Controlling the Growth and Assembly of Silver Nanoprisms**

Transcription

1 DOI: /adfm Controlling the Growth and Assembly of Silver Nanoprisms** By Jianhui Zhang,* Huaiyong Liu, Peng Zhan, Zhenlin Wang, and Naiben Ming A novel water/polyvinylpyrrolidone (PVP)/n-pentanol ternary system is successfully developed to realize the controlled growth and assembly of silver nanoprisms using a one-step synthesis for the first time. The highly shape-selective growth and assembly of nanoprisms is based on the unique water/pvp/n-pentanol interface. It is seen that the cooperative assembly of nanoprisms with PVP greatly affects the size-dependent nanoprism plasmon bands. The plasmon bands of the assemblies are closer to those of bulk silver than to those of nanoprisms. The method is simple and versatile, and is successfully extended to prepare Au and Pd nanoprisms, lamellar ZnO particles, and layered ZnO assemblies with high purity. The method may provide powerful technology for shape-selective synthesis and assembly of lamellar nanoparticles with novel structures and functions in nanotechnology. 1. Introduction Controlling the size, shape, and assembly of inorganic nanoparticles is of both fundamental and technological interest because it provides effective strategies for tuning the electronic, magnetic, optical, optoelectronic, and catalytic properties of materials. [1] Like organic molecules, size- and shape-controlled nanoparticles can be taken as building blocks for constructing an extended solid-state structure with predesigned properties. [2] Silver nanoplates have attracted great interest because of their unique optical properties and applications in photonics, optical sensing, and biological labeling. [3 23] Four kinds of shape-selective synthesis strategies, including photoinduced conversion, [5 11] thermal transformation, [12 15] micellar-solution methods, [16 21] and solvent-induced shape-evolution methods, [22] have been reported to prepare Ag nanoplates. Recently, the micellar-solution method modified by a centrifugation process [23] has been reported to improve the size selection of Ag nanodisks. Until now, all the reported methods are limited for the preparation of Ag nanoplates, and only a few of them [6,8,11,16,20,21,23] can continuously modify the Ag nanoplate size over a wide range. Here a novel method based on the unique water/polyvinylpyrrolidone (PVP)/n-pentanol interface is reported. Our method can not only be used to synthesize triangular Ag nanoplates (nanoprisms) with controlled edge length in a wide range of nm, but it can also be used to assemble Ag nanoprisms into nanoprisms PVP composites with remarkable shapes, including butterfly-like, quasi-cubic, and cubic morphologies by a one-step process. Furthermore, our method has been successfully extended to prepare Au and Pd nanoprisms, lamellar ZnO particles and layered ZnO assemblies. As a surfactant, PVP can be used to form a uniform transparent solution from the immiscible water and n-pentanol phases in the water/n-pentanol volume ratio range As we know, in the water/n-pentanol system, PVP tends to exist completely in the water phase. [24] So in the water/pvp/n-pentanol system, the water-phase region near the n-pentanol interface should be enriched in PVP. Recently, the existence of a layered structure in the water solution of high PVP concentration has been confirmed. [25] The proposed layered structure consists of alternating layers of water and PVP with bound water. Thus it can be deduced that PVP enables the immiscible water and n-pentanol phases to form a stable solution by bounding water in the (water/water-pvp) n /n-pentanol (WWPN) interface region. Motivated by the successful synthesis of Ag nanoplates in the lamellar bilayer phase region of the octylamine water binary system, [26] we have successfully explored the unique layered WWPN interface to realize the shape-selective growth of Ag nanoprisms by using PVP as the main reductant. In an aqueous PVP solution containing impurities or an initiator, the crosslinking reaction of PVP can be induced by extended heating. [24] This phenomenon has been simultaneously exploited to prepare nanoprisms PVP composite assemblies. [*] Prof. J. H. Zhang, H. Y. Liu, P. Zhan, Prof. Z. L. Wang, Prof. N. B. Ming National Laboratory of Solid State Microstructures Department of Physics, Nanjing University Nanjing, (P.R. China) zhangjh@nju.edu.cn [**] J.H.Z acknowledges financial support from project of the NSFC, and H. Dong, X. N. Zhao, and A.Q. Xu for their help with the SEM and TEM measurements. 2. Results and Discussion 2.1. Synthesis of Ag Nanoprisms In brief, a solution of AgNO 3 in ethanol was added to the water/pvp/n-pentanol ternary solution in a sealed conical flask under stirring. Then the reaction solution was heated in a constant-temperature oven for 48 h at 95 C and the Ag nanoprisms were obtained. Figure 1 shows six transmission electron WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17,

2 Figure 2. XRD patterns of the samples shown in Figure 1d (a) and f (b). Figure 1. TEM images of Ag nanoprisms with average edge lengths of a) 24 ± 4 nm, b) 36 ± 5 nm, c) 52 ± 5 nm, d) 72 ± 6 nm, e) 92 ± 9 nm, and f) 120 ± 11 nm prepared by using 0.2 g, 0.4 g, 0.5 g, 0.8 g, 1.1 g, and 1.5 g of PVP, respectively. microscopy (TEM) images of typical Ag nanoparticles prepared using different amounts of PVP. As seen in the figure, the dominant products were nanoprisms. When 0.2 g of PVP was used, most of the nanoprisms had an average edge length of 24 ± 4 nm and thickness (evaluated from the self-assembled column or stack of the particles) of 10 ± 1 nm and they were generally truncated nanoprisms with round tips, even some nanodisks rather than nanoprisms were observed. The edge length of the nanoprisms could be readily tuned continuously to 120 nm by simply increasing the PVP amount. As seen in Figure 1b f, nanoprisms of average edge lengths of 36 ± 5 nm, 52 ± 5 nm, 72 ± 6 nm, 92 ± 9 nm, and 120 ± 11 nm were obtained by using 0.4 g, 0.5 g, 0.8 g, 1.1 g, and 1.5 g of PVP, respectively. In all cases, 200 particles were measured to get the average size. Although the edge length increases with increasing PVP amount, the thickness of the nanoprisms essentially does not change. However, the size distribution of the nanoprisms became broad, and the snips (a snip being the part removed from a tip of the nanoprisms) became more obvious and diverse. As seen in Figure 1e and f, the differences in shape and size of the snips can be found not only between the nanoprisms but also among the three tips of an individual nanoprism. It should be noted that these extremely deformed nanoprisms were omitted in evaluating the average edge length of the nanoprisms. Figure 2 shows the X-ray diffraction (XRD) patterns for the typical samples shown in Figure 1d and f. Only the diffraction peaks arising from the (111) and (222) lattice planes of facecentered cubic (fcc) silver (JCPDS 4 783) were observed in the XRD patterns. This clearly demonstrates that the silver particles bound by the (111) planes are the main constituent of the composite particles. High-resolution TEM (HRTEM) was performed on an individual nanoprism of the sample shown in Figure 1f to further investigate the crystallographic structure of Figure 3. a) HRTEM image of the edge of a nanoprism in the sample shown in Figure 1f. Inset: corresponding SAED pattern. As seen in the inset, the strong spots (square) could be indexed to the allowed {220} reflection, and the inner spots (circle) with a weak intensity corresponding to the formally forbidden (1/3){422} reflection. b) HRTEM image of a nanoprism that is on its side. the nanoprisms. Figure 3a shows a HRTEM image, as well as the corresponding selected-area electron diffraction (SAED) pattern obtained from a single Ag nanoprism lying flat on the support film, with the electron beam perpendicular to the triangular facets. The fringes are separated by 2.5 Å, which can be ascribed to the (1/3){422} reflection that is generally forbidden for an fcc lattice. [11,12,20] In this image it can be also seen that near the edge, the crystalline structure presents almost no defects and the roughness of the edges is basically at an atomic scale. The sixfold rotational symmetry displayed by the diffraction spots implies that the triangular faces are representing the {111} planes. Two sets of spots can be identified based on the d-spacing: the set with a spacing of 1.4 Å results from the {220} reflection of fcc Ag. The inner set with a spacing of 2.5 Å is believed to originate from the forbidden (1/3){422} reflection. [5 7,11 14,16,18 20] The HRTEM image shown in Figure 3b is a side view of one nanoprism near its tip. The regularity and single-crystalline structure is obvious, with constant interplane distances. As in a recent report, [11] the tip is round rather than angle-shaped such as is proposed in theoretical calculations, [4 6,27] which may slightly influence the ultimate optical response. Adv. Funct. Mater. 2007, 17, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Figure 4. a) Extinction spectra (shifted vertically for clarity) for the ethanol solutions of different-sized nanoprisms (1 8 edge lengths: 24 ± 4 nm, 36 ± 5 nm, 52 ± 5 nm, 64 ± 6 nm, 72 ± 6 nm, 92 ± 9 nm, 105 ± 10 nm, and 120 ± 11 nm). b) The in-plane dipole resonance absorption peak as a function of the nanoprism edge lengths and PVP amount. The formation of triangular silver nanoplates was also reflected by typical extinction spectra. Figure 4a shows the extinction spectra for ethanol solutions of nanoprisms with different average edge lengths. Every spectral line has three distinctive peaks, a long wavelength large peak, a weaker broad peak, and a small but sharp peak. According to the theoretical calculations, [4 6,23,27,28] these peaks can be assigned to the inplane dipolar, in-plane quadrupolar, and out-of-plane quadrupolar plasmon-resonance bands, respectively. As usual, the quadrupolar in-plane and dipolar out-of-plane resonances can not clearly be distinguished. [3 23,27] Here the ill-defined quadrupolar in-plane resonance band is too weak for a quantitative discussion. The out-of-plane quadrupolar plasmon-resonance band is located around 333 nm and is basically constant and independent of the nanoprism size. This wavelength value is slightly different from recent theoretical calculations [27] (338 nm) and experimental results [11] (330 nm). This can be attributed to the difference in surrounding mediums used between our experiment (ethanol) and the previous reports (isooctane [27] or D 2 O [11] ) because the resonance bands of Ag nanoplates red-shift with increasing refractive index (n) of the surrounding medium. [4,11,13,27] As in previous reports, [6,8,11,16,20,21,23,27] the in-plane dipole plasmon-resonance band is very sensitive to the variations in nanoprism size. As the nanoprism edge length increases from 24 nm to 120 nm, the dipole resonance-band position shifts from 475 nm to 891 nm. As shown in Figure 4b, there is a quasilinear correlation of the nanoprism edge length and the position of the inplane dipole plasmon band. Similar results have been reported by Jin et al. [6] and Bastys et al. [11] Compared with their results, [6,11] the dipole in-plane resonance position of the nanoprisms produced here has a medium wavelength value. For example, for nanoprisms with average edge length of 120 nm and thickness of 10 nm, the positions of the dipole in-plane resonance band are 856 nm, 891 nm, and 1076 nm reported by our group, Jin et al., [6] and Bastys et al., [11] respectively. Obviously, using ethanol (n = 1.359) rather than water (n = 1.33) as the surrounding medium is the main reason for us to obtain the higher resonance position than Jin et al. The even higher resonance position obtained by Bastys et al. may arise from the fact that the nanoprisms prepared by them have smaller average snip values than those synthesized by us. It has been shown that the dipole in-plane resonance band decreases in wavelength with increasing snip values. [4 6,8,12,13,23,27] We also note that the dipole in-plane plasmon-resonance band of our nanoprisms broadens with increasing nanoprism-edge length. This phenomenon can mainly be explained by the following two reasons. 1) The increase of the size distribution. 2) The enhancement of the differences in shape and value of the snips between the nanoprisms and among the three tips of an individual nanoprism. The size-dependent in-plane plasmon band can also easily be tuned by changing the PVP amount. As shown in Figure 4b, with increasing PVP amount, the in-plane dipole plasmon-resonance peak initially shifts to a higher wavelength, reaches a maximum value at ca. 1.5 g of PVP, after which the peak moves to a lower wavelength again. All the experiments were repeated three times or more, and similar results for all experiments were obtained. However, the actual wavelength value of the in-plane dipole plasmon peak for the repeated processes fluctuated in the experimental range of ±(x/5 75) nm. Here x is the wavelength value of the in-plane dipole plasmon peak Assembly of Ag Nanoprisms The assembly of Ag nanoprisms was realized in the same way as the actual synthesis of Ag nanoprisms by prolonging the heating time to around 60 h or above. Figure 5 shows the scanning electron microscopy (SEM) and TEM images of assemblies with different sizes and shapes. As shown in Figure 5a and b, the assemblies made using 1.7 g of PVP possessed a beautiful regular-shaped, butterfly-like morphology. The sideview image reveals that these composite particles consisted of eight symmetrical corners in space. With varying amounts of PVP and water, the eight symmetrical corners changed from cuspate to round, and merged to one cube eventually. Correspondingly, butterfly-like, quasi-cubic-like, and cubic composite particles with different sizes (Fig. 5c k) were obtained depending on the amounts of PVP and water used. The surface of the composite assemblies was sensitive to the amount of PVP. The samples shown in Figure 5f and g are of almost identical size but they have different surfaces, one is smooth, and the other is rough. In order to further investigate the inner structures of these particles, they were further characterized by TEM. As shown in Figure 5h, the assemblies shown in Figure 5f had a uniform amorphous PVP-coating layer on the surface. Some typical nanoprisms around the core implied that the core was composed of a lot of nanoprisms. To be able to have a clear look at the core structure, a strong electron beam was used to focus on the core. Unfortunately, the assembly de WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17,

4 Figure 5. SEM and TEM images of nanoprisms-pvp assemblies with different sizes and shapes. a d) Butterfly-like particles: a,b) 1.2 ± 0.1 lm, 1.7 g of PVP, 3.9 ml of water, (b) is the magnified image of the sample shown in (a). Inset: corresponding side view image. c) 0.6 ± 0.05 lm, 6 g of PVP, 1.5 ml of water, d) 0.43 ± 0.04 lm, 3 g of PVP, 1.5 ml of water. e) Quasi-cubic particles (0.89 ± 0.07 lm, 1.1 g of PVP, 3.9 ml of water.). f k) Cubic particles: f) 0.5 ± 0.04 lm, 4.5 g of PVP, 3.9 ml of water, g) 0.6 ± 0.06 lm, 0.6 g of PVP, 3.9 ml of water. h,j) Corresponding high-magnification TEM images obtained under weak electron beam of the samples shown in (f) and (g), respectively. The inset of (j) shows the corresponding SAED pattern. i) High-magnification TEM image obtained under strong electron beam of the sample shown in (f). k) 0.3 ± 0.02 lm, 0.6 g of PVP, 1.5 ml of water. formed quickly, but more nanoprisms in the core became discernable (see Fig. 5i), which confirmed that the assembly was composed of nanoprisms and PVP. As shown in Figure 5g and j, both the SEM and TEM images clearly indicate that the rough assemblies shown in Figure 5g have a rough surface composed of different facets of nanoprisms, in other words, the nanoprisms in the assemblies are randomly oriented. The polycrystalline-like diffraction spots in the SAED pattern (inset of Fig. 5j), obtained by aligning the electron beam perpendicular to one facet of an individual assembly, also suggest that there are a lot of nanoprisms in each assembly. As seen in Figure 5j, some of the nanoprisms on the surface of the rough assembly flaked off under sonication. The amorphous PVP protective layer around some of the freestanding nanoprisms was discernable, suggesting that the rough assemblies were also composed of PVP and nanoprisms. XRD patterns (see Fig. 6) were recorded for the samples shown in Figure 5f and g to analyze their crystalline structures. Like the XRD patterns shown in Figure 2 for the pure Ag nanoprisms, particularly strong (111) and (222) diffraction peaks were observed for both the assemblies, suggesting that the Ag nanoprisms form the main constituent of the composite assemblies. However, the XRD spectrum for the sample in Figure 5f shows only two peaks whereas the one for the sample in Figure 5g shows many peaks. This suggests that the nanoprisms in the assemblies in Figure 5g are well oriented in the same direction, and those in the assemblies in Figure 5f are randomly oriented. This may be the reason why the surface of the cubes in Figure 5f appears more regular than that of the cubes in Figure 5g. The extinction spectra of the assemblies shown in Figure 5f and g and their corresponding nanoprisms before being assembled were also recorded. As shown in Figure 7, in both the assemblies, except for a new band around 240 nm which may arise from PVP, [24] only the weak, wide, and ambiguous bands around the distinctive extinction bands of their corresponding nanoprisms were observed. In other words, the assemblies show a characteristic surface plasmon resonance (SPR) similar to that of bulk silver. The variations in the SPR might arise from the interprism interactions caused by the cooperative assembly of nanoprisms with PVP Mechanism of the Synthesis and Assembly of Ag Nanoprisms In order to study the mechanism of the synthesis and assembly of Ag nanoprisms, time-dependent extinction spectra and TEM images of a series of reaction solutions were investigated to monitor the growth and assembly processes of the Ag nanoprisms. We chose the reaction solution containing 1.7 g of PVP as an example to demonstrate the reaction process. After 5 min of stirring, the solution turned from colorless to yellow, and one band corresponding to the dipole resonance of silver nanospheres appeared around 415 nm (Fig. 8a), and the presence of silver nanospheres was confirmed by TEM. As the heating proceeded, this band decreased in intensity but remained essentially unchanged in wavelength, and the other characteristic bands of triangular silver nanoplates gradually appeared and increased in intensity, the solution turned from Figure 6. XRD patterns of the assemblies shown in Figure 5f (a) and g (b). Adv. Funct. Mater. 2007, 17, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 Figure 7. a) Extinction spectra (shifted vertically for clarity) for the ethanol solutions of the assemblies (1) shown in Figure 5f and the corresponding Ag nanoprisms (2) before being assembled. b) Extinction spectra for the ethanol solutions of the assemblies (1) shown in Figure 5g and the corresponding Ag nanoprisms (2) before being assembled. Figure 8. a) Time evolution of extinction spectra for the reaction solution containing 1.7 g of PVP. It should be noted that the spectrum after 60 h is for the ethanol solution of the product after 60 h, and not for the reaction solution after 60 h. b d) TEM images for the typical product extracted from the reaction solution of (a) after b) 24 h, c) 52 h, and d) 60 h of heating. The inset of (d) shows the corresponding high-magnification image of one particle shown in (d). The arrows in (b) and (d) mark the amorphous shapeless lumps. yellow via red to green, showing the growth of nanoprisms. After 24 h of heating, the typical Ag nanoprisms were formed, and tiny assemblies were observed. Figure 8b shows the TEM image for the typical product after 24 h of heating. After 48 h of heating, the solution turned blue, the initial band disappeared completely, and the extinction spectra were characterized by the peaks of pure triangular silver nanoplates. By increasing the heating time to 52 h, the solution remained blue, but the in-plane dipole plasmon-resonance peak blue-shifted slightly and broadened, and the in-plane quadrupolar plasmonresonance peak became undetectable. The corresponding product formed in this period of time was extracted and characterized by TEM. As seen in Figure 8c, most of the nanoprisms were deformed and marked by a variety of snips. Some particles even looked more like nanodisks rather than nanoprisms. In general, when the heating time was prolonged to 60 h, the solution changed from blue to colorless in half an hour, and the products deposited at the bottom of the vessel. All distinctive extinction bands of triangular silver nanoplates disappeared. The corresponding TEM image (Fig. 8d) shows that the product was dominated by butterfly-like assemblies. The extinction spectrum for the butterfly-like assemblies, like the assemblies shown in Figure 5g and f, shows only the weak, wide, and ambiguous bands around the distinctive extinction bands of their corresponding nanoprisms, except for a new band around 230 nm corresponding to PVP. When repeating the experiment time after time, we found that assemblies began to form during the growth process of nanoprisms, but the assembly speed was very slow. Around 60 h of heating, the formation of the assemblies suddenly became very fast and finished in half an hour in general. As shown in a previous report, [24] the crosslinking polymerization of PVP can be initiated by extended heating in the presence of impurities or initiators. Here we think the cooperative assembly of PVP and nanoprisms is induced by the crosslinking polymerization of PVP, and happens on a large scale after long-time heating. The Ag nanoprisms may play the role of the initiator for the crosslinking polymerization of PVP. It is worth noting that the amorphous shapeless lumps marked by arrows in Figure 8b and d may be the product arising from the crosslinking polymerization of PVP. Based on the above discussions and the WWPN interface, a model is proposed to explain the growth and assembly of the Ag nanoprisms. As shown in Scheme 1, the WWPN interface is created at first, and then the ethanol solution of AgNO 3 is added into the n-pentanol phase. As the reaction solution is heated, Ag+ ions diffuse into the WWPN region, and are reduced to Ag nuclei. PVP selectively adsorbs on Ag nuclei through coordination bonding of the O and N atoms of the pyrrolidone ring [1f,24,29] of PVP on the specific crystallographic planes of Ag. With further diffusion of Ag + ions, the reduction of Ag + ions happens on the specific crystallographic planes of the Ag nuclei without PVP or with less adsorption of PVP, and the Ag nuclei anisotropically grow into nanoprisms. As in a previous report, [13] the growth of Ag nanoprisms here may be associated with a melting-like process happening on the initial WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17,

6 Scheme 1. Schematic growth and assembly procedure of the Ag nanoprisms in the WWPN interface region. ly formed Ag nuclei. During the seeding growth process of the nanoprisms, the cooperative assembly of nanoprisms with PVP, induced by the crosslinking polymerization of PVP initiated by extended heating, [24] begins to take place very slowly. Prolonging the heating time deforms the nanoprism shape and marks partial nanoprisms with a variety of snips such as missing tips. In a previous report, [13] an enhanced deformation of nanoprisms was also seen after prolonged heating. We may, therefore, assume that the enhanced deformation of the nanoprisms seen here can also be associated with the durative melting-like process caused by continuous heating after the completion of the nanoprism growth. The actual assembly of the nanoprisms after extended heating (around 60 h) is fast (generally the assembly is finished within half an hour). To confirm the reaction mechanism, a series of experiments were carried out to prove that the growth and assembly of the nanoprisms happens in the WWPN region, and is controlled by an interaction between the Ag nanoparticle surfaces and PVP. [1f,24,29] First, it was found that modifying the WWPN interface by varying the amount of water or PVP or stirring the reaction system greatly affected the growth and assembly of nanoprisms. 3.9 ml of water was found to be optimal for preparing nanoprisms. A large deviation from 3.9 ml results in incomplete seeding growth. Only spherical silver nanoparticles and irregular Ag PVP composite assemblies were obtained without water. By fixing the amount of water at 3.9 ml, and the PVP amount at ca. 1.5 g, the nanoprisms reached their maximum size, and their corresponding assemblies were butterfly-like particles with maximum size (ca. 1.3 lm). Varying the amount of PVP resulted in smaller nanoprisms, and their corresponding assemblies were also reduced in size and changed in shape from butterfly-like via quasi-cubic to cubic. The disturbing of the WWPN interface by stirring at regular intervals greatly enlarged the average edge length of the nanoprisms to 320 nm, but the obtained nanoprisms tended to be polydisperse and possessed broad extinction bands (Fig. 9a,c). Stirring during the whole synthesis process almost destroyed the nanoprism growth, and irregular silver nanoplates with broad extinction bands were the main product (Fig. 9b,d). These extinction bands are broadened mainly by the large size (Fig. 9c) and shape (Fig. 9d) distribution of the Ag nanoplates. As shown in Figure 9c, the absorption of ethanol solvent also broadens the Figure 9. TEM images of the samples prepared from a reaction solution containing 1.5 g of PVP, 3.9 ml of water, and 3.0 ml of ethanol solution of AgNO 3 (0.6 wt %) after 6 days of heating by stirring at a) regular intervals (the stirring time and interval are 5 min and 24 h, respectively, or b) throughout the whole reaction process. The inset of (a) shows the corresponding SAED pattern. c,d) Corresponding extinction spectra for the samples in (a) and (b), respectively. The extinction spectrum of the ethanol solvent is also shown in (c). Adv. Funct. Mater. 2007, 17, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

7 extinction bands. Furthermore, the assembly of the nanoprisms with PVP was largely delayed, and the final composite assemblies were dominated by shapeless particles. Second, it is well known that the citrate anion has a greater coordination power than that of PVP. We carried out experiments using an aqueous solution of citric acid instead of distilled water during the synthesis so that the interaction between the Ag nanoparticle surfaces and PVP was replaced with that between the nanoparticle surfaces and citrate anions. This greatly reduced the nanoprism population in the final product, and dramatically changed the shape and size of the corresponding assemblies. As shown in Figure 10a,b dumbbell-like mesoporous Ag PVP composite particles were produced by introducing citric acid. All the composite particles were polycrystalline in nature, which could be confirmed by SAED, and they were covered with uniform mesopores. By changing the experimental conditions, mesoporous pod-like and nest-like composite particles were also prepared (Fig. 10c and d). As for the solid composite assemblies, all the characteristic plasmon bands of Ag were ambiguous in the spectra for the mesoporous composite assemblies. J. H. Zhang et al./controlling the Growth and Assembly of Silver Nanoprisms Figure 10. SEM images of the mesoporous Ag-PVP composite particles synthesized using an aqueous solution of citric acid. a,b) Dumbbell-like particles, 2.1 g of PVP, 2.2 ml of aqueous solution of C 6 H 6 O 7 H 2 O (0.19 M), (b) is the magnified image of the sample shown in (a). c) Pod-like particles, 3.0 g of PVP, 3.9 ml of aqueous solution of C 6 H 6 O 7 H 2 O (0.19 M). d) Nest-like particles, 6.0 g of PVP, 3.9 ml of aqueous solution of C 6 H 6 O 7 H 2 O (0.19 M). The insets of (a) and (d) show the corresponding SAED pattern and a magnified image, respectively Extension of the Method to the Synthesis of Au and Pd Nanoprisms, Lamellar ZnO Particles, and Layered ZnO Assemblies There are three advantages to our method. 1) The operation is simple, and the experimental conditions are mild. 2) The shape selectivity is high. 3) The layered WWPN interface created here is versatile, and suitable for the growth and assembly of other lamellar materials. As shown in Figure 11a and b, triangular Au and Pd nanoplates with high purity could also successfully be synthesized. The corresponding SAED patterns and XRD patterns (Fig. 11f) confirm the synthesis of fcc Au (JCPDS 4 784) and Pd (JCPDS ) nanoplates bounded by triangular (111) facets. Our method can also be extended to prepare lamellar inorganic oxides and their corresponding assemblies. Hexagonal lamellar ZnO plates could be grown as shown in Figure 11c. The hexagonal symmetry of the diffraction spots in the corresponding SAED pattern (inset) obtained by aligning the electron beam perpendicular to the hexagonal facets of an individual plate suggests that each ZnO plate is a single crystal bounded by hexagonal (001) facets. By reasonably modifying the experimental parameters, layered ZnO assemblies with double-layer (Fig. 11d) and multiple-layer (Fig. 11e) structures could also be Figure 11. SEM and TEM images of a) Au and b) Pd nanoprisms, c) hexagonal ZnO plates, and d,e) layered assemblies of ZnO plates. The insets of (a d) are the corresponding SAED patterns. f) XRD patterns for the samples shown in a (1), b (2), d (3), and e (4) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17,

8 prepared. Their XRD patterns (Fig. 11f) show that the growth of all the as-prepared layered ZnO assemblies occurs preferably along the (002) orientation. 3. Conclusions In summary, controlling the growth and assembly of silver nanoprisms has been realized by using the unique layered WWPN interface. The plasmon-resonance properties of the nanoprisms produced here are size dependent, and can be changed greatly by the cooperative assembly of nanoprisms with PVP. Characteristic SPR similar to that of bulk silver has been found for the assemblies. The excellent advantages of our method in shape selectivity, self-assembly, simple manipulation, and high-purity and large-quantity yields make it a powerful technology to explore the synthesis and assembly of other lamellar nanostructures with new physicochemical properties and applications. The present experiments have shown that our method can be extended to prepare Au and Pd nanoprisms, lamellar ZnO particles, and layered ZnO assemblies with double-layer and multiple-layer structures in high purity and large quantities. 4. Experimental 4.1. Materials Silver nitrate (AgNO 3, AR, 99.8 %), gold chloride (HAuCl 4 H 2 O, AR, %), polyvinylpyrrolidone (PVP, number-average molecular weight M n = g mol 1 ), anhydrous ethanol (AR, 99.7 %), zinc nitrate (Zn(NO 3 ) 2 6 H 2 O, AR, 99 %), and sodium hydroxide (NaOH, AR, 99 %) were used as received. TEM images were taken with a JEM-200CX (JEOL, Japan) or Technai F20 (Philips, The Netherlands) transmission electron microscope. Sample colloids were placed on carbon-coated copper grid. SEM was performed on a LEO1530 VP (or XL30, Philips, Netherlands) scanning electron microscope. Extinction spectra were recorded on a U-3410 spectrophotometer (Hitachi, Tokyo, Japan). Sample colloids were dispersed in ethanol for the measurements. Quartz cells with about 1 mm path length were used in the experiment. X-ray powder diffraction (XRD) patterns were recorded using an X-ray diffractometer (Rigaku D/max-RA, Japan) with Cu Ka radiation (k = Å) Synthesis of Ag Nanoprisms and Nanoprism-PVP Composites In a typical synthesis, the amount of PVP was dissolved in 60 ml of n-pentanol in a conical flask. To avoid the evaporation of the reaction solution, the flask was sealed with a ground stopper during the whole reaction process except when adding the reaction reagents. To this solution, 3.9 ml of distilled water was added with stirring for 30 min at room temperature. 3 ml of an ethanol solution of AgNO 3 (0.6 wt %) was added next, with stirring for another 5 min. Then the reaction solution in the sealed flask was put in a constant-temperature oven and heated at 95 C for 48 h. The reaction product was washed with ethanol three times by centrifugation and ultrasonication, and the Ag nanoprisms were obtained. The nanoprism PVP assemblies were prepared in the same way as the Ag nanoprisms themselves by only prolonging the heating time to 60 h or longer Synthesis of Au and Pd Nanoplates Au and Pd nanoplates were prepared in a similar way as the Ag nanoprisms by changing the PVP amount to 4.5 g for Au, or 1.5 g for Pd, and replacing the ethanol solution of AgNO 3 with 1.5 ml ethanol solution of M HAuCl 4 4 H 2 O for Au, or 3 ml ethanol solution of saturated PdCl 2 for Pd Synthesis of Lamellar ZnO Plates and Layered ZnO Assemblies 0.8 ml of an aqueous solution of NaOH (0.15 M) was added to 20 ml of n-pentanol containing 1.0 g of PVP in a conical flask under stirring at room temperature. To avoid the evaporation of the reaction solution, the flask was sealed with a ground stopper during the whole reaction process except for adding the reaction reagents. After 30 min, 0.6 ml of ethanol solution of Zn(NO 3 ) 2 6 H 2 O (0.17 M) was added with stirring for another 30 min. 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