Four diamond materials are shown: ‘Diamond black’ made of polycrystalline nanostructured carbon (top right); the same material before nanostructuring (top left); an intrinsic single crystal (bottom left); and a single crystal doped with boron (bottom right). Photo: A. Chemin/HZB.Using just the energy of sunlight, photoelectrodes can convert the greenhouse gas carbon dioxide (CO2) into methanol or nitrogen into valuable fertilizer. Now, a team led by Tristan Petit at Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Germany has shown that diamond materials could be ideally suited for such photoelectrodes.
By combining X-ray spectroscopic techniques at HZB’s BESSY II with other measurement methods, the team succeeded in precisely tracking which conversion processes are excited by light and revealed the crucial role played by the surface of diamond materials. They report their findings in a paper in Small Methods.
At first glance, lab-grown diamond materials have little in common with their namesakes in the jewellery shop. They are often opaque, dark and don’t look spectacular at all. But even if their looks are unimpressive, they show promise in many different applications, such as brain implants, quantum sensors and computers, as well as metal-free photoelectrodes in photo-electrochemical energy conversion. Such metal-free electrodes are fully sustainable and made of carbon only, they degrade little over time compared with metal-based photoelectrodes, and they can be industrially produced.
Diamond materials are suitable for use as metal-free photoelectrodes because when excited by light they can release electrons in water, thereby triggering chemical reactions that are difficult to initiate otherwise. A concrete example is the reduction of CO2 to methanol, which turns the greenhouse gas into a valuable fuel. It would also be exciting to utilize diamond materials to convert molecular nitrogen (N2) into nitrogen fertilizer (NH3), as this would use much less energy than the Haber-Bosch process.
However, diamond electrodes oxidize in water and it had always been assumed that their oxidized surfaces no longer emit electrons. In addition, the bandgap of diamond is in the UV range (at 5.5eV), so visible light is unlikely to be sufficient to excite the electrons. Despite this, however, previous studies have recorded puzzling emissions of electrons from visible light excitation. In their study, Petit and his team now uncover new insights and give cause for hope.
Arsène Chemin, a postdoctoral researcher in Petit's team, studied samples of diamond materials produced at the Fraunhofer Institute for Applied Solid State Physics in Freiburg, Germany. The samples were engineered to facilitate the CO2 reduction reaction. They were doped with boron to ensure good electrical conductivity and nanostructured to provide huge surface areas for enhancing the emission of charge carriers such as electrons.
Chemin used four X-ray spectroscopic methods at BESSY II to characterize the surfaces of the diamond samples and the energy needed to excite specific electronic surface states. Then he used the surface photovoltage measured in a specialized laboratory at HZB to determine which of these states are excited and how the charge carriers are displaced in the samples. In addition, he measured the photoemission of electrons by the samples in either air or in liquid. By combining these results, he was able, for the first time, to draw a comprehensive picture of the processes that take place on the surfaces of the diamond samples after excitation by light.
"Surprisingly, we found almost no difference in the photoemission of charges in liquid, regardless of whether the samples were oxidized or not," says Chemin. This shows that diamond materials are well suited for use in aqueous solutions. Excitation with visible light is also possible: in the case of the boron-doped samples, violet light (3.5 eV) proved sufficient to excite the electrons.
"These results are a great cause for optimism," says Chemin: "With diamond materials we have a new class of materials that can be explored and widely used." What’s more, the basic approach of combining these different spectroscopic methods could also lead to new breakthroughs in other photoactive semiconductor materials.
This story is adapted from material from HZB, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.