Dye-assisted dispersion of single-walled carbon nanotubes for solution
fabrication of NO2 sensors
M. M. Ramli, W. Zhang, S. R. P. Silva, and S. J. Henley a)
Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford, Surrey
GU2 7XH, United Kingdom
Direct golden orange dye molecules were used as a dispersing agent to produce suspensions of
single-walled carbon nanotubes (SWCNTs) in water. Uniform, thin film networks were
fabricated by vacuum filtration using different concentrations of SWCNT and transferred
subsequently to glass substrates. The dispersion efficiency was compared to other surfactants.
Measurement of the sheet resistance as a function of SWCNT concentration showed a transition
from 2D percolation to 3D conduction behaviour when the concentration of SWCNTs exceeded
0.001 mg/mL. The electrical response to NO2 gas exposure was investigated as a function of
temperature and an optimum response was observed at 200 °C.
a)
Electronic mail: s.henley@surrey.ac.uk
Carbon nanotubes (CNTs) have attracted considerable interest due to their unique
properties and vast potential in electronic applications.1-3 For example, they show extreme
sensitivity towards changes in their local chemical environment due to their large surface area
and hollow geometry.4-6 It has been demonstrated that CNTs are able to detect very small
concentrations of molecules, including industrial pollutants such as NO2.7 The ability to
monitor NO2 in the air is important as this toxic gas can detrimentally influence the growth of
agriculture and can cause respiratory problems in humans. Long term exposure may also
decrease human lung function.8
Currently, metal oxide semiconductor and solid electrolyte sensors are used to detect
NO2. However, these types of sensors require high operating temperature (300°C to 600°C)
1
in order to achieve the required chemical reactivity between molecules and the sensor
materials. In contrast, CNT based sensors can operate at much lower temperatures, thus
lowering the device power consumption in particular off currents and standby energy. The
adsorption of electron-withdrawing NO2 molecules onto a CNT can cause charge transfer
between the CNT and the molecules.9 However, the poor solubility of CNTs in liquid media
still remains a problem for the low cost fabrication of gas sensing devices. Non-covalent
functionalization of CNTs to promote their solubility is preferable to covalent bonding as the
pristine sp2 hybrid carbon bonding is preserved.10 Surfactants such as sodium
dodecylbenzenesulfonate (NaDDBS)11, sodium dodecyl sulphate (SDS)12,
13
and toluene14
were used to uniformly dispersed SWCNTs in solution. The suspension of SWCNTs was also
achieved by DNA coating as reported by Tu et al.,15 and with proteins where significant
improvement of mechanical and thermal properties of polymer/SWCNT composites was
observed.16 Organic solvents have also been used to make CNTs soluble as reported by
Ganter et al. 17 and Bahr et al.18 In this study, we have used an organic dye to disperse single
wall carbon nanotubes (SWCNTs) in water. This process forms stable suspension of
SWCNTs by the formation of a colloidal system due to reduction of the hydrophobicity of the
SWCNTs when the dye molecules are attached. The suspension was then vacuum filtrated to
produce a set of percolated network films. This dispersion method with vacuum filtration
process demonstrates an efficient method to produce uniform, thin film SWCNTs networks.
We also demonstrate good electrical response of the gas sensing devices indicating that the
dye coating does degrade the sensing ability. In fact, a molecular layer of dye may absorb
and bind the NO2 molecules as shown by Johnson et al.19, possibly enhancing the sensitivity.
In this work, the SWCNTs used were produced by the chemical vapour deposition
process, and were purchased from NanoLab, US (initial purity > 50%). Direct golden orange
dye (Color Index: 40215) obtained from Kemtex Education Supplies (UK) was used in order
2
to form a stable colloidal system of SWCNTs in water. Mixtures of dye (10 mg) and
SWCNTs (10mg) in deionized water (100 ml) were sonicated in a QS 10 ultrasonic bath
(Ultrawave Ltd. UK) for 60 minutes. Then, the mixture was centrifuged at 4000 x g for 50 –
60 minutes. The supernatant was then decanted carefully to remove the larger SWCNT
bundles together with catalyst particles. The resulting purified mixture was then filtered using
0.1 µm polycarbonate membranes and rinsed with water. The concentration of the final
SWCNTs solution after decanting process was determined by weighing the dry product of
SWCNTs. The SWCNT suspension was then further diluted with deionised water to produce
a set of different concentration that varied from 0.0001 mg/ml up until 0.025 mg/ml.
There are several methods to produce uniform films of SWCNTs including the
vacuum filtration method.20 This simple and reproducible method was used to filter the
SWCNTs suspension through polycarbonate membrane (0.1 µm pore size). As the solvent
passes through the pores, the nanotubes are trapped on the membrane surface thus forming an
interconnected network. Here, 0.5 ml of the each solution was sucked through a membrane,
coating a circular area with a diameter of 13mm. Hence, the density of the nanotube networks
can be controlled by varying the concentration of the suspension filtered through the
membrane. The electrical resistance of a set of the SWCNT network films on the membranes
were measured using computer controlled Keithley 2400 sourcemeter at room temperature.
Gold electrodes (3mm spacing) were evaporated onto the film through a shadow mask, prior
to measurements. A second set of films (without gold electrodes) was then moistened with
deionised (DI) water, and the nanotube coated side was pressed against the surface of a glass
substrate. The SWCNT film was allowed to dry and adhere to the substrate at room
temperature under a 1 kg weight for at least 10 h. The weight was removed and the
membrane dissolved in N-Methyl-2-pyrrolidone (NMP) (using a series of pure baths) to leave
the SWCNT film on the glass substrate. The films were then rinsed with DI water and baked
3
for 5 minutes at 90°C. The adhesion of the SWCNT thin film was found to be sufficiently
strong to prevent delamination during processing. These method were repeated for all the
concentration and yielded almost 100% of the film been transferred. Gold electrodes were
then applied to these films in order to produce the gas sensor devices. No degradation of the
film conductivity was observed after the transfer process. The SWCNT devices were then
placed in a sealed chamber and exposed to 50ppm NO2 in dry air, flowing at 0.5L/min, to
investigate the sensing characteristics. The effect of the sample temperature on the sensitivity
and recovery time of this sensor was also studied. The sensors were tested at three different
temperatures from room temperature to 300°C.
The optical absorption of the solutions was measured before film fabrication using a Cary
5000 UV-Vis-NIR spectrometer. The Raman spectra of the dispersed material were measured
using a Renishaw microRaman spectrometer, with 514 nm excitation. Drops of the solutions
were delivered onto silicon dioxide substrates and dried on a hot plate to evaporate the water
to prepare these samples.
4
FIG. 1. (a) (color online) Photograph displaying SWCNTs suspensions; (i) pure dye solution (0.1 mg/ml), (ii)
0.05 mg/ml SWCNTs, (iii) 0.005 mg/ml SWCNTs, and (iv) 0.0001 mg/ml SWCNTs. (b) Optical absorption
spectrum of pristine SWCNTs (0.005 mg/ml) and dye assisted dispersions
Figure 1 (a) shows the physical state of the SWCNT suspension. The color of the
solution becomes darker as the concentration increased. To prove there was no evidence of
settling, the image of the SWCNT suspension was taken 6 months after it was made. This
stability is comparable to that observed by others groups.21 Figure 1 (b) shows the optical
absorption spectra of the dye molecules in the SWCNT suspension. The ringed area in the
plotted graph shows a peak due to the presence of dye molecules, indicating that the dye was
attached to the tubes and remained attached during the filtration and re-dispersion process.
5
The dye molecules used contain benzene rings and nitrogen-nitrogen double bonds
(N=N), allowing the delocalisation of electronic charge and the formation of an extended πconjugate system (the chemical structure of dye molecule is shown inset in Figure 1(b)). The
SWCNTs have delocalised π-electrons along the nanotube axis.22 In water, dye molecules
(Dye-SO3Na) become dissociated and yield dye molecular anions (Dye-SO3-) and sodium
cations (Na+). When mixing with SWCNTs, dye molecular anions orient themselves in a
fashion that benzene rings face toward the CNT surfaces and initiate π-π stacking.23 As a
result, dye molecular anions are adsorbed on CNTs and form negative charges on the
SWCNT surfaces. By electrostatic forces, sodium cations were attracted and closely attach to
the surface of SWCNT. On the outer layer, free ions with a high concentration of Na+ exist.
The strong repulsive forces provided by this electrical double layer, enable individual
SWCNTs to be dispersed in water with the formation of colloids. The mixing mechanism
between dye molecules and SWCNT was also similar to the hybridization of single stranded
DNA and SWCNTs,24 where the hybridization process is driven by strong van der Waals
forces between the faces of nitrogen-based molecules (nucleobases) and the SWCNT
sidewalls. called π-π stacking interaction. The suspension was then kept in the storage and
after a few months, this well dispersed suspension was still very stable with no signs of
SWCNT flocculation or settling at the bottom of vials.
The Raman spectra of pristine SWCNT and dye-assisted dispersion are shown in
Figure 2. The pristine SWCNTs show typical D-mode at 1330 cm-1 and G-mode at 1590 cm-1
with overtone of D-mode, 2D at 2660 cm-1. SWCNT also showed radial breathing mode
(RBM) band at low frequency range at ~ 155 cm-1.25 Figure 2 (inset) also shows the Raman
spectra of dye molecules. The presence of dye molecules on the SWCNT suspension were
shown in circle [Fig. 1 (a)] where the same peaks were detected.
6
FIG. 2. (color online) Raman spectra for (a) dye coated SWCNTs, (b) for pristine SWCNTs
and for (inset) dye molecules.
Before, examining the electrical properties of the networks, it was important to determine the
morphology of the dye coating and the extent of SWCNT bundling in the films.
Figure 3(a)
& (b) show a comparison of AFM images the raw SWCNTs and the dye-SWCNTs dried onto
a glass surface. The raw tubes are bundled together, but the dye dispersed tubes appear more
isolated and show evidence of a significant surface coating. Figure 3(C) shows an AFM
image of a dye-SWNT network after transfer to the glass, where the larger-scale surface
morphology is dominated by clumps of the dye.
The conductivity of CNT networks are attributed to the formation of a continuous path that
allows efficient electrical transport. This kind of phenomenon can best be described by
percolation theory.20 The CNTs network appears randomly oriented with no preferential
direction. The CNTs tend to be attracted to each other and became bundled through van der
7
Walls interactions. Percolation theory for a 2D network requires only that the conducting path
has a length that is much greater than the CNT diameter,26 thus the overall conductivity will
depend critically on the number of conductive paths through the network and the typical
number of bundled junctions encountered on a given path.
FIG. 3. (color online) AFM image of (a) bundles of raw SWCNTs, (b) dye-functionalised
SWCNTs, showing isolated tubes and surface coating with the dye and (c) the dye-SWCNT
network after transfer to the glass.
8
FIG. 4. (color online) Electrical Resistance of different SWCNT films prepared using
different dispersing agents. Dashed lines are merely guides for the eye and the arrows
indicate the approximate position of the conversion from 2D to 3D conduction.
Figure 4 shows a plot of SWCNT films resistance as a function of solution concentration
varying from 0.0001 mg/ml to 0.03 mg/ml. For comparison, the SWCNTs were also
dispersed with the common surfactant SDS12, 13 at the same concentration as the dye and with
DI water. The dye dispersed SWCNTs produced a visibly more stable solution than the
surfactant with fewer aggregates. The settling of SWCNTs in this surfactant was confirmed
by visual inspection. Using pure water did not yield stable solutions. As shown in Figure 4,
the resistance of dye dispersed film were significantly lower than for the SDS dispersions,
with percolation at a higher concentration. This indicates the dye dispersion has formed more
uniform SWCNT network films than the SDS dispersion and more conducting tubes even at
similar concentrations. As expected, the resistance of films produced using raw SWCNT
9
solutions was very high compared to dye and SDS dispersion of SWCNTs due to its poor
solubility and significant, rapid, aggregation.
For the dye-dispersed SWCNTs, the sheet resistance of the network would be expected to be
inversely proportional to the concentration of CNT used if a 3D network was formed. As
shown in Figure 4, the sheet resistance for concentrations from 0.0001 mg/ml until 0.001
mg/ml were very high, but decreased very rapidly with increasing concentration. This is due
to the fact that fewer continuous conducting paths are formed at lower concentrations. Thus
2D percolated conduction is observed where, on average, we can consider there to be no
more than one single layer of tubes (or bundles of tubes) in any region. When the
concentration of SWCNTs was higher, (above 0.001 mg/ml) the sheet resistance was
approximately inversely proportional to the concentration. From this point onwards, the thin
film networks demonstrate a 3D (bulk) conduction behaviour. Here, when the concentration
of the SWCNT solutions increases, more layers of tubes are formed resulting in lower film
resistances. As it can be seen from figure 4, the point at which this change occurs is at a
higher concentration for the SDS dispersions, indicating that that clumping or settling may be
occurring here. To examine this further, the standard percolation equation, as given below,
was applied to determine the critical exponent of the films20.
R  (x – xc)n
Here, the x is the concentration of SWCNTs, xc is the critical concentration of SWCNTs and n
is critical exponent. The critical exponent was extracted from 2D and 3D regime of film
resistance giving n = - 1.6 and n = - 0.9, respectively.
Having demonstrated that the dye-assisted dispersion route is suitable for producing
electrically conductive films, it was important to determine if the dye coating significantly
affected the films for gas sensor applications. Figure 5 shows the NO2 sensing properties
10
(resistance versus time) for the SWCNT network film formed at 0.005mg/ml when exposed
to 50ppm NO2 gas in flowing air. The device was periodically exposed to clean dry air flow
for 1 h to record base values of device resistance and 2 min in NO2 environment to register
sensing signal before the dry air was introduced again to let the device recover. These steps
were repeated for three cycles and the signals showed good reproducibility. Figure 5(a)
shows a typical response at room temperature. Although a strong response was observed,
with the sensitivity around ~20% was recorded, the recovery was slow. Also, the device
never fully recovered to its baseline resistance before exposure in the timescales tested. The
device was tested subsequently at higher temperatures. When the device was exposed at
200°C [Fig. 5 (b)], the sensitivity was increased to ~50% and the response time was
improved by 30 seconds to 60 seconds. A further increment in the temperature to 300°C [Fig.
5 (c)] produced an even more rapid recovery (~10 mins), but the absolute response was
degraded compared to 200°C. The response time was calculated around 60-70 seconds and
the sensitivity was ~45%. At elevated temperatures, the resistance tends to recover more
quickly compared to when the device is exposed at room temperature due to the fact that the
greater available thermal energy increases the rate of detachment of a physisorbed molecule.
When the device was exposed to NO2 gas, it is believed that an oxidation process occurs,
where SWCNTs lose electrons to the NO2. The electron-withdrawing power of the NO2
molecule is very strong due to the existence of unpaired electrons. When electrons are
transferred to the NO2 molecules the majority carrier (holes) density6 in the SWCNTs
increases thus increasing the conductivity of the sensors.
In summary, stable and well dispersed suspensions of SWCNTs in water were
achieved by mixing SWCNTs with dye molecules. Raman spectroscopy showed that no
damage or modification to the SWCNTs structure occurred using this dye assisted dispersion
process. Using vacuum filtration, thin electrically percolated nanotube films were produced
11
from these dispersions. It is believed that the presence of dye molecules on the nanotube
surfaces may have an effect on the sensitivity of the devices towards NO2 exposure as the π-π
stacking of dye molecules on SWCNT surfaces may change their surface properties and
therefor the charge complex formed between SWCNTs and NO2 molecules could be altered.
However, despite a significant coating of the dye molecules, the films produced still
functioned well as a gas sensor, although the ultimate sensitivity may be decreased. Looking
at the S/N ratio for the gas data, a best-case sensitivity of 100ppb could be estimated, in line
with other works from the literature.27 Formation of thinner networks may further enhance
the sensitivity of an area which needs further investigation. This dye dispersion method also
demonstrated to produce better percolated networks compared to dispersion in SDS.
The authors are supported by the Engineering and Physical Sciences Research
Council (EPSRC). MMR is also supported by Ministry of Higher Education, Malaysia, and
University Malaysia Perlis. The authors would like to thank Mr. M. Nordin for useful
discussions.
12
FIG. 5. Resistance changes for 50ppm NO2 exposures of a device formed from a 0.005mg/ml
solution at (a) room temperature, (b) 200°C and (c) 300°C.
13
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