The reduction of molecular oxygen at single crystal rutile electrodes

THE REDUCTION OF MOLECULAR OXYGEN AT SINGLE CRYSTAL RUTILE ELECTRODES B. PARKINSON Ames Laboratory,

DOE, Iowa State University, U.S.A.

and FRANCO DECKER,J. F. JULIKO* and M. ABRAMOVICH IF UNICAMP, Campinas 13 100, Brazil

and H~LIO C. CHAGAS I.Q., USP, SBo Paula, Brazil (Revised version received20 August1979)

Abstract - The reduction of molecular oxygen was investigated on single crystal rutile electrodes. Cyclic voltammetry and a rotating ringdisk electrodewereemployed to elucidate the mechanism and energetics of the reaction. It was found that oxygen reduction proceeds through a surface species at potentials positive of the flatband potential in the electrolytes used. It was also demonstrated that the majority of the reaction proceeds via a 4-electron pathway while a small percentage of the oxygen is reduced by two electrons to hydrogen peroxide.

INTRODlJCl-ION There has been considerable interest in the last few years in the use of semiconducting oxide electrodes to accomplish the photoelectrolysis of water[l]. The rutile form of TiO, has been the most extensively studied of these materials. However, relatively little attention has been paid to redox processes on this material in the dark. Bard and others[2-41 have studied the behaviour of a variety of redox couples in the dark in both non+queous[2, 41 and aqueous[3] electrolytes. Morisaki et ar[S] noticed current for oxygen reduction on TiO, and also reported a photoeffect on the current. Kraeutler and Bard[4] also observed that the reduction of oxygen occurs at potentials considerably positive of the flatband potential and was simultaneously occurring with water oxidation when the electrode was illuminated with uv light. Recently Clechet et al have shown that H,OI is a product of oxygen reduction on polycrystalline anodic films of TiO,[6]. In this paper we present a detailed investigation of the reduction of O2 on single crystal rutile electrodes. A variety of electrochemical techniques have been applied to elucidate the mechanism and energetics involved in the reaction. EXPERIMENTAL Single crystal TiOl wafers 1 mm thick were cut from a boule supplied by Materials Research Co. and polished to optical grade. Circular samples for rotating *Assistant Professor at Universidade Federal do CearP, Brasil. (De@0 de Fisica). Fellow of PICD - Plan0 Institucional de Capacitaclo

de D-tes.

ring disk electrodes {RRDE) were cut out of wafers by means of a diamond drill and lathe-polished to a perfect circular shape. After reducing the samples at 800°C for 20min in a 10% Hz flowing gas mixture, contact was provided by rubbing Ga-In alloy on the back crystal surface prior to attaching a Cu wire or a steel shaft (RRDE) by means of Ag paste. The stationary electrode sample (SE) was then incorporated into a polyester capsule, leaving a front surface of 0.17 cm2 free for contact with the electrolyte ; while samples for RRDE were carefully mounted within a Teflon-sealed brass annulus with a Pt ring on top (disk radius r1 = 0.275 cm, inner and outer ring radii r2 = 0.316crn and r3 = 0.452cm, respectively) the gap within ring and disk being filled with epoxy. In both cases the face exposed to electrolyte was a (001) crystallographic plane. All samples were etched for about 15 s in a HF/HNO, solution[S] before each experiment. The electrolytical cell was a standard twocompartment cell, with a flat quartz window on the TiO, side for the experiments with light. A saturated calomel electrode (SCE) was the reference, and a 10cm2 Pt foil was the counter-electrode. For the electrolyte preparation commercially available reagents from Baker (KOH) and Carlo Erba (others) were employed. Electrolytes were deaereated using ultrapure N, gas (White Martins Co.) as delivered. A potentiostat, improving a scheme published in an earlier work[9], was assembled and coupled with a triangle wave generator for the experiments with the SE. The mechanical and electronic system for the experiments with the RRDE was previously built from one of the authors and is described elsewhere[iO]. Hewlett-Packard 7004B X-Y recorders and Tektronix 7613 store oscilloscope were used for recording current voltage curves.

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00

-10 Potential

/V

w

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see

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Fig. 1. Cyclic voltammograms at 0.25 V/s of TiOz : (1) in airsaturated 1 M KOH ; (2) in the same eIectrolytedeaaeated.

/,-

RESULTS

A nicely shaped cathodic wave, attributed to the irreversible reduction of dissolved molecular oxygen, was observed at potentials close to the flatband potential EFB when scanning the potentiaI of the TiO, electrode in air saturated solutions (Fig. 1). It WS_ observed that this wave was enhanced by exposing the crystal to alkaline solutions (1 M NaOH, 1 M KOH). After this initial treatment in base, the enhancement of the wave was not diminished by subsequent routine etching of the crystal surface with concentrated acid, nor by operation of the electrode in dimerent electrolytes. Cyclic voltammograms showed that the peak current I, increased linearly with the square root of the scan rate, in electrolytes of different pH (Fig. 2). A similar behaviour was observed for the irreversible reduction of [Fe(CN),]‘at the same TiO, electrode, in the same supporting electrolytes but

02

0.4

06

20

40

Squme root of s-p

60

mte/mVhs-&

Fig. 2. The reduction of oxygen at TiOz. Peak current dependence on potential sweep rate, in air saturated: A : 1 M NaOH, pH 13; 0:

I

1 M Na,S04 + 0.01 M NaOH,pH : 1 M Na,SO,, pH 6.

11.5;

without the presence of 0,. These results for [Fe(CN)6]3reduction are in agreement with those reported previously by Bard et 431. When the system was purged of oxygen a much smaller reversible wave was apparent in the same potential region as the O2 and Fe(CN);’ reductions were observed, (Fig. I). The symmetry and the linear dependence of peak currents (anodic and cathodic) on scan rate (Fig. 3) were indicative of a surface controlled phenomena. The pH dependence of the peak height

0.8

IO

12

sweep raw/vsFig. 3. The reduction of surface species at TiO,. Peak current dependence on potential sweep rate, in deaereated: A : 1 M KOH, pH 13.5 ; 0 : 0.5 M N+HPO, + 0.5 M Na,PO,, pH 11.4; 0: 1 M KC1 + lo-’ M HCI. pH 2; . : 0.5 M CH,COOH + 0.5 M CH,COONa, pH 4.3.

523

Tbe reduction of molecular oxygen at single crystal rutile electrodes

2

0

4

8

6

IO

14

12

PH

Fig. 4. pH dependenceof surface species reduction peak current (solution deaereated. sweep rate l.OV/s) (Fig. 4) closely followed

the pH dependence of the magnitude of the 0, reduction current (not shown). Figure 5 illustrates two interesting experimental facts: first, the peak potentials EP for both 0, and [Fe(CN)J3reduction and for the cathodic surface wave in the same electrolyte are nearly equal and all appear to be positive to the flatband potential EFB of TiO, aa measured by Wrigbton et af[ll]. Second, I&. shifts toward negative potential when the electrolyte PI-I is increased, paralleling the 59mV per unit EPB shift. Experiments in very acid electrolytes were hampered by hydrogen evolution which obscures the surface wave and oxygen reduction. Further experiments with the rotating Pt-ring TiO,disk electrode (RRDE) showed that the limiting current for both 0s and [Fe(CN)6]3-- are directly proportional to the square root of the rotation speed, although at high speed the experimental I, for oxygen reduction is systematically smaller than the current predicted from the Levich plot drawn for rotation speeds up to 2000 rev/mm (Fig. 6). During the RRDE experience it was also observed, that the magnitude of the current due to the surface process in the N-purged electrolyte was independent of the rotation speed. From the slope of the lines in Fig. 6 the ring collection efficiency (NJ was determined to be 0.37. This number is - 10 per cent less than the number

calculated from the geometry of the system[6], however the experimental number was used in subsequent calculations. The number of electrons (n) involved in O1 and [Fe(CN),13reduction reactions was calculated using the Levich equation as written in [12] and Kolthoffs data[14] for the concentration and diiusion coefficient of oxygen in similar electrolytes. While [Fe(CN),]‘+ [Fe(CN),]“was confirmed to be a one electron reaction at the TiO, electrode, the calculated n for the O2 reduction reaction was 3.8. The 5 per cent difference from the expected value confirmed a previous hypothesis that the oxygen reduction at TiOz might partially occur through a peroxide intermediate. The platinum ring electrode was then employed to ascertain the extent of peroxide formation in this system. 400

300

Q a_ 200 2

T-----7

5 .-P c E 2

loo

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-100

I

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Fig. 5.pH dependenceof reduction peak potentials E, (sweep rate 80mV/s).- Solid line : Aatband potential of 30, OS pH[li];

i.

Oxygen E,:

0:

[Fe(CN),]‘-

surfa&z species E,; -e:

E,.

.mot

4

6

of rotatkm fr8CJuency

8

Aizf

Fig. 6. The reductionof O2 and [Fe(CN),]’ - at TiOz RRDE. Levich plots for: I : 0, reduction in 1 M KOH; l :

[F~(cN),]~reduction in 0.5 M Na,HPO, + OSM Na,PO,; 0 : Fe(CN)z- oxidation at Pt ring in 0.5 M Na,HPO,

+ 0.5 M Na,PO,.

524

B. PARKINSON

et al

/

Potential V

vs see

Fig. 7. Detection of H,O, formation during 0, reduction. A : reduction of 0, at TiO, disk electrode. B : correspondinglimitingcurrentfor HzOz oxidationat,theringclectrode. 1 M KOH. 2: fresuencyf= 4Hz; 3: f= 9Hz; 4:f = 16Hz.

Figure 7 shows the results of a ring-disk experiment. The current at the ring is due to the oxidation of H,Oz back to OL at 0.15 V (the potential where this current was found to be maximum) and shows a peak at low rotation speeds and low disk overpotentiat. The disk current, due to O2 reduction reaches a transport limited plateau. DISCUSSION

Recently considerable attention has been paid to correlating the energy of semiconductor band-edges to the energy level distribution of redox species in solution, in order to elucidate whether charges originating from the valence band or the conduction band of the semiconductor are transferred across the junction directly, or through some intermediate levels, ie surface states[2, 31. The possibility that charge transfer can take place via surface states was demonstrated in acetonitrile solutions by Frank and Bard[Z]. Also in aqueous solution the presence of surface states was claimed as probable, with considerable but indirect experimental evidence to support their existence. Arguments in this sense were given by Noufi et a&3] and Dutoit et a![ 141, who found anomalously large Tafel slopes (compared to that predicted from a model[lS] for a conduction band to redox level process) for the reduction of several couples at TiO,. Recently Laser and Gottesfeld[lB] have demonstrated the existence of surface states within the band gap on a TiOl film electrode with the use of a two beam spectroscopic method. In some cases[5, 111 the presence of surface states was suggested in order to justify the nearly 100 per cent efficient photo-oxidation of water at TiOl by a process which should occur only through an isoenergetic charge transfer. Surface states were also indicated to be one of the causes of the frequency dependence and non-linear behaviour of Mott-Schottky plots by several authors[l7-191. An experimental demonstration was given recently by Tomkiewin[20]. Our experiments on TiO, in deaereated solutions

show that some surface species undergo a reversible electron transfer in the same potential range where the irreversible reduction of oxygen is observed. It is our contention that those species mediate the oxygen reduction at Ti02 surface. From cyclic voltammograms in deaereated 1 M KOH we were able to calculate the number of charging-discharging states corresponding to such surface species to be about (3 x 10’4)cm-2. This number is very sensitive to surface pre-treatment as well as on the electrolyte used. A limited number of active surtace sites could also account for the less than theoretical collection efficiency of the RRDE. Surface pre-treatment with strong base, for example, enhanced the oxygen reduction reaction at TiO,; at the same time, it also causes adeviation from linearity ofthe Mott-Schottky plots for the TiO, samples, an effect which Tomkiewicz[20] has shown to be due to charged surface species. The reduction waves for the surface species, O2 and [Fe(CN),Jas observed by cyclic voltammetry shift together towards negative potentials as the electrolyte pH increases, paralleling the 60 mV per pH unit shift of EFm This suggests the surface species is involved in a protonation equilibrium similar to that observed for regular oxide semiconductor surfaces, where protonation of dangling bonds has been proposed to explain the EFB dependence on pH. However, neither the energy distribution of those surface states nor their communication to the bulk of the semiconductor are really well known. Our results show that the current for the surface species, O1 and [Fe(CN),]‘reduction waves starts positive of E,, by about 0.5 V in the different electrolytes. Therefore a relatively high energy surface state (less than OSeV below conduction band edge), with an efficient communication with the conduction band via a tunnelling process would explain the easy charge-discharge mechanism. Tunnelling processes could readily occur in the highly reduced TiOz samples used in this study. As pointed out from Noufi et ar[3] even couples as positive as O3 and [Fe(CN)J3may have a reasonable overlap with a high energy state due to their large

The reduction of molecular oxygen at single crystal rutilt electrodes energy distribution, caused by the high energy of interaction with water. Charge transfer to such couples would result in a mass-transfer limited behaviour, as observed for O2 and [Fe(CN)6]3reductions, and would deplete electrons from the surface states, then removing the origin of the reversal anoclic wave appearing in voltammogranuns in deaereated supporting electrolytes. The oxygen reduction reaction is known to be kinetically slow at most electrodes and we found that at TiOz its rotation speed dependence at negative potentials is not as well behaved as that of [Fe(CN),]‘-. As an example, in cyclic voltammetry some reverse anodic wave is observed at high sweep rates showing an incomplete reaction of the surface state lying electrons with oxygen or oxidation of H,Oz back to Oz. In RRDE experiments some bending in the Levich plots for O2 reduction, not observed for [Fe(CN),13-, occurs at high rotation speeds. We have demonstrated that oxygen reduction at TiO, in alkaline solutions proceeds primarily through a Celectron reaction. Therefore a mechanism involving more than one step is invoked. We found also that some H201 is produced during 0, reduction at Ti03. We believe that the small amount of H,O, detected is a product in a reaction path parallel to that in which O2 is reduced to water without H,O, as an intermediate. Therefore the mechanism that we propose for oxygen reduction at TiOz should not differ too much from the mechanism proposed by Damjanovic et a/[217 for the 0, reduction on Pt in alkaline solution, where H,O, is not included as an intermediate in the reaction path. Plots of the ratio of disk current to ring current against the inverse of the square root of rotation speed are similar to those shown in reference [2i]. Following the results given by cyclic voltammetry, active sites at semiconductor surface, called S, are taken in account in the reaction mechanism. The first step of the oxygen to water reduction sequence is therefore: 2s+o,+2so

(1)

followed by the quasi-equilibrium steps : 250 + 2e- + H,O it 2SOH + 20H-

(2)

ZSOH + 2e- + 2s + 20H-.

(3)

On the other hand, hydrogen peroxide production at TiOz would follow a parallel 2 electron reaction path : (1) 2s + 0, + so

(2) 2S0 + 2e- + H,O ti 2SOH + 20H2SOH e 25 + H,O,

(4)

followed by its chemical or electrochemical reduction to water or hydroxide ion: H,O,

-+O,

+ H,O

H,O,

+ 2e- + 20H-.

(5)

or (6)

Experiments are under way with the RRDE technique to determine the extent of H,O, production during II,0 oxidation at some illuminated semicon-

52.5

ductor electrodes where the reactions may proceed through high energy intermediates due to the availability of highly reactive holes.

CONCLUSIONS

The reduction of oxygen at TiO, electrodes was studied by employing several electrochemical techniques. Surface species have been shown to mediate this reaction and estimates of the corresponding number of states and the energies of these states have been given. Small amounts of hydrogen peroxide were detected during the oxygen reduction reaction. A mechanism to illustrate the reaction paths of oxygen to water, with and without hydrogen peroxide as a side product, has been proposed. Acknowledgemenrs - This work was Partially supported by grants from FAPESP and FINEP. The authors wish to thank Dr. R. Brenzikofer for his collaboration before and during this work.

REFERENCES

1. M. Tomkiewicz and H. Fay, J. appl. Phys. l&1(1979) ; A. J. Nozik, Proc. 2nd World Hydrogen Congo. 1217, Ziirich (1978) and references therein. 2. S. N. Frank and A. J. Bard, J. Am. them. Sot. 97. 7427 (1975). 3. R. N.‘Nouti, P. A. Kohl, S. N. Frank and A. 1. Bard, J. eleczrochem. Sot. 125. 246 f1978). 4. B. Kraeutler and A. J. Bard. Nouo. J. Chemie 3.31 11979). 5. H. Morisaki, M. Hariya and K. Yarawa, Ap~l.~phyk Let;. _1(1. -. 7 11977) 6. F. Clechet, C. Nartelet, J. R. Martin and R. Olier, Electrochim. Acta 24. 457 (1979). 7. W. J. Alhery and M. L. Hit&man, Ring-disc Electrodes p. 84, Clarendon, Oxford (1971). and references therein. B. A. Fujishima, K. Honda and S. Kikuchi, Chem. Sot. Japan 72, 108 (1974). 9. D. D. Macdonakl, Trransient Techniques in Electrochemistrv D. 32. Plenum. New York (1977). 10. H. c_Ch&as, Ci&ia e Cult& 29,601 (1977). 11. J. M. Bolts and M. S. Wrighton, J. whys. Chem. 80.2641 (1976). 12. E. Gileadi, E. Kirowa-Eisner and J. Penciner, Interfacbl Electrochemistry. an Experimental Approach, p. 309, Addison-Wesley,London (1975). __ 13. Kolthoff. J. Am. them. Sot. 63. 1013 11941). Ber. Bun14. E. C. Dutoit. F. Cardon and’W. P. ‘Go&, senges. Phys. Chem., 76, 475 (1972). 15. H. Gerischor, Physica! Chemistry : An Advanced Treatise Vol. 9A, Academic Press, New York (1970). 16. D. Laser and S. Gottesfcld,J. electrochem. Sot. 124, 475 (1979). 17. E. C. Dutoit, R. L. Van Meirhaeghe and F. Cardon, Ber. Bunsenges. Phys. Chem. 79, 1206 (1975). 18. M. Tomkiewicz, Semiconductor Liquid Junction Solar Ce!ls D. 92: Efecrr. Sac.. 77-3. 99 (1977). 19. H. G&&h&, S&r Energy Cdnv&ion. !Fopics in Applied Physics Vol. 31, Springer, Berlin (1979). 20. M. Tomkiewicz, J. elecrrochem. Sot. 126, 1505 (1979). 21. A. Damjanovic, M. A. Genshaw and 1. O’M. Bockris, J. e[ecrrochem. Sm. 114, 1107 (1967). .---I-