An MoSxStructure with High Affinity for Adsorbate Interaction

Exploring the growth and oxidation of 2D-TaS2on Cu(111)

In this work, the growth and stability towards O₂exposure of two dimensional (2D) TaS₂on a Cu(111) substrate is investigated. Large area (∼1 cm²) crystalline 2D-TaS₂films with a metallic character are prepared on a single crystal Cu(111) substrate via a multistep approach based on physical vapor deposition. Analytical techniques such as Auger electron spectroscopy, low energy electron diffraction, and photoemission spectroscopy are used to characterize the composition, crystallinity, and electronic structure of the surface. At coverages below one monolayer equivalent (ML), misoriented TaS₂domains are formed, which are rotated up to±13orelative to the Cu(111) crystallographic directions. The TaS₂domains misorientation decreases as the film thickness approaches 1 ML, at which the crystallographic directions of TaS₂and Cu(111) are aligned. The TaS₂film is found to grow epitaxially on Cu(111). As revealed by low energy electron diffraction in conjunction with an atomic model simulation, the (3 × 3) unit cells of TaS₂match the (4 × 4) supercell of Cu(111). Furthermore, the exposure of TaS₂to O₂, does not lead to the formation of a robust tantalum oxide film, only minor amounts of stable oxides being detected on the surface. Instead, the exposure of TaS₂films to O₂leads predominantly to a reduction of the film thickness, evidenced by a decrease in the content of both Ta and S atoms of the film. This is attributed to the formation of oxide species that are unstable and mainly desorb from the surface below room temperature. Temperature programmed desorption spectroscopy confirms the formation of SO₂, which desorbs from the surface between 100 and500 K.These results provide new insights into the oxidative degradation of 2D-TaS₂on Cu(111).

Low-Temperature Synthesis Strategy for MoS2 Slabs Supported on TiO2(110)

MoS2 supported on oxides like TiO2 has a broad range of applications. The atomic structure of this system is therefore very useful to study. Previous research work in this area has made use of high-temperature synthesis methods, while the preparation of an MoS2/TiO2 in very important applications, such as catalysis, makes use of a low-temperature synthesis method. In this work, we investigate a low-temperature synthesis strategy for MoS2 slabs supported on rutile TiO2(110). Using scanning tunneling microscopy and X-ray photoelectron spectroscopy, we demonstrate that not only flat MoS2 slabs with irregular shapes but also MoSx stripes with a large number of coordinatively unsaturated Mo atoms are formed. In particular, it becomes evident that, for atomic structural characterization of MoS2/TiO2 and similar oxide-supported systems grown by low-temperature synthesis methods, the surface structure of the support becomes highly relevant.

Catalytic C2H2 synthesis via low temperature CO hydrogenation on defect-rich 2D-MoS2 and 2D-MoS2 decorated with Mo clusters

Rational design of novel catalytic materials used to synthesize storable fuels via the CO hydrogenation reaction has recently received considerable attention. In this work, defect poor and defect rich 2D-MoS₂ as well as 2D-MoS₂ decorated with Mo clusters are employed as catalysts for the generation of acetylene (C₂H₂) via the CO hydrogenation reaction. Temperature programmed desorption is used to study the interaction of CO and H₂ molecules with the MoS₂ surface as well as the formation of reaction products. The experiments indicate the presence of four CO adsorption sites below room temperature and a competitive adsorption between the CO and H₂ molecules. The investigations show that CO hydrogenation is not possible on defect poor MoS₂ at low temperatures. However, on defect rich 2D-MoS₂, small amounts of C₂H₂ are produced, which desorb from the surface at temperatures between 170 K and 250 K. A similar C₂H₂ signal is detected from defect poor 2D-MoS₂ decorated with Mo clusters, which indicates that low coordinated Mo atoms on 2D-MoS₂ are responsible for the formation of C₂H₂. Density functional theory investigations are performed to explore possible adsorption sites of CO and understand the formation mechanism of C₂H₂ on MoS₂ and Mo₇/MoS₂. The theoretical investigation indicates a strong binding of C₂H₂ on the Mo sites of MoS₂ preventing the direct desorption of C₂H₂ at low temperatures as observed experimentally. Instead, the theoretical results suggest that the experimental data are consistent with a mechanism in which CHO radical dimers lead to the formation of C₂H₂ that presents an exothermic desorption.

Substoichiometric Molybdenum Sulfide Phases with Catalytically Active Basal Planes

Molybdenum sulfide (MoS₂) is widely recognized for its catalytic activities where the edges of the crystals turn over reactions. Generating sulfur defects on the basal plane of MoS₂ can improve its catalytic activity, but generally, there is a lack of model systems for understanding metal-centered catalysis on the basal planes. Here, we synthesized a new phase of substoichiometric molybdenum sulfide (s-MoS_x) on a sulfur-enriched copper substrate. The basal plane of s-MoS_x contains chemically reactive Mo-rich sites that can undergo dynamic dissociative adsorption/desorption processes with molecular hydrogen, thus demonstrating its usefulness for hydrogen-transfer catalysis. In addition, scanning tunneling microscopy was used to monitor surface-directed Ullmann coupling of 2,8-dibromo-dibenzothiophene molecules on s-MoS_x nanosheets, where the 4-fold symmetric surface sites on s-MoS_x direct C-C coupling to form cyclic tetramers with high selectivity.