Letter pubs.acs.org/NanoLett
Shape-Controlled Synthesis of Palladium Nanocrystals: A Mechanistic Understanding of the Evolution from Octahedrons to Tetrahedrons Yi Wang,†,‡ Shuifen Xie,† Jingyue Liu,§ Jinho Park,† Cheng Zhi Huang,‡ and Younan Xia*,† †
The Wallace H. Coulter Department of Biomedical Engineering, School of Chemistry and Biochemistry, and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ Education Ministry Key Laboratory on Luminescence and Real-Time Analysis, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China § Department of Physics, Arizona State University, Tempe, Arizona 85287, United States S Supporting Information *
ABSTRACT: Palladium octahedrons and tetrahedrons enclosed by eight and four {111} facets have been synthesized from cuboctahedral Pd seeds by using Na2PdCl4 and Pd(acac)2, respectively, as the precursors. Our mechanistic studies indicate that the cuboctahedral seeds were directed to grow into octahedrons, truncated tetrahedrons, and then tetrahedrons when Pd(acac)2 was used as a precursor. In contrast, the same batch of seeds only evolved into octahedrons with increasing sizes when the precursor was switched to Na2PdCl4. The difference in growth pattern could be attributed to the different reduction rates of these two precursors. The fast reduction of Pd(acac)2 led to a quick drop in concentration for the precursor in the very early stage of a synthesis, forcing the growth into a kinetically controlled mode. In comparison, the slow reduction of Na2PdCl4 could maintain this precursor at a relatively high concentration to ensure thermodynamically controlled growth. This work not only advances our understanding of the growth mechanism of tetrahedrons but also offers a new approach to controlling the shape of metal nanocrystals. KEYWORDS: Nanocrystal, palladium, tetrahedron, precursor, reaction kinetics
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nanocrystals with a variety of shapes and the possible correlations between the nanocrystals with different shapes. In contrast to many other basic shapes (e.g., cubic, octahedral, cuboctahedral, spherical, and rod- or barlike) commonly taken by single-crystal metal nanocrystals, the tetrahedral shape has been elusive and rarely explored. Like an octahedron, the surface of a tetrahedron is also completely covered by {111} facets. However, the much larger (by 1.3 times) surface area to volume ratio relative to an octahedron makes the tetrahedral shape less favorable in a thermodynamically controlled synthesis. There are a few reports on the synthesis of metal nanocrystals with a tetrahedral shape but their growth mechanisms are yet to be elucidated. For example, El-Sayed and co-workers reported the first synthesis of Pt tetrahedrons as a mixture with Pt cubes when K2PtCl4 was reduced by H2 in the presence of polyacrylate as a colloidal stabilizer.6 Kaneda and co-workers reported the synthesis of Pd tetrahedrons by using tetranuclear Pd cluster as the precursor
hape control has proven to be a powerful means for tailoring and controlling the properties of metal nanocrystals and thus optimizing their performance in a broad range of applications related to photonics, electronics, catalysis, sensing, and biomedicine.1 To this end, metal nanocrystals with a variety of different shapes have been achieved over the past decade,2 and the growth mechanisms for some of these shapes have been established through the use of seed-mediated synthesis. For example, starting from single-crystal cubic or cuboctahedral seeds, nanocrystals with a variety of shapes including cubes, truncated cubes, cuboctahedrons, truncated octahedrons, and octahedrons could all be obtained by altering the surface free energies of different facets with a capping agent and thus the growth rates along different directions.3 As for pentagonal rods or wires, they could be obtained via axial growth along the 5-fold axes of decahedral seeds.4 In addition, nanocrystals with concave and convex surfaces could be generated by confining the growth selectively to the corners/ edges and side faces of seeds, respectively.5 Despite these successful demonstrations, it is still challenging to elucidate the growth pathways that lead to the formation of metal © 2013 American Chemical Society
Received: March 9, 2013 Revised: April 2, 2013 Published: April 9, 2013 2276
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Figure 1. (a) Schematic illustration showing the formation of Pd octahedrons and tetrahedrons, respectively, by using Na2PdCl4 and Pd(acac)2 as the precursors for seed-mediated growth. (b−d) Typical TEM images of the Pd cuboctahedral seeds, octahedrons, and tetrahedrons. The insets show the corresponding samples at a higher magnification with the scale bars being 5 nm. The 20 nm scale bar applies to all other images.
without the involvement of any reductant and stabilizer.7 Huang and co-workers recently synthesized Pt tetrahedrons by taking advantage of the selective capping effect of a peptide engineered with a specific sequence.8 Zheng and co-workers and Yan and co-workers prepared Pd concave tetrahedrons and Pt−Pd tetrahedrons, respectively, by introducing formaldehyde into a hydrothermal reaction.9 All of these studies were unable to decipher the growth pathway that led to the formation of tetrahedrons because it was essentially impossible to separate the nucleation and growth steps in these one-pot syntheses. Here we demonstrate that seed-mediated growth could be used to obtain Pd nanocrystals with a tetrahedral or octahedral shape depending on the precursor. The use of presynthesized seeds with the same size, shape, crystallinity, surface capping agent, dispersion medium, and particle concentration allowed us to single out one specific parameter for study.10 As a model system, Pd cuboctahedrons of 5 nm in size were synthesized using a polyol method and then used as seeds for growth. Except for the use of different precursors, all other experimental conditions including temperature, reductant, solvent, and the concentrations of seeds/reagents were kept the same. When Pd(acac)2 was used as a precursor, we obtained octahedrons, truncated tetrahedrons, and then tetrahedrons at different stages of growth. In contrast, the same seeds only evolved into octahedrons with increasing sizes when we switched the precursor from Pd(acac)2 to Na2PdCl4. We also investigated the mechanism responsible for the formation of Pd nanocrystals with a tetrahedral shape and found that it was related to the much faster reduction rate of Pd(acac)2 relative to Na2PdCl4. Figure 1a shows a schematic that compares the evolution pathways from single-crystal, cuboctahedral seeds of Pd to octahedral and tetrahedral nanocrystals when Na2PdCl4 and Pd(acac)2 were used as the precursors, respectively. The Pd cuboctahedrons were 5 nm in size and synthesized by reducing Na2PdCl4 with ethylene glycol as a solvent and reductant at 160 °C in the presence of poly(vinyl pyrrolidone) (PVP) as a stabilizer. Figure 1b shows a typical transmission electron
microscopy (TEM) image of the as-obtained cuboctahedral seeds of Pd with a purity approaching 100%. The Pd cuboctahedrons were enclosed by a mix of both {111} and {100} facets on the surface. We conducted the seed-mediated growth of Pd nanocrystals by introducing different precursors into a polyol system containing both the Pd seeds and PVP. In a typical synthesis, the Pd seeds and PVP were first dissolved in tetraethylene glycol (TTEG) at 140 °C under magnetic stirring. Here TTEG acted as both a solvent and a reductant while PVP served as a colloidal stabilizer. The two different Pd precursors, Na2PdCl4 and Pd(acac)2, were dissolved in TTEG and then quickly injected into the reaction mixtures with a pipet, respectively. The color of the solution was gradually darkened with time after the injection of both precursors, indicating that the precursor compounds were reduced to Pd atoms by TTEG at the used temperature. After 1 h, we obtained Pd nanocrystals with distinct shapes in the two syntheses conducted in parallel. As shown by TEM image in Figure 1c, Pd octahedrons enclosed by eight {111} facets were obtained in the presence of Na2PdCl4 as a precursor with an average edge length of 12.8 ± 1.3 nm. In comparison, Pd tetrahedrons enclosed by four {111} facets were obtained under the same condition, except for the use of Pd(acac)2 as precursor (Figure 1d). The average edge length of the Pd tetrahedrons was 14.5 ± 1.0 nm. Clearly, the difference in final shape for the Pd nanocrystals obtained through the seed-mediated growth should be attributed to the different precursors involved since all other parameters, including the seeds, temperature, reductant/solvent, and the concentrations of reagents, were kept the same. In order to understand how Pd nanocrystals with distinct shapes were formed when the two different precursors were used, we analyzed the products sampled at different reaction times by TEM. In this case, aliquots were taken out from the reaction mixture at different time points in the early stage (≤10 min) of a standard synthesis after the introduction of Pd precursor. Figure 2a−c shows TEM images and the 2277
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Figure 2. TEM images of Pd nanocrystals obtained in the early stages of standard syntheses when (a−c) Na2PdCl4 and (d−f) Pd(acac)2 were used as the precursors, respectively. (a,d) 0.5, (b,e) 1.5, and (c,f) 10 min. The insets show 3D models of the corresponding products obtained at different time points. The 20 nm scale bar applies to all images.
Figure 3. Structural analyses of the Pd tetrahedrons and truncated tetrahedrons obtained in a standard synthesis: (a) HAADF-STEM image of a Pd tetrahedron; (b) HAADF-STEM image of a Pd truncated tetrahedron; (c) typical TEM images of individual Pd truncated tetrahedrons at different orientations; and (d) 3D models of truncated tetrahedrons at different orientations matching the TEM images in (c). The insets in (a) and (b) show the corresponding FT patterns for the Pd tetrahedron and truncated tetrahedron.
energy of {100} facets relative to {111} facets.11 Once the nanocrystals were fully enclosed by {111} facets, the octahedral shape would not change any more with the increase of reaction time, while the edge length gradually increased from 5.5 ± 0.5 nm at t = 0.5 min to 6.7 ± 0.6 and 9.0 ± 0.8 nm for t = 1.5 and 10 min, respectively.
corresponding three-dimensional (3D) models of the Pd nanocrystals obtained at different time points when Na2PdCl4 was used as a precursor. At t = 0.5 min, Pd octahedrons with slight truncation at corners were observed. This result confirmed that the growth was mainly limited to the ⟨100⟩ directions in the initial stage due to the higher surface free 2278
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When Pd(acac)2 rather than Na2PdCl4 was used as precursor, the cuboctahedral seeds were immediately transformed into octahedrons with an average edge length of 7.1 ± 0.5 nm at t = 0.5 min (Figure 2d). The growth mode during this period of time was similar to the case of Na2PdCl4 in which the {100} facets grew at a relatively faster rate than the {111} facets. Different from the case of Na2PdCl4, only four of the {111} facets of an octahedron could further grow in the following step to generate a truncated tetrahedron enclosed by eight {111} facets with unequal surface areas at t = 1.5 min (Figure 2e). Afterward, sharp corners started to appear on the truncated tetrahedron to form a perfect tetrahedron at t = 10 min (Figure 2f). It should be pointed out that the edge length of a tetrahedron obtained at the end of a synthesis was exactly twice that of an octahedron obtained at t = 0.5 min (14.5 ± 1.0 nm vs 7.1 ± 0.5 nm). This result corresponds to an ideal situation for the formation of the smallest tetrahedron from an octahedral seed, as illustrated in Figures S1 and S2 of the Supporting Information. It indicates that the overgrowth only occurred on four of the eight {111} facets of an octahedron formed at the initial stage as there was essentially no growth for the other four {111} facets. In addition to the mechanistic study of growth pathway, we have also optimized the amount of precursor added into the solution in an effort to obtain Pd tetrahedrons with the highest possible purity and uniformity, as well as to avoid the generation of Pd nanoparticles via homogeneous nucleation (Supporting Information Figure S3). We further characterized the Pd tetrahedrons and truncated tetrahedrons shown in Figures 1d and 2e by high-resolution high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM). As shown in Figure 3a,b, the atomic lattices of a tetrahedron and truncated tetrahedron can be clearly resolved. The lattice fringe spacing of 1.9 and 2.3 Å marked on their surfaces can be indexed to the {200} and {111} reflections of face-centered cubic Pd, respectively. The Fourier transform (FT) patterns (insets) obtained from selected areas of the corresponding nanocrystals indicated that they were single crystals sitting against a plane perpendicular to the [110] zone axis, confirming that the {111} facets were exposed on the surfaces of both Pd tetrahedrons and truncated tetrahedrons. In order to better appreciate the shape of a truncated tetrahedron, we obtained TEM images of individual nanocrystals at different orientations relative to the electron beam (Figure 3c,d, together with their corresponding 3D models). Owing to the unequal surface areas of the eight {111} facets on a truncated tetrahedron, its profiles were quite different under TEM when it was sitting on the grid with different orientations. Figure S4 in the Supporting Information also shows TEM images of individual Pd tetrahedrons at different orientations. To better understand how the reaction kinetics of a synthesis was correlated with the type of precursor, we measured the percentages of metal ions remaining in the reaction solution as a function of time using inductively coupled plasma mass spectrometry (ICP-MS). As shown in Figure 4a, the reduction of Na2PdCl4 to Pd atoms was relatively slow and the percentage of Pd2+ ions remaining in the solution only decreased to 79.1% at t = 0.5 min and then to 63.6, 44.7, 19.7, and 4.8% at 1.5, 3.0, 5.0, and 10 min, respectively. In contrast, the conversion of Pd(acac)2 into Pd atoms was much faster than the Na2PdCl4 under the same condition, particularly in the early stage. The percentage of Pd2+ ions in the solution quickly dropped to 22.9% at t = 0.5 min and was then maintained at a level of ca.
Figure 4. Comparisons of the reaction kinetics when the two different precursors were used: (a) plots showing the percentages (determined by ICP-MS) of Pd2+ remaining in the reaction solutions as a function of reaction time; and (b) photographs showing changes to the solution color with increasing reaction time for standard syntheses with no cuboctahedral Pd seeds.
2.2% within 10 min. This result was also consistent with the solution colors observed at different time points. As shown in Figure 4b (top trace), the solution gradually changed from a deep yellow color to light brown, deep brown, and black within 10 min when Na2PdCl4 was reduced by the polyol. However, the solution quickly turned to light brown at 0.5 min, deep brown at 1.5 min, and dark at 3 min when Pd(acac)2 was reduced by the polyol (Figure 4b, bottom trace). The Pd seeds were not added in these control experiments to better observe the color changes as the precursors were reduced. These observations confirmed that Pd(acac)2 could be reduced at a much faster rate than Na2PdCl4 under the same condition, which could be attributed to a relatively weak binding force of the [acac]− ligand to Pd2+ relative to Cl−, and thus a faster release rate of Pd2+ from the Pd(acac)2. In another set of experiments (Supporting Information Figures S5 and S6), we also demonstrated that halides such as Cl−, Br−, and I− could all replace the [acac]− ligand in the precursor of Pd(acac)2 by forming relatively more stable complexes and thus slow down the reduction of Pd2+ ions into Pd atoms. As a result, Pd 2279
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corners/edges rather than the side faces of the Pd octahedron formed at the early stage owing to the higher surface energy of these sites than the side faces. Then, the adatoms could migrate to the side faces through surface diffusion due to the relatively high temperature used.14 When Pd(acac)2 was used as a precursor, the adatoms could only migrate to some of the eight side faces of an octahedron due to the fast reduction rate and the relatively low concentration of available Pd2+ ions, resulting in the formation of a tetrahedral shape enclosed by four {111} facets. In contrast, when the precursor was switched to Na2PdCl4, the slow reduction rate and the availability of Pd2+ ions around a Pd octahedral seed at a relatively high concentration allowed them to migrate to all eight facets of the Pd seed, leading to the formation of a Pd octahedron. Taken together, we can conclude that it was the difference in reduction rate for Na2PdCl4 and Pd(acac)2 that led to the formation of products with thermodynamically and kinetically controlled shapes (i.e., octahedrons and tetrahedron), respectively. In summary, we have demonstrated the effect of precursor on the reaction kinetics and thus growth pathway of a seedmediated synthesis. When Pd(acac)2 was used as a precursor, Pd cuboctahedral seeds could be directed to evolve into octahedrons, truncated tetrahedrons, and finally tetrahedrons as the reaction proceeded. In contrast, the same Pd seeds could only evolve into octahedrons when Na2PdCl4 was added into the reaction system as precursor. This study clearly demonstrates that nanocrystals enclosed by the same type of facet but in different shapes could be obtained by manipulating the reduction kinetics of a precursor. This work greatly advances our understanding of the growth mechanism for nanocrystals with a tetrahedral shape. We believe this strategy based upon the use of an appropriate precursor to manipulate the reaction kinetics could also be extended to cover other metals and even other types of inorganic materials.
nanocrystals with shapes other than tetrahedrons were obtained in these cases. A similar observation was also reported by Zheng and co-workers, where the stability of [PdI4]2− was found to be higher than Pd(acac)2 and the morphologies of Pd nanocrystals generated by reducing Pd(acac)2 with DMF were drastically varied with the addition of I−.12 It was also found that [acac]− did not have any effect on the reaction kinetics when Na2PdCl4 was used as a precursor, further suggesting the stronger binding energy of Cl− with Pd2+ ions than [acac]−. In another experiment, we introduced Cl− into the reaction system at t = 1.5 min after most of the Pd(acac)2 had already been reduced. As shown in Supporting Information Figure S7, most of the nanocrystals in the products were still Pd tetrahedrons. This result indicated that the ligand exchange between [acac]− and Cl− might occur on a slower time scale relative to the reduction of Pd(acac)2. As such, the growth pathway from octahedron to tetrahedron was not altered. A tetrahedron can only be formed under a kinetically controlled condition because it has a much higher (1.3 times) surface area to volume ratio than an octahedron. As we discussed before, the newly formed Pd atoms resulting from the reduction of a Pd precursor tended to be deposited on the {100} facets of a Pd cuboctahedral seed due to the difference in surface free energy between the {100} and {111} facets. It is worth noting that the edge length of the octahedrons obtained in the early stage of a synthesis involving Pd(acac)2 was relatively larger than the octahedrons obtained with Na2PdCl4 as the precursor (7.1 ± 0.5 nm vs 5.5 ± 0.5 nm at t = 0.5 min, as shown in Figure 2). This difference suggests that more precursor had been reduced to Pd atoms and subsequently included into the Pd octahedrons for the case of Pd(acac)2 due to its faster reduction rate. This observation was consistent with the ICP-MS data shown in Figure 4a. For the syntheses involving Pd(acac)2 and Na 2 PdCl 4 as the precursors, respectively, the percentages of remaining Pd precursor in the solutions were 22.9 and 79.1% at t = 0.5 min. As such, the Pd atoms newly formed through the reduction of Pd(acac)2 would not be able to nucleate on all of the eight {111} facets of a Pd octahedron. Instead, only four of the eight {111} facets of an octahedron could be involved in the heterogeneous nucleation and growth (as illustrated in Figure 1a and Figure S1 in the Supporting Information). Once a cluster (or nucleus) of Pd atoms had been created on a certain face of a seed, the reduction of precursor in the following step would preferentially occur at this site rather than on other regions due to the lower energy barrier.13 The site-localized growth could be retained as long as the reduction rate is faster than the surface diffusion rate of adatoms.14 Further growth of the four {111} facets eventually resulted in the formation of Pd tetrahedrons. In contrast, when Na2PdCl4 was used as a precursor, the availability of this precursor around a Pd octahedral seed at a relatively high concentration in the early stage of a synthesis allowed the nucleation to occur on all eight facets of the Pd seed. As a result, all of the eight {111} facets could undergo further growth, leading to the formation of Pd octahedrons with gradually increasing sizes. Even after the concentration of Pd2+ ions had dropped to a very low level at a later stage of the synthesis, the slow reduction rate of this precursor relative to surface diffusion of adatoms still could not lead to localized growth and thus formation of octahedrons.14 The formation of a tetrahedron from an octahedron can also be explained using another possible mechanism in which the newly formed Pd atoms were preferentially deposited at the
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Experimental details and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by a grant from NSF (DMR1215034) and startup funds from Georgia Institute of Technology. Y.X. was partially supported by the World Class University (WCU) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R32-20031). As a jointly supervised Ph.D. student from Southwest University, Y.W. was also partially supported by a Fellowship from the China Scholarship Council (CSC).
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