Oxidized rare earth-iron compounds in photoassisted electrolysis of water

Oxidized rare earth-iron compounds in photoassisted electrolysis of water

Mat. Res. Bull., Vol. 18, pp. 389-395, 1983. Printed in the USA. 0025-5408/83/040389-07503.00/0 C o p y r i g h t © 1983 Pergamon P r e s s Ltd. OXID...

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Mat. Res. Bull., Vol. 18, pp. 389-395, 1983. Printed in the USA. 0025-5408/83/040389-07503.00/0 C o p y r i g h t © 1983 Pergamon P r e s s Ltd.

OXIDIZED RARE EARTH-IRON COMPOUNDS IN PHOTOASSISTED ELECTROLYSIS

OF WATER

A. Shamsi and W. E. Wallace Department of Chemistry University of Pittsburgh Pittsburgh, PA 15260

(Received J a n u a r y 21, 1983; Communicated by W. B. White)

ABSTRACT Anodes for use in a photoelectrolysis cell can be prepared by heating rare earth (R) iron compounds (RFe2, RFe3, R2FeI7 and R6Fe23) in air using the flame of a Fisher burner. For comparison, iron oxide and titanium oxide were also prepared by heating iron and titanium metal in air. The current-voltage (I-V) properties of these materials were measured in aqueous solution of i M NaOH. The flatband potentials (Vfb) were determined with respect to a fixed potential (SCE) by extrapolating the I-V curves to zero current. Anodic photocurrents were generated by light with energy exceeding the band gap of the photoanode material. Photoelectrolysis of water occurred, as evidenced by the evolution of oxygen from the RFeO x electrodes and hydrogen from the Pt electrode. Most of these electrodes were found to be stable under experimental conditions.

Introduction Solar photoelectrolysis is currently a topic of considerable interest. To achieve an efficiency such that it constitutes a promising procedure for solar energy conversion it is necessary to have a semiconducting electrode that has (i) a suitable band structure such that the photoelectrochemical cell operates at zero or small bias and (2) has a band gap that optimizes the use of the solar spectrum. In addition, the electrode must be chemically inert so as to be stable under the conditions of cell operation. Many investigations of light-sensitive materials are under way, growing out of the pioneering photoelectrochemical work of Fujishima and Honda. I

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A. SHAMSI, e t al.

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They discovered that sunlight electrolyzes water into H 2 and 02 in a photocell involving n-TiO 2 electrodes. However, a small bias voltage was required. In later work, e.g., that of Wrighton et al., 2 using SrTiO 3 photoanodes, it was shown that an external bias was not required. Butler and Ginley noted 3 that the external bias needed for the operation is a function of the flatband potential, which is related to the electron affinity of the photoanode. They pointed out that systems with low electron affinity result in a negative flatband potential, and this required less applied potential for the decomposition of water. Rauh et al. prepared 4 some low electron affinity oxide semiconductors by doping the compounds SrTiO3, BaTi03, CaTiO 3 and TiO 2 with 3d elements. While these photoanodes displayed some unfavorable characteristics (large dark current, chemical instability, etc.), one favorable feature was the observation that the Co 3+, Cr 3+ and Fe 3+ substituted compounds respond to a higher wavelength light and thus afford the promise of hi~her efficiency in solar photolysis devices. Bolts and Wrighton measured J (by the differential capacitance method) the flatband potential of n-TiO2, Sn02, SrTi03, KTaO 3 and KTa0.77Nb0.2303 electrodes and found that the plots of flatband potential versus pH are linear with the slope of 0.059 V/pH. In addition, they showed a correlation between the flatband potential and the onset for the photoanodic currents (02 and H 2 evolution at the electrodes). These several aspects of the photoelectrolysis of H20 and many others have been reviewed by Nozik 6 and by Butler and Ginley. 7 Fe203 is currently receiving considerable attention because it is stable under typical operating conditions and with a bandgap energy of about 2.2 eV it is able to absorb more energy of the solar spectrum than other semiconductor oxides. 8 However, the valence band of Fe203 is more positive than the potential of H+/H2 redox couple and it, therefore, requires an external bias for hydrogen production. Since the threshold voltage for hydrogen production is a function of the electron affinity of the semiconductor and is related to the electronegativity of the constituent atom, 9 the bias voltage needed can be reduced by appropriate doping. Recent studies of oxidized intermetallic compounds containing rare earths suggested an alternative mode of forming photoanodes. Surface analyses of a number of rare earth intermetallic compounds indicate the presence of an oxidized surface layer which is in contact with the bulk intermetallic. I0-12 The latter is a powerful reducing agent. Its affinity for oxygen could render the surface oxides nonstoichiometric and hence conductive. Thus, it appeared that oxidized RFe x systems (where R = a rare earth) might be of interest as novel photoanodes, in a manner somewhat like that of the oxidized systems which have proved 13,14 to be significant as novel supported catalysts. Also, the surface iron oxide might contain rare earth as a dopant, making its flatband potential more favorable for use as a photoanode. The present work was undertaken to subject these ideas to experimental test. Experimental Procedures Electrode Preparation The RFe~ systems were prepared by techniques that are standard in this laboratory. I~ They were powdered and then pressed into disks (~ 0.3 cm x 1 cm in diameter) and annealed in a sealed quartz tube (10-3 torr) for one week at about IO00°C to assure the formation of a single-phase material. The disks then were polished to a mirror finish with a fine emery paper. Prior to oxidation the disks were cleaned and degreased in methanol and were then oxidized by placing them in the flame of a Fisher burner for about 15 min. Temperature of the burner varied between II00-1400°C. The

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391

sample was red hot during the heating period, and it was found upon cooling that a dark grey oxide had formed on the surface of the sample. The flame oxidation appears to be a good method, perhaps the best method, among several ways of forming oxide film on metal. 16 Ohmic contact between a copper wire and electrode was established by rubbing a Ga-In alloy onto one face of the electrode. The copper wire was attached by applying silver epoxy. The electrode and wire were then sealed in plexiglass so that the electrical contact was placed outside the solution. Electrochemical Measurements The three-electrode cell configuration, a potentiostat and an x-y recorder were used to record the current-voltage curve. The three electrodes were working electrodes, a Pt foil (i cm x 1 cm) counter-electrode, and a SCE reference electrode with a luggin probe. Illumination was provided by a 150 w xenon lamp (Model 6137 Oreil Co.). The electrolyte was 1 M NaOH prepared with deionized water. Results and Discussion Current-voltage (l-V) curves of iron oxide, titanium oxide and oxides of light rare earth intermetallic compounds were determined in the dark and under illumination. Data are shown in Pigs. 1-3. The I-V curve for the iron oxide shows that there is a large rise in anodic current starting about -0.45 V (SCE) in 1 M NaOH, which may be regarded as the flatband potential (Vfb) . The flatband potential determined thusly is reproducible to about 0.05 V. The dark current in the potential region -0.5 and +0.8 V relative to the standard calomel electrode (SCE) is essentially zero. Oxygen and hydrogen I

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i

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~

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,

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~\

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i

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Ho Fe 2 0 x

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FIG. 1 Current vs. potential curves for FeO and several oxidized RFe 2 systems. Potential is measured against the saturated calomel electrode. - oxidized Fe, - - - oxidized DyFe2, ..... oxidized HoFe 2. R e curves marked with d give the dark current. Data for oxidized ErFe 2 (not shown) were essentially identical with those for oxidized HoFe 2.

392

A. SHAMSI, et al.

2.4

!

I

"

I

I

Vol. 18, No. 4

I

30x ErFe30x

Dy Fe

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~

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0.8

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Potentiol, v, vs SCE FIG. 2 Current vs. potential curves for oxidized DyFe 3 ( ) and ErFe 3 (- - -). Potential are measured against the SCE. Dark current curves are marked w i t h a d.

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<~

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~

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Potential, v, vs SCE FIG. 3 Current vs. potential curves for oxidized Ho6Fe23 ( ), Er6Fe23 ( .... ) and Er2Fel7 (- - -). Potentials are measured against the SCE. Dark current curves are marked with a d.

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RARE EARTH-IRON COMPOUNDS

393

gas evolution occurs at the electrodes, and it was quite noticeable as the anodic current increased. Therefore it follows that the reaction is photoassisted. The following criteria were used to provide information in regard to the stability of the electrodes: (I) absence of large hysteresis in the I-V curve; (2) observation of gas evolution; (3) absence of anodic dark current at voltages less than the thermodynamic voltage for 02 evolution; (4) reproducibility of data; (5) absence of noticeable changes after testing when examined with an optical microscope; (6) absence of significant weight changes after testing. 17 The iron oxide and titanium oxide electrodes were found to be stable under the experimental condition. The TiO x electrode exhibited a large photocurrent rise starting at about -0.78 (SCE) and continued with increasing positive potential to about 1.0 V (SCE). The onset photocurrent is regarded as the flatband potential (Vfb) of the electrode. The dark current is nearly zero between -0.8 and 1.0 V (SCE), and the electrode is stable under the experimental conditions. The square of photocurrent vs. applied potential (SCE) is a straight line whose intercept on the potential (SCE) axis should be the flatband potential. This is nearly the same as the onset of photocurrent. Gas evolution occurs promptly on the electrode surface with rise of the anodic current and is quite noticeable. It was observed that the onset of photocurrent shifted to more negative potentials with increase of the pH of the electrolyte solution. Electrochemical measurements of oxidized rare earth iron electrodes (Figs. 1-3) indicated that, except for those mentioned below, the electrodes were stable under the experimental conditions (i M NaOH). They showed a large anodic current rise at about -0.35 V (SCE). The currents observed with these electrodes are lower than those of the currents obtained with an iron oxide electrode at the same applied potential and light intensity. Oxygen gas evolution was quite noticeable at the surface of these oxidized rare earth-iron electrodes with rise of anodic currents. The dark current is slightly larger for the oxidized HoFe 2 and ErFe 2 electrodes, possibly because of some reaction with the original materials which are coated with the oxide film. The oxidized Er2Fel7 showed a larger dark current than the other electrodes. This may be caused by the high concentration of iron in the intermetallic compound and separation of two oxides (iron and rare earth oxide) during the oxidation treatment. This separation may facilitate the reaction of the electrolyte with the intermetallic compounds which underlie these oxides. Bolts et al. noted 5 that the anodic current onset from the current-voltage curves is close to the value of the flatband potential, and the current-voltage curves are a good indication of the band-bending energy (EB). Therefore, extrapolating to zero current, the corresponding potential is measured (with respect to the fixed potential of the SCE) as the flatband potential. The flatband potentials (Efb) in i M NaOH obtained in this way are listed in Table I, along with values for Fe203 and TiO 2 obtained independently. The oxidized intermetallic compounds were examined by x-ray diffraction. Results were not entirely clear because some of the peaks overlapped. The x-ray diffraction pattern indicated that the iron in the RFe x systems had oxidized to a mixture of FeO and Fe203. X-ray diffraction gives information only about the bulk-phase. Since the semiconducting properties of the electrodes depend on the composition and structure of the surface, it seemed desirable to use AES to examine their surfaces. This technique was employed with oxidized ErFe2, Er6Fe23 and Ho2Fel7. Depth profiling indicates that the concentration of oxygen and iron increased during the sputtering, and mapping showed that high intensity Fe peaks correlated with high intensity oxygen peaks. This indicates that oxygen occurs together with Fe. The AES results strongly suggest that iron exists in the surface in the form of iron

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TABLE 1 Flatband Potentials __(Efb) in i M NaOH

ErFe20 x

ErFe30 x

Er6Fe230x

DyFe20 x

DyFe30x

-0.39

-0.27

-0.37

-0.36

-0.38 TiO 2

Efb(VOlts) a

HoFe20 x

Ho6Fe230 x

-0.38

-0.38

Efb(VOlts) a a. b.

FeO x

TiO 2

Fe203

-0.45

-0.78

-0.32 b

-0.90 b

vs the SCE from ref. 3.

oxide. The AES spectra showed that there is carbon on the surface and that this is removed during the argon sputtering. Sputtering for 25 min. did not indicate any rare earth signal. Thus these elements are not present to any significant extent on the surface. The increasing Fe and oxygen signals are possibly caused by the removal of carbon. The flatband potential for the oxidized iron intermetallic is very close to that of Fe203, suggesting that this material (i) is the photoconductive species and (2) constitutes the majority of the surface. Thus the results from the flatband measurements and AES are in good agreement. The results indicated that oxidizing these iron Intermetallics is a new way of preparing the Fe203 electrode. References i. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii. 12.

13. 14. 15.

A. Fujishima, K. Honda and S. Kikuchi, J. Chem. Soc. Japan 72, 108 (1969). M. S. Wrighton, A. B. Ellis, P. T. Wolczanskl, D. L° Morse, H. B. Abrahamson and D. S. Ginley, J. Am. Chem. Soc. 98, 2774 (1976). M. A. Butler and D. S. Ginley, J. Electrochem. Soc. 125, 228 (1978). R. D. Rauh, J. M. Buzby, T. F. Reise and S. A. Alkaitis, J. Am. Chem. Soc. 83, 2221 (1979). J. M. Bolts and M. S. Wrighton, J. Phys. Chem. 80, 2641 (1976). A. J. Nozik, Ann. Rev. Phys. Chem. 2_99, 189 (1978). M. A. Butler and D. S. Ginley, J. Materials Science 15, i (1980). M. A. Butler and D. S. Ginley, J. Appl. Phys. 48, 3070 (1977). M. A. Butler and D. S. Ginley, Chem. Phys. Lett. 47, 319 (1977). A. Moldovan, A. Elattar, W. E. Wallace and R. S° Craig, J. Sol. State Chem. 25, 23 (1978). H° C. Siegmann, L. Schlapbach and H. R. Brundle, Phys. Rev. Lett. 40, 972 (1978). W. E. Wallace, A. Elattar, H. Imamura, R. S. Craig and A. G. Moldovan, in The Science and Technology of Rare Earth Materials, W. E. Wallace amd E. C. Subbarao, eds. Academic Press, Inc., New York (1980), p. 329. H. Imamura and W. E. Wallace, J. Catal. 65, 127 (1980). A. Elattar, T. Takeshita, W. E. Wallace and R. S. Craig, Science 196, 1093 (1977). E. B. Boltich, W. E. Wallace, F. Pourarian and S. K. Malik, J. Mag. Mag. Mat. 25, 295 (1982).

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A. Fujishima and K. Kohayakawa, J. Electrochem. Soc. 122, 1487 (1976). H. H. Kung, H. S. Jarrett, A. W. Sleight and A. Ferretti, J. Appl. Phys. 48, 2463 (1977).