BESSY II: Surface analysis of catalyst particles in aqueous solutions
Der Mikrostrahl ist ein schnell fließender Flüssigkeitsstrom, der so dünn ist, dass er nur eine extrem schwache Dampfwolke erzeugt. Photonen und Partikel können die Oberfläche des Strahls erreichen und verlassen, ohne mit den Dampfmolekülen zusammenzustoßen.
In einem feinen Strahl schießt die Flüssigkeit mit suspendierten Metall-Oxid-Nanopartikeln durch das Röntgenlicht. So lassen sich chemische Reaktionen an den Grenzflächen zwischen festen Metall-Oxid-Partikeln und flüssigem Elektrolyt untersuchen.
In a special issue on the liquid jet method, a team reports on reactions of water molecules on the surfaces of metal oxide particles. The results are relevant for the development of efficient photoelectrodes for the production of green hydrogen.
Green hydrogen can be produced directly in a photoelectrochemical cell, splitting water with solar energy. However, this requires the development of super-efficient photoelectrodes that need to combine many talents at the same time. They must be excellent at converting sunlight into electricity, remain stable in acidic or basic water, act as catalysts to promote the splitting of water into hydrogen and oxygen, and be cheap, abundant and non-toxic.
The large material class of metal oxides comes into question. It is difficult to find out what really happens at the interfaces between the solid metal oxide electrodes and the aqueous electrolyte. This is because standard X-ray analysis does not work to investigate processes on samples in liquid environments. One of the few suitable methods are experiments with a liquid jet: an extremely fine jet of liquid in which nanoparticles of metal oxide are suspended. The jet shoots through the X-ray light of BESSY II, and the interference of the evaporated molecules with the measurement data is negligible (more in the foreword to the special issue).
Dr. Robert Seidel is an expert on this liquid jet method, which is the subject of a special issue of Accounts of Chemical Research. He was invited to be the guest-editor of the issue and to report also on new experiments at BESSY II that he conducted with Dr. Hebatallah Ali and Dr. Bernd Winter from the Fritz Haber Institute. They investigated two important model systems for photoelectrodes: Nanoparticles of iron oxide (hematite, α-Fe2O3, and anatase (titanium oxide or TiO2) in aqueous electrolytes with different pH values. Hematite and anatase in suspension are photocatalytic model systems. These are ideal for studying the solid/electrolyte interface at the molecular level and for exploring the chemical reactions at electrode-electrolyte interfaces.
Insights into interactions at surfaces with water molecules
“We used resonant photoelectron spectroscopy (PES) to identify the characteristic fingerprints of different reactions. This allowed us to reconstruct which reaction products are formed under different conditions, particularly as a function of pH.” The key question: How do the water molecules react with or on the nanoparticle surfaces?
In fact, how acidic or how basic an electrolyte is makes a big difference, Seidel noted. “At low pH, the water molecules on the surface of hematite tend to split. This is not the case with anatase, where water molecules are adsorbed on the surface of the TiO2 nanoparticles,” says Seidel. A basic pH value is required for water molecules to break down on the anatase nanoparticles. “Such insights into surface interactions with water molecules are only possible with this liquid-jet method,” says Seidel.
The spectra also revealed ultra-fast electron transitions between the metal oxide and the (split) water molecules on the surface. The results provide insights into the first steps of water dissociation and help to clarify the mechanisms of light-induced water splitting on metal oxide surfaces.
The special issue ‘Applications of Liquid Microjets in Chemistry
The editorial starts with a intriguing comparison: “Pour a glass of water, and bring it right up to your nose, one centimeter away. What don’t you see? About 3 million molecules intervene along a line between the tip of your nose and the surface of the water. Imagine an X-ray photon or charged or neutral particle trying to reach or escape the surface but first colliding with some of these interloping molecules in ways that scramble information about their interactions with interfacial and deeper water molecules.”
What is so vividly described in the foreword to this special issue has long been a major problem. It was not until 1988 that the liquid jet method, introduced by Manfred Faubel, Stephan Schlemmer and Jan Peter Toennies, made it possible to study water surfaces without these disturbances. The microjet is a fast-flowing stream of liquid so narrow that it produces only an extremely dilute vapour cloud. Photons and particles can reach and leave the surface of the jet without colliding with the vapour molecules. A special issue of the journal Accounts of Chemical Research now presents exciting new results using this method. HZB researcher Dr Robert Seidel was invited to be the guest editor.
- The future of corals – what X-rays can tell usThis summer, it was all over the media. Driven by the climate crisis, the oceans have now also passed a critical point, the absorption of CO2 is making the oceans increasingly acidic. The shells of certain sea snails are already showing the first signs of damage. But also the skeleton structures of coral reefs are deteriorating in more acidic conditions. This is especially concerning given that corals are already suffering from marine heatwaves and pollution, which are leading to bleaching and finally to the death of entire reefs worldwide. But how exactly does ocean acidification affect reef structures?
Prof. Dr. Tali Mass, a marine biologist from the University of Haifa, Israel, is an expert on stony corals. Together with Prof. Dr. Paul Zaslansky, X-ray imaging expert from Charité Berlin, she investigated at BESSY II the skeleton formation in baby corals, raised under different pH conditions. Antonia Rötger spoke online with the two experts about the results of their recent study and the future of coral reefs.
- 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.