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Facile synthesis of gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering Kailin Long a , Deyang Du a , Xiaoguang Luo a , Weiwei Zhao b , Zhangting Wu a , Lifang Si a , Teng Qiu a,∗ a
Department of Physics and Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 211189, PR China Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, School of Mechanical Engineering, Southeast University, Nanjing 211189, PR China b
a r t i c l e
i n f o
Article history: Received 22 April 2014 Received in revised form 13 May 2014 Accepted 19 May 2014 Available online xxx Keywords: Surface-enhanced Raman scattering Three-dimensional nanostructures Surface plasmons
a b s t r a c t This work reports a facile method to fabricate gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering (SERS) application. The effects of reaction parameters on the shape, size and surface morphology of the products are systematically investigated. The as-prepared 3D hierarchical structures have the advantage of a large surface area available for the formation of hot spots and the adsorption of target analytes, thus dramatically improving the Raman signals. The finite difference time domain calculations indicate that the pine-needle-like model pattern may demonstrate a high quality SERS property owing to the high density and abundant hot spot characteristic in closely spaced needle-like arms. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS) has drawn substantial attention since its discovery in 1974 [1]. The SERS is manifested as an enhancement by many orders of magnitude of the intensity of Raman radiation by molecules bound to nano-rough metal surfaces and nano-structured metal systems such as colloidal clusters of noble metals [2]. Theories of SERS that immediately followed its discovery considered the effect has predominantly an electromagnetic (EM) origin that arises from an increase in the local optical field exciting the molecule and multiplicative amplification of the re-radiated Raman scattered light [2–4]. This optical enhancement is commonly associated with the excitation of surface plasmon oscillations in most SERS systems [5]. Currently, a great deal of research effort in SERS mainly focuses on the controlled and reproducible fabrication of metallic nanostructures that can produce hot geometries where the molecules are appropriately and predictably located for giant Raman enhancement [6–8]. Traditionally, the sensitivity of simple 2D SERS substrates remains modest owing to a limited number of hot spots (usually below 105 ) [9–11]. To increase the sensitivity of SERS substrates, 3D hierarchical micro/nanostructured materials, assemblies using
∗ Corresponding author. Tel.: +86 25 52090600. E-mail address: [email protected] (T. Qiu).
nanoparticles [12–14], nanorods [15,16] and nanobelts [17–21] as building blocks, have been suggested as active SERS substrates with the advantage of having a large surface area available for the formation of hot spots and the adsorption of target analytes. Furthermore, 3D hierarchical micro/nanostructures have highly curved, sharp surface features with dimensions of less than 100 nm. This increases the localized EM field up to a hundredfold and it is referred to as the “lightning rod” effect [10,20,22,23]. Consequently, many fabrication strategies are employed to synthesize these 3D hierarchical structures with tunable size and well-defined morphologies. Though these approaches have shown feasible strategies to produce 3D nano-architectures, they both involve the addition of surfactants or organics which leads to potential process-related hazards or low production efficiency [24–27]. On the other hand, among these approaches, the self-assembly strategy, especially under low temperature solution environment, where low-dimensional building units spontaneously aggregate into high-dimensional architectures, is more attractive because of its low cost, facile procedure, and mild reaction conditions [28,29]. Therefore, developing a simple self-assembly method without any surfactants and/or organics to fabricate 3D hierarchical micro/nanostructures is still necessary. In this paper, we demonstrate a facile technique to the largescale synthesis of gold coated copper(II) hydroxide [Cu(OH)2 ] pine-needle-like micro/nanostructures without the addition of any surfactants and organics. The effects of reaction parameters
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Please cite this article in press as: K. Long, et al., Facile synthesis of gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.133
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Fig. 1. Schematic illustrating the fabrication process for the PAuNS: (a) pretreated copper foil substrate; (b) the solution process; (c) PCuNS; (d) PAuNS; and (e) representative SEM image of PAuNS (left), and pine needles taken from the pine tree (right). The “CuCl2 –KOH” in (b) means: firstly, the copper foil was immersed into a CuCl2 aqueous solution; then the copper foil was taken out from the solution and rinsed with deionized water; finally, the copper foil was immersed into an aqueous solution of KOH.
on the shape, size and surface morphology of the products are systematically investigated. A possible formation mechanism of the 3D micro/nanostructures which involves a self-assembly process is proposed. The as-prepared 3D micro/nanostructures show extremely high sensitivity (up to 10−8 M) in SERS detection of a target analyte, Rhodamine 6G (R6G). The topography was further tuned to optimize the enhancement factor by adjusting Cu(OH)2 reaction time and gold deposition. 2. Material and methods 2.1. Large-scale synthesis of pine-needle-like Cu(OH)2 micro/nanostructures (PCuNS) The procedure for the synthesis of PCuNS was performed as follows: First, a piece of copper foil was ultrasonically cleaned in acetone, and subsequently with deionized water to remove surface impurities. The fresh copper foil was then immersed into a CuCl2 aqueous solution (∼37.5 wt%). After 10 min, the copper foil was then taken out from the solution and rinsed with deionized water. Finally, the copper foil was immersed into an aqueous solution of KOH (10 wt%) (see Fig. 1(a) and (b)). After a given reaction time, the copper foil coated with the product film was taken out, washed with deionized water and dried in air. 2.2. Synthesis of gold coated PCuNS (PAuNS) A gold film was deposited on the PCuNS in a dc sputtering system at room temperature, as shown in Fig. 1(c) and (d). PAuNS (Fig. 1(e)) were obtained by using different sputtering times, between 20 and 260 s with a current of 0–10 mA. 2.3. Instrumentation and data acquisition Scanning electron microscopy (SEM) (FEI INSPECT F50), equipped with X-ray energy dispersive spectroscopy (EDS) capabilities, was used to investigate the structures. The Raman
measurements were performed on a Jobin Yvon LabRAM HR800 micro-Raman spectrometer with the 514 nm laser line at room temperature. An area ∼3 m in diameter was probed by a 50× objective lens and the incident power at the sample was 0.05 mW. The signal collection time was 20 s. In order to evaluate the capability of Raman enhancing of the product, a R6G water solution was used. To allow molecule adsorption, the substrate was maintained for 30 min in the R6G solution and then taken out and rinsed thoroughly. The acquisition time and laser power were uniform for all Raman spectra. The SERS spectra were recorded from multiple sites on the substrate surface to confirm reproducibility. Similar SERS spectral characteristics such as enhancement, position, and relative intensity of the bands were determined from various locations to confirm large area production of uniform geometries. 3. Results and discussion 3.1. The effect of reaction parameters on the surface structure of PCuNS The morphologies of PCuNS are reaction-time dependent, as shown in Fig. 2. At the early stage, the sample was composed of a large amount of irregular protuberances (Fig. 2(a)). One can see protuberances that are ∼2 m in size from the high magnification image. When the reaction-time proceeded to 2 min, the needlelike micro/nanostructures are found to grow sparsely out of the protuberance, as shown in the inset of Fig. 2(b). With a further extension of the reaction-time from 2 min to 5 min, the structures became much thicker and bamboo-shoot-like rods formed (Fig. 2(c)). After reaction for 30 min, no protuberances remained and the sample was composed entirely of 3D pine-needle-like micro/nanostructures (Fig. 2(d)) [30–32]. An average diameter of these 3D structures was measured to be ∼5 m. The entire structure of the architecture is built from several dozen needles with smooth surfaces and the needles had a variety of shapes and sizes. These needles were ∼200 nm thick and ∼600 nm wide and connected to each other through the center to form 3D pine-needle-like
Please cite this article in press as: K. Long, et al., Facile synthesis of gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.133
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Fig. 2. SEM images of the sample surface after immersion in the solution for: (a) 30 s; (b) 2 min; (c) 5 min; (d) 30 min; (e) 40 min; and (f) 125 min. Scale bars: 6 m. The inset (left) shows the SEM images at high magnification, scale bars: 2 m. The inset (right) shows the model of the micro/nanostructure. The additional inset in (d) shows EDS spectrum of the as-prepared PAuNS.
structures. The chemical composition of as-synthesized products was observed using EDS spectrum (see the inset of Fig. 2(d)). The EDS spectrum demonstrates that the products are made of Cu and O and the atomic ratio of Cu to O is approximately equal to 1:2, confirming that the products are Cu(OH)2 . Interestingly, from the magnified SEM images (see the inset of Fig. 2(d)), one can find that the growth seems to emanate from a common nucleation point with numerous needles sharing the same central core, and the diameter of the needles becomes larger with a longer reaction period. Throughout the whole process, we could easily obtain four kinds of precursor samples: (1) protuberances; (2) protuberances and needles; (3) protuberances, needles, and a 3D pine-needle-like structure; and (4) a 3D pine-needle-like structure. Such a process is a so-called two-stage growth process, which involves a fast nucleation of amorphous primary particles followed by a slow aggregation and crystallization of primary particles [33–35]. All samples obtained here exhibit pine-needle-like morphology, although a further increase in the reaction time (40 min and 125 min) blurred the structure of PCuNS, shown in Fig. 2(e) and (f). Hence, we consider 30 min is an appropriate reaction-time to obtain high quality structures of PCuNS. From this point, the reaction-time is the key factor in the formation of the pine-needlelike structures. In order to investigate the role of concentration of KOH in the formation of the 3D pine-needle-like structures, a series of experiments were conducted by varying the solvent, while other experimental parameters were kept the same. KOH concentration can also strongly influence the Cu(OH)2 micro/nanostructures, as presented in Fig. 3. We varied the KOH concentration from 1 wt% to 15 wt% while keeping temperature (room temperature)
and reaction-time (30 min) constant. At a lower KOH concentration of 1 wt%, the final morphology was Cu(OH)2 micro/nano-ribbons (Fig. 3(a)). An increase in the mass fraction of KOH (5 wt%) resulted in the formation of 3D plate-like micro/nano structures (Fig. 3(b)). The dominant products were 3D plate-like microflowers, and only a few dispersed micro/nanoplates were found in the samples. Further increasing the mass fraction of KOH to 10 wt% produced a large amount of perfect 3D pine-needle-like micro/nanostructures (Fig. 3(c)). And the surface of needles of these nanostructures turned smooth gradually with the plates becoming longer and longer. When the KOH concentration was increased to 15 wt%, blurring and filling (the spaces of two adjacent needles) in the flower-shaped nanostructures can be observed (Fig. 3(d)). Presumably, with increasing mass fraction, the nanoscale corrugation of the pine-needle-like nanostructures becomes less obvious, and only some protrusions on the surfaces are observed. Since the contributions can strongly increase the SERS effect which originates from cross-junctions between tilted and/or horizontally oriented needles, we adopt 10 wt% concentration for the growth value. 3.2. Possible formation mechanism The evolution of various morphologies such as plates, needles and 3D pine-needle-like micro/nanostructures, can be explained on the basis of the combination and concentration of the precursors. Based on the above experimental results, we propose a possible formation mechanism for the 3D pine-needle-like micro/nanostructures. The whole process is illustrated schematically in the inset of Fig. 2. In the heterogeneous nucleation stage,
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Fig. 3. SEM images of the Cu(OH)2 nanostructures prepared under different mass concentrations of KOH by keeping other reaction parameters constant (at room temperature for 30 min). (a) 1 wt%; (b) 5 wt%; (c) 10 wt%; and (d) 15 wt%.
Cu2+ coordinated with KOH to rapidly produce copper hydroxyl, which precipitated to become the nuclei and quickly grew into the primary particles. This promotes the random formation of nuclei on the surface of the Cu. A large number of primary irregular particles were instantaneously formed in this stage. In the second stage, as the growth and concentration of irregular particles reach a so-called metastable state in the reaction system, the asformed structures tend to aggregate into protuberances (see the inset of Fig. 2(a)). And these protuberances became the core of the pine-needle-like structure. The protuberances continued to grow by combining with the remaining primary particles, probably due to Ostwald ripening [36]. It is known that different growth rates of the crystal faces determine the ultimate morphology of the nanomaterial. The growth rates of Cu(OH)2 needles along the different direction can be of great difference. In the following stage, the needle-like structures grew gradually out of the protuberances. With increasing reaction time, needles became much thicker and longer, finally forming the 3D pine-needle-like structure. Although many kinds of such 3D structures have been reported, several factors, such as crystal-face attraction, associated with the aggregate, van der Waals forces, hydrophobic interactions, and hydrogen bonds, may have various effects on the self-assembly. Hence, the detailed formation mechanism in this synthesis still needs further investigation.
thought to be necessary for efficient SERS enhancement. Thus, to provide SERS activity, we coated gold onto these PCuNS to duplicate the morphology, i.e. we fabricated PAuNS. Here, we used the samples with the sputtering time of 80 s for demonstration. Raman activity of these PAuNS samples was evaluated by detecting the spectra of target analyte R6G, with a 10−5 M water solution, as featured in Fig. 4(a). For comparison, R6G adsorbed on gold coated particles, corresponding to Fig. 2(a), exhibits weak R6G Raman bands. The peaks from 400 to 900 cm−1 are attributed to R6G signals. Accumulation times (20 s) and the laser power (0.05 mW) are
3.3. SERS performances of the 3D PAuNS substrates
Fig. 4. (a) SERS spectra of 10−5 M R6G adsorbed on the PAuNS (red) and that adsorbed on the gold coated particles (blue); (b) SERS signal (at 612 cm−1 ) comparison of R6G adsorbed on the PCuNS and the corresponding PAuNS (t = 80 s). Scale bars: 2 m. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
are especially interested in these thorny We micro/nanostructures as their nanoscale features are currently
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Fig. 5. (a) Typical SEM image of PAuNS: t = 80 s. The inset in (a) shows one pineneedle-like arm of the PAuNS; (b) typical simulated EM-field distribution maps of the PAuNS. The 2 V/m in the EM field does not represent the maximum electric field strength but is artificially set to provide better intuitive models of the EM field.
the same for all the acquiring. A main peak (612 cm−1 ) intensity of PAuNS is more than 42 times stronger than that of corresponding PCuNS, as shown in Fig. 4(b). The enhancement can be qualitatively explained by that PAuNS are assembled on the copper foil substrate with a very high density and with many horns. Hence, R6G molecules in abundant hot spots between closely spaced pine-needle-like arms lead to intensive SERS enhancement. To assess the contributions of the PAuNS geometries to the SERS intensities, local EM fields were calculated using commercial finitedifference time-domain (FDTD) software (CST MWS 2009). A planar and cross-sectional view of the calculated radial EM field components of the PAuNS (t = 80 s) is displayed in Fig. 5. A pine-needle-like model pattern (elliptical shape, the major axis and minor axis are 591 nm and 193 nm, respectively) using dimensional parameters illustrated in Fig. 5(a). The radiation at 514 nm is assumed to be normal to the sample surface. In the contour plots of the near EM field distributions shown in Fig. 5(b), one can find that numerous hot spots exist near the interstitial areas of the pine-needle-like arms. We can achieve the largest enhanced factor value of ∼106 for the pine-needle-like model pattern. This exceeds the average enhanced factor (∼104 ) over the entire PAuNS by several orders of magnitude because most of the optical excitations are localized in these hot spots. To further explore the role of gold layer on the SERS properties of PAuNS, we increased the sputtering time of gold from 20 s to 260 s to tailor the sizes of the gold film as illustrated in Fig. 6(a). Fig. 6(b) shows corresponding SERS spectra of the PAuNS formed by using different gold sputtering times. The variation in the SERS signal intensity at 612 cm−1 is presented in Fig. 6(c). The intensity of the R6G Raman signal increases when the sputtering time goes from 20 to 140 s, but decreases when extending the sputtering duration. The excessive Raman enhancement can be mainly associated with the effect of resonance coupling between neighboring pine-needlelike arms. It can be observed that the Raman enhancement observed from the sample when t = 140 s is larger than that from the others. Such strong enhancement in that sample can be attributed to the fact that the pine-needle-like arms are assembled with a favorable gap configuration [37]. Comparisons among the spectra reveal the existence of different SERS enhancements, especially the quantitative intensity comparison from the selected band (612 cm−1 ). It should be noted that the SERS intensity fluctuation, from spot to spot on a sample plate, was approximately 24.4% from the average value. All the results fall inside the control limits, indicating that the discrepancy is within the acceptable level. Impressively,
Fig. 6. (a) Schematic of PAuNS tailored by tAu ; (b) SERS spectral comparison of 10−5 M R6G adsorbed on the PAuNS by adjusting the sputtering time tAu (from 0 s to 260 s); (c) intensity profile of the peak at 612 cm−1 as a function of tAu ; (d) SERS spectra of different R6G concentrations adsorbed on the PAuNS of (I) 10−5 M, (II) 10−6 M, (III) 10−7 M, (IV) 10−8 M, and (V) 10−9 M, demonstrating the intensity variation of the SERS signal at 612 cm−1 .
when the as-prepared PAuNS (t = 140 s) are applied for the SERS detection of R6G, the sensitivity is extremely high, where R6G with a concentration of 10−8 M can be easily tracked (Fig. 6(d)). From a R6G concentration of 10−5 M to 10−9 M, we can see very sharp R6G peaks with very limited noise. Hence, these 3D PAuNS SERS substrates will potentially find applications to detect and analyze other biomolecules or dangerous chemicals. 4. Conclusion In summary, we report a facile technique to the large-scale synthesis of PAuNS substrates without the addition of any surfactants and organics. The 3D hierarchical micro/nanostructured surface shows high sensitivity in SERS detection of the target analyte, R6G, with a detection limit up to 10−8 M. The FDTD calculations indicate that the pine-needle-like model pattern may demonstrate a high quality SERS property owing to the high density and abundant hot spot characteristic in closely spaced pine-needle-like arms. We believe that such 3D pine-needle-like structures could serve as better substrates for SERS applications and provide an excellent candidate for SERS analysis. Acknowledgments This work was jointly supported by the National Natural Science Foundation of China under Grant no. 51271057, the Natural Science Foundation of Jiangsu Province, China, under Grant no. BK2012757, and the Program for New Century Excellent Talents in University of Ministry of Education of China under Grant no. NCET-11-0096. References [1] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Raman spectra of pyridine adsorbed at a silver electrode, Chem. Phys. Lett. 26 (1974) 163–166.
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Please cite this article in press as: K. Long, et al., Facile synthesis of gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.133
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Facile synthesis of gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering Kailin Long a , Deyang Du a , Xiaoguang Luo a , Weiwei Zhao b , Zhangting Wu a , Lifang Si a , Teng Qiu a,∗ a
Department of Physics and Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 211189, PR China Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, School of Mechanical Engineering, Southeast University, Nanjing 211189, PR China b
a r t i c l e
i n f o
Article history: Received 22 April 2014 Received in revised form 13 May 2014 Accepted 19 May 2014 Available online xxx Keywords: Surface-enhanced Raman scattering Three-dimensional nanostructures Surface plasmons
a b s t r a c t This work reports a facile method to fabricate gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering (SERS) application. The effects of reaction parameters on the shape, size and surface morphology of the products are systematically investigated. The as-prepared 3D hierarchical structures have the advantage of a large surface area available for the formation of hot spots and the adsorption of target analytes, thus dramatically improving the Raman signals. The finite difference time domain calculations indicate that the pine-needle-like model pattern may demonstrate a high quality SERS property owing to the high density and abundant hot spot characteristic in closely spaced needle-like arms. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS) has drawn substantial attention since its discovery in 1974 [1]. The SERS is manifested as an enhancement by many orders of magnitude of the intensity of Raman radiation by molecules bound to nano-rough metal surfaces and nano-structured metal systems such as colloidal clusters of noble metals [2]. Theories of SERS that immediately followed its discovery considered the effect has predominantly an electromagnetic (EM) origin that arises from an increase in the local optical field exciting the molecule and multiplicative amplification of the re-radiated Raman scattered light [2–4]. This optical enhancement is commonly associated with the excitation of surface plasmon oscillations in most SERS systems [5]. Currently, a great deal of research effort in SERS mainly focuses on the controlled and reproducible fabrication of metallic nanostructures that can produce hot geometries where the molecules are appropriately and predictably located for giant Raman enhancement [6–8]. Traditionally, the sensitivity of simple 2D SERS substrates remains modest owing to a limited number of hot spots (usually below 105 ) [9–11]. To increase the sensitivity of SERS substrates, 3D hierarchical micro/nanostructured materials, assemblies using
∗ Corresponding author. Tel.: +86 25 52090600. E-mail address: [email protected] (T. Qiu).
nanoparticles [12–14], nanorods [15,16] and nanobelts [17–21] as building blocks, have been suggested as active SERS substrates with the advantage of having a large surface area available for the formation of hot spots and the adsorption of target analytes. Furthermore, 3D hierarchical micro/nanostructures have highly curved, sharp surface features with dimensions of less than 100 nm. This increases the localized EM field up to a hundredfold and it is referred to as the “lightning rod” effect [10,20,22,23]. Consequently, many fabrication strategies are employed to synthesize these 3D hierarchical structures with tunable size and well-defined morphologies. Though these approaches have shown feasible strategies to produce 3D nano-architectures, they both involve the addition of surfactants or organics which leads to potential process-related hazards or low production efficiency [24–27]. On the other hand, among these approaches, the self-assembly strategy, especially under low temperature solution environment, where low-dimensional building units spontaneously aggregate into high-dimensional architectures, is more attractive because of its low cost, facile procedure, and mild reaction conditions [28,29]. Therefore, developing a simple self-assembly method without any surfactants and/or organics to fabricate 3D hierarchical micro/nanostructures is still necessary. In this paper, we demonstrate a facile technique to the largescale synthesis of gold coated copper(II) hydroxide [Cu(OH)2 ] pine-needle-like micro/nanostructures without the addition of any surfactants and organics. The effects of reaction parameters
http://dx.doi.org/10.1016/j.apsusc.2014.05.133 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Long, et al., Facile synthesis of gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.133
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Fig. 1. Schematic illustrating the fabrication process for the PAuNS: (a) pretreated copper foil substrate; (b) the solution process; (c) PCuNS; (d) PAuNS; and (e) representative SEM image of PAuNS (left), and pine needles taken from the pine tree (right). The “CuCl2 –KOH” in (b) means: firstly, the copper foil was immersed into a CuCl2 aqueous solution; then the copper foil was taken out from the solution and rinsed with deionized water; finally, the copper foil was immersed into an aqueous solution of KOH.
on the shape, size and surface morphology of the products are systematically investigated. A possible formation mechanism of the 3D micro/nanostructures which involves a self-assembly process is proposed. The as-prepared 3D micro/nanostructures show extremely high sensitivity (up to 10−8 M) in SERS detection of a target analyte, Rhodamine 6G (R6G). The topography was further tuned to optimize the enhancement factor by adjusting Cu(OH)2 reaction time and gold deposition. 2. Material and methods 2.1. Large-scale synthesis of pine-needle-like Cu(OH)2 micro/nanostructures (PCuNS) The procedure for the synthesis of PCuNS was performed as follows: First, a piece of copper foil was ultrasonically cleaned in acetone, and subsequently with deionized water to remove surface impurities. The fresh copper foil was then immersed into a CuCl2 aqueous solution (∼37.5 wt%). After 10 min, the copper foil was then taken out from the solution and rinsed with deionized water. Finally, the copper foil was immersed into an aqueous solution of KOH (10 wt%) (see Fig. 1(a) and (b)). After a given reaction time, the copper foil coated with the product film was taken out, washed with deionized water and dried in air. 2.2. Synthesis of gold coated PCuNS (PAuNS) A gold film was deposited on the PCuNS in a dc sputtering system at room temperature, as shown in Fig. 1(c) and (d). PAuNS (Fig. 1(e)) were obtained by using different sputtering times, between 20 and 260 s with a current of 0–10 mA. 2.3. Instrumentation and data acquisition Scanning electron microscopy (SEM) (FEI INSPECT F50), equipped with X-ray energy dispersive spectroscopy (EDS) capabilities, was used to investigate the structures. The Raman
measurements were performed on a Jobin Yvon LabRAM HR800 micro-Raman spectrometer with the 514 nm laser line at room temperature. An area ∼3 m in diameter was probed by a 50× objective lens and the incident power at the sample was 0.05 mW. The signal collection time was 20 s. In order to evaluate the capability of Raman enhancing of the product, a R6G water solution was used. To allow molecule adsorption, the substrate was maintained for 30 min in the R6G solution and then taken out and rinsed thoroughly. The acquisition time and laser power were uniform for all Raman spectra. The SERS spectra were recorded from multiple sites on the substrate surface to confirm reproducibility. Similar SERS spectral characteristics such as enhancement, position, and relative intensity of the bands were determined from various locations to confirm large area production of uniform geometries. 3. Results and discussion 3.1. The effect of reaction parameters on the surface structure of PCuNS The morphologies of PCuNS are reaction-time dependent, as shown in Fig. 2. At the early stage, the sample was composed of a large amount of irregular protuberances (Fig. 2(a)). One can see protuberances that are ∼2 m in size from the high magnification image. When the reaction-time proceeded to 2 min, the needlelike micro/nanostructures are found to grow sparsely out of the protuberance, as shown in the inset of Fig. 2(b). With a further extension of the reaction-time from 2 min to 5 min, the structures became much thicker and bamboo-shoot-like rods formed (Fig. 2(c)). After reaction for 30 min, no protuberances remained and the sample was composed entirely of 3D pine-needle-like micro/nanostructures (Fig. 2(d)) [30–32]. An average diameter of these 3D structures was measured to be ∼5 m. The entire structure of the architecture is built from several dozen needles with smooth surfaces and the needles had a variety of shapes and sizes. These needles were ∼200 nm thick and ∼600 nm wide and connected to each other through the center to form 3D pine-needle-like
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Fig. 2. SEM images of the sample surface after immersion in the solution for: (a) 30 s; (b) 2 min; (c) 5 min; (d) 30 min; (e) 40 min; and (f) 125 min. Scale bars: 6 m. The inset (left) shows the SEM images at high magnification, scale bars: 2 m. The inset (right) shows the model of the micro/nanostructure. The additional inset in (d) shows EDS spectrum of the as-prepared PAuNS.
structures. The chemical composition of as-synthesized products was observed using EDS spectrum (see the inset of Fig. 2(d)). The EDS spectrum demonstrates that the products are made of Cu and O and the atomic ratio of Cu to O is approximately equal to 1:2, confirming that the products are Cu(OH)2 . Interestingly, from the magnified SEM images (see the inset of Fig. 2(d)), one can find that the growth seems to emanate from a common nucleation point with numerous needles sharing the same central core, and the diameter of the needles becomes larger with a longer reaction period. Throughout the whole process, we could easily obtain four kinds of precursor samples: (1) protuberances; (2) protuberances and needles; (3) protuberances, needles, and a 3D pine-needle-like structure; and (4) a 3D pine-needle-like structure. Such a process is a so-called two-stage growth process, which involves a fast nucleation of amorphous primary particles followed by a slow aggregation and crystallization of primary particles [33–35]. All samples obtained here exhibit pine-needle-like morphology, although a further increase in the reaction time (40 min and 125 min) blurred the structure of PCuNS, shown in Fig. 2(e) and (f). Hence, we consider 30 min is an appropriate reaction-time to obtain high quality structures of PCuNS. From this point, the reaction-time is the key factor in the formation of the pine-needlelike structures. In order to investigate the role of concentration of KOH in the formation of the 3D pine-needle-like structures, a series of experiments were conducted by varying the solvent, while other experimental parameters were kept the same. KOH concentration can also strongly influence the Cu(OH)2 micro/nanostructures, as presented in Fig. 3. We varied the KOH concentration from 1 wt% to 15 wt% while keeping temperature (room temperature)
and reaction-time (30 min) constant. At a lower KOH concentration of 1 wt%, the final morphology was Cu(OH)2 micro/nano-ribbons (Fig. 3(a)). An increase in the mass fraction of KOH (5 wt%) resulted in the formation of 3D plate-like micro/nano structures (Fig. 3(b)). The dominant products were 3D plate-like microflowers, and only a few dispersed micro/nanoplates were found in the samples. Further increasing the mass fraction of KOH to 10 wt% produced a large amount of perfect 3D pine-needle-like micro/nanostructures (Fig. 3(c)). And the surface of needles of these nanostructures turned smooth gradually with the plates becoming longer and longer. When the KOH concentration was increased to 15 wt%, blurring and filling (the spaces of two adjacent needles) in the flower-shaped nanostructures can be observed (Fig. 3(d)). Presumably, with increasing mass fraction, the nanoscale corrugation of the pine-needle-like nanostructures becomes less obvious, and only some protrusions on the surfaces are observed. Since the contributions can strongly increase the SERS effect which originates from cross-junctions between tilted and/or horizontally oriented needles, we adopt 10 wt% concentration for the growth value. 3.2. Possible formation mechanism The evolution of various morphologies such as plates, needles and 3D pine-needle-like micro/nanostructures, can be explained on the basis of the combination and concentration of the precursors. Based on the above experimental results, we propose a possible formation mechanism for the 3D pine-needle-like micro/nanostructures. The whole process is illustrated schematically in the inset of Fig. 2. In the heterogeneous nucleation stage,
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Fig. 3. SEM images of the Cu(OH)2 nanostructures prepared under different mass concentrations of KOH by keeping other reaction parameters constant (at room temperature for 30 min). (a) 1 wt%; (b) 5 wt%; (c) 10 wt%; and (d) 15 wt%.
Cu2+ coordinated with KOH to rapidly produce copper hydroxyl, which precipitated to become the nuclei and quickly grew into the primary particles. This promotes the random formation of nuclei on the surface of the Cu. A large number of primary irregular particles were instantaneously formed in this stage. In the second stage, as the growth and concentration of irregular particles reach a so-called metastable state in the reaction system, the asformed structures tend to aggregate into protuberances (see the inset of Fig. 2(a)). And these protuberances became the core of the pine-needle-like structure. The protuberances continued to grow by combining with the remaining primary particles, probably due to Ostwald ripening [36]. It is known that different growth rates of the crystal faces determine the ultimate morphology of the nanomaterial. The growth rates of Cu(OH)2 needles along the different direction can be of great difference. In the following stage, the needle-like structures grew gradually out of the protuberances. With increasing reaction time, needles became much thicker and longer, finally forming the 3D pine-needle-like structure. Although many kinds of such 3D structures have been reported, several factors, such as crystal-face attraction, associated with the aggregate, van der Waals forces, hydrophobic interactions, and hydrogen bonds, may have various effects on the self-assembly. Hence, the detailed formation mechanism in this synthesis still needs further investigation.
thought to be necessary for efficient SERS enhancement. Thus, to provide SERS activity, we coated gold onto these PCuNS to duplicate the morphology, i.e. we fabricated PAuNS. Here, we used the samples with the sputtering time of 80 s for demonstration. Raman activity of these PAuNS samples was evaluated by detecting the spectra of target analyte R6G, with a 10−5 M water solution, as featured in Fig. 4(a). For comparison, R6G adsorbed on gold coated particles, corresponding to Fig. 2(a), exhibits weak R6G Raman bands. The peaks from 400 to 900 cm−1 are attributed to R6G signals. Accumulation times (20 s) and the laser power (0.05 mW) are
3.3. SERS performances of the 3D PAuNS substrates
Fig. 4. (a) SERS spectra of 10−5 M R6G adsorbed on the PAuNS (red) and that adsorbed on the gold coated particles (blue); (b) SERS signal (at 612 cm−1 ) comparison of R6G adsorbed on the PCuNS and the corresponding PAuNS (t = 80 s). Scale bars: 2 m. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
are especially interested in these thorny We micro/nanostructures as their nanoscale features are currently
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Fig. 5. (a) Typical SEM image of PAuNS: t = 80 s. The inset in (a) shows one pineneedle-like arm of the PAuNS; (b) typical simulated EM-field distribution maps of the PAuNS. The 2 V/m in the EM field does not represent the maximum electric field strength but is artificially set to provide better intuitive models of the EM field.
the same for all the acquiring. A main peak (612 cm−1 ) intensity of PAuNS is more than 42 times stronger than that of corresponding PCuNS, as shown in Fig. 4(b). The enhancement can be qualitatively explained by that PAuNS are assembled on the copper foil substrate with a very high density and with many horns. Hence, R6G molecules in abundant hot spots between closely spaced pine-needle-like arms lead to intensive SERS enhancement. To assess the contributions of the PAuNS geometries to the SERS intensities, local EM fields were calculated using commercial finitedifference time-domain (FDTD) software (CST MWS 2009). A planar and cross-sectional view of the calculated radial EM field components of the PAuNS (t = 80 s) is displayed in Fig. 5. A pine-needle-like model pattern (elliptical shape, the major axis and minor axis are 591 nm and 193 nm, respectively) using dimensional parameters illustrated in Fig. 5(a). The radiation at 514 nm is assumed to be normal to the sample surface. In the contour plots of the near EM field distributions shown in Fig. 5(b), one can find that numerous hot spots exist near the interstitial areas of the pine-needle-like arms. We can achieve the largest enhanced factor value of ∼106 for the pine-needle-like model pattern. This exceeds the average enhanced factor (∼104 ) over the entire PAuNS by several orders of magnitude because most of the optical excitations are localized in these hot spots. To further explore the role of gold layer on the SERS properties of PAuNS, we increased the sputtering time of gold from 20 s to 260 s to tailor the sizes of the gold film as illustrated in Fig. 6(a). Fig. 6(b) shows corresponding SERS spectra of the PAuNS formed by using different gold sputtering times. The variation in the SERS signal intensity at 612 cm−1 is presented in Fig. 6(c). The intensity of the R6G Raman signal increases when the sputtering time goes from 20 to 140 s, but decreases when extending the sputtering duration. The excessive Raman enhancement can be mainly associated with the effect of resonance coupling between neighboring pine-needlelike arms. It can be observed that the Raman enhancement observed from the sample when t = 140 s is larger than that from the others. Such strong enhancement in that sample can be attributed to the fact that the pine-needle-like arms are assembled with a favorable gap configuration [37]. Comparisons among the spectra reveal the existence of different SERS enhancements, especially the quantitative intensity comparison from the selected band (612 cm−1 ). It should be noted that the SERS intensity fluctuation, from spot to spot on a sample plate, was approximately 24.4% from the average value. All the results fall inside the control limits, indicating that the discrepancy is within the acceptable level. Impressively,
Fig. 6. (a) Schematic of PAuNS tailored by tAu ; (b) SERS spectral comparison of 10−5 M R6G adsorbed on the PAuNS by adjusting the sputtering time tAu (from 0 s to 260 s); (c) intensity profile of the peak at 612 cm−1 as a function of tAu ; (d) SERS spectra of different R6G concentrations adsorbed on the PAuNS of (I) 10−5 M, (II) 10−6 M, (III) 10−7 M, (IV) 10−8 M, and (V) 10−9 M, demonstrating the intensity variation of the SERS signal at 612 cm−1 .
when the as-prepared PAuNS (t = 140 s) are applied for the SERS detection of R6G, the sensitivity is extremely high, where R6G with a concentration of 10−8 M can be easily tracked (Fig. 6(d)). From a R6G concentration of 10−5 M to 10−9 M, we can see very sharp R6G peaks with very limited noise. Hence, these 3D PAuNS SERS substrates will potentially find applications to detect and analyze other biomolecules or dangerous chemicals. 4. Conclusion In summary, we report a facile technique to the large-scale synthesis of PAuNS substrates without the addition of any surfactants and organics. The 3D hierarchical micro/nanostructured surface shows high sensitivity in SERS detection of the target analyte, R6G, with a detection limit up to 10−8 M. The FDTD calculations indicate that the pine-needle-like model pattern may demonstrate a high quality SERS property owing to the high density and abundant hot spot characteristic in closely spaced pine-needle-like arms. We believe that such 3D pine-needle-like structures could serve as better substrates for SERS applications and provide an excellent candidate for SERS analysis. Acknowledgments This work was jointly supported by the National Natural Science Foundation of China under Grant no. 51271057, the Natural Science Foundation of Jiangsu Province, China, under Grant no. BK2012757, and the Program for New Century Excellent Talents in University of Ministry of Education of China under Grant no. NCET-11-0096. References [1] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Raman spectra of pyridine adsorbed at a silver electrode, Chem. Phys. Lett. 26 (1974) 163–166.
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Please cite this article in press as: K. Long, et al., Facile synthesis of gold coated copper(II) hydroxide pine-needle-like micro/nanostructures for surface-enhanced Raman scattering, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.133