Langmuir 1996,11, 4309-4312
4309
Polarography and Voltammetry of Mixed Titanium(IV) Oxidehron(II1) Oxide Colloids Michael Heyrovsky" and Jaromir Jirkovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejikova 3, 182 23 Prague 8, Czech Republic
Miroslava Struplova-BartaEkova Department of Physical Chemistry, Faculty of Science, Charles University, 128 40 Prague 2, Czech Republic Received January 9, 1995. In Final Form: August 3, 1995@ The polarographic and voltammetric study of mixed titanium oxideliron oxide colloids shows that the composition and presumably also the structure of the colloidal particles vanes with their size. Iron seems to prevail in the smallest particles; titanium dominates in the surface layers ofthe bigger ones. Polarographic maxima indicate the particle size. The electroreduction of the trivalent iron in the particles proceeds as a bulk irreversible reaction, while the reduction of the titanium component remains, as in pure Ti02 particles, a partly reversible surface process. In the colloids with low iron content, the reduction of the both electroactive parts occurs independently of each other; however, when the iron content exceeds 50%, the reduction of Fe(II1) dominates the mechanism o f the Faradaic process.
Introduction The advantages of Ti02 as a semiconductor material in physicochemical measurements, particularly in studies on photocatalysis, are well-known and have been reviewed in several articles (see, e.g. refs 1 and 2). One of its disadvantages in practical photochemical applications is a poor absorption of sunlight. It had been observed3that doping a Ti02 single crystal with Fe(II1)introduces a small amount of visible absorption. A positive effect of Fe(II1) upon the photoelectrochemical properties of Ti02 was established in a systematic study of mixed iron(I1I)l titanium(IV) oxide electrode^.^ With the aim of improving the optical properties of Ti02 colloids, mixed titanium oxideliron oxide particles were ~ r e p a r e dand , ~ it was found that with increasing proportion of iron the absorption edge of the particles shifts toward visible light. It was also discovered6,' that doping of Ti02 with Fe(II1) prolongs considerably the lifetime of electron-hole pairs generated by photoexcitation of the semiconductor in the band gap region. The mixed titanium oxideliron oxides thus became a n interesting object of research. Continuing in our studies of aqueous colloidal solutions of Ti02 by means of electrolysis with renewed mercury electrode^,^,^ we have examined a series of samples of Ti02 colloids containing Fe(II1)in proportions from 0.01to 75%. Such samples represent an electroactivematerial of higher complexity. We believe that our results presented in this paper have demonstrated the utility of polarography and @
Abstract publishedinAdvanceACSAbstracts, October 1,1995.
(1)Finklea, H. 0. Titanium Dioxide and Strontium Titanate. In Semiconductor Electrodes;Finklea, H. O., Ed.; Studies in Physical and Theoretical Chemistry; Elsevier: New York,1988;Vol. 55,Chapter 2, p 43. (2) Fox, M. A.; Dulay, M. T. Chem. Rev. 1933,93,341. (3)Faughnan, B.W.;Kiss, 2. J. Phys. Rev. Lett. 1968,21,1331. (4)Danzfuss, B.;Stimming, U.J.Electroanal.Chem. 1984,164,89. (5)Bahnemann, D. Isr. J. Chem. 1993,33,115. (6) Moser, J.;Gratzel, M.; Gallay, R. Helv. Chim. Acta 1987, 70, 1596. (7) Gratzel, M.; Howe, R. F. J.Phys. Chem. 1990,94,2566. (8) Heyrovsky, M.; Jirkovsky, J. Langmuir, first of four papers in this issue. (9)Heyrovskjr, M.; Jirkovsky, J., StruplovA-BartBEkovB, M. L a n g muzr, third of four papers in this issue.
voltammetry for characterizing this form of matter and for a better understanding of its properties.
Experimental Section The mixed titanium oxiddiron oxide colloids were prepared in a procedure analogous to that for preparing pure Ti02 colloids described previou~ly.~ Tic14 was added into cooled solutions of FeC13 under vigorous stirring. "he concentration of FeCl3 was varied according t o the percentage of Fe(II1) intended t o be incorporated into the colloidalparticles. The rest ofthe operations was identicalwith those for pure TiOz;individual samples were prepared in the form of powders. By means of X-ray fluorescence analysis, it was confirmedlo that the actual composition of the samples agreed with the expected one within 2%. The mean diameter of the titanium oxideliron oxide particles was determined by transmission electron microscopyloas 5 nm. In order to verify that Fe(II1)did not dissolve from the particles into the solution in form of Fe3+,the colloid was salted out by an excess of NaC104; in the remaining solution no Fe3+could be detected colorimetrically above the detection limit of 5 ppm. For voltammetry with the rotating disk electrode (RDE)a golden disk of 5-mm diameter fured in a Teflon holder was used. The disk was driven by a special mechanical device constructed in the mechanical workshop of the Institute of Physical Chemistry. Other instruments and procedures were the same as in our previous work.g The ferric compounds used were FeC13.6HzO of analytical purity (Lachema Brno) for preparing them in colloids and Fe(ClO&*xHzO(Johnson Matthey) for preparing Fe(II1)solutions without C1- ions. Other reagents were the same as those mentioned earlier.9 "he composition of the mixed colloids is given in the percent iron of the total amount of metal atoms in the colloid.
Results and Discussion Voltammetry with the Rotating Disk Electrode (RDE). The electroreduction of trivalent iron in noncomplexing aqueous solutions begins in the potential region positive of the anodic dissolution of mercury,ll and we found that the same applies to the trivalent iron built ~~
~~~~
(10)Bockelmann, D. Ph.D. Dissertation, University of ClausthalZellerfeld, 1993. (11)Meites, L.;Zuman, P.; Narayanan, A.; Rupp, E. B. CRCHandbook Series in InorganicElectrochemistry;CRC Press: Boca Raton, FL, 1983; Vol. 111, p 40.
0 1995 American Chemical Society 0743-746319512411-4309$09.0010 1
4310 Langmuir, Vol. 11, No. 11, 1995
Figure 1. Cyclic voltammogram ofFe(II1)reduction in a mixed colloid on a rotatinggold disk electrode. Solution 10 mM HC104 8.3 mM mixed titanium oxideliron oxide colloid containing 62.5%Fe. Scan rate 5 mV-s-l; rotation speed 850 rpm. Full line: forward scan. Dashed line: reverse scan. Points: logarithmic analysis of the forward scan curve.
Heyrovsk5 et al.
+
in the colloidal particles. At the utmost positive potential that can be reached by mercury electrodes, i.e., a t about +0.38Vvs SCE, the rate ofreductionofFe(II1)is controlled by diffusion of the electroactive species to the electrode surface. Hence with the mercury electrode we could only follow the effects of experimental conditions upon the cathodic limiting diffusion current due to the reduction of Fe(II1)in the colloid. In order to learn about the initial phase of reduction of Fe(II1) in the mixed particles and to measure the half-wave potential (Ellz), we applied a rotating golden disk as a working electrode, which allowed us to reach potentials sufficiently positive for that purpose. In Figure 1the cyclic voltammetric curve is presented of the reduction of Fe(II1) from a mixed titanium oxide1 iron oxide colloid and the logarithmic analysis8 of the ascending branch of the curve. Ell2 of that reduction is about +0.55 V (vs SCE), and, as in the reduction of SnO2 and TiOz colloids, the curve is drawn-out over a wide potential range. The voltammogram displays a marked hysteresis; when the potential of the RDE returns from the final-more negative-to the initial-more positivevalue, the reduction process takes place a t a slower rate than during the forward scan. The logarithmic analysis of the ascending branch of the curve consists of two straight lines: the reciprocal slope of the first line is 175 mVAog unit; that of the second line is 270 mVAog unit. These results indicate what was observed also with the pure Ti02 particle^:^ in course of the electroreduction the protonated colloids lose their protective positive charge and get adsorbed a t the electrode surface in the form of an agglomerate. Once the electrode becomes completely covered, the adsorbed layer inhibits further electrode process. This is presumably the situation occurring during the reverse scan of the cyclic voltammogram, giving rise to the hysteresis. The two straight lines of the logarithmic analysis with different slopes correspond to different reaction rates a t the partly and at the fully occupied electrode surfaces, respectively. In polarographyholtammetry of polydisperse colloidal solutions the drawn-out shape of the curves is due primarily to the dependence of the redox potential of the particles on their size;s the inhibitive effect of the agglomerated adsorbed colloids on the electrode reaction leads to a similar result. The electroreduction of ferric ion on a rotating t'F disk electrode in aqueous perchloric acid solution12takes place in approximately the same potential region (Eu2 = +0.5 V) as the electroreduction of Fe(II1)in our mixed colloids; only the slope of the voltammetric curve is considerably steeper for the true than for the polydisperse colloidal solutions, as is the rule.s (12) Suzuki, J . Bull. Chem. SOC.Japn. 1970,43,755.
Figure 2. Polarographic curves (mean current) of mixed titanium oxiddiron oxide colloidswith various iron content: (a) 10 mM HClO,; (b-f) 10 mM HC104 and 5 mM mixed titanium oxiddiron oxide colloid containing (b) 0%, (c) 12.5%,(d) 25%, (e) 50%, and (f) 75% Fe(II1). The electron transfer to Fe(II1) in the colloidal particle is not preceded by the reduction of Hfas in the case of the pure Sn0213 and TiOZ9colloids, and hence the direct reduction of smaller mixed titanium oxideliron oxide particles is expected to take place a t more negative potentials than the reduction of bigger ones.14J5At higher concentrations of the colloid the inhibition of the electrode process shifts the reduction of particles of any size toward negative potentials. An experimental study to confirm the dependence of the reduction potential on the particle size would hence require systematic recording of voltammograms with the rotating disk electrode of solutions with the lowest concentrations of the colloid in order to minimize the autoinhibitive effect. Such measurements have yet to be done.
Polarography with the Dropping Mercury Electrode (DME). Polarographic Curves. Polarographic curves of the Ti02 colloids containing varying amounts of Fe(II1) are shown in Figure 2. (The maxima appearing on curves e and f a r e discussed separately below.) The limiting current a t the positive side of the potential scale depends linearly on the concentration of the particles as well as on the iron content of the colloid. The changes of the limiting current when the height of the mercury reservoir over the tip of the capillary is varieds proves that the current is controlled by the rate of diffusion of the particles to the electrode. At the negative side, in the potential region where the T i 0 2 part of the particles undergoes reduction, the limiting current shows a nonlinear dependence on the particle concentration as well as on the iron content (Figure 3). A nonlinear dependence of the limiting current on concentration has been observed already with the pure Ti02 colloidsg and explained by the inhibitive action of the reduction product, the agglomerated particles, adsorbed at the electrode. The nonlinear dependence on the iron content is presumably due to the gradual increase of the zero point of the charge (zpc) of the colloid5from pH 5.1 for pure Ti02 to pH 7 for particles containing 50%Fe(II1); the pH,, of pure Fez03 colloid is 7.5. In the electroreduction of the Ti02 colloid participate only protonated particles; the change of the zpc of the colloid in a given solution affects the protonation, the coagulation, and hence the whole electrode process. (13) Heyrovsky, M.; Jirkovsky, J.; Muller, B. R. Langmuir, second of four papers i n this issue. (14)Plieth, W. J. J.Phys. Chem. 1982,86, 3166. (15)NedeljkoviC, J. M.; NenadoviC, M. T.; MiCiC, 0. I.; Nozik, A. J. J. Phys. Chem. 1986,90, 12.
Titanium(IV) Oxide f Iron(III) Oxide Colloids
C(mM) Figure 3. Dependence of the polarographic limiting current measured at -1.2 V on the concentration of the mixed titanium oxidefiron oxide colloidswith various Fe(II1)contents: (a)12.5%; (b) 25%; (c) 50%; (d) 75%. 0'
5
r Figure 4. Polarographic curves (mean current) of Fe(II1) reduction with the characteristic maximum: (a) 7.5 mM mixed titanium oxidefiron oxide colloid containing 50% Fe(II1) in 10 mM HC10,; (b) 0.75 mM FeCls in 10 mM HC104. Another reason for the nonlinearity is the basically different character of the electron transfer reaction to the Fe20s and to the Ti02 colloid. While in the reduction of Fe(II1) in the colloidal phase, the electrons from the electrode go to the conduction band and apparently stay in the bulk of the particle, the reduction of the protonated Ti02 colloid is basically a surface p r o c e ~ s .At ~ positive potentials the reduction ofthe mixed titanium oxidefiron oxide particles thus proceeds as a bulk process, whereas at negative potentials it acquires a parallel surface reaction path. With unequal rates of the two processes the increasing iron content will make the net electron transfer a t negative potentials more prevailingly a bulk rather than a surface reaction. Polarographic Maxima. On the polarographic curves of the mixed titanium oxideliron oxide colloids with a n iron content of 25%and higher there appear characteristic maxima of bizarre shapes, as can be seen in Figure 2. These maxima, typical for solutions of ferric ions (Figure 41, are due to combined electrolytic, interfacial and hydrodynamic factors.16J7 The fact that the mixed particles containing Fe(II1) produce this specific effect indicates that there is a close similarity in the electrochemical behavior of these particles and the ferric ions in true solutions. The maxima observed with the mixed titanium oxidel iron oxide colloids are produced by the smallest size (16)Heyrovsky, J.; Kfita, J. Principles of Polarography; Academic Press: New York,1966; p 429. (17)Guidelli, R. J.Electroanal. Chem. 1979, 100, 711.
Langmuir, Vol. 11, No. 11, 1995 4311
Figure 5. Effect of C1- on the polarographic curve (mean current) of the mixed colloid: (a) 10 mM HC1; (b and c) 10 mM mixed titanium oxidefiron oxide colloid containing 25%Fe(II1) in (b) 10 mM HCl and ( c ) 10 mM HCIO4. particles, as can be concluded from the properties of maxima. The polarographic maxima are suppressed by low concentrations of surface active substances.16 Higher agglomerates of the colloids are more adsorbable and hence act as more efficient maxima suppressors. In a test of stability of the mixed titanium oxidefiron oxide colloid containing 50% iron we stirred its 10 mM solution in 5 mM HC104 by a continuous passage of nitrogen bubbles. After 90 min of stirring, the limiting current, proportional to the total concentration of the active colloids, decreased by 3.6%due to agglomeration enhanced by stirring. At the same time the polarographic maximum decreased by 24%of its original height, suppressed by the proportional increase of the higher agglomerates in the solution, irrespective of their degree of electroactivity. Particles of a sufficiently big size will not have maxima on their polarographic curves a t all; hence, the smallest particles-ultimately the ferric ions-will produce the largest maxima. Effect of Electrolytes. In concentrated solutions of electrolytes the mixed titanium oxidehron oxide colloids precipitate; however, the general enhancement of the polarographic current by initial additions of electrolytes, characteristic for pure Ti02 colloid^,^ does not occur here. This is presumably because for the reduction of the trivalent iron in the particle no direct contact with the electrode is necessary; the electrons are transferred to the particle by tunneling, and the structure of the double layer affects the reduction process to a considerably less extent than when the pure Ti02 particle is reduced. This conclusion is supported by the strictly linear dependence of the limiting current on the concentration of the mixed colloid, even in the absence of supporting electrolyte, a t variance with the pure Ti02 colloid. Also the instantaneous current-time curves recorded in the course of the drop life in solutions of mixed colloids reveal that adsorption plays gradually less role in the electrode process when the iron content in the particles increases. In presence of chloride ions in the solution the current is markedly lower in the whole potential range, as shown in Figure 5; the positive part of the curve is about twice more affected by C1- than the negative one. The negative chloride ions apparently attach themselves to the positive colloidal particles, lower their effective positive charge, and enhance thus their agglomeration. The iron atoms localized a t the surface of the particles are capable of binding chloride ions more efficiently than the titanium atoms. In the process of gradual agglomeration the Ti02 part of the colloids tends to prevail on the surface and maintains its electroactivity. The thiocyanate ions also enhance agglomeration of the mixed titanium oxidefiron oxide colloids; on the other hand,
4312 Langmuir, Vol. 11, No. 11, 1995
in acidic medium they accelerate thermal dissolution of Fez03 particles:18 the former effect would decrease the polarographic limiting current, while the latter would increase it-as a result a n addition of SCN- ions into the solution leads only to a slight decrease of the polarographic wave. Structure of the Mixed Colloids. In order to test the stability of the mixed titanium oxideliron oxide colloids with respect to a slow dissolution of the iron component, we subjected several samples of a 2 mM solution of the colloid containing 62.5%iron in 20 mM HC104 to dialysis and centrifugation. During the dialysis one sample precipitated, and in the remaining solution no polarographic activity could be detected, which proved that no ferric (or ferrous) ions dissolved from the colloid. In the other samples after separation, the fractions containing smaller particles showed less or no polarographic activity of the Ti02 component of the colloid, as compared with the original solution. This result seems to indicate that the composition or the structure of the mixed particles is not uniform for all sizes, as if the smaller particles consist primarily of iron oxide and if the titanium dioxide tends to prevail in the surface layer in the particles of bigger size. It has been reported6 that the Fe(II1)-doped Ti02 colloids with a low iron content consist of a mixture of anatase and an amorphous phase; a t higher iron concentration the two components of the particles can presumably either exist independently in different forms or enter into some aggregates of a particular structure. At any rate, in the range of the Ti(IV):Fe(III)proportions studied by us the mixed titanium oxideliron oxide colloids do not behave as species of characteristic uniform properties.
Voltammetry with the Hanging Mercury Drop Electrode (HMDE). Voltammetric Curves. The current-potential curves ofthe mixed colloid start a t positive potentials by a decreasing tail of diffusion-controlled cathodic current due to the reduction of trivalent iron in the particles and continue by a n increase to a round maximum a t about - 1.0V, corresponding to the reduction of the Ti02 component of the colloid. With particles of iron content higher than about 10%and with higher scan rates, two more current peaks appear on the voltammetric curve: one steep and prominent a t about -0.2 V and one wide and flat around -0.5 V (Figure 6). The former peak occurs in the region of the polarographic maximum (cf. Figure 51, and from its behavior it seems that it is caused by similar factors, i.e., not expressedly by the structure of the particle but more by the conditions of its reaction a t the electrode. The latter peak occurs in the region of the potential of zero charge on mercury, around -0.50 V, and is probably connected with some change in the electrode/solution interface in the course of the electroreduction. For more definite conclusions about the origin of the two peaks further study is necessary. (18)Regazzoni, A. E.; Blesa, M. A. Langmuir 1991,7,473.
Heyrovski et al.
h
-a2
-0.6
-10
EN)
Figure 6. Linear voltammetric curves of the mixed colloid. Solution of 2 mM mixed titanium oxidehron oxide colloid containing 62.5% Fe(II1) in 10 mM HC104. Scan rates (from below): 5, 10, 20, 50, and 100 mV.s-l.
Figure 7. Comparison of cyclic voltammograms of a pure Ti02 and a mixed titanium oxidehron oxide colloid: (a) 10 mM Ti02 colloid in 10 mM HClO,; (b) 2 mM mixed titanium oxideliron oxide colloid containing 62.5% Fe(II1)in 10 mM HC104. Scan rate 50 mV*s-'.
Electrode Process. From the results of cyclic voltammetry of the mixed colloids with the scan rates up to 200 m V c l the reduction of Fe(II1) in the particles appears as a n irreversible reaction, and also the two additional cathodic peaks are apparently due to irreversible processes (Figure 7). The partial reversibility of reduction of the Ti02 components is practically unaffected by the simultaneously occurring reduction of the Fe(II1) component; only the latter reduction taking place over the entire potential span shifts the whole voltammogram to the cathodic side. This seems to support the idea that the two processes are of basically different nature.
Acknowledgment. Our sincere thanks are due to Dr. J a n Weber of the Institute of Physical Chemistry for carrying out the measurements with the RDE. LA950020S
