Revisiting the Twin Plane Re-entrant Edge Growth Mechanism at an

28 Jul 2014 - It was determined that a twin plane re-entrant edge mechanism was responsible for the growth process at an atomic scale. Moreover, this ...
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Revisiting the Twin Plane Re-entrant Edge Growth Mechanism at an Atomic Scale by Electron Microscopy Zhiyang Yu,†,‡ Xin Fu,§ and Jing Zhu*,† †

Beijing National Center for Electron Microscopy, School of Materials Science and Engineering, The State Key Laboratory of New Ceramics and Fine Processing, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, China ‡ Center for Advanced Materials and Nanotechnology, Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States § General Research Institute for Nonferrous Metals, Beijing 100088, China S Supporting Information *

ABSTRACT: We conducted an extensive electron microscopy study on surface and defect structures of boron suboxide/ suboxycarbide platelets by examining them under various imaging conditions, e.g., side-view and top-view perspectives. It was determined that a twin plane re-entrant edge mechanism was responsible for the growth process at an atomic scale. Moreover, this thorough investigation provided an opportunity to resolve several critical issues regarding this otherwise wellknown growth mechanism for metallic nanostructures. In this study, the platelets contained multiple {001} twin lamellae parallel to basal planes and their side faces were mainly enclosed by {101} facets. Vertical growth was heterogeneously nucleated of {001}-type growth twins that were confined at the corners. In lateral growth, nucleation sites were greatly extended to twin reentrant edges around all side faces. No secondary growth twins were introduced during lateral growth because {101}-type side faces were not twin planes. This work clearly establishes that surface structures of twinned platelets determine nucleation behaviors on basal/side faces and thus control the final morphology, which is relevant to the shape control of metal nanoplates.

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INTRODUCTION Manipulation of the size, shape, chemical composition, and defect structure of metal nanostructures has attracted much attention in recent years.1−4 Extensive research has been conducted to produce hexagonal/triangular nanoplates, which are excellent substrates for surface-enhanced Raman scattering (SERS).5−8 It is generally accepted that these nanoplates contain high densities of microtwins and stacking faults parallel to their large flat surfaces.9,10 As schematically shown in Figure 1, the basal surfaces of hexagonal/triangular nanoplates are comprised of {111}FCC (FCC, face-centered cubic) planes. Two or more parallel twin lamellae would cause all the side faces to have A-type geometry with a convex ridge and B-type geometry with a concave trough. The troughs (re-entrant edges) serve as active sites for the deposition of metallic clusters, so that the platelets can maintain a high rate of growth along the side faces. The cooperative growth of troughs around all six edges prevents them from growing out and hence sustains fast lateral growth in two dimensions. This growth mechanism has been narrated in the face-centered cubic (FCC) system and termed the twin plane re-entrant edge (TPRE) growth mechanism.11,12 Although this mechanism was proposed more than 50 years ago, the detailed growth process is not yet fully understood. For instance, in real nanoplatelets, there would be multiple twinned segments (more than two) in the fabricated platelets if the segments were sufficiently large,13−18 except the ones produced © 2014 American Chemical Society

Figure 1. Schematic showing the cross-section structure and ridge− trough side face structure of a platelet with two parallel {111}FCC twin planes.

Received: April 14, 2014 Revised: July 24, 2014 Published: July 28, 2014 4411

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Figure 2. (a) Typical SEM image showing a variety of boron-rich platelets grown on a glassylike substrate. “Tips” of platelets were the acute corners suspended in the air, while the ones attached to the substrate were defined as “roots”. (b) Schematic drawing of a typical platelet. All the platelets had large flat (001)r surfaces, and growth perpendicular to the platelets was termed vertical growth. Lateral growth occurred in the [100]r and [010]r directions. To gain full insight into the distribution of defect structures, both top-view TEM characterization (c) and side-view TEM characterization (d) were conducted.

precisely by the seed-mediated growth method.8 These multiple lamellae were most likely the result of growth twins that were successively introduced during the growth. No experimental approaches had been adopted to understand how growth twins were introduced during vertical growth. Moreover, there have been contradictory reports about the orientation of side faces. Experimentally, both {111}FCC14,19−21 and {100}FCC21 planes have been observed for different platelets fabricated under different processing conditions. If {111}FCC planes were stable as side faces, it was not clearly understood why growth twins were rare on these faces in lateral growth compared to the high densities of growth twins on basal (111)FCC planes. The nucleation of a new twinned segment on basal faces led to the creation of kinks. It was believed that atoms would attach to these kinks rather than flat basal surfaces and by doing so advance. The process was termed step-flow growth in surface science. It was not very clear if these step-flow events were strictly epitaxial. To the best of our knowledge, no systematic experimental approaches have been performed to address these critical questions at an atomic scale. A cross-section view of twin lamellae parallel to basal faces and surface steps on basal planes is the key to these issues. There have been very few reports about the structure of nanoplates in cross-sectional view at an atomic scale. Occasionally, when the nanoplates were transferred onto carbon films, several nanoplates in the form of stacks would stand perpendicular to the grid. Even when some platelets were close to side-view orientation, it was not possible to analyze them especially when the lateral size of nanoplates was hundreds of nanometers, rendering them opaque to the electron beam. Consequently, only one or two nanoplates suitable for highresolution transmission electron microscopy (HRTEM) examinations were available in most research efforts. In these limited observations, microtwins and stacking faults parallel to (111)FCC basal surfaces were commonly observed.7,14−21 Sun presented a HRTEM image of a Ag nanoplate clearly showing the presence of multiple microtwins running parallel to the (111)FCC basal surface,17 on top of which a new twin segment has just been formed. Rodriguez-Gonzalez et al. observed bright and dark fringes in the top view of silver nanoprisms and

identified them as bending contours after aligning several nanoprisms to the edge-on condition.22 A fundamental understanding of the exact role of microtwins and surface steps in the growth process of twinned platelets in the TPRE mechanism has been severely hindered by these limited and often contradictory observations. Recently, we demonstrated that the growth of boron suboxide/suboxycarbide platelets was promoted by the TPRE mechanism.23,24 Their chemical compositions varied from B6O to BCO as confirmed by electron energy loss spectrometry (EELS) at high temperatures (>1300 °C).25 Most of the platelets were terminated by two sharp corners. The wedgeshaped corners were electron transparent when they were set to the edge-on direction as all the constituent elements (boron, carbon, and oxygen) have a smaller scattering cross section for electrons. This provided us with a great opportunity to systematically analyze the cross-section structure of twinned platelets. In this paper, we report a detailed HRTEM and highresolution scanning electron microscopy (HRSEM) characterization of 22 platelets in side-view perspective and more than 100 platelets in plane-view orientation. This structural information about the twinned platelets provides fundamental insight into the detailed growth process of the TPRE mechanism and resolves some of the outstanding issues outlined above.

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EXPERIMENTAL SECTION

Large quantities of platelets were synthesized by the chemical vapor deposition method23 from 1300 to 1500 °C. The starting precursors were BaO, B, and Fe3O4 powders at a 0.275:2.852:0.177 BaO:B:Fe3O4 mole ratio. The precursors were mixed and pressed into pellets at 4 MPa. After being sintered at an elevated temperature for 2 h, the pellets turned into gray porous pellets, on top of which large densities of platelets were found. The as-prepared platelets were carefully examined by JEOL 6301F field emission scanning electron microscopy (FESEM). The platelets were dispersed in ethanol and transferred onto transmission electron microscope grids for HRSEM imaging. To resolve fine growth features of platelets, a Hitachi 5500 field emission scanning electron microscope was utilized. An enormous amount of effort was made to locate the platelets standing perpendicular to commercial TEM 4412

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Figure 3. (a) Typical SEM images of several platelets showing macro steps on basal faces. All the macro steps can find their origin at their tips. Stepflow was followed with the nucleation at the corners to increase the thickness as indicated by the red arrows. Macro steps were also found to originate from the roots of platelets (b). The nucleation sites at the top or bottom face was confined to the acute corner of platelets (tips/roots), but these locations for nucleation were independent but not mutually exclusive. For example, macro steps were found to originate from both the tip and the root on the basal face as shown in panel c. Likewise, nucleation can occur both at the root of the top face and at the tip of the bottom face, as shown in panel d. (e−g) Nucleation events were found at (001)r basal faces, but they were quite rare. A HAADF image of a platelet with bright islands (as denoted by the boxes) on its basal faces is shown in panel e. High-magnification HAADF images showing the details of these islands are given in the inset. BF images of these islands are shown in panels f and g. grids (edge-on condition) with predesigned references for further TEM characterization. The edge-on platelets together with plan-view ones were studied with a JEOL 2011 transmission electron microscope. The chemical composition of platelets was analyzed with a JEOL 2010 field emission scanning electron microscope coupled with a Gatan imaging filter (GIF). A wide range of compositions from B6O to BCO were identified.23 We refer to these platelets as boron-rich platelets for the sake of simplicity. High-angle annular dark field (HAADF) images were recorded on a JEOL 2010 field emission scanning electron microscope.

along the [100]r and [010]r directions was accelerated by the TPRE mechanism, while vertical growth along the [001]r direction is not promoted. The platelets were transferred onto the carbon film, and thus, most of them will lay flat with their (001)r basal planes parallel to the carbon film. Thus, topview characterization was the most common perspective for platelets. It provided a convenient way to monitor lateral growth information, such as the lateral dimension of platelets and growth defects on side faces, if there were any (Figure 2c). However, the information about twin lamellae parallel to basal faces was lost in this perspective. To overcome this shortcoming, we located the platelets with (001)r basal faces close to edge-on condition and conducted a careful TEM study of their cross-section structures. Compared with those of metal nanoplatelets, the interactions of boron-rich platelets with the electron beam were much weaker. So even when (001)r basal faces were aligned to edge-on condition, the platelets were still electron transparent for hundreds of nanometers in the beam direction, as indicated in Figure 2d. Considering the fact that both tips and roots have wedge shapes, not only microtwins but also surface steps parallel to basal faces can be clearly resolved. The side-view perspective is important in revealing fine growth details, especially for vertical growth. Vertical Growth of Twinned Platelets. In addition to TEM characterization, high-resolution SEM imaging of platelets provided critical information for understanding the vertical growth process. On (001)r top faces, the occurrence of microsteps was common. These features were markers for nucleation and step-flow events during crystal growth history. We examined (001)r basal faces of hundreds of platelets, and a wide range of surface steps ranging from elemental steps to macro steps were observed. Advancement of these steps increased the thickness of platelets, as indicated by the red

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RESULTS AND DISCUSSION Top-View Characterization versus Side-View Characterization. A large amount of boron-rich platelets have been synthesized on a glassy substrate above 1300 °C, as shown in Figure 2a. All the platelets had large (001)r [the subscript r in the notation of planes or directions refers to the rhombohedral representation of the rhombohedral crystal structure; in the rhombohedral system, (001)r, (100)r, and (010)r planes are identical] basal faces and were a few hundred nanometers (100−600 nm) in thickness. The tips, defined as the acute corners suspended in the air, were usually capped with a tiny globule of catalysts. As the sintering temperature was increased, the density of such particles decreased. It appeared that the catalytic globules were consumed as growth continued because they were not detected in the high-temperature samples. The catalytic particles played a critical role in the nucleation of the platelets.23 Without these catalysts, the platelets cannot be produced. The roots of the platelets were connected with glassy substrates. A schematic drawing of a rhombohedral platelet is depicted in Figure 2b with the crystallographic orientation labeled. It has been demonstrated there were high densities of microtwins parallel to (001)r basal faces.23,26 Lateral growth 4413

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arrows and yellow arrows in panels a and b of Figure 3. Interestingly, the origin of these steps was traced back to the tips (Figure 3a) or the roots (Figure 3b). Between the tips and roots, no preferential nucleation sites were observed on basal planes. Rather, simultaneous nucleation on both sites was observed in certain platelets. An example of such an event is presented in Figure 3c. Nucleation events can also simultaneously take place at the bottom face and top face as shown in Figure 3d. It appeared that nucleation events of vertical growth were confined to tips or roots of basal faces. The HRTEM observations of platelets in side-view orientation provided more valuable information about both nucleation and step-flow events for vertical growth. Of the 22 platelets characterized in the side-view orientation, the nucleation of surface steps in the middle of (001)r basal faces was observed in only two cases. One such example is presented in Figure 3e−g. HAADF imaging of a platelet showed additional contrast on its basal surfaces as indicated by the boxes in Figure 3e. Close inspection of these regions revealed the presence of bright islands on (001)r basal planes as shown in the insets. The energy disperse spectrum (EDS) confirmed that these islands were rich in boron, carbon, and oxygen, with a minor amount of barium. Bright field (BF) images of these islands (Figure 3f,g) revealed that the islands were crystalline. Combining these results, we concluded that the islands can be treated as nuclei on (001)r basal faces. Nucleation on basal faces was rarely observed; this can be attributed to the energy barrier for nucleation on the surfaces being larger than that at the corners that were connected to catalysts. Among the platelets observed in the side view, seven of them had nanowires attached to tips or roots. Figure 4a shows a typical platelet with a tiny nanowire at its tip. High densities of surface macro steps were observed at its top/bottom face. These macro steps were traced back to the tip at the top face and the root at the bottom face. The atomic-resolution

HRTEM images at the edge-on condition provided a clear picture of the nucleation history (Figure 4b−d) and the subsequent step-flow events (Figure 4f,g). A tiny nanowire was observed standing at the tip as shown in Figure 4b. As one can see from the enlarged HRTEM image in Figure 4c, the nanowire consisted of a large amount of alternative (001) microtwins. A closer examination of the junction between the nanowire and the grown-out macro steps revealed that the macro steps shared exactly the same defect structure with the nanowire, as marked by the red boxes in Figure 3d. This experimental evidence directly demonstrates that the nanowire served as the nuclei for vertical growth and was the origin of twin lamellae parallel to basal faces; subsequent step-flow followed epitaxially with nucleation events. During the stepflow process, elemental steps with unit cell height tended to aggregate and form macro steps (see Figure 4a,e). The strip contrast as indicated by the green arrows in Figure 4e was indicative of (001)r-type microtwins or stacking faults. These strips ran parallel to basal faces and went all the way into the platelet. This was an indication that step-flow events on (001)r basal faces were epitaxial. High-resolution imaging of the macro steps gave us consistent information. As one can see in Figure 4f, all the microtwins were found to be parallel without the intersection of any secondary twin variant. For the other 15 platelets, the nuclei grew out to form macro steps. On the basis of our extensive SEM and TEM study of large quantities of platelets, we hypothesize that growth twins were integrated as twin lamellae by epitaxial step-flow. Although most step-flow events for vertical growth were epitaxial, here we found the lattice for each twin lamella parallel to basal faces showed disordered behaviors. Figure 5a−c shows an example of lattice disorder in the tip of a typical platelet. The diffraction pattern (Figure 5a) recorded at the tip shows pronounced streaking because of the presence of nanosized twin segments. Dark field images were recorded using g1 (Figure 5b) and g2 (Figure 5c) diffraction spots. The bright contrast in the diffracting segments was not homogeneous but interrupted with dark islands, as indicated by the white arrows. This was a strong indication that vertical growth was kinetic, and hence, lattice disorder was occasionally introduced. We believed the lattice disorder should be attributed to the nucleation events at the corners of platelets. The HRTEM examination of a tip and root from the same platelet is presented in Figure 5d to support this hypothesis. The thicknesses of twin segments were measured and are plotted as a function of their position. The histogram from the tip did not match that from the root as shown in Figure 5d. Lattice disorder, especially lattice rotation, was common for all the observed platelets. For example, when the tip of the platelet shown in Figure 5d was aligned to zone axis condition, the root would be 2° off the zone axis. Furthermore, while the top corner of the tip was oriented to the zone axis, the bottom corner was 0.5−1° off. On the basis of this evidence, it was clear that during the nucleation events at the tips or roots, lattice disorder was frequently brought in and finally integrated into platelets by epitaxial step-flow growth. Lateral Growth of Twinned Platelets. Top-view perspective can be applied to reveal lateral growth information directly if the side faces are flat, which was not the case as shown in Figure 6a. The (001)r-type twin lamellae terminated at side faces were responsible for the rough profile. On the contrary, side-view images provided more interpretable information about lateral growth. The side faces consisted of

Figure 4. (a) SEM image of a typical platelet standing perpendicular to the carbon film. Corresponding TEM images of nuclei at the tip (b−d) and macro steps on the (001)r basal faces (e and f) are shown at high magnifications. In the HRTEM images, “MT” stands for microtwins and “SF” stands for stacking fault. 4414

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Figure 5. (a) Diffraction pattern recorded at the tip of a typical platelet. The dark field (DF) images were obtained using g1 (b) and g2 (c) diffraction spots. White arrows indicated the growth faults in the twin segments. (d) HRTEM analysis of a tip and root recorded at a typical platelet. The low-magnification BF images are given in the inset. The thicknesses of each twin lamella, defined as D, were measured from the HRTEM images and are plotted in the histogram.

with the step-flow direction derived from the tips or the roots (see the colored arrows). These macro steps were believed to originate from twin re-entrant corners. Given that nucleation energy at re-entrant edges was greatly decreased,27 we believed the nucleation for lateral growth not only occurred at tips or roots but also was extended to twin re-entrant edges. Side-view HRTEM analysis was conducted to reveal their atomic structure. A typical platelet consisting of multiple (001)r twin segments is shown Figure 6f. Most twin lamellae were terminated by (111)r planes at the surfaces, as indicated by the dashed lines in Figure 6g. These (111)r planes were the projection of sample wedges but not the true side faces, which is clearly evident from the three-dimensional model of platelets shown in Figure 2b. To reveal the true side faces, several possible side face models have been proposed, and the features of their corresponding HAADF images were compared with experimental ones (see Figure S1 of the Supporting Information for more details). In the best match, the stable side faces were composed of {101)r planes; (100)r-type and (010)r-type side face models would produce reserve image features. For all the platelets we have observed in the side view, observation of {101)r as the terminal “surfaces” is dominant (>80%). Our results show that side faces of twinned platelets are completely different with basal planes; this was in direct contrast with the original TPRE model (both basal planes and side faces are {111} planes) that was proposed more than 50 years ago. The detailed nucleation and step-flow process of lateral growth on these {101}r planes still needed to ve investigated. Does nucleation introduce new twin variants on side faces during lateral growth? Is the following step-flow growth epitaxial? To understand the lateral growth process, we conducted top-view characterization on different platelets, with a representative TEM image shown in Figure S2 of the Supporting Information. Careful HRTEM examination of the edge of a typical platelet showed contrast variation in Figure S2 of the Supporting Information. The contrast variation actually

Figure 6. (a) Top-view SEM image of a typical platelet showing a rough side face profile. (b) A side-view observation helped resolve the side face profile. T denotes twinning. (c) Two representative platelets standing inclined on the supporting carbon film. The expanded views of side faces in the yellow boxes are shown in panels d and e. The red arrows indicate the step-flow growth originating from the tip and the yellow arrows the step-flow from the root. Step-flow in other directions is denoted by the white arrows. (f and g) Enlarged TEM images of the tip of the platelet in panel c showing the prevalence of (001)r-type microtwins. Note that most of the twin segments exposed (111)r free surface in the projective TEM image.

several alternating flat surfaces in Figure 6b. This was a typical indication of the presence of microtwins between these surfaces. Step-flow growth originated evidently from the root as indicated by the yellow arrow. A high-magnification SEM image of representative platelets is shown in Figure 6c, with well-resolved side faces (Figure 6d,e). Step-flow was followed by nucleation at the tips (see the yellow arrows in Figure 6d) or the roots (see the red arrows in Figure 6e). After a careful examination of the side faces shown in Figure 6d, we found a large fraction of macro steps with the growth direction (see the white arrows) perpendicular or even reversed 4415

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nucleation sites at re-entrant edges does not change much. The step-flow events on side faces are believed to be epitaxial. Similarly, elemental steps from growth centers pile up and form macro steps. It is worth mentioning that platelets tend to expose preferentially (101)r planes as their side faces. Because these types of planes are not twin planes, during the lateral nucleation (nucleation events B and C), no secondary growth twins will be introduced on side faces, and hence, parallel twinned cross sections in the platelets are maintained. Surface structures of twinned platelets are critical for determining nucleation behaviors on basal/side faces and thus controlling the plateletlike morphology of boron-rich platelets. This scenario can be directly extended to understand the evolution of the shape of metallic platelets. Truncated triangular prisms and rodlike tabular crystals were two basic forms of twinned platelets in colloid synthesis and solution synthesis.9 Truncated triangular prisms were commonly observed in metallic platelets. Among them, Zhang and coworkers reported {111}FCC and {100}FCC planes as stable side faces.21 Theoretically, it is difficult to form secondary twins on the sides faces with {111}/{001}/{111}FCC-type or {001}/ {111}/{001}FCC-type geometries as they are not energetically favorable. The prismlike morphology was thus maintained because of their side face geometries. This is consistent with the fact that experimentally, secondary twins have seldom been reported on truncated triangular prisms. In contrast with truncated triangular prisms, secondary twins have rarely been mentioned in metallic platelets. Similar cases have received attention in silver bromides and/or halide crystals.13,28 Millan et al. sliced silver bromide rod crystals and examined their surface/twin structure for the first time.13 They observed secondary twins in all the rod crystals, and a large fraction of surfaces were bounded by {111}FCC planes. Because {111}FCC planes were dominant in side faces, the {111}/{111}/{111}FCC-type trough−ridge geometry is feasible for the nucleation of secondary twins. A similar process was depicted by Lofton et al.9 in FCC-based metal platelets but received less attention. Combining previous studies of truncated triangular prisms and tabular crystals, we find it appears that the evolution of the shape of platelets with FCC structure is also directly related to their surface structures. Via the control of surface structures, it is expected that the morphology of twinned platelets can be tuned from truncated triangular prisms with a parallel twin structure to a rodlike tabular crystal with a nonparallel twinned cross section. We should mention that the mechanism described here is based on the assumption that growth twins are frequently introduced during growth, which is the case when the platelets are sufficiently large (approximately hundreds of nanometers to several micrometers).16,29 The chance of introducing growth twins during seed-mediated growth8 is low, and thus, their works are not relevant to our discussion.

came from the overlapping of alternative side faces. No microtwins or stacking faults were found at the corners as confirmed by HRTEM images and corresponding selected area diffraction patterns (SADPs). This suggested that both nucleation and step-flow events on (101)r side faces will not bring in new twin variants, contrasting somewhat with the continuous generation of alternative microtwins on basal planes. The “epitaxial” nucleation behavior can be rationalized by the fact that (101)r side faces were not twining planes and hence twining or stacking faults could not be introduced. Proposed Overall Growth Mechanism at an Atomic Scale. We revisit the TPRE mechanism by examining nucleation and the subsequent step-flow events for vertical growth and lateral growth, as shown in Figure 7. Vertical

Figure 7. Schematic showing vertical growth and lateral growth of twinned platelets. (001)r-type basal faces are colored purple and (101)r-type side faces sky blue.

growth starts with the formation of nuclei (defined as “nucleation event A”) confined at the acute corners that are connected to the catalyst (tip) or attached to the boron-rich liquid (root). Because nucleation takes place on (001)r basal faces, growth twins are frequently introduced by growth perturbation. These defect structures are integrated into platelets by epitaxial step-flow growth, as indicated by the yellow arrows in Figure 7. In certain instances, concurrent multiple nucleation events are triggered, elemental steps from which tend to aggregate and form macro steps. For lateral growth, in conjunction with the nucleation event at tips and roots (defined as nucleation event B), additional nucleation was observed at twinned re-entrant corners (defined as nucleation event C), as schematically shown in Figure 7. Compared to vertical growth, all the re-entrant twining boundaries in side faces serve as active sites for receiving boron-rich molecules, which strongly enhanced lateral growth. Although catalysts are introduced in our system, the general idea that the TPRE mechanism works by introducing more

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CONCLUSION We have performed an extensive electron microscopy structural study of boron-rich twinned platelets. In contrast to the usual top-view characterization, much effort has been spent to examine the cross section structure of 22 platelets by HRTEM at an atomic scale. On the basis of the characterization of hundreds of platelets, a general picture on the growth of twinned platelets has been presented at an atomic scale. During vertical growth, {001)r-type microtwins were regularly introduced on basal planes, which was the origin of twin lamellae in 4416

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(15) Xiong, Y. J.; Siekkinen, A. R.; Wang, J. G.; Yin, Y. D.; Kim, M. J.; Xia, Y. N. J. Mater. Chem. 2007, 17, 2600−2602. (16) Zhu, J. J.; Kan, C. X.; Li, H. C.; Cao, Y. L.; Ding, X. L.; Wan, J. G. J. Cryst. Growth 2011, 321, 124−130. (17) Sun, Y. G. Adv. Funct. Mater. 2010, 20, 3646−3657. (18) Sun, X. P.; Dong, S. J.; Wang, E. Angew. Chem., Int. Ed. 2004, 43, 6360−6363. (19) Lim, B.; Wang, J. G.; Camargo, P. H. C.; Jiang, M. J.; Kim, M. J.; Xia, Y. N. Nano Lett. 2008, 8, 2535−2540. (20) Germain, V.; Li, J.; Ingert, D.; Wang, Z. L.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 8717−8720. (21) Zhang, L.; Huang, C. Z.; Li, Y. F.; Li, Q. Cryst. Growth Des. 2009, 9, 3211−3217. (22) Rodriguez-Gonzalez, B.; Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Phys. Chem. B 2006, 110, 11796−11799. (23) Yu, Z. Y.; Jiang, J.; Yuan, J.; Zhu, J. J. Cryst. Growth 2010, 312, 1789−1792. (24) Yu, Z. Y.; Fu, X.; Yuan, J.; Lea, S.; Harmer, M. P.; Zhu, J. Cryst. Growth Des. 2013, 13, 2269−2276. (25) Yu, Z. Y.; Lea, S.; Yuan, J.; Zhu, J. Mater. Lett. 2010, 64 (22), 2541−2543. (26) Guan, Z.; Gutu, T.; Yang, J. K.; Yang, Y.; Zinn, A. A.; Li, D. Y.; Xu, T. T. J. Mater. Chem. 2012, 22, 9853−9860. (27) Lee, J. W.; Chung, U. J.; Hwang, N. M.; Kim, D. Y. Acta Crystallogr. 2005, A61, 405−410. (28) Jagannathan, S.; Chen, S.; Mehta, R. V.; Jagannathan, R. Phys. Rev. B 1996, 53, 9−11. (29) Sun, X. P.; Dong, S. J.; Wang, E. K. Langmuir 2005, 21, 4710− 4712.

platelets. For lateral growth, no growth twins were introduced at {101)r side faces. These contrasting nucleation behaviors were attributed to the difference in surface structures. Step-flow events from the nuclei at the top faces and side faces were mostly epitaxial, and elemental steps tended to form macro steps. Our thorough electron microscopy characterization provided us with an opportunity to revisit the details of growth of the TPRE mechanism. The proposed growth process at an atomic scale suggests that the entire process is more complex than the original description of the TPRE mechanism. This work unambiguously establishes that surface structures on basal/side faces are directly correlated with their individual nucleation behavior, providing fundamental insight into the evolution of the shape of metal nanoplates, which is believed to follow the TPRE mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

Additional HRTEM images and schematics. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86 10 62794026. Fax: +86 10 62772507. Funding

This work is financially supported by the National 973 Project of China and the Chinese National Natural Science Foundation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work made use of the resources of the National Center for Electron Microscopy (Tsinghua University). Z.Y. thanks his colleagues, Dr. Animesh Kundu and Prof. Martin P. Harmer, at Lehigh University for their long-term support, valuable discussions, and great effort in revising the manuscript.

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dx.doi.org/10.1021/cg500514c | Cryst. Growth Des. 2014, 14, 4411−4417