Langmuir 1986,2, 559-561
559
Overlayer-Support Interactions Associated with the Formation of a Chemically Modified Interface: The Nickel Ferrocyanide Derivatized Nickel Electrode Linda J. Amos, Michael H. Schmidt, Sujit Sinha, and Andrew B. Bocarsly* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received January 22, 1986. I n Final Form: J u n e 10, 1986 The anodization derivatization of nickel ferrocyanide on a nickel electrode is shown to be sensitive (with respect to surface coverage)to the electrode potential and the electrode crystal face. The epitaxial alignment of the Ni(100) face and the nickel ferrocyanide lattice promotes surface derivatization, while the mismatch of the surface layer and the Ni(ll1) face inhibits formation of a surface overlayer. Electrode potential is found to play a dual role. First, the Ni2+concentration in the interfacial region depends on this parameter; thus both reaction rate and electrode coverage are affected. Second, at higher electrode potentials (>1.0 V vs. SCE), reconstruction of the nickel surface to a surface which promotes epitaxial growth is observed. In the area of electrochemistry, crystal face effects typically are small compared to other phenomena associated with the electrode-electrolyte interface. A major exception to this statement involves submonolayer processes associated with electrodeposition and specific ion ads0rption.l We report here a strong macroscopic crystal face effect associated with the multiple-layer chemical derivatization of the nickel electrode. Recently we reported that polycrystalline nickel electrodes could be chemically derivatized with a nickel ferrocyanide matrix by anodizing the nickel electrode in the presence of aqueous ferricyanide.2 Electrochemicalg5 and spectroscopic6 investigations of these systems suggested that the surface material consisted of a cubic lattice containing cyanides which bridged iron and nickel centers to generate octahedral sites for both metals in analogy to Prussian Blue. Data available indicated the surface to be highly structured, suggesting that the surface material WBB polycrystalline in n a t ~ r e . ~ fWe ' further found that there was a strong relationship between the observed electrochemistry of such interfaces and the exact structure associated with the i n t e r f a ~ e . ~Since the microstructure developed at the interface was found to be a function of the synthetic procedure employed and since it is well established that the growth of crystalline material is an environmentally sensitive process, we have hypothesized that the nature of the nickel substrate might play a predominant role in the final microstructure of the nickel ferrocyanide lattice. In order to test this hypothesis we have considered the kinetics of surface derivatization on polycrystalline nickel substrates and observed the derivatization process on three different single-crystal faces of the nickel electrode: the (loo), ( l l l ) , and (110) faces. As discussed below, these experiments indicate a strong correlation between the crystal face employed and the ability to generate a nickel ferrocyanide derivatized interface.
Experimental Section Single-crystalnickel was obtained from Atomergic Chemmetals. The orientation of the samples was determined by single-crystal X-ray diffraction after being polished to a mirror finish with 1-pm diamond paste. The Ni samples were employed as the working electrode in a standard three-electrode cell which contained a Pt counter electrode and an SCE reference electrode. Derivatization was achieved6 by potentiostating electrodes having a 1.40 cm2exposed face in an aqueous 0.005 M potassium ferricyanide solution for 5 min. Once derivatized,the electrodes
* Author to whom correspondence
should be addressed.
were removed from the electrolyte, rinsed with distilled water, and wiped dry. Cyclic voltammograms of the electrodes in 0.1 M sodium nitrate electrolyte using a 100 mV/s scan rate were then obtained. Surface coverages were determined by integrating the cyclic voltammogram with respect to time. A Pine Instruments RDE-4 potentiostat was utilized to control the potential during derivatization and to obtain cyclic voltammograms of the derivatized electrodes. A Phillips-Norelco X-ray diffractometer fitted with a copper anode operating at 40 kV and 20 mA was used to obtain X-ray powder diffractograms. The derivatized Ni electrode was taped to a glass slide, placed in the diffractometer sample area, and scanned from 28 = 16' to 9 4 O at 1 deg per min. A 2-s time constant, 500 counts per second, and 0.8% suppression were employed.
Results and Discussion We have previously reported5 on coverage effects at the derivatized nickel electrode associated with solution K,Fe(CN), concentration, time of reaction, and electrode anodization potential. In the current study the Fe(CN)63concentration dependence has been removed by carrying out all derivatization processes at fixed concentration. A concentration regime between and M was previously observed to yield a linear solution concentration vs. coverage r e ~ p o n s e .Unlike ~ Fe(CN),3- concentration, both reaction time and electrode potential might be expected to affect the crystal structure and the degree of crystallinity of the final surface overlayer. In order to explore this possibility, studies on polycrystalline electrodes were carried out. As demonstrated in Figure la, for a variety of solution concentrations there exists a pseudo-fist-order relationship between the fractional number of unreacted surface sites (1 - a) (a = r/r0where I'O is the maximum coverage possible at the concentration of [Fe(cN),]% and potential employed) and the reaction time. Therefore the reaction kinetics can be described by eq 1. From these data the d(1 - a) -- kobsd(1 - a) (1) dt kinetic dependence on solution Fe(CN)63-is obtained by (1)See, for example: White, J. E.; Soriaga, M. P.; Hubbard, A. T. J . Electroanal. Chem. 1984,177, 89 and references therein. (2)Bocarsly, A. B.; Sinha, S. J. Electroanal. Chem. 1982,130,1319. (3)Bocarsly, A. B.; Sinha, S. J. Electroanal. Chem. 1982,140,167. (4)Sinha, S.;Humphrey, B. D.; Fu, E.; Bocarsly, A. B. J.EZectroanaL Chem. 1984,162,351. ( 5 ) Sinha, S.; Humphrey, B. D.; Bocarsly, A. B. Inorg. Chem. 1984,23,
203. (6) Humphrey, 88,736.
B. D.; Sinha, S.; Bocarsly, A. B. J. Phys. Chem. 1984,
0743-7463f 86f 2402-0559$0l.50/0 0 1986 American Chemical Society
Amos et al.
560 Langmuir, Vol. 2, No. 5, 1986 0.4
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noted Fe(CN)83-electrolyte concentrations. All reactions were carried out at 1.2 V vs. SCE. (b) Plot of kobd as obtained from the slopes of the lines in part a vs. Fe(CN)63-concentration. concentration as shown in plotting k,bsd vs. Fe(CN):Figure lb, to yield a final kinetic expression: -d(l - CY) = k ( i - a)[Fe(CN)!-] + k'(l - a) (2) dt On the basis of the data in Figure 1,values of k and k'are found to be 0.8 M-I s-l and 4 X s-l, respectively. The first term on the right-handed side of eq 2 is as expected for an elementary bimolecular reaction. This is consistent with previously reported data2,5indicating surface derivatization involves a precipitation-type reaction in which appropriate concentrations of both the ferricyanide and nickel ions in the interfacial reaction layer are necessary to carry out the deposition of a nickel ferrocyanide surface
layer. The second term is zero order in [Fe(CN),I3- and therefore appears to involve a direct surface interaction.' Presumably this second kinetic term mechanistically contains the ability of the nickel surface to template the final nickel ferrocyanide structure. The effects of this term will only be observed for concentrations of Fe(CN):- such that k[Fe(CN)6I3- 5 k'. For the reported rate constants this places an upper limit on [Fe(CN),I3- = 5 mM. Since this value is compatible with the previous coverage vs. concentration study and minimizes the time needed to reach maximum coverage, this concentration was chosen for the single-crystal experiments. Integration of eq 2 indicates that for the selected parameters a 300-s derivatization period allows the surface reaction to go to completion. Thus, this time interval was employed for the single-crystal studies. For fixed Fe(CN)63-concentration and reaction times, a reproducible relationship is observed between surface coverage and electrode potential (Figure 2). At potentials less than -0.6 V vs. SCE, minimal surface coverage is observed, due to the low Ni2+concentration in the interfacial region. Potentials in excess of 1.8 V vs. SCE lead to O2evolution inducing a large scatter in the coverage data and an apparent plateauing of the coverage vs. potential relationship. Consistent with a precipitation mechanism based on the concentration of reactants in the interfacial region, increases in Fe(CN):concentration cause the coverage potential curve to shift to higher coverages for a fixed potential. Since generation of Ni2+involves disruption of the nickel surface, reconstruction of the surface may be expected to occur during anodization. In order to investigate this possibility, X-ray patterns of the single-crystal electrodes were obtained before and after anodization in the Fe(CN):--containing electrolyte. These data indicate that at potentials around 0.6 vs. SCE reconstruction is minimal. However, at potentials above 1.0 V vs. SCE reconstruction of the (111)and (110) faces occurs, while the (100) face does not undergo major changes. (7) This type of kinetic expression is typically found in heterogeneous catalysis where reactant adsorption proceeds via a zero-order process until surface saturation occurs followed by a process which is first order in reactant.O On the basis of this analogy it appears likely that the initial process associated with surface formation involves a bonding-type interaction between Fe(CN):- and surface Ni. However, at this point it is impossible to rule out a simple precipitation reaction as the dominate mechanistic feature. In this case the surface templating effect would be associated with variations in dimensional stabilization and adhesion between NiFe(CN),2- and the various nickel surfaces employed.
Interactions Associated with Interface Formation
Langmuir, Vol. 2, No. 5, 1986 561
Table I. Effect of Electrode Potential and Electrode Crystal Face an Derivative Coverage'sb derivation potential crystal face Ni(100) Ni(ll0) Ni(ll1)
0.6 V vs. SCE 1.2 V vs. SCE 1.8 V vs. SCE 1.24 X 2.26 X loF9 1.59 X 10" 1.46 X 4.09 X lo4 1.01 X lo4 7.18 X lo4
-
a Values reported in mol/cm2 of geometric electrode area. Monmol/cm2 based on the conolayer coverage is estimated at figuration shown in Figure 3. bEach value represents the average of three to five trials as needed to gain reproducibility.
In order to discriminate between the interfacial concentration of Ni2+as a function of the electrode face employed and a templating effect associated with reconstruction of the crystal face, the surface coverage of anodized single-crystal nickel electrodes has been examined. The data at three potentials spanning the linear potential region identified in Figure 2 (0.6, 1.2, and 1.8 V vs. SCE) are reported in Table I. Results obtained a t 0.6 V vs. SCE show a strong preference for surface derivative growth on the (100) face. Within the limits of detection (one monolayer), overlayer growth is not observed on the (100) or (110) faces. Increasing the potential to 1.2 V vs. SCE induces overlayer production on all three of the crystal faces. However, growth on the (111) face is still suppressed when compared to that on (100) and (110) faces. Further increases in potential to 1.8 V vs. SCE cause increased coverages on all faces, consistent with a simple Ni2+concentration effect in the interfacial region. However, once again, the (111) face is found to inhibit overlayer growth when compared with the (100) and (110) faces. Thus, independent of potential, the (111) face exhibits an inhibiting effect on the formation of the nickel ferrocyanide derivative while the (100) face clearly promotes such activity. These variations in reactivity can be understood by considering geometric interactions of the various nickel crystal faces with the nickel ferrocyanide lattice. X-ray powder patterns obtained from bulk nickel ferrocyanide samples indicate this complex to have a cubic structure with a lattice parameter of approximately 10 Figure 3 illustrates the geometric arrangement of the nickel atoms on the various crystalline faces investigated. Superimposed on these geometries is the projection of the unit cell of the nickel ferrocyanide lattice. From this figure it can be seen that the nickel (100) face provides a cubic structure spacially matched to the nickel ferrocyanide lattice. The nickel-nickel spacing of 2.5 A provides a 4 X 4 arrangement of the nickel atoms in register with the iron-iron spacing of the nickel ferrocyanide surfaces. Thus, the epitaxial growth of nickel ferrocyanide is possible on this surface, apparently promoting the derivatization process. On the other hand, neither the (110) nor the (111) faces of the nickel surface can be brought into alignment with the nickel ferrocyanide lattice. This is particularly obvious for the case of the nickel (111) surface which is mismatched with the surface layer lattice by a 45O rotation. Consistent (8) The nickel ferrocyanide lattice parameter (Fe to Fe distance) is found to depend on the exact alkali cations incorporated in the structure and the iron oxidation state.1° However,at most, these changes cause a 0.1-Avariation in the lattice parameter. That the surface-confined material is structurally similar to bulk nickel ferrocyanide is demonstrated by X-ray scattering directly off the derivatized electrode. While confirming the crystal structure and lattice parameters found for the bulk samples, the surface data are of a lower quality. Therefore, a better estimate of the overlayer lattice dimension is obtained by using bulk samples. (9) Benson, S.W. The Foundations of Chemical Kinetics; McGraw Hill: New York, 1960; p 13.
( 1 00)
Figure 3. Projection of the nickel ferrocyanide lattice on to the various nickel crystal faces investigated. Note the (100) face yields an epitaxial environment while the other two faces do not. The iron-iron distance employed is 10 A, while the closest-packed nickel-nickel distance (center to center distance) on the (100) face is 2.5 A.
with the lack of epitaxy are the lowered coverages observed for this sqface independent of potential and the inability to obtain any significant coverage at potentials where reconstruction does not proceed. The (110) face indicates that the requirement for an epitaxial environment is rather strict in that along one dimension of the (110) face a molecular arrangement identical with the (100) face is found. Only the second dimension is spacially out of register with the nickel ferrocyanide lattice having a nickel to nickel spacing of 3.5 A. At the higher potentials tested, however, the coverages associated with the (110) face become equal to or exceed those associated with the (100) face. Pressumably, this is not only due to the reconstruction of the (110) face to the (100) face but to the hill and valley arrangement of the (110) face. This microstructural irregularity is expected to lead to a higher concentration of Ni2+ in the interfacial region due to an electropolishing effect. The data presented suggest that an epitaxial environment not only enhances the capability of generating ferrocyanide overlayers on the nickel surface but is a necessary requirement for such reactivity. Thus, the microstructural demands which the underlying substrate impose on the chemical derivatizing layer must be considered an important and controlling parameter in the synthesis of highly structured interfaces. Turning this statement around, we suggest that it may be possible to produce chemically derivatized interfaces of specific microstructural design by employing carefully chosen metal substrates to act as templates for the final interfacial structure. Since our studies indicatelo a strong correlation between the lattice parameters of surface attached cyanometallate complexes and the energetics of interfacial charge transfer, such a capability could allow for the rational design of reaction-specific electrode interfaces. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant CHE-8317081. We thank Professor S. L. Bernasek for supplying the single-crystal nickel samples and for helpful conversations. We also thank C. Kulick for assistance in obtaining X-ray data. (10) Schmidt, M. H. B.A. Thesis, Princeton University,Princeton, NJ,
1985.
