Nanodiamonds as photocatalysts

Doped Diamond Foam.

Doped Diamond Foam. © P. Knittel/Fraunhofer IAF

Diamond nanomaterials are considered hot candidates for low-cost photocatalysts. They can be activated by light and can then accelerate certain reactions between water and CO2 and produce carbon-neutral "solar fuels". The EU project DIACAT has now doped such diamond materials with boron and shown at BESSY II how this could significantly improve the photocatalytic properties.

Climate change is in full swing and will continue unabated as long as we do not succeed in significantly reducing CO2 emissions. For this we need all the options. One idea is to return the greenhouse gas CO2 to the energy cycle: CO2 could be processed with water into methanol, a fuel that can be excellently transported and stored. However, the reaction, which is reminiscent of a partial process of photosynthesis, requires energy and catalysts. If we succeed in using this energy from sunlight and developing light-active photocatalysts that are not made of rare metals such as platinum, but of inexpensive and abundantly available materials, there would be a chance of "green" solar fuels being produced in a climate-neutral way.

Diamond Nanomaterials need UV for activation

A candidate for such photocatalysts are so-called diamond nanomaterials - these are not precious crystalline diamonds, but tiny nanocrystals of a few thousand carbon atoms that are soluble in water and look more like black slurry, or nanostructured "carbon foams" with high surface area. In order for these materials to become catalytically active, however, they require UV light excitation. Only this spectral range of sunlight is rich enough in energy to transport electrons from the material into a "free state". Only then solvated electrons can be emitted in water and react with the dissolved CO2 to form methanol.

Can doping help?

However, the UV component in the solar spectrum is not very high. Photocatalysts that could also use the visible spectrum of sunlight would be ideal. This is where the work of HZB-scientist Tristan Petit and his cooperation partners in DIACAT comes in: modelling the energy levels in such materials, performed by Karin Larsson in Uppsala University, shows that intermediate stages can be built into the band gap by doping with foreign atoms. Boron, a trivalent element, appears to be particularly important.

Experiments at BESSY II show: yes, but...

Petit and his team therefore investigated samples of polycrystalline diamonds, diamond foams and nanodiamonds. These samples had previously been synthesized in the groups of Anke Krüger in Würzburg and Christoph Nebel in Freiburg. At BESSY II, X-ray absorption spectroscopy was used to precisely measure the unoccupied energy states where electrons could possibly be excited by visible light. "The boron atoms  present near the surface of these nanodiamonds actually lead to the desired intermediate stages in the band gap," explains Ph.D student Sneha Choudhury, first author of the study. These intermediate stages are typically very close to the valence bands and thus do not allow the effective use of visible light. However, the measurements show that this also depends on the structure of the nanomaterials.

Outlook: Morphology and doping with P or N

"We can introduce and possibly control such additional steps in the diamond bandgap by specifically modifying the morphology and doping," says Tristan Petit. Doping with phosphorus or nitrogen could also offer new opportunities.

Published in Journal of Materials Chemistry A (2018):Combining nanostructuration with boron doping to alter sub band gap acceptor states in diamond materials; Sneha Choudhury, Benjamin Kiendl, Jian Ren, Fang Gao, Peter Knittel, Christoph Nebel, Amélie Venerosy, Hugues Girard, Jean-Charles Arnault, Anke Krueger, Karin Larsson & Tristan Petit

DOI: 10.1039/c8ta05594g

EU-Project DIACAT: https://www.diacat.eu/

arö


You might also be interested in

  • Best Innovator Award 2023 for Artem Musiienko
    News
    22.03.2024
    Best Innovator Award 2023 for Artem Musiienko
    Dr. Artem Musiienko has been awarded a special prize for his groundbreaking new method for characterising semiconductors. At the recent annual conference of the Marie Curie Alumni Association (MCAA) in Milan, Italy, he received the MCAA Award for the best innovation. Since 2023, Musiienko has been carrying out his research project with a postdoctoral fellowship from the Marie Sklodowska Curie Actions in Antonio Abate's department, Novel Materials and Interfaces for Photovoltaic Solar Cells (SE-AMIP).
  • Fertilisation under the X-ray beam
    Science Highlight
    19.03.2024
    Fertilisation under the X-ray beam
    After the egg has been fertilized by a sperm, the surrounding egg coat tightens, mechanically preventing the entry of additional sperm and the ensuing death of the embryo. A team from the Karolinska Institutet has now gained this new insight through measurements at the X-ray light sources BESSY II, DLS and ESRF. 
  • Neutron experiment at BER II reveals new spin phase in quantum materials
    Science Highlight
    18.03.2024
    Neutron experiment at BER II reveals new spin phase in quantum materials
    New states of order can arise in quantum magnetic materials under magnetic fields. An international team has now gained new insights into these special states of matter through experiments at the Berlin neutron source BER II and its High-Field Magnet. BER II served science until the end of 2019 and has since been shut down. Results from data at BER II are still being published.