2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO)

The International Conference Centre Birmingham

20-23 August 20112, Birmingham, United Kingdom

Chemical Sensing with Multiwalled Carbon Nanotube

Dena Saadat', Ravi Silva2, and Paul Watts3

'Department of Electrical Engineering, University of Massachusetts Lowell, MA, 2 Advanced Technology Institute, University of Surrey, UK

Email: dena_saadat@student.uml.edu •• s.silva@surrey.ac.uk2.p.c.p.watts@gmail.com3

Abstract-The fundamental contributions of junction

resistances in mutli-walled carbon nanotube (MWCNT) mats,

namely the resistances between the tubes, play a key role in

allowing these nanostructures to be utilized as sensors via

affecting their sensitivity to external gas environments and water

vapours. In this work, we use MWCNTs functionalized in acetone

(on a glass substrate) by pulse valve deposition to investigate their

potential as chemical sensors. In a series of experiments we have

grown and characterised samples in various chemical

environments (cycling water vapour in a vacuum environment)

and report that the MWCNT mats exhibit fast and high-quality

responses. We examine in some detail the parameters affecting

the chemical-sensing responses, such as the thickness of the films,

heat treatment (temperature) and residual solvents. We find that

by increasing the thickness of the films, the ratio of the junction

resistance to the tube resistance (the resistance along the tube)

decreased, owing to the parallel arrangement of tubes.

Concomitant with this effect was a decrease in the samples'

responses due to lower current diffusion and the role of junction

resistance in thicker films.

Multi-walled carbon nanotubes, chemical sensing, nanosensors

I. INTRODUCTION

THE discovery of carbon nanotubes (CNTs), with unique electrical and mechanical properties, opened a new world

of efficient devices on truly nanoscopic scales. Carbon nanotubes, discovered by Iijima in 1991 [1], consist of rolled sheets of graphite, with one sheet yielding a single-wall CNT (SWCNT) and multiple sheets producing multi-wall CNTs (MWCNTs). The unique electrical and mechanical properties of these carbon sheets make them potentially attractive for being used as sensors. Their high surface area can be exploited to enhance the efficiency of sensors [2], with large numbers of CNTs (on a single substrate) giving rise to correspondingly higher sensitivities - a feat that can be exploited to construct accurate nanosensing devices. The increased surface area available for interaction with gas atoms may also lead to the attainment of improved signal-to-noise ratios (SNRs) [3]. Gas sensors based on CNTs are typically low cost and capable of operating at room temperature because of their enhanced sensitivity compared to, e.g., ordinary semiconductor schemes that in general operate above 200°K. Further, CNTs can now be produced readily over large areas and can, therefore, be

September 31,2011. Chemichal Sensing with Multi Walled Carbon Nanotube has presented at university of surrey. DenaSaadat. (Phone:+ 17744440724;e-mail: dena_saadat@student.uml.edu).

readily available for high chemical-sensitivity studies and nanodevices.

It has been reported that as SWCNTs are exposed to different chemical environments, such as N02, NH3 or water, their electrical resistance varies due to physisorbed molecules that act as dopants. This change in electrical resistance forms the basis for a sensing system. For instance, this technique could be deployed for gas sensing in order to detect potentially hazardous (toxic) gases in air. Recent research [3] shows that SWCNTs can be doped from the chemical environment that surrounds them. This was aided by the fact that in those configurations about two-thirds of the SWCNTs are semi­conducting, whereas MWCNTs are predominantly metallic. However, MWCNTs could potentially be more appealing than SWCNTs because they are relatively inexpensive and can be used for longer periods of time compared to SWCNTs, owing to having a greater number layers compared to their single­wall counterparts [4]. This simple reason should motivate a detailed assessment into the realistic potential of MWCNTs for chemical-sensing applications.

Previous experimental works have shown that the efficiency of gas sensors based on functionalized MWCNTs can be considerably affected by such parameters as the temperature, nature of used gases and junction resistances. It has, further, been found that when MWCNTs are added to, e.g., tungsten trioxide (W03) films to detect a gas, the surface area for interaction with the gas increased [5], which suggests that MWCNTs could in principle be used to improve on existing thin-film gas sensors. Another experiment has showed that MWCNTs can also be used for detecting N02, and it has likewise been established that acid functionalization can further increase the attained sensitivities (responses) [6]. Yoon et al. experimented on fabricating chemical sensors employing MWCNTs as the active sensing element, and showed that parameters such as temperature, electron charge transfer and the nature of the surface interaction between the gas and CNTs influenced the response of the MWCNTs sensors [7].

Normally, at room temperature CNTs are metallic. By decreasing the temperature their character may change to non­metallic. This transition temperature where the characteristics of CNTs are changed is known as T*. Fischer et al. showed that T* for SWCNT-rope occurs at low temperature, while for CNT mats the transition occurs at higher temperature [8]. In [9] it has been reported that this characteristic temperature also depends on the ratio of the resistance between the tubes Gunction resistance, RJ) to the resistance along the tubes (nanotube resistance, RNT). By increasing the ratio of RJ to RNT, the temperature T* may assume higher values. For

temperatures lower than T* the resistivity is dominated by the 'junction' resistance RJ, while at higher temperatures the resistivity is mainly determined by the nanotube resistance RNT. The above works, thus, already help to clearly illustrate the vital role played by the junction resistance of CNTs in detennining their electrical characteristics.

II. METHODOLOGY

In this section, the basic experimental techniques used throughout this work are presented. These include the preparation of the MWCNTs solution, deposition of MWCNTs with the pulse valve technique, scanning electron microscopy (SEM) to characterize the morphology of CNTs, and 1- V probe measurement to investigate their electrical resistance.

MWCNTs need to be in a solution form if they are to be suitable for being deposited on an appropriate substrate like glass. The first step in this stage is the acidification of the tubes by mixing them with sulphuric acid and nitric acid in order to obtain carboxylic-acid functionalized MWCNTs. After the reaction takes place, a centrifuge was used to separate the water and acid from the functionalized tubes when the centrifuge action had finished, the sample's pH was measured. If the pH was neutral it meant that there was no acid, so the water was removed from the sample and the carboxylic-acid MWCNTs remained. We used a filter equipment to make the sample more pure and then left it in the oven to dry at 150°C.

After the carboxylic MWCNTs are prepared, they are sprayed on a substrate (glass) by pulse valve deposition. Pulse valve deposition allows us to produce a homogenous thin film of CNTs. The method consists of spraying CNT solutions under vacuum on the substrate. To this end, a pulse valve containing the CNT solution (in this case MWCNTs) in acetone is mounted in a vacuum chamber. Employing the pulse valve deposition technique we have produced samples containing carboxylic MWCNTs in acetone on glass substrate. Seven films with different thicknesses were generated in this way. An exemplary result is illustrated in Fig. 1, showing the morphology of the thinnest film of COMWCNT that was prepared with the above methodology.

The next stage is investigating the electrical resistance of the samples. The gas-sensing response of the CNT composites was investigated using I-V measurements. Figure 2 shows the experimental setup for the gas-sensing measurement As

2

MWCNT films are exposed to different chemical environments, their electrical resistance correspondingly changes. To measure the resistance changes of samples under different gas environments,two-point probe 1- V measurements were conducted. To investigate the conductivity of the CNTs a Keithley 487 Picoammeter/V oltage source was used. To apply a voltage and measure the current circulating through the sample we used the "487 1- V control" software package [16].

- - - --­

."'''''.-�

(a) (b)

Fig 2. (a) Keithley 2425 Source Meter, (b) Gas chamber. The black arrows indicate the sample position. The red arrow indicates the vacuum outlet and the yellow arrow indicates the gas input.

Figure 2 shows the experimental setup used in the CNT gas sensing. By injecting alternating water vapour inside the gas chamber and then returning the sample to vacuum, the electrical resistance of the MWCNT mats was changed, recorded and analysed.

We investigated, in particular, the response of the sample given by the following relation:

(1)

Where Ro is the resistance of the sample in the out­gassed/vacuum state, and Rg is its resistance when exposed to the various gas environments, such as water vapour [11]. To investigate the role of junction resistance, the thicknesses of the films were accordingly changed. By varying the amount of time in the pulse-valve technique, we fabricated seven films with progressively (from sample 1 to 7) increased thicknesses. A mixture of carboxylic MWCNTs and acetone was used in the preparation of the samples. Their response was investigated in two situations, before and after heat treatment, and by periodically exposing them to water vapour and vacuum every 60 s. Heat treatment was preformed at 110°C for one hour. The results of our experiments, which led to detailed understanding the role herein played by the junction resistances, are presented in the following section.

III. EXPERIMENTAL RESULTS

We sought to measure the resistances and responses of the seven homogeneous films during the vacuum/gas cycles, both, before and after heat treatment. An exemplary result of our measurements is illustrated in Fig. 3. We find that during the first vacuum cycle (0- 60 s) the resistance of the samples remains constant with time, but is followed by a sharp increase

when the gas enters the chamber. After the gas is removed, the resistance (and response) of each sample slowly returns to the value it had during the fIrst cycle. Thereafter, the same periodic phenomenon continues during every vacuum/gas cycle. From Fig. 3(b) we see that the samples with the smallest thicknesses (largest resistances) exhibit the largest responses. For instance, the response of sample-l (that had the smallest thickness of all seven samples) assumes a maximum value of 39, i.e. the resistance of this sample increases by 39% when the gas interacts with the sample. From Fig. 3 it is also evident that similar trends pertain to all other six nanotube fIlms.

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Fig 3. (a) Comparison between the Resistances before heat treatment (b) Comparison between Responses before heat treatment Green circle shows the peak profile during the water vapour exposure

3

(a) - after heat treatment for 1050 Jl(ohm - after heat treatment for 5250 I K ohm - after h e at treatment for 8.8Jl(ohm - after heat treatment for 44 lkohm - afterheat treatment for 123Jl(ohm - after heat treatment for 478Jl(ohm

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Figure 4 reports the corresponding results after heat treatment. Here, the treatment was performed in an oven for one hour, and the samples were then left in vacuum for 24 hours. Close inspection of the results in Fig. 4 reveals that, after heat treatment, the resistances of the seven fIlms have decreased compared with the corresponding ones of Fig. 3. By contrast, we have found - as shown in Fig. 4(b) - that the responses of the samples after heat treatment are larger compared to those of Fig. 3(b). However, in both cases (before and after), the general trends in the variation of the resistances/responses with time during the vacuum/gas cycles are indeed very similar.

Upon comparing the resistances/responses of all corresponding samples, before and after heat treatment, we may arrive at the following 'universal' conclusions:

Both before and after the treatment, the resistances and responses of the nanotube fIlms monotonically decrease with increasing fIlm thickness;

• The response of each sample as a function of its resistance follows a universal functional dependence, both before and after the heat treatment, as clearly illustrated in Fig. 5. The fact that the responses vary as a function of the corresponding resistances following a general pattern that can be a priori predicted, is a critical aspect that allows us to arrive at generalized conclusions concerning the chemical-sensing ability of the herein investigated multiwalled carbon nanotube mats.

• Response before heat treatment -Logresponse before heat treatment 45 40 3 5

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Fig 5. (a) Responses vary with resistance before heat treatment; (b) Responses vary with resistance after heat treatment.

A summary of the actual experimental values reported in Figs. 3-5 is presented in Fig. 6. It is clearly seen that by increasing the thickness of each nanostructured mat its resistance and response decrease, both, before and after heat treatment. In particular, before heat treatment, the measured resistances decreased from 5350 Kn (film 1) to 8.8 Kn (film 2), whereas the responses correspondingly decreased from 39 to 7. After heat treatment, we found that the films' resistances decrease from 1800 Kn (film 1) to 5.5 Kn (film 7), and the responses drop from 47 to 9. Note from the results of Fig. 6 that film 5 did not seem to obey this trend, possibly because of either an inaccuracy in the experimental measurements or the unusual structure of the MWCNT network in this film classifying it into the RJ ;::; RNT case (see also later). Inspection of Fig. 6 further leads to the conclusion that the films' heat treatment resulted in a reduction of their resistances but, interestingly, to an increase of their responses. For instance, in the thinnest film (1) the resistance decreased from 5250 Kn to 1800 Kn, while the response increased from 39 to 47.

Before /teat (reatment After /teat treatme"t

ID Film res;stfll1celKO response Film resistance KO response

Film 1 5150 39 1800 47 Film 2 1050 27 341 30 Film 3 478 18 140 31 Film 4 350 14 130 15 FilmS 113 70':-Film 6 44 18 11 Film 7 8.8 5.5

4

Fig 6. This table shows film resistance and response before and after heat treatment

IV. CONCULUTION

Several films of MWCNT on glass were prepared by pulse valve deposition. By investigating the electrical resistance of these samples, which were cycled between vacuum and water vapour environments, we have found that the MWCNT mats can act as good gas sensors, featuring a fast and significant response. These nanostructures were operational even at low temperatures, in contrast with other semiconductor sensors. Furthermore, they worked with enhanced efficiency and lower power consumption. We have found that heat treatment and film thickness considerably affect the films' electrical resistance.

ACKNOWLEDGMENT

This work was conducted at the Advanced Technology Institute of University of Surrey. The authors would also wish to thank Dr Kosmas Tsakmakidis (bnperial College London) for helpful discussions and help with the preparation of the manuscript.

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