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THE STUDY OF RUTHENIUM OXIDATION BY PULSED FIELD DESORPTION MASS
SPECTROMETRY
G. Chuah, D. Cocke, N. Kruse, G. Abend, T. Kessler, J. Block
To cite this version:
G. Chuah, D. Cocke, N. Kruse, G. Abend, T. Kessler, et al.. THE STUDY OF RUTHENIUM OXIDATION BY PULSED FIELD DESORPTION MASS SPECTROMETRY. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-359-C2-363. �10.1051/jphyscol:1986255�. �jpa-00225689�
THE STUDY OF RUTHENIUM OXIDATION BY PULSED FIELD DESORPTION MASS SPECTROMETRY
G.K. CHUAH, D.L. COCKE, N. KRUSE*, G. ABEND**, T. KESSLER** and J.H. BLOCK**
Department of Chemistry, Texas R&U University, College Station, Texas 77843, U.S.A.
*I n s t i t u t fur Technische Chemie der T.U. Berlin, Strasse des 17.
Juni 228, D-1000 Berlin 12, F.R.G.
* *F r i t z - H a b e r - I n s t i t u t der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, F.R.G.
Résume - Les intermédiaires de surface impliqués dans l'oxydation du Ru à 700 K ont été examinés par spectromêtrie de masse de désorption de champ puisée. Les résultats sont comparés avec ceux obtenus par SMIS, spectromê- trie de masse de sonde atomique et spectromêtrie de masse d'évaporation thermique. Des ions chargées simples et doubles de Ru, RuO, RuO, et RuO,
2 + 3 +
aussi bien que Ru20 et RUpO, , sont observées. La présence d'une couche prêoxydêe sur la pointe de Ru s'est avérée être nécessaire avant le formation de RuOg apparaisse. RuO, était formé pour une température supérieure à 60C K à 10 PaOg. A 600 K, une haute mobilité des espèces d'oxyde a été observée.
Abstract - The surface intermediates involved in the oxidation of ruthenium at 700 K were examined by pulsed field desorption mass spectrometry (PFDMS).
The results were compared with those from SIMS, atom probe and thermal vapo- rization mass spectrometry. Sinoly and doubly charged ions of Ru, RuC, RuO,,
2+ " 2+
and Ru03 as well as Ru20 and R u203 were observed. The presence of a pre- oxidized layer on the ruthenium tip was found to be necessary before any RuO, formation occurred. RuO, was formed at temperatures above 600 K at 10 Pa 0-3 2- At 600 K, the oxide species were observed to be highly mobile.
I - INTRODUCTION
The chemisorption of oxygen on ruthenium has been extensively studied with various surface sensitive techniques /l/. However, little is known about the oxidation at high temperatures. This is surprising in view of the great importance of the metal for applications in catalysis. The practical use is limited because of the tendency of the metal to form volatile oxides, RuO, and RuO«. Kim and Winograd / 2 / used XPS and found that RuO, may be present as a defect structure of Ru02- Thermal desorption
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986255
JOURNAL DE PHYSIQUE
s p e c t r a o f oxygen f r o m r u t h e n i u m c o n t a i n e d f e a t u r e s which a r e p o s s i b l y r e l a t e d t o s u r f a c e o x i d e s o r t h e i r p r e c u r s o r s which r e l e a s e oxygen /3/. Cranstoun e t a l . /4,5/
performed s t u d i e s on t h e o x i d i z e d metal by r e a n s o f f i e l d i o n ~ i c r o s c o p y and atom probe t i m e - o f - f l i g h t Kass s p e c t r o m e t r y . The a u t h o r s i d e n t i f i e d a f i l m o f RuO o f s e v e r a l t e n l a y e r t h i c k n e s s as a c r y s t a l l i n e o x i d e . I n a d d i t i o n , 0' and RuO:' i o n s ( x = 1 ... 3 ) have been d e t e c t e d .
I n c o n t r a s t t o t h i s , SIMS a n a l y s i s o f t h e o x i d e between 300 and 700 K y i e l d e d o;,
RUO', and Ru20' i o n s /6/.
I n o r d e r t o e x a v i n e t h e i n t e r m e d i a t e stages o f o x i d e f o r m a t i o n we employed p u l s e d f i e l d d e s o r p t i o n mass spectrornetry (PFDMS). T h i s v e t h o d has a l r e a d y been used i n s t u d i e s o f metal o x i d a t i o n /7/.
I 1 - EXPERIMENTAL
The i m p l i c a t i o n s o f t h e PFDFS t e c h n i q u e f o r s t u d y i n g t h e k i n e t i c s o f chemical r e a c t - i o n s a t s u r f a c e s have been d e s c r i b e d i n d e t a i l elsewhere /8,9/. F a s t n e g a t i v e p u l s e s ( u p t o 2 0 kV a m p l i t u d e , a few 100 ns w i d t h , r e p e t i t i o n r a t e s up t o 100 kHz) a r e ap- p l i e d t o a c o u n t e r e l e c t r o d e i n f r o n t o f a r u t h e n i u m f i e l d e m i t t e r t i p . The f i e l d p u l s e s desorb t h e p a r t i c l e s , adsorbed a t t h e e m i t t e r s u r f a c e , i n a c o n t r o l l e d r a n - n e r . The r e s p e c t i v e i o n s a r e c h e m i c a l l y analyzed by t i m e - o f - f l i g h t mass s p e c t r o - m e t r y . The Ru t i p was spark e r o s i o n c u t f r o m a b o u l e s u p p l i e d by M a t e r i a l s Research C o r p o r a t i o n , and e l e c t r o l y t i c a l l y etched. C l e a n i n g o f t h e t i p s u r f a c e c o u l d be achieved i n s i t u b y f i e l d e v a p o r a t i o n .
111 - RESULTS AND DISCUSSION
F o r t h e ~ e a s u r e m e n t s , t o be r e p o r t e d here, we p r e t r e a t e d t h e Ru f i e l d e m i t t e r i n s i t u w i t h oxygen a t 900 K, 1.3 X 10-' Pa, a t l e a s t 2 hours.
A f t e r c o o l i n g down t o 700 K, f i e l d d e s o r p t i o n under dynamic oxygen s u p p l y a t 1.3 X 1 0 - ~ Pa y i e l d e d mass s p e c t r a which c o n t a i n e d v a r i o u s k i n d s o f s i n g l y and d o u b l y charged i o n s . Besides t h e f i e l d evaporated Run+ i o n s , we d e t e c t e d Ru 0 up t o
X Y
r u t h e n i u m t r i o x i d e and i t s dimer. The R U O ~ " i o n s o r i g i n a t e from t h e t ~ g h t l y bound o x i d e l a y e r as we1 l as from more weakly bound Ru02 molecules, as w i l l be c'iscussed below.
The f o r m a t i o n o f an o x i d e phase was a l s o observed by Cranstoun e t a l . /4/ under oxy- gen t r e a t m e n t a t 900 K.
The dependence o f t h e i o n i n t e n s i t i e s on t h e ( p u l s e d ) d e s o r p t i o n f i e l d s t r e n g t h i s shown i n f i g . 1. The i n t e n s i t i e s o f Ruon++, Ru20n+ and Ru203" have been summed u p as suboxide s p e c i e s . A c c o r d i n g t o t h e d i f f e r e n t f i e l d s t r e n p t h dependence we d i s t i n -
+ + + +
g u i s h suboxides, m o l e c u l a r i o n s (Ru02 and Ru03), and c l u s t e r i o n s (Ru204, Ru205, Ru20;). A t l o w f i e l d s t r e n g t h s , t h e m o l e c u l a r i o n s dominate. They appear c o n s i d e r - a b l y below t h e o n s e t s o f t h e c l u s t e r s and t h e suboxides. Thus we c o n c l u d e t h a t b o t h
i- +
moderate f i e l d s t r e n g t h , and decreases f u r t h e r o n . The c l u s t e r ions Ru204 , Ru205 ,
t -+
and Ru206 behave i n a s i m i l a r manner a s Ru03 , i . e . t h e i r i n t e n s i t i e s f i r s t i n - c r e a s e due t o an i n c r e a s i n g i o n i z a t i o n p r o b a b i l i t y , pass a maximum and then decrease.
This i n t e n s i t y decrease c o i n c i d e s with t h e sharp o n s e t of t h e suboxide i o n s . Due t o
desorption field strength [ V h m l
Fiq. 1 - Variation of pulsed desorption f i e l d s t r e n g t h . Oxide ions w i t h l e s s than two oxygen a t o m per ruthenium atom a r e sum~ed up. The c a l i b r a t i o n of t h e f i e l d s t r e n g t h s c a l e i s not very a c c u r a t e . Conditions: Tip temperature 700 K , oxygen pres- sure 1.3 X 1om3 Pa, monitored area 2 ntr 2 , (0001) o r i e n t e d ; desorption pulse width 300 ns; r e a c t i o n time between pulses 100 u s .
t h e removal of oxygen containing p a r t i c l e s from t h e s u r f a c e (by t h e f i e l d p u l s e s ) ,
JOURNAL DE PHYSIQUE
t h e o x i d a t i o n s t a t e o f t h e oxide l a y e r i s considerably lowered a t t h e h i g h e s t f i e l d strengths.
The h i g h i n t e n s i t i e s o f t h e suboxide i o n s and t h e R U O ~ ' can o n l y p a r t l y be explained by impingement o f gaseous O2 and subsequent r e a c t i o n w i t h t h e Ru s u r f a c e i n t h e moni- t o r e d area. There must be an a d d i t i o n a l supply by d i f f u s i o n o f s u r f a c e species i n t o t h e monitored area a t t h e apex of the t i p . A t t h e shank o f t h e e m i t t e r t i p t h e f i e l d
s t r e n g t h i s low, so t h a t t h e e q u i l i b r i u m oxide s t r u c t u r e i s n o t d i s t u r b e d by t h e f i e l d pulses, and weakly bound Ru02 and Ru03 r o l e c u l e s a r e formed. Along t h e p a t h from t h e shank t o t h e apex the chemical p o t e n t i a l o f t h e oxide l a y e r varies, so t h a t Ru03 and a l s o p o s s i b l y o t h e r oxide species, l i k e Rue2, d i f f u s e t o t h e apex. There, a t oxygen d e f i c i e n t s i t e s , they nay decompose. However, t h e decomposition and i n - corporaton i n t o t h e oxide l a y e r does n o t r e s t o r e i t s h i g h o x i d a t i o n s t a t e , because t h e f i e l d pulses c o n t i n u o u s l y remove oxide ions.
F u r t h e r c o n f i r m a t i o n o f t h e m o b i l i t y o f oxide molecules i s obtained from t h e f o l l o w - i n g observations:
- The oxide i o n i n t e n s i t y i s kept h i g h f o r several ~ i n u t e s a f t e r removing t h e oxygen gas supply, by pumping down q u i c k l y below 1 0 - ~ Pa. The lower the desorp- t i o n f i e l d strength, t h e slower t h e decrease o f t h e i o n i n t e n s i t y .
- No s u b s t a n t i a l b l u n t i n g o f t h e f i e l d e m i t t e r has been observed, although a few ten thousand ruthenium o x i d e ions were eetected, which corresponds t o several hundred l a y e r s . This observation can o n l y be understood i n t e r n s o f m o b i l e species ensuring t r a n s p o r t t o the apex o f t h e t i p , i n s t e a d o f removal o f l a t - t i c e l a y e r s a t t h e apex.
- Reaction t i m e v a r i a t i o n a t a h i g h d e s o r p t i o n f i e l d s t r e n g t h r e v e a l s a steep i n - crease o f t h e RuC3 surface c o n c e n t r a t i o n a f t e r a c e r t a i n delay. This delay time, about 10 trs a t 30 V/nm, increases a t higher f i e l d s t r e n g t h s . I t takes a longer t i m e t o r e s t o r e t h e o x i d e l a y e r , when t h e l a y e r Kore s t r o n g l y deviates from t h e h i g h o x i d a t i o n s t a t e . A f t e r t h e delay period, t h e i n t e n s i t y increases by two orders o f magnitude w i t h i n one order o f magnitude t i m e v a r i a t i o n . This steepness i s evidence f o r a d i f f u s i o n supply from t h e shank i n t o the monitored area, r a t h e r than f o r a k i n e t i c l i m i t a t i o n o f t h e f o r m a t i o n process.
The preceding d i s c u s s i o n confirms Ru03 as a ~ o b i l e r o l e c u l e . However, t h e d e t e c t i o n o f h i g h RuO ' i n t e n s i t i e s a t low f i e l d s t r e n g t h s ( f i g . l ) , and t h e presence o f RUO;,
besides Ru03 *+ , a f t e r removing t h e oxygen gas supply, support t h e existence o f a mo- b i l e Ru02 molecule. Cnly a t h i g h e r f i e l d strengths, where suboxides a r e desorbed, Ru03 may decompose and t r a n s f e r oxygen i n t o t h e oxide l a y e r . A t low f i e l d strengths, however, t h e h i g h Ru02 i n t e n s i t y cannot s o l e l y be explained by decomposition o f t
RuOg.
The continuous increase o f the Ru02 i n t e n s i t y i n f i g . t 1 - as compared t o t h e de- c r e a s i n g t r e n d o f R U O ~ + - i s due t o an i n c r e a s i n g d e s o r p t i o n o f Ru02 from t h e more +
A possible mobility of suboxide species remains an open question. Our measurements gave no indication, e i t h e r f o r or against mobility. The existence of suboxide ions could e a s i l y be understood by decomposition of mobile Rue2 and Ru03 a s well as reaction of the ruthenium with impinging oxygen. Both reactions seem t o occur.
The behaviour of the c l u s t e r species i n f i g . 1 indicates t h a t these trolecules a r e formed in a consecutive surface reaction from mobile Ru02 and Ru03.
When the surface concentration of these rolecules decreases a t high f i e l d strengths, the concentration of the c l u s t e r species has t o decrease likewise.
The existence of c l u s t e r s with more than two oxygen atoms per ruthenium atom co- incides w i t h the observation of an amorphous l i k e higher oxidation s t a t e layer by Winograd / 2 / . I t i s l i k e l y t h a t Ru02 and Ru03 a c t as intermediate species in the reaction towards the v o l a t i l e Ru04 molecule. A t the reaction conditions choosen f o r the reported measurements, no R U O ~ + has been detected. However, when decreasing the surface temperature to 400.. .450 K a t 1.3 X 1oa3 Pa oxygen pressure and low f i e l d strength, we obtained mass spectra with a considerable amount of Ru04 (besides +
R U O ~ ~ ' and RUO~"', n=1,2). The formation of the v o l a t i l e Ru04 rcolecule on a high oxidation s t a t e oxide i s in accord with thermodynamic considerations. The lower tem- perature possibly influenced the oxidation s t a t e of the oxide layer; the main reason f o r the observation of RUO~', however, i s the increase of the mean l i f e t i m e of Ru04 and, consequently, i t s concentration a t the surface. Mobile molecules with high oxi- dation s t a t e s e x i s t on an oxidized ruthenium surface, even a t low oxygen pressures.
This observation confirms t h a t the processes a t the i n t e r f a c e between bulk
ruthenium and the oxide phase a r e rather slow, hence t h e i r r a t e s a r e accessible to the pulsed f i e l d desorption technique.
ACKNOWLEDGEMENTS
This work was p a r t i a l l y supported by Sonderforschungbereich 6 a t the Freie Universi- t a t Berlin. G. K. C. g r a t e f u l l y acknowledges a fellowship of the Max-Planck-Gesel l -
schaft .
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