Effect of High-Humidity Aging on Performance of Tungsten Oxide-Type Aromatic Compound Sensors

WO 3 -type toluene gas sensors have been shown to be affected by humidity. The high-humidity aging effect is investigated on these sensors. High-humidity aging treatment is found to be effective against the effects of humidity on WO 3 -type toluene sensors. This humidity independence can be explained in the terms of a change in the surface conditions of WO 3 grains.


Introduction
The sick building syndrome is caused by harmful volatile organic compounds (VOCs) even if the concentration of the VOCs is at the ppb level.Aromatic compounds are widely known to be an important cause of the sick building syndrome.(3)(4)(5) The Ministry of Health, Labour and Welfare of Japan stipulates that the concentration of toluene in indoor room air should be less than 70 ppb. (6)It is therefore desirable to use a series of high-sensitivity VOC sensors such as aromatic compound sensors.
Metal oxides are some of the best materials for use as VOC sensors for a simple gas sensor system.We previously reported on high-sensitivity WO 3 -type gas sensors for aromatic compounds. (7)The selective sensing properties of WO 3 -type gas sensors can be controlled on the basis of temperature.9)(10)(11)(12) It has been reported that the gas-sensing response of 700°C-annealed WO 3 is not affected by humidity for ppm-level NO 2 detection at low temperatures. (13)However, humidity has an effect on WO 3 in cases of H 2 S (13) and aromatic compound detection, i.e., combustion mechanism.The sensor response of WO 3 to aromatic compound gases is decreased in high-humidity atmospheres.This problem needs to be resolved in order to realize the operation of WO 3 -type gas sensors under ambient conditions.We have reported that high-humidity aging improves the immunity to humidity of the sensor responses of Pt, Pd, and Au/SnO 2 systems in the case of combustion gas sensing. (14)In this study, we investigated improvements in the stability of the gas-sensing properties of WO 3 under various humidity conditions.

Synthesis of WO 3 films as aromatic sensors
9)(10)(11)(12) An aqueous solution of (NH 4 ) 10 W 12 O 41 •5H 2 O was neutralized by a dilute nitric acid solution yielding H 2 WO 4 precipitate.The precipitate was thoroughly washed with deionized water, dried, and then dispersed into ethylene glycol forming a suspension.A microdrop of the suspension was dropped on a 175×175 µm 2 area of a silicon substrate with a platinum heater and platinum comb-type electrodes, with a gap of 2 µm, using a micromanipulator.This was then dried and calcined at 400°C for 3 h.(10)(11)(12)

High-humidity aging
High-humidity aging of the WO 3 films was carried out in pure air with a relative humidity (RH) up to 90% at room temperature.The humid air had a flow rate of 250 mL/min.The elements were heated to 500°C and maintained at this temperature for approximately 2 weeks using a Pt heater.

Gas sensing properties
The gas sensing properties of the WO 3 films were experimentally determined using a flow apparatus.Synthetic air (Japan Fine Products "G1" grade) was flowed into a water bubbler.The RH of the synthetic air was set at 25, 50, and 75% at 20°C by controlling the temperature of the water bubbler.The films were heated to 400°C using the Pt heater.Toluene vapor was generated using a Gastec PD-1B gas permeator.The concentration of the toluene gas was controlled to below 70 ppb.The total flow was always 200 mL/min.The sensor response is defined using eq.( 1), where S, R a , and R g denote the sensor response magnitude, resistance in pure humid artificial air, and resistance in toluene gas with humid artificial air, respectively.

Sensing properties of the WO 3
The dynamic normalized resistance responses of the WO 3 films before high-humidity aging for 70 ppb toluene gas at 400°C for humidity values of 25, 50, and 75%RH are shown in Fig. 1.All the resistance profiles are normalized at the initial change in gas flow from pure air to air with toluene vapor.It is seen that the WO 3 films exhibit distinct responses to toluene vapor.A decrease in resistance of WO 3 occurs when toluene vapor combustion takes place on the surface of the WO 3 grains.Oxygen molecules from the ambient atmosphere are adsorbed onto the surfaces of the WO 3 grains and form an electron depletion layer.(17) The resistance of the WO 3 films decreased with increasing toluene concentration, and almost reached saturation in at least 1 min.The resistance of the nonaged WO 3 films caused by 70 ppb toluene gas decreased when the humidity was increased.Thus, the sensor response of the WO 3 films depends on humidity, and it was confirmed that the combustion effect of WO 3 is disturbed by moisture.Figure 2 shows the dynamic resistance response of the WO 3 films after high-humidity aging.A series of resistance measurements of the aging-treated WO 3 also show distinct responses to toluene gas.For the aging-treated WO 3 , the decrease in resistance caused by the toluene gas does not change much when the humidity was increased.All the profiles in Fig. 2 are almost identical.Thus, the sensor response of the aging-treated WO 3 is independent of humidity.
The sensor response (S) of the WO 3 films to 35 and 70 ppb toluene gas under different humidity conditions is summarized in Fig. 3.The plots show that the sensor response of the nonaged WO 3 films depends significantly on humidity.The sensor response to 70 ppb toluene gas under 75%RH is smaller than the sensor response to half the concentration, i.e., 35 ppb toluene gas, under 25%RH.The nonaged WO 3 film cannot be used to monitor the exact toluene concentrations within the levels that cause the sick building syndrome.The aging-treated WO 3 film shows good gas sensing performance because the sensor response to 35 and 70 ppb toluene gas barely changes even when the humidity changes.Consequently, these results prove that high-humidity aging treatment is effective for achieving independence from humidity effects for the WO 3 -type toluene sensor.

High-humidity aging effects
The high-humidity aging treatment is also effective for the resistance increase of WO 3 in pure air.In the case of 75%RH, the resistances of the nonaged and aging-treated WO 3  at the initial change in gas flow from pure air to air with toluene vapor are 486 kΩ and 1.68 MΩ, respectively.It is indicated that the high-humidity aging treatment promotes the oxidation to WO 3 , i.e., decreasing carrier electrons into WO 3 .It is therefore considered that the sensor responses of the WO 3 films are mostly improved by the high-humidity aging treatment, as shown in Fig. 3.
Jimenez et al. have reported the humidity dependence of the response of WO 3 with its structural defects. (13)The surface morphology of WO 3 depends on the annealing temperature.The WO 3 annealed at 400°C has structural defects, which correspond to oxygen deficiency.The amount of structural defects is decreased with the increase in annealing temperature (from 400 to 700°C, for 5 h).In this report, the responses of WO 3 to hydrogen sulfide are almost equal under more than 50%RH conditions, and WO 3 under low-or no-humidity condition shows higher response than that under more than 50%RH condition.Therefore, the oxidation of WO 3 is insufficient for the humidity independence of the sensor responses.
In the case of the high-humidity aging, the responses of WO 3 to the combustion gas, toluene, are almost equal in the range of 25-75%RH.Therefore, the humidity dependence is affected by the high-humidity aging effect as well as the structural effect.We propose a high-humidity aging mechanism to improve the gas-sensing properties of WO 3 .This humidity independence effect can be explained by the change in the surface conditions of the WO 3 grains.Figure 4 shows the surface of the WO 3 grains under different conditions.In the case of the nonaged WO 3 , under a high-humidity condition, the sensor response to toluene is decreased compared with that under the low-humidity condition, since the adsorbed water competes with oxygen at water-adsorbable sites.Therefore, the humidity change affects toluene combustion, i.e., the sensor response, for the WO 3 sensor.On the other hand, heat treatment of WO 3 under a high-moisture condition promotes the adsorption of water molecules, which bind to the WO 3 framework as hydroxyl groups, and the water-adsorbable sites are therefore blocked.Since the amount of water adsorbed is not changed under different humidity conditions, the sensor response of the aging-treated WO 3 is stable.
We have reported that the high-humidity aging treatment is also effective for palladium-loaded SnO 2 , but not effective for the non-palladium-loaded SnO 2 . (14)It is considered that palladium plays an important role in the formation of hydroxyl groups on the surface of SnO 2 . (14)However, the high-humidity aging treatment is effective for pristine WO 3 , that is, non-palladium-loaded WO 3 .The surface condition of WO 3 seems to be different from that of SnO 2 , although the reasons are not completely elucidated; the conditions should be studied in the future.

Conclusions
The sensor response of the nonaged WO 3 depends on humidity conditions.The highhumidity aging improves the humidity independence of the sensor responses of the WO 3 systems to toluene gas.The aging-treated WO 3 film shows responses to the toluene gas, which are independent of humidity from 25 to 75%RH.We propose a mechanism for the effects of the high-humidity aging on the basis of this phenomenon.

Fig. 1 .
Fig. 1.Normalized resistance profiles of nonaged WO 3 films in humid artificial air with and without toluene gas.Toluene concentrations are (a) 70 ppb and (b) 35 ppb.The normalization of resistance is based on the resistance at 5 min.

Fig. 2 .Fig. 3 .
Fig. 2. Normalized resistance profiles of WO 3 films after high-humidity aging in humid artificial air with and without toluene gas.Toluene concentrations are (a) 70 ppb and (b) 35 ppb.The normalization of resistance is based on the resistance at 5 min.