Novel environmentally friendly inorganic red pigments ...and the synthesis condition, and the most...

Journal of Advanced Ceramics

2015, 4(1): 39–45 ISSN 2226-4108

DOI: 10.1007/s40145-015-0129-1 CN 10-1154/TQ

Research Article

www.springer.com/journal/40145

Novel environmentally friendly inorganic red pigments based on

calcium bismuth oxides

WENDUSU, Tetsuro YOSHIDA, Toshiyuki MASUI, Nobuhito IMANAKA*

Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka,

Suita, Osaka 565-0871, Japan

Received: July 24, 2014; Revised: September 18, 2014; Accepted: September 22, 2014

© The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract: Novel environmentally friendly inorganic red pigments based on calcium bismuth oxide,

Ca3(Bi1xREx)8O15 (0 ≤ x ≤ 0.09; RE = Sc3+, Er3+, Y3+, Ho3+ and Dy3+), were successfully synthesized

and the color properties were characterized. The color of these pigments depended on the composition

and the synthesis condition, and the most vivid red color was obtained for the Ca3(Bi0.93Y0.07)8O15

sample calcined thrice at 800 ℃ for 2 h. The L* and a* values corresponding to brightness and red

chromaticity for this pigment were 45.6 and +30.6, respectively, which were greater than those of a

commercially available Fe2O3 pigment (L* = 38.4, a* = +29.5). Since the present pigment is composed

of nontoxic and safe elements, it should be an attractive alternative to the conventional Fe2O3

pigment.

Keywords: calcium bismuth oxide; environmentally friendly; red pigment; solid solution; cation

complexation method

1 Introduction

Inorganic pigments are applied in a wide range of

products such as paints, ceramics, porcelains, plastics,

enamels and glasses, due to their high thermal and

ultraviolet stability compared to organic pigments [1].

In particular, inorganic red pigments are in demand

because of their wide applicability. For decades,

several inorganic red pigments such as red lead (Pb3O4

or 2PbO·PbO2), cadmium red (CdSe·CdS) and red

mercury sulfide (HgS) were popularly used in many

products. However, the use of these compounds has

been restricted because they contain the hazardous

elements (Pb, Cd, Se and Hg), which are harmful not

only to human health but also to the environment.

On the other hand, red iron oxide (Fe2O3) has

been widely known as an environmentally friendly

inorganic red pigment for many uses, but its color

performance is unsatisfactory in comparison to those

of the conventional toxic pigments. Several studies

have been carried out on new environmentally friendly

inorganic red pigments [2–10], but these pigments

cannot exceed the color of the nontoxic red iron oxide

(Fe2O3). Although perovskite-type oxynitrides

Ca1xLaxTaO2xN1+x have also been reported as

nontoxic inorganic red pigments, toxic and flammable

ammonia gas is required for the synthesis of these

pigments [11]. Furthermore, these oxynitride

compounds are decomposed to oxides at 420 ℃ and

higher temperatures [11], accompanying with

generation of toxic nitrogen oxides.

Because of this situation, we focused on calcium

bismuth oxide (Ca3Bi8O15), as a base material for novel

* Corresponding author.

E-mail: [email protected]

J Adv Ceram 2015, 4(1): 39–45


40

environmentally friendly inorganic red pigment, where

bismuth is often mistaken for a toxic element, but it is

nontoxic. In fact, inorganic salts and oxides of bismuth

have been confirmed to be virtually nontoxic in some

medical references [12–14]. The coloring mechanism

of trivalent bismuth ion-based semiconductors like

Ca3Bi8O15 is based on a charge-transfer transition from

the valence band of a hybrid orbital of the Bi6s and O2p

orbitals to the conduction band of Bi6p orbital [15–17].

It is expected that the color of Ca3Bi8O15 can be tuned

by introducing other elements into the Ca3Bi8O15

lattice to control the lattice size, because the extent of

the Bi6s–O2p orbital hybridization in the valence band

should depend on the interionic distance between the

Bi3+ and O2 ions.

In this study, therefore, we synthesized

Ca3(Bi1xREx)8O15 (0 ≤ x ≤ 0.09; RE = Sc3+, Er3+, Y3+,

Ho3+ and Dy3+) samples in an attempt to find a novel

environmentally friendly inorganic red pigment.

Introduction of smaller rare earth ions (ionic radii: Sc3+

0.0885 nm [18], Er3+ 0.103 nm [18], Y3+ 0.104 nm [18],

Ho3+ 0.1041 nm [18] and Dy3+ 0.1052 nm [18]) into

the Bi3+ (0.117 nm) [18] sites of Ca3Bi8O15 generates

lattice shrinking, which reduces the bandgap energy by

modifying the band width of Bi6s–O2p hybrid orbital.

The color properties were characterized and the

composition and the calcination condition of them

were optimized to obtain the most vivid red hue.

2 Experimental

CaCO3 (Wako Pure Chemical Industries Ltd., 99.95%),

Bi2O3 (Kishida Chemical Co. Ltd., 99.9%), Sc2O3

(Kojundo Chemical Laboratory Co. Ltd., 99.9%),

Er2O3 (Shin-Etsu Chemical Co. Ltd., 99.9%), Y2O3

(Shin-Etsu Chemical Co. Ltd., 99.99%), Ho2O3

(Shin-Etsu Chemical Co. Ltd., 99.9%), Dy2O3

(Shin-Etsu Chemical Co. Ltd., 99.9%), citric acid

(Wako Pure Chemical Industries Ltd., 98.0%) and

nitric acid (Kishida Chemical Co. Ltd., specific gravity

1.38, density 60%) were used for the sample

preparation as received without further purification.

The Ca3(Bi1xREx)8O15 (0 ≤ x ≤ 0.09; RE = Sc3+,

Er3+, Y3+, Ho3+ and Dy3+) pigments were synthesized

by a cation complexation method using citric acid. An

aqueous solution of 1 mol·dm3 of citric acid was

added into a stoichiometric mixture of 0.1 mol·dm3

Ca(NO3)2, 0.5 mol·dm3 Bi(NO3)3 and 0.1 mol·dm

3

RE(NO3)3 (RE = Sc3+, Er3+, Y3+, Ho3+ and Dy3+)

aqueous solutions. The molar ratio of citric acid to the

total amount of the metal cations was adjusted to 3:2.

After stirring at 80 ℃ for 12 h, the mixed solution was

heated at 190 ℃ until the solvent was vaporized. The

resulting solid was calcined at 800 ℃ for 2 h to afford

the product. All samples were gently ground in an

agate mortar before characterization.

The samples were characterized by X-ray powder

diffraction (XRD, Rigaku, SmartLab) with Cu K

radiation (40 kV and 30 mA) to identify their crystal

structures. The lattice volumes of the samples were

calculated from the XRD peak angles, which were

refined using -Al2O3 as a standard. The sample

compositions were determined by X-ray fluorescence

spectroscopy (XRF, Rigaku ZSX-100e). The optical

reflectance was measured using an ultraviolet–visible

(UV–Vis) spectrometer with an integrating sphere

attachment (Shimadzu, UV-2600 and ISR-2600) using

barium sulfate as a reference, where the illuminants

were a deuterium arc lamp (D2 lamp) and a halogen

lamp (tungsten-iodine lamp), and the angles of the

incident lights were 0° for the sample beam and 8° for

the reference beam. The bandgap energies of the

samples were estimated from the absorption edge of

the absorbance spectra represented by the Kubelka–

Munk function, f(R) = (1 R)2/(2R), where R is the

reflectance [19,20]. The color properties of the samples

were evaluated in terms of the CIE L*a*b* system using

a chromometer (Konika-Minolta, CR-400) using the

standard illuminant C, which represented average day

light with a correlated color temperature of 6774 K.

This apparatus employs a diffused illumination and

vertical light receiving system, which is designed to

illuminate a specimen from all directions with almost

equal brightness, and the detector receives the light

vertically reflected from the specimen. Before the

measurements, the apparatus was calibrated using a

white calibration plate. The samples were pressed into

green pellets without any additives. The color

coordinate data were measured for 5 times and the

average values were calculated. The standard deviation

was 0.07, indicating that the experimental error can be

ignored. The L* parameter represents the brightness or

darkness of a color relative to a neutral grey scale, and

the a* (the red–green axis) and b* (the yellow–blue axis)

parameters express the hue of a color qualitatively.

J Adv Ceram 2015, 4(1): 39–45


41

3 Results and discussion

The compositions of all samples were determined by

the XRF measurement, and they were in good

agreement with the charge ratio of the starting

mixtures. Figure 1 shows the XRD patterns of the

Ca3(Bi0.93RE0.07)8O15 (RE = Sc3+, Er3+, Y3+, Ho3+ and

Dy3+) samples. A single Ca3Bi8O15 phase is observed

for the Ca3(Bi0.93RE0.07)8O15 (RE = Sc3+, Er3+ and Y3+)

samples, and there are no extra lines due to other

compounds or impurity phases. In contrast, for the

Ca3(Bi0.93RE0.07)8O15 (RE = Ho3+ and Dy3+) samples, a

Bi1.6Ca0.4O2.8 phase is observed as an impurity in

addition to the Ca3Bi8O15 phase. Table 1 lists

lattice volumes of the Ca3Bi8O15 phase in the


Dy3+) samples calculated from the diffraction angles in

the XRD patterns. The lattice volume of Ca3Bi8O15 is

decreased by substituting Sc3+, Er3+, Y3+, Ho3+ and

Dy3+ ions for Bi3+, because the ionic radii of these rare

earth ions (Sc3+ 0.0885 nm [18], Er3+ 0.103 nm [18],

Y3+ 0.104 nm [18], Ho3+ 0.1041 nm [18] and Dy3+

0.1052 nm [18]) are smaller than that of Bi3+

(0.117 nm) [18]. These results indicate that solid

solutions with the Ca3Bi8O15 structure are successfully

obtained in a single-phase form for the

Ca3(Bi0.93RE0.07)8O15 (RE = Sc3+, Er3+ and Y3+) samples.

In contrast, the Bi1.6Ca0.4O2.8 impurity is observed for

the Ca3(Bi0.93RE0.07)8O15 (RE = Ho3+ and Dy3+)

samples, although the lattice volumes for these

samples are smaller than that of Ca3Bi8O15 indicating

the formation of solid solutions.

Figure 2 depicts the UV–Vis diffuse reflectance

spectra for the Ca3(Bi0.93RE0.07)8O15 (RE = Sc3+, Er3+,

Y3+, Ho3+ and Dy3+) samples. Strong optical absorption

behavior was observed for all samples at wavelengths

shorter than 560 nm, including the green light region

(490–560 nm). As a result, these samples are red,

because green is a complementary color to red.

Furthermore, the absorption intensity is dependent on

the sample compositions. Among these pigments,

Ca3(Bi0.93Y0.07)8O15 absorbs the green light most

efficiently compared with the others.

The L*a*b* color coordinate data of the


Dy3+) samples are tabulated in Table 2. The color

parameters L*, a* and b* are affected by introducing the rare earth ions into the Bi3+ site in the Ca3Bi8O15

lattice. In particular, the a* value which corresponds to

the red chromaticity in the positive direction for these

samples becomes greater than that of the non-doped

Ca3Bi8O15 sample (a* = +23.2). Among them,

Ca3(Bi0.93Y0.07)8O15 pigment exhibits the most brilliant

red hue, of which the a* value is +28.2. These results

clarify that the partial substitution of the Bi3+ ions in

the Ca3Bi8O15 lattice with the smaller rare earth ions

Fig. 1 XRD patterns of the Ca3(Bi0.93RE0.07)8O15

(RE = Sc3+, Er3+, Y3+, Ho3+ and Dy3+) samples.

2 (°)

Intensity (a.u.)

Table 1 Lattice volume of the samples

Sample Lattice volume (nm3)

Ca3Bi8O15 34.243

Ca3(Bi0.93Sc0.07)8O15 33.700

Ca3(Bi0.93Er0.07)8O15 33.899

Ca3(Bi0.93Y0.07)8O15 33.958

Ca3(Bi0.93Ho0.07)8O15 33.986

Ca3(Bi0.93Dy0.07)8O15 34.073

Fig. 2 UV–Vis diffuse reflectance spectra of the


Dy3+) samples.

Wavelength (nm)

Reflectance (%)

Wavelength (nm)

Reflectance (%)

J Adv Ceram 2015, 4(1): 39–45


42

(Sc3+, Er3+, Y3+, Ho3+ and Dy3+) is effective to enhance

the red hue of Ca3Bi8O15, and the most vivid red color

is obtained for Ca3(Bi0.93Y0.07)8O15 (a* = +28.2), which

is obtained in a single Ca3Bi8O15 phase without any

impurity.

Subsequently, the amount of Y3+ ions introducing

into the Bi3+ site of Ca3Bi8O15 is optimized to give the

most vivid reddish hue. Figure 3 depicts the XRD

patterns of the Ca3(Bi1xYx)8O15 (0 ≤ x ≤ 0.09) samples.

The Ca3Bi8O15 structure is obtained in a single-phase

form for the samples in which Y3+ content is 7 mol%

or less, and no diffraction peaks of impurities or other

compounds are evident in the patterns. On the other

hand, Bi1.8Y0.2O3 is observed as a secondary phase for

the samples in which Y3+ content is more than 7 mol%.

The lattice volumes for the Ca3Bi8O15 phase in

Ca3(Bi1xYx)8O15 (0 ≤ x ≤ 0.09) are summarized in

Table 3. The lattice volume decreases linearly with

increasing the amount of Y3+ ions in the single-phase

region (0 ≤ x ≤ 0.07), but it becomes approximately

constant in the multi-phase region (0.08 ≤ x ≤ 0.09).

These results suggest that solid solutions are

successfully formed for the samples with compositions

of Ca3(Bi1xYx)8O15 (0.01 ≤ x ≤ 0.07), and the

solubility limit of Y3+ is 7 mol% for this system.

Table 3 Lattice volume of the samples

Sample Lattice volume (nm3)

Ca3Bi8O15 34.243

Ca3(Bi0.99Y0.01)8O15 34.175

Ca3(Bi0.97Y0.03)8O15 34.116

Ca3(Bi0.95Y0.05)8O15 34.042

Ca3(Bi0.93Y0.07)8O15 33.958

Ca3(Bi0.92Y0.08)8O15 33.949

Ca3(Bi0.91Y0.09)8O15 33.944

Figure 4 illustrates the UV–Vis diffuse reflectance

spectra for the Ca3(Bi1xYx)8O15 (0 ≤ x ≤ 0.09)

samples. Strong optical absorption is observed for all

samples at wavelengths between 490 nm and 560 nm

(i.e., in the green light region), and therefore, these

samples are also red, as the reason mentioned above.

Furthermore, the absorption intensity in the green light

(490–560 nm) increases with increasing the amount of

Y3+ ions in the single-phase region (0 ≤ x ≤ 0.07),

while it decreases in the two-phase region (0.08 ≤ x ≤

0.09). Among these pigments, the Ca3(Bi0.93Y0.07)8O15

sample absorbs the green light most efficiently.

The L*a*b* color coordinate data and optical

bandgap energies (Eg) for Ca3(Bi1xYx)8O15 (0 ≤ x ≤

0.09) are listed in Table 4. The a* values corresponding

to the red chromaticity of these pigments are

dependent on the composition, and the highest a* value

is obtained for Ca3(Bi0.93Y0.07)8O15 (a* = +28.2). The

optical bandgap energies of the Ca3(Bi1xYx)8O15

(0.01 ≤ x ≤ 0.09) samples decrease with increasing

the Y3+ concentration in the single-phase region

(0.01 ≤ x ≤ 0.07), and the bandgap energy of

Ca3(Bi0.93Y0.07)8O15 (2.25 eV) is the lowest. This is

caused by the enhancement in the hybridization effect

of Bi6s–O2p orbital by introducing Y3+ ions into the

Table 2 L*a*b* color coordinate data of the samples

Sample L* a* b*

Ca3Bi8O15 52.1 +23.2 +34.2

Ca3(Bi0.93Sc0.07)8O15 41.9 +27.3 +29.4

Ca3(Bi0.93Er0.07)8O15 47.9 +27.4 +36.6

Ca3(Bi0.93Y0.07)8O15 43.5 +28.2 +32.3

Ca3(Bi0.93Ho0.07)8O15 44.3 +26.3 +31.6

Ca3(Bi0.93Dy0.07)8O15 45.8 +26.4 +32.5

Fig. 3 XRD patterns of the Ca3(Bi1xYx)8O15

(0 ≤ x ≤ 0.09) samples.

Intensity (a.u.)

2 (°)

:Bi1.8Y0.2O3


Ca3(Bi1xYx)8O15 (0 ≤ x ≤ 0.09) samples.

Reflectance (%)

Wavelength (nm)

Reflectance (%)

Wavelength (nm)

J Adv Ceram 2015, 4(1): 39–45


43

Ca3Bi8O15 lattice, where the lattice volume is shrunk

(Table 3) to decrease the bandgap due to broadening of

the conduction band. As a result, the green light

absorption increases with the increasing amount of Y3+

ions in Ca3(Bi1xYx)8O15 (0 ≤ x ≤ 0.07), as already

shown in Fig. 4. On the other hand, the bandgap

energies increase in the two-phase region

(0.08 ≤ x ≤ 0.09) where the Bi1.8Y0.2O3 impurity is

observed. From these results, it is considered that the

Bi6s–O2p hybridization effect in Bi1.8Y0.2O3 is

significantly less than that of Ca3Bi8O15, and therefore,

the apparent bandbap energies for the multi-phase

samples are larger than those for the single-phase

samples. Accordingly, generation of the Bi1.8Y0.2O3 phase is inadequate to improve the redness of

Ca3Bi8O15.

Furthermore, calcination condition is another

important factor which affects the color properties of

ceramic pigments. Especially, repetitive calcination is

an effective way to obtain homogenous solid solutions

for ceramic compounds. In order to further increase the

red hue of Ca3(Bi0.93Y0.07)8O15, dependence of the

number of calcination in the synthesis process on the

pigment color is also discussed. Figure 5 shows the

XRD patterns of Ca3(Bi0.93Y0.07)8O15 calcined 1–4

times at 800 ℃ for 2 h. A single phase of the Ca3Bi8O15

structure is observed for all samples, and no peaks of

other impurities are evident, even if the calcination

(800 ℃ for 2 h) is repeated 4 times. As listed in Table

5, the lattice volume of Ca3(Bi0.93Y0.07)8O15 decreases

with repeating the calcination number up to 3, but

it becomes constant after the 4th calcination.

These results indicate that solid solution of

Ca3(Bi0.93Y0.07)8O15 is formed homogenously, when it

is calcined thrice or more at 800 ℃ for 2 h.

UV–Vis diffuse reflectance spectra of

Ca3(Bi0.93Y0.07)8O15 calcined 1–4 times at 800 ℃ for 2

h, and for comparison, that of the commercial Fe2O3

pigment (MR-320A, Morishita Bengara Kogyo

Co. Ltd.) are illustrated in Fig. 6. Among the

Ca3(Bi0.93Y0.07)8O15 samples synthesized with the

different repeat number of calcination, the strongest

optical absorption in the green light region at

wavelengths between 490 nm and 560 nm is observed

for the sample calcined thrice at 800 ℃ for 2 h and the

Bi6s–O2p orbital hybridization becomes the greatest in

this sample. Consequently, this pigment exhibits the

most brilliant red hue (a* = +30.6), and this value is

almost constant after the 4th calcination as listed in

Table 6.

In comparison with the commercial Fe2O3 pigment,

Table 4 L*a*b* color coordinate data and bandgap

energy (Eg) of the samples

Sample L* a* b* Eg (eV)

Ca3Bi8O15 52.1 +23.2 +34.2 2.30

Ca3(Bi0.99Y0.01)8O15 43.0 +25.2 +27.4 2.26

Ca3(Bi0.97Y0.03)8O15 43.6 +26.0 +30.2 2.25

Ca3(Bi0.95Y0.05)8O15 42.8 +26.8 +30.1 2.25

Ca3(Bi0.93Y0.07)8O15 43.5 +28.2 +32.3 2.25

Ca3(Bi0.92Y0.08)8O15 46.8 +26.9 +35.2 2.29

Ca3(Bi0.91Y0.09)8O15 50.5 +26.8 +40.3 2.37

Fig. 5 XRD patterns of Ca3(Bi0.93Y0.07)8O15 calcined 1–4

times at 800 ℃ for 2 h.

Wavelength (nm)

Intensity (a.u.)

2 (°)

Table 5 Lattice volume of the Ca3(Bi0.93Y0.07)8O15

calcined 1–4 times at 800 ℃℃ for 2 h

Sample (number of calcination times) Lattice volum (nm3)

Ca3(Bi0.93Y0.07)8O15 (1) 33.958

Ca3(Bi0.93Y0.07)8O15 (2) 33.848

Ca3(Bi0.93Y0.07)8O15 (3) 33.733

Ca3(Bi0.93Y0.07)8O15 (4) 33.733


Ca3(Bi0.93Y0.07)8O15 calcined 1–4 times at 800 ℃ for

2 h and the commercial Fe2O3 pigment.

Wavelength (nm)

Reflectance (%)

Wavelength (nm)

Reflectance (%)

J Adv Ceram 2015, 4(1): 39–45


44

the Ca3(Bi0.93Y0.07)8O15 pigment calcined thrice at

800 ℃ for 2 h reflects red light (605–750 nm) more

significantly (Fig. 6). As a result, both brightness and

redness for the Ca3(Bi0.93Y0.07)8O15 pigment (L* = 45.6,

a* = +30.6) are greater than those for the commercial

one (L* = 38.4, a* = +29.5) (Table 6). This is due to a

difference in the coloring mechanism of Fe2O3 with

that of the present Ca3(Bi0.93Y0.07)8O15 pigment. For the

Fe2O3 pigment, the bandgap transition and some d–d

transitions of Fe3+ ions occur simultaneously. As a

result, optical absorption is occurred in almost all the

visible part of the spectrum. In contrast, for the

Ca3(Bi0.93Y0.07)8O15 pigment, only the bandgap

transition occurs, as mentioned in the introduction.

Consequently, the Ca3(Bi0.93Y0.07)8O15 pigments reflect

red light more significantly, in spite of the fact that the

absorption in the green light region is less than that of

the commercial Fe2O3, as illustrated in Fig. 6. In

comparison with the Ca1xLaxTaO2xN1+x (x = 0.9) [11],

the redness value of the present Ca3(Bi0.93Y0.07)8O15

pigment (a* = +30.6) is much smaller than that of

Ca1xLaxTaO2xN1+x (x = 0.9, a* = +49.3). However,

the brightness of the present Ca3(Bi0.93Y0.07)8O15

pigment (L* = 45.6) is much larger than that of

Ca1xLaxTaO2xN1+x (x = 0.9, L* = 31.6) [11], probably

due to the difference of partial nitriding in the latter

compound.

4 Conclusions

The Ca3(Bi1xREx)8O15 (0 ≤ x ≤ 0.09; RE = Sc3+,

Er3+, Y3+, Ho3+ and Dy3+) samples are successfully

synthesized as novel environmentally friendly

inorganic red pigments through a cation complexation

method using citric acid. The redness value of these

pigments is dependent on the ionic species of rare

earths introduced into the Bi3+ site of the Ca3Bi8O15

lattice and the number of calcination in the synthesis

process. Among these pigments, the most brilliant red

hue is obtained for Ca3(Bi0.93Y0.07)8O15 calcined thrice

at 800 ℃ for 2 h, and this pigment exhibits a brighter

red color than the commercial Fe2O3 pigment, as the

brightness and redness values for this pigment (L* =

45.6, a* = +30.6) are greater than those for the

commercial Fe2O3 (L* = 38.4, a* = +29.5). Since the

present pigment consists of nontoxic elements (Ca, Bi,

Y and O), it should be an effective alternative to the

conventional Fe2O3.

Acknowledgements

This work was supported by the Development of

Alternative Technology for Hazardous Chemical

Substances and Development of Novel Environment-

and Human-friendly Inorganic Pigments for Three

Primary Colors (FY2010-2014) programs of the New

Energy and Industrial Technology Development

Organization (NEDO) and the Ministry of Economy,

Trade and Industry, Japan (METI).

Open Access: This article is distributed under the

terms of the Creative Commons Attribution License

which permits any use, distribution, and reproduction

in any medium, provided the original author(s) and the

source are credited.

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Novel environmentally friendly inorganic red pigments ...and the synthesis condition, and the most...

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