23P012-O
Synthesis of Nano-Carbon Fine Particles in Low-Temperature Plasma andEvaluation of their Gas Adsorption Properties
低温プラズマ中でのナノカーボン微粒子の成長とガス吸着特性の評価
Yasuaki Hayashi, Yoshifumi Kimura, Yasuhiro Masaki and Ryuhei Yamada
林 康明, 木村佳史，正木康寛，山田竜平
Department of Electronics, Kyoto Institute of Technology
Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
京都工芸繊維大学電子システム工学専攻 〒606-8585 京都市左京区松ヶ崎御所海道町

Nano-carbon fine particles were synthesized by plasma-assisted hot-filament chemical vapor deposition. Theirproperties of hydrogen thermal-desorption (TDS), nitrogen adsorption-isotherm, specific surface area, and hydrogenadsorption-isotherm were evaluated. The results showed that the height of a peak appearing between 100 and 200 ℃
for synthesized carbon fine particles in TDS was one order of magnitude higher than that for CoMoCAT, that thenitrogen specific surface area was 31 m2/g, and that the volume of hydrogen adsorbed on synthesized fine-particles at
atmospheric pressure was 2.2 and 0.13 wt% for 77 K and room temperature, respectively. They suggest that synthesizedcarbon fine-particles have a higher capacity for hydrogen storage.

1. Introduction
Because nano-caron fine particles like carbon
nanotubes (CNTs) generally have large surface
area per unit weight, they are expected to be used
for gas-adsorption material, especially, hydrogenstorage material.
Some papers have been
published that carbon single-walled carbon
nanotubes (SWNTs) show high capacity ofhydrogen storage [1-3], while different results
were also reported [4-6]. Since the structure andthe hydrogen-storage mechanism of nano-carbon
materials are complicated, the precise andrepeatable data may be difficult to obtain.
Therefore manysided analyses should be carriedout for the evaluation of hydrogen-storage
property of nano-carbon.
Wehavedevelopedanew
chemical-vapor-deposition (CVD) method of gas
phase synthesis of carbon fine particles containing
catalytic metal and SWNTs applying aradio-frequency (RF) glow discharge plasma alongwith hot-filaments (plasma-assisted hot-filament
CVD), for the purposes of longer growth andstructure modification of SWNTs by suspension ina reaction zone for a long time [7-9].
In this paper, we present the results of synthesisof carbon fine particles including SWNTs and theirproperties of hydrogen thermal-desorption, nitrogenadsorption-isotherm, specific surface area, andhydrogen adsorption-isotherm.

RF plasma was generated between groundedfilaments and an RF-induced cupper plate.
Filaments were heated up to 1800-2000 ℃ .
40-60 % ethylene diluted in hydrogen containing
the vapor of ferrocene ( Fe(C5H5)2 ) was allowed toflow toward the hot-filaments. The pressure in thechamber was maintained at 2.66-5.32 kPa (20-40Torr). The plate of RF electrode was set 15 mm
downstream from the hot filaments and placed
perpendicular to the flow of reaction gas. Powerup to 50 W was applied to the RF plate with thefilaments heated for about one hour. Preparedcarbon fine-particles were collected on an
upper-plate, which also serves as RF electrode, anda bottom-plate.

2. Experimental
Fig.1 shows the schematic diagram of the RF
glow discharge plasma system with hot-filaments.

Thermal-desorption spectroscopy (TDS) of theproperties of synthesized fine particles wereevaluated by the use of a qaudrapole mass analyzer

Fig.1. Schematic of plasma-assisted hot-filamentchemical vapor deposition system.

(QMS). Carbon fine particles of 200 mg were put
on a silica boat in a hot-wall silica tube.
Temperature was raised at the constant rate of 10oC/min. The change of partial pressure for M/q(the ratio of mass to charge) = 2 to 200 wasmeasured during temperature increase.
The nitrogen and hydrogen adsorption-isothermsof synthesized fine particles were measured by thevolumetric method using nitrogen gas in a small
stainless cell at the temperature of 77 K and roomtemperature (Fig.2).

beforehand baked in vacuum at 400 oC for 5 h,along with those for commercialized SWNTs(CoMoCAT) and multi-walled carbon nanotubes(MWNTs) for comparison. While synthesized fineparticles show smaller adsorption of nitrogen thanSWNTs, they show larger adsorption than MWNTs.
Specific surface areas were determined by the BET
plot of the adsorption isotherms to be 832, 157, and
31 m2/g for SWNTs, synthesized fine particles, andMWNTs, respectively.
Figure 4 shows a hydrogen adsorption isotherm
at 77 K for synthesized fine particles, which werebeforehand baked under the same conditions asabove. Since the saturated vapor pressure at 77 K
cannot be defined for hydrogen because of theexcess of the critical temperature, Fig.4 is plottedagainst equilibrium evaporation pressure.

Fig.2. Schematic of adsorption-isotherm measurement
system.

3. Results and Discussion
Fine particles including SWNTs were baked in
vacuum at 1000 oC for 5 h and immersed inhydrogen gas at atmospheric pressure and room
temperature for 15 h, the hydrogen gas wasevacuated, and then TDS was carried out. The
height of a peak appearing between 100 and 200 ℃
for synthesized carbon fine particles was one orderof magnitude higher than that for CoMoCAT, whichis commercialized SWNT. The hydrogen storage
capacity of synthesized fine particles was calculated
to be 0.01 wt% by the integration of the peakdetermined by TDS appearing at around 200 oC.

Fig.3. N2 adsorption isotherms for synthesized fineparticles, SWNTs and MWNTs.

Figure 3 shows a nitrogen adsorption isotherm
for synthesized fine particles, which wereFig.4. H2 adsorption isotherm for synthesized fineparticles.

The volume of hydrogen adsorbed onsynthesized fine-particles at atmospheric pressure(1013 hPa = P0) and 77 K was comparable to that ofnitrogen and was calculated to be 2.2 wt%. The
volume of hydrogen adsorbed on synthesizedfine-particles at atmospheric pressure and room
temperature was also measured and calculated to be0.13 wt%. The results suggest that synthesizedcarbon fine-particles have a higher capacity forhydrogen storage.
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