Synthesis and Characterization of Microscale Gold Nanoplates Using

Synthesis and Characterization of Microscale Gold Nanoplates Using Langmuir Monolayers of Long-Chain Ionic Liquid Xiangtao Bai, Liqiang Zheng,* Na Li, Bin Dong, and Hongguo Liu* Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3840–3846

ReceiVed May 24, 2008

ABSTRACT: Triangular, hexagonal, and truncated triangular single-crystal gold nanoplates are successfully synthesized under Langmuir monolayers of long-chain ionic liquid molecules 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) through interfacial reduction of AuCl4- by formaldehyde gas. The Au nanoparticles are characterized using transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and UV-vis spectroscopy. It is found that the size of the Au plates varies from several hundred nanometers to several micrometers, up to 20 µm, and the thickness is ca. 35 nanometers. The atomically flat planar surfaces of the Au nanoplates correspond to {111} planes and the lateral surfaces are {110} planes. The concentration of HAuCl4 aqueous solution strongly influences the formation of the Au nanoplates. The formation of the nanoplates should be attributed to the preferential adsorption of 1-hexadecyl3-methylimidazolium cations onto the {111} planes of Au nuclei and the connection of small, triangular nanoplates. Introduction Au nanoparticles have potential applications in fabricating nanoscale electronic and optical devices. They also have been used in catalysis and biochemical applications.1,2 The special chemical and physical properties of nanoparticles are closely related to their size and shape.3 For example, spherical gold nanoparticles show a surface plasmon absorption band at about 520 nm, shifting slightly to longer wavelengths with increasing particle size.4 Nanosheets offer new possibilities for both fundamental studies and technological applications.5,6 In flat gold particles of triangular shape, there will be a new band at about 860 nm due to the surface plasmon band.7 Therefore, shape control is an alternative tool to adjust optical or catalytic properties of the materials, and gold nanoplates are of particular interest because of their potential applications in the areas of electrochemistry and new nanodevices.8 Although the syntheses of spherical nanoparticles have been well developed for many kinds of materials, fabrication of an anisotropic nanocrystal is still a challenge for researchers in this field. Many shape-controlled gold nanoparticles such as rods, wires, cubes, disks, and other special morphologies have been successfully synthesized.9,10 However, few systems of large, smooth-surfaced gold nanodisks or planar nanoparticles have been observed.11-16 In addition, all of the as-prepared planar nanoparticles mentioned above are several hundred nanometers in diameter. Nanoplates with diameters larger than 1 µm are comparatively uncommon. For example, Cai17 and co-workers synthesized single-crystal gold nanoplates, with triangular, hexagonal, or truncated triangular shapes on the basis of a polyol process. These nanoplates were from several to tens of micrometers across and tens of nanometers thick. Many techniques have been exploited to prepare shape-controlled gold nanoparticles, including polyol synthesis,17 microwave-polyol synthesis,14 laser ablation,12 seed-mediated growth,18 lyotropic liquid crystal (LLC),19 hydrogel templates,20 layer-by-layer assembly (LBL),7 and the Langmuir-Blodgett technique.21 Among these methods, Langmuir-Blodgett films are the most * Corresponding author. Phone: +86-531-88366062. Fax: +86-531-88564750. E-mail: [email protected](L.Z.); [email protected](H.L.).

effective way to control the molecular orientation and packing at the molecular level. The well-defined Langmuir film medium provides an opportunity to control the size of nanoparticles. Besides, the highly ordered nature of the films can also lead to control over the shape and even orientation of the particles. Although all the above methods yield fairly monodispersed nanoparticles, their major drawback is that they are multistep processes and involve the use of a phase transfer agent, and an additional capping agent. One method that uses Langmuir monolayers to prepare metal nanoparticles in one step at the air-water interface has been reported.22 The metallic ion in the subphase is attracted by oppositely charged Langmuir monolayers through electrostatic interactions and then chemically reacts with external compounds. CdS23,24 CdSe,25 PbS,26,27 and Ag22 nanoparticles have been synthesized at the air-water interface in this way. To our knowledge, this technique is generally used to prepare metal compounds but seldom used to prepare crystal metal.22-27 Furthermore, nanoparticles prepared in this way are usually spherical. Anisotropic nanoplates prepared in this way are comparatively uncommon in the literature.26 However, we were able to use the Langmuir monolayer technique to obtain many Au nanoplates with clear edges. Ionic liquids (ILs) are a class of green solvents. ILs composed of the 1-alkyl-3-methylimidazolium cation [Cnmim]+ have been extensively studied in the field of synthesis28 and catalytic reactions.29,30 By changing the alkyl chain length or the anion, the properties can be varied over a wide range. Short-chain ILs and recently long-chain ILs have been extensively studied. The long-chain ILs resemble traditional surfactants and may be regarded as amphiphilic ILs. Our group has studied the surface activity of three kinds of long-chain ILs in aqueous solution.31 Both the adsorption efficiency (pC20) and the effectiveness of surface tension reduction (Πcmc) were rather larger than those reported for traditional ionic surfactants, and the critical micelle concentration (cmc) values were somewhat lower than those for typical cationic surfactants. And recently, the advantages of amphiphilic IL derivatives in the introduction of ordered selforganized structures have been reported. For example, longchain ILs have been used as synthetic templates in the preparation of mesoporous silica.32-34 But the use of ILs as

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Figure 1. Molecular structure of CnmimBr (n ) 16).

templates in nanoparticle synthesis has been scarcely reported, and there have been no previous reports about their ability to form Langmuir monolayers at the air-water interface. In this article, we report the properties of the film formed of [C16mim]Br and use it as a shape-control template. Through a simple Langmuir monolayer route, we obtained single crystalline Au, triangular, hexagonal, or truncated triangular in shape. In this process, HAuCl4 was reduced by formaldehyde at the air-water interface. The electron diffraction patterns reveal that these well crystallized nanoplates are all single-crystalline with a preferential growth direction along the Au {111} plane. Experimental Section Chemicals. C16mimBr (molecular structure is shown in Figure 1) was synthesized according to the literature method.31,35,36 All chemicals related to the synthesis were purchased from ACROS and used as received. In a typical synthesis, an excess of 1-hexadecylbromine (33.41 g, 0.128 mol) was mixed with 1-methylimidazole (10.26 g, 0.125 mol). The mixture was put into a 250 mL flask, refluxed at 75-80 °C for 48 h, then cooled to room temperature. Dichloromethane was removed by using a rotary evaporator under reduced pressure, leaving a white waxy solid. The product was further purified by recrystallization from ethyl acetate at least four times and dried under vacuum for 24 h. The purity of the product was ascertained by an 1HNMR spectrum in CDCl3. HAuCl4 was purchased from Shanghai Chemical Reagent Co. Ltd. and used as received. All other chemicals were analytical grade and used as received. Synthesis and Characterization of Au Nanoplates. The pressure-area (Π-A) isotherm was recorded on a NIMA 611 (Britain) rectangular trough. The subphase concentrations of HAuCl4 aqueous solutions were 10-5, 10-4, and 10-3 mol/L. After evaporating the spreading solvent for 15 min, the Langmuir monolayers were compressed at room temperature at a continuous barrier speed of ca. 10 cm/min. For the formation of Au nanoplates, an appropriate amount of HAuCl4 aqueous solution, calculated according to the Π-A isotherm, was added into a chemostat, and then an appropriate amount of C16mimBr chloroform solution was spread onto the surface of the HAuCl4 aqueous solution with a microsyringe. After the solvent was allowed to evaporate for 15 min, it formed a monolayer with a certain surface pressure. Then the chemostat was put into a desiccator (use as a reaction container) with some aqueous formaldehyde at the bottom. The system was allowed to react for one day at ambient temperature. After the reaction, the nanoparticles formed at the air-water interface were transferred onto Formvar-covered copper grids, carbon-coated copper grids, and quartz slides by the Langmuir-Schafer method for characterization with transmission electron microscopy (TEM) (JEM-100CX II (JEOL)), high resolution TEM (HRTEM) (JEM-2100), scanning electron microscopy (SEM) (JEOL JSM-7600F), and UV-vis spectroscopy (HITACHI U-4100).

Results and Discussion Π-A Isotherms. Figure 2a shows the surface pressure-area (Π-A) isotherms of C16mimBr monolayers formed on the surface of pure water and over different concentrations of HAuCl4 in aqueous solution. C16mimBr cannot form a stable monolayer on the pure water surface, possibly because of dissolution in water; but stable monolayers can be formed on the aqueous HAuCl4 solution surfaces. By extrapolation of the slopes of the solid-phase parts of the curves to zero pressure, the limiting molecular areas of C16mimBr in the monolayers are measured to be 45, 60, and 95 Å2 when the concentrations of the subphase

Figure 2. Surface pressure vs area isotherms of monolayers with different subphase concentration (a) and molecular area vs time plot (b).

are 10-5, 10-4, and 10-3 mol/L, respectively. The molecular area increases with the increase of the subphase concentration. This may be attributed to the adsorption of AuCl4- ions on the monolayers due to the electrostatic interaction between the AuCl4- anions and the C16mim+ cations. The amount of AuCl4ions adsorbed increases with increasing subphase concentration. It can also be seen that the collapse pressures of the three isotherms are all above 25 mN/m, indicating that all three monolayers are very steady and can be used as synthesis templates. This is also shown by the molecular area-time plot shown in Fgure 2b, which is obtained by compressing the monolayer to a surface pressure of 20 mN/m and maintaining it for several hours. We can clearly see that the molecular area only reduced by about 3 Å2 (from about 33 Å2 to 30 Å2) when the pressure is maintained for about 2200 s (from about 1200 to 3400 s). This may be taken as direct evidence of the stability of the monolayer. Formation of Au Nanoplates. Figure 3a shows a typical TEM image of gold nanocrystals recovered after 12 h of reaction when the subphase concentration is 10-5 mol/L. The products are mainly spherical in shape, about 50 nm in diameter, and nearly monodispersed. When the reaction time is prolonged to 24 h under the same conditions (Figure 3b), the diameter of the spheres do not increase much; only the size dispersion became more diverse. When the concentration of the subphase is increased to 10-4 mol/L, many regularly shaped nanoparticles (Figure 4a-c) are abundant in the samples. Nearly perfect microscale hexagonal, triangular, and truncated triangular nanocrystals are seen in Figure 4a-c. The beautiful and regular interference fringes on all of the nanoplates may indicate that these plates are very thin and may be bending. RodriguezGonzalez et al.37 investigated the bending contours in silver nanoprisms. They believed that these bending contours stem from slight variations in the angle formed between atomic planes of the same particular plane (hkl). Some of the fringes were star-like with well-defined crossing points. Because of Rodriguez-Gonzalez’s work, the central point of such star-like

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Figure 3. TEM images of gold nanoparticles measured 12 h (a) and 24 h (b) after preparation when the subphase concentration was 10-5 mol/L, and the surface pressure was 10 mN/m.

Figure 4. TEM images of gold nanoparticles measured when the subphase concentration was 10-4 mol/L (a-c) and HRTEM images of a final nanoplate (d). The measured distance is three times the distance of {422}. The inset shows the corresponding electron diffraction pattern of a single hexagonal Au nanoplate. The strongest spots (square) could be indexed to the allowed {220} reflection, the outer spots (circle) with the weakest intensity could be assigned to the allowed {422} reflection, and the inner spots (triangle) with a weaker intensity corresponded to the formally forbidden (1/3){422} reflection.

fringes corresponded to a zone axis of the crystal. Figure 4d shows a typical HRTEM image of a nanoplate oriented in the ξ direction. HRTEM results confirmed that all of the nanoplates have the same crystallographic structure. The fringes are separated by 2.50 Å, which can be ascribed to the (1/3){422} reflection that is generally forbidden for a face-centered cubic (fcc) lattice.17,38,39 The lattice parameter calculated from this image is 4.082 Å, in agreement with a prior report (a ) 4.079 Å, PDF No. 4-784).40 The inset shows the related SAED pattern obtained by focusing the electron beam on a nanoplate lying flat on the TEM grid. The SAED pattern reveals that a hexagonal symmetry diffraction spot pattern is generated, demonstrating that the Au nanoplate is a single crystal with a preferential growth direction along the Au {111} plane.41 Three sets of spots can be identified based on d-spacing. The set with a spacing of

1.41 Å is due to the {220} reflection of fcc Au. It indicates that the prepared nanoplates are single-crystalline with {111} lattice planes as the basal planes. The outer set with a lattice spacing of 0.81 Å can be indexed to the {422} Bragg reflection. These two sets of reflection are both allowed by an fcc lattice. The inner set with a lattice spacing of 2.41 Å is believed to originate from the forbidden 1/3{422} reflection. This forbidden reflection has also been observed in Ag or Au nanostructures in the form of thin plates or films bounded by atomically flat surfaces. According to the results of Pileni et al.,42 such 1/3{422} forbidden reflections observed on the plate-like structures of Au or Ag should be attributed to (111) stacking faults lying parallel to the (111) surface and extending across the entire nanosheet. It should be pointed out that the 1.41 Å d spacing of the {220} planes is beyond the resolution of our microscope,

Microscale Gold Nanoplates

Figure 5. SEM images of the products that were the same with Figure 4.

and hence, the {220} fringes are not observed in the HRTEM images. This assignment is consistent with a geometrical model, in which each triangular nanoplate is bound by two {111} planes as the top and bottom faces and three {100} planes as the side faces. Such features indicate the faceted morphology of the gold microplates and their inherent anisotropy, which is important since many optical and electronic properties depend on the orientation of crystal materials.43 These assignments are similar to the results previously reported for triangular nanoplates of Ag and Pd.39,44 The morphology of the products is further characterized by scanning electron microscopy (SEM). Figure 5 shows a typical SEM image of the nanoplates. In Figure 5, we can clearly see that the product is microscale in diameter and nanoscale in thickness; the thickness of the microplate is about 35 nm. Although we can see only the lateral face, it is still clear that

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the surface of the plates is very smooth, consistent with the SAED pattern mentioned above. This phenomenon is ubiquitous in our products. In order to investigate how the concentration of the subphase affects the morphology of the products, we increased the subphase concentration to 10-3 mol/L. As shown in Figure 6, the TEM images reveal that the morphology of the products does not change anymore. It only seems that these nanoplates are a little thicker and much bigger. The diameters of the plates (diagonal length for hexagonal and side length for triangular) reach the micrometer scale. When the surface pressure is 10 mN/m, as shown in Figure 6a-b, the diameters of the microplate are about 4-5 µm after 12 h (Figure 6a) and 7-8 µm after 36 h (Figure 6b). The size increases slightly with a further increase of the reaction time, but not very obviously. Instead, those plates become somewhat more uniform in size. We also studied the effect of surface pressure on the morphology of the products. When the surface pressure is 20 mN/m, the diameters of the microplates are also about 4-5 µm after 12 h of reaction, similar to that of Figure 6a. When the reaction time is 24 h (Figure 6c), very large nanoplates with a diameter of about 20 µm can be found, but it is rare. Such large Au nanoplates are very rare in published papers. The SAED reveals that all of these microplates are single crystal with high crystallinity, which is similar to that in Figure 4d. By summing up these results above, we can conclude that the concentration of the subphase strongly influences the formation of the Au nanostructures. When the concentration is low, isotropic nanoparticles are mainly formed. With increasing reaction time, these nanoparticles do not exhibit any changes in morphology. The average diameter of the particles does not increase, but some of the individual particles may grow larger. When the concentration reaches a certain level (for our experiments at about 10-4 mol/L), some anisotropic nanoplates

Figure 6. TEM images of gold nanoparticles prepared when the subphase concentration was 10-3 mol/L, and the surface pressure was 10 mN/m (a-b) and 20 mN/m (c).

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Figure 7. UV/vis spectrum of the Au nanoparticles suspended on a quartz substrate.

with hexagonal, triangular, and truncated triangular shapes appear. But if the concentration is further increased, no changes in morphology emerge. In this case, the diameters of the nanoplates increased gradually with reaction time, but not greatly. In fact, we obtained some very large nanoplates about 20 µm in diameter. It is well known that Au nanostructures usually display a very intense color because of the surface plasmon resonance (SPR). We see this clearly at the surface of the subphase after reaction. The products display a very beautiful golden color. It has been reported that a solution of spherical colloidal Au is red,45,46 but interestingly, our dispersoid is bright yellow in color when using ethanol as a dispersant. This also demonstrates that the SPR band of Au nanoparticles strongly depends on the size and shape of the nanoparticles.4 Figure 7 shows the absorption spectrum of the resulting Au particles deposited on a quartz substrate. For anisotropic metallic nanoparticles, it is well known that two or more plasmon resonances,47 the transverse and longitudinal bands, are expected. The spectrum of our product shows two distinct plasmon absorption bands around 630 and 990 nm. Spherical Au particles show an SPR band at approximately 520

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nm that is usually red-shifted to longer wavelengths with increasing particle size.7 The one at about 630 nm may be attributed to this band. The band around 990 nm can be attributed to the longitudinal plasmon band, reflecting the anisotropic nature of the gold nanoplates. The band is broad because of the relatively high polydispersity, both in size and shape, which implies quite a broad range of possible resonance frequencies.48 An asymmetrical shape may result from the existence of smaller gold plates in the colloidal system.49 This result is consistent with the TEM images. Dong and coworkers41 synthesized micrometer-scale single-crystalline Au plates of nanometer thickness by a wet-chemical route. The UV/ vis spectrum shows that there are two SPR bands located at about 680 and 925 nm, which arise from the longitudinal plasmon resonance of Au particles. They believe that this provides evidence for the formation of anisotropic Au particles, which is consistent with our results. El-Sayed et al.3,4 reported that the absorption band at 520 nm was the transverse mode of the surface plasmon absorption, and the longitudinal mode absorbing around 800 nm was attributed to the rod-like nanoparticles. Formation Mechanism of Au Nanoparticles. In our experiment, the IL, C16mimBr, serves as both template and stabilizer for the products. AuCl4- ions are reduced according to the following equation:

3HCHO + 4AuCl4- + 3H2O f 4Au0 + 3CO2 + 12H+ + 16Cl- (1) Au atoms are obtained by reducing AuCl4- ions in the Langmuir monolayer with formaldehyde during the reaction. When the quantity of Au atoms reaches supersaturation, they will begin to nucleate and grow into gold nanoclusters in the monolayer. Though the exact role of the Langmuir monolayer is still not very clear, here we suggest that the IL, C16mimBr, in the

Figure 8. Some typical TEM images of crystal growth by connection between two nanoplates (a) and HRTEM evidence (b, see arrows); c shows a schematic illustration for the formation of large nanoplates by connecting small triangular nanoplates of the same size.

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monolayer preferentially adsorbs on the sites of the {111} planes of Au nuclei or that in some other way it greatly decreases the surface energy of the {111} plane, which leads to preferential growth along the 〈111〉 directions. Moreover, as a result of the special environment of the monolayer and the inherent nature of gold crystals, the formation of triangular or hexagonal Au nanoplates is favored. However, many flat triangular and hexagonal nanoplates are large, but thin and with uniform thickness are observed. It is unbelievable that such morphologies come from conventional nucleation and growth. On this basis, we believe that the formation mechanism of the gold nanoplate is as described below. Because small nanoplates have relatively high surface energy, when a certain quantity of small nanoplates is produced, some small nanoplates will connect together along the {110} lateral planes. This leads to the formation of very large, but thin, triangular or hexagonal nanoplates or microplates; for example, see Figure 8. Figure 8a shows a typical connection between two triangular and truncated triangular plates at the initial stage of the reaction. A bend contour that runs through the two connected plates can be seen,and as described above, this bend contour may stem from slight variations in the bending of the flat plane; therefore, the interference fringes show that the connection between the two nanoplates is very compact. This is consistent with the HRTEM evidence as shown in Figure 8b (see arrows). From this image, we can clearly see the junction between two sets of lattice fringes of the {111} planes. At the boundary shown by arrow 1, the lattice fringes are matching very well, showing perfect lattice fringes of the {111} planes. In fact, we can hardly distinguish the two plates that connected together; only some differences of the contrast gradient can be seen. This may be because of the difference of the thickness between the two plates. However, at the boundary shown by arrow 2, we can obviously see the defects of the lattice fringes. The angle between the two fringes as shown in the image is about 55°, smaller than that of the {111} planes (60°). This may be because of the growth defects when the two plates are connected together. This is a very favorable evidence for our connection mechanism; it indicates that the proposed mechanism is probably correct. If this defect does not exist, there will be a perfect Au nanoplate. Figure 8c shows a schematic illustration of this possible mechanism. Such connections should be thermodynamically more stable and hence will occur spontaneously during the reaction. In addition, such a growth mechanism has also been illustrated by Cai et al. and Mirkin et al.,17,49 although direct proof was not given. Pileni et al.50,51 have studied how the self-ordered silver nanocrystals grow into triangular single crystals. They show that nanocrystals selfassembled at long-range in 2D and in 3D may grow to planar nanoparticles. The crystallinity of these nanoplates is rather good. They indicate that smaller nanoparticles can connect into larger nanoplates of single crystals. This is consistent with our conclusion. Conclusions In summary, our study has demonstrated that IL, C16mimBr, can form stable Langmuir monolayers on the HAuCl4 aqueous solution surfaces. These monolayers can be used as soft templates to prepare Au nanoplates. Gold particles are formed under the films of positively charged C16mim+ cations transferred from the subphase of aqueous HAuCl4. TEM and SEM measurements show that the shape of the gold crystals is

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influenced by the subphase concentration to some extent. HRTEM reveals that the planar surfaces of the Au nanoplates correspond to {111} planes and that the lateral surfaces are {110} planes. UV/vis spectra also show that the products are plate-like and anisotropic. The predominantly plate-like particles are mainly nearly perfect hexagonal, triangular, and truncated triangular nanocrystals, of micrometer scale in diameter. We also obtained a few very large naonoplates about 20 µm in diameter. Acknowledgment. We are grateful to the National Natural Science Foundation of China (No. 50472069 and 20773081) and National Basic Research Program (2007CB808004).

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