Synthesis and optimization of Layered Double Hydroxides (LDH) microstructures using microfluidic methods

Abstract

In the last two decades, microfluidic technologies have been largely used to control selfassembly processes and for materials synthesis. Undoubtedly, over this time, microfluidic devices have proved to be new advanced synthetic tools, leading to products that are different from the ones obtained in conventional bulk synthesis, for example, generating new structures with a particular morphology and/or different crystallinity. The main reason for these interesting outcomes, it is that microfluidic devices allow for a fine control of reaction-diffusion (RD) processes in chemical processes, i.e. in microfluidic devices mixing only occurs through diffusion. Layered Double Hydroxides (LDHs) are a class of 2D anionic materials with a huge variety of applications due to their properties that include, for example, magnetism and porosity, among others. In the literature, LDHs have been generated via conventional turbulent mixing yielding in most cases to crystalline powders with an undefined particle shape. The main goal of this project is to study the formation of LDHs using microfluidic devices, primarily we focused our investigations on two systems, Zn(II)/Cr(III)-LDH and Ni(II)/Co(II)-LDH. We wanted to investigate if RD conditions can trigger the formation of new crystalline shapes and/or morphologies. We studied a range of different conditions, and we identified some that could lead to fiber formation. When we found the suitable conditions, the products synthesized were characterized by microscopy and spectroscopy techniques. Their textural properties and morphologies were examined by optical images and scanning electron microscopy (SEM), their crystallinity was analyzed by X-ray powder diffraction (XRD), and finally, FTIR-ATR spectra was done to study the molecular vibrations of functional groups. On the one hand, the results obtained indicate that the Zn/Cr-LDH fibers are crystalline and that they present the hydrotalcite structure like the Zn/Cr-LDH powder generated in bulk synthesis. On the other hand, the Ni/Co-LDH fibers do not have intense diffraction peaks in the XRD patterns, compared to the bulk, but depending on the synthetic conditions used during the microfluidic synthesis, it was possible to obtain a pure simonkolleite structure (α-Co(II) hydroxide) or a brucitic phase (β-Co(II) hydroxide). It is important to note here that while wiith our microfluidic approach it is possible to control the generation of the α-Co(II) hydroxide or β-Co(II) hydroxide, with bulk synthetic approaches this possibility is by no means straightforward