MICROSCOPIC TECHNIQUES
It is known that microscopic techniques such as scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM) are widely employed to characterize the crystal structures,
for example, morphologies as plates, rods, discs, cubes, ellipsoids,
among others.In a usual transmission mode, the
infrared beam passes through the sample, and the transmittance
or absorbance of IR radiation as a function of wavenumber or
wavelength is measured (Ryczkowski 2001).Image contrast can be
achieved by elastic and inelastic scattering through interaction
of the incident primary electron beam with the specimen
and is divided into three classes: absorption contrast (sample
image forming polycrystalline monophasic) (Mannheimer
2002); diffraction contrast (the scattered electrons can be
arranged in the form of spots or rings, according to the
Bragg's Law) (Mannheimer 2002; Canevarolo Jr 2004); phase
contrast (obtaining of crystalline lattice images, range: 1 nm)
(Canevarolo Jr 2004).From these
crystallographic structures, the crystallographic descriptions were
established, such as orthorhombic, hexagonal, cubic, octahedral,
tetrahedral, and the atomic coordinates of polymorphous iron
oxides (Cornell and Schwertmann 2000); (ii) energy dispersive
X-ray (EDX) spectroscopy: the X-rays are emitted during the
interaction with the electron beam, allowing the determination
of chemical elements in the samples.The results are shown as
spectra, which consists of a series of peaks representing the type
and quantity of each chemical element of samples (Cornell and
Schwertmann 2000; Dedavid et al. 2007); (iii) Fourier transform
infrared spectroscopy (FT-IR) has been used as a technique
for the investigation of the characteristic functional groups of
iron oxides and characterization of precursors from different
synthesis methods.As a result of the electron beam interaction with the
sample surface, a series of radiation are emitted: (i) secondary
electrons, which are emitted from the sample atoms with lowenergy (typically 50 eV or less); (ii) backscattered electrons
(the reflection of images with topography contrast); (iii) X-ray
detectors (used for chemical analysis) (Canevarolo Jr 2004).Spectroscopic techniques
provide additional information to identify the chemical composition:
(i) X-ray diffraction can be performed to obtain the crystalline
structure of particles (angle position, width and intensity).