Synthesis and Characterization of Ca1-x Eu x ZrO3 as Environmentally Friendly Inorganic Yellow Pigments.

Eu2+-doped calcium zirconates, Ca1-x Eu x ZrO3 (0 ≤ x ≤ 1), were synthesized as novel environmentally friendly inorganic yellow pigments by the conventional solid-state reaction method. The crystal structure, morphology, optical properties, and color were characterized. The Eu2+-doped samples strongly absorbed blue light in the wavelength range of 435-480 nm, which was caused by the 4f-5d allowed transition of Eu2+. The color of the sample gradually became brilliant yellow with increasing the Eu2+ content. Among the samples synthesized in this study, the brightest yellow color was obtained with the Ca0.7Eu0.3ZrO3 (a* = +11.5 and b* = +70.7) sample. Compared with the commercially available praseodymium yellow pigment (a* = -3.28, b* = +70.3), the yellowness value (b*) of Ca0.7Eu0.3ZrO3 was comparable and the redness value (a*) was higher. As a result, this pigment exhibited a reddish yellow color as compared with praseodymium yellow. In addition, this pigment was chemically stable. Therefore, the Ca0.7Eu0.3ZrO3 pigment has the potential to become a novel environmentally friendly inorganic yellow pigment.

Introduction

Inorganic pigments have a wide range of applications such as in ceramics, plastics, and glasses due to their high thermal stability and hiding power. In particular, yellow pigments have high visibility and are applied to paints for road markers and warning sign boards. However, several industrial yellow pigments such as cadmium yellow (CdS), chrome yellow (PbCrO4), and nickel titanium yellow (TiO2–NiO–Sb2O3) contain the harmful elements (e.g., Cd, Pb, Cr, and Sb) for the human body as well as the environment. Therefore, development of novel yellow pigments without toxic elements is required, and a number of studies have been reported by several researchers.[1−9]

Because of this situation, we focused on a divalent europium (Eu2+) ion as a yellow coloring source. Eu2+ has been generally used as an activator for phosphor materials.[10−18] KBaGdSi2O7:Eu2+, Sr8MgLn(PO4)7:Eu2+ (Ln = Y and La), and Li2CaSiO4:Eu2+ as the examples of the Eu2+-doped phosphors absorb the visible light in the wavelength range from 350 to 450 nm due to the electronic transition between 4f and 5d orbitals.[16−18] The energy level of the 5d orbital of Eu2+ is strongly affected by the surrounding crystal field. Accordingly, the absorption wavelength due to the 4f–5d allowed transition depends on the host crystal structure. In the case of phosphor, the concentration of Eu2+ is controlled by about 1 mol % to prevent concentration quenching, and the optical absorption is weak because of the small Eu2+ content. Therefore, it is considered that the coloration of the samples can be seen by further increasing the content of Eu2+.

In this study, we selected perovskite-type calcium zirconate (CaZrO3) as a host material for the environmentally friendly yellow pigment, which is composed of only nontoxic elements. In addition, CaZrO3 has a high melting point (Tm = 2365 °C) and sufficient chemical stability.[19,20] Hence, this compound is well-known as a mother for rare-earth-doped phosphors, such as CaZrO3:RE3+ (RE3+ = Sm3+, Eu3+, Gd3+, Tb3+, and Tm3+).[21−23] When Eu2+ is doped into the Ca2+ site, the crystal field around the Eu2+ ion should be strong because the ionic radius of Ca2+ (0.112 nm for 8 coordination) is smaller than that of Eu2+ (0.125 nm for 8 coordination).[24] As a result, the crystal field splitting of the 5d orbitals of Eu2+ becomes large, and the transition energy of Eu2+ from the 4f to 5d orbitals is expected to be small, corresponding to the wavelength of visible light. That is, it is expected that a part of visible light will be absorbed, and the sample will be colored. Therefore, Ca1–EuZrO3 (0 ≤ x ≤ 1) pigments were synthesized by using a conventional solid-state reaction technique, and the optical and color properties were characterized.

Results and Discussion

X-Ray Powder Diffraction and Field-Emission-Type Scanning Electron Microscopic Image

Figure shows the X-ray powder diffraction (XRD) patterns of the Ca1–EuZrO3 (0 ≤ x ≤ 1) samples. Single-phase perovskite structures were obtained in the x = 0 and 0.1 samples. A small amount of ZrO2 was detected as an impurity when x was in the range 0.2 ≤ x ≤ 0.4, but the samples were obtained in almost single phase. However, no perovskite-type structure was obtained with EuZrO3 (x = 1), and only diffraction peaks attributed to the Eu2Zr2O7 phase were observed.

Figure 1

XRD patterns of the Ca1–EuZrO3 (0 ≤ x ≤ 1) samples.

CaZrO3 has an orthorhombic perovskite structure with the symmetry of the Pnma (no. 62) space group.[25−27] In the CaZrO3 structure, the coordination number (CN) values of Ca2+ and Zr4+ are 8 and 6, respectively. The lattice volumes of the Ca1–EuZrO3 (0 ≤ x ≤ 0.4) samples were calculated from the diffraction peak angles. The composition dependence of the lattice volume of Ca1–EuZrO3 (0 ≤ x ≤ 0.4) is shown in Figure . The lattice volume increased with increasing the Eu2+ concentration. This indicates that Ca2+ (ionic radius: 0.112 nm for CN = 8)[24] ions were partially substituted with larger Eu2+ (ionic radius: 0.125 nm for CN = 8)[24] ions. The lattice volume of EuZrO3 estimated using the fitted straight line in Figure was 0.2785 nm3, which was larger than that reported by Viallet et al. (0.2756 nm3).[28] This result suggests that some Eu3+ ions were also introduced into the Zr4+ site of Ca1–EuZrO3, because the ionic radius of Eu3+ (0.0947 nm for CN = 6) is larger than that of Zr4+ (0.072 nm for CN = 6).[24]

Figure 2

Composition dependence of the lattice volume of the Ca1–EuZrO3 (0 ≤ x ≤ 0.4) samples.

Figure shows field-emission-type scanning electron microscopy (FE-SEM) images of Ca1–EuZrO3 (x = 0.1, 0.2, 0.3, and 0.4) samples. Particles about 1.0 μm in size were observed in these samples, although they were partially fused since the calcination temperature was as high as 1400 °C.

Figure 3

FE-SEM images of Ca0.9Eu0.1ZrO3 (a), Ca0.8Eu0.2ZrO3 (b), Ca0.7Eu0.3ZrO3 (c), and Ca0.6Eu0.4ZrO3 (d).

X-Ray Photoelectron Spectra

The X-ray photoelectron spectra (XPS) of the Ca0.7Eu0.3ZrO3 sample is shown in Figure . The spectrum in Figure a provided information on the chemical state of Eu2+ and Eu3+ in the near-surface region. The peaks at 1123.8 and 1154.0 eV were attributed to the Eu2+ 3d5/2 and 3d3/2 configurations, while the strong peaks observed at 1133.8 and 1163.6 eV corresponded to the Eu3+ 3d5/2 and 3d3/2 lines, respectively.[29] These results indicated that Eu2+ and Eu3+ coexisted in the sample. The ratio of Eu2+ to Eu3+ was estimated to be 1:4 based on the deconvoluted peak area. Therefore, the dominant oxidation state of europium ions was trivalent on the near-surface region of the Ca0.7Eu0.3ZrO3 particles synthesized. The small peak at 1144.2 eV corresponded to a multiplet satellite.[30,31]

Figure 4

XPS of Eu 3d (a) and O 1s (b) in the Ca0.7Eu0.3ZrO3 sample.

As seen in Figure b, the O 1s peak was a doublet. The peak on the lower binding energy side (M–O) at 528.7 eV corresponded to oxygen in the lattice, while that on the higher energy side (VO) at 530.3 eV was assigned to the defect oxygen.[32] The high relative intensity of the defect oxide peak indicates that several defects existed in Ca0.7Eu0.3ZrO3 because both Eu2+ and Eu3+ coexisted in the lattice.

Rietveld Analysis

The Rietveld analysis of the XRD patterns of the Ca0.7Eu0.3ZrO3 sample was carried out to investigate the occupancy of the Eu2+/3+ ions in the Ca2+ and the Zr4+ sites. The Rietveld refinement profile of the sample is shown in Figure , and the crystallographic data and structural refinement parameters are summarized in Tables and 2, respectively. Figure shows the crystal structure of Ca0.7Eu0.3ZrO3 illustrated using the VESTA program based on the crystallographic data from the Rietveld refinement.[33] As shown in Table , the low R-factors were obtained for the sample. The Rietveld analysis revealed that the Eu2+/3+ ions occupied both Ca2+ and Zr4+ sites and most of the Eu ions were located in the Ca2+ site, as seen in Table . These results were in agreement with the results in Figure . The ratio of Eu2+ to Eu3+ was 12:1 in the bulk crystal lattice in contrast to 1:4 on the surface determined by XPS. The precision occupancy at the O1 site was 0.93, indicating that oxide anion vacancies were formed in the crystal lattice, and the XPS results show that this behavior was more pronounced on the surface.

Figure 5

Observed (black cross) and calculated (red line) patterns for the Rietveld refinement from the XRD pattern of the synthesized Ca0.7Eu0.3ZrO3 sample as well as the difference profile (bottom blue line) between the observed and calculated patterns. The vertical bars represent the Bragg reflection peak.

Table 1

Crystallographic Parameters of Ca0.7Eu0.3ZrO3 Obtained from Rietveld Refinement Analysisa

lattice parameter		R-factor
a/nm	0.579220(6)	Rwp	2.226
b/nm	0.808365(8)	Re	1.762
c/nm	0.564839(6)	S	1.263
V/nm3	0.264470(5)	RF	1.308

Crystal symmetry: orthorhombic, space group: Pnma (no. 62), number of formula units per unit cell: Z = 4.

Table 2

Refined Structural Parameters of the Ca0.7Eu0.3ZrO3 Samples from Rietveld Refinement using XRD Data Obtained at Room Temperaturea

atom	site	g	x	y	z	Biso/Å2
Ca	4c	0.7225b	0.04498(14)	1/4	0.4900(3)	0.66
Eu1(Eu2+)	4c	0.2775(6)	=x(Ca)	=y(Ca)	=z(Ca)	0.66
Zr	4a	0.9775b	0	0	0	0.23
Eu2(Eu3+)	4a	0.0225b	0	0	0	0.23
O1	4c	0.929(7)	0.4639(9)	1/4	0.5966(8)	0.51
O2	8d	1	0.2887(7)	0.0573(4)	0.2052(7)	0.51

The isotropic atomic displacement parameters (Biso) of calcium, zirconium, and oxygen sites were fixed to 0.66, 0.23, and 0.51 Å2, respectively, with reference to the literature.[25]

The occupancies (g) of the Ca and Zr sites were linearly constrained: g(Ca) = 1 – g(Eu1), g(Eu2) = 0.3 – g(Eu1), and g(Zr) = 1 – g(Eu2).

Figure 6

Crystal structure obtained by the Rietveld analysis for Ca0.7Eu0.3ZrO3 (Eu2+: yellow, Eu3+: blue).

The atomic arrangement in the bulk region is neatly aligned than that in the surface region, and there are fewer lattice defects. On the surface of a particle, on the other hand, there are a lot of uncoupled hands (i.e., dangling bonds). The electronic states for dangling bonds are different from those for the aligned structure in the bulk region. In the case of micron-sized particles, the bulk structure is much thicker than the surface structure. Therefore, the electronic states of micron-sized particles are generally different between surface and bulk regions. Additionally, it is previously reported that Eu2+ ions in the outermost layer were easily oxidized to Eu3+.[34] In this work, the sizes of the particles obtained were of the order of micrometers, as seen in the FE-SEM images (Figure ). From the results of the XPS and Rietveld analyses, it was evidenced that Eu2+ and Eu3+ species coexisted in the sample, and the former was dominant within the bulk and the latter was dominant on the surface.

Reflectance Spectra

The UV–vis reflectance spectra of the Ca1–EuZrO3 (0 ≤ x ≤ 0.4) pigments are depicted in Figure . As shown in Figure a, high reflectance was observed in the visible light region for the nondoped CaZrO3 (x = 0) sample. In contrast, the Eu2+-doped Ca1–EuZrO3 (0.1 ≤ x ≤ 0.4) samples strongly absorbed blue light in the wavelength range from 435 to 480 nm, which is the complementary color of yellow. This optical absorption band was originated by the allowed transition between 4f and 5d orbitals of Eu2+.[16−18,35]

Figure 7

UV–vis reflectance spectra (a) and enlarged spectra from 350 to 500 nm (b) and 550 to 800 nm (c) of the Ca1–EuZrO3 (0 ≤ x ≤ 0.4) samples.

An enlarged view of reflectance spectra from 350 to 500 nm is shown in Figure b. In the x range of 0.1 ≤ x ≤ 0.3, the spectral curve shifted to the longer wavelength side as the Eu2+ concentration increased. The red shift of the reflectance curve was caused by the increase in the absorption intensity depending on the Eu2+ content. In the case of the x range of x > 0.3, on the other hand, the spectrum did not shift because the crystal field strength around the Eu2+ ions became weaker due to the lattice expansion. Accordingly, there is a trade-off between the red shift due to the increase in the Eu2+ concentration and the blue shift due to the expansion of the crystal lattice. The results of the reflectance spectra suggest that in Ca1–EuZrO3, the red shift effect due to the increase in the absorption intensity of the 4f–5d transition was dominant at 0.1 ≤ x ≤ 0.3, but this effect was offset by the blue shift effect attributed to the reduction of the crystal field energy around Eu2+ ions at x > 0.3.

Figure c is an enlarged view of the wavelength region above 550 nm. The reflectance at wavelengths above 600 nm was reduced by Eu2+ doping. This behavior was attributed to the presence of the lattice and surface defects. As already discussed on the XPS and Rietveld analyses, oxide anion deficiencies were formed by the dissolution of Eu3+ into the Zr4+ site of Ca0.7Eu0.3ZrO3. The optical absorption above 600 nm was associated with F-type centers, namely oxide anion defects with two trapped electrons.[36,37]

Chromatic Properties

The L*a*b*Ch° color coordinate data for the Ca1–EuZrO3 (0 ≤ x ≤ 0.4) pigments are summarized in Table , and the photographs of these pigments are displayed in Figure . The brightness value (L*) decreased as the amount of Eu2+ increased. The redness (a*) and yellowness (b*) values increased in a positive direction. The hue angle values (h°) of the Eu2+-doped Ca1–EuZrO3 (0.1 ≤ x) pigments were located in the yellow region of 70 ≤ h° ≤ 105. As seen in Figure b, blue light absorption due to the 4f–5d transition was saturated in the x range of 0.3 or more, and yellow light reflection was observed most strongly at x = 0.3. Accordingly, the Ca0.7Eu0.3ZrO3 pigment showed the highest b* and C values among the Ca1–EuZrO3 (0 ≤ x ≤ 0.4) samples synthesized in this study. This means that this pigment exhibited the brightest yellow color.

Table 3

Chromatic Parameters for the Ca1–EuZrO3 (0 ≤ x ≤ 0.4) Samples

x	L*	a*	b*	C	h°
0	96.4	–0.39	+2.24	2.27	99.9
0.10	83.5	+4.61	+52.7	52.9	85.0
0.20	74.1	+10.8	+63.6	64.5	80.4
0.25	71.8	+12.0	+68.5	69.5	80.1
0.30	71.3	+11.5	+70.7	71.6	80.8
0.35	68.8	+10.8	+69.5	70.3	81.2
0.40	68.6	+10.3	+69.1	69.9	81.5

Figure 8

Photographs of the Ca1–EuZrO3 (0 ≤ x ≤ 0.4) samples.

The color coordinate data of the Ca0.7Eu0.3ZrO3 pigment was compared with those of the conventional yellow pigments on the market such as BiVO4 (Dainichiseika Color & Chemicals Mfg.), PbCrO4 (NIC) and ZrSiO4:Pr (Kawamura Chemical), as summarized in Table . The photographs of these pigments are also displayed in Figure . Although the yellowness value (b*) for the Ca0.7Eu0.3ZrO3 (b* = +70.7) fell short of those for BiVO4 and PbCrO4, it was almost equivalent to that of ZrSiO4:Pr. In addition, the redness value (a*) for the Ca0.7Eu0.3ZrO3 (a* = +11.5) pigment was higher than those of the commercial yellow pigments. In other words, the present pigment possesses the significant feature of a warmer reddish yellow color that is not found in the conventional pigments,[38−40] although this pigment contains expensive europium.

Table 4

Chromatic Parameters of the Ca0.7Eu0.3ZrO3 and Commercial Yellow Pigments

pigments	L*	a*	b*	C	h°
Ca0.7Eu0.3ZrO3	71.3	+11.5	+70.7	71.7	80.8
BiVO4	93.3	–15.7	+80.3	81.8	101
PbCrO4	89.9	+1.12	+96.5	96.5	89.3
ZrSiO4:Pr	83.5	–3.28	+70.3	70.4	92.7

Figure 9

Photographs of Ca0.7Eu0.3ZrO3 and commercial yellow pigments.

Thermal and Chemical Stability Tests

The thermal and chemical stabilities of the Ca0.7Eu0.3ZrO3 pigment were evaluated using the powder sample. To check the thermal stability, this sample was heated in an aluminum silicate (mullite) crucible at 300 and 500 °C for 3 h in an air atmosphere and naturally cooled to room temperature. The acid/base resistance of the Ca0.7Eu0.3ZrO3 pigment was tested in 4% CH3COOH and 4% NH4HCO3 aqueous solutions, and the pigment was soaked in the acid solution and the base solution, respectively. After allowing them to stand at room temperature for 2 h, the pigments were washed with deionized water and ethanol and then dried at ambient temperature. The chromatic coordinate data of the samples after the thermal and chemical stability tests are summarized in Table . Unfortunately, thermal stability of this sample was insufficient, and the original color disappeared after heating the Ca0.7Eu0.3ZrO3 pigment at 300 °C and above in air. This result was attributed to that Eu2+ was oxidized to Eu3+ by heating in air. The color degradation was caused by the significant decrease in the absorption intensity of 4f–5d transition. However, the present pigment can be unproblematically used to the general application such as paints because common pigments have been mostly used at about room temperature.[41,42] On the other hand, the color was almost unchanged after the leaching test in the acid and the base solutions. Therefore, the Ca0.7Eu0.3ZrO3 pigment has chemical stability.

Table 5

Color Coordinates Data of the Ca0.7Eu0.3ZrO3 Sample before and after Thermal and Chemical Stability Tests

treatment	L*	a*	b*	C	h°
as synthesized	71.3	+11.5	+70.7	71.7	80.8
300 °C	96.1	–0.24	+3.76	3.77	93.7
500 °C	96.0	–0.30	+4.06	4.07	94.2
4% CH3COOH	68.6	+12.4	+68.8	69.9	79.8
4% NH4HCO3	70.2	+11.9	+69.2	70.2	80.2

Conclusions

Eu2+-doped calcium zirconates, Ca1–EuZrO3 (0 ≤ x ≤ 1), were synthesized using a solid-state reaction method as environmentally benign inorganic yellow pigments. The samples strongly absorbed the blue light at the wavelengths of 435–480 nm by the 4f–5d transition of Eu2+. As a result, the samples showed bright yellow, and the brightest yellow color was obtained with the Ca0.7Eu0.3ZrO3 (a* = +11.5, b* = +70.7) sample. The yellowness value (b*) of this pigment was almost equal to that of the commercially available praseodymium yellow pigment. In addition, the redness value (a*) was larger than that of the commercial one. Accordingly, the Ca0.7Eu0.3ZrO3 pigment exhibited a warmer reddish yellow color. Although heat resistance of this pigment was not enough, this pigment has chemical stability. Since Ca0.7Eu0.3ZrO3 is composed only of nontoxic elements, it is expected to be one of the environmentally friendly inorganic yellow pigment series.

Experimental Section

Materials and Methods

The Ca1–EuZrO3 (0 ≤ x ≤ 1) samples were synthesized by a conventional solid-state reaction method. CaCO3 (99.5%), Eu2O3 (99.99%), and ZrO2 (98.7%) powders were used as the starting materials. Stoichiometric amounts of each reagent were mixed in an agate mortar. The mixtures were calcined in an alumina boat at 1400 °C for 10 h in a flow of 5% H2–95% N2 mixed gas. Finally, the samples were ground in an agate mortar before characterization.

Characterization

The samples synthesized were characterized by the following methods. X-ray fluorescence spectroscopy (Rigaku, ZSX Primus) measurements indicated that the sample compositions were in good agreement with the nominal stoichiometric compositions of the starting mixtures. The crystal structure of the sample was identified by XRD (Rigaku, Ultima IV) with Cu Kα radiation, operated with voltage and current settings of 40 kV and 40 mA, respectively. The XRD data were collected by scanning a 2θ range of 20–80°. The lattice volume was calculated with the CellCalc Ver 2.20 software from the refined XRD peak angles using α-Al2O3 as a standard. Rietveld refinement of the resulting XRD patterns obtained in the 2θ range from 10° to 120° was performed by the RIETAN-FP software package to determine the precise crystal structure and also to investigate the occupancy of Eu2+/3+ at Ca2+ and Zr4+ sites in Ca0.7Eu0.3ZrO3.[43] From the Rietveld analysis, the final reliability factors (R-factors) were obtained: weighted pattern R-factor (Rwp), R-expected factor (Re), goodness-of-fit (S), and R-structure factor (RF). XPS (ULVAC-PHI, PHI5000 Versa Prove II) was measured using Al Kα radiation to investigate the electronic state of Eu and O in the Ca0.7Eu0.3ZrO3 sample.

The size and morphology of the Ca1–EuZrO3 (x = 0.1, 0.2, 0.3, and 0.4) samples were observed using FE-SEM (JEOL, JSM-6701F). The optical reflectance spectra of the samples were measured with an ultraviolet–visible–near infrared spectrometer (JASCO, V-770) with reference to a standard white plate. The color properties of the samples were evaluated on the Commission Internationale de ĺÉclairage (CIE) L*a*b*Ch° system using a colorimeter (Konica-Minolta, CR-300). The L* parameter describes the brightness of a color with respect to neutral grayscale, and the a* (the red–green axis) and b* (the yellow–blue axis) parameters quantitatively describe the color. The chroma parameter (C), which represents the color saturation of the pigment, is calculated by the following formula: C = [(a*)2 + (b*)2]1/2. The parameter h° ranges from 0 to 360 (70 ≤ h° ≤ 105 means yellow) and is calculated by the formula, h° = tan–1(b*/a*).