Polarography and Voltammetry of Aqueous Colloidal TiO2 Solutions

Jan 9, 1995 - (1) Henglein, A. Topics in Current Chemistry·, Springer Verlag: Berlin, 1988; Vol. 143 ..... a white turbidity in the originally clear ...
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Langmuir 1995,11, 4300-4308

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Polarography and Voltammetry of Aqueous Colloidal Ti02 Solutions Michael Heyrovsky" and Jaroslav Jirkovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejj.kova 3, 182 23 Prague 8, Czech Republic

Miroslava gtruplova-BartaEkova Department of Physical Chemistry, Faculty of Science, Charles University, 128 40 Prague 2, Czech Republic Received January 9, 1995. I n Final Form: August 3, 1995@ In acidic aqueous solutions the protonated T i 0 2 colloid undergoes a complex process of electroreduction. It starts by reduction of protons on the surface of the particles; in cyclic voltammetrywith a fast scan rate this reaction is reversible. In the absence of an excess of a supporting electrolyte the particles, deprived of the stabilizing positive charge in the electroreduction process, aggregate and adsorb at the electrode and hinder the electrode reaction. Added electrolyte removes this hindrance and produces an increase of the electrolytic current. The hydrogen atoms generated at the Ti02 surface are able to reduce Ti(1V) to Ti(II1). At more negative potentials a direct reduction of surface hydroxo complexes of TiW) takes place. The mechanism and the potential of electroreduction of Ti02 colloids are strongly affected by ligands which form complexes with titanium atoms on the particle surface.

Introduction The Ti02 colloids belong a t present to the most frequently studied microheterogeneoussystems, especially in photocatalysis. Their physical and chemical properties were the subject of several reviews (e.g. refs 1-4). To our knowledge the only published information concerning the behavior of Ti02 colloids toward macroscopic electrodes is on their irreversible oxidation near f 0 . 4 V (vs SCE) a t an optical rotating disc e l e ~ t r o d e . ~ Renewed mercury electrodes provide an especially convenient tool for studying electrolytic reactions, particularly reductions, of species in solutions. In polarographic studies of true solutions of Ti(1V) compounds in water, it has been found that the titanyl cation, T i O P , is irreversibly reduced a t low pH values in the potential region around -0.85 V (vs SCE).6i7J0J2The complexes where Ti(1V) is strongly bound by anionic ligands have their half-wave potentials (E112)by some 0.4 V more p o s i t i ~ e , and ~ ~ ~their J ~ reductions are reversible, provided the pH of the solution is sufficiently low to suppress the tendency of titanium compounds for hydrolysis. Those strongly acidic solutions in which the complexes are in a fast equilibrium with Ti4+(aq)ions give polarographic reversible waves near the potential of the SCE.gJo The reversibility of the Ti(1V) reductions in 10 M HzS04 and in an acidic thiocyanate solution was proved also by the Abstract published inAdvance ACSAbstracts, October 1,1995. (1) Henglein, A. Topics in Current Chemistry; Springer Verlag: Berlin, 1988; Vol. 143, p 115. (2) Griitzel, M.Heterogeneous Photochemical Electron Transfer;CRC Press: Boca Raton, FL, 1989; p 87. (3) Serpone, N.; Lawless, D.; Terzian, R.; Meisel, D. In Electrochemistry in Colloids and Dispersions; Mackay, R., Texter, J., Eds.; VCH Publishers: New York, 1992; p 399. (4) Bahnemann, D. W. Zsr. J. Chem. 1993, 33, 115. (5)Albery, W. J.; Bartlett, P. N.; Porter, J. D. J . Electrochem. SOC. 1984, 131,2896. (6) Zeltzer, S . Collect. Czech. Chem. Commun. 1932,4, 319. (7) Kalousek, M. Collect. Czech. Chem. Commun. 1939, 11, 578. (8) Pecsok, R. L. J.Am. Chem. SOC.1951, 73, 1304. (9) Lingane, J. J.; Kennedy, J. H. Anal. Chim. Acta 1966, 19, 294. (10) Gierst, L.; Lienard, G. Ric. Sci. Suppl. A 1967, 27, 53. (11)Tribalat, S.; Delafosse, D. Anal. Chim. Acta 1958, 19, 74. (12) Habashy, G. M.Collect. Czech. Chem. Commun. 1960,25,3166. @

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ac oscillographic polarography,13 i.e. by a technique comparable with v01tammetry.l~In a solution of Tic14 in a~etonitrilel~ an addition of water causes a decrease of the polarographic reduction wave of Ti(1V) due to hydrolysis and to subsequent precipitation of TiOz. Obviously, bonding the Ti atom to oxygen makes the electroreduction of the ensuing compounds difficult and irreversible. Moreover, the Ti02 molecule appears as electroinactive at mercury electrodes due to its insolubility in aqueous solutions. In this respect there is a close resemblance between Ti02 and Sn02.16 We found that the conclusion about inactivity of Ti02 is valid only for true solutions, as in aqueous acidic solutions Ti02 displays quite a complex electroactivity when it is prepared in the form of ultrasmall colloid^.'^ Also here exists a basic similarity in the polarographic1 voltammetricbehavior of T i 0 2 and SnO2 colloids, although each of the compounds has its distinct specificities. In the present paper we bring the results of our study of the colloidal solutions by electrolysis with the dropping mercury electrode (DME) and the hanging mercury drop electrode (HMDE). In the course of our research we could only confirm our previous experience, that a combined use of these two electrodes is invaluable for gaining insight into nontrivial electrode processes. For this introductory study we used, of all the electrochemical techniques, the basic simple methods of dc polarography and of linear and cyclic voltammetry. We believe that the information provided in our paper will contribute to better understanding of the properties of the colloidal system which is being ever more widely studied. (13) Habashy, G. M. In Advances in Polarography; Longmuir, I. S., Ed.; Pergamon Press: New York, 1960; Vol. 3, p 868. (14) Heyrovsky, J.; Kota, J. Principles Of POlQrOgrQphy;Academic Press: New York, 1966; p 497. (15) Kolthoff, I. M.;Thomas, F. G. J . Electrochem. SOC.1964, 111, 1065. (16) Heyrovsky, M.; Jirkovsky, J.; Muller, B. R. Langmuir, second of four apers in this issue. (17) ltruplov6, M.;Jirkovsky, J.; Heyrovsky, M.In ESEAC'92 Book of Abstracts, 4th European Conference on Electroanalysis, Noordwijkerhout, 1992, Addenda.

0 1995 American Chemical Society

Aqueous Colloidal Ti02 Solutions

Experimental Section Of the reagents used the Tic14 was chemically pure (Fluka); all the other were analytically pure (pa): HClO4 (Merck);LiC104, NaC104.Hz0, Mg(C104)2, Ba(C104)2*3HzO(Fluka); HC1, LiC1, NaC1, KCl, MgC12.6Hz0, CaC12, BaCl2.2Hz0, AlC13.6Hz0, KBr, KI, KSCN (Lachema Brno). Water was redistilled in a quartz apparatus. Mercury for the electrodes was of purity “for polarographic analysis” (Sluiba vyzkumu, Prague). The ultrasmall Ti02 colloids were prepared by the procedure based on hydrolysis of TiC14:183.5 mL of Tic14 was slowly added to 900 mL of bidistilled water cooled to 1 “C under intensive stirring. After standing for 30 min the Ti02 colloidal solution thus formed was subjected to dialysis through a Spectrapor membrane against water which was continued until its pH value reached 2.5. Then the volume was completed by water to 960 mL. The analyticalconcentrationofTiO2in the resulting solution was 33 mM. When kept in a refrigerator at 4 “C, the solution remained relativelystable over severalyears. The mean diameter of the Ti02 particles prepared in this way was between 2 and 3 nm, which corresponds to the contents of several hundred molecules per particle. For some applicationsit is advantageous to prepare powder samplesofthe particlesby vacuum evaporation of the solution; the powder is reversibly soluble, forming clear colloidal solutions. During evaporation the vacuum was maintained under 1.3 kPa and the temperature of the water bath under 30 “C. The concentrated solution was subsequently brought to dryness in an exsiccator over silica gel. The colloid particles in the powderhave the crystallinestructure ofanatase.18 The solutions for measurement were prepared either by weighing the appropriate amount of the powder and dissolving it in the acid of required concentration or by adding the stock solution into the acid. In both cases the freshlyprepared solutions show a slight temporary change of optical absorbanceover several hours of standingat room temperaturewhich is due to an increase in the particle size. For that reason the solutions were always prepared 1day in advance and allowed to stand overnight at the temperature of the laboratory; after that time a dynamic equilibrium was attained in the solution with respect to the particle size. Compared with the SnOz colloids,16the Ti02 solutions are much more stable as such as well as toward electrolytes. A 0.4 mM solution of Ti02 in 0.1 MHClO4 when warmed slowly under stirring up to 60 “C and then cooled again to room temperature gives the same polarogram as before the heating. A 10 mM sample ofTiO2 in 10 mM HC104 doesnot precipitatewhen NaC104 is added to a concentration as high as 1M and when the solution is stirred for several hours by a passage of nitrogen bubbles. Subjecting a solution to polarographic or voltammetric analysis does not alter its behavior to any measurable extent; the curves repeated with one sample several times are exactlyreproducible (in voltammetry the condition of reproducibilityis renewing the electrode before repeating the measurement). Electrochemical measurements were carried out by means of the apparatus and by the procedures described in our previous paper.16 As there, also in this paper the values of potentials are given versus the saturated calomel electrode(SCE). The acidity ofthe solutionswas checkedby the OP-205/1pH meter (Radelkis, Budapest) with a glass electrode. For the temperature dependences the U4 thermostat (MLW Priifgerate-Werk, Freithal, Medingen) was used; the normal measurements were done at room temperature (22 k 1 “C). The absorption spectra were measured with the Specord M 42 instrument (Carl Zeiss, Jena) in 1-cm quartz cuvettes.

Langmuir, Vol. 11, No. 11, 1995 4301

Figure 1. Polarographic reduction wave of the 10 mM Ti02 colloid in 10 mM HClO4 and 0.5 M LE104 and its “logarithmic analysis”. The curve is reproduced as the contours of maxima and minima of the current oscillationswith minimum damping. The values of i and il for the logarithmic analysis have been taken from the mean of the oscillations.

Results Polarography. Polarographic Wave. Acidic solutions of colloidal Ti02 in concentrations of 1 mM and higher yield a well defined drawn-out cathodic wave in dc polarography with the DME (Figure 1). The half-wave potential (Elm)of the wave varies, according to conditions, within -0.95 f 0.1 V, the Elm and the shape of the wave depend on the size ofthe particleslg and on the composition

of the solution. In solutions of lower concentrations (Ti02 under 1mM and simultaneously HC104 under 1M) the wave shows the tendency to separate into two waves of E m around -0.8 and -1.0 V, respectively. With 1 mM Ti02 in 10 mM HClOI freshly prepared by diluting the stock solution the logarithmic analysislgof the reduction wave with Ell2 = -0.95 V gives a straight line with a reciprocal slope of 225 mVAog unit. (This value stands in sharp contrast to the 55 mVAog unit for the above mentioned T i W ) reversible reductions in true solutions; cf. ref 19). When the 1mM T i 0 2 solution in 10 mM HC104 is prepared by dissolving the powder, the Evz of its polarographic wave is -1.03 V and the logarithmic analysis consists oftwo linear branches, the more negative one with the reciprocal slope of 95 mVAog unit. Effect of Polydispersity of Colloids. From the stock solution 10 mL of 10 mM Ti02 in 10 mM HC104 was prepared and subjected to dialysis through the Spectrapor membrane against 20 mL of pure 10 mM HC104. After 23l/2 h there appeared some Ti02 in the acid on the other side of the membrane. The height of the polarographic wave in this solution (i.e., in the acid on the other side of the membrane) corresponded to one-sixth of the height obtained with the initial solution, its Ell2 was -0.92 V as compared to -0.945 V of the initial solution, and the reciprocal slope of its linear logarithmic analysis was 170 mVAog unit, whereas the same parameter for the initial solution was 215 mVAog unit. Another portion of the 10 mM colloidal Ti02 solution in 10 mM HC104 was centrifuged in an ultracentrifuge for 1h at the speed of 60 000 rpm. At the end the upper and lower layers of the solutions in the cells were collected separately and analyzed polarographically. The polarographic wave of the solution from the upper parts of the cells was 14% less and that from the lower parts was 17% higher than that of the initial solution. The polarographic wave of the thicker solution from the lower parts had Ell2 = -1.04 V, by 59 mV more negative than the wave of the initial solution and by 50 mV more negative than the wave of the thinner solution from the upper parts. The logarithmic analysis of the wave obtained with the thicker solution consisted of two distinct linear sections, the more negative one with the reciprocal slope of 145 mVAog unit. The more positive section had the same slope as the straight line of the logarithmic analysis of the solution

(18)Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988,92,5196.

(19)Heyrovskg, M.; Jirkovsky, J . Langmuir, first of four papers in this, sissue.

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Figure 3. Instantaneouscurrent-times curves recorded with DME at - 1.0 V in 10 mM HClO4 solutions containing colloidal T i 0 2 in the following concentrations (mM): (a) 1.2; (b) 3; (c) 5; (d) 10. The dashed line marks the blank current in 10 mM HC104.

Figure 2. Dependence of the height of the polarographic reduction wave on the concentrationofthe Ti02 colloid in 1,10, and 100 mM HC104, respectively, as denoted on the curves.

from the upper parts of the cells, i.e., the reciprocal of 240 mVAog unit, whereas the initial solution gave the value of 215 mVAog unit. Effect of Concentration of Ti02 and H+. With increasing Ti02 concentration, its polarographic wave increases; up to 1mM this increase is linear, and then the rate ofincrease of the wave-height with concentration slows down. The polarographic reduction current of Ti02 depends to a large extent also on the acidity of the solution. Figure 2 shows the concentration dependence of the limiting current in 1, 10, and 100 mM HC104. In the 10 mM acid the wave reaches its limiting height a t about 5 mM of TiOz. With more concentrated acid solutions the current is higher and in 100 mM HC104the limiting height is not attained even in the presence of 10 mM of TiOz. When the acid concentration is increased, the polarographic wave becomes steeper and shifts toward positive potentials. With 10 mM Ti02 dissolved first in 5 mM and then in 500 mM HC104,the E m shifts by +110 mV and the reciprocal slope of the linear logarithmic analysis changes from 220 to 190 mVAog unit. When plotted as a function of pH, the Euz of the Ti02 wave gives a straight line with a slope of -58 mV/pH unit. Nature of the Polarographic Wave. The factor controlling the limiting current a t the DME can be determinedlg by testing the dependence of the wave-height on the mercury pressure h. We found that, in the case where the limiting current is a linear function of Ti02 concentration (Le., in solutions below 2 mM), it is directly proportional to h112,which means that there the current is controlled by the rate of diffusion of the Ti02 particles to the electrode. This conclusion was supported also by the temperature dependence of the polarographic wave: the temperature coefficient of the limiting current was found to be 1.2%ldeg, somewhat lower than that for diffusion current in true solutions (about 1.6%/deg),which can be explained by slower diffusion in the colloidal dispersion. In the cases where the wave-height reached its concentration limit, the result was ambiguous: the experimental points were found to fit a linear dependence either on h with a large intercept on the ordinate or on hl” with a large intercept on the abscissa; there the factor determining the current is obviously an adsorption process strongly affecting the primarily diffision-controlled Faradaic reaction a t the electrode surface. These conclusions were confirmed by a study of the course of instantaneous current during the growth of the mercury drop. The current-time curves for the lower concentrations of Ti02 followed approximately the course given by the Ilkovit equationlgfor the limiting diffusion-controlled current a t the DME, Le., the relation i = Kd,tu6;with the concentration increasing to 10 mM the curves gradually acquired the

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Figure 4. Relative change of height of the Ti02 reduction wave with additions of strong electrolytes, to the solution of 10 mM Ti02 in 10 mM HClO4 added: (a) 0, HC104; +, NaC104; 0, LiC104. (b)0,NaC104; +, NaCl. (c) 0,KI; +, KBr. (d)0,LiC1; 0 CaC12;+, AlCl3. io = limiting current without and i = with addition of the electrolyte to the solution.

basically different course of the adsorption current, i = k,,t-l13, controlled by adsorption of the reaction product a t the surface of the DMEZ0(Figure 3). Effect of Electrolytes. The polarographic wave of the Ti02 colloid depends also strongly on the kind and concentration of neutral (meaning pH-neutral) electrolytes present in the solution. With a fresh 1mM Ti02 colloid in 10 mM HC104 prepared from the stock solution, the polarographic wave is gradually shifted to positive potentials when NaC104 is added. When the NaC104 concentrationreaches 0.5 M, the total shift ofEm amounts to 95 mV. The wave a t the same time becomes steeper: in 0.5 M NaC104 the reciprocal slope of its logarithmic analysis has decreased from 225 to 190mVAogunit.When strong neutral electrolytes are added in 0.1 M or higher concentration to the Ti02 solutions, the height of the polarographic wave remains linearly dependent on the colloid concentration throughout the tested concentration range and no limits of the wave-height, shown in Figure 2, occur. A marked feature of the polarography of colloidal Ti02 is the dependence of the wave-height on the “indifferent” electrolyte. With the addition of NaC104 the limiting current of the wave first increases by up to 120%, and above 0.5M NaC104 it begins to decrease. The effect of the strong electrolytes is more pronounced with higher concentrations of TiOZ. In Figure 4a the comparison is given of the effect on the wave-height caused by perchloric acid with that oflithium and sodium perchlorates; Figure ~~

(20) BrdiEka, R. 2.Elektrochem. 1942, 48,278.

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Figure 6. Effect of KSCN on the polarographic mean current of colloidal TiOz. Solutions: (a) 10 mM HClO4; (b) 10 mM Ti02 in 10 mM HC104; (c) 0.02 M KSCN in solution sub-b; (d) 0.04 M KSCN in solution sub-b. '1

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Figure 6. Effect of salicylic acid (SA) on the colloidal Ti02 solution: 1mM Ti02 in 10mM HC104 with gradually increased concentration of SA. Experimental points: 0, polarographic limiting current (left scale); x , optical absorbance at 1 = 400 nm (right scale). Solution containing 2 mM SA is turbid and makes froth when stirred.

4b shows the difference between the effects of sodium perchlorate and sodium chloride. The chloride ions differ in their effect from bromide and iodide ions as well (Figure 412). With electrolytes ofthe same anion also the kind and especially the charge of the cation play a role in the effect on the polarographic wave of the T i 0 2 colloid (Figure 4d). In absorption spectroscopy the addition of electrolytes did not cause any shift or other change of the absorption band of colloidal TiO2. In the presence of strong electrolytes of a certain critical concentration a n aggregation of the Ti02 colloid begins to take place in the bulk, manifested by the appearance of a white turbidity in the originally clear solution. This critical concentration is specific for each electrolyte. The onset of the aggregation was observed in a 10 mM Ti02 solution containing 10 mM HC104 when the electrolyte concentration exceeded 0.2 M for MgC12 and AlC13,0.3 M for LiC1, CaC12, and BaC12, 0.8 M for Mg(C10412 and Ba(C104)2,and 1 M for NaC1, KC1, KBr, KI, LiC104, and NaC104. The turbidity was clearly noticeable visually; in spectroscopy the Ti02 absorbance got increased due to light dispersion on the aggregated particles. Specific Ionic Effects. A specific effect is observed after a n addition of the SCN- ion in the solution (Figure 5). Already a small amount of KSCN produces a large increase of the polarographic wave and a simultaneous shift to positive potentials of the H+ ion reduction from the supporting acid. A different effect appears when salicylate ion is added to colloidal Ti02 solutions : a decrease of the polarographic wave. In absorption spectra an optical charge-transfer absorption band is observed which increases linearly with increasing salicylate concentration. Figure 6 shows the gradual increase of light absorption at the wavelength of

Figure 7. Polarographic (DME) and voltammetric (HMDE) curves of a 10 mM Ti02 colloid solution in 10 mM HC104. Polarographic curve recorded with minimum damping of oscillations;voltammetric curves with scan rates 5,10,20, and 50 mV-s-', respectively; current increasing in the same order.

400 nm and a simultaneous decrease of the polarographic limiting current recorded with the same solution when its salicylate content increases. Effect on the Reduction of H+ on Mercury. In TiOz colloidal solutions the reduction of the hydrogen ion on mercury from the supporting acid takes place in a diffusion-controlled wave with close to -1.6 V. Additions of Ti02 make the height of this wave increase linearly with Ti02 concentration. The Ell2 of the hydrogen wave shifts a t the same time slightly positively, by about 10 mV per 10-fold increase of concentration. In a 1mM HC104solution in 0.1 M LiC104the wave increase is such as if each individual Ti02 molecule contributed on average about 3.5 electrons to the limiting current of Hf reduction. In this solution the increment ofthe mean limiting current by Ti02 addition is about 20 times higher in the potential region of proton reduction on mercury than in the region of the Ti02 wave, more positive by 0.6 V. Voltammetry with the HMDE. Basic Voltammogram. Figure 7 shows the comparison of the dc polarographic and linear voltammetric curves of the same solution of the Ti02 colloid. The voltammograms are characterized by round and rather flat humps instead of the peaks typical for true solutions.1g Moreover,the shape of the curve is determined also by the scan rate and by the size of the HMDE: with a slower scan and smaller drop, the conditions of the process of diffusion change gradually from the linear to the spherical modez1and the humps become even flatter, approaching the shape of a polarographic wave. The dependence of the height of the hump on u , the rate of potential change (or scan),'g indicates that while in a dilute acid a t low Ti02 concentrations (below 1mM), the current is controlled by diffusion and, a t concentrations above 1mM, by adsorption, in the presence of an excess of an electrolyte the current is purely diffusion-controlled in a wide concentration range. Anodic Current. On a cyclic voltammogram of the colloidal Ti02 solution there appears an anodic Faradaic current in the course of the reverse scan, in the potential region around -0.8 V. The region of the anodic current becomes wider when the scan rate is increased (Figure 8); on the other hand, with the rate of 2 mVfs no anodic Faradaic current is apparent on the reverse branch of the voltammogram. If after reaching the negative limit of the polarization span the electrode potential is held constant for some time before the reverse scan is released, the anodic current increases and extends over a wider range of potentials, on the negative side to -1.2 V. The increase of the anodic current eventually reaches a limit ~~~

(21)Nicholson, R. S.;Shain, I. Anal. Chem. 1964, 36, 706.

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Figure 11. Cyclic voltammograms of a 10 mM Ti02 colloidal solution in 10 mM HClOd with different concentrations (mM) of NaC104 denoted on the curves. Scan rate 50 mV.s-'.

Figure 8. Cyclic voltammograms of 10 mM Ti02 in 10 mM HC104 recorded with scan rates (mV-s-l)denoted on the curves.

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Figure 12. Cyclic voltammograms of a 0.4 mM Ti02 colloidal solution in (a) 10 mM HClOd and (b) 100 mM HC104, at 22 and 40 "C, respectively. Scan rate 50 mV*s-l.

Figure 9. Cyclic voltammograms of 1mM HC104 solution in 0.1 M LiC104 with increasing concentrations of colloidal TiOz. Scan rate 50 mV.s-'; concentrations ofTiO2 (mM): (a)0.77; (b) 1.3; (c) 3; (d) 5.5.

Figure 10. Cyclic voltammograms of a 10 mM Ti02 colloidal solution in HClO4 of different concentrations (mM): (a) 10; (b) 22; (c) 50; (d) 100; (e) 200; (0 500. Scan rate 100 mV-s-'.

after the potential has been kept a t the negative span limit longer than 60 s. Effect of Experimental Conditions. With increasing concentration of Ti02 the cathodic current increases while the curve changes its shape; the anodic current increases to a lesser extent and only in the region around -0.8 V, while in the more negative region it decreases (Figure 9). The acidity of the solution has a qualitatively similar effect on the cyclic voltammogram as the concentration of the colloid (Figure 10); only when the acid concentration exceeds 0.5 M does the anodic current aquire the trend

to decrease with continuing acidity increase, and in a 1 M acid both branches of the voltammogram consist of only cathodic current (cf. Figure 12b). With gradual additions of neutral electrolytes the voltammograms of Ti02 solutions provide a sequence of curves similar to those in the case of increasing concentrations of Ti02 or of an acid, with the difference being that the anodic current around -0.8 V does not practically change. The effect of electrolytes hence shows essentially in the increase of the cathodic current. Warming of the Ti02 colloidal solutions has the stronger effect on the cyclic voltammograms the higher is the H+ ion concentration. Higher temperature obviously affects the electrode process in the same way as higher acidity, as demonstrated in Figure 12a,b. Effects ofAnions. In comparing the effects of electrolytes on the cyclic voltammograms of Ti02 solutions, we observedthat some anions play a special role in the studied electrode processes. In the presence of chloride ions, e.g., the anodic current increases over the negative part of the voltammogram. The addition of thiocyanate ion causes a n increase of both, cathodic and anodic, currents on the voltammetric curve, produces a new redox pair of humps a t -0.5 V, and shifts markedly the reduction of H+ ions on Hg to positive potentials (Figure 13). The salicylate anion has an effect opposite to most of the above ions in that it leads to a decrease of both, cathodic and anodic, currents, while producing a similar pair of humps a t -0.5 V as thiocyanate (Figure 14). Effect on H+Reduction on Mercury. The reduction of H+ ions on a Hg electrode takes place in a negative irreversible peak (Figure 15). As in polarography, the voltammetric curve also shows an almost 20 times higher effect ofTiO2 addition to the basic solution in the potential range of H+ ion reduction on Hg than in the range of the T i 0 2 reduction a t - 1.05V. When the Ti02 colloid is added

Aqueous Colloidal Ti02 Solutions

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A

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Figure 13. Cyclic voltammograms of a 10 mM Ti02 colloidal solution in 10mM HClO4 (a)and ofthe same solution containing 100 mM KSCN (b). Scan rate 50 mV.s-l. Figure 16. Cyclic voltammograms of 1.3 mM Ti02 colloidal solution in 1 mM HClOd (a and a’) and of the same solution containing 0.1 M LiC104 (b and b’). The curve of each solution is recorded with two different sensitivities(dashed curves with sensitivity 10 times higher than the full curves).

disappear; on the other hand, the current due to the reduction of the colloid increases (Figure 16).

Figure 14. Cyclic voltammograms of a 1 mM colloidal Ti02 solution in 20 mM HC104 with 0.05 mM (-1 and 1 mM (- -) salicylic acid. Scan rate 100 mV-s-l.

Figure 16. Cyclic voltammograms of 1mM HC104in 0.1 M LiC104 (a) and of the same solution containing 5.5 mM T i 0 2 colloid (b). Scan rate 50 mV.s-l.

to a dilute solution of the acid, the curve with the peak due to H+ion reduction is shifted to positive potentials, on the foot of the curve the current corresponding to the reverse scan crosses the ascending part of the curve, and in the potential region negative of the peak the curve is distorted (Figure 16). The height of the Hf reduction peak first decreases, and only when the concentration of Ti02 exceeds that of the acid, the peak increases linearly with the colloid additions. If to a dilute solution of T i 0 2 in a dilute acid a n excess of a neutral electrolyte is added, the peak corresponding to hydrogen ion reduction on Hg decreases considerably and the distortions on the curve

Discussion Effect of Particle Size on Its Electroreduction. The polarographic results confirmed what we found already with the Sn02 colloids16 and what we discussed in the introductory paper:19smaller protonated colloidal particles get reduced a t less negative potentials. This information follows from the polarograms recorded after dialysis as well as after centrifuging the TiOz solutions. Our data suggest that in the course of the centrifugation apparently some aggregation of the particles also takes place. The E m of the polarographic waves of solutions containing smaller particles are more positive, and after the separation from the bigger particles by dialysis the waves are also steeper, or the reciprocals oftheir logarithmic analyses are less than those pertaining to the initial distribution of particles. This is because their size dispersion is smaller, and hence the potential range of their electrode reactions is less expanded. The comparison ofabsorption spectra or of polarograms of two T i 0 2 solutions of identical concentration prepared once by diluting the stock solution and once by dissolving the powdered colloid shows that the particles in the powder are of bigger size. Further down we consider the direct reduction of Ti02 particles by electrons. In such a process which is not preceded by protonation the smaller particles should be reduced a t more negative potentials than the bigger ones (cf. discussion in ref 19). It is possible that the wide extension of the anodic current to negative potentials in cyclic voltammetry (as a t high scanning rates or in presence of C1-, e.g.1reflects the particle size distribution in oxidation reactions following the primary direct reduction of the colloids. Electroreduction of H+ on the Ti02 Surface. Similarly as in the case of SnO2 the electroreduction of the Ti02 colloid depends strongly on H+ concentration, and the shift of the corresponding wave toward positive potentials with increasing acidity is even more pronounced than for SnO2. This indicates that the electrode process is connectedwith a pH-dependent redox reaction, as shown below. We assume that the first reaction of the protonated Ti02 particles, when they come into contact with the

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Surface reactions 1-3 and 5 constitute a pH-dependent reversible system which explains the Nernstian 58-mV shift of Ell2 with the change of the solution acidity by a unit of pH. {TiO,} Hf {Ti02}H+ (1) Inhibition of the ElectrodeProcessby Aggregates. By undergoing the reduction of their surface protons, the { TiO,}H+ e{ Ti0,)H (2) particles are being deprived of the stabilizing charge. As a result in the course of electrolysis, the partly reduced Here again the brackets { } denote schematically the colloids aggregate a t the electrode surface. In that way, colloidal phase, and the symbols attached to them stand in general, the electroreduction of protonated oxide for the species bound to the particle surface. In eq 2 we particles is accompanied, particularly in dilute solutions, put the two-way arrow, as the electroreduction of H+ on by generation of colloidal aggregates which hinder the a compact TiOz electrode is a reversible p r o c e s ~ . By ~ ~ - ~ ~electrode reaction. The inhibitive effect of the particle the oxidation reaction in (2) we would explain the anodic agglomerates can be clearly seen, e.g., in Figure 16, curve current in the potential region around -0.8 V, which a': there in the course of H+ reduction a t the Hg surface persists under varied experimental conditions (cf. Figures the increase of pH around the electrode leads to neutral8-13). The potential of the electrode process (1) (2) ization of all protons, free in the solution as well as bound depends on the size of the particle, as discussed in our to the particles. Stripped of the protective charge, the previous paper,lgand on the state ofthe particle surface-it particles aggregate and adsorb a t the electrode. The is presumably affected by species specifically adsorbed inhibition of the electrolytic reaction by the adsorbed layer from the solution, as our experiments with SCN- ions distorts the voltammetric curve in a characteristic way. indicate; however, a systematic study of Hf reduction on In this manner the nonlinear concentration dependences Ti02 colloids has yet to be done. of the heights of the polarographic waves (Figure 2) and It was observedz4that a n equilibrium gets established of the voltammetric peaks or humps (Figure 9) can be between Ti(1V) and Ti(II1) when Ti02 is heated in the explained by the autoinhibitive nature of the electrode atmosphere of hydrogen in the presence of Pt; the reducing process; this accounts also for the change of the shape of agent, atomic hydrogen, obviously participates in that polarographic and voltammetric curves with increasing equilibrium. Added in proof: A reversible reduction of Ti02 concentration (two separate reduction steps merging highly dispersed Ti02 by gaseous hydrogen could be into a single drawn-out one), as the autoinhibition followed in a zeolite matrix by reflectance spectroscopy increases. (G. Grubert, Diploma Thesis, University ofBremen, 1995). Effect of Electrolytes. The relative stability of the If this applies also to the situation at the Ti02 particle Ti02 colloids allowed us to study the marked changes of surface held a t negative potentials, the H atom formed in the polarographic and voltammetric currents caused by the nascent state in the Faradaic process (2) will be able additions of indifferent electrolytes into the solution to reduce the surface Ti(1V) species to Ti(II1): (Figure 4). As absorption spectra of the studied solutions did not show any changes, it was obvious that the cause {TiO,}H {TiO,)OTiOH (3) of the effect had to be sought in the electrode/solution interface. where OTiOH represents one ofthe surface Ti02 molecules In a solution containing a low concentration of ions, the which got reduced by the electrolytically produced hypositively charged colloidal particles come into direct drogen. At more negative potentials the electrolytic contact with the metallic surface of the negatively charged evolution of gaseous hydrogen at the particle surface could electrode. (Mercury electrode bears a negative charge in compete with that reaction: the potential range negative of its point of zero charge, which in aqueous HC104 solutions lies a t about -0.52 V {TiO,)H H+ e- {TiO,} H, (4) vs SCE.) The primary reduction product, the partly reduced Ti02 colloid rid of its protective charge, remains However, because of kinetic complications due to the then sticking to the electrode and blocks its surface. In parallel reactions and to the autoinhibition, it can come such a case the instantaneous current on the DME is into effect only a t high concentrations of H+ or a t high controlled by the adsorption of the product, and when the temperatures, where irreversible reaction 4 prevails in a adsorption is fast, the course of the i-t curve follows the wide range of potentials, as shown in Figures 10 and 12. relation for the adsorption current;20Le., i = as The trivalent titanium formed in the surface reduction was experimentally observed (Figure 3). With gradual will be also amenable to electrooxidation: additions of a strong electrolyte to the solution, the electrical double layers build up from the diffusion to the {TiO,}OTiOH H,O {TiO,}OTi(OH), e- H+ compact structure a t the particles as well as a t the electrode, and the proportion of the compact t o the diffusion (5) part of the double layers gradually increases. When the negatively charged electrode, is the reduction of protons on the Ti02 surface, as in the case of the SnOz colloids:

-

+

+

-

+

-

+ +

+

-

-

+

+ +

That oxidation, we suppose, takes place under the conditions when the anodic current extends beyond the peak due to oxidation of atomic hydrogen toward a more negative potential region (cf., e.g., Figure 8); the wide potential range of the electrooxidation process in the polydisperse solution may be due to the dependence of redox potential on particle size.lg (22) Koudelka, M.; Monnier, A,;Augustynski,J. J.Electrochem. SOC. 198, 131, 745. (23) Finklea, H. 0.InSemiconductorElectrodes; Finklea, H. O., Ed.; Studies in Physical and Theoretical Chemistry; Elsevier: New York, 1988; VOl. 53, p 81. (24) Gall, H.; Manchot, W. Chem. Ber. 1925,58B,482.

protonated Ti02 particle approaches the electrode in a sufficiently concentrated electrolyte solution, the immediate particle/electrode contact will be prevented by the counterions in their double layers and the transfer of the Faradaic electron will take place by tunneling through the two double layers. In this way the products of aggregation, the partly discharged particles, forming a t a distance from the electrode, will not automatically stick to it, and the inhibitive effect on the electrode reaction will be diminished. This interpretation of the electrolyte effect is in agreement with the cyclic voltammograms (Figure 1l),which show that it is prevailingly the primary, i.e., the cathodic, current which increases on addition of

Aqueous Colloidal Ti02 Solutions

Langmuir, Vol. 11, No. 11, 1995 4307

electrolytes. In polarography with the DME it can be followed from the course of the i-t curves how the blocking of the electrode surface by the particle aggregates sets in ever later when the electrolyte concentrationis increased, until it disappears from the curve and the current follows a monotonous increase typical for uninhibited electrode processes. Hence in this indirect manner-from the suppression of the inhibitive effect in polarography and voltammetry-conclusions cgn be drawn on the state of the double layer of colloidal particles. Further additions of the electrolyte into the solution of Ti02 colloids lead eventually to an aggregationofparticles in the bulk of the solution, the well-known flocculation of hydrophilic sols. Consequently, the concentration of electroactive colloids in the solution decreases and so decreases the polarographicor the voltammetric current. An advancing flocculation gives finally rise to a visible turbidity in the solution. The degree of lowering of the polarographidvoltammetric current and the appearance of the turbidity agree well with the Hofmeister series of efficiency ofvarious ions in bringing about the flocculation of sols. The additions of strong acids into the Ti02 solutions have a multiple effect upon the reduction current. The acids act as electrolytes in the formation of the electrical double layers. Besides, their protons stabilizing the oxidetype colloids, keep the pH of the solution low, which limits the aggregation also in the vicinity of the electrode, and, above all, undergo electroreduction in the electrode process. Reduction of Ti02 Surface Hydroxo Complex. With solutions containingTi02 particles in concentrations less than 1 mM, the polarographic limiting current due to H+reduction on the particle surface increases to another reduction wave when the electrode potential approaches -1.0 V. We assume that in that potential region the energy is reached that is necessary for reduction of the surface complex OTi(0H)Zexisting at the interface of the in water insoluble Ti02 with aqueous solutions: { TiO,}

+ H,O * { TiO,}OTi(OH),

(6)

The reduction of this complex {TiO,}OTi(OH),

+ e + H+ * {TiO,}OTiOH + H,O (7)

is the reverse of anodic reaction 5.

Electrolytic Evolution of Hz on Hg Electrodes. This reaction, a classical example of a totally irreversible process, can be catalyzed by adsorbed insoluble hydroxides of some metals, such as aluminum.25 In Figures 9 and 15 it can be seen that the adsorption of aggregated Ti02 particles also acts in that way : on addition of Ti02 colloid to an acidic solution the current beyond -1.2 V, due to reduction of H+ ions on mercury, shifts towards positive potentials (Figure 9); the catalytic activity explains also the high values of the limiting current of H+ion reduction produced by additions ofthe T i 0 2 colloid (Figure 15).From a certain degree of electrode coverage the catalyst acts simultaneously as an inhibitor of the electrode process. The approximate compensation of the two opposite effects leads to the quasi-diffusion-controlled character of the limiting current. Catalysis of the electrolytic evolution of hydrogen on mercury is obviously the reaction which contributes to the increase of current at negative potentials in solutions containing larger colloidal particles and thus causes the split of the logarithmic analysis of the polaro-

graphic waves into two linear parts: the more positive, less steep part corresponds to the reduction of protons on the surfaces of the polydisperse particles, while the more negative, steeper part pertains to the catalyzed reduction of protons on mercury.

Specific Surface Redox Reactions on Ti02 Colloids. Special situations occur when in the solution appear species which are ready to form chemical bonds with the titanium atoms at the particle surface. Such species get specifically adsorbed at the colloid and thus provide possibilities for electron trapping. On the Ti02 surface facing aqueous solutions, bound to the surface Ti atoms, there are -OH groups of which some can be exchanged for anions from the solution and some have acidic characternZ6 Chloride anions, e.g., probably get exchanged for the surface OH- and create thus a partly reversible surface redox system operating in the potential region around -0.9 V, as our results indicate: an addition of chlorides causes a slight increase of the cathodic and a marked increase of the anodic currents ofthe cyclic voltammogram in that region. The following reactions presumably take place there: {TiO,}

{TiO,)OTi(OH)Cl+ e- * {TiO,)OTi(OH)

+ C1-

(8)

(9)

In true aqueous solutions Ti(IV)forms a series of complexes with increasing numbers of C1 atoms when the concentration of C1- in the solution increases;27in the same tendency their electroreduction becomes gradually more Similarly the SCN- ions get bound to the Ti02 surface and produce effects similar to C1- and some more (Figure 13): the cathodic and the anodic currents increase, the reduction of H+ ions on Hg is shifted strongly toward positive potentials, and a new pair of humps, cathodic and anodic, appear around -0.5 V. In true solutions the SCN- ion reacts with Ti(1V) like Cl-,29,30and also the electroreduction of the ensuing complexes is a n a l o g ~ u s . ~ J ~ The colloidal Ti02 particle with the -SCN group bound to its surface is an efficient catalyst of electrolyticevolution of hydrogen on mercury, as can be seen in Figures 5 and 13. Free SCN- is not adsorbed at a negatively charged Hg surface and hence cannot act catalytically. However, when bound to the adsorbable Ti02 particle, the -SCN group containing the catalytically active S and N atoms31 has all reasons to be the cause of the observed lowering ofthe hydrogen overvoltage. It is possible that the increase of the cathodic current is partly due to a catalytic effect of the bound -SCN groups on the hydrogen evolution at the Ti02 surface as well. The redox pair ofhumps at -0.5 V presumably corresponds to the redox reaction of a part of the colloid which has dissolved under the effect of a higher concentration ofthe ligand and reacts as a titanium thiocyanate complex in molecular dispersion. A different effect occurs with salicylate ions present in the solution. It has been shown32that salicylate forms surface complexes with colloidal TiOn. The polarographic cathodic and the cyclic voltammetric cathodic and anodic (26)Boehm, H.P. Discuss. Faraday SOC.1971,52,264. (27)Rumpf, M. E.Ann. Chim. (Paris) 1937,8 , 456. (28)Strubl, R. Collect. Czech. Chem. Commun. 1938,10, 475. (29)Delafosse, D.Compt. Rend. 1953,236,2313. (30)Schmitz-Du Mont, 0.;Ross, B. Angew. Chem. 1984,76, 304. (31)Stackelberg, M. v.; Hans, W.; Jensch, W. 2. Elektrochem. 1968, 62. , 839. ~~(32)Moser, J.; Punchihewa, S.; Infelta, P. P.; Gratzel, M. Langmuir 1991, 7,3012. ~~

(25)Heyrovsky, M. Collect. Czech. Chem. Commun. 1980,25,3120.

+ C1- + H+ * {TiO,}OTi(OH)Cl

4308 Langmuir, Vol. 11, No. 11, 1995

Heyrovsky et al. Scheme 1

PH

_+H++OH-

{TiOZjOTi +e-

-OH-

-1

(TiOP)

-H20

+H+ e (Ti02)H+

1 1 +H~O

ll

"

+H+ +e-

(Ti021 + HZ

It

-H+ E2 -1 .OV -e-

{TiOzjOTiOH

-I

+e-

-H+

4

1

Scheme 2 currents decrease when salicylic acid is added to the + Y2-- 2 OHsolution (Figures 6 and 14). This is the sign that at the particle surfaces the salicylate ligand occupies both -OH I -Y2-+2H20-2H+ groups and forms a strong surface complex not reducible X / / +X- - OHin the potential range available under given conditions. (TiOz)OTi {TiOz}OTiY (TiOdOTi The simultaneous increase of optical absorbancy in the -X- +H20-H+ charge-transfer band (Figure6) pertaining to the t i t a n i d salicylate electron donorlacceptor interaction indicates that the orientation of the bound salicylate favors the charge transfer localized a t the particle surface. In &I {TiOzjOTiOH aqueous solutions Ti(1V) and salicylic acid form comwhich were studied also p o l a r ~ g r a p h i c a l l y . ~ ~ ~tion ~ of particles on the electrode surface. Addition of The appearance of the pair of humps a t -0.5 V (Figure ligands which get specifically bound to the surface of Ti02 14) can be interpreted as a redox reaction of titanium/ can either increase or decrease the electron transfer in salicylate complexes dissolved from the particles into the the electrode process. The Ti02 colloids act catalytically solution analogously to the above case of thiocyanates. on the electrolytic evolution of Hz on mercury. From analysis ofpolarographic curves information can be gained Conclusions on the state of the colloidal particles in the solution. As in the electroreduction of SnOz colloids, the first On the basis of our results, we propose a simplified cathodic reaction on the surface of Ti02 particles in acidic scheme of the processes taking place in electroreduction media is the reduction of H+ ions; however, due to much of Ti02 particles. For the sake of space the scheme is higher stability of T i 0 2 colloidal solutions many more presented in two parts (Schemes 1and 2) coupled by the processes can be studied with them than with the SnOz surface redox reaction of the Ti(1V) and Ti(II1) hydroxo solutions. The proton reduction takes place as a partly complexes. The individual reactions have been discussed reversible process on the surface of Ti02 particles; at high in the text above. As there, the brackets { } stand for the H+concentrations or a t high temperatures the irreversible colloidal phase, and the chemical symbols adjacent to them HZ formation prevails a t negative potentials. An increase represent a species bound to the particle surface from the of cathodic current brought about by additions of elecsolution side or a molecule singled out in the surface layer; trolytes is caused by a double-layer effect on the aggregaX i s a monodentate and Y a bidentate ligand. The mean potentials correspondingto the individual electron transfer (33)Sommer, L.Collect. Czech. Chem. Commun. 1957,22,453. reactions are denoted by E with indicated approximate (34)Dutt, N.K.;Goswami, N. 2.Anorg. Allg. Chem. 1959,289,258. (35)Babko, A. K.;Volkova, A. I.; Getman, T. E. Zh. Neorg. Khim. values for polydisperse solutions.

-

4

1962,7,145,1121. (36)Habashy, G. M.J . Electroanal. Chem. 1964,8, 237.

LA950011J