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Ultrasonic Intercalation of Gold Nanoparticles into Clay Matrix in the Presence of Surface-Active Materials. Part I: Neutral Polyethylene Glycol Valentina Belova,* Daria V. Andreeva, Helmuth Mo¨hwald, and Dmitry G. Shchukin Max-Planck Institute of Colloids and Interfaces, D14476 Potsdam, Germany ReceiVed: October 23, 2008
The combined application of polyethylene glycol and ultrasonic irradiation has been investigated as a possible method to synthesize intercalated Au/clay nanocomposites. Twenty minutes of ultrasonic irradiation on the Na+-montmorillonite clay matrix in the presence of polyethylene glycol suffices to introduce polymer molecules into the lamellar space of the clay, increasing the basal distance between clay layers. Additional ultrasonic treatment allowed the successful replacement of the intercalated polyethylene glycol by gold nanoparticles within the interlamellar space. The incorporation of gold nanoparticles in the clay matrix under ultrasound is twice as efficient with PEG as without it. Eventually, spherical-shaped gold nanoparticles of 6-8 nm diameter spread homogeneously within the nanocomposites. The structures of the produced nanocomposites have been studied by small and wide-angle X-ray scattering, transmission electron microscopy, scanning electron microscopy, BET, Fourier transform infrared measurements, and thermogravimetric analysis. 1. Introduction Nowadays nanocomposites are used in a broad variety of technical and scientific fields because they possess useful mechanical and chemical properties such as high stiffness to weight ratio, high strength, low permeability, and reduced flammability.1-3 Nanocomposite systems can be derived from different materials. Among these, clays are widely applied because their layered structure with high active surface area and cation exchange capacities has advantages for nanocomposite production.4,5 The common approach for the fabrication of the composites is first to swell clay to increase the interlamellar space. During the swelling process, water molecules bind to the surface of Na+-montmorillonite and penetrate inside the layers. As a result, the distance between the layers can rise up to 200%. It is known that polymer molecules also have the potential to increase the interlamellar space of Na+-montmorillonite.6-8 The distance of clay minerals treated with polymer solutions depends on the size of the used polymers. Another aspect is that clays containing hydrated Na+ ions easily interact with hydrophilic polymers.9-13 There are three principal methods for preparing polymer/clay composites depending on the materials and processing techniques. These are in situ polymerization,14,15 solvent methods,16,17 and polymer melt intercalation.18,19 For example, according to ref 20, polyethylene oxide has been intercalated into clay layers. The composites used in this study had high-surfactant loading. Their d spacing varied from 1.4 to 1.8 nm, depending on the different loading.21 Smectite adsorbs one or two layers of the surfactants in its interlayer space. The intercalation of PEO in Na-MMT and Na-hectorite allowed the stoichiometric incorporation of one or two polymer chains between the silicate layers, increasing the intersheet spacing from 0.98 to 1.36 and 1.71 nm, respectively.22 However, polymer intercalation generally is very slow, requiring times of hours to days. * Corresponding author. Phone: +49 (0) 331-567-9235. Fax: +49 (0)331567-9202. E-mail:
[email protected].
A possible way to improve and accelerate the incorporation of polymers into clay layers is the application of high-intensity ultrasonic treatment on the suspension of the clay mineral in the presence of polymer molecules. Ultrasonic irradiation can significantly decrease the experimental times.23,24 We find two different interlamellar distances strongly depending on the sonication time.25 Ultrasonic irradiation can also create a hydrophilic clay surface. In a previous study,26 it has been proven that ultrasonic treatment for intercalating Au nanoparticles into clay nanocomposites is a very effective tool. The analysis showed that clay samples loaded from 4.2 wt % gold colloid solution are saturated by Au nanoparticles after only 40 min sonication. In this work, a neutral polymer, polyethylene glycol (PEG), is used to increase the hydrophilic characteristics of the clay surface. Ultrasonic intercalation of the PEG molecules into the clay matrix increased the interlamellar space from 1.88 to 7.01 nm. Following that, replacement of PEG molecules by Au nanoparticles has been successfully performed under ultrasonic irradiation. This Article is structured as follows: First, the experimental details are described. The results are then presented in two parts using the following analytical techniques: small and wide-angle X-ray scattering (SAXS and WAXS), transmission and scanning electron microscopy (SEM and TEM), Fourier transform infrared measurements (FTIR), BET surface area analysis, and thermogravimetric analysis (TGA). 2. Experimental Section 2.1. Materials. All reagents are analytical grade and used without additional purification. Polyethylene glycol (PEG 600) was supplied by Alfa Aesar GmbH & Co. KG. The sodiummontmorillonite, hydrogen tetrachloroaurate(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 prepared using deionized Millipore Milli-Q water. 2.2. Characterization Methods. The thermogravimetric analysis of the polymer/clay and nanogold/clay composites was
10.1021/jp8093929 CCC: $40.75 2009 American Chemical Society Published on Web 03/13/2009
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Figure 1. Scheme of ultrasonic intercalation of gold nanoparticles into the clay matrix in the presence of PEG.
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. TEM studies were carried out on a JEOLJEM 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 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 using the Barrett-Joyner-Halenda (BJH) model involving the use of the Halsey equation.27 The Fourier transform infrared measurements were carried out with a Bruker Hyperion 2000 IR microscope 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 were 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 method28 as described in a previous study.26 The gold nanoparticles are very stable over a long time (i.e., 4 months) in aqueous solution without any sign of agglomerates. They are spherical with a diameter of approximately 6 ( 0.5 nm. The intercalation of nanogold into clay layers comprised two steps (Figure 1). The first step was the preparation of the polymer/clay composites for increasing the interlamellar space between the clay layers. Distilled water has been used as solvent for intercalating the polyethylene glycol into the clay matrix. 0.04 g of Na+-montmorillonite has been swollen in 250 mL of distilled water for 48 h, removing excess water. Next, 40 mL of 0.4% PEG solution has been added to the swollen clay under ultrasonic treatment (20 kHz, 500 W) for intercalating the PEG molecules into the layered clay. Sonication has been performed by using the ultrasonic processor VCX 505 (Sonics & Materials, Newtown, U.S.). The intercalation progress has been monitored for different periods (1, 5, 10, 15, 20, 30, 40 min). The second step was the replacement of the intercalated PEG by the Au nanoparticles. Three Au colloid-clay mixed solutions have been prepared with Au content (A1 4.5 wt %; A2 8.5 wt %; A3 12 wt %) by adding different amounts of Au nanoparticles to PEG/clay composite with vigorous stirring. The solutions were then transferred to a sonication flask and sonicated at 20 kHz, 500 W for 40 min. The precipitated nano-
Figure 2. SAXS pattern of (a) initial Na+-montmorillonite; (b) PEG/ clay composite at 10 min of sonication; (c) PEG/clay composite at 20 min of sonication; and (d) PEG/clay composite at 30 min of sonication. Because of probably different clay content in the samples, the absolute intensities cannot be compared.
Au intercalated product has been separated by centrifugation, washed with water, and dried under vacuum overnight. Afterward, the samples were calcinated under an air atmosphere at 800 °C for 4 h to finalize the nancocomposite production. 3. Results and Discussion Part 1. Preparation of the Polymer/Clay Composites: PEG Intercalation. PEG molecules have been intercalated into the clay matrix by applying ultrasound treatment, which increases the interlamellar space between clay layers. The described analyses confirmed the presence of PEG molecules in the clay matrix. 3.1. Small-Angle X-ray Scattering and Microscopic Characterization. SAXS and TEM analyses are important tools for better understanding the internal structure of PEG/clay composites.29,30 Changes in interlamellar spacing of Na/PEG loaded montmorillonite have been studied by using SAXS in the angle range of 1° < 2Θ < 10°. A wavelength (λ) equal to 0.154 nm has been used for all SAXS measurements. The average interlayer distances d of the PEG/clay composite are illustrated as a function of sonication time in Figure 2. The SAXS patterns of the composites show that the d-spacing of initial Na+-montmorillonite is 1.09 nm. The basal distance of Na+-montmorillonite swollen in distilled water without ultrasonic treatment is d ) 1.54 nm. After 20 min of sonication in PEG solution, d equals 1.88 nm. Furthermore, at this time, an additional peak appears at d ) 7.01 nm. The presence of this peak indicates the opening of the interlamellar space of the clay
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Figure 3. TEM images of (a) initial Na+-montmorillonite; (b) PEG/clay at 5 min of sonication; (c) PEG/clay at 10 min of sonication; (d) PEG/ clay at 20 min of sonication; and (e) PEG/clay at 30 min of sonication.
layers by stretching between silicate sheets. One also notes that the diffraction peaks are very broad, indicating that the periodicity extends only some layers spacing. The same effect has been found in the other work in which platinum nanoparticles have been intercalated.31 For longer sonication time (>20 min), the intensity of both peaks decreases.
Figure 3 illustrates the TEM images of PEG/clay composites. It can be seen by comparing Figure 3a-c (i.e., composites at 0, 5, and 10 min sonication time) and Figure 3d (i.e., 20 min sonication time) that at 20 min of sonication the intercalated multilayer crystallites are present. Subsequent increases of ultrasonic treatment time (to 30 or 40 min) induce partial
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Figure 4. SEM images of (a) initial Na+-montmorillonite; (b) PEG/clay at 20 min of sonication; and (c) PEG/clay at 30 min of sonication.
Figure 5. FTIR spectra of the (a) pure PEG; (b) initial Na+-montmorillonite; (c) PEG/clay at 5 min of sonication; (d) PEG/clay at 10 min of sonication; (e) PEG/clay at 20 min of sonication; and (f) PEG/clay at 30 min of sonication.
exfoliation of the layers (Figure 3e). Furthermore, the SAXS patterns of PEG/clay composites sonicated for more than 20 min also confirm the partial exfoliation of layers. So, according to SAXS and TEM analysis, 20 min of sonication provides the highest extension of the interlamellar spacing without undesired exfoliation of clay layers and decomposition of the Na+-montmorillonite structure. SEM also confirms that 20 min time of ultrasonic treatment is optimal: Figure 4 shows the SEM of the PEG/clay composite at 0, 20, and 30 min sonication time. The initial Na+montmorillonite (Figure 4a) is characterized by a tightly layered clay surface without signs of defects. With increasing sonication time (at 20 min), the interlamellar space gradually enlarges due to the forced intercalation of PEG (Figure 4b). The distance between the layers increased without causing any damage to the surface. However, further increase of sonication to 30 min leads to the destruction of the clay structure and agglomeration of the dense particles (Figure 4c).
3.2. FTIR and BET Measurements. The FTIR spectra of the pure PEG, initial Na+-montmorillonite, and treated PEG/ clay composites at different sonication times are presented in Figure 5 and summarized in Table S1 (see Supporting Information). For the FTIR spectrum of the pure PEG, the peaks in the area from 1700 to 1300 cm-1 correspond to -CH bending of the molecules. The sharp band at 1110 cm-1 in the PEG spectrum corresponds to C-O symmetric stretching of PEG. Peaks at 940 and 835 cm-1 are due to the C-C stretching. As compared to the initial spectrum (Figure 5b), the intensity of the strong absorption band at ∼1087 cm-1 representing the asymmetric stretching vibration of the Si-O-Si groups of the tetrahedral sheet (Figure 5b-d) decreases by 10% after sonication of the PEG/clay composites. The same effect observed more pronounced at ∼3621 and ∼3447 cm-1 is due to the -OH stretching vibration on the surface of Na+-montmorillonite. The intensities of the lines at 1635 and at ∼1560 cm-1 corresponding to the bending of the HO-H water in the silica matrix decrease
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J. Phys. Chem. C, Vol. 113, No. 14, 2009 5385 TABLE 1: Characteristics of PEG/Clay and Au Nanoparticle/Clays Nanocomposites
Figure 6. Pore size distribution of (a) initial Na+-montmorillonite; (b) PEG/clay at 10 min of sonication; (c) PEG/clay at 20 min of sonication; and (d) PEG/clay at 30 min of sonication.
with increasing sonication time. This indicates that PEG molecules replace water molecules due to the cavitation process. The peak at ∼779 cm-1 corresponds to Si-O stretching vibration with increasing sonication time. New peaks corresponding to the H-bonded OH stretch of PEG appear at 2400 cm-1 even after 5 min of sonication. The maximum intensity of these new peaks is observed after 20 min of sonication, suggesting that the PEG molecules completely attach to the clay surface. Furthermore, the peaks persist with extending the cavitation process, indicating that the clay is fully saturated with polymer. It can be seen from the data summarized that the CH2 symmetric and asymmetric stretching vibrations of PEG bound on Na+-montmorillonite lie at 2850 and 2921 cm-1, respectively, which are the same wavelengths of pure PEG. The peaks become broader and intense, which demonstrates ion exchange between polymer and functional groups of clay. The observed peaks indicate interaction between the OH- functional groups of PEG and the Si-OH groups at the clay surface via hydrogen bonding. The FTIR spectra of the PEG/clay composites reveal the peaks associated with pure PEG and the Na+-montmorillonite peaks. The FTIR result reveals the successful incorporation of PEG into the clay structure. Adsorption isotherms for the initial clay and the PEG/clay composites (presented in Figure S1a,b (see Supporting Information)) are characteristic of typical L-type adsorption32,33 that represents a system where the adsorbate is strongly attracted by the adsorbent. The saturation value of this process is characterized by the “plateau” of the isotherm. The maximum polymer insertion into the clay is calculated by averaging the points on the isotherm “plateau”. It has been found that the maximum adsorption of PEG into Na+-montmorillonite occurs at 20 min of sonication, and a further increase of the sonication time leads to the decomposition of the clay composite and 30% PEG desorption. Figure 6 illustrates the pore diameter of clay as a function of pore volume for different sonication times. For short sonication times (i.e.,
