Photon, Electron, and Ion Probes of Polymer Structure and Properties

polymers often contain, besides C, H, and 0, elements of Group VII. (halogens), Group VI (S, ... states has been the fact that photoelectron line ener...
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14 Identification of Chemical States by Spectral Features in X-Ray Photoelectron Spectroscopy C. D . W A G N E R

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W h i l e t h e p r i n c i p a l i n t e r e s t i n t h i s symposium i s t h e a p p l i c a t i o n o f s u r f a c e a n a l y t i c a l t e c h n i q u e s to p o l y m e r i c systems, p o l y m e r s o f t e n c o n t a i n , b e s i d e s C , H , and 0 , e l e m e n t s o f Group V I I ( h a l o g e n s ) , Group V I ( S , S e , T e ) , Group V ( N , P , A s , S b ) , and sometimes Group I V ( S i , G e , S n ) . I n p o l y m e r t e c h n o l o g y t h e s t u d y o f polymers i n heterophase systems i s i n c r e a s i n g l y i m p o r t a n t , w i t h p r o c e s s i n g a d d i t i v e s , f i l l e r s , p i g m e n t s , and i n t e r f a c e s i n c o m p o s i t e s i n v o l v i n g many o t h e r e l e m e n t s . F o r these reasons i t seems a p p r o p r i a t e t o d i s c u s s t h i s s u b j e c t i n q u i t e a g e n e r a l w a y . From t h e e a r l i e s t p r e s e n t a t i o n s o f t h i s t e c h n i q u e ( 1 ) i t was c l e a r t h a t a p r i n c i p a l v a l u e o f t h e method i s t h e i n f o r m a t i o n i t f u r n i s h e s on t h e e n v i r o n m e n t o f t h e e m i t t i n g a t o m , o b t a i n e d by o b s e r v i n g the exact energy o f the p h o t o e l e c t r o n s . Chemical s h i f t s i n t h i s e n e r g y a r e s u b s t a n t i a l (2^) f o r c a r b o n , s u l f u r , and n i t r o g e n . The range i n p h o t o e l e c t r o n e n e r g y f o r t h e s e e l e m e n t s i n o r g a n i c systems i s 7-8 e V . Since line positions after charge r e f e r e n c i n g a r e o r d i n a r i l y d e t e r m i n a b l e w i t h an e r r o r l e s s t h a n 0 . 5 e V , t h e t e c h n i q u e h a s been o f g r e a t v a l u e i n a n a l y s i s o f polymer s u r f a c e s . The p r i n c i p a l limitation i n i d e n t i f i c a t i o n of chemical s t a t e s h a s b e e n t h e f a c t t h a t p h o t o e l e c t r o n l i n e e n e r g y i s a one p a r a m e t e r s y s t e m . M o r e o v e r , c h e m i c a l s h i f t s i n some e l e m e n t s a r e v e r y s m a l l ; f o r example t h o s e o f Z n , A g , C d , I n , and t h e a l k a l i m e t a l s and a l k a l i n e e a r t h s encompass r a n g e s o f l e s s t h a n two e l e c t r o n v o l t s , so t h a t f o r t h e s e e l e m e n t s t h i s t e c h n i q u e h a s l i t t l e use. C l e a r l y t h e r e i s much i n c e n t i v e t o f i n d other s p e c t r a l f e a t u r e s t h a t p r o v i d e i n f o r m a t i o n on c h e m i c a l s t a t e . Shakeup S a t e l l i t e s . A n u n u s u a l f e a t u r e i n many s p e c t r a i s t h e shakeup s a t e l l i t e , a l i n e o r l i n e s u s u a l l y s e v e r a l eV l o w e r i n k i n e t i c energy than the p a r e n t p h o t o e l e c t r o n l i n e . I t a r i s e s when the p h o t o e l e c t r i c t r a n s i t i o n has a s i g n i f i c a n t p r o b a b i l i t y o f g e n e r a t i n g a f i n a l i o n i n an e x c i t e d s t a t e . The e x t r a e n e r g y i n the e x c i t e d s t a t e i s r e f l e c t e d i n the energy s e p a r a t i o n o f t h e s a t e l l i t e from t h e m a i n l i n e . A common e x a m p l e i n o r g a n i c systems 0097-6156/81/0162-0203$05.00/0 © 1981 American Chemical Society Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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i s t h a t due to the TT* TT t r a n s i t i o n . In this instance the e x c i t a t i o n e n e r g y i s i n t h e r a n g e 5 . 5 - 8 . 5 eV (_3); a w e l l - k n o w n example i s t h a t o f p o l y s t y r e n e ( F i g . 1 ) . The s a t e l l i t e is o b s e r v e d when t h e c a r b o n atom i s p a r t o f a p i - b o n d e d s y s t e m , e . g . compounds w i t h benzene r i n g s , ( 4 ) , p y r r o l e , t h i o p h e n e (^5), p y r i d i n e , f u r a n , or even d i o l e f i n i c or m o n o - o l e f i n i c s y s t e m s (6^). The i n t e n s i t y o f the s a t e l l i t e c a n be as much as 15% o f the p a r e n t peak. S i m i l a r s a t e l l i t e s have b e e n o b s e r v e d w i t h a number o f other compounds, s u c h as f o r m a l d e h y d e , c a r b o n s u b o x i d e (_7)> t e t r a c y a n o q u i n o d i m e t h a n e ( 8 ) and m e t a l c a r b o n y l s . The s a t e l l i t e s h a v e been o b s e r v e d w i t h the l i n e s o f the h e t e r o a t o m s as w e l l as w i t h C i s , w i t h s i m i l a r , b u t n o t i d e n t i c a l , e n e r g y s e p a r a t i o n s and intensities. The e n e r g y s e p a r a t i o n and i n t e n s i t y a r e u n i q u e f o r l i n e s from each atom i n e a c h c h e m i c a l s t a t e . Shakeup s a t e l l i t e s a r e o f t e n more i n t e n s e w i t h p a r a m a g n e t i c i n o r g a n i c i o n s , s u c h as t h o s e o f t h e f i r s t t r a n s i t i o n s e r i e s , t h e r a r e e a r t h s , and the a c t i n i d e s . Those o f the f i r s t t r a n s i t i o n s e r i e s c a n be q u i t e c o m p l e x ( F i g . 2 ) , w i t h m u l t i p l e s a t e l l i t e s (multiple excited states) possible. Frost, Ishitani, and M c D o w e l l (9) f o r c o p p e r compounds and M a t i e n z o e t a l (10) and Tolman et a l ( J J J f o r n i c k e l compounds have f u r n i s h e d rather c o m p r e h e n s i v e d a t a on s a t e l l i t e e n e r g i e s and i n t e n s i t i e s . The p a r a m a g n e t i c o c t a h e d r a l and t e t r a h e d r a l n i c k e l c o m p l e x e s e x h i b i t shakeup p a t t e r n s , w h i l e the d i a m a g n e t i c s q u a r e p l a n a r ones do not. W i t h c o p p e r t h e p a r a m a g n e t i c c u p r i c s t a t e s have shakeup s a t e l l i t e s w h i l e t h e d i a m a g n e t i c c u p r o u s s t a t e s do n o t . The r u l e i s n o t i n v a r i a b l e , h o w e v e r , and s a t e l l i t e s a r e sometimes o b s e r v e d w i t h d i a m a g n e t i c s p e c i e s , b u t u s u a l l y i n much l o w e r i n t e n s i t y . W i t h the t r a n s i t i o n m e t a l s t h e e x c i t a t i o n mechanism u s u a l l y i n v o l v e s charge t r a n s f e r w i t h the l i g a n d . B r i s k and B a k e r ( 1 2 ) s u m m a r i z e the d i s c u s s i o n s on mechanism t h a t have a p p e a r e d . R a r e e a r t h i o n s a l s o e x h i b i t c o m p l e x shakeup s a t e l l i t e p a t terns. T h a t o f Ce3d i n CeO^ i s e s p e c i a l l y n o t e w o r t h y i n t h e f a c t t h a t two s a t e l l i t e s f o r e a c h o f Ce3d« and C e 3 d ^ * a p p e a r , one o f them more i n t e n s e t h a n t h e p r i m a r y l i n e and s e p a r a t e d from i t by as much as 16 eV ( F i g . 3 ) . A c t i n i d e compounds a l s o e x h i b i t many s a t e l l i t e s in their spectra. Wide use o f t h e s e f e a t u r e s in a n a l y t i c a l work a w a i t s c o l l e c t i o n o f t h e s e s p e c t r a i n a c o m p r e hensive review. 2

Multiplet Splitting. I n t e r a c t i o n o f a core vacancy r e s u l t i n g from the p h o t o e l e c t r i c p r o c e s s w i t h u n p a i r e d e l e c t r o n s i n a v a l e n c e s h e l l induces m u l t i p l e t s p l i t t i n g i n the l i n e s c o r r e s p o n d i n g to the e m i t t e d e l e c t r o n . T h u s , t h e 3s l e v e l i n t h e t r a n s i t i o n metals e x h i b i t r e l a t i v e l y simple s p l i t t i n g s , often o f s e v e r a l e V , s p e c i f i c f o r each c h e m i c a l s t a t e (_13). The 2p l e v e l s a r e s p l i t i n t o m u l t i p l e l i n e s , and t h e e f f e c t o f t h e i r c o n v o l u t i o n i s to w i d e n t h e a p p a r e n t s p l i t o f t h e d o u b l e t ( 1 4 ) . F r o s t , et a l ( 1 5 ) t a b u l a t e the 2p s p l i t t i n g f o r c o b a l t compounds, w h i c h v a r i e s i n t h e r a n g e 1 4 . 6 t o 1 6 . 1 e V . T h e r e s h o u l d be s i m i l a r v a r i a b i l i t y

Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

WAGNER

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Identification of Chemical States by XPS

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Perkin-Elmer Corporation

Figure 1. Shakeup line for the C-ls line in polystyrene (2)

Perkin-Elmer Corporation

Figure 2. Examples of shakeup lines in the Cu-2p spectrum (2). Energy separation of the 2p lines is 19.8 eV.

Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTON, ELECTRON, AND ION PROBES

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BINDING ENERGY. eV Perkin-Elmer Corporation

Figure 3.

Shakeup spectra in the Ce-3d lines of eerie oxide (2)

*

Figure 4.

9.2eV

Multiplet splitting effects in the Cr-2p doublet

Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

WAGNER

Identification of Chemical States by XPS

207

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f o r N i , F e , Mn, and C r . Chromate i o n and c h r o m i c o x i d e a r e r e a d i l y d i s t i n g u i s h e d by t h i s e f f e c t ( F i g . 4 ) as w e l l as by t h e A l l o f these f e a t u r e s , the b i n d i n g e n e r g y o f the shakeup s a t e l l i t e s and t h e m u l t i p l e t s p l i t t i n g phenomena, h a v e the advantage t h a t c o r r e c t i o n for s t a t i c charge i s u n n e c e s s a r y . C o r e - T y p e Auger L i n e s , and C h e m i c a l S t a t e P l o t s . The u t i l i t y o f Auger l i n e s i n i d e n t i f i c a t i o n o f c h e m i c a l s t a t e s i s b e i n g r e c o g n i z e d . About h a l f o f the n a t u r a l l y - o c c u r r i n g elements p r e s e n t i n s o l i d s t a t e m a t e r i a l s e x h i b i t A u g e r l i n e s i n XPS s p e c t r a w i t h A l or Mg X - r a d i a t i o n ( F i g . 5 ) . O f t h e s e , t w e n t y two e x h i b i t c o r e - t y p e Auger t r a n s i t i o n s ( f i n a l v a c a n c i e s i n c o r e l e v e l s ) and s i x t e e n have v a l e n c e - t y p e A u g e r t r a n s i t i o n s . Coret y p e Auger groups i n the s p e c t r a o f n e i g h b o r i n g elements are v e r y s i m i l a r , d i f f e r i n g o n l y i n m a g n i t u d e o f l i n e s p a c i n g , and h a v e a t l e a s t one l i n e t h a t i s n a r r o w and i n t e n s e . Chemical s h i f t s i n t h e s e l i n e s a r e d i f f e r e n t from and o r d i n a r i l y l a r g e r t h a n t h o s e o f p h o t o e l e c t r o n l i n e s o f t h e same e l e m e n t s (16). When l i n e e n e r g i e s o f t h e most i n t e n s e p h o t o e l e c t r o n l i n e a r e p l o t t e d a g a i n s t t h o s e o f t h e most i n t e n s e or s h a r p A u g e r l i n e f o r compounds o f t h e same e l e m e n t , t h e r e s u l t i n g t w o - d i m e n s i o n a l a r r a y becomes much more useful for i d e n t i f y i n g chemical s t a t e s . An e x a m p l e i s shown i n F i g . 6 for copper. C u p r i c compounds a r e a l l l o c a t e d a t h i g h e r photoelectron binding energies and have identifying shakeup lines. C u p r o u s f o r m s , on t h e o t h e r h a n d , h a v e v i r t u a l l y t h e same p h o t o e l e c t r o n b i n d i n g e n e r g y as e l e m e n t a l c o p p a r , and no s h a k e u p satellite lines. They a r e d i s t i n g u i s h a b l e , h o w e v e r , by t h e A u g e r energy. S i x unknown s a m p l e s c o n t a i n i n g C u , C I , S , and 0 , as w e l l as o t h e r m e t a l s , gave l i n e e n e r g i e s d e n o t e d by t h e c i r c l e s , and i n d i c a t e c o p p e r i s p r o b a b l y p r e s e n t as Cu S, o r p o s s i b l y C u ^ O , b u t c l e a r l y n o t C u C l o r Cu m e t a l . Many o f t h e c h e m i c a l s t a t e p l o t s embodying p r e s e n t k n o w l e d g e o f l i n e e n e r g i e s h a v e b e e n a s s e m b l e d and p u b l i s h e d (17), i n c l u d i n g F , Na, Mg, Cu, Z n , Ge, A s , Se, A g , C d , I n , S n , S b , T e , and I . One f e a t u r e o f t h e s e p l o t s i s o f a d d i t i o n a l i n t e r e s t . The d i a g o n a l g r i d r e p r e s e n t s t h e sum o f t h e A u g e r k i n e t i c e n e r g y and t h e p h o t o e l e c t r o n b i n d i n g e n e r g y , w h i c h we c a l l t h e m o d i f i e d A u g e r P a r a m e t e r (17), : 4x1

f