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 2 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.