Sonochemical Insertion of Silver Nanoparticles into Two-Dimensional

Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center for. AdVanced Materials and Nanotechnology, Bar-Ilan...
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J. Phys. Chem. C 2007, 111, 11161-11167

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Sonochemical Insertion of Silver Nanoparticles into Two-Dimensional Mesoporous Alumina Sayan Bhattacharyya, Alexandra Gabashvili, Nina Perkas, and A. Gedanken* Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center for AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel ReceiVed: March 18, 2007; In Final Form: May 25, 2007

Nanocomposites of Ag nanoparticles/mesoporous γ-Al2O3 were synthesized by sonochemical treatment of the precursors. The sonicated product consisted of Ag nanoparticles dispersed in the bayerite [Al(OH)3]/ boehmite [AlO(OH)] matrix. Upon calcination under argon, the Ag nanoparticles were found to be incorporated in a mesoporous structure of γ-Al2O3. For a solid containing 3.7 wt % Ag nanoparticles, the nanoparticles remained on the surface of mesoporous alumina and hence BET surface area increased as compared to pristine γ-Al2O3, whereas for 10.5 wt % Ag nanoparticles, the surface area decreased. HRTEM studies corroborated this fact and showed that, at higher Ag concentrations, Ag nanoparticles blocked the pores and also increased the diameter of the pores of mesoporous alumina. The products with the 3.7 wt % silver concentration had uniform pores with narrow pore size distribution of the pores, although the TEM pictures indicated wormhole channel motifs. The formation of the mesoporous structure was governed by the templating behavior of the organic groups (mainly formic acid) attached to the alumina nanoparticles. The shape of the pores closely resembled the two-dimensional hexagonal mesoporous structure with the P6mm space group, as observed from small-angle X-ray diffraction experiments. Diffuse reflection optical spectra at 27 °C showed an absorption band (419-424 nm) due to the surface plasmon of Ag inside the ceramic matrix.

1. Introduction Over the years, the combination of ceramic matrix and metal nanoparticles for functional nanocomposites was known to be a potential candidate for optical, electrical, catalytic, thermal, and mechanical applications.1 Composites such as Pd/Al2O3,2 Au/mesoporous silica,3 Cr and Si doped alumina nanowires,4 Ag/hollow silica,5 and Ag/Al2O36 are among the systems that have been studied. In this context, the various forms of Ag/ alumina nanocomposites are studied for their electrical properties1 and linear and nonlinear optical properties,9 and are useful for a variety of purposes such as catalysis,7,8 floating-gate memory devices,10 and cutting tool inserts.11 However, to our knowledge, until now there have been no literature reports on the nanocomposites of silver with mesoporous alumina. Mesoporous materials (MSPM) were first discovered in 1992.12 They have a large surface area, nanometer-size pores, and a narrow pore size distribution. These materials have special importance in today’s science and technology since they are applicable to a variety of fields such as catalysis, gas purification, and the synthesis of nanomaterials.13 Many different methods have been employed for incorporating the nanoparticles into MSPM.14 Recently, the nanocomposites of silver halides and mesoporous alumina showed very high ionic conductivity.15 In this paper, we have studied the insertion of Ag nanoparticles into the mesoporous alumina matrix with the sonochemical treatment of the respective precursors. The advantage of sonochemistry for the insertion of nanoparticles into mesoporous structures has been discussed in the literature.14 In the sonochemical technique, bubbles are created by ultrasound radiation. When the bubbles collapse near a solid surface, they create microjets and shock waves. These energetic jets throw the asprepared nanoparticles onto the mesostructure at a very high * Corresponding author. E-mail: [email protected].

speed. Hence, the nanoparticles are spread homogeneously in the mesoporous matrix. In our case, the porous bayerite and the inserted Ag nanoparticles are synthesized simultaneously from two precursors. The subsequent annealing of the products converts the bayerite phase into mesoporous γ-Al2O3, with the Ag nanoparticles trapped inside. The mechanism of the formation of the pores is discussed herein. 2. Experimental Section 2.1. Synthesis. All reagents (chemically pure) were purchased from Aldrich and used as received. In this work, three products are taken into consideration: A0, γ-Al2O3; A1, γ-Al2O3 + 3.7 wt % Ag; and A2, γ-Al2O3 + 10.5 wt % Ag. The weight percent Ag was relative to γ-Al2O3 concentration. The synthesis of γ-Al2O3 has already been discussed in the literature.16 The insertion of Ag nanoparticles into γ-Al2O3 was done by an ultrasound-assisted reduction method. For the synthesis of A1 and A2 samples, 1 g of aluminum triisopropoxide (Al-isp) was placed in 30 mL of water, into which 8.5 mL of formic acid was slowly introduced, completely dissolving the alkoxide by constant stirring. The Al-isp solution was then transferred to a 100 mL sonication flask and Ar(g) purged the solution for 1 h to remove traces of O2/air. A 0.02 M AgNO3 solution (the Ag concentrations in the solution phases were 5.3 and 10.5 wt % for A1 and A2, respectively) was injected into the solution dropwise, followed by the dropwise addition of ethylene glycol (EG) (AgNO3 solution:EG ) 10:1). The reaction mixture was then irradiated with a high-intensity direct immersion ultrasonic horn (Ti horn, 20 kHz, 600 W) for 2 h at 30 °C under flow of an Ar-H2 mixture (95:5). A 20 mL volume of a 25 wt % aqueous solution of ammonia was added to the reaction slurry during the initial 5 min of sonication. The sonication flask was placed in a cooling bath, maintaining a constant temperature of 30 °C during the experiment. The resulting solid product was

10.1021/jp072152n CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007

11162 J. Phys. Chem. C, Vol. 111, No. 30, 2007 separated by centrifugation, thoroughly washed with ethanol, and dried under vacuum overnight. The reaction progress was monitored as a function of time, and 2 h was found to be the optimum sonication time. With shorter sonication times, the reaction was found to be incomplete. The dried product (AA, before calcination) was ground and calcined under an argon atmosphere at 700 °C for 4 h. The A0 sample was prepared under similar conditions, without the addition of a AgNO3 solution and ethylene glycol. 2.2. Characterization. The silver content was determined by volumetric titration with KSCN according to the Foldgard method, with the first step being the dissolution of the silver in HNO3.17 It is assumed that the HNO3 reacted with all the silver, including the silver inside the mesopores. The samples were identified by X-ray diffraction (XRD) analysis with a Bruker D8 diffractometer with Cu KR radiation. The oxidation state of the components was determined by XPS on a KRATOS AXIS HS spectrometer with Al KR radiation. The C 1s (binding energy (Eb) ) 285.0 eV) peak was chosen as a reference line for the calibration of the energy scale. Transmission electron microscopy (TEM) studies were carried out on a JEOL-JEM 100 electron microscope. High-resolution TEM (HRTEM) images were obtained by employing a JEOL-2010 device with a 200 kV accelerating voltage. The thermogravimetric analysis (TGA) data were collected on a Mettler TGA/SDTA 851 under N2 atmosphere at 10 °C/min heating or cooling rates. Nitrogen isotherms were measured at 196 °C, using a Micrometrics (Gemini 2375) analyzer, after outgassing 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 with the Halsey equation.18 The carbon, hydrogen, and nitrogen contents in the uncalcined and calcined products were measured by C, H, N analysis (Eager 200). The Fourier transform infrared (FTIR) measurements were carried out with a Varian spectrophotometer at room temperature with KBr pellets. Each pellet contained 3 mg of the sample and 200 mg of KBr (FTIR grade). The diffuse reflection optical spectra were recorded on a CARY 100 Scan UV spectrometer in a 300-900 nm wavelength range. 3. Results and Discussion The silver concentrations in the nanocomposites, as measured by volumetric titration, were obtained as 3.7 and 10.5 wt % with respect to γ-Al2O3 in the A1 and A2 samples, respectively. In A1, the number was less than the amount of Ag (5.3 wt %) in the solution phase in the sonication cell. Hence, it was not possible to insert all of the silver from AgNO3 into the alumina matrix in A1, unlike in A2 where all the silver is found in the solid product. Both the Ar-H2 gaseous mixture and the aqueous ammonia solution were used to obtain the Ag nanoparticles, since the absence of any one of them produced a lesser amount of silver. The HNO3 treatment was performed in boiling water for 50 min. This fully dissolved the metallic silver, and the titration with KSCN estimated the total Ag content in the nanocomposites. Figure 1 shows the XRD pattern of the products. The sonochemically prepared dried product, A2, before calcination (AA) displays only a silver phase. Ag crystallizes in the face-centered-cubic (fcc) structure with a Fm3hm space group and a lattice parameter a ) 4.0772 Å. The measured 2θ values match well the data in JCPDS 87-0720. It was reported previously that the sonication of an Al-isp solution gives an amorphous phase, which has been identified as aluminum hydroxide (bayerite).16 The bayerite/boehmite phase is not observed in Figure 1a. However, the bayerite/boehmite phase

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Figure 1. XRD patterns of (a) AA, (b) A0, (c) AA calcined in air, (d) A1, and (e) A2.

Figure 2. TGA trace of AA in N2 atmosphere. (inset) Calcined product at 550 °C in Ar(g) atmosphere.

transforms into γ-Al2O3 upon calcination at 700 °C for 4 h under air and/or inert atmosphere. Figure 1b shows the XRD pattern of the product (A0) from the calcination of a pure bayerite phase under argon atmosphere. γ-Al2O3 was crystallized in the fcc structure with a lattice parameter a ) 7.9392 Å. To study the decomposition of the sonicated products with the calcination temperature, TGA traces were performed, and a representative TGA trace of AA is shown in Figure 2. The TGA measurements were carried out up to 1000 °C. All three samples show a singlestep weight-loss pattern, with the weight loss ending at 550 °C. The bayerite phase converts to the alumina phase at ∼550 °C, losing water molecules. The relative cumulative weight losses are 37%, 38%, and 39% for A0, A1, and A2 before calcination. These values approximately match ∼35% of the weight loss of three water molecules from 2 mol of bayerite to give γ-Al2O3. This slight difference might be due to the significantly less amount of absorbed water in the sonicated products. However, from Figure 2 (inset) it is seen that the calcined product at 550 °C contains a crystalline phase. Looking at the changes in the XRD of samples calcined at 550 and 700 °C, we believe that the 550 °C sample is not fully crystallized and still contains amorphous alumina. Hence, 700 °C is observed to be the appropriate temperature at which to obtain the crystalline phases. We have performed calcination experiments both under air and under inert atmosphere. The Ag nanoparticles in the sonicated product (AA) might get oxidized to Ag2O under air. However, even if this happens, the Ag2O decomposes to Ag and O2 at temperatures >100 °C. Another report claims that Ag2O decomposes to Ag during the sintering process at ∼240 °C.1 However, in our case the calcination process under air atmo-

Ag Nanoparticle Insertion into Mesoporous γ-Al2O3

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Figure 3. (a) TEM micrograph of AA, (b) Ag particle size distribution in AA, (c) TEM micrograph of A1, and (d) TEM micrograph of A2. The insets in (c) and (d) display a distinct view of the Al2O3 nanoparticles.

sphere yields Ag2O and γ-Al2O3 (Figure 1c). Ag2O crystallizes in the monoclinic structure with lattice parameters a ) 5.8592 Å, b ) 3.4842 Å, and c ) 5.4995 Å. However, calcination under argon atmosphere at 700 °C for 4 h yields the desired Ag and γ-Al2O3 phases (Figure 1d,e). The XPS results of the representative A2 sample confirm the coexistence of silver and γ-Al2O3. The Ag 3d5/2 spectrum is very scattered and centered at 367.3 eV. The scattered spectrum also implies the presence of a lesser amount of Ag at the surface. In the literature, the Ag 3d5/2 peak for metallic silver has been reported at a low Eb of 367.6 eV19 and also at the high value of 368.6 eV.20 In our case, the lower Eb of the Ag 3d5/2 peak definitely confirms the presence of Ag in the zero valence state. The O 1s peak is sharp and centered at 531.5 eV, which closely approximates the O 1s Eb value of Al2O3 in the literature.21 The Al 2p3/2 peak corresponding to γ-Al2O3 is centered at 74.3 eV. This is the typical Eb value of Al2O3 reported in the literature.21-23 The TEM images of the AA, A1, and A2 samples are shown in Figure 3. It is seen that the Ag nanoparticles are uniformly dispersed in the amorphous bayerite/boehmite matrix for AA (Figure 3a). It is also seen that a few darker particles (Ag) are distributed within the lighter particles (bayerite/boehmite), both of them having nearly the same size. The Ag particles are crystalline (as observed from XRD) and nearly spherical. The average particle diameter (D) deduced from Figure 3b with Scion software is 4.2 nm with σ(D) ) (0.5 nm. The TEM images of A1 and A2 samples are shown in parts c and d, respectively, of Figure 3, where we observe the porous structure typically detected for mesoporous materials.24 The long-range order in the pore structure is not very clear. Rather, the samples exhibit wormhole channel motifs. The γ-Al2O3 nanoparticles are 4.2 ( 0.2 nm in A1 and 5.0 ( 0.5 nm in A2. Moreover, the pore size is slightly larger in A2, ∼5 nm, than in A1. Although the pore size distribution in both samples is reasonably uniform, the distribution appears to be more ordered in A1 than in A2. The Ag nanoparticles are not observed in the TEM pictures of A1 and A2. The change in the pore size could be triggered by other factors such as EG concentration and pH. In our work, these parameters are kept constant in all the samples. Hence, the change in pore size is explained only on the basis of the variation of Ag concentration in the subsequent discussions. The HRTEM pictures are investigated in order to visualize the pore structure and the position of silver nanoparticles within the mesoporous structure. Figure 4a shows the sonicated product

Figure 4. HRTEM images of (a) A0 (before calcination), (b) A1, (c) A2 (showing a single pore), (d) arrangement of sheets of alumina nanoparticles in the mesporous structure formation in A2, and (e) an individual Ag nanoparticle within the mesoporous structure in A2. (f) SAED pattern of A2.

of A0 before calcination. It can be seen that the bayerite/ boehmite phase consists of micropores ∼2 nm in diameter. The pores are distributed uniformly over the whole matrix and are in accordance with the following BET results. Figure 4b shows the pore structure of A1, where the pore is observed to be around 3.7 nm. The lattice fringes are observed on certain alumina particles, but are not seen clearly on others. This unclear image is due to a coating of the organic ligands on the top of the nanoparticles. This is clear from Figure 4c, which shows a single pore in A2. Here the lattice fringes are not observed, due to a thicker coating of the organic layer (investigated by later FTIR studies). From this picture, the pore diameter is estimated to be 5.5 nm. However, clear lattice fringes are observed in Figure 4d, where the alumina nanoparticles arrange themselves around the mesopores. Here both the alumina nanoparticles and the pores are ∼5 nm. Hence, the pore diameter in A2 is higher than in A1. The distance between the two lattice planes in γ-Al2O3 is 0.458 nm, according to the (111) plane in JCPDS 50-0741. The thickness of the pore walls is not evident in the images. The alumina nanoparticles are nearly hexagonal in shape. The silver nanoparticles are not on the same plane, but lie on different sheets, one over the other. The formation of the alumina sheets might occur during the calcination process. Thus, the HRTEM images show a two-dimensional (2-D) mesostructure of alumina, which is not commonly dealt with in the literature. Figure 4e shows the position of the Ag nanoparticles within the mesoporous structure in A2. The arrow indicates an individual silver nanoparticle. The Ag nanoparticle lies partially inside the mesopore, blocking it halfway. The lighter contrast of the Ag nanoparticle is due to the position inside the pore

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Figure 5. N2 adsorption (filled symbols) and desorption (open symbols) isotherms.

structure, below the alumina nanoparticles. The lattice fringes of the Ag nanoparticles are situated 0.235 nm apart and correspond to the (111) plane of Ag [JCPDS 87-0720]. Figure 4f shows the selected area electron diffraction pattern (SAED) of Figure 4e and shows the particular diffraction points. The 3-5 nm pore size illustrated in the TEM and HRTEM pictures is not the only evidence claiming that γ-Al2O3 is mesoporous in structure. This is also revealed from the N2 adsorption/desorption isotherms (Figure 5). For the A0, A1, and A2 samples, the nitrogen sorption displays a type IV isotherm. The hysteresis loop is of the H2 type, which is a clear signature of its mesoporous nature.25 H2 loops indicate that the distribution of pore size and shape is not well-defined. The nature of the hysteresis loops also indicates that the mechanisms of the adsorption and desorption processes occurring inside the pores are different. The AA sample shows the type H4 loop, which can be attributed to the narrow slitlike pores. The bayerite/ boehmite phase might also be microporous. Table 1 gives the values of the surface area, pore volume, pore diameter, and pore size distribution of all the samples. The BET measurements were repeated three times, and the results were found to be precise and reproducible. The pores of the mesoporous γ-Al2O3 have a narrow and uniform size distribution, as is seen in Figure 6. The incorporation of Ag has an interesting influence on the porosity and the pore size of the mesoporous γ-Al2O3. The surface area increases drastically for A1, as compared with A0, and then reduces abruptly for A2 (inset, Figure 6). For A0, the surface area, 130(1) m2/g, resembles the pure mesoporous alumina, without any silver. The pore diameter remains almost equal for A0 and A1 despite the incorporation of 3.7% Ag and is between 3.3 and 3.5 nm. When more silver is added in A2, the surface area decreases, while the pore diameter increases by ∼2 nm. For the sonicated product (AA), the surface area is the smallest, compared to the calcined products, since the bayerite/boehmite phase might contain the micropores but does not have a mesoporous structure. The appreciable surface area [121(1) m2/g] is due to the uniform dispersion of Ag nanoparticles on the bayerite/boehmite phase. The pore volume of AA is sufficiently low, while the pore diameter is comparable to that of the calcined products.

Figure 6. Pore size distribution. (inset) Variation of surface area and pore volume with Ag concentration.

Figure 7. Small-angle XRD for (a) A0, (b) A1, and (c) A2. (inset) Enlarged view of the small-angle region in A1.

The mesoporosity of γ-Al2O3 is again confirmed by smallangle X-ray diffraction (SAXRD) measurements (Figure 7). The SAXRD pattern of A1 shows three peaks, where the main peak (2θ ) 0.92°) corresponds to (100) and the other two weak peaks at 2θ ) 1.09° and 1.80° correspond to (110) and (200) reflections, respectively (Figure 7, inset), and are in close approximation to a 2-D hexagonal mesoporous structure with the P6mm space group. This is in accordance with the three peaks usually observed in mesoporous alumina.26 A d100 distance of 9.6 nm is calculated using the Bragg equation, a distance that might signify the distance between the two pores, which includes the two radii, twice the thickness of the wall, and the space between the pores. However, the hexagonal mesoporous structure is not evident from the TEM images; rather a nonuniform distribution of the mesopores is observed with wormhole-like channel motifs. In addition, the distance between two pores may not be constant throughout. It is noteworthy that when the Ag concentration is increased, the intensity of the main peak (100) decreases, and no appreciable shift in the peak position is observed. However, with the increase in the pore

TABLE 1: Pore Characteristics sample name A0 AA A1 A2

BET surf. area (m2/g)

pore vol (cm3/g)

pore diam (D) (nm)

fwhm of D (nm)

130 ( 1 121 ( 1 232 ( 2 141 ( 1

0.17 ( 0.02 0.08 ( 0.02 0.26 ( 0.01 0.24 ( 0.01

3.5 ( 0.1 3.0 ( 0.2 3.3 ( 0.2 5.1 ( 0.3

0.6 ( 0.0 0.6 ( 0.1 0.3 ( 0.0 1.4 ( 0.1

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Figure 8. FTIR spectra of AA, A1, and A2.

diameter by 1.8 nm (BET measurements) from A1 to A2, the (100) peak is not affected, signifying that the reproducible distance is unchanged. This is possible if we assume that modifications in the pore wall thickness and the space between the pore walls have occurred. The BET and SAXRD results, arising from the mesoporous alumina, can be explained with the help of a pore formation mechanism drawn from C, H, N analysis, FTIR measurements, and possible schematic diagrams. The uncalcined products contain carbon, hydrogen, and nitrogen, with the nitrogen concentration being the smallest. After calcination, nitrogen is not present. Moreover, after calcination the carbon content decreases by 93-95%. The C and H sources are the initial reactants formic acid, Al-isp, and EG, whereas the N source is the aqueous ammonia solution. FTIR measurements are carried out for the uncalcined and calcined products. Figure 8 shows the FTIR spectra for AA, A1, and A2. In AA, the band at 3235 cm-1 can be attributed to the stretching vibration of the free, intra-/intermolecular bonded OH group from EG. The shoulder bands at 2880-3068 cm-1 could be assigned to the N-H stretching of the amide group. It can be pointed out that, during the sonication process, aqueous ammonia reacts with formic acid to form formamide (HCONH2). The peaks at 2336 and 2361 cm-1 can be attributed to the HCOOH+ species formed during sonication. The peak at 1579 cm-1 can be due to either the C-O symmetrical stretching of -COOH or to the coupling of NH deformation and C-N stretching modes of the amide group. The shoulder peaks at 1344-1458 cm-1 represent the C-O symmetrical stretching mode of -COOH. The peaks at 1042 and 1072 cm-1 are due to the formyl radical. The small peak at 881 cm-1 is due to Al-O stretching. In A1 and A2, the bands and peaks due to OH and amide groups disappear. Additional peaks appear at 2900 and 2987 cm-1, which are attributed to O-H stretching of carboxylic acid dimers. The peaks at 2336 and 2361 cm-1 (HCOOH+) become stronger. The peaks due to the C-O symmetrical stretching of the -COOH group become scattered, whereas the peak at 1048 cm-1 (formyl radical, HCOO-) is distinct. Initially, when formic acid was added to Al-isp, it hydrolyzes completely to form (HCOO)3Al. Hence, in the sonication cell, the (HCOO)3Al species undergoes transformation to Al(OH)3/ AlO(OH). However, in this process, one or more formyl radicals (HCOO-) remain attached to aluminum with the help of weak van der Waals interactions, as schematically shown in Figure

Figure 9. Schematic representation of organic ligands and pore structures.

9a. The formamide might also get attached to aluminum with a similar weak bond, as shown in Figure 9b. Moreover, in the presence of EG, the OH groups remain free or attached to Al with intra-/intermolecular bonds (Figure 9c). All the different weakly bonded organic radicals separate Al(OH)3/AlO(OH) nanoparticles from each other and prevent agglomeration. A self-assembled structure leads to the formation of micropores in the uncalcined products, both in the presence and in the absence of Ag. The Al(OH)3/AlO(OH) nanoparticles are held together with weak attractive interaction, such as H-bonding and van der Waals forces. The explanation for the formation of the pores is in good agreement with the HRTEM image of the uncalcined product of A0 (Figure 4a). When the sonicated products are calcined, the organic ligands decompose incompletely, leaving the pore structure more defined. The carboxylic acid dimers/polymers observed in FTIR studies are mainly responsible for keeping the alumina nanoparticles apart and maintaining the pore structure (Figure 9d). The formic acid monomers form chains in the form of polymers and remain interlinked with one another. A cohesive attraction results from the weak intermolecular forces and keeps the alumina nanoparticles attached to the carboxylic acid network (Figure 9e). This preserves the mesoporous structure between the alumina nanoparticles. The organic network structure remains inside the pores, as well as outside. The pores are interlinked and are separated by one or more alumina nanoparticles. Thus, the mesoporous structure of γ-Al2O3 is governed by the selfassembly process of formic acid and formyl acid radicals. Recently, citric acid has been shown to play a major role in the formation of pore structure in mesoporous titania/silica.27 At this point, it is important to clarify a few points (Table 1) as to what happens when Ag is incorporated into the mesoporous γ-Al2O3 matrix: (a) The pore diameter remains nearly constant for A0 and A1 (3-3.5 nm), but increases to 5.1 nm for A2. (b) The distribution of pore size is narrow for A0 and A1, but a wide distribution is observed for A2. (c) The pore volume increases from A0 to A1, and decreases slightly for A2 (inset,

11166 J. Phys. Chem. C, Vol. 111, No. 30, 2007 Figure 6). (d) The BET surface area increases drastically from A0 (Ag ) 0%) to A1 (Ag ) 3.7%) and then decreases for A2 (Ag ) 10.5%) (inset, Figure 6). In the sonochemical synthesis, the silver nanoparticles are formed by the generation of H• and OH• radicals from water and EG molecules by the absorption of ultrasound. The H• radicals act as reducing species and trigger the Ag+ f Ag0 reduction.28 However, side reactions (not related to sonochemistry) occur as follows: (I) HCOOH + NH4OH f HCOONH4 + H2O; (II) HCOONH4 + AgNO3 f HCOOAg + NH4NO3; (III) 2HCOOAg f 2Ag + HCOOH + CO2. Thus, Ag nanoparticles are formed by the sonication process, as well as by the result of reaction III. When the sonicated products are calcined, the organic framework is broken down, and the Ag nanoparticles are placed between the alumina nanoparticles, blocking the pores partially or fully. There is also a partitioning of the Ag nanoparticles between the mesopores and the surface of γ-Al2O3 (Figure 4e). When the amount of Ag nanoparticles is only 3.7% (A1), the deposition of a lesser amount of silver does not affect the pore structure, and the pore diameter does not change appreciably from A0 to A1. When the amount of silver is increased to 10.5%, a larger amount of Ag nanoparticles blocks the pores partially/fully. In due course, the alumina spherical nanoparticles around the pore structure are held together less firmly by the intermolecular hydrogen bond/van der Waals forces by the formic acid polymeric network. Hence, in A2 the larger amount of Ag nanoparticles is inserted partially onto the mesopores and can force the alumina nanoparticles to increase the pore diameter nonuniformly. The amount of silver is only 10.5%, and hence, only a limited number of pores are increased in diameter, leading to a wide distribution in the pores’ diameters. This explains (a) and (b). In addition, due to the microjet and cavitation phenomena in the sonochemical process, the injection of Ag nanoparticles may create additional pores in the Al(OH)3/AlO(OH) structure, pores that are retained even after calcination. This explains the increase in pore volume from A0 to A1. With the partial blocking of the pores by silver in A2, the pore volume decreases slightly. This explains (c). The drastic increase in surface area [102(1) m2/g] from A0 to A1 is due to the presence of Ag nanoparticles (with a high surfaceto-volume ratio) on the surface of mesoporous γ-Al2O3 in A1. This signifies that 3.7% of the Ag nanoparticles is equal to the same mass fraction, and in 1 g of sample, 0.037 g of Ag represents 102 m2, i.e., ∼2757 m2/g. Previously it was observed that the deposition of Fe/Fe oxide nanoparticles on the surface of silica and C-spherules increases the surface area appreciably.29,30 The decrease in the surface area from A1 to A2 is explained by (i) the increase in the size of γ-Al2O3 nanoparticles from 4.2 ( 0.2 nm in A1 and 5.0 ( 0.5 nm in A2 and (ii) the partial blocking of the mesopores due to a larger amount of silver. The larger amount (10.5%) of ∼5 nm Ag nanoparticles (in A2) is sufficient to block the 5.1 nm pores and decrease the surface area (Figure 9f). The decrease in the surface area upon the insertion of nanoparticles inside the pores of the matrix has already been studied by our group.31 Hence, it can be inferred that the Ag nanoparticles strongly influence the mesoporous structure of γ-Al2O3. This is a novel finding in this research work. The characterization of the Ag nanoparticles inside alumina is done by diffuse reflection optical spectroscopy (DRS). Figure 10 shows the DRS spectra for all the samples at 27 °C. There is a broad absorption band in the 350-650 nm range. According to the Mie theory, the absorption band arises due to the excitation of the surface plasmon vibrations of Ag inside the ceramic matrix.32 The Ag nanoparticles exhibit characteristic

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Figure 10. DRS spectra at 27 °C.

surface plasmon bands centered at 419, 424, and 424 nm for AA, A1, and A2 samples, respectively. This optical property is sensitive to many factors such as concentration, size, shape of the particles, and the dielectric environment, i.e., interaction with the surrounding matrixes.32,33 A sharp surface plasmon resonance at ∼400 nm is observed in the case of well-dispersed uniform spherical Ag nanoparticles.34 In our case, the surface plasmon band of the Ag nanoparticles is slightly red-shifted and broadened. Similar observations have been reported in the literature.35-37 These spectral changes could be attributed to the interaction of the agglomerated Ag nanoparticles with the dielectric medium of the bayerite/alumina matrix. The DRS spectra of A1 and A2 are almost identical and indicate that the dielectric environments of Ag nanoparticles in A1 and A2 are almost the same. The peak for AA is slightly broader than those for A1 and A2, and this may be due to the difference in the dielectric environments of bayerite and alumina matrixes in which the Ag nanoparticles are trapped. 4. Conclusions In conclusion, the nanocomposites of Ag and γ-Al2O3 are prepared via sonochemistry and the subsequent calcination under Ar(g) atmosphere at 700 °C for 4 h. TEM pictures indicate the uniform dispersion of Ag nanoparticles in the bayerite matrix (sonicated product AA), and wormhole channel motifs are observed for Ag-inserted mesoporous alumina. For 3.7 wt % Ag (A1), the BET surface area is more than that of γ-Al2O3 (A0) since a lesser amount of Ag nanoparticles stays on the surface. For 10.5 wt % Ag (A2) the surface area decreases again, since the Ag nanoparticles partially/fully block the pores, thus increasing the pore size. These facts are supported by HRTEM images. FTIR studies show that formic acid groups (polymers) are responsible for holding the alumina nanoparticles apart and maintaining their mesoporous structure. This paper reports on an interesting finding in the study of nanoparticle insertion in a mesoporous matrix, wherein the nanoparticles change the pore structure of the mesoporous matrix. The 2-D mesostructure is not a common feature, and only a very few such examples are known. Acknowledgment. The authors acknowledge the EC for financial assistance in carrying out this research work through the Napolyde Project, Contract No. NMP2-CT-2005-515846. Helpful discussions with Prof. Miron Landau (Ben-Gurion University) are duly acknowledged. Supporting Information Available: XPS spectra and electron paramagnetic resonance (EPR) data. This material is available free of charge via the Internet at http://pubs.acs.org.

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