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. 2018 Aug 3;9:3056. doi: 10.1038/s41467-018-05352-9

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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 — In situ SERS study of Ag NP-photocatalyzed C₂H₄ epoxidation. a Schematic of experimental setup, where a home-made reaction flow cell is mounted in an optical microscope for in situ SERS study. The SERS monitoring was carried out under the flow of C₂H₄ in air, which serves as the source of oxygen. b Pie chart of activity across 220 Ag NP emitters, showing three classes: non-responsive NP emitters (~60%), those exhibiting graphenic carbon formation (23%), and those showing graphenic carbon formation along with C₂H₄ oxidation species (17%). A time series of SERS spectra, under photocatalytic reaction conditions, is shown by a waterfall plot for representative cases of c a non-responsive Ag NP emitter, d a Ag NP emitter that shows the formation of only graphenic carbon, and e a Ag NP emitter showing formation of both graphenic carbon and reactive species related to C₂H₄ oxidation species. Panels c–e share the same color scale and y-axis title. SERS spectra were acquired continuously at a rate of 0.2 s per frame and are shown normalized. The instant when C₂H₄ flow was introduced (and sample refocused) is labeled as on. A representative unnormalized SERS spectrum (t = 160 s slice, t = 64 s slice, t = 240.2 s slice from water-fall plots in c, d, e, respectively) demonstrates f a non-responsive Ag NP emitter, g presence of graphenic carbon, indicated by SERS peaks associated with D, G, 2D, 2G, and D + G vibrational modes, and h presence of both graphenic carbon and reaction species, respectively. Each key vibrational mode is labeled with the peak wavenumber indicated in cm⁻¹. Panels f–h and j share the same y-axis title. i Representative ×60 CCD images of a single laser-excited Ag NP emitter within the spectrograph slit (top, scale bar = 20 μm) and white light-illuminated wide-field region of a Ag NP monolayer (middle, scale bar = 20 μm), and a ×100 color camera dark-field scattering image of the Ag NP monolayer (bottom, scale bar = 10 μm). j Comparison of SERS spectra (single time-slice) obtained from a single Ag NP emitter (two representative cases) with those from a Ag NP monolayer (two representative cases), demonstrating the necessity for single-emitter-level spectroscopy. Spectra are shown stacked