electromagnetic modulation of carbon nanotube...
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PAPER www.rsc.org/materials | Journal of Materials Chemistry
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Electromagnetic modulation of carbon nanotube wetting
Yi-Fan Li,a Chia-I Hung,a Hsin-Fu Kuo,a Sen-Hong Syue,a Wen-Kuang Hsu,*a Shin-Liang Kuob
and Shu-Chuan Huangb
Received 1st June 2009, Accepted 6th August 2009
First published as an Advance Article on the web 27th August 2009
DOI: 10.1039/b910793b
NH3$H2O treated carbon nanotubes form an electric double layer in the presence of an electric field and
tube wetting is therefore improved. Proton concentration on tube surfaces can be further modulated
by a Lorentz force and is verified by multi-transition of an hydrophobic into an hydrophilic phase.
1. Introduction
A recent study on carbon nanotubes (CNTs) known as graphene
sheets rolled into a seamless cylinder has largely focused on
solution properties and it is now understood that wetting of
conjugated surfaces is dominated by a dispersive force and tube
curvature induces an extra contribution of polar energy to the
critical surface tension.1 CNTs can be hydrophilized via
oxygenation whereas surface wetting is improved at the expense
of severe lattice damage.2 In this work, an electromagnetic
modification of CNTs is demonstrated and the procedure
involves a generation of an electric double layer (EDL) on CNT
surfaces via NH3$H2O treatment. We find that protons accu-
mulate at nanotube surfaces in the presence of an electric field
and the liquid–droplet interface is therefore dominated by polar
character. The proton concentration on tube surfaces can be
finely tuned by a Lorentz force and is supported by the multi-
transition of an hydrophobic into an hydrophilic phase.
2. Experimental
Single-walled CNTs (SWCNTs, 1.1–1.4 nm in diameter and 1–4
mm in length, 90% purity) made by electric arc discharge of Ni–Yi
embedded graphite rods3 are heated in an ambient environment
(300 �C, 40 min) to remove carbonaceous impurities and oxidized
SWCNTs are subsequently dispersed in deionized water (DW)
containing 1 wt% of sodium dodecyl sulfate (SDS).4 SWCNT
dispersion (1 mg mL�1) is transferred onto a polyethylene tere-
phthalate (PET) film via roller printing and SDS is removed by
rinsing the SWCNT coated film with DW. Electrical connections
are made by depositing silver paste onto nanotube film edges via
a masking technique and the film temperature is measured by
a digital thermocouple (Agilent-34970A) attached underneath.
Comparative experiments were carried out on ammonia treated
SWCNT films and the procedure is as follows. SWCNTs are
deposited onto PET film by a similar method to that described
above and NH3$H2O (Showa, 28%) is sprayed onto the
composite film.5–7 The treated film is placed in a fume cupboard
for 1 h to remove excess ammonia and subsequently subjected to
aDepartment of Materials Science and Engineering, National Tsing HuaUniversity, HsinChu, 30013, Taiwan. E-mail: [email protected];Fax: +886-3-5722366; Tel: +886-3-5715131-35399bResearcher Department of Polymer Hybrid MCL, Industrial TechnologyResearch Institute, Hsinchu, 30013, Taiwan. E-mail: [email protected];Fax: +886-3-5732346; Tel: +886-3-5732452
7694 | J. Mater. Chem., 2009, 19, 7694–7697
wetting tests. An optical microscope equipped with a charged
coupled device (CCD) camera is employed to measure film
wettability and DW is selected as the testing droplet. Ethylene
glycol (EG) known to possess a greater dispersive character is
also studied for comparison.5
3. Results and discussion
Fig. 1a shows the experimental set up and the contact angle (CA)
measurement is carried out within an area of 1 � 1 cm2. The
SWCNT-coated PET film shows an excellent flexibility (Fig. 1b)
and transmittance at the visible-light regime has been estimated
to be 65% (Fig. 1c). Fig. 1d displays the coating of SWCNTs and
the sheet resistance revealed by the four-probe technique is 4500
� 500 U sq�1 before and 680 � 20 U sq�1 after SDS removal.
Fig. 2a and 2f show DW and EG droplets on untreated SWCNT
film and the CA is found to be 93.1� and 69.7� (Table 1); the
CA(DW) > CA(EG) being a consequence of the stronger dispersive
interaction of the conjugated CNT surfaces with the ethylene
moiety of EG (Table 2).5 The CA on ammonia treated film
decreases to 83.1� for DW and 56.3� for EG, corresponding to
10.7% and 19.2% reductions (Fig. 2b and 2g). We have calculated
Fig. 1 Experimental set-up (a), SWCNT coated PET film (b), UV-VIS
spectra of SWCNT-coated PET film (c), SEM image of the coating of
SWCNTs (d).
This journal is ª The Royal Society of Chemistry 2009
Fig. 2 CAs of deionized water (DW, left column) and ethylene glycol
(EG, right column) droplets on pristine SWCNT film (a)(f), NH3$
H2O-treated SWCNT film (b)(g), NH3$H2O-treated SWCNT film
with applied voltage at 10 V (c)(h), NH3$H2O-treated SWCNT film with
applied voltage at 12 V (d)(i), NH3$H2O-treated SWCNT film with
an applied voltage at 14 V (e)(j). The anode is situated at the left-hand
side of the droplets.
Table 2 The gdlv, gp
lvand glv
of DW and EG
Liquid gdlv (mN m�1) gp
lv (mN m�1) glv (mN m�1)
DW 21.8 51 72.8EG 33.8 14.2 48
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the surface tension of the SWCNT film according to the Owens–
Wendt model and the equation is expressed below,8,9
1þ cos q ¼ 2ffiffiffiffiffiffigd
sv
q ffiffiffiffiffiffigd
lv
pglv
!þ 2
ffiffiffiffiffiffig
psv
p ffiffiffiffiffiffig
plv
pglv
!(1)
where g represents the overall surface tension and is equivalent to
the sum of the dispersive (gd) and polar (gp) components; the
former is contributed to by the van der Waals interaction and,
the hydrogen bonds and dipole–dipole interaction for the latter.
Before ammonia treatment SWCNTs are dominated by the
dispersive component and the gdsv/gp
svratio is 4.1, consistent with
a hydrophobic nature found in as-made SWCNTs (i.e. gdsv/gp
sv> 1
for hydrophobic and gdsv/gp
sv # 1 for hydrophilic).8,9 For
ammonia treated film the gdsv/gp
svratio is reduced to 3.17 and, the
polar and dispersive components are enhanced by 62.7% and
Table 1 CA and calculated surface tensions of SWCNT film and DW and E
Samples CA(DW) (�) CA(EG) (�)
Untreated 93.1 69.7Treated 83.1 56.310 V 41.6 33.212 V 38.3 30.314 V 35 25.8
This journal is ª The Royal Society of Chemistry 2009
24.47% (Table 1). According to previous FTIR data, immersion
of SWCNTs into ammonia water results in NH3 adsorption and
adsorbed molecules are found to interact with the tube via a long
pair on the N-atoms.6 This is supported by a large blue-shift of
the umbrella mode and broadening of the N–H bending
modes and the adsorption energy has been estimated to be
comparable with hydrogen bonding character,7 accounting for
the gpsvenhancement detected here. It is noteworthy that adsorbed
NH3 essentially behaves as an electron donor and tends to
capture a proton from the attached droplet in the presence of an
electric field.5 This yields an EDL and Coulomb attraction (Fatt)
established between NH4+ and a negatively charged SWCNT
film is therefore expected to facilitate the droplet wetting and
permeation. Fig. 2c and 2h show DW and EG droplets on treated
films with an applied voltage of 10 V, corresponding to 5� 105 V mm�1.
We find that the DW droplet is dragged by the electric field and
the asymmetric structure becomes apparent when the field
intensity increases (Fig. 2c–e). In contrast, the EG droplet
remains beaded, indicative of a lower gplv at the tube–liquid
interface. Compared with the untreated film, the CA on
NH3$H2O-treated film is reduced by 55% for DW and 51% for
EG at 10 V whereas the gpsv is promoted by a factor of ten
(Table 1). CA reduction by Joule heating is unlikely because
droplet vaporization is absent and the film temperature as
revealed by a thermocouple fluctuates only by�2 �C. CA further
decreases as the applied voltage increases and reaches 35� and
25.8� at 14 V for DW and EG. In total, the gpsv is promoted by
a factor of 11 and the gdsv/gp
svratio decreases to 0.18–0.2 at 10–
14 V. Additional evidence in support of EDL formation on tubes
comes from the fact that an electric field applied to ammonia
untreated SWCNT film causes only a 2–3% CA reduction and gpsv
remains unchanged.
CA change however is irreversible and protons do not detach
from tube surfaces upon field reversing, possibly due to (i) liquid
permeation into the SWCNT film and (ii) g[ Fatt.5 According
to the Right-Hand rule application of the magnetic field
perpendicular to the electric field produces a Lorentz force and
charged particles are accelerated in the same orientation as the
electric field. In the next experiments, we find that the electric
field induced proton accumulation at the SWCNT surfaces can
be deferred or accelerated by an external magnetic field,
depending on the applied direction. For example, a reverse
G
gdsv (mN m�1) gp
sv (mN m�1) gsv (mN m�1)
17.77 4.29 22.0722.12 6.98 29.119.62 47.49 57.119.48 50.33 59.819.89 52.22 62.11
J. Mater. Chem., 2009, 19, 7694–7697 | 7695
Fig. 3 A reverse magnetic field application to an electrically connected
electric double layer (EDL)-SWCNT device (a), a forward magnetic field
application to an electrically connected EDL-SWCNT device (b), CA
variation of deionized water (DW) and ethylene glycol (EG) versus
magnetic field intensity in the presence of a constant electric field
(5 � 105 V mm�1) (c).
Fig. 4 Electromagnetic field controlled CAs from the hydrophobic to
the hydrophilic phase and corresponding images: DW (top panel) and
EG (lower panel).
7696 | J. Mater. Chem., 2009, 19, 7694–7697
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application of the magnetic field produces an upward Lorentz
force and protons therefore remain dispersed in the droplet
(Fig. 3a). Proton precipitation is accelerated at forward magnetic
field and gpsvenhancement, hence CA reduction, becomes signif-
icant (Fig. 3b). Fig. 3c shows the magnetic field modulated CA
and the electric field intensity is set at 5 � 105 V mm�1. At zero
magnetic-field the CA is 42� for DW and 33.4� for EG, consistent
with the value detected at 10 V (Table 1). Upon applying the
magnetic field the CA decreases significantly with increasing field
intensity at forward application and we obtain 38.7� and 30.4� at
90 Oe, 36.3� and 28.5� at 140 Oe, and 30.8� and 23.4� at 200 Oe
for DW and EG. In contrast, the CA decrease is insignificant
at reverse application and measurements give 47� and 37.2� at
�90 Oe, 56.7� and 41.5� at�140 Oe, and 70.5� and 47.4� at�200
Oe for DW and EG respectively, i.e. the CA decrement shrinks as
Fig. 5 Volumetric flow rate (Vf) vs. magnetic field intensity (a), variation
of gpsv, gd
sv, and gsv with magnetic field (b), and variation of the calculated
surface energy with the applied magnetic force (c).
This journal is ª The Royal Society of Chemistry 2009
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the field intensity increases. Results are reproducible when the
magnetic field is repeatedly tuned from 200 Oe to �200 Oe, and
vice versa. The Lorentz force induced concentration gradient of
the proton at the interface is also supported by the fact that the
gdsv/gp
svremains around 3.17 when the electric field is switched off
and the device experiences a magnetic force only. Fig. 4 plots CA
vs. Lorentz force controlled gpsv and transition of the hydro-
phobic into the hydrophilic phase is now divided into eleven
steps, including field-free CA on untreated (1) and treated (2)
films, the electric field controlled CA on treated film (6,8,10) and
the electromagnetic modulated CA on treated film (3,4,5,7,9,11).
This outcome supports the fine tuning of carbon nanotube
wetting by an electromagnetic field.
The Lorentz force controlled wetting can be further charac-
terized by plotting volumetric flow rate (Vf) vs. magnetic field
(insert, Fig. 5a) and droplet permeation was recorded by a CCD
camera equipped with location marking and frame-to-frame
software. First, the Vf increases with forward application and
field reversing leads to Vf reduction, verifying the Lorentz force
controlled wetting. Second, the Vf(EG) is greater than Vf(DW). In
the current study, the SWCNT film thickness is controlled at
0.5–0.7 mm and the pore size distributes around 9.1–9.5 nm.5
Therefore, various Vf due to different penetration paths is
unlikely. Here we believe that the Vf(EG) > Vf(DW) originates from
a larger gd in EG and is supported as follows. First, protons
accumulate mostly at the film surface and dispersive character
essentially remains in the embedded tubes. Accordingly, a faster
Vf is anticipated for EG permeation. Second, the gd is the sum of
gdlv and gd
sv at zero magnetic field and the calculation gives 44.42
mN m�1 for EG and 31.42 mN m�1 for DW (Tables 1 and 2).
Upon application of the magnetic field, the gdlv remains
unchanged and gdsv increases by 8.38 mN m�1 at �200 Oe, thus
giving gd ¼ 51.8 mN m�1 for EG and 39.8 mN m�1 for DW at
�200 Oe (Fig. 5b). This is consistent with Vf(EG) > Vf(DW). It is
noteworthy in Fig. 5b that the gsv curve exhibits a similar trend
This journal is ª The Royal Society of Chemistry 2009
to gpsv at both forward and reverse fields, indicative of a gp
sv
dominated interface. Compared with the electric field controlled
process at 10 V (Fig. 2c–e & 2h–j) the Vf obtained at �200 Oe is
lower by 74% and 85% and is greater by 111% and 180% at
200 Oe for DW and EG respectively, again verifying a good
control of CNT wetting by the Lorentz force. Fig. 5c shows the
field modulated surface energy and the gap between gpsv and gd
sv
reaches 163% at the reverse field (i.e. gdsv increases by 96.2% and
gpsv decreases by 66.8% at �200 Oe). As described above the gd
sv
originates from the van der Waals interaction and cannot
be altered by field application. Accordingly, a small decrease in
gdsv at the forward field is attributable to the screening effect by
accumulated protons (Fig. 5c). When protons detach, the
hydrophobic CNT surfaces are exposed and gpsv increases
accordingly, consistent with Fig. 5b.
Acknowledgements
We thank the National Science Council of Taiwan for the
financial support (NSC-97-2112-M-007-014-MY2).
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