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

Gd3+-doped cerium vanadates, Ce1−xGdxVO4 (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 Ce1−xGdxVO4 (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 Ce4f and V3d orbitals. The absorption spectrum of Ce1−xGdxVO4 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, Ce0.80Gd0.20VO4 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%).


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 oen 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 nally 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 reect NIR light in this region, and pigments of various colors that reect NIR light have been reported. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The NIR reection properties of various color pigments (white, yellow, blue, etc.) are generally superior to those of black pigments, because these pigments tend to reect 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 articial heat exhausted by air conditioning increases. 21 The heat island effect is a serious problem in the world. By using NIR reective 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) 2 O 3 and Fe 2 TiO 4 have been reported as NIR-reective black pigments. 4,22 However, (Fe, Cr) 2 O 3 contains toxic chromium, and the NIR-reective properties of both materials are not sufficient.
This study focused on cerium vanadate (CeVO 4 ) as the host material for environmentally friendly NIR-reective black pigments composed of only low toxic elements. It has been reported that CeVO 4 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 reect 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 eld strength around V 5+ , because the split width of the V 3d orbitals corresponds to the crystal eld 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 shis to the longer wavelength side when smaller ions are doped into the Ce 3+ site.
As a dopant to modify the crystal eld strength, Gd 3+ (ionic radius: 0.1053 nm), 25 which is smaller than Ce 3+ (ionic radius: 0.1143 nm), 25 was selected. When Gd 3+ ions are doped into the Ce 3+ 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 1Àx Gd x VO 4 (0 # x # 0.30) solid solutions were synthesized in this study. Their NIR reection and color properties were characterized, and the composition was optimized to satisfy both sufficient blackness and high NIR reectance.

Materials and methods
The Ce 1Àx Gd x VO 4 (0 # x # 0.30) powder samples were synthesized by a conventional solid-state reaction method. Stoichiometric amounts of CeO 2 (FUJIFILM Wako Pure Chemical Industries, Ltd., 99.5%), V 2 O 5 (FUJIFILM Wako Pure Chemical Industries, Ltd., 99.0%), and Gd 2 O 3 (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 uorescence spectroscopy (XRF; Rigaku, ZSX Primus) were in good agreement with the nominal stoichiometric compositions of the starting mixtures. The crystal structure was identied by X-ray powder diffraction (XRD; Rigaku, Ultima IV) with Cu-Ka radiation (40 kV and 40 mA). The sampling width and the scan speed were 0.02 and 6 min À1 , respectively. The lattice volume was calculated with the CellCalc Ver. 2.20 soware from the XRD peak angles rened using a-Al 2 O 3 as a standard. The morphology of the Ce 1Àx Gd x VO 4 (x ¼ 0, 0.10, 0.20 and 0.30) particles was investigated using a eldemission-type scanning electron microscope (FE-SEM; JEOL, JSM-6701F).
The optical reectance spectra were measured using an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrometer (JASCO, V-770 with an integrating sphere attachment). Barium sulphate (BaSO 4 ) and polytetrauoroethylene (PTFE) powders were used as reference samples in the visible and NIR light regions, respectively. NIR solar reectance (R) was calculated according to the American Society for Testing and Materials (ASTM) Standard G173-03 and estimated by the following formula: where r(l) is the spectral reectance obtained from the experiment and i(l) is the standard solar spectrum (W m À2 nm À1 ). The NIR reectance value (R) was expressed as the integral of the product of the observed spectral reectance 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- 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:

Results and discussion
X-ray powder diffraction (XRD) and eld-emission-type scanning electron microscopic (SEM) image  Fig. 2 shows the FE-SEM images and particle size distributions of the Ce 1Àx Gd x VO 4 (0 # x # 0.30) samples. The average particle size increased with increasing the Gd 3+ concentration; the size increased from 1.99 mm (x ¼ 0) to 3.49 mm (x ¼ 0.30). This particle growth was promoted by the Gd 3+ doping, because the melting point of GdVO 4 (1800 C) 31 is lower than that of CeVO 4 (1832 C). 32

Reectance spectra
The UV-Vis-NIR reectance spectra of the Ce 1Àx Gd x VO 4 (0 # x # 0.30) samples are shown in Fig. 3(a). An enlarged view of the reectance 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 eld energy around V 5+ was increased by the lattice contraction, the spectral curve was shied to the longer wavelength side by the Gd 3+ doping. Actually, the band gap energy was decreased by the Gd 3+ doping, as shown later in Table 2. As a result, the color of the sample gradually changed from reddish brown to black as the Gd 3+ content increased.
In the range of x from 0 to 0.20, the absorption wavelength shied to the longer wavelength side with increasing the amount of Gd 3+ . Since the ionic radius of Gd 3+ is smaller than that of Ce 3+ , partial replacement of Ce 3+ with Gd 3+ causes the crystal lattice to contract and the crystal eld energy near V 5+ to increase. This red shi 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 shied to the shorter wavelength side as the Gd 3+ content increased. This blue shi was due to the narrowing of the energy band of the Ce 4f orbitals due to the decrease in the Ce 3+ 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 ¼ 0.20. When Gd 3+ was doped, the NIR reectivity 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 mm with increasing the Gd 3+ concentration. Therefore, the decrease in the NIR reectivity 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 reectance
The L*a*b*C color coordinate data, bandgap energies (E g ) and NIR solar reectance (R) of the Ce 1Àx Gd x VO 4 (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 Gd 3+ . When 0.25 # x, on the other hand, the L*, a*, b*, and C values of Ce 1Àx Gd x VO 4 (0.25 # x) were almost the same as those of the x ¼ 0.20 sample. These results are consistent with the reection 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 Gd 3+ content. The darkest black was obtained with Ce 0.80 -Gd 0.20 VO 4 among all samples. As mentioned in Fig. 3(a), the optical reection in the NIR light region (700-2500 nm) was reduced by the partial substitution of Ce 3+ with Gd 3+ , resulting in the decrease in the R value. However, the Ce 0.80 Gd 0.20 VO 4 sample, which exhibited the darkest black of all samples, showed a sufficient high R value (66.3%). From these results above, Ce 0.80 Gd 0.20 VO 4 is considered the best black pigment with good NIR-reective property.

Chemical stability tests
The chemical stability of the Ce 0.80 Gd 0.20 VO 4 pigment was evaluated using the powder sample. The acid/base resistance of the Ce 0.80 Gd 0.20 VO 4 pigment was tested in 4% CH 3 COOH and 4% NH 4 HCO 3 aqueous solutions, and the pigment was soaked in the acid solution and the base solution, respectively. Aer 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 aer the chemical stability test are summarized in Table 3. The color was almost unchanged aer the leaching test in the acid and the base solutions. Therefore, the Ce 0.80 Gd 0.20 VO 4 pigment has chemical stability.

Comparison with commercially available pigments
The UV-Vis-NIR diffuse reectance spectrum and the color parameters for the optimized Ce 0.80 Gd 0.20 VO 4 pigment were   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 0.80 Gd 0.20 VO 4 pigment showed higher reectance in the NIR wavelength region and higher R values than those of the commercially available pigments. Therefore, we can conclude that Ce 0.80 Gd 0.20 VO 4 is a novel black pigment with sufficient blackness and NIR-reective property.

Conclusions
The Ce 1Àx Gd x VO 4 (0 # x # 0.30) solid solutions were synthesized by a conventional solid-state reaction method with the aim of developing a new NIR-reective black pigment. In the samples with the x range of 0 to 0.20, the spectral curve shied to the longer wavelength side because the energy gap between the Ce 4f and the V 3d orbitals decreased as the crystal eld energy around V 5+ 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 reectance (R) were obtained at Ce 0.80 Gd 0.20 VO 4 . The R value for this sample was 66.3% and the L*a*b*C color parameters were L* ¼ 22.2, a* ¼ +5.71, b* ¼ +2.77, and C ¼ 6.35, respectively. In particular, the R value was signicantly higher than those of the conventional commercially available black pigments (R < 50%). Ce 0.80 Gd 0.20 VO 4 has both sufficient blackness and NIR-reective property, which could make this pigment an effective alternative to the conventional black pigments for thermal shielding.

Conflicts of interest
There are no conicts to declare.