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Amorphous and crystalline IrO2 thin films as potential stimulation electrode coatings
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Applied Surface Science 254 (2008) 5164–5169 www.elsevier.com/locate/apsusc
Amorphous and crystalline IrO2 thin films as potential stimulation electrode coatings Sachin S. Thanawala a,d,*, Ronald J. Baird b, Daniel G. Georgiev b,c, Gregory W. Auner a,b a Department of Biomedical Engineering, Wayne State University, Detroit, MI 48202, United States Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI 48202, United States c Department of Electrical Engineering and Computer Science, University of Toledo, Toledo, OH 43606, United States d Greatbatch, Inc., Medical Solutions, Clarence, NY 14031, United States b
Received 19 October 2007; received in revised form 14 January 2008; accepted 6 February 2008 Available online 10 March 2008
Abstract Amorphous and crystalline iridium oxide thin films with potential use as coating materials for stimulation electrodes were studied. Characterization of these films by cyclic voltammetry and impedance spectroscopy has revealed a considerable decrease in impedance and an increase in charge capacity of iridium oxide thin films after an electrochemical activation process in 0.9% NaCl solution. The surface morphology of these films was studied by scanning electron microscopy. The two types of IrO2 films were also compared under conditions relevant to applications as stimulation electrodes. The results indicate that amorphous IrO2 films have significantly higher charge storage capacity and lower impedance than crystalline IrO2 films. This makes the amorphous films a preferable coating material for stimulation applications. # 2008 Elsevier B.V. All rights reserved. Keywords: Iridium oxide; Stimulation electrodes; Coatings; Impedance spectroscopy; Scanning electron microscopy
1. Introduction Medical implant devices that employ electrodes for electrical stimulation of cardiac or neural cells have been in use for several decades. The earliest example of such an implant is the heart pacemaker which was first successfully implanted in 1958 [1]. Among other devices that have been recently developed for treatment of diseases and restoration of lost function are retina stimulators [2–4] for treatment of visual impairment and cochlear implants [5] for treatment of hearing impairment. A wide variety of materials have been tested as electrode materials and electrode coatings for stimulation applications including platinum [6] and its alloys [7], titanium nitride [8], fractal iridium [9] and iridium oxide [10]. Recently, iridium and its oxides have shown the most promising results as functional coatings for stimulating electrodes [1,10,11]. Studies have shown that electrodes coated
* Corresponding author at: Greatbatch, Inc. 10,000 Wehrle Dr. Clarence, NY 14031, USA. Tel.: +716 759 5340; fax: +716 759 5430. E-mail address: [email protected] (S.S. Thanawala). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.054
with electrochemically activated iridium oxide can inject between 4 and 10 times the charge of platinum and its alloys with iridium [11]. Iridium oxide belongs to a new class of electrode material called ‘‘valence change oxides’’, i.e. this material injects charge using a reversible charge injection reaction which is governed by a valence-change within the layer of the material. Iridium oxide thin films have also found considerable research interest for applications such as oxygen diffusion barrier materials in random access memory devices [12,13]. Iridium oxide (IrO2) layers can be formed by anodic electrochemical activation of iridium metal (AIROF) [10,11,14,15], electrodeposition (EIROF) [16,17] onto a bulk material, thermal decomposition of an iridium salt (TIROF) [18], and by reactive sputtering using an iridium target (SIROF) [1,19,20]. Although activated or anodic IrO2 films are widely used as electrode coatings for stimulation applications [10,11,15], these films have a problem with long term stability [21,22]. The charge delivery capacity of these films increases with the increase in the number of activation cycles (due to increase in volume of oxide), and then after reaching a maximum it decreases sharply [19,23,24]. This decrease in the charge
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delivery capacity can be attributed to irreversible decrease of Ir3+/Ir4+ conversion kinetics (main valence change redox process) a process referred to as irreversible film aging [25]. Hence, sputtered IrO2 films (SIROF) have gained importance as an alternative material to AIROF for functional stimulation applications [1,19,26]. The electrochemical activity of SIROF is very similar to that of AIROF. It is remarkably reversible in nature, and it also increases during potential cycling [19,27]. The main advantage of SIROF as compared to AIROF is that it is more mechanically stable and corrosion resistant [19,21,22], which makes it an ideal candidate as electrode coating material. In this study amorphous IrO2 films (deposited at room temperature) and crystalline IrO2 films were made by reactively sputtering an Ir target using a pulsed-DC magnetron sputtering system. Electrochemical impedance spectroscopy and cyclic voltammetry techniques were used to study the electrochemical properties of these SIROF thin films. 2. Experimental A pulsed-DC sputtering system was used for deposition of iridium oxide thin films onto borosilicate glass substrates. Based on our previous study on the deposition parameters of the pulsed-DC reactive sputtering process [28], a pulsing frequency of 100 kHz and a reverse bias duration of 2 ms were chosen to deposit the IrO2 thin films in this study. The Ir sputtering target had a diameter of 2 in. (purity 99.9%). A thin titanium adhesion layer (10 nm) was first deposited in the same sputtering chamber. Iridium oxide films were reactively sputtered in Ar/O2 plasma. The O2/(O2 + Ar) mixing ratio (OMR) for the deposition was maintained at 17% to ensure that the Ir target is not oxidized. Prior to deposition the chamber was evacuated to a base pressure of 105 Pa by a mechanical pump first and then by a cryogenic pump. The sputtering power was fixed at 150 W. The sputtering pressure was maintained at 0.66 Pa. Based on the results from our previous study [28] the substrate temperature for deposition of crystalline IrO2 thin films was maintained at 400 8C, and the amorphous films were deposited at room temperature. The IrO2 film thickness was measured by profilometry and found to be about 100 nm. The crystallinity of films deposited at a substrate temperature of 400 8C, was measured by X-ray diffraction (XRD) (theta-2theta scans) in a Rigaku-Rotaflex RU2000 diffractometer using Cu Ka radiation and the stoichiometry of the films was confirmed using Rutherford backscattering measurements [28]. A Renishaw InVia micro-Raman system was used to perform Raman measurements on the amorphous and crystalline IrO2 thin films in a back scattering geometry at room temperature. The spectra were excited by 514.5 nm Ar-ion laser. The laser power at the sample surface was about 1 mW which is below the levels at which any laser damage could be observed. The laser radiation was focused to a spot size of about 3 mm in diameter on the sample surface. In order to reproduce the physiological environment, all electrochemical measurements were performed in physiological saline solution (unbuffered aqueous 0.9% NaCl). All measurements were performed at room temperature with the
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geometric area (as opposed to the electrochemically active surface area) of the test sample being 0.1 cm2. A standard threeelectrode glass cell with a silver–silver chloride (Ag/AgCl) reference electrode (Bioanalytical Systems) was used for the measurements. The counter electrode was a high area Pt foil. All potentials are reported versus the Ag/AgCl electrode. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the thin film surfaces. The cyclic voltammetry and impedance spectroscopy measurements were carried out using a Gamry potentiostat system (model PCI4). The SIROF’s were subjected to a repetitive potential cycling at scan rate of 100 mV/s for 500 cycles in the potential range 1.0 to 1.0 V, a process called electrochemical activation [19]. The cyclic voltammograms were recorded before and after the electrochemical activation within the water window for IrO2, i.e. 0.6 and +0.8 V at a scan rate of 100 mV/ s. The anodic and cathodic charge storage capacities (CSC) of the test samples were calculated by integrating the respective cyclic voltammograms. The ac impedance spectra were measured in the frequency range of 0.1 Hz–100 kHz using a sinusoidal perturbation of 10 mV rms. A high resolution field emission scanning electron microscope (Hitachi S4800) was used to study the morphology of the film surfaces before and after the electrochemical activation procedure. 3. Results and discussion Fig. 1(a) shows the XRD pattern for the IrO2 thin film reactively sputtered onto BSG substrate at a substrate temperature of 400 8C. As seen from the figure the IrO2 films have a well defined poly-crystalline structure at the substrate temperature of 400 8C, dominated by (2 0 0) reflection, which is consistent with the results observed by other groups [20] and results obtained from our previous study [28]. For the room temperature reactive sputtering amorphous IrO2 is expected to form and so we did not observe any sharp peaks in the XRD pattern (not shown here). Fig. 1(b) shows the Raman spectra of the IrO2 films reactively sputtered onto BSG substrates at room temperature and at a substrate temperature of 400 8C respectively. The major Raman modes Eg, B2g and A1g are located at 561, 728 and 752 cm1 respectively [28]. The B2g and A1g modes at 728 and 752 cm1 are not resolved and form a broad peak dominated by A1g. The spectrum of the roomtemperature-deposited films shows noticeably wider peaks due to poor crystallinity or, rather, the lack of it. We can also observe a weaker mode at 365 cm1 in the amorphous film spectrum. This mode was observed in our previous experiments and we have attributed it to vibrations of molecular units in localized non-stoichiometric phases [28]. Fig. 2(a) shows the Bode plots of the electrochemical impedance spectra of the amorphous and crystalline IrO2 thin films before and after the electrochemical activation procedure respectively. As seen from the figure, the IrO2 thin films show a typical capacitive behavior in the low frequency range. It can also be seen from Fig. 1(a) that the magnitude of the impedance of the unactivated amorphous IrO2 film is considerably lower than that of the unactivated crystalline IrO2 film. The cut-off
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Fig. 1. A XRD pattern of IrO2 thin film reactively sputtered onto BSG substrate at a substrate temperature of 400 8C (a), Raman spectra of IrO2 thin films reactively sputtered onto BSG substrates at room temperature and a substrate temperature of 400 8C (b). (The spectra are normalized to the maximum intensity in that particular spectrum).
frequency for capacitive behavior of crystalline IrO2 film is at 500 Hz, whereas that of amorphous IrO2 film is at 100 Hz. This reflects an increase in the measured capacitance of the amorphous IrO2 film. Table 1 gives the values of the film capacitances calculated from the impedance data. It is interesting to note from Fig. 2(a) that the impedance of the crystalline IrO2 thin film exhibits consistently higher impedance compared to the amorphous IrO2 film, even in the high frequency range and is about 60–70 V as opposed to 40–50 V for the amorphous IrO2 thin film. Fig. 2(b) shows the impedance phase for these respective samples. The phase is also frequency dependant closing towards 0 at higher frequencies and near 90 for lower frequencies indicating a high-pass capacitive behavior of the films. Owing to their amorphous and highly porous nature, amorphous IrO2 films act like a random network of channels for easy access of ionic species into the bulk material, providing
Fig. 2. Bode plots of different IrO2 thin films, in 0.9% NaCl solution (a) shows the magnitude and (b) shows the phase of the complex impedance as a function of frequency for various IrO2 films. The solid lines are before activation and the dashed lines are after activation.
temporary bonding sites for these ionic species [22]. Amorphous IrO2 thin films have previously been reported to form gel-like materials when they come in contact with aqueous electrolytes (such as saline or body fluids) [19]. These gel-like materials contain cross-linked chains of oxygenbridged hydroxyl–aqua–iridium complexes which can be easily penetrated by water and ions such as H+, OH and Na+ [19]. The increase in water uptake within the film results in increase in thickness of the oxide layer. On the other hand, crystalline IrO2 thin films possess a smooth surface with reduced porosity and well-defined rutile tetragonal lattice structure [20]. The lower porosity and the high degree of crystallinity of the films has the effect of reducing ionic conductivity of the films, making it difficult for water and ionic species to penetrate within the bulk material [22]. In other words, crystalline films rarely form a gel-like material when in contact with aqueous electrolytes. Crystalline SIROF is reported to have similar electronic conductivity as compared to its amorphous counterpart [22]. Since the electrical conductivities are similar, it is clear that structure plays a very important role in influencing the ionic conductivity of these films.
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Table 1 Values of interfacial capacitance and a from the fitting of the impedance data for different samples; also anodic and cathodic charge storage capacities calculated by integrating the cyclic voltammograms Sample
Cf (mF/cm2)
Unactivated amorphous IrO2 thin film Electrochemically activated amorphous IrO2 thin film Unactivated crystalline IrO2 thin film Electrochemically activated crystalline IrO2 thin film
0.37 2.61 0.034 0.27
a 0.77 0.86 0.79 0.82
CSCa (mC/cm2)
CSCc (mC/cm2)
3.74 18.9 0.6 1.8
4.14 14.14 0.27 1.6
Interfacial capacitance (Cf), Anodic charge storage capacity (CSCa), cathodic charge storage capacity (CSCc).
Fig. 2(a) also shows the electrochemical impedance spectra of the electrochemically activated samples. Here we can see that the impedance of the films has been reduced considerably after the electrochemical activation procedure. Similar trends have been observed by other groups studying the electrochemical properties of sputtered amorphous IrO2 films [19]. It is important to note the consistency of lower impedance of the amorphous IrO2 film as compared to the impedance of crystalline IrO2 film even after the electrochemical activation process. The cut-off frequency of samples coated with activated amorphous IrO2 films for capacitive behavior has shifted to a lower frequency value of 10 Hz and for the activated crystalline IrO2 film it has shifted to about 100 Hz indicating a rise in the measured interfacial capacitance which can be verified from Table 1. The impedance of samples coated with activated crystalline and amorphous IrO2 films in the high frequency range is about 55–45 V and 30–40 V respectively. A similar trend is observed in the impedance phase as is evident from Fig. 2(b). This type of impedance spectrum is usually modeled by a double layer capacitance in parallel with polarization resistance in series with the electrolyte resistance. The equivalent circuit for such a model is shown in Fig. 3, where Rs represents the electrolyte resistance, Rp is the polarization of the sample and we have used the constant phase element (CPE) instead of a double layer capacitance to represent the non-ideal capacitive behavior of the impedance. The CPE impedance is represented as: Z CPE ¼
1 QðivÞa
(1)
where i is the imaginary number, v is the angular frequency, Q is a constant and a is a parameter that has a value between 0 and 1 [29]. In case of an ideal capacitance a = 1, and Q is the capacitance. Recently, the CPE behavior has been widely studied and discussed in literature [29–31]. It has been argued that CPE behavior is a result of surface roughness or any other structure of the surface (porosity, fractal nature) [32]. This
Fig. 3. Equivalent circuit of the electrode/electrolyte system.
equivalent circuit and model were found to be satisfactory in representing the electrode–electrolyte interfaces studied in this work. An important observation that can be made from the impedance spectra in Fig. 2(a) is that the reduction in impedance (in the lower frequency range) of the crystalline IrO2 film (6 times) after activation is almost two times larger than the reduction in the impedance of the amorphous IrO2 film (3 times) after activation. This could be attributed to the fact that the electrochemical activation procedure leads to a decrease in the crystallinity of the IrO2 films as revealed by the SEM images in Fig. 4 (a) and (b). Fig. 4 (a) is a high resolution field emission SEM image showing the morphology of the crystalline IrO2 film prior to the electrochemical activation procedure. Fig. 4(b) shows the morphology of the same crystalline IrO2 film after the electrochemical activation procedure (500 cycles of activation). Both the FE-SEM images are taken at the same instrument magnification of 100 kX. A comparison of both images suggests that the crystal size has reduced after the film was activated for 500 cycles. This raises the important question whether increasing the activation cycles (more than 500 cycles) for crystalline IrO2 film will further decrease the impedance of the film due to further grain size reduction and possibly, partial amorphization. Studies have shown that after 500 cycles of activation the amorphous films, there is saturation in the reduction of the impedance and an increase in charge storage capacity (CSC) [19]. Although further, more detailed studies of the electrochemical behavior are needed, such processes do not appear to take place in case of crystalline IrO2 films. Table 1 presents the values of the interfacial capacitance calculated by fitting the impedance spectra for different samples under study. As seen from the table the capacitance of amorphous IrO2 films is 0.37 mF/cm2 before activation which is significantly higher than the previously reported values for AIROF [8]. The value increases to 2.61 mF/cm2 after activation which is comparable to previously reported values for activated amorphous SIROF [19]. The increase in the value of interfacial capacitance shows that the activation process leads to an increase in the active surface area of the electrode material which is highly desirable in a stimulation electrode application. It can also be noted from the table that after the activation process the amorphous IrO2 film shows an increase in the value of a (0.86) which represents a more ideal capacitive behavior (a = 1). This highlights the significance of using CPE instead of capacitance for modeling the interfaces. The a value for the crystalline IrO2 film has also increased to 0.82 suggesting a
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Fig. 4. High resolution FE-SEM images of crystalline IrO2 thin films before electrochemical activation (a), and after electrochemical activation (b); taken at a magnification of 100 k.
more ideal capacitive behavior of these films. The increase in the value of a can be attributed to the increase in surface roughness and porosity of the films. Fig. 5 shows cyclic voltammograms (CV) of amorphous and crystalline IrO2 thin films before and after the electrochemical activation process. It can be clearly seen from the figure that the activation process has lead to an increase in charge storage capacities of the films (increase in the area under the curves). As seen from the equivalence of the area under the anodic and cathodic scans (for amorphous IrO2 thin film), the valence change of iridium oxide is easily reversed. In physiological solutions like 0.9% NaCl, the Ir3+/Ir4+ redox peaks are broad as compared to those observed in non-physiologic, high-buffer concentration solutions like H2SO4 [11,14,15]. A comparison of the CV curves for amorphous and crystalline IrO2 thin films shows that the charge storage capacity of crystalline IrO2 thin films is significantly less than that of amorphous IrO2 thin films (before and after the activation process). A closer look at the CV curves of crystalline IrO2 thin films indicates the absence of a current peak at 0.5 V which is clearly observed in the CV curves for amorphous IrO2 thin films; this suggests that the crystalline films do not undergo the valence change reaction in
Fig. 5. Cyclic voltammograms of amorphous and crystalline iridium oxide thin films before and after electrochemical activation process taken in 0.9% NaCl solution at a scan rate of 100 mV/s.
the whole volume of the film. This can be attributed to the poor ionic conductivities of the crystalline IrO2 thin films. Table 1 also presents results from the cyclic voltammetry experiments. It can be clearly seen that the activation process has increased the charge storage capacity of the films. The charge storage capacities calculated from the time integral along anodic (CSCa) and cathodic (CSCc) current directions for the amorphous IrO2 films are 3.74 and 4.14 mC/cm2 respectively before activation and 18.9 and 14.14 mC/cm2 respectively after the activation process. This increase in the charge storage capacity indicates that the activation process has lead to an increase in the volume of the oxide layer by water uptake, which eventually increases the roughness and porosity [19]. The charge storage capacity values for crystalline films are also presented in Table 1. As seen from the Table 1 the amorphous IrO2 films have a significantly higher charge storage capacity before and after activation than the crystalline IrO2 films making them preferable electrode coating materials in stimulation applications. Comparing the charge storage capacities of crystalline IrO2 films before and after the activation process shows a significant increase in the values of CSC leading to the conclusion that the activation process has considerably increased the ionic conductivity of the crystalline films. Hence we can speculate that further increase in the number of activation cycles will result in a further increase in ionic conductivity of these films, resulting in the formation of more porous and less crystalline structure. Two major requirements for a good stimulating electrode coating are high charge storage capacity and low impedance in the frequency range of interest for stimulation (10 Hz–1 kHz) [19]. Amorphous SIROF appear to satisfy both of these requirements. The charge storage capacities of these films have increased to about 18.9 mC/cm2 (anodic) and 14.14 mC/cm2 (cathodic) after the activation process. These films show a constant low impedance of about 40–50 V in the frequency range of 100 Hz–100 kHz and after activation their impedance drops to about 30–40 V extending from 10 Hz to 100 kHz. The CV and EIS results provide the necessary information needed for selection of a good stimulating electrode coating material. However, the information obtained from the EIS measurements gives a better representation about the material under study as it provides both high and low frequency response
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as compared to the CV results which only provides the low frequency response [8]. The EIS data also provides an estimate of the interfacial capacitance which is a measure of polarizability of the electrode material. 4. Conclusions Results from impedance spectroscopy and cyclic voltammetry measurements on amorphous and crystalline SIROF have been compared and suggest that amorphous IrO2 films are preferable materials for stimulation electrode coatings. It is concluded that the thin film material structure influences the ionic conductivities of the films. Amorphous SIROF owe their good ionic conductivities due to their amorphous phase. In order to accurately fit the electrochemical impedance spectra it is advisable to use CPE instead of simple capacitance for modeling the electrode/electrolyte interfaces. The electrochemical activation procedure significantly increases the charge storage capacities and at the same time considerably reduces the impedance of both amorphous and crystalline IrO2 thin films. The reduction in impedance and the increase in charge storage capacity of the crystalline IrO2 films upon activation, suggests that the activation procedure causes an increase in the ionic conductivity of the crystalline films. Based on the SEM results it can be speculated that the activation process leads to an increase in porosity of the crystalline films eventually making them lose their crystalline structure. A further detailed study is required in order to investigate the effect of a large number of activation cycles on the structure of crystalline IrO2 thin films. Acknowledgements This work was supported in part by the Michigan Life Science Corridor (MLSC) grant GR-358, the National Science Foundation-Integrative Graduate Education and Research Traineeship Program (NSF-IGERT) grant DGE-9870720 and funds from the DTE Energy Foundation. The authors would like to acknowledge Hitachi for the high resolution field emission SEM images. References [1] B. Wessling, W. Mokwa, U. Schnakenberg, J. Micromech. Microeng. 16 (2006) 142.
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Applied Surface Science 254 (2008) 5164–5169 www.elsevier.com/locate/apsusc
Amorphous and crystalline IrO2 thin films as potential stimulation electrode coatings Sachin S. Thanawala a,d,*, Ronald J. Baird b, Daniel G. Georgiev b,c, Gregory W. Auner a,b a Department of Biomedical Engineering, Wayne State University, Detroit, MI 48202, United States Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI 48202, United States c Department of Electrical Engineering and Computer Science, University of Toledo, Toledo, OH 43606, United States d Greatbatch, Inc., Medical Solutions, Clarence, NY 14031, United States b
Received 19 October 2007; received in revised form 14 January 2008; accepted 6 February 2008 Available online 10 March 2008
Abstract Amorphous and crystalline iridium oxide thin films with potential use as coating materials for stimulation electrodes were studied. Characterization of these films by cyclic voltammetry and impedance spectroscopy has revealed a considerable decrease in impedance and an increase in charge capacity of iridium oxide thin films after an electrochemical activation process in 0.9% NaCl solution. The surface morphology of these films was studied by scanning electron microscopy. The two types of IrO2 films were also compared under conditions relevant to applications as stimulation electrodes. The results indicate that amorphous IrO2 films have significantly higher charge storage capacity and lower impedance than crystalline IrO2 films. This makes the amorphous films a preferable coating material for stimulation applications. # 2008 Elsevier B.V. All rights reserved. Keywords: Iridium oxide; Stimulation electrodes; Coatings; Impedance spectroscopy; Scanning electron microscopy
1. Introduction Medical implant devices that employ electrodes for electrical stimulation of cardiac or neural cells have been in use for several decades. The earliest example of such an implant is the heart pacemaker which was first successfully implanted in 1958 [1]. Among other devices that have been recently developed for treatment of diseases and restoration of lost function are retina stimulators [2–4] for treatment of visual impairment and cochlear implants [5] for treatment of hearing impairment. A wide variety of materials have been tested as electrode materials and electrode coatings for stimulation applications including platinum [6] and its alloys [7], titanium nitride [8], fractal iridium [9] and iridium oxide [10]. Recently, iridium and its oxides have shown the most promising results as functional coatings for stimulating electrodes [1,10,11]. Studies have shown that electrodes coated
* Corresponding author at: Greatbatch, Inc. 10,000 Wehrle Dr. Clarence, NY 14031, USA. Tel.: +716 759 5340; fax: +716 759 5430. E-mail address: [email protected] (S.S. Thanawala). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.054
with electrochemically activated iridium oxide can inject between 4 and 10 times the charge of platinum and its alloys with iridium [11]. Iridium oxide belongs to a new class of electrode material called ‘‘valence change oxides’’, i.e. this material injects charge using a reversible charge injection reaction which is governed by a valence-change within the layer of the material. Iridium oxide thin films have also found considerable research interest for applications such as oxygen diffusion barrier materials in random access memory devices [12,13]. Iridium oxide (IrO2) layers can be formed by anodic electrochemical activation of iridium metal (AIROF) [10,11,14,15], electrodeposition (EIROF) [16,17] onto a bulk material, thermal decomposition of an iridium salt (TIROF) [18], and by reactive sputtering using an iridium target (SIROF) [1,19,20]. Although activated or anodic IrO2 films are widely used as electrode coatings for stimulation applications [10,11,15], these films have a problem with long term stability [21,22]. The charge delivery capacity of these films increases with the increase in the number of activation cycles (due to increase in volume of oxide), and then after reaching a maximum it decreases sharply [19,23,24]. This decrease in the charge
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delivery capacity can be attributed to irreversible decrease of Ir3+/Ir4+ conversion kinetics (main valence change redox process) a process referred to as irreversible film aging [25]. Hence, sputtered IrO2 films (SIROF) have gained importance as an alternative material to AIROF for functional stimulation applications [1,19,26]. The electrochemical activity of SIROF is very similar to that of AIROF. It is remarkably reversible in nature, and it also increases during potential cycling [19,27]. The main advantage of SIROF as compared to AIROF is that it is more mechanically stable and corrosion resistant [19,21,22], which makes it an ideal candidate as electrode coating material. In this study amorphous IrO2 films (deposited at room temperature) and crystalline IrO2 films were made by reactively sputtering an Ir target using a pulsed-DC magnetron sputtering system. Electrochemical impedance spectroscopy and cyclic voltammetry techniques were used to study the electrochemical properties of these SIROF thin films. 2. Experimental A pulsed-DC sputtering system was used for deposition of iridium oxide thin films onto borosilicate glass substrates. Based on our previous study on the deposition parameters of the pulsed-DC reactive sputtering process [28], a pulsing frequency of 100 kHz and a reverse bias duration of 2 ms were chosen to deposit the IrO2 thin films in this study. The Ir sputtering target had a diameter of 2 in. (purity 99.9%). A thin titanium adhesion layer (10 nm) was first deposited in the same sputtering chamber. Iridium oxide films were reactively sputtered in Ar/O2 plasma. The O2/(O2 + Ar) mixing ratio (OMR) for the deposition was maintained at 17% to ensure that the Ir target is not oxidized. Prior to deposition the chamber was evacuated to a base pressure of 105 Pa by a mechanical pump first and then by a cryogenic pump. The sputtering power was fixed at 150 W. The sputtering pressure was maintained at 0.66 Pa. Based on the results from our previous study [28] the substrate temperature for deposition of crystalline IrO2 thin films was maintained at 400 8C, and the amorphous films were deposited at room temperature. The IrO2 film thickness was measured by profilometry and found to be about 100 nm. The crystallinity of films deposited at a substrate temperature of 400 8C, was measured by X-ray diffraction (XRD) (theta-2theta scans) in a Rigaku-Rotaflex RU2000 diffractometer using Cu Ka radiation and the stoichiometry of the films was confirmed using Rutherford backscattering measurements [28]. A Renishaw InVia micro-Raman system was used to perform Raman measurements on the amorphous and crystalline IrO2 thin films in a back scattering geometry at room temperature. The spectra were excited by 514.5 nm Ar-ion laser. The laser power at the sample surface was about 1 mW which is below the levels at which any laser damage could be observed. The laser radiation was focused to a spot size of about 3 mm in diameter on the sample surface. In order to reproduce the physiological environment, all electrochemical measurements were performed in physiological saline solution (unbuffered aqueous 0.9% NaCl). All measurements were performed at room temperature with the
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geometric area (as opposed to the electrochemically active surface area) of the test sample being 0.1 cm2. A standard threeelectrode glass cell with a silver–silver chloride (Ag/AgCl) reference electrode (Bioanalytical Systems) was used for the measurements. The counter electrode was a high area Pt foil. All potentials are reported versus the Ag/AgCl electrode. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the thin film surfaces. The cyclic voltammetry and impedance spectroscopy measurements were carried out using a Gamry potentiostat system (model PCI4). The SIROF’s were subjected to a repetitive potential cycling at scan rate of 100 mV/s for 500 cycles in the potential range 1.0 to 1.0 V, a process called electrochemical activation [19]. The cyclic voltammograms were recorded before and after the electrochemical activation within the water window for IrO2, i.e. 0.6 and +0.8 V at a scan rate of 100 mV/ s. The anodic and cathodic charge storage capacities (CSC) of the test samples were calculated by integrating the respective cyclic voltammograms. The ac impedance spectra were measured in the frequency range of 0.1 Hz–100 kHz using a sinusoidal perturbation of 10 mV rms. A high resolution field emission scanning electron microscope (Hitachi S4800) was used to study the morphology of the film surfaces before and after the electrochemical activation procedure. 3. Results and discussion Fig. 1(a) shows the XRD pattern for the IrO2 thin film reactively sputtered onto BSG substrate at a substrate temperature of 400 8C. As seen from the figure the IrO2 films have a well defined poly-crystalline structure at the substrate temperature of 400 8C, dominated by (2 0 0) reflection, which is consistent with the results observed by other groups [20] and results obtained from our previous study [28]. For the room temperature reactive sputtering amorphous IrO2 is expected to form and so we did not observe any sharp peaks in the XRD pattern (not shown here). Fig. 1(b) shows the Raman spectra of the IrO2 films reactively sputtered onto BSG substrates at room temperature and at a substrate temperature of 400 8C respectively. The major Raman modes Eg, B2g and A1g are located at 561, 728 and 752 cm1 respectively [28]. The B2g and A1g modes at 728 and 752 cm1 are not resolved and form a broad peak dominated by A1g. The spectrum of the roomtemperature-deposited films shows noticeably wider peaks due to poor crystallinity or, rather, the lack of it. We can also observe a weaker mode at 365 cm1 in the amorphous film spectrum. This mode was observed in our previous experiments and we have attributed it to vibrations of molecular units in localized non-stoichiometric phases [28]. Fig. 2(a) shows the Bode plots of the electrochemical impedance spectra of the amorphous and crystalline IrO2 thin films before and after the electrochemical activation procedure respectively. As seen from the figure, the IrO2 thin films show a typical capacitive behavior in the low frequency range. It can also be seen from Fig. 1(a) that the magnitude of the impedance of the unactivated amorphous IrO2 film is considerably lower than that of the unactivated crystalline IrO2 film. The cut-off
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Fig. 1. A XRD pattern of IrO2 thin film reactively sputtered onto BSG substrate at a substrate temperature of 400 8C (a), Raman spectra of IrO2 thin films reactively sputtered onto BSG substrates at room temperature and a substrate temperature of 400 8C (b). (The spectra are normalized to the maximum intensity in that particular spectrum).
frequency for capacitive behavior of crystalline IrO2 film is at 500 Hz, whereas that of amorphous IrO2 film is at 100 Hz. This reflects an increase in the measured capacitance of the amorphous IrO2 film. Table 1 gives the values of the film capacitances calculated from the impedance data. It is interesting to note from Fig. 2(a) that the impedance of the crystalline IrO2 thin film exhibits consistently higher impedance compared to the amorphous IrO2 film, even in the high frequency range and is about 60–70 V as opposed to 40–50 V for the amorphous IrO2 thin film. Fig. 2(b) shows the impedance phase for these respective samples. The phase is also frequency dependant closing towards 0 at higher frequencies and near 90 for lower frequencies indicating a high-pass capacitive behavior of the films. Owing to their amorphous and highly porous nature, amorphous IrO2 films act like a random network of channels for easy access of ionic species into the bulk material, providing
Fig. 2. Bode plots of different IrO2 thin films, in 0.9% NaCl solution (a) shows the magnitude and (b) shows the phase of the complex impedance as a function of frequency for various IrO2 films. The solid lines are before activation and the dashed lines are after activation.
temporary bonding sites for these ionic species [22]. Amorphous IrO2 thin films have previously been reported to form gel-like materials when they come in contact with aqueous electrolytes (such as saline or body fluids) [19]. These gel-like materials contain cross-linked chains of oxygenbridged hydroxyl–aqua–iridium complexes which can be easily penetrated by water and ions such as H+, OH and Na+ [19]. The increase in water uptake within the film results in increase in thickness of the oxide layer. On the other hand, crystalline IrO2 thin films possess a smooth surface with reduced porosity and well-defined rutile tetragonal lattice structure [20]. The lower porosity and the high degree of crystallinity of the films has the effect of reducing ionic conductivity of the films, making it difficult for water and ionic species to penetrate within the bulk material [22]. In other words, crystalline films rarely form a gel-like material when in contact with aqueous electrolytes. Crystalline SIROF is reported to have similar electronic conductivity as compared to its amorphous counterpart [22]. Since the electrical conductivities are similar, it is clear that structure plays a very important role in influencing the ionic conductivity of these films.
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Table 1 Values of interfacial capacitance and a from the fitting of the impedance data for different samples; also anodic and cathodic charge storage capacities calculated by integrating the cyclic voltammograms Sample
Cf (mF/cm2)
Unactivated amorphous IrO2 thin film Electrochemically activated amorphous IrO2 thin film Unactivated crystalline IrO2 thin film Electrochemically activated crystalline IrO2 thin film
0.37 2.61 0.034 0.27
a 0.77 0.86 0.79 0.82
CSCa (mC/cm2)
CSCc (mC/cm2)
3.74 18.9 0.6 1.8
4.14 14.14 0.27 1.6
Interfacial capacitance (Cf), Anodic charge storage capacity (CSCa), cathodic charge storage capacity (CSCc).
Fig. 2(a) also shows the electrochemical impedance spectra of the electrochemically activated samples. Here we can see that the impedance of the films has been reduced considerably after the electrochemical activation procedure. Similar trends have been observed by other groups studying the electrochemical properties of sputtered amorphous IrO2 films [19]. It is important to note the consistency of lower impedance of the amorphous IrO2 film as compared to the impedance of crystalline IrO2 film even after the electrochemical activation process. The cut-off frequency of samples coated with activated amorphous IrO2 films for capacitive behavior has shifted to a lower frequency value of 10 Hz and for the activated crystalline IrO2 film it has shifted to about 100 Hz indicating a rise in the measured interfacial capacitance which can be verified from Table 1. The impedance of samples coated with activated crystalline and amorphous IrO2 films in the high frequency range is about 55–45 V and 30–40 V respectively. A similar trend is observed in the impedance phase as is evident from Fig. 2(b). This type of impedance spectrum is usually modeled by a double layer capacitance in parallel with polarization resistance in series with the electrolyte resistance. The equivalent circuit for such a model is shown in Fig. 3, where Rs represents the electrolyte resistance, Rp is the polarization of the sample and we have used the constant phase element (CPE) instead of a double layer capacitance to represent the non-ideal capacitive behavior of the impedance. The CPE impedance is represented as: Z CPE ¼
1 QðivÞa
(1)
where i is the imaginary number, v is the angular frequency, Q is a constant and a is a parameter that has a value between 0 and 1 [29]. In case of an ideal capacitance a = 1, and Q is the capacitance. Recently, the CPE behavior has been widely studied and discussed in literature [29–31]. It has been argued that CPE behavior is a result of surface roughness or any other structure of the surface (porosity, fractal nature) [32]. This
Fig. 3. Equivalent circuit of the electrode/electrolyte system.
equivalent circuit and model were found to be satisfactory in representing the electrode–electrolyte interfaces studied in this work. An important observation that can be made from the impedance spectra in Fig. 2(a) is that the reduction in impedance (in the lower frequency range) of the crystalline IrO2 film (6 times) after activation is almost two times larger than the reduction in the impedance of the amorphous IrO2 film (3 times) after activation. This could be attributed to the fact that the electrochemical activation procedure leads to a decrease in the crystallinity of the IrO2 films as revealed by the SEM images in Fig. 4 (a) and (b). Fig. 4 (a) is a high resolution field emission SEM image showing the morphology of the crystalline IrO2 film prior to the electrochemical activation procedure. Fig. 4(b) shows the morphology of the same crystalline IrO2 film after the electrochemical activation procedure (500 cycles of activation). Both the FE-SEM images are taken at the same instrument magnification of 100 kX. A comparison of both images suggests that the crystal size has reduced after the film was activated for 500 cycles. This raises the important question whether increasing the activation cycles (more than 500 cycles) for crystalline IrO2 film will further decrease the impedance of the film due to further grain size reduction and possibly, partial amorphization. Studies have shown that after 500 cycles of activation the amorphous films, there is saturation in the reduction of the impedance and an increase in charge storage capacity (CSC) [19]. Although further, more detailed studies of the electrochemical behavior are needed, such processes do not appear to take place in case of crystalline IrO2 films. Table 1 presents the values of the interfacial capacitance calculated by fitting the impedance spectra for different samples under study. As seen from the table the capacitance of amorphous IrO2 films is 0.37 mF/cm2 before activation which is significantly higher than the previously reported values for AIROF [8]. The value increases to 2.61 mF/cm2 after activation which is comparable to previously reported values for activated amorphous SIROF [19]. The increase in the value of interfacial capacitance shows that the activation process leads to an increase in the active surface area of the electrode material which is highly desirable in a stimulation electrode application. It can also be noted from the table that after the activation process the amorphous IrO2 film shows an increase in the value of a (0.86) which represents a more ideal capacitive behavior (a = 1). This highlights the significance of using CPE instead of capacitance for modeling the interfaces. The a value for the crystalline IrO2 film has also increased to 0.82 suggesting a
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Fig. 4. High resolution FE-SEM images of crystalline IrO2 thin films before electrochemical activation (a), and after electrochemical activation (b); taken at a magnification of 100 k.
more ideal capacitive behavior of these films. The increase in the value of a can be attributed to the increase in surface roughness and porosity of the films. Fig. 5 shows cyclic voltammograms (CV) of amorphous and crystalline IrO2 thin films before and after the electrochemical activation process. It can be clearly seen from the figure that the activation process has lead to an increase in charge storage capacities of the films (increase in the area under the curves). As seen from the equivalence of the area under the anodic and cathodic scans (for amorphous IrO2 thin film), the valence change of iridium oxide is easily reversed. In physiological solutions like 0.9% NaCl, the Ir3+/Ir4+ redox peaks are broad as compared to those observed in non-physiologic, high-buffer concentration solutions like H2SO4 [11,14,15]. A comparison of the CV curves for amorphous and crystalline IrO2 thin films shows that the charge storage capacity of crystalline IrO2 thin films is significantly less than that of amorphous IrO2 thin films (before and after the activation process). A closer look at the CV curves of crystalline IrO2 thin films indicates the absence of a current peak at 0.5 V which is clearly observed in the CV curves for amorphous IrO2 thin films; this suggests that the crystalline films do not undergo the valence change reaction in
Fig. 5. Cyclic voltammograms of amorphous and crystalline iridium oxide thin films before and after electrochemical activation process taken in 0.9% NaCl solution at a scan rate of 100 mV/s.
the whole volume of the film. This can be attributed to the poor ionic conductivities of the crystalline IrO2 thin films. Table 1 also presents results from the cyclic voltammetry experiments. It can be clearly seen that the activation process has increased the charge storage capacity of the films. The charge storage capacities calculated from the time integral along anodic (CSCa) and cathodic (CSCc) current directions for the amorphous IrO2 films are 3.74 and 4.14 mC/cm2 respectively before activation and 18.9 and 14.14 mC/cm2 respectively after the activation process. This increase in the charge storage capacity indicates that the activation process has lead to an increase in the volume of the oxide layer by water uptake, which eventually increases the roughness and porosity [19]. The charge storage capacity values for crystalline films are also presented in Table 1. As seen from the Table 1 the amorphous IrO2 films have a significantly higher charge storage capacity before and after activation than the crystalline IrO2 films making them preferable electrode coating materials in stimulation applications. Comparing the charge storage capacities of crystalline IrO2 films before and after the activation process shows a significant increase in the values of CSC leading to the conclusion that the activation process has considerably increased the ionic conductivity of the crystalline films. Hence we can speculate that further increase in the number of activation cycles will result in a further increase in ionic conductivity of these films, resulting in the formation of more porous and less crystalline structure. Two major requirements for a good stimulating electrode coating are high charge storage capacity and low impedance in the frequency range of interest for stimulation (10 Hz–1 kHz) [19]. Amorphous SIROF appear to satisfy both of these requirements. The charge storage capacities of these films have increased to about 18.9 mC/cm2 (anodic) and 14.14 mC/cm2 (cathodic) after the activation process. These films show a constant low impedance of about 40–50 V in the frequency range of 100 Hz–100 kHz and after activation their impedance drops to about 30–40 V extending from 10 Hz to 100 kHz. The CV and EIS results provide the necessary information needed for selection of a good stimulating electrode coating material. However, the information obtained from the EIS measurements gives a better representation about the material under study as it provides both high and low frequency response
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as compared to the CV results which only provides the low frequency response [8]. The EIS data also provides an estimate of the interfacial capacitance which is a measure of polarizability of the electrode material. 4. Conclusions Results from impedance spectroscopy and cyclic voltammetry measurements on amorphous and crystalline SIROF have been compared and suggest that amorphous IrO2 films are preferable materials for stimulation electrode coatings. It is concluded that the thin film material structure influences the ionic conductivities of the films. Amorphous SIROF owe their good ionic conductivities due to their amorphous phase. In order to accurately fit the electrochemical impedance spectra it is advisable to use CPE instead of simple capacitance for modeling the electrode/electrolyte interfaces. The electrochemical activation procedure significantly increases the charge storage capacities and at the same time considerably reduces the impedance of both amorphous and crystalline IrO2 thin films. The reduction in impedance and the increase in charge storage capacity of the crystalline IrO2 films upon activation, suggests that the activation procedure causes an increase in the ionic conductivity of the crystalline films. Based on the SEM results it can be speculated that the activation process leads to an increase in porosity of the crystalline films eventually making them lose their crystalline structure. A further detailed study is required in order to investigate the effect of a large number of activation cycles on the structure of crystalline IrO2 thin films. Acknowledgements This work was supported in part by the Michigan Life Science Corridor (MLSC) grant GR-358, the National Science Foundation-Integrative Graduate Education and Research Traineeship Program (NSF-IGERT) grant DGE-9870720 and funds from the DTE Energy Foundation. The authors would like to acknowledge Hitachi for the high resolution field emission SEM images. References [1] B. Wessling, W. Mokwa, U. Schnakenberg, J. Micromech. Microeng. 16 (2006) 142.
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