Spectroscopic investigation on the molecular and electronic structure of [Mo3S13]2−, a discrete binary transition metal sulfur cluster

Mo3S13 Cluster-Based Cathodes for Rechargeable Magnesium Batteries: Reversible Magnesium Association/Dissociation at the Bridging Disulfur along with Sulfur-Sulfur Bond Break/Formation

Multivalent cation batteries are attracting increasing attention in energy-storage applications, but reversible storage of highly polarizing multivalent cations is a major difficulty for the electrode materials. In the present study, charge-delocalizing Mo3S13 cluster-based materials (crystalline (NH4)2Mo3S13 and amorphous MoSx) are designed and investigated as cathodes for rechargeable magnesium batteries. Both of the cathodes show high magnesium storage capacities (296 and 302 mAh g-1 at 100 mA g-1) and superior rate performances (76 and 80 mAh g-1 at 15 A g-1). A high area loading of 3.0 mg cm-2 could be achieved. These performances are of the highest level compared with those of reported magnesium storage materials. Further mechanism study and theoretical computation demonstrate the magnesium storage active sites are the bridging disulfur groups of the Mo3S13 cluster. The valence state of bridging disulfur decreases/increases largely during magnesiation/demagnesiation along with breaking/formation of the sulfur-sulfur bond, which makes the Mg-association/dissociation highly reversible. The sulfur-sulfur bond breaking and formation provides high reversible capacities. Prominently, the valence state increase and sulfur-sulfur bond formation of the bridging disulfur during charge weakens the bonding with Mg2+, significantly assisting the magnesium dissociation. The present study not only develops high-performance magnesium storage cathode materials but also demonstrates the importance of constructing favorable magnesium storage active sites in the high-performance cathode materials design. The findings presented herein are of great significance for the development of electrode materials for the storage of multivalent cations.

Surface Anchoring and Active Sites of [Mo3S13]2- Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution

Achieving light-driven splitting of water with high efficiency remains a challenging task on the way to solar fuel exploration. In this work, to combine the advantages of heterogeneous and homogeneous photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate [Mo₃S₁₃]^2– clusters onto photoactive metal oxide supports to act as molecular co-catalysts for photocatalytic water splitting. We demonstrate that strong and surface-limited binding of the [Mo₃S₁₃]^2– to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S₂^2– groups, upon which Mo–O–Ti bonds with the hydroxylated TiO₂ surface are established. The heterogenized [Mo₃S₁₃]^2– clusters are active and stable co-catalysts for the light-driven hydrogen evolution reaction (HER) with performance close to the level of the benchmark Pt. Optimal HER rates are achieved for 2 wt % cluster loadings, which we relate to the accessibility of the TiO₂ surface required for efficient hole scavenging. We further elucidate the active HER sites by applying thermal post-treatments in air and N₂. Our data demonstrate the importance of the trinuclear core of the [Mo₃S₁₃]^2– cluster and suggest bridging S₂^2– and vacant coordination sites at the Mo centers as likely HER active sites. This work provides a prime example for the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and gives the incentive to explore other thio(oxo)metalates.

Performance and Mechanism of Photoelectrocatalytic Activity of MoSx/WO3 Heterostructures Obtained by Reactive Pulsed Laser Deposition for Water Splitting

This work studies the factors that affect the efficiency of the photoelectrochemical hydrogen evolution reaction (HER) using MoS_x/WO₃ nano-heterostructures obtained by reactive pulsed laser deposition (RPLD) on glass substrates covered with fluorinated tin oxide (FTO). Another focus of the research is the potential of MoS_x nanofilms as a precursor for MoO_z(S) nanofilms, which enhance the efficiency of the photo-activated oxygen evolution reaction (OER) using the MoO_z(S)/WO₃/FTO heterostructures. The nanocrystalline WO₃ film was created by laser ablation of a W target in dry air at a substrate temperature of 420 °C. Amorphous MoS_x nanofilms (2 ≤ x ≤ 12) were obtained by laser ablation of an Mo target in H₂S gas of varied pressure at room temperature of the substrate. Studies of the energy band structures showed that for all MoS_x/WO₃/FTO samples, photo-activated HER in an acid solution proceeded through the Z-scheme. The highest photoelectrochemical HER efficiency (a photocurrent density ~1 mA/cm² at a potential of ~0 V under Xe lamp illumination (~100 mW/cm²)) was found for porous MoS_4.5 films containing the highest concentration of catalytically active sites attributed to S ligands. During the anodic posttreatment of porous MoS_x nanofilms, MoO_z(S) films with a narrow energy band gap were formed. The highest OER efficiency (a photocurrent density ~5.3 mA/cm² at 1.6 V) was detected for MoO_z(S)/WO₃/FTO photoanodes that were prepared by posttreatment of the MoS_x~3.2 precursor. The MoO_z(S) film contributed to the effective photogeneration of electron–hole pairs that was followed by the transport of photoelectrons from MoO_z(S) into the WO₃ film and the effective participation of holes possessing strong oxidation ability in the OER on the surface of the MoO_z(S) film.

Reactive Pulsed Laser Deposition of Clustered-Type MoSx (x ~ 2, 3, and 4) Films and Their Solid Lubricant Properties at Low Temperature

We studied the tribological properties of amorphous molybdenum sulfide (MoSx) thin-film coatings during sliding friction in an oxidizing environment at a low temperature (-100 °C). To obtain films with different sulfur contents (x ~ 2, 3, and 4), we used reactive pulsed laser deposition, where laser ablation of the Mo target was performed in H2S at various pressures. The lowest coefficient of friction (0.08) was observed during tribo-testing of the MoS3 coating. This coating had good ductility and low wear; the wear of a steel counterbody was minimal. The MoS2 coating had the best wear resistance, due to the tribo-film adhering well to the coating in the wear track. Tribo-modification of the MoS2 coating, however, caused a higher coefficient of friction (0.16) and the most intensive wear of the counterbody. The MoS4 coating had inferior tribological properties. This study explored the mechanisms of possible tribo-chemical changes and structural rearrangements in MoSx coatings upon contact with a counterbody when exposed to oxygen and water. The properties of the tribo-film and the efficiency of its transfer onto the coating and/or the counterbody largely depended on local atomic packing of the nanoclusters that formed the structure of the amorphous MoSx films.

New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study

A new decomposition mechanism for molybdenum dialkyldithiocarbamate, a friction modifier used in engine lubricants, has been proposed.

Synthesis and structural characterization of the novel cluster compound

Reaction of [Mo3Y(mu-S)3(dtp)4(H2O)] (Y = O, S; dtp = S2P(OC2H5)2(-)) with HgI2 gave the novel compound [[Mo3S7(dtp)3]4 x I][(HgI3)3] x 4H2O (1), which contains a [[Mo3S7(dtp)3]4 x I] tetramer and (HgI3)-. Compound 1 has been characterized by IR, Raman, UV/Vis, and NMR spectroscopy and single-crystal X-ray diffraction analysis. It is shown that this formation process can be referred to as a new cluster reaction. The structure and spectroscopic data of the tetramer is also compared with that of the related discrete cluster [Mo3S7(dtp)3 x I]. Crystal data: space group F23, a = 26.786(3) A, V = 19218.7(4) A3, Z = 4, R = 0.059.

Structure and Reactivity of [Mo(3)-&mgr;(3)S-(&mgr;S(2))(3)](4+) Complexes. Quantum Chemical Calculations, X-ray Structural Characterization, and Raman Spectroscopic Measurements

A series of compounds containing the [Mo(3)-&mgr;(3)S-(&mgr;S(2))(3)-(dtc)(3)](+) complex (dtc = diethyldithiocarbamate) with the anions I(-) (1), I(-) and Br(-) (2), S(2)(-) (3), ClO(4)(-) (4), NO(3)(-) (5), and SO(4)(2)(-) (6) was prepared and characterized by elemental analysis, NMR, IR, and Raman spectroscopy, and FAB mass spectrometry. The previously reported crystal structure of 1 was reinvestigated. The X-ray analysis revealed the incorporation of CH(2)Cl(2) in the crystal having the composition [Mo(3)S(7)(dtc)(3)]I.0.5CH(2)Cl(2) (1a), which was in contradiction to the previous protocol. The corresponding ClO(4)(-) compound (4a) is isotypic. Crystal data: C(15.5)H(31)Cl(2)Mo(3)N(3)O(4)S(13), orthorhombic space group Aba2, a = 25.816(5) Å, b = 17.761(4) Å, c = 16.250(3) Å, Z = 8. For 1a, 4a, 6, and the previously analyzed 2 and 3 the crystal structures revealed characteristic interactions between the anions X and the three axial (out-of-plane) sulfur atoms S(ax) of the disulfido bridges. The Raman data showed a significant decrease of the S(eq)-S(ax) stretch resonance frequency in the order 4, 5, 6 > 1 > 3. This decrease is paralleled with a slight increase of the S(eq)-S(ax) bond length and with a significant shortening of the X.S(ax) distances when compared to the sum of the corresponding van der Waals radii. A comprehensive quantum chemical study, using both density functional theory and semiempirical calculations, revealed that for hard counterions such as NO(3)(-) and ClO(4)(-) the S(ax).X interactions can be understood in terms of an almost entirely electrostatic interaction, whereas for soft nucleophiles such as I(-) and S(2)(-) significant covalency is observed. In addition, the general reaction of [Mo(3)S(7)](4+) complexes with a nucleophile was modeled. With regard to the side-on bonding of the &mgr;-S(2) groups to Mo, the calculations indicated a significantly higher bond energy for the axial (out-of-plane) sulfur atoms, explaining the much higher lability of the sulfur atoms in the equatorial (in-plane) position. Analogous differences for the ligating atoms of the peripheral ligands, having a cis and trans position with respect to &mgr;(3)-S, are less pronounced.