Rhodium Decahedral Nanocrystals: Facile ... - Wiley Online Library

DOI: 10.1002/cnma.201700327 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

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Rhodium Nanocrystals

Rhodium Decahedral Nanocrystals: Facile Synthesis, Mechanistic Insights, and Experimental Controls Sujin R. Lee,[a] Madeline Vara,[a] Zachary D. Hood,[a, b] Ming Zhao,[a] Kyle D. Gilroy,[c] Miaofang Chi,[b] and Younan Xia+*[a, c, d] intensive research has been conducted with a focus on maximizing the utilization efficiency of Rh by elucidating which nanocrystal shape (i. e., facet or surface atomic structure) gives rise to the best catalytic performance in terms of activity and selectivity.[1c] To establish a correlation between the catalytic performance and shape of nanocrystals, a number of protocols have been developed for generating Rh nanocrystals with a single-crystal structure and a variety of different shapes such as cubes,[3] octahedra,[4] tetrahedra,[5] and multipods.[6] The availability of Rh nanocrystals with well-defined and controllable shapes has led to a number of remarkable structure-activity discoveries. For example, tetrahedral Rh nanocrystals enclosed by {111} facets were found to be six times more active towards the hydrogenation of anthracene as compared to spherical nanoparticles.[5] In addition to singlecrystal nanocrystals, those with multiply-twinned internal structures (e. g., decahedra and icosahedra) have generated considerable interest owing to their unique physical and chemical properties. It was reported that the size-dependent lattice strain caused by internal twin defects could be utilized to further increase the catalytic performance of nanocrystals towards specific reactions.[7] For example, Pd@Pt core-shell decahedral nanocrystals were found to be superior in activity towards the oxygen reduction reaction when compared with other Pd@Pt core-shell nanocrystals (e. g., cubic and octahedral).[1h] Moreover, the introduction of twin defects into the internal structure of a nanocrystal redefines the underlying symmetry, providing access to new shapes with diversified properties.[8] For example, nanocrystals with a penta-twinned structure have a unique uniaxial symmetry that can be employed to template the formation of one-dimensional nanostructures including nanorods and nanowires. Despite the interesting properties and potential applications, it has been challenging to synthesize Rh nanocrystals with a multiplytwinned structure and in reasonable yields. This difficulty could potentially be attributed to the relatively high twin-defect energy of Rh (145 mJ · m 2) as compared with Au (26 mJ · m 2), Ag (16 mJ · m 2), and Pd (106 mJ · m 2).[9] After reviewing the literature, we only found a few reports on the synthesis of Rh nanocrystals with a multiply-twinned structure. In 2010, our group reported the synthesis of starfishlike Rh nanocrystals with five twinned arms by preventing oxidative etching in a polyol synthesis.[10] More recently, we successfully synthesized thirty-fold twinned Rh icosahedra with well-defined {111} facets by optimizing the reaction condi-

Abstract: Decahedral nanocrystals have received great attention owing to their unique symmetry and strain-energy distribution. In contrast to other noble metals, it has been difficult to synthesize decahedral Rh nanocrystals. We report a robust, one-pot method based on polyol reduction for the facile synthesis of Rh decahedral nanocrystals in high purity, with sub-20 nm sizes. The success of the synthesis relied on our ability to manipulate reduction kinetics by systematically tuning experimental parameters. We found that the yield of Rh decahedral nanocrystals could be maximized by optimizing: i) the concentration of Rh(acac)3 (metal precursor); ii) the molecular weight and amount of poly(vinyl pyrrolidone) (colloidal stabilizer/capping agent); and iii) the chain length of the polyol (solvent/reducing agent), with tetraethylene glycol being the best. We believe the mechanisms elucidated herein can be extended to other syntheses to produce metal nanocrystals with multiply twinned structures. Among the platinum-group metals (PGMs), Rh is widely used as a catalyst in a wide variety of reactions including hydrogenation, hydroformylation, NOx reduction, CO oxidation, and hydrogen production.[1] Its excellent catalytic performance, combined with its strong resistance to heat and corrosion, make it a material of choice for many vital industrial processes. However, similar to the case of Pt, the extremely low abundance of Rh in the earth’s crust and its increasing price have become a major concern for all existing and emerging applications built upon Rh-based catalysts.[2] In recent years,

[a] S. R. Lee, M. Vara, Z. D. Hood, M. Zhao, Prof. Y. Xia+ School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States E-mail: [email protected] [b] Z. D. Hood, Dr. M. Chi Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, USA [c] Dr. K. D. Gilroy, Prof. Y. Xia+ The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States [d] Prof. Y. Xia+ School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States [+] The authors declare no competing financial interest. Supporting information for this article is available on the WWW under https://doi.org/10.1002/cnma.201700327 ChemNanoMat 2018, 4, 66 – 70

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tions.[11] In a typical synthesis, benzyl alcohol containing Rh(acac)3, PVP (MW 40 kDa), and 4-benzoylbenzoic acid was heated to 195 8C under vigorous stirring for 3 h. The success of this reaction was attributed to the deceleration of the initial reduction rate of a Rh precursor to facilitate the formation of multiply-twinned seeds. By slightly increasing the initial reduction rate, it might be possible to change the products from icosahedra to decahedra. Our group recently demonstrated that this trend could be deterministically achieved in a polyol synthesis, where Pd decahedra could be readily produced by introducing an additive to slightly increase the initial reduction rate of a polyol synthesis developed for Pd icosahedra.[12] Inspired by the strong correlation between the initial reduction rate and the expressed internal twin-defect structure, we expected that this strategy could be extended to the synthesis of Rh decahedra. Herein, we report a facile synthesis of Rh decahedral nanocrystals with well-defined {111} facets in high uniformity and purity. To generate the Rh decahedra, we found that the reduction kinetics needed to be carefully optimized by varying the reaction rate while maintaining a high concentration of poly(vinyl pyrrolidone) (PVP). We found that faster reaction rates were necessary to promote the formation of decahedra over icosahedra, but if the reaction rate was too fast, nanocrystals with a single-crystal structure would be formed preferentially. By optimizing the concentrations of Rh(acac)3 (precursor), PVP (MW 10 kDa, colloidal stabilizer and capping agent), and tetraethylene glycol (TTEG, solvent and reducing agent), we were able to tightly control the reduction kinetics of the synthesis to reproducibly form Rh decahedra with purities approaching 90%. In a standard protocol, a solution of Rh(acac)3 in TTEG was rapidly injected into a preheated solution of PVP in TTEG. Figure 1A shows a typical transmission electron microscopy (TEM) image of the as-obtained Rh decahedral nanocrystals. They displayed an average size of 15.2 0.8 nm (see Figure S1 for an illustration showing how the size of an individual particle was defined and measured). Rh nanocrystals with a decahedral structure could be obtained in 80–90% purity, with the remaining 10–20% adopting {111}-bound shapes such as thin triangular nanoplates or icosahedra. Figure 1B shows TEM images of individual Rh decahedra in two common orientations, together with the corresponding models. Figure 1, C and D, show a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the Rh decahedra, and a high-resolution TEM image of an individual decahedron, respectively. As marked by the orange lines in both figures, we could easily resolve the five twin planes. The lattice spacing between parallel fringes was about 2.2 A˚, in agreement with the expected spacing between the (111) planes of face-centered cubic (fcc) Rh. In order to elucidate the mechanism involved in the formation of Rh decahedra, we sampled and characterized the intermediate products at different stages of a standard synthesis. After 10 min into the synthesis (Figure 2A), Rh decahedra with an average size of 4.8 nm were obtained. We believe that these particles seeded the formation of larger decahedra with ChemNanoMat 2018, 4, 66 – 70

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Figure 1. (A) TEM image of Rh decahedra prepared using the standard procedure. (B) TEM images and schematic models of a decahedron in two different orientations (axial and side view). (C) HAADF-STEM image of the Rh decahedra. (D) High-resolution TEM image taken from an individual particle in sample (A) where the twin defects are marked with orange lines. (Inset scale bar in (B): 5 nm)

Figure 2. TEM images of Rh decahedra prepared using the standard procedure, except that the reaction time was changed to (A) 10 min, (B) 1 h, (C) 2 h, and (D) 24 h, respectively.

sizes of 7.9, 14.2, and 16.7 nm at t = 1, 2, and 24 h, respectively (Figure 2, B–D). This result suggests that the size of the Rh decahedra can be readily controlled by simply varying the reaction time. Considering these results, we believe that the unique twin structure of the Rh decahedra was formed during homogeneous nucleation. Subsequently, the decahedra contin-

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ued to grow into larger nanocrystals over time through the constant deposition of newly formed Rh atoms, which increased their size without changing the internal twin structure. We chose to use TTEG because it has a longer hydrocarbon chain compared to ethylene glycol (EG) or diethylene glycol (DEG).[13] The longer hydrocarbon chain of TTEG allows us to control the reduction kinetics more precisely owing to the weaker reducing power. In addition, the high boiling point (275 8C) of TTEG makes it an ideal solvent for conducting syntheses at high reaction temperatures (e. g., 260 8C), whereas EG and DEG are less useful due to their lower boiling points at 197 and 245 8C, respectively. To verify the stronger reducing power of DEG relative to TTEG, we compared the products obtained using a standard protocol conducted at 240 8C in TTEG and DEG, respectively. When the TTEG was used, the yield of Rh decahedra increased, as shown in Figure S2A, a result that can be attributed to the relatively slower reduction kinetics associated with TTEG. When the reaction rate was accelerated through the use of DEG, a mixture of single crystals, plates, icosahedra, and small decahedra were obtained, as shown in Figure S2B. We also evaluated the role of PVP in the present synthesis. This polymer is widely used in nanocrystal syntheses as a capping agent or growth modifier, colloidal stabilizer or dispersant, and/or reducing agent.[14] To determine the effect that PVP has on the formation of Rh decahedra, we conducted a series of syntheses with different concentrations of PVP while keeping all the other parameters fixed. When a relatively low amount of PVP (e. g., 25 mg) was used, the major products were Rh icosahedra with an average size of 13 nm, together with some aggregated plates (see Figure 3A). As indicated by the yellow arrows in the TEM image, Rh decahedra were the minor product. The purity of decahedra could be increased to 45, 64, and 90% as the amount of PVP was increased to 100, 600, and 1200 mg, respectively, see Figure 3, B–D. This result is in agreement with previous work that demonstrated the effectiveness of PVP in stabilizing the {111} facets of Rh nanocrystals.[1a] Interestingly, PVP has also been identified as an effective capping agent in the synthesis of {111}-bound Au nanocrystals.[15] In that work, it was demonstrated that Au truncated tetrahedra, icosahedra, and decahedra, respectively, would be formed when gradually increasing the concentration of PVP. In addition to the concentration of PVP used, the molecular weight of PVP also played a pivotal role in directing the synthesis. By utilizing the standard protocol except for the use of PVP with molecular weights of 40 kDa and 55 kDa, respectively, we obtained Rh particles with irregular shapes and aggregates, see Figure S3, A and B. This result indicated that using PVP with a proper molecular weight played a crucial role in generating and stabilizing the resultant Rh decahedra. In addition, this observation is in agreement with a previous study in which PVP with a higher molecular weight was found to give rise to a relatively slower reduction rate, affecting the growth rate of the nanocrystals.[16] As such, by optimizing the amount of 10 kDa PVP, together with the other experimental conditions, ChemNanoMat 2018, 4, 66 – 70

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Figure 3. TEM images of Rh nanocrystals obtained using the standard protocol, except that the amount of PVP was changed to (A) 25, (B) 100, (C) 600, and (D) 1200 mg, respectively. The yellow arrows in (A) point to Rh decahedra, demonstrating that they only represent a small fraction of the product.

we could obtain Rh decahedra in high purity at 250 8C. Figure 4A shows a TEM image and the corresponding UV/vis spectrum. Specifically, the experiment was carried out by adding 5 mg of preheated Rh(acac)3 to a 3.5 mL TTEG solution containing 500 mg of PVP, and then maintaining the solution at 250 8C for 1 h. The resultant Rh decahedra displayed a well-defined spectral peak at around 265 nm, which we attribute to localized surface plasmon resonance. Optical signals in the UV-region have previously been observed for Rh nanocrystals with multipodal and cubic shapes.[6,17] We simulated the UV/Vis spectrum of an individual 15-nm Rh decahedron in water (n = 1.33) using the discrete dipole approximation (DDA) method (see the SI for simulation details and Figure S4 for the reproduced Rh dielectric constant). Figure 4, B and C, show the simulation results from when the k-vector is parallel and perpendicular to the 1D axis of the decahedron, respectively. The peak positions and overall profiles matched well with the experimentally measured spectrum in Figure 4A. The relatively broad peak observed in the experimental measurements could be attributed to both the natural asymmetry of the decahedra, and/or the variation in size of the Rh nanocrystals along the polydispersity present in the sample. It is important to note that Rh nanocrystals, in contrast to those composed of Au, Ag or their alloys,[18] have extinction peaks and profiles that are not strongly unique to the shape, see Figure S5 for a comparison between a Rh sphere, cube, and decahedron with sizes of 15 nm. Therefore, only TEM measurements serve as a viable method for identifying the shape and dispersity of Rh nanocrystals.

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Figure 5. TEM images of Rh decahedra obtained using the standard protocol, except that the concentration of Rh(acac)3 was changed to (A) 6.25, (B) 13.0, (C) 18.75, and (D) 25.0 mM, respectively. The yellow arrows indicate incomplete decahedra. (Inset scale bar: 10 nm)

once again suggesting that a relatively high concentration of PVP is necessary for generating Rh decahedra in high purity. The influence of reduction rate on the synthesis of Rh decahedra was further investigated by changing the pH of the polyol. Generally, higher pH values can enhance the reducing power of a polyol system.[19] To decrease the reducing power, we lowered the pH by introducing HCl into the TTEG solution. As shown in Figure S6, A and B, the introduction of HCl to TTEG resulted in the formation of plates and some twinned structures, including icosahedra and decahedra. It is worth pointing out that despite their high surface energy, the formation of thin nanoplates is favored when the reduction rate is slowed down. In contrast to acidic conditions, when the reaction was carried out in alkaline conditions, Rh nanocrystals with a mix of different structures resulted, especially those that appear to be of single-crystal structure, see Figure S6C. These observations are in agreement with our previous work based on Pd nanocrystals,[20] where slow, moderate, and fast rates of reduction gave rise to plates and decahedra, icosahedra, and single-crystals, respectively. In summary, we have demonstrated a polyol synthesis of Rh decahedra in high purity with controllable average sizes in the range of 5–28 nm. Because TTEG has a relatively weaker reducing power as compared to other polyols, the reaction rate could be easily tuned to induce the formation of multiplytwinned structures. We also discovered that the use of PVP with a relatively low molecular weight of 10 kDa and at high concentrations relative to that of the Rh precursor not only stabilized the decahedral seeds against aggregation, but also promoted the formation of uniform Rh nanocrystals with a decahedral structure. As a result, we could successfully

Figure 4. (A) Experimentally measured UV/Vis spectrum of Rh decahedra, with the inset showing a typical TEM image. Calculated UV/Vis spectra for an individual 15-nm Rh decahedron in water (n = 1.33) when the k-vector penetrates the (B) axial and (C) side orientation as shown by the models in the respective insets.

We also systematically investigated the effect that Rh precursor concentration has on the final shape and structure of the product. Figure 5 shows TEM images of the samples prepared by altering the amount of Rh(acac)3 added into the reaction mixture. When we decreased the concentration of Rh(acac)3 from 12.5 to 6.25 mM, we obtained Rh decahedra with an average size less than 8 nm. When the concentration of Rh(acac)3 was increased to 13.0, 18.75, and 25 mM, the final products contained Rh decahedra with an average size of 15, 23, and 28 nm, respectively, but the overall yield of Rh decahedra decreased as the concentration of Rh(acac)3 was increased. The final products contained a mixture of triangular plates, icosahedra, and what appeared to be incomplete decahedra (as indicated by yellow arrows in Figure 5, C and D). We believe that increasing the concentration of Rh(acac)3 from 12.5 to 18.75 and 25 mM unfavorably changed the mass ratio of the Rh precursor to PVP from 1 : 100 to 1 : 67, and 1 : 50, respectively. In these cases, we postulate that the amount of PVP is not enough to promote the formation of Rh decahedra,

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acknowledges the support of Graduate Research Fellowship from the NSF under Grant No. DGE-1148903 and the Georgia Tech-ORNL Fellowship.

synthesize Rh decahedra in high yields. By investigating the parameter-space within this polyol synthesis of Rh nanocrystals, we were able to pinpoint the key variables that could be used to modulate the reduction kinetics to guide the formation of Rh decahedra. This experimental approach can be further extended to Rh nanocrystals with diverse geometries or to other noble metals to generate nanocrystals with a decahedral shape. Thanks to the presence of unique multiply-twinned internal structure, we expect that Rh decahedra will be promising candidates as heterogeneous catalysts for a variety of reactions.

Conflict of Interest The authors declare no conflict of interest. Keywords: rhodium · decahedra · kinetics · nanocrystals · twin defects

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Chemicals. Rh(acac)3 (97%), poly(vinyl pyrrolidone) (PVP, with molecular weights of 10, 40, and 55 kDa), and tetraethylene glycol (TTEG) were all obtained from Sigma-Aldrich. All the chemicals were used as received. Standard protocol for the synthesis of Rh decahedra. For the synthesis of Rh decahedra with an average size of 15.2 0.8 nm, 5 mL of TTEG containing PVP (MW 10 kDa, 1000 mg) was preheated in air at 260 8C under magnetic stirring for 20 min. Subsequently, 2 mL of TTEG containing 10 mg of Rh(acac)3 was added in one shot using a pipette. The reaction was continued at 260 8C for 3 h. The resulting solution was cooled down to room temperature, and Rh decahedra were collected by centrifugation at 12 000 rpm for 20 min and washed with acetone and ethanol several times to remove TTEG and excess PVP. The nanocrystals were re-dispersed in deionized water for further use. Instrumentation. Transmission electron microscopy (TEM) images were obtained with an HT7700 microscope (Hitachi) operated at 120 kV. High-angle annular dark-field scanning TEM (HAADF-STEM) was performed using a cold-field emission Hitachi HD2700 aberration-corrected STEM operated at 200 kV. High-resolution TEM images were obtained with a JEOL JEM 2200FS STEM/TEM microscope at an acceleration voltage of 200 kV equipped with a CEOS probe corrector (Heidelberg, Germany) to provide nominal resolution of ~ 0.07 nm.

Supporting Information Available: TEM images of Rh decahedra prepared using the standard protocol but with systematic variations in one of the key experimental parameters. Modelling and simulation details are also provided. This material is available free of charge via the Internet at http:// http://onlinelibrary.wiley.com.

Acknowledgements This work was supported in part by a grant from the NSF (DMR 1506018) and start-up funds from Georgia Tech. Microscopy work was performed at the Georgia Tech’s Institute for Electronics and Nanotechnology (IEN), a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS1542174). A portion of the microscopy research was also conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Z.D.H. gratefully

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Manuscript received: November 3, 2017 Accepted Article published: November 10, 2017 Version of record online: December 4, 2017

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