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Sensors and Materials, Vol. 35, No. 11 (2023) 3851–3861
MYU Tokyo
S & M 3454

Sensing Behavior of Adsorption/Combustion-type
Gas Microsensors to Various Alcoholic Vapors
Takeo Hyodo,1* Kazunori Nagae,1 Taro Ueda,1Takahiko Sasahara,2 and Yasuhiro Shimizu1
1Graduate School of Engineering, Nagasaki University,

1-14 Bunkyo-machi, Nagasaki 852-8521, Japan

2Gas Equipment R&D Center, Yazaki Energy System Corporation,

23 Minamikajama, Futamata-cho, Tenryu-ku, Hamamatsu-shi, Shizuoka 431-3393, Japan(Received March 24, 2023; accepted July 25, 2023)

Keywords: adsorption/combustion-type gas microsensor, alcohol, dynamic response, static response,
volatile organic compound

Adsorption/combustion-type gas microsensors utilizing γ-Al2O3 co-loaded with m wt%
noble metal and 10 wt% metal oxide [mN/10MO/γ-Al2O3, N: noble metal (Pd or Pt), MO: metaloxide (CeO2 or Bi2O3)] were fabricated, and their sensor-signal profiles to five types of alcoholic
vapor (ethanol, 1-propanol, 1-butanol, 1-pentanol, and 1-hexanol) were investigated and analyzed
by differential and integral processing. The sensor-signal profiles largely depended on the types
of N and MO as well as the amount of N loaded (m). The dynamic response arises from the flash
catalytic combustion of the alcohols at 450 °C and their related components (partially oxidized
and/or condensed products) adsorbed at 150 °C on the sensing film, and the 10Pt/10CeO2/γ-Al2O3
and 10Pd/10CeO2/γ-Al2O3 sensors showed the largest dynamic responses to 1000 ppm ethanoland 1-hexanol, respectively, among all the sensors tested. These sensors showed sufficiently
large dynamic responses even to 10 ppm alcohols. The differential processing of their sensorsignal profiles was valuable in precisely finding slight changes and differences in their sensing
behaviors and providing considerable information for data processing.

1.

Introduction

Various types of gas sensor (principal sensing material: semiconductor,(1–5) solid
electrolyte,(6–10) polymer,(11,12) optical fiber,(13) acoustic resonator,(14) and so on) have beendeveloped to detect volatile organic compounds (VOCs) sensitively and selectively. Among
various candidates, our group has specifically focused on improving the VOC-sensing propertiesof adsorption/combustion-type gas microsensors during the last two decades.(15–23) The
adsorption/combustion-type gas microsensors are fabricated by micro-electromechanical system(MEMS) technology. As they can be easily miniaturized with a low power consumption as well
as a low heat capacity, they are adequately capable of working under dynamic temperature*Corresponding author: e-mail: hyodo@nagasaki-u.ac.jp

https://doi.org/10.18494/SAM4403

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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operation to detect a low concentration of VOCs. The large amounts of VOCs are firstly
adsorbed on the sensing materials at low temperatures (e.g., room temperature (RT)–150 °C),
and subsequently, they burn instantaneously upon pulse heating up to elevated temperatures(e.g., 250–450 °C), which is the origin of the large dynamic response to VOCs. Therefore, thestrict compositional and morphological controls of the sensing materials [generally, noble metal(N)-loaded γ-Al2O3] were indispensable to effectively oxidize VOCs adsorbed on the surface
during the flash heating-up period. We previously demonstrated that the co-loading with Au andPd on γ-Al2O3 by impregnation and the mixing of thermally conductive αAl2O3 to the basesensing material, γ-Al2O3, were effective in improving the ethanol sensing properties.(18) In
addition, the high-dispersed loading with core(Au)/shell(Pd) nanoparticles on γ-Al2O3 byutilizing the sonochemical reduction of Au and Pd chlorides with polyethylene glycol
monostearate considerably enhanced the sensing properties to ethanol, toluene, and n-hexane.(19)Moreover, the co-loading of Pt with one of the various types of metal oxide on γ-Al2O3 largely
improved the catalytic combustion behavior of VOCs over the oxide surface and thus enhanced
the VOC sensing properties.(20–23) On the basis of these previous studies, the effects of theloading of noble metal (Pd or Pt) with metal oxide (Bi2O3 or CeO2) on γ-Al2O3 on the sensing
properties to five types of alcoholic vapor (ethanol, 1-propanol, 1-butanol, 1-pentanol, and1-hexanol) have been investigated in this study.

2.

Experimental Procedure

2.1

Preparation of sensing materials

Aluminum secondary butoxide [Al(sec-OC4H9)3, ca. 4.2 × 10−2 mol] and behenic acid
(C21H43COOH, ca. 1.0 × 10−2 mol) were mixed with 1-propanol (ca. 300 cm3), and then deionizedwater (ca. 10 cm3) was added to the solution. After the solution was stirred at RT for 45 h tohydrolyze Al(secOC4H9)3, it was solvothermally treated at 110 °C for 1 h (CEM Corp., MARS5).
The white precipitate obtained was washed with ethanol, and then it was fired at 700 °C for 2 h
in ambient air, to obtain γ-Al2O3 powder. Subsequently, an appropriate amount of bismuth(III)nitrate pentahydrate [Bi(NO3)3·5H2O] or cerium(III) nitrate hexahydrate [Ce(NO3)3·6H2O] wasmixed with the γ-Al2O3 powder in a small amount of deionized water. After they were
evaporated to dryness, the precipitates obtained were fired at 700 °C for 1 h in air. The MOloaded γ-Al2O3 powder [MO: metal oxide (Bi2O3 or CeO2)] obtained was denoted as lMO/γAl2O3 [l: amount of MO loaded, 10 (wt%) in this study]. Furthermore, Pd or Pt was loaded onto
the 10MO/γ-Al 2 O3 powder by impregnation using palladium(II) nitrate dihydrate[Pd(NO3)2·2H2O] or chloroplatinic(IV) acid hexahydrate, respectively, followed by annealing at
500 °C for 1 h in ambient air. The N-loaded 10MO/γ-Al2O3 powder obtained [N: noble metal (Pdor Pt)] was denoted as mN/10MO/γ-Al2O3 [m: amount of N loaded, 1, 5, 10 (wt%) in this study].

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2.2

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Fabrication of sensors and measurement of sensing properties to various alcoholic
vaporsA schematic drawing of the adsorption/combustion-type gas sensor is shown in Fig. 1(a). A
microsensor chip was fabricated by MEMS technology. Three thin insulating layers (SiO2,
Si3N4, and HfO2) were deposited on a silicon substrate with a diaphragm structure, and a coupleof Pt heaters, which were covered with a thin alumina film, were formed just on the diaphragm
structure. The sensing or reference material, i.e., unloaded γ-Al2O3 powder, was mixed with anorganic vehicle [a mixed solution of di-n-butyl phthalate (ca. 36.4 g) and terpineol (ca. 53.6 g)
containing polyvinyl butyral resin (ca. 10 g, mean polymerization degree: 700)], followed by ball
milling for 30 min. The paste containing an mN/10MO/γ-Al2O3 or unloaded γ-Al2O3 powder wasapplied over the Pt heaters of the microsensor chip, as a sensing or reference film, respectively,by the drop coating technique employing an air-pulse fluid dispenser (Musashi Eng., Inc., MS10DX) with a syringe of suitable size, and then the microsensor chips attached with these films
were fired at 700 °C for 2 h in ambient air. Hereafter, the sensor is referred to as a sensing
material (mN/10MO/γ-Al2O3).
The sensing properties to alcoholic vapors were measured in an acrylic chamber (innervolume: ca. 50 dm3), in which five types of target alcoholic vapor (ethanol, 1-propanol, 1-butanol,
1-pentanol, and 1-hexanol; concentration range: 10–1000 ppm) were evaporated with a compact
heater. The sensing and reference films with Pt heaters (heat-sensitive elements) of mN/10MO/γAl2O3 sensors were incorporated in a bridge circuit with two fixed resistive elements. All
sensors were operated with a mode of pulsed temperature heating [high-temperature operation
at 450 °C for 0.4 s after low-temperature (LT) operation at 150 °C for 9.6 s with a cycle of 10 s]
by applying a rectangular bridge voltage. Figure 1(b) shows a typical sensor-signal profile ofadsorption/combustion-type gas sensors. Alcohols and their related components [partiallyFig. 1. (Color online) (a) Schematic drawing of adsorption/combustion-type gas sensor and (b) typical sensorsignal profile with definition of three types of response.

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oxidized and/or condensed products (intermediates)] in air adsorb on both the sensing andreference materials during the LT period at 150 °C. It is expected that the molecules adsorbed on
the sensing material will easily burn upon pulse heating up to 450 °C. Therefore, a sensor-signal
profile generally has one dynamic signal, which arises from the flash catalytic combustion of thealcohols and their related components adsorbed on the sensing film, and a subsequent static
signal, which arises from the general catalytic combustion of the alcohols during pulse heating at
450 °C. The general response ΔVMAX was defined as the maximum of the difference (ΔV)
between output voltages in air containing the alcohols (Valcohol) and that in air (Vair). Two
additional responses calculated by integrating the sensor-signal profiles, i.e., the approximately
integrated dynamic response (IDR) and approximately integrated static response (ISR), were also
defined as shown in Fig. 1(b). Moreover, the representative sensor-signal profiles weredifferentiated with respect to time, especially to elucidate the behavior of the dynamic signal.

3.

Results and Discussion

3.1

General response and integrated dynamic and static responses

Figure 2 shows sensor-signal profiles of the 10N/10MO/γ-Al2O3 sensors to 1000 ppm ethanol
in air. The dynamic responses of the 10Pt/10MO/γ-Al2O3 sensors to ethanol were clearly larger
than those of the 10Pd/10MO/γ-Al2O3 sensors. This indicates that the Pt nanoparticles loaded
onto the 10MO/γ-Al2O3 surface effectively burned ethanol and its related components [partially
oxidized and/or condensed products (intermediates)] adsorbed on the surface during the LT
period at 150 °C when the operating temperature instantaneously rose from 150 to 450 °C. In
addition, the co-loading of 10 wt% CeO2 onto γ-Al2O3 with N largely enhanced both the
dynamic and static responses in comparison with the co-loading of 10 wt% Bi2O3 onto γ-Al2O3
with N, regardless of the type of N (Pd or Pt). On the other hand, the dynamic and static
responses of 10Pt/10MO/γ-Al2O3 sensors to ethanol in this study were larger than those of
1Pt/10MO/γ-Al2O3 sensors (1Pt/10MO/γ-Al2O3: 10MO/γ-Al2O3 loaded with 1 wt% Pt by
utilizing the sonochemical reduction technique)(20,23) under the same operating conditions, but
the difference was not as large as expected from the difference in the amount of Pt between

Fig. 2.

(Color online) Sensor-signal profiles of 10N/10MO/γ-Al2O3 sensors to 1000 ppm ethanol in air.

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them. This result probably shows that the dispersibility of Pt nanoparticles loaded by theimpregnation technique on the 10MO/γ-Al2O3 surface is inferior to that of Pt nanoparticles
loaded by the sonochemical reduction technique.
Figure 3 shows the sensor-signal profiles of all 10N/10MO/γ-Al2O3 sensors to 1000 ppm
large-molecular-weight alcohols (1-propanol, 1-butanol, 1-pentanol, and 1-hexanol) in air. The
dependences of ΔVMAX, IDR, and ISR on the number of carbon atoms of alcohols are also
summarized in Fig. 4. Theoretically, the heat of combustion of alcohols increases with themolecular weight (i.e., the number of carbon atoms) of alcohols.(24) ΔV represents an output
voltage at a transient time, which is derived from the combustion rate of each alcohol, while theintegral of ΔV represents the sum of the combustion heat of the alcohols during a period. Thus, it
is expected that the magnitude of responses (especially ISR) of adsorption/combustion-type gas
sensors will increase with the number of carbon atoms of the alcohols, and thus, it is dependent
on the catalytic combustion behavior of the alcohols over the 10N/10MO/γ-Al2O3 surface. In
addition, ΔVMAX and IDR are affected also by the number of carbon atoms of the alcohols andtheir related components adsorbed on the 10N/10MO/γ-Al2O3 surface during the LT period at
150 °C. These factors showed that the observed magnitudes of both the dynamic and static
responses of the 10N/10MO/γ-Al2O3 sensors were largely dependent on the types of N and MO.
As for the 10Pt/10CeO2/γ-Al2O3 sensor, the increase in the number of carbon atoms in alcohols
decreased ΔVMAX, delayed the time at which the maximum value of the dynamic profile
appeared (e.g., ca. 10 ms and ca. 35 ms from flash heat-up for ethanol and 1-hexanol,
respectively), and enhanced the tailing of the dynamic profile. Therefore, the shape of the
dynamic response broadened with an increase in the number of carbon atoms of alcohols, andthe IDR to 1-butanol was the largest of those to all alcohols. Moreover, the ISRs of the

(a)

(c)

(b)

(d)

Fig. 3. (Color online) Sensor-signal profiles of 10N/10MO/γ-Al2O3 sensors to 1000 ppm (a) 1-propanol, (b)1-butanol, (c) 1-pentanol, and (d) 1-hexanol in air.

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Fig. 4. (Color online) Dependences of (a) ΔVMAX, (b) IDR, and (c) ISR of 10N/10MO/γ-Al2O3 sensors on thenumber of carbon atoms of alcohols.

10Pt/10CeO2/γ-Al2O3 sensor to 1-butanol and 1-pentanol were also relatively large in comparison
with those to ethanol and 1-propanol. These results indicate that the co-loading of Pt and CeO2
on the γ-Al2O3 surface was not effective in burning the large-molecular-weight alcohols andtheir related components adsorbed on the surface at 150 °C, especially under the dynamic
operation.
The ΔVMAX of the 10Pt/10Bi2O3/γ-Al2O3 sensor tended to be saturated with an increase in thenumber of carbon atoms of alcohols, and ΔVMAX to large-molecular-weight alcohols was larger
than that of the 10Pt/10CeO2/γ-Al2O3 sensor. However, IDR gradually increased with the numberof carbon atoms of alcohols owing to the delay and widening of the dynamic profiles, which issimilar to the behavior of the dynamic responses of the 10Pt/10CeO2/γ-Al2O3 sensor. Thus, theIDRs of the 10Pt/10Bi2O3/γ-Al2O3 sensor to 1-pentanol and 1-hexanol were larger than those of
the 10Pt/10CeO2/γ-Al2O3 sensor. ISR also gradually increased with the number of carbon atomsof alcohols, and the ISR of the 10Pt/10Bi2O3/γ-Al2O3 sensor to 1-hexanol was comparable to thatof the 10Pt/10CeO2/γ-Al2O3 sensor.
The 10Pd/10Bi2O3/γ-Al2O3 sensor showed the smallest ΔVMAX, IDR, and ISR among all the
sensors. However, all the values gradually increased with the number of carbon atoms ofalcohols, and its ΔVMAX, IDR, and ISR became close to those of the 10Pt/10CeO2/γ-Al2O3 sensor.

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On the other hand, the 10Pd/10CeO2/γ-Al2O3 sensor maintained the relatively large ISRs to all
the alcohols, which hardly depended on the number of carbon atoms, while its ΔVMAX and IDR
abruptly increased with the number of carbon atoms of alcohols, and the sensor showed thelargest ΔVMAX and IDR to 1-butanol, 1-pentanol, and 1-hexanol among all 10N/10MO/γ-Al2O3
sensors. These results indicate that the co-loading of Pd and CeO2 on the γ-Al2O3 surface was
effective in burning the large-molecular-weight alcohols and their related components adsorbedon the surface at 150 °C, especially under the dynamic operation.
Considering these results, the types of N and MO and their combination have a significant
impact on the magnitude of responses to these gases, and hereafter, the catalytic combustion
behavior of these alcohols over the 10N/10MO/γ-Al2O3 surface should be investigated to
optimize the composition and microstructure of N and MO loaded on the γ-Al2O3 surface.
3.2

Differential sensor-signal profiles

Figure 5 shows the differential sensor-signal (DSS) profiles of 10Pt/10CeO2/γ-Al2O3 and10Pd/10CeO2/γ-Al2O3 sensors to 1000 ppm ethanol and 1-hexanol in air. As dΔV/dt originatesfrom the change in the combustion rate of each alcohol with time, the differential processing of
the sensor-signal profiles of these sensors is of considerable significance in precisely
understanding the slight changes and differences in their behaviors. The DSS profile of theFig. 5. (Color online) Differential sensor-signal profiles of (a) 10Pt/10MO/γ-Al2O3 and (b) 10Pd/10MO/γ-Al2O3
sensors to 1000 ppm ethanol and 1 hexanol in air.

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10Pt/10CeO2/γ-Al2O3 sensor to ethanol, which showed the largest ΔVMAX to ethanol, consists ofa large, narrow, and positive signal and a subsequent broad negative signal, which indicates thatthe large amount of ethanol and its related components adsorbed abruptly during the LT period
as well as efficiently burned on the surface under the dynamic operation. The DSS profile to1-hexanol of the 10Pt/10CeO2/γ-Al2O3 sensor mainly had two small, broad, and positive signals
for 50 ms from flash heat-up, because the small amount of 1-hexanol and its related components
adsorbed during the LT period inefficiently burned on the surface. The positive and negative
signals in the DSS profile of the 10Pt/10Bi2O3/γ-Al2O3 sensor to ethanol were smaller than thoseof the 10Pt/10CeO2/γ-Al2O3 sensor. The 10Pt/10Bi2O3/γ-Al2O3 sensor had positive and negative
signals also in the DDS profile to 1-hexanol. However, the time at which the maximum value ofthe positive signal appeared markedly delayed and its shape largely broadened, which means that
1-hexanol and its related components adsorbed during the LT period were relatively difficult toburn on the surface in comparison with ethanol and its related components.
The DSS profile of the 10Pd/10Bi2O3/γ-Al2O3 sensor to 1-hexanol, which was larger than thatto ethanol, was comparable to the DDS profile of the 10Pt/10CeO2/γ-Al2O3 sensor to 1-hexanol.
The DSS profile of the 10Pd/10CeO2/γ-Al2O3 sensor to ethanol was also too small to be
sufficiently analyzed. However, the 10Pd/10CeO2/γ-Al2O3 sensor had a large positive signal anda subsequent large negative signal to 1-hexanol, which was slower than that of the 10Pt/10CeO2/
γ-Al2O3 sensor to ethanol. This indicates that a large amount of 1-hexanol and its related
components adsorbed on the 10Pd/10CeO2/γ-Al2O3 surface during the LT period, and they were
efficiently burned under the dynamic operation.
As mentioned above, the differential processing of the sensor-signal profiles of these sensors
to alcohols provides much information on the sensing behavior. Thus, we expect that thedifferences in these DSS profiles will be useful in identifying the types of alcohol by various
data processing techniques with modern Internet-of-Things (so-called IoT) technology.
3.3

Concentration dependences of sensor responses

Figure 6 shows the concentration dependences of the IDR and ISR of 10Pt/10CeO2/γ-Al2O3,
10Pd/10CeO2/γ-Al2O3, and 10Pd/10Bi2O3/γ-Al2O3 sensors to all alcohols in air. The IDRs of all
sensors to all alcohols tend to be saturated with an increase in concentration as shown by
auxiliary lines, and such dependences of the IDRs on the concentration almost correspond to
those of other adsorption/combustion-type gas microsensors in previous papers.(15,16,19,20) Thisbehavior is one of the lines of evidence that the limited numbers of alcohols and their related
components could be adsorbed on the surface at 150 °C, and they burned during the flash
heating-up period from 150 to 450 °C. In addition, the IDRs of the 10Pt/10CeO2/γ-Al2O3 sensor
to 10 ppm 1-pentanol and 1-hexanol were larger than those of the 10Pd/10CeO2/γ-Al2O3 sensor,
even though the IDRs of the 10Pt/10CeO2/γ-Al2O3 sensor to 1000 ppm 1-pentanol and 1-hexanol
were smaller than those of the 10Pd/10CeO2/γ-Al2O3 sensor. This might be caused by thedifferences in the amounts of 1-pentanol, 1-hexanol, and their components adsorbed on thesurface under the LT period at 150 °C, which are expected to depend on their oxidation activities

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Fig. 6. (Color online) Concentration dependences of IDR and ISR of (a) 10Pt/10CeO2/γ-Al2O3, (b) 10Pd/10CeO2/γAl2O3, and (c) 10Pd/10Bi2O3/γ-Al2O3 sensors to all alcohols in air.

against the different concentrations of alcohols at 150 °C. In addition, the thickness of the
10N/10CeO2/γ-Al2O3 sensing film of the sensors is also considered to be one of the importantfactors that affect the magnitude of IDR, because the Pt heater (heat-sensitive element) is locatedbeneath the sensing film. On the other hand, the ISRs of all sensors to all alcohols increased
approximately linearly with the concentration. This is because the alcohols directly burned overthe surface during the static operation at 450 °C under pulse heating, which approximately
corresponds to that of general catalytic combustion-type gas sensors.(25)
3.4

Effects of amount of N on sensing properties to alcohols

Figure 7 shows the sensor-signal profiles of mN/10CeO2/γ-Al2O3 sensors (N: Pd, Pt, m: 1, 5,10) to 10, 100, and 1000 ppm ethanol and 1-hexanol in air. The magnitude of responses(especially ΔVMAX and IDR) decreased with a decrease in the amount of N. The tendencies of
ΔVMAX and IDR were prominent in the high concentration range of both ethanol and 1-hexanol.
The behavior indicates that the highly concentrated alcohols are not efficiently oxidized at 450°C on the surface of mN/10CeO2/γ-Al2O3 with the smaller amount of N and that the amounts ofalcohols and their related compounds adsorbed at 150 °C are relatively small on the surface.
Moreover, the time at which the maximum value of the dynamic response appeared markedly
delayed, and the shapes of the sensor-signal profiles to more concentrated 1-hexanol largely
changed with a decrease in the amount of Pt, in the case of the sensor-signal profiles of the
mPt/10CeO2/γ-Al2O3 sensors. In contrast, the shapes of the sensor-signal profiles of the
mPd/10CeO2/γ-Al2O3 sensors to both ethanol and 1-hexanol were not markedly dependent on theconcentration. These results also show that the loading of Pd on the 10CeO2/γ-Al2O3 surface iseffective in enhancing the sensing properties to 1-hexanol as well as the oxidation activity of
1-hexanol.

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Fig. 7. (Color online) Sensor-signal profiles of mN/10CeO2/γ-Al2O3 sensors [N: (a) Pd, (b) Pt, m: 1, 5, 10] toethanol and 1-hexanol (10, 100, and 1000 ppm) in air.

4.

Conclusions

The sensing properties of mN/10MO/γ-Al2O3 sensors to five types of alcohol (ethanol,
1-propanol, 1-butanol, 1-pentanol, and 1-hexanol) were investigated in this study. The loading of
Pt on the 10CeO2/γ-Al2O3 surface largely improved ΔVMAX and IDR to ethanol, whereas it was
not effective in enhancing ΔVMAX and IDR to large-molecular-weight alcohols (1-pentanol and1-hexanol) as we had expected. Instead, the loading of Pd on the 10CeO2/γ-Al2O3 surface
considerably increased ΔVMAX and IDR to large-molecular-weight alcohols, especially 1-hexanol,
probably owing to the relatively large oxidation activity. These sensors showed sufficiently large
responses (especially ΔVMAX and IDR) even to 10 ppm alcohols, and the magnitudes and shapesof the sensor-signal profiles were largely dependent on the combination of N and MO as well as
the amount of N of the mN/10MO/γ-Al2O3 sensors. The differential processing of their sensorsignal profiles considerably provided much information on the sensing behavior.

Acknowledgments
This work was supported by JSPS KAKENHI Grant Number JP21H01626.

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