Synthesis of Silver Nanoplates by Two-Dimensional Oriented

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Synthesis of Silver Nanoplates by Two-Dimensional Oriented Attachment Zhun Liu,† Hu Zhou,† Young S. Lim,‡ Jung-Hoon Song,§ Longhai Piao,*,† and Sang-Ho Kim*,† †

Department of Chemistry, Kongju National University, Chungnam 314-701, Korea Green Ceramics Division, Korea Institute of Ceramic Engineering and Technology, 233-5 Gasan-dong, Geumcheon-gu, Seoul 153-801, Korea § Department of Physics, Kongju National University, Chungnam 314-701, Korea ‡

S Supporting Information *

ABSTRACT: Synthesis of silver nanoplates was studied in the modified polyol method, where the nucleation and seed stage occurred in a poly(ethylene glycol) (PEG)−water mixture solution, and the growth stage happened in the PEG environment. The morphological evolution of nanoplates was characterized using UV, SEM, and TEM. Interestingly, plane nanostructures with unusual jagged edges were finally formed in our modified polyol method. Using TEM, we observed the medium state of fusion between two nanoplates, resulting in generating unusual jagged edges. Therefore, a novel twodimensional oriented attachment occurred in our modified polyol method, which involves smaller nanoplates as the building blocks. Further control experiments showed that the presence of water could break this kinetic preferred reactivity, leading to the formation of nanoparticles.

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INTRODUCTION Controlled growth and assembly leading to anisotropic nanostructures inspired widespread interests in the field of nanomaterials synthesis,1−3 since the properties of nanoparticles change with their sizes or shapes. In the past, solution-based methods have been widely investigated for controlled growth of nanoparticles, where various capping agents were often required.3−5 A new approachcontrolled assembly of nanoparticlesis now one of the most attractive techniques, which involves an oriented attachment mechanism.2,6,7 Compared with the classical atom/molecularmediated crystallization, the oriented attachment method uses nanoparticles with common crystallographic orientations as the building blocks to form more complex nanostructures such as arrow-, zigzag-, and tetrapod-shaped nanocrystals.8 This method affords solution processability potential applications in low-cost integrated systems.9 Ag nanoplates have received intensive attention due to their potential applications in many fields from molecular detection10 to various engineering applications,11,12 such as optoelectronics and sensing. At present, many methods are available for generating two-dimensional (2D) Ag nanostructures with different sizes and shapes;13 however, the solution-phase chemical reduction remains a basic strategy due to low cost and high quality. Such methods and modified versions thereof usually involved the use of citrate as a capping agent because it can selectively bind to {111} facets, thus maintaining a platelike shape,5,14 or the use of weak reducing agents (hydrazine,15 N,N-dimethylformamide (DMF),16 and polyacrylamide17) to achieve kinetic control for generating a planar structure. For © 2012 American Chemical Society

these methods, generation of Ag nanoplates was generally thought to be a result of atom-mediated crystallization. In this study we report a novel 2D oriented attachment growth of Ag nanoplates in the modified polyol method. The polyol method was often used to synthesize spherical Ag nanoparticles in the past;18 where higher molecular weight PVP (Mw = 55 000 g/mol) and higher reduction temperature were applied. Our modified version used lower molecular weight PVP (Mw = 10 000 g/mol) and lower reduction temperature (108 °C). What is more, we changed the nucleation and growth procedurethe nucleation and seed stage occurred in a PEG− water mixture solutionand the 2D oriented attachment growth stage occurred in the PEG environment through evaporation of water. These changes therefore led to the formation of anisotropic planar structures. Interestingly, those plane nanostructures with unusual jagged edges were observed. Further investigations showed that the formation of unusual jagged edges was the result of 2D oriented attachment between smaller nanoplates. Moreover, our work revealed that this 2D oriented attachment process was driven by the kinetically preferred reactivity at the edges of nanoplates.

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EXPERIMENTAL SECTION

Materials. Silver nitrate (AgNO3, 99.9%), poly(vinylpyrrolidone) (PVP, Mw = 10 000), poly(ethylene glycol) (PEG, Mw = 400), ethylene glycol (99.8%), ethanol, and acetone. Silver nitrate was from Kojima Chemicals Co., and other reagents were purchased from Received: December 14, 2011 Revised: May 30, 2012 Published: May 30, 2012 9244

dx.doi.org/10.1021/la301721y | Langmuir 2012, 28, 9244−9249

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Figure 1. Ag nanoplates with the unusual jagged borders, characterized by SEM (a), TEM (b), XRD (c), and UV−vis spectra (d).

Figure 2. Morphological evolution of Ag nanoplates during the reaction process was characterized by optical images (b), UV−vis spectra (c), and SEM (d). The schematic illustration is shown in panel a. In panels b and c, a drop of reaction solution was diluted in ethanol. In panel d, the reaction solutions were diluted in the mixture solvent and centrifuged to obtain SEM samples; all the scale bars are 1 μm. the AgNO3 aqueous solution (preheating at 80 °C) was injected to the vigorously stirred hot PEG solution. After injection, the reacting system was quickly heated up to 98 °C at 10 °C/min. Two hours later, evaporation of water was almost complete, and then the temperature was increased up to 108 °C. To study the morphological evolution, 1 mL of the solution was removed from the reaction system after different reaction times. The samples taken at different times were collected by centrifugation and washed three times with the mixture

Sigma-Aldrich Co. All chemicals were used as received without any further purification. Aqueous AgNO3 solution was prepared with ultrapure water, which was produced using an Ultrapure Water System. Synthesis of Silver Nanoplates. In a typical procedure, PVP (0.80 g) was dissolved in PEG-400 (5.0 mL) in a three-neck bottle. This process was facilitated by magnetic stirring and heating at 80 °C. AgNO3 (0.80 g) was dissolved in distilled water (5.0 mL), and then 9245

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solvent (ethanol and acetone by 2:1 vol/vol) to remove excess PVP and PEG. Then the final precipitates were dispersed in ethanol by ultrasonication. Characterization. The UV−vis absorption spectra were obtained on a Scinco-S4100 spectrophotometer. All peaks are normalized at maximum of the main plasmon band. TEM images and electron diffraction patterns were captured on a JEOL 3000F transmission electron microscope operated at 300 kV. SEM images were taken on a S-4800 field emission scanning electron microscope (Hitachi, Tokyo, Japan) operated at an accelerating voltage of 10 kV. XRD data were recorded on a Rigaku DMAX 2000 X-ray diffractometer with Cu Kα radiation (λ = 1.541 78 Å). XRD samples were prepared by drying the dispersion of nanoplates on a piece of glass.

gradually red-shifted to the near-infrared region, indicating an increase of nanoplate size. The SEM images of the as-synthesized Ag nanoplates at different reaction times are depicted in Figure 2d. At t = 4 h, various triangle, hexagonal, and irregularly polygonal nanoplates were formed, having an average edge length of 100 nm. As shown in the highlights, the isosceles trapezoid shows a bottom edge of 200 nm and a top edge of 100 nm, suggesting the fusion of three triangles of edge length 100 nm. At t = 8 h, Ag nanoplates became polydisperse. Larger irregular nanoplates show an edge length >100 nm. On further reaction, the smaller nanoplates nearly disappeared, and the larger ones markedly increased both in size and in quantity. At t = 24 h, we noted that all the edges are nearly smooth and straight, suggesting the occurrence of Ostwald ripening2 in the edges. Interestingly, the thickness of nanoplates is nearly invariable after 4 h, ∼25 nm (estimated from the titled nanoplates in the SEM images). Figure 3 shows TEM images of products obtained at different stages. Figure 3a shows a typical silver seed with clear

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RESULTS AND DISCUSSION As shown in SEM and TEM images (Figure 1a,b), the Ag nanoplates obtained at 8 h from the typical synthetic process exhibit the unusual jagged borders, which are different from the previously reported Ag nanoplates of triangular, hexagonal, or disk shape.15,19 These jagged borders suggest the occurrence of 2D oriented attachment growth between the edge surfaces of adjacent nanoplates. Note that the corner angles are usually very sharp, and some of them are acute angles, showing that these jagged morphologies would originate from truncation and fusion of triangular modules. Furthermore, the bend contour in TEM images (marked by arrow) indicates that the Ag nanoplate is single crystalline. In addition, the Ag nanoplates synthesized here usually have an edge length of 300−1000 nm. The XRD pattern (Figure 1c) is dominated by the (111) peak of silver, indicative of its {111} faces parallel to the substrate. The typical UV−vis spectra (Figure 1d) exhibits one distinct peak located at 340 nm and a broad absorption band around 690 nm, which in coincidence with previous studies20 were attributed to the out-of-plane quadrupole and in-plane dipole excitation, respectively. Those results from XRD and UV−vis spectra consistently indicate the planar structure of the silver nanoplates. To study the morphological evolution, 1 mL of the solution was removed from the reaction system after different reaction times and was characterized by UV−vis absorption spectroscopy, SEM, and TEM. First, we noticed a fascinating change of color when these solutions were diluted in ethanol (Figure 2b). Such a change thus suggested the red shift of their plasmon band with increasing reaction time. It is known that surface plasmon properties of noble metal nanoparticles are extremely sensitive to their size and shape.1,21 Therefore, the change of color in the solutions suggested the morphological evolution of the products with the reaction time. Figure 2c shows UV−vis absorption spectra of the products at different reaction times. At the reaction time t = 1 h, the solution gave a single narrow peak centered at 405 nm, suggesting the presence of very small Ag seeds.22 Two hours after commencing reaction, this peak shifted to 420 nm accompanied by a weak shoulder at 340 nm. The occurrence of the weak shoulder indicates that these seeds became anisotropic in the case of nanoplates growth process.23 At t = 4 h, four obvious peaks appeared at 340, 420, 490, and 660 nm. This shows that a change of shape occurred for the products spherical seeds were transformed to nanoplates. The plasmon resonance line width is much higher than indicated by theoretical calculations, mainly attributable to the ensemble of the different sizes and geometry of nanoplates.20 On further incubating the reaction solution, the in-plane dipole resonance

Figure 3. TEM images of Ag seeds (a) and Ag nanoplates (b−d). In panel a, the fringes of seeds with a separation of 2.4, 2.0, and 1.4 Å corresponded to the {111}, {200}, and {220} planes, respectively. In panels b and c, oriented attachment occurred between two adjacent nanoplates. In panel d, high-resolution images corresponded to spot A in (c).

crystalline lattice fringes. The fringe spacing of 2.4, 2.0, and 1.4 Å corresponded to the {111}, {200}, and {220} planes, respectively.24 We also observed the {111} twin planes in the seed, which might imply the kinetic control of growth for the seeds. Many investigations have suggested that the generation of {111} twin planes and stacking faults in seeds were key factors governing plate growth.24−26 Note that the spherical seeds were obtained from the reaction solution at t = 1 h. The size of seeds was estimated to be 6 nm, suggesting slow growth in the seed stage. Figure 3b,c exhibits oriented attachment between two adjacent nanoplates. The initial stage in Figure 3b was shown by the coherent crystal fringes occurred in two adjacent nanoplates, and the medium stage in Figure 3c was demonstrated by the presence of partial fusion between them. The inserted electron diffraction pattern in Figure 3c was recorded on these two adjacent nanoplates. They were shown in the state of single crystals, which was a manifestation of oriented attachment. Figure 3d shows high-resolution TEM images of the fusion region (from spot A in Figure 3c). The 9246

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nanoparticles were obtained (Figure 4b). Owing to the higher viscosity of the longer polyols chain, the diffusion and the growth process slowed down significantly at relatively low temperature.4 In stage III (ca. >4 h), the oriented attachment growth became a dominant way to form more complex plane nanostructures, as confirmed by TEM (Figure 3b−d). Oriented attachment has been widely demonstrated in the synthesis of complex nanomaterials.2,7,9,29,30 This mechanism uses smaller nanoblocks to build a larger and more complex nanostructure, which is different from the classical atom/molecular-mediated crystallization. This directed-assembly process is generally thought to be driven by preferred crystal face reactivity2 or dipole moments in the nanocrystals.31 For example, Yin and Alivisatos8 pointed out that oriented attachment often results from sequential elimination of a high-energy facet, forming complex nanostructures such as arrow or zigzag rods. Here, the edges of Ag nanoplates were highly thermodynamic unstable26 and thus possibly were easily fused. Recent molecular dynamics simulations32 showed that the edge−edge attachment had a much higher probability than the face−face attachment for nanoplates due to the lower energy barrier at the small side face. People did not fully understand oriented attachment-based growth kinetics until now, since the microscopic growth process was often not easy to observe directly.7 Simply, oriented attachment growth often involves two reaction steps: diffusion and coalescence of nanoblocks. The motion of nanoblocks in a fluid is due to Brownian motion and fluid convection, which is much slower than the diffusion of molecules.33 For that reason, the oriented attachment growth and the Ostwald ripening occur simultaneously in the diffusioncontrolled crystal growth. We clearly observed the Ostwald ripening in the late period of stage III, which smoothed all the rough edges of nanoplates and eliminated the interface gap between the nanoplates. In this case, once two or more nanoplates effectively collided to form a complex; nanoplates rotation and desorption of surface species (PVP) were required for further bonding in a proper orientation. Nanoplates rotation was believed to be the result of the reduction of high-energy faces, which led to a perfectly matching orientation. This process has been investigated extensively both theoretically and experimentally during the orientation of aggregate particles.6,7,33 To understand the effect of solvents, synthesis was carried out under refluxing of water. As a result, only nanoparticles were obtained (Figure 4c); water seems to break their coalescence as a surface pollute. A recent investigation by Xia et al.34 on Pt and Au nanoparticles found that oriented attachment growth and recrystallization occur in the less polar solvents but do not happen in the strongly polar water solution. They suggested that a higher energy barrier exist in water which strongly solvates negatively charged particles by hydrogen bonding and prevents the particles from approaching one another close enough to adhere and fuse together. Thus, the PEG environment was required for this oriented attachment process. In additional study on the effect of light, the control experiments were done in the dark environment. The oriented attachment also occurred in a dark environment (Figure 4d), and thus fusion was insensitive to light. This was different from previous report31 that the fusion of nanoprisms occurs in an edge-selective manner using the plasmon excitations. Photoexcitation of metal yields ballistic “hot holes” that oxidize the

fringes with a separation of 2.5 Å correspond to the 1/3 {422} reflection, which is generally forbidden for a face-centered-cubic (fcc) lattice.25 Meanwhile, the {220} fringes separated by 1.4 Å were observed along the perpendicular direction. The wellresolved interference fringe patterns confirm the single crystalline structure at this spot. This is supported by the FFT image directly obtained from the combined single nanoplate (inset of Figure 3d). We will now focus on the possible growth mechanism of Ag nanoplates using this novel method. As demonstrated above, three main morphological stages occurred during the reaction process. Such a morphological evolution is illustrated in Figure 2a. In stage I, the twined spherical seeds are formed (ca.