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Underpotential deposition of lead on quasi-spherical and faceted gold nanoparticles
Journal of Electroanalytical Chemistry xxx (xxxx) xxx–xxx
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Underpotential deposition of lead on quasi-spherical and faceted gold nanoparticles C. Jeyabharathia,⁎, M. Zanderb, F. Scholza a b
Institute of Biochemistry, Felix-Hausdorff-Str. 4, University of Greifswald, 17487 Greifswald, Germany Institute of Geology and Geography, Friedrich-Ludwig-Jahn-Str. 17A, University of Greifswald, 17487 Greifswald, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Gold Nanoparticles Underpotential deposition Lead Crystallinity
The underpotential deposition of lead was studied on spherical and faceted gold nanoparticles. On 21.7 nm spherical nanoparticles, a single stripping peak from Au(111) face was observed at −0.2 V, whereas on 82 nm faceted gold nanoparticles the peak split in to two at −0.2 V and − 0.18 V vs Ag/AgCl (3 M KCl). The roughening of the Au(111) terraces by electrochemical treatment leads to the suppression of the spike. It appears that the extent of surface crystallinity of nanoparticles causes the observed differences in the upd-stripping behaviour. The splitting of the Pb-upd stripping peak from Au(111) face can be used as an indication for the surface crystallinity of gold nanoparticles.
1. Introduction Underpotential deposition has been used to probe the surface structure as it is sensitive to the steps and terraces of stepped single crystal surfaces [1–4]. Underpotential deposition of lead (Pb-upd) or copper (Cu-upd) on Au, Ag, Pt etc. [5] has been studied in detail as it provides unique finger-prints of the surfaces. Pb-upd has been successfully applied to decipher the surface structure of gold nanostructures [6]. The structural information thus obtained from Pb-upd has been used to understand electrode processes such as oxygen reduction [6], methanol oxidation [7,8], glucose oxidation [9], etc., which are surface sensitive reactions [10–12]. Still, the wealth of information available in the single crystal gold research has not been fully applied to complex systems like bulk gold and gold nanoparticle surfaces [13] and thus more efforts are necessary. We performed Pb-upd on spherical gold nanoparticles and faceted gold nanoparticles. The results revealed that on spherical particles, a Pb-upd stripping peak was observed at −0.2 V related to the Au(111) face, whereas a “doublet peak” [2,6] (a peak at − 0.2 V vs. Ag/AgCl (3 M KCl) and a spike at − 0.18 V vs. Ag/AgCl (3 M KCl)) like on an Au(111) single-crystal electrode was observed on facetted Au nanoparticles. Schultze et al. [1] reported that the doublet peak results from slow kinetics of lead stripping. Based on STM studies, Tao et al. [14] showed that island formation and coalescence lead to the splitting of the adsorption peaks into two, but an explanation for the splitting of stripping peak was not provided. In the light of our result it appears that with smaller terraces with more steps and defects, a doublet peak was not observed even at 0.1 V s− 1 scan rate, but it was
⁎
observed on a faceted particle at 0.02 V s− 1. Also, the electrochemical cleaning by deposition/dissolution of PbO2 film [8] resulted in the suppression of the spike at 0.02 V s− 1. Possibly, the well oriented nanocrystals have well-grown terraces of Au(111) that resulted in a doublet peak similar to the extended Au(111) terraces and the cleaning potential cycles could eventually introduce such a degree of roughness on the terraces, that the spike at −0.18 V vs. Ag/AgCl (3 M KCl) is suppressed. 2. Experimental Perchloric acid (Merck, 70%, 99.999% purity relating to trace metals), Sodium perchlorate (Alfa-Aesar, 98.0–102.0%), lead perchlorate (Sigma-Aldrich, ≥ 99.995%), potassium nitrate (Merck, Emsure® for analysis), but-2-yne-1,4-diol (Sigma-Aldrich) were used as obtained. All the solutions were prepared using Millipore water (18.2 MΩ cm) and the temperature was maintained at 25 °C. Gold nanoparticles (AuNPs) were synthesized using but-2-yne-1,4diol (BD) solutions as a reducing agent as reported earlier [15]. To get spherical nanoparticles, the solution composition was: 0.25 mM HAuCl4 + 2.5 mM BD (1:10 ratio). For faceted nanoparticles, we used 0.25 mM HAuCl4 + 0.175 mM BD (1:0.7 ratio). HAuCl4 solutions, standardised by ICP-MS, were prepared by dissolution of gold in aqua regia [16]. The AuNPs were characterized by transmission electron microscopy (TEM) using 120 kV, JEM 1210 microscope. A three-electrode cell configuration connected to Autolab potentiostat (from Metrohm) was used for electrochemical experiments in
Corresponding author. E-mail address: [email protected] (C. Jeyabharathi).
http://dx.doi.org/10.1016/j.jelechem.2017.10.011 Received 17 May 2017; Received in revised form 1 September 2017; Accepted 3 October 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Jeyabharathi, C., Journal of Electroanalytical Chemistry (2017), http://dx.doi.org/10.1016/j.jelechem.2017.10.011
Journal of Electroanalytical Chemistry xxx (xxxx) xxx–xxx
C. Jeyabharathi et al.
quiescent solution. A glassy carbon (GC) electrode (Ø = 3 mm) was used as working electrode. The electrode was mechanically polished using 1 μm, 0.3 μm and 0.05 μm Al2O3 slurries on a polishing pad (from Buhler) successively, cleaned well by sonication in Millipore water for 2 min and finally rinsed with Millipore water. The AuNPs were deposited on the GC electrode by drop casting and drying. The spherical and faceted gold nanoparticles deposited on GC electrode are denoted as sp-AuNPs/GC and ft-AuNPs/GC, respectively. A GC rod (from Metrohm) was used as a counter electrode, and an Ag/AgCl (3 M KCl) electrode (from Metrohm) as a reference (E0 = 0.210 V vs SHE). The reference electrode was separated from the electrolyte by KNO3-agar salt bridges (polyethylene tubing filled with the salt-gel). The Pb-upd experiment was performed in deaerated solution of 0.1 M NaClO4 + 0.01 M HClO4 mixture containing 1 mM Pb(ClO4)2 solutions. The potential was fixed at − 0.42 V for 60 s to obtain the underpotential deposit and the deposit was stripped by employing anodic potential sweeps at a scan rate of 0.02 V s− 1. In cyclic experiments, a scan rate of 0.02 V s− 1 was used for cathodic deposition and anodic dissolution of upd-Pb. Unless otherwise mentioned, the electrolytes were deaerated by bubbling N2 for 20 min before the measurements and a N2 blanket was maintained throughout the experiment.
3. Results and discussions The Fig. 1A & B show the representative TEM images of the AuNPs synthesized by reducing HAuCl4 with but-2-yne-1,4-diol [15]. The 1:10 ratio of reactants produced almost quasi-spherical (less faceted) nanoparticles (Fig. 1A), while 1:07 ratio produced well-faceted nanoparticles (Fig. 1B). The mean particle size/mean edge length of quasi-spherical nanoparticles was 21.7 ± 3.5 nm/10.3 ± 0.7 nm and of larger faceted nanoparticles was 82 ± 27 nm/10.3 ± 0.7 nm. The morphology of the faceted nanoparticles was not very regular. The surface characteristics of these particles deposited on a GC electrode were assessed by Pb-upd. Fig. 2A & B show the upd-Pb deposition and a dissolution curves recorded on sp-AuNPs/GC and ftAuNPs/GC (as-deposited and cleaned) at a scan rate of 0.02 V s− 1. The cleaning was performed as proposed by Hernandez et al. [8] with PbO2 film deposition and dissolution during potential cycling between 0.5 V to 1.6 V vs Ag/AgCl (3 M KCl) at a scan rate of 0.1 V s− 1 for 10 cycles in acidic solutions. The cycles for the PbO2 formation and dissolution comprised also the potentials for gold oxidation and gold oxide reduction (CV is not shown). On as-deposited sp-AuNPs/GC, the Pb-upd curve showed two stripping peaks, 1a and 1a′ at −0.35 V and 0.017 V corresponding to terraces (< 4-atom width) and steps of Au(110), respectively [2], and a
Fig. 2. Pb-upd curves recorded on as-deposited (black curve) and cleaned (red curve) gold nanoparticles at a scan rate of 0.02 V s− 1: (A) sp-AuNPs/GC and (B) ft-AuNPs/GC. The arrow indicates the disappearance of the spike at −0.18 V. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
peak 2a at − 0.2 V that is related to the Au(111) face. However, on asdeposited ft-AuNPs/GC, we noticed a spike 2a′ at − 0.18 V and two other tiny peaks at − 0.28 V and − 0.12 V in addition to those observed on spherical particles. The latter two peaks (3a and 3a′) were ascribed
Fig. 1. TEM micrograph of AuNPs: (A) spherical and (B) faceted nanoparticles. Scale bar = 20 nm.
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to the Au(100) face [6]. The doublet peak (both 2a and 2a′) look like those observed on a single crystalline Au(111) face [2] and on gold nanorods [6]. A cleaned sp-AuNPs/GC electrode showed similar voltammetric features with a slight decrease of peak currents in comparison to the as-deposited electrode. This could be attributed to the decrease in electrochemically active area due to the dissolution of gold during the cyclic potential treatment [17]. The full-width half-maximum (FWHM) value of the Au(111) peak on as-deposited sp-AuNPs/ GC and cleaned sp-AuNPs/GC electrodes was 0.032 V to 0.025 V, respectively. However, on the ft-AuNPs/GC electrode, the spike (2a′) at − 0.18 V of the Au(111) face disappeared after cleaning cycles, i.e., the doublet peak with FWHM of 0.054 V was changed to a single peak with an edge at −0.2 V with FWHM of 0.021 V. The FWHM values of the peak (excluding the spike) have been reported between 0.012 V to 0.038 V on a single crystal gold electrode [18]. Schultze et al. [1] showed the evolution of the spike in the stripping of the upd-Pb on an Au(111) single crystal face. They used high scan rates up to 10 V s− 1, where the peaks strongly shifted due to ohmic drop, and where the peak at − 0.2 V was more diminished than the more positive peak (our spike). We studied the effect of scan rate at such values, where no ohmic distortion occurs. The results are shown in Fig. 3A & B. On as-deposited sp-AuNPs/GC (Fig. 3A) the spike 2a′ at − 0.18 V was not observed and the FWHM value increased from 0.032 at 0.02 V s− 1 to 0.042 V at 0.1 V s− 1. On the ft-AuNPs/GC electrode (Fig. 3B & C), we observe a pronounced scan rate dependence of the spike, indicating a slower kinetics of the Pb stripping as observed by Schultze et al. [1] This shows that the quality of the Au(111) face is very similar to the single crystal Au(111) face. Though the intensity of the peak 2a′ is increased upon increasing scan rate, however the intensity of the peak 2a is not changed much, which should be independent on the scan rate as expected on Au(111) terraces [1]. For further elucidation of the Pb-upd behaviour, we performed experiments with a variation of switching potentials. Fig. 4A & B show the Pb-upd curves recorded by switching either the anodic or cathodic upper potential limits. The redox couples 1a/1c, 1a′/1c′ and 2a/2c are reversible. The spike 2a′ is associated with the adsorption peak at 2c′ and is also reversible process as revealed from cyan curve by switching the anodic upper potential limit at − 0.1 V (Fig. 4A). However, the cathodic wave denoted as 2c′ + 4c is due to a complex adsorption process at which both 2c′ and an irreversible adsorption of lead (4c) could take place. When we limit the cathodic potentials between − 0.23 V and −0.16 V (Fig. 4B), we observe the peak 4a with decreasing current intensity, but it disappears when we limit the potential at 0.13 V (blue curve, Fig. 4B). Hence, the wave 2c′ + 4c is not entirely related to the peak 4a (stripping of irreversibly adsorbed lead [19]) and it could be a combined adsorption of Pb which leads to the stripping peaks at 2a′ and 4a. The mechanism of the adsorption and stripping of Pb-upd on the Au (111) face was studied by Green et al. [20], who showed that the step decoration with Pb and Pb-island formation on terraces take place at positive potentials (at 2c′) before the Pb-islands coalesce to form a full monolayer at more negative potentials. Tao et al. [14] also reported similar observation in that the surface coverage vs. potential plot showed a plateau at 0.25 monolayer. This suggests that the Pb-islands are stable before they coalesce at more negative potentials. Possibly, the Pb-islands are stabilized by the formation of oxides and hydroxides on top of the islands as shown by Gewirth et al. [19]. We expect similar adsorption processes on the ft-AuNPs as we observe the typical voltammetric profiles. The spike 2a′ on ft-AuNPs/GC resembles the characteristic spike on the Au(111) single crystal face. This suggests that well-defined Au(111) terraces are formed on the faceted nanoparticles as a result of a favoured growth due to its lowest surface energy among the low-index faces [21]. The sp-AuNPs/GC do not exhibit the spike, which suggests that the surface is less-crystalline because of its smaller size than that of the ft-AuNPs. We do not observe the spike on the spAuNPs even at 0.1 V s− 1, although a slight increase in FWHM from
Fig. 3. Pb-upd stripping curves on (A) sp-AuNPs/GC, (B) ft-AuNPs/GC at different scan rates and (C) ft-AuNPs/GC normalized by scan rate.
0.032 at 0.02 V s− 1 to 0.042 at 0.1 V s− 1 is noticed. This may indicate a lack of extended Au(111) terraces. Hamelin [2] showed that as the width of Au(111) terrace decreased in the direction of (111)–(110) zone, the spike disappeared. Hence, the larger terrace width in the ftAuNPs could cause the appearance of the spike and the higher FWHM of the doublet peak. However, smaller width of the terraces in sp-AuNPs resulted in a single Pb stripping peak with smaller FMHM. It is known that co-adsorption of strongly adsorbing anions with adatoms at positive potentials and the replacement of anions by adatoms at more 3
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10 cycles could causes the roughening [25]. Köntje et al. [26] recently showed the surface patterning on Au(111) single crystal electrode with islands and holes of one atomic step in height as a result of 1–2 initial repetitive oxide formation and reduction cycles and larger holes with 10 cycles. As our cleaning treatment also involved a similar oxide formation and reduction (up to 10 cycles), we expect a similar roughening/patterning process leading to the defective Au(111) terraces. This could have in turn caused the decrease of current intensity of the spike. These results suggest that the splitting of the Pb-upd stripping peak from the Au(111) face can be used as an indication for the surface crystallinity of polyoriented gold nanoparticles. 4. Conclusions Quasi-spherical and faceted gold nanoparticles showed distinctive Pb-upd behaviour of Au(111) face: (a) sp-AuNPs exhibits only one stripping peak; (b) ft-AuNPs exhibits a splitting of the peak in to two: the main stripping peak, 2a is from Au(111) terraces on both sp- and ftAuNPs; the spike 2a′ seems to be formed only on well-grown Au(111) terraces. The occurrence of the spike, 2a′ at a slightly more positive potential on ft-AuNPs/GC is a result of kinetic effects and is affected by the size of the Au(111) terraces. Possible roughening of the Au(111) terrace during upd processes and during cleaning cycles that lead to the drop in the spike current also indicates the dependence of the spike formation on the larger Au(111) terraces. The splitting of the Pb-upd stripping peak from Au(111) faces can be used as an indication for the surface crystallinity of polyfaced gold nanoparticles. References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 4. Pb-upd curves recorded on ft-AuNPs/GC at a scan rate of 0.05 V s− 1: (A) at different anodic switching potentials and (B) at different cathodic switching potentials.
[9] [10] [11]
negative potential to form a sub-monolayer of adatoms leads to the distinct stripping peaks due to phase transitions. For example, Cu upd on Au(111) sulfate ions co-adsorb with Cu that resulted in two peaks, whereas only one peak was obtained in perchloric acid [22]. In our case, we have weakly adsorbing perchlorate ions. Seo et al. [23] showed that the desorption of perchlorate ions precedes the adsorption of Pb. So, we do not think that our spike is a result of such phase transitions. Instead, as discussed earlier, we attribute it to the size of the terraces on which island formation and coalescence takes place. As the reported value of average size of Pb-islands on Au(111) face was ca. 4–15 nm [14,19], we could expect that the Pb island formation and coalescence is more feasible on ft-AuNPs with 28.9 nm edge length than that of the sp-AuNPs with 10.3 nm edge length. Scan rate effect on ftAuNPs showed that the observation of the spike is due to a kinetic effect as observed on a single-crystal Au(111) face. Interestingly, the spike current is decreased after the cleaning cycles. Alloying and dealloying of Pb with Au during the Pb adsorption and stripping processes could lead to the roughening of the Au terraces [24]. Also, repetitive cycling in the gold oxidation and reduction window in acidic solution up to
[12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23] [24] [25] [26]
4
J.W. Schultze, D. Dickertmann, Surf. Sci. 54 (1976) 489. A. Hamelin, J. Electroanal. Chem. 165 (1984) 167. A. Hamelin, J. Lipkowski, J. Electroanal. Chem. 171 (1984) 317. R. Parsons, G. Ritzoulis, J. Electroanal. Chem. 318 (1991) 1. E. Herrero, L.J. Buller, H.D. Abruña, Chem. Rev. 101 (2001) 1897. J. Hernández, J. Solla-Gullón, E. Herrero, A. Aldaz, J.M. Feliu, J. Phys. Chem. B 109 (2005) 12651. Y. Chen, S. Milenkovic, A.W. Hassel, ChemPhysChem 11 (2010) 2838. J. Hernández, J. Solla-Gullón, E. Herrero, A. Aldaz, J.M. Feliu, Electrochim. Acta 52 (2006) 1662. S. Hebié, K.B. Kokoh, K. Servat, T.W. Napporn, Gold Bull. 46 (2013) 311. J.M. Feliu, E. Herrero, Contributions to Science, 6 (2010), p. 161. B. Maestro, J.M. Ortiz, G. Schrott, J.P. Busalmen, V. Climent, J.M. Feliu, Bioelectrochemistry 98 (2014) 11. A. Kuzume, E. Herrero, J.M. Feliu, E. Ahlberg, R.J. Nichols, D.J. Schiffrin, Phys. Chem. Chem. Phys. 7 (2005) 1293. C. Jeyabharathi, P. Ahrens, U. Hasse, F. Scholz, J. Solid State Electrochem. 20 (2016) 3025. N.J. Tao, J. Pan, Y. Li, P.I. Oden, J.A. DeRose, S.M. Lindsay, Surf. Sci. 271 (1992) L338. C. Jeyabharathi, P. Esakki Karthik, K.L.N. Phani, RSC Adv. 4 (2014) 7780. C. Jeyabharathi, U. Hasse, P. Ahrens, F. Scholz, J. Solid State Electrochem. 18 (2014) 3299. S. Cherevko, A.A. Topalov, A.R. Zeradjanin, I. Katsounaros, K.J.J. Mayrhofer, RSC Adv. 3 (2013) 16516. O.A. Oviedo, L. Reinaudi, S.G. García, E.P.M. Leiva, F. Scholz (Ed.), Underpotential Deposition: From Fundamentals and Theory to Applications at the Nanoscale, Springer International Publishing, Cham, 2016, p. 91. C.H. Chen, N. Washburn, A.A. Gewirth, J. Phys. Chem. 97 (1993) 9754. M.P. Green, K.J. Hanson, R. Carr, I. Lindau, J. Electrochem. Soc. 137 (1990) 3493. D. Seo, C.I. Yoo, I.S. Chung, S.M. Park, S. Ryu, H. Song, J. Phys. Chem. C 112 (2008) 2469. L. Blum, D.A. Huckaby, M. Legault, Electrochim. Acta 41 (1996) 2207. M. Seo, M. Yamazaki, J. Electrochem. Soc. 151 (2004) E276. M.P. Green, K.J. Hanson, Surf. Sci. Lett. 259 (1991) L743. T. Kondo, J. Zegenhagen, S. Takakusagi, K. Uosaki, Surf. Sci. 631 (2015) 96. C. Köntje, D.M. Kolb, G. Jerkiewicz, Langmuir 29 (2013) 10272.
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Underpotential deposition of lead on quasi-spherical and faceted gold nanoparticles C. Jeyabharathia,⁎, M. Zanderb, F. Scholza a b
Institute of Biochemistry, Felix-Hausdorff-Str. 4, University of Greifswald, 17487 Greifswald, Germany Institute of Geology and Geography, Friedrich-Ludwig-Jahn-Str. 17A, University of Greifswald, 17487 Greifswald, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Gold Nanoparticles Underpotential deposition Lead Crystallinity
The underpotential deposition of lead was studied on spherical and faceted gold nanoparticles. On 21.7 nm spherical nanoparticles, a single stripping peak from Au(111) face was observed at −0.2 V, whereas on 82 nm faceted gold nanoparticles the peak split in to two at −0.2 V and − 0.18 V vs Ag/AgCl (3 M KCl). The roughening of the Au(111) terraces by electrochemical treatment leads to the suppression of the spike. It appears that the extent of surface crystallinity of nanoparticles causes the observed differences in the upd-stripping behaviour. The splitting of the Pb-upd stripping peak from Au(111) face can be used as an indication for the surface crystallinity of gold nanoparticles.
1. Introduction Underpotential deposition has been used to probe the surface structure as it is sensitive to the steps and terraces of stepped single crystal surfaces [1–4]. Underpotential deposition of lead (Pb-upd) or copper (Cu-upd) on Au, Ag, Pt etc. [5] has been studied in detail as it provides unique finger-prints of the surfaces. Pb-upd has been successfully applied to decipher the surface structure of gold nanostructures [6]. The structural information thus obtained from Pb-upd has been used to understand electrode processes such as oxygen reduction [6], methanol oxidation [7,8], glucose oxidation [9], etc., which are surface sensitive reactions [10–12]. Still, the wealth of information available in the single crystal gold research has not been fully applied to complex systems like bulk gold and gold nanoparticle surfaces [13] and thus more efforts are necessary. We performed Pb-upd on spherical gold nanoparticles and faceted gold nanoparticles. The results revealed that on spherical particles, a Pb-upd stripping peak was observed at −0.2 V related to the Au(111) face, whereas a “doublet peak” [2,6] (a peak at − 0.2 V vs. Ag/AgCl (3 M KCl) and a spike at − 0.18 V vs. Ag/AgCl (3 M KCl)) like on an Au(111) single-crystal electrode was observed on facetted Au nanoparticles. Schultze et al. [1] reported that the doublet peak results from slow kinetics of lead stripping. Based on STM studies, Tao et al. [14] showed that island formation and coalescence lead to the splitting of the adsorption peaks into two, but an explanation for the splitting of stripping peak was not provided. In the light of our result it appears that with smaller terraces with more steps and defects, a doublet peak was not observed even at 0.1 V s− 1 scan rate, but it was
⁎
observed on a faceted particle at 0.02 V s− 1. Also, the electrochemical cleaning by deposition/dissolution of PbO2 film [8] resulted in the suppression of the spike at 0.02 V s− 1. Possibly, the well oriented nanocrystals have well-grown terraces of Au(111) that resulted in a doublet peak similar to the extended Au(111) terraces and the cleaning potential cycles could eventually introduce such a degree of roughness on the terraces, that the spike at −0.18 V vs. Ag/AgCl (3 M KCl) is suppressed. 2. Experimental Perchloric acid (Merck, 70%, 99.999% purity relating to trace metals), Sodium perchlorate (Alfa-Aesar, 98.0–102.0%), lead perchlorate (Sigma-Aldrich, ≥ 99.995%), potassium nitrate (Merck, Emsure® for analysis), but-2-yne-1,4-diol (Sigma-Aldrich) were used as obtained. All the solutions were prepared using Millipore water (18.2 MΩ cm) and the temperature was maintained at 25 °C. Gold nanoparticles (AuNPs) were synthesized using but-2-yne-1,4diol (BD) solutions as a reducing agent as reported earlier [15]. To get spherical nanoparticles, the solution composition was: 0.25 mM HAuCl4 + 2.5 mM BD (1:10 ratio). For faceted nanoparticles, we used 0.25 mM HAuCl4 + 0.175 mM BD (1:0.7 ratio). HAuCl4 solutions, standardised by ICP-MS, were prepared by dissolution of gold in aqua regia [16]. The AuNPs were characterized by transmission electron microscopy (TEM) using 120 kV, JEM 1210 microscope. A three-electrode cell configuration connected to Autolab potentiostat (from Metrohm) was used for electrochemical experiments in
Corresponding author. E-mail address: [email protected] (C. Jeyabharathi).
http://dx.doi.org/10.1016/j.jelechem.2017.10.011 Received 17 May 2017; Received in revised form 1 September 2017; Accepted 3 October 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Jeyabharathi, C., Journal of Electroanalytical Chemistry (2017), http://dx.doi.org/10.1016/j.jelechem.2017.10.011
Journal of Electroanalytical Chemistry xxx (xxxx) xxx–xxx
C. Jeyabharathi et al.
quiescent solution. A glassy carbon (GC) electrode (Ø = 3 mm) was used as working electrode. The electrode was mechanically polished using 1 μm, 0.3 μm and 0.05 μm Al2O3 slurries on a polishing pad (from Buhler) successively, cleaned well by sonication in Millipore water for 2 min and finally rinsed with Millipore water. The AuNPs were deposited on the GC electrode by drop casting and drying. The spherical and faceted gold nanoparticles deposited on GC electrode are denoted as sp-AuNPs/GC and ft-AuNPs/GC, respectively. A GC rod (from Metrohm) was used as a counter electrode, and an Ag/AgCl (3 M KCl) electrode (from Metrohm) as a reference (E0 = 0.210 V vs SHE). The reference electrode was separated from the electrolyte by KNO3-agar salt bridges (polyethylene tubing filled with the salt-gel). The Pb-upd experiment was performed in deaerated solution of 0.1 M NaClO4 + 0.01 M HClO4 mixture containing 1 mM Pb(ClO4)2 solutions. The potential was fixed at − 0.42 V for 60 s to obtain the underpotential deposit and the deposit was stripped by employing anodic potential sweeps at a scan rate of 0.02 V s− 1. In cyclic experiments, a scan rate of 0.02 V s− 1 was used for cathodic deposition and anodic dissolution of upd-Pb. Unless otherwise mentioned, the electrolytes were deaerated by bubbling N2 for 20 min before the measurements and a N2 blanket was maintained throughout the experiment.
3. Results and discussions The Fig. 1A & B show the representative TEM images of the AuNPs synthesized by reducing HAuCl4 with but-2-yne-1,4-diol [15]. The 1:10 ratio of reactants produced almost quasi-spherical (less faceted) nanoparticles (Fig. 1A), while 1:07 ratio produced well-faceted nanoparticles (Fig. 1B). The mean particle size/mean edge length of quasi-spherical nanoparticles was 21.7 ± 3.5 nm/10.3 ± 0.7 nm and of larger faceted nanoparticles was 82 ± 27 nm/10.3 ± 0.7 nm. The morphology of the faceted nanoparticles was not very regular. The surface characteristics of these particles deposited on a GC electrode were assessed by Pb-upd. Fig. 2A & B show the upd-Pb deposition and a dissolution curves recorded on sp-AuNPs/GC and ftAuNPs/GC (as-deposited and cleaned) at a scan rate of 0.02 V s− 1. The cleaning was performed as proposed by Hernandez et al. [8] with PbO2 film deposition and dissolution during potential cycling between 0.5 V to 1.6 V vs Ag/AgCl (3 M KCl) at a scan rate of 0.1 V s− 1 for 10 cycles in acidic solutions. The cycles for the PbO2 formation and dissolution comprised also the potentials for gold oxidation and gold oxide reduction (CV is not shown). On as-deposited sp-AuNPs/GC, the Pb-upd curve showed two stripping peaks, 1a and 1a′ at −0.35 V and 0.017 V corresponding to terraces (< 4-atom width) and steps of Au(110), respectively [2], and a
Fig. 2. Pb-upd curves recorded on as-deposited (black curve) and cleaned (red curve) gold nanoparticles at a scan rate of 0.02 V s− 1: (A) sp-AuNPs/GC and (B) ft-AuNPs/GC. The arrow indicates the disappearance of the spike at −0.18 V. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
peak 2a at − 0.2 V that is related to the Au(111) face. However, on asdeposited ft-AuNPs/GC, we noticed a spike 2a′ at − 0.18 V and two other tiny peaks at − 0.28 V and − 0.12 V in addition to those observed on spherical particles. The latter two peaks (3a and 3a′) were ascribed
Fig. 1. TEM micrograph of AuNPs: (A) spherical and (B) faceted nanoparticles. Scale bar = 20 nm.
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to the Au(100) face [6]. The doublet peak (both 2a and 2a′) look like those observed on a single crystalline Au(111) face [2] and on gold nanorods [6]. A cleaned sp-AuNPs/GC electrode showed similar voltammetric features with a slight decrease of peak currents in comparison to the as-deposited electrode. This could be attributed to the decrease in electrochemically active area due to the dissolution of gold during the cyclic potential treatment [17]. The full-width half-maximum (FWHM) value of the Au(111) peak on as-deposited sp-AuNPs/ GC and cleaned sp-AuNPs/GC electrodes was 0.032 V to 0.025 V, respectively. However, on the ft-AuNPs/GC electrode, the spike (2a′) at − 0.18 V of the Au(111) face disappeared after cleaning cycles, i.e., the doublet peak with FWHM of 0.054 V was changed to a single peak with an edge at −0.2 V with FWHM of 0.021 V. The FWHM values of the peak (excluding the spike) have been reported between 0.012 V to 0.038 V on a single crystal gold electrode [18]. Schultze et al. [1] showed the evolution of the spike in the stripping of the upd-Pb on an Au(111) single crystal face. They used high scan rates up to 10 V s− 1, where the peaks strongly shifted due to ohmic drop, and where the peak at − 0.2 V was more diminished than the more positive peak (our spike). We studied the effect of scan rate at such values, where no ohmic distortion occurs. The results are shown in Fig. 3A & B. On as-deposited sp-AuNPs/GC (Fig. 3A) the spike 2a′ at − 0.18 V was not observed and the FWHM value increased from 0.032 at 0.02 V s− 1 to 0.042 V at 0.1 V s− 1. On the ft-AuNPs/GC electrode (Fig. 3B & C), we observe a pronounced scan rate dependence of the spike, indicating a slower kinetics of the Pb stripping as observed by Schultze et al. [1] This shows that the quality of the Au(111) face is very similar to the single crystal Au(111) face. Though the intensity of the peak 2a′ is increased upon increasing scan rate, however the intensity of the peak 2a is not changed much, which should be independent on the scan rate as expected on Au(111) terraces [1]. For further elucidation of the Pb-upd behaviour, we performed experiments with a variation of switching potentials. Fig. 4A & B show the Pb-upd curves recorded by switching either the anodic or cathodic upper potential limits. The redox couples 1a/1c, 1a′/1c′ and 2a/2c are reversible. The spike 2a′ is associated with the adsorption peak at 2c′ and is also reversible process as revealed from cyan curve by switching the anodic upper potential limit at − 0.1 V (Fig. 4A). However, the cathodic wave denoted as 2c′ + 4c is due to a complex adsorption process at which both 2c′ and an irreversible adsorption of lead (4c) could take place. When we limit the cathodic potentials between − 0.23 V and −0.16 V (Fig. 4B), we observe the peak 4a with decreasing current intensity, but it disappears when we limit the potential at 0.13 V (blue curve, Fig. 4B). Hence, the wave 2c′ + 4c is not entirely related to the peak 4a (stripping of irreversibly adsorbed lead [19]) and it could be a combined adsorption of Pb which leads to the stripping peaks at 2a′ and 4a. The mechanism of the adsorption and stripping of Pb-upd on the Au (111) face was studied by Green et al. [20], who showed that the step decoration with Pb and Pb-island formation on terraces take place at positive potentials (at 2c′) before the Pb-islands coalesce to form a full monolayer at more negative potentials. Tao et al. [14] also reported similar observation in that the surface coverage vs. potential plot showed a plateau at 0.25 monolayer. This suggests that the Pb-islands are stable before they coalesce at more negative potentials. Possibly, the Pb-islands are stabilized by the formation of oxides and hydroxides on top of the islands as shown by Gewirth et al. [19]. We expect similar adsorption processes on the ft-AuNPs as we observe the typical voltammetric profiles. The spike 2a′ on ft-AuNPs/GC resembles the characteristic spike on the Au(111) single crystal face. This suggests that well-defined Au(111) terraces are formed on the faceted nanoparticles as a result of a favoured growth due to its lowest surface energy among the low-index faces [21]. The sp-AuNPs/GC do not exhibit the spike, which suggests that the surface is less-crystalline because of its smaller size than that of the ft-AuNPs. We do not observe the spike on the spAuNPs even at 0.1 V s− 1, although a slight increase in FWHM from
Fig. 3. Pb-upd stripping curves on (A) sp-AuNPs/GC, (B) ft-AuNPs/GC at different scan rates and (C) ft-AuNPs/GC normalized by scan rate.
0.032 at 0.02 V s− 1 to 0.042 at 0.1 V s− 1 is noticed. This may indicate a lack of extended Au(111) terraces. Hamelin [2] showed that as the width of Au(111) terrace decreased in the direction of (111)–(110) zone, the spike disappeared. Hence, the larger terrace width in the ftAuNPs could cause the appearance of the spike and the higher FWHM of the doublet peak. However, smaller width of the terraces in sp-AuNPs resulted in a single Pb stripping peak with smaller FMHM. It is known that co-adsorption of strongly adsorbing anions with adatoms at positive potentials and the replacement of anions by adatoms at more 3
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10 cycles could causes the roughening [25]. Köntje et al. [26] recently showed the surface patterning on Au(111) single crystal electrode with islands and holes of one atomic step in height as a result of 1–2 initial repetitive oxide formation and reduction cycles and larger holes with 10 cycles. As our cleaning treatment also involved a similar oxide formation and reduction (up to 10 cycles), we expect a similar roughening/patterning process leading to the defective Au(111) terraces. This could have in turn caused the decrease of current intensity of the spike. These results suggest that the splitting of the Pb-upd stripping peak from the Au(111) face can be used as an indication for the surface crystallinity of polyoriented gold nanoparticles. 4. Conclusions Quasi-spherical and faceted gold nanoparticles showed distinctive Pb-upd behaviour of Au(111) face: (a) sp-AuNPs exhibits only one stripping peak; (b) ft-AuNPs exhibits a splitting of the peak in to two: the main stripping peak, 2a is from Au(111) terraces on both sp- and ftAuNPs; the spike 2a′ seems to be formed only on well-grown Au(111) terraces. The occurrence of the spike, 2a′ at a slightly more positive potential on ft-AuNPs/GC is a result of kinetic effects and is affected by the size of the Au(111) terraces. Possible roughening of the Au(111) terrace during upd processes and during cleaning cycles that lead to the drop in the spike current also indicates the dependence of the spike formation on the larger Au(111) terraces. The splitting of the Pb-upd stripping peak from Au(111) faces can be used as an indication for the surface crystallinity of polyfaced gold nanoparticles. References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 4. Pb-upd curves recorded on ft-AuNPs/GC at a scan rate of 0.05 V s− 1: (A) at different anodic switching potentials and (B) at different cathodic switching potentials.
[9] [10] [11]
negative potential to form a sub-monolayer of adatoms leads to the distinct stripping peaks due to phase transitions. For example, Cu upd on Au(111) sulfate ions co-adsorb with Cu that resulted in two peaks, whereas only one peak was obtained in perchloric acid [22]. In our case, we have weakly adsorbing perchlorate ions. Seo et al. [23] showed that the desorption of perchlorate ions precedes the adsorption of Pb. So, we do not think that our spike is a result of such phase transitions. Instead, as discussed earlier, we attribute it to the size of the terraces on which island formation and coalescence takes place. As the reported value of average size of Pb-islands on Au(111) face was ca. 4–15 nm [14,19], we could expect that the Pb island formation and coalescence is more feasible on ft-AuNPs with 28.9 nm edge length than that of the sp-AuNPs with 10.3 nm edge length. Scan rate effect on ftAuNPs showed that the observation of the spike is due to a kinetic effect as observed on a single-crystal Au(111) face. Interestingly, the spike current is decreased after the cleaning cycles. Alloying and dealloying of Pb with Au during the Pb adsorption and stripping processes could lead to the roughening of the Au terraces [24]. Also, repetitive cycling in the gold oxidation and reduction window in acidic solution up to
[12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23] [24] [25] [26]
4
J.W. Schultze, D. Dickertmann, Surf. Sci. 54 (1976) 489. A. Hamelin, J. Electroanal. Chem. 165 (1984) 167. A. Hamelin, J. Lipkowski, J. Electroanal. Chem. 171 (1984) 317. R. Parsons, G. Ritzoulis, J. Electroanal. Chem. 318 (1991) 1. E. Herrero, L.J. Buller, H.D. Abruña, Chem. Rev. 101 (2001) 1897. J. Hernández, J. Solla-Gullón, E. Herrero, A. Aldaz, J.M. Feliu, J. Phys. Chem. B 109 (2005) 12651. Y. Chen, S. Milenkovic, A.W. Hassel, ChemPhysChem 11 (2010) 2838. J. Hernández, J. Solla-Gullón, E. Herrero, A. Aldaz, J.M. Feliu, Electrochim. Acta 52 (2006) 1662. S. Hebié, K.B. Kokoh, K. Servat, T.W. Napporn, Gold Bull. 46 (2013) 311. J.M. Feliu, E. Herrero, Contributions to Science, 6 (2010), p. 161. B. Maestro, J.M. Ortiz, G. Schrott, J.P. Busalmen, V. Climent, J.M. Feliu, Bioelectrochemistry 98 (2014) 11. A. Kuzume, E. Herrero, J.M. Feliu, E. Ahlberg, R.J. Nichols, D.J. Schiffrin, Phys. Chem. Chem. Phys. 7 (2005) 1293. C. Jeyabharathi, P. Ahrens, U. Hasse, F. Scholz, J. Solid State Electrochem. 20 (2016) 3025. N.J. Tao, J. Pan, Y. Li, P.I. Oden, J.A. DeRose, S.M. Lindsay, Surf. Sci. 271 (1992) L338. C. Jeyabharathi, P. Esakki Karthik, K.L.N. Phani, RSC Adv. 4 (2014) 7780. C. Jeyabharathi, U. Hasse, P. Ahrens, F. Scholz, J. Solid State Electrochem. 18 (2014) 3299. S. Cherevko, A.A. Topalov, A.R. Zeradjanin, I. Katsounaros, K.J.J. Mayrhofer, RSC Adv. 3 (2013) 16516. O.A. Oviedo, L. Reinaudi, S.G. García, E.P.M. Leiva, F. Scholz (Ed.), Underpotential Deposition: From Fundamentals and Theory to Applications at the Nanoscale, Springer International Publishing, Cham, 2016, p. 91. C.H. Chen, N. Washburn, A.A. Gewirth, J. Phys. Chem. 97 (1993) 9754. M.P. Green, K.J. Hanson, R. Carr, I. Lindau, J. Electrochem. Soc. 137 (1990) 3493. D. Seo, C.I. Yoo, I.S. Chung, S.M. Park, S. Ryu, H. Song, J. Phys. Chem. C 112 (2008) 2469. L. Blum, D.A. Huckaby, M. Legault, Electrochim. Acta 41 (1996) 2207. M. Seo, M. Yamazaki, J. Electrochem. Soc. 151 (2004) E276. M.P. Green, K.J. Hanson, Surf. Sci. Lett. 259 (1991) L743. T. Kondo, J. Zegenhagen, S. Takakusagi, K. Uosaki, Surf. Sci. 631 (2015) 96. C. Köntje, D.M. Kolb, G. Jerkiewicz, Langmuir 29 (2013) 10272.