Birth and Growth of Octapod-Shaped Colloidal Nanocrystals Studied

Sep 6, 2011 - The growth of octapod-shaped colloidal nanocrystals is analyzed in detail by electron tomography. The 3D shape of the starting cubic ...
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Birth and Growth of Octapod-Shaped Colloidal Nanocrystals Studied by Electron Tomography Rosaria Brescia,* Karol Miszta, Dirk Dorfs, Liberato Manna, and Giovanni Bertoni Istituto Italiano di Tecnologia (IIT), Via Morego 30, 16163 Genova, Italy

bS Supporting Information ABSTRACT: The growth of octapod-shaped colloidal nanocrystals is analyzed in detail by electron tomography. The 3D shape of the starting cubic berzelianite Cu2 xSe seeds is studied, and their evolution into the final structure composed of a sphalerite CdSe core and eight wurtzite CdS pods is followed. The 3D reconstruction shows markedly different shapes of the pods, four of them having sharp tips and four flat ends. Also, the combination of the tomography results with high-resolution and energy-filtered transmission electron microscopy analyses leads to a precise identification of the structure and composition of the final particles. An interpretation of the different shapes of the pod ends is given based on the intrinsic anisotropy of both the sphalerite CdSe and the wurtzite CdS crystal structures and the different reactivity of their facets in the growth environment.

’ INTRODUCTION Over the past decade, considerable progress has been made in the synthesis of colloidal nanocrystals of a wide variety of materials with controlled sizes and shapes and in the development of applications based on them.1,2 This progress is also being fostered by the concomitant refinement of many analytical tools for studying the structural, morphological, and compositional features of objects on the nanoscale.3 5 A detailed knowledge of the shape of a nanocrystal can help clarify its growth mechanism6,7 and in addition is important for calculating its electronic and optical properties8,9 and for predicting/modeling the formation of ordered superstructures when using the nanocrystal as a building block.10 12 Because of the small size of colloidal nanocrystals, the identification of their 3D shape requires a volume reconstruction with nanometer resolution. The technique of choice in this case is tomography based on electron microscopy.13 Whereas single transmission electron microscopy (TEM) images only provide 2D projections, reaching sub-Angstrom resolution in the case of high-resolution TEM (HRTEM), volume reconstructions can be performed starting from a series of TEM images acquired at different tilt angles. Nowadays this technique is widely employed to identify the 3D structure of various objects with spatial resolution limited to ∼1 nm. In the case of crystalline nanoparticles, both scanning TEM (STEM) high-angle annular dark field (HAADF) and energyfiltered TEM (EFTEM) imaging can be used for electron tomography because these modes satisfy the projection requirement necessary for reconstruction due to the predominantly incoherent scattering of the collected electrons.13 On the basis of the nature of the particular analyzed system, either one of the two techniques is preferred.14 Electron tomography has already been used to study various nanocrystals with peculiar shapes;9,15 17 r 2011 American Chemical Society

however, only a few studies have exploited this tool, in conjunction with other microscopy techniques, to monitor the shape evolution during the synthesis of nanoparticles.6 Recently, our group developed the synthesis of branched, octapod-shaped colloidal nanocrystals consisting of a central CdSe core and eight CdS “pods” departing from it.18 The synthesis of these particles proceeds via seeded growth starting from Cu2 xSe nanocrystals in the cubic berzelianite phase, with a radius of 10 15 nm. These undergo a rapid cation exchange and are converted to sphalerite CdSe nanocrystals, the latter preserving the shape and size of the starting Cu2 xSe nanocrystals. These relatively large CdSe seeds, with well-developed {111} facets, promote the nucleation and growth of hexagonal wurtzite CdS pods on all of their eight facets. The aim of the present work is to address the following aspects related to these octapods: (i) Because sphalerite CdSe has only tetrahedral symmetry, four of the eight {111} facets of the CdSe seeds should have lower reactivity than the other {111} facets. How does this translate into the growth rate and detailed morphology of the various CdS pods? (ii) What is the morphology of the pods in the early stages of octapod development? (iii) Does the initial seed preserve its initial shape in the final octapod? To this aim, we reconstructed, by combining electron tomography and HRTEM, the 3D shape of the starting cubic berzelianite Cu2 xSe seeds as well as their evolution into CdSe/CdS octapods. In the 3D reconstruction of the octapods, four pods have sharp tips, whereas the remaining four have flat ends, which is in line with the different growth rates of the various crystal facets of Received: July 2, 2011 Revised: August 31, 2011 Published: September 06, 2011 20128

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wurtzite in the specific growth environment. This difference could be observed already in the very early stages of octapod growth. In addition to this, we combined data from tomography, HRTEM, and EFTEM to identify precisely the structure and composition of the final particles. An outcome of these latter analyses is for instance that the initial seed preserves its size and shape in the fully developed octapod.

’ MATERIALS AND METHODS The TEM analyses were performed using a JEOL JEM2200FS microscope operated at 200 kV equipped with a CEOS aberration corrector for the objective lens (point resolution 0.9 Å). The microscope has an in-column Omega energy filter (0.8 eV energy resolution) and a STEM unit (GATAN Digiscan II) with an HAADF detector, with a point resolution of 1.9 Å in STEMHAADF mode. The preparation procedure of the starting Cu2 xSe seeds and the details of the one-pot CdSe(core)/CdS(pods) octapod synthesis are thoroughly described in the Supporting Information. The samples for HRTEM-EFTEM analyses were prepared by depositing ∼10 μL of the solution onto copper grids with an ultrathin carbon layer on top of a lacey carbon net. For the acquisition of the tilt series for tomography, one drop of each solution was deposited onto a 1.5  1.5 mm2 square copper grid coated with a thin carbon layer. The grid was then mounted onto a purposely designed sample holder for tomography (FISCHIONE model 2030). For the acquisition of tilt series of 2D images, the STEM-HAADF mode was preferred over the single-window EFTEM mode due to the lower contamination and reduced beam damage.14 The tilt series were acquired in the widest possible angular range (maximum allowed in the employed system: 70 to +70) with a 2 sampling. The smallest condenser lens aperture (2.7 mrad) was used for improved depth of focus. Alignment based on cross-correlation and sample tomograms (without fiducial markers) was applied to the tilt series using the IMOD software.19 The volume reconstruction was then performed starting from the aligned tilt series via a combination of weighted back-projection (WBP) and simultaneous iterative reconstruction technique (SIRT) using the plug-in TomoJ of ImageJ.20,21 After initial WBP, a few tens of SIRT iterations were carried out until convergence was reached. The reported surface rendering was performed using the UCSF Chimera package.22 ’ RESULTS HRTEM images of the Cu2 xSe nanocrystals (in the cubic berzelianite phase) used as starting seeds for the growth of octapods show shapes compatible with 2D projections of regular octahedrons with slight truncation (Figure 1a). The average size of the particles is typically 16 ( 3 nm, as determined from conventional TEM or STEM-HAADF images (e.g., see Figure 1a,b). However, this size was evaluated as the width of 2D projections of randomly oriented 3D nanoparticles, assuming a spherical shape that clearly does not correspond to reality. The exact 3D shape of the Cu2 xSe seeds was indeed obtained by an electron tomographic reconstruction ( 50 to +62 tilt series, 2 Å spot). The isosurface of the reconstructed volume of one representative nanocrystal oriented along two significant orientations is reported in Figure 1c,d. The geometry of the particle is close to a regular octahedron limited by equilateral triangular facets. (See the model in the insets in the Figures.) Since the crystal structure of the seeds, determined by HRTEM and

Figure 1. (a) HRTEM image of a cubic berzelianite Cu2 xSe nanoparticle viewed along the [110] zone axis. (See the fast Fourier transform in the top inset.) The observed projection corresponds to a regular octahedron with a slight truncation along the [001] direction and negligible truncation along the remaining two Æ100æ directions. (See the dotted lines in the model in the bottom inset.) (b) STEM-HAADF image of a group of Cu2 xSe nanocrystals. (c,d) Isosurface rendering of the volume of one representative particle (indicated with the arrow in panel b) reconstructed by means of electron tomography, viewed along the (c) [001] and (d) [110] directions of the cubic structure. The insets show the corresponding projections of the geometric model of a regular octahedron, without the slight truncations visible at the corners.

XRD,18 is cubic, the lateral facets of the octahedron can be identified as the {111} triangular sections (with edge l ≈ 13 nm ) of a cube. In good agreement with the HRTEM images, the reconstructed particle exhibits a slight truncation of some of the corners, visible in several seeds (Figure 1). This truncation, when more pronounced, can give rise to the previously reported truncated octahedral shape.18 The following discussion is based on the assumption that the prevailing shape adopted as a base for pods growth is a simple octahedron, with negligible truncation, because this explains the shape of the final particles. The most marked truncation observed leads indeed to ∼10% surface reduction of the complete {111} triangular facets, which can be assumed to have only a negligible effect on the following pod growth. (See the following section) The reconstructed shape of the Cu2 xSe particles is in agreement with the one already reported for nanocrystals of similar size based on cubic crystal structures.10,11,14 The shape of each particle in TEM/STEM images can now be recognized as a particular 2D projection of a slightly truncated octahedron, and an average value can be estimated for well-defined dimensions. For instance, the average length of the edge of the triangular facets determined from Figure 1b is l = 12 ( 2 nm. Whereas tomography cannot provide statistical data, a more reliable indication of the average size of the particles is obtained from 2D TEM/STEM images of several seeds. Moreover, one always has to keep in mind that a volume reconstruction has a lower spatial resolution compared with the one of individual projections.13 20129

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Figure 2. (a) HRTEM image of a “baby” octapod in an aliquot collected after 2 min from the start of the CdS pod growth, highlighting the formation of wurtzite CdS pods elongated along their Æ0001æ direction, with flat ends (F) opposed to sharp tips (T), around the central core. The dashed contour highlights the outer profile of the epitaxial CdS(pod)/CdSe(core)/CdS(pod) heterostructure. Sphalerite CdSe-related lattice planes are not visible because of the superposition of the CdS pods onto the CdSe core in the selected orientation. (The lattice planes of pods with their Æ0001æ direction perpendicular to the plane are shown in the particle center.) (b) Tomographic reconstruction of one particle from the same sample, exhibiting the presence of sharp and flat ends in opposite pods and (c) corresponding top and bottom views showing the three-fold symmetry, with additional faceting in some cases. (See the movie in the Supporting Information.)

Figure 3. (a) HRTEM image of one octapod from sample 1 with arms clearly showing either flat ends (F) or sharp tips (T). Two opposite pods exhibit the [5 2 7 3] pattern of wurtzite CdS (see insets on top left and bottom right), confirming the identical orientation of opposite pods with respect to each other. In the inset on top right, the combination of EFTEM elemental maps of S (yellow) and Se (magenta) in one octapod of the same sample shows the presence of a Se-containing core in the center of the particles and S-rich pods. (b) HRTEM image of one particle of sample 1 with one pod pointing upward (and the opposite one pointing downward), showing the triangular cross-section of the pod. The fast Fourier transform (inset) of the area including the vertical pods shows that they are directed along the direction of wurtzite CdS.

For the first octapod sample, an aliquot was extracted after 2 min of reaction. (See the Supporting Information.) During this time, the Cu2 xSe nanoparticles had undergone complete cation exchange so that they had turned into sphalerite CdSe; in addition, CdS pods had already started growing on top of them. The HRTEM images of these nanocrystals recorded at favorable orientations (e.g., see Figure 2a) indicate the presence of short single crystal wurtzite CdS pods around the core and elongated along their Æ0001æ direction. (See also Figure SI-1 of the Supporting Information.) Moreover, different shapes of the opposite pods are observed (Figure 2a): one pod exhibits a

sharpened tip, whereas the opposite pod has a flat end. An epitaxial structure is formed when moving from one pod to the opposite pod through the core. (See the contour drawn in Figure 2a.) It is clear from tomographic reconstructions ( 70 to +70 tilt series, 7 Å spot) of particles of this sample (Figure 2b,c) that the octapod shape is not yet well-defined at this stage of growth; nevertheless, the markedly different morphology in opposite pods can be identified. (See the movie in the Supporting Information.) One pod has a triangular pyramidal shape, whereas the opposite pod has a dome shape, with triangular base and a rather flat end. This clearly reflects 20130

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Figure 5. (a) Schematic 3D crystal model (not in scale) of an octahedral sphalerite CdSe core and two opposite wurtzite CdS pods with triangular section starting from it (e.g., corresponding to the pods highlighted in blue in the tomographic reconstruction in the inset, obtained from a particle of sample 1). The model emphasizes the faceting and the relative orientation of the two opposite pods relative to each other and with respect to the core. (b,c) Atomic arrangement and bonding of Cd and S atoms on the (0001) and (1 0 1 1) surfaces of wurtzite CdS, respectively.

Figure 4. (a) STEM-HAADF images of three significant projections of a pair of octapods of sample 1 and (b) corresponding orientations of the electron tomography reconstruction of the volume of the particles (common scale in all panels). (c) Enlarged view of the volume reconstruction of one octapod of the same sample and (d) one octapod of sample 2. (e) Schematic model of the geometry of an octapod, with the corresponding crystallographic facets of wurtzite CdS indicated for two opposite pods. The lateral facets are indexed as {1 0 1 0} for the sake of simplicity despite their small off-axis angle. In panel c, the arrow indicates an irregularly faceted pod.

the different reactivity of the opposite {111} facets of the CdSe seeds, having a sphalerite structure and therefore only tetrahedral symmetry, and of the opposite {0001} facets of the wurtzite CdS pods, lacking inversion symmetry. For representative samples of “fully-grown” octapods, several syntheses were carried out, and aliquots were collected from the reaction flask at longer reaction times (sample 1 collected after 10 min reaction under the same conditions as the 2 min aliquot; sample 2 collected after 7 min in slightly different reaction conditions, see the Supporting Information). The epitaxial orientation between core and pods of these particles was already reported in our previous work (CdSesphalerite(111)[211]//CdSwurtzite(0001)[1 0 1 0]).18 In this stage of growth, some of the pods clearly show sharp tips and others flat ends (Figure 3), as in the “baby” octapods. In addition, the HRTEM analysis of an

individual octapod provides evidence that pairs of opposite CdS pods, one exhibiting a sharp tip and the other one exhibiting a flat end, share the same crystal orientation (Figure 3a). EFTEM elemental maps were acquired with the three-windows method for S (L edge at 165 eV, 10 20 eV slit width) and Se (L edge at 1436 eV, 60 eV slit width) on “grown-up” octapods to deduce the shape of the CdSe core in the final particles. For this aim, the most favorable orientation is the Æ100æ direction of CdSe, with octapods having four pods touching the film and the remaining four pointing upward (inset in Figure 3a). The combination of the maps of S and Se, evaluated for several particles from samples 1 and 2, indicates that the projection of the CdSe core is a square with a side l of 12 ( 1 nm. (See also Figure SI-2 of the Supporting Information.) The shape corresponds to the projection of a simple octahedron made of cubic CdSe along its Æ100æ direction, and the edge size is compatible with the average l value estimated for the initial Cu2 xSe seeds. By analyzing HRTEM images of octapods with two pods vertically aligned, we can deduce that the CdS pods grow along their Æ0001æ directions and have a triangular section, with only slight faceting on three additional sides (Figure 3b). Volume reconstructions of representative “grown-up” octapods of the samples 1 and 2 are reported in Figure 4 ( 70 to +70 tilt series, 7 Å spot). For sample 1, the tomographic reconstruction of a pair of octapods is compared with the original STEM-HAADF images in some significant projections of the tilt series as a verification of the fidelity of the 3D reconstruction (Figure 4a,b). Single HAADF projections, as HRTEM images, clearly evidence the different shapes of pods that are opposite to 20131

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The Journal of Physical Chemistry C each other with respect to the central region of the octapod. The present 3D reconstructions of octapods of the two samples, characterized by rather different dimensions (Figure 4 c,d), indeed show that the structure can be viewed as a superposition of two tetrapods, one with flat-ending pods and one with tipending pods, sharing the same center. All pods have a welldefined triangular section, starting from the base up to the tip, with a ∼70 angle between the axes of neighboring pods. In particular, the particles from sample 1 exhibit a tapering shape, with the three lateral facets forming a small angle (//CdSe < 111>). (See Figure 5a.) The observed triangular section of the pods is a further proof that the truncation of the octahedral Cu2 xSe seeds is negligible for the sets of samples analyzed here. Alternatively, in the presence of appreciable truncation of the octahedral seeds, the {111} facets would rather be hexagonal, leading to a predominantly hexagonal section of the CdS pods, already reported for wurtzite CdSe(core)/wurtzite CdS(shell) nanorods25 and for the pods of sphalerite CdSe(core)/wurtzite CdS(pods) tetrapods.26 In the octapods, if one pod is limited by vicinal surfaces of three of the six {1 0 1 0} planes of the wurtzite structure, approximately perpendicular to the base, the opposite pod develops vicinal surfaces of the three remaining {1 0 1 0} planes. (See Figures 4e and 5a.) Since the {1 0 1 0} surfaces are nonpolar, even the observed low off-angle of the lateral facets (tapered shape of the pods) does not significantly change their polarity. Some of the pods show an irregular shape with respect to the one described here, for instance, additional faceting (e.g., the pod indicated with the arrow in Figure 4c). This can be attributed to occasional irregular faceting in the seeds

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compared with the simple octahedron (e.g., appreciable truncation). Moreover, the tip geometry is compatible with the formation of three {1 0 1 1} facets (Figure 5a), appearing at the beginning of the pod growth, as suggested by the pyramidal tips of the particle from the 2 min aliquot (Figure 2). HRTEM analyses of both the “baby” and the “grown-up” octapods show that the whole pod core pod structure can be viewed as an epitaxial CdS-CdSe-CdS heterostructure. The observed arrow shape (highlighted by the contour in Figure 2a and visible in Figures 3 and 4) is similar to what was already reported for wurtzite CdSe and CdS rods.27,28 The 3D reconstruction demonstrates that opposite CdS pods do not form a single crystal because they develop different faceting, but the epitaxial relationship between them is mediated by the intervening CdSe seed. In a recent work,7 phase image reconstruction of HRTEM images of zinc-blende ZnSe/wurtzite ZnS tetrapods (synthesized with a solvothermal method) allowed to identify the Zn-terminated (111) facets of the initial octahedral ZnSe core as the starting ones for the growth of the four pods because of their higher reactivity in the synthesis environment. In those particles, the pods point toward the ZnS directions. In our “grown-up” CdSe/CdS octapods, the presence of the eight pods complicates the retrieval of the phase image so that the starting core facets of the two types of pods cannot be identified unambiguously. Nevertheless, a plausible model of the morphology of the octapods can be derived by considering the intrinsic asymmetry of the wurtzite structure and the results reported for surface and binding energy calculations that simulate in part a growth environment similar to the one involved in the present octapod synthesis.27,29 In the (0001) and the (0 0 0 1) polar surfaces of CdS, the Cd atoms expose one and three dangling bonds, respectively, suggesting a possibly lower reactivity for the (0001) facet with respect to the opposite (0 0 0 1) facet in our synthesis environment. In this scheme, the wurtzite (0 0 0 1) facet is unstable and quickly replaced by the more stable {1 0 1 1} facets, which effectively slow down the growth rate along the [0 0 0 1] direction, whereas in the opposite direction the (0001) facet is stable enough to survive during the evolution of the pod because it can be well passivated by phosphonic acid molecules.27,29 Both the (0001) facet and the {1 0 1 1} facets are polar and do not differ much from each other in terms of atomic arrangements. Most likely, in our octapods, all polar facets are Cd-terminated because Cd atoms can be efficiently passivated by phosphonic acid/phosphonate molecules in the present reaction environment. Then, the only difference between the two types of Cdterminated polar facets is that in the (0001) facet all Cd atoms have only one dangling bond, whereas in the {1 0 1 1} facets half of the Cd atoms have one dangling bond and the other half have two dangling bonds. (See Figure 5b,c.) The experimental evidence that both the (0001)-facet terminated pods and {1 0 1 1}-facet terminated pods have the same length indicates that the {1 0 1 1} facets should have a growth rate that is slower than that of the (0001) facet because the overall surface area of the three {1 0 1 1} facets that we see in pods with sharp tips is larger than the surface area of the single (0001) facet in the flat-ended pods. In other words, the larger surface area of the three combined {1 0 1 1} facets with respect to the single (0001) facet must be compensated by a faster growth rate of the latter because the two types of pods are growing at comparable rates along the [0 0 0 1] and [0 0 0 1] directions. It is likely that this type of faceting, which equilibrates growth rates along the [0 0 0 1] and [0 0 0 1] 20132

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The Journal of Physical Chemistry C directions, can be well-developed if the pod diameter is large, that is, if it is at least 10 15 nm as in the present case, because the seeds used to grow the octapods are in this size range. In the more traditional seeded grown CdSe/CdS rods, the sizes of the usually employed CdSe seeds are smaller (3 6 nm), consequently resulting in rods with smaller diameters.3,30 In those rods, the CdSe seed is typically located closer to one tip of the rod as a result of unbalanced growth rates along the [0 0 0 1] and [0 0 0 1] directions. Marked faceting at the tips was generally not reported for those rods. In conclusion, in the present work, the evolution of octapodshaped nanoparticles is investigated by STEM-HAADF tomography, starting from the Cu2 xSe seeds and ending at the final octapods. The regular octahedral shape of the initial seeds shows eight almost complete triangular facets, each of them acting as nucleation site for the growth of the CdS pods, following the Cu Cd ion exchange. The final morphology of the octapods can be seen as direct evolution of the seeds where the latter ones, after conversion to CdSe, do not show appreciable variation of the initial shape and size. The eight pods grow perpendicular to the {111} facets of the seeds and adopt them as bases for the growth of pods with triangular cross-section. Moreover, opposite pods show markedly different ends, indicating an anisotropic behavior in the growth environment. This can be explained within a simple description of different growth behaviors along opposite Æ0001æ directions of wurtzite CdS. The derived detailed knowledge of the shape and structure of these new branched nanocrystals is evidently necessary for future studies on their physical properties.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of the seed and octapod synthesis conditions, additional HRTEM and EFTEM images of the analyzed particles, and animations of the volume reconstructions. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge financial support from European Union through the FP7 starting ERC grant NANO-ARCH (contract number 240111). R.B. and G.B thank Sergio Marras for helpful discussions.

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