Excess oxygen in delafossite CuFeO2+δ: Synthesis, characterization, and applications in solar energy conversion

As an alternative to high-temperature oxidation, the present study demonstrates a new approach to the intercalation of excess O into 3R-delafossite CuFeO2, based on a non-equilibrium hydrothermal synthesis. At lower temperatures, or during the initial stage of this hydrothermal reaction, a large amount of excess O remains between the self-assembled block layers, owing to the incomplete reduction of propionaldehyde in the hydrothermal process. This work not only confirmed the presence of this excess O and quantified the excess O concentrations, but also established the mechanism by which non-stoichiometric CuFeO2+delta is generated, on the basis of experimental observations and theoretical calculations. Normally, highly crystalline CuFeO2+delta prepared using the conventional hydrothermal process only contains low levels of excess O, but the present work shows that this problem can be mitigated by employing a microwave-assisted hydrothermal reaction. This technique can rapidly synthesize high quality 3R-delafossite CuFeO2+delta containing elevated concentrations of excess O. Electronic structures calculated based on density functional theory showed that CuFeO2+delta should exhibit half-metallic ferromagnetism and multiple band gaps. Systematic experimental characterization and theoretical calculations were used to elucidate the manner in which excess O significantly affects the microstructure of this material, while enhancing the electrical, optical and electrochemical properties of CuFeO2. As a result of these enhancements, this material is suitable for the improvement of solar energy conversion. Photoelectrochemical trials and data from the photocatalytic performances confirm these properties, suggesting that non-stoichiometric 3R-delafossite CuFeO2+delta compounds have potential applications in solar energy conversion.