Roles of Twin Defects in the Formation of Platinum Multipod

(e) unsymmetrical tripod, (f) tetrapod, (g) monopod, (h) bipod, and (i) tripod. The reaction temperature was 160 °C, and the reaction time was 60 min...
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J. Phys. Chem. C 2007, 111, 14312-14319

Roles of Twin Defects in the Formation of Platinum Multipod Nanocrystals Sean Maksimuk, Xiaowei Teng, and Hong Yang* Department of Chemical Engineering, UniVersity of Rochester, GaVett Hall 206, Rochester, New York 14627 ReceiVed: June 18, 2007; In Final Form: July 27, 2007

This paper describes the synthesis of platinum multipods from platinum acetylacetonate in the presence of adamantanecarboxylic acid (ACA), hexadecylamine (HDA), and 1,2-alkanediol. Regular cubes and a range of other shapes can be generated in these reaction mixtures using diphenyl ether as the solvent and at a reaction temperature ranging from 160 to 200 °C. The formation of both planar and three-dimensional multipods of platinum can be attributed to the twin defects in the seed crystals. High-resolution transmission electron microscopy (HR-TEM) and electron diffraction (ED) data of platinum multipods show the stacking fault plays a key role in the reduction of symmetry in face-centered cubic metals such as platinum and enables the formation of mono-, bi-, tri-, and multipods of metal nanocrystals. The final shapes of the nanocrystals depend on both the type and number of defects, which can be changed by varying the reaction conditions such as the ACA/HDA molar ratio, the type of diols, the reaction time, and the temperature. High-aspect-ratio multipods of platinum can be generated by using 1,2-dodecanediol. The mechanisms that govern the formation of platinum multipods should be applicable for making other metal multipods.

Introduction The synthesis of geometrically controlled nanocrystals is very important for tailoring unusual properties.1-7 The recent demonstration of extraordinary catalytic properties by the Pt3Ni (111) surface and tetrahexahedral Pt nanocrystals suggests that the crystal phase is pivotal for the high performance of nanometersized metal catalysts.8,9 Shape-controlled low-dimensional crystals can also be used to build nanometer-scaled architectures, which may add extra functionality to the nanostructures.4,10-18 For a platonic nanocrystal, a given shape is enclosed by certain crystal facets that are the direct result of preferred growth from a nucleus. For example, a perfect cube nanoparticle of a facecenter cubic (fcc) platinum or gold metal is bound by six (100) surfaces, whereas a tetrahedral crystal is enclosed by four (111) surfaces.19-22 The nonhydrolytic colloidal synthesis is arguably the most-successful method for controlling the shapes of nanocrystals.3,7,23-27 Although the methods for generating simple and highly symmetrical shapes of nanoparticles have been developed for several noble metals, the formation of low-dimensional forms including one-dimensional (1D) nanorods or nanowires are not obvious for fcc metals. To reduce the symmetry in a nanocrystal of an fcc metal, the defect needs to be introduced at an early stage of the growth.28 The introduction of defects enables the reduction of symmetry and becomes a powerful approach for controlling the directions of crystal growth and the final morphology of the nanocrystals.28 It has been known for a long time that screw dislocations in metals lead to a whisker-like morphology. The stacking faults in many fcc metals can lead to the formation of platelets because of the energetically favorable nucleation centers along the long axis of the crystal.29-35 Decahedral nanocrystals, which are defective in themselves, can grow into nanorods or nanowires * Corresponding author. E-mail: [email protected]. Telephone: (585) 275-2110. Fax: (585) 273-1348.

under a favored kinetically controlled condition.29,36,37 It has also been suggested that tetrapod nanocrystals of II-VI semiconductors form from a multiple-twin wurtzite nucleus.38 The use of defects in controlling the morphology during a colloidal synthesis nevertheless is still rather limited owing to the fact that defects are generally difficult to control. Although a few classes of low-dimensional metal nanostructures including nanowires and multipods have been made during recent years, the formation mechanism, particularly the involvement of defects in seed crystals, has not been given much attention. Fivefold twin in silver or gold is perhaps the best-studied defect that has been used in controlling the formation of uniform nanorods and nanowires.28 Recently, we have presented our preliminary study on the role of crystal twinning in the formation of planar tripods of platinum.39 In this paper, we describe several types of platinum shapes and the effect of twin defects in their formation. Experimental Section 1. Materials. Platinum acetylacetonate (Pt(acac)2) was purchased from Gelest. 1-Adamantanecarboxylic acid (ACA, 99%), 1,2-hexadecanediol (HDD, 90%, tech. grade), 1-hexadecylamine (HDA, 90%), diphenyl ether (DPE, 99%), and 1,2-dodecanediol (DDD, 90%) were purchased from Aldrich. All chemicals and reagents were used as received. 2. Synthesis. In a standard procedure, Pt(acac)2 (100 mg or 0.25 mmol), ACA (90 to 270 mg, 0.50 to 1.5 mmol), HDD (1.6 g or 6.2 mmol), and HDA (2 g or 8.2 mmol) were mixed with DPE (1 mL or 6.3 mmol) in a 15-mL three-neck roundbottom flask equipped with a magnetic stirrer. The synthesis was carried out under argon atmosphere using the standard Schlenk line technique. The reaction flask was immersed in a glycerol bath set at 130 °C, and the reaction mixture turned into a transparent yellowish solution at this temperature. The flask was then transferred to a second glycerol bath set at a designed temperature, typically at 160 °C unless specified

10.1021/jp074724+ CCC: $37.00 © 2007 American Chemical Society Published on Web 09/13/2007

Twin Defects in Platinum Multipod Nanocrystals otherwise. A Chemglass ETC Temperature Controller was used to control the temperature of the oil bath. The reaction time varied from 5 min to over 12 h. The nanocrystals were separated by dispersing the 200-µL reaction mixture with 0.8 mL of chloroform and 1 mL of ethanol, followed by centrifugation at 5000 rpm for 5 min. This procedure was repeated three times to wash away the excess reactants and capping agents. The final powders were dispersed in chloroform. To examine the effect of a given reaction condition, such as reaction time, temperature, reactant type, and ratio, on the morphology of Pt multipods, we changed only the specific parameter while keeping the others the same. Aliquots of 200 µL of reaction mixtures were retrieved after the designed time periods to monitor the growth. Further experimental details are given in the following subsections. 2.1. Effect of Reaction Time. The typical reaction time was 1 h after the flask was transferred to the glycerol bath at 160 °C. To examine the effect of reaction time on the shapes of Pt multipods, we took out reaction mixture aliquots of 200 µL at 5, 10, 15, 30, 120, and 720 min, respectively. The molar ratio of the reaction mixture was kept at 1 Pt(acac)2:4 ACA:24 HDD:33 HDA:25 DPE. 2.2. Effect of Molar Ratio between Pt(acac)2/ACA. The amount of ACA was adjusted to investigate its effects on the formation of multipods. The molar ratio between Pt(acac)2 and ACA tested was set at 1:n, where n was equal to 2, 4, and 6. The reaction time varied from 20 min to about 11 h. 2.3. Effect of Adamantine Functional Group. In this test, DDD was used in lieu of HDD. In a typical synthesis, DDD (1.82 g or 9 mmol) and ACA (270 mg or 1.5 mmol) were used; that is, the molar ratio was kept at 1 Pt(acac)2:6 ACA:36 DDD:33 HDA: 25 DPE. The reaction temperature, that is, the temperature of the second glycerol bath, was set at 160 °C. 2.4. Effect of Reaction Temperature. Nanocrystals were obtained at the molar ratio of 1 Pt(acac)2:4 ACA:24 HDD:33 HDA:25 DPE. The reaction temperature was at 160, 180, and 200 °C, respectively. Under each temperature, the reaction time was 20, 40, and 60 min, respectively. 3. Characterization. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were recorded on a JEOL JEM 2000EX microscope at an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) images were recorded on either a FEI Tecnai F-20 microscope operating at 200 kV or a Hitachi HD-2000 STEM. TEM specimens were prepared by drop-casting a suspension of platinum nanocrystals in chloroform onto carbon-coated copper grids. The UV-vis-NIR spectrum was recorded on a Perkin-Elmer Lambda 900 spectrometer. Results and Discussion In these reaction systems, DPE was used as the solvent, ACA and HDA as the capping agents, and HDD or DDD as the reducing agent. All of the reaction precursors were dissolved at 130 °C, but this temperature was too low for the formation of platinum nanocrystals. The reaction proceeded when the temperatures were raised to above 160 °C. The use of an oil bath was important to achieve homogeneous heating of the reaction mixture. We recently found a set of conditions under which Pt(acac)2 could be reduced to platinum metal at 160 °C without the use of heterogeneous species.39,40 Under these conditions, growth of nanocrystals, governed most likely by the reaction kinetics rather than thermodynamics, became possible, leading to the formation of anisotropic shapes.3 Figure 1 shows the TEM image of the product obtained at a Pt(acac)2/ACA molar ratio of 1:3 after a reaction time period

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Figure 1. TEM image of the product obtained at 160 °C after reaction for 60 min. The Pt(acac)2/ACA molar ratio was 1:3.

Figure 2. Collage of TEM images of various shapes of platinum nanocrystals obtained at a Pt(acac)2/ACA molar ratio of 1:3 or 1:4: (a) cubes, (b) symmetrical planar tripod, (c) cis-bipod, (d) trans-bipod, (e) unsymmetrical tripod, (f) tetrapod, (g) monopod, (h) bipod, and (i) tripod. The reaction temperature was 160 °C, and the reaction time was 60 min. All scale bars are 50 nm.

of 60 min. Monopods, bipods, tripods, and other multipods, together with cubes, were obtained from the same mixture. A quarter of the total nanocrystal population had a planar tripod shape. Most of these tripods had approximately σ3h symmetry, whereas unsymmetrical tripods could also be found. The size of the tripod was tuneable by adjusting the Pt(acac)2/ACA molar ratio, where high ACA content yields tripods with low aspect ratios.39 The tripods reached an average arm length of 200 nm and a width of 13 nm at 120 min. Figure 2 shows representative TEM images of major shapes of these Pt nanocrystals formed at a Pt(acac)2/ACA ratio of 1:3 or 1:4. These Pt nanocrystals could roughly be classified into four major categories: cube (60% of the total population, Figure 2a), symmetrical planar tripod (24%, Figure 2b), various multipods with a rod-like center (10%, Figure 2c to 2f), and multipods with a sphere-like core (6%, Figure 2g-i). At a Pt(acac)2/ACA ratio of 1:3 and a reaction time of 60 min, the cubes had an average edge length of 20 nm, whereas the branch of symmetrical planar tripods had an average width of about 9 nm and a length of 170 nm. If the arms of planar tripods could be approximated as a cylindrical rod, then the atomic ratio of platinum between cubes and planar tripods in the product was then estimated to be about two, which meant that Pt(acac)2 reacted and formed mostly the multipods. Those multipods with a rod-like center could be cis- and trans-bipods, unsymmetrical tripods, and tetrapods. Those multipods with a spherical core could be monopods, bipods, tripods, and various other rod-based

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Maksimuk et al.

Figure 4. ED patterns of (a) cis-bipod and (b) tetrapod with the rectangular centers. The corresponding TEM images are shown in the insets. All scale bars are 50 nm.

Figure 3. (a) ED pattern of a single symmetrical planar tripod. A closeup image of the 2-20 spot is shown in the inset. (b-d) HR-TEM images showing the lattices of different sections of the tripod: (b) triangular core, (c) a branch with lattice and growth directions at 30°, and (d) a branch with lattice and growth directions at 90°. (e) The schematic of an ED pattern from a single planar tripod showing the shape-factor-intensity-modified reciprocal lattice points that consist of three disks.

morphologies. These observations suggest that at least three or four different modes of formation exist in such reaction systems. It is clear that the nanocubes were the single-crystalline product from the fast growth along the directions of fcc platinum.19 Electron diffraction (ED) was used to analyze the symmetrical planar tripod, Figure 3a. The ED pattern of a single tripod showed a sixfold symmetry, indicating that the zone axis was along the direction. The d-spacing calculated from the ED spots matched those of fcc platinum. An interesting feature of this pattern is that the formally forbidden spots of 1/3 (422) diffractions in a single crystal were observed. This observation suggests that there should be at least one stacking fault, or twin plane, in the (111) plane perpendicular to the electron beam.30,31,34,35 This feature has been observed for triangular and hexagonal plates of different fcc metals. Another important aspect of this ED was the sixfold streaking pattern of individual diffraction spots (Figure 3a, inset). Such a pattern can be explained in terms of the shape factor of the ED. A single rod-shaped particle lying perpendicular to the

electron beam can give a twofold streaking pattern.41 This rises from the observation that the short axis gives a broad diffraction spot, whereas the long axis gives rise to a narrow diffraction spot. Thus, the direction perpendicular to the long axis of the ED spot can be assigned as the growth direction of the rod. The phenomenon of the broadening of the ED spot is analogous to X-ray diffraction of small crystals where the small crystals have broad diffraction peaks. The streaking of the diffraction spots can also be explained in reciprocal space terms. Reciprocal lattice points are modified by the shape factor where a rodshape particle (1D structure) gives rise to a reciprocal lattice point that is the shape of a disk (2D structure), often called rel-disks for reciprocal lattice disks.41 A tripod is composed of three arms 120° apart; thus, the shape-factor-intensity-modified reciprocal lattice point is three rel-disks separated by 120°, Figure 3e. It should be noted that for clarity the illustration of Figure 3e shows only a single reciprocal lattice point and a portion of the Ewald’s Sphere, a sphere with a radius that is equal to the inverse radiation wavelength and goes through the origin of this lattice, and the shape factor is arbitrary. The portion where the Ewald’s sphere intersects with the reciprocal lattice point gives rise to the diffraction spot. These results along with HR-TEM analysis indicate that the tripods grow along the coplanar [2-1-1], [-12-1], and [-1-12] or [-211], [1-21], and[11-2] directions, where the two sets are not equivalent. Further analysis based on TEM data (see below) suggests that the tripods grow along the equivalent coplanar [2-1-1], [-121], and [-1-12] directions. ED was also performed on a cis-bipod and a tetrapod with rod-like centers, Figure 4. The SAED of the cis-bipod was a superposition of and zone axes, whereas the ED of the planar tetrapod was a superposition of the and zone axes. These two types of superposition of zone axes have been observed for Ag and Au nanorods with a fivefold symmetry along the long axis of the nanorod where the nanorod grows from a decahedron particle.29,36,37 This observation suggests that a fivefold symmetric crystal exists in the bipod and tetrapod. To investigate the detailed structure of the bipod, we performed electron diffraction on an entire single bipod and only on the branches. Figure 5 shows the TEM image of a single bipod, its corresponding ED patterns for the entire nanostructure, and the two individual branches. The ED pattern of the whole bipod shows a superposition of the and zone axes, similar to that shown in Figure 4a. The ED from the branches, however, only shows a pattern from the zone axis. These results indicate that the fivefold twin crystal did not exist in the branch regions, but only in the rod-like central core. The ED patterns from the branches show the “forbidden” spots of 1/3 (422) diffractions, suggesting that a twin plane exists

Twin Defects in Platinum Multipod Nanocrystals

Figure 5. (a) TEM image and (b-d) the SAED patterns of (b) whole, (c) top, and (d) bottom arms of a bipod.

Figure 6. (a) HRTEM image of the core (left), middle section (center), and tip (right) of a monopod, the SAED pattern of (b) the whole and (c) arm of the monopod, and (d) two simulated ED patterns of the zone axis offset by 141°. The inset in b shows the monopod used for the SAED study. The scale bar in the inset is 50 nm.

perpendicular to the electron beam. This observation indicates that the arms of the bipods grow in a fashion similar to that of the tripods. The apparent similarity of the branches in terms of length, width, and the kink morphology of the edges also implies that the branches should be related to the tripods where only a single twin plane existed. HR-TEM was used to study an individual monopod. The three panels in Figure 6a show the spherical core, middle section, and tip of a monopod. The core of the monopod was multiply twinned with a twin plane extending into the arm (Figure 6a, left panel). The middle section of the monopod shows a twin plane running through the middle along the long axis of the monopod. Both the (111) fringe (2.27 Å) and (200) fringe (1.96 Å) were observed. These two sets of fringes met at a 141° angle and created a herringbone-like structure along the growth direction of the monopod. The HR-TEM image of a tip of the monopod also shows (111) fringes and reveals the rounded profile of the tip. ED was performed on a single monopod, similar to the one used for HR-TEM analysis, in which the twin plane could be observed. The TEM image (Figure 6b, inset) of the monopod used for ED analysis shows a twin plane parallel

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Figure 7. Schematic of the classification of nanocrystals based on the number of twin planes: (a) zero, (b) single, (c) five, and (d) multiple. Scale bars are 20 nm if not labeled in the images.

to the growth direction of the rod. Most of these ED spots show a twofold streaking pattern arising from the anisotropic shape of the monopod (Figure 6b). Figure 6c shows the ED pattern from the arm of the monopod, and Figure 6d shows a superposition of two simulated ED patterns with a zone axis of offset by a 141° angle. With a (111) twin plane in the monopod and a zone axis of the ED pattern, the growth direction of the monopods are [11-2]. This direction is perpendicular to the streaking observed in the ED patterns. From the above analysis, we could classify the observed shapes by the number of twin planes in the seed or core crystals, Figure 7. In the absence of a twin plane, there is relatively fast growth rate along the directions in comparison to yielded nanocubes, Figure 7a. These nanocubes could eventually form octapods over a long time because of the selective growth along the directions.3 At a reaction temperature of 160 °C and a Pt(acac)2/ACA molar ratio of 1:3, the octpods shown in Figure 7a took about 120 min to form. Seeds with a single twin plane bound with the (111) face resulted in planar tripods, Figure 7b. From a fivefold twinned particle or a decahedron, a short fivefold twinned rod formed, similar to those observed in Au and Ag nanorods. If the growth along the long axis stopped, then the branches could grow 30° from the long axis of the fivefold twinned rod, Figure 7c. It appears that the growth of these branches had a growth mode similar to that of the symmetrical planar tripods, resulting in multipods with two to four branches and a rod-like center, Figure 7c. Other multiply twinned particles including icosahedrons resulted in monopod or other multipod structures with sphere-like cores, Figure 7d. The branches of these nanocrystals resulted from the twin-induced growth where a single twin plane in the arm extends into the sphere as seen in the HR-TEM image, Figure 6a. Our data indicates that the growth of the nanocrystals was sufficiently slow at 160 °C and the evolution of the shapes could be monitored by taking the samples at various reaction times. Figure 8 shows a series of TEM images of the Pt nanocrystals that were sampled at reaction times from 5 min to 12 h using the reaction mixture at a Pt(acac)2/ACA molar ratio of 1:4. At 5 min, all particles were faceted and seed crystals of various

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Figure 8. TEM images of nanocrystals formed at the reaction time of (a) 5 min, (b) 10 min, (c) 15 min, (d) 30 min, (e) 2 h, and (f) 12 h, respectively. The reaction temperature was 160 °C, and the Pt(acac)2/ ACA molar ratio was 1:4.

morphologies could be observed. The cuboctahedron crystal labeled A could lead to the formation of cube. A threefold symmetric nanocrystal, labeled B in Figure 8a, was a precursor to the symmetrical planar tripod. Multiply twinned particles, labeled C and D in Figure 8a, could be easily distinguishable because of the inherent contrast difference within the nanocrystals. At 10 min, cubes, labeled A and symmetrical planar tripods, labeled B, could be observed, Figure 8b. Branches began to grow into bipods from a rod-like center, crystal C in Figure 8b. Multiply twinned particles were observed at this stage, crystal D in Figure 8b. After 15 min, the different morphologies of nanocrystals could be easily distinguishable. The average length of the branches of the multipods reached 100 nm by 30 min. The branches of symmetrical planar tripods exceeded 200 nm by 2 h. The slow transformation from cube to octapod began to show as the growth in the eight directions continued. Tripods and other multipods apparently maintained their overall shape and dimension by 12 h, although they had a rounded profile as compared to those formed at 2 h. Such surface smoothing could be due to a ripening process.3 This time-dependent shape evolution also provides experimental evidence on how the tripods formed from the seed crystals with twin planes. Figure 9 shows the proposed formation pathway of planar tripods and the observed TEM images at the corresponding stages. Single or multiple crystal twinning parallel along the direction is responsible for the frequently observedtriangularorhexagonalplatesofvariousfccmetals.30-32,34,35 These plates grow parallel to the twin plane because of the

Maksimuk et al. formation of troughs as a result of crystal twinning. The troughs act as preferential nucleation sites, and thus fast growth is observed along the trough.32-35 Such anisotropic growth has also been theoretically simulated.42 In this study, the initial seed crystal of a tripod was a bicrystal with a single twin plane in the (111) plane and all of the surfaces were bound by (111) planes, stage 1. Three troughs (orange) and three ridges (blue) formed as a consequence. Growth along the three troughs could lead to the enclosure of the troughs and the formation of a triangular crystal, stage 2. Selective nucleation on the tips of the triangular crystal regenerated the troughs, stage 3. Growth along one direction is shown in the illustration for clarity. Growth along the troughs, or the coplanar [2-1-1], [-12-1], and [-1-12] directions, has also been attributed to the formation of germanium dendrites.43 Furthermore, the nucleation barrier on the trough was calculated to be 39% of the nucleation on a flat (111) face.33 Thus, nucleation along the direction of trough was energetically favorable compared to other nucleation events. The selective nucleation on the tips of the triangular crystal would be a result of a concentration gradient of Pt monomers where the monomers were unsaturated near the center of planar tripod, similar to a model for pseudo two-dimensional crystallization of shape-controlled nanocrystal arrays.44 This concentration gradient was a major cause for the selective nucleation at the places that were furthest from the center of the nanocrystal.5 Regeneration of the trough led to its enclosure along with growth around its surrounding areas, stage 4. A new regeneration trough could be created on the side of the arms (orange), but growth was limited because it was adjacent to a ridge. This growth process occurred for all three tips and led to a tripod with kinks on the edges, as observed for all planar tripods formed at early stages (15 to 120 min), stage 5. After the reaction precursors exhausted, the tripods underwent a ripening process where the edges could get smoothened, stage 6. This was in contrast to previous observations of Pt multipods in the presence of Ag(acac), which underwent a ripening process to spherical nanocrystals at about 20 min.40 Such a difference could be the result of slow growth kinetics in the absence of catalytic species in the solutions. The effect of the ACA concentration was investigated to examine its role in shape control. Figure 10 shows the TEM images of the products at different times with Pt(acac)2/ACA ratios of 1:2 and 1:6, respectively. In these experiments, the amount of Pt(acac)2 and all other precursors except ACA were kept the same as those above. Because there was an excess amount of HDA in all of the solutions we examined, the growth along different directions should be affected mostly by the competitive binding on the surfaces between ACA and HDA.45,46 Noticeably, by decreasing the ACA amount tripods could no longer be observed, Figure 10a and b. Unlike the above cases, the nanocrystals had multipods originating from a core, resembling to some extent the nanoflowers reported previously.39 Some multipods with long branches were observed occasionally, which could be a result of the aforementioned twin-induced growth. Long branches, however, were rather rare, possibly because of the lack of twinning events, and only nanoflowers formed and no nanocubes formed under this condition. When the Pt(acac)2/ACA ratio was changed to 1:6, planar tripods reappeared but with low length-to-width aspect ratios (Figure 10c and d). This observation can be explained by the enhanced capping ability by the surfactants of the tips of planar tripods, thus enhancing the relative growth rate along other directions. The displacement of an adamantine-containing ligand with a straight alkane chain surfactant on metal surfaces could be an

Twin Defects in Platinum Multipod Nanocrystals

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Figure 9. Schematic illustrations and the corresponding TEM images of a proposed growth process for a symmetrical planar tripod. Scale bars are 10 nm for stages 1-5, and 100 nm for stage 6.

Figure 10. TEM images of Pt nanocrystals obtained at a Pt(acac)2/ ACA molar ratio of (a and b) 1:2 and (c and d) 1:6, respectively.

important factor in determining the morphology of nanocrystals in these systems.45,46 The average branch length of planar tripods was about 64 nm, and the width was 11.4 nm at 1 h (Figure 10c). By 11 h, ripening caused the smoothening of the nanocrystals while the overall shape and dimension were maintained. The large population of cubic shapes shows the ability of capping agents to bind to (100) facets because the cube was the thermodynamically stable shape for single crystals. The effect of temperature on the formation of Pt nanocrystals was investigated at a Pt(acac)2/ACA molar ratio of 1:4. Figure 11 shows the TEM images of Pt nanocrystals formed at 180 °C. All of the nanocrystals are branched with similar arm

Figure 11. TEM images of representative nanocrystals obtained at a Pt(acac)2/ACA molar ratio of 1:4 and a reaction temperature of 180 °C after (a) 20, (b) 40, and (c) 60 min, respectively. (d) ED pattern of an octapod obtained at 40 min and the corresponding TEM image. The scale bar is 20 nm for that in the inset of panel d.

lengths. Although it was difficult to analyze the nanocrystals because of their three-dimensionality, most of the nanocrystals had more than four branches and the shapes were not welldefined. The average length of the branches was 24 nm, and the width was 9 nm. By 60 min, the ripening process smoothed out the surfaces of the Pt nanocrystals. Note that long branches were seldom observed when the Pt(acac)2/ACA ratio was 1:4. This observation further suggests that twin-induced growth is involved in the growth of the long arms in this system. Octapods were among the types of multipods observed. Figure 11d shows

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Figure 12. TEM images of representative nanocrystals obtained at a Pt(acac)2/ACA molar ratio of 1:4 and a reaction temperature of 200 °C after (a) 20 and (b) 60 min, respectively.

Figure 13. (a) TEM image and (b) UV-vis-NIR spectrum of representative multipods obtained at the Pt(acac)2/ACA/DDD/HDA/ DPE molar ratio of 1:6:36:33:25 and a reaction temperature of 160 °C.

an ED pattern of an octapod, the shape that could be confirmed from the difference in contrast between the core and the branches on the TEM image viewed along the zone axis. The diffraction pattern indicates that the octapod was single-crystal and should be derived from a nanocube as discussed above. The ED results further suggest that the branches grow in eight equivalent directions. Thus, at this condition the growth along the directions was enhanced dramatically, most likely because of the weak binding between the capping agents and the (111) planes of platinum at higher temperatures. Figure 12 shows the TEM images of Pt nanocrystals formed at 200 °C using a reaction mixture having a Pt(acac)2/ACA ratio of 1:4. At 20 min, selective growth along the directions could still be observed, Figure 12a. The high temperature resulted in a fast nucleation rate, and few monomers per nanocrystal were available for growth, resulting in even shorter branches than those formed at a temperature of 180 °C or lower. At 200 °C, Pt(acac)2 could also be directly reduced by longchain amine. This temperature was also close to that for the decomposition of Pt(acac)2. Thus, the shape control via a kinetically controlled process became less-likely, particularly after a relatively long reaction time period. In the current reaction system, the multipods turned into thermodynamically stable sphere-like nanocrystals by 60 min, Figure 12b. Interestingly, the dissolution process appeared to occur in the middle section of the branches. This observation suggests that the type of faceted surfaces shown in Figure 9 at stages 4 or 5, and highenergy edges could dissolve first. The high length-to-width aspect ratio multipods with dendrites were synthesized by using DDD in lieu of HDD at a Pt(acac)2/ ACA:DDD molar ratio of 1:6:36 and a reaction temperature of 160 °C. Most of the nanostructures obtained at 160 min were three-dimensional hyperbranched multipods, and some planar tripod structures could also be observed, Figure 13a. Although the formation mechanism was not completely clear, the branches

Maksimuk et al. were most likely the result of the aforementioned twin-induced growth. The use of DDD, a short-chain diol, could induce twinning events at a much higher frequency than that of HDD, rendering all of the nanocrystals with twin planes and twininduced branch growth and the formation of hyperbranched multipods. The UV-visible-NIR spectrum of these long hyperbranched multipods is shown in Figure 13b. The longitudinal and transverse plasmon bands of the one-dimensional metal nanostructures should give rise to double extinction peaks.4 A broad peak with a maximum at 1380 nm could correspond to the longitudinal plasmon oscillation of the platinum multipods.47 Conclusions Shape control of the nanocrystals of Pt, an fcc metal, has been investigated using ACA and HDA as capping agents. HDD or DDD could have the dual functions of reducing and capping agents in these reaction systems. The shapes of Pt nanocrystals formed depended on the number and type of twin planes in seed crystals. With an adamantine-based capping agent together with HDA, several types of morphologies form. Single-crystal seeds with no defect yielded cubic and octapod nanocrystals. Seeds with a single twin in the (111) plane led to planar tripods, whereas decahedrons favored the formation of bipods to tetrapods with rod-like cores. Other multiple-twin seeds can be used to create monopods to multipods with a sphere-like cores. The growth mechanism of planar tripods involved troughs and ridges created along the (111) twin plane. Selective growth regenerates the troughs, leading to growth along three coplanar [2-1-1], [-12-1], and [-1-12] directions. Branches that grow from decahedron and other multiple-twin platinum cores are a result of the twin-trough model. Changes in the amount of ACA and reaction temperature can dramatically alter the twinning events and growth kinetics along given directions. These changes lead to the formation of various branched structures, such as octapods and other branched nanostructures. The use of crystal twinning as a strategy to control the growth of a large variety of multipods of fcc metals is relatively unexplored except for the commonly observed nanorods, nanowires, and nanoplates. With the identification of proper surface-specific or preferred capping agents, the methods described in this work should be applicable to other fcc metals where crystal twinning has regularly been observed. Because such nanocrystals are bound by the same or few limited types of crystal planes, they may show interesting face-specific catalytic properties. The uniform and aspect-ratio tunable planar and three-dimensional multipods can also be good building blocks in constructing self-assembled hierarchical nanostructures. Acknowledgment. This work was supported in part by the grants from the National Science Foundation (CAREER Award, DMR-0449849) and the Environmental Protection Agency (EPA-STAR R831722). This work made use of the FEI Tecnai F-20 microscope at Cornell Center for Materials Research (CCMR) supported by NSF (DMR-0520404) and the Hitachi HD-2000 STEM at the Center for Nanostructure Imaging, University of Toronto, funded by Canada Foundation of Innovation and Ontario Innovation Trust. We thank Mr. John Grazul of CCMR for the help in running the HR-TEM and Mr. Shengchun Yang for valuable discussions. References and Notes (1) Somorjai, G. A.; Contreras, A. M.; Montano, M.; Rioux, R. M. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10577-10583.

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