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
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
Near edge X-ray Absorption Spectroscopy is too Often Neglected Conduction band structure Shifts in the edge
Resources Available Online on XAS XAFS Training Module
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)
Near-edge Information Oxidation State Direct fingerprinting Theory-backed (eg FEFF 8) Reaction pathways Statistical tools LUMO occupancy Polarization alignment to MO
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
Norm absorption [au] Near-edge Information Reaction Pathways 5 0 15 10 05 00 77 77 77 Photon energy [kev]
Near-edge Information LUMO occupancy
Norm absorption [au] In-situ XAS experiment X-ray out Nylon screws 15 Kapton tape Li metal Load 10 05 charging Separator Thin Al foil current collector with cathode powder on top Gasket 00 5 0 15 10 83 83 836 Photon energy [kev] 838 Electrode plates X-ray in Atomic and electronic structure can be measured as a function of the state of charge
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 35 3 0 1 3 5 Capacity (mah)
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 1 8 1 6 1 1 1 0 0 8 0 6 0 0 S h ift in N i K -e d g e w ith d e lith ia tio n 0 0 8330 830 8350 8360 8370 8380 8390 E n e rg y (e V ) Ni is certainly oxidized, but this picture needs a closer look
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 35 3 0 1 3 5 Capacity (mah)
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 1 8 1 s t C h a rg in g N i X A N E S p s ta te s 1 6 1 1 3 7 V 3 9 V 1 V 3 V 1 0 0 8 0 6 0 0 is o s b e s tic p o in t 0 0 8330 830 8350 8360 8370 8380 8390 E n e rg y (e V ) What does the isosbestic point indicate?
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
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 1 8 1 s t C h a rg in g N i X A N E S p s ta te s 1 6 1 1 3 7 V 3 9 V 1 V 3 V 1 0 0 8 0 6 0 0 Non-isosbestic point!! is o s b e s tic p o in t 0 0 8330 830 8350 8360 8370 8380 8390 E n e rg y (e V )
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 1 6 1 1 3 7 V 3 9 V 1 V 3 V 1 0 0 8 0 6 0 0 Nearly isosbestic point!! is o s b e s tic p o in t 0 0 8330 830 8350 8360 8370 8380 8390 E n e r g y ( e V ) Isosbestic point indicates a single reaction Ni is oxidized
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 1 6 1 6 1 1 3 5 V 3 9 V 3 V 5 V 1 1 3 7 3 9 1 3 1 0 1 0 0 8 0 8 0 6 0 6 0 0 0 0 0 0 650 6560 6580 6600 660 0 0 650 6560 6580 6600 660 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
Norm absorption [au] Norm absorption [au] X-ray absorption near-edge structure of Co 5 0 0 15 10 15 05 00 13 ev 10 77 77 77 Photon energy [kev] 7695 77 7705 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]
Norm absorption [au] Principal Component Analysis: Representation of XAS Spectra as an m x n Matrix 5 0 15 10 05 00 77 77 77 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 1 1 3 m 1 n n 3 n mn
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 31 3 3 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
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 1 6 1 b x = 0 1 1 0 8 0 1 0 0 0 0 8 0 6 0 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 = 0 1 1 6 1 0 8 0 0 0 x = 0 3 7 0 1 6 1 x = 0 7 0 0 0 8 0-0 7 6 8 7 6 9 7 7 0 7 7 1 7 7 7 7 3 7 7 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 0 0 7 6 9 7 7 0 7 7 1 7 7 7 7 3 7 7 E n e rg y (K e V ) Q P R P R Q Decreasing Li conc in cathode
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 1 1 10 08 06 0 01 018 05 037 00 07 055 063 070 A B 006 005 00 003 because if the d-band occupancy of Co changes with increased delithiation, then this peak amplitude should increase 0 s to d 00 000 7685 7690 7695 7700 7705 7710 7715 770 Energy (kev) 00 001 7688
FT [ '(k)] Theoretical fit to real-space data 5 x in Li (1-x) CoO = 0 15 10 05 01 018 05 037 00 07 055 063 070 Co-O Co-Co & Co-Li Second cluster First cluster 00 0 1 3 5 6 R (Å)
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 6 0 1 8 1 6 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 1 0 1 0 0 3 0 0 5 0 6 0 7 x in L i ( 1 - x ) C o O
Ex-situ Study of Li Intercalation in Ag V O Particularly useful in medical applications High power delivery Non-toxic
F T (k * (k )) Ag K-edge analysis of Li x Ag V O 0 0 0 A g -A g A g fo il Ag V O Li 0 7 Ag V O 0 0 1 5 Li 1 3 Ag V O Li 5 5 9 Ag V O 0 0 1 0 0 0 0 5 A g -O (S V O ) 0 0 0 0 0 1 3 5 6 R (Å ) FT { } at the silver k-edge of the samples Features of SVO are well resolved from those of metallic Ag
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 1 0 1 0 1 0 0 9 0 8 0 8 A g V O 0 7 L ia g V O 0 8 A g V O 0 7 L ia g V O 0 7 0 6 0 6 1 3 L ia g V O 5 5 9 L ia g V O A g A g V O 0 6 1 3 L ia g V O 5 5 9 L ia g V O A g fo il 0 5 0 0 0 0 0 3 0 0 0 0 0 1 5 6 5 8 5 5 0 5 5 E n e rg y (k e V ) 0 0 0 1 3 5 6 0 0 5 6 5 8 5 5 0 5 5 5 5 5 5 6 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
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 3 1 0 3 80 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 80 90 300 310 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 50 560 580 Photon energy (ev)
Sof X-ray XAS: LUMO occupancy LUMO occupancy Orientation of molecular orbitals
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 0 1 6 0 1 0 1 0 1 0 0 0 8 0 0 6 3 C O o n : P t 0 0 0 0 0 0 0 Grazing angleangle; σ enhanced 1 P t Ru 75 5 P t Ru 50 50 80 90 300 310 30 330 E n e rg y (e V ) 1 Magic-angle; no orientation effects 0 5 0 10 0 3 0 8 0 80 90 300 310 30 330 E n e rg y (e V ) 5 0 5 0 6 0 1 0 0 C O o n : P t 80 90 300 310 30 330 E n e rg y (e V ) 0 P t 75 Ru 5 0 3 P t 50 Ru 50 15 0 0 1 0 Normal angle-angle; π* enhanced 10 5 0 0 C O o n : 80 90 300 310 30 330 E n e rg y (e V ) P t P t Ru 75 5 P t Ru 50 50 80 90 300 310 30 330 E n e rg y (e V ) 0 80 90 300 310 30 330 E n e rg y (e V )
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