(Invited) Microscopy and Spectroscopy on Thin-Films for (Electro-)Catalysis
Authors: Delen Guusje , Mandemaker Laurens Dirk Bernardus , Fu Donglong , Wijten Jochem H.J. , Weckhuysen Bert Marc 
Published: February 2020

Porous materials are of high importance, a.o. in the field of catalysis. A typical example of relevant and well-known crystalline, microporous materials is zeolites. These stable aluminosilicates possess tuneable Lewis and Brønsted acidity, in combination with microporous channels and/or cages allowing for size and shape selectivity. As a result, these zeolites are used for many catalytic applications, including large industrial processes such as fluid catalytic cracking (FCC).

Another class of highly crystalline porous solids, namely metal-organic frameworks (MOFs), have gained a lot of attention over the last few decades. These materials, built-up of metal nodes and organic linkers, are interesting due to their huge versatility: material properties, such as pore size and Lewis acidity, can be tuned to its specific application by simply tailoring its building blocks. Comparable to zeolites, MOFs can be used for a wide range of applications, such as catalysis, separation and in electronic (sensing) devices.

Of both zeolites and MOFs, a large number of framework structures are known. Most of these compounds have been developed through the process of trial-and-error and/or educated guesses. Ultimately, one wishes to apply true rational design to the development of catalysts for novel applications. The required fundamental understanding of crystal growth needed for such purposes is unfortunately still lacking in literature. To study nucleation and growth behaviour, a model system apt for near-field characterisation is needed. As a result, 2D thin-film structures have been developed to facilitate high control over, and easy analysis of, crystal growth. Additionally, by converting these porous materials into thin-films, the gate-way to (membrane) specific applications is opened.

To gain fundamental insights into catalytic thin-films, and to establish structure-performance correlations, we apply multiple (in-situ) characterization techniques. Initially, hetero-epitaxial growth processes were probed to uncover crystal nucleation and growth mechanisms. This real-time in-situ research was performed on HKUST-1 surface-mounted MOFs, or SURMOFs. During a one-pot synthesis between metal-ions and linkers, the surface topology was continually measured using atomic force microscopy (AFM). On a single (10 x 10 µm2) AFM spot we could follow the formation of MOF islands over multiple timeframes. Using a tailor-made script, these (growing) islands were labelled and tracked over all timeframes. From this, a (temperature-dependent) growth rate could be established. Next to structural information, the chemical nature of the formed MOF grains was uncovered using infra-red reflection absorption spectroscopy (IRRAS) at grazing angles, a highly sensitive technique providing data from monolayer coverage and up. Additionally, a novel photo-induced force microscopy (PiFM, or AFM-IR) technique was applied to these SURMOFs, which is able in distinguishing grain specific chemical nature of the nm-sized MOF islands.

This knowledge of nucleation and growth behaviour can be applied to the production of high-quality SURMOFs. Whereas the direct synthesis provides a facile medium for in-situ AFM investigations, a layer-by-layer synthesis offers high control over SURMOF fabrication quality. This synthesis, in which the growth phases of the inorganic and organic compounds are separated, is a perfect model system for the systematic study of growth parameters and their effect of resulting SURMOF quality. Using IRRAS, we can fully identify peaks in the HKUST-1 SURMOF spectrum, including precursor species, which were previously unknown in literature. Additionally, in contrast to previously posed growth mechanisms, our (AFM-)IR studies show growth to occur through a solution-mediated mechanism. Moreover, AFM-IR can be used to directly probe defect formation on the nm-scale, allowing for catalytic structure-performance correlations. In accordance with the found mechanism, existing synthesis procedures can be optimized in terms of quality and speed.

For zeolite ZSM-5, an innovative approach to enhance catalytic performance is to prepare homo-epitaxially grown zeolite ZSM-5 as continuous, oriented films with accessibility to only the straight zeolite channels. Applying advanced characterization techniques, such as tip-enhanced Raman spectroscopy (TERS), single-molecule fluorescence microscopy as well as grazing incidence characterization methods, we can explore the synthesis mechanisms, as well as study 3-D diffusion and related reactivity through a single pore orientation of zeolite ZSM-5 down to the level of a single molecule. Furthermore, we show the methanol-to-hydrocarbons reaction on ZSM-5 thin-films can be studied with AFM-IR. AFM-IR is highly sensitive to the surface chemical structure with regards to Al3+ framework incorporation as well as hydrocarbon formation, providing a more complete nanoscale picture of the structural and catalytic information. These combined case studies show the strength of microscopy and spectroscopy tools in understanding and improving catalytic materials.

Figure 1