plasma - enhanced chemical vapor deposition carbon nanotubes for ethanol gas sensors

Shih *4:* * Department of Materials Science and Engineering, National Tsing Hua University Hsinchu, 30013, Taiwan, ROC > Department of Materials Science and Engineering, National Chung

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Diamond & Related Materials

journal homepage: www.elsevier.com/locate/diamond

DIAMOND

RELATED MATERIALS

Plasma-enhanced chemical vapor deposition carbon nanotubes for ethanol gas sensors

Chia-Te Hu ?, Chun-Kuo Liu *, Meng-Wen Huang ”, Sen-Hong Syue ?, Jyh-Ming Wu‘, Yee-shyi Chang °,

Jien-W Yeh *, Han-C Shih *4:*

* Department of Materials Science and Engineering, National Tsing Hua University Hsinchu, 30013, Taiwan, ROC

> Department of Materials Science and Engineering, National Chung Hsing University Taichung, 40227, Taiwan, ROC

© Materials Science and Engineering, Feng Chia University Taichung, 40724, Taiwan, ROC

4 Institute of Materials Science and Nanotechnology, Chinese Culture University Taipei, 11114, Taiwan, ROC

Available online 12 November 2008

Keywords:

Carbon nanotubes

Ethanol

Conductance

Gas sensor

Adsorption

Surface modification

Carbon nanotubes (CNTs) have been fabricated by microwave plasma-enhanced chemical vapor deposition for detecting the presence of ethanol vapor The conductance of the CNTs decreases when the sensors are successively exposed to ethanol vapor at room temperature The surface of the CNTs was modified in oxygen plasma to elevate the detection sensitivity for ethanol Successful utilization of CNTs in gas sensors may open

a new window for the development of novel nanostructure gas devices

© 2008 Elsevier B.V All rights reserved

1 Introduction

The remarkable structural, electrical, mechanical, and chemical

properties of carbon nanotubes (CNTs) have generated great interest

in various fields [1-5] Of special interest is the gas adsorption

property that allows CNTs to be made as new gas materials, depending

on their large surface area [6-8] and hollow geometry Several models

that account for the gas sensor applications of CNTs have been

reported recently [9-16] Hyeok et al [17] fabricated a gas sensor from

a nanocomposite by polymerizing pyrrole monomers with single wall

carbon nanotubes (SWCNTs) Polypyrrole (Ppy) was prepared by a

simple and straightforward in-situ chemical polymerization of pyrrole

mixed with SWCNTs, and the sensor electrodes were formed by spin-

casting SWCNT/Ppy onto pre-patterned electrodes They found that

the sensitivity of the nanocomposite was about ten times higher than

that of Ppy The SWCNT bundles could be nanodispersed, which may

increase the specific surface area of the coated Ppy and thereby further

increase the sensitivity One of the most difficult aspects of CNT gas

sensing is that most techniques are based on SWCNT field-effect

transistors or require UV light irradiation to desorb the detected gas

molecules Furthermore, multiwall carbon nanotubes (MWCNTs) are

not very sensitive to ambient gases [12-15] Our objective in this

investigation is to modify the surface of MWCNTs for elevating the

detection sensitivity of an ethanol gas sensor It is therefore necessary

* Corresponding author Department of Materials Science and Engineering, National

Tsing Hua University Hsinchu, 30013, Taiwan, ROC Tel.: +886 3 5715131x33845;

fax: +886 3 5710290

E-mail address: hcshih@mx.nthu.edu.tw (H.-C Shih)

0925-9635/S - see front matter © 2008 Elsevier B.V All rights reserved

doi:10.1016/j.diamond.2008.10.057

to upgrade the surface structure of MWCNTs precisely in order to realize the applications of the devices However, only a few papers have investigated the use of MWCNTSs for ethanol detection Liang et al [16] reported a resistance sensor fabricated from MWCNTs coated with a thin tin-oxide layer and found that the barrier height between the tin- oxide grains on the MWCNTs varies for different gases so that the sensor resistance changes markedly, which makes the sensitivity of the sensors great enough for real applications In addition, the Shottky barrier between tin-oxide grains and MWCNTSs is very low, such that electrons conduct in the MWCNTs with low resistance

So far, only noise studies on gas sensors at elevated temperatures have been carried out Wan et al [18] fabricated a gas sensor from a

ZnO oxide nanowire; its sensitivity at an ethanol concentration of

100 ppm increased sharply as the temperature increased from 200 to

300 °C, mainly due to the enhanced reaction between the ethanol and

the absorbed oxygen at an elevated temperature This model, however, cannot be applied under room temperature conditions Therefore, a more generalized model is required In this paper, we present the results of ethanol gas detection by MWCNTs at room temperature and use a distinct oxygen plasma treatment to enhance the sensitivity It was found experimentally that the sensitivity of MWCNT gas sensors varies significantly with various gas concentra- tions at room temperature after oxygen plasma modification In order

to perform our study, the structure was systematically analyzed by field emission scanning electron microscopy (FE-SEM), and further quantitative analysis was conducted by high-resolution transmission electron microscopy (HRTEM) In the sensor structure, MWCNT growth occurred on an aluminum oxide substrate that was strongly held by two alligator clamps acting as electrodes for the conducting current This assembly is highly sensitive to the presence of the

1

0 se

Z ` 35 sec

SY

Fig 1 FE-SEM images of CNTs treated in oxygen plasma at a microwave power of 600 W for (a) 0 s; showing the amorphous carbon clamped on the surface of CNTs, (b) 5 s, (c) 20 s, (đ) 35 s, (e) 60 s, and (f) 90s

ethanol molecules The conductance of the sensors adjusts itself when

they are exposed to ethanol gas

2 Experimental

In this work, microwave plasma enhanced chemical vapor

deposition (MPECVD) was used to synthesize the carbon nanotubes

An iron-containing compound was used as the catalyst, which was

first coated onto a non-conductive aluminum oxide substrate using a

sol-gel method to promote the growth of carbon nanotubes The

substrate was placed in the MPECVD chamber, where a mixture gas of

J

methane and hydrogen (1:10) was introduced and simultaneously decomposed by the microwaves to synthesize carbon-related materi- als During this period, the pressure was kept at 20 Torr with a microwave power of 1.5 kW at 650 °C as measured by a thermocouple (CA) After the growth of the CNTs, the microwave chamber was

cooled down to room temperature, and surface modification treat-

ment was subsequently started The oxygen plasma treatments for the MWCNTs were conducted under the following conditions: oxygen flow rate of 20 sccm, operating pressure of 0.5 Torr, microwave power

of 600 W, process duration of 5-35 s, and average sample temperature

of about 30 °C It was demonstrated that the treatment result was

Fig 2 HRTEM images of CNTs treated in oxygen plasma for (a) 0 s; denoting the amorphous carbon clamped on the surface of CNTs, (b) 20 s; showing the removal of the amorphous

directly related to the excited species density, which was controlled by

the treatment duration The morphology of the specimens was

examined by field emission scanning electron microscopy (FE-SEM,

JSM-6500F) High-resolution electron microscopy (HRTEM, JEOL JEM-

2010) was performed at 200 kV with a point resolution of 0.19 nm and

a lattice resolution of 0.1 nm

Adsorption isotherm experiments were performed at -261.17 °C

with a Micromeritics ASAP 2000 accelerated surface and porosimetry

analysis system using N2 gases on the MWCNT samples with masses of

~0.2 g Before each adsorption isotherm test, the nanotube bundles

to ensure complete desorption of adsorbates All measurements were

conducted at -261.17 °C; the saturation pressure po for N2 at this

temperature is 760 Torr Gas-sensing experiments were carried out

using a volt-amperometric technique During the experiment, the

MWCNT-based gas sensor was placed in a sealed chamber with an

electrical feedthrough Pure ethanol gas flowed through the sealed

chamber while the electrical properties of the MWCNTs were

monitored All such measurements were taken at 25 °C

3 Results and discussion

Fig 1 shows the surface morphology of the CNTS with various

modification durations from the SEM analysis Clearly visible in Fig 1(a)

are the amorphous carbon layers deposited on the surface, connecting

with each other, which probably result from incomplete carbon atomic

piling When CNTs were treated for 5 and 20 s, the amorphous carbon

domains are eliminated, and the tubes become smoother and cleaner as

shown in Fig 1(b) and (c) After 60 and 90s, as seen in Fig 1(e) and (f), the

carbon nanotubes become ambiguous and one can not distinguish a

single carbon nanotube from the rest From the results, it can be seen

that the oxygen plasma treatment can remove the fragile parts like

amorphous carbon, but too much treatment would eventually destroy

the CNTs

High-resolution transmission electron microscopy (HRTEM) played

a vital role, as it is the only technique that allows for real space imaging

of atomic distribution in nanoparticles, particularly when the particle

size is small With consideration of the particle shape symmetry,

HRTEM can be used to determine the 3D shape of small particles

although the image is a 2D projection of a 3D object The grown carbon

nanotubes for TEM analysis were separated from the substrate and

then ultrasonicated in ethanol After ultrasonic treatment, a drop of

liquid was then sprayed onto a carbon-coated copper grid The CNTs

were analyzed by TEM to confirm that they were really CNTs and not

carbon fibers Fig 2 reveals that the CNTs have an inner diameter of

~8 nm, an outer diameter of ~23 nm, and the distance between wall

layers is ~0.33 nm Comparison of the images in Fig 2(a) and (b)

indicates that the oxygen plasma modification exerted to the surface of

the CNTs can remove impurities as well as amorphous carbons This is

due to the fact that the amorphous carbons are easily oxidized under

the oxygen plasma species Unfortunately, we were not able to obtain

the microstructure when we continuously applied the oxygen

plasma treatment for a duration longer than 20 s, as shown in

Fig 2(c) This is because ion bombardments may cause the creation

of vacancies and interstitials in the MWCNTs The surface structure,

including defects and amorphous carbons on the carbon nanowires,

is removed due to the ion sweeping, which leads to an increase in

diameter The results from TEM in combination with SEM figures

reveal that the surface of the CNTs can be indeed modified by oxygen

plasma treatment, but prolonged treatments are harmful and can

destroy the outside walls

Using the Brunauer-Emmett-—Teller (BET) method, the adsorption

isotherms of Nj on the MWCNTs were measured for all the samples

Fig 3 Adsorption and desorption isotherms of Nz on modified MWCNTs at -261.17 °C;

(a

(b

(d)

200

150

100

50

250

200

150

100

50

250

200

150

100

50

250

200

150

100

50

—s—ads

—s=— des

5

Relative Pressure (p/pạ)

—m— adS

—e— des

|

Ề

f

Ụ

Relative Pressure (p/p,)

—s—ads

—e— des

(0.458,42.4) (0.732,61.15)

0.0

Relative Pressure (p/p,)

—— ads

—e— des

i

,

|

=

C j

(0.3938,34.2) (0.73,54.2) -

Relative Pressure (p/p,)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 200 400 600 300 1000 1200

50+

\

¢

é

=

oS

oS

Ss

fh C,H;OH

concentration(ppm)

Fig 4 Adsorption isotherm curves for various modified CNTs at (a) low relative 140+

pressure, (b) medium relative pressure, and (c) high relative pressure

at -26117 °C Results showed that all of the adsorption isotherms

a

(at -26117 °C) for the MWCNTs are close to Type IV [19-20], ie.,

Oxygen Specific surface Monolayer Single point Micropore area 0 200 400 600 800 1000 1200

treating capacity: plpu 0.1984 (mˆJg)

ime (s) m (cele ) Fig 5 Three cycles of response-recovery characteristics of the MWCNTs exposed to

0 91.3 21.0 90.0 14.5 various ethanol concentrations for distinct modification for (a) 0 s, (b) 5 s, (c) 20 s, and

when p/p,*0.06-0.73, and the amount adsorbed increases steeply after

p/P 0.73, as shown in Fig 3 Moreover, the overall enlarged adsorption

isotherms can be divided into three parts, A, B, and C, as shown in Fig 4,

which indicates that the capabilities of the CNTs to absorb gases under

various oxygen plasma treatments follow the order: 20 s>5 s>O s>35s,

This curve is in qualitative agreement with the SEM and TEM observations

for the same result As the duration of the modification increases, the gas

adsorption property enhances Unfortunately, the gas volume adsorption

drops dramatically for prolonged exposure in oxygen plasma This effect

might be due to the defects and amorphous carbons, which decrease the

gas adsorption capability Applying the BET method to the data in Fig 3

yields the monolayer adsorption capacity (V,,) and the specific surface

area of adsorption (A,), where molecular cross-sectional areas of 91.3, 97.0,

102.5, and 90.1 m?/g were used for N2 during the respective duration of

—====

Electrical current in air

CạH;=O

H

Electrical current in alcohol

350L

300}

=, |

250L

< 200L

‘5 150L

2 |

C,H;OH concentration(ppm)

3.8+ |—=—0 sec

36L [eve 35sec

< 3.4 |

S

& 3.0L

= [

8 2.8L

5 2.6}

24L oo

C;H;OH concentration(ppm)

Fig 6 (a) In air, negatively charged oxygen adsorbates cover the surface of the CNTs and make

the CNTs become hole-doped because of oxygen’s electron affinity In ethanol gas, oxygen

adsorbates react with the adsorbed ethanol molecules attached by hydrogen bonds, trap

electrons, and cause the current to decrease (b) Changes in sensitivity as a function of MWCNT

oxygen plasma modification, i.e., 0, 5, 20, and 35 s Table 1 summarizes the

resulting V,, and A, values of the MWCNTs

Fig 5 gives the electrical property curves of the MWCNT-based

sensors in a sealed chamber, which were evacuated to 10°° Torr by a

turbo pump We plot a typical time evolution of the conductance at a temperature of 25 °C for a MWCNT-based sensor successively exposed to

10 ppm, 20 ppm, and 30 ppm ethanol gas The dotted line represents the variation of ethanol concentration, which corresponds to the dropping time It can be seen that as ethanol gas with a concentration of 10 ppm

was introduced for 60 s, the conductance of the gas sensor decreases

from its original electrical potential to the electrical potential due to adsorption As the ethanol gas was removed, conductance of the gas sensor was lost Similar behavior occurred at other concentrations of ethanol gas (20 ppm and 30 ppm) The sensing mechanism is surface conduction modulated by adsorbed gas molecules; the electrical conductivity depends strongly on surface states produced by molecular adsorption, which result in space-charge layer changes and band

modulation In our new sensors based on carbon nanotubes, a large

fraction of the atoms are present at the surface and the surface properties become paramount Oxygen is known to have good charge transfer to planar defected graphite, especially in the presence of catalytic metallic particles Carbon nanotubes can become hole-doped in the presence of adsorbed oxygen after oxygen plasma treatment because of the electron affinity of oxygen [21] Owing to the interesting layered structure, especially with ethanol intercalated, it reacts with C—O layers through hydrogen bonds to charge the conductivity of the sensors and causes the current to decrease, as shown in Fig 6(a) Furthermore, the sensitivity was highest for the specimen treated for 20 s with oxygen plasma In

order to realize the sensitivity of various surface modification durations,

the sensitivity (S) can be calculated by S=Cyir/Cgas, where C,ir is the conductivity of the surrounding air, and C,a, is the conductivity when ethanol is introduced Fig 6(b)-(d) illustrates the sensitivity of various

surface modification durations As we can see, the sensitivity of the

specimen with 20 s of oxygen plasma treatment is superior to that of the

other specimens, which were treated for other time durations The

enhanced sensitivity may arise from the promoted surface-to-volume ratio of CNTs and eliminate the unsteady performance of non-treated CNTs (Fig 5(a)) Moreover, once the ethanol is introduced, the conductivity signal drops dramatically after the modification It might

be that the defects and amorphous carbons decrease the gas adsorption activation and that oxygen plasma treatment will however overcome this disadvantage But we are able to decrease the sensitivity if too much oxygen plasma treatment is applied

4 Conclusions

The MWCNT-based gas sensors made by MPECVD for detecting ethanol molecules have been studied at a ppm-level at room temperature The conductance of the sensors decreases when the sensors are exposed

to ethanol gas Development of the oxygen plasma modification lies at the heart of success in enhancing the sensitivity It is expected that this technique will come into use as its utilization is more widely appreciated These studies have so far proven that CNTs can detect ethanol gas remarkably, and further experiments on gas sensor devices still remain to

be conducted

Acknowledgment The authors would like to thank the National Science Council of the Republic of China for support of this research under contract NSC95- 2221-E-034-020-MY2

References

[1] YH Yang, CY Wang, U.S Chen, W,J Hsieh, Y.S Chang, H.C Shih, J Phys Chem C

111 (2007) 1601-1604

3] S.H Tsai, CT Shiu, §.H Lai, H.C Shih, Carbon 40 (2002) 1597-1617

{4] LH Chan, K.H Hong, S.H Lai, XW Lin, H.C Shih, Thin Solid Film 423 (2002) 27-32

15] CH Wang, HY Du, YT Tsai, CP Chen, CJ Huang, LC Chen, K.H Chen, H.C Shih,

J Power Sources 171 (2067) 55-62

{B] Kay H Baughman, Changxing Cui, Anvar A Zakhidov, Zafar Iqbal, Joseph N Barisci,

Geoff M Spinks, Gordon G Wallace, Alberto Mazzoldi, Danilo De Rossi, Andrew G

Ringler, Oliver faschinski, Siegmar Roth, Miklos Kertesz, Science 2841999) 1340-1344

17| Chunming Niu, Enid Kk Sichel, Robert Hoch, David Moy, Howard Tennent, Appl

Phys Lett, 70 (1997) 1480-1482

{8] Philippe Serp, Massimiliano Corrias, Philippe Kalck, Appl Catal A: Gen, 253 (2003)

{O] PE Qi, 0 Vermesh, M Grecu, A Javey, Q Wang, HJ Dai, S Peng, iC] Cho, Nano Lett

3 (2003) 347

[10] J Li, YE Lu, Q Ye, ML Cinke, J) Han, M Meyyappan, Nano Lett 3 (2003) 929

[HT] EP Novak, ES Snow, EJ, Houser, D Park, |.L Stepnowski, R.A McGill, Appl Phys

Lett 83 (2003) 4026

[12] O.K Varghese, P.D Kichambre, D Gong, K.G Ong, E.C Dickey, CA Grimes, Sens, Actuators,B 81 (2001) 32

[13] A Modi,N Koratkar, £ Lass, B.Q Wei, P.M Ajayan, Nature (London) 424 (2003) 171, [14] L Valentini, L Lozzi, C Cantalini, | Armentano, JM Kenny, L Ottaviano, § Santucci, Thin Solid Films 436 (2603) 95,

[15] ¥.M Wong, W.P Kang, LL Davidson, A Wisitsora-at, KL Soh, Sens Actuators, B

93 (2003) 327

[16] V.X Liang, YJ Chen, TH Wang, Appl Phys Lett 85 (2004) 4

[17] K.H An, 8.Y, Jeong, H.R Hwang, V.H Lee, Adv Mate 16 (2004) 12

[18] Q, Wan, QH Li, Yj Chen, TH Wang, Appl Phys Lett 84 (2004) 12

[19] CM Yang, K Kaneko, M Yudasaka, S fijima, Physica B 323 (2002) 140 [20] E Frackowiaka, K Metenier, V Bertagna, EF Beguinb, Appl Phys Lett 77 (2000) 2421-2423

[21] Philip G Collins, Keith Bradley, Masa Ishigami, A Zettl, Science 287 (2000) 1801.