Photochemical deposition of Pt on CdS for H₂ evolution from water: Markedly enhanced activity by controlling Pt reduction environment

Pt is one of the most effective cocatalysts used in hydrogen evolution photocatalysts in water splitting. However, because Pt is an expensive noble metal, the efficient and optimum use of Pt is of high importance. In this work, the effect of loading conditions for Pt nanoparticles on the surface of CdS as a model photoabsorber was studied to reveal the important parameters for obtaining Pt cocatalyst of high activity. The activities of as-prepared Pt/CdS samples were evaluated in H₂ evolution from photocatalytic water splitting using Na₂S and Na₂SO₃ as sacrificial reagents. It was found that the solution environment during Pt photoreduction played a critical role. Pt/CdS photocatalyst with Pt reduced in NaOH alkaline solution exhibited a markedly higher H₂ evolution activity (∼1300 μmol h^-1) than samples obtained in acidic or neutral solution (<50 μmol h^-1). Further materials characterization revealed that, in alkaline solution, Pt⁴⁺ species in the precursor (H ₂PtCl₆·xH₂O) can be well reduced to metallic Pt⁰, which shows high hydrogen evolution activity. On the other hand, in acidic or neutral solution, Pt⁴⁺ species can be only partially reduced to Pt²⁺ species, which are deposited on the surface of CdS. It is suggested that the promotional effect of alkaline conditions on Pt reduction is attributable to enhanced hydrolysis of PtCl₆ ^2- to species containing fewer Cl^- ions, such as Pt(OH)₅Cl^2- and Pt(OH)₆^2-, which are then more easily reduced.