Graphene metal organic framework composites and their potential applications 2

27

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=log
10
(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
28

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).

 Violet: 400 - 420 nm
 Indigo: 420 - 440 nm
 Blue: 440 - 490 nm
 Green: 490 - 570 nm
 Yellow: 570 - 585 nm
 Orange: 585 - 620 nm
 Red: 620 - 780 nm

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 transitions gives a
29

combination of overlapping lines. This appears as a continuous absorption band. Possible
electronic transitions of p, s, and n electrons are as shown
5
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
30

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.
31


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.
32


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.
33


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
34

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.
35

2.1.6 Raman Spectroscopy
Raman spectroscopy is a spectroscopic technique based on inelastic scattering of
monochromatic light from a laser source.
18
It is a spectroscopic technique used to observe
vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic
scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near
infrared, or near ultraviolet range.
19
The laser light interacts with molecular vibrations, phonons
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 technique
21
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.
E
binding
= E
photon
- (E
kinetic
+  )
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 E
o
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,
41

since at some point the diffusion layer has grown sufficiently above the electrode so that the flux
of reactant to the electrode is not fast enough to satisfy that required by the Nernst equation. In
this situation the current begins to drop just as it did in the potential step measurements. In fact
the drop in current follows the same behaviour as that predicted by the Cottrell equation.
The voltammogram was recorded at a single scan rate. If the scan rate is altered the
current response also changes. The Figure 2.13 shows a series of linear sweep voltammograms
recorded at different scan rates for an electrolyte solution containing only Fe
3+
.

Figure 2.13 a series of linear sweep voltammograms recorded at different scan rates

2.2.2 Cyclic Voltammetry
Cyclic voltammetry (CV) is very similar to LSV. In this case the voltage is swept
between two values (Figure 2.14, left) at a fixed rate, however when the voltage reaches V
2
the
scan is reversed and the voltage is swept back to V
1
.
27

42


Figure 2.14 (left) the voltage swept between two values at a fixed rate for CV. (right) A typical cyclic
voltammogram

A typical cyclic voltammogram recorded for a reversible single electrode transfer
reaction is shown in Figure 2.14, right. The forward sweep produces an identical response to

that seen for the LSV experiment. When the scan is reversed we simply move back through the
equilibrium positions gradually converting electrolysis product (Fe
2+
back to reactant (Fe
3+
). The
current flow is now from the solution species back to the electrode and so occurs in the opposite
sense to the forward seep but otherwise the behaviour can be explained in an identical manner.
For a reversible electrochemical reaction the CV recorded has certain well defined
characteristics.
I) The voltage separation between the current peaks is

43

II) The positions of peak voltage do not alter as a function of voltage scan rate
III) The ratio of the peak currents is equal to one

IV) The peak currents are proportional to the square root of the scan rate

Figure 2.15 Cyclic voltammogram at different scan rates
As with LSV the influence of scan rate is explained for a reversible electron transfer
reaction in terms of the diffusion layer thickness. The CV for cases where the electron transfer is
not reversible show considerably different behaviour from their reversible counterparts. The
Figure 2.16 shows the voltammogram for a quasi-reversible reaction for different values of the
reduction and oxidation rate constants.
44


Figure 2.16 cyclic voltammogram for different values of the reduction and oxidation rate constants

The first curve shows where both the oxidation and reduction rate constants are still fast,
however, as the rate constants are lowered the curves shift to more reductive potentials. Again
this may be rationalised in terms of the equilibrium at the surface is no longer establishing so
rapidly. In these cases the peak separation is no longer fixed but varies as a function of the scan
rate. Similarly the peak current no longer varies as a function of the square root of the scan rate.
2.2.3 Rotating disc electrode (RDE)
The RDE was the first widely-used (solid) hydrodynamic electrode and is still the most
popular.
28
A disc electrode is set in an insulating rod, which is rotated at a constant frequency in
solution. The solution drag on the rotating surface results in a vortex as shown in Figure 2.17.
The electrode and rod need to have perfect cylindrical symmetry so that the electrode does not
wobble on its axis when it rotates. This does not present technical complications for electrodes
with radii of several millimetres but would be a serious problem when fabricating rotating disc
45

microelectrodes. This means that the most practical RDEs have radii of a few millimetres.
Moreover the rotation speed is also limited by this constraint - a maximum practical rotation
speed is about 50Hz.

Figure 2.17 Flow profile at a rotating disc electrode.
29
Image reproduced from reference 29.

One way around this problem would be to fabricate a rotating microring electrode. The
diameter of the insulator inside the ring could be made large to reduce eccentricities in the
rotation whist the ring itself could be made very thin to achieve high rates of mass transport.
Although a microring (analogous to a microband) does not reach a steady-state under diffusion-
only conditions, the convection due to rotation would ensure a steady-state response.
The first mathematical treatment of convection and diffusion towards a rotating disk

electrode was given by Levich. Considering the case where only the oxidized form of a molecule
(or ion) of interest is initially present in the electrochemical cell, the cathodic limiting current
(i
LC
) observed at a rotating disk electrode is given by the Levich equation :
30

46

i
LC
= 0.620 n F A (D
O
)
2/3
ν
-1/6
C
O
ω
1/2

In terms of the concentration (C
O
) of the oxidized form in the solution, the Faraday
constant (F = 96485 coulombs per mole), the electrode area (A), the kinematic viscosity of the
solution (ν), the diffusion coefficient (D
O
) of the oxidized form, and the angular rotation rate (ω).
Alternatively, when the solution initially contains only the reduced form, the Levich equation for

the anodic limiting current (i
LA
) can be written as
i
LA
= 0.620 n F A (D
R
)
2/3
ν
-1/6
C
R
ω
1/2

where the concentration term (C
R
) and diffusion coefficient (D
R
) are for the reduced form rather
than the oxidized form.
2.2.4 Rotating Ring-Disk Electrode (RRDE)
Soon after the rotating disk electrode was developed, the idea of putting a ring electrode
around the disk electrode was introduced, and the rotating ring-disk electrode was born.
31
In this
“ring-disk” geometry, the overall axial flow pattern initially brings molecules and ions to the
disk electrode.
32

Then, the subsequent outward radial flow carries a fraction of these molecules
or ions away from the disk electrode and past the surface of the ring electrode. This flow pattern
allows products generated (upstream) by the half reaction at the disk electrode to be detected as
they are swept (downstream) past the ring electrode (Figure 2.18).
Two of the key parameters which characterize a given ring-disk geometry are the
collection efficiency and the transit time.
33
The collection efficiency is the fraction of the
material from the disk which subsequently flows past the ring electrode, and can be expressed as
47

a fraction between 0.0 and 1.0 or as a percentage. Typical ring-disk geometries have collection
efficiencies between 20% and 30%. The transit time is a more general concept indicating the
average time required for material at the disk electrode to travel across the gap between the disk
and the ring electrode. Obviously, the transit time is a function of both the gap distance and the
rotation rate.

Figure 2.18 (a) Rotating Ring-disk device image (b) Rotating ring-disk electrode (c) Rotating Ring-Disk
Voltammograms at Various Rotation Rates.
34
Image reproduced from reference 34.
Once the collection efficiency value has been established empirically for a particular
RRDE, it can be treated as a property of that particular RRDE, even if the RRDE is used to study
a different half reaction in a different solution on a different day. Although the empirically
measured collection efficiency (N
empirical
) is a ratio of two currents with opposite mathematical
signs (anodic and cathodic), the collection efficiency is always expressed as a positive number.
N
empirical

= - i
LIMITING, RING
/ i
LIMITING, DISK

48

2.2.5 BET surface area
BET theory aims to explain the physical adsorption of gas molecules on a solid surface
and serves as the basis for an important analysis technique for the measurement of the specific
surface area of a material. In 1938,
35
Stephen Brunauer, Paul Hugh Emmett, and Edward Teller
published an article about the BET theory in a journal[1] for the first time; "BET" consists of the
first initials of their family names. The concept of the theory is an extension of the Langmuir
theory, which is a theory for monolayer molecular adsorption, to multilayer adsorption with the
following hypotheses: (a) gas molecules physically adsorb on a solid in layers infinitely; (b)
there is no interaction between each adsorption layer; and (c) the Langmuir theory can be applied
to each layer. The resulting BET equation is expressed :
36


p and p
0
are the equilibrium and the saturation pressure of adsorbates at the temperature of
adsorption, v is the adsorbed gas quantity (for example, in volume units), and v
m
is the monolayer
adsorbed gas quantity. c is the BET constant.
Considering multilayered gas molecule adsorption, where it is not required for a layer to

be completed before an upper layer formation starts. Furthermore, we have five assumptions to
calculate BET surface area:
1. Adsorptions occur only on well-defined sites of the sample surface (one per molecule)
2. The only considered molecular interaction is the following one: a molecule can act as a single
adsorption site for a molecule of the upper layer.
49

3. The uppermost molecule layer is in equilibrium with the gas phase, i.e. similar molecule
adsorption and desorption rates.
4. The desorption is a kinetically-limited process, i.e. a heat of adsorption must be provided:
4.1. these phenomenon are homogeneous, i.e. same heat of adsorption for a given molecule layer.
4.2. it is E
1
for the first layer, i.e. the heat of adsorption at the solid sample surface
4.3. the other layers are assumed similar and can be represented as condensed species, i.e. liquid
state. Hence, the heat of adsorption is E
L
is equal to the heat of liquefaction.
5. At the saturation pressure, the molecule layer number tends to infinity (i.e. equivalent to the
sample being surrounded by a liquid phase)

Figure 2.19 nitrogen adsorption and desorption diagram

2.2.6 Gas chromatography–mass spectrometry (GC-MS)
GC-MS is a method that combines the features of gas-liquid chromatography and mass
spectrometry to identify different substances within a test sample.
37
Applications of GC-MS
include drug detection, fire investigation, environmental analysis, explosives investigation, and
identification of unknown samples. GC-MS can also be used in airport security to detect

50

substances in luggage or on human beings. Additionally, it can identify trace elements in
materials that were previously thought to have disintegrated beyond identification. GC-MS has
been widely heralded as a "gold standard" for forensic substance identification because it is used
to perform a specific test. A specific test positively identifies the actual presence of a particular
substance in a given sample. A non-specific test merely indicates that a substance falls into a
category of substances. Although a non-specific test could statistically suggest the identity of the
substance, this could lead to false positive identification.

Figure 2.20 (a) GC-MS schematic (b) The insides of the GC-MS, with the column of the gas
chromatograph in the oven on the right.
38
Image reproduced from reference 38.


The GC-MS is composed of two major building blocks: the gas chromatograph and the
mass spectrometer (Figure 2.20).
39
The gas chromatograph utilizes a capillary column which
depends on the column's dimensions (length, diameter, film thickness) as well as the phase
51

properties (e.g. 5% phenyl polysiloxane). The difference in the chemical properties between
different molecules in a mixture will separate the molecules as the sample travels the length of
the column. The molecules are retained by the column and then elute (come off of) from the
column at different times (called the retention time), and this allows the mass spectrometer
downstream to capture, ionize, accelerate, deflect, and detect the ionized molecules separately.
The mass spectrometer does this by breaking each molecule into ionized fragments and detecting
these fragments using their mass to charge ratio.

40