Materials Chemistry and Physics 69 (2001) 176–179 WO 3 thin film sensor prepared by sol–gel technique and its low-temperature sensing properties to trimethylamine Maosong Tong, Guorui Dai ∗ , Dingsan Gao 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 (WO 3 ) thin film has been prepared on a porcelain tube with gold comb-type electrodes by using a sol–gel technique with WCl 6 as 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 TiO 2 -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 WO 3 thin films via a sol–gel process, using WCl 6 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 WO 3 thin film was prepared by a sol–gel technique using WCl 6 as a precursor. WCl 6 (99.95%) was dissolved in isopropanol at a ratio of 5g/100ml 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: S0254-0584(00)00389-8 M. Tong et al./ Materials Chemistry and Physics 69 (2001) 176–179 177 air at 120 ◦ C for 15min. 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 −7 Pa 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 284.6eV) was used. 2.3. Measurements of gas sensing properties WO 3 were 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 m 3 and kept under a continuous flow of fresh air for 10min before measurement. The operating voltage (V H ) was supplied to either of the coils for heating the sensors and the circuit voltage (V C = 10 V) was supplied across the sensors and the load resistor (R L = 2k) connected in series. The signal voltage across the load, which changed with sort and concentration of gas, was measured. The gas sensitivities to TMA, C 2 H 5 OH gas, gasoline, CH 4 , 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 = V g /V a , where V g and V a are 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 WO 3 (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 4p 1/2 ,4p 3/2 ,4d 3/2 ,4d 5/2 , 4f 5/2 ,4f 7/2 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 WO 3 crystal. As shown in Fig. 2(c), the pho- toelectron peaks of W 4f 5/2 andW4f 7/2 are found to lie at 37.1 and 35.20 eV, respectively and are contributed to lattice tungsten in the WO 3 crystal. 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 WO 3 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 178 M. Tong et al./Materials Chemistry and Physics 69 (2001) 176–179 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 5/2 and W4f 7/2 peaks. that the WO 3 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 ∼3at70 ◦ 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 sensitivity 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. NH 3 , gasoline, C 2 H 5 OH, CH 4 and CO at the same concen- tration of 1000ppm and to water vapor. These experimental results indicate that the WO 3 thin film sensor is very sensi- tive to TMA at 50–85 ◦ C, but not sensitive to NH 3 , gasoline, C 2 H 5 OH, CH 4 and CO and water vapor. These results in- dicate that the WO 3 thin 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. M. Tong et al./ Materials Chemistry and Physics 69 (2001) 176–179 179 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 WO 3 thin 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 WO 3 thin films can be prepared by a sol–gel process using WCl 6 as a precursor. The WO 3 thin film sensor provides a high sensitivity, excellent selectivity and quick re- sponse behavior at 70 ◦ C to TMA. These results indicate that the WO 3 thin film sensor prepared by the sol–gel technique can be used to detect TMA gas, e.g. the freshness of the fish. 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