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. 2018 Dec 21;9:5422. doi: 10.1038/s41467-018-07850-2

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — Synthesis and structural characterizations. a Illustration of preparation process of single iron atomic sites supported on a nitrogen, phosphorus and sulfur co-doped hollow carbon polyhedron (Fe-SAs/NPS-HC). b Transmission electron microscopy (TEM) image of Fe-SAs/NPS-HC predecessor, marked as zeolitic imidazolate framework-8 (ZIF-8)/Fe@poly(cyclotriphospazene-co-4,4′-sulfonyldiphenol) (PZS). Scale bar, 2 μm. Inset: high-resolution TEM (HRTEM) image of ZIF-8/Fe@PZS, indicating that the thickness of the PZS shell is approximately 20 nm; scale bar, 100 nm. c TEM image of Fe-SAs/NPS-HC. Scale bar, 500 nm. d High angle annular dark field scanning TEM (HAADF-STEM) image of Fe-SAs/NPS-HC. Scale bar, 200 nm. e The enlarged HAADF-STEM image and corresponding element maps (Fe: yellow, C: blue, N: cyan, S: orange, P: green). Scale bar, 100 nm. f, g Aberration-corrected (AC) HAADF-STEM image and enlarged image of Fe-SAs/NPS-HC catalyst. Scale bar, 5 nm (f); 1 nm (g)