Hydrothermal synthesis of a doped Mn-Cd-S solid solution as a visible-light-driven photocatalyst for H2 evolution.

The effect of metal doping (i.e., with Cr, Fe, Ni, Cu, Zn, Ag and Sn) on the crystal structure of hydrothermally synthesized Mn(1-x)Cd(x) S (where x≈0.1) is studied with the aim of enhancing photocatalytic activity. In contrast to the low-crystalline, undoped solid solution Mn(1-x)Cd(x)S, Ni doping yields a well-crystallized wurtzite-type Mn-Cd-S solid solution, which precipitates as planar hexagonal facets of several hundred nanometers in size, together with much larger grains of α-MnS (>10 µm). By removing inactive α-MnS through sedimentation, a single phase with composition of Ni(0.01)Mn(0.56)Cd(0.43)S is obtained successfully. The Ni doping achieved a threefold higher photocatalytic activity for H(2) evolution from a 0.1 M Na(2)S/0.5 M Na(2)SO(3) solution under visible-light irradiation (λ≥420 nm). The apparent quantum yield of 1 wt % Pt-loaded Ni(0.01)Mn(0.56)Cd(0.43)S measured at λ=420 nm reached 25 %. The enhanced photocatalytic activity is most likely the result of a decreased concentration of defects, responsible for electron-hole recombination, in the active solid-solution phase and a slightly higher bandgap energy (2.4 eV).