CO2 Reduction under Periodic Illumination of ZnS

High Resolution Image Download MS PowerPoint Slide The photoreduction of CO 2 to formate (HCOO – ) in sphalerite (ZnS) aqueous suspensions is systematically studied in the presence of Na 2 S hole scavenger. A series of cut-on filters at λ cut-on ≥ 280, 295, 305, 320, and 400 nm are used to measure the reaction rate of formate production. The dependence of the measured reaction rates on λ cut-on indicates that a wavelength of λ = 345 nm is associated with the actual bandgap of the semiconductor nanocrystallites suspended in water. The results from apparent quantum yield measurements during periodic illumination experiments suggest that (1) valence-band holes on the surface of ZnS disappear within deciseconds due to the oxidation of the scavenger while simultaneously pumping electrons to the conduction band, (2) excited electrons in the conduction band of ZnS are transferred to CO 2 to produce the intermediate CO 2 •–, and (3) CO 2 •– abstracts a proton from water and undergoes further photoreduction on the surface of ZnS in an overall time scale for steps 2 + 3 of a few milliseconds. The separation of both process merges at ∼29 ms because it decreases exponentially with a drop in Na 2 S accompanied by a less negative surface potential. The behavior of the reaction rate at variable pH resembles the fraction of dissolved CO 2, discarding the direct participation of bicarbonate and carbonate in the reaction. Combined chromatographic, mass spectrometry, and spectroscopic studies provide new insights to understand the role of surface chemistry on the photoreduction of CO 2 on ZnS nanocrystals.