ZIF‑8 Vibrational Spectra: Peak Assignments and Defect Signals

Zeolitic
imidazolate framework (ZIF-8) is a promising
material
for gas separation applications. It also serves as a prototype for
numerous ZIFs, including amorphous ones, with a broader range of possible
applications, including sensors, catalysis, and lithography. It consists
of zinc coordinated with 2-methylimidazolate (2mIm) and has been synthesized
with methods ranging from liquid-phase to solvent-free synthesis,
which aim to control its crystal size and shape, film thickness and
microstructure, and incorporation into nanocomposites. Depending on
the synthesis method and postsynthesis treatments, ZIF-8 materials
may deviate from the nominal defect-free ZIF-8 crystal structure due
to defects like missing 2mIm, missing zinc, and physically adsorbed
2mIm trapped in the ZIF-8 pores, which may alter its performance and
stability. Infrared (IR) spectroscopy has been used to assess the
presence of defects in ZIF-8 and related materials. However, conflicting
interpretations by various authors persist in the literature. Here,
we systematically investigate ZIF-8 vibrational spectra by combining
experimental IR spectroscopy and first-principles molecular dynamics
simulations, focusing on assigning peaks and elucidating the spectroscopic
signals of putative defects present in the ZIF-8 material. We attempt
to resolve conflicting assignments from the literature and to provide
a comprehensive understanding of the vibrational spectra of ZIF-8
and its defect-induced variations, aiming toward more precise quality
control and design of ZIF-8-based materials for emerging applications.