Characterization techniques

3.3.1 Scanning electron microscopy

Scanning electron microscopy (SEM) is an imaging technique that produces an image by focusing a beam of accelerated electrons on a sample. Unlike the light microscope, the SEM can produce higher resolution images of the sample because it uses beam of electrons (instead of light) - which have a much lower wavelength compared to light. The electron beams are produced by an electron gun, accelerated and focused on specific area of the sample by travelling through electromagnetic fields and magnetic lens. When electron beam impinges on a given sample, different signals which includes: secondary electrons, back-scattered electrons, light and X-rays are produced. In SEM, the secondary and back-scattered electrons being produced are detected by a detector and analysed to produce the sample’s image.3

In this thesis, the sample’s surface morphology was analysed using SEM on a Carl Zeiss EVO MA10 system with incident voltage of 5 to 30 kV.

3.3.2 Transmission electron microscopy

Transmission electron microscopy (TEM) is very similar to the SEM in the sense that both imaging techniques uses a beam of accelerated electrons to image a sample. Unlike SEM, the electron beams are transmitted through an ultrathin sample. Because the transmitted electron bears information pertinent to a sample’s internal structure, they are magnified and focused onto an imaging device and a detector picks up the signal. Three types of transmitted electrons are generated namely: elastic scattered, inelastic scattered and unscattered electrons. Depending on a sample’s required contrast, bright field and dark field images can be generated using unscattered and scattered electrons, respectively. In addition to using TEM to analyse the internal structure of a sample, it can also be used to produce a diffraction pattern of a sample using a TEM imaging technique known as selected area electron diffraction (SAED).4

In this thesis, the samples TEM images were taken on a Phillips CM100 under an accelerating voltage of 100 kV.

3.3.3 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is a surface technique that can be used for the elemental analysis of a sample and also for identification of the chemical and electronic states of the constituent elements. In this technique, the sample’s surface is being irradiated by a beam of X-rays in an ultra-high vacuum environment. During analysis, the incident X-rays excite the sample, resulting in the ejection of the sample’s inner shell electron. An electron energy analyzer measures the intensity of the photoelectron and difference between the energy of the incident X-

ray and that of the photoelectron (binding energy). Based on this data, the exact elemental composition and states of the sample’s elements can be determined.5

In this thesis, XPS spectra of the sample was generated on an X-ray photoelectron spectroscopy(XPS, PHI Quantera SXM) using monochromatic Al KαX-rayradiation.

3.3.4 Magic Angle Spinning Nuclear Magnetic Spectroscopy

The magic angle spinning nuclear magnetic spectroscopy (MAS NMR) is a solid-state NMR used for identifying and studying individual nuclei present in a molecule or compound. Like NMR, MAS NMR analysis of elements in the periodic table is based on the fact that all elements possess at least one magnetically active isotope with a corresponding nonzero nuclear spin (which is greater than ½ in most cases) and nuclear quadrupole moments. In MAS NMR analysis, samples are irradiated with an intense magnetic field, specific nuclei of the solid sample generate resonant electromagnetic field frequency which is typically expressed as the chemical shift (δ). The emergence of characteristic peaks at a particular chemical shift helps to identify the specific chemical groups in a sample including its corresponding chemical environment.6

In this thesis, 11B NMR MAS measurement was undertaken at 11.7 T using a Brucker Avance 500 wide-bore spectrometer operating at a Larmor frequency of 160.47 MHz equipped with a 4 mm Bruker probe with a rotor spinning at 12 kHz. The spectrum was referenced using BF3(OEt)2 (δ= 0 ppm). The spectrum was acquired using a single pulse experiment at 90o _{with a} pulse of 83 μs and a recycle delay of 1 s.

3.3.5 X-ray diffraction

X-ray diffraction (XRD) is a spectroscopic technique that is commonly used for analyzing and characterizing the crystal structure of a sample. An X-ray-irradiated sample produces intense

diffraction peaks corresponding to specific lattice planes of the sample. Peaks are observed in the XRD spectra when constructive interference occurs among the diffracted X-rays. The condition only occurs when Bragg’s law (equation 3-1) is satisfied.7

nλ = 2d sinθ (3-1) In this thesis, the XRD spectra of the samples was generated using a PANalytical Empyrean X-ray diffractometer operated at room temperature, with Cu Kα radiation (λ = 1.5418 Å) and nickel filter. The scanning range was 10-80o at step size of 0.0070.

3.3.6 Raman spectroscopy

This is a technique for detecting molecules and studying the structural transformation they undergo during a chemical reaction. In this technique, the sample of interest is irradiated with a monochromatic light (typically a laser) resulting in the polarization of the sample’s electron cloud. In this instance, some of the incident light is absorbed by the sample’s molecule, excitation to higher vibrational (phonon) states of the molecule occurs. When relaxation of the excited electron to its ground state or lower vibrational energy occurs, emission of a photon in a direction different from that of the incident light occurs in a process termed scattering. Only emitted photons that undergo inelastic scattering results in the generation Raman bands, which are used for identifying a sample.8

In this thesis, Raman spectra were collected using a He-Ne laser wavelength of 633 nm (laser power of 17 mW) on a confocal Raman imaging system (Horiba Jobin Yvon LabRam ARAMIS).

3.3.7 Fourier Transform Infrared spectroscopy

This technique is used for detecting functional groups present in a material of interest and studying the evolution of structural bonds in such material due to the absorption or emission of infrared spectra. However, only compounds that possess molecular structures which give rise to change in dipole antisymmetric vibration are considered infrared-active. Typically, a molecular vibrational transition occurs when samples are irradiated with a photon of wavelength in the mid infrared range (2.5 - 15.4 μm). These molecular vibrational transitions result in changes in bond length (stretch vibration) and deformations either by bending or twisting. The vibrational transitions are recorded as spectra comprising of a plot of either absorbance or transmittance against wavenumber.9

In this thesis, Fourier Transform Infrared spectroscopy (FT-IR) spectra were obtained using an Agilent Cary 630 FT-IR Spectrometer operated in the total attenuated reflectance mode.