Novel near-infrared reflective black inorganic pigment based on cerium vanadate

Abstract

Gd³⁺-doped cerium vanadates, Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30), were synthesized as near-infrared (NIR) reflective black pigments by a conventional solid-state reaction method. Crystal structure, particle size, optical properties, and color of the samples were characterized. The Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) samples were obtained in a single-phase form and the lattice volume decreased with increasing Gd³⁺ concentration. Optical absorption below 630 nm was observed in all samples, which corresponded to the charge-transfer transition between Ce_4f and V_3d orbitals. The absorption spectrum of Ce_1−xGd_xVO₄ was shifted to the longer wavelength side as the Gd³⁺ content increased, because of the increase in the crystal field around V⁵⁺ due to the lattice shrinkage. As a result, the sample color gradually changed from dark brown to black with increasing Gd³⁺ content. Among the samples synthesized in this study, Ce_0.80Gd_0.20VO₄ absorbed visible light with wavelengths shorter than 650 nm and exhibited the darkest color. Furthermore, this black pigment showed a sufficient NIR reflectance value (R = 66.3%), which was higher than those of the commercially available products (R < 50%).

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Introduction

The urban heat-island effect gives rise to the ambient temperature in an urban area being higher than that in the surrounding countryside. This phenomenon often adversely affects the habitability of many cities, including the high-power consumption of air conditioners, poor air quality, and human health in the summer.¹ Natural sunlight consists of 5% ultraviolet light (UV; 280–400 nm), 43% visible light (Vis; 400–700 nm), and 52% near-infrared light (NIR; 700–2500 nm).² The photon energy obtained by absorbing light is converted into vibration energy and finally into thermal energy. In addition, NIR light in the wavelength range of 700 to 1300 nm plays the most important role in heat generation.³ Therefore, in order to mitigate heat storage, it is desirable for the material to reflect NIR light in this region, and pigments of various colors that reflect NIR light have been reported.^4–18

The NIR reflection properties of various color pigments (white, yellow, blue, etc.) are generally superior to those of black pigments, because these pigments tend to reflect NIR light as well as visible light.^19,20 Common black pigments such as carbon black basically absorb not only visible light but also NIR light to store heat. Generally, they are painted on roads and exterior walls, so in urban areas where the proportion of roads and exterior walls is higher than that of green areas, the temperature rises, and the artificial heat exhausted by air conditioning increases.²¹ The heat island effect is a serious problem in the world. By using NIR reflective black pigment on the road surface and the wall of the building, it can be expected to alleviate the phenomenon. Several compounds such as (Fe, Cr)₂O₃ and Fe₂TiO₄ have been reported as NIR-reflective black pigments.^4,22 However, (Fe, Cr)₂O₃ contains toxic chromium, and the NIR-reflective properties of both materials are not sufficient.

This study focused on cerium vanadate (CeVO₄) as the host material for environmentally friendly NIR-reflective black pigments composed of only low toxic elements. It has been reported that CeVO₄ shows optical absorption due to a Ce_4f–V_3d charge transfer transition and its optical band gap energy is 1.8 eV.^23,24 Therefore, this vanadate compound should exhibit a dark brown color and reflect NIR light. The wavelength of the optical absorption caused by the Ce_4f–V_3d charge transfer transition can be controlled by adjusting the crystal field strength around V⁵⁺, because the split width of the V_3d orbitals corresponds to the crystal field strength, which is enhanced by the lattice contraction. Since the absorption wavelength depends on the energy difference between the Ce_4f and the V_3d orbitals, the optical absorption wavelength shifts to the longer wavelength side when smaller ions are doped into the Ce³⁺ site.

As a dopant to modify the crystal field strength, Gd³⁺ (ionic radius: 0.1053 nm),²⁵ which is smaller than Ce³⁺ (ionic radius: 0.1143 nm),²⁵ was selected. When Gd³⁺ ions are doped into the Ce³⁺ site, the split width of the V_3d orbital should be widened due to lattice shrinkage. The color of the material is expected to change from dark brown to black due to the decrease in the energy between the Ce_4f and the V_3d orbitals. Therefore, Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) solid solutions were synthesized in this study. Their NIR reflection and color properties were characterized, and the composition was optimized to satisfy both sufficient blackness and high NIR reflectance.

Experimental

Materials and methods

The Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) powder samples were synthesized by a conventional solid-state reaction method. Stoichiometric amounts of CeO₂ (FUJIFILM Wako Pure Chemical Industries, Ltd., 99.5%), V₂O₅ (FUJIFILM Wako Pure Chemical Industries, Ltd., 99.0%), and Gd₂O₃ (Shin-Etsu Chemical Co., Ltd., 99.9%) were mixed in an agate mortar. The mixtures were calcined in an alumina crucible at 900 °C for 6 h in an air atmosphere. Finally, the samples were ground in an agate mortar before characterization.

Characterization

The powder samples obtained were characterized by the following methods. The chemical composition of the samples analyzed by X-ray fluorescence spectroscopy (XRF; Rigaku, ZSX Primus) were in good agreement with the nominal stoichiometric compositions of the starting mixtures. The crystal structure was identified by X-ray powder diffraction (XRD; Rigaku, Ultima IV) with Cu-Kα radiation (40 kV and 40 mA). The sampling width and the scan speed were 0.02° and 6 min⁻¹, respectively. The lattice volume was calculated with the CellCalc Ver. 2.20 software from the XRD peak angles refined using α-Al₂O₃ as a standard. The morphology of the Ce_1−xGd_xVO₄ (x = 0, 0.10, 0.20 and 0.30) particles was investigated using a field-emission-type scanning electron microscope (FE-SEM; JEOL, JSM-6701F).

The optical reflectance spectra were measured using an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrometer (JASCO, V-770 with an integrating sphere attachment). Barium sulphate (BaSO₄) and polytetrafluoroethylene (PTFE) powders were used as reference samples in the visible and NIR light regions, respectively. NIR solar reflectance (R) was calculated according to the American Society for Testing and Materials (ASTM) Standard G173-03 and estimated by the following formula:

where r(λ) is the spectral reflectance obtained from the experiment and i(λ) is the standard solar spectrum (W m⁻² nm⁻¹). The NIR reflectance value (R) was expressed as the integral of the product of the observed spectral reflectance and the solar irradiance divided by the integral of the solar irradiance, both integrated in the range of 700 to 2500 nm. The band gap energies of the samples were calculated from the absorption edge of the absorbance spectrum represented by the Kubelka–Munk function, f(R) = (1 − R)²/2R, where R is reflectance.²⁶ The color property was evaluated in terms of the Commission Internationale de l'Éclairage (CIE) L*a*b*C system using a colorimeter (Konica-Minolta, CR-400). The L* parameter indicates the brightness or darkness of a color relative to the neutral gray scale, and the a* (the red-green axis) and the b* (the yellow-blue axis) parameters represent the color quantitatively. The chroma parameter (C) represents the color saturation of the pigments and is calculated according to the following formula: C = [(a*)² + (b*)²]^1/2.

Results and discussion

X-ray powder diffraction (XRD) and field-emission-type scanning electron microscopic (SEM) image

Fig. 1 shows the XRD patterns of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) samples. It has been reported that CeVO₄ has tetragonal^27,28 and monoclinic structures.^28–30 In all samples, the diffraction pattern was indexed to that of the tetragonal CeVO₄ phase and obtained in a single-phase form. The lattice volumes of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) samples are listed in Table 1. The lattice volume monotonically decreased as the Gd³⁺ concentration increased. These results indicated that Ce³⁺ ions (0.1143 nm)²⁵ in the CeVO₄ lattice were partially substituted with smaller Gd³⁺ ions (0.1053 nm)²⁵ and the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) solid solutions were successfully synthesized in a single-phase form.

Fig. 1 XRD patterns of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) samples.

Table 1 Lattice volumes of Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30)

x	Lattice volume/nm^3
0	0.3551
0.10	0.3529
0.15	0.3514
0.20	0.3501
0.25	0.3491
0.30	0.3478

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Fig. 2 shows the FE-SEM images and particle size distributions of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) samples. The average particle size increased with increasing the Gd³⁺ concentration; the size increased from 1.99 mm (x = 0) to 3.49 mm (x = 0.30). This particle growth was promoted by the Gd³⁺ doping, because the melting point of GdVO₄ (1800 °C)³¹ is lower than that of CeVO₄ (1832 °C).³²

Fig. 2 FE-SEM images and particle size distributions of Ce_1−xGd_xVO₄; x = 0 (a and e), x = 0.10 (b and f), x = 0.20 (c and g), and x = 0.30 (d and h).

Reflectance spectra

The UV-Vis-NIR reflectance spectra of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) samples are shown in Fig. 3(a). An enlarged view of the reflectance spectra in the visible light region (300–750 nm) is also depicted in Fig. 3(b). All samples were found to strongly absorb visible light with wavelengths shorter than 650 nm, and this optical absorption was attributed to the Ce_4f–V_3d charge transfer transition.²³ Since the crystal field energy around V⁵⁺ was increased by the lattice contraction, the spectral curve was shifted to the longer wavelength side by the Gd³⁺ doping. Actually, the band gap energy was decreased by the Gd³⁺ doping, as shown later in Table 2. As a result, the color of the sample gradually changed from reddish brown to black as the Gd³⁺ content increased.

Fig. 3 Reflectance spectra of the UV-Vis-NIR (a) and UV-Vis (b) regions of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) samples.

Table 2 Color coordinates, bandgap energies (E_g) and NIR solar reflectance (R) of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) powder samples

x	L*	a*	b*	C	Eg/eV	R/%
0	26.8	+11.1	+6.62	12.9	1.69	77.5
0.10	27.3	+8.57	+5.14	9.99	1.67	74.1
0.15	25.1	+6.99	+3.72	7.92	1.63	70.0
0.20	22.2	+5.71	+2.77	6.35	1.60	66.3
0.25	23.2	+6.14	+3.24	6.94	1.62	67.0
0.30	23.1	+6.17	+3.18	6.99	1.62	66.0

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In the range of x from 0 to 0.20, the absorption wavelength shifted to the longer wavelength side with increasing the amount of Gd³⁺. Since the ionic radius of Gd³⁺ is smaller than that of Ce³⁺, partial replacement of Ce³⁺ with Gd³⁺ causes the crystal lattice to contract and the crystal field energy near V⁵⁺ to increase. This red shift of the spectral curve was due to a decrease in the transition energy between the Ce_4f and the V_3d orbitals, caused by an increase in the split width of the V_3d orbitals. In the x region of x ≥ 0.20, on the other hand, the spectral curve shifted to the shorter wavelength side as the Gd³⁺ content increased. This blue shift was due to the narrowing of the energy band of the Ce_4f orbitals due to the decrease in the Ce³⁺ concentration, leading to an increase in the Ce_4f–V_3d charge transfer energy. In other words, there is a trade-off relationship between the increase in the splitting width of the V_3d orbitals and narrowing of the Ce_4f orbitals. The boundary between the x regions, where the former and latter effects were dominant, was approximately x = 0.20.

When Gd³⁺ was doped, the NIR reflectivity of the sample decreased. Based on the Mie scattering theory,³³ light scattering capacity is generally maximized when the particle size is approximately equal to the wavelength of the corresponding light. As already shown in Fig. 2, the average particle size of the sample increased from 1.99 to 3.49 μm with increasing the Gd³⁺ concentration. Therefore, the decrease in the NIR reflectivity was caused by an increase in the number of particles that were too large in size compared to the wavelength of NIR light (700–2500 nm).³⁴

Chromatic properties and NIR solar reflectance

The L*a*b*C color coordinate data, bandgap energies (E_g) and NIR solar reflectance (R) of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) powder samples are summarized in Table 2. The photographs of them are also displayed in Fig. 4. In the x region of x ≤ 0.20, the brightness (L*), redness (a*), yellowness (b*) and chroma (C) values were decreased by doping with Gd³⁺. When 0.25 ≤ x, on the other hand, the L*, a*, b*, and C values of Ce_1−xGd_xVO₄ (0.25 ≤ x) were almost the same as those of the x = 0.20 sample. These results are consistent with the reflection behavior in the red light (600–750 nm) region shown in Fig. 3. The sample color gradually changed from reddish brown to black with increasing the Gd³⁺ content. The darkest black was obtained with Ce_0.80Gd_0.20VO₄ among all samples. As mentioned in Fig. 3(a), the optical reflection in the NIR light region (700–2500 nm) was reduced by the partial substitution of Ce³⁺ with Gd³⁺, resulting in the decrease in the R value. However, the Ce_0.80Gd_0.20VO₄ sample, which exhibited the darkest black of all samples, showed a sufficient high R value (66.3%). From these results above, Ce_0.80Gd_0.20VO₄ is considered the best black pigment with good NIR-reflective property.

Fig. 4 Photographs of the Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) pellets made from the powder samples.

Chemical stability tests

The chemical stability of the Ce_0.80Gd_0.20VO₄ pigment was evaluated using the powder sample. The acid/base resistance of the Ce_0.80Gd_0.20VO₄ 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 chemical stability test are summarized in Table 3. The color was almost unchanged after the leaching test in the acid and the base solutions. Therefore, the Ce_0.80Gd_0.20VO₄ pigment has chemical stability.

Table 3 Color coordinates of Ce_0.80Gd_0.20VO₄ before and after chemical stability test

Treatment	L*	a*	b*	C	R/%
As synthesized	22.2	+5.71	+2.77	6.35	66.3
4% CH3COOH	21.3	+5.31	+2.62	5.92	69.1
4% NH4HCO3	21.4	+5.38	+2.78	6.06	72.3

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Comparison with commercially available pigments

The UV-Vis-NIR diffuse reflectance spectrum and the color parameters for the optimized Ce_0.80Gd_0.20VO₄ pigment were compared with those of several commercially available NIR-reflective black pigments such as Black 6350 (iron and chromium oxide; Asahi Kasei), Black 6301 (manganese and bismuth oxide, Asahi Kasei), and MPT-370 (calcium, manganese, and titanium oxide, Ishihara Sangyo). The UV-Vis-NIR results are shown in Fig. 5 and the color parameters are summarized in Table 4. The photographs of these pigments are also demonstrated in Fig. 6. As evidenced from these results, the present Ce_0.80Gd_0.20VO₄ pigment showed higher reflectance in the NIR wavelength region and higher R values than those of the commercially available pigments. Therefore, we can conclude that Ce_0.80Gd_0.20VO₄ is a novel black pigment with sufficient blackness and NIR-reflective property.

Fig. 5 UV-Vis-NIR reflectance spectra of various black pigments.

Table 4 Color coordinates and NIR solar reflectance (R) of various black pigments

Pigment	L*	a*	b*	C	R/%
Ce0.80Gd0.20VO4	22.2	+5.71	+2.77	6.35	66.3
Black 6350	27.0	+0.78	+3.53	3.62	44.7
Black 6301	25.0	+0.42	+0.97	1.06	40.1
MPT-370	25.7	+0.59	−0.48	6.35	49.0

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Fig. 6 Photographs of Ce_0.80Gd_0.20VO₄ (a), Black 6350 (b), Black 6301 (c), and MPT-370 (d) pellets made from the powder samples.

Conclusions

The Ce_1−xGd_xVO₄ (0 ≤ x ≤ 0.30) solid solutions were synthesized by a conventional solid-state reaction method with the aim of developing a new NIR-reflective black pigment. In the samples with the x range of 0 to 0.20, the spectral curve shifted to the longer wavelength side because the energy gap between the Ce_4f and the V_3d orbitals decreased as the crystal field energy around V⁵⁺ increased due to the lattice shrinkage. As a result, the color of the sample gradually changed from reddish brown to black. Both adequate blackness and high NIR solar reflectance (R) were obtained at Ce_0.80Gd_0.20VO₄. The R value for this sample was 66.3% and the L*a*b*C color parameters were L* = 22.2, a* = +5.71, b* = +2.77, and C = 6.35, respectively. In particular, the R value was significantly higher than those of the conventional commercially available black pigments (R < 50%). Ce_0.80Gd_0.20VO₄ has both sufficient blackness and NIR-reflective property, which could make this pigment an effective alternative to the conventional black pigments for thermal shielding.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI (Grant Numbers JP19K05668 and JP20H02439).

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