Supporting Information. Observing Solid-state Formation of Oriented Porous. Functional Oxide Nanowire Heterostructures by in situ

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1 Supporting Information Observing Solid-state Formation of Oriented Porous Functional Oxide Nanowire Heterostructures by in situ TEM Jo-Hsuan Ho,+, Yi-Hsin Ting,,+, Jui-Yuan Chen,+, Chun-Wei Huang, Tsung-Chun Tsai, Ting-Yi Lin, Chih-Yang Huang, and Wen-Wei Wu *,,, Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC. Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu31040, Taiwan, ROC Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan. Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan. + These authors contributed equally to this work 1

2 * corresponding List of contents. Table S1 Plane distance comparison of the critical planes of ZnO and Fe 3 O 4 Table S2 Physical properties of ZnO and Fe 3 O 4 Table S3 Ionic radius of Zn +, O 2-, Fe 2+, Fe 3+ Figure S1 Schematic illustrations of the preparation for in situ observation sample. Figure S2 Schematic illustrations of the fabrication process of ZnO/Fe 3 O 4 heterostructured nanowires. Figure S3 TEM and SEM images of ZnO nanowires and Fe metal pads before and after annealing. Figure S4 The EDS mapping of void structures with plate-like and zigzag-like showed the element distribution of Fe, Zn, O and Au after cation exchange reaction. Figure S5 EELS analysis of Fe 3 O 4 region in Fe 3 O 4 /ZnO nanowire heterostructure. Figure S6 In situ TEM images of solid-state diffusion reaction in Fe 3 O 4 /ZnO hetero-nanowire with plate-like voids. Figure S7 Schematic illustration and detailed calculation for the volume and packing density of ZnO and Fe 3 O 4, respectively. Figure S8 TEM images of the nanowire. Figure S9 Ex situ and in situ TEM images of solid-state diffusion reaction in Fe 3 O 4 /ZnO hetero-nanowire with zigzag-like hollow voids. Figure S10 Schematic illustrations of the ledge diffusion mechanism in the Fe 3 O 4 /ZnO nanowire heterostructure with zigzag-like hollow voids. Figure S11 TEM, HRTEM images and corresponding FFT of ZnO/Fe 3 O 4 nanowire heterostructure with zigzag-like hollow voids. Movie S1 In situ observation video of the void formation process in a Fe 3 O 4 /ZnO hetero-nanowire with plate-like voids at 800K. 2

3 Supplementary Tables Table S1. Plane distance comparison of the critical planes of ZnO and Fe 3 O ZnO Fe 3 O 4 d-spacing theoretical value (nm) actual value (nm) Table S2. Physical properties of ZnO and Fe 3 O 4 ZnO Fe 3 O 4 crystal structure wurtzite cubic inverse spinel space group P6 3 mc Fd-3m lattice constant (nm) a = c = 0.52 a = melting point (K) volume (Å 3 ) cell density (%) Table S3. Ionic radius of Zn +, O 2-, Fe 2+, Fe 3+ 3 Element Ionic radius (Å) Zn O Fe Fe (CN=4) / (CN=6) Reference: (1) García-Martínez, O.; Rojas, R. M.; Vila, E.; de Vidales, J. L. M., Microstructural characterization of nanocrystals of ZnO and CuO obtained from basic salts. Solid State Ionics 1993, 63-65, (2) Okamura, A.; Nakamura, S.; Tanaka, M.; Siratori, K., Mössbauer Study of the Impurity Effect of In3+ and Cr3+ in the High Temperature Phase of Fe3O4. Journal of the Physical Society of Japan 1995, 64 (9), (3) Shannon, R., Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallographica Section A 1976, 32 (5),

4 Supplementary Figures Supplementary Figure S1 Figure S1. Schematic illustrations of the preparation for in situ observation of the sample. (a) Transport the ZnO nanowires onto the specimen. (b) Coat with photoresist. (c) Use e-beam exposure system to remove the photoresist. (d) Deposit Fe metal 300-nm-thick as the reactant and Au metal 30-nm-thick for protecting Fe from oxidation. 4

5 Supplementary Figure S2 Figure S2. Schematic illustrations of the fabrication process of a ZnO/Fe 3 O 4 heterostructure nanowire. (a) A single-crystalline ZnO nanowire synthesized by a vapor-liquid-solid method. (b) Fe metal pads deposited on ZnO nanowire. (c) Iron atoms start diffusing to ZnO lattice at 800 K. (d) Zn sites were replaced by iron atoms and ZnO were transformed to Fe 3 O 4, forming a ZnO/Fe 3 O 4 heterostructured nanowire. 5

6 Supplementary Figure S3 Figure S3. TEM and SEM images of a ZnO nanowire and Fe metal pads before and after annealing. 6

7 Supplementary Figure S4 Figure S4. The EDS mapping of void structures showed the element distribution of Fe, Zn, O and Au after cation exchange reaction. The void structures with plate-like and zigzag-like revealed the same results. Gold layer did not diffuse into the nanowires; instead, it just protected the Fe pad from oxidation. 7

8 Supplementary Figure S5 Figure S5. EELS analysis of the Fe 3 O 4 region in the Fe 3 O 4 /ZnO nanowire heterostructure. (a) The colored points show the area where the analysis was performed,and the corresponding results are shown in (b). 8

9 Supplementary Figure S6 Figure S6. In situ TEM images of the solid-state diffusion reaction in a Fe3O4/ZnO hetero-nanowire with plate-like voids. 9

10 Supplementary Figure S7 Figure S7. Schematic illustration and detailed calculation for the volume and packing density of ZnO and Fe 3 O 4, respectively. (a) Wurtzite ZnO. (b) Inverse spinel Fe 3 O 4. 10

11 Supplementary Figure S8 Figure S8. TEM images of the nanowire. (a) ZnO nanowire before annealing. (b) Fe 3 O 4 /ZnO hetero-nanowire after annealing. 11

12 Supplementary Figure S9 Figure S9. Ex situ and in situ TEM images of the solid-state diffusion reaction in a Fe3O4/ZnO hetero-nanowire with zigzag-like hollow voids. Colored arrows show the diffusion path of the atomic ledges. Electronic irradiation would indeed accelerate the reaction because of the heating supplement. However, the electronic irradiation would be the minor factor in our in situ experiments. The ex situ experiments took place in the region without beam exposure because the overall specimen was annealed at 800K to induce the exchange reaction between Zn atoms and Fe atoms. (a-b) The ex situ experiments took place in the region without beam exposure because the overall specimen was annealed at 800K to induce the exchange reaction between Zn atoms and Fe atoms. (c-f) The in situ TEM images, captured from the real-time video, showed that ledge diffusion of a few atomic layers occurred at the void region. 12

13 Supplementary Figure S10 Figure S10. Schematic illustrations of the ledge diffusion mechanism in the Fe 3 O 4 /ZnO nanowire heterostructure with zigzag-like hollow voids. (a) Fe atoms diffuse through the surfaces of the nanowires. (b) Fe atoms diffuse into the ZnO lattice. (c) The cation exchange reaction occurs. (d) Layer-by-layer transformation completed. 13

14 Supplementary Figure S11 Figure S11. TEM, HRTEM images and corresponding FFT of ZnO/Fe 3 O 4 nanowire heterostructure with zigzag-like hollow voids. (a-c) TEM images of the ZnO/Fe 3 O 4 nanowire heterostructure with zigzag-like hollow voids. Green and orange dotted lines marked in (b,c) highlight the special oriented planes of the voids. (d) High-magnification TEM image of the hollow voids,showing the Fe 3 O 4 lattice and the amorphous region. (e) Corresponding FFT diffraction pattern of (d) with the zone axis [211]. Green and orange circles mark the specially oriented 11 1 and 022 planes in the zigzag-like hollow voids. 14

15 Supplementary video Supplementary Movie S1 The video shows the plate-like void formation process of thefe 3 O 4 /ZnO nanowire heterostructure at 800K and is played at 2 times the actual speed. The movie indicates that the plate-like voids form along aspecific orientation, which is the close-packed plane of Fe 3 O 4. 15