Chapter 2: Methods
2.3 Composition and Morphology Characterisation
Scanning Electron Microscopy (SEM) uses an electron beam to obtain morphological and chemical information from sample surfaces. Electrons from the beam can be bounced off the surfaces (backscattered electrons, ‘BSE’), or cause more electrons (secondary electrons, ‘SE’) and other types of radiation (e.g. X-rays) to be emitted by the surfaces. These can be detected and analysed to reconstruct useful micrographs and spectra.
Ability to backscatter electrons increases with increasing atomic number. Contrast between phases that have different elemental compositions can be seen in BSE micrographs. This is used in this thesis to supplement X-ray diffraction phase analysis. Micrographs constructed from SE information on the other hand give good morphological resolution, and are used in this thesis to study the shape of particles and grains within materials. X-rays that are emitted from the sample after exposure to the electron beam can be collected and quantified to pinpoint the identity and quantity of specific elements. In Energy Dispersive X-ray Spectroscopy (EDXS), a spectrum of X-rays is collected and analysed for detection of broad number of elements. The Wavelength Dispersive X-ray Spectroscopy (WDXS) method focuses on quantifying specific X-ray wavelengths to more accurately determine quantities of specific elements. This is slow, but especially useful if the elements of interest have overlapping spectral peaks307.
From morphological analysis of nanoparticles in Chapter 6, the particles were spread in a thin layer directly onto carbon tape on a sample stub. For BSE and EDXS analysis, the samples were polished through sequentially finer sandpapers (from 240 grit commercial grade sandpaper, to 1 μm diamond lapping & polishing film) by hand until the surface was smooth, with washing and sonicating between. The samples were then directly mounted onto a sample stub using carbon tape. Manual polishing was used instead of thermal or chemical etching in order to circumvent issues with preferential leeching of bismuth and vanadium (higher volatility and solubility than other component elements). For WDXS, samples were set in hard resin then polished sequentially though 1200, 2400 and 4000 grit sandpapers, before finally polishing with 6, 3, then 1 μm diamond paste, all using an Allied MetPrep3 grinding/polishing system. All samples were coated in carbon prior to analysis.
Data presented in Chapters 3 - 7 were collected at the Centre for Advanced Microscopy, ANU.
An Hitachi 4300 SE FESEM in conjunction with an Oxford Instruments INCA X-MAX EDXS Analysis system and backscattered electron detector were used for Chapters 3 – 6 for elemental analysis and general imaging. The EDXS data were obtained at a voltage of 15 kV. A Zeiss Ultrapluss was used in Chapter 6 in which a higher resolution was required for particle imaging. The secondary electron imaging mode was utilized at a lower voltage (1.5 kV). In Chapter 7, a JEOL JXA-8530F Plus Electron Probe Microanalyzer was used to obtain WDXS data and thus better resolution of overlapping Ti and V characteristic X-ray peaks. A voltage of 15 kV was used on this instrument.
2.3.2 X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy is in some ways the reverse of EDXS mentioned above. This method hinges on shining an X-ray source (Ehv) on a sample surface and detecting the energies of electrons which are ejected (EKE), then calculating their ‘binding energy’ (EBE) from the known spectrometer work function (ϕspectrometer) per:
𝐸𝐸𝐵𝐵𝐵𝐵 =𝐸𝐸ℎ𝑣𝑣− 𝐸𝐸𝐾𝐾𝐵𝐵 − 𝜙𝜙𝑠𝑠𝑤𝑤𝑠𝑠𝑖𝑖𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 (2.6)
The peaks in binding energy in the XPS spectra are specific to element types, orbitals, and valence states. The structure of the valence band can also be analysed with this method and some information about the relationship of the valence band (and any gap states) to the fermi level can be determined. This technique is limited to information from the surface to a depth of 1-10 nm in the sample308.
Samples for XPS analysis were in pellet or powder form, with pellets being polished in hexane before measurement. The data were collected using a Thermo Scientific ESCALAB250Xi spectrometer with an Al Kα X-ray source at UNSW Sydney. The spectra were then fit using the Thermo Scientific ‘Avantage’ program to the reference C 1s = 284.8 eV in order to establish relative proportions of valence states and atomic percentages of each detected element. This method is used in this thesis to get an idea about how elemental incorporation influences defect formation and the band structure. While extremely useful, it should be noted that information gathered comes only from the sample surface. Other techniques are used to supplement this information.
2.3.3 Mӧssbauer Spectroscopy
Another spectroscopic technique which provides useful chemical information is 57Fe Mӧssbauer spectroscopy. This technique is used in this thesis to confirm the iron valence, see if the samples are magnetic and examine if iron occupies different sites within the crystal. This method works by exposing the sample to gamma radiation produced by a 57CoRh source. The 40
wavelength of the radiation is altered by linear accelerator, which moves the source through a range of velocities. This creates a Doppler Effect, the outcome of which is that the sample is scanned through a range of closely related wavelengths. Iron nuclei in the sample which are of the same isotope (57Fe) will absorb gamma rays when a resonance condition is reached309. This can occur at different velocities depending on the environment of the iron in the sample. The 57Fe nucleus has a ground spin state of I = 1/2 and an excited state of I = 3/2. These energy levels experience Quadrupole Splitting (QS), leading to a doublet being observed in paramagnetic systems. The magnitude of the energy difference between the split excited states of the nuclei is influenced by the electric field gradient, in turn dictated by parameters such as the coordination number, type of ligands and crystallographic site. Magnetic Hyperfine Splitting can occur when the iron nucleus is surrounded by a magnetic field, such that a sextet of possible transitions at different energies can be seen for magnetic samples. The shift of all peaks with respect to the normal velocity of the source, is referred to as the Isomer Shift (IS). These shifts arise from effective s-electron density around the nucleus. The Fe2+ has a larger shift than Fe3+ due to screening by d-electrons307.
57Fe Mӧssbauer spectra were collected at room temperature using a standard constant- acceleration spectrometer in a transmission geometry using a 57CoRh source (UNSW Canberra). Samples were powdered and mixed with boron nitride (BN) for a final concentration 13 – 16 mg/cm2 of the compound under test on the sample holder. An α-Fe foil standard was also measured for calibration. Spectra were fitting using IMSG2011 310 to extract the isomer shift, quadrupole shift, Half Width Half Maximum (HWHM) and in the case of more than one species being present, their relative proportions, for each material.
Together with the diffraction methods, these microscopic and spectroscopic studies provide a picture of the nature of the structure and bonding in each of the materials. Techniques used to measure the properties of whole samples are detailed below.