Synthesis of ZnO inverse opals with high crystalline quality by a three-dimensional colloidal crystal template-assisted hydrothermal method over a seed layer

Abstract

In this paper, zinc oxide (ZnO) inverse opal structures were fabricated on a seed layer using three-dimensional (3D) colloidal crystal templates. The 3D colloidal crystal templates hindered the diffusion of zinc ions during hydrothermal growth thereby restricting the formation of porous ZnO structures at typical Zn²⁺ concentrations leading to rods. Instead of porous ZnO microstructures, ZnO nanobowl films were exclusively grown using the 3D colloidal templates on the photoresist-patterned seed layer at the initial Zn²⁺ concentrations. 3D porous ZnO with different filling fractions were successfully created by a hydrothermal method at a Zn²⁺ concentration of 0.15 M with different adjuvants. The addition of sodium citrate yielded ZnO having inverse opal structures and showing variable reflection peaks according to the photonic stop band, which were controlled by the diameter of the template colloidal spheres. ZnO inverse opals prepared with sodium citrate as an adjuvant also showed superior ultraviolet photoluminescence intensity values compared to their adjuvant-free and polyethylenimine counterparts. The hydrothermal method resulted in ZnO inverse opal structures with significantly enhanced crystalline quality compared to those prepared by electrodeposition methods. ZnO inverse opals prepared by large-diameter colloidal crystal templates showed slightly better properties than those prepared using small-diameter templates. The ZnO inverse opals conformally coated by a thin layer of TiO₂ were used as photoanodes for water splitting. The ZnO inverse opals prepared by the hydrothermal method showed significantly higher photocurrent values than those synthesized by electrodeposition. These ZnO inverse opals showed maximum photocurrent values of ca. 0.9 mA cm⁻² comparable to the best results of other ZnO-based microstructures.