Article pubs.acs.org/JPCC
Rapid Synthesis of Silver Nanowires and Network Structures under Cuprous Oxide Nanospheres and Application in Surface-Enhanced Raman Scattering Ming Chen, Chengjiao Wang, Xiujuan Wei, and Guowang Diao* College of Chemistry and Chemistry Engineering, Yangzhou University, Yangzhou, Jiangsu, 225002, People’s Republic of China Key Laboratory of Environmental Materials & Environmental Engineering of Jiangsu Province, Yangzhou University, Yangzhou, Jiangsu, 225002, People’s Republic of China S Supporting Information *
ABSTRACT: Crystalline silver nanowires, with diameters of 50−500 nm and lengths up to tens of micrometers, have been successfully synthesized by a simple wet chemical route by using cuprous oxide nanospheres as a reductant and directional agent. The products are characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and UV−vis absorption spectroscopy. The two-dimensional netlike nanostructure is composed of several silver nanowires. The possible mechanism for the formation of silver nanowires is discussed. It is found that the architecture of silver crystals is drastically influenced by the concentration of the precursors and the reaction temperature. The experimental results reveal that the Cu2O nanospheres might play the two roles during the growth process of silver nanowires. Except for a reducing agent, Cu2O nanospheres act as a growth substrate to induce the formation of silver nanowires and a two-dimensional netlike nanostructure. Furthermore, the obtained two-dimensional netlike silver nanostructure can be used as surface-enhanced Raman scattering (SERS) substrates with high SERS activity and stability for detecting Rhodamine 6G (R6G) molecules. The analytical enhancement factor on the two-dimensional netlike silver nanostructure substrate is about 8 × 1010. Compared with other morphologies of silver substrates, it is found that the twodimensional netlike silver nanowires exhibit the highest SERS sensitivity. Hence, SERS substrates of the two-dimensional netlike silver nanowires described in this work have potential applications in chemical and biological analysis as well as medical detection.
1. INTRODUCTION Recently, much effort has been devoted to the controlled synthesis of one-dimensional (1D) nanostructured materials, such as nanorods, nanobelts, and nanowires. In particular, onedimensional (1D) metal nanostructures have received extensive attention because of their unique electrical, optical, thermal, and mechanical properties and the potential applications in electronics, photonics, (bio)chemical sensing and imaging, catalysis, etc.1−3 Especially, 1D silver nanostructures have been widely studied for their highest electrical and thermal conductivity among all metals and strong surface plasmon resonance largely dependent on their size and shape.4−8 Such nanomaterials have a wide range of applications, including surface-enhanced Raman spectroscopy, efficient catalysts for chemical reactions, sensing, electrochemistry, and so on. © 2013 American Chemical Society
Many methods have been developed to prepare 1D silver nanostructures. To obtain silver nanowires, a variety of hard (or rigid) and soft (or flexible) templates have been widely used. The hard templates generally consist of highly ordered materials, such as carbon nanotubes,9,10 zeolite,11 anodized alumina,12 and silica.13,14 Soft templates are usually micelles with certain shapes formed from DNA chains,15,16 ionic liquids,17 and high-concentration surfactants.18−22 However, the exploration of simple methods for the synthesis of metal nanowires is still an interesting research field. Moreover, it would be highly desirable to fabricate 1D silver nanostructures Received: May 8, 2013 Revised: June 2, 2013 Published: June 6, 2013 13593
dx.doi.org/10.1021/jp404563h | J. Phys. Chem. C 2013, 117, 13593−13601
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Figure 1. (A) XRD, (B) XPS, (C) UV−vis spectrum, and (D) SEM image of silver nanowires obtained at [AgNO3] = 0.02 M.
including the concentration of the precursor and the reaction temperature, have great influence on the morphology of crystalline silver. Furthermore, the obtained two-dimensional netlike silver nanocomposites can be used as SERS substrates with high activity SERS for detecting Rhodamine 6G (R6G) molecules. Different morphologies of silver substrates have great differences on the SERS activities of R6G molecules. SERS substrates described in this work exhibit the high SERS activity and excellent stability, and have potential applications in chemical and biological analysis as well as medical detection.
with hierarchical architectures in solution because architectural manipulation of 1D silver nanostructures could further endow them with unique properties and promising applications. It remains a significant challenge to develop a facile, mild, and effective method for creating two- or three-dimensional structures, such as two-dimensional nanoplates,23−25 and three-dimensional nanocubes,26−28 and dendritic silver nanostructures.29−32 The chemical synthesis of metal silver mainly utilizes the reduction of silver ions by various reducing agents, such as polyol,17 sodium borohydride,18,19,29 trisodium citrate,21 glucose,22,26 and ascorbic acid.23 As far as we know, preparation of silver nanowires using cuprous oxide nanospheres as a reducing agent has not been reported. Surface-enhanced Raman scattering (SERS) has been shown to be a useful tool for the detection of low-concentration analytes, sometimes even achieving single-molecule sensitivity. The origin of SERS enhancement is attributed to two mechanisms. The first one is based on an electromagnetic effect. The electromagnetic field at or near laser-irradiated noble metal particle surfaces is enhanced as a result of localized surface plasmon excitation, leading to more intense Raman scattering from molecules near or adsorbed onto the particle surface.33 The SERS intensity is, in this case, strongly sensitive to particle size, shape, composition, arrangement, and surface structure. The second mechanism is a chemical effect, which involves specific interactions between analyte molecules and the metal particles.34 To date, the main challenge is how to prepare SERS-active substrates that can provide great detection sensitivity and be stable, reproducible, inexpensive, and easy to prepare. Among all of the SERS-active substrates, silver (Ag) shows a superior SERS performance and various Ag nanostructures, such as nanoparticles,35 nanowires,36,37 nanorods,38 nanoplates,39 and dendritic silver,40,41 have been investigated as highly sensitive substrates. In this paper, we report a simple wet chemical route to synthesize crystalline silver nanowires through the redox reaction between silver ions and Cu2O nanospheres. Neither a template nor a polymer stabilizer is required in this process. Studies found that some related experimental parameters,
2. EXPERIMENTAL SECTION 2.1. Materials. AgNO3 (Shanghai Chemical Reagents Company) and other chemical reagents are analytical reagent grade and used as commercially available. Cuprous oxide nanospheres were prepared by the reduction of CuSO4 in psulfonated calix[8]arene aqueous solution using hydrazine as a reducing agent.42 Cuprous oxide nanospheres were characterized by TEM and XRD and are presented in Figure S1 (Supporting Information). Double distilled water is used to prepare all solutions. 2.2. Characterization. The X-ray powder diffraction (XRD) was taken by a M03XFH (MAC Science) X-ray diffractometer operated at a voltage of 40 kV and a current of 40 mA with Cu Kα radiation (λ = 0.154056 nm). The morphologies of Ag were characterized by transmission electron microscopy (TEM) by a Philips TECNAI12 TEM using an accelerating voltage of 120 kV. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL-2010 transmission electron microscope. The Ag specimens for taking scanning electron microscopy (SEM) were coated with gold. SEM was taken by a Hitachi S4800 operating at an accelerating voltage of 20 kV. The UV−vis spectra were taken by a Shimadzu UV-2501 double-beam spectrophotometer with a stoppered quartz cell with a 1 cm path length, l. Raman spectra were recorded by a Renishaw in Via Raman microscope using a 532 nm laser excitation source 13594
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with 25 mW power and a 2 μm diameter spot size using a single 60 s accumulation unless otherwise stated. 2.3. Preparation of Silver Nanowires. A typical synthesis of silver nanowires was as follows: The pH of the AgNO3 solution was adjusted by addition of a little HNO3. The value of the pH of the AgNO3 solution was 5. AgNO3 (50 mL, 0.01 mol·L−1) was heated to a boil. A 1 mg portion of Cu2O was dispersed in 4 mL of water, which was ultrasonically vibrated for 15 min. A 4 mL aliquot of Cu2O (0.25 mg·L−1) aqueous solution was then quickly added into the above AgNO3 solution. The reaction system was stirred intensively at 100 °C for 10 min; then, it was cooled to room temperature. The black precipitate was separated from the solution by centrifugation at 8000 rpm for 5 min. It was then dispersed again in a small amount of deionized water and collected by centrifugation again. Repeated operations were done in order to remove the residual reagents. At last, the precipitate was dried at 40 °C in a vacuum oven for 24 h and characterized by XRD, TEM, SEM, and UV−vis. Ag spherical nanoparticles, Ag nanorods, and dendritic Ag were prepared according to references reported previously.43,21,44 2.4. Preparation of Silver Nanowires on the Glass Sheet. A 1 mg portion of Cu2O was dispersed in 1 mL of ethanol, which was ultrasonically vibrated for 15 min. The prepared solution was spread out on the surface of a glass sheet and dried under ambient conditions at room temperature. After about 15 min, a brick red solid film was obtained. A 50 mL (0.02 mol·L−1) portion of AgNO3 was then heated to a boil. The glass sheet with the Cu2O film was added into the above AgNO3 solution. The reaction was carried out at 100 °C for 10 min. The color of the film turned into black. At the end, the glass sheet was dried at 40 °C in a vacuum oven for 24 h and characterized by SEM. 2.5. SERS Measurements of R6G on Two-Dimensional Netlike Silver Nanowires. R6G solution was diluted to various concentrations ranging from 1 × 10−10 to 1 × 10−16 M with methanol. A 50 μL aliquot of each solution was then dropped onto two-dimensional netlike silver nanowires, and the solvent was allowed to evaporate under ambient conditions. Finally, a Raman spectrometer was used to measure the SERS activities of these silver substrates directly. In this experiment, more than three SERS-active substrates of each silver sample were prepared, and at least 10 different points on each substrate were selected to detect the R6G probes, to verify the stability and reproducibility of these SERS-active substrates.
obtained by dispersing the silver nanowires in water under sonication. The spectrum exhibits a broad plasmon peak centered at 415 nm. It is a well-known surface plasmon resonance of silver, the transverse plasmon band, which is expected for anisotropic metallic nanoparticles, including silver nanorods.45 Figure 1D presents a representative SEM image of the silver product, which apparently consists of unordered silver nanowires up to several tens of micrometers in length. The wirelike nanostructures are actually silver nanowires typically ranging from 80 to 150 nm in width and about 50−75 nm in thickness. To further investigate the morphology of the silver nanowires, TEM images are shown in Figure 2. In Figure 2A,
Figure 2. (A) TEM image of the obtained silver nanowires. (B) Electron diffraction pattern of silver nanowires. (C) HRTEM image for silver nanowires obtained at [AgNO3] = 0.02M.
a netlike two-dimensional nanostructure is composed of several wirelike silver nanowires. The selected area electron diffraction (SAED) pattern (Figure 2B) confirms the single-crystalline nature of the silver nanowire crystals. Image C in Figure 2 shows a typical high-resolution transmission electron microscopy image recorded from the silver nanowire crystals. The fringe spacing is determined to be about 0.235 nm, which is close to the (111) lattice spacing of Ag crystals, indicating that the growth direction of the silver nanowires is preferential in the [111] direction.44 The HRTEM images show that the individual silver nanowire is a single crystal. The XRD pattern and UV−vis absorption spectra of the silver nanowires at different concentrations of AgNO3 are shown in Figure S2 (Supporting Information). From the XRD pattern of the samples (Figure S2A, Supporting Information), all the diffraction peaks corresponded to face-centered cubic (fcc) Ag. In UV−vis absorption spectra of the silver nanowires (Figure S2B, Supporting Information), a broad plasmon peak centered at 415 nm is unchanged, but the absorbency of the silver nanowires decreased gradually with the increase of the concentration of AgNO3. The morphology of the resulting silver products depends strongly on the concentration of the AgNO3. When the other conditions are kept constant, if the concentration of AgNO3 is 0.01 M, only stubby rodlike silver nanowires with diameters of about 50 nm are observed (Figure 3A). When 0.05 M AgNO3 solution is used, silver nanowires with diameters of about 70 nm are orderly assembled and form a two-dimensional netlike nanostructure (Figure 3B). As the AgNO3 concentration increases to 0.1 M, a two-dimensional netlike nanostructure is
3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of Silver Nanowires. Figure 1 shows the XRD pattern, XPS, UV−vis, and SEM of the silver nanowires. A typical XRD pattern of the sample is shown in Figure 1A. From the XRD pattern of the sample, there are five peaks with 2θ values of 38.2, 44.4, 64.6, 77.4, and 81.7 corresponding to (111), (200), (220), (311), and (222) crystal planes of face-centered cubic structure of metallic Ag (JCPDS File, No. 04-0783). The lattice constant calculated from this pattern is 4.087 Å, which is in good agreement with the value in the literature (a = 4.086 Å).44 Additionally, XPS is used to detect the specimen. As shown in Figure 1B, the peaks corresponding to the core-level 3d of Ag at 368.55 and 374.52 eV, respectively, are seen. These values compare well with the reported values for Ag 3d levels in the elementary substance. Figure 1C shows the ultraviolet−visible (UV−vis) absorption spectrum of the silver nanowires, which is 13595
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(Figure 4B). Even if the reaction temperature rises to 50 °C, the nanospheres structure still appears in the reaction system, and large amounts of irregular slices are simultaneously shown in Figure 4C. When the temperature increases to 75 °C, a few quantities of silver nanowires are obtained, but a small quantity of nanospheres still clearly display in Figure 4D. From the results of the above experiment, the morphology of the silver products intensely relies on the reaction temperature and a lower temperature is not beneficial to the heterogeneous nucleation and growth of silver nanowires. 3.2. Growth Mechanism for the Two-Dimensional Netlike Silver Nanocomposites. From the above experimental results, the possible crystal growth mechanism of silver nanowires is discussed. As is known already, the {111}, {100}, and {110} surfaces of face-centered cubic structured metal particles are different, not only in their surface atom densities but also in their electronic structure, bonding, and possibly chemical reactivities.46 Like most face-centered crystal metals, metal silver has a cubic octahedral shape dominated by the {111} faces and the {100} faces, because these faces have the lowest energies. A general order of the surface energies for different faces of the face-centered cubic metals may hold, γ{111} < γ{100} < γ{110}.46 That means that more energy is released by adding a silver atom to the {100} faces or the {110} faces than to {111} faces during crystal growth.47 These facets, which have a higher surface energy compared to the others, have a great tendency to adsorb other material, such as surfactant, polymer, and other substrates. In this case, the surface of reductant Cu2O nanospheres was modified by p-sulfonated calix[8]arene.42 Therefore, the negative electricity of Cu2O nanospheres would be inclined to adsorb Ag+ ions, which are reduced on the surface of Cu2O nanospheres and generate small silver nanoparticles. Active silver nanoparticles then have a great tendency to adsorb on the surface of Cu2O nanospheres, which play the role of a growth substrate to induce the formation of silver nanowires and a twodimensional netlike nanostructure. To confirm the function of Cu2O nanospheres a as growth substrate, Cu2O nanospheres are immobilized on the surface of a glass sheet to form the film and the reaction is carried out on the Cu2O film with different concentrations of AgNO3. From Figure 5A, some Ag nanosheets grow on the Cu2O film at lower concentration of
Figure 3. SEM and TEM images of the obtained silver nanowires obtained in different concentrations of AgNO3: (A) 0.01, (B) 0.05, (C) 0.1, and (D) 0.2 M.
well-assembled and the diameter of the silver nanowires is increased to about 100 nm (Figure 3C). When a 0.2 M AgNO3 solution is used, larger-sized silver flakes from the interlink of some nanowires are formed (Figure 3D). Therefore, the concentration of precursor has an important effect on the product morphology, and the size of the wire will increase with the precursor concentration. Except for the effect of the concentration of AgNO3, the morphology of the resulting silver products also strongly depends on the reaction temperature. At the lower reaction temperature (5 °C), as shown in Figure 4A, the morphology of the products is nanospheres and the surface of the sphere changes to rough. It is indicated that, at lower temperature, silver products grow on the surface of Cu2O nanospheres to form a core/shell (Cu2O/Ag) structure. When the reaction temperature is 25 °C, the morphology of the products is still nanospheres and the surface of sphere is much more rough
Figure 5. SEM images of silver nanowires obtained on the Cu2O film at different concentrations of AgNO3: (A) 0.01, (B) 0.02, (C) 0.05, and (D) 0.1 M.
Figure 4. SEM and TEM images of the products obtained in different temperatures: (A) 5, (B) 25, (C) 50, and (D) 75 °C. 13596
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On the basis of the above discussion, a proposed schematic illustration for the growth process of silver nanowires is shown in Figure 7. It can be seen that the morphologies of the final
AgNO3. However, with the increase of precursor concentration, it is evident that unordered silver nanowires grow on the surface of the Cu 2 O film (Figure 5B,C). When the concentration of AgNO3 increases to 0.1 M (Figure 5D), the growth of silver nanowires is much more orderly and silver nanowires tend to form the nano-folium. All in all, from the above experiments, it is powerfully demonstrated that the Cu2O nanospheres may be applied as the growth substrate to induce the formation of silver nanowires. A detailed time course study is expected to provide direct evidence for the growth mechanism. Therefore, we try to search for the detailed formation process of silver nanowires at different times. At the first 30 s, small silver nanoparticles with different diameters are shown in Figure 6A. With the extension
Figure 7. Schematic illustration of the experimental mechanisms for the crystal growth at different temperatures and precursor concentrations.
products are greatly dependent on the experimental conditions, such as precursor concentration and temperature. Maybe at lower temperature, the small Ag nanoparticles grown at first aggregate on the surface of Cu2O nanospheres easily in order to decrease the surface free energy of the system to form the core/ shell (Cu2O/Ag) structure (as shown in Figure 4A,B). A proposed schematic illustration for the growth process of products at lower temperature is shown in Figure 7A. With the increase of temperature (at 50 and 75 °C), some Ag nanosheets and nanowires grow on Cu2O nanoparticles (as shown in Figure 4C,D), and a proposed growth process of products at a middling temperature is shown in Figure 7B. At the highest temperature (at 100 °C) in this case, the oxidizability of Ag+ is improved based on the Nernst equation, which leads to the rapid decomposition of Cu2O nanospheres into small Cu2O nanoparticles (as shown in Figure 6D). The rates of the heterogeneous nucleation are increased with the rise of temperature, resulting into the large amount of silver nanoparticles’ generation. Owing to the decomposition of Cu2O nanospheres, silver nanoparticles have no suitable surface to absorb to decrease the surface free energy. Thus, the large amount of small nanoparticles grows into silver nanowires according to Ostwald ripening (as shown in Figure 6C). Simultaneously, small Cu2O nanoparticles decomposed from Cu2O nanospheres would adsorb on the high energy lattice face of Ag to induce the formation of silver nanowires. Moreover, high temperature enhances the thermal motion velocity of Ag+, Ag, and Cu2O nanoparticles, which will bring about the change of the adsorption sites and growth sites. With the disappearance of Cu2O nanoparticles, the silver nanowires no longer grew along the unique axis to develop into the straight nanowires. As a result, irregular fold lines and two-dimensional netlike nanostructures are obtained finally. Therefore, a proper
Figure 6. TEM images of the products obtained at different reaction times: (A) 0.5, (B) 1, and (C, D) 2 min.
the reaction time, the morphology of the product presents the irregular nanowires with inhomogeneous diameters (Figure 6B). These silver nanowires are inclined to connect to each other and further self-organize to form a netlike twodimensional nanostructure (double-headed arrows in Figure 6B). Furthermore, a large number of tiny nanoparticles (
