In Situ X-ray Scattering Guides the Synthesis of Uniform PtSn Nanocrystals

Transcription

1 Supporting Information In Situ X-ray Scattering Guides the Synthesis of Uniform PtSn Nanocrystals Liheng Wu,, Amanda P. Fournier, Joshua J. Willis,, Matteo Cargnello,,, * and Christopher J. Tassone, * Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94305, United States *Corresponding authors: mcargnello@stanford.edu, tassone@slac.stanford.edu S1

2 Experimental Section Chemicals. Oleylamine (70%), 1-octadecene (90%), and Sn(acac) 2 Cl 2 (acac = acetylacetonate, 98%) were purchased from Sigma-Aldrich. Pt(acac) 2 (98%) was purchased from Acros Organics. Hexanes and 2-propanol were purchased from Fisher Scientific. All chemicals were used as received without further purification. Synthesis of PtSn nanocrystals (NCs) from metal salts. The synthesis was performed in the custom-made flask reactor shown in Figure S mg of Pt(acac) 2 (0.15 mmol) and 58.5 mg of Sn(acac) 2 Cl 2 (0.15 mmol) were mixed with 1-octadecene (7 ml) and oleylamine (3mL) under a gentle flow of N 2. The mixture was magnetically stirred and heated up to 110 o C, and kept at this temperature for another 30 min under continuous N 2 flow to remove air and moisture. Then under a blanket of N 2, the mixture was heated up to 280 o C at a heating rate of ~15 o C min -1. SAXS/WAXS patterns were consecutively collected with an acquisition frequency of each pattern every 11 s (5 s of exposure time and 6 s of data collecting and storing time) during the heating process. The reaction mixture was kept at 280 o C for 30 min. After the reaction, the flask was cooled down to room temperature by removing the heating tape. The obtained NCs were precipitated with 2-propanol (20 ml) and collected by centrifugation (8000 rpm, 3 min). The NCs were washed again with hexanes/2-propanol (5 ml of hexanes was used to disperse these NCs followed by adding 20 ml of 2-propanol to precipitate them) and re-dispersed in 5 ml of hexanes for further characterization. Synthesis of Pt NCs. The Pt NCs were synthesized in a lab Schlenk line following a previously reported procedure. 1 Pt(acac) 2 (100 mg), 1-octadecene (10 ml), oleic acid (1 ml), and oleylamine (1 ml) were mixed under magnetic stirring and degassed at 120 o C under continuous N 2 flow for 20 min to remove air and moisture. Then the temperature was raised to 180 o C and the flask was kept under N 2 blanket. A diluted solution of Fe(CO) 5 in hexanes (0.1 ml, prepared by adding 0.05 ml of Fe(CO) 5 in 0.45 ml of hexanes) was injected into the reaction mixture. The reaction solution was kept at 180 o C for 30 min before it was cooled down to room temperature. The obtained NCs were purified similarly to the synthesis of PtSn NCs and re-dispersed in 5 ml of hexanes for further use. S2

3 Synthesis of PtSn NCs from seed-mediated growth. The seed-mediated growth was probed in situ using the custom-made flask reactor. In a typical procedure, 8 ml of oleylamine and 49 mg of Sn(acac) 2 Cl 2 (0.13 mmol) were mixed in the in situ reactor under magnetic stirring and degassed at 110 o C under N 2 flow for 20 min to form a clear solution. The solution was cooled down to 80 o C. 25 mg of Pt NCs (0.13 mmol) in 2 ml of hexanes was added in the solution. The temperature was then raised to 110 o C under N 2 flow for another 20 min to remove the hexanes. Then under a blanket of N 2, the reaction mixture was heated to 280 o C at a rate of ~15 o C min -1 and kept at 280 o C for 30 min. During the heating process, SAXS/WAXS patterns were consecutively collected from 110 o C. After the reaction, the heating tape was turned off and the reaction mixture was cooled down to room temperature. Similar separation and purification steps were followed to purify the NCs and the final NCs were re-dispersed in 5 ml of hexanes for further characterization. Synchrotron X-ray measurements. In situ SAXS/WAXS measurements were carried out at Beamline 1-5 of Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory. The wavelength λ of the X-ray was Å (15 kev) and the beam spot size is 500µm 500 µm. The exposure time for each frame was set to be 5 s. The SAXS patterns were collected using a Rayonix 165 SX CCD area detector and the WAXS patterns were collected using a Pilatus 100K detector. The sample-to-detector distance for SAXS was calibrated to be mm using a silver behenate standard. 2D SAXS data were integrated into 1D scattering curve I (q) presented as a function of scattering vector q = (4 /λ)sin(θ/2), where λ is the wavelength of the incident X-ray and θ is the scattering angle. The absolute intensity calibration was performed using a glassy carbon standard. 2 The sample-to-detector distance for WAXS was calibrated to be mm with a horizontal tilt of o using a lanthanum hexaboride standard. Additional ex situ X-ray diffraction measurements of the Pt and PtSn NCs were carried out at Beamline 11-3 of SSRL (photon energy = 12.7 kev, sample-to-detector distance = mm). Data analysis. In situ SAXS data were analyzed using the Irena package from the Advanced Photon Source ( 3 In situ WAXS data were analyzed using a custom written code in python. The scattering data were analyzed after standard background subtraction. For the synthesis of PtSn NCs from reduction of the two metal S3

4 precursors, the SAXS/WAXS patterns of 1-octadecene in the flask reactor were first collected at various temperatures ranging from 30 o C to 280 o C at the same exposure time of 5 s for each frame and used as the background. For the seed-mediated synthesis of PtSn NCs using Pt NC seeds, the scattering signal from the clear Sn(IV) solution in the flask reactor at 110 o C before the injection of Pt NCs were collected as the background. During the seed-mediated growth, as more and more Sn incorporated inside Pt NCs, the shape of Pt-Sn NCs changed from initial cubes into more faceted polyhedrons. However, the scattering in the low q region is not shape-dependent and NC shape only changes the periodicity of the oscillation and its intensity. 4 Moreover, for the colloidal NC solution, the volume fraction ( ) of NCs can be determined by the equation 1 =, which is not dependent on NC shape. The shape independence was also validated by the results that very similar NC volume for the initial Pt NCs were obtained from SAXS fitting using either cube or sphere form factor. Therefore, to determine NC volumetric change during the seed-mediated growth, we used a sphere form factor to fit the SAXS patterns neglecting the shape change. The NC volumetric increase during the seed-mediated growth was then calculated based on the diameter derived from the sphere form factor fitting. For the in situ WAXS, the peak intensities and positions were calculated by Gaussian fitting of the Pt (111) and (200) peaks and the PtSn (102) and (110) peaks. Electron microscopy characterization. Transmission electron microscopy (TEM) images were collected on a FEI Tecnai transmission electron microscope equipped with an Orius CCD camera. High-resolution aberration-corrected TEM images, high-angle annular dark field scanning transmission electron microscopy (STEM) images, and energy dispersive X-ray spectroscopy (EDS) maps were recorded using an FEI Titan environmental transmission electron microscope equipped with a spherical aberration corrector in the image forming lens, a Gatan OneView camera, an Oxford Xmax SDD EDX detector. Both instruments were operated in high vacuum at an accelerating voltage of 200 kv. The TEM samples were prepared by drop-casting NC diluted hexane dispersions onto TEM grids. References S4

5 1. Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. H. A General Approach to the Size- and Shape- Controlled Synthesis of Platinum Nanoparticles and Their Catalytic Reduction of Oxygen. Angew. Chem. Int. Ed. 2008, 47, Zhang, F.; Ilavsky, J.; Long, G. G.; Quintana, J. P. G.; Allen, A. J.; Jemian, P. R. Glassy Carbon as an Absolute Intensity Calibration Standard for Small-Angle Scattering. Metall. Mater. Trans. A 2010, 41a, Ilavsky, J.; Jemian, P. R. Irena: Tool Suite for Modeling and Analysis of Small-Angle Scattering. J. Appl. Crystallogr. 2009, 42, Senesi, A.; Lee, B. Scattering Functions of Polyhedra. J. Appl. Crystallogr. 2015, 48, S5

6 Supplementary Figures Figure S1. (a) Schematic of the flask reactor used for the in situ studies. (b) A photograph of the reactor. (c) A photograph of the experimental setup for in situ small-angle and wide-angle X-ray scattering (SAXS/WAXS) at Beamline 1-5 of Stanford Synchrotron Radiation Lightsource. S6

7 Figure S2. (a) Temperature profile for the synthesis of PtSn nanocrystals through reduction of Pt(acac) 2 and Sn(acac) 2 Cl 2 precursors. (b) Temperature profile for the synthesis of PtSn nanocrystals through seedmediated growth using pre-formed Pt nanocrystals. In both cases, the reaction time was set as t = 0 s at 110 o C when the reaction mixture started to be heated up. S7

8 Figure S3. (a) WAXS pattern of the NCs at 66 s during the synthesis from the reduction method. The peaks correspond to the (111) and (200) diffraction peaks of face-centered cubic Pt phase (JCPDS ), suggesting the formation of monometallic Pt NCs. (b) WAXS pattern of the NCs at 429 s during the synthesis. The peaks correspond to the diffraction peaks of hexagonal intermetallic PtSn phase (JCPDS ), suggesting the formation of intermetallic PtSn NCs. S8

9 Figure S4. (a) SAXS pattern (black) of the as-synthesized Pt NCs used for the seed-mediated synthesis and its corresponding fit using a cube form factor (red). (b) Size distribution of the Pt NCs measured from TEM (blue, 6.0 ± 0.6 nm) and SAXS (red, 5.7 ± 0.8 nm). S9

10 Figure S5. SAXS fitting of some representative patterns at different reaction times during the seedmediated synthesis using pre-formed Pt NCs. The black plots are experiments results and the red plots are the corresponding fits. S10

11 Intensity (a.u.) PtSn(101) Pt(111) PtSn (102) PtSn (110) Pt (200) Reaction time (s) q (Å -1 ) Figure S6. Real-time WAXS patterns from the seed-mediated synthesis of PtSn NCs. The time interval for each measurement is 11 s (5 s of exposure time and 6 s of data collecting and storing time), and there are 180 patterns shown in this figure. S11

12 Figure S7. High-resolution TEM image of an intermediate nanocrystal collected at reaction time 450 s. The core shows Pt domain with (111) lattice fringes and the shell shows some PtSn crystalline domains with (101) lattice fringes. S12

13 Figure S8. STEM-EDS line scan crossing an individual PtSn NC. (a) STEM image of the NC scanned. (b) Corresponding intensity profile across the line position shown in (a). S13

14 Intensity (a.u.) (101) (102) (111) (110) (202) (200)(201) (200) (220) PtSn (212) (214) Pt (311) (222) q (Å -1 ) Figure S9. Ex situ X-ray diffraction patterns of the initial Pt NCs and final PtSn NCs synthesized from the seed-mediated approach. The reference peaks of Pt (JCPDS ) and PtSn (JCPDS ) are included. S14

15 Intensity (a.u.) (111) (200) Reaction time (s) q (Å -1 ) Figure S10. Real-time WAXS patterns from the seed-mediated synthesis of Pt 4 Sn NCs. The time interval for each measurement is 11 s, and there are 170 patterns shown in this figure. S15

16 Figure S11. (a) High-resolution X-ray diffraction pattern of the obtained Pt 4 Sn NCs. (b) Enlarged patterns of the (111) diffraction peaks (black) and the corresponding Gaussian fits (red). Compared to the initial Pt NCs, the diffraction peaks shift to lower scattering vector q, indicating increased lattice parameter by alloying with Sn. The full width at half maximum ( q ) is measured to be Å -1 for Pt and Å -1 for Pt 4 Sn. The average crystallite size (D) can be derived from the Scherrer equation =, where the shape factor K = 0.94 for cubic particles. The average crystallite size is calculated to be 5.5 nm for Pt NCs and 6.1 nm for Pt 4 Sn NCs. The increase of crystallite size is in agreement with the volumetric increase measured by SAXS and size increase measured by TEM. S16

17 Figure S12. STEM-EDS line scan crossing an individual Pt 4 Sn NC. (a) STEM image of the NC scanned. (b) Corresponding intensity profile across the line position shown in (a). S17

18 Figure S13. Preliminary results on ex situ synthesis of PdSn and PdCu nanocrystals using Pd nanocrystal seeds. (a) TEM image of the Pd nanocrystal seeds. (b) Size histogram of the Pd nanoparticles (11.1 ± 1.1 nm). (c) TEM image of the PdSn nanocrystals synthesized using the Pd nanocrystal seeds. (d) Size histogram of the PdSn nanocrystals (13.3 ± 0.8 nm). The size increased by 2.2 nm compared to the Pd nanocrystal seeds, corresponding to a volumetric increase by 72%. (e) Energy-dispersive X-ray spectroscopy spectra of the PdSn nanocrystals showing the formation of bimetallic PdSn nanocrystals. The Cu signal is from the Cu TEM grid. (f) TEM image of the PdCu nanocrystals synthesized using the Pd nanocrystal seeds. (g) Size histogram of the PdCu nanocrystals (13.9 ± 0.9 nm). The size increased by 2.8 nm, corresponding to a volumetric increase by 96%. (e) Energy-dispersive X-ray spectroscopy spectra of the PdCu nanocrystals showing the formation of bimetallic PdCu nanocrystals. The Ni signal is from the Ni TEM grid. S18