Adsorption and partial charge transfer at diamond electrodes—I. Phenomenology: an impedance study

E&ctmchbnica Acta. Vol. 37, No. 5, pp.

Printi in Glut alitin.

973-9978, 1992

00134686/92 S5.W+O.o0 Q 1992.PcrgamonReu pk.

ADSORPTION AND PARTIAL CHARGE TRANSFER AT DIAMOND ELECTRODES-I. PHENOMENOLOGY: AN IMPEDANCE STUDY ALLA SAKHAROVA,*

LAJOSNYnCost$ and YURI bSKOV*

A.N. Frumkin Institute of Electrochemistry, 117071 Moscow V-71, Leninsky Prospekt 31, Russia tInst.itute for Atomic Energy Research, Central Research Institute for Physics, H-1525 Budapest, P.O. Box 49, Hungary

*The

(Receiued 19 September 1991) Abatraet-Impedance spectra of semiconducting polycrystalline and single crystal diamond specimens were studied in aqueous electrolytes under ideally polarizable (blocking) and non-polarizable conditions. The dispersive impedance of the samples in the blocking case is probably unrelated to the roughness of the polycrystalline films and appears as an inherent property of the interface. Different samples are seen to have different electrocatalytic activity. Diffusion-controlled partial charge transfer was proven to take place on the most active specimens which are promising candidates for non-corrosive, dimensionally stable photoelectrodes. Key wor&: semiconductor, roughness, constant phase impedance.

INTRODUCTION Diamond, being extraordinarily stable chemically, is undoubtedly a promising electrode material. It may well prove suitable for non-corrosive, dimensionally

stable @hoto)electrodes when prepared in a conductive and electrocatalytically active state. Unlike other carbon materials (if graphite, glassy carbon), its electrochemical investigation has just begun. Both natural and synthetic diamond is an insulator in its virgin state (bandgap N 5.5 eV). Yet, it acquires electrical conductivity upon doping by some impurities such as boron. Polycrystalline 6hns grown by CVD in the hydrocarbon-hydrogen phase[l] may be obtained sufficiently conductive to enable one to make electrochemical studies, when deposited at a temperature which is a bit higher than that necessary to produce insulating films. This material is a p-type semiconductor, although the detailed mechanism of conductance and the nature of the acceptor impurity is not yet clear. Such fihns are under investigation in this work. Photoelectrochemical properties of the synthetic diamond films were studied for the first time in Ref. [2]. Diamond proved to be a fairly stable electrode in aqueous solutions, with quite reproducible properties, sensitive to visible and near-uv light. Its photosensitivity is presumably due to some lightabsorbing impurities introduced in the course of material growth. The photoelectrochemical behaviour is governed by the process of the current carrier photogeneration and their separation in the space charge region at the semiconductor/solution interface. The flatband potential was determined from the /I $Author to whom correspondence should be addressed.

squared cathodic photocurrent vs. potential plot. Moreover, oxygen chemisorption strongly influences the electrode properties such as fIatband potential. To gain insight into the bulk electrical conductance of the .polycrystalline 6lms, the impedance of the solid-state system was studied[3] in which the film was provided with two contacts. The tungsten substrate and silver paste (or aquadag) contacts to the film proved to be ohmic. In the complex plane the impedance of these lihns was represented by a rather perfect semicircle, and could be simulated by a simple equivalent circuit consisting of two resistances and a capacitance. In some cases, however, indications for a somewhat depressed semicircle were found. Analysis of the frequency dependence of the impedance based on the “effective medium” model resulted in the conclusion that the film is nonhomogeneous, consisting of moderately conductive diamond crystal&s separated by resistive/capacitive barriers of the intercrystalline boundaries (probably amorphous carbon), the content of the latter being a few per cent of the total film volume. Specific resistance of the crystallites and that of the intercrystalline boundaries were estimated to be 10’ and 10’ R cm, respectively. As a consequence the electrode surface can be visualized as that of a truly diamond semiconductor, with minor semi-insulating spots of some other carbon phase. Impedance of the insulating diamond m (deposited at a slightly lower temperature~unlike that of the “conductive” films described above-was purely capacitive, with the capacitance being practically equal to the geometric capacity of the silver contacts. The understanding of charge transfer processes at this interface necessitated a thorough impedance study. 913

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A. SAKHAROVAet al.

Fig. 1. SEM

pictures of the polycrystalline diamond films. Top, faceted; bottom, globular morphology. The magnification is identical on the two pictures.

EXPERIMENTAL

Polycrystalline samples and a single crystal specimen was used. The polycrystalline films were kindly donated by Prof. B. Spitsyn, Institute of Physical Chemistry, Academy of Sciences of U.S.S.R., Moscow. They were deposited onto tungsten substrate by crystallization from the methane-hydrogen phase at a low pressure[l] at lOSO-1250°C. Linear dimensions of the substrate were 8 x 6 mm, and the film thickness was N 10 pm. In the film electrodes the

scanning micrographs revealed two types of surface morphology: globular and faceted (Fig. 1). The film surface was cleaned by heating in air at 400-45O”C. The single crystal specimen was obtained by crystallization from a carbon-saturated liquid metal, and boron doping was done by Prof. A. Gontar of the Institute of Extra-Hard Materials, Kiev. The globular and the faceted samples will be called type N (nanocrystals) and type M (microcrystals), respectively. Since the surfaces are irregular, the microscopic surface is probably considerably higher

Impedance spectraof diamond electrodes

than the apparent, macroscopic one. The single crystal film is smooth, and will be referred to as type S. The diamond 6lms and the tungsten substrate were insulated by Ceresine wax except for an exposed 6lm portion of a few millimetres square. All data are normalized to 1 cm* apparent area. Analytical grade chemicals were used without further purification. In some of the experiments performed in the supporting electrolyte without electroactive species, the electrolyte was deaerated by high-purity N2 bubbling, and the solution was kept under N,-atmosphere during the measurements. Since the role of dissolved oxygen was found to be insignificant, inert atmosphere was not used in the majority of the experiments. The measurements were done in standard threeelectrode cells with the calomel electrode used as a reference. All potentials quoted refer to the calomel scale. Cyclic voltammograms were recorded by a Solarton 1286 potentiostat, a Hewlett-Packard (HP) 709OA measurement-plotting system, controlled by an HP 86B desktop calculator. The impedance measurement setup consisted of a Solartron 1286 electrochemical interface, and a Solartron 1250 frequency response analyser controlled by an HP 86A desktop calculator. Data were saved on disk and transferred to an HP Vectra PC for fitting. To extract the electrochemically relevant parameters from the raw impedance data, impedance spectra of various assumed equivalent circuits were fitted to the measured data. This is by no means a straightforward task since the problem is non-linear, and therefore an iterative least mean square fit algorithm must be used. A further complication arises from the fact that the impedance, 2, is a complex function of the frequency w. In other words, parameter estimation requires the simultaneous fit of the Re(Z) vs. o and the Zm(Z) vs. w functions which depend on the same parameters (except for the series resistance, which appears in the real part only). In view of this, Marquardt’s gradient-expansion algorithm[4] was used with appropriate modifications. The x2 function, its derivatives with respect to the parameters and the curvature matrix were modified appropriately. The measured data points were weighted by assuming constant relative error. The full variance-covariance matrix was calculated in each iteration so parameter errors and correlations could be determined correctly. The algorithm was implemented in Pascal and run on IBM ATcompatible desktop machines. RESULTS AND DISCUSSION

Several electrodes of type M and N were studied: they had been produced under slightly different circumstances. Under ideally polarixable conditions, they all behaved in a consistent manner, and the observed differences can probably be attributed to the differences in their true, microscopic area. In the experiments with the electroactive substances present, some of the electrodes exhibited much higher charge transfer currents than the others: they apparently possess more pronounced electrocatalytic activity. The main attention will be focused on these particular

975

specimens. Altogether several hundred impedance spectra were recorded under different experimental conditions. The blocking case

As supporting electrolytes, 1 M KCl, 1 M HCl or 1 M NaNO, were used. In the absence of electroactive substances, all the diamond samples behaved as ideally polarizable ones. The impedance could be well approximated by a constant phase element (CPE), which has the form Z

1 =a(io)“*

The value of the exponent for all the polycrystalline films was found to be remarkably constant, stable in time and falling in the a = 0.80 f 0.04 range. The exponent a was seen to depend weakly on pH, sina it slightly changed on going from 1 M KCl to 1 M HCl, but the difference was hardly significant. The single crystal specimen, under similar conditions, was characterized by an a value of 0.75 f 0.15 which is practically the same as for the polycrystalline samples. The u value is, however, markedly different for the different electrodes. The exact numerical comparison of the cr values is difficult, sina the units of this parameter depend on a. For a = 1, e is a capacitana, while for a = 0, u plays the role of the inverse resistance. Generally, Q has some intermediate unit Fan”-’ cm-*. Sina the a parameter is basically constant, it is not really inconsistent to compare Q values corresponding to slightly different a. All u values we quote in the paper assume that impedance is measured in R cm*, and the frequency in I-Ix, and we omit its cumbersome unit. For type N electrodes, the typical range of u values is 2 x 10b4-4 x lo-‘, that for type M is 1 x 10-L 2 x 10e5 while for the single crystal 1 x IO-*2 x 10m6.Were a equal to unity, these numbers would correspond to values between 1 and 4OOpFcm-*. The apparently high values and the differences can probably be attributed to different surface roughness and microscopic area. In an attempt to clarify the reasons for the frequency dispersion, several additional series of experiments were carried out. The potential dependence of the impedance was measured to see whether this is a Mott-Schottky type impedance. In fact, no dramatic change was observed (Fig. 2) in the neighbourhood of the flatband potential, and by changing the potential by almost 1.5 V, the u value changed by about a fattor of two only [from (2.9 f 0.2) x 10e4 at -700 mV to (1.8 f 0.3) x 10e4 at +800 mV]. a was found to be 0.81 f 0.02, independent of the applied potential. This weak dependence points to roughness effects which is expected on the basis of the morphology. Therefore, measurements were performed by varying the specific resistivity of the supporting electrolyte to see whether we have RC coupling which should be the case if roughness is responsible for dispersion, In this case the resistive impedana of the electrolyte adjacent to the irregular surface couples with the capacitana of the electrode to produce a dispersive impedana which often leads to CPE behaviour[Sl and the parameter u should then depend on the

A. SAKHAROVA

916

5 r

et

al.

(a)

+ZW>

O -2

I

I

I

I

-I

0

I

2

Log

ei

I 3

Fig. 4. Equivalent

circuits,

(a) modified

Randles, (b)

Grafov-Ukshe.

Frq (Hz)

Fig. 2. Impedance spectra at different potentials measured under blocking conditions in 1 M NaNO, . Potential values from top to bottom: -700, - 100, and +800 mV.

have been the reason why some (apparently “bad”) contacts produced CPE behaviour in the solid state measurements. To settle this issue, solid state measurements with proper blocking contacts should specific resistivity, p, of the solution as o ocp’-I. be performed. Apart from the trivial series resistance change, no There are two reasons, however, why we think the such effect was observed (Fig. 3). Here a is again above explanation is improbable, and that the disconstant (0.83 f 0.02), and u changes only slightly on persion is not of geometrical origin, ie it is not going from distilled water [a = (1.0 k 0.1) x 10m4] connected to roughness. The first is that the single to u = (l.l’+ 0.1) x 10m4 in 0.01 M NaNO, to o = crystalline sample, which is known to have a smooth (1.3 f 0.1) x 10e4 in 1 M NaNO,. surface exhibits also a CPE with a comparable a value Although it is theoretically possible that RC and, second, that CPE is observed over seven orders coupling due to roughness does occur, but both of magnitude in frequency. Were this behaviour the resistive and capacitive domains are in the connected to roughness, one would expect a much solid phase. The specific resistivity of diamond narrower domain and pronounced‘ cut-offs corre(104Rcm for the crystallites, 1O’Rcm for the intersponding to the smallest and the largest irregularities. crystalline boundaries) may exceed the resistivity of Since spatial cut-offs transform roughly linearly even the distilled water. As a consequence, the RC into frequency-domain ones, one expects roughnesscoupling effect does not show up with aqueous based dispersion in as many decades of frequency as solutions. It would not be detected in the solid we have between the smallest and largest spatial junctions either, because the impedance of the good features. ohmic contacts simply shunts this element. This may Inhomogeneous capacitance distribution (produced by the semiconducting and the insulating islands) might serve as an alternative explanation but an unrealistically wide distribution would be required to explain the appearance of the CPE in such a wide range of frequencies[6]. Consequently, impedance dispersion leading to CPE behaviour is probably an inherent property of the interface, and/or it might be caused by surface states[7] or by anomalous dielectric relaxation[8].

5r

Redox reactions

01 -3

I

I

I

I

I

I

I

-2

-I

0

I

I

2

3

4

5

Log Frq

(Hz)

Fig. 3. Impedance spectra measured in electrolytes of different conductivity. Electrolytes: distilled water (top curve), 0.01 M NaNO, (middle), and 1 M NaNO, (bottom curve).

For investigating charge transfer reactions, as a fast redox couple, the ferro/ferricyanide was used in equimolar concentrations. Cyclic voltammograms (cus) on type M and N samples showed normal behaviour (ie peak position at the expected potential, peak current proportional to solute concentration and to the square root of the sweep rate). One remarkable feature of the cv curves was that they showed a considerable capacitive distortion. Type N electrodes had generally higher currents than type M. On most of type S, only the capacitive component could be seen. This can be ascribed probably to a

lmpedance~ofdiamoadeI~

ditference in true surface area and/or electrocatalytic activity of the different spe&nens. Impedance behaviour of the different electrodes corresponded qualitatively to that observed in the vohammograms: sampIes with high currents gave low impedances. Determining the proper equivalent circuit (ie to see what processes are present) proved to be fairly complicated, however. The requirements for the correct equivalent circuit are rather strict. As we shah see, the mere fact that the impedance spectrum of a particular equivalent circuits fits the data over many orders of magnitudes of frequency does not necessarily mean that the particular equivalent circuit is cormct. Moreover, circuits with different topology (therefore describing different mechanisms) can lead to exactly the same frequency dependence. Therefore, impedance data alone are not enough to identify the processes. Additional experimental parameters should be varied, and the fitted parameters must show a reasonable dependence on them. For a fast redox couple, one expects the Randlescircuit as a first approximation: a semicircle plus a Warburg tail. For fast charge transfer the introduction of the CF’E instead of the double layer capacitance is necessary in accordance with the blocking case (Fig. 4a). By fitting the impedance of this circuit to the measured spectra, good fits were obtained. If the reaction rate is slow, and diffusion is not ratedetermining, one gets a semicircle only: this is roughly the case with the electrodes which show a very low rate of charge transfer. Although the fit is perfect, the parameters depend on solution concentration in an unacceptable way. The charge transfer resistance and the Warburg parameter shows a fractional power dependence on solute concentration with an exponent = 1.22 f 0.11: this is unacceptable for an equimolar couple. Moreover, the u values are extremely high and they also depend on the solute concentration with a non-integer exponent 0.86 f 0.05. Since these dependences were very hard to rationalize, we conclude that the modified Randles-type equivalent circuit of Fig. 4a does not give a satisfactory description of the processes occurring at the diamond electrodes. The low-frequency part of the spectra shows a conventional (ie 45”) Warburg-limit. This can be explained in at least two ways: either diffusion is really semi-iniinite (which means roughness does not alter the kinetics of diffusion), or it is not, but two special effects cancel (see Ref. [9]) so the classical 45” behaviour is fortuitous. Accordingly, we tried to fit de Levie’s expression describing the impedance of a rough surface where diffusion and charge transfer are both taking place, but the result was that the measured spectra cannot be fitted by the skewed arc which is de Levie’s prediction for this case. Adsorption seems to be the only remaining plausible possibility. However, it cannot be a simple adsorption, because then we would have simply an extra capacitance. Adsorption, however, must be reversible, since several experiments aimed at detecting some irreversible adsorption layer remained unsuccessful. Based on the treatments of Frumkin and MelikGaikaxyan[lO], and of Lorenx and Salie[l 11,Grafov

977

and UkshHl2] calculated the impedance of a system where the redox reaction occurs in two steps, via an adsorbed intermediate state. The oxidized and the reduced form of the couple is thought to diITuse to the electrode and transfer partial charges n, and n,, respectively, to form the adsorbed state A with a chemical potential & so that n, + n2 = n, the total number of electrons transferred in the reaction. This assumption leads to the equivalent circuit of Fig. 4b with a negative cross-term. Accordingly, the impedance of the various elements of the circuit in Fig. 4b are given by R

RT - ” - V,,n;F*

c,, = n:Fqii

RT WI, =

R==-

n:F%,o~

C,2=n,n2F2pi1

RT v,n;F2

&=&+& 12

1 1 -_=-+-_. CL czz

1 G2

Here, V,, and V, are the rates characterizing the Red x= A or the A + Ox processes. The impedance of the above expression was found to fit the measured spectra fairly well (Fig. 5). We remark that, according to the best of our knowledge, this is the first experimental work where the above partial charge transfer model was found to be valid. The reaction steps are indeed unsymmetrical since n, = 0.35 f 0.05. Also, V, is much higher than VI,,, and they both were found to be strictly proportional to solute concentration. On the other hand, then, and $A values are independent of solute concentration as expected in the equimolar solutions used. The double layer impedance (thought to be simply capacitive in the model) cannot be reliably determined from the

O-1 Log Frq (Hz) Fig. 5. Impedance spectra measured at dierent fcrro/ ferricyanide concentrations. From top to bottom: 1,2,4,8. and 16mM solute concentration. Solid lines show the impedance as cahlated by the equivalent circuit 4b.

978

A. S-OVA

6OooO ? 4

m/z Fig. 6. Impedance of the single crystalline sample under blocking conditions (upper curve) and in the presence of 16 mM of ferri/ferrocyanide (lower curve).

data because of its modest contribution to the total impedance under the present circumstances. The formulae in equation (2) are symmetrical with respect to the indices 1 and 2 so we cannot say whether the index 1, for example, corresponds to the Red +A or the A +0x part of the reaction. To distinguish between these possibilities, further experiments are planned in which the Red/Ox ratio is varied. The description offered by the partial charge transfer model is not perfect, however. We have some systematic deviations (eg in the low frequency phase angle), but this can probably be attributed to the fact that this model treats the double layer as a simple capacitance, not a CPE, which is now the case. Up till now, mainly type-N electrodes have been discussed and a fast redox couple only. For the majority of type-M electrodes, the Grafov-Ukshe picture does not work, ie adsorption does not seem to influence the charge transfer rate, which is, on the other hand, much slower on these specimens. In contrast, the modified Randles-circuit (Fig. 4a) is a fairly good approximation. For both type N and M, a slower redox couple (FeCl,/FeCl,) gives a semicircle only, ie diffusion is not rate-determining, and adsorption does not seem to play an important r81e. For the single crystal electrode the charge transfer impedance (Fig. 6) looks simple although the rate is extremely slow. In this case no equivalent circuits were found which were able to fit the data. CONCLUSIONS 1. In the blocking case all electrodes show a dispersive impedance, the parameters of which are practically independent of electrolyte specific resistivity,

et al.

pH, and potential. The appearance of the CPE is probably not a roughness effect, and not caused by capacitance distribution either. Since it is not observed in the solid state junctions, it may be some inherent property of the diamond-aqueous electrolyte interface. 2. With a fast redox couple on type N electrodes, diffusion-controlled partial charge transfer was shown to take place via an adsorbed intermediate. 3. With a fast redox couple on type M electrodes, adsorption does not seem to play a significant rale and a simple one-step, diffusion-controlled charge transfer takes place. 4. With slower redox couples diffusion does not limit the transfer rate any more on either type of electrodes, and the impedance is basically that of a simple charge transfer resistance and a CPE. 5. The rate of the charge processes on the single crystal are extremely low: therefore probably the amorphous regions in the polycrystalhne films are responsible for electrocatalysis. The reaction mechanism is also different from the films and occurs in an unknown manner. 6. Some of the various polycrystalline flhns show high electrocatalytic activity and they are thus promising candidates for special electrochemical applications. More work is needed to clarify, however, the connection between film growth parameters and electrochemical activity as well as to the close identification of the various steps observed in the coupled

diffusion-adsorption-reaction

case.

Acknowledgements-The authors are greatly indebted to T. Pajkossy, B. Grafov, and A. Sevastyanov for enlightening discussions. REFERENCES 1. B. V. Spitsyn, L. L. Bouilov, and B. V. Detjaguin, J. Cryst. Growth 52, 219 (1981). 2. Yu. V. Pleskov, A. Ya. Sakharova, M. D. Krotova, L. L. Bouilov, and B. V. Sdtsvn. _ _. J. electroanal. Chem. 218, 19 (1987). 3. A. Ya. Sakharova, A. E. Sevastyanov, Yu. V. Pleskov, G. L. Teplitskaya, V. V. Surikov, and A. A. Voloshin, Elektrokhimiya 27, 263 (199 1). 4. P. R. Bevinaton. Data Reduction and Error Analvsis for the Physica~Sci~nces, p. 235. McGraw-Hill, N& York (1969). 5. R. Scheider, J. phys. Chem, 79, 127 (1975). 6. B. Kurityka and R. de Levie, J. electroanal. Chem., in p2.S. 7. G. Nogami, J. electrochem. Sot. 129, 2219 (1982). 8. A. K. Jonscher, Phys. Slat. Sol. 32a, 665 (1974). 9. R. de Levie, J. electroanal. Chem. 281, 1 (1990). 10. A. N. Frumkin and V. I. Melik-Gaikazyan, Do&l. Akad. Nauk. SSSR 78, 855 (1951). 1I. W. Lorenz and G. Salie, 2. Phys. Chem. (Zeipzig) 21% 259 (1961). 12. B. M. Grafov and E. A. Ukshe, Alternating Current Electrochemical Circuits, (in . Russian). Nauka, Moscow (1973).