Raman spectra of polyparaphenylene-based carbon prepared at low heat-treatment temperatures

Article abstract of DOI:10.1063/1.115718

Raman spectroscopy was used to characterize the structural development of polyparaphenylene (PPP) based carbon at various heat treatment temperatures (THT), with an emphasis on the low THT regime. Experimental data show that PPP-based carbons undergo a smooth transiiton from a mostly PPP polymer-type structure for heat treatments below ～750°C to a graphitized structure at a heat treatment temperature of 2700°C. These results give insight into the unusually high specific capacities observed in lithium-PPP systems which are of particular interest for application to Li ion battery technology.

Full text of DOI:10.1063/1.115718

Raman spectra of polyparaphenylenebased carbon prepared at low heattreatment
temperatures
M. J. Matthews, X. X. Bi, M. S. Dresselhaus, M. Endo, and T. Takahashi
Citation: Applied Physics Letters 68, 1078 (1996); doi: 10.1063/1.115718
View online: http://dx.doi.org/10.1063/1.115718
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Raman spectra of polyparaphenylene-based carbon prepared
at low heat-treatment temperatures
M. J. Matthews,a) X. X. Bi, and M. S. Dresselhaus
Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. Endo and T. Takahashi
Faculty of Engineering, Shinshu University, Nagano 380, Japan
͑
Received 12 July 1995; accepted for publication 15 December 1995͒
Raman spectroscopy was used to characterize the structural development of polyparaphenylene
PPP͒ based carbon at various heat treatment temperatures ͑THT͒, with an emphasis on the low
͑
THT regime. Experimental data show that PPP-based carbons undergo a smooth transiiton from a
mostly PPP polymer-type structure for heat treatments below ϳ750 °C to a graphitized structure at
a heat treatment temperature of 2700 °C. These results give insight into the unusually high specific
capacities observed in lithium-PPP systems which are of particular interest for application to Li ion
battery technology. © 1996 American Institute of Physics. ͓S0003-6951͑96͒01808-0͔
Experimentally, graphite under ambient conditions has
Raman spectra were taken at ambient temperatures using
an upper limit for the accommodation of intercalated lithium
a standard backscattering geometry from a variety of
samples, each having a different THT. An excitation wave-
length of 4880 Å was produced by an argon ion laser source
capable of supplying 20–100 mW of power. In order to pre-
vent damage to the samples, an optical filter was used to
reduce the incident power to 0.6 mW. Several locations on
the sample surface were probed for each sample to ensure
reproducibility of each scan. Scattered light was focused into
a double grating monochromator fitted with a 300 ␮m slit.
Scattered intensities were measured using a high-resolution
CCD camera.
1
atoms up to the stoichiometry LiC . Through the introduc-
6
2
tion of dopants in graphitic systems, or the application of
3
high pressures, larger amounts of Li may be accommodated
into these carbon structures. This has created a great amount
of interest in the practical application of LiC systems which
x
may be used as active anode materials in Li ion batteries
with enhanced cell capacities.4 Although many types of dis-
ordered carbon structures have been investigated for this pur-
pose, most systems show only a modest improvement over
GICs in terms of lithium affinity. However, it has been re-
cently shown that some disordered carbons prepared at rela-
tively low THT , such as heat-treated coal tar pitch or phe-
nolic resin, but particularly PPP-based carbons, can
accommodate very large amounts of lithium with Li:C ratios
,5
The Raman spectrum of non-heat-treated PPP has been
studied extensively.9
–11
Theoretical calculations have been
used to predict the various observed IR and Raman modes of
the aromatic ground state of PPP. It can be shown that planar
PPP molecules with D2h symmetry give rise to 18 in-plane
spectroscopic modes at kϭ0 which have symmetries 5Ag
ϩ5B ϩ4B ϩ4B . Of these, the A and B modes are
5
close to 1:2. Details about the mechanism for lithium stor-
age in these carbonaceous systems prepared at such low
THT are not yet clear. In this letter, we present results on the
structural characteristics of heat-treated PPP from a partially
dehydrogenated polymeric structure at THTϭ650 °C to a gra-
phitic structure for THTϭ2700 °C. In particular, we focus on
the special nature of the carbon structure near
THTϭ700 °C, at which the highest Li affinity properties have
previously been reported.5
1
g
2u
3u
g
1g
Raman active while the B2u and B3u modes are IR active.
Experimentally, 3 strong A modes have been observed from
g
resonant Raman spectroscopy ͑RRS͒ at 1220, 1280, and
Ϫ1
Ϫ1
12
1
600 cm , with the 1600 cm line being the strongest.
4
On the other hand, graphitic samples which possess D6h
space group symmetry, yielding 6 nonzero mode frequencies
13
Polyparaphenylene was synthesized using the Kovacic
which can be enumerated as 2B ϩ2E ϩA ϩE .
2
g
2g
2u
1u
6
method producing granular PPP samples which were shaped
Only the two in-plane E2g modes are Raman active and in
into disks roughly 0.2 mm thick and 12 mm in diameter.
Conventional electric and graphite resistance furnaces were
used to heat treat the samples to various temperatures be-
tween 600 and 3000 °C in an atmosphere of high purity ar-
gon gas for 15 min. The pristine PPP polymer precursors can
be characterized as a linear ͑para-͒ assembly of phenyl
groups ͑C H ͒ , which arrange themselves in an approxi-
graphitic samples these modes produce peaks near 1582 and
Ϫ1
4
2 cm , and in the presence of disorder, will be accompa-
Ϫ1 14
nied by a disorder-induced peak near 1360 cm
.
Figure 1 displays Raman spectra and least-square fitting
results for samples heat treated at 650, 700, 750, 800, 1000,
1
500, 2000, and 2700 °C ͑hereafter referred to as HT650,
6
4 n
HT700, etc.͒. It is interesting to note that, although a strong
luminescence is expected for PPP using visible excitation
wavelengths, this effect was negligible for all samples except
HT650, which showed a relatively strong luminescence
background in the corresponding Raman spectra ͑this back-
ground was subtracted off, yielding the line shape shown in
Fig. 1͒. For low-heat-treatment samples, four primary peaks
mately planar fashion, with slightly alternating twists out-of-
plane due to the steric repulsion between hydrogen atoms
7
along the chain. Upon heat treatment, these polymeric
chains are expected to crosslink and form extended sheets as
hydrogen atoms are expelled.8
a͒Electronic mail: ibo@mgm.mit.edu
are observed near 1248, 1275, 1350, and 1610 cm . The
Ϫ1
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1

FIG. 2. Observed shift in the high frequency C–C stretch mode ͑᭹͒ as a
function of heat treatment temperature ͑left-hand side͒ and the correspond-
ing full width at half-maximum linewidth ͓͑᭺͒, right-hand side scale͔.
samples HT650–HT750 as corresponding to slightly shifted
Ϫ1
1
220 and 1280 cm modes of PPP combined with a 1335
Ϫ1
cm quinoidlike inter-ring stretching mode due to a contrac-
tion in inter-ring bond length as the PPP chains are converted
into graphitic ribbons. The 1335 cm could also be due to a
Ϫ1
FIG. 1. Raman spectra for PPP heat treated to various values of THT indi-
cated to the left-hand side of the Raman trace.
‘‘bridging’’ of the aromatic rings along the chain by, on av-
erage, more than one C–C bond, which has been observed to
610 cmϪ1 peak was fit to a Breit–Wigner–Fano ͑BWF͒ line
have the effect of stiffening the inter-ring A mode of PPP
g
1
for the case of methylene bridging groups.16 For
shape, while the others were well represented by Lorentz-
ians. As THT is increased from 650 to 700 °C, only three
lines appear, with frequencies near 1260, 1330, and 1610
THTϾ750 °C, this low frequency complex quickly forms into
Ϫ1
a single peak centered near 1360 cm , consistent with a
Ϫ1
disorder-induced peak for highly defective graphite.
Nonetheless, the progression from a PPP-dominated
spectra to that of disordered carbon is clearly shown in Fig.
cm , although a better fit is obtained if a small, but broad,
Ϫ1
Lorentzian peak near 1350 cm is included in the analysis.
Ϫ1
It would appear that the 1248 and 1275 cm peaks observed
2
. Starting with the spectrum for the HT650 sample and pro-
in the HT650 spectrum have diminished and combined as a
result of heat treatment to 700 °C, thus giving the appearance
gressing up to the spectrum for the HT800 sample, we see
the intensities in the wave number range 1200 to 1300
Ϫ1
of a single peak at 1260 cm , which was fit to a Lorentzian
Ϫ1
cm which are completely ascribed to PPP phonons, gradu-
line shape. Upon further heat treatment above 700 °C, the
multiple-peaked spectrum quickly reduces to the two-peaked
structure commonly seen in partially graphitized
ally decrease as a function of increasing THT. Furthermore,
the high frequency peak may be attributed to an interplay
between the high frequency A mode of PPP and the E
1
4,15
materials.
We see that for the HT750 sample, a very
g
2g2
broad peak which may be fit reasonably well to either the
four peak fit used for low-heat-treatment samples, or a two
peak fit consisting of BWF and Lorentzian line shapes for
mode of graphite. Indeed, both modes are expected to result
in similar frequency shifts with increasing THT, since their
origins both lie in the C–C stretch mode of sp2 bonded
carbon. Thus we should expect a smooth downshift from
modes near 1600 cm to modes near 1580 cm , and this
behavior is indeed observed in Fig. 2. Throughout the full
heat treatment temperature range from 650 to 2700 °C, it
seems reasonable that peaks originating from each species
are present, but due to their mutual proximity in frequency
and variability of the respective domain sizes with THT, sepa-
rate peaks cannot be resolved.
Ϫ1
peaks near 1600 and 1360 cm , respectively, suggestive of
Ϫ1
Ϫ1
the admixture of local PPP and graphitic regions. A structure
Ϫ1
near 1260 cm is still visible for the HT750 sample, appear-
Ϫ1
ing as a shoulder on the more prominent 1330 cm peak.
Ϫ1
Although the 1610 cm peak remains virtually unshifted for
samples HT650, HT700, HT750, and HT800, we see a
gradual decrease in intensity with respect to the other struc-
Ϫ1
tures around 1300 cm . For samples heat treated above
8
00 °C ͑i.e., HT800–HT2700͒, only two peaks appear in the
The interesting properties of HT700 samples with re-
spect to the doping of lithium may then be due to the unique
structure produced at the crossover point, signaling the car-
bonization of PPP. Along with the evidence presented here
from vibrational studies for a transformation threshold near
700 °C for ‘‘Kovacic’’ PPP, characterization of electrochemi-
cally prepared polyphenylene films shows a similar structural
behavior under heat treatment.11 In these studies, a drastic
increase in evolved H and CH is observed near 700 °C with
Ϫ1
spectrum, one near 1355 cm and the other near 1600
cm , typical of disordered carbons.
Ϫ1
15
These results are consistent with the idea that at low
THT, domains of both PPP-type and graphite-type structures
are present in the Raman spectra. Upon increasing THT, the
PPP nature of the material is drastically reduced, and the
features associated with the graphitized structure become in-
creasingly pronounced. In Fig. 1, we tentatively identify the
three low frequency peaks observed for spectra taken from
2
4
peaks at ϳ750 °C, as well as an increase in room-
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Appl. Phys. Lett., Vol. 68, No. 8, 19 February 1996 Matthews et al. 1079
On: Thu, 27 Nov 2014 09:36:50

Raman spectra of heat treated PPP show evidence of
both PPP and graphite structures. At low heat treatment tem-
peratures, PPP is more prominent in the Raman spectra. Heat
treatment above 700 °C drastically removes the majority of
the PPP character of the material, so that above 750 °C the
material shows a spectrum characteristic of a common disor-
dered carbon. Near a heat treatment of 700 °C, we find that
the peaks corresponding to PPP just begin to diminish pro-
ducing a partially carbonized species. Thus, the carbon struc-
ture present at a THT of 700 °C could be of a highly relaxed
nature with C–C distances able to accommodate larger
amounts of Li than typical GICs.
FIG. 3. A possible model for pyrolysis of PPP near 700 °C. The original
PPP chains are horizontal showing a depletion of hydrogen atoms ͑small
dots͒ and partially formed interchain bonds.
The authors thank Dr. Don Heiman for helpful discus-
sions and generous use of optical equipment at Francis Bitter
National Magnet Laboratory. Portions of this work were sup-
ported by NSF Grant 95-10093-DMR. M.J.M. gratefully ac-
knowledges the support of a NSF Fellowship covering
1994–1997.
temperature electrical conductivity by 7 orders of magnitude,
both indicating the onset of carbonization in these films.
A possible model for the accommodation of large
amounts of Li in samples prepared at 700 °C may be sur-
mised from these data. In graphite layers, the C–C bond
lengths are 1.42 Å giving a distance between hexagon cen-
ters of 2.46 Å somewhat shorter than the interatomic dis-
1
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2
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0
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11
hexagons represent fully aromatic structures derived from
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On: Thu, 27 Nov 2014 09:36:50
1