Jakob Kibsgaard

Size-dependent structure of MoS2 nanocrystals

Molybdenum disulphide nanostructures are of interest for a wide variety of nanotechnological applications ranging from the potential use of inorganic nanotubes in nanoelectronics to the active use of nanoparticles in heterogeneous catalysis. Here, we use atom-resolved scanning tunnelling microscopy to systematically map and classify the atomic-scale structure of triangular MoS2 nanocrystals as a function of size. Instead of a smooth variation as expected from the bulk structure of MoS2, we observe a very strong size dependence for the cluster morphology and electronic structure driven by the tendency to optimize the sulphur excess present at the cluster edges. By analysing of the atomic-scale structure of clusters, we identify the origin of the structural transitions occurring at unique cluster sizes. The novel findings suggest that good size control during the synthesis of MoS2 nanostructures may be used for the production of chemically or optically active MoS2 nanomaterials with superior performance.

Building an appropriate active-site motif into a hydrogen-evolution catalyst with thiomolybdate [Mo3S13]2− clusters

Identifying and understanding the active sites responsible for reaction turnover is critical to developing improved catalysts. For the hydrogen-evolution reaction (HER), MoS2 has been identified as an active non-noble-metal-based catalyst. However, only edge sites turnover the reaction because the basal planes are catalytically inert. In an effort to develop a scalable HER catalyst with an increased number of active sites, herein we report a Mo-S catalyst (supported thiomolybdate [Mo3S13](2-) nanoclusters) in which most sulfur atoms in the structure exhibit a structural motif similar to that observed at MoS2 edges. Supported sub-monolayers of [Mo3S13](2-) nanoclusters exhibited excellent HER activity and stability in acid. Imaging at the atomic scale with scanning tunnelling microscopy allowed for direct characterization of these supported catalysts. The [Mo3S13](2-) nanoclusters reported herein demonstrated excellent turnover frequencies, higher than those observed for other non-precious metal catalysts synthesized by a scalable route.

Size Threshold in the Dibenzothiophene Adsorption on MoS2 Nanoclusters

In hydrodesulfurization (HDS) of fossil fuels, the sulfur levels are reduced by sulfur extraction from hydrocarbons through a series of catalyzed reaction steps on low-coordinated sites on molybdenum disulfide (MoS(2)) nanoclusters. By means of scanning tunneling microscopy (STM), we show that the adsorption properties of MoS(2) nanoclusters toward the HDS refractory dibenzothiophene (DBT) vary dramatically with small changes in the cluster size. STM images reveal that MoS(2) nanoclusters with a size above a threshold value of 1.5 nm react with hydrogen to form so-called sulfur vacancies predominately located at edge sites, but these edge vacancies are not capable of binding DBT directly. In contrast, MoS(2) nanoclusters below the threshold perform remarkably better. Here, sulfur vacancies form predominantly at the corner sites, and these vacancies show a high affinity for DBT. The results thus indicate that very small MoS(2) nanoclusters may have unique catalytic properties for the production of clean fuels.

High-quality Fe-doped TiO2 films with superior visible-light performance

We report on high-quality polycrystalline Fe-doped TiO2 (Fe–TiO2) porous films synthesized via one-step electrochemical oxidation. We demonstrate that delicate properties such as the impurity concentration and the microstructure that strongly influence the performance of the material for photovoltaic and photocatalysis applications can be controlled by adjusting the electrolyte composition. Compared to Fe-doped TiO2 films prepared with traditional phosphate- or silicate-based electrolytes, our newly synthesised Fe–TiO2 films contain solely Fe dopants, which results in excellent photocatalytic and photovoltaic performance under visible light irradiation.

Electrocatalysis of water oxidation by H2O-capped iridium-oxide nanoparticles electrodeposited on spectroscopic graphite.

Electrocatalysis of water oxidation by 1.54 nm IrOx nanoparticles (NPs) immobilized on spectroscopic graphite electrodes was demonstrated to proceed with a higher efficiency than on all other, hitherto reported, electrode supports. IrOx NPs were electrodeposited on the graphite surface, and their electrocatalytic activity for water oxidation was correlated with the surface concentrations of different redox states of IrOx as a function of the deposition time and potential. Under optimal conditions, the overpotential of the reaction was reduced to 0.21 V and the electrocatalytic current density was 43 mA cm(-2) at 1 V versus Ag/AgCl (3 M KCl) and pH 7. These results beneficially compete with previously reported electrocatalytic oxidations of water by IrOx NPs electrodeposited onto glassy carbon and indium tin oxide electrodes and provide the basis for the further development of efficient IrOx NP-based electrocatalysts immobilized on high-surface-area carbon electrode materials.