Gold flails by electrochemical deposition: The role of gelatin

Electrochimica Acta 56 (2011) 1485–1489

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Gold flails by electrochemical deposition: The role of gelatin Jamil Elias ∗ , Pierre Brodard, Martine G.C. Vernooij, Johann Michler, Laetitia Philippe EMPA, Swiss Federal Laboratories for Materials Science and Technology, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland

a r t i c l e

i n f o

Article history: Received 15 June 2010 Received in revised form 23 September 2010 Accepted 25 September 2010 Available online 29 October 2010 Keywords: Electrochemical deposition Gold nanostructures Gelatin Additives Surface-enhanced Raman spectroscopy

a b s t r a c t In this paper, we report on novel flail-like gold architectures obtained by a simple and template-free electrochemical method. The addition of gelatin plays a major role in the fabrication of this original structure and might open new possibilities for electrochemical deposition of other metals. The size of the flail-like gold structures can be varied very precisely from 250 nm to 8.5 ␮m by controlling the electrodeposition time. Importantly, as a preliminary application, such flail-like gold structures show a strong SERS effect associated with their geometry and size. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, there has been a tremendous interest in the fabrication of gold nano/microstructures with different morphologies due to their great potential for technological applications such as superhydrophobicity [1], catalysis [2], surface-enhanced Raman spectroscopy (SERS) [3], and sensors [4]. The intrinsic properties of gold nano/microstructures can be controlled by tailoring their size, composition, and, especially, shape [5]. For instance, colloid chemistry [6] has been widely used to synthesize nano/microparticles of different shapes and of different metals, in particular gold [7,8]. Nevertheless, dispersed gold colloidal nano/microparticles obtained by this approach cannot be used directly in applications because they need to be extracted from the solution phase. Hence, an additional procedure is necessary to disperse and immobilize them onto a solid substrate. On the other hand, electrochemical deposition allows synthesizing gold nano/microstructures directly on conductive substrates [9], which could facilitate their practical applications as nanobuilding blocks in nanodevices, and therefore making this approach a more suitable, controllable and promising alternative to colloid chemistry. In the past few years several strategies have been developed to electrodeposit gold nano/microstructures on conductive substrates with various shapes: flowers [10], hedgehogs [11], bunch-of-grapes [12], dendrites [13], etc. Because of the attractive

∗ Corresponding author. Tel.: +41 33 228 36 27; fax: +41 33 228 44 90. E-mail address: [email protected] (J. Elias). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.090

properties obtained with these different morphologies, the synthesis of gold architectures with tailored shape and size is still a challenge [14]. Here we present a simple electrochemical deposition approach to fabricate, on transparent conductive oxide (TCO) substrates, sizecontrolled flail-like gold (FG) nano/microstructures with pyramidal spikes on their surface. This original gold morphology obtained by the electrochemical deposition technique is reported for the first time, and the addition of gelatin to the gold bath is shown to play a major role in the synthesis. The growth mechanism of the FG is discussed, and their dimension is shown to be easily controlled by the electrodeposition time. We think that this approach is a potential candidate for developing diverse nanodevices and can represent a novel synthesis pathway for many applications. As a primary example, we have studied the SERS activity of the FG structures by probing the Raman response of a deposited monolayer of p-mercaptoaniline (pMA).

2. Experimental FG nano/microstructures were prepared on Transparent Conductive Oxide (TCO) substrates (glass/SnO2 :F, 10 /sq, Solaronix) by means of electrochemical deposition from a gold bath of 10 g l−1 potassium dicyanoaurate(I) (Puramet 402, Doduco) with a small amount of gelatin (2 wt%), pH 7.3. This pH is the one obtained with the as-prepared solution. The optimized temperature for obtaining FG nano/microsturcture was 55 ◦ C. Before electrodeposition, the bare TCO was cleaned thoroughly by sonication in acetone,

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Fig. 1. (a) and (b) SEM images of FG structures at different magnifications. The inset in (a) shows a single spike. The inset in (b) shows a picture of a real flail taken from internet.

ethanol and isopropanol at 70 ◦ C. The cleaned TCO was then used as a working electrode in a three-electrodes electrochemical cell, with a Pt spiral wire as the counter electrode and an Ag/AgCl reference electrode. The electrodeposition was performed at a constant electrode potential (−0.8 V) and the deposition time was varied between 2.5 and 50 min. It is important to note that this applied potential has been determined after cyclic voltammetry studies (not shown here). An Autolab PGSTAT-30 was used for the electrodeposition. SERS spectra were recorded with a reflective-type micro-Raman spectrometer (Renishaw Ramascope) equipped with a HeNe laser operating at 632.8 nm as the excitation source (laser power ∼ 1–2 mW, 100× objective, laser spot diameter ∼ 1 ␮m). The acquisition time for each spectrum was 30 s, and each measurement was repeated twice at two different locations. In order to test the SERS response of the FG, we deposited a monolayer of pMA by immersion in a 100 ␮M ethanol solution for 3 h, followed by a thorough rinsing with ethanol. Morphology and crystallinity of the FG are characterized by scanning electron microscopy (SEM, Hitachi S-4800), electron backscatter diffraction (EBSD, in a Zeiss SEM DSM 962 with OIMDC 5.21) and X-ray diffraction (XRD, CuK-alpha radiation, Siemens). For EBSD analysis, a FG structure was cut in half with focussed ion beam (FIB, Tescan Lyra) using a 770 pA probe current beam operating at 30 keV. EBSD patterns were then acquired on the resulting cross section with a lateral resolution of 50 nm and presented in an orientation map. The energy dispersive X-ray (EDX) spectrum was obtained with a Genesis 4000 EDAX in SEM.

3. Results and discussion Fig. 1 shows the SEM images of FG nano/microstructures electrodeposited during 20 min on TCO substrate at different magnifications. A low magnification image (Fig. 1a) indicates that the substrate is covered by a uniform and well-dispersed array of rough microspheres with an average diameter of about 4.5 ␮m. Fig. 1b shows a higher magnification image of an individual structure which consists of a microsphere with a surface homogeneously and randomly covered by well-faceted pyramidal spikes (inset of Fig. 1a shows a high magnification image of a single spike). In order to characterize the crystallographic orientation of the electrodeposited gold, XRD measurements were performed on the TCO substrate covered by the gold electrodeposit obtained in Fig. 1a. Five diffraction lines are observed in the XRD pattern at 38.2◦ , 44.5◦ , 64.7◦ , 77.6◦ and 81.8◦ (Fig. 2a). For the face-centered cubic (fcc) structure of metallic Au with the space group Fm3m (JCPDS, card no. 04-0784), these diffraction lines are assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes, respectively. The ratio between the maximum intensities of the (1 1 1) and the (2 0 0) diffraction lines (3.0) is considerably larger than described in the JCPDS card (1.9), indicating that the as-prepared gold deposit has a preferential orientation perpendicular to {1 1 1} planes. The XRD results suggest also that no other phases or impurities are present. This was confirmed by an EDX analysis made on a single FG. The spectrum (Fig. 2b) shows that although gelatin was used in the electrodeposition solution, only peaks corresponding to Au are detected.

Fig. 2. (a) XRD and (b) EDX of the FG array shown in Fig. 1. For EDX, the structures are dispersed on a copper grid for the characterization.

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Fig. 3. (a) EBSD orientation map on a FIB-cut FG (inset: inverse pole figure colour key, representing the crystal direction perpendicular to the map). (b) Secondary electron image of a bi-pyramid. Orientation of the {1 1 1} planes obtained by EBSD (white dotted lines represent the crystal planes). The analyzed spike is marked by a white arrow in (a). Insets (I) and (II) are pole figures of <1 1 1> of both parts of the pyramid, red dots are poles to the {1 1 1} planes, black solid and dotted lines are traces of the relevant [1 1 1] directions and (1 1 1) planes, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The intrinsic crystal structure of the pyramidal spikes on the surface of FG was investigated by EBSD. Within the EBSD map (Fig. 3a), grain boundaries (misorientation > 5◦ ) are indicated by black lines and twins by white lines. The grains are colour coded (inset of Fig. 3a) according to the colour key corresponding to the directions that are perpendicular to the map. The EBSD map illustrates that the interior structure of the FG central sphere is constituted of fibrous grains with various different crystallographic orientations that have radially grown from the centre (Fig. 3a). The pyramids

are of either single or bi-crystalline nature. Crystallographic planes within the pyramids are calculated from the EBSD data set. It can be observed in pole figures of the pyramids that the growth direction of the pyramids is perpendicular to {1 1 1} and that the faces of the pyramids are parallel to <1 1 1> (Fig. 3b). Growth twins can be observed in both pyramidal spikes and fibrous grains. In order to gain further insight into the growth mechanism of FG nano/microstructures, the evolutions of both morphology and dimensions as a function of deposition time and gelatin content

Fig. 4. SEM images of gold deposits obtained with (a–f) and without (g–i) gelatin and at different deposition times. (a) and (g) 2.5 min, (b) and (h) 5 min, (c) and (i) 10 min, (d)–(f): 15, 20 and 50 min, respectively. The insets correspond to magnified SEM images. Note that the scales bars are not the same in the insets.

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Fig. 5. (a) Evolution of pyramids height (black squares) and spheres diameter (red circles) as a function of electrodeposition time. (b) Current density–time curves recorded during electrodeposition without (black curve) and with 2 wt% of gelatin (red curve). The inset of (b) shows the j vs. t2 curve resulting from the current density–time curve obtained with 2 wt% of gelatin. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

were investigated by SEM, and the results are summarized in Fig. 4. The deposition time was varied between 2.5 and 50 min (Fig. 4a–f) while all the other parameters (temperature, potential, etc.) were kept constant. Low magnification images show that large numbers of homogeneously dispersed nano/microstructures with average diameters ranging from 250 nm to 8.5 ␮m (see insets of Fig. 4a–f) have been obtained. In addition, FG with pyramids on their surfaces were obtained even at relatively short time of deposition (e.g. Fig. 4a, 2.5 min). We observed that not only the sphere diameter but also the pyramid size increase by increasing the deposition time. The time evolution of the height of these pyramids as well as the diameter of the spheres have been calculated from a statistical analysis of SEM images and summarized in Fig. 5a. This study reveals that the growth rates of the spheres and the spikes seem to be different: the pyramids height growth rate is more than 3 times slower than that of the spheres diameter. On the other hand, the linear sizetime dependence shown in Fig. 5a indicates that the growth kinetics of instantaneously formed spherical Au nano/microparticles is controlled by the charge transfer at the Au/electrolyte interface. In this case, the corresponding current transient for the deposition with gelatin (red curve) shown in Fig. 5b should follow a parabolic j–t2 dependence [15]. This has been confirmed by the parabolic shape of the j–t2 curve shown in the inset of Fig. 5b. The gelatin effect on the growth of Au was also investigated. Without gelatin added to the gold bath, no FG structures were obtained (Fig. 4g–i). The deposit consists of large, angular grains with a smooth surface (inset of Fig. 4g) and a diameter 20 times larger (5 ␮m vs. 0.25 ␮m) than the FG nanostructures obtained with the same time of deposition but without gelatin (2.5 min, Fig. 4a). As the time increases, these microspheres become bigger and bigger in a relatively short time, producing a compact layer in about 10 min (Fig. 4h and i). The effect of the concentration of gelatin was also studied. Below 2 wt% in the bath, gelatin has no effect on the morphology and no FG were observed. On the other hand, with gelatin concentrations significantly higher than 2 wt%, no Au deposits are obtained. This can be due to the increase of the solution viscosity (gelatin is partially dissolved), leading to a dramatic decrease of the current density. Therefore, the concentration of gelatin has to be finely optimized (around 2 wt%, ±0.5 wt%) in order to produce FG. Based on the above results, we present a tentative mechanism for the electrochemical formation process of the FG structures. Initially, Au nuclei are formed via electron transfer reaction on the substrate through the reduction of KAu(CN)2 following this reaction: Au(CN)2 − + e− → Au + 2CN− As a consequence, a large quantity of gold nuclei will be instantaneously and randomly formed on the bare TCO substrate. These

nuclei constitute gold nanoelectrodes, negatively charged due to the cathodic potential applied for electrodeposition (−0.8 V). In general, gelatin is derived from collagen, which has a triple-helical structure and is soluble in water at temperature higher than 37.5 ◦ C [16]. In our case, the temperature used for the electrodeposition was 55 ◦ C, which leads to a complete denaturation process of gelatin molecules and conversion from triple-helical structure to a random coil configuration [17]. At the pH used in our solution (∼7.3), the gelatin molecules are positively charged because this value is lower than its isoelectric point (∼pI 9.0) [18]. Therefore, after the formation of gold nanoelectrodes negatively charged in the nucleation step, and during the first growth stage, the positively charged gelatin molecules adsorb onto the gold surface, leading to a slow growth rate of the gold structures compared to the case without gelatin. This is supported by the big decrease of the current density (the deposition rate), as seen in the current density–time curve (Fig. 5b). In order to prove our consideration about the role of the pH on the charging of gelatin molecules, an electrodeposition has been performed by using a solution with pH 10 (>pI). As a result, no FG structure has been obtained. This can be related to the negative charge of gelatin molecules at this pH (>pI), preventing their adsorption on the gold surface deposited on the cathode. As already mentioned, the Au spheres growth rate is about 3 times higher than the pyramids one (Fig. 5a). This could be explained in terms of face-selective adsorption of gelatin. As shown from the structural studies in Figs. 2a and 3a and b, the pyramids are always constituted of single or bicrystals with facets that are perpendicular to {1 1 1}, whereas the spheres are polycrystalline and composed of fibrous grains. In general, the {1 1 1} facets are thermodynamically the most stable among the possible facets of the fcc crystals. Therefore, it is reasonable to assume that the crystals with the {1 1 1} facets are preferentially grown at a very slow rate. In addition, Tian et al. [19] reported that anisotropic nanostructures such as pyramids could be readily obtained by operating at a slow reduction rate. Therefore, we think that the local slow growth rate of the nanopyramids resulting from a selective adsorption of gelatin on the {1 1 1} facets is the key to the formation of the unique FG. Though the mechanism is still open for further investigation, the present results represent the first simple and template-free route to obtain large amounts of FG nano/microstructures with controlled dimensions. As a first potential application of the FG [20], we studied their SERS response in the presence of a test molecule. The SERS effect is an enhancement of the Raman scattering intensity than can be separated in two main contributions: a physical effect caused by an electromagnetic field (EF-SERS) increase due to the excitation of localized surface plasmons in metallic nanostructures, and a chemical effect caused by a charge transfer (CT-SERS) from the metal

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Fig. 6. (a) Raman spectrum of pMA on a single FG (green) and on a multi-FG cluster (blue), (b) SERS intensity of two peaks as a function of the spheres size. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

to the adsorbed molecules [21]. Fig. 6a shows the Raman spectrum of a pMA monolayer on a cluster of more than five FG with a diameter of 418 nm (blue) and on a single FG of the same size (green). The Raman intensity is massively enhanced on the cluster of FG, whereas no SERS effect is observed on a single FG. In fact, we observed that the SERS effect increases with the number of FG in the cluster, with a plateau from about 5 FG. This has been observed on FG of all sizes, and confirmed by similar measurements with another molecule (Brilliant Cresyl Blue instead of pMA). This shows that the SERS effect is localized in so-called hot-spots, that is contact points between the FG. Such nanoscale sites are known to generate intense local electromagnetic fields and therefore produce a very efficient EF-SERS effect [21]. The Raman spectrum of pMA is composed of peaks with a1 (1086, 1188 and 1593 cm−1 ) and b2 (1151, 1322, 1400, 1452 and 1570 cm−1 ) symmetry [22]. A preferential enhancement of the b2 modes (as compared to the a1 modes) has been ascribed to CT-SERS [23]. In Fig. 6b, we show the evolution of the SERS effect with the increasing size of FG for two modes at 1593 cm−1 (a1 ) and 1452 cm−1 (b2 ). We observe an exponential decay of the Raman intensity as the FG grow, with almost the same decay constant for both peaks. Since we do not observe a preferential enhancement of the b2 modes, we conclude that EF-SERS is dominating, in accordance with the hot-spots effect mentioned above. However, the number of hot-spots is the same for clusters with the same number of FG, whatever their size. Therefore, we think that the reason why the SERS effect decreases with the FG size is due to the correlated size of the spikes, which act as nanoscale spacers between the gold spheres, thus generating efficient hotspots [24,25]. By looking carefully at the data, we observe that with a FG diameter of 3 ␮m, the SERS effect has already decreased by more than an order of magnitude. At this diameter, the spikes have a size which corresponds approximately to the wavelength of the light ( = 633 nm). It is well known that EF-SERS is a near-field phenomenon that occurs only in subwavelength metallic junctions [26]. Therefore, big FG with large spikes (size > ) cannot come close enough to each other to generate hot-spots, and their SERS activity is very low. 4. Conclusions We have successfully prepared new gold architectures by simple and template-free electrochemical deposition on TCO substrate. The dimension of the FG can be easily controlled by controlling the deposition time. The addition of gelatin plays a major role to obtain

this original structure and might open new possibilities for electrodeposition of other metals. As a potential application, we found that FG present a strong SERS effect due to the massive electromagnetic field enhancement localized in the nanoscale gaps generated between the spheres by the subwavelength spikes, allowing chemical identification of species in the micromolar range. Future work will investigate the possible use of these substrates for applications in sensors and superhydrophobic surfaces. Acknowledgements The authors thank Dr. Rudy Ghisleny for FIB experiments and Dr. Christoph Niederberger for fruitful discussions. This work was supported by the NanoGold project, Switzerland. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

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