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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.

We present a Raman study of the effects induced by pressure, thermal treatments, and O2loading in MCM41, MSU-H, and MSU-F representative mesoporous silica. We compared the starting powders with the mechanically pressed tablets produced applying pressures of ∼0.2 and ∼0.45 GPa. The spectra of the three untreated tablets evidence that the main value of the Si–O–Si angle decreases and that in the MCM41 and the MSU-H Si–O–Si hydrolysis occurs, whereas such a process is absent or much less efficient in the MSU-F. Despite their different networks, the three powders tend to crystallize in cristobalite when treatments are at 1000 °C. The MCM41 and MSU-H tablets exhibit behavior similar to their starting powders, whereas the MSU-F tablets tend to form the tridymite crystalline phase. Such a finding could be related by the differences in the Si–O–Si hydrolysis, occurring during the tablets production. Finally, we inserted O2molecules within the interstices of the SiO2wall only in MSU-F. By monitoring the O2content as a function of the delay time from the loading end, we provide evidence that the O2remains in the interstices for a sufficiently large time to modify the material properties by inserting small molecules and not only through the surface functionalization.

An experimental study on the diffusive equilibrium value of interstitial O2in silica nanoparticles was carried out on samples with average particles diameter 40, 14, and 7 nm. The investigation was performed by measuring the concentration of interstitial O2by Raman and photoluminescence techniques. The dependence of diffusive equilibrium concentration on pressure and temperature was investigated in the pressure range from 0.2 to 76 bar and in the temperature range from 98 to 244 °C. The equilibrium concentration of interstitial O2follows Henry’s law at pressures below 13 bar whereas a departure from this model is observed at higher pressures. In particular, O2concentration saturates above about 60 bar, reaching a temperature-independent value for fixed particles size. The concentration of dissolved O2, under fixed thermodynamical conditions, is size dependent, being lower in smaller particles. These results evidence an effect of interaction among diffusing species in the dynamics of transport and give support to a shell-like model for the distribution of O2in nanoparticles.

The O2diffusion process in silica nanoparticles is experimentally studied in samples of average radius of primary particles ranging from 3.5 to 20 nm and specific surface ranging from 50 to 380 (m2/g). The investigation is done in the temperature range from 98 to 177 °C at O2pressure ranging from 0.2 to 66 bar by measuring the interstitial O2concentration by Raman and photoluminescence techniques. The kinetics of diffusion can be described by the Fick’s equation with an effective diffusion coefficient depending on the temperature, O2pressure, and particles size. In particular, the dependence of the diffusion coefficient on the pressure and nanoparticles size is more pronounced at lower temperatures and is connected to morphological and physical factors.

We studied the emission of the O2molecules embedded in fumed silica (amorphous silicon dioxide) nanoparticles differing for diameters and specific surface. By using a 1064 nm laser as a source we recorded both the O2emission and the Raman signal of silica. Our experimental data show that the O2emission/Raman signal (at 800 cm–1) ratio decreases with increasing the specific surface for both the as-received and the loaded samples. By performing a thermal treatment (600 °C for 2 h) we modified the structure and the water content of the smallest nanoparticles without observing any significant change in the O2emission/Raman signal ratio. Our data are explained by a shell model showing that the O2emission is essentially due to the molecules entrapped in the core of the nanoparticles, whereas the contribution due to the surface shell, having a thickness of about 1 nm, is negligible because of its high content of Si–OH groups that introduce nonradiative relaxation channels or because of the very low content of molecules trapped in this thin region.

An experimental study of the O2diffusion process in nanoparticles of amorphous SiO2in the temperature range from 98 to 157 °C was carried out by Raman and photoluminescence techniques. We studied O2diffusion in high purity silica nanoparticles with a mean diameter of 14, 20, and 40 nm detecting the outgassing of molecules trapped during the manufacturing. The kinetics of diffusion is well described for all the investigated nanoparticles by the Fick’s equation proving its applicability to nanoscale systems. The diffusion coefficient features an Arrhenius law temperature dependence in the explored temperature range, and the diffusion coefficient values are in good agreement with extrapolation of Arrhenius law from higher temperature studies.

We report an experimental investigation by electron paramagnetic resonance (EPR) spectroscopy on the properties of the E′γcenters induced by β-ray irradiation in nanoparticles of amorphous SiO2(fumed silica) with mean diameters from 7 up to 40 nm. We found that the E′γcenters are induced in all the fumed silica types in the dose range 4–400 kGy. They are characterized by an EPR line shape similar to that observed in common bulk silica materials independently on the particle diameter. Moreover, the E′γcenter concentration decreases on decreasing of the particle size for each given dose. Our findings are interpreted in terms of a shell-like model of nanoparticles in which it is assumed that stable E′γcenters can be mainly induced in the inner part of the particles, whereas those induced in the surface shell are supposed to be essentially unstable and rapidly disappear after irradiation. Furthermore, we have found that the same shell-like model naturally explains the actual radiation resistance observed for nanoparticles which has been previously tentatively attributed to the high O2content of the materials.

We report an experimental study on the emission properties of O2molecules loaded by a thermal diffusion process at 200 °C into high-purity silica nanoparticles with mean diameters of 7 and 40 nm. The embedded O2features a singlet to triplet emission band peaked at 1272 nm in agreement with the band observed for bulk silica materials. The photoluminescence excitation spectra have been determined in the visible and in the infrared range and are characterized by narrow bands peaked at 691, 764, and 1069 nm, respectively. By comparison of the transition energies, the vibrational quanta have been determined for the ground and for both the excited states; the values found are lower than the corresponding energies reported for the O2gaseous molecule. The singlet to triplet emission lifetime has been also evaluated and shows a value lower than that typically reported for bulk silica. Our results show that even if the main spectroscopic properties of O2embedded in nanometric silica are analogous to those of free O2and O2embedded in bulk silica, the nanometric host imposes differences related to the dynamics of the electronic states.

We report an experimental investigation by Raman and infrared (IR) absorption spectroscopies on the structural modifications induced by isochronal thermal treatments on amorphous SiO2nanoparticles (fumed silica). In particular, three different commercial types of this material, characterized by particle mean diameters of 7, 14, and 40 nm, were subjected to thermal treatments from 100 up to 1000 °C. We found that some properties of fumed silica, such as the SiOSi mean bond angle, ring size distribution, and surface adsorbed water content, are drastically different from those of common bulk silica materials and intimately related to the particles’ dimension. The SiOSi mean bond angle, probed by the main Raman line peaked at about 440 cm−1, is modified by thermal treatments above 400 °C and tends toward typical values of bulk silica materials, whereas the three-membered ring population, probed by the D2line peaked at about 600 cm−1, changes but does not reach bulk silica features. The surface adsorbed water content, estimated by IR measurements, gradually decreases, starting from 100 °C. The peculiar properties of fumed silica, which suggest a strained atomic network structure, together with the investigation of its modifications induced by thermal treatments, are interpreted in terms of a shell-like model of the constituting particles. In particular, the model assumes that each particle comprises a surface shellcharacterized by a network structure highly strained and a core shellwith a less strained structure more similar to that of bulk silica. This model, discussed on the basis of our experimental results, suggests that the structural property modifications induced by thermal treatments into the surface shell are related to the dehydroxylation process and to the buildup of particle-to-particle linking (sintering effects), whereas the structural modifications into the core shell arise from the network relaxation activated by thermal treatments.