A simple way to fabricate pure anatase 2D TiO2 IO monolayer: structure, color control and its application in electrochromism

Abstract

Pure anatase two dimensional (2D) TiO₂ inverse opal (IO) films, consisting of a highly ordered hexagonal-patterned structure, are synthesized from various sized polystyrene spheres (PS) as colloidal template simply coupled with TiOSO₄ aqueous solution as TiO₂ precursor using a “dynamic-hard-template infiltration” strategy. Herein, the TiOSO₄ solution is directly infiltrated into the interstices of the 2D self-assembled PS opal template at an air/water interface resulting in a TiOSO₄/PS opal composite film floating on the surface of water which was further deposited onto ITO or silicon substrates. Calcination of the obtained opal composite films at temperatures ranging from 300 to 550 °C resulted in 2D TiO₂ IO films with various pore sizes having an inverse moth's eye structure. Based on EDS measurements, sulfur ions S⁶⁺ were detected in the IO films calcined up to 550 °C. In order to eliminate these S⁶⁺ ions and obtain pure anatase 2D TiO₂ IO, aqueous immersion was performed after calcination without disturbance of the IO ordered structure. Surface morphology, crystal phase and optical transmittance of the TiO₂ IO films, were concurrently investigated by SEM, Raman and UV-vis-NIR. Owing to their precisely adjustable structure, the obtained TiO₂ 2D IO films exhibited structural colors varying from pale purple, to blue, to polychrome as the array period increases. The films obtained on ITO substrates were successfully used as active electrodes in the fabrication of electrochromic (EC) devices.