Teresa Valdés-Solís

In a previous work by the authors (A. Fernández-Pérez el al., Room temperature sintering of polar ZnO nanosheets: I-evidence, 2017, DOI: 10.1039/C7CP02306E), polar ZnO nanosheets were stored at room temperature under different atmospheres and the evolution of their textural and crystal properties during storage was followed. It was found that the specific surface area of the nanosheets drastically decreased during storage, with a loss of up to 75%. The ZnO crystals increased in size mainly through the partial merging of their polar surfaces at the expense of narrow mesoporosity, in a process triggered by the action of moisture, oxygen and, in their absence, by light. In the present work, a set of spectroscopic techniques (FTIR, Raman and XPS) has been used in an attempt to unravel the mechanism behind this spontaneous sintering process. The mechanism starts with the molecular adsorption of water, which takes place on Zn atoms close to oxygen vacancies on the (100) surface, where H₂O dissociates to form two hydroxyl groups and to heal one oxygen vacancy. This process triggers the room temperature migration of Zn interstitials towards the outer surface of the polar region. What were previously interstitial Zn atoms now gradually occupy the mesopores, with interstitial oxygen being used to build up the O sublattice until total occupancy of the narrow mesoporosity is achieved.

Polar ZnO nanosheets of a high specific surface area (∼120 m² g⁻¹) were subjected to storage under different atmospheres at room temperature and analyzed for changes in their textural and crystal properties. During their storage under laboratory conditions (in closed transparent polypropylene vials kept under the light of the laboratory on worktop tables) the nanosheets lost up to 75% of their specific surface area in approximately two months, with most of the loss occurring during the first two weeks. The narrow mesoporosity (∼5 nm pore size) became filled with ZnO during the process. No loss or gain in weight was detected. The loss of specific surface area took place under all of the atmospheres assayed, in the following order: moist air (with or without light) > moist CO₂-free atmosphere (with or without light and/or oxygen) > dry CO₂-free oxygen-containing atmosphere (with or without light) > dry inert atmosphere (with light) > dry inert atmosphere (in the dark). During storage the ZnO crystals grew mainly by the partial merging of their polar surfaces in a process triggered by the action of moisture, oxygen and, in the absence of these two agents, light. The mechanism of this intriguing phenomenon will be analyzed in detail in the second part of this work.

ZIF-8 is a zeolitic imidazolate framework with very good thermal and chemical stability that opens up many applications that are not feasible by other metal–organic frameowrks (MOFs) and zeolites. Several works report the adsorption properties of ZIF-8 for strategic gases. However, despite the vast experimental corpus of data reported, there seems yet to be a dearth in the understanding of the gas adsorption properties. In this work we provide insights at a molecular level on the mechanisms governing the ZIF-8 structural deformation during molecular adsorption. We demonstrate that the ZIF-8 structural deformation during the adsorption of different molecules at cryogenic temperature goes beyond the gas-induced rotation of the imidazolate linkers. We combine experimental and simulation studies to demonstrate that this deformation is governed by the polarizability and molecular size and shape of the gases, and that the stepped adsorption behavior is defined by the packing arrangement of the guest inside the host.