Shape Evolution of Flag Types of Silver ... - ACS Publications

Oct 19, 2010 - Center of Advanced Instrumental Analysis, Kyushu University, Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Cryst. Growth Des. , 2010, 10 ...
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DOI: 10.1021/cg101041m

Shape Evolution of Flag Types of Silver Nanostructures from Nanorod Seeds in PVP-Assisted DMF Solution

2010, Vol. 10 5238–5243

Masaharu Tsuji,*,†,‡ Xinling Tang,‡ Mika Matsunaga,† Yoshinori Maeda,‡ and Midori Watanabe§ †

Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580, Japan, Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan, and §Center of Advanced Instrumental Analysis, Kyushu University, Motooka, Nishi-ku, Fukuoka 819-0395, Japan

‡

Received August 8, 2010; Revised Manuscript Received September 29, 2010

ABSTRACT: Shape evolution of novel flag types of silver nanostructures has been studied using a two-step reduction method. In the first step, pentagonal Ag nanorods were prepared by polyol reduction of AgNO3 in ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP). In the second step, AgNO3 was reduced in N,N-dimethylformamide (DMF) in the presence of Ag nanorod-seeds and PVP. Then, trapezoid and triangular platelike structures having {111} facets were evolved from long side {100} facets of Ag nanorods. At higher AgNO3 concentrations, tetrahedral flags having {111} facets were evolved from equilateral triangular plates. The crystal structures of flag types of Ag nanostructures are determined, and their growth mechanisms are discussed on the basis of TEM, SEM, SAED, and TEM-EDS data. It was found that tetrahedral flags were grown either through the formation of two equilateral triangular plates connected on a nanorod or through the crystal growth of triangular pyramidal structures over triangular plates. The present results demonstrate that Ag nanorods can be used as new seeds for the unique flag types of Ag nanostructures.

Introduction Recently, control of metallic nanostructures has been the focus of intensive research because of their shape-dependent chemical and physical properties.1,2 Among them, silver nanostructures have attracted considerable attention, mainly as a result of their remarkable optical properties and numerous applications in fields such as catalysis, surface plasmonics, surface enhanced Raman scattering, and chemical and biological sensing.1,3-5 Different chemical and physical properties of metallic crystals arise from different shapes and surface orientation. Therefore, the controllable preparation of nanocrystals with different shapes and exposed surfaces is very important and challenging. It is known that favorable facets of Ag nanostructures produced from reduction of AgNO3 in ethylene glycol (EG) and N,N-dimethylformamide (DMF) are different.1,3-10 Therefore cubes, right-triangular bipyramids, and pentagonal rods and wires having {100} facets are formed preferentially in EG, whereas triangular and hexagonal plates, decahedra, and icosahedra having {111} facets are dominantly formed in DMF. We have recently studied shape conversion from Ag cubes and right-triangular bipyramids having {100} facets to octahedra and triangular single-twin plates having {111} facets using a two-step process with EG and DMF as the reductant and solvent.8 After Ag cube and bipyramid seeds were prepared in EG, shape and size controlled syntheses of octahedra and triangular plates were possible in DMF. An advantage of this two-step method is that shape controlled syntheses of Ag nanocrystals having {111} facets is possible. In the present study, we applied the above two-step method to prepare new Ag nanostructures. When Ag nanorods are *To whom correspondence should be addressed. E-mail: tsuji@cm. kyushu-u.ac.jp. pubs.acs.org/crystal

Published on Web 10/19/2010

used as seeds, new flag types of Ag nanostructures can be produced. The shapes of Ag flags can be controlled by changing the concentration of AgNO3 in the second step. The growth mechanism of each structure is discussed on the basis of transmission electron microscopic (TEM), scanning electron microscopic (SEM), selected area electron diffraction (SAED), and TEM-energy dispersed X-ray spectroscopic (EDS) data. Experimental Section Preparation of Ag Nanorod Seeds. In our preliminary experiments using mixtures of short Ag nanorods and long Ag nanowires, new flag type Ag nanostructures are produced from Ag nanorods and little shape changes are observed for long Ag nanowires. Therefore, we decided to use Ag nanorods as new seeds. Although the preparation method of long Ag nanowires has been well established,1,3-10 no method for the preparation of short Ag nanorods has been known. Thus, at first we attempted to find a new technique for the preparation of Ag nanorods in high yield, which is necessary for the preparation of monodispersed Ag nanostructures in the second step. In the first step, Ag nanorods were prepared as seeds using oilbath heating in EG. Thirty millimolar AgNO3 and 265 mM polyvinylpyrrolidone (PVP: Mw = 40,000) were dissolved in 19.4 mL of EG solution, and the mixture was heated from room temperature to 140 °C, and 10 mM NaCl (0.6 mL) was injected drop-by-drop into the solution at a flow rate of 0.6 mL/min. The roles of NaCl for the preparation of anisotropic Ag nanostructures such as rods and cubes have been discussed in our previous papers.5f The final concentrations of AgNO3, NaCl, and PVP were, respectively, 29.1, 0.30, and 257 mM. Then the solution was heated to 160 °C and held at this temperature for 10 min. After naturally cooling down to room temperature, the Ag seeds were separated and obtained from an EG/C2H5OH solution by centrifuging the colloidal solution at 12,000 rpm for 10 min and 10,000 rpm for 10 min three times. Small amounts of Ag cubes and bipyramids involved in the solution of seeds were removed by centrifugal separation. r 2010 American Chemical Society

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Figure 1. TEM images of Ag rod seeds and flag types of Ag nanostructures prepared in DMF at various [AgNO3]2/[AgNO3]1 ratios. Preparation of Flag Type Ag Nanostructures. As the second step, 1 mL of a DMF solution of Ag seed involving 0.015 mmol of Ag was added to a DMF solution (15 mL) containing 2-400 mM of AgNO3 and 100 mM of PVP (Mw = 40,000). Hereafter, [AgNO3]1 and [AgNO3]2 stand for the AgNO3 concentration in the first and second steps, respectively. Then the solution was heated in an oil bath at 140 °C for 2 h. The gas atmosphere and the sampling process significantly influence the morphology of Ag nanoparticles in the second step. Until the heating ends, caps of a three necked flask must be closed. If the reagent solution was exposed to fresh air for sampling the product solution below 2 h, the yield of tetrahedral flag decreases greatly, probably due to effects of a small amount of O2 dissolved in solution. Product solutions were centrifuged at 10,000 rpm for 20 min. The precipitates were collected and then redispersed in deionized water. For TEM (JEM-2000XS and 2100F; JEOL) and SEM (SU-8000; Hitachi) observations, samples were prepared by dropping colloidal solutions of the products on carbon-coated Cu or Au grids. Absorption spectra of the product solutions were measured in the ultraviolet (UV)-visible (Vis)-nearinfra (NIR) region using a Shimadzu UV-3600 spectrometer.

Results and Discussion TEM and SEM Images of Flag Type Ag Nanostructures. Figure 1a shows TEM images of Ag seeds prepared in the first step, where monodispersed Ag nanorods having average lengths of 1040 ( 230 nm and average diameters of 22 ( 4.1 nm are obtained in high yield. When AgNO3 was added to Ag nanorod seeds, significant shape evolution was observed as shown in Figure 1b-f. At low [AgNO3]2/[AgNO3]1 molar ratio of 4-150, trapezoid platelike flags are grown up from side position of Ag nanorods. Above the [AgNO3]2/[AgNO3]1 ratio of 150, the growth rate of flags increases more rapidly. With increasing the [AgNO3]2/[AgNO3]1 ratio from 150 to 300, trapezium flags are grown up to equilateral triangular flags. Further increasing the [AgNO3]2/[AgNO3]1 ratio from 300 to 400, triangular plates change to pyramidal structures. The average edge sizes of flags were not constant in the [AgNO3]2/[AgNO3]1 ratio range of 4-400. At the [AgNO3]2/[AgNO3]1 ratios of 4 and 20, plates start to form from all side edges of Ag nanorods. On the other hand, there are some parts in Ag nanorods where no flag type structures appear from the side facets in the [AgNO3]2/[AgNO3]1 ratio range 150-400. The average edge lengths of trapezoid, triangular, and pyramidal flags in Figure 1d, e, and f were 796 ( 151, 829 ( 173, and 393 ( 69 nm, respectively.

Figure 2. Typical SEM images of flag types of Ag nanostructures prepared at [AgNO3]2/[AgNO3]1 ratios of 150 and 400.

To examine product structures in more detail, SEM, TEM-SAED, and TEM-EDS data of various flag structures are observed. Figure 2 shows typical SEM images of trapezoid, triangular, and triangular-pyramidal products obtained at [AgNO3]2/[AgNO3]1 molar ratios of 150 and 400 (see also other SEM images in Supporting Information, Figure S1). It is clear from SEM images that trapezoid and triangular flags are platelike structures. When SAED patterns of triangular plates are measured, besides hexagonal spots corresponding to the {220} reflections of the

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Figure 3. (a) TEM image of a triangular platelike flag; (b) its SAED pattern for the red circle in part a; (c) TEM image of a trapezoid particle; and (d) its high-resolution TEM image for the red circle in part c.

Figure 4. (a) TEM, (b) TEM-EDS, and (c) line analysis data of flag-type Ag nanostructures.

face-centered-cubic (fcc) Ag, weak hexagonal spots corresponding to forbidden 1/3{422} diffraction are observed (Figure 3a and b). These results indicate that electron beams are perpendicular to the {111} facets of Ag triangular plates and a twin plane exists in the center of the plane.6,11 The existence of twin planes in triangular plates and tetrahedral flags can also be confirmed from contrast changes of facets in SEM images due to the presence of twin boundaries (dotted red lines in Figure S2 in Supporting Information). Figure 3c

and 3d shows high-resolution-TEM images of a typical platelike flag. The appearance of interference fringe patterns on the flat facets attests to the single crystallinity of the silver plate, which is consistent with the observed spotlike SAED patterns (Figure 3b). The fringes with a spacing of 0.25 nm could be assigned to 3  {422} lattice spacing of the fcc silver crystal.11-13 TEM-EDS and its line analysis data obtained at an [AgNO3]2/[AgNO3]1 molar ratio of 400 indicated that both platelike and pyramidal flag structures consisted of

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pure Ag atoms (Figure 4 and Figure S3 in Supporting Information). The Au component observed in Figure S3b arises from Au grids used for the TEM-EDS observation. Crystal Structures and Growth Mechanisms of Flag Type Ag Nanostructures. On the basis of SEM, TEM, and TEMEDS data, it seems that tetrahedral nanocrystals are formed from pentagonal Ag nanorods via triangular plates. It is wellknown that Ag pentagonal rods arise from multiple decahedral particles so that five twin planes exist in nanorods ((a-c) in Figure 5).1,3,5,6 In order to obtain information about the crystal structure of triangular pyramidal flags, we calculated the length and angle of tetrahedral Ag nanostructures formed from these two equilateral triangular planes with an internal angle of 72° using the cosines laws ((e) in Figure 5 and Figure S4 in Supporting Information). It was found that the edge length of new triangles is larger than that of equilateral triangles by a factor of only 1.8% and an internal angle of new isosceles triangles is 61°. This implies

Figure 5. Crystal structures of decahedron, nanorod, and trapezoid and triangular platelike and tetrahedral flags. The length and angle of the tetrahedral flag (ABCD) from two equilateral triangular plates (ABC and ADC) are shown in part e.

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that the resulting two new triangles are very close to equilateral triangles. There is some uncertainty in the internal angle of pentagonal nanorods due to the crystal defect of decahedral particles (Figure 5a).1,3 It is therefore reasonable to assume that a nearly regular tetrahedron can be formed from two neighboring equilateral triangular planes with an internal angle of 72°. It is known that there are two cases for crystal structures of triangular plates in fcc crystals.14 They are formed through hexagonal particles (Figure S5 in Supporting Information). If {111} growth is preferentially inhibited, triangular plates (green triangle) are formed; a similar condition is met when {100} growth is preferentially inhibited (blue triangle). Triangular plates bounded by {111} facets are grown to a tetrahedral structure having {111} facets (case a in Figure S5), whereas those bounded by {100} facets are grown to a pyramidal structure having {100} facets (case b in Figure S5). Only the former case leading to tetrahedral flags was observed under the present experimental conditions. The driving force of the present shape evolution is the difference in favorable facets in EG and DMF owing to selective adsorption of PVP on {100} and {111} facets, respectively. Ag nanorods with {100} facets are formed preferentially in EG, whereas favorable facets of Ag nanostructures in DMF are {111} facets. Thus, twin Ag triangular plates surrounded by two wide top and bottom {111} facets and four narrow side {111} facets are formed (Figure 5d). After that, tetrahedral flags having four {111} facets are grown at higher [AgNO3]2/ [AgNO3]1 molar ratios. On the basis of SEM data, we can deduce the growth mechanisms of tetrahedral flags from pentagonal nanorods (Scheme 1). Initially one twin plate is formed from a side edge of an Ag nanorod and a regular triangular twin plate is formed via a trapezoid plate. There are two routes for the shape evolution of tetrahedral flags from regular triangular flags. One route (route 1a in Scheme 1) is the formation of two triangular plates from side facets of pentagonal Ag nanorods and then crystal growth from one plate to the other one occurs to bury the internal space of two triangular plates (red circles in Figure S6 in Supporting Information).

Scheme 1. Growth Mechanisms of Flag-Types of Ag Nanostructures from Ag Nanorods in DMF

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Figure 6. UV-Vis-NIR spectra of Ag seeds and flag-type Ag nanostructures obtained at various [AgNO3]2/[AgNO3]1 ratios.

The other route (route 1(b) in Scheme 1) is the crystal growth of triangular pyramidal structures over triangular plates leading to tetrahedral flags (blue circles in Figure S6). The branching ratios of routes 1(a):1(b) were about 50:50 under the present experimental conditions, as shown in Figure S6. At a high [AgNO3]2/[AgNO3]1 ratio of 400, a small amount of flags with twin tetrahedra are obtained (Figure 2f ). On the basis of SEM data, there are two growth routes for the formation of this structure from a single tetrahedral flag. One route is the formation of a second triangular plate (route 1(c) in Scheme 1 and green circles in Figure S6), and the other one is formation of a triangular plate from a different side which is not connected with nanorods (route 1(d) in Scheme 1 and yellow circles in Figure S6). From these intermediate structures, twin tetrahedral flags are grown. In general, when a new facet of Ag crystal starts to form from its seeded structures having well-defined facets, Ag0 atoms are deposited on their intermediate structures until unstable facets involved in the intermediates completely disappear, as reported for the stepwise growth of decahedral and icosahedral particles.9 The formation of decahedron and icosahedron proceeds through one-by-one stepwise growth of tetrahedral units, except for the final stage where the simultaneous growth of multiple facets was observed for the formation of complete decahedron and icosahedron. Simultaneous crystal growth of two or more facets gives less stable intermediates than those arising from crystal growth of one facet. Therefore, crystals give intermediates where only one unstable truncated facet is present. Under the present conditions, one side twin plain in pentagonal Ag nanorods becomes active in the initial stage and the second twin plane starts to grow after one plane is formed completely. This mechanism also holds for the formation of a tetrahedral flag. After the formation of a triangular platelike flag, the formation of a tetrahedral flag from a platelike flag occurs via routes 1(a) and 1(b). No direct crystal growth from rods to tetrahedral flag without the formation of triangular platelike flag occurs. This means that crystal growth along one direction is generally favorable so that the crystal growth to the second direction takes place after that to the first direction is completed. It is known that decahedra consist of five tetrahedral units and pentagonal nanorods are produced through decahedra as seeds (Figure 5a-c).1,3-6 It is generally difficult to prepare one and two tetrahedral Ag nanostructures because the formation of decahedra and pentagonal nanorods from tetrahedral particles is too fast to detect tetrahedral

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parrticles.1,3,4 Thus, the present technique is a new promising method for the preparation of large sizes of Ag tetrahedra on pentagonal Ag nanorods. UV-Vis-NIR Spectra of Flag Type Ag Nanostructures. The extinction spectra of various shapes and sizes of Ag nanoparticles have been observed, and the observed surface plasmon resonance (SPR) bands have been assigned on the basis of theoretical calculations.1,3,5,10,15-19 The extinction spectra of Ag seeds and flag type products were measured in the UV-Vis-NIR region (Figure 6). The SPR band of Ag seeds consists of a main peak at ca. 385 nm with a weak shoulder peak at ca. 350 nm. The main SPR peaks at 385 and the subpeak at 350 nm are ascribed to the out-of-plane dipole and out-of-plane quadrupole resonances of the {100} side walls of the Ag nanowires, respectively.16 When long Ag nanowires are produced, a long tail band is observed above ca. 450 nm.5,10,16 This tail band can be ascribed to the overlapping of the in-plane quadrupole and dipole resonance modes of the Ag nanowires.16 It is known that silver plates give the main SPR bands above ca. 500 nm, and they shift to red and become broad with increasing edge lengths.15,17,18 The red shifts of the major SPR peak from 385 to 410 nm and an increase in the intensity of the SPR bands above ca. 450 nm with increasing the [AgNO3]2/[AgNO3]1 ratio from 4 to 150 can be explained by the formation of trapezoid and triangular platelike structures. Further increasing the [AgNO3]2/[AgNO3]1 ratio to 300, shapes of flags change from platelike flags to tetrahedral flags. The SPR band of silver tetrahedron with an edge length of ca. 150 nm has been observed in the 300-800 nm region with a peak at ca. 670 nm.19 The edge length of tetrahedral flags was 300-500 nm, which is much larger than that of the reported tetrahedra. It seems therefore that the SPR peak further shifts to red for tetrahedral flags with larger sizes of tetrahedral crystals. Actually, the longerwavelength component above 400 nm becomes strong and spectra give rather flat extinction in the 400-1200 nm region with a broad peak at ca. 850 nm, when tetrahedral flags are formed at an [AgNO3]2/[AgNO3]1 ratio of 400. In the present study, we found that platelike and tetrahedral flag types of Ag nanostructures give broader and flatter extinction in the Vis-NIR region than those of known Ag nanostructures.1,3-5,8-10,15-19 Therefore, flag type Ag nanostructures are promising materials having new optical properties in a wide wavelength range. Conclusion We succeeded in the preparation of new flag types of Ag nanostructures through a two-step method using Ag nanorods as seeds. The product shapes of flags could be tuned from trapezoid and triangular plates to tetrahedral structures by changing the [AgNO3]2/[AgNO3]1 ratio. It was found that tetrahedral flags were grown either through the formation of two equilateral triangular plates connected on a nanorod or through the crystal growth of triangular pyramidal structures over triangular plates. The driving force of significant shape evolution is ascribed to the changes in favorable facets of Ag nanostructures from {100} in EG to {111} in DMF. Although various kinds of seeds have been used for the shape and size controlled syntheses of metallic nanostructures, we found here that Ag nanorods can be used as new seeds. Thus, the present technique gives a new method for the shape and size controlled syntheses of metallic nanostructure from nanorods. In addition, from the detailed analyses of shape evolution from rods to two-dimensional plates and three-dimensional tetrahedra, we

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can obtain fundamental information on crystal growth mechanisms from one-dimensional metallic nanostructures. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (B) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT, No. 22310060), by Kyushu University G-COE (novel carbon resource sciences), and the Management Expenses Grants for National Universities Corporations from the MEXT. Supporting Information Available: SEM and TEM-EDS data, calculation methods of lengths and angles of the tetrahedral Ag flag, and crystal structures and growth mechanism of hexagonal fcc particles to triangular particles. This material is available free of charge via the Internet at http://pubs.acs.org.

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