2844
Anal. Chem. 1986, 58, 2844-2847
ACKNOWLEDGMENT We thank J‘ Munger9 Engineering Science, California Institute of Technology, for collecting and supplying the fogwater samples.
(23) McDowell, W. L.; Dasgupta, P. K. Atmos. Environ. 1984, 18, 2209-2216. (24) McDowell, W. L.; Dasgupta, P. K., unpublished results, Texas Tech University, 1983. (25) Paul, K. R.; Gupta, V. K. Atmos. Environ. 1983, 17, 1773-1777. (26) Dasgupta. P. K. Atmos. Environ. 1984, 18, 477-478. (27) Haugland, R. P. Molecular Probes Handbook of Fluorescence Probes; Molecular Probes: Junction City, OR, 1965. (28) Nara, Y.; Tsuzimura, K. Bunseki Kagaku 1973, 2 2 , 451-452. (29) Takahashi. H.; Nara, Y., Tsuzimura, K. Agric. Biol. Chem. 1978, 4 0 , 2493-2494. (30) Takahashi. H.; Nara, Y., Tsuzimura, K. Agric. Biol. Chem. 1978, 4 2 , 769-774. (31) Machich, M.; Takahashi, T.; Itoh, K.; Sekine, T.; Kanaoka. Y. Chem. Pharm. Bull. 1978, 2 6 , 596-604. (32) Nara, Y.; Tsuzimura, K. Agric. Biol. Chem. 1978, 4 2 , 793-798. (33) Takahashi, H.; Nara, Y.; Meguro, H.; Tsuzimura, K. Agric. Biol. Chem. 1979, 4 3 , 1439-1445. (34) Takahashi, H.; Yoshida. T.; Meguro, H. Bunseki Kagaku 1981, 30, 339-34 1. (35) Takahashi, H.; Nara, Y.; Yoshida, T.; Tsuzimura, K.; Meguro, H. Agric. Biol. Chem. 1981, 4 5 , 79-85. (36) Meguro, H.; Takahashi, C. Anal. Lett. 1983, 16(A20), 1625-1632. (37) Ruzicka, J.; Hansen, E. H. Flow Injection Analysis; Wiley: New York, 1981. (38) Richards, L. W.; Anderson, J. A,; Blumenthal, D. L.; McDonald, J. A,; Kok, G. L.; Lazrus, A. L. Atmos. Environ. 1983, 17, 911-914. (39) Munger, J. W.; Jacob, 0. J.; Hoffmann, M. R. J . Atmos. Chem. 1984, 1 , 335-350. ( 4 0 ) Munger, J. W.; Tiller, C., Hoffmann, M. R. Science 1986, 2 3 1 , 247-249. (41) Grosjean, D.; Wright, B. Atmos. Environ. 1983, 17, 2093-2096. (42) Jacob, D. J.; Waldman, J. M.; Haghi, M.; Hoffman, M. R.; Flagan, R. C. Rev. Sci. Instrum. 1988, 5 6 , 1291-1293.
LITERATURE CITED Adams. D. F.; Farwell, S. 0.; Robinson, E.;Pack, M. R.; Bamesberger, W. L. Environ. Sci. Technol. 1981, 15, 1493-1498. Sze, N. D.; KO, M. K. W. Atmos. Environ. 1980, 14, 1223-1239. American Public Health Association, Intersociety Committee Methods of Air Sampling and Analysis, 2nd ed.; APHA: Washington, DC, 1977. Dasgupta, P. K. Atmos. Environ. 1984, 18. 1593-1599. Dasgupta, P, K.; McDowell. W. L.; Rhee, J . 4 . Analyst (London) 1986, 11 I , 87-90. Tanner, R. L.; Markovits, G. Y.; Ferreri. E. M.; Kelly, T. J. Anal. Chem. 1986. 5 8 , 1857-1865. Dasgupta, P, K.; Gupta, V. K. Environ. Sci. Technoi. 1986, 2 0 , 524-526. Hwang, H.; Dasgupta, P. K. Anal. Chem. 1988, 5 8 , 1521-1524. West, P. W.; Gaeke, G. C. Anal. Chem. 1956, 2 8 , 1816-1819. Fed. Regist. 1971, 36(84), 8187-8191. Dasgupta, P. K.; DeCesare. K.; Ullrey, J. C. Anal. Chem. 1980, 5 2 , 1912-1922. Dasgupta. P. K.; DeCasare, K. B. Atmos. Environ. 1982, 16, 2927-2934. Kok, G. L.; Gitlin, S. N.; Lazrus. A. L. J . Geophys. Res. 1986, 91. 2801-2804. Deister, U.; Neeb, R.; Helas, G.; Warneck, P. J . Phys. Chem. 1986, 9 0 , 3213-3217. Dong, S.; Dasgupta. P. K. Atmos. Environ. 1986, 2 0 , 1635-1637. Dasgupta, P. K. Air Pollut. Contr. Assoc. J. 1981, 3 1 , 779-782. Genfa, 2.;Dong, S. Fenxi Huaxue 1984, 12, 418-420. Kok, G. L.; Gitlin, S. N.; Gandrud, B. W.; Lazrus, A. L. Anal. Chem 1984, 5 6 , 1993-1994. Pearce, A. G. E. Histochemistry, 3rd ed.; Little Brown; Boston, MA, 1968; Vol. 1, Chapter 13. Miksch, R. R.; Anthon, D. W.; Fanning, L. 2.;Hollowell, C. D.; Revzan, K.; Glanville, J. Anal. Chem. 1981, 5 3 , 2118-2123. Irgum, K. Anal. Chem. 1985, 57, 1335-1338. Dasgupta, P. K. Anal. Chem. 1981, 5 3 , 2084-2087.
RECEIVED for review April 21, 1986. Accepted July 14, 1986. We gratefully acknowledge the support of the Electric Power Research Institute through RP 1630-28 for making this work possible.
Measurement of Electron Diffusion Coefficients through Prussian Blue Electroactive Films Electrodeposited on Interdigitated Array Platinum Electrodes B. J. Feldman’ and Royce W. Murray* Kenan Laboratories
of
Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514
Electron mobility In electroactive polymers, measured as the dlffuslon coefflclent D,, can be measured by deposltlng the polymer over fingers and Insulating gap of an Interdigitated array electrode (IDA) and adjusting the electrode potentials so as to oxidize and reduce the polymer fllm at opposing, adjacent finger electrodes. A parallel plate theory for De and the llmlting current flow through the polymer fllm in the IDA gap Is evaluated with Prusslan Blue as the exemplary electroactlve material. The theory accurately describes experlmental behavior for electrodeposited films thick enough to give unlform coatings within the 2.5-pm gap but thinner than the 2.5-pm gap.
Of special interest is the coating of such electrodes with electroactive films. Wrighton and co-workers have shown that microarray electrodes coated with conducting organic polymers such as poly(pyrro1e) ( I , 2 ) and poly(viny1ferrocene) (5)mimic the behavior of solid-state diodes (5)and transistors (1-4) and can act as chromatographic detectors (4). We have shown (8) that an interdigitated array (IDA) electrode coated with Prussian Blue exhibits understandable voltammetry and electrochromism in gas-phase media. Wohltjen and co-workers (IO, 11) have also described gas phase sensor experiments with solid-state “chemiresistors” constructed by applying Langmuir-Blodgett films of nickel phthalocyanines to IDA’S. Coated microarray electrodes and IDA’S additionally find important applications to fundamental electron transport studies, including conductivity as a function of potential for conducting organic polymers (1-4) and the environmental (solvent, ions) dependence of electron hopping rates in crystalline Prussian Blue (8). Whether an IDA coated with an electroactive polymer is used for practical or for more theoretical purposes, a proper measurement of the electron conductivity of the polymer is
Microlithographically defined microarray (1-5) and interdigitated array (6-21) electrodes have been recently shown to have impressive utility in electrochemistry and in sensors. ‘Present address: IBM Almaden Research Center, 650 Harry Rd., K34/802, San Jose, CA 95120-6099. 0003-2700/86/0358-2844$01.50/0
C
1986 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 58, NO. 13. NOVEMBER 1986
denirahle. Electroactive materials that contain mobile counterions can be characterized hy a dc electron conductivity, u. (Sad),or an electmn diffusion d i c i e n t , De (a2 s-'), which is related (12) to sethrough p (F m-9,the redox capacitance per unit volume. Determination of uaor De from IDA limiting currents is not, however, straightforward because the "open faced" IDA geometry cannot he treated as simply as earlier electrode/film/electrode sandwiches (13, 14). We recently presented (6) a theoretical model (eq 3, vide infra) for the limiting current flowing between two parallel plate electrodes separated by distance d and contacting a polymer film of thickness t in the interelectrode gap. This model is attractive because it allows determination of De without independent measurement of the electrode area, polymer film thickness, or concentration of the electroactive material. Our aim in thispaper is 2-fold (i) to describe experimental techniques for verifying that film thickness and homogeneity conform to the standards required for use of eq 3 and, (ii) to determine the range of film thicknesses over which eq 3 is usefully applicable. To this end we have used a combination of electmchemistry, step profdometry, and optical and electmn microscopy on IDA's coated with varying thicknesses of Prussian Blue. This electroactive material was chosen b e c a u s e of the ease of deposition of uniform films of known concentration and density. EXPERIMENTAL SECTION Fabrication and mounting of IDA's and Prussian Blue deposition onto them have been described previously (6). Electrochemistry was performed with an ARDE 3 hipotentiostat (Pine Instruments) in a glass fritted, N, degasahle threecompartment cell using Pt auxilsry and SSCE reference electrodes. All electroehemical potentials are reported vs. the NaCI-saturated saturated calomel electrode (SSCE). D. was evaluated through two electrochemical measwementa. First, the charge, Q, required to reduce the Prussian Blue film from its blue Fe(III/II) form to the clear Fe(II/II) form was determined. This was done hy integrating the cyclic voltammetric Fe(III/II) Fe(lI/II) peak obtained by m n i n g the potentials of both terminals of the IDA simultaneously from 0.5 V to 4 . 2 V vs. SSCE at 5 mV/s. Secondly, we measure the steady-state current, i, generated hy seanning the potential of one IDA terminal from 0.5 V to 4 . 2 V at 5 mV/s while maintaining the other terminal at 0.5 V. The limiting current ill. of the resulting voltammogram is used with Q to obtain De from eq 3. Profilometry on IDA'S was performed with an Alpha-Tencor 1M) step profiler with 12.5-pm stylus at a scan rate of 0.01 mm/s. SEM micrographs were obtained with an IS1 Model DS-130, and optical microscopy was performed with a Zeiss universal microscope.
-
RESULTS AND DISCUSSION
IDA D , Equation. The interdigitated array (IDA) electrode we have used (6)is constructed of 3.5 pm wide, 0.3 p m high (h) Pt finger electrodes which are separated by 2.5 pm wide insulatinggaps (d)of borosilicate g h substrate. Twenty Pt fingers emanate from one contact pad and 21 from the other. (The contact pads are masked during electrochemical experiments.) An SEM micrograph showing two (of the total of 41) fingers of an uncoated IDA is shown in Figure 1A. Coating of the IDA fingers and gaps with a film of Prussian Blue of thickness t, is illustrated in the schematic cross-sectional diagram in Figure 1B. The electrochemistryof Pmian Blue has been studied (15, 16) and includes a well-defined, Fe(II/II) reduction wave centered a t reversible Fe(III/II) 0.2 V M. SSCE in KNO, electrolyte. We will briefly describe the derivation (6)of the IDA De equation to aid in clarity of presentation. If the potentials of both of the coated IDA terminals are scanned through the Prussian Blue reduction, the charge collected, Q, is given hy
-
Q = nFpNltC
(1)
2845
A
B
-..-
d -
W I. (A) SEM micrograph ( n m l incidence. magnificafbn6600) an uncuated IDA. 161 Schematic diaaram of Prussia; Blue co&d interelectrode gap; d ='2.5 pm. h = 032 pm.
F
showin0 IArw finaers of t = 0.12-2.38 pm.
where n and F have the usual significance, p is the center to center electrode spacing (6 pm), N is the number of finger electrodes (41), 1 is the finger length (1-2 mm), and C is the concentration of electroactive Fe(III/I) sites in the Prussian Blue. The product pN1 represents the projected area of the electrodeposited coating. Equation 1 assumes that the Prussian Blue deposit is uniformly thick over fingers and gaps and does not significantly extend beyond the immediate area of the IDA. When the potential of one IDA terminal is swept through the Pruenian Blue Fe(III/II) Fe(II/II) reduction while the other terminal is maintained a t 0.5 V. a steady-state voltammogram is developed in which Prussian Blue is reduced and reoxidized a t opposite finger electrodes (6, 8). If the elect r o d e / P m i a n Blue/electrode assemhly is approximated hy multiple parallel plate electrodes of height t, separated by a film filled gap of dimension d , then application of Fick's first law to the electron diffusion problem gives the steady-state limiting current, ih, as
-
ili, = nF(N - l)tlCD,/d
(2)
We believe eq 2 is more appropriate than an alternative model (5)that substitutes the f w e r height, h, for t in eq 2, or another model (9)that contains a multiplicative geometrical constant ( k ) for which no exact theory is available. combination of eq 1and eq 2 gives the IDA D. equation
De = i,dpN/Q(N - 1)
(3)
Film Homogeneity. The fmt assumption in the derivation of eq 1, that the film is uniform and homogeneous, was examined with optical and electron microscopy and hy step profilometry. Figure 2A shows an SEM micrograph of a Prussian Blue coated IDA viewed down the finger axis at 70° from normal incidence. The cracks in the coating are incidental and are a consequence of f h shrinkage associated with drying in the lod torr SEM environment. The SEM indicates that the thin, 0.68 pm thick Prussian Blue film is relatively evenly deposited over both Pt fingers and gaps. Figure 28, an SEM micrograph of the edge of the same array confirms that the electrodeposited film is confined to the array boundary (as evidenced hy cessation of the cracking pattern) and does not spread very far onto the surrounding borosilicate glass, so that the product plN in eq 1reasonably represents
2846
ANALYTICAL CHEMISTRY, V M . 58. NO. 13, NOVEMBER 1986
a
A
0
t (urn)
1
I
3 '
0
crroy edge ,e
Flpure 2. SEM mlnographs 01 Russian Blue coated IDA. film thickness atop Fi finger is 0.68 pm: (A) 70' from normal lncldence. magntrkation 8300; (6)m i krcldence, magnincamn 5610, showing outermost Pt linger 01 Russian Blue coated IDA.
A
2--
-t --
OS-=
I
-
Flgure 4. (A) I , (0) and D. (0)as a function 01 R w h n Blue thickness, for ths Fe(lIl1ll) Fe(IIII1) reductkm h 0.5 M KN03. pH 4. Opposite IDA terminals are potentlostated at 0.5 and -0.2 V. (B) Schematic diagram showing expected cross-sectional profiks of film at different thicknesses in regions A. B. and C (see text).
moved, the gap between two adjacent electrodes is increased to 8.5 pm, which is large enough (17) for interrogation by the profilometer stylus. SEM evidence also shows that melted fingers roll up into metal spheres 1-3 p m in diameter, leaving little residual metal in the gap.) These profilometer results quantitate the inference from the SEM data that the Prussian Blue is evenly coated over both fingers and gaps in the IDA
-...
I . .
In addition, the expected Prussian Blue thickness can be calculated ( I S ) from the electrochemically determined quantity of Fe(IlI/II) present and the lattice constant of the Prussian Blue crystal tdcd = Q(10.26 X
10")3N0/4nFA
(4)
where 10.26 A is the Prussian Blue lattice constant (19).NO is Avogardro's number, A is the macmcopic (papa plus lingers) cmz),n = 1, and F is the Faraday area of the array (1.54 x constant. The constant 4 arises from the presence of four electroreductively active Fe atnms in a h i a n Rlue unit cell. From Q = 36.8 pC for Figure BB, teal&is found to be 0.40 wn, in fairlv mod ameement with the Drofilometw results (0.46 and O.&-pm). Verification of Euuation 3. The easential wumDtion of eq 2, that the IDA &ent flux can be approximated as*flowing through a f h of height t and of width d, sandwiched between two parallel plate electrodes, leads to the prediction that De should he independent o f t . We tested this prediction by incrementally increasing the quantity of Prussian Blue deposited on a given IDA, measuring ill,, the steady-state FeFe(II/II) current which could he passed by the (III/II) deposited film, and calculating De a t each incremental film thickness. These results are shown in Figure 4A, where ih (0) and De ( 0 )are plotted vs. the charge Q extracted from the electrodeposited Prussian Blue film (as aasayed by cyclic voltammetry). iu, and De are also plotted vs. the corresponding calculated (eq 4) film thickness, tdd. We see that the limiting current ib (0) increases linearly with t d until t d c d approaches 1 pm and then levels off. D. ( 0 )increases somewhat with tdd a t very low coverages (tealed < 0.25 pm), then becomes nearly constant (0.25 pm < tealed< 1pm), and
-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
finally declines (tcdcd > 1 pm). The results of Figure 4A can be rationalized by invoking three thickness regimes, labeled A, B, and C in Figure 4A and schematically diagramed (to scale) in Figure 4B. In region A, the film deposited in the gap is so thin that there may be “low spots” or holes in the film, so that De is artificially depressed. In region B, the Prussian Blue deposit has achieved a uniform depth in the gap and yet is still thin enough that eq 2 is reasonably accurate, and now the calculated De is nearly constant as ideally predicted by eq 3. The boundary between region B and C is conceptually expected, but its exact location is somewhat arbitrarily chosen. In region C, the coating may still be uniform but is now so thick that the (solution) outermost parts of the film in the gap, which are most remote from the fingers, carry little current. Under this circumstance, eq 2 breaks down and De is again artificially depressed; this appears to occur in Figure 4A above about 1 pm. This experiment shows that reliable, constant De values can be obtained through use of eq 3 with our IDA for Prussian Blue films between 0.25 and 1pm thick. In fact, De changes by only a factor of 2 over the thickness range 0.12-2.38 pm, indicating that eq 3 is a reasonable approximation even over a much broader thickness range. Thus, an electroactive coating deposited to a depth greater than the finger height, h, but less than or equal to the gap width, d , can still carry considerable current. On this basis we believe eq 3 is to be preferred over the previous model (5),but approximate values can be obtained by either. Ideally, the De vs. film thickness curve should be evaluated for any new electroactive coating and IDA configuration since the thickness range boundaries of regions A-C of Figure 4A over which eq 3 works properly may vary with the particular materials. In practice, the range of useful film thickness may depend on the characteristics of the deposition process of the polymer (or crystal). Kinetic and spatial aspects of the electrodeposition of electroactive polymers have been documented only for a few materials. For example, we have observed (6) that deposition of quite thin coatings (0.1 pm) of poly[O~(bpy)~(vpy)~](Cl0~)2 (bpy is 4,4’-bipyridine, vpy is 4-vinylpyridine) onto the Pt IDA fingers is accompanied by deposition of polymer both over the gap and also over some 20 pm beyond the edge of the array’s outermost finger. In contrast, it is possible (2) in the deposition of poly(Nmethylpyrrole) to localize it atop one microarray electrode, without spreading across a gap which is only 1.4 pm wide to another electrode (2). (It has been demonstrated (3-5) that, in the course of electrodeposition a t one finger electrode, potentiostating adjacent finger electrodes at potentials inimical to polymer deposition will keep them free of polymer.) (A reviewer suggested that adventitious electroactive impurities dissolved in the contacting solution might enhance ik, because solution diffusion coefficients are so much larger than De. This is an interesting point, but charge flux involves concentration as well as diffusion coefficient terms, and film concentrations are, contraveningly, typically very large (1M or more). In the present case, for example, an unrealistically large concentration, ca. 1 mM, of a dissolved impurity with diffusion coefficient lo3 times larger than De would be required to produce a charge flux equivalent, on a cross-sectional basis, to that in the Prussian Blue film.) Gaining an improved fundamental understanding of microlevel spatial aspects of polymer film electrodeposition will
2847
be important both to the use of array electrodes in the study of De and to the design of polymer microstructures that involve junctions between different electroactive materials. A complete picture will have to account for a complex mixture of processes, which would include (1) the nature (mechanisms) of the electropolymerization process [possibilities include (i) surface localized addition of activated monomer to a growing polymer film, (ii) repeated solution-phase dimerization and oligomerization of activated monomers with subsequent nucleation and precipitation, and (iii) initiation of chain polymerization by a single activated monomer], (2) strong radial transport effects (edge diffusion) a t the edges of microscopically sized electrode elements of the array, (3) counterion influences on the solubility or insolubility of forming pofymer strands, and (4) morphological aspects (density, porosity) of the growing film. The shape of an electrodeposited film may depend on which process is dominant. For example, mechanism (i) may result in films that are thicker on the electrodes than in the gaps, whereas mechanisms (ii) and (iii) should result in greater uniformity. Controlling polymer electrodeposition will involve manipulation of the following experimental variables: electrode size and shape, monomer concentration, the solvent and electrolyte, and the nature of the potential/time program applied to the array electrode elements during electrodeposition. Thus far, desired film qualities have usually been obtained by semiempirical manipulation of the experimental variables; eventually a fundamental basis should emerge.
ACKNOWLEDGMENT Scanning electron microscopy assistance by R. Kunz is gratefully acknowledged. Registry No. Prussian Blue, 12240-15-2. LITERATURE CITED White, H. S.; Kittiesen, G. P.: Wrighton, M. S . J. Am. Chem. SOC. 1984, 106, 5375. Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. SOC. 1984. 106. 7389. PauL’E. W.’; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 8 9 ,
1441. Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 8 9 , 5133. Kittiesen, G. P.; White, H. S.; Wrighton, M. S . J. Am. Chem. SOC. 1985, 107, 7373. Chidsey, C. E. D. Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 58, 601. Chidsey, C. E. D.; Murray, R. W. Science 1986, 231, 25. Feidman, B. J.; Murray, R. W., unpublished work. Sanderson, D. G.; Anderson, L. B. Anal. Chem. 1985, 5 7 , 2388. Barger, W. R.; Wohltjen, H.; Snow, A. W. Proceedings of the International Conference on Sensors and Actuators-Transducers. 1985. Philadelphia, PA, June 1985. (11) Wohitjen, H.; Barger, W.; Snow, A.; Jarvis, N. L. I€€€ Trans. Electron Devices 1985, € 0 - 3 2 , 7. (12) Chidsey, C. E. D.; Murray, R. W. J. Phys. Chem. 1986, 9 0 , 1479. (13) Pickup, P. G.;Kutner, W.: Leaner, C. R.; Murray, R. W. J. Am. Chem. SOC. 1984, 106, 1991. (14) Feidman, B. J.; Burgmayer, P.; Murray, R. W. J. Am. Chem. SOC. 1985, 107, 872. (15) Neff, V . D. J . Nectrochem. SOC. 1978, 125, 886. (16) Itaya, K.; Uchida, I.;Neff, V. D. Acc. Chem. Res., in press. (17) Alpha -step Model 100 frofiler Manual, Tencor Instruments, Mountain View, CA, 1984 p 23. (18) Siperko, L. M.; Kuwana, T. J. Nectrochem. SOC. 1983, 130, 396. (19) Ludi, A.; Gudel, H. V. Struct. Bonding (Berlin) 1973, 1 4 , 1.
RECEIVED for review February 26,1986. Accepted July 9,1986. This research was supported in part by grants from the Office of Naval Research and the National Science Foundation.
