Controlling the spin of electrons opens up future scenarios for applications in spin-based electronics (spintronics), for example in data processing. It also presents new opportunities to control the selectivity and efficiency of chemical reactions. The researchers recently presented their first successes with the example of splitting water to produce “green” hydrogen and oxygen. A joint project involving working groups from the Center for Soft Nanosciences at the University of Münster (Germany) and from the University of Pittsburgh Institute of Chemistry (Pennsylvania, Professor David Waldeck) now has the task of promoting the systematic development of spin. – Selective catalyst materials. To this end, the researchers linked the catalytic activity of various spin-polarized inorganic materials to direct measurements of spin selectivity. The focus is on oxide materials that have been intentionally grown with a chiral structure. In addition, the researchers also want to investigate the origin of spin polarization in these chiral materials. The results of a preliminary study of chiral copper oxide layers in . have now been published ACS nano magazine.
Results in brief
The team of German and American researchers first examined chiral oxide catalysts – which in this case consist of thin layers of chiral copper oxide on a thin film of gold. The measured data show that the sine polarization of the electrons depends on the layers from which the electrons come. The team considers two effects to be responsible for this: the contrast-induced spin selectivity (CISS) effect and the magnetic arrangement in the chiral layers. The results are intended to aid in the future production of spin-selective oxide catalytic materials, thus improving the efficiency of chemical reactions.
Fuel cell example: unwanted electron spin reduces efficiency
Explanation of the background: why the electron spin is important in the following example. In fuel cells, hydrogen and oxygen react with each other to form water, releasing electrical energy in the process. The hydrogen may have previously been produced through a reverse process, which breaks water molecules down into hydrogen and oxygen. The energy required for this can be provided by electrical power from renewable energy sources or directly through sunlight, so that in the future hydrogen can serve as an energy source in a power cycle designed to be carbon dioxide.2-neutral.
What hinders any large-scale commercialization of this concept – for example, in fuel cell electric cars – is, among other things, low efficiency. A lot of energy must be used to break down water molecules, which means that it is currently less expensive to use that energy directly to recharge a car battery. This lower efficiency in breaking down water molecules is a result not only of the excessive voltage required to develop oxygen at the anode of the electrolytic cell, but also of producing unwanted by-products such as hydrogen peroxide and electronically excited oxygen. Due to their high reactivity, these byproducts can also attack the electrode material. Both byproducts occur in a state called the singular state, in which the spins of electrons participating in molecular bonds in an antiparallel position are aligned with each other. In the desired product of the reaction – oxygen in the electronic ground state – this is not the case because it forms a triplet state with parallel-aligned spins, thus generating only one spin direction helps to reach this desired oxygen state.
A new approach: the oxide catalyst produces the required electron spin
This is a novel approach because it involves the rotation of the adsorbed radicals on the surfaces of the catalysts, from which the byproducts are formed, in parallel. Such parallel alignment of the electron cycles can be achieved by using a chiral material. In this case, the transmission of electrons through the electrodes could be as a result of the Quebec effect, or through a structural change in the oxide, spin-selective. As a result, the formation of molecules in the unwanted single state is suppressed and hydrogen production is increased.
While researchers have successfully demonstrated selective rotational stimulation, there is still no complete understanding of the origin of the Quebec effect. The selective transfer of electrons through helical—and thus also—molecules has been demonstrated. However, recent studies show that spin-selective transition also occurs in inorganic and non-chiral inorganic materials. Inorganic rotational filtration surfaces are more chemically stable than chiral molecular layers and allow greater current density in the context of rotational selective catalysis.
The current study in detail
In the now-published study, lead author Paul Mullers, a doctoral student at the University of Munster, examined chiral copper oxide films just a few nanometers thick that had previously been electrochemically deposited in helical form on thin gold substrates by researchers from Pittsburgh. UV laser pulses were used to stimulate the photoelectrons from the samples and their average spin polarization (in a spin polarimeter based on “Mott scattering”) was measured. Depending on whether the samples were struck from the front oxide-covered side or from the reverse side, in the process, electrons with different energies were emitted from the gold substrate or from the oxide films themselves, in different proportions. By correlating the energy distribution with the measured spin polarization values, the Munster researchers showed that electrons from both layers are polarized at different ranges.
Electrons are filtered from the gold substrate, with respect to its spin, by the Quebec effect as they pass through the chiral layer. Electrons from chiral copper oxide display opposite chiral polarization, and in the case of films thicker than 40 nm, these copper oxide electrons predominate. Additional measurements by the working group led by Professor Heiko Wende at the Department of Physics at the University of Duisburg-Essen indicate that this reflects a magnetic arrangement in the chiral layers that has not been observed in non-chiral oxide films with the same composition.
In order to pursue this hypothesis, the experimental setup in Munster will be extended by the possibility of directly measuring the spin polarization of electrons depending on their energy. Continuing from the now published study, further measurements on copper and cobalt oxide films will not only allow a clear distinction between the two polarization mechanisms, but also which inorganic inorganic catalysts will be specifically engineered.
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Spin-polarized light emission from CuO Chiral catalyst thin films
The date the article was published
August 9, 2022
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