Effects of Anions on Electrochemical Reactions of Silver Shells on

Article pubs.acs.org/JPCC

Effects of Anions on Electrochemical Reactions of Silver Shells on Gold Nanorods Yuki Hamasaki,† Naotoshi Nakashima,†,‡,§ and Yasuro Niidome*,†,‡ †

Department of Applied Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan World Premier International (WPI), Research Center International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § JST-CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan ‡

S Supporting Information *

ABSTRACT: Redox reactions of the silver shells of Au−Ag core− shell nanorods were performed in five electrolytes (KBr, KCl, K2HPO4/KH2PO4, KNO3, and KClO4). The spectral changes indicated oxidation and reduction of the silver shells. Silver shells were dissolved in the positive sweeps of the cyclic voltammogram measurements. In the presence of phosphate, deposition of metallic silver on the electrode was suppressed, and silver shell deposition on gold nanorods was observable. In the other electrolytes, the silver ions were deposited as metal silver nanoparticles on the ITO plate.

1. INTRODUCTION Anisotropic noble metal nanoparticles have been widely studied because of their remarkable optical properties that come from surface plasmon (SP) bands in the visible and near-infrared (near-IR) regions.1,2 Because the SP bands originate from collective motions of free electrons in nanoparticles, their shape, size, and aggregation intensively affect the optical properties. Control of these parameters is important to design preferable optical properties in noble metal nanoparticles. There are many papers describing the preparation of uniform and shape-controlled metal nanoparticles. For example, gold nanorods are typical anisotropic gold nanoparticles that have uniform shapes.3 A variety of characterizations and applications of gold nanorods have been previously reported.4−9 Their intense extinction bands in near-IR regions contributed biorelated applications of those.10−14 Gold nanoprisms, which are triangular nanosheets of gold, are another popular anisotropic nanoparticle.15−18 The gold nanoprisms also show their SP bands in the near-IR region and are expected to be used for unique self-assembling nanostructures due to their unique shapes. Versatile surface modification of gold using thiol molecules contributed controlled delivery of gold nanoparticles to a tissue.14,19−23 Self-assembling of nanoparticles is also possible using surface modification.24−28 The assembling of gold nanoparticles induced dramatic spectral changes and could be used for analysis of biorelated molecules.16,22,29,30 Chemical stabilities and uniformity in their shapes are the advantages of gold nanoparticles. These contribute to making precise control of optical properties; however, in the case of gold, there are not so many variations of shapes other than spherical, rod-shaped, and triangular nanoparticles. Silver nanoparticles show remarkable optical properties in visible regions and hold much attention for application in © 2012 American Chemical Society

catalyst, surface-enhanced Raman scattering (SERS), and some types of sensors. It is known that silver nanoparticles can have a variety of shapes. Wires,31−35 cubes,36−39 polyhedra,40−42 and nanoprisms15,43 and so on were reported. Because of this “polymorphism” of silver nanoparticles, colloidal silver nanoparticles tend to have various shapes. Methodology for obtaining uniform silver nanoparticles has been an issue in the characterization and application of silver nanoparticles. Citrate reduction44 and poly-ol methods35,38,45 are known as useful methods to obtain uniform silver nanoparticles; however, in spite of intensive efforts, the size distribution and shape of silver nanoparticles are not as good as those of gold nanoparticles. Silver shell formation on gold nanoparticles is a possible method to tune the shape of silver nanoparticles because gold nanoparticles can be prepared in a very uniform way.46−49 Using gold nanorods as core nanoparticles, we have produced anisotropic Au−Ag core−shell nanorods48,50 that exhibited the optical properties of “silver nanorods”. Our core−shell nanorods are very uniform and show four SP bands that are assignable to specific SP oscillations.9,51−53 Electrochemical reactions of silver are a convenient way to design the shapes of silver nanoparticles because the redox reactions of silver are controllable in solution. For example, deposition of silver ions has been used to obtain silver nanoparticle films on a silicon electrode,54 while electrochemical etching of silver nanopatterns has been used to Special Issue: Nanostructured-Enhanced Photoenergy Conversion Received: June 30, 2012 Revised: October 27, 2012 Published: November 8, 2012 2521

dx.doi.org/10.1021/jp306469s | J. Phys. Chem. C 2013, 117, 2521−2530

The Journal of Physical Chemistry C

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Figure 1. Extinction spectra of Au−Ag nanorods in colloidal solution (dashed lines) and deposited on an ITO plate (solid lines). The ITO plates were immersed in KBr (a), KCl (b), K2HPO4/KH2PO4 (c), KClO4 (d), and KNO3 (e) solutions (0.1 M).

control their optical properties.55 If we can design an electrochemical reaction of uniform silver nanoparticles, it will be a useful method to obtain preferable optical properties. In this work, we deposited Au−Ag core−shell nanorods on transparent ITO plates and performed electrochemical reactions of the silver shells. Redox reactions of silver in an aqueous solution are strongly affected by the type of electrolyte used and the surface modification of the nanorods. We examined the electrochemical oxidation of the silver shells and reductive deposition of silver on ITO plates in five kinds of electrolytes (KBr, KCl, K2HPO4/KH2PO4, KNO3, and KClO4). The spectral changes accompanying the redox reactions were monitored using a multichannel spectrophotometer. In situ observation revealed the details of the oxidation and the deposition of the silver shells.

further CTAC solution (80 mM, 10 mL). A AgCl microparticle solution (0.1 mL), which was prepared by the addition of silver nitrate (17 mg) to a CTAC solution (10 mL, 80 mM), was added to this mixed solution. Then, a sodium hydrochloride solution (0.5 M, 0.1 mL), to control the pH, was added to start the shell formation. The resultant core−shell nanorods measured 31 ± 6 and 63 ± 4 nm in transverse and longitudinal directions, respectively. The core−shell nanorods were centrifuged twice and dispersed in water to decrease the concentration of CTAC. After the third centrifugation, the nanorods were dispersed in a solution of poly(styrene sulfonate) (PSS, Mw 70 000, Aldrich) to obtain polyanionic surfaces. The excess PSS was removed by centrifugation, and the nanorods were redispersed in water. An ITO plate was exposed to 3-amino-propyltriethoxysilane (APTES) to generate a surface with polycations. The APTES-treated substrates were immersed in an aqueous solution of the PSS-wrapped core−shell nanorods to deposit them on the cationic surfaces. The surface resistance of the ITO plates (GEOMATEC Co. Ltd.) was 7.3 ± 0.5 Ω/□. The nanorod-deposited plates were cut into about 9-mm wide strips and set in an optical cell with an optical path length of 1 cm. A standard electrode (SCE) and a counter electrode (Pt wire) were also set in the cell and sealed with a polymer film. Cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) were performed using a potentiostat (HZ-5000, Hokuto). Before CV measurements, the electrolyte was purged with nitrogen gas. In situ spectroscopy of the nanorod-deposited plates was performed using a multichannel spectrophotometer (MCPD-7700, Otsuka

2. EXPERIMETNAL SECTION Gold nanorods were obtained from Dainihon Toryo Co. Ltd., with sizes of about 10 and 50 nm in the transverse and longitudinal directions, respectively. Silver shells were prepared on gold nanorods according to the method described in a previous paper.53,56 Gold nanorods in a hexadecyltrimethylammonium bromide (CTAB) solution were centrifuged at 15 000g for 10 min, and the precipitates were redispersed in a hexadecyltrimethylammonium chloride (CTAC) solution (80 mM). This procedure was repeated twice, and the concentration of the gold nanorods was then adjusted to 33 nM (0.22 mM as atomic Au). The absorbance of the nanorod solution was 0.5 at 900 nm. The nanorod solution (0.4 mL) and an ascorbic acid solution (100 mM, 0.5 mL) were added to a 2522



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Denshi) at measurement intervals of 0.1 s. For EQCM measurements, AT-cut quartz crystals with gold electrodes (Hokuto, 10 MHz) were used. The wavelength range of the measurements was 317.3−1110 nm. After a cathodic sweep, the potential of the nanoroddeposited plate was set at its natural potential. Under this condition, reduced silver was deposited on the plate. The plate was then washed with a small amount of water, dried in air, and observed using FE-SEM (JSM-6701, JEOL).

3. RESULTS AND DISCUSSION 3.1. Deposition of Gold−Silver Core−Shell Nanorods on ITO Plates. In Figure 1, extinction spectra of the core−shell nanorods in colloidal solutions (dashed lines) and on the ITO plates (solid lines) are shown. To obtain the spectra of the solid lines, the ITO plates were immersed in the electrolyte solutions (100 mM, KBr (a), KCl (b), K2HPO4/KH2PO4 (c), KClO4 (d), KNO3 (e)). The colloidal solutions (dashed lines) showed four extinction peaks in the visible region, assignable to SP bands of the core−shell nanorods.51−53 The five nanoroddeposited ITO plates also showed four extinction peaks, and the longitudinal SP bands at around 550 nm shifted to the longer-wavelength region. It was shown that the core−shell nanorods were deposited on the ITO plates retaining their optical properties without forming aggregates, and the longitudinal SP bands were responsive to the high reflex indices of the ITO. A typical scanning electron microscopy (SEM) image of the nanorod-deposited ITO plate is shown in Figure 2. The white rod-shaped spots are the deposited core− Figure 3. CVs of the nanorod-deposited ITO plates in KBr (a), KCl (b), K2HPO4/KH2PO4 (c), KClO4 (d), and KNO3 (e). The solid, dashed, dotted, and dot-dashed lines show the first, second, third, and fourth sweeps, respectively.

perchlorate (d) and nitrate (e), the peaks at 0.26 V vs SCE originated from the reduction of Ag+ ions. Second cathodic peaks were found in the cases of the phosphate (c, 0.035 V vs SCE), the perchlorate (d, 0.055 V vs SCE), and the nitrate (e, 0.135 V vs SCE). As shown in the Supporting Information (Figure S1), in situ spectroscopic measurements in these potential ranges showed that no spectroscopic change of the SP bands was induced around these peaks. It was found that these peaks did not originate from redox reactions of the silver shells. The dashed lines in Figure 3 denote the CVs of the second sweeps. The decreases in the peak currents indicated a decrease in silver at the electrode surfaces after the first sweeps. Some of the silver ions, which were produced by the first anodic sweeps, diffused into the bulk and were not deposited by the following cathodic sweeps. The following CV measurements showed further decrease in the peak currents, but the largest decreases were found between the first and the second sweeps. The decreases in the redox currents depended on the type of electrolyte. In the cases of bromide (a) and chloride (b) solutions, the decrease was not as remarkable as those for the phosphate (c), perchlorate (d), and nitrate (e) solutions. It was observed that the silver ions in the halogen solutions (a, b) were mostly retained on the electrodes, but in the other three electrolytes (c, d, e), much larger amounts of silver ions diffused into the bulk, and the peak currents decreased less than half as much as those of the first sweeps. Extinction spectra of the nanorod-deposited plates were recorded during the CV measurements at intervals of 0.1 s and are shown in Figure 4. The sweep rates were 80 mV/s. In the

Figure 2. SEM image of an as-prepared nanorod-deposited plate.

shell nanorods. A few aggregated core−shell nanorods were found in the image. It was consistent with the spectroscopic properties in Figure 1. 3.2. CV Measurements and in Situ Spectroscopy. CVs of the nanorod-deposited ITO plates are shown in Figure 3. KBr (a), KCl (b), potassium phosphate (pH = 7, K2HPO4/ KH2PO4) (c), KClO4 (d), and KNO3 (e) were used as supporting electrolytes (100 mM). The sweep rate was 80 mV/ s. The first and the second sweeps are shown as solid and dashed lines, respectively. The redox reactions of the silver shells were strongly dependent on the anions. The standard reduction potentials of the silver ions were: AgBr, 0.071 (−0.17); AgCl, 0.222 (−0.019); and Ag+, 0.80 (0.56) V vs NHE (vs SCE).57 The onsets of the redox reactions in Figure 3 were consistent with the order of the standard potentials. The bromide (a) and chloride (b) ions gave AgX2− (X = Br− or Cl−) rather than the neutral salts (AgX).58 In the case of phosphate (c), the formation of silver phosphate (Ag3PO4), insoluble in water,59 should be taken into account. In the cases of 2523



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Figure 4. Extinction spectral changes of nanorod-deposited ITO plates in KBr (a, b), KCl (c, d), K2HPO4/KH2PO4 (e, f), KClO4 (g, h), and KNO3 (i, j). Left column (a, c, e, g, i): oxidation of the silver shells. Right column (b, d, f, h, j): reduction of silver ions.

silver ions and interrupting direct contact of the nanorods to the electrodes. The degree of oxidation of the silver shells was dependent on the sweep ranges of the CV measurements, how the silver ions were able to diffuse into the bulk solution, and how frequently the PSS-wrapped nanorods contacted on the electrodes. In the perchlorate solution (g, h), the SP bands almost disappeared after the anodic sweep (g), and large spectral changes were observed in the cathodic sweep (h). It is probable that the wide sweep range (0.8 V) and the frequent contact of the nanorods on the electrode gave the large spectral changes in the perchlorate solution. Meanwhile, in the other

all electrolytes, sweeps to the anodic side (a, c, e, g, i) monotonically decreased the extinction peaks of the core−shell nanorods. This indicated the oxidation of the silver shells. In contrast, in the sweeps to the cathodic side (b, d, f, h, j), increase of the extinction, which corresponded to the deposition of metal silver, was observed. It should be noted that some of the core−shell nanorods were not oxidized during the anodic sweeps. After the anodic sweeps in Figure 4(a, c, e, g, i), the SP bands of the core−shell nanorods were observable. This probably originated from the PSS layers on the nanorod surfaces limiting the diffusion of 2524



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Figure 5. Difference spectra between anodic and cathodic sweeps in KBr (a), KCl (b), K2HPO4/KH2PO4 (c), KClO4 (d), and KNO3 (e). These spectra were assignable to the metal silver deposited by the cathodic sweeps. The solid and dashed lines were the spectra of the first and the second sweeps, respectively.

could not find a SP band of the core−shell nanorods (Figure 1). SEM images taken after the reductive sweeps showed that larger spherical nanoparticles were generated on the ITO plates (Figure 6). SEM−EDX indicated that these spherical particles

four electrolytes, some of the core−shell nanorods were retained on the ITO plates, keeping their rod shapes. During the oxidation processes in bromide (Figure 4(a)) and chloride (Figure 4(b)) solutions, the extinction peaks shifted to shorter wavelengths. In contrast, the spectral changes observed in Figure 4(c, d, e) showed shifts to the longer wavelengths. This indicated that the silver shells were partially oxidized and that their changes of shape had different pathways depending on the type of electrolytes. Using the extinction spectra obtained during the in situ measurements, we could obtain difference spectra between the extinction spectra at the natural potential after the cathodic sweeps and those after the anodic sweeps. For example, in the case of the KCl solution, two spectra were obtained at 0 V vs SCE after sweeps to anodic (to +0.20 V vs SCE) and cathodic (to −0.20 V vs SCE) sides. The spectra in Figure 5 are the difference spectra obtained in solutions of KBr (a), KCl (b), K2HPO4/KH2PO4 (c), KClO4 (d), and KNO3 (e) for the extinction spectra of the metallic silver that was electrochemically deposited by the cathodic sweeps. The solid and dashed lines indicate spectra obtained after the first and the second sweeps, respectively. The chloride showed the same spectra as those of the bromide, while the spectra of the nitrate were the same as those of the perchlorate. In the case of the chloride electrolyte (Figure 5(b)), the electrochemically deposited silver showed two peaks at 420 and 500 nm. The former corresponded to the SP band of isolated spherical silver nanoparticles. For the latter wavelength, we

Figure 6. SEM images of the nanorod-deposited plates after cathodic sweeps in KBr (a), KCl (b), K2HPO4/KH2PO4 (c), KClO4 (d), and KNO3 (e). 2525



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Figure 7. Extinction spectral changes (a, b) and difference spectra (c, d) of the nanorod deposited ITO substrate during constant potential electrolysis in KCl (100 mM). The potentials of the plates were set to be +100 mV vs SCE (a, c) and the reductive electrolysis at 0 mV vs SCE (b, d), respectively. The images are SEM images after the oxidative (e) and reductive (f) electrolysis.

and nitrate (Figure 6(d, e)) showed some larger nanoparticles, indicating that some of the Ag+ ions dispersed on the electrode surfaces and formed these larger particles as well as the silver shells. Thus, the spectra shown in Figure 5(d, e) are attributed to overlaps of three species: isolated silver nanoparticles (at 420 nm), larger particles (at around 520 nm), and silver shells (at 420 and around 550 nm). In contrast, the difference spectra in phosphate (Figure 5(c)) showed two comparable peaks at 430 and 560 nm, but the peak intensities were smaller than those observed in the other electrolytes. It is notable that the second sweeps gave the same spectrum as that of the first sweep, in spite of the diffusion of silver ions into bulk during the CV measurements, as shown in Figure 3(c). This indicates that the deposition of the silver ions was independent of the amount of silver on the electrode. It is probable that silver ions around the nanorods were electrochemically reduced and deposited as silver shells. Among the five kinds of electrolytes, only in the phosphate solution, an insoluble salt, Ag3PO4, formed during the CV measurements.59 The insoluble Ag3PO4 must have contributed to the electrochemical and the spectroscopic responses of the silver ions. From the SEM image of the ITO plate in phosphate (Figure 6(c)), there were very few larger silver particles and many “rounded” shaped nanorods. The rounded nanorods may have been partially oxidized core−shell nanorods, as discussed above. 3.3. Constant Potential Electrolysis. The CV measurements applied overpotentials, up to +0.5 and −0.3 V vs SCE, to

consisted of silver (see Supporting Information, Figure S2). Thus, the peak at around 500 nm probably originated from these larger silver nanoparticles. The second sweep showed the larger peak at around 500 nm. It is probable that repeated sweeps produced large silver nanoparticles on the ITO plate. Many rod-shaped particles were observed in Figure 6(a, b, c), but these did not have sharp rectangular shapes like the original nanorods in Figure 2. The “rounded” nanorods were the partially oxidized core−shell nanorods as mentioned above. In the case of bromide (Figure 5(a) and Figure 6(a)), the same CVs and SEM images as those of the chloride were obtained. Thus, deposition of silver on gold nanorods as silver shells did not occur in the cases of KBr and KCl electrolytes. In the phosphate, the perchlorate, and the nitrate solutions (Figure 5(c, d, e)), the difference spectra showed the peaks in the longer-wavelength regions at around 560 nm. These peak positions were consistent with those of the SP bands of the core−shell nanorods on the ITO plates (Figure 1). This strongly suggested that the peaks at 550−560 nm originated from silver shell formation on the gold nanorods. In the case of the perchlorate and nitrate (Figure 5(d, e)), the peaks at around 420 nm were larger than that in the phosphate. Formation of silver nanoparticles on the ITO plates in the perchlorate and the nitrate probably resulted in these larger peaks at around 420 nm. It should be noted that the second sweeps (dashed line) in Figure 5(d) showed smaller spectral changes. SEM observations of the ITO plates in perchlorate 2526



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Figure 8. Extinction spectral changes (a, b) and difference spectra (c, d) of the nanorod deposited ITO substrate during constant potential electrolysis in K2HPO4/KH2PO4 (PO4−: 100 mM). The potentials of the plates were set to be +250 mV vs SCE (a, c) and the reductive electrolysis at +150 mV vs SCE (b, d), respectively. The images are SEM images after the oxidative (e) and reductive (f) electrolysis.

(AgCl2−); however, the reduction of the silver ions did not form silver shells on gold nanorods. Rapid diffusions of AgCl2− going out to the PSS layers should be taken into account. Figure 8 shows extinction spectral changes (a, b) and difference spectra (c, d) during the constant potential electrolysis in a K2HPO4/KH2PO4 solution and SEM images before (e) and after (f) the electrolysis. The oxidative electrolysis at +250 mV vs SCE decreased the extinction of the core−shell nanorods (a, c), but the degree of the decrease was not as intense as those in the KCl solution (Figure 7). The SEM observation (Figure 8(e)) showed a few core−shell nanorods even after the electrolysis. These probably resulted from the original core−shell nanorods insulated from the electrode or partially oxidized core−shell nanorods that were wrapped with insoluble Ag3PO4. The Ag3PO4 layer interrupted electronic contact between the silver shells and suppressed their further oxidation. This is consistent with the spectral changes seen in Figure 8(a, c), in which the SP bands were retained after 900 s of electrolysis. Reductive electrolysis at +150 mV vs SCE increased the extinction (Figure 8(b)) and resulted in the same difference spectra as those observed during the CV measurements, but the final spectrum observed after 900 s of electrolysis showed a broadened profile in the near-IR region. It was found that the constant potential electrolysis resulted in aggregation of silver nanoparticles, and the degree of the spectral changes was not as remarkable as that observed in chloride (Figure 7(b, d)). Thus, the insoluble Ag3 PO4

oxidize and reduce the silver shells. That would be disadvantageous to make precise control of the electrochemical deposition of silver. Here, we attempted constant potential electrolysis of the silver shells in KCl and K2HPO4/KH2PO4 electrolytes (100 mM). Taking into account the CVs in Figure 3, the potentials to oxidize and reduce silver were set to be +100 and 0 mV vs SCE in a KCl solution, respectively. In a K2HPO4/KH2PO4 solution, the potentials were set to be +250 and 150 mV vs SCE for oxidation and reduction, respectively. Figure 7 shows extinction spectral changes (a, b) and difference spectra (c, d) during the constant potential electrolysis in a KCl solution. SEM images before (e) and after (f) the electrolysis are also shown in Figure 7. The oxidative electrolysis showed a dramatic decrease of extinction bands of core−shell nanorods (a, c). After 900 s of electrolysis, a peak that could be assigned to a longitudinal SP band of gold nanorods was found at around 900 nm. It was found that the constant potential electrolysis in the KCl solution almost completely oxidized the silver shells. The SEM image after the electrolysis (Figure 7(e)) showed thin gold nanorods on the ITO surface. This also indicated the efficient oxidation of silver shells. In the reductive electrolysis at 0 mV vs SCE, an increase of extinction at around 430 nm was observed. This corresponded to the SP bands of the silver particles that were found in the SEM images (Figure 7(f)). The core−shell nanorods were wrapped with polyanionic PSS layers. The PSS layers potentially suppress the diffusion of anionic silver ions 2527



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the amount of the core−shell nanorods on the QCM tips was much larger than that on the ITO plates. In the case of the chloride (a, b), the repeated CV measurements (a) gave new oxidation peaks at around 0.12 and 0.15 V vs SCE. These peaks were assignable to the oxidation of Ag to AgCl2− and AgCl, respectively,60 and indicated the deposition of insoluble AgCl on the QCM tip. The EQCM measurements (b) supported the deposition of AgCl. That is, when Ag oxidized on the electrode between 0.14 and 0.2 V vs SCE, the mass of the electrode increased due to the deposition of AgCl. In the cathodic sweeps between 0 and −0.2 V vs SCE, the reduction of AgCl to Ag decreased the mass of the electrode. This originated from the release of Cl− from the electrode surface. Repeating the CV measurements increased the mass, which indicated that some of the AgCl was retained on the electrode after the cathodic sweeps. These responses were consistent with our previous EQCM measurements in a AgCl2− solution.60 In the case of the ITO electrode (Figure 3), the amount of core−shell nanorods was much smaller than that on the QCM tip. The CVs in Figure 3(b) did not show deposition of AgCl because the concentration of Ag+ at the electrode surface was insufficient to form AgCl instead of AgCl2−. This was not contradictory to the model where oxidized Ag on the ITO plates formed AgCl2− and diffused into the bulk, as discussed above. In the case of the phosphate solution, no peak arose during the CV measurements (c), but the peak intensities decreased. This is the same as that observed in Figure 3(c). The EQCM measurements in Figure 9(d) indicated a decrease in mass during the anodic sweeps and an increase during the cathodic sweeps. Repeating CV measurements indicated a stepwise decrease in mass. This directly indicated diffusion of silver ions into the bulk and was consistent with the CV results in Figures 3(c) and 9(c), which showed decreases in the peak currents with repeating CV measurements. The diffused silver ions probably formed Ag3PO4, most of which did not deposit on the ITO electrode but diffused away into the bulk. In the first sweeps (black solid line), 17 ng of silver was released from the electrode, and about 10 ng was deposited. Thus, the amount of silver ions around the nanorods was 10 ng, and it was irrelevant of the cycle numbers of CV measurements. Even in the fourth sweeps, the amount of the silver deposition was about 10 ng. This is consistent with the reversible spectral changes observed in the cathodic sweeps of Figures 4(f) and 8(b, d).

suppressed the oxidation of the silver shells, prevented them from forming silver nanoparticles during reductive electrolysis, and electronically insulated them from the electrode. 3.4. EQCM Measurements. EQCM measurements were used to evaluate the diffusion and deposition of silver ions during their electrochemical reactions. Gold electrodes on the QCM tips were modified with 4-aminothiophenol to obtain cationic surfaces. The PSS-wrapped core−shell nanorods were deposited on the tips before CV and EQCM measurements. Figure 9 shows the CVs (a, c) and mass changes (b, d) of QCM tips on which the core−shell nanorods were deposited. The currents in the CV measurements were much larger than those in the cases of the ITO glasses (Figure 3). This indicates that

4. CONCLUSIONS CV measurements and constant potential electrolysis were performed on Au−Ag core−shell nanorods on ITO plates. In situ spectroscopy showed that the redox reactions of the silver shell were strongly affected by the anion type. In the presence of phosphate, the observed spectral changes of the Au−Ag core−shell nanorods were reversible. The formation of insoluble Ag3PO4 suppressed the oxidation of the silver shells and the deposition of silver nanoparticles on the ITO plates. In the other anion solutions, silver ions diffused into the bulk and were deposited as larger silver nanoparticles. This reversible silver shell formation, which originated from the silver ions around the nanorods, was observable because the deposition of silver nanoparticles was suppressed in the phosphate solution. It was found that the electrochemical responses of the core− shell nanorods could be controlled by the diffusion of silver ions and electrochemical deposition of metallic silver. Further surface modification of the core−shell nanorods and electrode surfaces using polymers and/or amphiphilic mole-

Figure 9. CVs (a, c) and mass changes (b, d) of the nanoroddeposited EQCM tips in KCl (a, b) and K2HPO4/KH2PO4 (c, d). The solid, dashed, dotted, and dot-dashed lines show the first, second, third, and fourth sweeps, respectively. 2528



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cules would affect the crystal growth of the silver on an electrode. The resulting combination of surface modification and controllable redox reaction will open up a new methodology to design functional silver nanoparticles that have electrochemically tunable optical properties.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +81 92 802 2841. E-mail: [email protected]. kyushu-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a KAKENHI Grant-in-Aid for Scientific Research (B) (No. 21350110) (for NN), Grantin-Aid for Scientific Research (B) (No. 21651053) (for YN), the Global COE Program “Science for Future Molecular Systems”, and World Premier International Research Center Initiative (WPI), from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Effects of Anions on Electrochemical Reactions of Silver Shells on

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