CVD-Grown Graphene Modified with Aryl Groups by Electroreduction of Corresponding Diazonium Salts

Graphene has been widely studied material because of its interesting properties (for example large surface area, high conductivity, good mechanical, electronic, optical, thermal and chemical properties). It is well-known that high-quality graphene can be prepared by mechanical exfoliation of highly oriented pyrolytic graphite, although this method is not suitable for large scale applications. According to the literature, chemical vapor deposition (CVD) is one of the most promising approach in producing high quality and large-scale graphene. Graphene has been widely grown by CVD on nickel or copper substrate. Also, it is well known that graphene synthesis by CVD yields a multi-layer graphene on the Ni surface.¹ Based on the recent report,² we can talk about multi-layer graphene if this material consist small number (between two and about 10) of graphene layers.

Graphene is also a good model substrate to be modified because of its interesting properties. A very attractive method for the modification of electrode surfaces is the electrochemical reduction of aryldiazonium salts. This method is based on the formation of an aryl radical which reacts with the electrode surface giving a strong bond between substrate and organic layer.³ Numerous studies have been reported about spontaneous functionalization of various diazonium salts to modify graphene-based substrates (including CVD-grown graphene). However, according to our knowledge, the electrochemical grafting of CVD-grown graphene electrodes by diazonium reduction is relatively rare. Therefore, the aim of this study was to investigate the electrochemical grafting of graphene-based electrodes via electrochemical reduction of the corresponding diazonium salts. The synthesis of graphene was performed by CVD onto Ni foil and the functionalization of CVD-grown graphene with aryl groups was carried out in a specially designed three-electrode cell set-up using cyclic voltammetry (CV). Furthermore, the CVD-grown graphene on Ni foil electrografted with aryl groups was characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, atomic force microscopy (AFM) and CV.

First of all, the quality of CVD-grown graphene on Ni foil was evaluated by using different surface analytical methods. The high-resolution scanning electron microscopy, Raman spectroscopy and AFM results revealed almost defect-free multilayer graphene formation on Ni foil.⁴ Next, the samples of CVD-grown graphene on Ni foil were electrografted either with azobenzene (AB), 9,10-anthraquinone (AQ), 2–methyl–4–([2–methylphenyl]azo)benzene (GBC), 2,5–dimethoxy–4–([4–nitrophenyl]azo)benzene (FBK),  4-nitrophenyl (NP) and 4-bromophenyl (PhBr) groups using pre–synthesized azobenzene diazonium salt, 9,10-anthraquinone-1-diazonium salt and commercially available Fast Garnet GBC sulfate salt, Fast Black K salt, 4-nitrobenzenediazonium and 4-bromobenzenediazonium salt, respectively (Scheme 1).

The electrochemical grafting of the CVD-grown graphene electrodes by diazonium reduction was confirmed by a wide range of techniques. For example, the XPS results confirmed the presence of corresponding functional groups on CVD graphene electrodes. The AFM images of aryl-modified CVD graphene electrodes showed clearly that the surfaces of all electrodes were covered with granular layer. Furthermore, while NP and AQ groups are electrochemically active, the CV behavior of the NP- and AQ-modified  CVD graphene electrodes was studied in Ar-saturated 0.1 M KOH and a quasi-reversible response was observed. These results are significant because the aryl-modified CVD graphene electrodes can be used for various applications, including electrocatalysis and electroanalysis. 

References
1. Y. Zhang, L. Zhang, and C. Zhou, Acc. Chem. Res., 46, 2329, 2013.
2. A. Bianco, H.-M. Cheng, T. Enoki, Y. Gogotsi, R.H. Hurt, N. Koratkar, T. Kyotani, M. Monthioux, C.R. Park, J.M.D. Tascon, and J. Zhang, Carbon, 65, 1, 2013.
3. M. Delamar, R. Hitmi, J. Pinson, and J.-M. Savéant, J. Am. Chem. Soc., 114, 5883, 1992.
4. E. Kibena, M. Mooste, J. Kozlova, M. Marandi, V. Sammelselg, and K. Tammeveski, Electrochem. Commun., 26, 35, 2013.