- Email: [email protected]
Fluorination of carbon blacks: an X-ray photoelectron spectroscopy study
PERGAMON
Carbon 37 (1999) 1891–1900
Fluorination of carbon blacks: an X-ray photoelectron spectroscopy study IV. Reactivity of different carbon blacks in CF 4 radiofrequency plasma T. Shirasaki a , F. Moguet a , L. Lozano a , A. Tressaud a , *, G. Nanse b , E. Papirer b a
` Condensee ´ de Bordeaux, CNRS, Av. Dr. A. Schweitzer, 33608 Pessac Cedex, France Institut de Chimie de la Matiere Institut de Chimie des Surfaces et Interfaces, CNRS, 15 rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, France
b
Received 17 July 1998; accepted 4 March 1999
Abstract The effect of CF 4 -plasma enhanced fluorination on the surface modification of carbon blacks has been examined using XPS. Three different types of carbon blacks have been studied: a thermal black, a furnace black and a high electrical conducting black. The analysis of the XPS spectra of fluorinated carbon black samples indicates that all fluorine atoms, fixed at the surface and in a subsuperficial zone of the particles, are covalently linked to carbon atoms. The influence of the physicochemical properties and morphology of these three types of carbon blacks on the fluorination reaction has also been investigated. The proportion of different types of fluorinated carbon atoms, i.e. on one hand CFx surface and border groups of graphitic bulk domains for which the planar configuration of the graphene layers is preserved together with the sp 2 character of C, i.e. structures of type I, on the other hand polyalicyclic perfluorinated structures in which sp 3 C form puckered layers similar to those of covalent fluorographites, i.e. structures of type II, and also the F / C ratio of the fluorinated groups are related to the surface morphology and depend on the microstructural organization of particles. When the microstructure ordering and graphitic character of the carbon increase, the size of the ordered graphitic domain also increases. At the same time the density, the size of defects and proportion of protonated sp 3 C entities bridging the graphene layers decrease. As a consequence, the proportion of carbon atoms, potentially able to form perfluorinated CF 2 and CF 3 groups, decreases. The relative contribution of those groups is appreciably higher in fluorinated compounds which are derived from carbon blacks with a lower structural order. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon black; B. Halogenation, Activation; C. Photoelectron spectroscopy; D. Reactivity
1. Introduction The studies on the reactivity of carbon materials with fluorine have been mostly devoted to graphite and graphitized carbons [1]. Fluorination of carbon blacks has received less attention. Watanabe et al., analyzing the XPS spectra of these materials treated with gaseous F 2 , have identified three types of C–F bonds [2,3]. In a more recent paper, Nakahara et al. have compared the fluorination of carbon blacks with that of active carbons [4]. We have undertaken a general study on the fluorination mechanisms of different types of carbon blacks which are *Corresponding author.
greatly affected by characteristics such as specific surface area, surface structure, coherence length of the graphitic domains. In previous papers, the effect of F 2 -gas fluorination on several types of carbon blacks was presented [5–7]. We established a fitting procedure of the XPS spectra [5] and discussed the influence of the oxidative activation of the starting materials and of the fluorination conditions on the amount of fixed fluorine and on the nature of carbon– fluorine bonds [6]. We showed that the fixed fluorine is essentially located at the surface of the particle or in its immediate vicinity. We also discussed the influence of morphology for several types of carbon blacks [7]. Moreover, the efficiency of the plasma-enhanced fluorination (PEF) to modify the surface properties of materials using
0008-6223 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00066-4
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
1892
radiofrequency (r.f.) plasmas of fluorinated gases has been clearly demonstrated elsewhere [8–10]. In order to investigate the influence of the physicochemical characteristics of carbon blacks upon PEF, three types of carbon blacks were selected: a thermal black, a furnace black and a high electrical conducting black. These carbon blacks have different morphological characters and different structural orders. X-ray photoelectron spectroscopy has been used to characterize the modifications occurring at the surface of these materials after CF 4 treatments in r.f. plasma conditions. Furthermore the influence of activation of the carbon blacks and of the X-ray irradiation during XPS measurements will also be investigated. Finally, the results of PEF will be compared with those obtained with F 2 -gas conditions.
2. Experimental procedure
2.1. Carbon black samples Three types of carbon blacks were selected: a thermal black (MT N990 from Degussa), a furnace black (Corax N115 also from Degussa) and a high electrical conducting black (Printex XE-2). The MT thermal black is obtained at a rather low temperature (11008C). It is formed of particles with diameters of several hundred nm. The number of surface defects is high and the reactivity is therefore important in spite of a low specific surface area. N115 furnace black is formed at 14008C. It exhibits a higher nanoporosity and a better crystallinity than MT black. XE-2 black appears as a by-product in the gasification process of mineral oils at 20008C. The peculiar morphology of this latter material, i.e. hollow particles with rather well organized domains, is due to the high concentration of oxidant and the length of the reaction zone which gives rise to an unusual after-treatment process. Although the coherence length along the c-direction is more or less similar in all materials (Lc51.25 nm), it can be noted that the lowest value of d 002 is found for XE-2, where d50.348 nm (to be compared to that of N115 for instance, where d50.367 nm) and evidences the better organization of this material which is due to a higher formation temperature. The active surface area (ASA), corresponding to the border of the graphitic layers, allows to calculate the most reactive part of the surface during the reaction with fluorine.
The mean particle size, the specific surface area (from BET), the absolute ASA values and the contribution of ASA to the specific surface (in %) of the three types of carbon blacks are collected in Table 1. Although the absolute value of ASA increases with increasing specific surface area and therefore is the largest for XE-2 sample, it can be noticed that, when referred to the specific surface area, an inversion is observed, (ASA / specific surface) ratio being highest for the furnace black.
2.2. Activation procedure The carbon black samples were used either ‘as-received’ (na-samples, for non-activated) or activated (a-samples) by heating in air at 5008C during 2 h in the case of N990 and N115 samples. Due to its peculiar morphology, XE-2 samples cannot be activated in similar conditions and were used ‘as-received’ only. It has been pointed out that fixation of fluorine by XE-2 is not enhanced by activation at 4508C [14].
2.3. Plasma-enhanced fluorination procedure The fluorination in r.f.-plasma conditions was carried out in a S.E. 80 Barrel Plasma Technology System. CF 4 gas was excited by a r.f. source at 13.56 MHz. The reactor consisted of two aluminum barrel electrodes which were coated with alumina (Al 2 O 3 ). The inner electrode on which the sample was placed was connected to the r.f. source and the outer one was grounded. A primary vacuum was obtained by a 40 m 3 h 21 Edwards E2M40-type pump equipped with a liquid nitrogen condenser which trapped the residual gases. The gas was introduced in the inner part of the reactor and then dissociated by electron impacts occurring between the two electrodes. Neutral species and radicals diffused from this plasma zone to the center of the reactor where they reacted with the sample. The chamber was thermostatically controlled and a temperature lower than 1008C was maintained during the process. Several tunable parameters could be adjusted during the PEF experiments, namely:
• the inlet precursor composition CF 4 1d O 2 , with (0# d (%)#25). However, due to an important etching effect on XE-2 samples, the addition of O 2 has been
Table 1 Characteristics of carbons blacks Type
Mean particle size (nm)
BET surface area (m 2 / g)
ASA (m 2 / g)
ASA / specific surface (%)
MT N990 Corax N115 Printex XE-2
320 25 30
8 145 1000
0.35 8.80 24.00
4.4 6.1 2.4
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
• • • •
abandoned, in order to have the same experimental conditions for all samples, inlet gas flow: 8#Q (cm 3 min 21 )#16, total pressure: 25#p (mTorr)#200, r.f. power: 40#P (W)#110, reaction duration: 10#T (min)#300.
Taking into account previous experiments of PEF on various types of carbon materials, the following optimized conditions could be established for powdered samples of several hundred mg; CF 4 gas flow rate: 8 cm 3 min 21 ; p: 200 mTorr; P: 80 W; T : 60 min [14]. Several experiments have also been carried out using the conventional F 2 -gas fluorination at room temperature, in order to compare the two different fluorination routes. The experimental procedure was described in a previous paper [7]. Rough information on the reaction completion was obtained from the weight uptake after fluorination.
2.4. XPS analysis Information concerning the spectrometer and experimental details was developed in Part I [5]. The spectrometer is a LEYBOLD LHS 11. The ionizing radiation (K a1a2 of Mg) was provided by a non-monochromatic X-ray source working at an acceleration tension of 10 kV and an emission current of 30 mA. The sample (powder) was pressed on an indium support and the analyzed surface area was 7 mm 2 . The high resolution spectra of carbon (C1s peak), of fluorine (F1s peak) and oxygen (O1s peak) were recorded in the fixed analyzer transmission mode (FAT) with a pass energy of 30 eV, after various durations of X-ray exposure in order to evidence a possible evolution under irradiation. The experimental spectra were fitted using the DS 100 processing program from LEYBOLD. On fitting the C1s envelope, the ‘graphitic’ component corresponding to structural units is restored by an asymmetrical profile. Details concerning this procedure are given in a previous paper [11].
3. Results and discussion
3.1. Plasma-enhanced fluorination of N115 furnace carbon black 3.1.1. Fluorination of a non-activated sample Fig. 1 shows the fitted high resolution C1s and F1s spectra of the plasma-fluorinated na-N115 carbon black. The assignments for the different components of C1s and F1s envelopes that we individualize are given in Table 2. The complete C1s spectra has been fitted into 10 components in order to take into account the different types of bonds present in the materials. This procedure has been proposed in the previous parts of the study (Parts I to III) and details of the fitting can be found there. Such an
1893
assumption has been confirmed on several classes of materials exhibiting varied characteristics: high electrical conducting-, thermal-, and furnace carbon blacks, either as-received or activated, and treated in various fluorination conditions. The C1s envelope is formed of two major peaks. The peak at lower binding energy (BE), located at 284.3 eV, can be assigned to the component C1s (1) corresponding to non-functionalized sp 2 and sp 3 C atoms that are not affected by fluorination. The area of component (1), which is noted AC1s (1), represents 42% of the total area of the C1s envelope. As in previous studies, this peak will be taken as an internal reference to define the energy separation with the other (i) components: DC i C 1 . Areas of the different components and BE separations are collected in Table 3. The peak at higher BE, C1s (6), which is located at 288.6 eV is assigned to carbon atoms that are covalently linked to a fluorine atom at the surface and border of the graphitic-like domains without any change in the sp 2 conformation of the carbon in the bulk (type I structure). Consequently, the C–C bond length does not significantly differ from that of the initial carbon. Between components (1) and (6), the envelope can be fitted into four components corresponding to C atoms that are not directly bound to F atoms. The shift induced by the presence of F atoms in b position of a given C atom, i.e. bound to its first neighbors, has been evaluated to about 0.660.2 eV [12] and is roughly additive. The assignment of these components are given in Table 2. The components C1s (i), with i$6, are attributed to carbon atoms that are covalently bound to F. The amount of such C in the layer explored by XPS can be evaluated from the ratio SC1s (i) (6#i#10) / SAC1s (total). This ratio is equal to 36%. In order to identify the components shifted by the charge effect and to differentiate the various forms of fluorinated C that contribute to the C1s (i) (i$6) components, we took the F1s peak as a reference. As a matter of fact, the chemical shift of F1s core level is less dependent on the x value of CF x groups, with x51, 2, 3, than the C core level. In type I fluorinated structure (i.e. where F atoms are covalently bound to the C atoms of the surface or subsuperficial zones of particles which keep their graphitic character) F atoms belonging to either CF, CF 2 or CF 3 groups contribute to the same component of the spectrum. This component, F1s (6), is located at 687.6 eV, and the difference in BE between corresponding components (6) of the F1s and C1s spectra is: DF 6 C 6 5 399.1 eV. F atoms of CF, CF 2 and CF 3 groups belonging to polyalicyclic perfluorinated structures of type II, in which sp 3 C skeleton forms puckered layers similar to those of covalent graphite fluorides (CF) n [13], contribute to the same component, F1s (8). This component is shifted by 2.0 eV relative to F1s (6) in the direction of increasing BE. The corresponding fluorinated entities are those with the highest BE. Consequently, the components with even higher BE, i.e. with i.8, can be attributed to fluorinated
1894
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
Fig. 1. Fitted high resolution C1s and F1s spectra of plasma-fluorinated furnace carbon black (na-N115).
domains in which the charge effect is locally more pronounced (differential charging). Covalent bonding between C and F atoms modifies locally the electrical conductivity of the sample. In order words, the key-point will be the extension of the thickness of the fluorinated islands. When the dimensions of these domains increase, the positive space charges that appear in these areas during the photoemission process can be only partially neutralized by photoelectrons and secondary electrons emitted from neighboring or underlying nonfunctionalized areas. The value of the BE separation between component (1) and the high energy components
of C1s spectrum, i.e. DC i C 1 , with i$6, will thus depend on the degree of fluorination of the sample. The component C1s (7) at 289.8 eV corresponds mostly to CF groups of type I with CF 2 groups as nearest neighbors. F atoms of these groups are assigned to component (6) of the F1s spectrum. The peak whose fitted component is located at 290.7 eV, C1s (8), can be attributed to CF 2 groups of fluorinated structure of type I, whose F atoms contribute to F1s (6) and also to CF groups of polyalicyclic perfluorinated structures of type II, whose F contribute to the component (8) of F1s envelope. Shoulder peaks (9) and (10) correspond mostly to CF n
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
1895
Table 2 Energy shifts and assignments of the different components of C1s and F1s spectra Component
DC i C 1 (eV)
Assignment
C1 C2
– 1.0
C3
1.8
C4
2.6
C5
3.360.1
C6 C7 C8
4.260.2 5.460.3 6.360.4
C9
7.960.4
C 10
9.660.4
Non-functionalized sp 2 and sp 3 C atoms, not affected fluorination Aliphatic non-functionalized sp 3 C in a non-fluorinated environment (areas modified by ‘plasma effect’) Non-functionalized sp 2 C in b position to one F atom Non-functionalized sp 2 C or sp 3 C in b position to one or two F atom(s), respectively sp 2 C bound to an oxygen atom (phenol, phenyl ether) Non-functionalized sp 2 C or sp 3 C in b position to two or three F atoms, respectively Oxygenated sp 3 C in a non-functionalized environment (CH x –OH, or C–O–C) Oxygenated sp 3 C in b position to a F atom Non-functionalized sp 3 C in b position to at least three F atoms sp 2 C ‘semi-ionic’, bound to intercalated F atom (very weak contribution) sp 2 C covalently linked to an F atom in type I structure CF groups of type I structure in b position of CF 2 groups CF 2 groups of type I structure CF groups of type II structure CF 3 groups of type I structure CF 2 groups of type II structure CF 3 groups of type II structure Plasmon
Component
DF i F 6 (eV)
Assignment
F6 F8 F9 F 10
– 2.060.1 3.560.1 5.960.1
F F F F
atoms atoms atoms atoms
in in in in
type type type type
I structure II structure II structure with charge effect II structure with higher charge effect
groups of type II and are associated with components (9) and (10) of F1s (see Table 2). The corresponding zones are strongly affected by charge effects. From the analysis of the XPS spectra of fluorinated, non-activated N115 carbon black samples, we conclude that all fluorine atoms, fixed at the surface and in the subsuperficial zone of the carbon black particles, are covalently linked to carbon atoms. The majority of the structures of the fluorinated islands are of type I where the planar conformation of the graphene layers is preserved.
However fluorinated islands of type II structure also exist and some of them show a significant charge effect. We also conclude that there are no fluorine atoms trapped in the micropores of the superficial zone of particles.
3.1.2. Influence of activation of starting materials F / C ratio deduced from the mass uptake resulting from the fluorination of activated N115 carbon black is three times more important than that of non-activated sample (0.094 vs. 0.032), whereas the proportion of fluorinated
Table 3 XPS spectra fitting results of fluorinated na-N115 carbon black Component
DC i C 1 (eV)
AC1s (i) (%)
Component
DF i F 6 (eV)
AF1s (i) (%)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C 10
– 1.0 1.8 2.6 3.3 4.4 5.5 6.4 8.1 9.9
43 7 6 5 4 17 2 10 5 1
F6 F8 F9 F 10
– 2.1 3.5 5.9
79 15 5 1
1896
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
carbon in the layer analysed by XPS has increased weakly (4164% vs. 3663.5%). The F / C ratio deduced from XPS spectra has therefore increased from 0.52 to 0.60. Fig. 2 shows the fitted high resolution C1s and F1s spectra of plasma-fluorinated a-N115 carbon black. The shape of the envelopes are similar to those of the nonactivated sample, but the relative intensities of the components are slightly different (cf. Table 3). The energy gaps between component C1s (6) and component C1s (1) do not differ noticeably for the activated and non-activated samples (4.5 instead of 4.4). It implies that the mean thickness and dimensions of the fluorinated islands are of the same order of magnitude.
The relative decrease of AF1s (i) (8#i#10) in the case of the activated sample suggests that the proportion of fluorinated entities of type II structure slightly decreased in the case of the activated N115 whereas the amount of fixed fluorine increased. The activated sample has a specific surface area of 421 m 2 g 21 instead of 145 m 2 g 21 for the non-activated one. The activation causes the opening of the micropores and facilitates the penetration of fluorine atoms. The preceding results suggest that the fluorine radicals have not only reacted with C of the surface and the subsuperficial zone of the particles, but have diffused inside the carbon black because the activation has gener-
Fig. 2. Fitted high resolution C1s and F1s spectra of plasma-fluorinated activated furnace carbon black (a-N115).
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
ated mesopores and / or intraparticle cavities (preferential oxidation of the C of the less ordered core zone of the particle). The CF x entities which are formed by reaction of these F radicals with C of this zone are not detected by XPS.
3.1.3. Influence of irradiation exposure during XPS measurements Due to specific experimental conditions that are required to obtain useful spectra, i.e. high power of X-ray source and relatively long exposure durations, the following behavior has been observed. When the fluorinated area of the sample is the seat of appreciable differential charging, all component peaks of the C1s envelope, with the exception of C1s (1), shift back toward lower BE values as the irradiation duration increases (the charging effect diminishes with increasing irradiation exposure, a typical behavior of non-conducting samples when analysed by XPS). The energy gap DC1s(i)C1s(1) decreases progressively and the shape of the envelope is altered. As an example, the C1s envelopes of the spectra from a non-activated N115 fluorinated sample taken respectively after 30, 110 and 300 min of exposure are compared in Fig. 3. It can be noted that the C1s component corresponding to non-fluorinated areas, which extend almost up to the external surface, is not associated with any charge effect and is not shifted. On the other hand, C1s (i$6) components which correspond to fluorinated domains are shifted significantly. The F / C ratio and relative intensities of C1s (i$6) components of perfluorinated carbons also decrease during the irradiation, whereas the corresponding intensities of C1s (1,i,5) increase. The following assumption can be raised: under X-ray irradiation, C–CF x or CF x (x52, 3) groups are broken with evolution of gaseous fluorinated species or fluorocarbon groups. This degradation is mostly induced by secondary electrons emitted by the X-ray window, and in a lesser extent by thermal effects since the X-ray source is very close to the sample. 3.2. Influence of morphology and physicochemical characteristics of carbon blacks Fig. 4 shows the fitted C1s and F1s spectra of three different types of carbon blacks which were plasma-fluorinated in the following conditions: CF 4 flow rate: 8 cm 3 min 21 , pressure: 200 mTorr, power: 80 W, duration: 60 min. F / C ratios and some salient features deduced from the relative areas of fitted XPS spectra are summarized in Table 4. The F / C ratios calculated by mass uptake are low, but differ strongly from one type of carbon black to the other, i.e. 0.008 for MT N990, 0.032 for N115, and 0.183 for XE-2. The tendency seems to depend on the specific surface area and active surface area. For example, XE-2
1897
which has the largest surface area shows the highest F / C ratio by mass uptake. The ratio F / C calculated from XPS and the ratio [AC1s (i$6) /AC1s (tot)] which shows the proportion of carbon atoms which are directly bound to fluorine atoms differ not so drastically when compared to the low F / C ratios obtained from mass uptake. They are both related to the microstructural characteristics of the carbon black (degree of microstructural order and graphitic character of the surface and subsuperficial zone of the particles). It should be noted that these values are more important for MT N990 and XE-2 than that for N115. MT N990 thermal black and XE-2 high electrical conducting black react more readily with F radicals on the surface and superficial zone of the particles than does N115 furnace black. This result could be expected for the thermal black since it has lower degree of microstructural order and graphitic character. In the case of XE-2, the apparent higher reactivity of this carbon black is certainly related to its particular morphology, i.e. to highly accessible surface areas. The components F1s(i) (8#i#10) of the F1s envelope correspond essentially to F participating in type II structures. The ratio of the sum of areas of these components to the total area of the F1s envelope gives insight into the relative contribution of type II structures. It increases in the following order: 0.118 for MT N990, 0.124 for N115, and 0.335 for XE-2 (Table 4). The ratio [AF1s (tot) /AC1s (i$6)] gives insight into the relative importance of perfluorinated CF x (x.1) entities. It increases as the mean number of F linked to a C atom increases. It decreases in the following order: MT N990.N115.XE-2 (cf. Table 4). These ratios and accordingly the relative importance of the different types of fluorinated carbons, are also related to the surface morphology and structure of the carbon black. In the case of the MT thermal black, only few structural entities of type II are formed. On the other hand, there are more perfluorinated C of types CF 2 and CF 3 than in the case of N115 and XE-2. Owing to its lower structural order, the proportion of border sp 2 C and of protonated sp 3 C which can bond to one or more F atoms are much higher in the volume explored by XPS in the thermal black than in the furnace black. In the case of the XE-2 high electrical conducting black, which has a higher organized surface structure and graphitic character, once the ‘reactive’ sp 2 C which are present at the surface and border of the graphitic domains have fixed an F atom, further fixation of fluorine can only occur at the non-protonated sp 2 C of the graphene layers: this implies the destruction of the graphitic structure. Theoretically polyalicyclic perfluorinated entities can only form at relatively high temperatures (.3508C). When the heterogeneous reaction between solid C and fluorine atoms or radicals is performed in the ‘static bed’ mode, as it was the case, it is highly probable that the reaction conditions will differ from one zone to another one in the reaction mechanism. It is practically impossible to fix and control
1898
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
Fig. 3. Dependence of C1s and F1s spectra on irradiation time during XPS measurements for plasma-fluorinated furnace carbon black (na-N115).
precisely the experimental parameters (especially the temperature) at the microscopic scale. Fluorination of C is a highly exothermic process. Accordingly, it can be expected that the temperature of the reaction zone may reach quite high values at the nanometric scale so that perfluorinated
groups of type II can be locally formed. On the other hand from 400 to 4508C some C–F bonds are broken. The thermal stability of the perfluorinated entities which can be formed decreases when the structural order and the graphitic character of the carbon materials decrease [3].
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
1899
Fig. 4. Fitted high resolution C1s and F1s spectra of plasma-fluorinated MT N990, N115 and XE-2 (non-activated).
Table 4 F / C ratios and salient features of different fluorinated na-carbon blacks
F / C (mass uptake) F / C (XPS) [AC1s (i) (6#i#10) /AC1s (total)] [AF1s (i) (8#i#10) /AF1s (total)] [AF1s (total) /AC1s (i)] (6#i#10)
MT N990
N115
XE-2
0.008 0.85
0.032 0.50
0.183 0.62
0.47
0.36
0.51
0.118
0.214
0.335
1.78
1.40
1.22
Accordingly, it can be expected that the proportion of entities of type II is significantly lower in fluorinated thermal black than in fluorinated XE-2.
3.3. Comparison between r.f. plasma and F2 -gas fluorination The shape of the C1s envelopes of fluorinated carbon blacks obtained by both methods are not so different from each other. Moreover, when fitting the F1s envelope of the XPS spectra of F 2 -gas fluorinated N115 furnace black a component, F1s (4), was individualized, which was assigned to physisorbed F 2 [6]. In the case of MT N990 thermal black and XE-2 high electrical conducting black, such a component was not observed. In addition to fluorine species that are covalently bound to carbon of the surface and subsuperficial zones of the particles, non-reacted F 2 can thus exist in the micropores of the superficial zone of N115 particles. In the case of plasma fluorination, the F1s envelope shows only components corresponding to covalently bound
1900
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
F atoms for the three types of carbon blacks. Fluorine radicals are so active that they immediately react to give covalent carbon–fluorine bonds in the surface or superficial zone of the particles. The formed radicals being unstable out of plasma conditions, no F species can be trapped.
4. Conclusions We have investigated by XPS the plasma fluorination of three types of carbon blacks: a thermal black, a furnace black and a high electrical conducting black. The amount of fluorine that reacts with the surface of these carbon blacks depends on the reactivity of superficial and subsuperficial zones of the materials, i.e. on the morphology (microstructural order) and on the organization (graphitic character) of these particles. This trend is similar to that already observed in F 2 -gas treated carbon blacks. From the analysis of the XPS spectra of fluorinated, non-activated N115 carbon black samples, we conclude that most of F fixed is covalently bound to ‘reactive’ C of the external and superficial part of the particles. The majority of the structure consists therefore in fluorinated islands of type I structure where the planar conformation of the graphene layers is preserved. However, fluorinated islands of type II structure also exist, some of them exhibiting a significant charge effect. The main effect of the activation of the initial carbons is to create conditions that favor the diffusion of F species within the bulk of the materials. We also conclude that there are no fluorine atoms physisorbed in the micropores of the superficial zone of particle after PEF treatment, whereas trapped fluorine atoms have been detected in the case of F 2 -gas fluorination. The differences in morphological characters, structural order and graphitic characters of the three types of carbon
blacks affect the ratio of type I / type II structures of the fluorinated carbons. For higher structural organization of the particles, i.e. for XE-2 blacks, the proportion of type II increases and the F / C ratio in fluorinated functional groups (CFx ) with x51, 2, 3, decreases. In this case the transformation into polyalicyclic perfluorinated structures of (CF) n composition, similar to covalent graphite fluoride, is achieved more easily whereas in carbon blacks with lower crystallinity, the presence of numerous defects leads preferentially to CF 2 and CF 3 entities.
References [1] Nakajima T. Fluorine–carbon and fluoride–carbon materials, New York: Marcel Dekker, 1995. [2] Watanabe N, Nakajima T, Touhara H. In: Graphite fluorides, Amsterdam: Elsevier, 1988, p. 65. [3] Watanabe N, Kita Y, Mochizuki O. Carbon 1979;17:359. [4] Nakahara M, Ozawa K, Sanada Y. J Mater Sci 1994;29:1646. [5] Nanse´ G, Papirer E, Fioux P, Moguet F, Tressaud A. Carbon 1997;35:175. [6] Nanse´ G, Papirer E, Fioux P, Moguet F, Tressaud A. Carbon 1997;35:371. [7] Nanse´ G, Papirer E, Fioux P, Moguet F, Tressaud A. Carbon 1997;35:515. [8] Turban G. Ph.D. Thesis, University Nantes, 1981. [9] Plumb IC, Ryan KR. Plasma Chem Plasma Process 1986;6:205. [10] Magro C, Tressaud A, Lozano L, Hudakova N, Cardinaud C, Turban G. J Mater Sci 1994;29:4225. [11] Papirer E, Lacroix R, Donnet JB, Nanse´ G, Fioux P. Carbon 1994;32:1341. [12] Clark DT, Fast WJ, Kilcast D, Musgrave WKR. J Polym Sci 1973;2:389. [13] Nakajima T, Watanabe N. Graphite fluorides and carbon– fluorine compounds, Boca Raton, FL: CRC Press, 1991. [14] Moguet F. Doctorate Thesis, University Bordeaux I, 1996.
Carbon 37 (1999) 1891–1900
Fluorination of carbon blacks: an X-ray photoelectron spectroscopy study IV. Reactivity of different carbon blacks in CF 4 radiofrequency plasma T. Shirasaki a , F. Moguet a , L. Lozano a , A. Tressaud a , *, G. Nanse b , E. Papirer b a
` Condensee ´ de Bordeaux, CNRS, Av. Dr. A. Schweitzer, 33608 Pessac Cedex, France Institut de Chimie de la Matiere Institut de Chimie des Surfaces et Interfaces, CNRS, 15 rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, France
b
Received 17 July 1998; accepted 4 March 1999
Abstract The effect of CF 4 -plasma enhanced fluorination on the surface modification of carbon blacks has been examined using XPS. Three different types of carbon blacks have been studied: a thermal black, a furnace black and a high electrical conducting black. The analysis of the XPS spectra of fluorinated carbon black samples indicates that all fluorine atoms, fixed at the surface and in a subsuperficial zone of the particles, are covalently linked to carbon atoms. The influence of the physicochemical properties and morphology of these three types of carbon blacks on the fluorination reaction has also been investigated. The proportion of different types of fluorinated carbon atoms, i.e. on one hand CFx surface and border groups of graphitic bulk domains for which the planar configuration of the graphene layers is preserved together with the sp 2 character of C, i.e. structures of type I, on the other hand polyalicyclic perfluorinated structures in which sp 3 C form puckered layers similar to those of covalent fluorographites, i.e. structures of type II, and also the F / C ratio of the fluorinated groups are related to the surface morphology and depend on the microstructural organization of particles. When the microstructure ordering and graphitic character of the carbon increase, the size of the ordered graphitic domain also increases. At the same time the density, the size of defects and proportion of protonated sp 3 C entities bridging the graphene layers decrease. As a consequence, the proportion of carbon atoms, potentially able to form perfluorinated CF 2 and CF 3 groups, decreases. The relative contribution of those groups is appreciably higher in fluorinated compounds which are derived from carbon blacks with a lower structural order. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon black; B. Halogenation, Activation; C. Photoelectron spectroscopy; D. Reactivity
1. Introduction The studies on the reactivity of carbon materials with fluorine have been mostly devoted to graphite and graphitized carbons [1]. Fluorination of carbon blacks has received less attention. Watanabe et al., analyzing the XPS spectra of these materials treated with gaseous F 2 , have identified three types of C–F bonds [2,3]. In a more recent paper, Nakahara et al. have compared the fluorination of carbon blacks with that of active carbons [4]. We have undertaken a general study on the fluorination mechanisms of different types of carbon blacks which are *Corresponding author.
greatly affected by characteristics such as specific surface area, surface structure, coherence length of the graphitic domains. In previous papers, the effect of F 2 -gas fluorination on several types of carbon blacks was presented [5–7]. We established a fitting procedure of the XPS spectra [5] and discussed the influence of the oxidative activation of the starting materials and of the fluorination conditions on the amount of fixed fluorine and on the nature of carbon– fluorine bonds [6]. We showed that the fixed fluorine is essentially located at the surface of the particle or in its immediate vicinity. We also discussed the influence of morphology for several types of carbon blacks [7]. Moreover, the efficiency of the plasma-enhanced fluorination (PEF) to modify the surface properties of materials using
0008-6223 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00066-4
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
1892
radiofrequency (r.f.) plasmas of fluorinated gases has been clearly demonstrated elsewhere [8–10]. In order to investigate the influence of the physicochemical characteristics of carbon blacks upon PEF, three types of carbon blacks were selected: a thermal black, a furnace black and a high electrical conducting black. These carbon blacks have different morphological characters and different structural orders. X-ray photoelectron spectroscopy has been used to characterize the modifications occurring at the surface of these materials after CF 4 treatments in r.f. plasma conditions. Furthermore the influence of activation of the carbon blacks and of the X-ray irradiation during XPS measurements will also be investigated. Finally, the results of PEF will be compared with those obtained with F 2 -gas conditions.
2. Experimental procedure
2.1. Carbon black samples Three types of carbon blacks were selected: a thermal black (MT N990 from Degussa), a furnace black (Corax N115 also from Degussa) and a high electrical conducting black (Printex XE-2). The MT thermal black is obtained at a rather low temperature (11008C). It is formed of particles with diameters of several hundred nm. The number of surface defects is high and the reactivity is therefore important in spite of a low specific surface area. N115 furnace black is formed at 14008C. It exhibits a higher nanoporosity and a better crystallinity than MT black. XE-2 black appears as a by-product in the gasification process of mineral oils at 20008C. The peculiar morphology of this latter material, i.e. hollow particles with rather well organized domains, is due to the high concentration of oxidant and the length of the reaction zone which gives rise to an unusual after-treatment process. Although the coherence length along the c-direction is more or less similar in all materials (Lc51.25 nm), it can be noted that the lowest value of d 002 is found for XE-2, where d50.348 nm (to be compared to that of N115 for instance, where d50.367 nm) and evidences the better organization of this material which is due to a higher formation temperature. The active surface area (ASA), corresponding to the border of the graphitic layers, allows to calculate the most reactive part of the surface during the reaction with fluorine.
The mean particle size, the specific surface area (from BET), the absolute ASA values and the contribution of ASA to the specific surface (in %) of the three types of carbon blacks are collected in Table 1. Although the absolute value of ASA increases with increasing specific surface area and therefore is the largest for XE-2 sample, it can be noticed that, when referred to the specific surface area, an inversion is observed, (ASA / specific surface) ratio being highest for the furnace black.
2.2. Activation procedure The carbon black samples were used either ‘as-received’ (na-samples, for non-activated) or activated (a-samples) by heating in air at 5008C during 2 h in the case of N990 and N115 samples. Due to its peculiar morphology, XE-2 samples cannot be activated in similar conditions and were used ‘as-received’ only. It has been pointed out that fixation of fluorine by XE-2 is not enhanced by activation at 4508C [14].
2.3. Plasma-enhanced fluorination procedure The fluorination in r.f.-plasma conditions was carried out in a S.E. 80 Barrel Plasma Technology System. CF 4 gas was excited by a r.f. source at 13.56 MHz. The reactor consisted of two aluminum barrel electrodes which were coated with alumina (Al 2 O 3 ). The inner electrode on which the sample was placed was connected to the r.f. source and the outer one was grounded. A primary vacuum was obtained by a 40 m 3 h 21 Edwards E2M40-type pump equipped with a liquid nitrogen condenser which trapped the residual gases. The gas was introduced in the inner part of the reactor and then dissociated by electron impacts occurring between the two electrodes. Neutral species and radicals diffused from this plasma zone to the center of the reactor where they reacted with the sample. The chamber was thermostatically controlled and a temperature lower than 1008C was maintained during the process. Several tunable parameters could be adjusted during the PEF experiments, namely:
• the inlet precursor composition CF 4 1d O 2 , with (0# d (%)#25). However, due to an important etching effect on XE-2 samples, the addition of O 2 has been
Table 1 Characteristics of carbons blacks Type
Mean particle size (nm)
BET surface area (m 2 / g)
ASA (m 2 / g)
ASA / specific surface (%)
MT N990 Corax N115 Printex XE-2
320 25 30
8 145 1000
0.35 8.80 24.00
4.4 6.1 2.4
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
• • • •
abandoned, in order to have the same experimental conditions for all samples, inlet gas flow: 8#Q (cm 3 min 21 )#16, total pressure: 25#p (mTorr)#200, r.f. power: 40#P (W)#110, reaction duration: 10#T (min)#300.
Taking into account previous experiments of PEF on various types of carbon materials, the following optimized conditions could be established for powdered samples of several hundred mg; CF 4 gas flow rate: 8 cm 3 min 21 ; p: 200 mTorr; P: 80 W; T : 60 min [14]. Several experiments have also been carried out using the conventional F 2 -gas fluorination at room temperature, in order to compare the two different fluorination routes. The experimental procedure was described in a previous paper [7]. Rough information on the reaction completion was obtained from the weight uptake after fluorination.
2.4. XPS analysis Information concerning the spectrometer and experimental details was developed in Part I [5]. The spectrometer is a LEYBOLD LHS 11. The ionizing radiation (K a1a2 of Mg) was provided by a non-monochromatic X-ray source working at an acceleration tension of 10 kV and an emission current of 30 mA. The sample (powder) was pressed on an indium support and the analyzed surface area was 7 mm 2 . The high resolution spectra of carbon (C1s peak), of fluorine (F1s peak) and oxygen (O1s peak) were recorded in the fixed analyzer transmission mode (FAT) with a pass energy of 30 eV, after various durations of X-ray exposure in order to evidence a possible evolution under irradiation. The experimental spectra were fitted using the DS 100 processing program from LEYBOLD. On fitting the C1s envelope, the ‘graphitic’ component corresponding to structural units is restored by an asymmetrical profile. Details concerning this procedure are given in a previous paper [11].
3. Results and discussion
3.1. Plasma-enhanced fluorination of N115 furnace carbon black 3.1.1. Fluorination of a non-activated sample Fig. 1 shows the fitted high resolution C1s and F1s spectra of the plasma-fluorinated na-N115 carbon black. The assignments for the different components of C1s and F1s envelopes that we individualize are given in Table 2. The complete C1s spectra has been fitted into 10 components in order to take into account the different types of bonds present in the materials. This procedure has been proposed in the previous parts of the study (Parts I to III) and details of the fitting can be found there. Such an
1893
assumption has been confirmed on several classes of materials exhibiting varied characteristics: high electrical conducting-, thermal-, and furnace carbon blacks, either as-received or activated, and treated in various fluorination conditions. The C1s envelope is formed of two major peaks. The peak at lower binding energy (BE), located at 284.3 eV, can be assigned to the component C1s (1) corresponding to non-functionalized sp 2 and sp 3 C atoms that are not affected by fluorination. The area of component (1), which is noted AC1s (1), represents 42% of the total area of the C1s envelope. As in previous studies, this peak will be taken as an internal reference to define the energy separation with the other (i) components: DC i C 1 . Areas of the different components and BE separations are collected in Table 3. The peak at higher BE, C1s (6), which is located at 288.6 eV is assigned to carbon atoms that are covalently linked to a fluorine atom at the surface and border of the graphitic-like domains without any change in the sp 2 conformation of the carbon in the bulk (type I structure). Consequently, the C–C bond length does not significantly differ from that of the initial carbon. Between components (1) and (6), the envelope can be fitted into four components corresponding to C atoms that are not directly bound to F atoms. The shift induced by the presence of F atoms in b position of a given C atom, i.e. bound to its first neighbors, has been evaluated to about 0.660.2 eV [12] and is roughly additive. The assignment of these components are given in Table 2. The components C1s (i), with i$6, are attributed to carbon atoms that are covalently bound to F. The amount of such C in the layer explored by XPS can be evaluated from the ratio SC1s (i) (6#i#10) / SAC1s (total). This ratio is equal to 36%. In order to identify the components shifted by the charge effect and to differentiate the various forms of fluorinated C that contribute to the C1s (i) (i$6) components, we took the F1s peak as a reference. As a matter of fact, the chemical shift of F1s core level is less dependent on the x value of CF x groups, with x51, 2, 3, than the C core level. In type I fluorinated structure (i.e. where F atoms are covalently bound to the C atoms of the surface or subsuperficial zones of particles which keep their graphitic character) F atoms belonging to either CF, CF 2 or CF 3 groups contribute to the same component of the spectrum. This component, F1s (6), is located at 687.6 eV, and the difference in BE between corresponding components (6) of the F1s and C1s spectra is: DF 6 C 6 5 399.1 eV. F atoms of CF, CF 2 and CF 3 groups belonging to polyalicyclic perfluorinated structures of type II, in which sp 3 C skeleton forms puckered layers similar to those of covalent graphite fluorides (CF) n [13], contribute to the same component, F1s (8). This component is shifted by 2.0 eV relative to F1s (6) in the direction of increasing BE. The corresponding fluorinated entities are those with the highest BE. Consequently, the components with even higher BE, i.e. with i.8, can be attributed to fluorinated
1894
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
Fig. 1. Fitted high resolution C1s and F1s spectra of plasma-fluorinated furnace carbon black (na-N115).
domains in which the charge effect is locally more pronounced (differential charging). Covalent bonding between C and F atoms modifies locally the electrical conductivity of the sample. In order words, the key-point will be the extension of the thickness of the fluorinated islands. When the dimensions of these domains increase, the positive space charges that appear in these areas during the photoemission process can be only partially neutralized by photoelectrons and secondary electrons emitted from neighboring or underlying nonfunctionalized areas. The value of the BE separation between component (1) and the high energy components
of C1s spectrum, i.e. DC i C 1 , with i$6, will thus depend on the degree of fluorination of the sample. The component C1s (7) at 289.8 eV corresponds mostly to CF groups of type I with CF 2 groups as nearest neighbors. F atoms of these groups are assigned to component (6) of the F1s spectrum. The peak whose fitted component is located at 290.7 eV, C1s (8), can be attributed to CF 2 groups of fluorinated structure of type I, whose F atoms contribute to F1s (6) and also to CF groups of polyalicyclic perfluorinated structures of type II, whose F contribute to the component (8) of F1s envelope. Shoulder peaks (9) and (10) correspond mostly to CF n
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
1895
Table 2 Energy shifts and assignments of the different components of C1s and F1s spectra Component
DC i C 1 (eV)
Assignment
C1 C2
– 1.0
C3
1.8
C4
2.6
C5
3.360.1
C6 C7 C8
4.260.2 5.460.3 6.360.4
C9
7.960.4
C 10
9.660.4
Non-functionalized sp 2 and sp 3 C atoms, not affected fluorination Aliphatic non-functionalized sp 3 C in a non-fluorinated environment (areas modified by ‘plasma effect’) Non-functionalized sp 2 C in b position to one F atom Non-functionalized sp 2 C or sp 3 C in b position to one or two F atom(s), respectively sp 2 C bound to an oxygen atom (phenol, phenyl ether) Non-functionalized sp 2 C or sp 3 C in b position to two or three F atoms, respectively Oxygenated sp 3 C in a non-functionalized environment (CH x –OH, or C–O–C) Oxygenated sp 3 C in b position to a F atom Non-functionalized sp 3 C in b position to at least three F atoms sp 2 C ‘semi-ionic’, bound to intercalated F atom (very weak contribution) sp 2 C covalently linked to an F atom in type I structure CF groups of type I structure in b position of CF 2 groups CF 2 groups of type I structure CF groups of type II structure CF 3 groups of type I structure CF 2 groups of type II structure CF 3 groups of type II structure Plasmon
Component
DF i F 6 (eV)
Assignment
F6 F8 F9 F 10
– 2.060.1 3.560.1 5.960.1
F F F F
atoms atoms atoms atoms
in in in in
type type type type
I structure II structure II structure with charge effect II structure with higher charge effect
groups of type II and are associated with components (9) and (10) of F1s (see Table 2). The corresponding zones are strongly affected by charge effects. From the analysis of the XPS spectra of fluorinated, non-activated N115 carbon black samples, we conclude that all fluorine atoms, fixed at the surface and in the subsuperficial zone of the carbon black particles, are covalently linked to carbon atoms. The majority of the structures of the fluorinated islands are of type I where the planar conformation of the graphene layers is preserved.
However fluorinated islands of type II structure also exist and some of them show a significant charge effect. We also conclude that there are no fluorine atoms trapped in the micropores of the superficial zone of particles.
3.1.2. Influence of activation of starting materials F / C ratio deduced from the mass uptake resulting from the fluorination of activated N115 carbon black is three times more important than that of non-activated sample (0.094 vs. 0.032), whereas the proportion of fluorinated
Table 3 XPS spectra fitting results of fluorinated na-N115 carbon black Component
DC i C 1 (eV)
AC1s (i) (%)
Component
DF i F 6 (eV)
AF1s (i) (%)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C 10
– 1.0 1.8 2.6 3.3 4.4 5.5 6.4 8.1 9.9
43 7 6 5 4 17 2 10 5 1
F6 F8 F9 F 10
– 2.1 3.5 5.9
79 15 5 1
1896
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
carbon in the layer analysed by XPS has increased weakly (4164% vs. 3663.5%). The F / C ratio deduced from XPS spectra has therefore increased from 0.52 to 0.60. Fig. 2 shows the fitted high resolution C1s and F1s spectra of plasma-fluorinated a-N115 carbon black. The shape of the envelopes are similar to those of the nonactivated sample, but the relative intensities of the components are slightly different (cf. Table 3). The energy gaps between component C1s (6) and component C1s (1) do not differ noticeably for the activated and non-activated samples (4.5 instead of 4.4). It implies that the mean thickness and dimensions of the fluorinated islands are of the same order of magnitude.
The relative decrease of AF1s (i) (8#i#10) in the case of the activated sample suggests that the proportion of fluorinated entities of type II structure slightly decreased in the case of the activated N115 whereas the amount of fixed fluorine increased. The activated sample has a specific surface area of 421 m 2 g 21 instead of 145 m 2 g 21 for the non-activated one. The activation causes the opening of the micropores and facilitates the penetration of fluorine atoms. The preceding results suggest that the fluorine radicals have not only reacted with C of the surface and the subsuperficial zone of the particles, but have diffused inside the carbon black because the activation has gener-
Fig. 2. Fitted high resolution C1s and F1s spectra of plasma-fluorinated activated furnace carbon black (a-N115).
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
ated mesopores and / or intraparticle cavities (preferential oxidation of the C of the less ordered core zone of the particle). The CF x entities which are formed by reaction of these F radicals with C of this zone are not detected by XPS.
3.1.3. Influence of irradiation exposure during XPS measurements Due to specific experimental conditions that are required to obtain useful spectra, i.e. high power of X-ray source and relatively long exposure durations, the following behavior has been observed. When the fluorinated area of the sample is the seat of appreciable differential charging, all component peaks of the C1s envelope, with the exception of C1s (1), shift back toward lower BE values as the irradiation duration increases (the charging effect diminishes with increasing irradiation exposure, a typical behavior of non-conducting samples when analysed by XPS). The energy gap DC1s(i)C1s(1) decreases progressively and the shape of the envelope is altered. As an example, the C1s envelopes of the spectra from a non-activated N115 fluorinated sample taken respectively after 30, 110 and 300 min of exposure are compared in Fig. 3. It can be noted that the C1s component corresponding to non-fluorinated areas, which extend almost up to the external surface, is not associated with any charge effect and is not shifted. On the other hand, C1s (i$6) components which correspond to fluorinated domains are shifted significantly. The F / C ratio and relative intensities of C1s (i$6) components of perfluorinated carbons also decrease during the irradiation, whereas the corresponding intensities of C1s (1,i,5) increase. The following assumption can be raised: under X-ray irradiation, C–CF x or CF x (x52, 3) groups are broken with evolution of gaseous fluorinated species or fluorocarbon groups. This degradation is mostly induced by secondary electrons emitted by the X-ray window, and in a lesser extent by thermal effects since the X-ray source is very close to the sample. 3.2. Influence of morphology and physicochemical characteristics of carbon blacks Fig. 4 shows the fitted C1s and F1s spectra of three different types of carbon blacks which were plasma-fluorinated in the following conditions: CF 4 flow rate: 8 cm 3 min 21 , pressure: 200 mTorr, power: 80 W, duration: 60 min. F / C ratios and some salient features deduced from the relative areas of fitted XPS spectra are summarized in Table 4. The F / C ratios calculated by mass uptake are low, but differ strongly from one type of carbon black to the other, i.e. 0.008 for MT N990, 0.032 for N115, and 0.183 for XE-2. The tendency seems to depend on the specific surface area and active surface area. For example, XE-2
1897
which has the largest surface area shows the highest F / C ratio by mass uptake. The ratio F / C calculated from XPS and the ratio [AC1s (i$6) /AC1s (tot)] which shows the proportion of carbon atoms which are directly bound to fluorine atoms differ not so drastically when compared to the low F / C ratios obtained from mass uptake. They are both related to the microstructural characteristics of the carbon black (degree of microstructural order and graphitic character of the surface and subsuperficial zone of the particles). It should be noted that these values are more important for MT N990 and XE-2 than that for N115. MT N990 thermal black and XE-2 high electrical conducting black react more readily with F radicals on the surface and superficial zone of the particles than does N115 furnace black. This result could be expected for the thermal black since it has lower degree of microstructural order and graphitic character. In the case of XE-2, the apparent higher reactivity of this carbon black is certainly related to its particular morphology, i.e. to highly accessible surface areas. The components F1s(i) (8#i#10) of the F1s envelope correspond essentially to F participating in type II structures. The ratio of the sum of areas of these components to the total area of the F1s envelope gives insight into the relative contribution of type II structures. It increases in the following order: 0.118 for MT N990, 0.124 for N115, and 0.335 for XE-2 (Table 4). The ratio [AF1s (tot) /AC1s (i$6)] gives insight into the relative importance of perfluorinated CF x (x.1) entities. It increases as the mean number of F linked to a C atom increases. It decreases in the following order: MT N990.N115.XE-2 (cf. Table 4). These ratios and accordingly the relative importance of the different types of fluorinated carbons, are also related to the surface morphology and structure of the carbon black. In the case of the MT thermal black, only few structural entities of type II are formed. On the other hand, there are more perfluorinated C of types CF 2 and CF 3 than in the case of N115 and XE-2. Owing to its lower structural order, the proportion of border sp 2 C and of protonated sp 3 C which can bond to one or more F atoms are much higher in the volume explored by XPS in the thermal black than in the furnace black. In the case of the XE-2 high electrical conducting black, which has a higher organized surface structure and graphitic character, once the ‘reactive’ sp 2 C which are present at the surface and border of the graphitic domains have fixed an F atom, further fixation of fluorine can only occur at the non-protonated sp 2 C of the graphene layers: this implies the destruction of the graphitic structure. Theoretically polyalicyclic perfluorinated entities can only form at relatively high temperatures (.3508C). When the heterogeneous reaction between solid C and fluorine atoms or radicals is performed in the ‘static bed’ mode, as it was the case, it is highly probable that the reaction conditions will differ from one zone to another one in the reaction mechanism. It is practically impossible to fix and control
1898
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
Fig. 3. Dependence of C1s and F1s spectra on irradiation time during XPS measurements for plasma-fluorinated furnace carbon black (na-N115).
precisely the experimental parameters (especially the temperature) at the microscopic scale. Fluorination of C is a highly exothermic process. Accordingly, it can be expected that the temperature of the reaction zone may reach quite high values at the nanometric scale so that perfluorinated
groups of type II can be locally formed. On the other hand from 400 to 4508C some C–F bonds are broken. The thermal stability of the perfluorinated entities which can be formed decreases when the structural order and the graphitic character of the carbon materials decrease [3].
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
1899
Fig. 4. Fitted high resolution C1s and F1s spectra of plasma-fluorinated MT N990, N115 and XE-2 (non-activated).
Table 4 F / C ratios and salient features of different fluorinated na-carbon blacks
F / C (mass uptake) F / C (XPS) [AC1s (i) (6#i#10) /AC1s (total)] [AF1s (i) (8#i#10) /AF1s (total)] [AF1s (total) /AC1s (i)] (6#i#10)
MT N990
N115
XE-2
0.008 0.85
0.032 0.50
0.183 0.62
0.47
0.36
0.51
0.118
0.214
0.335
1.78
1.40
1.22
Accordingly, it can be expected that the proportion of entities of type II is significantly lower in fluorinated thermal black than in fluorinated XE-2.
3.3. Comparison between r.f. plasma and F2 -gas fluorination The shape of the C1s envelopes of fluorinated carbon blacks obtained by both methods are not so different from each other. Moreover, when fitting the F1s envelope of the XPS spectra of F 2 -gas fluorinated N115 furnace black a component, F1s (4), was individualized, which was assigned to physisorbed F 2 [6]. In the case of MT N990 thermal black and XE-2 high electrical conducting black, such a component was not observed. In addition to fluorine species that are covalently bound to carbon of the surface and subsuperficial zones of the particles, non-reacted F 2 can thus exist in the micropores of the superficial zone of N115 particles. In the case of plasma fluorination, the F1s envelope shows only components corresponding to covalently bound
1900
T. Shirasaki et al. / Carbon 37 (1999) 1891 – 1900
F atoms for the three types of carbon blacks. Fluorine radicals are so active that they immediately react to give covalent carbon–fluorine bonds in the surface or superficial zone of the particles. The formed radicals being unstable out of plasma conditions, no F species can be trapped.
4. Conclusions We have investigated by XPS the plasma fluorination of three types of carbon blacks: a thermal black, a furnace black and a high electrical conducting black. The amount of fluorine that reacts with the surface of these carbon blacks depends on the reactivity of superficial and subsuperficial zones of the materials, i.e. on the morphology (microstructural order) and on the organization (graphitic character) of these particles. This trend is similar to that already observed in F 2 -gas treated carbon blacks. From the analysis of the XPS spectra of fluorinated, non-activated N115 carbon black samples, we conclude that most of F fixed is covalently bound to ‘reactive’ C of the external and superficial part of the particles. The majority of the structure consists therefore in fluorinated islands of type I structure where the planar conformation of the graphene layers is preserved. However, fluorinated islands of type II structure also exist, some of them exhibiting a significant charge effect. The main effect of the activation of the initial carbons is to create conditions that favor the diffusion of F species within the bulk of the materials. We also conclude that there are no fluorine atoms physisorbed in the micropores of the superficial zone of particle after PEF treatment, whereas trapped fluorine atoms have been detected in the case of F 2 -gas fluorination. The differences in morphological characters, structural order and graphitic characters of the three types of carbon
blacks affect the ratio of type I / type II structures of the fluorinated carbons. For higher structural organization of the particles, i.e. for XE-2 blacks, the proportion of type II increases and the F / C ratio in fluorinated functional groups (CFx ) with x51, 2, 3, decreases. In this case the transformation into polyalicyclic perfluorinated structures of (CF) n composition, similar to covalent graphite fluoride, is achieved more easily whereas in carbon blacks with lower crystallinity, the presence of numerous defects leads preferentially to CF 2 and CF 3 entities.
References [1] Nakajima T. Fluorine–carbon and fluoride–carbon materials, New York: Marcel Dekker, 1995. [2] Watanabe N, Nakajima T, Touhara H. In: Graphite fluorides, Amsterdam: Elsevier, 1988, p. 65. [3] Watanabe N, Kita Y, Mochizuki O. Carbon 1979;17:359. [4] Nakahara M, Ozawa K, Sanada Y. J Mater Sci 1994;29:1646. [5] Nanse´ G, Papirer E, Fioux P, Moguet F, Tressaud A. Carbon 1997;35:175. [6] Nanse´ G, Papirer E, Fioux P, Moguet F, Tressaud A. Carbon 1997;35:371. [7] Nanse´ G, Papirer E, Fioux P, Moguet F, Tressaud A. Carbon 1997;35:515. [8] Turban G. Ph.D. Thesis, University Nantes, 1981. [9] Plumb IC, Ryan KR. Plasma Chem Plasma Process 1986;6:205. [10] Magro C, Tressaud A, Lozano L, Hudakova N, Cardinaud C, Turban G. J Mater Sci 1994;29:4225. [11] Papirer E, Lacroix R, Donnet JB, Nanse´ G, Fioux P. Carbon 1994;32:1341. [12] Clark DT, Fast WJ, Kilcast D, Musgrave WKR. J Polym Sci 1973;2:389. [13] Nakajima T, Watanabe N. Graphite fluorides and carbon– fluorine compounds, Boca Raton, FL: CRC Press, 1991. [14] Moguet F. Doctorate Thesis, University Bordeaux I, 1996.