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Aggregative Growth in the Size-Controlled Growth of Monodispersed Gold Nanoparticles Peter N. Njoki, Jin Luo, Martha M. Kamundi, Stephanie Lim, and Chuan-Jian Zhong* Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902 Received May 12, 2010. Revised Manuscript Received June 24, 2010 This article describes the findings of an investigation of the aggregative growth mechanism for the formation of gold nanoparticles in aqueous solutions under ambient conditions with high monodispersity (2% RSD) over a wide range of particle sizes (10-100 nm). The utilization of the gold nanoparticles synthesized by this simple, reproducible growth mechanism has recently been demonstrated for establishing the size correlation for the surface plasmon resonance optical and surface-enhanced Raman scattering spectroscopic properties. The particle size, morphology, and optical properties of the nanoparticles produced at different stages of the growth processes were determined as a function of control parameters such as the reaction time and seed/precursor concentrations. The results have allowed us to establish a quantitative correlation between the growth size and the seed/precursor concentrations for the precise control of nanoparticle sizes. The kinetic measurements have demonstrated a polycrystalline character for the grown particles, a bimodal size distribution in the early stage of growth, sigmoidal kinetic behavior for the growth, and a correlation of the nucleation parameters with the particle size and distribution. These findings provided important indicators for the operation of an aggregative growth mechanism in the particle size growth and have important implications in understanding interparticle aggregation and coalescence in nanoparticle formation and growth under similar conditions.
Introduction Our recent study has shown that gold nanoparticles synthesized by seeded growth in aqueous solutions under ambient conditions exhibited high monodispersity for establishing the size correlation of the surface plasmon resonance optical and surface-enhanced Raman scattering spectroscopic properties.1 This simple, reproducible method can produce gold nanoparticles of almost any desired sizes in the range of 10-100 nm diameter with a size monodispersity of up to 2% relative standard deviation.1 This correlation is important for exploring their chemical, bioanalytical, biomedical, optical, and nanotechnological applications where the electronic, optical, and chemical/biological properties are very dependent on the size, shape, and size monodispersity.1,2 Although there are numerous methods known for the synthesis of gold nanoparticles,3 an understanding of the detailed mechanism in the control of the size, shape, and monodispersity in a wide size range is often lacking. A number of synthesis methods have been *To whom correspondence should be addressed. E-mail: cjzhong@ binghamton.edu. (1) (a) Njoki, P. N.; Lim, I-I. S.; Mott, D.; Park, H.-Y.; Khan, B.; Mishra, S.; Sujakumar, R.; Luo, J.; Zhong, C. J. J. Phys. Chem. C 2007, 111, 14664. (b) Zhong, C. J.; Njoki, P. N.; Luo, J. U.S. Patent 7,524,354, 2009. (2) (a) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974. (b) Chen, J.; Lim, B.; Lee, E. P.; Xia, Y. Nano Today. 2009, 4, 81. (3) (a) Jana, N. R.; Geatheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782. (b) Skrabalak, S. E.; Xia, Y. ACS Nano 2009, 3, 10. (c) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I. Langmuir 2003, 19, 4831. (d) Njoki, P. N.; Luo, J.; Wang, L.; Maye, M. M.; Quaizar, H.; Zhong, C. J. Langmuir 2005, 21, 1623. (4) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. J. Am. Chem. Soc. 2007, 129, 13939. (5) Sau, T. K.; Pal, A.; Jana, N. R.; Wang, Z. L.; Pal, T. J. Nano. Res. 2001, 3, 257. (6) Haiss, W.; Thanh, N. T. K.; Aveyard, J.; Fernig, D. G. Anal. Chem. 2007, 79, 4215. (7) (a) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203. (b) Goia, D. V.; Matijevic, E. Colloids Surf., A 1999, 146, 139. (8) (a) Hengelin, A. J. Phys. Chem. 1993, 97, 5457. (b) Jana, N. R.; Murphy, C. J. Chem. Mater. 2001, 13, 2313. (9) (a) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726. (b) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306.
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reported for the preparation of Au nanoparticles using seeds in aqueous solutions.3-11 For example, Au nanoparticles with size ranging from 5 to 40 nm were synthesized using Au seeds prepared by the borohydride reduction of Au salt in the presence of citrate as a capping agent, followed by growth in the presence of cetyltrimethylammonium bromide and ascorbic acid.3a,b Other examples involve the synthesis of Au nanoparticles of 10-20 nm size at elevated temperature using acrylate as both a reducing and a capping agent3c,d and the photochemical seeded growth of Au nanoparticles of 5-100 nm size in the presence of TX-100 (Triton) X-100 poly(oxyethylene)iso-octylphenyl ether and ascorbic acid.5 Seeding methods have often been used for growing larger Au nanoparticles (20 - 100 nm) in the presence of ascorbic acid or hydroxylamine.7-14 In seeded growth, the basic perception of the growth mechanism was that Au(III) was reduced on the surface of a smaller nanoparticle by citrate or hydroxylamine as the reducing agent at room temperature. This type of growth mechanism has been considered for the synthesis of Au nanorods (20-100 nm) in the presence of trace silver ions.10,11 In addition to considering the reduction of Au(III) on the surfaces of Au seeds by a reducing agent for the growth of larger particles in most previous reports, Ostwald ripening has often been considered to be another general driving force for the growth of particles. In Ostwald ripening, smaller particles dissolve preferentially with subsequent crystallization onto larger particles,15 which involves nucleation and growth processes for the growth of larger particles from smaller particles. However, little has been known with respect to the fundamental question of how the (10) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414. (11) Khanal, B. P.; Zubarev, E. R. J. Am. Chem. Soc. 2008, 130, 12634. (12) Bri~nas, R. P.; Hu, M.; Qian, L.; Lymar, E. S.; Hainfeld, J. F. J. Am. Chem. Soc. 2008, 130, 975. (13) Yu, K.; Kelly, K. L.; Sakai, N.; Tatsuma, T. Langmuir 2008, 24, 5849. (14) Perrault, S. D.; Chan, W. C. W. J. Am. Chem. Soc. 2009, 131, 17042. (15) Stella, A.; Cheyssac, P.; Kofman, R. In Science and Technology of Thin Films; Matacotta, F. C., Ottaviani, G., Eds.; World Scientific: River Edge, NJ, 1996; Vol. 57, p 57.
Published on Web 07/15/2010
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formation of larger nanoparticles correlates with the initial, intermediate, and final size distributions in terms of the growth parameters. For initial and final size distributions, our earlier and recent studies demonstrated that the interparticle aggregative coalescence was an important factor that was operative in the size growth of gold16 and copper nanoparticles17 under thermally activated growth processes in organic or molten solvents. In the latest work by Buhro and co-workers,18 a detailed experimental and theoretical study of the thermal coarsening kinetic process of thiolate-capped Au nanoparticles in the presence of tetraoctylammonium bromide led to a major new insight into the aggregative nucleation and growth mechanism. The aggregation parameters are shown to control the final particle size and size distribution, which are substantiated by the observation of polycrystalline particles, the observation of early-stage bimodal size distributions, the observation of sigmoidal growth kinetics, and the close correlation of the nucleation parameters with the particle sizes and distributions. This insight also ruled out the operation of Ostwald ripening in the early stages of particle growth. In this article, the results of a detailed investigation of the seeded growth for the formation of highly monodispersed gold nanoparticles in aqueous solutions at room temperature were described to assess the growth mechanism.1 Insights into the growth mechanism were gained by the determination of the size and optical properties as a function of growth parameters such as the reaction time and the seed/precursor concentrations, demonstrating the operation of the aggregation growth mechanism in this simple, reproducible growth process.
Experimental Section Chemicals. Hydrogen tetrachloroaurate (HAuCl4, 99%) and sodium acrylate (H2CdCHCO2Na, 97%) were purchased from Aldrich and used as received. Water was purified with a Millipore Milli-Q water system. Synthesis and Seeded Growth. Gold nanoparticle seeds (10-30 nm) were synthesized by reacting aqueous solutions of hydrogen tetrachloroaurate and sodium acrylate at room temperature.1a,3c For example, in the synthesis of 30 nm nanoparticle seeds, an aqueous solution of HAuCl4 (2.0 10-4 M) was mixed with sodium acrylate (12.0 10-3 M) and the mixture was stirred for 3 days at controlled room temperature. The resulting solution displayed a deep-red color characteristic of the formation of gold nanoparticles. Gold nanoparticles with diameters larger than the seeds were prepared by seeded growth via the reduction of AuCl4- in the presence of presynthesized Au seeds.1 The basic element of the synthesis protocols involved seed formation and seeded growth using a combination of reducing and capping agents, including sodium citrates, sodium acrylates, and acrylic acids. The actual sizes of the resulting nanoparticles depend on the seed sizes and the precursor concentrations, a detailed discussion of which is reported below. Briefly, seeds of Au nanoparticles of between 20 and 30 nm diameter were first prepared. The seeds then underwent a seeded growth reaction in the presence of HAuCl4 under a range of controlled concentrations of the reducing and capping agents to form large Au nanoparticles (up to ∼60 nm). To synthesize larger Au nanoparticles (up to ∼100 nm), larger seeds (e.g., 60 nm seeds grown from seeded growth) were used in (16) (a) Zhong, C. J.; Zhang, W. X.; Leibowitz, F. L.; Eichelberger, H. H. Chem. Commun. 1999, 1211. (b) Maye, M. M.; Zheng, W. X.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. Langmuir 2000, 16, 490 and references therein.(c) Maye, M. M.; Zhong, C. J. J. Mater. Chem. 2000, 10, 1895. (d) Schadt, M. J.; Cheung, W.; Luo, J.; Zhong, C. J. Chem. Mater. 2006, 18, 5147. (17) Mott, D.; Yin, J.; Engelhard, M. H.; Loukrakpam, R.; Chang, P.; Miller, G.; Bae, I.-T.; Das, N. C.; Wang, C.; Luo, J.; Zhong, C. J. Chem. Mater. 2010, 22, 261. (18) Shields, S. P.; Richards, V. N.; Buhro, W. E. Chem. Mater. 2010, 22, 3212.
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combination with controlled concentrations of HAuCl4 and H2CdCHCO2Na. The particle size was controlled by varying the concentration of the seeds and the concentration of AuCl4-. In addition, control of the pH of the reaction solution and the reaction temperature is essential to the control of seeded growth. In most cases, the resulting particles displayed high monodispersity in the targeted size range. In some cases, a small fraction of particles with smaller sizes were separated by centrifugation. Instrumentation and Measurements. The morphological, optical, and spectroscopic properties of gold nanoparticles of different sizes were examined using the following measurements. Ultraviolet-visible (UV-vis) spectra were acquired with an HP 8453 spectrophotometer. The spectra were collected over the range of 200-1100 nm. Other spectra were taken with a HP 8452 spectrophotometer in the range of 200-900 nm. Transmission electron microscopy (TEM) was performed at 100 kV from a Hitachi H-7000. The nanoparticle samples that were dissolved in water were drop cast onto a carbon-coated copper grid sample holder, followed by natural evaporation at room temperature.
Results and Discussion Size Correlation with Precursor Concentration. Figure 1 shows a typical set of TEM micrographs and size distribution for samples of the nanoparticle seeds and the seed-grown particles. The two examples of the seed-grown gold nanoparticles, with average sizes of 60 and 90 nm, were obtained by using 30 and 60 nm particles as seeds, respectively. In addition to the size controllability, a key morphological feature of the seed-grown particles is the high monodispersity (Figure 1d). The high monodispersity is evidenced by the very small relative standard deviation. (For example, the relative standard deviation for 80 nm particles with a standard deviation of 2.1 nm is 2.6%.) It is an experimental fact that the large particles are not perfectly spherical in shape. For the purpose of size counting, we assumed a spherical shape and used the particle area in the TEM image to determine the size. Although the overall morphology of these nanoparticles appears to be spherical, faceted edges along the outlines of many nanoparticles are clearly observable, which is consistent with the polycrystalline properties of the gold nanoparticles. The surface plasmon (SP) resonance band appeared at 524 nm for 30 nm seed nanoparticles, whereas the wavelength of the SP bands was shown to increase with particle size.1a This dependence was shown by the increase in the wavelength (λmax) at the maximum absorbance of the SP band from 519 to 569 nm in the size range of 10-100 nm. In general, the increase in the concentration of the reducing agent, when the concentration was very low (3 mM and constant during the seeded growth process, the particle size growth was found be very controllable and to display a clear dependence on the concentration of AuCl4-. Figure 2 shows a representative set of data for the relationship between the growth thickness of the particles and the concentration of AuCl4- for size growth as a function of AuCl4concentration from two different seed sizes: (a) 32 and (b) 61 nm. A gradual increase in thickness is evident for the seeded growth of both the 30 nm and 60 nm seeds. This size-concentration relationship was further assessed by considering a theoretical model based on an initial assumption of 100% conversion efficiency for seeded growth. On the basis of a spherical model for an Au nanoparticle with a radius r (cm) and DOI: 10.1021/la1019058
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Figure 1. (a-c) TEM micrographs for 30 nm seeds and seed-grown 60 and 90 nm nanoparticles. (d) Size distribution for particles from 30 to 90 nm. The top panel illustrates the size growth corresponding to the particles shown in the TEM images.
Figure 2. Size growth (expressed as growth thickness, d ) as a function of AuCl4- concentration for the seeded growth from seeds with a diameter of 2r: (a) 32 and (b) 61 nm.
a growth thickness d (cm), the mass balance between the seedgrown particles and the concentration of AuCl4- (CmM), 4π 4π N ðr þ dÞ3 - r3 F ¼ C V M ð1Þ 3 3 yields the following d-r correlation d ¼ r
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CV 3 1 þ 2:49 -1 N r3
ð2Þ
where N is the number of Au nanoparticles in the total volume, F is density of Au (18.9 g/cm3), M is the molecular weight of Au (197 g/mol), C is the concentration of Au precursor (mol/cm3), 13624 DOI: 10.1021/la1019058
and V is the total reaction volume. Note that this equation is consistent with those reported previously,5,19 where the final diameter of the particles is related to the diameter of seeds and the concentration of Au used. In Figure 3, the calculated and the theoretical fitting results are compared with the experimental data. Although values for the reaction volume (Vexpt), the particle radius (rexpt), and the number of Au nanoparticles added as seeds (Ninitial) can be experimentally determined, the actual (or effective) number of nanoparticle seeds that have participated in seeded growth (Neff) may not be 100% of Ninitial. Considering this factor for the evaluation of the efficiency of seeded growth based on values of Vexpt, rexpt, and Ninitial, a coefficient R, Vexpt ð3Þ R ¼ 2:49 Neff rexpt 3 was introduced. For example, for growing 32-61 nm particles using 32.8 nm seeds (1.1 1011 NPs/mL), the addition of 25 mL of seeds to a 125 mL synthesis solution (i.e., 2.8 1012 NPs) would theoretically yield R = 23.3, which corresponds to the solid line in Figure 3A(a). By fitting the experimental data in the full concentration range and in the concentration range of
