2402
J. Phys. Chem. 1983, 87, 2402-2405
Photo- and Electroluminescence Spectra from an n-Ti02 Semiconductor Electrode As Related to the Intermediates of the Photooxidation Reactlon of Water Yoshlhiro Nakato, Akira Tsumura, and Hiroshl Tsubomura Department of ChemiStty, Faculty of Engineering Sclence, Osaka University* Toyonaka, Osaka 560, Japan (Received: September 17, 1982; I n Final Form: January 3, 1983)
Photo- and electroluminescence spectra of an n-type Ti02 electrode in aqueous solutions were measured as functions of the electrode potential and the solution pH, together with the current-potential curves. The photoluminescence spectrum showed a relatively sharp band peaked at 1.47 eV. From the quenching of the photoluminescenceby reductants in solution, this band was concluded to arise from an oxidative surface species, called x1.47, acting as an intermediate of the photooxidation reaction of water. It was shown that the species X1.47 is originally absent at an n-TiOz surface, but produced and accumulated by illumination under anodic bias. It was also suggested, from a comparison of the electroluminescence spectra in HzOzsolutions with those in S2OS2-solutions, that the species x1.47can be assigned to a kind of OH. radical surface adduct. The origins of other luminescence bands peaked at higher energies appearing in the electroluminescencespectra are also discussed briefly.
Introduction The chemical origins of the surface states and their relation to intermediates of the oxygen evolution reaction have, The photoelectrolysis of water into hydrogen and oxygen however, still been quite obscure. By using a spin-trapping with semiconductor electrodes has been studied extensively method, Bard et al.,' recently reported that OH. radicals in view of the solar energy conversion.'-s So far, several are photoanodically produced during illumination of TiOz metal oxides such as n-type TiO,, SrTiO,, or Fe203have particles in aqueous solutions. been found to photooxidize water into O2and H+without We have studied the current-potential curves, the difcorrosion, but they unfortunately have too wide band gaps ferential capacitances, and the luminescence spectra of or too low surface band energies to decompose water into semiconductor electrodes such as n-Gap, n-CdS, n-ZnO, H2and O2with visible light under no external bias. Other and n-TiO, and discussed the results in relation to the semiconductors which have appropriate band gaps and precursors or intermediates of the photoanodic reacsurface band energies, such as n-GaP or n-CdS, are all t i o n ~ . ' In ~ ~the ~ present paper we will report on the results photocorrosive in aqueous solutions. of detailed studies on the electro- and photoluminescence The photodecomposition of water seems to proceed via spectra of an n-TiO, electrode. a series of elementary reactions, including very active surface intermediates. To elucidate the reaction mechaExperimental Section nism is therefore very important for finding an appropriate The luminescence properties of the n-TiOz electrodes, system. especially the dependence of the photoluminescence inIt was pointed out by several workers that surface states tensity on the electrode potential, were sensitive to the are present in n-TiO, electrodes from measurements of procedure of electrode preparation. The final procedure current-potential e l e c t r o l ~ m i n e s c e n c e or ~~-~~ that we have adopted is as follows: Single crystal wafers photol~minescence'~spectra, photoresponses in subof TiO,, cut parallel to the c axis, were obtained from band-gap excitations,'"" interfacial capacitances,18and Nakazumi Earth Crystals Corp. They were polished with the effect of adding reductants on the oxygen evol~tion.'~" alumina powder, etched in concentrated sulfuric acid containing 20% K2S04at ca. 250 "C for 1 h, annealed in the air at 1300 "C for 5-7 h, reduced under hydrogen (1) Fujishima, A.; Honda, K. Nature (London) 1972,238, 37-8. (2) Nozik, A. J. Annu. Reu. Phys. Chem. 1978, 29, 189-222. atmosphere at 600-700 "C for 0.5-2 h to get n-type sem(3) Maruska, H. P.; Ghosh, A. K. Sol. Energy 1978,20, 443-58. iconductivity, and then etched again in the same solution (4) Gerischer, H. Top. Appl. Phys. 1979, 31, 115-72. as above for 1 h. The n-TiO, electrode was prepared by (5) Bard, A. J. J. Photochem. 1979, 10, 59-75; Science 1980, 207, 139-44. making an ohmic contact on one face of the wafer with (6) Wrighton, M. S. Acc. Chem. Res. 1979, 12, 303-10. vacuum-evaporated indium metal, attaching a copper wire (7) Tomkiewicz, M.; Fay, H. Appl. Phys. 1979, 18, 1-28. with silver paste, and then covering the face with epoxy (8) Scaife, D. E. Sol. Energy 1980, 25, 41-54. (9) Frank, S. N.; Bard, A. J. J. Am. Chem. SOC.1975, 97, 7427-33. resin. (10) Wilson, R. H. J. Electrochem. SOC.1980, 127, 228-34. Photoluminescence was emitted when the electrode was (11) Salvador, P. J. Electrochem. SOC.1981, 128, 1895-900. illuminated at 365 nm from a high-pressure mercury lamp (12) Nodi, R. N.; Kohl, P. A.; Frank, S. N.; Bard, A. J. J. Electrochem. SOC.1978,125, 246-52. (13) Morisaki, H.; Yazawa, K. Appl. Phys. Lett. 1978, 33, 1013-5. (14) Nakato, Y.; Tsumura, A.; Tsubomura, H. Chem. Phys. Lett. 1982, 85, 387-90. (15) Morisaki, H.; Hariya, M.; Yazawa, K. Appl. Phys. Lett. 1977, 30, 7-9. (16) Laser, D.; Gottesfeld, S. J. Electrochem. SOC.1979, 126, 475-8. (17) Mavroides, J. G. 'Semiconductor Liquid-Junction Solar Cells"; Heller, A., Ed.; The Electrochemical Society: Princeton, NJ, 1977; pp 84-91. (18) Kobayashi, K.; Aikawa, Y.; Sukigara, M. Chem. Lett. 1981, 679-80. 0022-3654/83/2087-2402$0 1.50/0
(19) Fujishima, A.; Inoue, T.; Watanabe, T.; Honda, K. Chem. Lett.
1978, 357-60. Fujishima, A.; Inoue, T.; Honda, K. J . Am. Chem. SOC. 1979, 101, 5582-8. (20) Kobayashi, T.; Yoneyama, H.; Tamura, H. J . Electroanal. Chem. 1981, 122, 133-45. (21) Jaeger, C. D.; Bard, A. J. J.Phys. Chem. 1979, 83, 3146-52. (22) Nakato. Y.: Tsumura. A.: Tsubomura, H. J . Electrochem. Soc. 1980, 127, 1502-6; 1981, 128, 1300-4; Chem. Lett. 1981, 127-30; 1981, 383-6; ACS Symp. Ser. 1981, No. 146, 145-58; Bull. Chem. SOC.Jpn. 1982,55, 3390-3.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 13, 1983 2403
n-TIO, Semiconductor Electrode Xinm
500
7W
600
800
10 1
IA)
- 0.5
-1.0
U VS. SCE I V Flgure 2. Dependence of the peak intensity of the photoluminescence spectrum (I,) and the photocurrent density on the electrode potential (U) in a solution containing (a) 0.05 M H,SO, (-), (b) 0.1 M Na,S04 and phosphate buffer (pH 6.6) (- - -), (c) 0.1 M NaOH (. e ) . The sweep rate was in the range of 30-100 s/V.
up)
3.0
2. 5
2.0
1.5
h u i eV
Figure 1. Photo- and electroluminescence spectra of an n-TiO, eleclrode: (A) photoluminescencespectrum observed at 0.0 V vs. SCE in a 0.05 M H2S04sdution; (B) electroluminescencespectrum observed at -3.0 V vs. SCE In a solution containing 1.0 M Na2S208and 0.1 M carbonate buffer (pH 7.1): (C) electroluminescence spectrum observed at -2.5 V vs. SCE in a solution of 0.1 M Na,S04 and 0.9 M H202(pH
I
-* O
10
u v s . SCE / v 0.5
J IPL
9.0).
combined with a Shimadzu-Bausch-Lombmonochromator and appropriate light filters. The luminescence spectra were measured by using a Jobin-Yvon H20 monochromator and a Hamamatsu TV R316 or R712 photomultiplier cooled at -20 "C.The observed spectra were corrected for the spectral sensitivity of the photomultiplier, which was measured by use of an Eppley thermopile. As mentioned previously,14 the luminescence spectra of various semiconductor electrodes corrected in the same way as above agreed well with those reported by other workers. Electroluminescence was emitted when the electrode was kept under cathodic potentials by use of a potentiostat in an aqueous electrolyte solution containing an oxidant such as S2OS2-or H202. Such electroluminescencesfrom n-type semiconductors were reported to arise from radiative recombination between holes injected by the oxidants and electrons in the conduction band.12g14,22-24 The luminescence spectra were measured with the same detector system as used for the photoluminescence spectra. The current-potential curves were obtained with a Nikko-Keisoku NPGS-301 potentiostat. Solutions were prepared by use of deionized water and reagent-grade chemicals. The concentration unit, mol/dm3, is abbreviated as M in the present paper.
Results Figure 1A shows a photoluminescence spectrum of an n-Ti02electrode in an aqueous solution. A relatively sharp luminescence band was observed with a peak at 1.47 eV. The spectrum was essentially unchanged by both the electrode potential and the solution pH. Figure 2 shows the dependence of the peak intensity of the photoluminescence spectrum (IpL) on the electrode (23) Beckmann, K.H.; Memming, R. J.Electrochem. SOC. 1969,116, 368-73. (24) Pettinger, B.; Schoeppel, H.-R.; Gerischer, H. Ber. Bunsenges. Phys. Chem. 1976,80, 849-55.
- 0.5
I
0.5 UVS.
SCE / V
Flgure 3. Dependence of the photoluminescence intensity (I,) and the photocunent density on the electrode potential (U) in a solution of 0.05 M HS , O, (-), and the same in a solution of 0.05 M H,SO, and 0.5 M hydroquinone (- - -). The sweep rate was 60 s/V. The solution was stirred.
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1.0
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B
8
5 0.5
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0.0 0.0
0.01
0.1 1.0 Reductant Concentration I m ~ l d m - ~
10
Flgure 4. Photoluminescence intensity observed at 0.0 V vs. SCE in a 0.05 M H,S04 solution as a function of the concentrations of various reductants added.
potential (v) at pH 1.2 (-), 6.6 (---), and 13.2 as compared with the photocurrent density GP). The reported flat-band potential (UfiY5is also indicated by arrows. I t is seen that the luminescence intensity has a maximum near the potential where the photocurrent starts to appear in each solution. When hydroquinone was added to the solution, the photoluminescence was strongly quenched, and the photocurrent was much enhanced at the electrode potentials (.-e),
(25) Tomkiewicz, M. J. J. Electrochem. SOC.1979, 126, 1505-10.
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Nakato et al.
The Journal of Physical Chemistry, Vol. 87, No. 13, 1983
“fb
-0.5
0.5
-I
J I
lo
- 0.5
I
0.5
u v s . SCE I V
Figure 5. Photoluminescence intensity (IR) and the photocurrent density vs. potential curves In a 0.05 M H,SO, solution in the fkst few sweeps of the electrode potential. The illumination and the sweep were started at a potential indicated by an arrow. The sweep rate was 30 s/V.
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illumination off on
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0
I
/
7w
1.7
u vs.
SCE /
800
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1.5
v
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Flgure 8. (A) Transient dark cathodic current (shaded part) on sweeping the electrode potential from positive to negatlve at a rate of 3 s/V after illumination at 0.0 V vs. SCE for 60 s In a 0.1 M NaOH solution. (6)Spectrum of a transient luminescence emitted durlng the flow of the above transient cathodlc current (see text).
slightly less negative than the f’lat-band potential, as shown in Figure 3. More or less similar effects were observed by adding other reductants. Figure 4 shows the photoluminescence intensity as a function of the concentrations of various reductants added. All the IpL vs. U , and the jp vs. U curves shown in Figures 2 and 3 were observed when a stationary state was attained after several cyclic sweeps of the electrode potential under illumination. When such a once-illuminated n-Ti02electrode was either kept in the dark for more than 1day or kept at a very strongly cathodic potential and then illuminated at a cathodic potential, the photoluminescence intensity in the first sweep of potential toward the positive was smaller than those of the later backward or forward sweeps, as shown in Figure 5. The onset potential of the photocurrent in the first sweep was also somewhat more negative than those of the later sweeps. It was reported by Wilsonlo that, when an n-Ti02electrode was illuminated at an anodic potential and the potential was swept in the dark toward the negative, a transient cathodic current was observed at potentials slightly less negative than the flat-band potential (Ufi). We reproduced his result (the shaded part in Figure 6A) and also found that a transient luminescence was emitted during the flow of this transient cathodic current. The spectrum of the transient luminescence was measured by illuminating the electrode in a 0.1 M NaOH solution at 0.0 V vs. SCE for 60 s with the photocurrent of 50 pA/cm2 flowing, then turning the light off, and shifting the electrode potential instantaneously to -0.8 V vs. SCE. The initial intensity of a spikelike luminescence emitted was
Figure 7. Potential dependence of the intensity of an electroluminescence spectrum (I,) and the dark cathodic current density ( j c ) for an n-TIO, electrode. SdM lines were observed in a 0.9 M Na,S,O, solution at pH 4.3, and broken lines were observed In a solution of 0.1 M Na,SO, and 0.9 M HO , , at pH 5.2. The sweep rate was 30 s/V.
recorded at every 20 nm. The luminescence spectrum thus obtained is shown in Figure 6B. It is seen that it agrees well with the photoluminescence spectrum shown in Figure 1A. Solid lines in Figure 7 show the potential sweep of the electroluminescence intensity (IEL)for an n-Ti02electrode in a 0.9 M Na2S208solution at pH 4.3, as compared with the dark cathodic current density &). The In, vs. U curve was unchanged during many cyclic sweeps of the potential. The electroluminescence was weak, only detected in highly negative potentials. Similar IELw. U curves were obtained in solutions of other pH values. The electroluminescence spectrum observed in this case is shown in Figure 1B. As the signal-to-noise ratio is only 3-7 at the luminescence peak, the spectral shape in Figure 1B should be regarded as quite approximate. It is seen that the spectrum in Figure 1B is very much different from that in Figure 1A. Gradual increase in the luminescence intensity was observed with increasing solution pH. Broken lines in Figure 7 show the intensity of the electroluminescence spectrum and the dark cathodic current density in a solution of 0.1 M Na2S04and 0.9 M H202at pH 5.2. As seen from Figure 7, the electroluminescence in the H202solution was much stronger than that in the Na2S20ssolution and started to appear near the onset potential of the cathodic current. A similar IEL vs. U curve was obtained at pH 9.0. The electroluminescence spectrum observed is shown in Figure 1C. It was independent of the pH and rather similar to the photoluminescence spectrum shown in Figure 1A.
Discussion It was reported in a previous letter14that the photoluminescence spectra of n-GaP, n-CdS, and n-ZnO electrodes arise mostly from electronic transitions in the bulk of the semiconductors, while the electroluminescence spectra of the electrodes arise mainly from electronic transitions at the surface. It is clear, however, that the photoluminescence spectrum of the n-Ti02 electrode arises from the surface, because it is effectively quenched by reductants in solution (Figures 3 and 4). The electron-hole recombinations in the bulk of the n-Ti02 semiconductor may occur nonradiatively, or radiatively in wavelengths longer than 1000 nm. The entire electroluminescence spectra (Figure 1, B and C) are also thought to arise from the surface. The IpLvs. U curves in Figure 2 can be explained by assuming that the luminescence band at 1.47 eV is assigned
The Journal of Physical Chemistry, Vol. 87, No. 13, 1983 2405
n-TiO, Semiconductor Electrode
I
I
V.B.
n-Ti02
Electrolyte (pH 1.0)
Figure 8. Schematic energy level diagram at an n-TiO,/solution interface: (C.B.) conduction band, (V.B.) valence band, (EF)Fermi level, hole, and (e)electron. H2Q is hydroquinone and Q is benzoquinone.
(e)
to a recombination luminescence between the electrons in the conduction band and an oxidative surface species, hereafter called x1.47,produced by photogenerated holes (Figure 8). The weakening in the luminescence at too anodic potentials is explained by the decrease in the surface density of the electrons in the conduction band due to the increased upward band bending, and the weakening at the cathodic region is explained by the decrease in the density of the species x1.47caused by the recombination of the photogenerated holes with the electrons in the bulk owing to the decreased band bending. The quenching of the photoluminescence spectrum by hydroquinone or other reductants (Figures 3 and 4) can be explained by assuming that the species x1.47is more or less reduced by these reductants. In Figure 8, the standard redox potentials (eo) for the reductants are compared with an approximate energy level (vacant level) distribution for x1.47, which was estimated from the spectral energy of the luminescence band at 1.47 eV and the above assignment, in the same way as reported for other semiconductors.'4 It can be seen that, as the eo value for the reductant gets more negative, the more effective is the quenching of the photoluminescence. I t is also seen that the efficiency of the luminescence quenching largely changes with the reductants having the eo values in the relatively narrow range of potential in the neighborhood of the estimated energy level for X1.47. These results support the above-mentioned explanation for the luminescence quenching. It was reported by Fujishima et al.19 that the oxygen evolution at an n-Ti02 electrode under illumination was suppressed by reductants in solution in the order of C1< Br-
