Chapter 2: Experimental Techniques
2.2. Characterization techniques
2.2.1. Morphological and structural characterization
Fig. 2.3. Photographs of the p Zirconia balls.
possible since electron wavelengths are much smaller than photon wavelengths and the depth of field is much higher. The electron wavelength depends on the electron velocity or the accelerating voltage as
=
= = .
√ nm (2.1) Thus, for a 30 kV acceleration voltage, the resolution is extremely high.
Fig. 2.4. Photograph of the Field Emission Scanning Electron Microscope (Sigma Zeiss, Germany).
A photograph of the FESEM (Sigma Zeiss, Germany) used in the present study is shown in Fig. 2.4. In the FESEM, electrons are emitted from a field emission source under extreme vacuum. The vacuum allows the electrons movement along a column without scattering which helps to prevent discharges inside the instrument. The field emission source is tungsten filament (cathode) with sharp tip which is placed in a huge electrical potential gradient. The significance of extremely thin and sharp tip (diameter 10-100 nm) is that an electric field can be concentrated to an extreme level so that the work function of the material is lowered and electrons can leave the cathode. After emission, the electrons are accelerated by the two anodes. An accelerating voltage (0.5- 30 kV) between the cathode and anode is commonly used. This voltage combined with beam diameters determines the resolution of the image. As the voltage increases, better point-to-point resolution can be obtained. Because
of the smaller size of the electron source, the beam produced by this emitter is about 1000 times smaller than that in a standard scanning electron microscope (SEM), which markedly improves the image. The beam is collimated by electromagnetic condenser lenses, focused by an objective lens, and scanned across the surface of the sample by electromagnetic deflection coils. The primary imaging method is by collecting emitted secondary electrons that are released by the sample. A secondary electron detector is placed near to the specimen. By correlating the sample scan position with the resulting signal, an image is formed on the screen that is strikingly similar to what would be seen through an optical microscope. The FESEM is equipped with a special objective or focusing lens that projects the magnetic field below the lens. Very high resolution is obtained by shortening the specimen–lens distance and using a specially designed in–lens. The distance is shortened by placing the specimen in the lens magnetic field. In this case, secondary electron detector is placed above the objective in–lens (called as in–lens detector), which makes difference in the image compared to the conventional image of the secondary electron detector. Very high resolution and contrast can be obtained by using in–lens detector. The type of electron source is the main difference between SEM and FESEM. In SEM, electrical current is used to heat up the filament and when the heat is enough to overcome the work function of the filament, electron escape from the material.
For the sample preparation of the FESEM imaging, as-synthesized TiO2 powder was directly mounted on the FESEM stub using a carbon tape. Here the carbon tape is used as adhesive to stick the powder and also provides an electrical conduction path to the sample.
B. Transmission Electron Microscopy (TEM)
TEM is one of the best characterization techniques of nanomaterials in which structural information can be acquired by high resolution imaging close to the atomic level (0.2 nm) as well as by electron diffraction. The high resolution TEM (HRTEM) known as lattice imaging, gives the structural informations and presence of defects or dislocations. The growth orientation and lattice spacing can be studied from the lattice fringe image. The crystallographic information about the nanomaterials such as crystal structure (cubic, tetragonal, hexagonal or monoclinic etc.), crystallinity (single crystalline, poly-crystalline or amorphous) can be drawn from the selected area electron diffraction (SAED) patterns. In
case of a crystalline material, electron diffraction will only occur at specific angles which are the characteristic features for the crystal structures present. Moreover, elemental and chemical composition analysis down to sub-nanometer scale can be acquired with additional detector such x-ray detector which is known as energy dispersive x-ray (EDX) analysis. Fig.
2.5 shows the schematic diagram of a TEM. It works on the principle of optical projection;
when an object is placed in front of a light source, its image is enlarged and shadow is created on the screen placed far distance behind this object. Electrons emitted from an electron gun are accelerated to high voltages (typically 100 to 400 kV) and focused on the sample by a number of condenser lenses.1 We used a lanthanum hexaboride (LaB6) crystal for thermionic electron emission. The emitted electrons pass through a series of lenses to be focused and scanned across the sample. The sample is placed on a small copper grid a few mm (~3 mm) in diameter. The static beam has a diameter of a few microns. The sample must be sufficiently thin (a few tens to a few hundred nm) to be transparent to electrons. The transmitted and forward scattered electrons form a diffraction pattern in the back focal plane and a magnified image in the image plane. With additional lenses, either the image or the diffraction pattern is projected onto a fluorescent screen for viewing or photographic recording.
Fig. 2.5. Schematic diagram of Transmission Electron Microscope.
In our present study, a TEM (JEM2100, JEOL, Japan) operating at 200 kV with high resolution CCD camera (Gatan, USA) is used for the normal TEM and HRTEM imaging.
Sample for the TEM imaging was prepared by well dispersing the nanostructured samples in an ethanol solvent. A transparent dispersion of nanostructures was prepared by long time high frequency sonication and then drop casted on the carbon coated copper grid containing few hundreds of square shaped hollow meshes with dimension of ~1 µm. After normal dry for prolong time this grid is used for TEM imaging. Improved resolution of the lattice image is obtained after processing of fast Fourier transformation (FFT) using 'Digital Micrograph (Gatan, USA)' image analysis software. In addition, we used SAED pattern to get crystallographic information about the TiO2 nanostructures.
C. Powder x-ray diffraction (XRD)
Powder XRD is the most widely used non–destructive technique for general crystalline material characterization. The XRD patterns provide information on crystal phases, lattice parameters, crystallite size and strain in the material. In XRD, a collimated beam of x–rays, with wavelength λ = 0.5–2.0 Å, is incident on a sample and is diffracted by the atoms of the crystalline phases in the sample according to 'Bragg's law',
2 = (2.2)
where d is the spacing between atomic planes in the crystalline phase, θ is the angle of incidence of the x–ray beam with the atomic plane, n represents the order of diffraction (we consider only the first order diffraction, n =1, because the second order peaks are mostly difficult to detect experimentally). The intensity of the diffracted x–rays is measured as a function of the diffraction angle 2θ. In powder samples, the crystallites are oriented in various possible orientations, giving reflections from all possible planes. However, the preferred orientation of the crystallites can occur. In an x-ray diffraction pattern, the position and intensity of the diffraction peaks are characteristic for the crystallographic structure and the atomic composition of the material. In case of a multi-phase composition, the resulting pattern is a combination of the patterns of all structures present. Phase identification can be done by matching the XRD pattern with reference patterns of pure substances. Owing to the huge data bank available from JCPDS Powder Diffraction Files covering practically every phase of every known material, crystal phase of the sample is identified from the peak
positions of the diffractogram. Homogeneous or uniform elastic strain in the (hkl) direction can also be calculated from the shift in the diffraction peak positions, and the dhklspacing of the unstrained crystal. From the peak shapes and width of the diffraction peak, average crystallite size can be calculated.
Fig. 2.6. Photograph of the X-ray Diffractometer ( Rigaku RINT 2500, TTRAX III).
X–ray diffraction patters of the synthesized nanostructure samples were obtained using a commercial XRD (TTRAX III, Rigaku 2500) using a Cu Kα1 (λ = 1.5406 Å) radiation. A photograph of the XRD instrument in our laboratory is shown in Fig. 2.6. All measurements were carried out at an accelerated voltage of 50 kV and tube current of 200 mA. The scanning step size was 0.01°. The exact peak position and full width half maxima (FWHM) of the XRD peak is obtained from the Lorentzian fitting to the experimental data, using following expression
= +
()! # ! (2.3)
Where is the offset constant, $%, & and ' are the peak position, FWHM and area, respectively. FWHM gives the crystallite size (using Scherrer formula2) and lattice strain is calculated from the shift in $% using Williamsons-Hall method.3, 4
2.2.2. Optical characterization