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Raman Fingerprints of Graphene Produced by Anodic Electrochemical Exfoliation The University of Manchester Research Raman Fingerprints of Graphene Produced by Anodic Electrochemical Exfoliation DOI: 10.1021/acs.nanolett.0c00332 Document Version Accepted author manuscript Link to publication record in Manchester Research Explorer Citation for published version (APA): Nagyte, V., Kelly, D. J., Felten, A., Picardi, G., Shin, Y., Alieva, A., Worsley, R. E., Parvez, K., Dehm, S., Krupke, R., Haigh, S. J., Oikonomou, A., Pollard, A. J., & Casiraghi, C. (2020). Raman Fingerprints of Graphene Produced by Anodic Electrochemical Exfoliation. Nano Letters. https://doi.org/10.1021/acs.nanolett.0c00332 Published in: Nano Letters Citing this paper Please note that where the full-text provided on Manchester Research Explorer is the Author Accepted Manuscript or Proof version this may differ from the final Published version. If citing, it is advised that you check and use the publisher's definitive version. General rights Copyright and moral rights for the publications made accessible in the Research Explorer are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Takedown policy If you believe that this document breaches copyright please refer to the University of Manchester’s Takedown Procedures [http://man.ac.uk/04Y6Bo] or contact [email protected] providing relevant details, so we can investigate your claim. Download date:08. Oct. 2021 Raman Fingerprints of Graphene Produced by Anodic Electrochemical Exfoliation Vaiva Nagyte1, Daniel J. Kelly2, Alexandre Felten3, Gennaro Picardi1, YuYoung Shin1, Adriana Alieva1, Robyn E. Worsley1, Khaled Parvez1, Simone Dehm4, Ralph Krupke4,5, Sarah J. Haigh2, Antonios Oikonomou6,7, Andrew J. Pollard8, Cinzia Casiraghi*1 1 Department of Chemistry, University of Manchester, Manchester M13 9PL, UK. 2 Department of Materials, University of Manchester, Manchester M13 9PL, UK. 3 Synthesis, Irradiation and Analysis of Materials (SIAM), University of Namur, Belgium 4 Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany 5 Department of Materials and Earth Sciences, Technische Universität Darmstadt, Darmstadt 64287, Germany 6 National Graphene Institute, University of Manchester, Manchester M13 9PL, UK. 7 The Institute of Photonic Sciences, Castelldefels 08860, Spain 8 National Physical Laboratory, Teddington, Middlesex, TW11 0LW, UK *E-mail:[email protected] Electrochemical exfoliation is one of the most promising methods for scalable production of graphene. However, limited understanding of its Raman spectrum as well as lack of measurement standards for graphene strongly limit its industrial applications. In this work we show a systematic study of the Raman spectrum of electrochemically exfoliated gra- phene, produced using different electrolytes and different types of solvents in varying amounts. We demonstrate that no information on the thickness can be extracted from the shape of the 2D peak as this type of graphene is defective. Furthermore, the number of defects and the uniformity of the samples strongly depend on the experimental conditions, including post-processing. Under specific conditions, formation of short conductive trans-polyacetylene chains has been observed. Our Raman analysis provides guidance for the community on how to get information on defects coming from electrolyte, temperature and other experimental conditions, by making Raman spectroscopy a powerful metrology tool. KEYWORDS: graphene, Raman spectroscopy, electrochemical exfoliation, X-ray photoelectron spec- troscopy, Atomic force microscopy, Transmission electron microscopy. 1 Graphene, the first discovered 2-dimensional (2d) material, is attracting strong research interest because of its outstanding properties, which can be exploited in a wide range of applications.1 One of the most used techniques for large scale production of graphene is based on solvent assisted exfoliation,2–5 which allows preparation of 2d crystal dispersions. In particular, the most common techniques are liquid-phase exfoliation (LPE)6 and electrochemical exfoliation (ECE).7 Although both LPE and ECE offer a simple, low cost and scalable way to produce graphene, commercialization of graphene-based products is still limited due to the lack of metrology standards.8–13 In order to overcome this problem, the community has introduced some guidelines to establish universal practices to allow a better classification of solu- tion-processed 2d materials, based on specific parameters, such as thickness, size and surface chemis- try.10,11 However, characterization of such materials is currently very challenging because dispersions typically contain nanosheets with a wide distribution in size and thickness, in contrast to traditional col- loidal dispersions. Moreover, such distributions strongly depend on the exfoliation method and parame- ters used.9 Raman spectroscopy is a simple, low cost and widely used technique for the characterization of gra- phene.14–16 The Raman spectrum of graphene is composed of two main features, the G and the 2D peaks, which lie at ~1580 cm-1 and ~2700 cm-1, respectively.17 The full width at half maximum of the 2D peak (FWHM(2D)) of 25 - 30 cm-1 is typically used to identify a single-layer of graphene.17 Howev- er, graphene produced by different methods can show very different Raman spectra: in the case of solu- tion-processed graphene, the Raman analysis has to take into account other factors that can alter the Raman spectrum, such as the interaction with the solvent, the reduced flake size (e.g. effect of edges), defect formation, and the random re-stacking (i.e. loss of AB stacking of the starting graphite) of the flakes. These effects give rise to strong changes in the Raman spectrum, as compared to that measured on graphene produced by micro-mechanical exfoliation (MME).17,18 For example, graphene produced 2 by LPE typically has FWHM(2D) above 50 cm-1 6,19–23 in contrast to ~25 cm-1 for MME graphene17. Furthermore, the D peak at ~1350 cm-1, is also observed in graphene produced by LPE.6,19–23 In our group we have developed a simple protocol for qualitatively assessing the thickness distribution of graphene dispersions produced by LPE.20–23 Nevertheless, the Raman characterization of ECE gra- phene has been rather limited: transmission electron microscopy (TEM) and atomic force microscopy (AFM) are typically used to analyze the graphene flake’s morphology,24 while the chemical composition and crystal structure are typically evaluated by X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD).25,26 It was observed that there is a correlation between XPS analysis on oxy- gen-containing groups, layer spacing from XRD data and the D and G peak intensity ratio (I(D)/I(G)).27,28 Thus, I(D)/I(G) has been proposed as a parameter to define the quality of ECE graphene and optimal ECE process conditions.29 However, it is known that this ratio does not depend linearly on defect density,30–32 and it changes with the excitation energy.31 Thus, inaccurate conclusions could be derived by analyzing I(D)/I(G) only or by comparing I(D)/I(G) measured in different works using dif- ferent laser excitations. Indeed, it was already observed that graphene samples having similar C/O ratios can have rather different I(D)/I(G) values in some cases.33 Furthermore, XPS and XRD are bulk charac- terization techniques, while Raman spectroscopy can easily probe individual graphene sheets as the typ- ical flake size is well above the laser spot size (~500 nm). Most of the research on characterization of ECE graphene34–37 reports Raman spectra of a few selected flakes, which may not be representative of the dispersion – a statistical analysis is needed.9 Moreover, the Raman spectrum of an ECE graphene flake is compared with that of graphite to show successful exfoliation of graphene,38,39 but no further analysis of the Raman features is performed. Only Wang et al.40 presented Raman spectroscopy results combined with optical contrast analysis to evaluate the statistical distribution of the graphene sheet thickness. However, the optical contrast may vary depending on the degree of functionalization of gra- phene and intercalation stage. 3 In this work we show a systematic Raman spectroscopy analysis of graphene produced by ECE under different conditions. We show that the Raman analysis developed for LPE graphene cannot be applied to graphene obtained by ECE. In particular, the thickness of graphene produced by anodic ECE cannot be extracted by Raman spectroscopy. We propose a detailed approach to characterize ECE graphene, which allows qualitative determination of the level of defects in ECE flakes and the uniformity of the material. Furthermore, we show that the properties of such graphene are strongly sensitive to post- processing, further highlighting the importance of characterizing the material during every step of pro- duction, which is the key to enable batch to batch reproducibility and industrial uptake. RESULTS AND DISCUSSION To evaluate the influence of the electrolyte and the solvent on ECE graphene properties, 12 samples were prepared under different conditions (Table 1 and Figure S2.1). Note that each sample was repeated at least twice to ensure reproducible results (Section S7.4). Graphite electrodes were first electrochemi- cally exfoliated (See Methods in Section S1) using different electrolytes:
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