spin-coated thin films of sio2–wo3 composites for detection

Spin-coated thin films of SiO2– WO3 composites for detection ofXusheng Wang, Go Sakai, Kengo Shimanoe, Norio Miura, Noboru Yamazoe * Department of Materials Science and Technology, Gradu

Spin-coated thin films of SiO2– WO3 composites for detection of

Xusheng Wang, Go Sakai, Kengo Shimanoe, Norio Miura, Noboru Yamazoe *

Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu Uni 6ersity, Kasuga-shi, Fukuoka816, Japan

Received 21 May 1997; received in revised form 18 August 1997; accepted 20 August 1997

Abstract

Thin films of SiO2– WO3composites with various SiO2contents (0 – 20 wt%) were prepared by means of sol-gel method The grain size of WO3 decreased with increasing SiO2 content, resulting in an increase in NO2 sensitivity On the other hand, the response transient to NO2was brought to be the quickest at 5% SiO2 The thin film of SiO2(5%) – WO3was far more sensitive

to NO2than the thin film or sintered block of pure WO3 It could detect dilute NO2(0.1 – 2 ppm) in air at 350°C sensitively and fairly selectively The NO2sensing characteristics were stable over the whole test period of 10 days © 1997 Elsevier Science S.A

Keywords: Sol-gel; Thin films; NO2sensor; WO3; SiO2; AFM

1 Introduction

From an environmental concern, there has been an

increasing demand for sensory detection of nitrogen

oxides (NOx: NO and NO2), which are typical air

pollutants released from combustion facilities and

auto-mobiles Especially NO2 is highly toxic to human

nerves and respiratory organs so that high-sensitivity

detection of it is desired for air quality monitoring

Various solid-state sensors to detect NO2 and/or NO

have been proposed by using semiconducting oxides

[1 – 4], solid electrolytes [1,5] and SAW devices [6]

However, the detection of NO2 in the vicinity of

envi-ronmental standard (40 – 60 ppb in Japan) has remained

to be a challenge Recently, semiconductor sensors

using WO3 have been proved to be very sensitive to

NO2[2,3,7 – 10] The NO2sensitivity has been shown to

be improved by using fine particles of WO3 [3] or thin

films of WO3 prepared by vacuum evaporation [8], RF

sputtering [9], or sol-gel methods [10,12] These reports

suggest that further improvements in NO2 sensitivity

may be possible by controlling the microstructure of

WO3 Compared with the other techniques, sol-gel

methods allow one to control relatively easily the

parti-cle size and dispersion of components in composite

films, as illustrated for silica thin films [11] In this paper we examined the NO2sensing properties of SiO2 -mixed WO3 composite thin films prepared by a sol-gel method

2 Experimental

Thin films were fabricated on an alumina substrate (unpolished, 9 × 13 mm2) with comb-shaped gold elec-trodes (area 9 × 9 mm2) by a spin-coating technique Coating solutions were prepared as follows Te-traethoxysilane was mixed with ethanol and water (1:4:4 in molar ratio), together with a trace amount of HCl catalyst, and stirred for 30 min This solution (transparent) was further mixed with an aqueous solu-tion (2.5 wt%) of ammonium paratungstate ((NH4)10W12O41· 5H2O) to designated proportions A small amount of each coating solution thus obtained was dropped on the substrate, allowed to spread over

by spinning (1500 rpm, 20 s) and dried at 350°C for 10 min The same procedures were repeated 15 times to increase the film thickness up to about 0.8 mm, before firing was carried out at 550°C for 1 h The SiO2 content was set to 0, 5, 10 or 20 wt% to WO3 and is denoted like SiO2(5%) – WO3 hereafter

Thin films were characterized by means of XRD analysis (Riguku 4011) and AFM observation

* Corresponding author Tel.: + 81 92 5837539 fax: + 81 92

5752318.

0925-4005/97/$17.00 © 1997 Elsevier Science S.A All rights reserved.

PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 8 6 - 4

(Nanoscope IIIa; Digital Instruments Inc.) Sensing

characteristics were measured in a conventional flow

apparatus (gas flow rate 100 cm3min− 1) in the

temper-ature range of 250 – 450°C Gas sensitivity (S) was

defined as Rg/Ra, where Ra and Rg are the electrical

resistances in air and in the sample gas, respectively

3 Results and discussion

3.1 Microstructure

Fig 1 shows the XRD patterns of a series of SiO2–

WO3films prepared, together with that of the uncoated

substrate The XRD peaks other than those of Al2O3

(substrate) and Au (electrode) were all assigned to a

monoclinic phase of WO3 (JCPDS 43-1035), the

inten-sity pattern being very similar to that of the WO3

powder obtained from ammonium paratungstate by

pyrolysis It was also obvious that the SiO2 component

included was amorphous The XRD peaks of WO3

tended to become broader with increasing SiO2content

The average grain size of WO3was estimated from the

full width at half maximum of the strongest peak (200)

at 2u=24.2° based on Sherrer’s equation As shown in

Fig 2, the grain size decreased with an increase in SiO2

content, indicating that the growth of WO3 grains was

suppressed by SiO2

Fig 2 Average grain size of WO3evaluated from XRD as a function

of SiO2content.

Fig 3 shows AFM images of the thin films The bright spots, corresponding to the grains of WO3, were

50 – 100 and 30 – 50 nm in diameter for WO3 (a) and SiO2 (20%) – WO3 (b), respectively These values are in fair agreement with those obtained from the XRD peak above, if one considers the tip size (15 nm in diameter)

of AFM probe

3.2 NO2 sensing characteristics

The electrical resistance of each film in air (Ra) and in 0.4 ppm NO2-containing air (Rg) as well as the resulting sensitivity to 0.4 ppm NO2 are shown as a function of

temperature in Fig 4 As already reported [7 – 10], Rg was always larger than Ra, indicating anionic adsorp-tion of NO2on the surface of WO3(n-type oxide) At a

fixed temperature, both Ra and Rg tended to increase with an increase in SiO2content except that SiO2(5%) –

WO3had larger Ra and Rgvalues than WO3 However,

Rgwas more dependent on SiO2content than Ra, giving

Fig 1 XRD patterns of WO3, WO3+ 5wt% SiO 2 , WO3+

10wt% SiO2and WO3+ 20wt% SiO 2 thin films deposited on alumina

substrates with Au electrodes Fig 3 AFM images of thin films (a): WO , (b): SiO (20%) – WO

Fig 4 NO2sensing properties of SiO2– WO3thin films as a function

of temperature SiO2content: 0% (1), 5% (2), 10% (3), 20% (4) (a):

Resistance in air (Ra) or 0.4 ppm NO2 containing air (Rg); (b):

sensitivity to 0.4 ppm NO2in air.

seen to consist of two steps, although such steps be-came less visible at 0.2 and 0.4 ppm NO2 and totally disappeared at 0.8 ppm and above The same two-step behavior at low NO2concentrations was also observed with the other films, and is considered to appear from the coexistence of strong and weak adsorption of NO2

as described shortly

The correlations between normalized resistances (Ra/

Rg) and NO2concentrations for the films of pure WO3 and SiO2(5%) – WO3 at 350°C are shown in Fig 6

Rg/Ra was almost linear to the NO2 concentration and the slope of SiO2(5%) – WO3was almost twice as steep

as that of WO3, indicating the higher sensitivity of the

former film Since Rg/Ra should be unity at zero con-centration of NO2, the correlations should have a sharp inflection in the low concentration range as schemati-cally illustrated by a dotted line Qualitatively speaking, such an inflection can be accounted for by assuming two types of NO2 adsorption, i.e one type which is strong in bonding but small in capacity and the other which is weak but large in capacity It is considered that the saturation of the first type adsorption at a low concentration of NO2 gives rise to the inflection men-tioned above It follows that these two types differ in adsorption kinetics, giving rise to the two-step response transients at lower NO2 concentrations Correspond-ingly the replots of the same data in a log-log scale (Fig 7) gave the correlations, the slopes of which were close to unity in the higher NO2 concentration range and decreased with decreasing NO2 concentration For comparison, the data of the sintered block type element

of WO3 measured at 300°C are also indicated in the same figure Roughly speaking, the sensitivity decreases

to about 1/2 with a rise of temperature by 50°C Taking this into account, the present thin film of SiO2(5%) –

WO3 is estimated to have higher sensitivity than the sintered block of WO3 by more than one order of magnitude

3.3 Selecti 6ity and stability

A practical NO2sensor should be resistant enough to interferences by coexistence gases Thus the sensing properties of SiO2(5%) – WO3 to 0.4 ppm NO2 in air were examined under the coexistence of 100 ppm CO,

100 ppm CO2, or 5000 ppm H2O at 350°C Fig 8 shows

the influence of these gases on Ra and Rg These gases

tended to reduce Ra slightly or significantly (H2O),

while such tendencies were less pronounced for Rg

values As seen from Fig 8, the NO2sensing properties were fairly stable to the coexistence of CO and CO2 However, the interference by water vapor was signifi-cant, needing further investigations to overcome it

To examine the stability of NO2 sensing perfor-mance, the SiO2(5%) – WO3 film was exposed repeat-edly to 0.4 ppm NO at a frequency of three times a

rise to an increase in sensitivity (S) at a fixed

tempera-ture With lowering temperature, Rg increased more

steeply than Ra, leading to a rather sharp increase in S.

As seen from Fig 4, S tails off at about 500°C when

temperature is increased On the other hand, lowering

temperature resulted in a sharp loss in the rates of

response and recovery As a trade-off between the

sensitivity and the response kinetics, an operating

tem-perature of 350°C was selected in the subsequent study

Table 1 summarizes some sensing characteristics of

the composite films to 0.4 ppm NO2 in air at 350°C

The sensitivity (S) tended to increase gradually with

increasing SiO2content The times of 90% response and

90% recovery were rather large in all cases, being 5 – 7

and 7 – 15 min, respectively Nevertheless SiO2(5%) –

WO3showed the shortest recovery time of 7 min From

this feature as well as the rather high sensitivity, this

film was selected for further investigations of NO2

sensing properties

Fig 5 shows the response and recovery transients of

SiO2(5%) – WO3 film on switching on and off various

concentrations of NO2 in air between 0.1 and 2.0 ppm

at 350°C The response transient to 0.1 ppm NO is

Table 1

Sensing characteristics of SiO 2 – WO 3 thin films at 350°C

90% Recovery time (min)

Resistance (Ra , V) Sensitivity a(Ra/Rg ) 90% Response time (min) Film

SiO2(5%) – WO3 3.2×10 5 8.1

1.9×10 6

SiO2(20%) – WO3

a To 0.4 ppm NO2in air.

day for 10 days As shown in Fig 9, both Rg and Ra

were fairly stable over the whole test period This

confirms that the film is stable enough at least for a

short term

3.4 Imprecation of SiO2– WO3 composite films

In the present preparation conditions of SiO2– WO3

composite films, SiO2 was deposited through a sol-gel

process [3] On the other hand, it was confirmed by

XRD analysis that WO3took form through the

crystal-lization of WO3· 0.33H2O from the solution followed

by its dehydration It is considered that the dispersion

of fine particles of SiO2 hinders the grain growth of

WO3· 0.33H2O or WO3, leading to a decrease in WO3

grain size with increasing SiO2 content (Fig 2) It is

rather striking that the decreasing behavior of grain size

is exactly reflected by the sensitivity behavior of the

films (Table 1) This means that the sensitivity to NO2

is primarily determined by the grain size of WO3 in agreement with literature [3] The higher sensitivity of the SiO2– WO3 films than that of the WO3 sintered block previously reported [2] by about one order of magnitude or more may also be ascribable primarily to

a difference in grain size Apart from the grain size of

WO3, the dispersion of SiO2 would also change the microstructure of the film, e.g porosity and pore size distribution which affects the rate of response In this respect, the SiO2(5%) – WO3 film showing the shortest response and recovery times appears to have the mi-crostructure most porous for NO2molecules to diffuse inside and outside However, the response kinetics were not quick enough even with this film The microstruc-ture would depend not only on the SiO2 content but also on many other factors, e.g SiO2 particle size, agglomeration state and film thickness, which are all related with the methods and conditions of film prepa-ration Further studies to optimize the microstructure from these viewpoints would be necessary

As mentioned above, incorporation of SiO2 in the

WO3 sensing film to form SiO2– WO3 composite was fairly effective in suppressing the grain growth of WO3 and in enhancing the porous structure These effects have lead to higher sensitivity and quicker response,

Fig 5 Response transients of SiO2(5%) – WO3thin film on turning

on and off various concentrations of NO diluted in air.

Fig 6 Normalized resistance of thin films under exposure to various concentrations of NO in air (350°C).

Fig 7 Normalized resistance vs concentration correlations in a

log – log scale for WO3-based thin films (350°C) and sintered block

(300°C).

Fig 9 Short-term stability of electrical resistance of SiO 2 (5%) – WO 3

film sensor in air and in 0.4 ppm NO 2 at 350°C.

Acknowledgements

This work was partially supported by a Grant-in-Aid for Scientific Research from The Ministry of Ed-ucation, Science, Sports and Culture of Japan, and a grant from Steel Industry Foundation for the Ad-vancement of Environmental Protection Technology

References

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respectively Although the NO2 sensing performance

achieved here are not good enough, still it is expected

that environmental monitoring of NO2 (standard 40

ppb) would be possible by elaborating such composite

films further

4 Conclusions

The preparation of SiO2– WO3 thin films by a

sol-gel method was useful for enhancing the NO2 sensing

characteristics The grain growth of WO3 was

sup-pressed more intensively with increasing SiO2 content,

whereas the film microstructure was brought to be

most easily accessible by NO2 molecules at 5wt%

SiO2 The SiO2(5%) – WO3 film showed fairly good

sensing performances to NO2 in air in the range of

0.1 – 2 ppm

Fig 8 Influences of coexistent CO2, CO or H2O on the resistance of

SiO2(5%) – WO3film in air (Ra) and in 0.4 ppm NO2-containing air

(R) at 300°C.

[11] X Yao, L Zhang, S Wang, Pore size and pore-size distribution

control of porous silica, Sensors and Actuators B 24/25 (1995)

347 – 352.

[12] L Armelao, R bertoncello, G Granozzi, G Depaoli, E

Ton-dello, G Battaglin, High purity WO3sol-gel coatings: synthesis

and characterization, J Mater Chem 4 (1994) 407 – 411.

Biographies

Xusheng Wang has been an associate professor at

Xidian University of China since 1993 He received the

MSc degree in 1985 from the university He has been

with Xian Jiaotong University since 1994 Now he is

with Kyushu University as a visiting researcher His

current research interests the ferroelectric and

semicon-ducting functional materials and devices

Go Sakai has been a research associate at Kyushu

University since 1996 He received the BEng degree in

Applied Chemistry in 1991 and a DEng degree in 1996

from Kyushu University His current research works is

focused on the development of chemical sensor based

on semiconductive oxides

Kengo Shimanoe has been a research associate at

Kyushu University since 1995 He received the BEng

degree in Applied Chemistry in 1983 and MEng degree

in 1985 from Kagoshima University and Kyushu Uni-versity, respectively He joined the advanced material and technology laboratory of Nippon Steel Corp and has studied the electronic characterization on semicon-ductor surface and the electrochemical reaction on ma-terials He received a DEng degree in 1993 from Kyushu University His current research interests in-clude the development of chemical sensors and ECD as well as the analysis of solid surface

Norio Miura has been an Associate Professor at

Kyushu University since 1982 He received the BEng degree in Applied Chemistry in 1973, MEng degree in

1975 from Hiroshima University and the DEng degree

in 1980 from Kyushu University His current research concentrates on development of new chemical sensors

as well as other electrochemical functional devices such

as ECD and secondary batteries

Noboru Yamazoe has been a professor at Kyushu

University since 1981 He received the BEng degree in Applied Chemistry in 1963 and a DEng degree in 1969 from Kyushu University His current research interests include the development and application of functional inorganic materials