Instrument at BESSY II shows how light activates MoS2 layers to become catalysts
A new instrument at BESSY II can be used to study molybdenum-sulfide thin films that are of interest as catalysts for solar hydrogen production. A light pulse triggers a phase transition from the semiconducting to the metallic phase and thus enhances the catalytic activity.
© Martin Künsting /HZB
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.
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.