Metal-Organic Framework Crystal Growth in Microgravity

Crystals grown in microgravity have been shown to be larger in size, to be more uniform in size and morphology, and to contain fewer structural defects relative to Earth-grown crystals. This is due to the homogenous, diffusion-controlled growth in microgravity that results from the lack of gravity-induced convection and sedimentation. Metal-organic frameworks (MOFs) are a unique class of materials that consist of a central metal ion coordinated to organic ligands that extend to form two- or three-dimensional structures. MOFs have promising applications in electrocatalysts, photovoltaics, thermoelectrics, and electrical energy storage materials due to their tunable chemical, electrical, and mechanical properties. Persistent microgravity provides an ideal environment to study the growth of larger, higher-quality MOF crystals that will lead to more sustainable microelectronics.<br/>The International Space Station (ISS) National Laboratory offers a unique platform in persistent microgravity that enables the elimination of gravity-driven forces to elucidate the fundamental mechanisms of growth, morphology, and defect generation in MOF crystals. An improved understanding of the fundamental mechanisms of MOF crystal formation will lead to the growth of higher-quality MOF crystals both in space and on Earth. Several organic and inorganic crystals have been grown onboard the ISS, including proteins, semiconductors, and graphene aerogels.<br/>In this work, we will introduce the underlying physical phenomena of crystal growth in microgravity. We will present information on current ISS National Lab-sponsored research projects that are growing HKUST-1 3D MOF crystals, and hexaaminotriphenylene (HITP) and hexaaminobenzene (HIB) linker-based 2D MOF crystals. We will also discuss translational lessons learned from microgravity experiments that inform and direct terrestrial research and manufacturing. Finally, we will present opportunities for future microgravity experiments and access to ISS facilities through the ISS National Lab.