Highly efficient twinned Mn_xCd_1-xS homojunction photocatalyst modified by noble metal-free Ni₁₂P₅ for H₂ evolution under visible light

A series of twin-phase Mn_xCd_1-xS solid solutions with zinc blende (ZB) and wurtzite (WZ) structures were prepared by an alkaline hydrothermal method in this work. When using 0.35 M Na₂SO₃/0.25 M Na₂S solution as a sacrificial agent under visible light (λ ≥ 420 nm), the H₂ production activity of T-Mn_0.5Cd_0.5S is 33.4 mmol h⁻¹ g⁻¹, significantly higher than that of ZB-Mn_0.5Cd_0.5S (15.4 mmol h⁻¹ g⁻¹) or WZ-Mn_0.5Cd_0.5S (18.2 mmol h⁻¹ g⁻¹). The photocatalytic activity of T-Mn_0.5Cd_0.5S is further enhanced by introducing Ni₁₂P₅ as a cocatalyst, in which the H₂ evolution rate of 0.2% Ni₁₂P₅/T-Mn_0.5Cd_0.5S can reach 96.5 mmol h⁻¹ g⁻¹, 2.89 and 1.77 times than that of pure T-Mn_0.5Cd_0.5S and 0.5% Pt/T-Mn_0.5Cd_0.5S (54.6 mmol h⁻¹ g⁻¹), respectively. The crystalline phase, microstructure, elemental composition, optical and optoelectronic properties of the samples were characterized by XRD, SEM, TEM, HRTEM, XPS, UV–vis, PL and electrochemical workstation. The results show that the formation of a homojunction with interlaced energy band inhibits carrier recombination in the bulk phase of T-Mn_0.5Cd_0.5S. In addition, Ni₁₂P₅ can substitute for the noble metal as a cocatalyst to reduce the overpotential and improve the separation rate of electrons and holes to enhance H₂ evolution. This work provides reference for building homojunction photocatalysts and loading inexpensive transition metal phosphides instead of noble metals.