The removal of CO2 from the atmosphere is a major environmental issue. Artificial "fixation" of CO2, i.e., its incorporation in organic molecules such as formic acid, requires a catalyst. Bulk CdS and ZnS surfaces catalyze CO2 fixation in the presence of light, but CdSe surfaces do not. However, Cd-rich CdSe nanocrystals below a certain critical size are efficient photo-catalysts. We report first-principles calculations that reveal distinct roles played by several different aspects of the nanoscale.
On flat stoichiometric CdSe surfaces a CO2 molecule physisorbs and is no more reactive than in the free state. At a Cd vacancy, however, strong chemisorption occurs, and the molecule draws extra electron density from the back bonds to become negatively charged. The barrier for desorption is ~0.3 eV suggesting that, even at room temperature, CO2 molecules would be constantly chemisorbing and desorbing. If a chemisorbed molecule could desorb and carry an extra electron with it, it would be highly reactive (Fig 1). The energy cost is high, however, (1.3 eV, see Fig. 2a). Photoexcitation, which excites electrons to the conduction bands is essential for the catalytic process to occur. Doping the crystal n-type in the calculation reduces the energy cost to only 0.4 eV (Fig. 2c). It is now that a nanocrystal enters the scene as an absolute necessity. The energy gap of a nanocrystal increases with decreasing size (Fig. 2d). The critical diameter to enable the free flow of crystal electrons to desorbing CO2 molecules is estimated at about 3.5 nm, which compares well with the experimental value of 5 nm (Fig. 2e).
The calculations show first, that catalysis can occur away from a surface by generation of mobile reactive species, and second, that the most important role of the nanoscale is the opening of the band gap. This understanding may lead to effective implementation of semiconductor nanocrystals as "artificial leaves" to alleviate global warming and the depletion of fossil fuels.