X-ray Absorption at the Near-edge and Its Applications

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1 X-ray Absorption at the Near-edge and Its Applications Faisal M Alamgir faisal@msegatechedu School of Materials Science and Engineering, Georgia Institute of Technology

2 Cartoon of XAS ln(i 0 /I t ) or I f /I 0 measured by ionization chambers in XAS I 0 at of X-rays (synchrotron) Incident Photon

3 Near edge X-ray Absorption Spectroscopy is too Often Neglected Conduction band structure Shifts in the edge

4 Resources Available Online on XAS XAFS Training Module

5 Structure from Each Constituent Element of a Glass System Nature, Vol 39 6 January 006, p 19-5 Atomic specificity Local scattering Dual information: a) the softness of neighboring atoms (XANES) and b) their position (EXAFS)

6 Near-edge Information Oxidation State Direct fingerprinting Theory-backed (eg FEFF 8) Reaction pathways Statistical tools LUMO occupancy Polarization alignment to MO

7 How Complicated is an XAS Experiment? Hutch I 0 I t I r Window Focusing mirror(s) Double crystal monochromator (mono) (Entrance/Exit) beam defining aperture Piezoelectric crystal Harmonic rejection mirror Ionization chamber Fluorescence detector Sample stage

8 Norm absorption [au] Near-edge Information Reaction Pathways Photon energy [kev]

9 Near-edge Information LUMO occupancy

10 Norm absorption [au] In-situ XAS experiment X-ray out Nylon screws 15 Kapton tape Li metal Load charging Separator Thin Al foil current collector with cathode powder on top Gasket Photon energy [kev] 838 Electrode plates X-ray in Atomic and electronic structure can be measured as a function of the state of charge

11 C e ll P o te n ti a l (V ) Time-Resolution of In-situ XAS Experiment Li[LiNiMn]O Cathode vs Li First Charges: 033 ma to 8 V 5 second charge to 8 V 5 First charge to 8 V Capacity (mah)

12 N o rm a liz e d A b s o rp tio n XAS of Ni K-edge from 37 V - 8 V 1 s t C h a rg in g N i X A N E S p s ta te s S h ift in N i K -e d g e w ith d e lith ia tio n E n e rg y (e V ) Ni is certainly oxidized, but this picture needs a closer look

13 C e ll P o te n ti a l (V ) In-situ XAS Experiment: Narrower Focus on a Single Plateau Li[LiNiMn]O Cathode vs Li First Charges: 033 ma to 8 V 5 second charge to 8 V 5 First charge to 8 V Capacity (mah)

14 N o rm a liz e d A b s o rp tio n XAS of Ni K-edge from 37 V - 3 V: 1 st plateau s t C h a rg in g N i X A N E S p s ta te s V 3 9 V 1 V 3 V is o s b e s tic p o in t E n e rg y (e V ) What does the isosbestic point indicate?

15 Isosbestic Points If the reactants and the products have equal light absorbtion coefficient at a specific energy (ie αa = αb = α), and the analytical concentration remains constant, then we have an isosbestic point For the reaction: A B, where one mol of reactants produces one mol of products, the analytical concentration is the same at any point in the reaction: c A + c B = c The absorption coefficient is: μ = p (α A c A + α B c B ) = p α (ca+ cb )= p α c Hence, the absorption coefficient at that point remains constant

16 N o rm a liz e d A b s o rp tio n XAS of Ni K-edge from 37 V - 3 V: 1 st plateau s t C h a rg in g N i X A N E S p s ta te s V 3 9 V 1 V 3 V Non-isosbestic point!! is o s b e s tic p o in t E n e rg y (e V )

17 N o r m a liz e d A b s o r p tio n XAS of Ni K-edge from 37 V - 3 V during nd charge 1 8 n d C h a rg in g N i X A N E S p s ta te s V 3 9 V 1 V 3 V Nearly isosbestic point!! is o s b e s tic p o in t E n e r g y ( e V ) Isosbestic point indicates a single reaction Ni is oxidized

18 N o rm a liz e d A b s o rp tio n N o rm a liz e d A b s o rp tio n XAS of Mn K-edge from 35 V - 5 V: 1 st & nd charge 1 s t C h a rg in g M n X A N E S p s ta te s n d C h a rg in g M n X A N E S p s ta te s V 3 9 V 3 V 5 V E n e rg y (e V ) E n e rg y (e V ) Mn gets reduced in the 1 st charge; -ve shift in binding energy! In the nd cycle Mn participates in the charge compensation process

19 Norm absorption [au] Norm absorption [au] X-ray absorption near-edge structure of Co ev Photon energy [kev] The white line of the near-edge undergoes a shift of ~13 ev Photon energy [kev] similar shift for a Li x Ni 08 Co 0 O cell has been reported [Johnson and Kropf]

20 Norm absorption [au] Principal Component Analysis: Representation of XAS Spectra as an m x n Matrix Photon energy [kev] An XAS spectrum can be represented as a series of coordinates An XAS data set can be expressed as an m x n matrix 1 31 m m 1 n n 3 n mn

21 PCA Formulation and Interpretation in XAS 1 1 n a a 1 a 1 n 1 n b 1 b b n v 0 0 w w 1 w 1 n n c 31 c 3 c 3 n 0 v 0 w 1 w w n v ij 0 0 v nn w n 1 w n w nn m 1 m mn m m 1 m m m mn [A] = [B] [v] [w] t Data Components Eigenvalues weighting Purely statistical Provides an objective view of chemical transformation during cycling Useful as a preliminary analysis prior to invoking physical knowledge

22 N o rm a liz e d a b s o rp tio n N o rm a liz e d a b s o rp tio n PCA: Components and Reconstruction 1 6 ******** e x p e rim e n ta l re c o n s tru c te d u s in g p rin c ip a l c o m p o n e n ts 1 a b x = p rin c ip a l c o m p o n e n t 1 p rin c ip a l c o m p o n e n t p rin c ip a l c o m p o n e n t 3 C o X A N E S a t x = x = x = E n e rg y (K e V ) Three principal components found Sufficient to reconstruct data at various stages of delithiation Two possible reaction pathways E n e rg y (K e V ) Q P R P R Q Decreasing Li conc in cathode

23 Normalized Absorption The So-Called Pre-Edge: Co K-edge in Li (1-x) CoO 16 C x in Li (1-x) CoO = The Co K-edge XANES in Li (1-x) CoO as a function of x The s to d transition and s to p transition (A, B and C) are labeled Co does appears to be involved in charge recompensation A B because if the d-band occupancy of Co changes with increased delithiation, then this peak amplitude should increase 0 s to d Energy (kev)

24 FT [ '(k)] Theoretical fit to real-space data 5 x in Li (1-x) CoO = Co-O Co-Co & Co-Li Second cluster First cluster R (Å)

25 N e a re s t n e ig h b o r d is ta n c e s o f C o ( Å ) Theoretical fit to real-space data C o n e ig h b o r O x y n e ig h b o r 1 O x y n e ig b o r The atomic structure around Co shows charge compensation occurs first by formation of O holes (for x<05) XANES shows us that for x>05, charge compensation occurs through Co d-hole formation x in L i ( 1 - x ) C o O

26 Ex-situ Study of Li Intercalation in Ag V O Particularly useful in medical applications High power delivery Non-toxic

27 F T (k * (k )) Ag K-edge analysis of Li x Ag V O A g -A g A g fo il Ag V O Li 0 7 Ag V O Li 1 3 Ag V O Li Ag V O A g -O (S V O ) R (Å ) FT { } at the silver k-edge of the samples Features of SVO are well resolved from those of metallic Ag

28 N o rm a l A b s o rb tio n (a u ) P e rc e n ta g e N o rm a liz e d a b s o rp tio n XANES Using SVO at Ag and V K-edges A g V O 0 7 L ia g V O 0 8 A g V O 0 7 L ia g V O L ia g V O L ia g V O A g A g V O L ia g V O L ia g V O A g fo il E n e rg y (k e V ) E n e rg y (K e V ) A m o u n t o f L ith iu m (m o l) The XANES spectra at the V K-edge, unlike the case of Ag K- Linear edge Combination XANES, show analysis no isosbestic of Ag K-edge points XANES Arrows corroboration indicate points of where phase the segregation plots fail to from intersect Ag XANES isosbestically

29 X-ray Absorption X-ray Absorption Soft X-ray XAS: Low-Z Atom-Specific Information a Schematic of the Origin of the NEXAFS Signal b Molecular Orientation: NEXAFS Polarization Anisotropy Auger Electron (top 5 nm) Fluorescent Photon (top 00 nm) Continuum States 1s molecular orbitals h polarized r E r E H - C - H - C - H- C-H H- C-H H- C-H H- C-H H - C - H - C - Net C-C Vector C - H π* C - H π* 90 C C 300 Incident Photon Energy (ev) C C r E r E E parallel to sample 310 H H H H Net C-H Vector 1 0 E perpendicular to sample Incident Photon Energy (ev) c SEM Cross section of a Thin-film Li ion Battery d NEXAFS Study of Chemical Bonding at the interface between Thin-film LiCoOCathode and the Electrolyte 1 μm Photon energy (ev)

30 Sof X-ray XAS: LUMO occupancy LUMO occupancy Orientation of molecular orbitals

31 N o rm a liz e d A b s o rp tio n P artial E lectron Y ield N o rm a liz e d A b s o rp tio n P artial E lectron Y ield N o rm a liz e d A b s o rp tio n P artial E lectron Y ield Sof X-ray XAS: LUMO occupancy C O o n : P t Grazing angleangle; σ enhanced 1 P t Ru 75 5 P t Ru E n e rg y (e V ) 1 Magic-angle; no orientation effects E n e rg y (e V ) C O o n : P t E n e rg y (e V ) 0 P t 75 Ru P t 50 Ru Normal angle-angle; π* enhanced C O o n : E n e rg y (e V ) P t P t Ru 75 5 P t Ru E n e rg y (e V ) E n e rg y (e V )

32 Concluding Remarks Consider using XANES/NEXAFS in glass research You can obtain oxidation state information molecular orbital information time-resolved information Lends itself to statistical analysis tools like PCA and LC