Design of reddish-brown iron-doped SrMoO4 pigments with high NIR reflectance for cool coating applications

The synthesis of colored near infrared (NIR) reflective pigments offers a promising approach for cool coating applications. In the present investigation, reddish-brown Sr 1-x Fe x MoO 4 (x = 0-0.5) pigments were synthesized by the hydrothermal method. Powder X-ray diffraction (PXRD) results showed phase purity, while Fourier Transform Infrared (FT-IR) spectra showed O-Mo-O vibrations of the molybdate framework. SEM and TEM studies showed particle agglomeration with good crystallinity. The presence of Sr 2+ , Mo 5+ /Mo 6+ , and Fe 2+ /Fe 3+ states and lattice oxygen was confirmed by XPS results. Upon Fe addition, the optical band gap decreases from 4.22 eV to 1.87 eV, resulting in a color change from white to reddish brown, while maintaining Near Infrared (NIR) reflectance (82.36-58.43%). Additionally, the pigment exhibits good chemical durability and excellent thermal stability. Among the series, Sr 0.8 Fe 0.2 MoO 4 (SMOF2) composition showed an average temperature reduction at the surface and 3.6 °C at the interior temperature under IR lamp irradiation. Under outdoor solar exposure (real-time study), Sr 0.8 Fe 0.2 MoO 4 (SMOF2) exhibited a favorable thermal response compared with the reference coatings. These results demonstrate the potential of Fe-doped SrMoO 4 as an environmentally benign pigment platform for next-generation cool-roof and cool-coating applications.