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Charge transfer in No2AsF6 intercalated graphite
Synthetic Metals, 8 (1983) 177 - 181
177
C H AR GE T R A N S F E R IN NO2AsF 6 I N T E R C A L A T E D G R A P H I T E
L. MATTIX*, J. MILLIKEN, H. A. RESING, J. MINTMIRE and D. C. WEBER Naval Research Laboratory, Code 6122, Washington, DC 20375 (U.S.A.)
S u m mar y Using the m e t h o d of Schumacher and Slichter, we have obtained the density o f states (DOS) at the Fermi energy for a stage 1 graphite-AsF6intercalation c o m p o u n d . This c o m p o u n d was form ed via the reaction of the nitronium salt, NO:AsF 6, with highly oriented pyrolytic graphite. The theoretical value calculated from the known charge transfer and the theoretical DOS curve o f pure graphite agrees well with our experimental value. This shows th at the DOS for pure graphite can be used in the S c h u m a c h e r Slichter ex p er ime nt t o determine the charge transfer in graphite intercalation c o m p o u n d s of u n k n o w n chemistry, such as the AsFs-intercalation comp o u n d (as others have assumed!). Our subsequent measurements of the charge transfer in stage 1 and stage 2 AsFs-intercalated graphite agree with previous results.
Introduction It is widely believed that AsF s oxidizes the graphite lattice by the reaction [1]: 3AsF s + 2e-
) 2AsF 6- + AsF 3
A conclusive determination of the e x t e n t of conversion of AsF s to AsF 6 and AsF 3 has n o t y e t been made. Previous studies of AsF s com pounds, involving arsenic X-ray pre-absorption edge [1 ], XPS [2] metallic reflectance [3], de Haas-van Alphen effect [4, 5], S h u b n i k o v - d e Haas effect [5], conductivity magnetoresistance [6], and spin magnetic susceptibility [7], have resulted in fractional charge transfer (f) estimates which range from 0.15 to 0.66 electrons per intercalant molecule. Most of these studies depended critically on the validity of a rigid band model (or some extension of it) for the c o m p o u n d ' s electronic structure. The magnetic susceptibility m e t h o d , which gave a value of f = 0.24 for stage 1 and f = 0.48 for stage 2, actually used the density of states (DOS) curve of pure graphite. It is essential that this rigid band model be tested with a c o m p o u n d of known f before definitive statements can be made about the e x t e n t of the AsF s reaction. *Present address: Department of Chemistry and Physics, Norfolk State University, 2401 Corprew Avenue, Norfolk, VA 23504, U.S.A. 0379-6779/83/$3.00
© Elsevier Sequoia/Printed in The Netherlands
178 A direct intercalation of the AsF6- ion would provide such a test, since the value of f for the resulting c o m p o u n d would be 1.0 electron per AsF 6- complex. Following the lead of Moran et al. [9], we have intercalated AsF6- into highly oriented pyrolitic graphite (HOPG) by reacting it with the nitronium salt, NO2AsF 6. A measurement of the spin magnetic susceptibility has yielded the value of the DOS at the Fermi energy for the c o m p o u n d of interest.
Experimental The spin magnetic susceptibilities were measured at room temperature using the m e t h o d of Schumacher and Slichter [8]. The absolute intensity of the conduction electron spin resonance absorption was determined by direct comparison with the 19F nuclear resonance absorption signal in the same sample and at the same frequency. Thus both resonances were observed by sweeping the external magnetic field without changing any of the r.f. parameters. The absolute susceptibility was calculated from the simple Curie law, XN = N J ( J + l ) ( ~ s h ) 2 / 3 k T
where 7N is the nuclear gyromagnetic ratio and k is Boltzmann's constant. The absolute spin susceptibility becomes XE = (~E/TS )(IE ~IN )XS
where 7E is the electron gyromagnetic ratio, with IE and IN being the integrated electronic and nuclear resonance absorption intensities, respectively. Measurements were made at 29 MHz with a standard marginal oscillator spectrometer employing field modulation and phase sensitive detection. The spectra were recorded and integrated with a Nicolet 1074 signal averager. All samples were prepared from HOPG starting materials. Pure NO2AsF6 was prepared by the new m e t h o d of Moran et al. [9], and checked with X-ray diffraction. Two different methods were used to prepare the graphite intercalation compounds of AsF6-; both yielded the same results. The first m e t h o d , by Billaud et al. [10], used an h-shaped Pyrex reaction vessel equipped with a Teflon-in-glass vacuum stopcock. The vessel was placed in a dry box and charged with 100 mg NO2AsF 6 and HOPG (6 × 7 × 1 mm3). In the vacuum system, nitromethane (2 ml, dried over P4010) was distilled into the reactor and the resulting NO2AsF6 solution poured onto the graphite. After 1 - 2 h, the reaction ceased and the intercalated graphite was washed several times with pure nitromethane. In the second m e t h o d , 1 - 2 ml of nitromethane (dried over P4010) were placed in a 10 ml weighing bottle in a glove bag with N2 atmosphere. The HOPG sample (6 × 7 × 1 mm 3) was immersed and NOzAsF 6 was added slowly to prevent sample cleavage from rapid intercalation. A saturated solution of NO2AsF 6 in nitromethane was achieved over a period of ~ 4 h.
179 T h e r e a c t i o n was t e r m i n a t e d w h e n t h e s a m p l e t h i c k n e s s a n d c o l o r i n d i c a t e d i n t e r c a l a t i o n to stage 1. T h e s a m p l e was t h e n w a s h e d in p u r e n i t r o m e t h a n e a n d sealed (in n i t r o m e t h a n e ) in r e c t a n g u l a r P y r e x t u b i n g . T o ensure t h a t t h e stage was m a i n t a i n e d , all m e a s u r e m e n t s w e r e carried o u t in an excess o f p u r e n i t r o m e t h a n e . T h e N M R / E P R coil was w o u n d d i r e c t l y o n t h e s a m p l e t u b e .
Results a n d c o n c l u s i o n s A t y p i c a l E P R s p e c t r a is s h o w n in Fig. 1. B o t h N M R and E P R linewidths w e r e ~ 0 . 5 gauss ( p e a k - t o - p e a k derivative). T h e r a t i o R = Ie/In a n d b o t h t h e e x p e r i m e n t a l a n d t h e o r e t i c a l D O S at t h e F e r m i energy ( N ( E F)) is s h o w n f o r each c o m p o u n d in T a b l e 1. T h e e x p e r i m e n t a l values o f N ( E F ) w e r e o b t a i n e d a s s u m i n g t h a t t h e m e a s u r e d s u s c e p t i b i l i t y was d u e to t h e t e m p e r a t u r e i n d e p e n d e n t Pauli p a r a m a g n e t i s m , f o r w h i c h Pe = PB2N(EF), w h e r e PB is t h e B o h r m a g n e t o n a n d N ( E F ) includes b o t h signs o f spin. O u r a s s u m p t i o n was c o n f i r m e d b y s u b s e q u e n t m e a s u r e m e n t s at 145 ° and 77 ° , w h i c h have s h o w n n o e v i d e n c e o f localized e l e c t r o n i c m o m e n t s . F o r stage 1 and stage 2 AsF si n t e r c a l a t e d H O P G , o u r e x p e r i m e n t a l results agree w i t h t h o s e o f W e i n b e r g e r et al. [ 7 ] . TABLE 1 Compound
R
N(E F )*
N(EF )**
C20AsF62.5CH3NO2 CsAsF5 C16AsF s
5.16 + 0.52 1.70 + 0.23 3.29 + 0.44
0.086 + 0.009 0.059 + 0.008 0.057 +- 0.008
0.079 ---
*Experimental (states per eV per carbon atom). **Theoretical {states per eV per carbon atom).
//
Fig. 1. A typical EPR derivative absorption signal (A) and absorption curve (B).
180 0.15
0,i0
~
0,05~ ~'
0,15
;
I
I
I
I
t
I
I
I
~ ' ~ 0.i0 ,< o
~ 0,05
0 -2.0
-i,o
o ENERGY
1,0
2,o
(EV)
Fig. 2. The d e n s i t y o f states calculated from a o n e d i m e n s i o n a l tight-binding m o d e l and its first integral from the pure graphite Fermi energy.
For the AsF6- compound in which f is known to be unity, theoretical estimates of the DOS at the Fermi energy, based on (a) the rigid band model and (b) the DOS calculations for pure graphite, are in agreement with our experiment. This partially validates both t h e rigid band model applied to these acceptor intercalation compounds and the DOS curves used; we used the DOS curve given by Weinberger e t al. [7], as well as one calculated here and shown in Fig. 2. But in another sense we have constructed an experimental density of states curve, which can serve as an interpolation function in the region of the Fermi energy for these graphite intercalation compounds. Thus, for those compounds for which f is not known, we n o w have a valid density of states curve, which can be used with the SchumacherSlichter experiment to find the charge transfer (f). For stage 1 AsF s in graphite f = 0.26, and for stage 2, f = 0.50 electrons per intercalant molecule. Acknowledgements The authors thank Dr. M. Rubinstein for the use o f his electromagnet for this investigation. One of us (L. M.) was supported by a National Research Council-Naval Research Laboratory Post-doctoral Fellowship.
181 References 1 2 3 4 5 6 7 8 9 10
N. Bartlett, B. McQuillan and A. S. Robertson, Mater. Res. Bull., 13 (1978) 1259. M. J. Moran, J. E. Fischer and W. R. Salaneck, J. Chem. Phys., 73 (1980) 629. L. R. Hanlon,E. R. Falardeau and J. E. Fischer, Solid State Commun., 24 (1977) 377. Y. Iye, O. Takahashi and S. Tanuma, Solid State Commun., 33 (1980) 1071. R. S. Markiewicz, H. R. Hart, L. V. Interrante and J. S. Kasper, Solid State Commun., 35 (1980) 513. C. Zeller, L. A. Pendrys and F. L. Vogel, J. Mater. Sci., 14 (1979) 2241. B. R. Weinberger, J. Kaufer, A. J. Heeger, J. E. Fischer, M. Moran and N. A. W. Holzwarth, Phys. Rev. Lett., 41 (1978) 1417. R. T. Schumacher and C. P. Slichter, Phys. Rev., 101 (1956) 58. M. J. Moran, G. R. Miller, R. A. DeMarco and H. A. Resing, to be published. D. Billaud, A. Pron and F. L. Vogel, Synth. Met., 2 (1980) 177.
177
C H AR GE T R A N S F E R IN NO2AsF 6 I N T E R C A L A T E D G R A P H I T E
L. MATTIX*, J. MILLIKEN, H. A. RESING, J. MINTMIRE and D. C. WEBER Naval Research Laboratory, Code 6122, Washington, DC 20375 (U.S.A.)
S u m mar y Using the m e t h o d of Schumacher and Slichter, we have obtained the density o f states (DOS) at the Fermi energy for a stage 1 graphite-AsF6intercalation c o m p o u n d . This c o m p o u n d was form ed via the reaction of the nitronium salt, NO:AsF 6, with highly oriented pyrolytic graphite. The theoretical value calculated from the known charge transfer and the theoretical DOS curve o f pure graphite agrees well with our experimental value. This shows th at the DOS for pure graphite can be used in the S c h u m a c h e r Slichter ex p er ime nt t o determine the charge transfer in graphite intercalation c o m p o u n d s of u n k n o w n chemistry, such as the AsFs-intercalation comp o u n d (as others have assumed!). Our subsequent measurements of the charge transfer in stage 1 and stage 2 AsFs-intercalated graphite agree with previous results.
Introduction It is widely believed that AsF s oxidizes the graphite lattice by the reaction [1]: 3AsF s + 2e-
) 2AsF 6- + AsF 3
A conclusive determination of the e x t e n t of conversion of AsF s to AsF 6 and AsF 3 has n o t y e t been made. Previous studies of AsF s com pounds, involving arsenic X-ray pre-absorption edge [1 ], XPS [2] metallic reflectance [3], de Haas-van Alphen effect [4, 5], S h u b n i k o v - d e Haas effect [5], conductivity magnetoresistance [6], and spin magnetic susceptibility [7], have resulted in fractional charge transfer (f) estimates which range from 0.15 to 0.66 electrons per intercalant molecule. Most of these studies depended critically on the validity of a rigid band model (or some extension of it) for the c o m p o u n d ' s electronic structure. The magnetic susceptibility m e t h o d , which gave a value of f = 0.24 for stage 1 and f = 0.48 for stage 2, actually used the density of states (DOS) curve of pure graphite. It is essential that this rigid band model be tested with a c o m p o u n d of known f before definitive statements can be made about the e x t e n t of the AsF s reaction. *Present address: Department of Chemistry and Physics, Norfolk State University, 2401 Corprew Avenue, Norfolk, VA 23504, U.S.A. 0379-6779/83/$3.00
© Elsevier Sequoia/Printed in The Netherlands
178 A direct intercalation of the AsF6- ion would provide such a test, since the value of f for the resulting c o m p o u n d would be 1.0 electron per AsF 6- complex. Following the lead of Moran et al. [9], we have intercalated AsF6- into highly oriented pyrolitic graphite (HOPG) by reacting it with the nitronium salt, NO2AsF 6. A measurement of the spin magnetic susceptibility has yielded the value of the DOS at the Fermi energy for the c o m p o u n d of interest.
Experimental The spin magnetic susceptibilities were measured at room temperature using the m e t h o d of Schumacher and Slichter [8]. The absolute intensity of the conduction electron spin resonance absorption was determined by direct comparison with the 19F nuclear resonance absorption signal in the same sample and at the same frequency. Thus both resonances were observed by sweeping the external magnetic field without changing any of the r.f. parameters. The absolute susceptibility was calculated from the simple Curie law, XN = N J ( J + l ) ( ~ s h ) 2 / 3 k T
where 7N is the nuclear gyromagnetic ratio and k is Boltzmann's constant. The absolute spin susceptibility becomes XE = (~E/TS )(IE ~IN )XS
where 7E is the electron gyromagnetic ratio, with IE and IN being the integrated electronic and nuclear resonance absorption intensities, respectively. Measurements were made at 29 MHz with a standard marginal oscillator spectrometer employing field modulation and phase sensitive detection. The spectra were recorded and integrated with a Nicolet 1074 signal averager. All samples were prepared from HOPG starting materials. Pure NO2AsF6 was prepared by the new m e t h o d of Moran et al. [9], and checked with X-ray diffraction. Two different methods were used to prepare the graphite intercalation compounds of AsF6-; both yielded the same results. The first m e t h o d , by Billaud et al. [10], used an h-shaped Pyrex reaction vessel equipped with a Teflon-in-glass vacuum stopcock. The vessel was placed in a dry box and charged with 100 mg NO2AsF 6 and HOPG (6 × 7 × 1 mm3). In the vacuum system, nitromethane (2 ml, dried over P4010) was distilled into the reactor and the resulting NO2AsF6 solution poured onto the graphite. After 1 - 2 h, the reaction ceased and the intercalated graphite was washed several times with pure nitromethane. In the second m e t h o d , 1 - 2 ml of nitromethane (dried over P4010) were placed in a 10 ml weighing bottle in a glove bag with N2 atmosphere. The HOPG sample (6 × 7 × 1 mm 3) was immersed and NOzAsF 6 was added slowly to prevent sample cleavage from rapid intercalation. A saturated solution of NO2AsF 6 in nitromethane was achieved over a period of ~ 4 h.
179 T h e r e a c t i o n was t e r m i n a t e d w h e n t h e s a m p l e t h i c k n e s s a n d c o l o r i n d i c a t e d i n t e r c a l a t i o n to stage 1. T h e s a m p l e was t h e n w a s h e d in p u r e n i t r o m e t h a n e a n d sealed (in n i t r o m e t h a n e ) in r e c t a n g u l a r P y r e x t u b i n g . T o ensure t h a t t h e stage was m a i n t a i n e d , all m e a s u r e m e n t s w e r e carried o u t in an excess o f p u r e n i t r o m e t h a n e . T h e N M R / E P R coil was w o u n d d i r e c t l y o n t h e s a m p l e t u b e .
Results a n d c o n c l u s i o n s A t y p i c a l E P R s p e c t r a is s h o w n in Fig. 1. B o t h N M R and E P R linewidths w e r e ~ 0 . 5 gauss ( p e a k - t o - p e a k derivative). T h e r a t i o R = Ie/In a n d b o t h t h e e x p e r i m e n t a l a n d t h e o r e t i c a l D O S at t h e F e r m i energy ( N ( E F)) is s h o w n f o r each c o m p o u n d in T a b l e 1. T h e e x p e r i m e n t a l values o f N ( E F ) w e r e o b t a i n e d a s s u m i n g t h a t t h e m e a s u r e d s u s c e p t i b i l i t y was d u e to t h e t e m p e r a t u r e i n d e p e n d e n t Pauli p a r a m a g n e t i s m , f o r w h i c h Pe = PB2N(EF), w h e r e PB is t h e B o h r m a g n e t o n a n d N ( E F ) includes b o t h signs o f spin. O u r a s s u m p t i o n was c o n f i r m e d b y s u b s e q u e n t m e a s u r e m e n t s at 145 ° and 77 ° , w h i c h have s h o w n n o e v i d e n c e o f localized e l e c t r o n i c m o m e n t s . F o r stage 1 and stage 2 AsF si n t e r c a l a t e d H O P G , o u r e x p e r i m e n t a l results agree w i t h t h o s e o f W e i n b e r g e r et al. [ 7 ] . TABLE 1 Compound
R
N(E F )*
N(EF )**
C20AsF62.5CH3NO2 CsAsF5 C16AsF s
5.16 + 0.52 1.70 + 0.23 3.29 + 0.44
0.086 + 0.009 0.059 + 0.008 0.057 +- 0.008
0.079 ---
*Experimental (states per eV per carbon atom). **Theoretical {states per eV per carbon atom).
//
Fig. 1. A typical EPR derivative absorption signal (A) and absorption curve (B).
180 0.15
0,i0
~
0,05~ ~'
0,15
;
I
I
I
I
t
I
I
I
~ ' ~ 0.i0 ,< o
~ 0,05
0 -2.0
-i,o
o ENERGY
1,0
2,o
(EV)
Fig. 2. The d e n s i t y o f states calculated from a o n e d i m e n s i o n a l tight-binding m o d e l and its first integral from the pure graphite Fermi energy.
For the AsF6- compound in which f is known to be unity, theoretical estimates of the DOS at the Fermi energy, based on (a) the rigid band model and (b) the DOS calculations for pure graphite, are in agreement with our experiment. This partially validates both t h e rigid band model applied to these acceptor intercalation compounds and the DOS curves used; we used the DOS curve given by Weinberger e t al. [7], as well as one calculated here and shown in Fig. 2. But in another sense we have constructed an experimental density of states curve, which can serve as an interpolation function in the region of the Fermi energy for these graphite intercalation compounds. Thus, for those compounds for which f is not known, we n o w have a valid density of states curve, which can be used with the SchumacherSlichter experiment to find the charge transfer (f). For stage 1 AsF s in graphite f = 0.26, and for stage 2, f = 0.50 electrons per intercalant molecule. Acknowledgements The authors thank Dr. M. Rubinstein for the use o f his electromagnet for this investigation. One of us (L. M.) was supported by a National Research Council-Naval Research Laboratory Post-doctoral Fellowship.
181 References 1 2 3 4 5 6 7 8 9 10
N. Bartlett, B. McQuillan and A. S. Robertson, Mater. Res. Bull., 13 (1978) 1259. M. J. Moran, J. E. Fischer and W. R. Salaneck, J. Chem. Phys., 73 (1980) 629. L. R. Hanlon,E. R. Falardeau and J. E. Fischer, Solid State Commun., 24 (1977) 377. Y. Iye, O. Takahashi and S. Tanuma, Solid State Commun., 33 (1980) 1071. R. S. Markiewicz, H. R. Hart, L. V. Interrante and J. S. Kasper, Solid State Commun., 35 (1980) 513. C. Zeller, L. A. Pendrys and F. L. Vogel, J. Mater. Sci., 14 (1979) 2241. B. R. Weinberger, J. Kaufer, A. J. Heeger, J. E. Fischer, M. Moran and N. A. W. Holzwarth, Phys. Rev. Lett., 41 (1978) 1417. R. T. Schumacher and C. P. Slichter, Phys. Rev., 101 (1956) 58. M. J. Moran, G. R. Miller, R. A. DeMarco and H. A. Resing, to be published. D. Billaud, A. Pron and F. L. Vogel, Synth. Met., 2 (1980) 177.