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UV-shielding properties of zinc oxide-doped ceria fine powders derived via soft solution chemical routes
Materials Chemistry and Physics 75 (2002) 39–44
UV-shielding properties of zinc oxide-doped ceria fine powders derived via soft solution chemical routes Ruixing Li a , Shinryo Yabe b , Mika Yamashita b , Shigeyosi Momose b , Sakae Yoshida c , Shu Yin a , Tsugio Sato a,∗ a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan b Kose Co., 48-18 Sakae-cho, Kita-ku, Tokyo 114-0005, Japan c Nippon Inorganic Colour and Chemical Co. Ltd., 3-14-1 Funado, Itabashi-ku, Tokyo 174-0041, Japan
Abstract Nanoparticles of zinc oxide-doped ceria (3–7 nm diameter) were synthesized by soft solution chemical processes at 40 ◦ C and pH 6. Some zinc ions remained in the solution but cerium ions precipitated completely at pH 6. The solubility limit of ZnO in Ce1−x Znx O2−x was determined to be x = 0.561. The oxidation catalytic activity of ceria effectively decreased by doping with zinc oxide. The photocatalytic activity of undoped ceria and zinc oxide-doped ceria for oxidation of phenol was much smaller than that of titania. Zinc oxide-doped ceria showed excellent UV absorption and transparency in the visible ray region in a similar manner as that of undoped ceria. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ceria; Zinc oxide; Solid solution; Fine powder; UV-shielding; Catalytic activity
1. Introduction Since the UV-ray included in solar light deteriorates some organic materials and causes damaging effects on human’s health, various organic and inorganic sun-care products have been developed. Generally, organic UV absorbers show effective UV-B (290–320 nm) absorption property, but with modest UV-A (320–400 nm) absorbing ability. They pose a safety problem when used at high concentrations. Fine powders of ceria, titania and zinc oxide have characteristics ideal for use as a broad-spectrum inorganic ultraviolet (UV) radiation blocking material in personal-care products [1–4]. Fine particles of titania and zinc oxide are effective inorganic sunscreens which are popularly used in the cosmetic industry nowadays. Their high refractive indices, however, can make the skin look unnaturally white when incorporated into the products. Additionally, their high photocatalytic activity facilitates the generation of reactive oxygen species [5], which can oxidize and degrade other ingredients in the formulation. On the other hand, ceria possessing a lower refractive index is relatively transparent to visible light and appears natural on the skin without imparting an excessively pale white look, but has excellent ultraviolet radiation absorption properties [2]. Further, the photocatalytic activity ∗ Corresponding author. Tel./fax: +81-22-217-5597. E-mail address: [email protected] (T. Sato).
of ceria is generally much lower than those of titania and zinc oxide. However, because of the high catalytic activity for oxidation of organic materials, ceria has seldom been used commercially as a sunscreen material. The catalytic activity of ceria can be related with the oxygen evolution and absorption equilibrium reaction shown by Eq. (1). 3+ 1 CeO2 Ce4+ 1−x Cex O(2−x)/2 䊐x/2 + 4 xO2
(1)
It is well known that the ideal r(Mn + )/r(O2− ) ionic size ratio in MO8 eight coordinations oxide is 0.732. In the case of fluorite structure ceria, r(Ce4+ )/r(O2+ ) is 0.703, which is smaller than that of the ideal value, indicating that Ce4+ is not large enough to stabilize fluorite structure. To take on more stable eight coordinations of fluorite structure, some of Ce4+ would have a tendency to be reduced to Ce3+ which has a larger ionic radius than Ce4+ as shown by Eq. (1). Accompanying with this reaction, oxygen molecules are released to form oxygen vacancies. Therefore, it is expected that the evolution of oxygen may be depressed by doping with metal ion possessing both larger ionic size and lower valence than Ce4+ to stabilize fluorite structure and shift the equilibrium shown by Eq. (1) to the left-hand side, respectively. Actually, in a previous paper [3], it was reported that the oxidation catalytic activity of ceria was greatly decreased by doping with calcium ion possessing larger ionic size and lower valence.
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It was also reported that the particle size and colour of ceria greatly depended on the solution pH, i.e., the particle size of ceria prepared around pH 6 was much smaller than that prepared above pH 8 [3]. Further, the sample prepared around pH 6 was whitish, while that prepared above pH 8 is yellowish. Doping calcia with ceria in acidic solution is, however, difficult because of large solubility of calcium ion in water. Zinc oxide is a candidate material to overcome this problem, since zinc oxide is white and its solubility in water is much lower than that of calcia. In the present study, a series of tests were conducted to investigate the synthesis and photochemical properties of nanoparticles of zinc oxide-doped ceria.
2. Experimental procedures Undoped and zinc oxide-doped cerias were synthesized via soft solution chemical routes at 40 ◦ C as follows. At first 2.8 M sodium hydroxide solution (M = mol dm−3 ) and appropriate quantities of CeCl3 –ZnCl2 mixed solution (Ce3+ + Zn2+ = 1 M) were simultaneously dropped into stirred deionized water at 40 ◦ C, then a 2 M hydrogen peroxide solution was added and the final pH value was adjusted to 6 with NaOH and/or HCl solutions. The chemical reactions may be expressed by Eqs. (2) and (3). (1 − x)CeCl3 + xZnCl2 + (3 − x)NaOH → Ce1−x Znx (OH)3−x + (3 − x)NaCl
(2)
Ce1−x Znx (OH)3−x + 21 (1 − x)H2 O2 → Ce1−x Znx O2−x + (2 − x)H2 O
heated castor oil. The photocatalytic activity was evaluated by measuring the degree of decomposition of phenol by irradiating a 100 W high pressure mercury arc (λ > 290 nm) to 500 cm3 of 0.5 mM phenol solution dispersing 0.25 g of sample powder at 60 ◦ C with bubbling 250 cm3 min−1 of air. The UV-shielding property was evaluated by measuring the transmittance of a thin film, of 0.0125 mm thickness, containing the sample powder with an UV–visible spectrophotometer, where 2 g sample, 4 g nitrocellulose, 10 g ethyl acetate, and 9 g butyl acetate were mixed uniformly using a paint shaker and zirconia balls (5φ). The inductively coupled plasma atomic emission spectrometer (ICP-AES) was employed to determine the contents of Zn and Ce elements in the end supernatant solution and powder sample, where the powder sample was dissolved in a hot HNO3 –HCl mixed solution.
3. Results and discussion 3.1. Preparation of zinc oxide-doped ceria Zinc oxide and calcia-doped ceria powders were prepared with a Zn/(Zn + Ce) and Ca/(Ca + Ce) molar ratios of 0.2 at various final pHs. The amounts of zinc ions, calcium ions and cerium ions which remained in the supernatant solutions are shown in Fig. 1 as a function of final pH. The amount of calcium ions that remained in the solution significantly increased with decreasing solution pH and it might be concluded that the solution pH should be maintained above 12 to precipitate calcium ions completely. On the other hand, a small amount of zinc ions remained at pH 6 and could be
(3)
After centrifugation, the clear supernatant solution was decanted and the precipitate was washed with deionized water and methanol, and then dried at 85 ◦ C overnight. The crystalline phase was identified by XRD using graphite monochromatized Cu K␣ radiation. The differential thermal analysis was carried out in air at a heating rate of 10 ◦ C min−1 . The size distribution and shape of the particles were observed using a transmission electron microscope (JEOL, JEM-2000EX11). The specific surface area (BET) was measured by nitrogen sorption analysis (NOVA 1000-TS, Quantachrome Co.). The catalytic activity for oxidation of organic materials was determined by the conductometric determination method (Rancimat method) [6–8] using castor oil as an oxidizing material as follows. The powder sample (1 g) was mixed with 10 g of castor oil and maintained at 120 ◦ C with bubbling 500 cm3 min−1 of air, where the air was introduced into deionized water attached to the electric conductivity measurement cell. The oxidation catalytic activity was evaluated by measuring the increase in electric conductivity of deionized water by trapping volatile molecules coming from the oxidation of
Fig. 1. Amounts of (a) calcium ion, (b) zinc ion and (c) cerium ion remained in the supernatant solution at various solution pHs with Ca/(Ca + Ce) and Zn/(Zn + Ce) atomic ratio of 0.2 at 40 ◦ C.
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Fig. 2. XRD patterns of ceria formed with CeCl3 + ZnCl2 mixed solutions containing (a) 0, (b) 10, (c) 60, (d) 70 and (e) 90 mol% of ZnCl2 at 40 ◦ C and final pH 6. (䊏) CeO2 , (䊉) Zn(OH)2 , () ZnO, (䉲) ZnCl2 ·4Zn(OH)2 ·H2 O.
completely precipitated above pH 7. Further, the amount of cerium ions remained in the solution was negligibly small above pH 6. Ceria powders were prepared using mixed CeCl3 –ZnCl2 solutions containing 0–90 mol% ZnCl2 . The single fluorite-phase was observed by XRD for less than 60 mol% of nominal Zn2+ content as shown in Fig. 2. The peaks corresponding to zinc compounds such as ZnO, Zn(OH)2 and ZnCl2 4Zn(OH)2 H2 O appeared above 70 mol% Zn2+ and those of fluorite-phase ceria thoroughly disappeared at 90 mol% Zn2+ concentration. Both undoped ceria and zinc oxide-doped cerias, consisted of spherical nanoparticles (3–7 nm in diameter) as displayed in Fig. 3(a) and (b). The Zn/(Ce + Zn) atomic ratios in the starting solutions and the end powders consisting of the single phase fluorite structure as shown by XRD, specific surface areas and particle sizes determined by BET method are listed in Table 1. According to the results shown in Table 1, the maximum value of zinc ion content, i.e., the solubility limit x in Ce1−x Znx O2−x was determined as 0.561. The calculated values of particle size were almost identical to those measured by TEM.
The lattice parameter is extremely sensitive to the chemical composition; therefore, it is widely used to determine the solubility limit of solid solutions. Fig. 4 shows the variation of the lattice parameter with zinc ion content in the powder. It was reported that the lattice constant of calcia-doped ceria increased linearly with increasing calcia content up to the solubility limit, i.e., 15–20 mol% calcia since larger Ca2+ simply substitutes for Ce4+ [3,4,9]. On the other
Table 1 Zn/(Zn + Ce) atomic ratios in the starting solution and the powders precipitated, particle sizes and specific surface areas of the samples Zn/(Zn + Ce) atomic ratio Added
Precipitated
0 0.1 0.2 0.3 0.4 0.5 0.6
0 0.078 0.156 0.247 0.348 0.454 0.561
Crystallite size (nm)
Specific surface area (m2 g−1 )
7.0 7.2 5.9 5.8 5.3 6.4 6.3
119 117 114 148 165 139 145
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Fig. 3. Transmission electron micrographs of (a) undoped CeO2 and (b) Ce0.844 Zn0.156 O1.844 .
hand, it is seen that the lattice constant of zinc oxide-doped ceria decreased at first at x = 0.078, then increased with increasing zinc ion content up to x = 0.454, and thereafter decreased again to x = 0.561. Consequently, it was difficult to determine the solubility limit of zinc according to
the conventional Vegard’s law. These complicated profiles may suggest the complicated alloying situation of zinc ion in ceria. When a metal ion of different valence is dissolved in a solid, the crystal lattice must compensate for the resulting charge imbalance. A lower valence metal oxide,
Fig. 4. Lattice constants of Ce1−x Znx O2−x solid solutions as a function of x. ( ) Ce1−x Znx O2−x (present study), (䊊) Ce1−x Znx O2−x [3], (䊏) Ce1−x Znx O2−x [9].
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e.g., MO can be incorporated into CeO2 via three different mechanisms as follows: CeO2
MO MCe + VO + OxO CeO2
+ Mi + 2OxO 2MO MCe CeO2
+ Cei + 2OxO CexCe + 2MO 2MCe
(4) (5) (6)
These can be referred to as vacancy compensation, dopant interstitial compensation and cerium interstitial compensation, respectively [10]. It was reported that vacancy compensation is the favourable solution mechanism for large cations, whereas for the small dopant cations some compensation via dopant interstitial may occur [10]. Furthermore, if defect clusters are formed, the solution energy is decreased further. It is known that the ionic size mismatch (r: ionic radius (r dopant − r host )/rhost ) between Zn2+ and Ce4+ is −7.22%. Therefore, zinc ion seemed to be incorporated via both vacancy compensation and cation interstitial compensation mechanisms. The net variation in the lattice parameter of CeO2 by doping with a smaller metal ion can be a combination of the lattice contraction due to the substitution of a smaller ion for Ce4+ and the lattice expansion due to the formation of interstitial cation [10,11]. In the case of zinc ion-doped ceria, these two different effects seemed to result in the complicated change in lattice dimension. For the simplicity, however, the expression of Ce1−x Znx O2−x was used in the present study. 3.2. Oxidation catalytic activity Fig. 5 shows the oxidation catalytic activity of Ce1−x Znx O2−x determined by the Ranshimat method for the air oxidation of caster oil at 120 ◦ C. It is seen that the conductivity
Fig. 5. Time dependence of the electric conductivity by the Ranshimat test for Ce1−x Znx O2−x solid solutions with: (a) x = 0, (b) x = 0.078, (c) x = 0.156, (d) x = 0.247, (e) x = 0.348, (f) x = 0.454, (g) x = 0.561 and (h) blank.
increased rapidly after some induction periods, indicating that the oxidation of castor oil proceeded via radical reaction mechanism. The induction period above which the electric conductivity rapidly changed was increased at first by doping with 7.8–34.8 mol% zinc oxide and then slightly decreased. Usually, specific surface area is a very important factor to the catalytic activity. However, no clear relationship between the oxidation catalytic activity and specific surface area was observed according to the data shown in Table 1 and Fig. 5. These results suggested that the decrease in oxidation catalytic activity might be mainly due to the formation of oxygen defect and stabilization of a fluorite structure by doping with zinc oxide.
Fig. 6. Degradation of phenol by UV light (λ > 290 nm) irradiation from a 100 W high pressure mercury lamp in the presence of 0.25 g of zinc oxide-doped ceria and titania (Degussa P-25) powders in 500 ml of 0.5 mM phenol solution at 60 ◦ C with bubbling 250 cm3 min−1 air. (䊐) Undoped CeO2 , ( ) Ce0.844 Zn0.156 O1.844 , (䊊) Ce0.439 Zn0.561 O1.439 , (䊉) TiO2 (Degussa P-25).
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Fig. 7. Transmittance spectra of 0.0125 mm thickness films of undoped and zinc oxide-doped ceria. (- - -) Undoped CeO2 , (—) Ce0.844 Zn0.156 O1.844 , (— — —) Ce0.652 Zn0.348 O1.652 .
3.3. Photocatalytic activity Fig. 6 shows the photocatalytic activities of zinc oxidedoped cerias together with that of titania (Degussa P-25). Phenol was completely oxidized within 3 h in the presence of titania. On the other hand, more than 90% of phenol remained even after 6 h in the presence of undoped and zinc oxide-doped ceria, indicating that the photocatalytic activity of both undoped and zinc oxide-doped ceria is much lower than that of titania. The low photocatalytic activity of ceria may be due to the existence of oxygen defects formed by reaction (1) and doping with zinc oxide since it is known that the oxygen defects plays an important role to enhance the recombination reaction of photoinduced electrons and holes [12]. 3.4. UV-shielding properties Fig. 7 shows the optical transmittance of undoped and 7.8–34.8 mol% zinc oxide-doped cerias from 250 to 750 nm wavelength for a 0.0125 mm thickness of thin film. The transmittance spectra of all samples were almost identical, indicating high transparency in a visible light region above a 400 nm and high UV-shielding property below a 400 nm. Therefore, it might be concluded that doping with zinc oxide did not result in the loss of UV-shielding ability of ceria even at a high zinc oxide content of 34.8 mol%.
4. Conclusion From the present results, the following conclusions may be drawn. (1) Nanoparticles (3–7 nm) of zinc oxide-doped
ceria were prepared via soft solution chemical routes at 40 ◦ C and pH 6. (2) Zinc ions completely precipitated at pH 7 and partially remained in the solution at pH 6, whereas the amount of cerium ions remained in the solution at pH 6 was negligibly small. (3) The solid solubility limit of x in Ce1−x Znx O2−x with the fluorite structure was 0.561. (4) Doping with zinc oxide resulted in a decrease in the catalytic activity of ceria for the oxidation of organic materials. (5) The photocatalytic activity of undoped ceria and zinc oxide-doped ceria for oxidation of phenol was much smaller than that of titania. (6) Zinc oxide-doped ceria showed as excellent UV absorption and transparency in the visible ray region in a similar manner as that of the undoped ceria. References [1] T. Masui, K. Fujiwara, K. Machida, G. Adachi, Chem. Mater. 9 (1997) 2197. [2] S. Yabe, S. Momose, J. Soc. Cosmet. Chem. Jpn. 32 (1998) 372. [3] S. Yabe, S. Momose, M. Yamashita, K. Tahira, S. Yoshida, K. Hasegawa, S. Yin, T. Sato, Int. J. Inorg. Mater. 3 (2001) 1003. [4] R. Li, S. Yabe, M. Yamashita, S. Momose, S. Yoshida, S. Yin, T. Sato, Solid State Ionics, in press. [5] R. Cai, K. Hashimoto, K. Ito, Y. Kubota, A. Fujishima, Bull. Chem. Soc. Jpn. 64 (1991) 1268. [6] A. Tschope, W. Liu, M. Flytzani-Stephanopoulos, J.Y. Ying, J. Catal. A 157 (1995) 42. [7] M. Haneda, T. Mizushima, N. Kakuta, A. Ueno, Y. Sato, S. Matsuura, K. Kasahara, M. Sato, Bull. Chem. Soc. Jpn. 66 (1994) 1279. [8] M.W. Laubli, P.A. Bruttel, J. Am. Oil Chem. Soc. 63 (1986) 792. [9] W. Huang, P. Shuk, M. Greenblatt, Chem. Mater. 9 (1997) 2240. [10] L. Minervini, M.O. Zacate, R.W. Grimes, Solid State Ionics 116 (1999) 339. [11] P. Li, I. Chen, J.E. Penner-Hahn, T. Tien, J. Am. Ceram. Soc. 74 (1991) 958. [12] S. Torlaschi, U. Barcucci, G. Zorzella, Ind. Vernice. 24 (1970) 9.
UV-shielding properties of zinc oxide-doped ceria fine powders derived via soft solution chemical routes Ruixing Li a , Shinryo Yabe b , Mika Yamashita b , Shigeyosi Momose b , Sakae Yoshida c , Shu Yin a , Tsugio Sato a,∗ a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan b Kose Co., 48-18 Sakae-cho, Kita-ku, Tokyo 114-0005, Japan c Nippon Inorganic Colour and Chemical Co. Ltd., 3-14-1 Funado, Itabashi-ku, Tokyo 174-0041, Japan
Abstract Nanoparticles of zinc oxide-doped ceria (3–7 nm diameter) were synthesized by soft solution chemical processes at 40 ◦ C and pH 6. Some zinc ions remained in the solution but cerium ions precipitated completely at pH 6. The solubility limit of ZnO in Ce1−x Znx O2−x was determined to be x = 0.561. The oxidation catalytic activity of ceria effectively decreased by doping with zinc oxide. The photocatalytic activity of undoped ceria and zinc oxide-doped ceria for oxidation of phenol was much smaller than that of titania. Zinc oxide-doped ceria showed excellent UV absorption and transparency in the visible ray region in a similar manner as that of undoped ceria. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ceria; Zinc oxide; Solid solution; Fine powder; UV-shielding; Catalytic activity
1. Introduction Since the UV-ray included in solar light deteriorates some organic materials and causes damaging effects on human’s health, various organic and inorganic sun-care products have been developed. Generally, organic UV absorbers show effective UV-B (290–320 nm) absorption property, but with modest UV-A (320–400 nm) absorbing ability. They pose a safety problem when used at high concentrations. Fine powders of ceria, titania and zinc oxide have characteristics ideal for use as a broad-spectrum inorganic ultraviolet (UV) radiation blocking material in personal-care products [1–4]. Fine particles of titania and zinc oxide are effective inorganic sunscreens which are popularly used in the cosmetic industry nowadays. Their high refractive indices, however, can make the skin look unnaturally white when incorporated into the products. Additionally, their high photocatalytic activity facilitates the generation of reactive oxygen species [5], which can oxidize and degrade other ingredients in the formulation. On the other hand, ceria possessing a lower refractive index is relatively transparent to visible light and appears natural on the skin without imparting an excessively pale white look, but has excellent ultraviolet radiation absorption properties [2]. Further, the photocatalytic activity ∗ Corresponding author. Tel./fax: +81-22-217-5597. E-mail address: [email protected] (T. Sato).
of ceria is generally much lower than those of titania and zinc oxide. However, because of the high catalytic activity for oxidation of organic materials, ceria has seldom been used commercially as a sunscreen material. The catalytic activity of ceria can be related with the oxygen evolution and absorption equilibrium reaction shown by Eq. (1). 3+ 1 CeO2 Ce4+ 1−x Cex O(2−x)/2 䊐x/2 + 4 xO2
(1)
It is well known that the ideal r(Mn + )/r(O2− ) ionic size ratio in MO8 eight coordinations oxide is 0.732. In the case of fluorite structure ceria, r(Ce4+ )/r(O2+ ) is 0.703, which is smaller than that of the ideal value, indicating that Ce4+ is not large enough to stabilize fluorite structure. To take on more stable eight coordinations of fluorite structure, some of Ce4+ would have a tendency to be reduced to Ce3+ which has a larger ionic radius than Ce4+ as shown by Eq. (1). Accompanying with this reaction, oxygen molecules are released to form oxygen vacancies. Therefore, it is expected that the evolution of oxygen may be depressed by doping with metal ion possessing both larger ionic size and lower valence than Ce4+ to stabilize fluorite structure and shift the equilibrium shown by Eq. (1) to the left-hand side, respectively. Actually, in a previous paper [3], it was reported that the oxidation catalytic activity of ceria was greatly decreased by doping with calcium ion possessing larger ionic size and lower valence.
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It was also reported that the particle size and colour of ceria greatly depended on the solution pH, i.e., the particle size of ceria prepared around pH 6 was much smaller than that prepared above pH 8 [3]. Further, the sample prepared around pH 6 was whitish, while that prepared above pH 8 is yellowish. Doping calcia with ceria in acidic solution is, however, difficult because of large solubility of calcium ion in water. Zinc oxide is a candidate material to overcome this problem, since zinc oxide is white and its solubility in water is much lower than that of calcia. In the present study, a series of tests were conducted to investigate the synthesis and photochemical properties of nanoparticles of zinc oxide-doped ceria.
2. Experimental procedures Undoped and zinc oxide-doped cerias were synthesized via soft solution chemical routes at 40 ◦ C as follows. At first 2.8 M sodium hydroxide solution (M = mol dm−3 ) and appropriate quantities of CeCl3 –ZnCl2 mixed solution (Ce3+ + Zn2+ = 1 M) were simultaneously dropped into stirred deionized water at 40 ◦ C, then a 2 M hydrogen peroxide solution was added and the final pH value was adjusted to 6 with NaOH and/or HCl solutions. The chemical reactions may be expressed by Eqs. (2) and (3). (1 − x)CeCl3 + xZnCl2 + (3 − x)NaOH → Ce1−x Znx (OH)3−x + (3 − x)NaCl
(2)
Ce1−x Znx (OH)3−x + 21 (1 − x)H2 O2 → Ce1−x Znx O2−x + (2 − x)H2 O
heated castor oil. The photocatalytic activity was evaluated by measuring the degree of decomposition of phenol by irradiating a 100 W high pressure mercury arc (λ > 290 nm) to 500 cm3 of 0.5 mM phenol solution dispersing 0.25 g of sample powder at 60 ◦ C with bubbling 250 cm3 min−1 of air. The UV-shielding property was evaluated by measuring the transmittance of a thin film, of 0.0125 mm thickness, containing the sample powder with an UV–visible spectrophotometer, where 2 g sample, 4 g nitrocellulose, 10 g ethyl acetate, and 9 g butyl acetate were mixed uniformly using a paint shaker and zirconia balls (5φ). The inductively coupled plasma atomic emission spectrometer (ICP-AES) was employed to determine the contents of Zn and Ce elements in the end supernatant solution and powder sample, where the powder sample was dissolved in a hot HNO3 –HCl mixed solution.
3. Results and discussion 3.1. Preparation of zinc oxide-doped ceria Zinc oxide and calcia-doped ceria powders were prepared with a Zn/(Zn + Ce) and Ca/(Ca + Ce) molar ratios of 0.2 at various final pHs. The amounts of zinc ions, calcium ions and cerium ions which remained in the supernatant solutions are shown in Fig. 1 as a function of final pH. The amount of calcium ions that remained in the solution significantly increased with decreasing solution pH and it might be concluded that the solution pH should be maintained above 12 to precipitate calcium ions completely. On the other hand, a small amount of zinc ions remained at pH 6 and could be
(3)
After centrifugation, the clear supernatant solution was decanted and the precipitate was washed with deionized water and methanol, and then dried at 85 ◦ C overnight. The crystalline phase was identified by XRD using graphite monochromatized Cu K␣ radiation. The differential thermal analysis was carried out in air at a heating rate of 10 ◦ C min−1 . The size distribution and shape of the particles were observed using a transmission electron microscope (JEOL, JEM-2000EX11). The specific surface area (BET) was measured by nitrogen sorption analysis (NOVA 1000-TS, Quantachrome Co.). The catalytic activity for oxidation of organic materials was determined by the conductometric determination method (Rancimat method) [6–8] using castor oil as an oxidizing material as follows. The powder sample (1 g) was mixed with 10 g of castor oil and maintained at 120 ◦ C with bubbling 500 cm3 min−1 of air, where the air was introduced into deionized water attached to the electric conductivity measurement cell. The oxidation catalytic activity was evaluated by measuring the increase in electric conductivity of deionized water by trapping volatile molecules coming from the oxidation of
Fig. 1. Amounts of (a) calcium ion, (b) zinc ion and (c) cerium ion remained in the supernatant solution at various solution pHs with Ca/(Ca + Ce) and Zn/(Zn + Ce) atomic ratio of 0.2 at 40 ◦ C.
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Fig. 2. XRD patterns of ceria formed with CeCl3 + ZnCl2 mixed solutions containing (a) 0, (b) 10, (c) 60, (d) 70 and (e) 90 mol% of ZnCl2 at 40 ◦ C and final pH 6. (䊏) CeO2 , (䊉) Zn(OH)2 , () ZnO, (䉲) ZnCl2 ·4Zn(OH)2 ·H2 O.
completely precipitated above pH 7. Further, the amount of cerium ions remained in the solution was negligibly small above pH 6. Ceria powders were prepared using mixed CeCl3 –ZnCl2 solutions containing 0–90 mol% ZnCl2 . The single fluorite-phase was observed by XRD for less than 60 mol% of nominal Zn2+ content as shown in Fig. 2. The peaks corresponding to zinc compounds such as ZnO, Zn(OH)2 and ZnCl2 4Zn(OH)2 H2 O appeared above 70 mol% Zn2+ and those of fluorite-phase ceria thoroughly disappeared at 90 mol% Zn2+ concentration. Both undoped ceria and zinc oxide-doped cerias, consisted of spherical nanoparticles (3–7 nm in diameter) as displayed in Fig. 3(a) and (b). The Zn/(Ce + Zn) atomic ratios in the starting solutions and the end powders consisting of the single phase fluorite structure as shown by XRD, specific surface areas and particle sizes determined by BET method are listed in Table 1. According to the results shown in Table 1, the maximum value of zinc ion content, i.e., the solubility limit x in Ce1−x Znx O2−x was determined as 0.561. The calculated values of particle size were almost identical to those measured by TEM.
The lattice parameter is extremely sensitive to the chemical composition; therefore, it is widely used to determine the solubility limit of solid solutions. Fig. 4 shows the variation of the lattice parameter with zinc ion content in the powder. It was reported that the lattice constant of calcia-doped ceria increased linearly with increasing calcia content up to the solubility limit, i.e., 15–20 mol% calcia since larger Ca2+ simply substitutes for Ce4+ [3,4,9]. On the other
Table 1 Zn/(Zn + Ce) atomic ratios in the starting solution and the powders precipitated, particle sizes and specific surface areas of the samples Zn/(Zn + Ce) atomic ratio Added
Precipitated
0 0.1 0.2 0.3 0.4 0.5 0.6
0 0.078 0.156 0.247 0.348 0.454 0.561
Crystallite size (nm)
Specific surface area (m2 g−1 )
7.0 7.2 5.9 5.8 5.3 6.4 6.3
119 117 114 148 165 139 145
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Fig. 3. Transmission electron micrographs of (a) undoped CeO2 and (b) Ce0.844 Zn0.156 O1.844 .
hand, it is seen that the lattice constant of zinc oxide-doped ceria decreased at first at x = 0.078, then increased with increasing zinc ion content up to x = 0.454, and thereafter decreased again to x = 0.561. Consequently, it was difficult to determine the solubility limit of zinc according to
the conventional Vegard’s law. These complicated profiles may suggest the complicated alloying situation of zinc ion in ceria. When a metal ion of different valence is dissolved in a solid, the crystal lattice must compensate for the resulting charge imbalance. A lower valence metal oxide,
Fig. 4. Lattice constants of Ce1−x Znx O2−x solid solutions as a function of x. ( ) Ce1−x Znx O2−x (present study), (䊊) Ce1−x Znx O2−x [3], (䊏) Ce1−x Znx O2−x [9].
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e.g., MO can be incorporated into CeO2 via three different mechanisms as follows: CeO2
MO MCe + VO + OxO CeO2
+ Mi + 2OxO 2MO MCe CeO2
+ Cei + 2OxO CexCe + 2MO 2MCe
(4) (5) (6)
These can be referred to as vacancy compensation, dopant interstitial compensation and cerium interstitial compensation, respectively [10]. It was reported that vacancy compensation is the favourable solution mechanism for large cations, whereas for the small dopant cations some compensation via dopant interstitial may occur [10]. Furthermore, if defect clusters are formed, the solution energy is decreased further. It is known that the ionic size mismatch (r: ionic radius (r dopant − r host )/rhost ) between Zn2+ and Ce4+ is −7.22%. Therefore, zinc ion seemed to be incorporated via both vacancy compensation and cation interstitial compensation mechanisms. The net variation in the lattice parameter of CeO2 by doping with a smaller metal ion can be a combination of the lattice contraction due to the substitution of a smaller ion for Ce4+ and the lattice expansion due to the formation of interstitial cation [10,11]. In the case of zinc ion-doped ceria, these two different effects seemed to result in the complicated change in lattice dimension. For the simplicity, however, the expression of Ce1−x Znx O2−x was used in the present study. 3.2. Oxidation catalytic activity Fig. 5 shows the oxidation catalytic activity of Ce1−x Znx O2−x determined by the Ranshimat method for the air oxidation of caster oil at 120 ◦ C. It is seen that the conductivity
Fig. 5. Time dependence of the electric conductivity by the Ranshimat test for Ce1−x Znx O2−x solid solutions with: (a) x = 0, (b) x = 0.078, (c) x = 0.156, (d) x = 0.247, (e) x = 0.348, (f) x = 0.454, (g) x = 0.561 and (h) blank.
increased rapidly after some induction periods, indicating that the oxidation of castor oil proceeded via radical reaction mechanism. The induction period above which the electric conductivity rapidly changed was increased at first by doping with 7.8–34.8 mol% zinc oxide and then slightly decreased. Usually, specific surface area is a very important factor to the catalytic activity. However, no clear relationship between the oxidation catalytic activity and specific surface area was observed according to the data shown in Table 1 and Fig. 5. These results suggested that the decrease in oxidation catalytic activity might be mainly due to the formation of oxygen defect and stabilization of a fluorite structure by doping with zinc oxide.
Fig. 6. Degradation of phenol by UV light (λ > 290 nm) irradiation from a 100 W high pressure mercury lamp in the presence of 0.25 g of zinc oxide-doped ceria and titania (Degussa P-25) powders in 500 ml of 0.5 mM phenol solution at 60 ◦ C with bubbling 250 cm3 min−1 air. (䊐) Undoped CeO2 , ( ) Ce0.844 Zn0.156 O1.844 , (䊊) Ce0.439 Zn0.561 O1.439 , (䊉) TiO2 (Degussa P-25).
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Fig. 7. Transmittance spectra of 0.0125 mm thickness films of undoped and zinc oxide-doped ceria. (- - -) Undoped CeO2 , (—) Ce0.844 Zn0.156 O1.844 , (— — —) Ce0.652 Zn0.348 O1.652 .
3.3. Photocatalytic activity Fig. 6 shows the photocatalytic activities of zinc oxidedoped cerias together with that of titania (Degussa P-25). Phenol was completely oxidized within 3 h in the presence of titania. On the other hand, more than 90% of phenol remained even after 6 h in the presence of undoped and zinc oxide-doped ceria, indicating that the photocatalytic activity of both undoped and zinc oxide-doped ceria is much lower than that of titania. The low photocatalytic activity of ceria may be due to the existence of oxygen defects formed by reaction (1) and doping with zinc oxide since it is known that the oxygen defects plays an important role to enhance the recombination reaction of photoinduced electrons and holes [12]. 3.4. UV-shielding properties Fig. 7 shows the optical transmittance of undoped and 7.8–34.8 mol% zinc oxide-doped cerias from 250 to 750 nm wavelength for a 0.0125 mm thickness of thin film. The transmittance spectra of all samples were almost identical, indicating high transparency in a visible light region above a 400 nm and high UV-shielding property below a 400 nm. Therefore, it might be concluded that doping with zinc oxide did not result in the loss of UV-shielding ability of ceria even at a high zinc oxide content of 34.8 mol%.
4. Conclusion From the present results, the following conclusions may be drawn. (1) Nanoparticles (3–7 nm) of zinc oxide-doped
ceria were prepared via soft solution chemical routes at 40 ◦ C and pH 6. (2) Zinc ions completely precipitated at pH 7 and partially remained in the solution at pH 6, whereas the amount of cerium ions remained in the solution at pH 6 was negligibly small. (3) The solid solubility limit of x in Ce1−x Znx O2−x with the fluorite structure was 0.561. (4) Doping with zinc oxide resulted in a decrease in the catalytic activity of ceria for the oxidation of organic materials. (5) The photocatalytic activity of undoped ceria and zinc oxide-doped ceria for oxidation of phenol was much smaller than that of titania. (6) Zinc oxide-doped ceria showed as excellent UV absorption and transparency in the visible ray region in a similar manner as that of the undoped ceria. References [1] T. Masui, K. Fujiwara, K. Machida, G. Adachi, Chem. Mater. 9 (1997) 2197. [2] S. Yabe, S. Momose, J. Soc. Cosmet. Chem. Jpn. 32 (1998) 372. [3] S. Yabe, S. Momose, M. Yamashita, K. Tahira, S. Yoshida, K. Hasegawa, S. Yin, T. Sato, Int. J. Inorg. Mater. 3 (2001) 1003. [4] R. Li, S. Yabe, M. Yamashita, S. Momose, S. Yoshida, S. Yin, T. Sato, Solid State Ionics, in press. [5] R. Cai, K. Hashimoto, K. Ito, Y. Kubota, A. Fujishima, Bull. Chem. Soc. Jpn. 64 (1991) 1268. [6] A. Tschope, W. Liu, M. Flytzani-Stephanopoulos, J.Y. Ying, J. Catal. A 157 (1995) 42. [7] M. Haneda, T. Mizushima, N. Kakuta, A. Ueno, Y. Sato, S. Matsuura, K. Kasahara, M. Sato, Bull. Chem. Soc. Jpn. 66 (1994) 1279. [8] M.W. Laubli, P.A. Bruttel, J. Am. Oil Chem. Soc. 63 (1986) 792. [9] W. Huang, P. Shuk, M. Greenblatt, Chem. Mater. 9 (1997) 2240. [10] L. Minervini, M.O. Zacate, R.W. Grimes, Solid State Ionics 116 (1999) 339. [11] P. Li, I. Chen, J.E. Penner-Hahn, T. Tien, J. Am. Ceram. Soc. 74 (1991) 958. [12] S. Torlaschi, U. Barcucci, G. Zorzella, Ind. Vernice. 24 (1970) 9.