3D10

HNO3 Adsorption Effect on the Magnetism of Nanographite in
Activated Carbon Fibers(Dept. of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology)٤ Si-Jia Hao, Kazuyuki Takai and Toshiaki Enoki

1. Introduction
Activated carbon fibers (ACFs) are featured with a network of nanographite domains, and each ofthem consists of a stack of 3–4 nanographene sheets with a mean size of about 3 nm. Different from thenanotubes and fullerene, nanographene possesses a two-dimensional extended electronic structure of!-electrons with open edges. According to theoretical calculations and experimental results, a strongly
spin-polarized nonbonding !-electron state called edge state exists in a zigzag edge. This gives rise tounconventional nanomagnetic features to the nanographene. Between the nanographite domains, thepresence of slit-shaped nanopores in ACFs results in the ability to accommodate large amounts guest
molecules which interact with nanographite. Thus the host-guest interaction plays an important role inmodifying the electronic and magnetic properties of the host nanographite.
In this research, the HNO3 adsorption effect on the magnetism of nanographite in activated carbon
fibers is investigated.

2. Experimental
Phenol-based ACFs (Kuraray Chemicals) with specific surface areas of 2000 m2/g were heat-treated at
473 K for 24 h in vacuum of 10–6 Torr to remove adsorbed foreign gaseous species, and were then sealed ina Pyrex tube. The concentration of HNO3 in ACFs is controlled by the mass of ACFs and the volume of
HNO3 gas vaporized from concentrated nitric acid (>97%), which was purified by the freeze-pump-thaw
method prior to the adsorption process. Magnetic susceptibility measurements were carried out using aSQUID magnetometer (Quantum Design MPMS5) in the field of 1 T between 2 and 300 K with about 20
mg of each sample. ESR measurements were performed with a conventional X-band ESR spectrometer(JEOL JES-TE200) for typically 2 mg samples sealed in a quartz sample tube in the temperature range
between 3.5 and 300 K.

3. Results and Discussion
The susceptibility of pristine ACFs andHNO3-ACFs can be described in terms of a combinationof a temperature-dependent Curie-Weiss term "C and atemperature-independent term "0 in the wholetemperature range and the temperature range below
ca.200 K, respectively, as represented by "obs = "C + "0with "C = C/(T – #), where C is the Curie constant, and #is the Weiss temperature (Fig.1 (a)). Fig.2 (a) shows thatthe spin concentration calculated from Curie constantdecreases as the concentration of HNO3 in the ACFs
increases. As the strong oxidation ability, HNO3Fig.1. The temperature dependence of theproduct of "cT for HNO3-ACFs (a) and the NOdecomposition contribution (b).

Fig.2 The HNO3 concentration dependence of thespin concentration (a) and additional magnetic
moment (""cT) contribution at 300K (b) in
HNO3-ACFs

Fig.3 The HNO3 concentration dependence ofg-value (a) and line width (b) obtained from ESR
measurement at 3 K and 300K.

molecules are subjected to charge transfer with nanographite domains, and then the edge-state spin density
decreases in a manner of two-step process; charge transfer with the exterior nanographene sheets of thenanographite in the first step, and intercalation into the galleries between nanographene sheets of thenanographite in the second step.
Above ~200 K, the magnetic moment of HNO3–ACFs shows an additional contribution, which grows
when the temperature is elevated as shown in Fig.1 (b). The additional contribution is ascribed to thecontribution of magnetic NO molecules. Above 231K the melting point of HNO3), the decomposition ofHNO3 vapor can be represented by the overall equation 4HNO3!4NO2+2H2O+2O* accompanied with theequilibrium 2NO2!2NO+O2. Therefore, the decomposition reaction of HNO3 results in the creation ofmagnetic NO molecules with S=1/2, which are condensed into the nanopores surrounded by nanographite
domains. The decrease in the magnetic moment upon the lowering of the temperature is brought about by
the dimerization reaction, 2NO!(NO)2, in which the strong antiferromagnetic exchange interaction
between NO molecules is responsible for the stabilization of the singlet spin state in (NO)2 at low
temperature. Although HNO3 decomposition is accompanied with the generation of oxygen, the magnetic
O2 molecules (S=1) are not observed in the "cT-T plots. This suggests that the evolved oxygen works tooxidize the edge of nanographene. In Fig.2 (b), the additional magnetic moment contribution at 300K
shows a maximal peak at HNO3/C~0.027, mainly because the decomposition effect of HNO3 molecules.
When HNO3/C<0.027, NO is the major decomposition products and the amount of NO increases with theacid content. When HNO3/C>0.027, the amount of NO is decreased because the oxidization effect to form
NO2.
The HNO3 adsorption into ACFs gives rise to the decrease in linewidth and the increase in the g-value
at both 3 K and 300 K as shown in Fig.3. The increase in the g-value reflects the enhancement of spin-orbit
coupling caused by the interaction between nanographene and the HNO3 molecules [1]. The decrease of theESR linewidth can be attributed to the enlarged distance between edge-state spins due to the reduced spin
density upon the increase in HNO3 in ACFs.

Reference:[1] Nakajima, M., K. Kawamura, et al. (1988). Journal of the Physical Society of Japan 57(5): 1572-1575.