Supporting Information

Transcription

1 Supporting Information Highly Uniform Platinum Icosahedra Made by the Hot Injection-Assisted GRAILS Method Wei Zhou,, Jianbo Wu, and Hong Yang *, Department of Chemical & Biomolecular Engineering, University of Illinois at Urbana-Champaign, 600 S. Mathews Avenue, MC-712, Urbana, Illinois 61801, United States School of Chemistry and Environment, Beihang University, Beijing , P. R. China S 1

2 Experimental Details: Synthesis of Pt Icosahedra. All chemicals were purchased from Aldrich and used as received except those specified otherwise. In a typical procedure, 20 mg of platinum acetylacetonate (Pt(acac) 2, Strem Chemicals, purity: 98 %), 1 ml of dodecylamine (DDA, 98 %), and 50 μl of oleic acid (OA, 90 %) were mixed and preheated at 160 ºC to make the precursor solution. The mixture of 14 mg of Y(acac) 3 H 2 O (Strem Chemicals, 99.9 %), 1 ml of DPE (diphenyl ether, 99 %), and 9 ml of DDA was degassed in a 25-mL flask under the protection of argon gas for about 10 min. Next, carbon monoxide gas (99.98%, Specialty Gases of America, Inc.) was introduced and the CO-saturated solution was heated at 210 ºC for 15 min at a flow rate of 120 cm 3 /min (OMEGA FMA-A2305) at a pressure of 10 psi, followed by injecting the precursor solution with a syringe. The mixture in the closed flask turned dark within 1 min, which was kept at 210 ºC with the magnetic stirring and under the atmosphere of CO for 30 min. The black precipitate was washed with 10 ml of chloroform for 3 times and collected by centrifugation at 6500 rpm for 8 min. The procedure for the synthesis of Pt icosahedra at larger size but lower population is as the following: 20 mg of Pt(acac) 2, 50 μl of OA and 1 ml of DDA were mixed and preheated at 135 ºC to form the precursor solution. Another mixture of 1 ml of DPE and 9 ml of DDA were degassed in a 25-mL flask under the protection of argon for 10 min, and then bubbled with CO gas for 15 min at 210 ºC. The precursor solution was injected into the CO-saturated solution by a syringe. The mixture was kept at 210 ºC with the magnetic stirring under the atmosphere of CO for 30 min. The obtained product was washed and collected. The particles made were mainly composed of larger icosahedra, twinned nanorods, and nanocubes. Synthesis of Pt Nanocubes. The procedure is similar to that for the synthesis of Pt icosahedra, except the two solutions were made by standard mixing, instead of hot injection. A mixture of Pt(acac) 2 (20 mg), Y(acac) 3 H 2 O (14 mg), DDA (10 ml), OA (50 μl), and DPE (1 ml) was bubbled with CO gas for 15 min after degassing by vacuum. Then, the reaction flask solution was transferred to a preheated oil bath at 210 ºC. The mixture was under a dynamic flow of CO gas throughout. The synthetic mixture began to turn black about 5 min after the flask was transferred to the oil bath. It was kept in the oil bath for additional 30 min. The final product was washed with chloroform for several times using the same procedure as stated above. Synthesis of Pt Octapods. The procedure is similar to that for the synthesis of Pt icosahedra, except there was no degassing step. Synthesis of Pt Multipods. The procedure is similar to that for the synthesis of Pt icosahedra, except CO gas was not used. Other Control Experiments for Understanding the Role of Oxygen on the Formation of Icosahedra. The as-made Pt icosahedra were kept in the reaction mixture after the completion of reaction, followed by bubbling oxygen instead of carbon monoxide at the same temperature. An aliquot of the mixture (1 ml) with Pt nanoparticles (NPs) were drawn from the flask by syringes and collected in separate vials after O 2 was bubbled for 0, 15, and 30 min, respectively. The mixtures with Pt NPs were diluted by chloroform, and centrifuged at 6500 rpm for 8 min. The transparent supernatants were obtained, while black powder products precipitates at the bottom of the vials. Caution: the reaction solution might suddenly boil when O 2 was bubbled for 30 min or a longer time. S 2

3 Preparation of Carbon-Supported Catalysts. Carbon black (Vulcan XC-72) was used as the support for Pt icosahedral catalysts. In a standard procedure, 28 mg of carbon black was mixed with 7 mg of Pt icosahedral NPs in 14 ml of chloroform, followed by sonication for 30 min. The mixture was stirred overnight and the NPs were separated by centrifugation at 5000 rpm for 5 min. The carbon-supported catalysts were then re-dispersed in n-butylamine at 0.5 mg-catalyst/ml solvent and kept on stirring for three days. After the reaction, 5 ml of the suspension was mixed with 5 ml methanol and the NPs were separated by centrifuge at 5000 rpm for 5 min. The collected power samples were washed by 10 ml of methanol for two additional times. The final product was naturally dried under the protection of Argon gas and then sealed in the vial for further use. Electrocatalytic Measurement. A three-electrode cell system was used. The working electrode was a glass-carbon rotating disk electrode (RDE) with an area of cm 2. A platinum foil of 1 cm 2 was used as the counter electrode with a HydroFlex hydrogen electrode in a separate compartment as reference. The mass of metal in Pt/C catalyst was tested by thermogravimetric analysis (TGA) heating to 600 ºC at a ramp rate of 10 ºC/min. To make a catalysts ink, 5 mg of Pt/C catalyst was added in a mixture of 8 ml of de-ionized water, 2 ml of isopropanol, and 50 μl of 5% Nafion (v:v:v=8:2:0.05). A small volume of this catalyst ink (60 μl) was dropped on the RDE to make the working electrode. The electrochemical active surface area (ECSA) was calculated from the adsorption of hydrogen species in the range between 0.05 and 0.4 V in the cyclic voltammetry (CV) curves. The CV test was conducted in an Ar-saturated, aqueous solution of perchloric acid (0.1 M, HClO 4, prepared with Millipore water, 18.2 MΩ at 25 ºC). The test was performed at room temperature with a scan rate of 50 mv/s. Oxygen reduction reaction (ORR) activities were measured in the same HClO 4 solution purged with O 2 for 30 min before and during the test with a positive scan at 10 mv/s at a rotating rate of 1600 rpm. Characterization. The structure and crystal phase analyses were carried out by X-ray powder diffraction (XRD, Rigaku DMAX). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were carried out by a JEOL 2100 Cryo microscope at an accelerating voltage of 200 kv. A JEOL 2100 F TEM was used to do the quantitative energy dispersive X-ray (EDX) measurement under STEM mode equipped with energy-dispersive X-ray spectroscopy. The JEOL 2010 F TEM was used for electron energy loss spectroscopy (EELS) characterization. The TEM specimen was prepared by dispersing the particle suspension in chloroform and drop cast onto a carbon film-supported copper grid. S 3

4 Figure S1. TEM micrograph of Pt icosahedra with a shape selectivity of ~95%. Figure S2. (a) STEM image and (b) the corresponding EDS pattern of Pt icosahedra in the selected region shown in the rectangular area in Figure S2a. The inset shows the atomic and weight percentages of Pt and Y elements. The peak for Y L line (~2 kev) overlapped in part with that for Pt L line [R1, R2]. S 4

5 Y M4,5 Intensity (a.u.) Energy loss (ev) Figure S3. EELS spectrum for yttrium element (Y M 4,5 ) from a single Pt icosahedron shown in the inset. The peaks at 162 and 165 ev can be indexed to M 4,5 lines for Y 3+, respectively [R3, R4]. The small peaks from 155 to 160 ev can be assigned to Y 3d 5/2 and Y 3d 3/2 of Y 3+, respectively [R5, R6]. Figure S4. (a) TEM micrograph of the Pt icosahedra mixed with cubes and twinned rods. The sample was made under the same condition as those for the synthesis of Pt icosahedra, except no addition of Y(acac) 3. The shape selectivity for icosahedron was about 50 %. (b) TEM micrograph of Pt icosahedra with an average edge length of 18 nm (population: ~50 %), mixed with nanocubes and nanorods (~150 nm in length, population: ~25 %). These nanocrystals were obtained using a similar synthetic protocol as the one for sample shown in Figure S4a, except that the precursor solution was heated at 135 C instead of 160 C. S 5

6 Figure S5. (a) TEM image of icosahedra with edge length of ~5.5 nm. The synthesis followed mostly the preparation of Pt icosahedra. The mixed solution was kept at 210 C under CO atmosphere for 5 min instead of 30 min after injection. It was then moved out from the oil bath and cooled under CO gas for 20 min. The shortened reaction time did hinder the growth of icosahedra. (b) TEM image of the sample composed of cubes, some rods and reduced icosahedra. The process followed the preparation of Pt icosahedra without CO gas. After the mixture turned black upon injection, CO was added and the reaction maintained for 30 min. This control experiment was carried out to study the formation of seeds with hot injection in the absence of CO. Figure S6. TEM micrographs showing the shape evolution of Pt icosahedra at different stages upon exposure to O 2 gas. S 6

7 Figure S7. HRTEM micrograph of a short rod showing the twin plane. The d-spacing is 0.21 nm, which can be assigned to the (111) plane of Pt. Figure S8. TEM micrograph of various other branched nanostructures. Unmarked scale bar: 20 nm. These samples were synthesized by allowing the reactions to take place in the mixture of DDA and Pt(acac) 2 at different temperatures without any protective gas. S 7

8 Figure S9. TEM micrograph of the commercial Pt/C reference catalysts showing the Pt NPs with an average diameter of ~3 nm. Figure S10. TEM micrographs of Pt icosahedra/c catalysts after (a) 200 and (b) 2500 CV cycles, respectively. S 8

9 References: (R1) Wang, Q., Lian, G.; Dickey, E. C. Acta Mater. 2004, 52, (R2) Grigis, Ch.; Schamm, S. Ultramicroscopy 1998, 74, (R3) Wilke, A.; Yang, J.-M.; Kim, J. W.; Seifarth, O.; Dietrich, B.; Giussani, A.; Zaumseil, P.; Storck, P.; Schroeder, T. Surf. Interface Anal. 2011, 43, (R4) Klimiankou, M.; Lindau, R.; Möslang, A. Micron 2005, 36, 1-8. (R5) Gulino, A.; Fragalà, I. J. Mater. Chem. 1999, 9, (R6) Dou, Y.; Egdell, R. G.; Walker, T.; Law, D. S. L.; Beamson, G. Surf. Sci. 1998, 398, S 9