New-generation integrated devices based on dye-sensitized and perovskite solar cells for energy harvesting and storage are significantly important for self-powering systems and portable/wearable electronics.

First-principles calculations were performed to study the stabilities, electronic structures, and chemical activities of various CuO surfaces for the understanding of the gas-induced change of conductance of CuO nanowires. It was found that CuO(111) and CuO(1 j 11) have the lowest surface energies under ambient conditions and hence should be the most preferential facets of CuO nanowires. While band gaps of these surfaces are narrower than that of bulk CuO, they maintain the semiconductor feature. Adsorption of oxidizing gas such as O 2 or NO 2 on the CuO(111) and CuO(1 j 11) surfaces induces metallic behavior, and molecules gain electrons from the substrates. These two effects result in an increase of hole density and hence enhance the surface conductivity of CuO nanowires as observed in our experiments. On the contrary, adsorption of H 2 O molecules on CuO(111) not only widens the band gap but also donates electrons to the surface, which leads to reduction of surface conductivity. In addition, we found that CuO(111) is potentially an efficient catalyst for CO oxidation through the Mars-van Krevelen mechanism. † Part of the "D. Wayne Goodman Festschrift".