Sensors and Actuators B 24-2.5 (1995) 548-551
Aqueous ozone detector using In203 thin-film semiconductor sensor Tadashi Takada a, Hiromasa
Tanjou a, Tatsuo Saito a, Kenji Harada b
‘New Cosmos Ekctric Co., Ltd., 2-5-4 Mitsuya-naku, Yodogawa-ku, Osaka 532, Japan b Fuji Electric Co., Ltd., 1 Fuji-machi, Hino-&, Tokyo 19I, Japan
Abstract A detector for continuous determination of a trace amount of aqueous ozone, utilizing an ozone-extraction process from liquid (water) into the gas phase and an In,O, thin-film semiconductor ozone gas sensor, has been developed. It has many advantages, including high sensitivity to aqueous ozone and high aqueous ozone selectivity in the presence of various kinds of interfering compounds in water. It is able to detect 0.5 ppb of aqueous ozone with a sufficient response. The response to 1 ppm of free chlorine in water is smaller than that to 10 ppb of aqueous ozone. It is revealed that the high sensitivity and the high selectivity are attributed in considerable part to the higher extraction rate of ozone from water into the gas phase than those of other compounds in water. This detector is somewhat affected by a change in water temperature. The watertemperature correction has been studied. A precise determination of aqueous ozone has been performed in the watertemperature range l&40 “C with an improved detector equipped with the correction process. Keywords: Aqueous ozone detector; Gas sensors; Indium oxide; Thin-film sensors
1. Introduction Recently ozone has been attracting the attention of the world as a clean oxidizing agent. Ozonized water is used as a disinfectant, a deodorizing agent and a decolorizing agent in various fields, such as pools, fish farms, hospitals, food and integrated-circuit (IC) industries as well as water supplies. Tbe widespread use of ozonized water requires a highly sensitive and highly selective aqueous ozone detector. A conventional method for aqueous ozone determination is iodometry, which is based on the reduction of ozone by the iodide ion. However, in this method, interferences from strong oxidizing agents, such as oxidized forms of manganese, chromium (VI), chlorine and bromine, are of concern. Also lower concentrations of aqueous ozone below 0.1 ppm are hardly detectable with sufficient accuracy by iodometry. For the past several years, ozone gas sensors have been developed using In20, semiconductor thin films [l-7]. Among them, an Fe-added In,O, thin film, on whose surface SiOt is deposited by the chemicalvapour deposition CVD method, shows excellent properties in practical use, namely, high sensitivity to low concentrations of ozone below 0.1 ppm, high ozone selectivity, wide working range (3 ppb-10 ppm), good reliability, good long-term stability, ease of operation 0925-4005/95/$09.50 8 1995 Elsevier Science S.A. All rights resewed SSDI 0925-4005(94)01415-E
and low price . For example, sensitivities to ozone and other gases are shown in Fig. 1. Utilizing this ozone gas sensor and an ozone extraction process from liquid (water) into the gas phase, a method without any reagent, for continuous determination of a trace amount of aqueous ozone, has been developed [B-13]. It was found that this method had many advantages, such as high sensitivity, continuous determination, easy maintenance and especially elimination of interferences from various kinds of compounds present in water [lo]. But the reading of this detector was somewhat influenced by a change in water temperature. In this study we examined particularly where the high sensitivity to aqueous ozone
0.1 1 10 loo looo Concentration ofVarious Gases(ppm )
Fig. 1. Sensitivities to various gases at 420 “C sensor temperature for Fe-added In,4 thin film.
et al. I Senrors andActuators
and the high aqueous ozone selectivity were generated, and the extraction process was revealed to be a key to their production. A water-temperature correction was also studied . It was clarified that it was caused by the water-temperature dependence of the ozone extraction rate from water into the gas phase and the absolute humidity dependence of the semiconductor gas sensor. A precise determination of aqueous ozone was performed in the water-temperature range MO “C with an improved detector equipped to carry out the correction process.
2. Experimental A schematic representation of the apparatus used in the present work is illustrated in Fig. 2. An aqueous ozone sample is introduced continuously into an extraction vessel, where aqueous ozone is transferred from water into the gas phase. Sample water is bubbled into the vessel with clean air flowing at a rate of 30 cm min-I. In this process, ozone is extracted from ozonized water into the gas phase. After this, the air, captured ozone gas, is mixed with dry air at the same flow rate in order to cut down the absolute humidity to half and consequently reduce the water-temperature correction, which originates partly from the absolute humidity dependence of the gas sensor. The clean air and the dry air are obtained by passing room-air through an activated charcoal column and a silica gel column, respectively. The ozone concentration in the mixed air is continuously monitored with an In,O, thin-film semiconductor gas sensor. The determined ozone concentration is indicated after some calculations by the central processing unit (CPU), including the present watertemperature correction. The determination of ozone and free chlorine in water was performed by iodometry or absorptiometry. The concentrations of ozone or hypochlorous acid, HClO, in the gas phase were also determined by iodometry after capture of ozone or HClO in 2% potassium iodide solutions. The aqueous ozone samples below 100 ppb were prepared by dilution
B 24-25 (1995) 548-551
of known ozonized waters with distilled water. The concentration of the sample was determined by the ratio of the flow rate of the ozonized water to that of the distilled water.
3. Results and discussion The sensitivity to aqueous ozone for an In,O, semiconductor gas sensor equipped with the present detector is shown in Fig. 3. The sensitivity is described in terms of the resistance ratio RslRo, where Rs and R. denote the sensor resistances to sample water and distilled water, respectively. The temperature of the sample water was held at 18 “C! within 1 “C during the experiments of Figs. 3 and 4.0.5 ppb of ultra-trace aqueous ozone was measured with a sensitivity RJR,=2.75. It was revealed that the ozone-extraction process from water into the gas phase was the key to the high sensitivity to aqueous ozone. The relation between aqueous ozone concentration dissolved in water (ppm in mass ratio) and ozone concentration volatilized into the gas phase (ppm in volume ratio) is shown in Fig. 4. 30 ppm of ozone gas was transferred into the gas phase from 0.5 ppm of aqueous ozone in the present apparatus. This high ozone extraction rate brought about the high sensitivity to aqueous ozone. The thinfilm semiconductor gas sensor used in the present study has high sensitivity to ozone gas, as shown in Fig. 1,
102 10’ 100 Concentrationof AqueousOzone (ppb)
Fig. 3. Concentration dependence of aqueous ozone sensitivity at 18 “C water temperature for the present aqueous ozone detector.
Fig. 2. Schematic representation detector.
IJ of the present
and HCIO in Water @pm i mass Rtio)
Fig. 4. Correlation between concentrations of O3 and HClO dissolved in water and those volatilized to the gas phase (pH, 7.OkO.2; water temperature, 18 “C).
Z Takada et al. I Sensors and Actuators B 24-25 (1995) 548-551
and it enables one to detect 1 ppb ozone in the gas phase . Therefore, this method might enable one to determine 0.1 ppb of aqueous ozone with sufficient accuracy. The relation between the concentration of free chlorine in water, which is regarded as the main interfering compound in water, and that of HClO in the gas phase is also shown in Fig. 4. About 3 ppm of HClO gas was transferred from 0.5 ppm of free chlorine in water into the gas phase. The extraction rate of ozone from water into the gas phase was 10 times larger than that of HClO. It was noted that a high aqueous ozone selectivity was also generated in the extraction process. Moreover, the sensitivity to ozone gas for the semiconductor sensor is sufficiently higher than that to HClO gas, as shown in Fig. 1, where measurement of the sensitivity to HClO gas was simultaneously carried out in the experiment of Fig. 4. Consequently the interference from the same concentration of free chlorine in water was below 1% of the indicated aqueous ozone concentration, at least at 18 “C water temperature. Interferences from other compounds in water are negligibly small [lo]. The reading of this detector was somewhat affected by a change in water temperature. A process of watertemperature correction was studied. The response of the gas sensor v*,[CIc,]at any water temperature x “C! is converted into the response V;[C;] at a reference water-temperature a “C by the following transformation equation, which is reported elsewhere :
where C; = (P/D”) (PJC#$.&“fi. Da/Ox denotes the ratio of ozone extraction rate from water into the gas phase at a “C to that at x “C, and the next experimental relation between the sensor response V, and the concentration C, is accepted in the gas phase: V,= ([email protected]
~.Terms A and B in Eq. (1) denote the corrections originating from the absolute humidity dependence of the sensor response and the water-temperature dependence of the ozone extraction rate, respectively. A calibration curve at a “C is obtained from the above l$ and data for aqueous ozone concentrations C, by the least-squares method, where C, is measured by iodometry: v; = ac,p
A determination of C, is performed from data for x and v*, as follows: Pg is converted into Pg by the transformation Eq. (1); C, is then derived from I$ by the calibration Eq. (2). Experiments for the water-temperature correction were carried out in an incubator. Measurements were made for aqueous ozone samples ranging from 0 to 40
0.5 [ 0
Fig. 5. Water-temperature
10 20 30 Water Temperature( 0
Fig. 6. Correlation bchveen the reading of the present detector and iodometry.
“C in water temperature. The water temperature was held constant within 1 “C during each measurement, and the temperature difference between the water and the atmosphere in the incubator was always within a few degrees centigrade. The results for the watertemperature correction are shown in Fig. 5, which were obtained by using five semiconductor gas sensors and repeating 11 heat cycles. It was clarified that the present aqueous ozone detector had no water-temperature dependence in the water-temperature range O-40 “C. The results show some scatter, which maybe partly attributed to experimental errors of within 5% in the iodometry. The reading of the present detector as a function of the aqueous ozone concentration, determined by iodometry, is shown in Fig. 6. The regression equation was written as follows: Y= 1.008X-0.013, with’s correlation coefficient of 0.996. Thus, readings of the aqueous ozone detector were in excellent agreement with the results of iodometry in the water-temperature range O-40 “C.
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T Takada er al. I Senrors and Actuators B 24-25 (1995) 548-551
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