Supporting information for. Amine-Assisted Synthesis of Concave Polyhedral Platinum Nanocrystals Having {411} High-Index Facets

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1 Supporting information for Amine-Assisted Synthesis of Concave Polyhedral Platinum Nanocrystals Having {411} High-Index Facets Xiaoqing Huang, Zipeng, Zhao, Jingmin Fan, Yueming Tan, Nanfeng Zheng* State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen , China Experimental Details Reagents: Hexachloroplatinic acid (H PtCl 6, AR), PVP (K30, AR), methylamine solution (30%), ethylamine, n-butylamine and N, N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The water used in all experiments was ultrapure. All reagents were used as received without further purification. Synthesis of concave Pt nanocrystals: In a typical synthesis of concave Pt nanocrystals, H PtCl 6 (0 mg/ml, 0.5 ml), poly(vinylpyrrolidone) (K30, 00.0 mg) and 0.1 ml methylamine solution (30%) were mixed together with 10 ml N, N-dimethylformamide. The resulting homogeneous yellow solution was transferred to a Teflon-lined stainless-steel autoclave with a capacity of 0 ml. The sealed vessel was heated from room temperature to 160 C in around 30 min and kept at this temperature for 10.5 h before it was cooled to S1

2 room temperature. The products were precipitated by acetone, separated via centrifugation and further purified by an ethanol-acetone mixture. Characterization: TEM (including high-resolution transmission electron microscope, HRTEM) studies were performed on a TECNAI F-30 high-resolution transmission electron microscope operating at 300 kv. The samples were prepared by dropping ethanol dispersion of samples onto 300-mesh carbon-coated copper grids and immediately evaporating the solvent. SEM studies were performed on a Hitachi S4800 scanning electron microscope with a field emission electron gun. The samples were prepared by dropping ethanol dispersion of samples onto Si substrates and immediately evaporating the solvent. Electrochemical Measurements: Ethanol dispersion of purified nanoparticles was deposited on a glassy carbon electrode to obtain the working electrodes after the solvent is dried under ambient condition. A saturated calomel electrode (SCE) and a platinum foil were used as the reference and counter electrode, respectively. For the electrooxidation of formic acid, the cyclic voltammgrams were recorded at a sweep rate of 50 mv/s in 0.5 M H SO M formic acid. For the electrooxidation of ethanol, the cyclic voltammgrams were recorded at a sweep rate of 50 mv/s in 0.1 M HClO M ethanol. Before cyclic voltammetry measurements, 10 cycles of potential sweeps between -0.5V and 1.3V were applied. S

3 a b c Figure S1. Large-area (a), enlarged (b), and tiled (c) scanning electron microscope (SEM) images of the as-prepared concave Pt nanocrystals. S3

4 Figure S. The as-prepared Pt concave structure can be better described by excavating out a tetragonal-pyramid from each square (100) face of a cube. Each cut-out pyramid has a square (100) base face and four exposed (411) faces. Overall an octapod particle has 4 identical kite-like {411} faces, each of which is composed of two coplanar isosceles triangles with apex angles of 50.5 and 86.6 sharing the same base. The detailed structural models for the as-synthesized concave Pt nanocrystals are as follows: (a) an ideal geometrical model of the concave Pt nanocrystal having {411} facets. (b) the polyhedron net of the concave polyhedral structure. The arrow specified the folded directed the red edges. (c) the model showing how a tetragonal-pyramid is excavated out from each square (100) face of a cube to form the concave structure. (d) the model of an excavated tetragonal-pyramid. S4

5 Figure S3. Tilted TEM images (a) and corresponding ideal geometrical models (b) of the concave Pt nanocrystals. S5

6 a b c d Figure S4. Typical transmission electron microscope (TEM) images of the as-prepared concave Pt nanocrystals in different magnifications. S6

7 Figure S5. Atomic models of (311) (a), (511) (b) and (411) (c) surfaces in side and top view. An ideal Pt (n -1, 1, 1) surface consists of an ordered staircase of steps separated by (111) step edges. The corresponding terrace-step notation for a Pt (n -1, 1, 1) surface is Pt (S)-[n(100) (111)]. n represents the width in the number of (100) terrace atoms. An ideal (411) facet of fcc Pt is periodically composed of (311) and (511) steps. S7

8 Figure S6. TEM image of Pt nanocrystals collected from the reaction with the same condition used in the synthesis of concave Pt nanocrystals but in the absence of methylamine. S8

9 Figure S7. The detailed structural models for the concave nanocrystals with smaller degree of concavity: (a) an ideal 3D geometrical model of the Pt nanocrystal with less concavity. (b) the model showing how a tetragonal-pyramid is excavated out from each square (100) face of a cube to form the less concave structure. (c) the polyhedron net of the concave polyhedral structure. The arrow specified the folded directed the red edges. (d) models of concave structures oriented in [100] direction and with various degree of concavity. Overall a less-concave polyhedron has 4 identical {411} trapezoid faces and 4 identical right-angled isosceles triangle. The degree of the concavity (D c ) of the Pt nanocrystals is defined as the ratio of the opening diameter to the apex-to-apex diameter of the concave nanocrystals. The degree of the concavity (D c ) varies with the portion of concave area on each (100) facets of the cube defined by the eight apexes of the octapod. The angle of concavity does not change with D c. The concave surfaces of the nanocrystals having different D c are still (411). However, the percentage of (411) facets decreases with D c. When the degree of concavity is equal to 1, S9

10 100% of the surface is (411) when the degree of concavity is 1. When D c is 0.5, the percentage of (411) facets is ~87% of the total exposure surface. The calculations about the portion of {411} faces (S {411} %) in the total exposed surface area of a concave nanocrystal are detailed as follows: In the model of the less concave polyhedron shown below the plane FCEGH is one of the {411} faces and the plane ACF is one of the {100} facets. The concave polyhedron has θ 1 =θ =19.5 and θ 3 = For a concave polyhedron, the total {411} surface area S {411} = 4(S CEGF + S EGH ) and the total exposed surface area = 4(S ACF + S CEGF + S EGH ). For simplicity, we assume that L AB = a and D c = x, then L CD = ax 1 LAC = a (1 x ) LCF = a(1 x) ax LCE = sinθ ax H CEGF = LCE sinθ = ax LGE = LCF + LCE cos θ = a(1 x) + tanθ 1 1 ax H EGH = LGE tan θ3 = { a( 1 x ) + }tanθ3 tan θ 1 1 S ACF = LAC = { a( 1- x) } ax ax SCEGF = ( LCF + LGE ) H CEGF = { a ( 1 x ) + } tanθ 1 1 ax So S EGH = LGE H EGH = { a( 1- x) + } tanθ 3 4 tanθ { 411 } % SCEGF + SEGH S = SACF + SCEGF + SEGH ax{ a( 1 x) x} { a( 1- x) x} = 1 { a( 1- x) } + ax{ a( 1 x) x} { a( 1- x) x} x x = (0 < x 1) x x If x=0, the concavity could not be observed directly through the TEM. S10

11 Figure S8. TEM image of Pt nanocrystals collected from the reaction with the same condition used in the synthesis of concave Pt nanocrystals but prepared by supplying different amount of methylamine: (a) 0.0 ml, (b) 0.04 ml, (c) 0.10 ml and (d) 0.0 ml. S11

12 100 Transmission (%) methylamine Concave Pt nanocrystals Wavelength (cm -1 ) Figure S9. Fourier transform infrared (FT-IR) characterization for the as-prepared concave Pt nanocrystals. S1

13 a b c d Figure S10. TEM images of concave Pt nanocrystals prepared by replacing methylamine with (a) ethylamine, (b) n-butylamine, (c) 4-methylpiperidine and (d) trimethylamine. S13

14 Figure S11. TEM image of Pt nanocrystals collected from the reaction with the same condition used in the synthesis of concave Pt nanocrystals but prepared by replacing methylamine with methanol. Figure S1. TEM image of nanocrystals collected from the reaction with the same condition used in the synthesis of concave Pt nanocrystals but in the absence of PVP. S14

15 Figure S13. TEM image of the concave Pt nanocrystals produced in a.75-hr reaction with the use of 0.10 ml methylamine solution. Figure S14. TEM image of the concave Pt nanocrystals produced in a reaction run in a flask attached to a Schlenk line under constant N flow. S15

16 . Figure S15. Representative TEM image of the commercial Pt black purchased from Aldrich. Figure S16. Representative TEM image of the commercial Pt/C (E-TEK) S16

17 Figure S17. Electrocatalytic stability test of concave Pt nanocrystals, commercial Pt black and Pt/C with oxidation potential of 0.63 V (vs SCE) in 0.5 M H SO M HCOOH solution. (a) chronoamperometric cruves, (b) the corresponding chronoamperometric curves normalized by the initial current density. S17

18 a i / ma Initial After 500 cycles a loss of 3.1% in ECSA after 500 cycles b i / ma E / V vs SCE 1.0 Initial 0.5 After 500 cycles c i / ma a loss of 1.9% in ECSA after 500 cycles E / V vs SCE Initial After 500 cycles -1.0 a loss of 6.6% in ECSA after 500 cycles Scan rate:50mv/s E / V vs SCE Figure S18. Electrocatalytic stability test of concave Pt nanocrystals, commercial Pt black and commercial Pt/c. CV curves for (a) concave Pt nanocrystals, (b) commercial Pt black and (c) commercial Pt/C before and after 500 cycles of accelerated stability tests. The tests were carried out in 0.5 M H SO 4 solutions with the cyclic potential sweeping between -0.5 and 0.8 V at a sweep rate of 50 mv/s. S18

19 a b c Figure S19. TEM images of the concave Pt nanocrystals after electrocatalysis examinations (a), being thermally treated at 50 C for 5 hr in air (b), and being stored in ethanol for 4 months (c). j / mamg Concave Pt Pt black Pt/C E / V vs SCE Figure S0. Mass activity of the as-prepared concave Pt nanocrystals, commercial Pt black, and Pt/C in 0.5 M H SO M HCOOH solution at a scan rate of 50 mv/s. S19

20 Table S1 Detailed data on electrocatalytic oxidation of formic acid on the commercial Pt black, Pt/C and the as-prepared concave Pt nanocrystals produced in the 11-hr reaction with 0.10 methylamine solution. The geometric area of the glassy carbon electrode used for the deposition of the catalysts is cm. J p : current density for the postive scan, J n : current density for the negative scan. Catalysts Catalyst loading (μg) Specific ECSA (m /g) area J p (ma/cm ) area J n (ma/cm ) mass J p (ma/mg) mass J n (ma/mg) Concave Pt Pt black Pt/C Table S Comparison of peak current densities of concave Pt nanocrystals with different sizes. The Pt nanocrystals were produced in 5.0-, 8.0-, and 11-hr reactions (see Figure 3 for TEM images) in 0.5 M H SO M HCOOH solution at a scan rate of 50 mv/s. J p : current density for the postive scan, J n : current density for the negative scan. Reaction time for the catalyst synthsis Average apex-to-apex diameter area J p (ma/cm ) area J n (ma/cm ) 5 hr 31 ± hr 59 ± hr 70 ± S0

21 Table 3 Comparison of peak current densities of the Pt nanocrystals with various concavities. The Pt nanocrystals were prepared by supplying different amount of methylamine solution (30%) (see Figure S8 for TEM images) in 0.5 M H SO M HCOOH solution at a scan rate of 50 mv/s. J p : current density for the postive scan, J n : current density for the negative scan. Amount of methylamine for the catalyst synthsis Morphological feature of the products Average apex-to-apex diameter (nm) Degree of Concavity (D c ) of the concave particles area J p (ma/cm ) area J n (ma/cm ) 0.0 ml About half are cubes and another half are less concave particles. 3 ± 8 ~ 0.30 (average for concave partilces only) ml 0.10 ml All are concave but have small concavity. All are concave but have large concavity. 41 ± 7 ~ ± 15 ~ S1