DOI: 10.1021/cg1005456
Formation of Highly Thin, Electron-Transparent Gold Nanoplates from Nanoseeds in Ternary Mixtures of Cetyltrimethylammonium Bromide, Poly(vinyl pyrrolidone), and Poly(ethylene glycol)
2010, Vol. 10 3694–3698
Akrajas Ali Umar,*,† Munetaka Oyama,‡ Muhamad Mat Salleh,† and Burhanuddin Yeop Majlis† †
Institute of Micronegineering and Nanoelectronics, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia, and ‡Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto, 615-8520 Japan Received April 25, 2010; Revised Manuscript Received June 9, 2010
ABSTRACT: A simple approach to grow highly thin and electron-transparent gold nanoplates with a micrometer range edgelength dimension is demonstrated. The thin nanoplates were obtained by realizing an effective two-dimensional (2D) crystal growth of the nanoseed in a ternary surfactant mixture of cetyltrimethylammonium bromide (CTAB), poly (vinyl pyrrolidone) (PVP), and poly (ethylene glycol) (PEG). By simply controlling the concentration ratio of these three surfactants in the growth solution, high yield triangular and hexagonal nanoplates with thicknesses as low as ca.18.2 MΩ). The solution was aged undisturbed at room temperature for 2 h prior to further use. Second, the growth solution was prepared by adding 0.5 mL of 10 mM HAuCl4, 0.5 mL of 0.01 M ascorbic acid, and 20 μL of 1 M NaOH into 25 mL of aqueous solution of ternary mixture surfactants, namely, 10 mL of 0.1 M
cetyltrimethylammonium bromide (CTAB, WAKO Chemicals), 5 mL of 1 mM poly(vinyl pyrrolidone) (PVP) (M.W. ∼ 55000, Aldrich), and 10 mL of 1 mM poly(ethylene glycol) (MW ca. 200, WAKO Chemicals), with stirring. This solution was called the standard solution and used throughout the work or otherwise stated later. If this solution was used, final solution will contain CTAB, PVP, and PEG concentrations of 40, 0.2, and 0.4 mM, correspondingly. In this work, the effect of the concentration of individual surfactant on the nanocrystal growth was also studied by changing the concentration to below or above the optimum concentration while keeping the concentration of other surfactants unchanged. To grow the nanoplates, 0.5 mL of the as-prepared nanoseed particles solution was added into the growth solution and shaken properly. Then, the solution was left undisturbed for 8 h at 28 °C for a growth process of the nanoseed particles. The morphology of the grown nanocrystals was studied by a field emission scanning electron microscopy (FESEM) carried out using a JSF 7400F JEOL (Japan) FESEM machine.
3. Results and Discussion Figure 1A,B shows a typical FESEM image of the gold nanoplates prepared using the present procedure with CTAB, PVP, and PEG concentrations of 40 mM, 0.2 mM, and 0.4 mM, respectively. As can be seen from the figure, ultrathin gold nanoplates with an edge-length dimension of up to ca. 1 μm have been successfully synthesized. The morphology of the nanoplates was interestingly found to adopt the hexagonal or truncated-hexagonal and triangular structures with a yield percentage up to ca. 50 and 40%, respectively. Meanwhile, for the byproduct, the faceted nanoparticles, such as trihedral
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pyramid, decahedral, icosahedral, etc., was found to dominate the overall yield (Figure 1A). On the nanoplates products, judging from the image, it can be confirmed that the nanoplates are a very thin structure. It is proved by the existence of very low-contrast characteristics of the nanoplates when compared to the faceted nanoparticles byproduct. This characteristic was further confirmed from the FESEM ability and by comparison with our previous observation of Au nanoplates.13 In the SEM measurements, we fixed the acceleration voltage of the electron beam to 5.0 keV. From the known equation,14 the penetration depth of the electron beam can be estimated as ca. 52 nm if we assume a pure gold target. However, because we observe the reflection and secondary beams in the SEM measurement, the observed surfaces actually show the overlapping, or transparent, images should be thinner than 50 nm. In our previous result,13 Au nanoplates were formed on ITO surfaces. In this case, the thickness of the plates was proven to be 15-25 nm by AFM measurements, but the SEM images showed much less penetration of the electron beam in comparison with the present results. Although the SEM observations do not show the exact thickness of the materials, owing to the images showing the stacking and transparency of Au nanoplates and the easy access to the buried morphology as in Figure 1, the thickness of the nanoplates can be estimated to be 10 nm, compared to the present results (Figure 2A). In the present study, we actually used this previous recipe for the nanoplates formation, but with the addition of PEG. To our surprise, highly thin gold nanoplates with an edge-length dimension much larger compared to those prepared without PEG could be obtained. On the basis of these results, it can be worth assuming that the PEG should also play a certain important function in the
Ali Umar et al.
Figure 2. (A) FESEM image of the gold nanoplates prepared using a standard growth solution in the absence of PEG and (B) in the absence of PVP. Scale bar is 1 μm.
promotion of 2D crystal growth. So, in the presence of PVP, a well-known 2D crystal growth promoter,13 it was able to facilitate “a much effective” 2D crystal growth of the gold nanoseed. Thus, highly thin and large edge-length dimension nanoplates could be formed. In this work, to obtain a clearer function of the PEG in this process, we first examined the nanocrystal growth with the absence of PVP in the solution (CTAB and PEG only). The result is shown in Figure 2B. As can be seen from the image, in the absence of PVP, no nanoplate structures were formed, but a micrometer-size quasi-2D structure, namely, networked flakelike microcrystal, formed instead. The formation of such morphology, namely, a flakelike structure, in the absence of PVP strongly inferred that the PEG likely also has a peculiar function for projecting 2D nanocrystal growth of the nanoseeds, and, as has been mentioned early, with PVP, it may impressively promote the formation of highly thin nanoplates with perfect morphology instead. However, for a specific application that requires the gold nanoflakes structures, the present recipe could be used. For the optimum concentration for CTAB, PVP, and PEG that promotes the formation of highly thin nanoplates, the concentration of 40 mM, 0.2 mM, and 0.4 mM for CTAB, PVP, and PEG, correspondingly, can be used. If the concentration of an individual chemical is below or above this value, different nanocrystal growth characteristics were obtained. For example, for the case of PVP, its concentration should be in the range of 0.1-0.3 mM. A lower or higher concentration may produce a considerably thick and limited number of
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Figure 4. FESEM image of the gold nanocrystals prepared using the standard growth solution with a CTAB concentration of (A) 20 mM and (B) 60 mM. Scale bar is 100 nm.
Figure 3. FESEM images of gold nanostructures prepared using a standard growth solution with a PVP concentration of (A) 0.2 mM, (B) 0.1 mM, and (C) 0.4 mM. Scale bar is 100 nm.
nanoplates as well as faceted nanoparticles instead. The results are shown in Figure 3. Meanwhile, for CTAB, its concentration should be in the range of 40-50 mM. If a lower concentration is used, for example, 20 mM, the products were only faceted gold nanoparticles without the presence of any platelets morphology. In contrast to the lower concentration, if a higher concentration is applied, for example, 60 mM, large-scale thin nanoplates were produced. However, due to an active Oswald annealing in the presence of a high concentration of CTAB, the shape of the gold nanoplates becomes rounded. This also caused the nanoplates size to become smaller compared to those prepared using an optimum concentration. The result is shown in Figure 4.
Finally, for the case of PEG, relatively moderate concentration was necessary for the formation of highly thin gold nanoplates. Its typical concentration should be ca. ∼0.4 mM. At lower concentration or in the absence of PEG, as has been noted earlier, the nanoplates were typical of the product; however, their thickness was relatively higher. In contrast to the results from the lower concentration, higher concentration may produce a limited number of smaller edge-length size nanoplates. And at a relatively higher concentration, faceted gold nanoparticles without the presence of platelet morphology become the main product. The result is shown in Figure 5. At present, the actual mechanism of the formation of highly thin gold nanoplates is not yet well-understood. However, the following facts could be considered: First, the PVP should be present in the reaction for the projection of gold nanoplates morphology; if not large-scale flakelike structures will be obtained. Second, the PEG was essential and its concentration should be relatively higher for the projection of highly thin, electron-transparent, and large-size gold nanoplates. A lower concentration only produced relatively thick and smaller gold nanoplates. Third, the CTAB concentration should be relatively higher in the reaction; a lower concentration only promoted the formation of faceted-gold nanoparticles, though the effective combination function of CTAB, PVP, and PEG surfactant in the nanocrystal growth process via a selective adhesion to the nanocrystals planes was assumed to be the major driving force for the formation of these
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nanoseed that leads to the formation of highly thin, electrontransparent nanoplates structure. At this stage, the specific function of individual surfactant as well as their combined characteristics in the projection of highly thin gold nanoplates is still not understood yet and is under investigation. By understanding the chemistry behind the process, much thinner and larger edge-length dimension nanoplates might be produced in the future. Thus, novel properties will be produced and their use in currently existing applications will also be expanded. Concerning the detailed structures of thin nanoplates, we are planning to observe XRD and HRTEM as the next stage. Acknowledgment. We thank the Kyoto Nanocluster project for financial support. We also acknowledge part financial support from the Ministry of Higher Education of Malaysia and the Universiti Kebangsaan Malaysia under Project Grant UKM-GUP-NBT-08-25-086 and UKM-RRR1-07-FRGS00372009.
References
Figure 5. FESEM images of the gold nanocrystal prepared using the standard growth solution with PEG concentrations of (A) 0.48 mM and (B) 0.8 mM. Scale bar is 100 nm.
structures. The basic idea of the process has been reported previously.16 4. Conclusions In the present study, a simple method for preparing highly thin and electron-transparent gold nanoplates has been demonstrated using an aqueous ternary mixture of CTAB, PVP, and PEG. By simply optimizing the concentrations of CTAB, PVP, and PEG in the reaction, triangular and hexagonal gold nanoplates with a yield as high as ca. 90% and thickness as low as ca. < 5 nm could be successfully obtained. The edge-length dimension of the nanoplates was found to be able to extend to up to ca. 1 μm. PVP was considered as the key material for the projection of 2D crystal growth, and in the presence of PEG, it may facilitate a highly effective 2D crystal growth of the
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