A wide variety of semiconductor systems have proven capable of harnessing near-ultraviolet and visible light and using that energy to drive the reduction of carbon dioxide through photocatalysis or photoelectrochemistry. While p-type semiconductors serve as photocathodes, it is possible to perform CO2 reduction with n-type semiconductors as dark cathodes. Conversely, p-type semiconductors can be used as dark anodes. Unlike particles, in which the majority charge carrier is transferred to the opposite interface on the particle to perform the other half-cell reaction, photoelectrochemical cells require the semiconductor electrodes to be connected to an auxiliary electrode via an external circuit. Semiconductors modified with molecular catalysts take advantage of the selectivity and tunability of molecular catalyst systems as well as the advantages of heterogeneous catalysts including utilization of lower amounts of expensive catalysts, high concentrations of the catalysts at the reaction site, and easy separation of the catalyst from the reaction mixture.
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