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, Cd 2 P 3 Cl, Cd 4 P 2 Cl 3 , Cd 3 PCl 3, and Cd 7 P 4 Cl 6 . 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 Cd 2 P 3 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 Cd 7 P 4 Cl 6 has the highest activity and Cd 3 PCl 3 the lowest. The apparent quantum yield is highest in Cd 7 P 4 Cl 6 (20.1 %) even without the assistance of a co‐catalyst. The HER activity can be understood on the basis of photoelectrochemical measurements.