Understanding divergent behaviors in the photocatalytic hydrogen evolution reaction on CdS and ZnS: a DFT based study

Abstract

It has been a long time that divergent behaviors were observed in many photocatalytic hydrogen evolution reactions (HER) on CdS and ZnS although the two photocatalysts have similar compositions and structures. For example, CdS itself is inactive and loading of cocatalysts is indispensable to achieve high efficiency of hydrogen evolution, but the reverse is true for ZnS. The underlying reasons are still unclear to date. The Volmer reaction of HER on catalysts is H⁺ + e⁻ + * → H*, and its free energy (ΔG_H* = ΔE_H* + ΔE_ZPE − TΔS + eU; the adsorption energy, zero-point energy, entropy and potential energy are on the right side) is a good theoretical descriptor of the electrocatalytic HER activity from the electrocatalytic HER theory. In this paper, we firstly determined the most stable CdS and ZnS(110) termination under the conditions of photocatalytic HER, i.e., pure (110), by calculating the free energies of three reactions related to H₂O dissociation on (110). Then we rationalized these behaviors by calculating the free energy of H* adsorption on pure and Pt loaded CdS and ZnS(110) at different pH. The performance of photocatalytic HER on CdS and ZnS was found to be determined jointly by the free energy of H* adsorption and the conduction band minimum (CBM) of the photocatalysts. On pure (110) with large ΔG_H*, the photocatalytic HER is favored on ZnS due to its higher CBM; on Pt loaded (110) with small ΔG_H*, the photocatalytic HER is favored on CdS due to its lower CBM. These results well explained the divergent behaviors observed in the photocatalytic HER on CdS and ZnS.