HKUST-1 is a metal–organic framework (MOF) which plays a significant role in both applicative and basic fields of research, thanks to its outstanding properties of adsorption and catalysis but also because it is a reference material for the study of many general properties of MOFs. Its metallic group comprises a pair of Cu2+ ions chelated by four carboxylate bridges, forming a structure known as paddle-wheel unit, which is the heart of the material. However, previous studies have well established that the paddle-wheel is incline to hydrolysis. In fact, the prolonged exposure of the material to moisture promotes the hydrolysis of Cu–O bonds in the paddle-wheels, so breaking the crystalline network. The main objective of the present experimental investigation is the determination of the details of the structural defects induced by this process in the crystal, and it has been successfully pursued by coupling the electron paramagnetic resonance spectroscopy with other more commonly considered techniques, such as X-ray diffraction, surface area estimation, and scanning electron microscopy. Thanks to this original approach we have recognized three stages of the process of decomposition of HKUST-1, and we have unveiled the details of the corresponding equilibrium structures of the paddle-wheels at the atomic scale level

We report an experimental investigation by Raman spectroscopy of the decomposition process of Metal-Organic Framework (MOF) HKUST-1 upon exposure to air moisture (T=300 K, 70% relative humidity). The data collected here are compared with the indications obtained from a model of the process of decomposition of this material proposed in literature. In agreement with that model, the reported Raman measurements indicate that for exposure times longer than 20 days relevant irreversible processes take place, which are related to the occurrence of the hydrolysis of Cu-O bonds. These processes induce small but detectable variations of the peak positions and intensities of the main Raman bands of the material, which can be related to Cu-Cu, Cu-O, and O-C-O stretching modes. The critical analyses of these changes have permitted us to obtain a more detailed description of the process of decomposition taking place in HKUST-1 upon interaction with moisture. Furthermore, the reported Raman data give further strong support to the recently proposed model of decomposition of HKUST-1, contributing significantly to the development of a complete picture of the properties of this considerable deleterious effect.

The structural instability in a humid environment of the majority of metal− organic frameworks (MOFs) is a challenging obstacle for their industrial-scale development.Recently, two water-resistant MOFs have been synthetized, STAM-1 and STAM-17-OEt. They both contain copper paddle wheels, like the well-known water-sensitive HKUST-1, but different organic linkers. The crystal lattice of both the MOFs undergoes a phase transition upon interaction with water molecules. Their unusual flexibility allows the controlled breaking of some interpaddle wheel Cu−O interactions in the so-called crumple zones, with a mechanism called hemilability, which is considered to have a crucial role for the stability toward water. In this work, we present a detailed investigation on the different effects of water exposure on the local and long-range structures of HKUST-1, STAM-1, and STAM-17-OEt. Electron paramagnetic resonance (EPR) spectroscopy has allowed us to characterize the different phases occurring during hydration of each MOF. In particular, we have identified and portrayed the moment of the adsorption of the first water molecule on each copper ion and shown that such soft hydration lead to a similar reversible evolution in all of the three MOFs. This aspect unveiled that the bulk water stability of the MOFs studied is unimportant at this early stage, whereas with a higher degree of hydration (more than few hours in our experimental conditions), we observe the three MOFs embarked on different paths, here carefully described. The evolution of HKUST-1 is not reversible because of its well-known tendency to hydrolysis, but, in contrast, we proved the reversibility of the water effects in STAM-1 and STAM-17-OEt even at the atomic scale level. Furthermore, for the first time, we report a Raman characterization of both STAM-1 and STAM-17-OEt, for each phase of the hydration. The data also include X-ray diffraction, nuclear magnetic resonance measurements, and Brunauer−Emmett−Teller surface area calculations of all the samples.

Metal–organic frameworks (MOFs) are getting closer to finally being used in commercial applications. In order to maximize their packing density, mechanical strength, stability in reactive environments, and many other properties, the compaction of MOF powders is a fundamental step for the application field of research of these extraordinary materials. In particular, HKUST-1 is among the most promising and studied MOF. Contrary to what reported so far in the literature, here we prove that the tableting of HKUST-1 powders without any damage of the lattice is possible and easy to get. For the first time, this kind of investigation has been performed exploiting its peculiar magnetic properties with the aid of electron paramagnetic resonance spectroscopy. Indeed, they have allowed us to explore in detail all the smallest changes induced in the paramagnetic paddle-wheel units by the application of the mechanical pressure on the material. This original approach has permitted us to unveil the main source of structural instability of HKUST-1 during compaction, that is, the water molecules adsorbed by the powdered sample before tableting and finally to establish a proper compaction protocol. Our conclusions are also fully supported by the results obtained with powder X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, water sorption isotherms, and surface area estimation with the Brunauer–Emmett–Teller method, which prove that the tablet of HKUST-1 obtained by this new protocol actually preserves the crystal structure and porosity of the pristine powders. A morphological characterization has also been conducted with a combined use of optical and atomic force microscopies.

Hybrid halide perovskites are soft materials processed at room temperature, revolutionary players in the photovoltaic field. Nowadays, investigation of the nature and role of defects is seen as one of the key challenges toward full comprehension of their behavior and achievement of high device stability under working conditions. We reveal the reversible generation, under illumination, of paramagnetic Pb3+ defects in CH3NH3PbI3, synthesized in ambient conditions, induced by the presence of Pb–O defects in the perovskite structure that may trap photogenerated holes, possibly mediated by the concomitant oxidation and migration of ions. According to the mechanism that we hypothesize, one charge is trapped for each paramagnetic center generated; thus, it does not contribute to the photocurrent, potentially limiting the solar cell performance. Our study, based on combined experimental/theoretical approach, reveals the dynamic evolution of the perovskite characteristics under illumination that needs to be considered when investigating the material physical–chemical properties.

A HKUST-1 metal–organic framework was crystallized in the NH2-modified mesostructured silica FDU-12 in order to improve its structural stability upon water exposure.

We present an experimental investigation focused on the local structural changes taking place around Cu2+ ions in metal–organic framework (MOF) HKUST-1 for different times of exposure to air by XAFS (X-ray absorption fine structure). The analysis involves both XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) regions around the Cu K-edge. Starting from the paddle-wheel structures proposed in literature, a more detailed description of the geometrical environment of Cu2+ ions has been found. In particular, the paddle-wheel structure of a fresh sample, which means a pristine HKUST-1 material with a single water molecule weakly adsorbed on each Cu2+ ion, has been disclosed for the first time. Furthermore, after 20 days of exposure to air, relevant structural changes with respect to the pristine sample have been evidenced. An activation process has demonstrated that these local changes are totally reversible, in agreement with a recent model of the decomposition process of HKUST-1 proposed in literature.

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