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Electrochemical and Spectroscopic Studies of Graphene ... · PDF file Electrochemical and Spectroscopic Studies of Graphene Nanoflakes with Functionalised Edges Mailis Maria Lounasvuori

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  • Electrochemical and Spectroscopic Studies of

    Graphene Nanoflakes with Functionalised Edges

    Mailis Maria Lounasvuori

    Thesis submitted in partial fulfilment of the requirements for the degree of

    Doctor of Philosophy

    UNIVERSITY COLLEGE LONDON

    February 2017

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    Declaration

    I, Mailis Maria Lounasvuori, confirm that the work presented in this thesis is my own.

    Where information has been derived from other sources, I confirm that this has been

    indicated in the thesis.

    Signature: ………………………………………………………………….

    Date: ……………………….

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    Abstract

    The influence of surface functional groups on the electrochemical performance of

    carbon electrodes was studied by using graphene nanoflakes (GNF), a well-defined

    carbon nanomaterial. After characterisation with different techniques, GNF were used

    to modify a boron-doped diamond (BDD) electrode and the influence of different edge

    terminations on various redox probes was investigated using cyclic voltammetry (CV).

    The outer-sphere redox probe ferrocenemethanol (FcMeOH) was found to be

    unaffected by the presence of GNF at the electrode surface, confirming that GNF do

    not inhibit electron transfer. When proton-coupled electron transfer was investigated, it

    was shown that the acid-terminated GNF acted as a non-solution proton source and

    sink.

    The [Fe(CN6)] 3−/4− redox couple was found to be quasi-reversible and independent of

    electrolyte pH at clean BDD and BDD modified with amide-terminated GNF. When

    GNF were decorated with COOH functionalities, the reaction became less reversible

    and pH-dependent. The reaction was also directly influenced by the electrolyte

    concentration, with low concentrations causing the reaction to become more

    irreversible.

    Potential-induced dissociation of the carboxylic acid edge groups on GNF was

    investigated with in situ spectroelectrochemistry combining potentiostatic control with

    attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR).

    Applying a negative electrode potential led to the deprotonation of both electrode-

    immobilised groups and species in solution. Acid dissociation was driven by an

    increase in interfacial cation activity at the electrode surface that lowered the apparent

    pKa of all species at or near the electrode.

    Different methods of GNF attachment on the electrode surface were explored,

    including direct attachment to gold via thiol edge groups and EDC-mediated amidation

    reaction to form covalent bonds with a self-assembled monolayer (SAM) on gold.

    Scanning tunnelling microscopy (STM) was used to verify the presence and probe the

    orientation of GNF at the surface.

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    Contents

    Declaration ................................................................................................................... 2

    Abstract ....................................................................................................................... 3

    Contents ....................................................................................................................... 4

    List of Symbols ............................................................................................................. 9

    List of Abbreviations ................................................................................................... 11

    List of Figures ............................................................................................................. 13

    List of Schemes .......................................................................................................... 19

    List of Tables .............................................................................................................. 20

    List of Appendix Figures ............................................................................................. 22

    List of Publications...................................................................................................... 23

    1 Introduction.......................................................................................................... 24

    1.1 Graphene ..................................................................................................... 24

    1.1.1 Graphene synthesis ............................................................................... 28

    1.1.2 Graphene Oxide..................................................................................... 30

    1.1.3 Graphene Functionalisation ................................................................... 32

    1.2 Electrochemistry of Graphene ...................................................................... 33

    1.3 Graphene Nanoflakes................................................................................... 36

    1.4 Acid-base Properties of Graphene-related Materials..................................... 40

    1.4.1 Controlling the Protonation State of Electrode-Immobilised Species ...... 41

    1.5 Methods of immobilising GNF on Electrode .................................................. 43

    1.5.1 Drop-coating .......................................................................................... 43

  • Contents

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    1.5.2 Spin-coating........................................................................................... 43

    1.5.3 Self-assembly ........................................................................................ 44

    1.6 Aim and Scope of the Thesis ........................................................................ 45

    References for Chapter 1 ........................................................................................ 47

    2 Experimental Theory and Techniques.................................................................. 56

    2.1 Electrochemistry ........................................................................................... 56

    2.1.1 Cyclic Voltammetry ................................................................................ 59

    2.1.2 Differential Pulse Voltammetry ............................................................... 61

    2.2 Infrared Spectroscopy................................................................................... 64

    2.2.1 Attenuated Total Reflectance................................................................. 66

    2.3 Scanning Tunnelling Microscopy .................................................................. 68

    2.4 X-ray Photoelectron Spectroscopy................................................................ 70

    2.5 Transmission Electron Microscopy ............................................................... 71

    References for Chapter 2 ........................................................................................ 73

    3 Characterisation of GNF ...................................................................................... 74

    3.1 Introduction................................................................................................... 74

    3.2 Experimental Methods .................................................................................. 75

    3.2.1 Preparation of Complexed GNF............................................................. 76

    3.2.2 X-ray Photoelectron Spectroscopy......................................................... 76

    3.2.3 Transmission Electron Microscopy......................................................... 76

    3.2.4 pH Titration ............................................................................................ 77

    3.2.5 Electrochemical Experiments ................................................................. 77

    3.2.6 ATR-FTIR .............................................................................................. 79

    3.2.6.1 Stability of Aqueous Suspension of GNF ........................................ 79

    3.2.6.2 Solution-Phase Characterisation of GNF ........................................ 79

    3.3 Results and Discussion ................................................................................ 80

    3.3.1 Transmission Electron Microscopy......................................................... 80

    3.3.2 X-ray Photoelectron Spectroscopy......................................................... 81

    3.3.3 Infrared Spectroscopy............................................................................ 83

  • Contents

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    3.3.3.1 Stability........................................................................................... 86

    3.3.4 pH Titration ............................................................................................ 86

    3.3.5 In Situ pH Studies Monitored with Infrared Spectroscopy....................... 89

    3.3.6 Electrochemistry of GNF without Redox Probes..................................... 91

    3.3.7 Electrochemistry of FcMeOH at GNF-Modified Electrode....................... 95

    3.3.8 Electrochemistry of Hydroquinone/Benzoquinone at GNF-Modified Electrode.............................................................................................. 100

    3.3.8.1 pH-Dependence of the Q/H2Q Reaction ....................................... 102

    3.3.8.2 Exploring the Mechanism for Hydr

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