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MYU Tokyo
S & M 1177

Solid Oxide Amperometric CO Sensor
Prepared by DC Sputtering
Tatsumi Ishihara,1,2* Toshikazu Mori,1 Jun Iwata,1 Hidehisa Hagiwara,1,2 and Shintaro Ida1,2
1

Department of Applied Chemistry, Faculty of Engineering, Kyushu University
Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
2International Institute for Carbon-Neutral Energy Research (WPI-I2CNER),
Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan(Received November 20, 2015; accepted January 28, 2016)

Keywords: solid oxide amperometric sensor, CO sensor, carbon neutral energy

An amperometric CO sensor using a solid oxide electrolyte was studied, and it was found that theoxygen pumping current increased with increasing CO concentration when Au(10 wt%)/In1.9Sn0.1O3
and RuO2(10 wt%) were used for cathode and anode, respectively, and La0.9Sr0.1Ga0.8Mg0.2O3 wasused as the electrolyte. In this study, the electrode was prepared by DC sputtering. Although themorphology of the deposited electrode film was dense, the sensor responded sensitively to CO
if the electrode was prepared by DC sputtering. Coating RuO2/La0.6Sr0.4CoO3 powder on a RuO2
electrode prepared by sputtering effectively increased sensitivity because of the increased surface
area and oxidation activity, and a sensitivity to CO of ca. 19 μA/decade was achieved, which is ca.4
times higher than that of powder painted electrodes (4.5 μA/decade) at 673 K in our previous study[Sens. Actuators, B 223 (2016) 535].

1.

Introduction

A selective and reliable CO sensor is highly important in various areas because of safety issues
due to the high toxicity of CO. Therefore, several types of CO sensor have been proposed, andsome are already commercially available.(1) Among them, a semiconductor type sensor, namely,
SnO2 modified with additives, is widely used for monitoring CO in rooms,(2–6) and also solid
electrolyte type sensors have also been widely studied.(7–12) However, the most serious issue
with the semiconductor-type sensor is low selectivity. Therefore, catalytic combustion-type
and electrochemical-type CO sensors are also widely used for detection of CO. In the case ofelectrochemical CO sensors, selective detection of CO can be achieved; however, maintenance ofthe sensor is required relatively often (at least once per year). Recently, the ability to detect CO
in exhaust gas from a gas water heater has been strongly demanded after an accident caused byincomplete combustion. However, applying a semiconductor-type CO sensor which monitors the
combustion state is very difficult, and there is an urgent requirement for the development of analternative CO sensor that is highly selective and can detect CO directly in an exhaust gas line.(1)To meet this demand, a CO sensor using a solid state electrolyte was proposed and studied
intensively.(1) Solid electrolyte-type gas sensors generally have better selectivity for the gas
species of interest. Therefore, an O2 sensor using yttria- stabilized ZrO2 has been widely used for*

Corresponding author: e-mail: ishihara@cstf.kyushu-u.ac.jp
ISSN 0914-4935 © MYU K.K.

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monitoring PO2 in engine exhaust gas.(1) On the other hand, mixed potential type sensors have been
attracting much attention recently because they offer a wider range of detectable gas species.(7) To
date, there have been many reports on mixed potential-type sensors.(8–12) In our previous studies,
we investigated the amperometric type sensor based on changes in the oxide ion pumping current
in a LaGaO3-based oxide, and high sensitivity as well as selectivity was exhibited by this typesensor against CH4, C3H6, and NO based on the choice of electrode catalyst.(13–17) The detection
mechanism of this sensor was reported in our previous study.(14) However, we applied +1 V to theactive electrode for oxidation, and oxygen was electrochemically pumped from the inactive to theactive electrode to oxidize the sensed gas. Since the oxygen partial pressure on the active electrodewas decreased by the oxidation reaction of the pumped oxygen, the driving force for oxygen
pumping was increased, which caused the oxide ion current also to increase upon exposure to a gas
like CO. We noted that the electrode overpotential of the active electrode decreased upon exposure
to CO, which might have been caused by decreased PO2 around the active electrode. This may be
another reason for increased oxygen pumping current when the sensor was exposed to CO. In thisstudy, we investigated the amperometric CO sensor based on the same detection mechanism,(17,18) i.e.,a combination of active and inactive electrode catalysts for CO oxidation. The effects of electrodedeposition by sputtering were studied along with miniaturization and reproducibility of the sensor.

2.

Experimental Methods

A solid electrolyte of La0.9Sr0.1Ga0.8Mg0.2O3 (denoted as LSGM) was prepared by sintering
LSGM powder, which was prepared using a conventional solid state reaction method.(19,20) The
diameter and thickness of the disk were 17 and 0.4 mm, respectively, unless otherwise noted. RuO2
and Au/In1.9Sn0.1O3 (ITO) were deposited on each face of the LSGM disk by DC sputtering (Pascal
Co. Ltd.). Dense disks of RuO2 and Au/ITO (10 wt% Au/In0.927Sn0.073O1.5) were used as the targetof anode and cathode, respectively. Deposition of each electrode was performed at 773 K with 12
W constant power mode for 4 h. The deposited electrode was square with a 5 mm length. After
deposition of electrodes, Pt lead electrodes were deposited on each by DC sputtering, and Pt leadlines were connected to the Pt lead electrodes using Pt paste followed by calcining at 873 K for 1 h.
A schematic image of the prepared sensor element is shown in Fig. 1. To increase the sensitivity,
RuO2/La0.6Sr0.4CoO3 (LSC) (1:9) was painted on the sputtered RuO2 electrode. ITO and LSC werealso prepared by the solid state reaction method using metal nitrates or oxides as starting chemical
reagents. Before preparation of a slurry, the electrode powder was calcined at 873 K for 6 h.

PtPt線
line

Pt
lead
Pt電極

RuO2

スパッタ電極Sputteredelectrode
Spatteredelectrode

Au line
Pt線

LSGM electrolyte

Au leadTop image(a)

Pt lead

Au/ITO

Vertical Image(b)

Fig. 1. Schematics of the prepared sensor element using Au/ITO and RuO2 electrodes: (a) top view and (b) crosssectional view.

Sensors and Materials, Vol. 28, No. 4 (2016)273

In the measurement assembly, the sensor element was placed in a gas flow chamber in which
either reference air or sample CO (50–3000 ppm), which is made by mixing 1% CO in He withair, was fed at 100 ml/min. Change in current density was measured by the dc two-probe method,and Pt and Au wires were used as lead wires for the active and the inactive electrodes, respectively.
Pt and Au meshes were also used as current correctors for the active and inactive electrodes,
respectively, and were set on the lead electrode deposited by sputtering using commercial Pt and Au
pastes. In the amperometric mode, 1 V dc was always applied using a potentiostat/galvanostat (Hokuto
Denko HA301) as the RuO2 based electrode was always positive. The voltage and current were
measured with a digital multimeter (Advantest, Model R645A) and/or digital electrometer (Advantest,
Model R8240). The sensitivity of the sensor was defined as a current change against the COconcentration of one order of magnitude. The sensing property was measured several times withseveral elements and the reproducibility of the sensitivity was confirmed (Deviation of sensitivity is
within 5%).

3.

Results and Discussion

Figure 2 shows scanning electron microscopy (SEM) images of Au/ITO electrodes preparedby DC and radio frequency (RF) sputtering. In both methods, an Au/ITO film was successfully
deposited on an LSGM film, and EDX analysis suggested that the composition of the film wasalmost the same as 10 wt%/In1.9Sn0.1O3. However, the morphology of the electrode differed
between DC and RF sputtering. As shown in Fig. 2, a more dense and smooth morphology of the
Au/ITO electrode was obtained by DC sputtering compared with that from RF sputtering. A tighter
contact was also achieved on the Au/ITO electrode deposited by DC sputtering. Therefore, we used
DC sputtering for preparation of the Au/ITO and RuO2 electrodes.
Figure 3 shows SEM images of the fractured surface of Au/ITO and RuO2 electrodes. It
is obvious that flat and uniform thicknesses of electrodes were deposited successfully by DC
sputtering. Although a small gap was observed on the Au/ITO electrode, this could have been
formed when fracturing the surface. The thickness of the deposited electrode was estimated to be

Fig. 2.

SEM images of Au/ITO electrodes prepared by (a) DC and (b) RF sputtering.

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Sensors and Materials, Vol. 28, No. 4 (2016)

Fig. 3.

SEM images of the fractured surface of (a) Au/ITO and (b) RuO2 electrodes.

ca. 1mm for both Au/ITO and RuO2 electrodes. It is also noted that a dense film was deposited on
both electrodes in spite of the columnar shape that tends to be deposited in sputtering.
Figure 4 shows typical responses to CO at 773 K on the sensor with RuO2 and Au/ITO film
electrodes. Obviously, oxygen pumping current was increased upon exposure to CO, and thechange in current increased with increasing CO concentration. The increase in current upon
exposure to CO on the sensor using the electrode deposited by sputtering is the same as that using
Au/ITO and RuO2/LSC prepared using the powder painting method. Therefore, the detection
mechanism seems to be the same for both sensors. Figure 5 shows the current change of the sensor
as a function of CO concentration at temperatures from 573 to 773 K. At each temperature, it isobvious that oxygen pumping current increased with increasing CO concentration, so CO can bedetected by observing the oxygen pumping current. The sensitivity to CO increased with increasing
operating temperature, and at 773 K, a sensitivity of 93.2 μA/decade to CO was achieved. Thissensitivity is 10 times larger than that of the sensor prepared by hand painting in our previous study.(21)In Fig. 5, the reproducibility of the sensor data is also shown, and it is obvious that reproducibilityof the sensor is reasonably high. Considering the sensitivity, higher operating temperatures are
suitable; however, even at 673 K, it is obvious that reasonable linearity was observed between
oxygen pumping current and CO partial pressure.
In our previous study, this type sensor was highly selective to CO; however, only oxygen
strongly affects the oxygen pumping current. Therefore, the influence of oxygen on this sensor withelectrodes deposited by sputtering was studied. Figure 6 shows the response to O2 of the sensor
at 673 K. Upon exposure to low partial pressure of oxygen, current was immediately decreased
and then gradually increased. When the atmosphere was returned to air, the current returned to theoriginal level after the overshoot. Impedance measurements suggested that this decrease in oxygen
pumping current was caused by an increased overpotential of the Au/ITO electrode. However, thechange in current due to O2 was not large for the sensor using Au/ITO and RuO2 electrodes preparedby sputtering. The current is shown in Fig. 6(b) as a function of oxygen partial pressure. In thisfigure, current change in response to O2 at 773 K is also shown. At 673 K, dependence of current
change on PO2 was not large, particularly, in a PO2 range higher than 10%, which is the expected PO2
range for combustion control in small steam boilers. In contrast, at 773 K, sensitivity to O2 wasgreatly increased (404 μA/decade), so the optimum operating temperature for this sensor could bearound 673 K.

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773K

1.05
1.04

0.10
1030ppm
1030ppm

1.03

1.01

0.12

Current change / mA

Current / Base Current

1.06

1.02

0.00256000ppm 6000ppm
3000ppm
3000ppm

500ppm500ppm1.00

0.06

0.00151.08 μA/decade0.001093.2 μA/decade1.16 μA/decade573K1st573K2nd673K1st673K2nd773K1st773K2nd0.00050.0000
1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

log(CO concentration / ppm)

0.04

12.6 μA/decade12.5 μA/decade

0

0.00
1.8

1000 2000 3000 4000 5000 6000 7000 8000

2.0

2.2

Time/second

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

log(CO concentration / ppm)

Fig. 4. Typical response to CO at 773 K on thesensor with RuO2 and Au/ITO film electrodes. Blackis the first response, and gray indicates the second
trial for reproducibility.

Fig. 5. Change in current of the sensor using Au/
ITO and RuO2 as a function of CO concentration
at temperatures from 573 to 773 K. “1st” means
response in the first trial, and “2nd” means responsein the second trial for reproducibility.

0.0

1.6

673K
400℃

-0.1

1.2

5%

1%

10%

Current change / mA

1.4

Current / Base Current

92.3 μA/decade

0.02

0.99
0.98

0.08

573K1 st573K2 nd0.0020Current change / mA

1.07

15%

1.0

0.8

34.5 μA/decade

400℃
673K
500℃
773K

-0.2
-0.3
-0.4

404 μA/decade

-0.5
-0.6

0.6

0

1000

2000

3000

4000

5000

0.0

0.2

0.4

0.6

0.8

Time/secondlog(O2 concentration / %)

(a)

(b)

1.0

1.2

Fig. 6. (a) Response to O2 on the sensor at 673 K and (b) change in current of the sensor as a function of oxygen
partial pressure from 573 to 773 K.

To increase sensitivity to CO, coating the RuO2 electrode with RuO2/LSC powder wasinvestigated. A schematic image of the sensor is shown in Fig. 7. A powder of RuO2/LSC was
hand-painted on the Pt lead wire deposited on the RuO2 film electrode. After coating, the sensor
was calcined at 873 K for 30 min before measurement. Figure 8(a) shows a typical response curve
to CO for this sensor at 773 K. It is obvious that the oxygen pumping current was increased andreturned to its original level within 60 s after expose to CO and air. Therefore, considering thetime required to exchange the atmosphere in the measurement chamber, the response of this sensor
was reasonably fast. We also observed that the change in oxygen pumping current increased
with increasing CO partial pressure. However, at 6000 ppm CO, which is an excessively high276

Sensors and Materials, Vol. 28, No. 4 (2016)

RuO2 + LSC

Pt sputtered line Sputtered electrodePt lead line

RuO2

Pt lead line

Pt sputtered line

Pt lead line

LSGM electrolyte

Pt lead line

Pt sputtered line

(a)

(b)

Schematics of the sensor with RuO2/LSC powder coated on RuO2: (a) top view and (b) cross-sectional

Fig. 7.
view.

1.20

0.30

6000ppm

773K

1.15
3000ppm

1.101030ppm

1.05

873K
773K
673K
573K

0.25

Current change / mA

Current / Base Current

10 wt% Au-ITO

500ppm100ppm1.00

198.5 µA/decade

0.20
0.15

67.1 µA/decade

0.10
19.2 µA/decade

0.05

1.96 µA/decade

0.00
0

1000

2000

3000

4000

5000

Time/second

(a)

6000

7000

8000

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

log(CO concentration / ppm)(b)Fig. 8. (a) Typical response curve to CO of the sensor using RuO2/LSC-coated RuO2 and Au/ITO electrodes at 773 K.
(b) Oxygen pumping current as a function of CO from 673 to 873 K.

concentration, the current achieved a maximum and gradually decreased with time, i.e., the socalled annealing phenomena in the response was observed. This may be explained by the reactionof CO with the active oxygen initially existing on the surface of the RuO2 film electrode followedby using oxygen, which was supplied by the electrochemical pump. The relationship between
oxygen pumping current and CO partial pressure of the sensor using the RuO2/LSC powder-coated
RuO2 film electrode is shown in Fig. 8(b). Obviously, the oxygen pumping current monotonicallyincreased with increasing CO concentration. Compared to the slopes of lines in Fig. 5, those in Fig.
8 are much larger at all operating temperatures, and this suggests that the sensitivity of the sensor
can be increased by coating the RuO2 film electrode with RuO2/LSC powder. For example, thesensitivity to CO at 673 K was increased from 12.6 to 19.5 μA/decade. Although contact between
RuO2 electrode was greatly improved by preparing it using sputtering, the surface area was muchsmaller than that of the powder-coated one. Therefore, by coating with RuO2/LSC powder, reaction
sites for CO oxidation can be increased, resulting in increased sensitivity to CO. At 973K, an
oxidation reaction also occurred on Au/ITO, the inactive electrode. Therefore, the difference inPO2 between anode and cathode became smaller at excessively high operating temperatures. In any

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Sensors and Materials, Vol. 28, No. 4 (2016)

case, it is clear that the oxygen pumping current of the sensor with RuO2 and Au/ITO film preparedby sputtering can be used for detection of CO in exhaust gas. Because it is expected that theminiaturization of the sensor may also be easily achieved, reasonable response and high sensitivityof the sensor prepared with sputtering is highly attractive.

4.

Conclusion

This study revealed that the oxygen pumping current increased with increasing CO concentration
when Au(10 wt%)/ITO and RuO2(10 wt%) were used for cathode and anode, respectively, andLSGM was used as the electrolyte. In this study, electrodes were prepared by DC sputtering andtheir sensitivity to CO was much improved by good contact between electrolyte and electrode.
Although the morphology of the deposited electrode film was somewhat dense, the oxygen pumping
current in the sensor responded sensitively to CO. Coating RuO2/LSC powder on RuO2 electrodes
prepared by sputtering is effective for increasing sensitivity to CO and, considering the sensitivity
to O2, the optimum operating temperature seems to be 673 K.

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