Highly dispersed (Cr, Sb)-co-doped rutile pigments of cool color with high near-infrared reflectance
Introduction
The heat build-up during summer caused by solar radiation negatively affects comfort in the built environment. This condition is worse in densely urbanized areas because of the “urban heat island” effect, which produces temperatures 3 °C–5 °C higher than those in neighboring rural areas [1]. Air conditioning is usually used to adjust the ambient temperature; however, such a tool significantly increases energy consumption. The adoption of cool roofs may serve as an alternative to air conditioning [1], [2], [3], [4], [5], [6]. Cool roofs with high near-infrared (NIR) reflectance can reflect a significant fraction (52%) of the solar energy that arrives as NIR radiation [5], thereby reducing the heat gain at the roof surface. Consequently, cool roofs can improve internal thermal comfort in buildings without air conditioning.
Cool roofing products are made of highly reflective cool materials and can remain cooler than traditional materials when irradiated [5], [7]. White pigments with high solar reflectance are often used as cool materials. However, the necessity for cool nonwhite products arose because the aesthetics of dark colors is usually preferred [5]. Dark pigments also have high NIR reflectance [8], [9], [10], [11], [12], although they absorb visible light. Mo6+- and Pr4+-doped yttrium cerate pigments exhibit a bright yellow hue with NIR solar reflectance exceeding 90% [8]. The yellow pigment BiVO4 doped with Ta has an NIR solar reflectance of up to 86.6% [12], and this pigment doped with Ca and Zn exhibits a high b* of >90 [13]. The blue pigments YMnO3 [11] and SrCuSi4O10 doped with Fe and with La and Li [10] also present high NIR reflectance, respectively.
Rutile TiO2, a white pigment with high solar reflectance, is a widely used cool material. Cr and Sb co-doped TiO2 presents brilliant orange yellow [14]; hence, it may be used as a nonwhite cool pigment. In our previous work, we synthesized yellow rutile TiO2 pigments at low temperature; however, the produced pigments had a large size of 1 μm–10 μm and irregular morphology [14]. Small particles have higher NIR reflectance than large particles [15]. Therefore, highly dispersed Cr and Sb co-doped TiO2 pigments with small particles were prepared in the present study to obtain high NIR reflectance.
Section snippets
Materials
Dispersants (Kelong Chemical): Polyethylene glycol (PEG 300, PEG 600, PEG 1000, PEG 1000), Polyacrylamide (PAM, 3000000), Polyvinylpyrrolidone (PVP-K30); titanium tetrachloride(TiCl4, Sinopharm Chemical Reagent Co. Ltd., China.), polyurethane (PU) paint (Dü Fang, Acryl PU-Klalack), High-density polyethylene (HDPE, PetroChina Co. Ltd. of Lanzhou Petrochemical Company), chromium chloride(CrCl3), antimony chloride(SbCl3), ammonia(28%) were used as received.
Synthesis of (Cr, Sb)-co-doped TiO2 pigments
Rutile pigments of cool color were
Results and discussion
We previously reported that Cr-doped rutile pigments can be facilely synthesized at a low temperature of 700 °C; however, these pigments presented irregular morphology and small sizes ranging from 1 μm to 10 μm [14]. In the present work, dispersants were employed to improve the morphology and size distribution. As shown in Fig. 1, highly dispersed pigments were obtained by adding the dispersant PEG 1000. The amount of dispersant is important to obtain regular morphology and narrow size
Conclusion
Highly dispersed (Cr, Sb)-co-doped rutile TiO2 pigments were prepared by adding dispersants during precursor hydrolysis. The as-prepared pigments presented spherical morphology and narrow size distribution (0.5 μm–1 μm). The pigments had high luminosity and b* values. The formation of hexavalent chromium and oxygen defects can be avoided by co-doping with Cr and Sb. The co-doped pigments had high NIR reflectance because of the absence of Cr6+ in the NIR range. The temperature in the inner
Acknowledgments
The work described in this paper was supported by a grant from “China Postdoctoral Science Foundation funded project (2013M531925)” and “Fundamental Research Funds for the Central Universities (XCJK2014C038)”.
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