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Facile Fabrication of Photoelectrochemical Assemblies Consisting of Gold Nanoparticles and a Tris(2,2′-bipyridine)ruthenium(II)-Viologen Linked Thiol Yutaka Kuwahara, Tsuyoshi Akiyama, and Sunao Yamada* Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan Received April 18, 2001. In Final Form: July 6, 2001 Facile fabrication of a modified electrode with gold nanoparticles and a tris(2,2′-bipyridine)ruthenium(II)-viologen linked thiol (RuVS) is described. First, the gold nanoparticles (14 ( 1 nm) were precipitated onto the gold electrode in an aqueous solution. Then, this electrode was immersed into a acetonitrile solution containing RuVS, to immobilize it via Au-S bonding. Scanning electron micrographs show agglomeration and island formation of the gold particles on the electrode and 1-3 µm thickness of the deposition. In cyclic voltammetry, a redox couple due to one-electron reduction of the viologen moiety was clearly observed and its intensity increased with increasing the extent of deposition. The anodic photocurrent from the modified electrode ascribed to ruthenium-complex-sensitized redox cycles was clearly observed and was more than 15-fold larger as compared with that of the flat electrode (without gold particles).
Introduction Substantial progress has been made toward fabrication of noble metal (especially gold) nanoparticle-functional molecule superstructures and understanding of their properties, because of their potential applications to chemical, optical, and electronic devices.1 For these purposes, fabrication of the assemblies on conductive supports is of great importance. Gold nanoparticles may be passive structural elements, so that their superstructures can offer porous (high surface area) electrodes, where the local microenvironments can be frequently controlled by organic cross-linking elements to keep interparticle conductivity.2 In addition, gold nanoparticles show unique optical characteristics in the visible region due to surface plasmon oscillations;3 this is advantageous for optical characterization of the gold nanoparticle materials at various environments. Therefore, the assembly of gold nanoparticles on conductive supports is expected to yield well-defined systems of unique (photo)electrochemical properties by the combination with molecular photoactive or electroactive components. In fact, the combination of gold nanoparticles and electrochemically redox-active molecules (such as viologen and quinine) on the conductive supports has been achieved using molecular cross-linkers by Au-S bonding2a,4 and electrostatic interactions.5 As to photoelectrochemically active species, on the other hand, electron donor-acceptor pairs can be very useful photoinduced electron-transfer units.6 Recently, Au-S selfassembled monolayers of electron donor-acceptor pairs or dyads7-9 and triads10 on the gold electrode surface have shown efficient photon-to-current conversion. Quite recently, Willner et al.5f-h reported an alternative method of fabricating multistructures consisting of gold nanoparticles and a porphyrin-bispyridinium or a ruthenium (1) See for example: (a) Nanosystems, Molecular Machinery, Manufacturing and Computation; Drexler, K. E., Ed.; Wiley: New York, 1992. (b) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 1852. (2) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (b) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (3) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410-8426 and references therein. (4) Gittins, D. I.; Bethell, D.; Nichiols, R. J.; Schiffrin, D. J. Adv. Mater. 1999, 11, 737-740.
complex-bispyridinium dyad by electrostatic interactions and their photocurrent responses. To increase the photocurrent efficiency of those assemblies, buildup of donor-acceptor electrode ordering and increase in the density of donor-acceptor units are basically important. In this regard, the use of covalent Au-S bonding (self-assembling) may be superior to the above-described electrostatic method to obtain spatial donor-acceptor electrode alignment; the usefulness of the Au-S bonding has been verified in the monolayer assemblies.7-10 Also, fabrication of the modified electrode with a large area may be practically important. From these viewpoints, we preliminarily investigated a facile method of constructing the assemblies of gold nanoparticles and a tris(2,2′-bipyridine)ruthenium(II)-viologen linked thiol (5) (a) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 13131317. (b) Blonder, R.; Sheeney, L.; Willner, I. Chem. Commun. 1998, 1393-1394. (b) Kharitonov, A. B.; Shipway, A. N.; Willner, I. Anal. Chem. 1999, 71, 5441-5443. (c) Lahav, M.; Gahai, R.; Shipway, A. N.; Willner, I. Chem. Commun. 1999, 1937-1938. (d) Kharitonov, A. B.; Shipway, A. N.; Willner, I. Anal. Chem. 1999, 71, 5441-5443. (e) Lahav, M.; Shipway, A. N.; Willner, I. J. Chem. Soc., Perkin Trans. 2 1999, 1925-1931. (f) Lahav, M.; Gabriel, T.; Shipway, A. N.; Willner, I. J. Am. Chem. Soc. 1999, 121, 258-259. (g) Shipway, A. N.; Lahav, M.; Willner, I. Adv. Mater. 2000, 12, 993-998. (h) Lahav, M.; Heleg-Shabtai, V.; Wasserman, J.; Katz, E.; Willner, I.; Du¨rr, H.; Hu, Y.-Z.; Bossmann, S. H. J. Am. Chem. Soc. 2000, 122, 11480-11487. (6) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer; Wiley-VCH: New York, 1993. (7) (a) Yamada, S.; Kohrogi, H.; Matsuo, T. Chem. Lett. 1995, 639640. (b) Yamada, S.; Koide, Y.; Matsuo, T. J. Electroanal. Chem. 1997, 426, 23-26. (c) Koide, Y.; Terasaki, N.; Akiyama, T.; Yamada, S. Thin Solid Films 1999, 350, 223-227. (d) Terasaki, N.; Akiyama, T.; Yamada, S. Chem. Lett. 2000, 668-669. (8) (a) Kondo, T.; Ito, T.; Nomura, S.; Uosaki, K. Thin Solid Films 1996, 284/285, 652-655. (b) Yanagida, M.; Kanai, T.; Zhang, X.-Q.; Kondo, T.; Uosaki, K. Bull. Chem. Soc. Jpn. 1998, 71, 2555-2559. (c) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367-8368. (d) Kondo, T.; Kanai, T.; Iso-o, K.; Uosaki, K. Z. Phys. Chem. 1999, 212, 23-30. (e) Kondo, T.; Yanagida, M.; Zhang, X.-Q.; Uosaki, K. Chem. Lett. 2000, 964-965. (9) (a) Akiyama, T.; Imahori, H.; Ajawakom, A.; Sakata, Y. Chem. Lett. 1996, 907-908. (b) Imahori, H.; Ozawa, S.; Ushida, K.; Takahashi, M.; Azuma, T.; Ajavakom, A.; Akiyama, T.; Hasegawa, M.; Taniguchi, S.; Okada, T.; Sakata, Y. Bull. Chem. Soc. Jpn. 1999, 72, 485-502. (10) (a) Imahori, H.; Yamada, H.; Ozawa, S.; Ushida, K.; Sakata, Y. Chem. Commun. 1999, 1165-1166. (b) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. J. Phys. Chem. B 2000, 104, 2099-2108. (c) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100-110.
10.1021/la010566g CCC: $20.00 © 2001 American Chemical Society Published on Web 08/18/2001
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Figure 1. SEM photographs of the RuVS-AuP/Au electrodes. The RuVS-AuP/Au electrodes are prepared by precipitation of the gold particles from the colloidal solutions: 10 (a, e), 20 (b, f), 30 (c, g), and 50 mL (d, h). Cross-sectional pictures are (e), (f), (g), and (h).
(RuVS) via Au-S bonding on the surface of a gold electrode, intended for larger photocurrent generation.
Experimental Section The preparation procedure of RuVS has been described previously.7b Chlorauric acid (HAuCl4) and other chemicals were used as received. Cyclic voltammetric (CV) and differential pulse voltammetric (DPV) measurements of a modified (working) electrode were carried out using a platinum counter electrode and a Ag/AgCl (sat. KCl) reference electrode in aqueous solutions containing 0.1 M NaClO4. Photocurrent measurements were carried out in an aqueous NaClO4 (0.1 M) solution by using a three-electrode photoelectrochemical cell, consisting of the modified electrode, the Ag/AgCl (sat. KCl) reference electrode, and the platinum counter electrode. All measurements were carried out in the presence of 0.05 M triethanolamine (TEOA) as a sacrificial reagent under a nitrogen atmosphere.7 An aqueous solution of colloidal gold nanoparticles was prepared according to the previous method.11 Briefly, an aqueous solution of chlorauric HAuCl4 (25 mg/190 mL) and 10 mL of 1% sodium citrate solution was added to the boiling solution. The mean diameter of the particles, analyzed by transmission electron microscopy (TEM), was 14 ( 1 nm. A gold electrode was prepared by vacuum deposition of titanium followed by gold onto a glass plate (1.5 × 0.9 × 0.1 cm) at 300 °C.12 The gold electrode was placed at the bottom of a flat-bottomed glass vessel (diameter 3.2 cm), and a certain volume (10, 20, 30, 50 mL) of the above-described colloidal solution was added. Subsequent addition of a certain volume (1, 2, 3, 5 mL) of a saturated aqueous NaClO4 solution caused precipitation of the gold particles onto the surface of the gold electrode; the solution became colorless. After being left overnight, the electrode was immersed into water for washing, followed by drying with a stream of nitrogen gas. Accordingly, the gold nanoparticles (11) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (12) Naotaka, H.; Ye, S.; Uosaki, K. Colloids Surf., A 1999, 154, 201208.
Figure 2. Absorption spectra of the RuVS/Au (a) and RuVSAuP/Au (b-e) electrodes. The RuVS-AuP/Au electrodes are prepared by precipitation of the gold particles from the colloidal solutions: 10 (b), 20 (c), 30 (d), and 50 mL (e). were immobilized on the gold electrode, denoted as AuP/Au electrode (AuP: gold nanoparticle). As a next step, the AuP/Au electrode was immersed into a CH3CN solution of RuVS (1 × 10-3 M for monomer unit) for 3 days. After withdrawal, the electrode was rinsed with CH3CN and dried under nitrogen gas, to obtain the RuVS-gold nanoparticle assembled electrode, denoted as the RuVS-AuP/Au electrode. In a similar manner, RuVS was immobilized on the surface of the gold electrode (without gold nanoparticles), denoted as the RuVS/Au electrode.
Results and Discussion Typical scanning electron microscopy (SEM) photographs of the samples prepared from 10, 20, 30, and 50 mL of the colloidal solutions on the gold electrode are shown in Figure 1. All pictures show that the particles tend to aggregate and form as islands on the gold electrode (bright sites). It is also clear that the bare part (dark sites) of the gold electrode decreases with increasing the number of precipitated gold particles and the deposited structure tends to become macroscopically homogeneous. The crosssectional picture reflects the morphology of a part of the precipitated layer. The thickness of the sample tends to increase as the volume of the colloidal solution used increases from ∼1 to ∼3 µm. Absorption spectra of the RuVS-AuP/Au electrode prepared at different volumes of colloidal solutions (10, 20, 30, 50 mL) are shown in Figure 2. If the particle size is assumed to be 14 nm (corresponding to ∼83 000 atoms), the extinction coefficient of the particle is estimated to be
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Figure 3. Cyclic voltammograms of the RuVS/Au (a) and RuVS-AuP/Au (b-e) electrodes (scan rate, 0.1 V s-1). The RuVS-AuP/Au electrodes are prepared by precipitation of the gold particles from the colloidal solutions: 10 (b), 20 (c), 30 (d), and 50 mL (e). The inset shows differential pulse voltammograms in (a) and (e) (scan rate, 0.02 V s-1).
as large as ∼3 × 108 M-1 cm-1 particle-1.13 However, the characteristic plasmon band at ∼530 nm in the colloidal solution no longer appears and a remarkably broad structure lasting as long as near-infrared regions is observed. This is clearly due to high aggregation of the particles rather than interparticle plasmon coupling,13,14 while the broad band in the 400-500 nm region may be predominantly composed of the absorption band of the ruthenium complex moiety (∼1.3 × 104 M-1 cm-1 at 460 nm). Cyclic voltammograms of the modified electrode in the 0 to -0.8 V region are shown in Figure 3. The redox couple characteristic of the first reduction of the viologen moiety was clearly observed in the -0.3 to -0.6 V region. Its peak current was proportional to the scan rate, indicating immobilization of RuVS. The half-wave reduction potential is independent of the scan rate as is verified from the DPV analysis (inset of Figure 2). Thus, coulometric analysis of the oxidation wave provides the number of immobilized RuVS. Typically, the value of 3.1 × 1015 molecules cm-2 (calculated as the area of the flat electrode) is obtained when 50 mL of the colloidal solution is used (Figure 3e); in this case, the number of deposited gold particles on the electrode is estimated to be 1.9 × 1013 particles cm-2 (as the area of the flat electrode). This translates to an average number of ∼160 immobilized RuVS per gold particle. In the case of the flat electrode (Figure 3a), the surface coverage of RuVS is 1.8 × 1014 molecules cm-2. Thus, more than a 15-fold increase in the number of immobilized RuVS could be achieved.15 Photocurrent action spectra of the modified electrodes in the presence of TEOA (5 × 10-2 M) at 0 V are shown in Figure 4. Each action spectrum showed a clear band (13) (a) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529-3533. (b) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410-8426. (14) Grabarm, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735-743 and references therein. (15) X-ray photoelectron spectroscopy results also indicated the formation of Au-S bonding (162.1 eV S(2p3/2)) in the RuVS-AuP/Au sample. No appreciable peaks due to free S species were detectable.
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Figure 4. Photocurrent action spectra of the RuVS/Au (a) and RuVS-AuP/Au (b-e) electrodes. The RuVS-AuP/Au electrodes are prepared by precipitation of the gold particles from the colloidal solutions: 10 (b), 20 (c), 30 (d), and 50 mL (e). Conditions: [TEOA] ) 5 × 10-2 M, E ) 0 V vs Ag/AgCl, ∆λ ) (16 nm. The solid line shows the absorption spectrum of RuVS in CH3CN. The inset shows the relationship between the photocurrent at 460 nm and the number of RuVS evaluated from cyclic voltammetry.
in the ∼400 to ∼500 nm region, which matched well with the corresponding absorption band of the Ru-complex moiety in solution. These observations indicate that the photocurrent is induced by photoexcitation of the tris(2,2′-bipyridine)ruthenium moiety. Typically, photocurrent intensities at 460 nm are 4.6 µA mW-1 cm-2 for (e) (calculated as the area of the flat electrode) and 0.31 µA mW-1 cm-2 for (a). The IPCE (incident photon to photocurrent efficiency) value for (e), as determined from the photocurrent (A cm-2) at 460 nm (∆λ ) (16 nm) in the three-electrode photoelectrochemical cell, was 0.8%. While the photocurrent is larger for a larger amount of deposited gold particles, it tends to saturate above 30 mL of the colloidal solution used. This must be in part due to insufficient penetration of the incident light into thicker multistructure assemblies, as is suggested from very low transmittance of the light in the absorption spectral measurements (Figure 2). Also, the effects of diffusion of the electrolyte and TEOA in the multistructure electrode should be taken into account. In conclusion, we reported a facile method for the fabrication of a multistructure assembly consisting of gold nanoparticles and a photoactive dyad via covalent Au-S bonding on the surface of a gold electrode. The method has enabled a more than 1 order of magnitude increase in the absolute photocurrents. Skillful optimization of the particle size, film thickness, and deposition and immobilization conditions will certainly increase the photocurrent responses, and the work is now in progress. Acknowledgment. The present study was partially supported by the Grants-in-Aid for Scientific Research (No. 11167266 and No. 12450347) from the Ministry of Education, Science, Sports and Culture, Japan. The authors also thank Professor N. Kimizuka of our department for SEM measurements and the Center of Advanced Instrumental Analysis, Kyushu University, for NMR measurements. LA010566G
