Langmuir 1992,8, 650-657
650
Investigation of the Structure-Reactivity Relationship in the Pt/MxCdFe(CN)s Modified Electrode System Carmela H. Luangdilok, Douglas J. Arent, and Andrew B. Bocarsly' Department of Chemistry, Frick Laboratory, Princeton University, Princeton, New Jersey 08544-1009
Robert Wood ATT-ERC, Princeton, New Jersey 08540 Received June 24, 1991. I n Final Form: September 24, 1991 Cadmium ferrocyanide modifying layers on platinum electrodes are found to be polycrystalline by scanning electron microscopy and X-ray powder pattern analysis, with a typical crystallite size of 1bm. Such layers are electroactive, with cation exchange at interstial lattice sites tightly coupled to charge transport processes. Charge transfer dynamics are affected by both the intrinsic mobility of interstial cations in the [CdFe(CN)6I2-/-surface overlayer and cation-induced structural reorganization of the overlayer. Energy dispersive X-ray analysis of interfaces which have undergone triangular wave potential cycling (cyclic voltammetric) in supporting electrolytes containing various alkali cations shows that, in general, these cations are exchanged into the surface lattice interstial (mobile) sites. One important exception is the case of a sodium nitrate containing electrolyte in which Cd2+ions are found in a portion of these sites. Thismolecular configuration optimizesthe observed charge transfer dynamics. A comparison of variations in powder pattern data as a function of potential cycling time (multiple cyclic voltammograms) shows specific correlations between structural changes and electrochemical reactivity. Several researchers have observed for Prussian Blue type modified electrodes that electron transport is coupled to cation transportand that ion transport is strongly related to the surface microstructure. From the variation in cyclic voltammetric features of Prussian Blue modified electrodes measured in different supporting electrolytes, it was shown that cations of smaller Stokes radii penetrate more ea~ily.l-~ Recent work of Ikeshoji and Iwasaki on in situ X-ray diffraction measurements of Prussian Blue packed layers on electrodes4has provided strong evidence for dynamic structural changes occurring as ions penetrate into the Prussian Blue lattice during the redox reaction. In the cyclic voltammograms of [NiFe(CN)6I2-/- derivatized nickel electrodes, the redox potentials of the iron species have been found to vary linearly with the radii of the supporting electrolyte cation is cases where monovalent cations were em loyed.5-8 For this system, the observed shift of 600 mV/ radii has been associated with variations in the crystal field surrounding the iron centers due to shifts in the lattice parameters of the surface confined material induced by intercalation of the various alkali cations. In general, two types of ion-lattice effects have been suggested: ion mobility is a function of lattice channel size and, conversely, lattice structure can be modified by intercalated ions. The work described here deals with the investigation of the structure of the [CdFe(CN)612-/- surface derivative
g:
on platinum and its correlation with electrochemical reactivity. All the available electrochemical data suggest the existence of a cation-dependent structural reorganization of the [CdFe(CN)612-/-lattice.gHowever, no direct observation of this phenomenon has been achieved. Thus, the structural and morphological study of these derivatized electrodes is of value in order to clearly elucidate the influence of mobile interstitial ions on the overlayer structure. X-ray diffraction patterns were taken of the modified surfaces previously scanned in different electrolytes. Scanning electron micrographs and optical micrographs of these surfaces were also employed to correlate electrochemical behavior with surface morphology. X-ray diffraction powder patterns have previously been shown by severalgroups+l3to be a powerful technique to investigate the structure of electrode surfaces. One potential application of modified electrodes is their use as voltammetric ion sensor^.^^-^^ This approach differs from the widely used potentiometric ion selectiveelectrode (ISE),opening up the possibility for miniaturization and improved limits of detection. However, a detailed knowledge of the variation in electrochemical response with intercalation of cations into the surface overlayer is essential to the successfuldevelopment of this application. An understanding of the relationship between intercalated cation and overlayer structure, both on a micro- and
~~
* Author to whom correspondence ahould be addressed.
(1) Itaya, K.; Ataka, T.; Toahima, S.J. Am. Chem. SOC.1982, 104, 4767. (2) Itaya, K.; Uchida, I.; Neff, V. D. Acc. Chem. Res. 1986, 19, 162. (3) Crumbliss, A. L.; Slugg, P. S.; Morosoff, N. Znorg. Chem. 1984,23, 4701. ( 4 )Ikeshoji, T.; Iwasaki, T. Znorg. Chem. 1988, 27, 1123. (5) Sinha, S.; Humphrey, B. D.; Bocarsly, A. B. I n o g . Chem. 1984,23, 203. (6) Humphrey, B. D.; Sinha, S.;Bocarsly, A. B. J.Phys. Chem. 1987, 91, 586. (7) Humphrey, B. D.; Sinha, S.;Bocarsly, A. B. J. Phys. Chem. 1984, 88,7361. (8) Bocarsly, A. B.; Sinha, S. J. Electroanal. Chem. Interfacial Electrochem. 1982,137, 157.
(9) Arent, D. J. Thesis, Princeton University, 1987. (10) Chianelli, R. R.; Scanlon, J. C. J. Electrochem. SOC.1978, 125, 1563. (11) Dahn, J. R.; Py, M. A.; Haering, R. R. Can.J.Phys. 1982,60,307. (12) McKinnon, W. R.; Dahn, J. R. Solid State Commun. 1984, 52, 245. (13) Fleischmann, M.; Oliver,A.; Robinson, J. Electrochim. Acta 1986, 31, 899. (14) Herren, F.; Fischer, P.; Ludi, A.; Halg, W. Znorg. Chem. 1980,19, 956. (15) Ludi, A,; Gudel, H. V. Struct. Bonding (Berlin)1973,14, 1. (16) Day, P.; Herren, F.; Ludi, A.; Gudel, H. V.; Hulliger, F.; Givord, D. Helu. Chim. Acta 1980, 63. (17) Thomas, K. N., Baldwin, R. P. Anal. Chem. 1989,61,2594. (18) Deakin, M. R.; Byrd, H. Anal. Chem. 1989, 61, 290.
0743-7463/92/2408-0650$03.00/0 0 1992 American Chemical Society
PtlM,CdFe(CNh Modified Electrode System macroscopic level, provides a direct link between the electrochemicaland the analytical response of this system. The first proposed structure for a polynuclear transition metal cyanide, MAdMB(CN),I.xH20,was deduced by Keggin and Miles from Prussian Blue powder diffraction data.lg The model consisted of a three-dimensional polymeric network having alternating iron(I1) and iron(111) nuclei located on face centered cubic lattice sites, bridged by cyanide ligands (a NaCl Structure). Thismodel has become accepted as the basic structure of Prussian Blue and most of its analogues. The lattice parameter, UO, which is the FelLC-N-FellLN-C-Fe*l distance, is approximately 10 A. The open spaces in the cube (body centered or face centered cubic) formed by this lattice are occupied by water and cations as needed for charge balance. Several studies of the bulk material have shown that the cubic framework can undergo distortion. Noncubic structures have been reported for some transition-metal hexacyanometalatea. For instance, monoclinic symmetry was reported for Zn3[Fe(CN)&, and cobalt(I1) hexacyanoferrate(I1) containing no alkali cations was said to have tetragonal sy”etry.20s21 Until recently, all of these structural have concentrated on bulk phase^.^"^^ These studies have shown that the MANCMB framework is highly defective. A complicated and variable stoichiometry is typical for this classof compounds due to vacancies a t the MAsites. The observed structure is found to be strongly dependent on the synthesis conditions. A key concern in the proposed solid-state structural models of transition-metal cyanides is their ability to explain the widely recognized cation-exchange properties of these materials. In general, cations are exchanged from the interstial, body centered, or face centered locations. For example, cobalt(I1)hexacyanoferrate(II) has been used for separations of trace amounts of cesium from urine and of 13’Cs from nuclear waste solid^.^^?^^ Most investigations of the bulk structure have dealt mainly with the influence of transition metal ions, sited in the lattice or the properties of the formed crystallite^.^^ Ceranic20was one of the early researchers who realized that the mobile interstitial ions can play a structural role in ion exchange reactions. This observation is essential in explaining the observed selectivity for monovalent cations. In a separate study, V ~ l ’ k h i nanalyzed ~~ the zeolitic nature of mixed ferrocyanides of a number of transition metals including cadmium (19) Keggin, J. F.; Miles, F. D. Nature 1936, 577. (20) Ceranic, T. Z. Naturforsch., B Anorg. Chem., Org. Chem. 1978,
336,1484. (21) Cola, M.; Valentini, M. T. g. Inorg. Chem. Lett. 1972,8, 5. (22) Buser, H. J.; Schwarzenback, D.; Peter, W.; Ludi, A. Inorg. Chem. 1977,16, 2604. (23) Beall, G. W.; Milligan, W. 0.; Korp, J.; Bernal, I. Inorg. Chem. 1977,16, 2715. (24) (a) Maer, K.; Beasley, M. L.; Collins, R. L.; Milligan, W. 0. J.Am. Chem. SOC.1968,90, 3201. (b) Shriver, D. F.; Shriver, S. A.; Anderson, S. A. Inorg. Chem. 1965,4,725. (c) Brown, D. B.; Shriver, D. F. Znorg. Chem. 1968, 7,77. (d) Brown, D. B.; Shriver, D. F. Znorg. Chem. 1969, 8, 37. (25) Shriver, D. F. Struct. Bonding 1966,1, 32. (26)Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition Metals; Academic Press: New York, 1976. (27) Chadwick,B. M.; Sharpe, A. G. Advances inznorganic Chemistry and Radiochemistry;Academic Press: New York and London, 1966; Vol. 8, p 83. (28) Kuznetsov, V. G.;Popova, Z. V.; Seifer, G. B. Russ. J.Znorg. Chem. 1970,15, 956. (29) Wolberg,A. Acta Crystallogr., Sect.& Struct. Crystallogr. Cryst. Chem. 1969, B25, 161. (30) Boni, A. L. AnaZ. Chem. 1966,38, 89.
(31)Prout, W. E.; Russell, E. R.: Groth, H. J. J. Inorg. Nucl. Chem. 1965, 27, 473. (32) Vol’khin, V. V.; Schulgar, M. B.; Zilberman, M. V. Zzu. Akad. Nauk SSSR, Neorg. Mater. 1971, 7, 77.
Langmuir, Vol. 8, No. 2, 1992 661 ferrocyanide and concluded that if the crystal structure is incompatible with the size of the exchanging cations, the ferrocyanides hardly interact with the cations at all, However, in a number of cases ion exchange can lead to a change in the lattice constants of the ferrocyanides or even to lattice rearrangement in accordance with the requirement of the absorbed cation. Unit cell volume, the presence of lattice defeds, and the interaction between the exchange cations and the water molecule framework of the lattice were cited as important parameters determining the ion-exchange property. Experimental Section Sample Preparation. Platinum foil electrodes were derivatized with [CdFe(CN)61Z-/-using a procedure similar to that employed by KuwanaS3for the formation of [CuFe(CN)6]*-/-on SnOz and Schneemeyer for the production of [NiFe(CNIel2-/on platinum. Prior to derivatization,the platinum electrodewas cleaned by treatment in 0.5 M HzSO, for 10 s and rinsed with deionized water. The electrode was then placed in an aqueous solution containing -4 X M Cd(NO& and stepped from -0.5 to -1.0 V vs SCE. After 30 s at -1.0 V vs SCE, the electrode was linearly scanned (100 mV/s) to 0.0 V vs SCE. This process was repeated until a reproducible i-E plot was obtained. A cadmium-containing deposit was generated by holding the electrode at -1.0 V vs SCE for times varying from 30 s to 2 min. After being rinsed in distilled water, the electrode was soaked in 0.05 M K3Fe(CN)8for times varying from 6 min to several hours to produce the [CdFe(CN)#-/- derivative. Prior to X-ray diffractogramsbeing taken, the electrodes were rinsed in deionized water and dried. In order to investigatethe effect of the various supporting electrolyte cations, the derivatized foils were divided into three parts and separately cycled in 1M NaNOa, 1M KNOa and 0.5 M CSNO3 aqueous electrolytes, respectively. Authentic,bulk cadmium ferrocyanidesampleswere prepared by mixing a 2:l molar ratio of Cd(N0)3)2and K$‘e(CN)ein water and washing the precipitate formed with water several times. Electrochemistry. All experiments were carried out in a standard three-electrode cell employing an SCE reference electrode and a large area platinum counterelectrode. Cyclic voltammogramswere recorded directly on a Houston Instruments 2000 X-Y recorder using a PAR 173 potentiostat/PAR 175 universal programmer. X-ray Powder Patterns. A Phillips-Norelco X-ray diffractometer or a Syntag PADV powder X-ray diffractometer fitted with a copper anode operating at 40 kV and 20 mA was used to obtain X-raydiffractograms. The derivatizedelectrodewas taped to a glass slide, placed in the sample area, and scanned from 20 = 10 to 70 at 1 deg/min. A 5-8time constant, 100 counts/s and 0% suppression were employed. The platinum substrate diffraction peaks were used as an internal standard. In most cases, the foils were rotated 90° and the diffraction pattern was again taken. Electrodes were placed in the reduced state prior to Xray analysis. The Lazy Pulverix program was employed to calculate the theoretical X-ray diffraction pattern. By we of this program combined with the Appleman and Evans Unit Cell Refinement Program, the lattice parameters as well as symmetry that would best fit the observed diffractionpattern were obtained. Scanning Electron Microscopy (SEM). SEM data were obtained using an Amray 100 instrument operating with an accelerating voltage of 20 kV. A Tracor Northern 2000 energy dispersiveX-ray (EDX)system provided elementalanalysis. Prior to SEM analysis samples were sputter-coatedwith 75 A of gold. Control experiments indicated that this process did not distort the surface morphology. The Au coating was found to minimize electron beam damage to the surface.
Results and Discussion Structure Determination. Scanning electron micrographs of Pt electrodes held at reducing potentials in a (33) Siperko, L. M., Kuwana, T. J.Electrochem. SOC.1983,130,396. (34) Schneemeyer, L. F.; Spengler, S.E.; Murphy, D. W. Znorg. Chem. 1985, 24, 3044.
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652 Langmuir, Vol. 8, No. 2, 1992
1
3
I sI
I Figure 1. Representative scanning electron micrograph of platinum electrode derivatized with Cd(0H)Z.
Cd(N03)2electrolyte show a polycrystalline deposit consistingof hexagonal platelets (Figure 1). X-ray diffraction data show these polycrystals are composed of Cd(OH)2. This is not surprising since oxidation of Cd metal into the white hydroxide readily occurs in neutral or basic aqueous solution.35 Distinct changes in the SEM image are observed upon soaking a Cd(OH)2 modified electrode in a &Fe(CN)6 solution (Figure 2). At short reaction times (Figure 2a), the quantity of hexagonal Cd(OH)2 platelets is observed to decrease with the formation of ill-defined microcrystallites. Diffuse reflectance Fourier transform infrared (FTIR) analysis of such surfaces indicates the appearance of vibrational transitions in the 2020-2070 cm-l region. These spectral features have previously been associated with36the bridging cyanide stretches of surfaceconfined Cd [Fe(CN)6]2-/-. More prolonged reaction times lead to the formation of distinct interfacial crystallites having an orthorhombic appearance (Figure 2b) and the completeabsence of structures indicativeof Cd(OH)2.The SEM data are indicative of complete conversion of the surface layer to cadmium ferrocyanide. Consistent with this conclusion,EDX analysis of these surfaces shows the presence of cadmium, iron, and potassium. The diffraction pattern of a derivatized Pt foil taken immediately after prolonged soaking in the &Fe(CN)6 solution and washing with deionized water is shown in Figure 3a. This diffraction pattern is found to be almost identicalto the diffractogramobtained from bulk cadmium ferrocyanide which was formed by mixing a 2:l molar ratio of Cd(N03)2 and &Fe(CN)6 in aqueous solution. However, the broad feature observed between 28 = 20" and 25" is absent in the authentic bulk sample. This feature signals the presence of some amorphous material in addition to a crystalline phase on the "as prepared" electrode. As shown in parts b and c of this figure, this broad diffraction peak disappears upon cycling the electrode potential through the FeI1/Fe1I1 redox potential independent of the electrolyte employed. We, therefore, do not associate this feature with the electrochemical response of the system. Presumably, the redox cycling process causes crystallization of the initially amorphous material. The pattern shown in Figure 3a could not be indexed (35) Hampson, N. A.; Latham, R. S. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1973; Vol. 1, p 165. (36) Rubin, H.D.;Arent, D. J.; Humphrey, B. D.; Bocarsly, A. B. J. Electrochem. Soc. 1978,125,1563.
Figure 2. Scanning electron micrographs of platinum electrodes derivatized with [CdFe(CN)6l2-/-. The micrographs were obtained prior to any electrochemicalcycling. After a short reaction time in a 5 mM &Fe(CN)s solution a combination of hexagonal Cd(OH)2 crystallites and ill-definedK[CdFe] (CN)6particles is observed (part a). Longer reaction times lead to well-defined crystallites of K[CdFe(CN)6] exhibiting an orthorhombichabit (part b). No unreacted Cd(OH)2 is apparent.
to satisfy a cubic system.37 However, the d values obtained are consistent with an orthorhombic structure. The best fit to the diffractionpattern was observedwhen a primitive orthorhombic lattice was assumed with lattice parameters a = 10.1 A, b = 10.9 A,and c = 9.5 A. Table I compares the power pattern data to the theoretically expected Xray diffraction pattern. Assignment of the hkl values to the diffraction pattern in Figure 3a was based on this simulation. Note that in the orthorhombic system, the interplanar spacing equation37 is -=1 d2,,,,
h2+E+t a2 b2 c2
The surface overlayer formed upon derivatization must not have a perfect orthorhombic symmetry as judged by the absence of some reflections. The intense (200) platinum substrate peak in the region around 28 = 46" obscures some 01the reflections. The expected cadmium ferrocyanide reflections at 28 = 27.5", 38.5", and 40" associated with the (131), (113), and (313) planes may be absent due to their low intensity and the limited signal to noise ratio available. More puzzling are the absence of (37) Azaroff, L. V.; Buerger, M. J. The Powder Method in X-ray Crystallography; McGraw-Hill Book Co., Inc.: New York, 1958.
Langmuir, Vol. 8, No. 2, 1992 653
R I M , CdFe(CN)6 Modified E 1ect rode System
40
35
30
25
20
30
25
20
1
b) Na
40
40
35
30
35
28
25
Figure 4. EDX data for the cadmium ferrocyanide electrode cycled in 0.5 M KN03, 0.5 M CsN03 supporting electrolyte.
20
Figure 3. Diffraction patterns of cadmium ferrocyanide GZrivatized platinum electrodes: (a) taken immediately after derivatization without electrochemical redox cycling, (b) after cycling in 1 M NaN03 supporting electrolyte until no further increase in the cyclic voltammogram was observed, (c) after at least 20 scans in 1 M KN03. All the scans were initiated at 0.0 V vs SCE with a scan rate of 100 mV/s. Table I. Experimental and Calculated Diffraction Pattern Results Based on a Primitive Orthorhombic Latticea theoretical observed 28, deg intensity, intensity, Miller indices (hk0 (10.25') arb. units arb. units 111 020 200 002 220 022 202 131 311 222 040 400 240 004 133 420 313
15 16.5 17.5 18.5 24 25 25.5 27.5 29.5 30.5 33 35.5 37.5 38 38.5 39.5 40
3.0 5.4 5.4 5.4 3.9 3.2 2.5 0.6 0.3 1.9 1.6 1.2 1.6 2.2 0.2 1.1 0.1
3.4 0.86 3.2 0.8 0.3 1.0 1.9
1.5
a The calculated pattern is based on the Lazy Pulverix program with Q = 10.1 A, b = 10.9 A, and c = 9.5 A. Normalized to the calculated pattern using the (022)peak.
peaks associated with the (020), (002), (222), (240), and (004) reflections. While the (020) and (002) reflections may be mixed with the (200) reflection,the other reflections should be observable. Likewise, the ability to observe the very low intensity reflection associated with the (311) plane is surprising. In general, the observed intensities to not correlate well with that predicted, suggesting that preferential ordering is occurring. This is consistent with the SEM data presented in Figure 2 which shows that the crystallite long axis tends to lie perpendicular to the plane of the electrode. Such ordering would account for both the peak intensity pattern observed and the missing reflections. In addition, there are features in Figure 3a which suggestthat other crystalline planes may be present
to a limited extent. For example, the scattering centered at 28 = 33' is extremely broad and asymmetric, suggesting the presence of multiple reflections in this region. Such reflections are not expected for an orthorhombicstructure. Influence of Cation on Structure. EDX analysis of modified electrodes which have been potential cycled through the surface confined iron redox potential in the presence of either K+or Cs+ions indicatesthe intercalation of these ions into the interfacial layer. Typical EDX data for intercalation employing an electrolyte containing both K+ and Cs+ are shown in Figure 4. In this case, both ions are observed to enter the cyanometalate lattice. Potential cycling of a modified interface in an electrolyte containing Na+ yields a very different result from that observed for the cations mentioned above. In this case, EDX analysis does not provide a signal for sodium. While this may be associated with the small EDX cross section for Na+, the ratio of the cadmium to iron EDX signals is found to increase by a factor of 2.4 in this system when compared to either the K+ or Cs+ (or mixed K+/Cs+) intercalated interfaces, suggesting that Na+ supporting electrolytes produce a population of mobile Cd2+ ions rather than mobile Na+ ions in the [CdFe(cN)6l2-/-lattice. We have recently presented chemical evidence supporting this conclusion.38 Intercalation of various cations into the [CdFe(CN)J 2lattice is found to have little effect on the position of the diffraction features (see Figure 3);however, major changes in diffraction intensity are observed. This is most pronounced for the case of K+intercalation (Figure 3c) where increases in the scattering features at 28 = 17.5' (d = 5.1 A) and 28 = 35.4' (d = 2.5 A) are found to dominate the diffractogram upon redox cycling of the electrode. Redox cycling of a modified electrode in a NaN03 supporting electrolyte (Figure 3b) also introduces relative intensity shifts of the diffraction features (as discussed below) are observed. The lack of observableshifts in reflection planes suggeststhat the lattice parameters are relatively invariant with cation intercalation, although the intensity variations indicate structural changes are occurring. (38) Hildalgo-Luangdilok,C.; Wood, R.; Bocarsly, A. B. J. Phys. Chem. 1990, 94,
654 Langmuir, Vol. 8, No. 2, 1992
Luangdilok et al.
0.6
0.0
1.0
Figure 5. Typical cyclic voltammetric responses of [CdFe(CN)6I2-/-derivatized Pt foil electrodes in different supporting electrolytes: (a) 1 M KNO,; (b) 0.5 M CsNOa; (c) 1 M NaN03.
Interaction of Surface Microstructure andcharge Transfer Processes. The nature of the supporting electrolyte and, therefore, the cation resident in the [CdFe(CN)J2-/- overlayer can strongly affect both the energetics and dynamics of interfacial charge transfer.58 A key concern in this study is whether or not the features identified in the structural analysis can be attributed to the observed change in the electrochemistry. For example, the possibility that the observed electrochemistry is associatedwith only a small fraction of the surface material must be considered. If this is the case, the actual electroactive material may not even be detected using SEM and EDX techniques. Integration of cyclic voltammogram area with respect to time yields the coverage of electroactive material. On the average, we find that electrode coverages require a thickness of slightly under 1 pm. A SEM survey analysis based on data similar to that given in Figure 2 (edge on images were also evaluated) suggests a typical crystallite size of slightly less than 1 pm with 80-100 7% surface coverage. Thus, the electrochemical results must pertain to the material being imaged by SEM. Since the SEM results mirror those obtained by X-ray, we concluded that the structural and electrochemicalanalysis are being carried out (for the most part) on the same molecular population. Representative cyclic voltammetric responses of [CdFe(CN)612-/-derivatizedplatinum foil electrodes in different supporting electrolytes are shown in Figure 5. These results are very nonideal in the presence of either K+ or Cs+supporting cations, while a Na+ supporting electrolyte provides a relatively ideal voltammetric response. Similar to the previously reported behavior of surface-confined [NiFe(CN)612-/-,the iron half-wave potential ( E 1 4 shifts positive as the supporting cation radius increases (Figure 6). Although typically Ell2 values are directly equated with standard redox potentials (ER') for surface-confined complexes, this practice is questionable in the present case for K+or Cs+supporting electrolytes due to the high degree of nonideality presented by the cyclic voltammetric data. V ~ l ' k h i nhas ~ ~ previously reported ERO values for bulk K,CdFe(CN)s (+0.58 V vs SCE) and bulk Cs,CdFe(CN)G (+0.80 V vs SCE) using redox titration techniques. A comparison of E1p values reported here with Vol'khin's E R O values indicates reasonable agreement. It is, therefore, concluded that surface confinement of cadmium ferro-
O
D
7
-
.
+
cs
.'L,
I 0 1.00
;
I . 40
,
;
,
;
1.80
;
,
;
.
;
2. 20
IONIC R A D W (A0)
Figure 6. Variation in half-wave potential of surface attached [CdFe(CN)612-/-with the alkali cation in the supporting electrolyte. A linear relationship between the surface redox potential and the Pauling univalent radii of the alkali cation is observed.
cyanide does not significantlyperturb the associatedcharge transfer thermodynamics. Additionally, the nonideality of the cyclic voltammetric results apparently does not invalidate in the present case the equality of E112 and ERO. In all supporting electrolytes utilized, a time evolution of the cyclicvoltammetric response is observed on repeated cycling. In K+-containing electrolytes, a net apparent decrease in electroactive surface confined [Cd[Fe(CN)61s/is observed as illustrated in Figure 5 and Table 11. A similar result is obtained when a CsN03 electrolyte is employed. In distinction, voltammetric cycling in a NaNO3-containing electrolyte (generatingmobile interstial Cd2+in the surface overlayer) causes an apparent increase in the amount of electroactive surface confined material (see Figure 5c and Table 11). In all cases, the change in the apparent surface coverage is not associated with the physical loss (or gain) of material from the electrode surface, since changing the supporting electrolyte cation in the absence of dissolved ferricyanide can reverse the observed trend. For example, as shown in Table 11,addition of CsN03 to an electrolyte containing only KN03 causes an apparent growth in the amount of surface confined electroactive material. For all systems examined, repeated voltammetric cycling
PtlM,CdFe(CN)G Modified Electrode S y s t e m
Langmuir, Vol. 8, No. 2, 1992 655
Table 11. Electrochemically Monitored Surface Coverage*
with nearest neighbor (sitesite) interactions.40Decreasing peak width can be traced to attractive nearest neighbor interactions. This type of variation is consistent with an cationb increased degree of crystallinity in the overlayer. The Na+ 13 29.9 10 diffraction data given in Figure 3 confirms this correlation. K+ 5.6 2.5 10 A comparison of the intense (400) reflection for the unK+ 5.6 0.90 32 cs+ 24 18 10 cycledversus 1M N d 0 3 electrolyte cycled electrodeyields 14.9 50 cs+ 24 a slight decrease in the diffraction peak width indicating K+/Cs+ 0.44 12.2 15 increased crystallite size. Increased crystallinity is exObtained by integration with respect to time of the area under pected to lead to better definition of the zeolitic channels, the anodic cycle voltammetric peak. In all cases the electrolyte was thereby enhancing supporting cation transport through composed of the nitrate salt(s) of the indicated cation. An ionic the cadmium ferrocyanidelattice. This electrochemically strength of 1 M was maintained. Electrolyte composition is 0.5 M induced structural variation is expected to both bring more KN03 and 0.5 M CsN03. iron centers into redox communication with the electrode and increase the charge transfer diffusion coefficient as Table 111. Apparent Diffusion Coefficients for the Oxidation of [ C ~ F ~ ( C N ) E . I ~ is observed. The electrochemical variations observed in multiple cationa Do1/2/d,bs-112 DO: cm2/2 cyclic voltammetric scans are found to be accompanied by Na+ 3.19 3.8 x 10-lo changes in the diffraction powder pattern. These redoxK+ 0.55 1.5 x lo-" dependent changes (see Figure 3) are dependent on the cs+ 0.71 7.6 x lo-" alkali cation present in the electrolyte (and, therefore, the All supporting electrolytes are composed of the nitrate at 1 M ion exchanging species) and in the cases studied to date concentration. These values were extracted from the slope of do not involve shifts in the crystallite lattice parameters. integrated Cottrell plot of the early time data (t < 250 ma).' Values Typical results for a modified electrode potential cycled of d were obtained from integration of cyclic voltammetric data and in a 1 M NaN03 supporting electrolyte (10 cycles) are the known crystal structure of CdFe(CN)B2-. shown in Figure 7. On preliminary inspection, a new diffraction feature at 20 = 29.5' appears to have been produced a stabilized interface in which no further change produced, along with a general variation in the intensities in the voltammetric response is observed. Therefore, of the other scattering features. However, referring back intercalation of various cations has the effect of bringing to Figure 3a it can been seen that the 28 = 29.5O peak is (or excluding) a fraction of the surface confined material into charge transfer communication with the ele~trode.3~ present in the "as made" interface. The lack of this reflection, assigned to the (311) plane (d = 3.0 A), is due This conclusion can be understood in terms of changes in to the lowered sensitivity utilized to collect the data in the crystal structure of the [CdFe(CN)e12-/-lattice with Figure 7. Therefore, while multiple cyclic voltammetric intercalated cation. Consistent with this hypothesis, scans serve to enhance the (311) reflection intensity, this chroncoulometric analysis of derivatized electrodes,which process does not introduce a new reflection plane. Pohad been previously cycled in specific alkali cation tential cycling in a l M KN03 containing electrolyte, containing supporting electrolytes until a stable response likewise (Figure 8), does not introduce new diffraction was observed, yielded cation specific diffusion limited features; rather, major changes in relative scattering behaviors7 Analysis of early time chroncoulometry data intensity are observed for the reflections at 28 = 17.5O and utilizing the procedures previously found applicable to 2e = 35.50. cyanometalate modified electrodes7produced the Cotrell The integrated intensity of a reflection is dependent on type diffusion Coefficients reported in Table 111. Electhe Bragg diffraction angle, 8, the structure factor F,the trolytes yielding a poor cyclic voltammetric response were multiplicity factor P, and a thermal vibrational faction found to have very small apparent diffusion coefficients. e-2Mas given by eq 2. The Bragg angle influences both the However,cycling in N d 0 3 electrolyte causesthe apparent diffusion coefficientto increase by approximatelyan order of magnitude. We have previously demonstrated for the analogous CdRu(CN)4(bipyridine) derivatized interface sin2e cos e that the apparent diffusion coefficient is associated with both electron and ion transport through the cyanometvalue of the thermal vibrational function (which decreases alate layer.37 While it is difficult to separate out these as 20) and the trigonometric term (Lorentz polarization two processes, both are expected to be affected by a change factor). With respect to the present discussion, in which in the cyanometalate phase. However, electron transport intensity variations for a given reflection (and thus value is presumably only affected by the average iron to iron of 8) are observed upon charge transfer cycling, eq 2 distance, while the detailed geometry of the structure (and indicates that these changes can be directly related to the related water content) impacts on the diffusivity of variations in the square of the structure factor. Although cations in the overlayer region. The available X-ray data it is difficult to directly compare data from different indicate the iron-iron distance is relatively constant, electrode samples because of questions related to stansuggestingthat variations in diffusion rates are associated dardization of observed diffraction intensity from sample with cation transport. to sample, two features stand out in comparing the unSupport for the conclusion that cyclic voltammetric cycled electrode to the NaN03 cycled electrode (Figure 7) cycling primarily affects cation transport is obtained from and to the calculated powder pattern (Table I). Indethe data provided in Figure 5c. This set of cyclic voltampendent of the cation exposure history the (220) reflection mograms indicates a concomitant decrease in the voltais less intense than expected and the (420) reflection is mmetric full width at half height with continued potential more intense than predicted. Similarly,the (220) reflection scanning. This parameter has previously been associated is approximately that expected based on the Lazy Pulvix initial coverage, mol/cm2 x 109
final coverage, mol/cm2 x 109
no. of voltammetric cycles
(39) Bocarsly,A. B.;Amos,L. J.; Duggal, A.; Mirsky,E.;Ragonessi, P.; Fitzgerald-Bocarsly,P. A. Anal. Chem. 1988, 60, 245.
(40) Laviron,E. J . Electroanal. Chem. Interfacial Electrochem. 1981, 122, 31.
Luangdilok et al.
656 Langmuir, Vol. 8, No. 2,1992 UNCYCLED ¶
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Figure 8. Relative peak intensities of the different reflections in the diffractograms obtained upon potential cycling in a KN03 supporting electrolyte: (a) prior to redox cycling; (b)after seven redox cycles in 1M KN03; (c) after 32 redox cycles in the same electrolyte. The vertical scale has been adjusted to keep the most intense reflections on scale. The apparent disappearance of several reflections is actually due to the extensive growth of the (200) and the (400) reflection intensities. m rzo c22 a22
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Figure 7. Relative peak intensities of the different reflections in the diffractogramsobtained upon potential cycling in a NaN03 supporting electrolyte as compared to the diffractogram of the uncycled electrode: (a) prior to redox cycling; (b)after ten redox cycles in 1M NaN03; (c) comparison of observed intensities to predicted intensities. Key: D, calculated pattern;m, intensities prior to cycling; A,intensities after cycling in "03 supporting electrolyte. analysis, but the (400)reflection is always more intense than expected. Both observations involvesparallel planes which have a common element along the h crystallographic axis. In both cases redox cycling in a sodium nitrate supporting electrolyte servesto accentuate these anomalies as shown in Figure 7. Given that redox cycling in NaN03 gives rise to a population of interstitial cadmium ions, a heavy metal center, and, thus, a good scatterer, these data suggest that the production of interstitial Cd2+constructs improved reflection planes consistent with an orthorhombic structure. While atomic variations in scattering cross section may totally explain the observed intensity variation, a second possibility is that the Cd2+ion exchange site may not occur at the same lattice site as K+ or Na+. A similar observation has been reported for fajasite type zeolites.41 This hypothesis is consistent with electroneutrality demands that on average every other unit cell contains a Cd2+ with intervening cells containing only solvent. This is in distinction to the alkali cation saturated structure in which each unit cell contains a cation. The quality of the available diffraction data do not allow for a definitive structural assessment. (41) Baker, M.D.; Godber, J.; Ozin, G. A. J . Am. Chem. SOC.1985,107,
3033.
In contrast to the structural and electrochemical variations observed when a NaN03 supporting electrolyte is employed, no new diffraction features are observed to occur when KN03 is employed as the supporting electrolyte. Instead, as the area under the cyclic voltammogram is observed to decrease, the d = 5.1A and d = 2.5 A reflecting planes are found to intensify (Figure 8). The increase in the intensity of these two diffraction peaks could be explained by the increase in the incorporation of K+inside the diffraction unit cells. Simultaneously, a rearrangement of atoms within the unit cell might be occurring such that certain reflections are eliminated completely. With just two peaks in the diffraction pattern, conversion to a distorted cubic lattice cannot be ruled out. In the literature, cubic symmetry was r e p ~ r t e dfor ~ ~bulk ~~~ samples of KzCdFe(CN)s ( a = 10.03 A) and K1.78Cdl~lFe~~(CN)6. However, some deviations from the cubic structure have been suggested. The K to Cd ratio is important in determining the lattice structure. It must be noted that, if some kind of preferential orientation is occurring with continuous cycling in 1M KNO3, eq 2 would be in~alid.~'This is considered likely because the two reflections correspond to the parallel (200) and (400) planes. It is interesting that this phenomenon is observed only in the K+-containing electrolyte. The X-ray diffraction pattern of bulk cadmium ferrocyanide formed by gradually mixing a 1:l molar ratio of Cd(NO& and K4Fe(CN)6 also showed a very similar pattern. That is, these two reflections are most prominent. This structure apparently servesto immobilize the potassium ions limiting the ability of the [CdFe(CN)612-/-overlayerto support an ion current and, thereby, electronic charge transport. This (42) Rigamonti, E. Gam. Chim. Ital. 1938, 68, 803.
PtlM,CdFe(CN)G Modified Electrode System structural effect is manifest electrochemically by a decreased apparent charge transport diffusion coefficient and a decrease in the fraction of surface confined material in redox communication with the electrode.
Conclusion The structural changes brought about by the incorporation of the different cations can explain most of the observed electrochemical reactivity. Relative diffusion coefficientsfor the surface layer as a function of supporting electrolyte obtained from the early time coulometric response have shown that the apparent charge transfer rate in Na+ electrolyte is increased compared to pure K+or Cs+-containingsupporting electrolytes. This indicates that the ability of the surface overlayer to transport charge varies depending on the electrolyte cation. From the Xray results, it can be inferred that the structure and crystallinity of the overlayer changes to accommodate the interstial cations. Increased incorporation of K+appears to result in a breakdown of the lattice crystallinity. This explains the limited charge transport capabilities of the potassium loaded surface overlayer. On the other hand, the Na+-cycled electrodes show diffraction patterns indicative of a more crystalline structure. The reorganization in these surfaces appears to be such that the electrode potential (i.e. redox) induced changes in the structure of the overlayer facilitates electron motion and concomitant cation exchange. The main difference observed by redox cycling these cadmium ferrocyanide derivatized platinum electrodes in different supporting electrolyte cations is in the relative peak intensities of the various reflections. These peak intensity variations are determined by the type of cation incorporated, the amount of cation (or cadmium to cation
Langmuir, Vol. 8, No. 2,1992 667 ratio) as well as by the different lattice sites that are vacated or occupied by the ions. There are also strong indications that a reorganization in the lattice is occurring to accommodate the cations. A drastic change in the symmetry is probable, particularly in the case of cyclingthese modified electrodes in pure 1M KN03 for a prolonged amount of time. Taken as a while, the data provided suggest that the principles involved in the rational design of chemically modified charge transfer interfaces have to encompass both the chemical nature of the surface confined redox species and the spatial arrangement of such interfacial species. The effect of interfacial microstructure on heterogeneous charge transfer appears most acute for interfaces which exhibit a high degree of self-assemblyvia crystal organization. In the [CdFe(CN)slW - system, the high degree of order on the mesoscopic scale produces an interface having charge transfer thermodynamics and kinetics which are dynamically coupled to variations in crystal structure. As such, a feedback mechanism exists in which crystal structure is varied as a function of charge transfer and, in turn, the structure so produced regulates rates of charge transport via control over cation mobility and diffusivity.
Acknowledgment. The Department of Energy, Office of Basic Energy Sciences, is thanked for support of this work under Grant DE-FG02-86ER13438. Ms. Gayatri Seshadri is thanked for technical support. Registry No. [CdFe(CN)6I2-, 107282-79-1; [CdFe(CN)sl-, 18703-34-9;Pt,7440-06-4;Na, 7440-23-5;K, 7440-09-7;Cs, 744046-2; NaN03, 7631-99-4; KNOB,7757-79-1; CSN03, 7789-18-6; Cd(OHh, 21041-95-2;KaFe(CNh, 13746-66-2.
