Intrinsic and Extrinsic Responses of ZIF-8 under High Pressure: A Combined Raman and X-ray Diffraction Investigation

ZIF-8 is a prototypical porous material in the family of zeolitic imidazolate frameworks, and its structural stability is of great significance for a wide range of practical applications. Here, the intrinsic and extrinsic structural and mechanical responses of nanosized ZIF-8 perturbed by high external pressure have been comparatively investigated based on our in situ Raman measurements in different pressure environments, accompanied by synchrotron X-ray diffraction measurements and results from previous reports. An improvement of both the chemical and crystallographic stability of ZIF-8, rather than an intrinsic quick deterioration, can be observed when penetrating guest molecules get into the pores. Spectral evidence demonstrates that the C=C section of the imidazolate ring is an optimal interaction site for the penetrating methanol, water, and nitrogen molecules. We further find that water molecules can cleave Zn-N bonds to bring linker-associated defects to ZIF-8. When compressed in the non-penetrating silicone oil as a pressure transmitting medium (PTM) or when no PTM is used, irreversibility is observed, while samples show a highly reversible transformation both in the short-range and long-range regions upon decompression from a partially or entirely amorphous state in penetrating PTM. Furthermore, the released samples transform to a dense bulk state in the absence of penetrating PTM instead of preserving the original nanosized particles when penetrating pressure transmitting fluids is present. Moreover, the compressible behavior of ZIF-8 varies and is found to be strongly influenced by the size and polarity of molecules in the pores. The present results indicate that the stability, compressibility, and reversibility of ZIF-8 under pressure are closely related to the pressure environments and the characteristics of the guest molecules in the PTM. Thus, this study sheds light on the available mechanical modifications and selectable applications of ZIF-8 as gas absorbents for the future.