Supporting Information for. Revealing Surface Elemental Composition and Dynamic Processes

Transcription

1 Supporting Information for Revealing Surface Elemental Composition and Dynamic Processes Involved in Facet-dependent Oxidation of Pt 3 Co Nanoparticles via in-situ Transmission Electron Microscopy Sheng Dai 1, Yusheng Hou 2, Masatoshi Onoue 2, Shuyi Zhang 1,3, Wenpei Gao 1, Xingxu Yan 1, George W. Graham 1,3, Ruqian Wu 2, Xiaoqing Pan 1, 2, * 1 Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, United States 2 Department of Physics and Astronomy, University of California Irvine, Irvine, CA, 92697, United States 3 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48109, United States *Corresponding author xiaoqing.pan@uci.edu

2 1. EDS line scan result at the {100} and {111} surfaces of Pt 3 Co nanoparticles Figure S1. EDS line scan analysis of the initially disordered Pt 3 Co NP surfaces. (a) HAADF-STEM image of an initially disordered Pt 3 Co NP along the <110> zone axis. Yellow and green dashed lines indicate the EDS scan path at the {111} and {100} surfaces, respectively. (b-c) EDS line scan result showing the {111} surface of Pt 3 Co NPs has a relatively higher content of Co than the {100} surface. The intensities in (b) and (c) are normalized to the maximum value of Pt for comparison. 2. EELS measurement on the oxide layers of the oxidized Pt 3 Co nanoparticles Besides the imaging analysis of the oxide layers on the oxidized Pt 3 Co nanoparticls, EELS measurement was carried out for the further confirmation of the +2 valence state of the Co in the oxide layers. Since the EELS signals taken in the gas cell are relatively weak due to the electron scattering by the window membranes, the EELS measurement was carried out ex-situ on the Pt 3 Co samples, which were annealed under one atmosphere of oxygen at 350 ºC for more than 10 minutes, the same condition we used in the in situ experiment. Figure S2a presents the HAADF-STEM image of an ex-situ treated Pt 3 Co nanoparticle, showing the visible oxide layers on the {111} surfaces. EEL spectra #1 was collected from the oxide layers on the Pt 3 Co nanoparticle, as indicated by the red point in Fig. S2a. As a comparison, EEL spectra #2 was collected from the bulk of an initially disordered Pt 3 Co nanoparticle (as indicated by the blue point in Fig. S2b)

3 under the same acquisition conditions. Figure S2c presents the EEL spectra #1 and #2 showing the O K-edge and Co L-edges, respectively. For comparison, the two spectra are aligned to the maximum intensity of the Co L 3 -edges for the further analysis. Figure S2. EELS measurement on oxidized Pt 3 Co and initial Pt 3 Co nanoparticles. (a) HAADF-STEM image of a Pt 3 Co nanoparticle after ex-situ oxygen annealing treatment, showing the oxide layers on (111) surfaces. (b) HAADF-STEM image of an initially disordered Pt 3 Co nanoparticle. (c) EEL spectra taken from the red point in (a) (spectrum #1) and the blue point in (b) (spectrum #2), respectively. (d) Reference EEL spectra 1 of metallic Co, CoO and Co 3 O 4. Firstly, the +2 valence of Co of the oxide layer can be determined by analyzing the O K-edge. As shown in spectra #1, the O K-edge shows a characteristic peak at 540 ev. The strong intensity of O, which is comparable to the intensity of Co confirms

4 the existence of oxygen in the oxide layer, although the oxygen atoms are invisible in the HAADF-STEM images. In addition, according to the reference spectra 1 (Fig. 2Sd), the O K-edge can work as a characteristic feature to determine the valence state of Co in the different cobalt oxides. As shown in Fig. S2d, a main difference of the O K-edge in CoO and Co 3 O 4 is a significant increase in the O K-edge prepeak intensity in Co 3 O 4 due to the larger number of unoccupied Co 3d states compared to CoO [1]. Clearly, no strong O K-edge prepeak was observed in spectra #1, thus ruling out the possibility of Co 3 O 4. By contrast, both the shape and peak position of the O K-edge of spectrum #1 are very similar to those of the reference CoO spectrum. Therefore, the oxide layers on the (111) surfaces of the oxidized Pt 3 Co nanoparticles can be determined as CoO. Secondly, the +2 valence of Co in the oxide layer can further be confirmed by analyzing the Co L-edges. As shown in Fig. S2c, the Co L 3 peak position of the oxide layer (spectrum #1) shows a shift to the high energy side compared to that of the Pt 3 Co (spectrum #2). This trend is in agreement with Co L 3 peak shift of CoO compared to metallic Co in reference spectra (Fig. 4d). In addition, Co L 3 /L 2 -ratio is always considered as a main feature for the determination of the Co valence. Although the value of Co L 3 /L 2 ratio may deviate in different instruments due to the acquisition conditions (e.g. camera length), the relative L 3 /L 2 ratio of cobalt oxides should still follow the relationship, based on the published data 1-3, as followings: ( L / L ) > ( L / L ) > ( L / L ) 3 2 CoO 3 2 Co 3 2 Co3O4 In our results, the Co L 3 /L 2 -ratio of the oxide layer (~2.3) is definitely larger than that of the Pt 3 Co (~1.6), consistent with the relationship ( L / L ) ( / ) 2+ > L L Co 3 2 Co Therefore, the valence of cobalt in the oxide layer can be confirmed as +2.

5 3. In situ observation of the high mobility of Pt atoms/clusters in oxygen Figure S3. In-situ HAADF-STEM images showing the high mobility of Pt atoms from Pt 3 Co nanoparticles in an oxygen environment. (a) Visible Pt atoms/clusters (indicated by white arrows) were attaching on the surface of the Pt 3 Co nanoparticle. (b) After an elapsed time of 64 s, the smooth surface was formed through diffusion of Pt atoms. Inset shows the comparison of the particle profile in (a) and (b), indicating the volume of the particle was increasing due to the Pt atom diffusion.

6 4. Discussion on the electron beam effect In order to minimize the electron beam effect, a relatively small probe current 20 pa was used for imaging 4, while the probe current for normal STEM imaging is between pa 5, 6. Meanwhile, the magnification was always kept below 8 M and the acquisition time was less than 16 seconds. Considering the probe size (1.3 Å) and our imaging condition (a pixel dwell time of 15 µs/pixel and a pixel size of 0.04 Å 2 ), the electron dose we used was about 10 4 e/å 2. The current density of our in-situ experiment is estimated to be 15.1 pa/å 2, which is about 1~2 orders of magnitude lower than typical values for high resolution STEM imaging 7, 8. In addition, it should be noted that our Pt 3 Co nanoparticles were enclosed in a gas cell. The beam intensity is actually reduced by a factor of approximately 3.0 due to scattering from the Si 3 N 4 membranes (about nm thick) before it interacts with the sample, reducing the possibility of a beam effect even further 9. Figure S4. Comparison of the same Pt 3 Co nanoparticle in 500 Torr of N 2 under 350 C, before (a) and after (b) an elapsed time of 600 seconds. The imaging condition was used as same as in Figures 2 and 3 in the manuscript. Moreover, one comparative experiment was also done to confirm the Pt atom diffusion is induced by the effect of oxygen environment and not the electron beam irradiation. This time, we placed the original Pt 3 Co nanoparticles in an inert gas phase

7 (500 Torr of N 2 ) under 350 C, instead of the O 2 environment. The STEM imaging condition (e.g. beam intensity, acquisition time, etc.) was the same as we used under the oxygen environment in order to make a comparison. Here, Figure S4 shows the HAADF-STEM images of the same Pt 3 Co nanoparticle before and after an elapsed time of 600 seconds in 500 Torr of N 2 under 350 C. It is clear that no obvious surface reconstruction was found even after a longer time in the N 2 environment. The Pt atoms on {100} surface did not show a mobility as high as that in the oxygen environment. Neither the additional layer growth nor the alternation of saw-tooth steps took place on {100} surface this time (see marked {200} planes #1~#30). In other words, the surface configuration of Pt 3 Co nanoparticle was relatively stable in 500 Torr of N 2 under 350 C, with the imaging condition we used. This evidence demonstrates that the observed surface reconstruction in Figures 2 and 3 in the manuscript is truly the result of the oxygen environment, not the electron beam irradiation. 5. Details about first principle calculations Calculations based on density functional theory (DFT) were carried out with the Vienna Ab-initio Simulation Package (VASP) along with the projector augmented wave (PAW) method The exchange and correlation interactions among electrons were treated at the level of the spin-polarized generalized gradient approximation (GGA), using the Perdew-Burke-Ernzerhof (PBE) functional 16. An energy cutoff of 600 ev was used for the plane-wave basis expansion. In order to mimic the possible surfaces for the disordered Pt 3 Co nanoparticle, we constructed every possible surface model within a reasonable unit cell, as shown in Fig. S3, and added the vacuum gap of about 15.0 Å thick between the periodic slabs. During the geometry optimization, the middle three layers of all slabs were fixed, while all other layers were completely relaxed. A Monkhorst-Pack k-point mesh17 was used to evaluate integrals in the reciprocal space. The criteria for structural optimization are (1) the atomic force

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