Langmuir 1995,11, 4293-4299 4293

Polarography and Voltammetry of Aqueous Colloidal SnOs Solutions

Michael Heyrovsky" and Jaromir Jirkovsky

Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej3.kova 3, 182 23 Prague 8, Czech Republic

Bernd R. Muller

Institute of Inorganic, Analytical, and Physical Chemistry, University of Berne, Freiestrasse 3, 3000 Berne 9, Switzerland

Received January 12, 1995. In Final Form: August 3, 1995@

In aqueous HClO4 solutions the protonated colloidal SnO2 particles undergo a two-stage reduction at renewed mercury electrodes, the second stage being discernible at voltammetric curves. First a re reduced the protons on the surface of the colloid; t h e hydrogen atoms thus formed reduce subsequently SnO2 to metallic tin. At more negative potentials then prevails a direct reduction of hydroxo complexes of tin on t h e surface of the particles to metal. A side product of both reactions, an aggregate of SnO2 colloid deprived of its stabilizing positive charge, adsorbs at the electrode surface and hinders fur ther electroreduction.

Introduction Tin dioxide has been frequent ly used in spectroelec-

t rochemistry for its exquisite property of providing thin transparent and electrically conduct ing layers on various material~.l-~ Besides, as a wide-gap semiconductor it has a lso proved t o be an appropriate electrode material for photoelectrochemical investigations.6-11 At present there a p p e a r s the tendency to combine all its advantages in using SnO2 in the form of colloids for applications as diverse as gas sensing12 and solar energy conversion.13 F o r the latter the highly porous s t r u c t u r e of surface films, made of sintered colloidal semiconductor particles, appears especially at t ract ive, as it c a n be easily modified by various sensitizers. l4

As a result of the m a n y studies the electrochemical propert ies of the bulk p h a s e SnO2 are known; however, not much research has been done o n the electrochemistry of the SnO2 colloid^.'^ Our present paper is intended as a contribution toward filling that gap. F o r an introductory approach t o the new field, we used the basic techniques of d c polarography and linear and cyclic vol tammetry. The polarographic and vol tammetr ic studies of colloids proved t o provide an insight in to the stabi l i ty and the

@ Abstract published inAdvanceACSAbstracts, October 1,1995. (1) Kuwana, T.; Darlington, R. K.; Leedy, D. W. Anal. Chem. 1964,

36. 2023. (2) Laitinen, H. A.; Vincent, C. A.; Bednarski, T. M. J . Electrochem.

(3) Mollers, F.; Memming, R. Ber. Bunsen-Ges. Phys. Chem. 1973, SOC. 1968,115, 1024.

77 R7Q . . , - .1. (4) Winograd, N.; Kuwana, T. Spectroelectrochemistry at Optically

Transparent Electrodes. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1974; Vol. 7, p 1.

( 5 ) Armstrong, N. R.; Lin, A. W. C.; Fujihara, M.; Kuwana, T. Anal. Chem. 1976,48, 741. (6) Mollers, F.; Memming, R. Ber. Bunsen-Ges. Phys. Chem. 1972,

76, 469. (7) Gleria, M.; Memming, R. Z. Phys. Chem. (Munich) 1975,98,303. (8) Wrighton, M. S.; Morse, D. L.; Ellis, A. B.; Ginley, D. S.;

(9) Memming, R.; Schroppel, F. Chem. Phys. Lett. 1979, 62, 207. (10) Ghosh, P. K.; Spiro, T. G. J . Am. Chem. SOC. 1980, 102, 5543. (11) Frippiat, A.; Kirsch-De Mesmaeker, A. J . Electrochem. SOC. 1987,

(12) Ando, M.; Suto, S.; Suzuki, T.; Tsuchida, T.; Nakayama, C.;

(13) Bedja, I.; Hotchandani, S.; Kamat, P. V. J . Phys. Chem. 1994,

(14) Ford, W. E.; Rotgers, M. A. J. J . Phys. Chem. 1994, 98, 3822. (15) Mulvaney, P.; Grieser, F.; Meisel, D. Langmuir 1990, 6, 567.

Abrahamson, H. B. J . Am. Chem. SOC. 1976, 98,44.

134, 66.

Miura, N.; Yamazoe, N. J . Mater. Chem. 1994, 4 , 631.

98, 4133.

corrosion processes of the part ic les in solution when charged t o var ious potentials.

Experimental Section All solutions were prepared from reagents of purity grade "pro

analysi" without further purification and from water, either redistilled from a quartz still or purified by the Millipore Milli- Q/RO system, with specific conductivity of less than 5 x 10-8 S/cm.

The SnOz colloidal solution was prepared by hydrolysis of SnC14 in a modified procedure described by Mulvaney et al.I5 A 3-mL aliquot of freshly distilled SnC14 was added dropwise under stirring to 1 L of water. The ensued solution was left standing in the dark for 24 h to allow for the formation of a white amorphous precipitate. The water above the precipitate was then decanted, and the precipitate was washed with water until its pH rose to 6. Thus treated, the precipitate was then peptized by a dropwise addition of 1 M NaOH which gave rise to a clear colloidal solution of SnOz. The solution was then subjected to dialysis against water until pH 7.5 was attained. The colloidal solutions prepared in this way, colorless and perfectly clear, were found to be stable for several years, irrespective of their concentration.

The SnOz solutions used in our experiments were polydisperse; the transmission electron microscopy (TEM) revealed an average particle diameter of 2.5 nm. For the TEM measurement we let 1 or 2 drops of 6.5 mM solution dry up with 1 drop of acetone on a carbon film as a support. The pictures taken with the magnification 1:1.39 x lo6 showed crystal planes ofthe individual particles which indicated that the particles were basically minute single crystals of cassiterite structure.

The stability of the colloidal SnOz solutions in the presence of electrolytes is rather poor. In 1 M NaOH, NaC104, KC1, or HC1 SnOz precipitates immediately after addition. Mulvaney e t al.15 report their unsuccessful attempts to stabilize SnOz in solutions below pH 5. Of all acids we found only HClO4 to be compatible with the colloid to a certain extent, depending on the concentration. In HClO4less concentrated than 1 M the dissolved SnOz colloids were precipitating in a matter of hours to minutes-the more dilute acid the faster the precipitation. In 1 M HC1O4 the dilute SnOz solutions (1 mM to 50 pM) were stable for several days, more concentrated ones were more stable, and a 24 mM solution in 1 M HC104, when kept in refrigerator in a dark bottle, was yielding unchanging spectral and electro- chemical characteristics for more than 2 years; that one was used as our stock solution. The solutions which were found relatively stable by spectral as well as by electrochemical methods yielded perfectly reproducible polarographic and voltammetric curves; undergoing electrolysis with renewed mercury electrodes did not change the properties of the solutions.

0743-746319512411-4293$09.00/0 0 1995 American Chemical Society

4294 Langmuir, Vol. 11, No. 11, 1995 Heyrovskj et al.

Figure 1. Polarographic reduction wave of 0.5 mM SnO2 colloid in 1 M HC104 and its “logarithmic analysis”. The curve is reproduced in contours connecting maxima and minima, respectively, of the oscillations of the current recorded with minimum damping (cf. Figure 3a); for the logarithmic analysis the mean of the oscillations was taken.

Polarographic and voltammetric measurements were carried out by the PA4 polarograph with the static mercury drop electrode SMDE 1 and the 4103 and 4106 XY recorders of “Laboratorni pfistroje”, Prague, and by the Polarecord 626 with the VA stand 663, the multimode electrode, and the VA scanner of “Metrohm”, Herisau. We restricted our measurements to the basic dc techniques. The instantaneous current-time curves were recorded by means of the Endim 622.01 XY recorder (Messap- paratewerk, Schlotheim i.Thiir.) with the 631.01 time base generator. For the dropping mercury electrode (DME) a glass capillary with its end bent by 90” and cut under the angle of 45” was used to ensure a complete renewal of the solution around the capillary tip by slight convection caused by the falling mercury drops. Before each measurement was started, the solution was deaerated in the cell by a stream of bubbles of pure nitrogen; during the measurement nitrogen was passed above the solution. In the three-electrode system a small F’t sheet was used as auxiliary electrode and a saturated calomel electrode (SCE) or a silver-silver chloride in saturated KCl solution was used as the reference electrode. In the present paper all values of electrochemical potential are referred to SCE. All measurements were carried out at room temperature, i.e., at 22 f 1 “C.

Results Polarographic Measurements. The colloidal SnO2

solutions in HC104 give a single, well defined polarographic reduction wave with the half-wave potential (Em) between -0.80 and -0.85 V, depending on the composition of the solution (Figure 1). The “logarithmic analysis’’ of the wave,16J7 Le., the plot of the log[& - i)hI versusE (where i is the mean current, id the mean limiting diffusion current, and E the potential of the electrode), provides a straight line (cf. Figure 1). For solutions in 1 M HC104 the slope of this line does not change with SnO2 concen- tration, its reciprocal value being 160 mVAog unit; with HC104 concentration decreasing to 0.1 M this value decreases to 81 mVAog unit while Ell2 shifts to negative potentials by 15 mV.

The limiting current of the SnO2 reduction wave is directly proportional to the colloid concentration up to 5 mM; above 5 mM the dependence bends toward a limit attained at about 20 mM Sn02 (Figure 2). When tested

(16) Heyrovskjr, J.; KPlta, J. Principles of Polarography; Academic

(17) Heyrovskjr, M.; Jirkovskjr, J. Langmuir, previous paper in this Press: New York, 1966; pp 129, 213.

issue.

; [SnoJ (mM) OO 2 4 G

Figure 2. Concentration dependence of the polarographic limiting current due to electroreduction of colloidal SnOz in 1 M HC104.

A n

Figure 3. Instantaneous current-time curves of the 10 mM SnO2 colloid in 1 M HC104 recorded with DME at constant potentials given under each curve. The dots mark the tU6 course of a diffusion-controlled current at the DME according to the IlkoviE equation.16

for the effect of mercury pressure (given by the height h of the level of mercury in the reservoir above the tip of the capillary) upon the limiting current,ls the wave obtained in 20 mM solution showed a linear increase with h; on the other hand the wave in 0.5 mM SnO2 grew linearly with h lI2, In polarography a diffusion-controlled current in- creases linearly with hU2, whereas a linear increase of the wave with h indicates a current controlled by adsorption. With concentrations of SnOz increasing from 0.2 to 2 mM the Ell2 shifted to positive potentials by about 20 mV.

The time course of the instantaneous current a t the growing DME recorded at successively increasing negative values of constant potential in the region of limiting current (Figure 3) confirms the result of the wave-height dependence on h: in the electroreduction there appears an adsorption process which blocks the electrode surface and prevents further increase of the mean current.

The wave-height of SnO2 is also a linear function of the concentration of the acid: from 0.1 to 1 M HC104 the limiting current of 0.5 mM SnOz increases about 3 times; the E1/2 shifts a t the same time in the positive direction by 20 mV.

Polarograms of Sn02 solutions in 1 M HC104, which had stood for several hours in the electrolytic cell a t room temperature, showed a small round maximum on top of the limiting current a t -1.05 V (Figure 4a); with higher SnO2 concentration it was more pronounced.

Voltammetry with the Hanging Mercury Drop Electrode (HMDE). The voltammetric curves of the SnO2 solutions in HC104 show a prominent reduction peak (Figure 4); the potential of this peakE, shifts from -0.85 to -1.00 V when the rate of potential scan u increases from 5 to 200 mVw’ (Figure 5).

When the HC104 concentration changes, the peak height changes, while E, remains constant. With increasing concentration of HC104 from 0.1 M the peak increases less than linearly to a maximum in 0.5 M acid, and then it decreases; on its ascending part a shoulder of constant height gradually develops and shifts to positive potentials (Figure 6).

(18) Ref 16; p 86.

Aqueous Colloidal SnO2 Solutions Langmuir, Vol. 21, No. 11, 1995 4295

Figure 4. Comparison of a polarographic (a) and a voltam- metric (b) curve of the 0.5 mM SnO2 colloid in 1 M HC101. Both curves show a catalytic current at -1.05 V. The polarographic curve was recorded with minimum damping; the voltammetric scan rate was 50 mV.s-'.

E (VI -0.6 -0.8 4.0

Figure 5. Voltammetric curves of the 0.6 mM SnOz colloid in 1 M HClOl recorded with different scan rates (mV*s-l), given on the curves.

The dependence of the peak height on the SnOz concentration is also nonlinear, tending toward a limit which is reached at concentration above 10 mM. On the other hand, the height of the shoulder attains its limit already in 1 mM SnOz solution.

When tested for the effect ofthe rate ofpotential change u upon the height of the peak,lg the 20 mM solution of SnOz shows a linear dependence on u , while in the 0.5 mM solution the peak increases linearly with u~'~. After the shoulder has reached its limiting height a t 1 mM concentration, it increases linearly with u. Similarly as in polarography in the test of the dependence of wave height on h, in voltammetry a linear dependence of the

Figure 6. Voltammetric curves (scan rate, 100 mV-s-') of the 1.14 mM SnOz colloid in HClO4 of different concentrations: (a) 0.1 M (- -1; (b) 0.2 M (-1; (c) 0.5 M ( -a ) ; (d) 1 M (- -); (e) 2 M (-).

Figure 7. Cyclic voltammograms ofthe 1.14 mM SnOz colloid in 1 M HCIOl recorded with different scan rates (mV.s-'): (a) 10 (-); (b) 20 (...); (c) 50 (- -); (d) 100 (-).

peak height on u1I2 is the sign of a diffusion-controlled current and a peak increasing linearly with v signalizes an adsorption control of the current.

The voltammograms of the SnOz colloid in 1 M HC104 show an additional cathodic current of the shape of a polarographic wave in the potential region negative of the main peak, a t about -1.05 V (cf. Figure 4b), which decreases and ultimately disappears with increasing scan rate (Figure 5). Such a kind of dependence is characteristic of the currents due to a catalytic evolution of hydrogen.20

The cyclic voltammograms of SnOz colloidal solutions yield one sharp anodic peak at about -0.45 V and a less prominent round maximum a t +0.20 V. The latter does not appear when the scan rate is slower than 20 mVwl (Figure 7). When the HMDE is polarized by several subsequent voltammetric cycles, there appears on the curve a new small cathodic peak on the second and further cycles a t -0.42 V which is reversible to the sharp anodic peak (Figure 8). This pair of peaks corresponds to the redox system Sn(II)/Sn(Hg), well studied on the HMDE.21,22 The more positive round anodic peak is due to an irreversible electrooxidation of Sn(I1) to Sn(IV). The fact that there appears no cathodic counterpart to that peak on the second cycle indicates that the primarily produced form of Sn(IV) undergoes a fast inactivation step-pre- sumably a hydrolytic reaction.

On the voltammetric curves of cycles repeated on one electrode surface with scanning rates less than 50 m V w l the heights of both, the shoulder and the main cathodic peak, gradually decrease and the peak shifts to negative potentials (cf. Figure 8).

To study the effect of accumulation of the SnOz colloid on the HMDE, the electrode was maintained, after the circuit had been closed, a t the initial potentialE,. For the

(20) Ref 16; p 410. (21) Phillips, S. L.; Shain, I. Anal. Chem. 1962, 34, 262. (22) Bond, A. M. Anal. Chem. 1970,42, 1165. (19) Ref 16; p 505.

4296 Langmuir, Vol. 11, No. 11, 1995 Heyrovsky et al.

Figure 8. Five successive cyclic voltammograms of the 1.14 mM SnOz colloid in 1 M HC104 recorded with one drop of the HMDE (scan rate, 50 mV-s-l), the order of curves (the main peak gradually decreasing): (1) (-); (2) (- -1; (3) (-a); (4) (-1; (5) (- -).

f, A .1 -

-0.6 -0.8 -10 E(V)

Figure 9. Accumulation of the SnOz colloid on one drop of the HMDE at the initial potential Ei = -0.10 V while waiting for (a) 0, (b) 30, (c) 60, and (d) 120 s prior to scan: voltammetric curves of the 0.5 mM SnOz colloid in 1 M HClO,; scan rate, 50 mV*s-'.

first series of experiments Ei was -0.10 V, and for the second series, -0.60 V. After a waiting period of SUC- cessively prolonged periods of time from 0 to 120 s, the scan was applied and the voltammogram recorded. Both series of experiments gave quantitatively equal results: while the main cathodic peak was steadily increasing with a prolonged time of accumulation, the shoulder on its positive side had reached its limiting height already after 30 s (Figure 9).

In 1 M and 0.5 M HClOl solutions the SnOz colloids are fairly stable and their voltammograms change little with time: the height of the main reduction peak would decrease by lO%/h. However, on the voltammogram of a solution containing 0.5 M HCl04 and 0.5 M NaC104 the main reduction peak of SnOz decreases in the matter of minutes, indicating the salting-out effect of the colloid by the neutral electrolyte. The stability of the SnO2 colloid in 0.1 M supporting acid was tested by a prolonged stirring of the solution by a passing stream of nitrogen bubbles. The result of this test is shown in Figure 10 (in 0.1 M HC104 only one simple reduction peak appears-cf. Figure 6): after prolonged stirring the reduction peak increases, widens, and shifts to negative potentials. Solutions of the SnOz colloid in 0.01 M HC104 are slightly turbid, and on their voltammograms of irregular and changing shapes the reduction peak, larger than in more acidic solutions, appears a t more negative potentials; the typical anodic peak still persists unchanged on the cyclic voltammogram.

As a contrast to the voltammetric behavior of colloidal SnOz, Figure 11 shows the cyclic voltammogram of Sn-

~~ ~

(23) Lingane, J. J. J. Am. Chem. SOC. 1946, 67, 919. (24) Kovalenko, P. N.; Lektorskaya, N. A. Zuuod. Lab. 1960,16,924. (25) Phillips, S. L.; Morgan, E. Anal. Chem. 1961, 33, 1192.

Figure 10. Effect ofthe time of stirring on the voltammogram ofthe0.5mhTSnOzcolloidin0.1MHC10~(scanrate, 5OmV-s-'). The time of stirring after preparing the solution in the cell and before recording the curve: (a) 5 min; (b) further 5 min; (c) further 10 min; (d) further 20 min.

Figure 11. Cyclic voltammogram of 0.1 mM SnC14 in 1 M HC1 saturated with NH&l (scan rate, 50 mV*s-l).

(IV) in a concentrated chloride solution where a strong stannic chlorocomplex is formed.23

Discussion

Electrolytic Reactions of Sn(W. In Figure 7 we can see that the tetravalent tin is produced at the electrode in a noncomplexing solution at +0.2 V, however, on a cyclic voltammogram this anodic peak does not have any cathodic counterpart-the primarily formed Sn(1V) turns rapidly into an electroinactive form. SnO2, which is the probable short-lived intermediate in this inactivation reaction, is obviously not reducible in the normally accessible potential range. Tetravalent tin is stable and electroreducible in homogeneous aqueous solutions only when bound in stable complexes by ligands like halides,23 oxalate, citrate, tartarateF4 pyrogall01,~~ or 3-mercapto- propionic acid.26 Figure 11 shows the cyclic voltammo- gram of a solution containing Sn(IV) in the form of the SnCl62- complex. The first, round cathodic peak corre- sponds to a relatively slow irreversible reduction of Sn- (IV) to Sn(II), and the second, sharp cathodic peak, to a fast reversible reduction of a Sn(I1) to Sn amalgam. The latter peak and its anodic counterpart are split due to separate reactions of free and complexed Sn(I1) ions, respectively. The picture of electroactivity of Sn(IV) bound

~

(26) Phillips, S. L.; Toomey, R. A. Anal. Chem. 1966, 37, 607.

Aqueous Colloidal SnOs Solutions

in the form of colloidal Sn02, as provided by polarography and voltammetry, is different.

Electroreduction of H+ on a SnOz Surface. An essential condition of a reproducible polarographidvol- tammetric behavior of the colloidal SnOz solution is a sufficiently high concentration of hydrogen ions. As mentioned above, reasonably stable solutions can be prepared by adding HC1O4 to an aqueous SnOz colloid to final concentrations of 1 M HC104 and higher than 0.5 mM Sn02. When the electrode dipped in these solutions is polarized to negative potentials, the reduction current begins to rise between -0.5 and -0.6 V, where mercury in a HC104 solution has its potential of zero charge. Beyond this potential in the negative direction, it acquires an increasingly negative charge which attracts electro- statically the protonated particles.

The overvoltage for the electrolytic evolution of hydrogen on the surface of SnO2 is obviously considerably lower than on the surface of mercury. On thin layer Sn02 electrodes in acidic media the electrolytic reduction of hydrogen takes place a t potentials more negative than -0.5 V; the primary electrochemical reaction is then followed by chemical reduction of the tin dioxide electrode surface by atomic hydrogen to metallic tin.2,5s27 In general chemistry it has been known28 that tin dioxide is reduced to tin when heated in a stream ofhydrogen or when treated with zinc and hydrochloric acid. The atomic hydrogen primarily produced at an SnOz electrode is presumably able to carry out the reduction equally well:

Langmuir, Vol. 11, No. 11, 1995 4297

gradual reduction of one SnO2 molecule a t the particle surface

4H+ + 4e- - 4H (1)

SnO, + 4H- Sn + 2H20 (2)

Our experimental results seem to prove that the same processes as on SnOz films occur also on the surface of colloidal particles. There the above net reaction starts presumably as

{ SnO,} + H+ - { SnO,}H+ (3)

where the brackets { 1 denote the colloidal phase and the symbol adjacent to the bracket represents the species attached to the surface of the colloidal particle.

On coming into conducting contact with the electrode, the particles acquire its potential which they maintain while in contact. At potentials more negative than -0.5 V the protons attached to the the particle surface accept electrons, originating from the metallic electrode and transferred to them along the n-semiconductor surface, and turn into hydrogen atoms

{SnO,}Hf + e- - {SnO,}H (4)

In distinction from H+ ion electroreduction on Ti02 surface29 we have no experimental evidence of reversibility of reaction 4; however, this may be due to the high rate of the follow-up heterogeneous chemical process formally expressed as a sequence of surface reactions describing

(27) Muller, B. Ph.D. Dissertation, University of Hamburg, 1993, p

(28) Mellor, J. W. A Comprehensive Treatise on Inorganic and

(29) Heyrovsky, M.; JirkovskJi, J.; Struplovl-BartlEovl, M. Langmuir,

139.

Theoretical Chemistry; Longmans: London, 1927; Vol. 7, p 399.

following paper in this issue.

{Sn02}H - {SnO,}SnOOH (5)

{SnO,}SnOOH + H+ - +H{SnO,}SnOOH ( 6 )

+H{SnO,}SnOOH + e- - H{SnO,}SnOOH (7)

H(Sn02}Sn00H - {SnO,}Sn(OH), (8)

etc., until

{SnO,}Sn(OH2), - {SnO,}Sn + 2H,O (9)

and, a t the Hg electrode

{SnO,}Sn + Hg - {SnO,} + Sn[Hgl (10)

The atom oftin produced on the particle surface by reaction 9 in contact with the electrode dissolves readily in mercury in the form of tin amalgam Sn[Hgl.

The whole process is presumably faster than the reaction which would lead to the evolution of gaseous hydrogen

{ SnO,}H + Hf + e- - { SnO,} + H, (1 1)

The formation of the shoulder on the positive side of the main voltammetric reduction peak confirms that the first reaction in the cathodic process on the SnOz surface is the reduction of hydrogen. When the concentration of HCIOI is increased from 0.1 M (cf. Figure 61, the colloidal particles in the solution become more protonated and as such they get their surface protons reduced at less negative poten- tials. However, in the course of the electroreduction the protons are consumed and the protective positive charge of the colloids involved in the process is diminished. Simultaneously the pH in the solution layer surrounding the electrode increases; when it approaches 4.3, the isoelectric point of Sn02,15 the colloid begins to precipitate in the diffusion layer and that slows down the increase of the current with the potential. As a result, the curve gains the shape of a shoulder.

Direct Electroreduction of Surface Hydroxocom- plexes of Sn. At potentials more negative than the H+ reduction on the SnO2 surface, the cathodic current on the voltammogram increases to the main peak. According to the experimental results this steep increase is presum- ably due to the transfer of electrons from the electrode directly to the surface atoms of S n W ) which in contact with aqueous media form hydroxocomplexes, the simplest type of which can be formulated as

{ SnO,}OSnO + H,O * { SnO,)OSn(OH), (12)

The electrode reaction thus changes from hydrogen atom mediated to a direct electron transfer surface reduction process. The change from the one-electron to the four- electron electrode reaction explains the appearance of the prominent peak on the voltammetric curve at the onset of the direct reduction. The situation at the mercury electrode with its potential becoming gradually more negative is also well illustrated by the sequence of instantaneous current-time curves recorded with the DME in the region of the polarographic mean limiting current, shown in Figure 3. After the limiting diffusion current for the hydrogen ion reduction from the particle surface has been reached, the direct reduction of the surface hydroxo complex joins in as a parallel process,

4298 Langmuir, Vol. 11, No. 11, 1995

which at more negative potentials gradually takes over the control of the current. As proved by cyclic voltammetry (cf. Figures 7 and 8 ) and by the shift ofE, of the main peak to negative potentials with increasing scan rate30 (cf. Figures 5 and 71, this reduction is irreversible. Since at the limiting current of H+ reduction the preprotonation of SnOz colloids a t the electrode surface does not come into consideration, the main voltammetric peak does not shift with increasing concentration of the acid (cf. Figure 6). However, the net reduction of SnOz is accompanied also by consumption of protons, i.e., by an increase of pH in the solution around the electrode according to the net process

(SnO,)OSn(OH), + 4e- + 4H' - {SnO,}Sn + 3H,O

(13)

Heyrouskj et al.

reverse branch of the curve appears the typical peak of the anodic current which is due to the electrolytic dissolution of the tin amalgam. If the potential of the electrode is held at -0.65 V for a gradually prolonged time before starting the reverse scan, the anodic peak correspondingly increases. Hence in two different po- tential regions-that of the shoulder and that of the main peak-the same product, metallic tin, is formed from SnOz in two different processes: a t less negative potentials in the chemical reduction by hydrogen atoms and a t more negative potentials in direct electroreduction by electrons. In a study of microheterogeneous electron transfer in aqueous colloidal SnOz solutions of pH > 9 Mulvaney et al.15 concluded that lattice reduction was slightly favored over proton reduction. In the electron transfer process on mercury electrodes in acid solutions proton reduction is favored over lattice reduction as long as the electrode potential does not reach the negative region where the lattice reduction prevails. At any rate, both reduction processes lead to the same product.

In electrochemistry the problem has been often dis- cussed whether in a particular electrolytic process the reduction proceeds via hydrogen atoms or via electrons (see, e.g., refs 31 and 32). The present results show that the mechanism by which the reduction actually proceeds depends above all on the potential of the electrode. On electrode material with a low hydrogen overvoltage the reduction by hydrogen is possible when the potential is not too negative; in a more negative potential region, in general, the direct reduction by electrons is likely to take place.

Catalytic Electroreduction of H+ Ions on the Hg Electrode. The cathodic current at -1.05 V, observed on polarographic as well as on voltammetric curves (Figures 3,4 , and 7), is caused by the catalytic evolution of hydrogen on the surface of the tin amalgam which is formed as a product of the electrode reaction. Phillips and Shain21 found that the hydrogen overvoltage on a tin amalgam is markedly lower than on pure mercury. At the DME, the catalytic current appears when large protonated colloidal particles arrive at the electrode and the protons are reduced at their surface as well as on the surface of the DME containing simultaneously formed tin amalgam. At the HMDE the catalysis occurs in a situation when the rates of the two ways of the SnOz reduction are favorably tuned together with the scan rate and the inhibiting action of the deprotonated colloid.

State of the Colloidal Solution. Figure 10 demon- strates that higher agglomeration of particles, promoted by stirring the solution, shifts the reduction potential of the colloids in a negative direction; i.e., that smaller protonated particles are more easily reduced than bigger 0ne5.I~ This further means that more drawn-out current- potential curves are a sign of a wider particle size dispersion. In general, it appears that diluting the supporting acid decreases the stability of the solution; the results of the logarithmic analysis of polarographic waves indicate that the acid dilution leads primarily to better homogenization of the particle dispersion with only a slight increase of aggregation. On the other hand, increasing concentration of the acid increases the propor- tion of smaller particles in the solution, which results in an increase of the total Faradaic current17 and a shift of E112 to positive potentials. In solutions standing for longer time at room temperature, high SnOz aggregates are formed with relatively low net charge which contribute

or

{SnO,}OSn(OH), + 4e- + H 2 0 - {SnO,}Sn + 4 0 H - (14)

so the colloid approaching the electrode gets neutralized and precipitates a t the electrode surface. Hence, as the i-t curves in Figure 3 clearly show, the steep initial increase of the current is followed by a decrease due to the blockage of the electrode surface. In the mean polaro- graphic current the two extremes cancel each other out-that is the reason why on the polarographic curves only one continuous wave appears and the two stages of the electrode process, so distinctly marked at the volta- mmetric and i-t curves, are not discernible.

Inhibition of Electroreduction by Aggregated Colloids. The mechanism of electroreduction of the SnOz colloid thus includes the autoinhibitive process of the electrode passivation by the adsorbed precipitate. The experiments with accumulation on the HMDE have shown that even the unreduced, i.e., positively charged, colloidal particles (presumably especially the large size ones) adsorb spontaneously on the positively charged electrode surface in spite of the electrostatic repulsion (in fact in this case we should rather consider the electrostatic attraction between the positive colloids and the negative ions in the solution side of the electrode double layer). The inhibitive effect of the byproduct of electroreduction is also evident from the curves recorded with repeated voltammetric scans (cf. Figure 8). This explains the nonlinear concen- tration dependence of the heights of the polarographic wave and of the voltammetric shoulder and peak, and also their dependence on h and u , respectively. At low concentrations of SnOz the inhibition is relatively small and the limiting currents are diffusion-controlled, whereas a t high concentrations the currents are limited by the adsorption of the precipitate a t the electrode surface. As the precipitation is caused by the increase of pH in the solution around the electrode, the acidity of the supporting electrolyte has an enhancing effect on the reduction current-therefore the increase of the polarographic and voltammetric response with increasing concentration of

Two Parallel Paths of One Reduction Process. Cyclic voltammetry provides proof that metallic tin is produced at the very beginning of the electrolytic process when Hf ions are reduced a t the particle surface: if, while going toward negative potentials, the scan is stopped at the very foot of the cathodic peak (in 0.1 or 0.2 M HC104) or shoulder (in 0.5 or 1 M HC104) at -0.65 V, then on the

HC104.

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

(31) Stackelberg, M. v.; Weber, P. 2. Elektrochem. 1952, 56, 806. (32) Muller, 0. H. InAdvances in Polarography; Longmuir, I . S . , Ed.;

Pergamon Press: New York, 1960; Vol. 1, p 251.

Aqueous Colloidal SnOz Solutions

less to the main electrolytic current but due to their higher adsorptivity bring about the catalytic reduction of Hf ions a t negative potentials.

Conclusions For the electrolytic processes which the SnOz colloidal

particles undergo at mercury electrodes we suggest the following simplified scheme where the brackets { } symbolize the colloidal and [ I the metallic phases:

The reaction starts a t the surface of the semiconductor particle when this touches the electrode under the potential El. The protons carried by the colloid get reduced, and the atomic hydrogen thus generated reduces

Langmuir, Vol. 11, No. 11, 1995 4299

immediately the nearest surface molecules of SnO2 through three hypothetical intermediates down to metallic tin. The atoms of tin appearing in this way on the surface ofthe particle in contact with the electrode dissolve readily in mercury and form tin amalgam. At the more negative potential E2 the particle in touch with the electrode is able to accept the electrons by the surface hydroxo complex. The rapidly transferred electrons produce again metallic tin on the particle surface, and the net process consumes the same amount of protons as the first reaction. The difference is that the rate of electron transfer is now faster and more potential dependent than before and that the protons enter into the reaction only after the electrons. The byproduct of both electrode reactions, the aggregate of SnO2 particles deprived of protective charge, adsorbs at the surface of the mercury electrode and acts as a hindrance to the diffusion-controlled reduction of SnOz colloids to tin amalgam.

The polarographic and voltammetric curves due to the above outlined processes contain information about the state of the colloidal solutions. So far this information is of qualitative nature; in order to gain quantitative data on the solution properties, further systematic research is necessary, preferably with monodisperse colloidal solutions.

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