Narrow band gap cuprous/cupric oxide thin films prepared via sol-gel methods for the electrochemical reduction of CO2

A convenient method for preparing cupric and cuprous oxide thin films from a common sol-gel approach is described. The method consists in depositing a thin film of a copper nitrate precursor embedded in a hydroxypropyl cellulose matrix followed by calcination at 500 degrees C. It was found that cupric oxide (CuO) thin films were produced if the calcination step was carried in air, while cuprous oxide (Cu2O) thin films were predominantly produced if the calcination was carried under a constant-flow rate nitrogen stream. Interestingly, the Cu2O films were shown to be doped with carbon and nitrogen atoms, with the dopant concentration being related to the nitrogen flow rate used during the calcination step. Importantly, the doping induces a decrease in band gap to lower values than the 2.2eV benchmark for intrinsic Cu2O. The conductive properties of the doped Cu2O remained in line with literature values as long as doping was kept minimal. The deposition method of such films is compatible with conductive transparent oxide for application to photo-electroreduction of CO2.