Synthetic mechanism discovery of monophase cuprous oxide for record high photoelectrochemical conversion of CO₂ to methanol in water

Precise control of the oxidation state of transition-metal oxides, such as copper, is important for high selectivity of CO₂ reduction in an aqueous condition to compete with the reduction of water. The phase of copper oxide nanofibers was controlled by predictive synthesis, which controls the nanoscale gas−solid reaction by considering thermodynamics and kinetics. The driving force of the phase transformation between the different oxidation states of copper oxide is calculated by comparing the Gibbs free energy of each of the oxidation states. From the calculation, the kinetically processable window for the fabrication of Cu₂O in which monophase Cu₂O can be fabricated in a reasonable reaction time scale is discovered. Herein, we report the monophase Cu₂O nanofiber photocathode, which photoelectrochemically converted CO₂ into methanol with over 90% selectivity in an aqueous electrolyte, and a hierarchical structure is developed to optimize the photoactivity and stability of the electrode. Our work suggests a rational design of the calcination strategy for precisely controlling the oxidation states of transition metals that can be applied to various applications in which the phase of the materials plays an important role.