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Study on the two dealloying modes in the electrooxidation of Au–Sn alloys by in situ Raman spectroscopy
Electrochimica Acta 54 (2009) 1102–1108
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Study on the two dealloying modes in the electrooxidation of Au–Sn alloys by in situ Raman spectroscopy Shu Chen a , Youping Chu b , Jufang Zheng b , Zelin Li a,b,∗ a Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China b Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
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Article history: Received 10 March 2008 Received in revised form 28 June 2008 Accepted 5 July 2008 Available online 25 July 2008 Keywords: In situ Raman spectroscopy Dealloying Au–Sn alloys Electrooxidation SERS
a b s t r a c t The electrochemical processes in dealloying of Au–Sn alloys in a solution of 2 mol dm−3 HCl have been first investigated in detail by means of in situ potential-dependent and time-resolved Raman spectra. Two dealloying modes were found occurring within different potential regions in the electrooxidation of Au–Sn alloys. One is the mode known as classical dealloying, where Sn is selectively dissolved; and the other a so-called quasi-dealloying mode found here, in which Au re-deposits automatically after simultaneous dissolution with Sn. Meanwhile, nanoporous gold, thin layers of gold nanoparticles stacked on the surface, and colloidal gold in the solution can be prepared from the Au–Sn alloys simply by an electrochemical control of potential. Moreover, the quasi-dealloying manner of Au–Sn alloys has also been grafted onto a pure Au electrode with a tin overlayer by electrodeposition to construct the SERS substrate conveniently. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Alloys dissolve either selectively or simultaneously. Dealloying occurs in the former case, where the active component is selectively removed from the alloys chemically or electrochemically, leaving behind the noble one porous, which is an efficient way to fabricate nanoporous metals (NPMs). Binary alloys like (Ag, Cu)–Au [1–5], (Zn, Al, Ni, Mn, Zr)–Cu [6–11], (Ag, Ni, Cu)–Pd [12–14], Cu–Pt [15], (Cu, Al, Zn)–Ni [16,17], Mg–Cd [18], Zn–Ag [19] are examples of classical dealloying ever reported. Their high surface-to-volume ratio and open porosity make NPMs attractive for vast applicability in catalysis [20], microfluidic flow control [21], sensors [22] and surface enhanced Raman spectroscopy (SERS) [23], etc. It appears that the selective electrodissolution mode can be transferred into the mode of simultaneous electrodissolution under certain conditions in such systems as Cu–Zn [24], Ag–Pd, Cu–Pd, Ni–Pd [12,13], Fe–Cr [25], Cu–Ni [26,27] and so on. However, simultaneous mode is generally unwanted for dealloying.
∗ Corresponding author at: Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Lushan Road, Changsha 410081, China. Tel.: +86 579 82283897; fax: +86 579 82282595. E-mail address: [email protected] (Z. Li). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.07.044
The anodic electrooxidation behaviors of Au–Sn alloys in a 2 mol dm−3 HCl solution are presented here for the first time, which have been investigated by the measurements of electrochemical techniques, in situ Raman spectroscopy and ex situ scanning electron microscopy (SEM). Besides the classical dealloying into nanopores in the lower potential region, another kind of so-called anodic “quasi-dealloying”, is especially emphasized. It appears in the higher potential region, where both components can be simultaneously dissolved into Au(III) and Sn(II) species. However, only the gold species (AuCl4 − ) is capable of getting back to the surface as gold nanoparticles (GNPs) by immediately followed chemical reactions in stoichiometric proportion between the two dissolved species. The final effect of it is equivalent to that of the classical dealloying but via a different mechanism. In this way, gold nanoparticles can be obtained on the electrode surface as well as in the solution as colloid. It is the reductivity of dissolved Sn(II) that makes the Au–Sn unique in dealloying contrasted with other Au–Me alloys. Based on the quasi-dealloying mechanism, a new procedure has been developed to prepare the SERS substrate. Another interesting point we would like to emphasize in this work is that the physicochemical processes involved in the dealloying were studied in situ with a confocal Raman microscope by varying the experimental parameters of anodic potential, alloy component, medium, and laser position. These two dealloying modes have been revealed clearly by the in situ potentialdependent and time-resolved Raman spectroscopy.
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2. Experimental Normal electrochemical measurements were carried out in a conventional H-type glass cell with three electrodes by using CHI 660C electrochemical station (Chenhua, Shanghai, China). The working electrodes were disks made of metal wires of Au70 Sn30 at.% (1 mm diameter), pure Au and Sn (both in a diameter of 2 mm, purity ≥99.99%) purchased from Goodfellow, while the Au40 Sn60 at.% working disk (3 mm diameter) can be easily selfmade from the wires of Au70 Sn30 and pure Sn because of their lower melting point (below 300 ◦ C). Prior to use the electrode was polished with 1000# metallographic paper and successively cleaned with ultrasonic waves in ultra-pure water. Taking into account the different electrode areas, the current is translated into current density (j/mA cm−2 ) for reader’s convenience. The potential quoted here is versus the saturated mercurous sulfate electrode (SMSE). All experiments were performed at room temperature (∼20 ◦ C) and solutions were freshly prepared under nitrogen purging with analytical grade chemicals and ultra-pure water (from a water purification system of Millipore Corp., USA). After electrolysis under potential control, the gold colloids produced in the quasi-dealloying were mixed with 5 mmol dm−3 PVP (polyvinylpyrrolidone, average molecular weight 10,000) to prevent the nanoparticles from agglomeration, sequentially centrifugated at 12,000 rpm, rinsed with absolute ethanol twice, and redispersed ultrasonically in an ounce of ethanol. Drops of the dispersed colloid were placed on the carbon-coated copper grid and glass slide, respectively, for the detections of particle sizes and composition. A sol of Au for contrastive experiments, signed as Sample I, was prepared by simply adding 5 cm3 0.01 mol dm−3 KBH4 drop by drop into 5 cm3 0.005 mol dm−3 HAuCl4 [28,29] under vigorous stirring for 10 min without adding any stabilizer and then by centrifugation aggregated GNPs were ready for the next test of Raman spectra. Images of shape and size were measured by a Hitachi S-4800 filed-emission scanning electron microscope (FE-SEM) at 5 or 10 kV accelerating voltage, or performed on a transmission electron microscope (TEM) with Tecnai G2 20ST operated at 200 kV accelerating voltage. The atomic ratio of gold and tin was checked by energy-dispersive X-ray (EDX) spectra before and after dealloying. XRD patterns were collected on a Philips PW 3040/60 powder diffractometer equipped with a Philips Analytical X’Celerator, using Cu K␣ radiation in a 2 range from 30 to 80◦ with a scan rate of 0.2◦ s−1 . The working voltage of the instrument was 40 kV and the current was 40 mA. Optical absorbance spectra of colloids were registered in the wavelength range from 400 to 700 nm using the UV–vis spectrophotometer (Nicolet Evolution 500 from Thermo). Raman measurements with 632.8 nm excitation were performed with a Renishaw RM1000 confocal microscope in a self-designed spectroscopic cell made by Teflon with a quartz window [30]. The objective was of long working distance (8 mm) with 50× magnification and the laser power was about 3 mW on the sample.
3. Results and discussion 3.1. Electrochemical behaviors Fig. 1 shows cyclic voltammograms (CVs) performed in 2 mol dm−3 HCl at freshly polished Sn, Au, Au70 Sn30 and Au40 Sn60 electrodes, respectively, with an upper limit of 1.2 V, which is negative to the oxygen evolution potential in order to prevent the oxidation of Sn(II) by oxygen bubbles. Au dissolution begins at about 0.3 V (roughly indicated by the vertical dotted line) and reaches a maximum current density around 0.6 V, followed by passivation positive to 0.9 V (Fig. 1(b)). Moreover, the curves cross in
Fig. 1. Cyclic voltammograms (scan rate 100 mV s−1 ) in 2 mol dm−3 HCl for electrodes of (a) pure Sn, (b) pure Au, (c) Au70 Sn30 , and (d) Au40 Sn60 , respectively. Typical current oscillations for Au–Sn alloys at 0.95 V are shown in the insert of parts (c) and (d). The dotted vertical line at 0.3 V performs as a glancing line dividing the regions between classical dealloying and quasi-dealloying.
the CV around the transition region of gold between the active and passive states, forecasting current oscillations as our group ever reported [30]. Sn keeps on mass active electrodissolution from −1.0 V on with steep current increase till 0.3 V in the CV (Fig. 1(a)). It has been proposed that pure Sn was electrodissolved solely into Sn(II) in HCl [31] Sn(0)(s) → Sn(II)(aq) + 2e−
(i)
In the backward scan, reductive peaks below −1.0 V can be observed, corresponding to the Sn deposition. Turn to the CVs of Au–Sn alloys in Fig. 1(c) and (d), significant electrodissolution of Sn occurs around 0 V with much less anodic current than that in the pure Sn (Fig. 1(a)) due to alloying with Au. However, the alloys are far from passivation between 0 and −0.8 V (Fig. 1(d)) or −0.6 V (Fig. 1(c)) where the gold is inert, noting the large scale of current density (100 mA cm−2 ) in Fig. 1. In the more positive potential range (on the right of the dotted vertical line), the alloys present quite similar behaviors to the pure Au in Fig. 1(b), such as crossed CVs and current oscillations (the insert of Fig. 1(c) and (d), yet the current in the passivating range positive to 0.9 V is somewhat higher than that in the pure Au, indicating that the passive film is less protective after alloying. The cathodic peak for the reduction of AuCl4 − at 0 V decreases with the increase of Sn ratio in the alloys from Fig. 1(b)–(d), due to the chemical consumption of AuCl4 − produced at higher potentials by more dissolved Sn(II). No reduction peaks of AuCl4 − can be observed either by reversing the scan at potentials less than 0.3 V (not showing) since gold does not
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dissolve below that potential, which also identified by in situ Raman spectra (Section 3.2). These facts suggest that there are two kinds of potential-dependent dealloying for Au–Sn alloys, i.e. without and with co-dissolving of gold in the lower and higher potential ranges, respectively. That is what we called classical dealloying and quasidealloying. 3.2. In situ Raman spectroscopy to reveal the dealloying processes To better understand the physicochemical processes involved in the electrooxidation of Au–Sn alloys, in situ Raman spectra have been taken by varying the potential, electrode component, medium, and laser position. Fig. 2(a) and (b) shows the Raman spectra for the two alloys in 2 mol dm−3 HCl, and the potential control is such that it was first increased (the bottom) and then at the open state for a period of time (the upper). The doublet at 322 and 346 cm−1 for the complex AuCl4 − appears by 0.4 V for Au70 Sn30 (the bottom of Fig. 2(a)), in agreement well with our previous results [30] where pure Au dissolution commences nearby. The intensity of the doublet for the higher Sn-containing alloy Au40 Sn60 is much weaker even at 0.6 V (the bottom of Fig. 2(b)), indicating the quicker consumption of AuCl4 − by more abundant Sn(II) via the reaction 2AuCl4 − (aq) + 3Sn(II)(aq) → 2Aunano(s) + 3Sn(IV)(aq) + 8Cl− (aq)
(ii)
An alternative mechanism is thermodynamically possible via the reaction 2AuCl4 − (aq) + 3Sn(0)(s) → 2Aunano(s) + 3Sn(II)(aq) + 8Cl− (aq)
(iii)
The ion reaction (ii) between Sn(II) and Au(III) in solution phase should be more dominant than reaction (iii) between aqueous
Au(III) with Sn in alloy phase, in view of also that Sn(II) can be produced electrochemically (i) and chemically (iii). There is a very strong companion peak appearing at 269 cm−1 (the bottoms of Fig. 2(a) and (b)) for the adsorbed Cl− (Aunano –Cl− ad ) on the roughed gold [30] evidenced the SERS effect of the newly produced gold nanopores and/or nanoparticles (Aunano ). When the circuit was switched to the open state (the uppers of Fig. 2(a) and (b)), the doublets disappear due to the removal of AuCl4 − from the surface via the chemical reduction as well as diffusion. Meanwhile, the Au–Cl− ad stretching at 269 cm−1 diminishes gradually and a new feature appears around 326–328 cm−1 . However, only the doublet fades away by diffusion in the case of pure gold in 2 mol dm−3 HCl (Fig. 2(c)) [30]. It can be reasonably figured out that the peak at 328 cm−1 must be from the adsorption of Sn species on the Au most probably Sn(II) complexes with Cl− , noticing also that it is the sole peak below 0.3 V in Fig. 2(a) and (b) where only classical pore-forming dealloying occurs. More experiments have been performed to support the guess on the origin of the peak at 328 cm−1 . By mixing the as-prepared GNPs (Sample I from the reaction of KBH4 with HAuCl4 as described in Section 2) with drops of 2 mol dm−3 HCl, there is a SERS peak at 275 cm−1 for the adsorbed Cl− on the gold nanoparticles (Fig. 2(d)). Then, by dropping solutions of Sn(II) (2 mol dm−3 HCl solution containing 0.1 mol dm−3 SnCl2 ) and Sn(IV) (2 mol dm−3 HCl solution containing 0.1 mol dm−3 SnCl4 ) on the GNPs (Sample I) separately, different phenomena were observed (Fig. 2(d)): the peak at 275 cm−1 for adsorbed Cl− is replaced by that at 326 cm−1 for Sn(II), yet holds for Sn(IV). These results provide a piece of direct evidence for the origin of the peak at 328 cm−1 , and it is no doubt from the adsorption of Sn(II) complexes with Cl− . However, the
Fig. 2. In situ Raman spectra by varying the potential, electrode component and medium. (a) Au70 Sn30 , (b) Au40 Sn60 and (c) Au in 2 mol dm−3 HCl, and (d) gold sol (Sample I) for comparison with Sn(II) (0.1 mol dm−3 SnCl2 + 2 mol dm−3 HCl) or Sn(IV) (0.1 mol dm−3 SnCl4 + 2 mol dm−3 HCl) solution. All of the collection time for recording a single spectrum is 20 s.
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exact coordination form and adsorption mode of Sn(II) complexes are out of the scope of this paper, and we do not further pursue them here. The results also indicate that the adsorption of Sn(IV) complex (probably SnCl6 2− ) is so weak that it could not be detected with SERS even if it is present from the chemical reaction (ii) and the following electrooxidation reaction on the gold surface Sn(II)(aq) → Sn(IV)(aq) + 2e−
(iv)
It would be worthwhile showing how to distinguish the two kinds of dealloying ways more unambiguously with the in situ Raman spectroscopy. It is very interesting to find that the quasidealloying mode can be monitored simply by locating the laser at position 2 (Fig. 3a) rather that at position 1 as did in the above Raman measurements. The position 2 is on the insulating wrap of the disk electrode about 500 m far from the edge of the embedded metal. For the quasi-dealloying of Au40 Sn60 at a higher potential of 0.6 V, the SERS band at 272 cm−1 for the Aunano –Cl− ad can be observed at the position 2 after 50 s under that potential control and its intensity gets stronger with time as seen from the time-resolved
Fig. 3. In situ Raman spectra by changing the laser position from 1 to 2. Position 1 in (a) is focused on the metal center of the electrode as did in Fig. 2, but position 2 in (a) that on the insulating wrap (ca. 500 m far from the edge of the metal embedded) was chosen here in part (b) and the time-resolved spectra in (b) were taken with Au40 Sn60 alloy at 0.6 V and then at the open circuit state.
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Raman spectra (the bottom of Fig. 3b). The band at 272 cm−1 is replaced by the band at 329 cm−1 (the upper of Fig. 3b) while the potential changes from 0.6 V to the opening circuit (ca. −0.3 V), indicating that the adsorption of Sn(II) species is stronger than Cl− at the much lower open circuit potential. These facts indicate that the gold nanoparticles produced from the reaction of co-dissolved Au(III) and Sn(II) can drift to the position 2 from the electrode surface and stack on the wrap in a short period of time. While in the classical dealloying potential range (below 0.3 V), we cannot observe these peaks at the position 2, because only Sn dissolves there resulting in nanopores on the surface of alloys. 3.3. Surface morphology in the classical- and quasi-dealloying regions By controlling the potential for a few minutes in the lower potential region (0.2 V), where only Sn dissolves in the classical dealloying way, familiar nanoporous structures can be obtained from the Au40 Sn60 and Au70 Sn30 alloy, as shown in Fig. 4(a), (b) and (f). Dealloying is likely while the nobler component is below 80 at.% according to the percolation theory [32], and that is true here. With the formation of nanopores, the nobler component is built up at the surface from Au40 Sn60 to Au90 Sn10 in 200 s (lines A and C in Fig. 5) detected by EDX. Selective dissolution of Au–Sn alloys could also proceed chemically in the same medium but with a much slower rate. While immerging the alloy (Au40 Sn60 ) in 2 mol dm−3 HCl without application of an external voltage, the surface is only slightly chemically etched even for 3600 s (Fig. 4(e)) ended with a component of Au49 Sn51 (line B of Fig. 5). So the contribution from the chemical dissolution is minor during dealloying electrochemically here. The open circuit potentials (OCPs) of Sn, Au40 Sn60 and Au70 Sn30 in 2 mol dm−3 HCl are ca. −1, −0.9 and −0.6 V, respectively, and electrodissolution of Sn commences while the potential is slightly larger than those OCPs (Fig. 1). However, more accurate evaluation for the critical potential Ec [32–34] of the selective dealloying of Au–Sn alloys needs more detailed studies such as through empirical current threshold [32], polarization curves [32,33] and steady-state current [34], which is not the concern here. More emphasis of us is placed on the quasi-dealloying in the higher potential region, where the two components co-dissolve into AuCl4 − and Sn(II) complexes and gold nanoparticles form immediately by the redox reaction of the two dissolved species. The produced GNPs can pile up to a thin layer on the electrode surface and disperse into the solution as colloids. Fig. 4(c) and (d) shows the top and side SEM images, respectively, for the stacked thin layer of gold nanoparticles on the surface. It can be seen from the images that these nanoparticles are rather uniform and densely packed on the substrate, consisting of purely gold as indicated by EDX (line D in Fig. 5). The layer from the quasi-dealloying also shows nanoporous structure. Most of the GNPs dispersed into the solution and a thread of aubergine stream can be observed coming from the Au–Sn anode during quasi-dealloying at higher potential region (e.g. 0.6 V), resulting in a deep purple colloid. This observation suggests that the electrochemically co-dissolved Au(III) and Sn(II) species can form nanoparticles very rapidly before their going far off the electrode surface, which might offer an expeditious method for preparing clean gold colloids continuously according to the required amount from the simultaneous electrodissolution of Au–Sn alloys. The asprepared colloidal GNPs are spherical with diameters below 40 nm as ascertained with TEM (Fig. 6a). A broad peak appears around 540 nm in the UV–vis absorption spectrum (Fig. 6b). The red-shift with respect to the usual peak position [29] of gold colloids at 520 nm is probably due to the larger particle size and the adsorp-
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Fig. 4. Typical surface SEM images of Au40 Sn60 alloy treated in 2 mol dm−3 HCl by (a) classical dealloying at 0.2 V for 200 s with nanoporous gold, by (c) quasi-dealloying at 0.6 V for 200 s with stacked gold nanoparticles, and by (e) dipping in 2 mol dm−3 HCl for 1 h with slightly eroded surface. The insets in (a) and (c) shows larger magnification. The side view of samples (a) and (c) are shown in (b) and (d), respectively. (f) A typical image for the classical Au70 Sn30 dealloying at 0.2 V for 200 s.
tion of Sn(II) species. The synthesized GNPs shows no detectable Sn species by the analysis of EDX (line D of Fig. 5) and XRD (Fig. 6c) because both EDX and XRD are not sensitive enough as SERS in detecting (sub)monolayer adsorption of Sn species. 3.4. Facile fabrication of SERS substrate by grafting the quasi-dealloying
Fig. 5. The EDX spectra for the Au40 Sn60 alloy: (A) the original alloy material, (B) chemically etched sample, (C) classical dealloying sample, and (D) quasi-dealloying sample, (B)–(D) correspond to the SEM images (e), (a) and (c) in Fig. 4, respectively.
We have grafted the quasi-dealloying manner of Au–Sn|HCl into Au|Sn|HCl to fabricate gold SERS substrate according to the following procedure. Firstly, deposit a thin Sn layer on the polished Au surface by controlling a cathodic potential at −1.1 V for 20 s with a bath of 2 mol dm−3 HCl + 0.1 mol dm−3 SnCl2 , and then co-dissolve the Sn overlayer with Au underneath in 2 mol dm−3 HCl by controlling the potential at 0.6 V for 20 s. A thin layer of slightly aggregated GNPs sticking to the gold surface can be obtained (Fig. 7a) in a similar way to the quasi-dealloying. Calculations and experiments have shown that it is possible to achieve large electromagnetic field enhancement at particle junctions of
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Fig. 8. A sketch map summarizing the Au–Sn anodic electrooxidation: (i) classical dealloying of Au–Sn alloy to gold nanopores, (ii) quasi-dealloying of Au–Sn and a graft of quasi-dealloying by electrodepositing a thin layer of Sn on the pure Au surface (Au|Sn) to the GNPs stacked on the surface or dispersed into the electrolyte as colloid. Fig. 6. The typical (a) TEM image, (b) UV–vis spectrum, and (c) XRD pattern of the as-prepared colloidal GNPs from Au40 Sn60 by holding the potential at 0.6 V for 1000 s.
aggregates [35,36]. In addition, other two manners that we ever employed to prepare the SERS substrate prove to be incompetent, such as Sn|(HAuCl4 + HCl) and Au|(SnCl2 + HCl). Either the base metal (Sn) is too active to apply anodic potentials and to stay in acidic solutions (along the former manner) or the gold nanoparticles yielded are poor adherence to the surface of the gold electrode (the latter case). Among others one obvious reason for the drawback in the latter case of Au|(SnCl2 + HCl) is that the reaction (ii) that produces gold nanoparticles apparently takes place at positions averagely farther off the surface than the case of co-dissolution of Au|Sn overlayer or Au–Sn alloy in HCl (both AuCl4 − and Sn(II) simultaneously come from the surface of the substrate). To test the SERS activity of the prepared substrate, pyridine was chosen as the probe molecule. High quality SERS spectra of absorbed pyridine were observed indeed (Fig. 7b) with characteristic peaks [37,38] located at 628, 1009, 1033, 1066, 1213, and 1591 cm−1 and maximum intensity at −1.0 V, which are consistent with literature. 4. Conclusions Potential-dependent and time-resolved in situ Raman spectra have been successfully applied to study the dealloying processes of Au–Sn alloys in the HCl medium. It has been revealed that there are two dealloying modes depending on the applied potential: (i) classical dealloying occurs in the lower potential region forming gold nanopores by selectively dissolving Sn; and (ii) quasi-dealloying appears in the higher potential region producing gold nanoparticles by the redox exchange reaction of Sn(II) and AuCl4 − from the simultaneous dissolution of Au–Sn alloys. The gold nanoparticles can stack on the electrode surface or disperse into the solution as gold colloid (as shown in Fig. 8). Furthermore, the quasi-dealloying mode can be grafted on a pure gold substrate with an electrodeposited Sn layer to prepare a thin layer of GNPs, showing strong SERS activity for pyridine molecules. Acknowledgments
Fig. 7. (a) The typical SEM morphology of stacked GNPs on Au electrode treated by the grafted quasi-dealloying method (see the text in Section 3.4 for details), and (b) in situ Raman spectra for the adsorption of pyridine on the GNPs covered electrode with potential decrease in the solution of 0.1 mol dm−3 KCl plus 0.01 mol dm−3 pyridine. The exposure time for CCD was 10 s.
Financial support of this research from National Natural Science Foundation of China (20673103, 20373063) is gratefully acknowledged.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Study on the two dealloying modes in the electrooxidation of Au–Sn alloys by in situ Raman spectroscopy Shu Chen a , Youping Chu b , Jufang Zheng b , Zelin Li a,b,∗ a Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China b Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
a r t i c l e
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Article history: Received 10 March 2008 Received in revised form 28 June 2008 Accepted 5 July 2008 Available online 25 July 2008 Keywords: In situ Raman spectroscopy Dealloying Au–Sn alloys Electrooxidation SERS
a b s t r a c t The electrochemical processes in dealloying of Au–Sn alloys in a solution of 2 mol dm−3 HCl have been first investigated in detail by means of in situ potential-dependent and time-resolved Raman spectra. Two dealloying modes were found occurring within different potential regions in the electrooxidation of Au–Sn alloys. One is the mode known as classical dealloying, where Sn is selectively dissolved; and the other a so-called quasi-dealloying mode found here, in which Au re-deposits automatically after simultaneous dissolution with Sn. Meanwhile, nanoporous gold, thin layers of gold nanoparticles stacked on the surface, and colloidal gold in the solution can be prepared from the Au–Sn alloys simply by an electrochemical control of potential. Moreover, the quasi-dealloying manner of Au–Sn alloys has also been grafted onto a pure Au electrode with a tin overlayer by electrodeposition to construct the SERS substrate conveniently. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Alloys dissolve either selectively or simultaneously. Dealloying occurs in the former case, where the active component is selectively removed from the alloys chemically or electrochemically, leaving behind the noble one porous, which is an efficient way to fabricate nanoporous metals (NPMs). Binary alloys like (Ag, Cu)–Au [1–5], (Zn, Al, Ni, Mn, Zr)–Cu [6–11], (Ag, Ni, Cu)–Pd [12–14], Cu–Pt [15], (Cu, Al, Zn)–Ni [16,17], Mg–Cd [18], Zn–Ag [19] are examples of classical dealloying ever reported. Their high surface-to-volume ratio and open porosity make NPMs attractive for vast applicability in catalysis [20], microfluidic flow control [21], sensors [22] and surface enhanced Raman spectroscopy (SERS) [23], etc. It appears that the selective electrodissolution mode can be transferred into the mode of simultaneous electrodissolution under certain conditions in such systems as Cu–Zn [24], Ag–Pd, Cu–Pd, Ni–Pd [12,13], Fe–Cr [25], Cu–Ni [26,27] and so on. However, simultaneous mode is generally unwanted for dealloying.
∗ Corresponding author at: Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Lushan Road, Changsha 410081, China. Tel.: +86 579 82283897; fax: +86 579 82282595. E-mail address: [email protected] (Z. Li). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.07.044
The anodic electrooxidation behaviors of Au–Sn alloys in a 2 mol dm−3 HCl solution are presented here for the first time, which have been investigated by the measurements of electrochemical techniques, in situ Raman spectroscopy and ex situ scanning electron microscopy (SEM). Besides the classical dealloying into nanopores in the lower potential region, another kind of so-called anodic “quasi-dealloying”, is especially emphasized. It appears in the higher potential region, where both components can be simultaneously dissolved into Au(III) and Sn(II) species. However, only the gold species (AuCl4 − ) is capable of getting back to the surface as gold nanoparticles (GNPs) by immediately followed chemical reactions in stoichiometric proportion between the two dissolved species. The final effect of it is equivalent to that of the classical dealloying but via a different mechanism. In this way, gold nanoparticles can be obtained on the electrode surface as well as in the solution as colloid. It is the reductivity of dissolved Sn(II) that makes the Au–Sn unique in dealloying contrasted with other Au–Me alloys. Based on the quasi-dealloying mechanism, a new procedure has been developed to prepare the SERS substrate. Another interesting point we would like to emphasize in this work is that the physicochemical processes involved in the dealloying were studied in situ with a confocal Raman microscope by varying the experimental parameters of anodic potential, alloy component, medium, and laser position. These two dealloying modes have been revealed clearly by the in situ potentialdependent and time-resolved Raman spectroscopy.
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2. Experimental Normal electrochemical measurements were carried out in a conventional H-type glass cell with three electrodes by using CHI 660C electrochemical station (Chenhua, Shanghai, China). The working electrodes were disks made of metal wires of Au70 Sn30 at.% (1 mm diameter), pure Au and Sn (both in a diameter of 2 mm, purity ≥99.99%) purchased from Goodfellow, while the Au40 Sn60 at.% working disk (3 mm diameter) can be easily selfmade from the wires of Au70 Sn30 and pure Sn because of their lower melting point (below 300 ◦ C). Prior to use the electrode was polished with 1000# metallographic paper and successively cleaned with ultrasonic waves in ultra-pure water. Taking into account the different electrode areas, the current is translated into current density (j/mA cm−2 ) for reader’s convenience. The potential quoted here is versus the saturated mercurous sulfate electrode (SMSE). All experiments were performed at room temperature (∼20 ◦ C) and solutions were freshly prepared under nitrogen purging with analytical grade chemicals and ultra-pure water (from a water purification system of Millipore Corp., USA). After electrolysis under potential control, the gold colloids produced in the quasi-dealloying were mixed with 5 mmol dm−3 PVP (polyvinylpyrrolidone, average molecular weight 10,000) to prevent the nanoparticles from agglomeration, sequentially centrifugated at 12,000 rpm, rinsed with absolute ethanol twice, and redispersed ultrasonically in an ounce of ethanol. Drops of the dispersed colloid were placed on the carbon-coated copper grid and glass slide, respectively, for the detections of particle sizes and composition. A sol of Au for contrastive experiments, signed as Sample I, was prepared by simply adding 5 cm3 0.01 mol dm−3 KBH4 drop by drop into 5 cm3 0.005 mol dm−3 HAuCl4 [28,29] under vigorous stirring for 10 min without adding any stabilizer and then by centrifugation aggregated GNPs were ready for the next test of Raman spectra. Images of shape and size were measured by a Hitachi S-4800 filed-emission scanning electron microscope (FE-SEM) at 5 or 10 kV accelerating voltage, or performed on a transmission electron microscope (TEM) with Tecnai G2 20ST operated at 200 kV accelerating voltage. The atomic ratio of gold and tin was checked by energy-dispersive X-ray (EDX) spectra before and after dealloying. XRD patterns were collected on a Philips PW 3040/60 powder diffractometer equipped with a Philips Analytical X’Celerator, using Cu K␣ radiation in a 2 range from 30 to 80◦ with a scan rate of 0.2◦ s−1 . The working voltage of the instrument was 40 kV and the current was 40 mA. Optical absorbance spectra of colloids were registered in the wavelength range from 400 to 700 nm using the UV–vis spectrophotometer (Nicolet Evolution 500 from Thermo). Raman measurements with 632.8 nm excitation were performed with a Renishaw RM1000 confocal microscope in a self-designed spectroscopic cell made by Teflon with a quartz window [30]. The objective was of long working distance (8 mm) with 50× magnification and the laser power was about 3 mW on the sample.
3. Results and discussion 3.1. Electrochemical behaviors Fig. 1 shows cyclic voltammograms (CVs) performed in 2 mol dm−3 HCl at freshly polished Sn, Au, Au70 Sn30 and Au40 Sn60 electrodes, respectively, with an upper limit of 1.2 V, which is negative to the oxygen evolution potential in order to prevent the oxidation of Sn(II) by oxygen bubbles. Au dissolution begins at about 0.3 V (roughly indicated by the vertical dotted line) and reaches a maximum current density around 0.6 V, followed by passivation positive to 0.9 V (Fig. 1(b)). Moreover, the curves cross in
Fig. 1. Cyclic voltammograms (scan rate 100 mV s−1 ) in 2 mol dm−3 HCl for electrodes of (a) pure Sn, (b) pure Au, (c) Au70 Sn30 , and (d) Au40 Sn60 , respectively. Typical current oscillations for Au–Sn alloys at 0.95 V are shown in the insert of parts (c) and (d). The dotted vertical line at 0.3 V performs as a glancing line dividing the regions between classical dealloying and quasi-dealloying.
the CV around the transition region of gold between the active and passive states, forecasting current oscillations as our group ever reported [30]. Sn keeps on mass active electrodissolution from −1.0 V on with steep current increase till 0.3 V in the CV (Fig. 1(a)). It has been proposed that pure Sn was electrodissolved solely into Sn(II) in HCl [31] Sn(0)(s) → Sn(II)(aq) + 2e−
(i)
In the backward scan, reductive peaks below −1.0 V can be observed, corresponding to the Sn deposition. Turn to the CVs of Au–Sn alloys in Fig. 1(c) and (d), significant electrodissolution of Sn occurs around 0 V with much less anodic current than that in the pure Sn (Fig. 1(a)) due to alloying with Au. However, the alloys are far from passivation between 0 and −0.8 V (Fig. 1(d)) or −0.6 V (Fig. 1(c)) where the gold is inert, noting the large scale of current density (100 mA cm−2 ) in Fig. 1. In the more positive potential range (on the right of the dotted vertical line), the alloys present quite similar behaviors to the pure Au in Fig. 1(b), such as crossed CVs and current oscillations (the insert of Fig. 1(c) and (d), yet the current in the passivating range positive to 0.9 V is somewhat higher than that in the pure Au, indicating that the passive film is less protective after alloying. The cathodic peak for the reduction of AuCl4 − at 0 V decreases with the increase of Sn ratio in the alloys from Fig. 1(b)–(d), due to the chemical consumption of AuCl4 − produced at higher potentials by more dissolved Sn(II). No reduction peaks of AuCl4 − can be observed either by reversing the scan at potentials less than 0.3 V (not showing) since gold does not
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dissolve below that potential, which also identified by in situ Raman spectra (Section 3.2). These facts suggest that there are two kinds of potential-dependent dealloying for Au–Sn alloys, i.e. without and with co-dissolving of gold in the lower and higher potential ranges, respectively. That is what we called classical dealloying and quasidealloying. 3.2. In situ Raman spectroscopy to reveal the dealloying processes To better understand the physicochemical processes involved in the electrooxidation of Au–Sn alloys, in situ Raman spectra have been taken by varying the potential, electrode component, medium, and laser position. Fig. 2(a) and (b) shows the Raman spectra for the two alloys in 2 mol dm−3 HCl, and the potential control is such that it was first increased (the bottom) and then at the open state for a period of time (the upper). The doublet at 322 and 346 cm−1 for the complex AuCl4 − appears by 0.4 V for Au70 Sn30 (the bottom of Fig. 2(a)), in agreement well with our previous results [30] where pure Au dissolution commences nearby. The intensity of the doublet for the higher Sn-containing alloy Au40 Sn60 is much weaker even at 0.6 V (the bottom of Fig. 2(b)), indicating the quicker consumption of AuCl4 − by more abundant Sn(II) via the reaction 2AuCl4 − (aq) + 3Sn(II)(aq) → 2Aunano(s) + 3Sn(IV)(aq) + 8Cl− (aq)
(ii)
An alternative mechanism is thermodynamically possible via the reaction 2AuCl4 − (aq) + 3Sn(0)(s) → 2Aunano(s) + 3Sn(II)(aq) + 8Cl− (aq)
(iii)
The ion reaction (ii) between Sn(II) and Au(III) in solution phase should be more dominant than reaction (iii) between aqueous
Au(III) with Sn in alloy phase, in view of also that Sn(II) can be produced electrochemically (i) and chemically (iii). There is a very strong companion peak appearing at 269 cm−1 (the bottoms of Fig. 2(a) and (b)) for the adsorbed Cl− (Aunano –Cl− ad ) on the roughed gold [30] evidenced the SERS effect of the newly produced gold nanopores and/or nanoparticles (Aunano ). When the circuit was switched to the open state (the uppers of Fig. 2(a) and (b)), the doublets disappear due to the removal of AuCl4 − from the surface via the chemical reduction as well as diffusion. Meanwhile, the Au–Cl− ad stretching at 269 cm−1 diminishes gradually and a new feature appears around 326–328 cm−1 . However, only the doublet fades away by diffusion in the case of pure gold in 2 mol dm−3 HCl (Fig. 2(c)) [30]. It can be reasonably figured out that the peak at 328 cm−1 must be from the adsorption of Sn species on the Au most probably Sn(II) complexes with Cl− , noticing also that it is the sole peak below 0.3 V in Fig. 2(a) and (b) where only classical pore-forming dealloying occurs. More experiments have been performed to support the guess on the origin of the peak at 328 cm−1 . By mixing the as-prepared GNPs (Sample I from the reaction of KBH4 with HAuCl4 as described in Section 2) with drops of 2 mol dm−3 HCl, there is a SERS peak at 275 cm−1 for the adsorbed Cl− on the gold nanoparticles (Fig. 2(d)). Then, by dropping solutions of Sn(II) (2 mol dm−3 HCl solution containing 0.1 mol dm−3 SnCl2 ) and Sn(IV) (2 mol dm−3 HCl solution containing 0.1 mol dm−3 SnCl4 ) on the GNPs (Sample I) separately, different phenomena were observed (Fig. 2(d)): the peak at 275 cm−1 for adsorbed Cl− is replaced by that at 326 cm−1 for Sn(II), yet holds for Sn(IV). These results provide a piece of direct evidence for the origin of the peak at 328 cm−1 , and it is no doubt from the adsorption of Sn(II) complexes with Cl− . However, the
Fig. 2. In situ Raman spectra by varying the potential, electrode component and medium. (a) Au70 Sn30 , (b) Au40 Sn60 and (c) Au in 2 mol dm−3 HCl, and (d) gold sol (Sample I) for comparison with Sn(II) (0.1 mol dm−3 SnCl2 + 2 mol dm−3 HCl) or Sn(IV) (0.1 mol dm−3 SnCl4 + 2 mol dm−3 HCl) solution. All of the collection time for recording a single spectrum is 20 s.
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exact coordination form and adsorption mode of Sn(II) complexes are out of the scope of this paper, and we do not further pursue them here. The results also indicate that the adsorption of Sn(IV) complex (probably SnCl6 2− ) is so weak that it could not be detected with SERS even if it is present from the chemical reaction (ii) and the following electrooxidation reaction on the gold surface Sn(II)(aq) → Sn(IV)(aq) + 2e−
(iv)
It would be worthwhile showing how to distinguish the two kinds of dealloying ways more unambiguously with the in situ Raman spectroscopy. It is very interesting to find that the quasidealloying mode can be monitored simply by locating the laser at position 2 (Fig. 3a) rather that at position 1 as did in the above Raman measurements. The position 2 is on the insulating wrap of the disk electrode about 500 m far from the edge of the embedded metal. For the quasi-dealloying of Au40 Sn60 at a higher potential of 0.6 V, the SERS band at 272 cm−1 for the Aunano –Cl− ad can be observed at the position 2 after 50 s under that potential control and its intensity gets stronger with time as seen from the time-resolved
Fig. 3. In situ Raman spectra by changing the laser position from 1 to 2. Position 1 in (a) is focused on the metal center of the electrode as did in Fig. 2, but position 2 in (a) that on the insulating wrap (ca. 500 m far from the edge of the metal embedded) was chosen here in part (b) and the time-resolved spectra in (b) were taken with Au40 Sn60 alloy at 0.6 V and then at the open circuit state.
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Raman spectra (the bottom of Fig. 3b). The band at 272 cm−1 is replaced by the band at 329 cm−1 (the upper of Fig. 3b) while the potential changes from 0.6 V to the opening circuit (ca. −0.3 V), indicating that the adsorption of Sn(II) species is stronger than Cl− at the much lower open circuit potential. These facts indicate that the gold nanoparticles produced from the reaction of co-dissolved Au(III) and Sn(II) can drift to the position 2 from the electrode surface and stack on the wrap in a short period of time. While in the classical dealloying potential range (below 0.3 V), we cannot observe these peaks at the position 2, because only Sn dissolves there resulting in nanopores on the surface of alloys. 3.3. Surface morphology in the classical- and quasi-dealloying regions By controlling the potential for a few minutes in the lower potential region (0.2 V), where only Sn dissolves in the classical dealloying way, familiar nanoporous structures can be obtained from the Au40 Sn60 and Au70 Sn30 alloy, as shown in Fig. 4(a), (b) and (f). Dealloying is likely while the nobler component is below 80 at.% according to the percolation theory [32], and that is true here. With the formation of nanopores, the nobler component is built up at the surface from Au40 Sn60 to Au90 Sn10 in 200 s (lines A and C in Fig. 5) detected by EDX. Selective dissolution of Au–Sn alloys could also proceed chemically in the same medium but with a much slower rate. While immerging the alloy (Au40 Sn60 ) in 2 mol dm−3 HCl without application of an external voltage, the surface is only slightly chemically etched even for 3600 s (Fig. 4(e)) ended with a component of Au49 Sn51 (line B of Fig. 5). So the contribution from the chemical dissolution is minor during dealloying electrochemically here. The open circuit potentials (OCPs) of Sn, Au40 Sn60 and Au70 Sn30 in 2 mol dm−3 HCl are ca. −1, −0.9 and −0.6 V, respectively, and electrodissolution of Sn commences while the potential is slightly larger than those OCPs (Fig. 1). However, more accurate evaluation for the critical potential Ec [32–34] of the selective dealloying of Au–Sn alloys needs more detailed studies such as through empirical current threshold [32], polarization curves [32,33] and steady-state current [34], which is not the concern here. More emphasis of us is placed on the quasi-dealloying in the higher potential region, where the two components co-dissolve into AuCl4 − and Sn(II) complexes and gold nanoparticles form immediately by the redox reaction of the two dissolved species. The produced GNPs can pile up to a thin layer on the electrode surface and disperse into the solution as colloids. Fig. 4(c) and (d) shows the top and side SEM images, respectively, for the stacked thin layer of gold nanoparticles on the surface. It can be seen from the images that these nanoparticles are rather uniform and densely packed on the substrate, consisting of purely gold as indicated by EDX (line D in Fig. 5). The layer from the quasi-dealloying also shows nanoporous structure. Most of the GNPs dispersed into the solution and a thread of aubergine stream can be observed coming from the Au–Sn anode during quasi-dealloying at higher potential region (e.g. 0.6 V), resulting in a deep purple colloid. This observation suggests that the electrochemically co-dissolved Au(III) and Sn(II) species can form nanoparticles very rapidly before their going far off the electrode surface, which might offer an expeditious method for preparing clean gold colloids continuously according to the required amount from the simultaneous electrodissolution of Au–Sn alloys. The asprepared colloidal GNPs are spherical with diameters below 40 nm as ascertained with TEM (Fig. 6a). A broad peak appears around 540 nm in the UV–vis absorption spectrum (Fig. 6b). The red-shift with respect to the usual peak position [29] of gold colloids at 520 nm is probably due to the larger particle size and the adsorp-
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Fig. 4. Typical surface SEM images of Au40 Sn60 alloy treated in 2 mol dm−3 HCl by (a) classical dealloying at 0.2 V for 200 s with nanoporous gold, by (c) quasi-dealloying at 0.6 V for 200 s with stacked gold nanoparticles, and by (e) dipping in 2 mol dm−3 HCl for 1 h with slightly eroded surface. The insets in (a) and (c) shows larger magnification. The side view of samples (a) and (c) are shown in (b) and (d), respectively. (f) A typical image for the classical Au70 Sn30 dealloying at 0.2 V for 200 s.
tion of Sn(II) species. The synthesized GNPs shows no detectable Sn species by the analysis of EDX (line D of Fig. 5) and XRD (Fig. 6c) because both EDX and XRD are not sensitive enough as SERS in detecting (sub)monolayer adsorption of Sn species. 3.4. Facile fabrication of SERS substrate by grafting the quasi-dealloying
Fig. 5. The EDX spectra for the Au40 Sn60 alloy: (A) the original alloy material, (B) chemically etched sample, (C) classical dealloying sample, and (D) quasi-dealloying sample, (B)–(D) correspond to the SEM images (e), (a) and (c) in Fig. 4, respectively.
We have grafted the quasi-dealloying manner of Au–Sn|HCl into Au|Sn|HCl to fabricate gold SERS substrate according to the following procedure. Firstly, deposit a thin Sn layer on the polished Au surface by controlling a cathodic potential at −1.1 V for 20 s with a bath of 2 mol dm−3 HCl + 0.1 mol dm−3 SnCl2 , and then co-dissolve the Sn overlayer with Au underneath in 2 mol dm−3 HCl by controlling the potential at 0.6 V for 20 s. A thin layer of slightly aggregated GNPs sticking to the gold surface can be obtained (Fig. 7a) in a similar way to the quasi-dealloying. Calculations and experiments have shown that it is possible to achieve large electromagnetic field enhancement at particle junctions of
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Fig. 8. A sketch map summarizing the Au–Sn anodic electrooxidation: (i) classical dealloying of Au–Sn alloy to gold nanopores, (ii) quasi-dealloying of Au–Sn and a graft of quasi-dealloying by electrodepositing a thin layer of Sn on the pure Au surface (Au|Sn) to the GNPs stacked on the surface or dispersed into the electrolyte as colloid. Fig. 6. The typical (a) TEM image, (b) UV–vis spectrum, and (c) XRD pattern of the as-prepared colloidal GNPs from Au40 Sn60 by holding the potential at 0.6 V for 1000 s.
aggregates [35,36]. In addition, other two manners that we ever employed to prepare the SERS substrate prove to be incompetent, such as Sn|(HAuCl4 + HCl) and Au|(SnCl2 + HCl). Either the base metal (Sn) is too active to apply anodic potentials and to stay in acidic solutions (along the former manner) or the gold nanoparticles yielded are poor adherence to the surface of the gold electrode (the latter case). Among others one obvious reason for the drawback in the latter case of Au|(SnCl2 + HCl) is that the reaction (ii) that produces gold nanoparticles apparently takes place at positions averagely farther off the surface than the case of co-dissolution of Au|Sn overlayer or Au–Sn alloy in HCl (both AuCl4 − and Sn(II) simultaneously come from the surface of the substrate). To test the SERS activity of the prepared substrate, pyridine was chosen as the probe molecule. High quality SERS spectra of absorbed pyridine were observed indeed (Fig. 7b) with characteristic peaks [37,38] located at 628, 1009, 1033, 1066, 1213, and 1591 cm−1 and maximum intensity at −1.0 V, which are consistent with literature. 4. Conclusions Potential-dependent and time-resolved in situ Raman spectra have been successfully applied to study the dealloying processes of Au–Sn alloys in the HCl medium. It has been revealed that there are two dealloying modes depending on the applied potential: (i) classical dealloying occurs in the lower potential region forming gold nanopores by selectively dissolving Sn; and (ii) quasi-dealloying appears in the higher potential region producing gold nanoparticles by the redox exchange reaction of Sn(II) and AuCl4 − from the simultaneous dissolution of Au–Sn alloys. The gold nanoparticles can stack on the electrode surface or disperse into the solution as gold colloid (as shown in Fig. 8). Furthermore, the quasi-dealloying mode can be grafted on a pure gold substrate with an electrodeposited Sn layer to prepare a thin layer of GNPs, showing strong SERS activity for pyridine molecules. Acknowledgments
Fig. 7. (a) The typical SEM morphology of stacked GNPs on Au electrode treated by the grafted quasi-dealloying method (see the text in Section 3.4 for details), and (b) in situ Raman spectra for the adsorption of pyridine on the GNPs covered electrode with potential decrease in the solution of 0.1 mol dm−3 KCl plus 0.01 mol dm−3 pyridine. The exposure time for CCD was 10 s.
Financial support of this research from National Natural Science Foundation of China (20673103, 20373063) is gratefully acknowledged.
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