BaCO3 complex pigments with impressive near infrared reflectance and excellent color performance

Solar Energy Materials and Solar Cells 199 (2019) 99–107

Contents lists available at ScienceDirect

Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat

Yellow β-Bi2O3/BaCO3 complex pigments with impressive near infrared reflectance and excellent color performance

T

Jian Zou∗, Ziyi Yu School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Complex pigments NIR reflectance Cool pigments Bi2O3 Yellow pigments

Yellow Bi2O3/BaCO3 complex pigments were prepared by thermolysis of precursors from the precipitation of Bi2O2CO3 on BaCO3 particles at low temperature between 350 and 410 °C. The synthesized powders were analyzed by colorimetry, UV-Vis-NIR spectroscopy, X-ray diffraction, and scanning electron microscopy. The pigments were found to have composite phases consisting of β-Bi2O3 and BaCO3. The addition of BaCO3 substantially reduced the consumption of Bi2O3 and maintained comparable or improved color properties and NIR reflectance compared to pure β-Bi2O3. The complex pigments prepared at a 1:1 composition of Bi2O3/BaCO3 exhibited excellent color performance, with L* = 95.63, b* = 79.85, and h° = 90.48 (closed to ideal yellow color, h° = 90), as well as impressive NIR reflectance of up to 96.51%. Finally, an approximately 7.6 °C decrease in temperature was observed for the inner surface of ceramic tiles coated with the cool pigments relative to blank polyurethane coating.

1. Introduction During the summer, the heat build-up caused by solar irradiance results in a significant increase in energy consumption due to air conditioning and a decrease of indoor comfort. Cool roofing products based on solar reflective materials can decrease cooling energy consumption whilst improving indoor comfort in buildings without air conditioning [1,2]. White pigments, which can maximize the reflectance in visible and near infrared (NIR) spectra, are often used as cool materials. However, the need for colored cool products has increased due to aesthetics [3] as well as their anti-glare [4,5] and stain resistance [6] properties. In fact, nonwhite products can also exhibit a good solar reflective performance by maximizing reflectance in NIR region [7]. Currently, some attempts are being adopted to further develop cool colored pigments [8–12]. Yellow is an important color in the pigments industry, with yellow pigments being widely applied in daily products. Thus, much research has focused on developing yellow solar reflective pigments [12–17]. Yellow pigments based on bismuth compounds exhibit some attractive pigmentary properties and high solar reflectance [17–23], and are environmentally friendly due to the absence of toxic elements such as Pb, Cr, and Cd, which are often used in traditional yellow pigments. Bi4Zr3O12 has a comparable yellowness value with that of the commercial praseodymium yellow pigment [18]. The pale-yellow color

∗

nano pigment Bi4Ti3O12 presents a high NIR reflectance of 95%, with a better thermal performance than white TiO2 pigment [19]. Monoclinic bismuth vanadate (BiVO4) has a band gap of 2.14–2.51 eV and good visible absorption [20,21], which results in a vivid yellow color, and its pigmentary performances can be further enhanced by the substitution of Ta/P into the vanadium sites [20], Ca/Zn into the Bismuth sites [21], or co-substitution of vanadium and bismuth sites by CaMoO4 [17] or (LiLa)1/2MoO4 [22]. These pigments have an even high yellowness value of over 90% and 97% NIR reflectance. However, it has also proven difficult to control the pigmentary color of BiVO4 [22], with major variations in color value being reported in some studies [17,23]. In general, solid phase method at reaction temperatures of 800 °C or above are used to synthesize these yellow pigments based on bismuth compounds [17–23]. These conditions resulted in a large consumption of energy for their synthesis as well as a high cost, proving an obstacle for the large-scale application of BiVO4 yellow pigments [23]. Bismuth oxides (Bi2O3) have the band gaps of 2.1–2.82 eV depending on their crystallographic phase [24,25], and β-Bi2O3 has a band gap of approximately 2.4 eV [26], closed to that of monoclinic BiVO4 [20,21]. Therefore, β-Bi2O3 maybe have a similar pigmentary color with BiVO4. Kusumoto prepared a Nb2O5 doped β-Bi2O3 solid solution by the conventional ceramic method at 800 °C, which exhibited a brilliant reddish yellow color [27]. In addition, β-Bi2O3 has a high refractive index [28] and should therefore have high NIR

Corresponding author. E-mail address: [email protected] (J. Zou).

https://doi.org/10.1016/j.solmat.2019.04.031 Received 25 October 2018; Received in revised form 30 March 2019; Accepted 26 April 2019 Available online 01 May 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.

Solar Energy Materials and Solar Cells 199 (2019) 99–107

J. Zou and Z. Yu 2500

reflectance. Herein, yellow pigments based on β-Bi2O3 were prepared by thermolysis method at low temperatures of between 350 °C and 410 °C. In order to reduce the consumption of expensive Bi2O3, BaCO3 was added to prepare Bi2O3/BaCO3 complex pigments. Our previous works indicated that complex pigments had comparable color properties and enhanced NIR reflectance to that of the pure colored components [29,30]. The Bi2O3/BaCO3 complex pigments prepared herein are also expected to have good color properties and high NIR reflectance.

∗ Rsol / NIR =

∫sol / NIR r (λ ) i (λ ) dλ 2500

∫sol / NIR i (λ ) dλ

where r(λ) is the spectral reflectance obtained from the experiment and i(λ) is the standard solar spectrum (Wm−2 mm−1) determined from the ASTM Standard G173-03. The solar irradiance reflective spectrum of pigments was obtained in accordance with the value of r(λ)i(λ). 0.2 g of pigments were dispersed into 10 g of polyurethane (PU) paint with ultrasonic treatment for 2 min. The PU paint was then coated onto ceramic tiles. The temperature of the pigment-coated ceramic tiles was measured directly by a digital thermometer with a temperature detector under irradiation by an infrared bulb (Philips Infrared PAR38E 150 W).

2. Experimental 2.1. Materials

3. Results and discussions Barium chloride (BaCl2), bismuth ternitrate (Bi(NO3)3), ammonium carbonate ((NH4)2CO3), calcium chloride (CaCl2), sodium sulfate (Na2SO4), ethylene glycol and polyurethane (PU) paint (Dü Fang, Acryl PU-Klalack) were used as received.

Pure β-Bi2O3 can be synthesized by the thermal decomposition of some bismuth compounds, including Bi2O2CO3 [33,34] or a Bi (Ш)glycerol complex [26]. In this paper, the Bi2O2CO3 was also used as the precursor for the synthesis of β-Bi2O3 pigments. The pure Bi2O2CO3 precursor (PDF #41–1488) decomposed into pure β-Bi2O3 (PDF#27–0050) at 350 °C and to pure α-Bi2O3 phase (PDF#65–2366) when annealed at 400 °C (Fig. 1a). Hu et al. [34] observed that Bi2O2CO3 decomposed gradually into β-Bi2O3 from 280 °C to 320 °C, and that pure β-Bi2O3 was gradually converted to monoclinic α-Bi2O3 when the treatment temperature was increased from 350 °C to 500 °C. This temperature is much lower than the one at which β-Bi2O3 was formed by cooling high temperature cubic phase δ-Bi2O3 at approximately 650 °C [33]. Fig. 1b shows the visible absorbance spectra of pure Bi2O3. These two samples both exhibited intense absorption in the visible region beyond 400 nm. In contrast to the monoclinic α-Bi2O3 phase (400 °C), the spectrum of the tetragonal β-Bi2O3 phase (350 °C) showed a red shift. Additionally, β-Bi2O3 had a threshold at approximately 520 nm, while α-Bi2O3 sample showed the absorption threshold at approximately 430 nm, indicating that the β-Bi2O3 phase has better absorption in the visible region compared to α-Bi2O3. The sample band gaps, determined by the optical absorption near the band edge, were estimated by the following equation [26,34]:

2.2. Synthesis of pigments Preparation of BaCO3. BaCl2 was dissolved into deionized water to obtain 1 M solution. An equimolar (NH4)2CO3 solution was added dropwise into BaCl2 solution of the same concentration under vigorous stirring, and a white suspension was immediately obtained. The suspension was stirred for 4 h at 40 °C. A white precipitate was obtained after several cycles of filtering and washing processes. The BaCO3 powder was obtained by drying the precipitate at 105 °C for 4 h. CaCO3 was prepared by the same process substituting BaCl2 by CaCl2. Additionally, BaSO4 was obtained by substituting (NH4)2CO3 by NaSO4. Preparation of complex pigments. BaCO3 powder (2 g) was redispersed into 50 mL of the (NH4)2CO3 solution under vigorous stirring. Bi(NO3)3 ethylene glycol solution (0.2 M) was added dropwise into the BaCO3 suspension at different Bi:Ba ratios (0.25:1,0.5:1,0.75:1,1:1 and 1:1.25). In the BaCO3 suspension, (NH4)2CO3 was added in slight excess such that Bi(NO3)3 was completely precipitated to form Bi2O2CO3. The suspension was stirred for 4 h at 40 °C. A white precipitate was obtained after several cycles of filtering and washing processes. The Bi2O2CO3/ BaCO3 composite powders were obtained by drying the precipitate at 105 °C for 4 h. Finally, yellow complex pigments were obtained by annealing the Bi2O2CO3/BaCO3 composite powder at 350–410 °C for 1.5 h. Pure Bi2O3 was prepared without BaCO3 powders, and BaCO3 was replaced by BaSO4 or CaCO3 to prepare Bi2O3/BaSO4 or Bi2O3/ CaCO3 complex pigments.

αhv = K (hv − Eg )n/2 Where α, hv, K and Eg are the absorption coefficient, the photo energy, a constant, and band gap energy, respectively; n is 1 for Bi2O3 [26,34]. Here, the absorption coefficient was replaced with the absorbance ( A ) of samples as obtained from Fig. 1b. The Eg value of α-Bi2O3 and β-Bi2O3 samples was estimated to be 2.86 and 2.43 eV, respectively, based on the plots of (Ahv)2 versus hv in Fig. 1c. The narrower energy gap of β-Bi2O3 leads to a red shift of optical absorption relative to α-Bi2O3, indicating that β-Bi2O3 will present better colour. As show in the set of Fig. 1c, β-Bi2O3 (350 °C) had a high luminosity (L* = 95.08), as well as a much higher yellow hue value (b* = 75.05) than one of α-Bi2O3 (b* = 19.48), which were also much higher than ones of Cr doped TiO2 yellow pigments [15]. Additionally, the a* value of the as-prepared β-Bi2O3 sample was only −0.94, indicating that this pigment has a nearly pure yellow hue. These color properties compared favorably with some commercial high-grade yellow pigments such as BiVO4 [17] and Pr-yellow pigments [17]. Additionally, the pigment consists of thin flakes of approximately 2 μm (Fig. 1d), which is a suitable size for pigments [35]. The results above therefore indicate that βBi2O3 should prove to be an excellent pigment. Bi2O3 has a high reflective index [28]. This would result in high reflection in the non-absorption light region. The reflectance of β-Bi2O3 increased dramatically near the optical band edge of approximately 520 nm (Fig. 1e and f), as determined by the energy gap of β-Bi2O3 in Fig. 1b. When the wavelength exceeded 540 nm, the reflectance increased to above 90%. Obviously, the reflectance in whole NIR region

2.3. Characterization The pigment phases were analyzed by X-ray diffraction (XRD) using CuKa radiation (36 kV, 20 mA) in an XD-6 diffractometer (Beijing Pgeneral). The particle morphologies were characterized by scanning electron microscopy (SEM-EDS; Quanta x50 FEG) and transmission electron microscopy (TEM; FEI Tecnai G2 F20, acceleration voltage of 200 kV). X-Ray photoemission spectroscopy (XPS) was performed using a Thermo ESCALAB 250Xi instrument equipped with a monochromatic Al anode X-ray gun. UV–Vis spectra were recorded on a spectrophotometer with an integrating sphere (Shmadzu UV-2550); BaSO4 was used as the reference sample. The color of the pigments was evaluated by measuring the L*, a*, and b* parameters using a 3NH spectrophotometer NS810 (Shenzhen 3NH). Micrographs were obtained using an Olympus CX31 instrument. UV-Vis-NIR spectra were recorded on a spectrophotometer with an integrated sphere (Hatachi U-4100). The solar reflectance (R*sol, 280–2500 nm) and NIR reflectance (R*NIR, 700–2500 nm) were calculated in accordance with the ASTM Standard G173-03, as described elsewhere [31,32], by the formula: 100

Solar Energy Materials and Solar Cells 199 (2019) 99–107

J. Zou and Z. Yu

Fig. 1. XRD (a) of samples, visible absorbance spectra (b) and plot of (Ahv)2 versus hv (c) for Bi2O3 at 350 and 400 °C, SEM (d), UV-Visible-NIR reflectance spectrum (e), and solar irradiance reflective spectrum based on AM 1.5 solar spectrum w/37° global tilt (f) of the Bi2O3 at 350 °C.

pure Bi2O2CO3 decomposed directly into α-Bi2O3 at 400 °C [34]. For the BaCO3/Bi2O3CO3 complex precursor, no α-Bi2O3 phase was observed despite increasing the annealing temperature up to 410 °C. The addition of BaCO3 may have restricted the growth of Bi2O2CO3 during its formation process, resulting in the formation of smaller sized Bi2O2CO3 particles. Greaves et al. [33] observed that small sized Bi2O2CO3 particles decomposed into β-Bi2O3 at approximately 400 °C. A series of BaCO3/Bi2O3CO3 complex pigments were also produced by changing the ratio of Bi:Ba (Fig. 2c). With the increase in Bi content, the peaks of the BaCO3 phase gradually decreased, while those of βBi2O3 were simultaneously enhanced, suggesting the formation of βBi2O3/BaCO3 composites of different compositions. The variation of composition of the prepared complex pigments would lead to a variation in their optical and color properties. The optical absorbance spectra of the complex pigments showed an intense absorption in the visible region from 400 to 520 nm (Fig. 3). This was attributed to the intrinsic absorption of β-Bi2O3 [26,34]. With regards to annealing temperature (Fig. 3a), the absorbance of pigments was enhanced with the increase in temperature, with the change being more

was over 90%. The NIR (700–2500 nm) reflectance value (R*NIR) was estimated to be 94.74% based on the NIR reflectance spectrum in Fig. 1f. Therefore, β-Bi2O3 could potentially be used as a cool pigment. Previous studies indicated that complex pigments consisting of inexpensive white components and expensive color components had better color properties and an enhanced NIR reflectance than the pure pigments [29,30]. Herein, cheaper BaCO3 powders were used to prepare β-Bi2O3/BaCO3 complex pigments in order to minimize the use of Bi2O3. The XRD patterns of the complex precursor, composed of BaCO3 and Bi2O3CO3, were obtained (Fig. 2a). The XRD results in Fig. 1 indicated that Bi2O3CO3 decomposed into β-Bi2O3 at 350 °C, and that BaCO3 remained stable at that same temperature. Therefore, β-Bi2O3/ BaCO3 complex pigments are expected to be obtained by annealing this precursor at above 350 °C. Indeed, β-Bi2O3/BaCO3 complex pigments were obtained by annealing the precursor. β-Bi2O3 and BaCO3 co-existed in the products annealed at temperatures from 350 °C to 410 °C (Fig. 2b), with no other phases being observed in these pigments. These results are different from one of pure Bi2O2CO3 in Fig. 1 and a previous report [34], where 101

Solar Energy Materials and Solar Cells 199 (2019) 99–107

J. Zou and Z. Yu

Fig. 2. XRD patterns of precursors at a 1:1 ratio of Bi:Ba (a), samples with 1:1 ratio of Bi:Ba annealed at different temperature (b) and products with different Bi/Ba ratio at 400 °C(c). The vertical lines at the bottom of the figure represent the relative peak intensities at respective 2θ values corresponding to the Bi2O2CO3 (green), BaCO3 (red) and β-Bi2O3(blue)phases. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Bi:Ba ratios, a similar change was observed (Fig. 3b), wherein the absorbance increased and there was an absorption band edge with a red shift with the increase in Bi content. The Eg value of the complex

obvious at lower temperatures. Additionally, the absorption band edge showed a red shift when the precursor was treated at higher temperatures. Regarding the composition of complex pigments with different

Fig. 3. Optical absorbance spectra of samples (a) with a 1:1 ratio of Bi:Ba annealed at different temperatures and (b) at different Bi:Ba ratios at 400 °C. Plot of (Ahv)2 versus hv of samples at a 1:1 ratio of Bi:Ba annealed at different temperatures (c) and at different Bi:Ba ratios at 400 °C (d). 102

Solar Energy Materials and Solar Cells 199 (2019) 99–107

J. Zou and Z. Yu

Table 1 The Colourimetric parameters, energy gap (Eg), NIR (700–2500 nm) and solar reflectance value (R*) of complex pigments. samples

∗

L a∗ b∗ C* ho Eg (eV) Bi2O3 content (wt %) R*NIR (%) R*sol (%)

Temperature (oC)

Composition (Bi:Ba)

350

380

390

400

410

0.25:1

0.5:1

0.75:1

1:1

1.25:1

96.17 −2.04 65.76 65.79 91.78 2.47

96.07 −2.06 71.49 71.52 91.65 2.45

94.97 −1.18 78.02 78.03 90.86 2.43

95.63 −0.67 79.85 79.85 90.48 2.43

92.61 0.15 73.24 73.34 89.88 2.42

96.83 −3.24 60.27 60.35 93.08 2.49 22.79 94.90 84.40

96.76 −2.58 64.11 64.16 92.31 2.47 37.11 96.67 84.58

96.53 −2.22 71.07 71.05 91.79 2.45 46.96 95.46 82.27

95.63 −0.67 79.85 79.85 90.48 2.43 54.14 96.51 83.45

95.37 −1.22 73.64 73.65 90.95 2.42 59.61 96.05 81.17

besides the expected phases of β-Bi2O3 and BaSO4. For the Bi2O3/ CaCO3 and Bi2O3/BaSO4 samples, Bi2O3 reacted with CaCO3 and BaSO4 to form new phases, resulting in the decrease or even absence of the colored β-Bi2O3 phase, and therefore, in a reduced color performance. Similarly, some unexpected phases were also observed in Bi2O3/BaCO3 samples following annealing at 450 °C (Fig. S2), and its color properties were also reduced to values almost equal to those of Bi2O3/BaSO4 samples (Table S1). These results indicate that the β-Bi2O3 phase determines the color performance of complex pigments and, further, that only BaCO3 is suitable for the preparation of complex pigments. The chemical composition of the complex pigment prepared at a 1:1 ratio of Bi:Ba was investigated by EDS and XPS. The Bi, Ba, O, and C elements were detected in the EDS spectrum (Fig. 4a), and their presence was confirmed by XPS (Fig. 4b). The elemental ratio observed was consistent with the expected ones. Ba3d peaks were resolved into two components, Ba3d3/2 and Ba3d5/2 (Fig. 4c), where these peaks (Ba3d5/2 at 779.9 eV and Ba3d3/2 at 795.3 eV) corresponded to barium ion (II) [39]. The Bi4f peaks were also resolved into two components, Bi4f5/2 and Bi4f7/2 (Fig. 4d), located at 164.2 and 158.9 eV, respectively; these two peaks were assigned to those of trivalent Bi [40] and, therefore, no pentavalent Bi existed in the complex pigments. Therefore, the yellow color of the complex pigments stems from the trivalent Bi compound, which is β-Bi2O3, as confirmed by XRD in Fig. 2. The ratio of Bi:Ba was estimated to be 1.14:1 by EDS, which is close to the value of 1:1, yet a much higher ratio of 2.52:1 was estimated by XPS. The difference between EDS and XPS may have been due to the special structure of the complex pigments. Fig. 5 shows the morphology and structure of the complex pigments at the various compositions. A remarkable morphological feature of these complex pigments is that they consist of thin flakes and rods or granules, which is markedly different from pure β-Bi2O3 (Fig. 1d). The pure β-Bi2O3 was composed of thin flakes, much larger than those in the complex pigments. Therefore, the thin flakes observed in the composites belonged to the β-Bi2O3 phase, whereas the rods or granules belonged to the BaCO3 phase. At a low Bi: Ba ratio (Fig. 5a and b), the BaCO3 rods were distinctive and had a smooth surface. With the increase in Bi: Ba ratio, the complex pigments mainly consisted of irregular aggregates, with few rods presenting a rough surface (Fig. 5c–e). The TEM image of the aggregates showed thin flakes enwrapping granules and rods (Fig. 5f), indicating that β-Bi2O3 exists in the outer layer of the complex particles. And it is this special structure which resulted in a higher ratio of Bi:Ba in XPS results of the complex pigments in Fig. 4. It is obvious that β-Bi2O3/BaCO3 complex pigments have an excellent color performance, superior to that of pure β-Bi2O3. It is particularly noteworthy that the content of Bi2O3 in the complex pigments can be substantially reduced while the good color performance is maintained. In our previous works [29,30], we indicated that the special core-shell structure of complex pigments endowed them with an enhanced color performance because the outer colored component can maximize the visible absorption. In the current work, the BaCO3

pigments treated at different temperatures and of different composition were also estimated based on the plots of (Ahv)2 versus hv in Fig. 3c and d, respectively. The Eg value decreased gradually with the increase in temperature and Bi content. When the annealed temperature was elevated to 410 °C from 350 °C, the Eg value decreased to 2.42 from 2.47 eV (Table 1); similarly, when the ratio of Bi:Ba increased from 0.25:1 to 1.25:1, the Eg value also dropped to 2.49 from 2.42 eV. These changing trends agree with one of the absorption spectra. The absorption spectra showed that all samples had an intense absorption in the visible region (Fig. 3), indicating that they were brightly colored. As shown in Table 1, even for the sample annealed at 350 °C, good color properties were obtained, with a high luminosity (L* = 96.17) and a yellow component (b*) of 65.76. When the annealing temperature was elevated to 380 °C, the b* value increased beyond 70, then increasing further to a maximum of 79.85 at 400 °C. Therefore, the β-Bi2O3/BaCO3 complex pigment showed optimal color properties when at an annealing temperature of 400 °C, which is slightly higher than that of pure β-Bi2O3. Pigment composition also determines the color properties of complex pigments. With the increase of bismuth, the yellow component value b* increased from 60.27 for the sample with a composition of Bi:Ba = 0.25 to a maximum value of 79.85 for that with a 1:1 ratio of Bi:Ba. The theoretical Bi2O3 content in these two complex pigments was only of 22.79 wt% and 54.14 wt%, respectively. Interestingly, they both exhibited good color properties, and especially the latter, which showed a better color performance relative to the pure β-Bi2O3 pigments in Fig. 1 and a comparable performance with available yellow pigments [15,17,18,20]. All of the complex pigments produced herein showed a high luminosity L* above 90, a high chroma C* of above 60, and a hue angle h° value close to 90°, which lies within the yellow region of the cylindrical color space (h° = 70–105 for yellow) [36–38]. An ideal yellow color is determined by high lightness (L* > 60), yellowness (b* > 50), high purity (C* = 0–100), and a huge angle of h° = 90° [36,37]. Obviously, these color parameters indicate that the complex pigments have the characteristics of an ideal yellow pigment. It is also noted that the color properties, especially the hue angle of these pigments, are comparable with the values of industrial pigments such as PbCrO4 (87.8°), BiVO4 (95.8°), CdS (93.6°), and praseodymium yellow (92.7°) [36,37]. Therefore, these complex pigments present with a brilliant yellow color and are ideal yellow pigments. In addition, we tried to substitute BaSO4 or CaCO3 for BaCO3 to prepare the complex pigments. Regrettably, their color properties were substantially reduced (Table S1). No pigments showed a yellow component b* above 50, showing values even lower than the minimum of βBi2O3/BaCO3 complex pigments (Table 1). The UV–Vis absorption spectra showed that Bi2O3/CaCO3 or Bi2O3/BaSO4 complex pigments had a lower absorption in the visible region, with a blue shift relative to β-Bi2O3/BaCO3 pigments (Fig. S1). As shown in the XRD patterns in Fig. S2, the CaBi6O11 phase existed predominantly in Bi2O3/CaCO3 complex pigments, with new phases in the Bi2O3/BaSO4 complex samples 103

Solar Energy Materials and Solar Cells 199 (2019) 99–107

J. Zou and Z. Yu

Fig. 4. The chemical composition of the 1:1 Bi:Ba sample. (a)EDS spectrum, (b) XPS survey, high resolution XPS spectra of Ba3d (c), and Bi4f (d).

reason that the complex pigments have good color properties while the consumption of Bi2O3 can be markedly reduced. The special structure of complex pigments was confirmed by an optical micrograph map (Fig. 6a) that exhibited an almost uniform yellow hue, indicating that the surface of the complex pigments contained colored β-Bi2O3 phase particles; this is in agreement with the SEM and TEM results (Fig. 5). The micro-structure which the colored βBi2O3 components located at outer layer of complex pigment particles maximizes the visible absorption of β-Bi2O3 because it can avoid shielding of visible light from non-colored components [30]. Therefore, the β-Bi2O3/BaCO3 complex pigments presented a brilliant yellow color, unlike the mixed pigments which presented a lighter color

particles and ammonium carbonate were initially dispersed in the solution, with the Bi2O2CO3 precursor being subsequently deposited on the surface of BaCO3 upon addition of the Bi3+ solution. Ultimately, the white BaCO3 components may be surrounded by the yellow colored βBi2O3 to form β-Bi2O3/BaCO3 complex pigments following annealing. Then formation of the complex pigments is depicted in Scheme 1. This special structure was supported by the SEM and TEM results (Fig. 5), and is completely different from that of the mixed pigments prepared by directly mixing the colored and white components [29,30,41]. The mixed pigments presented a lighter color relative to the pure colored components [29,30,41] because the white components shield part of the colored components and absorb some visible light [30]. This is the

Fig. 5. SEM images of Bi2O3/BaCO3 complex pigments at varying Bi:Ba ratios, namely (a) 0.25:1, (b) 0.5:1, (c) 0.75:1, (d) 1:1, and (e) 1.25:1, and TEM image of the 1:1 Bi:Ba sample (f). 104

Solar Energy Materials and Solar Cells 199 (2019) 99–107

J. Zou and Z. Yu

reducing with increasing wavelength, especially in the NIR region beyond 1600 nm. These changes were also observed in previous studies [19,43], and the observations were attributed to the absorption of resin [43]. In fact, PU is often used to prepare transparent films without a distinct absorption in NIR region [44]. Therefore, for the PU β-Bi2O3/ BaCO3 coating herein, other factors were responsible for the resulting lowered reflectance in the NIR region. The particle size of pigments also determines their reflectance. Jeevanandam et al. [45] observed that metal oxide nanoparticles had a higher NIR reflectance from 750 to 1300 nm relative to their micro-particles, yet the inverse was observed at wavelengths beyond 1300 nm and the difference increased with increasing wavelength. This paralleled very closely the β-Bi2O3/BaCO3 coating in Fig. 8a. Song et al. [46] also observed that the NIR reflectance of the coating they assessed increased as the particle size of the pigments increased. Therefore, complex pigments would present smaller particle size in PU coating relative to β-Bi2O3/BaCO3 powders, suggesting that β-Bi2O3/BaCO3 can be well dispersed into the PU coating and is suitable for application in cool coatings. The PU painting coating preserved a high solar NIR irradiance reflection, with a distinct decrease occurring only at 1500–2500 nm (Fig. 8b), in agreement with the reflectance results (Fig. 8a). However, the irradiance at 1500–2500 nm accounted for a very low proportion in solar NIR irradiance spectrum (Fig. 8b). Thus, the PU coating retained an R*NIR and R*sol of 84.47% and 72.07%, respectively. Studies by Kolokotroni et al. [47] showed that energy savings in a cooled office could be achieved when a roof reflectivity was 60–70%. An initial solar reflectance of 25% can meet the solar reflectance target for steep-sloped roofs [48]. The inner surface temperature of tiles varied with irradiation time (Fig. 8c). After 20 min of irradiance, an even temperature was reached for the blank tile and the pigment-coated PU film, with a corresponding temperature of 68.8 °C and 61.2 °C, respectively. Therefore, the complex pigment coat remained approximately up to 7.6 °C cooler than the pure PU coating.

Scheme 1. A plausible mechanism of formation of complex pigments.

[29,30]. β-Bi2O3/BaCO3 was shown to have a bright yellow color (Fig. 6b), and therefore a high tinting strength. To illustrate this, a film of polyurethane (PU) was painted with the bright yellow 2 wt% complex pigments (Fig. 6c) without any further processing. The PU film exhibited a homogeneous color, indicating that the complex pigments can be homogeneously dispersed directly in the organic matrix. The results above show that the complex pigments preserved the good color performance of the pure materials. Additionally, it is expected that they will also have a high reflectance in the NIR region. All complex pigments showed a similarly high reflectance (Fig. 7a) to that of pure β-Bi2O3 pigment (Fig. 1e), exceeding 90% for the optical spectrum. Therefore, the complex pigments maintained a high optical reflection. The solar irradiance reflection spectra are presented in Fig. 7b. Compared with the standard of global tilt, the complex pigments showed a much lower solar irradiance reflection spectrum at the wavelength of below 530 nm, attributed to the intrinsic absorption of βBi2O3 (Fig. 7a); at the wavelength of 530–2500 nm, especially in the NIR region of 700–2500 nm, the spectra of the complex pigments almost coincided with one of global tilt, indicating that the complex pigments also had a high NIR irradiance reflection. The NIR (700–2500 nm) reflectance value (R*NIR) of the complex pigments was estimated to be approximately 95% based on the NIR solar reflectance spectrum (Table 1). For colored pigments, an intrinsic absorption in the visible region results in a reduced visible reflection, and therefore NIR reflection is often used to evaluate the solar reflection performance [29,30]. NIR irradiance (700–2500 nm) accounts for approximately 52% of energy in the solar irradiance spectrum [42]. A high NIR reflection can significantly reduce the heat build-up, resulting in lower temperatures on the surface of objects [15]. Moreover, β-Bi2O3/BaCO3 complex pigments showed a high solar reflectance value (R*sol, 280–2500 nm) of over 80% (Table 1), indicating that β-Bi2O3/BaCO3 complex pigments can be used as cool pigments. To assess the heat reflection efficiency of complex pigments, a coating was painted on the ceramic tiles by mixing a 2 wt% complex pigment into PU paint. The UV-Vis-NIR diffuse reflection spectrum (Fig. 8a) showed that the coating had a high reflectance in the NIR region below 1000 nm. Some absorptions also occurred at 1200 nm, 1400 nm, 1700 nm, 1900 nm, and 2300–2500 nm, with the reflectance

4. Conclusions Herein, yellow Bi2O3/BaCO3 complex pigments were prepared by thermolysis of the precursor at low temperatures (350 °C–410 °C). βBi2O3 is the colored component of the prepared pigments. The complex pigments exhibited excellent color properties, close to those of ideal yellow pigments. The prepared complex pigments have a high NIR reflectance of above 95%, and the paint coating based as-prepared pigments efficiently lowered the temperature of the ceramic surface by 7.6 °C. BaCO3 could reduce the use of Bi2O3, in order to allow the relatively low-cost manufacture of these pigments.

Acknowledgments This research was funded by Chongqing Research Program of Basic Research and Frontier Technology (cstc2016jcyjA0027).

Fig. 6. Optical micrograph (a; scale bar 100 μm) and photograph (b) of the samples prepared at a 1:1 Bi:Ba ratio. Polyurethane film painted with the complex pigments (c). 105

Solar Energy Materials and Solar Cells 199 (2019) 99–107

J. Zou and Z. Yu

Fig. 7. UV-Vis-NIR diffuse reflection spectra (a) and solar irradiance reflection spectra based on AM 1.5 solar spectrum w/37° global tilt (b) for complex pigments with different ratio of Bi: Ba.

Fig. 8. UV-Vis-NIR diffuse reflection spectrum of the pigment film (a), solar irradiance reflective spectrum based on AM 1.5 solar spectrum w/37° global tilt (b), and inner surface temperatures of ceramic tiles coated with a pigment PU coating vary with irradiation time for film based on complex pigments with 1:1 ratio of Bi:Ba.

Appendix A. Supplementary data

[5] H. Gonome, M. Baneshi, J. Okajima, A. Komiya, S. Maruyam, Controlling the radiative properties of cool black-color coatings pigmented with CuO submi- cron particles, J Quant.Spectrosc.Radit. Transf. 132 (2014) 90–98. [6] X. Xue, J. Qin, J. Song, J. Qu, Y. Shi, W. Zhang, Z. Song, L. Jiang, J. Li, H. Guo, T. Zhang, The methods for creating energy efficient cool gray building coatings-Part I: preparation from white and black pigments, Sol. Energy Mater. Sol. Cells 130 (2014) 587–598. [7] R. Levinson, P. Berdahl, H. Akbari, W. Miller, I. Joedicke, J. Reilly, Y. Suzuki, M. Vondran, Methods of creating solar-reflective nonwhite surfaces and their application to residential roofing materials, Sol. Energy Mater. Sol. Cells 91 (2007) 304–314. [8] S. Jose, A. Prakash, S. Laha, S. Natarajan, M.L. Reddy, Green colored nano-pigments derived from Y2BaCuO5: NIR reflective coatings, Dyes Pigments 107 (2014) 118–126. [9] L. Yuan, A. Han, M. Ye, X. Chen, L. Yao, C. Ding, Synthesis and characterization of environmentally benign inorganic pigments with high NIR reflectance: lanthanumdoped BiFeO3, Dyes Pigments 148 (2018) 137–146. [10] D. Schildhammer, G. Fuhrmann, L. Petschnig, H. Schottenberger, H. Huppertz, Synthesis and optical properties of new highly NIR reflective inorganic pigments RE6Mo2O15 (RE=Tb, Dy, Ho, Er), Dyes Pigments 140 (2017) 22–28. [11] S. Jose, A. Jayaprakash, S. Laha, S. Natarajan, K.G. Nishanth, M.L.P. Reddy,

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solmat.2019.04.031. References [1] G.M. Revel, M. Martarelli, M. Emiliani, A. Gozalbo, M.J. Orts, M.A. Bengochea, L.G. Delgado, A. Gaki, A. Katsiapi, M. Taxiarchou, I. Arabatzis, I. Fasaki, S. Hermanns, Cool products for building envelope-part I: development and lab scale testing, Sol. Energy 105 (2014) 770–779. [2] C. Ferrari, A. Libbra, A. Muscio, C. Siligardi, Design of ceramic tiles with high solar reflectance through the development of a functional engobe, Ceram. Int. 39 (2013) 9583–9590. [3] A. Synnefa, M. Santamouris, K. Apostolakis, On the development, optical properties and thermal performance of cool colored coatings for the urban environment, Sol. Energy 81 (2007) 488–497. [4] K.L. Uemoto, N.M.N. Sato, V.M. John, Estimating thermal performance of cool colored paints, Energy Build. 42 (2010) 17–22.

106

Solar Energy Materials and Solar Cells 199 (2019) 99–107

J. Zou and Z. Yu

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19] [20]

[21] [22]

[23]

[24]

[25]

[26]

[27] [28]

[29]

[30]

and tunable color properties, Ceram. Int. 42 (2016) 8198–8205. [31] Q. Gao, X. Wu, Y. Fan, Q. Meng, Color performance and near infrared reflectance property of novel yellow pigment based on Fe2TiO5 nanorods decorated mica Composites, Dyes Pigments 146 (2017) 537–542. [32] B. Huang, Y. Xiao, C. Huang, J. Chen, X. Sun, Environment-friendly pigments based on praseodymium and terbium doped La2Ce2O7 with high near-infrared reflectance: synthesis and Characterization, Dyes Pigments 147 (2017) 225–233. [33] C. Greaves, S.K. Blower, Structural relationships between Bi2O2CO3 and β-Bi2O3, Mater. Res. Bull. 23 (1998) 1001–1008. [34] R. Hu, X. Xiao, S. Tu, X. Zuo, J. Nan, Synthesis of flower-like heterostructured βBi2O3/Bi2O2CO3 microspheres using Bi2O2CO3 self-sacrifice precursor and its visible-light-induced photocatalytic degradation of o-phenylphenol, Appl. Catal. B Environ. 163 (2015) 510–519. [35] R.A. Eppler, Colorants for ceramics, in: R.E. Kirk, D.F. Othmer (Eds.), Encyclopedia of Chemical Technology, vol. 8, Wiley, New York, 1998, pp. 877–892. [36] V.S. Vishnu, S. Jose, M.L. Reddy, Novel environmentally benign yellow inorganic pigments based on solid solutions of samarium–transition metal mixed oxides, J. Am. Ceram. Soc. 94 (2011) 997–1001. [37] A.K.V. Raj, P.P. Rao, S. Sameera, V. James, S. Divya, Synthesis of novel nontoxic yellow pigments: Sr2Ce1-xTbxO4, Chem. Lett. 43 (2014) 985–987. [38] P. Sulcova, M. Trojan, Thermal synthesis of the (Bi2O3)1–x(Er2O3)x pigments, J. Therm. Anal. Calorim. 88 (88) (2007) 111–113. [39] S. Sengupta, A. Ghatak, S. Sett, M. Sreemany, S. Bysakh, B. Ghosh, A.K. Raychaudhuri, Restoration of perovskite phase in the top layer of thin BTO film by plasma treatment and annealing, J. Phys. D Appl. Phys. 51 (2018) 085304. [40] A.Shiraishi Wendusu, N. .Takeuchi, T. Masuiaand, N. Imanaka, Novel environment friendly inorganic red pigmentsbased on Bi4V2O11, RSC Adv. 5 (2015) 44886–44894. [41] L. Jiang, X. Xue, J. Qu, J. Qin, J. Song, Y. Shi, W. Zhang, Z. Song, J. Li, H. Guo, T. Zhang, The methods for creating energy efficient cool gray building coatingsPartII: preparation from pigments of complementary colors and titanium dioxide rutile, Sol. Energy Mater. Sol. Cells 130 (2014) 410–419. [42] R. Levinson, P. Berdahl, H. Akbari, Solar spectral optical properties of pigments–Part II: survey of common colorant, Sol. Energy Mater. Sol. Cells 89 (2005) 351–389. [43] A.E. Smith, M.C. Comstock, M.A. Subramanian, Spectral properties of the UV absorbing and near-IR reflecting blue pigment YIn1-xMnxO3, Dyes Pigments 133 (2016) 214–221. [44] Z. Chen, Y. Gao, L. Kang, C. Cao, S. Chen, H. Luo, Fine crystalline VO2 nanoparticles: synthesis,abnormal phase transition temperatures and excellent optical properties of a derived VO2 nanocomposite foil, J. Mater. Chem. 2 (2014) 2718–2727. [45] P. Jeevanandam, R.S. Mulukutla, M. Phillips, S. Chaudhuri, L.E. Erickson, K.J. Klabunde, Near infrared reflectance properties of metal oxide nanoparticles, J. Phys. Chem. C 111 (2007) 1912–1918. [46] J. Song, J. Qin, J. Qu, Z. Song, W. Zhang, X. Xue, Y. Shi, T. Zhang, W. Ji, R. Zhang, H. Zhang, Z. Zhang, X. Wu, The effects of particle size distribution on the optical properties of titanium dioxide rutile pigments and their applications in cool nonwhite coatings, Sol. Energy Mater. Sol. Cells 130 (2014) 42–50. [47] M. Kolokotroni, B.L. Gowreesunker, R. Giridharan, Cool roof technology in London: an experimental and modelling study, Energy Build. 67 (2013) 658–667. [48] R. Levinson, H. Akbari, P. Berdahl, K. Wood, W. Skilton, J. Petersheim, A novel technique for the production of cool colored concrete tile and asphalt shingle roofing products, Sol. Energy Mater. Sol. Cells 94 (2010) 946–954.

YIn0.9Mn0.1O3-ZnO nano-pigment exhibiting intense blue color with impressive solar reflectance, Dyes Pigments 124 (2016) 120–129. A. Moghtada, A. Shahrouzianfar, R. Ashiri, Facile synthesis of NiTiO3 yellow nanopigments with enhanced solar radiation reflection efficiency by an innovative onestep method at low temperature, Dyes Pigments 139 (2017) 388–396. A.K.V. Raj, P.P. Rao, T.S. Sreena, T.R.A. Thara, Pigmentary colors from yellow to red in Bi2Ce2O7 by rare earth ion substitutions as possible high NIR reflecting pigments, Dyes Pigments 160 (2019) 177–187. D. Schildhammer, G. Fuhrmann, L. Petschnig, N. Weinberger, H. Schottenberger, H. Huppertz, Synthesis and characterization of a new high NIR reflective ytterbium molybdenum oxide and related doped pigments, Dyes Pigments 138 (2017) 90–99. J. Zou, P. Zhang, C. Liu, Y. Peng, Highly dispersed (Cr, Sb)-co-doped rutile pigments of cool color with high near-infrared reflectance, Dyes Pigments 109 (2014) 113–119. B. Huang, Y. Xiao, H. Zhou, J. Chen, X. Sun, Synthesis and characterization of yellow pigments of Bi1.7RE0.3W0.7Mo0.3O6 (RE=Y, Yb, Gd, Lu) with high NIR reflectance, ACS Sustain. Chem. Eng. 6 (2018) 10735–10741. S. Sameera, P.P. Rao, S. Divya, A.K.V. Raj, T.R.A. Thara, High IR reflecting BiVO4CaMoO4 based yellow pigments for cool roof applications, Energy Build. 154 (2017) 491–498. T. Tsukimori, R. Oka, T. Masui, Synthesis and characterization of Bi4Zr3O12 as an environment friendly inorganic yellow pigment, Dyes Pigments 139 (2017) 808–811. P. Meenakshi, M. Selvaraj, Bismuth titanate as an infrared reflective pigment for cool roof coating, Sol. Energy Mater. Sol. Cells 174 (2018) 530–537. L.S. Kumari, P.P. Rao, A.N.P. Radhakrishnan, V. James, S. Sameera, P. Koshy, Brilliant yellow color and enhanced NIR reflectance of monoclinic BiVO4 through distortionin VO43- tetrahedral, Sol. Energy Mater. Sol. Cells 112 (2013) 134–143. T. Masui, T. Honda, Wendusu, N. Imanaka, Novel and environmentally friendly (Bi,Ca,Zn)VO4 yellow pigments, Dyes Pigments 99 (2013) 636–641. S. Sameera, P. Prabhakar Rao, V. James, S. Divya, A.K.V. Raj, Influence of (LiLa)1/ 2MoO4 substitution on the pigmentary properties of BiVO4, Dyes Pigments 104 (2014) 41–47. L. Guan, J. Fan, Zhang, Y. Guo, H. Duan, Y. Chen, H. Li, H. Liu, Facile preparation of highly cost-effective BaSO4@BiVO4 core-shell structured brilliant yellow pigment, Dyes Pigments 128 (2016) 49–53. Y. Wang, Y. Wen, H. Ding, Y. Shan, Improved structural stability of titanium-doped β-Bi2O3 during visible-light-activated photocatalytic processes, J. Mater. Sci. 45 (2010) 1385–1392. Y. Sun, W. Wang, L. Zhang, Z. Zhang, Design and controllable synthesis of α-/γBi2O3 homojunction with synergetic effect on photocatalytic activity, Chem. Eng. J. 211 (2012) 161–167. Y. Yan, Z. Zhou, Y. Cheng, L. Qiu, C. Gao, J. Zhou, Template-free fabrication of αand β-Bi2O3 hollow spheres and their visible light photocatalytic activity for water purification, J. Alloy. Comp. 605 (2014) 102–108. K. Kusumoto, Synthesis of Bi2O3–Nb2O5 solid solutions for environmental-friendly reddish yellow pigments, J.Ceram.Soc. Jap. 124 (2016) 926–928. C.L. Gomez, O. Depablos-Rivera, P. Silva-Bermudez, S. Muhl, A. Zeinert, M. Lejeune, S. Charvet, P. Barroy, E. Camps, S.E. Rodil, Opto-electronic properties of bismuth oxide films presenting different crystallographic phases, Thin Solid Films 578 (2015) 103–112. W. Zheng, J. Zou, Synthesis and characterization of blue TiO2/CoAl2O4 complex pigments with good colour and enhanced near-infrared reflectance properties, RSC Adv. 5 (2015) 87932. J. Zou, W. Zheng, TiO2@CoTiO3 complex green pigments with low cobalt content

107