Mixed Redox-Couple-Involved\nBornite Phase Cu<sub>5</sub>FeS<sub>4</sub> as Efficient and Robust\nCocatalysts for Greatly Enhanced\nVisible-Light Photocatalytic Activities

Design and conception of a competent photocatalyst for\nwater splitting\nand photodegradation are critical for energy transformation and environmental\nremediation. Herein, for the first time, we demonstrated that mixed\nredox-couple-involved bornite phase Cu<sub>5</sub>FeS<sub>4</sub> could\nact as an efficient and robust cocatalyst for graphitic C<sub>3</sub>N<sub>4</sub> (g-C<sub>3</sub>N<sub>4</sub>), which enables the achievement\nof considerably boosted visible-light photocatalytic activities in\nboth H<sub>2</sub> evolution and pollutant degradation reactions.\nUnder visible-light irradiation, the optimized g-C<sub>3</sub>N<sub>4</sub>/Cu<sub>5</sub>FeS<sub>4</sub> heterojunction presented a\nremarkably increased H<sub>2</sub> production rate of 27.92 μmol\ng<sup>–1</sup> h<sup>–1</sup> (without involving any\nnoble metals) and methyl orange (MO) photodegradation rate of 2.18\nmin<sup>–1</sup> g<sup>–1</sup> compared with those\nof g-C<sub>3</sub>N<sub>4</sub> and Cu<sub>5</sub>FeS<sub>4</sub>.\nTo be more specific, these rates are ∼15 and 3 times higher\nthan those of pure g-C<sub>3</sub>N<sub>4</sub> in these two reactions,\nwhere the pure Cu<sub>5</sub>FeS<sub>4</sub> did not show any activities.\nThe enhanced photocatalytic performance was identified to be due to\nthe presence of a “mixed redox-couple” of Cu(I)–S–Fe(III),\nwhich enhanced charge separation efficiency between g-C<sub>3</sub>N<sub>4</sub> and Cu<sub>5</sub>FeS<sub>4</sub>, consequently facilitating\nthe overall reaction kinetics. Overall, our work not only demonstrates\nthe immense potential of using mixed redox-couple-involved bornite\nphase Cu<sub>5</sub>FeS<sub>4</sub> as the cocatalyst in photocatalysis\nbut also expedites the designing and discovering of novel photocatalytic\nsystems based on the proposed concept of the mixed redox-couple.