Semiconductors are materials that can absorb light and separate electrons and holes to perform electrochemical reactions. This property can be utilized to convert solar energy into chemical energy by reducing water to hydrogen or carbon dioxide to hydrocarbons at p-type semiconductors, whereas n-type semiconductors can conduct oxidative reactions such as water oxidation.
In the Bocarsly Group, recent work has focused on a class of semiconductors with the delafossite structure (ABO2), particularly those with CuI in the A site. These materials are less subject to photodegradation than many other p-type semiconductors, such as gallium phosphide, due to the predominance of trivalent B-site atom acceptor states in the conduction band, though the presence of small amounts Cu 3d character in the conduction band can lead to instability.
One of the delafossites studied was Mg-doped CuFeO2, which has a band gap of 1.36 eV and a conduction band edge of -1.1 V vs. SCE.1 This semiconductor was able to reduce CO2 directly starting at an underpotential of ~400 mV and, at -0.9 V vs. SCE, yielded formate with a Faradaic efficiency of 10%, with the balance likely going to H2 formation. Though some Cu0 is detected after extended electrolysis, it is not responsible for the production of formate.
A similar material, CuRhO2, having a band gap of 1.9 eV and conduction band edge of -1.5 V vs. SCE, was studied in the context of water reduction.2 In a 1 M NaOH solution under argon, H2 production decreased substantially over time and the surface of the electrode had significant amounts of Cu0. However, when the solution was saturated with air, the electrode yielded H2 consistently over a 9 hour period. The dissolved oxygen improved electrode stability by preventing the decomposition of the CuRhO2 into rhodium oxide and metallic copper.
1. Gu, J.; Wuttig, A.; Krizan, J.W.; Hu, Y.; Detweiler, Z.D. Cava, R.J.; Bocarsly, A.B. Mg-Doped CuFeO2Photocathodes for Photoelectrochemical Reduction of Carbon Dioxide. J. Phys. Chem. C 2013, 117 (24), 12415-12422.
2. Gu, J.*; Yan, Y.*; Krizan, J. W.; Gibson, Q. D.; Detweiler, Z. M.; Cava, R. J.; Bocarsly, A. B. p-Type CuRhO2 as a Self-Healing Photoelectrode for Water Reduction under Visible Light. J. Am. Chem. Soc. 2014, 136 (3), 830-833.