Article pubs.acs.org/Langmuir
Electron Transfer to Covalently Immobilized Keggin Polyoxotungstates on Gold Mustansara Yaqub,† James J. Walsh,‡ Tia E. Keyes,*,‡ Anna Proust,*,§ Corentin Rinfray,§ Guillaume Izzet,§ Timothy McCormac,*,† and Robert J. Forster*,‡ †
Dundalk Institute of Technology, Co. Louth, Ireland School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland § Institut Parisien de Chimie Moléculaire, UMR CNRS 8232 - UPMC Univ Paris 06, Université Pierre et Marie Curie, 4 place Jussieu, case courrier 42, F-75252 Paris Cedex 05, France ‡
ABSTRACT: Spontaneously adsorbed monolayers have been formed on gold electrodes using a Keggin polyoxotungstate with covalently attached alkanethiol linkers of two different lengths. Films of both polyoxotungstates show two welldefined reduction processes associated with the polyoxotungstate centers where the ionic liquid, [BMIM][BF4], acts as supporting electrolyte. The surface coverages are both less than that expected for a close-packed monolayer. For the short and long linkers, the voltammetric response can be described in terms of the Butler− Volmer response involving a surface confined species using standard heterogeneous electron transfer rate constants of 170 and 140 s−1 for the first reduction and 150 and 100 s−1 for the second reduction processes, respectively. The rate of electron transfer to a solution phase redox probe, ferrocyanide, is significantly more sensitive to the length of the linker than the rate of electron transfer to the tungstate centers. This behavior probably arises due to potential-induced changes in the film structure.
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INTRODUCTION Polyoxometalates, POMs, are an important class of anionic, inorganic, metal oxide clusters which display great diversity in their structure and composition.1,2 Their very high stability toward redox cycling makes them attractive candidates for applications in different areas such as photocatalysis,3,4 electrochromism,5,6 medicine,7 and sensing.8,9 Reliable approaches to surface immobilization of stable polyoxometalate thin films is important across a wide range of their applications in particular in their exploitation in solar cells.10 There have been significant advances recently in the synthesis and derivitization of POMs leading to a significant range of robust “designer” components for “bottom-up” materials synthesis. However, there is still a major challenge to devise general methods for constructing functional nanoscale architectures from these versatile building blocks. For example, to create functional architectures and devices, it must be possible to control intercomponent separation and the orientation of the building blocks. Moreover, the design phase is greatly simplified if immobilization of POMs onto various surfaces does not significantly alter their redox or optical properties,11 so that these properties can be reliably translated from solution to the interface-based device.12 Interfaces have been modified using POMs by the deposition of Langmuir−Blodgett monolayers,13 drop-casting,14,15 sol−gel synthesis,16 and layer-by-layer selfassembly (LBL).17,18 For example, we recently reported on thin film assembly of metallopolymer with polyoxometalate to give stable films for photocurrent generation at ITO.3 There have © 2014 American Chemical Society
also been elegant examples of monolayers in which the POM is chemically linked to the surface,19−24 but synthesizing POMs with appropriate functional groups for immobilization remains challenging.25 In this contribution, in a drive toward generating thin films suitable for photoelectrocatalytic applications, we report on the properties of Keggin-type polyoxotungstate monolayers (Scheme 1) on gold substrates formed via a Au−S linkage. Monolayers have been formed using two alkyl linkers of different lengths that possess thioacetyl termini that can bind to gold electrodes. Significantly, while both POMs are redox-active when dissolved in aqueous or organic solvents, spontaneously adsorbed monolayers do not exhibit any well-defined redox processes when in contact with these conventional supporting electrolyte solutions. In sharp contrast, in the ionic liquid [BMIM][BF4], well-defined voltammetric responses associated with the POMs are observed allowing the dependence of surface coverage, cyclic voltammetric behavior, and rate of heterogeneous electron transfer on alkyl chain length have been studied. The rate of heterogeneous electron transfer is sufficiently high to allow the oxidation state of the film to be switched within a few milliseconds. Significantly, we have used the intrinsic redox couples of the polyoxometalates and a solution phase redox probe to investigate the effect of potential Received: January 10, 2014 Revised: March 24, 2014 Published: March 26, 2014 4509
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Scheme 1. Synthetic Routes to the Thioester-Terminated Targets 3 and 5
CH2−CH2−CH3), 1.43 (sextuplet, JH−H = 7.5 Hz, 32H, N−CH2− CH2−CH2−CH3), 1.0 (t, JH−H = 7.5 Hz, 48H, N−CH2−CH2−CH2− CH3). 31P NMR (CD3CN, 121 MHz): δ = −10.75 (s + d, JSn−P = 24.0 Hz). Elemental analysis for C73H151N4O40PSnW11 (%): Calculated: C 22.50, H 3.91, N 1.44. Found: C 21.82, H 3.77, N 1.44. Step 2: Synthesis of [TBA]4[PW11O39SnC6H4CCCH2SC(O)CH3] (3). Diisopropyl azodicarboxylate (DIAD) (20.2 mg, 4 equiv, 0.1 mmol) was added to an ice-cold solution of triphenylphosphine (26.2 mg, 4 equiv, 0.1 mmol) in freshly distilled THF (1 mL) under an argon atmosphere. After vigorous stirring for 30 min the mixture was cannulated into a cold solution of 2 (100 mg, 1 equiv, 0.025 mmol) and thioacetic acid (7.6 mg, 4 equiv, 0.1 mmol) in dry, distilled DMF (500 μL) over a period of 15 min. The mixture was then allowed to stir for 2 days at room temperature, and the product was precipitated by addition of excess diethyl ether. A crude oily product was obtained by centrifugation. The oil was triturated with diethyl ether three times and redissolved in a small volume of acetonitrile. Excess (ca. 1 g) tetrabutylammonium bromide was added to this solution, and the product was precipitated by addition of excess ethanol. Yield: 63 mg, 63%. IR (KBr pellet cm−1): vC−H 2962 (m), vC−H 2935 (sh), vC−H 2847 (m), vCO 1694, vC−H 1483 (m), vC−H 1380 (w), vP−O 1070 (s), vWO 963 (s), υW−O−W 886 (s), W−Oc−W 802 (s), 662 (w), vP−O 514 (m), 381 (s) (α-isomer), 333 (w) (α-isomer). NMR: 1H NMR (CD3CN, 300 MHz): δ = 7.68 (d + dd, JH−H = 8.0 Hz, JSn−H = 96.0 Hz, 2H, Ar− H), 7.49 (d + dd, JH−H = 8.0 Hz, JSn−H = 34.0 Hz, 2H, Ar−H), 3.93 (s, 2H, −CH2−SCOCH3), 3.14 (m, 32 H, N−CH2−CH2−CH2−CH3), 2.40 (s, 3H, −CO−CH3), 1.65 (m, 32 H, N−CH2−CH2−CH2−CH3), 1.43 (sextuplet, JH−H = 7.5 Hz, 32H, N−CH2−CH2−CH2−CH3), 1.0 (t, JH−H = 7.5 Hz, 48H, N−CH2−CH2−CH2−CH3). 31P NMR (CD3CN, 121 MHz): δ = −10.75 (s + d, JSn−P = 24.0 Hz). Elemental analysis for C75H153N4O40PSSnW11 (%): Calculated: C 22.78, H 3.90, N 1.42. Found: C 22.66, H 3.82, N 1.46. Synthesis of Thioacetate Modified POM with Long Alkyl Chain. [TBA]4[PW11O39SnC6H4CC(CH2)4SC(O)CH3] (5) Two-Step Procedure. Step 1: Synthesis of [TBA]4[PW11O39SnC6H4CC(CH2)4OH] (4).
on the orientation of the adsorbate. The distance tunneling parameters obtained suggest that the orientation of the adsorbate relative to the electrode depends on the applied potential. The implications of these observations for the development of practical devices is considered.
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EXPERIMENTAL SECTION
Materials. Unless otherwise stated, chemicals were reagent grade and were used as received. The precursor organometallic polyoxotungstostannate [TBA]4[PW11O39SnC6H4I] (1) was synthesized as described previously.26 S-Hex-5-yn-1-yl ethanethioate was synthesized adapting a published procedure.27 Synthesis of Thioacetate-Modified POM with Short Alkyl Chain: [TBA]4[PW11O39SnC6H4CCCH2SC(O)CH3] (3). Step 1: Synthesis of [TBA]4[PW11O39SnC6H4CCCH2OH] (2). Under an argon atmosphere, a mixture of [TBA]4[PW11O39SnC6H4I] (1, 1000 mg, 0.25 mmol, 1 equiv), Pd(PPh3)2Cl2 (10.5 mg, 0.015 mmol, 6%), and CuI (1 mg, 0.015 mmol, 6%) was prepared in 15 mL of distilled DMF. Propargyl alcohol (28 mg, 0.5 mmol, 2 equiv) was added after gently bubbling argon through the solution for 10 min. Freshly distilled triethylamine (505.6 mg, 5 mmol, 20 equiv) was added to the mixture with vigorous stirring. The reaction mixture was allowed to stir under a blanket of argon overnight at room temperature, and the product was precipitated using excess diethyl ether. A crude oily product was obtained by centrifugation. The oil was triturated with ethanol and then redissolved in a small volume of acetonitrile. Excess (ca. 1 g) tetrabutylammonium bromide was added, and the product was precipitated by addition of excess ethanol. Yield: 530 mg, 53%. IR (KBr pellet, cm−1): vC−H 2962 (m), vC−H 2934 (sh), vC−H 2847 (m), vC−H 1483 (m), 1381 (w), νP−O 1069 (s), νWO 963 (s), υW−O−W 886 (s), W−Oc−W 813 (s), vP−O515 (m), 381 (s) (α-isomer), 333 (w) (αisomer). NMR: 1H NMR (CD3CN, 300 MHz): δ = 7.70 (d + dd, JH−H = 8.5 Hz, JSn−H = 96.0 Hz, 2H, Ar−H), 7.53 (d + dd, JH−H = 8.5 Hz, JSn−H = 34.0 Hz, 2H, Ar−H), 4.40 (d, J = 6.0 Hz, 2H, −CH2−OH), 3.16 (m, 32 H, N−CH2−CH2−CH2−CH3), 1.65 (m, 32 H, N−CH2− 4510
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Alumina powders of sizes 0.05, 0.3, and 1.0 μm were received from CH Instruments. Water was purified using a Milli-Q water purification system (18 MΩ cm−1). Film Formation. Gold working electrodes (2 mm, CH Instruments) were successively polished with 1.0, 0.3, and 0.05 μm alumina powder followed by sonication in water. The electrodes were then immersed in mild piranha solution (a mixture of 30% H2O2 and concentrated H2SO4 in a 1:3 ratio) for 10 min. (Caution! Care should be taken while treating with piranha solution as it is very reactive toward the organic compounds and is highly corrosive.) The electrodes were then rinsed with Milli-Q water and finally cycled in a 0.5 M H2SO4 solution from −0.400 to +1.400 V (vs Ag/AgCl) at a scan rate of 100 mV s−1 for 10 cycles. The electrodes were again rinsed with Milli-Q water immediately after cycling. Monolayers were then fabricated by immersing the clean electrode in 4 mM solutions of POM in acetonitrile for 24 h. After this time the electrodes were rinsed carefully with pure solvent (acetonitrile) and dried under a stream of N2. Electrochemistry. Electrochemical measurements were carried out using a CHI 660 potentiostat and a conventional three-electrode cell. The working electrode was a 2 mm diameter gold macrodisk, the counter electrode was a large area Pt wire, and the reference electrode was Ag/AgCl (aqueous) or a silver wire in contact with 0.01 M AgNO3 (nonaqueous). For voltammetric measurements in the [BMIM][BF4] ionic liquid, a Pt wire was employed as the pseudoreference electrode, which was calibrated versus the cobaltocene/cobaltocenium (Cc/Cc+) redox process.28 All voltammetric measurements were carried out at room temperature after deoxygenating with N2 for 15 min. Raman Spectroscopy. Raman spectra were collected on a Horiba Jobin Yvon HR800 UV spectrometer with 785 nm excitation. A 50× microscope objective was used to focus the laser beam onto a gold macrodisk electrode covalently modified with a layer of POM. A 600 lines/mm diffraction grating was employed, providing data at 0.5 cm−1 resolution. The x-axis was calibrated versus the Rayleigh line (0 nm) and the phonon mode from a silicon wafer (520 cm−1).
[TBA]4[PW11O39SnC6H4I] (1, 100 mg, 0.025 mmol, 1 equiv), Pd(PPh3)2Cl2 (2.6 mg, 3.7 μmol, 15%), CuI (0.7 mg, 3.7 μmol, 15%), and 5-hexyn-1-ol (7.4 mg, 0.075 mmol, 3 equiv) were dissolved in 1.5 mL of freshly distilled DMF under a blanket of argon. Freshly distilled triethylamine (51 mg, 0.5 mmol, 20 equiv) was added to the solution after 3 min of gentle argon bubbling. The reaction mixture was stirred vigorously under an argon atmosphere overnight at room temperature. Excess (ca. 1 g) tetrabutylammonium bromide was added, and the product was precipitated using excess diethyl ether. A crude oily product was obtained by centrifugation. The oil was dissolved in a small volume of acetonitrile and precipitated by addition of excess ethanol. Yield: 59 mg, 59%. IR (KBr pellet, cm−1): vC−H 2962 (m), vC−H 2936 (sh), vC−H 2874 (m), vC−H 1483 (m) and 1380 (m), νP−O 1069 (s) and 1030, vWO 963 (s), υW−O−W 886 (s), W−Oc−W 813 (s), vP−O 515 (m), 380 (s) (α-isomer), 333 (m) (α-isomer). NMR: 1H NMR (CD3CN, 300 MHz): δ = 7.66 (d + dd, JH−H = 8.0 Hz, JSn−H = 96.0 Hz, 2H, Ar−H), 7.48 (d + dd, JH−H = 8.0 Hz, JSn−H = 34.0 Hz, 2H, Ar−H), 3.57 (d, J = 5.5 Hz, 2H, −CH2−OH), 3.16 (m, 32H, N−CH2−CH2−CH2−CH3), 2.53 (t, JH−H = 5.5 Hz, 1H, −CH 2 −OH), 2.49 (m, 2H, CC−CH 2 − ), 1.65 (m, 36H, N−CH2−CH2−CH2−CH3 + −CH2−CH2−), 1.43 (sextuplet, JH−H = 7.5 Hz, 32H, N−CH2−CH2−CH2−CH3), 1.0 (t, JH−H = 7.5 Hz, 48H, N−CH2−CH2−CH2−CH3). 31P NMR (CD3CN, 121 MHz): δ = −10.75 (s + d, J Sn−P = 24.0 Hz). Elemental analysis for C76H157N4O40PSnW11 (%): Calculated: C 23.17, H 4.02, N 1.42. Found: C 22.68, H 3.83, N 1.47. Step 2: Synthesis of [TBA]4[PW11O39SnC6H4CC(CH2)4SC(O)CH3] (5). 40.4 mg of diisopropyl azodicarboxylate (DIAD) (4 equiv, 0.2 mmol) was added dropwise over 5 min to an ice-cold solution of triphenylphosphine (42.5 mg, 4 equiv, 0.2 mmol) in THF (1 mL) under an argon atmosphere. After vigorous stirring for 30 min under ice cooling, a solution of thioacetic acid (15.2 mg, 4 equiv, 0.2 mmol) and 4 (200 mg, 1 equiv, 0.05 mmol) in 500 μL of DMF was cannulated into the DIAD solution over a period of 5 min under vigorous stirring. The reaction mixture was allowed to stir for 2 days at room temperature under a blanket of argon. The crude product was precipitated by addition of excess diethyl ether and collected by centrifugation. The powder product was dissolved in a small quantity of acetonitrile and was precipitated using an excess of ethanol to give an off-yellow powder. Yield: 161 mg, 81%. One-Step Procedure. [TBA]4[PW11O39SnC6H4I] (1, 150 mg, 0.038 mmol, 1 equiv), Pd(PPh3)2Cl2 (4.0 mg, 5.7 μmol, 15%), CuI (1.1 mg, 5.7 μmol, 15%), and S-hex-5-yn-1-yl ethanethioate (17.7 mg, 0.11 mmol, 3 equiv) were dissolved in 3.0 mL of freshly distilled DMF under a blanket of argon. Freshly distilled triethylamine (76 mg, 0.76 mmol, 20 equiv) was added to the solution after 3 min of gentle argon bubbling. The reaction mixture was stirred vigorously under an argon atmosphere overnight at room temperature. Excess (ca. 1 g) tetrabutylammonium bromide was added, and the product was precipitated using excess diethyl ether. A crude oily product was obtained by centrifugation. The oil was dissolved in a small volume of acetonitrile and precipitated by addition of excess ethanol. Yield: 120 mg, 79%. IR (KBr pellet, cm−1): vC−H 2962, vC−H 2935, vC−H 2874, vCO 1686, vC−H 1483, vP−O 1069, vP−O 1030, vWO 963, υW−O−W 886, W−Oc−W 813, vP−O 514, 381 (α-isomer), 333 (α-isomer). NMR: 1H NMR (CD3CN, 300 MHz): δ = 7.66 (d + dd, JH−H = 8.0 Hz, JSn−H = 96.0 Hz, 2H, Ar−H), 7.48 (d + dd, JH−H = 8.0 Hz, JSn−H = 34.0 Hz, 2H, Ar−H), 3.16 (m, 32 H, N−CH2−CH2−CH2−CH3), 2.95 (t, JH−H = 7.0 Hz, 2H, −CH2−S−), 2.48 (t, JH−H = 7.0 Hz, 2H, −CC− CH2−), 2.33 (s, 3H, −COCH3), 1.65 (m, 36 H, N−CH2−CH2− CH2−CH3 + −CH2−CH2−), 1.43 (sextuplet, JH−H = 7.5 Hz, 32H, N− CH2−CH2−CH2−CH3), 1.00 (t, JH−H = 7.5 Hz, 48H, N−CH2−CH2− CH2−CH3). 31P NMR (CD3CN, 121 MHz): δ = −10.75 (s + d, JSn−P = 24.0 Hz). Elemental analysis for C78H159N4O40PSSnW11 (%): Calculated: C 23.44, H 4.01, N 1.40. Found: C 23.47, H 3.97, N 1.49. 1-Butyl-3-methylimidazolium Tetrafluoroborate [BMIM][BF4] (Sigma-Aldrich, ≥98.5% (HPLC)) was used as purchased; H2SO4, 95−97%, H2O2, HClO4, tetrabutylammonium hexafluorophosphate, cobaltocenium (98%, Strem Chemicals). All other chemicals were of reagent grade and were used as received unless otherwise stated.
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RESULTS AND DISCUSSION Synthesis. As illustrated in Scheme 1, two thioesterterminated hybrid polyoxometalates [TBA] 4 [PW 11 O 39 SnC 6 H 4 CCCH 2 SC(O)CH 3 ] (3) and [TBA]4[PW11O39SnC6H4CC(CH2)4SC(O)CH3] (5) have been synthesized, with either a short or a long link between the POM backbone and the grafting function. A common strategy allowing the postfunctionalization of POMs relies on coupling the versatile iodoaryl-functionalized POM platform [TBA]4[PW11O39SnC6H4I] (1)26 to an organic tether bearing a terminal alkyne function through a Sonogashira cross-coupling reaction. In keeping with this strategy, we attempted to synthesize the short alkynylthioester (3-acetylthio-1-propyne) using a reported synthetic procedure29 and to couple it to compound 1. However, numerous side products were obtained together with compound 3 probably due to the low stability of the short alkynyl thioester. A two-step procedure involving the prior formation of the alcohol-terminated hybrid [TBA]4[PW11O39SnC6H4CCCH2OH] (2) followed by a Mitsunobu reaction was thus developed. In the case of the long chain, both synthetic pathways could be implemented successfully, i.e., either directly from reaction between compound 1 with the corresponding alkynylthioester or in a two-step procedure involving the formation of the alcoholterminated [TBA]4[PW11O39SnC6H4CC(CH2)4OH] (4). The ability of thioesters to form films on metals such as gold is well-known,30,31 and the POMs were immobilized by placing either a gold macrodisk working electrode or a piece of silicon wafer coated with evaporated gold into a 4 mM solution of the 4511
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the surface and immobilization will also change symmetry and polarizability of the molecule. As expected, the POM-based modes are relatively significantly weaker in the SAM as this unit will be remote from the surface. The modes at 990, 215, and 230 cm−1 are reduced in intensity, as is the P−O stretch at 1070 cm−1. The alkyne CC symmetrical stretch modes observed at approximately 2240 and 2276 cm−1 shift to higher energy to approximately 2320 and 2356 cm−1 in each SAM, indicating shortening/strengthening of this bond on monolayer formation. The influence is the same in each monolayer regardless of the linker length. The modes at 350 and 370 cm−1; 430, 480, and 510 cm−1; and in particular 1150 and 1190 cm−1 are all increased in relative intensity in the surfaceenhanced spectra. These modes are all associated with the organic linker moiety. The organo-tin modes barely evident in the powder appear at approximately 550 cm−1 in the film. Also, the number of additional modes observed upon immobilization is greater for the POM with the longer linker, suggesting a variety of different binding modes, adsorptions, or side reactions. Cyclic Voltammetry. Voltammetry can provide insights into the energetics and dynamics of heterogeneous electron transfer that underpins successful applications from electrocatalysis to solar energy conversion. The nature of the solvent,37 including ionic liquids,38 profoundly affects the voltammetry of polyoxometalates; e.g., the formal potentials shift to significantly more positive values in aqueous media compared to acetonitrile due to its stronger Lewis acid character. Moreover, the mechanism by which the redox state of a solid layer can be switched when in contact with an ionic liquid has been described.39 For these monolayers the impact of solvent is especially pronounced, and while both POMs are redox-active when dissolved in aqueous or organic solvents, the films show no discernible redox activity when voltammetrically cycled in contact with a conventional aqueous or organic solvent-based supporting electrolyte solution. This result most likely arises from the reductions moving outside the available potential window. In sharp contrast, as shown in Figure 2, in the ionic liquid [BMIM][BF4], well-defined voltammetric responses associated with the POMs are observed.
thiolated POM in acetonitrile overnight, followed by rinsing with acetonitrile and drying in air. Raman Spectroscopy. Spectroscopies, such as XPS32 and Raman,33 can provide significant insights into the structure of adsorbed films. The intensity of the Raman spectral profile of a given analyte can be significantly enhanced when the species is adsorbed on or near a plasmonic metal surface, in particular gold, and when the incident excitation wavelength overlaps with the plasmon absorption frequency of the metal.34 Thus, surface-enhanced Raman spectroscopy, SERS, has been extensively used to characterize SAMs and has the sensitivity to allow films where the surface coverage, Γ, is less than a single monolayer (≲10−10 mol cm−2) to be studied.35 The detection of a SERS signal is aided when the gold electrode substrate has been electrochemically roughened prior to adsorption in order to both increase the surface area and create local plasmonic nanostructures at the metal interface. Figure 1 illustrates the Raman spectra of both POM-thiols as both solids and after adsorption onto a gold electrode.
Figure 1. Raman spectra of the long and short linker POM as solids and thin films. The solids were dispersed in KBr disks at approximately 10% w/w. In all cases, excitation was at 785 nm.
Figure 1 compares the Raman spectroscopy, excited at 785 nm, of solid samples of [TBA] 4 [PW 11 O 39 SnC 6 H 4 C CCH2SC(O)CH3] (3) (short chain POM) and [TBA]4[PW11O39SnC6H4CC(CH2)4SC(O)CH3] (5) (long chain POM) with SAMs of these materials on gold. Raman of the solid samples reveal, as expected, that the spectra of both POMs are broadly similar and are dominated by the terminal W−O stretch at approximately 990 cm−1, which is characteristic of Keggin-type polyoxotungstates.36 Other W−O stretch and bend modes are visible at 915 and 215 cm−1, respectively. The P−O asymmetric stretch is weak and appears at approximately 1070 cm−1. In addition, weak modes associated with pdisubstituted aryl linker (1580 cm−1), CC symmetrical stretch (2230 and 2270 cm−1), C−S (715−750 cm−1), and CH3 (2880, 2950, and 1460 cm−1) could also be observed. The acyl carbonyl stretch expected at approximately 1700 cm−1 was not observed in either solid or SAM form most likely due to the large dipole moment of this group. Significant changes occur to the spectra upon adsorption of each complex to the roughened Au electrode. These very significant changes provide good evidence of surface immobilization as this will cause strongest surface enhancement of vibrational modes of moieties close to
Figure 2. Scan rate dependence of the voltammetry for a film of the short linker POM on an Au macrodisk electrode in deaerated [BMIM][BF4]. Scan rates from top to bottom are 7, 6, 5, 4, 3, and 2 V s−1. The reference electrode was a Ag wire calibrated vs the Cc/Cc+ redox couple. The surface coverage is 1.2 × 10−11 mol cm−2. 4512
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be obtained by fitting the full voltammograms obtained at high scan rates where the rate of heterogeneous electron transfer influence the response. Figures 3 and 4 show the background
The redox potentials of the surface confined and solution phase species dissolved in MeCN are similar. Figure 2 shows the two redox couples labeled as I/I′ and II/II′ with E1/2 values of −1.128 and −1.436 V vs Cc/Cc+, respectively. For an ideally reversible surface-confined redox couple, a zero peak-to-peak separation, ΔEp, is expected. For the two couples, ΔEp values of 39 and 55 mV are observed even at very slow scan rates, suggesting that some processes, e.g., redox-induced structural changes or ion pairing, are kinetically slow relative to the time scale of the experiment. However, for both monolayers, the peak current increases linearly with increasing scan rate for values up to 30 V s−1, indicating that the rate of heterogeneous electron transfer is fast in these systems.40 The full width at half-maximum values, 110 ± 10 mV, are somewhat larger than the value of 90.6 mV expected for a surface-confined species undergoing a one-electron-transfer system where there are no lateral interactions between adsorbates. This result suggests that there are weak destabilizing interactions between the adsorbates. The surface coverages, Γ, as estimated from the charge passed after correction for double layer charging, were 1.2 ± 0.1 × 10−11 and 3.7 ± 0.2 × 10−12 mol cm−2 for the short and long linker systems, respectively. This coverage is significantly lower than that expected for a close-packed monolayer, approximately 1 × 10−10 mol cm−2. The surface coverage of both linker lengths does not increase for deposition times longer than 24 h; i.e., the submonolayer coverages do not reflect slow adsorption rates. The low coverages may arise from not all adsorbates being electroactive, but incomplete hydrolysis of the thioester prior to assembly may also be important. That the limiting surface coverage depends on the linker length suggests that they are not fully extended away from the electrode; i.e., the adsorbate may lie close to the electrode surface, and its apparent area of occupation depends on the linker length. This issue is addressed in more detail later through investigations of the distance-dependent rate of heterogeneous electron transfer. The two Γ values correspond to mean molecular areas of occupation of approximately 15 and 45 nm2 for the short and long linkers, respectively. These areas suggest that the adsorbates are separated by distances of the order of 1−4 nm. The stability of these films toward electrochemical cycling in [BMIM][BF4] was also probed. After 100 redox cycles, the films of the short and long linker showed a 5% and 7% loss in peak current, respectively. This high degree of stability is important for practical applications and is consistent with gold−sulfur bonding. The kinetics of heterogeneous electron transfer across the electrode−film interface are of critical importance in determining the practical utility of these redox active films, and there have been many significant contributions to understanding distance-dependent heterogeneous electron transfer.41,42 The anodic and cathodic peak potentials do not shift with increasing scan rate, ν, for 50 < ν < 1000 mV s−1. Even in the absence of ohmic drop, at higher scan rates the peak potentials shift progressively to increasingly more positive and negative potentials, respectively. The shift in the peak potentials with increasing scan rate is not ideally linear; i.e., there is not a sharp transition at which the rate of heterogeneous electron transfer influences the voltammetry, most likely due a distribution of rate constants corresponding to different adsorbate orientations. However, the voltammetry indicates that the standard heterogeneous electron transfer rate constant is of the order of 150 s−1. A better insight into the electron transfer dynamics can
Figure 3. Cyclic voltammogram for a film of the short linker POM on an Au macrodisk electrode in deaerated [BMIM][BF4] (solid line) at a scan rate of 30 V s−1. The open circles denote the fit obtained using the Butler−Volmer formulation of electrode kinetics for a surface confined reactant where the standard heterogeneous electron transfer rate constants are 170 and 150 s−1 for the redox processes centered at approximately −0.95 and −1.26 V, respectively. The reference electrode was a Ag wire calibrated vs the Cc/Cc+ redox couple. The surface coverage is 1.2 × 10−11 mol cm−2.
corrected cyclic voltammograms obtained at a scan rate of 30 V s−1 for the POMs with the short and long linkers, respectively. These figures show that two well-defined reduction processes are observed for both POMs and that a significant ΔEp exists which does not arise from iR drop. For both linkers, the ΔEp
Figure 4. Cyclic voltammogram for a film of the long linker POM on an Au macrodisk electrode in deaerated [BMIM][BF4] (solid line) at a scan rate of 30 V s−1. The open circles denote the fit obtained using the Butler−Volmer formulation of electrode kinetics for a surface confined reactant where the standard heterogeneous electron transfer rate constants are 140 and 100 s−1 for the redox processes centered at approximately −1.05 and −1.35 V, respectively. The reference electrode was a Ag wire calibrated vs the Cc/Cc+ redox couple. The surface coverage is 3.7 × 10−12 mol cm−2. 4513
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value for the second reduction is larger than the first, suggesting that its rate of heterogeneous electron transfer is lower. Moreover, the ΔEp values for the shorter linker are smaller than those observed for the longer linker, suggesting a faster rate of electron transfer. These figures also show the best fits obtained using the Butler−Volmer formulation of electrode kinetics for a surface confined redox couple. This simple model is capable of reproducing the general peak shapes, peak currents, and peak potentials. For the short and long linkers, the best fit is obtained using standard heterogeneous electron transfer rate constants of 170 and 140 s−1 for the first reduction and 150 and 100 s−1 for the second reduction processes, respectively. These values are consistent with that recently reported for a similar POM grafted onto glassy carbon by a dediazonation procedure and disclose the effect of the linker.28 While the rate of electron transfer for the shorter linker is faster than the long linker as expected, given that the difference in electron transfer distance would be of the order of 3 nm (assuming linkers fully extended), the difference in k° values is strikingly small; i.e., an apparent distance dependence tunneling parameter, β, is approximately 0.06 and 0.12 Å−1 for the first and second redox processes, respectively. These very low apparent β values suggest that the POM moieties lie flat against the surface at least at the negative potentials of the POM-based redox processes. Blocking Experiments. The voltammetric data indicate that the surface coverage is less than a close-packed monolayer. An insight into the distribution of the adsorbates on the surface can be obtained by probing the ability of the films to block electron transfer to an electroactive species, such as ferrocyanide and ruthenium hexamine, in solution. Figure 5
metric response at the bare electrode can be modeled as a reversible redox process under semi-infinite linear diffusion control with a standard heterogeneous electron transfer rate constant, k°, of 5 × 10−3 s−1 using the Butler−Volmer formalism of electrode kinetics. Figure 5 shows that the response of the short linker monolayer fitted with the optimal k° value of 3.5 × 10−4 s−1. There are slight deviations between the experimental and theoretical responses most likely reflecting either a range of different redox potentials (microenvironments) or rate constants (orientations) for individual adsorbates. Similarly, the response for the long linker can be fitted using a lower k° of 2 × 10−5 s−1. Significantly, the experimental rate of heterogeneous electron transfer depends exponentially on the electrode−probe separation as defined by the POM and a distance-dependent tunneling factor, β, of 1.1 Å. This value is entirely consistent with that observed for electron tunneling across aliphatic bridges.43 The significant difference in the β factors obtained from the electron transfer rates to the POM and ferrocyanide in solution may suggest that the film structure, and hence its barrier properties, depends on the applied potential. Significantly, in contrast to ferrocyanide, the voltammetry of [Ru(NH3)6]3+ in this case is not affected by the presence of the film. The redox couple maintained its reversibility with no variation in peak-to-peak separation value and peak currents. While recognizing that both ferrocyanide and ruthenium hexamine are charged, given that the effective sizes of these two probes are not very different, these results strongly suggest that electrostatic repulsion rather than physical exclusion dominates the blocking behavior observed for ferrocyanide.
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CONCLUSIONS
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AUTHOR INFORMATION
Polyoxotungstates containing covalently linked, surface-active thiols with different linker lengths have been adsorbed onto gold electrodes and their voltammetry explored in the ionic liquid [BMIM][BF4]. Both films exhibit well-defined POMbased redox processes and are stable to electrochemical cycling. The surface coverage of both materials is less than that expected for a dense monolayer. The rate of heterogeneous electron transfer across both linker lengths for both POMbased redox processes has been determined using cyclic voltammetry. The rates are in the range of 100−200 s−1, indicating that the redox state of the film can be switched within a few milliseconds which is important for catalysis, solar energy conversion, and sensor development. Significantly, the distance-dependent tunneling parameter, β, for the two successive POM redox processes is very low, approximately 0.06 and 0.12 Å−1, suggesting that at negative potentials the POM moieties lie close to the electrode surface. In sharp contrast, tunneling to a solution phase species such as ferrocyanide exhibits a β factor of 1.1 Å−1, which is consistent with tunneling across a saturated aliphatic spacer. These results suggests that the film structure depends markedly on the applied potential.
Figure 5. Cyclic voltammogram for a 1 mM solution of K3[Fe(CN)6] in aqueous 0.1 M KCl electrolyte at a gold electrode modified with a film of the short linker POM (solid line) at a scan rate of 5 V s−1 The open circles denote the best fit obtained for a surface confined species using the Butler−Volmer formulation of electrode kinetics and a standard heterogeneous electron transfer rate constant, k°, of 3.5 × 10−4 s−1.
shows a cyclic voltammogram of a 1 mM solution of K3[Fe(CN)6] in aqueous 0.1 M KCl as electrolyte at a gold electrode modified with a film of the short linker POM. The redox response for the ferrocyanide is less reversible at the POM-coated electrode, suggesting a slower rate of heterogeneous electron transfer. This result is significant since a decrease in the current, but no change in the reversibility would be expected if the film consisted of patches of densely packed monolayer interspersed with unmodified regions. The voltam-
Corresponding Authors
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[email protected] (T.E.K.).
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[email protected] (T.M.).
[email protected] (R.J.F.). dx.doi.org/10.1021/la4048648 | Langmuir 2014, 30, 4509−4516
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work arises from a project funded by Science Foundation under Grant No. 07/RFP/MASF386 and 10/IN.1/B3021. A.P. and T.Mc.C. acknowledge the French ‘Ministère des Affaires Etrangères et Européennes’ for a Ulysses-PHC funding.
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