Gold Colloids from Cationic Surfactant Solutions. 1. Mechanisms That

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Gold Colloids from Cationic Surfactant Solutions. 1. Mechanisms That Control Particle Morphology Epameinondas Leontidis,*,† Konstantina Kleitou,† Tasoula Kyprianidou-Leodidou,† Vlasoula Bekiari,‡ and Panagiotis Lianos‡ Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus, and Engineering Science Department, University of Patras, GR-26500 Patras, Greece Received August 29, 2001. In Final Form: January 23, 2002 The mechanism of formation of gold particles by reduction of AuIII in solutions of alcyltrimethylammonium chloride surfactants was studied in the absence and in the presence of NaCl. AuIII anions interact strongly with trimethylammonium cations forming insoluble ion pairs (Torigoe et al. Langmuir 1992, 8, 59). Above the surfactant critical micelle concentration, the ion pairs are solubilized in the micelles returning to the solution. Gold particles were produced by photochemical reduction of the clear micellar solutions. The coupling between surfactant aggregation and inorganic crystallization phenomena in these systems was investigated using transmission electron microscopy (TEM), UV-vis, and time-resolved fluorescence spectroscopy. At concentrations close to the phase boundary of the L1 phase with the lyotropic liquid crystalline phases many gold particles have a threadlike morphology, as previously noted by Esumi et al. (Langmuir 1995, 11, 3285). The presence of NaCl modifies the micellar size and affects the gold crystallization process in surprising and unexpected ways, as evidenced by intermediate structures observed by TEM. Our observations support the idea that the formation of threadlike gold particles occurs primarily through a combination of crystal aggregation and specific crystal face stabilization and not through templating mechanisms.

I. Introduction Gold colloids have fascinated science from the times of the alchemists. Although the modern era of research on gold colloids was initiated by Michael Faraday 150 years ago,1 the scientific interest has always been strong. In recent years, the emergence of nanotechnology and the evolution of modern analytical methods that allow easy observation and manipulation of nanoparticles have created an explosion of interest in the synthesis and study of gold clusters and colloids.2 A major current research theme is to tailor the size and shape of the particles at will using colloid chemistry methods. While size-control and stabilization have been demonstrated in numerous cases, and ingenious new methods for the production and stabilization of gold nanoparticles have been reported,3 only a limited number of papers dealing with effective shape control of gold particles exists in the literature. Controlling the shape of nanoparticles is technologically important, since the optical, electronic, magnetic, and * To whom all correspondence should be addressed. † University of Cyprus. ‡ University of Patras. (1) Faraday, M. Philos. Trans. R. Soc. London, Ser. A 1857, 147, 145. (2) (a) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (b) Henglein, A. Chem. Rev. 1989, 89, 1861. (c) Schmid, G. Chem. Rev. 1992, 92, 1709. (d) Wang, Z.-L. Adv. Mater. 1998, 10, 13. (e) Templeton, A. C.; Wuelfing, P. W.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (3) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801. (b) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3486. (c) Templeton, A. C.; Chen, S.; Gross, M.; Murray, R. W. Langmuir 1999, 15, 66. (d) Han, M. Y.; Quek, C. H.; Huang, W.; Chew, C. H.; Gan, L. M. Chem. Mater. 1999, 11, 1144. (e) Warner, M. G.; Reed, S. M.; Hutchison, J. E. Chem. Mater. 2000, 12, 3316. (f) Bronstein, L. M.; Chernyshov, D. M.; Valetsky, P. M.; Wilder, E. A.; Spontak, R. J. Langmuir 2000, 16, 8221. (g) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (h) Zhou, Y.; Itoh, H.; Uemura, T.; Naka, K.; Chujo, Y. Chem. Commun. 2001, 613. (i) Yonezawa, T.; Yasui, K.; Kimizuka, N. Langmuir 2001, 17, 271. (j) Shon, Y.-S.; Wuelfing, P. W.; Murray, R. W. Langmuir 2001, 17, 1255.

catalytic properties of a nanomaterial often depend critically not only on particle size but also on particle shape.4 In the case of gold, for example, it was recently demonstrated that the longitudinal plasmon resonance shifts to higher wavelengths with increasing aspect ratio of the particles5 and also that nanoparticles with a high aspect ratio exhibit a strong fluorescence enhancement.6 Forcing colloidal particles to acquire a nonspherical, elongated shape is no easy matter as it involves a fight against thermodynamics, which dictates that the minimum free energy structure is that with the minimum specific area, hence a sphere, a cube, or some other compact shape. Forcing gold particles to acquire shapes with high aspect ratios using wet chemical routes has been demonstrated only in a handful of experimental systems to date. In all these cases it is either certain or possible that a templating mechanism is operating. The rodlike gold particles, prepared in the pores of alumina membranes by electrochemical reduction, are a clear demonstration of the success of a hard template.7 Formation of rodlike gold particles by an electrochemical reduction method in a surfactant solution was demonstrated by the group of (4) (a) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (b) Dirix, Y.; Bastiaansen, C.; Caseri, W.; Smith, P. Adv. Mater. 1999, 11, 223. (c) Dickson, R. M.; Lyon, L. A. J. Phys. Chem. B 2000, 104, 6095. (5) (a) Van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G. J.; Scho¨nenberger, C. J. Phys. Chem. 1997, 101, 852. (b) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (c) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (6) Mohamed, M. B.; Volkov, V.; Link, S.; El-Sayed, M. A. Chem. Phys. Lett. 2000, 317, 517. (7) (a) Tierney, M. J.; Martin, C. R. J. Phys. Chem. 1989, 93, 2878. (b) Foss, C. A.; Tierney, M. J., Martin, C. R. J. Phys. Chem. 1992, 96, 9001. (c) Foss, C. A.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963. (d) Hornyak, G. L.; Patrissi, C. J.; Martin, C. R. J. Phys. Chem. B 1997, 101, 1548. (e) Al-Rawashdeh, N. A. F.; Sandrock, M. L.; Seugling, C. J.; Foss, C. A. J. Phys. Chem. B 1998, 102, 361. (f) Van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G. J.; Scho¨nenberger, C. Langmuir 2000, 16, 451.

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Wang8 and postulated to depend on some unidentified templating mechanism. Gold particles formed with the previous two methods have been used in very interesting experiments, including thermal and laser-shape annealing9 and examination of the crystal faces by high-resolution transmission electron microscopy (TEM),10 while they have shown interesting light propagation,4c absorption,5 fluorescence,6 and self-assembly11 properties. A third method to produce long, threadlike gold particlessalthough not exclusivelyswas advanced by Esumi and Torigoe in a series of publications.12 In their method, gold particles are produced by photochemical reduction in the presence of cationic surfactant micelles. The authors expressed the opinion that the templating effect of rodlike micelles is essential for the formation of particles with very large aspect ratios in these systems.12b,e Despite the reported successful attempts to modify gold crystal shapes in surfactant solutions,8,12 little is known about the actual mechanism behind shape control. Relatively few recent studies are concerned with the formation mechanism of gold colloids,13 even though such studies have appeared in much older pioneering work.14 While important investigations using the γ-irradiation method have focused on the reduction steps of the reaction AuCl4f Au0 f Aun,15 gold particle formation by photoreduction in the presence of surfactants and electrolytes is little understood. We have decided to further investigate the formation of gold particles in trimethylammonium surfactant solutions to achieve a better understanding of the crystallization process in the presence of surfactant aggregates. The solution of cationic trimethylammonium surfactants is simpler than other systems that have been used so far. In addition, these surfactants appear to play a rather unique role in crystallizing systems, as they are involved in a variety of cases, in which rodlike particles are formed. The most notable example is the synthesis of a large range of mesoporous zeolites, which is thought to involve either cylindrical micelles or the organization of micelles into (8) (a) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. J. Phys. Chem. B 1997, 101, 6661. (b) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.C.; Wang, C. R. Langmuir 1999, 15, 701. (9) (a) Mohamed, M. B.; Ismail, K. Z.; Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 9370. (b) Link, S.; Burda, C.; Mohamed, M. B.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 1999, 103, 1165. (c) Mohamed, M. B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 10255. (d) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226. (e) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C.-J. Langmuir 2000, 16, 490. (10) (a) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Surf. Sci. 1999, 440, L809. (b) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (c) Wang, Z. L.; Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 5417. (11) (a) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635. (b) Dujardin, E.; Hsin, L.-B.; Wang, C. R. C.; Mann, S. Chem. Commun. 2001, 1264. (12) (a) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (b) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (c) Esumi, K.; Megumi, N.; Aihara, N.; Usui, K. New J. Chem. 1998, 22, 719. (d) Esumi, K.; Hara, J.; Aihara, N.; Usui, K.; Torigoe, K. J. Colloid Interface Sci. 1998, 208, 578. (e) Kameo, A.; Suzuki, A.; Torigoe, K.; Esumi, K. J. Colloid Interface Sci. 2001, 241, 289. (13) (a) Chen, S.; Templeton, A. C.; Murray, R. W. Langmuir 2000, 16, 3543. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313. (c) Okitsu, K.; Yue, A.; Tanabe, S.; Matsumoto, H.; Yobiko, Y. Langmuir 2001, 17, 7717. (14) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (15) (a) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (b) Mosseri, S.; Henglein, A.; Janata, E. J. Phys. Chem. 1989, 93, 6791. (c) Gachard, E.; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Belloni, J. New J. Chem. 1998, 22, 1257. (d) Henglein, A.; Meisel, D. Langmuir 1998, 14, 7392. (e) Henglein, A. Langmuir 1999, 15, 6738. (f) Bronstein, L.; Chernyshov, D.; Valetsky, P.; Tkachenko, N.; Lemmetyinen, H.; Hartmann, J.; Fo¨rster, S. Langmuir 1999, 15, 83.

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bundles during the gelation process.16 Other cases include the electrochemical method of Wang discussed before,8 the production of silver through reduction by ascorbic acid,17 and the formation of nanowires of CdS and Mo3Se3-.18 We would like to understand the formation mechanism of the rodlike or threadlike particles. Is this really a templating mechanism, requiring the presence of long micelles or not? There is much discussion about “soft” templating phenomena in surfactant solutions in the literature. Important work by the group of Pileni, which examines the formation of rodlike Cu particles in surfactant lyotropic phases, is a related example.19 Recent evidence from Pileni’s group suggests that templating is not the only and sometimes probably not the dominant force operating in the particle production process.19c,d A second issue that we address for the first time in this work is the effect of additional electrolyte on gold crystallization. Ions present in such solutions may not only affect micellar sizes and shapes and gold particle interactions but also interfere with the photochemical reduction process and affect the activity and solubility of various components. Finally, ions affect the coordination chemistry of AuIII, a fact that is overlooked in recent literature. To examine electrolyte effects on gold particle growth, we have conducted parallel experiments in the presence and absence of excess NaCl. Wishing to keep the crystallizing system as simple as possible, we have opted to avoid stabilizers or chemical reducing agents and have used photochemical reduction almost exclusively in this work. Photochemical reduction has been used as a method for the preparation of gold colloids in several recent publications.12,20 Its main disadvantage is that it does not permit efficient nucleation control: nuclei are continuously formed, as long as gold precursor ions exist in solution. We make extensive use of TEM to investigate gold particle formation. TEM has been used to study gold crystal formation already from the early years of its invention14 and is a necessary tool in gold nanoparticle research.21 The main disadvantage of TEM is that one must remove the solvent before viewing the particles, which sometimes leads to aggregate formation on the TEM grid.22 Finally, time-resolved fluorescence spectroscopy with pyrene as a fluorescent probe is used to examine potential changes in the surfactant aggregates present in our solutions. Fluorescence was preferred over the more (16) (a) Beck, J. S.; Vartuli, J. C. Curr. Opin. Solid State Mater. Sci. 1996, 1, 76. (b) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (17) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (b) Jana, N. R.; Gearheart, L.; Murphy. C. Adv. Mater. 2001, 13, 1389. (18) (a) Chen, C.-C.; Chao, C.-Y., Lang, Z.-H. Chem. Mater. 2000, 12, 1516. (b) Messer, B.; Song, J. H.; Huang, M.; Wu, Y.; Kim, F.; Yang, P. Adv. Mater. 2000, 12, 1526. (19) (a) Pileni, M.-P.; Gulik-Krzywicki, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Langmuir 1998, 14, 7359. (b) Pileni, M.-P.; Ninham, B. W.; Gulik-Krzywicki, T.; Tanori, J.; Lisiecki, I.; Filankembo, A. Adv. Mater. 1999, 11, 1358. (c) Filankembo, A.; Pileni, M.-P. J. Phys. Chem. B 2000, 104, 5865. (d) Pileni, M.-P. Langmuir 2001, 17, 7476. (20) (a) Yonezawa, Y.; Sato, T.; Ohno, M.; Hada, H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1559. (b) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129. (c) Sato, T.; Ito, T.; Iwabuchi, H.; Yonezawa, Y. J. Mater. Chem. 1997, 7, 1837. (d) Moriguchi, I.; Fujiyoshi, N.; Sakamoto, R.; Teraoka, Y.; Kagawa, S. Colloids Surf., A 1997, 126, 159. (e) Ravaine, S.; Fanucci, G. E.; Seip, C. T.; Adair, J. H.; Talham, D. R. Langmuir 1998, 14, 708. (f) Zhou, Y.; Wang, C. Y.; Zhu, Y. R.; Chen, Z. Y. Chem. Mater. 1999, 11, 2310. (g) Han, M. Y.; Quek, C. H. Langmuir 2000, 16, 362. (21) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (22) (a) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1078. (b) Haidara, H.; Mougin, K.; Schultz, J. Langmuir 2001, 17, 659. (c) Maillard, M.; Motte, L.; Pileni, M.-P. Adv. Mater. 2001, 13, 200.

Gold Colloids from Cationic Surfactant Solutions

conventional scattering methods for reasons that will become apparent below. II. Experimental Section Chemicals. Tetrachloroauric acid (HAuCl4), its sodium salt (NaAuCl4), pyrene, and dodecyl- (DTAC) and hexadecyltrimethylammonium chloride (CTAC) were obtained from Fluka. All other chemicals were obtained from Sigma, Aldrich, or Fluka. All were of analytical grade and used without further purification. Solution Preparation. Dilute solutions were simply made by mixing appropriate volumes of separate aqueous solutions of AuCl4- (5 mmol/L), surfactant (usually 0.1 M), and electrolyte. For concentrated solutions (25-45 wt % surfactant), we dissolve the appropriate amount of solid surfactant within an aqueous solution containing the gold salt and the electrolyte. Concentrated DTAC solutions were found to have a density close to that of pure water. For DTAC, which was used much more extensively, a solution of 35 wt % is roughly 1.3 M. For the concentrated surfactant solutions, we have kept the molar ratio, rg, of gold to surfactant equal to 1/60, which we prove below to be sufficient for complete solubilization of the gold-surfactant ion pair in the micelles. Fresh solutions were used for reduction; they were protected to avoid partial gold reduction by ambient light. Gold(III) Reduction. The photochemical reduction of the gold solutions was carried out under air, using a high-pressure 400-W Hg lamp. Although some oxidation of newly formed gold nanocrystals is possible and has been reported in some cases,15e,23 gold is much less sensitive than other metals. Previous photochemical work with gold was also made mostly under air.12,20 Three to four milliliters of the solutions were placed in small glass vials with a wide opening. The vials were held inside a jacketed beaker that contained water thermostated at 25 °C. This precaution was found necessary, since the powerful Hg lamp can heat up the samples significantly. Loss of water by evaporation during the reduction process can be particularly troublesome, especially for concentrated surfactant solutions that are close to the phase boundaries with lyotropic phases. After the prescribed reduction time, each vial was removed, capped with Parafilm, covered with aluminum foil, and stored in a dark place, prior to further analysis. We have also occasionally reduced AuIII with sodium borohydride or hydrazine, but we invariably found that, in the absence of specific stabilizers, chemical reduction proceeds always in a fast, uncontrollable way and produces large aggregates. Spectroscopic Measurements. UV-vis absorption spectra of gold-containing surfactant solutions were obtained on a Shimadzu UV-160A or on a UV-1601 spectrometer. FTIR spectra of the ion pairs that precipitate upon mixing equal amounts of surfactant cations and AuCl4- were measured with the KBr pellet method on a Shimadzu FTIR-8900 spectrometer. Time-resolved fluorescence measurements were made by registering fluorescence decay profiles with the photon-counting technique, using a homemade nanosecond hydrogen flash lamp and ORTEC electronics. Electron Microscopy. TEM was performed at the Institute of Neurology and Genetics, Cyprus, on a JEOL-1010A instrument, with an acceleration voltage of 80 kV. Drops of the solutions were put on Formvar-coated grids and left there a few minutes for water evaporation. Concentrated solutions (25-45 wt % surfactant) were diluted with sufficient amounts of water, before being examined by TEM.

III. Results III.1. On Ion-Pair Formation between DTAC and AuCl4- and the Coordination of AuIII. To understand the gold reduction results presented below, we must first consider two important points. The first is the formation of insoluble 1:1 ion pairs between AuCl4- and cationic surfactants,12,24 a general phenomenon exhibited by other complex ions of heavy metals,25 which has been effectively used for the production of mesostructured materials from alkylamines.26 (23) Weaver, S.; Taylor, D.; Mills, G. Langmuir 1996, 12, 4618.

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Esumi et al. have observed that the ion pairs formed between CnTAC and AuCl4- can be solubilized in the micelles formed in excess surfactant. Because we would like to work with clear solutions and avoid precipitates that would hinder our mechanistic investigation, we identified the surfactant-to-gold ratio, beyond which complete dissolution of the precipitated ion-pairs occurs, in the absence and in the presence of 1 M NaCl. We have prepared solutions with a AuCl4- concentration of 1 mmol/L and surfactant concentrations smaller than 0.1 M. For small surfactant concentrations the final solutions were turbid as expected. Precipitation of the formed ion pairs was enhanced by centrifugation. The FTIR spectrum of the precipitate clearly showed all the expected bands of the dodelyltrimethylammonium ion. Increasing surfactant concentration above the critical micelle concentration (cmc), we observed a gradual dissolution of the precipitate in the supernatant after centrifugation, with a corresponding increase of AuCl4- absorbance. The positions of the two principal gold bands are originally located at 221 and 295 nm in the absence of NaCl and at 228 and 312 nm in the presence of NaCl. This is a proof that AuCl4- exists in its hydrolyzed form in the absence of NaCl in dilute DTAC solutions. However, with increasing DTAC concentration the gold-surfactant adducts return to the solution and the gold peaks are now found at 235 and 332 nm (Figure 1a). This implies that the goldsurfactant adduct is solubilized intact in the micelles formed above the cmc, as already discussed by Esumi et al.12b,e In Figure 1b we plot the absorbance of the solution at the new band maximum (332 nm) and observe that (a) the absorbance starts increasing above the cmc (which is equal to 20 mmol/L for DTAC at 25 °C),27 and (b) levels off at a surfactant concentration of 60-70 mmol/L. Observation (a) is a strong indication that we are dealing with a solubilization phenomenon involving the micelles. Observation (b) indicates that the ratio of micellized surfactant to AuIII required for complete solubilization of the ion pairs is roughly equal to ([DTAC]plateau - cmc)/ [AuCl4-] ≈ 50. Given the aggregation numbers of DTAC micelles (50-100, see below), we anticipate that each micelle contains on average one to two gold-surfactant adducts. In the ensuing experiments we have kept the ratio rg ) [AuIII]/[DTAC] equal to 1/60, with two goals in mind: (a) that all AuIII species be associated with the surfactant micelles, and (b) that the solutions be clear and do not contain precipitates that would hinder the mechanistic investigation. The second important point to consider is that AuCl4is partly hydrolyzed through the following reaction:28-30

AuCl4- + H2O h AuCl3OH- + H + + ClThe equilibrium constant for this reaction at 298 K was reported to be equal to 2.4 × 10-6 M2 in dilute aqueous (24) (a) Buslaeva, T. M.; Sinitsyn, N. M.; Samarova, L. V.; Koteneva, N. A. Rus. J. Inorg. Chem. 1989, 34, 882. (b) Moriguchi, I.; Fujiyoshi, N.; Sakamoto, R.; Teraoka, Y.; Kagawa, S. Colloids Surf., A 1997, 126, 159. (25) (a) Yonezawa, T.; Tominaga, T.; Richard, D. J. Chem. Soc., Dalton Trans. 1996, 783. (b) Yonezawa, T.; Toshima, N.; Wakai, C.; Nakahara, M.; Nishinaka, M.; Tominaga, T.; Nomura, H. Colloids Surf., A 2000, 169, 35. (26) (a) Antonelli, D. M.; Ying, J. Y. Chem. Mater. 1996, 8, 874. (b) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 426. (c) Antonelli, D. M.; Nakahira, A.; Ying, J. Y. Inorg. Chem. 1996, 35, 3126. (d) Liu, P.; Moudrakovski, I. L.; Liu, J.; Sayari, A. Chem. Mater. 1997, 9, 2513. (e) Do, J.; Jacobson, A. J. Chem. Mater. 2001, 13, 2436. (27) Malliaris, A.; Lang, J.; Zana, R. J. Colloid Interface Sci. 1986, 110, 237.

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Figure 1. UV absorption investigation of ion-pair formation between DTA+ and AuCl4. All solutions contained 1 mmol/L AuCl4-. (a) UV-vis spectra of solutions containing 0.2 mmol/L DTAC (dashed line), 0.2 mmol/L DTAC and 1 M NaCl (dashedand-dotted line), and 80 mmol/L (solid line) DTAC. (b) Solution absorbance at 332 nm as a function of DTAC concentration for [DTAC] > cmc. A solubilization plateau can be seen above 60 mmol/L.

solutions,29 although a smaller value (7 × 10-7 M2) has also been reported.30 Further chloride substitution by hydroxide ions in the coordination sphere of gold is possible at very dilute solutions, while water may also play the role of ligand. Chemical equilibrium calculations show that in the absence of electrolyte and at small surfactant concentrations, a significant percentage of AuIII must be in the form of AuCl3OH- or of even more strongly hydrolyzed complexes, a fact corroborated by the spectra in Figure 1a. This may have an impact on the photoreduction process, since each complex species is characterized by a different reduction potential, while a different reduction pathway may be followed in each case. However, at the high DTAC and NaCl concentrations used in the experiments presented in this work, there is always abundant chloride in the system, which should force most of the AuIII to be in the AuCl4- form. We have measured the pH of the 35% DTAC solutions and found it to be 2.37 in the absence of NaCl and 2.45 in the presence of NaCl. The overall AuCl4- concentration being 0.022 M, these pH values imply that the degree of hydrolysis of AuCl4before reduction may be as high as 20%.31 III.2. Time Evolution of Gold Particle Formation in the Absence and Presence of NaCl. According to recent results by Esumi’s group,12e threadlike gold par(28) Elding, L. I.; Gro¨ning, A.-B. Acta Chem. Scand. A 1978, 32, 867. (29) Carlsson, L.; Lundgren, G. Acta Chem. Scand. 1967, 21, 819. (30) Chateau, H.; Gadet, M.-C.; Pouradier, J. J. Chim. Phys. 1966, 63, 269.

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ticles are obtained by irradiation in DTAC solutions, provided that the surfactant concentration is in excess of 20 wt %. To examine the formation mechanism of threadlike gold particles in concentrated DTAC solutions, we have undertaken experiments, in which identical samples containing 35 wt % DTAC and having ratio rg ) 1/60 were irradiated for different periods of time. A second set of samples identical to the first, but also containing 0.65 M NaCl, was investigated in a similar way. The sequence of TEM pictures presented in Figure 2 was obtained for the salt-free case. Initially, small irregular aggregates of gold nuclei are observed. The individual nuclei are smaller than 5 nm in size, their aggregates being of the order of 50 nm (Figure 2a). After 10 min of irradiation, the aggregates have undergone a sintering process (as manifested by their irregular contours) and continue growing to sizes of about 30 nm (Figure 2b). One can observe numerous such compact particles with a relatively narrow size distribution. At 15 min (picture available in the supporting material) a distribution of spherical particles with sizes ranging from 15 to 75 nm is observed. A wider size distribution is indeed expected as new nuclei are continuously formed in this system, because of the continuous irradiation. The TEM picture obtained from a sample after 35 min of irradiation is revealing. In a collection of spherical particles, we now find some particles with rodlike shape and with morphology strongly suggestive of linear particle aggregation (pointed by arrows in Figure 2c). At the same time, we observe new nuclei continuously forming in the system. After 90 min the system has evolved a number of particle morphologies ranging from spherical to rodlike, with a few large, thin, triangular or polygonal particles that are often observed in gold reduction work and are known to result from Ostwald ripening (Figure 2d).32 For longer irradiation times the percentage of rodlike particles increases. In the presence of excess NaCl, the TEM pictures are quite remarkable and present a completely different evolution of the crystallizing system (Figure 3). At times smaller than 15 min we observe the formation of thin, irregular surfactant-based structures, as evidenced by their low electron contrast (Figure 3a). These structures appear to evolve into delicate dendritic forms after 30 min of irradiation, which break up on the microscope grid (Figure 3b). We believe that these dendrites are genuine structures in the solution and not artifacts obtained upon drying of the solution on the TEM grids. Dendritic structures in similar systems are usually associated with diffusion-limited particle aggregation processes.33 After 1 h of irradiation time we observe numerous, surprising, large, crosslike structures, with segments as long as 1 µm (Figure 3c). Nucleation of small spherical gold particles goes on at the same time. Figure 4 is a close-up of one of these remarkable structures, the formation of which has not been observed before to our knowledge. The structures contain gold atoms (hence the contrast under the TEM), but they also contain surfactant, as evidenced by the fact (31) AuCl4- and AuCl3OH- in these systems are expected to be bound on DTA+ and do not exist free in solution. However, assuming that the equilibrium constant for the reaction in water (2.4 × 10-6 M2) still holds and neglecting ionic activity coefficients or the fact that the water concentration is not 55.5 M, we obtain a hydrolysis degree for AuCl4equal to only 1.0% in the presence of 1 M DTAC and without excess NaCl. A much higher hydrolysis degree is implied by the pH measurements (≈10-2.4/0.022 ) 18%). (32) (a) Bruche, B. Kolloid-Z. 1960, 170, 97. (b) Milligan, W. O.; Morriss, R. H. J. Am. Chem. Soc. 1964, 86, 3461. (33) (a) Servan, S. T. Chem. Commun. 1998, 351. (b) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. Adv. Mater. 1999, 11, 850.


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Figure 2. TEM pictures showing the time evolution of gold-particle formation in salt-free solutions containing 35% DTAC and AuCl4- with rg ) 1/60: (a) 5 min; (b) 10 min; (c) 35 min; (d) 90 min.

that they disintegrate under electron-beam focus (see Supporting Information). These structures are reproducible and always appear in this system, although sometimes at shorter or longer irradiation times, depending on the amount of material in the sample, fluctuations in lamp intensity, etc. For longer irradiation times we observe the formation of very long threadlike gold particles, along with a smaller amount of particles of other morphologies, as was also observed in the absence of NaCl. When NaCl is present, however, the percentage of threadlike particles

is much larger, and their length increases dramatically (Figure 3d). The sequence of structures in Figure 3 and the dramatic differences in the crystallization evolution with and without NaCl strongly contradict the idea of gold particle templating by rodlike micelles. In fact, we have here the proof that surfactant structures other than micelles are present and may contribute to the reduction mechanism. III.3. Photochemical Gold Reduction Experiments in CTAC Solutions. Esumi et al.12b,e have performed

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Figure 3. TEM pictures showing the time evolution of gold-particle formation in solutions containing 0.65 M NaCl, 35% DTAC, and AuCl4- with rg ) 1/60: (a) 15 min; (b) 30 min; (c) 1 h; (d) 8 h.

extensive gold photoreduction experiments in CTAC solutions; therefore we will not present our own results here. Esumi et al. attributed the formation of threadlike particles to a templating effect of the large micelles formed in this system above a surfactant concentration of roughly 1.2 M,12,34 while a sphere-to-rod transition for CTAC is already documented at 7.5 wt %.34c In the presence of

high NaCl concentrations, CTAC has been reported to form flexible, wormlike micelles.34b,c However, DTAC does not form large rodlike micelles, even at high external NaCl concentration,34a since it has a much shorter alkyl chain.34 (34) (a) Ozeki, S.; Ikeda, S. Bull. Chem. Soc. Jpn. 1981, 54, 552. (b) Imae, T.; Ikeda, S. Colloid Polym. Sci. 1987, 265, 1090. (c) Lee, Y. S.; Surjadi, D.; Rathman, J. F. Langmuir 1996, 12, 6202.


Langmuir, Vol. 18, No. 9, 2002 3665

since it can quench monomer fluorescence by excimer formation. Usually, pyrene is solubilized at concentrations comparable with the concentration of micelles, and it is accepted that its distribution among micelles follows Poisson statistics.36 We have used pyrene decay kinetics to count micelles in solution by analyzing monomer fluorescence decay profiles with the following model:39

I(t) ) I0 exp{-A2t - A3[1 - exp(-A4t)]}

(1)

A2 ) 1/τ0, where τ0 is the decay time in the absence of quenching; A3 ) [P]/[M], where [P] is the pyrene concentration and [M] is the micellar concentration; A4 ) kq, which is the pseudo-first-order intramicellar quenching rate constant. The above parameters are valid if we accept that there is no quencher migration among micelles.39 Given the pyrene concentration [P], the micellar concentration [M] can be directly obtained by fitting the model of eq 1 to the decay profile. The micellar aggregation number N, a measure of the micellar size, can also be calculated by the following formula for a specific surfactant concentration cs

N)

Figure 4. Close-up of one of the crosslike particles of Figure 3c.

In our experiments with CTAC we have found that rodlike gold particles are formed only after considerably longer irradiation times than is the case with DTAC, even in the presence of excess NaCl. This relative difficulty of forming rodlike gold particles in CTAC systems contradicts the micellar templating model. To further assess the micellar templating idea, we have performed dynamic fluorescence measurements to measure how the DTAC and CTAC micelles develop in the presence of NaCl and AuCl4- and also upon gold particle formation. To our knowledge such extensive measurements have not been performed before in the presence of AuIII anions and gold particles. We have preferred the dynamic fluorescence method to more conventional light scattering, because the complexity of the structures observed in the presence of gold particles (Figures 2 and 3) would render the light scattering results impossible to interpret. III.4. Time-Resolved Pyrene Fluorescence Probing. The number of micelles in a given aqueous micellar solution can be counted by time-resolved fluorescence probing. A micelle-bound fluorophore is introduced in the micellar solution, together with a micelle-bound quencher. By analyzing the intramicellar quenching kinetics using standard models, one can deduce the number of micelles, given the number of quencher molecules and provided that a certain statistics of quencher distribution is adopted. A standard probe of the aqueous micellar environment, used for a long time by numerous researchers, is pyrene, a hydrophobic molecule that binds to the dispersed phase in aqueous micellar solutions.36-38 In the case of aqueous micelles, pyrene is both the fluorophore and the quencher, (35) (a) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (b) Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds. Micelles, Membranes, Microemulsions, and Monolayers; Springer-Verlag: New York, 1994. (36) Infelta, P. P.; Gratzel, M. J. Chem. Phys. 1979, 70, 179. (37) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1981, 84, 100.

cs - cmc [M]

(2)

where cmc is the critical micelle concentration and [M] is the micellar concentration. Another important parameter to be deduced from eq 1 is the intramicellar quenching rate, kq. Increase or decrease of kq is an index of structural variations in the micelles. Combination of kq and N can offer another useful parameter that characterizes micellar structure, the so-called microviscosity of the micellar environment, given by

ηi ≈ 1/Vkq

(3)

V being the effective micellar volume, V ) (n + 1)N, where n is the number of carbon atoms in the surfactant alkyl chain.40 This correlation holds true for spherical or spheroidal micelles.40 The advantage of pyrene fluorescence in the systems studied here is that it focuses on the surfactant aggregates, while scattering methods would be strongly affected by the presence of the gold particles and would not yield useful information about micellar shape and size. We have verified that pyrene fluorescence is not affected by the presence of gold salts and particles and that quenching occurs only through excimer formation. We have calculated [M], kq, N, and ηi by pyrene time-resolved fluorescence analysis, and the results are tabulated in Tables 1 and 2, for DTAC and CTAC, respectively. Inspection of Table 1 reveals that the variation of DTAC concentration in the range 0.05-1.0 M has a small effect on micellar size, which increases with surfactant concentration. Indeed, N was found to vary from 55 to 68. These values are close to those found previously by others.41 Addition of NaCl, in the range 0.1-1.0 M caused a further increase of N, which in the case of 1.0 M DTAC and 1.0 M NaCl was 96. The increase of the micellar size is accompanied by an analogous decrease of the intramicellar quenching rate, which is expected, since reaction efficiency decreases when (38) Alargova, R. G., Kochijashky, I. I.; Sierra, M. L.; Zana, R. Langmuir 1988, 14, 5412. (39) Zana, R. Surfactant Solutions: New Methods for Investigation; Marcel Dekker: New York, 1987. (40) Anthony, O.; Zana, R. Langmuir 1996, 12, 3590. (41) Wikander, G.; Eriksson, P.-O.; Burnell, E. E.; Lindblom, G. J. Phys. Chem. 1990, 94, 5964.

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Table 1. Data Obtained by Analysis of Pyrene Decay Profiles in Aqueous DTAC Micelles in the Absence and Presence of Additives

Table 2. Data Obtained by Analysis of Pyrene Decay Profiles in Aqueous CTAC Micelles in the Absence and Presence of Additives

intramicellar quenching micellar rate surfactant NaCl AuCl4- micelle micellar microconcn concn concn concn aggregation constant viscosity (107 s-1) (M) (M) (mM) (mM) no. (N)a ni

intramicellar quenching micellar rate surfactant NaCl AuCl4- micelle micellar microconcn concn concn concn aggregation constant viscosity (107 s-1) (M) (M) (mM) (mM) no. (N)a ni

0.05 0.07 0.40 0.60 1.00

0 0 0 0 0

0 0 0 0 0

No Additives 0.54 55 0.91 55 6.80 56 9.40 62 14.4 68

1.8 1.7 1.6 1.5 1.4

0.08 0.08 0.09 0.08 0.08

Addition of NaCl 0.86 58 0.66 76 14.8 66 14.2 69 11.1 88 10.2 96

1.5 1.1 1.2 1.1 0.97 0.84

0.09 0.10 0.10 0.10 0.09 0.10

0.07 0.07 1.0 1.0 1.0 1.0

0.1 1.0 0.1 0.3 0.6 1.0

0 0 0 0 0 0

0.07 0.07 0.07 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.0 1.0 1.0 0.6 1.0 1.0 1.0 1.0 1.0 1.0

0.07 0.20 0.60 6.0 0.07 0.10 0.20 0.40 0.60 6.0

Addition of AuCl40.64 78 0.60 83 0.59 85 9.1 108 10.2 96 9.7 101 9.4 104 9.1 108 8.8 111 8.4 117

1.1 1.0 1.0 0.75 0.84 0.84 0.76 0.75 0.75 0.74

0.09 0.10 0.10 0.09 0.09 0.10 0.09 0.10 0.09 0.10

0.07 0.07 0.07 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.0 1.0 1.0 0.6 1.0 1.0 1.0 1.0 1.0 1.0

0.07 0.20 0.60 6.0 0.07 0.10 0.20 0.40 0.60 6.0

UV Treatment 0.74 67 0.66 76 0.63 79 7.8 125 8.1 121 7.9 124 7.9 124 7.7 127 7.4 132 0.7 138

1.20 1.10 0.97 0.62 0.71 0.65 0.62 0.61 0.58 0.56

0.10 0.09 0.10 0.10 0.09 0.10 0.10 0.10 0.10 0.10

a Micellar aggregation numbers, N, have been calculated by adopting that the cmc is 20 × 10-3 M.

the effective reaction volume increases. On the contrary, the effective microviscosity of the micellar phase increased only slightly in going from the pure micellar phase to the one containing salt but remained, otherwise, unaffected by the change in the micellar size. These results indicate that the present micellar system does not suffer any important structural variations, either by changing surfactant concentration or by introducing additional ionic strength. The micelles do increase in size and decrease in number, but they remain in the same structural domain, that of the small spheroidal micelles. In addition, the small micellar-charge neutralization effected by the DTA+AuCl4pairs and the additional ionic strength induces a further small increase in the micellar size, without changing the micellar structural domain, i.e., the micelles remain small and spheroidal. Finally, after UV irradiation of the solutions and the formation of gold colloidal particles, we have detected a further decrease in the number of micelles, which gives even higher N values. However, the unchanged ηi values suggest that the micellar structural domain did not change even after formation of colloidal gold. Part of the surfactant may participate in the stabilization of the metal particles, but most of it is still expended in the formation of small spheroidal micelles. The case of CTAC micelles is very similar, as seen in Table 2. CTAC micelles keep their size practically un-

0.07 0.40 0.75

0 0 0

0 0 0

No Additives 0.82 84 4.3 93 7.9 95

0.61 0.59 0.54

0.16 0.14 0.15

0.75 0.75 0.75

0.1 0.3 1.0

0 0 0

Addition of NaCl 7.8 96 7.6 98 7.4 101

0.57 0.57 0.51

0.14 0.14 0.15

0.75 0.75 0.75 0.75

1.0 1.0 1.0 1.0

0.07 0.10 0.40 0.60

Addition of AuCl47.4 101 7.4 102 7.4 102 7.4 102

0.51 0.54 0.50 0.50

0.14 0.14 0.15 0.15

0.75 0.75 0.75 0.75

1.0 1.0 1.0 1.0

0.07 0.10 0.40 0.60

UV Treatment 7.4 102 7.4 102 7.4 102 7.4 102

0.51 0.54 0.50 0.50

0.15 0.15 0.15 0.15

a Micellar aggregation numbers, N, have been calculated by adopting that the cmc is 1.6 × 10-3 M.

changed in the presence of AuCl4- or reduced gold, even when NaCl is present. Micellar size increases gradually with NaCl concentration, in agreement with the literature.41,42 The large wormlike CTAC micelles reported in other works34 were obtained at conditions of very high external salt concentrations (2-4 M NaCl), used apparently because they are relevant to the production of mesoporous zeolites. We have observed by TEM that high NaCl concentrations lead to significant salting-out of the surfactant, while NaCl itself is hard to dissolve in concentrated surfactant solutions (see TEM picture in Supporting Information), and we have opted to avoid such high salt concentrations. The general conclusion of the fluorescence experiments is that in the range of system compositions that we have worked there exists no well-defined transition to rodlike micelles. The micellar aggregation number shows a slow, gradual increase with surfactant and gold concentration, but the most elongated micelles observed in these systems cannot have aspect ratios larger than 2-2.5.43 IV. Final Discussion and Conclusions The formation of gold particles in solutions containing trialkylammonium surfactants is a very complex process. We have focused on the formation of threadlike gold particles, initially reported in the interesting series of papers by Torigoe and Esumi.12 Most of the evidence, which we have obtained from TEM and fluorescence spectroscopy, suggests that the rodlike or threadlike particle morphologies that dominate the distribution at long irradiation times do not originate from a templating mechanism. Time-resolved fluorescence results show that the surfactant micelles remain spheroidal even at 35% (42) (a) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022. (b) Magid, L. J.; Han, Z.; Warr, G. G.; Cassidy, M. A.; Butler, P. D.; Hamilton, W. A. J. Phys. Chem. B 1997, 101, 7919. (43) Assuming spherocylinders with area per surfactant head equal to 40 Å2, one deduces a micellar radius of 13 Å. The aggregation number increases from 55 (for spheres) to 125 (for spheroids). From these data one obtains a cylinder length of 34 Å, which yields an overall aspect ratio of 2.3.


surfactant and in the presence of 0.6-1 M NaCl, in systems that produce a large percentage of threadlike gold particles. Some of the TEM evidence (see, e.g., Figure 2c) strongly suggests that threadlike particles evolve through a one-dimensional aggregation process. In fact, Figure 2 suggests that a hierarchy of aggregation phenomena occurs: First, nuclei are aggregated and sintered into compact primary particles, then these particles themselves aggregate to form rodlike structures. Such two-level colloid aggregation processes have recently been observed and studied in the case of gold spheroids,44 although aggregation appears to be strongly directional in the present system. The remarkable sequence of structures observed in the presence of NaCl further weakens the micellar templating argument. The experimental fact that gold and surfactant concentration must be above certain thresholds for the threadlike particles to form12 is a further argument in favor of the aggregation hypothesis. Why would such an aggregation pattern be favored in these systems? The phenomenon of crystal shape (or “habit”) modification by tailor-made substances is well-known and has been studied for many years by the crystal-growth community.45 Recent work by the group of Alivisatos46 suggests that rodlike particle shapes can be obtained when a surfactant present in the system adsorbs specifically on a particular face of a growing crystal and stabilizes it, allowing the crystal to grow in different directions only. Surfactants are thus postulated to act as tailor-made additives. Production of silver nanowires in the presence of additives was also recently reported and assumed to occur because of preferential stabilization of specific crystal faces.47 To generalize the previous ideas, one might propose that growth by aggregation occurs also under a similar limitation. Faces on which the surfactant adsorbs strongly would be better stabilized than other faces. The aggregation would proceed by joining particles along faces, which are not properly stabilized.48,49d The resulting aggregates would then show defects, which however can be efficiently healed, as was demonstrated in fundamental work by the group of El-Sayed.10c The rodlike particles produced by the Wang et al. method8 were shown to be dominated by the theoretically unstable {110} faces, their axial growth direction being 〈112〉.10a It may well be that we are looking at a similar growth phenomenon, but detailed highresolution TEM work would be needed to clarify this issue. There has been much discussion in recent literature about the growth of crystals by primary crystal aggregation.49 It has been shown in several systems that the unique particle morphologies observed by electron microscopy are a result of particle aggregation.48,49 In our system, surfactant molecules are used to stabilize an everincreasing number of gold particles. The total percentage (44) (a) Adachi, E. Langmuir 2000, 16, 6460. (b) Adachi, E. Langmuir 2001, 17, 3863. (45) (a) Addadi, L.; Berkovitch-Yellin, Z.; Weissbuch, I.; van Mil, J.; Shimon, L. J. W.; Lahav, M.; Leiserowitz, L. Angew. Chem., Int. Ed. Engl. 1985, 24, 466. (b) Sugimoto, T.; Itoh, H.; Mochida, T. J. Colloid Interface Sci. 1998, 205, 42. (46) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 215. (47) Liu, S., Yue, J.; Gedanken, A. Adv. Mater. 2001, 13, 656. (48) (a) Penn. R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549. (b) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (c) Penn, R. L.; Oskam, G.; Strathmann, T. J.; Searson, P. C.; Stone, A. T.; Veblen, D. R. J. Phys. Chem. B 2001, 105, 2177. (49) (a) OcaZa, M.; Rodriguez-Clemente, R.; Serna, C. J. Adv. Mater. 1995, 7, 212. (b) Goia, D. V.; Matijevic´, E. New J. Chem. 1998, 22, 1203. (c) Privman, V.; Goia, D. V.; Park, J.; Matijevic´, E. J. Colloid Interface Sci. 1999, 213, 36. (d) Adair, J. H.; Suvaci, E. Curr. Opin. Colloid Interface Sci. 2000, 5, 160. (e) van Hyning, D. L.; Klemperer, W. G.; Zukoski, C. F. Langmuir 2001, 17, 3128.

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Figure 5. Schematic of the processes leading to the production of threadlike gold particles: (a) ion-pair precipitation and subsequent solubilization in micelles; (b) formation of gold particles by UV irradiation, their stabilization by surfactant bilayers, and one-dimensional particle growth by aggregation of gold particles.

of surfactant expended to stabilize the particles is, however, rather small. We have made a rough estimate of the amount of surfactant adsorbed on gold particles for the 35% DTAC and 0.022 M AuCl4- solution mostly used in this work. Assuming that all gold atoms can be found in particles with 25 nm radius and that the surfactant (with molecular length 15 Å) forms bilayers on the particles50,51 with average cross-sectional area 40 Å2/ molecule, we find that only 0.1% of the surfactant molecules adsorbs on the particles. The surfactant micelles do not change their size or shape in a significant way as the surfactant and gold concentrations in the system increase, even at 1 M NaCl, as indicated by the fluorescence decay results. The most remarkable results of the present investigation were obtained in the presence of a high NaCl concentration in the surfactant solution. The obvious effect of NaCl is to increase the ionic strength and decrease electrostatic repulsion between micelles and particles, thus enhancing aggregation phenomena. However, this cannot be so important, since the ionic strength due to DTAC is already high. The unexpected evolution of the system through the crosslike structures of Figures 3c and 4 implies that the surfactant may be salted out to a certain extent, because of the high salt concentration, forming a sequence of structures. The development of the gold crystals, through (50) (a) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281. (b) Shen, L.; Laibinis, P. E.; Hatton, T. A. Langmuir 1999, 15, 447. (51) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368.

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continuous nucleation, disrupts and eventually terminates the crystal growth of the surfactant. Some of the significant NaCl effect must then be due to the drop in surfactant solubility. NaCl also slows down the photochemical formation of zerovalent gold, since it provides additional excess of Cl- ions, which affects some of the photochemical reactions.23 A question mark still remains regarding the potential effect of Cl- concentration on AuIII coordination. For the present system this effect should not be very significant, since the dissociation of DTAC already provides a high Cl- concentration. Our current understanding of the sequence of events during gold-particle production in this system is presented schematically in Figure 5. There we depict the initial precipitation of the gold-surfactant ion pair, its solubilization in the micelles, the gradual micellar growth observed by fluorescence, the initial formation of secondary gold particles, and their directional aggregation under the influence of preferential surfactant binding, to form long linear aggregates. The present work demonstrates once more that extensive mechanistic investigations are required to clarify inorganic crystallization phenomena in the complex surfactant systems increasingly used today in materials science applications.

Leontidis et al.

Acknowledgment. We are grateful to Professor Paul Smith and Dr. Walter Caseri (Institut fu¨r Polymere, ETH, Zu¨rich) for their strong interest in this project, for their support in the early stages (initial experiments were performed in their laboratory), and for some preliminary TEM measurements. We are grateful to Ms. Konstantina Kapnisi, who performed exploratory experiments for her Undergraduate Diploma Thesis. We are finally grateful to Dr. Kyriacos Kyriacou and Mr. Andreas Zenios of the Department of Microscopy of the Institute of Neurology and Genetics (Nicosia, Cyprus) for support with the TEM measurements. This work was partly supported by research grants from the University of Cyprus to E.L. and by the Greek-French Cooperative Research Program (Platon), where Cyprus participated as a Mediterranean Partner. Supporting Information Available: Images of gold particles produced after irradiation in surfactant and both salt and salt-free solutions and melting of crosslike structures by electron beams. This material is available free of charge via the Internet at http://pubs.acs.org. LA011368S

Gold Colloids from Cationic Surfactant Solutions. 1. Mechanisms That

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