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Investigation of interfacial capacitance of Pt, Ti and TiN coated electrodes by electrochemical impedance spectroscopy
Biomolecular Engineering 19 (2002) 67 /71 www.elsevier.com/locate/geneanbioeng
Investigation of interfacial capacitance of Pt, Ti and TiN coated electrodes by electrochemical impedance spectroscopy A. Norlin a,b, J. Pan a,*, C. Leygraf a a
Department of Material Science and Engineering, Royal Institute of Technology, Drottning Kristinas Va¨g 51, SE-100 44 Stockholm, Sweden b St. Jude Medical AB, Veddestava¨gen 19, SE-175 84 Jarfalla, Sweden
Abstract Electrochemical processes at the electrode /electrolyte (body fluid) interface are of ultimate importance for stimulating/sensing electrode function. A high electrode surface area is desirable for safe stimulation through double-layer charging and discharging. Pt and Pt /Ir alloys have been the most common electrode materials. The use of TiN coating as the surface layer on the electrode has found increasing interest because of its metal-like conductivity, excellent mechanical and chemical properties, and the fact that it can be deposited with a high surface area. In this work, electrochemical impedance spectroscopy (EIS), which is a sensitive and nondestructive technique and widely used for characterization of electrical properties of electrode /electrolyte interfaces, was applied to investigate pure Pt and Ti, and TiN coated electrodes exposed to a phosphate-buffered-saline (PBS) solution. Platinized Pt and Ti were also studied for comparison. The capacitance value of the electrodes in PBS was obtained through quantitative analysis of the EIS spectra. The results reveal that the capacitance of the TiN coated electrodes with a rough surface is several hundreds times higher than that of a smooth Pt surface. Platinization of Ti can also increase the capacitance to the same extent as platina. EIS has been shown to be a powerful technique for characterization of stimulating/sensing electrodes. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical impedance spectroscopy; Stimulating/sensing electrodes; Interfacial capacitance; Effective surface area; TiN coating; Platinization
1. Introduction In pacemaker applications, stimulating/sensing electrodes transfer electric pulses from the pacemaker to the heart muscle to generate an artificial cardiac stimulation, and sense the cardiac response, which is then analyzed by the electronics in the pacemaker. To avoid irreversible reactions on the electrode surface, which may lead to degradation of the electrode or undesired reaction products, it is desirable that the stimulation pulses are transferred through interfacial charging / discharging processes. For stimulation, the electrode needs to exhibit capacitive characteristics with enough charging magnitude to overcome the so-called pacing threshold. For sensing, the electrode has to have low polarization, otherwise the cardiac signals might not be detected correctly. There is a dilemma of simultaneously
* Corresponding author. Tel.: /46-8-7906739; fax: /46-8-208284 E-mail address: [email protected] (J. Pan).
optimizing both pacing and sensing parameters. These parameters are to a large extent dependent on the material, surface structure of the electrode and electrochemical processes taking place at the electrode/electrolyte interface. Design of the electrode tip is very important for the stimulating/sensing behavior. Smooth cardiac electrodes were used in early pacing applications. In modern pacemakers, the electrodes are small so that the output impedance is high for the battery, and therefore the current drain is low and the battery life is long. In addition, rough-surfaced electrodes are used to increase the effective surface area. A larger effective surface area gives a higher capacitance, and also a lower polarization. Pt and Pt /Ir alloys have been the most common electrode materials because of their biocompatibility, electrical, mechanical and chemical properties. The use of TiN coating on the electrode has found increasing interest because of its metal-like conductivity, excellent mechanical and chemical properties, and the fact that it can be deposited with a high surface area.
1389-0344/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 9 - 0 3 4 4 ( 0 2 ) 0 0 0 1 3 - 8
68
A. Norlin et al. / Biomolecular Engineering 19 (2002) 67 /71
In pacing literature, the electrode/electrolyte interface is traditionally described as a capacitor connected in parallel with a resistor, and pure electrical elements are used for modelling the system [1 /3]. The capacitance is the so-called Helmholtz double-layer capacitance due to a build up of an electrochemical double-layer at the interface. The resistance is the polarization resistance (sometimes called Faraday impedance [2]). In reality, the capacitive behavior of most solid /liquid interfaces deviate from that of an ideal capacitor. This has to be taken into account when accurately describing and modelling the system. Moreover, since stimulation pulses are high frequency signals (about 3000 Hz), whereas cardiac signals are in relatively low frequency (about 100 Hz) region, [1] the characterization of electrochemical impedance behavior of the electrode/ electrolyte interface in the relevant frequency range is of vital importance for electrode performance. Electrochemical impedance spectroscopy (EIS), a sensitive and non-destructive technique, has been widely used in recent decades for characterization of various kinds of electrode/electrolyte interfaces. Resistive and capacitive characteristics of the interface can be obtained by analysis of the EIS spectra, usually based on a physical model and an equivalent electrical circuit describing the system. Non-ideal capacitive response can be taken into account by using a constant phase element (CPE). In this work, EIS was applied to investigate pure Pt and Ti, TiN coated electrodes (TiN/Ti, TiN/Pt/Ir), and platinized Pt and Ti, exposed to a phosphate-buffered-saline (PBS) solution. The capacitance value of the electrodes in PBS was obtained through fitting the EIS spectra to the most appropriate equivalent circuit. The results of TiN coated electrodes and porous platina are compared to that of a smooth Pt and Ti surfaces.
2.1.3. TiN/Ti and TiN/Pt/Ir The thin disc samples, commercially available, were supplied by Hereus. The rough, grain-like, crystalline TiN coatings were deposited by physical vapour deposition on Ti and Pt /Ir substrates. Analysis by Auger electron spectroscopy (AES) revealed the stoichiometry of the coating to be TiN0.8. Depth profiling performed by alternating AES analysis and 60 s argon ion sputtering etching cycles showed this stoichiometry to be constant through the 10 mm thick coating. The surface has a black appearance. Fig. 1 shows a SEM photograph of the rough surface.
2.1.4. Platinized Pt The Pt foil was cleaned with diluted HNO3, ethanol and acetone and placed in a test cell filled with 5 ml electrolyte 0.01 M H2PtCl6/0.01 M HCl. Platinization was performed with a constant current of 10 mA/cm2 for 30 min. The platinized surface was rinsed with deionized water.
2.1.5. Platinized Ti The wet polished (up to 4000# grit paper) Ti sample was etched in 15% oxalic acid for 60 min and in 96% sulphuric acid for 2 s, and then washed in deionized water [4]. The Ti surface was platinized with a constant current of 5 mA/cm2 for 100 minutes in a 0.0035 M H2PtCl6/0.01 M HCl solution. Thereafter it was rinsed for 6 h in stirred hot water (cooling to room temperature) and electrochemical cleaned by voltametric cycling at a scan rate of 1 V/s between O2 and H2 evolution.
2. Experimental 2.1. Materials 2.1.1. Pt A Pt (99.9%) foil of 0.025 mm thickness was purchased from Sigma Aldrich. Prior to the EIS measurement, the foil was pre-treated with diluted HNO3 and then cleaned with ethanol and acetone to remove any contamination on the surface. This bright Pt surface was used as the reference smooth surface. 2.1.2. Ti Disc samples of 1 mm thickness were made from commercially pure Ti (grade 2, Johnson Matthey). The surface was wet polished successively to 4000# grit paper, and then rinsed in deionized water and cleaned with ethanol and acetone before EIS tests.
Fig. 1. SEM photograph of the TiN coated electrode.
A. Norlin et al. / Biomolecular Engineering 19 (2002) 67 /71
69
2.2. Electrolyte A PBS solution was used as electrolyte for all EIS measurements. The solution contains similar ion concentrations as blood, see Table 1. 2.3. Electrochemical cell and instrument The EIS experiments were performed using a threeelectrode electrochemical cell, with a Pt mesh as counter electrode and an Ag/AgCl reference electrode. All EIS spectra were collected using an electrochemical interface (Solatron 1287) and a frequency response analyser (Solartron 1250), which are controlled by a computer with ZPlot software (Scribner Associates, Inc.). The measurements were performed at the open circuit potential over a frequency range from 1 /104 to 5 / 10 3 Hz. The perturbation amplitude was 10 mV. Measurements were performed at room temperature.
3. Results and discussion 3.1. EIS spectra and interpretation The EIS spectra of most samples exhibit one time constant impedance behavior, i.e., one capacitive response in parallel with a resistive response, typical for simple metal-electrolyte interfaces. In Fig. 2 examples of the spectra are shown in Bode format, with the impedance modulus and the phase angle plotted versus frequency. In the Bode plot, the impedance modulus at different frequencies can be directly read from the spectra, and the capacitive behavior is indicated by the increase in the impedance with decreasing frequency. In high frequency region, the impedance is independent of frequency; it is the resistance of the electrolyte between the sample and the reference electrode. At the low frequency limit, the impedance is attributed to the polarization resistance of the sample in the electrolyte. This type of spectra is usually interpreted by using a simple model consisting of the double-layer capacitance in parallel with the polarization resistance, in addition to the electrolyte resistance. The model can be represented by the equivalent circuit shown in Fig. 3, where Re is the resistance of the electrolyte; Rp is the polarization resistance of the sample in the electrolyte. CPE is the constant phase element; it is used instead of a capaci-
Table 1 Ion composition in PBS Na
K
H
Cl
PO43
0.17 M
0.02 M
0.05 M
0.15 M
0.03 M
Fig. 2. Bode plots for Pt (upper graph) and TiN/Pt /Ir (lower graph) in PBS solution.
Fig. 3. Equivalent circuit of the electrode/electrolyte systems, where Re is the resistance of the electrolyte, Rp is the polarisation resistance and CPE is the constant phase element of the interface.
tance to account for the non-ideal capacitive response. The CPE impedance is represented by: ZCPE
1 Q(iv)h
(1)
where i is the imaginary number, v the angular frequency, Q is a constant and h is a mathematic expression (0 0/h 0/1) [5]. In the case of ideal capacitor, h /1 and Q is the capacitance. The physical origin of the CPE is widely discussed in literature [6,7]. It has been well recognized that a CPE may arise from some fractal nature of the surface, i.e., surface roughness [8,9]. The simple model and equivalent circuit were found to be satisfactory for describing the electrode/electrolyte interfaces investigated in the work. Simulated spectra using fit parameters are also shown in Fig. 2 for
A. Norlin et al. / Biomolecular Engineering 19 (2002) 67 /71
70
comparison with the measured spectra, indicating a high quality of fitting. The good agreement between the measured and simulated spectra validates the choice of this simple physical model. 3.2. Capacitance and effective surface area The results of spectra fitting for all samples investigated are summarized in Table 2. The data are the mean values for three parallel measurements. For simplicity in later discussion, C (capacitance) is shown instead of Q of the CPE. The values of interfacial capacitance C and polarization resistance Rp in the table are referred to geometric area. Rs is the electrolyte resistance which is mainly determined by the solution and the cell set up. It varies only from a few tens to about one hundred ohms, and is not of interest in this study; therefore it is not included in the table. As electrochemical parameters, the interfacial capacitance and polarization resistance are directly related to the effective surface area. The double-layer can be assumed to act as a parallel-plate capacitor; its capacitance is given by: C oo 0
rA
(2)
d
where o is dielectric constant of the medium and o 0 the dielectric permittivity of vacuum, d is the distance between the charged layers, and r is the ratio of effective surface area Aeff to geometric area A . As can be seen, the interfacial capacitance is proportional to the effective surface area. Similar to an electric resistor, the polarization resistance is inversely proportional to the surface area if other parameters remain to be the same [1]. However, the polarization resistance is also strongly influence by surface condition, e.g., presence of a thin passive film. In common practice, the capacitance is used to evaluate the effective surface area. In Table 2, r was obtained by normalizing the capacitance to that of pure Pt. The real surface area of a rough surface is difficult to define. In practice, surface roughness parameters are
often used to evaluate a rough surface, and different methods such as BET and cyclic voltammetry are used to determine the surface area. However, the results may be quite different because of different principles in the methods. In this case, the interfacial capacitance is essential for this particular application, and the electrical effective surface area determined by EIS is the most relevant information. 3.3. Comparison between different electrodes It can be seen in Table 2, for pure Pt with a smooth surface in PBS, the capacitance is quite low and the polarization resistance is high. The h value (0.90) of the CPE is lower than 1, indicating some deviation from ideal capacitive response. For polished Ti, the initial capacitance may be high, however, a thin passive oxide film spontaneously forms on Ti may result in a decreased capacitance with time. Compared to the pure Pt, all rough-surfaced electrodes exhibit a significantly higher capacitance and a lower polarization resistance. The increase in the effective surface area are several hundreds times or even a thousand times for TiN coated electrodes. The TiN/Pt/Ir electrode exhibits a higher capacitance than the TiN/Ti electrode. This may be due to different thickness and/or different surface roughness of the TiN coating in the two cases. It is well known that platinization of Pt can increase the effective surface area remarkably. The purpose of this work was not to optimise the platinization process, nevertheless, the results in this work show that platinization of Ti can also increase the effective surface area to the same extent as platina. The increased effective surface area results in an increased interfacial capacitance as well as a low polarization, which is favourable for stimulating/sensing performance. Meanwhile, the rough-surfaced electrodes exhibit a decreased exponential factor (h value down to 0.74 /0.78) of CPE, which indicates more deviation from ideal capacitive response (h /1). This emphasizes the importance of using CPE instead of capacitance for modelling the electrode/electrolyte interface.
4. Conclusions Table 2 Fitting results from the EIS spectra Sample Pt Ti TiN/Pt /Ir TiN/Ti Platinized Pt Platinized Ti
C (F/cm2) 5
6.0 10 1.6 10 4 6.6 10 2 3.5 10 2 1.7 10 2 2.5 10 2
h 0.90 0.79 0.78 0.74 0.81 0.86
Rp (V cm2) 5
2.2 10 6.7 104 3.9 103 6.4 103 3.3 103 2.1 104
r (Aeff/A ) 1 / 1100 580 280 410
Based on the results from this investigation, it can be concluded that, EIS is a powerful technique for characterization of stimulating/sensing electrodes, as it reveals both capacitive and resistive parts of the impedance over a wide frequency range. It is appropriate to use CPE instead of capacitance in the model to describe the electrode/electrolyte interface. Deposition of a rough TiN coating on Ti and Pt /Ir surface lead to several hundreds times increase in the effective surface area. Platinization of Ti can also lead to an increase in
A. Norlin et al. / Biomolecular Engineering 19 (2002) 67 /71
the surface area similar to platina. The increased effective surface area results in an increased interfacial capacitance.
Acknowledgements The Swedish Research Council for Engineering Sciences and St. Jude Medical AB are acknowledged for the financial support to this research project.
71
References [1] Boltz A, Schaldach M, Hubmann M. Electrocardiography 1991;13(2):714. [2] de Voogt WG. Am J Cardiol 1999;83:187. [3] Ellenbogen KA, Key GN, Wilkoff BL, editors. Clinic cardiac pacing and defibrillation, 2nd ed.. W.B. Saunders, 2000:17 /52. [4] Iniesta J, Gonza´lez-Garcı´a J. J Mater Chem 1999;9:3141. [5] Song H-K, Hwang H-Y, Lee K-H, Dao LH. Electrochim Acta 2000;45:2241. [6] Zoltowski P. J Electroanal Chem 1998;443:149. [7] La´ng G, Heusler KE. J Electroanal Chem 1998;457:257. [8] Wang Y-B, Yuan R-K, Willander M. Appl Phys A 1996;63:481. [9] Koch DL, Sangani AS. J Electrochem Soc 1991;138:475.
Investigation of interfacial capacitance of Pt, Ti and TiN coated electrodes by electrochemical impedance spectroscopy A. Norlin a,b, J. Pan a,*, C. Leygraf a a
Department of Material Science and Engineering, Royal Institute of Technology, Drottning Kristinas Va¨g 51, SE-100 44 Stockholm, Sweden b St. Jude Medical AB, Veddestava¨gen 19, SE-175 84 Jarfalla, Sweden
Abstract Electrochemical processes at the electrode /electrolyte (body fluid) interface are of ultimate importance for stimulating/sensing electrode function. A high electrode surface area is desirable for safe stimulation through double-layer charging and discharging. Pt and Pt /Ir alloys have been the most common electrode materials. The use of TiN coating as the surface layer on the electrode has found increasing interest because of its metal-like conductivity, excellent mechanical and chemical properties, and the fact that it can be deposited with a high surface area. In this work, electrochemical impedance spectroscopy (EIS), which is a sensitive and nondestructive technique and widely used for characterization of electrical properties of electrode /electrolyte interfaces, was applied to investigate pure Pt and Ti, and TiN coated electrodes exposed to a phosphate-buffered-saline (PBS) solution. Platinized Pt and Ti were also studied for comparison. The capacitance value of the electrodes in PBS was obtained through quantitative analysis of the EIS spectra. The results reveal that the capacitance of the TiN coated electrodes with a rough surface is several hundreds times higher than that of a smooth Pt surface. Platinization of Ti can also increase the capacitance to the same extent as platina. EIS has been shown to be a powerful technique for characterization of stimulating/sensing electrodes. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical impedance spectroscopy; Stimulating/sensing electrodes; Interfacial capacitance; Effective surface area; TiN coating; Platinization
1. Introduction In pacemaker applications, stimulating/sensing electrodes transfer electric pulses from the pacemaker to the heart muscle to generate an artificial cardiac stimulation, and sense the cardiac response, which is then analyzed by the electronics in the pacemaker. To avoid irreversible reactions on the electrode surface, which may lead to degradation of the electrode or undesired reaction products, it is desirable that the stimulation pulses are transferred through interfacial charging / discharging processes. For stimulation, the electrode needs to exhibit capacitive characteristics with enough charging magnitude to overcome the so-called pacing threshold. For sensing, the electrode has to have low polarization, otherwise the cardiac signals might not be detected correctly. There is a dilemma of simultaneously
* Corresponding author. Tel.: /46-8-7906739; fax: /46-8-208284 E-mail address: [email protected] (J. Pan).
optimizing both pacing and sensing parameters. These parameters are to a large extent dependent on the material, surface structure of the electrode and electrochemical processes taking place at the electrode/electrolyte interface. Design of the electrode tip is very important for the stimulating/sensing behavior. Smooth cardiac electrodes were used in early pacing applications. In modern pacemakers, the electrodes are small so that the output impedance is high for the battery, and therefore the current drain is low and the battery life is long. In addition, rough-surfaced electrodes are used to increase the effective surface area. A larger effective surface area gives a higher capacitance, and also a lower polarization. Pt and Pt /Ir alloys have been the most common electrode materials because of their biocompatibility, electrical, mechanical and chemical properties. The use of TiN coating on the electrode has found increasing interest because of its metal-like conductivity, excellent mechanical and chemical properties, and the fact that it can be deposited with a high surface area.
1389-0344/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 9 - 0 3 4 4 ( 0 2 ) 0 0 0 1 3 - 8
68
A. Norlin et al. / Biomolecular Engineering 19 (2002) 67 /71
In pacing literature, the electrode/electrolyte interface is traditionally described as a capacitor connected in parallel with a resistor, and pure electrical elements are used for modelling the system [1 /3]. The capacitance is the so-called Helmholtz double-layer capacitance due to a build up of an electrochemical double-layer at the interface. The resistance is the polarization resistance (sometimes called Faraday impedance [2]). In reality, the capacitive behavior of most solid /liquid interfaces deviate from that of an ideal capacitor. This has to be taken into account when accurately describing and modelling the system. Moreover, since stimulation pulses are high frequency signals (about 3000 Hz), whereas cardiac signals are in relatively low frequency (about 100 Hz) region, [1] the characterization of electrochemical impedance behavior of the electrode/ electrolyte interface in the relevant frequency range is of vital importance for electrode performance. Electrochemical impedance spectroscopy (EIS), a sensitive and non-destructive technique, has been widely used in recent decades for characterization of various kinds of electrode/electrolyte interfaces. Resistive and capacitive characteristics of the interface can be obtained by analysis of the EIS spectra, usually based on a physical model and an equivalent electrical circuit describing the system. Non-ideal capacitive response can be taken into account by using a constant phase element (CPE). In this work, EIS was applied to investigate pure Pt and Ti, TiN coated electrodes (TiN/Ti, TiN/Pt/Ir), and platinized Pt and Ti, exposed to a phosphate-buffered-saline (PBS) solution. The capacitance value of the electrodes in PBS was obtained through fitting the EIS spectra to the most appropriate equivalent circuit. The results of TiN coated electrodes and porous platina are compared to that of a smooth Pt and Ti surfaces.
2.1.3. TiN/Ti and TiN/Pt/Ir The thin disc samples, commercially available, were supplied by Hereus. The rough, grain-like, crystalline TiN coatings were deposited by physical vapour deposition on Ti and Pt /Ir substrates. Analysis by Auger electron spectroscopy (AES) revealed the stoichiometry of the coating to be TiN0.8. Depth profiling performed by alternating AES analysis and 60 s argon ion sputtering etching cycles showed this stoichiometry to be constant through the 10 mm thick coating. The surface has a black appearance. Fig. 1 shows a SEM photograph of the rough surface.
2.1.4. Platinized Pt The Pt foil was cleaned with diluted HNO3, ethanol and acetone and placed in a test cell filled with 5 ml electrolyte 0.01 M H2PtCl6/0.01 M HCl. Platinization was performed with a constant current of 10 mA/cm2 for 30 min. The platinized surface was rinsed with deionized water.
2.1.5. Platinized Ti The wet polished (up to 4000# grit paper) Ti sample was etched in 15% oxalic acid for 60 min and in 96% sulphuric acid for 2 s, and then washed in deionized water [4]. The Ti surface was platinized with a constant current of 5 mA/cm2 for 100 minutes in a 0.0035 M H2PtCl6/0.01 M HCl solution. Thereafter it was rinsed for 6 h in stirred hot water (cooling to room temperature) and electrochemical cleaned by voltametric cycling at a scan rate of 1 V/s between O2 and H2 evolution.
2. Experimental 2.1. Materials 2.1.1. Pt A Pt (99.9%) foil of 0.025 mm thickness was purchased from Sigma Aldrich. Prior to the EIS measurement, the foil was pre-treated with diluted HNO3 and then cleaned with ethanol and acetone to remove any contamination on the surface. This bright Pt surface was used as the reference smooth surface. 2.1.2. Ti Disc samples of 1 mm thickness were made from commercially pure Ti (grade 2, Johnson Matthey). The surface was wet polished successively to 4000# grit paper, and then rinsed in deionized water and cleaned with ethanol and acetone before EIS tests.
Fig. 1. SEM photograph of the TiN coated electrode.
A. Norlin et al. / Biomolecular Engineering 19 (2002) 67 /71
69
2.2. Electrolyte A PBS solution was used as electrolyte for all EIS measurements. The solution contains similar ion concentrations as blood, see Table 1. 2.3. Electrochemical cell and instrument The EIS experiments were performed using a threeelectrode electrochemical cell, with a Pt mesh as counter electrode and an Ag/AgCl reference electrode. All EIS spectra were collected using an electrochemical interface (Solatron 1287) and a frequency response analyser (Solartron 1250), which are controlled by a computer with ZPlot software (Scribner Associates, Inc.). The measurements were performed at the open circuit potential over a frequency range from 1 /104 to 5 / 10 3 Hz. The perturbation amplitude was 10 mV. Measurements were performed at room temperature.
3. Results and discussion 3.1. EIS spectra and interpretation The EIS spectra of most samples exhibit one time constant impedance behavior, i.e., one capacitive response in parallel with a resistive response, typical for simple metal-electrolyte interfaces. In Fig. 2 examples of the spectra are shown in Bode format, with the impedance modulus and the phase angle plotted versus frequency. In the Bode plot, the impedance modulus at different frequencies can be directly read from the spectra, and the capacitive behavior is indicated by the increase in the impedance with decreasing frequency. In high frequency region, the impedance is independent of frequency; it is the resistance of the electrolyte between the sample and the reference electrode. At the low frequency limit, the impedance is attributed to the polarization resistance of the sample in the electrolyte. This type of spectra is usually interpreted by using a simple model consisting of the double-layer capacitance in parallel with the polarization resistance, in addition to the electrolyte resistance. The model can be represented by the equivalent circuit shown in Fig. 3, where Re is the resistance of the electrolyte; Rp is the polarization resistance of the sample in the electrolyte. CPE is the constant phase element; it is used instead of a capaci-
Table 1 Ion composition in PBS Na
K
H
Cl
PO43
0.17 M
0.02 M
0.05 M
0.15 M
0.03 M
Fig. 2. Bode plots for Pt (upper graph) and TiN/Pt /Ir (lower graph) in PBS solution.
Fig. 3. Equivalent circuit of the electrode/electrolyte systems, where Re is the resistance of the electrolyte, Rp is the polarisation resistance and CPE is the constant phase element of the interface.
tance to account for the non-ideal capacitive response. The CPE impedance is represented by: ZCPE
1 Q(iv)h
(1)
where i is the imaginary number, v the angular frequency, Q is a constant and h is a mathematic expression (0 0/h 0/1) [5]. In the case of ideal capacitor, h /1 and Q is the capacitance. The physical origin of the CPE is widely discussed in literature [6,7]. It has been well recognized that a CPE may arise from some fractal nature of the surface, i.e., surface roughness [8,9]. The simple model and equivalent circuit were found to be satisfactory for describing the electrode/electrolyte interfaces investigated in the work. Simulated spectra using fit parameters are also shown in Fig. 2 for
A. Norlin et al. / Biomolecular Engineering 19 (2002) 67 /71
70
comparison with the measured spectra, indicating a high quality of fitting. The good agreement between the measured and simulated spectra validates the choice of this simple physical model. 3.2. Capacitance and effective surface area The results of spectra fitting for all samples investigated are summarized in Table 2. The data are the mean values for three parallel measurements. For simplicity in later discussion, C (capacitance) is shown instead of Q of the CPE. The values of interfacial capacitance C and polarization resistance Rp in the table are referred to geometric area. Rs is the electrolyte resistance which is mainly determined by the solution and the cell set up. It varies only from a few tens to about one hundred ohms, and is not of interest in this study; therefore it is not included in the table. As electrochemical parameters, the interfacial capacitance and polarization resistance are directly related to the effective surface area. The double-layer can be assumed to act as a parallel-plate capacitor; its capacitance is given by: C oo 0
rA
(2)
d
where o is dielectric constant of the medium and o 0 the dielectric permittivity of vacuum, d is the distance between the charged layers, and r is the ratio of effective surface area Aeff to geometric area A . As can be seen, the interfacial capacitance is proportional to the effective surface area. Similar to an electric resistor, the polarization resistance is inversely proportional to the surface area if other parameters remain to be the same [1]. However, the polarization resistance is also strongly influence by surface condition, e.g., presence of a thin passive film. In common practice, the capacitance is used to evaluate the effective surface area. In Table 2, r was obtained by normalizing the capacitance to that of pure Pt. The real surface area of a rough surface is difficult to define. In practice, surface roughness parameters are
often used to evaluate a rough surface, and different methods such as BET and cyclic voltammetry are used to determine the surface area. However, the results may be quite different because of different principles in the methods. In this case, the interfacial capacitance is essential for this particular application, and the electrical effective surface area determined by EIS is the most relevant information. 3.3. Comparison between different electrodes It can be seen in Table 2, for pure Pt with a smooth surface in PBS, the capacitance is quite low and the polarization resistance is high. The h value (0.90) of the CPE is lower than 1, indicating some deviation from ideal capacitive response. For polished Ti, the initial capacitance may be high, however, a thin passive oxide film spontaneously forms on Ti may result in a decreased capacitance with time. Compared to the pure Pt, all rough-surfaced electrodes exhibit a significantly higher capacitance and a lower polarization resistance. The increase in the effective surface area are several hundreds times or even a thousand times for TiN coated electrodes. The TiN/Pt/Ir electrode exhibits a higher capacitance than the TiN/Ti electrode. This may be due to different thickness and/or different surface roughness of the TiN coating in the two cases. It is well known that platinization of Pt can increase the effective surface area remarkably. The purpose of this work was not to optimise the platinization process, nevertheless, the results in this work show that platinization of Ti can also increase the effective surface area to the same extent as platina. The increased effective surface area results in an increased interfacial capacitance as well as a low polarization, which is favourable for stimulating/sensing performance. Meanwhile, the rough-surfaced electrodes exhibit a decreased exponential factor (h value down to 0.74 /0.78) of CPE, which indicates more deviation from ideal capacitive response (h /1). This emphasizes the importance of using CPE instead of capacitance for modelling the electrode/electrolyte interface.
4. Conclusions Table 2 Fitting results from the EIS spectra Sample Pt Ti TiN/Pt /Ir TiN/Ti Platinized Pt Platinized Ti
C (F/cm2) 5
6.0 10 1.6 10 4 6.6 10 2 3.5 10 2 1.7 10 2 2.5 10 2
h 0.90 0.79 0.78 0.74 0.81 0.86
Rp (V cm2) 5
2.2 10 6.7 104 3.9 103 6.4 103 3.3 103 2.1 104
r (Aeff/A ) 1 / 1100 580 280 410
Based on the results from this investigation, it can be concluded that, EIS is a powerful technique for characterization of stimulating/sensing electrodes, as it reveals both capacitive and resistive parts of the impedance over a wide frequency range. It is appropriate to use CPE instead of capacitance in the model to describe the electrode/electrolyte interface. Deposition of a rough TiN coating on Ti and Pt /Ir surface lead to several hundreds times increase in the effective surface area. Platinization of Ti can also lead to an increase in
A. Norlin et al. / Biomolecular Engineering 19 (2002) 67 /71
the surface area similar to platina. The increased effective surface area results in an increased interfacial capacitance.
Acknowledgements The Swedish Research Council for Engineering Sciences and St. Jude Medical AB are acknowledged for the financial support to this research project.
71
References [1] Boltz A, Schaldach M, Hubmann M. Electrocardiography 1991;13(2):714. [2] de Voogt WG. Am J Cardiol 1999;83:187. [3] Ellenbogen KA, Key GN, Wilkoff BL, editors. Clinic cardiac pacing and defibrillation, 2nd ed.. W.B. Saunders, 2000:17 /52. [4] Iniesta J, Gonza´lez-Garcı´a J. J Mater Chem 1999;9:3141. [5] Song H-K, Hwang H-Y, Lee K-H, Dao LH. Electrochim Acta 2000;45:2241. [6] Zoltowski P. J Electroanal Chem 1998;443:149. [7] La´ng G, Heusler KE. J Electroanal Chem 1998;457:257. [8] Wang Y-B, Yuan R-K, Willander M. Appl Phys A 1996;63:481. [9] Koch DL, Sangani AS. J Electrochem Soc 1991;138:475.