Inelastic neutron scattering study of proton dynamics in carbon blacks

Carbon Vol. 34, No. 7, pp. 903-908, 1996 Copyright 0 1996 Elsevier Science Ltd Printed inGreatBritain. All rightsreserved 0008-6223/96 $15.00+ 0.00

Pergamon SOO0t.b6223(96)00042-5

INELASTIC

NEUTRON SCATTERING STUDY OF PROTON DYNAMICS IN CARBON BLACKS

P. ALBERS,~‘* G. PRESCHER,~ K. SEIBOLD,~ D. K. Rossb and F. FILLAUX~ “Degussa AG, Corporate Research Functions, P.O. Box 1345, D-63403 Hanau, Germany bDepartment of Physics, University of Salford, Joule Laboratory, Salford M5 4WT, England ‘Laboratoire de Spectrochimie Infrarouge et Raman, Centre National de la Recherche Scientifique, 2 rue Henry-Dunant 94320 Thiais, France (Received 1 November 1995; accepted in revised form 12 February 1996) Abstract-Inelastic neutron scattering spectra from 16 to 4000 cm-’ of various carbon blacks (rubber blacks, graphitized black, post-oxidized gas black) and activated carbon at 30 K are presented. They reveal new aspects of proton dynamics in these samples. The nearly proton-free graphitized sample is similar to pure graphite. All spectra reveal a continuum of intensity which is attributed to the recoil of free protons inserted in the basic structural units. On the top of the continuum, more or less well-defined bands are attributed to protons either chemically bound at the border of the basic structural units or, in

the case of the post-oxidized gas black, trapped in topological defects. The activated carbon is dominated by bands due to OH groups. The integrated intensities are not simply related to proton concentrations given by analytical

techniques.

Copyright

0 1996 Elsevier Science Ltd

Key Words-Inelastic

neutron

scattering,

carbon

black, proton

hydrogen on the nanostructure of carbonaceous materials is well-known [S]. Variations in hydrogen content have been predominantly seen as surface related properties of carbon blacks [9] but it is still not absolutely clear whether hydrogen in the bulk has any additional impact on the in-rubber performance of the carbon aggregates. Experiments on charcoal and carbon blacks have revealed that about 30% of the integral hydrogen content is retained in the material after evacuation at 1200°C. The other hydrogen atoms are desorbed as H,O or HZ entities. It was suggested that a part of the hydrogen is associated with carbon atoms in the interior of the carbon particles [l,lO]. This conclusion is supported by recent results from secondary ion mass spectrometry (SIMS) under quasistatic or dynamic conditions. This technique revealed that the formation of fragment ions such as CH-, C,Hand especially C,H; is not strictly correlated to, e.g. the relative amounts of chemically active surfacehydrogen, as determined by chemical titration techniques [ 1 l] and photoelectron spectrometry (XPS), at the topmost atomic layer [ 12,131. Fragmentation profiles which were obtained by eroding the surface of carbon blacks by means of inert gas ion bombardment showed a decrease of the CHand C,Hsignals whereas the relative intensity of the C,H; fragment increased. It follows that considerable amounts of hydrogen are inside the material [ 121. However, the chemical nature of these hydrogenous entities and their localization are largely unknown. Therefore, the physico-chemical properties of hydrogen inside the structure of the microcrystalline carbon black particles deserve further studies. Inelastic neutron scattering (INS) has been shown to be a straightforward method for investigating

1. INTRODUCTION

Numerous parameters govern the technical properties of carbon blacks. In particular the micromorphology of the surface, the crystallinity and the amount of hydrogen-containing structures at the edges of the basic structural units can play an important role with respect to the reinforcing potential of rubber blacks. During the production of carbon blacks in a reactor, the formation and growth of carbon particles is associated with a dehydrogenation process C,H, + O2 -+ carbon black + CO + CO, + H2 + HZ0 (1) The hydrogen content of carbon blacks is in the range of about 0.01-0.7 wt% [l]. The feedstock composition, the furnace temperature and the quench parameters govern the crystallite size and therefore the relative amounts of crystallite edges at the surface. Edges provide the active sites in terms of surface energy and adsorption activity as determined by inverse gas chromatography [ 2,3]. According to the new microstructural model of carbon blacks [4], proposed on the basis of scanning tunnelling microscopy (STM) [S] and other techniques, edges could be of importance with respect to the interaction between the surface of a carbon black particle and the polymer chains in the reinforcement of rubber materials. The terminating hydrogen atoms at the edges of the lattice planes and the degree of order/disorder of the basic structural units were considered to contribute to the in-rubber performance of carbon blacks [4,6]. Partial graphitization of carbon blacks leads to a decrease of the reinforcing properties [7]. Furthermore, the influence of

*To whom correspondence

should

dynamics.

be addressed. 903

904

P. ALBERSrt trl.

proton dynamics in carbonaceous materials such as bituminous and anthracite coals [14-161. The scattering cross-section of the hydrogen atom is more than one order of magnitude greater than those of other common atoms (e.g. C, 0, N,...) so this technique is uniquely suited to the observation of vibrational spectra of hydrogenous species in non-hydrogenous matrices. Even for concentrations of the hydrogenous species as low as - 1 wt% the matrix can be regarded as virtually transparent. Therefore, even samples as thick as - 1 mm or more can be studied with INS which effectively probes the whole sample. There are no selection rules and, in principle, band intensities are simply related to the amount of the corresponding hydrogenous species, Previous INS work on coals [ 14-161 revealed the presence of bound as well as free protons inside this material. Whilst chemically bound protons give broad bands essentially due to C-H-vibrations, free protons give a continuum of intensity due to recoil. Coals of different ranks contain different proportions of free and bound protons [ 14,151. Simultaneous measurements over large energy and momentum transfer ranges revealed that the recoil spectrum can be modelled with a gas of free (i.e. unbound) protons (effective mass 1 amu). H’ entities are supposedly intercalated in the basic structural units, whilst the associated electrons are delocalized in the conduction band of the graphite-like structure [ 161. Since coals and carbon blacks are formed under quite different conditions, it is interesting to compare proton dynamics in these materials. INS studies of various carbon blacks reported in this paper give a deeper insight into the proton dynamics for different grades of technical carbon blacks. Free protons are observed. 2. EXPERIMENTAL

INS spectra of wet-pelletized furnace blacks (Coraxs N550, N220, and EBllO, EBlll), and nonpelletized, “fluffy” samples (N234 and FW200, Degussa) were recorded. They were selected with respect to their different properties and technical applications. The furnace blacks Coraxa’ N550, N220, N234 and the EB samples, which are modified versions of the carbon black grade N220, are typical rubber blacks. The Corax’@ N550 is used in tyre carcasses, mechanical rubber goods and extrusion compounds. The N220 is used in, e.g. tyre treads for trucks; N234 is used in tyre treads for high-speed cars, railroad pads or conveyor belt covers. The surface concentrations of oxygen on these blacks obtained with X-ray photoelectron spectroscopy (XPS) range from about 0.8-2.0% of area. The concentrations of non-extractable sulfur on the surface were in the range of 0.220.8% of area. Wet-pelletized samples were extracted for 24 hours using a methanol/water mixture to remove sulfurcontaining pelletizing agents. XPS reveals that only

chemically bound sulfur species with binding energy around 163-164eV were left on the surface. Afterwards, the samples were dried and extracted with toluene for 24 hours to remove tar and extractable organic components. The gas black FW2008 has a comparatively high surface area. It is used as a black pigment, e.g. for jet-black coatings and plastics. The surface has been postoxidized under controlled conditions to establish enhanced amounts of surface functional groups [ 171. XPS revealed the presence of about 15% of surface oxygen, predominantly in chemical active groups such as carboxylic and phenolic structures (pH - 2.5). SIMS investigations suggest that the controlled surface oxididation leads to serious modifications in the selvedge regions and to the hydrogen containing structures [ 121. The dry-pelletized and the fluffy samples were extracted with toluene only. A sample of the hard furnace black N234 was graphitized at 3000 K in an inert atmosphere for 48 hours (Ringsdorff-Werke, Bonn, Germany). A sample of activated carbon (steam-activated pine wood, powdered. washed with HCI and water according to a standard procedure [ 131 and dried irt tutto) was measured. This sample was not extracted with organic solvents. The carbon blacks are specified in Table 1 according to the corresponding ASTM number or trade name and nitrogen surface area. Furthermore, the mean crystallite size determined by statistical evaluations of transmission electron micrographs and the integral hydrogen concentrations measured with a Leco RH-402 analyzer at a heating rate of 4 KS ~‘. are given. The INS spectra were measured using the TFXAspectrometer (time focused crystal analyzer) at the spallation neutron source (ISIS) at the Rutherford Appleton Laboratory, UK. The spectrometer allows geometric time focusing and software energy focusing, giving a rather flat energy dependence of the resolution of around 3% in AE;‘E. Each sample (- 10 g) was wrapped in thin aluminium foil in the form of a flat plate and was loaded into a refrigerator at 30 K. The samples were not compressed. A counting time of about 8 hours for each sample was found to be the best compromise between statistics and beam time available. Spectra were renormahzed to 20 g of sample in the beam. 3. INS SPECTRA

In Fig. 1 (a-f) the INS spectra of the furnace blacks are compared. NB: the intensity scale of the spectrum of the graphitized N234 (Fig. I (a)) is expanded. Figure 2 (a, b) shows the spectra of the activated carbon and the oxidized gas black. The INS spectrum of the N234 graphitized black (Fig. l(a)) is very similar to that of pure graphite. The sharp band at - 100 cm-i is due to lattice modes, The INS spectra of the rubber blacks, pelletized or

Inelastic

neutron

scattering

study of proton

Table 1. Characterization

of the carbon

dynamics samples

INS integrated Particle Name

(3;;)

N550 N220 EBllO EBlll N234 fluffy FW200 Activated carbon

42 115 118 118 122 460 1250’

in carbon

blacks

and INS intensities intensities

(au)

sizeb

(nm)

Total

46 24 25 25 20 12 f

283Ok24 3390_+28 3671 k26 3140+20 3680_+ 30 10130+50 1466Ok45

Continuum 1990*35 2275 k 29 2525 k 28 216Ok23 2590_+40 6780+70 9960 k 60

905

Bands

Graphitized black subtracted

H” (ppm)

840 k 35 1115*40 1146+38 980&30 1090+50 3350+80 4700+75

1310&30 1870&33 2150532 1620_+27 216Ok34 8607&51 1324Ok47

3103 4458 4010 4189 3835’ 7680 14550

a Nitrogen surface area, determined according to Brunauer/Emmet/Teller. b Medium particle size, determined by transmission electron microscopy. ’ Elemental analysis. d Hydrogen value after graphitization 290 ppm. e With respect to the BET constant being >200 the “BET” surface of this micorporous material according to DIN 66131 is given only for orientation. r Not determined.

0

1000 Energy

2000 Transfer

3000 (cm’)

4000 Energy

Transfer

(cm~‘)

1000 Energy

2000 Transfer

3000 (cm’)

0 0

1000 Energy

2000 Transter

3000 (cm])

4000

4000

I I

0

’ 0

J

1000 2000 Enci-gy Transfer

3000 (cm-‘)

4000

0

1000 Energy

2000 transfer

3000 (cm’)

4000

Fig. 1. INS spectra of furnace blacks: (a) Corax N234, graphitized; (b) Corax N234 (original, fluffy); (c) Corax N550; (d) Corax N220; (e) EBllO; (f) EBlll. The spectra were recorded at 30 K and were scaled to 20 g of sample.

P. ALBERSIY~~.

906

_I

IO00 Energy

2000 Transfer

3000 (CC’)

-1000

3000

100(1

(b)X

0

1000 2000 Enerclv 2. Tranrfer

(cm

’1

Fig. 2. INS spectra of activated carbon (derived from pine wood), (a) and of the gas black FW200 (fluffy), (b). The spectra were recorded at 30 K and were scaled to 20g of sample. not,

in

principle

are

similar

to

each

other

(Fig.

1

(b-f)) and to the INS spectra of coals [ 14,151. They reveal rather broad bands, as expected for such complex materials of great disorder. These bands are superimposed on a continuum with almost constant intensity over the whole spectral range. The following interpretation can be proposed on the basis of previous work on coals. The continuum of intensity is due to proton recoil. It reveals the existence of protons which are not chemically bound to any atom. They are free in as much as there is no evidence for any bound vibrational state in a local potential. It has been shown previously [16] that these protons behave as a gas of free (unbound) particles with mass 1 amu. The broad bands, on the other hand. correspond to vibrations of protons chemically bound to heavy atoms (C, 0, S,...). According to the assignment scheme proposed for coals the prominent bands at 880 and 1170 cm-’ correspond to aromatic CH bending modes. The dip of intensity at - 1000 cm-’ can be regarded as specific to relatively large polyaromatic species with graphite-like structure. The broad bands centered at 2900- 3100 cm-’ correspond to proton stretching modes. The sharp bands at - 100 cm-i are due to lattice modes of the basic structural units with graphite-like structure. For the various carbon blacks, significant differences appear in the 1300-1500 cm- ’ region where bending modes of non-conjugated CH groups are

anticipated. The concentration of non-conjugated groups is a maximum for the Corax N220 whose spectrum (see Fig. l(d)) resembles that of lignite (Fig 1 in ref. [ 141). The rather weak band at - 220 cm-’ is tentatively assigned to the CH, torsional mode. The other spectra (Fig. 1(b. c. e, f ) are similar to those of coals with higher ranks. However, for carbon blacks the sharp bands at 4 100 cm- ’ are always well separated from the broad density of states at low frequency whilst these bands are barely visible for coals. The graphitization degree is greater for carbon blacks than for coals. The INS spectrum of the finely dispersed gas black FW200 (fluffy sample) is quite different (see Fig. 2(b)). There is still a continuum extending over the whole frequency range with constant intensity which is due to proton recoil. But, on the top of this continuum, it is impossible to distinguish any band structure similar to those discussed above in the proton bending mode region. Instead of this, there is a broad band, structureless and asymmetric, whose intensity arises at - 500 cm-i and decreases smoothly to vanish at - 2000 cm i. There is virtually no dip of intensity at - 1000 cm-‘. A very weak proton stretching band can be distinguished at - 3100 cm-i. Only a broad density of states is observed at low frequency. The rather sharp band at 100 cm-’ due to graphitelike entities is barely visible. This sample is extremely disordered and there is virtually no basic structural units of significant enhanced spatial extent such as in graphite or the furnace blacks. The broad band between 500 and 2000 cm-’ does not correspond to protons bound to polyaromatic entities as for the other carbon blacks or coals. It seems that these protons are trapped and experience a broad distribution of local potentials, presumably, related to a broad distribution of rather small carbonaceous entities. The existence of chemical bonds such as C-H. O-H, S-H, etc., cannot be ascertained from the spectrum in Fig. 2(b). However, XPS, chemical titration techniques and pH measurements reveal a comparatively high amount of chemically active groups at the surface of this colour black [6,12]. This INS spectrum suggests that the chemically active groups are trapped protons rather than OH groups such as carboxylic or phenolic entities. This point has to be studied in detail by additional INS experiments on unoxidized and oxidized blacks. The INS spectrum of the dried activated carbon (Fig. 2(a)) is quite different from all the previous ones. This is due to the high porosity and adsorption capacity of this material and to the fact that this sample was dried but not thoroughly extracted. The spectrum is dominated by rather strong bands at 560 and 50 cm-i which may correspond to OH containing functional groups and water molecules with icelike structure. The features at 50 cm 1 might be acoustic modes. The librational modes in ice appear in the range of 500-850 cm-i [ 181. The weak bands between 1600 and 1750 cm ’ are consistent with OH bending modes. The shoulders at - 900 cn~‘, the

Inelastic

neutron

scattering

study of proton

band at 1200 cm-’ and the dip of intensity at 1050 cm-’ suggest a large proportion of basic structural units. The lattice modes anticipated at 100 cm-’ are presumably hidden by the intense peak at 50 cm-‘. There is also a continuum of intensity due to proton recoil. The ice-like structure with the sharp band at 50 cm-’ is rather surprising since water in the pores should not organize into crystalline ice. Therefore, either the water molecules are not inside the pores, (i.e. they are mostly on the surface), or the occupied pores are large enough to accomodate a multidimensional arrangement of water molecules. A final interpretation will require additional neutron work on activated carbons as done in studies on the vibrational properties of ice in porous Vycor glass containing pores of about 3 nm [19].

4. DISCUSSION

INS spectra reveal a striking difference between the furnace blacks (Fig. 1 (b-f)) on the one hand, and the gas black (Fig. 2 (b)) on the other. These proton dynamics reflect extremely different structures for the carbonaceous matrix. By analogy with coals, rubber blacks can be seen to consist of basic structural units with graphite-like structures, clearly identified with their lattice modes at - 100 cm-‘. Most of the bound protons form CH chemical bonds at the border of these polyaromatic entities. These protons give typical features such as bands at 880 and 1170 cm-l and the dip of intensity at - 1000 cm-‘. The graphite-like domains are sufficiently extended to have electronic band structures. However, in contrast to the ideal semiconductor graphite structure, there are additional electrons in the conducting band of the basic structural units. The charge-compensating protons are delocalized and behave as free particles. Their INS signature is recoil. The origin of these delocalized electrons and protons can be seen in the formation of sp3 singularities in the basic structural units during the production process. These singularities transform to sp’ states by releasing delocalized electrons and protons. Proton recoil is also observed for the gas black sample (Fig. 2(b)). Therefore, some protons and electrons are delocalized over graphite-like structures. The absence of features characteristic of protons bound at the border of these graphite-like domains suggest the existence of localized defects. These may arise from topological defects due to small non-sixmember-rings or chemical active groups in the domains or at the border. Electrons released by singularities in these defects are delocalized over only a limited number of atomic sites to create local potentials which trap the protons. The broad distribution of proton frequencies reflect the great variety of topological defects in the basic structural units. Supposedly, this is at least partly a consequence of the postoxidation treatment. Another remarkable piece of information provided by the INS spectra concerns dynamical interactions

dynamics

in carbon

blacks

907

between protons and the carbonaceous matrix. The sharp lattice mode at - 100 cm-’ is almost unaffected by protons in rubber blacks (Fig. l(b-f)) compared to the graphitized proton-free sample (Fig. l(a)). Spectra subtractions confirm a nearly perfect compensation in this region. Therefore, proton dynamics, either bound or free, are totally independent of the lattice dynamics. As opposed to this, the postoxidized gas black gives great intensity below - 100 cm-’ which partially hides the lattice modes. However, dynamical coupling between protons and lattice is rather unlikely for this sample. This great intensity could be due to tunnelling like transitions in local potentials with multiple minima. Further measurements at lower energy and high resolution should address this point. The INS cross-section represents the amount of scatterers in the beam, provided that multiple scattering events are negligible. The integrated elastic and inelastic scattering intensity is roughly proportional to the hydrogen concentration but the integrated inelastic intensity depends on the hydrogen bonding. For similar bonding, the integrated INS-intensity is proportional to the hydrogen content. For carbon blacks, the concentration of protons given by the elemental analysis and other techniques is much less than 1 wt% and, consequently, the intensity scattered by carbon atoms is non-negligible. This intensity corresponds approximately to the spectrum of the N234 graphitized sample which contains virtually no significant amounts of protons (Fig. 1 (a)). Therefore, difference spectra obtained after subtraction of the spectrum of the N234 sample reflect essentially the proton contribution. The corresponding INS intensities integrated from 16 to 4000 cm-’ are given in Table 1. For the rubber blacks the measured intensities are proportional to the proton concentrations within 20% accuracy. This is similar to previous estimates for coals [ 141. The physical and chemical analyses of the FW200 gas black, on the other hand, give about twice as many protons but the scattered intensity is about three times greater than for rubber blacks. Therefore, INS suggests that the proton concentration in this sample is largely underestimated. In contrast to INS, most of the techniques used to measure the proton concentration are much more sensitive to protons at the surface than in the bulk. In previous work, direct comparison of the proton stretching mode intensities for coals and perylene was used to estimate the relative amounts of free and bound protons [14,15]. This approach is not relevant for carbon blacks. The stretching bands are very weak and their intensities cannot be measured accurately. A direct estimate from the observed INS intensities for free and bound protons is not straightforward. Many phenomena like multiple scattering, phonon wings, etc., which contribute to the observed spectra are difficult to model. Therefore, the relative amounts of bound and unbound protons are not simply related to the integrated INS intensities of the bands and the continuum which are given in Table 1.

908

P.

ALB~KS

A simple comparison of the spectra of rubber blacks and coal (anthracite from La Mure) [ 14,151 reveals that the relative intensities for bound and free protons are quite similar. Following these evaluations which were made for coals and perylene, it is thus likely that the relative concentrations, namely about 80% and 20% of bound and free protons. respectively. are rather similar for all these samples. It is important to stress that there is apparently no clear correlation between the relative amounts of free and bound protons, on the one hand, and the crystallite size on the other. TEM studies allow the rough estimation that a black such as N220 contains about 10” and N550 about 10” crystallites/g. The mean number of C atoms per crystallite are between 10J This is much greater than and lo’, respectively. estimated for the mean size of the basic structural units in coals. The observed ratios for free and bound protons in carbon blacks of various origins suggest that they contain basic structural units of rather similar size, largely independent of the furnace temperature. In contrast to amorphous hydrogenated carbon films [20] or graphites, INS data on carbon blacks give no evidence for the presence of significant amounts of molecular hydrogen.

5. CONCLUSION INS reveals proton dynamics in various carbon blacks at rather low hydrogen concentrations. As previously observed for coals, free recoiling and chemically bound hydrogen are distinguished. All the rubber blacks are composed of basic structural units with a graphite-like structure. Bound protons are likely to be at the border of these domains. Free protons are inserted in these domains and their associated electrons are supposedly delocalized in the electronic conduction band. These samples are quite similar to anthracite. The post-oxidized gas black FW200 is quite different. It appears to be much more disordered. Free protons are still observed and this suggests that the basic structural units are still sufficiently extended to have a graphite-like electronic band structure. Bound protons, on the other hand, give a broad band which is tentatively related to a broad distribution of trapping sites caused by topological defects in the basic structural units. Only comparatively low amounts of C-H or O-H bonds are observed in spite of the very small particle size and the rather high surface concentrations of carboxyl, phenolic or lactolic groups. As anticipated, the INS spectrum of the activated

et trl.

carbon is dominated by OH vibrations and free protons. Finally, for rubber blacks, the integrated INS intensities are roughly in line to the estimates of proton concentrations from other analytical techniques. However, for the post-oxidized gas black the scattered intensity is twice as great as anticipated. This result casts some doubt on the ability of analytical techniques to measure accurately all the protonic species in carbonaceous materials. ilck,lor\/rt/g~~~?Imta-We would like to thank the SERC and the RAL (UK) for access to the ISIS neutron facility. One of us (P.A.) wants to thank the support from the European Union HCM programme. REFERENCES I. J.-B. Donnet,

R. C. Bansal

and

B&k: Scirnc~ and Technolo~e~,p.

M. J. Wang, Ctrrhon

178. Marcel Dekker. New York (1993). 2. S. Wolff, M. J. Wang and J.-B. Donnet, Ruhhrr Chew. Trchnol. 64, 714 (1991). I