Direct growth of silver nanostructures on zinc substrate by a modified galvanic displacement reaction

Solid State Communications 149 (2009) 1755–1759

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Direct growth of silver nanostructures on zinc substrate by a modified galvanic displacement reaction Sa Lv a , Hui Suo a , Xu Zhao b , Chunxu Wang a , Shengyu Jing a , Tieli Zhou a , Yanan Xu a , Chun Zhao a,∗ a

Joint State Key Laboratory on Integrated Optoelectronics, Jilin University, Changchun 130012, People’s Republic of China

b

College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China

article

info

Article history: Received 12 May 2009 Received in revised form 20 June 2009 Accepted 18 July 2009 by F. Peeters Available online 23 July 2009 PACS: 73.61.At 79.60.Jv Keywords: A. Metals A. Nanostructures E. Galvanic displacement

abstract This paper reports a modified galvanic displacement approach for the synthesis of Ag nanostructures with different morphologies. During the process, AgNO3 as starting material is reduced using zinc foil and this is followed by suitable thermal treatment. The reaction time, concentration of the AgNO3 aqueous solutions and thermal treatment temperature directly influence the morphologies of Ag nanostructures. X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and UV–visible spectra are used to characterize the products obtained. Furthermore, a representative experiment using rhodamine (R6G) as the probe molecule confirms that the Ag nanostructure shows strong surface-enhanced Raman scattering (SERS) activity. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, metal nanostructures have attracted considerable attention on account of their widespread potential applications in electronics, photography, catalysis, magnetism and information storage [1,2]. In particular, Ag nanostructures continue to arouse intense scientific interest due to their practical uses in ultra-trace chemistry, biological sensing, surface plasma resonance (SPR) and surface-enhanced Raman scattering (SERS) [3,4]. The intrinsic properties of metal nanostructures are associated with their shape and size [5]. Therefore, substantial efforts have been focused on modifying the synthesis strategies that could provide the desired control of the morphology of Ag nanostructures. So far, Ag nanowires, nanobelts, nanorods, nanocubes, nanodendrites and nanoplates have been constructed by a variety of approaches, including seed-mediated growth, the polyol process, electrochemical deposition, use of soft templates and photochemical synthesis, etc. [6–10]. Most recently, galvanic displacement as a facile, efficient method has been widely adopted for depositing Ag nanostructures directly on solid substrate. Galvanic displacement is a type of electroless deposition process that utilizes the difference

∗

Corresponding author. Tel.: +86 431 85168241; fax: +86 431 88499134. E-mail address: [email protected] (C. Zhao).

0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.07.026

of the standard electrode potentials of various elements, leading to spontaneous reaction without using any additives or complicated procedures. However, most galvanic displacements involve the reduction of silver salts with metal or semiconductor substrate by immersing the substrate into the silver salt solution at room temperature. The reaction is inevitably effected by many external conditions, such as temperature, immersion rate, ultrasonic vibration and concentration gradient, which usually lead to uneven deposition [11]. To resolve these problems, Sun et al. have employed a method of dropping the AgNO3 solution on the surface of the substrate to directly grow Ag nanostructures at room temperature [3,12,13]. In our previous work, we made a detailed comparison of the growth processes and mechanisms for Ag nanostructures prepared by solution-dropping and solution-immersion routes. The results confirmed that it was difficult to control the deposition rate in the solution-immersion route. Moreover, a reaction rate that is too fast is unfavorable for anisotropic growth of Ag nanostructures [14]. In this communication, we describe the employment of a modified galvanic displacement reaction to directly grow Ag nanostructures on zinc substrate, which was realized by dropping AgNO3 solution on zinc surface followed by suitable thermal treatment. Furthermore, the Ag nanostructure obtained, as SERS substrate material, was also investigated by using rhodamine (R6G) as the probe molecule.

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Fig. 1. (a) Low magnification and (b) high magnification FE-SEM images; (c) a TEM image; ((d), (e)) SAED patterns; (f) the XRD pattern of the Ag nanoflowers.

2. Experimental section In a typical procedure, AgNO3 aqueous solution (0.02 M) was preheated at 50 ◦ C for 10 min, then a droplet of the above AgNO3 solution was dropped onto the surface of the previously cleaned zinc foil. After that, the zinc foil was transferred into a sealed container with a black cap to minimize the effect of light illumination and water evaporation. The whole reaction process was kept at 50 ◦ C for 5 h. The resulting zinc foil was then thoroughly rinsed with ethanol and deionized water and dried under nitrogen. The products obtained were characterized using X-ray diffraction (XRD, Shimadzu XRD 6000, Cu Kα radiation), a scanning electron microscope (SEM, JEOL FESEM JEM-6700F), a transmission electron microscope (TEM, HITACHI H-8100), selected area electron diffraction (SAED), UV–visible (Shimadzu UV-2550) and Raman spectrometers (Renishaw 1000, excited with a 514.5 nm Ar+ laser). 3. Results and discussion In our experiments, Ag nanostructures were prepared by reduction of AgNO3 with Zn according to the reaction equation 2+ 0 Zn0(s) + 2Ag+ (aq) → 2Ag(s) + Zn(aq) . The reaction principle was discussed in our previous paper [14]. Fig. 1 shows the characterized images of typical product obtained by a procedure of dropping 0.02 M AgNO3 solution onto zinc substrate followed by thermal treatment at 50 ◦ C for 5 h. As revealed by a low magnification SEM image (Fig. 1(a)), highly uniform Ag nanoflowers are distributed on the zinc substrate over a large area. Fig. 1(b) is a corresponding magnified image, clearly showing that the nanoflowers are constructed from many nanorods with a mean diameter of 60 nm. The area outlined in Fig. 1(b) is a representative image of the end of the nanorods, which exhibits a pentagonal crystalline structure. The results are further confirmed in Fig. 1(c) using TEM by scraping

off the black films on the zinc foil and ultrasonic dispersal in ethanol. The corresponding SAED patterns shown in Fig. 1(d) and (e) are taken at the tip and root sections of the nanorods, ¯ respectively. The appearance of the (111), (020) and (200) facets indicates that the Ag rods have twinning structure. Combining with the SEM result as shown in Fig. 1(b), it can be concluded that the Ag nanorods should be of fivefold-twinned structure bounded by five {111} facets, with the growth direction indexed to be along [110] as reported in the literature [15]. Fig. 1(f) shows a representative XRD pattern of the Ag nanoflowers. As can be seen, all of the peaks can be readily indexed to face-centered cubic (fcc) Ag (JCPDS card no. 04-0783), in addition to the peaks marked with asterisks being attributed to the zinc substrate (JCPDS 04-0831). No characteristic peaks of other impurities are observed, indicating high purity of the samples. The temporal evolution of the Ag nanoflowers was investigated and the corresponding SEM images of the samples synthesized from 10 min, 1 h, 5 h, and 9 h are presented in Fig. 2. During the early reaction stage (10 min), the structures obtained were spherical aggregates of nanoplates with the thickness of 25–35 nm (Fig. 2(a)). When the reaction time was increased to 1 h, Ag microspheres consisting of many short branches with sharpened tips were achieved (Fig. 2(b)). Further increasing the reaction time to 5 h resulted in highly uniform and dense Ag nanoflowers on a large scale (Fig. 2(c)), and the inset in Fig. 2(c) gives a magnified representative image of individual Ag nanoflowers comprising many nanorods, also evident from Fig. 1(a) and (b). With the reaction time extended to 9 h, it is observed from Fig. 2(d) that numerous Ag nanoparticles with diameter of about 30 nm formed net-like structures. The effect of the AgNO3 concentration on the morphologies of Ag nanostructure was further examined. When the AgNO3 concentration is 0.2 M under otherwise identical conditions, dendritic Ag

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Fig. 2. FE-SEM images of Ag nanostructures obtained at different reaction times: (a) 10 min; (b) 1 h; (c) 5 h; (d) 9 h.

Fig. 3. FE-SEM images of Ag nanostructures synthesized with different AgNO3 concentrations: (a) 0.2 M; (b) 0.002 M.

nanostructure was produced in high density with a length of about 3.5 µm (Fig. 3(a)). The inset in Fig. 3(a) indicates that the dendrite is a hierarchical structure sprawling to several generations with apparent symmetry. The dendrite growth is attributed to the oriented attachment and the appropriate concentration gradient of the Ag nanocrystals [11]. Ag nanoflowers were obtained with AgNO3 concentration of 0.02 M, as shown in Fig. 1(a) and (b). When the AgNO3 concentration is decreased to 0.002 M, quasi-spherical structures comprised of Ag nanoparticles with a lumpy surface were produced (Fig. 3(b)). Different thermal treatment temperatures influence the fabrication of Ag nanostructures. Low temperature (25 ◦ C) led to the formation of aggregates of Ag nanoparticles with the diameters in the range of 115–140 nm (Fig. 4(a)). This probably results from the reduced surface activity at low temperature, which hinders the preferential growth [16]. Once the temperature was raised to 75 ◦ C, the reaction proceeded much more quickly. Hence, a large quantity of initial Ag nuclei were rapidly formed. These unstable Ag

atoms diffuse and coalesce to form Ag clusters and gradually grow larger as more Ag atoms are deposited, as shown in Fig. 4(b). A similar mechanism was proposed for the Sn/HAuCl4 galvanic reduction system [17]. In our reaction system, the reaction rate is controllable through modulating the reactant concentration and thermal treatment temperature. The relatively slow deposition rate leads to anisotropic growth of the Ag nanostructure. In the initial stages of the reaction, accompanying the continuous generation and growth of Ag nuclei, quasi-flower Ag structures are produced as aggregates of Ag nanoparticles (supporting information, Fig. S1). The initially formed nanoparticles serve as starting nucleation points for producing Ag nanoplates. The nanoplates exhibit twinned bicrystalline structure, which is bounded by five {100} side faces and capped at both ends by five {111} faces [18]. The nanorods are obtained from the nanoplates with a specific elongation along the common [110] growth direction. As the reaction proceeds, short branches grow into long rods and form Ag nanoflowers. A similar

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Fig. 4. FE-SEM images of Ag nanostructures formed at different thermal treatment temperatures: (a) 25 ◦ C; (b) 75 ◦ C.

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Raman Intensity (a.u.)

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Fig. 5. (a) UV–visible absorption spectrum and (b) SERS spectrum of the Ag nanoflowers.

formation mechanism has been proposed in a previous report [15]. When the reaction time is overlong, net-like structures are probably formed through polymerization and fusion of adjacent nanostructures [3]. Increasing temperature over a wide range keeps the nucleation and growth in kinetic control, but the corresponding reduction rate is too fast to favor anisotropic growth [19]. As a result, consideration of both the appropriate reaction rate and the time is necessary for obtaining Ag nanostructures with controllable morphologies. The UV–visible absorption spectrum of the as-prepared Ag nanostructure is shown in Fig. 5(a). The peak at around 350 nm is an optical characteristic of bulk Ag [6] and the absorption broadband around 670 nm can be assigned to an in-plane dipole resonance [20]. SERS measurement was performed with an excitation wavelength of 514.5 nm and R6G as the probe molecule. A typical SERS spectrum is shown in Fig. 5(b) and a significant enhancement signal was observed for R6G solution with a concentration as low as 10−6 M. The peaks corresponding to R6G at 1184, 1308, 1362, 1507, 1512, 1572, 1596 and 1649 cm−1 were obviously detected, which is consistent with the previous report [21]. The signal enhancement could be associated with possible local electrical field enhancement at the tip of the nanorods. A similar case was proved in a recent report [22]. 4. Conclusion In summary, different morphologies of Ag nanostructures have been directly grown on zinc foil by means of a modified galvanic displacement reaction followed by suitable thermal treatment. The Ag nanostructure obtained could serve as an excellent active

substrate material for SERS detection of low concentration target molecules. We believe that this facile method can be extended to fabricating other kinds of metal or semiconductor materials for practical applications in nanodevices. Acknowledgement This work was financially supported by the National Natural Science Foundation of China, No. 20773044. Appendix. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ssc.2009.07.026. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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