On the Role of Surface Diffusion in Determining the Shape Or Morphology of Noble-Metal Nanocrystals

On the role of surface diffusion in determining the shape or morphology of noble-metal nanocrystals

Xiaohu Xiaa,1, Shuifen Xiea,1, Maochang Liua,1, Hsin-Chieh Pengb, Ning Luc, Jinguo Wangc, Moon J. Kimc,d, and Younan Xiaa,b,e,2

aThe Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA 30332; bSchool of Chemistry and Biochemistry and eSchool of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332; cDepartment of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080; and dDepartment of Nanobio Materials and Electronics, World Class University, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea

Edited by Gabor A. Somorjai, University of California, Berkeley, CA, and approved March 19, 2013 (received for review December 18, 2012) Controlling the shape or morphology of metal nanocrystals is central a cubic seed is determined by the ratio between the rates for to the realization of their many applications in catalysis, plasmonics, atom deposition and surface diffusion (Vdeposition/Vdiffusion). When and electronics. In one of the approaches, the metal nanocrystals are Vdeposition/Vdiffusion >> 1, surface diffusion can be ignored and grown from seeds of certain crystallinity through the addition of thereby the growth will be largely confined to the corner sites along atomic species. In this case, manipulating the rates at which the the <111> directions, resulting in the formation of Pd octapods atomic species are added onto different crystallographic planes of (Fig. 1B, i). On the contrary, when Vdeposition/Vdiffusion << 1, the a seed has been actively explored to control the growth pattern of growth will be dominated by surface diffusion and be switched to a seed and thereby the shape or morphology taken by the final the <100> and <110> directions as most of the adatoms at the product. Upon deposition, however, the adsorbed atoms (adatoms) corners can quickly migrate to edges and side faces of a cubic seed, may not stay at the same sites where the depositions occur. Instead, promoting the formation of a cuboctahedron as the final product they can migrate to other sites on the seed owing to the involve- (Fig. 1B, iv). Similar arguments can also be applied to the situations ment of surface diffusion, and this could lead to unexpected where the ratios of Vdeposition/Vdiffusion are between these two deviations from a desired growth pathway. Herein, we demon- V V

extremes. For example, when deposition/ diffusion is slightly larger CHEMISTRY strated that the growth pathway of a seed is indeed determined by than 1, a small portion of the adatoms at the corners will migrate the ratio between the rates for atom deposition and surface to the edges (which are relatively more active than the side faces diffusion. Our result suggests that surface diffusion needs to be – due to a lower coverage density for the Br ions) of a seed, leading taken into account when controlling the shape or morphology of to the formation of Pd concave nanocubes (Fig. 1B, ii). When metal nanocrystals. Vdeposition/Vdiffusion is slightly less than 1, some of the adatoms will stay at corners while the rest can diffuse to both edges and side seeded growth | shape control | noble metals faces of a seed. As a result, the final product will be an enlarged Pd nanocube with slight truncations at the corners (Fig. 1B, iii). urface diffusion is a general process that involves the motion of We conducted a set of experiments based on seed-mediated Sadsorbed atoms (adatoms), molecules, or atomic clusters on growth to validate the proposed mechanisms. The growth involved the surface of a solid material (1, 2). Over the past decades, it has the use of Pd nanocubes as seeds in an aqueous solution, with emerged as an important concept in many areas of surface science, L-ascorbic acid (AA), Na2PdCl4, and poly(vinyl pyrrolidone) (PVP) including catalysis, epitaxial growth, and electromigration of voids serving as the reductant, Pd precursor, and stabilizer, respectively. (3–7). Here, we demonstrated that surface diffusion also plays In a standard synthesis, an aqueous Na2PdCl4 solution was injected a pivotal role in determining the growth pathway of a seed and thus using a syringe pump into an aqueous suspension containing AA, fi the shape or morphology taken by the nal product in a solution- PVP, and Pd seeds that were hosted in a glass vial at room tem- phase synthesis of metal nanocrystals. Fig. 1 schematically illus- perature (∼22 °C) under magnetic stirring (see Materials and trates four possible pathways for the growth of a cubic seed. As Methods for experimental details). For this synthesis, Na2PdCl4 is a model system, we focused on Pd nanocubes with slight truncation supposed to be immediately reduced into Pd atoms by AA upon at corners and edges, together with six side faces passivated by – addition into the reaction solution due to the strong reduction chemisorbed Br ions. In the following discussion, we refer to them power of AA (14, 15). As such, the concentration of the newly as “Pd cubic seeds” for simplicity. We chose them as seeds for two formed Pd atoms in the reaction solution and thereby Vdeposition will major reasons: (i) they had a well-defined shape, together with be mainly determined by the injection rate for Na2PdCl4 solution a set of low-index facets on the surface (8, 9); and (ii) their side – that can be readily controlled through the use of a syringe pump. faces are blocked by Br ions to ensure selective deposition of Because surface diffusion is a thermally promoted process with its atoms onto the corner sites during seed-mediated growth (10–12). rate increasing with temperature (2), Vdiffusion can be adjusted by These two distinctive features allowed us to easily track the de- presetting the oil bath to a specific temperature. Collectively, the position of atoms and their surface diffusion during a growth roles played by V and V can be separated from each fi diffusion deposition process by analyzing the shape or morphology of the nal product. other for investigation by varying the reaction conditions. The newly formed Pd atoms resulting from the reduction of a Pd precursor are expected to deposit at the corners of a cubic seed – because the side faces are blocked by the chemisorbed Br ions Author contributions: X.X., S.X., and Y.X. designed research; X.X., S.X., and M.L. per- (Fig. 1A, 1). Upon deposition, there will be two different options formed research; N.L., J.W., and M.J.K. contributed new reagents/analytic tools; X.X., for these adatoms: staying at the corner sites or migrating to other S.X., M.L., H.-C.P., and Y.X. analyzed data; and X.X. and Y.X. wrote the paper. sites, including edges and side faces, through surface diffusion The authors declare no conflict of interest. (Fig. 1A, 2 and 3). It should be pointed out that only surface dif- This article is a PNAS Direct Submission. fusion was allowed here to move atoms from corners to edges and 1X.X., S.X., and M.L. contributed equally to this work. side faces of a seed during growth. Other mechanisms such as 2To whom correspondence should be addressed. E-mail: [email protected]. Ostwald ripening (13) were not considered because the side – This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. faces of a seed were blocked by Br ions. The growth pathway of 1073/pnas.1222109110/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1222109110 PNAS Early Edition | 1of5 Downloaded by guest on September 25, 2021 respectively. The sizes (defined as the distance between two op- posite {100} facets of a nanocrystal as illustrated in Fig. S3)of these four different types of Pd nanocrystals were measured to be 15.5, 18, 20.5, and 23 nm, respectively. Therefore, the growth of 15-nm cubic seeds was mainly confined to the <111> directions at 0 °C (Fig. 2A) and almost no deposition was found along the <100> directions. When the reaction temperature was increased from 0 to 75 °C (Fig. 2D), however, the growth was switched to the <100> and <110> directions because the size of the cuboctahedrons (i.e., 23 nm) was consistent with that of a cuboctahedron produced through exclusive growth on the {100} and {110} facets of a 15-nm cubic seed (Fig. S4) (17, 18). These results imply that, for the current system, the critical reaction temperatures for all of the adatoms to stay at corners sites or diffuse to the edges and side faces of a cubic seed are 0 and 75 °C, respectively. In the second set of experiments, we adjusted the injection rate for Na2PdCl4 solution while keeping the reaction temperature the same as the standard synthesis (22 °C). In these cases, the ratio of Vdeposition/Vdiffusion should increase as the injection rate for Na2PdCl4 solution increases because Vdiffusion is fixed. According to the proposed mechanisms (Fig. 1), it is not difficult to argue that Pd nanocrystals with shapes and sizes similar to those shown in Fig. 2 would also be obtained by decreasing the injection rate for Na2PdCl4 solution. As expected, Pd nanocrystals with shapes ranging from octapods to concave nanocubes, nanocubes with slight truncation at corners, and cuboctahedrons were indeed obtained when the injection rate for Na2PdCl4 solution was decreased from 1.5 to 0.75, 0.5, and 0.25 mL/h, respectively (Fig. S5). These results again supported our proposed mechanisms. Inter- estingly, the Pd octapods shown in Fig. 2A were found to transform into concave nanocubes (Fig. S6B) and then truncated nanocubes (Fig. S6C) after annealing in an aqueous solution at 80 °C for Fig. 1. Effect of surface diffusion on the growth pattern of a Pd cubic seed. 10 and 20 d, respectively. Similar shape transformations from Schematics of (A) three different options for the Pd atoms added to the corner cubes to cuboctahedrons have also been observed in our previous site of a Pd cubic seed whose side faces are capped by Br− ions, and (B) dif- studies when Pd nanocubes were annealed in an electron micro- ferent pathways and the corresponding shapes or morphologies expected for − the growth of a Pd cubic seed under four different conditions. The size of Br ions was reduced relative to Pd atoms to show the surface structure clearly.

Results and Discussion Validation of the Proposed Mechanism. As shown by the trans- mission electron microscopy (TEM) images in Fig. S1, the Pd seeds were nanocubes with an average edge length of 15 nm, which – were prepared in the presence of Br as a capping agent as reported previously (16). A close examination indicates that a small portion of the cubes were slightly elongated to become bars with an average aspect-ratio of 1.1. Because their side faces were − still bound by {100} facets and covered by Br ions, they should not affect our discussion on surface diffusion as long as their aspect ratios were close to 1. In our discussion, we also refer to them as “Pd cubic seeds” for simplicity. As indicated by the X-ray photo- electron spectroscopy (XPS) spectra shown in Fig. S2 for Pd cubes and octahedrons, only Pd{100} facets or the side faces of the Pd – cubic seeds were covered by chemisorbed Br ions. In the first set of experiments, we varied the reaction temperature while the injection rate for Na2PdCl4 solution was kept the same as the standard synthesis (0.5 mL/h). In this case, the ratio of Vdeposition/Vdiffusion is expected to decrease with the increase of reaction temperature because Vdeposition is fixed. If the proposed mechanisms are correct, Pd nanocrystals with all four different shapes or morphologies shown in Fig. 1B,i–iv, should be obtained as the reaction temperature is increased. As shown in Fig. 2, we Fig. 2. Four distinctive types of Pd nanocrystals that were obtained at different reaction temperatures. TEM images of Pd nanocrystals prepared using indeed obtained Pd octapods (Fig. 2A), concave nanocubes (Fig. B C the standard procedure except for the variation in reaction temperature: (A) 2 ), nanocubes with slightly truncation at corners (Fig. 2 ), and 0, (B) 22, (C) 50, and (D) 75 °C. [Scale bar (applies to all images), 50 nm.] Insets cuboctahedrons (Fig. 2D) from the same batch of Pd cubic seeds show TEM images of individual nanocrystals at a higher magnification. [Scale when the reaction temperature was set to 0, 22, 50, and 75 °C, bar (applies to all Insets), 5 nm.]

2of5 | www.pnas.org/cgi/doi/10.1073/pnas.1222109110 Xia et al. Downloaded by guest on September 25, 2021 scope chamber at 500 °C for 1 h or when silver (Ag) nanocubes were annealed in ethylene glycol at 145 °C for 2 h (19, 20). These results were also consistent with the mechanisms described in Fig. 1, where Vdeposition was essentially zero whereas Vdiffusion was relatively large.

Kinetics of Surface Diffusion. To better understand the role played by surface diffusion in the shape evolution of a Pd cubic seed during growth, we studied the kinetics of surface diffusion. By comparing the TEM images of Pd nanocrystals obtained at different reaction temperatures (Fig. 2) with that of the initial cubic seed (Fig. S1), we could roughly estimate the total number (Ndiffusion;seeFig. S3 for detailed calculation) of adatoms that had diffused to the edges and side faces of an individual cubic seed in the course of growth. Fig. 3 shows a curve generated by plotting Ndiffusion as a function of reaction temperature (Temp, in degrees Celsius). It can be seen that Ndiffusion increased with the reaction temperature while the slope of the curve decreased as Temp was increased. From surface chemistry studies, it is known that the kinetics of surface diffusion can be understood in terms of adatoms residing at adsorption sites on a surface and moving between adsorption sites by a hopping or jumping process (2). The diffusion coefﬁcient (D) that measures the rate of spreading of an adatom across a surface can be expressed as an Arrhenius-like equation as follows: À Á D = D0 exp −Ediff=RT ; [1] CHEMISTRY

where D0 is the diffusion preexponential factor, Ediff is the poten- tial energy barrier to diffusion, R is the ideal gas constant, and T is the absolute temperature (in kelvin). At lower T or short diffusion time (t), the number of adatoms that escape from the (111) face (i.e., Ndiffusion) will be proportional to sqrt(D), a quantity that is Fig. 4. Effect of seed size on surface diffusion. TEM images of (A)Pdcubic more or less linear with T for certain values of Ediff. However, N will be saturated as T becomes large and/or the dimen- seeds with an average edge length of 6 nm and (B) the corresponding diffusion cuboctahedrons grown from these seeds. All of the reaction conditions were sions of the (111) face become much less than sqrt(2Dt). Remark- N fl kept the same as those in the standard synthesis except for the differences in ably, such a trend for the calculated diffusion was also re ected size (6 vs. 15 nm) for the seeds and injection volume (0.5 vs. 3.0 mL) for the N Temp experimentally in Fig. 3: diffusion increased with at low Na2PdCl4 solution. [Scale bar (applies to both images), 20 nm.] Insets show TEM temperatures (0–50 °C) and became saturated at high temper- images of individual nanocrystals at a higher magnification. (Scale bar, 2 nm.) atures (>75 °C). This observation implies that the atoms depos- ited on the edges and side faces of a cubic seed during growth mainly came from the corner sites through surface diffusion. timescale of surface diffusion. To examine the effect of seed size, we conducted a set of experiments by replacing the 15-nm Pd cubes Effect of Seed Size on Surface Diffusion. Besides the reaction tem- with 6-nm Pd cubes as the seeds, with all other parameters being perature, seed size was also found to have an impact on the kept the same as in the standard synthesis except that the injection volume of Na2PdCl4 solution was reduced from 3.0 to 0.5 mL. Fig. 4 shows TEM images of the Pd nanocubes of 6 nm in edge length that were used as the seeds and the corresponding Pd cuboctahedrons grown from these 6-nm cubic seeds, respectively. The size of the cuboctahedrons was consistent with the size of an octahedron produced via exclusive growth along <100> directions of a 6-nm cubic seed (Fig. S4). This result indicates that most of the adatoms at the corners of a cubic seed had diffused to the edges and side faces during growth. In contrast, the final product took a concave cubic morphology when 15-nm cubes were used as the seeds (Fig. 2B), suggesting that only a portion of the adatoms at the corners was able to diffuse to the edges and side faces of larger seeds. Although Vdiffusion was roughly the same for both the 6- and 15-nm cubic seeds, the time needed to diffuse from the corners to side faces was much shorter in the former case due to a shorter diffusion distance.

Extension from Pd–Pd to Pd–Pt Nanocrystals. We also extended the mechanistic understanding of surface diffusion to the synthesis of Pd–Pt bimetallic nanocrystals. Platinum (Pt) is an invaluable Fig. 3. Relationship between surface diffusion and reaction temperature. material due to its excellent performance as catalysts in a myriad The curve was generated by plotting the total amount of Pd atoms (Ndiffusion) – that had diffused to the edges and side faces of individual Pd cubic seeds of industrial processes (21 25). However, Pt is extremely expen- (shown in Fig. 2) as a function of reaction temperature (Temp). Error bars are sive due to its low content in the Earth’s crust and the continuous SEs with n = 20. growth of demand in the automobile industry. In general, such

Xia et al. PNAS Early Edition | 3of5 Downloaded by guest on September 25, 2021 a dilemma can be overcome by using two different approaches (or perpendicular to the [001] zone axis, implying an epitaxial re- a combination of them): (i) engineering the surface structure of lationship between the two metals. The energy dispersive X-ray catalytic Pt nanocrystals to maximize the specific activity; and (ii) (EDX) mapping (Fig. 5E) clearly shows a color difference between depositing a thin layer of Pt on the surface of a less expensive the core (red, Pd) and shell (green, Pt), confirming a bimetallic metal such as Pd, which is only one-third of the price for Pt, to core–shell structure and indicates that the Pd cubic seed was intact minimize the use of Pt. Our previous studies showed that the during the growth of Pt shell. The magnified HAADF-STEM (Fig. growth of Pt on Pd cubic seeds in an aqueous system at 60–100 °C 5F) clearly shows the atomic steps at the edge of a core–shell typically yielded Pd–Pt bimetallic nanodendrites with irregular Pt concave nanocube, implying the presence of high-index facets on branches randomly positioned at the corners and edges of Pd the surface. Line-scan EDX profiles along the edge-to-edge and cubic seeds (26–28). We believe that the relatively low reaction corner-to-corner directions of an individual Pd–Pt concave nano- temperature and thus small Vdiffusion might be responsible for the cube (Fig. 5 G and H) confirm that the out-extending corners and formation of Pt branches. In this regard, we expect that Pd–Pt edges were dominated by Pt, whereas the cubic core was essentially core–shell nanocrystals with well-defined surface can be obtained made of pure Pd. Notably, such Pd–Pt core–shell nanocrystals with if Vdiffusion is greatly enhanced. To this end, we conducted a syn- concave structures and high-index facets were rarely reported in thesis at elevated reaction temperatures by injecting Na2PtCl6 (a previous studies. It is worth pointing out that deposition of Pt precursor to elemental Pt) solution at a rate of 8.0 mL/h into atoms on Pd cubic seeds was found to mainly occur at the corner − ethylene glycol containing the 15-nm Pd nanocubes, AA, and Br sites as indicated by the HAADF-STEM image in Fig. S7. This (serving as seeds, reductant, and capping agents, respectively) that image was taken from a sample collected in the very early stage of was held in an oil bath preset to 160–200 °C (see Materials and a synthesis of Pd–Pt concave nanocubes (after the addition of only Methods for experimental details). It was found that Pd–Pt bi- 3.0 mL of Na2PtCl6 solution). This observation provides additional metallic multipods (Fig. 5A), concave nanocubes (Fig. 5B), and evidence to support our proposed mechanisms illustrated in Fig. 1. nanocubes (Fig. 5C) were obtained when the reaction tempera- Fig. S8 shows HAADF-STEM images of the Pd–Pt multipods and ture was set to 160, 180, and 200 °C, respectively. nanocubes shown in Fig. 5 A and C, respectively. Because the Pd–Pt concave nanocubes (Fig. 5B) have high-index Obviously, the key to coating a thin layer of Pt on Pd cubic seeds facets that usually provide an enhanced catalytic activity (29–31), was to increase Vdiffusion and thus decrease the ratio of Vdeposition/ we specifically characterized the individual Pd–Pt concave nano- Vdiffusion by elevating the reaction temperature. This strategy was cubes. As shown by the high-angle annular dark-field scanning also consistent with our proposed mechanism (Fig. 1). Compared TEM (HAADF-STEM) image in Fig. 5D, the brighter surface with the case of growth Pd on Pd cubic seeds, the temperature layer can be attributed to Pt, most of which is concentrated at the required for the Pt adatom to diffuse from corners to side faces was corners and edges of the Pd cubic seed. The selected area electron significantly higher. This difference might be related to the fol- diffraction (SAED) pattern from the same particle (Fig. 5D, Inset) lowing order in bonding energies: EPt–Pt (307 kJ/mol) > EPt–Pd (191 indicates that it was a piece of single crystal sitting against a plane kJ/mol) > EPd–Pd (136 kJ/mol) (26, 32). An additional energy

Fig. 5. Extension from Pd–Pd to Pd–Pt bimetallic nanocrystals. (A–C) TEM images of Pd–Pt bimetallic nanocrystals that were obtained by depositing Pt on Pd cubic seeds at: (A) 160 °C, (B) 180 °C, and (C) 200 °C. [Scale bar: in C (A–C), 50 nm.] Insets show TEM images at a higher magniﬁcation. (Scale bar: 5 nm.) (D–H) Characterizations of an individual Pd–Pt concave nanocube in B:(D) HAADF-STEM image and SAED pattern (Inset) recorded from the same particle; (E)EDX mapping of the same particle shown in D;(F) magniﬁed HAADF-STEM image of the region marked by a blue box in D;(G and H) line-scan EDX spectra of elemental Pd and Pt that were recorded along the edge-to-edge and corner-to-corner directions, respectively, of the same particle (see Insets for illustrations).

4of5 | www.pnas.org/cgi/doi/10.1073/pnas.1222109110 Xia et al. Downloaded by guest on September 25, 2021 Standard Procedure for the Growth of Pd Nanocrystals. barrier, that is, Ediff in Eq. 1, would involve in surface diffusion In a standard synthesis, because the newly formed Pt atoms prefer to stay at corners to 3.0 mL of aqueous Na2PdCl4 solution (1.2 mg/mL) was injected at a rate of form more stable Pt–Pt bonds rather than to diffuse to side faces 0.5 mL/h using a syringe pump into an 8.0-mL aqueous suspension con- and form less stable Pt–Pt bonds. Therefore, to facilitate surface taining the 15-nm Pd cubic seeds (∼2.3 × 1012 particles per mL), 60 mg of AA, diffusion, a higher reaction temperature is required to overcome and 50 mg of PVP that was hosted in a 30-mL glass vial at room temperature ∼ the relatively higher energy barrier to surface diffusion. ( 22 °C) under magnetic stirring. After injection of Na2PdCl4, the solution wasmaintainedwithstirringforanother5mintoallowthereactiontocom- Conclusion plete. The product was collected by centrifugation at 15,000 × g for 10 min We have demonstrated that surface diffusion plays an important and washed twice with water. role in shape-controlled synthesis of metal nanocrystals. It can be Synthesis of Pd–Pt Bimetallic Nanocrystals. In a typical synthesis, 13 mL of concluded that the relative rate of atom deposition over surface ethylene glycol containing the 15-nm Pd cubic seeds (∼4.6 × 1012 particles per diffusion determines the growth pathway of a seed and thus the ﬁ mL), 54 mg of KBr, 100 mg of AA, and 66.6 mg of PVP, was preheated at shape taken by the nal product. Using the overgrowth of Pd cubic 110 °C for 60 min in a three-neck ﬂask with magnetic stirring and then seeds as a model example, we found that the rate of surface dif- ramped to the desired temperature (160, 180, and 200 °C). Then, 12 mL of fusion increased with reaction temperature. The bonding energy Na2PtCl6·6H2O solution in ethylene glycol (0.5 mg/mL) was injected into the between metal atoms was also found to have an impact on surface preheated solution at a rate of 8 mL/h using a syringe pump. After the ad-

diffusion. Based on the mechanistic understanding, a number of dition of Na2PtCl6, the solution was maintained with magnetic stirring for Pd nanocrystals with controlled shapes could be easily prepared another 5 min to allow the reaction to complete. The product was collected from Pd cubic seeds by controlling the reaction conditions that by centrifugation at 15,000 × g for 10 min and washed twice with ethanol affect the rates of atom deposition and surface diffusion. As shown and thrice with water. in the present work, the growth mechanisms could be extended from Pd–Pd to a bimetallic system such as Pd–Pt. The mechanisms Characterizations. The samples were characterized by TEM using a JEOL mi- can also be used to explain the growth habits of other types of seeds croscope (JEM-1400) operated at 120 kV. The concentration of Pd ions was such as Rh and Pt cubic seeds observed in previous studies (10). determined using inductively coupled plasma mass spectrometry (Perkin- We believe that the mechanistic understanding can be potentially Elmer Elan DRC II), which could be converted to the particle concentration of extended to other systems involve various metal nanocrystals and Pd nanocrystals once the particle size and morphology had been resolved by other inorganic nanomaterials. TEM imaging. The XPS data were recorded using a Thermo K-Alpha spec- trometer with an Al Kα source (eV). HAADF-STEM and EDX analyses were CHEMISTRY Materials and Methods performed using a JEOL ARM200F with STEM Cs corrector operated at Preparation of 15- and 6-nm Pd Cubes to Be Used as the Seeds. The 15-nm cubic 200 kV. Pd seeds were prepared using a previously reported protocol with some minor modiﬁcations (16). In a typical synthesis, 11 mL of an aqueous solution ACKNOWLEDGMENTS. This work was supported in part by National Science containing 105 mg of PVP (molecular weight of ∼55,000), 60 mg of AA, Foundation Grant DMR-1215034 and start-up funds from Georgia Institute 500 mg of KBr, and 57 mg of Na PdCl was heated at 80 °C under magnetic of Technology. Y.X. was also supported by the World Class University (WCU) 2 4 Program from Ministry of Education, Science and Technology (MEST) stirring for 3 h and then cooled down to room temperature. After centri- through National Research Foundation (NRF) Grant R32-20031. As jointly fugation and being washed thrice with water, the seeds were dispersed in supervised PhD students from Xiamen University and Xi’an Jiaotong Univer- 11 mL of water for further use. The protocol for synthesizing the 6-nm cubic sity, respectively, S.X. and M.L. were partially supported by Fellowships from Pd seeds was similar to the aforementioned procedure, except that the the China Scholarship Council. M.J.K. was supported by the WCU Program amount of KBr was reduced from 500 to 5 mg. from MEST through NRF Grant R31-10026.

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