Graphene metal organic framework composites and their potential applications 2

The principles of materials characterization techniques using Fourier-transform Infra-red spectroscopy FTIR, UV-Vis absorbance spectroscopy, atomic force microscopy AFM, scanning electro

Chapter 2: Experimental Methods

Introduction

This chapter presents the principles of the analytical techniques used in the characterization of the physical and chemical properties of graphene and graphene-based hybrid material The principles of materials characterization techniques using Fourier-transform Infra-red spectroscopy (FTIR), UV-Vis absorbance spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) are briefly introduced here Techniques for checking applications of the composites such as cyclic voltammetry (CV), linear sweep voltammetry (LSV), rotating disk electrode (RDE), rotating ring disk electrode (RRDE), GC/MS, BET surface area are also discussed

2.1 Materials Characterization

2.1.1 UV-Vis spectroscopy

An ultraviolet-visible (UV-Vis) Spectrophotometer is used to determine the absorption or transmission of UV-Vis light (180 to 820 nm) by a sample.1 In this region of electromagnetic spectrum, molecules undergo electronic transitions UV-Vis spectrophotometer is used for quantitative concentration measurement of absorbing materials based on the calibration curves of the material The concentration of an analyte in solution can be determined by measuring the

absorbance at a particular wavelength and applying Beer-Lambert’s law: A=log10 (I 0/I) = c L

where A is the measured absorbance, I 0 is the intensity of the incident light at a given

wavelength, I is the transmitted intensity, L the pathlength through the sample, and c the

concentration of the absorbing species For each species and wavelength, ε is a constant known

as the molar absorptivity or extinction coefficient This constant is a fundamental molecular property in a given solvent, at a particular temperature and pressure The Beer-Lambert Law is useful for characterizing many compounds but does not hold as a universal relationship for the concentration and absorption of all substances.2

When white light passes through or is reflected by a colored substance, a characteristic portion of the mixed wavelengths is absorbed The remaining light will then assume the complementary color to the wavelength(s) absorbed.3 This relationship is demonstrated by the color wheel shown on the right Here, complementary colors are diametrically opposite each other A common feature of all these colored compounds, displayed below, is a system of

extensively conjugated pi-electrons (Figure 2.1)

Figure 2.1 the color wheel spectrum covered UV- vis –IR region

Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy The spectrum of a molecule containing these chromophores is complex.4 This is because the superposition of rotational and vibrational transitions on the electronic transitio ns gives a

combination of overlapping lines This appears as a continuous absorption band Possible electronic transitions of p, s, and n electrons are as shown5 in the Figure 2.2

Figure 2.2 electronic transitions of species containing p, s, and n electrons.5 Image reproduced from reference 5

2.1.2 Fourier-transform Infra-red Spectroscopy (FTIR)

The mid infra-red (IR) spectral range (2.5-25 μm) is the most accessible and the richest in providing structural information The absorption bands in this frequency domain form a molecular fingerprint, thereby allowing the detection of compounds and the deduction of structural details.6 This is important for this thesis work to confirm the success of functionalization of graphene-based materials The initial IR instrument is based on dispersive spectrometers that functions in a sequential mode Subsequently, Fourier – transform infra-red (FTIR) spectrophotometer emerged to overcome the limitations encountered with dispersive instruments FTIR is capable of simultaneous analysis of the full spectral range using interferometry, the interferometer produces a unique type of signal which has all of the infrared frequencies “encoded” into it Interferometers employ a beam splitter which takes the incoming infrared beam and divides it into two optical beams One beam reflects off of a flat mirror which

is fixed in place The other beam reflects off of a flat mirror which allows this mirror to move a

very short distance (~ few mm) away from the beam splitter.7 The two beams reflect off of their

respective mirrors and are recombined when they meet back at the beam splitter (Figure 2.4) As

the path of one beam is fixed and the other is constantly changing as its mirror moves, the signal which exits the interferometer is the result of these two beams “interfering” with each other The resulting signal is called an interferogram which has information about every infrared frequency which comes from the source As the measured interferogram signal cannot be interpreted directly, Fourier transformation is performed by the computer which presents the spectral

information in a plot of absorbance (or transmission) versus the wave number (Figure 2.3)

Figure 2.3 Schematic of processing of interferogram using Fourier-Transform (FFT) calculations to

produce an IR spectrum.8 Image reproduced from reference 8

For the work presented in this thesis, FTIR samples were prepared by KBr pellet The samples is mixed with KBr powder and pressed into a pellet using a 13mm die set with a force of 10-tonne exerted by a bench top hydraulic press

Figure 2.4 Schematic diagram of an interferometer, configured for FTIR.9 Image reproduced from reference 9

2.1.3 Atomic Force Microscopy (AFM)

The AFM is a high-resolution type of scanning probe microscope, with a resolution of less than a nanometer The image is gathered by scanning the sample surface with a sharp probe

at the end of a micro-scale cantilever.10 Atomic resolution can be obtained by reducing the contact force to ~10 − 9 N This is less than most interatomic forces, limiting tip induced sample deformation and contact area, which allows the imaging of single atoms Estimating the ionic bond energy ≤10 eV, a van der Waals bonding energy of ≤10 meV, and a repulsive force acting of a distance of Δ ≈ 0.2 Å Figure 2.5 shows a schematic view of AMF device When the tip is brought into sample surface, repulsion forces between the tip and the atomic shells of the sample lead to a deflection of the cantilever providing a true 3D surface profile Samples viewed

by AFM do not require any special treatments that would irreversibly change or damage the sample and can work perfectly well without the need for vacuum

Figure 2.5 Schematic of AFM.11 Image reproduced from reference 11

2.1.4 Scanning Electron Microscopy (SEM)

Scanning electron microscope (SEM) uses electron beam for imaging.12 The schematic

diagram of SEM instrumentation is shown in Figure 2.6 A beam of electrons is produced by

heating of a metallic filament The electron passes through the electromagnetic lenses which focus and direct the beam down towards the sample As it hits the sample, photons and electrons are ejected from the sample Detectors collect the secondary or backscattered electrons, and convert them to a signal The most common detection mode, secondary electron imaging (SEI), can produce very high-resolution images of a sample surface in this thesis SEM micrographs have a large depth of field because of the very narrow electron beam thereby has the capability to characterize three-dimensional structure of a sample In this thesis for samples that are non-conductive, sputtering them with platinum improves the resolution of the image

Figure 2.6 Schematic diagram of SEM instrumentation.12 Image reproduced from reference12.

2.1.5 Transmission electron microscopy (TEM)

The first transmission electron microscopy (TEM) was built by Max Knoll and Ernst Ruska in 1931.13 While SEM imaging is due to the secondary or backscattered electrons, TEM imaging is based on the transmitted electrons that interact with the sample as it passes through TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or

to be detected by a sensor such as a CCD camera (Figure 2.7)

TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small de Broglie wavelength of electrons.14 This enables the instrument's user to examine fine detail even as small as a single column of atoms, which is tens of thousands times

smaller than the smallest resolvable object in a light microscope TEM forms a major analysis method in a range of scientific fields, in both physical and biological sciences TEMs find application in cancer research, virology, materials science as well as pollution, nanotechnology, and semiconductor research.15 At smaller magnifications TEM image contrast is due to absorption of electrons in the material, due to the thickness and composition of the material At higher magnifications complex wave interactions modulate the intensity of the image, requiring expert analysis of observed images Alternate modes of use allow for the TEM to observe modulations in chemical identity, crystal orientation, electronic structure and sample induced electron phase shift as well as the regular absorption based imaging

Figure 2.7 TEM Image of our COOH Functionalized Nanotubes COOH-MWNTS.16,17

Image reproduced from reference 16,17.

or other excitations in the system, resulting in the energy of the laser photons being shifted up or down The shift in energy gives information about the vibrational modes in the system The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud and the bonds of that molecule.20 For the spontaneous Raman Effect, which is a form of light scattering, a photon excites the molecule from the ground state to a virtual energy state When the molecule relaxes it emits a photon and it returns to a different rotational or vibrational state The difference in energy between the original state and this new state leads to a shift in the emitted photon's frequency away from the excitation wavelength (Figure 2.8). The Raman Effect, which is a light scattering phenomenon, should not be confused with absorption (as with fluorescence) where the molecule is excited to a discrete (not virtual) energy level

Figure 2.8 Energy level diagram showing the states involved in Raman signal

36

2.1.7 X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique21 that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material.22 XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 1 to 10 nm of the material being analyzed XPS requires ultra-high vacuum (UHV) conditions XPS is one of the most versatile and generally applicable surface spectroscopic techniques used for a myriad of application, from catalyst characterization to fundamental physics of adsorbate ionization XPS measures the elemental composition, empirical formula, chemical state and electronic state of the elements of a material To obtain XPS spectra, the sample/material is irradiated with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the material being

analyzed Figure 2.9 presents the schematic diagram of X-ray photoemission process

Figure 2.9 Schematic drawing of the X-ray photoemission process of core-level electrons

37

The electron binding energy of each of the emitted electrons can be determined by the equation below since the energy of an X-ray with particular wavelength is known

Ebinding = Ephoton - (Ekinetic +  )

where E binding is the binding energy (BE) of the electron, E photon is the energy of the X-ray photons being used, E kinetic is the kinetic energy of the electron as measured by the instrument

and φ is the work function of the spectrometer A typical XPS spectrum is a plot of the number

of electrons detected (Y-axis) versus the binding energy of the electrons detected (X-axis) Each element produces a characteristic set of XPS peaks at characteristic binding energy values that directly identify each element that exist in or on the surface of the material being analyzed

Figure 2.10 Basic components of a monochromatic XPS system.23 Image reproduced from reference 23

2.1.8 X-ray diffraction (XRD)

X-ray diffraction (XRD) is a non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of a substance In materials with a crystalline structure, X-rays scattered by ordered features will be scattered coherently in certain

38

directions meeting the criteria for constructive interference leading to signal amplification

(Figure 2.11) The conditions required for constructive interference are determined by Bragg’s

law: n = 2d sin with corresponding to X-ray wavelength, d refers to the distance between

the lattice planes and corresponds to the angle of incidence with the lattice plane.24 Where a mixture of different phases is present, the resultant diffractogram is formed by addition of the individual patterns From X-ray diffraction, a wealth of structural, physical and chemical information about the material investigated can be obtained A host of application techniques for various material classes is available, each revealing its own specific details of the sample studied

Figure 2.11 (a) XRD pattern formed when X-rays are focused on a crystalline material (b) Each dot,

called a reflection, forms from the coherent interference of scattered X-rays passing through the crystal (c) XRD-Diagram.25 Image reproduced from reference25

2.2 Techniques used for application

2.2.1 Linear Sweep Voltammetry

In linear sweep voltammetry (LSV) a fixed potential range is employed much like potential step measurements.26 However in LSV the voltage is scanned from a lower limit to an

upper limit as shown below (Figure 2.12)

39

Figure 2.12 linear sweep voltammogram

The voltage scan rate (v) is calculated from the slope of the line Clearly by changing the

time taken to sweep the range we alter the scan rate The characteristics of the linear sweep voltammogram recorded depend on a number of factors including:

 The rate of the electron transfer reaction(s)

 The chemical reactivity of the electroactive species

 The voltage scan rate

In LSV measurements the current response is plotted as a function of voltage rather than

time, unlike potential step measurements For example in the Fe 3+ /Fe 2+ system;

The scan begins from the left hand side of the current/voltage plot where no current flows As the voltage is swept further to the right (to more reductive values) a current begins to flow and eventually reaches a peak before dropping To rationalise this behaviour we need to consider the influence of voltage on the equilibrium established at the electrode surface If we

consider the electrochemical reduction of Fe 3+ to Fe 2+, the rate of electron transfer is fast in

40

comparison to the voltage sweep rate Therefore at the electrode surface equilibrium is established identical to that predicted by thermodynamics We may consider from equilibrium electrochemistry that the Nernst equation:

The relationship between concentration and voltage (potential difference): where E is the

applied potential difference and Eo is the standard electrode potential So as the voltage is swept

from V 1 to V 2 the equilibrium position shifts from no conversion at V 1 to full conversion at V 2 of the reactant at the electrode surface The exact form of the voltammogram can be rationalised by

considering the voltage and mass transport effects As the voltage is initially swept from V 1 the equilibrium at the surface begins to alter and the current begins to flow:

The current rises as the voltage is swept further from its initial value as the equilibrium position is shifted further to the right hand side, thus converting more reactant The peak occurs,