Truncated Tetragonal Bipyramidal Crystal Growth of Calcium Squarate Metal–Organic Framework Controlled by the Addition of α-Hydroxy Carboxylates

The morphological control of crystals is a critical design strategy for maximizing the functional performance of metal–organic frameworks (MOFs). Herein, we focus on modulating the crystal growth of a calcium squarate (Ca-squarate) MOF, chemically formulated as Ca(C4O4)·xH2O─a promising candidate for CO2 capture. This MOF typically crystallizes in a needle-like morphology (aspect ratio (width/length) range: 0.1–0.3) due to preferential growth along the c-axis of its tetragonal lattice; however, effective strategies to control its morphology are lacking. We report a novel approach to systematically tune its morphology using carboxylate additives. We demonstrate that α-hydroxy carboxylates specifically induced a morphological transition from an anisotropic needle-like form to a more isotropic cubic-like one─a truncated tetragonal bipyramidal crystal growth. With the addition of α-hydroxy carboxylates, the average aspect ratio significantly increased to a maximum of 0.73. Combined experimental and theoretical analyses revealed that this morphological evolution was driven by a “molecular mimicry” mechanism, where the additive mimicked the coordination geometry of the squarate ligand. This mimicry led to a site-selective competitive adsorption on the (001) and high-index facets, effectively suppressing the intrinsic anisotropic growth. These findings provide new guidelines for the rational design of MOF morphologies to enhance material properties.