J. Plasma Fusion Res. SERIES, Vol. 8 (2009)

Enhancement of Microplasma Generated in Water
by Adding Carbon Nanotubes
Usama KHALED*, Kiminobu IMASAKA and Junya SUEHIRO

Graduate School of Information Science and Electrical Engineering, Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan(Received: 31 August 2008 / Accepted: 10 December 2008)

In this study, microplasma was generated in the distilled water, single-walled carbon nanotubes (SWCNTs) andgraphite powder suspensions by using pulsed streamer discharge technique respectively. The phenomenon of microplasma
in these suspensions was detected by using ICCD camera. Photograph of microplasma in the SWCNT suspension revealed
that the microplasma was very stable and number of the discharge channel was dramatically increased, while the discharge
in both of distilled water and graphite powder suspension were weak and unstable. It was experimentally concluded thatthe carbon shape and size have a direct effect on microplasma enhancement in water. Then the electric field at the tip ofSWCNT was calculated by using simple plane to plane electrodes model. According to the theoretical results, the induced
electric field was significantly enhanced when SWCNT exists on electrode. These results suggest that addition of carbon
nanotubes in water could be useful for inception and activation of microplasma. In addition, enhancement of microplasma
could represent an effective method for water purification processes such as sterilization and removal of organic pollutants
and bacteria.
Keywords: microplasma, carbon nanotube, streamer discharge, aspect ratio, solubilizationand plane electrodes which immersed in distilled waterby using a home-made pulsed power generator based on aBlumlein transmission line as shown in Fig.1 [3]. The gap
length between electrodes, applied pulse voltage, andpulse repetition rate were 14mm, 40kV, and 15Hz
respectively.

1. Introduction

CNT is one of the very promising nanomaterials invariety fields. The poor solubility of CNTs in water ororganic solvents limits their use in many potential
applications, especially in biochemical science andcomposite material engineering. We have previously
revealed that H� and O* radicals produced bymicroplasma (pulsed streamer discharge) in water played
an important role in the water solubilization of SWCNTs[1]. A pulsed streamer discharge is a nonthermal plasmaand can produce high energy electrons, a high electricfield, active chemical species, ultraviolet rays and shockwaves in water [2]. Sun et al. reported that the formationof chemical active species due to streamer corona
discharge was very dependent on physicochemicalparameters such as pH, conductivity of the water andadditives [3]. We have also reported that the phenomenaof microplasma and the radicals intensity were criticallyaffected by solution electrical conductivity [4].
In the present study, effects of adding SWCNTs towater on microplasma phenomenon were investigated. Inorder to clarify effects of shape and size of carbon
materials, graphite powders were also employed forcomparison.

2. Experimental set-up

Fig.1

The microplasma was produced between a thin wire

Experimental set-up.

author’s e-mail: usamakhaled2003@yahoo.com

603©2009 by The Japan Society of Plasma
Science and Nuclear Fusion Research

U. Khaled et al., Enhancement of Microplasma Generated in Water by Adding Carbon Nanotubes

In order to clarify effect of adding small amount ofcarbon materials on microplasma enhancement, theSWCNTs or graphite powders were suspended in water atconcentration varied from less than 1μg/mL to 10μg/mL.
SWCNT had 1-2 nanometers diameter and 2-5
micro-meters of length, while the size of graphite powder
typically ranges from the order of several micro-meters
and up to sub mille-meters. An ICCD camera was used todetect microplasma in each case of water, graphite
powders and SWCNTs suspensions at different
concentrations. A spectrometer was used in order tomeasure the optical emission spectrum intensity fromproduced microplasma.

50

Applied voltage (kV)

403020ICCD Expousure time
10300 ns

0-0.1

00.1

0.2

0.3

0.4

0.5

Time (�s)Fig.2 Applied voltage waveform.

3. Results and Discussion

The pulsed power generator could generate a squarepulsed voltage, which was applied to a thin wire electrode.
Fig.2 shows applied voltage waveform to wire electrodewith a pulsed width of 300ns. The exposure time of ICCD
camera was set at the same pulse duration time. First ofall, it was observed from ICCD images that microplasma
which produced in distilled water was very weak,
unstable along the wire electrode, the number ofmicroplasma channels were low and its length were very
short as shown in Fig.3 (a). By adding a small amount ofgraphite powder of about 10μg/mL concentration todistilled water, microplasma developed and the number of
streamer channels was little increased compared to
distilled water case as shown in Fig.3 (b). WhenSWCNTs, of same concentration of graphite powder
suspension, was added to distilled water, microplasma
became stronger, streamer channels was dramaticallyincreased, propagated vertically toward plane electrodeand its length became longer as shown in Fig.3 (c). For
more clarification, an Image-J software program wasused in order to process images of microplasma
photographs shown in Fig.3. The processed images areshown in Fig.4. It provides relationship between
microplasma intensity and distribution of streamer
discharge channels along the wire electrode and candistinguish the difference and stability of developed
microplasma in each suspension. Moreover, it is possible
to evaluate the number of streamer channels. Fig.5 givesa reasonable proof that number of streamer channels
which were counted from Fig.4 was highly affected byadding carbon nanomaterials. Actually, a clear differenceof microplasma enhancement by adding SWCNTs towater was confirmed at low concentrations which lessthan 1μg/mL. The number of channels, in case ofSWCNTs suspension, was two or three times higher thanthat in case of graphite powder suspension under thesame powder concentration in water.
In order to verify the effect of treatment bymicroplasma on particle size of carbon nanomaterials,
Fig.6 shows effective particle size distribution for both(a)PlaneelectrodeH.V wire electrode

5 mm(b)(c)

Microplasma intensity (a.u.)

Fig.3

Typical ICCD images of microplasma in(a) distilled water, (b) Graphite powder
suspension (10μg/mL) and (c) SWCNTs
suspension (10μg/mL).

(a)

(b)

(c)

Streamer channels position along the wire (mm)Fig.4 Image processed of microplasma intensity in(a) distilled water, (b) Graphite powder
suspension and (c) SWCNTs suspension.

604

U. Khaled et al., Enhancement of Microplasma Generated in Water by Adding Carbon Nanotubes

texture according to SEM and TEM observations [1,5,6].
From the results shown in Fig.6, it was clear thatmicroplasma treatment effectively reduced the particle
size of graphite powder and SWCNTs. Also, graphitepowders had a larger effective particle size than SWCNTs
even after microplasma treatment. It is clear that
SWCNTs had smaller and intensive particle range after
microplasma treatment compared to graphite powderparticles. The initial effective particle size of treatedgraphite powder was reduced from ~100μm to ~10μm.
While, the initial effective particle size of treated
SWCNTs was reduced from ~100μm to ~0.3μm. Thisdecrease on effective size of SWCNTs was due to theeffect of water-solubilization of SWCNTs by the
microplasma treatment [1].
Fig.7 shows optical emission spectrum of the
microplasma in water and SWCNTs suspension. It has
been found that emission intensity in distilled water wasvery weak. When a low concentration around 10μg/mL of
SWCNTs powder was added into distilled water,
emission spectrum intensity was increased significantly.
This phenomenon would be due to strong electric field
which could be built up by adding SWCNTs. It leads to
produce higher electron energy which could enhance theexcitation and ionization of water molecules and resultant
formation of radicals such as H� and O*. The other reason
which clarifies increasing spectrum intensity is due toincreasing number and length of produced streamer
channels once adding SWCNTs into water as shown
before in Fig. 3 and 4. Mechanism of enhancement effectof microplasma by SWCNTs would depend on size effectof SWCNT and graphite powder. Due to that aspect ratio(length/diameter) of SWCNTs was very high and its ends
were very sharp, so electric field might be enhanced by
SWCNTs and consequently a high emission intensity ofmicroplasma was generated compared to that of
distilled water. Many parameters, including the aspectgraphite powder and SWCNTs in water, which wasobtained using an optical particle size analyzer(SALD-7100, Shimadzu, Japan) before and after
microplasma treatment. The equipment analyzes aneffective particle size according to diffraction andscattering optical elements assuming a spherical particle.
It was previously reported that SWCNTs after themicroplasma treatment had a well-dispersed mesh-like

Number of channels

20SWCNTs

16128Graphite
400

Fig.5

246Concentration (µg/mL)

810

Relationship between SWCNTs and graphite
powder suspension concentration and numberof produced microplasma channels.
4035(a)Before treatment
After treatment

Frequency (%)

3025201510500.1

4035

110100Effective particle diameter (Pm)

(b)

1000

Before treatment
After treatment

SWCNTs suspension
Distilled water

Emission intensity (a.u.)Frequency (%)

3025201510500.1

Fig.6

110100Effective particle diameter (Pm)H (656nm)
�O (777nm)
*

1000
200

Effective Particle size distribution of (a)SWCNTs and (b) graphite powder in water
before and after microplasma treatment
respectively.

300

400

500

600

700

800

900 1000

Wavelength (nm)

Fig.7

605

Emission spectrum in distilled water andSWCNTs suspension (10μg/mL).

U. Khaled et al., Enhancement of Microplasma Generated in Water by Adding Carbon Nanotubes

of conductivity of water. Fig.9 shows the relation
between microplasma treatment time and water
conductivity. The water was treated by microplasma forone hour treatment time, the applied pulsed voltage was80kV and the concentration of SWCNTs was 0.1 mg/ml.
The water conductivity without adding SWCNTs wasincreased with the treatment time because some ionicspecies such as NO2- and NO3- were produced in water bythe discharge. On the other hand, the water conductivity
was further increased during the discharge when theSWCNTs were added in water. This result indicated that
the SWCNTs were well solubilized and dispersed inwater by the discharge and in addition of production ofionic species mentioned above, many current passes by
the dispersed SWCNTs would be formed in water. As theresult, conductivity of water was increased.

ratio of nanotubes, the anode-cathode distance, CNT with
open/close caps, intertube distance and so on, caninfluence the emission properties of the CNTs [7-15].
We discuss a possible enhancement effect of electricfield by adding SWCNTs in water. Although a thin wire
and plane electrodes were used in experiments, forsimplicity of electric field calculation, we assumed atheoretical model consists of plane to plane electrodes
system, in which SWCNT exists on a plane electrode.
Electric field was calculated at tip of SWCNT. A simple
formula to estimate the electric field enhancement, E at
the apex of a single, straight SWCNT was given as thefollowing expression [15]E � � 0 � E0(1)�0 �h�� 3.5

Electric field at tip of SWCNT (MV/mm)

where, E0 is a uniform electric field and �0 is the fieldenhancement factor which given by the followingequation(2)

where, h is the SWCNT length, and � is its radius. The
uniform electric field strength for the field between twoplates can be calculated by

E 0 � Va d(3)where, Va is the potential difference placed across plates
and d is the gap length between electrodes. Fromequations (1)-(3) the electric field at the tip of SWCNT
could be expressed by�h� VE � �� � 3.5 �� � a��� d0.25

SWCNT Length 5 �m, Dia 1nm
SWCNT Length 2 �m, Dia 2nm
SWCNT Length 5 �m, Dia 2nm
Uniform field (without SWCNT)

0.2

Anode

0.15

V=Va2�d0.1

hV=0Cathode

0.05
02

4

68

101214

Gap length between electrodes (mm)Fig.8(4)Fig. 8 shows relation between electric field at tip ofSWCNT, E and gap distance between electrodes, d withdifferent aspect ratios of SWCNT. It shows a clear
comparison between electric field calculations in case ofSWCNT presence and electric field when SWCNT does
not exist. From Fig.8, it was found that electric field
when SWCNT exist between electrodes was higher
compared to that without SWCNT. Electric field was very
high at the tip of SWCNT and decreased gradually byincreasing gap distance between electrodes. Also, electricfield of high aspect ratio of SWCNT was higher thanlower SWCNT aspect ratio. Therefore, induced electricfield could be greatly enhanced if SWCNTs with large
aspect ratio was added to water. These calculation results
suggest that addition of SWCNTs into water enhances theelectric field at the tip of SWCNT and enhanced electricfield leads to the increase of intensity of microplasma andnumber of streamer channels.
One of the other significant effects when SWCNTs
added in water on the discharge phenomena is increasing

Relation between electric field at tip ofSWCNT and gap length.

Water conductivity (�S/cm)

12Conductivity-without SWCNT
Conductivity-with SWCNT in water

10864200102030

405060Treatment time (min)Fig.9606

Effect of adding SWCNTs on the waterconductivity.

U. Khaled et al., Enhancement of Microplasma Generated in Water by Adding Carbon Nanotubes

4. Conclusions

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[3]

This study presents an observation of microplasma
phenomena when SWCNTs were added into distilledwater. It was observed that the microplasma intensity andnumber of streamer discharge channels were stable andenhanced by adding a little amount of SWCNTs to thewater. Therefore, it would be very useful to enhance themicroplasma generation. In addition, theoretical analysis
was revealed that the electric field was enhanced byadding SWCNTs compared to that without SWCNTs.
These results are very important and suggest that theenhancement of microplasma could represent an effective
method for many applications of microplasma treatment
in water such as water purification processes and removalof organic pollutants and bacteria.

Acknowledgments

This work was partly supported by Grants-in-Aid for
Scientific Research from Japan Society for the Promotionof Science (Nos.20360184 and 20560303) and by the
research granted from Iketani Science and Technology
Foundation, The Murata Science Foundation and Shorai
Foundation for Science and Technology.
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