Researchers decipher the structure of promising organic metal structures

A class of materials called organic metal structures, or MOFs, has sparked considerable interest in recent years for a variety of potential energy-related applications, especially since researchers discovered that these typically insulating materials could also be made electrically conductive.

Thanks to the extraordinary combination of porosity and conductivity of MOFs, this discovery has opened up the possibility of new applications in batteries, fuel cells, supercapacitors, electrocatalysts and specialized chemical sensors. But the process of developing specific MOF materials that possess the desired characteristics has been slow. This is largely due to the fact that it has been difficult to understand their exact molecular structure and how it affects the properties of the material.

Now, researchers from MIT and other institutions have found a way to control the growth of crystals of different types of MOF. This made it possible to produce crystals large enough to be probed by a battery of tests, allowing the team to finally decode the structure of these materials, which resemble the two-dimensional hexagonal lattices of materials such as graphene.

The findings are described in the journal today Natural materials, in a paper by a team of 20 people at MIT and other universities in the United States, China and Sweden, led by WM Keck, Mircea Dincă energy professor in MIT’s Department of Chemistry.

Since conductive MOFs were first discovered a few years ago, says Dincă, many teams have worked to develop versions for many different applications, “but no one had been able to achieve a material structure with so many details.” The better you understand the details of those structures, he says, “it helps you design better materials and much faster. And that’s what we’ve done here: we’ve provided the first detailed crystal structure at atomic resolution.”

The difficulty in growing crystals that were large enough for such studies, he says, lies in the chemical bonds within the MOFs. These materials are made up of a lattice of metal atoms and organic molecules that tend to form into crooked needle-like or thread-like crystals, because the chemical bonds that connect the atoms in the plane of their hexagonal lattice are more difficult to form and more difficult to break. Conversely, the bonds in the vertical direction are much weaker and therefore continue to break and reform at a faster rate, causing structures to lift faster than they can expand. The resulting thin crystals were too small to be characterized by most of the available tools.

The team solved the problem by modifying the molecular structure of one of the organic compounds in the MOF to change the balance of electron density and how it interacts with the metal. This reversed the imbalance in bond strengths and growth rates, thus allowing for the formation of much larger crystal sheets. These larger crystals were then analyzed using a battery of high-resolution diffraction-based imaging techniques.

As is the case with graphene, finding ways to produce larger sheets of material could be a key to unlocking the potential of this type of MOF, Dincă says. Initially, graphene could only be produced using duct tape to peel layers of a single atom from a block of graphite, but over time, methods have been developed to directly produce sheets large enough to be useful. The hope is that the techniques developed in this study can help pave the way for similar advances for MOFs, Dincă says.

“This basically provides a foundation and blueprint for creating large two-dimensional MOF crystals,” he says.

As with graphene, but unlike most other conductive materials, conductive MOFs have a strong directionality with respect to their electrical conductivity – they conduct much more freely along the plane of the material sheet than in the perpendicular direction.

This property, combined with the material’s high porosity, could make it a strong candidate for use as an electrode material for batteries, fuel cells or supercapacitors. And when its organic components have certain groups of atoms attached to them that bind to particular other compounds, they could be used as very sensitive chemical detectors.

Graphene and the handful of other known 2-D materials have opened up a wide range of research into potential applications in electronics and other fields, but those materials have essentially fixed properties. Because MOFs share many of the characteristics of these materials, but form a large family of possible variations with varying properties, they should allow researchers to design the specific types of materials needed for a particular use, says Dincă.

For fuel cells, for example, “you want something that has many active sites” for the reactivity over the large surface area provided by the facility with its open lattice, he says. Or, for a sensor to monitor the levels of a particular gas such as carbon dioxide, “you want something that is specific and doesn’t give false positives.” These types of properties can be engineered through the selection of the organic compounds used to create the MOFs, he says.

This story was republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site covering MIT research, innovation, and teaching news.