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)

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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

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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

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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.

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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.

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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 π-

conjugate 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.

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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

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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.

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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

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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

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(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

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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.

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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.

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