Carbon nanotubes as electron-conducting nanowires

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CHAPTER 3 RESULTS AND DISCUSSION*,1 SPECTROSCOPIC , MICROSCOPIC AND ELECTROCHEMICAL PROPERTIES OF IRON-PHTHALOCYANINE SINGLE-WALLED CARBON NANOTUBE BASED ELECTRODES

SAM formation strategies

The stepwise self-assembly strategy employed in the preparation of the main electrodes is schematically represented in the Scheme 3.1 following the established procedure. For simplicity, the gold electrodes modified with the cysteamine is represented as Au-Cys, the Au-Cys integrated with single-walled carbon nanotubes is represented as Au-Cys-SWCNT, while the Au-Cys-SWCNT integrated with the FeOHETPc as the Au-Cys-SWCNT-FeOHETPc.
Upon acid treatment, single-walled carbon nanotubes bear most of the carboxylic acid groups which are found at the end of these tubes, the edge-plane and or defect sites of the tubes. Few appearance of the functional groups at the defect sites of the walls are also possible as depicted in scheme 3.1. The fabrication of Au-Cys-SWCNT-FeTAPc followed a similar protocol. Also, the Au-FeOHETPc was prepared as reported by Ozoemena et al. [1], while the Au-FeTAPc electrode was fabricated using the method described by Somashekarappa et al. [2].

Atomic force microscopy characterization

AFM studies were conducted to give an insight to the surface morphologies of the formed SAMs and figure 3.1 exemplifies the 3-D AFM images of the Au-Cys (a), Au-Cy-SWCNT (b) and Au-Cys-SWCNT-FeOHETPc (c), indicating that the SAMs assume perpedincular orientations on the gold surfaces.
The needle-like protrusions are in agreement with several literature reports for SWCNT-based SAMs [3-5]. If we realise that as many as eight carboxyl groups might be present at each end of a ~1.3 nm-diameter SWCNT [4], it is reasonable therefore to assume that about eight amide bonds could be generated between each SWCNT and the modified gold surface, hence explaining the preferred vertical alignment of the SWCNT onto the gold surface.
The strong van der Waal’s attractive forces existing between carbon nanotubes should make SWCNTs assemble as bundles and not as individual. This is even more possible given that Au-Cys SAM (Figure 3.1(a)) also exist as bundles because of this attractive force. According to literature [3], SWCNTs prefer to assemble on gold surfaces as bundles of around 5 – 20 tubes. It is apparent from the images shown in figure 3.1(a) that the aligned SWCNTs did not assemble on the gold surface as individual tubes but as bundles.
Although, the ‘cut’ SWCNTs were not fractionalized before being immobilized onto the gold electrodes, it is interesting to observe that the heights of the vertically aligned SWCNT bundles still lie in the ~ 30 – 50 nm range, as observed in previous reports [3-5]. The AFM images obtained on subsequent coupling of the FeOHETPc onto the Au-Cys-SWCNT surface (reaction time 25 hr) (Figure 3.1(b)) expectedly shows similar bundled shapes of the Au-Cys-SWCNT. However, unlike in figure 3.1(b), the FeOHETPc showed more defined needle-like protrusions, with slight increase (ca. 2 nm) in the average bundle lengths, suggesting that the FeOHETPc species are linked to the ends of the SWCNTs (few binding on the defect sites of the sidewalls of the SWCNTs may not be completely ruled out). Li et al. [6] predicted from computer modelling that MPc containing eight peripheral substituents, R, (R = O(CH2)4CH3) has a diameter of 21 Å. In this work, where R = S(CH2)2OH, it means that the diameter of the FeOHETPc (if modelled as a circle) with standing (vertical) orientation as depicted in Scheme 1 may be assumed to be about 20 Å (i.e., ~ 2 nm). This value certainly explains the insignificant change in the length of Au-Cys-SWCNT-FeOHETPc compared to that of the Au-Cys-SWCNT shown by the AFM experiments.

XPS characterization

Having confirmed the formation of the SAMs on gold surfaces by AFM experiments, XPS was carried out to give some insights into the elemental compositional details of the self-assembled nanostructures. Figure 3.2 shows the survey X-ray photoelectron spectra for the full (Figure 3.2(a)) and the expanded portions of regions of most interests; sulphur (2p) (Figure3.2(b)), nitrogen (1s) (Figure 3.2(c) and carbon (1s) (Figure 3.2(d)) for the bare Au (i), Au-Cys (ii), Au-Cys-SWCNT (iii), and Au-Cys-SWCNT-FeOHETPc (iv) electrodes.
It is well recognized in XPS [6,7] that oxygen and carbon are almost always present on gold surfaces due to contaminations, the most reliable indicators for the formation of the SAMs studied here are the peaks due to sulphur and nitrogen, however, the carbon peaks give some insights into the presence of the CO and COOH. Calibrating the binding energies using the normal carbon 1s peak of adventitious carbon at 284.5 eV, the sulphur (2p) peak for both Au-Cys and Au-Cys-SWCNT appeared at 161.5 eV, assigned to the normal Au-S bond [6-12], while that for the Au-Cys-SWCNT-FeOHETPc was observed at 162.5 eV.
The 1 eV shift of the sulphur (2p) peak of the Au-Cys-SWCNT-FeOHETPc with increased peak intensity (Figure 3.2(b) (iv)) compared to those of the other SAMs suggest the presence of both gold bound (Au-S) and the sulphur of the peripheral substituents (-S(CH2)2OH) of the FeOHETPc species. The presence of these two types of sulphur for the Au-Cys-SWCNT-FeOHETPc was confirmed by a multiplexing experiment which revealed two peaks at ~161.5 and 162.7 eV for the Au-Cys-SWCNT-FeOHETPc and one peak at ~161.5 eV for the Au-Cys SAM. The cysteamine SAM showed two components for the nitrogen (1s) peak at 399.5 and 401 eV, which is in agreement with literature for cysteamine SAMs [9]. The nitrogen (1s) peak for the phthalocyanine is known to occur at either 398 or 400 eV [10], thus the appearance of sharp nitrogen (1s) peak for the FeOHETPc at 399.5 eV confirms the attachment of the FeOHETPc on the SWCNT. The nitrogen (1s) peak of the CONH for the SWCNT was observed at 400.5 eV. The different binding energies for the nitrogen (1s) of the different SAMs is indicative of the different environments where the nitrogen atoms occur. As would be expected, in terms of peak intensity and binding energy width, SWCNT exhibited relatively high concentrations of carbon Figure 3.2(d) (iii) and oxygen (not shown). Also, unlike the cysteamine SAM (Figure 3.2(d) (ii)), the SWCNT (Figure 3.2(d) (iii)) and FeOHETPc (Figure 3.2(d) (iv)) SAMs exhibited shoulders in the binding energy regions of 287.5 and 289.5 eV (see asterisked), attributed to the carbonyl and carboxylic groups [10,11] and suggesting the formation of the amide and ester bonds during the self-assembly. The Au-Cys-SWCNT-FeOHETPc exhibited a weak peak at the binding energy of ~ 718 eV (not shown) corresponding to the Fe(II) peak (Fe, 2p3/2) [12].

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Cyclic voltammetric characterization

Pretreatment of SWCNT-FeOHETPc

Unless otherwise stated, adsorption times of 18, 24 and 48 hr were adopted for the formation of the SAMs of Cys, SWCNT and FeOHETPc, respectively. It was dicovered that these SAMs could be formed within 8hr adsorption period; however, longer adsorption time was employed to permit for enhanced coverage of the SAMs on gold surface. To avoid possible oxidative desorption of the SAMs at high positive potential > 1.0 V vs Ag|AgCl, all voltammetric studies were restricted to the -0.20 to 1.0 V potential window. Previous studies [13,14] have shown that the first cyclic voltammetric scan of SAMs, especially the MPc SAM, sometimes differ from subsequent scans. Figure 3.3 presents cyclic voltammetric profiles of the various electrodes studied compared with the bare Au in 0.5M H2SO4. The modified electrode exhibited strong electrochemical stability, confirmation proved by reproducibility in 0.5M H2SO4 solution during repetitive scanning. Such stability is important for electrochemical studies and application of surface-confined thin films in aqueous solutions.
The redox couple observed for the Au-Cys-SWCNT is attributed to the four-electron process [15]. From the several reports on the electrochemistry of surface-confined phthalocyaninatoiron(II) complexes [16-18], the peaks at the +0.23V and +0.02V at the A-Cys-SWCNT-FeOHETPc electrodes were attribute to the Fe(III)/Fe(II) redox process. For the SWCNT and SWCNT-FeOHETPc films, the peak separations are greater than the ideal zero volts expected for surface immobilized species which may be attributed to the kinetic limitations or some electrostatic interactions of the molecules in the films.

Interfacial capacitance

The interfacial capacitances of the films were estimated from the non-Faradaic region of the CVs (~ +0.6V vs Ag|AgCl)) using the equation (3.1) [18,19]:
where CT= total capacitance, Ich = charging current, v = scan rates and A is the area of the electrode. The capacitances estimated from Figure 3.3 were 1.4 μFcm-2 (bare gold), 0.6 μFcm-2 (Au-Cys), 0.7 μFcm-2 (Au-Cys-SWCNT), 0.6 μFcm-2 (Au-Cys-SWCNT-FeOHETPc), 1.2 μFcm-2 (Au-SWCNTdd-FeOHETPc), 0.7 μFcm-2 (Au-Cys-SWCNT-FeOHETPc(no DCC),) 0.6 μFcm-2 (Au-Cys-FeOHETPc), and 0.5 μFcm-2 (Au-FeOHETPc). Au-SWCNTdd-FeOHETPc obtained by first preparing a nanotube bed by drop-coating SWCNT gave good FeOHETPc electrochemistry but with huge capacitive current. Various reports [20-22] have indicated that the electrochemistry of small redox-active molecules and proteins were observed through hydrophobic walls of CNTs. Also phthalocyanine complexes such as tetra-tert-butylphthalocyanines [23], FePc [24] and CoTAPc [25] have been observe to strongly adsorb onto CNT via π-π interactions. Thus, the observed electrochemistry of FeOHETPc on SWCNT bed electrode as seen in figure 3.3 should perhaps not be surprising as the bed position could easily allow direct association of the phthalocyanine ring of the FeOHETPc with the walls of the SWCNT via π-π interaction (although some covalent interactions cannot be completely ruled out). Note also that some electrochemistry of the FeOHETPc is observed at the Au-Cys-SWCNT-FeOHETPc in the absence of DCC, which is necessary for covalent linkages of the SWCNT and FeOHETPc. This indicates that there is significant non-specific adsorptions of the FeOHETPc on the SWCNT, the same explanation of close π-π interaction as for the bed position could also hold for this behaviour. The relatively smaller capacitive current of the Au-Cys-SWCNT-FeOHETPc, coupled to the faster electron transfer kinetics (further discussed in section 3.5) is advantageous to electroanalytical applications and underscores the preference of the aligned nanotube compared to electrode without DCC or its bed form on gold electrode.

Dedication 
Declaration 
Acknowledgements
Abstract.
Table of contents
Abbreviation
List of Figure
List of Schemes
List of Tables
SECTION A
CHAPTER 1 INTRODUCTION
1.1 General Overview of Thesis: Problem Statement
1.1.1 Self-assembly in electrode fabrication
1.1.2 Carbon nanotubes as electron-conducting nanowires
1.1.3 Metallophthalocyanines as electrocatalysts
1.1.4 Carboxylated ferrocenes as electrocatalysts
1.1.5 Aim of thesis
1.2 Overview of Electrochemistry
1.2.1 Basic concepts
1.2.2 The electrode-solution interface
1.2.3 Faradaic and Non-Faradaic process
1.2.4 Mass transport processes
1.3 Voltammetric techniques
1.3.1 Cyclic voltammetry
1.3.1.1 Reversible process
1.3.1.2 Irreversible process
1.3.1.3 Quasi-reversible process
1.3.2 Square wave voltammetry
1.3.3 Chronoamperometry
1.3.4 Linear sweep voltammetry
1.4 Chemically modified electrodes .
1.4.1 Methods of modifying electrode surface
1.4.1.1 Self-Assembly/chemisorption
1.4.1.1.1 Characterization of SAM-modified electrodes.
1.4.1.1.2 Application of SAM modified electrodes
1.4.1.2 Electrodeposition
1.4.1.3 Drop-dry method.
1.4.1.4. Dip-dry coating
1.4.1.5 Spin coating
1.4.1.6 Vapour deposition.
1.4.1.7 Langmuir-Blodgett .
1.4.1.8 Electropolymerisation
1.5 Organo-iron complexes and carbon nanotubes
1.5.1 Metallophthalocyanine modified electrodes.
1.5.2 General overview on ferrocene-derivatised self-assembled monolayers
1.5.3 Introduction to carbon nanotubes
1.6 Physico-chemical characterization of modified electrodes
1.6.1 Electrochemical Impedance Spectroscopy (EIS)
1.6.1.1 Basics of electrochemical impedance spectroscopy
1.6.1.2 Applications and data representation
1.6.1.3 Factors affecting rate of electron transfer.
1.6.2 Atomic Force Microscopy
1.6.3 Scanning Electron Microscopy
1.6.4 X-ray Photoelectron Spectroscopy
1.7 Background on the studied analytes
1.7.1 Potassium thiocyanat
REFERENCES
CHAPTER 2 EXPERIMENTAL
2.1 Introduction
2.2 Reagents and material
2.2.1 Functionalization of carbon nanotubes
2.3 Instrumentation
2.4 Electrode modification procedur
2.4.1 Electrode pre-treatment
2.4.2 Self-assembling technique
2.4.2.1 SWCNT-phthalocyanine based electrode.
2.4.2.2 SWCNT-ferrocene based electrodes
2.4.2.3 Nano-gold indium tin oxide electrode.
REFERENCES
SECTION B
CHAPTER 3 RESULTS AND DISCUSSION SPECTROSCOPIC , MICROSCOPIC AND ELECTROCHEMICAL PROPERTIES OF IRON-PHTHALOCYANINE SINGLE-WALLED CARBON NANOTUBE BASED ELECTRODES
3.1 SAM formation strategies
3.2 Atomic force microscopy characterization
3.3 XPS characterization
3.4 Cyclic voltammetric characterization
3.5 Electrochemical impedimetric characterization
REFERENCES
CHAPTER 4 ELECTROCATALYTIC PROPERTIES OF IRON-PHTHALOCYANINESWCNT BASED ELECTRODES:THIOCYANATE AS A MODEL ANALYTE
4.1 Square wave voltammetric detection of SCN-
4.2 Influence of scan rates on electrocatalysis of SCN-
4.3 Rotating gold disk electrode experiments
4.4 Chronoamperometric investigations
4.5 Real sample analysis with smoker’s saliva
REFERENCES
CHAPTER 5 MICROSCOPIC AND ELECTROCHEMICAL PROPERTIES OF FERROCENE SINGLE-WALLED CARBON NANOTUBES BASED ELECTRODES
5.1 SAM formation strategies.
5.2 Atomic force microscopic characterization
5.3 Electron transfer dynamics in 0.5 M H2SO4 solution
5.4 Electron transfer dynamics in a redox probe, [Fe(CN)6]4- /[Fe(CN)6]3-
5.4.1 Cyclic voltammetric characterization
5.4.2 Electrochemical impedimetric characterizatio
REFERENCES
CHAPTER 6 ELECTROCATALYTIC PROPERTIES OF FERROCENE SINGLEWALLED CARBON NANOTUBES BASED ELECTRODES: THIOCYANATE AS A MODEL ANALYTE
6.1 Square wave voltammetric detection of SCN-
6.2 Influence of scan rates on electrocatalysis of SCN-
6.3 Rotating gold disk electrode experiments
6.4 Chronoamperometric investigations
6.5 Gold nanoparticle-modified indium tin oxide electrode experiment
REFERENCES
CONCLUSIONS AND FUTURE PERSPECTIVE
APPENDIX
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