanoparticles by ultrasonic irradiation. This work provides original insight into the production of layered nanocomposites loaded with inorganic nanoparticles. The ultrasonic treatment accelerates the incorporation process dramatically. The anionic surfactant blocks the penetration of Au nanoparticles deep into the clay layers, which can be useful for creation of different devices, such as a nanocondenser system and sensors, and useful in optoelectronic fields, etc. Intercalation of Au nanoparticles in the clay matrix in the presence of positively charged surfactant under ultrasonic treatment provides high loading of the edges of the clay with Au nanoparticles.

Figure 14. TGA data: (A) (a) SDS/clay composite loaded from 4.5 wt % Au colloid solution (A1), (b) SDS/clay composite loaded from 8.5 wt % Au colloid solution (A2), (c) SDS/clay composite loaded from 12 wt % Au colloid solution (A3); (B) TGA data of (a) CTAB/ clay composite loaded from 4.5 wt % Au colloid solution (A1), (b) CTAB/clay composite loaded from 8.5 wt % Au colloid solution (A2), (c) CTAB/clay composite loaded from 12 wt % Au colloid solution (A3).

4. Summary and Conclusion Two different Au nanoparticle/clay nanocomposites were prepared by intercalation of Au nanoparticles under ultrasonic treatment into Na+-montmorillonite in the presence of negatively charged sodium dodecylsulfate or positively charged cetyltrimethylammonium bromide. It has been found that the surface active material plays a critical role for the final microstructure of the composites. It increases the interlamellar space of the clay and hydrophobizes the clay surface. The SAXS patterns reveal that at 15 min of sonication in the presence of SDS the interlamellar space increases from 1.74 to 6.41 nm. TEM analysis also proves the intercalated structure with preserved layer packing. SAXS patterns demonstrate the increase of the space between the clay layers from 2.17 to 4.9 nm also for the CTAB/clay composite. FTIR spectra of both surfactant/clay composites illustrate the presence of surfactant in the lamellar space interacting with the clay matrix. Furthermore, the BET measurement provides complete information about changes which occur during intercalation. The presence of SDS in the clay matrix permits deposit of a large amount of Au nanoparticles only on the edge of the clay and in voids created by ultrasonic treatment. The SAXS and XRD patterns of these composites clearly indicate the formation of an intercalated structure and an increase of the interlamellar space between the clay layers. The Au nanoparticles after intercalation have a spherical shape and diameter of 6 ( 2 nm. The intercalation of Au nanoparticles in CTAB/clay composite

Acknowledgment. The work was supported by the EU FP6 STREP project MatSILC (#33410) and the joint German-French project PROCOPE 2008 D0707578.The LEA Sonochemistry between CEA Marcoule and the MPIKG and by the Gay-Lussac/ Humboldt Award to Helmuth Mo¨hwald. The authors thank Dr. Hartmann and Rona Pitschke for helping with TEM measurements. Supporting Information Available: A table of FTIR bands of surfactants, clay, and clay composites, an image of the SDS/ clay composite, and plots of adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Pinnavaia, T. J. Science 1983, 220, 365–371. (2) Morikawa, Y. AdV. Catal. 1993, 39, 303–327. (3) Sivakumar, T.; Krithiga, T.; Shanthi, K. etc. J. Mol. Catal. A: Chem. 2004, 223, 185–194. (4) Katz, E; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042–6108. (5) Sanchez, C.; Soler-Illia, G.; Ribot, F. etc. Chem. Mater. 2001, 13, 3061–3083. (6) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599–5611. (7) Tricokot, Y. M.; Fendler, J. H. J. Am. Chem. Soc. 1984, 106, 7359– 7366. (8) Dekany, I.; Turi, L.; Tombacz, E.; Fendler, J. H. Langmuir 1995, 11, 2285–2292. (9) Belova, V.; Mo¨hwald, H.; Shchukin, D. G. Langmuir 2008, 24, 9747–9753. (10) Ray, S. S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539–1641. (11) Chen, B.; Evans, J. R. H.; Greenwell, C. etc Chem. Soc. ReV. 2008, 37, 568–594. (12) Juang, R.-S.; Lin, S.-H.; Tsao, K.-H. J. Colloid Interface Sci. 2002, 254, 234–241. (13) Chen, G.; Han, B.; Yan, H. J. Colloid Interface Sci. 1998, 201, 158–163. (14) Tahani, A.; Karroua, M. J. Colloid Interface Sci. 1999, 216, 242– 249. (15) Brahimi, B.; Labbe, P.; Reverdy, G. Langmuir. 1992, 8, 1908– 1918. (16) Patzko, A.; Dekany, I. Colloids Surf. 1993, 71, 299–307. (17) Gregg, S. J.; Sing, K. S. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (18) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. AdV. Mater. 1995, 7, 795–797.

JP900431X