Identifying Structure and Texture of Metal-Organic Framework Cu2(bdc)2(dabco) Thin Films by Combining X‑ray Diffraction and Quantum Mechanical Modeling.

Abstract

This study describes a strategy for unambiguously determining metal-organic framework (MOF) thin film structures, which is demonstrated for a pillar-layer MOF consisting of Cu paddlewheel nodes connected by benzene-1,4-dicarboxylate (bdc) linkers and 1,4-diazabicyclo[2.2.2]-octane (dabco) pillars. An initial structural model is derived by isostructural replacement from the material's Zn²⁺ analogue. This is followed by a structure optimization using density functional theory. The model is supported by comparing calculated and measured diffraction patterns and infrared spectra for two differently grown thin films. Key to verifying the structure and identifying the thin film texture are grazing incidence X-ray diffraction (GIXD) experiments with rotating samples. These probe the majority of reciprocal space and thus also allow a straightforward generation of pole figures for various diffraction peaks. Two types of films are prepared either by layer-by-layer deposition or by ceramic-to-MOF conversion. Both share the same phase but display clearly different textures: a uniplanar texture in the case of the layer-by-layer grown film and a distorted axial texture with an epitaxial alignment between MOF and Cu-(OH)₂ crystallites for the ceramic-to-MOF-converted film. The variations in the texture follow from differences in the substrate surfaces. Our findings highlight the potential of performing GIXD experiments on rotating samples (augmented by theoretical modeling) to (i) determine the texture of MOF thin films and (ii) to solve MOF crystal structures from thin film data even for strongly varying textures.