The Characterization of CVD-Grown Graphene Modified with Nitrophenyl Groups Using the Diazonium Reduction Method
Authors: Kibena Elo , Mooste Marek , Kozlova Jekaterina , Marandi Margus , Matisen Leonard , Niilisk Ahti , Sammelselg Väino , Tammeveski Kaido 
Published: February 2020

Graphene, the two-dimensional carbon allotrope, has been attracted much interest since its discovery by Novoselov, Geim and co-workers.1 One of the most promising route for synthesizing a large-area graphene is by chemical vapor deposition (CVD) on catalytic substrate (for example nickel).2 Often the multilayer graphene is formed on the Ni surface by CVD and recently, the morphological properties and electrochemical behavior of multilayer graphene was studied in our research group.3

The modification of carbon substrates via electrochemical reduction of diazonium salts has been an important research area.4 The electrografting has been widely performed in the narrow potential range where the reduction peak of aryldiazonium compound is observed, however the results have shown that the formed aryl layers are rather thin.4 Lately, a special grafting procedure based on diazonium chemistry was used in order to form thick aryl layers on different substrates.5 Recently, anthraquinone layers with high thickness were electrografted on graphene-based substrates using this special modification procedure.6 Considering the fact that the thickness of aryl layer depends on the modification procedure,5,6the main aim of this work was to form thin and thick 4-nitrophenyl (NP) layers on CVD-grown graphene on Ni foil (designated as Ni/Gra) using 4-nitrobenzenediazonium tetrafluoroborate (NBD) and to compare the morphological and electrochemical properties of differently modified Ni/Gra/NP electrodes. To our knowledge, the formation of thick NP films on Ni/Gra electrodes was applied for the first time.
For this work, the Ni/Gra electrodes were prepared in the Institute of Physics of the University of Tartu and further characterization of Ni/Gra samples by various surface analytical techniques confirmed the presence of good quality of multilayer graphene on Ni foil.3,7

To form thin and thick NP layers on Ni/Gra electrodes, three modification procedures were applied: (1) one potential cycle in the narrow potential range (0.6 to -0.5 V vs. SCE at a scan rate (ν) of 100 mV s–1), (2) 10 potential cycles in the narrow potential range (0.6 to -0.5 V vs. SCE, ν = 100 mV s–1) followed by holding the electrode at -0.2 V for 10 min and (3) 10 potential cycles in wider potential range (0.6 to -1.4 V vs. SCE, ν = 1 V s–1). All the modifications were carried out in acetonitrile containing 1 mM of NBD and 0.1 M tetrabutylammonium tetrafluoroborate as a base electrolyte. The characterization of NP-modified Ni/Gra electrodes was performed by cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy.
The experimental results revealed that the electrochemical behavior during electrografting was different when narrow or wider potential range was used.7 The XPS analysis confirmed the presence of NP layers on Ni/Gra surfaces and the Raman spectra revealed a small D peak after electrografting of Ni/Gra samples.7 The AFM images showed that the typical pattern of Ni/Gra was clearly visible in case of using narrow potential range during electrografting. When wider potential range was applied, the Ni/Gra was fully covered with granular structure and the characteristic folds and wrinkles of the underlying material were rarely visible.7 Additionally, the NP layer thickness was determined by AFM scratching method: 5, 20 and 30 nm for the Ni/Gra electrodes modified by procedures (1), (2) and (3).7These results indicate that to obtain thicker NP film on Ni/Gra electrodes, the potential cycling in a wider potential range is more preferable than using narrow potential range.
The importance of aryl-modified graphene is generally recognized and we do believe that the results obtained in the present work give further insight into their properties.
References
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4. J. Pinson, In Aryl Diazonium Salts: New Coupling Agents in Polymer and Surface Science; Chehimi, M. M., Ed.; Wiley: Weinheim, Germany, 2012; Chapter 1, pp 1-35.
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7. M. Mooste, E. Kibena, J. Kozlova, M. Marandi, L. Matisen, A. Niilisk, V. Sammelselg, and K. Tammeveski, 2014 (submitted).