J. Phys. Chem. C 2007, 111, 2533-2538
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Investigation of the Growth Process of Gold Nanoplates Formed by Thermal Aqueous Solution Approach and the Synthesis of Ultra-Small Gold Nanoplates Wan-Ling Huang, Chiu-Hua Chen, and Michael H. Huang* Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan ReceiVed: NoVember 2, 2006; In Final Form: December 1, 2006
The growth process of gold nanoplates formed by a thermal aqueous solution approach was investigated by withdrawing drops of a heated solution containing HAuCl4, cetyltrimethylammonium bromide (CTAB), and trisodium citrate and examined the intermediate products formed by a transmission electron microscopy analysis. The formation process proceeds extremely rapidly within seconds of reaction to form large aggregated structures, which are composed of wormlike structures just several nanometers in diameter. These tiny nanostructures are fused extensively toward the central region to form a thicker mass. The central core grows in size via incorporating the exterior wormlike nanostructures and eventually evolves into the plate morphology. Appropriate amounts of CTAB and trisodium citrate were also found to be critical to the formation of nanoplates. This synthesis approach has been extended to prepare ultrasmall triangular gold nanoplates with average widths of 40 ( 7 and 58 ( 10 nm. Because of their relatively uniform sizes, these tiny nanoplates can spontaneously self-assemble into some ordered 2-dimensional structures such as the hexagonally arranged pattern. UV-vis absorption spectroscopy showed that these nanoplates exhibit a strong absorption band at 590-602 nm and a weak and broadband centered at ∼775-900 nm.
Introduction Shape-controlled synthesis of gold nanostructures has recently received considerable attention because of the fundamental interest in generating anisotropic nanostructures for the examination of their optical and structural properties and the opportunities of using these novel nanostructures for a variety of applications. Among the various morphologies of gold nanostructures that have been reported, there has been a substantial increase in the number of studies on the synthesis of gold nanoplates in the recent years.1-18 Several different solution synthesis methods have been employed to prepare gold nanoplates and microplates, including biomolecule reduction of HAuCl4,1-3 seed-mediated synthesis at room temperature,4 polymer-assisted synthesis,5-13 microwave heating,13,14 and the non-polymer-directed thermal aqueous solution approach.15,16 From these studies, several possible growth mechanisms of gold nanoplates have been proposed. Some key factors such as the reagent concentrations and the reaction temperature and time necessary to enhance the nanoplate formation have been found. The most widely proposed growth mechanism generally involves with the confined growth of gold nanoparticles into a platelike morphology through surfactant capping or polymer adsorption onto the {111} faces of the developing nanoplates.8,13,19,20 The possible effects of reaction temperature, kinetics, and byproduct formation are less considered.4a,11 In another study on the use of lemongrass extract for the synthesis of triangular gold nanoprisms, it was revealed that spherical nanoparticles were formed initially, followed by an aggregation of these small particles into a quasitriangular shape and the subsequent formation of crystalline nanoplates.1a The presence of any effective structure-directing agent or driving force to guide the assembly of spherical gold nanoparticles is not clear. Another * To whom correspondence should be addressed. E-mail: hyhuang@ mx.nthu.edu.tw.
interesting issue regarding the growth mechanism of metal nanoplates is the shape control. From the observation of the bimodal growth of silver nanoprisms, Mirkin et al. have proposed light-induced fusion growth of silver nanoprisms to form larger triangular nanoplates through the assembly of existing smaller triangular nanoplates of a particular size.21 This mechanism, however, may not account for the formation of truncated triangular nanoplates and explain why few hexagonal nanoplates were produced. Despite these growth mechanisms proposed in the literature, few studies have actually examined the growth process of gold nanoplates at high solution temperatures in aqueous solution. The notion of nanoplate formation via surface capping of nanoparticles with surfactant molecules forming lamellar micellar structure even at high temperatures is doubtful, as micelles can become less stable at such temperatures (e.g., at or near the boiling temperature of water). In this study, we used a previously reported thermal aqueous solution approach to prepare gold nanoplates with widths from several hundreds of nanometers to a few micrometers.15 Since the reaction time was sufficiently long (that is, 40 min), a quick withdrawal of a few drops of the solution at different points during the reaction enables the observation of the intermediate products and offers a convenient way to examine the growth process of these nanoplates. Here interesting and surprising intermediate products are shown, and a somewhat different growth mechanism is given. The results also offer insights into the previously speculated possible formation of hexagonal nanoplates derived from the shape transformation of triangular and truncated triangular nanoplates. Another interesting aspect of this work is the preparation of ultrasmall gold nanoplates. Most of the literature on the synthesis of gold nanoplates obtained products that are hundreds of nanometers to tens of micrometers in width. Relatively few studies have successfully made nanoplates with widths of less
10.1021/jp0672454 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
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than 100 nm as the main product (usually in the 70-100 nm range).13,17 Papers reporting the formation of gold nanoplates with widths below 70 nm are rare.17 Our thermal aqueous solution approach was also able to make gold nanoplates with an average width of 85 ( 28 nm.15 In this study, we have extended the size range of this approach to synthesize ultrasmall triangular gold nanoplates with widths of 30-50 and 40-70 nm. The interest of this part of research is not simply the ultrasmall sizes of these nanoplates. With triangular nanoplates of this size range, their width distribution is narrower, and the chance of observing ordered 2-dimensional self-assembled structures from these nanoplates is substantially increased. These self-assembled structures are also presented, along with the UVvis absorption spectra of the ultrasmall nanoplates. Experimental Section Sodium citrate dihydrate (Mallinckrodt, trisodium salt), hydrogen tetrachloroaurate(III) trihydrate (Aldrich), and cetyltrimethylammonium bromide (CTAB, Aldrich) were used as received. Ultrapure deionized water (18.3 MΩ cm-1) was used for all solution preparations. For the investigation of the growth process of micrometersized gold plates and the preparation of ultrasmall gold nanoplates, our previously reported thermal aqueous solution synthesis approach was adopted with some modifications.15 First, 14.5 mL of deionized water was heated with stirring in a double-neck flask to 85 °C over a heating mantle. Then 0.5 mL of 2.5 × 10-2 M trisodium citrate solution was added. One minute later, another 10 mL solution containing 1.25 × 10-3 M HAuCl4 and 7.50 × 10-3 M CTAB and preheated to 50 °C was added to the trisodium citrate solution. To prevent the loss of water, the solution was refluxed by connecting one neck of the flask to a condenser, and another neck was sealed. Preheating of the two solutions before mixing reduces the reaction time and can give better control of the distribution of plate sizes. The resulting solution turned from orange to light yellow and then to colorless in 5 min and reflects the gradual formation of large gold structures. The reaction was allowed to proceed for a total of 40 min before removing the heating mantle from the flask. The final products were sandlike golden crystals suspended in the solution. To observe the intermediate products formed, a few drops of the heated solution were withdrawn with a pipet, transferred to a 200-mesh copper grid placed over a filter paper, and vacuum dried for the transmission electron microscopy (TEM) analysis. With the fast removal of heat in this tiny volume of solution, the growing gold nanocrystals in the solution should be captured. Snapshots of the nanoplate growth process can then be obtained. To synthesize ultrasmall gold nanoplates with widths of 4070 nm, 15 mL of 1.70 × 10-3 M trisodium citrate solution was first heated to 100 °C with stirring in a double-neck flask over a heating mantle. The solution was also refluxed to prevent the loss of water. Then 10 mL of an aqueous solution containing 1.25 × 10-3 M HAuCl4 and 7.50 × 10-3 M CTAB heated to 50 °C was injected into the hot trisodium citrate solution. The orange solution color turned colorless and then blue. After 10 min, the flask was separated from the heating mantle and cooled. To collect the nanoplate product, the top solution was slowly withdrawn with a pipet and centrifuged at 3500 rpm for 10 min in 5 mL portions (Hermle Z323 centrifuge). To further reduce the sizes of the gold nanoplates synthesized with widths of 30-50 nm, 7.5 mL of 1.70 × 10-3 M trisodium citrate solution was similarly heated to 100 °C with stirring in a double-neck flask over a heating mantle and refluxed. Then
Figure 1. SEM image of the gold microplates obtained after 40 min of reaction at 85 °C. Insets show SEM images of two microplates with one having 6 long edges and 6 short edges (upper right), and the other one having a more rounded triangular shape. Scale bar ) 5 µm for both. These are minor microplate shapes observed.
5 mL of an aqueous solution containing 1.25 × 10-3 M HAuCl4 and 7.50 × 10-3 M CTAB heated to 50 °C was injected into the hot trisodium citrate solution. The orange solution turned colorless and then purplish blue. After 5 min, the flask was separated from the heating mantle and cooled. To collect the nanoplates, the top solution was slowly withdrawn with a pipet and centrifuged at 4000 rpm for 15 min in 5 mL portions. The modest changes in the synthetic procedure for the preparation of these slightly different-sized nanoplates were implemented to obtain optimized nanoplate products. TEM images were acquired using either a JEOL 2000FX or a JEOL JEM-2010 microscope with an operating voltage of 200 kV for both. Scanning electron microscopy (SEM) characterization was performed on a JEOL JSM-6330F electron microscope. The UV-vis absorption spectra were taken using a JASCO V-570 spectrophotometer. Results and Discussion This study used the CTAB-assisted thermal aqueous solution approach to prepare micrometer-sized gold plates. The initial purpose was to identify the existence and the mechanism by which triangular nanoplates transform into hexagonal nanoplates. Our previous study showed that smaller gold nanoplates were largely triangular in shape (that is, nanoplates with widths around and below 100 nm), but a mixture of triangular, truncated triangular, and hexagonal nanoplates were produced when their sizes were increased to several hundreds of nanometers or larger.15 Thus, small amounts of the reaction solution were removed to examine the intermediate products and the nanoplate growth process. On the basis of the general growth mechanism proposed in the literature, it was assumed that small gold seed particles are formed first, and gold ions continuously add to the seeds under the templating effect of surfactant micelles or polymer to grow into a platelike morphology. The nanoplates then grow in size with time as the reaction proceeds. With this growth mechanism, one would likely only observe structurally well-defined nanoplates growing in size if intermediate products were removed and examined. The TEM results obtained in this study indicate that very different intermediate products are actually formed. Figure 1 shows a SEM image of the final microplates synthesized after 40 min of reaction. Most of the microplates are several micrometers in width and have a
Growth Process of Gold Nanoplates
Figure 2. (a and b) TEM images of the products collected after just 5 s of reaction at 85 °C. (c) An enlarged image of the lower left portion of panel b revealing the structural details of the outer region of the developing nanoplate. (d and e) TEM image and the corresponding SAED pattern of an interesting radial or dendritic gold nanostructure obtained after 10 s of reaction.
hexagonal shape. Triangular and truncated triangular microplates were also formed. In addition, few microplates with more unusual structures were observed (see insets of Figure 1), possibly because gold microplates with a round morphology is more favored than ones with sharp tips. The thicknesses of these plates should be in the range of a few tens of nanometers.15 Large faceted nanoparticles (hundreds of nanometers to over one micron in diameter) were also formed, as the reaction time was quite long for their continuous and fast growth. To follow the nanoplate growth process from the beginning, drops of the heated reaction solution were removed within seconds after the HAuCl4/CTAB solution was added to the trisodium citrate solution. Surprisingly exotic intermediate products were observed even at this very early stage. Figure 2a shows a TEM image of gold nanostructures formed after just 5 s of reaction. It is interesting to note that many irregularly shaped gold nanoparticles of a few nanometers in diameter already form and aggregate into relatively large masses of nanostructures about 50-150 nm in size at this very early stage. Fusion of some of these nanoparticles is already evident, especially near the central regions of these nanostructures. The driving force to form such aggregated nanostructures is unclear but should be related to the condition of high solution temperature, which favors the rapid formation of gold nanoparticles and facilitates particle fusion, the small particle sizes, and the capping action of the surfactant micelles. Figure 2b gives another TEM image of an intermediate product observed after just 5 s of reaction. Even at this early stage of the nanoplate formation process, a highly dense and extensive network of fused wormlike nanostructures has already been formed and rapidly grown to a size of several hundred nanometers. The central region of this big exotic structure appears to be thicker than the outer regions. Figure 2c shows a magnified image of an outer region (lower left portion of Figure 2b). An extensive growth of wormlike nanostructures with diameters of a few nanometers is revealed. These continuously winding and twisting nanostructures are more discernible toward the outermost regions; the tortuous nanostructures become fused together toward the inner central region. The wormlike gold nanostructures may be formed under the capping action of elongated CTAB micellar structures, but quickly the force to fuse and reorganize the gold atoms becomes
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Figure 3. (a) TEM image of the intermediate product observed after 30 s of reaction. (b) TEM image of a representative gold nanostructure obtained after 1 min of reaction. (c) TEM image of a hexagonal nanoplate formed after 3 min of reaction. (d) TEM image of an intermediate product collected after 6 min of reaction.
much greater at such high solution temperature and with a fast reaction rate. Figure 2d gives a TEM image of an unusual gold nanostructure found after only 10 s of reaction. This rather large and well-developed dendritic structure, although not necessarily a representative intermediate product regularly observed, does share common structural characteristics of the nanostructures observed at longer reaction times. It has a central core and an overall flat appearance, and the external sheetlike structures appear to concentrate toward the central region. A selectedarea electron diffraction (SEAD) pattern of the entire nanostructure is provided in Figure 2e. The ring diffraction pattern matched with that of gold, suggesting that this intermediate product is a polycrystalline gold nanostructure. The exotic nanostructures shown in Figure 2 presumably are not formed as a result of the drying process of the TEM grid or under the electron beam irradiation, since drying would not cause particle fusion and structural changes of the samples during the TEM analysis were not observed. The extensive formation of this dendritic or radial structure continues to be observed in samples collected after 30 s of reaction, and is displayed in Figure 3a. The darker central region appears to contain a dense assembly of fused gold nanoparticles and forms a large mass of several hundreds of nanometers in diameter. The outermost extended fine structures would disappear upon a prolonged electron beam irradiation. This observation suggests that these outermost fine structures are part of the residual feature left by the CTAB micelles upon solvent evaporation from the substrate. Figure 3b presents a TEM image of the product extracted after 1 min of reaction. Similar dendritic nanostructures were still observed to surround a dense and thick core. However, a close examination of these outer structures revealed that the aggregated wormlike particles have become more extensively fused structures. Some small disordered sheetlike structures can be found mingled with the fused mass. The reaction proceeds so rapidly that some nanoplates have emerged within a few min of reaction. Figure 3c shows a TEM image of a hexagonal nanoplate with a width of ∼450 nm
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Figure 4. (a) TEM image of an intermediate product acquired after 10 min of reaction. Inset gives the SAED pattern of this nanomaterial. (b and d) TEM images of the intermediate products obtained after 20 min of reaction. Inset shows the SAED pattern of the truncated triangular nanoplate in panel b. (c) Enlarged image of the nanoplate shown in panel b. (e) TEM image of a triangular microplate synthesized after 21 min of reaction. (f) TEM image of the edge portion of a hexagonal microplate synthesized after 30 min of reaction. Insets show the TEM image of this hexagonal microplate and its SAED pattern.
formed after 3 min of reaction. Although only few nanoplates have been collected in the reaction solution, it is not unusual for nanoplates to grow to such dimensions or larger within a few minutes of reaction at high solution temperatures.5a,14,16,20 Interestingly, this gold nanoplate seems to be composed of winding nanowire structures or some elongated nanostructures with diameters of several nanometers, and closely arranged to fill the entire nanoplate. The nanoplate surface also appears to be covered with surfactants. This unusual nanoplate structure has not been observed before, and may well be unique to gold nanoplates synthesized via this thermal aqueous solution approach. This finding suggests that these gold nanoplates are formed via a rapid and dense aggregation of elongated or wormlike nanostructures, followed by fusion of these tiny nanostructures and reorganization of the gold atoms. Anyway, it was found that most of the intermediate products observed have not developed into well-defined nanoplates at this point. Figure 3d is a TEM image of a product collected after 6 min of reaction. The degree of fusion and the size of the central solid mass have increased. The exterior aggregated wormlike nanostructures have fused into larger and more irregularly shaped structures. These TEM results show that the central core appears to grow in size via incorporating the exterior gold nanostructures. The nanoplate growth process continued in the following minutes. Figure 4a gives a TEM image of a sample collected after 10 min of reaction. Here the central mass has grown denser and larger to hundreds of nanometers in width, while the exterior aggregated nanostructures still surround the central core. The SAED pattern of this nanomaterial produced complete rings and indicates that the entire structure is polycrystalline gold due to the large amount of the exterior nanostructures. After 20 min of reaction, some nanoplates have completed their growth, but others may still be covered with the wormlike or branched nanostructures (Figure 4b). A close examination of this truncated triangular nanoplate reveals that its surface is covered with wormlike or dotted features (Figure 4c). These branched or dotted structures appear to be embedded within the plate, or just be on the plate surface giving this nanoplate a rough and bumpy surface contour. This structural characteristic is similar to that observed in the nanoplate shown in Figure 3c and provides further evidence of nanoplate growth through
Huang et al. fusion and incorporation of the exterior wormlike and branched nanostructures. The SAED pattern of this nanoplate results in strong spots and weak rings, identifying this nanoplate as largely single and crystalline gold. Some large faceted gold nanoparticles formed also contain the same exterior structural feature, suggesting that these large nanoparticles found in the final product sample are also synthesized by the same growth mechanism. Figure 4d shows another intermediate product formed after 20 min of reaction. The central core has a triangular shape and is expected to eventually develop into a sharp triangular nanoplate. Nanoplates with well-defined shapes were largely formed after 20 min of reaction. Figure 4e is a TEM image of a triangular nanoplate synthesized after 21 min of reaction. Clearly the edges of this nanoplate still possess rough surfaces, presumably because the exterior nanostructures have not completely incorporated into this nanoplate. Similar plate morphology with rough edges has also been recorded as an intermediate product in the synthesis of gold nanoplates with lemongrass extract.1a A hexagonal microplate obtained after 30 min of reaction was found to also possess such structurally imperfect edges (Figure 4f), although to a much lesser extent. The SAED pattern indicates that this gold microplate is a single crystal, and its surface bounded by {111} facets. With further reaction, sharp edges should be obtained. From this series of examination of the intermediate products, we can propose a general nanoplate growth process using the thermal aqueous solution approach. It was found that the nanostructure formation process proceeds extremely rapidly within seconds of reaction to form large aggregated structures, which are composed of wormlike or elongated winding structures just several nm in diameter. These tiny nanostructures are fused more extensively toward the central region to form a thicker mass. The central core then grows in size via incorporating the exterior wormlike and branched nanostructures. Although the gold atoms in the solution can still be continuously added to the exterior nanostructures, the general trend is that the exterior fine structures decrease their overall extended sizes relative to that of the central region. These exterior elongated nanostructures were found to be incorporated into and cover the entire developing nanoplates, and this may be the mechanism by which these nanoplates are grown. The nanoplate edges may be rough initially, but eventually the gold atom reorganization process may be complete to yield the final structurally welldefined nanoplate products. This investigation also did not find any evidence of structural transformation from triangular to hexagonal nanoplates. Once these plates are formed, they possess certain symmetrical shapes. We have also checked the products formed in the absence of CTAB surfactant. Mainly highly faceted gold nanocrystals with diameters of tens of nanometers and some small nanoplates with similar sizes as the faceted nanocrystals or larger were produced after 40 min of reaction. When the amount of trisodium citrate used was reduced by half, large faceted gold nanoparticles (hundreds of nanometers in diameter) were formed along with some microplates. These control experiments demonstrate that appropriate amounts of CTAB surfactant and trisodium citrate reductant are required for the optimal formation of gold plates. The same thermal aqueous solution approach has also been employed to prepare ultrasmall gold nanoplates. Figure 5 shows TEM images of the nanoplates formed with an average width of 58 ( 10 nm. These nanoplates predominantly exhibit triangular shape, consistent with previous observations of gold nanoplates of this size range.15 Because of their similar sizes
Growth Process of Gold Nanoplates
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Figure 5. (a and b) TEM images of the ultra-small gold nanoplates with an average width of 58 ( 10 nm. These triangular nanoplates can spontaneously self-assemble into some short-ranged 2-dimensional ordered patterns.
Figure 7. Size distribution plots of the gold nanoplate samples shown in Figures 5 (a) and 6 (b).
Figure 6. (a and b) TEM images of the ultra-small gold nanoplates with an average width of 40 ( 7 nm. Again, some ordered selfassembled structures can be observed.
and sharp triangular structure, these small nanoplates can spontaneously self-assemble into some ordered 2-dimensional structures, such as hexagonal structure (Figure 5a) and alternate up-and-down arranged linear structure (Figure 5b). Presence of some faceted nanoparticles can disrupt these ordered structures. Figure 6 gives the TEM images of the gold nanoplates with an average width of 40 ( 7 nm. The size distribution plots for gold nanoplates of the two size ranges are provided in Figure 7. These ultrasmall nanoplates have a relatively narrow size distribution and can also arrange into some ordered 2-dimensional structures. Interestingly, these nanoplates do not possess sharp edges, suggesting that gold nanoplates of this size range have a more rounded structure. This more rounded plate morphology creates interstitial voids and reduces the interactions among neighboring nanoplates. Thus, these ultra-small gold nanoplates cannot pack into tightly arranged hexagonal arrays as those larger triangular nanoplates do. There were also more faceted nanoparticles present that can interfere with the selfassembled structures formed by the nanoplates. The optical properties of these ultrasmall gold nanoplates were also characterized. Figure 8 presents the UV-vis absorption spectra of the gold nanoplates synthesized with average widths of 40 ( 7, 58 ( 10, and 80 ( 13 nm. The absorption spectrum for the nanoplates with an average width of 80 ( 13 nm was also added to show the progressive shifts in the surface plasmon
Figure 8. UV-vis absorption spectra of the synthesized ultrasmall gold nanoplates. (Black trace) Nanoplates with an average width of 40 ( 7 nm. (Gray trace) Nanoplates with an average width of 58 ( 10 nm. (Dotted trace) Nanoplates with an average width of 80 ( 13 nm. The dotted trace is added here to show the shifts in the SPR absorption bands with increasing nanoplate sizes.
resonance (SPR) absorption bands with increasing nanoplate sizes. These nanoplates were prepared the same way as that used to make nanoplates having widths of 40-70 nm but with a CTAB concentration of 8.75 × 10-3 M and a reaction time of just 5 min. Their TEM image and size distribution plot can be found in the Supporting Information. These nanoplates typically show two SPR absorption bands. For nanoplates with an average width of 40 ( 7 nm, a strong SPR absorption band at 590 nm and a weak and broadband centered at ∼775 nm were observed. These bands are characteristic SPR absorption bands for gold nanoplates of this size range.15 Also note that there is significant absorption from the spherical nanoparticles in the 520-550-nm range for all the samples, and exhibits as a shoulder band. For nanoplates with an average width of 58 ( 10 nm, the strong SPR absorption band shifts slightly to 602 nm. The long-wavelength SPR absorption band red-shifts to ∼900 nm, and become broader and with a higher absorbance value. Nanoplates with an average width of 80 ( 13 nm have a strong SPR absorption band at 620 nm. The long-wavelength SPR absorption band further red-shifts and also shows a slightly increased absorbance value relative to that of the other two
2538 J. Phys. Chem. C, Vol. 111, No. 6, 2007 samples. From these spectral data and the known spectral profiles for gold nanoplates and nanorods, the short-wavelength SPR absorption band at 590-620 nm is related to the thicknesses of the nanoplates (generally 3-6 nm for nanoplates with widths of 40 ( 7 nm and 5-10 nm for nanoplates with widths of 58 ( 10 nm). This band shows a smaller shift in the band position, as the change in the plate thickness is small. The longwavelength SPR absorption band is related to the widths of the nanoplates, and can show progressively red-shifted band position and increased absorbance in the near-infrared (NIR) region as the nanoplates increase in size.1b Possibly because these nanoplates are well-dispersed in solution and do not form partially stacked structures, no interparticle plasmon coupling band appears in the NIR region. Conclusion The growth process of gold plates formed by thermal aqueous solution approach has been studied. The results showed that fine wormlike nanostructures formed rapidly, and aggregated toward the center to create a fused mass. This central mass of polycrystalline gold structure grew in size by continuously incorporating the exterior dendritic gold nanostructures, and eventually adopting the plate morphology. Interestingly, the nanoplates seem to be composed of these wormlike nanostructures that are tightly arranged and fused together. Although this growth mechanism is completely different from what has been previously proposed, the results obtained suggest that a different growth process is indeed in operation under such a high solution temperature synthesis condition. Appropriate amounts of CTAB surfactant and trisodium citrate are also important factors in facilitating the growth of nanoplates. The transformation of nanoplate shape from triangular to hexagonal and truncated triangular structures has not been found. We have also extended this nanoplate growth approach to prepare ultrasmall triangular gold nanoplates with average widths of 40 ( 7 and 58 ( 10 nm. These are by far some of the smallest gold nanoplates ever been reported. Their small and uniform sizes enable the formation of ordered 2-dimensional structures. UV-vis absorption spectroscopy revealed that these tiny nanoplates exhibit a strong SPR absorption band at around 600 nm and a weak and broadband in the NIR region. Significantly, this study shows that the growth process of gold nanoplates under a high solution temperature condition can be quite complicated. In addition,
Huang et al. the roles templating surfactant and reducing agent, such as trisodium citrate, play in directing the formation of nanoplates should also take into account. Acknowledgment. This work was supported by a grant from the National Science Council of Taiwan (Grant NSC 94-2113M-007-012). Supporting Information Available: TEM image of the gold nanoplates with larger sizes and their width distribution plot. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nature Mater. 2004, 3, 482. (b) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Chem. Mater. 2005, 17, 566. (2) Liu, B.; Xie, J.; Lee, J. Y.; Ting, Y. P.; Chen, J. P. J. Phys. Chem. B 2005, 109, 15256. (3) Shao, Y.; Jin, Y.; Dong, S. Chem. Commun. 2004, 1104. (4) (a) Millstone, J. E.; M’etraux, G. S.; Mirkin, C. A. AdV. Funct. Mater. 2006, 16, 1209. (b) Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L.; Schatz, G. C.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 5312. (5) (a) Sun, X.; Dong, S.; Wang, E. Langmuir 2005, 21, 4710. (b) Sun, X.; Dong, S.; Wang, E. Angew. Chem., Int. Ed. 2004, 43, 6360. (6) Ah, C. S.; Yun, Y. J.; Park, H. J.; Kim, W.-J.; Ha, D. H.; Yun, W. S. Chem. Mater. 2005, 17, 5558. (7) Yamamoto, M.; Kashiwagi, Y.; Sakata, T.; Mori, H.; Nakamoto, M. Chem. Mater. 2005, 17, 5391. (8) Wang, L.; Chen, X.; Zhan, J.; Chai, Y.; Yang, C.; Xu, L.; Zhuang, W.; Jing, B. J. Phys. Chem. B 2005, 109, 3189. (9) Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 2002, 18, 3694. (10) Kim, J.-U.; Cha, S.-H.; Shin, K.; Jho, J. Y.; Lee, J.-C. AdV. Mater. 2004, 16, 459. (11) Xiong, Y.; Washio, I.; Chen, J.; Cai, H.; Li, Z.-Y.; Xia, Y. Langmuir 2006, 22, 8563. (12) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem., Int. Ed. 2004, 43, 3673. (13) Tsuji, M.; Hashimoto, M.; Nishizawa, Y.; Kubokawa, M.; Tsuji, T. Chem.-Eur. J. 2005, 11, 440. (14) Li, Z.; Liu, Z.; Zhang, J.; Han, B.; Du, J.; Gao, Y.; Jiang, T. J. Phys. Chem. B 2005, 109, 14445. (15) Chu, H.-C.; Kuo, C.-H.; Huang, M. H. Inorg. Chem. 2006, 45, 808. (16) Sun, X.; Dong, S.; Wang, E. Chem. Lett. 2005, 34, 968. (17) Porel, S.; Singh, S.; Radhakrishnan, T. P. Chem. Commun. 2005, 2387. (18) Ibano, D.; Yokota, Y.; Tominaga, T. Chem. Lett. 2003, 32, 574. (19) Chen, S.; Carroll, D. L. J. Phys. Chem. B 2004, 108, 5500. (20) Kan, C.; Zhu, X.; Wang, G. J. Phys. Chem. B 2006, 110, 4651. (21) Jin, R.; Cao, Y. C.; Hao, E.; Me´traux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487.
