Low Packing Density Self-Assembled Superstructure of Octahedral

Jun 6, 2011 - Erik Wetterskog , Alice Klapper , Sabrina Disch , Elisabeth Josten , Raphaël P. Hermann , Ulrich Rücker , Thomas Brückel , Lennart Be...
0 downloads 0 Views 5MB Size
Download PDF
LETTER pubs.acs.org/NanoLett

Low Packing Density Self-Assembled Superstructure of Octahedral Pt3Ni Nanocrystals Jun Zhang,†,# Zhiping Luo,‡,# Zewei Quan,† Yuxuan Wang,§ Amar Kumbhar,|| Detlef-M. Smilgies,*,^ and Jiye Fang*,†,§ †

Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, United States Microscopy and Imaging Center and Materials Science and Engineering Program, Texas A&M University, College Station, Texas 77843, United States § Materials Science and Engineering Program, State University of New York at Binghamton, Binghamton, New York 13902, United States Chapel Hill Analytical and Nanofabrication Laboratory, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States ^ Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, New York 14853, United States

)

‡

bS Supporting Information ABSTRACT: We present a structural study of Pt3Ni nanoctahedron superlattice, prepared through both drop-casting and controlled solvent evaporation approaches. In this superlattice system containing ∼10.6 nm side-length Pt3Ni nanoctahedra, we observed a body-centered cubic (bcc) packing structure in both local superlattices and statistically averaged superlattice ensembles using transmission electron microscopic tomography and grazing-incidence small-angle X-ray scattering techniques, respectively. Within the superstructure, it was directly observed that nanoctahedra are orientated along the superstructure axes through sharing their vertices. We found that this arrangement of a bcc superstructure with nanoctahedra connecting through their vertices is dependent on neither the processing pathway nor the substrate under our experimental conditions. With such a very low packing density and ultrahigh surface area, this type of self-organized superstructure possesses unique features for future applications. KEYWORDS: Pt3Ni nanoctahedra, self-assembly, low packing density, bcc, TEM tomography, GISAXS

T

hree-dimensional (3D) self-assemblies, also called superlattices or supercrystals, of metallic and/or semiconductor nanocrystals (NCs) are of great interest for the development of advanced materials with potential novel applications1 4 such as terahertz,5 mechanical,6 plasmonic,2 and surface-enhanced Raman spectroscopy substrate7 technologies. It is critically important to understand the structures of 3D superlattices consisting of NCs with various building block shapes.8,9 For identical nanospheres, it is well-known that the highest packing density is obtained in the face-centered cubic ( fcc) or hexagonal close packed (hcp) packing with 74.04%.10 The crystallographic orientation of crystalline nanospheres is random in the resultant superlattice. When these nanospheres are replaced by nonspherical NCs, however, unusal superlattices with a unique structure could possibly be generated.5,8,9 In an early study, Whetten et al. found body-centered cubic (bcc) packing consisting of nonspherical gold NCs with a cuboctahedral shape.11 More recently, it was found that superlattices consisting of nanocubes could adopt a simple cubic packing structure with a packing density of 100%, if the interparticle spacing is neglected.12 15 The assembly of nanoctahedra is more complicated and it usually results in a r 2011 American Chemical Society

number of complex superlattices.16 19 Note that there is no simple space-filling structure for octahedra in contrast to that for cubes. Moreover, nanoctahedra expose only {111} facets and would be able to produce a large area of highly active surface for oxygen reduction reaction (ORR) in electrocatalytic applications,20 if the ligands can be removed. The study of Pt3Ni nanoctahedron-based superstructures may be beneficial for energy-inspired exploration because of the ultrahigh ORR catalytic activity on the Pt3Ni {111} surface.21,22 Nanoctahedra expose only {111} facets and would be ideal to serve as large {111} surface area catalysts, if the ligands can be removed. In this work, we present a study of the superstructure of Pt3Ni nanoctahedron superlattices. We applied transmission electron microscopic (TEM) tomography and grazing-incidence small-angle X-ray scattering (GISAXS) to the investigation of superlattices containing ∼10.6 nm side-length Pt3Ni nanoctahedra. In order to study the superstructure composed of nonspherical Received: April 25, 2011 Revised: May 25, 2011 Published: June 06, 2011 2912

dx.doi.org/10.1021/nl201386e | Nano Lett. 2011, 11, 2912–2918

Nano Letters nanoparticles, knowledge about not only the conventional lattice type and lattice parameters but also the nanocrystals coordinates and orientation, as directly revealed through the TEM tomography, is essential to fully understand the superstructure. We observed a bcc packing structure in both local and bulk superlattices of the Pt3Ni nanoctahedra prepared through both dropcasting and controlled solvent evaporation approaches (Figure 1). This bcc superstructure containing Pt3Ni nanoctahedra is an arrangement with nanoctahedra connecting through their vertices, leaving a large amount of space (theoretically 66.67%) between the metallic cores. Experimental Section. Methods. a. Chemicals. Tungsten hexacarbonyl (99.9%), oleic acid (90%), and oleylamine (70%)

Figure 1. Flowchart of studies on Pt3Ni nanoctahedron superlattices. In the 2  2  2 bcc supercell, the nanoctahedra at the corner and center of the bcc unit cell are shown in different colors, exhibiting a large amount of space between metallic cores which is taken up by the ligands.

LETTER

are Sigma-Aldrich products and were used as received. Platinum(II) acetylacetonate (49.3 49.8% Pt), nickel(II) acetylacetonate (97%), absolute ethanol (200 proof), and anhydrous hexane (98.5%) were from Gelest, Alfa Aesar, AAPER and BDH, respectively, without further purification. b. Synthesis. A typical experiment can be described as follows: 0.010 g of nickel acetylacetonate, 0.020 g of platinum(II) acetylacetonate, 9.0 mL of oleylamine, and 1.0 mL of oleic acid were loaded into a three-neck flask and heated to 130 C under an argon stream. A 0.050 g portion of tungsten hexacarbonyl was then added into the solution, and the temperature was subsequently raised to 230 C and kept for 40 min with stirring. The resultant products were isolated by centrifugation and washed/redispersed with absolute ethanol/anhydrous hexane for several cycles. The Pt3Ni nanoctahedra were then subject to size selection and finally redispersed in hexane, forming a colloidal suspension. Assembly. a. Drop-Casting. One drop of the Pt3Ni nanoctahedra suspended in hexane was cast on a 200 mesh TEM grid (Ted Pella, 01801) which was preplaced on a piece of filter paper or on a piece of (111) surface-polished Si wafer in ambient conditions. The number of layers (thickness of a pattern) can be tuned by varying the concentration of Pt3Ni suspension. b. Controlled Solvent Evaporation. A TEM grid was horizontally held by a pair of self-closing TEM tweezers and submerged in a hexane suspension of Pt3Ni nanoctahedra in a vial. The solvent was allowed to evaporate in ambient conditions until the level of the colloidal solution was below the TEM grid. In the case of GISAXS samples, the same Si wafer was horizontally placed in a 10 mL vial which contains a sufficient amount of Pt3Ni nanoctahedron suspensions in hexane with a proper concentration. The vial was partially sealed and placed in an ambient environment until all of the solvent evaporated. It was found that a slow evaporation could produce structure-oriented superlattice with high quality.23 Our process required 3 4 days depending

Figure 2. STEM image and diffraction patterns of a drop-cast superlattice. (a) STEM image, showing layer-by-layer formation of a superlattice consisting of Pt3Ni nanoctahedra. (b d) The corresponding electron diffraction patterns of mono-, double-, and triple-layer regions labeled in (a), respectively. 2913

dx.doi.org/10.1021/nl201386e |Nano Lett. 2011, 11, 2912–2918

Nano Letters on the amount of colloidal suspension and other evaporation conditions. Composition Characterization. Compositions of the Pt3Ni NCs were analyzed using TEM-equipped energy dispersive X-ray spectroscopic (EDS) and inductively coupled plasma mass spectrometric (ICP-MS) techniques, and are presented in sections S1 and S2 in the Supporting Information, respectively. In both types of analyses, no trace of tungsten was determined. Superlattice Pattern Characterization. a. EM. TEM work was performed using Hitachi 7000 (110 kV), JEOL-2010 FEG (200 kV), and FEI Tecnai G2 F20 (200 kV). For a 3D reconstruction study, TEM tomography was carried out and a series of images were taken at different angles, and then reconstructed by the backprojection method using the FEI Xplore3D program. SEM images were recorded on a field emission scanning electron microscope (Carl Zeiss Supra 55VP). b. GISAXS. Measurements were performed at D1 station at the Cornell High Energy Synchrotron Source (CHESS), Cornell University. A 10 kV X-ray beam impinged onto the sample at an angle of 0.25. The beam size was defined with precision slits as 0.5 mm horizontal and 0.1 mm vertical. At grazing incidence the full length of the sample (typically 10 mm) along the beam is illuminated. Scattered X-rays were collected with a CCD camera at 660 mm from the sample. The intense scattering in the incident plane was blocked with a 3 mm diameter rodlike beam stop. c. ICP-MS. Analysis was conducted on Varian 810 ICP-mass spectrometer. Results and Discussion. The manipulation and analysis of superlattices demand a processing of high-quality spherical or nonspherical nano building blocks, such as nanoctahedra.16,24 26 An octahedron is a platonic solid that has 6 polyhedron vertices, 12 polyhedron edges, and 8 equivalent equilateral triangular faces.27 Among nanoctahedra, Pt3Ni is especially interesting due to its unusual ORR catalytic activity on {111} facets which are the surfaces of the octahedra.21,22 We have recently demonstrated a new synthesis strategy for the preparation of monodisperse Pt3Ni nanoctahedra through a coreduction of platinum(II) acetylacetonate and nickel(II) acetylacetonate in a mixed organic solvent of oleic acid and oleylamine in the presence of tungsten hexacarbonyl at high temperature.22 With this method, perfect Pt3Ni nanoctahedra with an average side length of 10.6 ( 0.3 nm were synthesized at 230 C22 and further assembled through two approaches (Figure 1). TEM and Tomography Study on a Drop-Casting Assembly. Figure 2 shows a representative TEM image with a layer-by-layer formed superlattice prepared through a drop-casting technique which illustrates an intermediate morphology of this superlattice evolution process, that is, from monolayer to doublelayer and to multilayer thin films. The image in Figure 2a was acquired in the scanning TEM (STEM) mode, so that the nanoctahedra exhibit bright contrast when showing the stacking features. In the monolayer assembly, most of the nanoctahedra were arranged into a 2D hexagonal pattern by contacting their {111} facets on the substrate. This ordered structure was further confirmed by a selected area electron diffraction (SAED) pattern with {220} diffraction spots presented in Figure 2b, determining that such an orientation is the most frequently observed arrangement in the monolayer assembled patterns. It was also observed that the nanoctahedron orientation in the double-layer region was different from that in the monolayer pattern. In the double-layer zone, two layers of nanoctahedra assemblies are overlapped. An

LETTER

Figure 3. Double-layer stacking structure in a drop-cast superlattice studied using a tomography technique: (a) TEM image; (b) SAED from this agglomerate showing textured structure that most of the nanoctahedra oriented along [110]; (c) stacking model of double layers, along the plane and side views; (d) reconstructed volume, with two cross section views, with top edges marked as AB and CD respectively, shown on the right side; (e, f) slice views of bottom and top layers respectively, at the heights as marked in (d). Nanoctahedra marked with solid and open dots correspond to those in (a). (g) Superimposition of (e) and (f) but the intensity is divided by 2.

SAED pattern presented in Figure 2c shows that the strongest diffraction spots are from {111} rather than {220}. This indicates that most of the building blocks sit on their {110} edges, instead of their {111} facets. In the triple-layer assembled pattern, the corresponding SAED pattern (Figure 2d) shows that the {111} diffraction dots are even much stronger than the intensity of the {220} reflections as compared with Figure 2c, indicating that most nanoctahedra in the third layer still sit on their {110} edges. Apparently, this orientation in the bi- and trilayer is stabilized by three-dimensional packing forces. For both cases, looking down along the projection direction, nanoctahedra are stacked on edges along their Æ110æ axis while contacting their vertices only along the Æ100æ axis which is perpendicular to the projection direction. For further study, a 3D reconstruction study using TEM tomography was carried out. A series of images were taken at different angles for the reconstruction. Figure 3a displays a TEM 2914

dx.doi.org/10.1021/nl201386e |Nano Lett. 2011, 11, 2912–2918

Nano Letters

LETTER

Figure 4. Triple-layer stacking structure in a drop-cast superlattice studied using a tomography technique: (a) TEM image, with an inset of SAED along [110]; (b) magnified image of (a) showing oriented [110] nanoctahedra, with spacing of 15.7 nm along the aligned direction; (c) structure models of the bcc packing unit cell, as well as plane and side views; (d) reconstructed volume, with two cross section views, with top edges marked as AB and CD, respectively, shown on the right side; (e g) slice view of bottom, middle, and top layers, respectively, at the heights as marked in (d); (h) superimposition of (e), (f), and (g) but the intensity is divided by 3.

image, where nanoctahedra marked with solid dots were lying on the bottom support film, and with open dots were in the top layer. A planar unit cell is outlined, with a = 15.7 nm along the vertical direction (projection direction), and the other direction √ is about 2a. Figure 3b is an SAED pattern from this area, showing a textured structure that most of the NCs oriented along [110]. A stacking model of the double layer, along the plane and side views, is shown in Figure 3c. Figure 3d illustrates a crosssectional view of the 3D reconstructed volume, and two cross section views with top edges marked as AB and CD, respectively, on the right panel. It is evident that this is a double layered structure, and the nanoctahedra of the top layer are located in the gap between nanoctahedra of the bottom layer that are the more stable positions to reduce the surface energy. Panels e and f of Figure 3 are the slices of bottom and top layers, respectively, at the heights as marked in Figure 3d. Nanoctahedra marked with solid and open dots correspond to those in Figure 3a. Figure 3g is a superposition of Figure 3e and Figure 3f but the intensity was divided by 2. The superposition produces two lines of paired bright spots within a unit cell, confirming the bright image spots appearing in the TEM image in Figure 3a are indeed the empty channels along the viewing direction. This can also be seen in the plane view model in Figure 3c. In the stacking structure of a triple-layer pattern, Figure 4a shows a TEM image with an inset of its SAED along [110]. A magnified image, showing oriented [110] nanoctahedra (rhombus

shape) with spacing of 15.7 nm along the aligned direction, is presented in Figure 4b. Figure 4c illustrates a structure model of the bcc packing unit cell, as well as plane and side views. The reconstructed volume, with two cross section views marked as AB and CD on the top edges, respectively (shown on the right side), is presented in Figure 4d. From this analysis, three stacked layers are recognizable. Similar to the double-layer structure, the nanoctahedra of two neighboring layers are shifted by half of the repeat spacing. The nanoctahedra thus form a bcc packing superlattice (refer to Figure 4c). Panels e, f, and g of Figure 4 show slices of the bottom, middle, and top layers, respectively, at the heights as marked in Figure 4d. With the outlined planar unit cells, the nanoctahedra of the top and bottom layers have equivalent positions, whereas nanoctahedra in the middle layer are located in the maximum space between them. Figure 4h is a superimposition of panels e g of Figure 4 but the intensity was divided by 3. The resulting bright image dot lines are consistent with the observed TEM image in Figure 4a. The triple-layer analysis suggests that such a superlattice of Pt3Ni nanoctahedra represents a bcc arrangement. TEM Study on a Solvent-Evaporated Assembly. In addition, we have investigated the multilayer patterns of Pt3Ni nanoctahedron superlattices prepared through a controlled solvent evaporation approach. For each [100]-, [110]- and [111]-projection orientation in a nanoctahedron superlattice (Figure 5), there should geometrically be more than one type of arrangement.16 In 2915

dx.doi.org/10.1021/nl201386e |Nano Lett. 2011, 11, 2912–2918

Nano Letters

LETTER

Figure 5. TEM projection images of monolayer Pt3Ni nanoctahedra: (a c) TEM images of [100]-, [110]-, and [111]-oriented Pt3Ni nanoctahedra, respectively; (d f) corresponding HRTEM image for each corresponding single nanoctahedron. The [100] projection image is a square, and the nanoctahedron sits on one of its octahedral vertices. The [110] projection image looks like a rhombohedron, and the nanoctahedron is orientated by placing one of its edges on the substrate. The [111] projection image shows a hexagon, and nanoctahedron is stabilized by contacting one of its equilateral triangular facets on the substrate.

)

)

)

the case of Pt3Ni nanoctahedra, however, we have only observed one type of arrangement in each orientation as shown in Figure 6a c, based on a multitude of other TEM spots on multilayer samples assembled on TEM grids via a solvent evaporation approach. Among the three orientations observed, the orientation exhibited in Figure 6b is the ordering that was found most frequently. These supercrystal orientations can be represented more clearly in their corresponding (negative) SAED patterns as shown in Figure 6d f, in which a group of >100 NCs was selected, taken from each multilayer assembly pattern. An intriguing feature is that these SAED patterns displayed discrete diffraction spots similar to those of a single crystal, instead of diffraction rings obtained normally from polycrystals. On the basis of this analysis, models of nanoctahedral stacking structures corresponding to the three TEM patterns are given in Figure 6g i. Through careful examination of the three assembly structures, it was determined that they actually correspond to the same bcc superlattice with [100], [110], and [111] projections, respectively. In other words, all of the nanoctahedra in such a superlattice have the same corner-to-corner orientation. The orientation correlation between the Pt3Ni nanoctahedra and the bcc superlattice can be described as follows: [100]superlattice [100]Pt3Ni-nanoctahedra, [110]superlattice [110]Pt3Ni-nanoctahedra, and [111]superlattice [111] Pt3Ni-nanoctahedra. A 3D structure of an octahedron presented in section S3 (Supporting Information) shows a building block in Figure 6g i which actually describes the same bcc superlattice structure. GISAXS Study. Although TEM images and tomography provided a clear insight about the superstructure of Pt3Ni nanoctahedron patterns locally, TEM alone is neither capable of probing the bulk structure of a whole superlattice grain nor capable of providing an unambiguous differentiation between projections of superlattice with different packing patterns. There is a critical need to fully capture the 3D bulk packing information of these assemblies over multiple length scales. To fill this gap, GISAXS has emerged as a powerful tool to confirm such 3D superstructures.23,28,29 Figure 7a (also see section S4 in the Supporting Information) shows a GISAXS pattern of a superlattice prepared

Figure 6. TEM images, diffraction patterns, and models of Pt3Ni superlattices formed through a controlled solvent evaporation processing approach: (a c) TEM images of Pt3Ni multilayer patterns with different orientations; (d f) the corresponding selected area electron diffraction (negative) pattern of superlattices shown in (a-c); (g-i) the corresponding bcc structural model of (a c) in their TEM projection directions. Projection directions: [100] for (a, d, g); [110] for (b, e, h); and [111] for (c, f, i).

through a controlled solvent evaporation approach, while section S5 (Supporting Information) shows a GISAXS pattern of Pt3Ni nanoctahedron films formed onto Si wafers by in-situ dropcasting at the beamline. With a well-controlled solvent (hexane) evaporation rate of a high-concentration Pt3Ni octahedral colloidal suspension (∼2 mg/mL), large-scale 3D superlattices consisting of Pt3Ni nanoctahedra could be assembled on a polished silicon wafer.12,16 SEM characterization revealed that the as-formed superlattices, with a size of at least micrometers in scale, possess multilayer structures with a high level of ordering (Figure 7b and section S6 in the Supporting Information). The observed GISAXS patterns on both samples show similar distinguishing scattering features and are unambiguously indexed with bcc superlattice structure with the (110) lattice plane parallel to the substrate (refer to Figure 7a and section S7 in the Supporting Information).30 In the substrate plane the orientation of the superlattice grains is random, i.e., they form a uniaxially aligned powder. Figure 7a shows the bcc indexation of the stronger spots. Weaker, barely discernible spots also follow the bcc pattern (refer to section S4 in the Supporting Information), but indices were not shown for clarity. At an incident angle of 0.25, films of up to a couple micrometers in thickness are still fully penetrated by the X-ray beam. Hence the observed scattering pattern represents a characterization of the full interior of the nanoctahedron deposit. We observed scattering from wellaligned bcc {110} crystallites yielding sharp diffraction spots. On the basis of a fit of these scattering features, we index the GISAXS patterns in section S4 (Supporting Information) as a (110) oriented bcc superlattice with a lattice constant of 15.7 nm, in good agreement with the TEM result. The in-situ cast film in 2916

dx.doi.org/10.1021/nl201386e |Nano Lett. 2011, 11, 2912–2918

Nano Letters

Figure 7. GISAXS pattern (a) and corresponding SEM image (b) of a Pt3Ni nanoctahedron sample prepared under controlled solvent drying resulting in micrometer-sized individual superlattices (refer to figures in sections S4 and S6 of the Supporting Information). The GISAXS image is shown as a function of the parallel (qpar) and perpendicular (qperp) scattering vector using a false-color logarithmic intensity scale, as indicated by the scale bar on the right edge of the image. Scattering spots can be unambiguously indexed as a bcc lattice with the (110) face parallel to the substrate and a lattice constant of 15.7 nm (refer to figure in section S7 of the Supporting Information). Miller indices are denoted below the corresponding spot. In addition low-index bcc powder rings are indicated. A small amount of the material follows the rings and is thus bcc, but without specific orientation relative to the substrate.

section S5 (Supporting Information) displays a larger lattice constant of 17.5 nm. In later in-situ experiments, we learned that apparently this film was not completely dried and the larger lattice constant was due to remnant solvent in the lattice. Apart from the well-defined spots due to the oriented superlattice, we also observed weak {110} and {220} conventional powder rings between the spots, indicative of a small amount of unoriented bcc superlattice grains.

LETTER

The combination of TEM, TEM topography, GISAXS, and SEM clearly determined a bcc packing superstructure formed by Pt3Ni nanoctahedra with a specific orientation on their lattice sites. The ideal unit cell of the bcc superstructure contains two nanoctahedra, resulting in a very low volume fraction of 1/3 with respect to the metal metallic cores (see section S8 in the Supporting Information). The remaining space is taken up by oleylamine ligands,22,31 anchored on the Pt3Ni facets by the amine head. The distance between {111} facets of any adjacent nanoctahedra is determined as 4.532 nm from the bcc model. We estimated that for a superlattice containing perfect nanoctahedra with a bcc superlattice constant of 15.7 nm and an average ligand molecular length of ∼2 nm, the ligands can at most fill 66.19% of the space between the metal cores (section S8 in the Supporting Information). In the TEM investigations, we found that any space between assembled Pt3Ni nanoctahedra is a little darker than the empty background, which indicates that the ligands fill the available space evenly, giving rise to a repulsive interactions to stabilize the self-organized superlattice with an extremely low packing density. We note that the observed bcc packing superstructure is closely related to the perfect space filling of the Archimedean truncated octahedron32 which corresponds to the Wigner Seitz cell of the bcc lattice. Thus, if the tips of these nanoctahedra are not perfectly formed but somewhat truncated, the packing density could be tunable, from 33.33% (for ideal nanoctahedra) to perfect space filling for Archimedean truncated nanoctahedra. In fact, the nanoctahedra we synthesized are slightly truncated. Therefore, the real packing density is somewhat higher than the ideal value of 33.33%. Using the tomography slice views (Figure 3e, Figure 3f, and Figure 4e g), among the total area of 156528 nm2 measured, the nanoctahedra area is 74662 nm2, occupying an area fraction of 47.7%. By supposing the slices are made randomly so that the area fraction equals to the volume fraction based on the stereological fundamental principle,33 the real packing density is thus estimated as 47.7%. This provides an option of varying the packing density by tuning the building block shape which is determined by using a Æ100æ/Æ111æ growth rate ratio, R.34 Conclusions. Well-defined Pt3Ni nanoctahedra self-assembled into superlattices through both drop-casting and controlled solvent evaporation approaches on TEM grids and Si wafers, respectively. TEM tomography and GISAXS were used to explore this low packing density superstructure, with regard to not only the lattice type and lattice parameter but also the nanocrystals coordinates and their orientations that were not explored previously. Our observations suggest that bcc is the most dominant arrangement in this system yielding a low-density packing of the NC cores, while the remaining volume is taken up, at least partially, by the oleylamine ligands. We were able to quantify this interstitial space directly using TEM tomography. The formation of such superlattices is dependent on neither the processing pathway (drop-casting versus controlled solvent evaporation) nor the substrate (TEM grid or silicon wafer) under our experimental conditions. This type of self-assembled Pt3Ni superlattices is expected to yield a promising metamaterial, because their unique low packing density—this is perhaps the lowest value achieved so far. To explore a potential application, development of techniques for removal of the organic capping ligands from the superlattice, such as controlled plasma annealing, will be the next challenge. 2917

dx.doi.org/10.1021/nl201386e |Nano Lett. 2011, 11, 2912–2918

Nano Letters

’ ASSOCIATED CONTENT

bS

Supporting Information. Information on the EDS analyses of Pt3Ni nanoctahedra, ICP-MS results of Pt3Ni nanoctahedra, 3D structure of an octahedron, optical micrograph and associated GISAXS pattern of a Pt3Ni NC film dried under controlled solvent evaporation conditions and in-situ drop-cast on a clean Si wafer, typical SEM images of Pt3Ni nanoctahedron containing superlattices, origin of the bcc (110) uniaxial diffraction pattern, and estimate of the ligand space-filling factor. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: D.-M. Smilgies, [email protected]; J. Fang, jfang@ binghamton.edu. Author Contributions #

These authors contributed equally to this work.

’ ACKNOWLEDGMENT This work was supported by NSF (DMR-0731382). CHESS was supported by the NSF & NIH/NIGMS via NSF award DMR0936384. Part of the structural modeling was done using MS Modeling 5.0 through Laboratory for Molecular Simulation, Department of Chemistry, Texas A&M University. We thank Dr. Kaikun Yang and Professor Howard Wang for their assistance in the SEM study. We also thank Dr. Hongzhou Yang and Prof. Shouzhong Zou for their support on ICP analysis.

LETTER

(15) Li, F.; Delo, S. A.; Stein, A. Angew. Chem., Int. Ed. 2007, 46, 6666–6669. (16) Lu, W.; Liu, Q.; Sun, Z.; He, J.; Ezeolu, C.; Fang, J. J. Am. Chem. Soc. 2008, 130, 6983–6991. (17) Niu, W.; Zheng, S.; Wang, D.; Liu, X.; Li, H.; Han, S.; Chen, J.; Tang, Z.; Xu, G. J. Am. Chem. Soc. 2009, 131, 697–703. (18) Chang, C.-C.; Wu, H.-L.; Kuo, C.-H.; Huang, M. H. Chem. Mater. 2008, 20, 7570–7574. (19) Xie, S.; Zhou, X.; Han, X.; Kuang, Q.; Jin, M.; Jiang, Y.; Xie, Z.; Zheng, L. J. Phys. Chem. C 2009, 113, 19107–19111. (20) Chen, S.; Ferreira, P. J.; Sheng, W.; Yabuuchi, N.; Allard, L. F.; Shao-Horn, Y. J. Am. Chem. Soc. 2009, 130, 13818–13819. (21) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493–497. (22) Zhang, J.; Yang, H.; Fang, J.; Zou, S. Nano Lett. 2010, 10, 638–644. (23) Hanrath, T.; Choi, J. J.; Smilgies, D.-M. ACS Nano 2009, 3, 2975–2988. (24) Song, Q.; Ding, Y.; Wang, Z. L.; Zhang, Z. J. J. Phys. Chem. B 2006, 110, 25547–25550. (25) Wang, Z. L. Adv. Mater. 1998, 10, 13–30. (26) Zheng, R.; Gu, H.; Xu, B.; Fung, K. K.; Zhang, X.; Ringer, S. P. Adv. Mater. 2006, 18, 2418–2421. (27) Torquato, S.; Jiao, Y. Nature 2009, 460, 876–880. (28) Saunders, A. E.; Ghezelbash, A.; Smilgies, D.-M.; M., B. S., Jr.; Korgel, B. A. Nano Lett. 2006, 6, 2959–2963. (29) Smith, D. K.; Goodfellow, B.; Smilgies, D. M.; Korgel, B. A. J. Am. Chem. Soc. 2009, 131, 3281–3290. (30) Smilgies, D.-M.; Blasini, D. R. J. Appl. Crystallogr. 2007, 40, 716–718. (31) Zhang, J.; Yang, H.; Yang, K.; Fang, J.; Zou, S.; Luo, Z.; Wang, H.; Bae, I.-T.; Jung, D. Y. Adv. Funct. Mater. 2010, 20, 3727–3733. (32) Chien, C. Acta Crystallogr. 1979, A35, 946–952. (33) Underwood, E. E. Quantitative Stereology; Addison-Wesley: Reading, MA, 1970. (34) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153–1175.

’ REFERENCES (1) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Nature 2010, 466, 474–477. (2) Tao, A. R.; Ceperley, D. P.; Sinsermsuksakul, P.; Neureuther, A. R.; Yang, P. Nano Lett. 2008, 8, 4033–4038. (3) Rogach, A. L.; Eychm€uller, A.; Hickey, S. G.; Kershaw, S. V. Small 2007, 3, 536–557. (4) Jones, M. R.; Macfarlane, R. J.; Lee, B.; Zhang, J.; Young, K. L.; Senesi, A. J.; Mirkin, C. A. Nat. Mater. 2010, 9, 913–917. (5) Chen, H.-T.; Padilla, W. J.; Cich, M. J.; Azad, A. K.; Averitt, R. D.; Taylor, A. J. Nat. Photonics 2009, 3, 148–151. (6) Tam, E.; Podsiadlo, P.; Shevchenko, E.; Ogletree, D. F.; Delplancke-Ogletree, M.-P.; Ashby, P. D. Nano Lett. 2010, 10, 2363–2367. (7) Liu, X.; Zhao, L.; Shen, H.; Xu, H.; Lu, L. Talanta 2011, 83, 1023–1029. (8) Quan, Z.; Fang, J. Nano Today 2010, 5, 390–411. (9) Haji-Akbari, A.; Engel, M.; Keys, A. S.; Zheng, X.; Petschek, R. G.; Palffy-Muhoray, P.; Glotzer, S. C. Nature 2009, 462, 773–777. (10) West, A. Basic Solid State Chemistry, 2nd ed.; John Wiley & Sons: Chichester, 1999. (11) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428–433. (12) Zhang, J.; Kumbhar, A.; He, J.; Das, N. C.; Yang, K.; Wang J.-Q.; Wang, H.; Stokes, K. L.; Fang, J. J. Am. Chem. Soc. 2008, 130, 15203–15209. (13) Hoogenboom, J. P.; Retif, C.; Bres, E. d.; Boer, M. v. d.; LangenSuurling, A. K. v.; Romijn, J.; Blaaderen, A. v. Nano Lett. 2004, 4, 205–208. (14) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821–823. 2918

dx.doi.org/10.1021/nl201386e |Nano Lett. 2011, 11, 2912–2918