Determining the Distributions of Components inside Metal-Organic Framework Thin Films with an Ar-Gas Cluster Ion Beam (Ar) and Ar Cosputter via Secondary Ion Mass Spectrometry.

Metal-organic frameworks (MOFs) are widely used as functional porous materials because of their high specific surface area, adjustable pore size, and functional groups in their structure. Understanding the spatial distribution of guest molecules inside MOFs may help further advance the development of MOFs and provide more insights into their application in various fields. However, analytical techniques that can directly obtain the distribution of organic guests inside MOF materials are scarce. In this work, the UiO-66 MOF was used as a model MOF to validate the experimental parameters for constructing an authentic depth profile with a time-of-flight secondary ion mass spectrometer (ToF-SIMS). In the analysis phase, pulsed C was used as the primary ion beam to generate molecular secondary ions. In the sputter phase, sets of Ar gas cluster ion beams (Ar-GCIB, Ar) with different energy densities (energy per atom, E/n = 2-20 eV/atom) and atomic Ar with different kinetic energies and current densities were used to cosputter the samples. The results show that when only Ar-GCIB is used, the sputtered ions cause less damage to the sample and preserve the chemical structure of the organic components as the E/n decreases. However, preferential sputtering occurs because the removal rate of inorganic nodes is much lower than that of the organic linkers of MOFs. Eventually, the inorganic components remaining on the surface prevent subsequent analysis. When cosputtered with Ar, the auxiliary atomic ions increase the sputter rate of the inorganic node, eliminate damage to the chemical structure, and alleviate the preferential sputtering between organic and inorganic components. Higher voltages and higher current densities (500 V, 5 × 10 A/cm) of Ar yielded the most realistic results. In summary, to obtain a realistic component distribution inside the MOF, the use of Ar-GCIB─Ar cosputtering is necessary. Based on low energy density (E/n = 4 eV/atom) of Ar-GCIB and optimized Ar cosputter, the distributions of inorganic nodes, organic linkers, and guest molecules inside the MOF films were reliably and thoroughly identified. This work presents a generalizable direct method for determining the distribution of molecules within MOF composites.