pubs.acs.org/Langmuir © 2009 American Chemical Society
Synthesis of Monodisperse Quasi-Spherical Gold Nanoparticles in Water via Silver(I)-Assisted Citrate Reduction Haibing Xia,*,†,‡ Shuo Bai,‡ J€urgen Hartmann,‡ and Dayang Wang*,‡ †
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China, and ‡Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany Received August 11, 2009. Revised Manuscript Received October 14, 2009
We demonstrate a simple and reproducible way to produce quasi-spherical Au nanoparticles (NPs) with a fairly narrow size distribution in water by rapidly adding a mixture solution of HAuCl4, sodium citrate, and a trace amount of silver nitrate into boiling water. The sizes of quasi-spherical Au NPs obtained increases from 12 ( 1 nm to 18 ( 3, 25 ( 3, and 36 ( 3 nm with decrease of the citrate concentration in a fairly linear way. The present protocol can efficiently minimize the effect of citrate to buffer the pH of the reaction media and thus change the type and reactive activity of auric ions and significantly speed up the nucleation and growth rate of Au NPs. The presence of Agþ ions can not only suppress the secondary nucleation but also reshape the polycrystalline Au NPs into a quasi-spherical shape. In the case of synthesis of Au NPs of sizes ranging from 10 to 36 nm, our approach efficiently makes up the shortages of the classical Turkevich method with respect to the reproducibility and uniformity of the NP size and shape.
Introduction Au nanoparticles (NPs), known since the times of ancient Romans, have been extensively studied because of their fascinating physicochemical properties and promising technical applications.1 Up to date, numbers of successful methods have been developed to synthesize Au NPs with different sizes and shapes in aqueous and organic media.1 Synthesis of Au NPs by reduction of tetracholoauric acid (HAuCl4) with sodium citrate in water has been developed by Turkevich et al. in 1951.2 The Turkevich method still remains the most commonly used one to synthesize Au NPs due to its fairly simple and environmentally benign procedure and its flexibility to tune the size of Au NPs from 10 to 150 nm by varying the molar ratio of citrate to HAuCl4.3a Despite countless studies of the Turkevich method in multifarious applications in the past more than 5 decades,3 however, its experimental protocol remains little changed;rapid addition of sodium citrate to hot aqueous solution of HAuCl4. As compared with those prepared in organic media, the Au NPs obtained in water by the Turkevich method have a broader distribution of size and shape in particular. The optimal size of Au NPs with an acceptable size distribution of 13-16% is in the range of 12-40 nm, but the NP shapes are nonuniform and irregular, such as quasi-sphere, ellipsoid, and triangle.3f This difficulty is attributed to the fact that the mechanism governing the nucleation and *Corresponding authors: e-mail:
[email protected] (H.X.), dayang.
[email protected] (D.W.); Fax þ49 331 5679202.
(1) (a) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (b) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Chem. Soc. Rev. 2008, 37, 1783. (2) (a) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (b) Turkevich, J.; Garton, G.; Stevenson, P. C. J. Colloid Sci., Suppl. 1954, 1, 26. (c) En€ust€un, Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317. (d) Turkevich, J. Gold Bull. 1985, 18, 86–91. (e) Turkevich, J. Gold Bull. 1985, 18, 125. (3) (a) Frens, G. Nature (London): Phys. Sci. 1973, 241, 20. (b) Takiyama, K. Bull. Chem. Soc. Jpn. 1958, 31, 944. (c) Chow, M. K.; Zukoski, C. F. J. Colloid Interface Sci. 1994, 165, 97. (d) Pei, L.; Mori, K.; Adachi, M. Langmuir 2004, 20, 7837. (e) Kumar, S.; Gandhi, K. S.; Kumar, R. Ind. Eng. Chem. Res. 2007, 46, 3128. (f) Kimling, J.; Maier, M.; Okenve, B.; Kotaidis, V.; Ballot, H.; Plech, A. J. Phys. Chem. B 2006, 110, 15700. (4) (a) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. J. Am. Chem. Soc. 2007, 129, 13939. (b) Rodríguez-Gonzalez, B.; Mulvaney, P.; Liz-Marzan, L. M. Z. Phys. Chem. (Munich) 2007, 221, 415.
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crystal growth of Au NPs in the Turkevich method is far more complicated than its simple experimental protocol suggests.2-4 Herein we modify the Turkevich method by rapidly adding a mixture solution of HAuCl4 and sodium citrate into boiling water. Significantly, the method presented allows reproducible preparation of Au NPs in water with a fairly narrow size distribution and a uniform quasi-spherical shape provided a trace amount of silver nitrate (AgNO3) is in the HAuCl4/citrate mixture. Note that uniform quasi-spherical Au NPs cannot otherwise be directly synthesized in water. The sizes of quasispherical Au NPs obtained increases from 12 ( 1 nm to 18 ( 3, 25 ( 3, to 36 ( 3 nm with decrease of the citrate concentration in a fairly linear way.
Experimental Section Hydrogen tetrachloroaurate(III) hydrate (99.9%, metals basis, Au >49%, min, item no. 12325) was purchased from Alfa Aesar, silver nitrate (99.9%, Art. 7908.1) from Carl Roth GmbH, and trisodium citrate dihydrate (Na3C6H5O7, 99%, product no. 398071) from Aldrich, and they were used as received. Prior to performing nanoparticle synthesis protocols, the glassware and stir bars were cleaned with aqua regia (3:1 v/v HCl (37%):HNO3 (65%)) solutions (caution: aqua regia solutions are dangerous and should be used with extreme care; never store these solutions in closed containers) and then rinsed thoroughly with H2O before use. The water in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ cm. The aqueous HAuCl4 solution (0.5 wt %) was prepared before use and stored in the fridge (þ4 °C). To synthesize monodisperse quasi-spherical Au NPs, the experimental protocol is as follows. 1 mL of HAuCl4 aqueous solution (0.5 wt %) and 42.5 μL of AgNO3 aqueous solution (0.1 wt %) were added to a given volume of citrate aqueous solution (1 wt %) under stirring. Water was added to bring the volume of the mixture solution to 2.5 mL. This mixture was incubated for 5 min before addition to hot water. The color of the mixture solutions remained little changed during incubation. Note that adding citrate solution to the HAuCl4/AgNO3 mixture solution may cause the color change from light yellow to orange, leading to black Au precipitates after
Published on Web 10/30/2009
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Article adding the resulting HAuCl4/AgNO3/citrate mixture solution to boiling water. 47.5 mL of water in a 100 mL flask was placed in the two-necked flask with condenser and heated to boil. After the water continues to boil for 10 min, the HAuCl4/citrate/AgNO3 mixture solution was quickly injected into the boiling water by pipet under vigorous stirring. The color of the reaction solution was changed quickly from colorless, grayish blue, purple, to ruby red within less than 1 min at the citrate concentration ranging from 2.97 10-2 to 6.10 10-3 wt %. But this color change took 10 min at the citrate concentration of 4.10 10-3 wt %. The reaction solution was further refluxed for 1 h under stirring to warrant formation of uniform quasi-spherical Au NPs. UV-vis absorption spectra were recorded with an Agilent 8453 UV-vis spectrophotometer. Quantitative analysis of temporal consumption of HAuCl4 was carried out following the method reported in the literature.5 Briefly, the reaction aliquots were added into preprepared and ice-cooled mixture solutions of NaCl (0.9 M) and HCl (0.1M) to quench the reaction and form colorless solutions at room temperature. AuCl4- ions shows a sharp absorption peak appeared at 314 nm due to the charge transfer to solvent band at pH 1.5 (Figure S1a).4 According to Beer’s law, the concentrations of AuCl4- ions were determined by the intensity of the absorption peak at 314 nm (Figure S1b). Transmission electron microscopy (TEM) images were obtained with a Zeiss EM 912 Omega microscope at an acceleration voltage of 120 kV. Dynamic light scattering (DLS) experiments were performed at room temperature and at a fixed angle of 173° on a Malvern Zetasizer Nano ZS equipped with 50 mW 533 nm laser and a digital autocorrelator. Prior to DLS measurement, the aqueous dispersions of as-prepared Au NPs were filtered through 0.2 μm polycarbonate membrane. The number-average values were used to compare the sizes and size distributions of the Au NPs as the number-average hydrodynamic sizes were more comparable to the particle sizes analyzed from TEM images.
Results and Discussion When sodium citrate is mixed with HAuCl4 in water at high temperature, the former is oxidized to sodium acetone dicarboxylate (SADC) while the latter is reduced to AuCl (Scheme 1). In parallel, depending on the pH of the reaction solution AuCl4ions (pH 3.3) are hydrolyzed into different types of auric precursor ions, AuCl3(OH)- (pH 6.2), AuCl2(OH)2- (pH 7.1), AuCl(OH)3- (pH 8.1), and Au(OH)4- (pH 12.9) ions, and their reactive activity decreases in the following sequence: AuCl4- > AuCl3(OH)- > AuCl2 (OH)2- > AuCl (OH)3- > Au(OH)4(Scheme 1).6,7 The recent study reported by Ji et al. has highlighted a non-negligible but less-recognized role of citrate that it can strongly buffer the pH of the reaction solution.4a As such, one can envision that upon addition of citrate one not just reduces HAuCl4 to AuCl but buffers the pH of the reaction solution from originally much acidic (pure HAuCl4) to less acidic and even neutral (HAuCl4/citrate mixture) depending on the amount of citrate added. Taking into account that both the nucleation and crystal growth of Au NPs are very fast at high temperature (less than 10 min at 100 °C), this buffer effect of citrate plus inhomogeneous mixing of the citrate solution with HAuCl4 solution should cause inhomogeneous nucleation, leading to a temporal overlap between nucleation and crystal growth and thus to a broad size distribution of Au NPs obtained. To minimize the pH buffer effect of citrate, here we rapidly added 2.5 mL of the mixture solution comprising 1.0 mL aqueous solution of HAuCl4 (0.5 wt %) and a given volume of citrate solution (1 wt %) to 47.5 (5) Gammons, C. H.; Yu, Y.; Williams-Jones, A. E. Geochim. Cosmochim. Acta 1997, 61, 1971. (6) Goia, D. V.; Matijevic, E. Colloids Surf. A 1999, 146, 139. (7) Fry, F. H.; Hamilton, G. A.; Turkevich, J. Inorg. Chem. 1966, 5, 1943.
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Xia et al. Scheme 1. Reactions Involved during Citrate Reduction of HAuCl4 for Nucleation and Growth of Au NPs
Table 1. pH Values of the Citrate/HAuCl4/AgNO3 Reaction System before and after Formation of Au NPs at the Different Concentrations of Citratea citrate 2.97 10-2 1.40 10-2 1.01 10-2 6.1 10-3 4.1 10-3 (wt %) pH 6.06 4.94 4.36 3.92 3.76 (before) pH 5.98 3.92 3.54 3.25 3.17 (after) a Note: the pH of the Milli-Q water used in the current work is 6.22.
mL of boiling water instead of adding citrate solution to boiling HAuCl4 solution. The final concentration of HAuCl4 was about 0.01 wt %, and the final citrate concentration was varied from 2.97 10-2 wt % to 1.40 10-2, 1.01 10-2, 6.10 10-3, and 4.10 10-3 wt % by varying the volume of citrate solution (1 wt %) from 1.5 mL to 0.7, 0.5, 0.3, and 0.2 mL. Note that in the case of the citrate concentration of 2.97 10-2 wt % our recipe was almost identical to that used in the Turkevich method for synthesis of 12-16 nm Au NPs. The pH of the reaction solutions used in our work was in the range of 3.3-6.2 so that only the very reactive AuCl3(OH)- ion exists in the reaction solution (Table 1).6 This was proposed to avoid the inhomogenous nucleation due to the coexistence of multiple auric precursor ions in the Turkevich method. The Turkevich protocol has a noticeable induction time of about 60 s before the dramatic decrease of the AuCl4- concentration due to oxidation of citrate to SADC by HAuCl4 (Figure 1). In contrast, our protocol shows a negligible induction time (Figure 1), suggesting a very fast formation of SADC. Chow and Zukoski have demonstrated that when 30% of AuCl4- ions is consumed, the color of the reaction solution suddenly turned from purple to red, corresponding to formation of Au NPs.3c Figure 1 shows that it takes about 4 min to consume 30% of AuCl4- ions in the Turkevich method but around 1 min in our Langmuir 2010, 26(5), 3585–3589
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Figure 1. Temporal evolution of auric ion concentration in the Turkevich protocol at the citrate concentration of 2.97 10-2 wt % (4) and in the present protocol at different concentrations of citrate: 2.97 10-2 wt % in the absence of Agþ (1) and 2.97 10-2 wt % (0), 1.40 10-2 wt % (O), 1.01 10-2 wt % (f), 6.10 10-3 wt % (b), and 4.10 10-3 wt % (9) in the presence of Agþ (8.5 10-5 wt %). The concentration of HAuCl4 was 0.01 wt %.
approach. This indicates a rapid nucleation and crystal growth of Au NPs. TEM imaging indicates that as compared with those obtained by adding citrate to hot HAuCl4 aqueous solution, the Au NPs, obtained by adding the HAuCl4/citrate mixture to boiling water, are more round in shape; less triangular or polygonal NPs were visible (Figure S2). The growth of Au NPs is due to disproportionation of AuCl, which is a rapid autocatalytic process catalyzed by the nuclei when they are formed (Scheme 1).1 The nuclei are not formed by collision of Au atoms derived from reduction of AuCl by statistic fluctuation according to LaMer theory.8 As initially proposed by Turkevich et al. and later supported by others,2,3 SADC and AuCl are coordinated into macromolecular complexes (Scheme 1), which consecutively coagulate into colloidally stable precursor particles, in which a number of Au nuclei are formed by disproportionation of AuCl. Although the nucleation of Au NPs is not governed by the LaMer fluctuation theory, the coagulation of macromolecular SADC/AuCl complexes should be induced by the concentration fluctuation. Matijevic et al. have demonstrated that rapid coagulation favors the formation of monodisperse spherical particles.9 Consequently, the rapid formation of a large amount of SADS should favor the formation of complex particles with a narrower size distribution.3f In parallel to the aforementioned reactions, SADC readily decomposes to acetone at high temperature, especially at a higher pH.10 Acetone can reduce auric precursor ions to AuCl,2,11 thus leading to the secondary nucleation and in turn broadening the size distribution of Au NPs obtained. This secondary nucleation should be minimized by fast reduction of citrate to SADC at a lower pH. The recent study demonstrates that Agþ ions can dramatically catalyze the oxidation of citrate to SADC under light.12 To further speed up formation of SADC, therefore, we added 42.5 μL aqueous solution of AgNO3 (0.1 wt %) into the HAuCl4/ citrate mixture prior to addition into boiling water; the concentration of Agþ ions in the reaction media was 8.5 10-5 wt %. Figure 1 shows that the presence of Agþ ions largely accelerates the consumption of the AuCl4- ions, and the consumption rate (8) LaMer, V. K.; Dinegar, R. H. J. Am. Chem. Soc. 1950, 72, 4847. (9) Privman, V.; Goia, D. V.; Park, J.; Matijevic, E. J. Colloid Interface Sci. 1999, 213, 36. (10) Wiig, E. O. J. Phys. Chem. 1928, 32, 961. (11) Li, G.; Lauer, M.; Schulz, M.; Boettcher, C.; Li, F.; Fuhrhop, J. H. Langmuir 2003, 19, 6483. (12) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313. (b) Wu, X.; Redmond, P. L.; Liu, H.; Chen, Y.; Steigerwald, M.; Brus, L. J. Am. Chem. Soc. 2008, 130, 9500.
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dramatically increases with the citrate concentration used. The AuCl4- ions are completely consumed in about 6 min at the citrate concentration of 2.97 10-2 wt % and in less than 2 min at the citrate concentration of 6.10 10-3 wt %. Exceedingly short induction (less than 30 s) is visible for all experimental runs in the presence of Agþ ions, suggesting fast oxidation of citrate to SADC. Figure 1 reveals that in the case of 2.97 10-2 wt % citrate used the concentration of AuCl4- ions continually decreases with the reaction time after the induction time in the absence of Agþ ions, whereas it just slightly decreases over 3 min and afterward rapidly declines in the presence of Agþ ions. (Note that in the case of using lower concentration of citrate the NP synthesis is too fast to monitor.) This suggests that the use of Agþ ions allow a good temporal separation of nucleation and crystal growth, thus narrowing the size distribution of the resulting NPs.13 TEM imaging demonstrates that our approach allows formation of uniform Au NPs with sizes increasing from 12 ( 1 nm to 18 ( 3, 25 ( 3, and 36 ( 3 nm with decrease of citrate concentration from 2.97 10-2 wt % to 1.40 10-2, 1.01 10-2, and 6.10 10-3 wt % (Figures 2 and 3). The size distribution remains little increased with the NP size. Of most importance is that Au NPs obtained by using our approach exhibit a uniform quasi-spherical shape until the citrate concentration is 6.10 10-3 wt %; nonspherical shaped NPs such as triangular ones are hardly observed (Figure 2). In contrast, the size distribution of Au NPs obtained using the Turkevich method is fairly broad;48 ( 18 and 62 ( 20 nm NPs were obtained at the citrate concentration of 1.01 10-2 and 6.10 10-3 wt %;and the shape becomes irregular with increase of the NP size (Figure 2e,f). In the present experimental protocol, as shown in Figure 1, the sharp drop of the AuCl4- concentration was also observed in the first 10 s in the absence of Agþ ions when the citrate concentration is 2.97 10-2 wt %. This to some degree narrowed the size distribution of the Au NPs so obtained; the size of the Au NPs obtained in the absence of Agþ ion was 12 ( 3 nm while it was 12 ( 1 nm in the presence of Agþ ions. Note that the traditional Turkevich method can sometimes show an initial sharp decrease of AuCl4- concentration at the citrate concentration of 2.97 10-2 wt %, leading to a narrower size distribution.4b Taken together, this highlights that 2.97 10-2 wt % is the best citrate concentration for growth of Au NPs by citrate reduction, less dependent on the experimental protocol. The elongated shape of Au NPs obtained by the Turkevich methods is usually characterized by nonzero extinction baseline at longer wavelength and the notable band asymmetry.3f In contrast, the Au NPs obtained by our method have a rather symmetric surface plasmon absorption band, and the absorption maximum shows a smaller red shift with the NP size, from 514 nm for 12 ( 1 nm NPs to 519 nm for larger than 18 ( 3 nm NPs (Figure 3a). In comparison, the surface plasmon absorption band of the Au NPs obtained via the Turkevich method red shifts with the particle size (Figure 3b). The surface plasmon absorption maximum (λmax) of Au NPs strongly depends on their aspect ratio (R):14 λmax ¼ 420 þ 95R Provided Au NPs keep the spherical shape, say, R = 1, their surface plasmon absorption band should be centered at 515 nm. When the NPs become elongated, the surface plasmon absorption band red shifts with R. This theoretical analysis is in a good (13) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (14) Huang, X.; Neretina, S.; El-Sayed, M. A. Adv. Mater. 2009, DOI: 10.1002/ adma.200802789.
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Figure 2. TEM images of Au NPs obtained by the present method (a-c) and the Turkevich method (d-f) at different citrate concentrations:
2.97 10-2 wt % (a, d), 1.01 10-2 wt % (b, e), and 6.10 10-3 wt % (c, f). The concentration of Agþ ions in the present method was 8.5 10-5 wt %. The concentration of HAuCl4 in both methods was 0.01 wt %.
Figure 3. (a) UV spectra of Au NPs obtained by the present approach at different concentrations of citrate: 2.97 10-2 wt % (black curve, 12 ( 1 nm), 1.40 10-2 wt % (red curve, 18 ( 3 nm), 6.10 10-3 wt % (green curve, 36 ( 3 nm), and 4.10 10-3 wt % (yellow curve, 48 ( 5 nm). The concentrations of Agþ ions and HAuCl4 were 8.5 10-5 and 0.01 wt %, respectively. (b) UV spectra of Au NPs obtained by the Turkevich approach at different concentrations of citrate: 2.97 10-2 wt % (black curve, 14 ( 4 nm), 1.40 10-2 wt % (red curve, 27 ( 9 nm), 1.01 10-2 wt %, (pink, 48 ( 18 nm), and 6.10 10-3 wt % (green curve, 62 ( 20 nm). The concentration of HAuCl4 was 0.01 wt %. 3588 DOI: 10.1021/la902987w
agreement with our experimental results, underlining the narrow size distribution and the uniform quasi-spherical shape of the Au NPs obtained by our approach. Fast formation of SADC catalyzed by Agþ ions can account for formation of the monodisperse size of Au NPs obtained in our work, but it cannot account for formation of the quasispherical shape. Since a gold NP is grown from several nuclei embedded in the SADC/AuCl complex particles rather than from one nucleus, the resulting NP is polycrystalline and its shape should be nonspherical.2,3 The comparison of Au NPs obtained with and without Agþ ions suggests that Agþ ions can reshape the irregular shape of polycrystalline Au NPs formed. Sanchez et al. have calculated that the underpotential deposition shift for Ag on the Au (111), (100), and (110) facet are 0.12, 0.17, and 0.28 eV.15 This suggests that Ag atoms, when obtained by citrate reduction of Agþ ions, prefer to deposit on the (110) and (100) facets on polycrystalline Au NPs.1b,16 Although the Ag layer can be oxidized and replaced by the Au ions, this preferential deposition of Ag may significantly slow down the growth rate of Au NPs on the (110) and (100) facets, thus rendering the shape of Au NPs quasi-spherical. Since citrate is a weaker stabilizing ligand, it is dynamically capped on Au NPs at a higher temperature. This mobile citrate capping should enable to deposition and decomposition of Ag on Au NPs, thus guaranteeing reshaping of polycrystalline Au NPs to quasi-spherical. We conducted the synthesis of Au NPs at room temperature to lower the mobility of the citrate capping, and the resulting Au NPs showed an oval shape after 11 h in the presence of Agþ ions (Figure S3a). In the absence of Agþ ions, Au NPs with much more elongated shape were obtained after 2 days (Figure S3b). This suggests the important role of Agþ ion to accelerate the formation of Au NPs and keep the NP shape round. (15) (a) Sanchez, C. G.; Del Popolo, M. G.; Leiva, E. P. M. Surf. Sci. 1999, 421, 59. (b) Rojas, M. I.; Sanchez, C. G.; Del Popolo, M. G.; Leiva, E. P. M. Surf. Sci. 2000, 453, 225. (16) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 22192.
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Figure 4. TEM images of Au NPs obtained by the present method at different concentrations of Agþ: 8.5 10-5 (a), 4.0 10-5 (b), and 2.0
10-5 wt % (c). The concentrations of HAuCl4 and citrate were 0.01 and 1.40 10-2 wt %, respectively. The insets are the histogram of the size distributions of the resulting Au NPs, analyzed by DLS.
Forming macromolecular SADC/AuCl complexes necessitates a minimum of 2 SADC to tether 3 AuCl (Auþ) (Scheme 1), corresponding to the citrate to HAuCl4 molar ratio of 0.67. When the citrate concentration used was 4.10 10-3 wt % in the current work, the molar ratio of citrate to HAuCl4 was 0.53. Therefore, the amount of citrate was not sufficient to reduce HAuCl4. Most of the AuCl4- ions should be consumed by acetone derived from decomposition of SADC, leading to a slower consumption of AuCl4- ions in the late course of the reaction (Figure 1). Formation of acetone led to inevitable secondary nucleation to broaden the size distribution. As shown in Figure S4, micrometer-sized Au precipitates with irregular shapes were obtained in the absence of Agþ ions. In contrast, the presence of Ag ions (8.5 10-5 wt %) led to formation of colloidally stable Au NPs of 48 ( 5 nm in size, although their shape was much more elongated than that of the NPs obtained at the citrate concentration higher than 6.10 10-3 wt %. This underlines the important role of Agþ ions to narrow the size and shape distribution of Au NPs in the present method. As compared with those obtained at higher citrate concentration, the Au NPs obtained at the citrate concentration of 4.10 10-3 wt % show asymmetric surface plasmon absorption band with the maximum absorption at 535 nm due to the elongated shape and the particle size increase (Figure 3a). In order to understand the crucial role of Agþ ions, we varied the amount of AgNO3 aqueous solution in the HAuCl4/citrate mixture solution from 42.5 μL to 20 and 10 μL, the corresponding final concentration of Ag ions in the reaction media from 8.5 10-5 wt % to 4.0 10-5 and to 2.0 10-5 wt %. Figure 4 shows that when the concentration of the Agþ ions decreases, Au NPs become less uniform in size and triangular NPs are clearly observed at the Agþ ion concentration of 2.0 10-5 wt %. DLS analysis reveals a clear bimodal size distribution of Au NPs obtained when the Agþ ion concentration dramatically decreases (Figure 4). Formation of smaller Au NPs underlines the secondary nucleation induced by acetone derived from SADC decomposition in the process of citrate reduction of HAuCl4 to Au NPs at the lower concentration of Agþ ions.
Conclusion In summary, we have successfully developed a new and simple strategy to reproducibly synthesize monodisperse quasi-spherical Au NPs in water via citrate reduction of HAuCl4. As compared with the commonly used Turkevich method, two distinct
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characters of our approach are (1) aqueous solutions of citrate and HAuCl4 are well mixed at room temperature and then rapidly added to boiling water and (2) the citrate/HAuCl4 mixture contains a trace amount of Agþ ions. Because of these two modifications, our methodology can (1) efficiently minimize the effect of citrate to buffer the pH of the reaction media and thus change the type and reactive activity of auric ions to guarantee a dominant presence of highly active AuCl3(OH)- ions, (2) significantly speed up the nucleation and growth rate of Au NPs due to Agþ ion catalyzed oxidation of citrate to SADC to largely narrow the NP size distribution, and (3) reshape the polycrystalline Au NPs into a quasi-spherical shape with the aid of Agþ ions due to their Au facet selective deposition. In the case of synthesis of Au NPs of sizes ranging from 10 to 40 nm, our approach efficiently makes up the shortages of the classical Turkevich method with respect to the reproducibility and uniformity of the NP size and shape. Our preliminary results indicate that citrate reduction of HAuCl4 may allow synthesis of monodisperse 5 nm Au NPs based on nucleation induced by HAuCl4 hydrolysis at high pH. Our ensuing tasks are to study the type of active metallic ions, mainly Au and Ag, and their hydrolysis kinetics at different pH, the nucleation mechanism including possible side reactions and secondary nucleation, and the crystal growth mechanism including the difference of the growth rate between different facets to synthesize metallic NPs with defined but varied sizes and shapes in water. Acknowledgment. Dr. Xia thanks the Alexander von Humboldt foundation for a research fellowship. We thank Prof. H. M€ohwald for helpful discussions. This work is financially supported by the Max Planck Society and Deutsche Forschungsgemeinschaft (WA 1704/4-1, MO283/38-1, and Cluster of Excellence “Unifying Concepts in Catalysis (EXC 314) coordinated by the Technical University of Berlin) and an EU-FP6 grant (BONSAI, LSHB-CT-2006-037639). Supporting Information Available: Details of measurement of the concentration of AuCl4- ions; TEM images of Au NPs synthesized by the Turkevich method and the present one in the absence of AgNO3 and by the present method at room temperature in the presence and absence of AgNO3; TEM images of Au NPs synthesized by the Turkevich method and the present one in the presence of AgNO3 at the citrate concentration of 4.1 10-3 wt %. This material is available free of charge via the Internet at http://pubs.acs.org.
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