Structural Features and HER activity of Cadmium Phosphohalides.

We have carried out a combined experimental and theoretical investigation of the structures and properties of a family of cadmium phosphochlorides with varying Cl/Cd and P/Cd ratios, Cd2 P3 Cl, Cd4 P2 Cl3 , Cd3 PCl3, and Cd7 P4 Cl6 . Their optical band gaps are in the visible region and the values are sensitive to the Cl/Cd and P/Cd ratios, leading to an increase and decrease, respectively. First-principles calculations were used to understand the bonding and electronic structures. All phosphochlorides except Cd2 P3 Cl possess direct band gaps. The calculated dielectric constants and Born effective charges illustrate the bonding, hybridization, and ionic character in these compounds. The band positions indicate the thermodynamic feasibility to perform water splitting. All systems can be used in the hydrogen evolution reaction (HER), where Cd7 P4 Cl6 has the highest activity and Cd3 PCl3 the lowest. The apparent quantum yield is highest in Cd7 P4 Cl6 (20.1 %) even without the assistance of a co-catalyst. The HER activity can be understood on the basis of photoelectrochemical measurements.