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ATOM-PROBE STUDY OF POLYPYRROLE : DISTRIBUTION OF DOPANTS AND OXYGEN
H. Kato, S. Uchikoga, O. Nishikawa
To cite this version:
H. Kato, S. Uchikoga, O. Nishikawa. ATOM-PROBE STUDY OF POLYPYRROLE : DISTRIBU-
TION OF DOPANTS AND OXYGEN. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-429-C7-
434. �10.1051/jphyscol:1986772�. �jpa-00225967�
ATOM-PROBE STUDY OF POLYPYRROLE
:DISTRIBUTION OF DOPANTS AND OXYGEN
H.KATO,
S.UCHIKOGA and
0.NISHIKAWA
Department of Materials Science and Engineering, The Graduate School at Nagatsuta, Tokyo Institute o f Technology. 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan
Abstract - The atom-probe analysis of BFq--doped polypyrrole(PP) revealed that the fresh PP layer polymerized electrochemically on a Pt tip has two regions: B-rich region and B-free region at the initial stage of oxidation. The thickness of these regions are found to be a few tens angstroms. The observed non-uniform B distribution can be attributed to the intrinsic profile of the fresh PP layer. The exclusion of boron from the fully oxidized PP layer may indicate the repulsive B-0 interaction.
I
- INTRODUCTION
Crystallization of most electrochemically polymerized conductive polymers is known to be low/l/ and the polymers are susceptible to oxidation/2/. Since the physical properties of the polymers depends on the structures and compositions, extensive studies have been conducted to clarify structmesby electron diffraction/l/, the dopant distribution by optical absorption/3/ and oxidation processes by measuring electric conductivity/3,4/
a n d e x a m i n i n g o p t i c a l a b s o r p t i o n / 4 /and ESR/5,6/. However, no microscopic study had been introduced until the high mass resblution atom-probe(A-P)/7/ successfully demonstrated its unique capability of clarifying the oxidation processes of C104-- doped polypyrrole(PP)"with the atomically microscopic point of view/8/.
Unexpectedly, the distribution of doped chlorine could not be investi- gated because no C1 ions were detected, possibly due to its high electronegativity and vapor pressure. Accordingly, the BF4--doped PP layers were prepared to clarify the dopant distribution for the present study. Non-uniform B distribution in the fresh PP layer and the repulsive B-0 interaction will be presented.
I1
- EXPERIMENTAL
The high mass resolution A-P/7/ with the background pressure of 3 x 1 0
-8Pa was used. The specimen, a thin PP layer, was prepared by electro- chemically polymerizing pyrrole monomers/2/ on a Pt needle with the tip radius of less than 1000 A. The electrolyte to dope BFq- is acetonitrile containing 0.1
Mpyrrole monomer and 0.1 M (Et)qNBFq.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986772
C7-430 JOURNAL DE PHYSIQUE
The t i p t e m p e r a t u r e was m a i n t a i n e d a t o r b e l o w 1 0 0 K d u r i n g A-P a n a l y s i s .
I11
-
BORON DISTRIBUTIONF i g u r e s 1 ( a ) , ( b ) and ( c ) show t h e i d e a l a n d f u l l y o x i d i z e d PP s t r u c t u r e s and t h e s t r u c t u r e a t t h e i n i t i a l s t a g e o f o x i d a t i o n , r e s p e c t i v e l y . The A-P a n a l y s i s o f a f u l l y o x i d i z e d ClOq--doped PP l a y e r r e v e a l e d t h a t t h e i n n e r s e c t i o n o f t h e PP l a y e r r e m a i n e d un- o x i d i z e d , i n d i c a t i n g t h e n o n - u n i f o r m o x i d a t i o n o f t h e PP l a y e r / 8 / . F i g u r e 2 is t h e m a s s s p e c t r u m o f a f r e s h PPBF4. The s p e c t r u m shows t h e f r a g m e n t s o f t h e p o l y m e r b a c k b o n e s u c h a s CHI CN, CNH a n d t h e f r a g m e n t s i n d i c a t i n g t h e i n i t i a l s t a g e o f o x i d a t i o n s u c h a s CO a n d COH.
E x c e p t f o r t h e d e t e c t i o n o f B i o n s , t h e s p e c t r u m i s q u i t e s i m i l a r t o t h a t o b t a i n e d f r o m t h e f r e s h PPC104/8/. T h e r e f o r e , t h e d e t e c t i o n o f CO a n d COH i o n s f r o m t h e f r e s h PPBF4 l a y e r e l i m i n a t e s t h e p o s s i b i l i t y o f o x i d a t i o n o f t h e f r e s h PP l a y e r by t h e r e l e a s e d o x y g e n o f d i s s o c i - a t e d ClO4- a n d i n d i c a t e s t h a t o x i d a t i o n t a k e s p l a c e w h i l e t h e PP l a y e r i s t r a n s f e r r e d f r o m t h e p r e p a r a t i o n chamber t o t h e A-PI e x p o s i n g t h e s p e c i m e n t o a i r f o r 3 h r s . The d e t e c t e d number o f B a n d C a t o m s i n d i c a t e s t h a t B c o n c e n t r a t i o n i n t h e PP l a y e r i s 3-4 p y r r o l e s p e r b o r o n .
I n o r d e r t o e x a m i n e t h e d o p a n t d i s t r i b u t i o n , f r a g m e n t i o n s w e r e p l o t t e d i n t h e o r d e r o f d e t e c t i o n . The a b s c i s s a o f F i g . 3 i s t h e number o f a l l f r a g m e n t i o n s d e t e c t e d a n d t h e o r d i n a t e i s t h e number o f t h e s p e c i f i e d f r a g m e n t i o n s . I f t h e d o p e d b o r o n i s u n i f o r m l y d i s t r i b u t e d i n t h e PP l a y e r , t h e number o f B i o n s d e t e c t e d i n c r e a s e s l i n e a r l y w i t h t h e t o t a l number o f d e t e c t e d i o n s . The i n t e r m i t t e d d e t e c t i o n s e q u e n c e o f B i o n s i n d i c a t e s t h e n o n - u n i f o r m B d i s t r i b u t i o n . The o b s e r v e d r e s u l t a g r e e s w i t h t h e c o n c l u s i o n d e d u c e d b y t h e d a t a o b t a i n e d by o p t i c a l a b s o r p t i o n 3 F i g u r e 3 a l s o i n d i c a t e s t h a t m o s t CO a n d COH i o n s a r e d e t e c t e d w h i l e n o B i o n s a r e f o u n d . The d e t e c t i o n s e q u e n c e o f B , C a n d CH a r e p l o t t e d , F i g . 4. T h e s e i o n s a r e d e t e c t e d f r o m t h e same r e g i o n o f t h e PP l a y e r . C o n t r a r y t o t h e C and CH i o n s , CN a n d CNH i o n s a r e d e t e c t e d f r o m t h e low B c o n c e n t r a t i o n r e g i o n s w h e r e CO a n d COH i o n s a r e d e t e c t - e d , F i g . 4 and 5.
+ - + -
F i g . 1 - The s t r u c t u r e s o f PP C104 a n d PP BF4
.
( a ) I d e a l s t r u c t u r e . ( b ) F u l l y o x i d i z e d s t r u c t u r e . ( c ) S t r u c t u r e a t t h e i n i t i a l s t a g e o f o x i d a t i o n . D o t s r e p r e s e n t u n p a i r e d e l e c t r o n s .Fig.
2- Mass spectrum of PPBF4. The specimen was exposed to air for
3hrs while it was transferred from the preparation chamber to the A-P.
The observed result suggests that the PP layer has two regions, B-rich region and B-free region possibly at the initial stage of oxidation because the detected CO, COH, CN and CNH ions are the designated fragments in Fig.
1(c). Although the non-uniform dopant distribution in the fresh PP layer was suggested by optical absorption/3/, the present study clarified that these regions are ultramicroscopically small and few tens angstrom thick, assuming the advancement of the analyzing section of 1 A by the detection of 10 ions/8/. The observed random distribution of B-rich and B-free regions raises an inevitable problem to understand the conduction of the polymers.
IV - REPULSIVE INTERACTION OF BORON AND OXYGEN
It is known that non-doped PP layer is susceptible to oxidation/3,9/
and the observed B-rich regions are oxygen-free. However, no B ions were found in the fully oxidized PP layer and the fragment ions such as CO and CNO were detected, Fig. 6. After the removal of the oxidized layer by field evaporation, many B ions were detected successively.
Existence of the high B concentration region under the fully oxidized layer can be explained as the result of the oxygen-induced B diffusion.
Under the high B concentration region a fresh PP region appeared and
C and CH ions were detected. When the A-P analysis reached the PP-Pt
interface, many ions with the masses corresponding to the pyrrole
dimers(PY) were detected, Fig.
6.C7-432 JOURNAL
DE
PHYSIQUETotal number of ions 50
40
V) c
-
030- n
t
$
20-lo
0
F i g . 3
-
D e t e c t i o n s e q u e n c e o f B , CO a n d COH i o n s o f F i g . 1.-
Bj
-...
COH .- - - - - -
- - . . A
-
8 - - T - -
. . - . -
I I I I I
,
I I100 200 300 400 500 600 700 800
Total number of ions
F i g . 4
-
D e t e c t i o n s e q u e n c e o f B , C a n d CH i o n s o f F i g . 1. T h e s e i o n s w e r e d e t e c t e d f r o m t h e B - r i c h r e g i o n .Fig. 5 - Detection sequence of B, CN and CNH ions of Fig. 1.
Total number of Ions
Fig. 6
-
Detection sequence of B, CO and PY ions from a heavily oxidized specimen.C7-434 JOURNAL DE PHYSIQUE
T h e p r e s e n t r e s u l t i n d i c a t e s t h a t B - f r e e r e g i o n s a r e p r e f e r e n t i a l l y o x i d i z e d a t t h e i n i t i a l s t a g e o f o x i d a t i o n a n d t h e n t h e r e p u l s i v e B-0 i n t e r a c t i o n i n d u c e s t h e d i f f u s i o n o f b o r o n o f t h e B r e g i o n t o t h e i n n e r s e c t i o n o f t h e PP l a y e r .
ACKNOWLEDGEMENTS
The s u t h o r s w i s h t o g r a t e f u l l y a c k n o w l e d g e t h e t e c h n i c a l h e l p s o f P r o f . H i r o s h i Kokado a n d M r . A. Nakazawa o f t h e G r a d u a t e S c h o o l a t N a g a t s u t a , Tokyo I n s t i t u t e o f T e c h n o l o g y .
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