Letter pubs.acs.org/NanoLett
Tilted Face-Centered-Cubic Supercrystals of PbS Nanocubes Zewei Quan,† Welley Siu Loc,† Cuikun Lin,‡ Zhiping Luo,§,▼ Kaikun Yang,∥ Yuxuan Wang,⊥ Howard Wang,∥ Zhongwu Wang,*,# and Jiye Fang*,†,⊥ †
Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, United States Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United States § Microscopy and Imaging Center and Materials Science and Engineering Program, Texas A&M University, College Station, Texas 77843, United States ∥ Department of Mechanical Engineering, State University of New York at Binghamton, Binghamton, New York 13902, United States ⊥ Materials Science and Engineering Program, State University of New York at Binghamton, Binghamton, New York 13902, United States # Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University, Ithaca, New York 14853, United States ‡
S Supporting Information *
ABSTRACT: We demonstrate a direct fabrication of PbS nanocube supercrystals without size-selection pretreatment on the building blocks. Electron microscopic and synchrotron small angle X-ray scattering analyses confirm that nanocubes pack through a tilted face-centered-cubic (fcc) arrangement, that is, face-to-face along the ⟨110⟩super direction, resulting in a real packing efficiency of as high as ∼83%. This new type of superstructure consisting of nanocubes as building blocks, reported here for the first time, is considered the most stable surfactant-capped nanocube superstructure determined by far. KEYWORDS: Self-assembly, PbS nanocubes, tilted face-centered-cubic, supercrystal, synchrotron X-ray scattering
S
building blocks, the packing structure of a supercrystal consisting of nonspherical ones is more complicated, and the highest packing efficiency generated may not always be 74%. In the case of tetrahedral particle packing, for example, even a subtle shape change can cause profound variation in the resulting ordered structures.20 As reported before, a simple cubic and 1D-shifted cubic structure could generally be observed from a supercrystal consisting of cubic nanobuilding blocks.21,22 Packing patterns with such superstructures will theoretically achieve a 100% packing efficiency23 if the capping ligands are neglected. In a real nanocube-based supercrystal, however, the surface-passivated organic ligands make any adjacent nanocubes “separated” with a ligand-filled interstice (soft gap), resulting in an actual packing efficiency of lower than 100%. In an oleylamine−oleic acid synthesis system, it is well-known that the average ligand length is around ∼2.0 nm.24 The question is, when assembled slowly enough, whether a supercrystal with ∼2.0 nm interstice of organic ligands will still remain an energetically favorable face-to-face simple cubic superstructure23 or not. In this work, we adopt PbS nanocube as a model system and investigate its assembled superstructures in a large length scale, using both synchrotron small angle X-ray scattering (SAXS)
elf-assembly, as a natural phenomenon, is the autonomous organization of components into patterns or structures without human intervention, which is common in nature with complex, functional, and self-assembled structures such as opals and magnetosomes.1−4 Due to fundamental chemical and physical interest,5 self-assembly has been attracting intensive attention at all length scales especially in the nanometer range since the first report of iron oxide supercrystals in 1989.6 Based on effects such as biorecognition, shape complementarity, size, and charge, various approaches of self-assembly have been demonstrated in this quickly developing area.7−13 As a direct outcome of size-controlled synthesis, most self-assembly preparations were using spherical nanoparticles as their building blocks.14−16 For spherical nanocrystals, face-centered-cubic ( fcc) and hexagonal-closed-pack (hcp) stacking structures offer the highest packing efficiency (74%) and are often observed, although other packing symmetries may also be determined by the presence of certain interactions. Progressive achievement in colloid chemistry makes it feasible to prepare many classes of well-defined nonspherical nanocrystals. This allows a possibility of fabricating nonspherical nanoparticle-based supercrystals with additional investigation on their superstructures. In fact, considerable development has been made in enumerating and characterizing the stacking structure of polyhedral shapes.17−19 Recently, various types of superlattices assembled from nonspherical building have been reviewed.19 In comparison with spherical © 2012 American Chemical Society
Received: June 21, 2012 Revised: July 11, 2012 Published: July 19, 2012 4409
dx.doi.org/10.1021/nl302324b | Nano Lett. 2012, 12, 4409−4413
Nano Letters
Letter
the as-prepared PbS nanocube as building blocks. Scanning electron microscopy (SEM) images of the as-formed PbS nanocube assemblies were taken on a field emission scanning electron microscope (FE-SEM, Carl Zeiss Supra 55 VP). Results and Discussion. Figure 1a shows a typical TEM image of the as-prepared PbS nanocubes (prior to the selfassembly) in monolayer, demonstrating that the nanocube building blocks have an average edge length of ∼13 nm. A high-
and electron microscopy (EM) techniques. An explicit analysis suggests that these ∼13 nm PbS nanocubes acted as building blocks of an ultra slow assembly form an fcc superstructure in their tilted positions, instead of a simple cubic symmetry predicted by theory. The identified tilted fcc superstructure was observed as the most stable macroscopic assembly and has never been reported. The insights gained in this study are not only interesting in understanding the structure of a cubic supercrystal with anisotropic colloidal building blocks but also significant in developing the concept of nonspherical superlattices. Experimental Section. Synthesis. Sulfur-oleylamine (SOAm) precursor solution (1 M) was prepared in a fume hood by dissolving 0.05 mol of sublimed sulfur powder (J. T. Baker) in oleylamine (OAm, Sigma-Aldrich, 70%, 50 mL) in an Erlenmeyer flask, and the mixture was heated on a hot plate at 100 °C. Next, 3 mmol of lead oxide (PbO, Alfa Aesar, 99.99%), 14 mL of diphenyl ether (DPE, Alfa Aesar, 99%), and 6 mL of oleic acid (OA, Sigma-Aldrich, 90%) were loaded into a roundbottom flask that was connected to a Schlenk line with dry argon (99.999%). The mixture was heated at 150 °C for 1 h and then cooled to room temperature, yielding a clear yellow solution containing 0.15 M Pb-oleate precursors. In a typical synthetic protocol, 10 mL of as-prepared Pboleate, 1.5 mL of OAm, and 7.5 mL of DPE were loaded into a three-neck round-bottom flask in sequence, and 1.5 mL of SOAm was injected into the flask when the mixture was heated to 180 °C under agitation. After 7.5 min, the heating source was removed, and the flask was immediately immersed in a cold water bath to terminate the growth of PbS nanocrystals. The crude suspensions were collected at 50−60 °C and washed with ethyl alcohol (Pharmco AAPER, 200 proof) followed by centrifugation for two cycles. The isolated PbS nanocrystals were redispersed in hexane to form a stable colloidal suspension for future self-assembly. No size-selection treatment was conducted. Self-Assembly. About 60 μL of concentrated PbS nanocube suspensions in hexane (around 0.1 M) was transferred into a 2 mL vial. The vial was then sealed with a plastic cap and kept at room temperature without intervention. It took around 4 weeks to evaporate the solvent completely. Such a slow evaporation led to a formation of nanocube assemblies deposited on the internal wall and bottom of the vessel. Characterization. Several pieces of as-formed PbS nanocube assemblies were transferred into a sample chamber (200 μm in diameter and 100 μm in thickness) seated on the flat surface of one transparent diamond window for synchrotron SAXS measurement. Instead of solid assemblies, the nanocube suspension in hexane was drop-cast onto a Kapton tape for several times and used for wide-angle X-ray scattering (WAXS) characterization. Both SAXS and WAXS measurements were conducted at B2 station of Cornell High Energy Synchrotron Source (CHESS) using an angle dispersive synchrotron X-ray technique. Using a double crystal monochromator, the white Xrays were collimated into monochromatic beam at a wavelength of 0.6888 Å and 0.48596 Å for SAXS and WAXS measurements, respectively.25 A large MAR345 detector was used to record the X-ray scattered signals from the samples. Using a Fit2D software, the two-dimensional images were integrated into patterns with intensity as a function of 2θ (deg) or Q (nm−1) for structural analysis. An FEI Tecnai G2 F20 ST operated at 200 kV was employed to collect transmission electron microscopy (TEM) images of
Figure 1. (a) TEM image of PbS nanocubes in monolayer before their self-assembly (the inset is a high-resolution TEM image of an individual nanocube); (b) WAXS pattern of PbS nanocubes without self-assembly; (c) SEM image of a typical area on surface of an assembled PbS supercrystal. 4410
dx.doi.org/10.1021/nl302324b | Nano Lett. 2012, 12, 4409−4413
Nano Letters
Letter
resolution TEM image (the inset of Figure 1a) shows a high crystallinity with an interplanar spacing of 0.297 nm. This observation confirms the formation of PbS nanocubes with a surface termination of {100} facets. As shown in Figure 1b, the WAXS study indicates that the as-prepared PbS nanocubes possess a typical fcc atomic structure (Fm3̅m). The formation of (100)-terminated PbS nanocubes is resulted from a decreased reaction rate along ⟨100⟩ directions during the crystal growth stage, selectively tuned by the capping ligands. As well-known, a colloidal nanocrystal is normally stabilized by organic ligands through a covalently capping on surface. Since the hydrocarbon length of either an oleic acid or oleylamine molecule (the capping ligand in this system) is about ∼2.0 nm, respectively, the distance between adjacent nanocubes in Figure 1a matches the capping molecular length very well. This implies that these PbS nanocrystals might be separated by repulsive forces from their steric ligands. Figure 1c illustrates a typical SEM image on surface of an assembled PbS supercrystal. Most of the explored areas on the assembled supercrystal surface exhibit the similar structure as indicated in Figure 1c, showing the presence of ∼13 nm nanocubes rather than the minor phase that contains small and “irregular” particles as shown in Figure 1a. Figure 1c suggests that the self-assembly is also a process of building block self-organization (vide infra). It should be noted that the as-prepared nanocubes are not monodisperse. A small amount of nanocrystals in other shapes and/or with different sizes were also observed in Figure 1a. It was known that a narrow size distribution is the critical requirement for a formation of nanocrystal superlattice. In the current system without a size-selection, however, ordered nanocube supercrystals with dimensions up to tens of micrometers could be harvested (Figure S1 in the Supporting Information), while in the same assembled specimen small pieces of disordered assemblies consisting of small amount of noncubic nanocrystals (
