Langmuir 1999, 15, 83-91
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Laser Photolysis Formation of Gold Colloids in Block Copolymer Micelles L. Bronstein,* D. Chernyshov, and P. Valetsky Nesmeyanov Institute of Organoelement Compounds, 28 Vavilov Street, Moscow 117813, Russia
N. Tkachenko and H. Lemmetyinen Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, FIN-33101, Tampere, Finland
J. Hartmann and S. Fo¨rster Max Planck Institute of Colloids & Interfaces, Kantstrasse 55, D-14513 Teltow-Seehof, Germany Received July 13, 1998. In Final Form: October 19, 1998 The laser photolysis of gold AuIII salts embedded in micelle cores of block copolymer micelles derived from polystyrene-poly-4-vinylpyridine was studied. Two types of polystyrene-poly-4-vinylpyridines having different block length have been employed, producing micelles with different properties. The influence of the type of gold salt, loading rate, presence of water, micelle characteristics, and some other parameters on the rate of reduction and gold colloid formation were investigated. The presence of water in the system containing HAuCl4‚3H2O was found to accelerate the AuIII reduction, while the gold colloid size was not affected (about 3 nm). The substitution of HAuCl4‚3H2O with AuCl3 results in much slower accumulation of AuI species with subsequent slower nucleation of gold colloids that results in bigger particles with mean diameter of 6.0 nm. Increasing the amount of metal compound was found to lead to an increase of the particle size.
Introduction Metal colloid formation in micelles of amphiphilic block copolymers in selective solvents is a promising new trend in modern material science and is described by a number of authors.1-12 Earlier, metal colloids were prepared in microsegregated domains of block copolymers films.13-16 In our preceding publications we have shown that metal colloid size and morphology can be controlled mainly by the choice of a reducing agent, and various chemical (1) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, (12), 1000. (2) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Proceedings of the ACS; Division of Polymeric Materials: Science and Engineering, Fall Meeting 1995, 73, 283. (3) Seregina, M.; Bronstein, L.; Platonova, O.; Chernyshov, D.; Valetsky, P.; Hartmann, J.; Wenz, E.; Antonietti, M. Chem. Mater. 1997, 9, 923. (4) Platonova, O. A.; Bronstein, L. M.; Solodovnikov, S. P.; Yanovskaya, I. M.; Obolonkova, E. S.; Valetsky, P. M.; Wenz, E.; Antonietti, M. Colloid Polym. Sci. 1997, 275, 426. (5) Antonietti, M.; Heinz, S. Nachr. Chem: Technol. Lab. 1992, 40, 308. (6) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Macromolecules 1996, 29, 3220. (7) Spatz, J. P.; Roescher, A.; Mo¨ller, M. Adv. Mater. 1996, 8 (4), 337. (8) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Macromolecules 1995, 29, 9 (9), 3220. (9) Moffit, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. (10) Moffit, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178. (11) Mayer, A. B. R.; Mark, J. E. Polym. Prepr. 1996, 74, 459. (12) Mayer, A. B. R.; Mark, J. E. J. Colloid Polym. Sci. 1997, 275, 333. (13) Sankaran, V.; Yue, J.; Cohen, R. E.; Schrock, R. R.; Silbey, R. J. Chem. Mater. 1993, 5, 1133. (14) Chan, Y. N. C.; Schrock, R. R. Chem. Mater. 1993, 5, 566. (15) Saito, H.; Okamura, S.; Ishizu, K. Polymer 1992, 33, 1099. (16) Chan, Y. N. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 24.
reducing agents were tested. The slowly working reductants, such as triethylsilane, were clarified to result predominantly in the formation of one metal particle per micelle which was called “cherry-morphology”, while the fast working reducing agents (NaBH4 or superhydride) lead to the formation of many small particles per micelle which was called “raspberry-morphology”.1 Therefore, particle nucleation and growth is controlled by variation of reducing agents. However, almost any chemical reducing agents have an obvious disadvantage, lying in side product formation throughout course of reduction. For instance, metal particles produced with the NaBH4 reduction are often covered with metal borides that change the properties of a particle surface. Mo¨ller and coauthors7 have studied the gold nanoparticle formation on a grid of an electron microscope under an electron beam which can be considered as a “pure” reductant, but all manipulations are restricted to this geometry, i.e., can be done solely on the thin film samples obtained on the grid. There are many examples of metal colloid formation under UV- or γ-irradiation in the presence of various stabilizing systems, such as microemulsions, polymer solutions, and others.17-23 In several publications laser photolysis and pulse radi(17) Henglein, A.; Holzwarth, A.; Mulvaney, P. J. Phys. Chem. 1992, 96, 8700. (18) Henglein, A.; Tausch-Treml, R. J. Colloid Interface Sci. 1981, 80, 84. (19) Marignier, J. L.; Beloni, J.; Delcourt, M. O.; Chevalier, J. P. Nature 1985, 317, 344. (20) Toshima, N.; Takahashi, T.; Hirai, H. Chem. Lett. 1985, 1245. (21) Yonezawa, Y.; Sato, T.; Ohno, M.; Hada, H. J. Chem. Soc., Faraday Trans. 1987, 83, 1559. (22) Torigoe, K.; Esumi, K. Langmiur 1992, 8, 59. (23) Sato, T.; Kuroda, S.; Takami, A.; Yonazawa, Y.; Hada, H. Appl. Organomet. Chem. 1991, 5, 261.
10.1021/la980868r CCC: $18.00 © 1999 American Chemical Society Published on Web 12/11/1998
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Table 1. The Characteristics of PS-b-P4VP Micelles
notations of PS-b-P4VP samples
NAa
NBb
Rh/nmc
Zd
I II IIce
70 145 145
183 123 123
46.3 66.5 66.5
99 551 551
a N is the degree of polymerization of the P4VP block. b N is A B the degree of polymerization of the PS block. c Rh is the hydrodynamic radius of micelles (from dynamic light scattering data). d Z is the aggregation number. e With cross-linked core.
olysis were employed for metal colloid formation.24,25 An advantage of those techniques is that the kinetics of transient states and their formation and decay can be examined allowing an understanding of the mechanism of metal colloid formation. In this paper we focused on the laser photolysis of Au(III) species imbedded in micelle cores of block copolymer micelles derived from polystyrene-poly-4-vinylpyridine (PS-b-P4VP). Experimental Section The PS-b-P4VP block copolymers are synthesized by anionic polymerization following the standard procedure.26 All reactions take place within a glass vacuum line. BuLi (12% solution in the hexane, Aldrich) is used to initiate the polymerization, which is performed at -55 °C in THF. The resulting polymers are twice precipitated in low boiling petroleum ether and dried in vacuo at 50 °C for 2 days. Polymer characterization was performed by gel permeation chromatography (GPC) measurements on the block copolymers in THF (60 °C) and DMF (70 °C). In addition, the relative composition of the block copolymers was controlled by 1H NMR and elemental analysis. Before light scaterring measurements, the block copolymer solutions were subjected to 10-fold filtering through a Millipore 0.45 µm PTEF filter, so they were free from outside impurities, which could influence the micelle formation. The block copolymer and micelle characteristics of the samples used in the present work are summarized in Table 1. Preparation of Gold Colloids in Block Copolymer Micelles. Incorporation of HAuCl4‚3H2O and AuCl3 inside block copolymer micelles was carried out under air by a method described in earlier publications.1,2 Solutions of PS-b-P4VP were prepared in a spectral grade toluene. Though both gold compounds are not soluble in toluene, their solubilization in block copolymer solutions proceeds very fast (for several hours) due to coordination with pyridine units. So for preparation of a metal compound loaded solution, a corresponding salt was added to block copolymer solutions in toluene under stirring until complete solubilization. For laser irradiation experiments, the samples were prepared in vacuum cuvettes. Oxygen removal was achieved by a four-time degassing procedure of frozen samples in vacuum at 10-5 bar. UV spectra were measured with UV-2501 PC (Shimadzu) UV-visible spectrophotometer. UV sample irradiation was carried out with pulsed eximer laser at a wavelength of 308 nm with a pulse duration of 30 ns and pulse energy of approximately 40 mJ. The laser radiation was focused on the sample resulting in an energy density of 200 mJ/cm2. It should be noted that not a whole sample was irradiated, but just a stripe of 2 × 10 mm. After each course of irradiation (24) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. J. Am. Chem. Soc. 1983, 105, 2574. (25) Ghosh-Mazumdar, A. S.; Hart, E. J. Radiation Chem. I 1968, 81, 193. (26) Antonietti, M.; Heinz, S.; Rosenauer, C.; Schmidt, M. Macromolecules 1994, 27, 3276.
sample was shaken. Pulse repetition rate was 2 Hz, and the exposition was roughly 120 J cm-2 min-1. Flash photolysis experiments were carried out with the system described in ref 27. In brief, samples were excited by the pulses of the eximer laser, a Xe-arc lamp and a monochromator were used to produce monitoring light at the designed wavelength, and a photomultiplier coupled with another monochromator was used for detection. Transient signals from a photomultiplier were recorded by a digital oscilloscope. When the samples were exposed to UV irradiation, the laser operated at the 10 Hz repetition rate. During the transient absorption measurements samples were excited at rates approximately 0.5 Hz or lower when long living species were measured. Transmission Electron Microscopy. Samples were prepared by evaporation of 10-4 mol/L toluene solutions under air. A drop of solution was placed on an electron microscope copper sample grid. After drying, electron micrographs of the sample were obtained with a Zeiss 912 Omega electron microscope. A magnification of 125 000 was used. Particle sizes were determined by processing from the photographs.
Results and Discussion Preparation of PS-b-P4VP Loaded with Gold Compounds. Previously we have shown that metal particle formation is strongly governed by the type of block copolymer micelles in selective solvents28 which, in turn, is determined by the length of polystyrene (PS) and poly4-vinylpyridine (P4VP) blocks.29,30 The cohesion energies of PS and P4VP in PS-b-P4VP differ strongly from each other, consequently PS-b-P4VP copolymers form relatively strongly segregated micelles whose critical micelle concentration is so small that they do not exhibit unimers in solution, unlike traditional surfactants.29 In this paper we will consider two types of block copolymers. The sample with a notation of (I) has a short core block as compared to the corona block, the aggregation number and the density of micelle core are low, and relatively weakly segregated micelles are formed. By contrast, the sample with a notation of (II) has two blocks of nearly the same length and very high aggregation number that results in very strongly segregated micelles. Therefore, the solubilization of the latter polymer has to be promoted by heating of the polymer solution. The micellar characteristics of two block copolymers were also presented in Table 1. Earlier, it was found28 that during the slow reduction with triethylsilane the polymer exchange between micelles happens due to the collision which results in an uneven distribution of particles through micelles: some micelles were empty, others contained two particles. To avoid this event, two experimental tricks were used: the micelles were slightly cross-linked with p-xylylene dibromide (5 wt % per 4VP units), and some percents of methanol were added to the solvent to increase the mobility inside micelle cores.28 Such experimental tricks avoided exchange between micelles, and strictly one particle per micelle was obtained. For comparison, we also cross-linked polymer II and studied flash photolysis (Table 1, notation IIc) also in these systems. Gold colloids in the PS-b-P4VP micelles can be prepared from different gold sources.1,2 Earlier we have described (27) Lemmetyinen, H.; Ovaskainen, H. R.; Nieminen, K.; Vaskonen, K.; Sychtchikova I. J. Chem. Soc., Perkin Trans. 1992, 2, 113. (28) Bronstein, L. M.; Antonietti, M.; Valetsky, P. M. Metal Colloids in Block Copolymer Micelles: Formation and Material Properties In Nanoparticles and Nanostructured Films: Preparation, Characterization and Applications; Fendler, J. H., Ed.; Wiley-VCH: Weinheim, New York, Chichester, Brisbande, Singapore, Toronto, 1998; p 145. (29) Fo¨rster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956. (30) Fo¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10 (3), 195.
Laser Photolysis of AuIII Salts
Figure 1. UV spectra of {I + HAuCl4‚3H2O} (N:Au ) 9:1) in toluene before irradiation (1) and after 5 min (2), 15 min (3), 30 min (4), 45 min (5), 60 min (6), 70 min (7), and 80 min of irradiation with pulse frequency of 2 Hz (8).
that introduction of HAuCl4‚3H2O into the PS-b-P4VP micelle cores proceeds easier than AuCl3, because in the former case just a protonation of nitrogen in vinylpyridine units controls the incorporation of the gold compound but not the coordination with gold ion.2 In both cases micelle size does not change after gold compound loading and even after its reduction.1,2 According to our observations, HAuCl4‚3H2O can be embedded in micelle cores even at molar ratio N:Au ) 1:1 (where N matches to the amount of 4VP units), while for AuCl3 a molar ratio N:Au ) 3:1 is the limit. Besides, HAuCl4‚3H2O carries three water molecules in each gold compound molecule that might influence the course of the particle formation. Kinetic examinations with the flash photolysis system should also allow the comparison of those two gold components. Steady-State Absorption of the PS-b-P4VP Block Copolymers Loaded with Gold Compounds under UV Irradiation. Figure 1 shows the UV spectra of {I + HAuCl4‚3H2O} solutions in toluene before and after the laser irradiation. The initial spectrum contains a shoulder at about 320 nm, which corresponds to the absorption of the gold compound coordinated with 4VP units. After 5 min of irradiation, the initially yellow solution turns colorless and the sample has a very weak absorption (Figure 1, curve 2) in the UV range. After the following 10 min of irradiation (total exposure was 15 min) the solution turns slightly pink and is characterized by an increase in the absorption at 300-600 nm range (Figure 1, curve 3). A continuation of laser irradiation (Figure 1, curves 4-8) causes a plasmon band at 520 nm and a strong absorption at shorter wavelengths, which is characteristic for small gold colloids.2 The addition of 0.5 wt % of water into initial solution, which had to provide a higher mobility of gold ions (possible dissociation), results in similar changes, but gold colloid formation goes on much faster. The final spectrum of such a solution (Figure 2, curve 2) is almost similar, showing an absorption maximum at 517 nm, but it has been achieved with 60 min of irradiation instead of 80 min for the water-free sample. The increase of the gold compound loading (molar ratio N:Au ) 3:1 as compared to N:Au ) 9:1) causes similar changes, but after 15 min of laser irradiation, when absorption in a whole
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Figure 2. UV spectra of gold colloids prepared by laser irradiation of the system {I + HAuCl4‚3H2O} (N:Au ) 9:1) (1), the system {I + HAuCl4‚3H2O+0.5% of H2O} (N:Au ) 9:1) (2), the system {I + HAuCl4‚3H2O + 0.5% of H2O} (N:Au ) 9:1) at the simultaneous kinetics measurements (3), the system {I + AuCl3} (N:Au ) 9:1) (4), the system {II + HAuCl4‚3H2O} (N:Au ) 9:1) (5), the system {I + HAuCl4‚3H2O} (N:Au ) 3:1) (6), and the system {II (cross-linked, methanol) + HAuCl4‚3H2O} (N: Au ) 9:1) (7).
Figure 3. UV spectra of the system {I + HAuCl4‚3H2O} (N:Au ) 3:1) before irradiation (1) and after 5 min (2), 15 min (3), 30 min (4), 45 min (5), 60 min (6), and 70 min of laser irradiation (7) with a pulse frequency of 2 Hz.
range increases, a pronounced shoulder at 425 nm appears in the spectrum (Figure 3, curve 3). In ref 24 such an absorption was observed, solely in transient spectra of gold colloids in microemulsions, and was attributed to Au0. The final state is characterized by a more pronounced plasmon band (λ ) 526 nm) and weaker absorption at shorter wavelengths that might be attributed to bigger particles (Figure 2, curve 6). Substitution of HAuCl4‚3H2O with AuCl3 as a gold source results in a much slower process. After 5 min of
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absorption spectra were observed for samples containing big particles (Figure 2, curves 4 and 6) after storage for 24 h testifying at the same time some aggregation process. Smaller particles remained stable for months. Therefore, we can say that static absorption spectra display certain differences in stepwise reduction of AuIII species and metal colloid formation which are mainly reflected in different rates of each step. Reaction kinetics at UV irradiation can be studied by measurements of transient absorption. Those measurements were carried out at different phases of the photoreduction. Flash Photolysis of PS-b-P4VP Loaded with Gold Compounds. According to ref 24, laser photolysis of HAuCl4 in water can be described as a sequence of reactions resulting in reduction of AuIII to Au0 and gold colloid formation. In ref 24 the following reaction set was proposed: hν
Figure 4. UV spectra of the system {I + AuCl3} (N:Au ) 9:1) before irradiation (1) and after 5 min (2), 15 min (3), 45 min (4), 60 min (5), 75 min (6), and 115 min (7) of laser irradiation with a pulse frequency of 2 Hz.
irradiation (Figure 4) a noticeable absorption is observed at 322 nm (absorption band for AuCl3 in this system), and the solution remains slightly yellow. After 15 min of irradiation bleaching takes place. The further increase of the absorption happens very slowly, and the final spectrum contains a very pronounced plasmon band at 531 nm that might be attributed to rather big particles (Figure 2, curve 4). Similar to the observations in ref 24, it was also observed that the final state of absorption spectra strongly depends on the course of the laser irradiation, i.e., on the delay time between subsequent stages of irradiation. The comparison of curves 3 and 4 in Figure 2 illustrates this dependence. Consequently, the subsequent steps of irradiation were carried out exactly in 10 min one after another to obtain comparable results for different samples (Figures 1-4). The use of strongly segregated micelles (II) instead of weakly segregated ones (I) loaded with HAuCl4‚3H2O at a molar ratio N:Au ) 9:1 does not induce changes in the UV spectra after corresponding stages of laser irradiation as compared to polymer (I), but the final colloids are characterized by weaker plasmon resonance at 512 nm (which appears as a shoulder on curve 5 in Figure 2). The cross-linking of micelle core does not result in noticeable changes (Figure 2, curve 7). Depending on the rate of a process, different exposures to UV radiation were necessary to finish the colloid formation (80 min for the system {I + HAuCl4‚3H2O} (N:Au ) 9:1) as compared to 70 min for the system {I + HAuCl4‚3H2O} (N:Au ) 3:1)). By particular experiments it was found that initial solution of {I + HAuCl4‚3H2O} (N:Au ) 9:1) is absolutely stable in the darkness and under natural light exposition for many months. The solution bleached after the first 5 min of UV irradiation and presumably containing AuI species did not show any changes being stored in the darkness, but became pink even after 20-30 h under natural light. Thus the reduction of AuI species happens only under light or laser irradiation. On the other hand, the increase of the absorption in the UV spectra took place even in the darkness overnight for the samples exposed to irradiation of 45 min or more, when we can expect that the aggregation of the primarily formed gold atoms to gold colloids already takes place. Slight red shifts of
(HAu3+Cl4) 98 (HAu3+Cl4)/
(1)
(HAu3+Cl4)/ f (HAu2+Cl3‚‚‚Cl)
(2)
(HAu2+Cl3‚‚‚Cl) f HAu2+Cl3 + Cl
(3)
2HAu2+Cl3 f HAu3+Cl4 + HAu+Cl2
(4)
hν
HAu+Cl2 98 Au0 + HCl + Cl
(5)
nAu0 f (Au0)n
(6)
where the first step of reduction of AuIII to AuII proceeds under UV irradiation, while disproportionation of AuII by second-order reaction takes place without irradiation (reaction). The chemical and dielectric environment of gold compound molecules in the present case of HAuCl4‚3H2O incorporated into the P4VP micelle core in toluene solution is rather different from that discussed in ref 24. Moreover, the coordination of gold ions to vinylpyridine units strongly changes their mobility. Therefore dramatic differences in the behavior of such a system might be expected. By FTIR study,2 it was shown that the incorporation of HAuCl4‚ 3H2O in P4VP is accompanied with the same spectral changes such as the protonation of 4VP units with HCl. That is why as a reference experiment sample I protonated with aqueous solution of HCl was examined by flash photolysis. It was found, that some transients can be observed. However, their absorption intensity is 10 times less than that for {I + HAuCl4‚3H2O} and it decays with a lifetime of 1 × 10-7 s-1 at wavelengths longer than 380 nm. At shorter wavelengths absorption of the sample is too strong, which made reliable measurements impossible. Time-resolved transient absorption spectra recorded for polymer I filled with HAuCl4‚3H2O (N:Au ) 3:1) at wavelength 340 nm before the first 5 min period of UV irradiation are shown in Figure 5. The signals are characterized by a fast stepwise increase in the absorption just after excitation followed by the formations of some absorbing species at wavelengths shorter than 380 nm with lifetimes of 3 × 10-6 s (Figure 5a). At the same time, the transient absorption of the sample decreases at wavelengths of 380 nm and longer. This step is followed by a decrease in absorption at all wavelengths with lifetime approximately 5 × 10-5 s. The absorbing species at UV (380 nm) practically no change
Laser Photolysis of AuIII Salts
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Figure 6. Transient absorption spectra for {I + HAuCl4‚ 3H2O (N:Au ) 3:1)}, recorded before the course of the irradiation. hν
-C6H5N{HAuIIICl4‚3H2O} 98 -C6H5N{HAuIIICl4‚3H2O}/ (7) -C6H5N{HAuIIICl4‚3H2O}/ f -C6H5N{HAuIICl3‚‚‚Cl‚3H2O} (8) The final complex decays first with an elimination of Cl with a lifetime equal to 5 × 10-5 s. It is assumed that the complex -C6H5N{HAuIICl3‚3H2O} cannot coordinate with three water molecules and releases one water molecule as the next step of the reaction providing a transient signal with a lifetime of 7 × 10-4 s. Since each AuII complex has a different absorption, formations and decays shown in Figure 5b can be explained by the following reactions
-C6H5N{HAuIICl3‚‚‚Cl‚3H2O} f -C6H5N{HAuIICl3‚3H2O} + Cl (9) -C6H5N{HAuIICl3‚3H2O} f -C6H5N{HAuIICl3‚nH2O} + (3 - n)H2O (10)
Figure 5. Transient absorption observed at 340 nm of the {I + HAuCl4‚3H2O} (N:Au ) 3:1) system before and after 30 min of irradiation measured at microsecond time domains (a and b) and observed sample bleaching at a few first shots (c).
in the absorption was detected. Finally, complete relaxation of all UV-absorbing species is obtained with a lifetime of roughly 5 × 10-2 s. The transient absorption spectra of the sample at few different delay times are shown in Figure 6. It is noticeable that samples exposed to UV radiation exhibit very weak intensity of the transient signal at 340 nm. The behavior of transients described above can be understood as follows. HAuCl4‚3H2O is in a nondissociated form and connected to the 4VP units; under illumination an excited complex -C6H5N{‚HAuCl4‚3H2O}* was formed and observed in the stepwise rise of the absorption. The increase of the absorption with a lifetime of 3 × 10-6 s can be ascribed to the formation of the complex containing AuII, which has a higher absorption at 340 nm than the AuIII species.25 The processes can be denoted as
where n ) 0-2. The decay of the AuII complex can proceed in two ways. First one is a disproportionation of two complexes obtained in (10) by reaction 11 with the formation of AuIII and AuI species characterized by lower absorption at 340 nm as compared to that of AuII. This is a traditional way to describe a decay of an unstable AuII species.24,25 Another way to explain these data is the photoinduced reduction of AuII to AuI. In principle the decay observed in Figure 5c with the lifetime of 0.05 s can be described by the reactions
2-C6H5N{HAuIICl3‚nH2O} f -C6H5N{HAuIIICl4‚3H2O} + -C6H5N{HAuICl2‚mH2O} (11) where m ) 0-1 hν
-C6H5N{HAuIICl3‚nH2O} 98 -C6H5N{HAuICl2‚mH2O} + Cl (12) If the concentration of gold in the micelle (gold loading) is decreased by a factor of 3 (N:Au ) 9:1), the spectra of the transients strongly change. Though the formation of AuII species after excitation pulse monitored at 340 nm has nearly the same formation rate (τ ) 6 ×10-6 s, because this reaction is independent of the concentration), the
88 Langmuir, Vol. 15, No. 1, 1999
decay of this species takes place with significantly longer lifetimes. This is especially true for the final step (11), which is characterized by a lifetime of approximately 20 s. If the disproportionation of AuII would be strictly a bimolecular reaction, the decrease of the concentration of gold salt by a factor of 3 had to decrease the reaction rate by a factor of 9. However, because for our particular system the reaction proceeds in a rather solid state and each gold compound molecule is immobilized by 4-VP units, the dependence of the reaction rate of the AuII decay on the gold compound concentration is obviously more complicated. The difference in lifetimes τ ) 20 s and τ ) 5 × 10-2 s of 3 orders of magnitude apparently reflects a rather high localization of the gold species. The difference in the rates of the last step may result in a different course of the reaction for lower loaded samples. The typical delay between the pulses during the UV irradiation was 0.5 s, while dark reactions lasted tens of seconds. Thus, during UV irradiation there can be a significant buildup of AuII by the UV pulses. In a toluene solution of PS-b-P4VP, HAuCl4‚3H2O is in a nondissociated form. To clarify the influence of water on gold species reduction in block copolymer micelles, 0.5 wt % of water was added to the system {I + HAuCl4‚ 3H2O} (N:Au ) 9:1). In this case, one can support dissociation of the gold salt. This event results in a very fast growth of the absorption after the pulse, which can be attributed to the AuII species formation characterized by a lifetime of 9 × 10-8 s. This species has a lifetime of τ ) 1.8 × 10-5 s, the same as species described in reactions 9 and 10. Because the formation and decay of AuII species are much faster in the system containing water than those in nonaqueous solution, it can be supposed that the presence of water in P4VP micelle cores facilitates the diffusion of ions and the disproportionation reaction. Unlike HAuCl4‚3H2O, which provides a protonation of the VP units, AuCl3 coordinates directly to nitrogen of the 4VP units. Such a coordination obviously causes higher localization and has to reduce the rates of bimolecular reactions. However, even the formation of the gold AuII species was found to proceed very slowly and can be characterized by two lifetimes: 2.1 × 10-5 and 2.5 × 10-4 s. For 1 ms we did not observe the decay of the absorption at 340 nm. The slower reduction of AuIII species in AuCl3 compared to that in HAuCl4 can be understood by considering a higher redox potential of {(C6H5N)n‚AuCl3}. Two lifetimes can be attributed to both the addition of the electron to AuIII and the subsequent rearrangement of the complex as shown in reactions 13 and 14:
Bronstein et al.
Figure 7. Transient absorbance observed at 340 nm in the system {II + HAuCl4‚3H2O} (N:Au ) 9:1) recorded before the first 5 min of irradiation.
Figure 8. Transient absorbances observed at 500 nm in the system {I + HAuCl4‚3H2O} (N:Au ) 3:1) after 3600 laser pulses.
Slower accumulation of the AuI species can be also the reason for slower nucleation that finally results in bigger particles. All transients discussed above were observed in weakly segregated block copolymer micelles (I). The strongly segregated block copolymer micelles (II) resulted in complicated transient decay curves (Figure 7) which did
not permit calculation of the lifetime. This might be explained by perturbations in the sample during laser pulse absorption. The kinetics of transients was studied through some stages of laser irradiation: for initial solutions when the buildup of AuII and its decay to AuI take place, and also after 5, 15, and 30 min of laser irradiation. For nearly all the samples, after 5 min of excitation the static absorption spectra show already some very low absorption which might be ascribed to AuI species. After 15 and 30 min of irradiation, the formation of gold colloids was observed. The transient absorption of the sample {I + HAuCl4} (N: Au ) 3:1) after 30 min of laser irradiation monitored at 500 nm is presented in Figure 8. One can see that very fast formation of the absorption is accompanied with two decays: τ ) 6 × 10-6 s (Figure 8a) and τ ) 4 × 10-5 s
Laser Photolysis of AuIII Salts
Figure 9. TEM micrograph of gold nanoparticles obtained in the system {I + HAuCl4‚3H2O} (N:Au ) 9:1) after finish of laser irradiation.
(Figure 8b). Presumably, the fast buildup is related to an excited state -C6H5N{HAuICl2‚mH2O}/ with the following reduction of the complex with two lifetimes: 1.1 × 10-5 and 1.3 × 10-4 s by the reaction
-C6H5N{HAuICl2‚mH2O}/ f -C6H5N + Au0 + HCl + Cl (15) Note that the last reaction is actually a multistage process, and presumably a sequence of decay processes can be expected. According to the transient decay measurements the maximum absorption is observed at 500 nm. The gold colloid formation from gold atoms according to eq 6 should proceed very slowly in such a system. Indeed even for higher loading (N:Au ) 3:1), the buildup proceeds with lifetimes 6.3 × 10-1 s. For this sample a shoulder in the static absorption spectrum at 420 nm was also observed. It is reminded that this feature of the static absorption spectrum was characteristic only for the higher gold compound loading (N:Au ) 3:1). The observed absorption at 420-460 nm might be attributed to some very small gold clusters with an organic environment. In this situation, a higher concentration of gold compound should facilitate the formation of such clusters. The progress of reduction and aggregation leads to metal colloids, and pronounced absorption shoulder at 420 nm almost disappears (Figure 3). The increase of the absorption at 540 nm was observed in the presence of water to occur with a lifetime of 1.9 × 10-3 s. Thus, the presence of polar solvent molecules in the micelle cores facilitates the gold nanoparticle growth but does not influence the final state (Figure 2, curves 1 and 2). The use of higher segregated micelles which are characterized by higher density of micelle cores also permits observation of the formation of the absorption at 460 nm (after preliminary 1800 pulses of irradiation) with a lifetime of 4.6 × 10-5 s. This is ascribed again to the
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Figure 10. TEM micrograph of gold nanoparticles obtained in the system {I + HAuCl4‚3H2O} (N:Au ) 3:1) after finish of laser irradiation.
cluster formation. It should be noted that although the first step of the reduction and nucleation strongly depends on the environment and on additives used, the final step of reduction proceeds rather similarly in all the samples. TEM Examination of Gold Colloids Prepared in PS-b-P4VP under Laser Photolysis. The rates of nucleation and growth processes are also reflected in the particle size and the particle size distribution. It is therefore interesting to relate the differences seen in transmission electron microscopy (TEM) micrographs (presented in Figures 9-14) directly to the kinetic scenarios developed above. The TEM micrographs of gold colloids prepared in the sample {I + HAuCl4‚3H2O} (N: Au ) 9:1) after laser irradiation exhibit small gold particles with a mean diameter of 3.0 nm and standard deviation of 10% (Figure 9). The gold particles of similar size were prepared by us earlier with the superhydride and NaBH4 reduction at molar ratio N:Au ) 3:1.1,2 Because block copolymer micelles (I) are comparably weakly segregated, they remain unseen under an electron microscope without additional staining. The increase of gold compound loading (N:Au ) 3:1) in the same polymer system results in an increase of the particle size to 4.5 nm with standard deviation of 12% (Figure 10). The use of AuCl3 instead of HAuCl4‚3H2O where the nucleation of gold particles was shown to proceed much slower, results in bigger particles with a mean diameter of 6.0 nm and standard deviation of 17% (Figure 11). For the system {I + AuCl3} (N:Au ) 9:1), it was shown that the course of the photolysis strongly affects the rate of the nucleation process. Figure 12 shows a TEM micrograph of gold colloids prepared in the system {I + AuCl3} (N:Au ) 9:1) after laser photolysis (the similar stages such as those for the previous sample) and subsequent kinetics measurements after three stages of photolysis. Very big particles can be seen with the mean size of about 18 nm and shapes different from spherical. Such pictures confirm a very slow growth of gold nanoparticles. The final colloids prepared
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Bronstein et al.
Figure 11. TEM micrograph of gold nanoparticles obtained in the system {(I) + AuCl3} (N:Au ) 9:1) after finish of laser irradiation.
Figure 13. TEM micrographs of gold nanoparticles obtained in the system {(II) + HAuCl4 × 3H2O} (N:Au ) 9:1) after finish of laser irradiation presented at different scales (a, b).
Figure 12. TEM micrograph of gold nanoparticles obtained in the system {(I) + AuCl3} (N:Au ) 9:1) after finish of laser photolysis (the similar stages as for Figure 11) and subsequent kinetics measurements after three stages of photolysis.
in such a way have in a UV spectrum a maximum at 549 nm (Figure 2). When strongly segregated micelles (II) are used with HAuCl4‚3H2O loading at molar ratio N:Au ) 9:1, the gold particles formed have a mean diameter of 2.8 ( 0.3 nm (Figure 13a). Here and in Figure 14, dark gray regions demonstrate micelle cores which are visible for block copolymer (II), while nearly black areas present gold nanoparticles located in micelle cores. Another interesting
feature of this sample is that it forms wormlike micelles on the electron grid which are filled with metal nanoparticles (Figure 13b). Formation of the wormlike or cylindrical micelles, instead of the spherical ones, was observed for several systems and explained by their thermodynamic state close to the superstrong segregation limit.30 A second look on the micrograph shows that particles are mainly located on the edges of micelle core where the nucleation obviously is most favorable. Crosslinking of the core and addition of methanol (II and IIc) does not change the particle size much (the mean diameter is 2.9 ( 0.4 nm, Figure 14); however the micelles filled with metal nanoparticles are completely spherical and do not form any cylinder micelles due to the lack of mobility in cross-linked micelle cores.
Laser Photolysis of AuIII Salts
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rene-poly-4-vinylpyridine has been studied. By static absorption spectra and flash photolysis, certain differences in stepwise reduction of AuIII species and metal colloid formation were observed. These differences mainly lie in different rates of each step. It was found that the addition of water into micelle cores makes the formation and decay of AuII species much faster because it provides dissociation and facilitates the diffusion of ions and disproportionation reaction. At the same time, the final state is not affected. The increase of the gold compound loading in the system or density of micelle core (for stronger segregated micelles) allows observation the buildup of an absorption maximum at 460 nm (after 15 min of irradiation) which was ascribed to the formation of gold clusters within the organic environment. Progress of reduction results in the disappearance of this absorption. The size of final colloids prepared under laser photolysis is mainly influenced by the type of gold compound and the loading rate, by the course of laser irradiation (time delays between subsequent stages of irradiation) and can be adjusted between 2.8 and 18 nm.
Figure 14. TEM micrograph of gold nanoparticles obtained in the system {(IIc) + HAuCl4 × 3H2O + methanol} (N:Au ) 9:1) after finish of laser irradiation.
Conclusion The laser photolysis of HAuCl4‚3H2O and AuCl3 in various block copolymer micelles derived from polysty-
Acknowledgment. L. Bronstein, D. Chernyshov, and P. Valetsky wish to thank the Russian Foundation for Basic Research (Grant No. 96-03-32335) for support of this research. J. Hartmann and S. Fo¨rster acknowledge the financial support provided by the Max Planck Society. H. Lemmetyinen is grateful for the support provided by the Technical Development Center of Finland. The very fruitful discussion with Professor M. Antonietti is highly appreciated. LA980868R
