Conductivity of clay-modified electrodes: alkali metal cation hydration

Jun 1, 1990 - Hexacyanoferrate(III) Transport in Coated Montmorillonite Clay Films. Effects of Water-Soluble Polymers. Jean-Marie Séquaris. Langmuir ...
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4998

J . Phys. Chem. 1990, 94, 4998-5004

Conductivity of Clay-Modified Electrodes: Alkali Metal Cation Hydration and Film Preparation Effects Samuel A. Lee and Alanah Fitch* Department of Chemistry, Loyola University of Chicago, Chicago, Illinois 60626 (Received: July 5, 1989: In Final Form: November IS, 1989) The conductivity of clay-modified electrodes was examined and found to correlate with hydration effects associated with the electrolyte cation. The hydration effects were observable in a stepwise fashion and may confer upon these films voltammetric sensitivity to electroinactivealkali-metal cations. The type of substrate electrode was shown to affect the overall conductivity of the electrode when the clay films had an initial disordered state. I t was also shown that the film behaved as a nonideal ultramicroelectrode array.

Introduction Clay-modified electrodes have been investigated for their use as photoelectrocatalytic devices,’ for biconductive films,2 and for their synthetic utility (shape ~electivity,~,~ compartmentalization of reactant^,^ acid catalysis,6 and redox catalysis’). Despite the number of reports on their potential uses, the role of clay film structure on diffusivity is not well understood. In an earlier communication,s we suggested that the observed current for Fe(CN)63-at a clay-modified Pt electrode could be explained in terms of the face-face pore size of two interacting negative plates (Figure IA). In this paper, we more fully examine the factors that affect charge transport of an electroactive anion in these cation-exchange clay films: hydration and osmotic effects driven by the anion and cation composition of the electrolyte, electrolyte concentration, method of preparation, electrode surface charge, and film thickness. We explore the application of the film sensitivity to electrolyte in terms of an electrochemical sensor. Experimental Section K,Fe(CN),, CaC12,NaCI, KCI, CsCI, LiCI, Na2S04,K2IrCI6 (Aldrich), and 0.1 M potassium phthalate buffer (VWR) were used as received. Clay (SWy-I, University of Missouri at Colombia, Department of Geology) was prepared by stirring I O g of clay in 200 mL of distilled, deionized water for 48 h. The clay was centrifuged at approximately 500 rpm on a lab top centrifuge for 1 h, and the supernatant was collected, freeze-dried, and resuspended in distilled, deionized water at a concentration of 5 g/L. This procedure results, approximately, in a size fraction less than 1 p m 9 A Pt electrode (5 X I0-j cm2) sealed in glass and an edgeoriented pyrolytic graphite electrode ( 1 /8-in. diameter, Union Carbide) sealed in Torr Seal (Varian) were used as substrate electrodes. The Pt electrode was treated by polishing with I-pm alumina and sonicating in distilled water to remove the alumina and was dried with a wipe. The graphite electrode was treated by polishing with Buehler microcloth 40-7208 cloth and rinsing with deionized water. The clay films on Pt were prepared in three ( 1 ) (a) Kamat, P. V. J . Electroanal. Chem. 1984, 163, 389. (b) Ghosh, P. K.; Bard, A. J. J . Am. Chem. SOC.1983, 105, 5691. (c) Ege, D.; Ghosh, P. K.;White, J. R.; Equey, J.-F.; Bard, A. J . J . Am. Chem. Soc. 1985, 107, 5644. (d) Ghosh, P. K.; Mau, A. W.-H.; Bard, A. J. J . Elecrroanal. Chem. 1984, 169, 315. (e) White, J. R.; Bard, A. J . J . Elecrroanal. Chem. 1986, 197, 233. (2) (a) Castro-Acuna, C. M.; Fan, F.-R. F.; Bard, A. J. J , Elecrroanal. Chem. 1987,234, 347. (b) Carter, M. T.: Bard, A . J . J . Elecrroanal. Chem. 1987, 229, 191. (3) Yamagishi, A,; Aramata, A. J . Chem. SOC.,Chem. Commun. 1984, 452. ._

(4) Fitch, A.: Lavy-Feder, A . ; Lee, S. A,; Kirsh. M. T. J . Phys. Chem. 1988, 92, 6665. (5) Rusling. J . F.; Shi, C.-N.: Suib, S . L. J . Elecrroanal. Chem. 1988, 245, 131. . _.

( 6 ) Inoue, H.; Yoneyama, H. J . Elecrroanal. Chem. 1987, 233, 291. (7) Oyama, N.; Anson, F. C. J . Electroanal. Chem. 1986, 199. 461 (8) Fitch, A.; Fausto. C. L. J . Electroanal. Chem. 1988, 257, 299. (9) Tanner, C. B.; Jackson, M. L. Soil Sci. SOC.Am Proc 1947. 12. 60

0022-3654/90/2094-4998$02.50/0

ways. Type A electrodes were prepared by drying a I-pL solution of the stock (total of 5 pg of clay) on the Pt surface rapidly at 100 “C for 10 min, followed by a 5-min air-drying and cooling period. The total area covered by the clay was approximately 0.05 cm2, resulting in films of 100 pg of clay/cm2. The dry film . ~ thickness of these electrodes was approximately 0.3 p ~ n Type B electrodes were prepared by applying 1 pL of the 5 g / L clay stock solution to the electrode which was rotated at 500 rpm for I O min. The total area covered was approximately equal to the area covered in the fast dry method. Type C electrodes were layered versions of type A electrodes. A single 1 - r L aliquot of the stock SWy-1 clay was dried on Pt and then a second I-pL aliquot added, and so on, to result in a thicker, layered, film. The graphite electrode clay film was oven-dried in the same manner as the Pt electrode. Type A clay films for X-ray diffraction were prepared by either oven-drying for I O min at 100 “C or air-drying 0.5 mg of clay over approximately I .6 cm2 of a glass microscopic slide. This results in a film somewhat thicker than that obtained on the electrode surface (300 vs 100 pg/cm2). Type B films for X-ray diffraction were prepared by spin-coating the clay with a Headway Research photoresist spinner (Model l-EC101D-R485) at 1500 rpm for IO min. The total area covered was larger than for the oven-dried film, resulting in thinner films. All films were left exposed to ambient temperatures and moisture for up to 48 h prior to X-ray analysis. X-ray diffraction data were obtained on a Scintag/USA TAD V 20 instrument with a Co Ka source operated at 1.788 A. An EGG PAR 273 potentiostat/galvanostat was used. For variable scan rate studies, the data were digitized and exported to Lotus for background subtraction and plotting. Otherwise, an EGG PAR Model 0091 X-Y recorder was used to obtain the cyclic voltammograms. Scan rates of 50 mV/s were used for all experiments unless otherwise specified. Potentials were measured against the SCE electrode. Unless otherwise specified, the prepared electrodes were placed in an N,-purged blank electrolyte and the potential was cycled between the initial and final values, at 50 mV/s, for 5 min. Immediately following exposure to the electrolyte, the electrodes were transferred to an N,-purged electrolyte solution containing 4 mM Fe(CN),3- or IrC164-,and a cyclic voltammogram was obtained. The use of a set electrolyte exposure period implies that the results are a “snapshot” of some film structure attained after 5 min of swelling. After obtaining the cyclic voltammogram, the electrode was checked visually, and, occasionally, with an optical microscope, for loss of the clay film. In general, no peeling of the films was observed. This procedure (complete electrode preparation) was performed in triplicate. The peak current was measured from the extrapolated capacitive base-line current. The capacitive currents were never more than 0.05 pA when a scan rate of 50 mV/s was used. The peak currents varied from 0.05 to 1.5 FA. In order to make reasonable current measurements where the film is nearly insulating, very large currents would be required for measurements at the bare electrode and for the clay-modified electrode exposed to dilute electrolyte

0 1990 American Chemical Society

Conductivity of Clay-Modified Electrodes

The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4999

I

02-1 x

B

A

Figure 1. (A) Face-to-face (stacked) orientation of clay. (B) Edge-toface (house of cards) orientation of clay. Interlaver Spacine (A) conc, M N a+

TABLE I:

-

4-1 1 0.25

0.25 0.25

-

0

5.5" 9.5 30.5 C

0.5

K+

Ca2+

3.2" 3.2 3.2 5.5

9.6b 9.6 9.6 9.6

-

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-.

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. (

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*

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0.3 .

0.2

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strength solutions. Large currents could be obtained at scan rates faster than 50 mV/s; however, larger scan rates would also result in distortions due to resistance within the film and larger capacitive currents. A scan rate of 50 mV/s was used as a compromise. Results are plotted as the ratio of the average maximum reduction current obtained (peak or plateau) at the clay-modified electrode to the reduction peak current obtained at the bare electrode. Ratios less than one imply that the anion Fe(CN):- is hindered by the negatively charged clay from reaching the electrode surface.

Results and Discussion Alkali Metal Cation Hydration Driven Conductivity of Clay-Modified Electrodes. In a clay-modified electrode, charge transport of an electroactive species which does not interact specifically with the clay will depend upon the average size of pores within the film. The average pore size of the film depends upon structure of the clay film present during the electrochemical experiment. The conductivity of the clay film was measured by observing the ratio of the maximum reduction (peak or plateau) currents for Fe(CN),3- obtained at a clay-modified electrode to the current obtained, in the same electrolyte, at a bare electrode. Since the anion Fe(CN):- does not specifically interact with the negatively charged clay,I0 the currents should be controlled by the total pore space of the film. The presence of the film blocks a portion of the electrode surface and the maximum reduction current is diminished. Earlier results* suggested that a clay film formed by rapid drying of the clay on the surface of Pt, when exposed to a solution of concentrated N a + ion, exhibited conductivity that could be explained in terms of the pore size defined by the face-face interlayer spacing (Figure 1A) of individual platelets. If this hypothesis is correct, then it is possible that the overall conductivity of the film could be controlled via judicious choice of the electrolyte, as it is known that hydration of the intercalated cation plays an important role in the face-face interlayer distance between two clay platelets." In order to test this hypothesis, type A SWy-I clay-modified Pt electrodes were exposed to a variety of electrolyte solutions and the ratio of the maximum Fe(CN)63- reduction currents was plotted. In Figure 2A, data for LiCI, CaCI,, CsCI, and KCI are shown. For CaCI,, CsCI, and KCI, the current ratios obtained are nearly constant over the entire electrolyte range investigated. The current ratios obtained in the presence of KCI are nearly 0. The Cs+exposed clay is completely insulating over the entire concentration range. The ratios obtained for CaCI, are roughly invariant with electrolyte concentration and are approximately 0.1 5. (Data for [Ca2+]> 1 M could not be obtained due to saturation of the salt, CaCI2. I t is expected, however, that the ratio would remain at the 0.15 level in this regime.) These results are entirely consistent (IO) Liu, H.-Y.; Anson, F. C . J . Electrochem. Soc. 1985, 184. 41 I . ( I I ) Sposlto, G.; Prost, R. Chem. Reu. 1982,82, 554.

0

2

4

6

8

1

0

[Cation;M] -112 Figure 2. (A) Plot of the current ratio of the maximum reduction current for 4 mM Fe(CN):-obtained a t a type A (oven-dried) SWy-I Pt electrode as a function of concentration of ( X ) LiCI, (0)CaCI2, (+) KCI, and (0)CsCI. (B) Plot of the ratio of the maximum reduction current for 4 m M Fe(CN)$- obtained at type A (oven-dried) SWy-I and bare Pt electrodes as a function of the electrolyte concentration: ( X ) NaCl electrolyte; (+) N a 2 S 0 4electrolyte.

with the changes in face-face distance measured by X-ray spectroscopyfor well-ordered (stacked) films. In Table I, literature data for face-face interlayer distances are given as a function of the electrolyte species and concentration. The data are presented as interlayer spacings. (Basal spacing minus the clay thickness, 9.5 A, gives the inner layer, or face-face, spacing.) For K+ in Wyoming montmorillonite,'2 the interlayer spacing is quite small due to the ease of dehydration of K+. Similar behavior is expected for the Cs+ ion.'3914 The spacing, for K', increases only slightly from 3.2 to 4.4 8, in going from 4 M KCI to pure H 2 0 . The single spacing with electrolyte concentration is attributed to a single layer of water held within the interlayer.', The final spacing depends upon the prior treatment of the clay. For CaZ+,only a single interlayer spacing of 9.6 A is observed, which is attributed to a two-layer hydrate.'5b Unlimited osmotic swelling does not occur. I 5d The ratio data for LiCl and for NaCl and N a 2 S 0 4are shown in Figure 2, A and B, respectively. The data for LiCl mirrors that for NaCl and Na2S04,which are, in turn, similar to the results published earlier.s The close correspondence between the data obtained for LiCl and NaCl is consistent with the similar dehydration energies of these ions. For both ions, there is a region of nearly insulating behavior in high electrolyte, followed by a region in which there is a sharp rise in the conductivity of the film as each freshly prepared clay-modified electrode is exposed to a different dilute electrolyte solution. The results for NaCl and Na2S04indicate that there is a difference between the conductivity obtained in the presence of CI- and SO4,-. When CI- is the counterion, the films are more insulating (current ratio of 0.02) than when is the counterion (current ratio of 0.15). This is consistent with X-ray diffraction data obtained by NorrishI2 (12) Norrish, K . Trans. Faraday SOC.1954, 18, 120-134. (13) (a) Sawhney, B. L. Clays Clay Miner. 1972,20,93.(b) Van Olphen, H. Clays Clay Miner. 1966, 14, 393. (c) Rich, C. 1.; Lutz, J. A. Jr. Soil Sci. SOC.Am. Proc. 1965,29, 167.

(14)Schultz, L. G.Clays Clay Miner. 1969, 17, 115-149. ( 1 5 ) (a) Glaeser, R.; Mering, J. C.R. Hebd. Seances Acad. Sci. 1968,267, 436-466. (b) Suquet. H.; De La Calle, C.; Pezerat, H. Clays Clay Miner. 1975,23,1-9. (c) Mering, J. Trans. Faraday Soc. 1946,42B,205-219. (d) MacEwan, D.M. C.; Wilson, M. J. In Crystal Structures of Clay Minerals and Their X-ray IdentiJcation; Brindley, G. W . , Brown, G., Eds.; Mineralogical Society: London, 1980;p 197.

5000

The Journal of Physical Chemistry, Vol. 94, No. 12, 1990

TABLE 11: X-ray Diffraction Data as a Function of Clay Film Preoaration clay film" type A , oven-dried type A, air-dried type B

d, A

22.7 22.7 22.7 20.3

cps 970 800 1300 760

18.6

750

d , 8, I , cps 11.3 11.3 11.1 10.1 12.0 11.1 11.5 11.5 11.3

1950 1950 2000 2300 4800 2550 910 870 650

d.

A

3.1

I , cps

390 300 400 300 300 300

3.1

3.1 3.1 3.1 3.1

Film preparation is described within the Experimental Section. b l

= intensity.

in which, for oriented (stacked) clays exposed to concentrated resulted in larger interlayer electrolyte, the presence of Sod2spacings than in the presence of Cl-. (It is interesting to note that the face-face spacing differences observed in CI- and may relate to the ability of clays to strongly adsorb tris(diimine) complexes in the presence of SO,' in excess of the cation-exchange capaci t ~ . ~ , ' ~ ) The X-ray data (Table I) for Na+-exchanged clays show similarities to the ratio data in that there is a collapse of the interlayer distance in concentrated Na+ and an expansion of the interlayer distance on dilution of Na+. For Wyoming montmorillonite, in the presence of concentrated Na+, an approximate 5.5-8, interlayer spacing is observed.I2 A face-face interlayer spacing of 9.5 8, is observed from 1 > [Na] > 0.25 M. At 0.25 M Na+, there is a jump to an interlayer spacing of 30.5 8,. In more dilute solutions of Na+, the spacing increases steadily with continued dilution of the electrolyte. These spacing changes are attributed respectively to a single hydration layer and to two hydration and three hydration layers of the cation. Following the third layer of hydration, osmotic swelling is predicted for Na+. Li+, having a similar energy of hydration, will behave like Na+. In general, the X-ray diffraction data, as represented by Table I, beautifully predict the macroscopic conductivity of the clay film with respect to Fe(CN)?- (Figure 2). The current ratios obtained in the presence of 2 M Na+ (0.02) and in all concentrations of K+ and Cs+ (0) are all consistent with the larger h drationcontrolled interlayer spacing of 5.5 8, for Na+ vs 3.2 for K+. The ratio obtained for Ca2+ is 0.15 and corresponds to the dual hydration layer interlayer distance of 9.6 8,. The jump observed in the current ratio for Na+ is remarkably consistent with the ratio that can be predicted from the X-ray diffraction data for Ca2+ and Na+. The current ratio of Na+ at the end of the jump compared to the current ratio for Ca2+ (0.40/0.15 = 2.7) compares well with the X-ray diffraction data for swelling in the presence of the three-layer Na+ vs two-layer Ca2+hydrate (30.5/9.6 = 3.2). The only discrepancies between the X-ray data and the current data are (a) the lack of resolution of the first and second hydration layers for Na+ in the current data and (b) the lack of osmotic or unlimited swelling of the clay when exposed to dilute Na+ or Li+ electrolyte. These experimental results are also consistent with those of Itaya and Bard,)' who found that clays propped open with a hydroxy AI or Fe bridge (pillaring) had an interlayer distance of 7 8,. This distance resulted in exclusion of Fe(CN),j- from the clay film. They also found that currents for Fe(bpy),2+ were suppressed at the clay-modified electrode in the presence of K+. In dilute Li+ and Na+ the correspondence between the X-ray diffraction data and the current data fails. The presence of a limiting value of the ratio cannot be ascribed to the loss of the film since the film remains intact as observed visually and, in a prior study,* by scanning electron microscopy. The data can, however, be explained by reference to the initial state of the film as measured by X-ray spectroscopy (Table 11). Type B clay films

8:

(16) Traynor, M. R.; Mortland. M. M.:Pinnavia, T. J. Clays Clay Miner. 11978, 26. 318.

( i 7 ) itaya. K.: Bard, 4.J . J Phy.s. Chem. 1985. 89. 5565

Lee and Fitch (spin-coated) show only an expected X-ray diffraction peak associated with a single spacing for the smectite. This is consistent with a well-oriented film structure and is anticipated from the method of preparation. Spinning of the electrode, by analogy to centrifugation, causes platelets to slide across each other in a face-face orientation parallel to the substrate surface.'*a,b Films prepared by centrifugation, however, can exhibit layering of size fractions.18c Type A films (either oven- or air-dried) show the presence of additional peaks centered at 18-22 and 3.1 8,. These additional peaks have been observed e l ~ e w h e r eand ' ~ are attributed to randomness within the film. This disorder can be due either to variable face-face interlayer distances (Figure 1 A) arising from differences in hydration or to a more random film orientation which contains edge-face structure (Figure 1 B). Edge-face structure was first noted in viscosity measurements of clay gels in the 1950~.~OA dilute electrolyte clay suspension exhibits edge-face structure due to the association of positive edges of broken crystals and the negative face surfaces of clays. The rapid drying of this house of cards structure can result in the retention of the structure.21a I n the viscosity data, a strong electrolyte caused a conversion of edge-face gel structure to face-face gel structure. Very dilute electrolyte concentrations did not destroy the edge-face gel structure. Edge-face behavior has been noted for clay films, in which swelling pressures associated with cross-linking (edge-face structure) forces are noted.21b*cThe force of an edge-face bond was found to be IOe5 N.2'bWhen crosslinking is present, swelling will depend on the prior treatment of the clay. Montmorillonite clays treated with Calgon (polymetaphosphate) swell more extensively than untreated clays due to the adsorption of Calgon at the edge faces and resultant disruption of edge-face bonds21b In the same way, swelling at low pH is less than at high pH due to the positive charge of the edge Hysteresis effects in compression-decompression curves have been attributed to the conversion of a clay film containing edge-face cross-linking to a film containing predominately face-face structure.?Ie The presence of electrolyte can shield the positive edge surfaces from the negative face surfaces. This would predict that the extent of swelling, in the dilute electrolyte region, would, paradoxically, increase with increasing electrolyte. Furthermore, the time dependence of swelling would be electrolyte dependent. These predictions are borne out by the ratio data. In Figure 3, the time dependence of the current ratios are plotted. For the type A electrode (oven-dried) exposed to a dilute (0.02 M NaCI) electrolyte solution, the swelling period is long. A type B electrode (spin-coated), similarly treated, shows a much larger current ratio which is time independent (no evidence of swelling). As expected, the rate of swelling of the type A electrode increases as the electrolyte concentration increases, as noted by the more rapid increase in the measured current ratio when the electrode is exposed to 0.3 and 0.7 M NaCI. In 0.3 M NaCI. a current ratio maximum is rapidly reached and very minimally decreases over the 3 0 4 1 1 observation period. In 0.7 M NaCl electrolyte, there is a rapid increase in the current ratio. There is an additional trend to a final film state which results in a lower current ratio. In 0.7 M NaCI, following the initial increase in current ratio, there is a slow decrease in the current. In 2.0 M NaCI, low current ratios are immediately apparent. (18) (a) Brown, G. J . Soil Sci. 1953, 4, 229. (b) Brown, G.; Brindley, G. W. In Crysral Strucrures of Clay Minerals and Their X-ray Identijcarion; Brindley. G. W. Brown, G., Eds.; Minerological Society: London. 1980; Chapter 5. (c) Gibbs, R . J. Am. Mineral. 1965, 50, 1941. (19) Van Groos, A. F. K.; Guggenheim, S. Am. Mineral. 1984, 69, 872. (20) (a) Van Olphen. H. J . Colloid Sci. 1962, 17, 660. (b) Van Olphen, H. Discuss. Faraday SOC.1951, 11, 82. (c) Van Olphen, H. J . Colloid Sci. 1964, 313. ( 2 1 ) (a) Van Olphen. H. An lnrroduction ro Clay Colloid Chemisfry, 2nd ed.; Wiley: New York, 1977; p 27. (b) Norrish, K.; Rausell-Colom, J. A. Clays Clay Miner. 1963, 10, 123. (c) Cebula, D. J.; Thomas, R. K.; Middleton, S.; Ottewill, R. H.; White. J , W. Clays Clay Miner. 1979. 27, 39. (d) Kowell, D. I,. Soil Sci. 1965, 100. 340. (e) Warkentin, B. U : Bolt, (2. H.: Miller. R . D Soil Sei. Sac. 4 m . Proc. 1957, 21. 495.

Conductivity of Clay-Modified Electrodes

The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5001

,

: ;I 0.8

I

e =

*

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0

7

/

=

0.5

r=zxz=2

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Min Figure 3. Ratio of currents obtained at clay-modified and bare Pt electrodes for 4 mM Fe(CN),'- as a function of time. Type A (oven-dried) SWy-I clay in NaCI: (0)2 M; ( 0 )0.7 M, (0)0.3 M, and (e) 0.02

M electrolyte. ( X ) Type A (oven-dried) SWy-1 clay in 0.3 M CaCI,. Type B (spin-coated) SWy-I clay in 0.02 M NaC1. Error bars for 2 M NaCl and 0.3 M CaCI2are smaller than the symbol. (-)

There was no time dependence observed at all. This trend, at long times, to smaller current ratios is consistent with a model of approaching face-face plates. The rate of approach is electrolyte dependent as predicted from Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory.22 If one uses the cation-exchange capacity of SWy-I of 0.74 mequiv/g4 and a Hammaker constant J,23 the barrier heights for the approach for mica of 2.2 X of two platelets in 0.3 and 2.0 M NaCl can be calculated to be 5 X IO4 and 0 J/m2, respectively. Data obtained for 0.3 M CaCI2, unlike that obtained for 0.3 M NaCI, mirrored the data for 2 M NaCl in having no time dependence. This is again consistent with DLVO theory, as embodied in the Schulz-Hardy rule, which states that equivalent energy barriers to flocculation are achieved for monovalent vs divalent ions at concentration ratios of 100 to 1.6. Thus, 0.3 M CaC12 is equivalent to a concentration of 18.7 M NaCI. At the 5-min observation period, in Figure 3, the current ratio rises from a low at 2 M NaCl to a peak extending from 0.7 to 0.3 M NaC1, followed by a decrease in the current ratio at more dilute concentrations. This is the trend in data observed for LiC1, NaCI, and N a 2 S 0 4shown in Figure 2. These data (X-ray diffraction and the electrolyte-controlled time dependencies of the current ratio) all are consistent with a film that has an initial cross-linked structure. Such a structure will inhibit observation of the osmotic swelling of the film in the 5-min observation period used here. In order to determine whether the face-face spacing in the "osmotic" swelling region could be observed electrochemically, type B films were prepared. Type B films, formed by spin coating, are well ordered (Table 11) and show no time dependence in their swelling in dilute electrolyte (Figure 3). The current ratios obtained for the spin-coated electrode were compared to the low angle diffraction data for the basal plane spacing obtained by Norrish.I2 Excellent agreement was obtained as seen in Figure 4 where the data of Norrish are reproduced. What is most astonishing is the observation of all three hydration states as measured macroscopically by the current ratios and the close correspondence in the dilute electrolyte region to linearity (correlation coefficient, #, of 0.992). The stepwise expansion of the interlayer region from 5.5 to 9.5 to 30.5 A via mono-, di-, and trilayer hydration states in the presence of Na+ implies that size exclusion films can be easily designed and reversible controlled. Furthermore, the measurement of discrete hydration states via the ratio method suggests that surface hydration forces may be investigated. The "osmotic" swelling region was well observed with the ratio method. This confirmation of Norrish's data is of interest due to the difficult nature of the low-angle X-ray diffraction exper(22) Adamson, A . W. Physical Chemisfry ofsurfaces; Wiley: Chichester, 1982; p 245. (23) Israelachvili. J . N.; Adams, G. E. Narure (London) 1976, 262, 774.

0

2

4

6

8

10

[Cation;M] -1/2 Figure 4. (A) Plot of the ratio of the maximum reduction for 4 mM

Fe(CN),3- at type B (spin-coated) SWy-1 and bare Pt electrodes as a function of NaCl concentration. (B) Plot of the basal plane spacing of oriented clay platelets as a function of NaCl ( X ) and Na2S04(0)concentration. Data reproduced from ref 12. iment. Interlayer spacings of > 1 1 A are measured at angles approaching a grazing angle (28 < So). Background counts are high and, since the penetration depth of the X-ray beam decreases with smaller 28,lScthe analysis of beam intensities is complicated. Furthermore, the interpretation of data in this region is not s t r a i g h t f o r ~ a r d . ~ To ' ~ , obtain ~ ~ ~ the average interlayer spacing in the low-angle region, an ideal distribution function of spacings is computed. Several assumptions must be made with respect to order within the sample, to the average electron density due to water in the interlayer region, and to the total distance between As a result, Norrish's data have not been interacting extensively replicated. It is of interest to obtain data in this region in order to determine whether long-range swelling is the result of overlapping ion atmospheres of two interacting plates (and or whether swelling is therefore dependent on clay charge)12*21,25 the result of a long-range perturbation of water structure due to surface adsorption of water.24 Long-range swelling is of practical interest in the control of waste water seepage. In this respect, our simple ratio data represent an elegant confirmation of a difficult experiment. This also suggests that the current ratio method may provide an alternative probe to surface forces of clays and similar layered materials. Sensors and Switches Based on Molecular Sieving of Fe( C N 6 ) - . The results in the prior section indicate that the conductivity of the film to the anion Fe(CN):- can be controlled by (24) (a) Viani, B. E.; Low, P. F.; Roth, C. B. J . Colloid Inrerface Sci. 1983, 96, 229. (b) Low, P. F.; Margheim, J . F. Soil Sci. SOC.Am. J . 1979, 43, 473. (c) Low, P. F. Soil Sci. SOC.Am. J . 1980, 44, 667. (d) Low, P. F. Soil Sci. SOC.Am. J . 1981, 45, 1074. (25) (a) Langmuir, 1. J . Chem. Phys. 1938,6,873. (b) Schofield, R. K. Faraday SOC.Trans. 1946,428. 219. (c) Bolt, G. H.; Miller, R. D. SoilSci. SOC.Am. Proc. 1955, 19, 285. (d) Callaghan, I. C.; Ottewill, R. H. Faraday Discuss. Chem. SOC.1974, 57, 110.

Lee and Fitch

The Journal of Physical Chemistry. Vol. 94, No. 12, 1990

5002

T 0.1

u~ i ,

J

.-c0

m

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0 00 0

20

60

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80

100

Min Figure 5. Ratio of currents obtained at type A (oven-dried) SWy-1 clay-modified and bare Pt electrodes in response to a repetitive perturbation of electrolyte concentrations between 0.02 and 1 M NaC1. Larger ratios are obtained in the presence of 0.02 M NaCI, while the smaller ratios are obtained in the presence 1 M NaCI.

the choice of the electroinactive cation which controls the microscopic pore size within the film. The large change in conductivity with the Na+ concentration suggests that a reversible electrochemical switch could be devised which is controlled by the electroinactive cation. The response of the SWy-1 type A clay-modified Pt electrode was monitored in response to a repetitive variation of electrolyte between 0.02 and 1 M NaCI. Sequential square wave impulses were observed (Figure 5 ) . This observation indicates that a reversible change in the porosity of the film (from insulating to conducting) occurs. The switching time in response to a variation in the electrolyte concentration is immediate on the time scale observed, as is the subsequent response to the falling electrolyte concentration. N o degradation in the signal is observed over a five-cycle switching period. These results suggests that there may be good long-term stability of the electrode. The sensitivity of these reversibly swelling clay films to "electrolyte strength" is similar to electrolyte effects observed at polymer-modified electrodes.26 In those films, charge hopping between immobilized redox-active sites can be affected by the rigidity of the polymer film imparted by electrostatic cross-linking. The cross-linking is controlled, in part, by electrolyte strength and, in part, by the solvation of the intercalated ion. Nonideal Microelectrode Array Behavior of Clay-Modified Electrodes. An attempt was made to measure the average pore size within the film in situ. If the conductivity of the film with respect to Fe(CN),3- is controlled via micropore size, then the film should behave as a microelectrode array. Microelectrode array models2' can be used to determine the average pore size in the film for comparison with face-face spacings predicted from the hydration model. For a cylindrical microelectrode array, when the diffusion layer distance is larger than the distance between individual microelectrodes, overlapping diffusion layers result (regime A). The observed current is of the same order of magnitude as that obtained at a bare electrode with no surface coverage. There is, however, an apparent decrease in the rate constant for heterogeneous electron transfer. As the scan rate is increased, the diffusion layers decrease and each microelectrode operates independently (regime B). Typical microelectrode plateau-shaped cyclic voltammograms are obtained. The magnitude of the current is proportional to the summed area of the individual microelectrodes. As the scan rate is increased still further, the diffusion layer can become smaller than the dimension of an individual microelectrode and linear diffusion (peak-shaped voltammogram) is again observed (regime C). The current is related to the (26) (a) Lasky, S. J.; Buttry, D. A. J . Am. Chem. SOC.1988, 110, 6258. (b) Faulkner, L. R. Presented at the Chicago Section Electrochemical Society, Dec 1988. (27) (a) Amatore, C.; Saveant, J . M.; Tessier, D. J . Elecrroanal. Chem. 1983, 146, 37. (b) Amatore, C.; Saveant, J. M.; Tessier, D. J . Ekctroanol. Chem. 1983, 147, 39. (c) Sabatani, E.; Rubinstein, 1. J . Phys. Chem. 1987, 91,6663. (d) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J. Elecrroanal. Chem. 1982, 138, 65

0.8

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Conductivity of Clay-Modified Electrodes observed (Figure 6, center). Working with 0.7 M NaCI, with an electrolyte equilibration time of 30 min (to avoid time-dependent signals, Figure 3), a transition from peak- to plateau-shaped cyclic voltammograms was obtained as a function of increased scan rate. At all scan rates, the calculated diffusion layer exceeded the dimensions of the modifying layer, so the changes in voltammetric shape are attributed to changes in the microarray behavior. Further confirmation of microelectrode array behavior comes from inspection of the magnitude of the limiting current as a function of scan rate. When the voltammogram is plateau shaped, the limiting current at the plateau is27b

The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5003

616

.-0 c Q

m 0.2

0.0

ili, = FSc0D/0.6Ro(1 -

(2) where S is the surface area of the bare electrode and co is the bulk solution concentration. Equation 2 indicates ilimis independent of scan rate. As a consequence, as one moves from regime A to regime B, the peak heights should deviate from a linear function with Y ' / * to become independent of Y. Such behavior is noted (Figure 6, bottom). Despite the fact that the appropriate peak shape changes were obtained, there is a discrepancy in the expected peak heights. For true regime A behavior, the peak height should be similar to that obtained at the bare electrode. Since our peak heights, at low Scan rates, do not correspond to those at the bare electrode (Figure 6, bottom), we conclude that the geometric surface area sampled by the overlapping diffusion layers of the microelectrode array, in the high electrolyte region, is not the same as the underlying geometric area of the electrode surface. This suggests that the micropores are not uniformly distributed through the clay film. The geometric area of the array is therefore unknown, and a quantitative determination of Ro, 8, and R,, the pore size, via electrochemical methods is not possible. The results in this section confirm that the electrode operates as a microelectrode array as would be predicted from the hydration-controlled porosity model of the clay film. Consistent with the method of film preparation, however, the array is not uniform in coverage of the underlying electrode surface. Electrode Surface Effects. The question arises as to whether the effects observed are truly the result of the bulk film clay platelet face-face interaction. An alternative possibility is that the effects observed could be the result of the interaction of the clay charged face surface with the electrode charged surface. In order to elucidate the effect that the electrode surface plays, we compared the current ratios obtained at a 5-pg type A electrode at pyrolytic edge oriented graphite in NaCl (pH 5.8) and in NaCl in the presence of phthalate buffer (pH 3.71). (The presence of changing amounts of NaCl had no effect on the pH.) Edge-oriented pyrolytic graphite has been shown to contain quinone-like functionalities which can be protonated and deprotonated.28 At pH 5.8, the edge surfaces should be deprotonated and negatively charged. At pH 3.71, the quinone-like sites on the edge surfaces should be neutral or positively charged. The effect of this charge variation has been noted in the electrochemical behavior of positively and negatively charged proteins.29 Similar to negatively charged proteins, the negatively charged plates should interact more strongly with the electrode surface at the graphite electrode in the higher pH electrolyte. In order to account for the effect of pH on the edge charge sites of the clay30 and to account for (28)(a) Panzer, R. E.; Elving, P. J. Electrochim. Acta 1975, 20,635. (b) Panzer, R. E.; Elving, P. J. J . Electrochem. Sac. 1972, 119, 864. (c) Blurton, K. F. Electrochim. Acta 1973, 18, 869. (d) Dong, S.;Kuwana, T. J . Elecrrochem. SOC.1984. 131. 813. (e) Hoogvliet, J. C.; Van den Beld, C. M. B.; Van den Poel, C. J . J . Electroanal. Chem. 1986, 201, 11. (f) Randin, J. P.; Yeager, E. J . Electroanal. Chem. 1975, 58, 313. (9) Evans, J. F.; Kuwana, T. Anal. Chem. 1977,49, 1632. (h) Deakin, M.R.; Stutts, K. J.; Wightman, R. M. J . Electroanal. Chem. 1985, 182, 112. (29) (a) Armstrong, F. A.; Hill, H. A. 0.;Oliver, 9. N.; Whitford, D. J . Am. Chem. Sac. 1985, 107, 1473. (b) Armstrong, F.A,; Cox, P. A.; Hill, H. A. 0.;Lowe, V. J.; Oliver, 9. N . J . Electroanal. Chem. 1987, 217, 331. (c) Bowden, E.F.; Hawkridge, F. M.; Blount, H. N. J . Electroanal. Chem. 1984, 161, 355. (30) (a) Schoonheydt, R. A.; Pelgrims, J.; Heroes, Y.; Uytterhoeven, J . B. Clay Miner. 1978, 13, 435. (b) Velghe, F.;Schoonheydt, R. A,; Uytterhoeven. J . B.; Peigneur, P.; Lunsford, J. H. J . Phys. Chem. 1977, 81, 1187.

j

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-log [Cation;M] -112 Figure 7. Plots of the maximum reduction current ratios for 4 mM Fe(CN),"- at type A (oven-dried) SWy-I electrodes in NaCI. (A) Pt electrode in ( X ) pH 5.8 and (+) p H 3.7 (buffered). (B) Pyrolytic graphite electrode in (0) pH 5.8 and ( 0 )pH 3.7 (buffered). (C) Type A (oven-dried) SWy-1 Pt electrode in NaCl in (0)4 mM Fe(CN):and ( X ) 4 mM lrCl,"-.

the effect of the K+ present in the phthalate buffer, we performed the same experiments at the Pt electrode (Figure 7A). No differences were noted for the currents obtained at Pt for the two different pH values, indicating that any differences obtained at the graphite electrode are related to the surface of the graphite. For graphite (Figure 7B), in the high electrolyte region, there are no differences between the data obtained for the two p H values. Additionally, the data at graphite mirror the data at Pt with the exception of the larger current ratios obtained in 2 to 1 M NaCl (approximately 0.1 at graphite vs 0.05 at Pt). There is a significant difference in the currents for the two different p H values in the low electrolyte regime. Larger currents are obtained at the negatively charged surface (high pH). These data are consistent with the fact that adherence of negatively charged face surfaces to the electrode surface in dilute electrolyte would be poorer for the more negatively charged electrode, and thus, more electrode surface is available to the anion. The dependence of the observed currents in the low electrolyte region on the fixed electrode charge suggests that the observed current might also be dependent upon the applied electrode potential. In Figure 7C, the results for IRC162-/3-(EO = 0.87 vs SHE31a)are compared with the results for Fe(CN)63-/4- ( E o = 0.36 vs SHE31b) at a fast-dried SWy-1 Pt electrode in NaCI. There are no notable differences in the results. This suggests that the interaction between the clay platelet and the electrode surface (31)(a) George, P.; Hanania, G. I. H.; Irvine, D. H. J . Chem. SOC.1957, 3048. (b) Murray, R. C.; Rock, P. A. Electrochim. Acta 1968, 13, 969.

5004

The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 M

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Lee and Fitch as the layered thickness of the film was increased. The dry thickness of a single 5-pg layer of clay film was previously found to be 0.3 p m 4 If the clay layers add sequentially, the largest dry film thickness examined was 1.5 pm. Assuming that the diffusion coefficient, D , of Fe(CN),3- (7 X IOd cm2/s3*)is equal to that in solution (free diffusion within the film), the diffusion layer distance is estimated from ( D t ) 1 / 2where , t is the time required to scan from the formal potential for reduction to the switching potential (515 mV). At a scan rate of 50 mV/s, this distance is approximately 85 pm, so, for all thicknesses, the diffusion layer resides outside of the film. This implies that the change in film thickness does not affect the region sampled (clay vs bulk solution) by the electrochemical experiment. The hypothesis of dual structural domains predicts that, at low electrolyte concentrations, the bulk film pore size is large enough that the electrode surface/clay interface structure controls the observed current. As a consequence, there should be no dependence of the current on the number of layers applied to the surface, l2, as observed for the fast-dried SWy-I Pt electrode (Figure 8B). In high electrolyte concentrations, the hypothesis predicts that the face-face pore size in the bulk film controls the observed current. Leddy and Vandenborgh33 have shown that for an electrode surface containing small pores there is a distance, lZ, dependence for the current, so that increased number of layers should diminish the currents. As expected, the current diminished as the film thickness increased.

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Added Figure 8. (A) Proposed models for dual domain structure at fast-dried, low electrolyte exposure (left), and type A (oven-dried), high electrolyte exposure (right), clay-modified electrodes. In low electrolyte (left) the current is determined by the immediate interaction between the electrode surface and clay platelets (d,, I ] ) . In high electrolyte (right) the current is determined by the bulk film face-face structure (d,, 1,). (B) Ratio of currents obtained at type C (oven-dried) S W y - l / P t electrodes in 0.02 M (+) and 0.7 M NaCl ( X ) as a function of the total micrograms of clay added in 5-pg sequential layers to the electrode surface.

depends upon the fixed charges at the electrode surface which is consistent with results obtained for charged proteins at electrode surfaces.29 Effects of Film Thickness. In the low electrolyte region, the results indicate that, for type A electrodes, electrode surface structural effects are important. In the high electrolyte region, the results are indicative of clay face-face surface structural effects. This suggests that a dual domain model is operative (Figure 8A). When the diameter of the pore size determined by the face-face clay platelet interaction, d Z , is small (high electrolyte strength), the current is controlled by d2 and the length 12. When bulk film pore size is large (edge-face or expanded face-face structure in dilute Na+), the current is controlled by the immediate electrode surface coverage pore dimensions, d,, which has a fixed distance, I,. To test this hypothesis, we measured the effect of thickness of the clay film on the observed currents using type C electrodes. Sequential amounts of 5 pg of SWy-1 clay were oven-dried on a Pt electrode surface. The sequential addition method was used to ensure that the electrode surface/clay interface was similar even

Summary We have demonstrated the unique response of a clay-modified electrode to hydration effects associated with the electrolyte cations. The hydration effects are observable in a stepwise fashion and may confer voltammetric sensitivity of these films upon electroinactiue cations such as the alkali-metal cations (K+, Cs', Rb') as well as for NH4+. The films may also find application as sensors to solvent c o m p o ~ i t i o nand ~ ~ gases. The stepwise expansion of the face-face spacing might also be used to impart molecular sieve selectivity. These areas of research are under exploration. The effects were shown to arise from the bulk film structure. The effect of the substrate electrode has been shown to be important for films whose swelling is inhibited by an initial disordered structure. The films behave as nonideal microelectrode arrays. Acknowledgment. This research was supported, in part, by N S F Grant CHE-8707710. The X-ray diffraction data were obtained in the lab of Dr. Joe Stucki, Department of Agronomy, University of Illinois, Champaign-Urbana. We are grateful for helpful talks with Dr. Stucki and Dr. Steve Guggenheim, Department of Geology, University of Illinois, Chicago, on clay X-ray diffraction studies. Registry No. Fe(CN),, 13408-62-3; CaCI,, 10043-52-4; KCI, 744740-7; CsCI, 7647- 17-8; LiCI, 7447-41-8; N a 2 S 0 4 , 7757-82-6; IrCI6, 608 18-94-2; Pt, 7440-06-4. (32) Oesterling, T. D.; Olson, C. L. Anal. Chem. 1967, 39, 1546. (33) Leddy, J.; Vandenborgh, N. E. J . Electroanal. Chem. 1987, 235, 299. (34) Olejnik, S.; Posner, A. M.; Quirk, J. P.Clays Clay Miner. 1974, 22, 361.