Novel environment-friendly inorganic red pigments based on (Bi, Er, Y, Fe)₂O₃ solid solutions

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

Abstract

Novel environmental-friendly inorganic red pigments, ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 < x ≤ 0.28, y = 0.20), were successfully synthesized using a conventional solid-state reaction method in order to further enhance the red hue of a ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ pigment, which was previously reported by our group. The color of the samples depended on their composition and the most brilliant red hue was obtained for ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃. The a* value corresponding to red chromaticity was +33.1 for ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃, and it was greater than those of previously reported ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ (a* = +30.9) and commercial Fe₂O₃ (a* = +28.9) pigments. Since the ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ pigment is composed of nontoxic elements (Bi, Er, Y, Fe, and O), it should be an attractive alternative to the conventional Fe₂O₃ pigments.

Keywords:

• Bismuth oxide
• Environment-friendly
• Red pigment
• Solid solution
• Cubic δ-Bi₂O₃ structure
• Solid-state reaction

1 Introduction

Inorganic pigments are applied in a wide variety of products such as paints, ceramics, plastics, enamels, and glasses, because they possess high thermal and UV stability [1]. In particular, inorganic red pigments are in demand because of their wide applicability. Several inorganic red pigments such as lead oxides (red lead; Pb₃O₄ or 2PbO·PbO₂), cadmium sulfoselenide red (CdSe·CdS), and mercuric sulfide red (HgS) were popularly used for many applications. However, the use of these compounds has been restricted because of the hazardous elements, such as lead (Pb), cadmium (Cd), and mercury (Hg), which are harmful not only to human health but also to the environment. On the other hand, 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. A number of studies have been done on new environmentally friendly inorganic red pigments [2–9], but the pigments cannot exceed the color of the nontoxic red iron oxide (Fe₂O₃). Although perovskite-type oxynitrides Ca_(1−x)La_xTaO_(2−x)N_(1+x) have also been reported as nontoxic inorganic red pigments, toxic ammonia gas is required for the synthesis of these pigments [10]. Furthermore, oxidation of these oxynitride compounds occurs at 420 °C or higher temperatures [10], where they decompose and generate toxic nitrogen oxides.

Because of this situation, we focused on bismuth oxide (Bi₂O₃), which has already been reported as a nontoxic and insoluble compound [11]. Bi₂O₃ exists in several polymorphic phases: a low-temperature stable α-phase (monoclinic structure), a high-temperature stable δ-phase (cubic fluorite structure), and a metastable β-phase (tetragonal structure) or γ-phase (body-centered cubic structure). At room temperature, Bi₂O₃ exists in a pale yellowish α-phase, which is stable up to 730 °C. At around 730 °C, a phase transition from the α-phase to the orange δ-phase occurs, and the δ-phase is stable up to the melting point (825 °C). The color of Bi₂O₃ originates in the charge-transfer transition from the valence band of a hybrid orbital of Bi6s and O2p to the conduction band of Bi6p [12].

In our previous study, we elucidate that the high-temperature δ-Bi₂O₃ phase was stabilized at room temperature by introducing Er³⁺ ions into the Bi³⁺ site, and that the red chromaticity was significantly increased by further dissolution of Fe³⁺ into the lattice to insert the Fe3d level as a new energy level below the Bi6P conduction band. In particular, it was found that a ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ pigment showed the highest redness value (a* = +30.9 in the CIE L*a*b* system) [13]. However, the band gap energy of the ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ pigment became too low because of significant lattice shrinkage, and the optical absorption extended into the red region. As a result, the red color was not so vivid, although it was higher than that of the commercial Fe₂O₃ pigment (a* = +28.9) [13]. In this study, therefore, a part of the Er³⁺ (0.103 nm) [14] site was substituted with larger Y³⁺ ions (0.104 nm) [14] to relax the excessive lattice shrinkage. Therefore, ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 < x ≤ 0.28, y = 0.20) samples were synthesized as advanced environmentally friendly red pigments. The color properties were characterized and the composition was optimized to produce the most vivid red hue.

2 Experimental

The ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 < x ≤ 0.28, y = 0.20) pigments were synthesized using a conventional solid-state reaction method. As is the case with the ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ pigment [13], double calcination was adopted in the synthesis process to obtain the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ solid solutions certainly. In the first process, (Bi_0.72Er_0.28−xY_x)₂O₃ solid solutions were prepared in a single-phase form of the cubic δ-Bi₂O₃ structure, and in the second one, Fe³⁺ ions were introduced into the (Bi_0.72Er_0.28−xY_x)₂O₃ lattice maintaining the single-phase δ-Bi₂O₃ structure. Stoichiometric amounts of Bi₂O₃, Er₂O₃ and Y₂O₃ powders were mixed in an agate mortar, followed by mechanical mixing using a planetary-type ball-milling apparatus (Pulverisette 7, Fritsch GmbH) for 3 h. The homogenous mixed powder was calcined at 800 °C for 10 h under an air atmosphere. Finally, the powder obtained was mixed with Fe₂O₃ in an agate mortar and was calcined at 500 °C for 10 h under an air atmosphere.

All the samples were gently ground in an agate mortar before characterization. The sample compositions were analyzed by X-ray fluorescence spectroscopy (XRF, Rigaku Supermini200). The samples were characterized by X-ray powder diffraction (XRD; Rigaku, SmartLab) using Cu-Kα radiation (40 kV, 30 mA) to identify the crystal structure. Optical reflectance spectra were obtained using a UV–vis spectrometer (Shimadzu UV-2600) with barium sulfate as a reference. The band-gap energies of the samples were determined 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 [15,16]. The color properties of the samples were estimated in terms of the CIE L*a*b* system with a colorimeter (Konica-Minolta CR-300). The parameter L* represents the brightness or darkness of a color relative to a neutral gray scale, while the parameters a* (the red–green axis) and b* (the yellow–blue axis) qualitatively express the color. The particle morphology of the samples was observed using a scanning electron microscope (SEM; Shimadzu, SS-550). The size distribution and mean particle size were evaluated by measuring the diameters of 200 particles on the SEM micrographs.

3 Results and discussion

The compositions of all samples determined by the XRF analysis were in good agreement with the theoretical values. To confirm the formation of the cubic δ-Bi₂O₃ structure, the (Bi_0.72Er_0.28−xY_x)₂O₃ (0 < x ≤ 0.28) samples, which were synthesized in the first step of the double calcination process, were measured using the XRD analysis. Fig. 1 shows the XRD patterns of the (Bi_0.72Er_0.28−xY_x)₂O₃ (0 ≤ x ≤ 0.28) samples. A single phase of the δ-Bi₂O₃-type structure was observed for all samples, and there were no extra lines due to other compounds or impurities. As summarized in Table 1, the cubic lattice parameter of the (Bi_0.72Er_0.28−xY_x)₂O₃ (0 ≤ x ≤ 0.28) samples increased linearly with increasing the amount of Y³⁺, because the ionic radius of Y³⁺ (0.104 nm) [14] is slightly larger than that of Er³⁺ (0.103 nm) [14]. These results indicate that the solid solutions were formed successfully for all samples.

Table 1 Cubic lattice parameters of the (Bi_0.72Er_0.28−xY_x)₂O₃ (0 ≤ x ≤ 0.28) samples with estimated standard deviation (esd).

Sample	Lattice parameter/nm
(Bi0.72Er0.28)2O3 [13]	0.54843(23)
(Bi0.72Er0.18Y0.10)2O3	0.54891(17)
(Bi0.71Er0.10Y0.19)2O3	0.54934(42)
(Bi0.72Er0.04Y0.24)2O3	0.54950(81)
(Bi0.72Y0.28)2O3	0.54982(25)

Fig. 1 XRD patterns of the (Bi_0.72Er_0.28−xY_x)₂O₃ (0 ≤ x ≤ 0.28) samples.

Subsequently, iron ion (Fe³⁺) was doped into the (Bi_0.72Er_0.28−xY_x)₂O₃ lattice to introduce a red chromogenic species, and the XRD patterns of the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 ≤ x ≤ 0.28, y = 0.20) samples were depicted in Fig. 2. Also in this case, a single-phase cubic δ-Bi₂O₃ structure was observed for all samples and no diffraction peaks of impurities were evident in the patterns. The cubic lattice parameters of ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ are summarized in Table 2. It is confirmed that the lattice parameters of the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ samples were larger than that of ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ [13] and were increased with increasing Y³⁺ content. These results indicate that solid solutions were successfully formed for the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 < x ≤ 0.28, y = 0.20) samples, as is the case with the (Bi_0.72Er_0.28−xY_x)₂O₃ samples in Table 1.

Table 2 Cubic lattice parameters of the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 ≤ x ≤ 0.28, y = 0.20) samples with estimated standard deviation (esd).

Sample	Lattice parameter/nm
((Bi0.72Er0.28)0.80Fe0.20)2O3 [13]	0.54457(41)
((Bi0.72Er0.18Y0.10)0.80Fe0.20)2O3	0.54522(11)
((Bi0.71Er0.10Y0.19)0.80Fe0.20)2O3	0.54559(23)
((Bi0.72Er0.04Y0.24)0.80Fe0.20)2O3	0.54603(10)
((Bi0.72Y0.28)0.80Fe0.20)2O3	0.54645(61)

Fig. 2 XRD patterns of the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 ≤ x ≤ 0.28, y = 0.20) samples.

Fig. 3 shows the UV–vis diffuse reflectance spectra for the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 ≤ x ≤ 0.28, y = 0.20) samples. Strong optical absorption in the green light region at wavelengths shorter than 560 nm was observed for the samples. All samples are red, because green is the complementary color to red. Although the green light absorption was gradually decreased with increasing the amount of Y³⁺, the red light reflection between 605 and 750 nm increased.

Fig. 3 UV–vis diffuse reflectance spectra for the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 ≤ x ≤ 0.28, y = 0.20) samples.

The L*a*b* color coordinate data and the apparent band-gap energies for the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 < x ≤ 0.28, y = 0.20) samples are listed in Table 3 with those of the previous ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ and commercially available Fe₂O₃ (MR-320A, Morishita Bengara Kogyo Co. Ltd.) pigments for comparison. The a* values, which correspond to the red chromaticity in the positive direction, of the Y³⁺-doped samples were larger than that of the previous ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ pigment (a* = +30.9). In addition, the a* value gradually increased with the Y³⁺ concentration in the ((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ (0 < x ≤ 0.24, y = 0.20) samples. Among these pigments, the ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ pigment shows the highest redness value, a* = +33.1. On the other hand, the apparent band-gap energies of the samples increased with the amount of Y³⁺ as intended. This is caused by the decrease in the hybridization effect of Bi6s–O2p orbital by introducing Y³⁺ into the ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ lattice (Table 2) to relax the excessive lattice shrinkage. As a result, the red light reflection was increased by the Y³⁺ doping into the ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ lattice (Fig. 3).

Table 3 L*a*b* color coordinate data and apparent band-gap energies for the samples.

Sample	L*	a*	b*	Eg/eV
((Bi0.72Er0.18Y0.10)0.80Fe0.20)2O3	42.2	+31.9	+42.0	2.26
((Bi0.71Er0.10Y0.19)0.80Fe0.20)2O3	45.8	+32.4	+45.3	2.26
((Bi0.72Er0.04Y0.24)0.80Fe0.20)2O3	42.5	+33.1	+43.0	2.27
((Bi0.72Y0.28)0.80Fe0.20)2O3	44.9	+32.5	+44.3	2.30
((Bi0.72Er0.28)0.80Fe0.20)2O3 [13]	41.2	+30.9	+38.5	2.25
Commercially available Fe2O3	38.9	+28.9	+25.3	2.21

Fig. 4 depicts the UV–vis diffuse reflectance spectra of ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃, ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃, and commercially available Fe₂O₃. Although the optical absorption in the green light region for these pigments was almost identical, the reflection in the red light region (605–750 nm) for ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ was significantly higher than those for the others. Therefore, the redness value (a*) of the present ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ pigment (a* = +33.1) was higher than those of previously reported ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ (a* = +30.9) and also commercially available Fe₂O₃ (a* = +28.9) as listed in Table 3.

Fig. 4 UV–vis diffuse reflectance spectra of ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃, ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃, and commercially available Fe₂O₃.

Fig. 5 illustrates SEM images and size distribution profiles of the previous ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ and the present ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ pigments. Average particle size of the present ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ pigment was about 1.32 μm, which was almost the same as that of the previous ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ (1.43 μm). These results indicate that the enhancement in the red hue of the present ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ pigment compared with that of the previous ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ pigment was caused by the relaxation of the excessive lattice shrinkage by substitution of Y³⁺ for Er³⁺ as mentioned above, and the contributions from the particle size on the pigment properties can be eliminated.

Fig. 5 SEM images and particle size distribution of ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ (a and b) and ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ (c and d).

4 Conclusions

((Bi_0.72Er_0.28−xY_x)_1−yFe_y)₂O₃ solid solutions were synthesized as environment-friendly inorganic red pigments to enhance the redness value of a ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ pigment which was previously reported by our group. The redness value of the pigment was increased by substituting a part of Er³⁺ ions with larger Y³⁺ ions by the relaxation of the excessive lattice shrinkage. Among these pigments, the most brilliant red hue was obtained for the ((Bi_0.72Er_0.04Y_0.24)_0.80Fe_0.20)₂O₃ pigment, and the red hue of this pigment (a* = +33.1) was more brilliant than that of the commercial Fe₂O₃ pigment (a* = +28.9) as well as that of ((Bi_0.72Er_0.28)_0.80Fe_0.20)₂O₃ pigment (a* = +30.9).

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).

Notes

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.