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Preparation and Structural Characterization of Novel Nanohybrids by Cationic 3D Silica Nanoparticles Sandwiched between 2D Anionic Montmorillonite Clay through Electrostatic Attraction Tzu-Fan Tseng and Jeng-Yue Wu* Department of Chemical Engineering, National Chung Hsing UniVersity, 250 Kuo-Kwang Road, Taichung, Taiwan, Republic of China ReceiVed: January 23, 2009; ReVised Manuscript ReceiVed: May 27, 2009
In our study, cationic colloidal silica was prepared using a silylation process to modify silica by chemically grafting it with (γ-aminopropyl)trimethoxysilane (APS) under acidic conditions. The silylation of silica by APS grafting was characterized by FTIR and solid 29Si NMR with its ζ potential measured for the confirmation of cationic nature for APS-modified silica (APS-silica). The strong electrostatic attraction of cationic APSsilica toward negatively charged Na+-montmorillonite (MMT) clay resulted in the formation of APS-silica/ MMT nanohybrids during the mixing process. The coagulated nanohybrids acquired different charges depending on the weight ratios (WRs) of APS-silica to MMT in the nanohybrids. The surface potential of nanohybrids also varies with the amount of APS-silica imparted when measured using a ζ potential analyzer. Images obtained by transmission electron microscopy and field-emission scanning electron microscopy reveal the supracolloidal structure of the nanohybrids. APS-silica nanoparticles, with a diameter of 22 nm, were embedded in two-dimensional MMT arrays that are approximately 300 nm × 300 nm in size. The ordered structure of the nanohybrid was demonstrated in a small-angle X-ray scattering study. Also, the wide-angle X-ray diffraction of nanohybrids was employed to analyze the interlayer distance of MMT in nanohybrids. The precipitates of coagulated nanohybrids were separated from solution and weighed to yield the “coagulation efficiency” on the basis of the feeding weight of APS-silica and MMT. Coagulation efficiencies over 97.6% are obtained for WRs between 2 and 7. The calculation using coagulation-efficiency data gives a saturated WR of 8.1 that agrees with that of the theoretical packing of APS-silica nanoparticles covering the MMT surface. In addition to determining the ordered supracolloidal structure, this research also presents a novel method to exfoliate the 2D MMT nanoclay with 3D nano-APS-silica colloids by a self-assembly process through electrostatic attraction. A comparison of the Brunauer-Emmett-Teller surface area of MMT in nanohybrids with the theoretical packing of silica on MMT concludes the biased layout of silica colloids on MMT with partial overlapping and void development in the interstices of nanohybrids. 1. Introduction Advanced materials based on the hybrids of inorganic nanoparticles and layered silicates are currently one of the most interesting topics of research.1-13 Among the layered silicates employed, as a result of its being well characterized,14,15 montmorillonite (MMT) clay has attracted the most fundamental and commercial interest for a wide range of applications in the preparation of nanocomposites.16-18 In MMT clay, the exchangeable cations located between clay galleries are easily replaced by other cations through ion exchange by electrostatic attraction. In addition to cations, other moieties such as surfactants can also be exchanged to render the MMT surface hydrophobic and result in the formation of organoclay.19-23 Recently, nanohybrids of anionic MMT and other cationic colloids such as zinc oxide,1 titanium oxide,2-4 palladium,5 cadmium sulfide,6 gold,7 and magnetite8,9 have been studied by in situ formation or postcolloid formation. A special hybrid film that can be used to fabricate the Langmuir-Blodgett (LB) composite film in a layer-by-layer approach has been presented.10 Also, the anionic silica was mixed with anionic MMT for the preparation of hybrid colloids by heterocoagulation with the aid of watersoluble cationic polymers or polymer electrolytes.11,12 Further * To whom correspondence should be addressed. E-mail: jywu@ dragon.nchu.edu.tw.
modification of silica colloids by silane coupling agents to impart a cationic nature to the silica surface, through which the direct interaction or coagulation with nano MMT platelets becomes possible, has also been studied by other research groups.13,24 Although the MMT/cationic silica hybrid has previously been prepared and reported,13 very few data regarding the mechanism and evolutionary structure of hybrid formation through heterocoagulation have been published. Hence, this research describes the formation mechanism of nanohybrids by the electrostatic interaction between anionic two-dimensional (2D) MMT and cationic three-dimensional (3D) silica using a novel selfassembly process. Figure 1 presents the preparation procedure for the modification of colloidal silica (Figure 1a) and the formation of (γ-aminopropyl)trimethoxysilane-modified silica (APS-silica)/MMT nanohybrids (NHBs) (Figure 1b). An ordered array of APS-silica on MMT clay can easily be established. Various characterization techniques, including Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), ζ potential analysis, wide-angle X-ray diffraction (WAXD), and Brunauer-Emmett-Teller (BET) surface area measurements, were used to provide insight into
10.1021/jp9007043 CCC: $40.75 2009 American Chemical Society Published on Web 07/07/2009
Preparation and Characterization of Novel NHBs
Figure 1. Schematic diagram for (a) the modification of colloid silica with APS and (b) the formation of self-assembled APS-silica/MMT nanohybrids.
the morphology and supracolloidal architecture of the novel and ordered APS-silica/MMT nanohybrids. 2. Experimental Section 2.1. Materials. APS from Shin-Etsu Co. was used without further purification. The purity of the APS reagent, as reported by the manufacturers, is greater than 98%. Colloidal silica (AS40) was obtained from DuPont as a 40 wt % aqueous dispersion of discrete silica spheres approximately 22 nm in diameter. Standard hydrochloric acid or sodium hydroxide aqueous solutions (0.1 N) were used for adjusting the pH value of aqueous solutions. Na-montonorillonite (MMT, Kunipia F) clay was purchased from Kunimine Industries with a specific surface area of 750 m2 g of clay-1 and a cationic exchange capacity (CEC) of 1.15 mmol g of clay-1. 2.2. APS Modification of Silica Nanoparticles. The concentrated colloidal silica solution was diluted with deionized water to 5 wt % aqueous solution, and the pH was adjusted to around 3.5. The APS (0.4 g) was added dropwise into the silica solution (400 mL, 5.0 wt %) under vigorous stirring with an adjusted pH value within 3-4. Silylation of APS and silica was carried out for 12 h in a 60 °C water bath. A Millipore ultrafiltration membrane disk with a molecular weight cutoff of 1000 and a pore size of 18 Å was dialyzed with deionized water to discard unreacted APS remaining in the supernatant. The solid content of modified silica was calculated by gravimetric analysis and then diluted to 1 wt % aqueous solution. 2.3. Preparation of APS-Silica/MMT NHBs. APS-silica solution (1 wt %) and MMT solution (1 wt %) were prepared separately and mixed directly at a wide range of weight ratios (WRs) for each solid, and the mixed solution was stirred for 4 h at room temperature. The pH of the APS-silica solution and MMT solution was adjusted to about 3.5 beforehand. On mixing the APS-silica solution and MMT solution, white to gray nanohybrid precipitates were obtained and then filtered and freeze-dried at -40 °C and 250 mbar to obtain dried powders. The “coagulation efficiency” was determined by the weight of the dried nanohybrid powders on the basis of the feed of solid silica and MMT. The details of the experiment are shown in Table 1 for nanohybrids with various WRs. 2.4. Characterization Techniques. Samples of APS-silica and unmodified silica, freeze-dried previously, were used to characterize the silylation of silica by FTIR spectroscopy performed in a Perkin-Elmer spectrophotometer, model Paragon 500. The specimens were prepared by using the KBr pressed disk technique. The solid-state 29Si NMR spectra of the APSsilica specimens were measured on a Bruker DSX 400 MHz spectrometer at room temperature. The ζ potential of silica colloid and APS-silica/MMT nanohybrids (about 50 ppm in
J. Phys. Chem. C, Vol. 113, No. 30, 2009 13037 Millipore water) was measured by a Nano-ZS analyzer from Malvern Instruments. The pH of the solution was controlled to the desired value by adding 0.1 N standard hydrochloric acid aqueous solution or 0.1 N standard sodium hydroxide aqueous solution. The TEM images were taken with a JEOL JEM-200CX microscope employing an accelerating voltage of 120 kV. The specimens were prepared by dropping a suitable volume of colloid solution onto a copper microgrid. Thin specimens with 60-80 nm thickness were prepared by embedding APS-silica/ MMT nanohybrid powder in epoxy resin and curing, followed by thin sectioning on an ultramicrotome, and supported on a copper microgrid for direct observations. The FESEM images were taken with a JEOL microscope (JSM-6700F). The samples were prepared by dropping APS-silica/MMT nanohybrid solution onto aluminum foil by a spin coater at 1500 rpm for 20 s and then drying in an oven at 60 °C. Then the samples were analyzed at an accelerating voltage of 3 kV after being coated with platinum for 30 s at 10 mA to produce a conducting surface. Powder wide-angle X-ray diffraction patterns were obtained on a Rigaku diffractometer D/MAX2000 equipped with a rotating anode (Cu KR radiation) operated at 40 kV and 30 mA. The diffraction data were collected with 2θ between 1.5° and 8° in a fixed mode with a scanning speed of 1 deg/min with step intervals of 0.01°. The specimens for SAXS were prepared by dropping several drops of APS-silica/MMT nanohybrid solutions onto aluminum foil and then drying for several days at room temperature to obtain a thin film. SAXS patterns were collected using a Rigaku 18 kW X-ray generator operated at 40 kV and 300 mA. The distance from the sample to detector is approximately 40 cm. The corrected data are presented as the intensity as a function of the magnitude of the scattering vector q, q ) (4π/λ) sin(θ), where 2θ is the scattering angle. The contents of C and N in the prepared materials were measured by elemental analysis using a Neraeous CHN-OS rapid analyzer. BET sorption data were undertaken by a Micromeritics ASAP 2010 for bare silica, APS-silica, MMT clay, and APSsilica/MMT nanohybrids to determine their surface area for comparison and discussion. 3. Results and Discussion 3.1. APS Modification of Silica Nanoparticles and Their Characterizations by FTIR, NMR, and ζ Potential Analysis. The FTIR spectra of bare silica and APS-silica prepared by a surface grafting with APS are shown in parts a and b, respectively, of Figure 2. The spectra of poly(aminosiloxane) on the silica, shown in Figure 2c, were obtained by digitally subtracting the absorbance contribution of the bare silica spectrum from that of the APS-silica. The absorption bands near 797 and 1052 cm-1 arise from new Si-O-Si linkages, indicating that the hydrolyzed APS had condensed with the silanol groups on the silica surface. The bands near 1558 and 1539 cm-1 arise from the deformation of an NH2 group, with strong hydrogen bonding and asymmetric deformation of NH3+ in the SiO- · · · H · · · NH2+ group on the silica surface.25 Bands at 3005 and 1415 cm-1 emerge after silylation and are caused by the CH stretching and CH2 bending of the APS, respectively.26 29 Si NMR spectroscopy of the APS-silica confirms the formation of interfacial siloxane bonds, as shown in Figure 3. The peaks corresponding to -57.7 and -68.0 ppm correspond to the Si(OH)R(OSi)2 (T2) and SiR(OSi)3 (T3) units, respectively, whereas bands corresponding to -91.7, -101.6, and -111.5 ppm are respectively assigned to the Si(OH)2(OSi)2 (Q2), Si(OH)(OSi)3 (Q3), and Si(OSi)4 (Q4) units.27,28 The presence
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TABLE 1: Compositions of APS-Silica/MMT Nanohybrids and the Coagulation Efficiency Data input of MMT
input of APS-silica
nanohybrid
MMTa (g)
MMT, wc (g)
APS-silicab (g)
APS-silica, ws (g)
WRc
coagulation efficiency, Fd (%)
fraction of APS-silica on the MMT surface, f (%)
NHB0.25 NHB0.5 NHB1 NHB2 NHB3 NHB4 NHB5 NHB7 NHB10 NHB15
16.0287 10.0203 10.1099 5.0336 5.0061 4.0317 3.0208 1.9992 1.5006 1.0557
0.1603 0.1002 0.1011 0.0503 0.0501 0.0403 0.0302 0.0200 0.0150 0.0106
4.0228 5.0201 10.0032 10.0272 15.0291 15.9888 15.0108 13.9949 15.0420 15.0023
0.0403 0.0502 0.1000 0.1003 0.1503 0.1599 0.1501 0.1399 0.1504 0.1500
0.25 0.50 1.00 2.00 3.00 4.00 5.00 7.00 10.00 15.00
39.3 91.5 95.1 97.6 98.5 99.5 99.5 99.9 74.1 54.1
81.4 57.6
a A 1 wt % MMT aqueous solution and weight of MMT. b A 1 wt % APS-silica aqueous solution and weight of APS-silica. c WR is the weight ratio of silica to MMT on a weight basis. d The coagulation efficiency was obtained by the weight of the dried APS-silica/MMT nanohybrid powder divided by the weight of the input of MMT and APS-silica.
Figure 2. FTIR absorbance spectra of (a) bare silica, (b) APS-silica, and (c) poly(aminosiloxane) on the silica surface.
Figure 3.
29
Si NMR spectrum of APS-silica.
of the T2 and T3 resonances suggests that a Si-O-C bond is formed between APS and the silanol groups on silica through hydrolysis and polycondensation reactions. After functionalization of silica with APS at low pH, the protonated amino groups on the silica positively charge its surface. The ζ potential data shown in Figure 4 clearly illustrate bare silica, APS-silica, and MMT at various pH levels. Between pH 2 and pH 11, the MMT clay exhibits a range of negative ζ
Figure 4. Effect of pH on the ζ potential of bare silica, APS-silica, and MMT.
potentials from -40 to -65 mV. The isoelectric point (IEP) of bare silica was measured to be pH 2.85. Above this pH, the bare silica bears a negative charge that increases with an increase of pH. Below pH 2.85, the bare silica bears a positive charge. However, the IEP of APS-silica measured at pH 8.67 indicates that its surface is positively charged between pH 2 and pH 8.67. The APS-silica acquires a negative charge only when the pH is higher than 8.67. Apparently, the successful grafting of APS onto bare silica completely changes the surface properties of silica. The bare silica is negatively charged within a pH range from 2.85 to 8.67, whereas the APS-silica is positively charged within the same pH range. (Note that the MMT charge is negative within this same pH range.) 3.2. Preparation of APS-Silica/MMT NHBs by Heterocoagulation. At pH 3.5, APS-silica has a ζ potential of +42 mV, whereas MMT has a ζ potential of -45 mV. By directly mixing the MMT solution with the APS-silica solution at this pH, cationic APS-silica interacted strongly with anionic MMT by electrostatic attraction to form nanohybrids that quickly settled down to the bottom of the test tube. This speedy heterocoagulation is due to multiple charge complex formation between the cationic silica particles and the negative MMT layers caused by the electrostatic interaction between the polyvalent sites. Various compositions of prepared nanohybrids are shown in Table 1. To understand the characteristics of these nanohybrids, we used ζ potential measurements, coagulation efficiency measurements, FESEM, TEM, SAXS, WAXD, and BET surface area measurements to identify their structures, morphologies, and characteristics.
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3.2.1. Coagulation Efficiency Analysis. Coagulation efficiency analysis measures the effect of the silica nanoparticle dosage on the formation of the nanohybrids. Figure 5 shows the coagulation efficiency of nanohybrids for NHB0.25-NHB15. The supernatant of sample NHB0.25 had a translucent and slightly yellow appearance after centrifugation because of the presence of excess MMT platelets relative to the APS-silica particles in the solution. The coagulation efficiency is only 39.3%. The supernatant of samples NHB0.5-NHB7 are colorless and transparent after centrifugation. The coagulation efficiency for these samples lies between 91.5% and 99.9%. This result implies that almost all APS-silica and MMT coagulated together. These APS-silica/MMT nanohybrids contain 68.3-13.4% MMT. After centrifugation, the supernatant of nanohybrids NHB10 and NHB15 exhibits a translucent and blue appearance due to the presence of excess silica colloids uncoagulated. The measured coagulation efficiencies for these samples decreased to 74.1% and 54.1%, respectively. The visual appearance of the supernatant and precipitate for various WRs ranging from 0.25 to 15 is illustrated in Figure S1 in the Supporting Information. Figure S1a shows that the number of single MMT platelets is greater than that of APSsilica colloids for NHB0.25. Hence, we obtained a translucent supernatant that contained bare MMT and coagulated nanohybrids in the bottom layer. For samples NHB0.5-NHB7, as shown in Figure S1b, a complete coagulation is achieved for nanohybrid formation, as evidenced by the clear and transparent appearance of the supernatant. However, an excess amount of APS-silica is found in the supernatant for samples NHB10 and NHB15. At these WRs, the amount of APS-silica colloids is beyond the saturated adsorption level of the MMT and the supernatant appeared blue in color because of the presence of a large excess of nano APS-silica colloids in solution. The relation between the coagulation efficiency (F) and the input weight of MMT and silica can be derived below as eq 1. Equation 1 describes the coagulation efficiency as the weight of the precipitated nanohybrids, consisting of MMT and coagulated silica, divided by the total input weight of MMT and silica. Equation 2
F)
Wc + Ws f Wc + Ws
(1)
f)
F-1 +F WR
(2)
which is the rearranged form of eq 1, calculates the weight percentage of silica adsorbed on the MMT surface on the basis of the amount of silica (designated as f). In these two equations, Wc is the input weight of MMT, Ws is the input weight of silica, and Ws/Wc ) WR. Using eq 2, f is calculated to be 81.4% and 57.6% for samples NHB10 and NHB15, respectively. Hence, we can obtain the saturation amount of APS-silica nanoparticles on the MMT surface on the basis of the unit weight of MMT as calculated by (WR)f. Values of 8.16 and 8.06 g of silica g of clay-1 are obtained for samples NHB10 and NHB15, respectively. Therefore, the saturated limit of APS-silica nanoparticles adsorbed on the MMT surface predicts a WR value of 8.1. This saturated WR is further compared with that estimated from the theoretical packing of APS-silica colloids on the MMT surface as shown in the Supporting Information. According to this model, a value of 9.5 is obtained for the saturated or maximum packing without deducting the surface area of the edge region
Figure 5. Coagulation efficiency of nanohybrids corresponding to various weight ratios of APS-silica to MMT. (All NHB samples were prepared at pH 3.5.)
of MMT. Detailed discussions can be found in the Supporting Information. This discrepancy in WR is reasonable since there will be a roughly 12-15% error if just one row of silica particles (22 nm) is not adsorbed on one face of MMT having a lateral width of 200 nm. 3.2.2. Structural Characterizations from the Morphology Study of APS-Silica/MMT Nanohybrids Using FESEM and TEM. In the FESEM images shown in Figure 6a, it is clearly observed that some APS-silica nanoparticles are adsorbed on the upper MMT surface in the top view. However, some outof-focus APS-silica nanoparticles are hidden below the single MMT layer and are embedded between adjacent MMT layers. We clearly observe a highly ordered arrangement of APS-silica nanoparticles on the MMT surface along with many single MMT layers covering the top of the APS-silica nanoparticles. The diameter of the APS-silica particles is around 22 nm, and the dimension of MMT is about 200-500 nm × 200-500 nm. By estimating the geometric arrangement, we find that each MMT face will adsorb about 100-400 APS-silica nanoparticles. The electrostatic attraction between countercharged colloids drives a self-assembly process that creates a highly ordered arrangement of APS-silica in the nanohybrids. This self-assembly behavior is a complex process that may be caused by the combined effect of electrostatic attraction in the dispersed state and capillary attraction in the drying procedure.29 Figure 6b shows TEM images of the dried APS-silica/MMT nanohybrids suspended in the aqueous solution. It also reveals the presence of a small portion of free APS-silica nanoparticles in the solution. Most of the APS-silica nanoparticles are adsorbed onto the MMT surface in an orderly manner. The APSsilica/MMT nanohybrid patches may contain a few layers of MMT alternated with silica that may constitute the multilayered MMT/APS-silica/MMT structure. In addition to the top view of the APS-silica/MMT nanohybrids observed in the FESEM image, a side view of the APSsilica/MMT nanohybrids also attracts our attention. We obtained this image with TEM by observing a thin section of nanohybrids embedded in epoxy resin (Figure 7). Again, we find a highly ordered layer-by-layer structure, which implies that the APSsilica and MMT layers are stacked one after another to form a 3D structure. The schematic for these self-assembled nanohybrids illustrates the supracolloidal “packing” of this layer-bylayer structure as shown in Figure S2 in the Supporting Information. Because of the polyvalent nature of cationic APS-
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Figure 7. TEM image of APS-silica/MMT nanohybrid thin sections (NHB2).
Figure 6. Morphology of APS-silica/MMT nanohybrids: (a) FESEM image of nanohybrids (NHB2); (b) TEM image of nanohybrids (NHB4).
silica nanoparticles and MMT platelets, the APS-silica colloids are adsorbed between two MMT layers, causing the d spacing of MMT to be greater than or nearly equal to the diameter of the embedded APS-silica colloids. 3.2.3. Order Characterization by SAXS. Figure 8a shows the diffraction intensity versus the diffraction angle (or q) for these particular APS-silica/MMT nanohybrids with various silica-toMMT WRs. The diffraction peaks in the table in Figure 8b indicate that, for pure APS-silica, the ratio of q is 1:1.62:2.44, which is similar to that of the ordered structure between the body-centered cubic (BCC; 1:21/2:31/2) and hexagonally packed cylindrical (Hex; 1:31/2:2) structures.30 The stacking of APS-silica during the drying process when preparing the nanohybrid can often result in nonperfect packing such as that observed in the BCC and Hex structures. For the different WRs used in our study, the diffraction pattern of nanohybrids is similar to that of pure APS-silica. Again, the q ratio of 1:1.83:2.84 corresponds to a mixed mode between Hex and lamellar (Lam; 1:2:3) ordered structures after MMT is introduced into the APS-silica colloidal arrays. The diffraction peaks exhibit a left shift, and a lower diffraction vector emerges
Figure 8. Small-angle X-ray scattering data for APS-silica and APSsilica/MMT nanohybrids (NHB2, NHB4, and NHB7): (a) q versus I, (b) q versus Iq2. The table in (b) represents the peaks of the curves.
that is assigned to the enlargement of the ordering distance for APS-silica sandwiched between adjacent MMT layers. 3.2.4. Effect of the WR on WAXD and ζ Potential Analysis of Nanohybrids. We gain further insight into the supracolloidal structure of nanonybrids by inspecting the d spacing of
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Figure 10. ζ potential of APS-silica/MMT nanohybrids with various weight ratios of APS-silica to MMT. (All NHB samples were prepared at pH 3.5.)
Figure 9. (a) WAXD pattern of various weight ratios of APS-silica to MMT. (b) For samples from NHB4 to NHB15, the intensity of the WAXD pattern was enlarged to 7.5 times that of the original data.
nanohybrids using WAXD. Figure 9a shows WAXD patterns of the various nanohybrids. For WRs below 2, all patterns exhibit a 5.86° diffraction peak, which corresponds to the 1.51 nm d spacing for MMT obtained by Bragg’s law.31 Increasing the WR, from NHB0.25 to NHB2.0, the intensity of this diffraction peak decreases. For WRs of less than 1, the diffraction peak is high but not sharp. The diffraction intensity decreases further for WRs greater than 2 with only a very small hump observed. When we magnify the intensity scale for NHB4 to NHB15 as the WR increases, much clearer diffraction patterns emerge in the diffraction angles between 4.5° and 6.5° and centered at around 5.8°. Because all the detectable peaks are located around 1.51 nm, we postulate that these peaks originate from MMT. Due to its ionic nature, the pristine MMT can stack together by the attraction force between the cationic edge and anionic surface during the drying process, creating a 1.2-1.5 nm d spacing that can be typically observed and correlated with the water content of MMT.32 This d spacing is very close to that of bare MMT in dried powders that contain adsorbed water molecules in the MMT gallery. Hence, it is suggested that an overlap of the edge side of MMT not covered by APS-silica colloids in the nanohybrid may occur. In addition, a small diffraction peak is observed at 2.6° for WRs greater than 4. The reason for this is unclear but is assumed to be related to silica particles only.
Figure 10 shows that the ζ potential of nanohybrids increases from a negative charge (-40.2 mV) to a positive charge (+40.0 mV) as the WR increases from below 3 to above 4. Zero-charge neutralization of the nanohybrids is achieved when the WR approaches 3.83 by interpolation. The absence of an electrical repulsion force can reduce the energy barrier during coagulation with a consequence of a high coagulation efficiency achieved at this WR by van der Waals attraction. However, either positively charged or negatively charged nanohybrids for NHB0.5 to NHB7 can also achieve a high (91.5% or 99.9%) coagulation efficiency. This is because the SiO2 nanoparticles and MMT have different shapes and multiple charges, which leads to the formation of APS-silica/MMT nanohybrids without the need of balancing all positive and negative charges. At a WR of 3.83, the nanohybrid has zero charge, and the anionic exchange capacity (AEC) for APS-silica is calculated to be roughly equal to the cationic exchange capacity (1.15 mmol g of clay-1) divided by 3.83, which is equal to 0.30 mmol g of silica-1. From the elemental analysis of C and N, the grafted APS on the silica surface can be calculated as millimoles per gram of silica.32 With 1.19 wt % C atoms measured, the amount of grafted APS on the APS-silica was calculated to be 0.33 mmol g of silica-1, which is quite close to 0.30 mmol g of silica-1. Hence, the calculated AEC for APS-silica by elemental analysis is very close to that estimated from the ζ potential for the grafted APS on the APS-silica. 3.2.5. BET Analysis of APS-Silica/MMT Nanohybrids. On the basis of the proposed nanohybrid structure shown in Figure S2 in the Supporting Information, the embedded APS-silica nanoparticles in the MMT gallery will facilitate the enlargement of the MMT d spacing in nanohybrids. Therefore, we explored the structure of these nanohybrids by BET surface analysis. Detailed experimental data by the BET surface area analysis for APS-silica, MMT, and nanohybrids can be found in Table 2. In general, for bare and nonswollen MMT, the nitrogen gas molecules cannot penetrate between the overlapped MMT galleries, but can penetrate into the void spaces of the individual MMT platelets pillared by APS-silica.33 Therefore, we analyzed the surface area of nanohybrids by BET analysis and investigated the percentage of “unoverlapped” MMT surface area. Figure 11 shows sorption isotherms for pure MMT, APS-silica, NHB2, NHB4, NHB7, and NHB10; these isotherms were used to calculate the specific surface area for nanohybrids by the desorption branch. Because the nanohybrids NHB2 to NHB7
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TABLE 2: BET Surface Area Analysis of APS-Silica/MMT Nanohybrids surface area (experiment)a (m2 g-1) sample MMT APS-silica NHB2 NHB4 NHB7 NHB10
surface area (calculation) (m2 g-1)
WR
adsorption
desorption, S
APS-silicac, S1
unoverlapped MMTd, S2
fraction of unoverlapped MMT surface area (%)e
2 4 7 8.1b
5.09 137.48 165.16 189.23 182.88 183.41
7.37 158.47 209.55 229.77 211.85 212.39
105.65 126.78 138.66 141.17
311.70 515.00 585.52 652.47
41.56 68.67 78.07 87.00
a The surface areas of the adsorption part and desorption part were obtained by the BJH method, surface area of pores between 8.5 and 1500.0 Å radius. b The weight ratio of NHB10 is 10, which is greater than the saturated WR of 8.1. Hence, only the saturated WR is used here. c S1 was calculated by 158.47(WR)/(WR + 1). d S ) (S1(WR) + S2)/(WR + 1). S2 was calculated as the unoverlapped MMT surface area. e Since the surface area of MMT is 750 m2 g-1, the percentage of unoverlapped MMT surface area can be calculated from S2 divided by 750 m2 g-1.
Figure 11. Nitrogen sorption isotherms of MMT, APS-silica, and APSsilica/MMT nanohybrids. For clarity, the isotherms of the samples are vertically displaced by 100, 200, 350, 500, and 650 cm3 g-1 for material APS-silica, NHB2, NHB4, NHB7, and NHB10, respectively.
have high coagulation efficiencies, they also facilitate the calculation of unoverlapped surface area of MMT in nanohybrids by a simple mathematical equation describing the surface area for bare MMT, APS-silica, and nanohybrids. On the basis of the law of additivity for the surface areas of both MMT and APS-silica in nanohybrids, the surface area S of the nanohybrid is given by
S)
S1(WR) + S2 WR + 1
(3)
where S1 is the surface area of APS-silica and S2 is that for MMT. Since the APS-silica particles are of spherical shape, they will not overlap with each other. Only the MMT platelets are considered to have the chance to overlap with each other due to their layer structure. By eq 3, the surface area of MMT alone in the nanohybrid can be calculated for NHB2 to NHB10. As shown in Table 2, the surface area of MMT is 311.7 m2 for NHB2, 515 m2 for NHB4, and 585.5 m2 for NHB7. For the nanohybrid NHB10 discussed in the coagulation efficiency analysis, the WR of 8.1 is adopted in the area calculation because the ratio of APS-silica to MMT exceeds the saturation limit, and 652.4 m2 is obtained for NHB10. This means that the surface area of MMT in the nanohybrid increases as the WR increases. This increasing trend of the surface area is explained by the pillaring effect of the MMT gallery due to the
Figure 12. Comparison of the unoverlapped MMT surface area measured by BET and predicted by proposed structural model b in Figure 13.
embedding of more APS-silica colloids, exposing more MMT surface area that can be measured by nitrogen sorption. At the same time, this also reminds us that some part of the adjacent MMT surfaces overlap with each other. The overlapping surface area is calculated as 750 m2 minus S2. The nanohybrid NHB10 with a WR greater than 8.1 should bear a 100% unoverlapped surface area of 750 m2 for MMT. However, the calculated surface area is 652.4 m2 and is only 87.00% that of the bare MMT. This may be due to the incomplete surface coverage of APS-silica particles on the MMT edge area because the MMT width cannot be an exact multiple of the APS-silica diameter. In addition, this incomplete coverage also affords a chance for the possible overlapping of adjacent MMT platelets. In Figure 12, the surface area of unoverlapped MMT measured by BET analysis is compared with the results of unoverlapped MMT on the basis of the theoretical packing of silica on MMT. As discussed before, the saturated APSsilica arranged on the MMT occurred at a WR of 9.5. Hence, the predicted percentage of the MMT surface covered by APS-silica can be roughly calculated as proportional to the WR if we assume that the packing of all silica on MMT is biased to one side instead of uniformly distributed across the entire surface. The dotted line in Figure 12 gives the predicted surface area (m2) for the unoverlapped MMT by WR/9.5 × 750. For nanohybrids with WR ) 2 and 4, the unoverlapped MMT surface area obtained by BET analysis is higher than that predicted by the theoretical packing of silica on MMT because not all APS-silica colloids pack
Preparation and Characterization of Novel NHBs
J. Phys. Chem. C, Vol. 113, No. 30, 2009 13043 ordered structure is further confirmed by small-angle X-ray scattering. SAXS diffraction patterns reveal the ordered structure for supracolloids between hexagonal and lamellar. Further studies by BET analysis on the surface area of MMT in nanohybrids suggest the biased packing model of silica on MMT and the presence of an overlapped MMT structure that are consistent with SAXS and wide-angle X-ray diffraction, respectively. Also, this study affords a novel method for the exfoliation of MMT by embedding cationic APSsilica in the MMT gallery for easy preparation of various polymer nanocomposites.
Figure 13. Proposed structural models for APS-silica/MMT nanohybrids: (a) uniform packing of APS-silica on MMT (0 < WR < 8.1), (b) biased packing of APS-silica on MMT (0 < WR < 8.1) (the slashes in MMT represent the location of overlapped MMT), (c) close packing model with defect (WR > 8.1).
biasedly on MMT. Increasing the WR will counteract this effect because more APS-silica particles will be available to coagulate ideally with MMT. Hence, the surface area data between the solid line and dotted line coincide very closely for nanohybrids with WRs of 7 and 8.1. Shown in Figure 13 are three ideal models for the structure packing of APS-silica on MMT in nanohybrids. Model a proposes a uniform packing of silica across the entire MMT surface. In this model, the MMT surface area calculated by eq 3 should be identical to that of exfoliated MMT for different WRs. However, this is not the case as discussed in previous sections since the MMT surface area increases with an increase of WR. Model b illustrates the biased packing of silica on MMT, with the slashed area representing the overlapped portion between adjacent MMT platelets. On the basis of the wide-angle XRD data, a diffraction peak at 5.86° (d spacing 1.51 nm) was observed. A d spacing of 1.51 nm is assigned to this partial overlapping of the unadsorbed MMT surface, and the peak intensity decreases as the WR increases from 0.5 to 7 and almost disappears when the WR is greater than 7. For WRs greater than the saturation packing of silica on MMT as shown in model c, most silica particles are packed closely on MMT with some imperfect arrangement of silica particles inside it. This model also conforms well to a mixed structure mode between hexagonal and lamellar packing as explained in structure ordering determined by SAXS data. 4. Conclusions We have successfully prepared positively charged APSsilica nanoparticles by grafting silica with APS at acidic conditions; the successful preparation is confirmed from FTIR, NMR, and ζ potential data. A self-assembled heterocoagulation mechanism causes these positively charged polyvalent APS-silica nanoparticles to adsorb strongly onto negatively charged MMT surfaces, forming highly ordered APS-silica/MMT nanohybrids in aqueous solution. Coagulation studies on nanohybrids show that a coagulation efficiency of over 97.6% is obtained for WRs between 2 and 7. For the prepared nanohybrids, the ζ potential increases from negative to positive potential at WRs greater than 3.86. The saturated WR of APS-silica to MMT is close to an experimental value of 8.1. This value is acceptable when compared with that of the ideal packing as 9.5 within experimental error. Morphology studies, based on TEM and FESEM images, indicate an organized structure of supracolloids composed of layered 3D APS-silica colloids embedded in 2D MMT was obtained. This
Acknowledgment. This study was supported by the National Science Council under Contract Number 95-2221-E005-087. We also thank the Ministry of Education in Taiwan for funding the Center for Advanced Industry Technology and Precision Processing, NCHU. Supporting Information Available: Schematic diagram of the formation of APS-silica/MMT nanohybrids (Figure S1), schematic representation of layer by layer APS-silica/MMT nanohybrids (Figure S2), and theoretical calculations of the saturation packing34 of APS-silica on the MMT surface in nanohybrids (text). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Nemeth, J.; Rodriguez-Gattorno, G.; Diaz, D.; Vazquez-Olmos, A. R.; Dekany, I. Langmuir 2004, 20, 2855–2860. (2) Mogyorosi, K.; Dekany, I.; Fendler, J. H. Langmuir 2003, 19, 2938– 2946. (3) Zhu, H. Y.; Orthman, J. A.; Li, J.-Y.; Zhao, J.-C; Churchman, G. J.; Vansant, F. F. Chem. Mater. 2002, 14, 5037–5044. (4) Ooka, C.; Akita, S.; Ohashi, Y.; Horiuchi, T.; Suzuki, K.; Komai, S.; Yoahida, H.; Hattori, T. J. Mater. Chem. 1999, 9, 2943–2952. (5) Kiraly, Z.; Veisz, B.; Mastalir, A.; Kofarago, Gy. Langmuir 2001, 17, 5381–5387. (6) Han, Z.; Zhu, H.; Bulcock, S. R.; Ringer, S. P. J. Phys. Chem. B 2005, 109, 2673–2678. (7) Hata, H.; Kubo, S.; Kobayashi, Y.; Mallouk, T. E. J. Am. Chem. Soc. 2007, 129, 1064–3065. (8) Bourlibos, A. B.; Karakassides, M. A.; Simopoulos, A.; Petridis, D. Chem. Mater. 2000, 12, 2640–2645. (9) Tombacz, E.; Csanaky, C.; Illes, E. Colloid Polym. Sci. 2001, 279, 484–492. (10) Umemura, Y. J. Phys. Chem. B 2002, 106, 11168–11171. (11) Jerez, J.; Flury, M.; Shang, J.; Deng, Y. J. Colloid Interface Sci. 2006, 294, 155–164. (12) Ariga, K.; Lvov, Y.; Ichinose, I.; Kunitake, T. Appl. Clay Sci. 1999, 15, 137–152. (13) Cezar, N.; Xiao, H. Ind. Eng. Chem. Res. 2005, 44, 539–545. (14) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; John Wiley & Sons: New York, Toronto, 1974; pp 1-28. (15) van Olphen, H. An Introduction to Clay Colloid Chemistry; John Wiley & Sons: New York, London, 1963; pp 59-82. (16) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, 28, 1–63. (17) Ray, S. S.; Okamoto, M. Prog. Polym. Sci. 2003, 1539–1641. (18) Utracki, L. A.; Sepehr, M.; Boccaleri, E. Polym. AdV. Technol. 2007, 18, 1–37. (19) Yilmaz, N.; Yapar, S. Appl. Clay Sci. 2004, 27, 223–228. (20) Starodoubtsev, S. G.; Lavrentyeca, E. K.; Khokhlov, A. R.; Allegra, G.; Famulari, A.; Meille, S. V. Langmuir 2006, 369–374. (21) Bujdak, J.; Lyi, N. Chem. Mater. 2006, 18, 2618–2624. (22) Heinz, H.; Krishnamoorti, R. A.; Fatmer, B. L. Chem. Mater. 2007, 19, 59–68. (23) Theng, B. A. G. The Chemistry of Clay-Organic Reactions; John Wiley & Sons: New York, Toronto, 1974; pp 84-127. (24) Xiao, H.; Cezar, N. J. Colloid Interface Sci. 2003, 267, 343–351. (25) Chiang, C.-H.; Ishida, H.; Koenig, J. L. J. Colloid Interface Sci. 1980, 74 (2), 196–404. (26) Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 3rd ed.; Brooks/Cole, Thomson Learning, Inc.: Florence, KY, 2001; pp 29-30. (27) Young, S. K.; Jarrett, W. L.; Mauritz, K. A. Polymer 2002, 43, 2311–2320.
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