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
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) 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 SnO2 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 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- +δH2O (1)
The electrode potential is, in this case
(2)
where M is the metal, are chemical potentials (the chemical potentials of pure phases are constant), s and l denote solid and liquid, and is the Galvani potential. and 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, 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
(3)
Rgs1000RB
(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 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 4mm2. Then, in order to prove experimental results, the simplified equivalent circuit of the SnO2 pH electrode has been simulated, where RB and CB are 8106Ω and 166.7pF, Cgs and Rgs are 23pF and 0.19351014Ω, 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 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 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 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 + B X (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.
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.