Metal-organic frameworks (MOFs), are crystalline, micro to mesoporous functional materials consisting of inorganic clusters interconnected by organic linkers. The possibility of tailoring the chemical functionality and pore size while maintaining the framework structure, a concept termed isoreticularity, makes these materials promising for adsorption based processes, such as heat pumps, catalysis, and particularly photo-catalysis, gas storage, drug delivery, sensing and imaging. To target those applications isoreticular MOFs composed of mixtures of linkers, referred to as multivariate MOFs or MixMOFs, is one of the latest achievements in the field. However, along with the benefits of multivariate MOF complexity comes the challenge of spatially resolving the distribution of the constituent building blocks within MOF crystallites. For example, crystal homogeneity is a prerequisite for advanced applications in catalysis or sensing but determining the chemical composition at the nanoscale in such materials remains elusive due to the limited spatial resolution of conventional techniques. This lack of spatially resolved information hinders fundamental understanding of these materials and consequently the ability to engineer them for greatest efficacy. In this work, the local chemical composition of individual MixMOF micro-crystals is determined for the first time with nanoscale resolution using the Photo Thermal Induced Resonance (PTIR) technique, a novel method that combines the lateral resolution of atomic force microscopy (AFM) with the chemical specificity of infrared (IR) spectroscopy. PTIR experiments show that MixMOFs isoreticular to In-MIL-68, made either directly from solution or by post-synthetic linker exchange are homogeneous down to a length scale of ≈ 100 nm. Additionally, we report an an in situ process for engineering anisotropic domains in MOFs with a concentration gradient occurring within ≈ 600 nm, as revealed by PTIR chemical maps.
Citation: Angewandte Chemie-International Edition
Pub Type: Journals
Nanoscale Chemical Imaging, MOFs, PTIR