Nitroxidation of H‑Terminated Si(111) Surfaces with Nitrobenzene and Nitrosobenzene

Ultrathin silicon oxynitride films have attracted substantial attention as gate dielectrics. In this work, we investigate a wet-chemistry approach to introduce a monolayer silicon oxynitride film by reacting H-terminated Si(111) surface with nitro- or nitrosobenzene. The bifunctional aromatic molecules serve as a source of oxygen and nitrogen, while phenyl ring remains intact after the reaction and can be used for further modifications or as a resist. Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to confirm surface reaction and to quantify surface coverage. Density functional theory (DFT) cluster calculations were employed to explore feasible reaction pathways, predict the vibrational spectra of possible reaction products, and compare the observed XPS binding energies with calculated N 1s core level energies. Substantial differences in reactions of these two molecules on silicon provide the opportunity to tune the nitroxidation process to achieve the desired levels of oxygen and nitrogen by chemical means at relatively mild conditions

Building Organic Monolayers Based on Fluorinated Amines on the Si(111) Surface

Functionalized silicon surfaces can serve as starting points for a wide variety of structures. Controlled introduction of fluorine-containing monolayers on silicon may affect a number of chemical and physical properties of silicon substrates. This approach becomes especially interesting when these monolayers are built based on the interaction of amino-functionalized fluoroorganics with chlorinated silicon single crystals. In this work, a carefully prepared H-terminated Si(111) surface is converted into Cl–Si­(111) by mild chlorination with PCl₅ and then reacted with trifluoroethylamine (TFEA) and <i>p</i>-fluoroaniline (pFA) using a wet-chemistry procedure in an oxygen-free environment. The surface species formed and the efficiency of the reactions are monitored by infrared spectroscopy and X-ray photoelectron spectroscopy, and complemented with density functional theory (DFT) studies. Although the reaction of TFEA can be optimized to form a nearly complete monolayer, the similar procedure with pFA results primarily in surface oxidation, despite similar reaction energy landscapes predicted by DFT. This difference is discussed based on the differences of adsorption geometries of the two amines on Cl-terminated Si(111) surfaces

Single-Atom-Based Vanadium Oxide Catalysts Supported on Metal–Organic Frameworks: Selective Alcohol Oxidation and Structure–Activity Relationship

We report the syntheses, structures, and oxidation catalytic activities of a single-atom-based vanadium oxide incorporated in two highly crystalline MOFs, Hf-MOF-808 and Zr-NU-1000. These vanadium catalysts were introduced by a postsynthetic metalation, and the resulting materials (Hf-MOF-808-V and Zr-NU-1000-V) were thoroughly characterized through a combination of analytic and spectroscopic techniques including single-crystal X-ray crystallography. Their catalytic properties were investigated using the oxidation of 4-methoxybenzyl alcohol under an oxygen atmosphere as a model reaction. Crystallographic and variable-temperature spectroscopic studies revealed that the incorporated vanadium in Hf-MOF-808-V changes position with heat, which led to improved catalytic activity

Increased Electrical Conductivity in a Mesoporous Metal–Organic Framework Featuring Metallacarboranes Guests

Nickel­(IV) bis­(dicarbollide) is incorporated in a zirconium-based metal–organic framework (MOF), NU-1000, to create an electrically conductive MOF with mesoporosity. All the nickel bis­(dicarbollide) units are located as guest molecules in the microporous channels of NU-1000, which permits the further incorporation of other active species in the remaining mesopores. For demonstration, manganese oxide is installed on the nodes of the electrically conductive MOF. The electrochemically addressable fraction and specific capacitance of the manganese oxide in the conductive framework are more than 10 times higher than those of the manganese oxide in the parent MOF

Increased Electrical Conductivity in a Mesoporous Metal–Organic Framework Featuring Metallacarboranes Guests

Nickel­(IV) bis­(dicarbollide) is incorporated in a zirconium-based metal–organic framework (MOF), NU-1000, to create an electrically conductive MOF with mesoporosity. All the nickel bis­(dicarbollide) units are located as guest molecules in the microporous channels of NU-1000, which permits the further incorporation of other active species in the remaining mesopores. For demonstration, manganese oxide is installed on the nodes of the electrically conductive MOF. The electrochemically addressable fraction and specific capacitance of the manganese oxide in the conductive framework are more than 10 times higher than those of the manganese oxide in the parent MOF

Effect of Redox “Non-Innocent” Linker on the Catalytic Activity of Copper-Catecholate-Decorated Metal–Organic Frameworks

Two new UiO-68 type of Zr-MOFs featuring redox non-innocent catechol-based linkers of different redox activities have been synthesized through a de novo mixed-linker strategy. Metalation of the MOFs with Cu­(II) precursors triggers the reduction of Cu­(II) by the phenyl-catechol groups to Cu­(I) with the concomitant formation of semiquinone radicals as evidenced by EPR and XPS characterization. The MOF-supported catalysts are selective toward the allylic oxidation of cyclohexene and it is found that the presence of in situ-generated Cu­(I) species exhibits enhanced catalytic activity as compared to a similar MOF with Cu­(II) metalated naphthalenyl-dihydroxy groups. This work unveils the importance of metal–support redox interactions in the catalytic activity of MOF-supported catalysts which are not easily accessible in traditional metal oxide supports

An Exceptionally Stable Metal–Organic Framework Supported Molybdenum(VI) Oxide Catalyst for Cyclohexene Epoxidation

Molybdenum­(VI) oxide was deposited on the Zr₆ node of the mesoporous metal–organic framework NU-1000 via condensed-phase deposition where the MOF is simply submerged in the precursor solution, a process named solvothermal deposition in MOFs (SIM). Exposure to oxygen leads to a monodisperse, porous heterogeneous catalyst, named <b>Mo-SIM</b>, and its structure on the node was elucidated both computationally and spectroscopically. The catalytic activity of <b>Mo-SIM</b> was tested for the epoxidation of cyclohexene. Near-quantitative yields of cyclohexene oxide and the ring-opened 1,2-cyclohexanediol were observed, indicating activity significantly higher than that of molybdenum­(VI) oxide powder and comparable to that of a zirconia-supported analogue (Mo-ZrO₂) prepared in a similar fashion. Despite the well-known leaching problem of supported molybdenum catalysts (i.e., loss of Mo species thus causes deactivation), <b>Mo-SIM</b> demonstrated no loss in the metal loading before and after catalysis, and no molybdenum was detected in the reaction mixture. In contrast, Mo-ZrO₂ led to significant leaching and close to 80 wt % loss of the active species. The stability of <b>Mo-SIM</b> was further confirmed computationally, with density functional theory calculations indicating that the dissociation of the molybdenum­(VI) species from the node of NU-1000 is endergonic, corroborating the experimental data for the <b>Mo-SIM</b> material

Effect of Redox “Non-Innocent” Linker on the Catalytic Activity of Copper-Catecholate-Decorated Metal–Organic Frameworks

Two new UiO-68 type of Zr-MOFs featuring redox non-innocent catechol-based linkers of different redox activities have been synthesized through a de novo mixed-linker strategy. Metalation of the MOFs with Cu­(II) precursors triggers the reduction of Cu­(II) by the phenyl-catechol groups to Cu­(I) with the concomitant formation of semiquinone radicals as evidenced by EPR and XPS characterization. The MOF-supported catalysts are selective toward the allylic oxidation of cyclohexene and it is found that the presence of in situ-generated Cu­(I) species exhibits enhanced catalytic activity as compared to a similar MOF with Cu­(II) metalated naphthalenyl-dihydroxy groups. This work unveils the importance of metal–support redox interactions in the catalytic activity of MOF-supported catalysts which are not easily accessible in traditional metal oxide supports