Atmospheric concentrations of the greenhouse gas carbon dioxide have increased dramatically over the past few decades. Plants can help lower these levels through photosynthesis, in which carbon dioxide (CO₂) is removed from the air and converted to harmless oxygen. Real vegetation requires light to perform this transformation, but new findings suggest that artificial leaves may one day facilitate this change--even in the dark.

In a paper to be published in Physical Review Letters, Ligen G. Wang, then at Oak Ridge National Laboratory, and colleagues describe having investigated why some substances can only catalyze the transformation of carbon dioxide into other organic molecules, a process known as fixation, under certain circumstances. Whereas big chunks of cadmium selenide (CdSe) fail to fix CO₂, cadmium-rich nanocrystals of the compound under a certain size jumpstart the reaction. The team's calculations shed light on why this is. As it turns out, the reaction does not take place directly on the metal surface. Instead, each carbon dioxide molecule attaches to the metal and gains an electron, making it highly reactive. Unlike large pieces, the tiny crystals enable this process because it requires less energy to transfer electrons from the metal to CO₂ molecules. These so-called radicals are subsequently freed and the fixation reaction proceeds away from the crystal's surface.

The scientists' calculations further show that if atoms of indium were added to the nanocrystals, the reaction would no longer need light energy in order to occur. This suggests that such nanocrystals could form the basis of artificial leaves. If so, in dark, carbon dioxide-rich regions such as smokestacks, these leaves could remove the harmful greenhouse gas before it reaches the atmosphere.