Structure and Properties of Cd₄P₂Cl₃, an Analogue of CdS

Substitution of the sulfide ions in CdS by aliovalent P^3- and Cl^- ions is known to markedly affect the electronic structure and properties, reducing the band gap of the semiconductor. The decrease in band gap arises because the P (3p) states occupy the top of the valence band while the Cl (3p) orbitals lie deep down in energy. Progressive substitution of S by equal proportions of P and Cl should result in CdP_0.5Cl_0.5 or Cd₂PCl. We have investigated the electronic structure of this compound and carried out first-principles calculations to understand the electronic structure and properties. Interestingly, Cd₄P₂Cl₃ is a semiconductor with a band gap of 2.36 eV comparable to that of CdS, and it exhibits a photoluminescence band at 580 nm similar to CdS. Its conduction band and valence band edges are appropriately placed with respect to the water redox potentials, for it to exhibit excellent hydrogen evolution by photochemical water splitting. Visible-light induced hydrogen evolution rate of 1007(±23) and 54(±4) μmol h^-1 g^-1 has been obtained in the presence and absence of sacrificial agent. Hydrogen evolution in the absence of any sacrificial agent and the absence of photocorrosion seems to be unique features of Cd₄P₂Cl₃.