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.

Polarons generally affect adversely the photochemical and photophysical properties of transition metal oxides. However, the excited-state dynamics of polarons are not fully established to date and thus require an atomistic understanding. We focus on α-Fe₂O₃ with photoexcitation, electron injection, and heterovalent doping as the small polaron models, and conduct simulations of ab initio adiabatic molecular dynamics (AIMD) and nonadiabatic molecular dynamics (NA-MD). The elaborately designed AIMD simulations show that localization of electron at a single Fe site is an adiabatic and ultrafast process within sub-15 fs. Fe₂O₃ doping with an electron or a Si and Ti dopant forms a localized electron polaron while photoexcitation forms localized electron and hole polarons simultaneously, leading to diverse electron–hole recombination dynamics. NA-MD simulations demonstrate that recombination of an electron polaron created by doping with a delocalized hole at the valence band maximum of α-Fe₂O₃ takes place around 5 ps, while recombination between a pair of small electron and hole polarons in photoexcited Fe₂O₃ delays to about 110 ps owing to weak NA coupling and fast decoherence process. The ultrafast formation of small electron polarons in α-Fe₂O₃ impedes the accumulation of delocalized holes in the valence band that directly participate in water oxidation at photoanodes. The detrimental effect can be partially circumvented in photoexcited Fe₂O₃ for slowing electron–hole recombination despite polarons may retain low charge mobility. These findings provide a fundamental understanding of the excited-state dynamics of small electron polaron in α-Fe₂O₃ and may help design efficient transition metal oxides photoanodes.