Saikat
Mandal
,
Sujatha K.
Arumugam
,
Suguna D.
Adyanthaya
,
Renu
Pasricha
and
Murali
Sastry
*
Materials Chemistry Division, National Chemical Laboratory, Pune–411 008, India
First published on 6th October 2003
In this paper we describe the facile synthesis of gold nanocrystals of variable morphology using aqueous foam as a template. The aqueous foams are formed by bubbling an aqueous solution of AuCl−4 ions electrostatically complexed with the surfactant cetyltrimethylammonium bromide (CTAB). The gold ions in the stable foam are then reduced by hydrazine vapours, this process leading to the formation of gold nanoparticles of spherical, flat plate and flake-like structures. The variation in morphology of the gold nanoparticles derived from the foam is believed to arise from the complex spatial structure of reaction sites in the foam. The foam-derived gold nanoparticles were analysed by UV-vis spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy.
An exciting and hitherto considerably underexploited dynamic biomimetic template for crystal growth is foam lamellae.32 Davey and co-workers have shown that stabilizing surfactants at the air-bubble/solution interface in foams could be used as nucleation centers for the growth of glycine and CaCO3 crystals.32 The foam provides a high surface area of gas bubbles dispersed in a liquid. The stabilizing surfactant or other amphiphile is adsorbed at the gas/liquid interface and hence offers the possibility that the liquid lamellae might be used as a template for growing a range of inorganic materials by novel chemistry. Since the thickness of the liquid lamellae is controlled by the drainage within the plateau borders of the foam, it also follows that the size of the resulting inorganic crystals may be controlled via the drainage rate. In this paper we describe a simple procedure for the large-scale synthesis of anisotropic gold nanoparticles in aqueous foams stabilized by the surfactant cetyltrimethylammonium bromide (CTAB). This is accomplished in a two-step process involving the electrostatic binding of AuCl−4 ions with the cationic CTAB molecules populating the air-bubble/solution interface followed by the reduction with hydrazine vapors in the final step. A highlight of this study is our observation that gold nanoparticles of variable morphology are formed within different regions of the foam. We believe this is due to the variation in structure across the foam presenting reaction cavities of variable thickness. Presented below are details of the investigation.
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Fig. 1 (A) UV-vis spectra of CTAB-capped gold nanoparticle solutions 1–4, corresponding to curves 1, 3, 2 and 4 respectively (see text for details). The inset shows pictures of test tubes of gold nanoparticle solutions in samples 1–4. (B) XRD pattern of a drop-coated film of CTAB-capped gold nanoparticles from sample 1 deposited on a glass substrate. |
Fig. 2A shows FTIR spectra of pure CTAB (curve 1) and CTAB-capped gold nanocrystals obtained from sample 1 (curve 2) in the spectral region 2500–3550 cm−1. The C–H symmetric and antisymmetric stretching vibration frequencies at 2850 and 2920 cm−1 are clearly seen for pure CTAB (curve 1) while these absorption bands appear at 2865 and 2944 cm−1 for gold nanocrystals prepared in the foam (sample 1). The presence of the methylene vibrational signatures in the gold nanoparticle sample (curve 2) clearly indicates the presence of CTAB and suggests a mechanism for the stabilization of the gold nanoparticles by a surface-bound layer of CTAB. The shift in the C–H symmetric and antisymmetric stretching vibrations of CTAB bound to the surface of the gold nanoparticles relative to pure CTAB indicates some disorder in the hydrocarbon chains of CTAB on the nanoparticle surface. In addition to the methylene vibration bands, resonances at 3390 cm−1 in curve 1 and at 3176 cm−1 and 3315 cm−1 in curve 2 are also seen in Fig. 2A.33 The broad band at 3390 cm−1 corresponds to N–H stretch vibrations from pure CTAB molecules and after binding to the gold nanoparticles this band is shifted to 3176 cm−1. The band at 3315 cm−1 in curve 2 (CTAB-capped gold nanoparticles) indicates the presence of a bilayer of CTAB on the nanoparticle surface wherein the secondary layer of CTAB exposes ammonium ions to water and stabilizes the particles. The band at 3435 cm−1 in curve 2 corresponds to the O–H stretching frequency of water solvating the surfactant CTAB. Fig. 2B shows the FTIR spectra of pure CTAB (curve 1) and CTAB-capped gold nanocrystals in sample 1 (curve 2) in the spectral region 1300–1700 cm−1. The doublet at 1462 and 1472 cm−1 in curve 1 (for pure CTAB) is attributed to the CH2 scissoring mode of vibration and indicates close-packing of the methylene chains. For the CTAB-capped gold nanocrystals, the splitting between the peaks in the doublet becomes smaller (in curve 2). This is due to loss in crystallinity of the hydrocarbon regions and may be due to the curvature of the nanoparticles. The band at 1600 cm−1 is assigned to the O–H bending mode of water around the bound headgroups. The FTIR data thus clearly indicates that the surfactant CTAB is bound to the gold nanoparticles synthesized in the foam and furthermore, that they stabilize the particles by forming an interdigitated bilayer structure. It is easy to see how such a structure could electrostatically stabilize the gold nanoparticles through exposure of ammonium ions on the surface. Recently, Sastry and co-workers demonstrated the formation of water-dispersible gold nanoparticles capped with surface-bound interdigitated bilayers of CTAB33 and it is likely that the gold nanoparticles of this study have a similar structure.
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Fig. 2 FTIR spectra recorded from drop-coated films of pure CTAB (curve 1) and CTAB-capped gold nanoparticles in sample 1 (curve 2) in the spectral ranges (A) 2500–3550 cm−1 and (B) 1300 –1700 cm−1 respectively. |
Figs. 3A and B show representative TEM micrographs recorded at different magnifications from a drop-cast film of CTAB-capped gold nanoparticles in sample 1. Highly anisotropic gold particles are seen that co-exist with a small percentage of nearly spherical particles. Another interesting observation is that the tape-like nanoparticles appear to originate from a flat surface (indicated by an arrow in Fig. 3A). Indeed, a number of flat gold particle assemblies exhibit this peculiar feature. This suggests a templating mechanism for the growth of the gold nanoparticles, a possibility being that the gold ions bind to CTAB monolayers at the bubble surface in the foams where gold nanocrystal nucleation and growth occur. The higher magnification image (Fig. 3B) shows the morphology of one aggregate of tape-like gold nanostructures in greater detail. The widths of the tapes are rather non-uniform and vary from 20–150 nm. An estimate of the thickness of the gold nanostructures could not be made but from the contrast in Fig. 3B, they appear to be thin. The inset in Fig. 3B shows the selected area electron diffraction (SAED) pattern of the gold nanoplates shown in Fig. 3B. The diffraction rings have been indexed in the figure and are clearly due to a face centered cubic (fcc) gold unit cell structure.
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Fig. 3 (A) and (B) Representative TEM micrographs recorded at different magnifications of drop-cast films of CTAB-capped gold nanoparticles in sample 1. The inset in (B) shows the selected area electron diffraction (SAED) pattern of the flat-gold nanoplates shown in the figure. |
Figs. 4A and B show representative TEM micrographs recorded at different magnification of a drop-cast film of gold nanoparticles in sample 2 (gold nanoparticles in the supernatant obtained by centrifuging the CTAB-capped gold nanoparticle solution formed in the foam). Highly irregular, flat gold nanoplates/flakes can be seen in these TEM micrographs. In comparison with the gold nanostructures of sample 1 (Fig. 3), the gold nanoparticles in the pellet appear to be thinner. The gold nanoflakes vary in size from 20 nm to 90 nm and appear to be made of triangular gold nanostructures. There was no evidence for the presence of spherical gold nanoparticles in this sample indicating that the centrifugation was successful in removing the spherical structures seen in the as-prepared CTAB-capped gold nanoparticles (Fig. 3). The inset in Fig. 4A shows the SAED pattern of the gold nanoflakes shown in Fig. 4. The diffraction rings in the figure are clearly due to a face centered cubic (fcc) gold unit cell structure. Further to the TEM measurements, the broadening of the plasmon resonance band observed in the UV-vis spectrum of the CTAB-capped gold nanoparticle solution 2 (Fig. 1A, curve 3) can be attributed to the flat, anisotropic structure of the nanoparticles and is not due to the aggregation of particles.
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Fig. 4 (A) and (B) Representative TEM micrographs recorded at different magnifications of drop-cast films of CTAB-capped gold nanoparticles in sample 2. The inset (A) shows the selected area electron diffraction (SAED) pattern of the gold nanostructures shown in the figure. |
Figs. 5A and B show representative TEM micrographs recorded at different magnification of a drop-cast film of CTAB-capped gold nanoparticles in sample 3. Only spherical gold nanoparticles are seen in these TEM micrographs. The spherical particles are fairly uniform and range in size from 8 nm to 70 nm. It is interesting to note that a fairly well-defined inter-particle separation exists in the assemblies and that they are not in direct physical contact. This provides further evidence for a protective CTAB sheath around the gold nanoparticles. Thus, centrifugation leads to the concentration of the relatively heavier spherical gold nanoparticles loosely bound to the nanoflakes/nanoplates in the pellet while the lighter gold nanoflake/nanoplate-like structures remain in the supernatant (Fig. 4).
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Fig. 5 (A) and (B) Representative TEM micrographs recorded at different magnifications of a drop-cast film of gold nanoparticles in sample 3. |
Figs. 6A and B show representative TEM micrographs recorded at different magnifications of a drop-cast film of CTAB-capped gold nanoparticles obtained in the control experiment (sample 4). Only spherical gold nanoparticles are seen in these TEM micrographs. The spherical particles vary in size from 10 nm to 60 nm. It is interesting to note that a fairly well-defined inter-particle separation exists in the assemblies and that they are not in direct physical contact. This provides the evidence for a protective CTAB sheath around the gold nanoparticles. The inset in Fig. 6B shows the selected area electron diffraction (SAED) pattern of the gold nanoparticles shown in Fig. 6. The diffraction rings have been indexed in the figure and are clearly due to a face centered cubic (fcc) gold unit cell structure. It is thus clear from the TEM analysis of the gold nanoparticles synthesized in the foam (Figs. 3–5) and those synthesized in solution in the presence of CTAB (Fig. 6) that it is not the surfactant that plays a role in determining the morphology of the nanoparticles. Instead it is the unusual structure of the reaction regions in the foam that causes the interesting morphology variations observed.
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Fig. 6 (A) and (B) Representative TEM micrographs recorded at different magnifications of drop-cast films of CTAB-capped gold nanoparticles in sample 4. The inset in Fig. 6B shows the selected area electron diffraction (SAED) pattern of the gold nanostructures shown in the figure. |
In Scheme 1 we illustrate a possible mechanism operative in the foam that could explain the differences in morphology of the gold nanoparticles grown. The liquid lamellae between bubbles in the foam may be considered to consist of two Langmuir monolayers of CTAB electrostatically complexed with AuCl−4 ions and therefore, the gold ions amenable to reduction by hydrazine are at the interface only. Region ‘a’ in Scheme 1 shows the liquid lamellae within the plateau borders of the foam wherein the thickness of this region is very small. It is likely that this is the nanoreactor where formation of flat gold nanoplates by hydrazine reduction of AuCl−4 ions occurs. Region ‘b’ shows the junction of four plateau borders where the reaction space for the formation of spherical gold nanoparticles is more extended and more isotropic. We believe the spherical gold nanoparticles form in this region of the foam. Much work is required before a full appreciation of the capabilities of the nanoreactors in foams is achieved. In the control experiment, the AuCl−4 ions were reduced by hydrazine vapors and capped with CTAB directly in bulk solution. TEM pictures of the control experiment show only spherical nanoparticles, with no indication of the presence of gold nanoflakes/nanotapes indicating that the complex geometry of reaction regions in foams as discussed above is crucial in directing the gold nanoparticle morphology variation.
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Scheme 1 Scheme showing the different regions in the foam believed to be involved in the synthesis of gold nanocrystals of variable shape. The regions ‘a’ and ‘b’ are described in the text in detail. |
In conclusion, the formation of gold nanocrystals stabilized by the surfactant cetyltrimethylammonium bromide (CTAB) in liquid foam has been described. The process of ion entrapment in the foam and their reduction results in the formation of gold nanostructures of variable morphology. The gold nanoparticle morphology varies from spherical to flat plates/flakes and simple centrifugation enables separation of the different shapes. This morphology control is believed to occur due to differences in the structure of the foam, which varies from highly anisotropic (liquid lamellae between bubbles) to extended and isotropic (junction between plateau borders). The combination of an extremely large interfacial templating area provided by the liquid lamellae in foams and the dynamic nature of the foam bubbles makes this method potentially exciting for the large-scale synthesis of other inorganic materials (nanomaterials) and minerals which is currently being attempted.
This journal is © The Royal Society of Chemistry 2004 |