gas sensing characteristics and porosity control of nanostructured films

The sensing films composed of nanotubes prepared at 230◦C showed a high sensor response to toluene at 500◦C as compared with those composed of TiO2 nanoparticles.. Scanning electron micro

Contents lists available atScienceDirect Sensors and Actuators B: Chemical

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / s n b

Gas sensing characteristics and porosity control of nanostructured films

Min-Hyun Seoa, Masayoshi Yuasab, Tetsuya Kidab, Jeung-Soo Huhc, Kengo Shimanoeb,∗, Noboru Yamazoeb

aDepartment of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

bDepartment of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

cDepartment of Materials Science and Metallurgy, Kyungpook National University, Daegu 702-701, South Korea

a r t i c l e i n f o

Article history:

Received 15 August 2008

Received in revised form 19 December 2008

Accepted 30 January 2009

Available online 10 February 2009

Keywords:

TiO 2 nanotube

VOC

Porosity control

Porous film

Hydrothermal treatment

a b s t r a c t

Preparation and morphology control of TiO2nanostructured films for gas sensor applications were inves-tigated To examine the effect of the morphology of sensing films on the sensing characteristics, TiO2with different morphologies, nanoparticles and nanotubes, were used for the film preparation TiO2nanotubes were prepared by a hydrothermal treatment of TiO2nanoparticles in a NaOH solution at 160, 200, and

230◦C for 24 h and subsequent washing with an HCl solution Uniform sized TiO2nanotubes of 1␮m in length and 50 nm in diameter were formed at 230◦C The sensing films composed of nanotubes prepared

at 230◦C showed a high sensor response to toluene at 500◦C as compared with those composed of TiO2

nanoparticles Scanning electron microscope (SEM) analysis and pore size distribution measurements indicated that the sensing films composed of the TiO2nanotubes had a high porous morphology with a peak pore size of around 200 nm, which can promote the diffusion of toluene deep inside the films and improve the sensor response The obtained results demonstrated the importance of microstructure con-trol of sensing layers for improving the sensitivity to large size molecules like volatile organic compounds (VOCs)

© 2009 Elsevier B.V All rights reserved

1 Introduction

Semiconductor gas sensors based on metal oxides have aroused

considerable interest owing to their high sensitivity to pollutant

gases, low cost, and small size[1–3] Various oxide materials such

as SnO2 [4], WO3 [5], TiO2 [6–9], and ZnO[10]have so far been

used for gas sensors Among them, TiO2is a well-known important

functional material used for a variety of applications such as

pho-tocatalysts[11], dye-sensitized solar cells[12], batteries[13], and

pigments[14] For gas sensor applications, it has been reported that

TiO2with a large surface area shows good sensing properties to CO

[7], H2[8], and NOx[9] The important feature of TiO2-based gas

sensors is that they can be operated at high temperature because

of the good chemical stability of TiO2

Recently, utilization of nanostructured TiO2for various devices

has attracted much attention due to prospects for upgrading

the device performance through the nanostructure control of

devices[15,16] Various nanostructured TiO2such as nanorods[17],

夽 Paper presented at the International Meeting of Chemical Sensors 2008

(IMCS-12), July 13–16, 2008, Columbus, OH, USA.

∗ Corresponding author Tel.: +81 92 583 7876; fax: +81 92 583 7538.

E-mail address:simanoe@mm.kyushu-u.ac.jp (K Shimanoe).

nanowires[18], nanosheets[19], and nanotubes[20–23]have been prepared by wet-chemical methods TiO2nanotubes were firstly reported by Kasuga et al in 1998[20] Since then, the chemical and physical properties of TiO2nanotubes have been studied intensively due to the ease of the preparation using a simple hydrothermal method The nanotubular architecture can achieve high specific surface area and thus TiO2 nanotubes prepared by a hydrother-mal method have been successfully utilized for photocatalysis[24], dye-sensitized solar cells[25], and lithium-ion batteries[26] TiO2 nanotubes and nanofibers prepared by anodization and electrospinning methods have been used for the detection of H2 [27,28], NO2[29], and water vapor[30,31] Varghese et al reported

a large change in the electric resistance of arrays of TiO2 nan-otubes in response to H2[27,28] However, the sensing mechanism

of nanostructured TiO2films composed of nanotubes or nanofibers has not yet been understood well It is thus of considerable interest and importance to examine the detailed gas sensing properties of nanostructured films based on TiO2nanotubes

For semiconductor gas sensors, the porosity of sensing films is

an important parameter[32–34]; porous sensing films can facilitate gas diffusion deep inside the films and reach high gas sensitivity In particular, the microstructure control is important to detect large size molecules such as volatile organic compounds (VOCs) Note that it is possible to prepare porous gas sensing films using TiO2 0925-4005/$ – see front matter © 2009 Elsevier B.V All rights reserved.

Fig 1 SEM images of the sensing films composed of (a) commercial TiO2 nanoparticles (P-25) and those hydrothermally treated at (b) 160, (c) 200 and (d) 230◦C (a’)–(d’) show the corresponding images after calcination at 600 ◦ C.

nanotubes because of their high anisotropic shape; packing tubular

particles would produce loosely-packed particulate films with

ran-domly distributed pores while hindering intimate contacts among

the particles Such a porous film is expected to show sensitivities

to large sized gas molecules like VOCs

In this study, we fabricated porous gas sensing films composed of

TiO2nanotubes prepared by a hydrothermal treatment and studied

the gas sensing properties of the porous TiO2 nanotubular films

The investigation was carried out with a particular emphasis on

the promotion of the gas sensitivity through the porosity control of gas sensing films

2 Experimental

TiO2nanotubes were prepared by a hydrothermal method as reported in the literature[18] 0.5 g of a TiO2commercial powder (Degussa P-25 (mean particle size: ca 20 nm)) was hydrothermally treated with 30 mL of a NaOH solution (10 mol/L) at 160, 200 and

Fig 3 Pore size distribution of the sensing films composed of (a) commercial TiO2

nanoparticles (P-25) and those hydrothermally treated at (b) 160, (c) 200 and (d)

230◦C They were calcined at 600◦C.

230◦C for 24 h in a Teflon-lined autoclave After the treatment, the

TiO2powder was washed with 50 mL of a HCl solution (0.2 mol/L)

under ultrasonic irradiation for 1 h Then, the obtained products

were filtered and dried to recover TiO2 nanotubes The resulting

products were characterized by X-ray diffraction (XRD) with Cu-K␣

radiation, scanning electron microscopy (SEM), and transmission

electron microscopy (TEM)

For electrical and sensing characterizations, TiO2 thick films

were fabricated by a screen-printing method By using a binary

dispersant of ␣-terpineol (95 mass%) mixed with ethyl cellulose

(5 mass%), the sensor material was converted into a paste, which

was screen-printed on an alumina substrate attached with a pair of

Au electrodes (line width: 180␮m, distance between lines: 90 ␮m,

sensing layer area: 64 mm2) After screen-printing, the fabricated

sensor devices were calcined at 600◦C for 1 h The porosity of the

films was measured by mercury porosimetry

The electrical and gas sensing properties of the films were tested

using CO, H2, ethanol, and toluene as target gases at 450–550◦C

Measurements were performed using a conventional gas flow

appa-ratus equipped with an electric furnace at a gas flow rate of

Fig 4 XRD patterns of (a) commercial TiO2 nanoparticles (P-25) and those

◦

Fig 5 XRD patterns of (a) commercial TiO2 nanoparticles (P-25) and those hydrothermally treated at (b) 160, (c) 200 and (d) 230 ◦ C They were calcined at

600◦C.

100 cm3/min The sensor response was defined as Rair/Rgas, where

Rairand Rgasare the electric resistances in air and that in a test gas, respectively

3 Results and discussion

3.1 Characterization of nanotubular TiO 2 films

Fig 1shows SEM images of the surface of TiO2thick films com-posed of TiO2nanotubes prepared by the hydrothermal treatment

at 160, 200, and 230◦C, together with that of a film composed of commercial TiO2 nanoparticles (P-25) The surfaces of the films were observed before and after calcination at 600◦C It is obvious that the morphology of the films was significantly changed after the hydrothermal treatment, depending on the treatment temperature The treatment at 160◦C produced heavily aggregated TiO2particles,

as shown inFig 1(b) On the other hand, the treatment at 200 and

230◦C resulted in the formation of tubular TiO2of 1␮m and 50 nm

in length and diameter, respectively, as shown inFig 1(c) and (d) With increasing the temperature of the hydrothermal treatment, the length of the tubes increased and the distribution of the diam-eter became narrower Nanotubes with a more uniform size were formed by the hydrothermal treatment at 230◦C The SEM images confirm that the films composed of the TiO2nanotubes are more porous than those composed of TiO2nanoparticles, as expected It

is also noted that the calcination induced no drastic change in the morphology of TiO2, but resulted in slight sintering of TiO2 aggre-gates and nanotubes obtained at 160 and 200◦C, respectively, as shown inFig 1(b’) and (c’)

The nanostructures of the TiO2 nanotubes calcined at 600◦C were observed by TEM, as shown inFig 2 The TEM observation

Table 1

Specific surface area of commercial TiO 2 nanoparticles (P-25) and those hydrother-mally treated at 160, 200 and 230◦C, together with the peak pore size of the sensing films composed of the particles They were calcined at 600◦C.

Sample Specific surface area (m 2 /g −1 ) Peak pore size (nm)

Fig 6 Electric resistance in air as a function of operating temperature for the thick

films composed of (a) commercial TiO 2 nanoparticles (P-25) and those

hydrother-mally treated at (b) 160, (c) 200 and (d) 230 ◦ C.

revealed that the tubular structure is stable even after calcination at

600◦C and TiO2aggregates obtained at 160◦C have no tubular

struc-ture The wall thickness of nanotubes prepared at 200 and 230◦C is

estimated to be ca 5 nm from the TEM images In addition, a

uni-form size distribution was confirmed for the nanotubes treated at

230◦C, suggesting that the optimum temperature of the

hydrother-mal treatment is 230◦C The mechanism of the nanotube formation

by a hydrothermal treatment has been reported as follows[23];

first, TiO2transformed into a layered compound of Na2Ti2O5·H2O

by NaOH The washing of the product with HCl results in the ion

agreement with the SEM results The pore size at the maximum pore volume for the films composed of TiO2nanoparticles, aggre-gates, and nanotubes is summarized inTable 1, together with the specific surface area of TiO2 measured by the B.E.T method The surface area of calcinated nanotubes was lower than that of TiO2 nanoparticles A peculiar feature was observed in the TiO2 aggre-gates obtained at 160◦C that show a high surface area but a small peak pore size

The crystal structure of the obtained TiO2nanotubes was ana-lyzed by XRD Fig 4 shows the XRD patterns of the products hydrothermally treated at different temperatures The patterns show that the crystal phase of TiO2 changed from a mixed phase

of rutile and anatase to that of H2Ti3O7 and anatase after the hydrothermal treatment, irrespective of reaction temperature The

Fig 7 Sensor responses to (a) CO (500 ppm), (b) H2 (500 ppm), (c) ethanol (47 ppm), and (d) toluene (50 ppm) gases in the temperature range of 450–550 ◦ C for the devices using commercial TiO nanoparticles (P-25) and those hydrothermally treated at 160, 200 and 230 ◦ C.

Fig 8 Sensor responses to CO (500 ppm), H2 (500 ppm), ethanol (47 ppm), and toluene (50 ppm) gases at 500 ◦ C for the devices using (a) commercial TiO 2 nanoparticles (P-25) and those hydrothermally treated at (b) 160, (c) 200 and (d) 230 ◦ C.

main peak for the TiO2nanotubes was assigned to H2Ti3O7

accord-ing to recent structural characterizations[35,36], although different

assignments of nanotubes to H2Ti2O5·H2O and H2Ti4O9·H2O have

also been reported[23,37].Fig 5shows the XRD patterns of the

products after calcination at 600◦C The crystallization of H2Ti3O7

to anatase occurred in the TiO2 nanotubes obtained at 200 and

230◦C, although the H2Ti3O7phase remained On the other hand,

for TiO2 aggregates obtained at 160◦C, the H2Ti3O7 phase

com-pletely transformed into the anatase phase after calcination It has

been reported that the thermal stability of the protonated titanate

phase is improved by the presence of sodium ions and the removal

of sodium ions promotes the thermal conversion to anatase[35] It

is suggested that the reaction of TiO2with NaOH is incomplete at

160◦C to produce partly Na-intercalated layered compounds on the

basis of the proposed mechanism discussed above and the SEM and

TEM results, and that the subsequent washing with HCl effectively

removed sodium ions from the layered compounds Consequently,

the low sodium content in aggregates obtained at 160◦C may assist

in the formation of the anatase phase

3.2 Gas sensing properties of nanotubular TiO 2 films

Fig 6shows the electrical resistances in air as a function of

temperature ranging from 450 to 550◦C for the fabricated films

composed of TiO2nanoparticles, aggregates, and nanotubes The

film using TiO2nanotubes obtained at 230◦C gave the highest

resis-tance, reflecting their higher porosity than the other films However,

the electric resistance of the other films was not correlated well

with their porosity This is because the electric resistance is

depen-dent on various parameters such as grain size, tube length, film

thickness, crystal structure, and physical parameters such as

car-rier density and effective mobility In addition, the observed thin

wall thickness (ca 5 nm) of the nanotubes may also be responsible

for the high electric resistance

The electric resistance decreased with increasing the operating

temperature, following the typical behavior of oxide

semiconduc-tor However, the electric resistance at lower than 500◦C exceeded

109, which is too high to be measured using a conventional elec-tric circuit Thus, the optimal operating temperature is judged to be

500◦C from a practical point of view

The sensor response of the fabricated films composed of TiO2 nanoparticles, aggregates, and nanotubes was examined against traces of CO, H2, ethanol, and toluene gases at 450, 500, and 550◦C,

as shown inFig 7 The fabricated sensors using TiO2(n-type semi-conductor) responded to the target gases by a decrease in the electric resistance Thus, the probable sensing mechanism is that

gases tested, despite their lower surface area than that of

com-mercial TiO2nanoparticles As noted above, for semiconductor gas

sensors, target gases diffuse in a sensing film through pores and

react with surface oxygen adsorbed on component particles to

induce the resistance change The concentration of the target gases

decreases inside the sensing film as they diffuse The component

particles located deep inside the film may remain intact or

inacces-sible for the target gases provided that the film is not sufficiently

porous This would lead to a decrease in the sensor response due to

a decrease in the utility factor of the sensing film or a decrease in

the accessibility of the target gas Thus, the effect of the porosity of

sensing films on the sensor response is more pronounced for target

gases with large molecular sizes As revealed by the pore size

dis-tribution measurements, the film using TiO2nanotubes prepared

at 230◦C has the peak pore size of around 200 nm, which is much

larger than those for the film using TiO2aggregates and nanotubes

prepared at 160 and 230◦C, respectively Such macropores can

pro-vide high-diffusivity paths for large toluene molecules and improve

the utility factor of the sensing film Thus, the observed particular

Fig 10 Response transients to toluene (50 ppm) at 500◦C for the devices using (a) commercial TiO 2 nanoparticles (P-25) and (b) those hydrothermally treated at

230 ◦ C.

increase in the sensor response to toluene for the film using TiO2 nanotubes prepared at 230◦C can be ascribed to the high porosity

of the nanotubular film, as is schematically shown inFig 9 In addi-tion, almost constant sensor responses to CO and H2were observed for the films using TiO2aggregates and nanotubes This is probably because more combustible gases than toluene are difficult to dif-fuse deep inside the films and tend to burn at the film surfaces even

if the porosity of the films is high Importantly, the selective detec-tion of toluene was achieved by the porosity improvement of the sensing film and the high temperature operation

There have been some papers reporting the response charac-teristics of TiO2-based sensors to VOCs Teleki et al reported the

sensor response of S = 6 to ethanol gas (30–300 ppm) in dry air at

400◦C for TiO2doped with Nb[39] On the other hand, Gessner et

◦

Fig 12 Sensor responses to toluene (50 ppm) for the TiO2 nanotubular films

cal-cined at (a) 600 and (b) 700 ◦ C.

al reported the sensor response of S = 4 to toluene gas (100 ppm)

in dry air at 400–500◦C[40] Although the preparation methods of

TiO2and the experimental conditions are different from those in

the present study, the reported sensor responses were lower than

our results Therefore, our approach of controlling the film pore size

is quite effective in improving the sensor performance to large size

molecules like ethanol and toluene

Fig 10shows the response transients to toluene (50 ppm) at

500◦C of the sensors using TiO2nanotubes prepared at 230◦C and

commercial TiO2 nanoparticles (P-25) Even if a difference in the

response transients is seen by both sensors, it seems not to be the

important difference because the speed of response and recovery

depends on dead volume in the equipment, as reported by Kida et

al.[41] According to a basic viewpoint of gas diffusion[42], it is

know that the diffusion through the microstructure affects sensor

response (Rair/Rgas) From the above, it is thought that the sensor

response of nanotubes observed in Fig 10is as high as that of

nanoparticles

To provide a further experimental evidence of supporting the

above discussions, the porosity of the nanotubular film was

con-trolled by calcination and then the sensor response was tested

again Fig 11 shows the SEM images of the film composed of

TiO2 nanotubes prepared at 230◦C after calcination at 600 and

700◦C From the SEM images, it appears that the porosity obviously

decreased due to sintering of nanotubes after calcination at higher

temperature As a result, the sensitivity to toluene gas significantly

decreased, as shown inFig 12 Thus, the results obtained indicate

again the importance of the microstructure control of sensing layers

for detecting large sized gas molecules

4 Conclusion

TiO2nanotubes of 1␮m in length and 50 nm in diameter were

formed by the hydrothermal treatment of TiO2nanoparticles with

NaOH at 200 and 230◦C Uniform sized nanotubes were obtained at

230◦C The tubular structure of the TiO2nanotubes was stable even

after calcination at 600◦C for 1 h The calcination of the nanotubes

resulted in the formation of the anatase phase A porous film with a

peak pore size of around 200 nm was successfully formed using the

TiO2nanotubes obtained at 230◦C The sensing film using the

nan-otubes exhibited higher sensor response to toluene at 500◦C than

CO and H2 The porous morphology of the nanotubular films

facili-tated diffusion of large sized molecules like toluene through pores

in the film, leading to the improved sensor response and selectivity

to toluene

Acknowledgement

This study was partly supported by NISSAN Science Foundation,

a Grant-in-Aid for Exploratory Research (No 19659402) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and by Environmental Research and Technology Develop-ment Fund from the Ministry of EnvironDevelop-ment of Japan

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National University in 1983 and 1985 He obtained his Ph.D in Electronic Materials from M.I.T (Massachusetts Institute of Technology) in 1993 His current research interests include gas sensor, odor sensing and medical application of these sensors.

Kengo Shimanoe has been a Professor at Kyushu University since 2005 He received

the B.E Degree in Applied Chemistry in 1983 and the M Eng Degree in 1985 from Kagoshima University and Kyushu University, respectively He joined Nippon Steel Corp in 1985, and received his Dr Eng Degree in 1993 from Kyushu University His current research interests include the development of gas sensors and other functional devices.

Noboru Yamazoe had been a Professor at Kyushu University since 1981 until he

retired in 2004 He received his M Eng Degree in Applied Chemistry in 1963 and his

Dr Eng Degree in 1969 from Kyushu University His research interests were directed mostly to the development and application of functional inorganic materials.