Expensive noble metals are frequently used to illuminate screens or convert solar energy into fuels. Chemists at the University of Basel have now succeeded in replacing these rare elements with a much less expensive metal. The new materials have properties that are very similar to those used in the past.
We’re all familiar with chromium from everyday uses like chromium steel in the kitchen and chrome-plated motorcycles. However, the element may soon be found in ubiquitous mobile phone screens or used to convert solar energy. Researchers at the University of Basel’s Department of Chemistry, led by Professor Oliver Wenger, have developed chromium compounds that can replace the noble metals osmium and ruthenium – two elements that are nearly as rare as gold or platinum – in luminescent materials and catalysts.
The team reports in Nature Chemistry that the luminescent properties of the new chromium materials are nearly as good as some of the osmium compounds used previously. In comparison to osmium, chromium is approximately 20,000 times more abundant in the earth’s crust – and much cheaper.
If the new chromium compounds are irradiated with a red lamp, the energy from the light can be stored in molecules which can then serve as a power source. Here, there’s also the potential to use our new materials in artificial photosynthesis to produce solar fuels.
Professor Oliver Wenger
The new materials are also proving to be effective photochemical catalysts, including processes triggered by light exposure, such as photosynthesis. This process is used by plants to convert sunlight energy into energy-rich glucose and other substances that serve as fuel for biological processes.
If the new chromium compounds are irradiated with a red lamp, the energy from the light can be stored in molecules which can then serve as a power source. “Here, there’s also the potential to use our new materials in artificial photosynthesis to produce solar fuels,” explains Wenger.
Tailor-made packaging for chromium
The researchers built the chromium atoms into an organic molecular framework of carbon, nitrogen, and hydrogen to make them glow and convert energy. This organic framework was designed to be particularly stiff, allowing the chromium atoms to be well packaged. This tailored environment aids in minimizing energy losses caused by unwanted molecular vibrations and optimizing luminescent and catalytic properties. The disadvantage of the new materials is that chromium requires a more complex framework than noble metals, necessitating additional research in the future.
Encased in its rigid organic framework, chromium proves to be much more reactive than noble metals when exposed to light. This paves the way for photochemical reactions that are otherwise difficult to initiate. A potential application could be in the production of active pharmaceutical ingredients.
Competition with other alternatives
For a long time, the search for sustainable and cost-effective materials that did not contain noble metals was primarily focused on iron and copper. Other research groups have already seen promising results with both of these elements, and chromium has been used in the past to make luminescent materials.
However, in many cases, the luminescent and catalytic properties of these materials lagged far behind those of materials containing rare and expensive noble metals, rendering them ineffective as a viable alternative. The new chromium materials are distinct because they contain a form of chromium that is particularly similar to noble metals, achieving luminescent and catalytic efficiencies that are comparable to those of materials containing such metals.
“At the moment, it seems unclear which metal will ultimately win the race when it comes to future applications in luminescent materials and artificial photosynthesis,” Wenger said. “What is certain, however, is that the postdocs Dr. Narayan Sinha and Dr. Christina Wegeberg have made important progress together.”
Following that, Wenger and his research team intend to develop their materials on a larger scale to allow for more extensive testing of potential applications. They hope to achieve light emission in various spectral colors ranging from blue to green to red by making additional improvements. They also want to improve the catalytic properties to get us one step closer to converting sunlight into chemical energy for storage, as in photosynthesis.
