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Evolution of K-, Rb- and Cs-graphite intercalation compounds surfaces under controlled oxidization: An XPS study
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Solid State Communications, Vol. 88, No. I, pp. 1-4, 1993. Printed in Great Britain.
0038-1098/93 $6.00+. 00 Pergamon Press Ltd
EVOLUTION O F K-, Rb- AND Cs-GRAPHITE INTERCALATION COMPOUNDS SURFACES UNDER CONTROLLED OXIDIZATION: AN XPS STUDY. B. Rousseau and H.Estrade-Szwarckopf Centre de Recherche sur la Mati~re Divis6e - C.N.R.S./Universitd. 45071 ORLEANS Cedex 2 , FRANCE
(Received 10 July 1993, acceptedfor publication 6 August 1993 by H. Kamimura) The evolution of XPS spectra of K-, Rb- and Cs-GICs of stage 1,2 and 3, when their cleaved surfaces are submitted to well controlled oxidization is presented. The results confirm the existence of the alkali surface species previously observed on clean surfaces and the surface evolution under oxidization is described through diffusion of the metallic atoms from inner interlayers onto the surface. A clear explanation may, thus, be given for the apparent superficial enrichment in alkali claimed in various previous papers.
1-Introduction Clean surfaces of heavy aikali-g_ra~hite intercalation oriented compounds (K- and Rb-GICs - and Cs-GICs 1), stage 1 to 3 have been yet thoroughly studied through XPS measurements. The observed surfaces were obtained by in-situ cleavage and kept in the cxceptionnally clean atmosphere of the spectrometer analysis chamber. Among others, some of former conclusions thus drawn, have to be recalled here. 1/ The alkali atoms located on the outermost surfaces present binding energies higher than the bulk ones, about 2 eV shift in stage 1, 1.2 eV in higher stages, whereas the outermost carbon atoms do not differenciate from the bulk ones. 2 / T h e surface stoichiometry is close to half of that of the bulk. This stoichiometry, as well as all the other characteristics of the XPS spectra described hereover, are very stable under vacuo, both at 100 and at 300 K, as long as the conditions remain clean. 3 / F o r all the studied compounds, whatever the nature of the intercalated metal or the stage of the compounds, in the carbon spectra, a small line is observed located at 285.6 eV (+0.1) and representing about 15 to 20 % of the total C l s intensity. We attributed this line to carbon atoms located in both superficial and volumic perturbed zones of the sample, we called "crumpled" zones. Those zones, which are not present in the pristine HOPG are created during the intercalation process and result from both this process and from topological necessity in higher stages. Conclusions 1/and 2 / a r e deeply modified as soon as some pollution occurs, accidently or willingly. In order to better understand the mechanism of these modifications, we performed XPS experiments on surfaces of K-, Rb- and Cs-GICs of stage 1 to 3, after having exposed them to a limited oxidization both at 100 K and at room temperature. In this paper, we present the results concerning the XPS spectra shape analysis and stoichiometry measurements on those modified surfaces. Conclusions will be drawn concerning the oxidization processes. 1-Experiments The experimental conditions have been yet exposed 1. The XPS spectra were obtained in a VG ESCALAB II spectrometer with a non-monochromatized Mg-Ka source. The observations are done on in-situ cleaved surfaces and kept for clean conditions, in the
exceptionnally clean atmosphere of the analysis chamber, in a vacuo pressure about 2-3. 10-ll torr. We followed the surface modifications after limited exposure to oxygen. Such an oxidization is defined by the number of pure oxygen langmuirs (pressure x exposure time = O LO2). This oxidization is performed in the preparation chamber of the spectrometer or, for the very first steps of oxidization, directly in the analysis chamber. The oxygen pressure was kept between 10-9 and 10-5 torr and we studied the 0.1 to 1000 LO 2 range for O. When operating in the analysis chamber, the sample could be kept at a constant temperature (circa 100 K or 300 K) whereas when operating in the preparation chamber, the sample had to be separated from its cooling support and its temperature could not been kept low during the exposure (Intermediate Temperature, IT). The successive oxidization steps were either cumulative or preceded by surface cleavages, but in both cases, the oxidization appears to be progressive. 3-Oxldized surfaces 3.1. XPS Peak shape modifications Under oxidization, the carbon peaks remain unchanged in position and in shape in stage 2 and 3 for all the studied O range. This insensitivity to oxidization is also observed in Cs stage 1 samples for low O values but for 0 > 10 L02 and at 300 K, the C l s peak shifts slightly from 284.6 to 284.8 eV and its shape gets more "round" close to its maximum, getting then like the higher stage 2 Cls peak (Figure 1 in 1). On the contrary, as previously noticed, in all the studied samples, the alkali spectra are deeply modified even by very slight oxidization: for a O value as low as 0.1 langmuir (i.e. after 10 seconds in a 10"s torr oxygen atmosphere!), the surface contributions which are high energy shifted relatively to the bulk peaks, are no longer detectable, i.e.: - in Cs-compounds, the "bump" previously observed on the high energy side of the Cs3d spectra disappears (Fig. 1); - in Rb compounds, the apparent intensity of the Rb3d3/2 component decreases noticeably (Fig.2); - in the K compounds, the K2p lines appear more hollowed between the two components of the doublet and on the high energy side of the K2pl/2 line. Simultaneous with the disappearance of the surface components, a new peak is now present, which is low energy shifted relatively to the bulk peaks which remain
AN XPS STUDY A.U.
Oxide
88, No.
slightly with increasing oxidization during the very first steps i.e. for 0 > 0.5 LO2, but for higher oxidization, they are quite stable and equal to 290, 110.2 and 724.8 eV respectively for the K, Rb and Cs compounds; those peaks are ahnost symmetrical and their gaussian broadening depends on the presence of several oxide compositions• A precise analysis, through XPS and UPS experiments, of the formed oxides nature will be published elsewhere for the Cs first stage compound 3 For high oxidization states, we verified by tilting the sample relatively to the analyser axis, and thus by favouring the superficial contribution to the XPS spectra, that the oxides are located in the outermost atomic layers. Taking account of the invariance of the observed bulk contribution and of the cleanness of the new surface obtained after tape cleavage (0.01 mm thick) we may conclude the oxygen not to penetrate deeply in the sample.
"x Normal ..~~.detection.
/ ,vo,.mo\ Crumpled ~ ' "
I
Vol.
3.2 Crumpled zones I
I
I
722
I
I
724 726 728 Binding Energy (eV)
730
Fig.l.Evolution of the Cs3d£,2 XPS peak of a Cs-GIC (stage 1) under oxidization at 100 K• The clean surface peak is decomposed into volume, crumpled zones and surface contributions• Note how the surface peak disappears quickly and how the oxide peak increases. The peak intensities are arbitrary on this figure. invariant as for their shape and position 1,2. The intensity of that new component increases with increasing oxidization as seen for instance on figure 3: in stage 3 Cs-GIC, for 0 = 0.25 LO2, it is greater than the "clean volume" peak. Obviously, this new alkali species, is an oxidized one. In stage 2 and 3 samples, because of greater energy separation from the clean volume peak, the oxidized peak appear more easily distinguished than in stage 1 sample• The oxidized alkali peaks binding energy decreases A.U.
Ox3/2.1~.. i 0x5/2 "~
/
:"
..
f •:
.
A.U
Normal detection
"
In other respects, during the first steps of oxidization, in stage 1 Cs-GICs, on the high energy side of the bulk alkali peak and between it and the surface contribution, a small peak is revealed as had yet been explained in 1. The intensity of this new peak seems to depend neither on the detection angle nor on the oxidization state: it is both a volume and a surface contribution and we attribute it to alkali atoms located in crumpled zones described in the introduction and put in evidence through the carbon spectra. In the Cs-higher stage compounds, this peak may be present but, unfortunately, its energy is almost equal to the bulk peak one and cannot be distinguished from it. On the contrary, in Rb-GICs stage 2 and 3, the presence of this peak is quite evident on oxidized surfaces• Its intensity is about 10% of the total Rb3ds/2 and as its binding energy is 112.7 eV, it may be clearly separated from the volume peak (112.1 eV) as soon as the surface contribution is destroyed due to the oxidization. As previously mentioned the K2p peak is not as easy to decompose and the presence of a peak issued from such a crumpled zone seems impossible to put in evidence• However the great
,~N, ~
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:
:
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Volume ~ r f a c e
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clean I
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Binding Energy (eV) Fig.2.Evolution of the Rb3d XPS spectra of a Rb-GIC (stage 3) under oxidization at intermediate temperature, (IT, see text). The clean spectrum is decomposed into volume (Vs/2 and V3/2), and surface+crumpled zones ((S+C)5/2 and (S+ C)3/2) doublet.
I
723
I
I
I
725 727 729 Binding Energy (eV)
I
73 l
Fig.3.Modification of the Cs3ds/2 XPS peak of a Cs-GIC (stage 3) under 0.25 LO2 at intermediate temperature (IT). Note how the surface peak disappears and how the new oxide peak is important.
1
V o l . 88, N o . 1 0.4 0.3 0.2
AN XPS STUDY
~
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300K
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similarity between the 3 heavy alkali-GICs, allows us to induce the presence of potassium atoms in the crumpled zones yet put in evidence in the Cls photopeak. 3.3.Peak intensity modifications Together with the shape modifications, another effect may be noticed during oxidization, and that is an important increase of the total alkali peak intensity, specially at 300 K, as had already been observed 4.5.6. In order to understand this effect, we studied the evolution of the area under the Cls, Cs3ds/2 and Ols peaks versus 0 both at 100 and at 300 K. On figure 4, we show, versus O (in fact vs logloO for visibility convenience), the intensities of the total C, Cs and O spectra measured, on a stage 1 Cs-GIC. We can see that for 0 = 1000 LO2, all the curves have reached their saturation limits. We may also notice that the alkali peak intensity increases at 300 K by a factor 34 whereas at 100 K, the factor is only 1.5. For the same O limit, the carbon peak intensity is divided by 3 whereas it remains constant at 100 K and the oxygen peak is twice as high at 300 K as at 100 K.
0.02
cs
_ O/Co
., , , . , . e - ~
~
.~ .--
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. ...-" ~ " " " .
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4.6
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300
7.8:1:1 4.8 3.5 3.0 1.4 I
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Both evolutions, increase of the Cs peak and decrease of the carbon ones, result in a very steep decrease of the atomic C/Cs ratii (Table 1). In this table, the values given for the C/Cs ratii have been calculated, assuming a constant Cs cross-section: we will see hereunder that this assumption is not fully justified during oxidization. On the same figure 4, we reported the results obtained on the stage 1 Cs-GIC not really at constant temperature as in the former described experiments: the sample being cooled (about 100 K) in the analysis chamber, is translated to the preparation chamber where it is submitted to oxygen atmosphere. During this treatment, its temperature increases undoubtly, depending on the time necessary for the wanted oxygen coverage to be achieved (till 1000 seconds). After this treatment at "intermediate temperature" (IT), the sample is transferred back to the analysis chamber where it is cooled again and observed by XPS. As seen on figure 4, the 3 (IT) intensity curves lie between the I00 K and the 300 K ones, as could be expected. This mode of oxidization has been used for the other studied GICs, Cs stage 2 and 3, Rb and K stage 1-3. In those samples, the evolution of the carbon, oxygen and metal peak intensities versus oxygen coverage is qualitatively similar to the one described here over for the Cs-GIC stage 1 as seen on figure 5 drawn for the stage 3 K-, Rb- and Cs-GICs. Table 2 gives the evolution of the apparent stoichiometry of stage 2-GICs assuming the alkali cross-section to be constant in spite of oxidization. Quantitatively, the situation depends on the nature of the alkali metal. K diffuses onto the surface more than the Rb and the Cs, and more oxygen is fixed on K- and on Cs-GICs than on Rb-ones. After decomposition of the alkali peaks into three contributions: the volume (which remains invariant in energy and in shape), the oxide and the crumpled zones, we may calculated a mean Table 2.Apparent stoichiometry C/M of K-, Rb- and Cs-GICs (stage 2) under oxidization at intermediate temperature (IT, see text) calculated as in Table 1.
0 _ M/Co 0.4
0
100
I
Fig.4.Evolution of the total area under Ols, Cs3d and Cls XPS spectra versus oxidization at 100 K, intermediate temperature (IT) and 300 K on a Cs-GIC (stage 1). The area have been normalized to Co, the area under the Cls peak of the clean surface and taking account of the different crosssections. The lines drawn through the dots are guides for
0.08 0.06 0.04
Table l.Apparent stoichiometry C/Cs of a Cs-GIC (stage 1) under oxidization at different temperatures. The CICs ratii are determined via the total area under the XPS spectra, assuming the cross-sections to remain constant in spite of oxidization.
l0
.
.
Cs
o,-0
.
I
100
I
1000
Fig.5.Evolution of the total area under oxygen and alkali XPS spectra versus oxidization at intermediate temperature (IT) of K-, Rb- and Cs-GICs (Stage 3). Same normalization as in riga with a Cls peak intensity almost constant.
0 LO 2
C/K
C/Rb
C/Cs
0 0.25 0.5 1.0 2 10 100
26±2 17.5 11.1 14.6
23 14.3 9.3 9.9
25_4-2
3.0 1.1
6.0 5.9
7.9 4.0 1.6 0.6
Voi. 8~, No. 1
AN XPS STUDY stoichiometry for the sole oxide: the saturation is obtained for a composition close to Rb20 and K20 whereas for the Cs-GICs, the limit is close to CsO. 4-Oxidization process discussion At room temperature, the intensity increase of the alkali peak versus oxidization suggests an increase of the amount of alkali and thus an enrichment of the surface. It has been shown by XPS and Auger studies on thin alkali films 7.8,9 that the cross sections of the metallic core levels increase when the surface is exposed to oxygen (probably because of disappearance of plasmon losses). Such an increase alone would be efficient enough to explain the total increase at low temperature and a great part of this increase at room or at intermediate temperature of the alkali signal on our GICs surfaces, without invoking any enrichment process. However, two facts suggest that segregation takes really place: 1/ the metal signal increase depends on temperature and not only on oxygen exposure; 2 / t h e carbon signal decreases a lot, specially at 300 K. Such a decrease is easily explained by a screening due to an overlayer for instance of sole oxygen; however at 100 K, the oxygen signal increases as would the thickness of such an overlayer, without any noticeable decrease of the carbon one. Thus, the sole oxygen cannot explain the screening of carbon signal. The real overlayer which explains the evolutions of the 3 species, metal, carbon and oxygen peaks is undoubtly an oxide overlayer, the thickness of which increases by diffusion of metal atoms from the deeper interlayers, until it invades all the surface, hidding both the carbon atoms and the inner volume peaks. This alkali diffusion is relatively inefficient at 100 K but is highly thermally activated. In the case of the stage 1 compounds, during such a diffusion process, the underlying interlayers become poorer and the carbon peak shape modifies from 1st stage characteristics sharp-pointed at EB= 284.7eV to that of 2nd stage round shaped and centered at EB= 284.8 eV as is experimentally observed. The alkali peak evolution of these underlying interlayers is more difficult to put in evidence. Indeed, all the higher stage alkali volume peaks have almost same binding energy and this one is very close to that of the crumpled zones one from which they cannot easily been distinguished: while the poorer underlying interlayer signal would increase during the
oxidization process, the crumpled zone signal would decrease following the decrease of the invariant volume one and the sum could appear apparently as constant. In all the higher stages, similarly, the evolution to poorer underlayers is not discernable. 5.Conclusion In this paper, we described the oxidization process in several steps: 1/ oxidization of the alkali superficial atoms with disappearance of their outermost high energy shifted contribution and appearance of an oxide peak low energy shifted; 2/ diffusion onto the surface from the deeper alkali intercalated layers. The oxygen driven alkali segregation towards the surface is present and similar in all the studied GICs: K, Rb and Cs of stage 1, 2 or 3. This diffusion is thermally activated and thus slowed down but not suppressed at low temperature. This migration of the alkali atoms towards the surface is certainly allowed through the cracks or frontiers present between the cristallites of the ex HOPGGICs. The chemical mechanism is not completely explicited, but it may be dominated by a dilution process of the oxides located in the grain boundaries. The enrichment in alkali of the surfaces under oxidization is certainly at the origin of the conclusion drawn by some authors following which, an electrostatic effect due to the upper limit of the crystal would produce such a surface segregation. If this was the rule, the intensity of the peak due to outermost clean alkali layer would increase and moreover all the higher stages would evolute towards a first stage, at least at surface: this is never observed. Under clean atmosphere, all the characteristics of the binary GICs are stable both as for their volume and their surface. However, if submitted to any slight pollution, all their superficial alkali species disappear and they all evolute towards superficial oxides, the stoichiometry of which depends on the nature of the alkali, on the degree of oxidization and on the temperature. At last, the evolution of the alkali peaks under the very first steps of oxidization confirms the presence of alkali atoms in the crumpled zones induced by the intercalation and put in evidence through the carbon photopeaks.
REFERENCES 1. H.Estrede-Szwarckopf and B.ROUSSEAU, J.Phys. Chem. Solids, 53 (1992) 419. 2. B.Rousseau and H.Estrede-Szwarckopf, Solid State Com., 85 (1993) 793. 3. M.Vayer, B.Roussean and H.Estrade-Szwarckopf, to be published. 4. M.E.Preil and J.E.Fischer, Synth. Metals, $ (1983) 149. 5. R.Schl6gl, P.Oelhafen and H.LGfmtherodt, 4th Inter.Carbon ConL, Baden-Baden 1986.
6. R.Schl6gl, V.Geiser, P.Oelhafen and H.J.G~ntherodt, Phys.Rev. B, 35 (1987) 6414. 7. M.L.Shek, X.Pan, M.Strongin and M.W.Ruckman, Phys. Rev. B, 34 (1986) 3741. 8. L.G.Penersson and S.E.Kadsson, Phys.Scripta, 16 (1977) 425. 9. M.Besan~on-Vayer, Thesis Universit6 de Nancy (1987). 10. M.Lagu~, D.Marchand and C.Fr6tigny, J.Vac.Sci.Technol., A5 (1987) 19-92.
Solid State Communications, Vol. 88, No. I, pp. 1-4, 1993. Printed in Great Britain.
0038-1098/93 $6.00+. 00 Pergamon Press Ltd
EVOLUTION O F K-, Rb- AND Cs-GRAPHITE INTERCALATION COMPOUNDS SURFACES UNDER CONTROLLED OXIDIZATION: AN XPS STUDY. B. Rousseau and H.Estrade-Szwarckopf Centre de Recherche sur la Mati~re Divis6e - C.N.R.S./Universitd. 45071 ORLEANS Cedex 2 , FRANCE
(Received 10 July 1993, acceptedfor publication 6 August 1993 by H. Kamimura) The evolution of XPS spectra of K-, Rb- and Cs-GICs of stage 1,2 and 3, when their cleaved surfaces are submitted to well controlled oxidization is presented. The results confirm the existence of the alkali surface species previously observed on clean surfaces and the surface evolution under oxidization is described through diffusion of the metallic atoms from inner interlayers onto the surface. A clear explanation may, thus, be given for the apparent superficial enrichment in alkali claimed in various previous papers.
1-Introduction Clean surfaces of heavy aikali-g_ra~hite intercalation oriented compounds (K- and Rb-GICs - and Cs-GICs 1), stage 1 to 3 have been yet thoroughly studied through XPS measurements. The observed surfaces were obtained by in-situ cleavage and kept in the cxceptionnally clean atmosphere of the spectrometer analysis chamber. Among others, some of former conclusions thus drawn, have to be recalled here. 1/ The alkali atoms located on the outermost surfaces present binding energies higher than the bulk ones, about 2 eV shift in stage 1, 1.2 eV in higher stages, whereas the outermost carbon atoms do not differenciate from the bulk ones. 2 / T h e surface stoichiometry is close to half of that of the bulk. This stoichiometry, as well as all the other characteristics of the XPS spectra described hereover, are very stable under vacuo, both at 100 and at 300 K, as long as the conditions remain clean. 3 / F o r all the studied compounds, whatever the nature of the intercalated metal or the stage of the compounds, in the carbon spectra, a small line is observed located at 285.6 eV (+0.1) and representing about 15 to 20 % of the total C l s intensity. We attributed this line to carbon atoms located in both superficial and volumic perturbed zones of the sample, we called "crumpled" zones. Those zones, which are not present in the pristine HOPG are created during the intercalation process and result from both this process and from topological necessity in higher stages. Conclusions 1/and 2 / a r e deeply modified as soon as some pollution occurs, accidently or willingly. In order to better understand the mechanism of these modifications, we performed XPS experiments on surfaces of K-, Rb- and Cs-GICs of stage 1 to 3, after having exposed them to a limited oxidization both at 100 K and at room temperature. In this paper, we present the results concerning the XPS spectra shape analysis and stoichiometry measurements on those modified surfaces. Conclusions will be drawn concerning the oxidization processes. 1-Experiments The experimental conditions have been yet exposed 1. The XPS spectra were obtained in a VG ESCALAB II spectrometer with a non-monochromatized Mg-Ka source. The observations are done on in-situ cleaved surfaces and kept for clean conditions, in the
exceptionnally clean atmosphere of the analysis chamber, in a vacuo pressure about 2-3. 10-ll torr. We followed the surface modifications after limited exposure to oxygen. Such an oxidization is defined by the number of pure oxygen langmuirs (pressure x exposure time = O LO2). This oxidization is performed in the preparation chamber of the spectrometer or, for the very first steps of oxidization, directly in the analysis chamber. The oxygen pressure was kept between 10-9 and 10-5 torr and we studied the 0.1 to 1000 LO 2 range for O. When operating in the analysis chamber, the sample could be kept at a constant temperature (circa 100 K or 300 K) whereas when operating in the preparation chamber, the sample had to be separated from its cooling support and its temperature could not been kept low during the exposure (Intermediate Temperature, IT). The successive oxidization steps were either cumulative or preceded by surface cleavages, but in both cases, the oxidization appears to be progressive. 3-Oxldized surfaces 3.1. XPS Peak shape modifications Under oxidization, the carbon peaks remain unchanged in position and in shape in stage 2 and 3 for all the studied O range. This insensitivity to oxidization is also observed in Cs stage 1 samples for low O values but for 0 > 10 L02 and at 300 K, the C l s peak shifts slightly from 284.6 to 284.8 eV and its shape gets more "round" close to its maximum, getting then like the higher stage 2 Cls peak (Figure 1 in 1). On the contrary, as previously noticed, in all the studied samples, the alkali spectra are deeply modified even by very slight oxidization: for a O value as low as 0.1 langmuir (i.e. after 10 seconds in a 10"s torr oxygen atmosphere!), the surface contributions which are high energy shifted relatively to the bulk peaks, are no longer detectable, i.e.: - in Cs-compounds, the "bump" previously observed on the high energy side of the Cs3d spectra disappears (Fig. 1); - in Rb compounds, the apparent intensity of the Rb3d3/2 component decreases noticeably (Fig.2); - in the K compounds, the K2p lines appear more hollowed between the two components of the doublet and on the high energy side of the K2pl/2 line. Simultaneous with the disappearance of the surface components, a new peak is now present, which is low energy shifted relatively to the bulk peaks which remain
AN XPS STUDY A.U.
Oxide
88, No.
slightly with increasing oxidization during the very first steps i.e. for 0 > 0.5 LO2, but for higher oxidization, they are quite stable and equal to 290, 110.2 and 724.8 eV respectively for the K, Rb and Cs compounds; those peaks are ahnost symmetrical and their gaussian broadening depends on the presence of several oxide compositions• A precise analysis, through XPS and UPS experiments, of the formed oxides nature will be published elsewhere for the Cs first stage compound 3 For high oxidization states, we verified by tilting the sample relatively to the analyser axis, and thus by favouring the superficial contribution to the XPS spectra, that the oxides are located in the outermost atomic layers. Taking account of the invariance of the observed bulk contribution and of the cleanness of the new surface obtained after tape cleavage (0.01 mm thick) we may conclude the oxygen not to penetrate deeply in the sample.
"x Normal ..~~.detection.
/ ,vo,.mo\ Crumpled ~ ' "
I
Vol.
3.2 Crumpled zones I
I
I
722
I
I
724 726 728 Binding Energy (eV)
730
Fig.l.Evolution of the Cs3d£,2 XPS peak of a Cs-GIC (stage 1) under oxidization at 100 K• The clean surface peak is decomposed into volume, crumpled zones and surface contributions• Note how the surface peak disappears quickly and how the oxide peak increases. The peak intensities are arbitrary on this figure. invariant as for their shape and position 1,2. The intensity of that new component increases with increasing oxidization as seen for instance on figure 3: in stage 3 Cs-GIC, for 0 = 0.25 LO2, it is greater than the "clean volume" peak. Obviously, this new alkali species, is an oxidized one. In stage 2 and 3 samples, because of greater energy separation from the clean volume peak, the oxidized peak appear more easily distinguished than in stage 1 sample• The oxidized alkali peaks binding energy decreases A.U.
Ox3/2.1~.. i 0x5/2 "~
/
:"
..
f •:
.
A.U
Normal detection
"
In other respects, during the first steps of oxidization, in stage 1 Cs-GICs, on the high energy side of the bulk alkali peak and between it and the surface contribution, a small peak is revealed as had yet been explained in 1. The intensity of this new peak seems to depend neither on the detection angle nor on the oxidization state: it is both a volume and a surface contribution and we attribute it to alkali atoms located in crumpled zones described in the introduction and put in evidence through the carbon spectra. In the Cs-higher stage compounds, this peak may be present but, unfortunately, its energy is almost equal to the bulk peak one and cannot be distinguished from it. On the contrary, in Rb-GICs stage 2 and 3, the presence of this peak is quite evident on oxidized surfaces• Its intensity is about 10% of the total Rb3ds/2 and as its binding energy is 112.7 eV, it may be clearly separated from the volume peak (112.1 eV) as soon as the surface contribution is destroyed due to the oxidization. As previously mentioned the K2p peak is not as easy to decompose and the presence of a peak issued from such a crumpled zone seems impossible to put in evidence• However the great
,~N, ~
~"
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/ O ! i d e ~ ~
detection
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,.--,-4-"-.b-..I . . :'-• I"",.
:
:
,,:
I..I, •.
: •
---J"
--J I 108
A,
:
"..
'~".: "~....., 1 LO 2 •, ,% ~
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92% I ll0
~,
I 112
,(S+C)3/2
t
114
Volume ~ r f a c e
0 25 L o
"~
clean I
116
Binding Energy (eV) Fig.2.Evolution of the Rb3d XPS spectra of a Rb-GIC (stage 3) under oxidization at intermediate temperature, (IT, see text). The clean spectrum is decomposed into volume (Vs/2 and V3/2), and surface+crumpled zones ((S+C)5/2 and (S+ C)3/2) doublet.
I
723
I
I
I
725 727 729 Binding Energy (eV)
I
73 l
Fig.3.Modification of the Cs3ds/2 XPS peak of a Cs-GIC (stage 3) under 0.25 LO2 at intermediate temperature (IT). Note how the surface peak disappears and how the new oxide peak is important.
1
V o l . 88, N o . 1 0.4 0.3 0.2
AN XPS STUDY
~
O/Co
300K
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2
eyes.
similarity between the 3 heavy alkali-GICs, allows us to induce the presence of potassium atoms in the crumpled zones yet put in evidence in the Cls photopeak. 3.3.Peak intensity modifications Together with the shape modifications, another effect may be noticed during oxidization, and that is an important increase of the total alkali peak intensity, specially at 300 K, as had already been observed 4.5.6. In order to understand this effect, we studied the evolution of the area under the Cls, Cs3ds/2 and Ols peaks versus 0 both at 100 and at 300 K. On figure 4, we show, versus O (in fact vs logloO for visibility convenience), the intensities of the total C, Cs and O spectra measured, on a stage 1 Cs-GIC. We can see that for 0 = 1000 LO2, all the curves have reached their saturation limits. We may also notice that the alkali peak intensity increases at 300 K by a factor 34 whereas at 100 K, the factor is only 1.5. For the same O limit, the carbon peak intensity is divided by 3 whereas it remains constant at 100 K and the oxygen peak is twice as high at 300 K as at 100 K.
0.02
cs
_ O/Co
., , , . , . e - ~
~
.~ .--
Rb
0.3 0.2 0.1
. ...-" ~ " " " .
0.25 0.5 1
0 0.25 0.5 1 2 10 26 100 1000
K
7.8±1 5.4 5.3 1.4
4.6
IT 7.4:tl 5.7 5.0 5.8 4.1 3.7 2.9 2.8 2.8
K
300
7.8:1:1 4.8 3.5 3.0 1.4 I
0.9
Both evolutions, increase of the Cs peak and decrease of the carbon ones, result in a very steep decrease of the atomic C/Cs ratii (Table 1). In this table, the values given for the C/Cs ratii have been calculated, assuming a constant Cs cross-section: we will see hereunder that this assumption is not fully justified during oxidization. On the same figure 4, we reported the results obtained on the stage 1 Cs-GIC not really at constant temperature as in the former described experiments: the sample being cooled (about 100 K) in the analysis chamber, is translated to the preparation chamber where it is submitted to oxygen atmosphere. During this treatment, its temperature increases undoubtly, depending on the time necessary for the wanted oxygen coverage to be achieved (till 1000 seconds). After this treatment at "intermediate temperature" (IT), the sample is transferred back to the analysis chamber where it is cooled again and observed by XPS. As seen on figure 4, the 3 (IT) intensity curves lie between the I00 K and the 300 K ones, as could be expected. This mode of oxidization has been used for the other studied GICs, Cs stage 2 and 3, Rb and K stage 1-3. In those samples, the evolution of the carbon, oxygen and metal peak intensities versus oxygen coverage is qualitatively similar to the one described here over for the Cs-GIC stage 1 as seen on figure 5 drawn for the stage 3 K-, Rb- and Cs-GICs. Table 2 gives the evolution of the apparent stoichiometry of stage 2-GICs assuming the alkali cross-section to be constant in spite of oxidization. Quantitatively, the situation depends on the nature of the alkali metal. K diffuses onto the surface more than the Rb and the Cs, and more oxygen is fixed on K- and on Cs-GICs than on Rb-ones. After decomposition of the alkali peaks into three contributions: the volume (which remains invariant in energy and in shape), the oxide and the crumpled zones, we may calculated a mean Table 2.Apparent stoichiometry C/M of K-, Rb- and Cs-GICs (stage 2) under oxidization at intermediate temperature (IT, see text) calculated as in Table 1.
0 _ M/Co 0.4
0
100
I
Fig.4.Evolution of the total area under Ols, Cs3d and Cls XPS spectra versus oxidization at 100 K, intermediate temperature (IT) and 300 K on a Cs-GIC (stage 1). The area have been normalized to Co, the area under the Cls peak of the clean surface and taking account of the different crosssections. The lines drawn through the dots are guides for
0.08 0.06 0.04
Table l.Apparent stoichiometry C/Cs of a Cs-GIC (stage 1) under oxidization at different temperatures. The CICs ratii are determined via the total area under the XPS spectra, assuming the cross-sections to remain constant in spite of oxidization.
l0
.
.
Cs
o,-0
.
I
100
I
1000
Fig.5.Evolution of the total area under oxygen and alkali XPS spectra versus oxidization at intermediate temperature (IT) of K-, Rb- and Cs-GICs (Stage 3). Same normalization as in riga with a Cls peak intensity almost constant.
0 LO 2
C/K
C/Rb
C/Cs
0 0.25 0.5 1.0 2 10 100
26±2 17.5 11.1 14.6
23 14.3 9.3 9.9
25_4-2
3.0 1.1
6.0 5.9
7.9 4.0 1.6 0.6
Voi. 8~, No. 1
AN XPS STUDY stoichiometry for the sole oxide: the saturation is obtained for a composition close to Rb20 and K20 whereas for the Cs-GICs, the limit is close to CsO. 4-Oxidization process discussion At room temperature, the intensity increase of the alkali peak versus oxidization suggests an increase of the amount of alkali and thus an enrichment of the surface. It has been shown by XPS and Auger studies on thin alkali films 7.8,9 that the cross sections of the metallic core levels increase when the surface is exposed to oxygen (probably because of disappearance of plasmon losses). Such an increase alone would be efficient enough to explain the total increase at low temperature and a great part of this increase at room or at intermediate temperature of the alkali signal on our GICs surfaces, without invoking any enrichment process. However, two facts suggest that segregation takes really place: 1/ the metal signal increase depends on temperature and not only on oxygen exposure; 2 / t h e carbon signal decreases a lot, specially at 300 K. Such a decrease is easily explained by a screening due to an overlayer for instance of sole oxygen; however at 100 K, the oxygen signal increases as would the thickness of such an overlayer, without any noticeable decrease of the carbon one. Thus, the sole oxygen cannot explain the screening of carbon signal. The real overlayer which explains the evolutions of the 3 species, metal, carbon and oxygen peaks is undoubtly an oxide overlayer, the thickness of which increases by diffusion of metal atoms from the deeper interlayers, until it invades all the surface, hidding both the carbon atoms and the inner volume peaks. This alkali diffusion is relatively inefficient at 100 K but is highly thermally activated. In the case of the stage 1 compounds, during such a diffusion process, the underlying interlayers become poorer and the carbon peak shape modifies from 1st stage characteristics sharp-pointed at EB= 284.7eV to that of 2nd stage round shaped and centered at EB= 284.8 eV as is experimentally observed. The alkali peak evolution of these underlying interlayers is more difficult to put in evidence. Indeed, all the higher stage alkali volume peaks have almost same binding energy and this one is very close to that of the crumpled zones one from which they cannot easily been distinguished: while the poorer underlying interlayer signal would increase during the
oxidization process, the crumpled zone signal would decrease following the decrease of the invariant volume one and the sum could appear apparently as constant. In all the higher stages, similarly, the evolution to poorer underlayers is not discernable. 5.Conclusion In this paper, we described the oxidization process in several steps: 1/ oxidization of the alkali superficial atoms with disappearance of their outermost high energy shifted contribution and appearance of an oxide peak low energy shifted; 2/ diffusion onto the surface from the deeper alkali intercalated layers. The oxygen driven alkali segregation towards the surface is present and similar in all the studied GICs: K, Rb and Cs of stage 1, 2 or 3. This diffusion is thermally activated and thus slowed down but not suppressed at low temperature. This migration of the alkali atoms towards the surface is certainly allowed through the cracks or frontiers present between the cristallites of the ex HOPGGICs. The chemical mechanism is not completely explicited, but it may be dominated by a dilution process of the oxides located in the grain boundaries. The enrichment in alkali of the surfaces under oxidization is certainly at the origin of the conclusion drawn by some authors following which, an electrostatic effect due to the upper limit of the crystal would produce such a surface segregation. If this was the rule, the intensity of the peak due to outermost clean alkali layer would increase and moreover all the higher stages would evolute towards a first stage, at least at surface: this is never observed. Under clean atmosphere, all the characteristics of the binary GICs are stable both as for their volume and their surface. However, if submitted to any slight pollution, all their superficial alkali species disappear and they all evolute towards superficial oxides, the stoichiometry of which depends on the nature of the alkali, on the degree of oxidization and on the temperature. At last, the evolution of the alkali peaks under the very first steps of oxidization confirms the presence of alkali atoms in the crumpled zones induced by the intercalation and put in evidence through the carbon photopeaks.
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