Rapid Flame Synthesis of Highly Efficient CuO-CuBi₂O₄ Heterojunction Photocathode for Improved Charge Separation and Light Capture Efficiency

The rapid flame annealing (FA) method has the advantages of convenience and rapidity with an instantaneous temperature rise and fall process. In this work, the influence of flame annealing duration on the front side and back side of CuBi₂O₄-based photocathodes was investigated, and photoelectrodes with variable compositions were obtained. A highly efficient CuO@CuO/CuBi₂O₄ photoelectrode was successfully obtained via a two-step FA method within a few seconds. Excellent hydrogen evolution reaction performance with a photocurrent of 2.3 mA cm^-2 @ 0.2 V_RHE in 0.1 M Na₂SO₄ (2.35 mA cm^-2 @ 0.6 V_RHE in 0.1 M KOH) electrolyte was obtained, which is the highest photocurrent of a CuBi₂O₄-based photoelectrode in a neutral electrolyte without a trapping agent so far. The light capture efficiency of the photoelectrode was greatly improved by the introduction of CuO, which formed a heterojunction with CuBi₂O₄ to promote the charge separation ability. Our work shows the universality of rapid flame annealing in the fabrication of materials, especially for temperature-sensitive compositions.