1834
J. Phys. Chem. 1987, 91, 1834-1842
TABLE VII: Frequency Dependence of the 31Pand 'Ha Relaxation Rates in NaDEHP/H,0/C,H6 Micelles
31P 36.5 MHz (TI-l)FH,n S'I s-I
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expt
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calcd'
expt
calcde
0.49 0.30 0.23
0.49
0.66
0.656
1.2
1.195
0.51
0.501 0.418
1.04
0.28 0.227 0.044
(Tl-l)CsA? s-I
1.09
1.05
IH, 90 MHz TI-', s-'
500 MHz
expt
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expt
caIcd
7.14
7.05
2.07
1.99
Fully protonated. deuteriated. Fully deuteriated. dContribution of the chemical shift anisotropy. CTaking7S = 1.62 X s and DR = 12.9 (see Figure 7). fCalculated with the two-step s, and 71 = 2 X lO-'Os. model with IS1 = 0.5, 7$ = 1.6 X by the slow overall and fast segmental motions. They have proposed a three-step model where the spectral densities are expressed as17922 J ( w ) = Sz1
7s
+ (S12- SZ)- 1 +
7i
+ (1 -S12)7r
(22)
the present case the motions of intermediate time scale between T, and Tf are the reorientation of the surfactant about the O,PO, bisector A M and possibly the motion of AM about the normal to the interface in the water evidenced in the case of lyotropic liquid crystal^.'^ Whatever the nature of these motions is, their existence accounts for the difference between the values of rf obtained from I3C and IH relaxations by using the two-step model.
Conclusion This work is a first attempt at interpreting in term of conformer populations the order parameters derived from the two-step model of Wennerstrom et al.Iqz in a reversed micelle of a double tailed surfactant. The zH and 13Crelaxations over wide frequency ranges are in apparent agreement with this model, provided that each chain has its own local director oriented toward the polar head. This orientation is obtained from a comparison of the relaxation data in Mn2+ doped and diamagnetic samples. The consistency between the results of the relaxation experiments on these systems is a good indication that the MnZf probe does not perturb appreciably the micellar structure. The other main point of this work is the determination of the density profile of surfactant chains. This is a specific application of the paramagnetic relaxation, allowing an estimate of the solvent penetration which is in fair agreement with experiment.
where SI and S represent the order left by motions of fast and intermediate time scales, respectively. As pointed out by these authors, eq 22 may be generalized to include more terms if there are more than three kinds of motions of different time scales. In
Acknowledgment. The authors are greatly indebted to Profs. Canet and Niry from Nancy University for the low-field ZH experiments and for stimulating discussions. T. Ahlnb is grateful to the National Swedish Board of Technical Development for a grant covering his stay in Saclay. Registry No. NaDEHP, 141-65-1.
(22) Davis, J. H.; Jeffrey, K. R.; Bloom, M. J . Magn. Reson. 1978, 29, 191.
(23) Chachaty, C.;Quaegebeur, J. P.; Caniparoli, J. P.: Korb, J. P. J . Phys. Chem. 1986, 90, 11 15.
+ w%,2
W2f12
pH-Sensitive Ni(OH),-Based Microelectrochemical Transistors Michael J. Natan,+ Daniel Bblanger,+Michael K. Carpenter,*f and Mark S. Wrighton*+ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39, and Electrochemistry Department, General Motors Research Laboratories, Warren, Michigan 48090 (Received: September I O , 1986)
Properties of arrays of closely spaced (1.2 pm) Au or Pt microelectrodes ( - 2 pm wide X 50 fim long X 0.1 pm high) coated with cathodically grown films of Ni(OH)* are reported. Electrical connection of microelectrodes by Ni(OH)* was verified by cyclic voltammetry. The ratio of anodic charge to cathodic charge in cyclic voltammograms for the Ni(OH), +NiO(0H) interconversion exceeds one. However, it is shown that excess charge in the anodic cyclic voltammetric wave for oxidation of Ni(OH)2 does not affect the conductivity of Ni(OH), films. The steady-state resistance of Ni(OH), connecting two microelectrodes has been measured as a function of potential from 0 to 0.7 V vs. SCE and was typically found to vary from lo7 to IO4 ohms. The measured resistance corresponds to a resistivity of approximately 30 ohm-cm in the oxidized state. The decrease in resistance is caused by electrochemical oxidation of insulating Ni(OH), to "conducting" NiO(0H). At fixed drain voltage, VD,the gate current, I,, and the drain current, I D , can be measured simultaneously as the gate voltage, VG,is varied at a given frequency. The frequency response is limited by the slow electrochemistry of Ni(OH), films. At Hz, Ni(OH),-based microelectrochemical transistors can amplify electrical power by a factor of a frequency of 3.8 X 20. The temperature dependence of ZD indicates an activation energy for conductivity in NiO(0H) of 23 f 2 kJ/mol at V, = 0.45 V vs. SCE. A pair of microelectrodes connected by Ni(OH)* functions as a pH-sensitive microelectrochemical transistor, because there is a pH dependence in the potential associated with the oxidation of Ni(OH),. The pH dependence of the transistor behavior is illustrated under dynamic and steady-state conditions; as the pH of a basic solution is increased, VGfor device turn on moves negative, in accord with the known pH dependence of the redox chemistry of Ni(OH),. Detection of a change in pH from 12 to 13 in a flowing stream was demonstrated with Ni(OH),-based microelectrochemical transistors.
-
-
In this article we report the properties of pH-sensitive Ni(OH)2-based microelectrochemical transistors, prepared by cathodic electrochemical deposition of Ni(OH)2 onto Au or Pt 'Massachusetts Institute of Technology. 'General Motors Research Laboratories.
0022-3654/87/2091-1834$01.50/0
microelectrode arrays. The oxidation of Ni(OH)2, usually written as Ni(OH), F= N i O ( 0 H ) + H+ + e(1) has been of great interest to electrochemists since the turn of the century, when Edison patented the use of Ni(OH)z as the anode 0 1987 American Chemical Society
Ni(OH)z-Based Microelectrochemical Transistors
The Journal of Physical Chemistry, Vol. 91, No. 7, 1987
SCHEME I: A Ni(OH),-Based Transistor That Turns On (ID> 0) When VG Is Moved from VG1,Where Ni(OH), Is Reduced and Insulating, to VG2,Where Ni(OH), Is Oxidized and Conducting' Potentiortat
TVAT
I
w
I/
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oevtcr "Off.
O w i c e "On'
o w i c e "Off'
Oevicr "On"
"The Ni(OH),-based device can also be turned on and off by varying the pH at fixed V,.
in alkaline electrochemical storage cells. Since then, a tremendous amount of research has been devoted to understanding this complex reaction.'v2 There are several methods of preparation of electroactive Ni(OH)2 films, including direct growth from Ni metal electrodes3 and electrodeposition onto conducting substrates by a variety of cathodic4 or anodic5 techniques. In addition to being an important battery electrode, Ni(OH)2 is an electrochromic material, becoming colored upon electrochemical oxidation.6 Like other electrochromic transition metal oxide^,^ coloration of Ni(OH)2 is accompanied by a significant increase in conductivity. This change in conductivity, coupled with the p H dependence of the electrochemical potential where oxidation to the conducting state occurs, and the ability to deposit Ni(OH), on closely spaced (1.2 pm) microelectrodes together provide the basis for a pH-dependent microelectrochemical transistor. In (1) Gunther, R.; Gross, S., Eds. Proceedings o f t h e Symposium on the Nickel Electrode; The Electrochemical Society: Pennington, NJ, 1984; Vol. 82-84. (2) (a) Halpert, G. J . Power Sources 1984,12, 177. (b) Olivia, P.; Leonaredi, J.; Laurent, J. F. J . Power Sources 1982, 8, 229. (c) Barnard, R.; Randell, C. F.; Tye, F. L. J . Appl. Electrochem. 1980, 10, 109. (d) Barnard, R.; Randell, C. F.; Tye, F. L. J . Appl. Electrochem. 1980, 10, 127. (3) (a) MacDougall, B.; Graham, M. J. J . Electrochem. SOC.1981, 128, 2321. (b) Wilhelm, S. M.; Hackerman, N. J . Elecfrochem. SOC.1981,128, 1668. (c) Glarum, S.H.; Marshall, J. H.J . Electrochem. SOC.1982, 129, 535. (d) Schrebler Guzman, R. S.;Vilche, J. R.; Arvia, A. J. J . Appl. Electrochem. 1979.9, 183. (e) Schrebler Guzman, R. S.; Vilche, J. R.; Arvia, A. J. J. Appl. Electrochem. 1979, 9, 321. (f) Schrebler Guzman, R. S.;Vilche, J. R.; A p i a , A. J. J. Electrochem. Soc. 1978, 128, 1578. (4) (a) Falk, S. U.; Salkind, A. J., Eds. Alkaline Storage Batteries; Wiley: New York, 1969. (b) McEwen, R. S. J . Phys. Chem. 1971, 75, 1782. (c) Briggs, G. W. D.; Wynne-Jones, W. F. K. Electrochim. Acta 1962, 7, 241. ( 5 ) (a) Tuomi, D. J . Electrochem. SOC.1965.112, 1. (b) Manandhar, K.; Pletcher, D. J . Appl. Electrochem. 1979, 9, 707. (c) Tench, D.; Warren, L. F. J . Electrochem. SOC.1983, 130, 869. (6) (a) McIntyre, J. D. E.; Peak, W. F.; Schwartz, G. P. Presented at the 21st Electronic Materials Conference, Boulder, CO, 1979; paper D4. (b) McIntyre, J. D. E. Presented at the 22nd Electronic Materials Conference, Cornell, NY, 1980; paper A l . (c) Lampert, C. M.; Omstead, T. R.; Yu, T. C. In Proceedings of S.P.I.E.-International Society of Optical Engineering, Lampert, C. M., Ed.; S.P.I.E.: Bellingham, WA, 1985; Vol. 562, p 15. (7) (a) Dautremont-Smtih, W. C. Displays 1982,3, 3. (b) DautremontSmith, W. C. Displays 1982, 3,67. (c) Agnihotry, S . A,; Saini, K. K.; Subhas Chandra Indian J . Pure Appl. Phys. 1986, 24, 19.
1835
principle, any of the proton-dependent electrochromic transition J* metal oxides, such as W03,7,8Nb205,799 IrOz,7J0M o O ~ , ~or RhO2,'*l2 that can be deposited onto closely spaced microelectrodes should exhibit pH-dependent transistor-like behavior. A study of pH-sensitive W0,-based microelectrochemical transistors, prepared by rf plasma deposition of polycrystalline WO, onto microelectrode arrays, has been completed. l 3 Closely spaced (1.2 pm) Pt or Au microelectrodes (- 2 pm wide X -50 pm long X -0.1 pm high) are useful in the study of the conductivity of transition metal oxides like Ni(OH)2 and Wo3,l3 whose conductivities are low compared to those of metals. They have also been useful in the study of conducting, electroactive organic polymers that can be derivatized onto Pt or Au. Polyp y r r ~ l e , polyaniline,15 '~ and po1y(3-methylthiophene)l6 all undergo conductivity changes accompanying electrochemical redox reactions of the polymers. Thus, microelectrochemical transistors based on conducting organic polymer-connected microelectrodes have already been demonstrated.'"I6 Derivatized microelectrode arrays have been also been useful in the demonstration of molecule- based diodes. I'J * Scheme I illustrates the operation of a microelectrochemical transistor based on conductivity modulation of Ni(OH)2. The extent of oxidation (and thus the extent of conductivity) is controlled by the gate potential, VG. The current associated with the faradaic processes controlled by V, is the gate current, I G . The current that flows between the microelectrodes (when there is a potential difference between them) as a result of the enhanced conductivity of the oxidized Ni(OH)2 film is termed the drain current, ID. The potential difference maintained between the microelectrodes is the drain voltage, VD. When V , equals VG', Ni(OH)2 is reduced (and insulating), and there is no current flowing in the drain circuit, I D = 0. When Ni(OH), is oxidized according to eq 1 by moving VGto VG2,the oxide is conducting, and for a finite VD,there is current in the drain circuit, I D > 0. The microelectrochemical transistors resemble solid-state trans i s t o r ~ , but ' ~ a key difference is that in the solid-state device I G is simply a capacitative current, rather than a faradaic current associated with actual electrochemistry. An important consequence of the faradaic processes occurring in microelectrochemical transistors is that factors which affect the redox chemistry can also affect I D . The redox potential of Ni(OH)2 is pH-dependent, and therefore I D is pH-dependent, as illustrated in Scheme I. For a fixed V, (near the redox potential of Ni(OH),) and fixed VD,a change in pH changes the ratio of (8) (a) Deb, S . K. Philos, Mag. 1973, 27, 807. (b) Faughnan, B. W.; Crandall, R. S.;Lampert, M. A. Appl. Phys. Lett. 1975, 27, 275. (c) Hersh, H. N.; Kramer, W. E.; McGee, J. H. Appl. Phys. Lett. 1975, 27, 646. (9) (a) Dyer, C. K.; Leach, J. S.J . Electrochem. SOC.1978, 125, 23. (b) Reichman, B.; Bard, A. J. J. Electrochem. SOC.1980, 127, 241. (10) (a) Buckley, D. N.; Burke, L. D. J . Chem. SOC.,Faraday Trans. 1 1975, 71, 1447. (b) Gottesfeld, S.;McIntyre, J. D. E.; Beni, G.; Shay, J. L. Appl. Phys. Lett. 1978, 33, 208. (11) (a) Arnoldussen, T. C. J . Electrochem. SOC.1976, 123, 527. (b) Rabelais, J. W.; Colton, R. J.; Guzman, A. M. Chem. Phys. Lett. 1974, 29, 131. (c) Colton, R. J.; Guzman, A. M.; Rabelais, J. W. J . Appl. Phys. 1978, 49, 409. (d) Dickens, P. G.; Birtill, J. J. J . Electron. Mater. 1978, 7 , 679. (12) (a) Gottesfeld, S. J . Electrochem. SOC.1980, 127, 272. (b) Burke, L. D.; O'Sullivan, E. J. M. J . Electroanal. Chem. 1978, 93, 11. (13) (a) Natan, M. J.; Mallouk, T. E.; Wrighton, M. S. J . Phys. Chem. 1987, 91, 648. (b) Natan, M. J. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1986. (14) (a) White, H. S.; Kittlesen, G. P.; Wrighton, M. S. J . Am. Chem. Soc. 1984,106, 5375. (b) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J . A m . Chem. SOC.1984, 106, 7389. (c) Kittlesen, G. P. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1985. (15) Paul, E. W.; Ricco, A. J.; Wrighton, M. S.J . Phys. Chem. 1985.89, 1441. (16) (a) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J . Phys. Chem. 1985,89, 5133. (b) Thackeray, J. W. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1986. (17) (a) Kittlesen, G. P.; White, H. S.;Wrighton, M. S. J . Am. Chem. Soc. 1985, 107, 7373. (b) Kittlesen, G. P.; Wrighton, M. S. J . Mol. Electronics 1986, 2, 23. (!8) Leventis, N.; Natan, M. J.; Schloh, M. 0.; Wrighton, M. S., unpublished results. (1 9) Sze, S. M. Physics of Semiconductor Deuices; Wiley: New York, 1981.
1836 The Journal of Physical Chemistry, Vol. 91 No. 7 , 1987
Natan et al.
~
The current densities used for galvanostatic deposition of Ni(OH), on SnO, were too low to control with accuracy on microelectrode arrays; for this reason, deposition on microelectrodes was performed using potentiostatic control. Adherent films were obtained by holding the potential of the microelectrodes between -0.75 and -0.80 V vs. SCE for 60-100 s in aqueous solutions containing either 0.01 M Ni(NO,),, 0.1 M Ni(N03),, 0.1 M Ni(N0,),/0.1 M KNO,, or 0.05 M Ni(N03),/0.05 M Na(N0,). After an initial spike, the cathodic current decreased during the deposition, indicative of formation of an insulating film of Ni(OH), on the microelectrodes. Ni(OH)* could be deposited selectively onto individual microelectrodes by holding adjacent electrodes, onto which Ni(OH), is not deposited, between 0.2 and Experimental Section. 0.3 V vs. SCE. When there was no decrease in the current during the deposition, the electrochemistry of the resulting films was Preparation of Sn02 Electrodes. F-doped SnO, and In-doped “poor” in that well-formed cyclic voltammograms for the Ni(OH), SnO,(ITO), used for optical measurements, were obtained from * NiO(0H) interconversioncould not be detected. In some cases, commercial sources. The optical transmittance of F-doped S n 0 2 the deposition procedure was performed twice before electrowas about 80% in the visible region. Transmittance spectra showed chemistry of Ni(OH), was observed. It has been observed that modulations due to optical interference from the SnO, film. The properties of Ni(OH), electrodes are critically dependent upon sheet resistivity of F-doped SnO, was 14 ohms/square as deterdeposition conditions.2-6 The conditions used here for micromined by four-point probe measurements. F-doped SnO, elecelectrodes typically produce films of Ni(OH)2 approximately trodes were cleaned electrochemically prior to Ni(OH)2deposition 0.5-1 .O pm thick. SnO, elecbrodes used for optical measurements by passing anodic, cathodic, and then anodic current at 1 mA/cm2 could also be derivatized in this manner. for 30 s each in 5 M KOH, and In-doped SnO, was cleaned by The electrochemical properties of films prepared with different successive sonication in H 2 0 , 2-propanol, and hexane. electrolyte solutions do not vary significantly. With 0.01 M Preparation of Microelectrode Arrays. The microelectrodes used in these experiments were of a design previously des~ribed.’~*’~ Ni(N03), as the electrolyte, films of Ni(OH), containing both 1y and 0 phases are occasionally formed.24 In such cases the waves Each chip consists of an array of eight parallel Au or Pt micollapse to a single peak after repeated cycling. Judging from croelectrodes on an insulating layer of Si3N4, surrounded by the location of the redox wave, the a phase is ultimately formed. macroscopic contact pads. The microelectrodes are 50 pm long, The best films are obtained when K N 0 3 or N a N 0 3 is added to 2.4 pm wide, and 0.1 pm thick and are separated by 1.2 pm. In the electrolyte; sharp single peaks are observed for both oxidation some cases, an additional layer of Si3N, was deposited to insulate and reduction of the Ni(OH)2 films obtained in such cases. the entire device except for the actual eight-wire array and the The derivatized microelectrodes were rinsed in triply distilled contact pads. When Si3N4was not used, the devices were insulated H 2 0or pH 12 C032-/C03H- buffer and characterized electrowith epoxy. The packaged microelectrode assemblies were then chemically in 1 M KOH. cleaned with an rf 0, plasma etch (150 W) for 5-8 min to remove Equipment. In situ transmittance measurements were obtained any residual photoresist or epoxy from the microelectrodes. The with a Bausch and Lomb Spectronic 2000 spectrophotometer. The microelectrodes were then cycled individually at 200 mV/s 4 or spectroelectrochemical cell was fashioned from a standard poly5 times each from -1.6 to -2.1 V vs. SCE in a 0.05 M pH 7 styrene cuvette with a I-cm path length. Time-dependent phosphate buffer. Hydrogen evolution at the negative potential transmittance at 500 nm was measured by a single-beam speclimit further cleans the microelectrodes and ensures reproducible trophotometer with components from Photon Technology Interelectrochemical behavior. The microelectrodes were then tested national, Inc., and included a Model HH 150 high-efficiency arc by examining their behavior in an 0.2 M LiCl solution containing lamp source with a 150-W xenon lamp (Osram) and water filter, 5 mM Ru(NH,)~’+. A well-defined current-voltage curve at 50 monochromator, sample compartment, and photomultiplier unit mV/s for the reduction of Ru(NH,):+ is characteristic of a “good” with a 1P28 photomultiplier tube (RCA). Switching measuremicroelectrode.” ments were recorded on a Busch Mark 200 strip chart recorder. Reagents. NaOH, KOH, LiOH, CsOH, NaNO,, K N 0 3 , Time-dependent optical absorbance at 500 nm was measured by KH2P0,, K2HP04,Na2C03, NaHCO,, and LiCl were used as a Hewlett-Packard 845 1A rapid scan spectrometer. Electroobtained from commercial sources. R u ( N H , ) ~ C ~and , Ni(N03), chemical instrumentation consisted of Pine Instruments RDE 4 (99.99%) were used as obtained from Alfa. High-purity 2bipotentiostats with Kipp and Zonen BD 91 X-Y-Y’-T recorders propanol and hexane were used. The H20used for all experiments for microelectrode experiments and PAR Model 173/ 175 or was Omnisolv HPLC grade. Unless stated otherwise, all basic Model 273 potentiostat/programmers with a Houston Instruments solutions contained KOH as the base. None of the solutions were Model 2000 X-Y recorder for S n 0 2 experiments. Thickness degassed. measurements were made using a Tencor Instruments surface Deposition of Ni(OH), onto Sn02 and Microelectrode Arrays. profiler. Flowing streams were produced and delivered to miSnO, electrodes for transmittance experiments were derivatized croelectrodes by the pumps of a Hewlett-Packard Model 1084-B with Ni(OH), by using a literature technique that was optimized.4a liquid chromatograph. Optical micrographs of derivatized miThe electrodes were immersed in 0.01 M Ni(N03),, and cathodic croelectrodes were obtained with a Polaroid camera mounted on galvanostatic deposition was performed at 0.04 mA/cmZ for 8 min a Bausch and Lomb MicroZoom optical microscope. to obtain the desired thickness. Adherent films of Ni(OH)2 were produced. The potential of the working electrode was found to Results and Discussion be approximately -0.7 to -0.8 V vs. S C E during the deposition. Characterization of Cathodically Deposited Ni( OH), on OpThe thickness of an 8-min deposition film was measured to be tically Transparent Electrodes and on Microelectrode Arrays. approximately 0.1 pm. Thin films of Ni(OH)2 cathodically deposited onto conducting, optically transparent F-doped SnO, were characterized by cyclic (20) Wrighton, M. S.;Thackeray, J. W.; Natan, M. J.; Smith, D. K.; Lane, voltammetry and optical measurements in 1 M KOH. The films G. A,; B€langer, D. Philos. Trans. R.SOC.London, Ser. E , in press. were prepared by galvanostatic deposition from solutions of 0.01 (21) Thackeray, J. W.; Wrighton, M. S.J . Phys. Chem. 1986, 90, 6674. M Ni(NO,),. The relationship between the cyclic voltammetry (22) Belanger, D.; Wrighton, M. S., submitted for publication in Anal. NiO(0H) to Ni(OH)2, causing a change in I,. Ni(OH)2-based microelectrochemical transistors can thus function as pH sensors in strongly basic solutions, where the oxide is very durable. Several pH-sensitive microelectrochemical transistors have recently been described,,O including those based on WO3,l3 on platinized poly(3-methylthi0phene),~~on ferrocyanide-loaded, protonated poly(4-~inylpyridine),~~ and on a viologen/quinone-based polymer.,, One important difference between Ni(OH)2 and the other active materials in pH-sensitive transistors is that Ni(OH), is especially rugged at pH 14, where none of the other materials can be employed, but it is unstable in acidic media, where the other devices operate.
Chem. (23) (a) Smith, D. K.; Lane, G. A,; Wrighton, M. S.J . A m . Chem. SOC. 1986, 108, 3522. (b) Smith, D. K.; Lane, G. A,; Wrighton, M. S., in prepara tion.
(24) (a) Bode, H.; Dehmelt, K.; Nitte, J. Electrochim. Acta 1966, 11, 1074.
Ni(OH),-Based Microelectrochemical Transistors '
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1837
The Journal of Physical Chemistry, Vol, 91, No. 7, 1987
I
Cyclic Voltammetry and Relative Transmittance a t 5 0 0 n m VI Potential at pH14 f o r a O I p m F i I m o f N i ( 0 H ) z o n Sn02
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,80% of its maximum ID in C2 s. While the magnitude of ID is limited at fixed VD by the resistance and, therefore, VG,the rate at which a particular ID is attained is determined by rate of faradaic processes in the gate circuit. Electron motion within the oxide is not thought to be limiting in the electrochemistry of Ni(OH), films. Rather, slow diffusion of protons in and out of the films limits the rate of charge transport.27 The proton diffusion coefficient, DH,during oxidation and reduction has been found to be 3.1 X 1O-Io and 4.6 X lo-” cm2/s, respecti~ely.~’Surprisingly, upon stepping to 0.1 V vs. SCE, ZD returns to zero in less than 2 s, although DH for reduction is smaller than for oxidation. However, charging and discharging are not symmetric processes, and this behavior has been noted with other oxides. For example, reduction of W 0 3 films is faster than oxidation of H,W03.25 We have obtained reproducible ZD-VG characteristics during the course of the characterization, a period of about 1 h. Evidence that Ni(OH)2-based microelectrochemical transistors function as electrical power amplifiers is illustrated in Figure 5, which shows the magnitudes and relationships of VG,ZG,and ID, for a slow triangular potential variation between 0 and 0.45 V vs. SCE. In these experiments, three adjacent, Ni(OH),-connected microelectrodes are used; ZG is recorded for the center micro-
N lo7 to lo4 ohms, as VGis varied from 0.0 to 0.7 V vs. SCE. These data were collected starting at 0 V vs. SCE and moving positive; some hysteresis is observed when measurements are initiated from 0.7 V, in accord with the separation of the oxidation and reduction waves for the Ni(OH), film. The measured resistance of different samples may vary from the data in Figure 3 by up to an order of magnitude, but the change in resistance for a particular sample never varies by more than 4 orders of magnitude. The resistance of Ni(OH),-connected microelectrodes in the oxidized, conducting state is much higher than for reduced W0313or oxidized conducting organic polymer^'^-'^ derivatized on microelectrodes. In the insulating state, the resistance, as for Wo3,I3is lower than for the conducting organic polymers.14-16 The low resistance of this semiconducting oxide in the “insulating” state may be a consequence of the high concentration of doping impurities in solution. A manifestation of the sharpness of the oxidation wave for Ni(OH),, compared to other redox active conducting materials, is the slope of the resistance-VG plot in Figure 3. Nearly the entire 3 order of magnitude change in resistance occurs between VG= 0.3 and 0.4 V vs. SCE, the sharpest change in resistance found to date for any conducting material derivatized onto microelectrodes. The resistance of Ni(OH),-connected microelectrodes per se is not an especially meaningful parameter, since it can only be referenced to the resistance of other redox active electronic conductors derivatized on closely spaced microelectrodes of the same geometry. Unlike the resistance, R,the resistivity, p, defined in eq 2, is a characteristic property of a material. In eq 2, 1 is R = p(l/A) (2) the length of a uniform conductor and A is its cross-sectional area. Assuming complete uniformity of oxidized Ni(OH), films, an approximate calculation of the resistivity can be made by using the well-defined microelectrode geometry. For typical Ni(OH),-based devices, the 50-pm-long microelectrodes are covered with a 0.5-1 .O-pm-thick film. The length is taken to be the 1.2-pm spacing between microelectrodes. Calculations reveal that, in the conducting state, the resistivity of cathodically deposited Ni(OH), is 20-40 ohmcm. This value is 5 or 6 orders of magnitude higher than for elemental metals and about equal to the resistivity of highly doped single-crystal semiconductors. The sample to sample variability in resistance is probably a result of differences in film
(28) Lofton, E. P.;Thackeray, J. W.; Wrighton, M. S. J . Phys. Chem. 1986,90, 6080.
The Journal of Physical Chemistry, Vol. 91, No. 7, 1987 1839
Ni(OH),-Based Microelectrochemical Transistors ’ h o s e R e l a t i o n s h l p B e t w e e n VG,
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