Deciphering the Space Charge Effect of the p–n Junction between Copper Sulfides and Molybdenum Selenides for Efficient Water Electrolysis in a Wide pH Range

Space charge transfer is crucial for an efficient electrocatalytic process, especially for narrow-band-gap metal sulfides/selenides. Herein, we designed and synthesized a core-shell structure which is an ultrathin MoSe2 nanosheet coated CuS hollow nanoboxes (CuS@MoSe2) to form an open p-n junction structure. The space charge effect in the p-n junction region will greatly improve electron mass transfer and conduction, and also have abundant active interfaces. It was used as a bifunctional electrocatalyst for water oxidation at a wide pH range. It exhibits a low overpotential of 49 mV for the HER and 236 mV for the OER at a current density of 10 mA·cm-2 in acidic pH, 72 mV for the HER and 219 mV at 10 mA·cm-2 for the OER in alkaline pH, and 62 mV for the HER and 230 mV at 10 mA·cm-2 for the OER under neutral conditions. The experimental results and density functional theory calculations testify that the p-n junction in CuS@MoSe2 designed and synthesized has a strong space charge region with a synergistic effect. The built-in field can boost the electron transport during the electrocatalytic process and can stabilize the charged active center of the p-n junction. This will be beneficial to improve the electrocatalytic performance. This work provides the understanding of semiconductor heterojunction applications and regulating the electronic structure of active sites.