3D07
Oxidation‐induced magnetism in Nano‐graphite(Department of Chemistry, Tokyo Institute of Technology1, Ookayama, Meguro, Tokyo 152‐8551, Japan,Denso Corporation2, Nisshin, 470‐0111, Japan) VATTAKATTURAMAKRISHNAN, Rajeevkumar1; TAKAI,Kazuyuki1; HAO, Si‐Jia1; HAYASHI, Hitoshi2; TAKASHIRO, Junichi1, KANEKO, Satoshi1, KIGUCHI, Manabu1;*ENOKI, Toshiaki1,[Introduction]

The study of carbon nano‐structures have received considerable reattention since the discovery ofgraphene in 2004.This relumed the passion for intense research in this area. This is largely because of theunusual physical and chemical properties exhibited by these carbon nano‐structures. The members of thisfamily include fullerenes, carbon nanotubes and graphene. Compared to the closed shell counterparts,
the open shell nanostructures such as nanographene shows unconventional electronic and magneticproperties. The edges of a nanographene sheet can be described in terms of armchair and zigzag edges.
Nanographene sheets with edges terminated with armchair are similar to the closed shell counter parts
such as fullerenes in terms of their magnetic properties whereas sheets terminated with zigzag edges
show edge state magnetism due to the presence of nonbonding electrons of π origin. There are several
attempts to bring in additional magnetic centers apart from the edge state spins. The first report in thisdirection is the ferromagnetism observed by the irradiation of highly oriented pyrrolytic graphite withproton. Recently, it has been established that defects can induce magnetism in carbon nano‐structures
which also leads to edge state magnetism. The adjacent carbon atoms in a graphene sublattice carries
opposite spins. When functional groups are connected to adjacent carbon atoms, equal number of carbon
atoms are missing from different sublattices. This leads to a net zero magnetic moment. When alternate
carbon atoms carry the functional groups, carbon atoms are removed from the same sublattice. This leads
to a sublattice imbalance and a net nonzero magnetic moment. With graphene oxide as a target material,
we varied the degree of oxidation in order to increase the number of defects. The robust nature ofgraphite towards conventional mild chemical reactions made us to select oxidation as the preferred
method for introducing defects. The electronic and magnetic properties of nanographene oxide wereinvestigated for activated carbon fibers (ACF), which consist of 3D disordered network of nanographite
domains (stacked nanographene sheets).
[Experiment]
A series of nanographite oxide samples were prepared by modified Hummer’s method using ACFs as thegraphite precursor, varying the degree of oxidation (O/C) by changing KMnO4/ACF ratio from 1 to 4.
Nanographite oxide samples with O/C>0.6 were dispersible in water whereas samples with O/C<0.6) wereinsoluble. These samples were characterized by ESR, SQUID, NEXAFS and elemental analysis. The ESRmeasurements were done at room temperature using X‐band spectrometer. The magnetic susceptibilities
were measured with a SQUID magnetometer in the temperature range 2‐300 K with an applied magnetic
field of 1T. The magnetization measurements were done at 2K, 10K and 300K in the field range from 0 to7T. Carbon K‐edge NEXAFS measurements were done at the X‐ray beam line BL‐7A at Photon Factory‐
Institute of Materials Structure Science, KEK Tsukuba.
[Results and Discussion]
The NEXAFS spectrum of the pristine ACFs before oxidation is described in terms of the π* conduction
band contribution (285.7 eV) and the nonbonding edge state contribution (284.5 eV), the latter of which ispositioned around the Fermi level. On the basis of the analysis with the deconvolution, the NEXAFS
spectrum of the nanographite oxide revealed the presence of an additional state (oxidation induced state)(285.1 eV) near the Fermi level apart from the edge states. This oxidation‐induced state is suggested tohave nonbonding nature as it is positioned at the Fermi level. The intensity of the π* peak of theconduction electrons decreased from 2.1 to 0.78(the ratio of π* peak intensity to edge jump intensity
at 340 eV is taken as the intensity, see Fig.1a) upon increasing the degree of oxidation. This implies thatthe robust π network can be destroyed locally by employing modified Hummers method. The intensity ofthe edge state decreased from 0.62 to 0.16. The intensity of the oxidation‐induced state increased from
0.0 to 0.34 as we increased the degree of oxidation. It should be noted that the contribution of theoxidation‐induced state surpasses around O/C=0.3.

The observed susceptibility is described in terms of Curie‐Weiss type spin susceptibility s with anegative Weiss temperature and the temperature independent orbital susceptibility orb for all thesamples having different oxygen contents. From the analysis of the susceptibility, as we move from
O/C~0.04 to 0.6, the absolute value of orb decreased from 8x10‐7 emu.g‐1 to 1 x10‐7 emu.g‐1. This indicatesthat the extended π‐electronic structure is destroyed successively as the oxidation proceeds, in goodagreement with the NEXAFS results which indicates the decrease in the contribution of the π* conduction
band (Fig. 1a). The spin density (Ns) increased steeply from 0.4x1020 g‐1 to 5x1020 g‐1, which is more thanten time of that of pristine ACFs, and the absolute value of Weiss temperature decreased from 5 K to 2 K(Fig. 1c). Taking account of the presence of a crossover point, at which the major contribution of themagnetic nonbonding state is changed from the edge state to the oxidation‐induced state, the steep
increase in the spin concentration is caused by the creation of nonbonding states induced by oxidation,
while the contribution of the edge state is declining (Fig. 1a). The decrease in the Weiss temperature uponoxidation is understood by that the edge state spins having itinerant nature are interacting with eachother through conduction π‐electron‐mediated exchange interaction, whereas the localized spins of theoxidation‐induced state are isolated and free from exchange interaction. This difference is observed in theESR line width change upon oxidation (Fig. 1f). The peak to peak width (ΔHpp) decreased from 2 mT to 0.7
mT as we move from O/C~ 0.04 to 0.6. This suggests that the edge‐state spins coupled through exchangeinteraction are affected by spin fluctuations, in contrast to the negligible such effect in the spins ofoxidation‐induced state.
For the oxidized samples, the observed magnetization curves deviates from the behavior ofnoninteracting S=1/2, and has values larger than those expected from S=1/2 Brillouin curve in the low
field range below ca.20000 Oe in Fig.2. This trend is emphasized upon the increase in the degree ofoxidation. This suggests the presence of ferromagnetic interaction acting between localized spins ofoxidation‐induced nonbonding states. In the sample most highly oxidized O/C=0.6, the average strength of
the ferromagnetic interaction is estimated as the coefficient of internal field, α=1.9×103 Oe.emu‐1.g. It isconsidered that the spins strongly coupled through ferromagnetic interaction are free from thesurrounding. Selective hydroxylation of benzenoid rings and unzipping of all‐benzenoid nanographenesheets into oxygen‐joined fragments
are the possible candidates for the observed magnetism.
mT2.0

Hppf21.5

orb0-3e orb (emu.g-1)x 10-7-6

1.5

SMagnetization (emu/g)

1.0

Seffectived

1.0

(K)

0.5-1-2-3-400

NsSpin Density (g )x10

Intensity

2.0

1.0
0.5

NEXAFS

0.2

aoxidation-induced state
O/C

0.4

40000

60000

Figure 2. The black dots show the experimental
magnetization data for nanographite oxide sample
with O/C=0.6. The solid line shows the fit using
3
ferromagnetic internal field of α=1.9 × 10
‐1Oe.emu .g. The dashed line shows the fit withBrillouin function assuming S=1/2.

* peakedge state

20000

Field(H,Oe)

b20

2

1.5

1c

Weiss Temp.

-14

M (Brillouin Fit)
M (Internal Field)
M (Experimental)

0.6

Figure 1. The variation of the edge state, oxidation‐induced state, π* peak, spin density, Weisstemperature, orbital susceptibility and peak topeak width (ΔHpp) with O/C ratio.