Hollow silica nanospheres synthesized by one-step etching method to construct optical coatings with controllable ultra-low refractive index

It is generally believed that silica nanospheres deposited on substrates by sol-gel method to form coatings with low refractive index, if hollow structures are introduced into silica nanospheres, the obtained coatings can easily have an ultra-low refractive index (<1.15). However, with the currently reported techniques, facile and large-scale preparation of hollow silica nanospheres (HSNs) with diameters suitable for transparent optical coatings in a one-step method is hard to realize. Therefore, it is crucial to propose a one-step strategy to prepare HSNs colloids, and also to achieve controllable morphologies. Stöber silica nanospheres were used as initial nanospheres, and diameters of nanospheres were controlled by the concentration of ammonia. Nanospheres of different diameters were etched using different concentrations of HF, and the structural transformation of nanospheres before and after etching was recorded. Etched nanospheres were deposited into coatings, refractive index was measured and appropriate coatings were selected to design and prepare a double-layer coating with broadband antireflection performance. Experiments indicated that nanospheres of different sizes exhibited different structural transformations after being etched by HF, and smaller nanospheres were aggregated and larger ones were hollowed out. It was the first time that HSNs have been prepared by one-step etching method. The refractive index of etched nanospheres was 1.387–1.057. The double-layer broadband antireflective coating designed and prepared using coatings with refractive index of 1.057 and 1.339 as upper and bottom layers respectively had an average transmittance of 99.5% in the visible region and exhibited a good antifogging property and weather durability.