N-Doped Graphene Oxide Nanoparticles Studied by EPR
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Applied Magnetic Resonance
ORIGINAL PAPER
N‑Doped Graphene Oxide Nanoparticles Studied by EPR Francesco Tampieri1,3 · Matteo Tommasini2 · Stefano Agnoli1 · Marco Favaro1,4 · Antonio Barbon1 Received: 21 June 2020 / Revised: 9 September 2020 / Accepted: 25 September 2020 © The Author(s) 2020
Abstract Graphene-derived materials attract a great deal of attention because of the peculiar properties that make them suitable for a wide range of applications. Among such materials, nano-sized systems show very interesting behaviour and high reactivity. Often such materials have unpaired electrons that make them suitable for electron paramagnetic resonance (EPR) spectroscopy. In this work we study by continuous wave and pulse EPR spectroscopy undoped and nitrogen-doped graphene quantum dots (GQD) with a size of about 2 nm. The analysis of the spectra allows identifying different types of paramagnetic centers related to electrons localized on large graphenic flakes and molecular-like radicals. By hyperfine spectroscopies on nitrogen-doped samples, we determine the hyperfine coupling constant of paramagnetic centers (limited-size π-delocalized unpaired electrons) with dopant nitrogen atoms. The comparison of the experimental data with models obtained by density functional theory (DFT) calculations supports the interpretation of doping as due to the insertion of nitrogen atoms in the graphene lattice. The dimension of the delocalized regions in the flakes observed by pulse EPR is of about 20–25 carbon atoms; the nitrogen dopant can be classified as pyridinic or graphitic.
* Antonio Barbon [email protected] 1
Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Via Marzolo 1, 35131 Padova, Italy
2
Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
3
Present Address: Biomaterials, Biomechanics and Tissue Engineering Group, Department of Materials Science and Engineering, Technical University of Catalonia (UPC), c. Eduard Maristany 16, 08019 Barcelona, Spain
4
Present Address: Helmholtz‑Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Hahn‑Meitner‑Platz 1, 14109 Berlin, Germany
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F. Tampieri et al.
1 Introduction Graphene [1] is a flat and regular structure that is arousing technological interest for its peculiar properties [2, 3]. Graphene-derived materials include a large variety of materials, with quite different characteristics that is possible to tune by tuning the synthesis parameters and the presence of heteroatoms in the atomic lattice. Beside the large interest on the material itself, we note that only few techniques are available for the characterization of graphene-like materials [4]; in this respect, electron paramagnetic resonance (EPR) spectroscopy is an appropriate method to study them because such materials exhibit paramagnetic centers [5, 6]. Paradoxically, most of the interest in graphene-like materials derives from the possibility of pro
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