Instrument at BESSY II shows how light activates MoS2 layers to become catalysts

Thin films of molybdenum and sulfur belong to a class of materials that can be considered for use as photocatalysts. Inexpensive catalysts such as these are needed to produce hydrogen as a fuel using solar energy. However, they are still not very efficient as catalysts. A new instrument at the Helmholtz-Berlin Zentrum’s BESSY II now shows how a light pulse alters the surface properties of the thin film and activates the material as a catalyst.

MoS₂ thin films of superposed alternating layers of molybdenum and sulfur atoms form a two-dimensional semiconducting surface. However, even a surprisingly low-intensity blue light pulse is enough to alter the properties of the surface and make it metallic. This has now been demonstrated by a team at BESSY II.

Enhanced catalytic activity in the metallic phase

The exciting thing is that the MoS₂ layers in this metallic phase are also particularly active catalytically. They can then be employed, for example, as catalysts for splitting of water into hydrogen and oxygen. As inexpensive catalysts, they could facilitate the production of hydrogen – an energy carrier whose combustion produces no CO₂, only water.

New at BESSY II: SurfaceDynamics@FemtoSpeX

Physicist Dr. Nomi Sorgenfrei and her team have constructed a new instrument at BESSY II to precisely measure the changes in samples using temporally-resolved electron spectroscopy for chemical analysis (trESCA) when irradiating the samples with low-intensity, ultra-short light pulses. These light pulses are generated at BESSY II using femtosecond time-slicing (femtoslicing) and are therefore both low intensity and extremely short duration. The new instrument, named SurfaceDynamics@FemtoSpeX, can also rapidly obtain meaningful measurements of electron energies, surface chemistry, and transient alterations using these low-intensity light pulses.

Observation of the phase transition

Analysis of the empirical data showed that the light pulse leads to a transient accumulation of charge at the surface of the sample, triggering the phase transition at the surface from a semiconducting to a metallic state.

“This phenomenon should also occur in other representatives of this class of materials, the p-doped semiconducting dichalcogenides, so it opens up possibilities of influencing functionality and catalytic activity in a deliberate way”, Sorgenfrei explains.