Mn²⁺ Doping in In₂S₃ Increases Photoanodic Activity for Photoelectrochemical Water-Splitting

The photoelectrochemical (PEC) water-splitting process emerges as a significant pathway for solving global energy demands for hydrogen (H₂) generation. Here, a visible light-active semiconductor, indium sulfide (In₂S₃), is explored. The interconnected nanosheets of In₂S₃ can generate a photocurrent density of 3.18 mA/cm² at 1.2 V versus RHE in Na₂SO₄. To improve the PEC activity of In₂S₃, Mn²⁺ is incorporated into In₂S₃ through an in-situ hydrothermal method. Mn-In₂S₃ exhibits a photocurrent density of 4.49 mA/cm² at 1.2 V versus RHE. It demonstrates a separation efficiency of 51.2%, 1.58 times higher than In₂S₃ (32.4%) at 1.2 V versus RHE. Mn-doping facilitates the charge transportation, keeping excitons apart, suppressing their recombination. Mott-Schottky analysis indicates 3.23 times carrier density enhancement in Mn-In₂S₃. For practical applicability, the PEC performance of In₂S₃ and Mn-In₂S₃ is also determined in 3.5 wt% saline water. The Mn-In₂S₃ shows the photocurrent density of 5.23 mA/cm² at 1.2 V versus RHE, ∼1.5 fold higher than the bare In₂S₃ (3.40 mA/cm²). Doping with Mn²⁺ effectively alters the band edge positions of In₂S₃, further supported by ultraviolet photoelectron spectroscopy (UPS) analysis, which indicates upward and downward shifting of the valence and conduction bands, respectively, resulting in narrowing of the band gap.