Photocatalytic behavior of cerium titanates, CeTiO4 and CeTi2O6 and their composite powders with SrTiO3

Journal of Alloys and Compounds 376 (2004) 262–267

Photocatalytic behavior of cerium titanates, CeTiO4 and CeTi2 O6 and their composite powders with SrTiO3 Shinya Otsuka-Yao-Matsuo∗ , Takahisa Omata, Manabu Yoshimura Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, Yamada-oka 2-1, Suita 565-0871, Japan Received 18 April 2003; received in revised form 7 January 2004; accepted 7 January 2004

Abstract Several cerium titanates of the Ce–Ti–O system exhibit yellowish, reddish, and grayish color, because they absorb visible light (λ > 410 nm). Among them, Ce2 Ti2 O7 , Ce2/3 TiO3 , and Ce4 Ti9 O24 , with mainly Ce3+ , and CeTiO4 and CeTi2 O6 , with mainly Ce4+ , were prepared and their diffuse reflectance spectra recorded, together with commercial CeTiO4 (PB). The X-ray diffraction (XRD) pattern of CeTiO4 prepared by the oxidation of Ce2 Ti2 O7 at temperatures as low as 673 K differed from that of CeTiO4 (PB). The diffuse reflectance spectra of CeTiO4 , CeTi2 O6 , and CeTiO4 (PB) indicated that the energy differences among the impurity levels in the energy band gap were relatively large; therefore, the photobleaching of methylene blue aqueous solution sensitized by CeTiO4 and CeTi2 O6 powders and the respective composite powders with SrTiO3 was examined with irradiation of Xe discharge light. The composite powder of CeTiO4 with SrTiO3 was found to exert a relatively high photocatalytic activity under visible light irradiation (λ > 420 nm) using a UV-cut filter. © 2004 Elsevier B.V. All rights reserved. Keywords: Cerium titanate; Photocatalyst; Composite particle; Photobleaching; Methylene blue

1. Introduction Photocatalytic reaction sensitized by TiO2 [1,2] and other semiconductor materials [3,4] has attracted extensive interest as a potential way of solving energy and environmental issues. Most investigations have focused on anatase-type TiO2 [5], because it exerts a relatively high photocatalytic activity under irradiation of light with wavelength λ < 390 nm and high chemical stability. Titanium oxides have another advantage of relatively low cost. To induce photocatalytic reactions under visible light (λ > 410 nm), an increasing number of investigators have recently doped nonmetal elements, e.g., N, S, and C, in the oxygen ion site of TiO2 [6–9]. In view of the reproducible production of sample powders, complex oxides involving titanium must be investigated. For instance, several cerium titanates exist in the Ce–Ti–O system. Three cerium titanates, i.e., Ce2 TiO5 , Ce2 Ti2 O7 , and Ce4 Ti9 O24 , involving mainly Ce3+ , which were recently ∗ Corresponding author. Tel.: +81-6-6879-7461; fax: +81-6-6879-7461. E-mail address: [email protected] (S. Otsuka-Yao-Matsuo).

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synthesized, were reddish-brown, reddish-brown, and chestnut, respectively, as reported in the previous paper [10]. It has been reported that perovskite-type Ce2/3 TiO3 involving mainly Ce3+ exhibits a metal-like behavior [11]. It has been reported that cerium titanates, CeTiO4 and CeTi2 O6 , with mainly Ce4+ , were formed when annealing cerium titanates with Ce3+ in air at high temperatures [12]; the latter was yellow colored. The structure of CeTiO4 has not been clarified, but CeTi2 O6 possessed brannerite structure [12,13]. According to our recent work [14,15], the oxidation of a pyrochlore-related structure involving a cation possessing two valence states, e.g., Ce3+ and Ce4+ , may proceed smoothly at a low temperature, maintaining the ordered arrangement of the constituent cations [16]. In the present study, according to this empirical principle, we prepared yellow CeTiO4 by the oxidation of the precursor Ce2 Ti2 O7 . Yellow CeTiO4 is also commercially available; however, it was found that the structure differed from that of the CeTiO4 prepared in this study. Therefore, the commercial CeTiO4 is distinguished here as CeTiO4 (PB). All of the compounds mentioned above absorb visible light (λ > 410 nm). The objective of the present work is to examine whether some of the titanates with the composition (xTi /xCe )
1 exert photocatalytic activity. Very recently, two of the authors [17] found

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that the addition of SrTiO3 powder enhanced the photocatalytic activity of TiO2 several times. Therefore, the photocatalytic behavior of the composite powders of the cerium titanates with SrTiO3 has been examined.

263

the present study, because in a previous study [18] the maximum composite effect was observed at around z = 30 mass% of Sr(Zr0.90 Y0.10 )O3–δ on the composite system of TiO2 –Sr(Zr0.90 Y0.10 )O3–δ , where the addition of Sr(Zr0.90 Y0.10 )O3–δ enhanced the photocatalytic activity of TiO2 .

2. Experimental 2.2. Evaluation of the photocatalytic activity 2.1. Materials The photocatalytic activity of the sample powders was evaluated from the photobleaching of methylene blue aqueous solutions [18–20]. Methylene blue aqueous solution of 2 × 10−5 mol dm−3 was prepared; its maximum absorbance value around the wavelength of 664 nm lay between 1.50 and 1.55. The aqueous solution (100 ml) with the sample powders (0.20 g) was loaded in a glass container (28 cm2 ) and then set in cooling water. The aqueous solution was stirred with a magnetic stirrer. The maximum absorbance value of the aqueous solution around the wavelength of 664 nm may change slightly due to change in temperature induced by the cooling system. After 1 min had passed, the irradiation of 500 W Xe discharge light above the aqueous solution was started. After a preselected time had passed, 12 ml of the solution was aspirated and subjected to centrifugation. The optical absorption spectrum for the supernatant solution was recorded using a double-beam spectrophotometer (Hitachi U4000). At an appropriate interval of time, a UV-cut filter (Suruga Seiki L42) was inserted, and the photobleaching of methylene blue under visible light (λ > 420 nm) was examined.

3. Experimental results and discussion

015 220, 221 006 214 222 024 016 401 223, 224 215, 025, 216

013 004 202 210 014 020 212 005 203, 204

012 200, 201

002

011

(a) Ce2Ti2O7

211

Fig. 1 shows the X-ray diffraction pattern for the Ce2 Ti2 O7 powder synthesized in this study, compared with the calculated one. Previously, the compound Ce2 Ti2 O7

Intensity, I / arb. units

Powdered raw materials were CeO2 (4N) supplied from Anan Kasei Co. Ltd., and TiO2 (anatase-type, 4N) purchased from Rare Metallic Co. Ltd. For Ce2 Ti2 O7 , Ce2/3 TiO3 , and Ce4 Ti9 O24 , the raw materials were weighed in their respective preselected ratios considering ignition losses, and mixed using a planetary ball mill with 80 cm3 pots made of partially stabilized zirconia. In order to ensure thorough mixing of the powders, ethanol was added during the mixing operation. The dried mixtures were spread on a Pt–10%Rh sheet and calcined at 1423 K for 10 h for Ce2 Ti2 O7 and at 1373 K for 48 h for Ce2/3 TiO3 in Ar + 1%H2 gas. The calcined powders were mixed again using a planetary ball mill and pressed into 17.2 mm-diameter disks under 100 MPa. The disks were subsequently sintered at 1523 K for 20 h for Ce2 Ti2 O7 and at 1573 K for 48 h for Ce2/3 TiO3 , respectively, in Ar + 1%H2 gas. For Ce4 Ti9 O24 , the disk made of the mixed raw materials was sintered at 1473 K for 20 h in Ar + 1%H2 gas. The disks were crushed to obtain the sample powders. The crystalline phases obtained were identified by a powder X-ray diffraction (XRD) method (Mac Science, MXP18 , Cu K␣ radiation using a curved graphite receiving monochromator). The Ce2 Ti2 O7 and Ce4 Ti9 O24 powders were loaded into a Pt–10%Rh crucible and oxidized in O2 gas at 673 K for 10 h and at 1273 K for 10 h, respectively, to attain a single phase of CeTiO4 and phases involving mainly CeTi2 O6 . Very fine powder of SrTiO3 , whose nominal particle size was 50 nm, was purchased from TPL. Inc. According to the SEM observation, the particle aggregate was approximately 2 ␮m. According to the XRD analysis, the powder contained a trace of anatase-type TiO2 . As seen from the diffuse reflectance spectrum, photoexcited electronic transitions among impurity levels were not observed; therefore, the amount of impurity in the SrTiO3 powder was negligible. The SrTiO3 (TPL) powder was used for the composite powders with the cerium titanates. The anatase-type TiO2 powder (Ishihara Sangyo, ST-01), which is known to have a relatively high photoactivity, was used for comparison. The preparation of the composite powders with SrTiO3 was made in a simple manner. After the two kinds of powders were mixed in mass ratios of (100-z): z, they were fired at 873 K for 1 h without pelletizing and then milled lightly for 5 min using the zirconia mortar. In this paper, the mixing compositions are denoted as CeTiO4 –zSrTiO3 , for example, where z indicates the mass percent of SrTiO3 added. The mixing composition z = 30 was adopted in

(b) calculated

10

20 30 40 Diffraction angle, 2 / degree

50

Fig. 1. X-ray diffraction pattern for synthesized Ce2 Ti2 O7 , compared with the calculated one. (a): Ce2 Ti2 O7 ; and (b): calculated on the basis of Ca2 Nb2 O7 structure with space group of P21 ; (a = 0.7776 nm, b = 0.5515 nm, c = 1.2999 nm, β = 98.36◦ ) [10].

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Fig. 2. X-ray diffraction pattern for synthesized CeTiO4 , compared with those for Ce2 Ti2 O7 and CeTiO4 (PB). (a): Ce2 Ti2 O7 ; (b): CeTiO4 obtained by annealing of Ce2 Ti2 O7 in O2 at 673 K for 10 h; and (c): CeTiO4 (PB) purchased from Pfaltz&Bauer Inc.

was synthesized by the reaction between CeO2 and Ti2 O3 powders loaded in a silica–platinum ampoule sealed at both ends after evacuation [10]; in some cases, a trace of CeO2 was found. As seen in Fig. 1, the compound Ce2 Ti2 O7 synthesized from CeO2 and TiO2 in Ar + 1%H2 gas was a single phase; no other phases were observed. Fig. 2 shows the X-ray diffraction pattern for the CeTiO4 powder obtained by the oxidation of Ce2 Ti2 O7 powder, compared with those for the precursor Ce2 Ti2 O7 and CeTiO4 (PB) purchased from Pfaltz & Bauer Inc. As seen in Fig. 2(b), after oxidation, the main diffraction peaks due to Ce2 Ti2 O7 disappeared completely. As an additional experiment, the authors prepared CeTiO4 powder by the oxidation of Ce2 Ti2 O7 at 873 K for 10 h; the XRD patterns for the CeTiO4 powders oxidized at 673 and 873 K agreed completely. The authors also confirmed in a preliminary thermobalance experiment that oxygen can be intercalated in the precursor Ce2 Ti2 O7 phase at temperatures as low as 573 K. Thus, we concluded that a single CeTiO4 phase was obtained in the present study, although the crystal structure of the CeTiO4 phase has yet to be identified. Bazuev and co-workers [12] previously prepared the precursor for CeTiO4 by the reaction of CeO2 and TiO2 powders with carbon at 1773 K; they noted that the phase obtained was CeTiO3 . By heating the CeTiO3 in air at 723–773 K for 6–8 h, they obtained a green poorly crystallized CeTiO4 so that only the main peak at around 2θ = 30◦ could be observed in the XRD pattern. The crystallinity of the CeTiO4 obtained in the present study was fairly high. This may result from the fact that the CeTiO4 phase is formed by oxygen insertion in the Ce2 Ti2 O7 phase, retaining the arrangement of Ce and Ti ions. The present result is in line with the appearance of κ-type CeZrO4 [14–16] and κ-related SnNbO4 [21] which were obtained by the oxidation of pyrochlore-type Ce2 Zr2 O7 and Sn2 Nb2 O7 . The crystal structure of the CeTiO4 powder synthesized in this study was completely different from that of the pur-

Fig. 3. X-ray diffraction patterns for synthesized CeTi2 O6 , compared with the calculated one. (a): CeTi2 O6 ; and (b): calculated on the basis of brannerite structure with space group of C2/m; (a = 0.984 nm, b = 0.375 nm, c = 0.691 nm, β = 119.15◦ ) [10] (䉲): rutile-type TiO2 ; (䊏): CeO2 .

chased CeTiO4 (PB). The reason for this is not clear because the production method of the commercial CeTiO4 (PB) is unknown. The XRD pattern of synthesized Ce4 Ti9 O24 agreed well with that previously reported [10]. Fig. 3 shows the X-ray diffraction pattern for the CeTi2 O6 phase involving a trace amount of TiO2 phase, obtained by the oxidation of Ce4 Ti9 O24 . The precipitation of a trace amount of CeO2 together with TiO2 implies that the mutual diffusion of Ce4+ and Ti4+ in the powder, i.e., the reaction between CeO2 and TiO2 may be slow. The XRD pattern of synthesized perovskite-type Ce2/3 TiO3 agreed with that previously reported [22]. Fig. 4 shows the diffuse reflectance spectra of the cerium titanates prepared in this study, compared with those of TiO2 (ST-01), SrTiO3 (TPL) and CeTiO4 (PB). The Ce2 Ti2 O7 , Ce2/3 TiO3 , Ce4 Ti9 O24 , CeTiO4 , and CeTi2 O6 , prepared in this study were reddish brown, black, brown, yellow, and pale yellow, respectively; CeTiO4 (PB) was

120 SrTiO3(TPL)

Diffuse reflectance, Rd / %

264

100

CeTi2O6

TiO2(ST-01)

80

CeTiO4(673K) CeTiO4(873K)

60 CeTiO4 (PB)

40

Ce2Ti2O7 Ce4Ti9O24

Ce2/3TiO3

20 0

300

400

500

600

Wave length,

700

800

900

1000

/ nm

Fig. 4. Diffuse reflectance spectra of the cerium titanates used in this study, compared with those of TiO2 (anatase-type, ST-01) and SrTiO3 . The SrTiO3 was purchased from TPL Inc., CeTiO4 (673 K) and CeTiO4 (873 K) were prepared by oxidation at 673 and 873 K, respectively.

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1.8

1.6 0h 1h 2h 3h 4h 5h 6h

1.2 1.0

0.6

1.2 1.0

0.6 0.4 0.2 0.0

0.4

0

1

2

3

4

5

6

Irradiation time, t / h

0.2 0.0

1.4

0.8 Absorbance,

/ arb.units Absorbance,

0.8

/ arb.units

starting solution

1.6 1.4

265

300

400 500 Wave length,

600 700 / nm

800

Fig. 5. Variation in the absorption spectrum of methylene blue aqueous solution sensitized by CeTiO4 powder under irradiation with 500 W Xe discharge light. The concentration of methylene blue in the starting solution was 2.0 × 10−5 mol dm−3 . 0–2 h: irradiation using UV-cut filter (L42); 2–3 h: without irradiation; 3–4 h: irradiation using UV-cut filter (L42); 4–5 h: without irradiation; 5–6 h: unfiltered irradiation.

pale orange. Ce2/3 TiO3 exhibited metal-like behavior. For the CeTiO4 and CeTi2 O6 with Ce4+ , the energy difference among the impurity levels in the energy band gap was relatively large. Thus, the cerium titanates with mainly Ce4+ were supplied as sample powders for the photobleaching of the methylene blue aqueous solution. In Fig. 4, the diffuse reflectance spectrum for CeTiO4 prepared at 873 K is shown with that for CeTiO4 prepared at 673 K; no distinct difference between the two spectra was observed. For the cerium titanates, the defects related to oxygen, e.g., oxygen vacan× cies such as VO •• , VO • , and VO , oxygen at interstitial sites ×   such as Oi , Oi , and Oi , and holes on the lattice oxygen such as OO • , may be considered as color centers. Detailed discussion on the defects inducing photoexcitations requires further experiments using many samples. Fig. 5 shows the variation in the absorption spectrum of methylene blue aqueous solution under irradiation with Xe discharge light, when the CeTiO4 powder oxidized at 673 K was dispersed in the aqueous solution. When the ST-01 commercial powder of anatase-type TiO2 was used, the characteristic absorption peak of the methylene blue at around 664 nm was decreased upon the irradiation and slightly shifted toward a shorter wavelength [18,20]; finally, the solution became colorless. Therefore, in Fig. 6, the maximum absorbance in the wavelength range between 600 and 664 nm is plotted against time, where the data for the cerium titanates are compared with those for TiO2 (ST-01). A decrease in the absorbance at 0 min, which implies a strong adsorption of methylene blue on the sample powder, was observed for CeTiO4 (PB). The authors previously found that

Fig. 6. Change in the maximum absorbance in the wavelength range of 600–664 nm of methylene blue sensitized by CeTiO4 , CeTiO4 (PB), and CeTi2 O6 powders, compared with that by TiO2 (ST-01) under irradiation with 500 W Xe discharge light. Solid line indicates unfiltered irradiation. Chain line and bold arrow indicate irradiation with visible light with λ > 420 nm using UV-cut filter (L42, Suruga Seiki Co. Ltd.). Dotted line indicates the interruption of irradiation. (䊉), (䉲): CeTiO4 ; (䊏): CeTiO4 under the conditions of MB 50 ml with 0.1 g sample; (䊊): CeTiO4 (PB); ( ): CeTi2 O6 ; (夽): TiO2 (anatase-type ST-01) powder dried at 413 K; and (䉫): blank test without sample powder.

this phenomenon occurs for the sample powder consisting of two-phase mixtures [17,18]; therefore, we did not carry out further examination. The photobleaching rates for the CeTiO4 and CeTi2 O6 sample powders were not high. When the CeTiO4 powder kept in an evacuated desiccator for several days was used as the sample powder, the photobleaching rate indicated with 䉲 in Fig. 6 became low, similar to that without irradiation. Thus, we found that the CeTiO4 powder was attached onto the upper sidewall of the glass container. That is, the CeTiO4 powder had a hydrophobic surface. It is not clear whether the hydrophobic property was brought by the light irradiation. However, if so, the behavior of CeTiO4 is in contrast with that of TiO2 , which becomes hydrophilic upon light irradiation [23,24]. To attain a better stirring condition of the aqueous solution, the quantities of the liquid and powder, respectively, were reduced to one-half. Then, as shown with 䊏 in Fig. 6, the photobleaching of methylene blue proceeded smoothly upon irradiation with Xe discharge light. Fig. 7 shows variations in the maximum absorbance of the methylene blue aqueous solution in the wavelength range between 600 and 664 nm with time, when the composite powders of CeTiO4 –zSrTiO3 and CeTi2 O6 –zSrTiO3 with z = 30 mass% were used as samples; the present results for TiO2 (ST-01), SrTiO3 (TPL), CeTiO4 , and CeTi2 O6 and the previous result for TiO2 –zSrTiO3 with z = 30 mass% [17] are shown for comparison. Evaluated photobleaching rates for various sample powders are shown in Table 1. Because the slight decrease in the maximum absorbance was observed without irradiation, the photobleaching rates were determined from the difference in the slopes between the irradiation and its interruption. Very recently, two of the au-

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/ arb.units

1.6 1.4 1.2 1.0

Absorbance,

0.8 0.6 0.4 0.2 0.0

0

1

2

3

4

5

6

Irradiation time, t / h Fig. 7. Change in the maximum absorbance in the wavelength range of 600–664 nm of methylene blue sensitized by composite powders of CeTiO4 and CeTi2 O6 with SrTiO3 , compared with those by CeTiO4 , CeTi2 O6 , SrTiO3 , TiO2 (ST-01), and TiO2 –SrTiO3 powders, under irradiation with 500 W Xe discharge light. Solid line indicates unfiltered irradiation. Chain line and bold arrow indicate irradiation with visible light with λ > 420 nm using UV-cut filter (L42, Suruga Seiki Co. Ltd.). Dotted line indicates the interruption of irradiation. (䊉): CeTiO4 –zSrTiO3 composite, (z = 30 mass%); (䉲): CeTi2 O6 –zSrTiO3 composite, (z = 30 mass%); (䊏): TiO2 –zSrTiO3 composite, (z = 30 mass%) [17]; (䊊): CeTiO4 ; ( ): CeTi2 O6 ; (䊐): TiO2 powder dried at 413 K; (夽): SrTiO3 ; and (䉫): blank test without sample powder.

thors [18] reported that the addition of Sr(Zr0.90 Y0.10 )O3–δ to TiO2 increased several times in photocatalytic activity for the photobleaching of methylene blue. The result has been explained using a model for the flow of photogenerated electrons and holes through the heterogeneous junction between the composite particles. The charges flowing through the junction must contribute to spatial separation of the photogenerated charges, which results in the inhibition of the recombination of the holes and electrons [18,25,26]. The result for the TiO2 –zSrTiO3 composite has been interpreted in line with that for TiO2 –zSr(Zr0.90 Y0.10 )O3–δ . In Fig. 7, two phenomenological features of the composite powders appeared, which are very clear and important. First, the photobleaching rate for the composite was higher than that for the respective constituent oxides. The difference in the photobleaching rates of CeTiO4 –zSrTiO3 and SrTiO3 powders was small. Table 1 Photobleaching rates of methylene blue aqueous solution sensitized using various sample powders Sample powder

Photobleaching rate, ∆abs min−1 Unfiltered

CeTiO4 CeTiO4 CeTiO4 –zSrTiO3 CeTi2 O6 CeTi2 O6 –zSrTiO3 SrTiO3 TiO2 TiO2 –zSrTiO3

0.8 1.6 6.6 1.1 3.2 5.8 35.5 156.3

× × × × × × × ×

10−3

10−3 10−3 10−3 10−3 10−3 10−3 10−3

Remarks

Filtered 1.3 1.9 1.3 0.2 0.4 0.8 0.5 1.0

× × × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

Hydrophobic Strong stirring z = 30 mass% z = 30 mass% A trace TiO2 z = 30 mass%

However, one can note that the CeTiO4 phase acts as the photocatalyst in the composite and the SrTiO3 surrounded by it assists the spatial separation of the charges. The composite containing a smaller amount of SrTiO3 exerted slightly but clearly higher photocatalytic activity than did CeTiO4 alone. Second, the absorbance of the methylene blue aqueous solution at 0 min, i.e., after dispersing sample powders, became lower than that of as-prepared solution. The flow of charges through the heterogeneous junctions may lead to the accumulation of charges on the particle surface, which must adsorb the methylene blue ions; the adsorption must result in a slight decrease of the methylene blue concentration in the solution. The decrease in the absorbance thereafter with irradiation could be attributed to the photobleaching of methylene blue, because the decrease in the absorbance almost stopped upon the interruption of irradiation, and further, with successive irradiation, the decrease in the absorbance proceeded again. The feature of the composite, described above, appeared for the CeTiO4 –zSrTiO3 powder rather than CeTi2 O6 –zSrTiO3 . In addition, under visible light irradiation (λ > 420 nm) using a UV-cut filter, CeTiO4 –zSrTiO3 appeared to exert slightly higher photocatalytic activity than SrTiO3 alone. The recombination of the photogenerated charges via defects related to oxygen vacancies should be reduced. In the case of CeTiO4 formed by oxygen insertion into Ce2 Ti2 O7 , trace oxygen vacancies may be contained. Precise control of the oxygen concentration in CeTiO4 may be needed to enhance the photocatalytic activity. For the composite composed of two oxides with different Fermi levels, the diffusion potential appears around the heterogeneous junction [27]. The present results suggest that the diffusion potential may be useful for restraining the recombination of the photogenerated electrons and holes.

4. Conclusions Several cerium titanates were prepared and their optical absorption spectra were recorded. On the basis of the result, the photocatalytic activity of CeTiO4 and CeTi2 O6 , and their composite powders with SrTiO3 was examined for the photobleaching of methylene blue aqueous solution. The results obtained were as follows, (1) A highly crystallized CeTiO4 could be obtained by oxidation of Ce2 Ti2 O7 at temperatures as low as 673 K. Its XRD pattern differed completely from that of commercial CeTiO4 (PB). (2) The formation of brannerite-type CeTi2 O6 by the oxidation of Ce4 Ti9 O24 was accompanied with the precipitation of a trace amount of CeO2 together with TiO2 ; the reaction between CeO2 and TiO2 may be slow. (3) The oxides with mainly Ce3+ were deeply colored and absorbed light with longer wavelength than did the ox-

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ides with mainly Ce4+ . The former oxides appeared to be inadequate as photocatalysts. (4) The photocatalytic activities of the CeTi2 O6 and CeTiO4 (PB) may be weaker than that of CeTiO4 . The composite powder of CeTiO4 with SrTiO3 was found to exert a relatively high photocatalytic activity under visible light irradiation (λ > 420 nm). References [1] A. Fujishima, K. Honda, Bull. Chem. Soc. Jpn. 44 (1971) 1148. [2] A. Fujishima, K. Honda, Nature 238 (1972) 37. [3] K. Domen, J.N. Kondo, M. Hara, T. Takata, Bull. Chem. Soc. Jpn. 73 (2000) 1307. [4] A. Kudo, J. Ceram. Soc. Jpn. 109 (2001) S81. [5] A. Fujishima, T.N. Rao, A. Tryk, J. Photochem. Photobiol. C: Photochem. 1 (2000) 1. [6] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [7] H. Irie, Y. Watanabe, K. Hashimoto, Photocatalysis 9 (2002) 30. [8] T. Ohno, Y. Masaki, S. Hirayama, M. Matsumura, J. Catal. 204 (2001) 163. [9] T. Umebayashi, T. Yamaki, S. Tanaka, K. Asai, Photocatalysis 9 (2002) 32. [10] A. Preuss, R. Gruehn, J. Solid State Chem. 110 (1994) 363. [11] W.H. Jung, Condens. Matter 10 (1998) 8553.

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