wo3 thin film sensor prepared by sol–gel technique and its low-temperature sensing properties to trimethylamine

WO 3 thin film sensor prepared by sol–gel technique and itslow-temperature sensing properties to trimethylamine Department of Electronic Engineering, National Integrated Optoelectronics

WO 3 thin film sensor prepared by sol–gel technique and its

low-temperature sensing properties to trimethylamine

Department of Electronic Engineering, National Integrated Optoelectronics Laboratory,

Jilin University, Changchun 130023, P R China

Received 20 April 2000; received in revised form 23 June 2000; accepted 1 July 2000

Abstract

Tungsten oxide (WO3) thin film has been prepared on a porcelain tube with gold comb-type electrodes by using a sol–gel technique with WCl6as precursor The X-ray diffraction and X-ray photoelectron spectrum results indicate the crystallization of tungsten oxide occurs at temperatures higher than 500◦C The sensing characteristics of the thin film sensor to trimethylamine were measured A low-temperature,

high sensitivity, excellent selectivity, quick response and recover of thin film sensors to trimethylamine were found © 2001 Elsevier Science B.V All rights reserved

Keywords: Gas sensor; Sol–gel technique; Thin films; Trimethylamine; Tungsten oxide

1 Introduction

It is well known that the detection of fish freshness is

the most important problem in the fish-processing industry

One of the most widely used methods of testing freshness is

chemically measuring the breakdown products of adenosine

triphophate-related compounds in the fish’s tissue, which

re-quires a great deal of effort and much time [1] During the

deterioration of the fish after death, some gaseous species

such as trimethylamine (TMA), dimethylamine and

ammo-nia are given off The concentration of these gases increases

with the decreasing of the fish-freshness So the fish

fresh-ness can be monitored by a rapid, continuous, easy and

non-destructive way using a special gas sensor capable of

detecting TMA [2,3] The previous studies of this gas sensor

were concentrated on TiO2-based materials These sensors

are usually derived from ceramic processing and thick film

technology, however, they require high operating

tempera-ture, and are difficult to be miniaturized and incompatible

with the industrial technology of integrated circuits

Such problems can be overcome by thin films [4,5] The

thin film sensors can be fabricated by a number of

tech-niques, such as vacuum vaporization [6], radio frequency

(r.f.) sputtering [7], and chemical vapor deposition [8] It is

well known that the sol–gel technique has several

advan-∗Corresponding author Tel.:+86-431-8923189; fax: +86-431-8923907.

E-mail address: grdai@mail.jlu.edu.cn (G Dai).

tages, such as easy control of film thickness and porosity, the ability to produce thin films at low cost and at low tem-perature, and the ability to make homogeneous distribution

of the components This technique is very useful to enhance the gas sensitivity [9–11]

The aims of this work are to prepare WO3 thin films via a sol–gel process, using WCl6 as a precursor and test the sensing properties of thin films to TMA gas at the low operating temperature of 70◦C.

2 Experimental

2.1 Thin film preparation

The WO3 thin film was prepared by a sol–gel technique using WCl6as a precursor WCl6 (99.95%) was dissolved

in isopropanol at a ratio of 5 g/100 ml and stayed in dry air for 2 days Then the obtained sol was deposited onto the substrates by dip coating method Prior to the coating deposition the substrates were ultrasonically cleaned, first in acetone then in isopropanol and deionized water, and dried

at 100◦C The cleaned substrates were slowly dipped in the

solution at room temperature The films showed a sudden solution color change when they were gradually withdrawn from the solution This reflected that the hydrolysis reaction took place as soon as the coatings were exposed to air After the hydrolysis and condensation, the gel films were dried in 0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V All rights reserved.

PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 8 9 - 8

air at 120◦C for 15 min In order to obtain the crystalline

films, the heat treatment was performed on the dried films

at 150, 350 and 500◦C for 10 h.

2.2 Thin film characterization

The crystalline structures of the obtained thin films were

determined by X-ray diffraction (XRD) using a SIEMENS

D5005 apparatus with Cu K␣ radiation and Ni filter at room

temperature The data were collected by a step scanning

method for the diffraction angle range 20◦≤ 2θ ≤ 60◦, with

a step width of 0.05◦and a step time of 1 s The

composi-tion and oxidizing state of the thin films were investigated

using X-ray photoelectron spectroscopy (XPS), which were

carried out on an ESCALAB MARK-II XPS spectrometer

with Mg K␣ radiation The working pressure in the

analy-sis chamber was maintained below 5× 10−7Pa during the

specimen analysis To eliminate the effect of sample surface

charging the shift of the XPS peak of carbon (C 1s whose

binding energy is 284.6 eV) was used

2.3 Measurements of gas sensing properties

WO3were deposited between interdigital gold electrodes

on the outer wall of a ceramic tube The electrical contacts

were made with 0.05 mm gold wires attached to the gold

electrodes Then the deposited thin film was annealed at

500◦C for 10 h The thin film sensors were set up in a glass

test chamber with a volume of 0.18 m3 and kept under a

continuous flow of fresh air for 10 min before measurement

The operating voltage (VH) was supplied to either of the

coils for heating the sensors and the circuit voltage (VC=

10 V) was supplied across the sensors and the load resistor

(RL= 2 k) connected in series The signal voltage across

the load, which changed with sort and concentration of gas,

was measured The gas sensitivities to TMA, C2H5OH gas,

gasoline, CH4, CO, and water vapor were measured A given

amount of each gas was injected into the chamber and mixed

by a fan for 30 s The sensitivity to gases, S, is defined as

S = Vg/Va, where Vgand Vaare the voltage drops across

the load resistance in testing gases/air mixture and in air,

respectively

3 Results and discussion

3.1 The XRD and XPS analyses of the thin films

Fig 1 shows the X-ray diffraction patterns of the films

that were deposited on Si wafers and annealed at different

temperatures for 10 h The XRD pattern of the thin film

an-nealed at 150◦C does not show any definite structure So

the sample annealed at this temperature was in the

amor-phous state (see Fig 1(a)) When the sample was treated

at 350◦C, a few weak peaks can be observed in the XRD

Fig 1 X-ray diffraction patterns of the thin films annealed at different temperatures.

pattern and the result indicates that the sample was amor-phous or in a weak polycrystalline state (see Fig 1(b)) The diffraction pattern recorded for the sample heated at 500◦C

for 10 h shows many sharp peaks which can be attributed to (100), (200), (220), (221), (002) and (400) crystal planes of WO3 (see Fig 1(c)) No other crystalline phases could be detected in the film annealed at 500◦C for 10 h.

Fig 2 shows the XPS spectra of the films, which were deposited on Si substrates and annealed at 500◦C for 10 h.

The XPS survey scan spectrum of the sample is shown in Fig 2(a) The peaks of tungsten 4p1/2, 4p3/2, 4d3/2, 4d5/2, 4f5/2, 4f7/2 and oxygen 1s (O 1s) can be detected in the spectrum The photoelectron peaks of chloride cannot be observed in this spectrum This indicates that the films an-nealed at 500◦C for 5 h consist of only oxygen and tungsten

elements As shown in Fig 2(b), the photoelecton peak of

O 1s is found to lie at 530.2 eV, and is assigned to lattice oxygen in the WO3crystal As shown in Fig 2(c), the pho-toelectron peaks of W 4f5/2 and W 4f7/2are found to lie at 37.1 and 35.20 eV, respectively and are contributed to lattice tungsten in the WO3crystal

3.2 Gas sensing properties of the thin film sensors

In general, the sensitivity of the sensor is affected by the operating temperature Fig 3 shows the relationship between the gas sensitivity and the operating temperature of WO3 thin film sensor to TMA gas with various concentrations As shown in Fig 3, to 100 and 500 ppm (volume concentration)

of TMA, the optimum of the operating temperature is 70◦C.

To 700 and 1000 ppm TMA, the gas sensitivity reaches al-most the highest value at 70◦C When the operating

temper-ature is in the range of 70–85◦C, the sensitivity increases a

little With further increasing of the operating temperature, the sensitivity to TMA gas decreases These results mean

Fig 2 X-ray photoelectron spectra of the thin film annealed at 500 ◦C

for 10 h (a) Survey scan spectrum, (b) O 1s peak, and (c) W 4f 5/2and

W 4f 7/2peaks.

that the WO3 thin film can be used to detect TMA at low

temperature

Fig 4 shows the relationship between the gas sensitivity

and TMA concentrations at 70◦C: the gas sensitivity of the

sensor increases with TMA concentration The detectable

minimal concentration of TMA is 50 ppm and the gas

sen-sitivity is∼3 at 70◦C When the concentration of TMA is

higher than 100 ppm, the gas sensitivity increases linearly

with the TMA concentration

This type of thin film sensor was also tested for its

sen-sitivity to other gases Fig 5 shows its sensen-sitivity to TMA,

Fig 3 Operating temperature dependence of the gas sensitivity for the

WO 3 thin film sensor in TMA gas with different concentrations.

Fig 4 The relationship between the gas sensitivity and TMA concentration

at 70 ◦C.

NH3, gasoline, C2H5OH, CH4and CO at the same concen-tration of 1000 ppm and to water vapor These experimental results indicate that the WO3thin film sensor is very sensi-tive to TMA at 50–85◦C, but not sensitive to NH

3, gasoline,

C2H5OH, CH4and CO and water vapor These results in-dicate that the WO3thin film sensor prepared by a sol–gel

Fig 5 The relationship between the gas sensitivity and operating tem-perature to different gases at the same concentration of 1000 ppm and to water vapor.

Fig 6 The typical gas response characteristic of the WO 3 thin film sensor

to 500 ppm TMA at 70 ◦C.

method has a high sensitivity and selectivity to TMA at

70◦C.

Fig 6 shows the typical gas response characteristic of

the WO3thin film sensor After an introduction of 500 ppm

TMA gas, the response appears immediately The 90%

re-sponse time is 2.5 s and the 90% recovery time is 24 s We

also tested the response behavior of the sensor to TMA gas

of different concentrations The 90% response time to 100

and 1000 ppm TMA gas is 6.5 and 2 s, respectively The

90% recover time to 100 and 1000 ppm TMA gas is 21 and

30 s, respectively

4 Conclusions

Based on the present experiment results, we have shown

that WO3 thin films can be prepared by a sol–gel process

using WCl6 as a precursor The WO3 thin film sensor provides a high sensitivity, excellent selectivity and quick re-sponse behavior at 70◦C to TMA These results indicate that

the WO3thin film sensor prepared by the sol–gel technique can be used to detect TMA gas, e.g the freshness of the fish

Acknowledgements

This work was supported by the Chinese Natural Science Foundation (No.69776038)

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