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1 INFORMATION TO USERS The most advanced technology has been used to photograph and reproduce this manuscript from the microfilm master. UMI films the original text directly from the copy submitted. Thus, some dissertation copies are in typewriter face, while others may be from a computer printer. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyrighted material had to be removed, a note will indicate the deletion. Oversize m aterials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each oversize page is available as one exposure on a standard 35 mm slide or as a 17" x 23" black and white photographic print for an additional charge. Photographs included in the original manuscript have been reproduced xerographically in this copy. 35 mm slides or 6" x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. UMI Accessing the World's Information since North Zeeb Road. Ann Arbor. Ml USA

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3 Order N u m b er Electrochemical and thermal desorption analysis of glassy carbon and carbon fiber surfaces Fagan, Danny T., Ph.D. The Ohio State University, 1987 C opyright 1987 b y Fagan, Danny T. All rig h ts reserved. UMI 300 N. ZeebRd. Ann Arbor, MI 48106

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5 PLEASE NOTE: In all cases this material has been filmed in the best possible w ay from the available copy. Problems encountered with this docum ent have been identified here with a check mark V. 1. Glossy photographs or p ag es ^ 2. Colored illustrations, paper or p rin t 3. Photographs with dark background 4. Illustrations are poor copy 5. P ages with black marks, not original copy \ / 6. Print show s through as there is text on both sides of p a g e 7. Indistinct, broken or small print on several pages \ / 8. Print exceeds margin requirem ents 9. Tightly bound copy with print lost in sp in e 10. Com puter printout pages with indistinct print 11. P age(s) lacking when material received, an d not available from school or author. 12. Page(s) seem to b e missing in numbering only as text follows. 13. Two pages num bered. Text follows. 14. Curling and wrinkled p ag e s 15. Dissertation contains pages with print at a slant, filmed a s received 16. Other

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7 ELECTROCHEMICAL AND THERMAL DESORPTION ANALYSIS OF GLASSY CARBON AND CARBON FIBER SURFACES DISSERTATION P re s e n te d in P a r t i a l F u lf illm e n t o f t h e R equirem ents fo r th e D egree D octor o f P h ilosophy i n th e G raduate School o f The Ohio S ta te U n iv e rs ity By Danny T. Fagan * * * * * The Ohio S ta te U n iv e rs ity 1987 D is s e r ta tio n C om m ittee: A pproved by T heodore Kuwana R ich ard L. McCreery A lan G. M a r s h a ll a d v is o r D e p a rtm e n t o f C h e m is try

8 (o )c o p y rig h t by Danny T. Fagan 1987

9 ACKNOWLEDGMENTS X w ould l i k e to e x p re s s my a p p r e c ia t io n t o Ted Kuwana f o r h is encouragem ent and guidance th ro u g h o u t my graduate s tu d ie s. I enjoyed th e s tim u la tin g environm ent t h a t he m ain tain ed in our re s e a rc h group and t h e freed o m t h a t he e x te n d e d t o me in th e d e v e lo p m e n t o f t h i s r e s e a r c h p r o j e c t. X w ould a l s o l i k e t o th a n k a l l o f th e m em bers o f o u r r e s e a r c h g ro u p, p a s t and p r e s e n t, who p ro v id e d in v a lu a b le a s s is ta n c e and made g rad u ate s c h o o l m ore e n jo y a b le. S p e c ia l th a n k s a lso t o the D epartm ents of C hem istry and P harm aceutical C hem istry and th e C e n te r f o r B io A n a ly tic a l R e s e a rc h a t th e U n iv e r s ity o f K ansas where th e l a t t e r h a lf o f t h is re s e a rc h was c a rrie d o u t. To my w ife, L a u rie, I would lik e t o express my d e e p est g r a titu d e fo r h e r undying s u p p o r t and e n c o u ra g e m e n t e s p e c i a l l y d u rin g th e d i f f i c u l t tim e s. She was alw ays u n d erstan d in g, and w ith o u t her none o f t h i s would have been p o s s i b l e. And f i n a l l y, I w ould l i k e t o th a n k my p a r e n t s who alw ays encouraged b u t never fo rc e d my academ ic endeavors. ii

10 VITA January 6, 1961 Born - A lexandria, V irg in ia 1983 B.S., N orthern I l l i n o i s U n iv e rsity Summer 1981,1982,1983 B esearch Chem ist, Amoco Research C enter N a p e rv ille, I l l i n o i s 1983 Teaching A sso ciate Ohio S ta te U n iv e rsity Research A ssociate Ohio S ta te U n iv e rsity and U n iv e rsity o f Kansas PUBLICATIONS "Vacuum H e a t T re a tm e n t f o r A c tiv a tio n o f G lassy Carbon E le ctro d e s," F a g an, D.T.; Hu, I.- F.; Kuwana, T. A n a ly tic a l C h e m is try, 1985, 57, "T h erm al D e s o rp tio n A n a ly s is o f Carbon F ib e r s I. S u rfa c e Oxygen Complexes ", Fagan, D.T.; Kuwana, T. m an u scrip t in p re p a ra tio n. "T h erm al D e s o rp tio n A n a ly s is o f C arbon F ib e r s I I. A d s o rp tio n o f O rganic M olecules", Fagan, D.T; Kuwana, T., m an u scrip t in p re p a ra tio n. " c is -b is (1,2 - E th a n e d ia m in e )D if lu r o c h ro m iu m ( III) Io d id e," Fagan, D.T.; F r ig e r io, J.S.; Vaughn, J.W. In o rg a n ic S y n th e sis, 1986, 24, 185. MAJOR FIELD OF STUDY: A n a ly tic a l C hem istry iii

11 TABLE OF CONTENTS ACKNOWLEDGEMENTS... VITA... LIST OF TABLES... LIST OF FIGURES... LIST OF SYMBOLS... CHAPTER i i i i i v i v i i x i PAGE I. INTRODUCTION... 1 M o tiv a tio n... 2 S tru c tu re o f Carbon... 5 L i s t o f R e fe re n c e s I I. ACTIVATION OF GLASSY CARBON AND CARBON FIBER ELECTRODES USING THERMAL TREATMENT METHODS I n tr o d u c tio n E x p e rim en ta l R e s u lts and D is c u ssio n C onclusions and Summary L i s t o f R e fe re n c es I I I. THERMAL DESORPTION MASS SPECTROMETRY- INSTRUMENTATION 59 I n tr o d u c tio n Background In stru m e n ta l D e s c rip tio n L i s t o f R e fe re n c e s IV. FORMATION AND REMOVAL OF SURFACE OXYGEN COMPLEXES ON CARBON FIBERS I n tr o d u c tio n Background Exp e rim e n ta l R e s u lts and D is c u ssio n Summary and C o n c lu sio n s L i s t o f R e fe re n c e s iv

12 V. ADSORPTION OF ORGANIC MOLECULES ON CARBON FIBERS APPENDICES I n tr o d u c tio n Background E x p erim en tal R e su lts and D iscu ssio n Summary and C onclusions L is t o f R eferen ces A. Theory o f D e so rp tio n B. D eterm ination of Pumping Speed and Quadrupole Gain R eferences BIBLIOGRAPHY v

13 LIST OF TABLES TABLE PAGE 2-1. E le ctro ch em ical and XPS d a ta fo r p o lis h e d and VHT g la s s y carbon e le c tr o d e s E le ctro ch em ical and XPS d a ta fo r th e rm a lly tr e a te d g la s s y carbon e le c tr o d e s E le ctro c h e m ic a l d a ta fo r th erm ally tr e a te d and o xidized carbon f ib e r e le c tr o d e s L ite r a tu r e summary o f d e so rp tio n d a ta fo r v a rio u s ty p es o f c a rb o n Summary of i n i t i a l d e so rp tio n p ro d u cts from carbon f i b e r s Peak maximum tem p eratu res f o r the d e so rp tio n o f CO and C02 from carbon f ib e r s Amounts of CO and C02 desorbed from o x id iz e d carbon f i b e r s A c tiv a tio n energy o f d e so rp tio n v a lu e s fo r C02 d e so rp tio n from carbon f i b e r s L is t o f o rg a n ic m olecules u sed fo r a d s o rp tio n s t u d i e s Norm alized p e a k a re a s fo r benzene and p h en o l Norm alized p eak a re a s fo r a n ilin e and to lu e n e Normalized p e a k a re a s fo r cyclohexane and c y clo h exanol B 1. L is t o f m easured pumping speed and MS g ain f o r th e o rg an ic m olecules s tu d ie d vi

14 LIST OF FIGURES FIGURE PAGE 1-1. S tru c tu re o f v a rio u s c a rb o n s Co re -s h e a th model o f carbon f ib e r s tr u c tu r e E lectro ch em ical p robes f o r a c t i v i t y e v a lu a tio n Schem atic diagram s o f VHT and th erm al tre a tm e n t cham bers Diagram o f e le c tro c h e m ic a l c e l l f o r g la s s y carb o n E le ctro n m icrographs o f a s -re c e iv e d and p o lis h e d g la ssy c a rb o n C yclic voltammograms fo r FC a t a s - r e c e iv e d, p o lis h e d and VHT g la s s y carbon e le c tr o d e s C yclic voltammograms f o r AA a t p o lis h e d and VHT g la ssy carbon e le c tr o d e s C yclic voltammograms fo r AA a t g la s s y carbon e le c tro d e s h e a t tr e a te d a t v a rio u s te m p e ra tu re s Background CV c u rv e s fo r p o lis h e d and VHT g la ssy carbon e le c tr o d e s X-ray p h o to e le c tro n s p e c tra fo r p o lis h e d and VHT g la ssy c a rb o n DPV curv es fo r p o lis h e d and VHT g la s s y carbon e le c tr o d e s C yclic voltammograms fo r FC a t v a rio u s ly tr e a te d carbon f i b e r e le c tr o d e s E le ctro n m icrographs o f VHT carbon f i b e r s E le c tro n m icrographs o f o x id iz e d f i b e r s C yclic voltammograms fo r AA a t v a rio u s ly tr e a te d carbon f i b e r e le c tr o d e s vii

15 3-1. Schematic diagram o f sample heating c i r c u i t P lo t of h e a tin g r a t e v s. tim e for carbon f ib e r tem p eratu re in c r e a s e Diagram o f carbon f i b e r sam ple h o ld er Schematic diagram o f TDMS system Schem atic diagram o f sample dosing system P o ssib le s u rfa c e oxygen complexes on c a r b o n P re ssu re v s. tim e p l o t f o r oxygen complex form ation on carbon P o te n ia l energy diagram f o r chem icsorption o f oxygen on c a rb o n I n i t i a l d e s o rp tio n p ro d u c ts mass s p e c tra fo r PP and HTPP carbon f i b e r s S e rie s o f mass s p e c tr a re c o rd e d during f i r s t and second vacuum d egassing o f carbon f i b e r s D esorption r a t e v s. te m p e ra tu re curves for C02 e v o lu tio n D esorption r a t e v s. te m p e ra tu re curves fo r CO e v o lu tio n E le c tro n m icrographs f o r s e v e rly o x id ized f i b e r s D esorption r a t e v s. te m p e ra tu re curves for C02 d e so rp tio n a t v a rio u s h e a tin g ra te s P lo ts o f ln(b /T p2) and ln(n ) vs. 1/T fo r C02 d e so rp tio n from o x id iz e d f i b e r s D esorption r a t e vs. te m p e ra tu re curves for CH3CN and CH3OH Comparison o f d e so rp tio n c u rv e s for C02 and CH3CN P o ssib le r e a c tiv e carbon s i t e s and t h e i r analog arom atic compounds viii

16 D esorption r a t e v s. tem perature curv es f o r n-propylam ine... D esorption r a t e v s. tem perature cu rv es.f o r n -p ro p a n o l.... D esorption r a te v s. tem perature curv es f o r benzene... D esorption r a t e v s. tem perature curv es f o r phenol... D esorption r a t e v s. tem perature curves f o r a n ilin e and to lu e n e... D esorption r a t e v s. tem perature curves fo r cyclohexane... D esorption r a t e v s. tem perature curv es f o r c y c lo h e x a n o l.... P re ssu re v s.' tem perature fo r high and low pumping r a t e s... Diagram o f th e o r e tic a l system to determ ine e ffu sio n r a t e through an o r i f i c e... Diagram o f system c o n fig u ra tio n t o measure pumping speed... P lo t o f P2 v s. P^ fo r benzene... P lo t o f MS g a in v s. P^ f o r benzene ix

17 LIST OF SYMOBOLS am peres, a re a Angstrom a s c o rb ic a c id ra d iu s h e a tin g r a t e chronocoulom etry c y c lic voltam m etry c e n tim e te r d i f f e r e n t i a l p u lse voltam m etry a c tiv a tio n energy o f a d so rp tio n a c tiv a tio n energy o f d e so rp tio n p e a k p o t e n t ia l anodic p eak p o te n tia l e le c tro n e le c tr o n v o l t conductance f e r r i/f e r r o - c y a n id e c o rre c te d conductance gauge f a c to r r e l a t e d t o th e dependence o f d e so rp tio n r a t e on s u rfa c e coverage in n e r d iam e te r

18 i n. K k kc kj k a kv L inch p ro p o rtio n a lity c o n sta n t Boltzman c o n sta n t C laussing c o rre c tio n f a c to r k ilo jo u le s x -ra y em ission lin e used fo r XPS k ilo v o lts l i t e r s, leak r a t e 1 len g th M m min. ml mm mol msec m torr mv m/z N Np n o.d. P m olar c o n c e n tra tio n, m o lecu lar w eight mass m inute m i l l i l i t e r s m illim e te rs mole m illiseco n d s m i l l i t o r r m illiv o lts mass t o charge r a t i o d e so rp tio n r a te d e so rp tio n r a t e a t peak maximum tem perature r e a c tio n o rd e r, d e n s ity o u te r diam eter p re s s u re xi

19 Peq ppm p s i * P Q ads Qdl redox f i t R r S SCE s T To TP t UHV ua um usee uf e q u ilib ru iu m p re s s u re p a r t s p e r m illio n pounds p e r square in c h P - Peq c h a rg e, flu x charge due t o p h y siso rb ed m a te ria l n o n -fa ra d a ic c a p a c itiv e charge fa ra d a ic charge t o t a l background charge gas c o n sta n t c o r r e la tio n c o e f f ic ie n t pumping sp eed s a tu ra te d calom el e le c tro d e su rfa c e coverage tem p eratu re i n i t i a l tem p eratu re peak tem p eratu re tim e u ltr a - h ig h vacuum m icroam peres m icrom eters m icroseconds m icro farad V v o l t, volume V frequency f a c to r xii

20 mean m olecular speed peak i d e n t i f i e r p ro d u ct o f t r a n s f e r c o e f f ic ie n t and number o f e le c tro n s peak i d e n t i f i e r peak i d e n t i f i e r peak i d e n t i f i e r c h a r a a c te r is tic pumping speed = V/S xiii

21 CHAPTER X INTRODUCTION Overview The work p re se n te d in th is d is s e r a ta tio n re p re s e n ts a fundam ental i n v e s t i g a t i o n o f t h e s u r f a c e c o m p o s i tio n and p r o p e r i t e s o f c a rb o n a c e o u s m a t e r i a l s in te rm s o f t h e i r e le c tr o c h e m ic a l and a d s o r p tio n b e h a v io r. S p e c i f i c a l l y, tw o ty p e s o f carbon w ere in v e s tig a te d : g lassy carbon p la te s and carbon f ib e r s. Both m a te ria ls a re v itre o u s, n o n -g ra p h itic carbon. The a n a ly tic a l methods employed in t h i s s tu d y in c lu d e v a rio u s e le c tr o c h e m i c a l te c h n iq u e s, x - r a y p h o to e le c tro n sp ectro sco p y, scanning e le c tro n m icroscopy, and th e rm a l d e so rp tio n a n a ly s is. A s ig n if ic a n t p o rtio n o f t h is work involved the d e s ig n, c o n s tr u c tio n, and e v a lu a tio n o f a th e r m a l d e s o r p tio n m ass s p e c tr o m e tr y sy stem f o r th e s tu d y o f a d s o r p tio n on c a rb o n f i b e r s. A d s o rp tio n in t h i s c a s e in c lu d e s th e i r r e v e r s i b l e c h e m is o rp tio n o f oxygen to form su rfa c e fu n c tio n a l groups (oxygen complexes) and a lso th e a d so rp tio n o f o rg an ic m olecules. T h is i n tr o d u c to r y c h a p te r w i l l p r e s e n t th e m o tiv atio n fo r t h is work along w ith a d isc u ssio n o f th e e v o lu tio n o f th e study from g la s s y c a rb o n to c a rb o n f i b e r s. carbons w ill be p re se n te d. A d d i t io n a lly, th e s t r u c t u r e o f v a r io u s R e le v a n t b ack g ro u n d m a t e r i a l c o n c e rn in g e a c h a s p e c t o f t h i s p r o j e c t i s g iv e n in a l l s u b s e q u e n t c h a p te r s and w i l l n o t be re p e a te d h e re. 1

22 2 M otivation I n te r e s t in carbon r e s u lte d i n i t i a l l y from i t s im portance a s an e le c tro d e m a te ria l. P rio r to th e developm ent o f the dropping mecury e le c tro d e (DME), th e f ie l d o f e le c tro c h e m is try p ro g ressed slow ly due to problem s a s s o c ia te d w ith s o lid m e ta l e l e c t r o d e s ( i r r e p r o d u c i b i l - i t y ). The DME e f f e c t i v e l y e lim in a te d s e v e r a l o f th e s e p ro b le m s by p r o v id in g a c le a n, c o n tin u o u s ly re n e w a b le s u r f a c e (w ith e v e ry new drop) p r io r to each m easurem ent. A renew able su rfa c e was s ig n if ic a n t s in c e e le c tro n t r a n s f e r, b eing an i n t e r f a c i a l phenom enon, i s h ig h ly s e n s i t i v e t o th e s u r f a c e s t a t e o f th e e l e c t r o d e. H ow ever, th e DME s u f f e r e d fro m s e v e r a l d is s a d v a n ta g e s, t h e m ost s i g n i f i c a n t b e in g a l i m i t e d u s e f u l p o t e n t i a l r e g io n ; th e DME c a n n o t be used a t p o s i t i v e p o t e n t i a l s due to th e o x id a tio n o f m e rc u ry. A lso, m e c h a n ic a l d i f f i c u l t i e s a s so c ia te d w ith p roviding a continuous flow o f m ercury through a c a p illa r y reduced i t s u s e fu ln e ss. Noble m etal e le c tr o d e s, on th e o t h e r hand, w ere much e a s i e r t o h a n d le, and w ere u s a b le a t p o t e n t i a l s p o s i t i v e o f 0.0 v o l t s. H ow ever, P t and Au s u f f e r e d fro m sm a ll double la y e r reg io n s a s w e ll as h igh c o s t. F urtherm ore, i t was n e c e s s a ry t o c le a n th e s u r f a c e c a r e f u l l y to a c h ie v e f a s t e l e c t r o n t r a n s f e r. The d e v e lo p m e n t o f th e c a rb o n p a s te e le c t r o d e by Adams (1), and r e p o r ts concerning th e use o f g la ssy carbon and g ra p h ite a s e le c tro d e m a t e r i a l s (2-4) in th e l a t e 5 0 's and e a r l y 6 0 's, p r e c i p i t a t e d many s tu d ie s o f e le c tro c h e m is try a t carbon. Carbon o f f e r s s e v e ra l d i s t i n c t advantages t o Hg and P t, in c lu d in g a wide u sa b le p o te n tia l window in

23 3 th e o x id a tiv e r e g io n (0.0 t o c a. 2.0 V v s. SCE), e a se o f f a b r i c a t i o n in to e le c tro d e s, and low c o s t. The p r in c ip a l d isad v an tag e, how ever, i s th e need to c le a n and renew o r " a c t iv a t e " th e s u r f a c e in o r d e r to o b ta in r a p id e l e c t r o n t r a n s f e r. The a c t i v a t i o n t y p i c a l l y in v o lv e s p o lis h in g an d /o r e le c tro c h e m ic a l o x id a tio n. The causes o f a c tiv a tio n a r e n o t w e ll known, a lth o u g h a c o n s id e r a b le am ount o f w ork has b e e n done in t h i s area. T h is f a c t p ro v id e d th e m o tiv a tio n f o r th e f i r s t s ta g e o f t h i s work which i s p re se n te d in Chapter I I. In o rd e r to b e t t e r understan d th e c a u se s o f c a rb o n s u r f a c e a c t i v a t i o n, a s tu d y was i n i t i a t e d t o p rep are and c o n tro l g la ssy carbon (gc) su rfa c e s by c a re fu l p o lis h in g, vacuum h e a t tre a tm e n t (VHT), and s u rfa c e o x id a tio n. VHT was chosen a s a m ethod c a p a b le o f rem o v in g a d so rb e d m a t e r i a l (in c lu d in g s u r f a c e oxygen fu n c tio n a l groups) from a gc s u rfa c e. The r e s u ltin g e le c tro d e s were then e v a lu a te d in term s o f t h e i r e le c tro c h e m ic a l a c tiv ity. Of s p e c if ic i n te r e s t was the e f f e c t o f oxygen com plexes on a c tiv a tio n. The above study using VHT w ith gc was extended to in clu d e carbon f ib e r s owing to th e growing i n te r e s t i n t h e i r use as m ic ro e le c tro d e s (5). F ib e r s o f f e r s e v e r a l a d v a n ta g e s o v e r gc due to t h e i r s m a ll s i z e and low background c u rre n t; however, th e y are s tr u c tu r a lly s im ila r t o g c. In o r d e r to im p le m e n t t h i s s tu d y, a vaccum sy ste m w hich w o u ld p e rm it h e a tin g carbon f ib e r sam ples up to 1000 C was c o n stru c te d. The r e s u l t s o f vaccum h e a t t r e a tm e n t and s u r f a c e o x id a tio n o f c a rb o n f ib e r s in term s o f e le c tro c h e m ic a l a c tiv a tio n a re a lso p rese n ted in C hapter I I.

24 4 The d e v e lo p m e n t o f a th e r m a l d e s o r p tio n (TD) sy stem f o r c a rb o n f i b e r s i s d e s c r ib e d in d e t a i l in C h a p te r I I I. T his sy ste m w as expanded (beyond VHT) to in c lu d e m ass s p e c tr o m e tr y d e t e c t i o n o f d e so rb e d p r o d u c ts (TDMS), and a d o s in g sy stem was added to p e r m it c o n tr o l le d a d s o r p tio n o f o r g a n ic m o le c u le s on v a r i o u s ly p r e p a r e d s u r f a c e s. T hus, a v a r i e t y o f e x p e rim e n ts c o n c e rn in g th e s u r f a c e com position and p r o p e r tie s o f carbon f ib e r s was perform ed in c lu d in g rem oval and a n a ly s is o f m a te ria l i n i t i a l l y adsorbed an d /o r trap p ed on th e s u r f a c e (C h a p te r IV ), f o rm a tio n and rem o v a l o f c a rb o n -o x y g e n s u r f a c e f u n c t i o n a l g ro u p s (C h ap ter IV), and a d so rp tio n /d e so rp tio n o f o rg an ic s on oxygen f r e e and o x id ized carbon su rfa c e s (Chapter V). The a d s o r p tio n o f o r g a n ic s on c a rb o n f i b e r s was s tu d i e d to exam ine th e e f f e c t o f s u r f a c e tr e a tm e n t on th e a d s o r p tio n o f m o le c u le s w ith d i f f e r e n t f u n c t i o n a l g ro u p s to p ro v id e some i n d i r e c t e v id e n c e c o n c e rn in g th e c h e m ic a l n a tu r e o f s u r f a c e s i t e s on c a rb o n. In a d d itio n, long term g o a ls were d ire c te d a t e x p lo itin g th e a d so rp tiv e p r o p e r t i e s o f c a rb o n f o r th e d e v e lo p m e n t o f carb o n f ib e r - b a s e d s e n so rs, and th u s fundam ental in fo rm a tio n o f t h i s type was needed. O v e r a ll, t h i s w ork e v o lv e d from a stu d y o f g la s s y c a rb o n t o carbon f ib e r s. B esides th e in c re a sin g im portance o f carbon f ib e r s f o r e le c tro c h e m ic a l a p p lic a tio n s, t h i s e v o lu tio n o ccu rred in p a r t due to th e e a s e a t w h ich c a rb o n f i b e r s c o u ld b e h e a te d r e s i s t i v e l y up to 1000 C. G lassy carbon p la te s re q u ire la rg e am ounts o f c u rre n t to be h e a te d in t h i s way and th u s m ust be h eated in ste a d by ra d ia tio n. T his f a c to r co m p lic a tes th e e x p e rim en tal a s p e c ts o f TDMS. However, owing

25 5 to th e s tr u c tu r a l s im i la r i t y o f gc and carbon f ib e r s, i t was f e l t t h a t s tu d ie s o f f ib e r s would n o t compromise th e s ig n ific a n c e o f a g e n e ra l study o f v itre o u s carbon. The S tru c tu re o f Carbons Much o f th e m a t e r i a l p r e s e n te d b e lo w can b e found in tw o c o m p re h e n siv e m onographs one by J e n k in s and Kawamura (6) and t h e o th e r by Donnet and Bansal (-7). These a u th o rs, who a re very a c tiv e in th e a r e a o f c a rb o n r e s e a r c h th e m s e lv e s, h a v e a sse m b le d much o f t h e p e r t i n e n t i n f o r m a tio n c o n c e rn in g th e s tr u c u r e o f carb o n t h a t h a s appeared in th e l i t e r a t u r e in the p a s t two. decades. A ll c a rb o n a c e o u s m a t e r i a l s a r e made up o f e i t h e r diam ond o r g r a p h i t e s t r u c t u r e s. I n d ia m o n d, t h e c a rb o n i s b o n d e d in a te tr a h e d r a l arrangem ent w ith sp - h y b r id iz e d o r b i t a l s fo rm in g sig m a b onds. G r a p h ite, on th e o t h e r h a n d, p o s s e s s e s e x te n d e d s h e e ts o f hexagonal carbon rin g s w ith sp carbon (basal planes) th a t a r e lay e red in an ABA arrangem ent. The s e p a ra tio n betw een th ese sh eets i s ca. 3.4 A, and they a re h e ld to g e th e r only by weak e le c tr o s ta t ic a t t r a c t i o n. T h is s t r u c t u r e i s d e p ic te d i n F ig u re 1-1A. In a d d itio n to t h e b a s a l p la n e s, edge p la n e s a ls o e x i s t a t t h e ends o f th e a ro m a tic s h e e t s, w ith th e fo rm e r b e in g h ig h ly r e s i s t a n t to c h e m ic a l a t t a c k and t h e l a t t e r showing high r e a c tiv i ty. The c a rb o n s u se d in t h i s s tu d y a re n o n - g r a p h i tic, v i t r e o u s carbons form ed by th e c a rb o n iz a tio n o f high m o le c u la r w eight polym ers. In t h i s p ro c e ss, which re q u ire s tem p eratu res o f 1000 C or g r e a te r, th e polym er lo se s m ost o f i t s non-carbon co n te n t. The r e s u ltin g m a te ria l

26 6 F ig u re 1-1. D iagram o f (A) g r a p h i t e b a s a l p la n e s, (B) g l a s s y c a rb o n m i c r o f i b r i l, and (C) c a rb o n f i b e r s t r u c t u r a l m odels (ta k e n fro m r e f e r e n c e 6).

27 Typical strong confluence B!*vXv > W a W w. Weak confluence Figure 1-1.

28 8 i s am orphous and la c k s th e a n i s o t r o p i c p r o p e r i t e s o f g r a p h i t e. The s tr u c tu r e of t h i s m a te ria l i s n o t known to th e same degree o f accuracy a s i s g r a p h i t e o r diam ond due t o th e u n a v a i l a b i l i t y o f p u re s i n g l e c r y s ta ls. However, a c o n sid e ra b le body o f work has been done, and i t i s now g e n e ra lly b e lie v e d th a t th e v itre o u s carbon s tr u c tu r e i s made up o f co n d en sed a r o m a tic rib b o n s w hich a r e o r ie n t e d ran d o m ly and tan g le d in a com p licated manner (8). These ribbon s tr u c tu r e s, which a re diagram m ed i n F ig u re 1-1B, can be v ie w e d d i r e c t l y w ith h ig h re s o lu tio n tra n sm issio n e le c tro n m icroscopy (8). The th in rib b o n s a re made up o f g r a p h i t i c b a s a l p la n e s and a r e e v id e n t ly f a i r l y s t a b l e ow ing t o t h e i r r e s i s t a n c e to g r a p h i t i z a t i o n. T h ere i s l i t t l e convincing evidence fo r th e presence o f sp3 carbon. The ta n g le d rib b o n s t r u c t u r a l u n i t d e p ic te d i n F ig u re 1-1B i s re f e rre d to as a m ic r o f ib r il. T his u n it r e s u l t s from th e o v erlap o r m erg in g o f th e a fo re m e n tio n e d rib b o n s (to form w eak and s tr o n g co n flu en ces, re s p e c tiv e ly ) in to h ig h ly is o tr o p ic s tr u c tu r e s w ith v ery few " lo o s e e n d s." The la c k o f lo o s e en d s may e x p la in th e c h e m ic a l i n e r t n e s s o f g l a s s y c a rb o n. The m i c r o f i b r i l s a r e a tta c h e d to e a c h o th e r by th e edges o f th e ribbons w ith stro n g c o v a le n t bonds. These bonds e v id e n tly p re v e n t th e narrow g ra p h ite rib b o n s from moving in to th e ABA sta c k in g p a tte r n, th us p re v e n tin g g r a p h itiz a tio n (8). N o n - g r a p h itic c a rb o n f i b e r s p o s s e s s s t r u c t u r e s t h a t a r e v e ry s im ila r t o g la ssy carbon. I t is th o u g h t t h a t th e m ic r f i b r i ls a re long w inding rib b o n s, stack ed on to p o f each o th e r, t h a t a re w rin k led and t w i s t e d a lo n g t h e f i b e r a x is (9 ). R uland (10) o b s e rv e d t h a t th e

29 9 l a y e r in g in c a rb o n f i b e r s i s th e sam e a s i n g l a s s y c a rb o n. T h e re i s a ls o e v id e n c e f o r th e e x is te n c e o f n e e d l e - l i k e p o r e s w ith c r o s s se c tio n s o f ca. 15 A and len g th s o f > 300 A. The g e n e ra l o r ie n ta tio n o f the f ib e r s tr u c tu r e i s shown in Figure 1-1C. C arbon f i b e r s a re b e lie v e d t o be made up o f d i f f e r i n g in n e r and o u te r s t r u c t u r e s. In g e n e r a l, i t i s th o u g h t t h a t th e o u t e r s u r f a c e p o s s e s e s a s t r u c t u r e s i m i l a r t o, b u t d i s t i n c t from th e in n e r c o r e. The d iffe n c e may be in th e long range o rd e r o f the m a te r ia l, w ith th e in n er c o re being f a i r l y a n is o tro p ic and th e o u te r su rfa c e d isp la y in g is o tro p ic behavior. Johnson (11) proposed th e th re e -d im e n sio n a l model shown in Figure 1-2 which shows a d iffe re n c e in s tr u c tu r e and d e n s ity o f th e c a rb o n r ib b o n s n e a r th e c e n te r and th o s e n e a r th e s u r f a c e. Based on h ig h r e s o l u t i o n TEM w ork, i t was p ro p o se d t h a t th e f i b e r s t r u c t u r e i s made up o f an a ro m a tic b a se u n i t l e s s th a n 10 A in s i z e and a s s o c i a te d e d g e to edge in a p a r a l l e l m anner to fo rm " w rin k le d " arom atic la y e rs. The sh e e ts c o n s its o f e n ta n g led fo ld s, a s e v id e n t in th e f ig u r e, th a t a re p a r a l l e l to th e f ib e r a x is. G la s sy c a rb o n i s u s u a lly form ed from th e p y r o l y s i s o f p h e n o lic r e s i n s. Carbon f i b e r s, on th e o t h e r hand, a r e form ed from a w id e v a r i e t y o f m a t e r i a l s in c lu d in g p o l y a c r y l o n i t r i l e (PAN), c e l l u l o s e, p o ly v in y l c h l o r i d e, c o a l t a r p i t c h, p e tro le u m p i t c h, and p h e n o lic r e s i n, among o t h e r s. S u r p r i s i n g l y, f i b e r s p r e p a r e d from d i f f e r e n t p r e c u r s o r s have b e e n found t o have many common s t r u c t u r a l f e a t u r e s (12). The f i b e r s u se d i n th e p r e s e n t s tu d y w ere p y r o ly z e d from p u r i f i e d c o a l t a r p i t c h a t 1100 C u n d e r n itr o g e n. Some f i b e r s a l s o

30 10 F igure 1-2. Diagram o f "sh eath -co re" m odel o f carbon f i b e r s tr u c tu r e (taken from re fe re n c e 7).

31 11 r e c e iv e d a s u b s e q u e n t h e a t - t r e a t m e n t a t 3000 C u n d e r a rg o n by th e m anufacturer. These f ib e r s can be c la s s if i e d as Type XI f ib e r s, t h a t i s, having a low modulus o f e l a s t i c t y, b u t high stre n g th.

32 LIST OF REFERENCES (1) Adams, R.N. A n al. Chem , ^ 0, (2) L o rd, S.S. J r. ; R o g e rs, L.B. A nal. Chem , 26, 284. (3) G a y lo r, V.F.; E lv in g, P.J.; C onrad, A.L. A nal. Chem , 2 5, (4) Z i t t e l, H.E.; M i l l e r, F.J. A nal. Chem. 1965, 37, 200. (5) Edm onds, T.E. A n al. Chim. A cta 1985, 175, 1. (6 ) J e n k in s, G.M.; K aw am era, K. " P o ly m e ric c a rb o n s - c a rb o n f i b e r, g la s s, and char"; Cambridge U n iv e rsity P re ss: London, (7) Donnet, J.-B.; B ansal, R.C. "Carbon F ib ers", I n te r n a tio n a l F ib e r S c ie n c e and T echnology S e r ie s, M arcel D ekker: New Y ork, V ol (8 ) J e n k in s, G.M.; K aw am era, K. " P o ly m e ric c a rb o n - c a rb o n f i b e r, g la s s, and char"; Cambridge U n iv e rsity P re ss: London, 1976, pp (9) Jo h n so n, W.; W a tt, W. N a tu re , 215, 384. (10) R u la n d, W. P o ly m er 1969, 9, (11) B e n n e t t, S.C.; J o h n s o n, D.J. P r o c. 5 th L ondon C a rb o n a n d G r a p h ite C o n f., V ol 1, Soc. Chem. In d. London, , 377. (12) Donnet and B ansal, "Carbon F ib e rs"; I n te r n a tio n a l F ib e r Science and Technology S e rie s, M arcel Dekker: New York, 1984, Vol. 3, p

33 CHAPTER II ACTIVATION OP GLASSY CARBON AND CARBON FIBER ELECTRODES USING THERMAL TREATMENT METHODS In tro d u c tio n The w id e s p re a d u s e o f c a rb o n a c e o u s m a t e r i a l s (carb o n p a s t e, g ra p h ite, g la ssy carbon, and carbon fib e rs) a s e le c tro d e s h as re s u lte d in many s tu d ie s concerning the p re p a ra tio n o f e le c tro c h e m ic a lly a c tiv e s u r f a c e s. T h ese a c t i v a t i o n m ethods in c lu d e p o l is h i n g w ith an a b r a s iv e m a t e r i a l (1,2 ), e le c tr o c h e m ic a l o x id a tio n (3-5 ), in s i t u la s e r ir r a d ia tio n (6,7), h eatin g a t low p re s s u re (8,9), and tre a tm e n t w ith ra d io frequency plasm as (10). In th is, c o n te x t, "activ e " r e f e r s to a s u rfa c e t h a t e x h i b i t s a f a s t e l e c t r o n t r a n s f e r r a t e ( 1 ). T y p ic a lly, u n t r e a t e d c a rb o n e x h i b i t s slo w c h a rg e t r a n s f e r fo r many e le c tr o c h e m ic a l s y s te m s and th u s r e q u i r e s a c t i v a t i o n by one o f th e p r e tr e a tm e n t m eth o d s m e n tio n e d ab o v e. The d e g re e o f a c t i v a t i o n i s u s u a lly m onitored w ith a m olecule such a s a s c o rb ic a c id t h a t shows a h ig h d e g re e o f s e n s i t i v i t y to t h e s u r f a c e s t a t e o f th e e le c tr o d e (1, 12). V irtu a lly a l l of th e aforem entioned a c tiv a tio n tech n iq u es r e s u l t in an in c r e a s e i n th e am ount o f s u rfa c e oxygen f u n c t i o n a l g ro u p s ( r e f e r r e d to a s oxygen co m p lex es) and a l s o i n c r e a s e th e b ack g ro u n d c h a rg in g c u r r e n t. For t h i s r e a s o n, s u r f a c e ro u g h n e ss a n d oxygen f u n c t i o n a l i t y a r e o f te n used t o e x p la in a c tiv a tio n r e s u ltin g from a 13

34 14 p re tre a tm e n t method. In a d d itio n, oxygen complexes have a ls o been s u g g e s te d to m e d ia te e l e c tr o n t r a n s f e r, a f f e c t a d s o r p tio n and w e tta b ility, and play a r o le in c a ta ly t i c and e l e c t r i c a l p r o p e r tie s o f carb o n m a t e r i a ls (10,11,13-15). In order t o b e tte r u n d erstan d th e r o le th a t oxygen com plexes and m ic ro s c o p ic ro u g h n e s s have in t h e a c t i v a t i o n p r o c e s s, a s tu d y to c o n tr o l and e v a lu a te t h e s e tw o p a r a m e te r s f o r g la s s y c a rb o n (gc) su rfa c e s was i n i t i a t e d. T his work was perform ed j o i n t l y w ith Ing-Peng Hu in o u r r e s e a r c h g ro u p. My c o n t r i b u t i o n in c lu d e d d e s ig n and co n stru c tio n o f a high vacuum h e a tin g system fo r g la s s y carbon p la te s, m easurem ent o f th e e le c tr o c h e m i c a l a c t i v a t i o n u s in g f e r r i / f e r r o - cyanide (FC) and a sco rb ic acid (AA), and d e te rm in a tio n of th e su rfa c e oxygen c o n te n t o f the e le c tro d e w ith x-ray p h o to e le c tro n sp ectro sco p y (XPS). Other a sp e c ts o f t h is p r o je c t, in c lu d in g d e te rm in a tio n o f the b ack g ro u n d c h a rg e u s in g c h ro n o c o u lo m e try (CC) and d e te c tio n o f th e surface-bound red o x groups using d i f f e r e n t i a l p ulse voltam m etry (DFV), w i l l b e d is c u s s e d in t h e c o n te x t o f t h i s w o rk, b u t a r e d e s c r ib e d in d e ta il elsew h ere (16). Wightman a n d cow orkers (8,9) h a d p re v io u sly a c tiv a te d gc v i a h eat tre a tm e n t a t 550 C under low p re s s u re (m to rr range). Along w ith the a c t i v a t i o n, a s i g n i f i c a n t i n c r e a s e in t h e b ack g ro u n d c h a r g e was observed, p o s s ib ly due t o surface roughening an d /o r th e fo rm a tio n o f s u r f a c e oxygen c o m p le x e s due t o e x p o s u re o f th e h e a t t r e a t e d gc to a m b ie n t a i r p r i o r to c o o lin g. To e x p l o i t t h e u s e f u ln e s s o f th e r m a l tre a tm e n t in th e a c tiv a tio n p ro c e ss w hile avoid in g th e r e a c tio n o f gc

35 15 w ith oxygen a t e le v a te d t e m p e r a t u r e s, e le c t r o d e s w ere p r e p a r e d by vacuum h e a t tre a tm e n t (VHT) and cooled in vacuum p r io r t o exposure to am bient a i r. The r e s u lts show th a t t h i s method reduces th e amount o f s u r f a c e oxygen com plex p r e s e n t and lo w e rs t h e b a c k g ro u n d c h a rg in g c u r r e n t w h ile a t th e same tim e p r o v id in g an a c t i v e s u r f a c e f o r th e e le c tro o x id a tio n o f AA and th e re d u c tio n /o x id a tio n (redox) o f PC. The second s ta g e o f t h i s w ork in v o lv e d e x te n d in g th e VHT a c tiv a tio n method to n o n -g ra p h itic carbon f ib e r s. This ty p e o f f ib e r i s s i m i l a r in s t u c t u r e to g l a s s y c a rb o n, as d is c u s s e d i n C h a p te r I, b u t d isp la y s q u ite d if f e r e n t e le c tro c h e m ic a l b eh av io r in some c a se s. Carbon f i b e r s a r e now r o u t i n e l y u s e d a s m ic r o e le c tr o d e s (17), e s p e c i a l l y f o r i n v iv o e l e c t r o a n a l y s i s o f b i o l o g i c a l l y im p o r ta n t m o le c u le s (1 8,1 9 ), ow ing t o t h e i r s m a ll s iz e a n d re d u c e d b ack g ro u n d c h a rg in g c u r r e n t s. W hereas gc i s o f t e n a c t i v a t e d by p o l i s h i n g th e s u r f a c e w ith an a b r a s iv e m a t e r i a l, carb o n f i b e r s a r e r o u t i n e l y a c t i v a t e d by an e le c tr o c h e m i c a l o x id a tio n (1 9,2 0 ). W ith o u t t h i s tre a m e n t, th e e le c tro d e u s u a lly e x h ib its no redox c u rre n t f o r s o lu tio n s p e c ie s, in d ic a tin g t o t a l d e a c tiv a tio n. This i s c o n tra ry to th e case o f gc which shows some charge tra n s f e r even p r i o r to p re tre a tm e n t. S u r p r i s i n g l y, VHT d id n o t r e s u l t in s i g n i f i c a n t e n h a c e m e n t in a c t i v i t y fo r AA o x id a tio n o r f o r FC redox w ith carbon f ib e r s as i t did f o r gc. Thus, o th e r tre a tm e n t methods in c lu d in g gas p h ase o x id a tio n w ere a l s o e x p lo re d in o r d e r to b e t t e r u n d e rs ta n d t h e c a u s e s o f a c t i v a t i o n o f t h i s m a t e r i a l. In th e c a s e o f c a rb o n f i b e r s, a d i r e c t c o r r e la tio n betw een th e e x te n t of s u rfa c e o x id a tio n and a c t i v i t y was

36 16 observed. Of s p e c if ic i n t e r e s t was th e lin k betw een th e presence o f e l e c t r o a c t i v e s u r f a c e bound oxygen co m p le x es and e le c tr o c h e m i c a l a c t i v i t y. S cannin g e l e c t r o n m ic ro sc o p y (SEM) r e s u l t s s u g g e s t t h a t s e v e r e o x id a tio n r e s u l t s i n th e re m o v a l o f an o u te r s k in s t r u c t u r e from the f i b e r th a t may impede e le c tro n tra n s f e r. E valuation o f A c tiv ity In o r d e r to e v a l u a t e th e a c t i v i t y o f a c a r b o n e l e c t r o d e p r e t r e a t e d u n d e r d i f f e r e n t c o n d itio n s, i t i s n e c e s s a r y t o ch o o se a r e f e r e n c e e le c tr o c h e m ic a l sy ste m t h a t i s s e n s i t i v e t o th e s u r f a c e s t a t e of th e e le c tro d e. As m entioned p re v io u s ly, the e le c tro o x id a tio n o f AA has been w idely used f o r th is purpose and was a ls o employed in t h i s study. A scorbic acid undergoes a 2e, 2H+, o x id a tio n a t m ercury w ith a h a lf wave p o te n tia l o f ca. 250 mv vs. SCE in a c id ic media (21). E le c tr o n t r a n s f e r i s th o u g h t to o c c u r v ia an i n n e r s p h e re m echanism r e s u ltin g in a high s e n s i t i v i t y to th e su rfa c e s ta te (22). At carbon e l e c t r o d e s, h o w e v e r, an o v e r p o t e n t i a l due t o s u r f a c e d e a c t iv a t io n r e s u l t s in a s h if t i n the h a l f wave p o te n tia l t o more p o s itiv e v a lu e s ( in c r e a s e s o f up t o s e v e r a l h u n d red mv a r e o b s e rv e d ). As th e e le c tr o d e becom es a c t i v a t e d, th e p o t e n t i a l s h i f t s i n th e n e g a tiv e d ir e c tio n, moving c lo s e r to th e 250 mv lim it observed a t Hg. The e le c tr o c h e m i s tr y o f AA i s c o m p lic a te d s in c e th e o x id a tio n p r o d u c t u n d e rg o e s a r a p id h y d r o ly s i s (m aking th e sy ste m c h e m ic a lly i r r e v e r s i b l e ), th u s p re v e n tin g the d e te c tio n o f th e re d u c tiv e wave on th e c y c lic v o lta m m e try (CV) tim e s c a l e. F ig u re 2-1A show s th e s tr u c tu r e o f AA alo n g w ith th e e le c tro c h e m ic a l and chem ical re a c tio n s.

37 17 A ASCORBIC ACID B FERRI/FERR 0 - CYANIDE Fe(CN)g *32= Fe(CN)g'+e_ F ig u re 2-1. E le c tr o c h e m ic a l p r o b e s f o r s u r f a c e a c t i v a t i o n : (A) o x id a tio n o f a s c o rb ic a c id, (B) r e d u c t i o n / o x i d a t i o n o f f e r r i / f e r r o - cy an id e.

38 18 The m echanism o f a s c o r b a te o x id a tio n h a s b een d is c u s s e d by s e v e r a l a u th o rs (1 1, 2 2 ) and w i l l n ot be d isc u sse d here. F e r ri/fe rro -c y a n id e (FC) was chosen as a second p ro b e fo r su rfa c e a c t i v a t i o n (see F ig u re 2-1B ). U n lik e AA, FC i s th o u g h t to u ndergo e le c t r o n t r a n s f e r v i a an o u t e r s p h e re m echanism (23,24) and s h o u ld t h e r e f o r e b e l e s s s e n s i t i v e t o th e s u r f a c e s t a t e o f t h e e le c tr o d e. H ow ever, FC does e x h i b i t a s lo w e le c t r o n t r a n s f e r r a t e a t c a rb o n su rfa c e s u n le ss some a c tiv a tio n is perform ed p r i o r to u se (1). Since FC i s q u a s i- re v e r s ib le a t carbon, the e x te n t o f su rfa c e a c tiv a tio n can be e v a lu a te d by co m p arin g th e o b s e rv e d peak s e p a r a tio n, AEp betw een th e o x id a tio n and re d u c tio n waves to th e thermodynamic lim itin g value o f 56 mv f o r a le p r o c e s s (25). V e rif ic a tio n o f th e Presence o f Surface Oxygen I t w as a c e n t r a l p a r t o f t h i s p r o j e c t to d e te r m in e w hat e f f e c t su rfa c e oxygen com plexes had on the a c tiv a tio n o f carbon e le c tro d e s. VHT was used to remove surface oxygen w h ile p o lis h in g, e le c tro c h e m ic a l o x id a tio n, and g as p h a se o x id a tio n i n 0 2 w e re used t o in tr o d u c e oxygen.. T h e r e f o r e, a d e te c t i o n m ethod was n e e d e d t o i d e n t i f y th e c h a n g es i n th e s u r f a c e oxygen c o n te n t f o llo w in g t h e s e v a r io u s tre a tm e n ts. For th e gc p l a t e s, which w ere ap p ro x im ately 1.5 cm2 in a re a, XPS w as u s e d t o d e t e c t s u r f a c e o x y g e n. T h is t e c h n i q u e i n v o l v e s ir r a d ia tin g th e sam ple su rfa c e w ith low energy x -ra y s and d e te c tin g th e e m itte d p h o to e le c tro n s. The r e s u ltin g peak h e ig h ts an d /o r a re a s c a n be u se d t o q u a n t i t a t e th e am ount o f C-0 s u r f a c e g ro u p s. In th e

39 19 case o f th e carbon f ib e r s, th erm al d e so rp tio n mass sp ectro m etry (TDMS) w as used t o v e r i f y th e fo rm a tio n a n d /o r re m o v a l o f s u r f a c e oxygen complexes by re c o rd in g th e d e so rp tio n o f CO and C02- The in stru m e n ta l a s p e c ts o f th e TDMS sy ste m a r e p r e s e n te d i n C h a p te r I I I, and th e d e c o m p o s itio n o f s u r f a c e oxygen co m p lex es t o y i e l d CO and C0 2 i s d iscu ssed in Chapter IV. XPS was n o t used f o r the carbon f ib e r s due to th e r e l a tiv e ly la rg e sam pling a re a o f th e in stru m e n t. F u rth e r v e r if ic a tio n o f th e p resen ce o f su rfa c e oxygen was done w ith DPV. P r e v io u s ly, DPV h a s b een u se d to d e te c t t h e p r e s e n c e o f s u r f a c e bound q u in o n e - lik e f u n c t i o n a l g ro u p s (1). T hese s u r f a c e s p e c ie s a r e th o u g h t to p la y a r o l e i n th e m e d ia tio n o f e l e c t r o n tr a n s f e r f o r a sco rb ic acid. Exam ination o f S urface S tru c tu re and M icro p o ro sity Changes in th e m icroscopic su rfa c e a re a w ere probed by SEM w ith a low a c c e l e r a t i n g v o lta g e (26) t o enhan ce s u r f a c e to p o g ra p h y. In a d d itio n, m ic r o s c o p ic s u r f a c e a r e a was p ro b e d by m e a s u rin g th e background ch arg in g c u rre n t w ith a chronocoulom etry s te p experim ent. C o n tr ib u tio n s t o th e t o t a l b ack g ro u n d c h a rg e, Qt, r e s u lt- f r o m t h r e e so u rc e s, 2 t dl + ^ re d o x + ads (2. 1 ) where i s th e n o n -fa ra d a ic c a p a c itiv e charge which i s dependent on th e m icroscopic s u rfa c e a re a and Qrecjox is th e fa ra d a ic charge due to th e p r e s e n c e o f s u r f a c e a n d /o r s o l u t i o n red o x s p e c ie s (28). I n th e a b sen c e o f s o l u t i o n s p e c i e s, Qre<jox r e s u l t s o n ly from s u r f a c e bound (chem isorbed) redox groups. These redox groups a re l ik e ly to be oxygen

40 20 complexes in the form o f quinones. The f in a l te rm, Qa<j s» i s fa ra d a ic charge due to s p e c ie s weakly adsorbed on th e e le c tro d e su rfa c e and i s assumed to be n e g lig ib le in th e e le c tr o ly te s used. The c o n t r i b u t i o n s to Qt from Qd l and Qre(j ox can be s e p a r a te d by perform ing a s e rie s o f p o te n tia l ste p s over v a rio u s p o te n tia l re g io n s (w ith and w ith o u t su rface-b o u n d redox couples). T his work was done by Hu and i s d e scrib ed in d e ta il elesew here (16,29). Thus, a com parison betw een th e values o f Qd l f o r v ario u s tre a tm e n t m ethods can y ie ld some in fo rm a tio n concerning the d egree o f m icro sco p ic su rfa c e a re a. Experimental 1. S urface tre a tm e n ts For m ost of th e ex p erim en ts, gc was f i r s t p o lish e d t o a m irro r l i k e f i n i s h w ith an a lu m in a s l u r r y on a g ro u n d g la s s p l a t e u s in g NANOpure w a te r ( S y b r o n / B a r n s t e a d, B o s to n, MA) f o l l o w e d by u l t r a s o n i c a t i o n. The gc p l a t e was p o lis h e d u s in g a c i r c u l a r m o tio n w ith heavy hand p r e s s u r e f o r s e v e r a l m in u te s w ith 600 g r i t, 1. 0 um, 0.3 um and 0.05 um a lu m in a (B u e h le r L td., Lake B l u f f, IL o r B aik o w sk i I n t e r n a t i o n a l, C h a r lo t te, NC) s u c c e s s iv e ly. B etw een e a c h s te p, t h e e le c t r o d e w as r in s e d w ith NANOpure w a te r and u l t r a s o n i c a t e d f o r 5 m inutes. The gc was th e n used im m ediatly fo r maximum a c t i v i t y (1), o r e x p o sed t o room a i r t o d e a c t iv a t e th e s u r f a c e. S u b se q u e n t th e r m a l tre a tm e n ts o f gc in v o lv ed VHT a t C a t p re s s u re s o f le s s th a n 2 x 1 0 ~ t o r r, o r r e a c tio n w ith oxygen o r n itro g e n in a flo w in g stre a m (1-2 L /m in.) a t 400 o r 700 C.

41 21 I t was n o t p o s s ib le to p o lis h th e carbon f ib e r s due to t h e i r s iz e and f r a g i l i t y. The carbon f ib e r s were vacuum t r e a te d a t C o r C a t 2 x l0 8 t o r r o r l e s s. R e a c tio n s w ith oxygen w ere c a rrie d o u t a t C o r C under 200 m to rr o f oxygen. The e x p e rim e n ta l a p p aratu s and proceedure f o r gc i s d e scrib ed below. The ap p aratu s f o r tr e a tin g carbon fib e r s i s d e scrib ed in d e ta i l in Chapter I I I (therm al d e so rp tio n in stru m e n ta tio n ). 2. Thermal tre a tm e n t and r e a c tio n cham bers Two d if f e r e n t system s w ere used f o r th erm al tre a tm e n t o f g la ssy carbon a s diagrammed in F ig u re 2-2. The gc h e a t tre a tm e n t chamber was c o n s tru c te d o f a sta n d ard 2-3 /4 inch o.d. s ta in le s s s te e l 6 -way u l t r a - h ig h vacuum (UHV) c r o s s. A tu n g s te n w ire was wound a ro u n d a q u a r tz tu b e (1.6 mm i.d.) w hich was e n c lo s e d i n a l a r g e r q u a r tz tu b e (2.5 mm i.d.) to t r a p any e v a p o ra te d tu n g s te n. T h is a s se m b ly, w h ich i s diagam m ed i n F ig u re 2-2A, was i n s e r t e d i n t o th e OHV c r o s s in a h o riz o n ta l p o s itio n. The sq u a re shown in th e c e n te r o f th e in n e r tube re p re s e n ts th e gc sam ple p o s itio n d u r in g t r e a tm e n t. E l e c t r i c a l and th e rm o c o u p le c o n n e c tio n s w ere made w ith a vacuum fe e d th ro u g h (C e ra m a s e a l, In c. New L ebanon C e n te r, NY). The tu n g s te n w ir e was h e a te d r e s i s t i v e l y u s in g a v a r i a b l e tr a n s f o r m e r. H igh vacuum was e s t a b l i s h e d in th e cham ber w ith a B a lz e r s (Hudson, NH) TPD 050 tu r b o m o le c u la r pump a f t e r a n i n i t i a l ro u g h pum ping w ith a l i q u i d n itro g e n -c o o le d m o lecu lar s ie v e s o rp tio n pump. These pumping system s w ere chosen to avoid co n tam in atio n o f th e sam ple su rfa c e w ith o i l due t o backstream ing. The te m p e ra tu re was m easured w ith a Chrom el-alumel

42 22 HIGH VACUUM UHV FEEDTHROUGH TUNGSTEN WIRE GC SAMPLE THERMOCOUPLE WIRE QUARTZ TUBES ULTRA - TORR FITTING PROPANE FLAME GAS F igure 2-2. Schem atic diagram s o f VHT chamber (A) and gas flow system (B) fo r th e rm a lly tr e a tin g g la ssy carbon.

43 23 (type K) therm ocouple w ire and an Omega (Stam ford, CT) model 450 AKT d i g i t a l therm ocouple gauge. The h e a t tr e a tm e n t p ro c e d u re in v o lv e d i n s e r t i n g a p o lis h e d, d e a c tiv a te d gc sam ple in to th e s m a lle r tube o f the VHT chamber. t i p o f th e therm ocouple made d ir e c t c o n ta c t w ith th e gc su rfa c e. The The gc was h e a te d by th e r m a l r a d i a t i o n from th e tu n g s te n w ir e a f t e r e v a c u a tio n t o 2 x 10 6 t o r r. The te m p e ra tu re w as m a in ta in e d a t C (some experim ents w ere run a t low er and h ig h e r tem p eratu res) f o r 10 m in u te s a t a p r e s s u r e b e tw e en 2 and 8 x 10" 6 t o r r. F o llo w in g h e a t tre a tm e n t, th e gc was allow ed t o cool to 35 C b e fo re th e chamber was f i l l e d w ith n itro g e n and opened to the atm osphere. G la ssy carb o n was a ls o r e a c te d w ith oxygen in a flo w in g stre a m u s in g th e e x p e r im e n ta l a p p a r a tu s d iag ram m ed in F ig u re 2-2B. T h is system c o n s is te d o f a q u a rtz tube (2.5 cm o.d.) w ith Cajon (Macedonia, OH) u l t r a - t o r r f i t t i n g s on b o th e n d s. The g a s s tre a m e n te r e d a t one e n d and flo w e d o u t a t th e o t h e r, w h ic h a l s o s e rv e d a s th e e n tr a n c e p o r t fo r th e therm ocouple w ire s. The sam ple was h e a te d by d ir e c tin g th e flam e from a propane to r c h a t th e o u ter su rfa c e o f the q u a rtz tube d ir e c tly beneath th e gc sam ple. The oxygen r e a c t i o n p ro c e d u re in v o lv e d p l a c in g a p o lis h e d gc sam ple in th e r e a c tio n tube and a llo w in g 0 2 t o flow a t 1-2 L/m in. f o r s e v e r a l m in u te s p r i o r t o h e a tin g. The sa m p le w as th e n h e a te d t o e i t h e r 400 o r 700 C f o r 10 m in u te s and a llo w e d t o c o o l i n th e 0 2 s tr e a m. The sam p le was rem oved fro m th e cham ber f o r s u b s e q u e n t a n a ly s is by e le c tro c h e m is try o r XPS.

44 24 3. E le ctro c h e m ic a l m easurm ents A ll e le c tro c h e m ic a l m easurm ents were done w ith an in -house b u i l t t h r e e e le c tr o d e p o t e n t i o s t a t o f c o n v e n tio n a l d e s ig n o r a BAS CV-lb e le c tr o c h e m i s tr y in s t r u m e n t. F o r some o f th e e x p e r im e n ts, th e p o t e n t i o s t a t was i n t e r f a c e d t o an A pple l i e com puter u s in g a 12 b i t D/A and A/D in te r f a c e (In te ra c tiv e M icrow are Inc., S ta te C ollege, PA). S oftw are f o r CV, CC and DPV was w r itte n in o u r la b o ra to ry by I.-F. Hu. The e le c tr o c h e m ic a l c e l l used f o r th e e x p e rim e n ts w ith gc i s shown in F ig u re 2-3. The gc p la te was held i n place a t th e bottom of th e c e l l by a sm all clamp and se p a ra te d by a Viton o r s ilic o n e rubber g a s k e t w ith a m easured in n e r area o f 0.08 cm2. The e le c tro c h e m ic a l geo m etric a re a was cm a s determ ined from chronocoulom etry u sin g FC and the d iffu s io n c o e f f ic ie n ts re p o rte d by Adams (30). The a u x i l i a r y e le c t r o d e was P t gauze i s o l a t e d by a g l a s s f r i t and th e re fe re n c e was a Ag/AgCl(satd.) e le c tro d e. A ll p o te n tia ls re p o rte d in t h i s work a r e r e f e r e n c e d t o Ag/AgCl (sa td.). The re fe re n c e e le c tro d e w as s e p a r a te d fro m th e s o lu t i o n b y a V ycor f r i t and a L uggin c a p i l l a r y, fo rm in g a t r i p l e j u n c t io n as show n in t h e f i g u r e. T his arrangem ent reduced th e chance of Ag+ e n te rin g th e w orking s o lu tio n a n d a ls o re d u c e d t h e ir d ro p b e tw e e n t h e r e f e r e n c e and w o rk in g e le c tr o d e s (31). E le c tro c h e m istry w ith carbon f ib e r s w as done in a 10 ml beaker. The f i b e r sa m p le w as m ounted on t h e t i p o f a b a re w ir e w ith s i l v e r ep o x y ( T e c k n it, C ra w fo rd, N J). The end o f t h e f i b e r w as im m erse d in th e e l e c t r o l y t e t o a d e p th o f 5-9 mm, r e s u l t i n g i n a m ic r o c y lin d e r

45 25 F igure 2-3. Schem atic diagram o f e le c tro c h e m ic a l c e ll u sed fo r g la ssy c a rb o n e x p e rim e n ts : A) A g /A g C l(satd.) r e f e r e n c e, B) Ag w ir e, C) tu b e f o r N2 g a s c o n n e c tio n, D) c ra c k e d P t j u n c t i o n, E) V ycor j u n c t i o n, F) P t g auze a u x i l i a r y e l e c t r o d e, G) g l a s s f r i t, H) L uggin c a p i l l a r y, X) clam p, J) s ilic o n ru b b er o r V iton g a s k e t, K) g la s s y carb o n e le c tro d e, L) Nylon d is k w ith e l e c t r i c a l c o n ta c t.

46 26 e le c tro d e w ith a m easured geom etric su rfa c e a r e a of 5.2 to 9.3 x 10 3 cm 2 ( f i b e r d ia m e te r o f 33 um). T he Ag/AgCl (sa td.) r e f e r e n c e was p la c e d in a Luggin c a p illa r y and p o s itio n e d n e a r the end o f th e f ib e r. The a u x ilia r y e le c tro d e was a Pt gauze w ire im m ersed d ir e c tly i n to the e le c tr o l y te. 4. Carbon sam ples and re a g e n ts A ll g la s s y carbon e le c tro d e s u se d in th e s e s tu d ie s were 1.5 cm s q u a re s c u t from l a r g e r p l a t e s o f T o k a i (Tokyo, Japan) g rad e GC-20. Carbon f ib e r s (33 um in diam eter) w hich were p y ro ly zed from c o a l t a r p i tc h were su p p lie d by AVCO S p e c ia lty Products D ivision (Lowell, MA). T hese f i b e r s r e c e iv e d a 3000 C h e a t t r e a tm e n t u n d e r argon by th e m a n u f a c tu r e r and a r e p ro d u c e d to b e a n o n c r y s t a l l i n e, i s o t r o p i c carbon, s im i la r in s tr u c tu r e to g la ssy carbon. The Young's modulus is 5 t o 6 x 1 0 p s i. R e a g e n t g ra d e p o ta s s iu m f e r r ic y a n id e, a s c o rb ic a c id (J.T. Baker C h e m ic a l C o., P h i l l i p s b u r g h, N J ), KC1, NaOH, H.3 PO 4, a n d HN03 (M allin ck ro d t In c., P a ris, KY, F ish er S c i e n t if i c, F a ir law n, NJ o r MCB, C in c in n a ti, OH) w ere used th ro u g h o u t. F e rro c y a n id e o r f e r r ic y a n id e and a s c o rb ic a c id s o lu tio n s were p re p a re d d a ily w ith NANOpure w ater. A ll e le c tro c h e m is try w ith FC was done u sin g 1 mm s o lu tio n s c o n ta in in g 1 M KC1. A s c o rb ic a c id s o l u t i o n s w e re 1 mm i n 0.1 MpH 2.0 p h o s p h a te b u f fe r c o n ta in in g 0.1 M KC1 (the KC1 w as o m itte d fo r th e experim ents w ith carbon f ib e r s ). The o x y g e n u s e d i n t h i s s t u d y was L in d e ( S o m e r s e t, NJ) h y d ro carb o n -free grade (H2 0, C02, and CO < 3 ppm ).

47 27 5. S u rface a n a ly s is equipm ent X -ra y p h o to e le c tr o n s p e c tro s c o p y w as p e rfo rm e d w ith a P e r k in - Elmer PHI (Eden P r a ir ie, MN) model 548 Esca/Auger system equipped w ith a d u a l anode (Mg and Al) x -ray so u rce and c y lin d r ic a l m irro r a n a ly z e r. The r e s u l t s re p o rte d h e re in were a l l o b ta in e d w ith Mg kct r a d i a t i o n. Some ex p erim en ts were reco rd ed u s in g s ig n a l averaging under com puter c o n tr o l w ith an IBM AT com puter. S can n in g e le c tr o n m ic ro g ra p h s w ere o b ta in e d on e i t h e r an IS I (M ilp ita s, CA) model SX-30 scanning e le c tr o n m icroscope o r an H itach i (Tokyo, Japan) model S-570 scanning e le c tro n m icroscope equipped w ith a LaBg f ila m e n t. The a c c e l e r a t i o n v o lta g e was e i t h e r 5 o r 15 kv in a l l c a se s. R e su lts and D iscussion 1. G lassy carbon G lassy carbon, a s -re c e iv e d from th e m an u fa ctu rer, i s a lm o st n ever used a s an e le c tro d e m a te r ia l w ith o u t f i r s t p o lis h in g th e s u rfa c e to a m ir r o r lik e f in is h. To th e naked eye, th e u n tre a te d s u rfa c e appears to h a v e a d u l l sheen a n d some s c r a t c h e s a r e e v id e n t. SEM a n a l y s i s rev ecils h o le s, contam inant p a r t i c l e s, im p e rfe c tio n s, and s c ra tc h e s as e v id e n t in F ig u re 2-4A. A fte r p o l i s h i n g, th e s u r f a c e a p p e a r s a s in F ig u r e 2-4B. Many o f th e h o le s have b e e n re d u c e d in s i z e a n d /o r f i l l e d in and much o f th e d e b ris has been removed. The p rese n c e o f a f in e l y compacted carbon powder la y e r has been p roposed to e x i s t a f t e r su c h p o l i s h i n g (26). I t i s n o t c l e a r, h o w ev er, t h a t su ch a pow d er

48 28 F ig u re 2-4. Scanning e le c tr o n m icrographs o f (A) a s -re c e iv e d and (B) p o lis h e d g la s s y carbon (both m icrographs o f a r e o f th e same sam ple). The s o l i d b a r n e x t t o th e t h r e e d a s h e d l i n e s a t t h e b o tto m o f th e photographs r e p r e s e n ts um.

49 29 la y e r, i f i t e x is t s, can be removed by VHT. The CV f o r FC, r e c o rd e d w ith an a s - r e c e i v e d, u n p o lis h e d, e le c t r o d e, i s shown i n F ig u re 2-5 (dashed c u rv e ). N o tic e t h a t th e peak s e p a ra tio n in th is case is q u ite la rg e (300 mv a t a scan r a te o f lv /sec), in d ic a tin g th a t th e e le c tro d e i s d e a c tiv a te d (e x h ib its a slow c h a rg e t r a n s f e r r a t e ). A f te r VHT, h o w e v e r, th e p e a k s e p a r a tio n was re d u c e d t o c a. 75 mv ( s o l i d c u rv e in F ig u re 2-5 ), w hich com pares fa v o r a b ly to a p o lis h e d gc e le c tr o d e ( d o tte d c u rv e ). The s u r f a c e o f th e a s - r e c i e v e d gc sa m p le, as o b s e rv e d by SEM, a p p e a re d unchanged a f t e r VHT, ex c ep t fo r th e rem oval o f some d e b ris. P o lis h e d gc shows maximum a c t i v i t y f o r FC o r AA i f i t i s u sed im m e d ia te ly a f t e r p r e p a r a tio n. I f e x p o sed to room a i r o r o th e r s o u rc e s o f c o n ta m in a n ts, th e s u r f a c e i s g r a d u a l ly d e a c t iv a t e d a s in d ic a te d by an in c re a se in AE, th e s e p a ra tio n betw een th e anodic and Jr c a th o d ic p e a k m axim a f o r FC, o r a s h i f t i n Ep a, th e a n o d ic p e a k p o t e n t i a l maximum f o r AA t o more p o s i t i v e v a u le s. F or e x a m p le, th e d o tte d c u rv e i n F ig u re 2-6 shows th e o x id a tio n o f AA a t a p o l is h e d, d e a c tiv a te d gc e le c tro d e. I f t h i s d e a c tiv a te d e le c tro d e i s tr e a te d by VHT, th e CV shown by th e s o l i d c u rv e in t h e f ig u r e i s o b ta in e d. VHT has e f f e c tiv e ly a c tiv a te d th e su rfa c e as e v id e n t by the s h i f t in Epa to a m ore n e g a tiv e v a lu e. N o tic e a ls o t h a t th e p e a k c u r r e n t f o r th e e le c tro o x id a tio n in c re a s e s w ith e x te n t o f a c tiv a tio n. This i s due to a change i n th e k i n e t i c p a r a m e te r, a n,, an d p o s s ib l y th e o x id a tio n a mechanism as d isc u sse d by Hu (11). The dependance o f a c tiv a tio n on VHT te m p e ra tu re was exam ined fo r AA as shown in F igure 2-7. Curve (a) shows th e CV fo r AA a t a gc

50 Potential (mv vs Ag/AgCl) F ig u re 2-5. C y clic voltam m etry o f fe rric y a n id e (1.0 mm in 1.0 M KC1) f o r a s - r e c e i v e d (dashed l i n e ), f r e s h l y p o lis h e d ( d o tte d l i n e ), and vacuum h e a t t r e a te d (so lid lin e ) g la s s y carbon. R eference e le c tro d e : Ag/AgCl (sa td.), scan rate=*1.0 V /s e c, i= ua.

51 Potential (mv vs Ag/AgCI) F ig u re 2-6. C y c lic v o lta m m e try o f a s c o r b ic a c id o x id a tio n f o r p o l is h e d, ( d o tte d c u rv e ) and p o l is h e d, vacuum h e a t t r e a t e d ( s o l i d curve) g la s s y carbon. E x p erim en tal c o n d itio n s: 1.0 mm a s c o rb ic a c id i n 0.1 M p h o s p h a te b u f f e r (ph=2.0) w ith 0.1 M KC1, r e f e r e n c e : Ag/AgCI (sa td.), scan rate= 100 mv/sec., and i=6.33 ua.

52 32 e le c tr o d e t h a t r e c e iv e d a 520 C VHT, c u rv e (b) a 650 C VHT a n d curve (c) a 750 C VHT. C le a rly, as th e tem p e ra tu re i s in c re a s e d, th e ex te n t o f a c t i v a t i o n i s a ls o in c r e a s e d (750 C w as th e u p p e r l i m i t f o r o u r h e a tin g sy ste m ). S i m i la r r e s u l t s w ere o b s e rv e d f o r FC. P o lis h e d /d e a c tiv a te d gc e le c tro d e s ty p ic a lly g iv e peak se p a ra tio n s f o r FC o f g re a te r th a n 100 mv a t a scan r a t e o f 1 V /sec. A fte r VHT, th e s e v a lu e s were reduced to ca. 65 mv o r l e s s, which i s c lo s e t o the therm odynam ic lim it o f 59 mv f o r a one e le c tro n p ro c e ss. The peak s e p a ra tio n s f o r FC and E 's fo r AA o x id a tio n a r e su m m arized in T a b le 2-1 f o r th e v a r i o u s l y t r e a t e d s u rfa c e s. Changes in th e b ack g ro u n d c u r r e n t ( c u r r e n t i n th e a b s e n c e o f s o lu tio n redox sp ecies) due to VHT were exam ined by re c o rd in g a CV in b u f fe r only f o r p o lish e d and polished/vht e le c tro d e s. These r e s u lts a re shown in F igure 2-8. Curve (a) in the fig u re shows the background tra c e f o r a fre s h ly p o lis h e d gc e le c tro d e. The s e p a ra tio n betw een the fo rw a rd and r e v e r s e s c a n s i s c a. 10 ua in t h e 0 to 500 mv p o t e n t i a l r e g io n. N o tic e a ls o t h e p r e s e n c e o f a v e r y weak r e v e r s i b l e peak, c e n te r e d a t 250 mv, due t o th e re d o x o f a s u r f a c e b o u n d q u in o n e - lik e fu n c tio n a l groups (1). The background CV f o r a p o lis h e d, vacuum heat t r e a t e d gc e le c tr o d e i s show n by c u rv e (b ). Im m e d ia te ly o b v io u s i s the d ram a tic re d u c tio n in the background c u rre n t to about 2 ua. The peaks due to th e redox o f th e su rfa c e bound quinones a r e a ls o g re a tly dim in ish ed. The in c re a s e in c u rre n t a t p o te n tia ls g r e a t e r th a n 600 mv

53 33 5 < 3 Z Ui IT tr 3 U T11111II11n * n 11II11111II IT POTENTIAL E <mv) 50 F ig u re 2-7. C y c lic v o lta m m e try f o r a s c o r b ic a c id a t p o l is h e d, d e a c tiv a te d, g la ssy carbon w hich was vacuum h e a t tre a te d a t 520 C (a), 650 C (b ), and 750 C (c). The s o lu t i o n c o n d itio n s and s c a n r a t e a r e th e same a s d e scrib e d fo r F ig u re 2-6, re fe re n c e : Ag/AgCl(satd.).

54 Table 2-1 Treatment AEp, FC (mv) Epa, AA (mv) Q t (uf/cm ) O/C1 polished only 68±1 270± 5 117±1 polished, deactivated 10 8 ± ± ±0.05 polished, deactivated, followed by VHT ± ±0.01 FC=ferricyaride, AA=ascorbic acid Q j= to ta l background charge, VKT=vacuum h eat treatm ent 'u n c o rrec ted p ea k height ratios from C(18) and O ils) XPS signal

55 llll Mill N i l INI Mill Mil CURRENT <ua> >2ioO POTENTIAL E GnV) F ig u re 2-8. C y c lic v o lta ra m e try c u rv e s r e c o r d e d i n ph 2.0 p h o s p h a te b u f fe r f o r p o lis h e d (tra c e a) and p o lis h e d, vacuum h e a t tr e a te d (tra ce b) g lassy carbon. R eference: Ag/AgCl(satd.) scan rate= 0.5V /sec.

56 36 f o r b o th c u rv e s i s due to th e o x id a tio n o f e l e c t r o l y t e (H2 0 i n t h i s c a se ) and th e e le c tr o d e s u r f a c e (32). VHT may have th e e f f e c t o f reducing th e r a t e o f th is p ro c e ss s lig h tly. A f u r th e r in v e s tig a tio n o f th e background charge was done using c h ro n o c o u lo m e try. In t h i s e x p e rim e n t, th e p r e t r e a t e d e l e c tr o d e ( p o lis h e d o n ly o r polished/v H T ) was p la c e d in a ph 2.0 p h o s p h a te b u f f e r s o lu t i o n and th e p o t e n t i a l s te p p e d from 0 t o 600 mv. The e x tra p o la te d in te r c e p t o f th e r e s u ltin g Q vs. t ^ be th e t o t a l charge, Qt, fo r t h a t p o te n tia l s te p. 2 p lo t was assumed to The r e s u l t s confirm th e above c o n ten tio n th a t th e VHT reduces th e t o t a l background charge a s i n d ic a te d by th e v a lu e s o f Qt r e p o r te d in T ab le 2-1. N o tic e t h a t th e VHT method h as the e f f e c t o f reducing th e background charge by a f a c t o r o f 10 com pared to t h e p o lis h e d o n ly s u r f a c e. T h is m easu red c h a rg e in c lu d e s c o n t r i b u t i o n s from b o th f a r a d a ic p r o c e s s e s from s u r f a c e bound g ro u p s and n o n - f a r a d a i c p r o c e s s e s due t o th e d o u b le l a y e r ; h o w ev er, c o n t r i b u t i o n s from s o lu t i o n re d o x s p e c ie s and p h y siso rb ed m olecules were assum ed to be zero in b u ffe r o nly s lig h t ly. Hu perform ed a chronocoulom etry s te p experim ent which s e p a ra te d th e fa ra d a ic and n o n -fa ra d a ic charge (1 6,2 9 ). T hese r e s u l t s c o n firm th e f a c t t h a t th e a c t i v a t i o n o f gc by VHT d o es n o t r e s u l t i n an in c r e a s e i n e i t h e r th e n o n - f a r a d a ic o r f a r a d a i c c h a rg e (in f a c t, b o th a r e re d u c e d ). T h is s u g g e s ts t h a t th e s u r f a c e m ic r o p o r o s ity i s n o t in c re a se d by VHT, sin ce such an in c re a s e would r e s u l t in a h ig h e r nonf a r a d a ic charge. The p o lis h e d and VHT gc s u r f a c e s w ere a n a ly z e d by XPS to determ ine the e f f e c t o f VHT on th e amount o f su rfa c e oxygen. The

57 '3 7 r e s u l t s a r e shown i n F ig u re 2-9. The XPS s u rv e y s c a n shows th e binding energy vs. in te n s ity over a 1000 ev ran g e. The o n ly two peaks o b s e rv e d i n th e sp e c tru m a r e due t o oxygen (530 ev) a n d carbon (285 ev). The s m a ll p e a k a t 740 ev i s due t o an oxygen KW A uger t r a n s i t i o n (33). T ra ce (a) show s th e r e s u l t s f o r a p o lis h e d gc sam p le. In t h i s c a s e, th e u n c o rre c te d 0/C p e a k h e ig h t r a t i o was S u b se q u e n t h ig h r e s o l u t i o n e x p e rim e n ts, u s in g s i g n a l averaging, showed t h i s to correspond to 8-10% oxygen on th e s u rfa c e. The XPS sp e c tru m shown by t r a c e (b) was o b ta in e d f o r th e sam e gc e le c tr o d e a f t e r VHT. N o tic e t h a t th e 0/C r a t i o was d e c re a s e d t o an a lm o s t u n m e a s u r a b le l e v e l ( ), w h ic h i s e s tim a te d to c o rre s p o n d t o a b o u t 3% s u r f a c e oxygen. XPS w as a ls o u se d t o c o n firm t h a t th e c a rb o n s u r f a c e d id n o t co n tain any o th e r co n tam in ats in tro d u ced by p o lis h in g o r VHT. For gc p o lish e d w ith alum ina, an XPS s ig n a l fo r Al(2s) an d /o r Al(2p) a t ev i s i n d i c a t i v e o f alum inum c o n ta m in a tio n. The A1 p e a k s w ere n o t o b s e rv e d f o r th e p o lis h e d o r polished/v H T s a m p le s p r e p a r e d in t h i s s tu d y. A lso, th e a b sen c e o f W(4f) p eak s a t ev shows t h a t tu n g ste n was n o t d e p o site d on the carbon su rfa c e during VHT. To exam ine th e e f f e c t o f VHT on th e s u r f a c e bound q u in o n e fu n c tio n a l groups t h a t a re p re s e n t on a fre s h ly p o lish e d su rfa c e, DPV a n a l y s i s was p e rfo rm e d on gc sam p le s p r i o r t o and a f t e r VHT. The voltammograms were reco rd ed in ph 2.0 phosphate b u ffe r w ith c u rre n t sam p led 3.3 m sec a f t e r th e p u ls e and th e d c b a c k g ro u n d c u r r e n t

58 38 C Binding E n erg y (ev) F ig u r e 2-9. X -ray p h o t o e l e c t r o n s p e c t r a o f p o lis h e d g l a s s y c a rb o n b e fo re (tra c e a) and a f t e r (tra c e b) vacuum h e a t tre a tm e n t.

59 39 s u b tra c te d 4.0 usee p r i o r t o th e p u ls e. The r e s u l tin g voltammogram s th u s re p r e s e n t e v e n ts o c c u rrin g very n e a r the e le c tr o d e su rfa c e (redox o f su rfa c e bound g ro u p s). These r e s u l t s are shown in F ig u re 2-10 fo r p o l is h e d ( t r a c e a) a n d p o lish e d /V H T ( tr a c e b) g c. The tw o p e a k s a t 275 and 700 mv f o r t h e p o l is h e d o n ly s u r f a c e have b e e n p r e v io u s ly o b s e rv e d (1,3 4 ). The p eak a t 275 mv i s th o u g h t t o a r i s e fro m a q u i n o n e - l ik e s u r f a c e f u n c t i o n a l i t y a s d is c u s s e d ab o v e. VHT h a s th e e f f e c t o f r e d u c in g th e m a g n itu d e o f b o th p e a k s and a l s o s h i f t s th e p o t e n t i a l o f th e p e a k s t o m ore p o s i t i v e v a lu e s. The c h e m ic a l s tr u c tu r e o f th ese s u rfa c e groups is e v id e n tly a lte r e d b y VHT. Thus, VHT c l e a r l y r e s u l t s in a d e c re a se i n th e am ount o f su rfa c e oxygen p r e s e n t on th e su rfa c e o f gc w ith a c o n c u rre n t in c re a s e i n the e l e c t r o c h e m i c a l a c t v i t y. I n o r d e r t o d e te r m in e i f t h i s i n c r e a s e i s due to th e rem oval o f oxygen o r to th e th erm al tre a tm e n t, s e v e r a l gc e l e c t r o d e s w ere p r e p a r e d b y t r e a t m e n t in 0 2 o r N2 a t e l e v a te d t e m p e r a t u r e s u s in g t h e s y s te m shown i n F ig u re 2-2B. T he gc s a m p le s w ere p rep a re d by f i r s t p o lis h in g and d e a c tiv a tio n fo llo w e d by h e a tin g i n a f lo w in g s tr e a m o f 02 o r N2 a t 400 o r 700 C. The r e s u l t s a re sum m arized in Table 2-2. In t h i s c a se, a l l th e rm a l tre a tm e n ts had the e f f e c t o f i n c r e a s i n g th e am o u n t o f s u r f a c e oxygen p r e s e n t o n th e s u rfa c e (compared t o p o lis h e d only) a s d eterm in ed from th e c o rre c te d XPS p eak a r e a s f o r 0 ( l s ) and C (ls) p e a k a r e a s. The N2 h e a t t r e a tm e n t in c re a s e d th e oxygen c o n te n t s lig h t ly due to t r a c e am ounts o f oxygen o r due t o o x i d a t io n b y a d s o rb e d 0 2 o r H20 i n i t i a l l y p r e s e n t o n th e s u rfa c e.

60 Z niilmil ii iii in iiiiiim SO POTENTIAL E CmV) F ig u re D i f f e r e n t i a l p u ls e v o lta m m e tr y c u rv e s f o r p o l is h e d g la s s y c a rb o n b e f o r e ( t r a c e a) a n d a f t e r ( tr a c e b) vacuum h e a t t r e a tm e n t. E x p e rim e n ta l c o n d itio n s : 0.1 M phoshpate b u ffe r (ph=2.0) w ith 0.1 M KC1, DC scan r a te = 2 m V /sec, p u ls e a m p litu d e = 1 0 mv, p u l s e width=»60 m sec, p u l s e freq u en cy = 2 H z, s a m p lin g tim e= 3.3 msec a f t e r p u lse, re fe re n c e e le c tr o d e : A g /A g C l(sa td.).

61 Table 2-2 Treatment %0( 1 s)1 Epa, AA (mv) polished only GPO (400 C) GPO (620 C) NHT (700 C) A A = a s c o r b i c a c i d, G P O = g a s p h a s e o x i d a t i o n i n O, N H T = n i t r o g e n h e a t t r e a t m e n t 1c a l c u l a t e d f r o m c o r r e c t e d X P S p e a k a r e a s

62 42 As e v id e n t from th e r e p o r te d Ep a 's f o r AA, a l l o f th e th e r m a l tre a tm e n ts had th e e f f e c t o f im proving the a c tiv ity compared t o the p o lis h e d, d e a c t iv a t e d e le c tr o d e. The v a lu e s a re c lo s e to th o s e o b ta in e d f o r VHT. T hese r e s u l t s s u g g e s t t h a t th e r m a l t r e a tm e n t, r a t h e r th a n re m o v a l o f s u r f a c e oxygen, p la y s a g r e a t e r r o l e in a c tiv a tio n o f gc s u rfa c e s. 2. Carbon F ib e rs C arbon f i b e r s, a s r e c e iv e d from th e m a n u f a c tu r e r, show no e le c tr o c h e m ic a l a c t i v i t y f o r AA o r FC an d t h u s r e q u i r e som e p r e t r e a t m e n t. H ow ever, due t o t h e i r s m a ll s i z e and f r a g i l i t y, i t i s i m p r a c t ic a l t o p o l is h th e s u r f a c e a s was done f o r g c. T y p ic a lly, c arb o n f i b e r s a r e a c t i v a t e d by e le c tr o c h e m i c a l o x id a tio n (1 9,2 0 ). This tre a tm e n t in v o lv e s c y c lin g th e p o te n tia l, u s u a lly betw een 0.0 and ca. 2.0 V, a t a g iv e n fre q u e n c y. To e s t a b l i s h a c o m p a riso n b e tw e e n fib e r s and gc and b e tte r understand th e causes o f a c tiv a tio n, th e VHT e x p e rim e n ts w e re e x te n d e d t o in c lu d e a s tu d y o f c a rb o n f i b e r m ic ro e le c tro d e s. This work was done j o in t ly w ith Mike Weber in our group. The CV f o r FC a t a s -re c e iv e d, u n tre a te d carbon f ib e r s is shown in F ig u re 2-11A. No o x id a tio n o r r e d u tio n p e a k s a r e o b s e rv a b le. Follow ing e le c tro c h e m ic a l tre a tm e n t betw een 0.0 and 1.8 V a t 1 V /sec f o r 10 m i n u t e s, t h e CV show n i n F i g u r e 2-11C i s o b s e r v e d. E le ctro c h e m ic a l o x id a tio n s o f t h i s ty p e have been shown to in tro d u c e su rfa c e oxygen (4,5). The e le c tro c h e m ic a l tre a tm e n t has a c tiv a te d th e f ib e r s u rfa c e and r e s u lte d in a peak s e p a ra tio n o f mv (scan r a te

63 ua 0.4 ua 1.0 ua 1.0 ua F ig u re C yclic voltammograms f o r fe rric y a n id e (1.0 mm in 1.0 M KC1) a t (A) u n tre a te d, (B) vacuum h e a t tr e a te d, (C) e le c tro c h e m ic a lly o x i d i z e d, a n d (D) g a s p h a s e o x i d i z e d ( i n 0 2) c a r b o n f i b e r m ic r o e l e c t r o d e s. P o t e n t i a l s c a l e : V=200 mv, r e f e r e n c e e l e c t r o d e : Ag/AgCI (sa td.), scan r a te : 100 mv/sec.

64 44 o f 100 m V /m in.). I f a f i b e r i s t r e a t e d by VHT a t C (2 x 1 0 " 8 t o r r ), th e CV shown in F ig u re 2 - llb i s o b s e rv e d. C le a r ly, o n ly p a r t i a l a c tiv a tio n h as o c c u rre d, which i s m arkedly d if f e r e n t from th e r e s u l ts o b tain ed a t gc. A more severe VHT ( C) r e s u lts in no im p ro v em en t in th e e l e c tr o c h e m i s tr y o f FC. I f th e f i b e r i s r e a c te d w ith oxygen a t C fo llo w in g VHT, however, an a c tiv e e le c tro d e f o r th e re d o x o f FC i s o b ta in e d a s shown i n F ig u re 2-11D. The r e s u l t i n g p e a k s e p a r a tio n o f mv i s s l i g h t l y b e t t e r th a n t h a t r e s u ltin g from e le c tro c h e m ic a l o x id a tio n. N otice a lso t h a t th e shape o f th e voltammograms changes w ith th e v a r io u s t r e a tm e n ts. The sh a p e o f th e CV i n F ig u re 2 - llb f o r VHT f i b e r s i s a p p ro a c h in g a s te a d y s t a t e voltam m ogram a s o b se rv e d a t m icro d isk e le c tro d e s (35), w hereas th e o x id a tiv e tre a tm e n ts r e s u l t in p e a k -s h a p e d voltam m ogram s a s o b s e rv e d in F ig u re 2-11C and 2-11D. A c c o rd in g t o M atsu d a, h o w e v e r, i t i s n o t p o s s i b l e t o a c h ie v e p u re s te a d y s t a t e b e h a v io r a t m ic r o c y lin d e r e l e c tr o d e s (3 6 ), a s u se d in t h i s s tu d y. tre a tm e n ts The c a u s e i f t h e change i n sh a p e w ith th e o x id a tiv e i s n o t known, b u t i t may be due to a d s o rp tio n o r a su rfa c e a r e a e f f e c t (37). To b e t t e r u n d erstan d th e e f f e c t o f VHT and gas phase o x id a tio n on th e s u r f a c e, SEM a n a l y s i s w as done on u n t r e a te d and t r e a t e d c a rb o n f i b e r s. The r e s u l t s r e v e a l a sm ooth f e a t u r e l e s s s u r f a c e f o r a s - re c e iv e d f ib e r s. D uring VHT, however, e ru p tio n s o c c u rre d on th e f ib e r s u r f a c e a s o b s e rv e d i n th e m ic ro g ra p h shown i n F ig u re T hese s m a ll e r u p tio n s can b e l o c a l i z e d and a re so m e tim e s b ro k en o p e n,

65 F ig u r e S c a n n in g e l e c t r o n m ic ro g ra p h s o f vacuum h e a t t r e a t e d c a rb o n f i b e r s shown a t 250X (A) and 1200X (B) m a g n i f ic a tio n. The d o tte d lin e a t th e bottom o f th e photographs r e p re s e n ts 120 and 25 urn, r e s p e c tiv e ly.

66 4 6 depending on th e te m p e ra tu re o f VHT. I t ap p ears t h a t some o u te r s k in s t r u c t u r e h a s been expanded due t o th e r e l e a s e o f tr a p p e d g a s s e s d u rin g VHT. The p r e s e n c e o f a o u t e r s k in s u rro u n d in g an in n e r c o re has been proposed a s one p o s s ib le type o f s tr u c tu r e fo r some ty p es o f f ib e r s as d isc u sse d in C hapter I (38). A ccording to the m an u fa ctu rer, h o w ev er, s u r f a c e. th e s e f i b e r s hav e no c o a tin g o r r e s i n a p p lie d to t h e i r When a VHT f i b e r has r e c e iv e d a g as p h a s e o x id a tio n a s d e s c r ib e d a b o v e, th e s u r f a c e a p p e a rs a s shown i n th e m ic ro ra p h in F igure In t h i s c a se, a c le a r c o rro sio n o r p i t t i n g o f th e carbon su rfa c e has o ccu rred. I t i s proposed th a t t h i s p i t t i n g b e g in s a t th e e ru p ted s i t e s i n i t i a l l y produced by VHT. The a c tiv a tio n f o r FC observed a t th e gas phase o x id iz e d f ib e r s may be due to th e fo rm atio n o f s u rfa c e oxygen groups o r, more lik e ly, to th e exposure o f th e in n e r, more a c tiv e, carbon s tr u c tu r e. I t seems lik e ly t h a t th e o u te r su rfa c e o f th e carbon f ib e r s, as re c e iv e d, does n o t e x h ib it ra p id charge tra n s f e r. VHT fo llo w e d by o x id a tio n r e s u l ts in th e "o p en in g up" o f t h e in n e r c a rb o n, w hich i s d i f f e r e n t th a n o b s e rv e d f o r g c, i n w h ic h th e i n i t i a l p o l i s h i n g s te p w as u se d t o remove any i n i t i a l s u rfa c e la y e r. R e c a ll, how ever, th a t FC redox was p o s s ib le a t gc t h a t has re c e iv e d o n ly VHT. The r e s u l ts o f AA o x id a tio n a t carbon f ib e r e le c tro d e s re c e iv in g th e sam e t r e a t m e n t s a s d e s c r ib e d f o r FC a r e show n in F ig u re Once a g a in, no s ig n a l was observed a t th e u n tre a te d f ib e r (2-14A), and th e e le c tro c h e m ic a l o x id a tio n has th e e f f e c t o f a c tiv a tin g th e su rfa c e t o g iv e an Epa o f mv (2-14C) a t a s c a n r a t e o f 100 m V /sec. In

67 F ig u re Scanning e le c tr o n m icrographs o f vacuum h e a t tr e a te d and g a s p h a s e o x i d i z e d f i b e r s sh o w n a t 2200X (A) a n d 5000X (B) m a g n i f ic a tio n. The d o t te d l i n e a t th e b o tto m o f th e p h o to g ra p h s r e p r e s e n ts 13.6 and 6.0 urn, r e s p e c tiv e ly.

68 48 F igure C yclic voltam m ogram s fo r a sco rb ic a c id (1.0 mm in ph 2.0 p h o s p h a te b u f f e r ) a t (A) u n t r e a te d, (B) vacuum h e a t t r e a t e d, (C) e le c tro c h e m ic a lly o x id iz e d, and (D) gas phase o x id iz e d (in O2 ) carbon f i b e r s. The t h r e e t r a c e s in (D) r e p r e s e n t 3 t r i a l s fro m th e sam e b u n d le a f t e r o x id a tio n. P o t e n t i a l s c a l e : V=200 mv, r e f e r e n c e : Ag/AgCI (sa td.), scan r a t e : 100 mv/sec.

69 49 t h i s c a s e, how ever, VHT r e s u l t s in no en h an cem en t i n a c t i v a t i o n as observed by the CV i n Figure 2-14B. when th e f ib e r s a re re a c te d w ith oxygen as d escrib ed f o r FC, a c tiv a tio n was observed to vary in g degrees and w ith o u t good r e p r o d u c ib ility. The th re e se p a ra te CV's in Figure 2-1 4D fo r th re e f ib e r s from th e same bundle (carbon f ib e r s a re tre a te d i n a b u n d le o f s e v e r a l h u n d re d f i b e r s as d e s c r ib e d i n C h a p te r I I I ) i l l u s t r a t e t h is i r r e p r o d u c ib ility. R e c a ll t h a t FC a c tiv a tio n by th is m ethod show ed good r e p r o d u c i b i l i t y. F or a s i n g l e f i b e r o x id iz e d in a i r, the r e s u ltin g CV was s im ila r to th e s o lid curve in Figure 2-14D and was v e ry rep ro d u c ib le. I t i s assumed t h a t the v a r i a b i l i t y in AA a c tiv a tio n i s due to the s l i g h t d iffe re n c e s i n the e x te n t of s u rfa c e o x id a tio n depending on the p o s i t i o n o f th e f i b e r in th e b u n d le d u rin g t r e a t m e n t. These s m a ll d iffe re n c e s had no e f f e c t on th e a c tiv a tio n f o r FC, w hich showed good r e p r o d u c i b i l i t y r e g a r d l e s s o f th e f i b e r p o s ito n s in t h e b u n d le. In g e n e ra l, a s the e x te n t of s u rfa c e o x id a tio n in c re a s e d, th e a c tv ia tio n a l s o in creased. The r e s u l t s o f DPV a n a l y s i s, a s p r e v io u s ly d e s c r ib e d f o r gc r e v e a l t h a t th e e le c tr o c h e m i c a l o x id a tio n p ro d u c e s a s i g n i f i c a n t am ount o f su rfa c e bound redox groups a t ca. 325 mv. The DPV p l o ts fo r VHT fib e rs e x h ib ite d no peaks in the p o te n tia l reg io n o f 0.0 to 0.6 V. F o r gas p hase o x id iz e d s u rfa c e s, a s m a ll peak was observed w ith DPV a t mv, b u t was a l s o i r r e p r o d u c i b l e. F i b e r s t h a t e x h ib i te d th e maximum DPV response f o r s u rfa c e bound redox groups a ls o gave th e most n e g a tiv e Epa fo r AA, i n d i c a t i n g th e h ig h e s t d e g re e o f a c t i v a t i o n.

70 50 A p p a r e n t l y, a d i r e c t c o r r e l a t i o n e x i s t s b e tw e e n t h e r a p i d e le c tro o x id a tio n o f a sco rb ic a c id a t carbon f ib e r s and th e p rese n c e o f s u rfa c e -b o u n d oxygen g ro u p s t h a t a r e e l e c t r o a c t i v e a t p o t e n t i a l s s im ila r to the E fo r AA. E v id e n tly, th e e le c tro c h e m ic a l o x id a tio n Xr r e s u l ts in th e production o f more e le c tr o a c tiv e oxygen com plexes than does th e gas phase o x id a tio n in Oj. This was a ls o observed by Woodard (27), who claim ed th a t e le c tro c h e m ic a lly o x id ized f ib e r s co n ta in ed 10X more e le c tr o a c tiv e su rface oxygen th an did a i r o x id ized f ib e r s. C h ro n o c o u lo m e try e x p e rim e n ts w ere p e rfo rm e d, a s p r e v i o u s ly d is c u s s e d, to d e te rm in e Qt, th e t o t a l b ack g ro u n d c h a rg e f o r th e v a r i o u s ly t r e a t e d f i b e r s u r f a c e s. The r e s u l t s a r e su m m a riz ed in T a b le 2-3 a lo n g w ith th e o b s e rv e d p e a k s e p a r a tio n s f o r FC and th e Epa 's f o r AA. VHT had v e ry l i t t l e e f f e c t on th e c h a rg e, w h e re a s a l l o th e r tre a tm e n ts r e s u l t in an in c re a s e. The la r g e s t v a lu e 135 uf/cm2, was o b s e rv e d f o r th e e le c tr o c h e m i c a l ly o x id iz e d s u r f a c e ; a good p o r tio n o f w hich i s due t o f a r a d a i c c h a rg e from th e s u r f a c e bound re d o x g ro u p s t h a t a r e in tr o d u c e d by t h i s tr e a tm e n t (as e v id e n t from DPV). The v a lu e o f 80 uf/cm2 o b tain e d fo r f ib e r s r e a c te d w ith oxygen re p re s e n ts p r im a rily n o n -fa ra d a ic charge due to th e sm a ll amount o f su rfa c e fu n c tio n a l groups in tro d u c e d t h a t a re e le c tr o a c tiv e in th e 0.0 t o 0.6 V r e g io n. Thus, th e g a s p h a s e o x id a tio n does r e s u l t in an in c re a s e, to some e x te n t, in th e m icroscopic s u rfa c e roughness o f the f ib e r as compared to VHT, i f i t i s assumed t h a t th e o n ly c o n trib u tio n t o th e n o n - f a r a d a ic c h a rg e i s d i r e c t l y r e l a t e d t o t h i s m ic ro s c o p ic roughness. F u rth e r evidence f o r t h i s i s o b tain e d by th e o b se rv a tio n

71 51 Table 2-3 Treatment AEp, FC Epa, AA (mv) (mv) 1 (uf/cm2 ) 1. none 2. VHT (1000 C) 3. VHT (600 C) 4. GPO 114 ± ± 1 15 (650 C> 5. ECO 128± ± ± ± ± ± ± 3 FC=ferrocyanide, AA=ascorbic acid, Qt =total background charge VHT=vacuum heat treatment, GPO=gas phase oxidation in O2 ECO=electrochemical oxidation

72 52 o f s m a ll h o le s and p i t t s e v id e n t in th e e l e c t r o n m ic ro g ra p h s (se e F ig u re 2-13). Summary an d C o nclusions In o rd er t o d isc u ss th e a c tiv a tio n o f carbon e le c tr o d e s, e le c tro n tr a n s f e r i s assumed to o ccu r a t s p e c if ic su rface s i t e s w hich vary in r e l a tiv e amounts depending on th e su rfa c e tre a tm e n t. These s ite s may in c lu d e c a rb o n -o x y g e n s u r f a c e f u n c t i o n a l g ro u p s, a r o m a tic c a rb o n s i t e s, edge-type p la n e s, o r b a s a l d e fe c ts. A c tiv a tio n may a ls o o ccur due t o i n c r e a s e s in th e e f f e c t i v e s u r f a c e a re a due to an i n c r e a s e in th e m ic ro p o ro sity. D eactiv ated e le c tro d e s r e s u l t when an i n s u f f ic i e n t num ber o f s i t e s a r e p r e s e n t, o r th e s i t e s a r e somehow b lo c k e d, p re v e n tin g e le c tr o n t r a n s f e r. In th e c a s e o f g c, VHT was found to a c t i v a t e th e s u r f a c e o f a s - re c e iv e d and p o lis h e d /d e a c tiv a te d e le c tro d e s. Therm al tre a tm e n ts in f lo w in g s tre a m s o f oxygen a n d /o r n itr o g e n a ls o have th e e f f e c t o f e n h a n c in g th e a c t i v i t y. Thus i t i s p ro p o se d t h a t th e s e t r e a tm e n t m ethods r e s u lt in th e rem oval o f contam inants and produce a " p ris tin e " carbon su rfa c e which a llo w s ra p id e le c tro n t r a n s f e r. VHT a ls o reduces th e am ount o f s u r f a c e bound oxygen f u n c t i o n a l itie s, in d ic a tin g t h a t t h e i r presence i s n o t n e c e ssa ry i n o rd er t o achieve a c tiv a tio n. Thus i t a p p e a r s t h a t a th e r m a l tr e a tm e n t t o rem ove c o n ta m in a n ts a n d /o r a l t e r th e chem ical su rfa c e s tr u c tu r e s li g h t ly i s th e m ost im p o rta n t e f f e c t i n th e a c t i v a t i o n m echanism a t g c. The p o s s i b i l i t y t h a t s u rfa c e oxygen m ed iates e le c tr o n t r a n s f e r a t o th e r w is e c o n ta m in a te d carbon s t i l l e x is t s. The n a tu re o f th e se contam inants i s n o t known.

73 53 Since fre s h ly p o lis h e d gc d e a c tiv a te s w ith exposure to am bient a i r, i t may be a s im p le a d s o r p tio n o f g a s m o le c u le s t h a t b lo c k th e a c t i v e s i t e s. H ow ever, s in c e th e p o l is h i n g was done i n w a te r, and a l l tre a te d gc e le c tro d e s were im m ediatly im m ersed in e le c tr o l y te, i t i s p o s s ib le th a t th e h ig h ly a c tiv e s u rfa c e was s ta b liz e d o r p ro te c te d by aqueous m edia. The re d u c tio n in th e background charging c u rre n t a f t e r VHT i s due to th e rem oval o f su rfa c e bound oxygen groups th a t c o n trib u te to th e fa ra d a ic charge, and e v id e n tly to s tr u c tu r a l changes o r a n n ealin g th a t r e s u l t in a re d u c e d n o n - f a r a d a ic c h a rg e. From an a n a l y t i c a l sta n d p o in t, s m a lle r background c u rre n ts a re advantageous s in c e low er c o n c e n tra tio n s o f s o lu tio n o r adsorbed sp e c ie s can th e n be determ ined. I t i s n o t expected, -however, th a t s ig n if ic a n t s tr u c tu r a l changes can o c c u r a t th e s e te m p e r a tu r e s. T hese r e s u l t s i n d i c a t e t h a t th e a c tiv a tio n mechanism f o r VHT a t gc cannot be a ttr ib u t e d to an in c re a s e in th e m icro sco p ic roughness. The r e s u l ts f o r carbon f ib e r s a re q u ite d if f e r e n t. Im m ediately o b v io u s from th e r e p o r t e d p e a k s e p a r a tio n s f o r FC a n d th e E_'s f o r p a AA, c o u p le d w ith th e l i m i t e d a c t i v a t i o n by VHT, i s th e f a c t t h a t carbon f ib e r s do n o t e x h ib it the same degree o f a c tiv a tio n as does gc (although i t has been p o lish e d ). A pparently, carbon f ib e r s p o seses an o u te r s k in s t r u c t u r e w h ic h i s a form o f c a rb o n t h a t does n o t a llo w ra p id e le c tr o n tr a n s f e r. In o rd e r t o a c tiv a te the s u rfa c e by th erm al m e th o d s, i t i s n e c e s s a r y t o e ro d e aw ay o r a l t e r p a r t o f t h i s o u te r s t r u c t u r e, p o s s i b l y e x p o s in g an i n n e r, m ore a c t i v e, c a rb o n. T h is

74 54 ero sio n o ccu rs to th e g r e a te s t e x te n t when th e f ib e r s a re re a c te d w ith oxygen a t e le v a te d tem p e ra tu re s. F u rth e rm o re, th e a c t i v a t i o n o f c a rb o n f i b e r s f o r AA i s m ore d i f f i c u l t th a n i t i s f o r FC. F o r b o th m o le c u le s, e le c tr o c h e m ic a l o x id a tio n r e s u l t s i n good a c t i v i t y and a ls o th e p r e s e n c e o f a la r g e su rfa c e bound redox peak observed w ith DPV. When f ib e r s a re a c tiv a te d by re a c tio n w ith oxygen, how ever, th e DPV wave i s u s u a lly i l l d e fin ed o r q u ite s m a ll, s u g g e s tin g t h a t th e s u r f a c e oxygen fo rm ed i s n o t e le c tr o a c tiv e betw een 0.0 and 0.6 V. The redox o f FC a t t h i s s u rfa c e, how ever, show s good a c t i v i t y, w h e re a s AA o n ly show s r e a s o n a b le a c t i v i t y when th e DPV p e a k i s c l e a r l y p r e s e n t. T hus, f o r AA, th e p r e s e n c e o f s u r f a c e oxygen g ro u p s may f a c i l l i t a t e r a p id e l e c t r o n t r a n s f e r f o r AA a t c a rb o n f i b e r s u r f a c e s. F or FC, h o w ev er, th e p resen ce o f th e se e le c tr o c h e m ic a lly a c tiv e s u r f a c e oxygen g ro u p s i s n o t im p o rta n t, b u t r a th e r an in c re a s e in th e a v a il a b i l ity o f th e in n e r carbon s tr u c tu r e i s th e dom inant cause o f a c tiv a tio n. C le a rly, th e r e i s a d iffe re n c e in th e a c tiv a tio n mechanism f o r gc compared to carbon f ib e r s. However, f ib e r s a re th e m a te ria l o f choice f o r many a p p l i c a t i o n s due to t h e i r s m a ll s i z e. Adams (1 9 ), f o r exam ple, has e x p lo ite d t h e i r u s e f u ln e s s f o r i n v iv o e le c tr o c h e m i c a l a n a l y s i s i n r a t b r a i n s. F u r t h e r m o r e, t h e u s e o f f i b e r s a s e le c tr o c h e m ic a l d e t e c t o r s may r e s u l t in g r e a t e r s e n s i t i v i t y due to s m a l le r b ack g ro u n d c h a rg e. F o r th e s e r e a s o n s, among o t h e r s, i t was d e s ira b le t o in v e s tig a te th e fundam ental p r o p e r tie s o f carbon f ib e r s t o a g r e a t e r e x t e n t. T h is le d t o t h e d e v e lo p m e n t o f th e TDHS sy tem

75 5 5 (w hich i s d e s c r ib e d i n th e n e x t c h a p te r) t o s tu d y th e f o rm a tio n and re m o v a l o f s u r f a c e oxygen c o m p lex es and th e a d s o r p tio n o f o r g a n ic m olecules on carbon f ib e r su rfa c e s.

76 LIST OF REFERENCES (1) Hu, I. - F. f Karweik, D.H.; Kuwana, T. J. E le c tro a n a l. Chem. 1985, 188, 59. (2) T hornton, D.C.; Corby, K.T.; Spendel, V.A.; Jordan, J. ; R obbat, A. J r. ; R utstrom, D.J.; G ross, M.; R itz ie r, G. Anal. Chem. 1985, 57, 150. (3) Engstrom, R.C A n a l. Chem. 1982, 54, (4) Engstrom, R.C.; S tra s s e r, V.A. A nal. Chem. 1984, 56, 136. (5) C abaniss, G.E.; D iam antis, A.A.; Murphy, W.R.; L in to n, R.W.; Meyer, T. J. J^ Am. Chem. S o c. 1985, 107, (6) H ershenhart, E.;.McCreery, R.L. A nal. Chem. 1984, 56, (7) Poon, M.; M ccreery, R.L. A n a l. Chem. 1987, 5 9, (8) Wightman, R.M.; Deakin, M.R.; Kovach, P.M.; Kuhr, W.G.; S t u tts, K.J. J ^ E lectrochem. Soc. 1984, 131, (9) S tu tts, K.J.; Kovach, P.M.; K uhr, W.G.; Wightman, R.M. A nal. Chem. 1983, 55, (10) Evans, J. F. ; Kuwana, T. A n al. Chem. 1977, 49, (11) Hu, I. F. ; Kuwana, T. A nal. Chem. 1986, 58, (12) Zak, J. ; Kuwana, T. J. Am. Chem. S o c. 1982, 104, (13) Cenas, N.; R ozgaite, J. ; P ocius, A.; K ulys, J. J. E le c tro a n a l. Chem. 1983, 154, 121. (14) Gunasingham, H.; F le e t, B. A n aly st (London) 1982, 107, 896. (15) Golden, T.C.; Je n k in s, R.G.; O take, Y.; S caro n i, A.W. P ro c. E lectrochem. Soc. 1984, 8 4-5, 61. (16) Hu, I. - F. Ph.D. D is s e rta tio n, The Ohio S ta te U n iv e rs ity, Columbus, OH.,

77 57 (17) Edmonds, T.E. A n a l. Chim. A cta 1985, 175, 1. (18) Ponchon, J.- L.; C espuglio, R.; Gonon, F.; Jo u v e t, M.; P u jo l, J. - F. A n a l. Chem. 1979, 51, (19) Feng, J.-X.; B r a z e ll, M.; Renner, K.; K asser, R.; Adams, R.N. A nal. Chem., 1987, 59, (20) Gonon, F.-G.; F om barlet, G.M.; Buda, M.J.; P u jo l, F.- J. A nal. Chem., 1981, 53, (21) V avrin, Z. C o lle c t. Czech. Chem. Commun. 1949, 14, 367. (22) D ryhurst, G.; R adish, K.M.; S c h e lle r, F.; Rennenberg, R. "B io lo g ic al E le c tro c h e m istry " ; Academic P re ss: New York, 1984, Vol 1., pp (23) H ale, J.M. In "R eactions o f M olecules a t E le c tro d e s " ; Hush, N.S. Ed.; W ile y -In te rscie n c e : New York, (24) Meyer, T. J.; T aube, H. In o rg. Chem. 1968, 1_, (25) Bard, A.J.; F a u lk n er, L.R. "E lectrochem ical M ethods"; W iley- In te rsc ie n c e : New York, 1980, p p (26) Kazee, B.; W eisshaar, D.E.; Kuwana, T. A nal. Chem. 1985, 57, (27) Woodard, F.E.; McMackins, D.E.; Ja n sso n, R.E.W. E le c tro a n a l. Chem. 1984, 214, 303. (28) Bard, A.J.; F a u lk n er, L.R. "E lectrochem ical M ethods"; W iley- In te rsc ie n c e : New York, 1980, p (29) Fagan, D.T.; Hu, I. - F. ; Kuwana, T. A nal. Chem., 1985, 57, (30) Adams, R.N. " E le c tro c h e m istry a t S o lid E le c tro d e s " ; Marcel D ekker: New Y ork, 1969, 219. (31) Bard, A.J.; F a u lk n er, L.R. "E lectro ch em ical M ethods"; W iley- In te rs c ie n c e : New York, 1980, p. 24. (32) B je lic a, L.; P a rso n s, R.; R eeves, R.M. C ro a tia Chemica A cta, 1980, 53, 211. (33) M uilenberg, G.E. (Ed.) "Handbook o f X-ray P h o to e le c tro n S pectroscopy"; P erk in Elemer Corp: Eden P r a ir ie, MN, (34) L aser, D.; A r ie l, M. J. E le c tro a n a l. Chem., 1974, 52, 291.

78 58 (35) Wightman, R.M. A nal. Chem., , 1125A. (36) Aoki, K; Honda, K.; Tokuda, K.; M atsuda, H. J. E le c tro a n a l. Chem. 1985, 182, 269. (37) Adams, R.N., The U n iv e rsity o f Kansas, p e rso n a l communication, (38) Donnet, J.- B., B ansal, R.C. "Carbon F ib e rs ", I n te r n a tio n a l F ib er Science and Technology S e rie s, Marcel Dekker: New York, 1984, V o l. 3, p p

79 CHAPTER III THERMAL DESORPTION MASS SPECTROMETRY INSTRUMENTATION In tro d u c tio n S everal a n a ly tic a l m ethods have been a p p lie d to th e a n a ly s is of c a r b o n s u r f a c e s, i n c l u d i n g a c i d - b a s e t i t r a t i o n s ( 1-3 ), e l e c t r o c h e m i s tr y (4-6 ), IR s p e c tr o s c o p y (7-9 ), x - r a y p h o to e le c tr o n s p e c tro s c o p y ( ), and th e r m a l d e s o r p tio n (TD) a n a l y s i s ( ). M ost o f th e s e s t u d i e s h a v e fo c u s e d on u n d e rs ta n d in g th e c h e m ic a l n a tu r e o f th e oxygen f u n c t i o n a l g ro u p s (oxygen co m p lex es) t h a t a re formed on th e su rfa c e by some p re tre a tm e n t method. As d isc u sse d in C h a p te r I I, th e s e oxygen c o m p lex es may a f f e c t th e e le c tr o c h e m i c a l, a d s o r p tio n, c a t a l y t i c, and e l e c t r i c a l p r o p e r i t e s o f c a rb o n a c e o u s m a te ria ls ( ). The u se o f vacuum h e a t t r e a tm e n t t o t h e r m a lly d e s o rb s u r f a c e c o m p lex es and i m p u r i t i e s from g l a s s y c a rb o n, a s d is c u s s e d i n th e p rev io u s c h a p te r, m o tiv ated th e developm ent o f a th e rm a l d eso rp tio n mass sp e ctro m e try system t o s tu d y fu n d a m e n ta l p r o p e r t i e s o f oxygen com plex f o rm a tio n and re m o v a l on c a rb o n f i b e r s u r f a c e s. V itre o u s c a rb o n f i b e r s w e re s e l e c t e d o v e r g la s s y c a rb o n b e c a u s e o f th e e a se w ith w hich they can be r e s i s t i v e l y h e a te d, t h e i r im portance fo r u se as m ic ro ele ctro d es (see C hapter I I ), and t h e i r s tr u c tu r a l s i m i l a r i t y to g la s s y c a rb o n ( i e. n o n - g r a p h i tic ). A lso, th e b u lk o f th e l i t e r a t u r e c o n c e rn in g oxygen c o m p lex es on c a rb o n f i b e r s h a s b een l i m i t e d to 59

80 6 0 s tu d ie s concerning su rfa c e m o d if ic a tio n to im p ro v e t h e f i b e r / r e s i n bonding c h a r a c te r is tic s o f h ig h s tre n g th com posites (31). Very few fundam ental stu d ie s concerning th e s u rfa c e com position and p r o p e rtie s have been p ublished. T h erm al d e s o r p tio n, c o u p le d w ith m ass s p e c tr o m e tr y d e te c tio n (TDMS), has th e a n a ly tic a l c a p a b i l i t i e s to p r o v id e m ore in fo r m a tio n about th e form ation and rem oval of s u rfa c e oxygen complexes on carbon f ib e r s th an the o th e r techniq u es m entioned above. In a d d itio n, TDMS p r o v id e d a m ethod t o stu d y th e a d s o r p tio n o f o r g a n ic m o le c u le s a t c a r b o n s u r f a c e s t h a t w e re t r e a t e d in d i f f e r e n t w a y s. M ass sp e c tro m e try was chosen as th e d e te c tio n m ethod sin ce i t p ro v id e s a l a r g e a m o u n t o f i n f o r m a t i o n, i n c l u d i n g m /z d e t e r m i n a t i o n, f r a g m e n ta tio n p a t t e r n s, and i s o t o p i c r a t i o s. F urtherm ore, MS o f f e r s th e high s e n s i t iv i ty n e c essa ry to d e te c t d e so rp tio n p ro d u cts from low s u r f a c e a r e a f i b e r s a m p le s. The h ig h vacuum c o n d itio n r e q u ir e d fo r m ass s p e c tr o m e tr y p r o v id e s a c le a n e n v iro n m e n t w h ic h s e r v e s to p re se rv e th e in te g r ity o f a sam ple s u rfa c e p r i o r to a d so rp tio n an d /o r a n a l y s i s. T his i s e s p e c i a l l y im p o r ta n t f o r v a c u u m -d eg a sse d c a rb o n w hich r e a d ily re a c ts w ith oxygen and o th e r r e a c tiv e g a ses. Therm al d e so rp tio n m ethods have b e e n u se d e x te n s i v e l y t o s tu d y t h e a d s o r p tio n p r o p e r t i e s a n d s u r f a c e c o m p o s itio n o f c a rb o n a c e o u s m a t e r i a l s. In a t y p i c a l TD e x p e rim e n t, a h ig h s u r f a c e a re a c a rb o n sa m p le i s h e a te d u n d e r vacuum o r in a f lo w in g s tre a m o f i n e r t g as. M a te ria l t h a t i s c h e m ically bonded o r weakly adsorbed t o the s u rfa c e i s th e rm a lly removed and is o la te d f o r su b seq u en t a n a ly s is (13-15) or

81 61 d e te c te d d i r e c t l y by m ass s p e c tr o m e tr y ( ), g a s c h ro m a to g ra p h y (2 0 ), g r a v im e try (21) o r IR s p e c tr o m e tr y (22). In th e c a s e o f TD a n a ly s is o f carbon f ib e r s, m ost o f th e p rev io u s s tu d ie s have re p o rte d o n ly th e r e s i d u a l g a s a n a l y s i s o f th e d e s o rb e d p r o d u c ts, w ith no in fo rm a tio n given concerning th e number o f s i t e s, th e rm a l s t a b i l i t y, o r d e s o r p tio n k in e tic s (10,24-27). The TD w ork r e p o r t e d i n th e l i t e r a t u r e f o r c a rb o n o t h e r th a n f i b e r s has p r i m a r i l y been l i m i t e d to s t u d i e s o f f i n e l y g ro u n d o r powdered sam ples o f g ra p h ite o r carbon b lack. This i s in p a r t due to th e e x p e rim e n ta l d i f f i c u l t i e s a s s o c i a t e d w ith h e a tin g l a r g e (ca. 1 cm ) p ie c e s o f g r a p h i t e o r c a rb o n, e s p e c i a l l y i n th e p r e s e n c e o f oxygen o r i n vacuum. F u rth e rm o re, th e g ro u n d o r p o w dered sa m p le s p o s s e s s a h ig h s u r f a c e a r e a w h ich r e s u l t s in t h e d e s o r p tio n o f c o n sid e ra b le volumes o f gas. Thus, h ig h ly s e n s itiv e d e te c to rs a re n o t r e q u ir e d. The g r in d in g p r o c e e d u r e, h o w e v e r, h a s been shown to i n tr o d u c e i m p u r i t i e s w hich a f f e c t th e r a t e o f s u r f a c e com plex f o rm a tio n and may a l t e r th e num ber and ty p e o f s i t e s a t w h ic h th e s e com plexes form (32,33). I t was a c e n tr a l g o a l o f t h i s p r o je c t to develop a TDMS a n a ly s is m ethod which would p e rm it th erm al d eso rp tio n o f i n t a c t carbon f ib e r sam ples, th u s e lim in a tin g th e g rin d in g p ro c e ss. F urtherm ore, se v e ra l o th e r im p o rta n t f e a tu re s were d e s ire d which had r a r e ly, i f e v e r, been em ployed i n th e TD a n a l y s i s o f c a rb o n. T hese in c lu d e r a p i d h e a tin g c a p a b i l i t i e s d u rin g d e s o r p t io n t o im p ro v e s e n s i t i v i t y, th e u s e o f a dynamic (continuous rem oval o f desorbed m a te ria l) r a t h e r th an a s t a t i c

82 62 sy stem t o m in im iz e b a c k r e a c t i o n o f d e s o rb e d m a t e r i a l w ith th e su rfa c e, and th e a b i l i t y t o dose th e sam ple su rfa c e w ith an ad so rb an t m o le c u le t o s tu d y th e e f f e c t o f s u r f a c e tr e a tm e n t on a d s o r p tio n o f o r g a n ic m o le c u le s. T h is c h a p te r w i l l d is c u s s th e d e s ig n and developm ent o f such an in stru m e n t along w ith th e im p o rta n t a sp e c ts o f th e i n d iv i d u a l e x p e r im e n ta l com ponents (sample su p p o rt, tem p e ra tu re m easurem ent, vacuum system, e tc ). Background I f e n e rg y i s a p p lie d t o a sam p le s u r f a c e u n d e r vacuum o r in an i n e r t g a s s tr e a m, a d s o rb e d m a t e r i a l w i l l be r e l e a s e d i n t o th e g a s p h ase ( i.e. d e s o rb e d ). As th e te m p e r a tu r e i s in c r e a s e d, th e r a t e o f d e s o r p tio n re a c h e s a maximum and th e n f a l l s o f f a s th e s u r f a c e i s d e p le ted o f a d so rb a te, r e s u ltin g in a peak in th e d e so rp tio n r a t e vs. tem p e ra tu re p lo t (5). T his curve c o n ta in s in fo rm a tio n concerning th e n a tu r e o f th e i n t e r a c t i o n b e tw e en th e a d s o rb e d s p e c ie s and th e su rfa c e, in clu d in g : 1) th e number o f a d so rp tio n s i t e s, 2) th e th erm al s t a b i l i t y o f each o f the sp e c ie s desorbed, 3) the a c tiv a tio n energy o f d e so rp tio n (and o th e r k in e tic p a ram eters ) and 4) a b s o lu te o r r e l a tiv e su rface coverage ( ). The amount o f in fo rm a tio n a v a ila b le from a p a r t i c u l a r experim ent depends on th e com plexity o f th e su rfa c e, th e ad so rb a n t in te r a c tio n, and th e e x p e r im e n ta l te c h n iq u e s em ployed. Even i n th e c a s e o f c a re f u lly p rep ared m etal s in g le c r y s ta ls, th e d e so rp tio n r e s u l ts can be com plex (38). As e x p e c te d, th e r e s u l t s f o r p o l y c r y s t a l l i n e,

83 63 amorphous carbon, as d e scrib e d in t h i s work, show c e r ta in deg rees o f v a r i a b ility. Both c o v a le n tly bonded (chemtsorbed) s p e c ie s and th o se t h a t only e x p e rie n c e w eak e l e c t r o s t a t i c i n t e r a c t i o n s (p h y siso rb e d ) can be l i b e r a t e d from th e sa m p le d u rin g a TD e x p e rim e n t. In th e c a s e o f i r r e v e r s i b l e c h e m is o r p tio n (as, f o r e x a m p le, oxygen on c a rb o n ) th e term decom position r a th e r than d e so rp tio n i s more a c c u ra te sin c e th e s u r f a c e bound g ro u p i s d e s t r o y e d by t h e t h e r m a l t r e a t m e n t. D esorption, however, w i l l be r e ta in e d fo r s im p lic ity and to conform w ith th e l i t e r a t u r e. As m e n tio n e d above, en erg y m u st be a p p lie d to th e s u r f a c e in order f o r d e so rp tio n to o c c u r. T his energy can r e s u l t from 1) d ir e c t sam ple h e a tin g o r i r r a d i a t i o n w ith l i g h t (th e r m a l d e s o r p t io n ), 2) i r r a d i a t i o n w ith e l e c t r o n s ( e l e c t r o n s tim u la te d d e s o r p t io n ), 3) a p p lic a t i o n o f a h ig h e l e c t r i c a l f i e l d ( f i e l d d e s o r p t io n ), o r 4) bom bardm ent w ith sound w aves (phonon d e s o r p tio n ) (37). T h erm al d e s o r p t i o n m e th o d s a r e u s u a l l y th e s i m p l e s t t o c a r r y o u t e x p e r im e n ta lly, and o f t e n r e s u l t in l e s s com plex s p e c t r a. The e x p e rim e n ts d e s c r ib e d i n t h i s s tu d y r e l y on th e r m a l d e s o r p tio n v ia d i r e c t sam p le h e a tin g u s in g a l i n e a r te m p e r a tu r e ram p. L in e a r program m ed d e s o r p tio n i s th e m o st common m ethod em ployed (3 5,3 6 ), although th e re c ip ro c a l h e a tin g m eth o d s have a l s o been d e s c r ib e d in d e t a i l (3 5,3 9 ).

84 64 In stru m e n ta l D e sc rip tio n 1. L in e ar tem p e ra tu re program fo r d ir e c t sam ple h e a tin g. The carbon f ib e r sam ples used i n t h i s study have been d e scrib e d in C hapter I. They possess s u f f i c i e n t re s is ta n c e such t h a t a bundle o f f ib e r s o f approxim ately 1 cm in len g th can be co n v e n ie n tly heated t o C by th e a p p lic a tio n o f 2-3 A a t approxim ately 10 V. For t h i s purp o se, a Kepco (Flushing, NY) B ip o lar O p eratio n al Power S u p p ly /A m p lifie r w as u se d t o s u p p ly th e d i r e c t c u r r e n t. A l i n e a r v o lta g e ram p was p r o v id e d by pro g ram m in g th e pow er s u p p ly w ith a B io A n aly tical Systems (West L a y fa y e tte, IN) CV-1A-120 p o te n tio s ta t/f u n c tio n g e n e ra to r. F ig u re 3-1 shows a sch em atic diagram o f e l e c t r i c a l c i r c u i t. The l i n e a r i ty o f the tem p e ra tu re ramp i s shown in the p l o t in F igure 3-2. Here, th e o u tp u t o f th e fu n c tio n g e n e ra to r w as i n p u t d i r e c t l y i n t o th e y - a x is o f th e r e c o r d e r w ith th e x - a x i s reco rd in g th e tem p e ra tu re. Good l i n e a r i t y was o b s e rv e d b e tw e e n 100 and 900 C (r= ), w h ich i s th e r e g io n o f p r i n c i p a l i n t e r e s t f o r s u rfa c e oxygen com plex decom postion from carbon. The r a t e o f sa m p le h e a tin g can c o n v e n ie n tly be c o n t r o l l e d by a d ju s t in g th e sw eep r a t e (in m V /sec) on th e BAS CV-1A. A t y p i c a l v a lu e o f 50 m V /sec p ro d u c e d a l i n e a r h e a tin g ram p o f c a. 3 0 C /s e c. W ith h e a tin g r a t e s o f t h i s m a g n itu d e, e x p e rim e n ts a re so m e tim e s r e f e r r e d t o a s " f l a s h d e s o rp tio n " (35). F o r a l i n e a r h e a tin g r a t e, th e tem p e ra tu re, T, a t any tim e, t, i s given by th e r e la tio n : T = TQ + Bt (3.1) w here TQ i s th e i n i t i a l te m p e r a tu r e and B i s th e h e a tin g r a t e i n

85 BAS CV-IA CARBON FIBER SAMPLE I KEPCO DC SUPPLY/AMP. VACUUM SWEEP F ig u re 3-1. S c h e m a tic d iag ra m o f th e sam p le h e a tin g c i r c u i t. The o u tp u t from th e fu n c tio n g e n e ra to r (tr ia n g le wave) o f th e BAS CV-IA i s in p u t d ir e c tly in to th e Kepco power s u p p ly /a m p lifie r. An ammeter i s used to m o n ito r th e c u rre n t drawn by th e f ib e r sam ple. a\ in

86 T IH E US. TEMP FOR FIBER HEATING SAMPLE 422 <PYROLVZE» PITCH) a a 2 8. TIME < S E C > a 13 SLOPE = E B = p = B.fr- _L a. _i_ S TEMPERATURE (C ) L_ F igure 3-2. P lo t o f tim e vs. tem p e ra tu re f o r a 38 C /sec h e a tin g ramp u sin g th e system diagrammed in F ig u re 3-1. The re p o rte d s lo p e i s f o r th e p o rtio n o f th e curve in d ic a te d by the s o lid lin e.

87 67 d e g r e e s /s e c o n d. The h e a tin g r a t e an d i t s r e l a t i o n t o th e p e a k te m p e r a tu r e, T, th e sh a p e o f th e d e s o r p tio n c u rv e, and th e k i n e t i c XT p a ra m ete rs a re d isc u sse d in Appendix A. S e v e ra l a d v a n ta g e s r e s u l t from d i r e c t ohm ic h e a tin g o f th e sam pile. T h is m ethod m akes i t p o s s ib l e to p ro d u c e a te m p e r a tu r e in c re a s e w ith o u t an e x te rn a l h e a tin g so u rce ( c o il, furn ace, etc.) t h a t w ould have t o be lo c a t e d w ith in th e vacuum sy ste m. Thus, th e p o s s i b i l i t y o f d e s o r p tio n fro m e x tr a n e o u s s o u r c e s, w h ic h w ould b e h e a te d alo n g w ith th e s a m p le, i s m in im iz e d. D ir e c t h e a tin g a l s o a llo w s r e a c t i o n o f t h e sam p le w ith o x i d iz in g g a ses a t e le v a te d te m p e ra tu re s w h ich w ould o th e r w is e d e s tr o y a h e a tin g e le m e n t. F u rth e rm o re, h ig h h e a tin g r a t e s a re r e a d i l y a c h ie v a b le by d i r e c t r e s i s t i v e h e a tin g. 2. Carbon F ib e r Sample Holder Of p r i n c i p a l c o n c e rn in d e s ig n in g a sam p le s u p p o rt sy stem f o r TDMS i s m inim izing d e so rp tio n from s u rfa c e s o th e r than th e sample and red u c in g the h e a t lo ss due to p h y sic a l a tta c h m e n t. would suspend th e sam ple a d ia b a tic a lly i n "mid a i r. An id e a l su p p o rt Of co u rse, t h i s i s n o t p o s s i b l e, so an a l t e r n a t i v e m u st be d e v e lo p e d. A s c h e m a tic diagram of th e sam ple h o ld e r designed f o r t h i s w ork i s shown in F ig u re 3-3. The b a se i s an a lu m in a d i s k, 2.5 cm in d ia m e te r. A lum ina w as used because o f i t s h ig h e l e c t r i c a l r e s is ta n c e and th erm al s t a b i l i t y. The a lu m in a d i s k i s c o n n e c te d t o a s t a i n l e s s s t e e l p l a t e w hich i s a tta c h e d to a s t a i n l e s s b o l t. T h is b o l t can th e n be a tta c h e d t o th e sam p le t r a n s f e r r o d in t h e vacuum sy ste m ( d e s c r ib e d below in

88 68 5 SIDE VIEW TOP VIEW F ig u re 3-3. F r o n t and s id e v ie w s o f th e sam ple s u p p o rt. 1) C arbon f i b e r b u n d le, 2) Ta f o i l, 3) th e rm o c o u p le w ir e, 4) a lu m in a b a s e, 5) s t a i n l e s s s t e e l b o l t s f o r sam p le s u p p o rt and e l e c t i c a l c o n t a c t, 6 ) s ta i n le s s s te e l b o lt f o r a tta c h m e n t to l in e a r m otion feedthrough rod.

89 6 9 s e c tio n 4 ). The carbon sam ple is a bundle o f f ib e r s ap p ro x im ately 1 cm in le n g th h e ld t o g e t h e r a t th e e n d s by a s m a l l p ie c e o f mm t h i c k Ta f o i l (A lpha P r o d u c ts, D a n v e rs, MA). T an talu m was c h o sen b e c a u se o f i t s h ig h te m p e ra tu re s t a b i l i t y a n d m a l l e a b i l i t y. The b u n d le w as su sp en d e d 0. 6 cm from th e s u r f a c e o f th e a lu m in a by s t a i n l e s s s t e e l b o l t s (2.5 cm in le n g h t). The b o l t s p a s s th ro u g h holes in th e alum ina d isk and are used to p ic k up c u rre n t from copper w ires w ith in the vacuum system. 3. Thermocouple te m p e ra tu re m easurem ent. The two most common d e v ic e s used to m easure the tem p e ra tu re o f a sam ple d u rin g a TD e x p e rim e n t a r e th e o p t i c a l p y ro m e te r and th e therm ocouple. For t h i s w ork, a therm ocouple was chosen because th e sm all sam ple s iz e would n e c e s s ita te p la c in g o p tic s w ith in the vacuum system. In a d d itio n, c o n v en tio n al p y ro m e te rs o f f e r s lo w e r re s p o n s e tim es and much h ig h e r c o sts th a n therm ocouples. The r e q u ir e m e n ts f o r te m p e r a tu r e m easu rem en t i n th e th e r m a l d e so rp tio n a n a ly s is o f carbon f ib e r s in clu d e: 1 ) maximum tem p e ra tu re c a p a b i l i t y o f C, 2) f a s t r e s p o n s e tim e (w hich a llo w s h ig h h e a tin g r a t e s ), a n d 3) s t a b i l i t y i n o x id iz in g a tm o s p h e re s (w hich allo w s r e a c tio n o f th e carbon w ith oxygen a t e le v a te d te m p e ra tu re s). Chromel-Alumel therm ocouples (Type K) formed from sm all d ia m e te r w ire were found to s a t i s f y a l l o f th e se re q u ire m e n ts. Type K therm ocouples a r e c o n stru c te d o f N i-c r (Chromel) and N i-a l (A lum el) a l l o y s. They e x h i b i t h ig h th erm o p o w er a n d have good

90 70 o x id a tio n r e s is ta n c e up to 1260 C (40). O peration in m ild ly reducing a tm o s p h e re s, how ever,, l e a d s to t h e fo rm a tio n o f a g re e n o x id e o f chrom ium and a s u b s e q u e n t change in th erm o p o w e r. The u s a b le te m p e r a tu r e ra n g e f o r Type K th e rm o c o u p le s i s C (4 1 ), w hich in c lu d e s th e C r a n g e needed f o r th e d e s o r p tio n e x p e rim e n ts done in t h i s work. E rro rs i n tem p eratu re due to inhom ogeneity in th e w i r e c a n be a b o u t C, a n d e r r o r s d u e t o a r e v e r s i b l e m e ta llu rg ic a l i n s t a b i l i t y re g io n can account fo r e r r o r s o f 3-8 C (41). C onsidering th a t te m p e ra tu re s on th e o rd er o f oC a re recorded in a d e so rp tio n ex p erim en t, n e ith e r o f th ese e r r o r s were expected to be s ig n i f ic a n t. The th e rm o c o u p le s u s e d in t h i s s tu d y w ere c o n s tr u c te d by s p o t w e ld in g i n. d i a m e t e r c h r o m e l an d a lu m e l w i r e s (Omega E n g in e e r in g, S ta n fo r d, CT) t o form a bead u s in g a S e lect-a m p W elder (B el-a rt P roducts, Pequannock, NJ). The response tim e (tim e re q u ire d t o r e a c h 63.2% o f an in s ta n ta n e o u s te m p e r a tu r e change) o f su ch a system i s 0.15 seconds acco rd in g to th e m anufactu rer. T his re p re s e n ts an i d e a l c a s e ; i t i s e s tim a te d t h a t a re s p o n s e tim e o f l e s s th a n 0.5 seco n d s i s p o s s i b l e w ith t h e s y ste m d e s c r ib e d h e r e. A i n. d ia m e te r w ire, w hich o ffe rs a 3X im provem ent in th e response tim e, was fo und t o f a i l a t t e m p e r a tu r e s g r e a t e r th a n 1000 C and was t h e r e f o r e n o t u se d. To e x p l o i t t h e f a s t r e s p o n s e tim e and o p e r a te a t h ig h h e a tin g r a t e s, i t was n e c e s s a ry t o p la c e t h e th e rm o c o u p le i n d i r e c t c o n ta c t w ith a bundle o f carbon f i b e r s. In f a c t, th e p r in c ip a l m o tiv atio n fo r

91 71 u sin g a b undle, r a th e r than a s in g le f ib e r, was to a llo w tem p e ra tu re m e a s u r e m e n t u s in g a t h e r m o c o u p le. To a c c u r a t e l y m e a s u r e a tem p e ra tu re, th e mass o f th e m easuring d ev ice must be sm all compared t o th e sam p le such t h a t th e m e a su rin g d e v ic e does n o t a c t a s a h e a t sin k. Eor t h i s reaso n, a bare sm a ll d iam e te r w ire was used. I t would n o t be p r a c t i c a l to p la c e th e th e rm o c o u p le in a g l a s s s h e a th, a s i s som etim es done fo r powdered sam ples, in t h i s type o f a p p lic a tio n (42). The in. d ia m e te r th e rm o c o u p le w ire was c o n n e c te d to 30 g a u g e, t e f l o n s h e a th e d c h ro m e l-a lu m e l w ir e. T h is w ire w as c o ile d aro u n d th e sam p le t r a n s f e r ro d (d e s c rib e d i n th e n e x t s e c tio n ) and tra v e le d back and f o rth w ith th e sam ple. A C eram a-seal (New Lebanon C enter, NY) therm ocouple-vacuum feedthrough provid ed c o n ta c t between t h i s w ire and an Omega E n g in e e rin g TAC-386 th e r m o c o u p le -to -a n a lo g c o n v e rte r. T his device a m p lifie s th e v o lta g e from th e therm ocouple to g iv e an o u tp u t o f 1 mv/ C. The o u tp u t o f th e c o n v e r te r w as in p u t d i r e c t l y i n t o th e x - a x i s o f a H ouston (A u s tin, TX) m odel 2000 x -y re c o rd e r. 4. Vacuum system As d isc u sse d above, th erm al d e so rp tio n ex p erim en ts a re perform ed e ith e r under vacuum o r in a flo w in g stream (38). T his choice la rg e ly d e p e n d s on th e ty p e o f sam p le and th e d e t e c t o r u s e d. High s u r f a c e a r e a p o w d ers su ch a s c a rb o n b la c k and c a t a l y s t s le n d th e m s e lv e s to flo w in g stream experim ents and commonly employ th e rm a l c o n d u c tiv ity d e t e c t o r s. Low s u r f a c e a r e a s a m p le s, su c h a s f i l a m e n t s, a n d s in g le c r y s t a l m e ta ls a re ro u tin e ly exam ined under h ig h vacuum em ploying mass

92 72 sp e ctro m e try a s th e d e te c to r. The l a t t e r was used f o r t h i s study o f c a rb o n f i b e r s w ith c o n tin u o u s pum ping d u rin g d e s o r p tio n. Some TD experim ents have been done in a s t a t i c mode by a llo w in g th e desorbed p ro d u cts to b u ild up in the presence o f th e sam ple p r io r t o a n a ly sis (2 3,3 2,4 3 ). T h is ty p e o f a n a l y s i s i s u n d e s ir a b le, s in c e th e d e s o rb e d p r o d u c ts can i n t e r a c t w ith th e s u r f a c e a s t h e i r p a r t i a l p r e s s u r e i n c r e a s e s (17). T hus, th e TDMS sy ste m d e s c r ib e d h e re was o p e ra ted in a dynamic mode w ith continuous pumping to remove desorbed p ro d u cts as th ey were form ed. The TDMS system was c o n stru c te d o f s ta in le s s s te e l, u ltr a -h ig h vacuum, m e t a l - s e a l c o m p o n e n ts, and i s diagram m ed s c h e m a tic a lly in F ig u re 3-4. The sy ste m i s d iv id e d i n t o tw o s e c t i o n s. The m ain a n a ly s is chamber i s t h a t p o rtio n o f th e system to th e l e f t o f the gate v a lv e i n d i c a t e d by th e v e r t i c a l a rro w. In t h i s r e g io n, a B a lz e rs (Hudson, NH) QMG 511 q u a d ru p o le m ass s p e c tr o m e te r w as m ounted v e r t i c a l ly in d i r e c t l in e o f s ig h t w ith th e sam ple s u rfa c e. A lin e - o f - s ig h t geom etry was used to m inim ize d e so rp tio n in te r fe re n c e from th e chamber w a lls (44) and to enhance th e s e n s i t iv i ty. High to u l t r a - h ig h vacuum w as m a in ta in e d i n t h i s s e c t i o n w ith a L ey b o ld -H era eu s (E xport, PA) RPK 1500 c ry o pump (pumping speed o f 4500 L /s e c f o r H2 0). A c ry o pum ping was c h o sen b e c a u s e o f i t s h ig h pum ping r a t e and c l e a n l i n e s s (no m oving p a r t s ex p o sed t o vacuum, h e n c e no o i l 9 c o n ta m in a tio n ). The b a s e p r e s s u r e o f th e sy ste m w as 5X10 t o r r o r l e s s. The pum ping s p e e d f o r th e g a s e s d e t e c t e d, r e l a t i v e to th e h e a tin g r a t e, was s u f f i c i e n tly high t h a t th e mass s p e c tro m e te r sig n a l

93 73 F igure 3-4. Schem atic diagram o f th e TDMS system. 1) Quadrupole mass sp e c tro m e te r, 2) io n iz a tio n chamber o f quadrupole, 3) g a te v a lv e s, 4) c ry o pum p, 5) s o r p t i o n pump, 6 ) g l a s s v ie w p o rt on m ain a n a ly s i s cham ber, 7) g la s s v iew p o rt on in tr o d u c tio n /r e a c tio n cham ber, 8 ) sam ple h o l d e r, 9) gas i n t r o d u c t i o n p o r t, 1 0 ) e l e c t r i c a l fe e d th ro u g h s, 1 1 ) th e rm o c o u p le fe e d th ro u g h, 1 2 ) r o t a r y - l i n e a r m o tio n m a g n e tic a lly c o u p le d f e e d th ro u g h, 13) th e rm o c o u p le vacuum g a u g e. Not v i s i b l e in d ia g ra m : tu r b o m o le c u la r pump ( o p p o s ite v ie w p o r t 7) and i o n i z a t i o n vacuum gauge (opposite v iew p o rt 6 ). The g a te v a lv e w ith th e v e r t i c a l a r r o w s e p a r a t e s th e m a in a n a l y s i s c h a m b e r ( l e f t ) fro m t h e in tr o d u c tio n /r e a c tio n chamber.

94 FRONT VIEW I A/SA/V A M / 1 T T T 1 «u- ( 6 ) i i _ j X 3^ : F igure 3-4.

95 75 was d i r e c tly p ro p o rtio n a l t o the d e so rp tio n r a t e as in d ic a te d by th e peak shape (35,44) a s d isc u sse d in Appendix A. The s e c tio n t o th e r ig h t o f th e g a te v a lv e w ith th e arrow i s th e sam ple i n t r o d u c t i o n / r e a c t i o n cham ber. T his chamber can be is o la te d fro m t h e m a in c h a m b e r t o r e d u c e pum pdow n t im e a n d p r e v e n t c o n ta m in a tio n o f th e q u a d ru p o le d e t e c t o r d u r in g sam p le d e g a s s in g, o x id atio n o r dosing. Also, sin ce th e d e so rp tio n was c a rrie d o u t in a s e p e r a te ch am b er, b ack g ro u n d d e s o r p tio n fro m th e sy ste m w a lls was m inim ized. Sam ple t r a n s f e r b e tw e e n th e tw o c h a m b e rs, w ith o u t e x p o s u re t o a m b ie n t a i r, was done w ith a H u n tin g to n (M ountain V iew, CA) r o t a r y - lin e a r m otion m ag n e tic a lly coupled feed th ro u g h. This c o n s its o f a 3/4 in. d ia m e te r ro d t h a t was moved by e x t e r n a l m a g n e tic c o n ta c t. The p re v io u sly d e scrib e d sample su p p o rt i s a tta c h e d to th e end o f the rod and the therm oucouple w ire was c o ile d around i t. A 2 in ch len g th o f Teflon h e a t- s h r in k tu b in g was p laced around th e end o f th e ro d to hold th e w ire i n p la c e n e a r th e sam ple su p p o rt. E l e c t r i c a l c o n ta c t to th e sam ple h o ld e r was made v ia copper w ires ( e i t h e r 1 0 g a. w ir e o r 1 / 8 in. d i a. ro d ) t h a t w ere a tta c h e d t o e l e c t r i c a l feedthroughs. When the sam ple w ere moved in to p o s itio n in e i t h e r c h a m b e r, t h e tw o s t a i n l e s s s t e e l b o l t s on t h e b o tto m o f th e sam ple h o l d e r made c o n ta c t w ith th e c o p p e r w ir e s. The d e s ig n and d e v e lo p m e n t o f a t r a n s f e r sy ste m t h a t a llo w e d th e sam p le t o t r a v e l betw een th e two cham bers, w h ile p ro v id in g e l e c t r i c a l and therm ocouple c o n ta c ts, was th e m ost d i f f i c u l t e x p e rim e n ta l a s p e c t in th e design o f

96 76 th e TDMS system. High vacuum was m ain tain ed in the in tr o d u c tio n /r e a c tio n chamber w ith a B alzers TPU 50 tu rb o m o le c u la r pump. Rough pumping was achieved w ith a liq u id n itro g e n -c o o le d m olecular sie v e s o rp tio n pump (P erkin- Elm er, Eden P r a ir ie, MM). The in tr o d u c tio n /r e a c tio n chamber was a ls o equipped w ith a sample d o s in g sy ste m f o r a d m i ttin g m o le c u le s from g a s, l i q u i d, o r s o l i d sa m p le s. T h is system i s diagram m ed in F ig u re 3-5 and i s s i m i l a r in d e sig n to t h a t re p o rte d by Madix (45). The dosing system c o n s is te d o f a m ech an ically pumped vacuum lin e c o n stru c te d o f 1 /4 and 1/8 in ch o.d. s ta in le s s s t e e l tubing w ith b o th m o lecu lar siev e and liq u id n itro g e n tr a p s. Nupro (W illoughby, OH) b e llo w s v a lv e s is o la te d th e vacuum lin e from th e TDMS system. W hitey (Highland H eights, OH) b a ll v a lv e s were u se d i n a l l o t h e r a p p l i c a t i o n s in th e d o s in g s y s te m. The a d s o rb a n t sam ple was c o n tain ed in a g la s s v i a l t h a t was a tta c h e d to th e vacuum lin e v ia a Cajon (M acedonia, OH) u l t r a - t o r r f i t t i n g o r g la s s -to -m e ta l s e a l. This v i a l was made from a 5 ml v o lu m e tric p ip e tte. L iq u id sam p le s w e re f ro z e n o r c o o le d in t h e g la s s v i a l to m a in ta in a c o n s ta n t v a p o r p r e s s u r e in t h e vacuum l i n e f o r d o s in g. L ik e w is e, a s o l i d sa m p le c o u ld be a d m itte d a t room te m p e r a tu r e o r w arm ed to p r o v id e s u f f i c i e n t v a p o r p r e s s u r e. The a d s o rb a n t was a d m itte d in to th e TDMS sytem as a gas in d i r e c t l in e o f s i t e w ith th e f ib e r sample through 0.5 mm i.d. chrom atography tu b in g. The sam ple- t o - d o s in g tu b e d i s t a n c e w as a p p ro x im a te ly 5 mm. F o llo w in g sam ple d o s in g, th e a d s o rb a n t c o u ld b e f ro z e n i n t h e tu b in g w ith a l i q u i d

97 F ig u re 3-5. S c h e m a tic d iag ra m o f sa m p le d o s in g s y s te m. 1) Tube to m echanical pump, 2) m o lecu lar sie v e tra p, 3) liq u id n itro g e n tr a p, 4) therm ocouple vacuum gauges, 5) b a ll v alv es f o r gas lin e a tta ch m e n t, 6 ) g l a s s v i a l f o r d o s in g sa m p le, 7) m e te rin g v a lv e, 8 ) h ig h vacuum bellow s v a lv e s, 9) tu b in g to TDMS system, 10) c lo se -u p o f dosing tu b es in TDMS system, 10a) Swagelok c o n e c to rs, 10b) g a s feedthrough 10c) 0.5 mm i.d. c h ro m a to g ra p h y tu b in g, lod) c a rb o n f i b e r sam p le p o s i t i o n during d o s in g. 77

98 J =0= = t 7 8 t8 1 ffln l F igure

99 79 n itro g e n tra p t o quickly s to p th e flo w o f vapor m olecules im m ediately p r io r to d eso rp tio n. P re ssu re in th e main a n a ly s is chamber was m easured w ith a nude i o n i z a t i o n gauge o f th e B a y a r d -A lp e r t ty p e o p e ra te d by a G r a n v ille - P h i l l i p s (B o u ld e r, CO) s e r i e s 271 c o n t r o l l e r. P r e s s u r e i n th e in tro d u c tio n /re a c tio n cham ber and i n the dosing system was m easured w ith V arian (Lexington, MA) 801 therm ocouple gauge c o n tr o lle r and Type 0531 vacuum gauge tubes. E v e n tu ally, an io n iz a tio n gauge was added to the in tr o d u c tio n /re a c tio n chamber a ls o. 5. Quadrupole mass sp e ctro m e te r d e te c to r. As m entioned e a r l i e r, mass sp e c tro m e te rs o ffe r s e v e ra l advantages over o th e r types o f d e te c to rs commonly used f o r d eso rp tio n experim ents (th erm al c o n d u c tiv ity, flam e io n iz a tio n, gas chrom atograph) in clu d in g : 1 ) a continuous m onitoring o f a g iv en component d u rin g d e so rp tio n, 2 ) high s e n s i t iv i ty, and 3) sim u lta n e o u s m onitoring of s e v e ra l m ass peaks, as a fu n ctio n o f te m p e ra tu re. In a d d itio n, fra g m e n ta tio n p a tte r n s and is o to p ic r a tio s can be u sed to a id in the i d e n tif ic a tio n of desorbed p ro d u c ts. The TDMS system d e scrib e d here was b u i l t around a B alzers QMG 511 quadrupole mass sp e ctro m e te r w ith a 90 o f f ax is secondary e le c tro n m u l t i p l i e r (SEM). The SEM o f f e r s h ig h e r s e n s i t i v i t y th an a f a ra d a y cup, b u t i s s u s c e p tib le t o f lu c tu a tio n s in g a in w ith tim e and system p r e s s u r e. a n a ly s is. F o r t h i s r e a s o n, SEM d e t e c t i o n i s b e s t f o r q u a l i t a t i v e The 90 o f f - a x is arrangem ent o f th e d e te c to r ( r e la tiv e to th e q u a d ru p o le f ie l d ) o f f e r s e n h a n ced r e s o l u t i o n com pared t o th e

100 80 sta n d a rd geom etry by d is c rim in a tin g a g a in s t io n s t h a t a re n o t o f the sam e k i n e t i c e n e rg y. T h is in s tr u m e n t i s c a p a b le o f u n i t m ass r e s o lu tio n up to i t s maximum m /z of 511. The c o n tr o l u n it o f the quadrupole was in te r fa c e d to an Apple l i e com puter to allow r a p id scanning of a s e r ie s o f m ass s p e c tra d u rin g a d e s o r p tio n c y c le. An A/D c o n v e r te r ( I n t e r a c t i v e M ic ro w a re, S ta te C o lleg e, PA) was used t o send a +5.0V p u ls e to th e quadrupole c o n tro l u n i t to t r i g g e r the re c o rd in g o f a mass spectrum. The d a ta generated by th e q u a d ru p o le w as th en c o l l e c t e d th ro u g h t h e a n a lo g r e c o r d e r o u tp u ts and d i g i t a l i z e d by a D/A c o n v e r te r ( I n t e r a c t i v e M ic ro w a re). In t h i s w ay, th e q u a d ru p o le c o u ld o p e r a te a t much f a s t e r sc a n r a t e s (up t o 1 a m u /se c) t h a n i f a s t r i p c h a r t r e c o r d e r w ere em ployed. F u r th e r m o r e, a s e r i e s o f m ass s p e c tr a (up to 20) c o u ld be re c o rd e d c o n secutiv e ly and s to r e d on a m agnetic disk. The d a ta could th en be p l o tte d o u t i n a 3 -d im en sio n al tim e v s. m /z vs. s ig n a l in te n s ity p lo t u s in g a d i g i t a l p l o t t e r. The so ftw a re f o r th is p ro c e ss was w r itte n in o u r la b by I.- F. Hu.

101 LIST OF REFERENCES (1) Boehm, H.P.; D ieh l, E.; Heck, W; Sappok, R. Angew. Chem. I n t. Ed. 1964, 3, 669. (2) B arton, S.S.; B oulton, G.L.; H arriso n, B.H. Carbon 1972, 10, 395. (3) H iran n aiah, S.; S e th, R.L. "Advances in C a ta ly s is - Science and T echnology", W iley: New York, 1985, 205. (4) Hu, I. - F. ; Karweik, D.H.; Xuwana, T. J. E le c tro a n a l. Chem. 1985, 188, 59. (5) F e a te s, F.S.; Keep, C.W. T ra n s. F a r. Soc. 1970, 6 6, (6 ) Fagan, D.T.; Hu, I. - F. ; Kuwana, T. A nal. Chem. 1985, 557, (7) M attson, J.S.; Mark, H.B., J r. ; Weber, W.J., J r. A nal. Chem. 1969, 41, 355. (8 ) Nakamizo, M.; Kammereck, R.; W alker, P.L., J r. Carbon 1974, 12, 259. (9) S ujak, K.; Wlochowicz, A.; w ysocki, M. Acta Polym erica 1984, 35, 580. (10) P ro c to r, A.; Sherwood, P.M.A. J. E le c t. Spec. R el. Phenom. 1982, 39, 27. (11) Takahagi, T.; I s h i t a n i, A. Carbon 1984, 22, 43. (12) B arber, M.; S w ift, P.; Evans, E.L.; Thomas, I.M. N ature 1970, 22, (13) B an sal, R.C.; V a sto la, F. J. ; W alker, P.L., J r. Carbon 1970, 8, 443. (14) B arton, S.S.; H a rriso n, B.H.; D ollim ore, J. J. Phys. Chem. 1978, 82, 290. (15) Tremblay, G.; V a sto la, F. J. ; W alker, P.L., J r. Carbon 1978, 16,

102 82 (16 (17 (18 (19 (20 Sen, A.; Bercaw, J.E. J. Phys. Chem. 1980, 84, 465. P h i l l i p s, R.; V a sto la, P.J.; W alker, P.L., J r. Carbon 1970, 8, 197. Matsumoto, S.; Kanda, H.; S a to, Y.; S e tak a, N. Carbon 1977, 15, 299. Matsumoto, S.; S etaka, N. Carbon 1979, 17, 303. B re i, V.V.; Pavlov, V.V. Theo. Exp. Chem. 1979, 15, 253. (21 T ucker, B.G.; M ulcahey, M.R.P. 1969, 67, 274. Chem. S o c., T ra n s. F ar. Soc. (22 (23 (24 (25 (26 (27 (28 (29 (30 (31 (32 (33 (34 (35 Nayak, R.V.; Je n k in s, R.G. Am. Chem. Soc. Div. F u el Chem., P r e p rin ts o f Papers 1984, 29, 172. B arto n, S.S.; H a rriso n, B.H.; D ollim ore, J. J. Chem. Soc. F ar. T ra n s. I 1973, 69, D ra z l, L.T.; M escher, J.A.; H a ll, D.L. Carbon 1979, JL7, 375. Rand, B.; R obinson, R. Carbon 1977, 15, 257, 311. B a n sa l, R.C.; Chhabra, P.; P u r i, B.R. In d. J. Chem. 1980, 19A, F i tz e r, E.; Sturm, F.V.; W eiss, R. B ien. Conf. on Carbon- E xt. A bst. and Program. 1983, 16th, 494. Evans, J.F.; Kuwana, T.; Henne, M.T.; R oyer, G.P. J. E le c tro a n a l. Chem. 1977, 8 0, 409. Gunasingham, H.; F le e t, B. A nalyst 1982, 107, 896. Golden, T.C.; J e n k in s, R.G.; Otake, Y.j S c a ro n i, A.W. Proc. E lectrochem. S oc. 1984, 84-5, p p Donnet, J.R.; B ansal, R.C. "Carbon F ib e rs " I n te r n a tio n a l F ib e r S cience and Technology S e rie s, M arcel Dekker: New York, 1984; V ol. 3, pp V a sto la, F. J. ; W alker, P. J., J r. J. Chim. Phys. 1961, 20, 58. H arker, H.; H orsley, J.B.; Robson Carbon 1971, 9, 1. F a lc o n e r, J. L. ; Schwarz, J.A. C a t. R e v.-s c i. Eng. 1983, 25, 150. Redhead, P.A. Vacuum 1962, 12, 203.

103 83 (36) F a lc o n e r, J.L.; M adix, R.J. S u rfa c e S c i , 4 8, (37) Dawson, P.T.; W alker, P.C. In "E xperim ental Methods in C a ta ly tic R e s e a rc h ", V ol. 3, A n d erso n, R.B. and Dawson, P.T. Ed., A cadam ic P r e s s ; New Y o r k,1976, Ch. 6. (38) F a lc o n e r, J.L.; S ch w arz, J.A. C at. R ev.- S c i. Eng , 2 5, 143. (39) E h rlic h, G. J ^ Applied Phys. 1961, 32, 4. (40) Q uinn, T.J. "T em perature", Academic P r e s s : London, 1983, p (41) K in z ie, P.A. "T herm ocouple T e m p e ra tu re M easu rem en t", W iley: New York, 1973, pp (42) B u n n er, A.; H ucule, D.A. J. C a t a l y s i s , 5 6, 134. (43) D o llim o re, J.; Freedm an, C.M.; H a r r is o n, B.H.; Q uinn, D.F. Carbon 1970, 8, 587. (44) M c C a rro ll, B. J. A ppl. P hys. 1969, 4 0, 1. (45) M cc arty, J.; F a lc o n e r, J.; M adix, R.J. C a t a ly s is , 30, 235.

104 CHAPTER IV FORMATION AND REMOVAL OF SURFACE OXYGEN COMPLEXES ON CARBON FIBERS In tro d u c tio n For any i n t e r f a c i a l phenom enon, such as e l e c t r o n t r a n s f e r, a d so rp tio n, o r c a ta ly s is, the p h y s ic a l and chem ical s u rfa c e p ro p e r tie s o f th e s o l i d p h a se a r e o f te n th e p r i n c i p a l f a c t o r s t h a t a f f e c t th e outcom e o f th e i n t e r a c t i o n. In th e c a se o f c a rb o n, B.R. P u r i m akes the fo llo w in g s ta te m e n t in a review c h a p te r concerning su rfa c e oxygen com plexes (1 ): I t i s now known t h a t a lm o s t a l l ty p e s o f c a rb o n s a re covered w ith oxygen com plexes u n le ss s p e c ia l c a re is taken to e lim in a te them. These com plexes a re o f te n the so u rc e o f th e p r o p e r ty by w hich a carb o n becom es u se fu l o r e f f e c tiv e in c e r ta in re s p e c ts. F u rth e rm o re, s u r f a c e oxygen h a s been s u g g e s te d to a f f e c t th e w e tta b ility, a d so rp tio n b eh av io r, c a ta ly tic p r o p e r tie s, and e l e c t r i c a l p ro p e rtie s o f carbons (2) as d isc u sse d in Chapter I I. The im portance o f oxygen fu n c tio n a l groups (oxygen complexes) on th e su rfa c e o f carbon le d to th e developm ent o f a th erm al d e so rp tio n m ass s p e c tr o m e tr y (TDMS) s y s te m, as d e s c r ib e d in C h a p te r I I I. F ollow ing th e developm ent o f TDMS, a study was i n i t i a t e d to examine th e fu n d a m e n ta l a s p e c t s o f oxygen com plex fo rm a tio n and rem o v a l on c a rb o n f i b e r s u r f a c e s. T h is s tu d y em ployed TDMS to d e te rm in e th e p u r i t y o f " a s - r e c e iv e d " f i b e r s, a s w e ll a s to v e r i f y t h a t p r i s t i n e 84

105 85 carb o n was p ro d u c e d by vacuum d e g a s s in g p r i o r t o c h e m is o rp tio n o f oxygen. The fo rm atio n o f su rfa c e oxygen com plexes was id e n tif ie d by t h e i r th e r m a l d e c o m p o s itio n t o g iv e CO and C02. The r e s u l t i n g d eso rp tio n r a te vs. tem p eratu re curves p rovided in fo rm a tio n concerning th e num ber o f d e s o r p tio n s i t e s and th e th e r m a l s t a b i l i t y o f each s p e c ie s. F i n a l l y, th e a c t i v a t i o n e n e rg y of d e so rp tio n was measured fo r th e decom position o f oxygen complexes to give C02. F u rth e r m o tiv atio n f o r t h i s fundam ental study r e s u lte d from the lack o f in fo rm a tio n o f th is type in the l i t e r a t u r e fo r carbon f ib e r s. Most o f th e p rev io u s s tu d ie s o f th e carbon su rfa c e have been done on g r a p h i t iz e d c a rb o n b la c k. The a d s o r p tio n o f o r g a n ic m o le c u le s on carb o n f i b e r s w ith and w ith o u t s u r f a c e oxygen w as a ls o o f i n t e r e s t, and th u s in fo rm a tio n concerning su rfa c e oxygen complexes was n ecessary to i n te r p r e t t h i s d ata. Background F o rm atio n o f oxygen com plexes on carbon The study o f oxygen com plexes on carbon has been th e s u b je c t o f a m u ltitu d e o f in v e s tig a tio n s, d a tin g back a t le a s t to 1863 when Smith observed t h a t oxygen adsorbed on carbon could only be removed as C0 2 (3). When a c a rb o n s u r f a c e i s e x p o sed to oxygen, i r r e v e r s i b l e ch em iso rp tio n occurs and c o v a le n tly bonded su rfa c e fu n c tio n a l groups a re fo rm e d even a t room te m p e r a tu r e. T h ere i s no e v id e n c e f o r th e presence o f m o lecu lar oxygen once chem isorption has taken p la c e (4,5). L ikew ise, su rfa c e oxygen complexes can be formed by o th e r o x id a tiv e m ethods in c lu d in g w e t c h e m ic a l o x i d a tio n s u s in g n i t r i c a c id o r

106 86 p e rm a n g a n a te (6,7 ), e le c tr o c h e m i c a l o x id a tio n (8,9 ), and r a d io fre q u e n c y p la s m a t r e a tm e n ts ( 1 0, 1 1 ) among o t h e r s (see f o r e x a m p le, r e f. 1). A review o f su rfa c e o x id a tio n tre a tm e n ts fo r carbon f ib e r s i s g iv en by D onnet (12) c o n c e rn in g th e e f f e c t o f o x id a tio n on f i b e r / r e s i n bondin g c h a r a c t e r i s t i c s f o r th e p r e p a r a tio n o f h ig h s tre n g th com posites. In the p re s e n t stu d y, o x id a tiv e tre a tm e n ts were l i m i t e d to g a s p h a se r e a c t i o n w ith oxygen. T h is r e p r e s e n t s a c o n tr o l le d m ethod t o in tr o d u c e s u r f a c e oxygen and s e r v e s as a b a s is from which o th e r m ethods can be compared. Also, a co n sid e rab le body of l i t e r a t u r e e x is ts concerning the re a c tio n o f v a rio u s carbons w ith 0 2. I t i s a n t i c i p a t e d t h a t f u tu r e w ork on t h i s p r o j e c t w i l l be e x te n d e d to in c lu d e o t h e r m ethods f o r in tr o d u c in g s u r f a c e oxygen com plexes, e s p e c ia lly e le c tro c h e m ic a l o x id a tio n s. The c h e m ic a l s t r u c t u r e o f th e oxygen co m p lex es form ed on th e s u r f a c e o f c a rb o n i s n o t known. Many i n v e s t i g a t i o n s have a d d re s s e d t h i s p ro b le m ; h o w ev er, th e r e s u l t s a r e so m e tim e s in c o n c lu s iv e and g e n e ra lly r e ly on in d ir e c t evidence. N oneth eless, th e s tro n g e s t case f o r th e e x is t e n c e o f s p e c i f i c f u n c t i o n a l g ro u p s can be made f o r carb o n y l/q u in o n e, p h en o l, c a rb o x y lic a c id, and la c to n e groups (1,13-17). The p o s s ib le s u rfa c e s tr u c tu r e s o f th ese fu n c tio n a l groups a re d e p ic te d in F ig u re 4-1. R e g a rd le s s o f t h e i r s t r u c t u r e, oxygen co m p lex es decom pose upon th e r m a l t r e a tm e n t and e v o lv e CO and C02. C a rb o n y l-, q u in o n e -, and p h e n o l- ty p e g ro u p s have CO a s t h e i r d e so rp tio n p ro d u c t, w h ile c a rb o x y lic a c id and la c to n e groups r e s u l t in C02 e v o lu tio n (17). L acto n e-ty p e groups, however, may evolve both CO

107 87 '? Ti fl CARBONYL QUINONE \-O H OH ALCOHOL ' PHENOL / / CARBOXYLIC ACID \ 0h / \ L _ / \ LACTONE F ig u re 4-1. P o ssib le fu n c tio n a l groups p re s e n t on carbon su rfa c e s.

108 88 and C02 upon th e r m a l d e c o m p o s itio n b a sed on t h e i r p ro p o se d s u r f a c e s tr u c tu r e shown in F igure 4-1. The r a t e o f oxygen c h e m is o rp tio n h as b e e n s t u d i e d by s e v e r a l w o rk e rs ( ). In g e n e r a l, th e u p ta k e o f oxygen i s an a c t i v a t e d p ro c e s s and p ro c e e d s a t a r a p id r a t e i n i t i a l l y, fo llo w e d by a s u b s e q u e n t s lo w e r u p ta k e u n t i l s a t u r a t i o n c o v e ra g e i s re a c h e d ( g e n e r a lly 3% o f th e s u r f a c e f o r g r a p h i t i c c a rb o n s ). M ulcahey (18) observed t h a t a t l e a s t 2/3 o f the f in a l amount, o f s u rfa c e oxygen was formed d u rin g th e f i r s t m inute o f th e re a c tio n a t 500 C. W alker (17) a l s o o b s e r v e d a r a p i d i n i t i a l r a t e w h ic h w as a t t r i b u t e d to c h e m is o rp tio n a t s i t e s d i f f e r e n t th a n th o s e r e s p o n s ib le f o r th e s u b s e q u e n t s lo w e r u p ta k e. F ig u re 4-2 show s a p l o t o f oxygen c o n su m p tio n and com plex f o rm a tio n (as i n d ic a te d by CO and C02 p r o d u c tio n ) as a f u n c tio n o f tim e f o r a n o n - g r a p h i tic carbon (19). N o tic e t h a t th e oxygen c o n su m p tio n o c c u rs r a p i d l y f o r th e f i r s t 20 m in u te s o r so and th e n l e v e l s o f f. In t h e p r e s e n t s tu d y, oxygen ch em iso rp tio n s w ere c a rrie d out a t tim e s ran g in g from 1 to 45 m inutes. The te m p e r a tu r e o f t h e r e a c t i o n w ith oxygen h a s been show n to a f f e c t th e chem ical n a tu re o f th e oxygen com plexes form ed, although a m b ig u itie s e x i s t. O x id a tio n n e a r 400 C h a s been show n to in c r e a s e th e amount o f C02-d e so rb in g groups w h ile a ls o making th e su rfa c e more a c i d i c, b a s e d on t i t r a t i o n d a ta (2,2 1 ). A ls o, W alker (17) s u g g e s te d t h a t 400 C was th e te m p e r a tu r e a t w hich oxygen c h e m is o rp tio n was m axim ized. Higher tem p e ra tu re o x id a tio n was found to produce more CO d e s o rb in g g ro u p s w h ich a r e th o u g h t to a r i s e from b a s i c o r n e u t r a l

109 89 COMPLEX PRESSURE OXYGEN TIME (MIN.) F ig u re 4-2. R ate o f oxygen c o n su m p tio n a n d s u r f a c e oxygen com plex f o r m a tio n (as i n d ic a te d by CO and C02 e v o lu tio n ) a t 300 C on c a rb o n (taken from re fe re n c e 19).

110 90 complexes (22). Other r e p o r ts (23) in d ic a te th a t tem p e ra tu re s betw een 200 and 700 C a re more fa v o ra b le f o r th e fo rm atio n o f a c id ic groups, w h ile te m p e r a tu r e s above o r b elo w t h i s r e s u l t in a l e s s a c id i c s u rfa c e. S e v e r a l r e p o r t s c o n c e r n i n g t h e r e a c t i o n o f g r a p h ite and g ra p h itiz e d furnace b lack have in d ic a te d th a t th e amount o f CO evolved in c r e a s e d a s th e te m p e r a tu r e o f oxygen c h e m is o rp tio n in c r e a s e d (2 4,2 5 ). For e x a m p le, th e C0/C 02 r a t i o in c r e a s e d from 7.6 to 25.6 a s th e a d so rp tio n tem p e ra tu re in c re a se d from 200 to 500 C, re s p e c tiv e ly. No c le a r d iv is io n o f tem p e ra tu re e x is t s, however, fo r th e form ation o f one ty p e o f group over a n o th e r. In t h i s study, oxygen chem isorptions w ere done a t te m p e r a tu r e s n e a r 300 and 600 C. Thus, a c o m p a riso n c o u ld be made f o r oxygen com plex f o rm a tio n a t w id e ly d i f f e r e n t te m p e ra tu re s, b o th above and below 400 C. The term ch em iso rp tio n and o x id a tio n a r e o f te n u s e d sy n o n y m o u sly, as i n t h i s d i s s e r t a t i o n, h o w ev er, " o x id a tio n " i s so m e tim e s u se d to s u g g e s t th e rem o v a l o f c a rb o n by an o x id a tiv e p r o c e s s, e s p e c i a l l y a t h ig h e r te m p e r a tu r e s. C hem isorption o f oxygen, on th e o th e r hand, r e f e r s to th e ir r e v e r s ib le d is s o c ia tio n o f 02 m olecules to form oxygen com plexes. Depending on th e te m p e r a tu r e o f r e a c t i o n, b o th p r o c e s s e s a r e o c c u r r in g to some d e g re e. A sam p le o f th e o x id a tio n te m p e r a tu r e s e x am in ed in th e l i t e r a t u r e fo r v ario u s carbons i s shown in Table 4-1. Removal o f su rfa c e oxygen com plexes As i n d ic a te d above, s u r f a c e oxygen co m p le x es a r e rem oved from carbon by h e a tin g in an i n e r t atm osphere o r under vacuum. In g e n e ra l,

111 TABLE 4-1 T ype o f C arb o n ( \ b u r n o f f ) O x id a tio n Tem p. c co/co2 r a t i o Ed. CO (k J /m o l) Ed, CO, (k J /m o l) T, CO? C ) Tp. c 2 ( C) M e th o d / h e a t r a t e 1 Mode D e te c to r R e f. 1. GCB (16) , ,9 0 0, GCB (1 7 ) GCB (25) GCB ( 5) , GPB G r a p h i t e G r a p h ite G r a p h ite ,2 7 4, R 5 4,2 5 0, ,4 4, E dge G r CH PVD D iam ond , VCP I ,8 5 0 I I ,8 5 0 I I I , GC HTPP ,7 4 8,8 2 0, ,8 3 8,9 5 6, G C B - g r a p h itiz e d c a r b o n b l a c k, G F B = g ra p h itiz e d f u r n a c e b l a c k, C H 4«carbon p y r o ly z e d fro m m e th a n e, P V D»carbon p y r o ly z e d from p o l y v i n y l d i e n e, VCP= v i t r e o u s c a r b o n p o w d e r h e a t - t r e a t e d a t 1 ( ), 1 1 ( ) a n d ( ) C G O q la s s y c a r b o n, H T P P» h e a t- tr e a te d ( C ) p y r o ly z e d p i t c h c a r b o n f i b e r s R s r a d io l y z e d o x y g e n LPD/3 s t a t i c m an o m eter LPD/2 s t a t i c MS LPD/3 s t a t i c MS 26 LPD s t a t i c g r a v im e tr y IB 3 0 0,3 5 0,6 5 0 LPD/5 dynam ic 1R 24 M M 400 STEP s t a t i c MS 30 STEP s t a t i c m an o m eter 25 M ««L P D /3-8 d y n u o ic t h e m i s t o r 29 STEP dynam ic AES 5 LPD /5 s t a t i c p r e s s u r e L P D /300 s t a t i c MS LPD /20 dynam ic MS * «" 580 LPD /18 dynam ic MS m " " 600 M " t STEP dynam ic AES ,7 8 3 L PD /1000 d y n a m ic MS * 750 " " N L P D **linear pro g ram m ed d e s o r p t i o n, S T E P = step te m p e ra t u r e d e s. MS«mass s p e c t r o m e t r y, I R «i n f r a r e d s p e c t r o s c o p y, A ES»Auger e l e c t r o n s p e c t r o s c o p y,* «t h i s w ork * in d e g r e e s / s e c vo H*

112 92 oxygen complexes a re s ta b le up to th e tem p eratu re a t w hich th e y were fo rm e d (2 6 ). H o w ev er, v e r y l i t t l e d e c o m p o s i tio n o c c u r s a t te m p e r a tu r e s l e s s th an 250 C, an d m ost r e p o r t s i n d i c a t e t h a t s ig n if ic a n t d e so rp tio n o c c u rs only a t tem peratures above 400 C. The p ro d u ctio n of C02 g e n e ra lly occurs betw een 400 and 600 C, depending on th e type o f carbon and th e a n a ly s is method used (see T able 4-1). For CO, on th e o th e r hand, e v o lu tio n u s u a lly d o e s n o t b e g in u n t i l 600 C a n d can c o n tin u e up to 1000 C, i n d i c a t i n g a more s t a b l e s u r f a c e com plex. The th erm al decom position o f oxygen complexes to y ie ld CO an d /o r C02 i s th o u g h t t o r e s u l t i n th e g e n e r a tio n o f s u r f a c e s i t e s t h a t r e a d i l y f a c i l i t a t e th e fo rm a tio n o f oxygen c o m p lex es upon f u r t h e r exposure to oxygen (18,27). Kelemen (16), f o r example, suggested th a t a t h ig h c o v e ra g e s, the- d e c o m p o s itio n o f a d ja c e n t c a rb o n y l g ro u p s r e s u lts in th e e v o lu tio n o f CO and th e p roduction of s tr a in e d u n sta b le c a rb o n a to m ic c e n te r s w h ic h w i l l o x id iz e f u r t h e r. H ow ever, some re p o rts th e o riz e t h a t th e rm a l d e so rp tio n r e s u l ts in a re d u c tio n o f the number o f a v a ila b le re a c tiv e s ite s due to th e an n ealin g o f d e fe c ts or th e p ro d u ctio n o f s ta b le carb o n rin g s (16,25). I t is lik e ly t h a t both p ro c e sse s occur to some e x te n t. Surface h e te ro g e n e ity /m u ltip le a d so rp tio n s i t e s In g e n e ra l, i t is b e lie v e d t h a t oxygen ch e m iso rp tio n tak e s place a t d i s c r e t e s i t e s on th e s u r f a c e o f th e c a rb o n (1 7,1 8,2 2,2 6,2 8,2 9 ). T hese s i t e s in c lu d e b a s a l d e f e c t s, edge c a rb o n a to m s in e i t h e r th e a rm c h a ir o r z ig - z a g c o n f i g u r a t io n, and o t h e r u n i d e n t i f i e d r e a c t i v e

113 9 3 s i t e s (28,29). Oxygen c h e m is o rp tio n d o e s n o t o c c u r a t g r a p h i t i c b a sal p lan es in which d e fe c ts o r v acancies are a b sen t (5,19). The p resen ce o f d i s t i n c t s p e c ie s r e s u ltin g in th e decompostion o f oxygen c o m p lex es from v a rio u s s i t e s h a s been i n f e r r e d from low re s o lu tio n s te p d e so rp tio n experim ents (25,30), and have been observed d i r e c t l y u s in g h ig h e r r e s o l u t i o n l i n e a r program m ed d e s o r p tio n te c h n iq u e s (4,2 2,3 1 ). In t h i s s tu d y, th e num ber o f CO and C02 d e so rp tio n s i t e s was re a d ily determ ined by TDMS w ith th e appearance o f m u ltip le p e a k s i n th e d e s o r p tio n r a t e v s. te m p e ra tu re p l o t. The d i f f e r e n c e b e tw e e n tw o d i s c r e t e s p e c ie s c o u ld r e s u l t from th e decom position o f s im ila r fu n c tio n a l groups th a t e x i s t in s tr u c tu r a lly d i f f e r e n t e n v ir o n m e n ts, o r from th e d e c o m p o s itio n o f c o m p le te ly d i f f e r e n t f u n c t i o n a l g ro u p s w ith th e same d e s o r p tio n p r o d u c t (22). The a p p e a ra n c e o f m u ltip l e p e a k s f o r th e d e s o rp tio n o f a s in g le sp e cie s (such a s CO) w i l l be observed, provided th e energy d iffe re n c e b e tw e en d i s c r e t e s i t e s and th e r e s o l u tio n o f the a n a ly sis tech n iq u e em ployed i s s u f f i c i e n t l y h ig h (22,29). F o r s p e c ie s v e ry c lo s e in energy, the in d iv id u a l s i t e s w i l l n o t be re so iv e d, b u t r a th e r lo s t in th e p e a k w id th ( r e s u l t i n g in p e a k b ro a d e n in g ). I f a co n tin u u m o f s i t e e n e rg ie s e x is t s, a uniform d e so rp tio n r e s u l ts w ith no s p e c if ic peak s tr u c tu r e (29). A c tiv a tio n energy o f a d so rp tio n /d e so rp tio n The u p ta k e o f oxygen by c a rb o n i s an a c t i v a t e d p ro c e s s w ith an a c tiv a tio n energy a d s o rp tio n, Ea, o f kj/m ol (5,17,18,20). Once fo rm e d, th e oxygen com plex i s t h e r m a lly s t a b l e b e lo w i t s f o r m a tio n

114 94 te m p e ra tu re and i s re le a s e d a t h ig h er te m p e ra tu re s w ith an a c tiv a tio n e n e rg y o f d e s o r p t io n, o f k J /m o l. F ig u re 4-3 show s a p o t e n t i a l en erg y d ia g ra m f o r t h i s p r o c e s s - T h is s i g n i f i c a n t a c tiv a tio n energy o f d e so rp tio n r e s u lts from th e breakage o f C-C bonds t o r e l e a s e CO a n d /o r C02- The r a t e e q u a tio n f o r t h i s u n im o le c u la r p ro cess can be given by the A rrhenius e x p re ssio n (32,33): N(t) = - d s /d t = vnsnexp[-e d/rt] (4.1) w h ere N (t) i s th e d e s o r p tio n r a t e, s i s t h e s u r f a c e c o v e ra g e (m olecules/cm ^), vn i s the frequency o r p re -e x p o n e n tia l fa c to r, n is t h e r e a c t i o n o r d e r (e q u a l t o 1 in t h i s c a s e ), R i s th e g as c o n s ta n t an d T i s t h e te m p e ra tu re in d e g re e s K e lv in. F u r th e r e x p la n a tio n concerning t h is p ro c e ss is g iven in Appendix A. The v a lu e o f Ed i s r e a d i l y d e te rm in e d from th e d e s o r p tio n r a t e v s. tem p eratu re p l o ts recorded a t d if f e r e n t h e a tin g r a t e s as d isc u sse d i n A ppendix A. T a b le 4-1 l i s t s th e Ed v a lu e s r e p o r te d i n th e l i t e r a t u r e f o r v a rio u s carbons. Again, th e m a jo rity o f th is work has been done w ith g r a p h itic carbons. Experimental TDMS System The TDMS s y s te m h as b e e n d e s c r ib e d in d e t a i l i n C h a p te r I I I Scannning e le c tro n m icrographs were recorded on a H ita c h i model S-570 in stru m e n t (describ ed in C hapter I I). Carbon Fibers/O xygen/a dsorbants The c a rb o n f i b e r s u se d i n t h i s s tu d y w e re r e c e iv e d from Avco S p e c ia l ty P ro d u c ts D iv is io n (L o w e ll, MA). Two ty p e s o f f i b e r s w ere

115 9 5 > O c r UJ z UJ 1/ C C(0) REACTION COORDINATE F i g u r e 4-3. P o t e n t i a l e n e r g y d ia g r a m f o r t h e i r r e v e r s i b l e c h em isorption o f oxygen on carbon to produce s u rfa c e oxygen complex, C (0 ). B o th t h e a c t i v a t i o n e n e r g y o f a d s o r p t i o n, Ea, an d t h e a c t i v a t i o n e n e rg y o f d e s o r p t i o n, E^, a r e i n d ic a te d (a d a p te d fro m r e f e r e n c e 16).

116 96 in v e s tig a te d : p yrolyzed p i t c h (PP) and h e a t - t r e a t e d p y ro ly z e d p i tc h (HTPP). B oth w ere p y ro ly z e d from p u r i f i e d c o a l t a r p i t c h a t 1100 C under N2. The HTPP sam ples have undergone a subsequent 3000 C h e a t- tre a tm e n t under Ar (d u ratio n o f a few seconds only). According to the m a n u f a c tu r e r, th e s e f i b e r s do n o t have any ty p e o f c o a tin g o r r e s i n a p p lie d to them. The fib e r s a re m anufactured to be a n o n - c ry s ta llin e, v i tr e o u s ty p e c a rb o n. The Y oung's m odulus i s 5-6 x 106 p s i and th e d ia m e te r i s 33 urn. Linde (Som erset, NJ) h y d ro carb o n -free UHP grade oxygen (CO, C02, and H20 < 3 ppm each) was used fo r th e c h em iso rp tio n s. E xperim ental Procedure Two s e p a ra te methods were used to reco rd th e d a ta, depending on th e ty p e o f e x p e rim e n t p e rfo rm e d. To d e te r m in e w h at v a r i e t y o f fra g m e n ts w ere d e s o rb e d d u rin g sam p le h e a tin g, a s e r i e s o f mass s p e c tra w ere reco rd ed over a given m/z range as th e tem p e ra tu re o f the sam p le was in c r e a s e d. The r e s u l t i n g 3 - d im e n s io n a l p l o t o f m /z v s. tim e vs. s ig n a l in te n s ity was used as an i n i t i a l "survey" o f chem ical com position. I t d id n o t, however, in d ic a te th e tem p e ra tu re s a t which th e v a rio u s sp e c ie s were desorbed o r th e number o f d e so rp tio n s ite s. The second m ethod in v o lv e d m o n ito rin g a s in g l e m /z v a lu e a s a f u n c tio n o f te m p e r a tu r e. The r e s u l t i s a d e s o r p tio n r a t e vs. te m p e r a tu r e c u rv e. In t h i s c a s e, th e te m p e r a tu r e o f d e s o r p tio n, num ber o f d e s o r p tio n s i t e s, and r e l a t i v e am ount d e s o rb e d f o r t h a t p a r t i c u l a r sp e c ie s were determ ined.

117 97 The procedure and m ethod o f d a ta re c o rd in g fo r b o th experim ents a re o u tlin e d below. 1. I n i t i a l d e s o r p tio n p r o d u c ts. To d e te rm in e th e i n i t i a l d e so rp tio n p ro d u cts for "as receiv ed " f ib e r s, a new f ib e r bundle ( f i b e r s ) was m ounted i n th e vacuum s y ste m and e v a c u a te d t o 10 t o r r f o r h o u rs w ith o u t h e a t i n g. A c o m p u t e r - c o n t r o l l e d d e s o r p tio n s u rv e y scan w as p e rfo rm e d by r e c o r d in g 10 c o n s e c u tiv e sp e c tra over th re e se p e ra te m/z ran g es (1-51, 20-70, and ) w hile h eatin g th e sample from 50 t o 1000 C in 30 seconds. The scan r a t e was 1 amu/msec w ith a delay o f msec betw een each spectrum. 2. D e s o rp tio n o f S u rfa c e Oxygen C om plexes. S u rfa c e oxygen co m p lex es w ere fo rm ed on vacuum d e g a sse d f i b e r s by r e a c t i o n w ith oxygen an d t h e i r d e c o m p o stio n m o n ito red by reco rd in g the d eso rp tio n r a te vs. te m p e ra tu re for CO and C02 (m/z=28 o r 44, re s p e c tiv e ly ). The v ario u s s te p s can be o u tlin e d as fo llo w s: a. The sa m p le was vacuum d e g a sse d a t C f o r a t l e a s t 8 30 m in u te s a t a p r e s s u r e o f le s s th a n 5 x 10 t o r r and a llo w e d to c o o l to 70 C. b. The r e a c t i o n cham ber was i s o l a t e d from th e m ain a n a ly s i s chamber and flu sh e d 3 tim e s w ith oxygen and f i l l e d to 200 m to rr. The sam ple w as th e n h e a te d t o e i t h e r c a. 300 C o r 600 C f o r a s p e c i f i e d p erio d o f tim e. H e a tin g was th e n d is c o n tin u e d and th e cham ber *7 evacuated fo r 5 m inutes (to 10 t o r r o r le s s ). c. The sam ple was th e n moved in to d i r e c t l in e - o f - s ig h t w ith th e quadrupole in th e main a n a ly s is cham ber. The d e so rp tio n was recorded

118 98 w h ile h e a tin g t h e sa m p le from 50 to 1000 C a t c a. 3 0 /s e c and m o n ito rin g e i t h e r m /z=28 o r 44. The b a c k g ro u n d w as o b ta in e d by re c o rd in g th e d e so rp tio n a second tim e. R e s u lts and D isc u ssio n Vacuum d e g a ssin g - i n i t i a l d e so rp tio n p ro d u cts T h re e o f t h e te n s p e c t r a (to s i m p l i f y t h e d ia g ra m ) a re show n in F ig u re 4-4A and 4-4B f o r th e i n i t i a l d e s o r p tio n fro m PP a n d HTPP f i b e r s, r e s p e c t i v e l y. The f i r s t s p e c tru m was r e c o r d e d p r i o r to h e a tin g ; th e t h i r d was reco rd ed when the sam ple tem p e ra tu re reached appro x im ately 900 C. As i s e v id e n t in t h e f ig u r e, a w ide v a r i e t y o f low m o le c u la r w e ig h t C, H, and 0 fra g m e n ts a r e o b s e rv e d a s i n i t i a l d e s o r p tio n p r o d u c ts. T h is i s c a u se d by th e re m o v a l o f tr a p p e d c a r b o n iz a tio n p ro d u cts and th e decom position o f s u rfa c e oxygen com plexes i n i t i a l l y p r e s e n t on th e s u r f a c e (3 4 ). The p r e d o m in a n t p e ak s o b s e rv e d a s th e te m p e ra tu re is in c re a s e d a r e m/z = 1, 2, 18, 28, and 44, corresponding to H, H2, H20, CO and C02, re s p e c tiv e ly. These r e s u l ts a re sum m arized in T a b le 4-2. O th e r w o rk e rs h a v e o b s e rv e d s i m i l a r r e s u l t s f o r p o l y a c r y l o n i t r i l e (PAN) a n d g r a p h i t e f i b e r s (3 5,3 6 ). Q u a n t ita tiv e r e s u l ts a re d i f f i c u l t to determ ine due to f lu c tu a tio n s in th e g a in o f th e e l e c t r o n m u l t i p l i e r a n d th e pum ping s p e e d w ith tim e ; h o w e v e r, s e v e r a l e x p e rim e n ts w ith d i f f e r e n t sa m p le s i n d i c a t e d t h a t th e PP f i b e r s a lw a y s y i e l d m ore t o t a l e v o lv e d g a s e s th a n t h e HTPP f i b e r s during th e i n i t i a l vacuum degassing. This i s rea so n a b le c o n sid e rin g th a t th e HTPP f i b e r s have receiv e d an a d d itio n a l h e a t- tr e a tm e n t in an

119 M/Z M/Z F ig u re 4-4. I n i t i a l d e s o r p tio n p r o d u c ts f o r PP (A) and HTPP (B) f i b e r s r e c o rd e d from m /z = 20 t o 70. The f i r s t o f th e t h r e e c o n secu tiv e s p e c tra in each case was recorded p r io r to d eso rp tio n. At t h e t i m e o f t h e t h i r d s p e c t r a, t h e s a m p le t e m p e r a t u r e w as a p p ro x im a te ly 900 C. The scan r a t e was 1 m sec/am u w ith a 2500 m sec d elay betw een each scan.

120 Table 4-2 M/Z LIKELY IONS RELATIVE INTENSITY 1,2 H,H2 H 17 OH,NH3 M 18 H20 H 28,44 C0,C02 H 41,43,45 C2H2NH,C2H5N L c2h7n, ch3no 33,34 HS,H2S T Cl T HC1 L 64,66 S02 T 78,91 CgH6,C7H7 T H=high, M=moderate, L=low, T = tra c e

121 101 i n e r t atm osphere by the m an u factu rer. Some h ig h er m olecular w eight hydrocarbon frag m en ts, in d ic a tiv e of aro m a tic p re c u rs o rs, were observed in tr a c e am ounts, in c lu d in g m/z = 78 (CgHg) and m /z = 91 (C^H7). A pparently some a ro m a tic -lik e su rfa c e s p e c ie s can be r e l e a s e d upon h e a tin g in vacuum. These s p e c ie s may, h o w ev er, be due t o p h y s ic a l ly tr a p p e d m o le c u le s w hich w ere n ot e n tir e l y removed during th e c a rb o n iz a tio n p ro cess r a th e r than su rfa c e - bound aro m a tic groups. The e x is te n c e of n itro g e n -c o n ta in in g sp e c ie s i s suggested by the p re s e n c e o f m /z v a lu e s o f 17 (NH3 a n d /o r OH), and 41, 43, and 45 (C,H,N f r a g m e n ts ). N itro g e n g ro u p s a r e known to e x i s t in PAN-based fib e rs (37,38); however, p itc h f ib e r s do n o t sh are th e same chem ical s t r u c t u r e ( a c r y l o n i t r i l e g ro u p s). I t i s l i k e l y t h a t some n itr o g e n c o n ta in in g m olecules a re p re s e n t in th e co a l t a r p re c u rs o r from which- the f ib e r s a re made. The p r e s e n c e o f tw o o t h e r e le m e n ts was a ls o d e te c te d f o r b o th ty p e s o f f i b e r s. A peak a t m /z = 36, a lo n g w ith s m a lle r p e a k s a t m/z=35, 37, and 38, in d ic a te d th a t c h lo rin e -c o n ta in in g su rfa c e o r bulk groups w ere decomposing on h e a tin g to y ie ld Cl and HC1. The presence o f c h lo r i n e was c o n firm e d by m e a s u rin g th e i s o t o p i c r a t i o s. The 37C1/35C1 and H37C1/H35C1 peak h e ig h t r a t i o s gave an average v a lu e of 0.38, w hich is c lo s e to th e c h lo rin e n a tu r a l iso to p e abundance r a t i o o f B oth f i b e r t y p e s a l s o sh o w e d a p e a k a t m /z = 64 corresponding to S02. In th e case o f PP sam ples, w eaker peaks a t m/z = 33 an d 34 (HS and H2S, r e s p e c t i v e l y ) w ere a ls o o b s e rv e d. The

122 102 p resen ce o f b u lk o r su rfa c e s u lf u r i s n o t s u rp ris in g co n sid e rin g th a t th e p re c u rs o r m a te ria l is c o a l ta r p itc h. This type o f p itc h i s known to c o n ta in s u l f u r, a lth o u g h th e e x a c t form i s n o t known (39). I t should be n o ted th a t th e C1-, S-, and N -co n tain in g fragm ents, a s w e ll as th e a ro m a tic s, a re p re s e n t only in tr a c e q u a n titie s compared to CO and COj as in d ic a te d in Table 4-2. A second vacuum degassing removed most o f th e fragm ents which appeared in th e i n i t i a l spectrum fo r b o th f ib e r s as shown in F ig u re 4-5. N o tice t h a t a f t e r o n ly a few se c o n d s o f vacuum d e g a s s in g a t 1000 C, most o f the peaks in th e sp e c tra a re reduced to b a selin e w ith th e e x c e p tio n o f 28, 44, and 36. The p e a k s a t m /z=28 and 44 re m a in due t o the r e l a tiv e ly la rg e amount of su rfa c e oxygen complex i n i t i a l l y p re s e n t. S u rp ris in g ly, the p eak a t m /z = 36 re q u ire d a more e x te n siv e d e g a s s i n g f o r c o m p le te r e m o v a l. E v i d e n t l y som e c h l o r i n e f u n c t i o n a l itie s are in c o rp o ra te d in th e b u lk o f th e f ib e r or s tro n g ly bound to th e su rface and are n o t e a s ily removed. I t has been re p o rte d t h a t PAN f i b e r s a r e so m e tim e s c a rb o n iz e d i n an a tm o sp h e re o f HC1 v a p o rs to im p ro v e t h e y ie ld (40); h o w e v e r, i t i s n o t known i f su c h a tre a tm e n t had been a p p lie d t o the f ib e r s used in th is study. F u rth e r (30-60 m inutes) vacuum degassing a t 1000 C was s u f f ic ie n t to reduce a l l o f th e p e a k s in t h e i n i t i a l d e s o r p tio n s p e c tru m t o b a s e l i n e. A carbon s u rfa c e p rep ared in t h i s manner was assum ed to be contam inant- and o x y g en -free and w as used fo r th e subsequent o x id a tio n and dosing s tu d ie s.

123 103 A J W ^TLjV)JLa^««4 y*i ' * l ^ e ^ A i 'l n j \ * A A kd * Q - I I I... I.... I I... M/Z F ig u re 4-5 C om parison o f th e i n i t i a l d e s o r p tio n p r o d u c ts f o r PP f i b e r s d u rin g t h e f i r s t (A) and seco n d (B) vacuum d e g a s s in g. The s e r ie s o f 10 s p e c tr a shown were reco rd ed o v e r the tem p e ra tu re in te r v a l o f 200 t o 900 C. The d u ra tio n o f each d eg assin g was se c.

124 104 D esorption o f CO., The d e so rp tio n r a te vs. tem p e ra tu re curves f o r C02 (m/z=44) a re show n in F ig u re 4-6 fo r f i b e r s t h a t w ere p r e t r e a t e d u n d e r fo u r d i f f e r e n t c o n d itio n s. The d o tte d curve re p e se n ts d eso rp tio n from a vacuum d e g a sse d f i b e r t h a t w as n o t e x p o sed t o oxygen. The d ash ed c u rv e shows the d e so rp tio n o f C02 a f t e r oxygen ch em iso rp tio n a t 325 C f o r 5 m in u tes. The d ash ed -d o tted curve shows th e r e s u lts o f th e same c h e m is o r p tio n te m p e r a tu r e, b u t f o r an e x te n d e d tim e (45 m in.). The d e s o r p tio n o f C02 fro m a f i b e r sam ple o x id iz e d a t h ig h te m p e r a tu r e (640 C, 1 m in.) i s i n d ic a te d by th e s o l i d c u rv e. The r e s u l t s shown a r e fo r PP f i b e r s, h o w ev er, th e HTPP sa m p les e x h ib ite d th e same b e h a v io r w ith sm all s h if ts in T, the tem p e ra tu re o f the peak maximum. XT The a v e ra g e peak te m p e r a tu r e s a re su m m arized i n T able 4-3 f o r b o th ty p e s of f ib e r s. As e x p e c te d, th e vacuum d e g a sse d sam ple show ed no p e a k f o r C02 d e s o rp tio n, in d ic a tin g the ab sen ce o f s u r f a c e oxygen co m p le x es w ith C02 a s t h e i r th e r m a l d e c o m p o s itio n p r o d u c t. F o r f i b e r s e x p o sed to oxygen a t low tem p eratu re, however, tw o d i s t i n c t peaks w ere observed. T h u s, C02 w as p ro d u c e d from th e th e r m a l d e c o m p o s itio n o f l e a s t tw o d i s t i n c t su rfa c e com plexes o r from two s im ila r su rface com plexes in s t r u c t u r a l l y d if f e r e n t environm ents. For d isc u ssio n p u rp o ses, th e s e tw o p e a k s a re la b e le d <*(C02) and B(C02), c o rre s p o n d in g t o th e d e s o rp tio n maximums observed a t 535 C and 775 C, re s p e c tiv e ly. The r a t i o of th e peak a re a s, a ( c o 2 ) / 0(CO2), v a rie d from about 1.0 fo r th e 5 m inute o x id a tio n t o 2.5 f o r th e 45 m inute o x id a tio n. The extended

125 105 % TEMPERATURE, C F ig u re 4-6. D e s o rp tio n r a t e v s. te m p e r a tu r e f o r C02 e v o lu tio n fro m HTPP f ib e r s tr e a te d by vacuum d egassin g ( ), 325 C o x id a tio n f o r 5 min. 325 C o x id a tio n fo r 45 m in. ( - - ), and 640 C o x id a tio n fo r 1 min. ( ). The l in e a r h e a tin g r a te was ca. 30 C /sec.

126 Table R X N T p, C O T p. C O z T E M P, C a 0 y 6 a ( H T P P ) ( 1 1 ) ( 2 6 ) ( 8 ) ( 1 6 ) ( 2 6 ) ( 1 4 ) ( 1 5 ) ( 1 1 ) ( 1 9 ) ( 3 3 ) ( 3 2 ) ( 2 4 ) " " ( 1 2 ) ( 1 0 ) ( 8 ) ( 2 7 ) ( 1 9 ) ( 2 4 ) ( 1 2 ) ( 1 5 ) ( 1 3 ) ( 2 2 ) ( 1 1 ) ( 2 1 ) ( 2 6 ) ( 1 5 ) T h e v a l u e s i n p a r e n t h e s e s a r e s t a n d a r d d e v i a t i o n s H T P P = h e a t - t r e a t e d p y r o i y z e d p i t c h c a r b o n f i b e r s P P = p y r o l y z e d p i t c h c a r b o n f i b e r s T h e h e a t i n g r a t e w a s b e t w e e n a n d / s e c

127 107 o x i d a t io n tim e ( a t 325 C) r e s u l t s in an i n c r e a s e in t h e a p e a k in te n s ity r e l a tiv e t o th e 3 peak in te n s ity. The d e s o r p tio n o f CO2 from f i b e r s r e a c t e d w ith oxygen a t h ig h te m p e ra tu re (640 C, 1 min.) was m arkedly d i f f e r e n t a s in d ic a te d by th e s o l i d c u rv e in F ig u re 4-6. In t h i s c a s e, o n ly t h e 3(C02) p e a k w as o bserved (i.e., 0/ 8 = 0.0). The s u rfa c e oxygen group which decom poses t o g i v e t h e 0 (0 0 2 ) p e a k w as u n s t a b l e a t t h e h i g h e r r e a c t i o n te m p e r a tu r e. N o tic e a ls o t h a t th e t o t a l am ount o f C0 2 e v o lv e d ( p r o p o r t i o n a l t o t h e i n t e g r a t e d p e a k a r e a ) f o r a 1 mi n u t e c h e m iso rp tio n a t 640 C i s com parable to a 45 m inute c h em iso rp tio n a t 325 C. T h is r e f e c t s th e d i f f e r e n c e in th e r a t e o f s u r f a c e oxygen com plex f o r m a tio n a t th e tw o te m p e r a tu r e s. As th e a d s o r p t io n te m p e r a tu r e i s i n c r e a s e d, th e r a t e o f oxygen u p ta k e i s i n c r e a s e d. C le a r ly, th e r e l a t i v e am o u n ts o f th e tw o s u r f a c e c o m p le x e s can be c o n tr o lle d by changing th e o x id a tio n te m p e ra tu re and tim e. At 325 C th e a(c0 2 ) d eso rb in g s p e c ie s is m axim ized w h ile oxygen c h e m iso rp ito n a t 640 C r e s u l ts in a preponderance o f 3 (0 0 2 ) d eso rb in g groups. A b r i e f s ta te m e n t can be made concerning th e p o s s ib le i d e n t ity o f th e a and 3 d e s o rb in g g ro u p s. As d is c u s s e d e a r l i e r, th e tw o m o st l i k e l y s u r f a c e s p e c i e s c a p a b le o f p ro d u c in g C0 2 a r e l a c t o n e s and c a rb o x y lic a c id s. These groups c o n ta in two oxygen atom s bonded to one c a rb o n atom (se e F ig u re 4-1 ), and a r e th u s a b le t o e v o lv e C02 upon th e r m a l d e c o m p o s itio n. P e rh a p s i t i s r e a s o n a b le t o s u g g e s t t h a t th e 3(C02) d e s o rb in g g ro u p i s some ty p e o f s u r f a c e c a r b o x y lic a c i d g ro u p s in c e i t i s fo rm ed p r i n c i p a l l y a t th e h i g h e r te m p e r a tu r e.

128 108 C arboxylic a c id s re p re s e n t an advanced sta g e o f o x id a tio n compared to l a c t o n e s, a lc o h o l s, c a r b o n y ls, e t c. The alc02) p e a k m ig h t th e n be d e riv e d from a la c to n e -ty p e su rfa c e group. N otice th a t t h i s peak i s somewhat b ro ad e r than th e 6(0 0 2 ) peak. T his broadening may be due to th e b re a k a g e o f m u ltip l e s u r f a c e bonds (one C-C and one C-O) from a l a c t o n e - t y p e g ro u p, w h ereas th e n a rro w e r B(C02) peak re p re s e n ts th e b re a k a g e o f o n ly one C-C bond from a c a r b o x y lic a c id g ro u p. In a d d i t i o n, t h i s b ro a d n e s s may i n d i c a t e t h a t s e v e r a l s i m i l a r b u t s t r u c t u r a l l y (and t h e r e f o r e e n e r g e t i c a l l y ) d i s t i n c t l a c to n e - ty p e g r o u p s a r e p r e s e n t. I t i s i m p o r t a n t t o n o t e t h a t c o n c l u s i v e id e n t if i c a t io n o f th e se su rfa c e groups i s n o t p o s s ib le based on t h i s in fo rm a tio n a lo n e. F u rth e r evidence su p p o rtin g th e id e n tity o f th e se two groups i s given below in th e d isc u ssio n o f CO d e so rp tio n, and in th e fo llo w in g c h a p te r concerning th e a d so rp tio n o f o rg an ic s. D esorption o f CO The d e so rp tio n r a t e vs. tem p e ra tu re cu rv es f o r CO (m/z = 28) a re q u ite d if f e r e n t compared to th o se fo r C02r a s in d ic a te d in F igure 4-7 f o r b o th HTPP (A) and PP (B) f i b e r s. The u p p e r c u rv e s in th e f i g u r e (a) a r e f o r oxygen c h e m is o rb e d a t low te m p e r a tu r e s ( C, 5 min.) and th e low er curves (b) a re f o r high tem p e ra tu re ch em iso rp tio n ( C, 1 m in.). The dashed lin e s show th e d e so rp tio n from f ib e r s t h a t had n o t been exposed to oxygen. Once a g a in, th e vacuum degassed f i b e r s r e s u l t e d in no p e a k s i n th e d e s o r p tio n s p e c tru m. F o llo w in g oxygen c h e m is o r p tio n, h o w ev er, f o u r d e s o r p tio n p e a k s a r e o b s e rv e d, la b e le d a(c 0), 6(C0), y{c0), and S(CO), i n d i c a t i n g d e c o m p o stio n from

129 109 F ig u re 4-7. D e s o rp tio n r a t e v s. te m p e r a tu r e f o r CO e v o lu tio n from HTPP (A) and PP (B) f i b e r s. The c u rv e s la b le d (a) a r e f o r low te m p e r a tu r e o x id a tio n (ca. 330 C) and th e c u rv e s l a b l e d (b) a re f o r h ig h te m p e ra tu re o x id a tio n (640 C ). The d ash ed l i n e s show th e d e so rp tio n from vacuum degassed f ib e r s. The l in e a r h e a tin g r a te was c a. 3 0 C /se c.

130 M/Z 28 SIGNAL X4 _i < z o CO co CM u N s 5 X4 f H TEMPERATURE, C F igure

131 Ill fo u r se p a ra te s ite s. The maximum peak tem p e ra tu re s and th e r e la tiv e i n t e n s i t i e s o f th e f o u r s i t e s w ere fo u n d to v a ry c o n s id e r a b ly from r u n - t o - r u n and among d i f f e r e n t s a m p le s. A lso, th e sh a p e o f th e d e so rp tio n curves changed as th e f ib e r s were re p e a te d ly o x id ized and degassed, and a "stead y s ta te " d e so rp tio n p a tte rn was not a tta in e d fo r CO a s i t w as f o r C02. N o tic e, h o w ev er, t h a t th e a(co) p e a k a p p e a rs o n ly fo r th e low te m p e r a tu r e o x id iz e d f i b e r s a s was th e c a se f o r o(c02). F u rth e rm o re, b o th a p e a k s w ere o b s e rv e d to d e s o rb o v e r th e same tem p e ra tu re range ( C). This i s f u r th e r su p p o rt fo r t h e e x is te n c e o f a la c t o n e - t y p e s u r f a c e group w h ic h decom poses t o y i e l d both th e a peaks due to i t s dual f u n c tio n a lity. The v a r i a b i l i t y in CO d e s o r p tio n s u g g e s ts t h a t th e s u rfa c e com plexes desorbing t o y ie ld CO are form ed a t d if f e r e n t carbon s ite s th a n th o se f o r C02. S i te s t h a t a re e x p e c te d t o show c o n s id e r a b le v a r i a b i l i t y would in c lu d e d e fe c t s i t e s which a re re a d ily changed a s th e carbon su rfa c e undergoes vacuum d eg assin g and o x id a tio n. Repeated h ig h te m p e ra tu re t r e a tm e n t o f c arb o n h a s been s u g g e s te d t o " a n n e a l o u t' a c tiv e s ite s f o r oxygen ch em iso rp tio n (19). However, some types o f carbon, in clu d in g g ra p h itiz e d carbon b la c k, have been shown to have a h ig h e r c a p a c ity f o r oxygen c h e m is o rp tio n a f t e r a p r i o r o x id a tio n (2 6 ). Some o f t h i s v a r i a b i l i t y may a ls o be due t o th e re m o v a l o f an o u t e r c a rb o n s t r u c t u r e t o expose a s t r u c t u r a l l y d i f f e r e n t c o re m a t e r i a l. T here i s some e x p e r im e n ta l e v id e n c e, b a s e d on e l e c tr o n m ic ro sc o p y s t u d i e s, f o r th e re m o v a l o f such an o u t e r s k in s t r u c t u r e

132 112 fro m th e f i b e r s u r f a c e a f t e r p ro lo n g e d o r r e p e a te d o x id a tio n and d egassing t o expose an in n er c o re w ith s lig h t ly d if f e r e n t s tr u c tu r e. F ig u re 4-8 shows two e le c tro n m icrographs o f th e same f ib e r a re a a f t e r m u ltip l e o x id a tio n /d e g a s s in g c y c le s a t 2210X (A) and 4900X (B) m a g n i f ic a tio n. T h is r e p r e s e n t s an ad v an ced s ta g e o f o x id a tio n compared t o th e f ib e r sample shown p re v io u sly in F ig u re N otice t h e e x te n s iv e p i t t i n g and th e r e s i d u a l p l a q u e - l i k e s u b s ta n c e on th e s u r f a c e. T h is t h i n o u t e r c o v e r i n g i s p r o b a b l y some t y p e o f c a rb o n a c e o u s m a t e r i a l t h a t i s som ew hat d i f f e r e n t th an t h e b u lk s tr u c tu r e a s d isc u sse d in C hapter I. In g e n e ra l, CO d e so rp tio n h as been shown to r e s u l t in from two to f o u r s i t e s, d e p e n d in g on th e ty p e o f c a rb o n and th e a n a ly s i s m ethod u s e d. S e v e r a l w o rk e rs have r e p o r te d t h r e e o r f o u r s i t e s f o r CO d e s o r p tio n from g r a p h i t e (2 2,2 6,2 8,2 9,4 1 ). M atsum oto and S e ta k a o b s e rv e d tw o s i t e s f o r CO d e s o r p tio n from o x id iz e d v i t r e o u s c a rb o n (3 1 ). They a l s o r e p o r te d th e d e s o r p t io n o f CO and CO2 from o x id iz e d d ia m o n d p o w d e r a n d o b s e r v e d a t l e a s t f o u r s i t e s f o r o x y g e n c h e m iso rp tio n (4). I t seems l i k e l y th a t a v a rie ty o f carbon s i t e s i s c a p a b le o f a d s o rb in g oxygen t o form c o v a le n tly bound s u r f a c e com plexes, b o n d s, e t c. i.e., "arm -chair" and "zig-zag" fa c e s, d e f e c ts, u s a tu ra te d T h is v a r i e t y o f a d s o r p tio n s i t e s may le a d t o some v a r i a b i l i t y in th e d e s o r p tio n o f CO fro m d i f f e r e n t s a m p le s and d i f f e r e n t ty p e s of carbon. The r e l a t i v e am ounts o f CO and C02 desorbed a t th e two o x id a tio n te m p e r a tu r e s w ere d e te rm in e d by i n t e g r a t i n g t h e p e a k a r e a s and

133 F ig u re 4-8. S c a n n in g e l e c t r o n m ic ro g ra p h s o f a s e v e r e ly o x id iz e d c a rb o n f i b e r show n a t 2210X (A) and 4900X (B) m a g n i f ic a tio n. The d o tte d l i n e a t th e b o tto m o f th e p h o to g ra p h s r e p r e s e n t s 13.6 and 6.1 urn, r e s p e c tiv e ly.