Space laboratories conditions simulated on Earth allow synthesising high-quality 2D crystalline materials

by Virginia Greco

A team of researchers demonstrated that 2D porous crystalline molecular frameworks can be grown with excellent control over their morphology and homogeneity by using a custom-made microfluidic device. This approach recreates on Earth the microgravity environment of laboratories on the International Space Stations.

Chemical reactions are often performed under vigorous stirring to increase their efficiency. Such turbulent conditions, though, can cause the precipitation of some of the products, with relevant impact on the reaction performance. This is particularly critical for the synthesis of 2D or 3D functional crystals, such as metal-organic frameworks (MOF). In these processes, precipitation can lead to ungoverned and non-uniform growth and, therefore, poor control over the orientation, compactness, roughness and morphology of the final structure.

Recent studies performed on the International Space Station (ISS) demonstrated that the microgravity conditions present out there have a positive impact on the growth of crystals. In fact, the absence of convective mass transport processes favours the synthesis of larger samples of material, with less defects and an improved control over their morphology. Producing crystalline materials on the Space Station on a regular basis is, of course, unviable, as it is both non-practical and very expensive.

Recreating those conditions of microgravity in laboratories on Earth would be extremely convenient, and this is what the authors of a paper recently published in Advanced Materials have achieved, by employing custom-made microfluidic devices. These microfluidic devices have been developed by the group of Prof. Josep Puigmartí-Luis, from the University of Barcelona (UB). Prof. Puigmartí-Luis, Dr Daniel Ruiz-Molina, leader of the ICN2 Nanostructured Functional Materials Group, Dr Tiago Sotto Mayor, from Porto University, and Dr Raphael Pfattner, from the Institute of Materials Science of Barcelona (ICMAB-CSIC), have coordinated this multidisciplinary project, which also involved researchers from the ALBA Synchrotron, ETH Zurich, the University of Newcastle (Australia) and the “Sapienza” University of Rome (Italy). Noemí Contreras-Pereda, PhD student in Dr Ruiz-Molina’s group, is the first author of the paper.

The team of researchers synthesised a conductive bidimensional metal-organic framework –specifically Ni₃(HITP)₂, which is an exemplar case for growth controllability of MOFs– within a microfluidic device consisting of two substrates sandwiched with a thin silicon film. The solution including the reactants was injected into the device via inlet ports and the vapour-induced crystallization of the material was obtained by controlling the diffusion of ammonia gas and air. Different substrates where used (such as quartz and gold) and the result of the process was in each case the formation of a smooth, compact and defect-free continuous film, reaching a few-centimetre size.

While the same reaction performed in standard laboratory conditions leads to precipitation of some components and irregular growth, the use of this microfluidic device allows controlling the steps of the process and obtaining a greatly better outcome. This confirms the authors’ intuition, supported by numerical simulation, that the environment created in this device reproduces what observed in the experiments conducted on the ISS as an effect of microgravity conditions.

Other structures were synthetized by functionalising the surface in various ways or using different reactants, to explore the versatility of this approach. It proved efficient in growing homogeneous and large thin films of crystalline materials in a wide variety of substrates, with a remarkable control over their orientation and morphology. As such, it represents a useful tool for chemists, physicists and material scientists to produce high-quality samples of 2D functional materials in their laboratories.

Reference article:

Noemí Contreras-Pereda, David Rodríguez-San-Miguel, Carlos Franco, Semih Sevim, João Pedro Vale, Eduardo Solano, Wye-Khay Fong, Alessandra Del Giudice, Luciano Galantini, Raphael Pfattner, Salvador Pané, Tiago Sotto Mayor, Daniel Ruiz-Molina, and Josep Puigmartí-Luis, Synthesis of 2D Porous Crystalline Materials in Simulated Microgravity. Advanced Materials, June 2021. DOI: 10.1002/adma.202101777