Hydrothermal synthesis and characterization of novel yellow pigments based on V5+ doped BiPO4 with high near-infrared reflectance

In this study, a series of novel pigments based on V5+ doped BiPO4 have been prepared for the first time via a facile hydrothermal method and characterized using several analytical techniques, such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), ultraviolet-visible-near-infrared (UV-vis-NIR) spectrometry, the Commission International de l'Eclairage (CIE) L*a*b* color scales and thermogravimetry and differential thermal analysis (TG-DTA). The investigation demonstrated that the synthesized pigments of BiP1-xVxO4 (x = 0.00, 0.01, 0.05, 0.08, 0.10, 0.15) had a monazite-type phase structure and were about 0.25-2 m in size. Meanwhile, the substitution of V5+ for P5+ in BiPO4 resulted in the band gap of the pigments varying from 3.657 to 3.244 eV and its mechanism was explained by charge-transfer and energy band theory, while the color changed from white to yellow. More importantly, the V5+ doped pigments possessed high NIR reflectance (>72%) and NIR solar reflectance (75.64%) in the range 700-2500 nm. Moreover, coatings colored with synthetic pigments have higher NIR solar reflectance (78.59%) than conventional pigments. Additionally, the pigments showed good thermal/chemical stabilities in high-temperature/acid/alkaline tests. In conclusion, the pigments have the potential to be applied as cool pigments to reduce energy consumption.