Rapid\nFlame Synthesis\nof Highly Efficient CuO-CuBi<sub>2</sub>O<sub>4</sub> Heterojunction\nPhotocathode for Improved Charge\nSeparation and Light Capture Efficiency

The rapid flame annealing (FA) method\nhas the advantages\nof convenience\nand rapidity with an instantaneous temperature rise and fall process.\nIn this work, the influence of flame annealing duration on the front\nside and back side of CuBi<sub>2</sub>O<sub>4</sub>-based photocathodes\nwas investigated, and photoelectrodes with variable compositions were\nobtained. A highly efficient CuO@CuO/CuBi<sub>2</sub>O<sub>4</sub> photoelectrode was successfully obtained via a two-step FA method\nwithin a few seconds. Excellent hydrogen evolution reaction performance\nwith a photocurrent of 2.3 mA cm<sup>–2</sup> @ 0.2 V<sub>RHE</sub> in 0.1 M Na<sub>2</sub>SO<sub>4</sub> (2.35 mA cm<sup>–2</sup> @ 0.6 V<sub>RHE</sub> in 0.1 M KOH) electrolyte was obtained, which\nis the highest photocurrent of a CuBi<sub>2</sub>O<sub>4</sub>-based\nphotoelectrode in a neutral electrolyte without a trapping agent so\nfar. The light capture efficiency of the photoelectrode was greatly\nimproved by the introduction of CuO, which formed a heterojunction\nwith CuBi<sub>2</sub>O<sub>4</sub> to promote the charge separation\nability. Our work shows the universality of rapid flame annealing\nin the fabrication of materials, especially for temperature-sensitive\ncompositions.