Mapping metal distributions on the micrometre scale

10 000s of different, finely tuned MOF structures, built up from a wide range of metal atoms and organic linking materials, exist and their number is still growing. To adapt a MOF for a desired functionality or to develop new structures, it is important to know the spatial distribution of its building blocks. In a long-standing cooperation, scientists from the Materials Physics Division and the Sascha Ott group at Chemistry-Ångström have developed a method to measure this distribution without damaging the structure by using ion beams from a particle accelerator at the Tandem Laboratory.

The chemists modified a specific type of MOF crystal by immerging it into a solution containing nickel ions. The MOF is porous like a sponge so the solution diffuses into the crystal and nickel ions can bind to specific sites of the MOF.

A beam of helium ions, focussed down to 3 micrometres (= 0.003 mm), is then scanned across an individual MOF crystal, also only 10-20 micrometres wide. The helium ions scatter from the atoms in the sample into a detector, and the resulting signal is used to map the concentration of a single chemical element, for example nickel. New in this study is that the scientists also look at X-rays that are produced when the helium ions intrude into the sample. The X-rays can provide additional information about the bottom part of the MOF crystal that is hard to analyse otherwise.

With the newly developed method the scientists could show that the nickel forms a so-called core-shell structure with the highest amount of nickel at the MOF surface and a decreasing amount on the way to the centre. They could even determine how fast the nickel diffuses into the MOF structure and that in this case, molecular diffusion is the most important factor governing the nickel distribution.

While this nickel gradient might be very useful for applications in catalysis, it also makes the analysis more complicated. The measured signal from overhanging parts of the MOF crystals might look similar to that from a homogeneous shape with a nickel gradient. The researchers therefore built a computer model that includes both a concentration gradient and a shape correction. The model then tries to match the experimental data by finding optimum values for the nickel concentration and the MOF shape.

The developed method is not limited to the current system but can be used to study different MOF structures or transport processes of other metal ions.

Svenja Lohmann