Vibrational Fingerprinting of Defects Sites in Thin Films of Zeolitic Imidazolate Frameworks.

Surface-mounted metal-organic frameworks (SURMOFs) are crystalline films of MOFs and have garnered a great deal of attention in the past years. So far, thin-film MOF research has been mainly focused on the synthesis and the exploration of potential applications of these materials, while a detailed understanding of their growth is still lacking. In this report evidence is provided for the inter-grown nature of surface-mounted thin films of Zn-ZIF-8 (SURZIF-8; ZIF=zeolitic imidazolate framework). Two distinct SURZIF-8 thin films have been made through layer-by-layer (LBL) growth after applying 20 and 50 LBL cycles. They have been characterized with atomic force microscopy (AFM) and Raman micro-spectroscopy. A detailed analysis of the Raman mapping data, inter alia using principal component analysis (PCA), revealed the existence of phase boundaries within the 20-cycle thin film, while the 50-cycle thin film is chemically more homogeneous. To further analyze these chemical heterogeneities, density functional theory (DFT) calculations were performed of three theoretical models providing spectroscopic fingerprints of the molecular vibrations associated with the Zn-ZIF-8 thin films. Based on these calculations and the experimental data distinct vibrational markers indicative for the presence of defects sites were identified.

Behavior of a Metal Organic Framework Thin-Film at Elevated Temperature and Pressure as Studied with an Autoclave-Inserted Atomic Force Microscope.

Bridging the gap in studying surface reactions, processes, and morphology and measuring at (catalytic) relevant conditions is crucial for our understanding of the working principles of porous crystalline materials. Scanning tunneling microscopy is limited because of the required conductivity of the sample, whereas atomic force microscopy (AFM) is often challenging in use owing to the physical mechanism underlying the technique. Herein, we report a tailor-made autoclave-inserted AFM, able to measure at ∼20 bar and ∼110 °C. First, we show the ability to obtain nanometer resolution on well-defined test samples at before-mentioned conditions. Second, to demonstrate the possibilities of analyzing morphological evolutions at elevated temperatures and pressures, we use this setup to measure the stability of a surface-anchored metal-organic framework (SURMOF) in-situ at pressures of 1-20 bar in the temperature range between 20 and 60 °C. It was found that the showcase HKUST-1 material has a good physical stability, as it is hardly damaged from exposure to pressures up to 20 bar. However, its thermal stability is weaker, as exposure to elevated T damaged the material by influencing the interaction between organic linker and metal cluster. In-situ measurements at elevated T also showed an increased mobility of the material when working at such conditions. Combining the strength of AFM at elevated T and p with ex-situ AFM and spectroscopic measurements on this MOF showcases an example of how porous materials can be studied at (industrially) relevant conditions using the autoclave-inserted AFM.