2809
J. Phys. Chem. 1986, 90, 2809-2810
Photoassisted Water Decomposition by Ferroelectric Lead Zirconate Titanate Ceramics with Anomalous Photovoltatic Effects Yasunobu Inoue,*+Kiyoshi Sate,+ Kazunori Sato: and Hajime Miyamat Analysis Center, and Department of Chemistry, Technological University of Nagaoka, Nagaoka, Niigata 949-54, Japan (Received: February 28, 1986)
Ferroelectric lead zirconate titanate (PZT) ceramics with the polarization vector defined perpendicular to surface were employed, with and without Pt deposition, as photocatalysts for water decomposition under conditions in which either the positive or negative polar surface was irradiated with Xe light. The activity of H2 formation was 10-40 times higher for the positive than for the negative polar surface. It was shown that PZT is a useful photocatalyst and the activity differences between the oppositely polarized surfaces are associated with the inherent anomalous photovoltatic effects of the ferroelectrics.
Introduction Recently, we reported the substrate effects of the ferroelectrics upon the photocatalysis of semiconductive oxides when both materials are combined.' The ferroelectric polar surfaces of a poled LiNbO, single crystal enhanced the photocatalytic activity of the combined thin T i 0 2 films, compared with that of the nonferroelectric surface, and it was suggested that the polarization field gives rise to band bending in Ti02, which is effective for the separation and diffusion of photogenerated carriers. When transition-metal-atom-doped ferroelectrics or ferroelectric semiconductors possessing the defined direction of polarization vector, e.g., the direction perpendicular to the surface, are illuminated by light with enough energy to be excited, the behavior of the photogenerated carriers are strongly influenced by the polarization field. The most striking and intrinsic features in the ferroelectrics are the appearance of a stationary short circuit photocurrent and open circuit photovoltages larger than the band gap which reached the order of 100 V or more for a crystal a few millimeters long. These anomalous photovoltatic effects (APV effects) are evidently related to bulk polarization effects due to an asymmetric potential in the crystals and have been observed in BaTiO, ceramics,z ZnS single crystals,, and CdTe films.4 One of the most interesting ideas is to apply the polar direction-defined ferroelectrics to photocatalysts, because effective control of the photogenerated carriers can be expected, and hence to examine the APV effects upon photocatalysis. To our knowledge, this is the first study regarding the effects upon the photocatalytic reaction. In the present work, the piezoelectric ferroelectrics of Sr-doped lead zirconate titanate (PZT) ceramics and an undoped LiNb0, single crystal, both of which possess a polarization vector normal to the surface, were used for the photoassisted water decomposition reaction. We found that PZT is a useful photocatalyst and the APV effects give rise to the characteristic photocatalysis. Experimental Section The poled single crystal of undoped LiNbO, was the same as that used previo~sly,'~~ and 0.1- and 0.2-mm-thick disks (20-25 mm in diameter) of Sr-doped PZT ceramics were employed (PZT-1 and PZT-2, respectively). The Curie temperature of the PZT was 593 K. These ferroelectrics were polarized perpendicular to the surface, thus exposing the positive polar surface (+) at one end and the negative polar surface (-) at the opposite end. Before catalyst preparation, the ceramic samples were lightly dipped in an Fe(NO,), aqueous solution in order to remove Ag electrodes which were used for polarization. After the samples were ul(1) Inoue, Y.; Okamura, M.; Sato, K. J . Phys. Chem. 1985, 89, 5184. (2) Brody, P. S. Solid State Commun. 1973, 12, 673. (3) Lempicki, A. Phys. Reo. 1959, 113, 1204. (4) Pensak, L. Phys. Rev. 1958, 109, 601. (5) Inoue, Y.; Sato, K.; Suzuki, S. J. Phys. Chem. 1985,89, 2827. Inoue, Y.; Yoshioka, I.; Sato, K. J . Phys. Chem. 1984, 88, 1148.
0022-3654/86/2090-2809$01.50/0
TABLE I: Comparison of Photocatalytic Activity between the Positive and Negative Polar Surfaces (rate of H2 production) X IO7 mol h-' cm-2 ratio of ~
photocatalysts Pt/LiNbO, Pt/PZT-1 Pt/PZT-2 Pt/PZT-2" PZT-2
V(+) 1.4
2.9 3.3 1.5
V(-)
V(+)/V(-)
0.038 0.3 1.8
31
0.14
10.7
9.1 1.8
'Heat-treated at 623 K in 33.3 kPa of He for 30 min and cooled to room temperature without application of an electric field. trasonically washed in an acetone bath, the photocatalysts were prepared by depositing Pt metal films on both or either of the positive and negative polar surfaces. Pt was evaporated by Ar sputtering through a Mo mask and was deposited as a spot 0.15 mm in diameter in a regular array of 0.26 mm wide and 0.44 mm long in one unit. The average thickness of the Pt films was 6 nm. In the photoassisted decomposition of water, the photocatalyst was immersed fully in distilled and degassed water, and one side of the catalyst, viz., either the positive or negative polar surface, was irradiated by light. A 500-W Xe lamp (Ushio Denki Co. UXL-5OOD) was operated at 400 W and the light was filtered through a Toshiba UV-D33S filter (230-410 nm). The hydrogen evolved was gas chromatographically monitored and was taken as a measure of the photocatalytic activity. The photoresponse measurements of the ferroelectrics were made in air after the deposition of transparent Au electrodes by Ar sputtering in the direction normal to the polarization field. The Ag wire was connected to the electrodes with conducting resin. The photocurrents were determined by measuring the voltages drop across 106-107-Q resistors with an electrometer (Takeda Riken Co. T R 8651).
Results and Discussion Figure 1 shows the photoresponse of poled PZT and LiNbO, measured in air under illumination of filtered Xe light. In both ferroelectrics, an instantaneous photocurrent of reverse sign appeared upon light-on and -off, followed by drastic decreases. These transient currents were associated with pyroelectric currents. After a certain period, the photocurrent reached a stationary level of 1 X lo-' A/cm2 for the PZT-1 sample, whereas the current decreased to an extremely lower level of less than lo-" A/cm2 for LiNbO,. It is noted that the photocurrent in PZT remained unchanged when the resistors were varied. The open circuit photovoltages of PZT-1, which were measured with an electrometer with an impedance higher than lOI4 Q , were as high as 8.6 V, thus indicating an electric field of 8.6 X lo4 V/m. The preliminary results on the wavelength dependence of the photocurrent showed that the major light absorbing edge was present around 470 nm and extended to the UV region. From these 0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
AI
-54-
12as
Il __I t I ‘“I111
u
Ioos
I1 Y
I
I
Time
Figure 1. Photoresponse of polarized PZT and LiNb03: (A) PZT-I; (B) LiNbO,; ( 1 ) R = 2 X lo7 0;(2) R = 5 X IO6 R.
Reaction time/ h
Figure 2. H2production between the positive and negative polar surfaces: (0) Pt/(+)PZT-2; ( 0 ) Pt/(-)PZT-2; (0)(+)PZT-2; (W) (-)PZT-2.
findings, it is confirmed that the APV effects are prominent in the PZT ceramics used. Figure 2 shows the H2 production in photoassisted water decomposition. In the initial reaction time, there were induction periods of 30-60 min, which were followed by an H2 increase almost proportional to time. The values for the stationary photocatalytic activity are summarized in Table I. No appreciable activity was observed for both the polar surfaces of Pt/LiNbO,, which was in line with the near absence of photocurrents. Although oxygen evolution was negligibly small for both polar surfaces, the activity of PZT-2 for H2 formation was 10 times larger for the positive polar surface than for the negative polar surface. Such polarization-direction dependence was more pronounced for the thinner PZT-1: the activity of the positive polar surface was ca. 40 times as high as that of the negative surface. In another experiment using PZT-2 with Pt deposition on both polar surfaces, light illumination of the positive polar surface was
Letters continued until constant activity was attained, and then the reaction cell was turned so as to expose the negative polar surface to light. This in-situ turn resulted in a lower rate of H2evolution which is expected from the catalytic characteristics of the negative polar surface. When the PZT-2 catalyst was treated by heating in 33.3 kPa of H e at 623 K which is higher than the Curie temperature, followed by cooling to room temperature without applying an electric field, the photocatalytic activity of the positive polar surface remained almost unchanged, whereas that of the negative polar surface increased by a factor of 6: the activity difference between the oppositely polarized surfaces was remarkably reduced. It is of particular interest to see that PZT-2 without Pt deposition exhibited a considerably high rate of H 2 formation, approximately one-half that of the Pt/PZT-2, and that there is also a clear activity difference between the positive and negative polar surfaces. The appearance of the APV effects in the ferroelectrics is due to a microscopic asymmetric potential at the light absorbing center, since the effect is not observed in materials with a center of symmetry.6 In the experiments as shown in Figure 1, the photovoltatic currents flow in a direction opposite to the spontaneous polarization vector and hence it is reasonably considered that the polarization field is able to control the carrier movements and the density of the excited electron becomes much larger on a positive polar surface than on a negative surface under illumination. This situation apparently leads to a higher efficiency of H2 formation on the positive polar surface, which is consistent with the experimental observations. Additional evidence for the polarization-direction dependence of the catalytic activity was obtained from the results of a heat-treated PZT sample. Since heat treatment of ferroelectrics above the Curie temperature and cooling without poling give rise to random orientation of the polarization axis, it is expected that the contribution of the polarization effects to the photocatalysis becomes almost the same on the back and front surfaces. The experimental result is in agreement with this trend, as shown in Table I. From the APV effects, it is expected that the negative polar surfaces can be effective in the formation of oxygen as the oxidation reaction, since the photogenerated holes become rich at the surface. However, no significant amount of oxygen was obtained in the gas phase. This might be partly due to the formation of H202 as has been previously proposed on SrTi03’ and we need further study in this connection. In conclusion, the present study indicates that ferroelectrics such as PZT with APV effects are interesting materials as photocatalyst and, furthermore, suggests that the asymmetric center in piezoelectric crystals also contributes. Further study is in progress. Acknowledgment. This work was supported by a Grant-in-Aid for Energy Research (No. 59045060) and for Scientific Research (No. 59470002) from The Ministry of Education, Science and Culture. ( 6 ) Glass, A. M.; Von der Linde, D.; Auston, D. H.; Negran, T. J. J . Electron. Mater. 1975, 4 , 915. (7) Wrighton, M. S.;Ellis, A. B.; Woldzanski, P. T.; Morse, D. L.; Abrahamson, H. B.; Ginley, D. S.J . Am. Chem. Soc. 1976, 98, 2774.
