X-ray Absorption Spectroscopy Eric Peterson 9/2/2010

Transcription

1 X-ray Absorption Spectroscopy Eric Peterson 9/2/2010 Outline Generation/Absorption of X-rays History Synchrotron Light Sources Data reduction/analysis Examples Crystallite size from Coordination Number Linear combination analysis Lytle, F. W. (1999). "The EXAFS family tree: a personal history of the development of extended X-ray absorption fine structure." Journal of Synchrotron Radiation 6: Vonbordwehr, R. S. (1989). "A History of X-Ray Absorption Fine-Structure." Annales De Physique 14(4):

2 Extended X-ray Absorption Fine Structure (EXAFS) X-ray Absorption Near-Edge Structure (XANES ) X-ray Absorption Spectroscopy (XAS) EXAFS + XANES (XAFS) XANES XANES region No quantitative theory as of yet EXAFS region EXAFS region Can analyze quantitatively

3 William Conrad Roentgen Discovery of X-rays November 8, 1895 Mrs. Roentgen s hand-an early X-ray absorption experiment

4 Generation of X-rays Interaction of high energy electrons with the anode creates X-rays Any time that you combine high voltage with a vacuum, a significant amount of X-rays can be produced!

5 Generation of X-rays Bremsstrahlung Characteristic X-rays In reality there is a significant overlap of gamma and X-ray Energies (frequencies). Better to think of gamma rays being associated with transitions in the nucleus and X-rays associated with electronic transitions.

6 intensity A typical X-ray spectrum K Characteristic X-rays K Bremsstrahlung

7 Absorption of X-rays Beer-Lambert Law I I e 0 t In practice I e t I 0 Symbol SI unit Attenuation coefficient m -1 density g/m 3 Mass attenuation coefficient (what s usually tabulated) m 2 /g Incident x-ray intensity Transmitted x-ray intensity

8 Absorption contrast leads to image contrast absorption 1000 calcium (cm 2 /g) 100 carbon energy (KeV)

9 1900 Max Planck-Planck Postulate (1918 Nobel Prize Physics) E h hc 1905 Albert Einstein-Photoelectric Effect (1921 Nobel Prize Physics) Concerning an Heuristic Point of View Toward the Emission and Transformation of Light. Annalen der Physik 17 (1905):

10 absorption Incident X-rays Absorbing foil Change metal to change Cu K K, L series originally B,A series 1909 Charles Barkla-Systematic study of X-ray emission and absorption Plotted absorption of various Metals (y) vs. absorption in aluminum (x) E,

11 William Laurence Bragg and William Henry Bragg -Diffraction of X-rays by a crystal (1915 Nobel Prize Physics) n 2d sin

12 1913 Maurice de Broglie - The first x-ray spectrometer (rotating crystal) Use Bragg s Law to select from a white x-ray source (ie. A range of s) by varying

13 1913 Niels Bohr-Structure of the atom (1922 Nobel Prize Physics) 1913 Henry Mosley-Systematic relationship between characteristic x-ray frequencies in terms of Bohr atom.

14 Experimental capability as of ~1913

15 Bohr Atom Theoretical picture as of ~1913 Generate Characteristic X-rays

16 Edge represents the energy needed to transport an electron from the K shell into the continuum fluorescence Characteristic X-ray energy represents the energy lost by an electron falling from an outer shell to an inner shell Cu K Cu K edge Cu K 4 absorption Energy (KeV) 0.4

17 An aside Can use a Nickel foil to filter Cu K radiation (common practice in X-ray diffraction) Cu K Ni K edge Cu K edge 4 fluorescence absorption Energy (KeV) Cu K 0.4

18 chromium K-edges phosphorus 1918 Hugo Fricke- first description of absorption edge fine structure Reversed

19 1920 J Bergengren-absorption edge shifts with chemical valance in phosphorus 1920 Louis de Broglie-electron wave-particle duality (1929 Nobel Prize Physics) 1921 Erwin Schrödinger - Quantum mechanics (1933 Nobel Prize Physics)

20 Experimental observations regarding X-ray absorption edge fine structure circa 1930: What we know now: (i-iii) EXAFS measures something about the local structure surrounding the absorbing atom (iv) E-space to k-space transformation (v) A result of increasing Debye-Waller factors (atomic vibration) (vi) Thermal expansion in reciprocal (k) space

21 1931 Ralph Kronig-Modern X-ray absorption spectroscopy (Kronig structure) 1933 Hendrick Petersen (Kronig s Ph.D. student) - the EXAFS equation

22 1971 Sayers, Stern, and Lyttle- Fourier transform of the EXAFS equation to give a radial structure function

23 What s happening at the absorption edge:

24 EXAFS and XANES - sensitive probes of the chemical environment of the absorbing atom -XANES is especially sensitive to valence and coordination geometry -does not require a crystalline (or even solid) sample As of 1971 we could potentially do this: (E)) (r) (Å -3 ) E(eV) Something measured r (Å) Something ~real Still need more intense X-ray source though. Increased Energy (fine structure) resolution decreasing beam intensity increasing data collection time

25 Rotating crystal spectrometers only pass a small fraction of the bremsstrahlung Need a more intense source of bremsstrahlung X-rays Sealed source- Limited by heat generated by e - striking the anode Rotating anode 1947 Discovery of synchrotron light

26 Brookhaven National Synchrotron Light Source (NSLS) NSLSII Synchrotron light sources started becoming accessible ~1970 s

27 First Generation: Parasitic operation and storage rings Second Generation: Dedicated sources Third Generation: Optimized for brightness Fourth Generation: On the drawing boards Beam conditioning Monochromatic X-rays to sample Double crystal monochromator (n =2dsin ) White x-rays From ring Beamline X23A2 specifications: Energy Range kev Mono Crystal Resolution (ΔE/E) Flux Si(311) 2 x 10-4 monochromator bandpass ph/sec 10 kev, 100mA, 2.5 GeV) Spot Size (mm) 25H x 1.0V Total Angular Acceptance (mrad) 4

28 Experiment Hutch Ring Monochromator Beam Beam

29 sample I t I 0 Beam

30 X-RAY STORAGE RING PARAMETERS AS OF JULY 2009 Stored Electron Beam Energy 2.80 GeV Maximum Operating Current 300 ma Lifetime ~20 hours Circumference meters Beam Current (ma) :00:00 4:48:00 9:36:00 14:24:00 19:12:00 0:00:00 4:48:00 time

31 Analysis of XAS data

32 Analysis of XAS data

33 Analysis of XAS data XANES XANES No No quantitative quantitative theory theory as of yet as of yet Want to extract the wiggles EXAFS region EXAFS region Can analyze quantitatively Can analyze quantitatively (Thanks to Sayers, Stern, and Lyttle) XAS X-ray Absorption Spectroscopy XANES - X-ray Absorption Near Edge Spectroscopy EXAFS - Extended X-ray Absorption Fine Structure XAFS X-ray Absorption Fine Structure (XANES + EXAFS)

34 Extracting the EXAFS signal Post-edge line Pre-edge line

35 Extracting the EXAFS signal Background function

36 Extracting the EXAFS signal Normalized absorption edge

37 Extracting the EXAFS signal E(eV) k(å -1 )

38 Fourier transform window

39 Fourier transform of (k) (r) (Å -3 ) Absorbing atom (Pd in this case) Is at zero Å O Pd To a first approximation, the peaks correspond to nearest neighbor shells Peak positions are shifted about 0.5 Å smaller than the true shell radii. For example the true Pd-Pd 1 st shell distance is 2.8 Å r (Å) Data from Pd on alumina, heated 300 C 2 hours in H 2 /N 2 Sample is a mix of Pd metal and Pd oxide

40 Can fit the data in r-space -try different models -refine adjustable parameters ( r, C.N., Debye-Waller, etc.) (r) (Å -3 ) including PdO gives better fit r (Å) data Fit (Pd metal + PdO) Fit (Pd metal only)

41 EXAFS can measure average atomic coordination numbers (C.N.) C.N.=4 C.N.=6 C.N.=12 d nominal =0.8 nm C. N. average

42 Small particles have a large fraction of atoms on the surface (under-coordinated) relative to those in the bulk (C.N. 12 for F.C.C) R=particle radius r=nearest neighbor distance For spherical particles C.N diameter (nm)

43 C.N. /size correspondence is reasonable even for non-spherical particles C.N.=4 C.N.=6 d nominal =0.8 nm C.N.=12 d calc (5.7)=0.74 nm C. N. average

44 Pd particle size as a function of reduction temperature size nm reduction temperature C v s n XRD TEM EXAFS

45 XANES analysis using linear combinations of reference patterns XANES region is sensitive to valance, coordination 1.2 PdO on -alumina Palladium metal foil x 10 4

46 Linear combinations of reference patterns PdO sample Pd

47 Linear combinations of reference patterns Pd on -alumina Pd PdO

48 Linear combinations of reference patterns PdZn on -alumina PdO PdZn Pd

49 weighting factor PdO Pd PdZn PdO and Pd were not seen in XRD, but are apparent in XAS reduction temperature (K) Phase analysis mirrors catalytic behavior

50 Conclusions EXAFS provides a view of local structure/chemical environment of the absorbing atom EXAFS analysis can provide quantitative information regarding nearest neighbors, coordination number, atomic distances, and Debye-Waller factor/disorder Samples can be crystalline, amorphous, solid, liquid, or even gas Through linear combination analysis, XANES can also provide quantitative information, especially regarding absorbing atom valence and coordination environment Useful Software (free except for FEFF8 and FEFF9) Athena data reduction, linear combination analysis Artemis model refinement FEFF(6-9 ) absorption spectra simulation