New One Step Self-assembly Strategy of Large-Area Highly Ordered, Crack-Free 2D Inverse Opal Films of Transition Metal Oxides and Its Application to Fabrication of Bilayer Inverse Opal Films

A new one step method for self-assembly of sacrificial polystyrene (PS) spheres colloidal crystal with a transition metal oxide (TMO) precursor matrix material is developed to fabricate highly ordered, crack-free two-dimensional (2D) TiO2 and 2D WO3 inverse opal (IO) films over a centimeter scale. Opal composite monolayer PS/WO3 and PS/TiO2 films have been successfully generated via simultaneous assembly of polymeric colloidal spheres floating on aqueous TiO2 and WO3 precursor solutions, thereby avoiding the infiltration step of the TMO precursor solution into a preassembled opal template. Large-area crack-free 2D WO3 and 2D TiO2 IO films were subsequently obtained after the removal of the PS opal template by chemical method. Such new strategy avoids the need for liquid infiltration into a preassembled PS opal template and minimizes the associated large cracks and inhomogeneity that often occur during the fabrication of IO films. The obtained PS/WO3 and PS/TiO2 opal composite monolayer films were also used as building blocks for the fabrication of highly ordered bilayer (3D) IO films with homo- and hetero-structure via a bottom-up, layer bylayer route, through repeated operations of this new one step self-assembly method. Hence, four types of bilayer IO films were synthesized: homo-structural TiO2 and WO3, hetero-structural TiO2 (bottom layer)/WO3 (top layer) and WO3 (bottom layer)/TiO2 (top layer). The underlying mechanism of multilayer assembly that may account for the formation of large area crack-free multilayer films, is discussed. The electrochromic behavior of these 2D and 3D IO samples was investigated.