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

Photocatalytic hydrogen evolution(PHE) has gained massive attentionfor the development of renewable resources. Currently, prominent andstable PHE is still restricted by the limited utilization of photocatalyticcharge carriers, especially holes, of which the transfer rate is approximatelytwo to three orders of magnitude slower than that of electrons. Althoughit is widely accepted that surface holes can be consumed by electrondonors, the rational design of photocatalysts to speed up hole transferand understand the ultrafast photodynamics as well as the explorationof the effect of oxidation products is still highly urgent. Herein,cadmium sulfide (CdS), as the model photocatalyst, is investigatedfor prominent PHE based on hole modulation strategies. To acceleratethe surface reaction, H2S-saturated Na2S&Na2SO3 solution, which outperforms traditional holescavengers, is selected to remove the holes accumulating on the photocatalystsurface, while the oxidative cocatalyst palladium sulfide (PdS) issimultaneously impregnated on CdS to optimize the hole transfer process.Evidenced by femtosecond transient absorption spectroscopy (TAS) andband structure analysis, the ultrafast process (1-5 ps) highlyrelated with hole transfer from CdS to PdS is proved in the CdS/PdScomposite. More importantly, density functional theory (DFT) calculationsuggests that the hole accumulation site (PdS) is also the oxidationactive site where the active species (HS*) adsorbs on. Finally, theoxidation product composition is analyzed through Fourier transforminfrared (FTIR) spectroscopy, which indicates that the colorless andvalue-added product S2O3 (2-) protects the photocatalyst from the light shielding effect. Contributedby factors mentioned above, improved PHE efficiency (similar to 145.9mmol center dot g(-1)center dot h(-1)) is achievedon the CdS/PdS composite.