Fast Synthesis of MOF-5 Microcrystals Using Sol−Gel SiO₂ Nanoparticles

The work reports on the synthesis of an archetypal metal organic framework (MOF-5) microcrystals with a narrow size distribution using SiO2 nanoparticles with tailored surface chemistry as nucleating agents. The nanoparticles boost the reaction rate by up to an order of magnitude compared to the conventional MOF-5 solvothermal synthesis and can be successfully used as nucleation seeds for the selective growth of MOF-5 on specific substrates. These results are important fundamental advances toward the controlled scale-up of MOF synthesis and directed MOF growth on suitable supports

Influence of Temperature on the Photocatalytic Activity of Sol−Gel TiO₂ Films

The photocatalytic activity of TiO2 films synthesized via the sol−gel process has been measured as a function of UV irradiation time and substrate temperature. Fourier-transform infrared spectroscopy has been used to address the chemical changes in stearic acid and block copolymer Pluronic F127 films deposited on the photocatalytic surface. When the temperature of the photocatalytic substrate was raised above 50 °C, the removal of stearic acid from the surface was strongly affected by a process involving evaporation, whereas Pluronic F127 revealed a superior stability. Our study shows that heat enhances the photocatalytic activity, suggesting the importance of an accurate temperature control in photocatalytic efficiency measurements

Highly Ordered Self-Assembled Mesostructured Hafnia Thin Films: An Example of Rewritable Mesostructure

Self-assembly of supramolecular templates is a fascinating route that mimics natural processes to obtain porous organized materials. We have synthesized hafnia mesostructured films with a high degree of order, and the structure has been indexed as rhombohedral (R3̄m space group). The films have been revealed to be particularly interesting in their tunable state, immediately after solvent evaporation. We were able to modulate self-assembly under different conditions by changing the relative humidity in the deposition room and the intensity of the incident X-ray on the as-deposited film. The material has been shown to respond quickly and reversibly to changes in humidity in the deposition room, and the highly tunable state of hafnia films, before drying, has been used to “write” the structure through a high flux X-ray beam. The X-ray controlled reorganization path differs from those that have been traditionally observed because self-assembly can be achieved even in the absence of evaporation. Self-ordering, in this case, can be explained as an entropy-driven process, a general principle that is typical of biological systems

Complete Characterization of α-Hopeite Microparticles: An Ideal Nucleation Seed for Metal Organic Frameworks

This work reports on the structural and microstructural characterization of a new class of α-hopeite microparticles, which has recently been discovered as ideal seeding agents for the formation and functionalization of metal organic framework (MOF-5) crystals. The particles have been named desert rose microparticles (DRMs), as their morphology closely resembles that of the famous gypsum and Barite mineral. The DRMs form directly inside the MOF-5 precursor solution when a block copolymer surfactant, Pluronic F-127, is added in specific amounts. The particles formation is remarkably fast, and particles are observed to form within the first minute of reaction. The DRMs formation and growth has been monitored along a 3 h synthesis, until the first nuclei of MOF-5 start to appear on their surface. Electron microscopy, energy dispersive analysis, electron diffraction, FTIR, FT-Raman, and BET give an all-around description of the chemical and morphological features that give the DRMs their remarkable MOF-seeding capacity

Luminescent Porphyrinic Metal–Organic Frameworks for Oxygen Sensing: Correlation of Nanostructure and Sensitivity

Oxygen sensing capabilities of a series of luminescent zirconium-based porphyrinic metal–organic frameworks (MOFs) (PCN-222, PCN-223, and PCN-224) have been investigated. Design of experiments allowed identification of the most sensitive material, a PCN-224 MOF. In this material, luminescence in air is quenched by up to 7.3-fold with a bimolecular quenching constant kq of 56,000 Pa–1 s–1, which is significantly above the previously reported benchmark material of the same MOF type (kq of 34,000 Pa–1 s–1). The oxygen sensitivity increases in the order PCN-222 < PCN-223 < PCN-224 with effective Stern–Volmer (SV) constants in the range of 0.13 kPa–1 for PCN-222, 0.18 kPa–1 for PCN-223 and up to 0.45 kPa–1 for PCN-224-type materials. This effect is attributed to the increase in the porphyrin–porphyrin separation in these nanostructured materials that results in the increase in the fluorescence decay time and thus the probability of the quenching event. Defects in the nanostructure also are important and explain variations in the sensitivity within the MOFs of the same type. Among the investigated materials, those belonging to the PCN-223 type were found to be the most robust in terms of oxygen sensing capabilities with only little variation in the SV constants. The analogues of the PCN MOFs that utilize a Pt­(II) porphyrin as a building block were also prepared. It is shown that the same synthetic protocols established for the metal-free systems can be used for the metal complex. Analogous to the MOFs based on the metal-free porphyrin, the oxygen sensitivity increases from Pt­(II)­PCN-222 (KSV 30 kPa–1) via Pt­(II)­PCN-223 (KSV 40 kPa–1) to Pt­(II)­PCN-224 (KSV up to 87 kPa–1)

Bioactive MIL-88A Framework Hollow Spheres via Interfacial Reaction In-Droplet Microfluidics for Enzyme and Nanoparticle Encapsulation

Functional bio-MOF hollow spheres with controlled size in the 35–2000 μm range were successfully synthesized by interfacial reaction using a continuous-flow droplet microfluidic system in a single step and one-flow strategy. The architecture of MIL-88A frameworks was extended from single-shell to double-shell hollow spheres. Moreover, various functional nanoparticles (silica, cobalt, and UiO-66­(Zr) MOF) were directly encapsulated in the single-shell hollow spheres, while maintaining the functionality of the cargo. In particular, three kinds of enzymes (glycerol dehydrogenase, horseradish peroxidase, and acetylcholinesterase) were also encapsulated inside the single-shell hollow spheres under mild conditions. The catalytic activity and the superior recyclability of the encapsulated enzymes were demonstrated against free enzymes

Writing Self-Assembled Mesostructured Films with In situ Formation of Gold Nanoparticles

We have developed an integrated technology that allows patterning and formation of gold-nanoparticles in self-assembled mesoporous films in a single step; this technique allows fabrication of patterned structures with high aspect ratios in a direct, simple, and reproducible way. We have used high intensity X-rays that interact with the mesoporous matrix through different processes, by removing the organic templates, increasing the polycondensation of the organic−inorganic silica network, and partially removing the methyl groups bonded in the hybrid matrix. The final material, after patterning and chemical etching, exhibits gold nanoparticles of few nanometers homogeneously dispersed in the porous matrix

Highly Ordered “Defect-Free” Self-Assembled Hybrid Films with a Tetragonal Mesostructure

One-pot self-assembled hybrid films were synthesized by the cohydrolysis of methyltriethoxysilane and tetraethoxysilane and deposited via dip-coating. The films show a high “defect-free” mesophase organization that extends throughout the film thickness and for domains of a micrometer scale, as shown by scanning transmission electron microscopy. We have defined these films defect-free to describe the high degree of order that is achieved without defects in the pore organization, such as dislocations of pores or stacking faults. A novel mesophase, which is tetragonal I4/mmm (space group), is observed in the films. This phase evolves but retains the same symmetry throughout a wide range of temperatures of calcination. The thermal stability and the structural changes as a function of the calcination temperature have been studied by small-angle X-ray scattering, scanning transmission electron microscopy, and Fourier transform infrared spectroscopy. In situ Fourier transform infrared spectroscopy employing synchrotron radiation has been used to study the kinetics of film formation during the deposition. The experiments have shown that the slower kinetics of silica species can explain the high degree of organization of the mesostructure

Chemical Tailoring of Hybrid Sol−Gel Thick Coatings As Hosting Matrix for Functional Patterned Microstructures

A phenyl-based hybrid organic − inorganic coating has been synthesized and processed by hard X-ray lithography. The overall lithography process is performed in a two-step process only (X-rays exposure and chemical etching). The patterns present high aspect ratio, sharp edges, and high homogeneity. The coating has been doped with a variety of polycyclic aromatic hydrocarbon functional molecules, such as anthracene, pentacene, and fullerene. For the first time, hard X-rays have been combined with thick hybrid functional coatings, using the sol−gel thick film directly as resist. A new technique based on a new material combined with hard X-rays is now available to fabricate optical devices. The effect due to the high-energy photon exposure has been investigated using FT-IR and Raman spectroscopy, laser scanner, optical profilometer, and confocal and electron microscope. High-quality thick hybrid fullerene-doped microstructures have been fabricated