Iridium oxide-based microelectrochemical transistors for pH sensing

Sensors and Actuators B, 12 (1993) 225-230

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Iridium oxide-based microelectrochemical transistors for pH sensing K. P&ztorby*9 A. Sekiguchi, N. Shimo, N. Kitamura and H. Masuharaay* Microphotoconversion Projectt, ERATO, Research Development Corporation of Japan, 1280, Kami-izumi, Sodegaura, Chiba 299-02 (Japan) (Received July 3, 1992; in revised form December 22, 1992;accepted January 29, 1993)

Abstract A study on electrochemically deposited iridium oxide films on Au wire and a pair of Au microelectrodes is reported. The open-circuit potential of the deposited film on an Au wire has been studied by a series of measurements. The pH sensitivity of the film is shown to be -64 to -66 mV/pH, while the relevant extrapolated potential at pH = 0 ranges from 0.67 to 0.71 V. Iridium oxide-based microelectrochemical transistors have also been fabricated. The resistance of the film on a pair of Au microelectrodes changes by 1.5 orders of magnitude from 300 to 8 MR as the gate voltage is stepped from -0.3 to 0.3 V. The response time of the devicefor chemicalstimulation is less than 40 s and the device is demonstrated to be applicable as a pH sensor.

Introduction In the past ten years, new electrochemical devices based on conducting metal oxides, conducting polymers and redox polymers have been developed [ 11.A microelectrode array covered with poly(3-methylthiophene) [ 11,WO, [2] or Ni(OI-& [3] has been utilized to fabricate devices with specific sensitivity to 02, H2 or solution pH [ 11.The devices so far reported for pH sensing consist of a pair of closely spaced microelectrodes covered with redox active films, and pH is detected through a resistance change inside the film caused by electrochemical reactions. These devices are particularly useful as simple chemical sensors, and possess versatile applications to various research fields. As an example, the microenvironment in solution or films can be monitored by these devices, and the result can be fed back to control reactions. If the size of the device is reduced to the micrometer scale and coupled with other microelectrodes [4], the devices would be a potential means to sense microenviromnental conditions in minute volumes. Furthermore, an integrated chemical system, in

*Authors to whom correspondence should be addressed. TFive-yearproject, Oct. 1988~Sept. 1993.After Sept. 1993all correspondence should be sent to the permanent address of H.M. “Permanent address: Department of Applied Physics, Osaka University, 2-1, Yamadaoka Suita, Osaka 565, Japan. “On leave from Department of Electron Devices, Technical University of Budapest, Goldmann Gy. ter 3, H-1521 Budapest, Hungary.

09%4005/93/$6.00

which both reaction-inducing and -sensing sites are arranged arbitrarily on a microelectrode array, could be developed through these approaches. In order to test such possibilities, an important step is to prepare reliable and sensitive sensing materials and use them to fabricate devices. Among various sensing materials, iridium oxide (IrOz) is one of the possible candidates for such applications. 110~ shows promising behaviour in a wide range of applications and is utilized not only as a pH-sensing material [5- lo], but also as a display device [ 11, 121. It has also been reported that the oxide exhibits catalytic activity [13]. Indeed, IrO, films were applied to sensor-actuator systems for coulometric titration [ 71. IrO, films can be prepared by several methods and their reversible redox reactions have been well studied [14,15]. Electrochemical oxidation of an iridium wire or fihn by a potential-cycling [S, 61 or current-pulse method [7] produces so-called anodic iridium oxide film (AIROF). A sputtered iridium oxide film (SIROF) can be prepared by O2 plasma reactive sputtering [g-lo]. Thermal decomposition of an IrC13.3H20 ti [16], thermal oxidation of an iri$un-carbon !ihn [ 17, periodic reverse-current electrolysis (PRIROF) in an aqueous sulfatoiridate (III, IV) solution [ll] and anodic electrodeposition (AEIROF) [ 121 have so far been reported. Although various fabrication methods have been reported, AIROF and SIROF are mainly used for pHsensitive electrodes. Experimentally, AIROF is the easi-

@ 1993 ~ Elsevier Sequoia. All rights reserved

est to prepare. However, SIROF shows a more reproducible open-circuit potential [g-lo]. For preparing a reliable microdevice, we consider that the AEIROF is the most promising, since the anodic electrodeposition method is a very easy way to prepare IrO, films on various microelectrodes. In this paper, we report the preparation and characterization of electrochemically deposited IrO, films and their application to pH sensing.

Experimental Fabrication of Au microelectrodes

Parallel Au microelectrodes were fabricated on a Ta,O,-coated Si wafer by conventional UV photolithography. Each electrode was 200 pm long, 5 pm wide and 0.2 pm thick with 2 pm spacing between the electrodes. A Ta,O, film with a thickness of 0.1 pm was prepared on an Si wafer by thermal oxidation of an r.f. sputtered Ta film at 490 “C for 4 h under oxygen atmosphere. Cr was used as an adhesive layer between Ta,O, and Au. Both Cr and Au films were deposited by r.f. sputtering and patterned by photolithography and subsequent wet etching. The thickness of Cr or Au iilm was 30 nm or 0.2 w, respectively. The fabricated Au microelectrode was packaged into a printed circuit board and each electrode was connected to an external circuit through the relevant bonding pad. The Au electrodes, except for a reaction window, were covered with silicon rubber (TSE 392-W, Toshiba) for insulation. The electrodes were cleaned by potential cycling between - 1.5 and 2.0 V (versus saturated calomel electrode (SCE), 200 mV/s, > 10 cycles) in an aqueous 0.1 M K,HPO, solution. The fabricated microelectrodes were characterized by cyclic voltammetry in an aqueous solution of 5 mM Ru(NH,),C12 and 0.1 M KCl. The observed voltammograms showed sigmoidal curves and agreed satisfactorily with the reported ones [ 11.Devices whose resistance between the electrodes was higher than 10” R were used for experiments. Deposition of AEIROFs

IrO, films were electrochemically deposited on the Au m&electrodes by a potential-cycling method in an aqueous solution (100 ml) containing 0.15 g IrCl, (99.5%), 1 ml H,Oz (30%) and 0.5 g (COOH)2.2H20. The pH of the solution was adjusted to 10.5 with K2C0, [ 121.Au wires (Nilaco, 0.1 mm diameter) were coated with IrO, by the same procedures. Before deposition, the Au wires were rinsed in acetone and ethanol, and then cleaned ultrasonically in pure water. Although deposition of IrOz was not observed at a potential more negative than -0.2 V, smooth IrO, films were formed on Au microelectrodes and Au wires when the potential was cycled 80 times between -0.2 and +0.6 V at the

rate of 100 mV/s. The thickness of the film was 0.05 pm as estimated with a surface profiler (Surfcom, model E-MD-S53A, Tokyo Seimitsu). Electrochemical

measurements

Electrochemical deposition of IrOl films and cyclic voltammetry measurements were carried out with a PS-13 potentiostat and a FG-02 programmable function generator (Toho Technical Research), both controlled by a personal computer. All potentials were given with respect to an SCE, and measurements were carried out at room temperature (23 f 2 “C) in a Faraday cage under atmospheric conditions. Open-circdit potential measurements on the AEIROFcoated Au wires as a function of pH were performed in B&ton-Robinson buffer solution. The procedure was as follows. Measurements were started in an aqueous solution of 0.04 M H,PO,/CH,COOH/H,BO, (pH N 1.8), and 0.2 M NaOH solution was added to change the solution pH by a peristaltic pump (Mini-S 460, Ismatec), while the same amount of the solution was removed to maintain a constant liquid level. The solution was homogeneously mixed by a magnetic stirrer. The open-circuit potential of the AEIROF electrode in each pH was measured after a sufficient waiting time (at least 2 min) to stabilize the pH of the solution using a source measure unit (SMU, model 237, Keithley) with a typical input resistance of 1014Q. The pH of the solution was measured separately by a pH meter (model 671, Sibata) with a pH glass electrode (HGS-6005, TOA Electronics Ltd.) and SCE (Metrohm). The signal from the SMU was directly transferred to a computer while the output of the pH meter was sampled by an A/D converter (Keithley/ Metrabyte DAS-20).

Results and discussion Characterization of AEIROF on the Au wire

The electrochemically deposited iridium oxide film on an Au wire was characterized by cyclic voltammetry (CV) and the voltammogram was compared with that of the films prepared by constant-current electrolysis [ 121.A typical voltammogram of deposited AEIROF in a buffer solution of pH = 6.86 is shown in Fig. 1. This CV curve was highly reproducible without any noticeable change even after continuous cycling over one day. This is quite in contrast to the AEIROF prepared b constant-current electrolysis, which is electrochemically stable only after thermal annealing at 100 “C for 1 h. The film deposited by electrolysis was weak and easily removed from Au by applying a bias voltage of -0.8 V in 0.5 M H2S04. Yamanaka reported that an as-grown AEIROF prepared by electrolysis was amorphous in structure, while the crystallinity of IrO, increased by

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-60.0 -60.0

. ' ' -0.4 -0.2

' . ' . ' . 0.0 0.2 0.4 0.6

Potential

vs. SCE [V]

Fig. I, Typical cyclic voltammogram of the electrochemically deposited iridium oxide film on Au wire in pH = 6.86 buffer solution. Scan rate = 50 mV/s.

‘”

0.02t”““_l

Open-circuitpotential as a function of pH Figure 2 shows a typical open-circuit potenti&pH response of an AEIROF on an Au wire in a 0.04 M H,P0,/CH,COOH/HSB03 buffer solution. The opencircuit potential varied linearly with pH in a range from 1.8 to 12 and the slope of the fitted line (correlation coefficient = 0.9996) was -66 mV/pH with an interpolated potential of 0.68 V at pH = 0 (E,). The pH sensitivity is higher than the theoretical value of -59 mV/pH for a redox process involving one proton per electron, as expected from the ‘Nemst equation and the following redox reaction for iridium oxide: Ir02+H++e-=IrOOH

21r(OH)20m t H,O = IrO,(OH),0,3E

0.6

-0.2 ' 0

2

4

6

6

10

I 12

PH Fig. 2. Typical open-circuit potential-pH response of the electrochemically deposited iridium oxide film on Au wire. Upper panel represents residuals of the linear fitting.

thermal treatment [ 121. A reproducible CV of as-grown IrOp by the potential-cycling method may indicate that the crystallinity of the present h-0, is higher than that of the tilm prepared by constant-current electrolysis. CV curves are in good agreement with that of AEIROF [ 121,PRIROF [ 1l] and AIROF samples [6, 14, 151,and are similar in all important aspects to that of SIROF [S, 141.The charge exchanged during the redox reaction (AQ) was estimated to be 12 mC/cm* from the areas of the CV curve and electrode surface (0.03 cm’).

(1)

For an AIROF prepared by electrochemical oxidation, the pH sensitivity ranges from -60 to - 80 mV/pH [6], while that reported for a SIROF is -57 [lo] to -59.5 mV/pH [8]. The structure of the fihn prepared by electrochemical oxidation is expected to be very porous, since solvent molecules permeate from a solution phase to the Ir02 layer during oxidation. On the other hand, the him structure of a SIROF prepared by reactive sputtering in vacuum is less porous [8, lo]. Olthuis et al. have reported that the high pH sensitivity of an AIROF is ascribable to oxyhydroxide groups in the film: + 3H+ + 2e ( 2)

and this leads to the theoretical pH sensitivity of -88,5mV/pH [6]. The IrO, film deposited by the present potential-cycling method will be less porous than an AIROF but much more porous than a SIROF as expected from the deposition method of the film. The medium pH sensitivity of the present AEIROF ( -66 mV/pH) can be explained in terms of the porosity of the film and, therefore, the number of oxyhydroxide groups. This will be contirmed by the structural analysis of the films, which is in progress in our project. The scattering of the pH sensitivity with samples was very small. Three samples were measured within a week after fabrication without special storage conditions. The slope of the least-squares regression line for the open-circuit potential response varied from -64 to -66 mV/pH, with .E, ranging between 0.67 and 0.71 V. The potential-pH response of the AEIROF was highly reproducible despite the experiments in air, contrary to the results of an AIROF. The latter shows large fluctuations in pH sensitivity and E,,, depending on the oxidation state of the film [6]. The potential-pH response of the electrodes was measured over a period of a few weeks, with the sample electrodes being stored in distilled water under laboratory conditions between measurements. During the measurement period, the pH sensitivity and E,, increased by a few and about 100 mV, respectively, indicating further

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formation of oxyhydroxide groups inside the film. Although long-term drift measurements on the film are still in progress, we can conclude that the present AEIRQF is one of the promising materials for pH sensing. Characterization of Ir02-based microelectrochemical transistors

An iridium oxide film deposited onto Au microelectrodes was characterized by CV. The observed voltammdgram was similar to that on the Au wire (Fig. 1). According to the pH sensitivity of the IrOl-Au wire electrode described in the preceding Section, a microelectrochemical transistor can be fabricated based on the IQ-Au microelectrodes. The experimental set-up is shown in Fig. 3. The gate voltage (V,) provides a potential for redox processes against the source and drain microelectrodes, and a current flowing between two microelectrodes (ID) at a given drain voltage (V,) is inversely proportional to the resistance of the film according to Ohm’s law. The resistance of the AEIROF was measured as a function of Vc at pH = 6.86. In this experiment, the drain voltage was kept below 10 mV in order to prevent a non-uniform distribution of film resistance between the electrodes. The resistance at a given V, was calculated from the slope of the I-V curve measured by scanning V,, from 0 to 10 mV at a rate of 0.1 mV/s. The I- V curve at each VG was measured after waiting sufficient time to establish equilibrium inside the IrO, film at the new VGand zero VD. The results are summarized in Fig. 4. The resistance of the AEIROF changes from 300 to 8 Ma as V, is varied from -0.3 to 0.3 V. Using the geometrical data of the electrodes, the resistivity of the film was calculated to be 150 kfl cm, which was higher than that of a SIROF [lo] or W03 [2] by 1.5 or 2.5 orders of magnitude, respectively.

SourceMeasureUnit Fig. 3. Experimental set-up for the characterization oxide-based microelectrochemical transistors.

of iridium

.i

-0.3

-0.2

-0.1

0.0

V, vs.

0.1

0.2

0.3

SCE

Fig. 4. Resistance of an iridium oxide connected microelectrode pair as a function of V, in pH = 6.86 buffer solution.

-0.4

-0.2

0.0

Gate Voltage

0.2

(V,)

0.4

vs.

0.6

SCE [V]

Fig. 5. pH dependence of the steady-state drain current of iridium oxide-based transistor as a function of Vo (at fixed V, = 100 mV) in three bufier solutions.

In Fig. 5 a typical transfer characteristic of the device at V, = 100 mV is given for three buffer solutions whose maximum transconductance is 0.25 PA/V. The results demonstrate that the device can be used as a pH sensor. At fixed V, and VG, ID is solely determined by pH. All the data points were measured in equilibrium after a sufficient waiting time. To characterize the device further, the time responses for electrical and chemical stimulations were studied. Figure 6 shows the time responses of ID at V, = 100 mV when V, is changed from -0.2 V (non-conductive state) to 0.1 V (conductive state) in a buffer solution of pH = 6.86. The initial current peaks are not caused by a resistance change in the f&n but are due to the overshoot to establish the new state of the device. Reproducible

229

Time [set] Fig. 6. Time response of the drain current of an iridium oxidebased transistor. V, is changed repetitively from -0.2 to 0.1 V at fixed V, = 100mV in pH = 6.86 buffer solution.

6

I

I

I

1

7 85

-11

pH6.71

’

0

’

50

’

100 Time [set]

’

150

time response includes the contributions from both the response time of the device and the mixing time of the solution, it is concluded that the response time of the device is ~40 s. The present time response is comparable to that of a WOS- [2] or Ni(OH),-based [3] transistor; 40 or 95 s, respectively. For open-circuit potential measurements on the AIROF, however, a much faster response of 40 ms or 0.35 s has been reported for the fdm with AQ = 1 and 7 mC/cm’, respectively [6]. In our experiments AQ estimated from the CV curve of an AEIROF on microelectrodes is about SOmC/cm’, so that it will take a longer time to form a new equilibrium state in the case of the device with large AQ. In the VG potential range -0.4 to 0.7 V at V,, = 100 mV, the microelectrochemical transistor showed reproducible behaviour for longer than one week. However, the lifetime of the device was shortened by applying a V, more positive than 1.OV, which caused partial peeling of the film from the Au microelectrodes. The open-circuit potential is considered to be the result of surface processes, while an output signal of a microelectrochemical transistor is due to a resistance change in the whole film. Thus, microelectrochemical transistors are also useful for separation of surface and bulk processes in thin-film studies. The response of the drain current by changing VG or pH is fast enough (less than 1 min) that the present system can be practically applicable to a pH sensor.

’

200

Fig. 7. Tie response of the drain current of an iridium oxidebased transistor. The pH is changed from 1.81 to 6.71 at fixed VG = 0.1 V and V, = 100mV.

on/off switching was observed for more than 2 h. The time response of the device is limited by the diffusion time of protons in the IrO, 61m rather than by electron mobility inside the film. In Fig. 6, 1, reaches a steady state value of 5.5 nA within 50 s in the switching-on process (oxidation of the film) and returns to 0.2 nA with a slightly faster time constant (reduction of the film). Such an inequivalence of proton diffusion in oxidation and reduction processes has been reported for W03 [2] and Ni(OH), [ 31. It is worth noting that the difference in the time response between the oxidation and reduction processes in the present IrO, film is much smaller than that in W03 or Ni(OH), film. Figure 7 shows the time response of a drain current at V, = 100 mV and VG = 0.1 V when the pH is changed from 1.81 to 6.71. A large amount of 0.1 M NaOH solution was quickly injected into a 0.04 M H,P0,/CH,COOH/H3B0, solution to change the solution pH under vigorous stirring. Although the observed

Conclusions The pH sensitivity of an AEIROF was evaluated for IrO, on an Au wire based on open-circuit potential measurements. The films showed a linear potential-pH response in a pH range from 1.8 to 12, with a pH sensitivity of - 65 mV/pH. The interpolated potential at pH = 0 and the pH sensitivity of the AEIROF exhibited good reproducibility for different samples. It is concluded that electrochemical deposition of 110, offers an easy preparation technology with good sensor properties. The pH sensitivity of an AEIROF is independent of the oxidation state of the film, contrary to an AIROF, and higher than the value expected from the electrochemical reaction in eqn. (1). The results were discussed in terms of the porosity and chemical characteristics occurring in the IrO, film. On the basis of the present results, an IrO,-based pH-sensitive microelectrochemical transistor was fabricated and characterized. The resistance of the iridium oxide film on a pair of Au microelectrodes decreased by 1.5 orders of magnitude upon electrochemical oxidation According to the potential step experiments, the diffusion coefficient of protons inside AEIROFs in the oxidation process is slightly larger than that in the

230

reduction process. The response time of the device is about 40 s for a pH change and this value is suitable for application of the device to a pH sensor.

by thermal oxidation of iridium-carbon Elecfrochem. Sot., 134 (1987) 570-575.

composite films, J.

Biographies References M. J. Natan and M. S. Wrighton, in S. J. Lippard (ed.), Progress in Inorganic Chemisfry, Vol. 37, Wiley, New York, 1989, pp. 391-494. M. J. Natan, T. E. Mallouk and M. S. Wrighton, pH-sensitive WO,-based microelectrochemical transistors, J. Phys. Chem., 91 (1987) 648-654. M. J. Natan, D. Belanger, M. K. Carperter and M. S. Wrighton, pH-sensitive Ni(OH),-based microelectrochemical transistors, J. Phys. Chem., 91(1987) 1834-1842. D. Belanger and M. S. Wrighton, Microelectrochemical transistors based on electrostatic binding of electroactive metal Complexes in protonated poly(4-vinylpyridine): devices that respond to two chemical stimuli, Anal. Chem., 59 (1987) 1426-1432. L. D. Burke, J. K. Mulcahy and D. P. Whelan, Preparation of an oxidized iridium electrode and the variation of its potential with pH, .J. Electroanal. Chem., 163 (1984) 117-128. W. Olthuis, M. A. Robben, P. Bergveld, M. Bos and W. E. van der Linden, pH sensor properties of electrochemically grown iridium oxide, Sensors nnd Actua/ors B, 2 ( 1990) 247256. W. Olthuis, J. C. van Kerkhof, P. Bergveld, M. Bos and W. E. van der Linden, Preparation of iridium oxide and its application in sensor-actuator systems, Sensors and Actuators 8, 4 (1991) 151-156. 8 T. Katsube, I. Lauks and J. N. Zemel, pH-sensitive sputtered iridium oxide fihns, Sensors and Actuufors, 2 (1982) 399-410. 9 I. Lauks, M. F. Yuen and T. Dietz, Electrically free-standing IrO, thin-film electrodes for high temperature, corrosive environment pH sensing, Sensors and Actuators, 4 (1983) 375379. 10 M. J. Tarlov, S. Semancik and K. G. Kreider, Mechanistic and response studies of iridium oxide pH sensors, Sensors and Actuators, Bl (1990) 293-297. 11 T. Yoshmo, N. Baba and K. Arai, Electrochromic 110, thin filmsformed in sulfatoiridate (III, IV) complex solution by periodic reverse current electrolysis (PRIROF), Jpn. J. Appl. Phys., 26 (1987) 1547-1549. 12 K. Yamanaka, Anodicahy electrodeposited iridium oxide films (AEIROF) from alkaline solution for electrcchromic display devices, Jpn. J. Appl. Phys., 28 (1989) 632-637. 13 S. Gottesfeld and S. Srinivasan, Electrochemical and optical studies of thick oxide layers on iridium and their electrocatalytic activities for the oxygen evolution reactions, J. Electroanal. Chem., 86 (1978) 89- 104. M. F. Yuen, I. Lauks and W. C. Dautremont-Smith, pHdependent voltammetry of iridium oxide films, Solid State Ionics, II (1983) 19-29. V. Birss, R. Myers, H. Angerstein-Kozlowska and B. E. Conway, Electron microscopy study of formation of thick oxide films on Ir and Ru electrodes, J. Elecfrochem. Sot., 131 (1984) 1502-1510. R. M. Ianniello and A. M. Yacynych, Urea sensor based on iridium dioxide electrodes with immobilized urease, Anal. Chim. Acta, 146 (1983) 249-253. Y. Sato, K. Ono, T. Kobayashi, H. Wakabayashi and H. Yamanaka, Electrochromism in iridium oxide films prepared

Kdmlin Prisztor received his diploma in electrical engineering from the Technical University of Budapest in 1987. He worked at the Department of Electron Devices of the Technical University of Budapest from 1987 to 1990. He has worked at ERATO’s Microphotoconversion Project of the Research Development Corporation of Japan (JRDC) since 1990. His main interests are semiconductor technology, electron devices, chemical sensors and their applications in chemical systems. Mr P&or is a member of the Scientific Society for Telecommunications (Hungary).

Atsus!,riSekiguchi received his B.S. and MS. degrees in the College of Science and Engineering from Aoyama Gakuin University in 1980 and 1982, respectively. Mr Sekiguchi is a member of the American Vacuum Society, the Japan Society of Applied Physics, and the Chemical Society of Japan. Nobuo Shim0 received his B.E. and M.E. degrees in synthetic chemistry from Osaka University in 1973 and 1975, respectively, and joined Idemitsu Kosan Co., Ltd. From 1989, he has also worked at ERATO’s Microphotoconversion Project at the JRDC. His current interest is the application of laser chemistry to chemical industries and basic research on the chemistry in microfabricated and chemical functional&d materials. Mr Shimo is a member of the Chemical Society of Japan, the Japan Petroleum Institute, the Laser Society of Japan, the Japan Society of Applied Physics, the Japanese Photochemistry Association and the Silicon Photochemical Society of Japan. Noboru Kitamura graduated from Tokyo Metropolitan University in 1976. He received an M.S. and Ph.D. from Tokyo Institute of Technology (TIT) in 1978 and 1983, respectively. After working as a research associate in the Research Laboratory of Resources Utilization, TIT, from 1978 to 1988, he joined the Microphotoconversion Project, ERATO Programme, JRDC, as a technical manager. Hiroshi Masuhara graduated from Tohoku University in 1966 and received his Ph.D. in chemistry from Osaka University in 1971. He moved from Kyoto Institute of Technology to the Department of Applied Physics, Osaka University. Now he is a professor and the director of the Microphotoconversion Project, ERATO Programme.