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Size-Controlled Synthesis of Gold Nanoparticles via High-Temperature Reduction David A. Fleming and Mary Elizabeth Williams* Department of Chemistry, 152 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 Received December 4, 2003. In Final Form: February 26, 2004 Size and dispersity control of metallic nanoparticles is of fundamental necessity for their useful chemical implementation. A single-step synthesis of Au nanoparticles 4-15 nm in diameter and with low polydispersity is presented. Amine-induced reduction of a Au(I) precursor in the presence of coordinating ligands leads to nucleation of small particles which ripen to form larger particles. The rate of particle growth and subsequent aggregation is strongly dependent on the relative concentrations of protecting ligands and is monitored by both transmission electron microscopy and UV-visible absorption spectroscopy.
Introduction The development of methods capable of producing monodisperse nanoparticles of controlled size and shape is essential for many potential applications in chemistry1,2 and biology3 to be realized. Particles of the sub ∼100 nm size regime exhibit strong size-dependent optical,4 electronic,5 and magnetic6 properties, and as a result, controlling the particle size allows control over physical properties. This is seen most elegantly in fluorescent semiconducting nanocrystals composed of, for example, CdS or CdSe, where increasing particle size causes the emission wavelength to red-shift significantly.7 Several synthetic schemes have been developed to exert control over nanoparticle growth, including micellar confinement,8 reduction in the presence of stabilizing ligands,9 and digestive ripening.10 While effective for producing mono* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Haes, A.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 1059610604. (2) (a) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340-8347. (b) Roucox, A.: Sculz, J.; Patin, H. Chem. Rev. 2002, 102, 3757. (c) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir 2002, 18, 49214925. (3) Levy, L.; Sahoo, Y.; Kim, K.-S.; Bergey, E. J.; Prasad, P. N. Chem. Mater. 2002, 14, 3715-3721. (4) (a) Vollmer, M.; Kreibig, U. Optical Properties of Metal Clusters; Springer Series in Materials Science; Springer: Berlin, 1995. (b) Doremus, R. J. Chem. Phys. 1964, 40, 2389. (c) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564570. (5) (a) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. Science 1998, 280, 2098-2101. (b) Chen, S.; Murray, R. W.; Feldberg, S. W. J. Phys. Chem. B 1998, 102, 9898-9907. (c) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (6) (a) Gilbert, I.; Milla´n, A.; Palacio, F.; Falqui, A.; Snoeck, E.; Serin, V. Polyhedron 2003, 22, 2457-2461. (b) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin, O ¨ .; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125, 9090-9101. (7) (a) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475. (b) de Mello Donega, C.; Hickey, S. G.; Wuister, S. F.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2003, 107, 489-496. (8) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639-646. (9) (a) Murray, C. B.; Sun, S.; Doyle, H.; Betley, B. MRS Bull. 2001, 26, 985-991. (b) O’Brien, S.; Brus, L.; Murray, C. B. J. Am. Chem. Soc. 2001, 123, 12085-12086. (c) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (10) (a) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935-942. (b) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719-2724.
disperse samples of various sizes, all of these methods rely on systematic variation of starting conditions, so that separate preparation of each particle size is necessary. A one-pot synthesis capable of producing monodisperse particles of different sizes simply through the removal of fractions at different times is thus highly desirable. We report a single-step synthesis which allows the production of nanoparticles that evolve in size over time depending on specific capping ligand concentrations. A number of single-step methods have been previously developed which allow fraction removal leading to samples with different sizes. For example, nanoparticle seeding utilizes smaller, pre-prepared nanoparticles as nucleation centers for the formation of larger or core/shell structures and has been utilized for the formation of Au nanoparticles,11 Au/Ag12 nanoparticles, core/shell nanoparticles of composition Au/X (where X ) Bi, Sn, or In13), and magnetic nanoparticles.14 However, in each of these examples, seed formation prior to growth is essential, and as a result, an additional synthetic step is required. Alternatively, time-dependent particle growth has been observed during the synthesis of semiconducting nanoparticles in trioctylphosphine oxide (TOPO) and hexadecylamine (HDA),7,15 although metallic nanoparticle formation utilizing the same method has been largely unexplored. As a result, we have adapted semiconducting nanocrystal growth strategies to the formation of Au nanoparticles by using high temperatures and coordinating solvents to reduce a Au(I) complex. Results and Discussion Our strategy involves the reduction of Au(acac)PPh316 at elevated temperatures in solutions of varied TOPO and HDA concentrations (see the Supporting Information). In (11) (a) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726-728. (b) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306313. (c) Brown, K. R.; Lyon, L. A.; Audrey, R.; Reiss, B. D.; Natan, M. J. Chem. Mater. 2000, 12, 314-323. (d) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313-2322. (12) (a) Kim, Y.; Johnson, R. C.; Li, J.; Hupp, J. T.; Schatz, G. C. Chem. Phys. Lett. 2002, 352, 421-428. (b) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718-724. (c) Rivas, L.; Snachez-Cortes, S.; Garcı´a-Ramos, J. V.; Morcillo, G. Langmuir 2000, 16, 9722-9728. (13) ) Yu, H.; Gibbons, P. C.; Kelton, K. F.; Buhro, W. E. J. Am. Chem. Soc. 2001, 123, 9198-9199. (14) Sun, S.; Zheng, H. J. Am. Chem. Soc. 2002, 124, 8204-8205. (15) Reiss, P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2, 781-784. (b) Talpin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207-211. (16) Vincent, J.; Chicote, M.-T. Inorg. Synth. 1998, 32, 172-176.
10.1021/la0362829 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/18/2004
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Figure 1. UV-vis spectra of Au nanoparticles grown in a 50: 50 mixture of TOPO and HDA recorded at 20, 28, 38, 49, 65, and 83 min after the injection of Au(acac)PPh3. The Au surface plasmon peak steadily increases over the course of the reaction before precipitation occurs, while a significant red shift and absorbance decrease are also observed.
a typical synthesis, 0.050 g (8.95 × 10-5mol) of Au(acac)PPh3 was dissolved in 2 mL of diphenyl ether and injected into a mixture of TOPO and HDA held at 105 °C. The TOPO/HDA solution had previously been degassed at 105 °C under a vacuum and backfilled with Ar a minimum of three times. A gradual color change from pale yellow to pink to red to purple occurred, ending in particle precipitation and the formation of a metallic solid. The time frame in which this color change took place was dependent on the ratio of TOPO to HDA, with larger HDA concentrations leading to a faster color change due to increased particle formation and growth. Fractions (1 mL) were removed throughout the course of the reaction and injected into 19 mL of toluene to arrest particle growth, after which UV-vis analysis was performed. Particle purification was accomplished through centrifugation in a 50/50 mixture of toluene/methanol. Purified particles, which remained stable in solution for a period of many months with no observable change in size or dispersity, were redissolved in toluene and cast on transmission electron microscopy (TEM) grids for size analysis. Particle growth was monitored using UV-visible absorption spectroscopy, performed on a Cary Varian 500 spectrophotometer, by tracking the formation and gradual growth of the well-known surface plasmon peak11,17 associated with nanometer-sized Au particles. The UVvisible spectra, shown in Figure 1 for Au nanoparticles grown in a reaction mixture of 50% TOPO and 50% HDA, show that the plasmon peak gradually grows in and red shifts (from 516 to 547 nm) over the course of 83 min. The increase in absorbance intensity is indicative of Au nanoparticle growth, while the observed red shift is a result of aggregation that occurs as the particles become too large to be stabilized by the TOPO and HDA ligands. As aggregation begins, particle precipitation occurs, with a corresponding decrease and broadening of the plasmon absorbance (see the Supporting Information). While growth and subsequent aggregation are observed in each of the trials of varied TOPO/HDA ligand ratio, the rate of plasmon peak growth (i.e., the rate of nanoparticle growth) is controlled by the relative ratios of the ligands. To quantitatively determine the size of the particles produced as a function of time in each of these trials, transmission electron micrographs were obtained on a JEOL JEM 1200 EXII transmission electron microscope (17) Alvarez, M. M.; Khoury, J. T.; Schaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706-3712.
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Figure 2. Representative TEM image of Au nanoparticles removed from a 50:50 mixture of TOPO and HDA after 38 min. Particle size was calculated to be 10.2 ( 1.4 nm. The inset shows a histogram calculated from 465 particles measured from different areas on the TEM grid.
Figure 3. Evolution of particle size over time for samples containing 0% (b), 25% (9), 50% (2), and 75% ([) TOPO. Size increases until about 15 nm (closed symbols), at which point particles become destabilized and aggregation occurs (open symbols). At this point, particle dispersities also dramatically increase.
fitted with a Gatan Bioscan 792 camera and operated at 100 kV for each collected fraction. A representative TEM image of nanoparticles removed from a 50:50 mixture of TOPO and HDA 38 min after Au(acac)PPh3 injection is shown in Figure 2. The results of particle analysis for this sample (from 465 particles observed on different portions of the TEM grid) are displayed in the figure inset and show that the size distribution is Gaussian. Based on this statistical analysis, the as-prepared particles in Figure 2 have average diameters of 10.2 ( 1.4 nm (14% polydispersity). The time-dependent growth of particle size in TOPO/HDA solutions containing 0%, 25%, 50%, and 75% TOPO was monitored in sequential TEM images. Figure 3 plots the changes in particle size and polydispersity versus time for each ligand concentration and shows that particles initially grow to a diameter of ∼15 nm (closed symbols) with dispersities that decrease over time. For example, based on particle analysis of the TEM images, samples collected at short time intervals were determined to be 35-40% polydisperse, whereas samples obtained at later intervals (but before particle diameters exceeded 15 nm) were 13-15% polydisperse. In the trial containing 75% HDA/25% TOPO, the initially large size dispersity is mainly due to the presence of particles of two different
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diameters, producing a quasi-bimodal distribution in samples removed shortly after Au injection (see the Supporting Information for histograms). Although similar bimodal distributions are not observed for the other trials, this may be a result of the sampling rate at short time scales. The narrowing of size distributions (e.g., size “focusing”) during nanoparticle growth has been previously discussed and is thought to result from differences in particle growth rate, where smaller particles grow more rapidly than larger ones, narrowing the size distribution over time.18 In each of the trials shown graphically in Figure 3, at long time intervals the particles reach a diameter of ∼15 nm, after which extensive aggregation is evident in the TEM images. Aggregation leads to the formation of structures with diameters of ∼50 nm and with greater size deviation (open symbols in Figure 3). These structures are also less soluble in solution, precipitating shortly after formation. These observations are consistent with the redshifted and dampened plasmon bands seen in the UVvisible spectra for fractions collected at later time intervals (Supporting Information). It is widely understood that the nature of the ligands (size, charge, concentration, etc.) ultimately limits the maximum size of the particle that may be stabilized against aggregation.19 The observation that the Au nanoparticles consistently begin to aggregate at a diameter of 15 nm suggests that ligands stabilizing their surfaces are the same for each trial. Figure 3 also shows that the relative concentration of the TOPO and HDA ligands dramatically affects the rate of particle growth; as the concentration of HDA increases, the growth rate also increases. This is in contrast with the observed growth dynamics of semiconducting nanocrystals synthesized in TOPO-HDA mixtures, where increases in HDA concentration lead to decreased particle growth rates.15b However, in the case of Au nanoparticle formation, HDA plays the dual roles of both stabilizing ligand and reducing agent, giving rise to the concentration dependency observed in Figure 3. This is experimentally determined through a comparison of trials conducted in 100% HDA and 0% HDA (trial not shown). As seen in Figure 3, injection of the Au complex into pure HDA leads (18) Peng, X.; Wickman, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344. (19) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763-3772.
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to rapid nanoparticle formation and growth, with aggregation occurring after ∼20 min. In contrast, injection of the Au complex into pure TOPO failed to produce any observable particle growth over the course of 3 h. Aggregation and precipitation arise as a result of the ligands’ inability to adequately stabilize the particles above the 15 nm critical size limit regardless of the relative concentrations of HDA and TOPO. However, the rates of particle growth are strongly dependent on the ligand concentrations, and systematic study of this phenomenon is ongoing. Initial analysis indicates that particle growth cannot be fit to simple first-order kinetics. Instead, a mechanism involving particle growth as well as coalescence is proposed to account for observed behavior. Au(acac)PPh3 injection leads to nucleation of small particles, which eventually ripen10 to form larger particles. Particle growth succeeds in narrowing the measured size dispersities; bimodal size distributions evident at short times diminish and eventually disappear as smaller particles are incorporated into larger ones. Further support is evident in TEM images of aggregated samples, which exhibit a certain percentage of small, isolated particles not yet incorporated into the larger Au structures. Conclusion In conclusion, we report a single-step procedure for the synthesis of Au nanoparticles of variable sizes through high-temperature reduction in coordinating solvents. Variation of the relative concentrations of the coordinating HDA and TOPO ligands serves as a method for control of the nanoparticle growth rate. When the as-prepared particles have diameters less than 15 nm, they are relatively size monodisperse and are stable in organic solvents over a period of months. Acknowledgment. This work was supported in part by grants from the American Chemical Society Petroleum Research Fund and a National Science Foundation CAREER Award. Supporting Information Available: Complete experimental details, UV-vis spectra, particle TEM images, and complete dispersity data. This material is available free of charge via the Internet at http://pubs.acs.org. LA0362829
