Development of 2D and 1D ZnO Materials via Self-Assembly of Liquid-Crystalline Zinc Hydroxide Carbonate

Liquid crystals are soft materials that exhibit liquid-like fluidity and solid-like ordered structure. Various functional materials based on assembled structures of liquid crystals have been reported. The most common liquid crystals are molecular-based liquid crystals. Inorganic colloidal particles with anisotropic shapes such as rods and disks also form liquid-crystalline (LC) phases. Inspired by the formation process of biominerals such as seashells, bones, and teeth, we developed “biomineral-inspired colloidal liquid crystals”. These are organic/inorganic hybrid nanoparticles with rod and disk shapes. The organic components of the colloidal liquid crystals are acidic polymers that interact with inorganic cation species, and the inorganic components are calcium carbonate or calcium phosphate. Most of the biominerals are hybrids of calcium salts and biomacromolecules, while some ants and scorpion hybridize heavy metal elements such as zinc and manganese with biomacromolecules as their stings and fangs. Inspired by these biological materials, we aim to synthesize zinc-based colloidal liquid crystals. Herein, we report the synthesis of new colloidal liquid crystals based on zinc hydroxide carbonate (ZHC) nanoplates. Oriented zinc oxide (ZnO) materials obtained through self-assembly of ZHC nanoplates and subsequent thermal treatment are also described.<br/>The nanoplates of ZHC crystals were obtained by mixing aqueous Zn(NO₃)₂ and (NH₄)₂CO₃ solutions in the presence of poly(acrylic acid) (PAA) as an additive. The hybridization of PAA with ZHC crystals was supported by Fourier transform infrared (FTIR) spectras. The structures of the mesogenic ZHC nanoplates were clarified by transmission electron microscope (TEM) and X-ray diffraction (XRD) measurements. The TEM images showed that the nanoplates had polycrystalline structures. The cross-sectional TEM images and XRD patterns suggested a layered structure with stacked 2-nm thick ZHC nanosheets. A concentrated colloidal dispersion of ZHC showed a LC texture under polarizing optical microscope (POM). In small-angle X-ray scattering (SAXS) measurements, the LC phase showed anisotropic scattering, which suggests the presence of ordered structure in the colloidal dispersion. We assume that nanoplates of ZHC formed discotic nematic liquid crystals in their dispersions.<br/>We have achieved the development of 2D and 1D ZnO materials by simple methods utilizing the ZHC liquid crystals. The ZHC liquid crystals were macroscopically oriented by external stimuli. Transparent ZHC self-standing thin films were obtained by sandwiching the ZHC liquid crystals between fluorinated polymer sheets and drying them at ambient conditions. The POM images and XRD analysis showed that the ZHC nanoplates were oriented and stacked along the interface of the thin-film surface during drying process. The ZHC thin films were converted to self-standing ZnO thin films by annealing. The annealed samples maintained thin-film morphologies. Out-of-plane and in-plane XRD patterns suggested the <i>c</i> axis of ZnO crystals was oriented perpendicular to the thin-film surface. A nanoporous structure due to the water evaporation and the decomposition of PAA was observed by scanning electron microscope (SEM). 1D ZnO fibrous materials were synthesized by thermal treatment of ZHC fibers prepared through injecting the liquid crystals into acetone solvent. The POM images of ZHC fibers revealed the formation of macroscopically anisotropic structures. These results suggested that the ZHC-nanoplates planes were aligned parallel to the injecting direction due to the shear stress and rapid diffusion of water into acetone. The SEM observation showed the ZnO fibers had nanoporous structures as ZnO thin films. As these 2D and 1D ZnO materials have oriented, nanoporous, and polycrystalline structures, they are expected to be used for varistors and catalyst materials.