Letter Cite This: Anal. Chem. XXXX, XXX, XXX−XXX
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Size-Dependent Voltammetry at Single Silver Nanoelectrodes Hongmei Hua, Yong Liu, Dongmei Wang, and Yongxin Li* Anhui Key Laboratory of Chemo/Biosensing, College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, P.R. China
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S Supporting Information *
ABSTRACT: Understanding the kinetics and thermodynamics of electro-oxidation of metal nanostructures at single particles/electrodes level is extremely important. In this work, single silver nanoelectrodes with the radii down to 10 nm were fabricated by a laser-assisted pulling technique. Size-dependent voltammetry could be observed using prepared single silver nanoelectrodes with different radii, which involved a fast electron-transfer kinetics process through measuring electron-transfer rate and a favorable thermodynamics process from the negative potential shift with the decrease of radii. Our method provided a meaningful attempt to investigate electron-transfer kinetics and thermodynamics at single nanoparticles/nanoelectrodes level.
M
that only a single peak current at a given voltage could be recorded in a short time and no structural information could be revealed at all.11 It is also impossible to investigate the kinetics and thermodynamics process of electro-oxidation at the same time. On the other hand, the unique properties of nanoelectrodes, such as high mass transport rate, fast electrochemical responses, and small RC constants, have resulted in their wide applications, including fundamental electrochemical research and single cell analysis, as well as scanning electrochemical microscopy (SECM).12,13 Single nanoelectrodes with various geometries, such as nanodisk, nanowire, and nanopore, have been developed by our lab14,15 and other groups.16−19 Most of the single nanoelectrodes were prepared by sealing a gold or platinum microwire into glass tube or insulated materials (e.g., epoxy and wax) and then polishing the electrode until Pt or Au disk was exposed. Until now, challenges still exist to prepare nanoelectrodes made of metals other than Pt and Au (e.g., Ag, Hg) due to the unavailability of materials or fabrication techniques.20 In this work, single Ag nanoelectrodes (SAgNEs) with radii down to 10 nm were fabricated by an improved laser-assisted pulling method.14,17 Because silver (961.78 °C) has a relatively lower melting point than gold (1064.18 °C) and platinum (1773 °C) and has a significant mismatch between its thermal expansion coefficient and commercial glass capillary, it is very difficult to pull Ag ultrasharp tips without rupture and prepare nanoelectrodes. Herein, we have successfully fabricated SAgNEs with radii as low as 10 nm by changing the protocol and adjusting the pulling parameters (see Supporting
etal nanostructures, especially Ag nanostructures, have received tremendous attention due to their extensive applications in fundamental electrochemistry,1 bioanalysis and sensor fabrications, and surface-enhanced Raman spectroscopy (SERS).2 It is well-known that the properties of silver nanomaterials are related to their size, shape, and structures, and several groups have investigated the size effect of silver nanoparticles on their physical and chemical properties.3 For example, Compton and co-workers revealed that particle size and surface coverage of silver nanoparticles attached to a conductive electrode surface could affect the stripping voltammetry.1,3 Ivanova and Zamborini also observed that peak potential for silver nanoparticles oxidation was dependent on the size of silver nanoparticles.4 Despite many efforts having been made to address the size-dependent electrochemistry of silver nanomaterials, this issue is not yet understood clearly because the properties of silver nanomaterials modified on the electrodes surface are complicated; many factors, such as surface coverage, particle distribution, and the interaction of the particles with supporting materials, can affect the final results, and investigation of the properties of silver nanomaterials at the single nanoparticle level may be a good attempt. Recently, the stochastic collision amperometry (SCA) technique, developed by Xiao and Bard,5 has been extensively used to investigate the electro-oxidation of single Ag nanoparticles.1,6−10 For example, Zhang and co-workers and White and co-workers investigated the collision dynamics during the electrooxidation of single silver nanoparticles.6,7 Compton and co-workers9 developed a method for detecting silver nanoparticles in aqueous solution. Ma et al.10 observed potential-dependent current traces during the dynamic electrooxidation of single Ag nanoparticles. Although the SCA technique has achieved huge success at single silver nanoparticle electrochemistry, Anderson and Zhang pointed out © XXXX American Chemical Society
Received: June 12, 2018 Accepted: August 3, 2018 Published: August 3, 2018 A
DOI: 10.1021/acs.analchem.8b02644 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry
results,20 and thus, it is possible to use SAgNEs to investigate the size-dependent voltammetry. The fabricated SAgNEs were first used to investigate the electron-transfer kinetics through measuring the electrontransfer rate constant (k0) for the oxidation of Ru(NH3)63+. k0 is usually measured by means of the mass transfer rate of molecular to electrode surface.17 A hemispherical or very small radius of the disk electrode steady-state voltammetry is mainly caused by the diffusion of the material, and the condition can be expressed as21
Information). The continuity of pulled silver tips was checked by transmission electron microscopy (TEM), which could help us adjust the pulling parameters. Figure 1A shows the TEM
(D/k0a) ≥ 0.1
(1)
0
The value of k is generally in the range of 1 to 100 cm/s, which can be measured using nanosized electrode (1−100 nm) assuming a typical value of D (∼10−5 cm2/s). Figure 2A gives the voltammetric responses of Ru(NH3)6Cl3 at SAgNEs with different radii, which shows that the diffusion limit current (id) increases as the electrode size increases. The voltammograms in Figure 2A have been normalized (shown in Figure 2B) through dividing the corresponding value of limiting current, which can better reflect the relationship between the voltammetric response and the size of the nanoelectrode. From Figure 2B, it can be obviously observed that the halfwave potential, E1/2, shifts toward positive potential with the radius decrease of SAgNEs, which is consistent with previous results reported by us14,17 and other groups.16,22 The traditional Butler−Volmer electrode dynamics is one of the most classical formulas for studying electrode surface dynamics.22 Herein, we assume that the results of Figure 2 are consistent with BV dynamics, according to the equation of i−E
Figure 1. (A) TEM images of a pulled Ag tip with a radius of ∼35 nm. (B) FE-SEM image of a single Ag nanoelectrode with a radius of ∼30 nm.
image of a pulled silver/glass tip, in which it is clearly observed that silver wire with a radius of ∼35 nm is sealed inside the glass sheath uniformly and continuously. The surface morphology of the polished SAgNEs was checked by field emission scanning electron microscopy (FE-SEM; see Figure 1B), which showed that an exposed Ag with a radius of ∼30 nm was sealed well inside the glass sheath, and the geometrical shape was a kind of hemisphere, not a regular disk shape, which was different from Pt or Au nanoelectrodes.14,17 The reason may be due to the difference of glass capillaries used for Pt/Au nanoelectrodes (quartz)14,17 and this work (lime glass); the latter capillary is much softer than quartz. From the energy dispersive X-ray spectroscopy (EDS) result (shown in Figure S1), it could be determined that SAgNEs were fabricated successfully. The fabricated SAgNEs were further characterized by the electrochemical method, and the details were provided in the Supporting Information. From the well-defined cyclic voltammograms (CVs) of SAgNEs with different radii (Figure S2) and different scan rates (Figure S3) in Ru(NH3)63+ solution, it could be obtained that the SAgNEs were fabricated very well, and no obvious leakage or damage existed. The radii of SAgNEs could be estimated according to the diffusionlimited steady-state equation21 and also could be calculated from COMSOL simulation (Figures S4 and S5).15 The CVs and simulation results showed that the radii of SAgNEs were down to 10 nm, which was much smaller than previous
NFAC*k0 exp(1 − α)F(E − E 0′)/RT
i= 1+
0
ak D
[exp(− αF(E − E 0′)/RT ) + exp((1 − α)F(E − E 0′)/RT )]
(2)
where A is the surface area of the electrode, E ′ is the formal potential of the electrode, α is the electron-transfer coefficient, T is the absolute temperature, n is the number of electrons transmitted, F is the Faraday constant, and R is the ideal gas constant. The values of k0 and α can be obtained by measuring the parameters (E1/4 − E1/2) and (E1/2 − E3/4) shown in Figure 2A according to the method developed by Mirkin and Bard.22 Herein, E1/4, E1/2, and E3/4 are the electrode potentials where the current is equal to 1/4, 1/2, and 3/4 of the limiting value, respectively. The values of a and k0 are obtained (shown in Table 1) in Ru (NH3)63+ solution using SAgNEs with the radius of 11, 42, 60, 91, and 234, respectively. The average 0
Figure 2. (A) Voltammetric responses of single Ag nanoelectrodes with different radii in a 5 mM Ru(NH3)6Cl3 solution containing 0.2 M KNO3. (B) Normalized voltammograms shown in (A) through dividing the corresponding limiting current. Scan rate, 10 mV/s. B
DOI: 10.1021/acs.analchem.8b02644 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry Ag 0 → Ag + + 1e−
Table 1. Kinetic Parameters for the Oxidation of Ru(NH3)63+ at Ag Nanoelectrodes in H2O/0.2M KNO3 radii (nm)
ΔE1/4 (mV)
ΔE3/4 (mV)
k0 (cm/s)
α
234 91 60 42 10
31.2 29.3 31.1 34.0 39.0
41.0 43.9 32.5 43.0 51.3
1.4 1.6 4.8 4.6 5.9
0.13 0.33 0.35 0.41 0.46
(3)
From Figure 3A, it can be observed that Ep shifts negatively with the decrease of SAgNE size, which had been predicted theoretically by Henglein, Brus and co-workers, and Plieth23−25 ΔEp = −
2γVAg 1 zF r
(4)
where ΔEp is the difference in equilibrium oxidation potential between the SAgNE (Ep, SAgNE) and the bulk silver (Ep, bulk), γ is the surface tension, VAg is the molar volume (shown in the following section), z is transfer electron number (z = 1), F is Faraday constant, and r is the radius of SAgNE. From eq 4, it can be calculated that ∼24 mV shift will be expected for 14 nm SAgNE, but the experimental ΔEp (assuming 387 nm SAgNE as bulk silver) is ∼30 mV. A similar phenomena can be observed for 38, 115, and 190 nm SAgNEs, which was also observed by Ivanova and Zamborini,4 and the reason may be due to the difference between the theoretical value of γ and actual value.25 For comparison, the LSV collected from a 12.5 μm Ag electrode was also provided (shown in Figure S8), and its peak potential is close to 387 nm SAgNE. From Figure 3A, it can be observed that ASVs have a peak shape, not a traditional diffusion wave from micro/nanoelectrodes, which may be due to the limited amount of Ag+ ions released from electrode oxidation and the steady-state diffusion layer not
value obtained in this test is k0 = 3.4 ± 2.5 cm/s and a = 0.33 ± 0.2, which is lower than the results using single Au nanoelectrodes14 or single Pt nanoelectrodes.16,17 Because the reduction kinetics of Ru(NH3)63+ relies heavily on supporting the electrolyte, the chloride concentration is also an important effect.1 We think the lower value of k0 is due to the use of KNO3 as supporting electrolyte instead of KCl as in previous studies.14,16,17 In order to get the thermodynamics property for silver oxidation, SAgNEs with different radii were used to investigate the anodic stripping voltammetry (ASV). Figure 3A shows the linear stripping voltammograms (LSVs) in the range of 0.2 to 0.6 V for SAgNEs oxidation with the radii of 14, 38, 115, 190, and 387 nm with corresponding peak potential (Ep) values of 0.418, 0.423, 0.439, 0.446, and 0.448 V (vs Hg/HgO reference electrode), respectively. The anodic current corresponds to one electron oxidation reaction
Figure 3. (A) Linear sweep voltammograms (LSVs) obtained in 0.1 M PBS at the scan rate of 50 mV/s using SAgNEs with the radius of 14 nm (black line), 38 nm (red line), 115 nm (green line), 190 nm (blue line), and 387 nm (purple line), respectively. (B) LSVs using 45 nm SAgNE in 0.1 M PBS at the scan rate of 10 mV/s (black line), 50 mV/s (red line), 100 mV/s (green line), and 200 mV/s (blue line), respectively. (C) ASVs of single Ag nanoelectrode in a 0.1 M PBS solution containing 0.01 M KCl (black), 0.05 M KCl (red), 0.1 M KCl (green), and 0.2 M KCl (blue), respectively. Radius, ∼123 nm; scan rate, 10 mV/s; pH, 7.0. C
DOI: 10.1021/acs.analchem.8b02644 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry
small anodic peak at ∼0.44 V vs Hg/HgO, but a sharp and relatively large anodic peak at ∼0.16 V appears when the supporting electrolyte is 0.1 M KCl solution. Because the test conditions are the same except the supporting electrolyte, it can be determined that the difference of voltammograms should arise from the influence of anions. In KNO3 solution, SAgNE was oxidized to Ag+ and then diffused in aqueous solution. However, the products of oxidized Ag in KCl solution likely form AgCl (s) or other chloride complexes such as AgCl2−, which is favorable for the oxidation of silver and makes the peak potential shift negatively and peak current increase. To further investigate the effect of anions on the anodic voltammograms of SAgNEs, KCl solution with different concentrations was added into the PBS solution and the voltammograms were recorded in Figure 3C. Four stripping voltammograms of SAgNEs with different colors represents a PBS solution with an addition of 0.01 M KCl (black curve), 0.05 M KCl (red curve), 0.1 M KCl (green curve), and 0.2 M KCl (blue curve), respectively. From Figure 3C, it can be observed that, with the increase of KCl concentration, the peak potential shifts negatively gradually with the increase of peak current, indicating that the concentration of Cl− is one of the important parameters for the oxidation of SAgNEs. When SAgNEs were oxidized in a KCl solution, the following equation should take place
being easily formed during Ag+ ions’ diffusion from the electrode surface to bulk solution.26 It must be pointed out that this is the first direct evidence to prove this potential shift at single particle level, which excluded the effect of coverage and interaction of silver nanoparticles ensemble immobilized on substrate developed by Compton and co-workers and Ivanova and Zamborini.1,4 The amount of charge transferred (Qexp) during oxidation of SAgNEs can be obtained from the voltammograms shown in Figure 3A, and the result is listed in Table S1. From the data in Table S1, we also determined that the Ep was directly proportional to the logarithm of Qexp (Figure 3A, inset), which was developed by Brainina27 and demonstrated by Compton and co-workers and Ivanova and Zamborini experimentally.1,4 On the other hand, the amount of charge transferred from SAgNEs with different radii can be calculated, assuming the SAgNEs have a hemispherical shape. The actual volume of the SAgNEs (VAg) can be calculated using the following equation VAg =
2πr 3 ×f 3
(5)
where r is the radius of SAgNE and f is a correction factor, which is chosen as 0.5 in this work considering that the SAgNE is not a perfect hemisphere.1 On the basis of Faraday’s First Law, the total charge transferred during the entire oxidation of SAgNE can be calculated according to the following equation Q cal =
ρzF VAg Ar
Ag(s) + Cl−(aq) − e− = AgCl(s)
(7)
On the basis of the Nernst equation, eq 8, a linear relationship between peak potential of silver oxidation and the logarithm of Cl− concentration can be obtained, assuming the test is at room temperature and 1 atm, and the slope should be 59 mV assuming the oxidation involving one Cl− ion per Ag atom.
(6)
where ρ is the density of silver and Ar is the atomic mass of silver. The value of Qcal is also listed in Table S1. Compared the values of Qexp and Qcal obtained from Figure 3A and calculated from eq 6, we can observe that Qexp is larger than Qcal when the size of SAgNE is small (14 and 38 nm), indicating that Ag may be overoxidized or a small gap existed between the silver wire and glass, though no obvious peak shape appeared in the high speed voltammetric scan in Figure S3. However, when the electrode size is relatively large (radii 115 nm, 190 nm), the Qexp is smaller than Qcal, indicating the incomplete oxidation of SAgNEs, which is in agreement with the oxidation of silver nanoparticles ensemble.4 We also compared the limiting current Ru(NH3)63+ collected from SAgNEs before and after ASV oxidation (data not shown), and the ratio is ∼1.64, which is closed to the theoretical prediction (ihemisphere/idisk = 2π/4 = 1.57).21 The higher ratio value may be due to the overoxidized SAgNEs (recessed electrode formed). Therefore, the hemispherical-shaped electrode may be suitable for this discussion. The effect of scan rate on the peak potential of SAgNEs oxidation was also investigated, and the result was provided in Figure 3B and Table S2. It can also be observed that Ep shifts positive with increasing scan rate (from 10 to 200 mV/s), and the Ep is linearly proportional to the logarithm of scan rate (see Figure 3B, inset), which is in agreement with Brainina’s prediction27 and Ivanova and Zamborini’s experimental results.4 The effect of two kinds of supporting electrolytes, KNO3 and KCl, on the ASV of SAgNEs has been investigated because AgCl (s) or AgCl2− may be formed during this oxidation process using KCl solution as supporting electrolyte.28 Figure S6 gives the ASVs of SAgNE in a 0.1 M KNO3 solution (curve a) and 0.1 M KCl solution (curve b), respectively. In a 0.1 M KNO3 solution, the stripping voltammogram shows a relatively
E = Eθ −
0.059 log[Cl−] 1
(8)
From the peak potentials vs the concentration of Cl− shown in Figure 3C, the linear relationship between peak potential of silver oxidation and the logarithm of Cl− concentration can be plotted (see Figure 3C, inset), and a slope of 69.4 mV can be obtained, which is larger than the theoretical value (59 mV). This result may be due to the participation of more than one Cl− ion per Ag atom during the oxidation process, and AgCl2− or other higher halide complexes may be formed.1 The LSVs of SAgNEs with different radii in a 0.1 M KCl solution were also recorded (Figure S7), and the result showed that the peak potential had a similar shift compared to the result in the solution without KCl addition (Figure 3A). Overall, from the potential negative shift with the decrease of SAgNEs’ radii (shown in Figures 3 and S6), it can be determined that the small size of SAgNEs is thermodynamically favorable for silver electro-oxidation.
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CONCLUSIONS In conclusion, we prepared SAgNEs with radii down to 10 nm by improving the laser-assisted pulling technique. The prepared SAgNEs were used to investigate the thermodynamics and kinetics of electro-oxidation of the silver nanostructure. The results showed that fast electron-transfer kinetics took place with the radii decrease of SAgNEs through measuring k0. Negative potential shift was observed through recording the ASV of SAgNE oxidation with the decrease of electrode radii. The results also demonstrated that Cl− was D
DOI: 10.1021/acs.analchem.8b02644 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry
(17) Li, Y.; Bergman, D.; Zhang, B. Anal. Chem. 2009, 81, 5496− 5502. (18) Marquitan, M.; Clausmeyer, J.; Actis, P.; Cordoba, A. L.; Korchev, Y.; Mark, M. D.; Herlitze, S.; Schuhmann, W. ChemElectroChem 2016, 3, 2125−2129. (19) Katemann, B. B.; Schuhmann, W. Electroanalysis 2002, 14, 22− 28. (20) Noel, J.-M.; Velmurugan, J.; Gokmese, E.; Mirkin, M. V. J. Solid State Electrochem. 2013, 17, 385−389. (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 2001. (22) Mirkin, M. V.; Bard, A. J. Anal. Chem. 1992, 64, 2293−2302. (23) Henglein, A. J. Phys. Chem. 1993, 97, 5457−5471. (24) Redmond, P. L.; Hallock, A. J.; Brus, L. E. Nano Lett. 2005, 5, 131−135. (25) Plieth, W. J. J. Phys. Chem. 1982, 86, 3166−3170. (26) Li, Y.; Cox, J. T.; Zhang, B. J. Am. Chem. Soc. 2010, 132, 3047− 3054. (27) Brainina, K. Z. Talanta 1971, 18, 513−539. (28) Toh, H. S.; Batchelor-McAuley, C.; Tschulik, K.; Compton, R. G. Analyst 2013, 138, 4292−4297.
thermodynamically favorable for SAgNE oxidation. This is the first attempt to investigate the kinetics and thermodynamics of the electro-oxidation of metal nanostructure at single nanoparticle/nanoelectrode level, and we think this work may be helpful for deep understanding of the electro-oxidation of metal nanostructures.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b02644.
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Detailed experimental section, single Ag nanoelectrodes fabrication, tables, and supporting results (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 86-553-386-9302. Fax: 86-553-386-9303. ORCID
Yongxin Li: 0000-0001-5543-4242 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. Bo Zhang (University of Washington) for his useful suggestions and discussion. This work is financially supported by the National Natural Science Foundation of China (No.21775003, No.21375002), and the Foundation for Innovation Team of Bioanalytical Chemistry of Anhui Province.
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REFERENCES
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DOI: 10.1021/acs.analchem.8b02644 Anal. Chem. XXXX, XXX, XXX−XXX
