Hollow Rodlike MgF₂ with an Ultralow Refractive Index for the Preparation of Multifunctional Antireflective Coatings

Antireflective coatings with superhydrophobic, self-cleaning, and wide-spectrum high-transmittance properties and good mechanical strength have important practical value. In this research, hollow nanorod-like MgF₂ sols with different void volumes were prepared by a template-free solvothermal method to further obtain hollow nanorod-like MgF₂ crystals with an ultralow refractive index of 1.14. Besides, a MgF₂ coating with an adjustable refractive index of 1.10-1.35 was also prepared by the template-free solvothermal method. Then through the combination of base/acid two-step-catalyzed TEOS and hydroxyl modification on the surface of nanosilica spheres, the SiO₂ coating with good mechanical strength, a flat surface, and a refractive index of 1.30-1.45 was obtained. Double-layer broadband antireflective coatings with an average transmittance of 99.6% at 400-1400 nm were designed using the relevant optical theory. After the coating thickness was optimized by the dip-coating method, the double-layer antireflective coatings, whose parameters were consistent with those designed by the theory, were obtained. The bottom layer was a SiO₂ coating with a refractive index of 1.34 and a thickness of 155 nm, and the top layer was a hollow rodlike MgF₂ coating with a refractive index of 1.10 and a thickness of 165 nm. The average transmittance of the obtained MgF₂-SiO₂ antireflective coatings was 99.1% at 400-1400 nm, which was close to the theoretical value. The hydrophobic angle of the coating surface reached 119° at first, and the angle further reached 152° after conducting surface modification by PFOTES. In addition, because the porosity of the coating surface was only 10.7%, the pencil hardness of the coating surface was 5 H and the critical load Lc was 27.05 N. In summary, the obtained antireflective coatings possessed superhydrophobic, self-cleaning, and wide-spectrum high-transmittance properties and good mechanical strength.