Physicochemical characterization techniques

The following are the typically used physicochemical characterization techniques used in this thesis including polarized optical microscopy (POM), atomic force microscopy (AFM), scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectrometer, fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), specific surface area (SSA) analysis, tensile test and four-terminal sensing (4T sensing).

2.4.1.1 POM

Polarized optical microscopy (POM) is a kind of polarized light microscopy that usually cross- polarize light to observe the birefringence of samples. Usually two polarizers t placed at right angles to each other are used as filters in a POM, which is named as cross-polarized optical microscopy.

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The polarizer only allows the passing of light at a specific direction. As illustrated in Figure 2.2, only light waves with the same direction as the first polarizer could travel through it to encounter the samples. Afterwards, the passed wave is blocked by the second polarizer (also named as analyszer) that is placed perpendicular to the first one. Therefore, light waves passing through an isotropic specimen would experience no changes and cannot pass through the second polarizer, making it appear dark. On the other hand, light waves of one direction could be resolved into two individual wave components (named as O-ray and E-ray) by the anisotropic regions. Part of these waves would be able to pass through the second polarizer (analyser), showing bright regions. In this work, a Leica DM 6000 was used to detect the existence of liquid crystal phases in LCGO dispersion and its basic mixtures.

Figure 2.2 Schematic showing the working principles of POM for (a) isotropic and (b) anisotropic

specimen.[352]

2.4.1.2 AFM

Atomic force microscopy (AFM) utilizes a probe with a sharp tip to scan the specimen surface and record the forces between them. High resolution AFM images could be acquired to analyse the surface roughness as well as the thickness and size of thin-sheet samples. In this study, an Asylum MFP-3D AFM was applied to characterize the exfoliated LCGO and MoS2 nanosheets, which were

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2.4.1.3 SEM and EDS

The scanning electron microscope (SEM) is a common electron microscope that is used to capture the surface (and/or cross-sectional) morphology of samples. A high-energy beam of electrons was used to collect the signals that resulted from the interactions between electrons and atoms on the sample surface. Detecting secondary electrons emitted by atoms was the most commonly used mode to observe the surface topography while the produced X-rays can be recorded through the energy- dispersive X-ray spectrometer to analysis the elemental composition and distribution.

In this thesis, both the powder and film samples were conductive composites. They were directly fixed onto an aluminium holder by carbon conductive tape for the characterizations using a JEOL JSM7500 FA field emission SEM. SEM images were taken under a 5 kV acceleration voltage and a 10 μA emission current; while the EDS mapping images were collected with a 15.0 kV acceleration voltage and a 20 μA emission current.

2.4.1.4 TEM

Transmission electron microscopy (TEM) is a kind of electron microscopy that forms images using a beam of electrons to transmit through ultrathin samples. After being magnified and focused on imaging devices, TEM can produce extra high-resolution images for the characterizations of surface morphology and crystal structure. TEM can also be modified for EDS mapping with the assistance of an energy-dispersive X-ray spectroscopy (EDS) detector. In this thesis, samples were loaded onto a copper grid with holey carbon films. A 200 kV conventional LaB6 TEM (JEOL JEM-2010) was used to collect bright-field TEM images while a 200 kV probe corrected STEM (JEOL JEM- ARM200F) was applied for EDS mapping.

2.4.1.5 XRD

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The X-ray diffraction (XRD) technique uses an X-ray beam to interact with atoms in the crystal and collects the diffraction signals for chemical structure analysis. The diffraction patterns are unique for every element or compound, which could be used to confirm the atomic and molecular structure in a sample. As shown in the schematic arrangement of an XRD diffractometer (Figure 2.3), samples were fixed on a rotated sample holder which have an angle of θ with the beam. Incident angle of X- ray beam was uniformly changed while the diffraction signals detector receives a modified diffraction X-ray signal as a function of 2θ. Interlayer distance between atom layers (d) can be calculated through the following Bragg’s Law.[353]

d = 𝑛𝜆

2𝑠𝑖𝑛𝜃 2-1

where λ is the wavelength of X-ray beam, θ is the angle between X-ray beam and sample, n is the order of reflection (usually is 1).

In this work, a GBC MMA diffractometer with Cu Kα X-ray (λ=0.15406nm) was used to collect XRD spectra at a scan rate of 2 degrees per min.

2.4.1.6 XPS

X-ray photoelectron spectroscopy (XPS) was used to quantitatively analyze the sample surface by recording the kinetic energy and number of electrons that escaped from the surface of materials irradiated by an X-ray beam. The XPS spectra provides detailed information about the elemental composition as well as the chemical and electronic state of every element. In this study, a hemispherical energy PHOIBOS 100/150 analyser was applied to collect the XPS spectra for all samples. Al Kα radiation with photon energy of 1486.6 eV produced the X-ray at a voltage of 12 V and a power of 120 W. All XPS data were analysed using the software CasaXPS.

2.4.1.7 Raman

As a common spectroscopic technique, Raman spectroscopy is mainly used to study the vibration of molecules while rotational or some other low-frequency modes may be included.[354] _{A laser is}

the most common light source that is used in Raman spectroscopy. Based on the inelastic scattering of photons (i.e. Raman scattering), laser photons can interact with the molecular vibrations, phonons or other excitations in the system. The resultant energy shifts are recorded to give details about the vibrational modes in the system. In this thesis, all Raman spectra were collected by a confocal Raman spectrometer (Jobin Yvon HR800, Horiba) using a 632.8 nm diode laser.

2.4.1.8 FTIR

Fourier transform infrared spectroscopy (FTIR) is a spectroscopic technique used to obtain the infrared spectra of the absorption or emission of samples. Every absorption peak in FTIR spectra can be recognised as a certain molecular moiety, which can be used to identify chemical structures in various samples, especially the organic and polymer materials. In Chapter 3, a Shimadzu FTIR

Page | 73 Prestige-6821 (Shimadzu Scientific Instruments) was used to acquire the FTIR spectra of samples containing PPy or annealed PPy.

2.4.1.9 TGA

Thermogravimetric analysis (TGA) is a thermal analysis method that is carried out through recording and studying the weight variation curves of samples over a wide range of temperatures. Generally, the temperature is varied at a certain speed while the reaction atmosphere varies from vacuum, ambient air to insert gases such as nitrogen or argon. The results can indicate physical and chemical properties about the sample, including phase transitions, thermal decomposition, solid-gas reactions, and so on. Based on the atmosphere used and referencing known reactions, TGA can also be applied to determine the weight percentage of each component in composites. In this thesis, all TGA experiments were conducted on a thermogravimetric analyzer (Q500, TA instruments) under air flow.

2.4.1.10 BET

Specific surface area (SSA) data is a physical value that is defined as the total surface area of a material per unit of mass or volume. It can be used to reveal the structure and properties of materials. Brunauer-Emmett-Teller (BET) isotherm tests are applied to get the SSA data, in which nitrogen is the most widely used gaseous adsorbate due to its chemical stability with many materials. In this work, SSA was calculated from the BET measurements that were carried out by using a quantachrome corporation NOVA 1000 high speed gas sorption analyser or a Tristar II 3020 gas adsorption analyser (Micromeritics).

2.4.1.11 Tensile test

Tensile test is mainly applied to test the fracture strength of materials. During the stretching process with a certain velocity, the force is accurately measured as a function of time or elongation. The elongation can be used to analyze the elasticity of samples while the fracture force is used to calculate the fracture strength (P, MPa) using the following equation:

P=F/S 2-2 which F is the tensile force at the fracturing point in N; S is the cross-sectional area (mm2_{) calculated}

using the thickness and width of MSG films. In Chapter 5, a Shimadzu EZ mechanical tester was applied to conduct the tensile tests of rGO and MoS2/rGO films at a cross-head speed of 1 mm min- 1_.

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2.4.1.12 4T sensing probe

Figure 2.4 Schematic of a typical four-point sensing probe.[355]

A four-terminal sensing (4T sensing) probe is an electrical impedance measuring technique that uses two pairs of current-carrying and voltage-sensing electrodes to characterize the conductivity or resistance of samples (Figure 2.4).[355] _{The separation of current and voltage electrodes would}

neutralise contact resistance for precise measurement. Resistance is calculated from the measured voltage between connections 2 and 3 as well as the applied current between connections 1 and 4. Commonly, 4T sensing affords the resistance of samples in the unit of ohm per square, which can be used to calculate the conductivity (σ, S cm-1 _{or S m}-1_{) of film samples according to the following}

equation:[356]

σ =1

𝜌= 1

𝑅𝑡 2-3

where ρ is the bulk resistivity in Ω∙cm or Ω∙m, R is the sheet resistance in Ω/square, t is the film thickness in cm or m.