pubs.acs.org/Langmuir © 2010 American Chemical Society
Electrocatalytic Oxygen Reduction on Functionalized Gold Nanoparticles Incorporated in a Hydrophobic Environment Fakhradin Mirkhalaf*,† and David J. Schiffrin‡ †
Sonochemistry Centre, Faculty of Health and Life Sciences, Coventry University, Coventry, CV1 5FB, U.K., and ‡Chemistry Department, University of Liverpool, Liverpool, L69 7ZD, U.K. Received May 27, 2010. Revised Manuscript Received August 12, 2010
The electrocatalytic properties of gold nanoparticles covalently capped with a monolayer film of 1,4-decylphenyl groups for oxygen reduction in an alkaline solution have been studied. Functionalized nanoparticles were adsorbed on a film of the same capping ligand previously grafted to a glassy carbon electrode. The molecular film-nanoparticle assembly was characterized by cyclic voltammetry and XPS. It is shown that although the attachment of the capping ligand to the electrode surface blocks direct electron transfer, the metal centers of the incorporated nanoparticles provide sites for electron tunneling from the electrode surface thus leading to sites where oxygen reduction can take place. Rotating disk voltammetry shows that the oxygen reduction reaction follows mainly a peroxide formation channel on these nanostructured surfaces. The capping ligand greatly influences the reduction mechanism by establishing a local hydrophobic environment at the reaction centers within the film.
1. Introduction Metal nanoparticles have been extensively employed as electrocatalysts due to their large surface to volume ratios and high catalytic activity. The latter property is largely dependent on their shape and size and in addition, geometry and composition have a large influence on the selectivity for a desired reaction.1-3 Different types of stabilizers have been used for the synthesis of monolayer protected clusters (MPC) relying, for instance, on the use of polymers or by direct attachment of organic ligands. In the latter case, thiols have been extensively employed1-4 and more recently, the direct formation of strong carbon-metal bonds have been successfully realized following a two-phase method involving the simultaneous reduction of a metal salt and a diazonium compound.5 The formation of metal-carbon bonds avoids the use of capping ligands containing electroactive functionalities (e.g., thiols). This synthetic strategy has already been employed for the preparation of Au,5 Pt,5 Pd,6 Co,7 Ti,8 and Ru9 functionalized nanoparticles. Surprisingly, thiol-capped nanoparticles have been observed to exhibit electrocatalytic properties for reactions that are wellknown to be strongly surface sensitive, for instance, for CO and methanol oxidation.10,11 It was shown that small size MPC, when attached to an electrode surface, were both electrochemically *Corresponding author. E-mail:
[email protected]. Telephone: þ4424 7688 8624. Fax: þ44-24 7688 8173.
(1) Liz-Marzan, L. M.; Kamat, P. V. Nanoscale Materials; Kluwer Academic Pubs: Dordrecht, The Netherlands, 2003. (2) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27–36. (3) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (4) Murray, R. W. Chem. Rev. 2008, 108, 2688–2720. (5) Mirkhalaf, F.; Paprotny, J.; Schiffrin, D. J. J. Am. Chem. Soc. 2006, 128, 7400–7401. (6) Ghosh, D.; Chen, S. J. Mater. Chem. 2008, 18, 755–762. (7) Grass, R. N.; Athanassiou, E. K.; Stark, W. J. Angew. Chem., Int. Ed. 2007, 46, 4909–4912. (8) Ghosh, D.; Pradhan, S.; Chen, W.; Chen, S. Chem. Mater. 2008, 20, 1248– 1250. (9) Ghosh, D.; Shaowei, C. Chem. Phys. Lett. 2008, 465, 115–119. (10) Maye, M. M.; Lou, Y.; Zhong, C.-J. Langmuir 2000, 16, 7520–7523. (11) Lou, Y.; Maye, M. M.; Han, L.; Luo, J.; Zhong, C.-J. Chem. Commun. 2001, 473–474.
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accessible and catalytically active for oxidation reactions after electrochemical activation.11 These oxidation reactions involve the formation of surface gold oxides or the formation of oxidizing adsorbed oxygen species and it was unexpected to observe the coexistence of these groups as reaction centers with the thiolated moieties.12 The electrocatalytic properties of thiolate capped gold-platinum alloy nanoparticle assemblies attached to a glassy carbon electrode by an exchange-cross-linking-precipitation route have been studied and the importance of the capping ligand to prevent poisoning of the particle surface and possible aggregation has been highlighted.11 Other examples of enhancement of electrocatalytic reactions by MPC is the electrochemical oxidation of H2O213 and the oxidation of DOPA (3,4-dihydroxyphenylanaline) catalyzed by hydroxythiophenol capped nanoparticles. For the latter, the involvement of the capping ligand is likely.14 In addition, the incorporation of Au nanoparticles on self-assembled monolayers is known to increase the rate constant of electron transfer across the linkers, for example, for accessing the metal center of metalloenzymes15 or for the enhancement of the rate of complex reactions such as the electrocatalytic oxidation of ascorbic acid.16 There are currently extensive research efforts for developing efficient and selective electrocatalysts for the reduction of oxygen due to their importance in fuel cells. The oxygen reduction reaction follows different mechanisms depending on electrode materials and experimental conditions, leading to water and/or hydrogen peroxide.17-20 The reaction proceeds by a two- or a
(12) Zhong, C.-J.; Maye, M. M. Adv. Mater. 2001, 13, 1507–1511. (13) Patolsky, F.; Gabriel, T.; Willner, I. J. Electroanal. Chem. 1999, 479, 69–73. (14) Stolarczyk, K.; Palys, B.; Bilewicz, R. J. Electroanal. Chem. 2004, 564, 93–98. (15) Abad, J. M.; Gass, M.; Bleloch, A.; Schiffrin, D. J. J. Am. Chem. Soc. 2009, 131, 10229–10236. (16) Sivanesan, A.; Kannan, P.; John, S. A. Electrochim. Acta 2007, 52, 8118– 8124. (17) Adzic, R. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; pp 197-242. (18) Kim, J.; Gewirth, A. A. J. Phys. Chem. B 2006, 110, 2565–2571. (19) Yeager, E. Electrochim. Acta 1984, 29, 1527–1537. (20) Li, X.; Gewirth, A. A. J. Am. Chem. Soc. 2005, 127, 5252–5260.
Published on Web 08/27/2010
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four-electron mechanism on gold, depending on the surface orientation and/or experimental conditions.21,22 Two questions have been addressed in the present work, to determine whether metal-carbon functionalization of MPCs blocks the electrocatalytic reduction of oxygen and to establish if the local hydrophobic environment can be employed to direct the reduction product distribution since it is well-known that nonaqueous solvents stabilize superoxide and peroxide species formed in the reduction of oxygen.23-26 For this reason, gold nanoparticles capped with covalently attached 1,4-decylphenyl groups (AuDP) were employed as electrocatalysts for the reduction of oxygen. The particles were assembled within a film of decylphenyl groups (DP) covalently grafted to a GC electrode surface in the expectation of forming a hydrophobic film incorporating dispersed metal centers, i.e., the nanoparticles cores.
2. Experimental Methods 4-Diazonium decylbenzene fluoroborate (DDB) and Au nanoparticles capped with this ligand were synthesized according to a published procedure.5 Briefly, DDB was prepared by the diazotization of 4-aminodecylbenzene (Aldrich) in fluoroboric acid and sodium nitrite. Au nanoparticles were synthesized in a two-phase system by transferring AuCl4- into a toluene phase using DDB as the phase transfer agent and the AuCl4-DDBþ salt formed was subsequently reduced by contacting the toluene solution with aqueous 0.1 M NaBH4. The dark red toluene solution containing nanoparticles was further purified by passing through a silica column (0.014 μm, Aldrich). The AuDP nanoparticles were retained by the column and other impurities were eluted with toluene. The nanoparticles were then eluted with THF. HRTEM images were obtained with a JEOL 3000F microscope. Toluene and THF solutions of Au nanoparticles were spotted on carbon coated copper grids and dried in air. A glassy carbon (GC) disk electrode of 0.0707 cm2 area was employed for most of the electrochemical experiments. The electrode was polished to a mirror finish using increasingly finer aqueous alumina slurries (1.0, 0.3, and 0.05 μm, Buehler) prior to each experiment, followed by repeated sonication in water. The electrode was finally dried in a stream of high purity argon before surface modification. The electrode was mounted on the PTFE tip of a EDI101 rotator (Radiometer Analytical, France) and the rotation rate was controlled with a CVT101 control unit. All experiments were performed in a three-electrode glass cell with a graphite disk counter electrode and a saturated calomel reference electrode (SCE); all potentials are referred to this electrode. The potential was controlled with a Compactstat electrochemical interface operated with IviumSoft software (Ivium Technologies, Eindhoven, The Netherlands). All measurements were made at room temperature (21(1 °C). Milli-Q water was used throughout; KOH (Alfa Aesar, 99.99%), K3Fe(CN)6 (Fisher Scientific, Analytical reagent grade) were used as received. The metal particles were adsorbed on a decylphenyl film (DP) grafted on the GC electrode (GC-DP) by electrochemical reduction of DDB. The grafting method has been previously described for the attachment of diazonium derivatives to carbon and (21) Damjanovic, A.; Genshaw, M. A.; Bockris, J. O’M. J. Electroanal. Chem. 1967, 15, 173–180. (22) Markovic, N. M.; Tidswell, I. M.; Ross, P. N. Langmuir 1994, 10, 1–4. (23) Vasudevan, D.; Wendt, H. J. Electroanal. Chem. 1995, 392, 69–74. (24) Chin, D.-H.; Chiericato, G.; Nanni, E. J.; Sawyer, D. T. J. Am. Chem. Soc. 1982, 104, 1296. (25) Chevalet, J.; Rouelle, F.; Gierst, L.; Lambert, J. P. J. Electroanal. Chem. 1972, 39, 201–216. (26) Divisek, J.; Kastening, B. J. Electroanal. Chem. 1975, 65, 603–621. (27) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883–5884. (28) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201–207. (29) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370–378.
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Scheme 1. Schematic Description of the Modified Glassy Carbon Capped Nanoparticles Assemblya
a Drawing not to scale. Although a single monolayer is described in the scheme, it is likely that some degree of branching in the film will take place during grafting.
metallic surfaces.27-30 Briefly, the polished glassy carbon electrode was rinsed thoroughly and sonicated for 10 min in pure water; it was then left to dry in air and immersed in a 10 mM solution of DDB in acetonitrile containing 0.1 M tetrabutylammonium tetrafluoroborate ((TBA)BF4). Grafting was performed by cycling the potential between þ0.4 and -0.4 V three times and then keeping the potential at -0.2 V for 2 min to ensure complete grafting. The electrode was then rinsed with acetonitrile and further sonicated for 10 min in acetonitrile to remove physically adsorbed DDB. The modified electrode was then immersed in a Au-DP solution in THF for 4 h, rinsed with toluene, acetone, and ethanol, and dried in air. Scheme 1 shows a schematic description of the interface after the two-step electrode modification. XPS spectra were obtained with a Kratos Axis Ultra-DLD system (Kratos Analytical, vacuum less than 5 10-10 Torr), equipped with a delay-line detector and a monochromatic Al KR radiation source. In order to distinguish between the different binding environments of carbon, an indium tin oxide (ITO) surface was used to avoid the large C 1s signal from the substrate if glassy carbon was employed. The DP film was grown on this surface using the same method as described above for GC. The attachment of various organic molecules by the electroreduction of the corresponding diazonium cations on ITO coated glass surfaces has been previously reported.31 The XPS data were analyzed using CasaXPS software.
3. Results and Discussion 3.1. Nanoparticle Characterization. Figure 1a shows a TEM image of the synthesized Au nanoparticles capped with 1,4-decylphenyl groups (Au-DP). The average size was (7 ( 2) nm, similar to previous results.5 The average close contact separation between cores was (2.8 ( 0.3) nm. The length of the fully extended capping ligand is 1.6 nm, as estimated by molecular modeling approximately (Spartan software), indicating some degree of interdigitation between the capping molecules.5 Figure 1b illustrates an HRTEM image of a single nanoparticle. The image shows crystalline fringes for the Au core with a spacing of 2.4 ( 0.2 A˚. The lattice separation (d) for the different crystal orientations can be calculated from32 d = a/(h2 þ l2 þ k2)1/2 where a is the lattice constant (407.833 pm for Au33). For a FCC (111) crystalline (30) Laforgue, A.; Addou, T.; Belanger, D. Langmuir 2005, 21, 6855–6865. (31) Maldonado, S.; Smith, T. J.; Williams, R. D.; Barton, E.; Stevenson, K. J. Langmuir 2006, 22, 2884–2891. (32) Silbey, R. J.; Alberty, R. A. Physical Chemistry; John Wiley & Sons Inc.: New York, 2001, p 835. (33) Borg, R. J.; Dienes, G. J. The Physical Chemistry of Solids; Academic Press Inc.: San Diego, CA, 1992.
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Figure 2. First (solid line) and second (dotted line) grafting scans on a GC surface for the diazonium decylbenzene salt (DDB) in acetonitrile containing 0.1 M (TBA)BF4. Scan rate = 100 mV s-1.
Figure 3. Cyclic voltammograms of a GC electrode before (solid line) and after (dotted line) the attachment of the DP film in 0.1 M K3Fe(CN)6. SR = 100 mV s-1.
Figure 1. (a) TEM image of AuDP nanoparticles; the bar shows a resolution of 10 nm. (b) HRTEM image of a single nanoparticle on the edge of the grid; the bar shows a resolution of 2 nm.
structure, d=2.354 A˚, in close agreement with the value obtained from the HRTEM image (i.e., 2.4 A˚, Figure 1b and the Supporting Information). The presence of the capping molecules can be observed in Figure 1b as a shell with a thickness close to the length of the capping ligand (i.e., 1.6 nm). This layer could only be observed for nanoparticles present at the edge of the carbon support on the TEM grid; otherwise, it could not be distinguished from the support (see Supporting Information). It can be concluded that the alkyl chains form a compact shell on the Au cores. 3.2. Film Formation on GC. Figure 2 shows the first and second grafting scans of DDB on a polished GC electrode. The first scan shows a large reduction current which is greatly suppressed in the second scan due to the population of the surface by decylphenyl groups. Similar observations have been reported for the grafting of other diazonium derivatives and assigned to the blocking of the electrode surface by the grafted (34) Tammeveski, K.; Kontturi, K.; Nichols, R. J.; Potter, R. J.; Schiffrin, D. J. J. Electroanal. Chem. 2001, 515, 101–112. (35) Mirkhalaf, F.; Tammeveski, K.; Schiffrin, D. J. Phys. Chem. Chem. Phys. 2004, 6, 1321–1327. (36) Vaik, K.; Sarapuu, A.; Tammeveski, K.; Mirkhalaf, F.; Schiffrin, D. J. J. Electroanal. Chem. 2004, 564, 159–166.
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molecular film formed after the first scan.27,28,34-36 In order to check that the electrode surface was uniformly covered with a DP film, the reduction of Fe(CN)63- on the modified electrode was studied. Figure 3 compares cyclic voltammograms of a polished GC electrode with that of the same electrode grafted with DP, showing that the organic film completely blocks the electrode surface and that the film is free from pinholes and defect sites. This is due to the presence of long alkyl chain attached to the surface, as is also observed for self-assembled monolayers of long chain alkyl thiols on gold surfaces.37 AuDP nanoparticles were then incorporated into the DP film. 3.3. Au Nanoparticles-DP Hybrid Film Characterization. The presence of AuDP nanoparticles on the surface of a GC electrode grafted with DP and with Au nanoparticles incorporated in it (GC-DP(AuDP)) was confirmed by cyclic voltammetry in 0.1 M H2SO4 (Figure 4). Typical oxidation and reduction features for gold are observed demonstrating the presence of Au nanoparticles on the electrode surface. The oxide reduction peak presents, however, features indicating the presence of several oxides with slightly different reduction potentials. Most likely this reflects a spread in core dimensions and similar reduction potential dependencies has been recently observed.38 The relative area of gold available for reaction was calculated by integration of the current under the reduction wave and this is shown in the inset to Figure 4 from which a reduction charge of 7.0 10-4 C cm-2 is obtained. The charge required to form an oxide monolayer on (37) Brett, C. M. A.; Kresak, S.; Hianik, T.; Brett, A. M. O. Electroanalysis 2003, 15, 557–562. (38) Jirkovsky, J. S.; Halasa, M.; Schiffrin, D. J. Phys. Chem. Chem. Phys., in press.
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Figure 4. Cyclic voltammogram for a GC-DP(AuDP) electrode
in 0.1 M H2SO4 solution. Scan rate = 20 mV s-1. Inset: detail of the oxide reduction peak used to calculate the effective Au nanoparticle area in the DP film.
bulk gold is 386 μC cm-2,39 and therefore, the total surface of gold is approximately 0.2 cm2 per unit of geometric electrode area. These results indicate that the GC-DP(AuDP) electrode incorporates a large concentration of Au centers within the organic film with an effective surface area of the order of ∼20%. The nanoparticles incorporated within the DP film grafted on an ITO surface were further investigated by high resolution XPS. Figure 5 compares the HR-XPS spectra of an ITO slide modified with DP before and after dipping in an AuDP solution and followed by washing in a similar way as for the GC electrode. Figure 5a shows the C 1s spectral region before and after dipping in the nanoparticle solution. The ITO slide grafted with DP presents a high intensity C 1s peak centered at 281.4 eV and a small peak centered at 285 eV, corresponding to carbon in two different types of bonding. The first corresponds to an alkyl chain and the second to the phenyl ring attached to the ITO surface.31,40 When the nanoparticle assembly is incorporated within the film, in addition to the DP peaks, two low intensity further peaks and at the higher binding energies of 289 and ca. 292 eV are observed. These can be ascribed to carbon atoms bonded to an electron withdrawing center40 or to the π f π* shake up satellite band associated with the aromatic ring.41 It is proposed that these peaks correspond to carbon atoms of the capping ligand bonded to the Au cores. Figure 5b shows strong Au peaks when Au-DP nanoparticles are assembled on the DP film confirming the attachment of Au-DP nanoparticles on the DP film. It is suggested that van der Waals interactions between the capping ligand and the functionalized surface are responsible for the strong adsorption of the particles. 3.4. Electrocatalytic Oxygen Reduction. An example of a cyclic voltammogram of O2 reduction on the DP-modified GC electrode compared to that for bare GC is given in Figure 6. No reduction current is detected in the absence of oxygen. In contrast to the results for the hexacyanoferrate redox couple, although the DP film inhibits extensively O2 reduction, a small reduction current compared to a bare GC electrode is still observed. This can be attributed to the small size of the oxygen molecule that allows it to diffuse either through the DP film or through defect sites to reach the electrode surface. Although tunneling across the phenyl moieties cannot be excluded, this is unlikely. These experiments do not allow a distinction between tunneling and transfer across defects in the organic film. The reduction current (39) Tremiliosi-Filho, G.; Dall’Antonia, L. H.; Jerkiewicz, G. J. Electroanal. Chem. 2005, 578, 1–8. (40) Christ, B. V. Handbook of Monochromatic XPS Spectra, Vol 1, The Elements and Native Oxides; XPS International Inc.: Mountain View, CA, 1999. (41) Mackie, N. M.; Castner, D. G.; Fisher, E. R. Langmuir 1998, 14, 1227– 1235.
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Figure 5. XPS spectra of an ITO-DP electrode (dashed line) compared with that of ITO-DP(AuDP) (solid line) for (a) the C 1s region and (b) the Au 4f region.
Figure 6. Cyclic voltammograms of the GC-DP electrode in O2 (2, dotted line) and N2 (3, dashed line) saturated 0.1 M KOH compared with a bare GC electrode (1, solid line). SR = 100 mV s-1.
for this modified electrode is small compared to that of the bare GC electrode (Figure 6) but measurable, demonstrating the incorporation of O2 within the hydrophobic film. An estimate of the fraction of free GC surface available for the direct reduction of oxygen is given further on (see discussion on eq 2). The reduction of oxygen in alkaline solutions follows either a direct four-electron (reaction I) or an indirect two-step twoelectron mechanism leading to water (reactions II and III): k1
O2 þ 4e - þ 2H2 O sf 4OH k2
O2 þ 2e - þ H2 O sf HO2 - þ OH k3
HO2 - þ 2e - þ H2 O sf 3OH -
ðIÞ ðIIÞ ðIIIÞ
O2 reduction on polished GC stops at the two-electron stage leading to peroxide.35,36 However, Au supports both the two- and Langmuir 2010, 26(18), 14995–15001
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Figure 7. RDE voltammograms for oxygen reduction on glassy carbon electrodes: (1) unmodified GC; (2) GC modified with DPAu(DP); (3) bare GC after physical adsorption of Au(DP) by solvent evaporation. All the voltammograms were recorded in 0.1 M KOH at a rotation rate of 400 rpm.
four-electron reduction mechanisms depending on crystal orientation and/or surface composition.42 In order to investigate the effect of Au nanoparticles incorporated in the DP film on the electrochemical properties of this electrode (denoted as GC-DP(AuDP)), the reduction of oxygen on these two surfaces was compared. Rotating disk electrode (RDE) voltammograms for O2 reduction in alkaline solution for bare GC, GC-DP(AuDP) and GC(AuDP) are shown in Figure 7. The comparison between GC and GC-DP indicates that the attachment of the DP film leads to a strong inhibition of oxygen reduction as shown by a large negative shift of the onset reduction potential (Figure 6). However, when nanoparticles are incorporated within the film (sample GC-DP(AuDP)), the reduction potential is shifted to more positive values (Figure 7). This can be a consequence of mediation by the Au cores of electron tunneling from the electrode surface to the metal centers incorporated within the organic film. Besides this effect, charge transfer through the aryl-alkyl linker could be greatly enhanced by electron spillover from the core into the organic linker, as demonstrated by the linker length dependence of the electronic conductance of ruthenium nanoparticles capped with molecules of similar structure12 as those used in a previous5 and the present work. A comparison between the latter arrangement with that of AuDP nanoparticles simply adsorbed on the electrode surface by evaporation from their dispersion in THF (GC(AuDP)) shows that the presence of the grafted film provides an organized attachment layer onto which the AuDP particles can interdigitate and be strongly coupled to the electrode surface thus leading to more facile electron transfer due to electron tunneling to the particles within the organic film (Figure 7). Therefore, the former arrangement supports more facile O2 reduction. Long range electron transfer reactions between core-shell metal nanoparticles and electrode surfaces when they are separated by monolayer or multilayer films are well-known.43-46 For a self-assembled monolayer film of 11-mercaptoundecanoic acid, electron transfer to the hexacyanoferrate redox couple is strongly hindered by the film but electrostatic adsorption of Au nanoparticles greatly improves the rate of electron transfer even at (42) Strbac, S.; Adzic, R. R. J. Electroanal. Chem. 1996, 403, 169–181. (43) Zhao, J.; Bradbury, C. R.; Huclova, S.; Potapova, I.; Carra, M.; Fermin, D. J. J. Phys. Chem. B 2005, 109, 22985–22994. (44) Zhao, J.; Bradbury, C. R.; Fermin, D. J. J. Phys. Chem. C 2008, 112, 6832– 6841. (45) Chen, W.; Chen, S.; Ding, F.; Wang, H.; Brown, L. E.; Konopelski, J. P. J. Am. Chem. Soc. 2008, 130, 12156–12162. (46) Stolarczyk, K.; Bilewicz, R. J. Electrochim. Acta 2006, 51, 2358–2365.
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Figure 8. Rotating disk voltammograms for O2 reduction on a GC-DP(AuDP) electrode in O2 saturated 0.1 M KOH. Rotation rates: 200, 400, 800, 1500, 2500, 3500 rpm. The dashed curve represents a voltammogram for the same electrode in N2 saturated 0.1 M KOH at a rotation rate of 400 rpm.
low particle number density, with the Au particles behaving as an array of randomly distributed nanoelectrodes.43 Therefore, by controlling the number of nanoparticles on the modified surface, electron-transfer rate constants can be extensively altered. This effect has been discussed in terms of established models of tunneling across metal-insulator-metal junctions and these systems have been proposed as new types of nanocatalysts.43 Electron transfer between adsorbed nanoparticles and the metal electrodes have been found to be independent of the self-assembled layer thickness and the apparent kinetics of electron transfer is controlled by the electron exchange between the particles and the redox species rather than by charge transfer across the film.44 The charge-transfer resistance decreased by more than 2 orders of magnitude upon adsorption of Au nanoparticles and the kinetics of electron-transfer was independent of the distance between nanoparticles and electrode surface up to 6.5 nm.44 In the present study, the average distance between the metal cores and the electrode surface would be 2.8 nm for fully extended chains, similarly to the separation distance reported for Au-DP nanoparticles.5 This is due to the presence of decylphenyl groups in both the grafting film and the capping ligand. It is proposed that the facile electron transfer observed for oxygen reduction is due to the presence of the metallic nanoparticles cores acting as electron transfer mediators. Tunneling rates for distances corresponding to the fully extended chains could be very small (however, see refs 43 and 44), and the results presented here indicate extensive interdigitation involving the alkyl chains. The inclusion of AuDP nanoparticles into a film of DP leads to the formation of reaction centers and an increase in film conductance. This, and changes in the electron-transfer function, leads to facilitated electron hopping between nanoparticles within the film and the electrode surface. The electron transfer mechanism could be, however, more complex than suggested above as recently demonstrated by Chen et al.,45 who showed that nanoparticle-mediated intervalence electron transfer can contribute to electrode-nanoparticle coupling. 3.5. The Value of n. RDE voltammograms for GC-DP(AuDP) electrode at different rotation rates are shown in Figure 8. For potentials more negative than -0.8 V, these results were analyzed using the Koutecky-Levich (K.L.) equation:47 1 1 1 1 1 ¼ þ ¼ ð1Þ j jk jd nFkCOb 2 0:62nFDO2 2=3 ν - 1=6 COb 2 ω1=2 (47) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980.
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Figure 9. Koutecky-Levich plots for O2 reduction on a GC-DP(AuDP) electrode in 0.1 M KOH; the inset shows the calculated potential dependence of n. Data were from Figure 8.
where j is the current density, jk and jd are the kinetic and diffusion-limited current densities, respectively, n is the number of electrons transferred per O2 molecule, k is the overall rate constant for O2 reduction, F is the Faraday constant (96485 C b is the concentration mol-1), ω is the rotation rate in rad s-1, CO2 -6 of oxygen in the bulk (1.29 10 mol cm-3),48 DO2 is the diffusion coefficient of oxygen (1.73 10-5 cm2 s-1),48 and ν is the kinematic viscosity of the solution (0.01 cm2 s-1).49 The corresponding plots are shown in Figure 9. Some deviations from linearity at less negative potentials (e.g., -0.8 V) are observed, which are related to a more complex mechanism than that indicated by eq 1 (see below). However, parallel plots with decreasing intercepts indicate that the reaction becomes mass transfer controlled at more negative potentials. The value of n was calculated from the slopes of the K.L. plots in Figure 9 from which n =1.8 ( 0.1. n reaches a value close to two at sufficiently negative potentials (e.g., -1.2 V). The RDE data in Figure 8 shows an additional reduction wave at approximately -0.5 V. This additional wave has been repeatedly observed for the reduction of O2 on glassy carbon and has been ascribed to the electrocatalytic properties of native quinone groups present on the carbon surface.34-36 This can also be seen in the results shown in Figure 7. Thus, O2 reduction on these functionalized surfaces can proceed in parallel on two types of sites, on the Au cores and on quinone functionalities present on the carbon substrate. The two contributions were deconvoluted employing a nonlinear regression analysis (NLR). 3.6. Reduction Mechanism. The reaction on the quinone groups is considered to follow an electrochemical-chemical mechanism according to34 Q þ e - f Q• -
ðIVÞ
Q• - þ O2 f O2 • - þ Q
ðVÞ
2O2 • - þ H2 O f O2 þ HO2 - þ OH -
ðVIÞ
O2 • - þ H2 O þ e - f HO2 - þ OH -
ðVIIÞ
or
Q is the surface quinone species. Reaction V is the rate-determining step, and in this reaction model, the overall rate is determined by the surface concentration of Q•-.34 The superoxide anion (48) Ahlberg, E.; Falkenberg, F.; Manzanares, J. A.; Schiffrin, D. J. J. Electroanal. Chem. 2003, 548, 85–94. (49) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 82nd edn., CRC Press: Boca Raton, FL, 2001.
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Figure 10. Comparison of the NLR calculations with experimental data from Figure 8 at a rotation rate of 800 rpm. The arrow indicates the potential region where O2 reduction on the exposed native surface groups of GC takes place.
formed, O2•-, can disproportionate or be further reduced on the electrode surface (reactions VI and VII, respectively). The analysis is based on the Koutecky-Levich equation that has been previously described.34-36 In this case, two parallel reduction pathways are considered, reduction on the metal cores and on available native quinone groups as described by reactions IV-VII. The current density predicted by this model is given by: 1 1 ¼ j jD 1 2FCOb 2 k0 e - f RðE - E 0 Þ þ
1þe
2FCOb 2 k1 Γ1 0 0 f ðE - Eð1Þ Þ ðf =2ÞΔEð1Þ - f ðE - Eð1Þ Þ ðf =2ÞΔEð1Þ e
þe
e
ð2Þ 0
jD is the diffusionally controlled current density, E is the standard potential of the O2/HO2- couple (E0=-0.315 V vs SCE50); k0 is the electrochemical rate constant for oxygen reduction and R is the corresponding transfer coefficient; k1 is the rate constant for reaction V; Γ1 is the surface concentration of the native quinone species; E 0(1) is the standard potential of the surface Q/Q2- couple; ΔE(1) is the difference between the standard potentials of the Q•-/ Q2- and the Q/Q•- couples; F is the Faraday constant and f = F/RT. The data in Figure 8 was fitted to eq 2 by NLR (Origin software, vs 7, OriginLab Corporation) and Figure 10 shows an example of the fit for the results at 800 rpm; a very good fit of the data is observed. The value of jD was obtained from the regressions by setting it as a model parameter since this considers the whole current-potential dependence measured. The average error returned for jD was better than 0.2%. This approach returns a value of n that is an average over the whole potential range investigated. This is justified, in general, provided the value of n does not change significantly with potential, which is the case here as determined from the results shown in Figure 9. The average values of the model parameters obtained for all the rotation rates measured were: k0 = (7.1 ( 1.5) 10-5 cm s-1; R = 0 = (-0.572 ( 0.19 ( 0.01; Γ1k1=(3.4 ( 0.7) 10-4 cm s-1; E(1) 0.007) V, and ΔE(1) = (-0.27 ( 0.02) V. The calculated model parameters for the native surface quinone groups are very similar to those obtained for other functionalized surfaces. For example, for anthraquinone grafted GC 0 =(-0.56 ( 0.01) V and electrodes, the standard potential was E(1) ΔE(1) was (-0.25 ( 0.02) V,34 close to the values calculated from the present results. The largest difference is in the rate constant (50) Chen, W.; Chen, S. Angew. Chem., Int. Ed. 2009, 48, 4386–4389.
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component corresponding to the chemical step, which for bare GC was Γ1k1=(0.019 ( 0.002) cm s-1. This is an interesting result since it shows that, if the reaction of the semiquinone intermediate with oxygen (reaction V) is similar for both GC surfaces, a reasonable assumption, the available GC area for oxygen reduction at the DP modified surface is less than 2% of the geometric electrode area. It is not possible to speculate on the details of the carbon-organic functionality interface but these results give a reasonable explanation for the main features of the oxygen reduction kinetics in the whole potential range, i.e., two processes in parallel, one on free carbon sites and the other on Au nanoparticles. The oxygen reduction rate on the GC surface is greatly inhibited by the DP grafted film and therefore, the rate constant k0 calculated from eq 2 refers to the reduction of oxygen on the nanoparticles per unit geometric electrode area. Since the effective gold surface area is 20% of the geometric area, the intrinsic rate constant on gold is approximately 3.5 10-4 cm s-1. The oxygen reduction reaction on hexanethiol-functionalized nanoparticles has been recently investigated by Chen et al.50 The rate constants obtained can be compared with those derived from the nonlinear regression analysis (eq 2). The rate constant at E = -0.5 V vs Ag/AgCl/3 M NaCl for the largest cluster investigated by Chen et al. (Au140) was 5.9 10-3 cm s-1, and that calculated from the present results at this potential and referred to the same reference electrode and unit area of gold was approximately 1.8 10-3 cm s-1. The lower value obtained here could result from a particle size effect, in accordance with the observation of a large decrease in the rate constant with increasing core size.50 In addition, the larger capping ligand employed here can contribute to a decrease of oxygen reduction rate. In spite of the differences in the systems investigated (S-Au/C-Au bonding), the present results are comparable to those of Chen et al.50 The corresponding rate constant for clean Au particles was studied by Erikson et al.,51 and from their results and at the same potential, a rate constant of 0.07 cm s-1 was obtained. This demonstrates that, as expected, the attachment of organic functionalities results in a strong inhibition of the rate of electron transfer for oxygen reduction. The comparison of these data opens unusual questions regarding reactivity at the nanoscale. For instance, the rate constant for hexanethiol-capped Au11 clusters at the above potential is on the order of 0.06 cm s-1,50 close to that for a free Au nanoparticle surface. The origin of these effects is still unclear. The value of n is lower than that for bulk gold at all potentials indicating that a selective two-electron reduction leading to peroxide formation takes place on the DP(AuDP) modified electrodes. These results are in contrast with the two-step oxygen reduction mechanism observed for Au nanoparticles electrode(51) Erikson., H.; J€urmann, G.; Sarapuu, A.; Potter, R. J.; Tammeveski, K. Electrochim. Acta 2009, 54, 7483–7489. (52) Mirkhalaf, F.; Tammeveski, K.; Schiffrin, D. J. Phys. Chem. Chem. Phys. 2009, 11, 3463–3471.
Langmuir 2010, 26(18), 14995–15001
Article
posited on a nitrophenyl template recently reported.52 The difference between these results suggests that the local hydrophobic environment provided by the capping molecule and the attached layer on the carbon surface plays an important role in the mechanism of oxygen reduction by establishing a stabilizing environment for the reduction intermediates produced. The influence of a nonaqueous environment (e.g., solvents) in stabilizing the radical superoxide species and facilitating peroxide formation has been repeatedly observed.23-26 It has been proposed that the peroxide formed in the first step could be readsorbed on nanoparticles leading to further reduction,53 but no further reduction was observed in the present case. As mentioned above, it is proposed that this is due to the local nonaqueous environment created by the capping ligand that stabilizes the superoxide and peroxide formed.
4. Conclusions Gold nanoparticles capped with decylphenyl groups form nanocatalysts incorporated within a local hydrophobic environment consisting of a monolayer film grafted to GC. The DP film blocks the GC surface for direct electron transfer and thus the electrochemical reactivity observed for the modified electrode results from electron-tunneling from the electrode surface to the Au cores. The modified electrode supports the two-electron oxygen reduction leading to peroxide formation. It is proposed that the capping ligand provides a nonaqueous local environment that stabilizes superoxide and peroxide intermediates. The nanoparticles display selective electrocatalytic properties to yield peroxide due to the localized hydrophobic environment in which they are present. Acknowledgment. The support for part of this work by the European Union (NENA project, Contract No.: NMP3-CT2004-505906) is gratefully acknowledged. F.M. also thanks Prof Timothy Mason and Dr Mohammed Alarjah (Sonochemistry Centre, Coventry University, U.K.) for their help and support. TEM images were taken by Dr Lisa Carlson (Oxford Materials, Oxford University, U.K.) under the EPSRC Materials Equipment Access scheme (grant reference: EP/F01919X/1). XPS spectra were obtained by Dr D. Morgan (Wolfson Nanoscience Laboratory, Cardiff University, UK) under the EPSRC access scheme (Grant No. EP/F019823/1). Supporting Information Available: Figures showing the TEM image and the corresponding histogram for Au-DP nanoparticles and additional HRTEM images of single nanoparticles at the edge and center of the carbon support on the TEM grid. This material is available free of charge via the Internet at http://pubs.acs.org. (53) Schneider, A.; Colmenares, L.; Seidel, Y. E.; Jusys, Z.; Wickman, B.; Kasemo, B.; Behm, R. J. Phys. Chem. Chem. Phys. 2008, 10, 1931–1943.
DOI: 10.1021/la1021565
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