J. Phys. Chem. C 2008, 112, 659-665
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ARTICLES Microwave-Assisted Insertion of Silver Nanoparticles into 3-D Mesoporous Zinc Oxide Nanocomposites and Nanorods Sayan Bhattacharyya 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: July 30, 2007; In Final Form: October 29, 2007
The nanocomposite of mesoporous ZnO and Ag was synthesized by microwave radiation in a 15 min reaction conducted under an argon atmosphere. The 3.5 ( 0.4 nm Ag particles were inserted into the pores, changing the pore structure. The particles are also present as clusters on the surface of ZnO. The hexagonal ZnO nanodisks were stacked one on top of the other for the Ag:ZnO molar ratio of 0.13, whereas for Ag:ZnO ) 0.22, the nanodisks join in the form of nanorods, where Ag clusters catalyze nanorod formation. The mesoporous structure in the current study was prepared with the help of argon gas and ethylene glycol. The FTIR studies revealed the presence of the organic framework structure retained in the final products. The 14 ( 2 nm hexagonal ZnO nanodisks are bound together by this organic framework, forming pores of 3.2 ( 0.1 and 18.6 ( 0.2 nm in diameter. The BET surface area increased from 40 ( 0 (pristine ZnO) to 51 ( 1 m2/g (Ag:ZnO ) 0.22) due to formation of Ag clusters on the surface of ZnO with the increase in Ag concentration. Raman spectroscopy experiments indicate the modified surface characteristics of ZnO due to the porous structure and the presence of carbon in the products. The diffuse reflection optical absorption spectra show the ZnO excitonic absorption and Ag surface plasmon bands. The room-temperature photoluminescence experiments show emission bands assigned to the band edge exciton transitions in ZnO, oxygen defects, surface-deep traps, and impurity energy levels.
1. Introduction The steady and fast growing field of nanoscience and nanotechnology requires the design and fabrication of new composite nanomaterials, which can exhibit new functionality based on optical and electrical properties that are proximally close to the two functionally different components.1 In this regard, the interplay of metal nanoparticles and the metal oxide materials is of central interest in terms of various technological applications such as catalysts, gas sensors, and optical, electronic, and magnetic devices.2 Recently, research activities have been devoted to nanocomposites of silver and zinc oxide.3 Zinc oxide (ZnO) is an n-type direct band gap semiconductor (3.37 eV at room temperature) with interesting luminescent, piezoelectric, and photoconducting properties as well as a biosafe and biocompatible nature and antibacterial properties.4,5 ZnO nanomaterials have been reported in various forms such as nanohelices, nanocolumns, nanotubes, nanorods, nanowires, nanorings, nanobelts, and nanocrystals and as a wide range of porous materials.5,6 The mesoporous ZnO nanocrystals in the form of polyhedral drums, cages, and shells are useful for catalytic activities.7 Although there are a large number of reports on ZnO-Ag nanocomposites, until now, to the best of our knowledge, there have been no reports on the combination of silver and mesoporous ZnO as a composite material. The * To whom correspondence should be addressed. E-mail: gedanken@ mail.biu.ac.il.
mesoporous materials with a large surface area and high surfaceto-volume ratio are useful for a variety of applications.8 Hence, it is worth studying the behavior of the metal nanoparticles toward the pore structure of the host mesoporous material. In this paper, microwave radiation is applied to the synthesis of Ag-incorporated mesoporous hexagonal ZnO nanocomposites and nanorods. In the microwave chemical reactions, the reactant molecules undergo excitation with electromagnetic radiation. Microwave heating is able to heat the target compounds without heating the entire reaction vessel.9 A heterogeneous system (consisting of different precursor materials) undergoes selective heating in different parts of the material, leading to temperature gradients or domains. The hot spots are created due to the presence of zones with a higher temperature than the bulk of the liquid. These hot spots are subjected to a heat-transfer process between the domains, leading to a highly accelerated reaction rate. Hence, the mesostructure and as-prepared nanoparticles are synthesized simultaneously in a very short time, enabling the nanoparticles to be trapped on the mesoporous matrix. In our case, the Ag nanoparticles are found to be trapped inside the ZnO mesopores. They change the pore structure and also act as a catalyst in the formation of the nanorods. The mechanism of the formation of the pores is discussed herein with the help of FTIR studies. The porous structure is formed in the absence of any templating- or structure-directing agent, and hence, pore formation is a self-assembled process. The optical properties are discussed.
10.1021/jp0760253 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/28/2007
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2. Experimental Section 2.1. Materials and Characterization. All reagents were of the highest purity. Zinc(II) acetate dihydrate (C4H6O4Zn‚2H2O; Acros Organics 98%), silver nitrate (AgNO3; Aldrich), and ethylene glycol (EG; Aldrich) were used without further purification. The XRD measurements were carried out with a Bruker D8 diffractometer having Cu KR radiation. An inductively coupled plasma atomic emission spectrometer (ICP-AES) model “Spectroflame Modula E” from Spectro, Kleve, Germany, with a standard cross-flow nebulizer was used to carry out the ICP measurements. Samples were dissolved in dilute nitric acid. Analysis was conducted on portions of the solution, vs certified standards from Merck, by ICP-AES. The carbon contents were measured by C, H, and N analyses (Eager 200). 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 Barret-Joyner-Halenda (BJH) model with the Halsey equation.10 The high-resolution scanning electron microscopy (HRSEM) of the product was carried out on a JEOL-JSM 840 scanning electron microscope operating at 10 kV. Highresolution TEM (HRTEM) images were obtained by employing a JEOL-2010 instrument with a 200 kV accelerating voltage. The 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). An Olympus BX41 (Jobin-Yvon-Horiba) Raman spectrometer was employed using the 514.5 nm line of an Ar-ion laser as the excitation source to analyze the nanomaterials. The diffuse reflection spectroscopy (DRS) spectra were recorded on a CARY 100 Scan UV spectrometer in the 300-900 nm wavelength range. The PL spectra were recorded on an AmincoBowman-Series-2 Luminescence Spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The microwave-assisted reactions were carried out in a Kenwood-900 W domestic microwave oven, with a 2.45 × 109 Hz working frequency, modified with a refluxing system. In all of the experiments, the microwave oven worked in the following cycling mode: on for 21 s, off for 9 s, with the total power always at 900 W. This cycling mode was chosen in order to reduce the risk of superheating the solvent and prevent the violent “bump” boiling of the solvent. All reactions were conducted under a flow of argon. 2.2. Preparation of the Nanocomposite. A required stoichiometric amount of AgNO3, according to the Ag:ZnO molar ratio of 0.02, 0.05, 0.15, and 0.22 (in the solution phase), was dissolved in 1 mL of double-distilled water. A 2 g amount of zinc(II) acetate and 45 mL of EG were added to the aqueous AgNO3 solution in a 100 mL round bottomed flask and fitted to the refluxing system inside the microwave oven. The system was purged for 10 min with argon gas prior to switching on the microwave reactor. The reactions were conducted for 15 min under argon. In the postreaction treatment the product was centrifuged once at 9000 rpm with the mother liquid to separate the powder from the liquid and then washed a few times with water and ethanol at 20 °C and 9000 rpm. The product was dried overnight under vacuum. The pristine ZnO was prepared in a similar way without addition of the AgNO3 solution. 3. Results and Discussion Figure 1 illustrates the XRD patterns of the ZnO-Ag nanocomposite products. The XRD patterns were composed of ZnO and Ag phases. The ZnO and Ag phases matched their
Figure 1. XRD pattern of the nanocomposites.
Figure 2. (a) N2 adsorption/desorption isotherms: filled symbols, adsorption; open symbols, desorption. (b) Pore size distribution.
JCPDS cards nos. 89-0510 and 87-0720, respectively. The relative XRD intensity of the Ag phase increased with the increase in the amount of silver in the nanocomposites. The inductively coupled plasma (ICP) experiments confirmed the stoichiometry of the ZnO-Ag nanocomposite solid products according to the Ag:ZnO molar ratio of 0, 0.02, 0.05, 0.13, and 0.22 and will be designated as A0, A1, A2, A3, and A4, respectively, in the subsequent discussions in the paper. The molar ratio of Ag:ZnO in the solid state was very close to the solution ratio. This indicates that both precursors underwent their reactions (reduction and decomposition) at the same percentage. The N2 adsorption/desorption isotherms of the products are shown in Figure 2a. The nitrogen sorptions display a type IV isotherm.11 For A0, A1, and A2, the hysteresis loops are of the H3 type and do not exhibit any limiting adsorption at high p/p0. An H3-type hysteresis loop is mainly observed with an assemblage of particles forming an aggregate, giving rise to slitshaped pores, which might consist of a mixture of micropores and mesopores. For A3 and A4, the hysteresis loop is of an H2 type, which is a clear signature of its mesoporous nature. H2 loops indicate that the distribution of pore size and shape is not
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TABLE 1: Pore Characteristics sample name
BET surface area (m2/g)
pore volume (cm3/g)
pore diameter (D) (nm)
A0 A1 A2 A3 A4
40 ( 0 25 ( 1 35 ( 1 47 ( 0 51 ( 1
0.19 ( 0.01 0.24 ( 0.01 0.15 ( 0.01 0.12 ( 0.01 0.07 ( 0.01
3.2 ( 0.1, 18.6 ( 0.2 3.1 ( 0.0, 27 ( 0.3 3.1 ( 0.1, 8.1 ( 0.1 3.1 ( 0.1, 10.0 ( 0.1 3.1 ( 0.1, 5.5 ( 0.2
well defined. The nature of the hysteresis loops also indicates that the mechanism of the adsorption and desorption processes occurring inside the pores is different. Table 1 gives the values of the surface area, pore volume, and pore diameter of all the samples. The pores of the mesoporous structures have a mixture of small (3.1-3.2 nm) and big (5.5-27 nm) pore sizes, as seen in Figure 2b. The BET measurements were repeated three times, and the results were found to be precise and reproducible. The mesoporosity of ZnO is again confirmed by the smallangle X-ray diffraction (SAXRD) measurements (Figure 3). The SAXRD pattern of the products shows an asymmetric peak structure or a distinct shoulder on one side of the peak in the 2θ ) 2.08-2.31° range. The pore structure is in close approximation to a 2D hexagonal mesoporous structure with the P6mm space group, and the SAXRD peak corresponds to a (100) reflection. The SAXRD experiments reveal only the smaller pores, not the larger pores. The d100 distances of 4.1 ( 0.1, 3.8 ( 0.1, 3.9 ( 0.3, 4.0 ( 0.2, and 4.3 ( 0.1 nm are calculated using the Bragg equation for A0, A1, A2, A3, and A4, respectively, taking into consideration the highest intensity peaks. However, the presence of the peak asymmetry and/or shoulder peaks clearly suggests a bimodal distribution of the smaller pores. When the Ag concentration is increased, the intensity of the SAXRD peak (100) does not show any particular trend, although a shift in the peak position is observed. This signifies that with incorporation of Ag the reproducible distance (pore wall thickness and space in between the pore walls) is slightly modified. The HRSEM images of the products are shown in Figure 4. For A0 and A1, large aggregates are observed consisting of small spherical nanoparticles sized 30 ( 3 and 25 ( 1 nm, respectively. Similarly, for A2, the 12 ( 1 nm particles cling to each other to form balls of 350 nm in diameter. These aggregated structures indicate the origin of the porous nature of the materials. For A3 (Figure 4d,e) it is observed that the hexagonal nanodisks are stacked one on top of the other, while in case of A4 (Figure 4f,g) the hexagonal disks are found to be joined together to form nanorods with rough top-face and side edges. In A3, the 95 ( 5 nm hexagonal disks are stacked over each other to an approximate height of 165 nm. In A4, the nanorods have a diameter of 104 ( 9 nm and an aspect ratio of 2.8. Some of the nanorods are found to have holes ∼10 nm in diameter. The templated back-to-back stacking of ZnO hexagonal rings (disks with holes) is reported in the literature.12 However, in our case, the nanorod structures are composed of much smaller hexagonal or leaf-shaped nanodisks, as will be evident from HRTEM studies. The precise composition of the nanorods in A4 was confirmed by employing a highly sensitive, wave-dispersive X-ray analyzer (WDX), which is coupled to the HRSEM instrument. The sample was dispersed in ethanol and spread onto the silicon wafer. The selected area for the elemental dot mapping (WDX) is shown in Figure 4h. The Si, Zn, O, and Ag mappings are presented in Figure 4i-l, respectively. As expected, the intensity of the Si line drops in the sample region. The Zn, O, and Ag signals are detected within the position of the nanorods. The Ag signal is sufficiently strong,
Figure 3. Small-angle XRD of the nanocomposites. (inset) Variations in the surface area and pore volume with Ag concentration.
which implies that the Ag nanoparticles lie on top of the ZnO nanorods, probably at the broken and rough top faces and side edges. The inhomogeneous distribution of Ag nanoparticles on the ZnO nanorods can be related to a site-selective positioning of silver on ZnO. This is triggered via the small lattice mismatch of Ag on ZnO at the respective crystallographic plane.13 Hence, the Ag nanoparticles might form clusters at different positions of the ZnO nanorods, whereas nucleation of silver can take place at the energetically favored plane [e.g., the (101) plane of ZnO]. These Ag clusters act as nucleation sites for the growth of ZnO nanorods.14,15 Stacking of the nanodisks in A3 and formation of the hexagonal nanorods through the joining of the disks in A4, with an increase in the concentration of silver, clearly indicate that Ag is solely responsible for formation of ZnO nanorods. The TEM/HRTEM images in Figure 5 corroborate the morphologies observed in the HRSEM images. For A0, A1, and A2 (Figure 5a-c), aggregated balls with diameters of 230, 120, and 420 nm, respectively, are observed. The structures indicate wormhole channel motifs with no long-range order of the pore structure. The balls of A0, A1, and A2 are composed of smaller particles, 25 ( 2, 21 ( 2, and 13 ( 4 nm, respectively (Inset, Figure 5a-c). In A3, two balls with a diameter of ∼300 nm are found to lie on top of each other (Figure 5d). The balls in A3 are composed of hexagonal disks having a diameter of 14 ( 2 nm (Figure 5e). The 2D lattice fringes in the HRTEM image in Figure 5f signify the crystallinity of the ZnO nanostructure, and the distance between two fringes is 0.25 nm [(101) reflection in JCPDS card no. 89-0510]. The TEM image of A4 shows ellipsoidal structures (diameter ≈ 135 nm, length ≈ 270 nm) having a rough surface (Figure 5g). A closer view of the edge of the ellipsoidal structures reveals the leaf-shaped nanostructures having a diameter of ∼15 nm (Figure 5g, inset, h). The 2D lattice fringes of a single leaf-shaped nanostructure indicate the (101) reflection of the pure ZnO phase. Figure 5i shows the Ag nanoparticles (3.5 ( 0.4 nm) located on the side of a nanoleaf. The Ag nanoparticles can be clearly distinguished by its dark contrast. The selected-area electron diffraction (SAED) pattern of the region shown in Figure 5i shows the characteristic reflections of a wurtzite structure for ZnO and the cubic phase of Ag (Figure 5j), which was also found in the XRD pattern. Although formation of ZnO nanorods is governed by the catalytic action of Ag nanoparticles, the mesoporous nature of ZnO is achieved in the presence of argon gas and ethylene glycol. In the literature, polyethylene glycol has been used as a template to create porous structures.16 Porous zinc oxide
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Figure 4. HRSEM images of (a) A0, (b) A1, (c) A2, (d) A3, (e) A3 (another view), (f) A4 (lower magnification), and (g) A4 (higher magnification). (h) Selected image of A4 for X-ray dot mapping. X-ray dot mapping for (i) Si substrate, (j) Zn, (k) O, and (l) Ag on the selected 4h image.
nanomaterials were generally synthesized with the help of structure-directing agents or templates, such as triblock copolymers,17 while there is only one report on the template-free synthesis of porous ZnO.7 FTIR measurements were performed to investigate the mechanism of formation of pores in the asprepared ZnO. C, H, and N analyses show that the as-prepared products contain, on an average, 1.9 ( 0.5% carbon and 0.5 ( 0.1% hydrogen. Figure 6 shows typical FTIR spectra of the A0 and A4 products with nearly similar bands. The weak broad band at 3500-3900 cm-1 arises from the hydrocarboxyl and bidentate carbonate groups. The bands at 2850-2920 cm-1 are attributed to C-H stretching. The bands at and around 1398 and 1543 cm-1 resemble the symmetric (νsym) and asymmetric (νasym) CdO stretching in acetate groups, respectively. Detection of these bands indicates that the acetate groups are in the ionized form (COO-), where the two C-O bonds are identical with 1.5 bond order.18 However, when the carboxylic acids and their salts form complexes with metals, the frequency shifts (∆ν ) νasym - νsym) from 1400 and 1600 cm-1 and the shape of the stretching bands of the COO- group also change. In our case, this region in the FTIR spectra is noisy to determine the shift exactly. The bands at the 1200-1700 cm-1 region can be assigned to vibrational modes associated with esters that might be formed during the process. The bands around 1000 cm-1 arise due to the deformation and rocking modes of the methyl group.19 The bands at and below 700 cm-1 are typical for Zn-O stretching vibrations. Zinc acetate under microwave radiation in vacuum or inert atmosphere (argon) results in loss of acetic anhydride,20 leaving basic zinc acetate Zn4O(CH3CO2)6, which is a cluster compound having a tetrahedral structure. In the
absence of water in the media, esterification leads to formation of Zn4O(CH3CO2)6, which subsequently forms the (CH3CO2)2O entities. In the presence of water, the solvent moieties participate in the process and other esters are formed, which is manifested from the FTIR spectra. Anhydrous zinc acetate consists of a polymeric structure with zinc in a tetrahedral O4 coordination.21 The tetrahedrons are interlinked with each other by the acetate groups, and the zinc atoms are in octahedral coordination with the bidentate acetate groups. It is well known from the literature that metal-organic frameworks with multidentate linkers give rise to stable porous network-like structures.22 The water content in the zinc(II) acetate dihydrate and ethylene glycol solvent plays an important role in the synthesis of ZnO nanoparticles and helps to control formation of Zn2+ species. It is well known that a parasitic phase like a layered basic zinc acetate Zn5(OH)8(CH3COO)2‚2H2O phase, also known as hydroxy double salt (ZnHDS), can be prepared in the presence of water at 40-70 °C.23 Since the morphology of the placket-shaped Zn5(OH)8(CH3COO)2‚2H2O phase is preserved during its topotactic transformation into ZnO upon firing, formation of this species along with ZnO is likely to occur during microwave irradiation. However, in our case the characteristic (00l) diffraction band of the Zn5(OH)8(CH3COO)2‚2H2O phase is not detected in the XRD pattern below 2θ ) 20°. The microwave reaction in argon does not completely decompose the zinc(II) acetate dihydrate to ZnO but rather retains a small fraction of basic zinc acetate with a bidentate porous framework. The formed ZnO nanodisks are bound together in this organic framework forming a ball (Figure 5), and the voids in between the nanodisks generate the mesoporous structure. Interestingly, when microwave ir-
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Figure 6. FTIR spectra.
Figure 5. TEM images of (a) A0, (b) A1, (c) A2 (insets showing the smaller particles), (d) A3, and (e) A3, showing the hexagonal nanodisks. (f) HRTEM image of A3. (g) TEM image of A4 (inset shows the leafshaped structure). (h) HRTEM image of A4. (i) Ag nanoparticles in A4. (j) Selected-area electron diffraction (SAED) pattern of the region in Figure 5i.
radiation was carried out with zinc acetate under air, the zinc(II) acetate was decomposed completely to ZnO, retaining only a negligible amount of an organic porous framework, and the BET surface area was 6 m2/g. Hence, we can say that argon gas is the key factor in creating disorder and maintaining the porous framework in addition to ethylene glycol. From the BET experiments, the surface area of the pristine ZnO is 40(0) m2/g. With addition of silver up to an Ag:ZnO
molar ratio of 0.05, the surface area decreases. Higher concentrations of Ag lead to an increase of the surface area (Figure 3, inset). Thus, incorporation of Ag has an interesting influence on the porosity and pore size of the mesoporous ZnO. The smaller pores do not change appreciably in size, while the larger pores decrease in size with the increase in Ag concentration, which indicates the partial blocking of the pores of mesoporous ZnO with silver nanoparticles. Moreover, the pore volume is found to increase from A0 to A1 and decreases monotonously with higher Ag concentrations (Figure 3, inset). Recently, we studied the behavior of the pore dimensions of mesoporous γ-Al2O3 with insertion of Ag nanoparticles and found that Ag nanoparticles can partially/fully block the pores and also increase the pore size.24 In this study, the products of the four Ag concentrations were considered to find a suitable trend as to how the Ag nanoparticles behave in modifying the pore structure in this mesoporous ZnO matrix. From Table 1 it is clear that with the increase in Ag concentration (i) the BET surface area initially decreases, as compared to pristine ZnO, and then gradually increases, (ii) the smaller pores are not affected, while the pore diameter of the larger pores decreases on an average, and (iii) the pore volume initially increases and decreases after that. On the basis of the above studies, the observations regarding the pore size and structure are explained. The sudden drop in surface area from A0 to A1 and A2 occurs since the 3.5 ( 0.4 nm Ag particles partially block the smaller (3.1-3.2 nm) pores. With an increase in Ag concentration for A3 and A4, Ag nanoparticle clusters are present on the surface of mesoporous ZnO, which increases the surface area more than pristine ZnO. Previously it was observed that deposition of Fe/ Fe-oxide nanoparticles on the surface of silica and C-spherules increases the surface area appreciably.25 A small concentration of 3.5 ( 0.4 nm Ag particles does not affect the pore dimensions of the larger pores in A1. However, with the increase in Ag concentration from A2 to A4, the Ag nanoparticles lie on the larger pores, decreasing the pore diameter. Thus, there is a partitioning of the Ag nanoparticles between the mesopores and the surface of ZnO. Since the pore volume is the cross-section times the depth of the pore, the pore volume is also increased. However, with the partial blocking of the larger pores by silver in A2 to A4, a monotonous decrease in pore volume is observed. Here it is to be noted that the smaller (3.1-3.2 nm) pores in A4 have a very large volume as compared to the other products (Figure 2b). The explanation arises from the fact that in A4 the nanorods have very rough top face and side edges, which might result in the random creation of smaller pores with larger depth, and consequently, the Ag nanoparticles cannot penetrate the depth of the smaller pores in A4 (Figure 4g). Hence, with the help of microwave radiation, Ag nanoparticles are successfully inserted into the mesopores of ZnO and strongly influence the pore
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Figure 7. Raman spectra of the products.
structure. Previously, we studied the phenomena of Ag nanoparticle insertion into γ-Al2O3 with the help of microjets and cavitation phenomena in ultrasound irradiation, where the Ag nanoparticles were thrown into the pores, partially/fully blocking them.24 To the best of our knowledge, the occurrence of a similar phenomenon in microwave synthesis has not reported before and, hence, is a novel finding in this research work. Raman scattering experiments were carried out to determine any possible charge-transfer effects between the mesoporous matrix and the inserted metallic nanoparticles. Figure 7 shows the room-temperature Raman spectra of the products in the 2001700 cm-1 spectral region. In the case of embedded nanoparticles on porous materials and thin films, the surface is modified due to probable charge transfer between the individual species, which changes the optical Raman spectra.26-28 In A0, the Raman peak at 447 cm-1 is the E2 (high) mode of ZnO host, shifted from the reported 434 cm-1, while the peak at 331 cm-1 is the 2E2 (low) mode.28 These two peaks weaken with the increase in Ag concentration. The extra Raman modes in A0 and at 670 cm-1 for A2 to A4 are mainly the surface phonon modes. The Raman peak at 552 cm-1 in A3 is the E2 (low) + A1 (TO) mode of the ZnO matrix.29 The two peaks in the range 13401374 and 1589-1599 cm-1 are the well-known D and G bands of the disordered and graphitic carbon (of the retained top organic layer), respectively.30 Previously, in the Ag-containing products these peaks were attributed to vibration of the silver nanoclusters.27,31 However, we strongly believe these peaks are due to carbon since Ag cannot show vibrational Raman modes. In our case, even if the carbon content in all the products is nearly the same, surprisingly we do not see such peaks in A0 and A1. The optical absorption and photoluminescence spectra of the products are shown in Figure 8. In the DRS spectrum at 27 °C (Figure 8, inset), the ZnO excitonic absorption appears at 354, 363, 362, 363, and 360 nm for A0, A1, A2, A3, and A4, respectively. It is blue shifted, as compared to that of bulk ZnO (373 nm), which might be due to the effect of Ag on the band gap of ZnO. The absorption peaks are broad, which might be due to the disorder induced by the mesoporous structure. The Ag surface plasmon band appears at 429, 436, 430, and 429 nm for A1, A2, A3, and A4, respectively.24 According to the Mie theory, the absorption band arises due to excitation of the surface plasmon vibrations of Ag inside the host matrix.32 In our case, the Ag plasmon band is slightly red shifted, as
Bhattacharyya and Gedanken
Figure 8. Room-temperature PL spectra. (inset) DRS spectra at 27 °C.
compared to that at ∼420 nm for aggregated Ag colloids.33 These spectral changes could be attributed to the interaction of the clustered Ag nanoparticles with the dielectric medium of the mesoporous ZnO matrix.34 The room-temperature PL spectrum (λex ) 309 nm) of the products shows nearly similar bands for all Ag concentrations within the strong and broad emission band at 360-600 nm. The PL spectra exhibit two strong peaks at 394-396 and 420-422 nm with a distinct shoulder peak at 484-486 nm and a weak shoulder at 528531 nm for all products. An additional peak at 445-454 nm is observed for the products, other than A0. The peaks between 394 and 422 nm are associated with the band edge exciton transitions of ZnO.35 Formation of different shallow levels inside the band gap, due to the presence of interstitial zinc atoms, might give rise to red-shifted multiple peaks in this region.36 The emission peaks at 484-486 nm are related to the oxygen defects and associated with the blue-green emission due to emission from the excited 3T2 state (due to spin-orbit coupling with an excited singlet state) of the neutral VO0 center to the 1A1 ground state.36 The 484-486 nm emission can also result from surfacedeep traps, which is typical for porous ZnO nanostructures.19 The weak shoulder at 528-531 nm is the green emission band, which is usually attributed to the transition of a photogenerated electron from a dark level below the conduction band to a deeply trapped hole resulting from an oxygen vacancy.37 The luminescence observed from metal nanoparticles is very weak, as compared to semiconductor nanostructures, and hence not observed.38 However, impurity levels of silver can be created below the conduction band of ZnO from the surface plasmons arising from Ag interfaces. When the energy gap of the semiconductor is comparable to the surface plasmon energy at the metal/semiconductor interface, electron-hole pair recombinations emit photons into surface plasmon modes.39 Such a radiative recombination center is responsible for the 445-454 nm band. When this recombination center is nonradiative, new emission bands are not observed in the PL spectra.40 4. Conclusions Silver nanoparticles were inserted into mesoporous ZnO. The composite was synthesized using a domestic microwave oven in a 15 min reaction conducted under an argon atmosphere. The molar ratios of Ag:ZnO in the ZnO-Ag nanocomposite solid products were 0, 0.02, 0.05, 0.13, and 0.22. The mesoporosity
3-D Mesoporous Zinc Oxide Nanocomposites and BET surface area increased with the higher concentration of silver. Ag formed clusters on the surface of mesoporous ZnO and acted as a nucleation center for the ZnO nanorod formation. ZnO nanodisks were joined together to form the porous hexagonal nanorods. The pores were composed of two different sizes, 3.2 ( 0.1 and 18.6 ( 0.2 nm, for pristine ZnO. These pores were created in a self-assembled manner when the 14 ( 2 nm hexagonal ZnO nanodisks were arranged to form a pore in between the organic framework structure retained in the final products. Raman spectroscopy experiments reveal the modified surface characteristics of ZnO due to the porous structure and certain charge-transfer phenomena between Ag and ZnO. The optical absorption spectra reveal the ZnO excitonic absorption and Ag surface plasmon bands. Apart from the band edge exciton transitions in ZnO, oxygen defects, surface-deep traps, and electron-hole pair recombinations were evident from the PL spectra. This paper reports an interesting finding in the study of nanoparticle insertion in a mesoporous matrix, wherein the nanoparticles change the pore structure of the 3-D mesoporous matrix. Moreover, synthesis of the mesoporous matrix by microwave synthesis under argon is a novel finding in this research work. Acknowledgment. This research was supported by an EC grant to the LIDWINE Consortium through Contract No. NMP2CT-2006-026741 of the 6th EC Program. References and Notes (1) Shan, G.; Xu, L.; Wang, G.; Liu, Y. J. Phys. Chem. C 2007, 111, 3290. (2) Jedrecy, N.; Renaud, G.; Lazzari, R.; Jupille, J. Phys. ReV. B 2005, 72, 045430. (3) (a) Zeng, H.; Cai, W.; Li, J.; Liu, P. J. Phys. Chem. B 2005, 109, 18260. (b) Scrymgeour, D. A.; Sounart, T. L.; Simmons, N. C.; Hsu, J. W. P. J. Appl. Phys. 2007, 101, 014316. (c) Prokes, S. M.; Glembocki, O. J.; Rendell, R. W.; Ancona, M. G. Appl. Phys. Lett. 2007, 90, 093105. (d) Wood, A.; Giersig, M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 8810. (4) Ahn, B. D.; Kang, H. S.; Kim, J. H.; Kim, G. H.; Chang, H. W.; Lee, S. Y. J. Appl. Phys. 2006, 100, 093701. (5) Wang, Z. L.; Song, J. Science, 2006, 312, 242. (6) (a) Gao, P. X.; Ding, Y.; Mai, W.; Hughes, W. L.; Lao, C.; Wang, Z. L. Science 2005, 309, 1700. (b) Hu, P.; Liu, Y.; Wang, X.; Fu, L.; Zhu, D.; Chem. Commun. 2003, 1304. (c) Zhang, J.; Sun, L.; Liao, C.; Yan, C. Chem. Commun. 2002, 262. (d) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Angew. Chem. 2003, 42, 3031. (e) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (f) Qian, D.; Jiang, J. Z.; Hansen, P. L. Chem. Commun. 2003, 1078. (g) Ding, G. Q.; Shen, W. Z.; Zheng, M. J.; Fan, D. H. Appl. Phys. Lett. 2006, 88, 103106. (h) Shpeizer, B. G.; Bakhmutov, V. I.; Clearfield, A. Microporous Mesoporous Mater. 2006, 90, 81. (7) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (8) Wang, X.; Summers, C. J.; Wang, Z. L. AdV. Mater. 2004, 16, 1215. (9) Loupy, A. MicrowaVes in Organic Synthesis; Wiley-VCH: Weinheim, 2006. (10) Gregg, S. J.; Sing, K. S. Adsorption Surface Area and Porosity; Academic Press: London, 1982. (11) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, R.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.
J. Phys. Chem. C, Vol. 112, No. 3, 2008 665 (12) Li, F.; Ding, Y.; Gao, P.; Xin, X.; Wang, Z. L. Angew. Chem. 2004, 43, 5238. (13) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem. 2004, 43, 4774. (14) Tien, L. C.; Norton, D. P.; Pearton, S. J.; Wang, H.-T.; Ren, F. Appl. Surf. Sci. 2007, 253, 4620. (15) Zhu, Z.; Chen, T.-L.; Gu, Y.; Warren, J.; Osgood, R. M., Jr. Chem. Mater. 2005, 17, 4227. (16) (a) Bu, S.; Cui, C.; Liu, X.; Bai, L. J. Sol-Gel Sci. Technol. 2007, 43, 151. (b) Strano, M. S.; Zydney, A. L.; Barth, H.; Wooler, G.; Agarwal, H.; Foley, H. C. J. Membr. Sci. 2002, 198, 173. (17) (a) Polarz, S.; Orlov, A. V.; Schu¨th, F.; Lu, A. H. Chem. Eur. J. 2007, 13, 592. (b) Jaramillo, T. F.; Baeck, S.-H.; Kleiman-Shwarsctein, A.; McFarland, E. W. Macromol. Rapid Commun. 2004, 25, 297. (18) (a) Baigorri, R.; Garcı´a-Mina, J. M.; Gonza´lez-Gaitano, G. Colloid Surf. A 2007, 292, 212. (b) Chung, C.; Lee, M.; Choe, E. K. Carbohydr. Polym. 2004, 58, 417. (19) Song, R. Q.; Xu, A. W.; Deng, B.; Li, Q.; Chen, G. Y. AdV. Funct. Mater. 2007, 17, 296. (20) (a) Hiltunen, L.; Leskela, M.; Makela, M.; Niinisto, L. Acta Chem. Scand. 1987, 41, 548. (b) Capilla, A. V.; Aranda, R. A. Cryst. Struct. Commun. 1979, 8, 795. (21) Capilla, A. V.; Aranda, R. A. Cryst. Struct. Commun. 1979, 8, 795. (22) Eddaoudi, M.; Li, H.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 1391. (23) (a) Spanhel, L. J. Sol-Gel Sci. Technol. 2006, 39, 7. (b) Tokumoto, M. S.; Pulcinelli, S. H.; Santilli, C. V.; Briois, V. J. Phys. Chem. B 2003, 107, 568. (24) Bhattacharyya, S.; Gabashvili, A.; Perkas, N.; Gedanken, A. J. Phys. Chem. C 2007, 111, 11161. (25) (a) Ramesh, S.; Prozorov, R.; Gedanken, A. Chem. Mater. 1997, 9, 2996. (b) Pol, V. G.; Motiei, M.; Gedanken, A.; Calderon-Moreno, J.; Mastai, Y. Chem. Mater. 2003, 15, 1378. (26) Patakfalvi, R.; Diaz, D.; Jacinto, P. S.; Gattorno, G. R.; Berru, R. S. J. Phys. Chem. C 2007, 111, 5331. (27) Gangopadhyay, P.; Kesavamoorthy, R.; Nair, K. G. M.; Dhandapani, R. J. Appl. Phys. 2000, 88, 4975. (28) (a) Zhou, H.; Chen, L.; Malik, V.; Knies, C.; Hofmann, D. M.; Bhatti, K. P.; Chaudhary, S.; Klarr, P. J.; Heimbrodt, W.; Klingshirn, C.; Kalt, C. Phys. Status Solidi (a) 2007, 204, 112. (b) Wang, X. B.; Song, C.; Geng, K. W.; Zeng, F.; Pan, F. J. Phys. D: Appl. Phys. 2006, 39, 4992. (29) Millot, M.; Gonzalez, J.; Molina, I.; Salas, B.; Golacki, Z.; Broto, J. M.; Rakoto, H.; Goiran, M. J. Alloys Compd. 2006, 423, 224. (30) Pol, V. G.; Pol, S. V.; Perkas, N.; Gedanken, A. J. Phys. Chem. C 2007, 111, 134. (31) Hadad, L.; Perkas, N.; Gofer, Y.; Moreno, J. C.; Ghule, A.; Gedanken, A. J. Appl. Polym. Sc. 2007, 104, 1732. (32) Mie, G. Ann. Phys. 1908, 25, 377. (33) Canamares, M. V.; Garcia-Ramos, J. V.; Gomez-Varga, J. D.; Domingo, C.; Sanchez-Cortes, S. Langmuir 2005, 21, 8546. (34) (a) Zhang, L.; Shen, Y.; Xie, A.; Li, S.; Jin, B.; Zhang, Q. J. Phys. Chem. B 2006, 110, 6615. (b) Selvakannan, P. R.; Swami, A.; Srisathiyanarayanan, D.; Shirude, P. S.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2004, 20, 7825. (35) Tang, Q.; Zhou, W.; Shen, J.; Zhang, W.; Kong, L.; Qian, Y. Chem. Commun. 2004, 712. (36) Ischenko, V.; Polarz, S.; Grote, D.; Stavarache, V.; Fink, K.; Driess, M. AdV. Funct. Mater. 2005, 15, 1945. (37) Kim, C. G.; Sung, K.; Chung, T.-M.; Jung, D. Y.; Kim, Y. Chem. Commun. 2003, 2068. (38) (a) Boyd. G. T.; Yu, Z. H.; Shen, Y. R. Phys. ReV. B 1986, 33, 7923. (b) Lee, K. C. Surf. Sci. 1985, 163, L759. (39) Lai, C. W.; An, J.; Ong, H. C. Appl. Phys. Lett. 2005, 86, 251105. (40) Zhang, Y.; Zhang, Z.; Lin, B.; Fu, Z.; Xu, J. J. Phys. Chem. C 2005, 109, 19200.