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2006, 110, 14014-14019 Published on Web 07/01/2006
Molecular Mechanism of the Photochemical Generation of Gold Nanoparticles in Ethylene Glycol: Support for the Disproportionation Mechanism Susie Eustis and Mostafa A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: May 15, 2006; In Final Form: June 16, 2006
It is found that replacement of the chloride ions in tetrachloroauric acid with bulky bromide ions inhibits the formation of gold nanoparticles in the photochemical reduction in ethylene glycol. However, the addition of silver ions to either the bromide or the chloride auric acid solution is found to enhance the rate of gold nanoparticle formation. These results are found to be accounted for by the previously proposed mechanism (Eustis, S.; Hsu, H.-Y.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811) which involves disproportionation of the chloroauric complexes to generate free gold atoms and chloride ions. The steric effects of the bulky bromide ions inhibit the formation of the Au-Au bond needed in the electron transfer process of the disproportionation reaction. The addition of Ag+ ions results in the formation of insoluble silver halide, which shifts the disproportionation reaction toward the formation of gold atoms and thus the formation of gold nanoparticles.
Introduction In the field of nanoscience,1 a large number of gold nanoparticles have been synthesized and characterized.2-10 The detailed molecular (ionic) mechanisms of only a few have been revealed.10-17 In the bottom-up colloidal synthetic method of metal nanoparticles, thermal reduction of the metal ion takes place.2-17 The details of the reduction process in many of these synthetic methods are not elucidated and depend on the reducing agent, for example, citric acid,2-5 ascorbic acid,2 sodium borohydride,2,8 and alcohol.5,9,10,17 More complex is the detailed mechanism of the nanoparticle shape formation.2,18-31 In fewer synthetic methods, photochemistry is involved.17,32-46 Thus, while gold nanoparticles can be synthesized by reduction with ethylene glycol thermally at 160 °C,9,47,48 they can be photochemically generated at room temperature.17 In a previous publication,17 a simple mechanism is proposed for the photochemical synthetic process. The mechanism involves the photoexcitation of the starting material, [Au(3+)Cl4]-, which is then reduced to the unstable49 [Au(2+)Cl3]- by the solvent, ethylene glycol.17 This is followed by two disproportionation reactions.17,50-53 The [Au(2+)Cl3]- is unstable49 and rapidly forms17,50-53 [Au(1+)Cl2]- and [Au(3+)Cl4]-. The [Au(1+)Cl2]is a meta-stable species,54 which can undergo further disproportionation to form the starting material [Au(3+)Cl4]- and neutral gold, which forms gold nanoparticles in solution. Thus, the rate of formation of the gold nanoparticles is determined by the slow net disproportionation reaction slow
3[Au(1+)Cl2]- 98 [Au(3+)Cl4]- + 2Au0 + 2Cl0 fast
nAu 98 (Au)n * To whom correspondence should
[email protected].
(1) (2)
be
addressed.
E-mail:
10.1021/jp062972k CCC: $33.50
Previous support for this mechanism came from two observations. First is the reappearance of the absorption spectrum of the starting material,55-57 [Au(3+)Cl4]-, after irradiation ceases with a rate comparable to that of gold nanoparticle formation.3,7,17,58 Second is the observed dependence of the rate of formation of the gold nanoparticles on the composition and viscosity of water-ethylene glycol solvent. As the concentration of ethylene glycol increases in a water-ethylene glycol solvent mixture, the rate of formation of gold nanoparticles first increases and then decreases. The increase is explained by an increase in the rate of photoreduction by ethylene glycol. As the mole fraction of the viscous glycol increases, the rate of diffusion controlled bimolecular photoreduction and disproportionation reactions decrease. This is concluded from eq 1. In order for Aun+ to undergo disproportionation, two gold complexes need to interact through the formation of the Au-Au bond. This allows for electron transfer between the two gold ions. The rate of bond formation is expected to be sensitive to the size of the halogen ion attached to the gold ion. Replacing Cl- with Br- is expected to change the rate of the disproportionation processes and thus the rate of gold nanoparticle formation. In the present work, we found that no gold nanoparticle formation is observed when tetrabromoauric acid is used as the starting material. Another test of the previously proposed mechanism17 is the effect of the addition of Ag+ to the solution. From eq 1, the formation of Au0 (which forms the nanoparticles through eq 2) is accompanied by the production of halogen ions. The addition of Ag+ is expected to remove the halogen ions from solution, leading to a shift in the equilibrium in eq 1, which leads to an increase in the amount of Au0 produced. The rate of nanoparticle formation is then expected to increase by the addition of Ag+. This is observed for the reduction of both tetrachloroauric acid and tetrabromoauric acid. © 2006 American Chemical Society
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J. Phys. Chem. B, Vol. 110, No. 29, 2006 14015
Figure 1. Absorption spectra of HAuCl4 (black) and HAuBr4 (red) (0.023 M Au) in ethylene glycol in a 1 mm cuvette. The inset shows the absorption spectra of the same solutions in the presence of the filter. This shows that, in the excitation region used, the tetrabromoauric acid absorbs more light than the tetrachloroauric acid.
Experimental Section Tetrachloroauric acid trihydrate (HAuCl4) and tetrabromoauric acid trihydrate (HAuBr4) are purchased from Aldrich and used as received. Sodium bromide, sodium chloride, silver nitrate, ethylene glycol (EG), and poly(vinylpyrrolidone) (PVP) (MW ) 55 000) are used as received. Solutions in ethylene glycol are made up to contain 0.0023 M gold ions, using either the chloride salt or the bromide salt. The concentration of PVP in the final solutions is 0.010 M per repeat unit. Silver solutions have 3.50 × 10-4 M AgNO3 for a ratio of Au/Ag ions of 6.81. The pH is below 3 in all studies so that the hydrolysis equilibrium may be safely ignored. The sample is placed in a 1 cm × 1 cm × 3 cm quartz cuvette (or a 0.1 cm × 1 cm × 3 cm quartz cuvette for initial decrease of Au3+ studies) for irradiation with a xenon lamp (LPI-250, PTI power controller) operated at 50 W. In the light path are a lens to focus the light, aluminum screening to control the intensity, and a band-pass filter to control the wavelengths of irradiation (filter ∼250-400 nm). The power at the sample is ∼45 mW. Results Effect of Substituting HAuBr4 for HAuCl4 on the Initial Rate of Photoreduction and Gold Nanoparticle Formation. To further examine the photoreduction of HAuCl4 in ethylene glycol, the chloride ions are replaced with bromide ions. The absorption spectrum red shifts as the chloride is replaced with bromide, as shown in Figure 1. The absorption spectra are similar to those reported previously for AuCl4- and AuBr4with the two absorptions due to the electronic transitions.55-57,59-61 The absorption maxima are at 228 and 313 nm for AuCl4- and 255 and 382 nm for AuBr4-.55-57,59-61 The filter is the same band-pass filter used in our previous investigations17 to allow 250-400 nm light to reach the sample. Since the absorption spectrum red shifts in the bromide complex, a different amount of energy is imparted to the gold complex during photoirradiation. The integrated area of HAuBr4 absorbance after correction for the filter is shown in the inset of Figure 1 and is 2.3 times larger than the integrated area of the HAuCl4 absorption. Thus, the rate of reduction of HAuBr4 is expected to be faster than that of HAuCl4 if it is solely based on the rate of light absorption.
Figure 2. Changes in the absorption spectrum of HAuBr4 solutions with irradiation as a function of time (a) during irradiation and (b) after irradiation. The absence of a surface plasmon absorption in the region from 500 to 600 nm suggests that HAuBr4 does not form gold nanoparticles as rapidly as HAuCl4.
Solutions of HAuBr4 are irradiated to determine the effect of replacing chloride ions with bromide ions on the rate of the photochemical reduction process. The absorption spectra of this solution are shown in Figure 2a as a function of the irradiation time. The rate of decrease in the absorption intensity of the starting material is much slower for the bromide complex than for the chloride complex (see Figure 3a), even though the integrated absorption intensity is larger. If the concentration is increased, the rate of decrease of AuBr4- absorption does not change. Furthermore, no nanoparticle absorption is observed as determined from the observed continuous decrease of the absorbance at all wavelengths, including the 500-600 nm region where the strong gold nanoparticle plasmon absorption is expected. No nanoparticles are observed in the transmission electron microscopy (TEM) of these samples. Longer irradiation times do not generate gold nanoparticles. The rates of decrease of the absorption spectra due to ligand to metal charge transfer are followed as a function of irradiation time for HAuCl4 and HAuBr4. The rate of the disappearance of the absorption of the complex is calculated from the decrease of the absorption intensity to compare how the concentrations of these two complexes change with irradiation time. The rate of decrease of the absorption of AuCl4- and AuBr4-
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Figure 3. Effect of adding Ag+ ions on the rate of gold nanoparticle formation: time dependence of the absorption spectra of HAuCl4 solutions during irradiation (a) without silver and (b) with silver nitrate (3.5 × 10-4 M). TEM images of solutions of HAuCl4 after 36 min of irradiation (c) without silver and (d) with silver nitrate (3.5 × 10-4 M). The addition of Ag+ to the solution accelerates the rate of formation of the gold nanoparticles.
determined at their maximum absorption is -0.535 ((0.039) and -0.113 ((0.009), respectively. Thus, the change in the reaction rate is found to be slower in the bromide complex [Au(3+)Br4]- than the chloride complex [Au(3+)Cl4]-. The observed decrease in the rate of the bromide complex must then result from the rate of the photoreduction process of the excited state of Au3+ with the ethylene glycol. This could be a result of the steric effect due to the larger size of the bromide ion compared with the chloride ion or to an electronic effect due to differences in the charge density distribution of the excited states of the two complexes. In the irradiation of AuCl4-, nanoparticle growth takes place as a function of time after the irradiation is ceased. Figure 2b shows the absorption spectrum of AuBr4- after irradiation is stopped. The absorption intensity remains relatively constant with only a slight increase in the absorption of AuBr4- observed after 2 days. Effect of the Addition of Silver Ion on Gold Nanoparticle Formation. We investigated the effect of adding silver ions to
the solution before irradiation. Figure 3 shows the changes in the absorbance of HAuCl4 solutions as a function of the irradiation times with and without the addition of silver ions. Figure 3a is similar to that reported previously,17 where the absorbance of AuCl4- decreases and then gold nanoparticles form after most of the AuCl4- is reduced (as observed by the small absorption between 300 and 400 nm) when the plasmon resonance absorption of gold nanoparticles at 550 nm increases. Figure 3b shows that when silver ions are added to this solution and the solution is irradiated under the same conditions, the yield of nanoparticles is much larger as measured by the intensity of the plasmon resonance of the gold nanoparticles between 500 and 600 nm. Gold nanoparticles are formed in the first 6 min of irradiation with silver ions present instead of after 18 min without silver ions, as seen in Figure 3. This is an increase in the rate of nanoparticle formation by 300% as a result of the presence of Ag+. TEM images of the solutions shown in parts a and b of Figure 3 are shown in parts c and d of Figure 3, respectively. The size distribution and nanoparticle shapes
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Figure 4. (a) Changes of the absorption spectra of HAuBr4 in ethylene glycol solutions with the addition of silver nitrate during irradiation. The increase in the absorption in the 500-600 nm region suggests the formation of gold nanoparticle is enhanced in the presence of Ag+. (b) TEM image confirming the presence of gold nanoparticles.
formed in the two solutions are distinctly different. Some triangular and hexagonal shapes are observed in the HAuCl4 solution without Ag+ ions, but only a small percentage. Due to the acceleration of nanoparticle formation with the addition of silver ions, more small nanoparticles and more spherical nanoparticles are observed after the addition of Ag+ ions. Another important difference in solutions with silver ions present is that the absorption of [Au(3+)Cl4]- does not seem to decrease to the same extent as observed without silver ions present. This further suggests that disproportionation is being accelerated by the presence of the silver ions to form more Au0, observed as gold nanoparticles, and also reforming Au3+, leading to the slow apparent disappearance of [Au(3+)Cl4]- from solution. Figure 4a shows the changes in the absorption spectrum during the photoreduction of tetrabromoauric acid in the presence of Ag+. The first signs of gold nanoparticle formation are observed as the plasmon resonance of gold nanoparticles increases between 550 and 600 nm. This is different from the behavior observed in Figure 2a, in the absence of Ag+, where only a decrease in intensity is observed with the same initial concentrations of HAuBr4. TEM results confirm that gold nanoparticles are formed when AuBr4- is irradiated in the presence of silver ions in solution, as shown in Figure 4b. The nanoparticles formed from the HAuBr4 solution are similar in size to those formed from HAuCl4 with Ag+, but only spherical and oblong shapes are observed from the HAuBr4 solution. Many small nanoparticles are also observed in this sample due to the increase of the rate of nanoparticle formation by the addition of Ag+ ions. A blank experiment is carried out with only silver nitrate (3.50 × 10-4 M) and PVP in ethylene glycol. After only 6 min of irradiation, the characteristic plasmon resonance at ∼400 nm of silver spheres7 is observed in the solution. Silver is known to have a lower reduction potential than gold,15 leading to the possibility that the silver ions are a source of heterogeneous nucleation. Gold ions in solution are known to replace silver atoms, leading to the formation of gold nanoparticles.10 How-
ever, the reappearance of the [Au(3+)Cl4]- suggests that the disproportionation of the Au1+ is the dominant reaction in this system. Discussion of the Results and Proposed Mechanism The first principle observation is that replacing the Cl- ions with Br- ions in the starting material (tetrachloroauric acid) is found to have two effects: (1) to slow the photoreduction rate of the starting material and (2) to diminish the rate of nanoparticle formation. The second principle observation is that the addition of silver ions is found to accelerate the rate of formation of the gold nanoparticles formed from either the tetrachloroauric acid or tetrabromoauric acid. These observations can be explained in terms of (and thus supports) the previously proposed mechanism17 shown below hν
Au(3+)X4- 9 8 [Au(3+)X4-]* glycol [Au
(3+)
-
X4 ]* f Au
(2+)
-
X3 + X
(Excitation) (3) (Reduction) (4)
2Au(2+)X3- f Au(1+)X2- + Au(3+)X4- fast (Disproportionation) (5) 2Au(1+)X2- f Au(2+)X3- + Au0 + X- slow (Disproportionation) (6) 3Au(1+)X2- f Au(3+)X4- + 2Au0 + 2X(Net Disproportionation) (7) glycol
Au+m 98 Au0 (+m: +3, +2, +1) (Glycol Reduction) (8) nAu0 f (Au0)n rapid (Nanoparticle Formation) (9) where X is either Cl or Br. The rate of formation of the gold nanoparticles is determined by the rate of formation of the Au0. This in turn is determined
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by the rate of the slow net disproportionation process given by eq 7. This net reaction is slow because of the slow disproportionation52,62 reaction given in eq 6. This disproportionation reaction involves electron transfer between two gold ions while halide ions are attached to each gold ion. This bimolecular complex must involve a gold-gold bond through which the electron transfers, as suggested in the disproportionation reaction. For example, the disproportionation reaction in eq 6 can be rewritten as
halide insoluble salts with the halide ions that are produced together with Au0 atoms in the rate-determining disproportionation reaction. This shifts the equilibrium of this reaction, allowing for more Au0 (and thus gold nanoparticles) to be formed. Acknowledgment. We wish to acknowledge the chemistry division of NSF (Grant No. 0240380) for funding. We also wish to acknowledge the center for nanostructure characterization and fabrication at Georgia Tech used for TEM characterization of samples. References and Notes
Br-
Cl-
First, since the ion is larger in size than the ion, there will be a large energy barrier to the formation of the goldgold dimer with Br-.63 Furthermore, even if the dimer is formed, it will be relatively unstable due to the Br--Br- repulsion. These two factors decrease the concentration of the dimer complex and in turn decrease the rate of formation of Au0 (and thus the rate of gold nanoparticle formation). In support of the above explanation is the fact that the species Au2Cl6 is well-known in the literature.63-66 Although the Au2Br6 is reported to exist, the literature on this complex is less,63 suggesting that it is less stable and there is a larger barrier for its formation. The fact that the generation of a metal-metal bond can be formed even when chloride ions are attached to the gold suggests that a network structure of gold atoms can form which starts the nucleation process of the metal nanoparticles. This mechanism is also useful in explaining the effect of the silver ion. The silver ions can react with the halide ions formed in all the disproportionation reactions to form insoluble AgCl(s) or AgBr(s). The solubility products of silver chloride and silver bromide are 1.77 × 10-10 and 8.45 × 10-13, respectively.67 This suggests that when silver ions are present in the solution, they complex with the free halide ions in solution. When tetrachloroauric acid or tetrabromoauric acid is present in high concentration with the same gold/silver ratio used in this study, cloudy solutions form immediately upon the addition of the silver ions. This confirms the precipitation of the insoluble silver halide as a result of the rapid reaction between the silver ions and the halide ions in solution. The mechanism of the disproportionation reactions involves the production of halide ions together with the formation of Au0 (see reaction 6). The addition of Ag+ ions to the solution removes the halide ions from solution, thus driving the equilibrium to the right, which leads to the formation of Au0. This in turns accelerates the formation rate of gold nanoparticles, as observed experimentally. Conclusions We found that the Br substitution reduced the rate of the reduction of the starting material. This is proposed to result from the steric effect of the large Br- ion and the difference in the structure of the excited state. We also observed a large change in the nanoparticle yield and rate of formation upon either bromide substitution for the chloride in the starting acid or upon the addition of Ag+ to solution. These observations can be accounted for by the previously proposed mechanism. As the bromide ions are larger than the chloride ions, bromide ions are proposed to sterically slow the disproportionation rate of the AuX2-, which is the rate-determining step in the formation of Au3+ and gold nanoparticles. The effect of the addition of silver ions in accelerating the rate of formation of the gold nanoparticles is discussed in terms of the formation of silver
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