An Osaka University research team has created a new material based on gold and black phosphorous to produce clean hydrogen fuel using more of the sunlight spectrum.
One of the cleanest low-carbon fuels is hydrogen, which is able to react with oxygen to release energy and emit only water as the end product. However, it is difficult to obtain it because most hydrogen is locked into H2O or other molecules. Hydrogen can be acquired by splitting H2O, but this tends to use more energy than the produced hydrogen can give back in return, making it inefficient. Water splitting is often done through solar power (or solar-to-hydrogen conversion), done by converting sunlight to chemical energy, but only part of the UV spectrum is absorbed by traditional materials. The Osaka University research team developed a material which uses a broader spectrum of the sunlight, with the potential to increase the efficiency of solar-to-water conversion.
The material is a three-part composite which maximizes the potential to both absorb light and its efficiency for splitting water. The core of the material is a traditional titanium oxide semiconductor, but is covered with tiny gold nanoparticles. The gold covered titanium oxide semiconductor is then mixed with ultrathin sheets of the element black phosphorous (BP) which functions as a light absorber.
“BP is a wonderful material for solar applications, because we can tune the frequency of light just by varying its thickness, from ultrathin to bulk,” says Tetsuro Majima, the research team leader. “This allows our new material to absorb visible and even near infrared light, which we could never achieve with LTO alone.”
BP is stimulated by absorbing the large spectrum of energy, releasing electrons which are conducted to the gold nanoparticles in the titanium oxide semiconductor. The gold nanoparticles will then absorb visible light, separating some of its own electrons in the process. Those free electrons in both the gold nanoparticles and black phosphorus transfer into the titanium oxide semiconductor, where they function as an electric current to split water apart.
Hydrogen production utilizing this material is not only more efficient due to the broader spectrum of light absorption, but also due to the increase electron conduction which is caused by the interface between BP and the titanium oxide. The material is almost 60 times more active than liquid titanium oxide on its own.
“By efficiently harvesting solar energy to generate clean fuel, this material could help to clean up the environment,” says Majima. “Moreover, we hope our study of the mechanism will spur new advances in photocatalyst technology.”