1 Techniques to be covered Electron microscopy – SEM, TEM and EDAX Vibrational spectroscopy Optical spectroscopy Solid State NMR XRF, XANES, EXAFS XPS and UPS TGA, DTA and DSC Electron microscopy Electron microscopes offer the opportunity to see you sample and to analyze it – Can obtain high resolution images where you can “see” the atoms. TEM – Can get diffraction patterns from very small crystallites. This allows phase identification and indexing – Can obtain low resolution images suitable for examining particle size and morphology issues. SEM – Can obtain X-ray emission spectra and use them for chemical analysis – Can obtain electron energy loss spectra (EELS) and use them for determination of oxidation state etc.
Transcript
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Techniques to be covered� Electron microscopy
– SEM, TEM and EDAX� Vibrational spectroscopy� Optical spectroscopy� Solid State NMR� XRF, XANES, EXAFS� XPS and UPS� TGA, DTA and DSC
Electron microscopy� Electron microscopes offer the opportunity to see
you sample and to analyze it– Can obtain high resolution images where you can “see” the
atoms. TEM– Can get diffraction patterns from very small crystallites.
This allows phase identification and indexing– Can obtain low resolution images suitable for examining
particle size and morphology issues. SEM– Can obtain X-ray emission spectra and use them for
chemical analysis– Can obtain electron energy loss spectra (EELS) and use
them for determination of oxidation state etc.
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Effect of electrons hitting sample
� Backscattered electrons useful for SEM
� X-rays for analysis� Inelastically scattered
electrons for chemical information
� Diffracted and unscattered electrons for imaging
Scanning electron microscopy (SEM)
� Tightly focused electron beam is rastered across the sample
� Low energy secondary electrons scattered from the surface are used to form an image
� Good for imaging objects in 100 nm to several micron range
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Transmission electron microscopy (TEM)
� Used primarily for imaging and selected area electron diffraction– Needs very thin
samples
Selected area electron diffraction� An area of you sample can
be selected for examination in the TEM by using an aperture
� If an single crystallite is selected a diffraction pattern looking like an X-ray precession photograph is recorded– Can be used to identify
phase or help determine unit cell
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HRTEM of Rb0.1WO3
� Intergrowth of two different structures is clearly visible
X-ray analysis
� Bombarding you sample with electrons causes the specimen to fluoresce
� The X-ray emission lines are characteristic of the elements present in the sample
� Can do semiquantitative analysis using intensity of emitted X-rays
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EELS� Some of the electrons
passing through the sample lose energy as they excite atomic transitions for the various elements present in the sample
� Measuring an energy loss spectrum provides information on the elements that are present and in some cases oxidation state
Vibrational spectrocopy� Both IR and Raman spectra of solid can be useful
– However, in addition to getting bands that are characteristic of any “functional groups” that might be present you get low frequency vibrations associated with the collective motion of all the atoms/molecules in the solid
» These are called phonons
� Inelastic neutron scattering is particularly useful for studying phonons and any vibrations involving the movement of hydrogen atoms– Incoherent neutron scattering is very sensitive to hydrogen– Neutron scattering has no selection rules unlike IR and
Raman spectroscopy
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IR spectra of minerals
� Band at 3500 cm-1
characteristic of O-H� Bands 300 – 1500 cm-1
primarily due to oxyanions
� Lattice modes (phonons) < 300 cm-1
Optical spectroscopy�The UV/Vis absorption spectrum of a material
can be informative– Can measure excitonic transitions
» Involving color centers, d-d, f-f. 6s-6p for heavy metals like Pb(II)
– Charge transfer transitions– Transitions across band gap– Promotion of electron from localized orbital to
band
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Possible transitions for an AB solid
i) Excitonic transition
ii) Charge transfer transitions
iii) Localized orbital to band transition
iv) Transition across band gap
Solid State NMR
�NMR spectra of most solids consist of broad featureless humps if no special techniques are used– Broadening primarily caused by dipolar
coupling�High power decoupling can be used to
obtain useful spectra but the lines are still broad
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Chemical shift anisotropy� The chemical shift of a nucleus in a compound is
dependent on its environment and on its orientation in the magnetic field
� If a powder sample is used each crystallite will have a different orientation so broad lines will be observed as each different orientation has a different resonant frequency– This broadening is spans a wider frequency range than
what you get due to differences in chemical environment
Static powder line shapes� Complex line shape
due to chemical shift anisotropy– Can extract
components of chemical shift tensor
� Line shape is characteristic of the nuclear environments symmetry
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Magic angle spinning (MAS)
� Spinning the sample at high speed about the “magic angle” removes effect of chemical shift anisotropy and gives spectra that have separate peaks for distinct chemical environments
Spectrum of PCl5
�PCl5 consists of PCl4+ and PCl6
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Effect of varying spinning speed
� If the spinning speed is not bigger than the static NMR line width spinning side bands will be observed
29Si MAS NMR of aluminosilicates
�29Si NMR provides Si/Al ratio and other information
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Synchrotron radiation and X-ray absorption spectroscopy
� Both EXAFS (Extended X-ray Absorption Fine Structure)and XANES (X-ray Absorption Near Edge Structure) make use of the samples x-ray absorption spectrum– they provide information on the local environment of an atom– they require a tunable source of X-rays
� Synchrotrons provide tunable X-rays– they actually produce almost “white” radiation and any
wavelength can be selected from the “white” distribution
Synchrotron sources
� All charged particles emit EM radiation when they are accelerated
� A synchrotron X-ray source is a particle accelerator that produces radiation by accelerating electrons or positrons– does this by passing particle beam through a
magnetic field
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Producing Synchrotron Radiation
Synchrotron radiation
� High intensity� Plane polarized� Intrinsically collimated � Wide energy range� Has well defined time
structure
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EXAFS and XANES
� The X-ray absorption spectra of compounds show wiggles - EXAFS and XANES � EXAFS are are found in the spectra of all species
more complicated than a monatomic gas
The measurement of X-ray absorption spectra
� Can be done by transmission or fluorescence
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XANES
�XANES features can be though of as either arising from transitions between well defined states or as being due to multiple scattering– Energy of absorption edge and presence of “pre-
edge features” provide information on oxidation state and coordination geometry
XANES for Vanadium oxides
� Near edge features can tell you about site symmetry and edge position tells you about oxidation state
Data from Wong, Lytle, Messmer and Mylotte, Phys. Rev. B 30, 5596 (1984).
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Why is there any EXAFS?
� Outgoing photoelectron wave interferes with itself
What does the fine structure tell you?
� The coordination number of the absorber
– not very accurate
– can infer particle size for small particles
� The bond lengths around the central atom
– +- 0.02Å ?
� The types of atoms surrounding the absorber
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How do you extract the EXAFS from the raw data?
Getting the information out
� The simplest expression is
– χ(k) = (1/kR2) |f(π,k)| sin (2kR + ψ + 2δ)
– this applies to the one neighbor situation
– R provides structural information
– f(π) is different for each element so different elements can be distinguished
– ψ and δ are a complication
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Backscattering amplitudes
How do we analyze the data?
� There are two approaches– real space
– k space
� Fourier transforming χ(k) gives a radial distribution function– but distances are wrong unless correction factor is
applied
� Fit your structural model to χ(k)
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Fourier transformed data
Limitations of the technique
� Need a synchrotron� You must have a useable absorption edge� Sample concentration > 1mM ?
– can be used for solids, liquids or gases
� Limited distance probe� Peaks in RDF often overlap making data analysis
difficult
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Photoelectron spectroscopy� Irradiate sample with monochromatic photons� Electrons are ejected� Measure kinetic energy of ejected electrons� Allows you to calculate how tightly bound the
electrons were in the sample
KE = hν – BE
KE – kinetic energy
BE - binding energy
Types of PES� PES is done with both UV light from He discharge
lamps (UPES) and with soft x-rays typically emitted from Mg or Al targets (XPS)
� UPES has good energy resolution and provides detailed information about which orbitals/bands in the sample are occupied
� Peaks and peak positions in XPS can be used to identify which elements are present and distinguish oxidations states
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Sensitivity of XPS/ESCA to chemistry
XPS for sulfate and thiosulfate
PES spectra of the tungsten bronze Na0.7WO3
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Thermal analysis� Measuring the weight losses occurring on heating a
sample can provide valuable information about decomposition reactions that occur on heating– This is thermogravimetric analysis (TGA)
� Measuring the heat flow out of or into a sample on heating/cooling provides useful information on the phase transitions that the sample is undergoing– This can done by differential thermal analysis (DTA) or
differential scanning calorimetry (DSC)
Monitoring decompositions by TGA
� TGA can be used to follow decomposition processes– However, weight loss
temperatures depend upon temperature ramp rate and other conditions such as particle size
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DTA� The DTA experiment involves measuring the
temperature of your sample and a reference material that are both being heated in the same furnace– When the sample starts to absorb heat or gives out heat a
temperature difference will develop
DSC
�DTA is not a good method for quantitatively determining enthalpy changes
�DSC is much better– Typically your sample and the reference are in
separate furnaces and they are heated up so that the two samples are always at the same temperature
» Difference in heat required by sample and reference allows calculation of enthalpies
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DTA is an efficient method for determining phase diagrams
� As solid-solid and solid-liquid phase transformations involve heat changes, the phase transition temps can be determined by DTA
Irreversible and sluggish transitions � Examining samples on heating and cooling can detect
the presence of transitions that are irreversible by virtue of kinetics or thermodynamics– Also allows examination of hysteresis associated with
sluggish transitions
Transition a is irreversible and transitions b and c show hysteresis On cooling a glass forms
