Double-potentiostatic electrodeposition of Ag nanoflowers on ITO glass for reproducible surface-enhanced (resonance) Raman scattering application

Electrochimica Acta 67 (2012) 12–17

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Double-potentiostatic electrodeposition of Ag nanoflowers on ITO glass for reproducible surface-enhanced (resonance) Raman scattering application Juncao Bian, Zhe Li, Zhongdong Chen, Xiwen Zhang ∗ , Qian Li, Shan Jiang, Junhao He, Gaorong Han State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China

a r t i c l e

i n f o

Article history: Received 19 November 2011 Received in revised form 17 January 2012 Accepted 20 January 2012 Available online 10 February 2012 Keywords: Ag nanoflowers Double-potentiostatic method Electrodeposition SE(R)RS Reproducibility

a b s t r a c t Herein, we first report electrodeposition of dense (1.3× 109 cm−2 ) and uniform (RSD of size is about 11%) Ag nanoflowers on ITO glass by a double-potentiostatic method without any surfactant. Raman spectra showed that the Ag nanoflowers with multiple petals on ITO glass exhibited excellent SER(R)S ability. It could accommodate a considerable range of sample concentrations (10−10 M at least for rhodamine 6G). The smallest RSD of the intensities for 1651 cm−1 Raman band of rhodamine 6G was about 14.1%, indicating good reproducibility. Through the time-dependent morphological evolution investigation and high-resolution transmission electron microscopy (HRTEM) analysis, the formation of petals was ascribed to the slow reduction rate and high nucleation driving force by electrical field concentration. © 2012 Elsevier Ltd. All rights reserved.

Introduction Synthesis of nanosilver with various shapes is a promising strategy to tailor the surface plasmon resonance properties for the applications in areas such as photonics, sensing, and surface enhanced Raman scattering (SERS), etc. [1–3]. For instance, theoretical calculations and experimental results have both indicated that the SERS effect highly depends upon the shape and structure of metallic nanoparticles [4–6]. Particularly, the narrow gaps and sharp edges are rich in the three-dimensional Ag nanostructures, rendering them highly desirable for SERS application since the giant electromagnetic field are available in these areas [7]. While lots of approaches are achievable for the synthesis of dendritic, rosettelike, flowerlike and branched Ag nanoparticles in solution, multiple surfactants, organics or other metal powders are introduced, needing subsequent procedures for purifying the products [7–18]. On the other hand, the reproducibility for SERS of the solution-based particles is always disappointing. Hence, there is an increasing demand for the fabrication of Ag nanoparticles on substrates. To date, substrate-supported three-dimensional Ag nanostructures are mainly realized by direct chemical reduction of Ag ions by polyaniline (PANI) [19], replacement reaction using bulk materials (e.g. Si, Zn, Al) as substrates [20–22] and electrochemical methods [23–28]. However, the density and uniformity of these reported Ag nanostructures on substrates were unsatisfying,

∗ Corresponding author. Tel.: +86 571 88276234; fax: +86 571 87952341. E-mail address: [email protected] (X. Zhang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.01.073

which deteriorated the reproducibility and the efficiency for SER(R)S application. Therefore, direct construction of the dense and uniform three-dimensional nanosilver on substrate in a simple way is still a great challenge. Electrodeposition is attractive as it is versatile and simple to prepare metal nanoparticles. Compared with the other electrochemical methods, the double-potentiostatic method can realize a quick nucleation and slow particle growth process, which is crucial for deposition of uniform Ag nanoparticles on substrates. Nevertheless, this method is still rarely utilized in the synthesis of metallic nanostructures [29]. Herein, we first report single-step, green synthesis of uniform and dense Ag nanoflowers on ITO glass by a double-potentiostatic method without using any surfactant at room temperature. The morphology of the Ag nanoparticles could be tuned by varying the nucleation-potential. As-prepared Ag nanoflowers on ITO glass exhibited efficient surface-enhanced (resonance) Raman scattering (SE(R)RS) enhancement with excellent reproducibility. Experimental The electrodeposition of Ag nanoflowers were conducted at 26.5 ± 0.5 ◦ C in the aqueous electrolyte including 0.05 mM AgNO3 , 0.2 mM sodium citrate (C6 H5 Na3 O7 , CitNa) and 0.1 M KNO3 . During a typical double-potentiostatic electrodeposition process, the nucleation process was initially conducted at a more negative potential (named nucleation-potential) for short time. Then the growth process was extended at a more positive potential (named growth-potential) for long time, as was reported in our previous

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Fig. 1. SEM images of the samples deposited with CitNa at (a) −0.5 V (C1), (b) −0.4 V (C2), (c) −0.3 V (C3) and (d) without CitNa at −0.3 V for 100 s and −0.2 V for 3600 s. The insets in panel c and d are the size variation diagrams of the Ag nanoflowers. (e) EDS and (f) XPS spectrum of C3.

work [29]. Herein, the nucleation-potentials were set to be −0.5 V, −0.4 V and −0.3 V for 100 s, separately. The growth-potential was kept at −0.2 V for 3600 s. The Ag nanoflowers were deposited on cleaned ITO glass in a standard three-electrode system, where ITO glass was used as working electrode, Pt plate as counter electrode and saturated calomel electrode (SCE) as reference electrode. All the potentials are reported versus the SCE reference electrode. To avoid contamination of chloride ion from the SCE, the aqueous electrolyte was connected to the saturated potassium nitrate solution via a salt bridge filled with agaragar and potassium nitrate. The surface morphology of the Ag nanoflowers was obtained from Hitachi S4800 scanning electron microscopy (SEM). Transmission electron microscopy (TEM) and High-resolution TEM (HRTEM) with energy dispersive spectrometer (EDS) and selected area electron diffraction (SAED) were taken on a Philips-FEI Tecnai G2 F30 S-Twin TEM working at 300 kV. X-ray photoelectron spectroscopy (XPS) was performed on VG ESCALAB MARK II X-ray photoelectron spectrometer with Mg K␣ 1253.6 eV. The absorption spectra were recorded with a TU-1901 UV–vis spectrophotometer by using bare ITO coated glass as the reference. For SERS spectral assessment, as-prepared samples were soaked in a 10−6 , 10−8 and 10−10 M aqueous rhodamine 6G (R6G) and 10−6 M aqueous adenine solutions, respectively, for 3 h, then taken out, rinsed by deionized water and naturally dried. The Raman scattering measurements

were carried out on a Raman system (RENISHAW inVia) with confocal microscopy. The laser (514.5 nm, 0.025 mW) with the spot diameter of 1 ␮m was used and the acquisition time was 10 s. Results and discussion Fig. 1a–c shows that the surface morphology and structure of the Ag nanoparticles are highly dependent upon the nucleationpotential. To facilitate the description, the samples deposited at −0.5 V, −0.4 V and −0.3 V for 100 s and −0.2 V for 3600 s are referred to as C1, C2 and C3, respectively. As the nucleation-potential becomes more positive, the morphology of Ag nanoparticles gradually evolves from quasi-sphere into flower shape and the petal number of the nanoflowers increases. It should also be noted that the density of the nanoparticles decreases and their size increases. This phenomenon is because that less nucleation positions are activated as the nucleation is more positive, leading to more Ag ions available for the growth of the single particle [29]. To investigate the effect of CitNa on the morphology of the Ag nanoflowers, electrodeposition process without CitNa was also carried out. Comparing Fig. 1 c with d, and the insets (size variation of the Ag nanoflowers), it can be concluded that CitNa has no effect on the formation of Ag nanoflowers but improves the uniformity. It is attributed to that citrate played the role of complexant and could coordinate

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Fig. 2. (a) TEM image of a single nanoflower deposited with CitNa at −0.3 V for 100 s and at −0.2 V for 3600 s, (b) HRTEM image of a petal tip in panel a. Arrows indicate the attached Ag nanoparticles. (c) HRTEM image of an attached Ag nanoparticles, (d) TEM image of the Ag nanoparticles deposited at −0.8 V for 100 s and at −0.2 V for 3600 s.

with Ag ions, which declined the reduction rate of Ag ions [13]. EDS (Fig. 1e) and XPS (Fig. 1f) were obtained to further confirm the deposition of Ag. The Cu and C peaks in Fig. 1e come from the Cu grid used for characterization. To calculate the density of the Ag nanoflowers, the number of the nanoflowers was counted from the SEM image (×20,000), in which there were about 343 Ag nanoflowers. The length and width of the space in the SEM image were about

6342 nm and 4436 nm, respectively. The density was then obtained through dividing the number of the nanoflowers by the area of the space. The density of the Ag nanoflowers (C3) was calculated to be about 1.3 × 109 cm−2 , indicating their large density. To further investigate the structure of the flower-shaped Ag nanoparticles, TEM and HRTEM images of C3 were recorded. Fig. 2a shows the TEM image of an individual nanoflower of C3, in which

Fig. 3. SEM images of the Ag nanoflowers deposited at −0.3 V for 100 s and at −0.2 V for (a) 100 s, (b) 300 s, (c) 600 s and (d) 1200 s, separately, (e) TEM image of single nanoflower of the sample deposited at −0.3 V for 100 s and at −0.2 V for 100 s, (f) HRTEM of a joint in the square frame of panel e.

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Fig. 4. (a–c) UV–vis extinction spectra of C1–C3 with corresponding fitting curves, (d–g) Fitting curves as a function of nucleation-potential.

rod-like and branched petals can be observed. It should also be noted that there are some Ag nanoparticles attached to the petal tips. The HRTEM image of a petal tip in Fig. 2b clearly reveals lattice fringes, indicating the good crystalline. The fringe spacings are determined to be about 0.24 nm and 0.20 nm, which is well corresponding to (1 1 1) and (2 0 0) crystalline faces of Ag [8,14]. Fig. 2c shows an attached Ag nanoparticles in Fig. 2a. It is clear that the attached Ag nanoparticles and the petal tip have a different orientation, where the fringe spacings are about 0.20 nm and 0.24 nm, separately. Fig. 2d shows the Ag nanoparticles deposited under nucleation-potential of −0.8 V. It is obvious that the Ag nanoparticles are quasi-spherical. In order to understand the formation process of the nanoflowers, the time-dependent morphology evolution of C3 was investigated. Fig. 3a indicates that the asymmetric nanoparticles with some bulbous tips are formed in the very early growing stage (100 s). Then, as the growth time reaches 300 s (Fig. 3b), multiple petals emerge. Moreover, when the growth time is further prolonged to 600 s (Fig. 3c) and 1200 s (Fig. 3d), the diameter and length of the petals increase while their number seems to be unchanged. Fig. 3e shows the TEM image of the nanoparticles in the initial growing stage, where a particle linked to two smaller particles can be seen. One of the two joints is investigated by HRTEM (Fig. 3f), in which clear spacing lattice fringes can be observed. The fringe spacings are about 0.24 nm and 0.20 nm for the left and right side, respectively, which well match the (1 1 1) and (2 0 0) crystalline faces of Ag [8,14]. On the basis of the above results, a possible formation mechanism of the nanoflowers is proposed as follows. Initially, the Ag atoms were produced under −0.3 V. The nucleation driven force

was so small that the Ag atoms stacking in the (1 1 1) face was preferred as it has the lowest surface energy among the crystalline faces [7,10]. Due to the low reduction rate, the particle shape gradually deviated from sphere and anistropic growth was triggered. And sharp edges were produced [14]. Then the electrical field concentration occurred around the sharp edges, where a higher nucleation driving force was provided to form new nuclei whose atoms could stack in (2 0 0) [25,30]. More positive the nucleation potential was, more sharps edges were formed, leading to more petals. As the Ostwald ripening process proceeded under −0.2 V, (1 1 1) face stacking of Ag atoms happens. Then sharp edges were formed again and electrical field concentration induced new nuclei again, then flowers with branched petals could be produced. The process might repeatedly occur in this manner. It is believed that the nucleation potential plays the key role in the formation of flower-shaped Ag nanoparticles. In our previous work, only quasi-spherical Ag nanoparticles were formed when the nucleation-potential was more negative than −0.6 V. Anisotropic growth didn’t happen at the growth-potential of −0.2 V [29]. Because of the high nucleation driving force, the reduction rate of Ag ions was so high that quasi-spherical Ag nuclei were formed in the nucleation stage. Then the growth rates of the various directions of the Ag nuclei were similar and as-prepared Ag nanoparticles were quasi-spheres, as is shown in Fig. 2d. We speculate that once the anisotropic growth is triggered at low reduction rate in the nucleation stage, then the anisotropic growth will continue to the growth stage. Fig. 4a–c presents the extinction spectra of C1–C3. The broad resonance peaks are found to be a combined effect of several resonance

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Fig. 5. SERRS spectra of R6G aqueous solution (10−6 M) absorbed (a) on the surface of C1–C3 and (b) on the surface of C3 and Ag NAs in panel f, (c) SERS spectra of R6G aqueous solutions with different concentrations (10−6 , 10−8 , 10−10 M) absorbed on C3. (d) The Raman spectra of the bare ITO glass, “clean” Ag nanoflowers and the Ag nanoflowers immersed in 10−10 M R6G for 3 h, (e) SERS spectrum of adenine aqueous solution (10−6 M) absorbed on the surface of C3, (f) Ag NAs deposited at −0.3 V 100 s and −0.05 V 1800s (aqueous electrolyte including 0.5 mM AgNO3 and 0.1 M KNO3 ). (g–i) The normalized Raman signal intensities for the bands at 1651 cm−1 ,obtained from three different samples deposited at −0.3 V for 100 s and −0.2 V for 3600 s. 50 spots of every sample were examined.

peaks, which is clear for C1. The combined peaks are resolved into four Gaussian peaks, which are centered at around 370 nm, 420 nm, 510 nm and 650 nm. The peaks at around 420 nm and 510 nm can be attributed to the dipole mode of the Ag nanoparticles or the transverse mode of the rod-like petals [12]. The peaks at around 370 nm and 650 nm are due to the quadrupole mode of the nanoparticles and the longitudinal mode of the rod-like petals [7,12,29], respectively. As can be seen from Fig. 4d–g, all of the four peaks have red-shift as the nucleation-potential is more positive. The decreasing intensity of the quadrupole mode and the broadening peak widths of all peaks can be ascribed to the enhanced plasmon coupling of the adjacent nanoparticles [29,31]. The red-shift of the peaks centered at around 420 nm, 510 nm and 650 nm is due to the synergy effect of increasing size and plasmon coupling [29,32–34] Fig. 5a depicts the Raman spectra of R6G on the surface of C1, C2 and C3. The Raman bands at about 1651, 1574, 1509, 1363, 1312, 1182, and 1129 cm−1 are attributed to the Raman signal of R6G, which well match the previous report [35]. It should be noted that the Raman signal of R6G molecules was partially due to the resonance enhancement of the laser (514.5 nm) used [36]. Therefore, the Raman spectrum of R6G is actually the surface-enhanced

resonance Raman scattering (SERRS) signal. To further illustrate the enhancement ability of the Ag nanoflowers, a comparison of the SERRS ability between the Ag nanoflowers and the Ag nanoparticle arrays (NAs) with the comparable surface density of the Ag nanoparticles (shown in Fig. 5f) was conducted under the same characterization conditions. As is shown in Fig. 5b, the intensity of 1651 cm−1 band of R6G molecules absorbed on the surface of Ag nanoflowers is about 18 times larger than that of on the Ag NAs, revealing the strong enhancement ability of the Ag nanoflowers. Due to the complex morphology of the Ag nanoparticles as well as the resonance enhancement effect, it is difficult to obtain the accurate enhancement factor of C3 based on previous formula. Nevertheless, it is obvious that C3 has the highest enhancement efficiency. The SERRS signal intensity of C3 for the bands at 1651 cm−1 is about 2.5 times stronger than that of C2 and 6.5 times stronger than that of C1, even though the particle density of C3 is much lower than that of C1 and C2. This is because, compared with C1 and C2, C3 contains nanoflowers with more dense petals. The gaps between the neighboring petals generate giant electromagnetic fields which efficiently enhance the Raman signal of R6G [7]. To study the SERRS dynamical range of C3, R6G aqueous

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Fig. 6. SEM images of two samples deposited under the same conditions as C3.

solutions with different concentrations (10−6 , 10−8 , 10−10 M) are used as analyte, as is shown in Fig. 5c. The band at 1651 cm−1 is about 30 units for 10−10 M R6G, indicating that the number of hot spots for molecule adsorption is large enough to accommodate a considerable range of sample concentrations. Fig. 5d shows the Raman spectra of bare ITO glass, Ag nanoflowers on ITO glass and Ag nanoflowers on ITO glass immersed in 10−10 M R6G. Apart from the Raman bands of R6G at 164, 244, 313 and 370 cm−1 [35], only a weak peak at about 470 cm−1 is found which may be attributed to the carbon impurities. No ITO band appears possibly due to the low laser power. Hence, the ITO films and carbon impurities do not interfere with the low frequency mode of the target molecules. SERS signal of adenine, a small non-resonance molecule, was also collected to examine the SERS capability of the Ag nanoflowers. Evident adenine Raman peaks centered at about 730 and 1325 cm−1 , show in Fig. 5e, were found [7,14], indicating the good SERS ability of the Ag nanoflowers. As adenine is a nucleotide-based molecule, the direct detection of this molecule is very important for the biochemistry [37]. To evaluate the reproducibility of the SE(R)RS effect of the Ag nanoflowers, 50 spots were randomly chosen across C3 and measured under the same characterization conditions. Fig. 5g shows that the relative standard deviation (RSD) of the intensities for the 1651 cm−1 band (relative to the baseline) of the samples are about 14.7%. The electrodeposition process was then repeated for two times. And another two RSD, shown in Fig. 5h and i, are about 14.1% and 15.0%. The RSD of the three mean intensities of 1651 cm−1 band is about 2.2%, implying the reliability of the electrodeposition process. The SEM images of another two samples also show the similar surface morphologies, as is shown in Fig. 6. These results indicate the good reproducibility of the Ag nanoflowers for SER(R)S application. The difference in the intensity across the samples could be ascribed to the heterogeneity of the Ag nanoflowers or the number variation of the nanostructures among different spots.

Conclusions In summary, dense (1.3 × 109 cm−2 ) and uniform (RSD of size is about 11%) Ag nanoflowers with multiple petals have been successfully deposited on ITO glass by a double-potentiostatic method without surfactant. The formation of the nanoflowers was ascribed to the slow reduction rate and electrical field concentration, providing a high nucleation driving force for the formation of petals. As-deposited Ag nanoflowers with multiple petals on ITO glass exhibited excellent SE(R)RS enhancement compared with the other two substrates. It could accommodate a considerable range of sample concentrations. The smallest RSD for 1651 cm−1 Raman band of R6G was about 14.1%, indicating good reproducibility. The electrodeposition process also showed good reliability and can be used for processing other metals.

Acknowledgments This work was supported by the National Basic Research Program of China (973 Program) “2007CB613403” and Foundation of the scientific research base development (Engineering Research Center of the Education Ministry for the Surface and Structure Modification of Inorganic functional Materials) “KYJD09014”. References [1] H. Wei, Z. Wang, X. Tian, M. Käll, H. Xu, Nat. Commun. (2011) 1. [2] K.M. Mayer, J.H. Hafner, Chem. Rev. 111 (2011) 3828. [3] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Chem. Rev. 111 (2011) 3669. [4] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2003) 668. [5] N.R. Jana, T. Pal, Adv. Mater. 19 (2007) 1761. [6] A.J. Pasquale, B.M. Reinhard, L.D. Negro, ACS Nano 5 (2011) 6578. [7] L. Lu, A. Kobayashi, K. Tawa, Y. Ozaki, Chem. Mater. 18 (2006) 4894. [8] M. Zhang, A. Zhao, H. Guo, D. Wang, Z. Gan, H. Sun, D. Li, M. Li, CrystEngComm 13 (2011) 5709. [9] Y. Wang, P.H.C. Camargo, S.E. Skrabalak, H. Gu, Y. Xia, Langmuir 24 (2008) 12042. [10] L. Fan, R. Guo, Cryst. Growth Des. 8 (2008) 2150. [11] X. Li, J. Wang, Y. Zhang, M. Li, J. Liu, Eur. J. Inorg. Chem. (2010) 1806. [12] B.K. Jena, B.K. Mishra, S. Bohidar, J. Phys. Chem. C 113 (2009) 14753. [13] L. Hong, Q. Li, H. Lin, Y. Li, Mater. Res. Bull. 44 (2009) 1201. [14] Y. Han, S. Liu, M. Han, J. Bao, Z. Dai, Cryst. Growth Des. 9 (2009) 3941. [15] T. Liu, D. Li, D. Yang, M. Jiang, Langmuir 27 (2011) 6211. [16] J. Yang, R.C. Dennis, D.K. Sardar, Mater. Res. Bull. 46 (2011) 1080. [17] W. Ren, S. Guo, S. Dong, E. Wang, J. Phys. Chem. C 115 (2011) 10315. [18] G. Zhang, S. Sun, M.N. Banis, R. Li, M. Cai, X. Sun, Cryst. Growth Des. 11 (2011) 2493. [19] P. Xu, N.H. Mack, S.H. Jeon, S.K. Doorn, X. Han, H.L. Wang, Langmuir 26 (2010) 8882. [20] J. Fang, H. You, P. Kong, Y. Yi, X. Song, B. Ding, Cryst. Growth Des. 7 (2007) 864. [21] A. Gutes, C. Carraro, R. Maboudian, J. Am. Chem. Soc. 132 (2010) 1476. [22] P.R. Brejna, U. Sahaym, M.G. Norton, P.R. Griffiths, J. Phys. Chem. C 115 (2011) 1444. [23] N. Zhao, F. Shi, Z. Wang, X. Zhang, Langmuir 21 (2005) 4713. [24] Q. Zhou, S. Wang, N. Jia, L. Liu, J. Yang, Z. Jiang, Mater. Lett. 60 (2006) 3789. [25] C. Gu, T. Zhang, Langmuir 24 (2008) 12010. [26] B. Rezaei, S. Damiri, Talanta 83 (2010) 197. [27] S. Cherevko, X. Xing, C. Chung, Electrochem. Commun. 12 (2010) 467. [28] X. Qin, H. Wang, X. Wang, Z. Miao, Y. Fang, Q. Chen, X. Shao, Electrochim. Acta 56 (2011) 3170. [29] J. Bian, Z. Li, Z. Chen, H. He, X. Zhang, X. Li, G. Han, Appl. Surf. Sci. 258 (2011) 1831. [30] J. Fang, H. You, C. Zhu, P. Kong, M. Shi, X. Song, B. Ding, Chem. Phys. Lett. 439 (2007) 204. [31] D.D. Evanoff Jr., G. Chumanov, ChemPhysChem 6 (2005) 1221. [32] J.J. Mock, M. Barbic, D.R. Smith, D.A. Schultz, S. Schultz, J. Chem. Phys. 116 (2002) 6755. [33] P.K. Jain, S. Eustis, M.A. El-Sayed, J. Phys. Chem. B 110 (2006) 18243. [34] B. Choi1, H. Lee, S. Jin, S. Chun, S. Kim, Nanotechnology 18 (2007) 075706. [35] P. Hildebrandt, M. Stockburge, J. Phys. Chem. 88 (1984) 5935. [36] L. Jensen, G.C. Schatz, J. Phys. Chem. A 110 (2006) 5973. [37] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Chem. Rev. 99 (1999) 2957.