Electrodeposition of Biferrocene Derivative-Attached Gold Nanoparticles

Aug 12, 2003 - two-step one-electron oxidation of the biferrocene units of Aun-BFc in 0.1 M ... gold nanoparticles with an anthraquinone derivative (A...
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Langmuir 2003, 19, 8050-8056

Electrodeposition of Biferrocene Derivative-Attached Gold Nanoparticles: Solvent Effects and Lithographic Assembly Mami Yamada† and Hiroshi Nishihara* Department of Chemistry, School of Science, The University of Tokyo, Tokyo 113-0033, Japan Received May 27, 2003. In Final Form: July 8, 2003 The solvent effects of electro-oxidative deposition of octyl thiolate-stabilized gold nanoparticles with 2.3 ( 0.5 nm core diameter modified with biferrocene-terminated alkanethiolates on their surface (Aun-BFc) were investigated using cyclic voltammetry, UV-vis spectroscopy, scanning tunneling microscopy (STM), and electrochemical quartz crystal microbalance (EQCM) techniques. Consecutive potential scans causing two-step one-electron oxidation of the biferrocene units of Aun-BFc in 0.1 M Bu4NClO4-organic solvent [CH2Cl2, tetrahydrofuran (THF), toluene + acetonitrile (toluene/MeCN; 2:1 v/v)] solution under Ar produced the adhesive Aun-BFc film on an electrode. The deposition rate was higher in the order THF > toluene/ MeCN > CH2Cl2. The STM images indicated that the films prepared in CH2Cl2 or THF were likely to form domains with ∼80 and 170 nm diameters of the assembled Aun-BFc’s, respectively, as contrast particles were randomly connected in the film deposited in toluene/MeCN. The experimental results all suggested that the electrodeposition process was considerably influenced by solvent properties: the degree of assistance of the chemical reaction of BFc units and the affinity for the charged Aun-BFc’s. By utilizing scanning electrochemical microscopy (SECM), we constructed dot- and line-shape lithographic assemblies of Aun-BFc particles at a resolution of ∼50-100 µm.

Introduction Nanometer-sized metal nanoparticles have been one of the most compelling topics in the chemical,1 physical,2 and biological3 fields because of how strikingly different their characteristics are from those of bulk metal.4 These properties, attributable to the quantum size effect,5 push them to the level of novel desirable nanomaterials in the coming generation of electronics industry potential applications (magnetic,6 optical,7 electric,8 etc.). For these applications, the fixation of nanoparticles onto various substrates will be necessary to develop functional interfaces with advanced particle-particle interactions.9 We * To whom correspondence should be addressed. E-mail: [email protected]. Fax and telephone: +81-3-58414348. † Present address: School of Material Science, JAIST, Ishikawa 923-1292, Japan. (1) (a) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (b) Liu, J.; Ong, W.; Kaifer, A. E.; Peinador, C. Langmuir 2002, 18, 5981. (c) Rodriguez, J. A.; Liu, G.; Jirsak, T.; Hrbek, J.; Chang, Z. P.; Dvorak, J.; Maiti, A. J. Am. Chem. Soc. 2002, 106, 7729. (d) Ignacio, Q.; Yamada, M.; Kubo, K.; Mizutani, J.; Kurihara, M.; Nishihara, H. Langmuir 2002, 18, 1413. (e) Lou, Y. B.; Maye, M. M.; Han, L.; Luo, J.; Zhong, C. J. Chem. Commun. 2001, 473. (f) Yonezawa, T.; Imamura, K.: Kimizuka, N. Langmuir 2001, 17, 4701. (g) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (2) (a) Krahne, R.; Yacoby, A.; Shtrikman, H.; Bar-Joseph, I.; Dadosh, T.; Sperling, J. Appl. Phys. Lett. 2002, 81, 730. (b) Linden, S.; Kuhl, J.; Giessen, H. Phys. Rev. Lett. 2001, 86, 4688. (c) Portales, H.; Saviot, L.; Duval, E.; Fujii, M.; Hayashi, S.; Del Fatti, N.; Vallee, F. J. Chem. Phys. 2001, 115, 3444. (3) (a) Cai, H.; Xu, Y.; Zhu, N. N.; He, P. G.; Fang, Y. Z. Analyst 2002, 127, 803. (b) Mukherjee, P.; Ahmad, A.; Mandal, D.; Senapati, S.; Sainkar, S. R.; Khan, M. I.; Parishcha, R.; Ajaykumar, P. V.; Alam, M.; Kumar, R.; Sastry, M. Nano Lett. 2001, 1, 515. (c) Wang, J.; Xu, D. K.; Kawde, A. N.; Polsky, R. Anal. Chem. 2001, 73, 5576. (d) Moller, R.; Csaki, A.; Kohler, J. M.; Fritzsche, W. Langmuir 2001, 17, 5426. (e) Su, X. D.; Li, S. F. Y.; O’Shea, S. J. Chem. Commun. 2001, 755. (f) Keating, C. D.; Kovaleski, K. M.; Natan, M. J. J. Phys. Chem. B 1998, 102, 9404. (4) (a) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanparticles-Synthesis, Characterization, and Applications; Marcel Dekker Inc.: New York, Basel, 2002. (b) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843. (c) Schmid, G. Chem. Rev. 1992, 92, 1709. (5) (a) Chakravorty, D.; Banerjee, S.; Kundu, T. K. Appl. Surf. Sci. 2001, 182, 251. (b) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. Phys. Rev. B 1994, 50, 12052.

synthesized the octyl thiolate monolayer-protected metal nanoparticles (Mn-OT, M ) Au10 or Pd1d) modified with a thiol derivative of the redox species biferrocene (BFcS)11d by thiol exchange reaction11-13 between Mn-OT and BFcS, and we found the interesting phenomenon that the biferrocene-attached metal nanoparticles (Mn-BFc, M ) Au11a-d or Pd11e) could assemble on an electrode by twoelectron oxidation of BFc moieties. Conversely, we additionally showed that the electroreductive deposition of gold nanoparticles with an anthraquinone derivative (AunAQ)13 occurs by two-electron reduction of AQ sites. These findings led us to deduce that the metal nanoparticles (6) (a) Shemer, G.; Markovich, G. J. Phys. Chem. B 2002, 106, 9195. (b) Chen, D. H.; Hsieh, C. H. J. Chem. Mater. 2002, 12, 2412. (c) Fang, M.; Grant, P. S.; McShane, M. J.; Sukhorukov, G. B.; Golub, V. O.; Lvov, Y. M. Langmuir 2002, 18, 6338. (d) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (7) (a) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596. (b) Pivin, J. C.; Garcia, M. A.; Hofmeister, H.; Martucci, A.; Vassileva, M. S.; Nikolaeva, A.; Kaitasov, O.; Llopis, J. Eur. Phys. J. D 2002, 20, 251. (c) Maillard, M.; Giorgio, S.; Pileni, M. P. Adv. Mater. 2002, 14, 1084. (d) Kamat, P. V.; Barazzouk, S.; Hotchandani, S. Angew. Chem., Int. Ed. 2002, 41, 2764. (e) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature 2000, 405, 665. (f) Sagara, T.; Kato, N.; Toyota, A.; Nakashima, N. Langmuir 2002, 18, 6995. (8) (a) Graf, H.; Vancea, J.; Hoffmann, H. Appl. Phys. Chem. 2002, 80, 1264. (b) Braun, M.; Link, S.; Burda, C.; El-Sayed, M. Chem. Phys. Lett. 2002, 361, 446. (c) He, T.; Ma, Y.; Cao, Y.; Yang, W. S.; Yao, J. N. J. Electroanal. Chem. 2001, 514, 129. (d) Hicks, J. F.; Zamborini, F. P.; Osisek, A.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 7048. (9) (a) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (b) Stuart, H. R.; Hall, D. G. Phys. Rev. Lett. 1998, 80, 5663. (10) Brust, M.; Walker, M.; Bethell, D.; Shiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (11) (a) Yamada, M.; Tadera, T.; Kubo, K.; Nishihara, H. J. Phys. Chem. B. 2003, 107, 3703. (b) Yamada, M.; Nishihara, H. Eur. Phys. J. D, in press. (c) Yamada, M.; Nishihara, H. Chem. Commun. 2002, 2578. (d) Horikoshi, T.; Itoh, M.; Kurihara, M.; Kubo, K.; Hiroshi, N. J. Electroanal. Chem. 1999, 473, 113. (e) Yamada, M.; Quiros, I.; Mizutani, J.; Kubo, K.; Nishihara, H. Phys. Chem. Chem. Phys. 2001, 3, 3377. (12) (a) Labande, A.; Ruiz, J.; Astruc, D. J. Am. Chem. Soc. 2002, 124, 1782. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (13) (a) Yamada, M.; Tadera, T.; Kubo, K.; Nishihara, H. Langmuir 2001, 17, 2363. (b) Yamada, M.; Kubo, K.; Nishihara, H. Chem. Lett. 1999, 1335.

10.1021/la034915d CCC: $25.00 © 2003 American Chemical Society Published on Web 08/12/2003

Biferrocene Derivative-Attached Gold Nanoparticles

functionalized with multiple redox molecules can be regularly aggregated by multiple redox reactions near an electrode interface, resulting in the formation of a redox active metal nanoparticle film. Answering the fundamental question of why the electrochemical assembly of Mn-BFc (M ) Au or Pd) or Aun-AQ happens is important from both a basically scientific and an applied viewpoint, and the question is being discussed via the investigation of morphological qualities of the film and electrochemical measurements of Aun-BFc.11a We previously summarized that electrostatic interaction between BFc2+ moieties on a particle surface and electrolyte anions in CH2Cl2 solution triggers the voluntary self-assembly of charged particles because of their lattice energy involving electrostatic forces and van der Waals-like forces. In addition, at the potential range over E°′(BFc2+/BFc+) of BFc units, the exclusion of CH2Cl2 solvent molecules around Aun-BFc is observed, suggesting that solvent is one of the essential factors affecting the Aun-BFc film construction. This paper describes the solvent effects of Aun-BFc aggregation by introducing three different organic solvents, CH2Cl2, THF, and toluene + acetonitrile (toluene/ MeCN; 2:1 v/v), for electrodeposition. We have found that this electrodeposition system depends greatly on the choice of solvent species by STM observation of the films and electrochemical and spectroscopic measurements. In addition, the simple lithographic deposition comprising the assembly of Aun-BFc particles is demonstrated in a CH2Cl2 solution by applying scanning electrochemical microscopy (SECM). Experimental Procedures Chemicals. All solvents and reagents used for syntheses were of extrapure grade and were purchased from Kanto Chemicals or Tokyo Kasei. They were used as received, unless otherwise noted. Dichloromethane, THF, toluene, and MeCN used for electrochemical measurements were of HPLC grade (Kanto Chemicals). Bu4NClO4 (HPLC grade, Kanto Chemicals) was used after recrystallization from ethanol. Synthesis of BFc-Attached Au Nanoparticles (Aun-BFc). The whole procedure was as described in our previous reports.11a,d In brief, the modification of BFc units on the particle surface was performed by a typical substitution reaction12b of 1-(9-thiononyl1-one)-1′,1′′-biferrocene (BFcS) with Aun-OT in toluene at room temperature. The reaction was performed in a 25 mg/12 mL AunOT solution for 48 h with a 1:4 mole feed ratio of BFcS to octyl thiolate units on a Aun-OT surface. The solution was concentrated to a volume of ∼1 mL, following the addition of ethanol. The precipitate was filtered and washed with sufficient ethanol and acetone to remove excess BFcS and displaced octanethiol. No contamination of free thiol in the sample was confirmed by 1H NMR. The number of substituted BFcS ligands, θBFc, was 7.5, determined by the 1H NMR signals between BFc and methyl protons. The UV-vis spectrum of Aun-BFc in CH2Cl2 exhibited a broad absorption maximum of a surface plasmon band at 516 nm (max ) 7.7 × 105 M-1 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 an Agilent 453 UV-vis spectroscopy system. Cyclic Voltammetry. Cyclic voltammetry was carried out in a standard one-compartment cell under an argon atmosphere at 25 °C using a Pt-wire counter electrode and an Ag/Ag+ reference electrode [10 mM AgClO4 in 0.1 M Bu4NClO4-MeCN, E°′(Fc/ Fc+) ) 0.27 V vs Ag/Ag+ (Fc: ferrocene)] with a BAS CV-50W voltammetric analyzer. Electrodeposition of Aun-BFc on an indium-tin oxide (ITO)-coated glass electrode (1.0 × 1.5 cm2) or on highly ordered pyrolytic graphite (HOPG: 0.28 mm2) was carried out with consecutive scans between -0.3 and 0.9 V vs Ag/Ag+ in a solution of Aun-BFc in 0.1 M Bu4NClO4-organic

Langmuir, Vol. 19, No. 19, 2003 8051 Chart 1. Diagram of the SECM Apparatus for Electrodeposition of Aun-BFc.

solvent [CH2Cl2, THF, toluene/MeCN (2:1 v/v)]. The ITO electrodes were washed in ultrapure water (>18.2 MΩ cm-1) containing a protein remover for 5 min and cleaned sufficiently by ultrapure water and acetone for 5 min, respectively, under sonication before the experiment. The electrodeposited films were cleaned with each solvent after the deposition process and then dried under vacuum. Electrochemical Quartz Crystal Microbalance (EQCM) Measurements. A total of 6 MHz crystals were used in the EQCM measurements. The gold electrode (13 min in diameter) for EQCM measurement, which was definitely clean and sealed separately in a plastic package after vapor deposition of gold onto a quartz substrate, was purchased from Hokuto Denko and used as received as a working electrode. Only one side of the crystal was exposed to the electrolyte solution in a standard Teflon cell containing the oscillator circuit isolated from the solution. Measurements were carried out under an argon atmosphere at 25 °C using a Pt-wire counter electrode and an Ag/Ag+ reference electrode with a Hokuto HQ-101B QCM controller and an HZ3000 polarization system. Scanning Electrochemical Microscopy (SECM). The lithographic deposition of Aun-BFc was carried out as follows by adopting the standard SECM with a CCD camera of a Hokuto HV-402 in 10 µM Aun-BFc in 0.1 M Bu4NClO4-CH2Cl2, using a microelectrode of glass-coated Pt wire 7 mm in diameter (Hokuto Denko) as a counter electrode, a freshly prepared gold electrode 13 mm in diameter (Hokuto Denko) as a working electrode, and Ag wire as a reference electrode set in a one-compartment Teflon cell (Chart 1). First, the surface of the microelectrode was carefully aligned to be parallel to the surface of the working electrode, and the distance between the counter electrode and the working electrode was adjusted to be as short as possible (∼20 µm) by an XYZ stage controller after checking the CCD camera image. Then, the working potential was kept at 1.1 V by a potentiostat while the counter electrode in the X-Y position was moved normally to the surface of the working electrode at a speed of 122.7 µm/s controlled by a piezopositioner with a computer program. Transmission Electron Microscopy (TEM). TEM images of Aun-OT were obtained with a Hitachi HF-2000 microscope. Samples for TEM were prepared by placing a droplet of 4 µL of a 0.5 mg/mL Aun-BFc solution in CH2Cl2 onto standard carboncoated films on copper grids (600 mesh, Ohta Giken) and drying them in a vacuum overnight. Statistical treatment of at least

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Table 1. Data of Cyclic Voltammetry and UV-Vis Spectroscopy of Aun-BFc solvent

E°′1a (V)

E°′2a (V)

coverage of Aun-BFc’s in the film (×10-11 mol cm-2)/no. of layersc

THFb toluene/MeCNb CH2Cl2b

0.11 0.15 0.18

0.47 0.52 0.59

14/43 8.5/27 3.7/11

a Potentials refer to Ag/Ag+. b In 0.1 M Bu NClO -solvent at 4 4 ITO. c The monolayer coverage of Aun-BFc’s ) 3.2 × 10-12 mol cm-2.

200 particles in the enlarged TEM images gave the average and deviation of the particle core size by using the computer counting program Scion Image Release 4 (Scion Corporation). The prepared Aun-BFc has the average core diameter d ) 2.3 ( 0.5 nm with a rather small distribution according to TEM images, corresponding to 309 Au atoms of cuboctahedron core shape with an average of 92 octyl thiolate units adsorbed onto the one particle surface.14 Scanning Tunneling Microscopy (STM). STM images of electrodeposited film on HOPG were obtained with a PicoSPM (Molecular Imaging) controlled by a PicoScan (Molecular Imaging) at room temperature in air. The tip was 0.25 mm Pt/Ir (4:1) cut with a hardened steel cutter. The constant current range was 0.3-0.5 nA with a bias of 0.1-0.5 V.

Figure 1. UV-vis spectra and (inset) cyclic voltammograms in 0.1 M Bu4NClO4-CH2Cl2 at 100 mV/s between -0.3 and 0.9 V vs Ag/Ag+ of electrodeposited Aun-BFc films. The films were prepared in a solution of 5.2 µM Aun-BFc in 0.1 M Bu4NClO4THF (solid line), toluene/MeCN (dotted line), and CH2Cl2 (dashed line) at ITO at 100 mV/s between -0.3 and 0.9 V vs Ag/Ag+ with 75 scans.

Results and Discussion Electrodeposition of Aun-BFc in a Different Solvent at ITO. Three organic solvents, CH2Cl2, THF, and toluene/MeCN, were selected for electrochemical measurements of Aun-BFc, considering the stability of the solvent in the potential range of BFc oxidation (from -0.3 V to 0.9 V vs Ag/Ag+) and the solubility of Aun-BFc in the solvent. The cyclic voltammograms (CVs) of Aun-BFc in 0.1 M Bu4NClO4-solvent for each solvent at ITO exhibited two redox waves attributed to two-step one-electron oxidation of BFc units on a particle surface. The redox potentials are summarized in Table 1. The typical CV of Aun-BFc’s in the electrodeposition process where CH2Cl2 was adopted demonstrated a gradual increase of the peak currents by repeating the potential scans, indicating the accumulation of Aun-BFc’s on an electrode/solution interface to build up the Aun-BFc film, as shown in previous studies.11 We can observe a similar current increment in THF and toluene/MeCN solutions (Figure S1 of the Supporting Information). Figure 1 displays the UV-vis spectra and CVs of the Aun-BFc films arranged in each solution with the 75 potential scans on ITO. The UV-vis spectrum exhibits only a broad curve of Mie scattering without a distinctive SP band of Aun-BFc, because an SP band of a gold particle with a small core diameter below ∼5 nm is significantly damped.15 The coverage of the AunBFc films on ITO is estimated from the absorption intensity at 510 nm, assuming that the electrodeposited particles are densely packed with a spacing of 7.5 nm based on the STM data11a (vide infra) (Table 1). These values point out that the electrodeposition of Aun-BFc’s proceeds faster in the order THF > toluene/MeCN > CH2Cl2. The actual photos of the Aun-BFc films prove this order clearly with the color change of deposited Aun-BFc’s on ITO as the number of potential scans is increased (Figure 2). This deposition degree is consistent with the amount of redox charge in the CVs shown in the inset of Figure 1. The newly appearing peak at ∼0.3 V between the original redox waves was ascribed to decomposed (14) 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. (15) Mulvaney, P. Langmuir 1996, 12, 788.

Figure 2. Photographs of electrodeposited Aun-BFc films prepared in a solution of 5.2 µM Aun-BFc with 0.1 M Bu4NClO4 organic solvent at ITO at 100 mV/s between -0.3 and 0.9 V vs Ag/Ag+ with 3, 10, 25, 50, 75, and 120 cyclic scans from left to right.

chemical species of BFc2+ units generated by consecutive potential scans on an electrode.16 This peak can be recognized in all of the three CVs; however, it is especially conspicuous for the film made in toluene/MeCN solution, almost losing the original redox waves. This leads us to propose that the electrodeposition of Aun-BFc’s in toluene/ MeCN advances by involving the decomposition of almost all of the BFc species by two-electron oxidation, implying that the electrodeposition process of Aun-BFc in toluene/ MeCN is more complicated compared to that in THF or CH2Cl2. EQCM Measurement of the Aun-BFc Deposition Process. We used EQCM measurement to further elucidate the solvent effects in the deposition progress of AunBFc. Figure 3 displays the EQCM behavior of Aun-BFc at a gold-coated EQCM electrode in 0.1 M Bu4NClO4 combined with three organic solvents at the first scan. The concentration of Aun-BFc in each solution is adjusted to be the same in each case; the redox charge of BFc components estimated by the redox waves of CVs without a capacitance current is almost identical among the three. The E°′1 (for BFc+/BFc0) and E°′2 (for BFc2+/BFc+) of AunBFc agree well in Table 1 evaluated at ITO, and we refer to these values. As noted in the previous section, the CVs in THF and CH2Cl2 evince two pairs of an approximately reversible anodic and cathodic wave, while the extra cathodic wave at ∼0.3 V arises outstandingly in toluene/ (16) Kubo, K.; Kondow, H.; Nishihara, H. Electrochemistry 1999, 67, 1129.

Biferrocene Derivative-Attached Gold Nanoparticles

Langmuir, Vol. 19, No. 19, 2003 8053 Table 2. Data of EQCM Measurement of Aun-BFc solvent

∆F (Hz)

∆W (×10-8 g)

coverage of Aun-BFc’s in the film (×10-13 mol cm-2)/no. of layersa

THF toluene/MeCN CH2Cl2

144 46 6.0

234 74 9.7

227/7.1 74/2.3 9.5/0.30

a The monolayer coverage of Au -BFc’s ) 3.2 × 10-12 mol cm-2. n The estimated molecular weight of Aun-BFc ) 7.7 × 104.

Figure 3. Cyclic voltammograms (top) and ∆F-potential curves (bottom) of 5.2 µM Aun-BFc at a gold electrode in 0.1 M Bu4NClO4-THF (solid line), toluene/MeCN (dotted line), and CH2Cl2 (dashed line) at 100 mV/s between -0.3 and 0.9 V vs Ag/Ag+.

MeCN after a potential sweep over E°′2, intimating that the structure transformation of BFc2+ moieties occurs immediately in toluene/MeCN. This is caused by a nucleophilic reaction on the ferrocenium sites in the BFc2+ species of a MeCN solvent molecule,16 which has a cyano group and is known as a renowned metal complex ligand with high coordination ability, being relatively chemically reactive compared with the other solvents, THF and CH2Cl2.17 As for the frequency change (∆F)-potential curves, the common findings are as follows: (i) ∆F cannot be much discerned in the first electro-oxidation of the BFc units at E°′1, (ii) the frequency begins to fall off significantly at E°′2, and (iii) a continuous frequency decrease is observed until E°′2, when the potential scan is reversed at 0.9 V to the negative direction. A frequency decrease is equivalent to a mass increase on an EQCM electrode; these results mean that the electrochemical aggregation of Aun-BFc’s is induced only when a two-electron oxidized state is imposed on the BFc species in any solvent. But these ∆Fpotential curves also provide obvious evidence that the deposition process of Aun-BFc is diverse from that of each other solvent. First, the attitude of the ∆F-potential curve in toluene/MeCN in the potential range over E°′2 is palpably peculiar in comparison with those of the other two solvents, which is compatible with the CV results, namely, that the value of the frequency decrease over E°′2 is quite restrained in toluene/MeCN. The comparable values in THF and CH2Cl2 are regarded as nearly the same considering the E°′2 difference; that is, E°′2 in THF is more positive than that in CH2Cl2, which can produce a larger amount of Aun-BFc’s with BFc2+units, resulting in a slightly more ample frequency decrease. We infer that the prompt decomposition of BFc2+ moieties in toluene/MeCN inhibits the electron transfer through the redox species among Aun-BFc particles near an electrode/ electrolyte interface, which prevents an efficient electrochemical gathering of Aun-BFc’s on an electrode in toluene/ MeCN. (17) Shimura, T.; Aramaki, K.; Nishihara, H. J. Electroanal. Chem. 1996, 403, 219.

Figure 4. ∆F-time curves of 5.2 µM Aun-BFc at a gold electrode in 0.1 M Bu4NClO4-THF (a), toluene/MeCN (b), and CH2Cl2 (c) at 100 mV/s between -0.3 and 0.9 V vs Ag/Ag+ in the positive direction.

We also found that a significant frequency increase takes place in CH2Cl2 only after the electrode potential is inverted to be more negative than E°′2. This denotes that the Aun-BFc flocks onto an electrode are mostly redissolved into CH2Cl2 solution after BFc sites are returned to the neutral state by electroreduction. In contrast, in THF or toluene/MeCN, Aun-BFc’s with BFc2+ units are rallied onto an electrode too firmly to be desorbed. The gap of ∆F at the potential of -0.3 V represents the net amount of precipitated Aun-BFc’s on an electrode after a potential cycle, the data of which are summarized in Table 2, contemplating the Sauerbery equation of ∆F ) -C∆W, where C is the proportional constant that is dependent on the parameters of QCM electrode properties (e.g., the electrode area, quartz density, and quartz elasticity, and the value is equivalent to 6.17 × 107 Hz g-1 in this study), and ∆W is the increased weight on the electrode. The order of the values in Table 2 is THF > toluene/MeCN > CH2Cl2 according to the spectroscopic results. The supplementary aspect of the weight increase by multiple potential cycles (∼85 cycles) in EQCM measurement is prospected in Figure 4, where plots of ∆F versus time of potential scans are displayed. A negative spike derived from one potential cycle appears every 24 s. The bottom of the spike corresponds to the ∆F value at ∼0.6 V when the potential is reversed to the negative direction, and the maximum points at both ends of one negative spike concur with the start and the finish of each potential cycle at -0.3 V, respectively. Thus, the width of the line reflects the value of ∆F over E°′2, which is in the order THF > CH2Cl2 > toluene/MeCN, in agreement with Figure 3. At 2000 s after ∼85 scans, the degree of ∆F (THF > toluene/MeCN > CH2Cl2) is squared with the depicted results so far. In CH2Cl2, the electrodeposition amount of Aun-BFc’s is almost proportional to time () number of potential scans), while, in THF or toluene/MeCN, the deposition rate is relatively quick at the first stage of the deposition process by ∼400 s () ∼15 scans) and gradually becomes constant. EQCM Measurement of the Aun-BFc Film. EQCM measurement of the Aun-BFc film in Bu4NClO4-organic solvent has provided more information about the solvent efficacy for electrodeposition of Aun-BFc. The three samples of the Aun-BFc film were prepared under the same

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Figure 5. Cyclic voltammograms (top) and ∆F-potential curves (bottom) of a Aun-BFc film on a gold electrode in 0.1 M Bu4NClO4-THF (solid line), toluene/MeCN (dotted line), and CH2Cl2 (dashed line) at 30 mV/s between -0.3 and 0.9 V vs Ag/Ag+ in the positive direction with the first scan. The film was prepared in 5.2 µM Aun-BFc in 0.1 M Bu4NClO4-CH2Cl2 at 100 mV/s between -0.3 and 0.9 V vs Ag/Ag+ in the positive direction with 50 scans.

conditions in a Aun-BFc solution of 0.1 M Bu4NClO4CH2Cl2 by 50 potential scans. In Figure 5 (top), the CV of the Aun-BFc film for the first scan in Bu4NClO4-THF, -toluene/MeCN, and CH2Cl2 shows a two-step one-electron redox wave of BFc moieties with an additional wave at ∼0.3 V, as mentioned above. The amount of the active BFc sites in the film was calculated by the first redox charge in the figure, equal to 9.8 × 10-10 mol.11 The ∆Fpotential curves of the film in Figure 5 (bottom) suggest that the frequency decrease until the potential reaches ∼0.5 V in any solvent is interpreted by the introduction of the anion species into the Aun-BFc film by BFc+ units and the oxidized gold core; in contrast, the frequency increase after ∼0.6 V around E°′2 of the BFc units is dramatically detected only in CH2Cl2. We confirmed this kind of frequency increase in Bu4NPF6-CH2Cl2 solution reported in our previous study, which we ascribed to the release of the cation (Bu4N+) immobilized in the film among the particles matching 50% of the number of BFc sites, and the simultaneous exclusion of 7.2 CH2Cl2 solvent molecules per one BFc unit (54 molecules per one AunBFc) from the film by earning the polarization around the particles with BFc2+ units.11a The frequency increase in this study is 14 Hz, corresponding to 2.3 × 10-7 g, the value of which is covered only by the assumed cation species, 2.4 × 10-7 g, without the contribution of solvent molecules. This interpretation is unreasonable; if the frequency increase at E°′2 is primarily assigned to the release of immobilized cation species in the film, almost the same change of ∆F should be observed in any solvent. In THF or toluene/MeCN, a slight increase of ∆F at E°′2 can be ascertained, although the change is not distinguishable when comparing it with that in CH2Cl2 solution. It is plausible that this small shift (2 Hz in THF, 4 Hz in toluene/MeCN) contributes to the cation release from the film; namely, the impression of solvent exclusion in CH2Cl2 is predominant over the value of ∆F at E°′2. This elucidation presumes that the 8.2 CH2Cl2 solvent molecules per one BFc moiety are released from the film in this experiment, which is consistent with the revised values without the effect of the anion release of 8.5 CH2-

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Cl2 solvent molecules in the previous study. It should be noted that the amount of this frequency increase at E°′2 is slowly reduced as the decomposition of BFc sites progresses by piling up the potential scans. Finally, the simple slope downward is expected due to the metal core charging, which proves that the active BFc units are essential for the reversible movement of CH2Cl2 solvent molecules from the Aun-BFc film (Figure S2). The CH2Cl2 solvent molecules are excluded from the charged film because CH2Cl2 is the least polar solvent among the three, which diminishes its affinity for the Aun-BFc’s with BFc2+ units collecting polarity around the particle. We also mention that the frequency decrease, 12 Hz, at the starting potential at -0.3 V is recognized particularly in toluene/ MeCN, which is attributable to the irreversibly incorporated MeCN solvent molecules in the film by a coordinated MeCN to BFc2+ units, resulting in the decomposition and/ or a kind of polymerization by MeCN oxidation. STM Images of the Electrodeposited Aun-BFc Film Surface: Dependence of the Adopted Solvent and Morphological Features. We determined the morphological dependence of the Aun-BFc film on three types of solvent for electrodeposition by STM observations. STM images taken in the wide area 350 × 350 nm2 and their cross-sectional profiles of the Aun-BFc films at HOPG accumulated in each organic solvent by five cyclic scans are shown in Figure 6, showing tiny particles coating the electrode surface. We can detect the domainlike structure of Aun-BFc’s in the film deposited in CH2Cl2 and THF (circled by a dotted line in Figure 6A and B), while the film prepared in toluene/MeCN exhibits no such island of Aun-BFc’s but shows rather arbitrarily deposited networks. The domain size of Aun-BFc’s is 50-100 nm in Figure 6A and 100-200 nm in Figure 6B, suggesting that Aun-BFc’s tend to make a larger domain in THF than in CH2Cl2. Note that the STM observation reveals that the domain structure is slightly irregular depending on each domain; however, it is considerably uniform on average on a larger scale, as confirmed by AFM measurement.11a The difference in the domain size is explainable by the deposition rate discrepancy of the Aun-BFc accumulation, which proceeds more rapidly in THF than in CH2Cl2. The basis of the domain construction might be created at the two-electron oxidation state of BFc units; the Aun-BFc’s with BFc2+ units in THF are more profitable in lattice energy, involving electrostatic forces and van der Waalslike forces between the charged Aun-BFc’s and the electrolyte anion. We could not detect the solvent exclusion phenomenon in THF, which explains the difference of the deposition rate between the two solvents. It is feasible that the electrostatic interaction between the assembled charged Aun-BFc’s works more effectually through the THF solvent molecules with relatively high permeability, with the tight collective interactions among particles causing almost irreversible aggregation of Aun-BFc’s that can hardly be separated even if the BFc units are brought back to the neutral state. Concerning the film in toluene/ MeCN, the decomposition of BFc units by two-electron oxidation might hinder such a self-assembled arrangement of Aun-BFc’s without involving the efficacious lattice energy. Lithographic Deposition of Aun-BFc using the SECM Apparatus. In the last part of this study, we demonstrate the lithographic assembly of Aun-BFc by adopting SECM.18 The scanning electrochemical micro(18) (a) Bard, A. J.; Denuault, G.; Lee, H.; Mandeler, D.; Wipf, D. O. Acc. Chem. Res. 1990, 23, 357. (b) Shiku, H.; Takeda, T.; Yamada, H.; Matsue, T.; Uchida, I. Anal. Chem. 1995, 67, 312.

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Figure 6. STM image of the Aun-BFc film on HOPG electrodeposited in 5.3 µM Aun-BFc in 0.1 M Bu4NClO4-CH2Cl2 (A), THF (B), and -toluene/MeCN (C) between -0.3 and 0.9 V vs Ag/Ag+ at 100 mV/s by five cyclic potential scans and the typical crosssectional profile along the cross axis.

scope, the basic concept of which was first reported by Bard et al.,17a is an apparatus normally used for studying heterogeneous surface reactions. We used the SECM apparatus for the electrodeposition control of Aun-BFc, as shown in Chart 1. When the substrate potential is kept at 1.1 V in Aun-BFc solution in Bu4NClO4-CH2Cl2 where the electro-oxidative deposition of Aun-BFc occurs, the electronic density due to the faradaic current tends to gather at the area of the substrate where the distance to the counter electrode is the closest (the circled point in Chart 1). Figure 7 displays the CCD images of the lithographic deposition of Aun-BFc by using this method under several conditions. In each figure, we can clearly recognize the black precipitates deposited in a specific area. The simplest diagram is a “dot” described in Figure 7C, which is prepared by maintaining the deposition time for 5 s without moving the counter electrode. The diameter of the deposited domain is ∼50-80 µm, much enlarged relative to the electrode diameter, 7 µm. It is plausible that the redox reaction of Aun-BFc at the substrate is controlled by three-dimensional diffusion by using a microelectrode as a counter electrode, leading to the rather wide deposition area of the substrate. This consideration is also suitable with respect to the line drawing of Figure 7A, B, D, and E in ∼100 µm width deposited by moving the Pt counter electrode at 122.7 µm/s for 500 µm (A, B,

Figure 7. CCD image of the lithographic Aun-BFc deposition on a gold electrode substrate prepared in 10 µM Aun-BFc in 0.1 M Bu4NClO4-CH2Cl2 at the substrate potential 1.1 V by the SECM apparatus. Each line was deposited by moving the Pt counter electrode in a 7 µm diameter at 122.7 µm/s for 500 µm (A, B, and D) or 900 µm (E) with 10 scans. As for part D, the deposition time was maintained for 5 s without moving the counter electrode.

and D) or 900 µm (E) with 10 scans. As for the line length, the value is measured as ∼400-500 µm (A, B, and D) and ∼800-900 µm (E), which is reasonable compared to the

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practical moving distance. The starting point of some lines overlaps with a circle shape, which can be attributed to the time lag between applying the potential to the substrate and moving the counter electrode, and which could be overcome by improving the SECM technology. The detailed morphology of the lithographic Aun-BFc assemblies thus prepared interests us in additional ways; for example, we would like to determine the construction of the packed Aun-BFc particles, the surface, and the edge of the lines (dots). Furthermore, we believe that the variable parameters, such as the distance between a counter electrode and a substrate electrode, the scan speed, and the diameter of a counter electrode, could be applied to changing the line width and the deposition thickness to produce more precisely controlled shapes. These assemblies are under investigation in our laboratory, where we are attempting to use STM and AFM measurements with higher resolution. Conclusion The electrodeposition of a biferrocene (BFc)-functionalized gold nanoparticle with a 2.3 ( 0.5 nm core diameter (Aun-BFc) occurs in THF, toluene/MeCN, and CH2Cl2 electrolyte solutions by two-electron oxidation of BFc units to construct the Aun-BFc film on the electrode. The whole deposition process and the assembled film morphology of Aun-BFc’s depend on the employed solvent, which is summarized as follows: (i) the deposition in toluene/MeCN solution is more complicated in comparison with those of the other two solvents, and it is accompanied by immediate decomposition of BFc2+ units, resulting in a randomly combined organization of particles in the film; (ii) in THF and CH2Cl2 solution, the deposited film materializes as

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a self-assembled-like structure with lots of domains of Aun-BFc’s involving the lattice energy between the AunBFc’s with BFc2+ units and electrolyte anions; and (iii) the dissimilitude of the deposition rate in THF and CH2Cl2 solution and the domain size of the prepared film are ascribed to the affinity of the solvent for the charged Aun-BFc’s. We have also demonstrated the manipulation method of the substrate surface by applying SECM:19 the simple lithographic assembly of Aun-BFc particles is possible in the macroscopic range of micrometer order by utilizing this deposition system, which is applicable for the construction of multilayered gold nanoparticles with versatile morphology. Acknowledgment. The authors acknowledge S. Aoyagi and Y. Utsumi (Hokuto Denko Co. LTD) for SECM measurements. This work was financially supported by Grants-in-Aid for Scientific Research (No. 14204066) from the Ministry of Culture, Education, Science, Sports and Technology, Japan, and the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. Supporting Information Available: The CVs of Aun-BFc in 0.1 M Bu4NClO4-solvent for each organic solvent for electrodeposition (Figure S1) and the EQCM measurement of a Aun-BFc film on a gold electrode in 0.1 M Bu4ClO4-CH2Cl2 by multiple scans (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. LA034915D (19) (a) Bard, A. J.; Forouzan, F. J. Phys. Chem. B 1997, 101, 10876. (b) Kranz, C.; Gaub, H. E.; Wolfgang, S. Adv. Mater. 1996, 8, 634.