highly sensitive thin film nh3 gas sensor operating at room

The gas-sensitive composite thin film was prepared by using both commercially available multi-walled carbon nanotubes MWCNTs and nanosized SnO2 dispersion.. The effect of the preparation

Sensors and Actuators B 129 (2008) 888–895

Nguyen Van Hieua,b,∗, Luong Thi Bich Thuya, Nguyen Duc Chiena,b,c

aInternational Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), Viet Nam

bHanoi Advanced School of Science and Technology (HAST), Hanoi University of Technology (HUT), Viet Nam

cInstitute of Engineering Physics (IEP), Hanoi University of Technology (HUT), Viet Nam

Received 21 February 2007; received in revised form 26 September 2007; accepted 27 September 2007

Available online 13 October 2007

Abstract

A SnO2/MWCNTs composite-based NH3sensor working at room temperature was fabricated by thin film microelectronic technique The gas-sensitive composite thin film was prepared by using both commercially available multi-walled carbon nanotubes (MWCNTs) and nanosized SnO2 dispersion Microstructure and surface morphology of the composite were investigated and they revealed that the MWCNTs were still present and well embedded by SnO2particles in the composite powder as well as in the composite thin film at calcination temperatures up to 550◦C The effect of the preparation process of the sensitive composite thin film on gas-sensing properties was examined, and the preparation process parameters such as MWCNTs content, MWCNTs diameter, calcination temperature, and film thickness were optimized

At room temperature, the optimal composite sensor exhibited much higher response and faster response-recovery (less than 5 min) to NH3gas of concentrations ranging from 60 to 800 ppm, in comparison with the carbon nanotubes-based NH3sensor Based on the experimental observations,

a model of potential barrier to electronic conduction at the grain boundary for the CNTs/SnO2composite sensors was also discussed

© 2007 Elsevier B.V All rights reserved

Keywords: Nanocomposites; Carbon nanotubes; Gas sensors

1 Introduction

SnO2-based sensors have been extensively investigated since

they can detect a wide variety of gases with high sensitivity

and good stability at low production cost[1–3] However, like

other semiconductor type gas sensors, SnO2sensors should be

operated above room temperature, which brings about much

inconvenience for practical applications and sometimes it is even

unsafe for detecting combustion gases[4–6] Currently, SnO2

and noble metal doped SnO2-based sensors are commercially

available [7,8] Still, much effort has been made to improve

gas-sensitivity as well as to reduce operating temperature by

introducing dopants or decreasing SnO2 particle size to the

nanoscale (<10 nm)[2,4,5,9]

∗Corresponding author at: International Training Institute for Materials

Sci-ence (ITIMS), Hanoi University of Technology (HUT), No 1 Dai Co Viet Road,

Hanoi, Viet Nam Tel.: +84 4 8680787; fax: +84 4 8692963.

E-mail address:hieunv-itims@mail.hut.edu.vn (N Van Hieu).

Carbon nanotubes (CNTs) special geometry and their amaz-ing feature of beamaz-ing all surface reactamaz-ing materials offer great potential applications as gas sensor devices working at room temperature It has been reported that the CNTs are very sensi-tive to surrounding environment The presence of O2, NH3, NO2 gases and many other molecules can either donate or accept electrons, resulting in an alteration of the overall conductiv-ity [10,11] Such properties make CNTs ideal for nanoscale gas-sensing materials, and CNTs field effect transistors and conductive-based devices have already been demonstrated as gas sensors[12–15] However, the CNTs still have certain lim-itations for gas sensor application such as long recovery time, detection of limited gases, and strong influence of humidity and other gases

Recently, the combinations of metal oxides such as SiO2, TiO2, SnO2, and the CNTs have been paid much attention for various applications such as photocatalytic, anode materials for lithium-ion batteries as well as gas sensors[16–20] The combi-nation can be conducted by different ways such as SiO2/CNTs, TiO2/CNTs, and SnO2/CNTs composite[16–18], SnO2-coated CNTs[20], SnO2-filled CNTs[21], and SnO2-doped with CNTs 0925-4005/$ – see front matter © 2007 Elsevier B.V All rights reserved.

doi: 10.1016/j.snb.2007.09.088

[22] Gas sensors based on SnO2-coated CNTs and SnO2-doped

with CNTs have been reported by Liu et al.[20]and by Wei et

al.[22], respectively Recently, the composites of metal oxides

(SiO2, WO3, and SnO2)/CNTs for gas-sensing application have

also been reported[19,23,24] However, their gas-sensing

per-formance has not yet been much improved in comparison to

SnO2as well as CNTs-based sensors Furthermore, currently

reported composite sensors still operate at elevated

tempera-tures To the best of our knowledge, it seems that experimental

data on SnO2/CNTs composite-based gas sensors operated at

room temperature are still lacking

The NH3 gas sensors based on CNTs [25,26] and SnO2

[27–31]have been extensively investigated The SnO2-based gas

sensors can detect NH3gas with good sensitivity and

response-recovery time, but it only operates at elevated temperatures

In contrast, the CNTs-based sensors can detect NH3 gas at

room temperature, but their sensitivity is still low and

response-recovery time is still very long

In this paper, we present our current research on gas-sensing

properties of SnO2/MWCNT composites, in which we aim to

take advantage of both SnO2and CNTs to develop room

temper-ature gas sensors to detect NH3gas with much better response

and shorter response-recovery time, compared to those of the

sensors based on the SnO2or CNT material alone

2 Experimental

The SnO2dispersion (15% nanoparticles with particle size

of 10–15 nm dispersed in water) purchased from Chemat

Tech-nology Inc (US) [32] was used for the preparation of the

gas-sensing material Two kinds of MWCNTs with different

diameters (d < 10 and d = 60–100 nm), their lengths of 1–2␮m and their purity of 95% were used in this study (they were pur-chased from Shenzhen Nanotech Port Ltd Co., China [33]) The gas-sensing element based on a SnO2-MWCNTs compos-ite was fabricated in the following manners At the beginning, MWCNTs bundles and cetyltrimethyl ammonium bromide (C16TMAB A.R., Merck) were added and dispersed in the SnO2 dispersion by ultrasonic vibration for about 1 h to obtain a well-mixed suspension The immersion-probe ultrasonic with a high power up to 500 W (Model VC-505, Sonics, US) was used Then, the suspension of CNTs and SnO2nanoparticles was deposited

on the Pt interdigitated electrode by means of spin-coating The MWCNTs/SnO2composites with different MWCNTs con-tents were prepared for the sensitive thin film fabrication The thickness of the sensitive thin films was controlled by varying spin-coating speed The coating layer was dried in air for 24 h and subsequently calcinated for 1 h at different temperatures and conditions (vacuum or air atmosphere) The interdigitated elec-trode was fabricated using the conventional photolithographic method with a finger width of 100␮m and a gap size of 70 ␮m The fingers of interdigitated electrode were fabricated by sput-tering 10 nm Ti and 200 nm Pt on a layer of silicon dioxide (SiO2) with the thickness of about 100 nm thermally grown on top of a silicon wafer

The microstructure of the composite thin film was charac-terized by X-ray diffraction (XRD, Cu Ka radiation), using Bruker-AXS D5005 The morphology of the sensing layers was verified by field-emission scanning electron microscope (FE-SEM, 4800 Hitachi, Japan)

Fig 1 Apparatus for gas sensor testing.

The gas-sensing measurements were carried out as follows.

The sensor was first placed on a hot plate and electrically

con-nected by tungsten needles, and then all were loaded in a glass

chamber (see Fig 1) The desired NH3 gas concentrations,

obtained by mixing NH3 gas with air using a computerized

mass flow control system (AALBORG model

GFC17S-VALD2-A0200), were injected into the chamber subsequently The

injection of a certain amount of the mixed gas was accurately

controlled by a computer After a duration of time, the chamber

was purged with air and the experiment was repeated for another

cycles

The electrical resistance response during testing was

mon-itored by a precision semiconductor parameter analyzer

(HP4156A) The sensor response (S) for a given measurement

was calculated as follows: S = Rgas/Rair, where Rgasand Rairare

electrical resistances of the sensor in a tested gas and in air,

respectively

3 Results and discussion

3.1 Microstructure characterizations

The XRD patterns of blank SnO2, MWCNTs and MWCNTs/

SnO2 composite are compared in Fig 2 The most intense

two peaks of MWCNTs correspond to the (0 0 2) and (1 0 0)

reflections, respectively Only SnO2in crystalline phase can be

indexed from the patterns for SnO2and the composite It is

note-worthy that the characteristic peaks of MWCNTs can hardly

be identified from the patterns of the composite Although the

most intense peak of MWCNTs corresponding to (0 0 2)

reflec-tion overlaps the peak of crystalline tin oxide (1 1 0) reflecreflec-tion,

the composite presents a symmetric peak of the crystalline tin

oxide corresponding to (1 1 0) reflection in its diffraction

pat-terns Additionally, the other intense peak of MWCNTs due

to (1 0 0) reflection between 40◦and 50◦, where no peak can

be attributed to SnO2, is also absent for the composite This

Fig 2 XRD patterns of (a) SnO2 wt%, (b) MWCNTs wt%, and (c)

SnO2-10 wt% MWCNTs composites (Cu Ka radiation).

Fig 3 SEM images of (a) SnO2 wt% nanoparticles and (b) SnO2-10 wt% MWCNTs nanocomposites annealed at 550 ◦C in the vacuum of 10−2Torr.

observation can be hypothesized that the MWCNTs are well embedded in the SnO2matrix

The FE-SEM images of blank tin oxide and 10 wt% MWCNTs/SnO2 composite powder samples after heat treat-ment at 550◦C in vacuum (10−2Torr) are shown in Fig 3a and b, respectively Spherical fine particles (around 10 nm) were observed in the blank tin oxide sample This is just a rough esti-mation of the size of particles because of the limitation of the FE-SEM method One notes that the particle size of the tin oxide

in suspension solution was indicated by the producer to be less than 15 nm[32] As in the composite, it was found out that the CNTs disperse well and separate from each other clearly (see, Fig 3b) and CNTs are well embedded by spherical tin oxide nanoparticles Our sensing element is of a thin film type There-fore, the morphology of the composite thin film after the heat treatment at 550◦C in vacuum of 10−2Torr was also verified by the FE-SEM, and the result is shown inFig 4 It is observed that there are many fiber-like protrusions emerged from the SnO2 matrix, which may indicate that the CNTs are most embedded

in the SnO2 The CNTs on the surface of the composite thin film are also coated by SnO2nanoparticles as indicated in the inset ofFig 4 The diameter of the coated MWCNTs fibers is around 40 nm, which is larger than that of the pure MWCNTs

(d < 10 nm) It has been reported that there is a good attachment

of SnO2nanoparticles on CNTs due to the electrostatic inter-action between the tin oxide nanoparticles and the MWCNTs, which is quite strong so that the inner SnO nanoparticles

immo-Fig 4 SEM image of SnO2-10 wt% MWCNTs composites thin films annealed

at 550 ◦C in the vacuum of 10−2Torr.

bilized on the MWCNTs are stable[17,20,34,35]and agrees with

our experimental observations, where CNTs are well embedded

in the SnO2/MWCNTs composite of both powder and thin film

formations

3.2 Gas sensor characteristics

3.2.1 Sensor response at room temperature and sensing

mechanism

Fig 5plots a typical response curve to NH3gas at room

tem-perature of a 10 wt% MWCNTs/SnO2 composite sensor This

composite was calcined at 500◦C in vacuum of 10−2Torr The

response curve shows that the resistance of the sensor varies over

time with various cyclic tests It can be seen that the resistance

increases upon exposure to NH3gas and it returns to the original

value upon exposure to the air Since NH3is an electron donating

gas, the increase of the sensor resistance can be hypothesized that

the composite sensing layer behaves as a p-type semiconductor

We should note that the SnO2thin film cannot have resistance as

Fig 5 Sensor response of 10 wt%-MWCNTs/SnO2 composite sensor

calci-nated at 500 ◦C in the vacuum of 10−2Torr to different concentrations of NH3

gas.

low as that of the composite at room temperature Additionally,

it cannot respond to NH3gas at room temperature This implies that the response of the composite sensor should be mainly contributed by the MWCNTs, which have been well known to behave as a p-type semiconductor [12–15,25–28] Comparing with the CNTs-based NH3sensor[27,29]and the SnO2-based

NH3sensor [29–31]reported previously, as-synthesized com-posites SnO2/MWCNTs-based sensors have a higher response

to NH3gas at room temperature

The exact mechanism of the high response of the SnO2/MWCNTs composites as a sensing material is still not clear However, we speculate that the enhancement of the response to NH3gas of the composite sensors may result from the p–n hetero-junction formed by CNTs and SnO2 nanoparti-cles, which has been indicated by Wei et al.[22] The model

is similar to the p–n junctions of sensing materials, which have been investigated by several authors [36–39] The p–n semiconductor/SnO2gas sensor has been demonstrated to work

at room temperature They have proposed that the change in barrier height or in the conductivity of the SnO2sensitive layer may modulate the depletion layer at the p–n junction of the Si substrate This change of the depletion layer in the p–n junction, induced by the sensitive SnO2 layer, may cause the improve-ment in the performance of the gas sensor at low operating temperature

However, the SWCNTs-doped SnO2 sensor behaves as an n-type semiconductor, while our MWCNTs/SnO2 composite sensor behaves as a p-type semiconductor A plausible expla-nation is that the composite has a much higher CNTs content;

as a result, the major conducting carriers are the holes, which are mainly contributed by CNTs When the MWCNTs/SnO2 composites are exposed to NH3gas, NH3molecules may inter-act with the MWCNTs by replacing the pre-adsorbed oxygen [19,26,40], while NH3adsorbs and mutually interacts with oxy-gen on the surface of SnO2, resulting in oxidation of NH3gas

at the surface and removing the oxygen accordingly Therefore both of these effects can modulate the potential barrier of the hetero-junction formed by MWCNTs and SnO2and can change the conductivity of the composite material during the exposure to

NH3gas Apparently, this possible mechanism requires further experimental and theoretical investigations

Fig 6 is to show estimations of the response and recovery times of our best sensor, in which optimized parameters such

as MWCNTs content, thermal treatment condition and thick-ness were selected (will be shown later) In this figure, the time interval between measured points is 2 s It can be seen that the response-recovery time is less than 5 min.Fig 6also shows that the response occurred immediately after few seconds of gas-injection in the chamber The response time from A to B (Fig 6)

is the time needed for the gas in the testing chamber to become homogenous (seeFig 1) Previous reports have shown that the CNTs-based sensor can detect various gases at room temper-ature, but the response and recovery times are quite long, of the order of 1 h[14,25,26] This is hardly acceptable in prac-tice with CNTs-based sensors So, the use of SnO2/MWCNTs composites for the gas sensor can somehow overcome the problem

Fig 6 Response to NH3 of 15 wt%-MWCNTs/SnO2 composite sensor

calci-nated at 530 ◦C.

3.2.2 Effect of the MWCNTs content

It has been realized that when operated at room

tempera-ture SnO2-based sensors do not respond to NH3gas unlike the

MWCNTs-based sensors Therefore, it was predicted that the

content of MWCNTs in the composites strongly affected the

response of the composite sensors So various MWCNTs/SnO2

composites-based sensors, in which the MWCNTs content

(weight ratio of MWCNT to SnO2) was varied such as 5%,

10%, and 15%, were characterized Fig 7 shows the sensor

response versus NH3gas concentration of the composite sensors

with different MWCNTs contents It shows that the response of

the sensors depends strongly on the NH3gas concentration and

the slope (R/C) of the curve for linear fit is large enough

(0.03–0.05) for the gas sensor applications These values are

comparable with that previously reported for CNTs-based sensor

[14].Fig 7b plots the dependence of sensor response to 200 ppm

NH3gas on the MWCNTs content These sensors were annealed

at 530◦C in vacuum of 10−2Torr It can be seen inFig 7b that

the response of the sensor to NH3gas increases with increasing

MWCNTs content However, the relation between the response

and the NH3gas concentration is less linear in the case of high

MWCNTs content (see Fig.7a) So far we cannot increase the

MWCNTs content in the composite because the dispersion of a

higher MWCNT content in SnO2sol is not good enough and we

cannot get repeatable results To overcome the problem, further

studies are needed

3.2.3 Effect of the MWCNTs diameter

It was shown that the diameter of CNTs strongly affected

the electronic properties as well as gas-adsorption/desorption

behavior [41–43] Therefore, in this work, we also

stud-ied the effect of MWCNTs diameter on the response of the

MWCNTs/SnO2 composites-based sensor Fig 8 shows the

response of two composite sensors, which were fabricated by

using MWCNTs with diameters of lower than 10 nm and in

the range of 60–100 nm We observe that the composites using

MWCNTs with the larger diameter has higher response This

effect can be explained by the fact that the MWCNTs

embed-ded in SnO behave as nanochannels for the gas diffusion in

Fig 7 Effect of MWCNTs content on the response of the composite sensors: (a) the response of the different composites vs NH3 gas concentration and (b) the response to 200 ppm NH3 gas vs MWCNTs content.

Fig 8 Effect of the MWCNTs diameter on the response of the thin film com-posite sensors.

the composite materials So, larger diameter MWCNTs would

increase the number of gas molecules adsorbed on the material

The effect should be especially strong with the gas having larger

molecules like NH3 Specific surface area (SSA) of these CNTs

was examined by the BET method (data not shown here) The

SSA of MWCNTs with diameter of <10 and 60–100 nm were

242.2 and 45.2 m2/g, respectively In principle, the material with

a higher SSA would have a better gas response However, we

have observed an opposite effect for our case So, the SSA

fac-tor cannot be a piece of evidence for the difference in the sensor

response

3.2.4 Effect of thermal treatment conditions

This step was dedicated to investigate if any improvement

could be obtained in the detection of NH3 gas by

chang-ing the thermal treatment conditions In this experiment, the

composite sensors were calcinated at various temperatures of

400, 450, 500, and 550◦C The calcination at 500 and 550◦C

was carried out at vacuum of 10−2Torr to avoid the

burn-ing of CNTs because a thermal gravimetric analysis (TGA)

characterization (not shown here) pointed out that the

MWC-NTs in the composites started to burn out at temperature of

548◦C in the air. Fig 9a shows the response of the

sen-sors calcinated at different temperatures It clearly indicates

that the sensors calcinated at higher temperatures have much

higher response The sensor calcinated at 550◦C has the

high-est response in this experiment To be sure, we carried out

another experiment, in which the sensor was calcinated at a

temperature of 530◦C in vacuum of 10−2Torr As indicated

in Fig 9b, the response of this sensor is better than other

cases So, this can be considered as an optimized calcination

temperature

It has been reported in literatures[25–31]that SnO2/CNTs

composite sensors have better performance compared to the

SnO2and CNTs-based NH3 gas sensors So, we believe that

the contacts between SnO2nanoparticles and CNTs contribute

to the improvement of sensing performance of the composite

sensors Increasing the annealing temperature may result in the

improvement of the contact between SnO2 nanoparticles and

CNTs, and therefore, the sensing performance of the device

However, the higher calcinated temperature may also result in

burning of CNTs by residual oxygen or damaging of CNTs

structure, and thus the response decreases

3.2.5 Effect of the film thickness

It is well known that the thickness of the sensitive layer has a

great influence on the gas-sensing performance of thin film

sen-sors, which has provided a much better platform to produce high

performance gas sensors[44–49] In this work, we also explored

the effect of the thickness of the composite sensing layer on the

response, to find optimized thickness for the composite gas

sen-sor The sensing layer was fabricated by mean of spin-coating

The thickness of the film was therefore controlled by the spinner

speed as well as the deposition time.Fig 10shows the effect of

the composite film thickness on the sensor response It can be

seen that the sensor response to NH3gas of the SnO2/MWCNTs

composite gas sensor first increases as the thickness increases up

Fig 9 Effect of calcination temperature on the response of the composite sensor: (a) calcination temperatures from 400 to 550 ◦C with step of 50◦C and (b) calcination temperatures of 500, 530, and 550 ◦C.

to 400 nm but it decreases when the thickness further increases

to 600 nm

The result of previous studies by experiment and simula-tion on semiconductor oxide thin film showed that generally the response dropped as the thickness of the sensitive film increased

Fig 10 Effect of the film thickness on the response of the composite sensors.

[46,47] It was also concluded that this relation was prone to

dras-tic alterations with respect to the microstructural defects present

in the sensitive film employed Our experimental result shows

that if the MWCNTs/SnO2composite is too thin, the response

will be decreased Such gas-response dependent on the thickness

of the composite MWCNTs/SnO2thin film sensor has not been

clear so far It seems that the thin film composite cannot well

embed CNTs in the film due to the fact that CNTs have a

rela-tively large diameter, ranging from lower than 10 to 60–100 nm

Like semiconductor oxide gas sensors, the response of the

com-posite gas sensors could relate with the reactivity and diffusion

of gas molecules inside the gas-sensing layers[49] Therefore,

a increase in thickness of the thin film composite sensors results

in a decrease in the response due to the increase of the diffusion

length of gas[49]

4 Conclusion

A new composite MWCNTs/SnO2 thin film gas sensor

has been successfully developed with high response and good

response and recovery in detection of NH3 gas at room

tem-perature The composite sensor can solve the problems of

SnO2-based and carbon nanotubes-based sensors; the former

cannot detect NH3gas at room temperature and the latter has

very long recovery and response times in detection of NH3gas

at room temperature

The preparation of the MWCNTs/SnO2composite thin film

sensor was simple which both commercial SnO2nanoparticles

dispersion and MWCNTs were used; the fabrication process

involved the dispersion of MWCNTs in the SnO2 dispersion

using an ultrasonic high power immersion-probe and subsequent

spin-coating and thermal treatment

The response of the MWCNTs/SnO2composite thin film gas

sensor strongly depends on the preparation process of the

sen-sitive film The composite thin film with the MWCNTs content

of 15 wt%, the MWCNTs diameter of 60–100 nm, the

calcina-tion temperature of 530◦C under vacuum of 10−2Torr, and the

film thickness of 400 nm are optimal conditions This result also

implies that these conditions need to be optimized for practical

applications of the composites of semiconductor oxides/carbon

nanotubes as the gas sensors in general

The observations of the film morphology revealed that the

MWCNT bundles were embedded in the SnO2 nanoparticles

materials According to this result, a model of a potential barrier

to electronic conduction at the grain boundary for the composites

of CNTs/semiconductor oxide sensors is a plausible explanation

Acknowledgements

This work is financially supported by VLIR-HUT project,

Code AP05/Prj3/Nr03 The authors also acknowledge Grant

No 405006 (2006) from the Basic Research Program of the

Ministry of Science and Technology (MOST) and for partly

financial support from Third Italian-Vietnamese Executive

Pro-gramme of Co-operation in S&T for 2006–2008 under project

title, “Synthesis and Processing of Nanomaterials for Sensing,

Optoelectronics, and Photonic Applications”

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Biographies Nguyen Van Hieu received his MSc degree from the International Training

Institute for Material Science (ITIMS), Hanoi University of Technology (HUT)

in 1997 and PhD degree from the Department of Electrical Engineering, Univer-sity of Twente, Netherlands in 2004 Since 2004, he has been a research lecturer

at the ITIMS In 2007, he worked as a post-doctoral fellow, Korea University His current research interests include the nano-architectures of carbon nanotubes, oxide semiconductors and oxide semiconductor nanowires for chemical sensors.

Luong Thi Bich Thuy received the BS degree in physics at Hanoi University of

Education in 2004, and MSc degree in materials science from the International Training Institute of Material Science (ITIMS), Hanoi University of Technology (HUT), in 2006 Her research interest is the development of semiconductor oxide/carbon nanotubes composites gas sensors.

Nguyen Duc Chien received the engineering degree in electronic engineering

at Leningrad Electrotechnical University, Russian, in 1976, and the MSc and PhD in microelectronics at Grenoble Polytechnique University, France, in 1985 and 1988, respectively He has worked as associated professor at the Institute of Engineering Physics (IEP), Hanoi University of Technology (HUT) From 1989

to 1990 he worked as a visiting professor at the Grenoble University, France From 1992 to 2006 he was a vice director of the International Training Institute for Materials Science (ITIMS), HUT, where he established a Laboratory of Microelectronics and Sensors Since 2003 he has been the Director of the IEP, HUT His research interests include: characterizations and modeling of MOS devices, nanomaterials for chemical sensor, biosensor, optoelectronic materials and devices, and MEMS devices He has been a leader of many national research projects related to microelectronic devices and functional nanomaterials Dr Nguyen Duc Chien is a member of Physics Society of Vietnam and Vietnamese Materials Research Society.