Cuharuc, Anatolii S. (2015) Electron transfer on graphene and graphite : theoretical and experimental study. PhD thesis, University of Warwick.

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Abstract

The redox behaviour of several couples on highly oriented pyrolytic graphite (HOPG) and graphene has been studied using both micro- and macroscopic measurements supported by detailed analysis. The ET kinetics of Ru(NH3)6 3+/2+ , (ferrocenylmethyl)trimethylammonium (FcTMA2+/+ ), and ferrocenylcarboxylic acid (FcCOO-/0 ) was found fast, on the time-scale of voltammetric measurements, on a freshly cleaved HOPG surface, but on “aged” one, Ru(NH3)6 3+/2+ exhibited sluggish ET, showing quite unusual macroscopic cyclic voltammograms. The other two couples retained their fast response on an “aged” surface. The surface of “aged” HOPG is proposed to consist of graphene layers of different thicknesses, which, from graphite to monolayer graphene, have progressively diminishing capacity to support fast ET specifically for Ru(NH3)6 3+/2+ . Such a redox-selectivity correlates with the position of formal potentials of the three redox couples relative to the band structure of graphene, with Ru(NH3)6 3+/2+ being most close to the minimum in density of states (DOS) of undoped monolayer graphene.

Based on macroscopic voltammetry, low grade HOPG, whose surface is abundant with step edges, was found not to be redox-selective in the sense described above, meaning that its “aged” surface was as good as fresh one. It is highly likely that step edges being a type of crystal lattice defects, retain their capacity for fast ET, which correlates with their elevated DOS, and, thus, secure fast voltammetric response of low grade HOPG in macroscopic experiments on these complex “aged” surfaces.

IrCl6 2-/3- , Fe(CN)6 3-/4- and the three couples discussed above were all found to have fast electrochemistry on freshly cleaved surfaces of high grade HOPG. Estimated heterogeneous rate constants were > 0.1 cm s- 1 for Ru(NH3)6 3+/2+ and > 1.7 cm s- 1 for IrCl6 2-/3-and Fe(CN)6 3-/4 . This suggests that basal planes of graphite have, though low, but sufficient DOS to perform ET at a rate comparable with that on some metals. Furthermore, these results unarguably defy those opinions, that have been long circulating in the published literature, stating that basal plane of HOPG (and also sidewalls of carbon nanotubes) are nearly inert towards ET and that the defects were solely responsible for the observed electrochemical activity of this material.

The study of adsorption of three ferrocene derivatives on HOPG revealed FcTMA+ and FcCOO- adsorb weakly and approximately equally whereas zero-charged FcCH2OH adsorbs notably more strongly. The adsorption was studied with cyclic voltammetry and quantitative information was extracted from the experimental data basing on a simple dedicated theory developed in this thesis. The “aged” surface of high grade HOPG exhibited enhanced adsorption as compared to fresh one, but low grade HOPG did not show the difference. It is suggested that the highly flat surface of high grade HOPG (large terraces widths) facilitates formation of airborne contaminating film made of hydrocarbons on, which serves as a “trap” for ferrocene derivatives owing to their largely non-polar ferrocene moieties.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Graphite, Graphene, Electrochemistry, Nanochemistry, Nanotechnology
Official Date:	November 2015
Dates:	Date Event November 2015 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Unwin, Patrick R.
Extent:	xxxiii, 191 leaves : illustrations
Language:	eng

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