DOI: 10.1021/cg101220v
A Facile Synthesis of Two-Dimensional Dendritic Gold Nanostructures at the Air/Water Interface
2010, Vol. 10 4701–4705
Xiangtao Bai and Liqiang Zheng* Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, China Received August 9, 2010
ABSTRACT: Two-dimensional dendritic gold nanostructures are synthesized at the air/water interface with the assistance of a special additive, [camphorC2mim]Cl, which can form a precipitate with AuCl4-. Interestingly, the precipitate is essential to the formation of dendritic nanostructures. Transmission electron microscopy (TEM) characterization reveals that the ratio of [camphorC2mim]Cl to HAuCl4 rather than the concentration is crucial to the morphology of the obtained dendritic nanostructures. A reasonable mechanism based on diffusion-limited aggregation (DLA) theory is proposed according to the high resolution TEM (HRTEM) characterization. The obtained gold dendrites are shown to yield a large surface-enhanced Raman scattering (SERS) enhancement for rhodamine 6G.
*Corresponding author. E-mail:
[email protected]. Phone: þ86-53188366062. Fax: þ86-531-88564750.
Dendritic gold nanostructures also have been extensively studied. Chen et al. prepared bipodal, tripodal, and tetrapodal gold nanoparticles by using CTAB, HAuCl4, ascorbic acid, and NaOH. These nanoparticles can be seen as dendritic nanostructures, but they are relatively small (40 nm in length).8 Threedimensional (3D) gold dendrites with fractal nanostructure were prepared by Enomoto’s group using a seeding method in the presence of ascorbic acid.9 Hexatrimethylammomium bromide (HTAB) was used as the protecting agent and architecture soft template. Qin and co-workers reported that 3-fold symmetrical gold dendrites can be synthesized by the reaction between a zinc plate and a solution of HAuCl4 in the ionic liquid 1-butyl3-methylimidazolium hexafluorophosphate ([BMIM][PF6]).10 Recently, multifunctional 3D dendritic gold nanostructures have been synthesized by square wave potential pulse (SWPP) with an AuSn alloy in an electrolyte of NaOH or H2SO4.11 Electrochemical deposition can also be used for preparation of gold dendrites. For example, Xu et al.12 reported a templateless, surfactantless, simple electrochemical route to dendritic gold nanostructures. Moreover, this is also a fast and feasible way to modify an electrode with pristine gold nanostructures. Other groups have obtained dendritic nanostructures at the liquid/ liquid interface. For example, 3D dendritic gold nanostructures were prepared through an aqueous/organic interfacial reaction of hydrogen tetrachloroaurate aqueous solution and 3,4-ethylenedioxythiophene in dichloromethane by Lu et al.13 The air/ water interface is usually used to fabricate various nanostructures, but to the best of our knowledge, well-defined dendritic gold nanostructures have not been obtained at the air/water interface. Herein, we report the fabrication of two-dimensional (2D) dendritic gold nanostructures at the air/water interface with the assistance of 1-methyl-3-{(1S,2S,4S)-2-[(1,7,7-trimethylbicyclo[2.2.1]hept-2-yl)oxy]ethyl}imidazolium chloride ([camphorC2mim]Cl). [camphorC2mim]Cl can form precipitate with HAuCl4, and it is found that the precipitates are crucial to the formation of the dendritic nanostructures. The effects of the molar ratio of HAuCl4/[camphorC2mim]Cl and the concentration on the morphology are discussed. The obtained gold dendrites can be used as a substrate for Raman scattering, and they exhibited much higher surface enhancement than those of the widely used electrochemically roughed gold substrate. Experimental Part. Chemicals. Tetrachloroauric acid tetrahydrate (HAuCl4 3 4H2O, AR) was obtained from Shanghai Chemical Reagent Co. Ltd. and used without further purification.
r 2010 American Chemical Society
Published on Web 10/06/2010
Introduction. Size- and shape-controlled gold nanoparticles have received much attention during the past several decades due to their potential applications.1 Gold nanoparticles with different shapes, such as polyhedrals, irregular prismatics, cubes, ribbons, triangular nanoframes and nanoplates, nanosheets, etc., have been already successfully obtained.2 It is expected that dendritic nanostructures with rich surface nanostructures and large surface areas might exhibit superior physical and chemical characters as a result of the existence of more nanoscaled branches. Because of their special characteristics, such as their large surface area, good conductivity, and the ability to provide a natural framework for the study of disordered systems, the construction of dendritic nanostructures is of great significance.3 There have been many reports about the preparation of silver dendrites;4 however, it is interestingly that well-defined dendritic gold nanostructures have been reported only in recent years. Various methods have been employed for the synthesis of silver dendritic nanostructures. Silver dendritic nanostructures are traditionally formed on electrode surfaces by electrochemical deposition.5 The obtained silver fractal-like structures could readily be made luminescent with irradiances 3-5-fold less than silver island films.5 By using an ultraviolet irradiation photoreduction technique, Zhou et al. produced Ag dendrites in the presence of polyvinylalcohol (PVA).6 They found that the concentration of both AgNO3 and PVA has a significant effect on the formation and growth of the nanostructures. Raney nickel has been successfully used as both template and reducing agent in the synthesis of well-defined palladium and silver dendritic nanostructures with the assistance of ultrasonic waves.7 Wen et al.4b reported the synthesis of silver dendritic nanostructures using finely dispersed zinc microparticles as a reducing agent. Without using any templates or surfactants, phase-pure and well-defined silver nanodendrites can be synthesized in high yield (>85%) by this method. Then, by using zinc plate instead of microparticles, Fang et al.4c also obtained well-defined dendritic silver nanostructures in large-scale. The high resolution transmission electron microscopy (HRTEM) characterization demonstrated the transition from polycrystalline aggregates to a single crystal during silver dendritic growth. Dendritic silver crystals also have been synthesized in a simple mixed cetyltrimethylammonium bromide (CTAB)/sodium dodecyl benzyl sulfonate (SDBS) surfactant system by Fan and co-workers.4a
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[camphorC2mim]Cl was synthesized according to the literature.14 1-Dodecyl-3-methylimidazolium bromide ([C12mim]Br) was synthesized according to the literature and was described in our previous paper.15 All chemicals related to the synthesis were purchased from ACROS and used as received. All other chemicals were analytical grade and used as received. Synthesis and Characterization. HAuCl4 3 4H2O was first dissolved in deionized water to obtain a transparent solution. In a typical synthesis, an appropriate amount of [camphorC2mim]Cl was added to HAuCl4 aqueous solution and the solution became turbid immediately. The final concentrations of HAuCl4 and [camphorC2mim]Cl were kept at 1 10-3 M and 1 10-2 M, respectively, giving a HAuCl4 to [camphorC2mim]Cl molar ratio of 1:5. The suspension was kept standing for 24 h to deposit completely and then a portion (1 mL) of 0.01 M ice-cold NaBH4 aqueous solution was added carefully. After the reaction, films can be observed at the air/water interface and they were transferred onto Formvar-covered copper grids and quartz slides by the Langmuir-Schafer method for the characterization by transmission electron microscopy (TEM) (JEM-100CX II (JEOL)), high resolution TEM (HRTEM) (JEM-2100), UVvis spectroscopy (HITACHI U-4100), and surface-enhanced Raman scattering (SERS). In the experiment, the concentrations of HAuCl4 and [camphorC2mim]Cl were varied to obtain gold products with modified morphologies. UV irradiation and formaldehyde vapor were also used as the reducing agents for the controlled experiments. SERS Measurements. SERS measurements were carried out on a confocal microprobe Raman spectrometer (Jobin-Yvon HR800). Samples for SERS were prepared by drop casting 10 μL of 1 10-4 or 1 10-5 M R6G aqueous solution onto the quartz substrate. The reference sample was prepared by drop-casting 100 μL of 1 10-4 M R6G aqueous solution onto a quartz substrate and allowing the solvent to evaporate. The SERS measurements were performed with an excitation wavelength of 633 nm and power of 20 mW. Spectra were collected by focusing the laser line onto the sample using a 50 objective, providing a spatial resolution of about 1 μm. The data acquisition time was 30 s for one accumulation. In order to test the reproducibility, measurements at different positions were carried out for each sample.
Results and Discussion. Formation of Dendritic Gold Nanostructures. Although the complex formed by [camphorC2mim]þ
and AuCl4- cannot dissolve in water, films can be seen at the water surface after the addition of NaBH4. TEM observation shows that these films are composed of dendritic gold nanostructures. Figure 1 shows the typical TEM images of the obtained gold nanostructures when the concentration of HAuCl4 is kept at 1 10-3 M. When the mole ratio between HAuCl4 and [camphorC2mim]Cl (R) is 1:5, the obtained dendritic nanostructure is not a complete crystal, as shown in Figure 1a. These nanostructures are mainly well-organized nanosheets with dendritic shape or arrangement. Figure 1b shows the selected area electron diffraction (SAED) pattern obtained by focusing the electron beam on a dendritic nanostructure on the TEM grid. The diffraction rings correspond to {111}, {200}, {220}, {311}, and {331} from center to the outside, respectively. The clear bright rings formed by diffraction points reveal that the crystallinity of the dendritic nanostructures is very good. If R is decreased to 1:10, well-defined strip-like dendritic gold nanostructures can be obtained, as shown in Figure 1c,d. The length of the dendritic nanostructures can be up to several micrometers. From Figure 1d we can see that these dendritic nanostructures are formed by the fusion of some nanosheets. The width of the branches is not uniform, mainly between 5 and 15 nm. It should be pointed out that there are some precipitates at the bottom of the container after the reaction. TEM observations show that they are spherical or other shaped nanoparticles. In other words, the formation of
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Figure 1. TEM images of gold nanostructures obtained with HAuCl4 of 1 10-3 M and [camphorC2mim]Cl of 5 10-3 M (a and its SAED pattern (b), R = 1:5) and 1 10-2 M (c and d, R = 1:10).
Figure 2. TEM images of gold nanostructures obtained with HAuCl4 of 2 10-4 M and [camphorC2mim]Cl of 5 10-3 M (a and b, R = 1:25) and 1 10-2 M (c and d, R = 1:50).
dendritic nanostructures is less selective and there are also some byproducts. It was found that the amount of the byproduct can be controlled by the concentration of HAuCl4. When the concentration of HAuCl4 was reduced to 2 10-4 M, as we expected, the byproducts at the bottom are much less and dendritic gold nanostructures still can be obtained. When the concentration of [camphorC2mim]Cl is kept at 5 10-3 M to give a R value of 1:25, dendritic nanostructures also can be obtained, as shown in Figure 2a. The morphology does not show a significant difference under lower resolution. But under a higher resolution, the trunk hardly can be distinguished (Figure 2b). In other words, fractal growth is not obvious. The branches of the dendrite are connected together by fusion and a network nanostructure is obtained. But it is interesting that the width of the branches is much uniform and the average width of the branches is about 8 nm. When the concentration of [camphorC2mim]Cl is increased to 1 10-2 M and give a R value of 1:50, the obtained dendritic nanostructures looked more dense and the trunk can be distinguished clearly, as indicated in Figure 2c,d. If the concentrations of HAuCl4 and [camphorC2mim]Cl were reduced to 4 10-5 M and 2 10-3 M, respectively, keeping the R value of 1:50. Either before or after the reaction,
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Figure 3. TEM images of gold nanostructures obtained with HAuCl4 of 4 10-5 M and [camphorC2mim]Cl of 2 10-3 M (R = 1:50).
Figure 5. UV-vis-NIR spectrum of the dendritic gold nanostructures suspended on a quartz substrate.
Figure 4. HRTEM images of the obtained gold nanostructures.
there are no precipitates at the bottom of the solution. Besides, no films can be observed with naked eyes at the air/water interface. But interestingly, under the TEM characterization, dendritic nanostructures still can be observed (Figure 3), and the morphologies have no significant changes compared to Figure 2c,d. These results reveal that the morphology of the dendrites is mainly controlled by the ratio between HAuCl4 and [camphorC2mim]Cl rather than their concentrations. But the total concentration can control the amount of the byproduct. All the above results also indicate that the dendritic gold nanostructures can be obtained in a wide range of R values (from 1:5 to 1:50); only the characters of the obtained dendrite nanostructures have some difference, such as the density of the dendrite, the width of the branches, and the fusion between the branches. The obtained dendritic gold nanostructures were further characterized by HRTEM, as shown in Figure 4. Figure 4a shows a typical HRTEM image of one branch of the gold dendrites. The fringes are separated by 2.50 A˚, which can be ascribed to the (1/3){422} reflection that is generally forbidden for a face-centered cubic (fcc) lattice.16 The lattice parameter calculated from this image is 4.08 A˚, in agreement with a prior report (a = 4.079 A˚, JCPDS 04-0784). The hexagonal symmetry lattice fringes demonstrate that the branch of the dendrite is a planar single crystal with the (111) plane as the top surface. Twins can be observed at the terminal of some branches (Figure 4b), which is very common in gold nanoparticles. Besides, interface dislocations also can be observed (marked by white arrows, which will be discussed later). The optical properties of these dendritic gold nanostructures were characterized by UV-vis absorption spectroscopy. It is well-known that gold nanostructures usually display a very intense color because of the surface plasmon resonance (SPR). We see this clearly at the surface of the water after reaction and a very beautiful golden color can be observed. Figure 5 shows the absorption spectrum of the resulting gold dendrites deposited on a quartz substrate. Two sets of distinct plasmon absorption bands bordered by about 830 nm can be distinguished. Spherical Au particles show an SPR band at approximately 520 nm that is usually red-shifted to longer wavelengths with increasing particle size.17 So the band located between about 500 and 830 nm may be attributed to this band. The other band includes a sharp peak at about 920 nm and then a broad peak from about 970 nm to the near-IR region. This can be attributed to the longitudinal plasmon band, reflecting the anisotropic nature of the gold
nanostructures. The band is broad because of the polydispersity in the length and diameter of the “branches”, which may lead to a variety of sizes and aspect ratios.18 It should be pointed out that the lowest at about 825 nm is not a negative peak, but it is precisely the location of the baseline. Therefore, although the curve is very flat after 1000 nm, it is not the baseline but the absorption. Overlapping between the transverse band and the longitudinal band usually can be observed for gold nanodendrites,19 but they are separated in our case. This is indeed very interesting and should be further studied.
Formation Mechanism of the Dendritic Gold Nanostructures. When [camphorC2mim]Cl was added into the HAuCl4 solution, beside the precipitates at the bottom, some floaters can also be observed at the water surface. If these floaters were removed away from the water surface carefully, dendritic nanostructures hardly can be observed. We also use some other additives that cannot form precipitates with AuCl4-; for example, [C12mim]Br or CTAB, and no dendritic nanostructures can be observed, too. On the basis of the above results, we believe that the floater plays a key role in the formation of gold dendrites. Then the mechanism may be proposed as follows. As [camphorC2mim]-AuCl4 complex is insoluble in water and has a big hydrophobic group, some of the [camphorC2mim]-AuCl4 precipitates may float on the water surface. When NaBH4 was added, small gold nanoparticles will be formed at the air/water interface and they may act as nuclei. At the same time, the precipitates at the bottom will change into a large number of small nanoparticles, too. In the initial period of the reaction, the strong reduction environment and excessive [camphorC2mim]Cl will prevent them from growing to larger nanoparticles. Because they are very small and the reaction produces large quantities of gas, these small nanoparticles can rise to the water surface and provide raw materials for the growth of the nuclei. The growth of a dendritic nanostructure is usually explained by a diffusionlimited aggregation (DLA) model.20 It is believed that particles are released one by one from sites arbitrarily far from a central cluster, sticking irreversibly at the first contact with the growing cluster. Our system accords with this condition: the nuclei formed at the air/water interface may act as the central clusters and the raw nanoparticles are released from the bottom. Besides, since the existence of [camphorC2mim]Cl, dendritic nanostructures will be more inclined to form. Because of the steric hindrance of [camphorC2mim]Cl, the subsequently joined nanoparticles cannot be arranged closely, so large area nanocrystals (e.g., nanoplates) cannot be formed. As the DLA theory indicated, these nanoparticles will connect to the central cluster at a particular point and keep growing there. Besides, because this progress occurs at the air/water interface, 2D planar nanostructures
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Figure 6. TEM images of gold nanostructures obtained with UV irradiation (a) or formaldehyde vapor (b).
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Figure 8. (a, b) TEM images of terracelike nanostructures.
Figure 7. HRTEM images of an aggregate-attached branch: the interface dislocations.
rather than 3D nanostructures are more likely to form. Finally, 2D dendritic nanostructures can be formed. When the concentration of HAuCl4 is higher, a large number of small nanoparticles formed at the bottom in a short period and cannot migrate to the surface in time, so they grow up at the bottom of the solution and finally form a byproduct with various shapes. Therefore, a lower HAuCl4 concentration can reduce the amount of the byproduct. Besides, [camphorC2mim]Cl is excessive in all the experiments, as we mentioned above; the excessive [camphorC2mim]Cl can affect the connection of the nanoparticles on the central cluster, and thus the mole ratio between HAuCl4 and [camphorC2mim]Cl is important to the character of the dendritic nanostructures. The system was also reduced with UV irradiation or formaldehyde vapor, but no dendrites can be observed at the air/water interface and only irregular nanoparticles or various nanoplates can be obtained (Figure 6). This is because small nanoparticles reduced by UV irradiation or formaldehyde vapor cannot transfer from the bottom to the interface due to the absence of bubbles. This result also proves the speculation of the mechanism. As we mentioned in Figure 4b, interface dislocations also can be observed. In fact, this is a common phenomenon in the obtained dendritic structures (Figure 7a). The origin of these defects is likely a consequence of the growing mechanism itself. When crystals are formed by coalescence of grains at crystallographically specific surfaces, a small misorientation at the interface can result in dislocations. The white lines in Figure 7b highlight the presence of a series of edge dislocations. Dislocations formed at self-assembling interfaces are in agreement with earlier studies of “oriented attachment” on other systems.4c,21 The present results provide strong evidence that oriented attachment mechanism is a major path for the formation of the dendritic gold nanostructures. In addition, some terrace-like nanostructures also can be observed (Figure 8). The region of dark color displays no special appearance, only with higher particle concentrations (Figure 8b). This nanostructure may be formed by the following mechanism. As reported, patterned catalyst has been successfully used in the growth of horizontally aligned carbon nanotubes (CNT).22 Ordered catalysts can lead to the final regular arrays of CNT. In our system, the nuclei formed at the air/water interface can be arranged in straight lines for some external factors. The continued growth will be based on these nuclei lines, when the growth
Figure 9. Raman spectrum of solid R6G and SERS spectra of R6G on gold dendrites with different molecular concentrations.
reaches to the fore nuclei line, the growth will stop and the branches will fuse with the nuclei line. Then some lines with dark color will be formed and finally terracelike nanostructures can be observed. SERS Activity of the Dendritic Gold Nanostructures. It has been reported that gold nanostructures are excellent substrates for surface enhanced Raman scattering (SERS).23 AlvarezPuebla and co-workers found that various gold nanostructures can sustain surface plasmon resonances, making them suitable for plasmonic applications and optical enhancements such as SERS.24 The dendritic gold nanostructures described here were also used for fabricating SERS substrates. Rhodamine 6G (R6G) was chosen as the probe molecule due to its distinct spectral feature. Figure 9 shows the Raman spectrum of solid R6G and SERS spectra of R6G on gold dendrites with different mole concentrations. Specifically, vibrations assigned to C-C-C in-plane bending (614 cm-1), C-H out-plane bending (772 and 922 cm-1), C-H in-plane bending (1190 cm-1), and C-C stretching of the aromatic ring (1366, 1538, 1574, 1599, and 1650 cm-1) are enhanced greatly, as seen in other reports.25 When the concentration of R6G increases from 1 10-5 M to 1 10-4 M, the SERS intensity of these vibrations increased obviously. The surface enhancement factors (EF) were calculated using the following expression:26 EF ¼ ðISERS =Nads Þ=ðIbulk =Nbulk Þ where ISERS is the intensity of a vibrational mode in the surface enhanced spectrum, Ibulk is the intensity of the same mode in the Raman spectrum, Nads is the number of molecules adsorbed and sampled on the SERS-active substrate, and Nbulk is the number of molecules sampled in the bulk. When the concentration of R6G is 1 104 M, the EF for the band located at 614, 1365, and 1650 cm-1 can be calculated to be 2.89 105, 1.94 105, and 1.19 105. These values indicate the obtained gold dendrites can be used to fabricate SERS substrate with high activity.
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Conclusions. In summary, we develop a facile method for synthesis of 2D dendritic gold nanostructures at air/water interface with the assistance of [camphorC2mim]Cl. The sparingly solubility of [camphorC2mim]-AuCl4 complex in water and the resulting floaters formed at the water surface, as well as the ratio of [camphorC2mim]Cl to HAuCl4, are crucial to the formation of the dendritic nanostructures. The obtained gold dendrites are found to have a very strong SERS enhancement ability and can be used to fabricate SERS substrate with high activity. Acknowledgment. The authors are grateful for funding from the National Natural Science Foundation of China (No. 50972080) and the National Basic Research Program (2007CB808004 and 2009CB930101).
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