Abstract
Block copolymers (BCPs) that consist of chemically different components can self-assemble into various ordered nanostructures due to the incompatibility of constituted blocks. Among them, double gyroid (DG) phase with three-dimensional (3D) and bi-continuous networks is one of the appealing morphologies for practical applications due to its unique texture. In this study, we aim to take advantage of the degradable character of ester groups in polylactide-containing BCPs to create nanoporous polymeric materials with DG-forming nanochannels by hydrolysis of polylactide block in polystyrene-block-poly(L-lactide) PS-PLLA block copolymer. Subsequently, the nanoporous polymer can be served as a template for templated atomic layer deposition (ALD). Consequently, a variety of well-defined polymer/metal oxide nanohybrids can be fabricated. To acquire large-area, well-oriented nanostructured thin films from the self-assembly of the PS-PLLA, dip-coating is used in this study because of its low-cost and waste-free process. The thickness and morphologies of the coated film can be precisely controlled via this approach, giving well-ordered gyroid-forming bulk films with micrometer thickness. Consequently, templated ALD is carried out by using nanoporous PS from hydrolyzed PS-PLLA as a template to fabricate PS/ZnO DG nanohybrids. Unlike 1D structure, the 3D bi-continuous networks which have more complex interconnected channels require an optimization of processing parameters for ALD such as exposure time to the precursor gases, precursor partial pressure and deposition temperature. With the appropriate control of those factors, the gyroid-forming metal oxides can be successfully fabricated. Although well-defined nanohybrids can be synthesized via templated sol-gel reaction, electrochemical deposition, and electroless plating, it is difficult to control the reaction process to form complex morphologies, such as nanotubes with tunable thickness. As demonstrated in this study, micrometer-thick nanoporous DG ZnO with high porosity and high specific surface area can be successfully fabricated after calcination. Owing to the easy diffusion ability for pore-filling process, the templated ALD is very appealing for the fabrication of nanotubes with tunable thickness through the control of the number of ALD cycles. Moreover, there are many available recipes for the deposition of various metal oxides. As a result, alternating reaction process can be carried out for the formation of more complex morphologies, in particular core-shell morphologies. For instance, with the alternating ALD process with different precursors, ZnO@Al2O3 core-shell metal oxide alloys in PS matrix with DG-forming morphology can be obtained. For applications, nanoporous gyroid ZnO can be acquired after removal of PS template for fabricated PS/ZnO nanohybrids. One of the straightforward applications is the manufacture of a dye-sensitized solar cell (DSSC). As demonstrated in this study, the unique texture of the gyroid-forming nanoporous ZnO provides a good framework for the deposition of dyes to give higher efficiency for power conversion.