Zhang, Guohui (2016) Electrochemistry and applications of sp2 carbon materials : from graphite to graphene. PhD thesis, University of Warwick.

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Abstract

This thesis can be divided into three themes: (i) the electrochemistry of sp2 carbon materials, with a focus on graphite and graphene, where electron transfer (ET) kinetics and surface functionalisation were considered; (ii) methodology development for graphene transfer, to facilitate the fabrication of versatile tools for microscopy research and allow the properties of supported and suspended graphene to be readily assessed and compared; (iii) the electrowetting of graphite, providing a new mechanism for droplet actuation on a conducting surface with an applied electric field.

There is a large body of literature that the basal plane of highly oriented pyrolytic graphite (HOPG) is inert or has little electroactivity for outer-sphere redox couples and adsorbed species. Here, the model is revisited with the macroscopic ET kinetics studies of three classical (outer-sphere) redox couples on different grades of HOPG using a droplet-cell setup. It is shown that the ET kinetics for all of the redox species studied is fast on all grades of HOPG (comparable to metal electrodes), despite the low density of electronic states (DOS) on graphite. This is in line with the results where the ‘special’ redox couple, Fe3+/2+, associated with a slow kinetics, is tested. Moreover, localised surface mapping measurements of HOPG using scanning electrochemical cell microscopy (SECCM), reveal a relatively uniform activity on basal plane and step edges of HOPG towards Fe3+/2+, highlighting that the basal plane is electroactive and the major site for the ET kinetics of Fe3+/2+.

The next goal is to elucidate whether adsorbed electroactive anthraquinone-2,6-disulfonate (AQDS) can be used as a marker of step edges, previously regarded as the main electroactive sites of graphite. Step edges are shown to have little effect on the extent of adsorbed electroactive AQDS in macroscopic studies. The amount of adsorbed electroactive AQDS and the ET kinetics are independent of the step edge coverage, as determined by fast scan cyclic voltammetry-SECCM. Further, SECCM reactive patterning shows essentially uniform and high activity across the basal surface of HOPG, indicative of the dominance of basal plane in HOPG electroactivity.

Regarding the close relation between graphene and graphite, effort is put to introduce a polymer-free method for transferring chemical vapour deposition (CVD)-grown graphene, to arbitrary substrates, using an organic/aqueous biphasic configuration. Avoiding any polymeric contamination, graphene is coated on arbitrary substrates, such as atomic force microscopy (AFM) tips and transmission electron microscopy (TEM) grids, generating tools for conductive AFM and high resolution TEM imaging. Furthermore, electrochemical and wetting measurements at either a freestanding graphene film or a copper-supported graphene area, are readily made and compared.

As an example of the myriad potential applications of graphite, electrowetting is demonstrated at HOPG, using cyclic voltammetry, with significant changes in contact angle and relative contact diameter seen. These are comparable to the widely studied electrowetting-on-dielectric (EWOD) system, but over a much lower voltage range. Electrowetting is found to be due to the intercalation/de-intercalation of anions between the graphene layers of graphite, driven by the applied potential, providing a new mechanism for electrowetting and diversifying the means by which electrowetting can be controlled and applied.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Electrochemistry, Carbon, Graphite composites, Graphene -- Electric properties
Official Date:	September 2016
Dates:	Date Event September 2016 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Unwin, Patrick R.
Format of File:	pdf
Extent:	xxxv, 198 leaves : illustrations, charts
Language:	eng

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