Electrospun CuO Nanofibers: Stable Nanostructures for Solar Water Splitting

A novel synthetic approach for electrospinning of CuO nanofibers was applied to produce fibrous CuO photocathodes, calcined at different temperatures, in order to evaluate the impact of crystallinity and defect sites on solar-driven photoelectrochemistry. By careful optimization of the preparation conditions, stable electrospinning was achieved, allowing the fabrication of large quantities of highly crystalline, uniform CuO nanofibers. The as-spun fibers were calcined at 300 °C, 400 °C, 550 °C and 800 °C in air, and systematically characterized concerning their crystallographic and structural evolution. The correlation of the physicochemical properties with the PEC performance of CuO nanofiber photocathodes reveals a structure-property relationship: the higher the annealing temperature, the more developed are the crystalline domains of the nanofibers, which in turn result in better conductivity and less defect sites serving as trap states for the photo-excited charge carriers. Hence, the CuO nanofiber photocathodes annealed at 800 °C showed the highest photoresponse and stability of the here presented photocathodes, with no loss of the photocurrent after prolonged operation in aqueous electrolyte. Further improvement of the CuO photocathodes was realized by increasing the film thickness leading to photocurrents up to -0.16 mA at 0.4 V vs. RHE, however the stability of the thick photocathode remains a critical issue.