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2.3 X-Ray Methods

2.3.1 X-ray Powder Diffraction (XRD)  

X-ray diffraction (XRD) is one of the primary techniques used by mineralogists and solid state chemists for the characterization of crystalline solids and determination of their structure.  About 95% of all solid materials can be described as crystalline and when X-rays interact with a crystalline phase, a diffraction pattern is generated as a result of the interaction between the incident X-rays and the atomic architecture of the solid.

X-Ray Diffraction (XRD) methodology identifies the structure through the crystallographic phases of analysed material. Also it can be used to estimate the crystallinity degree and/or the purity of a phase. XRD data can also provide information about chemical composition. The development of this technique has allowed understanding of the structural configuration of adsorbent.

Each crystalline solid has unique atomic architecture and consequently has a unique characteristic X-ray powder pattern.  These patterns can be used as ‘fingerprints’ for identification of solid phases.  Once the material has been identified, X-ray crystallography may be used to determine its structure, i.e. how the atoms pack together in the crystalline state and the size and the shape of the unit cell. When X-rays interact with atoms in two lattice planes, and the path length difference between rays equals a whole multiple of the wavelength of the radiation, constructive interference occurs.   Bragg’s law describes the conditions for constructive interference in certain directions and the production of diffracted scattered X-rays:

                                           nλ = 2d sinθ                                                

Where        n = an integer,

λ = the wavelength of the X-rays

d = the spacing between 2 layers

θ = the angle between the incoming    X-ray and the atom layer.

Figure 2.6  An X-ray diffraction beam schematic representation showing the incident and scattered X-rays, from a pair of atoms in different lattice planes.

Figure 2.7 Schematic representation of a X-Ray Diffractometer

The X-ray source is on the left, the sample is in the middle and the detector is on the right. This is the basic relationship among the spacing between the lattice planes (d spacing), the wavelength and the angle (θ) of observation in a diffraction experiment. The angle between the incident and diffracted beams is 2θ degrees.

Figure 2.8  Photograph of the X-ray powder diffractometer

X-ray diffraction based methods are classifed into two as follows:

“SXRD Single crystal X-ray diffraction (SXRD) is a non-destructive analytical technique which provides detailed information to facilitate the determination of the structure of a material. The information collected includes: crystal symmetry, unit cell dimensions, details of site-ordering, atomic positions and space group.

PXRD Powder X-ray diffraction (PXRD) is a rapid analytical technique used for characterisation of crystalline materials. The sample is typically a powdered (polycrystalline) material which is composed of many small crystallites, thus it is possible that more than one crystal can satisfy Bragg’s law. Hence from a powder, the diffracted beams are cones of electron density reflected from the Miller planes, which resemble rings on the screen. These rings can be indexed and integrated to obtain a powder pattern. This technique is primarily used for phase identification as the powder pattern can be seen as a finger print of a specific structure.”

Figure 2.9 Different aspects of pattern diffraction that provide information about analysed material

2.3.2 X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is a commonly used surface sensitive chemical analysis technique that operates under ultra-high-vacuum.  When soft Xrays irradiate a sample surface, electrons will be ejected from valence and core levels of both surface and near surface atoms.  

When adsorbent samples to determine is being exposed to X-ray with enough energy, an inner-shell electron will be bombarded from the sample atoms, producing an ion in an excited state and XPS can detect the kinetic energy of the bombarded electron.  

Figure 2.10 The mechanism of photoelectron emission in X-ray Photoelectron Spectroscopy

The kinetic energies of ejected photoelectrons are not only characteristic of the atoms from which they are emitted, but can also provide information on the chemical states of those atoms. So the XPS can be used to qualitatively analyze samples. In a given experimental condition, number of emission electron is generally proportional to the concentration of emitters, therefore the XPS can also quantify the samples. The depth of samples studied in electron spectrum is less than 5 nm, eance XPS is a surface analysis method. Specific binding energy of each electron corresponds to a Gaussian peak, representing a type of functional group.

It should be noted that in measuring functional groups, the content percent (%) of other elements of functional groups to determine can be identified by XPS if the elements (such as C or O) have been given. The distribution of C and O structures can be derived from spectra. XPS can monitor oxygen-containing groups, but inaccurately quantify the groups.  The XPS technique analyses only the outer 1-10 nm of a sample because emitted photoelectrons lose kinetic energy as they travel through the sample.  Only photoelectrons generated in the outermost layers of the sample have a short enough escape path to reach the detector.