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Polarography and Voltammetry of Ultrasmall Colloids: Introduction to a New Field Michael Heyrovsky” and Jaromir Jirkovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejj.kova 3, 182 23 Prague 8, Czech Republic Received January 12, 1995. I n Final Form: August 3, 1995@ Small colloidal particles of a semiconductor nature yield polarographic and voltammetric diffusioncontrolled currents which differ, by the form as well as by the information contents, from the currents obtained with true solutions. The electrode reaction proceeds here either as a volume or as a surface process according to whether Faradaic charge is transferred to the conduction band and consumed in the volume of the particle or is transferred t o and consumed in the surface of the particle. In polydisperse colloidal solutions each particle contributes to the net current according to its size; different size implies different diffusion coefficient, different Faradaic charge, and different reductiodoxidation potential. Hence, in polarography the slope of the wave and the half-wave potential and in voltammetry the peak potential depend on the particle size distribution. Basic experimental material was gathered with ultrasmall SnOz, TiOz, and mixed TiOflez03 colloids. In the electroreduction ofprotonated SnOp and Ti02 colloids in acidic solutions, where the electrode process starts by reduction of the surface protons, the smaller particles are reduced at less negative potentials than the bigger ones.
Introduction In recent years there has been increasing attention paid, from various practical aspects, to the small colloidal particles of the semiconductors SnOz1-5 and However, not much has been known so far about the electrochemical activity of these particles. A group from the London Imperial College observed an anodic dark current and photocurrent in colloidal Ti02 solutions with the optical rotating disk electrode,l1JZbut, to our knowledge, nobody as yet examined these solutions with renewed mercury drop electrodes. We believed that such a study could provide useful information on the properties of semiconductor colloids, especially on their behavior in reduction processes. In the course ofour research we found that, apart from individual differences, the colloidal solutions of both compounds behave in a way which is new in polarographic and voltammetric experience. For that reason we decided to give first an introduction to this new field: to discuss the features which are characteristic and obviously common to ultrasmall electroactive colloids in general. In the next two papers,13J4 we report separately and in detail on the results obtained with tin Abstract published inAdvanceACSAbstracts, October 1,1995. (1)Feng,C. D.; Shimizu,Y.; Egashira, M. J . Electrochem. SOC.1994, 141, 220. (2)Orel, B.; Lavrencic Stangar, U.; Crnjak Orel, Z.; Bukovec, P.; Kosec, M. J . Noncryst. Solids 1994, 167,272. (3) Ando, M.; Suto,S.;Suzuki,T.;Tsuchida,T.; Nakayama, C.;Miura, N.; Yamazoe, N. J . Muter. Chem. 1994,4, 631. (4)Ford, W. E.;Rodgers, M. A. J. J . Phys. Chem. 1994,98,3822. (5)Bedja, I.; Hotchandani, S.; Carpentier, R.; Fessenden, R. W.; Kamat, P. V. J.Appl. Phys. 1994, 75,5444. (6)Micic, 0.I.; Zhang, Y.; Cromack, K. R.; Trifunac, A. D.; Thurnauer, M. C. J . Phys. Chem. 1993,97,13284. (7)Hagfeldt, A.; Didriksson, B.; Palmqvist, T.; Lindstrom, H.; Sodergren, S.; Rensmo, H.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 1994, 31,481. (8) Hashimoto, T.; Yoko, T.; Sakka, S. Bull. Chem. SOC.Jupn. 1994, 67,653. (9)Kay, A.; Humphry Baker, R.; Griltzel, M. J . Phys. Chem. 1994, 98,952. (10)Choi, W. Y.; Termin, A.; Hoffmann, M. R. Angew. Chem., Int. Ed. Engl. 1994,33, 1091. (11)Albery, W. J.;Bartlett, P. N.; Porter, J. D. J . Electrochem. SOC. 1984, 131,2892. (12)Albery, W. J.;Bartlett, P. N.; Porter, J. D. J . Electrochem. SOC. 1984,131, 2896. (13)Heyrovsky, M.; Jirkovskg, J.; Muller, B. R. Lungmuir, second of four papers in this issue. @
0743-7463/95/2411-4288$09.00/0
dioxide and titanium dioxide, respectively. The fourth paper15 in this series deals with the polarographic and voltammetric behavior of mixed titanium oxideliron oxide colloids.
Experimental Section The ultrasmall SnOz and Ti02 colloids were prepared in principle by hydrolysis of the tin and titanium tetrachlorides and by a followingdialysis of the tin dioxide and titanium dioxide colloidal solutions against pure water. The details of individual preparations are given in the respective papers. Although the resulting particles were all small, the solutions thus prepared were basically polydisperse. The mean diameter of the particles was about 2.5 nm, which means that they belonged to the socalled q-size category.16 With such dimensions there were on average about 200 molecules in one particle. Transmission electron microscopic measuremenst have revealed partial crystalline structure of the core ofthese microparticles:of cassiterite for Sn0217J*and of anatase for Ti02.19 The surface of the particles interacts strongly with water; as both stannic and titanic hydroxides are amphoters, the surface molecules react readily with acids as well as with bases. By the protonation and deprotonation reactions the particles acquire positive or negative charge, which represents an electrostatic stabilizing factor protecting the particles from further aggregation. The SnOz and Ti02 colloids were showing polarographic and voltammetric activity only in acidic solutions; hence, their protective charge was positive due to the protons from the solution, attached to the oxygen atoms of the surface molecules. The stability of the ultrasmall colloids depends, apart from the carefully adjusted pH value, also on the ionic composition and concentration of the solution. The SnOz colloids were found to be much more prone to precipitation in the presence of electrolytes than the Ti02 particles. All solutions were prepared from bidistilled water and from chemicals of analytical purity grade. The measurements were carried out with deaerated solutions. For working electrodes mercury of special purity “for polarography” was used. Polaro(14)Heyrovskg,M.; Jirkovsky, J.; $truplov&Bart6Ekova,M. Langmuir, third of four papers in this issue. (15)Heyrovsky, M.; Jirkovsky, J.; Struplovl-BartlEkova,M.Langmuir, fourth of four papers in this issue. (16)Fojtik, A,; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88,969. (17)Mulvaney, P.;Grieser, F.; Meisel, D. Lungmuir 1990, 6,567. (18)Muller, B. Ph.D. Dissertation, University of Hamburg, 1994. (19)Bahnemann, D. W.; Monig, J.;Chapmann, R. J. J . Phys. Chem. 1987,91,3782.
0 1995 American Chemical Society
New Study of Ultrasmall Colloids
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graphic and voltammetric measurements were carried out by means of commercial potentiostatic polarographs with threeelectrode systems; optical properties ofthe solutionswere followed spectrophotometrically. All values of potentials given in this and in the following papers are referred to the potential of the saturated calomel electrode (SCE).
Results and Discussion Qualitative Difference in Faradaic Processes of True and Colloidal Solutions. When pure SnC14 is added to a deaerated aqueous 1 M HC104 solution in a polarographic cell, no immediate effect appears on the polarographic curve. Although tetravalent tin bound in suitable complexes is easily reduced on mercury electrodes,20the molecules of SnOz produced from SnC14 by hydrolysis according to the equation SnC1,
+ 2H,O
-
SnO,
+ 4HC1
(1)
are obviously not reducible. Unstable in aqueous media in molecular dispersion, they form higher aggregates. After 20 h a t room temperature the solution acquires, through some intermediate stages, a polarographic activity typical for the specially prepared ultrasmall particles of Sn02;13 after longer standing, it then turns turbid and the polarographic picture becomes more complex. With TiC14, on the other hand, a n addition of one drop of the pure substance to 1M HC104initiates an immediate formation of white turbidity in the solution. At the same time on the polarogram appears a cathodic reversible wave of halfwave potential (Eld of about -0.3 V, which obviously corresponds to the reduction of the rest ofthe yet unreacted Tic14 molecules. As these are undergoing a relatively slow hydrolysis TiC1,
+ 2H,O
-
TiO,
+ 4HC1
(2)
the wave gradually decreases with time. After more than 6 h a considerably more negative drawn-out wave appears on the polarogram, characteristic of the specially prepared colloidal Ti02 solutions,14while the first wave continues to decrease. The described experiments indicate that the SnO2 and Ti02 molecules, insoluble and not electroactive as such in the potential range accessible in the aqueous medium on a mercury electrode, are reducible on the renewed mercury drop electrodes in aqueous solutions after they aggregate. The aggregation of the molecules into colloidal particles makes the substance, in compact state a semiconductor, amenable to a Faradaic process. In the electroreduction of colloids the electrons from the electrode are obviously not transferred to ionic or molecular orbitals of the individual molecules but to the electronic levels of the small semiconductor particles as such; these represent the elements of the solid semiconducting phase. That process is, of course, energetically basically different from the redox processes which take place with true solutions. The results of this kind are reminders of the research carried out by M i ~ k a ~on l -the ~ ~electroactivity of suspensions. He found that many insoluble compounds give polarographic currents in potential regions grossly different from those of the molecular/ionic redox reactions (20) Meites, L.; Zuman, P.; Rupp, E.; Fenner, T. L.;Narayanan, A. CRC Handbook Series in Inorganic Electrochemistry; CRC Press: Boca Raton, FL, 1987; Vol. VII, p 48. (21) Micka, K. Collect. Czech. Chem. Commun. 1956,21, 647. (22) Micka, K. Collect. Czech. Chem. Commun. 1957, 22, 1400. (23) Micka, K. In Advances in Polarography; Longmuir, I. S., Ed.; Pergamon Press: London, 1960; p 1182. (24) Micka, K. Collect. Czech. Chem. Commun. 1966, 30, 235. (25) Micka, K.; Kadlec, 0. Collect. Czech. Chem. Commun. 1966,31, 3837.
assumed for the compounds in the state of homogeneous dispersion. Moreover, chemically identical compounds of different structural modifications turned out to be reducible a t completely different potentials, and by a defect of crystal lattice new electroactivity could be introduced into solid samples. The spontaneous motion of the large size suspension particles is rather slow, and hence the solution had to be stirred in order to increase the rate of the particle transport to the electrode and thus to provide a measurable current. Our ultrasmall colloidal particles yield measurable currents in unstirred solutions; thanks to their small size, the diffusion flux to the electrode is suEciently fast so that no stirring is needed. However, the electrode processes of the colloids are presumably analogous to those of the suspensions. Diffusion Current of Colloidal Particles. In our studies on SnOz and Ti02 colloids, we verified, according to the usual criteria,26 that the limiting currents in polarography and in voltammetry are basically controlled by diffusion of the particles from the solution bulk to the electrode. The stabilizing charge which protects the colloids from further aggregation can give rise to a current component due to migration of the particles in the field of the diffuse part of the double layer; this complicating factor can be avoided by using a sufficient excess of a n appropriate supporting electrolyte. The electrolytic current controlled by the rate of diffusion depends primarily on concentration of the electroactive species, on its diffusion coefficient, and on the number of elementary charges transferred in the electrode process. In the case of a colloidal polydisperse solution, we are dealing with a n array of electroactive particles of various sizes, and the measured electrolytic current is then given by the sum of individual contributions from all groups of equal size particles according to the given particle size distribution. Diffusion-controlledcurrent to the dropping mercury electrode is expressed by the IlkoviE equation.27 For the mean limiting diffusion current (in amperes) of a polydisperse colloidal solution, it can be written, in the first approximation, in the form
where F is the Faraday charge in coulombs, m is the rate of flow of mercury through the capillary in grams per second, tl is the drop-time in seconds, n is the number of electrons transferred to or from the particle of a particular size of the electroactive species in i multiples of nl per mole (nl being the number of electrons transferred per one molecule), c is the concentration of particles of a particular size in a fraction of moles per cubic centimeters, D is the diffusion coefficient of the particle in squared centimeters per second, and the index i is the agglomeration number, i.e., the number of molecules per colloidal particle. The sum of all i fractions of particles of concentrations ci multiplied by the agglomeration numbers i gives the total concentration c in moles per cubic centimeters: Ccii =c
(4)
i
Figure 1 brings a schematic picture (corresponding approximately to our experimental results with TiOz; cf. (26) Heyrovsky, J.; Kfita, J. Principles of Polarography; Academic Press: New York, 1966; pp 86, 505. (27) Ref 26; p 77.
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0 t
2
3
4 d(nm)
Figure 1. Schematic representation of distribution of concentrations cLof Ti02 colloids according to their sizes,expressed by their agglomeration numbers i or diameters d (corresponding approximately to our experimental results).
Figure 3. Schematic representation of the dependence of the number ofelectrons ni transferred in a surfaceelectrode reaction of colloidal particles, on their agglomeration number i or diameter d (values calculated for nl = 1). band remain in the bulk of the particle. If the number of transferred electrons correspond to complete chemical reduction of all the molecules contained in the semiconductor particle, we can write for the limit oftotal chemical reduction of the particle
Figure 2. Schematic representation of the dependence of diffusion coefficient D,of colloidal particles on their agglomeration number i or diameter d (values calculated for D 1= cm2*s-’). ref 28) of the distribution of concentrations ci of groups of particles of equal size as a function of their size expressed by the agglomeration number i or by the particle diameter d , assuming, in first approximation, particles of spherical shape. According to the Stokes-Einstein law, the diffusion coefficient D of a particle is indirectly proportional to its radius. Figure 2 shows the course of dependence of the diffusion coefficients on the agglomeration number i or on the particle diameter d , as in Figure 1. The number of electrons n transferred between a n electrode and a molecule or ion of a substance dissolved in a true solution depends on the available discrete elec.tronic orbitals of the substance, and it is a small integer, constant over a certain, often considerable, potential range. The number of electrons transferred between an electrode and a colloidal particle of a particular size, on the other hand, is much higher and, in principle, may depend on the potential (in eq 3 for the sake of simplification it was considered as potential independent). In a n idealized case of a reduction, once the electrode attains the potential corresponding to the energy of the lower edge of the conduction band of the semiconductor particle, the electrons start entering the conduction band, which in the case of the ultrasmall particles consists of discrete levels.29 The extent ofthis transfer ofthe Faradaic charge to the particle depends on the detailed structure of the band levels and on the way in which this structure is changing while the electrons fill in the levels. We assume that all the electrons transferred to the conduction (28) Gratzel, M . Heterogeneous Photochemical Electron Transfer;CRC Press: Boca Raton, FL, 1989; p 89. (29) Henglein, A. Chem. Rev. 1989, 89, 1861.
However, it is likely that the number of electrons transferred from the electrode to a colloidal particle may exceed this limit. A decisive factor which determines the number of elementary charges transferred from the electrode to the particle is the occurrence and the density of its surface states (cf., e.g., ref 30). The results of our experiments with the mixed titanium(IV)/iron(III) oxide colloids15 indicate that the electroreduction of colloids can proceed either as a bulk or as a surface reaction, depending on whether the transfer of the electrons from the electrode ends in the volume (i.e., in the conduction band) or on the surface (i.e., in the surface states) of the particle. According to the former alternative, the Faradaic charge transferred to one particle in the case ofits total chemical reduction will be proportional to its volume, i.e., to i, a case outlined above (eq 5). In the latter alternative, ifwe consider that each molecule on the surface of the particle will be reduced by nl electrons but none of the molecules in the volume of the particle will be changed, we can write for spherical particles in a simplified approximation
(6) where VI is the volume and SIthe area which one molecule occupies on the particle surface. In the extreme case of a purely surface electrode reaction of the colloidal particle, the Faradaic charge will hence increase with P3,Le., proportionally with the particle surface area, as is shown schematically in Figure 3. The electrode processes which we studied with SnO2 and pure Ti02 colloids appeared to occur on the surface of the particles; only the presence of Fe(II1)in the mixed t i t a n i d i r o n oxide colloids introduced new features in the polarographic/voltammetric behavior of colloidal solutions, which we ascribed to the “volume” nature of the electroreduction of Fe(II1). In a surface electrode reaction when the electrons are transferred between the electrode and individual molecules localized on the surface of the particle, the principal rules of electrode thermodynamics and kinetics valid for true solutions can be applied; however, this is not so for (30)Holmes, P. J., Ed. The Electrochemistry of Semiconductors; Academic Press: London, 1962.
New Study of Ultrasmall Colloids the case of a pure volume reaction, when the electrons go straight to the conduction band of the colloid; in this direction systematic research is needed. From the above discussed simplified models it can be concluded that for polydisperse solutions with increasing size of the particles the diffusion current contributions will in any case decrease, and the decrease will be more pronounced for the surface-controlled than for the volumecontrolled electrode processes. This conclusion implies that the ultrasmall, or the quantum-sized, semiconductor particles produce the largest electrolytic current; however, this large current is a simple consequence of the small dimensions and not any specific manifestation of the “q” character of the particles. In a polydisperse solution the experimentally observed potential dependence of the limiting diffusion current is caused by the dependence of the reductiodoxidation potential of colloidal particles on their size. If observed with a monodisperse solution of ultrasmall colloids, a dependence of the limiting diffusion current on potential in dc polarography might be interpreted as being due to gradual occupation of discrete levels of the conduction band in q-particles; however, this has yet to be tested experimentally. ReductiodOxidation Potential of Colloidal Particles. In our studies on electroreduction of the SnO2 and Ti02 colloids we have observed, both by optical and by electrochemical methods, that the size and the size distribution of the particles change with time, due obviously to the spontaneous process of the aging of the colloids and that the rate of this change depends very strongly on experimental conditions. In general, the experimental factors supporting aggregation of the particles in the solution lead to a shift of the polarographic and voltammetric cathodic currents to more negative potentials. This effect seems to be at variance with the expected effect of size on the redox potential of small particle^.^^,^^ The SnO2 and Ti02 colloids were found to be electroactive only in acidic solutions where hydrogen ions provide their stabilizing charge. The initial phase of the electrode process of both these compounds is not a reduction of the particles, as such, but of the protons on their surface. It can be assumed that, in the n-semiconductors SnO2 and TiOz,similarly to those in metals, the chemical potential of the electrons and hence the electronegativity and electron donating tendency increases with the decreasing size of their particles. The smaller particles hence attract more positive hydrogen ions from the solution than the bigger ones, and the process ofproton electroreduction begins first on the particles where the surface concentration of protons is highest, i.e., on the smallest ones. As a result, the effect of the size of the particles on the reduction potential of the associated hydrogen ions becomes opposite to the effect on their electrochemical potential. Nozik et al.33observed hydrogen evolution on small particles of aqueous HgSe and PbSe colloids, which did not take place on the bulk phase of these compounds. Our experience has confirmed that the size of the particle determines the potential at which it enters into electrode reaction; the size distribution of the particles hence determines the polarographic half-wave potential as well as the slope of the polarographic wave of polydisperse solutions of electroactive colloids. Figure 4 shows schematically how a polarographic curve of such a solution is composed of contributions by groups of particles of equal size, according to a particular size distribution. (31)Plieth, W. J. J . Phys. Chem. 1982,86,3166. (32) Henglein, A. In Topics in Current Chemistry, Electrochemistry IZ;Steckhan, E., Ed.; Springer-Verlag: Berlin, 1988; Vol. 143,p 113. (33) NedeljkoviC, J. M.; NenadoviC, M. T.; MiCiC, 0. I.; Nozik, A. J. J . Phys. Chem. 1986,90,12.
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Figure 4. Polarographic wave of colloidal polydisperse solution of 5 x M SnOz in 1M HClOl with a schematic indication of some contributions by groups of homodisperse particles of different sizes to the total current (purposely out of scale).
The physical meaning of the half-wave potential (Ellz) of colloidal solutions differs from that in true solutions;34 in a monodisperse solution it would indicate the potential a t which the electrode reaction of the particle of a particular size takes place; in polydisperse solutions it denotes the approximate center of the potential region in which the particles of a given size distribution undergo the electrode process. As this distribution is subject to experimental conditions and to time, the E112of colloidal solutions is variable; however, it remains, within certain limits, a value characteristic of the given colloid. According to our results the values of Ell2 of colloidal solutions were around -0.82 V for SnOz and -0.95 for TiOz. The difference of Eu2 pertaining to the reduction of protons from the surface of the two colloids is in qualitative agreement with the difference in their isoelectric points, which is about 4.3 for Sn0217and about 5.1 for Ti02.35 Most markedly affected by the state of the colloidal solution is another important feature of a polarographic curve, the slope of the “wave”. In true solutions this slope is determined by the number of electrons n involved in the Faradaic process for reversible reactions and, in addition, by the transfer coefficient a for irreversible reactions. In homodisperse colloidal solutions the slope is hence expected to be very high due to the high values of the transferred elementary charge n,. In the case of polydisperse solutions, on the other hand, the slope of the polarographic wave is much smaller, as it is determined primarily by the size distribution of the particles and by the potential dependence of the rate of the particular electrode reaction of the particles of different sizes. The so-called logarithmic analysis of the polarographic wave,36 Le., the plot of log{(& - i)/i} as a function of potential of the electrode (where i d is the mean limiting diffusion current and i a mean current from the potential region of the ascending part of the wave), gives straight lines with reciprocal slopes of 60 and 30 mV per log unit for a oneand a two-electron reversible electrode reaction, respectively, in true solutions. When applied to polarograms of colloidal solutions, the logarithmic analysis yielded often straight lines as well; however, the reciprocals of their slopes were much higher. The freshly prepared solutions of S n 0 2colloids gave values around 150 mV, and for TiOz colloids the values were around 200 mV per log unit. The changes of these values and of the corresponding halfwave potentials with time and under varied experimental conditions contain implicit information about the actual state of the colloidal solution and its changes. For the linear voltammograms of colloids the RandlesSevEik equation,37expressing the peak (in amperes) of (34)Ref 26;p 133. (35)Bahnemann, D.W. Isr. J . Chem 1993.33.115. (36)Ref 26;pp 129, 213. (37) Ref 26,p 505.
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Figure 5. Voltametric curve ofcolloidal polydisperse solution M HClOd with a schematic indication of M Ti02 in 2 x of some contributions by groups of homodisperse particles of different sizes to the total current (purposely out of scale).
the diffusion controlled current-potential curve in the version for a n irreversible electrode reaction, can be written in the form
i, = 2.99
x
105SvY2C{h,(a,n,D,)”2~i} (A)
(7)
i
where S is the surface area (cm2)of the electrode, u is the rate ofpotential scan (VW’), n, is the number ofelectrons transferred to or from the elementary particle in the ratedetermining step of the electrode process (mol-l), and the other symbols have the same significance as in the polarographic case. A colloidal solution of uniform dispersion characterized by a constant agglomeration number would yield a simple voltammogram with a prominent peak characterizing the Faradaic reaction of the given particle. However, the voltammetric curve of a polydisperse colloidal solution is again the sum of individual contributions from all i groups of particles of equal size according to the given distribution (Figure 5 ) . Due to the spread along the potential axis of the individual elementary electrode processes of the particles according to their sizes, the resulting curve acquires unusual forms; above all the characteristic peak shape rounds off. The change of the shape of the voltammogram with changing experimental conditions provides more information about the solution than does the polarographic curve. Still more information is gained from a cyclic voltammogram which indicates the degree of reversibility of individual steps in electrolytic reactions of colloids. A rigorous study of electroactivity of a colloid would require a combined polarographic and voltammetric analysis of a set of different homodisperse colloidal solutions of one chemical species, each solution containing particles of different dimensions, beginning from the q-size. The technique of preparing monodisperse colloids has been already developed to high p e r f e c t i ~ n . ~ ~ Special Features of Electroreduction of SnOzand Ti02 Colloids in Acid Solutions. The small colloidal particles of SnO2 and T i 0 2 are reasonably stable in aqueous acidic as well as alkaline solutions; however, they show polarographic and voltammetric activity only in the former. In acidic solutions the protective charge of the particles is positive, provided by hydrogen ions. For electroactivity of these colloids it is important that no additional stabilizer is needed, as stabilizers often inhibit exchange of Faradaic charge between the particle and the electrode. The limiting diffusion current of colloidal SnOs and Ti02 increases with concentration of the colloid as
well as with increasing acidity of the solution. The surfaces of the Sn0239and Ti02*0semiconductors have a considerably lower hydrogen overvoltage than the surface of mercury. When the particle carrying protons on its surface comes into conducting contact with the mercury electrode polarized to potentials negative of the point of zero charge on mercury (about -0.52 V vs SCE in nonadsorbing electrolytes), the protons localized on the semiconductor accept electrons supplied from the metallic electrode and turn into hydrogen atoms. These then react chemically with the tin dioxide or titanium dioxide, which leads to substantial differences in electrode reactions of the two colloids. However, in both cases in the course of the electroreduction the stabilizing positive charge of the colloid is diminished and the particles gradually aggregate and adsorb a t the electrode surface, forming a n unreducible layer which hinders further electrode reaction. This autoinhibitive tendency in electroreduction of both SnOn and T i 0 2 colloids is the cause of nonlinear concentration dependence of their limiting currents tending toward a limit. At more negative potentials a further increase of the cathodic current over the first limiting value can be observed, which is presumably due to a direct electron transfer to the hydroxo complexes on the surface of the particles. At still more negative potentials the net electrode processes include reduction of the hydrogen ions from the solution, this time on the mercury surface partly covered by one of the reduction products, aggregated colloidal particles without protective charge. Such a state of the electrode acts catalytically on electrolytic evolution of hydrogen, and the Faradaic current is then generated under the combined inhibitive and catalytic effects. In the two followingpapers13J4we describe the behavior of colloidal solutions of SnO2 and TiO2, respectively, when subjected to electrolysis with renewed mercury drop electrodes, in terms of the principles presented above and with respect to individual differences of the two compounds. In the third paper15 we demonstrate how the polarographidvoltammetric behavior of the Ti02 colloid changes when various amounts of Fez03 are incorporated into its particles.
Conclusions Polarographic and voltammetric behavior of colloids of semiconductor nature differs essentially from that of true solutions, due mainly to the basic difference of the electron transfer process in the two systems. Important differences are expected to be found between the results of experiments with mono- and polydisperse solutions. After a systematic study of monodisperse colloids will be carried out, the methods of electrolysis with renewed mercury electrodes will become important tools in research of the colloidal state. Acknowledgment. The research on the subject reported in this series of papers has been carried out under Contract No. 203f9310250 provided by the Grant Agency of the Czech Republic. LA9500181 (38)MatijeviC, E. Langmuir 1994, 10, 8. (39)Laitinen, H. A,;Vincent, C. A.; Bednarski, T. M. J.Electrochem. SOC.1968,115, 1024. (40)H a m s , L. A,;Gerstner, M. E.; Wilson, R. H. J.EZectrochem. SOC. 1979,126, 850.
