Development of the tin oxide pH electrode by the sputtering method
Chung-We Pan, *Jung-Chuan Chou, **Tai-Ping Sun, and Shen-Kan Hsiung
Institute of Electronic Engineering, Chung Yuan Christian University, Chung-Li, Taiwan 320,
R.O.C.
* Institute of Electronic Engineering, National Yunlin University of Science and Technology,
Touliu, Taiwan 640,R.O.C.
** Institute of Electrical Engineering, National Chi Nan University, Nantou, Taiwan 545,
R.O.C.
Abstract
Different preparation methods of the pH electrode influence sensing characteristics, so
this research is attempted to investigate the tin oxide (SnO2) membrane as the metal oxide pH
electrode by sputtering method, whose pH-sensing device is based on the indium tin oxide
(ITO) glass substrate, and this research uses two point calibration method to prove the sensing
characteristic. Besides, the sensing area plays an important role in fabricating a pH electrode,
so this paper uses experimental result and simulation method to confirm the suitable sensing
area of the pH electrode. As indicated by results, enough sensing area increases the stability of
the pH electrode and 4mm2 has been chosen as the sensing area of the SnO2 pH electrode. In
addition, the SnO2 pH electrode has linear response and higher stability as it compares with
the commercial pH glass electrode. As indicated by these experimental results, an effective
SnO2 pH electrode has been presented by sputtering method.
Keywords: tin dioxide (SnO2)、 pH electrode、indium tin oxide (ITO) glass、commercial pH
meter

Corresponding author: Dr. Jung-Chuan Chou, Tel: (8865)5342601 ext.2101 Fax: (8865)5321719,
E-mail: choujc@pine.yuntech.edu.tw
Present address: Institute of Electronic Engineering, National Yunlin University of Science and Technology, 123, Sec.3,
University Rd. Touliu, Yunlin, Taiwan 640, R.O.C.
1
1. Introduction
The pH value is an important parameter for chemical assay, therefore, numerous papers
reported how to fabricate an pH electrode [1-5] and some metal oxide materials were used to
fabricate different kinds of pH electrodes, such as TiO2 [6], RuO2 [6], RhO2 [6], Ta2O5 [6],
IrO2 [6], PtO2 [7], SnO2 [6,8-12], etc. For these sensing materials, different methods for the
preparation of the oxide layer influenced the structure and electronic characteristics of the pH
electrode, and then the pH sensing characteristics of the pH electrode were changed with
changing the fabrication methods. According to Glab et al. [13], the potential-pH response of
iridium oxide electrode depended on the method used for fabricating them. Iridium oxide
electrodes prepared by thermal decomposition of iridium chloride on a titanium support
respond to pH with a Nernstian sensitivity of 59 mV/pH unit. The slope of the potential-pH
dependence for sputtering iridium oxide electrodes on stainless steel and tantalum as well as
for iridium oxide on an insert electrode of the Ruzicka Selectrode type was also 59 mV/pH
[13]. Significantly different behaviors have been reported for anodic iridium oxide film
(AIROF) electrodes [13]. These exhibited a linear super-Nernstian response, which was
between 62 and 77 mV/pH [13]. According to above descriptions, the proper fabrication
method is necessary for the pH electrode and it influences the sensing characteristics.
In 1984, Fog et al. [6] presented a lot of metal oxide electrodes, which were fabricated as
pH electrodes, and it indicated commercial tin oxide (SnO2) was one of the pH sensing
material, which pH sensitivity was about 48.6mV/pH. However, they described the SnO2 in
the doped form was not suitable as a pH electrode, but functions mainly as a redox electrode,
according to this paper, it was true that SnO2 was inferior as a pH sensor because it had much
narrower dynamic range than pH glass electrode or Si3N4 ISFET. However, Yin et al. [10]
presented the SnO2 pH electrode based on the indium tin oxide (ITO) glass, which had good
pH sensing characteristics, so the SnO2 pH electrode based on the indium tin oxide (ITO)
2
glass had different sensing characteristics with commercial SnO2 pH electrode. Hence, what
lead to the different sensing characteristics with the SnO2 pH electrode?
In this paper, in order to obtain suitable pH sensing characteristics of the SnO 2 pH
electrode, the SnO2 thin film has been fabricated by the sputtering method, and the deposition
conditions and package methods were used to improve the pH electrode. In the case of
reactive sputtering, it was well known that the microstructure and properties of the films were
strongly influenced by the deposition conditions, such as the total sputtering pressure, O2
percentage in the sputtering gases, substrate temperature and radio frequency power [14].
Therefore, the total sputtering pressure and O2 percentage in the sputtering gases were used to
observe the pH sensing characteristics of the SnO2 pH electrode. In addition, the two-point
calibration method was used to identify the sensing characteristics of the pH electrode.
Experimental results were investigated in this paper.
2.Experimental
2.1Chemicals
All reagents were of analytical grade and the commercial buffer solutions were used as
the test solutions. In addition, the commercial pH meter SP2200 measured the pH value of
test solution, which was purchased from Suntex Company. For the pH electrode, it included
two elements, one was the ITO glass that was supplied by the Wintek Corporation, and
another was SnO2 thin film, which was deposited by the radio frequency sputtering system.
2.2 Fabrication of pH-sensing electrode
The SnO2 thin film was used as the sensing membrane to detect the pH value in the test
solution, which has been prepared by radio frequency sputtering in the mixed sputtering gases
3
to deposit on the ITO glass. Mixed sputtering gases included O2 and Ar. The thickness of the
SnO2 thin film was about 2000 Å. After the thin film was deposited, the conducting line was
bound from the ITO layer and packaged by epoxy.
2.3 Measurement system
The readout circuit of this study was based on the commercial pH meter SP2200, whose
reference electrode was the commercial Ag/AgCl electrode S120C, which was obtained from
Sensorex Company.
3. Results and discussions
3.1 Theory analysis
According to Fog and Buck [6], a single phase interaction electrode may similarly be
envisaged, though no example of pH electrode involving oxygen-deficit phases has been
proven. By omitting the water of hydration, the surface mechanism can be expressed as
follows:
MOx+2δH++2δe-
MO x - +δH2O
(1)
The electrode potential is, in this case
1
F
 s   l  ( )( l H    s e  
' constant'
 sO
2

 lH 2O
2
)
RT
RT
lna l H  
lna s O
F
2F
(2)
where M is the metal,  s are chemical potentials (the chemical potentials of pure phases are
constant), s and l denote solid and liquid, and  is the Galvani potential.  s O and a s O are
the chemical potential and activity of oxygen in the solid phase, respectively. Therefore, the
pH sensitivity of the metal oxide electrode should yield a straight line with a Nernst slope,
4
whose slope is about 59.16 mV/pH at 25℃.
In addition, Janata [15] presented the equivalent circuit model for an ion sensor with
solid-state internal contact. In this paper, the equivalent circuit model has been modified to
describe the simplified equivalent circuit of the SnO2 pH electrode, whose diagram was
shown in Fig. 1. Referring to Fig.1, the formula of the time response has been calculated in
this paper. For different resistors of the sensing membrane, the formula of the time response
of the gate-to-source voltage (Vgs) is expressed as follows:
Rgs＞＞RB
Vgs  Vi
CB
t
t
exp (
)  Vi (1  exp( 
))
CB  Cgs
R B (C B  Cgs )
R B (C B  Cgs )
(3)
Rgs  1000RB
Vgs  Vi
R gs
CB
t
t
exp (
)  Vi
(1  exp( 
))
CB  Cgs
(R B //R gs )(C B  Cgs )
R gs  R B
(R B //R gs )(C B  Cgs )
(4)
where RB and CB were the resistor and capacitor of the SnO2 pH electrode, Rgs and Cgs were
the resistor and capacitor of the readout circuit, Vi and Vgs were the input signal and output
signal, and t was the response time.
Therefore, according to above principles, the resistor and capacitor plays an important
role in the response time of the pH electrode.
3.2 Fabrication
3.2.1 Sensing area
In 1980, Fujimoto et al. [5] presented the relationship between the slope constant and
the length of the pH-sensitive tip of the glass micro electrode and they suggested that
manufacturing a smaller tip would inevitably be accompanied by a critical reduction of the pH
5
response. For this reason, the sensing area of the sensor associates with the pH sensitivity and
enough sensing area should be discussed.
Hence, in this study, the experimental result of the SnO2 pH electrode was shown in
Fig.2, whose pH sensitivity was about 59.2 mV/pH as sensing area was larger than 4mm 2.
Then, in order to prove experimental results, the simplified equivalent circuit of the SnO2 pH
electrode has been simulated, where RB and CB are 8  106Ω and 166.7pF, Cgs and Rgs are
23pF and 0.1935  1014Ω, respectively, and response time of the SnO2 thin film is about
0.1sec. Moreover, as indicated by the experimental results, the pH sensitivity of the SnO2 pH
electrode is near ideal Nernst response as sensing area was about 4mm2, therefore, it has been
used as the standard parameters to simulate the sensing characteristics, which simulation
result was also shown in Fig.2. In Fig.2, the simulation results correspond with the
experimental results, hence, a suitable sensing area is necessary for the SnO2 pH electrode to
increase its stability and this study uses 4mm2 as the standard sensing area.
3.2.2 Deposition conditions
For typical pH electrode, different fabrication methods were presented to discuss the
sensing characteristics [16-21], because the fabrication methods influenced the sensing
characteristics of the pH electrode. In 1989, Glab et al. [13] presented different preparation
methods and the preparation methods influenced the sensing characteristics of the pH
electrode. Therefore, a suitable preparation method was necessary to design a good pH
electrode. Moreover, according to Gao et al. [14], the ZrO2 film was prepared by radio
frequency reactive sputtering at different O2 concentrations in the sputtering gases to discuss
how to fabricate the ZrO2 film and it described that the gas concentration influenced the
structure of the thin film, therefore, this paper controls the O2 concentrations to find a suitable
deposition conditions for SnO2 pH electrode, and in order to test the sensing characteristics
6
that the measurement system was based on the commercial pH meter.
For sputtering method, deposition conditions controlled the structure of the thin film,
such as deposition rate, substrate temperature, deposition pressure, reactive gas concentration,
thickness, etc. And this study controlled reactive gas concentration and deposition pressure to
design an effective pH electrode. To define the suitable deposition conditions by controlling
the reactive gas concentration, the deposition rate versus the concentration of O2 gas of the
SnO2 membrane was shown in Fig.3 and the pH sensitivity versus the concentration of O2 was
shown in Fig.4. In Fig.3, when the concentration of O2 gas was about 20%, the deposition rate
was stable. In addition, according to Fig.4, different O2 gas concentrations changed the pH
sensitivity of the pH electrode and 20% was the proper concentration. Therefore, this study
used 20% as deposition concentration to design the metal oxide pH electrode. In addition, the
deposition pressure also influenced the pH sensitivity of the SnO2 pH electrode, which results
were shown in Fig.5, and then 20 mtorr was chosen as the deposition pressure.
3.2.3 Electron spectroscopy for chemical analysis (ESCA)
As indicated by above experimental results, the sensing area and deposition conditions
of the SnO2 thin film have been discussed, hence, in order to confirm the oxidation state of
the SnO2 thin film, the electron spectroscopy for chemical analysis (ESCA) study was carried
out with Mg K  X-ray source, which measurement instrumentation was ESCA PHI 1600
obtained from Physical Electronics Company. Fig.6 shows the spectrum of the SnO2 thin film,
whose two separative signals at the binding energy values of 495.5 eV and 487.0 eV
corresponding to Sn 3d3/2 and Sn 3d5/2 levels, respectively.
3.2.4 Reproducibility
After choosing the fabrication processes of the SnO2 pH electrode, nine pH electrodes
7
have been fabricated to prove the reproducibility, which the sensing characteristic of the SnO2
pH electrode were shown in Fig.7 and Fig.8. As indicated by these results, the SnO2 pH
electrode has the near Nernst slope between pH 2 and pH 12, which is about 59.17 mV/pH.
Hence, by using these fabrication processes, a SnO2 pH electrode has been designed with
good pH sensing characteristics.
3.3 Two-point calibration method
Indeed, a suitable pH electrode has been designed by sputtering method. However, to
prove the sensing characteristics of the tin dioxide pH electrode, the commercial pH meter
was used as the readout system, which measurement system was shown in Fig.9. This
commercial pH meter used the glass electrode as the pH electrode and it used the two-point
calibration method to calibrate the sensing signal of the glass electrode. For the ideal glass
electrode, the pH sensitivity is about 59.16 mV/pH, but it changes with time and each glass
electrode has slightly different potentials at pH 7. Therefore, the two-point calibration method
is useful to increase the accuracy of the glass electrode, where calibration method includes
two steps as follows:
The first step:
This system detects signals of the glass electrode in pH 7 solution, and then it detects
signals of the glass electrode in pH 4 solution. After detecting signals, the linear function of
the sensing mechanism can be expressed as follows:
Y=A+BX
(3)
where Y is the output potential of the glass electrode, A is the potential of the glass
electrode in the pH 0 solution, B is the slope of the glass electrode, X is the pH value of the
glass electrode in the sample solution. After this process, the real signal of the glass electrode
is obtained and the value of B is about 59.2 mV/pH for the ideal glass electrode.
8
The second step:
After detecting the signal of the glass electrode, the potential of the glass electrode in
the pH 0 solution is changed to 0V by using the calibration method, and then the slope is
increased to near 100mV/pH. Therefore, the linear function of the sensing mechanism should
be changed as follows:
Y = A’ + B’ X
where A’ is 0V, and B’ is about 100 mV/pH. By using this method, the offset of the pH
electrode can be cancelled and the slope of each pH electrode should be the same.
According to this two-point calibration method, the commercial pH meter was used as
the readout system to identify the pH sensing characteristics of the SnO2 pH electrode, which
output signal was shown in Fig.10. In Fig.10, the HP4401A (Digit multimeter, purchased
from Agilent Company) was used to detect the output potential of the pH meter, which
linearity of the pH electrode is good and this method is effective. In addition, the comparison
between the SnO2 pH electrode and the commercial glass electrode was shown in Fig.11 and
listed in Table1. As indicated by these experimental results, the SnO2 pH electrode had good
sensing characteristics and it was useful to replace the glass electrode as a pH sensor.
4.Conclusions
Different preparation methods of the pH electrode influence sensing characteristics, so
this research is attempted to investigate the tin oxide (SnO2) membrane as the metal oxide pH
electrode by sputtering method, whose pH-sensing device is based on the indium tin oxide
(ITO) glass substrate, and this research uses two point calibration method to prove the sensing
characteristic. Besides, the sensing area plays an important role in fabricating a pH electrode,
so this paper uses experimental result and simulation method to confirm the suitable sensing
area of the pH electrode. As indicated by results, enough sensing area increases the stability of
9
the pH electrode and 4mm2 has been chosen as the sensing area of the SnO2 pH electrode. In
addition, a proper deposition conditions should be considered to fabricate the pH electrode by
the sputtering method, in which the concentration of reactive gas influences the deposition
rate and the reproduction of the thin film, so this study uses 20% as the concentration of O2
gas to increase the reproduction of the pH electrode. Moreover, the deposition pressure also
influences the pH sensing characteristics and this study observe 20 mtorr is the proper value.
After controlling the deposition condition of the SnO2 thin film, the pH sensitivity of the pH
electrode is about 59.17mV/pH, which is close to the Nernst equation. Hence, using suitable
sputtering method can control the sensing characteristics. In addition, in order to verify the pH
sensing characteristics, the commercial pH meter is used as the readout system and the two
point calibration method is used to prove the accuracy of the SnO2 pH electrode, which results
show this metal oxide electrode has good pH sensing characteristics. Therefore, this study
controls the sensing area and deposition conditions to design an effective SnO2 pH electrode.
Acknowledgement
This study was supported by National Science Council, The Republic of China under the
contracts NSC 92-2215-E-033-003.
10
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13
Biographies
Chung-We Pan was born in Ping-Tung, Taiwan, Republic of China, on
February 16, 1977. He received bachelor in electronic engineering from
Chung Yuan Christian University, Chung-Li, Taiwan, in 2000. Since 2001
he has been working toward the Ph.D. degree in the Department of
Electronic Engineering at Chung Yuan Christian University, Chung-Li,
Taiwan. His research interests include characterization of biosensors,
mainly of the ISFET sensor and its applications.
Jung-Chuan Chou was born in Tainan, Taiwan, Republic of China, on
July 13, 1954. He received the B.S. degree in physics from Kaohisung
Normal College, Kaohsiung, Taiwan, in 1976; the M.S. degree in
applied physics from Chung Yuan Christian University, Chung-Li,
Taiwan, in 1979; and the Ph.D degree in electronics from National
Chiao Tung University, Hsinchu, Taiwan, in 1988. He taught at Chung
Yuan Christian University from 1979 to 1991. Since 1991 he has worked as an associate
professor in the Department of Electronic Engineering at the National Yunlin University of
Science and Technology. From 1997 to 2002 he was Dean, office of Technology Cooperation
at the National Yunlin University of Science and Technology. And since 2002, he has been
Secretary-General at the National Yunlin University of Science and Technology. His research
interests are in the areas of amorphous materials and devices, electrographic photoreceptor
materials and devices, electronic materials and devices, sensor devices, and science education.
14
Tai-Ping Sun was born in Taiwan on March 20, 1950. He received the
B.S degree in electrical engineering from Chung Cheng Institute of
Technology, Taiwan, in 1974, the M.S degree in material science
engineering from National Tsing Hua University, Taiwan, in 1977, and
Ph.D. degree in electrical engineering from National Taiwan University,
Taiwan, in 1990. From 1977 to 1997, he worked at Institute of Science
and Technology, Republic of China, concerning the development of
Infrared device, circuit and system. He joined the Department of management information
system, Chung-Yu college of Business Administration since 1997 as an associated professor.
Since 1999 he has joined the Department of Electrical Engineering, National Chi Nan
University as a professor. And since 2001, he has been Secretary-General at the National Chi
Nan University. And his research interests are infrared detector and system, analog/digital
mixed-mode integrated circuit design, special semiconductor sensor and their applications.
Shen-Kan Hsiung was born on June 14, 1942. He received the B.S.
degree from Department of Electrical Engineering, National Cheng-Kung
University, in 1965, the M.S. degree from Department of Electronic
Engineering, National Chiao-Tung University, Taiwan, in 1968 and the
Ph.D. degree from Material Science Engineering of USC, U. S. A. in
1974.From 1974 to 1978, he was an associate professor in Department of
Electrical Engineering, Chung Yuan Christian University. Since 1978 he
has been a professor in Department of Electronic Engineering, Chung Yuan Christian
University. And since 2000, he has been President, at the Chung Yuan Christian University.
His current interests are electronic materials, amorphous thin films and semiconductor
sensors.
15
Figures and Table Caption
Fig.1 Equivalent circuit of the tin oxide pH electrode.
Fig.2 pH response with different sensing areas.
Fig.3 Deposition rate versus the concentration of O2 gas.
Fig.4 pH sensitivity versus the concentration of O2 gas.
Fig.5 pH sensitivity versus the reactive pressure.
Fig.6 XPS spectrum of the tin oxide thin film.
Fig.7 Output signal of the tin oxide pH electrode by using instrumental amplifier as readout
circuit.
Fig.8 pH sensitivity of the tin oxide pH electrode by using instrumental amplifier as readout
circuit.
Fig.9 Measurement system of the tin oxide pH electrode by using commercial pH meter as
readout system.
Fig.10 Output signal of the tin oxide pH electrode by using commercial pH meter as readout
system.
Fig.11 Comparison between the tin oxide pH electrode and commercial glass electrode.
Table.1 pH value of the tin oxide pH electrode by two point calibration method.
16
Sensor
Circuit
RB
CB
Vi
Cgs
Fig.1
17
Rgs
Vgs
60
pH sensitivity (mV/pH)
50
40
pH responses with different sensing areas
30
is the simulation results
is the expermental results
20
10
0
-5
0
5
10
15
20
25
2
Sensing area (mm )
Fig.2
18
30
35
40
Deposition rate of the tin oxide membrane
Deposition rate (nm/min)
14
12
10
8
6
5
10
15
20
Concentration of O2 gas (%)
Fig. 3
19
25
30
60
pH sensitivity (mV/pH)
59
58
57
56
55
54
5
10
15
20
Concetration of O2 gas (%)
Fig.4
20
25
60
pH sensitivity (mV/pH)
58
56
54
52
50
48
46
44
0
20
40
60
80
100
120
Reactive pressure (mtorr)
Fig.5
21
140
160
180
20000
Sn 3d5/2
18000
Intensity (arb. unit)
16000
Sn 3d3/2
14000
12000
10000
8000
6000
4000
2000
0
500
495
490
Binding energy (eV)
Fig.6
22
485
480
0.4
2.11
2.11
Output voltage (V)
0.3
4.03
0.2
4.03
6.05
0.1
0.0
6.05
7.78
-0.1
7.78
9.46
-0.2
-100
9.46
11.44
0
100
200
300
400
Time (sec)
Fig. 7
23
500
600
700
0.3
Output voltage (V)
0.2
0.1
0.0
-0.1
-0.2
pH sensitivity = 59.17 mV/pH
Number of sample = 9
-0.3
-0.4
2
4
6
pH value
Fig. 8
24
8
10
12
Temperature
sensor
pH meter
SP-2200
pH sensor
Reference
electrode
Agilent 34401A
Computer
Test solution
Fig. 9
25
Display
1.2
Output voltage (V)
1.0
SnO 2 /ITO glass
Slope = 97.45mV/pH
pH 0 = 0V
(After using two-point calibration method)
0.8
0.6
0.4
0.2
0.0
0
2
4
6
pH value
Fig. 10
26
8
10
12
pH value (by tin oxide electrode)
12
10
Measurement of pH value
By two point calibration method
Electrode is the tin oxide electrode
From six samples
8
6
4
2
2
4
6
8
10
pH value (by commercial glass electrode)
Fig. 11
27
12
Commercial pH meter
(pH)
Table1
Tin oxide pH electrode
(pH)
Variation
(pH)
2.07
4.08
5.96
7.76
9.63
11.62
2.07
4.05
5.97
7.78
9.66
11.58
0.00
-0.03
0.01
0.02
0.03
0.04
28