Synthesis and characterization of CeO2-ZrO2-Bi 2O3 solid solutions for environment-friendly yellow pigments

Article abstract of DOI:10.1246/cl.2006.1032

CeO₂-ZrO₂-Bi₂O₃ solid solutions have been synthesized by a citrate complex route. The resulting colored materials have been characterized as potential environment-friendly pigments. The yellow hue of the pigment was significantly enhanced by the introduction of zirconium and bismuth into the CeO₂ lattice, which produces intrinsic strain and a hybrid orbital of Bi_6s and O_2p, respectively. The yellow pigments obtained have been found to be effective alternatives to the conventional toxic colored materials. Copyright

Full text of DOI:10.1246/cl.2006.1032

1
032
Chemistry Letters Vol.35, No.9 (2006)
Synthesis and Characterization of CeO –ZrO –Bi O Solid Solutions
2
2
2
3
for Environment-friendly Yellow Pigments
ꢀ
Toshiyuki Masui, Shinya Furukawa, and Nobuhito Imanaka
Department of Applied Chemistry, Faculty of Engineering, Osaka University,
-1 Yamadaoka, Suita, Osaka 565-0871
2
(Received May 24, 2006; CL-060606; E-mail: imanaka@chem.eng.osaka-u.ac.jp)
9
CeO2–ZrO2–Bi2O3 solid solutions have been synthesized by
the bulk of the solid solution. Accordingly, in the preparation
of the CeO2–ZrO2–Bi2O3 pigments, the Ce:Zr ratio was fixed
to 55:45 and the molar ratio of bismuth has been adjusted from
5 to 25 mol % at intervals of 5 mol %.
a citrate complex route. The resulting colored materials have
been characterized as potential environment-friendly pigments.
The yellow hue of the pigment was significantly enhanced by
the introduction of zirconium and bismuth into the CeO2 lattice,
which produces intrinsic strain and a hybrid orbital of Bi6s and
O2p, respectively. The yellow pigments obtained have been
found to be effective alternatives to the conventional toxic
colored materials.
The CeO2–ZrO2–Bi2O3 solid solutions were synthesized
from nitrate precursors by complexing the cations with the
ꢁ3
citrate. After mixing the aqueous solutions of 1.0 molꢂdm Ce-
ꢁ
3
ꢁ3
(NO3)3, 1.0 molꢂdm ZrO(NO3)2, and 0.5 molꢂdm Bi(NO3)3,
adjusting the total amount of the cations to be 20 mmol, an aque-
ꢁ3
3
ous solution of 1.0 molꢂdm citric acid (30 dm ) was added.
The resulting solution was stirred at 353 K for 5 h. A solid ob-
tained was dried at 403 K and then heated at 573 K for 1 h and
successively at 1273 K for 1 h to obtain a yellow powder. The
compositions of the pigments analyzed by an X-ray fluorescence
spectrometer (Rigaku ZEX-100e) are summarized in Table 1.
X-ray powder diffraction (XRD) patterns of the pigments
were collected (Rigaku Multiflex, 40 kV and 50 mA, Ni-filtered
Cu Kꢀ). Optical reflectance spectra were obtained with a UV–
vis spectrometer (Shimadzu UV-2450) using barium sulfate as
a reference. Color properties were evaluated in terms of CIE
The use of nontoxic raw materials in the chemical industries
is becoming important to prevent health hazards and environ-
1
mental damage. In the field of pigments, it has been necessary
to develop environment-friendly materials to substitute for the
conventional pigments containing toxic elements (e.g., Cd, Co,
Cr, Hg, Pb, Sb, and Se). However, only a limited number of non-
toxic pigments can satisfy requirements of high thermal and
chemical stabilities particularly in the case of yellow pig-
ments.2
–6
ꢀ
ꢀ ꢀ
Among the several candidates for the alternative nontoxic
yellow pigments, CeO2 and its related materials have been at-
tracted because of the opacity, low toxicity, and high-tempera-
L a b system with a chroma meter (Minolta, CR-300). The
ꢀ
value L represents the brightness or darkness of the color as re-
ꢀ
lated to a neutral grey scale. The values a (the axis red-green)
ꢀ
ture stability.6 The coloring mechanism is based on the
charge-transfer band from O2p to Ce4f in the semiconducting
CeO2. The band gap between the anionic O2p valence band
and the cationic Ce4f conduction band can be modified by the
formation of solid solutions, which introduces an additional
electronic level between the valence band and the conduction
band. As a result, shift of the charge-transfer band is observed.
In this work, new pigments based on ternary CeO2–ZrO2–
Bi2O3 solid solutions have been synthesized. Although these
materials were studied in details in our laboratory as oxygen-
,7
and b (the axis yellow-blue) indicate the color hue. The BET
specific surface area was measured using nitrogen adsorption
(Mircometrics FlowSorb II 2300).
Figure 1 shows the X-ray powder diffraction results of the
CeO –ZrO –Bi O samples. A single phase of cubic fluorite
structure was obtained for the pigments in which the bismuth
content was 20 mol % or less, while in the sample that the
2
2
2
3
3þ
Bi content was 23 mol %, Bi O was observed as a secondary
2
3
phase. In the region forming the single fluorite phase, the XRD
pattern shifted to higher angles with the increase in the Bi
content. This result shows that solid solutions were successfully
8
storage materials for automotive exhaust catalysts, their color
properties have not been characterized. Here, we have character-
ized the color properties from the viewpoint of possible ecolog-
3þ
obtained. Although the ionic radius of Bi (0.117 nm) is larger
þ
4
4þ
than those of Ce (0.097 nm) and Zr (0.084 nm) in eight
10
ical inorganic yellow pigments. Doping of Zr4 into the CeO2
þ
coordination, the diffraction pattern of the CZB component
lattice produces intrinsic strain, which will affect the O2p valence
shifted to higher angles because of the formation of oxide anion
vacancies.
The effects of increasing Bi content on the color of the pig-
3þ
band. Furthermore, coexistence of Bi in the CeO2–ZrO2 lat-
tice will enhance the visible light absorption by the formation
of a hybrid orbital of Bi6s and O2p. The inorganic yellow pig-
ments presented in this study can be synthesized by a simple cit-
rate complex method in an atmospheric air, using nontoxic ele-
ments. It is expected, therefore, that the pigments can be inher-
ently safe in a workplace and waste-disposal sites and will give
no environmental deterioration.
In our previous study on the catalysis of the CexZr1ꢁxO2 sol-
id solutions, we found that those close to x ¼ 0:55 compatible
with a cubic symmetry showed the deepest yellow hue as well
as high redox activity due to the distortion of oxide anions in
Table 1. Composition of the CeO2–ZrO2–Bi2O3 pigments
Sample composition
Ce:Zr
Bi content/mol %
Ce0:56Zr0:44O2:0
56:44
56:44
56:44
55:45
54:46
55:45
0
5
Ce0:53Zr0:42Bi0:05O1:975
Ce0:50Zr0:40Bi0:10O1:95
Ce0:47Zr0:38Bi0:15O1:925
Ce0:43Zr0:37Bi0:20O1:9
Ce0:42Zr0:35Bi0:23O1:89
10
15
20
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Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.9 (2006)
1033
Figure 1. X-ray powder diffraction patterns of the CeO2–
ZrO2–Bi2O3 pigments.
Figure 2. Diffuse reflectance spectra obtained for various com-
positions of the CeO2–ZrO2–Bi2O3 solid solutions.
ꢀ
ꢀ ꢀ
Table 2. L a b color coordinate, optical energy gap (Eg), and
BET specific surface area (S.A.) of the CeO2–ZrO2–Bi2O3 pig-
ments
bands are made up by a hybrid orbital of Bi6s and O2p and the
visible light absorption corresponds to the transition from the
hybrid orbital to the conduction band. As a result, the CeO2–
ZrO2–Bi2O3 pigments absorb in the wavelength region below
500 nm with the decrease in the band gap energies, producing
a yellow color.
In conclusion, new inorganic yellow pigments, CeO2–
ZrO2–Bi2O3 solid solutions, were successfully synthesized.
It is suggested that the coloring mechanism is based on the
modification of the valence band by forming a hybrid orbital
of Bi6s and O2p. The CeO2–ZrO2–Bi2O3 solid solutions have
high thermal stability and are composed of all nontoxic ele-
ments. Therefore, they are expected to be new environment-
friendly pigments.
ꢀ
ꢀ
ꢀ
2
ꢁ1
Pigment
L
a
b
Eg/eV S.A./m ꢂg
Ce0:56Zr0:44O2:0
96.9 ꢁ3:66 14.3 3.26
6.8
4.0
2.2
0.8
0.9
0.7
Ce0:53Zr0:42Bi0:05O1:975 92.9 ꢁ6:65 43.5 2.73
Ce0:50Zr0:40Bi0:10O1:95 88.4 ꢁ2:15 56.3 2.64
Ce0:47Zr0:38Bi0:15O1:925 86.9
1.76 63.0 2.59
6.93 68.9 2.54
12.5 65.0 2.43
Ce0:43Zr0:37Bi0:20O1:9
83.0
Ce0:42Zr0:35Bi0:23O1:89 78.1
ment powders were summarized in Table 2. The optical energy
gaps (Eg) of the CeO2–ZrO2–Bi2O3 samples determined from
the reflectance spectra and the BET specific surface area are also
tabulated. When the bismuth content increases, there is only
This work was supported by the Industrial Technology
Research Grant Program in ’02 (project ID: 02A27004c) from
the New Energy and Industrial Technology Development
Organization (NEDO) based on funds provided by the Ministry
of Economy, Trade and Industry, Japan (METI).
ꢀ
a slight increase in the red component (a ), while the yellow
ꢀ
component (b ) increases drastically giving rise to progressive
brilliant hue of the samples. The most attractive yellow
color was obtained for Ce0:43Zr0:37Bi0:20O1:9. However, in the
Ce0:42Zr0:35Bi0:23O1:89 sample, the yellow hue faded owing to
the secondary phase formation.
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Figure 2 depicts the diffuse reflectance spectra obtained
for the CeO2–ZrO2–Bi2O3 solid solutions. The strong optical
absorption behavior was observed in all samples in the region
of shorter wavelength than 500 nm, which corresponds to the
blue region. As a result, the color of the samples becomes
yellow, because blue is a complementary color to yellow. The
position of the reflectance edge depends critically on the bismuth
content, and the maximum absorption in longer wavelengths
was observed at the composition of Ce0:43Zr0:37Bi0:20O1:9 in
comparison with the other samples, which corresponds to the
color coordinates in Table 2.
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In CeO2–ZrO2, the 4f valence cell of Ce4 is empty and that
þ
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1
1
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Published on the web (Advance View) August 12, 2006; doi:10.1246/cl.2006.1032