Environment-friendly pigments based on praseodymium and terbium doped La2Ce2O7 with high near-infrared reflectance: Synthesis and characterization

Dyes and Pigments 147 (2017) 225e233

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Environment-friendly pigments based on praseodymium and terbium doped La2Ce2O7 with high near-infrared reflectance: Synthesis and characterization Bin Huang a, b, Yu Xiao a, b, Chao Huang a, b, Jinqing Chen b, Xiaoqi Sun a, * a b

Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, Xiamen 361021, China School of Metallurgy and Chemical Engineering, Jiangxi University of Science & Technology, Ganzhou 341000, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 May 2017 Received in revised form 2 August 2017 Accepted 4 August 2017 Available online 5 August 2017

The environmentally friendly pigments based on Pr4þ and Tb4þ doped La2Ce2O7 have been synthesized via the Sol-Gel method and characterized using several analytic techniques, such as XRD, UVeviseNIR spectrophotometer and CIE L*a*b* (1976) color space. The investigation has demonstrated that the doping of Pr4þ or Tb4þ for Ce4þ in La2Ce2O7 resulted in the color changes from light yellow, soft orange to dark orange. Moreover, the band gaps of pigments decreased from 3.34 to 2.37 (Pr-doped) and 2.24 (Tbdoped). The synthesized pigments of La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) were disordered defect fluorite-type structure and showed good chemical stabilities in acid/alkaline tests. Interestingly, the Pr4þ doped pigment possessed high NIR solar reflectance (>72.47%) in the range 700e2500 nm with the color of dark orange (L* ¼ 46.87 a* ¼ 13.5 b* ¼ 13.4). The substitution of Tb4þ for Ce4þ changed the color to dark orange (L* ¼ 51.35 a* ¼ 15.94 b* ¼ 17.57), and decreased the NIR solar reflectance slightly from 99.11% to 87.41%. The applied studies of two kinds of pigments coating on the galvanized sheets exhibited nice colors with high NIR solar reflectance. The superior performances of pigments have rendered them competent as qualified exterior coating materials. © 2017 Elsevier Ltd. All rights reserved.

Keywords: La2Ce2O7 Environment-friendly Near-infrared reflectance Cool pigment

1. Introduction Inorganic pigments with high NIR reflectance possess great potential for easing the global energy crisis, due to their special properties of superior reflectivity for solar energy [1e3]. Nowadays, the urban heat island effect and global warming have led to the use of air conditioning more frequently, resulting in more energy consumption in the field of air conditioning consumption [4e6]. The use of NIR reflective pigments in exterior coloring applications on roofs and walls has been a positive method to keep the indoor temperature cool, thereby reducing the use of air conditioning in the city [7e10]. NIR reflective inorganic pigments could not only reduce the heat accumulation effectively, but also possess a variety of colors, which have attracted considerable interest [11e14]. Inorganic pigments are commonly used in many fields, i.e., building material, paint, plastics, vehicle and ink [15e17]. However, traditional inorganic near-infrared pigments contained heavy metal

* Corresponding author. E-mail address: [email protected] (X. Sun). http://dx.doi.org/10.1016/j.dyepig.2017.08.004 0143-7208/© 2017 Elsevier Ltd. All rights reserved.

elements, such as chromium, lead, cobalt, cadmium, antimony and selenium [18e20]. When the heavy metal content exceeds a certain level, it poses a serious threat to the environment and human health. As a result, pigments with heavy metal elements have been limited in many countries. Therefore, the colorful inorganic pigments with high NIR reflectance will replace traditional pigments based on heavy metals in the future [21,22]. Recently, a series of rare earth (RE) based NIR reflectance inorganic pigments have been proposed as viable alternatives to traditional toxic pigments [23e25]. The RE oxides possess excellent chemical and thermal stabilities, low toxicities and color diversities [26], they are suitable raw materials for the preparations of pigments. REs can not only be used as the main materials of pigments, but also be utilized as doped elements to modify the color of pigments [27]. Among the RE based pigments, complex metal oxides of Y2Ce2O7 [28e30] and Sm2Ce2O7 [31] were widely studied. The transition metal element doped pigments Y2Ce2-xMxO7 and Sm2Ce2-xMxO7 (M ¼ Fe, Mo) revealed bright yellow, which were utilized as substitutes for traditional industrialized toxic yellow pigments (PbCrO4, PbMoO4, Pb2Sb2O7 et al.) [32,33]. The colored RE elements of Pr, Tb were doped in Y2Ce2O7 or Sm2Ce2O7 to prepare

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the brick-red pigments, which possessed the high NIR solar reflectance with dark brightness in color. Lanthanum and cerium mixed oxide (La2Ce2O7) was used for turbine blade coating because of its low thermal conductivity and good thermal stability, together with good lattice matching with the substrate [34e37]. Up to now, there has been no report on the near-infrared reflection of La2Ce2O7. In this study, we focused on investigating the effects of Pr4þ and Tb4þ on the color hue and NIR reflectance of less-toxic La2Ce2O7 pigment. A series of new pigments with the general formula of La2Ce2-xMxO7 (M ¼ Pr, Tb) were obtained by Sol-Gel method and characterized by X-ray powder diffraction. The UVeviseNIR spectrophotometer and CIE L*a*b* color space were also used to assess the optical properties of prepared pigments. The novel pigments outperformed the NIR reflectivity of previously reported Pr or Tb doped Y2Ce2O7 pigments. Further potential of La2Ce2O7 pigments is waiting to be tapped. 2. Experimental section

2.3. Characterization techniques and instrumentation The crystalline structure of calcined pigment samples was identified by Rigaku Miniflex 600 XRD system using Cu-Ka radiation with continuous scanning mode at a rate of 15 /min ranges from 5 to 100 . Operating conditions of 40 kV and 15 mA were used to obtain the XRD pattern. The particle size distributions of the powders were monitored by dynamic light scattering (Nanobrook Omni, Brookhaven). The samples were ultrasonically dispersed in ethanol for testing the D10, D50 and D90 size distributions. The optical properties of pigments were characterized by a UVeviseNIR spectrophotometer (Agilent carry 5000, America) using polytetrafluoroethylene (PTFE) as a white standard. The samples were filled in the powder cell and performed in reflection mode with a resolution of 1 nm and the electromagnetic spectra range from 300 nm to 2500 nm. The NIR solar reflectance (R*) of the pigments and their coatings were calculated by the following formula:

Z

2500

2.1. Material The chemical reagents of La(NO3)3$6H2O, Ce(NO3)3$6H2O, Pr(NO3)3$6H2O, Tb(NO3)3$6H2O and ethylene glycol were analytical grade (purity>99%) and obtained from Aladding Reagent (Shanghai) Co. The citric acid was provided by Shandong Xiya chemical technology Co., Ltd. All of the starting materials were used without further purification. 2.2. General method for the preparation of doped pigments As a typical experimental process, all the pigments were synthesized by a Sol-Gel method. According to the formula of La2Ce2-xMxO7 (M ¼ Pr or Tb; x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5), stoichiometric proportions of the raw materials were dissolved in the adequate ethylene glycol (2 mL). At the same time, citric acid was further added to the ethylene glycol as the chelation agent, the molar rate of RE cation to citric acid was 1:2 (1.78 g in this study). An ultrasonic cleaner was utilized to dissolve the mixture quickly until the solution was clear. Subsequently, the mixture solution was heated at 80  C on a constant temperature magnetic stirrer to promote the polymerization reaction. A lot of bubbles were generated in the solution after 20 min and lasted for 5 min, then the wet polymerization gel was formed (accompanied by the polymerization, the citric acid and ethylene glycol esterification were reacted to produce esters and water). Afterwards, the wet gel was diverted to a vacuum oven, and dried at 80  C for 10 h. The obtained xerogel precursor was transferred into an agate mortar and grounded into powder. Finally, the resultant powders were calcined at 900  C for 10 h to prepare the pigment samples. The amount of raw materials has been listed in Table 1.

R* ¼

700

Z

rðlÞiðlÞdl

2500 700

iðlÞdl

where r(l) is the spectral reflectance obtained from the UVeviseNIR spectrophotometer and the i(l) is the standard solar spectral irradiance from ASTM G173-03 Reference Spectra (W*m2*nm1) [38e40]. The band gap was determined using absorbance spectra range from 300 nm to 700 nm (visible spectral range) and calculated from the absorption edge by equation E(ev) ¼ h$c/l ¼ 1239 (eV nm)/l (nm). The color performance of synthesized pigment was obtained using the spectrophotometer CS-580A (Hangzhou CHN Spec) with CLEDs light source. The CIE L*a*b* (1976) color space was used as recommended by the Commission Internationale de l’Eclairage (CIE). When a color is represented by CIE L*a*b* system, L* indicates the brightness value (black ¼ 0, white ¼ 100), a* for red (þ) and green (), and b* represents the yellow (þ) and blue () value. Furthermore, the L*C*H color model uses the same color space as L*a*b* system. But the saturation of the color is defined as C* ¼ [(a*)2þ(b*)2]1/2, and h ¼ tan1(b*/a*) indicates the hue angle. 3. Result and discussion 3.1. Powder x-ray diffraction analysis The XRD patterns of synthesized pigments La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) after calcination at 900  C for 10 h are depicted in Fig. 1. The XRD pattern of La2Ce2O7 shows the characteristic reflections of the disordered defect fluorite-type

Table 1 The amount of raw materials for preparing La2Ce2-xMxO7 (M ¼ Pr or Tb; x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5). x

La(NO3)3$6H2O/g

Ce(NO3)3$6H2O/g

Pr(NO3)3$6H2O/g

Tb(NO3)3$6H2O/g

0 0.01 0.05 0.10 0.20 0.30 0.40 0.50

1 1 1 1 1 1 1 1

1.0046 0.9996 0.9795 0.9544 0.9042 0.8539 0.8037 0.7535

0 0.0050 0.0251 0.0502 0.1005 0.1507 0.2009 0.2512

0 0.0052 0.0262 0.0523 0.1046 0.1569 0.2092 0.2616

B. Huang et al. / Dyes and Pigments 147 (2017) 225e233

Fig. 1. Powder X-ray diffraction patterns of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5).

structure, which has been reported by Zhang et al. [34]. Our XRD date are consistent with the previous research [41,42]. According to the XRD pattern (x ¼ 0), it is observed that the diffraction patterns are not CeO2 or La2O3 peaks, even though they are related to the CeO2 peaks of cubic fluorite phase (CeO2, a ¼ 5.403 Å; La2Ce2O7, a ¼ 5.5681 Å). Similar to the La2Ce2O7 nanoparticles synthesized by co-precipitation method reported elsewhere [35], there was no sign of La2Ce2O7 pyrochlore phase (Fd3m), and its space group is Fm3m. The fluorite structure with (111), (200), (220) and (311) planes are corresponding to diffraction peaks of 2q at 27.68 , 32.34 , 46.18 and 54.74 , respectively. Obviously, the stable solid solution La2Ce2O7 (Ce0.5La0.5O1.75) constituted of La2O3 in CeO2 has been obtained. With the increase of Pr-dopant concentration in La2Ce2-xPrxO7 (x > 0.01), no change in the XRD patterns can be observed. Thus, the structure of La2Ce2-xPrxO7 was not changed, all the diffraction lines were successfully matched to the fluorite-type phase. The lattice parameter of La2Ce2-xPrxO7 were calculated with jade6.5 software. As can be seen in Table 2, the lattice parameter (a ¼ b ¼ c) of La2Ce2O7 was calculated to be 5.5681 Å, while that of CeO2 with a cubic structure was 5.403 Å (JCPDS file No. 01-0898436). Despite the oxygen content in the La2Ce2O7 lattice is smaller than that in the CeO2 one, the lattice parameter of La2Ce2O7 was larger than CeO2 due to the ionic radius of La3þ (0.1032 nm, eightfold coordination) being greater than that of Ce4þ (0.097 nm, eightfold coordination). As the praseodymium content increased, the lattice parameter came to be smaller than that without doping. Ce and Pr were 3 þ and 4 þ oxidation states in the oxide, the radii of Ce4þ and Pr4þ (eight-fold coordination) were 0.097 nm and 0.096 nm. The slightly smaller radii of Pr-dopant ions led to the smaller lattice parameters. The lattice parameters of La2Ce2-xPrxO7 were around 5.55 Å with x

227

Fig. 2. Powder X-ray diffraction patterns of La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5).

range from 0.01 to 0.5, which proved that the presence of Pr in predominantly 4 þ oxidation states based on the ionic size consideration. The powder XRD patterns of Tb doped La2Ce2O7 compounds are exhibited in Fig. 2, and the lattice parameters are summarized in Table 2. Similar to the Pr-doped pigments, the Tb doped La2Ce2O7 pigments are also disordered defect fluorite-type structure. The lattice parameters are decreased with the increase of Tb doped amount, which can be attributed to the 4 þ oxidation state of Tb (eight-fold coordination, radius ¼ 0.076 nm) in the compounds. The decreased trend is corresponding with X-ray diffraction patterns evidenced by the shift in diffraction peaks to higher 2theta angle in comparison with the parent compound. 3.2. Particle size analyzing The particle sizes of La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) are shown in Figs. 3 and 4. All the samples reveal a homogeneous size distribution. The D10, D50 and D90 of La2Ce2O7 are 229.17 nm, 419.22 nm and 766.87 nm, respectively. The D50 of pigments doped with Pr are around 500 nm, and it is 400 nm embodied in La2Ce2-xTbxO7, which is probably due to the greater uniformity of sample grinding. The homogeneous size distribution of sample ensures its coating application with a uniform color [3]. 3.3. Optical properties of La2Ce2-xPrxO7 The reflectance spectra of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments in the UVevis region are presented in Fig. 5, and their corresponding absorbance spectra are demonstrated in Fig. 6. As can be seen from the spectra of La2Ce2O7

Table 2 The lattice parameter of the La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5). Lattice parameter (Å)

Pigment composition (x) 0

0.01

0.05

0.1

0.2

0.3

0.4

0.5

La2Ce2-xPrxO7 Residual error of fit (%)

5.5681 8.42

5.5633 8.85

5.5569 8.19

5.5524 8.60

5.5594 8.02

5.5555 8.71

5.5496 8.51

5.5564 8.68

La2Ce2-xTbxO7 Residual error of fit (%)

5.5681 8.42

5.5680 8.90

5.5637 8.09

5.5449 8.61

5.5444 8.75

5.5388 8.51

5.5332 8.79

5.5188 8.42

228

B. Huang et al. / Dyes and Pigments 147 (2017) 225e233

Fig. 3. Particle size distribution of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5).

compounds shown in Fig. 5, there exists a strong absorption band at 370 nm. Since La3þ and Ce4þ show d0 and f0 configurations in their stable oxidation states, there are no f-f or f-d transitions [29]. The UVevis diffuse reflectance spectra of La2Ce2O7 are mainly derived from the shifts of charge transfer transitions between O2p valence and Ce4f conduction band of Ce4þ. By doping La2Ce2O7 with Pr4þ ions, an additional 4f1 electronic energy level of Pr4þ is introduced between the O2 valence and the Ce4þ conduction band. The absorption edge of La2Ce2O7 at 370 nm shifts to longer wavelengths (470 nm) when the substitution of the Pr4þ chromophore ion is increased to 1.25% mol (x ¼ 0.05). As a consequence, the band gap reduces from 3.34 to 2.62 eV (see Table 5). At the same time, colors of the pigments are varied from light yellow to soft orange. With the increase of doped Pr4þ (x ranged from 0.1 to 0.5), the absorption edge of pigment is shifted to 521 nm, and the band gap is decreased to 2.37 eV. Accordingly, color changes of the pigment from soft orange to dark orange [43e45]. The detailed band gap values of the La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) is shown in Table 5. The NIR reflectance spectra of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments are illustrated in Fig. 7. As can be seen from the reflectance spectra, the parent pigment La2Ce2O7 possesses the highest NIR reflectance of 95.95%. With the increases of doped Pr4þ for Ce4þ in La2Ce2O7, the NIR reflectance is significantly

Fig. 4. Particle size distribution of La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5).

Fig. 5. UVevis reflectance spectra of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

reduced from 95.95% to 75.30% (x ¼ 0.5). Although the addition of Pr4þ reduces the reflectivity of pigments, the dopant enriches the color of the pigment from light yellow to soft orange or even dark orange, which contributes to meet the public demands for colors. In accordance with the ASTM standard G173-03, the NIR solar reflectance spectra of powdered La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments were determined and presented in Fig. 8. The detailed information of NIR reflectance spectra and NIR solar reflectance spectra are given in Table 3. As the NIR reflectance, the parent pigment La2Ce2O7 possesses the highest NIR solar reflectance of 99.11%, which means that the pigments absorb almost no NIR solar energy. All the pigments show a high NIR solar reflectance (dark orange, >70%), which are higher than those of Ce25Pr0.8MoOy orange-red pigments and Y2Ce1.5Pr0.5O7 brick-red pigments. The prepared pigments with high NIR solar reflectance revealed the potential to become good insulation materials [24,29]. 3.4. Optical properties of La2Ce2-xTbxO7 The reflectance spectra of La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments in the UVevis region and their corresponding absorbance spectra are depicted in Fig. 9 and Fig. 10, respectively. The reflectance spectra reveal a significant shift in the

Fig. 6. Absorption spectra of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

B. Huang et al. / Dyes and Pigments 147 (2017) 225e233

229

Fig. 7. NIR reflectance spectra of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

Fig. 9. UVevis reflectance spectra of La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

Fig. 8. Solar reflectance spectra of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

Fig. 10. Absorption spectra of La2Tb2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

absorption edges toward longer wavelengths (from 370 to 552 nm) for the terbium-doped pigments in comparison with the parent compound. Doping of Tb4þ for Ce4þ in La2Ce2O7 results in a systematic decrease in band gap from 3.34 to 2.24 eV with the increase in dopant concentration (Table 6 presents the detailed information of band gaps of pigments). As a result, the colors of the pigments change from light yellow to soft orange, and finally become dark orange. Interestingly, their color evolution is just close to the class of Pr-doped La2Ce2O7 pigments. The charge transfer band shifts to longer wavelength, the consequent decrease in band gap can be attributed to the O2p-Tb4f charge transfer transitions. Fig. 11 and Fig. 12 demonstrate the NIR reflectance spectra and the corresponding NIR solar reflectance spectra of La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) powder pigments. Table 4 lists the detailed counterpart information of NIR reflectance and solar reflectance values. With the doping amount of Tb4þ increases

from x ¼ 0 to 0.5, the average NIR reflectance spectra and NIR solar reflectance spectra are slightly reduced. As can be seen from the curves, no regular decline can be observed by increasing the doped terbium. When the Tb4þ doped amount arrives at 1.25% (x ¼ 0.05), the color of the pigment changes from light yellow to orange. On this condition, its NIR reflectance spectra is 95.17%, the value approaches that of the parent pigment (95.95%). It is worthwhile and interesting to mention that the NIR solar reflectance spectra of all the samples are greater than 87%, which is much higher than the pigments of La2Ce2O7 doped with praseodymium, although they have similar colors in the same miscellaneous. As mentioned above, the increase of terbium doped amount deepens the color of the pigment, but NIR solar reflectance spectra are not decreased too much. The interesting phenomenon promises a great potential for its application in insulating cool pigment materials.

Table 3 The values of NIR reflectance and solar reflectance of La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments. Pigments composition (x)

0

0.01

0.05

0.1

0.2

0.3

0.4

0.5

NIR reflectance of the pigments (%) Solar reflectance of the pigments (%)

95.95 99.11

90.54 94.27

89.34 91.56

86.95 88.77

83.52 83.95

81.00 80.13

76.06 74.28

75.30 72.47

230

B. Huang et al. / Dyes and Pigments 147 (2017) 225e233

Fig. 11. NIR reflectance spectra of La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

color hue of the pigments significantly, evidenced by the C* value sharply increased (>19.02). Hence, the color evolution of synthesized pigments varied from light yellow, soft orange, orange to dark orange (Fig. 13). It is evident from the L*a*b* value of Tb4þ doped La2Ce2O7 pigments listed in Table 6 that the color evolution is similar to that of Pr4þ doped La2Ce2O7 pigments. The lightness of Tb4þ doped pigments is gradually decreased with the increased amount of dopant. The substitution of Tb4þ for Ce4þ up to x ¼ 0.01 enhances the b* value from 9.93 to 25.28. The b* value is also regularly decreased from 25.28 to 17.57 with the more doped Tb4þ, which indicates that the Tb4þ bring yellowness to the pigments. With the increases of Tb4þ from x ¼ 0.01 to 0.5, yellow gradually comes to be weakened. The a* value reaches the maximum at x ¼ 0.3 (from 1.26 to 16.71), and decreases somewhat to 15.94 (x ¼ 0.5). On the other hand, the saturation (C*) of pigments is intensified (x ¼ 0 to 0.05), then is declined gradually from 27.20 to 19.02. The hue angles (H ) of both pigments (La2Ce2-xPrxO7 and La2Ce2xTbxO7) are found to be changed from yellow to red region (ranged from 44.79 to 97.24). As a result, the color of La2Ce2-xTbxO7 pigments is similar to La2Ce2-xPrxO7 pigments, changed from soft orange to dark orange, but the lightness is higher than La2Ce2xPrxO7 pigments with the same amount of dopant (Fig. 13). 3.6. NIR solar reflectance analysis of the pigments coatings

Fig. 12. Solar reflectance spectra of La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

3.5. Color performance The chromatic properties of La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments were evaluated by CIE 1976 L*a*b* color coordinate, which are summarized in Tables 5 and 6, respectively. Table 5 reveals that the more replacing of Ce4þ for Pr4þ decrease the L* value from 94.18 (x ¼ 0) to 46.87 (x ¼ 0.5), which means the colors of pigments come to be darker. The redness of the sample is enhanced when the Pr4þ doped amount increased from 0 to 5% (x ¼ 0.5) corresponding to the increasing trend of a* value from 1.26 to 16.16. As the Prdopant continues to be increased, a* value is decreased to be 13.5. Moreover, the substitution of Pr4þ up to x ¼ 0.01 enhances the b* value from 9.93 to 22.94, and then decreases to 13.4 with more Pr doping which indicates that the yellowness of the pigments is enhanced, but declines with more Pr4þ replacing for Ce4þ. On the other hand, the doped Pr improves the richness of

In order to evaluate the thermal insulation properties of the synthesized pigments as “cool pigments”, the typical pigments of La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.1, 0.3, 0.5) were selected to coat on a metal sheet roofing material like galvanized sheet, the weight ratio of pigment to alkyd resin (binder) was 1:1. The photographs of coatings are exhibited in Fig. 14, and the corresponding color coordinates of the coatings are given in Table 7. The color performances of La2Ce1.7Pr0.3O7 and La2Ce1.7Tb0.3O7 coatings are close to the recently reported Y2Ce1.7Pr0.3O7 [29] and Y2Ce1.8Tb0.2O7 [30] pigment coatings, respectively. Fig. 15 depicts the NIR reflectance spectra and solar reflectance of the coatings colored with La2Ce2-xPrxO7 and La2Ce2xTbxO7 (x ¼ 0, 0.01, 0.1, 0.3, 0.5) pigments respectively. It is obvious to find that the coatings greatly enhance the NIR reflectance and the corresponding solar reflectance of the galvanized sheet. The NIR solar reflectance of La2Ce2O7 is 83.26%, which is much higher than the bare galvanized sheet (19.18%). Moreover, the coating possesses the high NIR solar reflectance of 55.64% with a dark orange hue (La2Ce1.5Pr0.5O7), which indicates that the synthesized pigments possess the potential as a kind of cool pigment. Table 8 lists the detailed values of NIR reflectance and solar reflectance of the two types of coatings, the values clearly reveal that the Tb-doped coatings hold the higher NIR solar reflectance compared with Pr-doped coating at the same doped amount. 3.7. Acid/alkali resistance studies of the pigments The typical pigments of La2Ce1.6Pr0.4O7 and La2Ce1.6Tb0.4O7 were soaked in 10% HCl, HNO3, H2SO4 and NaOH solution, respectively,

Table 4 The values of NIR reflectance and solar reflectance of La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments. Pigments composition (x)

0

0.01

0.05

0.1

0.2

0.3

0.4

0.5

NIR reflectance of the pigments (%) Solar reflectance of the pigments (%)

95.95 99.11

93.06 96.54

95.17 96.52

94.34 95.19

89.72 89.86

92.93 91.34

88.79 88.11

90.77 87.41

B. Huang et al. / Dyes and Pigments 147 (2017) 225e233

231

Table 5 Color coordinates and band gap values of the La2Ce2-xPrxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments. x

L*

a*

b*

C*

H

Eg (eV)

0

94.18

1.26

9.93

10.01

97.24

3.34

0.01

74.32

11.64

22.94

25.72

63.1

3.09

0.05

64.62

14.83

22.8

27.20

56.96

2.62

0.1

60.35

15.61

21.46

26.54

53.97

2.54

0.2

54.29

16.16

19.18

25.08

49.89

2.45

0.3

50.23

14.88

16.43

22.17

47.84

2.42

0.4

49.35

14.55

15.88

21.54

47.51

2.40

0.5

46.87

13.5

13.4

19.02

44.79

2.37

Color performance

Fig. 14. Photograph of the coatings colored with La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.1, 0.3, 0.5) pigments. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 6 Color coordinates and band gap values of the La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments. x

L*

a*

b*

C*

H

Eg (eV)

0

94.18

1.26

9.93

10.01

97.24

3.34

0.01

75.14

12.24

25.28

28.09

64.17

3.01

0.05

66.73

15.92

24.89

29.55

57.40

2.59

0.1

61.93

15.36

22.02

26.85

55.11

2.49

0.2

57.34

15.88

20.80

26.17

52.65

2.39

0.3

55.77

16.71

19.93

26.01

50.03

2.33

0.4

54.33

16.38

19.43

25.41

49.87

2.31

0.5

51.35

15.94

17.57

23.72

47.79

2.24

Color performance

and evenly mixed for 10 min. Following filtering and washing the pigments with deionized water, then drying the mass of pigments. The color coordinates of the treated samples were measured and compared with the original sample respectively. As can be seen from Table 9, the total color difference (DE*) of all the samples is smaller than 1.56, which reveals the chemical stabilities of synthesized pigments for the acid/alkali tested [2]. 4. Conclusions In summary, the Pr4þ and Tb4þ doped La2Ce2O7 inorganic

Table 7 Color coordinates of the coatings colored with La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.1, 0.3, 0.5) pigments. Pigments

composition (x)

L*

a*

b*

C*

H

La2Ce2-xPrxO7

0 0.01 0.1 0.3 0.5

86.00 61.51 44.79 35.29 32.87

3.37 13.79 18.9 14.72 12.98

13.71 24.43 20.81 12.93 10.63

14.12 28.06 28.11 19.59 16.78

103.81 60.56 47.76 41.30 39.32

La2Ce2-xTbxO7

0.01 0.1 0.3 0.5

62.72 47.32 39.14 35.04

15.89 18.89 18.21 15.87

28.56 22.44 17.37 13.23

32.68 29.33 25.16 20.66

60.91 49.91 43.65 39.82

pigments have been synthesized for the first time via a Sol-Gel method. The pigments of La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) were of a disordered defect fluoritetype structure, the two types of pigments exhibited a color evolution from light yellow, soft orange to dark orange, which can be attributed to the incorporation of suitable chromophore metal ions (Pr4þ or Tb4þ) in the La2Ce2O7 crystal, and the charge transfer band of O2p-Ce4f shift to the longer wavelength. Furthermore, the Prdoped pigment possessed an outstanding NIR solar reflectance over 72.47% in the range of 700e2500 nm, and the value of Tbdoped pigments was 87.41%, which was pretty high with the dark orange color (L* ¼ 51.35, a ¼ 15.94, b ¼ 17.57). The pigments also showed the high NIR solar reflectance in their coatings on the galvanized sheet (>55.64% for Pr-doped pigments and >57.8% for Tb-doped pigments), which can be used as “cool pigments” applied in the building roofing and facade materials. Moreover, the pigments are chemically stable towards the acid/alkali testing. As a

Fig. 13. Photographs of La2Ce2-xPrxO7 and La2Ce2-xTbxO7 (x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments.

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B. Huang et al. / Dyes and Pigments 147 (2017) 225e233

Fig. 15. NIR reflectance spectra and solar reflectance of the coatings colored with La2Ce2-xPrxO7 (a, b) and La2Ce2-xTbxO7 (c, d), (x ¼ 0, 0.01, 0.1, 0.3, 0.5). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 8 The values of NIR reflectance and solar reflectance of La2Ce2-xMxO7 (M ¼ Pr or Tb; x ¼ 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) pigments. La2Ce2-xPrxO7

Pigments composition (x)

NIR reflectance (%) Solar reflectance (%)

La2Ce2-xTbxO7

0.01

0.1

0.3

0.5

0.01

0.1

0.3

0.5

81.05 83.26

69.24 62.37

69.15 61.78

63.00 58.39

60.77 55.64

70.28 65.61

68.85 64.34

67.97 61.72

66.33 57.80

Table 9 Color coordinates of the La2Ce1.6Pr0.4O7 and La2Ce1.6Tb0.4O7 pigments after acid/alkali resistance test. 10% Acid/alkali

HCl HNO3 H2SO4 NaOH a

Galvanized sheet

0

La2Ce1.6Pr0.4O7

20.11 19.18

result, the Pr4þ and Tb4þ doped La2Ce2O7 compounds with high NIR solar reflectance would be non-toxic, environment-friendly, chemically stable pigments for coating applications.

La2Ce1.6Tb0.4O7

DE*

L*

a*

b*

a

50.13 50.45 49.50 49.85

13.63 13.69 14.06 14.16

15.29 15.35 15.42 15.81

1.34 1.56 0.68 0.64

DE* ¼ [(DL*)2þ(Da*)2þ(Db*)2]1/2.

L*

a*

b*

DE*

53.67 54.03 54.33 54.42

15.65 15.88 16.04 16.31

18.78 18.54 19.06 19.34

1.18 1.06 0.50 0.15

Acknowledgements This work was supported by ‘Hundreds Talents Program’ from Chinese Academy of Sciences. National Natural Science Foundation of China (21571179), Science and Technology Major Project of Fujian Province (2015HZ0001-3) and Science and Technology Service

B. Huang et al. / Dyes and Pigments 147 (2017) 225e233

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