Ag Nanochains and

May 14, 2009 - Arunkumar Shanmugasundaram , Boppella Ramireddy , Pratyay Basak , Sunkara V. Manorama , and Sanyadanam Srinath. The Journal of ...
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A Controlled Method to Synthesize Hybrid In2O3/Ag Nanochains and Nanoparticles: Surface-Enhanced Raman Scattering Jimin Du, Liang Huang, and Zhiqiang Chen School of Chemistry and Chemical Engineering, Anyang Normal UniVersity, Anyang Henan, 455000, People’s Republic of China

Dae Joon Kang* BK 21 Physics Research DiVision, Department of Energy Science, Institute of Basic Science, SKKU AdVanced Institute of Nanotechnology and Center for Nanotubes and Nanostructured Composites, Sungkyunkwan UniVersity, Suwon 440-746, South Korea ReceiVed: December 14, 2008; ReVised Manuscript ReceiVed: April 23, 2009

Hybrid In2O3/Ag nanochains and nanoparticles were synthesized by the solvothermal reaction of indium nitrate and silver nitrate with or without PVP in an ethylene glycol solution at 230 °C for 20 h followed by annealing at 450 °C for 5 h. The compositions of the resulting In2O3/Ag nanochains and nanoparticles were confirmed by XRD, XPS, and EDS. The morphology was investigated with SEM and TEM. On the basis of the experimental results, a surfactant-induced formation mechanism was proposed to account for the growth processes. Hybrid In2O3/Ag nanochains and nanoparticles were confirmed by their surface-enhanced Raman scattering signal. Further, calculations were carried out with the density function theory to confirm the variation of the band structure of the In2O3/Ag nanocomposites compared to pure In2O3 compounds. 1. Introduction Controlled synthesis of hybrid materials made from semiconductors and metals has attracted considerable attention because the materials possess improved optical, electrical, and magnetic properties.1-3 With hybrid materials, it is possible to examine the interactions between the nanoscale components of the different materials.4 In addition, new technological applications based on some of the combinational characteristics of semiconductors and metals are possible5 including advanced sensing and imaging technologies.6-8 In addition, local electric fields at the interface between semiconductors and metals can be formed because of the electron transition due to their different work functions, which can improve the nonlinear response of adsorbed compounds leading to single molecule detection applications.9-11 Among the various nanocomposites, In2O3/Ag hybrid materials have attracted increasing attention on account of their unusual optical and electronic properties. Indium oxide is a very important n-type semiconductor with a high speed of carriers.12 Hence, considerable research has been carried out to develop novel optoelectronic devices13 and gas sensors14 using indium oxide nanostructures because of their high electric conductance and strong interactions with certain gas molecules.15 On the other hand, silver nanostructures display some novel activities in chemical and biological sensing, which are the result of surfaceenhanced Raman scattering, localized surface plasmon resonance,16 and metal-enhanced florescence.17 Hybrid nanocomposites made from a combination of In2O3 and Ag may have enhanced optical and electronic properties as well as new chemical activities. Therefore, in this study, we applied a simple route to produce In2O3/Ag nanochains and nanoparticles by using a solvothermal method with or without PVP surfactants * To whom correspondence [email protected].

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followed by annealing at 450 °C for 5 h. Interestingly, the Raman scattering was improved because a local electric field at the In2O3 and Ag interface was created due to the different work functions of indium oxide (4.3 eV) and silver (4.1 eV). 2. Experimental Selection In(NO3)3 · 5H2O (99.9%, purity), AgNO3 (99.9%, purity), In2O3 (99.9%, purity), ethylene glycol (99%, purity), polyvinylpyrrolidone-30K (PVP), and ethanol (99% purity) were purchased from Aldrich and used as received. In the typical synthesis of In2O3/Ag nanocomposites, 97.7 mg of In(NO3)3 · 5H2O and 254.8 mg of AgNO3 were mixed in 6 mL of ethylene glycol. After ultrasonic treatment for 20 min, the solution was loaded into a 25 mL stainless steel autoclave and maintained at 230 °C for 20 h. The reaction solution was then cooled to room temperature. The sediments on the bottom of the autoclave were collected and washed with ethanol four times and subsequently dried at 80 °C for 12 h. Finally, the obtained products were annealed for 5 h at 450 °C under an argon atmosphere with a flow rate of 30 sccm. To control the sample morphologies, experiments were carried out in the presence of 200, 150, 100, and 60 mg of the PVP surfactant while keeping the other reaction conditions constant. Powder X-ray diffraction (XRD) spectra of the samples were obtained on a powder X-ray diffractometer (D8 FOCUS 2200 V, BRUKER AXS) with Cu KR radiation (λ ) 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was performed with an Ulvac Phi Model 5500 with Mg KR radiation as the excitation source. Field-emission scanning electron microscopy (SEM) images were taken with field-emission SEM (JEOL JSM-7401F). Transmission electron microscopy (TEM) images at low and high magnifications as well as energy dispersive X-ray (EDS) spectra were obtained by using TEM (JEOL, JEM 2010) to observe the microstructures and compositions of the nanocom-

10.1021/jp811131t CCC: $40.75  2009 American Chemical Society Published on Web 05/14/2009

Hybrid In2O3/Ag Nanochains and Nanoparticles

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Figure 1. XRD patterns of the hybrid In2O3/Ag (a) nanochains and (b) nanoparticles.

posites. In addition, micro Raman spectroscopy was carried out with use of the 514 nm line of an Ar+ laser (Invia Basic, Renishaw). Quantum calculations were performed by using the density functional theory and CASTEP code. 3. Results and Discussion 3.1. X-ray Diffraction Patterns. The crystallinities of the as-prepared hybrid In2O3/Ag nanochains and nanoparticles were determined by XRD. The XRD results shown in Figure 1 reveal that the samples have similar diffraction peaks. The diffraction peaks marked with an asterisk can be indexed to the (222), (400), (440), (222), and (622) crystal planes of the body-centered cubic (bcc) In2O3 crystalline phase with a lattice constant of a ) 10.1 Å (JCPD 89-4595).18 The remaining diffraction peaks are safely assigned to the (111), (200), (220), (311), and (831) crystalline planes of face-centered cubic (fcc) silver (JCPDS card no. 4-783, a ) 4.08 Å).19 Thus, the as-synthesized In2O3/Ag nanochains and nanoparticles were confirmed to have mixed compositions of indium oxide and silver. 3.2. X-ray Photoelectron Spectra. To determine the compositions of the obtained products, quantitative XPS analysis was performed on the hybrid In2O3/Ag nanochains (A) and nanoparticles (B). Survey XPS spectra indicated that both A and B products consisted mainly of Ag, In, and O emission peaks, as shown in Figure 2a. The weak carbon emission peak present may result from the ex situ preparation process or the transfer process of the sample into the UHV chamber.20 For the individual XPS spectra, the double emission peaks observed at binding energies of 368.30 and 374.35 eV due to the spin-orbital coupling (Figure 2b) were assigned to Ag 3d3/2 and Ag 3d5/2 for metallic silver, respectively, which confirms the presence of Ag in both as-synthesized samples.21 Because of the spin-orbital splits, the In 3d5/2 and In 3d3/2 XPS peaks (Figure 2c) also have characteristic double peaks centered at binding energies of 444.95 and 452.55 eV, which indicate the presence of In2O3 in the samples.22 In addition, the separated O1s XPS spectra are especially asymmetric (Figure 2d). After deconvoluting the O1s asymmetric peak with a nonlinear leastsquares fit program by using Gauss-Lorentzian peak shapes, three O1s peaks were separated and indexed to oxygen anions (530.42 eV) from indium oxide,23 surface hydroxyl oxygen (531.50 ( 0.5 eV)24 of the absorbed water, and adsorbed O2 (533 ( 1 eV) from the ambient atmosphere25 (Figure 2e). On the basis of the XPS data, it can be concluded that both assynthesized products were composed of indium oxide and silver. 3.3. Morphologies. The morphology of the In2O3/Ag nanocomposites prepared in a 6 mL ethylene glycol solution with 97.7 mg of In(NO3)3 · 5H2O and 254.8 mg of AgNO3 at 230 °C for 20 h followed by annealing at 450 °C for 5 h was

Figure 2. XPS spectra of (a) survey, (b) Ag 3d, (c) In 3d, (d) O1s, and (e) O1s deconvolution of hybrid In2O3/Ag (A) nanochains and (B) nanoparticles.

characterized by SEM. The mixed In2O3/Ag nanocomposites delineated at a high yield to low-dimensional nanostructures such as nanochains with a length of up to several micrometers, as seen in the LRSEM image (Figure 3a). In addition, the HRSEM image (Figure 3b) indicates that the as-prepared In2O3/ Ag nanochains have a rough surface, which resulted from the

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Figure 3. (a) LRSEM, (b) HRSEM, (c, e) LRTEM, (d) HRTEM images, and (f) EDS spectra of the areas labeled area 1, 2, and 3 corresponding to the places in panel e of the hybrid In2O3/Ag nanochains. Both insets (upper and bottom) in panel d show the indexed FFT spectrum corresponding to In2O3 and Ag.

Hybrid In2O3/Ag Nanochains and Nanoparticles small In2O3 nanoparticles attached to the surface of larger silver nanochains. Meanwhile, the In2O3/Ag nanochains also showed crankle characteristics at the connecting positions, which are closely related to their growth mechanism. The LRTEM image of In2O3/Ag (Figure 3c) also shows nanochains with many In2O3 nanoparticles attached to coarse-surfaced silver nanochains. The HRTEM image (Figure 3d) reveals a clear color contrast between the center and edge positions. The darker contrast regions of each nanochain are possibly silver, whereas the light contrast regions may be In2O3. Further, the HRTEM image (Figure 3d) illustrates that the In2O3 nanoparticles have a crystal fringe spacing of 0.29 nm, which is consistent with the (222) crystal lattice of bcc In2O3. The silver nanochains (Figure 3e) are single crystals with a lattice fringe spacing of 0.24 nm, corresponding to the (111) crystal facets of fcc silver. Obviously, this image shows that the (222) lattice planes of In2O3 are parallel to the silver (111) lattice owing to their favorable lattice matching. The FFT images of indium oxide and silver (upper and bottom insets in Figure 3d) also confirm that the growth directions of indium oxide and silver are identical to the [111] crystal axis. Furthermore, the composition of the hybrid In2O3/ Ag nanochains was determined by EDS, as shown in Figure 3f. The EDS spectra labeled with areas 1, 2, and 3 (Figure 3f) corresponding to the regions of the TEM image (Figure 3e) were measured under a dark field. The EDS spectrum of area 1 in Figure 3f has only silver peaks, indicating the existence of uncoated silver with indium oxide nanoparticles. Through careful measurement of area 2 (Figure 3f), the EDS spectrum shows In and O peaks. Further, the spectrum of the interface regions of area 3 in Figure 3f demonstrates the presence of O, In, and Ag peaks. Hence, the EDS experimental results verify that In2O3 nanoparticles are formed on the surfaces of the silver nanochains. For the aim of tailoring the In2O3/Ag morphology, controlled experiments were conducted with 150 mg of PVP surfactant with otherwise identical reaction conditions. Interestingly, the In2O3/Ag products exhibited irregular nanoparticles with sizes ranging from ∼100 to 500 nm, as observed in the LRSEM images (Figure 4a). Clearly, many In2O3 nanoparticles of ∼20 nm were attached to the surface of silver, leading to the rough surface characteristics seen in the HRSEM image in Figure 4b. The LRTEM image (Figure 4c) elucidates that the In2O3 nanoparticles attached to the silver surface are lightly contrasted compared to those of the darker silver. The HRTEM image of the nanocomposite In2O3/Ag nanoparticles showed a marked interface between the In2O3 and Ag particles (Figure 4d), indicating the formation of hybrid In2O3/Ag. Further, The HRTEM image reveals two groups of parallel fringes with spacings of 0.29 and 0.24 nm, corresponding to the (222) and (111) planes of bcc indium oxide and fcc silver, respectively. In addition, their FFT images (upper and bottom insets in Figure 4d) from the selected areas of the HRTEM image indicate that the In2O3 and Ag growth directions are the same as the [111] crystal axis. For the sake of investigating the compositions of different areas, EDS experiments were carefully performed on the selected highlighted areas 1, 2, and 3 in the TEM image (Figure 4e). The EDS results confirmed that area 1 (Figure 4f) is made up of pure Ag peaks due to the presence of uncoated silver areas. In addition, area 2 of the EDS spectra (Figure 4g) is made up of mainly In and O, which correspond to In2O3 nanoparticles. Finally, area 3 (Figure 4h) is made up of O, In, and Ag peaks, which originate from hybrid In2O3/Ag nanoparticles. Overall, the hybrid In2O3/Ag nanoparticles were success-

J. Phys. Chem. C, Vol. 113, No. 23, 2009 10001 fully synthesized with many In2O3 nanoparticles coated on the silver surface. The amount of PVP was increased up to 200 mg while the other reaction conditions were held constant. The obtained products were examined by SEM (Figure S1a, Supporting Information), which shows that hybrid Ag/In2O3 nanoparticles with irregular shapes and nonuniform sizes were obtained. Clearly, many silver nanoparticles adhered to the surface of the In2O3 nanoparticles. When the amount of PVP was 100 mg, hybrid Ag/In2O3 nanostructures with irregular shapes and nonuniform sizes were obtained. Figure S1b in the Supporting Information shows a typical SEM image of the hybrid Ag/In2O3 indicating that the products consist of a large quantity of nanochains with short lengths formed from the self-assembly of nanoparticles. Meanwhile, many nanowires with diameters of ∼100 nm and lengths up to several micrometers are also seen among the products in some regions of the image. The experimental results revealed that the PVP surfactant plays an important role in changing the morphology of the hybrid Ag/ In2O3. 3.4. Growth Mechanism. Before annealing, the hybrid In2O3/Ag samples obtained with or without the PVP surfactant also showed the nanochains and nanoparticles motif, as shown in Figure S2 in the Supporting Information. Hence, the calcinations in ambient argon at 450 °C for 5 h improved their stability but did not destroy their original morphologies. On the basis of our experimental results, a surfactant-induced formation mechanism was proposed to explain the growth processes of the In2O3/Ag nanochains and nanoparticles. It is also worth noting that the propensity to form hybrid In2O3/Ag nanocomposites was attributed to the kinetically controlled crystal growth of the particles, where the silver nuclei simply formed before the indium oxide. Thus, it appears that the essential contribution of the silver nanostructures is to offer a low energy interface for heterogeneous nucleation of the In2O3 crystalline phase. In brief, the formation processes of the hybrid In2O3/Ag nanocomposites in the solution mixture can be described by using the reaction equations depicted below. First, ethylene glycol can be reversibly converted into aldehyde and water by reaction 1 in the presence of heat.26 Then, AgNO3 can be reduced by aldehyde to form silver through reaction 2. Finally, indium oxide can be produced on the surfaces of the produced silver particles through the in situ hydrolyzation of In(NO3)3, as shown in reaction 3. heating

HOCH2CH2OH y\z CH3CHO + H2O

(1)

CH3CHO + AgNO3 + H2O f Ag + HNO3 + CH3COOH (2) 2In(NO3)3 + 3H2O f In2O3 + 6HNO3

(3)

Without the presence of PVP in the reaction system, the ethylene glycol serves as both the reducing agent and surfactant during the reaction processes.27 Silver nuclei can be formed through the chemical reaction of silver nitrate and aldehyde, according to reaction 2. Consequently, the silver colloids can aggregate into larger particles to minimize their surface energy. With the reaction continued, silver atoms generated from the consumption of reaction reagents in the solution diffused to the surface of nuclei, forming metallic bonds with their neighbors

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Figure 4. (a) LRSEM, (b) HRSEM, (c, e) LRTEM, and (d) HRTEM images of nanocomposites the In2O3/Ag nanoparticles. Both insets (upper and bottom) in panel d show the indexed FFT spectra corresponding to In2O3 and Ag; (f, g, h) EDS from selected areas from the TEM image in panel e.

Hybrid In2O3/Ag Nanochains and Nanoparticles

Figure 5. Raman spectra of the hybrid In2O3/Ag (a) nanochains, (b) nanoparticles, and (c) pure In2O3; the yellow and blue lines are the sum and part of deconvoluted Raman scattering peaks of (a) sample.

to form the larger particles. During the growth processes, the grain growth exhibited anisotropy by diffusing themselves at active surface sites along the [111] crystal direction to link the silver nanoparticles into low-dimensional structures with some junctions (referred to as nanochains above) because ethylene glycol can control the growth rates of various facets of silver crystals by interacting with them through its induced effect.28 In the presence of PVP surfactant, In2O3/Ag nanoparticles with irregular shapes were obtained. In this case, the entire surface of the initially formed silver nuclei is covered with a thick layer of the PVP surfactant.29 As a result, the probability for surface addition of silver atoms is nearly identical and the growth of the seeds also proceeds in an isotropic manner to form irregular hybrid In2O3/Ag nanoparticles.30 According to the In2O3/Ag nanoparticles formation mechanism, low-dimensional nanostructures would be produced due to the anisotropic growth if the initially produced nuclei are not covered completely with the PVP surfactant. Fortunately, hybrid In2O3/Ag nanowires were successfully produced (Figure S3a,b, Supporting Information) in the presence of 60 mg of PVP surfactant when the other reaction conditions were held constant, proving that the surfactant-induced formation mechanism is suitable to explain the hybrid In2O3/Ag processes. 3.5. Surfaced-Enhanced Raman Spectroscopy. Once In2O3 starts attaching to Ag, the electrons must transition from Ag to In2O3 because the work function of Ag (4.1 eV) is lower than that of In2O3 (4.3 eV). As a matter of fact, the charge redistribution leads to a positively charged silver particle and negatively charged indium oxide, which can form a local electromagnetic field at the interface between In2O3 and Ag to enhance the Raman signal. To test this scenario, the Raman spectra for the hybrid (a) nanochains and nanoparticles of (b) In2O3/Ag and (c) pure In2O3 were recorded at room temperature and are shown in Figure 5. On the basis of the crystallography data, the bcc In2O3 crystal structure belongs to the Ia3j space group with the point group Th.31 According to the group theory analysis, the 52 optical modes have an irreducible representation as shown below.

Γopt ) 5A1g + 5E1g + 5E2g + 17Tg + 20Tu The Ag, E1g, E2g, and Tg modes are Raman active and the Tu modes are infrared active. Therefore, 32 active modes are expected in the Raman spectra of bcc In2O3. In fact, only four modes were observed in our samples, as seen in Figure 5. After

J. Phys. Chem. C, Vol. 113, No. 23, 2009 10003 careful deconvolution of the peaks of the (a) sample, the scattering shift modes with blue lines centered at ∼315 (E1g), 374 (E2g), 505 (A1g), and 603 (E2g) cm-1 can be assigned to the typical modes of bcc In2O3.32 As observed in Figure 6c, pure In2O3 compounds have weak Raman signals because the electrostatic forces induced by incident light are unable to overcome the elastic deformation force for bending to embody Raman activity. Compared with pure In2O3, hybrid In2O3/Ag nanochains and nanoparticles show surfaced-enhanced Raman spectra due to the formation of a local electric field at the interface between indium oxide and silver. In general, SERS can be elucidated by two well-known enhancement mechanisms: chemical and electromagnetic enhancement.32 Chemical enhancement focuses on the charge transition between metal and organic molecules. Electromagnetic enhancement focuses on the electron configuration. In this case, the positive behavior was left on the surface of the Ag nanocrystals, while In2O3 possesses the negative propensity after electrons were transferred from silver to indium oxide. Consequently, a strong local electromagnetic field was created at the interface between In2O3 and Ag, which can enhance the Raman resonance of indium oxide. By careful observation, the SERS intensity of the hybrid In2O3/ Ag nanochains is stronger than that of the In2O3/Ag nanoparticles. SERS is a very local phenomenon occurring at curves or in pores of a rough surface. In our case, the In2O3/Ag nanochains mainly display interconnecting crankle characteristics, which lead to the formation of the local electric field with strong intensity. Hence, when excited by incident radiation, a collective surface plasmon is trapped at many crankle positions, creating a huge local electric field. Therefore, it is normal for the hybrid In2O3/Ag nanochains to exhibit higher Raman intensity enhancement than the In2O3/Ag nanoparticles.33 We prepared several samples under the same preparation conditions and confirmed their properties using different characterization tools. The results indicated that the samples prepared under the same conditions showed the same properties consistently. 3.6. Calculations of Band Structure. To further verify the electron transition between In2O3 and Ag, the energy band structures of In2O3 and hybrid In2O3/Ag compounds were calculated by using density functional methods on the basis of their crystal structures, as shown in Figure S3a,c in the Supporting Information. The calculated band structure of In2O3 (Figure S4b, Supporting Information) shows that the top of the valence bands is nearly flat and close to the Fermi level (0.0 eV) with semiconductor characteristics. On the other hand, for the band structure of In2O3/Ag nanocomposites (Figure S4d, Supporting Information), the Fermi level was cut across the upper part of the hybrid bands, resulting in In2O3/Ag metallic properties due to the electron transition from silver to indium oxide.34 4. Conclusion In summary, hybrid In2O3/Ag nanochains and nanoparticles were successfully produced by utilizing a solvothermal method followed by annealing at 450 °C for 5 h. On the basis of our experimental results, a surfactant-induced formation mechanism was proposed to account for their growth behavior of hybrid In2O3/Ag nanochains and nanoparticles. Interestingly, the hybrid In2O3/Ag nanochains and nanoparticles exhibited surfaceenhanced plasmon resonance due to both the creation of local electrical fields and the interband transition between In2O3 and Ag. Likewise, hybrid In2O3/Ag nanochains and nanoparticles showed surfaced-enhanced Raman scattering due to the electric field created at the interface of indium oxide and silver.

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Acknowledgment. This work was supported by MEST through KICOS under Grant No. 2008-00656, through KRF under Grant No. KRF-2008-005-J00703, the KOSEF through CNNC at SKKU, and the WCU program through the KOSEF under Grant No. R31-2008-000-10029-0. Supporting Information Available: SEM images of the hybrid In2O3/Ag nanochains, nanoparticles, and nanowires, crystal structure and energy band structures of In2O3, and crystal structure and energy band structures of In2O3/Ag. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Buso, D.; Guglielmi, M.; Martucci, A.; Mattei, G.; Mazzoldi, P.; Sada, C.; Post, M. L. Cryst. Growth Des. 2008, 8, 744–749. (2) Buso, D.; Guglielmi, M.; Martucci, A.; Mattei, G.; Mazzoldi, P.; Sada, C.; Post, M. L. Nanotechnology 2006, 17, 2429–2433. (3) EychmFller, A. J. Phys. Chem. B 2000, 104, 6514–6528. (4) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426. (5) Bruchez, J. M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013–2016. (6) Lin, Y. W.; Tseng, W. L.; Chang, H. T. AdV. Mater. 2006, 18, 1381–1386. (7) Baron, T.; Fernandes, A.; Damlencourt, J. F.; De Salvo, B.; Martin, F.; Mazen, F.; Haukka, S. Appl. Phys. Lett. 2003, 82, 4151–4153. (8) Mueller, A. H.; Petruska, M. A.; Achermann, M.; Werder, D. J.; Akhadov, E. A.; Koleske, D. D.; Hoffbauer, M. A.; Klimov, V. I. Nano Lett. 2005, 5, 1039–1044. (9) Schroedter, A.; Weller, H.; Eritja, R.; Ford, W. E.; Wessels, J. M. Nano Lett. 2002, 2, 1363–1367. (10) Schlamp, M. C.; Peng, X.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837–5842. (11) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425–2427. (12) Weiher, R. L.; Ley, R. P. J. Appl. Phys. 1966, 37, 299–302. (13) Liu, F.; Bao, Mi.; Wang, K. L. Appl. Phys. Lett. 2005, 86, 213101– 213103. (14) Li, B.; Xie, Yi.; Jing, M.; Rong, G.; Tang, Y.; Zhang, G. Langmuir 2006, 22, 9380–9385.

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