Revealing the Importance of Hole Transfer: Boosting Photocatalytic Hydrogen Evolution by Delicate Modulation of Photogenerated Holes

Abstract

Photocatalytic hydrogen evolution (PHE) has gained massive attention for the development of renewable resources. Currently, prominent and stable PHE is still restricted by the limited utilization of photocatalytic charge carriers, especially holes, of which the transfer rate is approximately two to three orders of magnitude slower than that of electrons. Although it is widely accepted that surface holes can be consumed by electron donors, the rational design of photocatalysts to speed up hole transfer and understand the ultrafast photodynamics as well as the exploration of the effect of oxidation products is still highly urgent. Herein, cadmium sulfide (CdS), as the model photocatalyst, is investigated for prominent PHE based on hole modulation strategies. To accelerate the surface reaction, H2S-saturated Na2S&Na2SO3 solution, which outperforms traditional hole scavengers, is selected to remove the holes accumulating on the photocatalyst surface, while the oxidative cocatalyst palladium sulfide (PdS) is simultaneously impregnated on CdS to optimize the hole transfer process. Evidenced by femtosecond transient absorption spectroscopy (TAS) and band structure analysis, the ultrafast process (1–5 ps) highly related with hole transfer from CdS to PdS is proved in the CdS/PdS composite. More importantly, density functional theory (DFT) calculation suggests that the hole accumulation site (PdS) is also the oxidation active site where the active species (HS*) adsorbs on. Finally, the oxidation product composition is analyzed through Fourier transform infrared (FTIR) spectroscopy, which indicates that the colorless and value-added product S2O32– protects the photocatalyst from the light shielding effect. Contributed by factors mentioned above, improved PHE efficiency (∼145.9 mmol·g–1·h–1) is achieved on the CdS/PdS composite.