Electroreductive Deposition of Anthraquinone Derivative Attached Au

Electroreductive Deposition of Anthraquinone Derivative Attached Au Clusters: Optical Properties and Scanning Tunneling Microscopy Observation of the ...
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Langmuir 2001, 17, 2363-2370

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Electroreductive Deposition of Anthraquinone Derivative Attached Au Clusters: Optical Properties and Scanning Tunneling Microscopy Observation of the Electrodeposited Cluster Film Mami Yamada, Tamon Tadera, Kenya Kubo, and Hiroshi Nishihara* Department of Chemistry, School of Science, The University of Tokyo, Tokyo 113-0033, Japan Received November 27, 2000. In Final Form: February 9, 2001 Anthraquinone derivative modified Au clusters with a 2.2 nm core diameter prepared by a substitution reaction of octyl thiolate-covered Au clusters with 1-(1,8-dithiaoctyl)anthracene-9,10-dione (AQS) underwent two-step one-electron reduction in aprotic solvents, and the second reduction potential was significantly more negative than that of free AQS. The magnitude of this potential shift became larger for the clusters in association with increase in the number of exchanged anthraquinone-terminated thiolates due to the electronic interaction between the adjacent anthraquinone moieties on the Au cluster surface. By electrochemical reduction of the anthraquinone moieties, a redox-active Au cluster film was formed on the electrode. The energy of the surface plasmon band of the film on ITO was shifted with the change in the solvent refractive index from nd20 ) 1.33 to 1.62, and the V-shaped profile of the energy shift vs nd20 plots is inexplicable by Mie theory. The band was also blue-shifted by 12 nm by changing the potential in the negative direction between -1.0 and -2.0 V vs Ag/Ag+ in Bu4NClO4-MeCN. The scanning tunneling microscopy image revealed that spherical Au clusters aggregated to form an organized assembly at the early stage of the electrodeposition process.

Introduction 1 has seen

The study of nanometer-sized metal particles rapid advances in various areas, e.g., synthetic methods,2 theoretical problems on quantum size effects,3 and construction of high-dimensional particle arrays4 for application to electronic5 and photonic devices.6 In particular, much attention has been focused on alkyl thiolate-covered Au clusters, which are stable in air, soluble in nonpolar organic solvents, and fairly monodispersed.2a These * To whom correspondence should be addressed. Tel & Fax: +813-5841-8063. E-mail: [email protected]. (1) (a) Schmid, G. Chem. Rev. 1992, 92, 1709. (b) Scho¨n, G.; Simon, U. Colloid. Polym. Sci. 1995, 273, 101. (c) Wang, Z. L. Adv. Mater. 1998, 10, 13. (2) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Lin, X. M.; Sorensen, C. M. Chem. Mater. 1999, 11, 198. (c) Porter, L. A., Jr.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378. (d) Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 7409. (e) Reets, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401. (f) Duteil, A.; Schmid, G.; Zaika, W. M. J. Chem. Soc., Chem. Commun. 1995, 31. (g) Horswell, S. L.; Kiely, C. J.; O’Neil, I. A.; Schiffrin, D. J. J. Am. Chem. Soc. 1999, 121, 5573. (h) Chen, S.; Kimura, K. Chem. Lett. 1999, 233. (3) (a) Chen, S.; Murray, R. W. J. Phys. Chem. B 1998, 102, 9898. (b) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (c) Temlpeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (4) (a) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (b) Braun, P. B.; Osenar, P.; Tohver, V.; Kennedy, S. B.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121, 7302. (c) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angw. Chem., Int. Ed. Engl. 1997, 36, 10, 1078. (d) Teranishi, T.; Haga, M.; Shiozawa, Y.; Miyake, M. J. Am. Chem. Soc. 2000, 122, 17, 4237. (e) Shenton, W.; Davis, S. A.; Mann, S. Adv. Mater. 1999, 11, 6, 449. (f) Murakoshi, K.; Nakato, Y. Adv. Mater. 2000, 12, 11, 791. (5) (a) St. John, J.; Coffer, J. L.; Chen, Y.; Pinizzotto, R. F. J. Am. Chem. Soc. 1999, 121, 1888. (b) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848. (c) Lahav, M.; Shipway, A. N.; Willner, I. J. Chem. Soc., Perkin. Trans. 2 1999, 1925. (d) Hroves, J. T.; Ulman, N.; Boxer, S. G. Science 1997, 275, 651. (6) (a) Nasr, C.; Hotchandani, S.; Kim, W. Y.; Schmehl, H. J. Phys. Chem. B 1997, 101, 7480. (b) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065.

clusters exhibit a phenomenon of stepwise charging with single electrons,7 and their surface plasmon (SP) band, which relates to the generation of third-order nonlinear optical properties, can be varied by the surrounding electronic environment.3c,8 The thiolates on the cluster surface are also capable of facile modification with other thiols, which, through exchange reactions, can introduce functionality on the cluster surface.9a Several groups have reported on the syntheses9 and electrochemical properties of Au clusters modified with redox-active species such as ferrocene10 and anthraquinone (AQ).11 We have reported the synthesis and electrochemical behavior of biferrocene derivative modified Au clusters.12 Electro-oxidative deposition of the Au clusters was found when the biferrocene units were oxidized to the biferrocenium(2+) form, resulting in the formation of a stable and adhesive electroactive Au cluster film on the electrode. This result prompted us to examine the possibility of reductive electrodeposition using Au clusters modified with redox species that form multivalent anions, in an attempt to elucidate the mechanism of the electrochemical aggregation process and to expand the type of materials useful for electrodeposition, a technique employed in the (7) (a) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alverez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (b) Brousseau, L. C.; Shultz, D. A.; Feldheim, D. L. J. Am. Chem. Soc. 1998, 120, 7645. (c) Chen, S.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996. (d) Pietron, J. J.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 5565. (8) Ethler, T. T.; Malmberg, N.; Noe, L. J. J. Phys. Chem. B 1997, 101, 1268. (9) (a) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (b) Chen, S.; Huang, K. Langmuir 2000, 16, 2014. (c) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (10) (a) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663. (b) Labande, A.; Astruc, D. J. Chem. Soc., Chem. Commun. 2000, 1007. (11) (a) Ingram, R. W.; Murray, R. W. Langmuir 1998, 14, 4115. (b) Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1999, 103, 4440. (12) Horikoshi, T.; Itoh, M.; Kurihara, M.; Kubo, K.; Hiroshi, N. J. Electroanal. Chem. 1999, 473, 113.

10.1021/la001630h CCC: $20.00 © 2001 American Chemical Society Published on Web 03/23/2001

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fabrication of metal cluster array structures.4 In this investigation, we employed AQ derivative modified Au clusters because a -2 charge can be stored on an AQ moiety. It should be noted that the previous report of an AQ derivative modified Au cluster by Murray et al.11 did not mention the redox reaction of the AQ-/AQ2- couple nor the electrodeposition. We have actually found the electrochemical deposition phenomenon of Au clusters modified with an AQ derivative, 1-(1,8-dithiaoctyl)anthracene-9,10-dione (AQS), and preliminary results of the electrodeposition have been reported in a communication.13 In this paper, we report the details of the redox properties and the electrodeposition behavior of the AQ derivative-modified Au clusters (Aun-AQ). In addition, we report the optical and morphological features of the AunAQ film thus prepared. Experimental Section Chemicals. All solvents and reagents used for syntheses were of extrapure grade and were purchased from Kanto Chemicals and used as received, unless otherwise stated. Dichloromethane and acetonitrile used for UV-vis spectroscopy were HPLC grade (Kanto Chemicals). For electrochemical measurements, dichloromethane, acetonitrile, and toluene of HPLC grade (Kanto Chemicals) and tetra-n-butylammonium perchlorate (Bu4NClO4) of lithium battery grade (Tomiyama Chemicals) were used. Synthesis of 1-(1,8-Dithiaoctyl)anthracene-9,10-dione (AQS). The synthetic procedure was based on procedures outlined in the literature.14 1-Chloroanthraquinone (2.42 g, 10 mmol) and 1,6-hexanedithiol (3.01 g, 20 mmol) were mixed with NaH (60% oil dispersion, 1.80 g, 45 mmol) in THF (200 mL), and the mixture was stirred for 5.5 h at room temperature, poured into icewater (200 mL), and acidified (pH 4, aqueous HCl). The solution was extracted with ether, and the organic solution was washed with water then dried over Na2SO4. The solvent was evaporated in vacuo, and the residue was purified by column chromatography (silica gel, CH3CO2CH3/hexane ) 1:30 v/v); recrystallization from CHCl3-hexane gave orange crystals (1.69 g, 49% yield). Anal. Found: C, 67.1; H, 5.79; S, 18.7. Calcd for C20H20O2S2: C, 67.4; H, 5.66; S, 18.1. IR (KBr) νmax/cm-1: 2564 (S-H), 1664 (CdO). UV-vis (CH2Cl2) λmax/nm: 442, 306. 1H NMR (CDCl3, 270 MHz) δ/ppm: 1.48 (s, 1H), 1.48-1.71 (m, 6H), 1.83 (m, 2H), 2.55 (dd, 2H, J ) 7.7, 15.2 Hz), 3.00 (t, 2H, J ) 7.7 Hz), 7.67-7.83 (m, 4H), 8.21 (dd, 1H, J ) 2.9, 6.4 Hz), 8.29 (dd, 2H, J ) 2.0, 11.6 Hz). Synthesis of Octyl Thiolate-Covered Au Cluster (AunOT). The synthesis followed a standard procedure.2a In short, to a vigorously stirred solution of tetra-n-octylammonium bromide (12.0 g, 21.6 mmol) in 518 mL of toluene was added HAuCl4‚H2O (2.04 g, 4.97 mmol) in 155 mL of deionized water. The yellow HAuCl4‚H2O aqueous solution became clear quickly, and the toluene phase turned orange-brown as the AuCl4- was transferred into it. Octanethiol (0.710 g, 4.87 mmol) was added, and the resulting solution was stirred for 10 min at room temperature. To the vigorously stirred solution was added NaBH4 (1.96 g, 25.4 mmol) in 104 mL of deionized water, slowly over 2 min. The very dark organic phase was stirred for 3 h. The organic phase was collected, and the solvent was removed with a rotary evaporator below 50 °C. The black product was suspended in 2 L of ethanol and kept in a refrigerator at -17 °C overnight. It was collected by filtration on a membrane filter and washed thoroughly with ethanol and acetone (1.10 g, 80%). No contamination of free thiol in the product was confirmed by 1H NMR. Synthesis of AQ-Modified Au Cluster (Aun-AQ). The modification of AQ units on the particle surface was performed by a typical exchange reaction9a of AQS with Aun-OT in toluene solution for 48 h at room temperature (Scheme 1). The solvent was removed at reduced pressure, then the sample was collected by filtration and rinsed with ethanol and acetone to remove excessive AQS and displaced octanethiol, respectively. It was confirmed that the product contained no free AQS by 1H NMR (13) Yamada, M.; Kubo, K.; Nishihara, H. Chem. Lett. 1999, 1335. (14) Zang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1998, 14, 4115.

Yamada et al. Scheme 1

and the disappearance of the absorption peak due to ν(S-H) of AQS at 2564 cm-1. Spectroscopy. The 1H NMR spectra of the samples in CDCl3 were collected with a JEOL EX270 spectrometer. Infrared absorbance spectra were acquired using a JASCO FT/IR-620V spectrometer. UV-vis absorption spectra were recorded with a JASCO V-570 spectrometer. Transmission electron microscopy (TEM) images were obtained with a Hitachi HF-2000 microscope. Cyclic Voltammetry. Cyclic voltammetry was carried out in a standard one-compartment cell under an argon atmosphere at 25 °C using a platinum-wire counter electrode and an Ag/Ag+ reference electrode (10 mM AgClO4 in 0.1 M Bu4NClO4-MeCN, E0′(Fc/Fc+) ) 0.20 V vs Ag/Ag+ (Fc, ferrocene)) with a BAS CV50W voltammetric analyzer. Glassy carbon working electrodes 3 mm and 7 µm in diameter were polished with 0.3 mm alumina abrasive, followed by a rinse with distilled water and acetone before each experiment. Electrochemical studies were carried out in 0.1 M Bu4NClO4-toluene + MeCN (2:1 v/v). Electrodeposition of Aun-AQ (the number of exchanged AQ-terminated thiolates on the surface of an Au cluster, θAQ ) 26) was carried out at an indium-tin oxide (ITO)-coated glass electrode (1.0 × 1.5 cm2) with consecutive scans between -1.0 and -2.0 V vs Ag/Ag+ in a solution of the sample in both 0.1 M Bu4NClO4 + CH2Cl2 and 0.1 M Bu4NClO4-toluene + MeCN. The electrodeposited films were rinsed with CH2Cl2 or MeCN, respectively, after the preparation process and dried under vacuum. Spectroelectrochemical Measurements. The differences in the UV-vis spectra of the electrodeposited film at ITO were obtained by changing the potential from 0 to -2.0 V vs Ag/Ag+ with a Toho Technical Research PS-12 potentiostat in 0.1 M Bu4NClO4-MeCN in a three-compartment quartz cell under N2. The samples for the measurement of the refractive index effect and the charge effect were prepared by the cyclic potential scans for 50 and 80 times, respectively, between -1.0 and -2.0 V vs Ag/Ag+ in a solution of Aun-AQ (θAQ ) 26) in 0.1 M Bu4NClO4-CH2Cl2. Scanning Tunneling Microscopy (STM). The STM topographic image of the electrodeposited film on highly oriented pyrolytic graphite (HOPG) was taken at a constant current of 0.5 nA and a bias of 1 V with a Pt/Ir tip with PicoScan (Molecular Imaging). Two samples were prepared by the cyclic potential scans for 20 and 5 times, respectively, between -1.0 and -2.0 V vs Ag/Ag+ in a solution of Aun-AQ (θAQ ) 26) in 0.1 M Bu4NClO4-CH2Cl2 solution.

Results and Discussion Syntheses. A convenient method by which to synthesize alkyl thiolate-covered Au clusters in the liquid phase was reported by Brust et al., who used reduction of HAuCl4 by aqueous NaBH4 in the presence of dodecanethiol as the stabilizer in one organic phase.2a This synthetic procedure can produce Au clusters of a 2.2-2.5 nm core diameter with narrow size distribution, criteria essential for the effective appearance of the cluster’s characteristic features such as single electron charging3a and nonlinear optical properties. Some subsequent papers have reported that the physical nature of the particle depends on several

Electroreductive Deposition

synthetic conditions,15 including the mole ratio of alkanethiol/HAuCl4/NaBH4, temperature, and the alkanethiol chain length. The longer alkanethiol can separate the particles more clearly into individuals, preventing the aggregation. We employed octanethiol, which has the longest chain length that does not bury the redox sites of AQS in the alkyl chain on the cluster surface after the exchange reaction. High-resolution transmission electron microscopy (HRTEM) images of Aun-OT thus prepared determined that the average core diameter of the Au clusters was 2.2 nm (Figure S1, Supporting Information), corresponding to 309 Au atoms of a cuboctahedron core shape bearing approximately 95 octyl thiolate units on one cluster.15 The standard deviation of the core diameter was estimated at 0.5 nm, whose value was almost the same as those reported for the clusters prepared with dodecanethiol. The FT-IR spectrum showed the peaks due to methylene stretching vibration ν(C-H) of octyl thiolate at 2918 and 2848 cm-1, indicative of chain ordering in the solid state.16 The number of exchanged AQ-terminated thiolates on the Aun-OT surface, θAQ, was determined by the ratio of the integrals of the 1H-NMR signals between AQ and methyl protons at ca. 7.5 and 3.0 ppm, respectively. The θAQ value could be controlled by changing the mole ratio of AQS to Aun-OT. Three types of Aun-AQ with θAQ ) 12, 18, and 26 were prepared by 1:16, 1:4, and 1:1 mole feed ratios of AQS to octyl thiolate units on the cluster surface in the exchange reaction. The FT-IR spectrum of Aun-AQ showed that the chain ordering was sustained after the introduction of AQS on the cluster surface, given that no change in the ν(C-H) bands occurred. The new absorption peak, ν(CdO), due to the carbonyl stretching vibration of terminated AQ units, appeared at 1670 cm-1, which is slightly higher than that of free AQS (1664 cm-1). UVvis spectra of Aun-AQ with θAQ ) 26 exhibited a surface plasmon band at 516 nm (max ) 7.7 × 105 M-1 cm-1) and additional absorption bands derived from the AQ moiety at 444 nm (max ) 9.1 × 105 M-1 cm-1) and 316 nm (max ) 1.3 × 106 M-1 cm-1). Electrochemical Behavior of the Aun-AQ. Anthraquinone derivatives usually undergo two-step oneelectron reduction to radical anion (AQ•-) and dianion (AQ2-) in aprotic solution. The first reduction step of anthraquinone at ca. -1.2 V vs Fc/Fc+ is reversible, while the second step at ca. -2.0 V is generally irreversible due to slow electron transfer by electronic mediation among the radical anion species. The redox behavior of the second step of quinones reflects the effective electronic environments in some systems.17 We directed our attention to the micro self-assembly (SAM) system of the AQ thiol derivative on the Au cluster surface, which exists under intermediate conditions between rigid and nonrigid molecular assemblies. In this system, the AQ units can move rather freely compared with their movement in the SAM on the single-crystal flat surface. Many papers on the interactions among the adjacent redox centers in SAMs of organosulfur compounds, including quinone derivatives on the bulk Au surface, have been published.14,18 We investigated whether (15) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vochet, R. W.; Clark, M. R.; Londono, D.; Green. S. J.; Stokes, J. J.; Wignall, G. D.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (16) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 2, 1262. (17) Gupta, N.; Linschitz, H. J. Am. Chem. Soc. 1997, 119, 6384. (18) (a) Katz, E.; Willner, I. Langmuir 1997, 13, 2264. (b) Katz, E. Y.; Solov’ev, A. A. J. Electroanal. Chem. 1990, 291, 171. (c) Xu, J.; Chen, Q.; Swain, G. M. Anal. Chem. 1998, 70, 3146.

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Figure 1. Cyclic voltammograms of 107 µM AQS (a) and 10 µM Aun-AQ (θAQ ) 26) (b) at a 7 µm diameter GC disk in 0.1 M Bu4NClO4-toluene + MeCN (2:1 v/v) at 20 mV/s vs Ag/Ag+. Inset: cyclic voltammograms of 1.5 mM AQS (a) and 25 µM Aun-AQ (θAQ ) 26) (b) at a 3 mm diameter GC disk at 100 mV/s in the same electrolyte solution.

Figure 2. Cyclic voltammograms of 11 µM Aun-AQ (θAQ ) 12) (a), 20 µM Aun-AQ (θAQ ) 18) (b), and 11 µM Aun-AQ (θAQ ) 26) (c) at a 7 µm-diameter GC disk in 0.1 M Bu4NClO4toluene+MeCN (2:1 v/v) at 20 mV/s vs Ag/Ag+. Inset: cyclic voltammograms of the same Aun-AQ solution at a 3 mmdiameter GC electrode at 50 mV/s.

the AQ units on the cluster surface display some specific features analogous to those of SAM on a flat surface, by investigating the effects of θAQ on the second-step redox reaction. Cyclic voltammograms of AQS and Aun-AQ at a 7 µm diameter glassy carbon (GC) microelectrode in 0.1 M Bu4NClO4-toluene + MeCN (2:1 v/v) show a two-step oneelectron reduction wave with a limiting current, as displayed in Figure 1. The second reduction potential of Aun-AQ shifts in a more negative direction than that of AQS. Figure 2 demonstrates the dependence of the voltammetric behavior of Aun-AQ on θAQ. The magnitude of the second redox potential shift in the more negative direction is larger for the clusters with higher θAQ, which indicates that the second reduction is strongly affected by the interaction between the adjacent AQ moieties on the cluster surface. This shift is attributed to the decrease in

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Table 1. Electrochemical Properties of AQS and Aun-AQ (θAQ ) 12, 18, and 26) Measured by Microelectrode Voltammetrya compound

E°′1/Va

E°′2

AQS Aun-AQ (θAQ ) 26) Aun-AQ (θAQ ) 18) Aun-AQ (θAQ ) 12)

-1.24 -1.23 -1.23 -1.23

-1.56 -1.60 -1.58 -1.48

a

/Va

106Cdl,Au/(F/cm2) 106D/ 2 (cm /s) AQ0 AQ-1 AQ-2 12 5.2 5.9 9.1

16 8.0 7.2

32 12 9.4

71 29 27

Potentials refer to Ag/Ag+.

Figure 4. Cyclic voltammograms of 6.2 µΜ Aun-AQ (θAQ ) 26) at an ITO electrode (1.5 cm2) in 0.1 M Bu4NClO4-toluene + MeCN (2:1 v/v) at 100 mV/s vs Ag/Ag+. Numbers in the figure refer to those of cyclic scans.

Figure 3. Cyclic voltammograms of Aun-AQ (θAQ ) 26, 5.5 µM) at a 3 mm diameter GC disk in 0.1 M Bu4NClO4-toluene + MeCN (2:1 v/v) at different scan rates vs Ag/Ag+. Numbers in the figure refer to those of scan rate (mV/s) (inset). The plots of the anodic (squares) and cathodic (circles) current of the first redox wave vs the scan rate.

interactions between neighboring AQ centers brought about by the surface dilution of the inert octyl thiolate chain. Given that similar phenomena are reported for other quinone-thiol derivatives adsorbed on single crystalline gold,14 the SAM on the metal cluster is considered to retain the nature of the condensed phase at the solid-solution interface. The parameters obtained by using the microelectrode equations and double-layer charging equations11a are summarized in Table 1. The diffusion coefficient of AQS is 1.5-2.5 times larger than the value of Aun-AQ. It should be noted that the D values might be somewhat perturbed by the physical adsorption of Aun-AQ indicated by the cyclic voltammetry at a 3-mm-diameter GC electrode as described below. The double-layer charging capacitance of Aun-AQ, Cdl,Au, relating to the slope of ∆I/∆E, increases by the accumulation of negative charge on the AQ moiety, namely, Cdl,Au(AQ) at EAQ (see Figure 2) < Cdl,Au(AQ•-) at EAQ- < Cdl,Au(AQ2-) at EAQ2-. This implies that the doublelayer charging of the metal cluster can be controlled stepwise by the multiple redox molecules covering the surface. Neither the cathodic nor the anodic peak current for the first step reaction in the cyclic voltammograms of AunAQ at a 3-mm-diameter GC electrode is proportional to v1/2 (v is the potential scan rate), but both are linear to v, and the peak-to-peak separation is 32 mV, as shown in Figure 3. In the first cyclic scan, the anodic peak current is broad and smaller than the cathodic current, suggesting that the reduction of AQ moieties forms an adsorbed layer where the adjacent Aun-AQs interact more extensively.

Electrodeposition of Aun-AQ. Cyclic voltammograms for the electrodeposition of Aun-AQ with θAQ ) 26 in Bu4NClO4-toluene + MeCN (2:1 v/v) at an ITO-coated glass electrode are shown in Figure 4. Consecutive potential scans between -1.0 and -2.0 V vs Ag/Ag+ exhibit a gradual increase of the peak current. Appearance of only one redox wave indicates the existence of considerable interaction between the hydroxide groups of the ITO base and the AQ moieties of Aun-AQ and/or interparticle interaction among the Aun-AQs in the electrodeposition process. This current increase with irreversible waves is similar to that observed in a solution of Aun-AQ in 0.1 M Bu4NClO4-CH2Cl2, which results in the formation of a more adhesive and thicker Aun-AQ film on the ITO electrode than that made in the toluene/MeCN solution. This suggests that the solvent affects the electrodeposition mechanism and the morphological features of the film. The characteristic features of the cluster film shown below are given mainly for Aun-AQ film electrodeposited in a CH2Cl2 solution. Properties of the Electrodeposited Aun-AQ Film. The UV-vis spectrum of the Aun-AQ film electrodeposited in a CH2Cl2 solution shows broad absorption bands that grow in intensity with increase in the number of potential scans (Figure 5A). An absorption band appears at ca. 560 nm and gradually develops when the electrodeposition proceeds. This band can be regarded as the SP band, redshifted from the SP band of the solution of Aun-AQ in CH2Cl2 at 516 nm. This is attributed to the fact that the interparticle spacing becomes small compared to the incident wavelength, and a new feature corresponding to a collective Aun-AQ SP oscillation grows as the cluster coverage increases.19 The cyclic voltammogram of the electrodeposited AunAQ film exhibits a redox reaction of adsorbed species at E°′ ) -1.26 and -1.81 V vs Ag/Ag+, as shown in Figure 6. The peak current of the first redox wave is proportional to v and the peak-to-peak separation is 19 mV, indicating the redox properties of the surface-immobilized species when the film is thin. The anodic peak is smaller and broader, with a half-peak width of 101 mV, than the cathodic peak, as seen in the cyclic voltammogram of AunAQ in solution. Cyclic voltammograms of the electrodeposited films prepared with different numbers of consecutive scans are shown in Figure 5B. The peak-to-peak separation and half-peak width become larger with (19) Freeman, R. G.; Grabar, K. C., Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629.

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Langmuir, Vol. 17, No. 8, 2001 2367

Figure 7. Estimated coverage of the electrodeposited Aun-AQ films prepared under the same conditions given in Figure 5, calculated by the UV-vis absorption (squares) and the cyclic voltammograms (triangles).

One possible explanation for this phenomenon is that the surface local charge accumulation by the redox molecule on the cluster surface increases the local ionic strength and thus compacts the ionic atmosphere around the cluster particles, resulting in a decrease in electrostatic repulsion between the particles. Consequently, the van der Waalslike force between adjacent Aun-AQs near the electrode becomes dominant, and an aggregation of the cluster particles takes place. The coverage of Aun-AQ on the ITO is evaluated from both the electrochemical and the optical measurements, ΓCV and ΓUV, respectively. They are calculated by the following equations

Figure 5. UV-vis spectra (A) and cyclic voltammograms (B) of electrodeposited Au cluster films prepared in a solution of 6.2 µΜ Aun-AQ (θAQ ) 26) in CH2Cl2 at an ITO electrode (1.5 cm2) at 100 mV/s vs Ag/Ag+. Numbers in the figure refer to those of cyclic scans.

Figure 6. Cyclic voltammograms of the electrodeposited AunAQ film on ITO in 0.1 M Bu4NClO4-MeCN at different scan rates vs Ag/Ag+. Numbers in the figure refer to those of the scan rate in mV/s. Inset: the plots of the anodic (squares) and cathodic (circles) current of the first redox wave vs the scan rate.

increases in the film thickness, suggesting that film resistance prevents the electron transfer in the film. The previous results for biferrocene derivative-modified Au clusters and the present results can be theorized to be examples of a like-charge aggregation20 of metal nanoparticles; the phenomenon can hardly be explained by existing theories, including the well-known DLVO theory.21

ΓCV ) C/2SFθAQ (mol/cm2)

(1)

ΓUV ) A/1000 (mol/cm2)

(2)

where S (cm2) is the area of the electrode, C (C) is the charge for electrodeposition, F (C/mol) is Faraday constant, A is the absorbance at 516 nm in Figure 5A, and  (M-1 cm-1) is the molar extinction coefficient of the SP band at 516 nm for Aun-AQ in CH2Cl2, 7.7 × 105 M-1 cm-1. The values obtained using these equations are given in Figure 7. The values of ΓUV and ΓCV are almost equal below 40 scans, but ΓCV is significantly smaller than ΓUV with increases in the number of scans. This means that the film thickness grows constantly with repeated scans, but its electroactivity is gradually lost. This effect derives from the increase in the film resistance (suppression of electron transfer) and/or the decomposition of redox-active sites in AQ moieties. Assuming that the electrodeposited clusters are packed with a spacing of 8.5 nm diameter based on the STM data (vide infra), the coverage of the Aun-AQ monolayer Γmono is calculated as 2.51 × 10-12 (mol/cm2). The maximum number of Aun-AQ layers is ca. 200, corresponding to the film thickness of ca. 1 µm, if the monolayer thickness is 5 nm. Solvent Refractive Index Effects on Surface Plasmon Absorption of the Aun-AQ Film. The electrodeposited Aun-AQ film is useful to investigate the optical properties of a metal cluster in comparison with those of an individual particle in solution. We examined the specific spectral change in the SP band of the Aun-AQ film by varying the organic solvent medium. The SP band of noble metal sols is quite sensitive to the electronic environment around metal particles including (20) (a) Ke´kicheff, P.; Spalla, O. Phys. Rev. Lett. 1995, 75, 1851. (b) Lasen, A. E.; Grier, D. G. Nature 1997, 385, 230. (21) (a) Derjaguin, B. V.; Landau, L. Acta Physicochim. U.S.S.R. 1941, 14, 633. (b) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic colloids; Elsevier: Amsterdam, 1948.

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the refractive index of the solvent medium,22 but that of the alkanethiolate-covered Au clusters is assumed to be insensitive, because the cluster surface-solvent interface is strictly covered with alkyl thiolate chain and can hardly be affected by the surrounding field.3b Thus, on the basis of Mie theory,23 a precise explanation of the solvent refractive index influence on the SP band for the alkyl thiolate-covered Au clusters is possible only when the protection effect provided by the alkyl thiolate monolayer is considered, which induces the equation3c

λpeak2 λp2

) 12.2 + 2m +

2g(s - m) 3

(3)

where λpeak is the Au clusters’ plasmon wavelength, λp is the bulk gold plasmon wavelength ()131 nm), m is the optical dielectric function of the medium, s is that of the alkyl thiolate shell layer, and g is the volume fraction of the shell layer. The third term on the right side of eq 3 gives the evident differences between a coated and an uncoated particle and predicts that (1) the smaller metal core with longer alkyl thiolate chain gains the value of g, resulting in the red-shift of the SP band position, (2) an increase of both m and m also leads to the lowering of the SP band energy, so that a plot of λpeak2/λp2 vs m should have a slope of 2-(2g/3). The validity of this model is confirmed by the experiment in which the solution spectra of a dodecyl thiolate-covered Au cluster with a 5.2 nm core diameter revealed an 8 nm red-shift with an increase of solvent index from nd20 ) 1.38 (hexane) to nd20 ) 1.55 (o-dichlorobenzene). Our Aun-AQ film has the advantage that the solvent effects can be measured with an expanded range of refractive index values, which minimizes the error in selecting the SP band maximum, while the usage of solvent is restricted to several aprotic ones for the particle in solution because of its insolubility. Figure 8A shows the absorption spectra for the Aun-AQ film as a function of the solvent refractive index from nd20 ) 1.33 (top) to nd20 ) 1.62 (bottom). (Note that the absorbance is normalized for easy recognition of the spectral shift and is not discussed further.) The SP band maximum at 576 nm in water (nd20 ) 1.33) gradually shifts to the higher energy with increases in the refractive index value until it reaches 560 nm in chloroform (nd20 ) 1.44), which next moves toward the lower energy until it reaches 572 nm in carbon disulfide, where it has the largest refractive index value, nd20 ) 1.62. The SP band positions are plotted vs solvent refractive index values in Figure 8B, which yields a V-shaped curve with the minimum wavelength in chloroform (nd20 ) 1.44). This correlation cannot be explained by eq 3, that is, by the explanation that the SP band wavelength simply rises in proportion to the increase of the refractive index value. Additionally, it is notable that the magnitude of the wavelength shift from water (nd20 ) 1.33) to chloroform (nd20 ) 1.44) reaches to 16 nm. As the g value lowers with decreases in the cluster size and g ) 0.68 for the cluster with a 2.2 nm core diameter with a 1.0 nm octyl thiolate monolayer, a detectable change is hardly observed in the spectra of the cluster solution from hexane (nd20 ) 1.38) to benzene (nd20 ) 1.50). Therefore, the SP band dependence on solvent for the Aun-AQ film is incredibly large. These results may be explained by the fact that the SP band of the closely packed Aun-AQ film is defined as a collective oscillation fairly affected by the

cluster-cluster interaction and not identified as the precise SP band of one particle. Another interpretation of these results is that they are explained by the difference in affinity of the solvents to the nonpolar alkyl chain on the clusters, which perturb the solvent-cluster core interaction.24 Potential Dependence of the SP Band of the AunAQ film. The alkyl thiolate-covered Au cluster can be electrolytically charged by one-electron oxidation or by reduction with the applied potential,7 and this charging affects the energy of the SP band. The charge influence of the Aun-AQ film was investigated by observing the spectral change over shifts in the potential. The potential-dependent spectra shown in Figure 9 indicate that the λmax of the SP band is observed at 572 nm at -1.0 V vs Ag/Ag+ in 0.1 M Bu4NClO4-MeCN. The λmax value is shifted to a shorter wavelength by 12 nm, and the absorbance increases with shifts in the potential in the negative direction to -2.0 V. The positive potential shift from -1.0 to 0 V gives the longer λmax and lower absorbance. This feature of spectroscopic dependence on the electrode potential is quite related to the biferrocenemodified Au cluster film, which indicates that the SP band is slightly red-shifted with decreases in the absorbance, which shift the potential in the positive direction between -0.1 and 0.9 V vs Ag/Ag+. This spectral shift can be explained by one or two things: the charge accumulation at the terminal redox units, and/or the direct charging of

(22) Papavassiliou, G. C. Prog. Solid State Chem. 1980, 12, 185. (23) Mie, G. Ann. Phys. 1908, 25, 377.

(24) Elliot, D. J.; Furlong, D. N.; Grieser, F.; Mulvaney, P.; Giersig, M. Colloids Surf., A 1997, 130, 141.

Figure 8. (A) UV-vis spectra of the Aun-AQ film on ITO in variable solvents. Each spectrum was measured from the top to the bottom in water (nd20 ) 1.33), acetonitrile (nd20 ) 1.34), acetone (nd20 ) 1.36), ethanol (nd20 ) 1.36), hexane (nd20 ) 1.37), dichloromethane (nd20 ) 1.42), chloroform (nd20 ) 1.44), benzene (nd20 ) 1.50), o-dichlorobenzene (nd20 ) 1.55), and carbon disulfide (nd20 ) 1.62). (B) Plots of the maximum SP band wavelength vs the solvent refractive index.

Electroreductive Deposition

Figure 9. UV-vis spectra of the electrodeposited Aun-AQ film on ITO at given potentials from 0 to -2.0 V vs Ag/Ag+ in 0.1 M Bu4NClO4-MeCN. Numbers in the figure refer to those of the applied potential in V. Inset: the plots of the SP band maximum vs the applied potential.

the Au metal core. The latter effect should be dominant because of the potential shift from 0 to -1.0 V vs Ag/Ag+, where the redox reaction of AQ units does not occur for the Aun-AQ film, caused the blue-shift of the SP band by 10 nm (Figure 9 inset). The number of electrons provided per cluster by charging from -1.0 to -2.0 V vs Ag/Ag+ is estimated at 5.3, based on the consideration that consecutive single-electron-transfer processes with the potential spacing (∆V) are dependent upon the cluster capacitance (Cclus), ∆V ) e/Cclus.7a It is calculated that Cclus ) 0.96 aF by the equation Cclus ) 4π0r(r + d)/d, where  is the monolayer dielectric constant, r is the core radius, and d is the monolayer thickness. Assuming that

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the shift in the SP band position has occurred by the charge difference in the metal core, the final SP band position (λfinal) at -2.0 V after 5.3-electron provision from -1.0 V vs Ag/Ag+ is predicted at 567 nm by the convenient equation of λfinal/λinit ) (Ninit/Nfinal)1/2, where λfinal and λinit ()572 nm) are the SP band positions after and before the redox reaction of metal core, respectively, and Nfinal and Ninit are the numbers of free electrons per metal core after and before the charging, respectively.3c The discrepancy between the theoretical value of a 5 nm shift and the experimental value of a 12 nm shift should be due to the simplification of the theory that the particles exist individually in solution and that affects the charge accumulation at the terminal redox units. STM Images of the Aun-AQ Film. Many intensive trials for the construction of dimensionally ordered metal nanoparticles have been carried out in an attempt to make effective usage of some exceptional features of metal nanoparticles. The electrodeposition technique is a useful tool for fabricating desired structures and controlling the thickness of surface films. We thus carried out an investigation of the morphological aspects of the Aun-AQ films by STM measurements. Figure 10A is an STM image of the electrodeposited Aun-AQ film on the basal plane of an HOPG electrode. Slight roughness within a level of 40 nm corresponding to eight layers of Aun-AQ with a diameter of ca. 5 nm is seen over a wide region of 700 × 700 nm2. Close examination reveals that there are spherical Au clusters gathered on the aggregates. An enlarged image of the submonolayer cluster film electrodeposited with a smaller number of scans (five scans) exhibits that Aun-AQ aggregates closely to form an organized assembly at the early stage of the electrodeposition process (Figure 10B, left).

Figure 10. (A) An STM image of a multilayer Aun-AQ film electrodeposited on HOPG in the area of 700 × 700 nm2. (B) An STM image of a submonolayer Aun-AQ film on HOPG in the area of 50 × 50 nm2 with the cross-sectional profile along the solid line. The images were taken at constant current of 0.5 nA and a bias of 1 V.

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Small islands of the clusters are heterogeneously constructed at first on the surface, then covering the whole area, and the thickness of the film gradually grows. Clear images can be obtained without the current noise in every region of the surface, in contrast to the reported STM image of the cast alkyl thiolate-covered Au cluster film, which is less clear except along a HOPG lattice step, beside the edges of which the particles could lie.25 This suggests that the Aun-AQ in this electrodeposited film is more closely packed and attached tighter to the basal plane than those prepared by just casting. The cross-sectional view of the figure shows that the separation between the peak of the curve is 8.5 nm, which is a value much larger than the estimated maximal Aun-AQ value of 5.8 nm (the core diameter, 2.2 nm; octyl thiolate, 1.0 nm; and anthraquinone, 0.8 nm). This long intercluster distance was observed for the biferrocene-modified Au cluster film. We found that the type of anions in the electrolyte solution extensively effects the electrodeposition process for the biferrocene-modified Au cluster. Subsequently, we consider that counterions play an essential role in this electrodeposition system and that the countercation Bu4N+, in the case of the Aun-AQ film, gathers around the negatively charged AQ units on the surface of the AunAQ, acting as a spacer between adjacent Aun-AQ of the film (Figure 10B, right). Our preliminary XPS measurements, however, have not found detectable peaks related to nitrogen, and the existence and role of the electrolytes are currently under investigation. (25) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537.

Yamada et al.

Conclusion In this study we have presented the electrodeposition phenomenon of the anthraquinone-functionalized Au cluster with a 2.2 nm core diameter and a 1.0 nm octyl thiolate monolayer (Aun-AQ), on the surface of which the redox centers of AQ units were strongly affected by the interaction between the adjacent AQ moieties, which were analogous to the SAMs on bulk metal. The electrochemical adsorption of Aun-AQ proceeded onto the electrode by electroreduction of the terminated AQ with increase of the electrostatic force among negatively charged Aun-AQ. The SP band of the electrodeposited Aun-AQ film exhibited a unique shift with changes in the solvent refractive index or the applied potential; the shift can be evaluated by the collective aggregates of Aun-AQ, not by the individual Au cluster particle. This electrodeposition system would be applicable for a simple arrangement of ordered metal cluster heteronetworks, created by repeating the oxidative and/or reductive electrodeposition of redox-active clusters in solution. Moreover, construction of a variety of cluster films by the selection of metal, cluster core size, and attached redox species is an interesting possibility and is currently under investigation in our laboratory. Acknowledgment. This work was supported by Grants-in-Aid for scientific research (Nos. 09237101, 11309003, and 12874085) from the Ministry of Education, Science, Sports, and Culture, Japan. Supporting Information Available: The TEM image and the core diameter histogram of Aun-AQ. This material is available free of charge via the Internet at http://pubs.acs.org. LA001630H