Formation of Stable Radicals by Mechanochemistry and Their Application for Magic Angle Spinning Dynamic Nuclear Polarization Solid-State NMR Spectroscopy.

High-field magic angle spinning (MAS) dynamic nuclear polarization (DNP) is becoming a common technique for improving the sensitivity of solid-state nuclear magnetic resonance (SSNMR) by the hyperpolarization of nuclear spins. Recently, we have shown that gamma irradiation is capable of creating long-lived free radicals that are amenable to MAS DNP in quartz and a variety of organic solids. Here, we demonstrate that ball milling is able to generate millimolar concentrations of stable radical species in diverse materials such as polystyrene, cellulose, borosilicate glass, and fused quartz. High-field electron paramagnetic resonance (EPR) was used to obtain further insight into the nature of the radicals formed in ball milled quartz and borosilicate glass. We further show that radicals generated in quartz by ball milling can be used for solid-effect DNP. We obtained 29Si DNP enhancements of approximately 114 and 33 at 110 K and room temperature, respectively, from a sample of ball milled quartz.

Incommensurate transition-metal dichalcogenides via mechanochemical reshuffling of binary precursors.

A new family of heterostructured transition-metal dichalcogenides (TMDCs) with incommensurate ("misfit") spatial arrangements of well-defined layers was prepared from structurally dissimilar single-phase 2H-MoS2 and 1T-HfS2 materials. The experimentally observed heterostructuring is energetically favorable over the formation of homogeneous multi-principle element dichalcogenides observed in related dichalcogenide systems of Mo, W, and Ta. The resulting three-dimensional (3D) heterostructures show semiconducting behavior with an indirect band gap around 1 eV, agreeing with values predicted from density functional theory. Results of this joint experimental and theoretical study open new avenues for generating unexplored metal-dichalcogenide heteroassemblies with incommensurate structures and tunable physical properties.

Unprecedented generation of 3D heterostructures by mechanochemical disassembly and re-ordering of incommensurate metal chalcogenides.

Three-dimensional heterostructures are usually created either by assembling two-dimensional building blocks into hierarchical architectures or using stepwise chemical processes that sequentially deposit individual monolayers. Both approaches suffer from a number of issues, including lack of suitable precursors, limited reproducibility, and poor scalability of the preparation protocols. Therefore, development of alternative methods that enable preparation of heterostructured materials is desired. We create heterostructures with incommensurate arrangements of well-defined building blocks using a synthetic approach that comprises mechanical disassembly and simultaneous reordering of layered transition-metal dichalcogenides, MX2, and non-layered monochalcogenides, REX, where M = Ta, Nb, RE = Sm, La, and X = S, Se. We show that the discovered solid-state processes are rooted in stochastic mechanochemical transformations directed by electronic interaction between chemically and structurally dissimilar solids toward atomic-scale ordering, and offer an alternative to conventional heterostructuring. Details of composition-structure-properties relationships in the studied materials are also highlighted.

Multi-principal element transition metal dichalcogenides via reactive fusion of 3D-heterostructures.

Transition metal dichalcogenides combining multiple principal elements in their structures are synthesized via mechanochemical exfoliation and spontaneous reassembly of binary precursors into 3D-heterostructures that are converted into single-phase layered materials by high-temperature reactive fusion. Physical and chemical events enabling these transformations are summarized in the form of a conceivable reaction mechanism.

Luminescence properties of mechanochemically synthesized lanthanide containing MIL-78 MOFs.

Three metal-organic framework (MOF) compounds, Ln0.5Gd0.5{C6H3(COO)3}; Ln = Eu, Tb, and Dy with a MIL-78 structure, have been synthesized by a solvent-free mechanochemical method from stoichiometric mixtures of benzene 1,3,5-tricarboxylic acid, C6H3(COOH)3, also known as trimesic acid, and the respective lanthanide carbonates, Ln2(CO3)3·xH2O, Ln = Eu, Gd, Tb and Dy. MIL-78 (Ln0.5Gd0.5) shows the characteristic red, green, and yellow luminescence of Eu3+, Tb3+, and Dy3+, respectively. Efficient intramolecular energy transfer from the ligand triplet state to the excited states of Ln3+ ions can be observed. The lifetimes and quantum yields of these compounds are studied and discussed in detail. Among the three compounds, the Tb3+ containing compound shows the longest lifetime and highest quantum yield due to a smaller contribution from non-radiative decay pathways and better matching of the lowest triplet energy level of the benzenetricarboxylate ligand and the resonance level of Tb3+.

Dry mechanochemical synthesis of alane from LiH and AlCl3.

A mechanochemical process for the synthesis of alane (AlH3) starting from lithium hydride (LiH) and aluminium chloride (AlCl3) at room temperature and the underlying reaction pathway have been studied. In contrast to a conventional process using the same two reactants dissolved in diethyl ether, our approach enables a solvent-free synthesis, thereby directly leading to adduct-free alane. The method described here is quick and efficient, resulting in the quantitative conversion of all aluminium in the starting mixture to alane. Both the intermediate compounds formed during the reaction and the final products have been characterized by powder X-ray diffraction, solid-state (27)Al NMR spectroscopy, and temperature programmed desorption analysis of the as-milled mixtures. We show that excess LiH in the starting mixture (with an optimal ratio of 9LiH : 1AlCl3) is essential for the formation and stability of Al-H bonds, initially in the form of alanates and, eventually, as alane. Further processing of this mixture, gradually adding AlCl3 to reach the ideal 3LiH : 1AlCl3 stoichiometry, appears to restrict the local accumulation of AlCl3 during the ball-milling process, thereby preventing the formation of unstable intermediates that decompose to metallic Al and molecular hydrogen. We also demonstrate that under the milling conditions used, a moderate hydrogen pressure of ca. 300 bar is required to suppress competing reactions that lead to the formation of metallic Al at room temperature. The identification of the reaction intermediates at each stage of the synthesis provides significant insight into the mechanism of this solid-state reaction, which may potentially afford a more rational approach toward the production of AlH3 in a simple solvent-free process.

Facile synthesis and regeneration of Mg(BH4)2 by high energy reactive ball milling of MgB2.

We report direct hydrogenation of MgB(2) in a planetary ball mill. Magnesium borohydride, Mg(BH(4))(2), and various polyhedral borane anion salts have been synthesized at pressures between 50 and 350 bar H(2) without the need for subsequent isothermal hydrogenation at elevated temperature and pressure. The obtained products release ∼4 wt% H(2) below 390 °C, and a major portion of Mg(BH(4))(2) transforms back to MgB(2) at around 300 °C, demonstrating the possibility of reversible hydrogen storage in an Mg(BH(4))(2)-MgB(2) system.

Solid-state NMR study of Li-assisted dehydrogenation of ammonia borane.

The mechanism of thermochemical dehydrogenation of the 1:3 mixture of Li(3)AlH(6) and NH(3)BH(3) (AB) has been studied by the extensive use of solid-state NMR spectroscopy and theoretical calculations. The activation energy for the dehydrogenation is estimated to be 110 kJ mol(-1), which is lower than for pristine AB (184 kJ mol(-1)). The major hydrogen release from the mixture occurs at 60 and 72 °C, which compares favorably with pristine AB and related hydrogen storage materials, such as lithium amidoborane (LiNH(2)BH(3), LiAB). The NMR studies suggest that Li(3)AlH(6) improves the dehydrogenation kinetics of AB by forming an intermediate compound (LiAB)(x)(AB)(1-x). A part of AB in the mixture transforms into LiAB to form this intermediate, which accelerates the subsequent formation of branched polyaminoborane species and further release of hydrogen. The detailed reaction mechanism, in particular the role of lithium, revealed in the present study highlights new opportunities for using ammonia borane and its derivatives as hydrogen storage materials.