The role of the nanoscale

The role of the nanoscale in “artificial leaves” has been elucidated. Several semiconductor materials are known to catalyze the removal of excess airborne carbon dioxide in the presence of light and organic molecules, just like real leaves. For example, bulk surfaces of cadmium sulfide and zinc sulfide can photocatalytically fix CO₂ into an organic molecule. Cadmium selenide, however, can only accomplish that task in its Cd-rich nanocrystalline form. Three physicists at Oak Ridge National Laboratory and Vanderbilt University believe they have now found out why. In a series of parameter-free, first-principles calculations, they found that CO₂ is adsorbed only at Se vacancies and then becomes negatively charged and, potentially, more reactive. The CO₂ does not react on the surface; it needs to yank the extra electron out of the semiconductor, desorb, and become incorporated into another molecule elsewhere. For this scenario to occur, the extra electron must first be excited into the semiconductor’s conduction band, for example by shining light on it. Still, the energy cost is too high for the charged CO₂ to desorb from bulk CdSe. Enter the nanoscale. As a nanocrystal’s size decreases, its energy gap increases. Thus, electrons can flow freely to desorbing CO₂ molecules if the CdSe crystal is small enough. As a bonus, the theorists found that n-doping with indium might allow CO₂ fixation to take place without the need for light. (L. J. Wang, S. J. Pennycook, S. T. Pantelides, Phys. Rev. Lett. 89, 075506, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.075506 .)