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J. Phys. Chem. 1987, 91, 1292-1295
surface is a surprisingly mobile surface whose reorganization appears to be impeded by the interfacial solution. During both oxidation and reduction of silver, molecular clusters of silver are formed through a slow relaxation of the electrode surface following silver dissolution or deposition. Surface clusters, detected through LIL, and growth of macroscopic silver surface structures, detected with SHG, are found to continue into times well after the reduction of silver had reached its maximum and decreased to a negligible amount, with the clusters appearing first and then followed by the growth of macroscopic roughness. These results indicate that the silver electrode surface presents an evolving variety of atomic-level and macroscopic surface features for chemical interaction
with adsorbates or solution-phase species during and following and ORC. Consequently, silver cluster chemistry and evolving surface structure must be considered in describing the chemical behavior of the electrode and its interaction with solution-phase species. These studies emphasize the need for real time, in situ studies of electrode surface structure to determine its relation to electrode surface chemistry. In addition, this work opens the possibility of further temporal studies of atomic-level and macroscopic surface reconstruction events at the solid-liquid interface.
Acknowledgment. We thank R. L. Mortensen for his gift of laser equipment which made these studies possible.
Appllcations of the Quartz Crystal Microbalance to Electrochemistry. Measurement of Ion and Solvent Populations in Thin Films of Poly(vinylferrocene) as Functions of Redox State Pierre T. Varineau and Daniel A. Buttry* Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 (Received: October 20, 1986; In Final Form: January 15, 1987)
The quartz crsytal microbalance (QCM) technique is applied to a study of ion and solvent content within poly(viny1ferrocene) (PVF) as a function of redox state in various supporting electrolytes. It is concluded that in C10; and PF6- containingelectrolytes the oxidation of the film occurs with little and no change in solvent content of the film, respectively. These findings are in agreement with previously postulated phaselike behavior for this system. Multiple peaks are observed in the voltammetry of the PVF film in C1- containing electrolytes. Based on the concurrent QCM measurements the structure in the voltammetric response is proposed to result from the electrochemically induced dissolution or delamination of the film and a new charge transport (diffusional) situation which results from loss of the film from the surface. The QCM technique is shown to be a powerful tool for the study of processes which result in mass changes at solid/liquid interfaces, especially electrode surfaces.
Introduction
Electrodes coated with polymers are of practical and fundamental interest both because of the possibility of studying the redox chemistry (and associated processes) of the immobilized polymer and because of the control over electrode processes which can often be exerted through manipulation of surface structure. The many potential uses of such modified electrodes have been adequately identified,' and more will undoubtedly be found. Of the processes which influence their electrochemical response, the transport of solvent and ionic species within these structures is thought to have great impact on the kinetics and thermodynamics of the redox event. However, very little is presently known about even the qualitative aspects of these processes. The relative absence of adequate techniques is largely responsible for this situation. Several research groups have recently reported on the use of the quartz crystal microbalance (QCM) to study electrode surface processes.* The Q C M is a piezoelectric device widely used in the vacuum community for the determination of the mass of thin films deposited on its s ~ r f a c e . Under ~ some conditions it can be used to probe mass changes of the electrode surface (or structures (1) Murray, R. W. In Electroanalyiical Chemistry, Vol. 13, Bard, A. J., Ed.; Marcel Dekker: New York, 1984; p 191. (2) (a) Kaufman, J. H.; Kanazawa, K. K.; Street, G. B. Phys. Rev. Lett. 1984,53,2461. (b) Bruckenstein, S.; Shay, M. J . Electroanal. Chem. 1985, 188, 131. (c) Bruckenstein, S.; Swathirajan, S.J . Electrochim. Acta. 1985, 30, 851. (d) Melroy, 0.; Kanazawa, K.; Gordon 11, J. G.; Buttry, D Langmuir 1986, 2, 697. (3) Applicaiiom of Piezoelectric Quartz Crystal Microbalances. Methods and Phenomena, Vol. 7, Lu, C., Czanderna, A. W., Eds.; Elsevier: New York, 1984.
attached to the surface) which may (or may not) be electrochemically induced. It has two significant attributes. One is its ability to make the mass determination in situ, in conjunction with the electrochemical measurements. The second is its excellent sensitivity, being capable of measuring mass changes corresponding to submonolayer adsorption and desorption. We have undertaken to apply this powerful new tool to the elucidation of the redox processes which occur within microstructures on electrodes, with particular emphasis on ionic and solvent transport. In this Letter we report on some initial findings which speak to the question of the degree of solvent transport which occurs during oxidation of thin films of poly(viny1ferrocene) (PVF) immobilized on the electrode surface, and which reveal the nature of the processes responsible for the unusual multiple peaks often seen in the voltammograms of this (and other) system(s). Experimental Section A schematic of the apparatus used for the QCM/electrochemical experiment is shown in Figure 1. The microbalance
is comprised of a 5-MHz AT-cut quartz crystal which is driven at its resonant frequency with a feedback oscillator. The crystal is sandwiched between two vacuum deposited gold electrodes by using a standard keyhole electrode c~nfiguration.~One electrode is kept out of the solution by using an O-ring mounting, and the other is used as the working electrode. The piezoelectrically and electrochemically active areas are 0.28 and 0.34 cm2, respectively. The signal from the oscillator is sent to a Philips PM6654 fre(4) Bruckensiein, S.; Shay, M. Electrochim. Acta. 1985, 30, 1295
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The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 1293
Letters
7 1 OOOHz
D
4
C
\
Figure 1. Schematic of the experimental apparatus for electrochemical/QCM measurements: A, potentiostat; B, recorder or computer; C, frequency counter; D, oscillator; E, crystal.
quency counter which provides an analog output for the Kipp and Zonen BD 91 XYY' recorder. A locally built potentiostat was used for the electrochemical measurements. Both frequency and current were recorded as functions of potential. A HewlettPackard 4192A impedance analyzer was used to obtain the frequency spectrum of the crystals with and without deposited films. As mass is gained or lost from this electrode (e.g. as the result of some electrochemical process), the resonant frequency of the crystal changes in linear fashion according to the following equation:
Af = -Cfm (1) where Afis the frequency change, m is the mass per cm2 of the deposit, and C f is the proportionality constant for the crystal. In the present case C, is 56 H z rg-' cm2. The negative sign indicates that the resonant frequency of the crystal decreases by 56 H z for a mass increase of 1 pg cm-* of the electrode surface. PVF films were deposited onto the QCM electrodes from a dichloromethane solution containing 2 mM PVF (Polysciences) and 50 mM tetrabutylammonium tetrafluoroborate by oxidation a t 0.75 V vs. Ag/AgCl (saturated). This method is similar to one previously r e p ~ r t e d . ~Electrochemical deposition was used in preference to direct application of a solution of the polymer followed by evaporation due to the much better uniformity of the fiIm obtained from the former technique and the possibility of monitoring the frequency change during the deposition. The total frequency change observed during deposition was typically 3000 Hz, which corresponds to the mass of the polymer and any solvent or ions which are contained within it. Detailed studies of the frequency change during deposition of the film await further experiments. Following deposition the films were transferred to aqueous solution containing various supporting electrolytes for investigation.
Results and Discussion The use of the QCM for measurements on thick films of the type in this study represents an almost ideal application of the device because its sensitivity makes the frequency measurement one of the most accurately known quantities in the experiment (i.e. the greatest uncertainty is in the electrochemical measurements). Equation 1 is valid when the deposited film behaves as a rigid layer, i.e. when the film is perfectly elastic (with viscosity equal to zero) and thin enough so that the frequency change is negligible with respect to the resonant frequency. This represents the rigid layer approximation in the use of the QCM for mass measurements. In some cases this linear connection between mass and frequency is not observed for mass measurements of thick films. This can occur due to viscous damping of the acoustic shear wave within the polymer films6 Measurement of the frequency width of the resonance for the quartz crystal before and after film deposition using a network or impedance analyzer gives a measure of the seriousness of this problem. Broadening of the resonance results from this viscous loss. We observed no significant broadening for the PVF films used in this study, indicating rigid ( 5 ) M y , A.; Bard, A. J. J . A m . Chem. SOC.1978, 100, 3222.
( 6 ) 0 Donnell, M.; Busse, L. J.; Miller, J. G. In "Ultrasonics", Vol. 19 of Methods of Experimental Physics, Marton, L., Marton, C., Eds.; Academic Press: New York, 1981; p 29.
I
I
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I
0.2
I
I
I
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E, VOLTS vs. AgIAgCI
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Figure 2. (a) (A) Cyclic voltammogram of PVF on a gold electrode in 0.1 M KPF6. Scan rate = 10 mV/s. (B) Frequency curve obtained simultaneously with (A). (b) Plot of frequency vs. charge for a scan from 0.0 to 0.60 V and back for a PVF film in 0.1 M NaC10, + 0.1 M HC104. The film was thinner than that in a. Scan rate = 25 mV/s.
layer behavior and verifying the assumptions implicit in eq 1. Figure 2a shows the cyclic voltammogram along with the corresponding frequency response (CV/QCM) for a PVF film in 0.1 M KPF6. The scan rate is very low so that the film behaves as a thin layer, with essentially no contribution of diffusion to the electrochemical response.' The frequency is seen to decrease during the oxidation and then to reach a constant value following the cessation of Faradaic current flow. After the reduction on the return scan the frequency attains its original value, indicating gross reversibility to the mass change. If we assume the absence of supporting electrolyte sorption into the film in its reduced form, electroneutrality requires that the removal of each electron from the film result in the insertion of one anion (PF,-) into the film. C/cm2 was passed during the oxidation A total of 12.8 X of the film. This corresponds to 1.33 X mol/cm2 of inserted anions, given the above assumption, or a projected change in the g/cm2. By eq 1 the frequency mass of the film of 1.92 X change for this process should be 1093 Hz. The observed value is 1100 Hz. Thus, the QCM senses the mass increase associated with the insertion of anions into the film during oxidation. Comparison of the charge and frequency change for the oxidation indicates that charge compensation is achieved by anion insertion with essentially no accompanying solvent. This is perhaps not surprising in light of the very weak hydration of large anions such as PF6-, Clod-, and BF4- in aqueous solutions* and the known relative insolubility of their salts with organic cations in water. (7) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; Wiley: New York, 1980, p 406. (8) Hindman, J. C. J . Chem. Phys. 1962, 36, 1000.
1294 The Journal of Physical Chemistry, Vol. 91, No. 6,1987
Letters
f 1OOOHz
c
A
,
I
I
I
I
I
-0.2
0.0
0.2
0.4
0.6
I
E, VOLTS vs AgIAgCI Figure 3. (A) Cyclic voltammogram of PVF on a gold electrode in 0.1 M NaCIO,. Scan rate = 25 mV/s. (B) Frequency curve obtained simultaneously with (A).
I -0.2
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I
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0.2
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It is probably the insolubility of the ferrocenium polymer salt which induces it to remain on the electrode surface. Similar experiments on PVF films in 0.1 M NaC104 were also done. Acid was sometimes added to shift the gold oxide formation to more positive potentials, away from the ferrocene response. A plot of frequency vs. charge which is representative of the data obtained for both the PF, and C104- systems is shown in Figure 2b. The quantitative linear relationship between charge and mass for these systems is clearly indicated by this plot. (The slight disparity between the starting and ending values of frequency and charge result from a small amount of charge trapping (see below) due to the higher scan rate used for this experiment.) The C104experiments revealed that the frequency change for the oxidation process consistently gave larger values (by between 2 and lo%, depending on the film) than those calculated based on the charge consumption. We feel that these measurements have significance and indicate that small amounts of water enter the film upon oxidation in Clod- containing electrolytes. On a molar basis the amount of water incorporated into the film is approximately an order of magnitude less than the quantity of inserted anions. Thus, a small degree of solvent transport into the film is indicated on oxidation in this medium. However, the wave shape in C104- is quite similar to that in PF;, indicating that activity effects9 are similar for these two systems. Figure 3 shows a CV/QCM experiment for a thick film in 0.1 M NaC104 which was done at a scan rate at which the film behavior is in the finite diffusion regime (Le. both the finite thickness of the film and the diffusion process influence the shape of the voltammogramlo). The frequency decreases during the oxidation and continues to decrease as long as anodic current flows, even after reversal of the scan direction. This is predicted based on the shape of the voltammogram and is a reflection of the fact that for PVF the QCM senses the direction of current flow by measuring the consequent anion transport. The frequency did not return to its original value on completion of the cycle. There is also a discrepancy between the anodic and cathodic charges which corresponds quantitatively to the number of anions which the QCM indicates is left within the film. These observations indicate that at this scan rate all of the anodic charge is not captured during the negative scan, so that some of the anions inserted during the anodic process remain trapped with their conjugate ferrocenium sites inside the film. Holding the potential at the negative limit for a minute or so (depending on the film thickness) allows time for the charge to be harvested and the frequency to regain its original value. Strikingly different behavior is observed in C1- containing solutions. Figure 4 shows the results of a scan in l .O M NaC1. The frequency is seen to increase dramatically during the oxidation
from one constant level to another. On reduction the frequency decreases with approximately a square root of time dependence and does not return to its original value. The voltammogram exhibits two distinguishable redox processes, each with discernable peaks. A careful inspection of the frequency and current curves reveals that the increase in frequency during oxidation actually accompanies the more positive of the two voltammetric waves. Some films showed slight decreases in frequency during the oxidation just prior to the dramatic frequency increase. We propose that the increase in frequency is caused by the electrochemically induced dissolution or delamination of the oxidized PVF film from the electrode surface. The slight frequency decreases seen prior to dissolution for some films probably result from C1- insertion into the film before it is lost from the surface. Calculation of the frequency increase expected for loss of all of the PVF which is electrochemically accessible (Le. integration of the voltammogram in Figure 1, assuming 100%electroactivity) consistently indicated that a slightly larger increase (ca. 10%) should have been observed. There are several possible reasons for this such as the influence of surface roughness, incomplete dissolution or delamination, or a change in the solution phase viscous loading on the QCM. Schumacher et al.ll have shown that surface roughness can influence the frequency of the QCM because solvent trapped within pores on the surface acts as attached mass. In order for this to cause the above effect (observed frequency change too small), however, the surface of the Au electrode on the QCM would have to be rougher than that of the polymer. This seems quite unlikely. Incomplete dissolution or delamination such that small segments of the polymer remain attached to the surface remains a possibility, as does a change in viscous loading. The calculation based on eq 1 implicitly assumes that the viscous loading of the solution on the crystal remains constant during loss of the polymer from the surface. In fact, following this loss the solution layer near to the electrode (which contains a high concentration of dissolved or delaminated, oxidized PVF) is almost certainly more viscous than the 1.0 M NaCl solution present at the surface prior to the dissolution. Since the QCM senses not only the mass on its surface but also the density and viscosity of the solution within the hydrodynamic layer at its surface,12 one must take these values into account when calculating frequency changes due to polymer dissolution from the surface in unstirred solution. The expected higher viscosity of the polymer layer in solution indicates that a smaller frequency
(9) (a) Daum, P.; Murray, R. W. J . Phys. Chem. 1981, 85, 389. (b) Daum, P.; Murray, R. W. J . Electroanal. Chem. 1979, 103, 289. (IO) Andrieux, C. P.; Saveant, J. M. J. Elec?roanal. Chem. 1980,111, 377.
( 1 1 ) Schumacher, R.; Borges, G.; Kanazawa, K. K. Surf. Sci. 1985, 163, L621. (12) (a) See ref 6. (b) Kanazawa, K. K, preprint
Figure 4. (A) Cyclic voltammogram of PVF on a gold electrode in 1.0 M NaCI. Scan rate = 50 mV/s. (B) Frequency curve obtained simultaneously with (A).
J. Phys. Chem. 1987,91, 1295-1297 change is expected than that ca!culated neglecting this effect, in agreement with observations. Quantitative calculations would be very difficult due to the unavailability of density and viscosity data for such poly(viny1ferrocenium) chloride layers. Thus, the present data do not allow for differentiation between incomplete loss and viscosity effects. The square root of time behavior seen for the frequency decrease during the return scan is indicative of a mass deposition which is diffusion controlled. Reduction of the polymer previously lost from the surface causes its redeposition after diffusion delivers it back to the surface. Thus, both the frequency change seen during oxidation and the time dependence of the frequency change during reduction point to loss of polymer from the surface as a result of oxidation. The loss of the polymer film from the surface seen during the positive scan occurs just between the two peaks in the voltammogram. We have observed this effect in several films and attribute it to a change in the nature of the charge transport from finite to semiinfinite diffusion. The details of this process are difficult to quantitate because the rapid loss of polymer from the surface must change the concentration of the ferrocene sites near the electrode surface and the thickness of the polymer layer. In addition, a probable change in the site to site distance may deliver control of the charge transport rate from polymer lattice mobility, which was proposed to control the electron exchange rate by Daum et al.,9 to a true, physical diffusion of the polymer chains to the electrode surface from regions more distant but still within the layer of dissolved polymer near the surface. The fact that both the more positive oxidation process and the reduction process exhibit diffusion tails even though thin layer conditions would prevail under these conditions for a film in a medium which does not induce loss from the surface supports this interpretation. It is clear from the preceding discussion and from much previous work on charge transport within polymer films that polymer swelling and dissolution have a profound effect on the charge
1295
transport rate and therefore the nature of the electrochemical response of such films. We speculate that a large change in polymer film density, and therefore in the concentration of sites and the distribution of site to site distances, is the major cause of the double peaks in the PVF voltammogram. PVF has been studied in many other media, in which non-Nerstian behavior is often observed. Several mechanisms have been proposed to account for such behavior, including the existence of (interconvertible) redox moieties in different environments and therefore with different redox potential^,'^ solvent swelling and deswelling kinetic^,^ and interactions between redox sites which lead to non-Nemstian variations of their activities with potential.I4 While each of these explanations undoubtedly contains some element of reality depending on the medium in which the film is being investigated, we feel that the present study represents the first direct determination of the cause of the unusual features in the voltammetry of such a system. The QCM is a powerful tool for the study of electrochemical reactions which are accompanied by mass changes on the electrode surface or within thin films on the electrode surface. Its capability of measuring submonolayer mass changesZbgcshould be significantly enhanced in the near future by improvements in instrumentation. This, coupled with ease of use and relatively low cost, should lead to its wide acceptance as an invaluable component of the electrochemist’s repertoire of techniques. Furthermore, its ability to probe adsorption and desorption at solid/liquid interfaces other than electrode surfaces should find it wide use in other areas of surface science.
Acknowledgment. This work was supported by a grant from the Research Corporation. (13)Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. Interfacial Electrochem. 1980, 114, 89. (14)Laviron, E.;Roullier, L. J. Electroanal. Chem. 1980, 115, 65.
Size Quantization in Layered Semiconductor Colloids with Tetrahedral Bonding: HgI2 0. I. Micic,* M. T. Nenadovic, Boris Kidric Institute of Nuclear Sciences, Beograde, Yugoslavia 1 1 001
M. W. Peterson, and A. J. Nozik* Solar Fuels Research Division, Solar Energy Research Institute, Golden, Colorado 80401 (Received: November 1 1 , 1986)
Small-particle-size colloids of red Hg12show relatively sharp blue-shifted optical absorption peaks in the UV that are attributed to size quantization effects. Within the framework of a simple model that only considers transitions between the lowest quantum states of electrons and holes in Hg12, the optical data are consistent with crystallite structures containing up to four Hg12 layers with dimensions in the layer plane ranging from 10 to 26 A. The lowest two energy peaks could not be accommodated by particles containing only a single layer of Hg12 Published data on layered Pb12colloids are also believed to be incompatible with a single PbIz layer if more acceptable values of the effective masses for electrons and holes in PbIz are used in the analysis. Preliminary calculations indicate that the optical data are also consistent with a single particle size showing multiple transitions between higher energy quantum levels of electrons and holes. The possible existence of magic number in HgIz colloids is inconclusive based on the optical data and electron microscopy.
Quantization effects that arise from the confinement of charge carriers in small semiconductor particles are under active investigation.l-I6 Recently, Sandroff and c o - w ~ r k e r sreported ~~,~~ (1) Rossetti, R.;Nakahara, S.;Brus, L. E. J . Chem. Phys. 1983, 79, 1096. (2) Brus, L. E. J . Chem. Phys. 1983, 79,5566. (3)Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (4) Rossetti, R.; Ellison, J. L.; Bigson, J. M.; Brus, L. E. J. Chem. Phys. 1984.80, 4464.
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quantization effects in layered Pb12 colloids that produced three absorption peaks that were blue shifted by 1-2 eV from the ( 5 ) Williams, F.; Nozik, A. J. Nature (London) 1984, 31 1, 21. (6) Welles, H.; Koch, V.; Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 649. ( 7 ) Fojtik, A,; Weller, H.; Koch, V.;Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 969. ( 8 ) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. I. J . Phys. Chem. 1985, 89, 391.
0 1987 American Chemical Society
