Bimetallic (Ag)Au Nanoparticles Prepared by the Seed Growth Method

The Journal of Physical Chemistry C 0 (proofing), ... Chemical Reviews 0 (proofing), ... Fabrication of SERS-Active Patterned Gold Nanoparticle Films ...
0 downloads 0 Views 330KB Size
Download PDF
Langmuir 2004, 20, 3407-3415

3407

Bimetallic (Ag)Au Nanoparticles Prepared by the Seed Growth Method: Two-Dimensional Assembling, Characterization by Energy Dispersive X-ray Analysis, X-ray Photoelectron Spectroscopy, and Surface Enhanced Raman Spectroscopy, and Proposed Mechanism of Growth Ivana Srnova´-Sˇ loufova´,†,‡ Blanka Vlcˇkova´,*,‡ Zdeneˇk Bastl,§ and Thomas L. Hasslett| Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic, Department of Physical and Macromolecular Chemistry, Charles University, Hlavova 2030, 128 40 Prague 2, Czech Republic, J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇ kova 3, 182 23 Prague 8, Czech Republic, and Department of Chemistry, University of Toronto and Photonics Research Ontario, 80 St. George Street, Toronto M5S1A1, Canada Received June 26, 2003. In Final Form: December 9, 2003 Layered core-shell bimetallic silver-gold nanoparticles were prepared by overdeposition of Au over Ag seeds by the seed-growth method using tetrachloroauric acid, with hydroxylamine hydrochloride as the reductant. The effects of pH, reduction rate, and seeding conditions on the morphology and surface plasmon extinction of the bimetallic nanoparticles were investigated. Nanoparticles prepared by a rapid reduction in the neutral ambient and assembled into two-dimensional nanoparticulate films by adsorption of 2,2′bipyridine were characterized by energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, surfaceenhanced Raman scattering spectroscopy, and transmission electron microscopy. The results are consistent with Ag core and Ag/Au-alloyed shell composition of the nanoparticles. Evidence of the presence of Ag on the surface of the nanoparticles, of enrichment of the Ag/Au alloy shell by Ag toward or at the nanoparticle surface, and of modification of the nanoparticle surface by adsorbed chlorides is also provided. Reduction of the size of the Ag seeds, alloying of Ag and Au in the shell of the nanoparticles, and modification of their surfaces by adsorbed chlorides are tentatively attributed to positive charging of the nanoparticles during the electrocatalytic overdeposition of Au over Ag seeds.

Introduction Nanosized bimetallic colloidal particles exhibit unique optical, electronic, magnetic, and catalytic properties1-21 distinct not only from the bulk metals but also from the corresponding monometallic particles as well. Preparation * To whom correspondence should be addressed. E-mail: vlc@ natur.cuni.cz. Phone: 420-221951309. Fax: 420-224919752. † Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic. ‡ Department of Physical and Macromolecular Chemistry, Charles University. § J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic. | Department of Chemistry, University of Toronto and Photonics Research Ontario. (1) Turkevich, J.; Kim, G. Science 1970, 169, 873. (2) Schmid, G.; West, H.; Malm, J. O.; Bovin, J. O.; Grenthe, C. Chem. Eur. J. 1996, 2, 1099. (3) (a) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (b) Toshima, N.; Hirakawa, K. Polym. J. 1999, 31, 1127. (c) Harada, M.; Asakura, K.; Toshima, N. J. Phys. Chem. 1993, 97, 5103. (d) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91, 4111. (e) Seregina, M. V.; Bronstein, L. M.; Platonova, O. A.; Chernyshov, D. M.; Valetsky, P. M.; Hartmann, J.; Wenz, E.; Antonietti, M. Chem. Matter. 1997, 9, 923. (f) Lee, A. F.; Baddeley, C. J.; Hardacre, C.; Ormerod, R. M.; Lambert, R. M. J. Phys. Chem. 1995, 99, 6096. (g) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Zamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033. (4) Schmid, G.; Lehnert, A.; Malm, J. O.; Bovin, J. O. Angew. Chem., Int. Ed. Engl. 1991, 30, 874. (5) Liz-Marzan, L. M.; Philipse, A. P. J. Phys. Chem. 1995, 99, 15120. (6) (a) Yonezawa, T.; Toshima, N. J. Mol. Catal. 1993, 83, 167. (b) Schmid, G.; West, H.; Mehles, H.; Lehnert, A. Inorg. Chem. 1997, 36, 891.

and characterization of colloids (hydrosols) of bimetallic nanoparticles constituting various combinations of noble metals were the subject of numerous papers, e.g., Au(7) (a) Michaelis, M.; Henglein, A.; Mulvaney, P. J. Phys. Chem. 1994, 98, 6212. (b) Esumi, K.; Wakabayashi, M.; Torigoe, K. Colloids Surf., A 1996, 109, 55. (c) Silvert, P.-Y.; Vijayakrishnan, V.; Vibert, P.; HerreraUrbina, R.; Elhsissen, K. T. Nanostruct. Mater. 1996, 7, 611. (d) Torigoe, K.; Esumi, K. Langmuir 1993, 9, 1664. (8) Torigoe, K.; Nakajima, Y.; Esumi, K. J. Phys. Chem. 1993, 97, 8304. (9) (a) Aihara, N.; Torigoe, K.; Esumi, K. Langmuir 1998, 14, 4945. (b) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129. (c) Teo, B. K.; Keating, K.; Kao, Y. H. J. Am. Chem. Soc. 1987, 109, 3494. (d) Shi, H. Z.; Zhang, L. D.; Cai, W. P. J. Appl. Phys. 2000, 87, 1572. (e) Hostetler, M. J.; Zhong, C. J.; Yen, B. K. H.; Anderegg, J.; Gross, S. M.; Evans, N. D.; Porter, M.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 9396. (f) Han, W. S.; Kim, Y.; Kim, K. J. Colloid Interface Sci. 1998, 208, 272. (g) Papavassiliou, G. C. J. Phys. F: Metal Phys. 1976, 6, L103. (h) Lee, I.; Han, S. W.; Kim, K. Chem. Commun. 2001, 1782. (i) Cottancin, E.; Lerme, J.; Gaudry, M.; Pellarin, M.; Vialle, J. L.; Broyer, M.; Prevel, B.; Treilleux, M.; Melinon, P. Phys. Rev. B 2000, 62, 5179. (j) Chen, Y. H.; Yeh, C. S. Chem. Commun. 2001, 371. (k) Sato, G.; Kuroda, S.; Takami, A.; Yonezawa, Y.; Hada, H. Appl. Organomet. Chem. 1991, 5, 261. (10) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (11) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (12) Morriss, R. H.; Collins, L. F. J. Chem. Phys. 1964, 41, 3357. (13) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 1. (14) Treguer, M.; de Cointet, C.; Remita, H.; Khatouri, J.; Mostafavi, M.; Amblard, J.; Belloni, J. J. Phys. Chem. B 1998, 102, 4310. (15) (a) Lu, P.; Dong, J.; Toshima, N. Langmuir 1999, 15, 7980. (b) Takenaka, T.; Eda, K. J. Colloid Interface Sci. 1985, 105, 342. (16) Chen, Y. H.; Nickel, U. J. Chem. Soc., Faraday Trans. 1993, 89, 2479.

10.1021/la0302605 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/16/2004

3408

Langmuir, Vol. 20, No. 8, 2004

Pd,1-4 Au-Pt,4-6 Ag-Pd,7 Ag-Pt,5,8 and Ag-Au.9-21 Nanoparticles composed of free-electron-like metals such as Ag and Au are known to provide strong resonance optical responses to irradiation by light,22 which result in amplification of light-induced processes undergone by molecules localized on their surfaces, such as Raman scattering, giving rise to surface-enhanced Raman scattering (SERS).23 Optical properties of bimetallic nanoparticles comprised of Ag and Au are thus the subject of considerable interest. Comparison of the calculated and measured surface plasmon (SP) extinction spectra was frequently employed as one of the criteria of distinguishing between an alloyed and/or layered (core-shell) structure of the bimetallic Ag-Au nanoparticles. Recently, a strategy for extracting optical constants of the core and/ or the shell material of bimetallic Ag-Au nanoparticles from their measured SP extinction spectra was reported.21 Furthermore, it was shown that bimetallic Ag-Au nanoparticles can be employed as SERS-active surfaces.11,18 SERS enhancement as a function of the layered Ag-Au nanoparticle composition and the state of their aggregation was investigated using pyridine11,18 and other types of probe adsorbates.11 Strategies of Ag-Au nanoparticle preparation are based on reduction of the metal salts by radicals produced by pulse radiolysis as well as by chemical reductants. Since Ag and Au are miscible in all proportions, but differ in both redox potentials and surface energies, the results of a particular preparative strategy with respect to formation of either the alloyed or layered nanoparticle composition are not always readily predictable, and characterization of the composition of the resulting nanoparticles is thus of key importance. Alloyed Ag-Au nanoparticles were reported, e.g., in refs 9 and 10. Freeman et al.11 and Morriss and Collins12 prepared nanoparticles consisting of gold core and silver shell. Mulvaney et al.13 deposited gold onto radiolyticaly prepared silver seeds by irradiation of KAu(CN)2 solution, Treguer et al.14 prepared layered nanoparticles by radiolysis of mixed AuIII/AgI solution. Silver colloid with gold reduced in the surface layer was prepared by Chen and Nickel16 by mixing a solution of HAuCl4 with a Ag colloid and addition of a reductant (pphenylenediamine) in the second step. Both (Ag)Au and (Au)Ag core-shell particles were prepared by a two-step chemical reduction in refs 17 and 18. A two-step wet radiolytic synthesis resulting in a size-dependent spontaneous alloying within Au-core/Ag-shell nanoparticles and a photochemical approach to Au core-Ag shell nanoparticle preparation were recently reported.19 In ref 20, preparation, TEM (transmission electron microscopy) images and TEM image analysis and simulations of Ag-Au bimetallic nanoparticles with unusual core-shell TEM image contrasts were reported. The nanoparticles were prepared by overdeposition of varying amounts of Au over Ag seeds using a variation of the seedgrowth method employed by Turkevich and Kim1 for preparation of Au-plated Pd nanoparticles. Comparison (17) (a) Sinzig, J.; Quinten, M. Appl. Phys. A 1994, 58, 157. (b) Sinzig, J.; Radtke, U.; Quinten, M.; Kreibig, U. Z. Phys. D 1993, 26, 242. (18) Rivas, L.; Sanchez-Cortes, S.; Garcı´a-Ramos, J. V.; Morcillo, G. Langmuir 2000, 16, 9722. (19) (a) Shibata, T.; Bunker, B. A.; Zhang, Z.; Meisel, D.; Vardeman, C. F., II; Gezelter, J. D. J. Am. Chem. Soc. 2002, 124, 11989. (b) Mallik, K.; Mandal, M.; Pradhan, N.; Pal, T. Nano Lett. 2001, 1, 319-322. (20) Srnova´-Sˇ loufova´, I.; Lednicky´, F.; Gemperle, A.; Gemperlova´, J. Langmuir 2000, 16, 9928. (21) Moskovits, M.; Srnova´-Sˇ loufova´, I.; Vlcˇkova´, B. J. Chem. Phys. 2002, 116, 10435. (22) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (23) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783.

Srnova´ -S ˇ loufova´ et al.

of observed and simulated TEM image contrasts indicated a Ag core-Ag enriched Au shell composition of the nanoparticles.20 SP extinction spectra of the same set of (Ag)Au colloids/hydrosols (with the molar fraction of Au varied in the 0.1-0.8 range) were analyzed in detail in ref 21. For example, it was shown21 that the SP extinction maxima of the (Ag)Au hydrosols with higher molar fractions of Au are located between those of pure Au and Ag (a feature typical for SP extinction spectra of all AgAu bimetallic systems reported previously) only after an additional reduction by sodium borohydride. From extinction spectra of the hydrosols measured after the additional reduction, optical constants of the shell material of the core-shell nanoparticles were extracted. The results were found consistent with a Ag (or Ag-rich) core and Ag/Au alloy shell model of the bimetallic nanoparticles.21 In this paper, we focus on investigation of the mechanism of the Ag-Au nanoparticles growth during overdeposition of Au over Ag seeds by reduction of tetrachloroauric acid by hydroxylamine hydrochloride in the presence of the seeding Ag hydrosol nanoparticles which, according to the previous studies,20,21 results into reduction of the original Ag seeds (cores of the bimetallic particles) and alloying of Ag and Au in the nanoparticle shell. We investigate the effect of pH, of the rate of reduction, and of the seeding conditions in the above-mentioned reaction on the SP extinction of the resulting hydrosols and on the morphology of the nanoparticles as well as on the affinity of the nanoparticles toward two-dimensional assembling mediated by 2,2-bipyridine (bpy) molecules. Nanoparticles prepared by a rapid reduction in the neutral ambient and assembled into two-dimensional nanoparticulate films by adsorption of bpy were imaged by TEM and further characterized by energy-dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and SERS spectroscopy, yielding information about the overall particle composition and about composition of the shell and of the surface of the bimetallic nanoparticles. Finally, the mechanism of the nanoparticle growth tentatively explaining the specificity of the bimetallic nanoparticle composition, surface composition and morphology, and the observed anomalies and changes in the SP extinction of the bimetallic nanoparticle hydrosol under various conditions of preparation is proposed. Experimental Section Preparation of Mono- and Bimetallic Colloids. Ag colloid was prepared according to the preparation procedure II in ref 24. (Ag)Au bimetallic colloids with varying molar fractions of Au were prepared both by the preparation procedure reported earlier20,21 (PPI) and by its modifications (PPII-PPV). For the sake of clarity, all (Ag)Au colloids reported in this paper are labeled by the approximate value of the molar fraction of Au (0.1-0.8). The colloids prepared by the modified preparation procedures PPII-PPV are also labeled by a Roman number (IIV) denoting the procedure employed in their preparation. Preparation Procedure I. The seed-growth method based on a reduction of HAuCl4 by NH2OH‚HCl20 was used to grow the Au shells over Ag nanoparticle seeds. Briefly, to 12.5 mL of Ag colloid diluted with 30 mL of deionized distilled water, x mL of 6.25 × 10-3 M NH2OH‚HCl and x mL 4.65 × 10-4 M HAuCl4 were added dropwise (ca. 4 mL/min) by two separate pipets upon vigorous stirring. The pH value in this preparation was 6-7. The stirring was continued for 45 min. Calculated molar fraction of Au in the resulting colloids together with the corresponding values of x used in their preparation are listed in the upper part of Table 1. Approximate values of the former variable (0.1-0.8) are used for labeling of the colloids in the same manner as in the (24) Vlcˇkova´, B.; Mateˇjka, P.; Sˇ imonova´, J.; C ˇ erma´kova´, K.; Pancˇosˇka, P.; Baumruk, V. J. Phys. Chem. 1993, 97, 9719.

Bimetallic Nanoparticles

Langmuir, Vol. 20, No. 8, 2004 3409

Table 1. Preparation, TEM Image Analysis, and EDX Analysis of (Ag)Au Nanoparticles (Ag)Au colloid (calcd molar fraction of Au) 0.8 (0.80) 25 mLa

0.7 (0.70) 15 mL

0.6 (0.61) 10 mL

0.5 (0.54) 7.5 mL

0.4 (0.44) 5 mL

0.3 (0.35) 3.5 mL

SET I: Mean Diameter of Ag Nanoparticles in Parent Ag Colloid ... 9 ( 2 nm mean (Ag)Au particle diameter (nm) TEM 16 ( 2 12 ( 2 11 ( 2 10 ( 2 calcd 15.3 12.3 10.9 10.4 mean core diameter (nm) TEM 6(1 5(1 6(1 Au:Ag ratio (atomic) EDX 3.82 1.62 0.85 0.51 calcd 3.93 2.36 1.57 1.18 0.79 0.55 Cl:Ag ratio (atomic) EDX 0.30 0.23 0.14 0.06 calcda 68.48 41.10 27.40 20.55 13.70 9.59 SET II: Mean Diameter of Ag Nanoparticles in Parent Ag Colloid ... 10 ( 2 nm mean (Ag)Au particle diameter (nm) TEM 17 ( 3 15 ( 2 14 ( 3 12 ( 3 11 ( 3 calcd 17.0 15.0 13.7 12.1 11.6 mean core diameter (nm) TEM 6(2 6(1 5(1 5(1 a

0.2 (0.24) 2 mL

0.1 (0.14) 1 mL

10 ( 2 9.9 0.29 0.31 0.04 5.48

2.74

10 ( 3 10.9

10 (2 10.5

0.16

Mililiters of reagent (value of x). b The ratio calculated from the composition of the reaction mixture.

previous papers.20,21 (Ag)Au-0.3, -0.6, and -0.8 colloids were selected as the reference samples for comparison with those prepared by modifications of the PPI procedure. TEM images of nanoparticulate films prepared from the selected (Ag)Au colloids and SP extinction spectra of the parent colloids are shown in Figures 1 and 2, respectively. The complete set of the data was published elsewhere.20,21 Preparation Procedure II. The PPI preparation procedure was modified by adjusting the pH of the seeding Ag colloid from neutral to alkaline (pH ) 10-11) by 1 M KOH. (Ag)Au-0.8-II, (Ag)Au0.6-II, and (Ag)Au-0.3-II colloids were prepared. Preparation Procedure III: (Ag)Au-0.6-III colloid. The PPI procedure was modified by reduction of the rate of the reagents addition. The amounts of the reagents were the same as in PPI, but the rate of addition of NH2OH‚HCl and HAuCl4 was reduced to 75 µL/min (i.e., 1 drop/min). The preparation procedure lasted for 133 min. A partial sedimentation of particles from the colloidal solution was observed after ca. 120 min. Preparation Procedure IV: (Ag)Au-0.8-IV colloid. The PPI procedure was modified by interruption of the Au overdeposition at the stage of (Ag)Au-0.3 for 24 h. To (Ag)Au-0.3 colloid prepared according to PPI and aged for 24 h, 21.5 mL of 6.25 × 10-3 M NH2OH‚HCl and 21.5 mL of 4.65 × 10-4 M HAuCl4 were added dropwise (ca. 4 mL/min) by two separate pipets upon vigorous stirring. The stirring was continued for 45 min. Preparation Procedure V: (Ag)Au-0.6-V colloid. The PPI procedure was principally modified (reversed). Instead of adding the reagents to Ag seeds, the reagents were first mixed, and Ag seeds were added to this reaction mixture. Briefly, to 30 mL of deionized distilled water, 10 mL of 6.25 × 10-3 M NH2OH‚HCl and 10 mL of 4.65 × 10-4 M HAuCl4 were added dropwise (ca 4 mL/min) by two separate pipets upon vigorous stirring. The reaction mixture remained colorless, and no formation of a colloid was observed. After addition of the seeding Ag nanoparticles (12.5 mL of Ag colloid), the reaction mixture turned gray-violet. Stirring of the mixture was continued for 45 min. The resulting hydrosol was unstable and sedimented within 3 days. Additional reduction of (Ag)Au colloids by NaBH4 was carried out according to the procedure described in ref 21. (Au)Au colloid was prepared as an analogue of the (Ag)Au-0.8 colloid with respect to the amount of the seeding and the shell material (which, in this case, are both gold). The seeds were formed by Au colloid (hydrosol) nanoparticles prepared according to ref 25 (mean particle size 4.9 ( 0.9 nm), and the amount of reagents used for the overdeposition of Au over the seeds was the same as that in the (Ag)Au-0.8 PPI preparation. The colloid was dark red in all stages of the preparation procedure. SP extinction spectra show a symmetric band with maximum at 518 nm. TEM imaging of (Au)Au-bpy film showed well-developed gold particles quite uniform in size (12 ( 2 nm). Preparation and Deposition of Metal Colloid-bpy Films. Parent (multilayer) interfacial films consisting of metal nano-

Figure 1. TEM images of selected (Ag)Au colloid-2,2′bipyridine films: (a) (Ag)Au-0.3-bpy film; (b) (Ag)Au-0.6-bpy film; (c) (Ag)Au-0.8-bpy film.

(25) Srnova´-Sˇ loufova´, I.; Vlcˇkova´, B. Nano Lett. 2002, 2, 121. Vlcˇkova´, B.; Barnett, S. M.; Kanigan, T.; Butler, I. S. Langmuir 1993, 9, 3234.

particles covered by adsorbed bpy were prepared according to the preparation procedures originally developed for Ag colloid-

3410

Langmuir, Vol. 20, No. 8, 2004

Srnova´ -S ˇ loufova´ et al. vis measurements, carbon disks for EDX analysis, steel plates with sputtered carbon layer for XPS, and carbon-coated Cu grids for TEM and surface-enhanced Raman scattering (SERS). Furthermore, Ag colloid/HCl/bpy and Ag colloid/NaCl/bpy film were prepared as the reference samples for EDX analysis. In their preparation, the original procedure26 was modified by addition of 10 µL of 1 M HCl or NaCl to 2 mL of the parent Ag hydrosol. Details of the preparation and morphology of these films are reported elsewhere.28 Instrumentation. A JEOL-JEM 200 CX transmission electron microscope was employed for imaging of metal colloids and metal colloid-bpy films deposited on carbon-coated Cu grids. Deposited dried-drops of the metal hydrosols were imaged only provided that attempts to prepare a two-dimensional nanoparticulate film from the particular colloid failed. A JEOL SUPERPROBE 733 scanning electron microscope equipped with JXA 733 X-ray analyzer and KEVEX ∆ EDX spectrometer was used for chemical analysis. Metal colloid-bpy films deposited on carbon blocks of 5 mm in diameter and of several millimeters height were used as samples. A MR line of Au and LR line of Ag were selected for analysis. The error of the analysis was less than 20%. The presence of Cl (line KR) in the sample was detected in addition to the characteristic lines LR of Ag and MR of Au. The estimated error for Cl analysis was about 20-30%. Deposited Ag colloid/NaCl-bpy film, Ag colloid/HCl-bpy film, and (Au)Au-bpy film were analyzed as the reference samples for detection of Cl. The X-ray photoelectron and Auger spectra were measured using an ESCA 310 (Gammadata Scienta, Sweden) electron spectrometer equipped with a high-power rotating anode, wide-angle quartz crystal monochromator, and a hemispherical electron analyzer operated in a fixed transmission mode. All XPS measurements were performed using Al KR radiation and an analyzer pass energy of 150 eV. The pressure of residual gases in the analyzer chamber was typically 6 × 10-10 mbar. A series of Au (4f), Ag (3d), C (1s), O (1s), Na (1s), Cl (2p) and N (1s) photoelectron spectra and Ag (M45VV) Auger electron spectra were measured at 90° and 5° (or 10°) detection angles defined with respect to the surface plane of the supporting carbon substrate. Bulk Au and Ag standards were employed as the reference samples. The accuracy of the measured electron energies was (0.02 eV. Spectra were curve-fitted after subtraction of Shirley background29 using Gaussian-Lorentzian line shape. Quantification of the elemental concentrations was accomplished by correcting photoelectron peak intensities for their cross sections.30 SERS spectra of bpy from the selected areas of metal colloid-adsorbate films deposited on C-coated Cu grids were obtained by a commercial confocal Raman microspectrometer LabRam (ISA) consisting of an Olympus BX40 microscope coupled to a single grating imaging spectrometer equipped with a CCD detector. Spectra were excited with an Ar-pumped Ti-sapphire laser operating at 780 nm. SP extinction spectra were collected on a Hewlett-Packard HP 8462A diode array UV-vis spectrometer.

Results and Discussion

Figure 2. Surface plasmon extinction (UV-vis) spectra of selected (Ag)Au colloids: (A) (Ag)Au-0.3; (B) (Ag)Au-0.6; (C) (Ag)Au-0.8. The spectra were measured (- - -) immediately after addition of the reactants, (s) after accomplishment of the preparation procedure, and (‚ ‚ ‚) after an additional reduction by NaBH4. bpy films.26 Reassembling of the nanoparticulate film into a monolayer and its deposition on various substrates were accomplished by the procedures described in refs 20 and 27. The following supporting substrates were used: glass slides for UV(26) (a) Vlcˇkova´, B.; Barnett, S. M.; Kanigan, T.; Butler, I. S. Langmuir 1993, 9, 3234. (b) Solecka´-C ˇ erma´kova´, K.; Vlcˇkova´, B.; Lednicky´, F. J. Phys. Chem. 1996, 100, 4954. (27) Srnova´, I.; Vlcˇkova´, B.; Neˇmec, I.; Sˇ louf, M.; Sˇ teˇpa´nek, J. J. Mol. Struct. 1999, 482-483.

EDX Analysis of (Ag)Au Nanoparticles Prepared by PPI. EDX analysis of the (Ag)Au nanoparticles assembled into (Ag)Au colloid-bpy films was targeted to determination of the Au:Ag ratios and of the relative content of Cl within the particles. The results are summarized in Table 1. The differences between the calculated and experimentally obtained Au:Ag atomic ratios are less than 10%, which is within the estimated experimental error of the multiparticle EDX analysis. This agreement indicates that the constitution of the particles (28) Sˇ loufova´, I. Ph.D. Thesis, Charles University, Prague, 2000. Srnova´-Sˇ loufova´, I.; Sˇ isˇkova´, K.; Vlcˇkova´, B.; Sˇ teˇpa´nek, J. In Proceedings of the XVIIIth International Conference on Raman Spectroscopy (ICORS 2002), 25-30 August, Budapest, Hungary; Mink, J., Jalsovsky, G., Keresztury, G., Eds.; John Wiley and Sons, Ltd.: Chichester, England, 2002, pp 277-278. (29) Shirley, D. A. Phys. Rev. B 1972, 5, 4709. (30) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1971, 8, 129.

Bimetallic Nanoparticles

(at least as determined from their assembly) represents the ratio of the metals employed in the preparation procedure. Since no peak of Au(III) was observed in the UV-vis spectra of the colloids (Figure 2), we assume that Au(III) was quantitatively reduced during the preparation procedure. The amount of Ag remaining in the solution in the form of Ag+ (and/or of its complexes) is thus lower than ca. 10%. As no turbidity was observed during all the stages of colloid preparation, the presence of noticeable quantities of solid AgCl in the aqueous ambient can be excluded. The results of EDX analysis are in accord with the match between the calculated and measured particle sizes (within 3%) reported in ref 20. The characteristic peak of Cl was detected in EDX spectra of all samples of (Ag)Au-bpy films (Table 1), as well as in the reference samples of Ag colloid/NaCl-bpy (atomic ratio Cl:Ag ) 0.11) and Ag colloid/HCl-bpy (atomic ratio Cl:Ag ) 0.16) films. By contrast, no Cl was detected in (Au)Au colloid-bpy film. The absence of Cl in EDX analysis of the (Au)Au colloid-bpy film, in contrast to its presence in EDX of (Ag)Au colloid-bpy films leads to the following implications: First, the results of EDX show that no detectable amount of Cl was introduced into the (Au)Au colloid-bpy film upon both its formation and deposition. Hence, it is highly probable that Cl atoms detected by EDX in the (Ag)Au colloid-bpy films were also not introduced into the system during deposition, but were an inherent part of the parent (Ag)Au nanoparticle composition. Second, the fact that no detectable amount of Cl is bound to Au in (Au)Au colloid-bpy films indicates that the Cl atoms detected in (Ag)Au colloid-bpy films are bound selectively to silver. Since the possibility that AgCl might form a separate phase within the (Ag)Au-0.8 particles was excluded,20 it can be well asssumed that the Cl atoms are located on the nanoparticle surface. The detected Cl signal thus most probably belongs to adsorbed chlorides and/or complex ions containing chlorides as ligands, e.g., [AgCl2]-. In summation, the presence of Cl on the surface of (Ag)Au0.8 particles, in contrast to its absence on the surface of (Au)Au particles, indicates the presence of Ag on the surface of (Ag)Au particles. SERS Spectra of (Ag)Au Colloid-bpy Films Consisting of Nanoparticles Prepared by PPI. SERS spectral probing of (Ag)Au nanoparticle surfaces was accomplished by collecting SERS signal of adsorbed bpy from selected regions of deposited (Ag)Au colloid-bpy films. In the previous SERS spectral studies, two spectral forms of adsorbed bpy were identified on Ag nanoparticle surfaces modified by adsorbed chlorides in Ag hydrosol systems: form I (identified on unmodified as well as on chloride modified surfaces) and form III (identified solely on modified surfaces at high concentration of chlorides).31-33 SERS spectra obtained from (Ag)Au-0.7 colloid-bpy film using 780 nm excitation are shown in Figure 3A, together with the SERS spectra of bpy measured from Ag colloid-bpy (Figure 3B) and Au colloidbpy (Figure 3C) films. All spectra were baseline corrected. The common features of all the spectra are the bands at 766, 1014, 1306, and 1478 nm. These bands are the characteristic marker bands form I of bpy adsorbed on Ag.31-33 Observation of the same characteristic marker bands in the SERS spectrum of Au colloid-bpy film (Figure 3C) indicates the presence of form I of adsorbed bpy on (31) Kim, M.; Itoh, K. J. Electroanal. Chem. 1985, 188, 137. (32) Kim, M.; Itoh, K. J. Phys. Chem. 1987, 91, 126. (33) Srnova´-Sˇ loufova´, I.; Vlcˇkova´, B.; Snoeck, T. L.; Stufkens, D. J.; Mateˇjka, P. Inorg. Chem. 2000, 39, 3551.

Langmuir, Vol. 20, No. 8, 2004 3411

Figure 3. SERS spectra of (A) (Ag)Au-0.7 colloid-bpy film, (B) Ag colloid-bpy film, and (C) Au colloid-bpy film.

the surface of Au nanoparticles. No marker bands of form III were observed in this spectrum. The absence of form III on the surface of Au nanoparticles despite the presence of chlorides (originating from HAuCl4 used as the reactant in Au nanoparticle preparation) in the systems is understandable particularly in the light of the EDX results which have shown that chlorides, although present in the system, are not adsorbed on the Au nanoparticle surface. In Ag colloid-bpy films, Ag nanoparticles are also not modified by chlorides, since no residual chlorides were present in the system (the starting material of Ag nanoparticle preparation is AgNO3). Hence, form III is also absent in SERS of Ag colloid-bpy film (Figure 3B). In contrast to those, the presence the marker bands at 1032, 1321, and 1493 cm-1 in the SERS spectrum of (Ag)Au-0.7 colloid-bpy (Figure 3A) clearly indicates the presence of form III, which was identified on chloridemodified Ag surfaces.31-33 SERS spectral probing of (Ag)Au nanoparticles by adsorbed bpy molecules indicates the presence of Ag on the surface of (Ag)Au nanoparticles and occupation of a part of the available Ag adsorption sites by adsorbed chlorides. The results of SERS spectral probing are consistent with those obtained by EDX analysis. XPS Analysis of Ag(Au) Nanoparticles Prepared by PPI and Assembled in (Ag)Au Colloid-bpy Films. In contrast to EDX analysis, in which the collection depth of the signal largely exceeds the dimensions of the nanoparticles, the signal collection depth in the XPS experiment is ∼3λ, where λ ) 1.53 and 1.65 nm for Ag (3d) and Au (4f) electrons, respectively.34 The intensity of a photoemission line is given by a convolution of the concentration gradient and the exponential attenuation law. In our XPS experiments, the sampling depth of photoelectrons was ca. 4.7 nm. The samples for XPS were deposited monolayer nanoparticulate films/(Ag)Au colloid-bpy films/prepared from the selected samples of (Ag)Au colloids. The characteristics of the (Ag)Au nanoparticles investigated by XPS are given in Table 2. The Ag (3d) and Au (4f) spectra of the selected (Ag)Au nanoparticles and of pertinent bulk metals are displayed in parts A and B of Figure 4, respectively. Relative atomic concentrations of elements (Au, Cl, O, N, C and Na) with respect to Ag determined at 90° and 5°(or 10°) takeoff angles are listed in Table 3. With the exception of oxygen (34) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1991, 17, 911.

3412

Srnova´ -S ˇ loufova´ et al.

Langmuir, Vol. 20, No. 8, 2004 Table 2. TEM Image Analysis and XPS Analysis of (Ag)Au Nanopaticles sample

I (Ag)Au-0.8

II (Ag)Au-0.6

III (Ag)Au-0.4

molar fraction of Au original core diameter (nm) actual core diameter (nm) (Ag)Au particle diameter (nm) Au:Ag molar ratio of the particle calcd Au:Ag molar ratio of the shell calcda XPS analysis Au:Ag molar ratio Cl:Ag molar ratio

0.80 9 6 16 3.9 5.57

0.61 9 5 12 1.6 1.89

0.44 10 5 12 0.8 0.90

2.86 0.46

1.82 0.36

0.78 0.20

a Calculated using the actual Ag core diameter and assuming alloy shell constituted by Ag (the amount of which corresponds to the difference between the amount of Ag in the original Ag seed and in the actual Ag core) and Au (the amount of Au overdeposited) in the second reduction step.

Table 3. XPS Spectral Analysis of (Ag)Au Nanoparticles

sample III (Ag)Au-0.4 II (Ag)Au-0.6 I (Ag)Au-0.8

angle of detection (deg)

O

C

Cl

N

Na

Au

90 5 90 10 90 5

0.30 0.24 0.39 0.36 0.55 0.45

1.93 1.07 2.61 2.20 5.01 3.10

0.20 0.18 0.36 0.28 0.46 0.51

0.11 0.12 0.26 0.20 0.23 0.19

0.010 0.005 0.018 0.016 0 0.004

0.78 0.82 1.82 1.76 2.86 2.69

Table 4. X-ray Photoelectron and Auger Electron Spectral Characteristics of (Ag)Au Nanoparticles sample (Ag)Au-0.4 (III) (Ag)Au-0.6 (II) (Ag)Au-0.8 (I) Ag Au

Ag (3d5/2)

Au (4f7/2)

R′

Ep

368.15 (0.64) 84.12 (0.54) 725.68 2.94 367.95 (0.69) 84.03 (0.53) 725.45 2.79 367.95 (0.73) 84.06 (0.54) 725.47 2.65 368.24 (0.48) 726.00 3.71 84.00 (0.58)

W

R(Ag)

4.62 4.90 5.04 3.28 5.18

0.42 0.41 0.58 0.47

and carbon, the obtained concentrations of elements for samples II and III do not depend within the experimental error ((10%) on takeoff angle as expected for a layer of randomly oriented particles. By contrast, for sample I, the Au:Ag atomic ratio is markedly lower for the 5° takeoff angle than for the 90° one, which indicates enrichment of the surface layer(s) by Ag. Binding energies of Au (4f7/2) and of Ag (3d5/2) electrons and the half-width (fwhm) of the corresponding photolines were determined and compared to those of bulk Ag and Au samples. Furthermore, the values of the modified Auger parameter R′ defined as a sum of the 3d5/2 core level binding energy and kinetic energy of M5VV electrons were determined for Ag in nanoparticles and compared to the value for bulk silver.35 The results obtained show subtle positive shifts (∼0.1 eV) of the binding energies of Au (4f7/2) electrons compared to the bulk Au (Table 4). By contrast, the Ag (3d5/2) spectra of the nanoparticle samples, in comparison to the bulk Ag standard, are broadened and shifted toward lower binding energies. The R′ values also decrease. The observed shifts are consistent with those reported for Ag-Au alloys.36 An approximately linear red shift of plasmon energy (Ep) with increasing concentration of Au observed in the spectra of Ag (3d) electrons (Table 4) is also in accord with the data published for gold-silver alloys.10 Shown in Figure 4C are the spectra in the valence band region. The spectra recorded for the bimetallic nanoparticles cannot be obtained simply by combining the spectra of the constituent elements. The measured d-bandwidth (W) decreases with the increasing content of Ag (Table 4). (35) NIST X-ray Photoelectron Spectroscopy Database, ver. 2.0, US Department of Commerce; NIST: Gaithersburg, MD 20899, USA, 1997. (36) Watson, R. E.; Hudis, J.; Perlman, M. L. Phys. Rev. B 1971, 4, 4139.

This behavior is characteristic of Ag-Au alloys37,38 and points thus again to the formation of the alloy in the superficial region of the (Ag)Au nanoparticles. Most importantly, model simulations using the QUASES program package39 have shown that Ag is present not only in the core of the particles but also in the shell. Had all the Ag been present as a core, the shape of the silver photoemission lines would be more substantially affected by inelastic scattering of the photoelectrons traveling through the Au shell. The binding energy shifts as well as the structure of the valence bands, and the results of model simulation are consistent with the Ag core-Ag/Au alloy shell composition of the nanoparticles. Furthermore, analysis of the XPS spectra confirms the presence of Cl as an inherent part of the (Ag)Au colloid nanoparticle composition (Table 3). On the other hand, the presence of O can be caused either by oxidization of the surface Ag atoms upon exposure of the deposited film to the air or by emission from oxygen-containing groups present on the carbon substrate surface. The latter explanation is supported by the existence of correlation between intensities of O (1s) and C (1s) peaks and by their dependence on the takeoff angle, caused likely by the presence of voids in a layer of nanoparticles (Table 3). A fraction of the C (1s) signal and the N (1s) peak (Table 3) originate from the bpy adsorbate. In Table 2, the Au:Ag and Cl:Ag molar ratios determined by the XPS analysis are related to TEM images and TEM image analysis of the selected samples of (Ag)Au colloidbpy films. Samples I and II were prepared from set 1 (Table 1) of (Ag)Au colloids, and sample III was prepared from the set 2. Both the match between the calculated and experimentally determined particle sizes and the calculated and experimentally determined overall particle composition indicate that the total amount of Ag constituting the (Ag)Au nanoparticles is nearly the same as that in the parent (seeding) Ag colloid nanoparticles. The observed substantial decrease of average size of the (Ag)Au nanoparticle cores in comparison to the average size of the parent Ag seeds (Tables 1 and 2) clearly indicates that Ag is present also within the shell. The results of the TEM image analysis, which are in full accord with the results of the XPS spectral analysis and model simulations presented above, allowed us to calculate the expected composition of the nanoparticle shell under an assumption that the shell composition is homogeneous. The results are presented in Table 2 in terms Au:Ag molar ratios. Considering the ca. 4.7 nm sampling depths of photoelectrons and the results of the TEM image analysis of (37) Sham, T. K.; Bzowski, A.; Kuhn, M.; Tyson, C. C. Solid State Commun. 1991, 80, 29. (38) Nahm, U. T.; Oh, S. J.; Chung, S. M. J. Electron Spectrosc. Relat. Phenom. 1996, 78, 127. (39) Tougaard, S. Appl. Surf. Sci. 1996, 100/101, 1.

Bimetallic Nanoparticles

Figure 4. (A) Ag (3d) core level spectra of (1) bulk Ag, (2) (Ag)Au-0.4 sample, (3) (Ag)Au-0.6 sample, and (4) (Ag)Au-0.8 sample. The spectra are normalized to the same height, and the region with plasmons is magnified. (B) Au (4f) core level spectra of (1) bulk Au, (2) (Ag)Au-0.4 sample, (3) (Ag)Au-0.6 sample, and (4) (Ag)Au-0.8 sample. The spectra are normalized to the same height. (C) The photoelectron spectra of valence region of (1) bulk Au, (2) bulk Ag, (3) (Ag)Au-0.4 sample, (4) (Ag)Au-0.6 sample, and (5) (Ag)Au-0.8 sample. The spectra are normalized to the same height.

the samples, we can relate the results of the XPS analysis to the nanoparticle composition in the following manner. In the case of sample I with the average shell thickness of 5 nm, the measured intensity of the photoemission signal comes almost entirely from the shell region of the particles. Provided that the shell is a homogeneous alloy, there should be an agreement between the calculated composition of the shell and the Au:Ag molar ratio determined by XPS. However, the Au:Ag molar ratio determined by XPS

Langmuir, Vol. 20, No. 8, 2004 3413

is ca. two times lower (Table 2). This result indicates inhomogeneity of the alloy shell composition and its enrichment by Ag toward the surface. This interpretation appears to be also supported by a higher value of the Auger to photoemission line intensity ratio, R(Ag) (Table 4), compared to the homogeneous bulk Ag (due to the lower kinetic energy of Auger electrons, the depth sampled by these electrons is about half of that sampled by 3d photoelectrons; λ(MVV) ) 0.72 nm34). Samples II and III have the same size both of the core (5 nm diameter) and of the shell (3.5 nm thickness) but differ mutually in the shell composition, as indicated in Table 2. For these samples, the signal is collected from the overall shell thickness with some contribution of the signal from the Ag core of the particles. Therefore, even if the shell was a homogeneous alloy, the experimentally determined Au:Ag molar ratios are expected to be somewhat lower than the calculated ones due to the contribution of the signal from the Ag core. The fact that the experimentally observed Au:Ag molar ratios are, for both samples, lower than the calculated ones (Table 2) thus cannot be unequivocally related to localization of Ag at the surface (in contrast to the sample I). This interpretation is in agreement with a slightly lower value of I (MVV)/I (3d) ratio obtained for samples II and III compared to the bulk Ag value. Finally, a pronounced decrease of Au:Ag molar ratios was determined by XPS analysis (measurements at 90° takeoff angle) after heating of samples I and III to 180 °C: from 2.86 (Table 3) to 2.11 for sample I and from 0.78 (Table 3) to 0.49 for sample III. These results indicate that heating of the Ag core-Ag/Au alloy shell nanoparticles leads to an additional enrichment of the shell by Ag toward the nanoparticle surface. Proposed mechanism of Ag Core-Ag/Au Alloy Shell Nanoparticle Growth. The effect of modifications of the preparation procedure I (PPI) has been evaluated by comparison of SP extinction spectra and TEM images of selected samples of (Ag)Au colloids prepared by PPI (Figures 1 and 2) with those prepared by the modified procedures PPII-V (Figures 5 and 6). A description of the procedures and the notation of the resulting (Ag)Au hydrosols are given in the Experimental Section. At first, we consider the effect of the changes in the preparation protocol on the extinction spectra of the (Ag)Au colloids measured after the colloid preparation was accomplished. The maxima in the SP extinction spectra of (Ag)Au 0.5-0.8 colloids prepared by PPI, i.e., by a rapid dropwise addition of the reactants in the neutral ambient, are red shifted with respect to SP extinction of pure Au colloid (518-522 nm), as demonstrated in Figure 2 for (Ag)Au-0.6 and (Ag)Au-0.8 colloids with the SP extinction maxima at 534 and 540 nm, respectively. This anomaly, however, recedes after the additional reduction the colloids by sodium borohydride. The additional reduction causes a blue shift of the SP extinction maxima of all (Ag)Au 0.1-0.8 colloids in such a way that the SP extinction maxima of all additionally reduced (Ag)Au 0.1-0.8 colloids are located between those of pure Ag and pure Au colloids.21 In particular, for (Ag)Au-0.6 and (Ag)Au-0.8 colloids after the additional reduction, these maxima are at 506 and 522 nm, respectively (Figure 2). For the selected colloids prepared by PPII, i.e., by a rapid dropwise addition of the reactants in the alkaline ambient, the above-mentioned effects are much less pronounced, or rather, almost negligible. First of all, the SP extinction maxima of (Ag)Au 0.6-II and (Ag)Au 0.8-II are located at 510 and 506 nm, respectively, i.e., within those of pure Ag and Au colloids (Figure 5). Furthermore, additional reduction by sodium

3414

Langmuir, Vol. 20, No. 8, 2004

Srnova´ -S ˇ loufova´ et al.

Figure 5. (a) SP extinction spectra of (Ag)Au-II and (Ag)AuIIr (additionally reduced) colloids. (b-d) TEM images of (Ag)Au-II colloids: (b) (Ag)Au-0.8-II; (c) (Ag)Au-0.6-II; (d) (Ag)Au-0.3-II.

borohydride induces no shift of SP extinction maximum of (Ag)Au 0.8-II, and only a small shift to 494 nm for (Ag)Au 0.6-II (Figure 5). The SP extinction maximum of (Ag)Au 0.6-III (slow addition of reactants in the neutral ambient) at 512 nm (Figure 6, curve a) is also located between those of Ag and Au. On the other hand, interruption of the original PPI procedure at the 0.3 stage for 24 h yielded (Ag)Au 0.8-IV colloid with the SP extinction maximum at 538 nm (Figure 6, curve b), i.e., shifted toward the red from the Au plasmon in the same manner as for (Ag)Au0.8 (540 nm). It thus appears that the pronounced red shifts of SP extinction maxima toward the red from Au

Figure 6. (A) SP extinction spectra of the (Ag)Au-0.6-III colloid (curve a), the (Ag)Au-0.8-IV colloid (curve b), and the (Ag)Au0.6-V colloid (curve c). (B) TEM image of (Ag)Au-0.6-III-bpy film. (C) TEM image of (Ag)Au-0.8 IV-bpy film. (D) TEM image of (Ag)Au-0.6-V colloid.

plasmon are limited to procedures in which overdeposition of the shell over Ag seeds was carried out at a rapid rate of addition of the reactants in the neutral ambient. It should also be noted that no such shift is observed for the (Au)Au colloid (SP extinction maximum at 518 nm). The red shifts of SP extinction maxima are thus related to Ag as the seeding material in PPI. Analogously to the red

Bimetallic Nanoparticles

shifts in the SP extinction spectra measured after the preparation of the particular (Ag)Au colloid and discussed above, also observation of the blue color stage during the preparation procedure (caused by a dramatic shift of SP extinction maximum to 592 nm in the (Ag)Au-0.3 stage of PPI shown in Figure 2A) is limited to a rapid addition of the reactants in the neutral ambient and to employment of Ag as the seeding material. Furthermore, there is evidence that the Ag seeds do not remain intact in the course of Au deposition by PPI. In particular, TEM image analysis (Table 1 and ref 20) shows that the original Ag seeds (average diameter 9 nm) were reduced in size forming the 5-6 nm cores of the (Ag)Au 0.4-0.8 particles. The optical constants extracted from the SP extinction spectra of (Ag)Au 0.1-0.8 measured after the additional reduction indicate that the shells of these particles are constituted by an Ag/Au alloy.21 In accord with that, XPS spectra provide direct evidence of the presence of both Au and Ag in the shell material of (Ag)Au-0.8. Furthemore, XPS and SERS spectra indicate the presence of Ag on the surface of (Ag)Au-0.7 and (Ag)Au0.8, i.e., on the surface of (Ag)Au nanoparticles with the highest content of Au in the shell. EDX analysis indicates the presence of Cl on the surfaces of all (Ag)Au 0.1-0.8 particles (formation of AgCl phase was excluded by TEM image simulations in ref 20), while no Cl is detected for (Au)Au particles. We thus propose that overdeposition of Au over Ag seeds by PPI results in alloying of Ag and Au in the shell as well as in a positive charging of the nanoparticles in the course of this process. We also propose that the positive charging of the particles and its compensation by adsorption of negatively charged ions, which are present in the reaction mixture (in particular chlorides), is the major factor responsible for the abovementioned red shifts in the SP extinction spectra. We now consider a possible scenario how the positive charging (anodic polarization) of the nanoparticles in the course of PPI can occur. In the (Ag)Au colloid preparations, a more electropositive metal, Au, is deposited over a more electronegative metal, Ag: Au3+/Au, E0 ) +1.5 V; Ag+/Ag, E0 ) +0.8 V. 40 The bulk redox potential of the actual system involved in the preparation is AuCl4-/Au, E0 ) +1.0 V.40 Reduction of HAuCl4 by NH2OH‚HCl in the slightly alkaline ambient has been proved to proceed by an electrocatalytic mechanism, i.e., by electron transfer from the adsorbed reductant to adsorbed AuCl4- via the seeding nanoparticle. This mechanism ensures that reduction of HAuCl4 to Au0 proceeds entirely on the seeding metal nanoparticles11,41 and prevents formation of mixed colloids. In this paper, the results of PPV (carried out in the neutral ambient), namely, the observation that reduction of HAuCl4 by NH2OH‚HCl in this ambient does not proceed unless Ag seeds are injected into the reaction mixture, indicate that reduction of HAuCl4 by NH2OH‚ HCl during PPI proceeds electrocatalytically as well. The following possible mechanism of the electrocatalytic reduction can be envisaged: NH3OH+ which is the actual reducing agent in the neutral ambient (2NH3OH+ f 2e(40) Lide, D. R. Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC Press: New York, 1995. (41) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726.

Langmuir, Vol. 20, No. 8, 2004 3415

+ 4H+ + 2H2O + N2, -1.87 V)40 transfers electrons to AuCl4- (and/or to an reduction intermediate) via the Ag0n nanoparticle. Provided that the number of electrons transferred from the reductant to the particles is lower than of those transferred from the particles to the electron acceptor, electron depletion the Ag0n nanoparticle(s) can occur via the following processes:

I. Ag0n + AuCl4- f Ag0n-3(AgCl)3 + Cl- + Au0 a positive charging (anodic polarization) of Ag0n nanoparticle, compensated by adsorption of anions such as Cland/or their complex ions.

II. Ag0n + AuCl4- f Ag0n-3 + 3Ag+ + 4Cl- + Au0 an oxidative etching of the Ag0n nanoparticle. Both processess were reported earlier to proceed in the absence of a reducing agent and were employed for deposition of small amounts of Au over Ag. In the case of PPI, the above-mentioned processes would occur provided that the electron-transfer rate from the particle to AuCl4is faster than that from NH3OH+ to the particle and/or AuCl4- is adsorbed faster than NH3OH+. Conclusions Overdeposition of Au over Ag seeds (preprepared by reduction of silver nitrate by sodium borohydride, average diameter 9-10 nm) carried out by reduction of HAuCl4 by hydroxylamine hydrochloride in the neutral ambient under a rapid dropwise addition of the reagents results into formation of core-shell bimetallic nanoparticles with Ag (or Ag rich) cores (average size 5-6 nm) and Au/Ag alloy shells. For these particles with shell thicknesses of ca. 5 nm, enrichment of the shell composition by Ag toward and/or at the surface was determined by XPS. In accord with that, EDX and SERS spectral probing by bpy molecules indicated the presence of Ag on the surface of the nanoparticles. Unusual red shifts of SP extinction of the nanoparticles observed in the course and after the preparation are attributed to positive charging of the nanoparticles (compensated by adsorption of anions, as witnessed by detection of chlorides within the nanoparticles) during the electrocatalytic reduction. The anomalies in the SP extinction were canceled by an additional reduction. After preparations carried out without the additional reduction, assembling of the bimetallic nanoparticles into multilayer and their reassembling into monolayer nanoparticulate films mediated by bpy were readily accomplished. Acknowledgment. The authors thank Professor Martin Moskovits, Department of Chemistry, University of Toronto (present address: Department of Chemistry, University of California at Santa Barbara) for a generous loan of the Labram Raman microspectrometer for SERS spectral measurements and for valuable advise. Financial support by the 203/01/1013 and 203/02/P028 (I.S-S ˇ ) grants awarded by the Grant Agency of the Czech Republic are gratefully acknowledged. The study is a part of the longterm research program of the Faculty of Science, Charles University Prague, Grant No. MSM 113100001. LA0302605