Fig. 5.

Band energy diagram of selected semiconductors. These materials are commonly used for photoelectrochemical water splitting and CO₂ reduction. Redox potentials of key CO₂ reduction reactions are also included. In principle, water splitting and CO₂ reduction can take place on the same semiconductor material if the conduction band energy level is aligned with, or more negative than, the energy level of the targeted CO₂ methanation reaction (−0.24 V_NHE) and the valence band energy level is aligned with, or more positive than, the oxygen evolution reaction energy level (1.23 V_NHE). This is indicated by the position of each material relative to the vertical bar dividing the figure. The materials exhibiting unfavorable band alignment are included in the figure, as they are commonly used as light-absorbers in photoelectrochemical cells^{66, 98}—adapted from ref. ⁶⁶—Published by Wiley-VCH; and ref. ⁹⁸—Published by The Royal Society of Chemistry