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2.2

Composition studies

2.2.1 Energy Dispersive X-ray Spectroscopy (EDX)

EDX has become a useful tool for measuring the elemental composition of solid-state GO based materials. Generally an EDX system is attached to an SEM: the high energy beam of electrons is generated and fired at the sample in the same manner as SEM. However, for EDX, instead of looking to the secondary and backscattered electrons, interest lies with the emitted X-rays.

In EDX analysis, high energy incident electrons are used to eject core electrons, leaving behind an electron vacancy. An electron from an outer shell then falls down to this lower energy orbital, filling the vacancy. In doing so, the electron will release radiation equivalent to the energy difference between the orbitals (figure 2.14). The emitted radiation is detected and measured.

Fig. 2.14 Diagram showing the interaction of an incident electron with an atom in EDX analysis.

EDX works on the principle that every element has a unique atomic structure, and thus the energy of the X-rays detected will be characteristic of a specific element.20 Since the emission depth of X-rays can be several µm, dependent on imaging conditions,

Nucleus K Incident electron Secondary electron L M Kα radiation Electron vacancy M L K Nucleus

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measurements are generally representative of the bulk sample rather than surface topography.

While EDX is useful for identifying the elements present in a sample, no information on the chemical environment of the elements is given. As a result, measurements may be skewed by sample contamination. For example, as a hygroscopic material GO is prone to water contamination, thus the percentage of oxygen incorporation as measured by EDX may come from a mixture of adsorbed water and oxidative functionalities on the GO sheet. To improve the reliability of EDX, measurements are taken from at least four different areas of the same sample to give an averaged result with quoted standard deviation values. EDX requires next to no sample preparation for GO-based materials, using powdered samples deposited onto a sticky carbon tab. EDX measurements give the elemental composition to an accuracy of approximately ±1 at.%, and to ensure accuracy they should be calibrated against samples of known (and similar) composition.

2.2.2

X-ray Photoelectron Spectroscopy (XPS)

XPS is an alternative means of calculating the elemental composition, empirical formula, chemical state and electronic state of elements within a sample. The samples are irradiated with X-ray radiation of a known energy, which excites the core electrons in the samples, enabling them to escape (figure 2.15).

Fig. 2.15 Diagram showing the interaction of X-ray radiation with an atom in XPS. Nucleus

K

hv

Photoelectron

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The emitted photoelectrons are detected, and their kinetic energies are measured. Since the radiation energy (𝐸), and the kinetic energy (𝐸𝐾𝑖𝑛) of the photoelectrons is known, the binding energy (ɸ) of the material can be calculated from:

𝐸 = ɸ + 𝐸_𝐾𝑖𝑛 (2.3)

Due to their individual structure, every atomic orbital has a characteristic binding energy. Hence XPS uses the kinetic energy of emitted photoelectrons to quantitatively analyse the elemental composition of a sample.21 While the X-rays have good penetration, the depth of photoelectron emission is limited: only photoelectrons in the top 1–10 nm of the sample will have enough energy to escape from the material and thus be detected. XPS is therefore a surface sensitive technique and can suffer from contamination effects such as surface adsorbed water and adventitious carbon.

XPS can work with powdered samples and so the preparation for GO-based materials is minimal. XPS analysis can be used to produce survey scans which directly show the percentage composition of a material (figure 2.16). The accuracy of such measurements is typically quoted as within 10% of the atomic percent,22, 23 although more accurate quantification is possible by taking the scattering cross section of the elements into consideration.

Fig. 2.16 Example XPS survey scan showing photoelectric and Auger lines.

Not all peaks in an XPS spectrum are due to the incident electrons. As seen in figure 2.16, alongside photoelectric lines, the survey scan shows Auger peaks. These lines are caused

0 200 400 600 800 1000 1200 Binding Energy / eV C 1s O 1s N 1s F 1s O Auger Spectral background Si 2s Si 2p

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when a higher energy electron falls to fill the vacancy left by a photoelectron, and a second electron, with energy characteristic of the energy difference between the orbital shells, is emitted. This process is demonstrated schematically in figure 2.17.

Fig. 2.17 Diagram showing the emission of an Auger electron in XPS.

While the Auger electrons are responsible for some peaks in the survey scan, in XPS it is the line intensity and position of the photoelectric signals which are used to gather information on the sample. Core level spectra provide a more in depth study of the elements in a sample, revealing the chemical environment of an element based on chemical shifts. This information is used to study chemical modifications of GO-based materials, disclosing, for example, the proportion of carbon involved in C-O and C=O bonding. This provides a big advantage over EDX analysis.

Nucleus _K hv Photoelectron L M Nucleus _K KLL Auger Electron L M

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Fig. 2.18 Example C 1s core level spectrum of a GO-based material.

Figure 2.18 shows a representative C 1s core level spectrum of a GO-based material. The spectrum can be split into individual contributions from C=C / C-C, C-O and C=O binding environments, assigned according to chemical shift measurements via a peak fitting process using mixed Gaussian-Lorentzian (Voigt) lineshapes. For GO-type materials, it is generally accepted that the peak around 284.8 eV is a combination of sp2 and sp3 hybridised carbon- carbon bonds, although often the contributions cannot be definitively separated with confidence due to their clashing chemical shifts.24

Often a π-π* satellite can also be seen in C 1s spectra – satellites occur when the incident radiation promotes an electron to a higher energy level (a bonding to anti-bonding transition in this case) before photoionisation takes place.25 As a result the emitted electron has less kinetic energy, which is thus observed in the spectrum as a satellite peak with an apparently higher binding energy.

XPS has been used to characterise a variety of samples in this thesis, using the survey scans to determine changes in the elemental composition, and C 1s, S 2p, N 1s, Au 4f and Pd 3d core level spectra to gain information on the specific chemical environments. CasaXPS software was used for the XPS peak fitting, with mixed Gaussian-Lorentzian lineshapes and a Shirley background. The line widths and peak intensities were restricted to ensure that the peak fitting was reasonable, allowing quantitative analysis of the sample composition.

282 284 286 288 290 292 294 Binding Energy / eV C=C / C-C C-O C=O